8,301 research outputs found
Phase transformation B1 to B2 in TiC, TiN, ZrC and ZrN under pressure
Phase stability of various phases of MX (M = Ti, Zr; X = C, N) at equilibrium
and under pressure is examined based on first-principles calculations of the
electronic and phonon structures. The results reveal that all B1 (NaCl-type) MX
structures undergo a phase transition to the B2-structures under high pressure
in agreement with the previous total-energy calculations. The B1-MX structures
are dynamically stable under very high pressure (210-570 GPa). The
pressure-induced B2 (CsCl-type) MC phases are dynamically unstable even at high
pressures, and TiN and ZrN are found to crystallize with the B2-structure only
at pressures above 55 GPa. The first-order B1-to-B2 phase transition in these
nitrides is not related to the softening of phonon modes, and the dynamical
instability of B2-MX is associated with a high density of states at the Fermi
level.Comment: 9 pages, 4 figure
Coulomb Drag in the Exciton Regime in Electron-Hole Bilayers
We report electrical transport measurements on GaAs/AlGaAs based
electron-hole bilayers. These systems are expected to make a transition from a
pair of weakly coupled two-dimensional systems to a strongly coupled exciton
system as the barrier between the layers is reduced. Once excitons form,
phenomena such as Bose-Einstein condensation of excitons could be observed. In
our devices, electrons and holes are confined in double quantum wells, and
carriers in the devices are induced with top and bottom gates leading to
variable density in each layer. Separate contact to each layer allows Coulomb
drag transport measurements where current is driven in one layer while voltage
is measured in the other. Coulomb drag is sensitive to interlayer coupling and
has been predicted to provide a strong signature of exciton condensation. Drag
measurement on EHBLs with a 30 nm barrier are consistent with drag between two
weakly coupled 2D Fermi systems where the drag decreases as the temperature is
reduced. When the barrier is reduced to 20 nm, we observe a consistent increase
in the drag resistance as the temperature is reduced. These results indicate
the onset of a much stronger coupling between the electrons and holes which
leads to exciton formation and possibly phenomena related to exciton
condensation.Comment: 12 pages, 3 figure
Direct extraction of transversity and its accompanying T-odd distribution from the unpolarized and single-polarized Drell-Yan processes
The Drell-Yan (DY) processes with unpolarized colliding hadrons and with the
single transversally polarized hadron are considered. The possibility of direct
(without any model assumptions) extraction of both transversity and its
accompanying T-odd parton distribution functions (PDF) is discussed. For DY
processes measurements planned at GSI the preliminary estimations demonstrate
that it is quite real to extract both transversity and its accompanying T-odd
PDF in the PAX conditions
Dislocation-induced superfluidity in a model supersolid
Motivated by recent experiments on the supersolid behavior of He, we
study the effect of an edge dislocation in promoting superfluidity in a Bose
crystal. Using Landau theory, we couple the elastic strain field of the
dislocation to the superfluid density, and use a linear analysis to show that
superfluidity nucleates on the dislocation before occurring in the bulk of the
solid. Moving beyond the linear analysis, we develop a systematic perturbation
theory in the weakly nonlinear regime, and use this method to integrate out
transverse degrees of freedom and derive a one-dimensional Landau equation for
the superfluid order parameter. We then extend our analysis to a network of
dislocation lines, and derive an XY model for the dislocation network by
integrating over fluctuations in the order parameter. Our results show that the
ordering temperature for the network has a sensitive dependence on the
dislocation density, consistent with numerous experiments that find a clear
connection between the sample quality and the supersolid response.Comment: 10 pages, 6 figure
Test of the τ-model of Bose–Einstein correlations and reconstruction of the source function in hadronic Z-boson decay at LEP
Bose–Einstein correlations of pairs of identical charged pions produced in hadronic Z decays are analyzed in terms of various parametrizations. A good description is achieved using a Lévy stable distribution in conjunction with a model where a particle’s momentum is correlated with its space–time point of production, the τ-model. Using this description and the measured rapidity and transverse momentum distributions, the space–time evolution of particle emission in two-jet events is reconstructed. However, the elongation of the particle emission region previously observed is not accommodated in the τ-model, and this is investigated using an ad hoc modification
Superfluidity of electron-hole pairs in randomly inhomogeneous bilayer systems
In bilayer systems electron-hole (e-h) pairs with spatially separated
components (i.e., with electrons in one layer and holes in the other) can be
condensed to a superfluid state when the temperature is lowered. This article
deals with the influence of randomly distributed inhomogeneities on the
superfluid properties of such bilayer systems in a strong perpendicular
magnetic field. Ionized impurities and roughenings of the conducting layers are
shown to decrease the superfluid current density of the e-h pairs. When the
interlayer distance is smaller than or close to the magnetic length, the
fluctuations of the interlayer distance considerably reduce the superfluid
transition temperature.Comment: 13 pages, 3 figure
Electromagnetic force density in dissipative isotropic media
We derive an expression for the macroscopic force density that a narrow-band
electromagnetic field imposes on a dissipative isotropic medium. The result is
obtained by averaging the microscopic form for Lorentz force density. The
derived expression allows us to calculate realistic electromagnetic forces in a
wide range of materials that are described by complex-valued electric
permittivity and magnetic permeability. The three-dimensional energy-momentum
tensor in our expression reduces for lossless media to the so-called Helmholtz
tensor that has not been contradicted in any experiment so far. The momentum
density of the field does not coincide with any well-known expression, but for
non-magnetic materials it matches the Abraham expression
Stability of Sarma phases in density imbalanced electron-hole bilayer systems
We study excitonic condensation in an electron-hole bilayer system with
unequal layer densities at zero temperature. Using mean-field theory we solve
the BCS gap equations numerically and investigate the effects of intra-layer
interactions. We analyze the stability of the Sarma phase with \bk,-\bk
pairing by calculating the superfluid mass density and also by checking the
compressibility matrix. We find that with bare Coulomb interactions the
superfluid density is always positive in the Sarma phase, due to a peculiar
momentum structure of the gap function originating from the singular behavior
of the Coulomb potential at zero momentum and the presence of a sharp Fermi
surface. Introducing a simple model for screening, we find that the superfluid
density becomes negative in some regions of the phase diagram, corresponding to
an instability towards a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) type superfluid
phase. Thus, intra-layer interaction and screening together can lead to a rich
phase diagram in the BCS-BEC crossover regime in electron-hole bilayer systems
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