Category Archives: 2011

Unveiling a nematic quantum critical point in multi-orbital systems

Electronic nematicity proposed to exist in a number of transition metal materials can have different microscopic origins. In particular, the anisotropic resistivity and meta-magnetic jumps observed in Sr3Ru2O7 are consistent with an earlier proposal that the isotropic-nematic transition is generaically first order and accompanied by meta-magnetism when tuned by a magnetic field. However, additional striking experimental features such as a non-Fermi liquid resistivity and critical thermodynamic behavior imply the presence of an unidentified quantum critical point (QCP). Here we show that orbital degrees of freedom play an essential role in revealing a nematic QCP, even though it is overshadowed by a nearby meta-nematic transition at low temperature. We further present a finite temperature phase diagram including the entropy landscape and discuss our findings in light of the phenomena observed in Sr3Ru2O7.

Christoph M. Puetter,1 Sylvia D. Swiecicki,1 and Hae-Young Kee1, 2, ∗
1Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
2Canadian Institute for Advanced Research, Quantum Materials Program, Toronto, Ontario M5G 1Z8, Canada

Identifying spin-triplet pairing in spin-orbit coupled multi-band superconductors: application to Sr2RuO4

We investigate the combined effect of Hund’s and spin-orbit (SO) coupling on superconductivity in multi-orbital systems. Hund’s interaction leads to orbital-singlet spin-triplet superconductivity, where the Cooper pair wave function is antisymmetric under the exchange of two orbitals. We identify three d-vectors describing even-parity orbital-singlet spin-triplet pairings among t2g-orbitals,
and find that the three d-vectors are mutually orthogonal to each other. SO coupling further assists pair formation, pins the orientation of the d-vector triad, and induces spin-singlet pairings with a relative phase difference of π/2. In the band basis the pseudospin d-vectors are aligned along the z-axis and correspond to momentum-dependent inter- and intra-band pairings. We discuss quasiparticle dispersion, magnetic response, collective modes, and experimental consequences in light of the superconductor Sr2RuO4.

Christoph M. Puetter1 and Hae-Young Kee1, 2, ∗
1Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada
2Canadian Institute for Advanced Research, Quantum Materials Program, Toronto, Ontario M5G 1Z8, Canada

Hidden spin liquid in an antiferromagnet: Applications to FeCrAs

The recently studied material FeCrAs exhibits a surprising combination of experimental signatures, with metallic, Fermi liquid like speci c heat but resistivity showing strong non-metallic character. The Cr sublattice posseses local magnetic moments, in the form of stacked (distorted) Kagome lattices. Despite the high degree of magnetic frustration, anti-ferromagnetic order develops below TN  125K suggesting the non-magnetic Fe sublattice may play a role in stabilizing the ordering. From the material properties we propose a microscopic Hamiltonian for the low energy degrees of freedom, including the non-magnetic Fe sublattice, and study its properties using slaverotor mean eld theory. Using this approach we nd a spin liquid phase on the Fe sublattice, which survives even in the presence of the magnetic Cr sublattice. Finally, we suggest that the features of FeCrAs can be qualitatively explained by critical  flutuations in the non-magnetic sublattice Fe due to proximity to a metal-insulator transition.

Je rey G. Rau and Hae-Young Kee
Department of Physics, University of Toronto, Toronto, Ontario M5S 1A7, Canada