461 research outputs found
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
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
Superconducting properties of sulfur-doped iron selenide
The recent discovery of high-temperature superconductivity in single-layer
iron selenide has generated significant experimental interest for optimizing
the superconducting properties of iron-based superconductors through the
lattice modification. For simulating the similar effect by changing the
chemical composition due to S doping, we investigate the superconducting
properties of high-quality single crystals of FeSeS (=0, 0.04,
0.09, and 0.11) using magnetization, resistivity, the London penetration depth,
and low temperature specific heat measurements. We show that the introduction
of S to FeSe enhances the superconducting transition temperature ,
anisotropy, upper critical field , and critical current density
. The upper critical field and its anisotropy are strongly
temperature dependent, indicating a multiband superconductivity in this system.
Through the measurements and analysis of the London penetration depth and specific heat, we show clear evidence for strong coupling two-gap
-wave superconductivity. The temperature-dependence of
calculated from the lower critical field and electronic specific heat can be
well described by using a two-band model with -wave-like gaps. We find that
a -wave and single-gap BCS theory under the weak-coupling approach can not
describe our experiments. The change of specific heat induced by the magnetic
field can be understood only in terms of multiband superconductivity.Comment: 13 pages, 7 figure
Nanostructuring and surface hardening of structural steels by ultrasonic impact-frictional treatment
With two structural steels (the steels 50 and 09G2S) as examples, the paper studies the effectiveness of a new method of ultrasonic impact-frictional treatment (UIFT) for the hardening and nanostructuring of the surface layer with the variation of the tilt angle of the vibrating indenter and the treatment environment. It is demonstrated that treatment with tool tilt angles different from 90° and with the absence of a contact liquid results in the formation of a nanostructured surface layer with increased microhardness. © 2018 Author(s)
Synthesis and characterization of the new high pressure phases A Cu 3 v 4O 12 (A =Gd, Tb, Er)
New ACu3V4O12 (A=Gd, Tb, Er) phases have been prepared at high pressure and high-temperature conditions (P∼8-9 GPa, T∼1000°C) in a toroid-type high pressure cell. These compounds crystallize in the cubic symmetry with a perovskite-like structure. At ambient pressure, they are paramagnetic and have activation-type conductivity. The effect of high pressure (10-50 GPa) on the electrical properties of the materials was analyzed in the temperature range from 78 to 300 K. Pressure ranges of the transition from activation type to metallic conductivity have been determined. The crystal structure of ACu3V4O12 (A=Gd, Tb, Er) was found to be stable up to 50 GPa. © 2013 Copyright Taylor and Francis Group, LLC
Technical Review of Robotic Complexes for Underground Mining
The paper contains classifies robots for work in mines, the tasks they perform, compares developments in this area with a description of the difficulties and solutions which have found. Social and economic difficulties that often hamper the process of automation and robotization of underground mining are given. © 2020 Published under licence by IOP Publishing Ltd
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