Low Temperature Physics: 46, 538 (2020); https://doi.org/10.1063/10.0001059
Fizika Nizkikh Temperatur: Volume 46, Number 5 (May 2020), p. 638-652    ( to contents , go back )

Features of excess conductivity and a possible pseudogap in FeSe superconductors

A.L. Solovjov, E.V. Petrenko, L.V. Omelchenko

B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Nauky Ave., Kharkiv 61103, Ukraine
E-mail: solovjov@ilt.kharkov.ua

E. Nazarova, K. Buchkov

Georgi Nadjakov Institute of Solid State Physics, Bulgarian Academy of Sciences 72 Tsarigradsko shosse Blvd., Sofia, 1784, Bulgaria

K. Rogacki

W. Trzebiatowski Institute of Low Temperatures and Structure Research, PAS Wroclaw, 1410 PL-50-050, Poland
pos Анотація:

Received December 9, 2019, published online March 24, 2020

Abstract

The temperature dependence of the excess conductivity σ′(T) in three polycrystalline samples of the FeSe0.94 superconductor prepared by various technologies was studied. Obtained from the measurements, the temperature dependences of the parameter Δ*(Т) which in cuprates is associated with a pseudogap, were analyzed in the model of local pairs. At high temperatures, all three samples exhibit a high narrow maximum Δ*(Т) at Тs1 ~ 250 K, typical of magnetic superconductors. Below T ≈ 225 K, the dependences Δ*(T) become different. In almost the entire temperature range below Тs1, sample S2, prepared by the solid-state reaction method and not containing impurities, shows Δ*(Т) typical of Fe pnictides. An exception is an interval from the temperature of the structural transition Тs = 85 K to Тс, where Δ*(T) exhibits an atypical broad maximum. An analysis of the obtained dependence suggests the discovery of a pseudogap in this FeSe0.94 sample below Ts. Samples S1 containing 4 wt% Ag and S3, with nominal composition, but containing non-superconducting inclusions of the hexagonal phase, both obtained by partial melting, show identical Δ*(T), but different from S2. They reveal a number of features that correlate with temperatures at which features are observed on M(T) and the Hall coefficient RH(T) changes sign several times with decreasing T, indicating a change in the type of charge carriers in FeSe. The dependence Δ*(T) of sample S3 below Тs has practically no maximum, since non-superconducting impurities of the hexagonal phase prevent the formation of paired fermions in S3 near Тs. As a consequence, S3 has a minimum density of local pairs <nn> = 0.26, determined by comparing Δ*(ТG)/Δmax near Тс with the Peters–Bauer theory, and the dependence Δ*(Т) does not follow the theory. S1 has a maximal <nn> = 0.47, most likely due to the influence of Ag impurities. S2 does not contain impurities, and found <nn> ≈ 0.3 is the same as in YBa2Cu3O7–δ. Importantly, both dependences Δ*(Т) for S1 and S2 follow the theory in a wide temperature range.

Key words: FeSe chalcogenides, magnetism, excess conductivity, pseudogap state, magnetization, local pairs.

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