Criticality of n-vector quantum systems is studied, via a renormalization group treatment, assuming a pair interaction which decays as a power law involving a characteristics parameter θ. Within a “single”-expansion parameter scheme for finite values of θ, a novel fixed point appears for...

The spectral density method is used to study the thermodynamic properties of an anisotropic antiferromagnetic Heisenberg linear chain in an external magnetic field. Insecond order approximation without the decoupling procedure for the two particle correlation function, we are able to obtain many...

The paper is devoted to an investigation of the damping effects of the Fermion elementary excitations in the Jordan-Wigner representation of a one-dimensional anisotropic spin-12 model in an external field. This is realized on the basis of the “Gaussian ansatz”, recently proposed by Nolting...

We study the effects of correlated symmetry-breaking-like random fields on critical properties of a d-dimensional sine-Gordon model. A Wilsonian renormalization group analysis shows an unusual scenario in the absence of stable fixed points. The physical nature of the related runaway in the...

The low-temperature properties and crossover phenomena of d-dimensional transverse Ising-like systems within the influence domain of the quantum critical point are investigated solving the appropriate one-loop renormalization group equations. The phase diagram is obtained near and at d=3 and...

A reduction procedure, suggested for classical systems some years ago, is extended to systems with quantum-phase transitions with the aim to generate exactly solvable models capturing fluctuation effects beyond the mean field approximation. For the reduced isotropic m-vector quantum models, an...

A d-dimensional model of multicomponent interacting bosons with spin S and a magnetic moment in the presence of a magnetic field H is studied by means of a large-n limit treatment and a renormalization group approach, in the spirit of critical phenomena theory. Magnetic phase transitions and a...

A study of the d-dimensional classical Heisenberg ferromagnetic model in the presence of a magnetic field is performed within the two-time Green function’s framework in classical statistical physics. We extend the well known quantum Callen method to derive analytically a new formula for...

The two-time Green’s function equation of motion method is employed to explore the low-temperature properties and crossovers close to the field-induced quantum critical point of a d-dimensional spin- 1/2 easy-plane ferromagnet with longitudinal uniform interactions. This is performed, on the...

A method for exploring the destruction of long-range order in pure classical systems with continous symmetry is extended to quantum ferroelectrics in the presence of correlated random fields. This allows us to obtain a reliable estimate of the lower critical dimensionality in terms of the...