523 research outputs found
Nature of the Quantum Phase Transition in Clean, Itinerant Heisenberg Ferromagnets
A comprehensive theory of the quantum phase transition in clean, itinerant
Heisenberg ferromagnets is presented. It is shown that the standard mean-field
description of the transition is invalid in spatial dimensions due to
the existence of soft particle-hole excitations that couple to the order
parameter fluctuations and lead to an upper critical dimension . A
generalized mean-field theory that takes these additional modes into account
predicts a fluctuation-induced first-order transition. In a certain parameter
regime, this first-order transition in turn is unstable with respect to a
fluctuation-induced second-order transition. The quantum ferromagnetic
transition may thus be either of first or of second-order, in agreement with
experimental observations. A detailed discussion is given of the stability of
the first-order transition, and of the critical behavior at the
fluctuation-induced second-order transition. In , the latter is mean
field-like with logarithmic corrections to scaling, and in it can be
controlled by means of a expansion.Comment: 15 pp., revtex4, 6 eps figs; final version as publishe
Quantum critical behavior in disordered itinerant ferromagnets: Logarithmic corrections to scaling
The quantum critical behavior of disordered itinerant ferromagnets is
determined exactly by solving a recently developed effective field theory. It
is shown that there are logarithmic corrections to a previous calculation of
the critical behavior, and that the exact critical behavior coincides with that
found earlier for a phase transition of undetermined nature in disordered
interacting electron systems. This confirms a previous suggestion that the
unspecified transition should be identified with the ferromagnetic transition.
The behavior of the conductivity, the tunneling density of states, and the
phase and quasiparticle relaxation rates across the ferromagnetic transition is
also calculated.Comment: 15pp., REVTeX, 8 eps figs, final version as publishe
Electrons in an annealed environment: A special case of the interacting electron problem
The problem of noninteracting electrons in the presence of annealed magnetic
disorder, in addition to nonmagnetic quenched disorder, is considered. It is
shown that the proper physical interpretation of this model is one of electrons
interacting via a potential that is long-ranged in time, and that its technical
analysis by means of renormalization group techniques must also be done in
analogy to the interacting problem. As a result, and contrary to previous
claims, the model does not simply describe a metal-insulator transition in
() dimensions. Rather, it describes a transition
to a ferromagnetic state that, as a function of the disorder, precedes the
metal-insulator transition close to . In , a transition from a
paramagnetic metal to a paramagnetic insulator is possible.Comment: 13 pp., LaTeX, 2 eps figs; final version as publishe
Local versus Nonlocal Order Parameter Field Theories for Quantum Phase Transitions
General conditions are formulated that allow to determine which quantum phase
transitions in itinerant electron systems can be described by a local
Landau-Ginzburg-Wilson or LGW theory solely in terms of the order parameter. A
crucial question is the degree to which the order parameter fluctuations couple
to other soft modes. Three general classes of zero-wavenumber order parameters,
in the particle-hole spin-singlet and spin-triplet channels, and in the
particle-particle channel, respectively, are considered. It is shown that the
particle-hole spin-singlet class does allow for a local LGW theory, while the
other two classes do not. The implications of this result for the critical
behavior at various quantum phase transitions are discussed, as is the
connection with nonanalyticities in the wavenumber dependence of order
parameter susceptibilities in the disordered phase.Comment: 9 pp., LaTeX, no figs, final version as publishe
Anderson-Mott Transition in a Magnetic Field: Corrections to Scaling
It is shown that the Anderson-Mott metal-insulator transition of
paramagnetic, interacting disordered electrons in an external magnetic field is
in the same universality class as the transition from a ferromagnetic metal to
a ferromagnetic insulator discussed recently. As a consequence, large
corrections to scaling exist in the magnetic-field universality class, which
have been neglected in previous theoretical descriptions. The nature and
consequences of these corrections to scaling are discussed.Comment: 5pp., REVTeX, no figs, final version as publishe
Magnetic pair breaking in disordered superconducting films
A theory for the effects of nonmagnetic disorder on the magnetic pair
breaking rate induced by paramagnetic impurities in quasi
two-dimensional superconductors is presented. Within the framework of a
strong-coupling theory for disordered superconductors, we find that the
disorder dependence of is determined by the disorder enhancements of
both the electron-phonon coupling and the spin-flip scattering rate. These two
effects have a tendency to cancel each other. With parameter values appropriate
for Pb_{0.9} Bi_{0.1}, we find a pair breaking rate that is very weakly
dependent on disorder for sheet resistances 0 < R < 2.5 kOhm, in agreement with
a recent experiment by Chervenak and Valles.Comment: 6 pp., REVTeX, epsf, 2 eps figs, final version as publishe
Universal low-temperature tricritical point in metallic ferromagnets and ferrimagnets
An earlier theory of the quantum phase transition in metallic ferromagnets is
revisited and generalized in three ways. It is shown that the mechanism that
leads to a fluctuation-induced first-order transition in metallic ferromagnets
with a low Curie temperature is valid, (1) irrespective of whether the magnetic
moments are supplied by the conduction electrons or by electrons in another
band, (2) for ferromagnets in the XY and Ising universality classes as well as
for Heisenberg ferromagnets, and (3) for ferrimagnets as well as for
ferromagnets. This vastly expands the class of materials for which a
first-order transition at low temperatures is expected, and it explains why
strongly anisotropic ferromagnets, such as UGe2, display a first-order
transition as well as Heisenberg magnets.Comment: 11pp, 2 fig
Split transition in ferromagnetic superconductors
The split superconducting transition of up-spin and down-spin electrons on
the background of ferromagnetism is studied within the framework of a recent
model that describes the coexistence of ferromagnetism and superconductivity
induced by magnetic fluctuations. It is shown that one generically expects the
two transitions to be close to one another. This conclusion is discussed in
relation to experimental results on URhGe. It is also shown that the magnetic
Goldstone modes acquire an interesting structure in the superconducting phase,
which can be used as an experimental tool to probe the origin of the
superconductivity.Comment: REVTeX4, 15 pp, 7 eps fig
Long-range order versus random-singlet phases in quantum antiferromagnetic systems with quenched disorder
The stability of antiferromagnetic long-range order against quenched disorder
is considered. A simple model of an antiferromagnet with a spatially varying
Neel temperature is shown to possess a nontrivial fixed point corresponding to
long-range order that is stable unless either the order parameter or the
spatial dimensionality exceeds a critical value. The instability of this fixed
point corresponds to the system entering a random-singlet phase. The
stabilization of long-range order is due to quantum fluctuations, whose role in
determining the phase diagram is discussed.Comment: 5 pp., REVTeX, epsf, 3 eps figs, final version as published,
including erratu
Scaling approach to itinerant quantum critical points
Based on phase space arguments, we develop a simple approach to metallic
quantum critical points, designed to study the problem without integrating the
fermions out of the partition function. The method is applied to the
spin-fermion model of a T=0 ferromagnetic transition. Stability criteria for
the conduction and the spin fluids are derived by scaling at the tree level. We
conclude that anomalous exponents may be generated for the fermion self-energy
and the spin-spin correlation functions below , in spite of the spin fluid
being above its upper critical dimension.Comment: 3 pages, 2 figures; discussion of the phase space restriction
modified and, for illustrative purposes, restricted to the tree-level
analysis of the ferromagnetic transitio
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