1,333 research outputs found
A new correlator in quantum spin chains
We propose a new correlator in one-dimensional quantum spin chains, the
Emptiness Formation Probability (EFP). This is a natural generalization
of the Emptiness Formation Probability (EFP), which is the probability that the
first spins of the chain are all aligned downwards. In the EFP we let
the spins in question be separated by sites. The usual EFP corresponds to
the special case when , and taking allows us to quantify non-local
correlations. We express the EFP for the anisotropic XY model in a
transverse magnetic field, a system with both critical and non-critical
regimes, in terms of a Toeplitz determinant. For the isotropic XY model we find
that the magnetic field induces an interesting length scale.Comment: 6 pages, 1 figur
Density and spacings for the energy levels of quadratic Fermi operators
received: 2017-02-07 accepted: 2017-05-22 published: 2017-06-12 pubcorrected: 2017-06-13received: 2017-02-07 accepted: 2017-05-22 published: 2017-06-12 pubcorrected: 2017-06-13F.D.C., A.M., and F.M. acknowledge support from EPSRC Grant No. EP/L010305/1. F.D.C. acknowledges partial support from the Italian National Group of Mathematical Physics (GNFM-INdAM). A.M. acknowledges the support of the Leverhulme Trust Early Career Fellowship (ECF 2013-613)
The power of random measurements: measuring Tr(\rho^n) on single copies of \rho
While it is known that Tr(\rho^n) can be measured directly (i.e., without
first reconstructing the density matrix) by performing joint measurements on n
copies of the same state rho, it is shown here that random measurements on
single copies suffice, too. Averaging over the random measurements directly
yields estimates of Tr(\rho^n), even when it is not known what measurements
were actually performed (so that one cannot reconstruct \rho)
Optical study of the vibrational and dielectric properties of BiMnO3
BiMnO3 (BMO), ferromagnetic (FM) below Tc = 100 K, was believed to be also
ferroelectric (FE) due to a non-centro-symmetric C2 structure, until
diffraction data indicated that its space group is the centro-symmetric C2/c.
Here we present infrared phonon spectra of BMO, taken on a mosaic of single
crystals, which are consistent with C2/c at any T > 10 K, as well as
room-temperature Raman data which strongly support this conclusion. We also
find that the infrared intensity of several phonons increases steadily for
decreasing T, causing the relative permittivity of BMO to vary from 18.5 at 300
K to 45 at 10 K. At variance with FE materials of displacive type, no
appreciable softening has been found in the infrared phonons. Both their
frequencies and intensities, moreover, appear insensitive to the FM transition
at Tc
Quantum state tomography by continuous measurement and compressed sensing
The need to perform quantum state tomography on ever larger systems has
spurred a search for methods that yield good estimates from incomplete data. We
study the performance of compressed sensing (CS) and least squares (LS)
estimators in a fast protocol based on continuous measurement on an ensemble of
cesium atomic spins. Both efficiently reconstruct nearly pure states in the
16-dimensional ground manifold, reaching average fidelities FCS = 0.92 and FLS
= 0.88 using similar amounts of incomplete data. Surprisingly, the main
advantage of CS in our protocol is an increased robustness to experimental
imperfections
Synthesis and characterization of multiferroic BiMnO
We report on the high pressure synthesis of BiMnO, a manganite
displaying a "quadruple perovskite" structure. Structural characterization of
single crystal samples shows a distorted and asymmetrical coordination around
the Bi atom, due to presence of the lone pair, resulting in
non-centrosymmetric space group Im, leading to a permanent electrical dipole
moment and ferroelectric properties. On the other hand, magnetic
characterization reveals antiferromagnetic transitions, in agreement with the
isostructural compounds, thus evidencing two intrinsic properties that make
BiMnO a promising multiferroic material.Comment: 4 pages, 3 figure
Roots of the derivative of the Riemann zeta function and of characteristic polynomials
We investigate the horizontal distribution of zeros of the derivative of the
Riemann zeta function and compare this to the radial distribution of zeros of
the derivative of the characteristic polynomial of a random unitary matrix.
Both cases show a surprising bimodal distribution which has yet to be
explained. We show by example that the bimodality is a general phenomenon. For
the unitary matrix case we prove a conjecture of Mezzadri concerning the
leading order behavior, and we show that the same follows from the random
matrix conjectures for the zeros of the zeta function.Comment: 24 pages, 6 figure
Internal-strain mediated coupling between polar Bi and magnetic Mn ions in the defect-free quadruple-perovskite BiMnMnO
By means of neutron powder diffraction, we investigated the effect of the
polar Bi ion on the magnetic ordering of the Mn ions in
BiMnMnO, the counterpart with \textit{quadruple} perovskite
structure of the \textit{simple} perovskite BiMnO. The data are consistent
with a \textit{noncentrosymmetric} spacegroup which contrasts the
\textit{centrosymmetric} one previously reported for the isovalent and
isomorphic compound LaMnMnO, which gives evidence of a
Bi-induced polarization of the lattice. At low temperature, the two
Mn sublattices of the and sites order antiferromagnetically
(AFM) in an independent manner at 25 and 55 K, similarly to the case of
LaMnMnO. However, both magnetic structures of
BiMnMnO radically differ from those of LaMnMnO.
In BiMnMnO the moments of the sites form
an anti-body AFM structure, whilst the moments \textbf{M} of the
sites result from a large and \textit{uniform} modulation along the b-axis of the moments \textbf{M} in the
-plane. The modulation is strikingly correlated with the displacements of
the Mn ions induced by the Bi ions. Our analysis unveils a strong
magnetoelastic coupling between the internal strain created by the Bi
ions and the moment of the Mn ions in the sites. This is ascribed to
the high symmetry of the oxygen sites and to the absence of oxygen defects, two
characteristics of quadruple perovskites not found in simple ones, which
prevent the release of the Bi-induced strain through distortions or
disorder. This demonstrates the possibility of a large magnetoelectric coupling
in proper ferroelectrics and suggests a novel concept of internal strain
engineering for multiferroics design.Comment: 9 pages, 7 figures, 5 table
Singling out the effect of quenched disorder in the phase diagram of cuprates
We investigate the specific influence of structural disorder on the
suppression of antiferromagnetic order and on the emergence of cuprate
superconductivity. We single out pure disorder, by focusing on a series of
YEuBaCuO samples at fixed oxygen content
, in the range . The gradual Y/Eu isovalent substitution
smoothly drives the system through the Mott-insulator to superconductor
transition from a full antiferromagnet with N\'eel transition K at
to a bulk superconductor with superconducting critical temperature
K at , YBaCuO. The electronic properties are
finely tuned by gradual lattice deformations induced by the different cationic
radii of the two lanthanides, inducing a continuous change of the basal Cu(1)-O
chain length, as well as a controlled amount of disorder in the active
Cu(2)O bilayers. We check that internal charge transfer from the basal to
the active plane is entirely responsible for the doping of the latter and we
show that superconductivity emerges with orthorhombicity. By comparing
transition temperatures with those of the isoelectronic clean system we
deterime the influence of pure structural disorder connected with the Y/Eu
alloy.Comment: 10 pages 11 figures, submitted to Journal of Physics: Condensed
Matter, Special Issue in memory of Prof. Sandro Massid
Seesaw parametrization for n right-handed neutrinos
Introducing n right-handed neutrinos to the Standard Model yields, in
general, massive active neutrinos. We give explicit parametrizations for the
involved mixing and coupling matrices in terms of physical parameters for both
the top-down and the bottom-up approach for arbitrary n. Bounds on the complex
mixing angles in the bottom-up approach from perturbativity of the Yukawa
couplings to charged lepton flavor violation are discussed. As a novel
illustration of possible effects from n != 3, we extend the neutrino anarchy
framework to arbitrary n; we show that while the anarchic mixing angles are
insensitive to the number of singlets, the observed ratios of neutrino masses
prefer small n for the simplest linear measure.Comment: 12 pages, 4 figures. Added references and details about the sampling
algorith
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