46 research outputs found
Spin Structure Factor of the Frustrated Quantum Magnet Cs_2CuCl_4
The ground state properties and neutron structure factor for the
two-dimensional antiferromagnet on the triangular lattice, with uni-directional
anisotropy in the nearest-neighbor exchange couplings and a weak
Dzyaloshinskii-Moriya (DM) interaction, are studied. This Hamiltonian has been
used to interpret neutron scattering measurements on the spin 1/2 spiral
spin-density-wave system, Cs_2CuCl_4, [R. Coldea, et al., Phys. Rev. B 68,
134424 (2003)]. Calculations are performed using a 1/S expansion, taking into
account interactions between spin-waves. The ground state energy, the shift of
the ordering wave-vector, Q, and the local magnetization are all calculated to
order 1/S^2. The neutron structure factor, obtained using anharmonic spin-wave
Green's functions to order 1/S, is shown to be in reasonable agreement with
published neutron data, provided that slightly different parameters are used
for the exchange and DM interactions than those inferred from measurements in
high magnetic field.Comment: 14 pages, 6 eps figures, submitted to Phys. Rev.
Anisotropic pairing in the iron pnictides
We determine the anisotropy of the spin fluctuation induced pairing gap on
the Fermi surface of the FeAs based superconductors as function of the exchange
and Hund's coupling . We find that for sufficiently large ,
nearly commensurate magnetic fluctuations yield a fully gapped
-pairing state with small anisotropy of the gap amplitude on each
Fermi surface sheet, but significant variations of the gap amplitude for
different sheets of the Fermi surface. In particular, we obtain the large
variation of the gap amplitude on different Fermi surface sheets, as seen in
ARPES experiments. For smaller values of Hund's coupling incommensurate
magnetic fluctuations yield an -pairing state with line nodes. Such a
state is also possible once the anisotropy of the material is reduced and three
dimensional effects come into play.Comment: Revised and substantially extended version, accepted for publication
in Phys. Rev. B; 9 pages, 6 figure
PyMembrane: A flexible framework for efficient simulations of elastic and liquid membranes
PyMembrane is a software package for simulating liquid and elastic membranes
using a discretisation of the continuum description based on unstructured
triangulated two-dimensional meshes embedded in three-dimensional space. The
package is written in C++, with a flexible and intuitive Python interface,
allowing for a quick setup, execution and analysis of complex simulations.
PyMembrane follows modern software engineering principles and features a
modular design that allows for straightforward implementation of custom
extensions while ensuring consistency and enabling inexpensive maintenance. A
hallmark feature of this design is the use of a standardized C++ interface
which streamlines adding new functionalities. Furthermore, PyMembrane uses data
structures optimised for unstructured meshes, ensuring efficient mesh
operations and force calculations. By providing several templates for typical
simulations supplemented by extensive documentation, the users can seamlessly
set up and run research-level simulations and extend the package to integrate
additional features, underscoring PyMembrane's commitment to user-centric
design.Comment: 7 Figure
Dynamics at a smeared phase transition
We investigate the effects of rare regions on the dynamics of Ising magnets
with planar defects, i.e., disorder perfectly correlated in two dimensions. In
these systems, the magnetic phase transition is smeared because static
long-range order can develop on isolated rare regions. We first study an
infinite-range model by numerically solving local dynamic mean-field equations.
Then we use extremal statistics and scaling arguments to discuss the dynamics
beyond mean-field theory. In the tail region of the smeared transition the
dynamics is even slower than in a conventional Griffiths phase: the spin
autocorrelation function decays like a stretched exponential at intermediate
times before approaching the exponentially small equilibrium value following a
power law at late times.Comment: 10 pages, 8eps figures included, final version as publishe
The quantum phase transition of itinerant helimagnets
We investigate the quantum phase transition of itinerant electrons from a
paramagnet to a state which displays long-period helical structures due to a
Dzyaloshinskii instability of the ferromagnetic state. In particular, we study
how the self-generated effective long-range interaction recently identified in
itinerant quantum ferromagnets is cut-off by the helical ordering. We find that
for a sufficiently strong Dzyaloshinskii instability the helimagnetic quantum
phase transition is of second order with mean-field exponents. In contrast, for
a weak Dzyaloshinskii instability the transition is analogous to that in
itinerant quantum ferromagnets, i.e. it is of first order, as has been observed
in MnSi.Comment: 5 pages RevTe
Non-Hookean statistical mechanics of clamped graphene ribbons
Thermally fluctuating sheets and ribbons provide an intriguing forum in which
to investigate strong violations of Hooke's Law: large distance elastic
parameters are in fact not constant, but instead depend on the macroscopic
dimensions. Inspired by recent experiments on free-standing graphene
cantilevers, we combine the statistical mechanics of thin elastic plates and
large-scale numerical simulations to investigate the thermal renormalization of
the bending rigidity of graphene ribbons clamped at one end. For ribbons of
dimensions (with ), the macroscopic bending rigidity
determined from cantilever deformations is independent of the width
when , where is a thermal length scale,
as expected. When , however, this thermally renormalized
bending rigidity begins to systematically increase, in agreement with the
scaling theory, although in our simulations we were not quite able to reach the
system sizes necessary to determine the fully developed power law dependence on
. When the ribbon length , where is the -dependent
thermally renormalized ribbon persistence length, we observe a scaling collapse
and the beginnings of large scale random walk behavior
Order parameter symmetry and mode coupling effects at dirty superconducting quantum phase transitions
We derive an order-parameter field theory for a quantum phase transition
between a disordered metal and an exotic (non-s-wave) superconductor. Mode
coupling effects between the order parameter and other fermionic soft modes
lead to an effective long-range interaction between the anomalous density
fluctuations which is reflected in singularities in the free energy functional.
However, this long-range interaction is not strong enough to suppress disorder
fluctuations. The asymptotic critical region is characterized by run-away flow
to large disorder. For weak coupling, this asymptotic region is very narrow. It
is preempted by a wide crossover regime with mean-field critical behavior and,
in the p-wave case, logarithmic corrections to scaling in all dimensions.Comment: final version as publishe
Effect of prolonged precipitation on morphology and crystal struture of the bacterial nanocelulose/Fe3O4 composite
Cellulose is a biopolymer with a wide range of properties like biocompatibility, hydrophilicity, porosity, good mechanical properties, biodegradability and non-toxicity. The properties and application of cellulose based materials are related to the source of the cellulose production. Despite the fact that the plant cellulose is playing a leading role in obtaining cellulose fibers, it has been found that ecologically and economically, a better source for obtaining cellulose is by fermenting a particular strain of bacteria. Although bacterial nano cellulose (BCN) based materials can be used in numerous industries, from the paper and food industries to biomedicine, their application in electronics is limited because bacterial cellulose does not have conductive and ferromagnetic properties. Having this in mind in this research, the results of the development of nanocomposite materials based on BCN modified with Fe3O4 has been presented. The differences in the interaction of Fe3O4 nanoparticles and BCN obtained by varying precipitation parameters were investigated and the effect of reaction time was followed by SEM-EDS, XRD, and FTIR analysis. It has been found that this type of modifications of the initial BCN, enables development of new composite materials with superior properties, which can be used in various fields of electronics
Smeared phase transition in a three-dimensional Ising model with planar defects: Monte-Carlo simulations
We present results of large-scale Monte Carlo simulations for a
three-dimensional Ising model with short range interactions and planar defects,
i.e., disorder perfectly correlated in two dimensions. We show that the phase
transition in this system is smeared, i.e., there is no single critical
temperature, but different parts of the system order at different temperatures.
This is caused by effects similar to but stronger than Griffiths phenomena. In
an infinite-size sample there is an exponentially small but finite probability
to find an arbitrary large region devoid of impurities. Such a rare region can
develop true long-range order while the bulk system is still in the disordered
phase. We compute the thermodynamic magnetization and its finite-size effects,
the local magnetization, and the probability distribution of the ordering
temperatures for different samples. Our Monte-Carlo results are in good
agreement with a recent theory based on extremal statistics.Comment: 9 pages, 6 eps figures, final version as publishe
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