High-energy magnon dispersion and multi-magnon continuum in the two-dimensional Heisenberg antiferromagnet

We use quantum Monte Carlo simulations and numerical analytic continuation to study high-energy spin excitations in the two-dimensional S=1/2 Heisenberg antiferromagnet at low temperature. We present results for both the transverse and longitudinal dynamic spin structure factor S(q,w) at q=(pi,0) and (pi/2,pi/2). Linear spin-wave theory predicts no dispersion on the line connecting these momenta. Our calculations show that in fact the magnon energy at (pi,0) is 10% lower than at (pi/2,pi/2). We also discuss the transverse and longitudinal multi-magnon continua and their relevance to neutron scattering experiments.Comment: 4 page

Modulated phases in a three-dimensional Maier-Saupe model with competing interactions

This work is dedicated to the study of the discrete version of the Maier-Saupe model in the presence of competing interactions. The competition between interactions favoring different orientational ordering produces a rich phase diagram including modulated phases. Using a mean-field approach and Monte Carlo simulations, we show that the proposed model exhibits isotropic and nematic phases and also a series of modulated phases that meet at a multicritical point, a Lifshitz point. Though the Monte Carlo and mean-field phase diagrams show some quantitative disagreements, the Monte Carlo simulations corroborate the general behavior found within the mean-field approximation.We thank P. Gomes, R. Kaul, G. Landi, M. Oliveira, R. Oliveira, and S. Salinas for useful discussions and suggestions. P.F.B. was supported by Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP) and the Condensed Matter Theory Visitors Program at Boston University. N.X. and A.W.S. were funded in part by the NSF under Grant No. DMR-1410126. Some of the calculations were carried out on Boston University's Shared Computing Cluster. (Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Condensed Matter Theory Visitors Program at Boston University; DMR-1410126 - NSF)Accepted manuscrip

Thermodynamics of a gas of deconfined bosonic spinons in two dimensions

We consider the quantum phase transition between a Neel antiferromagnet and a valence-bond solid (VBS) in a two-dimensional system of S=1/2 spins. Assuming that the excitations of the critical ground state are linearly dispersing deconfined spinons obeying Bose statistics, we derive expressions for the specific heat and the magnetic susceptibility at low temperature T. Comparing with quantum Monte Carlo results for the J-Q model, which is a candidate for a deconfined Neel-VBS transition, we find excellent agreement, including a previously noted logarithmic correction in the susceptibility. In our treatment, this is a direct consequence of a confinement length scale Lambda which is proportional to the correlation length xi raised to a non-trivial power; Lambda ~ xi^(1+a) ~1/T^(1+a), with a>0 (with a approximately 0.2 in the model).Comment: 4+ pages, 3 figures. v2: cosmetic changes onl

Striped phase in a quantum XY-model with ring exchange

We present quantum Monte Carlo results for a square-lattice S=1/2 XY-model with a standard nearest-neighbor coupling J and a four-spin ring exchange term K. Increasing K/J, we find that the ground state spin-stiffness vanishes at a critical point at which a spin gap opens and a striped bond-plaquette order emerges. At still higher K/J, this phase becomes unstable and the system develops a staggered magnetization. We discuss the quantum phase transitions between these phases.Comment: 4 pages, 4 figures. v2: only minor change

Ground state projection of quantum spin systems in the valence bond basis

A Monte Carlo method for quantum spin systems is formulated in the basis of valence bond (singlet pair) states. The non-orthogonality of this basis allows for an efficient importance-sampled projection of the ground state out of an arbitrary state. The method provides access to resonating valence-bond physics, enables a direct improved estimator for the singlet-triplet gap, and extends the class of models that can be studied without negative-sign problems. As a demonstration, the valence bond distribution in the ground state of the 2D Heisenberg antiferromagnet is calculated. Generalizations of the method to fermion systems are also discussed.Comment: 4+ pages, accepted for publication in Phys. Rev. Let

Order-Disorder Transition in a Two-Layer Quantum Antiferromagnet

We have studied the antiferromagnetic order -- disorder transition occurring at $T=0$ in a 2-layer quantum Heisenberg antiferromagnet as the inter-plane coupling is increased. Quantum Monte Carlo results for the staggered structure factor in combination with finite-size scaling theory give the critical ratio $J_c = 2.51 \pm 0.02$ between the inter-plane and in-plane coupling constants. The critical behavior is consistent with the 3D classical Heisenberg universality class. Results for the uniform magnetic susceptibility and the correlation length at finite temperature are compared with recent predictions for the 2+1-dimensional nonlinear $\sigma$-model. The susceptibility is found to exhibit quantum critical behavior at temperatures significantly higher than the correlation length.Comment: 11 pages (5 postscript figures available upon request), Revtex 3.

Quantum Monte Carlo with Directed Loops

We introduce the concept of directed loops in stochastic series expansion and path integral quantum Monte Carlo methods. Using the detailed balance rules for directed loops, we show that it is possible to smoothly connect generally applicable simulation schemes (in which it is necessary to include back-tracking processes in the loop construction) to more restricted loop algorithms that can be constructed only for a limited range of Hamiltonians (where back-tracking can be avoided). The "algorithmic discontinuities" between general and special points (or regions) in parameter space can hence be eliminated. As a specific example, we consider the anisotropic S=1/2 Heisenberg antiferromagnet in an external magnetic field. We show that directed loop simulations are very efficient for the full range of magnetic fields (zero to the saturation point) and anisotropies. In particular for weak fields and anisotropies, the autocorrelations are significantly reduced relative to those of previous approaches. The back-tracking probability vanishes continuously as the isotropic Heisenberg point is approached. For the XY-model, we show that back-tracking can be avoided for all fields extending up to the saturation field. The method is hence particularly efficient in this case. We use directed loop simulations to study the magnetization process in the 2D Heisenberg model at very low temperatures. For LxL lattices with L up to 64, we utilize the step-structure in the magnetization curve to extract gaps between different spin sectors. Finite-size scaling of the gaps gives an accurate estimate of the transverse susceptibility in the thermodynamic limit: chi_perp = 0.0659 +- 0.0002.Comment: v2: Revised and expanded discussion of detailed balance, error in algorithmic phase diagram corrected, to appear in Phys. Rev.

Double-layer Heisenberg antiferromagnet at finite temperature: Brueckner Theory and Quantum Monte Carlo simulations

The double-layer Heisenberg antiferromagnet with intra- and inter-layer couplings $J$ and $J_\perp$ exhibits a zero temperature quantum phase transition between a quantum disordered dimer phase for $g>g_c$ and a Neel phase with long range antiferromagnetic order for $g<g_c$, where $g=J_\perp/J$ and $g_c \approx 2.5$. We consider the behavior of the system at finite temperature for $g \ge g_c$ using two different and complementary approaches; an analytical Brueckner approximation and numerically exact quantum Monte Carlo simulations. We calculate the temperature dependent spin excitation spectrum (including the triplet gap), dynamic and static structure factors, the specific heat, and the uniform magnetic susceptibility. The agreement between the analytical and numerical approaches is excellent. For $T \to 0$ and $g \to g_c$, our analytical results for the specific heat and the magnetic susceptibility coincide with those previously obtained within the nonlinear $\sigma$ model approach for $N\to \infty$. Our quantum Monte Carlo simulations extend to significantly lower temperatures than previously, allowing us to obtain accurate results for the asymptotic quantum critical behavior. We also obtain an improved estimate for the critical coupling: $g_c = 2.525 \pm 0.002$.Comment: 23 pages, 12 figure

'Americanization' and the drivers of the establishment and use of works councils in three post-socialist countries

We question notions of the ‘Americanization’ of employment relations in Slovenia, Slovakia and Croatia. First, we examine the roles of unions, the use of US strategic approach to Human Resource Management (SHRM), and management perceptions of their organizations’ innovativeness in the establishment of Works Council (WCs). Second, we employ the same variables in relation to the use of WCs for downward communication in these countries in comparison with what Amable (2003) terms the Continental European Coordinated Market Economy (CECME) of Austria, adding the CECMEs Germany and Norway as control variables. Union influence drives the adoption of WCs and their use for management downward communication. Hence, on our measures the three countries share features of the CECME category and have not been “Americanized”