12,014 research outputs found
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
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
Electronic structure of nanoscale iron oxide particles measured by scanning tunneling and photoelectron spectroscopies
We have investigated the electronic structure of nano-sized iron oxide by
scanning tunnelling microscopy (STM) and spectroscopy (STS) as well as by
photoelectron spectroscopy. Nano particles were produced by thermal treatment
of Ferritin molecules containing a self-assembled core of iron oxide. Depending
on the thermal treatment we were able to prepare different phases of iron oxide
nanoparticles resembling gamma-Fe2O3, alpha-Fe2O3, and a phase which apparently
contains both gamma-Fe2O3 and alpha-Fe2O3. Changes to the electronic structure
of these materials were studied under reducing conditions. We show that the
surface band gap of the electronic excitation spectrum can differ from that of
bulk material and is dominated by surface effects.Comment: REVTeX, 6 pages, 10 figures, submitted to PR
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
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
Unitarity bounds and RG flows in time dependent quantum field theory
We generalize unitarity bounds on operator dimensions in conformal field
theory to field theories with spacetime dependent couplings. Below the energy
scale of spacetime variation of the couplings, their evolution can strongly
affect the physics, effectively shifting the infrared operator scaling and
unitarity bounds determined from correlation functions in the theory. We
analyze this explicitly for large- double-trace flows, and connect these to
UV complete field theories. One motivating class of examples comes from our
previous work on FRW holography, where this effect explains the range of
flavors allowed in the dual, time dependent, field theory.Comment: 38 page
Tight lower bound to the geometric measure of quantum discord
Dakic, Vedral and Brukner [Physical Review Letters \tf{105},190502 (2010)]
gave a geometric measure of quantum discord in a bipartite quantum state as the
distance of the state from the closest classical quantum (or zero discord)
state and derived an explicit formula for a two qubit state. Further, S.Luo and
S.Fu [Physical Review A \tf{82}, 034302 (2010)] obtained a generic form of this
geometric measure for a general bipartite state and established a lower bound.
In this brief report we obtain a rigorous lower bound to the geometric measure
of quantum discord in a general bipartite state which dominates that obtained
by S.Luo and S.Fu.Comment: 10 pages,2 figures. In the previous versions, a constraint was
ignored while optimizing the second term in Eq.(5), in which case, only a
lower bound on the geometric discord can be obtained. The title is also
consequently changed. Accepted in Phys.Rev.
Additivity and multiplicativity properties of some Gaussian channels for Gaussian inputs
We prove multiplicativity of maximal output norm of classical noise
channels and thermal noise channels of arbitrary modes for all under the
assumption that the input signal states are Gaussian states. As a direct
consequence, we also show the additivity of the minimal output entropy and that
of the energy-constrained Holevo capacity for those Gaussian channels under
Gaussian inputs. To the best of our knowledge, newly discovered majorization
relation on symplectic eigenvalues, which is also of independent interest,
plays a central role in the proof.Comment: 9 pages, no figures. Published Versio
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
Bounds on Quantum Correlations in Bell Inequality Experiments
Bell inequality violation is one of the most widely known manifestations of
entanglement in quantum mechanics; indicating that experiments on physically
separated quantum mechanical systems cannot be given a local realistic
description. However, despite the importance of Bell inequalities, it is not
known in general how to determine whether a given entangled state will violate
a Bell inequality. This is because one can choose to make many different
measurements on a quantum system to test any given Bell inequality and the
optimization over measurements is a high-dimensional variational problem. In
order to better understand this problem we present algorithms that provide, for
a given quantum state, both a lower bound and an upper bound on the maximal
expectation value of a Bell operator. Both bounds apply techniques from convex
optimization and the methodology for creating upper bounds allows them to be
systematically improved. In many cases these bounds determine measurements that
would demonstrate violation of the Bell inequality or provide a bound that
rules out the possibility of a violation. Examples are given to illustrate how
these algorithms can be used to conclude definitively if some quantum states
violate a given Bell inequality.Comment: 13 pages, 1 table, 2 figures. Updated version as published in PR
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