13,615 research outputs found
Dimensional crossover in a layered ferromagnet detected by spin correlation driven distortions
Magneto-elastic distortions are commonly detected across magnetic long-range
ordering (LRO) transitions. In principle, they are also induced by the magnetic
short-range ordering (SRO) that precedes a LRO transition, which contains
information about short-range correlations and energetics that are essential
for understanding how LRO is established. However these distortions are
difficult to resolve because the associated atomic displacements are
exceedingly small and do not break symmetry. Here we demonstrate high-multipole
nonlinear optical polarimetry as a sensitive and mode selective probe of SRO
induced distortions using CrSiTe as a testbed. This compound is composed of
weakly bonded sheets of nearly isotropic ferromagnetically interacting spins
that, in the Heisenberg limit, would individually be impeded from LRO by the
Mermin-Wagner theorem. Our results show that CrSiTe evades this law via a
two-step crossover from two- to three-dimensional magnetic SRO, manifested
through two successive and previously undetected totally symmetric distortions
above its Curie temperature.Comment: 17 pages main text, 4 figures, 12 pages supplementary informatio
A smooth entropy approach to quantum hypothesis testing and the classical capacity of quantum channels
We use the smooth entropy approach to treat the problems of binary quantum
hypothesis testing and the transmission of classical information through a
quantum channel. We provide lower and upper bounds on the optimal type II error
of quantum hypothesis testing in terms of the smooth max-relative entropy of
the two states representing the two hypotheses. Using then a relative entropy
version of the Quantum Asymptotic Equipartition Property (QAEP), we can recover
the strong converse rate of the i.i.d. hypothesis testing problem in the
asymptotics. On the other hand, combining Stein's lemma with our bounds, we
obtain a stronger (\ep-independent) version of the relative entropy-QAEP.
Similarly, we provide bounds on the one-shot \ep-error classical capacity of
a quantum channel in terms of a smooth max-relative entropy variant of its
Holevo capacity. Using these bounds and the \ep-independent version of the
relative entropy-QAEP, we can recover both the Holevo-Schumacher-Westmoreland
theorem about the optimal direct rate of a memoryless quantum channel with
product state encoding, as well as its strong converse counterpart.Comment: v4: Title changed, improved bounds, both direct and strong converse
rates are covered, a new Discussion section added. 20 page
Toeplitz Inverse Covariance-Based Clustering of Multivariate Time Series Data
Subsequence clustering of multivariate time series is a useful tool for
discovering repeated patterns in temporal data. Once these patterns have been
discovered, seemingly complicated datasets can be interpreted as a temporal
sequence of only a small number of states, or clusters. For example, raw sensor
data from a fitness-tracking application can be expressed as a timeline of a
select few actions (i.e., walking, sitting, running). However, discovering
these patterns is challenging because it requires simultaneous segmentation and
clustering of the time series. Furthermore, interpreting the resulting clusters
is difficult, especially when the data is high-dimensional. Here we propose a
new method of model-based clustering, which we call Toeplitz Inverse
Covariance-based Clustering (TICC). Each cluster in the TICC method is defined
by a correlation network, or Markov random field (MRF), characterizing the
interdependencies between different observations in a typical subsequence of
that cluster. Based on this graphical representation, TICC simultaneously
segments and clusters the time series data. We solve the TICC problem through
alternating minimization, using a variation of the expectation maximization
(EM) algorithm. We derive closed-form solutions to efficiently solve the two
resulting subproblems in a scalable way, through dynamic programming and the
alternating direction method of multipliers (ADMM), respectively. We validate
our approach by comparing TICC to several state-of-the-art baselines in a
series of synthetic experiments, and we then demonstrate on an automobile
sensor dataset how TICC can be used to learn interpretable clusters in
real-world scenarios.Comment: This revised version fixes two small typos in the published versio
Coexistence of the topological state and a two-dimensional electron gas on the surface of Bi2Se3
Topological insulators are a recently discovered class of materials with
fascinating properties: While the inside of the solid is insulating,
fundamental symmetry considerations require the surfaces to be metallic. The
metallic surface states show an unconventional spin texture, electron dynamics
and stability. Recently, surfaces with only a single Dirac cone dispersion have
received particular attention. These are predicted to play host to a number of
novel physical phenomena such as Majorana fermions, magnetic monopoles and
unconventional superconductivity. Such effects will mostly occur when the
topological surface state lies in close proximity to a magnetic or electric
field, a (superconducting) metal, or if the material is in a confined geometry.
Here we show that a band bending near to the surface of the topological
insulator BiSe gives rise to the formation of a two-dimensional
electron gas (2DEG). The 2DEG, renowned from semiconductor surfaces and
interfaces where it forms the basis of the integer and fractional quantum Hall
effects, two-dimensional superconductivity, and a plethora of practical
applications, coexists with the topological surface state in BiSe. This
leads to the unique situation where a topological and a non-topological, easily
tunable and potentially superconducting, metallic state are confined to the
same region of space.Comment: 12 pages, 3 figure
The activation energy for GaAs/AlGaAs interdiffusion
Copyright 1997 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. This article appeared in Journal of Applied Physics 82, 4842 (1997) and may be found at
An upper limit for the water outgassing rate of the main-belt comet 176P/LINEAR observed with Herschel/HIFI
176P/LINEAR is a member of the new cometary class known as main-belt comets
(MBCs). It displayed cometary activity shortly during its 2005 perihelion
passage that may be driven by the sublimation of sub-surface ices. We have
therefore searched for emission of the H2O 110-101 ground state rotational line
at 557 GHz toward 176P/LINEAR with the Heterodyne Instrument for the Far
Infrared (HIFI) on board the Herschel Space Observatory on UT 8.78 August 2011,
about 40 days after its most recent perihelion passage, when the object was at
a heliocentric distance of 2.58 AU. No H2O line emission was detected in our
observations, from which we derive sensitive 3-sigma upper limits for the water
production rate and column density of < 4e25 molec/s and of < 3e10 cm^{-2},
respectively. From the peak brightness measured during the object's active
period in 2005, this upper limit is lower than predicted by the relation
between production rates and visual magnitudes observed for a sample of comets
by Jorda et al. (2008) at this heliocentric distance. Thus, 176P/LINEAR was
likely less active at the time of our observation than during its previous
perihelion passage. The retrieved upper limit is lower than most values derived
for the H2O production rate from the spectroscopic search for CN emission in
MBCs.Comment: 5 pages, 2 figures. Minor changes to match published versio
Discovery (theoretical prediction and experimental observation) of a large-gap topological-insulator class with spin-polarized single-Dirac-cone on the surface
Recent theories and experiments have suggested that strong spin-orbit
coupling effects in certain band insulators can give rise to a new phase of
quantum matter, the so-called topological insulator, which can show macroscopic
entanglement effects. Such systems feature two-dimensional surface states whose
electrodynamic properties are described not by the conventional Maxwell
equations but rather by an attached axion field, originally proposed to
describe strongly interacting particles. It has been proposed that a
topological insulator with a single spin-textured Dirac cone interfaced with a
superconductor can form the most elementary unit for performing fault-tolerant
quantum computation. Here we present an angle-resolved photoemission
spectroscopy study and first-principle theoretical calculation-predictions that
reveal the first observation of such a topological state of matter featuring a
single-surface-Dirac-cone realized in the naturally occurring BiSe
class of materials. Our results, supported by our theoretical predictions and
calculations, demonstrate that undoped compound of this class of materials can
serve as the parent matrix compound for the long-sought topological device
where in-plane surface carrier transport would have a purely quantum
topological origin. Our study further suggests that the undoped compound
reached via n-to-p doping should show topological transport phenomena even at
room temperature.Comment: 3 Figures, 18 pages, Submitted to NATURE PHYSICS in December 200
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