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Register a new userMagnetic properties of FeSe superconductor
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Added by
Gennadiy Grechnev E
Publication date
2012
fields
Physics
and research's language is
English
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A detailed magnetization study for the novel FeSe superconductor is carried out to investigate the behavior of the intrinsic magnetic susceptibility $chi$ in the normal state with temperature and under hydrostatic pressure. The temperature dependencies of $chi$ and its anisotropy $Delta chi=chi_{|}-chi_{bot}$ are measured for FeSe single crystals in the temperature range 4.2-300 K, and a substantial growth of susceptibility with temperature is revealed. The observed anisotropy $Delta chi$ is very large and comparable with the averaged susceptibility at low temperatures. For a polycrystalline sample of FeSe, a significant pressure effect on $chi$ is determined to be essentially dependent on temperature. Ab initio calculations of the pressure dependent electronic structure and magnetic susceptibility indicate that FeSe is close to magnetic instability with dominating enhanced spin paramagnetism. The calculated paramagnetic susceptibility exhibits a strong dependence on the unit cell volume and especially on the height $Z$ of chalcogen species from the Fe plane. The change of $Z$ under pressure determines a large positive pressure effect on $chi$ which is observed at low temperatures. It is shown that the literature experimental data on the strong and nonmonotonic pressure dependence of the superconducting transition temperature in FeSe correlate qualitatively with calculated behavior of the density of electronic states at the Fermi level.
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We report pair distribution function measurements of the iron-based superconductor FeSe above and below the structural transition temperature. Structural analysis reveals a local orthorhombic distortion with a correlation length of about 4 nm at temperatures where an average tetragonal symmetry is observed. The analysis further demonstrates that the local distortion is larger than the distortion at temperatures where the average observed symmetry is orthorhombic. Our results suggest that the low-temperature macroscopic nematic state in FeSe forms from an imperfect ordering of orbital-degeneracy-lifted nematic fluctuations which persist up to at least 300 K.
In high temperature cuprate superconductors, it is now generally agreed that the parent compound is a Mott insulator and superconductivity is realized by doping the antiferromagnetic Mott insulator. In the iron-based superconductors, however, the parent compound is mostly antiferromagnetic metal, raising a debate on whether an appropriate starting point should go with an itinerant picture or a localized picture. It has been proposed theoretically that the parent compound of the iron-based superconductors may be on the verge of a Mott insulator, but so far no clear experimental evidence of doping-induced Mott transition has been available. Here we report an electronic evidence of an insulator-superconductor transition observed in the single-layer FeSe films grown on the SrTiO3 substrate. By taking angle-resolved photoemission measurements on the electronic structure and energy gap, we have identified a clear evolution of an insulator to a superconductor with the increasing doping. This observation represents the first example of an insulator-superconductor transition via doping observed in the iron-based superconductors. It indicates that the parent compound of the iron-based superconductors is in proximity of a Mott insulator and strong electron correlation should be considered in describing the iron-based superconductors.
We have performed electrical resistivity measurements of a polycrystalline sample of FeSe$_{0.25}$Te$_{0.75}$, which exhibits superconductivity at $T_{rm c} sim 14$ K, in magnetic fields up to 55 T to determine the upper critical field $mu_{0}H_{rm c2}$. In this compound, very large slopes of $mu_{0}H_{rm c2}$ at the onset, the mid-point, the zero-resistivity temperatures on superconductivity are determined to be -13.7, -10.1, and -6.9 T/K, respectively. The observed $mu_{0}H_{rm c2}(T)$s of this compound are considerably smaller than those expected from the Werthamer-Helfand-Hohenberg model, manifesting the Pauli limiting behavior. These results suggest that this compound has a large Maki parameter, but it is smaller than that calculated for a weak-coupling superconductor, indicating a large superconducting gap of this compound as a strong-coupling superconductor.
We investigate the thermodynamic properties of FeSe under the in-plane magnetic fields using torque magnetometry, specific heat, magnetocaloric measurements. Below the upper critical field Hc2, we observed the field-induced anomalies at H1 ~ 15 T and H2 ~ 22 T near H//ab and below a characteristic temperature T* ~ 2 K. The transition magnetic fields H1 and H2 exhibit negligible dependence on both temperature and field orientation. This contrasts with the strong temperature and angle dependence of Hc2, suggesting that these anomalies are attributed to the field-induced phase transitions, originating from the inherent spin-density-wave instability of quasiparticles near the superconducting gap minima or possible Flude-Ferrell-Larkin-Ovchinnikov state in the highly spin-polarized Fermi surfaces. Our observations imply that FeSe, an atypical multiband superconductor with extremely small Fermi energies, represents a unique model system for stabilizing unusual superconducting orders beyond the Pauli limit.
We present the results of paramagnetic LDA band structure calculations: band dispersions, densities of states and Fermi surfaces, for the new iron based high-temperature superconductor LiOHFeSe. Main structural motif providing bands in the vicinity of the Fermi level is FeSe layer which is isostructural to the bulk FeSe prototype superconductor. The bands crossing the Fermi level and Fermi surfaces of the new compound are typical for other iron based superconductors. Experimentally it was shown that introduction of Fe ions into LiOH layer gives rise to ferromagnetic ordering of the Fe ions at T$_C$=10K. To study magnetic properties of [Li$_{0.8}$Fe$_{0.2}$OH]FeSe system we have performed LSDA calculations for $sqrt 5 times sqrt 5$ superlattice and found ferromagnetism within the Li$_4$Fe(OH) layer. To estimate the Curie temperature we obtained Fe-Fe exchange interaction parameters for Heisenberg model from our LSDA calculations, leading to theoretical value of Curie temperature 10.4K in close agreement with experiment.
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