923 research outputs found
Site-selective adsorption of naphthalene-tetracarboxylic-dianhydride on Ag(110): First-principles calculations
The mechanism of adsorption of the
1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) molecule on the Ag(110)
surface is elucidated on the basis of extensive density functional theory
calculations. This molecule, together with its perylene counterpart, PTCDA, are
archetype organic semiconductors investigated experimentally over the past 20
years. We find that the bonding of the molecule to the substrate is highly
site-selective, being determined by electron transfer to the LUMO of the
molecule and local electrostatic attraction between negatively charged carboxyl
oxygens and positively charged silver atoms in [1-10] atomic rows. The
adsorption energy in the most stable site is 0.9eV. A similar mechanism is
expected to govern the adsorption of PTCDA on Ag(110) as well.Comment: 8 pages, 4 figures, high-quality figures available upon reques
The elevated Curie temperature and half-metallicity in the ferromagnetic semiconductor LaEuO
Here we study the effect of La doping in EuO thin films using SQUID
magnetometry, muon spin rotation (SR), polarized neutron reflectivity
(PNR), and density functional theory (DFT). The SR data shows that the
LaEuO is homogeneously magnetically ordered up to its
elevated . It is concluded that bound magnetic polaron behavior does
not explain the increase in and an RKKY-like interaction is
consistent with the SR data. The estimation of the magnetic moment by DFT
simulations concurs with the results obtained by PNR, showing a reduction of
the magnetic moment per LaEuO for increasing lanthanum doping.
This reduction of the magnetic moment is explained by the reduction of the
number of Eu-4 electrons present in all the magnetic interactions in EuO
films. Finally, we show that an upwards shift of the Fermi energy with La or Gd
doping gives rise to half-metallicity for doping levels as high as 3.2 %.Comment: 7 pages, 11 figure
C-Field Methods for Non-Equilibrium Bose Gases
We review c-field methods for simulating the non-equilibrium dynamics of
degenerate Bose gases beyond the mean-field Gross-Pitaevskii approximation. We
describe three separate approaches that utilise similar numerical methods, but
have distinct regimes of validity. Systems at finite temperature can be treated
with either the closed-system projected Gross-Pitaevskii equation (PGPE), or
the open-system stochastic projected Gross-Pitaevskii equation (SPGPE). These
are both applicable in quantum degenerate regimes in which thermal fluctuations
are significant. At low or zero temperature, the truncated Wigner projected
Gross-Pitaevskii equation (TWPGPE) allows for the simulation of systems in
which spontaneous collision processes seeded by quantum fluctuations are
important. We describe the regimes of validity of each of these methods, and
discuss their relationships to one another, and to other simulation techniques
for the dynamics of Bose gases. The utility of the SPGPE formalism in modelling
non-equilibrium Bose gases is illustrated by its application to the dynamics of
spontaneous vortex formation in the growth of a Bose-Einstein condensate.Comment: 7 pages, 1 figure. Unedited version of chapter to appear in Quantum
Gases: Finite Temperature and Non-Equilibrium Dynamics (Vol. 1 Cold Atoms
Series). N.P. Proukakis, S.A. Gardiner, M.J. Davis and M.H. Szymanska, eds.
Imperial College Press, London (in press). See
http://www.icpress.co.uk/physics/p817.html v2: Added arXiv cross-reference
Variational quantum chemistry requires gate-error probabilities below the fault-tolerance threshold
The variational quantum eigensolver (VQE) is a leading contender for useful
quantum advantage in the NISQ era. The interplay between quantum processors and
classical optimisers is believed to make the VQE noise resilient. Here, we
probe this hypothesis. We use full density-matrix simulations to rank the noise
resilience of leading gate-based VQE algorithms in ground-state computations on
a range of molecules. We find that, in the presence of noise: (i) ADAPT-VQEs
that construct ansatz circuits iteratively outperform VQEs that use "fixed"
ansatz circuits; and (ii) ADAPT-VQEs perform better when circuits are
constructed from gate-efficient elements rather than physically-motivated ones.
Our results show that, for a wide range of molecules, even the best-performing
VQE algorithms require gate-error probabilities on the order of to
to reach chemical accuracy. This is significantly below the
fault-tolerance thresholds of most error-correction protocols. Further, we
estimate that the maximum allowed gate-error probability scales inversely with
the number of noisy (two-qubit) gates. Our results indicate that useful
chemistry calculations with current gate-based VQEs are unlikely to be
successful on near-term hardware without error correction.Comment: 17 pages, 8 figure
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