3,913 research outputs found
Quantized Lattice Dynamic Effects on the Spin-Peierls Transition
The density matrix renormalization group method is used to investigate the
spin-Peierls transition for Heisenberg spins coupled to quantized phonons. We
use a phonon spectrum that interpolates between a gapped, dispersionless
(Einstein) limit to a gapless, dispersive (Debye) limit. A variety of
theoretical probes are used to determine the quantum phase transition,
including energy gap crossing, a finite size scaling analysis, bond order
auto-correlation functions, and bipartite quantum entanglement. All these
probes indicate that in the antiadiabatic phonon limit a quantum phase
transition of the Berezinskii-Kosterlitz-Thouless type is observed at a
non-zero spin-phonon coupling, . An extrapolation from the
Einstein limit to the Debye limit is accompanied by an increase in for a fixed optical () phonon gap. We therefore conclude that the
dimerized ground state is more unstable with respect to Debye phonons, with the
introduction of phonon dispersion renormalizing the effective spin-lattice
coupling for the Peierls-active mode. We also show that the staggered spin-spin
and phonon displacement order parameters are unreliable means of determining
the transition.Comment: To be published in Phys. Rev.
Supersonic dislocations observed in a plasma crystal
Experimental results on the dislocation dynamics in a two-dimensional plasma
crystal are presented. Edge dislocations were created in pairs in lattice
locations where the internal shear stress exceeded a threshold and then moved
apart in the glide plane at a speed higher than the sound speed of shear waves,
. The experimental system, a plasma crystal, allowed observation of this
process at an atomistic (kinetic) level. The early stage of this process is
identified as a stacking fault. At a later stage, supersonically moving
dislocations generated shear-wave Mach cones
Fidelity recovery in chaotic systems and the Debye-Waller factor
Using supersymmetry calculations and random matrix simulations, we studied
the decay of the average of the fidelity amplitude f_epsilon(tau)=<psi(0)|
exp(2 pi i H_epsilon tau) exp(-2 pi i H_0 tau) |psi(0)>, where H_epsilon
differs from H_0 by a slight perturbation characterized by the parameter
epsilon. For strong perturbations a recovery of f_epsilon(tau) at the
Heisenberg time tau=1 is found. It is most pronounced for the Gaussian
symplectic ensemble, and least for the Gaussian orthogonal one. Using Dyson's
Brownian motion model for an eigenvalue crystal, the recovery is interpreted in
terms of a spectral analogue of the Debye-Waller factor known from solid state
physics, describing the decrease of X-ray and neutron diffraction peaks with
temperature due to lattice vibrations.Comment: revised version (major changes), 4 pages, 4 figure
Theory of Polaron Resonance in Quantum Dots and Quantum-Dot Molecules
The theory of exciton coupling to photons and LO phonons in quantum dots
(QDs) and quantum-dot molecules (QDMs) is presented. Resonant-round trips of
the exciton between the ground (bright) and excited (dark or bright) states
mediated by the LO-phonon alter the decay time and yield the Rabi oscillation.
The initial distributions of the population in the ground and the excited
states dominate the oscillating amplitude and frequency. This property provides
a detectable signature to the information stored in a qubit made from QD or QDM
for a wide range of temperature T. Our results presented herein provide an
explanation to the anomaly on T-dependent decay in self-assembled InGaAs/GaAs
QDMs recently reported by experiment.Comment: 30 pages, 8 figure
The Electron-Phonon Interaction of Low-Dimensional and Multi-Dimensional Materials from He Atom Scattering
Atom scattering is becoming recognized as a sensitive probe of the
electron-phonon interaction parameter at metal and metal-overlayer
surfaces. Here, the theory is developed linking to the thermal
attenuation of atom scattering spectra (in particular, the Debye-Waller
factor), to conducting materials of different dimensions, from quasi-one
dimensional systems such as W(110):H(11) and Bi(114), to quasi-two
dimensional layered chalcogenides and high-dimensional surfaces such as
quasicrystalline 2ML-Ba(0001)/Cu(001) and d-AlNiCo(00001). Values of
obtained using He atoms compare favorably with known values for the bulk
materials. The corresponding analysis indicates in addition the number of
layers contributing to the electron-phonon interaction that is measured in an
atom surface collision.Comment: 23 pages, 5 figures, 1 tabl
Critical Temperature Tc and Charging Energy Ec between B-B layers of Superconducting diboride materials MgB2 in 3D JJA model
The diboride materials MB2 (M = Mg, Be, Pb, etc.) are discussed on the basis
of the 3D Josephson junction array (JJA) model due to Kawabata-Shenoy-Bishop,
in terms of the B-B layers in the diborides analogous to the Cu-O ones in the
cuprates.
We propose a possibility of superconducting materials with the MgB2-type
structure which exhibit higher critical temperature Tc over 39K of MgB2.
We point out a role of interstitial ionic atoms (e.g., Mg in MgB2) as
capacitors between the B-B layers, which reduce the charging coupling energy in
JJA.Comment: 3 pages, 1 figure included; to be published in J. Phys. Soc. Jpn. 70,
No.10 (2001
Energy spectra for quantum wires and 2DEGs in magnetic fields with Rashba and Dresselhaus spin-orbit interactions
We introduce an analytical approximation scheme to diagonalize parabolically
confined two dimensional electron systems with both the Rashba and Dresselhaus
spin-orbit interactions. The starting point of our perturbative expansion is a
zeroth-order Hamiltonian for an electron confined in a quantum wire with an
effective spin-orbit induced magnetic field along the wire, obtained by
properly rotating the usual spin-orbit Hamiltonian. We find that the
spin-orbit-related transverse coupling terms can be recast into two parts W and
V, which couple crossing and non-crossing adjacent transverse modes,
respectively. Interestingly, the zeroth-order Hamiltonian together with W can
be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics.
We treat the V coupling by performing a Schrieffer-Wolff transformation. This
allows us to obtain an effective Hamiltonian to third order in the coupling
strength k_Rl of V, which can be straightforwardly diagonalized via an
additional unitary transformation. We also apply our approach to other types of
effective parabolic confinement, e.g., 2D electrons in a perpendicular magnetic
field. To demonstrate the usefulness of our approximate eigensolutions, we
obtain analytical expressions for the n^th Landau-level g_n-factors in the
presence of both Rashba and Dresselhaus couplings. For small values of the bulk
g-factors, we find that spin-orbit effects cancel out entirely for particular
values of the spin-orbit couplings. By solving simple transcendental equations
we also obtain the band minima of a Rashba-coupled quantum wire as a function
of an external magnetic field. These can be used to describe Shubnikov-de Haas
oscillations. This procedure makes it easier to extract the strength of the
spin-orbit interaction in these systems via proper fitting of the data.Comment: 13 pages, 11 figure
Anomalous Spin Polarization of GaAs Two-Dimensional Hole Systems
We report measurements and calculations of the spin-subband depopulation,
induced by a parallel magnetic field, of dilute GaAs two-dimensional (2D) hole
systems. The results reveal that the shape of the confining potential
dramatically affects the values of in-plane magnetic field at which the upper
spin subband is depopulated. Most surprisingly, unlike 2D electron systems, the
carrier-carrier interaction in 2D hole systems does not significantly enhance
the spin susceptibility. We interpret our findings using a multipole expansion
of the spin density matrix, and suggest that the suppression of the enhancement
is related to the holes' band structure and effective spin j=3/2.Comment: 6 pages, 4 figures, substantially extended discussion of result
Bubble Raft Model for a Paraboloidal Crystal
We investigate crystalline order on a two-dimensional paraboloid of
revolution by assembling a single layer of millimeter-sized soap bubbles on the
surface of a rotating liquid, thus extending the classic work of Bragg and Nye
on planar soap bubble rafts. Topological constraints require crystalline
configurations to contain a certain minimum number of topological defects such
as disclinations or grain boundary scars whose structure is analyzed as a
function of the aspect ratio of the paraboloid. We find the defect structure to
agree with theoretical predictions and propose a mechanism for scar nucleation
in the presence of large Gaussian curvature.Comment: 4 pages, 4 figure
A Hybrid model for the origin of photoluminescence from Ge nanocrystals in SiO matrix
In spite of several articles, the origin of visible luminescence from
germanium nanocrystals in SiO matrix is controversial even today. Some
authors attribute the luminescence to quantum confinement of charge carriers in
these nanocrystals. On the other hand, surface or defect states formed during
the growth process, have also been proposed as the source of luminescence in
this system. We have addressed this long standing query by simultaneous
photoluminescence and Raman measurements on germanium nanocrystals embedded in
SiO matrix, grown by two different techniques: (i) low energy
ion-implantation and (ii) atom beam sputtering. Along with our own experimental
observations, we have summarized relevant information available in the
literature and proposed a \emph{Hybrid Model} to explain the visible
photoluminescence from nanocrystalline germanium in SiO matrix.Comment: 23 pages, 8 figure
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