17,691 research outputs found
Structural and electronic properties of the graphene/Al/Ni(111) intercalation-like system
Decoupling of the graphene layer from the ferromagnetic substrate via
intercalation of sp metal has recently been proposed as an effective way to
realize single-layer graphene-based spin-filter. Here, the structural and
electronic properties of the prototype system, graphene/Al/Ni(111), are
investigated via combination of electron diffraction and spectroscopic methods.
These studies are accompanied by state-of-the-art electronic structure
calculations. The properties of this prospective Al-intercalation-like system
and its possible implementations in future graphene-based devices are
discussed.Comment: 20 pages, 8 figures, and supplementary materia
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
Singular value decomposition and matrix reorderings in quantum information theory
We review Schmidt and Kraus decompositions in the form of singular value
decomposition using operations of reshaping, vectorization and reshuffling. We
use the introduced notation to analyse the correspondence between quantum
states and operations with the help of Jamiolkowski isomorphism. The presented
matrix reorderings allow us to obtain simple formulae for the composition of
quantum channels and partial operations used in quantum information theory. To
provide examples of the discussed operations we utilize a package for the
Mathematica computing system implementing basic functions used in the
calculations related to quantum information theory.Comment: 11 pages, no figures, see
http://zksi.iitis.pl/wiki/projects:mathematica-qi for related softwar
Heat capacity of the quantum magnet TiOCl
Measurements of the heat capacity C(T,H) of the one-dimensional quantum
magnet TiOCl are presented for temperatures 2K < T < 300K and magnetic fields
up to 5T. Distinct anomalies at 91K and 67K signal two subsequent phase
transitions. The lower of these transitions clearly is of first order and seems
to be related to the spin degrees of freedom. The transition at 92K probably
involves the lattice and/or orbital moments. A detailed analysis of the data
reveals that the entropy change through both transitions is surprisingly small
(~ 0.1R), pointing to the existence strong fluctuations well into the
non-ordered high-temperature phase. No significant magnetic field dependence
was detected.Comment: 4 pages, 2 figure
Structural precursor to the metal-insulator transition in V_2O_3
The temperature dependence of the local structure of V_2O_3 in the vicinity
of the metal to insulator transition (MIT) has been investigated using hard
X-ray absorption spectroscopy. It is shown that the vanadium pair distance
along the hexagonal c-axis changes abruptly at the MIT as expected. However, a
continuous increase of the tilt of these pairs sets in already at higher
temperatures and reaches its maximum value at the onset of the electronic and
magnetic transition. These findings confirm recent theoretical results which
claim that electron-lattice coupling is important for the MIT in V_2O_3. Our
results suggest that interactions in the basal plane play a decisive role for
the MIT and orbital degrees of freedom drive the MIT via changes in
hybridization.Comment: 6 pages, 5 figures, 2 table
Generalized Induced Norms
Let ||.|| be a norm on the algebra M_n of all n-by-n matrices over the
complex field C. An interesting problem in matrix theory is that "are there two
norms ||.||_1 and ||.||_2 on C^n such that ||A||=max{||Ax||_2: ||x||_1=1} for
all A in M_n. We will investigate this problem and its various aspects and will
discuss under which conditions ||.||_1=||.||_2.Comment: 8 page
On the efficiency of Hamiltonian-based quantum computation for low-rank matrices
We present an extension of Adiabatic Quantum Computing (AQC) algorithm for
the unstructured search to the case when the number of marked items is unknown.
The algorithm maintains the optimal Grover speedup and includes a small
counting subroutine.
Our other results include a lower bound on the amount of time needed to
perform a general Hamiltonian-based quantum search, a lower bound on the
evolution time needed to perform a search that is valid in the presence of
control error and a generic upper bound on the minimum eigenvalue gap for
evolutions.
In particular, we demonstrate that quantum speedup for the unstructured
search using AQC type algorithms may only be achieved under very rigid control
precision requirements.Comment: 17 pages, no figures, to appear in JM
Two pressure-induced structural phase transitions in TiOCl
We studied the crystal structure of TiOCl up to pressures of =25~GPa at
room temperature by x-ray powder diffraction measurements. Two pressure-induced
structural phase transitions are observed: At 15~GPa emerges
an 22 superstructure with -axis unique monoclinic
symmetry (space group P2/). At 22~GPa all lattice
parameters of the monoclinic phase show a pronounced anomaly. A fraction of the
sample persists in the ambient orthorhombic phase (space group ) over the
whole pressure range.Comment: 5 pages, 5 figures; accepted for publication in Phys. Rev.
Vacuum entanglement governs the bosonic character of magnons
It is well known that magnons, elementary excitations in a magnetic material,
behave as bosons when their density is low. We study how the bosonic character
of magnons is governed by the amount of a multipartite entanglement in the
vacuum state on which magnons are excited. We show that if the multipartite
entanglement is strong, magnons cease to be bosons. We also consider some
examples, such as ground states of the Heisenberg ferromagnet and the
transverse Ising model, the condensation of magnons, the one-way quantum
computer, and Kitaev's toric code. Our result provides insights into the
quantum statistics of elementary excitations in these models, and into the
reason why a non-local transformation, such as the Jordan-Wigner
transformation, is necessary for some many-body systems.Comment: 4 pages, no figur
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
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