399 research outputs found
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/
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
Nonanalytic behavior of the spin susceptibility in clean Fermi systems
The wavevector and temperature dependent static spin susceptibility,
\chi_s(Q,T), of clean interacting Fermi systems is considered in dimensions
1\leq d \leq 3. We show that at zero temperature \chi_s is a nonanalytic
function of |Q|, with the leading nonanalyticity being |Q|^{d-1} for 1<d<3, and
Q^2\ln|Q| for d=3. For the homogeneous spin susceptibility we find a
nonanalytic temperature dependence T^{d-1} for 1<d<3. We give qualitative
mode-mode coupling arguments to that effect, and corroborate these arguments by
a perturbative calculation to second order in the electron-electron interaction
amplitude. The implications of this, in particular for itinerant
ferromagnetism, are discussed. We also point out the relation between our
findings and established perturbative results for 1-d systems, as well as for
the temperature dependence of \chi_s(Q=0) in d=3.Comment: 12pp., REVTeX, 5 eps figures, final version as publishe
Theory of Disordered Itinerant Ferromagnets I: Metallic Phase
A comprehensive theory for electronic transport in itinerant ferromagnets is
developed. We first show that the Q-field theory used previously to describe a
disordered Fermi liquid also has a saddle-point solution that describes a
ferromagnet in a disordered Stoner approximation. We calculate transport
coefficients and thermodynamic susceptibilities by expanding about the saddle
point to Gaussian order. At this level, the theory generalizes previous
RPA-type theories by including quenched disorder. We then study soft-mode
effects in the ferromagnetic state in a one-loop approximation. In
three-dimensions, we find that the spin waves induce a square-root frequency
dependence of the conductivity, but not of the density of states, that is
qualitatively the same as the usual weak-localization effect induced by the
diffusive soft modes. In contrast to the weak-localization anomaly, this effect
persists also at nonzero temperatures. In two-dimensions, however, the spin
waves do not lead to a logarithmic frequency dependence. This explains
experimental observations in thin ferromagnetic films, and it provides a basis
for the construction of a simple effective field theory for the transition from
a ferromagnetic metal to a ferromagnetic insulator.Comment: 15pp., REVTeX, 2 eps figs, final version as publishe
Local field theory for disordered itinerant quantum ferromagnets
An effective field theory is derived that describes the quantum critical
behavior of itinerant ferromagnets in the presence of quenched disorder. In
contrast to previous approaches, all soft modes are kept explicitly. The
resulting effective theory is local and allows for an explicit perturbative
treatment. It is shown that previous suggestions for the critical fixed point
and the critical behavior are recovered under certain assumptions. The validity
of these assumptions is discussed in the light of the existence of two
different time scales. It is shown that, in contrast to previous suggestions,
the correct fixed point action is not Gaussian, and that the previously
proposed critical behavior was correct only up to logarithmic corrections. The
connection with other theories of disordered interacting electrons, and in
particular with the resolution of the runaway flow problem encountered in these
theories, is also discussed.Comment: 17pp., REVTeX, 5 eps figs, final version as publishe
Theory of many-fermion systems II: The case of Coulomb interactions
In a recent paper (cond-mat/9703164) a general field-theoretical description
of many-fermion systems with short-ranged interactions has been developed. Here
we extend this theory to the case of disordered electrons interacting via a
Coulomb potential. A detailed discussion is given of the Ward identity that
controls the soft modes in the system, and the generalized nonlinear sigma
model for the Coulombic case is derived and discussed.Comment: 12 pp., REVTeX, no figs, final version as publishe
Phase-ordering dynamics in itinerant quantum ferromagnets
The phase-ordering dynamics that result from domain coarsening are considered
for itinerant quantum ferromagnets. The fluctuation effects that invalidate the
Hertz theory of the quantum phase transition also affect the phase ordering.
For a quench into the ordered phase a transient regime appears, where the
domain growth follows a different power law than in the classical case, and for
asymptotically long times the prefactor of the t^{1/2} growth law has an
anomalous magnetization dependence. A quench to the quantum critical point
results in a growth law that is not a power-law function of time. Both
phenomenological scaling arguments and renormalization-group arguments are
given to derive these results, and estimates of experimentally relevant length
and time scales are presented.Comment: 6pp., 1 eps fig, slightly expanded versio
Absence of electron dephasing at zero temperature
Dephasing of electrons due to the electron-electron interaction has recently
been the subject of a controversial debate, with different calculations
yielding mutually incompatible results. In this paper we prove, by means of
Ward identities, that neither a Coulomb interaction nor a short-ranged model
interaction can lead to phase breaking at zero temperature in spatial
dimensions d>2.Comment: 7 pp., LaTeX, no figs, final version as publishe
Tricritical behavior in itinerant quantum ferromagnets
It is shown that the peculiar features observed in the low-temperature phase
diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple
mean-field theory. The nature of the ferromagnetic transition changes from
second order to first order at a tricritical point, and in a small external
magnetic field surfaces of first-order transitions emerge which terminate in
quantum critical points. This field dependence of the phase diagram follows
directly from the existence of the tricritical point. The quantum critical
behavior in a nonzero field is calculated exactly.Comment: 4pp., 4 eps figure
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