Related papers: Order-Disorder Transition in a Two-Layer Quantum A…
We present an investigation of the effect of randomizing exchange strengths in the $S=1/2$ square lattice quasi-two-dimensional quantum Heisenberg antiferromagnet (QuinH)$_2$Cu(Cl$_{x}$Br$_{1-x}$)$_{4}\cdot$2H$_2$O (QuinH$=$Quinolinium,…
We consider the quantum ferromagnetic transition at zero temperature in clean itinerant electron systems. We find that the Landau-Ginzburg-Wilson order parameter field theory breaks down since the electron-electron interaction leads to…
The universal dynamic and static properties of two dimensional antiferromagnets in the vicinity of a zero-temperature phase transition from long-range magnetic order to a quantum disordered phase are studied. Random antiferromagnets with…
Quantum Monte Carlo simulations are used to study the magnetic and transport properties of the Hubbard Model, and its strong coupling Heisenberg limit, on a one-third depleted square lattice. This is the geometry occupied, after charge…
The crossover from two- to three-dimensional critical behavior of nearly antiferromagnetic itinerant electrons is studied in a regime where the inter-plane single-particle motion of electrons is quantum-mechanically incoherent because of…
The quantum ferromagnetic transition at zero temperature in disordered itinerant electron systems is considered. Nonmagnetic quenched disorder leads to diffusive electron dynamics that induces an effective long-range interaction between the…
We use quantum Monte Carlo simulations to study effects of disorder on the quantum phase transition occurring versus the ratio g=J/J' in square-lattice dimerized S=1/2 Heisenberg antiferromagnets with intra- and inter-dimer couplings J and…
We study the interplay between antiferromagnetism and superconductivity in a generalized infinite-$U$ Anderson lattice, where both superconductivity and antiferromagnetic order are introduced phenomenologically in mean field theory. In a…
Critical behavior of three-dimensional classical frustrated antiferromagnets with a collinear spin ordering and with an additional twofold degeneracy of the ground state is studied. We consider two lattice models, whose continuous limit…
The spin-1/2 antiferromagnetic Heisenberg systems are studied on three typical diamond-type hierarchical lattices (systems A, B and C) with fractal dimensions 1.63, 2 and 2.58, respectively, and the phase diagrams, critical phenomena and…
Discontinuous quantum phase transitions besides their general interest are clearly relevant to the study of heavy fermions and magnetic transition metal compounds. Recent results show that in many systems belonging to these classes of…
It is shown that a quasi two dimensional (layered) Heisenberg antiferromagnet with fully frustrated interplane couplings ({\it e.g.} on a body-centered tetragonal lattice) generically exhibits two thermal phase transitions with lowering…
We obtain the complete phase diagram of the antiferromagnetic $J_{1}$-$J_{2}$ model, $0\leq \alpha = J_2/J1 \leq 1$, within the framework of the $O(N)$ nonlinear sigma model. We find two magnetically ordered phases, one with N\' eel order,…
Field-induced magnetic ordering in the structurally disordered quantum magnets Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $x=8$% and 13%, is studied by means of neutron diffraction. The order parameter critical exponent is found to be…
We investigate the interplay between charge order and superconductivity near an antiferromagnetic quantum critical point using sign-problem-free Quantum Monte Carlo simulations. We establish that, when the electronic dispersion is…
Using the Resonant Valence Bond (RVB) concept and the Schwinger-boson mean field approximation, we investigate a two dimensional anisotropic Heisenberg antiferromagnet. We find that increasing the coupling ratio ($\alpha=J_{y}/J_{x}$), a…
We investigate the quantum phase transition of itinerant ferromagnets. It is shown that correlation effects in the underlying itinerant electron system lead to singularities in the order parameter field theory that result in an effective…
We present a numerical study of 2D random-bond Potts ferromagnets. The model is studied both below and above the critical value $Q_c=4$ which discriminates between second and first-order transitions in the pure system. Two geometries are…
We consider a two-layer Heisenberg antiferromagnet which can be either in the N\'{e}el-ordered or in the disordered phase at $T=0$, depending on the ratio of the intra- and interlayer exchange constants. We reduce the problem to an…
The quantum critical regime (QCR) of a two-dimensional (2D) disordered and a 2D clean dimerized spin-$\frac{1}{2}$ Heisenberg models are studied using the first principles nonperturbative quantum Monte Carlo simulations (QMC). In…