100,323 research outputs found
Quantum criticality of a planar Heisenberg ferromagnet in a transverse magnetic field
No full textA one-loop renormalization group treatment is used to investigate the quantum phase transition and the low-temperature critical properties of a planar Heisenberg ferromagnet in a transverse field. The phase diagram, the free energy density and the relevant critical exponents in the influence domain of the quantum critical point are obtained for dimensionalities d>2 in the temperature regime where the quantum fluctuations dominate. The d = 2 criticality is also studied decreasing the temperature with the magnetic field fixed at its critical value
Quantum tricriticality in transverse Ising-like systems
No full textThe 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 ≥ 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 φ = 1/(d−1)
to the new one φ = 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 φ = 1/2(d − 1) in the
quantum tricritical region
Unified static renormalization-group treatment of finite-temperature crossovers close to a quantum critical point
No full textA nonconventional renormalization-group (RG) treatment close to and below four dimensions is used to explore, in a unified and systematic way, 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 transformation to treat the resulting effective classical Ginzburg-Landau free energy functional. This allows us to perform a detailed study of criticality of the quantum systems under study. The emergent physics agrees, in many aspects, with the known quantum critical scenario. However, a richer structure of the phase diagram appears with additional crossovers which are not captured by the traditional RG studies. In addition, in spite of the intrinsically static nature of our theory, predictions about the dynamical critical exponent, which parametrizes the link between statics and dynamics close to a continuous phase transition, are consistently derived from our static results.
PACS:
64.60.Ak, 05.70.J
Magnetic-field-induced quantum criticality in a spin-S planar ferromagnet with single-ion anisotropy
No full textThe 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 . 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
Quantum-like criticality for a classical transverse Ising model in 4–ε dimensions
No full textWe investigate the low-temperature critical properties of a classical Ising model in a transverse field. This is performed by applying the conventional Wilson renormalization group to the related Ginzburg-Landau functional emerging from a Hubbard-Stratonovich transformation. Results in 4–ε dimensions provide a low temperature critical scenario very similar to that found for the quantum counterpart. Remarkably, we find that the discrimination between the two cases essentially lies on the different values of the shift exponent which characterizes the behavior of the phase boundary close to the zero-temperature critical point. This feature suggests that, decreasing the temperature, the observation of different renormalized critical exponents may signal activation of genuine quantum critical fluctuations. Copyright EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg 2010
Scaling functions for classical to quantum crossover in the transverse Ising model via an effective Wilsonian renormalization group approach in 4 – ε dimensions
No full textThe classical to quantum crossover, which occurs in d-dimensional transverse field Ising model-like systems decreasing the temperature to zero in the influence domain of the quantum critical point (QCP), is described by employing an effective Wilsonian renormalization group approach in 4 - ε dimensions. The basic ingredient of the treatment is the static action arising from a preliminary one-loop averaging over non-zero frequency modes, which enter the original quantum one. The crossover scaling functions for susceptibility and related thermodynamic quantities are obtained to first order in ε as explicit functions of the temperature and the applied magnetic field. In our static framework, which can be easily extended to other quantum systems exhibiting a critical line which terminates in a QCP, the suitable procedure for observing this type of crossover through genuine thermodynamic measurements is clarified consistently with available experiments. Remarkably, our basic idea and results may be usefully employed to explore also the dimensional crossover which takes place in classical Ising-like systems with slab or film geometry and, possibly, in other finite-size classical systems
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
A non-conventional approach to study the quenched impurity effects on quantum criticality
No full textA 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
Quantum critical scenario from an effective classical renormalization group treatment close to and below four dimensions
No full textWe suggest a general procedure to analyze quantum criticality for a wide variety of quantum systems of topical interest, close and below four dimensions. The idea is to apply the Wilsonian renormalization group philosopy to an effective classical functional derived from the general quantum action by averaging over degrees of freedom with non zero Matsubara frequencies. This allows to describe in an unified way all crossovers expected close to a quantum critical poin
Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units
No full textThe 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
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