Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bi-layer Ruthenate Sr3Ru2O7

by 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

Journal Club talk by Hyeonjin Doh (see also his notes !)
Summary below by Igor Fridman



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).

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.

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.

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.

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.

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.