180 research outputs found
Fingerprint of dynamical charge/spin correlations in the tunneling spectra of colossal magnetoresistive manganites
We present temperature-dependent scanning tunneling spectroscopy measurements
on () films with different degrees of biaxial
strain. A depletion in normalized conductance around the Fermi level is
observed both above and below the insulator-to-metal transition temperature
, for weakly as well as highly-strained films. This pseudogap-like
depletion globally narrows on cooling. The zero-bias conductance decreases on
cooling in the insulating phase, reaches a minimum close to and
increases on cooling in the metallic phase, following the trend of macroscopic
conductivity. These results support a recently proposed scenario in which
dynamical short-range antiferromagnetic/charge order correlations play a
preeminent role in the transport properties of colossal magnetoresistive
manganites [R. Yu \textit{et al}., Phys. Rev. B \textbf{77}, 214434 (2008)].Comment: 9 pages, 4 figure
On the Origin of the Double Superconducting Transition in Overdoped YBa2Cu3Ox
The superconducting transition in a single overdoped, detwinned YBa2Cu3Ox
(YBCO) crystal is studied using four different probes. Whereas the AC and DC
magnetic susceptibilities find a dominant transition at 88 K with a smaller
effect near 92 K, the specific heat and electrical resistivity reveal only a
single transition at 88 K and 92 K, respectively. Under hydrostatic pressures
to 0.60 GPa these two transitions shift in opposite directions, their
separation increasing. The present experiments clearly show that the bulk
transition lies at 88 K and originates from fully oxygenated YBCO; the 92 K
transition likely arises from filamentary superconductivity in a minority
optimally doped phase (< 1 %) of YBCO located at or near the crystal surface.Comment: 9 pages, 3 figure
Thermodynamic phase diagram and phase competition in BaFe2(As1-xPx)2 studied by thermal expansion
High-resolution thermal-expansion and specific-heat measurements were
performed on single crystalline BaFe2(As1-xPx)2 (0 < x < 0.33, x = 1). The
observation of clear anomalies allows to establish the thermodynamic phase
diagram which features a small coexistence region of SDW and superconductivity
with a steep rise of Tc on the underdoped side. Samples that undergo the
tetragonal-orthorhombic structural transition are detwinned in situ, and the
response of the sample length to the magneto-structural and superconducting
transitions is studied for all three crystallographic directions. It is shown
that a reduction of the magnetic order by superconductivity is reflected in all
lattice parameters. On the overdoped side, superconductivity affects the
lattice parameters in much the same way as the SDW on the underdoped side,
suggesting an intimate relation between the two types of order. Moreover, the
uniaxial pressure derivatives of Tc are calculated using the Ehrenfest relation
and are found to be large and anisotropic. A correspondence between
substitution and uniaxial pressure is established, i.e., uniaxial pressure
along the b-axis (c-axis) corresponds to a decrease (increase) of the P
content. By studying the electronic contribution to the thermal expansion we
find evidence for a maximum of the electronic density of states at optimal
doping
Superconductivity mediated by a soft phonon mode: specific heat, resistivity, thermal expansion and magnetization of YB6
The superconductor YB6 has the second highest critical temperature Tc among
the boride family MBn. We report measurements of the specific heat,
resistivity, magnetic susceptibility and thermal expansion from 2 to 300 K,
using a single crystal with Tc = 7.2 K. The superconducting gap is
characteristic of medium-strong coupling. The specific heat, resistivity and
expansivity curves are deconvolved to yield approximations of the phonon
density of states, the spectral electron-phonon scattering function and the
phonon density of states weighted by the frequency-dependent Grueneisen
parameter respectively. Lattice vibrations extend to high frequencies >100 meV,
but a dominant Einstein-like mode at ~8 meV, associated with the vibrations of
yttrium ions in oversized boron cages, appears to provide most of the
superconducting coupling and gives rise to an unusual temperature behavior of
several observable quantities. A surface critical field Hc3 is also observed.Comment: 29 pages, 5 tables, 17 figures. Accepted for publication in Phys.
Rev.
Pharmakokinetik systemischer, regionaler und lokaler Medikamente in der Therapie intraokularer Entzündungen
Zusammenfassung: In der Therapie intraokularer Entzündungen gibt es ein breites Spektrum entzündungshemmender Medikamente mit unterschiedlichen Wirkmechanismen. Die mit Abstand häufigste Anwendung finden Kortikosteroide. Die Mechanismen sind bei den meisten Medikamenten vor allem aus tierexperimentellen Studien recht gut verstanden. Schaut man sich demgegenüber die pharmakokinetische Evidenz dieser Medikamente am Auge an, so ist man überrascht, wie wenig fundierte Daten und Informationen es hierzu für das menschliche Auge im Allgemeinen und das entzündete Auge im Besonderen gibt. Denn es ist zu erwarten, dass die Bioverfügbarkeit eines Medikaments in einem entzündeten Auge durch die Schrankenstörung anders ist und schneller niedrigere intraokulare Wirkspiegel als in einem nicht entzündeten Auge durch den besseren Abtransport entstehen können. Dieser Beitrag soll einen aktuellen Überblick zu dieser Problematik und den aktuellen Kenntnisstand zunächst für Lokaltherapeutika, dann intravitreal/parabulbär verabreichte Medikamente und schließlich der immunsuppressiven Systemtherapie vermitteln
Phonon Mode Spectroscopy, Electron-Phonon Coupling and the Metal-Insulator Transition in Quasi-One-Dimensional M2Mo6Se6
We present electronic structure calculations, electrical resistivity data and
the first specific heat measurements in the normal and superconducting states
of quasi-one-dimensional M2Mo6Se6 (M = Tl, In, Rb). Rb2Mo6Se6 undergoes a
metal-insulator transition at ~170K: electronic structure calculations indicate
that this is likely to be driven by the formation of a dynamical charge density
wave. However, Tl2Mo6Se6 and In2Mo6Se6 remain metallic down to low temperature,
with superconducting transitions at Tc = 4.2K and 2.85K respectively. The
absence of any metal-insulator transition in these materials is due to a larger
in-plane bandwidth, leading to increased inter-chain hopping which suppresses
the density wave instability. Electronic heat capacity data for the
superconducting compounds reveal an exceptionally low density of states DEF =
0.055 states eV^-1 atom^-1, with BCS fits showing 2Delta/kBTc >= 5 for
Tl2Mo6Se6 and 3.5 for In2Mo6Se6. Modelling the lattice specific heat with a set
of Einstein modes, we obtain the approximate phonon density of states F(w).
Deconvolving the resistivity for the two superconductors then yields their
electron-phonon transport coupling function a^2F(w). In Tl2Mo6Se6 and
In2Mo6Se6, F(w) is dominated by an optical "guest ion" mode at ~5meV and a set
of acoustic modes from ~10-30meV. Rb2Mo6Se6 exhibits a similar spectrum;
however, the optical phonon has a lower intensity and is shifted to ~8meV.
Electrons in Tl2Mo6Se6 couple strongly to both sets of modes, whereas In2Mo6Se6
only displays significant coupling in the 10-18meV range. Although pairing is
clearly not mediated by the guest ion phonon, we believe it has a beneficial
effect on superconductivity in Tl2Mo6Se6, given its extraordinarily large
coupling strength and higher Tc compared to In2Mo6Se6.Comment: 16 pages, 13 figure
Optical and thermodynamic properties of the high-temperature superconductor HgBa_2CuO_4+delta
In- and out-of-plane optical spectra and specific heat measurements for the
single layer cuprate superconductor Hg-1201 at optimal doping (Tc = 97 K) are
presented. Both the in-plane and out-of-plane superfluid density agree well
with a recently proposed scaling relation rho_{s}=sigma_{dc}T_{c}. It is shown
that there is a superconductivity induced increase of the in-plane low
frequency spectral weight which follows the trend found in underdoped and
optimally doped Bi-2212 and optimally doped Bi-2223. We observe an increase of
optical spectral weight which corresponds to a change in kinetic energy of
approximately 0.5 meV/Cu which is more than enough to explain the condensation
energy. The specific heat anomaly is 10 times smaller than in YBCO and 3 times
smaller than in Bi-2212. The shape of the anomaly is similar to the one
observed in YBCO showing that the superconducting transition is governed by
thermal fluctuations.Comment: 11 pages, 13 figure
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