An universal line revealing an independence of spin fluctuation contributions to the heat capacity on impurity content and its nature is discovered in the helical phase of Mn(Co,Fe)Si. This situation declares an invariance of the heat capacity of spin subsystem under doping, which probably arises as a result of relative stiffness of the helical spin structure in respect to the impurity spins. On the other hand the situation drastically changes at the helical fluctuation region when no long range spin order exists. At low temperatures the spin fluctuation contributions to the heat capacity for a whole set of compositions of Mn(Co,Fe)Si are described by a single power expression with an exponent less then unity, which implies divergence of the ratio Cp/T at T→0. The current data are revealing that a singular quantum critical point does not exist in the system under study. In its place, one can see some sort of a quantum critical cloud covering a significant range of dopant concentrations.

The magnetoresistance of a well-characterized bulk FeSi sample has been studied. It is shown that after some complicated behavior at temperatures below 6K, the magnetoresistance of FeSi, becomes a regular function of temperature and magnetic field. Then, the magnetoresistance passes a maximum at about 12 K and gradually decreases, approaching slightly negative values and then zero. Thus, the observations suggest the existence of the negative component of magnetoresistance, which appears at about 12K and finally defines the negative values of both transverse and longitudinal magnetoresistance of FeSi at high temperatures. The negative component of magnetoresistance may be related to the topological features of the chiral structure of FeSi.

The transverse and longitudinal magnetoresistance (MR) of two samples of the topological chiral semimetal CoSi with different RRR was studied. It is shown that the Kohler rule works for the transverse MR. The Kohler rule is also fulfilled in the case of longitudinal MR at a low reduced magnetic field. A sharp deviation of longitudinal MR curve for sample with low RRR from the Kohler prediction at high fields reveals its tendency to a sign change at higher magnetic fields. The Shubnikov de Haas quantum oscillations were observed and analyzed in both perpendicular and parallel configurations of the current and magnetic field in sample CoSi 1 with RRR 9.33 at low temperatures.

We report on a comprehensive experimental and theoretical study of Fe 1 − x Rh x Ge compounds, within the entire concentration range x ∈ [0 . 0 − 1 . 0], using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions x ∈ [0 . 2 − 0 . 9]. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

The positron annihilation lifetimes were measured using a 48V positron source in noncentrosymmetric cubic single crystals of CoSi, FeSi and MnSi. The following lifetimes were determined from the positron annihilation time spectra: 168(1) ps for CoSi, 114(1) ps for FeSi and 111(1) ps for MnSi. For single-crystal CoSi, the positron annihilation lifetime was also determined with a 22Na positron source. For CoSi, the lifetimes obtained from different positron sources are consistent. The differences in the positron annihilation lifetimes in MnSi and FeSi, on the one hand, and in the Weyl semimetal CoSi, on the other hand, are possibly caused by the formation of a positron + electron bound state (positronium).

Transition metal silicides crystallizing in a B20 chiral structure whose space group P2(1)3 does not contain an inversion center feature a number of remarkable properties that have been studied for many decades. We analyze investigations of MnSi, FeSi, and CoSi, the most studied materials of this class, which are a sequence of a magnetic metal, a semiconductor, and a semimetal. Each of these materials exhibits the influence of spatial symmetry on certain features of the electronic and phonon spectra, some of which have been discovered quite recently.

Samples of (Mn1-xCox)Si with x = 0.15 and x = 0.17 were grown, and their physical properties: magnetization, magnetic susceptibility, resistivity, and heat capacity were studied. The data analysis included previous results at x = 0.057, 0.063, and 0.09. The doping of MnSi with Co completely destroys the helical phase transition, but it saves the helical fluctuation area normally situated slightly above the phase-transition temperature. This area, spreading from similar to 5 to 0 K does not change much with doping and forms some sort of helical fluctuation cloud, revealing the quantum critical properties: C-p/T -> infinity at T -> 0.

Positron annihilation lifetime spectroscopy (PALS) with the 48V isotope in Ti foil as a convenient positron source has been used to study the B20 single crystalsMnSi, FeSi and CoSi. The following positron lifetimes in single crystals were determined: 111(1) ps for MnSi, 114(1) for FeSi and 168(1) ps for CoSi. Italso has been established that the lifetimes in CoSi found with 22Na and 48V as positron sources are the same. The di erences in the lifetimes of MnSi, FeSi, onone side, and CoSi on the other, is attributed to the presence of Weyl fermions in single crystal CoSi.

We report on a comprehensive experimental and theoretical study of Fe_1-xRh_xGe compounds, within the entire concentration range x ∈[0.0 - 1.0], using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions x ∈[0.2 - 0.9]. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

We report on a comprehensive experimental and theoretical study of Fe$_{1-x}$Rh$_{x}$Ge compounds, within the entire concentration range $x \in \left[0.0 - 1.0\right]$, using X-Ray diffraction, small-angle neutron scattering, magnetometry and theoretical calculations. While FeGe and RhGe are single phase helimagnet and unconventional superconductor, respectively, an internal splitting of the crystallographic and magnetic states is found for intermediate compositions $x \in \left[0.2 - 0.9\right]$. A theoretical analysis of the stability of the two detected phases, together with the experimental data, indicate that this splitting preserves a common space group and occurs within single crystallites. Despite their apparent similarity, these two phases however display different magnetic structures, with distinct ferro- and helimagnetic character.

Various critical phenomena and critical points appearing in many-particle systems under the variation of external conditions (fields), including the temperature, pressure, and magnetic and electric fields are described. Laws of statistical physics are universal for any particular nature of particles and the considered phenomena can be observed in systems of macroparticles (colloids), atoms, molecules, and subnuclear particles (nucleons and quarks).

The specific heat, magnetization and thermal expansion of single crystals of antiferromagnetic insulator EuTe, measured at temperatures down to 2 K and in magnetic fields up to 90 kOe, demonstrate non trivial properties. The Neel temperature, being ∼ 9.8 K at H=0, decreases with magnetic field and tends to zero at ∼ 76 kOe, therefore forming a quantum critical point. The heat capacity and thermal expansion coefficient reveal λ-type anomalies at the second order magnetic phase transition at low magnetic fields, evolving to simple jumps at high magnetic fields and low temperatures, well described in a fluctuation free mean-field theory. The experimental data and the corresponding analysis favor the quantum concept of effective increasing space dimensionality at low temperatures that suppresses a fluctuation divergence at a second order phase transition.

We have grown and characterized three samples of Co doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results follow that impurities (Co and Fe) eliminate the first order phase transition peaks and spread the fluctuation maxima in such a way that its low temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low temperature branches of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The $d\rho/dT$ data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio $C_p/T$ and hence to the diverging effective electron mass. We found out that at large concentration of the dopant there are no distinct phase transition points. What we observe is a cloud of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0~K.

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.

The problem of negative heat capacity, discussed in a number of theoretical and experimental works dealing with weak itinerant magnets, is studied. A detailed analysis of the heat capacity of helicoidal itinerant magnet MnSi apparently demonstrates that the negative heat capacity in this case is a false effect because the interaction of the spin subsystem with other degrees of freedom is ignored in the analysis of the partial contributions to the heat capacity of the system. The negative heat capacity obtained by subtracting the electron and phonon components from the total heat capacity of the system is obviously an effective magnetic contribution, including the effects of spin–phonon coupling.

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.

We have grown and characterized three samples of Co doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results follow that impurities (Co and Fe) eliminate the first order phase transition peaks and spread the fluctuation maxima in such a way that its low temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low temperature branches of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The dρ/dT data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio C_p/T and hence to the diverging effective electron mass. We found out that at large concentration of the dopant there are no distinct phase transition points. What we observe is a cloud of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0 K.

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 report results of studying the magnetization, specific heat, and thermal expansion of a single crystal with nominal composition (Mn1-xFex)Si with x = 0.15. We found no thermodynamic evidence in favor of a second-order phase transition in this material. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e., approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.