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 $\rho_{xy}$(B) at low temperatures and some other peculiarities of FeSe. We observe the inversion of the $\rho_{xx}$ 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 FeSe1−x{}_{1-x}Sx{}_{x}

The field and temperature dependencies of the longitudinal and Hall resistivity have been measured for FeSe${}_{1-x}$S${}_{x}$ (x=0.04, 0.09 and 0.19) single crystals. The sample FeSe${}_{0.81}$S${}_{0.19}$ 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 FeSe${}_{0.81}$S${}_{0.19}$ 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 FeSe${}_{1-x}$S${}_{x}$ (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

Multiband effect in elastoresistance of Fe(Se,Te)

We have investigated the elastoresistance of two FeSe${}_{1-x}$Te${}_{x}$ (x about 0.4 - 0.5) compounds that have a close chemical composition but differ significantly in electronic properties. The first compound has a negative temperature coefficient of resistance and does not show any phase transitions other than superconducting. The elastoresistance of this compound approximately follows $1/T$ low as it usually occurs in Fe(Se,S) with metallic conductivity. The second compound has a metallic type of conductivity and in addition to the superconducting transition, there is also a phase transition at a temperature of about 30 K. The elastoresistance of the second compound is sign-reversing and can be approximated with the sum of two Curie-Weiss type terms with opposite signs and different critical temperatures which suggest a competition of contributions to the elastoresistance from different band valleys

Magnetotransport properties of FeSe in fields up to 50 T

© 2017 Elsevier B.V. A study of the magnetotransport properties of a high-quality FeSe crystal in a wide temperature range and in magnetic fields up to 50 T shows that the main electron-like and hole-like bands have very similar values of carrier density and mobility, indicating good electron-hole symmetry in this compound. In addition to the main two bands, there is also a tiny, highly mobile, electron-like band which is responsible for the non-linear behavior of ρxy(B) at low temperatures and some other peculiarities of FeSe. We observe the inversion of the ρxx temperature coefficient at a magnetic field higher than about 20 T which is an implicit confirmation of the electron-hole symmetry in the main bands

Highly mobile carriers in iron-based superconductors

© 2017 IOP Publishing Ltd Printed in the UK.The field and temperature dependencies of the resistivity and Hall effect are measured for FeSe1-xSx (x = 0.04, 0.09, and 0.19) single crystals. Sample FeSe0.81S0.19 does not show a transition to an orthorhombic phase and at low temperatures exhibits transport properties, which are very different from those of orthorhombic samples. The behavior of FeSe0.81S0.19 is well described by the simple two-band model with comparable values of the hole and electron mobilities. The characteristics of the low-temperature transport properties of the orthorhombic Fe(SeS) samples are largely determined by the presence of a small number of highly mobile carriers, which may originate from the local regions of the Fermi surface, presumably, nearby the Van Hove singularity points. Our results, for the first time, demonstrate a strong evolution of a tiny band of highly mobile electrons at a tetragonal to orthorhombic quantum phase transition. The behavior of this band can be the reason for the diverging nematic susceptibility, determined from elastoresistivity, which is considered one of the most intriguing phenomena in the physics of iron-based superconductors

Multiband Effect in Elastoresistance of Fe(Se,Te)

We have investigated the elastoresistance of two compounds that have a close chemical composition but differ significantly in electronic properties. The first compound has a negative temperature coefficient of resistance and does not show any phase transitions other than a superconducting one. The elastoresistance of this compound approximately follows the law which is a special case of the Curie-Weiss law, which is usually observed for Fe(Se,S) with metallic conductivity. The second compound has a metallic type of conductivity and, in addition to the superconducting transition, there is also a phase transition at a temperature of about 30 K. The elastoresistance of the second compound is sign-reversing and can be approximated with the sum of two Curie-Weiss-type terms with opposite signs and different critical temperatures. We attribute this behavior to the competition of contributions to the elastoresistance from different band valleys. These competing contributions may appear since the composition of our compound is close to the critical point at which the low-temperature ground state in the 11 series of iron-based superconductors changes from electronic nematic order to magnetic order. © Copyright2020 EPLA.This work was supported in part from the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST “MISiS” (K2-2020-008) and by Act 211 of the Government of Russian Federation, agreements 02.A03.21.0004 and 02.A03.21.0006 and by the Program of Competitive Growth of Kazan Federal University. We acknowledge support from Russian Foundation for Basic Research (Grants 20-02-00561 and ofi-m 17-29-10007) and Russian Science Foundation (Grant 19-42-02010)

Crystal growth, transport phenomena and two-gap superconductivity in the mixed alkali metal (K1-zNaz)xFe 2-ySe2 iron selenide

Using the self-flux technique we grew superconducting (K 1-zNaz)xFe2-ySe2 (z = 0.3) single crystals. EDX mapping revealed the uniform elements distribution on the crystal surface while XRD measurements indicate that the crystals are compositionally inhomogeneous on the nanoscale. The physical properties of the as-prepared sample are characterized by electrical resistivity, magnetization and specific heat measurements. Resistivity measurements show the onset of the superconducting transition at 33 K and zero resistivity at 31.7 K. The large upper critical field Hc2(0) was estimated as high as about 140 T for the in-plane field and 38 T for the out-of-plane field. The anisotropy of Habc2(0)/Hcc2(0) and coherence lengths ξab(0)/ξc(0) was found to be around 3.7. The pioneering studies by multiple Andreev reflections effect spectroscopy ("break-junction" technique) revealed the presence of two anisotropic superconducting gaps ΔL = (9.3 ± 1.5) meV, ΔS = (1.9 ± 0.4) meV, and provided a measurement of the ΔL(T) temperature dependence. The Bardeen-Cooper-Schrieffer (BCS) theory ratio ratio for the large gap 2ΔL/kBTbulkc ≈ 6.3 points to a strong electron-boson coupling in the "driving" condensate characterized by the ΔL order parameter. This journal is © the Partner Organisations 2014