Charge Density Waves and Superconductivity in 2H-NbSe_2

Journal club talk by Igor Fridman on June 3, 2010
(click here for slides in pdf format)
Summary by William Witczak-Krempa

References

[1] Kiss et al., Nat. Phys. 3, 720 (2007)

http://dx.doi.org/10.1038/nphys699

[2] Borisenko et al., PRL 102, 166402 (2009)

http://dx.doi.org/10.1103/PhysRevLett.102.166402

[3] Johannes et al., PRB 73, 205102 (2006)

http://dx.doi.org/10.1103/PhysRevB.73.205102

The interplay between density wave and superconducting order has been a

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.

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.

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 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.

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...

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.

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.