Fine details of crystal structure of archetypal CeB6 hexaboride with heavy fermions are studied at temperatures 85 and 500 K by precise X-ray diffraction technique. Small static Jahn-Teller distortions of a simple cubic lattice are observed at these temperatures, leading to emergence of (i) dynamic charge stripes along selected directions <110>, <100>, and <111> in the crystals in combination with (ii) vibrationally coupled pairs of Ce ions. Instead of a Currie-Weiss type behavior, the temperature dependence of magnetization M∼(T − TCrand)−0.8 with TCrand∼TQ∼3.3 K was deduced in a wide temperature range 5–800 K for various directions of external magnetic field, which indicates the Griffiths phase formation with nanosized clusters of magnetic ions in CeB6. Moreover, in contrast to the scenario of a single-ion Kondo lattice, when revealing the power-law behavior of the magnetic contribution ρm(T)∼T−0.4 to the resistivity in the range of 8–90 K, we conclude in favor of the regime of weak localization of charge carriers in CeB6. Fourier maps of electron density confirm the conclusion about the nanosized magnetic clusters of Ce ions in this archetypal strongly correlated electron system with unusual magnetic ground state.

For the first time, an electronic phase transition at Tc ≈ 340 K in a classical strongly correlated electron system CeB6 is discovered and studied in detail. It is shown that the observed anomalies in the resistivity, thermal conductivity, specific heat, magnetization, and lattice parameters apparently correspond to the formation of a substructural charge density wave (s-CDW) at Tc, as well as to a change in configurations of dynamic charge stripes in this metal with electronic instability.

Zero-field heat capacity of EuxYb1-xB6 (x = 0, 0.127, 1) family was investigated at temperatures 1.9 -300 K. The applied procedure of C(T) decomposition allowed to identify the Debye component resulted from the rigid cage of boron atoms (& UTheta;D & AP;1160 K), as well as the contribution from the quasilocal vibrational modes of rare-earth (RE) ions with Einstein temperatures to be different in end-point compounds: & UTheta;E(YbB6) & AP;91.6 K and & UTheta;E(EuB6) & AP;125 K. Our results also suggest the existence of additional low-temperature defect mode for the Eulow systems (x & LE; 0.127) which is related to & AP;1.15-1.3% vacancies at boron position. The magnetic contribution for EuB6 was analyzed in the framework of mean-field theory. The estimates show that 97% of spin entropy, associated with the 8S7/2-state of Eu2+ ions, is frozen out at TC. The finding that short-range magnetic ordering in the paramagnetic (PM) phase is limited by no more than 3% of EuB6 spin entropy should be taken into account in modern approaches explaining electronic and magnetic phase separation in this extraordinary compound.

The broad-band reflection spectra of YB 6 and YbB 6 hexaborides with Jahn–Teller instability of the boron cage have been measured at room temperature. An optical conductivity analysis has revealed, along with the Drude electronic components, heavily overdamped collective modes, which are notable in YB 6 for high dielectric contributions, Δε = 2000–5700. The fraction of nonequilibrium charge carriers in YB 6 , which is at the boundary of structural instability in the hexaboride family, reaches 85–90%, whereas this fraction in doped YbB 6 semiconductor is not higher than 25%. It has been shown that unlike the predictions of the topological Kondo insulator model, the surface “metallization” in Yb 2+ B 6 crystals can be explained by additional doping of a surface layer with Yb 3+ ions.

Magnetization M and magnetoresistance Δρρ(H,T) (MR) were studied and MR analyzed quantitatively in the complicated low temperature Néel phase of RB12 (R - Ho, Er and Tm) antiferromagnets with structural (originating from cooperative Jahn-Teller effect) and electronic (coming from dynamic charge stripes) instabilities. It is shown, that well below the Néel field (HN) the magnetoresistance is determined by the concurrence of carriers' scattering by spin-density waves (leading to a linear positive contribution Δρρ(H) ∼ H) from one side, and by ferromagnetic nanoscale clusters (leading to a linear negative MR) from the other. The development of different kind instabilities in the critical region just below HN results into the emergence of an additional negative quadratic MR term, attributed to the carriers' scattering on 4f-5d local electron density fluctuations. The anisotropy of both two linear and one quadratic MR components is analyzed in detail and compared for RB12 compounds with different magnetic structure.

A three-dimensional (H, φ, θ, T = 2 K) magnetic phase diagram has been constructed for the first time for the antiferromagnetic metal Er11B12 with an amplitude-modulated magnetic structure exhibiting an electronic instability (dynamic charge stripes). The boundaries determining the shape of the main magnetic phases in the H–φ–θ space are reconstructed from measurements of the magnetoresistance. The role of dynamic charge stripes, which suppress the indirect Ruderman–Kittel–Kasuya–Yoshida exchange between the magnetic moments of the nearest Er3+ ions, and of the single-ion anisotropy in the formation of a complicated multicomponent phase diagram of Er11B12 is discussed.

Detailed Hall effect measurements were carried out at helium temperatures of 2.1–4.2 K in the magnetically ordered phases of a Ho 0.8 Lu 0.2 B 12 antiferromagnet in a magnetic field of up to 80 kOe on single crystals with normal orientations n || [001] and n || [110]. Based on the analysis of the angular dependences of the Hall resistance, some new phase transitions in the antiferromagnetic state were found and the anomalies associated with the effects of interaction between dynamic charge stripes and an external magnetic field were revealed.

Precise X-ray diffraction, heat capacity, magnetoresistance and magnetization measurements have been carried out on high quality Tm1-xYbxB12 single crystals with the goal to reconstruct the H-T and angular H-phi magnetic phase diagrams in the (110) plane of these antiferromagnets (AF) with dynamic charge stripes and Yb-ion valence instability. The analysis developed here allowed us to conclude in favor of essential changes in the filamentary structure of fluctuating charges, and the emergence and variation of stripe-induced magnetic and charge transport anisotropy that are controlled by lowering the temperature and ytterbium doping. It was found that local charge and spin fluctuations on Yb-sites suppress strongly the complicated AF state. However, the magnetic anisotropy is conserved, and the only moderate modifications of the Maltese Cross -type magnetic phase diagrams are detected in the range x < 0.2. We argue that the AF ordering of Tm3+ magnetic moments is the main factor, which determines the anisotropy in the Ne ' el phase of Tm1-xYbxB12 with carrier-mediated magnetic indirect RKKY exchange renormalized significantly by quantum fluctuations of the electron density along 110 directions. PACS: 73.22.-f, 75.47.-m, 71.27. + a.

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.

We present a study of low temperature electron transport (resistivity, magnetoresistance and Hall coefficient) in SmB6 single crystals having different polar (100) and nonpolar (110) and (111) surfaces after mechanical polishing and chemical etching. The estimation of effective parameters for surface and bulk charge carriers allows us to conclude that surface conductivity is very sensitive to the method of surface treatment. The most pronounced change is observed for the polar (100) surface, for which the related Hall concentration of charge carriers at 2 K decreases more than by 2 orders of magnitude and the Hall mobility increases by a factor of 15 after etching these faces in diluted nitric acid. We suggest that the strong dependence of surface properties on the type of treatment may result from both topological protection, which is influenced by intrinsic defects or surface reconstruction, and band bending effects, which modulate the properties of the surface conduction layer in case of polar faces.

The galvanomagnetic characteristics of SmB6 single crystals are studied within the temperature range of 1.9‒3.6 K at different orientations of the crystal faces. As a result, the electrical resistivities of the surfaces corresponding to the (100), (110), (111), and (211) crystallographic planes are determined. It is shown that the effective parameters of charge carriers, which determine the surface conductivity in SmB6, depend both on the orientation of the surface and on the method of its processing. It is found that the etching of polished polar surfaces formed by (100) planes leads at 1.9 K to a decrease in the density and to an increase in the mobility of surface n-type charge carriers from 113/a2 and 1.12 cm2/(V s) to 0.76/a2 and 18 cm2/(V s), respectively (lattice parameter a ≈ 4.134 Å). For etched nonpolar surfaces corresponding to the (110) and (111) planes, the maximum density of surface charge carriers (per unit area of the surface Brillouin zone) is found to increase by factors of 2.3 and 3.9, respectively. It is proposed to use this parameter as a simple criterion to identify the features of electron transport due to the nontrivial topology of the band structure of SmB6.

We present the results of experimental studies on the synthesis by molecular-beam epitaxy of AlGaAs nanowires with InAs quantum dots. The morphological, structural, and optical properties of the grown nanostructures have been studied. It is important to note that the emission from quantum dots is observed in the wavelength range from 750 to 970 nm. Assumptions about the nature of short-wavelength emission from quantum dots are formulated. In particular, one of the reasons may be a significant desorption of indium atoms and the presence of gallium atoms in catalyst drops during the growth at a substrate temperature of 510◦C. The proposed technology opens up new possibilities for integration direct-gap III−V materials with a silicon platform for various applications in photonics and quantum communications.

A detailed investigation of contributions to the magnetization of nonmagnetic YB 6 , LaB 6 , and YbB 6 hexaboride single crystals has been performed, and a procedure for their separation has been proposed. It has been shown that a low value of electronic susceptibility χ e ( T ) in YB 6 and LaB 6 hexaborides seems to be associated with a small effective mass of band carriers, m * ~ 0.5 m 0 . As a result of this, the Pauli component and Landau diamagnetism cancel each other. It has been found that χ e ( T ) varies in the intervals T < T * ( T * ~ 50 K) and T > 150 K, which are attributed to an order–disorder transition below T * and a carriers-related contribution due to the Jahn–Teller structural instability of the boron frame work.

Fine details of crystal structure of ZrB12 two-gap superconductor are studied at low temperatures by precise x-ray diffraction technique. Small static Jahn-Teller distortions of a face centered cubic lattice are observed in the range 30–70 K, leading to emergence of two types singularities of electron density (ED). These are: (i) dynamic charge stripes along selected directions <110> and <112> in the crystal and (ii) triangular lattice of the ED antinodes located in the interstices of the boron lattice in the {111} planes. The second anomaly was detected for the first time, and it is very pronounced for ZrB12, being intensified at low temperatures. The anisotropy of the two-gap superconductivity in ZrB12 is confirmed by measurements of the low temperature heat capacity in a magnetic field directed along three principal axes in the crystal, H || [100], H || [110] and H || [111]. We conclude in favor of the magnetic field-induced anisotropy arising from the interaction of vortex lattice in this superconductor with fluctuating ED (charge stripes) of a two-type filamentary structure.

The present article highlights two aspects at the intersection between Rashba physics and topological matter. Topologically nontrivial matter has been in the focus for almost two decades. It depends strongly on spin–orbit coupling but, in contrast to large parts of modern solid state physics, strong electron correlation does not play a major role. In this context, SmB6 has been suggested as the first topological insulator driven by strong electron correlation and the first topological Kondo insulator. We review the important role of the Rashba splitting in determining that the observed surface states are not topological. Moreover, we point out that the Rashba splitting of SmB6 represents the extreme case of a large splitting in momentum space at a small Rashba parameter.

We investigate high-quality single-domain crystals of YbB 12 using the precise x-ray diffraction technique in combination with the low-temperature polarized THz–infrared spectroscopy and accurate magnetotransport measurements. It is 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 (2018) and ibid 362 , 32 (2018)] is actually a heterogeneous compound in the regime of electronic phase separation. Changes in the configuration of the discovered dynamic charge stripes are proposed 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 YbB 12 . We argue that the discovery of stripes in YbB 12 is fundamental, elucidating the nature of exotic dielectric state in Kondo insulators.

Inhomogeneous superconductivity in the high quality single crystals of ZrB12 (Tc = 6 K) has been studied using the heat capacity and x-ray diffraction (XRD) data. Evidence of two-band superconductivity with two branches of upper critical field Hc2(Tc) is obtained in a magnetic field applied along the [110] axis of the crystal. On the contrary, at H //[100], the only dependence Hc2(Tc) is observed. This finding is supplemented with the checkerboard-type patterns of the charge stripes in ZrB12 deduced from the detailed analysis of XRD data. These patterns are compared to the structure of the charge stripes in the weakly bound superconductor LuB12, whose Tc is 15 times lower than that of ZrB12. Probable nature of the two-gap superconductivity in ZrB12 with strongly enhanced characteristics is discussed.