We used low-temperature synchrotron x-ray diffraction to investigate the structural phase transitions of Fe${}_{1+y}$Te in the vicinity of a tricitical point in the phase diagram. A detailed analysis of the powder diffraction patterns and temperature dependence of the peak widths in Fe${}_{1+y}$Te showed that two-step structural and magnetic phase transitions occur within the compositional range $0.11$\le${}y$\le${}0.13$. The phase transitions are sluggish, indicating a strong competition between the orthorhombic and the monoclinic phases. We combine high-resolution diffraction experiments with specific heat, resistivity, and magnetization measurements and present a revised temperature-composition phase diagram for Fe${}_{1+y}$Te.

The presence of magnetism in potentially altermagnetic RuO 2 has been a subject of intense debate. Using broadband infrared spectroscopy combined with density-functional band-structure calculations, we show that the optical conductivity of RuO 2 , the bulk probe of its electronic structure, is best described by a nonmagnetic model. The sharp Pauli edge demonstrates the presence of a Dirac nodal line lying 45 meV below the Fermi level. An excellent match between the experimental and plasma frequencies underpins the weakness of electronic correlations. The intraband part of the optical conductivity indicates Fermi-liquid behavior with two distinct scattering rates below 150 K. Fermi-liquid theory also accounts for the temperature-dependent magnetic susceptibility of RuO 2 and allows a consistent description of this material as a paramagnetic metal.

The microscopic mechanism governing the zero-resistance flow of current in some iron-based, high-temperature superconducting materials is not well understood up to now. A central issue concerning the investigation of these materials is their superconducting gap symmetry and structure. Here we present a combined study of low-temperature specific heat and scanning tunnelling microscopy measurements on single crystalline FeSe. The results reveal the existence of at least two superconducting gaps which can be represented by a phenomenological two-band model. The analysis of the specific heat suggests significant anisotropy in the gap magnitude with deep gap minima. The tunneling spectra display an overall “U”-shaped gap close to the Fermi level away as well as on top of twin boundaries. These results are compatible with the anisotropic nodeless models describing superconductivity in FeSe.

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and high-resolution synchrotron x-ray powder diffraction investigations of single crystals Fe1+yTe (0.06 $\le${}y$\le${} 0.15) reveal a splitting of a single, first-order transition for y$\le${} 0.11 into two transitions for y$\ge${} 0.13. Most strikingly, all measurements on identical samples Fe1.13Te consistently indicate that, upon cooling, the magnetic transition at TN precedes the first-order structural transition at a lower temperature Ts. The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c axis display a small distortion close to TN due to a lattice striction as a consequence of magnetic ordering, and a much larger change at Ts. The lattice symmetry changes, however, only below Ts as indicated by powder x-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

The critical properties of orthorhombic Pr0.6Sr0.4MnO3 single crystals were investigated by a series of static magnetization measurements along the three different crystallographic axes as well as by specific heat measurements. A careful range-of-fitting-analysis of the magnetization and susceptibility data obtained from the modified Arrott plots shows that Pr0.6Sr0.4MnO3 has a very narrow critical regime. Nevertheless, the system belongs to the three-dimensional (3D) Heisenberg universality class with short-range exchange. The critical exponents obey Widom scaling and are in excellent agreement with the single scaling equation of state M(H,$\epsilon${})=|$\epsilon${}|$\beta${}f\ifmmode\pm\else\textpm\fi{}(H/|$\epsilon${}|($\beta${}+$\gamma${}); with f+ for T>Tc and f$-${} for T<Tc. A detailed analysis of the specific heat that account for all relevant contributions allows us to extract and analyze the contribution related to the magnetic phase transition. The specific heat indicates the presence of a linear electronic term at low temperatures and a prominent contribution from crystal field excitations of Pr. A comparison with data from literature for PrMnO3 shows that a Pr-Mn magnetic exchange is responsible for a sizable shift in the lowest lying excitation.

The structurally simplest Fe-based superconductor FeSe with a critical temperature $T_c\approx 8.5\mathrm{K}$ displays a breaking of the fourfold rotational symmetry at a temperature $T_s\approx 87\mathrm{K}$. We investigate the electronic properties of FeSe using scanning tunneling microscopy/spectroscopy (STM/STS), magnetization, and electrical transport measurements. The results indicate two energy scales: (i) $T^{*}\approx 75\mathrm{K}$, denoted by an onset of electron-hole asymmetry in STS, enhanced spin fluctuations, and increased positive magnetoresistance; and (ii) $T^{**}\approx 22–30 \mathrm{K}$, marked by the opening up of a partial gap of about 8 meV in STS and a recovery of Kohler's rule. Our results reveal the onset of an incipient ordering mode at $T^{*}$ and its nucleation below $T^{**}$. The ordering mode observed here, both in spin as well as charge channels, suggests a coupling between the spins with charge, orbital, or pocket degrees of freedom.