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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:

Kitaev materials often order magnetically at low temperatures due to the presence of non-Kitaev interactions. Torque magnetometry is a very sensitive technique for probing the magnetic anisotropy, which is critical in understanding the magnetic ground state. In this work, we report detailed single-crystal torque measurements in the proposed Kitaev candidate honeycomb magnet α-RuBr3, which displays zigzag order below 34 K. Based on angular-dependent torque studies in magnetic fields up to 16 T rotated in the plane normal to the honeycomb layers, we find an easy-plane anisotropy with a temperature dependence of the torque amplitude following closely the behaviour of the powder magnetic susceptibility. The torque for field rotated in the honeycomb plane has a clear six-fold periodicity with a saw-tooth shape, reflecting the three-fold symmetry of the crystal structure and stabilization of different zigzag domains depending on the field orientation, with a torque amplitude that follows an order parameter form inside the zigzag phase. By comparing experimental data with theoretical calculations we highlight the importance of relevant anisotropic interactions and the role of the competition between different zigzag domains in this candidate Kitaev magnet.

Abstract:

In the Kitaev honeycomb model, spins coupled by strongly-frustrated anisotropic interactions do not order at low temperature but instead form a quantum spin liquid with spin fractionalisation into Majorana fermions and static fluxes. The realization of such a model in crystalline materials could lead to major breakthroughs in understanding entangled quantum states, however achieving this in practice is a very challenging task. The recently synthesized honeycomb material RuI3 shows no long-range magnetic order down to the lowest probed temperatures and has been theoretically proposed as a quantum spin liquid candidate material on the verge of an insulator to metal transition. Here we report a comprehensive study of the magnetic anisotropy in un-twinned single crystals via torque magnetometry and detect clear signatures of strongly anisotropic and frustrated magnetic interactions. We attribute the development of sawtooth and six-fold torque signal to strongly anisotropic, bond-dependent magnetic interactions by comparing to theoretical calculations. As a function of magnetic field strength at low temperatures, torque shows an unusual non-parabolic dependence suggestive of a proximity to a field-induced transition. Thus, RuI3, without signatures of long-range magnetic order, displays key hallmarks of an exciting candidate for extended Kitaev magnetism with enhanced quantum fluctuations.

Abstract:

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressure-temperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. This high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities.