867 research outputs found
Simultaneous brachial diplegia and rotational vertigo due to combined spinal anterior and vertebrobasilar embolism
Approximation by point potentials in a magnetic field
We discuss magnetic Schrodinger operators perturbed by measures from the
generalized Kato class. Using an explicit Krein-like formula for their
resolvent, we prove that these operators can be approximated in the strong
resolvent sense by magnetic Schrodinger operators with point potentials. Since
the spectral problem of the latter operators is solvable, one in fact gets an
alternative way to calculate discrete spectra; we illustrate it by numerical
calculations in the case when the potential is supported by a circle.Comment: 16 pages, 2 eps figures, submitted to J. Phys.
Lower bounds for nodal sets of eigenfunctions
We prove lower bounds for the Hausdorff measure of nodal sets of
eigenfunctions.Comment: To appear in Communications in Mathematical Physics; revised to
include two additional references and update bibliographic informatio
Lower bounds for nodal sets of Dirichlet and Neumann eigenfunctions
Let \phi\ be a Dirichlet or Neumann eigenfunction of the Laplace-Beltrami
operator on a compact Riemannian manifold with boundary. We prove lower bounds
for the size of the nodal set {\phi=0}.Comment: 7 page
Spin Gap in the Single Spin-1/2 Chain Cuprate SrCaCuO
We report Cu nuclear magnetic resonance and muon spin rotation
measurements on the S=1/2 antiferromagnetic Heisenberg spin chain compound
SrCaCuO. An exponentially decreasing spin-lattice
relaxation rate 1/T indicates the opening of a spin gap. This behavior is
very similar to what has been observed for the cognate zigzag spin chain
compound SrCaCuO, and confirms that the occurrence of a
spin gap upon Ca doping is independent of the interchain exchange coupling
. Our results therefore generally prove the appearance of a spin gap in an
antiferromagnetic Heisenberg spin chain induced by a local bond disorder of the
intrachain exchange coupling . A low temperature upturn of 1/T evidences
growing magnetic correlations. However, zero field muon spin rotation
measurements down to 1.5 K confirm the absence of magnetic order in this
compound which is most likely suppressed by the opening of the spin gap.Comment: 5 pages, 4 figure
Heat kernels on curved cones
A functorial derivation is presented of a heat-kernel expansion coefficient
on a manifold with a singular fixed point set of codimension two. The existence
of an extrinsic curvature term is pointed out.Comment: 4p.,sign errors corrected and a small addition,uses JyTeX,MUTP/94/0
Magnetoelectric coupling in a frustrated spinel studied using high-field scanning probe microscopy
Below its N\'eel temperature, the frustrated magnet CdCrO exhibits an
antiferromagnetic spin-spiral ground state. Such states can give rise to a
sizable magnetoelectric coupling. In this report, we measure the electric
polarization induced in single-crystalline CdCrO by a large applied
magnetic field. Because the detection of a macroscopic polarization is hindered
by the structural domains in the tetragonal spin-spiral phase, we have
pioneered an alternative method of measuring polarization induced by high
magnetic fields, using electrostatic force microscopy. This method enables us
to measure polarization from nanometer sized areas of the sample surface, as
well as imaging how to charge inhomogeneities change with magnetic field.Comment: 5 pages, 3 figure
CeRuPO: A rare example of a Ferromagnetic Kondo lattice
We have determined the physical ground state properties of the compounds
CeRuPO and CeOsPO by means of magnetic susceptibility chi(T), specific heat
C(T), electrical resistivity rho(T), and thermopower S(T) measurements. chi(T)
reveals a trivalent 4f1 cerium state in both compounds. For CeRuPO a pronounced
decrease of rho(T) below 50K indicates the onset of coherent Kondo scattering
which is confirmed by enhanced S(T). The temperature and magnetic field
dependence of chi(T) and C(T) evidence ferromagnetic (FM) order at TC=15K.
Thus, CeRuPO seems to be one of the rare example of a FM Kondo lattice. In
contrast, CeOsPO shows antiferromagnetic order at TN=4.4K despite only minor
changes in lattice parameters and electronic configuration. Additional 31P NMR
results support these scenarios. LSDA+U calculations evidence a quasi two
dimensional electronic band structure, reflecting a strong covalent bonding
within the CeO and RuP layers and a weak ionic like bonding between the layers.Comment: accepted in Phys. Rev. B, high quality figures:
http://www.cpfs.mpg.de/~krellner
Parametrizations of density matrices
This article gives a brief overview of some recent progress in the
characterization and parametrization of density matrices of finite dimensional
systems. We discuss in some detail the Bloch-vector and Jarlskog
parametrizations and mention briefly the coset parametrization. As applications
of the Bloch parametrization we discuss the trace invariants for the case of
time dependent Hamiltonians and in some detail the dynamics of three-level
systems. Furthermore, the Bloch vector of two-qubit systems as well as the use
of the polarization operator basis is indicated. As the main application of the
Jarlskog parametrization we construct density matrices for composite systems.
In addition, some recent related articles are mentioned without further
discussion.Comment: 31 pages. v2: 32 pages, Abstract and Introduction rewritten and
Conclusion section added, references adde
Common effect of chemical and external pressures on the magnetic properties of RECoPO (RE = La, Pr)
We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO
materials performed by means of muon spin spectroscopy. Zero-field measurements
show that the electrons localized on the Pr ions do not play any role in
the static magnetic properties of the compounds. Magnetism at the local level
is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co
sublattice only. The increase of the chemical pressure triggered by the
different ionic radii of La and Pr, on the other hand, plays a
crucial role in enhancing the value of the magnetic critical temperature and
can be mimicked by the application of external hydrostatic pressure up to 24
kbar. A sharp discontinuity in the local magnetic field at the muon site in
LaCoPO at around 5 kbar suggests a sizeable modification in the band structure
of the material upon increasing pressure. This scenario is qualitatively
supported by \emph{ab-initio} density-functional theory calculations.Comment: 13 pages, 10 figure
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