Giant thermal Hall conductivity in the pseudogap phase of cuprate superconductors

Abstract

The nature of the pseudogap phase of cuprates remains a major puzzle. Although there are indications that this phase breaks various symmetries, there is no consensus on its fundamental nature 1 . Although Fermi-surface 2 , transport 3 and thermodynamic 4 signatures of the pseudogap phase are reminiscent of a transition into a phase with antiferromagnetic order 5,6 , there is no evidence for an associated long-range magnetic order. Here we report measurements of the thermal Hall conductivity κ xy in the normal state of four different cuprates (La 1.6-x Nd 0.4 Sr x CuO 4 , La 1.8-x Eu 0.2 Sr x CuO 4 , La 2-x Sr x CuO 4 , and Bi 2 Sr 2-x La x CuO 6+δ ) and show that a large negative κ xy signal is a property of the pseudogap phase, appearing with the onset of that phase at the critical doping p*. Since it is not due to charge carriersas it persists when the material becomes an insulator, at low doping -or magnons -as it exists in the absence of magnetic order -or phonons -since skew scattering

Thermal Hall conductance and a relative topological invariant of gapped two-dimensional systems

“…In this paper, we show that although thermal Hall conductance is not a well-defined bulk transport coefficient, the difference of thermal Hall conductances of two 2d materials 1 We use the notation κ A instead of the more standard κ xy to avoid confusion with the off-diagonal component of κ S which may be nonzero if rotational invariance is broken. 2 For gapped 2d systems at low temperatures, one can try to define thermal Hall conductance as the coefficient of the gravitational Chern-Simons term in the low-energy effective action [7][8][9]. As explained in [5], the energy current corresponding to the gravitational Chern-Simons term is of higher order in derivatives, in agreement with the above discussion.…”

“…There has been much theoretical as well as experimental interest in thermal Hall conductivity and conductance of various materials. Just to give a couple of recent examples: (1) thermal Hall conductance has been used to probe the non-Abelian nature of the ν = 5/2 FQHE state [1]; (2) an unusual behavior of thermal Hall conductivity at low temperatures was observed in cuprate superconductors in the pseudogap region [2]. Despite many theoretical works on the thermal Hall effect (see e.g.…”

Thermal Hall effect in square-lattice spin liquids: A Schwinger boson mean-field study

“…Recent studies of the metallic state of high-T c cuprate superconductors, such as La 1.6−x Nd 0.4 Sr x CuO 4 (Nd-LSCO), obtained by suppressing superconductivity using magnetic fields, indicate a very interesting trend in the thermal Hall coefficient [2] as a function of doping. On the overdoped side, with a hole doping of p > p * , where p * corresponds to the doping value where the pseudogap temperature vanishes, the thermal Hall conductivity κ xy obeys the WF law for low T .…”

“…The large κ xy observed at dopings with and without Néel order suggests that magnons are not responsible for this phenomenon. Further, Grissonnanche et al [2] argue that the observed magnitude of κ xy at low temperatures is too large to be explained by spin-scattered phonons. This prompts the rather intriguing possibility of emergent neutral excitations that are responsible for this unusual behavior.…”

“…However, Ref. 2 shows that the large negative κ xy persists even in the insulating phase as the doping p → 0. This is the extreme limit of breakdown of the WF law, as σ xy = 0.…”