1,060 research outputs found
Lattice vs. continuum theory of the periodic Heisenberg chain
We consider the detailed structure of low energy excitations in the periodic
spin-1/2 XXZ Heisenberg chain. By performing a perturbative calculation of the
non-linear corrections to the Gaussian model, we determine the exact
coefficients of asymptotic expansions in inverse powers of the system length N
for a large number of low-lying excited energy levels. This allows us to
calculate eigenenergies of the lattice model up to order order N^-4, without
having to solve the Bethe Ansatz equations. At the same time, it is possible to
express the exact eigenstates of the lattice model in terms of bosonic modes.Comment: 17 pages, 8 Figures. The latest version can be found at
http://www.physik.uni-kl.de/eggert/papers/index.htm
Wigner crystal vs. Friedel oscillations in the 1D Hubbard model
We analyze the fermion density of the one-dimensional Hubbard model using
bosonization and numerical DMRG calculations. For finite systems we find a
relatively sharp crossover even for moderate short range interactions into a
region with density waves as a function of density. The results show
that the unstable fixed point of a spin-incoherent state can dominate the
physical behavior in a large region of parameter space in finite systems. The
crossover may be observable in ultra cold fermionic gases in optical lattices
and in finite quantum wires.Comment: 6 pages, 6 figures. Published version. The most recent file can be
found at http://www.physik.uni-kl.de/eggert/papers/index.htm
Dynamics and decoherence in the central spin model using exact methods
The dynamics and decoherence of an electronic spin-1/2 qubit coupled to a
bath of nuclear spins via hyperfine interactions in a quantum dot is studied.
We show how exact results from the integrable solution can be used to
understand the dynamic behavior of the qubit. It is possible to predict the
main frequency contributions and their broadening for relatively general
initial states analytically, leading to an estimate of the corresponding decay
times. Furthermore, for a small bath polarization, a new low-frequency time
scale is observed.Comment: 4 pages, 2 figures. Published version. See also
http://www.physik.uni-kl.de/eggert/papers/index.htm
Dynamics and decoherence in the central spin model using exact methods
The dynamics and decoherence of an electronic spin-1/2 qubit coupled to a
bath of nuclear spins via hyperfine interactions in a quantum dot is studied.
We show how exact results from the integrable solution can be used to
understand the dynamic behavior of the qubit. It is possible to predict the
main frequency contributions and their broadening for relatively general
initial states analytically, leading to an estimate of the corresponding decay
times. Furthermore, for a small bath polarization, a new low-frequency time
scale is observed.Comment: 4 pages, 2 figures. Published version. See also
http://www.physik.uni-kl.de/eggert/papers/index.htm
Local Density of States for Individual Energy Levels in Finite Quantum Wires
The local density of states (LDOS) in finite quantum wires is calculated as a
function of discrete energies and position along the wire. By using a
combination of numerical density matrix renormalization group (DMRG)
calculations and analytical bosonization techniques it is possible to obtain a
good understanding of the local spectral weights along the wire in terms of the
underlying many-body excitations.Comment: 5 pages, 4 figures. v2 with additional data. The most recent version
can be found at http://www.physik.uni-kl.de/eggert/papers
Physicians' knowledge and continuing medical education regarding fitness to drive: a questionnaire-based survey in Southeast Switzerland
Valid information for physicians in Switzerland concerning knowledge and continuing education in traffic medicine is not available. Also, their attitude to the legally prescribed periodic driving fitness examinations is unclear. In order to gain more information about these topics, 635 resident physicians in Southeast Switzerland were sent a questionnaire (response rate 52%). In a self-estimation, 79% of the queried physicians claimed to know the minimal medical requirements for drivers which are important in their specialty. Statistically significant differences existed between the specialties, whereby general practitioners most frequently claimed to know the minimal medical requirements (90%). It appears that the minimal medical requirements for drivers are well known to the queried physicians. Fifty-two percent of the physicians favored an expansion of continuing education in traffic medicine. Such an expansion was desired to a lesser extent by physicians without knowledge of the minimal requirements (p < 0.001). A clear majority of the medical professionals adjudged the legally prescribed periodic driving fitness examinations as being an expedient means to identify unfit drivers. A national standardized form for reporting potentially unfit drivers to the licensing authorities was supported by 68% of the responding physicians. Such a form could simplify and standardize the reports to the licensing authoritie
Discretionary medical reporting of potentially unfit drivers: a questionnaire-based survey in Southeast Switzerland
In Switzerland, every physician has the right to report a patient that is potentially unfit to drive to the licensing authority without violating medical confidentiality. Verified information regarding physicians' attitudes concerning this discretionary reporting and the frequency of such reports are not available. In order to answer these questions, 635 resident physicians were sent a questionnaire. The response rate was 52%. On average, the responding physicians—for all specialties—reported 0.31 patients (SD 0.64, 95% CI 0.24-0.38) in the year before the survey and 1.00 patient (SD 1.74, 95% CI 0.81-1.20) in the past 5years. Seventy-nine percent of the responding physicians indicated knowing the current legal requirements for driving in Switzerland. In applied logistic regression analysis, only two factors correlate significantly with reporting: male sex (odds ratio 5.4) and the specialty "general medicine” (odds ratio 3.4). Ninety-seven percent of the physicians were against abolishing medical discretionary reporting and 29% were in favor of introducing mandatory reporting. The great majority of the questioned physicians supported the discretionary reporting of drivers that are potentially unfit to drive as currently practiced in Switzerland. The importance and the necessity of a regular traffic medicine-related continuing education for medical professionals are shown by the low number of reports per physicia
Attractively bound pairs of atoms in the Bose-Hubbard model and antiferromagnetism
We consider a periodic lattice loaded with pairs of bosonic atoms tightly
bound to each other via strong attractive on-site interaction that exceeds the
inter-site tunneling rate. An ensemble of such lattice-dimers is accurately
described by an effective Hamiltonian of hard core bosons with strong
nearest-neighbor repulsion which is equivalent to the model with
Ising-like anisotropy. We calculate the ground-state phase diagram for a
one-dimensional system which exhibits incompressible phases, corresponding to
an empty and a fully filled lattice (ferromagnetic phases) and a half-filled
alternating density crystal (anti-ferromagnetic phase), separated from each
other by compressible phases. In a finite lattice the compressible phases show
characteristic oscillatory modulations on top of the anti-ferromagnetic density
profile and in density-density correlations. We derive a kink model which
provides simple quantitative explanation of these features. To describe the
long-range correlations of the system we employ the Luttinger liquid theory
with the relevant Luttinger parameter obtained exactly using the Bethe
Ansatz solution. We calculate the density-density as well as first-order
correlations and find excellent agreement with numerical results obtained with
density matrix renormalization group (DMRG) methods. We also present a
perturbative treatment of the system in higher dimensions.Comment: 10 pages, 9 figure
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