Constraints on the merging of the transition lines at the tricritical point in a wing-structure phase diagram

We consider the phase diagram of a ferromagnetic system driven to a quantum phase transition with a tuning parameter p. Before being suppressed, the transition becomes of the first order at a tricritical point, from which wings emerge under application of the magnetic field H in the T-p-H phase diagram. We show that the edge of the wings merge with tangent slopes at the tricritical point

Dome of magnetic order inside the nematic phase of sulfur-substituted FeSe under pressure

The pressure dependence of the structural, magnetic, and superconducting transitions and of the superconducting upper critical field were studied in sulfur-substituted Fe(Se1-xSx). Resistance measurements were performed on single crystals with three substitution levels (x=0.043, 0.096, 0.12) under hydrostatic pressures up to 1.8 GPa and in magnetic fields up to 9 T and were compared to data on pure FeSe. Our results illustrate the effects of chemical and physical pressure on Fe(Se1-xSx). On increasing sulfur content, magnetic order in the low-pressure range is strongly suppressed to a small domelike region in the phase diagrams. However, Ts is much less suppressed by sulfur substitution, and Tc of Fe(Se1-xSx) exhibits similar nonmonotonic pressure dependence with a local maximum and a local minimum present in the low-pressure range for all x. The local maximum in Tc coincides with the emergence of the magnetic order above Tc. At this pressure the slope of the upper critical field decreases abruptly, which may indicate a Fermi-surface reconstruction. The minimum of Tc correlates with a broad maximum of the upper critical field slope normalized by Tc

Anisotropic physical properties and pressure dependent magnetic ordering of CrAuTe4

Systematic measurements of temperature-dependent magnetization, resistivity, and angle-resolved photoemission spectroscopy (ARPES) at ambient pressure as well as resistivity under pressures up to 5.25 GPa were conducted on single crystals of CrAuTe4. Magnetization data suggest that magnetic moments are aligned antiferromagnetically along the crystallographic c axis below TN=255 K. ARPES measurements show band reconstruction due to the magnetic ordering. Magnetoresistance data show clear anisotropy, and, at high fields, quantum oscillations. The Néel temperature decreases monotonically under pressure, decreasing to TN=236 K at 5.22 GPa. The pressure dependencies of (i) TN, (ii) the residual resistivity ratio, and (iii) the size and power-law behavior of the low-temperature magnetoresistance all show anomalies near 2 GPa suggesting that there may be a phase transition (structural, magnetic, and/or electronic) induced by pressure. For pressures higher than 2 GPa a significantly different quantum oscillation frequency emerges, consistent with a pressure induced change in the electronic states

Strong cooperative coupling of pressure-induced magnetic order and nematicity in FeSe

A hallmark of the iron-based superconductors is the strong coupling between magnetic, structural and electronic degrees of freedom. However, a universal picture of the normal state properties of these compounds has been confounded by recent investigations of FeSe where the nematic (structural) and magnetic transitions appear to be decoupled. Here, using synchrotron-based high-energy x-ray diffraction and time-domain Moessbauer spectroscopy, we show that nematicity and magnetism in FeSe under applied pressure are indeed strongly coupled. Distinct structural and magnetic transitions are observed for pressures, 1.0 GPa <= p <= 1.7 GPa, which merge into a single first-order phase line for p >= 1.7 GPa, reminiscent of what has been observed, both experimentally and theoretically, for the evolution of these transitions in the prototypical doped system, Ba(Fe[1-x]Co[x])2As2. Our results support a spin-driven mechanism for nematic order in FeSe and provide an important step towards a universal description of the normal state properties of the iron-based superconductors.Comment: (14 pages, 4 figures