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Abstract:

Electronic scattering is a powerful tool to identify underlying changes in electronic behavior and incipient electronic and magnetic orders. The nematic and magnetic phases are strongly intertwined under applied pressure in FeSe, however, the additional isoelectronic substitution of sulfur offers an elegant way to separate them. Here we report the detailed evolution of the electronic and superconducting behavior of ${\mathrm{FeSe}}_{0.96}{\mathrm{S}}_{0.04}$ under applied pressure via longitudinal magnetoresistance studies up to 15 T. At intermediate pressures, inside the nematic phase, the resistivity displays an upturn in zero magnetic field, which is significantly enhanced in the magnetic field, suggesting the stabilization of a spin-density wave phase, which competes with superconductivity. At higher pressures, beyond the nematic phase boundaries, the resistivity no longer displays any clear anomalies in the zero magnetic field, but an external magnetic field induces significant upturns in resistivity reflecting a field-induced order, where superconductivity and magnetic anomalies are enhanced in tandem. This study highlights the essential role of high magnetic fields in stabilizing different electronic phases and revealing a complex interplay between magnetism and superconductivity tuned by applied pressure in ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ .

Abstract:

These open access data reflect the data collected as part of the manuscript ”Drastic field-induced resistivity upturns as signatures of unconventional magnetism in superconducting iron chalcogenides”, by Z. Zajicek, I. Paulescu, et al, available on the pre-print server at https://arxiv.org/abs/2512.20862, and to appear in Physical Review B (2026). These ASCII data are associated with a detailed transport study as a function of temperature (2 to 300K) under applied pressure both in zero magnetic field and under applied fixed magnetic fields using a 16T Quantum Design PPMS. Electrical transport measurements were performed on two different high-quality FeSe0.96S0.04 single crystals (S1 and S2) using a standard four-probe configuration to determine the longitudinal resistivity, rho_xx, in the conducting (ab) plane. A maximum a.c. current of 1 mA was applied to the sample, and the magnetic field of up to 16T for sample S1 and 15 T for sample S2 is aligned along the c direction and perpendicular to the applied current, thus probing the transverse magnetoresistance. Transport measurements in constant magnetic fields for sample S2 were performed using different field polarities, and the data were symmetrized afterwards to eliminate any effect of mixing resistivity components. Experiments were carried out under applied pressure using a commercially available BeCu pressure cell from Quantum Design up to 20 kbar while slowly cooling and warming to 2 K at a rate of 0.5 K/min. Hysteresis studies were also performed using 0.25 K/min. Daphne 7373 was used as the pressurizing medium, which is hydrostatic up to 22 kbar. The pressure was determined in situ using the superconducting transition temperature of tin at a slow cooling rate of 0.02 K/min. Resistivity was measured as a function of temperature from a fixed pressure and magnetic field, and each raw data name contains information about the estimated pressure value. Temperature dependence transport studies for sample S2 were performed using different magnetic field polarities and the data were symmetrized afterwards to eliminate any effect of mixing resistivity components.

Abstract:

This study utilises linear-scaling density functional theory (DFT) and develops a new machine-learning potential for carbon and nitrogen (GAP-CN), based on the carbon potential (GAP20), to investigate the interaction between carbon self-interstitials and nitrogen-vacancy (NV) centres in diamond, focusing on their excited states and diffusion behaviour. From the simulated excited states, 'Bright', 'Spike', and 'Dark' defect configurations are classified based on their absorption spectrum features. Furthermore, machine learning molecular dynamics simulation provides insight into the possible diffusion mechanism of Ci and NV, showing that Ci can diffuse away or recombine with NV. The study yields new insight into the formation of NV defects in diamond for quantum technology applications.