5,057 research outputs found
Hopping and clustering of oxygen vacancies in SrTiO3 by anelastic relaxation
The complex elastic compliance s11(w,T) of SrTiO3-d has been measured as a
function of the O deficiency d < 0.01. The two main relaxation peaks in the
absorption are identified with hopping of isolated O vacancies over a barrier
of 0.60 eV and reorientation of pairs of vacancies involving a barrier of 1 eV.
The pair binding energy is ~0.2 eV and indications for additional clustering,
possibly into chains, is found already at d ~0.004. The anistropic component of
the elastic dipole of an O vacancy is Deltalambda = 0.026.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Supersolid phases of dipolar bosons in optical lattices with a staggered flux
We present the theoretical mean-field zero-temperature phase diagram of a
Bose-Einstein condensate (BEC) with dipolar interactions loaded into an optical
lattice with a staggered flux. Apart from uniform superfluid, checkerboard
supersolid and striped supersolid phases, we identify several supersolid phases
with staggered vortices, which can be seen as combinations of supersolid phases
found in earlier work on dipolar BECs and a staggered-vortex phase found for
bosons in optical lattices with staggered flux. By allowing for different
phases and densities on each of the four sites of the elementary plaquette,
more complex phase patterns are found.Comment: 11 pages; added references, minor changes in tex
Heavy Fermion Quantum Criticality
During the last few years, investigations of Rare-Earth materials have made
clear that not only the heavy fermion phase in these systems provides
interesting physics, but the quantum criticality where such a phase dies
exhibits novel phase transition physics not fully understood. Moreover,
attempts to study the critical point numerically face the infamous fermion sign
problem, which limits their accuracy. Effective action techniques and
Callan-Symanzik equations have been very popular in high energy physics, where
they enjoy a good record of success. Yet, they have been little exploited for
fermionic systems in condensed matter physics. In this work, we apply the RG
effective action and Callan-Symanzik techiques to the heavy fermion problem. We
write for the first time the effective action describing the low energy physics
of the system. The f-fermions are replaced by a dynamical scalar field whose
nonzero expected value corresponds to the heavy fermion phase. This removes the
fermion sign problem, making the effective action amenable to numerical studies
as the effective theory is bosonic. Renormalization group studies of the
effective action can be performed to extract approximations to nonperturbative
effects at the transition. By performing one-loop renormalizations, resummed
via Callan-Symanzik methods, we describe the heavy fermion criticality and
predict the heavy fermion critical dynamical susceptibility and critical
specific heat. The specific heat coefficient exponent we obtain (0.39) is in
excellent agreement with the experimental result at low temperatures (0.4).Comment: 5 pages. In the replacement, the numerical value for the specific
heat coefficient exponent has been included explicitly in decimal form, and
has been compared with the experimental result
Unidimensional model of the ad-atom diffusion on a substrate submitted to a standing acoustic wave I. Derivation of the ad-atom motion equation
The effect of a standing acoustic wave on the diffusion of an ad-atom on a
crystalline surface is theoretically studied. We used an unidimensional space
model to study the ad-atom+substrate system. The dynamic equation of the
ad-atom, a Generalized Langevin equation, is analytically derived from the full
Hamiltonian of the ad-atom+substrate system submitted to the acoustic wave. A
detailed analysis of each term of this equation, as well as of their
properties, is presented. Special attention is devoted to the expression of the
effective force induced by the wave on the ad-atom. It has essentially the same
spatial and time dependences as its parent standing acoustic wave
Hexagonal spiral growth in the absence of a substrate
Experiments on the formation of spiraling hexagons (350 - 1000 nm in width)
from a solution of nanoparticles are presented. Transmission electron
microscopy images of the reaction products of chemically synthesized cadmium
nanocrystals indicate that the birth of the hexagons proceeds without
assistance from static screw or edge dislocatons, that is, they spiral without
constraints provided by an underlying substrate. Instead, the apparent growth
mechanism relies on what we believe is a dynamical dislocation identified as a
dense aggregate of small nanocrystals that straddles the spiraling hexagon at
the crystal surface. This nanocrystal bundle, which we term the "feeder", also
appears to release nanocrystals into the spiral during the growth process.Comment: 4 pages, 5 figure
Formation and Stability of Cellular Carbon Foam Structures:An {\em Ab Initio} Study
We use ab initio density functional calculations to study the formation and
structural as well as thermal stability of cellular foam-like carbon
nanostructures. These systems with a mixed bonding character may be
viewed as bundles of carbon nanotubes fused to a rigid contiguous 3D honeycomb
structure that can be compressed more easily by reducing the symmetry of the
honeycombs. The foam may accommodate the same type of defects as graphene, and
its surface may be be stabilized by terminating caps. We postulate that the
foam may form under non-equilibrium conditions near grain boundaries of a
carbon-saturated metal surface
Kondo Quantum Dots and the Novel Kondo-doublet interaction
We analyze the interactions between two Kondo Quantum Dots connected to a
Rashba-active Quantum Wire. We find that the Kondo-doublet interaction, at an
inter-dot distance of the order of the wire Fermi length, is over an order of
magnitude greater than the RKKY interaction. The effects induced on the
Kondo-doublet interaction by the wire spin-orbit coupling can be used to
control the Quantum Dots spin-spin correlation. These results imply that the
widely used assumption that the RKKY is the dominant interaction between
Anderson impurities must be revised.Comment: 4 pages, 4 figs, accepted for publication in PRL. title changed and
text polishe
Current-spin coupling for ferromagnetic domain walls in fine wires
The coupling between a current and a domain wall is examined. In the presence
of a finite current and the absence of a potential which breaks the
translational symmetry, there is a perfect transfer of angular momentum from
the conduction electrons to the wall. As a result, the ground state is in
uniform motion. This remains the case when relaxation is accounted for. This is
described by, appropriately modified, Landau-Lifshitz-Gilbert equations.Comment: 4 pqges, no figure
Mirages and enhanced magnetic interactions in quantum corrals
We develop a theory for the interactions between magnetic impurities in
nanoscopic systems. The case of impurities in quantum corrals built on the
(111) Cu surface is analyzed in detail. For elliptical corrals with one
impurity, clear magnetic mirages are obtained. This leads to an enhancement of
the inter-impurity interactions when two impurities are placed at special
points in the corral. We discuss the enhancement of the conduction electron
response to the local perturbation in other nanoscopic systems.Comment: 7 pages, 5 figure
Renormalization of the spin-wave spectrum in three-dimentional ferromagnets with dipolar interaction
Renormalization of the spin-wave spectrum is discussed in a cubic ferromagnet
with dipolar forces at . First 1/S-corrections are considered in
detail to the bare spectrum , where is the spin-wave stiffness,
is the angle between and the magnetization and
is the characteristic dipolar energy. In accordance with previous
results we obtain the thermal renormalization of constants and
in the expression for the bare spectrum. Besides, a number of previously
unknown features are revealed. We observe terms which depend on azimuthal angle
of the momentum . It is obtained an isotropic term proportional to
which makes the spectrum linear rather than quadratic when and . In particular a spin-wave gap proportional to
is observed. Essentially, thermal contribution from the
Hartree-Fock diagram to the isotropic correction as well as to the spin-wave
gap are proportional to the demagnetizing factor in the direction of domain
magnetization. This nontrivial behavior is attributed to the long-range nature
of the dipolar interaction. It is shown that the gap screens infrared
singularities of the first 1/S-corrections to the spin-wave stiffness and
longitudinal dynamical spin susceptibility (LDSS) obtained before. We
demonstrate that higher order 1/S-corrections to these quantities are small at
. However the analysis of the entire perturbation series is still
required to derive the spectrum and LDSS when .Comment: 11 pages, 1 figur
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