689 research outputs found
Unexpected systematic degeneracy in a system of two coupled Gaudin models with homogeneous couplings
We report an unexpected systematic degeneracy between different multiplets in
an inversion symmetric system of two coupled Gaudin models with homogeneous
couplings, as occurring for example in the context of solid state quantum
information processing. We construct the full degenerate subspace (being of
macroscopic dimension), which turns out to lie in the kernel of the commutator
between the two Gaudin models and the coupling term. Finally we investigate to
what extend the degeneracy is related to the inversion symmetry of the system
and find that indeed there is a large class of systems showing the same type of
degeneracy.Comment: 13 pages, 4 figure
Floquet spin states in graphene under ac driven spin-orbit interaction
We study the role of periodically driven time-dependent Rashba spin-orbit
coupling (RSOC) on a monolayer graphene sample. After recasting the originally
system of dynamical equations as two time-reversal related
two-level problems, the quasi-energy spectrum and the related dynamics are
investigated via various techniques and approximations. In the static case the
system is a gapped at the Dirac point. The rotating wave approximation (RWA)
applied to the driven system unphysically preserves this feature, while the
Magnus-Floquet approach as well as a numerically exact evaluation of the
Floquet equation show that this gap is dynamically closed. In addition, a
sizable oscillating pattern of the out-of-plane spin polarization is found in
the driven case for states which completely unpolarized in the static limit.
Evaluation of the autocorrelation function shows that the original uniform
interference pattern corresponding to time-independent RSOC gets distorted. The
resulting structure can be qualitatively explained as a consequence of the
transitions induced by the ac driving among the static eigenstates, i.e., these
transitions modulate the relative phases that add up to give the quantum
revivals of the autocorrelation function. Contrary to the static case, in the
driven scenario, quantum revivals (suppresions) are correlated to spin up
(down) phases.Comment: 10 pages, 8 figures. Typos corrected. Accepted for publication in PR
Variational study of the nu=1 quantum Hall ferromagnet in the presence of spin-orbit interaction
We investigate the nu=1 quantum Hall ferromagnet in the presence of
spin-orbit coupling of the Rashba or Dresselhaus type by means of
Hartree-Fock-typed variational states. In the presence of Rashba (Dresselhaus)
spin-orbit coupling the fully spin-polarized quantum Hall state is always
unstable resulting in a reduction of the spin polarization if the product of
the particle charge and the effective -factor is positive (negative). In
all other cases an alternative variational state with O(2) symmetry and finite
in-plane spin components is lower in energy than the fully spin-polarized state
for large enough spin-orbit interaction. The phase diagram resulting from these
considerations differs qualitatively from earlier studies.Comment: 9 pages, 3 figures included, version to appear in Phys. Rev.
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
Hyperfine induced spin and entanglement dynamics in Double Quantum Dots: A homogeneous coupling approach
We investigate hyperfine induced electron spin and entanglement dynamics in a
system of two quantum dot spin qubits. We focus on the situation of zero
external magnetic field and concentrate on approximation-free theoretical
methods. We give an exact solution of the model for homogeneous hyperfine
coupling constants (with all coupling coefficients being equal) and varying
exchange coupling, and we derive the dynamics therefrom. After describing and
explaining the basic dynamical properties, the decoherence time is calculated
from the results of a detailed investigation of the short time electron spin
dynamics. The result turns out to be in good agreement with experimental data.Comment: 10 pages, 8 figure
Fast domain wall propagation under an optimal field pulse in magnetic nanowires
We investigate field-driven domain wall (DW) propagation in magnetic
nanowires in the framework of the Landau-Lifshitz-Gilbert equation. We propose
a new strategy to speed up the DW motion in a uniaxial magnetic nanowire by
using an optimal space-dependent field pulse synchronized with the DW
propagation. Depending on the damping parameter, the DW velocity can be
increased by about two orders of magnitude compared the standard case of a
static uniform field. Moreover, under the optimal field pulse, the change in
total magnetic energy in the nanowire is proportional to the DW velocity,
implying that rapid energy release is essential for fast DW propagation.Comment: 4 pages, 3 figures; updated version replace
Perturbative regimes in central spin models
Central spin models describe several types of solid state nanostructures
which are presently considered as possible building blocks of future quantum
information processing hardware. From a theoretical point of view, a key issue
remains the treatment of the flip-flop terms in the Hamiltonian in the presence
of a magnetic field. We systematically study the influence of these terms, both
as a function of the field strength and the size of the spin baths. We find
crucial differences between initial states with central spin configurations of
high and such of low polarizations. This has strong implications with respect
to the influence of a magnetic field on the flip-flop terms in central spin
models of a single and more than one central spin. Furthermore, the
dependencies on bath size and field differ from those anticipated so far. Our
results might open the route for the systematic search for more efficient
perturbative treatments of central spin problems.Comment: 7 pages, 3 figure
Graphene with time-dependent spin-orbit coupling: Truncated Magnus expansion approach
We analyze the role of ac-driven Rashba spin-orbit coupling in monolayer
graphene including a spin-dependent mass term. Using the Magnus expansion as a
semi-analytical approximation scheme a full account of the quasienergie
spectrum of spin states is given. We discuss the subtleties arising in
correctly applying the Magnus expansion technique in order to determine the
quasienergy spectrum. Comparison to the exact numerical solution gives
appropriate boundaries to the validity of the Magnus expansion solution.Comment: 8 pages, 4 figure
Quantum Correlations in Two-Fermion Systems
We characterize and classify quantum correlations in two-fermion systems
having 2K single-particle states. For pure states we introduce the Slater
decomposition and rank (in analogy to Schmidt decomposition and rank), i.e. we
decompose the state into a combination of elementary Slater determinants formed
by mutually orthogonal single-particle states. Mixed states can be
characterized by their Slater number which is the minimal Slater rank required
to generate them. For K=2 we give a necessary and sufficient condition for a
state to have a Slater number of 1. We introduce a correlation measure for
mixed states which can be evaluated analytically for K=2. For higher K, we
provide a method of constructing and optimizing Slater number witnesses, i.e.
operators that detect Slater number for some states.Comment: 9 pages, some typos corrected and introduction modified, version to
be published in Phys. Rev.
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