An End to the Drought of Quantum Spin Liquids

Lee, Patrick A.

doi:10.1126/science.1163196

Cited by 467 publications

(516 citation statements)

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“…This idea was revived after the discovery in 1986 of high-temperature superconductivity 5 . A QSL, as currently understood, does not fit into Landau's conventional paradigm of symmetry breaking phases 1,2,6,7 , and is arXiv:1607.02615v2 [cond-mat.str-el] 31 Jul 2017 2 instead an exotic state of matter characterized by spinon excitations and emergent gauge structures [1][2][3]6 . The search for QSLs in models and materials [8][9][10][11][12] has been partly facilitated by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem that may hint at the possibility of QSLs in Mott insulators with odd electron fillings and a global U(1) spin rotational symmetry [13][14][15] .…”

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confidence: 99%

“…4a, b as well as the E scans at a few given momentum points (Γ, M, K) in Fig. 4c.The broad continuum is an immediate consequence and strong evidence of spinon excitations in QSLs 1,7,12 . This differs from magnon-like excitations that would peak strongly at specific momenta in the reciprocal space, with or without static magnetic order 23,24 .…”

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confidence: 99%

“…This differs from magnon-like excitations that would peak strongly at specific momenta in the reciprocal space, with or without static magnetic order 23,24 . In general, the spinful excitations in QSLs are carried by deconfined spinons 1,7 . For most experimentally relevant QSLs, the spinons carry half-integer spins.…”

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Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate

Shen

et al. 2016

Nature

350

385

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A quantum spin liquid is an exotic quantum state of matter in which spins are highly entangled and remain disordered down to zero temperature. Such a state of matter is potentially relevant to high-temperature superconductivity and quantum-information applications, and experimental identification of a quantum spin liquid state is of fundamental importance for our understanding of quantum matter. Theoretical studies have proposed various quantum-spin-liquid ground states [1][2][3][4] , most of which are characterized by exotic spin excitations with fractional quantum numbers (termed 'spinon'). Here, we report neutron scattering measurements that reveal broad spin excitations covering a wide region of the Brillouin zone in a triangular antiferromagnet YbMgGaO 4 . The observed diffusive spin excitation persists at the lowest measured energy and shows a clear upper excitation edge, which is consistent with the particle-hole excitation of a spinon Fermi surface. Our results therefore point to a QSL state with a spinon Fermi surface in YbMgGaO 4 that has a perfect spin-1/2 triangular lattice as in the original proposal 4 of quantum spin liquids.In 1973, Anderson proposed the pioneering idea of the quantum spin liquid (QSL) in the study of the triangular lattice Heisenberg antiferromagnet 4 . This idea was revived after the discovery in 1986 of high-temperature superconductivity 5 . A QSL, as currently understood, does not fit into Landau's conventional paradigm of symmetry breaking phases 1,2,6,7 , and is arXiv:1607.02615v2 [cond-mat.str-el] 31 Jul 2017 2 instead an exotic state of matter characterized by spinon excitations and emergent gauge structures [1][2][3]6 . The search for QSLs in models and materials [8][9][10][11][12] has been partly facilitated by the Oshikawa-Hastings-Lieb-Schultz-Mattis (OHLSM) theorem that may hint at the possibility of QSLs in Mott insulators with odd electron fillings and a global U(1) spin rotational symmetry [13][14][15] .Indeed, a continuum of spin excitations has been observed in a kagome-lattice material ZnCu 3 (OD) 6 Cl 2 (refs 12,16). However, the requirement of the U(1) spin rotational symmetry, prevents the application of OHLSM theorem in strong spin-orbit-coupled (SOC) Mott insulators in which the spin rotational symmetry is completely absent. A recent theory addressed this limitation of the OHLSM theorem, arguing that, as long as time-reversal symmetry is preserved, the ground state of an SOC Mott insulator with odd electron fillings must be exotic 17 .The newly discovered triangular antiferromagnet YbMgGaO 4 (refs 18,19) displays no indication of magnetic ordering or symmetry breaking at temperatures as low as 30 mK despite approximately 4 K for the spin interaction energy scale. Because of the strong SOC of the Yb electrons, YbMgGaO 4 was the first QSL to be proposed beyond the OHLSM theorem 19 . The thirteen 4 f electrons of the Yb 3+ ion form the spin-orbit-entangled Kramers doublets that are split by the D 3d crystal electric fields [20][21][22] . At temperatures co...

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Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate

Shen

et al. 2016

Nature

350

385

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“…Quantum spin liquids (QSLs) are exotic states of matter in which spins are highly correlated but remain dynamic down to zero temperature due to strong quantum fluctuations [1,2]. Many distinct QSL states have been proposed theoretically [3,4] and classified according to their nonlocal (topological) properties [5].…”

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Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

et al. 2018

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The spin-1=2 triangular lattice antiferromagnet YbMgGaO 4 has attracted attention recently as a quantum spin-liquid candidate with the possible presence of off-diagonal anisotropic exchange interactions induced by spin-orbit coupling. Whether a quantum spin liquid is stabilized or not depends on the interplay of various exchange interactions with chemical disorder that is inherent to the layered structure of the compound. We combine time-domain terahertz spectroscopy and inelastic neutron scattering measurements in the field-polarized state of YbMgGaO 4 to obtain better insight of its exchange interactions. Terahertz spectroscopy in this fashion functions as a high-field electron spin resonance and probes the spin-wave excitations at the Brillouin zone center, ideally complementing neutron scattering. A global spin-wave fit to all our spectroscopic data at fields over 4 T, informed by the analysis of the terahertz spectroscopy linewidths, yields constraints on the disorder-averaged g factors and exchange interactions. Our results paint YbMgGaO 4 as an easy-plane XXZ antiferromagnet with the combined and necessary presence of subleading next-nearest neighbor and weak anisotropic off-diagonal nearest-neighbor interactions. Moreover, the obtained g factors are substantially different from previous reports. This work establishes the hierarchy of exchange interactions in YbMgGaO 4 from high-field data alone and thus strongly constrains possible mechanisms responsible for the observed spin-liquid phenomenology.

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“…3 However, it was suggested that for some antiferromagnetic materials, strong quantum fluctuations due to small spin magnitude and low dimensionality combined with geometric frustration may lead to quantum coherent, spin disordered ground states. 4,5 In such cases, a quantum fluid approach 6 is probably a better starting point to describe the low-energy physics instead of usual spin-wave or semiclassical approach.…”

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Fermionic theory for quantum antiferromagnets with spinS>12

Liu

Zhou

2010

Phys. Rev. B

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The fermion representation for S = 1 2 spins is generalized to spins with arbitrary magnitudes. The symmetry properties of the representation is analyzed where we find that the particle-hole symmetry in the spinon Hilbert space of S = 1 2 fermion representation is absent for S Ͼ 1 2 . As a result, different path-integral representations and mean-field theories can be formulated for spin models. In particular, we construct a Lagrangian with restored particle-hole symmetry, and apply the corresponding mean-field theory to one-dimensional ͑1D͒ S = 1 and S =3/ 2 antiferromagnetic ͑AFM͒ Heisenberg models, with results that agree with Haldane's conjecture. For a S = 1 open chain, we show that Majorana fermion edge states exist in our mean-field theory. The generalization to spins with arbitrary magnitude S is discussed. Our approach can be applied to higher dimensional spin systems. As an example, we study the geometrically frustrated S = 1 AFM on triangular lattice. Two spin liquids with different pairing symmetries are discussed: the gapped p x + ip y -wave spin liquid and the gapless f-wave spin liquid. We compare our mean-field result with the experiment on NiGa 2 S 4 , which remains disordered at low temperature and was proposed to be in a spin-liquid state. Our fermionic mean-field theory provide a framework to study S Ͼ 1 2 spin liquids with fermionic spinon excitations.

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An End to the Drought of Quantum Spin Liquids

Cited by 467 publications

References 11 publications

Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate

Evidence for a spinon Fermi surface in a triangular-lattice quantum-spin-liquid candidate

Hierarchy of Exchange Interactions in the Triangular-Lattice Spin Liquid YbMgGaO4

Fermionic theory for quantum antiferromagnets with spinS>12

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