1,651 research outputs found
Temperature-dependent Raman scattering of KTa1-xNbxO3 thin films
We report a Raman scattering investigation of KTa1-xNbxO3 (x = 0.35, 0.5)
thin films deposited on MgO and LaAlO3 as a function of temperature. The
observed phase sequence in the range from 90 K to 720 K is similar to the
structural phase transitions of the end-member material KNbO3. Although similar
in the phase sequence, the actual temperatures observed for phase transition
temperatures are significantly different from those observed in the literature
for bulk samples. Namely, the tetragonal (ferroelectric) to cubic
(paraelectric) phase transition is up to 50 K higher in the films when compared
to bulk samples. This enhanced ferroelectricity is attributed to biaxial strain
in the investigated thin films
Raman scattering investigation across the magnetic and MI transition in rare earth nickelate RNiO3 (R = Sm, Nd) thin films
We report a temperature-dependent Raman scattering investigation of thin film
rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a
metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic
Neel transition at TN. Our results provide evidence that all investigated
samples present a structural phase transition at TMI but the Raman signature
across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3
and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the
paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is
characterized by a pronounced softening of one particular phonon band around
420 cm-1. This signature is unusual and points to an important and continuous
change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which
stabilizes upon cooling at the magnetic transition. The observed behaviour
might well be a general feature for all rare earth nickelates with TMI =/ TN
and illustrates intriguing coupling mechanism in the TMI > T > TN regime.Comment: Revised & published versio
Interpretation of scanning tunneling quasiparticle interference and impurity states in cuprates
We apply a recently developed method combining first principles based Wannier
functions with solutions to the Bogoliubov-de Gennes equations to the problem
of interpreting STM data in cuprate superconductors. We show that the observed
images of Zn on the surface of BiSrCaCuO can only be understood
by accounting for the tails of the Cu Wannier functions, which include
significant weight on apical O sites in neighboring unit cells. This
calculation thus puts earlier crude "filter" theories on a microscopic
foundation and solves a long standing puzzle. We then study quasiparticle
interference phenomena induced by out-of-plane weak potential scatterers, and
show how patterns long observed in cuprates can be understood in terms of the
interference of Wannier functions above the surface. Our results show excellent
agreement with experiment and enable a better understanding of novel phenomena
in the cuprates via STM imaging.Comment: 5 pages, 5 figures, published version (Supplemental Material: 5
pages, 11 figures) for associated video file, see
http://itp.uni-frankfurt.de/~kreisel/QPI_BSCCO_BdG_p_W.mp
On a class of invariant coframe operators with application to gravity
Let a differential 4D-manifold with a smooth coframe field be given. Consider
the operators on it that are linear in the second order derivatives or
quadratic in the first order derivatives of the coframe, both with coefficients
that depend on the coframe variables. The paper exhibits the class of operators
that are invariant under a general change of coordinates, and, also, invariant
under the global SO(1,3)-transformation of the coframe. A general class of
field equations is constructed. We display two subclasses in it. The subclass
of field equations that are derivable from action principles by free variations
and the subclass of field equations for which spherical-symmetric solutions,
Minkowskian at infinity exist. Then, for the spherical-symmetric solutions, the
resulting metric is computed. Invoking the Geodesic Postulate, we find all the
equations that are experimentally (by the 3 classical tests) indistinguishable
from Einstein field equations. This family includes, of course, also Einstein
equations. Moreover, it is shown, explicitly, how to exhibit it. The basic tool
employed in the paper is an invariant formulation reminiscent of Cartan's
structural equations. The article sheds light on the possibilities and
limitations of the coframe gravity. It may also serve as a general procedure to
derive covariant field equations
Simulating Superconducting Properties of Overdoped Cuprates: the Role of Inhomogeneity
Theoretical studies of disordered -wave superconductors have focused, with
a few exceptions, on optimally doped models with strong scatterers. Addressing
recent controversies about the nature of the overdoped cuprates, however,
requires studies of the weaker scattering associated with dopant atoms. Here we
study simple models of such systems in the self-consistent Bogoliubov-de Gennes
(BdG) framework, and compare to disorder-averaged results using the
self-consistent-T-matrix-approximation (SCTMA). Despite surprisingly linear in
energy behavior of the low-energy density of states even for quite disordered
systems, the superfluid density in such cases retains a quadratic
low-temperature variation of the penetration depth, unlike other BdG results
reported recently. We trace the discrepancy to smaller effective system size
employed in that work. Overall, the SCTMA performs remarkably well, with the
exception of highly disordered systems with strongly suppressed superfluid
density. We explore this interesting region where gap inhomogeneity dominates
measured superconducting properties, and compare with overdoped cuprates.Comment: 12 pages, 9 figure
Existential witness extraction in classical realizability and via a negative translation
We show how to extract existential witnesses from classical proofs using
Krivine's classical realizability---where classical proofs are interpreted as
lambda-terms with the call/cc control operator. We first recall the basic
framework of classical realizability (in classical second-order arithmetic) and
show how to extend it with primitive numerals for faster computations. Then we
show how to perform witness extraction in this framework, by discussing several
techniques depending on the shape of the existential formula. In particular, we
show that in the Sigma01-case, Krivine's witness extraction method reduces to
Friedman's through a well-suited negative translation to intuitionistic
second-order arithmetic. Finally we discuss the advantages of using call/cc
rather than a negative translation, especially from the point of view of an
implementation.Comment: 52 pages. Accepted in Logical Methods for Computer Science (LMCS),
201
Effect of high pressure on multiferroic BiFeO3
We report experimental evidence for pressure instabilities in the model
multiferroic BiFeO3 and namely reveal two structural phase transitions around 3
GPa and 10 GPa by using diffraction and far-infrared spectroscopy at a
synchrotron source. The intermediate phase from 3 to 9 GPa crystallizes in a
monoclinic space group, with octahedra tilts and small cation displacements.
When the pressure is further increased the cation displacements (and thus the
polar character) of BiFeO3 is suppressed above 10 GPa. The above 10 GPa
observed non-polar orthorhombic Pnma structure is in agreement with recent
theoretical ab-initio prediction, while the intermediate monoclinic phase was
not predicted theoretically.Comment: new version, accepted for publication in Phys. Rev.
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