113 research outputs found
Atomistic spin dynamics of the CuMn spin glass alloy
We demonstrate the use of Langevin spin dynamics for studying dynamical
properties of an archetypical spin glass system. Simulations are performed on
CuMn (20% Mn) where we study the relaxation that follows a sudden quench of the
system to the low temperature phase. The system is modeled by a Heisenberg
Hamiltonian where the Heisenberg interaction parameters are calculated by means
of first-principles density functional theory. Simulations are performed by
numerically solving the Langevin equations of motion for the atomic spins. It
is shown that dynamics is governed, to a large degree, by the damping parameter
in the equations of motion and the system size. For large damping and large
system sizes we observe the typical aging regime.Comment: 18 pages, 9 figure
Optical spectroscopy and the nature of the insulating state of rare-earth nickelates
Using a combination of spectroscopic ellipsometry and DC transport
measurements, we determine the temperature dependence of the optical
conductivity of NdNiO and SmNiO films. The optical spectra show the
appearance of a characteristic two-peak structure in the near-infrared when the
material passes from the metal to the insulator phase. Dynamical mean-field
theory calculations confirm this two-peak structure, and allow to identify
these spectral changes and the associated changes in the electronic structure.
We demonstrate that the insulating phase in these compounds and the associated
characteristic two-peak structure are due to the combined effect of
bond-disproportionation and Mott physics associated with half of the
disproportionated sites. We also provide insights into the structure of excited
states above the gap.Comment: 12 pages, 13 figure
Zeros of Rydberg-Rydberg Foster Interactions
Rydberg states of atoms are of great current interest for quantum
manipulation of mesoscopic samples of atoms. Long-range Rydberg-Rydberg
interactions can inhibit multiple excitations of atoms under the appropriate
conditions. These interactions are strongest when resonant collisional
processes give rise to long-range C_3/R^3 interactions. We show in this paper
that even under resonant conditions C_3 often vanishes so that care is required
to realize full dipole blockade in micron-sized atom samples.Comment: 10 pages, 4 figures, submitted to J. Phys.
Synthesis and characterization of entangled mesoscopic superpositions for a trapped electron
We propose a scheme for the generation and reconstruction of entangled states
between the internal and external (motional) degrees of freedom of a trapped
electron. Such states also exhibit quantum coherence at a mesoscopic level.Comment: 4 pages, 1 figure, RevTeX (twocolumn
Charge self-consistent many-body corrections using optimized projected localized orbitals
In order for methods combining ab initio density-functional theory and
many-body techniques to become routinely used, a flexible, fast, and
easy-to-use implementation is crucial. We present an implementation of a
general charge self-consistent scheme based on projected localized orbitals in
the projector augmented wave framework in the Vienna Ab Initio Simulation
Package (VASP). We give a detailed description on how the projectors are
optimally chosen and how the total energy is calculated. We benchmark our
implementation in combination with dynamical mean-field theory: first we study
the charge-transfer insulator NiO using a Hartree-Fock approach to solve the
many-body Hamiltonian. We address the advantages of the optimized against
non-optimized projectors and furthermore find that charge self-consistency
decreases the dependence of the spectral function - especially the gap - on the
double counting. Second, using continuous-time quantum Monte Carlo we study a
monolayer of SrVO, where strong orbital polarization occurs due to the
reduced dimensionality. Using total-energy calculation for structure
determination, we find that electronic correlations have a non-negligible
influence on the position of the apical oxygens, and therefore on the thickness
of the single SrVO layer.Comment: 11 pages, 6 figure
Resolvent methods for steady premixed flame shapes governed by the Zhdanov-Trubnikov equation
Using pole decompositions as starting points, the one parameter (-1 =< c < 1)
nonlocal and nonlinear Zhdanov-Trubnikov (ZT) equation for the steady shapes of
premixed gaseous flames is studied in the large-wrinkle limit. The singular
integral equations for pole densities are closely related to those satisfied by
the spectral density in the O(n) matrix model, with n = -2(1 + c)/(1 - c). They
can be solved via the introduction of complex resolvents and the use of complex
analysis. We retrieve results obtained recently for -1 =< c =< 0, and we
explain and cure their pathologies when they are continued naively to 0 < c <
1. Moreover, for any -1 =< c < 1, we derive closed-form expressions for the
shapes of steady isolated flame crests, and then bicoalesced periodic fronts.
These theoretical results fully agree with numerical resolutions. Open problems
are evoked.Comment: v2: 29 pages, 6 figures, some typos correcte
Quantum non-demolition measurement of a superconducting two-level system
In quantum mechanics, the process of measurement is a subtle interplay
between extraction of information and disturbance of the state of the quantum
system. A quantum non-demolition (QND) measurement minimizes this disturbance
by using a particular system - detector interaction which preserves the
eigenstates of a suitable operator of the quantum system. This leads to an
ideal projective measurement. We present experiments in which we perform two
consecutive measurements on a quantum two -level system, a superconducting flux
qubit, by probing the hysteretic behaviour of a coupled nonlinear resonator.
The large correlation between the results of the two measurements demonstrates
the QND nature of the readout method. The fact that a QND measurement is
possible for superconducting qubits strengthens the notion that these
fabricated mesoscopic systems are to be regarded as fundamental quantum
objects. Our results are also relevant for quantum information processing,
where projective measurements are used for protocols like state preparation and
error correction.Comment: 14 pages, 4 figure
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