12 research outputs found
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
Random walks and polymers in the presence of quenched disorder
After a general introduction to the field, we describe some recent results
concerning disorder effects on both `random walk models', where the random walk
is a dynamical process generated by local transition rules, and on `polymer
models', where each random walk trajectory representing the configuration of a
polymer chain is associated to a global Boltzmann weight. For random walk
models, we explain, on the specific examples of the Sinai model and of the trap
model, how disorder induces anomalous diffusion, aging behaviours and Golosov
localization, and how these properties can be understood via a strong disorder
renormalization approach. For polymer models, we discuss the critical
properties of various delocalization transitions involving random polymers. We
first summarize some recent progresses in the general theory of random critical
points : thermodynamic observables are not self-averaging at criticality
whenever disorder is relevant, and this lack of self-averaging is directly
related to the probability distribution of pseudo-critical temperatures
over the ensemble of samples of size . We describe the
results of this analysis for the bidimensional wetting and for the
Poland-Scheraga model of DNA denaturation.Comment: 17 pages, Conference Proceedings "Mathematics and Physics", I.H.E.S.,
France, November 200
Exotic quantum phase transitions in systems with quenched disorder
The possibility of a quantum phase transition in a d-dimensional model with quenched disorder is analyzed via a renormalization group treatment without using the dimensionality epsilon(tau) of the imaginary time direction tau as an additional small expansion parameter. We work at the fixed physical value epsilon(tau) = 1 from the beginning, assuming the infinite correlation length in the timelike direction at temperature T = 0 as generated by an appropriate limit of a time-dependent random potential with long-range correlation in time. A stable fixed point is found to occur for realistic dimensionalities which, in the time-totally-correlated limit, is assumed to govern an exotic second order quantum phase transition in the original physical model. Also in the present approach a double expansion procedure is used but, more physically, we involve the extension of the random correlation in the tau direction rather than the artificial and doubtful expansion in small epsilon(tau)
Low temperature critical properties and crossovers of a spin 1/2 planar ferromagnet in 4-e dimensions
The low temperature critical properties and crossovers of a spin 1/2 planar ferromagnet in a longitudinal magnetic field are explored in 4-e dimensions in terms of an anisotropic action, suitable to describe the spin model in the low temperature regime.
Reentrant phenomena in a three-dimensional spin-1 planar ferromagnet with easy-axis single-ion anisotropy
The two-time Green function method is employed to explore the phase diagram and the magnetic-fieldinduced
quantum criticality of a three-dimensional spin-one planar ferromagnet with easy-axis singleion
anisotropy. We adopt the Tyablikov and Anderson-Callen decouplings for higher order exchange and
single-ion anisotropy Green functions, respectively. The central finding is that, within a characteristic
range of the anisotropy parameter values, reentrant phenomena occur in the phase diagram close to the
quantum critical point producing a sensible change of the conventional quantum critical scenario
Magnetic-field-induced quantum criticality in a spin-
The effects of single-ion anisotropy on quantum criticality in a
d-dimensional spin-S planar ferromagnet is explored by
means of the two-time Green’s function method. We work at the Tyablikov decoupling level
for exchange interactions and the Anderson-Callen decoupling level for single-ion
anisotropy. In our analysis a longitudinal external magnetic field is used as the
non-thermal control parameter and the phase diagram and the quantum critical properties
are established for suitable values of the single-ion anisotropy
parameter D. We find that the single-ion anisotropy has sensible
effects on the structure of the phase diagram close to the quantum critical point.
However, for values of the uniaxial crystal-field parameter below a positive threshold,
the conventional magnetic-field-induced quantum critical scenario remains unchanged
Magnetic-field-induced quantum criticality in a planar ferromagnet with single-ion anisotropy
We analyze the effects induced by
single-ion anisotropy on quantum
criticality in a d-dimensional spin-3/2 planar ferromagnet.
To tackle this problem we employ the two-time Green's function method,
using the Tyablikov decoupling for
exchange interactions and the Anderson-Callen decoupling
for single-ion anisotropy.
In our analysis the role of non-thermal control parameter which drives the
quantum phase transition is played by a longitudinal external magnetic
field. We
find that the single-ion anisotropy has sensible effects on the
structure of the phase diagram close to the quantum critical
point
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