We report on structural, magnetic and transport properties of a new set of the high-pressuresynthesized compounds Mn1−xRhxGe (0 ≤ x ≤ 1) with the chiral magnetic ordering. The magnetic and transport properties depend substantially on the concentration of rhodium (x) and the pressure. The saturation magnetic moment corresponds to a known high-spin value for pristine MnGe (x = 0) and decreases almost linearly with increasing concentration x. In addition, XMCD spectra taken at 10 K and 2 T indicate magnetic polarization of the Rh 4d electron states and Ge 4p states, which decreases with x, too. In rhodium rich compounds (x ≥ 0.5) the temperature of the magnetic ordering increases significantly with pressure, whereas in manganese rich compounds (x < 0.5) the temperature decreases. Three different tendencies are also found for several structural and transport properties. In the intermediate range (0.3 ≤ x ≤ 0.7) samples are semiconducting in the paramagnetic phase, but become metallic in the magnetically ordered state. We carried out ab initio density-functional calculations of Mn1−xRhxGe at various concentrations x and traced the evolution of electronic and magnetic properties. The calculation results are in good agreement with the measured magnetic moments and qualitatively explain the observed trends in transport properties.
Based on ab initio band structure calculations we formulate a general theoretical method for description of the temperature dependence of electric field gradient in solids. The method employs a procedure of averaging multipole electron density component (l = 0) inside a sphere vibrating with the nucleus at its center. As a result of averaging each Fourier component (K = 0) on the sphere is effectively reduced by the square root of the Debye-Waller factor [exp(−W )]. The electric field gradient related to a sum of K−components most frequently decreases with temperature (T ), but under certain conditions because of the interplay between terms of opposite signs it can also increase with T . The method is applied to calculations of the temperature evolution of the electric field gradients of pristine zinc and cadmium crystallized in the hexagonal lattice. For calculations within our model of crucial importance is the temperature dependence of mean-square displacements which can be taken from experiment or obtained from the phonon modes in the harmonic approximation. For the case of Zn we have used data obtained from single crystal x-ray diffraction. In addition, for Zn and Cd we have calculated mean-square displacements with the density functional perturbation treatment of the Quantum Espresso package. With the experimental data for displacements in Zn our calculations reproduce the temperature dependence of the electric field gradient very accurately. Within the harmonic approximation of the Quantum Espresso package the decrease of electric field gradients in Zn and Cd with temperature is overestimated. Our calculations indicate that the anharmonic effects are of considerable importance in the temperature dependence of electric field gradients.
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