12 research outputs found
Does the Two-Dimensional t-J Model have Hole Pockets?
We have calculated the high temperature series for the momentum distribution
function n_k of the 2D t-J model to 12th order in inverse temperature. By
extrapolating the series to T=0.2J we investigate the possibility of hole
pockets in the t-J model. We find no indication of hole pockets at an electron
density of n=0.9 with J/t=0.5 or J/t=1.0.Comment: 2 pages, 2 figures. Contribution to Spectroscopies of Novel
Superconductors 97, Cape Cod, M
Towards longitudinal data analytics in Parkinson's Disease
The CloudUPDRS app has been developed as a Class I med- ical device to assess the severity of motor symptoms for Parkinson’s Disease using a fully automated data capture and signal analysis pro- cess based on the standard Unified Parkinson’s Disease Rating Scale. In this paper we report on the design and development of the signal pro- cessing and longitudinal data analytics microservices developed to carry out these assessments and to forecast the long-term development of the disease. We also report on early findings from the application of these techniques in the wild with a cohort of early adopters
Magnetic short-range order in iron above Tc? Statistical mechanics with many-atom interactions
An effective spin Hamiltonian for iron has been derived previously from electronic
structure calculations; there are strong many-atom interactions. The exchange interactions Jij
depend on the surrounding magnetic order. We investigate the magnetisation and spin
correlation function in iron on this basis in two ways: by fitting the Jij to an analytically soluble
Hamiltonian (an extended spherical model) and by Monte Carlo simulation. Short-range manyatom
interactions do not lead to substantial short range order in the paramagnetic state; the
behaviour closely resembles that of the nearest-neighbour Heisenberg model. The main effect is
a slight distortion of the magnetisation curve below TC. Longer range oscillatory pair
interactions are needed for short-range order, but are not supported by the electronic structure
data. We conclude that calculations of the electronic energy in static configurations do not lead
to unusual short range order
SPIN AND CHARGE MODES OF THE t-J LADDER
The spin and charge excitations of the t--J ladder are studied by exact
diagonalization techniques for several electron densities. The various modes
are classified according to their spin (singlet or triplet excitations) and
their parity under a reflection with respect to the symmetry axis along the
chains and a finite size scaling of the related gaps is performed. At low
doping, formation of hole pairs leads to a spin gap for all ratios. This
phase is characterized by (at least) one vanishing energy mode {\it only} in
the charge bonding channel when consistent with the
existence of superconducting pairing correlations. At larger doping the spin
gap disappears. Although the anti-bonding modes remain gapped, low energy
and spin and charge bonding modes are found
consistent with a single band Luttinger scenario. At sufficient low electron
density and above a critical value of J/t we also expect another phase of
electron pairs with gapped spin excitations.Comment: 4 pages, 11 figs. included in a uuencoded compressed file
From wellness to medical diagnostic apps: the Parkinson's Disease case
This paper presents the design and development of the CloudUPDRS app and supporting system developed as a Class I medical device to assess the severity of motor symptoms for Parkinson’s Disease. We report on lessons learnt towards meeting fidelity and regulatory requirements; effective procedures employed to structure user context and ensure data quality; a robust service provision architecture; a dependable analytics toolkit; and provisions to meet mobility and social needs of people with Parkinson’s
Vibrational properties of phonons in random binary alloys: An augmented space recursive technique in the k-representation
We present here an augmented space recursive technique in the
k-representation which include diagonal, off-diagonal and the environmental
disorder explicitly : an analytic, translationally invariant, multiple
scattering theory for phonons in random binary alloys.We propose the augmented
space recursion (ASR) as a computationally fast and accurate technique which
will incorporate configuration fluctuations over a large local environment. We
apply the formalism to , Ni_{88}Cr_12} and
alloys which is not a random choice. Numerical results on spectral functions,
coherent structure factors, dispersion curves and disordered induced FWHM's are
presented. Finally the results are compared with the recent itinerant coherent
potential approximation (ICPA) and also with experiments.Comment: 20 pages, LaTeX, 23 figure
The cloudUPDRS app: a medical device for the clinical assessment of Parkinson's Disease
Parkinson's Disease is a neurological condition distinguished by characteristic motor symptoms including tremor and slowness of movement. To enable the frequent assessment of PD patients, this paper introduces the cloudUPDRS app, a Class I medical device that is an active transient non-invasive instrument, certified by the Medicines and Healthcare products Regulatory Agency in the UK. The app follows closely Part III of the Unified Parkinson's Disease Rating Scale which is the most commonly used protocol in the clinical study of PD; can be used by patients and their carers at home or in the community unsupervised; and, requires the user to perform a sequence of iterated movements which are recorded by the phone sensors. The cloudUPDRS system addresses two key challenges towards meeting essential consistency and efficiency requirements, namely: (i) How to ensure high-quality data collection especially considering the unsupervised nature of the test, in particular, how to achieve firm user adherence to the prescribed movements; and (ii) How to reduce test duration from approximately 25 minutes typically required by an experienced patient, to below 4 minutes, a threshold identified as critical to obtain significant improvements in clinical compliance. To address the former, we combine a bespoke design of the user experience tailored so as to constrain context, with a deep learning approach based on Recurrent Convolutional Neural Networks, to identify failures to follow the movement protocol. We address the latter by developing a machine learning approach to personalize assessments by selecting those elements of the test that most closely match individual symptom profiles and thus offer the highest inferential power, hence closely estimating the patent's overall score
Phase separation and stripe formation in the 2D t-J model: a comparison of numerical results
We make a critical analysis of numerical results for and against phase
separation and stripe formation in the t-J model. We argue that the frustrated
phase separation mechanism for stripe formation requires phase separation at
too high a doping for it to be consistent with existing numerical studies of
the t-J model. We compare variational energies for various methods, and
conclude that the most accurate calculations for large systems appear to be
from the density matrix renormalization group. These calculations imply that
the ground state of the doped t-J model is striped, not phase separated.Comment: This version includes a revised, more careful comparison of numerical
results between DMRG and Green's function Monte Carlo. In particular, for the
original posted version we were accidentally sent obsolete data by Hellberg
and Manousakis; their new results, which are what were used in their Physical
Review Letter, are more accurate because a better trial wavefunction was use
Superconductivity in the two dimensional Hubbard Model.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It is shown that triplet p-wave pairing never
occurs. Singlet d_{x^2-y^2}-wave pairing is strongly favoured and optimal
doping occurs when the van Hove singularity, corresponding to the flat band
part, lies at the Fermi level. Nearest neighbour antiferromagnetic correlations
play an important role in flattening the bands near the Fermi level and in
favouring superconductivity. However the mechanism for superconductivity is a
local one, in contrast to spin fluctuation exchange models. For reasonable
values of the hopping parameter the transition temperature T_c is in the range
10-100K. The optimum doping delta_c lies between 0.14 and 0.25, depending on
the ratio U/t. The gap equation has a BCS-like form and (2*Delta_{max})/(kT_c)
~ 4.Comment: REVTeX, 35 pages, including 19 PostScript figures numbered 1a to 11.
Uses epsf.sty (included). Everything in uuencoded gz-compressed .tar file,
(self-unpacking, see header). Submitted to Phys. Rev. B (24-2-95
Magnetism in iron around the Curie temperature A computational study
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