tag:blogger.com,1999:blog-32888709210478330842024-02-21T11:32:48.310-05:00Condensed Matter Journal Club at the University of TorontoThe purpose of this journal club is to create the opportunity to semi-informally discuss the large body of literature that as a combined group we read. For each week the idea would be to choose one leader who would thoroughly read a paper of general interest and present it to the group as a discussion piece. We hope this will be a safe environment (nominally no faculty allowed!) where all questions are welcome and treated equally...and answered by the group as well as is possible.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.comBlogger33125tag:blogger.com,1999:blog-3288870921047833084.post-49107884076531354862012-09-17T20:57:00.003-05:002012-09-19T10:20:08.044-05:00New season<div dir="ltr" style="text-align: left;" trbidi="on">
The condensed matter journal club kicked off the new season with a talk by Luke titled<i> Electronic correlations in iron superconductors: insight from optical spectroscopy.</i> Stay tuned for a summary of Luke's talk.<br />
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Igor has also started an email <a href="https://listserv.physics.utoronto.ca/mailman/listinfo/cmjc" target="_blank">listserv</a> which will be used to post updates. Subscribe to keep track of events.<br />
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An organization meeting was held last week where we came up with a list of topics and a schedule.<br />
Join the listserv to get a link to the schedule. Feel free to pick a topic (or add one you're interested in) and choose a date for your talk!<br />
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Vijay Shankarhttp://www.blogger.com/profile/17358403212781935899noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-70698237165805545912011-06-13T15:24:00.010-05:002011-06-13T15:41:07.855-05:00Blogon 1: Schwinger Bosons<a href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQB9u_IP6jKoVYU8dapcxrGO7ayasXifjiaYkX6iEJbi_fCnJWTGB2mwNsdpf9nNkLyUxC3HetKidzQUmFj-KR8x_GowU_H4t3pXYh0rtjx6sYrsk0TQfAFEAPuSShE1xj2FHwz_6K73M/s1600/IMG_0332.JPG" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img style="float:right; margin:0 0 10px 10px;cursor:pointer; cursor:hand;width: 320px; height: 240px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhQB9u_IP6jKoVYU8dapcxrGO7ayasXifjiaYkX6iEJbi_fCnJWTGB2mwNsdpf9nNkLyUxC3HetKidzQUmFj-KR8x_GowU_H4t3pXYh0rtjx6sYrsk0TQfAFEAPuSShE1xj2FHwz_6K73M/s320/IMG_0332.JPG" border="0" alt="" id="BLOGGER_PHOTO_ID_5617805254887333938" /></a><br /><div>Opening talk by Tyler Dodds on June 9th, 2011.<div><br /></div><div>Tyler presented a pedagogical talk on Schwinger Boson's with regards to Heisenberg models.<span><span></span></span> Representing spin in terms of boson operators, together with a constraint, was shown to naturally lead to a gapped Z2 spin liquid, or to a magnetically ordered state if the bosons condense.</div></div>Matthew Killihttp://www.blogger.com/profile/06802456060789594135noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-5400498974012877032010-09-07T14:23:00.004-05:002010-09-07T14:40:20.288-05:00Last meeting of the Summer: Bosonization II<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgpe3kFXxWOzPBjtdO4QzJ3xA1lyllX7AdE_nz_RTEbCvf08l5l1U4f5-DWmfsdtJbhAbgXW-CGrpXivnYXhyW7J4tHbC9Z2IiN-NuJY15lQ58MOPG_YijpjWsR06TdfUiuDMGIMdzOvRE/s1600/shunsuke.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgpe3kFXxWOzPBjtdO4QzJ3xA1lyllX7AdE_nz_RTEbCvf08l5l1U4f5-DWmfsdtJbhAbgXW-CGrpXivnYXhyW7J4tHbC9Z2IiN-NuJY15lQ58MOPG_YijpjWsR06TdfUiuDMGIMdzOvRE/s200/shunsuke.jpg" alt="" id="BLOGGER_PHOTO_ID_5514258743135576882" border="0" /></a><br />Journal club talk by Shunsuke Furukawa, continued from Part I.<br /><br />Furukawa-san discussed 1d spin chains, in particular, the XXZ model. Using the Jordan-Wigner transformation, the spin model was mapped to a system of interacting fermions. The interacting fermions were, in turn, mapped onto a bosonic field theory. Wrapping up, Furukawa-san discussed his own work in mapping out the phase diagram of the XXZ model.<br /><br />-----------------------------------------------------------------------<br /><br />The journal club is adjourned until the summer of 2011, barring exceptionally interesting developments/papers that members might want to discuss.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-68823264669165748332010-08-26T14:04:00.000-05:002010-09-07T14:23:43.677-05:00Of mean-fields and convergencesJournal club talk by guest speaker Ioannis Anapolitanos<br />from the Department of Mathematics, UofT, on 26 Aug, 2010.<br /><br />Reference: http://arxiv.org/abs/0904.4514<br /><br />We had a lively session of condensed matter mathematics, with Ioannis sketching the status of mean-field theories in mathematical physics. He introduced Hartree mean-field theory for bosonic systems systematically, drawing an analogy to centre of mass motion in classical mechanics.<br /><br />He then discussed the convergence of expectation values of general p-particle observables in Hartree mean field theory. At the end, he briefly outlined his own work which has improved the bound.<br /><br />There were interesting discussions about fermionic systems and mixed density matrices.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-44878897048245826722010-08-19T00:47:00.001-05:002010-09-07T14:10:53.897-05:00Introduction to Bosonization<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgH9n3xb6rQod_Kz1LHaAfGKs_5O8VmUtfVbuyBombNsatUYUgb-Ef80gOnm6BwyShe0wOboqBRSnpoW6XNVZP1A3zifQd8U0uDnzyTyd4DaD3cEGVSLAJIeHX1KQ0kbHiCLJMUk8skyfg/s1600/Shunsuke.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 146px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgH9n3xb6rQod_Kz1LHaAfGKs_5O8VmUtfVbuyBombNsatUYUgb-Ef80gOnm6BwyShe0wOboqBRSnpoW6XNVZP1A3zifQd8U0uDnzyTyd4DaD3cEGVSLAJIeHX1KQ0kbHiCLJMUk8skyfg/s200/Shunsuke.jpg" alt="" id="BLOGGER_PHOTO_ID_5508850224546827090" border="0" /></a>Journal club talk by Shunsuke Furukawa on August 19, 2010<br /><br />References:<br />D. Senechal, cond-mat/9908262<br />F. D. M. Haldane, Phys. Rev. B 25, 4925 (1982).<br />K. Nomura and K. Okamoto, J. Phys. A 27, 5773 (1994)<br /><br />Furukawa-san first introduced the idea of bosonization, broadly discussing notions of effective field theory and Luttinger parameter. He then demonstrated the correspondence between a free-fermion-model and a 1d bosonic field theory.<br />Coming up in Part II: Applications to interacting fermions and spin chains.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-68653115641446625292010-08-12T00:35:00.000-05:002010-08-26T10:38:18.240-05:00Nematicity in the Pseudogap Phase of the Cuprates<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgredS0t5KW_6LmxLXzjFNARW7GRL_OsdXCffxiF-HAvxsOBzpCe5Vwth5InSRx00RYXbVyWN9e4AjaxjjC5GpLDk3MrbahyHgUqbuk_VTxznbRzvCsHWcQv5CujBRMeFWg4llVCNaK0mc/s1600/christoph.jpg"><img style="float: right; margin: 0pt 0pt 10px 10px; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgredS0t5KW_6LmxLXzjFNARW7GRL_OsdXCffxiF-HAvxsOBzpCe5Vwth5InSRx00RYXbVyWN9e4AjaxjjC5GpLDk3MrbahyHgUqbuk_VTxznbRzvCsHWcQv5CujBRMeFWg4llVCNaK0mc/s200/christoph.jpg" alt="" id="BLOGGER_PHOTO_ID_5508846344435146978" border="0" /></a><br />Journal club talk by Christoph M. Puetter on August 12, 2010<br /><br />Reference:<br />M. Lawler et al., Nature 466, 347, (2010)<br />(http://www.nature.com/nature/journal/v466/n7304/full/nature09169.html)<br /><br />Christoph chose this paper, written by the founder of this Journal Club, for an interesting discussion this week. There were broad discussions on the Cuprate phase diagram, nematicity and STM measurements. A lively debate ensued about the key finding of the paper that the nematic order tracks the pseudogap.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-40231740246801492192010-08-05T00:27:00.000-05:002010-08-26T10:40:13.269-05:00Mechanics of energy transfer in light-harvesting antenna proteins: scale-dependent descriptionsJournal club talk by guest speaker Hoda-Hossein Nejad from the Department of Chemistry, UofT, on August 5, 2010<br /><br />Abstract:<br />Recent observations of persistent coherence in light-harvesting<br />antenna proteins pose some interesting questions regarding the role of<br />quantum effects in photosynthesis: Has nature chosen quantum mechanics as<br />an strategy for survival? How important are the quantum effects, and on<br />what scale are these effects significant?<br /><br />It is plausible to argue that a fully quantum treatment captures the<br />dynamics of energy transfer on the scale of a few chromophores, but is<br />both unnecessary and unrealistic for descriptions of long-range energy<br />transfer between multiple proteins. In this talk,, I will present two<br />different formulations of the exciton transport problem as applied to the<br />photosynthetic light-harvesting complex PE545: 1) A fully quantum<br />description on the scale of a single protein, which takes into account the<br />interference between pathways leading to a given site. 2) A mixed quantum<br />classical treatment on the scale of multiple proteins, in which energy<br />transfer is regarded as a stochastic hop between delocalized eigenstates.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-79647381967186357112010-07-29T00:21:00.000-05:002010-08-26T10:40:28.112-05:00Inverse Spin Galvanic Effect in the Interface between a Topological Insulator and a Ferromagnet<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiVcdEEtOvaDS5k7hyphenhyphenGgUiVPwuheicKYvs2jS8msVgf3ZrtnaqfSpOfOAToMOQMr2KQVfj0fGzXk-5wZ2XINb5Dp79E_leYf5cfXhf8xL2DL1vuJQW6Jksrw7P2rDhDV2n8v7vNH33OPgY/s1600/will.jpeg"><img style="float: right; margin: 0pt 0pt 10px 10px; cursor: pointer; width: 200px; height: 154px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiVcdEEtOvaDS5k7hyphenhyphenGgUiVPwuheicKYvs2jS8msVgf3ZrtnaqfSpOfOAToMOQMr2KQVfj0fGzXk-5wZ2XINb5Dp79E_leYf5cfXhf8xL2DL1vuJQW6Jksrw7P2rDhDV2n8v7vNH33OPgY/s200/will.jpeg" alt="" id="BLOGGER_PHOTO_ID_5508843745275584354" border="0" /></a><br />Journal club talk by William Witczak-Krempa on July 29, 2010<br /><br />References:<br />I. Garate, M. Franz, Phys. Rev. Lett. 104, 146802 (2010)<br /> <a href="http://prl.aps.org/abstract/PRL/v104/i14/e146802" target="_blank">http://prl.aps.org/abstract/<wbr>PRL/v104/i14/e146802</a><br /> arXiv: <a href="http://arxiv.org/abs/0911.0106" target="_blank">http://arxiv.org/abs/0911.0106</a><br /><br />William went over a recent PRL paper by I. Garate and M. Franz [1], in<br />which they explore the behaviour of a thin ferromagnetic layer coated on the surface of a 3d topological insulator. They suggest the<br />possibility of dissipationless current-induced magnetization reversal in the ferromagnet, which might have applications in the field of spintronics.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-32283037272496043582010-07-22T00:13:00.000-05:002010-08-26T10:40:44.468-05:00All you wanted to know about photonic crystals but were afraid to ask<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmvEV4fTdDL35fI74T6tBirRi6FjLWmODuiPzw0Ckrxmr-DNa3kTNeS2QcZ8km2MaiLHKs0Pp205Vdwb6z_628HzYuQYC7nVzXx71nDbjD2rKKORwSNB3yqHKXrRKnIRGczrxc4UTFJY4/s1600/wahtung.jpg"><img style="float: right; margin: 0pt 0pt 10px 10px; cursor: pointer; width: 198px; height: 200px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjmvEV4fTdDL35fI74T6tBirRi6FjLWmODuiPzw0Ckrxmr-DNa3kTNeS2QcZ8km2MaiLHKs0Pp205Vdwb6z_628HzYuQYC7nVzXx71nDbjD2rKKORwSNB3yqHKXrRKnIRGczrxc4UTFJY4/s200/wahtung.jpg" alt="" id="BLOGGER_PHOTO_ID_5508842074529094514" border="0" /></a><br />Journal club talk by Wah-Tung Lau on July 22, 2010<br /><br />The slides can be found <a href="http://www.physics.utoronto.ca/%7Egramach/WTLCMJCtalk.pdf">here</a>.<br /><br />References:<br />[1] S. John, Phys. Rev. Lett. 58 2486 (1987).<br />[2] S. John, "Localization of Light", Physics Today 44 32 (May 1991).<br />[3] S. Fan et. al., Phys. Rev. Lett. 78 3294 (1997).<br />[4] S. Johnson et. al., Optic Express 9 748 (2001).<br />[5] W. T. Lau et. al., Appl. Phys. Lett. 92 103106 (2008).<br />[6] Experiments: Prof. S. G. Johnson's slides at<br /><a class="fixed" href="http://ab-initio.mit.edu/photons/tutorial/L3-fab.pdf" target="_blank">http://ab-initio.mit.edu/photons/tutorial/L3-fab.pdf</a>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-1729334289634577052010-07-20T15:58:00.003-05:002010-07-20T16:11:34.054-05:00Spin Liquid state of the S=1/2 Heisenberg model on the anisotropic triangular lattice<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYxw12SRODBVCOw_NBf3ytcZfzfHzWVJaI3ZZT9_kjT4pv70IoVjpV_JdXKEPt0DhJiU2GC3nuVsPEdBxbHYzvl7Z3Pr4eVDw19R_U1WwrwwF6JgaZjsNFpujTK4fhm35qagt0Kwqc6Jo/s1600/dariush.JPG"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhYxw12SRODBVCOw_NBf3ytcZfzfHzWVJaI3ZZT9_kjT4pv70IoVjpV_JdXKEPt0DhJiU2GC3nuVsPEdBxbHYzvl7Z3Pr4eVDw19R_U1WwrwwF6JgaZjsNFpujTK4fhm35qagt0Kwqc6Jo/s200/dariush.JPG" alt="" id="BLOGGER_PHOTO_ID_5496099061945886514" border="0" /></a><br />Journal Club talk by Dariush Heidarian on July 8, 2010<br />(click <a href="http://www.physics.utoronto.ca/%7Egramach/dariush_CMJC_triang_Heisenberg.pdf">here</a> for slides in pdf format)<br /><br />Summary by Vijay Shankar Venkataraman<br /><br />References:<br /><br />Theoretical Papers:<br />1) D. Heidarian, S. Sorella, and F. Becca, Phys. Rev. B 80, 012404 (2009)<br />2) T. Pardini and R. R. P. Singh, Phys. Rev. B 77, 214433 (2008)<br />3) O. A. Starykh and L. Balents, Phys. Rev. Lett. 98, 077205 (2007)<br />4) S. Yunoki and S. Sorella, Phys. Rev. B 74, 014408 (2006)<br />5) Zheng Weihong, R. H. McKenzie, and R. P. Singh, Phys. Rev. B 59, 14367 (1999)<br />6) M. Bocquet, F. H. L. Essler, A. M. Tsvelik, and A. O. Gogolin, Phys. Rev. B 64, 094425 (2001).<br /><br />Experimental Papers:<br />1) R. Coldea, D. A. Tennant, A. M. Tsvelik, and Z. Tylczynski, Phys. Rev. Lett. 86, 1335 (2001)<br />2) Y. Shimizu, K. Miyagawa, K. Kanoda, M. Maesato, and G. Saito, Phys. Rev. Lett. 91, 107001 (2003).<br /><br />The existence of spin-liquids is still mired in controversy although there are various promising candidates. Prominent among these is a quasi-two-dimensional class of organic compounds comprised of dimers of an organic molecule. A single electron is localized on each dimer and the compound forms layers of triangular lattices.<br /><br />Dariush began with a quick review of the experimental results of Shimizu et al where these materials show no sign of long range order down to very low temperatures. He then introduced the anisotropic Heisenberg model on the triangular lattice with nearest-neighbour interactions but with exchange anisotropy. This led to a vigorous discussion on the possible types (Ferromagnetic/Antiferromagnetic) and strengths of coupling constants that would cause frustration. After the audience reached a consensus, Dariush gave a brief survey of previous theoretical work on the model.<br /><br />Dariush moved on to discussing his own work studying the Hamiltonian by using a variational wavefunction approach. After discussing different methods of doing a variational calculation, he showed us the different variational wavefunctions that he used in his work. Dariush then presented his results for the ground state energy and compared it with previous work.<br /><br />Discussing results for the spin-spin correlation function as a function of anisotropy for different lattice sizes, he showed how the structure factor scales as a function of lattice size for different anisotropies. Dariush then showed us the phase diagram as a function of anisotropy which told us that the variational calculation points towards the existence of a stable spin-liquid state in the anisotropic triangular lattice.<br /><br />Time had run out by this point and an excursion into the technicalities of the variational Monte Carlo calculation was reserved for another journal club meeting.Unknownnoreply@blogger.com1tag:blogger.com,1999:blog-3288870921047833084.post-87158970735783558472010-07-15T00:09:00.000-05:002010-08-26T11:01:29.043-05:00Imaging the Fano lattice to hidden order transition in URu_2Si_2<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj-Q2eHovvxl_6UEh4C2sTKIf9x7SPC8nY12ABE3UY5cEHhjZq-FXf5-ks-NRrcpInwEs0o6Htj6gKU7euD3JZDTOhyZO9EenCKMtZfoc-9RefAE0l1O1oZiBZoUSsLxXoO0IkNa6mvQDI/s1600/fazel.jpg"><img style="float: right; margin: 0pt 0pt 10px 10px; cursor: pointer; width: 200px; height: 160px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEj-Q2eHovvxl_6UEh4C2sTKIf9x7SPC8nY12ABE3UY5cEHhjZq-FXf5-ks-NRrcpInwEs0o6Htj6gKU7euD3JZDTOhyZO9EenCKMtZfoc-9RefAE0l1O1oZiBZoUSsLxXoO0IkNa6mvQDI/s200/fazel.jpg" alt="" id="BLOGGER_PHOTO_ID_5508839949883959042" border="0" /></a><br />Journal club talk by Fazel Fallah Tafti on July 15, 2010<br /><br />Reference:<br />Nature, 465, 570Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-58857837610254404432010-06-17T00:03:00.000-05:002010-08-26T10:49:46.174-05:00Quantum spin Hall effect: Dirac particle approach<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvJIBHkuYvbKMSFjran8b9gmLR-Lw9GwiAKbfgkBTtlSncJdHGbMVT6AAFbcGpXsum0qrtgy4eXu5nM_jRKvGY4wj5sBzfb2rsX99vPrMMG2q6m9Vyn4WO9gWPvRE84cHvOKiciozScmg/s1600/bohmjung.jpg"><img style="float: right; margin: 0pt 0pt 10px 10px; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhvJIBHkuYvbKMSFjran8b9gmLR-Lw9GwiAKbfgkBTtlSncJdHGbMVT6AAFbcGpXsum0qrtgy4eXu5nM_jRKvGY4wj5sBzfb2rsX99vPrMMG2q6m9Vyn4WO9gWPvRE84cHvOKiciozScmg/s200/bohmjung.jpg" alt="" id="BLOGGER_PHOTO_ID_5508838876372109346" border="0" /></a><br />Journal club talk by Bohm-Jung Yang on June 17, 2010<br />(Slides can be found <a href="http://www.physics.utoronto.ca/%7Egramach/BJCMJCtalk.pdf">here</a>)<br /><br />References:<br />(1) "The anomalous hall effect and magnetic monopoles in momentum<br />space", Z. Fang et al., Science, 302, 92 (2003)<br />(2) "Quantum spin hall effect and topological phase transition in HgTe<br />Quantum wells", B. Andrei Bernevig et al., Science, 314, 1757 (2006)<br />(3) "Topological quantization of the spin Hall effect in two-dimensional<br />paramagnetic semiconductors", X. -L. Qi et al, PRB, 74, 085308 (2006)<br />(4) "Topological insulators", M. Z. Hasan et al., Arxiv: 1002.3895Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-46333096136748233482010-06-03T19:14:00.000-05:002010-08-26T11:00:57.515-05:00Charge Density Waves and Superconductivity in 2H-NbSe_2<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjCY9lIOuoqFqN54etGKuZV06sGUgd8l2Zy1IVEROXbCcxg3FPILFdiBSM6KVMrN4xPLtPcNoxEhyphenhyphenScZLPbUrPezLd0c_q59iHuw8yTRjtGUGxZ9Snq2HA1aMjtFGhaVyscSuZGta4yDq8/s1600/igor4.JPG"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjCY9lIOuoqFqN54etGKuZV06sGUgd8l2Zy1IVEROXbCcxg3FPILFdiBSM6KVMrN4xPLtPcNoxEhyphenhyphenScZLPbUrPezLd0c_q59iHuw8yTRjtGUGxZ9Snq2HA1aMjtFGhaVyscSuZGta4yDq8/s200/igor4.JPG" alt="" id="BLOGGER_PHOTO_ID_5485711866382899410" border="0" /></a><br />Journal club talk by Igor Fridman on June 3, 2010<br />(click <a href="http://www.physics.utoronto.ca/%7Egramach/CMJCtalkIgorJun2010.pdf">here</a> for slides in pdf format)<br />Summary by William Witczak-Krempa<br /><br /><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">References</span></span></p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">[1] Kiss et al., Nat. Phys. 3, 720 (2007) </span> </span></p><p style="margin: 0pt;"><span style="font-size:100%;"><a href="http://dx.doi.org/10.1038/nphys699"><span style="color: rgb(0, 0, 0);"><u>http://dx.doi.org/10.1038/nphys699</u></span></a></span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">[2] Borisenko et al., PRL 102, 166402 (2009)<br /></span></span></p><p style="margin: 0pt;"><span style="font-size:100%;"><a href="http://dx.doi.org/10.1103/PhysRevLett.102.166402"><span style="color: rgb(0, 0, 0);"><u>http://dx.doi.org/10.1103/PhysRevLett.102.166402</u></span></a></span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">[3] Johannes et al., PRB 73, 205102 (2006)</span></span></p><p style="margin: 0pt;"><span style="font-size:100%;"><a href="http://dx.doi.org/10.1103/PhysRevB.73.205102"><span style="color: rgb(0, 0, 0);"><u>http://dx.doi.org/10.1103/PhysRevB.73.205102</u></span></a></span></p><p style="margin: 0pt;"><br /></p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">The interplay between density wave and superconducting order has been a</span></span></p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">subject of intense interest in systems ranging from the cuprates to the iron-pnictides. Along this line, Igor discussed the coexistence of charge density waves (CDWs) and superconductivity (SC) in the material 2H-NbSe_2. The main point was the comparison of two recent ARPES studies [1,2] that differ in their conclusions about the interplay of the two aforementioned orders. Although NbSe_2 has been long known to host both CDW and SC, the nature of the relationship between the condensates and the mechanism responsible for CDW remain under debate.</span></span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">An introduction concerning 2H-NbSe_2 was first given. We learned that it is a layered material with a two-layer periodicity. It becomes superconducting below T_c = 7.2 K, whereas the CDW order appears around T_CDW ~ 33 K. A DFT study by Johannes et al. [3] demonstrated that the Fermi surface (FS) is composed of three bands: two of them two-dimensional and one three-dimensional. See the slides or [3] for the detailed structure. The superconductivity, of s-wave type, exists on on all bands but the SC gap is not isotropic as established for e.g. by [1,2]. The H_c2 anisotropy is 3 with H_c2^c = 5 T and H_c2^ab = 15 T.</span></span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">To explain the CDW order, the simple Peierls mechanism was argued to be too naive. In the absence of a reliable microscopic model, a phenomenological approach is usually taken. We were told that there are two main candidates: some argue that the FS nesting leads to an enhancement of the charge susceptibility at the hot-spots. Others propose</span> that saddle points at the Fermi energy can be unstable against CDW formation. (Saddle points are van Hove singularities and lead to an enhanced density of states.) The arena was ready for the fight of the ARPES groups.</span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;">The first group/contender, Kiss et al. [1], have conducted ARPES measurements across both the CDW and SC transitions. Their results favour the saddle-point explanation: they found that there is CDW-induced spectral-weight depletion at K-points, which evolves into the largest SC gaps. These gaps also exhibit the highest electron-phonon coupling and Fermi velocity. They concluded, against the prevailing view, that the charge order enhances SC in this system. Love, not war...</span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">In the other corner, the ARPES collaboration of Borisenko et al. [2], reached different conclusions. First, they claim to have the first direct observation of the CDW gap, which opens in regions connected by CDW vectors, hence suggesting that the nesting mechanism is at work and not the saddle-point one. To invalidate the latter they pointed out that according to their data the CDW vectors are too short to connect the saddle-points along the \Gamma-K line, instead they connect Fermi “arcs” on the K-M line! They make the additional claim that there is a CDW pseudogap: in analogy with the cuprates the CDW gap persists in the normal state. The gap was claimed to increase with temperature, which caused some agitation in the crowd. Cookies were immediately distributed to reinstate order. It should be noted that the band structure obtained by [2] agrees very well with the first principles calculation of Johannes et al. [3]. Finally, and most importantly, the results of [2] lead to the conclusion that SC competes against the CDW order, instead of helping it. </span></span></p><p style="margin: 0pt;"> </p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);">Although the data of [2] seemed more reliable, the debate is not closed. Solid experimental proofs are needed as well as a better understanding of the microscopic mechanism on the theoretical side.The marathon continues.<br /></span></span></p><p style="margin: 0pt;"><br /></p><p style="margin: 0pt;"><span style="font-size:100%;"><span style="color: rgb(0, 0, 0);"><br /></span></span></p>Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-56649183501343161132010-06-02T16:28:00.003-05:002010-06-02T16:31:19.534-05:00Mean field theory for spin liquids: Large-N slave-fermion approach<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiqdzNTc9yJyNO1Do3Q6l2M5beuPbvVxHtORtJ60e4SMdFjr7YyTS37PYOJc8TLVl3N6gCOLTaGueFFAOUJ04bWbmkCWPYEUMifoUhZMk8RkaKxCNN5ZYLc0dh_m_NHTOld007aeYh2iEU/s1600/jeff.JPG"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEiqdzNTc9yJyNO1Do3Q6l2M5beuPbvVxHtORtJ60e4SMdFjr7YyTS37PYOJc8TLVl3N6gCOLTaGueFFAOUJ04bWbmkCWPYEUMifoUhZMk8RkaKxCNN5ZYLc0dh_m_NHTOld007aeYh2iEU/s200/jeff.JPG" alt="" id="BLOGGER_PHOTO_ID_5478292039491580274" border="0" /></a><br />Journal club talk by Jeffrey Rau on May 27, 2010<br />Summary to be posted soon.<br /><br />References:<br /><br />* L. Balents, Nature 464, 191 (2010)<br />-A nice survey of recent theoretical and experimental results for spin liquids<br /><br />* Affleck and Marston, PRB 37,3774 (longer version in PRB 39,11538)<br />* Quantum Field Theory of Many-Body Systems, X.G. Wen, Chapter Nine<br />-Reasonably pedagogical sources<br /><br />* Read and Sachdev, Nucl. Phys. B316, 609<br />-Not so pedagogical sourcesUnknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-81283705386376903192010-05-25T12:18:00.004-05:002010-05-25T12:39:39.419-05:00Experimental Evidence for Spin Liquid States<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgb0kN4J_kMyQ_5U2VnCI4_S79Dwj5CCwnvPwjIV4rBrXvXsy2-0RCVmCNUW2y8g3EjRR0byeDDqjtKd9rck6_FxiIjCV1Yq6oEF29MEcP8-HM_kgFzc5oFh3Hj8vkTnJlefkWAQchqVNY/s1600/andrea2.JPG"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEgb0kN4J_kMyQ_5U2VnCI4_S79Dwj5CCwnvPwjIV4rBrXvXsy2-0RCVmCNUW2y8g3EjRR0byeDDqjtKd9rck6_FxiIjCV1Yq6oEF29MEcP8-HM_kgFzc5oFh3Hj8vkTnJlefkWAQchqVNY/s200/andrea2.JPG" alt="" id="BLOGGER_PHOTO_ID_5475261306540281698" border="0" /></a><br />Journal Club talk by Andrea Lupascu<br />on May 20, 2010 (click <a href="http://www.physics.utoronto.ca/%7Egramach/CMjC_QSL_andrea_nodata.ppt">here</a> for ppt file)<br />Summary by R. Ganesh<br /><br /><br />References<br />Yamashita et al.: Nature Physics, 4, 459 (2008)<br />Nakatsuji et al.: Science, 309, 1697 (2005)<br />Helton et al., PRL 98, 107204 (2007)<br /><br />Andrea presented a crisp and topical journal club talk on Quantum Spin Liquids. After a broad defintion, she listed essential features of the QSL states. While there is no conclusive experimental signature, there are many experimental hints. She discussed candidate QSL states which all have antiferromagnetic correlations seen from negative Curie Weiss temperatures. All candidates are also Mott insulators (the candidates from the organic family are weakly Mott<br />insulating).<br /><br />Taking two prominent examples - Herbertsmithite with a Kagome lattice structure and an organic salt with a triangular lattice structure, Andrea discussed experimental pointers to a spin liquid ground state. In particular, she showed susceptibility and heat capacity data as a function of temperature. These quantities show no sign of ordering and point to the existence of a Fermi surface, even though the systems are Mott insulators as seen from resistivity. This Fermi surface is conjectured to be associated with spinon excitations of a spin liquid ground state.<br /><br />With this background, she presented her own results on an organic salt in which Mn ions form a 'star' lattice. Its large value of the \gamma coefficient makes it a promising candidate for a QSL. This led to copious and enjoyable discussion on issues such as the lattice structure, the phonon contribution to specific heat, other probes and possibility of ordering at lower temperatures.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-57706842467425544992010-05-18T09:56:00.005-05:002010-05-18T10:14:30.139-05:00Introduction to topological insulators<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhSXn8h5LqkhhV6EHi6qW3OnashSHgoYrHqZpOg_2smw37AXh_N68sigVOmTkQi0t4qAHykF573FtOuj4xROhkvQpZZFlbPbM7Lsw0AVkZh0-mtjs-DT_mSnHF68XZW15xIHmhyatTYEZ4/s1600/tingpong.jpg"><img style="margin: 0pt 0pt 10px 10px; float: right; cursor: pointer; width: 200px; height: 150px;" src="https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhSXn8h5LqkhhV6EHi6qW3OnashSHgoYrHqZpOg_2smw37AXh_N68sigVOmTkQi0t4qAHykF573FtOuj4xROhkvQpZZFlbPbM7Lsw0AVkZh0-mtjs-DT_mSnHF68XZW15xIHmhyatTYEZ4/s200/tingpong.jpg" alt="" id="BLOGGER_PHOTO_ID_5472628658506240898" border="0" /></a><br />Journal Club Talk by Ting Pong Choy on 13th May 2010<br />Summary by Fazel Fallah Tafti<br /><br />References:<br />Nature 452, 970-974 (2008)<br />Phys. Rev. Lett. 98, 106803 (2007)<br />Phys. Rev. B 74, 195312 (2006)<br />Phys. Rev. Lett. 95, 146802 (2005)<br /><br /><br />Ting Pong introduced two model topological insulators. The first model was a simple two band honey comb lattice with a Hamiltonian consisting of a NN hopping term and an "effective" spin-orbit NNN hopping term. This Hamiltonian can be decoupled in k space and the eigenvalues may be studied in two cases; (a) if the spin-orbit coupling is zero it simply describes the tight binding Graphene system with two Dirac points related through the inversion symmetry (b) if the spin-orbit coupling is non-zero a gap opens at both Dirac points with each band being doubly degenerate with spin up and spin down states. However due to very small spin-orbit coupling in C atoms, this is mostly a theoretical toy model with no experimental realization.<br /><br />The second model is a Cd(Hg)Te quantum well system composed of a thin HgTe slab sandwiched between two thick CdTe layers. The band structure of the two compounds are inverted with respect to each other. The band structure of the quantum well is similar to CdTe in the thin regime but once the thickness of the HgTe is raised above some critical value (6 nm) the bands are inverted and at d=dc the gap must close. This material has been experimentally tested by measuring the hall resistivity of the quantum well. Ting Pong showed that the Hamiltonian of this system is identical to the toy Graphene model mentioned above in small momentum limit.<br /><br />In the second half of the discussion, a robust definition of the topological insulator was given based on the spin Chern number assuming spin being conserved. The spin Chern number is always an integer and it is calculated by integrating the curl of a Berry phase term in k-space. This number can be calculated for each occupied state and the sum over all the occupied states gives the total spin Chern number which is proved to always be an integer. This number is even for a conventional band insulator and odd for a topological band insulator. A model calculation was done to show how one can derive the Chern number using the model Hamiltonian which preserves the time reversal symmetry. Time reversal symmetry was introduced as iSy.K where K is the complex conjugate operator. The calculation was done through a mapping from kx,ky space into theta,phi space using a Jacobian. The "topology" comes from the way we define this mapping.Unknownnoreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-19061048372693999432007-12-10T14:34:00.000-05:002007-12-10T14:40:27.571-05:00Dynamics of entanglement in two coupled qubitsby Vivek M. Aji, Joel E. Moore<br /><br />Journal club talk by Asma Al-Qasimi<br />Summary by So Takei<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-asma.jpg"><br /><br />Today's discussion was led by Asma Al-Qasimi from quantum<br />optics who has recently joined our journal club. She<br />presented an article by Vivek Aji and Joel Moore which<br />discusses the time-evolution of entanglement between two<br />interacting qubits each coupled to a dissipative bath<br />(arXiv:cond-mat/0312145). Asma began by identifying<br />entanglement as the defining difference between a quantum<br />and a classical computer. She then stressed the difficulty<br />in preserving entanglement and its extreme fragility to<br />various sources of decoherence. She emphasized the particular<br />importance of considering decoherence effects in solid state<br />qubits because they are always subject to interactions with<br />a multitude of quantum degrees of freedom. Indeed, solid state<br />qubits generally possess much shorter decoherence times relative<br />to qubits implemented in, for example, cavity quantum<br />electrodynamics and ion traps. The speaker then underlined the<br />main topic of the article: the time evolution of an entangled<br />state of two solid state qubits in the presence of dissipation.<br /><br />Asma proceeded by introducing the solid state qubit considered<br />in the article called the flux qubit, or the persistent current<br />qubit. This qubit consists of a superconducting loop with typically<br />three Josephson junctions that encloses a flux supplied by an<br />external magnetic field. By appropriately tuning the three<br />Josephson coupling energies and the magnetic field, the effective<br />low-energy physics of the system is engineered to possess two<br />metastable states, |0> and |1> with opposite circulating persistent<br />current. Tunnel splitting between the two states is determined by the<br />offset from (1/2)Phi_0 of the flux, where Phi_0 is the superconducting<br />flux quantum.<br /><br />The article mutually couples two flux qubits via a Heisenberg-type<br />coupling of strength J. Assuming that the source of dissipation<br />arises from the noise in the external flux, the article couples<br />the z-component of the pseudospin to a bath of oscillators, which<br />models the environment. The primary motivation of the work is<br />to investigate the two qubit system with entanglement of formation<br />as a diagnostic. At t=0, the qubits are prepared in an entangled<br />state |3> in Eq.2.<br /><br />The speaker then presented three main results from the work.<br />In the first case with J=0, the work reveals an interesting<br />speed up in the decay rate of entanglement relative to the<br />decoherence rate of individual qubits. The speed up factor<br />is explicitly derived to be 2/log(2). The decoherence rate is<br />found to be linearly proportional to temperature and the<br />strength of dissipation. A question was asked whether the<br />decay rate of entanglement is generally faster than the<br />decoherence rate. The speaker supported the generality of the<br />result by alluding to her previous studies of entanglement.<br /><br />Second, she discussed the time evolution of entanglement for<br />a fixed temperature T and various J's. The work finds that<br />for larger J entanglement is completely lost in finite time.<br />This result seemingly implies that J encourages loss of<br />entanglement. However, an intriguing revival of entanglement<br />for large J is observed for long times. A question was asked<br />whether coherence also behaves in a similar oscillatory fashion.<br />The answer was yes. However, it was noted during the discussion<br />that coherence is not necessarily completely lost even at<br />times where entanglement is zero.<br /><br />Finally, the presenter discussed the dynamics of entanglement<br />for a fixed J and various T's. Here, the dependence for the<br />second case is reproduced but for higher temperatures the long<br />time decay increases rapidly while the initial loss of<br />entanglement is hardly affected. A question was raised whether<br />qubits with Ising symmetry were considered. The answer was yes;<br />in the Ising case, no revival in entanglement is observed for<br />a fixed J and various T's.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-79102379742282884312007-11-28T14:40:00.000-05:002007-11-28T14:47:08.178-05:00Detection of geometric phases in superconducting nanocircuitsby Giuseppe Falci, Rosario Fazio, G. Massimo Palma, Jens Siewert and Vlatko Vedral<br /><br />Journal club talk by Nakyeon Hwang<br />Summary by Patrick Morales<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-nakyeon.jpg"><br /><br />This week's talk, given by Nakyeon, was on a proposed experimental setup to detect geometrical phases in a superconducting device. The proposed device is an asymmetric SQUID where the thickness of the tunneling barrier is different in the two arms of the SQUID. The quantum interference due to the geometrical phases could be detected by measuring the charge state of the asymmetric SQUID as the Hamiltonian is adiabatically evolved by varying the offset charge and the flux through the SQUID in a cyclical fashion. Quantum interferometery based on these geometric phases can be used to develop a new design of gates for quantum computation using charge qubits.<br /> <br />When a quantum mechanical system, is evolved adiabatically such that the phase and the amplitude of the wave function describing the system is varied in a cyclical manner, the resulting wave function may differ from the original wave function by a phase factor. This geometrical phase, or Berry phase, results from the geometrical properties of the parameter space of the Hamiltonian. One example of this is the Aharonov-Bohm phase picked up from a charged particle encircling a magnetic field. The Aharonov-Bohm effect results in a periodic modulation of the critical current of a conventional SQUID as a function of magnetic field and provides evidence of macroscopic phase coherence of the superconducting condensate. <br /><br />The Hamiltonian of an asymmetric SQUID operating in the charging regime, where the temperature is lower than the Josephson coupling energies of the junctions which in turn is much smaller than the charging energy of the SQUID contains two terms: one related to the charge of the SQUID and the other to the flux through it. The Hamiltonian can be swept through its phase space by applying a voltage across the SQUID, varying the charge of the SQUID, and by applying a perpendicular magnetic field to vary the flux through the SQUID. A non-trivial loop in phase space produces a Berry phase due to the asymmetry of the SQUID. The dynamical component of the phase can be subtracted by inverting the state of the SQUID and retracing the same path in phase space in the reverse direction. The resulting phase difference is due only to the geometrical component of the phase.<br /><br />Ultimately, the geometric phase in an asymmetric SQUID could be used in the design of gates for quantum computation. Two capacitively coupled asymmetric SQUIDS could be used to create a universal two qubit gate. The gate voltages and the magnetic fluxes of each SQUID could be set independently. The effective charging of the target qubit would depend on the state of the control qubit, resulting in a controllable phase shift of the target qubit.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-11001261353182840552007-11-20T17:47:00.000-05:002007-11-20T17:54:49.010-05:00Quantum critical behaviour in the superfluid density of strongly underdoped ultrathin copper oxide filmsby Iulian Hetel, Thomas R. Lemberger & Mohit Randeria<br /><br />Journal club talk by Ganesh Ramachandran<br />Summary by Brandon Ramakko<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-ganesh.jpg"><br /><br />This week's talk by Ganesh focused on experimental results for thin films of underdoped YBCO. To begin the discussion, he briefly explained the temperature-doping phase diagram for this HTSC. The transition in the overdoped regime of the superconducting dome can be explained by a mean-field transition but it is not clear what causes the transtion in the underdoped regime. This paper aims to explain the transition in this regime using their experimental results.<br /><br />The speaker then discussed a plot of superfluid density versus temperature for Helium films. The superfluid density is constant and drops discontinuously to zero at Tc. The size of this drop increases linearly. This is characteristic of a 2D Kosterlitz-Thouless-Berezinski transition. So if the plot of superfluid density for YBCO has the same bahaviour, you could conclude that it is indeed a 2D Kosterlitz-Thouless-Berezinski transition.<br /><br />The speaker went on to explain the two-coil mutual-inductance method used by the authors. The sample is placed between two coils and an AC current running through the first coil produces a flux which induces an EMF in the sample which drives a current. The flux due to the current in the sample goes through the second coil and by measuring the voltage in the second coil you can calculate the conductivity. Using London's equations you can get the superfluid density.<br /><br />The speaker then discusses the experiental plot of superfluid density versus temperature. There is no discontinuous drop as expected in a Kosterlitz-Thouless-Berezinski transition. The drop is probably smoothed out due to complexities such as the measurement being taken at a frequency of 50 kHz, inhomogeneities, vortex pinning and possible new physics. A line was added that seems to go through each curve where the density starts to decrease quickly. This seemed to be consistent over a range of doping values. The Tc values were determined from this fit.<br /><br />It was asked how thick the layers used were and the speaker replied that the thinnest sample was 2 layers thick. It was also asked why the behaviour of the density for T less than Tc was quadratic when it should be linear. This difference in behaviour might be due to disorder or the frequency of 50 kHz (intead of 0) being used.<br /><br />The speaker then discussed a plot of Tc versus superfluid density(T=0). The measurements show a linear relationship consistent with a 2D Kosterlitz-Thouless-Berezinski transition for small doping or low temperature. It was asked at what temperature it stops behaving linearly. The speaker aswered that at 10K, Tc behaves like the square root of the density. Josephson scaling near a quantum critical point (QCP) implies that the critical point in the underdoped regime is a 2D QCP. Since the transition is mediated by vortex-antivortex pairs, if these could be suppressed you could have a room temperature superconductor.<br /><br />Patrick Morales commented on the difficulty of the experiment. Having attemped similar experiments he said that the split coil experiment on a thin film is difficult because you have weak signals with large noise. You need a large homogeneous sample. He mentioned it is extremely difficult to get a nice uniform thin sample because YBCO does not grow well in the underdoped regime and it is very hard to remove oxygen in a homogeneous manner.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-34664187928715707732007-11-12T18:14:00.000-05:002007-11-12T18:52:03.359-05:00Dynamics of a Quantum Phase Transition in a Ferromagnetic Bose-Einstein Condensateby Bogdan Damski and Wojciech H. Zurek<br /><br />Journal club talk by Edward Taylor<br />Summary by Ganesh Ramachandran<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-ed.jpg"><br /><br />Ed chose an interesting paper studying dynamics through a quantum phase transition in a spinor condensate. The discussion brought out some general ideas/features of the dynamics of quantum phase transitions.<br /><br />Ed gave a lightning introduction to spinor condensates and wrote down the energy functional given in the paper. To put it in context, he described previous experimental study of the ferromagnetic-polar transition by Sadler et al.The phase diagram is known to have ferromagnetic, polar and normal(non-condensed) phases. Experiments have probed the dynamics as well as the detailed domain structure formed. The phase diagram is known to have ferromagnetic, polar and normal(non-condensed) phases.<br /><br />The Hamiltonian possesses three Bogoliubov modes. In the polar phase, one is gapless, corresponding to the broken U(1) symmetry of the BEC. In the ferromagnet, broken U(1) and rotational symmetries give 2 gapless modes. The only energy scale then, is $\Delta$, the excitation gap of the third mode. As we move away from the critical point where even this mode is gapless(?), $\Delta$ rises from zero and eventually saturates. This gives us two time scales which should determine the dynamics - the relaxation time $1/\Delta$ and the transition time taken until saturation.<br /><br />Tuning the rate of increase of magnetic field, we can explore an impulse regime and an adiabatic regime depending on which time scale dominates. The crossover between regimes occurs where the time scales are equal, which should scale as the one-third power of the 'quench time'. The numerical calculations in the paper do give this precise scaling.<br /><br />Ed drew a typical plot of the z-magnetization as a function of changing magnetic field or time. The plot showed that the order parameter moved away from zero toward the expected equilibrium value, only after a delay. This was identified as the crossover between impulse and adiabatic regimes. The delay thus read off, showed the expected 1/3 scaling except for values close to zero.<br /><br />Ed wound up with a neat quick summary, only to make way for a brisk discussion. Igor's question prompted a discussion on domain formation. Drawing a parallel to the early universe where the size of structures was limited by the speed of light, Ed brought out that the size of domains was given by the speed of sound. With two soundlike modes, for some reason, it is the slower mode velocity that plays a role. Kibble-Zurek theory predicts a 1/3 scaling for domain size, which has been observed in this paper.<br /><br />There was a question from Michael which brought out that the above considerations only hold for intermediate time scales, where the transition is non-adiabatic, but slow enough. Michael also pointed out that it would be interesting to explore the normal region just above the critical point.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-45963657210649514262007-10-31T14:11:00.000-05:002007-10-31T14:17:25.864-05:00A Spin Triplet supercurrent though the half-metallic ferromagnet CrO2by R. S. Keizer, S. T. B. Goennenwein, T. M. Klapwijk, G. Miao, G. Xiao and A. Gupta<br /><br />Journal club talk by Patrick Morales<br />Summary by Fazel Fallah Tafti<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-patrick.jpg"><br /><br />Superconductivity and ferromagnetism are competing ordered states<br />of matter but it appears that they can coexist. In this article we<br />learned about another example of SC in a FM material which exists in<br />the half-metallic material CrO2.<br /><br />The experiment is realized by growing a single crystal thin film of<br />CrO2 on TiO2 substrate (epitaxial growth) and patterning two<br />superconducting islands made of NbTiN on top it using organic resist<br />mask with conventional electron beam lithography.<br /><br />NbTiN is a conventional S-wave BCS superconductor and CrO2 is a<br />spin polarized half metallic ferromagnet i.e. it has a finite DOS at<br />the Fermi level for the up spin polarization but it shows a gap of ~<br />2eV in the down spin polarization.<br /><br />The supercurrent tunnels through the Josephson junction<br />NTN-CrO2-NTN which is shown in IV characteristic curve. Fraunhofer<br />pattern is observed for Ic as a function of induced magnetic field<br />which is regarded as another evidence of supercurrent tunneling<br />through the FM material.<br /><br />The distance between the two NTN leads is about 300nm which is much<br />higher than the coherence length expected for a FM metal. This is even<br />higher than the coherence length in most normal metals (~100 nm) which<br />is surprising.<br /><br />Such persistence of a supercurrent through a FM half-metal can be<br />justified by the idea of spin triplet cooper pairs. In this picture we<br />can still have the main features of the BCS theory but the pairing<br />mechanism in unconventional such that instead of singlet spin pairs we<br />have a pair made of two electrons with parallel spins.<br /><br />The question to be answered is that what are is the mechanism<br />behind singlet-triplet conversion at the junction between the normal<br />s-wave SC and CrO2. In fact there are a number of proposed mechanisms<br />for such phenomenon but they all agree on the fact that such process<br />has to be caused by an interplay between the exchange field of the FM<br />at the FM-SC junction and a non-homogeneous magnetic field across the<br />junction. In order to make a correct theory to describe such process<br />one has to include various important factors such as the effect of<br />domain walls at the interface, spin-orbit interactions in the SC,<br />Double exchange interaction in CrO2 and perhaps the effect of local<br />magnetic impurities.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-307542543722820932007-10-26T08:22:00.000-05:002007-10-26T08:29:12.269-05:00Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bi-layer Ruthenate Sr3Ru2O7by K. Iwaya, S. Satow, T. Hanaguri, N. Shannon, Y. Yoshida, S. I. Ikeda, J. P. He, Y. Kaneko, Y. Tokura, T. Yamada and H. Takagi<br /><br />Journal Club talk by Hyeonjin Doh (see also his <a href="http://docs.google.com/Present?docid=df5pjbfw_22hhkdv2">notes </a>!)<br />Summary below by Igor Fridman<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-hyeonjin.jpg"><br /><br />This week's talk by Hyeonjin focused on experimental results of tunneling experiments on the compound Sr3Ru2O7. This compound has garnered recent interest due to possible presence quantum criticalities. The paper discussed presents the first available spectroscopic measurements on this compound using Scanning Tunneling Spectroscopy (STS).<br /><br />To begin the discussion, Hyeonjin illustrated the physical properties of Sr3Ru2O7. The crystaline structure the compound is described by the A(n+1)B(n)O(3n+1) group, where n is the number of layers. The compound under discussion, with n=2 layers, is an intermediate between Sr2RuO3 (n=1) and SrRuO3 (n=inf). The related compounds are known in their ground states to be a spin triplet superconductor and an itinerant ferromagnet, respectively.<br /><br />The ground state of Sr3Ru2O7 is a paramagnetic Fermi liquid, with most of the electronic properties coming from the Ru4+ ions. Unlike the n=inf compound, this bi-layer compound shows no evidence of an FM transition. There is, however, a meta-magnetic transition at 5.5T, applied parralel to the ab plane, between states of low and high magnetization. This transition was observed previously in measurements of susceptibility, and also seen in the tunneling data. Another transition occurs when ia field between 7 and 8T is applied parallel to the c axis. In the present tunneling experiment, field was applied parallel to the c axis, and no transitions were observed at 7T and 8T. However, Hyeonjin pointed out that this could be due to the fact that crystals used in the previous study were much cleaner, having a residual resisitivity of Rho_0 ~0.4uOhm-cm, as opposed to the crystals used for this study with Rho_0 ~7uOhm-cm.<br /><br />The spectroscopy measurements revealed two peaks in the DOS close to the Fermi level, at ~ +/- 7 meV, as measured in zero-field at T = 560 mK. Hyeonjin suggested that this is consistent with the Stoner picture. Indeed, the authors calculate that spectroscopic evidence of Stoner-type metamagnetism should show up on an energy scale of ~1 meV at these temperatures. The authors also collected data over a range of fields from 0 to 11T, and found a change in amplitudes of the DOS peaks with increasing field.<br /><br />The authors analyze their data by looking at the amplitudes of various spectroscipic peaks as a function of magnetic field. The first effect is a rapid increase of the DOS at the Fermi level above the critical field. This confirms the c-axis metamagnetic transition. The second effect is a shift of the spectral weight from the lower energy peaks at 2 and -1 meV to the slightly higher energies of 4 and -3 meV above the critical field. Thus, there is a change in the field dependency of the DOS at the transition, which is evidence that the transition is a quantum critical point.<br /><br />Putting the magnetic properties aside, Hyeonjin also discussed an apparent DOS modulation seen in the STM images of the cleaved compound. By changing the Fermi level between 7 and 100 meV, the authors were able to image two different geometries on the surface. At low voltage bias, the authors show an apparent asymmetry in the DOS between neighboring Ru atoms. This is unexpected, since electrons should not see any difference between neighboring sites according to the present model. This modulation gave the authors a reason to believe that the orbital degrees of freedom might have an important role in this cleaved sample, but they are not sure this kind of modulation would survive in bulke. Thus far, this effect is not fully understood.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-26021006062662071892007-10-18T10:17:00.000-05:002007-10-18T10:32:01.977-05:00Dimer-Quadrupolar Quantum Phase Transition in the Quasi-1D Heisenberg model with Biquadratic Interactionby Kenji Harada, Naoki Kawashima and Matthias Troyer<br /><br />Journal club talk by Christoph Puetter<br />Summary below by Thomas Grzesiak<br /><br /><img src="http://www.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-cpuetter.jpg"><br /><br />Christoph Puetter introduced the paper "Dimer-Quadrupolar Quantum Phase Transition in the Quasi-1D Heisenberg model with Biquadratic Interaction" by Harada, Kawashima, and Troyer. He explained that the concept of a deconfined quantum critical point (DQCP) is only a few years old, and people are still looking for models which exhibit this kind of phase transition. This paper investigates 2 critical points in the phase diagram of the (anisotropic) bilinear biquadratic Heisenberg model with Jl the bilinear coupling strength and Jq for the biquadratic term. For |Jl|>> |Jq|, we get the familiar ferromagnetic<br />or Neel state depending on the sign of Jl. When the couplings are comparable in strength, the situation becomes more complex.<br /><br />To illustrate this point, Christoph began by considering 2 isolated sites and showed that a singlet is preferred for Jl < Jq < 0. He explained that once more sites are included the singlet condition can no longer be satisfied on bonds sharing the same site. The system therefore develops new phases such as the dimer phase (for &lambda << 1) and the spin nematic. (lambda is the anisotropy parameter in the 2d Heisenberg model considered here).<br /><br />The authors of the paper performed MC simulations for various lattice sizes L, and measured the Quadrupole-Quadrupole (for nematic) and Dimer-Dimer correlation functions, Gq and Gd. They examined the ratio G(L/2)/G(L/4). Tuning lambda they found that these ratios (Rq and Rd) become independent of L at lambda_critical, signalling the phase transition.<br /><br />It was asked why the above ratio was chosen. Daniel explained this was for the correlation length, to get a constant alpha independent of L (a power law). Christoph then focused on a special point on the nematic-dimer phase boundary (Y in the paper). He explained that with spin-rotational symmetry preserved and translational symmetry broken in the dimer phase, and vice versa in the nematic, a first order transition is expected. It is also expected that Rq and Rd should be 1 (why?), and that there is a cusp in the energy at the transition. However, the fact that this was not observed could be due to a DQCP, although other possibilities cannot be ruled out.<br /><br />It was asked why DQCP doesn't appear from the Neel side (point X in the paper)? If there are 2 ordered states with no relationship between the orderings, there should be a first order transition, according to Landau, yet this paper found a second order transition. The talk ended with another question, as to what a DQCP actually is (in a nutshell). Michael gave an example: in a magnetic phase has spin waves that are spin 1 excitations in a spin 1/2 system. They can split into 2 spin 1/2 excitations at a DQCP, ie they become deconfined. For the dimer phase, spins are bound into singlet states. At a DQCP, they would become deconfined, breaking the singlet.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-68023337162579237192007-10-02T08:47:00.000-05:002007-10-02T09:04:08.854-05:00Metallic Spin-Liquid Behavior of the Geometrically Frustrated Kondo Lattice Pr2Ir2O7by S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan<br /><br />Journal club talk by Daniel Podolsky<br />Summary below by Jean-Sebastien Bernier<br /><br /><img src="http://www2.physics.utoronto.ca/~mlawler/cmjc-images/cmjc-dpodolsky.jpg"> <br /><br />For the first CMP journal club of the fall semester, our discussion leader was Daniel Podolsky who presented a paper by Nakatsuji and coworkers on the possible observation of metallic spin liquid behavior in the geometrically frustrated Kondo lattice Pr$_2$Ir$_2$O$_7$.<br /><br />Daniel began by presenting the material under study. He pointed out that the conduction electrons come from Iridium (Ir$^{4+}$) and that the local moments (Pr$^{3+}$) occupy the sites of a pyrochlore lattice (network of corner sharing tetrahedra). He noted that due to crystal field splitting the spins of the local moments are effectively Ising-like and point along an axis passing through the center of each tetrahedron. Daniel also explained that the Ising nature of the local moments makes this system unfrustrated if the RKKY interaction is antiferromagnetic. An AF RKKY interaction seems to be indicated by the large temperature susceptibility data. However, if the system is unfrustrated, it is difficult to understand why no long-range-order develops at the Curie-Weiss temperature.<br /><br />Someone then asked how can we be sure that there is RKKY interaction in this system and not only superexchange. Daniel replied that the paper points out that $T^* = -20K$ is much higher than what should be expected from superexchange.<br /><br />Daniel continued by presenting susceptibility data. He highlighted the unusual $\chi ~ \ln T$ behavior at low temperature and that no anomalies due to a magnetic transition were detected in the susceptibility except for freezing at $T_f = 120 mK$.<br /><br />He then presented the resistivity data that shows the usual Kondo minimum and pointed out that $T_{Kondo} = 25 K$ is very large for such a poor metal.<br /><br />Someone then asked if this material was also a poor metal at high temperatures. Daniel replied that he did not know.<br /><br />Moving on, he presented that for $T_f = 120 mK < T < T_{CW} = 1.7 K$, the entropy and specific heat vary as $\sqrt{T}$ which leads to a very large entropy.<br /><br />Someone then asked if $S(0)$ is known. Daniel answered that only $\Delta S$ was presented in this paper.<br /><br />Finally, Daniel presented the global phase diagram put forward by the authors:<br /><br />$T_f < T < T_{CW}$: metallic spin-liquid with $S \sim C \sim \sqrt{T}$ <br />$T_{CW} < T < |T^*|$: Kondo screening, but incomplete since susceptibility diverges<br />$T > |T^*|$: decoupled Ir and Pr<br /><br />... and he noted that the presence of a metallic spin-liquid did not sound too convincing to him.<br /><br />Someone asked if any instability of the conduction electrons were observed. Daniel answered that there is no evidence in the data for such a phase transition.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0tag:blogger.com,1999:blog-3288870921047833084.post-36062923905966718172007-09-28T09:36:00.000-05:002007-09-28T10:20:44.546-05:00Spin phonon induced colinear order and magnetization plateaus in triangular and kagome antiferromagnets. Applications to CuFeO_2by Fa Wang and Ashvin Vishwanath<br /><br />Frustration, defined as a large degeneracy of ground states, is bound to be lifted by real world complications. In this paper, the authors propose that phonons, generally neglected in most studies of frustrated magnets, may turn out to be an important frustration relieving mechanism. Perhaps their strongest argument in suport of this thesis is their demonstration that coupling spins to Einstein phonons can explain the stability of an unexpected colinear magnetic ordering in the triangular lattice antiferromagnet CuFeO_2 and the existence of magnetization plateaus.Michael J. Lawler, Ph. D.http://www.blogger.com/profile/16919355143102631845noreply@blogger.com0