709 research outputs found
Detailed studies of non-linear magneto-optical resonances at D1 excitation of Rb-85 and Rb-87 for partially resolved hyperfine F-levels
Experimental signals of non-linear magneto-optical resonances at D1
excitation of natural rubidium in a vapor cell have been obtained and described
with experimental accuracy by a detailed theoretical model based on the optical
Bloch equations. The D1 transition of rubidium is a challenging system to
analyze theoretically because it contains transitions that are only partially
resolved under Doppler broadening. The theoretical model took into account all
nearby transitions, the coherence properties of the exciting laser radiation,
and the mixing of magnetic sublevels in an external magnetic field and also
included averaging over the Doppler profile. Great care was taken to obtain
accurate experimental signals and avoid systematic errors. The experimental
signals were reproduced very well at each hyperfine transition and over a wide
range of laser power densities, beam diameters, and laser detunings from the
exact transition frequency. The bright resonance expected at the F_g=1 -->
F_e=2 transition of Rb-87 has been observed. A bright resonance was observed at
the F_g=2 --> F_e=3 transition of Rb-85, but displaced from the exact position
of the transition due to the influence of the nearby F_g=2 --> F_e=2
transition, which is a dark resonance whose contrast is almost two orders of
magnitude larger than the contrast of the bright resonance at the F_g=2 -->
F_e=3 transition. Even in this very delicate situation, the theoretical model
described in detail the experimental signals at different laser detunings.Comment: 11 pages, 9 figure
Ensemble v-representable ab-initio density functional calculation of energy and spin in atoms: atest of exchange-correlation approximations
The total energies and the spin states for atoms and their first ions with Z
= 1-86 are calculated within the the local spin-density approximation (LSDA)
and the generalized-gradient approximation (GGA) to the exchange-correlation
(xc) energy in density-functional theory. Atoms and ions for which the
ground-state density is not pure-state v-representable, are treated as ensemble
v- representable with fractional occupations of the Kohn-Sham system. A newly
developed algorithm which searches over ensemble v-representable densities [E.
Kraisler et al., Phys. Rev. A 80, 032115 (2009)] is employed in calculations.
It is found that for many atoms the ionization energies obtained with the GGA
are only modestly improved with respect to experimental data, as compared to
the LSDA. However, even in those groups of atoms where the improvement is
systematic, there remains a non-negligible difference with respect to the
experiment. The ab-initio electronic configuration in the Kohn-Sham reference
system does not always equal the configuration obtained from the spectroscopic
term within the independent-electron approximation. It was shown that use of
the latter configuration can prevent the energy-minimization process from
converging to the global minimum, e.g. in lanthanides. The spin values
calculated ab-initio fit the experiment for most atoms and are almost
unaffected by the choice of the xc-functional. Among the systems with
incorrectly obtained spin there exist some cases (e.g. V, Pt) for which the
result is found to be stable with respect to small variations in the
xc-approximation. These findings suggest a necessity for a significant
modification of the exchange-correlation functional, probably of a non-local
nature, to accurately describe such systems. PACS numbers: 31.15.
Current-induced magnetization dynamics in disordered itinerant ferromagnets
Current-driven magnetization dynamics in ferromagnetic metals are studied in
a self-consistent adiabatic local-density approximation in the presence of
spin-conserving and spin-dephasing impurity scattering. Based on a quantum
kinetic equation, we derive Gilbert damping and spin-transfer torques entering
the Landau-Lifshitz equation to linear order in frequency and wave vector.
Gilbert damping and a current-driven dissipative torque scale identically and
compete, with the result that a steady current-driven domain-wall motion is
insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform
magnetization is found to be much more stable against spin torques in the
itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic
spin-transfer torque reminiscent of the spin pumping in multilayers is
identified and shown to govern the current-induced domain-wall distortion
Structural Properties and Relative Stability of (Meta)Stable Ordered, Partially-ordered and Disordered Al-Li Alloy Phases
We resolve issues that have plagued reliable prediction of relative phase
stability for solid-solutions and compounds. Due to its commercially important
phase diagram, we showcase Al-Li system because historically density-functional
theory (DFT) results show large scatter and limited success in predicting the
structural properties and stability of solid-solutions relative to ordered
compounds. Using recent advances in an optimal basis-set representation of the
topology of electronic charge density (and, hence, atomic size), we present DFT
results that agree reasonably well with all known experimental data for the
structural properties and formation energies of ordered, off-stoichiometric
partially-ordered and disordered alloys, opening the way for reliable study in
complex alloys.Comment: 7 pages, 2 figures, 2 Table
Optical properties and electronic structure of β′−NiAl
The optical constants and their temperature derivatives have been determined for β′−NiAl from absorption and thermoreflectance measurements in the energy range of 0.2-4.4 eV. The results are interpreted using the self-consistent energy bands of Moruzzi, Williams, and Gelatt. By comparing a calculated joint density of states with ε2, the imaginary part of the dielectric function, good overall agreement is found between theory and experiment. In contrast to earlier analyses, it is found that the 2.5-eV peak in ε2 is primarily due to direct interband transitions terminating near the Fermi surface. This new interpretation of the 2.5-eV feature is discussed in relation to previously reported concentration effects and the rigid-band model
Spin Screening and Antiscreening in a Ferromagnet/Superconductor Heterojunction
We present a theoretical study of spin screening effects in a
ferromagnet/superconductor (F/S) heterojunction. It is shown that the magnetic
moment of the ferromagnet is screened or antiscreened, depending on the
polarization of the electrons at the Fermi level. If the polarization is
determined by the electrons of the majority (minority) spin band then the
magnetic moment of the ferromagnet is screened (antiscreened) by the electrons
in the superconductor. We propose experiments that may confirm our theory: for
ferromagnetic alloys with certain concentration of Fe or Ni ions there will be
screening or antiscreening respectively. Different configurations for the
density of states are also discussed.Comment: 5 pages; 4 figures. to be published in Phys. Rev,
Four-terminal resistance of an interacting quantum wire with weakly invasive contacts
We analyze the behavior of the four-terminal resistance, relative to the
two-terminal resistance of an interacting quantum wire with an impurity, taking
into account the invasiveness of the voltage probes. We consider a
one-dimensional Luttinger model of spinless fermions for the wire. We treat the
coupling to the voltage probes perturbatively, within the framework of
non-equilibrium Green function techniques. Our investigation unveils the
combined effect of impurities, electron-electron interactions and invasiveness
of the probes on the possible occurrence of negative resistance.Comment: 10 pages, 7 figure
Spin-orbit coupling in ferromagnetic Nickel
We use the Gutzwiller variational theory to investigate the electronic and
the magnetic properties of fcc-Nickel. Our particular focus is on the effects
of the spin-orbit coupling. Unlike standard relativistic band-structure
theories, we reproduce the experimental magnetic moment direction and we
explain the change of the Fermi-surface topology that occurs when the magnetic
moment direction is rotated by an external magnetic field. The Fermi surface in
our calculation deviates from early de-Haas--van-Alphen (dHvA) results. We
attribute these discrepancies to an incorrect interpretation of the raw dHvA
data.Comment: 4 pages, 3 figures, submitted to PR
About the strength of correlation effects in the electronic structure of iron
The strength of electronic correlation effects in the spin-dependent
electronic structure of ferromagnetic bcc Fe(110) has been investigated by
means of spin and angle-resolved photoemission spectroscopy. The experimental
results are compared to theoretical calculations within the three-body
scattering approximation and within the dynamical mean-field theory, together
with one-step model calculations of the photoemission process. This comparison
indicates that the present state of the art many-body calculations, although
improving the description of correlation effects in Fe, give too small mass
renormalizations and scattering rates thus demanding more refined many-body
theories including non-local fluctuations.Comment: 4 pages, 4 figure
Geometric, electronic, and magnetic structure of CoFeSi: Curie temperature and magnetic moment measurements and calculations
In this work a simple concept was used for a systematic search for new
materials with high spin polarization. It is based on two semi-empirical
models. Firstly, the Slater-Pauling rule was used for estimation of the
magnetic moment. This model is well supported by electronic structure
calculations. The second model was found particularly for Co based Heusler
compounds when comparing their magnetic properties. It turned out that these
compounds exhibit seemingly a linear dependence of the Curie temperature as
function of the magnetic moment. Stimulated by these models, CoFeSi was
revisited. The compound was investigated in detail concerning its geometrical
and magnetic structure by means of X-ray diffraction, X-ray absorption and
M\"o\ss bauer spectroscopies as well as high and low temperature magnetometry.
The measurements revealed that it is, currently, the material with the highest
magnetic moment () and Curie-temperature (1100K) in the classes of
Heusler compounds as well as half-metallic ferromagnets. The experimental
findings are supported by detailed electronic structure calculations
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