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arXiv:1907.02847 · Physical review. B./Physical review. B

Spinon excitations in the quasi-one-dimensional S=12 chain compound Cs4CuSb2Cl12

T. Thao Tran, Christopher A. Pocs, Yubo Zhang, Michal Jerzy Winiarski, Jianwei Sun, Minhyea Lee, Tyrel M. McQueen

Published Jun 1, 2020• 26 citations
arXivHTMLPDF

Abstract

The $\mathrm{spin}\text{\ensuremath{-}}\frac{1}{2}$ Heisenberg antiferromagnetic chain is ideal for realizing one of the simplest gapless quantum spin liquids (QSLs), supporting a many-body ground state whose elementary excitations are fractional fermionic excitations called spinons. Here we report the discovery of such a one-dimensional (1D) QSL in $\mathrm{C}{\mathrm{s}}_{4}\mathrm{CuS}{\mathrm{b}}_{2}\mathrm{C}{\mathrm{l}}_{12}$. Compared to previously reported $S=\frac{1}{2}$ 1D chains, this material possesses a wider temperature range over which the QSL state is stabilized. We identify spinon excitations extending at $T>0.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with a large $T$-linear contribution to the specific heat, $\ensuremath{\gamma}=31.5(2)\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, which contribute itinerantly to thermal transport up to temperatures as high as $T=35\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At $T=0.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, we find a second-order phase transition that is unchanged by a ${\ensuremath{\mu}}_{0}H=5\phantom{\rule{0.16em}{0ex}}\mathrm{T}$ magnetic field. $\mathrm{C}{\mathrm{s}}_{4}\mathrm{CuS}{\mathrm{b}}_{2}\mathrm{C}{\mathrm{l}}_{12}$ reveals new phenomenology deep in the 1D QSL regime, supporting a gapped QSL phase over a wide temperature range compared to many other experimental realizations.

DOI
10.1103/physrevb.101.235107

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Spin 1/2 1D chains are ideal for realizations of quantum spin liquids (QSLs). Here we report the discovery of a QSL in Cs4CuSb2Cl12. Compared to previously reported S = 1/2 1D chains, it possesses a wider temperature range over which the QSL state is stabilized. We find Fermionic-like excitations extending from T = 0.8 K up to at least T = 5 K, with a large T-linear contribution to the specific heat, gamma = 31.5(2) mJ mol-1 K-2. At T = 0.7 K, there is a second-order phase transition, reminiscent of a superconducting transition, with a specific heat jump of delta C / (gammaTc) = 0.8(1), that is unchanged by a 5 T magnetic field. Thus Cs4CuSb2Cl12 reveals new phenomenology deep in the 1D QSL regime.
Coexisting localized and itinerant gapless excitations in a quantum spin liquid candidate 1T-TaS2_22​
To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS2_22​, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient γγγ and the finite residual linear term of the thermal conductivity in the zero temperature limit κ0/T=κ/T(T→0)κ_0/T=κ/T(T\rightarrow 0)κ0​/T=κ/T(T→0) are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses γγγ, whereas it enhances κ0/Tκ_0/Tκ0​/T. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS2_22​ provides new insights into the influence of quenched disorder on the QSL.
Many phenomena in magnetism can be described using classical models, yet magnetism is a quantum mechanical effect. In addition to thermal fluctuations, quantum fluctuations arise due to the quantum uncertainty of the ground state. These fluctuations are enhanced by low dimensionality and low spin. Spin chains show interesting excitations associated with quasiparticles. Model materials for spin chains can be realized in compounds where the interchain interactions are weak enough. This thesis presents neutron scattering studies of two model materials for spin chains with different spin symmetry. The spin2 Heisenberg antiferromagnetic chain CuSO4*5D2O has a main exchange of J ∼ 3 K, which permits to study its excitation spectrum up to temperatures of ten times the exchange. The spin2 Heisenberg magnet has been studied widely both theoretically and experimentally. The excitation spectrum of the spin2 Heisenberg antiferromagnet consists of quasiparticles called spinons, and calculations for it exist for T = 0 and as a function of temperature. A quantitative comparison between experimental data and theoretical models across a range of temperatures is presented. The excitation spectrum of CuSO4*5D2O is excellently described by the excitation spectrum of a spin2 Heisenberg antiferromagnetic chain at finite temperatures. Energy-temperature scaling is observed at temperatures up to 2J . For these temperatures the Luttinger liquid description of the spin2 Heisenberg antiferromagnetic chain is valid. A spinon continuum is confirmed at all measured temperatures. Therefore, even at ten times the main exchange (T ≈ 10 J), CuSO4*5D2O shows the behavior of a quantum magnet, and not of a classical system at infinite temperatures, for which one would expect single spin flips. The Ising chain is one of the most studied models in magnetism, one model compound for it is RbCoCl3. Optical spectroscopy measurements suggest a lower gap than for similar compounds, which makes it better suited for high resolution neutron spectroscopy. Magnetic correlations along the chains arise in RbCoCl3 below 80 K. A gapped continuum is observed, which arises from isolated antiferromagnetic Ising chains. Below the first magnetic ordering temperature TN1 ≈ 28 K bound states are observed, which arise due to correlations which form between the chains. Strong diffuse scattering leads to the conclusion that disorder is still present. At the second magnetic ordering transition, at TN2 ≈ 14 K, the bound states split as the correlations between the chains change. Fits indicate that the correlations change continuously as a function of temperature. Above TN2 the excitation spectrum of RbCoCl3 can be described by a spin1 2 Ising chain with an XY-component which is treated as a perturbation. Below TN2 the main features are reproduced correctly, but deviations from the details indicate that additional terms in the Hamiltonian may be necessary. This thesis was carried out at the Institut Laue-Langevin, the Laboratory for Neutron Scattering and Imaging at the Paul Scherrer Institute and ETH Zurich.
Motivated by recent experiments on the Heisenberg S=1/2S=1/2S=1/2 quantum spin liquid candidate material kapellasite, we classify all possible chiral (time-reversal symmetry breaking) spin liquids with fermionic spinons on the kagome lattice. We obtain the phase diagram for the physically relevant extended Heisenberg model, comparing the energies of a wide range of microscopic variational wave functions. We propose that, at low temperature, kapellasite exhibits a gapless chiral spin liquid phase with spinon Fermi surfaces. This two-dimensional state inherits many properties of the nearby one-dimensional phase of decoupled antiferromagnetic spin chains, but also shows some remarkable differences. We discuss the spin structure factors and other physical properties.
Quantum spin liquids (QSLs) are intriguing phases of matter possessing fractionalized excitations.Several quasi-two-dimensional materials have been proposed as candidate QSLs, but direct evidence for fractionalization in these systems is still lacking.In this paper, we show that the interplane thermal conductivity in layered QSLs carries a unique signature of fractionalization.We examine several types of gapless QSL phases-a Z 2 QSL with either a Dirac spectrum or a spinon Fermi surface, and a Uð1Þ QSL with a Fermi surface-and consider both clean and disordered systems.In all cases, the in-plane and c-axis thermal conductivities have a different power-law dependence on temperature because of the different mechanisms of transport in the two directions: In the planes, the thermal current is carried by fractionalized excitations, whereas the interplane current is carried by integer (nonfractional) excitations.In layered Z 2 and Uð1Þ QSLs with a Fermi surface, and in the disordered Z 2 QSL with a Dirac dispersion, the c-axis thermal conductivity is parametrically smaller than the in-plane one but parametrically larger than the phonon contribution at low temperatures.
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