Seebeck coefficient has been studied on the single crystals of metallic hexaborides RB 6 (R = La, Pr, Nd, Gd) at temperatures of 2–300 K. The experiment has shown that the signal is limited by the values | S | ≤ 1.5 μV/K for all tested materials. The data obtained for the systems LaB 6 and GdB 6 were approximated by phonon drag contribution caused by quasi-local (Einstein) mode with characteristic temperatures Θ E (LaB 6 ) ≈ 240 K and Θ E (GdB 6 ) ≈ 180 K. On the contrary, the crystalline electric field effect induces the inversion between negative and positive types of thermoelectricity, which complicates the simulation of phonon drag in the case of PrB 6 and NdB 6 .

We report the study of resistivity and transverse magnetoresistance of LaB6 and R(0.01)La(0.99)B(6 )diluted systems (R-Ce, Pr, Nd, Eu, Gd, and Ho) measured at temperatures between 1.8 K and 300 K in magnetic fields up to 82 kOe. Our data allow us to observe the influence of a wide class of impurities on the transport properties of LaB(6 )host material accompanied by various interesting anomalies including a crossover from positive (R-La, Pr, Nd, Eu, and Gd) to negative (R-Ce, Ho) regime of magnetoresistance below inversion points. The temperature dependences of zero-field resistivity are shown to be well described by a sum of (i) Debye term from rigid boron cages (Theta(D )approximate to 1160 K) and (ii) the contribution from quasilocal vibrations of La3+ ions (Theta(E1) approximate to 154-156 K and Theta(E2) approximate to 320-340 K). We register a low-T rise of resistivity for the materials containing R-Ce, Pr, Nd, and Ho centers. It is found that instead of logarithmic behavior rho(T)similar to -lnT, predicted by Kondo model, the scattering on magnetic impurities follows to power law rho(T)similar to T-alpha (alpha approximate to 0.2-0.49), which corresponds to the regime of weak localization of the charge carriers.

Seebeck effect in the crystalline samples of EuxYb1-xB6(x= 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2-300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (S-d=AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures Theta(E)(YbB6) approximate to 91 K and Theta(E)(EuB6) approximate to 122 K. The estimation of effective massm*of the charge carriers proves that increasing of Eu content induces crossover from 'heavy' holes withm(h)*(x <= 0.127) approximate to 0.3-0.36m(0)to 'light' electrons withm(e)*(x > 0.9) approximate to 0.12-0.13m(0)(m(0)-free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.

We present results of precision measurements of magnetoresistance of isotopically pure (HoB12)-B-11 at low temperatures 2 divided by 10 K in magnetic field up to 80 kOe of different orientation to the crystal axes. The data obtained revealed strong anisotropy of magnetoresistance and allowed us to reconstruct magnetic H-T phase diagrams for main crystallographic directions H parallel to [001], [110], and [111]. Analysis of magnetoresistance derivatives allowed to conclude in favor of two main magnetoresistance contributions. Among of them the negative quadratic component is attributed to charge carriers scattering on a magnetic clusters of Ho3+ ions (4f component) and positive linear one may be explained in terms of scattering on a spin density waves (5d component).

The anisotropy of charge transport was investigated in the antiferromagnetic II state of GdB6 from precise measurements of transverse magnetoresistance. Based on the data obtained we detected a complicated behavior of magnetoresistance curves which are characterized by the appearance of considerable hysteresis on the field and angular dependences below T-N2. Moreover it was shown that the system GdB6 is sensitive to cooling-warming prehistory. The data analysis allowed to reconstruct magnetic H-T phase diagram of GdB6 along main crystallographic directions (H parallel to < 001 >, < 110 >, < 111 >) and to propose additional phase transition inside AF II phase at H-1 approximate to 0.5 T.

Recent theoretical models and experiments suggest that SmB6 is a topological Kondo insulator. Here we report on heat capacity C(T) investigations of three single-crystalline SmB6 samples having different quality, of SmB6 single crystals doped with various concentration of trivalent lanthanum ions (Sm1-xLaxB6 solid solutions) and divalent ytterbium ions (Sm1-xYbxB6), and of samarium-deficient sintered samples(Sm1-xB6). Isosbestic (crossing) points, which can be considered as a feature of universality, were observed on C(T, x) dependencies for all investigated types of samples at Tiso(Sm1-xLaxB6) approximate to 17 K, Tiso(Sm1-xYbxB6) approximate to 31 K, and Tiso(Sm1-xB6) approximate to 21 K. In connection with this as well as with the detailed analysis of the low-temperature C(T) dependencies it can be assumed that by appropriate doping the properties of SmB6 may be changed/tuned. To obtain the evolution of the gap structure of SmB6 with changing doping, a simple phenomenological model based on rectangular densities of states was proposed. Additionally, for Sm1-xLaxB6 solid solutions which were available with a high La concentration, it was shown that the insulator-metal transition appears around x(La) approximate to 0.40.

The specific heat of high quality (LaB6)-B-N (N = 10, 11, natural) single crystals is investigated in a wide range of temperatures 2 similar to 300 K. The obtained data allow to estimate correctly (i) the electronic gamma. T term of specific heat (gamma approximate to 2.4 mJ/(mol.K-2)), (ii) the contribution from quasilocal vibrating mode of La3+ ions (Theta(E) approximate to 150 - 152 K), (iii) the Debye-type term from rigid boron cages (Theta(eta) approximate to 1160 +/- 40 K). Our data also suggest an additional defect-mode component (iv) which may be attributed to a contribution of 1.5% boron vacancies in LaB6. The obtained results may be interpreted in terms of formation of two level systems, which appear when La3+ ions are displaced from their centrosymmetric positions in the cavities of rigid boron cages, apart from randomly distributed boron vacancies in the LaB6 matrix.

Hall E ect in GdB6 M. Anisimova,∗, A. Bogach, V. Glushkov, S. Demishev, N. Samarin, N. Shitsevalova, A. Levchenko, V. Filippov, A. Kuznetsov, K. Flachbart, N. Sluchanko A. M. Prokhorov General Physics Institute of RAS, Vavilov str. 38, 119991 Moscow, Russia Moscow Institute of Physics and Technology, Institutskii per. 9, 141700 Dolgoprudnyi, Russia I. Frantsevich Institute for Problems of Materials Science NAS, Krzhyzhanovsky str. 3, 03680 Kiev, Ukraine Moscow Engineering Physics Institute, Kashirskoe sh. 31, 115409 Moscow, Russia Institute of Experimental Physics SAS, Watsonova 47, 040 01 Ko2ice, Slovakia

The Hall effect of GdB6 has been studied on high quality single crystals in the temperature range 2-150 K and in magnetic field of 1 T. The obtained data allow to detect anomalies in the antiferromagnetic (AF) phase including (i) a drastic enhancement of negative Hall coefficient below T-N1 approximate to 15.5 K and (ii) the appearance of an anomalous Hall effect at T-N2 approximate to 4.7 K. Possible scenarios of the AF ground state formation are discussed.

High precision measurements of the Seebeck coefficient S(T) were carried out on the single crystals of RB12 (R = Ho, Er, Tm, Lu) at temperatures 2-300 K. It was shown that the effects of phonon drag result from vibrations of rare earth ions (h(wE)) approximate to 10-33 meV) in the rigid framework structure of the B-12 clusters and determine the main contribution to thermopower at intermediate temperatures (30-300 K). The correlated behavior of transport parameters favors the appreciable enhancement of spin fluctuations in the sequence of magnetic compounds (HoB12-TmB12) when approaching to the valence instability state in YbB12. The giant increase in S(T) detected in the vicinity of the Neel temperature TN for HoB12, ErB12, and TmB12 Seems to result from the density of states renormalization caused by anti ferromagnetic ordering.

High precision measurements of magnetoresistance Delta rho/rho = f(T, H) and magnetization M(T, H) were carried out on single crystals of rare-earth do-decaboride ErB12 at temperatures in the interval 1.8-30 K in magnetic fields up to 70 kOe. The high accuracy of the experiments allowed us to perform numerical differentiation and analyze quantitatively the behavior of the derivative d(Delta rho/rho)/dH = f(T, H) and of the magnetic susceptibility X(T, H)= dM/dH in paramagnetic and magnetically ordered (antiferromagnetic, T-N approximate to 6.7 K and T-M approximate to 5.85 K) phases of ErB12. It was shown that negative magnetoresistance anomalies observed in present study in paramagnetic state of ErB12 may be consistently interpreted in the framework of a simple relation between resistivity and magnetization - Delta rho/rho similar to M-2.

High precision measurements of Hall RH(T) and Seebeck S(T) coefficients have been carried out for the first time on single crystals of rare earth dodecaborides RB12 (R - Ho, Er, Tm, Lu) at temperatures 1.8 - 300K. Low temperature anomalies associated with antiferromagnetic phase transitions in HoB12, ErB12 and TmB12 compounds have been detected on the temperature dependencies of RH(T) and S(T). The estimated values of charge carriers mobility allowed us to conclude about the appreciable influence of spin fluctuations on the charge transport in these compounds with B12 atomic clusters.

High precision measurements of Hall RH(T) and Seebeck S(T) coefficients have been carried out for the first time on single crystals of rare earth dodecaborides RB12 (R - Ho, Er, Tm, Lu) at temperatures 1.8 - 300K. Low temperature anomalies associated with antiferromagnetic phase transitions in HoB12, ErB12 and TmB12 compounds have been detected on the temperature dependencies of RH(T) and S(T). The estimated values of charge carriers mobility allowed us to conclude about the appreciable influence of spin fluctuations on the charge transport in these compounds with B12 atomic clusters.