97 research outputs found
Low-Temperature Quantum Critical Behaviour of Systems with Transverse Ising-like Intrinsic Dynamics
The low-temperature properties and crossover phenomena of -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
and several sets of critical exponents are determined which describe
different responses of a system to quantum fluctuations according to the way of
approaching the quantum critical point. The results are in remarkable agreement
with experiments for a wide variety of compounds exhibiting a quantum phase
transition, as the ferroelectric oxides and other displacive systems.Comment: 36 pages, 2 figures, accepted in Physica
Quantum tricriticality in transverse Ising-like systems
The quantum tricriticality of d-dimensional transverse Ising-like systems is
studied by means of a perturbative renormalization group approach focusing on
static susceptibility. This allows us to obtain the phase diagram for 3<d<4,
with a clear location of the critical lines ending in the conventional quantum
critical points and in the quantum tricritical one, and of the tricritical line
for temperature T \geq 0. We determine also the critical and the tricritical
shift exponents close to the corresponding ground state instabilities.
Remarkably, we find a tricritical shift exponent identical to that found in the
conventional quantum criticality and, by approaching the quantum tricritical
point increasing the non-thermal control parameter r, a crossover of the
quantum critical shift exponents from the conventional value \phi = 1/(d-1) to
the new one \phi = 1/2(d-1). Besides, the projection in the (r,T)-plane of the
phase boundary ending in the quantum tricritical point and crossovers in the
quantum tricritical region appear quite similar to those found close to an
usual quantum critical point. Another feature of experimental interest is that
the amplitude of the Wilsonian classical critical region around this peculiar
critical line is sensibly smaller than that expected in the quantum critical
scenario. This suggests that the quantum tricriticality is essentially governed
by mean-field critical exponents, renormalized by the shift exponent \phi =
1/2(d-1) in the quantum tricritical region.Comment: 9 pages, 2 figures; to be published on EPJ
Excitonic condensation in quasi-two-dimensional systems
We present a low energy model for the Bose-Einstein condensation in a
quasi-two-dimensional excitonic gas. Using the flow equations of the
Renormalization group and a model with the dynamical critical exponent
we calculate the temperature dependence of the critical density,
coherence length, magnetic susceptibility, and specific heat. The model can be
relevant for the macroscopic coherence observed in GaAs/AlGaAs coupled quantum
wells.Comment: 4 Revtex page
Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units
The field-induced quantum criticality of compounds with ferromagnetically
coupled structural spin units (as dimers and ladders) is explored by applying
Wilson's renormalization group framework to an appropriate effective action. We
determine the low-temperature phase boundary and the behavior of relevant
quantities decreasing the temperature with the applied magnetic field fixed at
its quantum critical point value. In this context, a plausible interpretation
of some recent experimental results is also suggested.Comment: to be published in Physics Letters
Critical behaviour of thin films with quenched impurities
The critical behaviour of thin films containing quenched random impurities
and inhomogeneities is investigated by the renormalization-group method. The
finite-size crossover in impure films has been considered on the basis of the
fundamental relationship between the effective spatial dimensionality and the
characteristic lengths of the system. The difference between the critical
properties of infinite systems and films is demonstrated and investigated. A
new critical exponent, describing the scaling properties of the thickness of
films with extended impurities has been deduced and calculated. A special
attention is paid to the critical behaviour of real impure films.Comment: 27 pages LaTex; figures are available in the journal varian
Thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet with long-range interactions via the spectral density method
The thermodynamic properties of a classical d-dimensional spin-S Heisenberg
ferromagnet, with long-range interactions decaying as and in the
presence of an external magnetic field, is investigated by means of the
spectral density method in the framework of classical statistical mechanics. We
find that long-range order exists at finite temperature for with
and for with , consistently with known theorems. Besides,
the related critical temperature is determined and a study of the critical
properties is performed.Comment: 27 pages, 2 figures, Submitted to Physica
Some basic aspects of quantum phase transitions
Several basic problems of the theory of quantum phase transitions are
reviewed. The effect of the quantum correlations on the phase transition
properties is considered with the help of basic models of statistical physics.
The effect of quenched disorder on the quantum phase transitions is also
discussed. The review is performed within the framework of the thermodynamic
scaling theory and by the most general methods of statistical physics for the
treatment of phase transitions: general length-scale arguments, exact
solutions, mean field approximation, Hubbard-Stratonovich transformation,
Feynman path integral approach, and renormalization group in the field
theoretical variant. Some new ideas and results are presented. Outstanding
theoretical problems are mentioned.Comment: 81 pages, Latex2e, 8 figures, Phys. Rep.(2003) in pres
Unified static renormalization-group treatment of finite-temperature crossovers close to a quantum critical point
A non conventional Renormalization Group treatment close and below four dimensions is used to explore the low temperature properties of a wide class of systems in the influence domain of their quantum critical point. The approach consists in a preliminary averaging over quantum degrees of freedom and a successive employment of the Wilsonian RG trasformation to treat the resulting effective Ginzburg-Landau free energy functional. Thia allows a detailed study of criticality of the quantum systems
A non conventional approach to study the quenched impurity effects on quantum criticality
A non conventional point of view is used to explore the competition between quenched disorder and quantum fluctuations in systems which exhibit a quantum phase transition in the clean limit. The approach consists in averaging over quantum degrees of freedom and next in applying the renormalization group transformation to the resulting effective classical random action. It emerges that, below four dimensions, the quantum criticality appears to be controlled by the classical random fixed point
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