522 research outputs found
Ferromagnetic and antiferromagnetic spin fluctuations and superconductivity in the hcp-phase of Fe
High purity iron, which transforms into the hcp phase under pressure, has
recently been reported to be superconducing in the pressure range 150-300 kBar
[shim]. The electronic structure and the electron-phonon coupling
() are calculated for hcp iron at different volumes. A
parameter-free theory for calculating the coupling constants
from ferromagnetic (FM) and antiferromagnetic (AFM) spin fluctuations is
developed. The calculated are sufficiently large to explain
superconductivity especially from FM fluctuations. The results indicate that
superconductivity mediated by spin fluctuations is more likely than from
electron-phonon interaction.Comment: (4 pages, 1 figure
Thermodynamics and tunneling spectroscopy in the pseudogap regime of the boson fermion model
Motivated by the STM experimental data on Bi_2 Sr_2 CaCU_2 O_{8+x} which
indicate the tunneling conductance asymmetry sigma(-V) not equal sigma(V), we
report that such a behavior can be explained in terms of the boson fermion
model. It has been shown in the recent studies, based on various selfconsistent
techniques to capture the many-body effects, that the low energy spectrum of
the boson fermion model is featured by an appearance of the pseudogap at T^* >
T_c. We argue that the pseudogap structure has to exhibit a particle-hole
asymmetry. This asymmetry may eventually depend on the boson concentration.Comment: 4 pages, 2 figures. submitted to Physica
Magnon Exchange Mechanism of Ferromagnetic Superconductivity
The magnon exchange mechanism of ferromagnetic superconductivity
(FM-superconductivity) was developed to explain in a natural way the fact that
the superconductivity in , and is confined to the
ferromagnetic phase.The order parameter is a spin anti-parallel component of a
spin-1 triplet with zero spin projection. The transverse spin fluctuations are
pair forming and the longitudinal ones are pair breaking. In the present paper,
a superconducting solution, based on the magnon exchange mechanism, is obtained
which closely matches the experiments with and . The onset of
superconductivity leads to the appearance of complicated Fermi surfaces in the
spin up and spin down momentum distribution functions. Each of them consist of
two pieces, but they are simple-connected and can be made very small by varying
the microscopic parameters. As a result, it is obtained that the specific heat
depends on the temperature linearly, at low temperature, and the coefficient
is smaller in the superconducting phase than in the
ferromagnetic one. The absence of a quantum transition from ferromagnetism to
ferromagnetic superconductivity in a weak ferromagnets and is
explained accounting for the contribution of magnon self-interaction to the
spin fluctuations' parameters. It is shown that in the presence of an external
magnetic field the system undergoes a first order quantum phase transition.Comment: 9 pages, 7 figures, accepted for publication in Phys.Rev.
Unusual condensates in quark and atomic systems
In these lectures we discuss condensates which are formed in quark matter
when it is squeezed and in a gas of fermionic atoms when it is cooled. The
behavior of these two seemingly very different systems reveals striking
similarities. In particular, in both systems the Bose-Einstein condensate to
Bardeen--Cooper-Schrieffer (BEC-BCS) crossover takes place.Comment: Lectures delivered at 8th Moscow school of Physics (33rd ITEP Winter
School of Physics
Spin fluctuations, electron-phonon coupling and superconductivity in near-magnetic elementary metals; Fe,Co,Ni and Pd
An investigation of possibilities for superconductivity mediated by spin
fluctuations in some elementary metals is motivated by the recent discovery of
superconductivity in the hcp high-pressure phase of iron.
The electronic structure, the electron-phonon coupling () and
the coupling due to spin-fluctuations () are calculated for
different phases and different volumes for four elementary metals. The results
show that such possibilities are best for systems near, but on the non-magnetic
side of, a magnetic instability. Fcc Ni, which show stable magnetism over a
wide pressure range, is not interesting in this respect. Ferro- and
antiferro-magnetic fluctuations in hcp Fe contribute to a relatively strong
coupling in the pressure range where superconductivity is observed. The absence
of fluctuations at large q-vectors makes fcc Pd only moderately interesting
despite its large exchange enhancement for q=0. Fcc Co at high pressure (
0.5 Mbar) behaves as an improved version of Pd, where the fluctuations extend
to larger q. The estimations of T, which reproduce the experimental
situation in Fe quite well, suggest a measurable T for the high-pressure
phase of fcc Co, while the estimate is lower for the ambient-pressure phase of
fcc Pd.Comment: 9 pages, 4 figures, 2 table
Nonadiabatic Landau Zener tunneling in Fe_8 molecular nanomagnets
The Landau Zener method allows to measure very small tunnel splittings \Delta
in molecular clusters Fe_8. The observed oscillations of \Delta as a function
of the magnetic field applied along the hard anisotropy axis are explained in
terms of topological quantum interference of two tunnel paths of opposite
windings. Studies of the temperature dependence of the Landau Zener transition
rate P gives access to the topological quantum interference between exited spin
levels. The influence of nuclear spins is demonstrated by comparing P of the
standard Fe_8 sample with two isotopically substituted samples. The need of a
generalized Landau Zener transition rate theory is shown.Comment: 5 pages, 6 figure
Dicke-Type Energy Level Crossings in Cavity-Induced Atom Cooling: Another Superradiant Cooling
This paper is devoted to energy-spectral analysis for the system of a
two-level atom coupled with photons in a cavity. It is shown that the
Dicke-type energy level crossings take place when the atom-cavity interaction
of the system undergoes changes between the weak coupling regime and the strong
one. Using the phenomenon of the crossings we develop the idea of
cavity-induced atom cooling proposed by the group of Ritsch, and we lay
mathematical foundations of a possible mechanism for another superradiant
cooling in addition to that proposed by Domokos and Ritsch. The process of our
superradiant cooling can function well by cavity decay and by control of the
position of the atom, at least in (mathematical) theory, even if there is
neither atomic absorption nor atomic emission of photons.Comment: 15 pages; 8 figure
Crossover from thermal hopping to quantum tunneling in Mn_{12}Ac
The crossover from thermal hopping to quantum tunneling is studied. We show
that the decay rate with dissipation can accurately be determined near
the crossover temperature. Besides considering the Wentzel-Kramers-Brillouin
(WKB) exponent, we also calculate contribution of the fluctuation modes around
the saddle point and give an extended account of a previous study of crossover
region. We deal with two dangerous fluctuation modes whose contribution can't
be calculated by the steepest descent method and show that higher order
couplings between the two dangerous modes need to be taken into considerations.
At last the crossover from thermal hopping to quantum tunneling in the
molecular magnet Mn_{12}Ac is studied.Comment: 10 pages, 3 figure
Berry's phase and Quantum Dynamics of Ferromagnetic Solitons
We study spin parity effects and the quantum propagation of solitons (Bloch
walls) in quasi-one dimensional ferromagnets. Within a coherent state path
integral approach we derive a quantum field theory for nonuniform spin
configurations. The effective action for the soliton position is shown to
contain a gauge potential due to the Berry phase and a damping term caused by
the interaction between soliton and spin waves. For temperatures below the
anisotropy gap this dissipation reduces to a pure soliton mass renormalization.
The gauge potential strongly affects the quantum dynamics of the soliton in a
periodic lattice or pinning potential. For half-integer spin, destructive
interference between soliton states of opposite chirality suppresses nearest
neighbor hopping. Thus the Brillouin zone is halved, and for small mixing of
the chiralities the dispersion reveals a surprising dynamical correlation: Two
subsequent band minima belong to different chirality states of the soliton. For
integer spin, the Berry phase is inoperative and a simple tight-binding
dispersion is obtained. Finally it is shown that external fields can be used to
interpolate continuously between the Bloch wall dispersions for half-integer
and integer spin.Comment: 20 pages, RevTex 3.0 (twocolumn), to appear in Phys. Rev. B 53, 3237
(1996), 4 PS figures available upon reques
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