Structural transformations and thermal stability of RhGe synthesized under high temperature and pressure.

Here, we study B20-type RhGe, a representative of a class of non-centrosymmetric monosilicides and monogermanides, which possess unique topological and magnetic properties important for many possible applications. The stability and phase transitions of the non-equilibrium B20-RhGe phase that can only be obtained under high pressure, are investigated theoretically usingab initiocalculations and experimentally by means of differential scanning calorimetry. For RhGe and, for comparison, for its analogue RhSi, we conducted an evolutionary search for low-energy polymorphic modifications at zero temperature and then performed simulations of their behavior at finite temperatures. The (P,T) conditions of stability for the found polymorphs are determined. Our calorimetric studies on high-pressure-synthesized RhGe samples allowed us to reveal peculiarities in thermal stability and heating-induced phase transformations. X-ray diffraction analysis and microstructure analysis of the samples were carried out before and after the heating. We also determined the specific heat from calorimetric measurements and compared the results with our calculations in the quasi-harmonic approximation.

The hyperfine magnetic fields and the effect of high pressure on the magnetic transition temperatures of the noncentrosymmetric FeRhGe2compound (B20).

Hyperfine parameters and the pressure dependence of the magnetic transition temperatures of FeRhGe2have been investigated. Sample has been prepared using high pressure-high temperature synthesis technique. FeRhGe2consists of two B20 structure phases with close lattice constants. The phase separation stays constant in the temperature range 4-300 K. The magnetic transition temperaturesTc1= 213 K andTc2= 135 K of FeRhGe2slightly increases with pressure in the range 0-4.5 GPa. We have compared this pressure dependence with some others compounds in the family Fe1-xRhxGe. The two phases in FeRhGe2have slightly different values of the hyperfine magnetic fields.

High-pressure single-crystal synchrotron diffraction study of MnGe and related compounds.

Single crystal synchrotron diffraction for pressures up to 50 GPa has revealed an essential difference in structural properties and compressibility of MnGe compared with Mn1-x Co x Ge and Mn1-x Fe x Ge solid solutions. A negative thermal expansion has been observed for MnGe at low-temperatures and high-pressures. The single crystal refinement has shown a discontinuous change of the atomic coordinates and Mn-Ge interatomic distances of MnGe in contrast to Mn0.1Co0.9Ge. These peculiarities of MnGe are likely to be associated with high-spin-low-spin transition. The relation between anisotropy of the coordination of Mn-atom and its magnetic moment is discussed.

Thermal expansion of monogermanides of 3d-metals.

Temperature dependent powder and single-crystal synchrotron diffraction, specific heat, magnetic susceptibility and small-angle neutron scattering experiments have revealed an anomalous response of MnGe. The anomaly becomes smeared out with decreasing Mn content in Mn1-x Co x Ge and Mn1-x Fe x Ge solid solutions. Mn spin state instability is discussed as a possible candidate for the observed effects.

Chiral properties of structure and magnetism in Mn(1-x)Fe(x)Ge compounds: when the left and the right are fighting, who wins?

Magnetic susceptibility measurements have shown that the compounds Mn(1-x)Fe(x)Ge are magnetically ordered through the whole range of concentrations x = [0.0,1.0]. Small-angle neutron scattering reveals the helical nature of the spin structure with a wave vector, which changes from its maximum (|k| = 2.3 nm(-1)) for pure MnGe, through its minimum (|k| → 0) at x(c) ≈ 0.75, to the value of |k| = 0.09 nm(-1) for pure FeGe. The macroscopic magnetic measurements confirm the ferromagnetic nature of the compound with x = x(c). The observed transformation of the helix structure to the ferromagnet at x = x(c) is explained by different signs of chirality for the compounds with x > x(c) and x

Interplay between magnetism and superconductivity and the appearance of a second superconducting transition in α-FeSe at high pressure.

We synthesized tetragonal α-FeSe by melting a powder mixture of iron and selenium at high pressure. Subsequent annealing at normal pressure results in removing traces of hexagonal ß-FeSe, formation of a rather sharp transition to a superconducting state at T(c)∼7 K, and the appearance of a magnetic transition near T(M) = 120 K. Resistivity and ac-susceptibility were measured on the annealed sample at hydrostatic pressure up to 4.5 GPa. A magnetic transition visible in ac-susceptibility shifts down under pressure and a resistive anomaly typical for a spin density wave (SDW) antiferromagnetic transition develops near the susceptibility anomaly. T(c), determined by the appearance of a diamagnetic response in susceptibility, increases linearly under pressure at a rate dT(c)/dP = 3.5 K GPa(-1). Below 1.5 GPa, the resistive superconducting transition is sharp, the width of transition does not change with pressure and, T(c), determined by a peak in dρ/dT, increases at a rate ∼3.5 K GPa(-1). At higher pressure, a giant broadening of the resistive transition develops. This effect cannot be explained by possible pressure gradients in the sample and is inherent to α-FeSe. The dependences dρ(T)/dT show a signature for a second peak above 3 GPa which is indicative of the appearance of another superconducting state in α-FeSe at high pressure. We argue that this second superconducting phase coexists with SDW antiferromagnetism in a partial volume fraction and originates from pairing of charge carriers from other sheets of the Fermi surface.

Structural properties of Y1-xYbxNi2B2C synthesized at high pressure: EXAFS data analysis.

Local structure of Y(1-x)Yb(x)Ni2B2C series synthesized at high pressure 8 GPa has been studied using EXAFS. Measurements were performed at the Ni K-edge in temperature range 5-300 K. The results show that the Debye-Waller factor for Ni-Ni bond in the parent YNi2B2C compound is characterized by the Einstein temperature O(E) = 350 K, while a minimum value O(E) = 300 K is reached for the compound with = 0.025, which has the highest critical temperature T(c) = 12.5K of the superconductive transition. This correlates with the further suppressing of superconductivity and with the appearance of the local magnetic moments in the investigated Y(1-x)Yb(x)Ni2B2C series for x > or = 0.05 compounds. Observed changes in the local electronic and the local crystal structure of this system as a function of Yb concentration and of temperature were explained in the frame of the band filling effect.