Abstract Quantum spin liquids (QSL) are prototypical examples of ground states with massive many-body entanglement. While an unambiguous realization of this long-sought-after state remains elusive, a growing number of materials candidates keep emerging with varieties of novel and exotic properties. Here, we report a combined experimental and theoretical study of a new antiferromagnet Ba 3 CuNb 2 O 9 which has an in-plane anisotropic exchange interaction arising from distorted triangular arrangements of magnetic Cu 2 + (spin-1/2) ions. We show an unconventional gapless QSL like ground state in this material which does not exhibit any signature of magnetic ordering down to 0.3 K, while a finite Curie–Weiss temperature ∼ − 41 K is evident. The gapless QSL state is supported by a high spin fluctuation, a power-law dependence of magnetic specific heat ( C m ∝ T γ ) and a unique scaling of magnetic susceptibility and specific heat with temperature and magnetic field respectively. Our ab-initio simulation also confirms the anisotropic exchange interaction which can only be explained via superexchange mechanism. While the combined results strongly suggest a random singlet state driven QSL behavior in present material, we strongly believe that the anisotropic exchange interaction has a possible role for this exotic ground state behavior.
Abstract Quantum spin liquids (QSL) are prototypical examples of ground states with massive many-body entanglement. While an unambiguous realization of this long-sought-after state remains elusive, a growing number of materials candidates keep emerging with varieties of novel and exotic properties. Here, we report a combined experimental and theoretical study of a new antiferromagnet Ba 3 CuNb 2 O 9 which has an in-plane anisotropic exchange interaction arising from distorted triangular arrangements of magnetic Cu 2 + (spin-1/2) ions. We show an unconventional gapless QSL like ground state in this material which does not exhibit any signature of magnetic ordering down to 0.3 K, while a finite Curie–Weiss temperature ∼ − 41 K is evident. The gapless QSL state is supported by a high spin fluctuation, a power-law dependence of magnetic specific heat ( C m ∝ T γ ) and a unique scaling of magnetic susceptibility and specific heat with temperature and magnetic field respectively. Our ab-initio simulation also confirms the anisotropic exchange interaction which can only be explained via superexchange mechanism. While the combined results strongly suggest a random singlet state driven QSL behavior in present material, we strongly believe that the anisotropic exchange interaction has a possible role for this exotic ground state behavior.
Motivated by recent experiments on the material Ba3NiSb2O9, we consider a spin-one quantum antiferromagnet on a triangular lattice with the Heisenberg bilinear and biquadratic exchange interactions and a single-ion anisotropy. Using a fermionic ``triplon'' representation for spins, we study the phase diagram within mean-field theory. In addition to a fully gapped spin-liquid ground state, we find a state where one gapless triplon mode with a Fermi surface coexists with d+id topological pairing of the other triplons. Despite the existence of a Fermi surface, this ground state has fully gapped bulk spin excitations. Such a state has linear in-temperature specific heat and constant in-plane spin susceptibility, with an unusually high Wilson ratio.
Experimental quest for the hypothetical "quantum spin liquid" state has recently met a few promising candidate materials including organic salts κ-(ET)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2, S=1/2 triangular-lattice Heisenberg antiferromagnets consisting of molecular dimers. These compounds exhibit no magnetic ordering nor the spin freezing down to very low temperature, while various physical quantities exhibit gapless behaviors. Recent dielectric measurements revealed the glassy dielectric response suggesting the random freezing of the electric polarization degrees of freedom. Inspired by this observation, we propose as a minimal model of the observed quantum spin-liquid behavior the S=1/2 antiferromagnetic Heisenberg on the triangular lattice with a quenched randomness in the exchange interaction. We study both zero- and finite-temperature properties of the model by an exact diagonalization method, to find that when the randomness exceeds a critical value the model exhibits a quantum spin-liquid ground state. This randomness-induced quantum spin-liquid state exhibits gapless behaviors including the temperature-linear specific heat. The results provide a consistent explanation of the recent experimental observations on organic salts.
Effect of quenched disorder on a quantum spin liquid state of triangular-lattice antiferromagnet 1T-TaS2
A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS2 is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted and electron irradiated crystals of 1T-TaS2. In pure 1T-TaS2, the linear temperature term of the heat capacity γT and the finite residual linear term of the thermal conductivity in the zero-temperature limit κ0/T≡κ/T(T→0) are clearly resolved, consistent with the presence of gapless spinons with a Fermi surface. Moreover, while the strong magnetic field slightly enhances κ0/T, it strongly suppresses γ. These unusual contrasting responses to magnetic field imply the coexistence of two types of gapless excitations with itinerant and localized characters. Introduction of additional weak random exchange disorder in 1T-Ta(S1−xSex)2 leads to vanishing of κ0/T, indicating that the itinerant gapless excitations are sensitive to the disorder. On the other hand, in both pure and Se-substituted systems, the magnetic contribution of the heat capacity obeys a universal scaling relation, which is consistent with a theory that assumes the presence of localized orphan spins forming random singlets. Electron irradiation in pure 1T-TaS2 largely enhances γ and changes the scaling function dramatically, suggesting a possible new state of spin liquid.
Coexisting localized and itinerant gapless excitations in a quantum spin liquid candidate 1T-TaS2
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, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient γ and the finite residual linear term of the thermal conductivity in the zero temperature limit 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 γ, whereas it enhances κ0/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 provides new insights into the influence of quenched disorder on the QSL.
We investigate the ground-state and finite-temperature properties of the bond-random s=1/2 Heisenberg model on a honeycomb lattice with frustrated nearest- and next-nearest-neighbor antiferromagnetic interactions, J1 and J2, by the exact diagonalization and the Hams--de Raedt methods. The ground-state phase diagram of the model is constructed in the randomness versus the frustration (J2/J1) plane, with the aim of clarifying the effects of randomness and frustration in stabilizing a variety of distinct phases. We find that the randomness induces the gapless quantum spin liquid (QSL)-like state, the random-singlet state, in a wide range of parameter space. The observed robustness of the random-singlet state suggests that the gapless QSL-like behaviors might be realized in a wide class of frustrated quantum magnets possessing a certain amount of randomness or inhomogeneity, without fine-tuning the interaction parameters. Possible implications to recent experiments on the honeycomb-lattice magnets Ba3CuSb2O9 and 6HB-Ba3NiSb2O9 exhibiting the gapless QSL-like behaviors are discussed.
In frustrated magnets, the experimental realization of a quantum disordered ground state (QDGS), especially quantum spin liquid (QSL) ground state, is a central topic in condensed matter physics.Several promising candidates were reported, and a number of QSL-like behavior, such as excitation continuum, Tlinear specific heat, and so on, were observed.However, whether the QSL-like behavior is owing to the synergy between spin frustration and quantum fluctuation or bond randomness effects has not been clarified yet.Several theoretical studies have demonstrated that the synergy effect between the magnetic frustration and the bond randomness suppresses magnetic ordering and induces a QSL-like state.This QSL-like state was proposed to be a random-singlet (RS) state, where not only nearest-neighbor spins but also distant spins form singlet pairs with random coupling energy.Since thermodynamic properties of RS and QSL are predicted to be similar to each other, it is important to elucidate the microscopic nature of randomness-induced QDGSs.However, its experimental verifications are still insufficient due to a lack of model materials.In this work, I synthesized polycrystalline samples of SrLaCuSbO 6 (SLCSO) and SrLaCuNbO 6 (SLCNO) by a solid-state reaction.Magnetic Cu 2+ ions with S = 1/2 Heisenberg spins form a square lattice with nearest-neighbor (J 1 ) and next-nearest-neighbor (J 2 ) exchange interaction.Thus, SLCSO and SLCNO were magnetically described as quasi-two-dimensional S = 1/2 J 1 -J 2 square lattice Heisenberg antiferromagnet (SLHAF).The temperature dependence of the magnetic susceptibilities of both compounds shows a broad maximum characteristic of an S = 1/2 SLHAF.However, both compounds undergo magnetic phase transitions at T N = 13.6 and 15.7 K for SLCSO and SLCNO, respectively, due to weak inter-layer interactions.Neutron powder diffraction measurements reveal contrasting spin structures in both compounds.The spin structures of SLCSO and SLCNO below T N are Néel antiferromagnetic and collinear antiferromagnetic, respectively.This result demonstrates that the nearest-neighbor interaction is dominant in SLCSO, whereas the next-nearest-neighbor interaction is dominant in SLCNO.The magnitude of the ordered moment was evaluated at 3.5 K to be m = 0.39(3) µ B for SLCSO and 0.37(1) µ B for SLCNO, which are significantly reduced from 1µ B expected in the classical spin case.The shrink of the ordered magnetic moment was caused by not only magnetic frustration effects but also bond randomness effects arising from the site disorder of Sr 2+ and La 3+ ions.Next, I synthesized the mixed system SrLaCuSb 1-x Nb x O 6 with 0.1 ≤ x ≤