44 research outputs found
Mixed state Hall effect in the multiphase superconductors
Hall effect below Tc in multiphase superconductors has been studied on
Bi-based superconductors. Samples with different relative content of 2212 and
2223 phase have been prepared. The phase content has been verified by X-ray
diffraction. Results show that while resistance and ac susceptibility is almost
insensitive to the content of 2212 phase, the qualitative behavior of the Hall
resistance is strongly influenced by the presence of both phases. Theoretical
calculation of Hall resistance has been made based on effective medium
approximation and compared with experimental results.Comment: 7 pages, 7 figure
Scaling of resistivities and guided vortex motion in MgB2 thin films
Longitudinal and transverse voltages have been measured on thin films of MgB2
with different superconducting transition widths. The study has been performed
in zero and non-zero external magnetic fields. The non-zero transverse voltage
has been observed in close vicinity of the critical temperature in zero
external magnetic field, while further away from Tc this voltage becomes zero.
In magnetic field it becomes a transverse voltage which is an even function
with respect to the direction of the field. The usual Hall voltage starts to
appear with increasing magnetic field and in large fields the even voltage
disappears and only the Hall voltage is measurable (i.e. the transverse even
voltage is suppressed with increasing magnetic field and increasing transport
current). New scaling between transverse and longitudinal resistivities has
been observed. This correlation is valid not only in the zero magnetic field
but also in nonzero magnetic field where transverse even voltage is detected.
Several models trying to explain observed results are discussed. The most
promising one seems to be guided motion of the vortices, though further
theoretical work will be required to confirm this
Transverse voltage in zero external magnetic fields, its scaling and violation of the time reversal symmetry in MgB2
The longitudinal and transverse voltages (resistances) have been measured for
MgB in zero external magnetic fields. Samples were prepared in the form of
thin film and patterned into the usual Hall bar shape. In close vicinity of the
critical temperature T non-zero transverse resistance has been observed.
Its dependence on the transport current has been also studied. New scaling
between transverse and longitudinal resistivities has been observed in the form
. Several models for explanation of the
observed transverse resistances and breaking of reciprocity theorem are
discussed. One of the most promising explanation is based on the idea of
time-reversal symmetry violation
Transverse voltage in high-Tc superconductors in zero magnetic fields
Longitudinal and transverse voltages have been measured in zero external
magnetic fields. In close vicinity of the superconducting transition nonzero
transverse voltage has been observed while far away from Tc, both above and
below no such voltage has been detected. The value of the transverse
resistivity depends on the value of the transport current. Several models have
been discussed taking into account also the penetration of self field due to
the applied transport current. It seems that observed results can be explained
using the Kosterlitz-Thouless model as a result of an unpairing of
vortex-antivortex pairs created below Tkt due to fluctuations. At Tkt free
vortices and antivortices are created and can contribute to a dissipation of
energy. Their movement should also be responsible for the observed nonzero
transverse voltage.Comment: 3 pages in Latex, 3 figs.ep
Electron magnetotransport in GaAs/AlGaAs superlattices with weak and strong inter-well coupling
We report on magnetotransport measurements in two MBE-grown GaAs/AlGaAs
superlattices formed by wide and narrow quantum wells and thin Si-doped
barriers subject to tilted magnetic fields. It has been shown that illumination
of the strongly coupled superlattice with narrow wells leads to reduction of
its dimensionality from the 3D to 2D. The illumination-induced transition is
revealed by remarkable change of magnetoresistance curves as compared to those
measured before illumination. The experimental data along with tight-binding
model calculations indicate that the illumination not only enhances the
electron concentration but also suppresses the electron tunneling through the
barriers.Comment: 3 pages, 3 figures, elsart/PHYEAUTH macros; presented on the LDSD
2007 Conference in the Caribbean Archipelago San Andres, Colombia. To be
published as a special issue of Microelectronics Journal (Elsevier
Magnetoresistance oscillations in GaAs/AlGaAs superlattices subject to in-plane magnetic fields
The MBE-grown GaAs/AlGaAs superlattice with Si-doped barriers has been used
to study a 3D-2D transition under the influence of the in-plane component of
applied magnetic field. The longitudinal magnetoresistance data measured in
tilted magnetic fields have been interpreted in terms of a simple tight-binding
model. The data provide values of basic parameters of the model and make it
possible to reconstruct the superlattice Fermi surface and to calculate the
density of states for the lowest Landau subbands. Positions of van Hove
singularities in the DOS agree excellently with magnetoresistance oscillations,
confirming that the model describes adequately the magnetoresistance of
strongly coupled semiconductor superlattices.Comment: 4 pages, 3 figures, elsart/PHYEAUTH macros; presented on the EP2DS-16
Conference in Albuquerque, New Mexico USA. To be published in Physica
In-plane Magnetic Field Dependent Magnetoresistance of Gated Asymmetric Double Quantum Wells
We have investigated experimentally the magnetoresistance of strongly
asymmetric double-wells. The structures were prepared by inserting a thin
AlGaAs barrier into the GaAs buffer layer of a standard
modulation-doped GaAs/AlGaAs heterostructure. The resulting
double-well system consists of a nearly rectangular well and of a triangular
well coupled by tunneling through the thin barrier. With a proper choice of the
barrier parameters one can control the occupancy of the two wells and of the
two lowest (bonding and antibonding) subbands. The electron properties can be
further influenced by applying front- or back-gate voltage.Comment: 4 pages, 5 figures, elsart/PHYEAUTH macros; to be presented on the
EP2DS-15 Conference in Nara, Japan. Revised version. To appear in Physica
Systematic study of Mn-doping trends in optical properties of (Ga,Mn)As
We report on a systematic study of optical properties of (Ga,Mn)As epilayers
spanning the wide range of accessible substitutional Mn_Ga dopings. The growth
and post-growth annealing procedures were optimized for each nominal Mn doping
in order to obtain films which are as close as possible to uniform
uncompensated (Ga,Mn)As mixed crystals. We observe a broad maximum in the
mid-infrared absorption spectra whose position exhibits a prevailing blue-shift
for increasing Mn-doping. In the visible range, a peak in the magnetic circular
dichroism blue shifts with increasing Mn-doping. These observed trends confirm
that disorder-broadened valence band states provide a better one-particle
representation for the electronic structure of high-doped (Ga,Mn)As with
metallic conduction than an energy spectrum assuming the Fermi level pinned in
a narrow impurity band.Comment: 22 pages, 14 figure