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

Abstract

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...

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

“…Indeed, classical Monte Carlo simulations for the instantaneous spin structure factor SðQÞ reveal that either large J AEAE 1 or relatively small J 2 ∼ 0.2J 1 yields correlations peaked at the M point of the triangular-lattice Brillouin zone [61]. Maximal correlations at the M point have been reported in all neutron scattering investigations to date [61][62][63]65]. In this regard, we note that Monte Carlo simulations with a large J AEAE 1 yield a strong modulation in SðQÞ from the first to the second Brillouin zone [61].…”

“…In the context of prevailing models our results suggest that both NNN and off-diagonal anisotropic exchange interactions are present in YbMgGaO 4 -with pseudodipolar terms subleading compared with the XXZ part of the model-and that this nominal set of exchanges may lie closer to a spin-liquid regime than previously thought [47,61]. When combined with the likely presence of exchange disorder, the extent of which remains to be determined, our work strongly constrains the nature of the YbMgGaO 4 ground state and points towards a more intricate underlying scenario than originally reported [62].…”

“…The immediate availability of single crystals [55] uncovered a QSL phenomenology in YbMgGaO 4 characterized by the absence of spin ordering or freezing down to T ¼ 100 mK in muon spin relaxation measurements [59], much lower than the Curie-Weiss temperature θ W ≈ −4 K, and a power-law behavior for the magnetic specific heat at low temperatures [55,60]. Perhaps the strongest evidence for a QSL in YbMgGaO 4 came from inelastic neutron scattering measurements in zero field that unraveled a broad continuum of magnetic excitations across the entire Brillouin zone [61][62][63]. This continuum has been interpreted as being composed of fractional excitations from a U(1) QSL state with spinon Fermi surface [42,44,62] or from a RVB-like state [63].…”

“…Perhaps the strongest evidence for a QSL in YbMgGaO 4 came from inelastic neutron scattering measurements in zero field that unraveled a broad continuum of magnetic excitations across the entire Brillouin zone [61][62][63]. This continuum has been interpreted as being composed of fractional excitations from a U(1) QSL state with spinon Fermi surface [42,44,62] or from a RVB-like state [63].…”

“…In fact, the nature of the dominant exchange interactions in YbMgGaO 4 is also controversial. While the overall planar anisotropy is clear from susceptibility measurements [55,62], both antiferromagnetic next-nearest-neighbor terms (J 2 ) [61] and nearest-neighbor anisotropic offdiagonal exchanges (so-called J AEAE 1 and J zAE 1 ) [43,65] have been proposed as extensions from the XXZ model. Comprehensively determining the exchange interactions in YbMgGaO 4 is of fundamental importance in deciphering the nature of its ground state.…”