825 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 $d\leq 3$ due to
the existence of soft particle-hole excitations that couple to the order
parameter fluctuations and lead to an upper critical dimension $d_c^+ = 3$. 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 $d=3$, the latter is mean
field-like with logarithmic corrections to scaling, and in $d<3$ it can be
controlled by means of a $3-\epsilon$ expansion.Comment: 15 pp., revtex4, 6 eps figs; final version as publishe

### Transport properties of clean and disordered superconductors in matrix field theory

A comprehensive field theory is developed for superconductors with quenched
disorder. We first show that the matrix field theory, used previously to
describe a disordered Fermi liquid and a disordered itinerant ferromagnet, also
has a saddle-point solution that describes a disordered superconductor. A
general gap equation is obtained. We then expand about the saddle point to
Gaussian order to explicitly obtain the physical correlation functions. The
ultrasonic attenuation, number density susceptibility, spin density
susceptibility and the electrical conductivity are used as examples. Results in
the clean limit and in the disordered case are discussed respectively. This
formalism is expected to be a powerful tool to study the quantum phase
transitions between the normal metal state and the superconductor state.Comment: 9 page

### Fluctuation-Driven Quantum Phase Transitions in Clean Itinerant Ferromagnets

The quantum phase transition in clean itinerant ferromagnets is analyzed. It
is shown that soft particle-hole modes invalidate Hertz's mean-field theory for
$d \leq 3$. A renormalized mean-field theory predicts a fluctuation-induced
first order transition for $1 < d \leq 3$, whose stability is analyzed by
renormalization group techniques. Depending on microscopic parameter values,
the first order transition can be stable, or be pre-empted by a
fluctuation-induced second order transition. The critical behavior at the
latter is determined. The results are in agreement with recent experiments.Comment: 4 pp., REVTeX, no 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
$d=2+\epsilon$ ($\epsilon\ll 1$) dimensions. Rather, it describes a transition
to a ferromagnetic state that, as a function of the disorder, precedes the
metal-insulator transition close to $d=2$. In $d=3$, a transition from a
paramagnetic metal to a paramagnetic insulator is possible.Comment: 13 pp., LaTeX, 2 eps figs; final version as publishe

### Electronic Transport at Low Temperatures: Diagrammatic Approach

We prove that a diagrammatic evaluation of the Kubo formula for the
electronic transport conductivity due the exchange of bosonic excitations, in
the usual conserving ladder approximation, yields a result consistent with the
Boltzmann equation. In particular, we show that an uncontrolled approximation
that has been used to solve the integral equation for the vertex function is
unnecessary. An exact solution of the integral equation yields the same
asymptotic low-temperature behavior as the approximate one, albeit with a
different prefactor, and agrees with the temperature dependence of the
Boltzmann solution. Examples considered are electron scattering from acoustic
phonons, and from helimagnons in helimagnets.Comment: Submitted to Physics E (FMQT08 Proceedings). Requires Elsevier style
file (included

### 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

### Properties of spin-triplet, even-parity superconductors

The physical consequences of the spin-triplet, even-parity pairing that has
been predicted to exist in disordered two-dimensional electron systems are
considered in detail. We show that the presence of an attractive interaction in
the particle-particle spin-triplet channel leads to an instability of the
normal metal that competes with the localizing effects of the disorder. The
instability is characterized by a diverging length scale, and has all of the
characteristics of a continuous phase transition. The transition and the
properties of the ordered phase are studied in mean-field theory, and by taking
into account Gaussian fluctuations. We find that the ordered phase is indeed a
superconductor with an ordinary Meissner effect and a free energy that is lower
than that of the normal metal. Various technical points that have given rise to
confusion in connection with this and other manifestations of odd-gap
superconductivity are also discussed.Comment: 15 pp., REVTeX, psfig, 2 ps figs, final version as publishe

### Nonanalytic Magnetization Dependence of the Magnon Effective Mass in Itinerant Quantum Ferromagnets

The spin wave dispersion relation in both clean and disordered itinerant
quantum ferromagnets is calculated. It is found that effects akin to
weak-localization physics cause the frequency of the spin-waves to be a
nonanalytic function of the magnetization m. For low frequencies \Omega, small
wavevectors k, and small m, the dispersion relation is found to be of the form
\Omega ~ m^{1-\alpha} k^2, with \alpha = (4-d)/2 (2<d<4) for disordered
systems, and \alpha = (3-d) (1<d<3) for clean ones. In d=4 (disordered) and d=3
(clean), \Omega ~ m ln(1/m) k^2. Experiments to test these predictions are
proposed.Comment: 4 pp., REVTeX, no fig

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