Heat transport in magnetic fields by quantum spin liquid in the organic insulators EtMe ₃ Sb[Pd(dmit) ₂ ] ₂ and κ -(BEDT - TTF) ₂ Cu ₂ (CN) ₃

Abstract

-Measurements of the low-temperature thermal conductivity collected on insulators with geometrical frustration produce important experimental facts shedding light on the nature of quantum spin liquid composed of spinons. We employ a model of strongly correlated quantum spin liquid located near the fermion condensation phase transition to analyze the exciting measurements of the low-temperature thermal conductivity in magnetic fields collected on the organic insulators EtMe3Sb[Pd(dmit)2]2 and κ − (BEDT − TTF)2Cu2(CN)3. Our analysis of the conductivity allows us to reveal a strong dependence of the effective mass of spinons on magnetic fields, to detect a scaling behavior of the conductivity, and to relate it to both the spin-lattice relaxation rate and the magnetoresistivity. Our calculations and observations are in a good agreement with experimental data.The organic insulators EtMe 3 Sb[Pd(dmit) 2 ] 2 and κ − (BEDT − TTF) 2 Cu 2 (CN) 3 have two-dimensional triangular lattices with the geometric frustration prohibiting the formation of spin ordering even at the lowest accessible temperatures T [1-5]. Therefore, these insulators offer unique insights into the physics of quantum spin liquids (QSL). Indeed, measurements of the heat capacity on the both insulators reveal a T -linear term indicating that the low-energy excitation spectrum from the ground state is gapless [1][2][3]. The excitation spectrum can be deduced from measurements of the heat conductivity κ(T ) in the low temperature regime. For example, at T → 0 a residual value in k/T signals that the excitation spectrum is gapless. The presence of the residual value is clearly resolved in EtMe 3 Sb[Pd(dmit) 2 ] 2 , while measurements of k/T on κ − (BEDT − TTF) 2 Cu 2 (CN) 3 suggests that the low-energy excitation spectrum can have a gap (a) Email: vrshag@thd.pnpi.spb.ru [4,5]. Taking into account the observed T -linear term of the heat capacity in κ − (BEDT − TTF) 2 Cu 2 (CN) 3 with the static spin susceptibility remaining finite down to the lowest measured temperatures [6], the presence of the spin excitation gap becomes questionable. Thermal conductivity probe elementary itinerant excitations and is totally insensitive to localized ones such as those responsible for Schottky contributions, which contaminates the heat capacity measurements at low temperatures [1][2][3][4][5]. The heat conductivity is formed primarily by both acoustic phonons and itinerant spinons, while the latter form QSL. Since the phonon contribution is insensitive to the applied magnetic field B, the elementary excitations of QSL can be further explored by the magnetic field dependence of k. Measurements under the application of magnetic field B of the heat conductivity κ on these insulators have exhibited a strong dependence of κ(B, T ) as a function of B at fixed T [4,5]. The obtained dependence at low temperatures rep-1

Spin-lattice decoupling in a triangular-lattice quantum spin liquid

“…Second, as the system is away from the QCP, the number of spin states is rapidly decreased, and consequently the spin–lattice interaction is weakened. This is compatible with the SCQSL model 25 , where the QSL has much similarity with heavy-fermion liquids.…”

“…On the other hand, κ is gradually increased by applying a magnetic field. A possible scenario is a model of a strongly correlated QSL (SCQSL) located near a fermion-condensation quantum-phase-transition point, where the QSL plays the role of heavy-fermion liquids placed into insulators 25 ; as a magnetic field is increased away from the QCP, the effective mass of spinons, m *, is reduced, leading to the increase of the spin thermal conductivity. In fact, we have recently determined the H – T phase diagram based on a scaling analysis of χ , where a QCP is present near the zero field (Fig.…”

Quantum Criticality of Spin Liquids in Novel Insulators and Magnets

“…On the other hand, it is suggested that the observed B-dependence implies that some spin-gaplike excitations coupling to the magnetic field are also present at low temperatures [45,46]. As a result, we face a serious problem in interpretation of the experimental data in a consistent way, including the B-dependence of the heat conductivity [14]. Representing a special case of QSL, SCQSL is a quantum state of matter composed of spinons-chargeless fermionic particles with spin 1/2 [12,13,35].…”

“…Recently, a natural object, the herbertsmithite ZnCu 3 (OH) 6 Cl 2 has been exposed as a S = 1/2 kagome antiferromagnet [5]. New experimental investigations have revealed its unusual behavior [6][7][8][9][10] suggesting that herbertsmithite can be viewed as HF compound [11][12][13][14]. High-quality single crystals of ZnCu 3 (OH) 6 Cl 2 were synthesized and characterized [9].…”

New state of matter: heavy-fermion systems, quantum spin liquids, quasicrystals, cold gases, and high temperature superconductors

“…( 6) [31]. TTF) 2 Cu 2 (CN) 3 [65,76,77]. Measurements of thermal transport are particularly salient in that they probe the low-lying elementary excitations of SCQSL in ZnCu 3 (OH) 6 Cl 2 and potentially reveal itinerant spin excitations that are mainly responsible for the heat transport.…”