814 research outputs found
Gauge theory description of glass transition
An analytical approach, which develops the gauge model of the glass
transition phenomenon, is suggested. It is based on the quantum field theory
and critical dynamics methods. The suggested mechanism of glass transition is
based on the interaction of the local magnetization field with the massive
gauge field, which describes frustration-induced plastic deformation. The
example of the three-dimensional Heisenberg model with trapped disorder is
considered. It is shown that the glass transition appears when the fluctuations
scale reaches the frustrations scale, and the mass of the gauge field becomes
equal to zero. The Vogel-Fulcher-Tammann relation for the glass transition
kinetics and critical exponent for non-linear susceptibility, , are derived in the framework of the suggested approach.Comment: 4 pages, 4 figures; Added references; correction
Magnetic transitions in CaMn7O12 : a Raman observation of spin-phonon couplings
The quadruple Calcium manganite (CaMn7O12) is a multiferroic material that
exhibits a giant magnetically-induced ferroelectric polarization which makes it
very interesting for magnetoelectric applications. Here, we report the Raman
spectroscopy study on this compound of both the phonon modes and the low energy
excitations from 4 K to room temperature. A detailed study of the Raman active
phonon excitations shows that three phonon modes evidence a spin-phonon
coupling at TN2 = 50 K. In particular, we show that the mode at 432 cm-1
associated to Mn(B)O6 (B position of the perovskite) rotations around the [111]
cubic diagonal is impacted by the magnetic transition at 50 K and its coupling
to the new modulation of the Mn spin in the (a,b) plane. At low energies, two
large low energy excitations are observed at 25 and 47 cm-1. The first one
disappears at 50 K and the second one at 90 K. We have associated these
excitations to electro-magneto-active modes
Kondo lattice on the edge of a two-dimensional topological insulator
We revisit the problem of a single quantum impurity on the edge of a
two-dimensional time-reversal invariant topological insulator and show that the
zero temperature phase diagram contains a large local moment region for
antiferromagnetic Kondo coupling which was missed by previous poor man's
scaling treatments. The combination of an exact solution at the so-called
decoupling point and a renormalization group analysis \`a la
Anderson-Yuval-Hamann allows us to access the regime of strong
electron-electron interactions on the edge and strong Kondo coupling. We apply
similar methods to the problem of a regular one-dimensional array of quantum
impurities interacting with the edge liquid. When the edge electrons are at
half-filling with respect to the impurity lattice, the system remains gapless
unless the Luttinger parameter of the edge is less than 1/2, in which case
two-particle backscattering effects drive the system to a gapped phase with
long-range Ising antiferromagnetic order. This is in marked contrast with the
gapped disordered ground state of the ordinary half-filled one-dimensional
Kondo lattice.Comment: 18 pages, 3 figures; fixed typos, updated reference
Full counting statistics of spin transfer through the Kondo dot
We calculate the spin current distribution function for a Kondo dot in two
different regimes. In the exactly solvable Toulouse limit the linear response,
zero temperature statistics of the spin transfer is trinomial, such that all
the odd moments vanish and the even moments follow a binomial distribution. On
the contrary, the corresponding spin-resolved distribution turns out to be
binomial. The combined spin and charge statistics is also determined. In
particular, we find that in the case of a finite magnetic field or an
asymmetric junction the spin and charge measurements become statistically
dependent. Furthermore, we analyzed the spin counting statistics of a generic
Kondo dot at and around the strong-coupling fixed point (the unitary limit).
Comparing these results with the Toulouse limit calculation we determine which
features of the latter are generic and which ones are artifacts of the spin
symmetry breaking.Comment: 9 pages, 3 eps figure
Low-energy properties of two-dimensional quantum triangular antiferromagnets: Non-perturbative renormalization group approach
We explore low temperature properties of quantum triangular Heisenberg
antiferromagnets in two dimension in the vicinity of the quantum phase
transition at zero temperature. Using the effective field theory described by
the matrix Ginzburg-Landau-Wilson model and the
non-perturbative renormalization group method, we clarify how quantum and
thermal fluctuations affect long-wavelength behaviors in the parameter region
where the systems exhibit a fluctuation-driven first order transition to a
long-range ordered state. We show that at finite temperatures the crossover
from a quantum theory to a renormalized two-dimensional classical
nonlinear sigma model region appears, and in this crossover region, massless
fluctuation modes with linear dispersion a la spin waves govern low-energy
physics. Our results are in good agreement with the recent experimental
observations for the two-dimensional triangular Heisenberg spin system,
NiGaS.Comment: 14 pages,7 figures, version accepted for publication in Physical
Review
Deep Spin-Glass Hysteresis Area Collapse and Scaling in the Ising Model
We investigate the dissipative loss in the Ising spin glass in three
dimensions through the scaling of the hysteresis area, for a maximum magnetic
field that is equal to the saturation field. We perform a systematic analysis
for the whole range of the bond randomness as a function of the sweep rate, by
means of frustration-preserving hard-spin mean field theory. Data collapse
within the entirety of the spin-glass phase driven adiabatically (i.e.,
infinitely-slow field variation) is found, revealing a power-law scaling of the
hysteresis area as a function of the antiferromagnetic bond fraction and the
temperature. Two dynamic regimes separated by a threshold frequency
characterize the dependence on the sweep rate of the oscillating field. For
, the hysteresis area is equal to its value in the adiabatic
limit , while for it increases with the
frequency through another randomness-dependent power law.Comment: 6 pages, 6 figure
Magnetic-glassy multicritical behavior of the three-dimensional +- J Ising model
We consider the three-dimensional model defined on a simple cubic
lattice and study its behavior close to the multicritical Nishimori point where
the paramagnetic-ferromagnetic, the paramagnetic-glassy, and the
ferromagnetic-glassy transition lines meet in the T-p phase diagram (p
characterizes the disorder distribution and gives the fraction of ferromagnetic
bonds). For this purpose we perform Monte Carlo simulations on cubic lattices
of size and a finite-size scaling analysis of the numerical results.
The magnetic-glassy multicritical point is found at , along the
Nishimori line given by . We determine the
renormalization-group dimensions of the operators that control the
renormalization-group flow close to the multicritical point, ,
, and the susceptibility exponent . The
temperature and crossover exponents are and , respectively. We also investigate the model-A dynamics, obtaining
the dynamic critical exponent .Comment: 17 page
Transient dynamics of the nonequilibrium Majorana resonant level model
The Majorana resonant level model (MRLM) describes the universality class of
the two-channel/terminal Kondo model at the Toulouse point as well as of a
resonant level between two half-infinite Tomonaga--Luttinger liquids. We
analyze the time evolution of the electric current and of the population
function after an instantaneous switching on of the tunneling coupling. We find
that the only timescale, which governs the relaxation of the initial dot
preparation is the inverse contact transparency , whatever the dot
offset energy , applied bias voltage or temperature. The voltage alone
determines the superimposed oscillatory behavior of the observables for weak
detuning . In the opposite case of strong detuning
a beating pattern emerges. For the current the finite
temperature plays the similar role as the hybridization. The dot population
function dynamics approaches that of a resonant () setup upon
increasing the voltage or/and temperature
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