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Detailed measurements of the Hall effect in the Ho0.8Lu0.2B12 antiferromagnetic compound (Néel temperature TN = 5.75 K) in magnetic fields up to 80 kOe oriented in the (110) plane at temperatures of 1.9–6.6 K are performed. It is established with the contribution separation procedure that the anisotropic positive contribution, which is responsible for the double inversion of the sign of the Hall resistance, dominates in the antiferromagnetic state of Ho0.8Lu0.2B12 in fields of 30–50 kOe. A sharp decrease in the amplitude of the isotropic negative contribution at the transition to the antiferromagnetic phase is found. The nature of the detected anomalies in the antiferromagnetic metal with dynamic charge stripes and spin-wave component of the magnetic structure is discussed.

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.

Seebeck coefficient has been studied on the single crystals of metallic hexaborides RB6 (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 LaB6 and GdB6 were approximated by phonon drag contribution caused by quasi-local (Einstein) mode with characteristic temperatures ΘE(LaB6) ≈ 240 K and ΘE(GdB6) ≈ 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 PrB6 and NdB6.

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.

The strongly correlated system $\mathrm{Ho}^{11}\mathrm{B}_{12}$ with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry, and heat capacity measurements at temperatures in the range of 3--300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field (CF) at holmium sites ${B}_{4}=\ensuremath{-}0.333$ meV and ${B}_{6}=\ensuremath{-}2.003$ meV (in Stevens notations), with an unconventional large ratio ${B}_{6}/{B}_{4}$ pointing to the dominant role of conduction electrons in the formation of a CF potential. The molecular field in the antiferromagnetic (AFM) state ${B}_{\mathrm{loc}}=(1.75\ifmmode\pm\else\textpm\fi{}0.1)$ T has been directly determined from the INS spectra together with short-range order effects detected in the paramagnetic state. A comparison of measured magnetization in diluted ${\mathrm{Lu}}_{0.99}{\mathrm{Ho}}_{0.01}{\mathrm{B}}_{12}$ and concentrated ${\mathrm{HoB}}_{12}$ single crystals showed a strong suppression of Ho magnetic moments by AFM exchange interactions in holmium dodecaboride. To account explicitly for the short-range AFM correlations, a self-consistent holmium dimer model was developed that allowed us to reproduce successfully field and temperature variations of the magnetization and heat capacity in the cage-glass phase of ${\mathrm{HoB}}_{12}$ in external magnetic fields.

The studies of high-quality single-domain crystals of YbB12 were carried out by the precise x-ray diffraction technique in combination with the low temperature polarized THz - infrared spectroscopy and accurate magnetotransport measurements. It has been shown for the first time that this archetypal strongly correlated system with a metal-insulator transition to a mysterious dielectric ground state with a metal Fermi surface (Science 362, 65-69 (2018) and ibid 362, 32-33 (2018)) is actually a heterogeneous compound in the regime of electronic phase separation. Changes in the configuration of the discovered dynamic charge stripes are investigated upon cooling, as a result, a conclusion is drawn in favor of a crossover between different patterns of the filamentary electronic structure penetrating the semiconducting matrix of YbB12. We argue that the discovery of stripes in YbB12 is fundamental, elucidating the nature of exotic dielectric state in Kondo insulators., Comment: 12 pages including 4 figures

The low temperature (T

Comparative analysis of Hall effect in substitutional solid solutions Ho0.5Lu0.5B12, Mn1 – xFexSi (0 ≤ x ≤ 1) and Eu1 – xGdxB6 (x < 0.04) is carried out at temperatures 2–300 K in magnetic fields up to 8 T. Anomalous contribution to the Hall effect $$\rho _{{xy}}^{{\text{A}}}$$ ~ $${{\rho }_{{xx}}}M$$ has been identified for systems with various types of magnetic frustration. The linear scaling $$\rho _{{xy}}^{{\text{A}}}$$ ~ ρxx is detected in the resistivity range ρxx ~ 0.01–1 mΩ cm lying outside the range of applicability of the classical model of asymmetric scattering. It is associated with the increase in the amplitude of spin fluctuations in the paramagnetic phase of the investigated compounds with noncollinear magnetic structure. The topological contribution to the Hall effect is extracted for Ho0.5Lu0.5B12 and Eu1 – xGdxB6. Its amplitude is found to vary from 80 nΩ cm (Ho0.5Lu0.5B12) to 7.5 μΩ cm (Eu0.97Gd0.03B6).

The high-frequency (60 GHz) electron paramagnetic resonance is studied in HoxLu1−xB12 single crystals at low temperatures (2–80 K) in an applied magnetic field up to 7 T within a wide doping range, 0.01 ≤ x ≤ 1. These compounds are characterized by the presence of a low-temperature cage glass phase with random displacements of magnetic holmium ions from the centrosymmetric position in B12 boron cuboctahedra. For the samples with x ≥ 0.1, it is demonstrated that the electron paramagnetic resonance in the form of a single broad line with a g-factor of about 5 appears in the cage glass phase at T 0.3, the pronounced broadening of the resonance line and a steep decrease in the g-factor related to antiferromagnetic correlations is observed on cooling at T < 30 K. The performed simulation of the electron paramagnetic resonance spectra suggests the dominant role of the exchange interaction effects and positional disorder in determining the characteristic features of spin dynamics in HoxLu1−xB12.

We have tested the conduction band electrons of lanthanum hexaboride that is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. We performed infrared spectroscopic, DC (direct current) resistivity and Hall-effect studies of LaB6 single crystals with various 10B and 11B isotope contents. We find that only a small amount of conduction electrons behave as Drude-type charge carriers while about 70% of the electrons are involved in collective oscillations of electron density coupled to vibrations of both the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions loosely bound to the lattice. We suggest that exactly these non-equilibrium conduction electrons determine the extraordinary low work function of thermoemission in LaB6.

Crystal structures, thermal and charge-transport (resistivity and Seebeck coefficient) characteristics of dodecaborides LuNB12 are studied to determine the role of boron isotopic composition N = 10, 11, natural. Small distortions of the fcc lattice are observed and attributed to cooperative Jahn-Teller effect in the boron sublattice. The Einstein and Debye temperatures, respectively for Lu and B atoms, are determined based on equivalent atomic displacement parameters (ADPs) refined as a part of structure model. Additionally, ADP values are separated into temperature dependent and independent components, arising from (i) thermal and (ii) zero temperature vibrations, together with (iii) static shifts of few atoms from lattice sites. Atomic disordering is most expressed in LunatB12 as follows from its ADP values and the Schottky anomalies in heat capacity. The largest static ADP components of LunatB12 are surprisingly combined with the lower resistivity and thermopower as compared to isotopically pure crystals. One may suppose the static shifts (defects) are centers of pinning facilitating formation of additional conductive channels (dynamic charge stripes), which reveal themselves on difference Fourier maps of electron density.

Experimental data on thermionic current density and electron work function of YB12 (100) at T = 1218 – 1978 K in high vacuum (p < 10-4 Pa) are first introduced. Temperature dependences of the thermionic current density and effective electron work function are presented without extrapolation to the zero-field currents because of the anomalous Schottky effect. The temperature dependences of theYB12 electron work function can be described by linear functions at certain temperature intervals. Preferential boron evaporation and additional ion bombardment by the residual gases ions and evaporated boron ions results in appearance of new phases depleted of boron on the YB12 surface. As a result, an YB4 – YB6 double layer on the YB12 single crystal surface has formed. The appearance of the new boride phases, depleted of boron, on the emitting surface causes an increase of thermionic current density compared with individual YB12.

The magnetic properties of substitutional solid solutions Eu0.9Yb0.1B6 have been investigated in the temperature range of 2–300 K in fields up to 5 T. The data obtained confirm that the state with the electronic and magnetic phase separation (typical of europium hexaboride) is implemented in Eu0.9Yb0.1B6 when the system metallization (TM ≈ 15 K) precedes the ferromagnetic ordering (TC ≈ 11.4 K). An analysis of the curves M(H) in the Belov–Arrott coordinates makes it possible to evaluate spontaneous magnetization Msp and zero field susceptibility χ0 and determine the character of their critical behavior near the Curie point. The calculated critical indices (γ ≈ 1.28 and β ≈ 0.34) are in agreement with the predictions of the three-dimensional Heisenberg model.

The magnetic properties of the topological Kondo insulator SmB6 are studied in the temperature range T T*, the magnetization qualitatively corresponds to the model of decay of Kondo singlets and is determined by the Pauli paramagnetism, which includes both the linear and nonlinear contributions. In the range T < T*, the Pauli paramagnetism becomes supplemented in the threshold manner by the contribution of localized magnetic moments. The performed estimate of effective magnetic moments arising at T < T* gives anomalously high values μ* ∼ (7–14) μB ion, far exceeding the values μ* ∼ (3–5) μB expected for a separate magnetic Sm3+ ion. This is a signature of the possible spin-polaron nature of localized magnetic moments in the topological Kondo insulator SmB6.

We have studied the normal and superconductive state characteristics (resistivity, Hall coefficient, heat capacity, and magnetization) of model strongly correlated electronic systems ${\mathrm{Lu}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{B}}_{12}$ 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 $l=5--140\phantom{\rule{0.16em}{0ex}}\AA{}$ and with a Cooper pair size changing nonmonotonously in the range 450--4000 \AA{}. The parent ${\mathrm{ZrB}}_{12}$ and ${\mathrm{LuB}}_{12}$ 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 $0.65\ensuremath{\le}{\ensuremath{\kappa}}_{1,2}\ensuremath{\le}6$. We argue in favor of the two-band scenario of superconductivity in ${\mathrm{Lu}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{B}}_{12}$ with gap values ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\sim}14\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\sim}6--8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with pairing corresponding to strong coupling limit $({\ensuremath{\lambda}}_{\text{e-ph}}\ensuremath{\sim}1)$ in the upper band, and to weak coupling $({\ensuremath{\lambda}}_{\text{e-ph}}\ensuremath{\sim}0.1--0.4)$ in the lower one. A pseudogap ${\mathrm{\ensuremath{\Delta}}}_{\text{ps-gap}}\ensuremath{\sim}60--110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is observed in ${\mathrm{Lu}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{B}}_{12}$ 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.

A comprehensive study of magnetoresistance and Hall effect has been performed for the set of the single crystals of nonmagnetic metal $\mathrm{Lu}{\mathrm{B}}_{12}$ with the Jahn-Teller instability of the boron cage and dynamic charge stripes forming along $\ensuremath{\langle}110\ensuremath{\rangle}$ direction. An anomalous positive contribution to Hall effect for a particular direction of magnetic field $\mathbf{H}//[001]$ is found in the single crystals of $\mathrm{Lu}{\mathrm{B}}_{12}$ of the highest quality. This contribution arising at ${T}_{\mathrm{E}}\ensuremath{\sim}150\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is shown to increase drastically when approaching the disordered ground state below ${T}^{*}\ensuremath{\sim}60\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. 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 (ARM) measurements are applied to reveal the origin of symmetry lowering in electron transport and the emergence of a huge number of magnetic phases in the ground state of the antiferromagnetic metal ${\mathrm{HoB}}_{12}$ with fcc crystal structure. By analyzing the polar H-\ensuremath{\theta}-\ensuremath{\varphi} magnetic phase diagrams of this compound reconstructed from the experimental ARM data, we argue that nonequilibrium electron density oscillations (dynamic charge stripes) are responsible for the suppression of the indirect Ruderman-Kittel-Kasuya-Yosida exchange along the ⟨110⟩ directions between the nearest neighboring magnetic moments of ${\mathrm{Ho}}^{3+}$ ions in this strongly correlated electron system.

Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T

Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T

Seebeck effect in the crystalline samples of Eu

In contrast to magnetic order formed by electrons' dipolar moments, ordering phenomena associated with higher-order multipoles (quadrupoles, octupoles, etc.) are more difficult to characterize because of the limited choice of experimental probes that can distinguish different multipolar moments. The heavy-fermion compound CeB6 and its La-diluted alloys are among the best-studied realizations of the long-range-ordered multipolar phases, often referred to as "hidden order". Previously the hidden order in phase II was identified as primary antiferroquadrupolar (AFQ) and field-induced octupolar (AFO) order. Here we present a combined experimental and theoretical investigation of collective excitations in the phase II of CeB6. Inelastic neutron scattering (INS) in fields up to 16.5 T reveals a new high-energy mode above 14 T in addition to the low-energy magnetic excitations. The experimental dependence of their energy on the magnitude and angle of the applied magnetic field is compared to the results of a multipolar interaction model. The magnetic excitation spectrum in rotating field is calculated within a localized approach using the pseudo-spin presentation for the Gamma8 states. We show that the rotating-field technique at fixed momentum can complement conventional INS measurements of the dispersion at constant field and holds great promise for identifying the symmetry of multipolar order parameters and the details of inter-multipolar interactions that stabilize hidden-order phases., Comment: 15 pages with 13 figures + Supplemental Material

The nature of charge transport and magnetization anisotropy was investigated in the ErB12 antiferromagnetic metal with both cooperative Jahn-Teller distortions of the fcc crystal structure and electronic instability (dynamic charge stripes). A butterfly pattern of the H-φ magnetic phase diagram in the (1 1 0) plane was reconstructed, including a large number of different magnetic phases separated from each other by radial and circular boundaries. It is argued that fluctuations of electron density are responsible for the suppression of the indirect Ruderman-Kittel-Kasuya-Yoshida (RKKY) exchange between the nearest neighbored Er3+ ions located along the 〈110〉 directions producing the magnetic phase diversity in ErB12.

The magnetic phase diagram of Tm1–xYbxB12 antiferromagnets is determined from the specific heat measurements performed at low and ultralow temperatures (0.07–10 K). Precise experimental studies of the features of the crystal structure at room temperature suggest that a metal−insulator transition occurring in Tm1–xYbxB12 with the growth of x is accompanied by a significant (by a factor of 2–6) increase in the amplitude of atomic displacements in the boron and rare-earth sublattices. There is also a signature of an instability arising in the electron configuration of ytterbium ions at this transition. At room temperature, near the quantum critical point at xc ≈ 0.25, anomalies in the structural characteristics of dodecaborides under study are observed.

The specific heat of Ho0.01Lu0.99B12 single crystals with dynamic charge stripes is studied at low and ultralow temperatures (0.07–20 K) in applied magnetic fields of 0–9 Т. It is shown that additional contributions to the specific heat appear at magnetic fields exceeding 1 T. These contributions depend on the orientation of the magnetic field and can be as large as 15% of the specific heat related to the subsystem of holmium ions in the cubic crystal field.

The electron spin resonance has been measured for the first time both in the paramagnetic phase of the metallic GdB6 antiferromagnet (TN = 15.5K) and in the antiferromagnetic state (T < TN). In the paramagnetic phase below T* ~ 70 K, the material is found to exhibit a pronounced increase in the resonance linewidth and a shift in the g-factor, which is proportional to the linewidth Δg(T) ~ ΔH(T). Such behavior is not characteristic of antiferromagnetic metals and seems to be due to the effects related to displacements of Gd3+ ions from the centrosymmetric positions in the boron cage. The transition to the antiferromagnetic phase is accompanied by an abrupt change in the position of resonance (from μ0H0 ≈ 1.9 T to μ0H0 ≈ 3.9 T at ν = 60 GHz), after which a smooth evolution of the spectrum occurs, resulting eventually in the formation of the spectrum consisting of four resonance lines. The magnetic field dependence of the frequency of the resonant modes ω0(H0) obtained in the range of 28–69 GHz is well interpreted within the model of ESR in an antiferromagnet with the easy anisotropy axis ω/γ = (H 0 2 +2HAHE)1/2, where HE is the exchange field and HA is the anisotropy field. This provides an estimate for the anisotropy field, HA ≈ 800 Oe. This value can result from the dipole−dipole interaction related to the mutual displacement of Gd3+ ions, which occurs at the antiferromagnetic transition.

Introducing of topological insulator concept for fluctuating valence compound – samarium hexaboride – has recently initiated a new round of studies aimed to clarify the nature of the ground state in this extraordinary system with strong electron correlations. Here we discuss the data of magnetic resonance in the pristine single crystals of SmB6 measured in 60 GHz cavity experiments at temperatures 1.8–300 K. The microwave study as well as the DC resistivity and Hall effect measurements performed for the different states of SmB6 [110] surface prove definitely the existence of the layer with metallic conductivity increasing under lowering temperature below 5 K. Four lines with the g-factors g ≈ 2 are found to contribute to the ESR-like absorption spectrum that may be attributed to intrinsic paramagnetic centers on the sample’s surface, which are robust with respect to the surface treatment. The temperature dependence of integrated intensity I(T) for main paramagnetic signal is found to demonstrate anomalous critical behavior I(T) ~ (T* − T)ν with characteristic temperature T * = 5.34 ± 0.05 K and exponent ν = 0.38 ± 0.03 indicating possible magnetic transition at the SmB6 [110] surface. Additional resonant magnetoabsorption line, which may be associated with either donor-like defects or cyclotron resonance mode corresponding to the mass m c ~ 1.2m0, is reported.

The transverse magnetoresistance of Ho0.8Lu0.2B12 dodecaboride with a cage glass structure is studied at low (2–10 K) temperatures. It is demonstrated that the isotropic negative magnetoresistance in this antiferromagnet is dominant within the broad temperature range near TN ≈ K. This contribution to the total magnetoresistance is due to the scattering of charge carriers by nanoclusters formed by Но3+ ions, and it can be scaled in the ρ = f(μ2effH2/T2 ) representation. It is found that the magnetoresistance anisotropy above (about 15% at 80 kOe) is due to the positive contribution, which achieves maximum values at the magnetic field direction close to H ║ [001]. The anisotropy of the charge carrier scattering is interpreted in terms of the cooperative dynamic Jahn−Teller effect at В12 clusters.

By measuring room temperature infrared (40–35000 cm–1) reflectivity of metallic LuB12 single crystals with different isotopic compositions (natB, 10B, 11B), we find that to model the spectrum we had to introduce, additionally to Drude free-carrier component, a broad excitation with unusually large dielectric contribution (Δe = 8000 ± 4000), which is characterized by a non-Lorentzian lineshape. It is suggested that the origin of the excitation is connected with cooperative dynamics of Jahn–Teller active B12 molecules producing quasilocal vibrations (rattling modes) of caged lutetium ions. The coupling of the Lu3+ rattling motions with the charge carriers of conduction band is proposed to be the reason of strongly damped character of the excitation.

Single crystals of LuB 12 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 sinθ/λ = 1.36 A −1 at 293 K, which has been reduced to 271 symmetry-independent reflections. Residual factors of the structure refinement in the Fm 3 ¯ m symmetry group are R 1(| F |)/w R 2(| F |) = 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.

Based on accurate x-ray structure analysis of $\mathrm{Gd}{\mathrm{B}}_{6}$ 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 $\mathrm{G}{\mathrm{d}}^{3+}$ pairs of about 3.3 \AA{} in size and with energy of quasilocal oscillations \ensuremath{\sim}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 $\mathrm{G}{\mathrm{d}}^{3+}$ ions was determined both from low-temperature heat capacity measurements and from the electron density distribution reconstructed from x-ray data.

Using Fourier-transform infrared spectroscopy and optical ellipsometry, room temperature spectra of complex conductivity of single crystals of hexaborides ${\mathrm{Gd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{B}}_{6},x(\text{Gd})=\text{0},0.01,0.1,0.78,1$, are determined in the frequency range $30--35000\phantom{\rule{4.pt}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. In all compounds, in addition to the Drude free-carrier spectral component, broad excitations are discovered with unusually large dielectric contributions $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}=7\phantom{\rule{0.16em}{0ex}}000--15\phantom{\rule{0.16em}{0ex}}000$ and non-Lorentzian line shapes. It is suggested that the origin of the excitations is connected with the dynamic cooperative Jahn-Teller effect of ${\mathrm{B}}_{6}$ clusters. Analysis of the spectra together with the results of dc and Hall resistivity measurements show that only $25--50%$ of the conduction band electrons are contributing to the free carrier ac conductivity, with the rest being involved in the formation of an overdamped excitation, thus providing a possible explanation, in terms of nonequilibrium (hot) electrons in these hexaborides, of both the remarkably low work function of thermoemission of ${\mathrm{Gd}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{B}}_{6}$ and the non-Fermi-liquid behavior in ${\mathrm{GdB}}_{6}$ crystals.

First detailed measurements of terahertz-infrared electrodynamic response of high-quality single-crystalline Tm 0.19 Yb 0.81 B 12 dodecaboride with dynamic charge stripes are performed at temperatures 5–300 K. Dramatic temperature changes in the dynamic conductivity spectra are observed, single-particle and collective excitations identified and their origin is discussed in terms of metal-insulator transition.

To understand mechanisms of high thermionic emission of LaB 6 we use infrared spectroscopy, DC resistivity and Hall-effect techniques to study conduction electrons state in LaB6 single crystals with different contents of 10B and 11B isotopes. We find that up to 70% of conduction electrons are strongly non-equilibrium due to involvement in the collective oscillations of electron density coupled to vibrations of Jahn-Teller unstable boron cage and rattling modes of La-ions. We claim that these non-equilibrium electrons determine the extraordinary low work function of thermoemission in LaB 6 .

The model strongly correlated electron system ${\mathrm{Ho}}_{0.8}{\mathrm{Lu}}_{0.2}{\mathrm{B}}_{12}$, which demonstrates a cooperative Jahn-Teller instability of the boron sublattice in combination with rattling modes of Ho(Lu) ions, dynamic charge stripes, and an unusual antiferromagnetic (AF) ground state, has been studied in detail at low temperatures by magnetoresistance (\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}), magnetization, and heat capacity measurements. Based on received results it turns out that the angular H-\ensuremath{\varphi}-T magnetic phase diagrams of this nonequilibrium AF metal can be reconstructed in the form of a ``Maltese cross''. The dramatic AF ground state symmetry lowering of this dodecaboride with fcc crystal structure can be attributed to the redistribution of conduction electrons. These leave the Ruderman-Kittel-Kasuya-Yosida oscillations of the electron spin density to participate in the dynamic charge stripes providing extraordinary changes in the indirect exchange interaction between magnetic moments of $\mathrm{H}{\mathrm{o}}^{3+}$ ions and resulting in the emergence of a number of various magnetic phases. It is also shown that the two main contributions to magnetoresistance in the complex AF phase, the (i) positive linear on magnetic field and the (ii) negative quadratic $\ensuremath{-}\mathrm{\ensuremath{\Delta}}\ensuremath{\rho}/\ensuremath{\rho}\ensuremath{\sim}{H}^{2}$ component can be separated and analyzed quantitatively, correspondingly, in terms of charge carrier scattering on the spin density wave ($5d$) component of the magnetic structure and on local $4f\ensuremath{-}5d$ spin fluctuations of holmium sites.

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 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bar{\varGamma }}$$\end{document}Γ¯ state to appear Rashba split and explain the prominent \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar X$$\end{document}X¯ state by a surface shift of the many-body resonance. We propose that the latter mechanism, which applies to several crystal terminations, can explain the unusual surface conductivity. While additional, as yet unobserved topological surface states cannot be excluded, our results show that a firm connection between the two material classes is still outstanding., Samarium hexahoride is argued to be a topological Kondo insulator, but this claim remains under debate. Here, Hlawenka et al. provide a topologically trivial explanation for the conducting states at the (100) surface of samarium hexaboride; an explanation based on Rashba splitting and a surface shift of the Kondo resonance.

Lutetium dodecaboride LuB12 is a simple weak-coupling BCS superconductor with critical temperature Tc = 0.42 K, whilst ZrB12 is a strong-coupling BCS superconductor with the highest critical temperature Tc = 6.0 K among this group of materials. In case of lutetium substitution by zirconium ions in LuB12 the crossover from weak- to strong-coupling superconductor can be studied. We have investigated the evolution of critical temperature Tc and critical field Hc in high-quality single crystalline superconducting samples of Lu(1-x)Zr(x)B12 (0 =< x =< 0.45) by measuring magnetic ac susceptibility between 1 K and 50 mK. To obtain this kind of experimental data, a new susceptometer was designed, constructed and tested, which can work in a wide temperature range of 0.05 K - 3 K in 3He-4He dilution refrigerator. The measurements with this new susceptometer revealed how Tc(x) and Hc(x) increases with increasing concentration of zirconium in Lu(1-x)Zr(x)B12 solid solutions as well as how their superconducting phase diagram develops., 6 pages, 4 figures, CSMAG conference contribution