Field-induced quantum phase transitions in the spin-1/2 triangular-lattice antiferromagnet Cs 2 CuBr 4 (original) (raw)
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Physical Review Letters, 2009
We report magnetocaloric and magnetic-torque evidence that in Cs2CuBr4 -a geometrically frustrated Heisenberg S = 1 2 triangular-lattice antiferromagnet -quantum fluctuations stabilize a series of spin states at simple increasing fractions of the saturation magnetization Ms. Only the first of these states -at M = 1 3 Ms -has been theoretically predicted. We discuss how the higher fraction quantum states might arise and propose model spin arrangements. We argue that the first-order nature of the transitions into those states is due to strong lowering of the energies by quantum fluctuations, with implications for the general character of quantum phase transitions in geometrically frustrated systems.
Ground states of a frustrated spin-12 antiferromagnet: Cs2CuCl4 in a magnetic field
Physical Review B, 2005
We present detailed calculations of the magnetic ground state properties of Cs2CuCl4 in an applied magnetic field, and compare our results with recent experiments. The material is described by a spin Hamiltonian, determined with precision in high field measurements, in which the main interaction is antiferromagnetic Heisenberg exchange between neighboring spins on an anisotropic triangular lattice. An additional, weak Dzyaloshinkii-Moriya interaction introduces easy-plane anisotropy, so that behavior is different for transverse and longitudinal field directions. We determine the phase diagram as a function of field strength for both field directions at zero temperature, using a classical approximation as a first step. Building on this, we calculate the effect of quantum fluctuations on the ordering wavevector and components of the ordered moments, using both linear spinwave theory and a mapping to a Bose gas which gives exact results when the magnetization is almost saturated. Many aspects of the experimental data are well accounted for by this approach.
Field-driven phase transitions in a quasi-two-dimensional quantum antiferromagnet
New Journal of Physics, 2007
We report magnetic susceptibility, specific heat, and neutron scattering measurements as a function of applied magnetic field and temperature to characterize the S = 1/2 quasi-two-dimensional frustrated magnet piperazinium hexachlorodicuprate (PHCC). The experiments reveal four distinct phases. At low temperatures and fields the material forms a quantum paramagnet with a 1 meV singlet triplet gap and a magnon bandwidth of 1.7 meV. The singlet state involves multiple spin pairs some of which have negative ground state bond energies. Increasing the field at low temperatures induces three dimensional long range antiferromagnetic order at 7.5 Tesla through a continuous phase transition that can be described as magnon Bose-Einstein condensation. The phase transition to a fully polarized ferromagnetic state occurs at 37 Tesla. The ordered antiferromagnetic phase is surrounded by a renormalized classical regime. The crossover to this phase from the quantum paramagnet is marked by a distinct anomaly in the magnetic susceptibility which coincides with closure of the finite temperature singlet-triplet pseudo gap. The phase boundary between the quantum paramagnet and the Bose-Einstein condensate features a finite temperature minimum at T = 0.2 K, which may be associated with coupling to nuclear spin or lattice degrees of freedom close to quantum criticality.
Quantum Stabilization of the 1/3-Magnetization Plateau in Cs2CuBr4
Physical Review Letters, 2009
We consider the phase diagram of a spatially anisotropic 2D triangular antiferromagnet in a magnetic field. Classically, the ground state is umbrella-like for all fields, but we show that the quantum phase diagram is much richer and contains a 1/3 magnetization plateau, two commensurate planar states, two incommensurate chiral umbrella phases, and, possibly, a planar state separating the two chiral phases. Our analysis sheds light on several recent experimental findings for the spin-1/2 system Cs2CuBr4.
Full ferromagnetic saturation of a two-dimensional quantum antiferromagnet
Applied Physics A: Materials Science & Processing, 2002
Cs 2 CuCl 4 is a 2D frustrated quantum magnet that has recently been shown to display a very unusual quantum spin liquid state. The excitations are not spin-1 magnons as observed in other un-frustrated 2D quantum magnets but instead are spinons carrying a fractional spin of 1/2, as predicted by a resonating-valence-bond picture. Here we use high magnetic fields to induce a transition from this fractional quantum spin-liquid phase to the fully-polarized phase where spins are ferromagnetically aligned along the field. In this field-induced phase all quantum fluctuations are quenched by the external field and the system is expected to behave like a classical magnet with spin-1 magnon excitations. Measurements are made in fields up to 12 T and temperatures below 0.2 K. Ferromagnetic saturation is observed at the critical field B c = 8.44(1) T a. Above B c the measured excitations lineshapes show well-defined, almost resolutionlimited peaks as expected for spin-1 magnons and are gapped throughout the zone. The gap to the lowest energy excitation decreases linearly upon decreasing field and closes at B c , below which the system orders into a cone phase. This transition provides an opportunity to study how ordered phases arise from the condensation of excitations.
Extreme sensitivity of a frustrated quantum magnet: Cs_{2}CuCl_{4}
Physical Review B, 2010
We report a thorough theoretical study of the low temperature phase diagram of Cs2CuCl4, a spatially anisotropic spin S = 1/2 triangular lattice antiferromagnet, in a magnetic field. Our results, obtained in a quasi-one-dimensional limit in which the system is regarded as a set of weakly coupled Heisenberg chains, are in excellent agreement with experiment. The analysis reveals some surprising physics. First, we find that, when the magnetic field is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. This is despite the fact that the inter-layer exchange coupling in Cs2CuCl4 is about an order of magnitude smaller than the weakest (diagonal) exchange in the triangular planes themselves. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions, heretofore neglected, of order a few percent or less of the largest exchange couplings. These interactions, which we describe in detail, induce entirely new phases, and a novel commensurate-incommensurate transition, the signatures of which are identified in NMR experiments. We discuss the differences between the behavior of Cs2CuCl4 and an ideal two-dimensional triangular model, and in particular the occurrence of magnetization plateaux in the latter. These and other related results are presented here along with a thorough exposition of the theoretical methods, and a discussion of broader experimental consequences to Cs2CuCl4 and other materials. ) ' ( J J
Phase Transitions of a Geometrically Frustrated Spin System CdCr2O4in Very High Magnetic Fields
Journal of the Physical Society of Japan, 2007
Phase transitions of the mixed spin-1/2 and spin-1 Ising-Heisenberg model on several decorated planar lattices consisting of interconnected diamonds are investigated within the framework of the generalized decoration-iteration transformation. The main attention is paid to the systematic study of the finite-temperature phase diagrams in dependence on the lattice topology. The critical behaviour of the hybrid quantum-classical Ising-Heisenberg model is compared with the relevant behaviour of its semi-classical Ising analogue. It is shown that both models on diamond-like decorated planar lattices exhibit a striking critical behaviour including reentrant phase transitions. The higher the lattice coordination number is, the more pronounced reentrance may be detected.
Quantum phase transitions in a three-dimensional frustratedS=1/2 Heisenberg antiferromagnet
Physical Review B, 1995
The spherically symmetric Green s function approximation is used to study a three-dimensional (3D) S = 2 antiferromagnet described by isotropic Heisenberg Hamiltonian. Detailed numerical calculations are carried out for the simple cubic lattice with antiferromagnetic nearestand next-nearest-neighbor exchange interactions (J&-J2 model). We obtained the phase diagram indicating two regions of temperature and frustration parameters p = J2/(J& +J2) where two diferent ordered states are possible. The absence of the long-range order (LRO) in 3D lattice even at zero temperature at some values of frustration is emphasized. At zero temperature with increasing frustration the system endures the second-order phase transitions from the Neel-type LRO state to the stripe-type LRO state through the spin liquid state.
Physical Review B, 2023
The spin-1/2 Heisenberg antiferromagnet on the frustrated diamond-decorated square lattice is known to feature various zero-field ground-state phases, consisting of extended monomer-dimer and dimer-tetramer ground states as well as a ferrimagnetic regime. Using a combination of analytical arguments, density matrix renormalization group (DMRG), exact diagonalization as well as sign-problem-free quantum Monte Carlo (QMC) calculations, we investigate the properties of this system and the related Lieb lattice in the presence of a finite magnetic field, addressing both the ground-state phase diagram as well as several thermodynamic properties. In addition to the zero-field ground states, we find at high magnetic field a spin-canted phase with a continuously rising magnetization for increasing magnetic field strength as well as the fully polarized paramagnetic phase. At intermediate field strength, we identify a first-order quantum phase transition line between the ferrimagnetic and the monomer-dimer regime. This first-order line extends to finite temperatures, terminating in a line of critical points that belong to the universality class of the two-dimensional Ising model.
Physical Review B, 2011
Recent experiments on the anisotropic spin-1/2 triangular antiferromagnet Cs2CuBr4 have revealed a remarkably rich phase diagram in applied magnetic fields, consisting of an unexpectedly large number of ordered phases. Motivated by this finding, we study the role of three ingredientsspatial anisotropy, Dzyaloshinskii-Moriya interactions, and quantum fluctuations-on the magnetization process of a triangular antiferromagnet, coming from the semiclassical limit. The richness of the problem stems from two key facts: 1) the classical isotropic model with a magnetic field exhibits a large accidental ground state degeneracy, and 2) these three ingredients compete with one another and split this degeneracy in opposing ways. Using a variety of complementary approaches, including extensive Monte Carlo numerics, spin-wave theory, and an analysis of Bose-Einstein condensation of magnons at high fields, we find that their interplay gives rise to a complex phase diagram consisting of numerous incommensurate and commensurate phases. Our results shed light on the observed phase diagram for Cs2CuBr4 and suggest a number of future theoretical and experimental directions that will be useful for obtaining a complete understanding of this material's interesting phenomenology.