70 research outputs found
Interface steps in field effect devices
The charge doped into a semiconductor in a field effect transistor (FET) is
generally confined to the interface of the semiconductor. A planar step at the
interface causes a potential drop due to the strong electric field of the FET,
which in turn is screened by the doped carriers. We analyze the dipolar
electronic structure of a single step in the Thomas-Fermi approximation and
find that the transmission coefficient through the step is exponentially
suppressed by the electric field and the induced carrier density as well as by
the step height. In addition, the field enhancement at the step edge can
facilitate the electric breakthrough of the insulating layer. We suggest that
these two effects may lead to severe problems when engineering FET devices with
very high doping. On the other hand steps can give rise to interesting physics
in superconducting FETs by forming weak links and potentially creating atomic
size Josephson junctions.Comment: 6 pages, 4 figures, submitted to J. Appl. Phy
Magnetic pair-breaking in superconducting (Ba,K)BiO_3 investigated by magnetotunneling
The de Gennes and Maki theory of gapless superconductivity for dirty
superconductors is used to interpret the tunneling measurements on the strongly
type-II high-Tc oxide-superconductor Ba1-xKxBiO3 in high magnetic fields up to
30 Tesla. We show that this theory is applicable at all temperatures and in a
wide range of magnetic fields starting from 50 percent of the upper critical
field Bc2. In this magnetic field range the measured superconducting density of
states (DOS) has the simple energy dependence as predicted by de Gennes from
which the temperature dependence of the pair-breaking parameter alpha(T), or
Bc2(T), has been obtained. The deduced temperature dependence of Bc2(T) follows
the Werthamer-Helfand-Hohenberg prediction for classical type-II
superconductors in agreement with our previous direct determination. The
amplitudes of the deviations in the DOS depend on the magnetic field via the
spatially averaged superconducting order parameter which has a square-root
dependence on the magnetic field. Finally, the second Ginzburg-Landau parameter
kappa2(T) has been determined from the experimental data.Comment: 11 pages, 5 figure
Anomalous magnetic field dependence of the thermodynamic transition line in the isotropic superconductor (K,Ba)Bi03
Thermodynamic (specific heat, reversible magnetization, tunneling
spectroscopy) and transport measurements have been performed on high quality
(K,Ba)BiO single crystals. The temperature dependence of the magnetic field
corresponding to the onset of the specific heat anomaly presents a
clear positive curvature. is significantly smaller than the field
for which the superconducting gap vanishes but is closely related to
the irreversibility line deduced from transport data. Moreover, the temperature
dependence of the reversible magnetization present a strong deviation from the
Ginzburg--Landau theory emphasazing the peculiar nature of the superconducting
transition in this material.Comment: 4 pages, 4 figures, 28 reference
Evidence for Nodal superconductivity in SrScFePO
Point contact Andreev reflection spectra have been taken as a function of
temperature and magnetic field on the polycrystalline form of the newly
discovered iron-based superconductor Sr2ScFePO3. A zero bias conductance peak
which disappears at the superconducting transition temperature, dominates all
of the spectra. Data taken in high magnetic fields show that this feature
survives until 7T at 2K and a flattening of the feature is observed in some
contacts. Here we inspect whether these observations can be interpreted within
a d-wave, or nodal order parameter framework which would be consistent with the
recent theoretical model where the height of the P in the Fe-P-Fe plane is key
to the symmetry of the superconductivity. However, in polycrystalline samples
care must be taken when examining Andreev spectra to eliminate or take into
account artefacts associated with the possible effects of Josephson junctions
and random alignment of grains.Comment: Published versio
Anisotropy of the Upper Critical Field and Critical Current in Single Crystal MgB
We report on specific heat, high magnetic field transport and
susceptibility measurements on magnesium diboride single crystals. The
upper critical field for magnetic fields perpendicular and parallel to
the Mg and B planes is presented for the first time in the entire temperature
range. A very different temperature dependence has been observed in the two
directions which yields to a temperature dependent anisotropy with 5 at low temperatures and about 2 near . A peak effect is observed
in susceptibility measurements for 2 T parallel to the axis and
the critical current density presnts a sharp maximum for parallel to the
ab-plane.Comment: 6 pages, 5 figure
The optical response of Ba_{1-x}K_xBiO_3: Evidence for an unusual coupling mechanism of superconductivity?
We have analysed optical reflectivity data for Ba_{1-x}K_xBiO_3 in the
far-infrared region using Migdal-Eliashberg theory and found it inconsistent
with standard electron-phonon coupling: Whereas the superconducting state data
could be explained using moderate coupling, \lambda=0.7, the normal state
properties indicate \lambda \le 0.2. We have found that such behaviour could be
understood using a simple model consisting of weak standard electron-phonon
coupling plus weak coupling to an unspecified high energy excitation near 0.4
eV. This model is found to be in general agreement with the reflectivity data,
except for the predicted superconducting gap size. The additional high energy
excitation suggests that the dominant coupling mechanism in Ba_{1-x}K_xBiO_3 is
not standard electron-phonon.Comment: 5 pages REVTex, 5 figures, 32 refs, accepted for publication in Phys.
Rev.
Momentum-resolved superconducting gap in the bulk of BaKFeAs from combined ARPES and SR measurements
Here we present a calculation of the temperature-dependent London penetration
depth, , in BaKFeAs (BKFA) on the basis of
the electronic band structure [1,2] and momentum-dependent superconducting gap
[3] extracted from angle-resolved photoemission spectroscopy (ARPES) data. The
results are compared to the direct measurements of by muon spin
rotation (SR) [4]. The value of , calculated with \emph{no}
adjustable parameters, equals 270 nm, while the directly measured one is 320
nm; the temperature dependence is also easily reproduced. Such
agreement between the two completely different approaches allows us to conclude
that ARPES studies of BKFA are bulk-representative. Our review of the available
experimental studies of the superconducting gap in the new iron-based
superconductors in general allows us to state that all hole-doped of them bear
two nearly isotropic gaps with coupling constants and
Suspended dry pick-up and flip-over assembly for van der Waals heterostructures with ultra-clean surfaces
Van der Waals heterostructures are an excellent platform for studying
intriguing interface phenomena, such as moir\'e and proximity effects. Surface
science techniques like scanning tunneling microscopy (STM) have proven a
powerful tool to study such heterostructures but have so far been hampered
because of their high sensitivity to surface contamination. Here, we report a
dry polymer-based assembly technique to fabricate van der Waals
heterostructures with atomically clean surfaces. The key features of our
suspended dry pick-up and flip-over technique are 1) the heterostructure
surface never comes into contact with polymers, 2) it is entirely solvent-free,
3) it is entirely performed in a glovebox, and 4) it only requires temperatures
below 130. By performing ambient atomic force microscopy and
atomically-resolved scanning tunneling microscopy on example heterostructures,
we demonstrate that we can fabricate air-sensitive heterostructures with
ultra-clean interfaces and surfaces. Due to the lack of polymer melting, the
technique is further compatible with heterostructure assembly under ultra-high
vacuum conditions, which promises ultimate heterostructure quality
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