13,635 research outputs found
An ultra-compact low temperature scanning probe microscope for magnetic fields above 30 T
We present the design of a highly compact High Field Scanning Probe
Microscope (HF-SPM) for operation at cryogenic temperatures in an extremely
high magnetic field, provided by a water-cooled Bitter magnet able to reach 38
T. The HF-SPM is 14 mm in diameter: an Attocube nano-positioner controls the
coarse approach of a piezo resistive AFM cantilever to a scanned sample. The
Bitter magnet constitutes an extreme environment for SPM due to the high level
of vibrational noise; the Bitter magnet noise at frequencies up to 300 kHz is
characterized and noise mitigation methods are described. The performance of
the HF-SPM is demonstrated by topographic imaging and noise measurements at up
to 30 T. Additionally, the use of the SPM as a three-dimensional dilatometer
for magnetostriction measurements is demonstrated via measurements on a
magnetically frustrated spinel sample.Comment: 6 pages, 5 figure
Exploring the phase diagram of the two-impurity Kondo problem
A system of two exchange-coupled Kondo impurities in a magnetic field gives
rise to a rich phase space hosting a multitude of correlated phenomena.
Magnetic atoms on surfaces probed through scanning tunnelling microscopy
provide an excellent platform to investigate coupled impurities, but typical
high Kondo temperatures prevent field-dependent studies from being performed,
rendering large parts of the phase space inaccessible. We present an integral
study of pairs of Co atoms on insulating Cu2N/Cu(100), which each have a Kondo
temperature of only 2.6 K. In order to cover the different regions of the phase
space, the pairs are designed to have interaction strengths similar to the
Kondo temperature. By applying a sufficiently strong magnetic field, we are
able to access a new phase in which the two coupled impurities are
simultaneously screened. Comparison of differential conductance spectra taken
on the atoms to simulated curves, calculated using a third order transport
model, allows us to independently determine the degree of Kondo screening in
each phase.Comment: paper: 14 pages, 4 figures; supplementary: 3 pages, 1 figure, 1 tabl
Controlled complete suppression of single-atom inelastic spin and orbital cotunnelling
The inelastic portion of the tunnel current through an individual magnetic
atom grants unique access to read out and change the atom's spin state, but it
also provides a path for spontaneous relaxation and decoherence. Controlled
closure of the inelastic channel would allow for the latter to be switched off
at will, paving the way to coherent spin manipulation in single atoms. Here we
demonstrate complete closure of the inelastic channels for both spin and
orbital transitions due to a controlled geometric modification of the atom's
environment, using scanning tunnelling microscopy (STM). The observed
suppression of the excitation signal, which occurs for Co atoms assembled into
chain on a CuN substrate, indicates a structural transition affecting the
d orbital, effectively cutting off the STM tip from the spin-flip
cotunnelling path.Comment: 4 figures plus 4 supplementary figure
Distinguishing Hope and Optimism: Two Sides of a Coin, or Two Separate Coins?
Structural equation modeling was used to test hypotheses about (a) the dimensionality of measures of dispositional hope (the Adult Hope Scale, AHS) and dispositional optimism (the Life Orientation Test, LOT), (b) the extent and source of conceptual overlap and divergence between hope and optimism, and (c) patterns of discriminant validity for each trait. Separate two-factor models best fit the hope (Agency and Pathways, r = .68) and optimism (Optimism and Pessimism, r = -.63) data. Analyzing the combined AHS and LOT data, a measurement model with separate, correlated second-order factors of Hope and Optimism ( r = .80) provided a better fit than did a higher-order model with a single second-order factor. Optimism correlated equally with both Agency and Pathways, whereas Pessimism was more strongly correlated with Agency than with Pathways. Confirming hypotheses, second-order Optimism had a stronger influence on the use of positive reappraisal as a coping strategy than did second-order Hope, whereas second-order Hope had a stronger influence on level of general self-efficacy than did second-order Optimism. We suggest that hope focuses more directly on the personal attainment of specific goals, whereas optimism focuses more broadly on the expected quality of future outcomes in general
Parallelizing Synthesis from Temporal Logic Specifications by Identifying Equicontrollable States
For the synthesis of correct-by-construction control policies from temporal logic specifications the scalability of the synthesis algorithms is often a bottleneck. In this paper, we parallelize synthesis from specifications in the GR(1) fragment of linear temporal logic by introducing a hierarchical procedure that allows decoupling of the fixpoint computations. The state space is partitioned into equicontrollable sets using solutions to parametrized games that arise from decomposing the original GR(1) game into smaller reachability-persistence games. Following the partitioning, another synthesis problem is formulated for composing the strategies from the decomposed reachability games. The formulation guarantees that composing the synthesized controllers ensures satisfaction of the given GR(1) property. Experiments with robot planning problems demonstrate good performance of the approach
Calibrated Sub-Bundles in Non-Compact Manifolds of Special Holonomy
This paper is a continuation of math.DG/0408005. We first construct special
Lagrangian submanifolds of the Ricci-flat Stenzel metric (of holonomy SU(n)) on
the cotangent bundle of S^n by looking at the conormal bundle of appropriate
submanifolds of S^n. We find that the condition for the conormal bundle to be
special Lagrangian is the same as that discovered by Harvey-Lawson for
submanifolds in R^n in their pioneering paper. We also construct calibrated
submanifolds in complete metrics with special holonomy G_2 and Spin(7)
discovered by Bryant and Salamon on the total spaces of appropriate bundles
over self-dual Einstein four manifolds. The submanifolds are constructed as
certain subbundles over immersed surfaces. We show that this construction
requires the surface to be minimal in the associative and Cayley cases, and to
be (properly oriented) real isotropic in the coassociative case. We also make
some remarks about using these constructions as a possible local model for the
intersection of compact calibrated submanifolds in a compact manifold with
special holonomy.Comment: 20 pages; for Revised Version: Minor cosmetic changes, some
paragraphs rewritten for improved clarit
Multiband theory of quantum-dot quantum wells: Dark excitons, bright excitons, and charge separation in heteronanostructures
Electron, hole, and exciton states of multishell CdS/HgS/CdS quantum-dot
quantum well nanocrystals are determined by use of a multiband theory that
includes valence-band mixing, modeled with a 6-band Luttinger-Kohn Hamiltonian,
and nonparabolicity of the conduction band. The multiband theory correctly
describes the recently observed dark-exciton ground state and the lowest,
optically active, bright-exciton states. Charge separation in pair states is
identified. Previous single-band theories could not describe these states or
account for charge separation.Comment: 10 pages of ReVTex, 6 ps figures, submitted to Phys. Rev.
Two universal results for Wilson loops at strong coupling
We present results for Wilson loops in strongly coupled gauge theories. The
loops may be taken around an arbitrarily shaped contour and in any field theory
with a dual IIB geometry of the form M x S^5. No assumptions about
supersymmetry are made. The first result uses D5 branes to show how the loop in
any antisymmetric representation is computed in terms of the loop in the
fundamental representation. The second result uses D3 branes to observe that
each loop defines a rich sequence of operators associated with minimal surfaces
in S^5. The action of these configurations are all computable. Both results
have features suggesting a connection with integrability.Comment: 1+12 pages. LaTeX. No figure
Negative thermal expansion in the plateau state of a magnetically-frustrated spinel
We report on negative thermal expansion (NTE) in the high-field,
half-magnetization plateau phase of the frustrated magnetic insulator CdCr2O4.
Using dilatometry, we precisely map the phase diagram at fields of up to 30T,
and identify a strong NTE associated with the collinear half-magnetization
plateau for B > 27T. The resulting phase diagram is compared with a microscopic
theory for spin-lattice coupling, and the origin of the NTE is identified as a
large negative change in magnetization with temperature, coming from a
nearly-localised band of spin excitations in the plateau phase. These results
provide useful guidelines for the discovery of new NTE materials.Comment: 6 pages, 2 figure
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