Thermal Hall Effect in a Phonon-Glass Ba3CuSb2O9

Abstract

A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba3CuSb2O9. The transverse thermal conductivity shows a power-law temperature dependence below 50 K, where a spin gap opens. We suggest that because of the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scattering of orphan Cu 2+ spins formed in the random domains of the Cu 2+ -Sb 5+ dumbbells in Ba3CuSb2O9.Hall measurements of metals are fundamental tools to investigate their physical properties and play important roles in the study of quantum phenomena, such as the quantum Hall effect or the anomalous Hall effect [1]. On the other hand, no Hall effect is expected in insulators because the Hall effect originates from conduction electrons. However, a Hall effect of charge-neutral excitations, which is observed as a thermal Hall effect (THE), has been predicted to occur in magnetic and non-magnetic insulators [2][3][4][5][6][7][8][9][10][11], thereby providing new insights into the research on quantum spin liquids and other frustrated materials. So far, three kinds of THEs have been reported in insulators: THEs of magnons in ordered magnets [12][13][14], spin excitations in disordered magnets [14][15][16], and phonon THEs [17,18]. Among them, the most studied case is the magnon THE which has been observed in ferromagnetic insulators [12][13][14] and is understood in terms of the Berry phase associated with the magnon bands [2,3]. The THEs of spin excitations in paramagnetic states have also been studied theoretically [2,4,5]; these THEs were reported recently in a spin ice compound Tb 2 Ti 2 O 7 [15], a ferromagnetic kagomé lattice system [14], and a frustrated kagomé lattice system [16].In contrast, reports on the THE of phonons have been limited to studies on a dielectric garnet Tb 3 Ga 5 O 12 (TbGG) [17,18]. The theoretical origin of the phonon THE has been discussed as a Raman-type interaction between phonons and large spins [6,7], a Berry curvature of phonon bands [8,9], and a resonant skew scattering of phonons by superstoichiometric Tb 3+ ions [10]. Experimentally, it has been determined that there is a large magneto-elastic coupling in TbGG [19,20]. Moreover, the smaller thermal conductivity of TbGG than that of other rare-earth gallium garnets indicates that there is strong scattering of phonons by the Tb 3+ ions [21,22]. These observations imply that the strong phonon scattering is important for the THE. However, the mechanism for the THE in TbGG is poorly understood because the measurement of the THE in TbGG has been limited only to ∼5 K [17,18]. Furthermore, because TbGG is paramagnetic at 5 K, it is impossible to separate the THE of phonons from that of spins. Thus, to investigate the phonon THE, it is crucial to observe the THE over a wide temperature range in a single crystal that has strong phonon scattering and no spin THE.In this Letter, we report the THE in Ba 3 CuSb 2 O 9 (BCSO) [23][24][25][26][27][28][29], which according to us is an ideal c...

Magnon Hall effect without Dzyaloshinskii–Moriya interaction

“…However, a closely related honeycomb-lattice antiferromagnetic material Bi 3 Mn 4 O 12 (NO 3 ) [12] shows evidence of magnetic order at a critical field of ∼ 6 Tesla consistent with a collinear Néel order [13]. Prior to this experimental report, we have already shown that honeycomb (anti)ferromagnet with a next-nearest-neighbour staggered DMI captures a negative κ xy and power-law temperature dependence κ xy ∝ T 2 [14,15] as recently seen in Ba 3 CuSb 2 O 9 [2]. In this regard, we believe that this correspondence between theory and experiment cannot be serendipitous.…”

“…This effect has been previously observed in collinear ferromagnetic materials with DMI [3][4][5] and pyrochlore spin liquid material [6]. In these recent reports, a transverse thermal Hall conductivity κ xy was observed in a strong magnetic field ∼ 15 Tesla applied perpendicular to the plane of the frustrated magnets [1,2]. The observed effect on the Kagomé volborthite is attributed to spin excitations in the spin liquid (SL) regime.…”

“…The honeycomb-lattice antiferromagnet Ba 3 CuSb 2 O 9 also shows a negative κ xy at the same magnetic field and a power-law temperature dependence κ xy ∝ T 2 [2]. It was suggested that the observed thermal Hall effect is a phonon Hall effect.…”

“…Recently, the experimental observation of thermal Hall effect of spin excitations has been reported in the frustrated Kagomé volborthite Cu 3 V 2 O 7 (OH) 2 ·2H 2 O [1] and frustrated honeycomb antiferromagnet Ba 3 CuSb 2 O 9 [2], with no signs of DMI. This effect has been previously observed in collinear ferromagnetic materials with DMI [3][4][5] and pyrochlore spin liquid material [6].…”

Frustrated spin- 12 molecular magnetism in the mixed-valence antiferromagnets Ba3MRu2O9Ziat1, Aczel2, Sinclair3 et al. 2017Phys. Rev. B314303View full textAdd to dashboardCite

“…More stoichiometric samples manage to preserve their room-temperature hexagonal symmetry down to much lower temperatures either through a dynamic JT effect [34] or else local distortions that nonetheless preserve the global symmetry of the structure and give rise to a random-singlet magnetic ground state [7]. The most recent experimental results, thermal conductivity measurements, on nearly stoichiometric single-crystal Ba 3 CuSb 2 O 9 point toward the local-distortion picture [35] which is consistent with the random singlet magnetic ground state and excitation gap [7]. Since a hexagonal to orthorhombic collective JT transition can be ruled out by the neutron diffraction results on the materials studied here, similar random distortions might then apply, and they may be ex- The high frequency data has been normalized so that the peak susceptibilities are equal, since the frequency response of the PPMS system is not perfectly flat.…”