8 research outputs found
The influence of depolarization field on dielectric and pyroelectric properties of ferroelectric films
Calculations of the spontaneous polarization (Ps), dielectric susceptibility (c) and pyroelectric coefficient (P) of the ferroelectric films have been performed in the thermodynamic phenomenological theory framework. The Euler-Lagrange equation determining polarization dependence on the film parameters and the external electric field was solved analytically under the boundary conditions with different extrapolation lengths at two surfaces, respectively. The depolarization field contribution was taken into account in the model of short-circuited mono-domain ferroelectric film, treated as perfect insulator. The detailed analysis of the aforementioned quantities, space distribution and their average values in two cases with and without depolarization field was carried out. It was shown that the depolarization field shifts critical temperature to smaller values and the critical thickness to bigger value in comparison to those obtained without accounting the depolarization field. Meanwhile average values of Ps, c and P dependences on the film parameters and temperature are similar to the corresponding dependences obtained without accounting the depolarization field. The depolarization field was shown to flatten Ps, c and P space distributions, which have the peculiarities otherwise (e.g. small maxima in c and P coordinate profiles near the film surfaces). It was shown that depolarization field influence in short-circuited film could be neglected when the film thickness or the extrapolation lengths in the boundary conditions are larger than the correlation length value
The enhancement of ferromagnetism in uniaxially stressed diluted magnetic semiconductors
We predict a new mechanism of enhancement of ferromagnetic phase transition
temperature in uniaxially stressed diluted magnetic semiconductors (DMS)
of p-type. Our prediction is based on comparative studies of both Heisenberg
(inherent to undistorted DMS with cubic lattice) and Ising (which can be
applied to strongly enough stressed DMS) models in a random field approximation
permitting to take into account the spatial inhomogeneity of spin-spin
interaction. Our calculations of phase diagrams show that area of parameters
for existence of DMS-ferromagnetism in Ising model is much larger than that in
Heisenberg model.Comment: Accepted for publication in Phys. Rev.
Suppression of carrier induced ferromagnetism by composition and spin fluctuations in diluted magnetic semiconductors
We suggest an approach to account for spatial (composition) and thermal
fluctuations in "disordered" magnetic models (e.g. Heisenberg, Ising) with
given spatial dependence of magnetic spin-spin interaction. Our approach is
based on introduction of fluctuating molecular field (rather than mean field)
acting between the spins. The distribution function of the above field is
derived self-consistently. In general case this function is not Gaussian,
latter asymptotics occurs only at sufficiently large spins (magnetic ions)
concentrations . Our approach permits to derive the equation for a
critical temperature of ferromagnetic phase transition with respect to
the above fluctuations. We apply our theory to the analysis of influence of
composition fluctuations on in diluted magnetic semiconductors (DMS) with
RKKY indirect spin-spin interaction.Comment: 6 pages, 2 figure
Energy scales and magnetoresistance at a quantum critical point
The magnetoresistance (MR) of CeCoIn_5 is notably different from that in many
conventional metals. We show that a pronounced crossover from negative to
positive MR at elevated temperatures and fixed magnetic fields is determined by
the scaling behavior of quasiparticle effective mass. At a quantum critical
point (QCP) this dependence generates kinks (crossover points from fast to slow
growth) in thermodynamic characteristics (like specific heat, magnetization
etc) at some temperatures when a strongly correlated electron system transits
from the magnetic field induced Landau Fermi liquid (LFL) regime to the
non-Fermi liquid (NFL) one taking place at rising temperatures. We show that
the above kink-like peculiarity separates two distinct energy scales in QCP
vicinity - low temperature LFL scale and high temperature one related to NFL
regime. Our comprehensive theoretical analysis of experimental data permits to
reveal for the first time new MR and kinks scaling behavior as well as to
identify the physical reasons for above energy scales.Comment: 7 pages, 6 figure
Direct evidence of soft mode behavior near the Burns' temperature in PbMgNbO (PMN) relaxor ferroectric
Inelastic neutron scattering measurements of the relaxor ferroelectric
PbMgNbO (PMN) in the temperature range
490~KT880~K directly observe the soft mode (SM) associated with the
Curie-Weiss behavior of the dielectric constant (T). The results
are treated within the framework of the coupled SM and transverse optic (TO1)
mode and the temperature dependence of the SM frequency at q=0.075 a* is
determined. The parameters of the SM are consistent with the earlier estimates
and the frequency exhibits a minimum near the Burns temperature (
650K)Comment: 6 figure