An observation of electron phase transition in SmB6 at low temperatures

In order to reveal the nature of the ground state of archetypal intermediate-valence compound SmB₆, a comprehensive study of its transport and magnetic properties was carried out on high-quality single crystals at temperatures of 1.8-300 K in magnetic fields up to 7 T. A drastic enhancement of negative magnetoresistance was observed below 14 K, with the maximum absolute value of Δρ/ρB²∼2.2×10⁻³ T⁻² at T≈5.2 K. This effect seems to be attributable to anomalous magnetic scattering of many-body (exciton-polaronic) complexes induced by fast valence fluctuations on Sm sites. The observed anomalies of magnetotransport, thermoelectric and magnetic characteristics are discussed in terms of electron phase transition to the coherent state of interacting many-body complexes occurring at T^*∼5 K.     

Thermal properties of rare earth dodecaborides

We have measured heat capacity and thermal expansion of rare earth dodecaborides REB₁₂ (RE=Y, Tb-Tm, Lu). YB₁₂ and LuB₁₂ are diamagnetics whereas the other dodecaborides are ordered antiferromagnetically. The amplitude of the heat capacity discontinuity at the Néel temperature and the shape of the heat capacity variation in the critical region for all these antiferromagnetics are characteristics for amplitude-modulated magnetic structures. In the ordered state TbB₁₂ reveals two first-order phase transitions, most likely due to magnetic structure changes. The heat capacity of ErB₁₂ just below the Néel point shows an anomaly of unclear origin. From the Schottky contribution to the heat capacity we have determined crystal field parameters. They are completely different than that is estimated from Point Charge Model.     

Bulk and surface electron transport in topological insulator candidate YbB6–δ

We report the study of transport and magnetic properties of the YbB_6–δsingle crystals grown by inductive zone melting. A strong disparity in the low temperature resistivity, Seebeck and Hall coefficients is established for the samples with the different level of boron deficiency. The effective parameters of the charge transport in YbB_6–δ are shown to depend on the concentration of intrinsic defects, which is estimated to range from 0.09% to 0.6%. The pronounced variation of Hall mobility μ_H found for bulk holes is induced by the decrease of transport relaxation time from τ ≈ 7.7 fs for YbB_5.994 to τ ≈ 2.2 fs for YbB_5.96. An extra contribution to conductivity from electrons with μ_H≈ –1000 cm² V^–1 s^–1 and the very low concentration n /n_Yb≈ 10^–6 discovered below 20 K for all the single crystals under investigation is suggested to arise from the surface electron states appeared in the inversion layer due to the band bending. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Anomalous thermopower in heavy-fermion compounds CeB<Subscript>6</Subscript>, CeAl<Subscript>3</Subscript>, and CeCu<Subscript>6 − <Emphasis Type="Italic">x</Emphasis></Subscript>Au<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

High-precision measurements of thermopower have been performed in a wide temperature range (2–300 K) for a series of cerium-based heavy-fermion compounds, including CeB₆, CeAl₃, CeCu₆, and substitutional solid solutions of the CeCu_{6 ? x} Au _x system (x = 0.1, 0.2). All compounds exhibit an unusual (logarithmic) asymptotic behavior of the temperature dependence of the Seebeck coefficient: S ∝ ?lnT. In the case of cerium hexaboride, this anomalous behavior of S(T) is accompanied by the appearance of weak-carrier-localization-mode asymptotics in the conductivity (σ(T) ∝ T ^0.39), while the paramagnetic susceptibility χ(T) and the effective mass of charge carriers m _eff(T) vary according to a power law (χ(T), m _eff(T) ∝ T ^?0.8) in the temperature interval T = 10–80 K. This behavior corresponds to renormalization of the density of states at the Fermi level. The observed anomalous behavior of thermopower in CeB₆ and other cerium-based intermetallic compounds is attributed to the formation of heavy fermions (many-body states in the metal matrix) at low temperatures.     

Surface Conductivity of the Ytterbium-Doped Topological Kondo-Insulator SmB6

Doklady Physics - The temperature dependence of the conductivity of the topological Kondo-insulator (TKI) Sm1 – xYbxB6 is investigated in the temperature range 2 &lt; T &lt; 300 K for...     

Neutron studies of crystal-field effects in PrB<Subscript>6</Subscript>

The roles of various physical mechanisms in the properties of the ground state of the Pr ion in PrB₆ are studied by measuring magnetic excitation spectra in the paramagnetic and magnetically ordered phases of polycrystalline PrB₆. The ground state of the Pr ion in the paramagnetic phase is experimentally found to be triplet Γ₅. During the transition into the magnetically ordered state, this triplet splits into three singlet levels. The results obtained do not exclude the appearance of an additional contribution to the splitting induced by a decrease in the local symmetry because of structural distortions.     

Anomalous hall effect in HoB12

Within a wide temperature range of 1.8–300 K, the Hall effect in holmium dodecaboride is investigated. The measurements of the angular dependences of the Hall resistance ρ_H(φ, T, H) for HoB₁₂ performed in a high magnetic field up to 80 kOe at helium and intermediate temperatures made it possible to separate the normal and anomalous contributions to the Hall effect. Analysis of the anomalous component behavior in the paramagnetic and Néel phases suggests the appearance of the 5d-state magnetic polarization (the spin polaron effect) and makes it possible to compare the revealed features of the Hall coefficient R _H(T, H) with the H-T magnetic phase diagram of HoB₁₂. Original Russian Text ? N.E. Sluchanko, D.N. Sluchanko, V.V. Glushkov, S.V. Demishev, N.A. Samarin, N.Yu. Shitsevalova, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 9, pp. 691–694.     

Features of the local structure of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Yb, Lu)

The parameters of the local crystal structure of dodecaborides RB₁₂ (R = Ho, Er, Tm, Yb, Lu) have been determined by extended X-ray absorption fine structure (EXAFS) spectroscopy. It has been shown that the vibrations of the rare-earth ion with respect to the boron cage are well described in the harmonic approximation. At the same time, the displacement of rare-earth ions from equilibrium positions of the crystal structure should be taken into account to determine the length of the R-B bond. The analysis of EXAFS spectra has revealed the displacement of 1–6% of rare-earth ions by about 0.2–0.3 Å in all compounds under investigation; this displacement at low temperatures results in the formation of a cage glass phase.     

Specific features of magnetoresistance during the antiferromagnet—paramagnet transition in Tm1 − x Yb x B12

The transverse magnetoresistance Δρ/ρ(H, T) of Tm_{1 ? x} Yb _x B₁₂ single crystals is studied in the ytterbium concentration range corresponding to the antiferromagnet-paramagnet transition in a magnetic field up to 80 kOe at low temperatures. A magnetic H-T phase diagram is constructed for the antiferromagnetic state of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions with x ≤ 0.1. The contributions to the magnetoresistance in the antiferromagnetic and paramagnetic phases of the dodecaborides under study are separated. Along with negative quadratic magnetoresistance -Δρ/ρ ∝ H ₂, the magnetically ordered phase of these compounds is found to have component Δρ/ρ ∝ H that linearly changes in a magnetic field. The negative contribution to the magnetoresistance of Tm_{1 ? x} Yb _x B₁₂ is analyzed in terms of the Yosida model for a local magnetic susceptibility.