110 research outputs found
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
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
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
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
Quantum critical behavior of disordered itinerant ferromagnets
The quantum ferromagnetic transition at zero temperature in disordered
itinerant electron systems is considered. Nonmagnetic quenched disorder leads
to diffusive electron dynamics that induces an effective long-range interaction
between the spin or order parameter fluctuations of the form r^{2-2d}, with d
the spatial dimension. This leads to unusual scaling behavior at the quantum
critical point, which is determined exactly. In three-dimensional systems the
quantum critical exponents are substantially different from their finite
temperature counterparts, a difference that should be easily observable.
Experiments to check these predictions are proposed.Comment: 14pp., REVTeX, 3 eps figs, final version as publishe
Order Parameter Description of the Anderson-Mott Transition
An order parameter description of the Anderson-Mott transition (AMT) is
given. We first derive an order parameter field theory for the AMT, and then
present a mean-field solution. It is shown that the mean-field critical
exponents are exact above the upper critical dimension. Renormalization group
methods are then used to show that a random-field like term is generated under
renormalization. This leads to similarities between the AMT and random-field
magnets, and to an upper critical dimension for the AMT. For
, an expansion is used to calculate the critical
exponents. To first order in they are found to coincide with the
exponents for the random-field Ising model. We then discuss a general scaling
theory for the AMT. Some well established scaling relations, such as Wegner's
scaling law, are found to be modified due to random-field effects. New
experiments are proposed to test for random-field aspects of the AMT.Comment: 28pp., REVTeX, no figure
Theory of helimagnons in itinerant quantum systems
The nature and effects of the Goldstone mode in the ordered phase of helical
or chiral itinerant magnets such as MnSi are investigated theoretically. It is
shown that the Goldstone mode, or helimagnon, is a propagating mode with a
highly anisotropic dispersion relation, in analogy to the Goldstone mode in
chiral liquid crystals. Starting from a microscopic theory, a comprehensive
effective theory is developed that allows for an explicit description of the
helically ordered phase, including the helimagnons, for both classical and
quantum helimagnets. The directly observable dynamical spin susceptibility,
which reflects the properties of the helimagnon, is calculated.Comment: 20 pp., 1 eps fig; corrects various typos and incorrect prefactors in
Phys Rev B versio
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
A metal-insulator transition as a quantum glass problem
We discuss a recent mapping of the Anderson-Mott metal-insulator transition
onto a random field magnet problem. The most important new idea introduced is
to describe the metal-insulator transition in terms of an order parameter
expansion rather than in terms of soft modes via a nonlinear sigma model. For
spatial dimensions d>6 a mean field theory gives the exact critical exponents.
In an epsilon expansion about d=6 the critical exponents are identical to those
for a random field Ising model. Dangerous irrelevant quantum fluctuations
modify Wegner's scaling law relating the conductivity exponent to the
correlation or localization length exponent. This invalidates the bound s>2/3
for the conductivity exponent s in d=3. We also argue that activated scaling
might be relevant for describing the AMT in three-dimensional systems.Comment: 10 pp., REvTeX, 1 eps fig., Sitges Conference Proceedings, final
version as publishe
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