12,692 research outputs found
Graphene on ferromagnetic surfaces and its functionalization with water and ammonia
Here we present an angle-resolved photoelectron spectroscopy (ARPES), x-ray
absorption spec-troscopy (XAS), and density-functional theory (DFT)
investigations of water and ammonia ad-sorption on graphene/Ni(111). Our
results on graphene/Ni(111) reveal the existence of interface states,
originating from the strong hybridization of the graphene {\pi} and
spin-polarized Ni 3d valence band states. ARPES and XAS data of the H2O
(NH3)/graphene/Ni(111) system give an information about the kind of interaction
between adsorbed molecules and graphene on Ni(111). The presented experimental
data are compared with the results obtained in the framework of the DFT
approach.Comment: accepted in Nanoscale Research Letters; 16 pages, 4 figures, 2 table
Quantum Nonlocal Boxes Exhibit Stronger Distillability
The hypothetical nonlocal box (\textsf{NLB}) proposed by Popescu and Rohrlich
allows two spatially separated parties, Alice and Bob, to exhibit stronger than
quantum correlations. If the generated correlations are weak, they can
sometimes be distilled into a stronger correlation by repeated applications of
the \textsf{NLB}. Motivated by the limited distillability of \textsf{NLB}s, we
initiate here a study of the distillation of correlations for nonlocal boxes
that output quantum states rather than classical bits (\textsf{qNLB}s). We
propose a new protocol for distillation and show that it asymptotically
distills a class of correlated quantum nonlocal boxes to the value , whereas in contrast, the optimal non-adaptive
parity protocol for classical nonlocal boxes asymptotically distills only to
the value 3.0. We show that our protocol is an optimal non-adaptive protocol
for 1, 2 and 3 \textsf{qNLB} copies by constructing a matching dual solution
for the associated primal semidefinite program (SDP). We conclude that
\textsf{qNLB}s are a stronger resource for nonlocality than \textsf{NLB}s. The
main premise that develops from this conclusion is that the \textsf{NLB} model
is not the strongest resource to investigate the fundamental principles that
limit quantum nonlocality. As such, our work provides strong motivation to
reconsider the status quo of the principles that are known to limit nonlocal
correlations under the framework of \textsf{qNLB}s rather than \textsf{NLB}s.Comment: 25 pages, 7 figure
Tsirelson bounds for generalized Clauser-Horne-Shimony-Holt inequalities
Quantum theory imposes a strict limit on the strength of non-local
correlations. It only allows for a violation of the CHSH inequality up to the
value 2 sqrt(2), known as Tsirelson's bound. In this note, we consider
generalized CHSH inequalities based on many measurement settings with two
possible measurement outcomes each. We demonstrate how to prove Tsirelson
bounds for any such generalized CHSH inequality using semidefinite programming.
As an example, we show that for any shared entangled state and observables
X_1,...,X_n and Y_1,...,Y_n with eigenvalues +/- 1 we have | + <X_2
Y_1> + + + ... + - | <= 2 n
cos(pi/(2n)). It is well known that there exist observables such that equality
can be achieved. However, we show that these are indeed optimal. Our approach
can easily be generalized to other inequalities for such observables.Comment: 9 pages, LateX, V2: Updated reference [3]. To appear in Physical
Review
Maximally entangled mixed states of two qubits
We consider mixed states of two qubits and show under which global unitary
operations their entanglement is maximized. This leads to a class of states
that is a generalization of the Bell states. Three measures of entanglement are
considered: entanglement of formation, negativity and relative entropy of
entanglement. Surprisingly all states that maximize one measure also maximize
the others. We will give a complete characterization of these generalized Bell
states and prove that these states for fixed eigenvalues are all equivalent
under local unitary transformations. We will furthermore characterize all
nearly entangled states closest to the maximally mixed state and derive a new
lower bound on the volume of separable mixed states
Pressure-induced insulator-to-metal transition in low-dimensional TiOCl
We studied the transmittance and reflectance of the low-dimensional
Mott-Hubbard insulator TiOCl in the infrared and visible frequency range as a
function of pressure. The strong suppression of the transmittance and the
abrupt increase of the near-infrared reflectance above 12 GPa suggest a
pressure-induced insulator-to-metal transition. The pressure-dependent
frequency shifts of the orbital excitations, as well as the pressure
dependences of the charge gap and the spectral weight of the optical
conductivity above the phase transition are presented.Comment: 4 pages, 6 figure
Induced magnetism of carbon atoms at the graphene/Ni(111) interface
We report an element-specific investigation of electronic and magnetic
properties of the graphene/Ni(111) system. Using magnetic circular dichroism,
the occurrence of an induced magnetic moment of the carbon atoms in the
graphene layer aligned parallel to the Ni 3d magnetization is observed. We
attribute this magnetic moment to the strong hybridization between C and
Ni 3d valence band states. The net magnetic moment of carbon in the graphene
layer is estimated to be in the range of per atom.Comment: 10 pages, 3 figure
Vibrating the QCD string
The large distance behaviour of the adiabatic hybrid potentials is studied in
the framework of the QCD string model. The calculated spectra are shown to be
the result of interplay between potential-type longitudinal and string-type
transverse vibrations.Comment: LaTeX2e, 9 pages, 2 Postscript figures, final version to appear in
Yad.Fi
CORE Technology and Exact Hamiltonian Real-Space Renormalization Group Transformations
The COntractor REnormalization group (CORE) method, a new approach to solving
Hamiltonian lattice systems, is presented. The method defines a systematic and
nonperturbative means of implementing Kadanoff-Wilson real-space
renormalization group transformations using cluster expansion and contraction
techniques. We illustrate the approach and demonstrate its effectiveness using
scalar field theory, the Heisenberg antiferromagnetic chain, and the
anisotropic Ising chain. Future applications to the Hubbard and t-J models and
lattice gauge theory are discussed.Comment: 65 pages, 9 Postscript figures, uses epsf.st
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