The presented studies of resistivity (rho), thermal conductivity (kappa) and specific heat (C) at low temperature 1.8-7 K in magnetic field up to 90 kOe made it possible to detect for the first time the exponential field dependences rho(H), kappa (-1)(H), C(H) similar to exp(-mu H-eff/k(B)T) of the charge transport and thermal characteristics in the so-called antiferroquadrupole (AFQ) phase of the archetypal heavy-fermion CeB6 hexaboride. From magnetoresistance measurements it is shown that in the AFQ state the effective magnetic moment varies in the range mu(eff)(T) = 1.4-1.9 mu(B), and its value is very close to mu(eff(tau)())(T) approximate to 2 mu(B), derived from the field dependence of the relaxation time tau(H) observed in the heat capacity and thermal conductivity experiments. The phenomenological model proposed here allowes us to attribute the magnetic moments to spin droplets (ferrons), that appear in the bulk AFQ phase of CeB6 crystals. The relevant electronic phase separation at the nanoscale, manifested by dynamic charge stripes, that leads to the formation of ferrons, was revealed from the analysis of low-temperature X-ray diffraction experiments using the maximum entropy method. We argue that the Jahn-Teller collective mode of B-6 clusters is responsible for the formation of charge stripes formation inducing transverse quasi-local vibrations of pairs and triplets of Ce ions, which leads to 4f-5d spin fluctuations providing spin-polarons (ferrons) in the CeB6 matrix.

Mn-based metallic helimagnets (MnSi and MnGe) were studied by detailed measurements of electrical resistivity at temperatures 2-300 K and magnetic fields up to 8.2 T. To interpret the data the procedure of rho(T) analysis was applied. The approach presented here is based on River-Zlatic model [N. Rivier, and V. Zlatic, J. Phys. F: Met. Phys. 2 , L87 (1972)]. The decomposition performed allowed identifying along with electron-phonon component the additional one caused by the scattering of electrons on localized spin fluctuations (LSF). It was shown that the last contribution plays significant role not only in paramagnetic (PM) but also in magnetically ordered states of compounds under investigation. Spin fluctuation temperature was estimated as theta lsf (MnSi) approximate to 120 K and theta lsf (MnGe) approximate to 250-300 K. This model may be applied to the resistivity analysis in other d-metallic systems.

We show that the separation of contributions to lowtemperature heat capacity and the Hall effect, carried out in Phys. Rev. Lett. 120 257206 (2018), Nat. Phys. 15 954 (2019), Phys. Rev. X 12 021050 (2022), leads to unfounded conclusions about (i) the formation of uncharged quasiparticles (Majorana fermions) and (ii) the transition, as the magnetic field increases, to the metallic state with heavy fermions in the YbB12 semiconductor with strong electronic correlations. We obtain an alternative explanation of the experimental data in terms of the filamentary structure of conducting channels in the semiconductor matrix of ytterbium-based dodecaborides. Such channels (charge stripes) are nanoscale electron-density inhomogeneities and form manybody states near the Fermi level.

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.

The results of magnetic measurements, electron spin resonance, heat capacity, resistivity, and magnetocaloric effect as well as density functional theory (DFT), are presented for the compound NdRh2 (MgCu2-type structure). This compound was synthesized at a pressure of 8 GPa and a temperature of 1700 K. The magnetic properties of the material are shown to be determined by the high-temperature (over 400 K) spin polarization of the 4d Rh electrons and by the ferrimagnetic (FiM) interaction of Rh and Nd magnetic subsystems. Concurrently, the spontaneous magnetization of the spin polarization of 4d Rh electrons is approximate to 0.01 mu B/Rh, while the Nd magnetic moment is approximate to 1.7 mu B/Nd. This leads to complicated magnetic behavior with long range FiM order at TC less than or similar to 7 K at zero field and with wide temperature range of spin fluctuations TC<T less than or similar to 50 K. Specific heat and resistivity data provide corroboration of the spin fluctuation regime with the spin fluctuation temperature Theta sf approximate to 28.5 K. The optimal critical parameters were identified as beta=1.18 +/- 0.02, gamma=0.85 +/- 0.02, and TC approximate to 40 K from modified Arrott plotting, which are distinct from any conventional universality class. The maximum magnetocaloric effect, when the magnetic field changes from 0 to 9 T, Delta H=9 T occurs at T approximate to 11 K and the magnetic entropy change reaches -Delta Sm=7.0 J (kg K)(-1). In our DFT calculations, the FiM arrangement was obtained, with values of magnetic moments of Nd and Rh. Additionally, the Fermi surface was constructed.

Nonmagnetic metal LuB_12 is known to exhibit considerable transport anisotropy, which was explained in literature by different mechanisms including possible formation of dynamic charge stripes below the point ∼ 150K. Here we study transport properties of solid solutions based on LuB_12 host compound with general formula R_xLu_1-xB_12 (R-Dy, Er, Tm, Yb, Lu) and with x ≤ 0.03. The experiment has been performed on single crystals of high quality in the temperature range 1.8 - 300K in magnetic fields up to 82kOe. The application of several models to the analysis of zero-field resistivity is discussed. A phenomenological description of large positive quadratic component of transverse magnetoresistance Δρ/ρ(H) = μ_D^2H^2, which dominates for all compounds under investigation, allows to estimate drift mobility exponential changes μ_D ∼ T^-α with the index α ≈ 0.95 - 1.46. In order to check the existence of additional channel of scattering, caused by probable presence of dynamic charge stripes, we performed the study of the anisotropy of magnetoresistance in Dy_0.01Lu_0.99B_12 and Tm_0.03Lu_0.97B_12 compositions including the measurements of the field scans with different current and field geometries. The data obtained allow us to confirm the fulfillment of semi-empirical Kohler's rule in a wide interval of temperatures 30 - 240K regardless of the orientation of current and magnetic field. This result was attributed as a proof of the absence of additional channel of scattering caused by stripes. We argue, that charge-transport anisotropy is originated in R_xLu_1-xB_12 due to the anisotropy of electron-phonon scattering on the one hand and the effects of Fermi surface (FS) topology (at low temperatures) on the other.

In this study, polycrystalline intermetallic compounds Ce3-xPd20+xSi6 (x = 0; 0.5; 1) as well as La-based analogues La3-xPd20+xSi6 (x = 0; 1) are synthesized. At the same time, R2Pd21Si6 (R=Ce, La) are new rare earth compounds presented here. An analysis of the crystal structure shows that the increase in x leads to partial (x = 0.5) or complete (x = 1) replacement of Ce/La ions by Pd in one of the two Wyckoff positions, namely, in position 4a. Ab initio DFT calculations demonstrate good agreement between the determined lattice parameters and those found experimentally for both Ce3-xPd20+xSi6 (x = 0; 0.5; 1) and La3-xPd20+xSi6 (x = 0; 1). A comparative analysis of resistivity and specific heat of Ce3-xPd20+xSi6 with x = 0; 0.5; 1 makes it possible to associate the Ce ions in the 8c position with Kondo behaviour in all three compositions, which manifests itself in an increase in resistivity with cooling, as well as in a broad peak of the magnetic contribution of specific heat. The Kondo temperature T-K approximate to 25 K obtained from the analysis of the temperature dependences of the heat capacity turns out to be the same for all Ce compounds. The appearance of cerium ions in the position 4a in compositions with x = 0 and x = 0.5 leads to an additional contribution to the specific heat, the value of which increases with decreasing temperature, as well as, to the formation of a coherent Kondo-lattice state with a maximum of the resistivity at T-coh approximate to 15 K, followed by the subsequent onset of negative magnetoresistance regime below T-inv approximate to 7 K. Apparently, this behaviour is associated with the appearance of spin and orbital correlations proceeding the quadrupole and antiferromagnetic transitions occurring at lower temperatures. Magnetization measurements show more localized character of magnetic moments of Ce ions at T = 2 K in Ce3Pd20Si6 as compared to the Ce2Pd21Si6.

Among of rare-earth (RE) hexaborides only two compounds SmB6 and YbB6 are discussed in literature to be members of a new class of 3D topological insulators. However, their ground states originate due to different physical mechanisms, including Kondo 4f-5d hybridization and 5d-2p band inversion, respectively. Here we report a comparative study of magnetotransport (resistivity and transverse magnetoresistance) measured on high quality single crystals of YbxSm1-xB6 and EuxSm1-xB6 solid solutions (x ≤ 0.05) at temperatures 1.7 − 300 K in magnetic fields up to 82 kOe. The choice of dopant was determined by the fact that the presence of magnetic/nonmagnetic (Eu2+/Yb2+) impurity in parent SmB6 matrix should lift/not lift the topological protection of surface states. Based on the two-gap paradigm the x-evolution of electron spectra in YbxSm1-xB6 and EuxSm1-xB6 was studied. Our data show that both the 4f lattice coherence (Eg) and the intrinsic gap (Ea) related to many-body states survive under RE doping at least for x ≈ 0.02 − 0.024. We also suggest that the point x(Eu) = 0.05 can be treated as an upper limit of the small gap closing in EuxSm1-xB6 materials. In YbxSm1-xB6 family a negative linear transverse magnetoresistance (TMR) was detected for the first time in the regime of surface conductivity (T < T* ≈ 5 K). The TMR anomaly at T* caused possibly by the topological protection of surface states in SmB6 is found to survive in Eu-doped compounds but disappears almost completely for Yb-doped compositions in the same fixed magnetic fields. This paradoxical observation is not consistent with general predictions of the topological Kondo insulator (TKI) model.

Type II superconductivity with Tc ∼ 6 K is discovered in LaB6. The critical fields are determined and the estimates for the coherence length ξ(0) ∼ 240 Å, the Ginzburg−Landau parameter κGL ∼ 2, and the electron–phonon coupling constant λe−ph ≈ 0.75 are obtained. Precision X-ray diffraction studies at T = 30 K reveal three-dimensional charge-stripe structures in LaB6. The scenario of superconductivity localized near filamentary channels with a fluctuating electron density arising in the lanthanum hexaboride matrix is discussed.

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.

The spatial spin modulated structure (SSMS) of the cycloid type present in bulk BiFeO3 prevents the linear magnetoelectric effect. One way to influence this structure is to reduce the crystal size to the nanoscale. Various opinions are circulating in the literature about the effect of nanocrystal size on SSMS, and to investigate this issue, we used a number of methods, with zero-field NMR (ZF NMR) spectroscopy at the forefront. ZF NMR spectroscopy enables the direct observation of the distribution profile of local fields on iron atoms and defines the SSMS presence and its properties. We also examined the synthesized samples using XRD, TEM, and magnetometry. We conclude that SSMS persists as the nanocrystal size decreases to the cycloid period and less, becoming more harmonic. This is accompanied by the change of the anisotropy type from an "easy axis" to an "easy plane". Magnetic measurements show a significant increase in the saturation magnetization, remanent magnetization, coercivity, and exchange bias of nanocrystals with sizes close to the cycloid period, which is probably associated with incomplete spin compensation in the case of an incomplete cycloid period. Despite the fact that SSMS is retained in the samples with decreased size, the magnetic properties experience a sharp increase up to applicable values.

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.

Angular measurements of the Hall resistivity and transverse magnetoresistance (MR) were utilized to probe at low temperatures the influence of nanoscale electronic phase separation (dynamic charge stripes) on the charge transport in single crystals of rare earth (RE) dodecaborides RB12 (R = Ho, Er, Tm, Lu). We studied several samples of every RB12 compound, each with different normal vectors n || [001], n || [110], n || [111] and n || [112] to the lateral surface of the sample, located in the same plane (1-10). The emergence of strong anisotropic components of both the Hall effect (HE), and MR was detected for all studied RB12. It turned out that the anomalies on the angular dependences of HE and MR are very similar for various RB12 with magnetic and non-magnetic RE ions, and that the amplitude of the anisotropic contributions is determined mainly by the concentration of impurities, which serve as pinning centers for the dynamic charge stripes. Depending on the charge carriers’ mobility µH in RB12 a crossover between two regimes of the anisotropic HE is observed with a threshold value µH ~ 960 cm2V-1s-1, corresponding to the mean free path [[EQUATION]], which may be attributed to some critical length of stripes in these conductors.

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.

Highly perfect FeBO3 and Fe0.91Ga0.09BO3 single crystals were studied in a wide temperature range using SQUID magnetometry. A theoretical model describing the temperature and field dependences of the magnetization of single crystals has been developed. It is found that even a small concentration of gallium, which is a diamagnetic impurity, substantially affects the magnetic properties of single FeBO3 crystals. In particular, the Fe0.91Ga0.09BO3 crystal differs from the pure FeBO3 phase in by a lower magnetic phase transition temperature and a higher antiferromagnetic susceptibility at low temperatures.