Hall Effect Anisotropy in the Paramagnetic Phase of Ho0.8Lu0.2B12 Induced by Dynamic Charge Stripes.

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.

Low-temperature infrared spectroscopy of the strongly correlated semiconductor Tm0.19Yb0.81B12with dynamic charge stripes.

Tm1-xYbxB12dodecaborides represent model objects for the studies of quantum critical behavior, metal-insulator transitions (MITs) and complex charge-spin-orbital-phonon coupling phenomena. In spite of intensive investigations, the mechanism of semiconducting ground state formation both in YbB12and in the Yb-based strongly correlated electron systems remains a subject of active debates. We have performed first systematic measurements of temperature-dependent spectra of infrared conductivity of Tm0.19Yb81B12at frequencies 40-35 000 cm-1and in the temperature range 10-300 K. Analysis of the temperature evolution of the observed absorption resonances is performed allowing to associate these with the cooperative dynamic Jahn-Teller instability of the boron sub-lattice. This ferrodistortive effect of B12-complexes induces the rattling modes of the rare earth ions leading to emergence of both the intra-gap mixed-type collective excitations and the dynamic charge stripes. We estimate the temperature-dependent effective mass of charge carriers and propose the scenario of transformation of the many-body states in the multiple relaxation channels. We attribute the MIT to the localization of electrons at the vibrationally coupled Yb-Yb pairs, which is accompanied by the electronic phase separation and formation of the nanoscale filamentary structure of electron density (stripes) in Tm1-xYbxB12compounds.

Fine details of crystal structure and atomic vibrations in YbB12 with a metal-insulator transition.

The crystal structure of single-crystal Kondo insulator YbB12 was studied at nine temperatures in the range 85-293 K based on X-ray diffraction data. Very weak Jahn-Teller distortions of the cubic lattice were detected at all temperatures, but did not require a revision of the structural model. Heat capacity and electrical conductivity of YbB12 single crystals were studied in the temperature range 1.9-300 K. It is found that both the structural parameters and the indicated physical properties have some specific features in the temperature range 120-160 K. The unit cell of YbB12 contracts when cooled below 160 K and expands at around 120 K. The temperature dependences of the equivalent atomic displacement parameters Ueq(T) are no longer monotonic around 140 K and should be modeled by two Einstein curves for Yb and two Debye curves for boron atoms above and below this temperature. As follows from the temperature behavior of the specific heat, coupled oscillations of Yb ions in a double-well potential lead to the appearance of a charge gap in the density of states and gradual deterioration in conductive properties of the crystal below 150 K. This metal-insulator phase transition is accompanied by a kink in the Ueq(T) curves and changes in the unit-cell values.

The lower symmetry electron-density distribution and the charge transport anisotropy in cubic dodecaboride LuB12.

High-quality single crystals of LuB12 are grown using the induction zone melting method. The x-ray data are collected at temperatures 293, 135, 95, 50 K. The crystal structure of LuB12 can be refined with record low R-factor in the cubic Fm [Formula: see text] m symmetry group despite reiterated observations of the cubic symmetry distortions both in the unit-cell values and in the physical properties. A peculiar computing strategy is developed to resolve this contradiction. True symmetry of the electron-density distribution in LuB12 is proved to be much lower than cubic as a result, which correlates very accurately with anisotropy of transport properties of LuB12.