675 research outputs found
Entanglement Spectra of Heisenberg Ladders of higher Spin
We study the entanglement spectrum of Heisenberg spin ladders of arbitrary spin length S in the perturbative regime of strong rung coupling. For isotropic spin coupling the the entanglement spectrum is, within first order perturbation theory, always proportional to the energy spectrum of the single chain with a proportionality factor being also independent of S. A more complicated situation arises for anisotropic ladders of higher spin S>=1 since here even the unperturbed ground state has a nontrivial entanglement spectrum. Finally we discuss related issues in dimerized spin chains
Dynamical polarizability of graphene beyond the Dirac cone approximation
We compute the dynamical polarizability of graphene beyond the usual Dirac
cone approximation, integrating over the full Brillouin zone. We find
deviations at ( the hopping parameter) which amount to a
logarithmic singularity due to the van Hove singularity and derive an
approximate analytical expression. Also at low energies, we find deviations
from the results obtained from the Dirac cone approximation which manifest
themselves in a peak spitting at arbitrary direction of the incoming wave
vector \q. Consequences for the plasmon spectrum are discussed.Comment: 8 pages, 6 figure
Entanglement in SU(2)-invariant quantum systems: The positive partial transpose criterion and others
We study entanglement in mixed bipartite quantum states which are invariant
under simultaneous SU(2) transformations in both subsystems. Previous results
on the behavior of such states under partial transposition are substantially
extended. The spectrum of the partial transpose of a given SU(2)-invariant
density matrix is entirely determined by the diagonal elements of
in a basis of tensor-product states of both spins with respect to a common
quantization axis. We construct a set of operators which act as entanglement
witnesses on SU(2)-invariant states. A sufficient criterion for having a
negative partial transpose is derived in terms of a simple spin correlator. The
same condition is a necessary criterion for the partial transpose to have the
maximum number of negative eigenvalues. Moreover, we derive a series of sum
rules which uniquely determine the eigenvalues of the partial transpose in
terms of a system of linear equations. Finally we compare our findings with
other entanglement criteria including the reduction criterion, the majorization
criterion, and the recently proposed local uncertainty relations.Comment: 7 pages, no figures, version to appear in Phys. Rev.
Entanglement spectra of coupled S=1/2 spin chains in a ladder geometry
We study the entanglement spectrum of spin-1/2 XXZ ladders both analytically
and numerically. Our analytical approach is based on perturbation theory
starting either from the limit of strong rung coupling, or from the opposite
case of dominant coupling along the legs. In the former case we find to leading
order that the entanglement Hamiltonian is also of nearest-neighbor XXZ form
although with an in general renormalized anisotropy. For the cases of XX and
isotropic Heisenberg ladders no such renormalization takes place. In the
Heisenberg case the second order correction to the entanglement Hamiltonian
consists of a renormalization of the nearest neighbor coupling plus an
unfrustrated next nearest neighbor coupling. In the opposite regime of strong
coupling along the legs, we point out an interesting connection of the
entanglement spectrum to the Lehmann representation of single chain spectral
functions of operators appearing in the physical Hamiltonian coupling the two
chains.Comment: 6 pages, 4 figures, published versio
Dielectric function of the semiconductor hole liquid: Full frequency and wave vector dependence
We study the dielectric function of the homogeneous semiconductor hole liquid
of p-doped bulk III-V zinc-blende semiconductors within random phase
approximation. The single-particle physics of the hole system is modeled by
Luttinger's four-band Hamiltonian in its spherical approximation. Regarding the
Coulomb-interacting hole liquid, the full dependence of the zero-temperature
dielectric function on wave vector and frequency is explored. The imaginary
part of the dielectric function is analytically obtained in terms of
complicated but fully elementary expressions, while in the result for the real
part nonelementary one-dimensional integrations remain to be performed. The
correctness of these two independent calculations is checked via Kramers-Kronig
relations.
The mass difference between heavy and light holes, along with variations in
the background dielectric constant, leads to dramatic alternations in the
plasmon excitation pattern, and generically, two plasmon branches can be
identified. These findings are the result of the evaluation of the full
dielectric function and are not accessible via a high-frequency expansion. In
the static limit a beating of Friedel oscillations between the Fermi wave
numbers of heavy and light holes occurs.Comment: 16 pages, 11 figures included. Update: Minor additions and
adjustments, published versio
Ballistic side jump motion of electrons and holes in semiconductor quantum wells
We investigate the ballistic motion of electrons and holes in III-V
semiconductor quantum wells with spin-orbit coupling and a homogeneous in-plane
electric field. As a result of a non-perturbative treatment of both of these
influences, particle wave packets undergo a pronounced side jump perpendicular
to the field direction. For wave packets of sufficient width the amplitude of
this motion can be estimated analytically and increases with decreasing field
strength. We discuss the scaling behavior of the effect and some if its
experimental implicationsComment: 4 pages, 3 figures include
Dielectric function of the semiconductor hole gas
We study the dielectric function of the homogeneous hole gas in p-doped
zinc-blende III-V bulk semiconductors within random phase approximation with
the valence band being modeled by Luttinger's Hamiltonian in the spherical
approximation. In the static limit we find a beating of Friedel oscillations
between the two Fermi momenta for heavy and light holes, while at large
frequencies dramatic corrections to the plasmon dispersion occur.Comment: 4 pages, 1 figure included. Version to appear in Europhys. Let
Spin- and entanglement-dynamics in the central spin model with homogeneous couplings
We calculate exactly the time-dependent reduced density matrix for the
central spin in the central-spin model with homogeneous Heisenberg couplings.
Therefrom, the dynamics and the entanglement entropy of the central spin are
obtained. A rich variety of behaviors is found, depending on the initial state
of the bath spins. For an initially unpolarized unentangled bath, the
polarization of the central spin decays to zero in the thermodynamic limit,
while its entanglement entropy becomes maximal. On the other hand, if the
unpolarized environment is initially in an eigenstate of the total bath spin,
the central spin and the entanglement entropy exhibit persistent monochromatic
large-amplitude oscillations. This raises the question to what extent
entanglement of the bath spins prevents decoherence of the central spin.Comment: 8 pages, 2 figures, typos corrected, published versio
Noncollinear Ferromagnetism in (III,Mn)V Semiconductors
We investigate the stability of the collinear ferromagnetic state in kinetic
exchange models for (III,Mn)V semiconductors with randomly distributed Mn ions
>. Our results suggest that {\em noncollinear ferromagnetism} is commom to
these semiconductor systems. The instability of the collinear state is due to
long-ranged fluctuations invloving a large fraction of the localized magnetic
moments. We address conditions that favor the occurrence of noncollinear
groundstates and discuss unusual behavior that we predict for the temperature
and field dependence of its saturation magnetization.Comment: 5 pages, one figure included, presentation of technical aspects
simplified, version to appear in Phys. Rev. Let
Spin-orbit interaction in symmetric wells with two subbands
We investigate the spin-orbit (s-o) interaction in two-dimensional electron
gases (2DEGs) in quantum wells with two subbands. From the Kane
model, we derive a new inter-subband-induced s-o term which resembles the
functional form of the Rashba s-o -- but is non-zero even in \emph{symmetric}
structures. This follows from the distinct parity of the confined states
(even/odd) which obliterates the need for asymmetric potentials. We
self-consistently calculate the new s-o coupling strength for realistic wells
and find it comparable to the usual Rashba constant. Our new s-o term gives
rise to a non-zero ballistic spin-Hall conductivity, which changes sign as a
function of the Fermi energy (), and can induce an unusual
\emph{zitterbewegung} with cycloidal trajectories \textit{without} magnetic
fields.Comment: v2: 4 two-column pages, 3 figures (added spin Hall conductivity and
self-consistent calculation
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