61,781 research outputs found
Electric field effect modulation of transition temperature, mobile carrier density and in-plane penetration depth in NdBa2Cu3O(7-delta) thin films
We explore the relationship between the critical temperature, T_c, the mobile
areal carrier density, n_2D, and the zero temperature magnetic in-plane
penetration depth, lambda_ab(0), in very thin underdoped NdBa2Cu3O{7-delta}
films near the superconductor to insulator transition using the electric field
effect technique. We observe that T_c depends linearly on both, n_2D and
lambda_ab(0), the signature of a quantum superconductor to insulator (QSI)
transition in two dimensions with znu-bar where z is the dynamic and nu-bar the
critical exponent of the in-plane correlation length.Comment: 4 pages, 4 figure
Large inverse tunneling magnetoresistance in CoCrFeAl/MgO/CoFe magnetic tunnel junctions
Magnetic tunnel junctions with the layer sequence
CoCrFeAl/MgO/CoFe were fabricated by magnetron sputtering
at room temperature (RT). The samples exhibit a large inverse tunneling
magnetoresistance (TMR) effect of up to -66% at RT. The largest value of -84%
at 20 K reflects a rather weak influence of temperature. The dependence on the
voltage drop shows an unusual behavior with two almost symmetric peaks at
mV with large inverse TMR ratios and small positive values around zero
bias
Implications of the isotope effects on the magnetization, magnetic torque and susceptibility
We analyze the magnetization, magnetic torque and susceptibility data of
La2-xSrxCu(16,18)O4 and YBa2(63,65)CuO7-x near Tc in terms of the universal
3D-XY scaling relations. It is shown that the isotope effect on Tc mirrors that
on the anisotropy. Invoking the generic behavior of the anisotropy the doping
dependence of the isotope effects on the critical properties, including Tc,
correlation lengths and magnetic penetration depths are traced back to a change
of the mobile carrier concentration.Comment: 5 pages, 3 figure
A consistent interpretation of the low temperature magneto-transport in graphite using the Slonczewski--Weiss--McClure 3D band structure calculations
Magnetotransport of natural graphite and highly oriented pyrolytic graphite
(HOPG) has been measured at mK temperatures. Quantum oscillations for both
electron and hole carriers are observed with orbital angular momentum quantum
number up to . A remarkable agreement is obtained when comparing
the data and the predictions of the Slonczewski--Weiss--McClure tight binding
model for massive fermions. No evidence for Dirac fermions is observed in the
transport data which is dominated by the crossing of the Landau bands at the
Fermi level, corresponding to , which occurs away from the point
where Dirac fermions are expected.Comment: 3 figure
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Situating multimodal learning analytics
The digital age has introduced a host of new challenges and opportunities for the learning sciences community. These challenges and opportunities are particularly abundant in multimodal learning analytics (MMLA), a research methodology that aims to extend work from Educational Data Mining (EDM) and Learning Analytics (LA) to multimodal learning environments by treating multimodal data. Recognizing the short-term opportunities and longterm challenges will help develop proof cases and identify grand challenges that will help propel the field forward. To support the field's growth, we use this paper to describe several ways that MMLA can potentially advance learning sciences research and touch upon key challenges that researchers who utilize MMLA have encountered over the past few years
Optimization of -Layer Systems for Josephson Junctions from a Microstructure Point of View
-layer systems are frequently used for Josephson junction-based
superconducting devices. Although much work has been devoted to the
optimization of the superconducting properties of these devices, systematic
studies on influence of deposition conditions combined with structural analyses
on the nanoscale are rare up to now. We have focused on the optimization of the
structural properties of -layer systems deposited on Si(111)
substrates with a particular focus on the thickness homogeneity of the
-tunnel barrier. A standard high-vacuum electron-beam deposition system
was used and the effect of substrate pretreatment, different Al-deposition
temperatures and Al-deposition rates was studied. Transmission electron
microscopy was applied to analyze the structural properties of the
-layer systems to determine the thickness homogeneity of the
layer, grain size distribution in the Al layers, Al-grain boundary
types and the morphology of the interface. We show that the
structural properties of the lower Al layer are decisive for the structural
quality of the whole -layer system. Optimum conditions yield an
epitaxial Al(111) layer on a Si(111) substrate with an Al-layer thickness
variation of only 1.6 nm over more than 10 and large lateral grain
sizes up to 1 . Thickness fluctuations of the -tunnel barrier are
minimized on such an Al layer which is essential for the homogeneity of the
tunnel current. Systematic variation of the Al-deposition rate and deposition
temperature allows to develop an understanding of the growth mechanisms
Strong lensing optical depths in a \LambdaCDM universe
We investigate strong gravitational lensing in the concordance CDM
cosmology by carrying out ray-tracing along past light cones through the
Millennium Simulation, the largest simulation of cosmic structure formation
ever carried out. We extend previous ray-tracing methods in order to take full
advantage of the large volume and the excellent spatial and mass resolution of
the simulation. As a function of source redshift we evaluate the probability
that an image will be highly magnified, will be highly elongated or will be one
of a set of multiple images. We show that such strong lensing events can almost
always be traced to a single dominant lensing object and we study the mass and
redshift distribution of these primary lenses. We fit analytic models to the
simulated dark halos in order to study how our optical depth measurements are
affected by the limited resolution of the simulation and of the lensing planes
that we construct from it. We conclude that such effects lead us to
underestimate total strong-lensing cross sections by about 15 percent. This is
smaller than the effects expected from our neglect of the baryonic components
of galaxies. Finally we investigate whether strong lensing is enhanced by
material in front of or behind the primary lens. Although strong lensing
lines-of-sight are indeed biased towards higher than average mean densities,
this additional matter typically contributes only a few percent of the total
surface density.Comment: version accepted for publicatio
Structural and magneto-transport characterization of Co_2Cr_xFe_(1-x)Al Heusler alloy films
We investigate the structure and magneto-transport properties of thin films
of the Co_2Cr_xFe_(1-x)Al full-Heusler compound, which is predicted to be a
half-metal by first-principles theoretical calculations. Thin films are
deposited by magnetron sputtering at room temperature on various substrates in
order to tune the growth from polycrystalline on thermally oxidized Si
substrates to highly textured and even epitaxial on MgO(001) substrates,
respectively. Our Heusler films are magnetically very soft and ferromagnetic
with Curie temperatures up to 630 K. The total magnetic moment is reduced
compared to the theoretical bulk value, but still comparable to values reported
for films grown at elevated temperature. Polycrystalline Heusler films combined
with MgO barriers are incorporated into magnetic tunnel junctions and yield 37%
magnetoresistance at room temperature
Influence of point defects on magnetic vortex structures
We employed micro-Hall magnetometry and micromagnetic simulations to
investigate magnetic vortex pinning at single point defects in individual
submicron-sized permalloy disks. Small ferromagnetic particles containing
artificial point defects can be fabricated by using an image reversal electron
beam lithography process. Corresponding micromagnetic calculations, modeling
the defects within the disks as holes, give reasonable agreement between
experimental and simulated pinning and depinning field values
On the occurrence of Berezinskii-Kosterlitz-Thouless behavior in highly anisotropic cuprate superconductors
The conflicting observations in the highly anisotropic Bi2Sr2CaCu2O8+x,
vidence for BKT behavior emerging from magnetization data and smeared 3D-xy
behavior, stemming form the temperature dependence of the magnetic in-plane
penetration depth are traced back to the rather small ratio, gsic+/gsic-=0.45,
between the c-axis correlation length probed above (+) and below (-) Tc, and
the comparatively large anisotropy. The latter leads to critical amplitudes
gsic0+,-which are substantially smaller than the distance between two CuO2
double layers. In combination with gsic+/gsic-=0.45 and in contrast to the
situation below Tc the c-axis correlation length gsic exceeds the distance
between two CuO2 double layers very close to Tc only. Below this narrow
temperature regime where 3D-xy fluctuations dominate, there is then an extended
temperature regime where the units with two CuO2 double layers are nearly
uncoupled so that 2D thermal fluctuations dominate and BKT features are
observable.Comment: 4 pages, 4 figure
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