1,947 research outputs found
Observing the Berry phase in diffusive conductors: Necessary conditions for adiabaticity
In a recent preprint (cond-mat/9803170), van~Langen, Knops, Paasschens and
Beenakker attempt to re-analyze the proposal of Loss, Schoeller and Goldbart
(LSG) [Phys. Rev. B~48, 15218 (1993)] concerning Berry phase effects in the
magnetoconductance of diffusive systems. Van Langen et al. claim that the
adiabatic approximation for the Cooperon previously derived by LSG is not valid
in the adiabatic regime identified by LSG. It is shown that the claim of
van~Langen et al. is not correct, and that, on the contrary, the
magnetoconductance does exhibit the Berry phase effect within the LSG regime of
adiabaticity. The conclusion reached by van~Langen et al. is based on a
misinterpretation of field-induced dephasing effects, which can mask the Berry
phase (and any other phase coherent phenomena) for certain parameter values.Comment: 25 pages, 9 figure
An analytical proof of Hardy-like inequalities related to the Dirac operator
We prove some sharp Hardy type inequalities related to the Dirac operator by
elementary, direct methods. Some of these inequalities have been obtained
previously using spectral information about the Dirac-Coulomb operator. Our
results are stated under optimal conditions on the asymptotics of the
potentials near zero and near infinity.Comment: LaTex, 22 page
Self-adjointness of Dirac operators via Hardy-Dirac inequalities
Distinguished selfadjoint extensions of Dirac operators are constructed for a
class of potentials including Coulombic ones up to the critical case,
. The method uses Hardy-Dirac inequalities and quadratic form
techniques.Comment: PACS 03.65.P, 03.3
Uniform Density Theorem for the Hubbard Model
A general class of hopping models on a finite bipartite lattice is
considered, including the Hubbard model and the Falicov-Kimball model. For the
half-filled band, the single-particle density matrix \uprho (x,y) in the
ground state and in the canonical and grand canonical ensembles is shown to be
constant on the diagonal , and to vanish if and if and
are on the same sublattice. For free electron hopping models, it is shown in
addition that there are no correlations between sites of the same sublattice in
any higher order density matrix. Physical implications are discussed.Comment: 15 pages, plaintex, EHLMLRJM-22/Feb/9
Hybridization and spin decoherence in heavy-hole quantum dots
We theoretically investigate the spin dynamics of a heavy hole confined to an
unstrained III-V semiconductor quantum dot and interacting with a narrowed
nuclear-spin bath. We show that band hybridization leads to an exponential
decay of hole-spin superpositions due to hyperfine-mediated nuclear pair flips,
and that the accordant single-hole-spin decoherence time T2 can be tuned over
many orders of magnitude by changing external parameters. In particular, we
show that, under experimentally accessible conditions, it is possible to
suppress hyperfine-mediated nuclear-pair-flip processes so strongly that
hole-spin quantum dots may be operated beyond the `ultimate limitation' set by
the hyperfine interaction which is present in other spin-qubit candidate
systems.Comment: 7 pages, 3 figure
Thermodynamic stability criteria for a quantum memory based on stabilizer and subsystem codes
We discuss and review several thermodynamic criteria that have been
introduced to characterize the thermal stability of a self-correcting quantum
memory. We first examine the use of symmetry-breaking fields in analyzing the
properties of self-correcting quantum memories in the thermodynamic limit: we
show that the thermal expectation values of all logical operators vanish for
any stabilizer and any subsystem code in any spatial dimension. On the positive
side, we generalize the results in [R. Alicki et al., arXiv:0811.0033] to
obtain a general upper bound on the relaxation rate of a quantum memory at
nonzero temperature, assuming that the quantum memory interacts via a Markovian
master equation with a thermal bath. This upper bound is applicable to quantum
memories based on either stabilizer or subsystem codes.Comment: 23 pages. v2: revised introduction, various additional comments, and
a new section on gapped hamiltonian
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