3,974 research outputs found
Classification of Reductive Monoid Spaces Over an Arbitrary Field
In this semi-expository paper we review the notion of a spherical space. In
particular we present some recent results of Wedhorn on the classification of
spherical spaces over arbitrary fields. As an application, we introduce and
classify reductive monoid spaces over an arbitrary field.Comment: This is the final versio
One dimensional Si-in-Si(001) template for single-atom wire growth
Single atom metallic wires of arbitrary length are of immense technological
and scientific interest. We describe a novel silicon-only template enabling the
self-organised growth of isolated micrometer long surface and subsurface
single-atom chains. It consists of a one dimensional, defect-free
reconstruction - the Haiku core, here revealed for the first time in details -
self-assembled on hydrogenated Si(001) terraces, independent of any step edges.
We discuss the potential of this Si-in-Si template as an appealing alternative
to vicinal surfaces for nanoscale patterning.Comment: 3 pages, 2 figure
Effect of bilayer coupling on tunneling conductance of double-layer high T_c cuprates
Physical effects of bilayer coupling on the tunneling spectroscopy of high
T cuprates are investigated. The bilayer coupling separates the bonding
and antibonding bands and leads to a splitting of the coherence peaks in the
tunneling differential conductance. However, the coherence peak of the bonding
band is strongly suppressed and broadened by the particle-hole asymmetry in the
density of states and finite quasiparticle life-time, and is difficult to
resolve by experiments. This gives a qualitative account why the bilayer
splitting of the coherence peaks was not clearly observed in tunneling
measurements of double-layer high-T oxides.Comment: 4 pages, 3 figures, to be published in PR
An experimental test of all theories with predictive power beyond quantum theory
According to quantum theory, the outcomes of future measurements cannot (in
general) be predicted with certainty. In some cases, even with a complete
physical description of the system to be measured and the measurement
apparatus, the outcomes of certain measurements are completely random. This
raises the question, originating in the paper by Einstein, Podolsky and Rosen,
of whether quantum mechanics is the optimal way to predict measurement
outcomes. Established arguments and experimental tests exclude a few specific
alternative models. Here, we provide a complete answer to the above question,
refuting any alternative theory with significantly more predictive power than
quantum theory. More precisely, we perform various measurements on distant
entangled photons, and, under the assumption that these measurements are chosen
freely, we give an upper bound on how well any alternative theory could predict
their outcomes. In particular, in the case where quantum mechanics predicts two
equally likely outcomes, our results are incompatible with any theory in which
the probability of a prediction is increased by more than ~0.19. Hence, we can
immediately refute any already considered or yet-to-be-proposed alternative
model with more predictive power than this.Comment: 13 pages, 4 figure
QKD with finite resources: secret key rates via R\'enyi entropies
A realistic Quantum Key Distribution (QKD) protocol necessarily deals with
finite resources, such as the number of signals exchanged by the two parties.
We derive a bound on the secret key rate which is expressed as an optimization
problem over R\'enyi entropies. Under the assumption of collective attacks by
an eavesdropper, a computable estimate of our bound for the six-state protocol
is provided. This bound leads to improved key rates in comparison to previous
results.Comment: 11 pages, 2 figure
Metal-insulator crossover in the Boson-Fermion model in infinite dimensions
The Boson-Fermion model, describing a mixture of tightly bound electron pairs
and quasi-free electrons hybridized with each other via a charge exchange term,
is studied in the limit of infinite dimensions, using the Non-Crossing
Approximation within the Dynamical Mean Field Theory. It is shown that a
metal-insulator crossover, driven by strong pair fluctuations, takes place as
the temperature is lowered. It manifests itself in the opening of a pseudogap
in the electron density of states, accompanied by a corresponding effect in the
optical and dc conductivity.Comment: 4 pages, 3 figures, to be published in Phys. Rev. Let
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