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Phononic-magnetic dichotomy of the thermal Hall effect in the Kitaev-Heisenberg candidate material Na$_2$Co$_2$TeO$_6$
Authors:
Matthias Gillig,
Xiaochen Hong,
Christoph Wellm,
Vladislav Kataev,
Weiliang Yao,
Yuan Li,
Bernd Büchner,
Christian Hess
Abstract:
Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the therm…
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Majorana fermions as emergent excitations of the Kitaev quantum spin liquid ground state constitute a promising concept in fault tolerant quantum computation. Experimentally, the recently reported topological half-quantized thermal Hall effect in the Kitaev material $α$-RuCl$_3$ seems to confirm the Majorana nature of the material's magnetic excitations. It has been argued, however, that the thermal Hall signal in $α$-RuCl$_3$ rather stems from phonons or topological magnons than from Majorana fermions. Here we investigate the thermal Hall effect of the closely related Kitaev quantum material Na$_2$Co$_2$TeO$_6$, and we show that the thermal Hall signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the transversal heat conductivity $κ_{xy}$ carries clear signatures of the phononic $κ_{xx}$, but changes sign upon entering the low-temperature, magnetically ordered phase. We systematically resolve the two components by considering the detailed temperature and field dependence of both $κ_{xy}$ and $κ_{xx}$. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in a Kitaev topological quantum spin liquid requires to disentangle phonon vs. magnetic contributions where the latter include potentially fractionalized excitations such as the expected Majorana fermions.
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Submitted 6 March, 2023;
originally announced March 2023.
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Frustration enhanced by Kitaev exchange in a $\boldsymbol{\tilde{j}_{\text{eff}}=\frac12}$ triangular antiferromagnet
Authors:
C. Wellm,
W. Roscher,
J. Zeisner,
A. Alfonsov,
R. Zhong,
R. J. Cava,
A. Savoyant,
R. Hayn,
J. van den Brink,
B. Büchner,
O. Janson,
V. Kataev
Abstract:
Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of…
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Triangular Heisenberg antiferromagnets are prototypes of geometric frustration, even if for nearest-neighbor interactions quantum fluctuations are not usually strong enough to destroy magnetic ordering: stronger frustration is required to stabilize a spin-liquid phase. On the basis of static magnetization and electron spin resonance measurements, we demonstrate the emergence of ${\tilde{j}_{\text{eff}}=\frac12}$ moments in the triangular-lattice magnet Na$_2$BaCo(PO$_4$)$_2$. These moments are subject to an extra source of frustration that causes magnetic correlations to set in far above both the magnetic ordering and Weiss temperatures. Corroborating the $\tilde{j}_{\text{eff}}=\frac12$ ground state, theory identifies ferromagnetic Kitaev exchange anisotropy as additional frustrating agent, altogether putting forward Na$_2$BaCo(PO$_4$)$_2$ as a promising Kitaev spin-liquid material.
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Submitted 3 November, 2021; v1 submitted 4 June, 2021;
originally announced June 2021.
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Magnetic interactions in the tripod-kagome antiferromagnet Mg$_{2}$Gd$_{3}$Sb$_{3}$O$_{14}$ probed by static magnetometry and high-field ESR spectroscopy
Authors:
C. Wellm,
J. Zeisner,
A. Alfonsov,
M. -I. Sturza,
G. Bastien,
S. Gaß,
S. Wurmehl,
A. U. B. Wolter,
B. Büchner,
V. Kataev
Abstract:
We report an experimental study of the static magnetization $M(H,T)$ and high-field electron spin resonance (ESR) of polycrystalline \MgGd, a representative member of the newly discovered class of the so-called tripod-kagome antiferromagnets where the isotropic Gd$^{3+}$ spins ($S = 7/2$) form a two-dimensional kagome spin-frustrated lattice. It follows from the analysis of the low-$T$ $M(H)$-curv…
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We report an experimental study of the static magnetization $M(H,T)$ and high-field electron spin resonance (ESR) of polycrystalline \MgGd, a representative member of the newly discovered class of the so-called tripod-kagome antiferromagnets where the isotropic Gd$^{3+}$ spins ($S = 7/2$) form a two-dimensional kagome spin-frustrated lattice. It follows from the analysis of the low-$T$ $M(H)$-curves that the Gd$^{3+}$ spins are coupled by a small isotropic antiferromagnetic (AFM) exchange interaction $|J| \approx$ 0.3\,K. The $M(H,T)$-dependences measured down to 0.5\,K evidence a long-range AFM order at $T_{\text{N}} = 1.7$\,K and its rapid suppression at higher fields $\geq 4$\,T. ESR spectra measured in fields up to 15\,T are analyzed considering possible effects of demagnetizing fields, single-ion anisotropy and spin-spin correlations. While the demagnetization effects due to a large sample magnetization in high fields and its shape anisotropy become relevant at low temperatures, the broadening of the ESR line commencing already at $T\lesssim 30$\,K may indicate the onset of the spin-spin correlations far above the ordering temperature due to the geometrical spin frustration in this compound.
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Submitted 7 December, 2020; v1 submitted 25 June, 2019;
originally announced June 2019.
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Signatures of low-energy fractionalized excitations in $α$-RuCl$_3$ from field-dependent microwave absorption
Authors:
C. Wellm,
J. Zeisner,
A. Alfonsov,
A. U. B. Wolter,
M. Roslova,
A. Isaeva,
T. Doert,
M. Vojta,
B. Büchner,
V. Kataev
Abstract:
Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut e…
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Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in $α$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization which -- most remarkably -- extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in $α$-RuCl$_3$ and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.
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Submitted 9 November, 2018; v1 submitted 2 October, 2017;
originally announced October 2017.