K. Flachbart

Fractional magnetization plateaus and magnetic order in the Shastry-Sutherland magnet TmB4.

We investigate the phase diagram of TmB4, an Ising magnet on a frustrated Shastry-Sutherland lattice, by neutron diffraction and magnetization experiments. At low temperature we find Néel order at low field, ferrimagnetic order at high field, and an intermediate phase with magnetization plateaus at fractional values M/M_(sat)=1/7,1/8,1/9,... and spatial stripe structures. Using an effective S=1/2 model and its equivalent two-dimensional fermion gas we suggest that the magnetic properties of TmB4 are related to the fractional quantum Hall effect of a 2D electron gas.

Magnetic and transport properties of TmB12, ErB12, HoB12 and DyB12

Abstract We present measurements of the magnetization and electrical resistivity of TmB 12 , ErB 12 and HoB 12 singlecrystalline samples as well as of a DyB 12 polycrystalline sample. Data were taken in the paramagnetic state as well as in the magnetically ordered regime. To our knowledge, this is the first systematic investigation of TmB 12 , ErB 12 , HoB 12 and DyB 12 on the magnetically ordered phase by measurements of magnetization and electrical resistivity. All samples investigated undergo antiferromagnetic ordering in the lower Kelvin temperature range. From the magnetization data, the Neel temperatures, the paramagnetic Curie temperatures, and the effective magnetic moments of the corresponding rare earth ions are determined. It is shown that the indirect exchange interaction of RKKY type is the dominating mechanism leading to the observed antiferromagnetic ordering. The measured temperature dependences of the electrical resistivity show a pronounced effect of superzone boundaries near Neel temperatures. The spin wave scattering of conduction electrons below T N is discussed, too.

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.

Energy gap of intermediate-valentSmB6studied by point-contact spectroscopy

We have investigated the intermediate valence narrow-gap semiconductor SmB6 at low temperatures using both conventional spear-anvil type point contacts as well as mechanically controllable break junctions. The zero-bias conductance varied between less than 0.01 mikrosiemens and up to 1 mS. The position of the spectral anomalies, which are related to the different activation energies and band gaps of SmB6, did not depend on the the contact size. Two different regimes of charge transport could be distinguished: Contacts with large zero - bias conductance are in the diffusive Maxwell regime. They had spectra with only small non-linearities. Contacts with small zero - bias conductance are in the tunnelling regime. They had larger anomalies, but still indicating a finite 45 % residual quasiparticle density of states at the Fermi level at low temperatures of T = 0.1 K. The density of states derived from the tunelling spectra can be decomposed into two energy-dependent parts with Eg = 21 meV and Ed = 4.5 meV wide gaps, respectively.

Electrical resistivity and superconductivity of LaB6 and LuB12

Abstract The electrical resistivity of LaB 6 and LuB 12 single crystals with high residual resistivity ratios has been measured down to 1.6 K. The temperature dependence below 30 K, caused by phonon scattering, shows for both materials a T n behaviour with n = 4.2 ± 0.1 for LaB 6 and n = 5.2 ± 0.1 for LuB 12 . From susceptibility measurements the superconducting transition temperature of LuB 12 was determined to be 0.44 K, for the LaB 6 sample with residual resistivity ratio of 160 no transition into the superconducting state was observed down to 5 mK.

Quantum oscillations and the Sermi surface of LuB12

To examine the Fermi surface of LuB12, measurements of the de Haas-van Alphen (dHvA) effect were made at temperatures between 0.35 and 2 K in magnetic fields up to 12 Tesla. Oscillations in the susceptibility occurred above 5 Tesla in any field direction relative to the single crystal sample. From the Fourier transform of the data obtained, we conclude the Fermi surface of both conduction bands to have multiple extremal cross sections. For some of these orbits, the temperature dependence of the dHvA signal was investigated to determine the corresponding cyclotron mass. For a better understanding, a Full Potential Linearized Augmented Plane Wave-(FLAPW-) band structure calculation was carried out and the shapes of the Fermi surfaces were determined. In addition, we investigated the transverse magnetoresistance as a function of the field and the field direction. Its anisotropy, as well as the Shubnikov-de Haas (SdH) oscillations occurring in certain geometries, are in agreement with the results of the dHvA measurements.

Low temperature resistivity of valence fluctuation compound SmB6

Abstract The electrical resistivity of SmB6 single crystal has been measured down to 50mK. At temperatures below 3 K a rather complicated behavior of electrical conductivity including the temperature activated and temperature non-activated term was observed. The activated conductivity term can be well described by σ = c exp [-( T 0 T ) x ] with x equal to or close to 1 and T0 = 2.68 K, while the non-activated conductivity is dominating below about 0.2 K and shows presence of the logarithmic Kondo-like contribution to resistivity. To explain observed results a model based on introducing a fine structure into the hybridization gap is proposed. This fine structure may consist of two narrow low-density bands with a gap between them of few tenths of meV wide.

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.

Point-contact spectroscopy of the electron-phonon interaction in single-crystal LaB6

Point-contact spectra of single-crystal LaB6 are obtained, yielding the energy positions of all the phonon modes up to 160 meV. A relatively strong anisotropy of the spectra is observed. It is related to the anisotropy of the phonon system. The point-contact electron-phonon interaction function and the point-contact electron-phonon interaction parameter are compared with published calculated data and a qualitative agreement is found. From the measured spectra the temperature dependence of the LaB6 electrical resistivity and heat capacity are calculated.

Samarium hexaboride is a trivial surface conductor

SmB6 is predicted to be the first member of the intersection of topological insulators and Kondo insulators, strongly correlated materials in which the Fermi level lies in the gap of a many-body resonance that forms by hybridization between localized and itinerant states. While robust, surface-only conductivity at low temperature and the observation of surface states at the expected high symmetry points appear to confirm this prediction, we find both surface states at the (100) surface to be topologically trivial. We find the