18 June 2019 Fırat Yılmaz


Speaker: Fırat Yılmaz, Fudan University
Title: Spontaneous thermal Hall conductance in superconductors with broken time-reversal symmetry
Date: June 18, 2019
Time: 14:30
Cookie & Tea: ENG B16, 14:15
Place: ENG B16
web: https://physics-seminars.ku.edu.tr/

The off-diagonal components of thermal conductance tensor, thermal Hall conductivities (THCs), have extensively been studied in recent condensed matter experiments to investigate fractionalized quantum spin liquids [1], and quantum Hall systems [2]. Under zero magnetic field, THCs spontaneously become non-zero for time-reversal symmetry (TRS) broken systems, and can have contributions from phonons, topologically protected edge states etc. Neglecting the phonon contribution, we focus on the additional bulk effect, impurity pair breaking in TRS broken superconductors. Inspired by Sr2RuO4 [3], the low temperature THC was calculated [4] for the chiral p-wave superconductors induced by isotropic impurities. Compared to topological part of THC, this contribution is found to be orders of magnitude larger as it scales with the density of states at the Fermi level. Motivated by other TRS broken superconductors, UPt3 [5], URu2Si2 [6] and SrPtAs [7], we extend the THC calculations to i. finite temperatures ii. other TRS broken order parameters, iii. anisotropic impurities.

For this study, the non-equilibrium quasiclassical (QC) Green’s function formalism is utilized. Non-zero THC components, K_ij , come from the vertex correction (anomalous Eliashberg propagator) term. This term is obtained by a systematic expansion of the QC transport equation in slowly varying bulk center of mass coordinate to first order. K_ij is obtained analytically around the gapless regime (ε → 0) for low temperatures and is numerically calculated for finite temperatures. Concerning the low temperature limit, the QC method can address the formation of impurity bands at low temperatures and allows for non-zero THCs meanwhile the Boltzmann kinetic equation approach fails to describe.

We find that the zero field K_xy is non-zero for both p- and d-wave pairing superconductors. Anisotropic impurities scatter the quasiparticles around the gapless regions of Fermi surface forming a momentum dependent impurity band. K_xy can be of the same order as the diagonal component (K_xx) for varieties of impurity potential strength δs, δp 6≠ {0, π/2}. We present our results and discuss the experimental implications.

*This work supported by Taiwan MOST with Project No: 107-2112-M001-035-MY3.
[1] Y Kasahara et al., Nature 559, 227231 (2018).
[2] M. Banerjee et al., Nature 559, 205210 (2018).
[3] Y. Maeno et al., J. Phys. Soc. Japan 81, 01109 (2012).
[4] S. Yip , Supercond. Sci. Technol. 29, 085006 (2016).
[5] J. A. Sauls , Adv. Phys. 43, p.113 (1994).
[6] A. P. Mackenzie and Y. Maeno, Rev. Mod. Phys. 75, 657 (2003).
[7] Biswas et al., Phys. Rev. B 87, 180503(R) (2013). 

Short Bio: Fırat Şerif Yılmaz is a postdoctorate researcher at Academia Sinica in Taiwan. He obtained his PhD degree from Bilkent University Physics Department under the supervision of Prof. Mehmet Ozgür Oktel. He conducted research on the cold atoms in optical lattices with artificial gauge fields. His recent work is mainly about the non-equilibrium quantum transport in unconventional superconductors. His leisure time interest is to develop and organize alternative education curriculum for high school students.