We have used the time differential perturbed angular correlation (TDPAC) spectroscopy to measure the electric and magnetic hyperfine fields in RhGe crystallized in the B20 cubic lattice structure and weakly doped with Hf (0.5-2 atomic %) in the temperature range from 5 K to 295 K. Two most commonly used In-111 ->( 111)cd and Hf-181 -> Ta-181, have been used. The experimental results combined with theoretical density functional calculations indicate that the In/Cd impurities substitute into the Ge-site whereas the Ta/Hf probes substitute into the Rh-site. It has also been found that the Ta/Hf impurity strongly distorts the local crystal environment, whereas the effect from the In/Cd probe is weak. There are no reliable evidences of the magnetic order in the studied alloys at low temperatures. (C) 2020 Elsevier B.V. All rights reserved.

A Tb 3+ :YAB crystal was grown using a high-temperature solution growth on dipped seeds technique. Polarized ground state absorption and fluorescence spectra from the 5 D 4 level, as well as a fluorescence decay curves of 5 D 4 and 5 D 3 levels, were recorded at room temperature. Radiative properties such as emission probabilities, branching ratios, and radiative lifetime were investigated within the theory of 4f-4f transition intensity in the case of the intermediate configuration interaction.

We present studies of neutron diffraction, electrical resistivity, magnetic susceptibility, magnetization, and specific heat of noncentrosymmetric YbNiC2. At normal pressure, YbNiC2 is a moderate heavy-fermion compound with Kondo lattice. At 16 K we observe an anomaly in the temperature dependence of specific heat which is ascribed to an abrupt valence change of Yb ions. At pressures above 7 GPa, the valence change and the Kondo lattice state are suppressed, and near a temperature of 10 K we detect the appearance of magnetic order. Above 5 GPa, the temperature dependence of the resistivity behaves similarly to other compounds of the CeNiC2-type family, indicating the formation of charge density waves. This is attributed to the nesting properties of the Fermi surface (FS) found within the density functional theory + dynamical mean field theory treatment. Our ab initio calculations also show that the valence of Yb in YbNiC2 at normal conditions is 2.85, which increases with temperature and pressure. The FS of YbNiC2 is anisotropic in shape in comparison with the 3D surface of YbCoC2.

We report on the study of the noncentrosymmetric ternary carbide YbCoC2. Our magnetization, specific heat, resistivity, and neutron diffraction measurements consistently show that the system behaves as a heavy-fermion compound, displaying an amplitude-modulated magnetic structure below the Neel temperature reaching T-N = 33 K under pressure. Such a large value, being the highest among the Yb-based systems, is explained in the light of our ab initio calculations, which show that the 4f electronic states of Yb have a dual nature, i.e., due to their strong hybridization with the 3d states of Co, 4f states expose both localized and itinerant properties.

The mesostructure of Mn1 – xFex Ge transition-metal monogermanides is studied by small-angle neutron scattering (SANS) and ultra-SANS in a wide concentration range of x = 0.0–1.0.It is shown that the main contribution to the scattering intensity for all concentrations x is made by scattering at crystallites with sharp boundaries and sizes of 1–10 μm, which is described by the squared Lorentzian function. An additional contribution to the scattering intensity as a result of scattering at an ensemble of defects is found as well, which is characteristic of manganese-rich samples. This contribution is well fitted by the power function Q–n with the exponent n = 3. The complementary scattering typical of iron-rich samples is described by an exponential function and also seems to be a part of scattering at sharp-boundary crystallites.

The search and exploration of new materials not found in nature is one of modern trends in pure and applied chemistry. In the present work, we report on experimental and ab initio density-functional study of the high-pressure-synthesized series of compounds Mn1-x(Co,Rh)(x)Ge. These high-pressure phases remain metastable at normal conditions, therewith they preserve their inherent noncentrosymmetric B20-type structure and chiral magnetism. Of particular interest in these two isovalent systems is the comparative analysis of the effect of 3d (Co) and 4d (Rh) substitution for Mn, since the 3d orbitals are characterized by higher localization and electron interaction than the 4d orbitals. The behavior of Mn1-x(Co,Rh)(x)Ge systems is traced as the concentration changes in the range 0 <= x <= 1. We applied a sensitive experimental and theoretical technique which allowed to refine the shape of the temperature dependencies of magnetic susceptibility chi(T) and thereby provide a new and detailed magnetic phase diagram of Mn1-xCoxGe. It is shown that both systems exhibit a helical magnetic ordering that very strongly depends on the composition x. However, the phase diagram of Mn1-xCox Ge differs from that of Mn1-xRhx Ge in that it is characterized by coexistence of two helices in particular regions of concentrations and temperatures.

We study the helimagnetic ground state of the MnGe cubic alloy using small-angle neutron scattering and a high-resolution method, the so-called MIEZE spectroscopy. Upon cooling below the Néel temperature T N = 170(5) K, we observe the proliferation of long-wavelength gapless spin fluctuations, concomitant with a continuous evolution of the helical correlation length. These fluctuations disappear at T com = 32(5) K when the helical period becomes commensurate with the lattice. We propose to describe this intermediate phase as a soliton lattice, promoting nonlinear collective modes, or phasons , over a large temperature interval. We discuss the possible relevance of our results to the previously observed magnetotransport anomalies. In this supplement, we provide details regarding sample preparation and characterization (Sec. I). The experimental methods used to obtain the results exposed in the main text, namely small-angle neutron scattering (Sec. II) and MIEZE spectroscopy (Sec. III) are also described. In Sec. IV, we reproduce the mean-field expressions used to calculate the expectation temperature-dependence of the ordered moment.

In the MnGe chiral magnet, the helimagnetic order and local moment collapse in two steps, showing the succession of high spin (HS) and low spin (LS) states as pressure increases. Here, we use high-pressure neutron diffraction to study the doped compounds Mn0.86Co0.14Ge and Mn0.9Rh0.1Ge, and show that the evolution of their microscopic magnetic properties is instead continuous. It means that the bulk HS-LS transition is a unique feature of pure MnGe, very sensitive to small changes of the band structure and easily suppressed by chemical substitution. On the other hand, the helimagnetic correlations appear to be strengthened by doping and survive up to larger pressures (approximate to 19 GPa, to be compared with approximate to 13 GPa). We discuss these results in the light of other disordered systems with remarkable properties, the so-called Invar alloys.

N. M. Chtchelkatchev, 2, 3, 4 M. V. Magnitskaya, 5, 1 V. A. Sidorov, L. N. Fomicheva, A. E. Petrova, and A. V. Tsvyashchenko Landau Institute for Theoretical Physics, Russian Academy of Sciences, 142432 Chernogolovka, Moscow Region, Russia Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, 108840 Troitsk, Moscow, Russia Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia Ural Federal University, 620002, 19 Mira str., Ekaterinburg, Russia Lebedev Physical Institute, Russian Academy of Sciences, 119991 Moscow, Russia (ΩDated: January 16, 2019)

We have studied the C14 hexagonal Laves phase of YbAg2 at normal conditions and under external pressure up to 19 GPa by the time-differential perturbed angular γ− γ correlation spectroscopy (TDPAC) using Cd probe nuclei. Under pressure the valence of Yb undergoes a two stage transition from 2.8 to 3. The two stage scenario is characterized by two distinct quadrupole frequencies of Cd probes in silver sublattice, monotonically increasing with pressure and saturating at 8 and 16 GPa. Our experimental data are compared with the density functional studies of the electron band structure of YbAg2, whose results are used for discussion and interpretation of these experiments. We have found that there are two different electric field gradients at inequivalent silver sites and that 4d-states of silver participate in metal bonding, allowing for the formation of the hexagonal Laves phase.

A novel metastable phase of DyGe2.85 synthesised in the AuCu3-structure under high pressure, has been studied by means of the magnetic susceptibility and electrical resistivity measurements (under the pressure P≤3.1 GPa), neutron powder diffraction and time-differential γ-γ perturbed angular correlations (TDPAC) using 111Cd nuclear probes. Two distinct phase transitions have been found in this compound as the temperature is lowered. We assign the first transition occurring at the temperature TCDW=80 K with charge density wave formation and the second transition at TN≈18 K with an antiferromagnetic spiral ordering of Dy magnetic moments, and discuss a close relationship between them.

We report on structural, magnetic and transport properties of a new set of the high-pressuresynthesized compounds Mn$_{1-x}$Rh$x$Ge ($0 \leq x \leq 1$) 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 ($x \geq 0.5$) 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 ($0.3 \leq x \leq 0.7$) samples are semiconducting in the paramagnetic phase, but become metallic in the magnetically ordered state. We carried out ab initio density-functional calculations of Mn$_{1-x}$Rh$_x$Ge 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.

AbstractThe mesostructure of transition-metal monogermanides Mn_1 – x Co_ x Ge is studied by small-angle neutron scattering in a wide range of concentrations x = 0–0.95. These compounds were synthesized under high pressure and are metastable under normal conditions. The experimental dependences I ( Q ) obtained for the whole series of samples in the range of transferred momenta (6 × 10^–2 nm^–1 < Q < 2.5 nm^–1) are described by the power dependence Q ^– n with an exponent n = 2.99 ± 0.02, uniquely related to the fractal properties of the system under study. The dependence obtained indicates that the superatomic structure of the samples is characterized by the presence of defects with a spatial organization described by a fractal model with a logarithmic dependence of the correlation function of the defect density. It is interesting to note that such defects are absent in the isostructural FeGe compound, i.e., the experimental dependences of the intensity are described well by the expression Q ^– n with an exponent n = 4.1 ± 0.1, which demonstrates the presence of crystallites with a uniform density distribution inside and a sharp boundary characterizing the surface.

The mesostructure of transition-metal monogermanides Mn 1 – x Co x Ge is studied by small-angle neutron scattering in a wide range of concentrations x = 0–0.95. These compounds were synthesized under high pressure and are metastable under normal conditions. The experimental dependences I ( Q ) obtained for the whole series of samples in the range of transferred momenta (6 × 10 –2 nm –1 < Q < 2.5 nm –1 ) are described by the power dependence Q – n with an exponent n = 2.99 ± 0.02, uniquely related to the fractal properties of the system under study. The dependence obtained indicates that the superatomic structure of the samples is characterized by the presence of defects with a spatial organization described by a fractal model with a logarithmic dependence of the correlation function of the defect density. It is interesting to note that such defects are absent in the isostructural FeGe compound, i.e., the experimental dependences of the intensity are described well by the expression Q – n with an exponent n = 4.1 ± 0.1, which demonstrates the presence of crystallites with a uniform density distribution inside and a sharp boundary characterizing the surface.

We study the evolution of helical magnetism in MnGe chiral magnet upon partial substitution of Mn for 3d-Co and 4d-Rh ions. At high doping levels, we observe spin helices with very long periods-more than ten times larger than in the pure compound-and sizable ordered moments. This behavior calls for a change in the energy balance of interactions leading to the stabilization of the observed magnetic structures. Strikingly, neutron scattering unambiguously shows a double periodicity in the observed spectra at x = 0.5 and >0.2 for Co- and Rh-doping, respectively. In analogy with observations made in smectic liquid crystals, we suggest that it may reveal the presence of magnetic "twist grain boundary" phases, involving a dense short-range correlated network of magnetic screw dislocations. The dislocation cores are here tentatively described as smooth textures, made of nonradial double-core skyrmions.

We have studied the C14 hexagonal Laves phase of YbAg$_2$ at normal conditions and under external pressure up to 19 GPa by the time-differential perturbed angular $\gamma-\gamma$ correlation spectroscopy (TDPAC) using $^{111}$Cd probe nuclei. Under pressure the valence of Yb undergoes a two stage transition from 2.8 to 3. The two stage scenario is characterized by two distinct quadrupole frequencies of $^{111}$Cd probes in silver sublattice, monotonically increasing with pressure and saturating at 8 and 16 GPa. Our experimental data are compared with the density functional studies of the electron band structure of YbAg$_2$, whose results are used for discussion and interpretation of these experiments. We have found that there are two different electric field gradients at inequivalent silver sites and that $4d$-states of silver participate in metal bonding, allowing for the formation of the hexagonal Laves phase.

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.

RhGe synthesized at high pressure is crystallized in noncentrosymmetric cubic structure of the B20 type. Measurements of the electrical resistivity and magnetization demonstrate a superconducting state below T$_c$ ~ 4.5 K and a weak ferromagnetism below T$_m$ ~ 140 K. Specific heat data confirm the bulk nature of superconductivity in this ferromagnetic superconductor. The superconducting region forms a dome on the P-T diagram with a maximum of T$_c$ near 4 GPa. Ab initio simulations suggest that the observed weak magnetization emerges from the pronounced spin polarization with magnetic quadrupole and toroidal moments located at Rh and Ge sites.