Critical-point anomaly in the EPR linewidth of two-dimensional antiferromagnets

Calculations are presented which satisfactorily explain the observed temperature and angular dependence of the EPR linewidth in the two-dimensional antiferromagnets Rb₂MnF₄ and K₂MnF₄ near T_N. These calculations are based on an earlier theory by Huber for the three-dimensional systems.     

Quantum percolation in two-dimensional antiferromagnets

The interplay of geometric randomness and strong quantum fluctuations is an exciting topic in quantum many-body physics, leading to the emergence of novel quantum phases in strongly correlated electron systems. Recent investigations have focused on the case of homogeneous site and bond dilution in the quantum antiferromagnet on the square lattice, reporting a classical geometric percolation transition between magnetic order and disorder. In this study we show how inhomogeneous bond dilution leads to percolative quantum phase transitions, which we have studied extensively by quantum Monte Carlo simulations. Quantum percolation introduces a new class of two-dimensional spin liquids, characterized by an infinite percolating network with vanishing antiferromagnetic order parameter.     

Spin orthogonality catastrophe in two-dimensional antiferromagnets and superconductors

We compute the spectral function of a spin S hole injected into a two-dimensional antiferromagnet or superconductor in the vicinity of a magnetic quantum critical point. We show that, near Van Hove singularities, the problem maps onto that of a static vacancy carrying excess spin S. The hole creation operator is characterized by a new boundary anomalous dimension and a vanishing quasiparticle residue at the critical point. We discuss possible relevance to photoemission spectra of cuprate superconductors near the antinodal points.     

Quantum criticality and percolation in dimer-diluted two-dimensional antiferromagnets

The S = 1/2 Heisenberg model is considered on bilayer and single-layer square lattices with couplings J1, J2, with each spin belonging to one J2-coupled dimer. A transition from a Néel to disordered ground state occurs at a critical value of g = J2/J1. The systems are here studied at their dimer-dilution percolation points p*. The multicritical point (g*,p*) previously found for the bilayer is not reproduced for the single layer. Instead, there is a line of critical points (g < g*, p*) with continuously varying exponents. The uniform magnetic susceptibility diverges as T(-alpha) with alpha element of [1/2,1]. This unusual behavior is attributed to an effective free-moment density approximately T(1-alpha). The susceptibility of the bilayer is not divergent but exhibits remarkably robust quantum-critical scaling.     

Approximation to the susceptibilities of two-dimensional antiferromagnets

Approximate susceptibilities of square-plane S = ∞ XY and $\text{S =}\text{1}\text{2}$ Ising antiferromagnets are calculated by using the new expansion method reported previously for the case of a square-plane S = ∞ Heisenberg antiferromagnet. The convergence of the new expansion is examined and it is shown that the present method gives a good approximation to the susceptibilities of all of the Heisenberg, XY and Ising antiferromagnets even below the temperature range where the result exhibits a pronounced broad maximum.     

Temperature dependence of EPR linewidth in one-dimensional magnets: A quasi-classical approach

An analytical expression for the EPR linewidth of an exchange-coupled 1/2-spin chain originating from uniform Dzyaloshinskii-Moriya interaction is derived using a quasi-classical approach. The expression successfully reproduces the results obtained by numerical quantum mechanical calculations based on Green’s function method at T > 2J/k _B.     

The magnetic order of two-dimensional anisotropic antiferromagnets

We study the two-dimensional quantum Heisenberg antiferromagnet on the square lattice with easy-axis exchange anisotropy by means of Green’s function approach within random phase and Callen’s approximations. The Néel temperature T_N, energy gap w₀ and staggered magnetization m are calculated. The theoretical predictions of T_N and w₀ for K₂NiF₄, Rb₂MnF₄, K₂MnF₄, Rb₂MnCl₄ and (CH₃NH₃)₂MnCl₄ fit well to the measured values. The power law behavior of is also investigated. The exponents β and ν for K₂NiF₄ are in excellent agreement with the experimental results.     

Entanglement and factorized ground states in two-dimensional quantum antiferromagnets

Making use of exact results and quantum Monte Carlo data for the entanglement of formation, we show that the ground state of anisotropic two-dimensional S=1/2 antiferromagnets in a uniform field takes the classical-like form of a product state for a particular value and orientation of the field, at which the purely quantum correlations due to entanglement disappear. Analytical expressions for the energy and the form of such states are given, and a novel type of exactly solvable two-dimensional quantum models is therefore singled out. Moreover, we show that the field-induced quantum phase transition present in the models is unambiguously characterized by a cusp minimum in the pairwise-to-global entanglement ratio R, marking the quantum-critical enhancement of multipartite entanglement.     

Two-magnon Raman scattering in two-dimensional antiferromagnets at finite temperatures

The temperature dependence of two-magnon Raman scattering in the quadratic-layer antiferromagnets K₂NiF₄ and K₂MnF₄ is found to agree with the higher-order Green function theory of Balucani and Tognetti. New experimental data on K₂MnF₄ are also presented.     

Critical damping of the uniform mode in easy-axis antiferromagnets: MnF2

We report calculations of the critical damping of the uniform spin wave mode in MnF₂. Evaluation of the damping rate using empirical values for the parameters leads to results in reasonable agreement with experiment for 0.1 K < T_N ? T < 1K.     

Spin dynamics and magnetic correlation length in two-dimensional quantum heisenberg antiferromagnets

The correlated spin dynamics and temperature dependence of the correlation length xi(T) in two-dimensional quantum (S = 1/2) Heisenberg antiferromagnets (2DQHAF) on a square lattice are discussed in light of experimental results of proton spin lattice relaxation in copper formiate tetradeuterate. In this compound the exchange constant is much smaller than the one in recently studied 2DQHAF, such as La2CuO4 and Sr2CuO2Cl2. Thus the spin dynamics can be probed in detail over a wider temperature range. The NMR relaxation rates turn out to be in excellent agreement with a theoretical mode-coupling calculation. The deduced temperature behavior of xi(T) is in agreement with high-temperature expansions, quantum Monte Carlo simulations, and the pure quantum self-consistent harmonic approximation. Contrary to the predictions of the theories based on the nonlinear sigma model, no evidence of crossover between different quantum regimes is observed.