63 research outputs found
Highly mobile carriers in orthorhombic phases of iron-based superconductors FeSeS
The field and temperature dependencies of the longitudinal and Hall
resistivity have been measured for FeSeS (x=0.04, 0.09 and
0.19) single crystals. The sample FeSeS does not show a
transition to an orthorhombic phase and exhibits at low temperatures the
transport properties quite different from those of orthorhombic samples. The
behavior of FeSeS is well described by the simple two
band model with comparable values of hole and electron mobility. In particular,
at low temperatures the transverse resistance shows a linear field dependence,
the magnetoresistance follow a quadratic field dependence and obeys to Kohler's
rule. In contrast, Kohler's rule is strongly violated for samples having an
orthorhombic low temperature structure. However, the transport properties of
the orthorhombic samples can be satisfactory described by the three band model
with the pair of almost equivalent to the tetragonal sample hole and electron
bands, supplemented with the highly mobile electron band which has two order
smaller carrier number. Therefore, the peculiarity of the low temperature
transport properties of the orthorhombic Fe(SeS) samples, as probably of many
other orthorhombic iron superconductors, is due to the presence of a small
number of highly mobile carriers which originate from the local regions of the
Fermi surface, presumably, nearby the Van Hove singularity points
Majority carrier type inversion in FeSe family and "doped semimetal" scheme in iron-based superconductors
The field and temperature dependencies of the longitudinal and Hall
resistivity have been studied for high-quality FeSeS (x up to
0.14) single crystals. Quasiclassical analysis of the obtained data indicates a
strong variation of the electron and hole concentrations under the studied
isovalent substitution and proximity of FeSe to the point of the majority
carrier-type inversion. On this basis, we propose a `doped semimetal' scheme
for the superconducting phase diagram of the FeSe family, which can be applied
to other iron-based superconductors. In this scheme, the two local maxima of
the superconducting temperature can be associated with the Van Hove
singularities of a simplified semi-metallic electronic structure. The
multicarrier analysis of the experimental data also reveals the presence of a
tiny and highly mobile electron band for all the samples studied. Sulfur
substitution in the studied range leads to a decrease in the number of mobile
electrons by more than ten times, from about 3\% to about 0.2\%. This behavior
may indicate a successive change of the Fermi level position relative to
singular points of the electronic structure which is consistent with the `doped
semimetal' scheme. The scattering time for mobile carriers does not depend on
impurities, which allows us to consider this group as a possible source of
unusual acoustic properties of FeSe
Magnetotransport properties of FeSe in fields up to 50T
Magnetotransport properties of the high-quality FeSe crystal, measured in a
wide temperature range and in magnetic fields up to 50 T, show the symmetry of
the main holelike and electronlike bands in this compound. In addition to the
main two bands, there is also a tiny, highly mobile, electronlike band which is
responsible for the non-linear behavior of (B) at low temperatures
and some other peculiarities of FeSe. We observe the inversion of the
temperature coeficient at a magnetic field higher than about 20 T
which is an implicit conformation of the electron-hole symmetry in the main
bands.Comment: MISM 201
Superconductivity, Electron Paramagnetic Resonance, and Raman Scattering Studies of Heterofullerides with Cs and Mg
In the present study, the results of investigation of physical properties of heterofullerides A3−xMxC60 (A=K, Rb, Cs, M=Be, Mg, Ca, Al, Fe, Tl, x=1,2); as well as RbCsTlC60, KCsTlC60, and KMg2C60 are described. All of the fullerides were synthesized by the exchange reactions of alkaline fullerides with anhydrous metal halides. Superconductivity was found in RbCsTlC60 and KCsTlC60
Sn delta-doping in GaAs
We have prepared a number of GaAs structures delta-doped by Sn using the
well-known molecular beam epitaxy growth technique. The samples obtained for a
wide range of Sn doping densities were characterised by magnetotransport
experiments at low temperatures and in high magnetic fields up to 38 T.
Hall-effect and Shubnikov-de Haas measurements show that the electron densities
reached are higher than for other delta-dopants, like Si and Be. The maximum
carrier density determined by the Hall effect equals 8.4x10^13 cm^-2. For all
samples several Shubnikov-de Haas frequencies were observed, indicating the
population of multiple subbands. The depopulation fields of the subbands were
determined by measuring the magnetoresistance with the magnetic field in the
plane of the delta-layer. The experimental results are in good agreement with
selfconsistent bandstructure calculations. These calculation shows that in the
sample with the highest electron density also the conduction band at the L
point is populated.Comment: 11 pages text (ps), 9 figures (ps), submitted to Semicon. Science
Tech
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Evidence of improvement in thermoelectric parameters of n-type Bi2Te3/graphite nanocomposite
Improvement of thermoelectric parameters is reported with graphite incorporation in n-type
Bi2Te3/graphite nanocomposite system. In-depth thermoelectric properties of nanostructured
Bi2Te3/graphite composites are probed both microscopically and macroscopically using X-ray
diffraction, Raman spectroscopy, inelastic neutron scattering and measurement of the temperature
dependence of thermal conductivity , Seebeck coefficient S, resistivity ρ, and carrier concentration
nH. Raman spectroscopic analysis confirms that graphite introduces defects and disorder in the
system. Graphite addition induces a large (17%) decrease of , originating from a strong phonon
scattering effect. A low lattice thermal conductivities L, value of 0.77 Wm-1K-1, approaching the
min value, estimated using the Cahill-Pohl model, is reported for Bi2Te3+1 wt% graphite sample.
Graphite dispersion alters the low energy inelastic neutron scattering spectrum providing evidence
for modification of the Bi2Te3 Phonon Density of States (PDOS). Improvement of the other
thermoelectric parameters, viz., Seebeck Coefficient and resistivity, is also reported. Theoretical
modeling of electrical and thermal transport parameters is carried out and a plausible explanation of
the underlying transport mechanism is provided assuming a simple model of ballistic electron
transport in 1D contact channels with two different energies
Electron mobility in selectively doped GaAs/Inx Ga1−xAs multiple quantum well structures
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