301 research outputs found
Current driven quantum criticality in itinerant electron ferromagnets
We determine the effect of an in-plane current flow on the critical
properties of a 2d itinerant electron system near a ferromagnetic-paramagnetic
quantum critical point. We study a model in which a nonequilibrium steady state
is established as a result of exchange of particles and energy with an
underlying substrate. the current gives rise not only to an effective
temperature equal to the voltage drop over a distance of order the mean free
path, but also to symmetry breaking terms of the form in the effective action. The effect of the symmetry breaking on
the fluctuational and critical properties is found to be small although (in
agreement with previous results) if rotational degrees of freedom are
important, the current can make the classically ordered state dynamically
unstable.Comment: 4 pages, published versio
Nonequilibrium quantum criticality in bilayer itinerant ferromagnets
We present a theory of nonequilibrium quantum criticality in a coupled
bilayer system of itinerant electron magnets. The model studied consists of the
first layer subjected to an inplane current and open to an external substrate.
The second layer is closed and subject to no direct external drive, but couples
to the first layer via short-ranged spin exchange interaction. No particle
exchange is assumed between the layers. Starting from a microscopic fermionic
model, we derive an effective action in terms of two coupled bosonic fields
which are related to the magnetization fluctuations of the two layers. When
there is no interlayer coupling, the two bosonic modes possess different
dynamical critical exponents z with z=2 (z=3) for the first (second) layer.
This results in multi-scale quantum criticality in the coupled system. It is
shown that the linear coupling between the two fields leads to a low energy
fixed point characterized by the larger dynamical critical exponent z=3. The
perturbative renormalization group is used to compute the correlation length in
the quantum disordered and quantum critical regimes. We also derive the
stochastic dynamics obeyed by the critical fluctuations in the quantum critical
regime. Comparing the nonequilibrium situation to the thermal equilibrium
scenario, where the whole system is at a temperature T, we find that the
nonequilibrium drive does not always play the role of temperature.Comment: 20+ pages, 3 figures; Revised version as accepted by PRB, added
figure of mean field phase diagra
Current-induced magnetization dynamics in disordered itinerant ferromagnets
Current-driven magnetization dynamics in ferromagnetic metals are studied in
a self-consistent adiabatic local-density approximation in the presence of
spin-conserving and spin-dephasing impurity scattering. Based on a quantum
kinetic equation, we derive Gilbert damping and spin-transfer torques entering
the Landau-Lifshitz equation to linear order in frequency and wave vector.
Gilbert damping and a current-driven dissipative torque scale identically and
compete, with the result that a steady current-driven domain-wall motion is
insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform
magnetization is found to be much more stable against spin torques in the
itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic
spin-transfer torque reminiscent of the spin pumping in multilayers is
identified and shown to govern the current-induced domain-wall distortion
Quantum Griffiths effects and smeared phase transitions in metals: theory and experiment
In this paper, we review theoretical and experimental research on rare region
effects at quantum phase transitions in disordered itinerant electron systems.
After summarizing a few basic concepts about phase transitions in the presence
of quenched randomness, we introduce the idea of rare regions and discuss their
importance. We then analyze in detail the different phenomena that can arise at
magnetic quantum phase transitions in disordered metals, including quantum
Griffiths singularities, smeared phase transitions, and cluster-glass
formation. For each scenario, we discuss the resulting phase diagram and
summarize the behavior of various observables. We then review several recent
experiments that provide examples of these rare region phenomena. We conclude
by discussing limitations of current approaches and open questions.Comment: 31 pages, 7 eps figures included, v2: discussion of the dissipative
Ising chain fixed, references added, v3: final version as publishe
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
Strongly-coupled quantum critical point in an all-in-all-out antiferromagnet
Dimensionality and symmetry play deterministic roles in the laws of Nature.
They are important tools to characterize and understand quantum phase
transitions, especially in the limit of strong correlations between spin,
orbit, charge, and structural degrees of freedom. Using newly-developed,
high-pressure resonant x-ray magnetic and charge diffraction techniques, we
have discovered a quantum critical point in Cd2Os2O7 as the all-in-all-out
(AIAO) antiferromagnetic order is continuously suppressed to zero temperature
and, concomitantly, the cubic lattice structure continuously changes from space
group Fd-3m to F-43m. Surrounded by three phases of different time reversal and
spatial inversion symmetries, the quantum critical region anchors two phase
lines of opposite curvature, with striking departures from a mean-field form at
high pressure. As spin fluctuations, lattice breathing modes, and quasiparticle
excitations interact in the quantum critical region, we argue that they present
the necessary components for strongly-coupled quantum criticality in this
three-dimensional compound
Pressure dependence of the magnetization in the ferromagnetic superconductor UGe_2
The recent discovery that superconductivity occurs in several clean itinerant
ferromagnets close to low temperature magnetic instabilities naturally invites
an interpretation based on a proximity to quantum criticality. Here we report
measurements of the pressure dependence of the low temperature magnetisation in
one of these materials, UGe_2. Our results show that both of the magnetic
transitions observed in this material as a function of pressure are first order
transitions and do not therefore correspond to quantum critical points. Further
we find that the known pressure dependence of the superconducting transition is
not reflected in the pressure dependence of the static susceptibility. This
demonstrates that the spectrum of excitations giving superconductivity is not
that normally associated with a proximity to quantum criticality in weak
itinerant ferromagnets. In contrast our data suggest that instead the pairing
spectrum might be related to a sharp spike in the electronic density of states
that also drives one of the magnetic transitions.Comment: to appear in Phys. Rev. Let
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