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A characteristic feature of the electronic structure of iron-based supercon-ductors is the shift of experimental electronic bands in comparison to the results of calculations. The temperature dependence of the band structure for FeSe can manifest the mechanism of such shifts, but different studies give opposite directions for these shifts in the centre of the Brillouin zone. In this paper, we report downward shift of both dxz and dyz bands in Z point within the temperature range 20160 K. Together with the results of evolution of the electronic structure in A point, such shifts should lead to a break of parity between electron and hole charge carriers that can be interpreted as an increase of electron-carrier density with increasing temperature. © 2018 G.V. Kurdyumov Institute for Metal Physics of N.A.S. of Ukraine. All rights reserved. ; This study was supported by the Ukrainian–German grant from the MES of Ukraine (Project M/20-2017) and RFBR grant No. 16-05-00938 supported crystal growth experiments. ; The work of D.A.Ch. was supported by the program 211 of the Russian Federation Government (agreement No. 02.A03.21.0006) and by the Russian Government Program of Competitive Growth of Kazan Federal University.

The binary synthetic compounds of Pt with chalcogens (O, S, Se, Te), pnictogens (As, Sb, Bi), and intermetallic compounds with Ga, In, and Sn of various stoichiometry were studied via X-ray absorption spectroscopy (XAS). The partial atomic charges of Pt in the compounds were computed using quantum chemical density functional theory (DFT) based methods: the Bader (QTAIM) method, and the density-derived electrostatic and chemical (DDEC6) approach. Strong positive correlations were established between the calculated partial atomic charges of Pt and the electronegativity (χ) of ligands. The partial charge of Pt in PtL2 compounds increases much sharply when the ligand electronegativity increases than the Pt partial charge in PtL compounds. The effect of the ligand-to-Pt atomic ratio on the calculated Pt partial charge depended on ligand electronegativity. The DDEC6 charge of Pt increases sharply with the growth of the number of ligands in PtSn (n = 1, 2; electronegativity χ(S) » χ(Pt)), weakly depends on the phase composition in PtTen (n = 1, 2; χ(Te) is slightly lower than χ(Pt)), and decreases (becomes more negative) with increase of the ligand-to-Pt ratio in intermetallic compounds with electron donors (χ(L) χ(L)) was overcompensated by the gain of the hybridized s-p electron density, which was confirmed by Pt L1-edge spectra analysis. As a result, the total electron density at the Pt site followed the electronegativity rule, i.e., it increased with the growth of the number of the ligands-electron donors. The empirical correlations between the Pt partial atomic charges and parameters of XANES spectral features were used to identify the state of Pt in pyrite, and can be applied to determine the state of Pt in other ore minerals. © 2021 by the authors. Licensee MDPI, Basel, Switzerland. ; This study was supported by RFBR grant No. 20-35-70049, the program 211 of the Russian Federation Government, agreement No. 02.A03.21.0006, by the Russian Government Program of Competitive Growth of Kazan Federal University, and by Ministry of Education and Science of the Russian Federation grant No. 075-15-2020-802.

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).

Due to their physical properties and potential applications in energy conversion and storage, transition-metal dichalcogenides (TMDs) have garnered substantial interest in recent years. Among this class of materials, TMDs based on molybdenum, tungsten, sulfur, and selenium are particularly attractive due to their semiconducting properties and the availability of bottom-up synthesis techniques. Here we report a method which yields high-quality crystals of transition-metal diselenide and ditelluride compounds (PtTe2, PdTe2, NiTe2, TaTe2, TiTe2, RuTe2, PtSe2, PdSe2, NbSe2, TiSe2, VSe2, ReSe2) from their solid solutions, via vapor deposition from a metal-saturated chalcogen melt. Additionally, we show the synthesis of rare-earth-metal polychalcogenides and NbS2 crystals using the aforementioned process. Most of the crystals obtained have a layered CdI2 structure. We have investigated the physical properties of selected crystals and compared them to state of the art findings reported in the literature. Remarkably, the charge density wave transition in 1T-TiSe2 and 2H-NbSe2 crystals is well-defined at TCDW ≈ 200 and 33 K, respectively. Angle-resolved photoelectron spectroscopy and electron diffraction are used to directly access the electronic and crystal structures of PtTe2 single crystals and yield state of the art measurements. © 2020 American Chemical Society. ; M.A.-H. acknowledges support from the VR starting grant 2018-05339 and KL1824/6. The crystal growth experiments were supported by the Russian Science Foundation, Project 19-12-00414. The work has been supported by the program 211 of the Russian Federation Government agreements 02.A03.21.0006 and 02.A03.21.0011, by the Russian Government Program of Competitive Growth of Kazan Federal University. We acknowledge MAX IV Laboratory for time on Beamline Bloch under Proposal 20190335. Research conducted at MAX IV, a Swedish national user facility, is supported by the Swedish Research council under contract 2018-07152 the Swedish Governmental Agency for Innovation Systems under contract 2018-04969, and Formas under contract 2019-02496. We acknowledge ARPES experiment support from Craig Polley (MAX IV), Maciej Dendzik (KTH) Antonija Grubisic-Cabo (KTH) and Oscar Tjernberg (KTH). H.R., D.P. and G.J.M. acknowledge the Swedish Research Council (2018-06465, 2018-04330) and the Swedish Energy Agency (P43549-1) for financial support.

Fluctuations of various order parameters are considered as the significant components of understanding the mechanism of high-Tc superconductivity. To study these fluctuations in the cuprate and Fe-based superconductors we use the joint measurements of direct current resistivity and non-resonant microwave absorption. Comparing data obtained with both methods allowed us to extract the contribution of dynamic charge density waves in La2−xSrxCuO4, superconducting fluctuations in Bi2Sr2Ca1−xYxCu2O8 and nematic fluctuations in FeTe1−xSex. © Kazan Federal University (KFU). ; Government Council on Grants, Russian Federation ; Government Council on Grants, Russian Federation ; Russian Academy of Sciences, RAS ; Kazan Federal University ; This work was supported by the Russian Academy of Sciences via the grant of Program 1.12 "Fundamental Problems of High-Temperature Superconductivity". The work of D.Ch. is supported by the program 211 of the Russian Federation Government, agreement No. 02.A03.21.0006 and by the Russian Government Program of Competitive Growth of Kazan Federal University. The work of A.V. has been supported by Act 211 of the Government of Russian Federation, Contracts No. 02.A03.21.0006 and No. 02.A03.21.0011.

We present measurements of the magnetic torque, specific heat and thermal expansion of the bulk transition metal dichalcogenide (TMD) superconductor NbS2 in high magnetic fields, with its layer structure aligned strictly parallel to the field using a piezo rotary positioner. The upper critical field of superconducting TMDs in the 2D form is known to be dramatically enhanced by a special form of Ising spin orbit coupling. This Ising superconductivity is very robust to the Pauli paramagnetic effect and can therefore exist beyond the Pauli limit for superconductivity. We find that superconductivity beyond the Pauli limit still exists in bulk single crystals of NbS2 for a precisely parallel field alignment. However, the comparison of our upper critical field transition line with numerical simulations rather points to the development of a Fulde-Ferrell-Larkin-Ovchinnikov state above the Pauli limit as a cause. This is also consistent with the observation of a magnetic field driven phase transition in the thermodynamic quantities within the superconducting state near the Pauli limit. © 2021, The Author(s). ; We thank U. Lampe for technical support. This work was supported by grants from the Research Grants Council of the Hong Kong Special Administrative Region, China (GRF-16302018, GRF-16303820, C6025-19G-A, SBI17SC14). M.A.-H. acknowledges the financial support from the Swedish Research Council (VR) under the project no. 2018-05393 and by the Arab-German Young Academy of Sciences and Humanities (AGYA). M.A.-H and D.C. acknowledge the financial support by the Megagrant 2020-220-08-6358 of the Government Council on Grants of the Russian Federation.

Utilizing a novel method with the resonance frequency of a LC circuit, we measured the superconducting anisotropy of single crystals of an Fe-based superconductor FeSe with applied magnetic field up to 16 T. We found that the temperature dependence of the upper critical field H c2 (T) of FeSe coincides with the Werthamer-Helfand-Hohenberg (WHH) model when taking the Maki parameter α into consideration, suggesting an important role played by spin-paramagnetic effect in suppressing the superconductivity. When temperature T → 0, the values of H c2,≈c (0) and H c2,≈ab (0) derived from the WHH fitting are close to and fall within the range of the Pauli limit, for field H 0 applied parallel to the c-axis and to the ab-plane, respectively. As compared with other typical iron-based high-T c superconductors, lower values of H c2 (0) and higher superconducting anisotropy Γ(0) were observed in FeSe. © 2019 Author(s). ; The work at Yangzhou Univ. was supported by Natural Science Foundation of China (NSFC) grant # 61474096 and by NSF of Jiangsu grant # BK20180889, and at Chinese Academy of Sciences by NSFC grants # 51477167 and 41527802. D.A.C. thanks supports by the program 211 of the Russian Federation Government (RFG), agreement 02.A03.21.0006 and by the Russian Government Program of Competitive Growth of Kazan Federal Univ. A.N.V. thanks supports by Russian Foundation for Basic Research Grant No. 17-29-10007, by the Ministry of Education and Science of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISiS (Grant No. K2-2017-084), and by Act 211 of RFG, agreements 02.A03.21.0004, 02.A03.21.0006, and 02.A03.21.0011.

The interplay of orbital and spin degrees of freedom is the fundamental characteristic in numerous condensed matter phenomena, including high-temperature superconductivity, quantum spin liquids, and topological semimetals. In iron-based superconductors (FeSCs), this causes superconductivity to emerge in the vicinity of two other instabilities: nematic and magnetic. Unveiling the mutual relationship among nematic order, spin fluctuations, and superconductivity has been a major challenge for research in FeSCs, but it is still controversial. Here, by carrying out 77Se nuclear magnetic resonance (NMR) measurements on FeSe single crystals, doped by cobalt and sulfur that serve as control parameters, we demonstrate that the superconducting transition temperature Tc increases in proportion to the strength of spin fluctuations, while it is independent of the nematic transition temperature Tnem. Our observation therefore directly implies that superconductivity in FeSe is essentially driven by spin fluctuations in the intermediate coupling regime, while nematic fluctuations have a marginal impact on Tc. © 2020, The Author(s). ; Semiconductor Research Corporation, SRC: 2018R1A5A6075964 ; Korea Basic Science Institute, KBSI: IBS-R014-D1 ; Kazan Federal University ; 2016K1A4A4A01922028 ; Government Council on Grants, Russian Federation ; Deutsche Forschungsgemeinschaft, DFG: BA 4927/ 2-1 ; National Research Foundation of Korea, NRF: EF86/7-1, BU887/25-1, 19-43-04129, NRF-2019R1F1A1057463 ; We acknowledge A. Chubukov for useful discussions. S.H.B. has been supported by the Deutsche Forschungsgemeinschaft (Germany) via DFG Research Grants BA 4927/ 2-1 and by the National Research Foundation of Korea (NRF-2019R1F1A1057463). D.V. E., B.B., I.M., and S.A. were supported by RSF-DFG project (no. 19-43-04129, BU887/25-1, EF86/7-1). D.V.E. and I.M. were also supported by VW foundation in the frame of the VW Trilateral Initiative. S.A. acknowledges financial support from Deutsche Forschungsgemeinschaft (DFG) via Grant No. DFG AS 523/4-1. The work at POSTECH was supported by Institute for Basic Science (no. IBS-R014-D1) and also by the National Research Foundation (NRF) of Korea through the SRC (no. 2018R1A5A6075964) and the Max Planck-POSTECH Center for Complex Phase Materials in Korea (MPK) (no. 2016K1A4A4A01922028). The work of DACh was supported by the program 211 of the Russian Federation Government, agreement No. 02.A03.21.0006, by the Russian Government Program of Competitive Growth of Kazan Federal University.

In iron-based superconductors the interactions driving the nematic order that breaks the lattice four-fold rotational symmetry in the iron plane may also facilitate the Cooper pairing, but experimental determination of these interactions is challenging because the temperatures of the nematic order and the order of other electronic phases appear to match each other or to be close to each other. Here we performed field-dependent 77Se-nuclear magnetic resonance (NMR) measurements on single crystals of iron-based superconductor FeSe, with magnetic field B 0 up to 16 T. The 77Se-NMR spectra and Knight shift split when the direction of B 0 is away from the direction perpendicular to the iron planes (i.e. B 0 ∥ c) upon cooling in temperature, with a significant change in the distribution and magnitude of the internal magnetic field at the 77Se nucleus, but these do not happen when B 0 is perpendicular to the iron planes, thus demonstrating that there is an orbital ordering. Moreover, stripe-type antiferromagnetism is absent, while giant antiferromagnetic spin fluctuations measured by the NMR spin-lattice relaxation gradually developed starting at ∼40 K, which is far below the nematic order temperature T nem = 89 K. These results provide direct evidence of orbital-driven nematic order in FeSe. © 2019 The Author(s). Published by IOP Publishing Ltd on behalf of the Institute of Physics and Deutsche Physikalische Gesellschaft. ; Chinese Academy of Sciences, CAS: 51477167, 41527802 ; Ministério da Educação e Ciência, MEC: 2-2017-084 ; National Natural Science Foundation of China, NSFC: 61474096, 1804291 ; BK20180890, 20180889 ; Russian Foundation for Basic Research, RFBR: 17-29-10007 ; Government Council on Grants, Russian Federation ; Work at YZU was supported by National Science Foundation of China (NSFC) (Grants #61474096 and 1804291) and NSF of Jiangsu (Grants #BK20180889 and BK20180890), and at CAS by NSFC (Grants# 51477167 and 41527802).DACthanks supports by the program 211 of the Russian Federation Government (RFG), agreement 02.A03.21.0006 and by the RFG Program of Competitive Growth of KFU.ANVthanks supports by Russian Foundation for Basic Research Grant#17-29-10007, by the Ministry of Education and Science of the RFG in the framework of ICP of NUST MISiS (Grant#K2-2017-084), and by Act 211 of RFG, agreements 02.A03.21.0004, 02.A03.21.0006, and 02.A03.21.0011.

Low-temperature scanning tunneling microscopy and spectroscopy has been used to image the vortex core and the vortex lattice in FeSe single crystals. The local tunneling spectra acquired at the center of elliptical vortex cores display a strong particle-hole asymmetry with spatial oscillation, characteristic of the quantum-limit vortex core. Furthermore, a quasihexagonal vortex lattice at low magnetic field undergoes noticeable rhombic distortions above a certain field ∼1.5 T. This field H∗ also reveals itself as a kink in the magnetic field dependence of the specific heat. The observation of a nearly hexagonal vortex lattice at low field is very surprising for materials with an orthorhombic crystal structure and it is in apparent contradiction with the elliptical shape of the vortex cores. These observations can be directly connected to the multiband nature of superconductivity in this material, provided we attribute them to the suppression of superconducting order parameter in one of the energy bands. Above the field H∗ the superconducting coherence length for this band can well exceed the intervortex distance which strengthens the nonlocal effects. Therefore, in addition to multiple-band effects, other possible sources that can contribute to the observed evolution of the vortex-lattice structure include nonlocal effects which cause the field-dependent interplay between the symmetry of the crystal and vortex lattice or the magnetoelastic interactions due to the strain field generated by vortices. © 2019 American Physical Society. ; Citrus Research and Development Foundation, CRDF ; Government Council on Grants, Russian Federation ; Russian Science Foundation, RSF: 17-12-01383, 18-72-10027 ; Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR ; Foundation for the Advancement of Theoretical Physics and Mathematics: 17-11-109 ; Ministero dellâ Istruzione, dellâ Università e della Ricerca, MIUR ; Kazan Federal University ; Office of Science, SC ; Division of Materials Sciences and Engineering, DMSE ; Russian Foundation for Basic Research, RFBR: 17-52-12044 ; Ministry of Education and Science of the Russian Federation, Minobrnauka ; Temple University, TU ; Argonne National Laboratory, ANL ; Nanjing University of Science and Technology, NUST: K2-2017-084 ; Drexel University ; The authors would like to acknowledge fruitful discussions with V. Kogan and T. Hanaguri. We also would like to acknowledge technical support during the early stage of these measurements from S. A. Moore. The work at Temple University, where low temperature scanning tunneling measurements were performed, was supported by US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division under Award No. DE-SC0004556. The work at Drexel University and at the M.V. Lomonosov Moscow State University was supported by the US Civilian Research and Development Foundation (CRDF Global). The work in Russia has been supported in part by the Ministry of Education and Science of the Russian Federation in the framework of the Increase Competitiveness Program of NUST MISiS Grant K2-2017-084, by Act 211 of the Government of Russian Federation, Contracts No. 02.A03.21.0004, No. 02.A03.21.0006, and No. 02.A03.21.0011 and by the Russian Government Program of Competitive Growth of Kazan Federal University. One of the authors (C.D.G.) would like to acknowledge partial support from MIUR (Ministry of Education, Universities and Research of the Italian Government). The work in IPM RAS (Nizhny Novgorod) was supported in part by the Russian Science Foundation (the calculation of the vortex-lattice characteristics Grant No. 18-72-10027; the calculation of the vortex-core deformation and the analysis of the experimental data Grant No. 17-12-01383), the Russian Foundation for Basic Research (Grant No. 17-52-12044), and Foundation for the Advancement of Theoretical Physics and Mathematics "BASIS" (Grant No. 17-11-109). The work at Argonne National Laboratory was supported by the US Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.

Through advanced experimental techniques on Formula Presented single crystals, we derive a pressure-temperature phase diagram. We find that Formula Presented increases to Formula Presented K with pressure up to Formula Presented GPa followed by a decrease to Formula Presented K at 21.2 GPa. The experimental results are reproduced by theoretical calculations based on density functional theory where electron-electron interactions are treated by a static on-site Hubbard Formula Presented on Cr Formula Presented orbitals. The origin of the pressure-induced reduction of the ordering temperature is associated with a decrease in the calculated bond angle, from Formula Presented at ambient pressure to Formula Presented at 25 GPa. Above 22 GPa, experiment and theory jointly point to the idea that the ferromagnetically ordered state is destroyed, giving rise first to a complex, unknown magnetic configuration, and at sufficiently high pressures a pure antiferromagnetic configuration. This sequence of transitions in the magnetism is accompanied by a well-detected pressure-induced semiconductor-to-metal phase transition that is revealed by both high-pressure resistivity measurements and ab initio theory. ©2022 American Physical Society. ; A.G. and M.A.-H. acknowledge financial support from the Carl Tryggers Foundation and the Swedish Research Council (VR) under Project No. 2018-05393. Support by the P220 291 program of the Government of Russia through Project No. 292 075-15-2021-604 and President of Russia through Project No. NSh-2394.2022.1.5 is acknowledged. G.H. and M.K. acknowledge support from the Czech Science Foundation (Project No. 20-08633X). J.V. acknowledges the support of Czech Research Infrastructures MGML (Project No. LM2018096). O.E. acknowledges financial support from the Knut and Alice Wallenberg Foundation, eSSENCE, SNIC, the VR, the Foundation for Strategic Research (SSF), and the ERC (Synergy Grant FASTCORR, Project No. 854843). Y.K. acknowledges financial support from the VR under Project No. 2019-03569. ...