575 research outputs found
Cotunneling signatures of Spin-Electric coupling in frustrated triangular molecular magnets
The ground state of frustrated (antiferromagnetic) triangular molecular
magnets is characterized by two total-spin doublets with opposite
chirality. According to a group theory analysis [M. Trif \textit{et al.}, Phys.
Rev. Lett. \textbf{101}, 217201 (2008)] an external electric field can
efficiently couple these two chiral spin states, even when the spin-orbit
interaction (SOI) is absent. The strength of this coupling, , is determined
by an off-diagonal matrix element of the dipole operator, which can be
calculated by \textit{ab-initio} methods [M. F. Islam \textit{et al.}, Phys.
Rev. B \textbf{82}, 155446 (2010)]. In this work we propose that
Coulomb-blockade transport experiments in the cotunneling regime can provide a
direct way to determine the spin-electric coupling strength. Indeed, an
electric field generates a -dependent splitting of the ground state
manifold, which can be detected in the inelastic cotunneling conductance. Our
theoretical analysis is supported by master-equation calculations of quantum
transport in the cotunneling regime. We employ a Hubbard-model approach to
elucidate the relationship between the Hubbard parameters and , and the
spin-electric coupling constant . This allows us to predict the regime in
which the coupling constant can be extracted from experiment
Gammaâray spectroscopy with singleâcarrier collection in highâresistivity semiconductors
With the standard planeâparallel configuration of semiconductor detectors, good Îłâray spectra can only be obtained when both electrons and holes are completely collected. We show by calculations (and experiments) that with contacts of hemispherical configuration one can obtain Îłâray spectra of adequate resolution and with signal heights of nearly full amplitude even when only one type of carrier is collected. Experiments with CdTe detectors for which the Â”Ï product for electrons is about 10^(3) times that of the holes confirm these calculations. The adoption of hemispherical contacts thus widens the range of highâresistivity semiconductors potentially acceptable for Îłâray detection at room temperature
Magnetic Anisotropy of Isolated Cobalt Nanoplatelets
Motivated in part by experiments performed by M.H. Pan et al. (nanoletters,
v.5, p 83, 2005), we have undertaken a theoretical study of the the magnetic
properties of two-monolayer thick Co nanoplatelets with an equilateral
triangular shape. The analysis is carried out using a microscopic Slater-Koster
tight-binding model with atomic exchange and spin-orbit interactions designed
to realistically capture the salient magnetic features of large nanoclusters
containing up to 350 atoms. Two different truncations of the FCC lattice are
studied, in which the nanoplatelet surface is aligned parallel to the FCC (111)
and (001)crystal planes respectively. We find that the higher coordination
number in the (111) truncated crystal is more likely to reproduce the
perpendicular easy direction found in experiment. Qualitatively, the most
important parameter governing the anisotropy of the model is found to be the
value of the intra-atomic exchange integral J. If we set the value of J near
the bulk value in order to reproduce the experimentally observed magnitude of
the magnetic moments, we find both quasi-easy-planes and perpendicular easy
directions. At larger values of J we find that the easy-axis of magnetization
is perpendicular to the surface, and the value of the magnetic anisotropy
energy per atom is larger. The possible role of hybridization with substrate
surface states in the experimental systems is discussed.Comment: 15 pages, 13 figure
Spectral properties of a generalized chGUE
We consider a generalized chiral Gaussian Unitary Ensemble (chGUE) based on a
weak confining potential. We study the spectral correlations close to the
origin in the thermodynamic limit. We show that for eigenvalues separated up to
the mean level spacing the spectral correlations coincide with those of chGUE.
Beyond this point, the spectrum is described by an oscillating number variance
centered around a constant value. We argue that the origin of such a rigid
spectrum is due to the breakdown of the translational invariance of the
spectral kernel in the bulk of the spectrum. Finally, we compare our results
with the ones obtained from a critical chGUE recently reported in the
literature. We conclude that our generalized chGUE does not belong to the same
class of universality as the above mentioned model.Comment: 12 pages, 3 figure
Antimony doping of Si layers grown by solid-phase epitaxy
We report here that layers of Si formed by solid-phase epitaxial growth (SPEG) can be doped intentionally. The sample consists initially of an upper layer of amorphous Si (~1 ”m thick), a very thin intermediate layer of Sb (nominally 5 Ă
), and a thin lower layer of Pd (~500 Ă
), all electron-gun deposited on top of a single-crystal substrate (1â10 Ω cm, p type, orientation). After a heating cycle which induces epitaxial growth, electrically active Sb atoms are incorporated into the SPEG layer, as shown by the following facts: (a) the SPEG layer forms a p-n junction against the p-type substrate, (b) the Hall effect indicates strong n-type conduction of the layer, and (c) Auger electron spectra reveal the presence of Sb in the layer
A new application of emulsions to measure the gravitational force on antihydrogen
We propose to build and operate a detector based on the emulsion film
technology for the measurement of the gravitational acceleration on antimatter,
to be performed by the AEgIS experiment (AD6) at CERN. The goal of AEgIS is to
test the weak equivalence principle with a precision of 1% on the gravitational
acceleration g by measuring the vertical position of the anni- hilation vertex
of antihydrogen atoms after their free fall in a horizontal vacuum pipe. With
the emulsion technology developed at the University of Bern we propose to
improve the performance of AEgIS by exploiting the superior position resolution
of emulsion films over other particle de- tectors. The idea is to use a new
type of emulsion films, especially developed for applications in vacuum, to
yield a spatial resolution of the order of one micron in the measurement of the
sag of the antihydrogen atoms in the gravitational field. This is an order of
magnitude better than what was planned in the original AEgIS proposal.Comment: 17 pages, 14 figure
New Class of Random Matrix Ensembles with Multifractal Eigenvectors
Three recently suggested random matrix ensembles (RME) are linked together by
an exact mapping and plausible conjections. Since it is known that in one of
these ensembles the eigenvector statistics is multifractal, we argue that all
three ensembles belong to a new class of critical RME with multifractal
eigenfunction statistics and a universal critical spectral statitics. The
generic form of the two-level correlation function for weak and extremely
strong multifractality is suggested. Applications to the spectral statistics at
the Anderson transition and for certain systems on the border of chaos and
integrability is discussed.Comment: 4 pages RevTeX, resubmitte
Coulomb correlations and coherent charge tunneling in mesoscopic coupled rings
We study the effect of a strong electron-electron (e-e) interaction in a
system of two concentric one-dimensional rings with incommensurate areas A_1
and A_2, coupled by a tunnel amplitude. For noninteracting particles the
magnetic moment (persistent current) m of the many-body ground state and first
excited states is an irregular function of the external magnetic field. In
contrast, we show that when strong e-e interactions are present the magnetic
field dependence of m becomes periodic. In such a strongly correlated system
disorder can only be caused by inter-ring charge fluctuations, controllable by
a gate voltage. The oscillation period of m is proportional to 1/(A_1 + A_2) if
fluctuations are suppressed. Coherent inter-ring tunneling doubles the period
when charge fluctuations are allowed.Comment: 4 pages, 4 eps figure
Distribution of level curvatures for the Anderson model at the localization-delocalization transition
We compute the distribution function of single-level curvatures, , for
a tight binding model with site disorder, on a cubic lattice. In metals
is very close to the predictions of the random-matrix theory (RMT). In
insulators has a logarithmically-normal form. At the Anderson
localization-delocalization transition fits very well the proposed novel
distribution with , which
approaches the RMT result for large and is non-analytical at small . We
ascribe such a non-analiticity to the spatial multifractality of the critical
wave functions.Comment: 4 ReVTeX pages and 4(.epsi)figures included in one uuencoded packag
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