Heat-capacity, X-ray diffraction, and resistivity measurements on a high-quality BaFe2As2 sample show an evolution of the magneto-structural transition with successive annealing periods. After a 30-day anneal the resistivity in the (ab) plane decreases by more than an order of magnitude, to 12 µΩcm, with a residual resistance ratio ∼36; the heat-capacity anomaly at the transition sharpens, to an overall width of less than K, and shifts from 135.4 to 140.2 K. The heat-capacity anomaly in both the as-grown sample and after the 30-day anneal shows a hysteresis of ∼0.15 K, and is unchanged in a magnetic field µ0H = 14 T. The X-ray and heat-capacity data combined suggest that there is a first order jump in the structural order parameter. The entropy of the transition is reported.

We investigate the anisotropic superconducting and magnetic properties of single-crystal RbEuFe4As4 using magnetotransport and magnetization measurements. We determine a magnetic ordering temperature of the Eu-moments of Tm = 15 K and a superconducting transition temperature of Tc = 36.8 K. The superconducting phase diagram is characterized by high upper critical field slopes of -70 kG/K and -42 kG/K for in-plane and out-of-plane fields, respectively, and a surprisingly low superconducting anisotropy of Γ = 1.7. Ginzburg-Landau parameters of κc ∼ 67 and κ ab ∼ 108 indicate extreme type-II behavior. These superconducting properties are in line with those commonly seen in optimally doped Fe-based superconductors. In contrast, Eu-magnetism is quasi-two dimensional as evidenced by highly anisotropic in-plane and out-of-plane exchange constants of 0.6 K and < 0.04 K. A consequence of the quasi-2D nature of the Eu-magnetism are strong magnetic fluctuation effects, a large suppression of the magnetic ordering temperature as compared to the Curie-Weiss temperature, and a cusp-like anomaly in the specific heat devoid of any singularity. Magnetization curves reveal a clear magnetic easy-plane anisotropy with in-plane and out-of-plane saturation fields of 2 kG and 4 kG.

SrxBi2Se3 and the related compounds CuxBi2Se3 and NbxBi2Se3 have attracted considerable interest, as these materials may be realizations of unconventional topological superconductors. Superconductivity with Tc ~3 K in SrxBi2Se3 arises upon intercalation of Sr into the layered topological insulator Bi2Se3. Here we elucidate the anisotropy of the normal and superconducting state of Sr0.1Bi2Se3 with angular dependent magnetotransport and thermodynamic measurements. High resolution x-ray diffraction studies underline the high crystalline quality of the samples. We demonstrate that the normal state electronic and magnetic properties of Sr0.1Bi2Se3 are isotropic in the basal plane while we observe a large two-fold in-plane anisotropy of the upper critical field in the superconducting state. Our results support the recently proposed odd-parity nematic state characterized by a nodal gap of Eu symmetry in SrxBi2Se3.

Results of high resolution x-ray diffraction experiments are presented for single crystals of the spin gap compound BaCuSi2O6 in the temperature range from 16 to 300 K. The data show clear evidence of a transition from the room temperature tetragonal phase into an incommensurately modulated orthorhombic structure below ∼100 K. This lattice modulation is characterized by a resolution limited wave vector qIC =(0,∼0.13,0) and its 2 nd and 3 rd harmonics. The phase transition is first order and exhibits considerable hysteresis. This observation implies that the spin Hamiltonian representing the system is more complex than originally thought.

There are very few materials that exhibit zero thermal expansion (ZTE), and of these even fewer are appropriate for electronic and optoelectronic applications. We find that a multifunctional crystalline hybrid inorganic-organic semiconductor, beta-ZnTe(en)(0.5) (en denotes ethylenediamine), shows uniaxial ZTE in a very broad temperature range of 4-400 K, and concurrently possesses superior electronic and optical properties. The ZTE behavior is a result of compensation of contraction and expansion of different segments along the inorganic-organic stacking axis. This work suggests an alternative route to designing materials in a nanoscopic scale with ZTE or any desired positive or negative thermal expansion (PTE or NTE), which is supported by preliminary data for ZnTe(pda)(0.5) (pda denotes 1,3-propanediamine) with a larger molecule.