496 research outputs found
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
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
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
. A renormalized mean-field theory predicts a fluctuation-induced
first order transition for , 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
Annealed disorder, rare regions, and local moments: A novel mechanism for metal-insulator transitions
Local magnetic moments in disordered sytems can be described in terms of
annealed magnetic disorder, in addition to the underlying quenched disorder. It
is shown that for noninteracting electron systems at zero temperature, the
annealed disorder leads to a new mechanism, and a new universality class, for a
metal-insulator transition. The transition is driven by a vanishing of the
thermodynamic density susceptibility rather than by localization effects. The
critical behavior near two-dimensions is determined, and the underlying physics
is discussed.Comment: 4 pp., LaTeX, no figs., final version as publishe
Breakdown of Landau-Ginzburg-Wilson theory for certain quantum phase transitions
The quantum ferromagnetic transition of itinerant electrons is considered. It
is shown that the Landau-Ginzburg-Wilson theory described by Hertz and others
breaks down due to a singular coupling between fluctuations of the conserved
order parameter. This coupling induces an effective long-range interaction
between the spins of the form 1/r^{2d-1}. It leads to unusual scaling behavior
at the quantum critical point in dimensions, which is determined
exactly.Comment: 4 pp., REVTeX, no figs, final version as publishe
Anomalous Density-of-States Fluctuations in Two-Dimensional Clean Metals
It is shown that density-of-states fluctuations, which can be interpreted as
the order-parameter susceptibility \chi_OP in a Fermi liquid, are anomalously
strong as a result of the existence of Goldstone modes and associated strong
fluctuations. In a 2-d system with a long-range Coulomb interaction, a suitably
defined \chi_OP diverges as 1/T^2 as a function of temperature in the limit of
small wavenumber and frequency. In contrast, standard statistics suggest
\chi_OP = O(T), a discrepancy of three powers of T. The reasons behind this
surprising prediction, as well as ways to observe it, are discussed.Comment: 4 pp, revised version contains a substantially expanded derivatio
Density Expansion for the Mobility in a Quantum Lorentz Model
We consider the mobility of electrons in an environment of static hard-sphere
scatterers, which provides a realistic description of electrons in Helium gas.
A systematic expansion in the scatterer density is carried to second order
relative to the Boltzmann result, and the analytic contribution at this order
is derived, together with the known logarithmic term in the density expansion.
It is shown that existing experimental data are consistent with the existence
of the logarithmic term in the density expansion, but more precise experiments
are needed in order to unambiguously detect it. We show that our calculations
provide the necessary theoretical information for such an experiment, and give
a detailed discussion of a suitable parameter range.Comment: 17pp., REVTeX, 7 figure attached as 8 postscript files, db/94/
Anomalous Pinning Fields in Helical Magnets: Screening of the Quasiparticle Interaction
The spin-orbit interaction strength g_so in helical magnets determines both
the pitch wave number q and the critical field H_c1 where the helix aligns with
an external magnetic field. Within a standard Landau-Ginzburg-Wilson (LGW)
theory, a determination of g_so in MnSi and FeGe from these two observables
yields values that differ by a factor of 20. This discrepancy is remedied by
considering the fermionic theory underlying the LGW theory, and in particular
the effects of screening on the effective electron-electron interaction that
results from an exchange of helical fluctuations.Comment: 4pp, 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
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