320 research outputs found
Temperature- and doping-dependent nanoscale Schottky barrier height at the Au/Nb:SrTiO3interface
We use ballistic electron emission microscopy to investigate prototypical Au/Nb-doped SrTiO3(NSTO) Schottky barrier diodes for different temperatures and doping levels. To this end, ultrathin Au overlayers are thermally evaporated onto TiO2-terminated NSTO single crystal substrates. We show that at room temperature, regardless of the nominal doping, rectification is controlled by a spatially inhomogeneous Schottky barrier height (SBH), which varies on a length scale of tens of nanometers according to a Gaussian distribution with a mean value of 1.29-1.34 eV and the standard deviation in the range of 80-100 meV. At lower temperatures, however, doping effects become relevant. In particular, junctions with a low Nb content of 0.01 and 0.05 wt. % show an 3c300 meV decrease in the mean SBH from room temperature to 80 K, which can be explained by an electrostatic analysis assuming a temperature-dependent dielectric permittivity for NSTO. In contrast, this model fails to predict the weaker temperature dependence of SBH for junctions based on 0.5 wt. % NSTO. Our nanoscale investigation demands to reassess conventional models for the NSTO polarizability in high-intensity electric fields. Furthermore, it contributes to the comprehension and prediction of transport in metal/SrTiO3junctions and devices
Anisotropic critical currents in FeSe0.5Te0.5 films and the influence of neutron irradiation
We report on measurements of the superconducting properties of FeSe05Te05
thin films grown on lanthanum aluminate. The films have high transition
temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T.
The temperature dependence of the upper critical field and the irreversibility
lines are steep and anisotropic, as recently reported for single crystals. The
critical current densities, assessed by magnetization measurements in a vector
VSM, were found to be well above 10^9 Am-2 at low temperatures. In all samples,
the critical current as a function of field orientation has a maximum, when the
field is oriented parallel to the film surface. The maximum indicates the
presence of correlated pinning centers. A minimum occurs in three films, when
the field is applied perpendicular to the film plane. In the forth film,
instead, a local maximum caused by c-axis correlated pinning centers was found
at this orientation. The irradiation of two films with fast neutrons did not
change the properties drastically, where a maximum enhancement of the critical
current by a factor of two was found
Highly effective and isotropic pinning in epitaxial Fe(Se,Te) thin films grown on CaF2 substrates
We report on the isotropic pinning obtained in epitaxial Fe(Se,Te) thin films
grown on CaF2 (001) substrate. High critical current density values larger than
1 MA/cm2 in self field in liquid helium are reached together with a very weak
dependence on the magnetic field and a complete isotropy. Analysis through
Transmission Electron Microscopy evidences the presence of defects looking like
lattice disorder at a very small scale, between 5 and 20 nm, which are thought
to be responsible for such isotropic behavior in contrast to what observed on
SrTiO3, where defects parallel to the c-axis enhance pinning in that directio
Tc=21K in epitaxial FeSe0.5Te0.5 thin films with biaxial compressive strain
High purity epitaxial FeSe0.5Te0.5 thin films with different thickness were
grown by Pulsed Laser Ablation on different substrates. By varying the film
thickness, Tc up to 21K were observed, significantly larger than the bulk
value. Structural analyses indicated that the a axis changes significantly with
the film thickness and is linearly related to the Tc. The latter result
indicates the important role of the compressive strain in enhancing Tc. Tc is
also related to both the Fe-(Se,Te) bond length and angle, suggesting the
possibility of further enhancement
Microscopic analysis of the chemical reaction between Fe(Te,Se) thin films and underlying CaF
To understand the chemical reaction at the interface of materials, we
performed a transmission electron microscopy (TEM) observation in four types of
Fe(Te,Se) superconducting thin films prepared on different types of substrates:
CaF2 substrate, CaF2 substrate with a CaF2 buffer layer, CaF2 substrate with a
FeSe buffer layer, and a LaAlO3 substrate with a CaF2 buffer layer. Based on
the energy-dispersive X-ray spectrometer (EDX) analysis, we found possible
interdiffusion between fluorine and selenium that has a strong influence on the
superconductivity in Fe(Te,Se) films. The chemical interdiffusion also plays a
significant role in the variation of the lattice parameters. The lattice
parameters of the Fe(Te,Se) thin films are primarily determined by the chemical
substitution of anions, and the lattice mismatch only plays a secondary role.Comment: 30 pages, 9 figur
Thermal properties of SmFeAs(O1-xFx) as probe of the interplay between electrons and phonons
A comparative study of thermal properties of SmFeAsO, SmFeAs(O0.93F0.07) and
SmFeAs(O0.85F0.15) samples is presented. Specific heat and thermal conductivity
show clear evidences of the spin density wave (SDW) ordering below TSDW 135 K
in undoped SmFeAsO. At low level of F-doping, SmFeAs(O0.93F0.07), SDW ordering
is suppressed and superconducting features are not yet optimally developed in
both specific heat and thermal conductivity. At optimal level of F-doping
SmFeAs(O0.85F0.15) anomalies related to the superconducting transition are well
noticeable. By a compared analysis of doped and undoped samples we conclude
that, despite F-doping modifies definitely the electronic ground state, it does
not substantially alter phonon and electron parameters, like phonon modes,
Sommerfeld coefficient, electro-phonon coupling. The analysis of the thermal
conductivity curves provides an evaluation of SDW and superconducting energy
gap, showing that phonons can suitably probe features of electronic ground
state
Demography of the bottlenose dolphin Tursiops truncatus (Mammalia: Delphinidae) in the Eastern Ligurian Sea (NW Mediterranean): quantification of female reproductive parameters
AbstractThe reproductive histories of 41 adult bottlenose dolphin females were analysed using photo-identification data collected between 2006 and 2014 in four sub-areas of the eastern Ligurian Sea (northwest Mediterranean). The Rapallo sub-area revealed the highest (highly significant) frequency of encounters (per unit effort) of reproductive females in association with young individuals, therefore emerging as a candidate nursery area in the region. The estimated fertility rate of adult females ranged between 290 and 407 births per 1000 individuals per year, higher than that of other known bottlenose dolphin populations, with a calving interval between 2.45 and 3.5 years. These results will be useful for projecting future trends of this (sub)population
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Temperature-dependent electric noise level in different iron-based superconductors
A detailed characterization of the voltage-noise properties has been performed in FeTe0.5Se0.5 epitaxial thin films and Co-doped BaFe2As2 bilayers, deposited by pulsed laser deposition. In all the samples analyzed, the experimental voltage-spectral density has a 1/f noise component. Different behaviors are observed for the bias current and temperature dependencies of this 1/f noise, and are related to specific structural and electric transport properties of the two materials
Influence of free charge carrier density on the magnetic behavior of (Zn,Co)O thin film studied by Field Effect modulation of magnetotransport
The origin of (ferro)magnetic ordering in transition metal doped ZnO is a still open question. For applications it is fundamental to establish if it arises from magnetically ordered impurity clusters embedded into the semiconducting matrix or if it originates from ordering of magnetic ions dilute into the host lattice. In this latter case, a reciprocal effect of the magnetic exchange on the charge carriers is expected, offering many possibilities for spintronics applications. In this paper we report on the relationship between magnetic properties and free charge density investigated by using Zinc oxide based field effect transistors, in which the charge carrier density is modulated by more than 4 order of magnitude, from 10(16) to 10(20) e(-)/cm(3). The magnetotransport properties are employed to probe the magnetic status of the channel both in pure and cobalt doped zinc oxide transistors. We find that it is widely possible to control the magnetic scattering rates by field effect. We believe that this finding is a consequence of the modulation of magnetization and carrier spin polarization by the electric field. The observed effects can be explained by the change in size of bound magnetic polarons that induces a percolation magnetic ordering in the sample
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