1,755 research outputs found
A Four-Unit-Cell Periodic Pattern of Quasiparticle States Surrounding Vortex Cores in Bi2Sr2CaCu2O8+d
Scanning tunneling microscopy is used to image the additional quasiparticle
states generated by quantized vortices in the high-Tc superconductor
Bi2Sr2CaCu2O8+d. They exhibit a Cu-O bond oriented 'checkerboard' pattern, with
four unit cell (4a0) periodicity and a ~30 angstrom decay length. These
electronic modulations may be related to the magnetic field-induced, 8a0
periodic, spin density modulations of decay length ~70 angstroms recently
discovered in La1.84Sr0.16CuO4. The proposed explanation is a spin density wave
localized surrounding each vortex core. General theoretical principles predict
that, in the cuprates, a localized spin modulation of wavelength L should be
associated with a corresponding electronic modulation of wavelength L/2, in
good agreement with our observations.Comment: 10 pages, 3 figure
Dirac Quantization of Parametrized Field Theory
Parametrized field theory (PFT) is free field theory on flat spacetime in a
diffeomorphism invariant disguise. It describes field evolution on arbitrary
foliations of the flat spacetime instead of only the usual flat ones, by
treating the `embedding variables' which describe the foliation as dynamical
variables to be varied in the action in addition to the scalar field. A formal
Dirac quantization turns the constraints of PFT into functional Schrodinger
equations which describe evolution of quantum states from an arbitrary Cauchy
slice to an infinitesimally nearby one.This formal Schrodinger picture- based
quantization is unitarily equivalent to the standard Heisenberg picture based
Fock quantization of the free scalar field if scalar field evolution along
arbitrary foliations is unitarily implemented on the Fock space. Torre and
Varadarajan (TV) showed that for generic foliations emanating from a flat
initial slice in spacetimes of dimension greater than 2, evolution is not
unitarily implemented, thus implying an obstruction to Dirac quantization.
We construct a Dirac quantization of PFT,unitarily equivalent to the standard
Fock quantization, using techniques from Loop Quantum Gravity (LQG) which are
powerful enough to super-cede the no- go implications of the TV results. The
key features of our quantization include an LQG type representation for the
embedding variables, embedding dependent Fock spaces for the scalar field, an
anomaly free representation of (a generalization of) the finite transformations
generated by the constraints and group averaging techniques. The difference
between 2 and higher dimensions is that in the latter, only finite gauge
transformations are defined in the quantum theory, not the infinitesimal ones.Comment: 33 page
Functional evolution of quantum cylindrical waves
Kucha{\v{r}} showed that the quantum dynamics of (1 polarization) cylindrical
wave solutions to vacuum general relativity is determined by that of a free
axially-symmetric scalar field along arbitrary axially-symmetric foliations of
a fixed flat 2+1 dimensional spacetime. We investigate if such a dynamics can
be defined {\em unitarily} within the standard Fock space quantization of the
scalar field.
Evolution between two arbitrary slices of an arbitrary foliation of the flat
spacetime can be built out of a restricted class of evolutions (and their
inverses). The restricted evolution is from an initial flat slice to an
arbitrary (in general, curved) slice of the flat spacetime and can be
decomposed into (i) `time' evolution in which the spatial Minkowskian
coordinates serve as spatial coordinates on the initial and the final slice,
followed by (ii) the action of a spatial diffeomorphism of the final slice on
the data obtained from (i). We show that although the functional evolution of
(i) is unitarily implemented in the quantum theory, generic spatial
diffeomorphisms of (ii) are not. Our results imply that a Tomanaga-Schwinger
type functional evolution of quantum cylindrical waves is not a viable concept
even though, remarkably, the more limited notion of functional evolution in
Kucha{\v{r}}'s `half parametrized formalism' is well-defined.Comment: Replaced with published versio
Unique gap structure and symmetry of the charge density wave in single-layer VSe
Single layers of transition metal dichalcogenides (TMDCs) are excellent
candidates for electronic applications beyond the graphene platform; many of
them exhibit novel properties including charge density waves (CDWs) and
magnetic ordering. CDWs in these single layers are generally a planar
projection of the corresponding bulk CDWs because of the quasi-two-dimensional
nature of TMDCs; a different CDW symmetry is unexpected. We report herein the
successful creation of pristine single-layer VSe, which shows a () CDW in contrast to the (4 4) CDW for the layers in
bulk VSe. Angle-resolved photoemission spectroscopy (ARPES) from the single
layer shows a sizable () CDW gap of 100 meV at the
zone boundary, a 220 K CDW transition temperature twice the bulk value, and no
ferromagnetic exchange splitting as predicted by theory. This robust CDW with
an exotic broken symmetry as the ground state is explained via a
first-principles analysis. The results illustrate a unique CDW phenomenon in
the two-dimensional limit
STM conductance of Kondo impurities on open and structured surfaces
We study the scanning tunneling microscopy response for magnetic atoms on
open and structured surfaces using Wilson's renormalization group. We observe
Fano resonances associated with Kondo resonances and interference effects. For
a magnetic atom in a quantum corral coupled to the confined surface states, and
experimentally relevant parameters, we observe a large confinement induced
effect not present in the experiments. These results suggest that the Kondo
screening is dominated by the bulk electrons rather than the surface ones.Comment: 6 pages, 6 figure
Quantum gravity effects in the CGHS model of collapse to a black hole
We show that only a sector of the classical solution space of the CGHS model
describes formation of black holes through collapse of matter. This sector has
either right or left moving matter. We describe the sector which has left
moving matter in canonical language. In the nonperturbative quantum theory all
operators are expressed in terms of the matter field operator which is
represented on a Fock space. We discuss existence of large quantum fluctuations
of the metric operator when the matter field is approximately classical. We end
with some comments which may pertain to Hawking radiation in the context of the
model.Comment: Latex, 26 pages, 1 figure, to appear in Phy. Rev. D (15
Coupled-barrier diffusion: the case of oxygen in silicon
Oxygen migration in silicon corresponds to an apparently simple jump between
neighboring bridge sites. Yet, extensive theoretical calculations have so far
produced conflicting results and have failed to provide a satisfactory account
of the observed eV activation energy. We report a comprehensive set of
first-principles calculations that demonstrate that the seemingly simple oxygen
jump is actually a complex process involving coupled barriers and can be
properly described quantitatively in terms of an energy hypersurface with a
``saddle ridge'' and an activation energy of eV. Earlier
calculations correspond to different points or lines on this hypersurface.Comment: 4 Figures available upon request. Accepted for publication in Phys.
Rev. Let
Surgical management and longterm follow-up of non-parasitic hepatic cysts
BACKGROUND: Despite the increasing use of laparoscopic techniques, the optimal surgical approach for cystic liver disease has not been well defined. This study aims to determine the optimum operative approach for these patients. METHODS: Data were identified from the Lothian Surgical Audit, case note review and general practitioner contact. Patients were contacted and asked to complete the SF-36 questionnaire on quality of life. RESULTS: A total of 102 patients (67 with simple cysts, 31 with polycystic liver disease [PCLD], four with cystic tumours) underwent 62 laparoscopic deroofings, 15 open deroofings, 36 resections and one liver transplant between June 1985 and April 2006. The median follow-up was 77 months (range 3–250 months). Morbidity and recurrent symptom rates after laparoscopic surgery were greater in PCLD patients compared with simple cyst patients, at 31% (four patients) vs. 15% (seven patients) and 85% (11 patients) vs. 29% (24 patients), respectively. Four patients with simple cysts and eight with PCLD required further surgery. All patients with simple cysts had comparable quality of life after surgery. Patients with recurrent symptoms after surgery for PCLD had a significantly better quality of life following laparoscopic deroofing than after resection. CONCLUSIONS: Most simple cysts can be managed laparoscopically, but there is a definite role for open resection in some patients. Open deroofing is the preferred approach for a dominant cyst pattern in PCLD, whereas resection is necessary for diffuse cystic disease
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