359 research outputs found
Importance of bath dynamics for decoherence in spin systems
We study the decoherence of two coupled spins that interact with a chaotic
spin-bath environment. It is shown that connectivity of spins in the bath is of
crucial importance for the decoherence of the central system. The previously
found phenomenon of two-step decoherence (Phys. Rev. Lett. {\bf 90}, 210401
(2003)) turns out to be typical for the bath with a slow enough dynamics or no
dynamics. For a generic random system with chaotic dynamics a conventional
exponential relaxation to the pointer states takes place. Our results confirm a
conjecture of Paz and Zurek (Phys. Rev. Lett. {\bf 82}, 5181 (1999)) that for
weak enough interactions the pointer states are eigenstates of the central
system.Comment: submitted to Physical Review Letter
Relaxation, thermalization and Markovian dynamics of two spins coupled to a spin bath
It is shown that by fitting a Markovian quantum master equation to the
numerical solution of the time-dependent Schr\"odinger equation of a system of
two spin-1/2 particles interacting with a bath of up to 34 spin-1/2 particles,
the former can describe the dynamics of the two-spin system rather well. The
fitting procedure that yields this Markovian quantum master equation accounts
for all non-Markovian effects in as much the general structure of this equation
allows and yields a description that is incompatible with the Lindblad
equation.Comment: arXiv admin note: text overlap with arXiv:1605.0660
Logical inference approach to relativistic quantum mechanics: derivation of the Klein-Gordon equation
The logical inference approach to quantum theory, proposed earlier [Ann.
Phys. 347 (2014) 45-73], is considered in a relativistic setting. It is shown
that the Klein-Gordon equation for a massive, charged, and spinless particle
derives from the combination of the requirements that the space-time data
collected by probing the particle is obtained from the most robust experiment
and that on average, the classical relativistic equation of motion of a
particle holds
Decoherence by a spin thermal bath: Role of the spin-spin interactions and initial state of the bath
We study the decoherence of two coupled spins that interact with a spin-bath
environment. It is shown that the connectivity and the coupling strength
between the spins in the environment are of crucial importance for the
decoherence of the central system. For the anisotropic spin-bath, changing the
connectivity or coupling strenghts changes the decoherence of the central
system from Gaussian to exponential decay law. The initial state of the
environment is shown to affect the decoherence process in a qualitatively
significant manner.Comment: submitted to PR
Photon and spin dependence of the resonance lines shape in the strong coupling regime
We study the quantum dynamics of a spin ensemble coupled to cavity photons.
Recently, related experimental results have been reported, showing the
existence of the strong coupling regime in such systems. We study the
eigenenergy distribution of the multi-spin system (following the Tavis-Cummings
model) which shows a peculiar structure as a function of the number of cavity
photons and of spins. We study how this structure causes changes in the
spectrum of the admittance in the linear response theory, and also the
frequency dependence of the excited quantities in the stationary state under a
probing field. In particular, we investigate how the structure of the higher
excited energy levels changes the spectrum from a double-peak structure (the
so-called vacuum field Rabi splitting) to a single peak structure. We also
point out that the spin dynamics in the region of the double-peak structure
corresponds to recent experiments using cavity ringing while in region of the
single peak structure, it corresponds to the coherent Rabi oscillation in a
driving electromagnetic filed. Using a standard Lindblad type mechanism, we
study the effect of dissipations on the line width and separation in the
computed spectra. In particular, we study the relaxation of the total spin in
the general case of a spin ensemble in which the total spin of the system is
not specified. The theoretical results are correlated with experimental
evidence of the strong coupling regime, achieved with a spin 1/2 ensemble
First-principles modelling of magnetic excitations in Mn12
We have developed a fully microscopic theory of magnetic properties of the
prototype molecular magnet Mn12. First, the intra-molecular magnetic properties
have been studied by means of first-principles density functional-based
methods, with local correlation effects being taken into account within the
local density approximation plus U (LDA+U) approach. Using the magnetic force
theorem, we have calculated the interatomic isotropic and anisotropic exchange
interactions and full tensors of single-ion anisotropy for each Mn ion.
Dzyaloshinskii-Moriya (DM) interaction parameters turned out to be unusually
large, reflecting a low symmetry of magnetic pairs in molecules, in comparison
with bulk crystals. Based on these results we predict a distortion of
ferrimagnetic ordering due to DM interactions. Further, we use an exact
diagonalization approach allowing to work with as large Hilbert space dimension
as 10^8 without any particular symmetry (the case of the constructed magnetic
model). Based on the computational results for the excitation spectrum, we
propose a distinct interpretation of the experimental inelastic neutron
scattering spectra.Comment: 8 pages, 2 figures. To appear in Physical Review
Modeling electronic structure and transport properties of graphene with resonant scattering centers
We present a detailed numerical study of the electronic properties of
single-layer graphene with resonant ("hydrogen") impurities and vacancies
within a framework of noninteracting tight-binding model on a honeycomb
lattice. The algorithms are based on the numerical solution of the
time-dependent Schr\"{o}dinger equation and applied to calculate the density of
states, \textit{quasieigenstates}, AC and DC conductivities of large samples
containing millions of atoms. Our results give a consistent picture of
evolution of electronic structure and transport properties of functionalized
graphene in a broad range of concentration of impurities (from graphene to
graphane), and show that the formation of impurity band is the main factor
determining electrical and optical properties at intermediate impurity
concentrations, together with a gap opening when approaching the graphane
limit.Comment: 17 pages, 17 figures, expanded version to appear in PR
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