1,573 research outputs found
Competing interactions of spin and lattice in the Kondo lattice model
The magnetic properties of a system of coexisting localized spins and
conduction electrons are investigated within an extended version of the one
dimensional Kondo lattice model in which effects stemming from the
electron-lattice and on-site Coulomb interactions are explicitly included.
After bosonizing the conduction electrons, is it observed that intrinsic
inhomogeneities with the statistical scaling properties of a Griffiths phase
appear, and determine the spin structure of the localized impurities. The
appearance of the inhomogeneities is enhanced by appropriate phonons and acts
destructively on the spin ordering. The inhomogeneities appear on well defined
length scales, can be compared to the formation of intrinsic mesoscopic
metastable patterns which are found in two-fluid systems.Comment: 9 pages, to appear in Jour. Superconductivit
Magnetic field enhanced structural instability in EuTiO_{3}
EuTiO_{3} undergoes a structural phase transition from cubic to tetragonal at
T_S = 282 K which is not accompanied by any long range magnetic order. However,
it is related to the oxygen ocathedra rotation driven by a zone boundary
acoustic mode softening. Here we show that this displacive second order
structural phase transition can be shifted to higher temperatures by the
application of an external magnetic field (increased by 4 K for mu_{0}H = 9 T).
This observed field dependence is in agreement with theoretical predictions
based on a coupled spin-anharmonic-phonon interaction model.Comment: 4 pages, 4 figure
Polaron Coherence as Origin of the Pseudogap Phase in High Temperature Superconducting Cuprates
Within a two component approach to high Tc copper oxides including polaronic
couplings, we identify the pseudogap phase as the onset of polaron ordering.
This ordering persists in the superconducting phase. A huge isotope effect on
the pseudogap onset temperature is predicted and in agreement with experimental
data. The anomalous temperature dependence of the mean square copper oxygen ion
displacement observed above, at and below Tc stems from an s-wave
superconducting component of the order parameter, whereas a pure d-wave order
parameter alone can be excluded.Comment: 7 pages, 2 figure
Oxygen-isotope effect on the superconducting gap in the cuprate superconductor Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta}
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the zero-temperature
superconducting energy gap \Delta_0 was studied for a series of
Y_{1-x}Pr_xBa_2Cu_3O_{7-\delta} samples (0.0\leq x\leq0.45). The OIE on
\Delta_0 was found to scale with the one on the superconducting transition
temperature. These experimental results are in quantitative agreement with
predictions from a polaronic model for cuprate high-temperature superconductors
and rule out approaches based on purely electronic mechanisms.Comment: 5 pages, 3 figure
Theory of Dynamic Stripe Induced Superconductivity
Since the recently reported giant isotope effect on T* [1] could be
consistently explained within an anharmonic spin-charge-phonon interaction
model, we consider here the role played by stripe formation on the
superconducting properties within the same model. This is a two-component
scenario and we recast its basic elements into a BCS effective Hamiltonian. We
find that the stripe formation is vital to high-Tc superconductivity since it
provides the glue between the two components to enhance Tc to the unexpectedly
large values observed experimentally.Comment: 7 pages, 2 figure
Screening in (d+s)-wave superconductors: Application to Raman scattering
We study the polarization-dependent electronic Raman response of untwinned
YBaCuO superconductors employing a tight-binding band
structure with anisotropic hopping matrix parameters and a superconducting gap
with a mixing of - and s-wave symmetry. Using general arguments we find
screening terms in the B^{\}_{1g} scattering channel which are required by
gauge invariance. As a result, we obtain a small but measurable softening of
the pair-breaking peak, whose position has been attributed for a long time to
twice the superconducting gap maximum. Furthermore, we predict
superconductivity-induced changes in the phonon line shapes that could provide
a way to detect the isotropic s-wave admixture to the superconducting gap.Comment: typos corrected, 6 pages, 3 figure
Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3
EuTiO3 (ETO) has recently experienced an enormous revival of interest because
of its possible multiferroic properties which are currently in the focus of
research. Unfortunately ETO is an unlikely candidate for enlarged
multifunctionality since the mode softening - typical for ferroelectrics -
remains incomplete, and the antiferromagnetic properties appear at 5.5K only.
However, a strong coupling between lattice and Eu spins exists and leads to the
appearance of a magnon-phonon-hybrid mode at elevated temperatures as evidenced
by electron paramagnetic resonance (EPR), muon spin rotation ({\mu}SR)
experiments and model predictions based on a coupled spin-polarizability
Hamiltonian. This novel finding supports the notion of strong
magneto-dielectric (MD) effects being realized in ETO and opens new strategies
in material design and technological applications.Comment: 9 pages, 4 figure
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