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.

The physical properties of rare-earth (RE) dodecaborides, characterized by a cage-glass crystal structure with loosely bound RE ions, are reviewed. These compounds are strongly correlated electron systems with simultaneously active charge, spin, orbital, and lattice degrees of freedom, which explains the complexity of all RB_12 compounds including antiferromagnetic (TbB_12-TmB_12) and nonmagnetic (LuB_12) metals, on one side, and the so-called Kondo insulator compound YbB_12 and Yb-based Yb_xR_1-xB_12 solid solutions, on the other. The development of the cooperative dynamic Jahn-Teller instability of the covalent boron network produces trigonal and tetragonal distortions of the rigid cage and results in the symmetry lowering of the fcc lattice in the dodecaborides. The ferrodistortive dynamics in the boron sub-lattice generates both the collective modes and quasilocal vibrations (rattling modes) of the heavy RE ions, causing a modulation in the charge-carrier density and the emergence of dynamic charge stripes. We consider their manifestations both in the properties of the nonmagnetic reference compound LuB_12 and in the phase diagrams of the RB_12 antiferromagnets that exhibit multiple magnetic phases with anisotropic field-angular phase diagrams in the form of the Maltese cross. We also discuss the metal-insulator transitions in YbB_12 and Yb-based dodecaborides in terms of the instability of the Yb 4f-electron configuration, which appears in addition to the Jahn-Teller instability of the boron cage, providing one more mechanism of the charge and spin fluctuations. The experimental results challenge the established Kondo-insulator scenario in YbB_12, providing arguments in favor of the appearance of Yb-Yb vibrationally coupled pairs which should be considered as the main factor responsible for the charge- and spin-gap formation.

Materials with low electron work function are of great demand in various branches of science and technology. LaB6 is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. A deep understanding of the physical mechanisms responsible for the extraordinary properties of LaB6 is required in order to optimize the parameters and design of thermionic elements for application in various electron-beam devices. Motivated by recent experiments on rare earth borides indicating a strong coupling of conduction electrons to the crystal lattice and rare earth ions, we have studied the state of electrons in the conduction band of lanthanum hexaboride by performing infrared spectroscopic, DC resistivity and Hall-effect studies of LaB6 single crystals with different ratios of 10B and 11B isotopes. We find that only a small amount of electrons in the conduction band behave as Drude-type mobile charge carriers while up to 70 the electrons are far out of equilibrium and involved in collective oscillations of electron density coupled to vibrations of the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions that are loosely bound to the lattice. We argue that exactly these non-equilibrium (hot) electrons in the conduction band determine the extraordinary low work function of thermoemission in LaB6. Our observation may guide future search for compounds with possibly lower electron work function.

Based on accurate x-ray structure analysis of GdB6 over the temperature range 85-300 K it has been shown that anomalously strong charge carrier scattering in the quantum diffusion regime of charge transport in this compound arises due to the formation of (i) dynamically coupled Gd3+ pairs of about 3.3 angstrom in size and with energy of quasilocal oscillations similar to 7-8 meV, and due to (ii) dynamic charge stripes along the [001] direction of the cubic lattice. It has been shown that the anharmonic approximation is appropriate when analyzing the static and dynamic components of the atomic displacement parameters of gadolinium. The barrier height of double-well potential of Gd3+ ions was determined both from low-temperature heat capacity measurements and from the electron density distribution reconstructed from x-ray data.

We have determined the superconducting and magnetic properties of four samples of LuxZr1-xB12 (x = 0.04, 0.07, 0.17, and 0.8) using muon spin rotation (mu SR) and magnetometry measurements. We observed a strong magnetic signal in both the mu SR and magnetometry data in one sample (x = 0.07), likely caused by the formation of static moments of size approximate to 1 mu(B) due to a clustering effect of the Lu3+ ions. In all other samples, we find only a small magnetic signal in the mu SR data thought to originate from boron nuclei in the B-12 cages. The superconductivity is found to evolve with x, with a decrease in x resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find the formation of nodes in the superconducting gap for x <= 0.17, providing a new example of an s-to-s + d-wave crossover in a superconductor.

Structure differences of isotopically different dodecaborides LuNB12 (N = 10, 11, natural) and their impact on thermal and charge transport characteristics of the crystals have been first discovered. Atomic displacement parameters (ADPs) of Lu and B atoms are described in terms of the Einstein and Debye models, respectively. Characteristic Einstein and Debye temperatures are calculated directly from the x-ray data and corresponding ADPs are separated into temperature dependent and temperature independent components. The first component is a measure of thermal atomic vibrations whereas the second one is a sum of zero vibrations and static shifts of some atoms from their crystallographic positions. Such a local disordering is more expressed in LunatB12 with 10B : 11B = 1 : 4 judging both from the large static ADP components and the Schottky anomalies in the heat capacity. Crystal structures are refined in Fm3-m group but certain distortions of the ideal cubic unit-cell values are observed in all three crystals under study due to cooperative Jahn-Teller effect. The distortions are mainly trigonal or mainly tetragonal depending on the isotope composition. Low symmetry distribution of electron density reveals itself in LunatB12 in the form of the dynamic charge stripes oriented in selected directions close to some of <110>. The large static ADP components of LunatB12 are surprisingly combined with high conductivity of the crystal. One may suppose the static shifts (defects) are centers of pinning facilitating formation of additional conductive channels.

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.

The magnetic response related to paramagnetic Meissner effect (PME) is studied in a high quality single crystal ZrB12 with non-monotonic vortex-vortex interactions. We observe the expulsion and penetration of magnetic flux in the form of vortex clusters with increasing temperature. A vortex phase diagram is constructed which shows that the PME can be explained by considering the interplay among the flux compression, the different temperature dependencies of the vortex-vortex and the vortex-pin interactions, and thermal fluctuations. Such a scenario is in good agreement with the results of the magnetic relaxation measurements.

We report the study of transport and magnetic properties of the YbB6–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 YbB6–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 μH found for bulk holes is induced by the decrease of transport relaxation time from τ ≈ 7.7 fs for YbB5.994 to τ ≈ 2.2 fs for YbB5.96. An extra contribution to conductivity from electrons with μH≈ –1000 cm2 V–1 s–1 and the very low concentration n /nYb≈ 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)

Single crystals of LuB12 of exceptionally high diffraction quality confirmed by an accurate X-ray structure analysis are grown by the modified crucibleless inductive floating zone melting. A thoroughly treated single crystal is used to collect a high-resolution data set up to sin theta/lambda = 1.36 angstrom(-1) at 293 K, which has been reduced to 271 symmetry-independent reflections. Residual factors of the structure refinement in the Fm (3) over bar m symmetry group are R1(vertical bar F vertical bar)/wR2(vertical bar F vertical bar) = 0.20/0.14% in a 13-parametric structure model. Revealed asymmetry of electron density distribution near average position of the Lu atom described using anharmonic displacement parameters. (C) 2016 Elsevier B.V. All rights reserved.

We report the study of spin relaxation in the Eu1-xGdxB6 (0x0.039) single crystals with the help of 60GHz electron spin resonance (ESR) technique. A drastic change in the linear slopes of the temperature dependences of the ESR linewidth is discovered in the paramagnetic phase of Eu1-xGdxB6. The corresponding crossover temperature T-0 is shown to decrease from T-0(x=0)approximate to 60K down to T-0(x=0.039)approximate to 15K under rising of Gd content. A non-bottlenecked Korringa relaxation is discussed as the main factor that governs spin dynamics in the unordered state of Eu1-xGdxB6 below T-0. Using of the band parameters extracted from static magnetic and transport data allows to estimate on-site exchange constant between localized spins and itinerant electrons, which is effectively tuned from 110meV for x=0 down to 43meV for x=0.039 under gradual filling of the Eu1-xGdxB6 conduction band.

We argue that the Anderson and Kondo models turn out to be irrelevant for the description of some strongly correlated electron systems and suggest the mechanism for the formation of many-body states (heavy fermions) being an alternative to the Kondo one. This mechanism involves the quantum tunneling of a heavy particle between the states in the double-well potential.