A. V. Bogach

Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB12

Precision measurements of the specific heat and spectral intensity I(ω) of Raman scattering for LuNB12 single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I(ω) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50−70 K, which appears when Lu3+ ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12–15 and 18–22 Å, respectively, have been estimated. The vibrational density of states of LuB12 has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB12 compound at T < T*, as well as the features of the formation of magnetic structures in RB12 antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB12.

Genesis of the anomalous Hall effect in CeAl2

The Hall coefficient RH, resistivity ρ, and Seebeck coefficient S of the CeAl2 compound with fast electron density fluctuations were studied in a wide temperature range (from 1.8 to 300 K). Detailed measurements of the angular dependences RH(ϕ T, H≤70 kOe) were performed to determine contributions to the anomalous Hall effect and study the behavior of the anomalous magnetic RHam and main RHa components of the Hall signal of this compound with strong electron correlation. The special features of the behavior of the anomalous magnetic component RHam were used to analyze the complex magnetic phase diagram H-T determined by magnetic ordering in the presence of strong spin fluctuations. An analysis of changes in the main contribution RHa (H, T) to the Hall effect made it possible to determine the complex activation behavior of this anomalous component in the CeAl2 intermetallic compound. The results led us to conclude that taking into account spin-polaron effects was necessary and that the Kondo lattice and skew-scattering models were of very limited applicability as methods for describing the low-temperature transport of charge carriers in cerium-based intermetallic compounds. The effective masses and localization radii of manybody states in the CeAl2 matrix were estimated to be (55–90)m0 and 6–10 Å, respectively. The behaviors of the parameters RH, S, and ρ were jointly analyzed. The results allowed us to consistently describe the transport coefficients of CeAl2.

Enhancement of band magnetism and features of the magnetically ordered state in the CeB6 compound with strong electron correlations

Precision measurements of transport and magnetic parameters of high-quality CeB6 single crystals are performed in the temperature range 1.8—300 K. It is shown that their resistivity in the temperature interval 5 K < T < T* ≈ 80 K obeys not a logarithmic law, which is typical of the Kondo mechanism of charge carrier scattering, but the law ρ ∝ T−1/η corresponding to the weak localization regime with a critical index 1/η = 0.39 ± 0.02. Instead of the Curie-Weiss dependences, the asymptotic form χ(T) ∝ T−0.8 is obtained for magnetic susceptibility of CeB6 in a temperature range of 15–300 K. Analysis of the field dependences of magnetization, magnetoresistance, and the Hall coefficient in the paramagnetic and magnetically ordered phases of CeB6 and comparison with the results of measurements of Seebeck coefficient, the inelastic neutron scattering coefficient, and EPR spectroscopy lead to the conclusion that the Kondo lattice model and skew scattering model cannot be used for describing the transport and thermodynamic parameters of this compound with strong electron correlations. On the basis of detailed analysis of experimental data, an alternative approach to interpreting the properties of CeB6 is proposed using (1) the assumption concerning itinerant paramagnetism and substantial renormalization of the density of electron states upon cooling in the vicinity of the Fermi energy, which is associated with the formation of heavy fermions (spin-polaron states) in the metallic CeB6 matrix in the vicinity of Ce sites; (2) the formation of ferromagnetic nanosize regions from spin polarons at 3.3 K < T < 7 K and a transition to a state with a spin density wave (SDW) at TQ ≈ 3.3 K; and (3) realization of a complex magnetic phase H-T diagram of CeB6, which is associated with an increase in the SDW amplitude and competition between the SDW and antiferromagnetism of localized magnetic moments of cerium ions.

Anomalies of magnetoresistance of compounds with atomic clusters RB12 (R = Ho, Er, Tm, Lu)

The magnetoresistance and magnetization of single-crystal samples of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Lu) have been measured at low temperatures (1.8–35 K) in a magnetic field of up to 70 kOe. The effect of positive magnetoresistance that obeys the Kohler’s rule Δρ/ρ = f(ρ(0, 300 K)H/ρ(0, T)) is observed for the nonmagnetic metal LuB12. In the magnetic dodecaborides HoB12, ErB12, and TmB12, three characteristic regimes of the magnetoresistance behavior have been revealed: the positive magnetoresistance effect similar to the case of LuB12 is observed at T > 25 K; in the range TN ≤ T ≤ 15 K, the magnetoresistance becomes negative and depends quadratically on the external magnetic field; and, finally, upon the transition to the antiferromagnetic phase (T < TN), the positive magnetoresistance is again observed and its amplitude reaches 150% for HoB12. It has been shown that the observed anomalies of negative magnetoresistance in the paramagnetic phase can be explained within the Yosida model of conduction electron scattering by localized magnetic moments. The performed analysis confirms the formation of spin-polaron states in the 5d band in the vicinity of rare-earth ions in paramagnetic and magnetically ordered phases of RB12 and makes it possible to reveal a number of specific features in the transformation of the magnetic structure of the compounds under investigation.

Is MnSi an itinerant-electron magnet? Results of ESR experiments

High-frequency (60 GHz) electron spin resonance (ESR) has been studied in manganese monosilicide, MnSi, single crystals. The measurements performed within the 4.2–300 K temperatures range at the applied magnetic field up to 70 kOe have demonstrated that the magnetic resonance in MnSi is due to localized magnetic moments of the Heisenberg type with the g-factor depending only slightly on temperature, g ∼ 1.9–2. At the same time, it has been found that the ESR linewidth is determined by spin fluctuations and can be quantitatively described in the wide temperature range (4.2 K < T < 60 K) in the framework of the Moriya theory using the SL(T) function. The revealed deviations from the model of weak itinerant-electron magnetism commonly used for the description of the magnetic properties of MnSi indicate a possible spin-polaron nature of the unusual magnetic properties of this strongly correlated metal.

Antiferromagnetic instability and the metal-insulator transition in Tm1 − xYbxB12 rare earth dodecaborides

The magnetic and transport characteristics of substitutional solid solutions of the Tm1−xYbxB12 rare earth dodecaborides have been investigated. The measurements performed in the wide temperature range 1.8−300 K on the high-quality single-crystalline samples make it possible to conclude that as x increases, antiferromagnetic instability develops with the quantum critical point (TN = 0) near x = 0.3 and relevant dielectrization of the electron structure in the range 0 ≤ x ≤ 0.8 takes place. With the results of the thermoelectric measurements in Tm1−xYbxB12, the activation energy values and their dependence on x have been determined.

Isotope effect in charge transport of LuB12

The galvanomagnetic properties of single-crystal samples with various isotopic boron compositions have been investigated for the first time for the normal state of superconductor LuB12 (Tc ≈ 0.44 K). Precision measurements of the resistivity, Hall coefficient, and magnetic susceptibility have been performed over a wide temperature range of 2–300 K in magnetic fields up to 80 kOe. A change of the charge transport regime in this nonmagnetic compound with metallic conduction is shown to occur near T* ≈ 50−70 K. As a result, a sharp peak with significantly different amplitudes for Lu10B12 and Lu11B12 is recorded in the temperature dependences of the Hall coefficient RH(T) near T*. A significant (about 10%) difference (in absolute value) of the Hall coefficients RH for the Lu10B12 and Lu11B12 compounds at helium and intermediate temperatures has been found and the patterns of behavior of the dependence RH(H) for T < T* in an external magnetic field H ≤ 80 kOe for Lu10B12 and Lu11B12 are shown to differ significantly. Analysis of the Curie-Weiss contribution to the magnetic susceptibility χ(T) leads to the conclusion about the formation of magnetic moments μeff ≈ (0.13−0.19)μB in each unit cell of the fcc structure of LuB12 compounds with various isotopic compositions. The possibility of the realization of an electronic topological 2.5-order transition near T* and the influence of correlation effects in the 5d-band on the formation of a spin polarization near the rare-earth ions in LuB12 is discussed.

Antiferro-quadrupole resonance in CeB6

We report experimental observation of a new type of magnetic resonance caused by orbital ordering in a strongly correlated electronic system. Cavity measurements performed on CeB 6 single crystals in a frequency range 60-100 GHz show that a crossing of the phase boundary T Q (B) between the antiferro-quadrupole and paramagnetic phases gives rise to development at T< T Q (B) of a magnetic resonance. The observed mode is gapless and correspond to g-factor 1.62.

Charge transport in HoxLu1-xB12: Separating Positive and Negative Magnetoresistance in Metals with Magnetic Ions

The magnetoresistance (MR)

Anomalous thermopower in heavy-fermion compounds CeB6, CeAl3, and CeCu6 − x Au x

\Delta \rho/\rho