92 research outputs found
Quantum criticality in inter-band superconductors
In fermionic systems with different types of quasi-particles, attractive
interactions can give rise to exotic superconducting states, as pair density
wave (PDW) superconductivity and breached pairing. In the last years the search
for these new types of ground states in cold atom and in metallic systems has
been intense. In the case of metals the different quasi-particles may be the up
and down spin bands in an external magnetic field or bands arising from
distinct atomic orbitals that coexist at a common Fermi surface. These systems
present a complex phase diagram as a function of the difference between the
Fermi wave-vectors of the different bands. This can be controlled by external
means, varying the density in the two-component cold atom system or, in a
metal, by applying an external magnetic field or pressure. Here we study the
zero temperature instability of the normal system as the Fermi wave-vectors
mismatch of the quasi-particles (bands) is reduced and find a second order
quantum phase transition to a PDW superconducting state. From the nature of the
quantum critical fluctuations close to the superconducting quantum critical
point (SQCP), we obtain its dynamic critical exponent. It turns out to be
and this allows to fully characterize the SQCP for dimensions .Comment: 5 pages, 1 figur
Absence of long-range order in a spin-half Heisenberg antiferromagnet on the stacked kagome lattice
We study the ground state of a spin-half Heisenberg antiferromagnet on the
stacked kagome lattice by using a spin-rotation-invariant Green's-function
method. Since the pure two-dimensional kagome antiferromagnet is most likely a
magnetically disordered quantum spin liquid, we investigate the question
whether the coupling of kagome layers in a stacked three-dimensional system may
lead to a magnetically ordered ground state. We present spin-spin correlation
functions and correlation lengths. For comparison we apply also linear spin
wave theory. Our results provide strong evidence that the system remains
short-range ordered independent of the sign and the strength of the interlayer
coupling
Fluctuations in a superconducting quantum critical point of multi-band metals
In multi-band metals quasi-particles arising from different atomic orbitals
coexist at a common Fermi surface. Superconductivity in these materials may
appear due to interactions within a band (intra-band) or among the distinct
metallic bands (inter-band). Here we consider the suppression of
superconductivity in the intra-band case due to hybridization. The fluctuations
at the superconducting quantum critical point (SQCP) are obtained calculating
the response of the system to a fictitious space and time dependent field,
which couples to the superconducting order parameter. The appearance of
superconductivity is related to the divergence of a generalized susceptibility.
For a single band superconductor this coincides with the \textit{Thouless
criterion}. For fixed chemical potential and large hybridization, the
superconducting state has many features in common with breached pair
superconductivity with unpaired electrons at the Fermi surface. The T=0 phase
transition from the superconductor to the normal state is in the universality
class of the density-driven Bose-Einstein condensation. For fixed number of
particles and in the strong coupling limit, the system still has an instability
to the normal sate with increasing hybridization.Comment: 10 pages, 8 figure
Superlight small bipolarons
Recent angle-resolved photoemission spectroscopy (ARPES) has identified that
a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis
polarized optical phonons is important in cuprate superconductors, in agreement
with an earlier proposal by Alexandrov and Kornilovitch. The estimated
unscreened EPI is so strong that it could easily transform doped holes into
mobile lattice bipolarons in narrow-band Mott insulators such as cuprates.
Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the
total energy, effective mass, pair radius, number of phonons and isotope
exponent of lattice bipolarons in the region of parameters where any
approximation might fail taking into account the Coulomb repulsion and the
finite-range EPI. The effects of modifying the interaction range and different
lattice geometries are discussed with regards to analytical
strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be
simultaneously small and light, provided suitable conditions on the
electron-phonon and electron-electron interaction are satisfied. Such light
small bipolarons are a necessary precursor to high-temperature Bose-Einstein
condensation in solids. The light bipolaron mass is shown to be universal in
systems made of triangular plaquettes, due to a novel crab-like motion. Another
surprising result is that the triplet-singlet exchange energy is of the first
order in the hopping integral and triplet bipolarons are heavier than singlets
in certain lattice structures at variance with intuitive expectations. Finally,
we identify a range of lattices where superlight small bipolarons may be
formed, and give estimates for their masses in the anti-adiabatic
approximation.Comment: 31 pages. To appear in J. Phys.: Condens. Matter, Special Issue
'Mott's Physics
Pressure induced FFLO instability in multi-band superconductors
Multi-band systems as intermetallic and heavy fermion compounds have
quasi-particles arising from different orbitals at their Fermi surface. Since
these quasi-particles have different masses or densities, there is a natural
mismatch of the Fermi wave-vectors associated with different orbitals. This
makes these materials potential candidates to observe exotic superconducting
phases as Sarma or FFLO phases, even in the absence of an external magnetic
field. The distinct orbitals coexisting at the Fermi surface are generally
hybridized and their degree of mixing can be controlled by external pressure.
In this Communication we investigate the existence of an FFLO phase in a
two-band BCS superconductor controlled by hybridization. At zero temperature,
as hybridization (pressure) increases we find that the BCS state becomes
unstable with respect to an inhomogeneous superconducting state characterized
by a single wave-vector q.Comment: 5 pages, 1 figur
Theory of laser-induced demagnetization at high temperatures
Laser-induced demagnetization is theoretically studied by explicitly taking
into account interactions among electrons, spins and lattice. Assuming that the
demagnetization processes take place during the thermalization of the
sub-systems, the temperature dynamics is given by the energy transfer between
the thermalized interacting baths. These energy transfers are accounted for
explicitly through electron-magnons and electron-phonons interaction, which
govern the demagnetization time scale. By properly treating the spin system in
a self-consistent random phase approximation, we derive magnetization dynamic
equations for a broad range of temperature. The dependence of demagnetization
on the temperature and pumping laser intensity is calculated in detail. In
particular, we show several salient features for understanding magnetization
dynamics near the Curie temperature. While the critical slowdown in dynamics
occurs, we find that an external magnetic field can restore the fast dynamics.
We discuss the implication of the fast dynamics in the application of heat
assisted magnetic recording.Comment: 11 Pages, 7 Figure
Nonlinear spin relaxation in strongly nonequilibrium magnets
A general theory is developed for describing the nonlinear relaxation of spin
systems from a strongly nonequilibrium initial state, when, in addition, the
sample is coupled to a resonator. Such processes are characterized by nonlinear
stochastic differential equations. This makes these strongly nonequilibrium
processes principally different from the spin relaxation close to an
equilibrium state, which is represented by linear differential equations. The
consideration is based on a realistic microscopic Hamiltonian including the
Zeeman terms, dipole interactions, exchange interactions, and a single-site
anisotropy. The influence of cross correlations between several spin species is
investigated. The critically important function of coupling between the spin
system and a resonant electric circuit is emphasized. The role of all main
relaxation rates is analyzed. The phenomenon of self-organization of transition
coherence in spin motion, from the quantum chaotic stage of incoherent
fluctuations, is thoroughly described. Local spin fluctuations are found to be
the triggering cause for starting the spin relaxation from an incoherent
nonequilibrium state. The basic regimes of collective coherent spin relaxation
are studied.Comment: Latex file, 31 page
Exchange coupling in Eu monochalcogenides from first principles
Using a density functional method with explicit account for strong Coulomb
repulsion within the 4f shell, we calculate effective exchange parameters and
the corresponding ordering temperatures of the (ferro)magnetic insulating Eu
monochalcogenides (EuX; X=O,S,Se,Te) at ambient and elevated pressure
conditions. Our results provide quantitative account of the many-fold increase
of the Curie temperatures with applied pressure and reproduce well the
enhancement of the tendency toward ferromagnetic ordering across the series
from telluride to oxide, including the crossover from antiferromagnetic to
ferromagnetic ordering under pressure in EuTe and EuSe. The first and second
neighbor effective exchange are shown to follow different functional
dependencies. Finally, model calculations indicate a significant contribution
of virtual processes involving the unoccupied f states to the effective
exchange.Comment: 4 pages, 6 figure
Magnetic ground state and multiferroicity in BiMnO
We argue that the centrosymmetric symmetry in BiMnO is
spontaneously broken by antiferromagnetic (AFM) interactions existing in the
system. The true symmetry is expected to be , which is compatible with the
noncollinear magnetic ground state, where the ferromagnetic order along one
crystallographic axis coexists with the the hidden AFM order and related to it
ferroelectric polarization along two other axes. The symmetry can be
restored by the magnetic field Tesla, which switches off the
ferroelectric polarization. Our analysis is based on the solution of the
low-energy model constructed for the 3d-bands of BiMnO, where all the
parameters have been derived from the first-principles calculations. Test
calculations for isostructural BiCrO reveal an excellent agreement with
experimental data.Comment: 5 pages, 5 figure
Effects of interatomic interaction on cooperative relaxation of two-level atoms
We study effects of direct interatomic interaction on cooperative processes
in atom-photon dynamics. Using a model of two-level atoms with Ising-type
interaction as an example, it is demonstrated that interparticle interaction
combined with atom-field coupling can introduce additional interatomic
correlations acting as a phase synchronizing factor. For the case of weakly
interacting atoms with , where is the interparticle
coupling constant and is the atomic frequency, dynamical regimes of
cooperative relaxation of atoms are analyzed in Born-Markov approximation both
numerically and using the mean field approximation. We show that interparticle
correlations induced by the direct interaction result in inhibition of
incoherent spontaneous decay leading to the regime of collective pulse
relaxation which differs from superradiance in nature. For superradiant
transition, the synchronizing effect of interatomic interaction is found to
manifest itself in enhancement of superradiance. When the interaction is strong
and , one-partice one-photon transitions are excluded and
transition to the regime of multiphoton relaxation occurs. Using a simple model
of two atoms in a high-Q single mode cavity we show that such transition is
accompanied by Rabi oscillations involving many-atom multiphoton states.
Dephasing effect of dipole-dipole interaction and solitonic mechanism of
relaxation are discussed.Comment: 34 pages, 8 figure
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