79 research outputs found
Reply to Comment on "Magnetization Process of Single Molecule Magnets at Low Temperatures"
This is the reply to a Comment by I.S.Tupitsyn and P.C.E. Stamp (PRL
v92,119701 (2004)) on a letter of ours (J.F.Fernandez and J.J.Alonso, PRL v91,
047202 (2003)).Comment: 2 LaTeX pages, 1 eps figure. Submitted to PRL on 20 October 200
Ground-state hyperfine structure of H-, Li-, and B-like ions in middle-Z region
The hyperfine splitting of the ground state of H-, Li-, and B-like ions is
investigated in details within the range of nuclear numbers Z = 7-28. The
rigorous QED approach together with the large-scale configuration-interaction
Dirac-Fock-Sturm method are employed for the evaluation of the
interelectronic-interaction contributions of first and higher orders in 1/Z.
The screened QED corrections are evaluated to all orders in (\alpha Z)
utilizing an effective potential approach. The influence of nuclear
magnetization distribution is taken into account within the single-particle
nuclear model. The specific differences between the hyperfine-structure level
shifts of H- and Li-like ions, where the uncertainties associated with the
nuclear structure corrections are significantly reduced, are also calculated.Comment: 22 pages, 11 tables, 2 figure
Transition frequency shifts with fine structure constant variation for Fe II: Breit and core-valence correlation correction
Transition frequencies of Fe II ion are known to be very sensitive to
variation of the fine structure constant \alpha. The resonance absorption lines
of Fe II from objects at cosmological distances are used in a search for the
possible variation of \alpha in cause of cosmic time. In this paper we
calculated the dependence of the transition frequencies on \alpha^2 (q-factors)
for Fe II ion. We found corrections to these coefficients from valence-valence
and core-valence correlations and from the Breit interaction. Both the
core-valence correlation and Breit corrections to the q-factors appeared to be
larger than had been anticipated previously. Nevertheless our calculation
confirms that the Fe II absorption lines seen in quasar spectra have large
q-factors of both signs and thus the ion Fe II alone can be used in the search
for the \alpha-variation at different cosmological epochs.Comment: 7 pages, submitted to Phys. Rev.
Dual kinetic balance approach to basis set expansions for the Dirac equation
A new approach to finite basis sets for the Dirac equation is developed. It
solves the problem of spurious states and, as a result, improves the
convergence properties of basis set calculations. The efficiency of the method
is demonstrated for finite basis sets constructed from B splines by calculating
the one-loop self-energy correction for a hydrogenlike ion.Comment: 14 pages, 1 tabl
QED corrections to the parity-nonconserving 6s-7s amplitude in Cs
The complete gauge-invariant set of the one-loop QED corrections to the
parity-nonconserving 6s-7s amplitude in Cs is evaluated to all orders
in using a local version of the Dirac-Hartree-Fock potential. The
calculations are peformed in both length and velocity gauges for the absorbed
photon. The total binding QED correction is found to be -0.27(3)%, which
differs from previous evaluations of this effect. The weak charge of
Cs, derived using two most accurate values of the vector transition
polarizability , is for and for . The first value
deviates by from the prediction of the Standard Model, while the
second one is in perfect agreement with it.Comment: 4 pages, 1 figure, 2 table
Coarsening of Disordered Quantum Rotors under a Bias Voltage
We solve the dynamics of an ensemble of interacting rotors coupled to two
leads at different chemical potential letting a current flow through the system
and driving it out of equilibrium. We show that at low temperature the
coarsening phase persists under the voltage drop up to a critical value of the
applied potential that depends on the characteristics of the electron
reservoirs. We discuss the properties of the critical surface in the
temperature, voltage, strength of quantum fluctuations and coupling to the bath
phase diagram. We analyze the coarsening regime finding, in particular, which
features are essentially quantum mechanical and which are basically classical
in nature. We demonstrate that the system evolves via the growth of a coherence
length with the same time-dependence as in the classical limit, -- the scalar curvature driven universality class. We obtain the
scaling function of the correlation function at late epochs in the coarsening
regime and we prove that it coincides with the classical one once a prefactor
that encodes the dependence on all the parameters is factorized. We derive a
generic formula for the current flowing through the system and we show that,
for this model, it rapidly approaches a constant that we compute.Comment: 53 pages, 12 figure
Nonlinear response of single-molecule nanomagnets: equilibrium and dynamical
We present an experimental study of the {\em nonlinear} susceptibility of
Mn single-molecule magnets. We investigate both their
thermal-equilibrium and dynamical nonlinear responses. The equilibrium results
show the sensitivity of the nonlinear susceptibility to the magnetic
anisotropy, which is nearly absent in the linear response for axes distributed
at random. The nonlinear dynamic response of Mn was recently found to be
very large and displaying peaks reversed with respect to classical
superparamagnets [F. Luis {\em et al.}, Phys. Rev. Lett. {\bf 92}, 107201
(2004)]. Here we corroborate the proposed explanation -- strong field
dependence of the relaxation rate due to the detuning of tunnel energy levels.
This is done by studying the orientational dependence of the nonlinear
susceptibility, which permits to isolate the quantum detuning contribution.
Besides, from the analysis of the longitudinal and transverse contributions we
estimate a bound for the decoherence time due to the coupling to the phonon
bath.Comment: 13 pages, 8 figures, resubmitted to Phys. Rev. B with minor change
Backward scattering of low-energy antiprotons by highly charged and neutral uranium: Coulomb glory
Collisions of antiprotons with He-, Ne-, Ni-like, bare, and neutral uranium
are studied theoretically for scattering angles close to 180 and
antiproton energies with the interval 100 eV -- 10 keV. We investigate the
Coulomb glory effect which is caused by a screening of the Coulomb potential of
the nucleus and results in a prominent maximum of the differential cross
section in the backward direction at some energies of the incident particle. We
found that for larger numbers of electrons in the ion the effect becomes more
pronounced and shifts to higher energies of the antiproton. On the other hand,
a maximum of the differential cross section in the backward direction can also
be found in the scattering of antiprotons on a bare uranium nucleus. The latter
case can be regarded as a manifestation of the screening property of the
vacuum-polarization potential in non-relativistic collisions of heavy
particles.Comment: 14 pages, 5 figure
Radiative and correlation effects on the parity-nonconserving transition amplitude in heavy alkaline atoms
The complete gauge-invariant set of the one-loop QED corrections to the
parity-nonconserving (PNC) amplitude in cesium and francium is evaluated to all
orders in using a local form of the Dirac-Fock potential. The
calculations are performed in both length and velocity gauges for the absorbed
photon and the total binding QED correction is found to be 0.27(3)% for Cs
and 0.28(5)% for Fr. Moreover, a high-precision calculation of the
electron-correlation and Breit-interaction effects on the 7 PNC amplitude
in francium using a large-scale configuration-interaction Dirac-Fock method is
performed. The obtained results are employed to improve the theoretical
predictions for the PNC transition amplitude in Cs and Fr. Using an average
value from two most accurate measurements of the vector transition
polarizability, the weak charge of Cs is derived to amount to . This value deviates by
from the prediction of the standard model. The values of the - PNC
amplitude in Fr and Fr are obtained to be 15.49(15) and
14.16(14), respectively, in units of i a.u.Comment: 28 pages, 8 tables, 2 figure
Relativistic calculations of the K-K charge transfer and K-vacancy production probabilities in low-energy ion-atom collisions
The previously developed technique for evaluation of charge-transfer and
electron-excitation processes in low-energy heavy-ion collisions [I.I. Tupitsyn
et al., Phys. Rev. A 82, 042701(2010)] is extended to collisions of ions with
neutral atoms. The method employs the active electron approximation, in which
only the active electron participates in the charge transfer and excitation
processes while the passive electrons provide the screening DFT potential. The
time-dependent Dirac wave function of the active electron is represented as a
linear combination of atomic-like Dirac-Fock-Sturm orbitals, localized at the
ions (atoms). The screening DFT potential is calculated using the overlapping
densities of each ions (atoms), derived from the atomic orbitals of the passive
electrons. The atomic orbitals are generated by solving numerically the
one-center Dirac-Fock and Dirac-Fock-Sturm equations by means of a
finite-difference approach with the potential taken as the sum of the exact
reference ion (atom) Dirac-Fock potential and of the Coulomb potential from the
other ion within the monopole approximation. The method developed is used to
calculate the K-K charge transfer and K-vacancy production probabilties for the
Ne -- F collisions at the F projectile
energies 130 keV/u and 230 keV/u. The obtained results are compared with
experimental data and other theoretical calculations. The K-K charge transfer
and K-vacancy production probabilities are also calculated for the Xe --
Xe collision.Comment: 16 pages, 4 figure
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