5,339 research outputs found
The effects of disorder in dimerized quantum magnets in mean field approximations
We study theoretically the effects of disorder on Bose-Einstein condensates
(BEC) of bosonic triplon quasiparticles in doped dimerized quantum magnets. The
condensation occurs in a strong enough magnetic field Hc, where the
concentration of bosons in the random potential is sufficient to form the
condensate. The effect of doping is partly modeled by delta - correlated
disorder potential, which (i) leads to the uniform renormalization of the
system parameters and (ii) produces disorder in the system with renormalized
parameters. These approaches can explain qualitatively the available
magnetization data in the Tl_(1-x)K_(x)CuCl_3 compound taken as an example. In
addition to the magnetization, we found that the speed of the Bogoliubov mode
has a peak as a function of doping parameter, x. No evidence of the pure Bose
glass phase has been obtained in the BEC regime.Comment: Includes 19 pages, 5 figure
Characteristics of First-Order Vortex Lattice Melting: Jumps in Entropy and Magnetization
We derive expressions for the jumps in entropy and magnetization
characterizing the first-order melting transition of a flux line lattice. In
our analysis we account for the temperature dependence of the Landau parameters
and make use of the proper shape of the melting line as determined by the
relative importance of electromagnetic and Josephson interactions. The results
agree well with experiments on anisotropic YBaCuO and
layered BiSrCaCuO materials and reaffirm the validity of
the London model.Comment: 4 pages. We have restructured the paper to emphasize that in the
London scaling regime (appropriate for YBCO) our results are essentially
exact. We have also emphasized that a major controversy over the relevance of
the London model to describe VL melting has been settled by this wor
Control of Superconducting Correlations in High-Tc Cuprates
A strategy to enhance d-wave superconducting correlations is proposed based
on our numerical study for correlated electron models for high-Tc cuprates. We
observe that the pairing is enhanced when the single-electron level around
(pi,0) is close to the Fermi level E_F, while the d-wave pairing interaction
itself contains elements to disfavor the pairing due to shift of the
(pi,0)-level. Angle-resolved photoemission results in the cuprates are
consistently explained in the presence of the d-wave pairing interaction. Our
proposal is the tuning of the (pi,0)-level under the many-body effects to E_F
by optimal design of band structure.Comment: 4 pages, 6 eps figure
Superconductivity and magnetism in RbxFe2-ySe2: Impact of thermal treatment on mesoscopic phase separation
An extended study of the superconducting and normal-state properties of
various as-grown and post-annealed RbxFe2-ySe2 single crystals is presented.
Magnetization experiments evidence that annealing of RbxFe2-ySe2 at 413 K, well
below the onset of phase separation Tp=489 K, neither changes the magnetic nor
the superconducting properties of the crystals. In addition, annealing at 563
K, well above Tp, suppresses the superconducting transition temperature Tc and
leads to an increase of the antiferromagnetic susceptibility accompanied by the
creation of ferromagnetic impurity phases, which are developing with annealing
time. However, annealing at T=488K=Tp increases Tc up to 33.3 K, sharpens the
superconducting transition, increases the lower critical field, and strengthens
the screening efficiency of the applied magnetic field. Resistivity
measurements of the as-grown and optimally annealed samples reveal an increase
of the upper critical field along both crystallographic directions as well as
its anisotropy. Muon spin rotation and scanning transmission electron
microscopy experiments suggest the coexistence of two phases below Tp: a
magnetic majority phase of Rb2Fe4Se5 and a non-magnetic minority phase of
Rb0.5Fe2Se2. Both microscopic techniques indicate that annealing the specimens
just at Tp does not affect the volume fraction of the two phases, although the
magnetic field distribution in the samples changes substantially. This suggests
that the microstructure of the sample, caused by mesoscopic phase separation,
is modified by annealing just at Tp, leading to an improvement of the
superconducting properties of RbxFe2-ySe2 and an enhancement of Tc.Comment: 13 pages, 12 figure
Realtime calibration of the A4 electromagnetic lead fluoride calorimeter
Sufficient energy resolution is the key issue for the calorimetry in particle
and nuclear physics. The calorimeter of the A4 parity violation experiment at
MAMI is a segmented calorimeter where the energy of an event is determined by
summing the signals of neighbouring channels. In this case the precise matching
of the individual modules is crucial to obtain a good energy resolution. We
have developped a calibration procedure for our total absorbing electromagnetic
calorimeter which consists of 1022 lead fluoride (PbF_2) crystals. This
procedure reconstructs the the single-module contributions to the events by
solving a linear system of equations, involving the inversion of a 1022 x
1022-matrix. The system has shown its functionality at beam energies between
300 and 1500 MeV and represents a new and fast method to keep the calorimeter
permanently in a well-calibrated state
First order phase transition of the vortex lattice in twinned YBa2Cu3O7 single crystals in tilted magnetic fields
We present an exhaustive analysis of transport measurements performed in
twinned YBa2Cu3O7 single crystals which stablishes that the vortex solid-liquid
transition is first order when the magnetic field H is applied at an angle
theta away from the direction of the twin planes. We show that the resistive
transitions are hysteretic and the V-I curves are non-linear, displaying a
characteristic s-shape at the melting line Hm(T), which scales as
epsilon(theta)Hm(T,theta). These features are gradually lost when the critical
point H*(theta) is approached. Above H*(theta) the V-I characteristics show a
linear response in the experimentally accessible V-I window, and the transition
becomes reversible. Finally we show that the first order phase transition takes
place between a highly correlated vortex liquid in the field direction and a
solid state of unknown symmetry. As a consequence, the available data support
the scenario for a vortex-line melting rather than a vortex sublimation as
recently suggested [T.Sasagawa et al. PRL 80, 4297 (1998)].Comment: 10 pages, 8 figures, submitted to PR
Onsager Loop-Transition and First Order Flux-Line Lattice Melting in High- Superconductors
Monte-Carlo simulations in conjunction with finite-size scaling analysis are
used to investigate the -phase diagram in uniaxial anisotropic high-
superconductors, both in zero magnetic field and in intermediate magnetic
fields for various mass-anisotropies. The model we consider is the uniformly
frustrated anisotropic Villain Model. In zero magnetic field, and for all
anisotropies considered, we find one single second order phase transition,
mediated by an Onsager vortex-loop blowout. This is the superconductor-normal
metal transition.A comparison with numerical simulations and a critical scaling
analysis of the zero-field loop-transition yields the same exponent of the loop
distribution function at the critical point. In the intermediate magnetic field
regime, we find two anomalies in the specific heat. The first anomaly at a
temperature is associated with the melting transition of the flux-line
lattice. The second anomaly at a temperature is one where phase coherence
along the field direction is destroyed. We argue that in the
thermodynamic and continuum limit. Hence, there is no regime where the flux
line lattice melts into a disentangled flux-line liquid. The loss of phase
coherence parallel to the magnetic field in the sample is argued to be due to
the proliferation of closed non-field induced vortex loops on the scale of the
magnetic length in the problem, resulting in flux-line cutting and
recombination. In the flux-line liquid phase, therefore, flux-lines appear no
longer to be well defined entities. A finite-size scaling analysis of the delta
function peak specific heat anomaly at the melting transition is used to
extract the discontinuity of the entropy at the melting transition.This entropy
discontinuity is found to increase rapidly with mass-anisotropy.Comment: 22 pages, 11 figures included, to be published in Phys. Rev. B, 57
xxx (1998
Quantum Nonlinear Switching Model
We present a method, the dynamical cumulant expansion, that allows to
calculate quantum corrections for time-dependent quantities of interacting spin
systems or single spins with anisotropy. This method is applied to the
quantum-spin model \hat{H} = -H_z(t)S_z + V(\bf{S}) with H_z(\pm\infty) =
\pm\infty and \Psi (-\infty)=|-S> we study the quantity P(t)=(1-_t/S)/2.
The case V(\bf{S})=-H_x S_x corresponds to the standard
Landau-Zener-Stueckelberg model of tunneling at avoided-level crossing for N=2S
independent particles mapped onto a single-spin-S problem, P(t) being the
staying probability. Here the solution does not depend on S and follows, e.g.,
from the classical Landau-Lifshitz equation. A term -DS_z^2 accounts for
particles' interaction and it makes the model nonlinear and essentially quantum
mechanical. The 1/S corrections obtained with our method are in a good accord
with a full quantum-mechanical solution if the classical motion is regular, as
for D>0.Comment: 4 Phys. Rev. pages 2 Fig
Possible new vortex matter phases in BSCCO
The vortex matter phase diagram of BSCCO crystals is analyzed by
investigating vortex penetration through the surface barrier in the presence of
a transport current. The strength of the effective surface barrier, its
nonlinearity, and asymmetry are used to identify a possible new ordered phase
above the first-order transition. This technique also allows sensitive
determination of the depinning temperature. The solid phase below the
first-order transition is apparently subdivided into two phases by a vertical
line extending from the multicritical point.Comment: 11 pages, 3 figures, accepted for publication in PR
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