Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe

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Robust superconductivity and fragile magnetism induced by the strong Cu impurity scattering in the high-pressure phase of FeSe

Z. Zajicek, S. J. Singh, and A. I. Coldea

Phys. Rev. Research 4, 043123 – Published 21 November 2022

Abstract

Superconductivity in FeSe is strongly enhanced under applied pressure and it is proposed to emerge from anomalously coupled structural and magnetic phases. Small impurities inside the Fe plane can strongly disrupt the pair formation in FeSe at ambient pressure and can also reveal the interplay between normal and superconducting phases. Here, we investigate how an impurity inside the Fe plane induced by the Cu substitution can alter the balance between competing electronic phases of FeSe at high pressures. In the absence of an applied magnetic field, at low pressures the nematic and superconducting phases are suppressed by a similar factor. On the other hand, at high pressures, above 10 kbar, the superconductivity remains unaltered despite the lack of any signature in transport associated to a magnetic phase in zero-magnetic field. However, by applying a magnetic field, the resistivity displays an anomaly preceding the activated behavior in temperature, assigned to a magnetic anomaly. We find that the high-pressure superconducting phase of FeSe is robust and remains enhanced in the presence of Cu impurity, whereas the magnetic phase is not. This could suggest that high- $T_{c}$ superconductivity has a sign-preserving order parameter in the presence of a rather glassy magnetic phase.

Received 7 June 2022
Revised 10 September 2022
Accepted 21 October 2022

DOI:https://doi.org/10.1103/PhysRevResearch.4.043123

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Electrical properties Impurities in superconductors Multiband superconductivity Superconducting phase transition

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Z. Zajicek ^*, S. J. Singh ^†, and A. I. Coldea ^‡

Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom

^*Corresponding author: zachary.zajicek@physics.ox.ac.uk
^†Current address: Institute of High-Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142, Warsaw, Poland.
^‡Corresponding author: amalia.coldea@physics.ox.ac.uk

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Vol. 4, Iss. 4 — November - December 2022

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Images

Figure 1
Zero-field transport properties of ${Fe}_{1 - x} {Cu}_{x} Se$ under pressure. (a) Temperature dependence of the resistivity as a function of applied pressure. (b) The evolution of the superconducting transition width, $Δ T_{c} = T_{on} - T_{off}$ (black circles), and the resistivity at 300 K (blue triangles) with pressure. Solid lines are guides to the eye. (c) Temperature-pressure phase diagram in zero magnetic field tuned by pressure. The nematic phase occurs at $T_{s}$ (blue triangles) and superconductivity has the offset temperature at $T_{c}$ (red squares). The phase diagram can be divided into three different regions: the low-pressure region up to $p_{1} = 10 kbar$ , inside the nematic phase; the high-pressure region above $p_{2} = 15 kbar$ , once the nematic phase is fully suppressed; and the intermediate pressure region between $p_{1}$ and $p_{2}$ . The shaded areas indicate different phase boundaries.
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Figure 2
The transport behavior in magnetic field of the ${Fe}_{1 - x} {Cu}_{x} Se$ under pressure. (a) The temperature dependence of the longitudinal resistivity in different magnetic fields at $p = 17 kbar$ . (b) The resistivity upturn at $T_{m}$ in 15 T at 17 kbar using various temperature sweep rates. The solid lines correspond to the warming sweeps whereas the dashed lines correspond to the cooling sweeps. (c) The temperature dependence of resistivity in a magnetic field of 15 T ( $H | | c$ ) for different applied pressures and (d) their corresponding derivatives. The value of $T_{m}$ is defined as the minimum in the derivative and indicated by arrows, similar to FeSe [12]. Curves are offset for clarity. Field dependence of (e) the longitudinal resistivity, $ρ_{xx}$ , and (f) the Hall resistivity, $ρ_{xy}$ at a pressure of $p = 13.3 kbar$ . (g) Mobility and (h) carrier density of each charge carrier (hole, $h$ , and electron, $e_{1}$ ) considering a two-band compensated model ( $n_{h} = n_{e_{1}}$ ) at different pressures. The two-band model at ambient pressure uses data below $< 7$ T and a three-band model could be used to described the full field window, as reported in Ref. [14] and shown in Fig. S7 in the SM [25]. Solid and dashed lines are guides to the eye.
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Figure 3
The pressure-temperature phase diagrams of Cu-substituted FeSe. The comparison between the different electronic phases of FeSe (open symbols) from Ref. [24] and Cu-FeSe (solid symbols) measured in (a) 0 T and (b) in 15 T for Cu-FeSe and 16 T for FeSe (after Ref. [24]). The shaded areas in all panels reflect the phase boundaries for Cu-FeSe, as described in Fig. 1. The solid lines in (b) are guides to the eye for FeSe. Nematic shaded areas in (a) and (b) are for Cu-FeSe, assuming a field-independent $T_{s}$ . (c) The scaling of the temperature-pressure phase diagram from (a) by using the reduced temperature $t = T / T_{s}$ , where $T_{s}$ is the nematic transition at ambient pressure for the two systems. The two shaded regions reflect schematically the two proposed superconducting regions with the boundaries given by the maximum value of $T_{c}$ .
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