Magnetic and non-magnetic phases of a quantum spin liquid

Abstract

A quantum spin-liquid phase is an intriguing possibility for a system of strongly interacting magnetic units in which the usual magnetically ordered ground state is avoided owing to strong quantum fluctuations. It was first predicted theoretically for a triangular-lattice model with antiferromagnetically coupled S = 1/2 spins. Recently, materials have become available showing persuasive experimental evidence for such a state. Although many studies show that the ideal triangular lattice of S = 1/2 Heisenberg spins actually orders magnetically into a three-sublattice, non-collinear 120° arrangement, quantum fluctuations significantly reduce the size of the ordered moment. This residual ordering can be completely suppressed when higher-order ring-exchange magnetic interactions are significant, as found in nearly metallic Mott insulators. The layered molecular system κ-(BEDT-TTF)(2)Cu(2)(CN)(3) is a Mott insulator with an almost isotropic, triangular magnetic lattice of spin-1/2 BEDT-TTF dimers that provides a prime example of a spin liquid formed in this way. Despite a high-temperature exchange coupling, J, of 250 K (ref. 6), no obvious signature of conventional magnetic ordering is seen down to 20 mK (refs 7, 8). Here we show, using muon spin rotation, that applying a small magnetic field to this system produces a quantum phase transition between the spin-liquid phase and an antiferromagnetic phase with a strongly suppressed moment. This can be described as Bose-Einstein condensation of spin excitations with an extremely small spin gap. At higher fields, a second transition is found that suggests a threshold for deconfinement of the spin excitations. Our studies reveal the low-temperature magnetic phase diagram and enable us to measure characteristic critical properties. We compare our results closely with current theoretical models, and this gives some further insight into the nature of the spin-liquid phase.

A two-dimensional spin liquid in quantum kagome ice

“…Previous efforts have focused largely on SU(2) Hamiltonians on kagome or triangular lattice materials 6,7 . In contrast, we propose to concentrate the search on the quantum spin ice pyrochlore materials, subject to an external field along the 18 not considered in equation (1), our study highlights the importance of the of J ±± term, which is present for instance in Yb 2 Ti 2 O 7 (ref.…”

“…Some, such as emergent gauge structures and fractional charges, are implicated in a wide range of future technologies like high-temperature superconductivity 1,2 and topological quantum computing 3 . It is therefore remarkable that, despite extensive examination of the basic theoretical ingredients required to promote a 2D QSL in microscopic models 4,5 , the state remains elusive, with only a few experimental candidates existing today 6,7 .…”

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

“…Their detection, however, remains a central challenge for condensed matter physics [6], and relies on the presence of quantum entanglement in their ground state and fractional quasiparticles in their excitation spectra. While the former can be checked by numerics [7], the latter can be experimentally detected by thermodynamic techniques [8][9][10] or spectroscopic probes such as inelastic neutron scattering [11][12][13][14][15][16][17] and electronspin resonance [18][19][20][21].…”

Power-law dependence of the optical conductivity observed in the quantum spin-liquid compoundκ-(BEDT-TTF)2Cu2(CN)Elsässer1, Wu2, Dressel3 et al. 2012Phys. Rev. B

“…9 Numerous theoretical and experimental work was performed during the last decade in order to explore mainly the thermodynamic and magnetic properties. [10][11][12][13][14][15] In the high-temperature range, the 1 H-NMR relaxation rate shows anomalies around 200 K, 8 the thermopower at 150 K, 16 microwave experiments exhibit a dielectric anomaly at 113 K, 17 while a peak in ǫ ′ (T ) occurs below 60 K in the radio-frequency range. 18 In addition, a low-temperature anomaly near 6 K has been observed in thermodynamic, 7,10 transport, 11 dielectric 17 and lattice 19 properties that has not been explained satisfactorily.…”