Nonmagnetic metal LuB_12 is known to exhibit considerable transport anisotropy, which was explained in literature by different mechanisms including possible formation of dynamic charge stripes below the point ∼ 150K. Here we study transport properties of solid solutions based on LuB_12 host compound with general formula R_xLu_1-xB_12 (R-Dy, Er, Tm, Yb, Lu) and with x ≤ 0.03. The experiment has been performed on single crystals of high quality in the temperature range 1.8 - 300K in magnetic fields up to 82kOe. The application of several models to the analysis of zero-field resistivity is discussed. A phenomenological description of large positive quadratic component of transverse magnetoresistance Δρ/ρ(H) = μ_D^2H^2, which dominates for all compounds under investigation, allows to estimate drift mobility exponential changes μ_D ∼ T^-α with the index α ≈ 0.95 - 1.46. In order to check the existence of additional channel of scattering, caused by probable presence of dynamic charge stripes, we performed the study of the anisotropy of magnetoresistance in Dy_0.01Lu_0.99B_12 and Tm_0.03Lu_0.97B_12 compositions including the measurements of the field scans with different current and field geometries. The data obtained allow us to confirm the fulfillment of semi-empirical Kohler's rule in a wide interval of temperatures 30 - 240K regardless of the orientation of current and magnetic field. This result was attributed as a proof of the absence of additional channel of scattering caused by stripes. We argue, that charge-transport anisotropy is originated in R_xLu_1-xB_12 due to the anisotropy of electron-phonon scattering on the one hand and the effects of Fermi surface (FS) topology (at low temperatures) on the other.

Among of rare-earth (RE) hexaborides only two compounds SmB6 and YbB6 are discussed in literature to be members of a new class of 3D topological insulators. However, their ground states originate due to different physical mechanisms, including Kondo 4f-5d hybridization and 5d-2p band inversion, respectively. Here we report a comparative study of magnetotransport (resistivity and transverse magnetoresistance) measured on high quality single crystals of YbxSm1-xB6 and EuxSm1-xB6 solid solutions (x ≤ 0.05) at temperatures 1.7 − 300 K in magnetic fields up to 82 kOe. The choice of dopant was determined by the fact that the presence of magnetic/nonmagnetic (Eu2+/Yb2+) impurity in parent SmB6 matrix should lift/not lift the topological protection of surface states. Based on the two-gap paradigm the x-evolution of electron spectra in YbxSm1-xB6 and EuxSm1-xB6 was studied. Our data show that both the 4f lattice coherence (Eg) and the intrinsic gap (Ea) related to many-body states survive under RE doping at least for x ≈ 0.02 − 0.024. We also suggest that the point x(Eu) = 0.05 can be treated as an upper limit of the small gap closing in EuxSm1-xB6 materials. In YbxSm1-xB6 family a negative linear transverse magnetoresistance (TMR) was detected for the first time in the regime of surface conductivity (T < T* ≈ 5 K). The TMR anomaly at T* caused possibly by the topological protection of surface states in SmB6 is found to survive in Eu-doped compounds but disappears almost completely for Yb-doped compositions in the same fixed magnetic fields. This paradoxical observation is not consistent with general predictions of the topological Kondo insulator (TKI) model.

Zero-field heat capacity of EuxYb1-xB6 (x = 0, 0.127, 1) family was investigated at temperatures 1.9 -300 K. The applied procedure of C(T) decomposition allowed to identify the Debye component resulted from the rigid cage of boron atoms (& UTheta;D & AP;1160 K), as well as the contribution from the quasilocal vibrational modes of rare-earth (RE) ions with Einstein temperatures to be different in end-point compounds: & UTheta;E(YbB6) & AP;91.6 K and & UTheta;E(EuB6) & AP;125 K. Our results also suggest the existence of additional low-temperature defect mode for the Eulow systems (x & LE; 0.127) which is related to & AP;1.15-1.3% vacancies at boron position. The magnetic contribution for EuB6 was analyzed in the framework of mean-field theory. The estimates show that 97% of spin entropy, associated with the 8S7/2-state of Eu2+ ions, is frozen out at TC. The finding that short-range magnetic ordering in the paramagnetic (PM) phase is limited by no more than 3% of EuB6 spin entropy should be taken into account in modern approaches explaining electronic and magnetic phase separation in this extraordinary compound.

A detailed study of charge transport in the paramagnetic phase of the cage-cluster dodecaboride Ho0.8Lu0.2B12 with an instability both of the fcc lattice (cooperative Jahn–Teller effect) and the electronic structure (dynamic charge stripes) was carried out at temperatures 1.9–300 K in magnetic fields up to 80 kOe. Four mono-domain single crystals of Ho0.8Lu0.2B12 samples with different crystal axis orientation were investigated in order to establish the singularities of Hall effect, which develop due to (i) the electronic phase separation (stripes) and (ii) formation of the disordered cage-glass state below T*~60 K. It was demonstrated that a considerable intrinsic anisotropic positive component ρanxy appears at low temperatures in addition to the ordinary negative Hall resistivity contribution in magnetic fields above 40 kOe applied along the [001] and [110] axes. A relation between anomalous components of the resistivity tensor ρanxy~ρanxx1.7 was found for H||[001] below T*~60 K, and a power law ρanxy~ρanxx0.83 for the orientation H||[110] at temperatures T < TS~15 K. It is argued that below characteristic temperature TS~15 K the anomalous odd ρanxy(T) and even ρanxx(T) parts of the resistivity tensor may be interpreted in terms of formation of long chains in the filamentary structure of fluctuating charges (stripes). We assume that these ρanxy(H||[001]) and ρanxy(H||[110]) components represent the intrinsic (Berry phase contribution) and extrinsic (skew scattering) mechanism, respectively. Apart from them, an additional ferromagnetic contribution to both isotropic and anisotropic components in the Hall signal was registered and attributed to the effect of magnetic polarization of 5d states (ferromagnetic nano-domains) in the conduction band of Ho0.8Lu0.2B12.

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

The galvanomagnetic characteristics of SmB6 single crystals are studied within the temperature range of 1.9‒3.6 K at different orientations of the crystal faces. As a result, the electrical resistivities of the surfaces corresponding to the (100), (110), (111), and (211) crystallographic planes are determined. It is shown that the effective parameters of charge carriers, which determine the surface conductivity in SmB6, depend both on the orientation of the surface and on the method of its processing. It is found that the etching of polished polar surfaces formed by (100) planes leads at 1.9 K to a decrease in the density and to an increase in the mobility of surface n-type charge carriers from 113/a2 and 1.12 cm2/(V s) to 0.76/a2 and 18 cm2/(V s), respectively (lattice parameter a ≈ 4.134 Å). For etched nonpolar surfaces corresponding to the (110) and (111) planes, the maximum density of surface charge carriers (per unit area of the surface Brillouin zone) is found to increase by factors of 2.3 and 3.9, respectively. It is proposed to use this parameter as a simple criterion to identify the features of electron transport due to the nontrivial topology of the band structure of SmB6.

We present the results of experimental studies on the synthesis by molecular-beam epitaxy of AlGaAs nanowires with InAs quantum dots. The morphological, structural, and optical properties of the grown nanostructures have been studied. It is important to note that the emission from quantum dots is observed in the wavelength range from 750 to 970 nm. Assumptions about the nature of short-wavelength emission from quantum dots are formulated. In particular, one of the reasons may be a significant desorption of indium atoms and the presence of gallium atoms in catalyst drops during the growth at a substrate temperature of 510◦C. The proposed technology opens up new possibilities for integration direct-gap III−V materials with a silicon platform for various applications in photonics and quantum communications.

A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of nonmagnetic metal LuB12 with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along < 110 > direction. An anomalous positive contribution to Hall effect for a particular direction of magnetic field H//[001] is found in the single crystals of LuB12 of the highest quality. This contribution arising at T-E similar to 150K is shown to increase drastically when approaching the disordered ground state below T-* similar to 60K. The Hall effect anomaly is shown to appear in combination with the peak of magnetoresistance. The various scenarios allowing for the topology of Fermi surface, anisotropy of relaxation time for charge carriers, and interaction of external magnetic field with the filamentary structure of fluctuating charge stripes are analyzed to explain the features of magnetotransport in this metal with inhomogeneous distribution of electron density. The origin of SdH oscillations, which are observed in this nonequilibrium metal with electron phase separation and strong charge carrier scattering, is discussed.

Precise angle-resolved magnetoresistance and magnetization measurements have revealed (i) strong charge transport and magnetic anisotropy and (ii) emergence of a huge number of magnetic phases in the ground state of TmB12 antiferromagnetic metal with fcc crystal structure and dynamic charge stripes. By analyzing the angular H-fi magnetic phase diagrams reconstructed from experimental angle-resolved magnetoresistance and magnetization data we argue that the symmetry lowering is a consequence of suppression of the indirect Ruderman- Kittel-Kasuya-Yosida (RKKY) exchange along 110 directions between nearest neighboring magnetic moments of Tm3+ ions and subsequent redistribution of conduction electrons to quantum fluctuations of the electron density (stripes). Magnetoresistance components are discussed in terms of charge scattering on the spin density wave, itinerant ferromagnetic nano-domains and on-site Tm3+ spin fluctuations.

Detailed measurements of the Hall effect in the paramagnetic phase of Ho0.8Lu0.2B12 antiferromagnet at the magnetic field up to 80 kOe in the temperature range of 1.9–300 K have been performed. It has been found that the transition to the cage glass phase (T < T * ~ 60 K) is accompanied by the appearance of a positive Hall resistance component in addition to that corresponding to the negative Hall effect. The amplitude and angular dependence of the former depend on the magnitude and direction of the applied magnetic field with respect to the crystallographic axes. The revealed anisotropy of the Hall effect in Ho0.8Lu0.2B12 is attributed to the interaction of charge carriers with dynamic charge stripes.

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity, and magnetization) of model strongly correlated electronic systems LuxZr1-xB12 with cooperative Jahn-Teller instability of the boron rigid cage and with dynamic charge stripes. It was found that these metals are s-wave dirty limit superconductors with a small mean free path of charge carriers l = 5-140 angstrom and with a Cooper pair size changing nonmonotonously in the range 450-4000 angstrom. The parent ZrB12 and LuB12 borides are type-I superconductors, and Zr to Lu substitution induces a type-I to type-II phase transition providing a variation of the Ginzburg-Landau-Maki parameter in the limits 0.65 <= kappa(1,2) <= 6. We argue in favor of the two-band scenario of superconductivity in LuxZr1-xB12 with gap values Delta(1) similar to 14K and Delta(2) similar to 6-8K, with pairing corresponding to strong coupling limit (lambda(e-ph) similar to 1) in the upper band, and to weak coupling (lambda(e-ph) similar to 0.1-0.4) in the lower one. A pseudogap Delta(ps-gap) similar to 60-110K is observed in LuxZr1-xB12 above T-c. We discuss also the possibility of anisotropic single-band superconductivity with stripe-induced both pair breaking and anisotropy, and analyze the origin of a unique enhanced surface superconductivity detected in these model compounds.

The evolution of wide-temperature range (4.2?300 K) electron transport in niobium-carbon nanocomposites was studied at niobium concentration range 0.15?0.35. It was found that electron transport in the nanocomposites has the features of universality, being expressed in the existence of two characteristic temperature intervals on the temperature dependences of conductivity. The crossover temperature between the intervals is in the range 20?30 K. Within each temperature interval, corrections to the conductivity are found to be as power-like ones. Power exponent p is characterized by the non-monotonic dependences on niobium concentration and varies in the ranges 0.5?1.4 and 0.2?1.4 in the low- and high-temperature intervals with a minimum at 0.30 and 0.27 of Nb content, respectively. The satisfactory description of electron transport in niobium-carbon nanocomposites was achieved within the model of the inelastic tunneling of the electrons between the metal grains in the framework of the effective medium approximation.

N.Sluchanko, 2, ∗ A.Bogach, N.Bolotina, V.Glushkov, 2 S.Demishev, 4 A.Dudka, V.Krasnorussky, O.Khrykina, K.Krasikov, V.Mironov, V.Filipov, and N.Shitsevalova Prokhorov General Physics Institute, Russian Academy of Sciences, 38 Vavilov Str., 119991 Moscow, Russia National University of Science and Technology (MISiS), 119049 Moscow, Russia Shubnikov Institute of Crystallography of Federal Scientific Research Centre Crystallography and Photonics of Russian Academy of Sciences, 59 Leninskii Ave., 119333 Moscow, Russia Moscow Institute of Physics and Technology (State University), 9 Institutskiy Per., 141700 Dolgoprudny, Russia Frantsevich Institute for Problems of Materials Science, National Academy of Sciences of Ukraine, 3 Krzhyzhanovsky Str., 03680 Kiev, Ukraine (Dated: July 10, 2017)

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 .

The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm1 – xYbxB6 is investigated in the temperature range 2 < T < 300 K for compositions with x ≤ 0.024. It is found that the ytterbium impurity most strongly affects the low-temperature (T < 20 K) electrical conductivity: when changing from x = 0 to x = 0.024, the activation energy of bulk conductivity decreases by 1.8 times from 4 to 2.2 meV, and the 2D surface conductivity increases by six times. The separation made for the contributions to the conductivity enables us to establish that the one-parameter scaling model can be used for describing the conductivity of 2D surface states in the Sm1 – xYbxB6 TKI in which both the interaction with phonons and the electron–electron scattering effects are taken into account.

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.