Thermal Hall conductivity in the cuprate Mott insulators Nd2CuO4 and Sr2CuO2Cl2

Abstract

The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La2CuO4 were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders – Nd2CuO4 and Sr2CuO2Cl2 – and show that two potential mechanisms can be excluded – the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO2 planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO2 planes.

Transport signatures of the pseudogap critical point in the cuprate superconductor Bi2Sr2−xLaxCuO6Lizaire1, Legros2, Gourgout3 et al. 2021Phys. Rev. BSelf Cite233160View full textAdd to dashboardCite

“…The lowtemperature trend of this thermal Hall conductivity κ xy cannot be explained by the simple motion of charged carriers, since κ xy and R H have opposite signs to T → 0. This large negative κ xy also persists down to p = 0, as confirmed in three cuprate Mott insulators-La 2 CuO 4 Nd 2 CuO 4 , and Sr 2 CuO 2 Cl 2 [22]. The phenomenon has since been attributed to phonons [23], because an equally large signal was seen for a current along the c direction, i.e., κ zy κ xy , and phonons are the only excitations in cuprates that travel as easily in directions parallel and perpendicular to the CuO 2 planes, as shown from the fact that κ zz κ xx in La 2 CuO 4 at low T [66].…”

“…The negative component of κ xy is clearly not due to charge carriers, and phonons are the most likely carriers. The fact that the magnitude of κ xy scales with the magnitude of phonon-dominated κ xx in the cuprate Mott insulators supports the view that phonons are responsible for the negative κ xy [22]. At T 20 K, the ratio |κ xy |/κ xx is equal to 0.30% in La 2 CuO 4 , 0.37% in Nd 2 CuO 4 , and 0.26% in Sr 2 CuO 2 Cl 2 [22], and the ratio |κ zy | /κ zz is equal to 0.48% in Nd-LSCO at p = 0.21.…”

“…This tells us that the pseudogap phase is a metal whose fermionic excitations carry heat and charge as do conventional electrons. Finally, a large negative thermal Hall conductivity, κ xy , has been observed in various cuprates below p [21,22], attributed to phonons that acquire chirality upon entering the pseudogap phase [23].…”

Implications of second harmonic generation for hidden order in Sr2CuO2Cl2

“…Although a large thermal Hall signal was reported in Ref. 16 which was attributed to a chiral contribution to the phonon thermal Hall effect (describable by a rank-3 tensor 29 ), to date, no anomalous signal has been reported. Besides the anomalous thermal Hall, there should also be a linear in field contribution to the longitudinal thermal conductivity.…”

“…15 As such, the SHG data points to the presence of a novel bulk electronic OP, reducing the MPG from mmm1 to mm m , the subgroup of mmm1 consistent with the newly discovered OP of 4/mm m magnetic symmetry. 12 Along the same direction, thermal Hall measurements indicate the presence of chirality 16 that is not consistent with a 4/mmm1 MPG. The physical realization of such an OP is however still unclear.…”