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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.

$\mathrm{Tm}{\mathrm{B}}_{4}$ is an anisotropic, metallic magnetic system with geometrical frustration of the Shastry-Sutherland type. Here an experimental study of the magnetocaloric effect (MCE) in Lu-doped $\mathrm{T}{\mathrm{m}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{u}}_{x}{\mathrm{B}}_{4}$ ($x=0.06$, 0.30), evaluated from the temperature dependence of heat capacity and magnetization curves at 2 K, is presented. The results are described within a theoretical model based on an extended Ising Hamiltonian which considers interactions up to the fourth-next-nearest neighbors. Model parameters were optimized to achieve the best match to the experimental results over the whole range of $\mathrm{L}{\mathrm{u}}^{3+}$ ion concentrations. After optimization a good quantitative agreement with the adiabatic temperature change and a good qualitative agreement with magnetization curves is obtained. Our study shows that the efficiency of the MCE can be tuned by dilution with nonmagnetic Lu ions. The theoretical model developed could be used to design new magnetocaloric materials.

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.

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

SmB

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 .

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

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)

We investigate the doping-induced changes in the electronic structure of ${\mathrm{CeB}}_{6}$ on a series of substituted ${\mathrm{Ce}}_{1\ensuremath{-}x}{R}_{x}{\mathrm{B}}_{6}$ samples ($R=\mathrm{La}$, Nd) using diffuse neutron scattering. We observe a redistribution of magnetic spectral weight across the Brillouin zone, which we associate with the changes in the Fermi-surface nesting properties related to the modified charge carrier concentration. In particular, a strong diffuse peak at the corner of the Brillouin zone ($R$ point), which coincides with the propagation vector of the elusive antiferroquadrupolar (AFQ) order in ${\mathrm{CeB}}_{6}$, is rapidly suppressed by both La and Nd doping, like the AFQ order itself. The corresponding spectral weight is transferred to the $X(00\frac{1}{2})$ point, ultimately stabilizing a long-range AFM order at this wave vector at the Nd-rich side of the phase diagram. At an intermediate Nd concentration, a broad diffuse peak with multiple local maxima of intensity is observed around the $X$ point, evidencing itinerant frustration that gives rise to multiple ordered phases for which ${\mathrm{Ce}}_{1\ensuremath{-}x}{\mathrm{Nd}}_{x}{\mathrm{B}}_{6}$ is known. On the La-rich side of the phase diagram, however, dilution of the magnetic moments prevents the formation of a similar $(00\frac{1}{2})$-type order despite the presence of nesting. Our results demonstrate how diffuse neutron scattering can be used to probe the nesting vectors in complex $f$-electron systems directly, without reference to the single-particle band structure, and emphasize the role of Fermi surface geometry in stabilizing magnetic order in rare-earth hexaborides.

For the last few decades, researchers have been intrigued by multipolar ordering phenomena while looking for the related quantum criticality in the heavy-fermion Kondo system Ce$_{1-x}$La$_{x}$B$_6$. However, critical phenomena induced by substitution level ($x$), temperature ($T$), and magnetic field ($B$) are poorly understood despite a large collection of experimental results is available. In this work, we present $T$-$B$, $x$-$T$, and $x$-$B$ phase diagrams of Ce$_{1-x}$La$_x$B$_6$ ($\mathbf{B}\parallel[110]$). These are completed by analyzing heat capacity, magnetocaloric effect (MCE), and elastic neutron scattering. A drastic increase of the Sommerfeld coefficient $\gamma_0$, which is estimated from the heat capacity down to 0.05 K, is observed with increasing $x$. The precise $T$-$B$ phase diagram which includes an unforeseen high-entropy region is drawn by analyzing the MCE for the first time in Ce$_{1-x}$La$_x$B$_6$. The $x$-$B$ phase diagram, which supports the existence of a QCP at $x>0.75$, is obtained by the same analysis. A detailed interpretation of phase diagrams strongly indicates positive correlation between the fluctuating multipoles and the effective electron mass., Comment: 11 pages, 8 figures

SmB6 has recently attracted considerable interest as a candidate for the first strongly correlated topological insulator. Such materials promise entirely new properties such as correlation-enhanced bulk bandgaps or a Fermi surface from spin excitations. Whether SmB6 and its surface states are topological or trivial is still heavily disputed however, and a solution is hindered by major disagreement between angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) results. Here, a combined ARPES and STM experiment is conducted. It is discovered that the STM contrast strongly depends on the bias voltage and reverses its sign beyond 1 V. It is shown that the understanding of this contrast reversal is the clue to resolving the discrepancy between ARPES and STM results. In particular, the scanning tunneling spectra reflect a low-energy electronic structure at the surface, which supports a trivial origin of the surface states and the surface metallicity of SmB6 .

Magnetocaloric effect (MCE) has drawn much attention because its magnetic cooling property enables refrigeration without producing noxious gas or using rapidly depleting resources. However, applications for everyday life are yet distant. In addition, we need to understand more about the practical aspect of the MCE. Here, we introduce a phenomenological model to explain the quasi-adiabatic MCE. Correction factors to the equilibrium thermodynamic feature implied by the entropy landscape are devised in analytic forms. To demonstrate the validity of the model, the MCE from two different materials is investigated. The recently discovered metallic paramagnet, YbPt 2 Sn, shows a linear and reversible MCE which is typical of a paramagnetic system and suitable for cryogenics without 3He. On the other hand, a complex-phase material, Ce 0.5 La 0.5 B 6 , exhibits a pronounced irreversible MCE especially across a magnetic phase boundary. A term that describes the field induced heating near a phase transition turns out to be essential in resolving the irreversible, non-equilibrium MCE.

In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn5 is similar to the recently discovered exciton peak in the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB6. The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a non-monotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields., 7 pages, 6 figures including one animation, accepted to Phys. Rev. B

In BCS-type superconductors Zr N B12 (T C≈6 K) and Lu N B12 (T C≈0.42 K) heat capacity C(T) and magnetization measurements have been carried out on high quality single crystals with various boron isotopes (with N=10, 11, and with natural composition of 10B and 11B). Parameters of the superconducting and normal states have been deduced from this study, allowing comparison between these two dodecaborides. It was shown that ZrB12 is a type-II superconductor in which the Ginzburg–Landau parameter varies in the range κ=0.8–1.12. A detailed analysis of specific heat in the normal state of R10B12, R11B12 and RnatB12 (where R=Zr, Lu) has revealed three Einstein type vibration modes of Zr4+-ions in ZrB12 with characteristic energies θ E1(Zr N B12)≈200 K and θ E2,3(Zr N B12)≈450 K, whereas only one quasi-local mode of Lu3+-ions in LuB12 with θ E1(Lu N B12)≈160–170 K was detected. This difference of vibration spectra has been found to be the main reason for the dramatic change of T C for ZrB12 and LuB12. Using strong coupling approximation, moreover, the Coulomb pseudopotential μ ∗, electron–phonon interaction constant λ ∗ and logarithmic average frequency ω ln were determined.

The specific heat of the ZrB12 compound in the normal and superconducting states (T C ≈ 6 K) has been studied in the 1.9–7 K temperature range for high-quality single crystals with different relative contents of boron isotopes. For Zr10B12, ZrnatB12, and Zr11B12 dodecaborides, the electron density of states and the electronphonon coupling constant, λe-ph ∼ 0.4, are found. The dependence of the thermodynamic and upper critical fields, as well as of the Ginzburg-Landau parameter (κ = 0.8–1.14) on temperature and isotope composition is determined. The results suggest the existence of the magnetic field induced phase transition at T* = 4–5 K, which is not related to the transition from type-I to type-II superconductivity. The possibilities of the existence of two-gap superconductivity and a structural phase transition at T* in zirconium dodecaboride are discussed.

Neutron spectroscopy results are presented for electron-doped YbB12-based systems. Electrondoping effects were obtained through the partial substitution of Yb for four-valence Zr, and through the introduction of a small amount of carbon in the boron sublattice of a YbB12 condo insulator. The different character of the original YbB12 gap-type magnetic-excitation-spectrum changes was observed.

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu1 − xCaxB6 (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) − ρ(H))/ρ(H) ≈ 7 × 105 detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu0.74Ca0.26B6 solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200−350 cm2/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu0.74Ca0.26B6 solid solution are comparable with the charge carrier mobility μH = 400−600 cm2/(V s) for the undoped EuB6 compound. The anomalous behavior of the transport and magnetic parameters of the Eu1 − xCaxB6 solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.

The thermodynamic, kinetic, and magnetic characteristics of (Yb, Zr)B12 have been investigated in detail in the temperature range from 4 to 300 K to reveal the effect of the band structure on the properties of the YbB12 Kondo insulator ground state. It is found that electron doping due to 20% substitution in the Yb sublattice significantly changes the properties of the low-temperature ground state of YbB12 Kondo insulator and only slightly affects the high-temperature spin-fluctuating state forming at T > 50 K.

The transport and magnetic properties of the Eu1-xCaxB6 (0x0.4) single crystals were studied in the wide ranges of temperatures (1.8-300 K) and magnetic fields (up to 8 T). The experimental data allow to identify a metal-insulator transition (MIT) at the critical Ca concentration xMIT≈0.3, which agrees well with the predictions of double exchange model (V.M. Pereira et al. Phys. Rev. Lett., 93 (2004) 147202). A significant enhancement of magnetoresistive effect is observed below 100K for Eu1-xCaxB6 compounds corresponding to the metallic side of the MIT (xxMIT). The drastic decrease of paramagnetic Curie temperature evaluated from the magnetic and magnetotransport data is discussed in terms of quantum MIT scenario recently proposed for this low carrier density system.

The magnetoresistance and magnetization of single-crystal samples of rare-earth dodecaborides RB12 (R = Ho, Er, Tm, Lu) have been measured at low temperatures (1.8–35 K) in a magnetic field of up to 70 kOe. The effect of positive magnetoresistance that obeys the Kohler’s rule Δρ/ρ = f(ρ(0, 300 K)H/ρ(0, T)) is observed for the nonmagnetic metal LuB12. In the magnetic dodecaborides HoB12, ErB12, and TmB12, three characteristic regimes of the magnetoresistance behavior have been revealed: the positive magnetoresistance effect similar to the case of LuB12 is observed at T > 25 K; in the range T N ≤ T ≤ 15 K, the magnetoresistance becomes negative and depends quadratically on the external magnetic field; and, finally, upon the transition to the antiferromagnetic phase (T < T N ), the positive magnetoresistance is again observed and its amplitude reaches 150% for HoB12. It has been shown that the observed anomalies of negative magnetoresistance in the paramagnetic phase can be explained within the Yosida model of conduction electron scattering by localized magnetic moments. The performed analysis confirms the formation of spin-polaron states in the 5d band in the vicinity of rare-earth ions in paramagnetic and magnetically ordered phases of RB12 and makes it possible to reveal a number of specific features in the transformation of the magnetic structure of the compounds under investigation.

Europium hexaboride single crystals have been investigated using the electron paramagnetic resonance at a frequency of 9.25 GHz in the temperature range 10–300 K. The magnetic phase separation of the spin system of europium ions Eu2+ is observed. The cause of the separation is the formation of the polarons of two types associated with the Kondo and anti-Kondo couplings of charge carriers of the valence and conduction bands, respectively, with the localized magnetic moments of Eu2+.

Based on low-temperature resistivity, heat capacity, and magnetization investigations, we show that the unusually strong suppression of superconductivity in ${\mathrm{Lu}}_{x}{\mathrm{Zr}}_{1\ensuremath{-}x}{\mathrm{B}}_{12}$ ($xl8%$) BCS-type superconductors is caused by the emergence of static spin polarization in the vicinity of nonmagnetic lutetium impurities. The analysis of the obtained results points to a formation of static magnetic moments with ${\ensuremath{\mu}}_{\mathrm{eff}}\ensuremath{\approx}6{\ensuremath{\mu}}_{B}$ per ${\mathrm{Lu}}^{3+}$ ion (${}^{1}{S}_{0}$ ground state, $4{f}^{14}$ configuration) incorporated in the superconducting ${\mathrm{ZrB}}_{12}$ matrix. The size of these spin-polarized nanodomains was estimated to be about 5 \AA{}.

${\mathrm{CeB}}_{6}$ is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with nonmagnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to ${T}_{\text{N}}$ upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.

High precision magnetoresistance (MR) Δρ/ρ(H,T) and magnetization M(H,T) measurements have been carried out for well known and typical strongly correlated electron system—cerium hexaboride. The detailed measurements have been fulfilled on single crystalline samples of CeB6 over a wide temperature range T ⩾ 1.8 K in magnetic fields up to 70 kOe. It was shown that the MR anomalies in the magnetic heavy fermion compound under investigation can be consistently interpreted in the frameworks of a simple relation between resistivity and magnetization—Δρ/ρ∼M2 obtained by Yosida [Phys. Rev. 107(1957)396]. A local magnetic susceptibility χ loc ( T , H ) = ( 1 / H * ( d ( Δ ρ / ρ ) / d H ) ) 1 / 2 was deduced directly from the MR Δρ(H,T) measurements and compared with the experimental data of magnetization M(H,T). The magnetic susceptibility dependences χloc(T,H) and χ(T,H) obtained in this study for CeB6 allow us to analyze the complicated H–T magnetic phase diagram of this so-called dense Kondo-system.

We report the results of a Seebeck effect study carried out on the single crystals of Eu1 − xCaxB6 (0 ≤ x ≤ 1) at temperatures 1.8–300 K. The Seebeck coefficient is shown to be determined by the sum of the linear term S = AT and a phonon-drag contribution, which is induced by the local phonon mode of europium (calcium) with the characteristic frequency 7.3 meV (9.1 meV). The crossover from negative (A ≈ −0.23 µV K−2 for x = 0) to positive (A ≈ +1.2 µV K−2 for x = 0.244) values of the diffusion thermopower indicates the smooth change of the band structure when approaching the hole-like conductivity found in Eu1 − xCaxB6 at x > xC ≈ 0.2.

Electron doping of the Kondo-insulator YbB12 has been achieved by substitution tetravalent Zr for Yb in the newly synthesized Yb0.8Zr0.2B12 solid solution and substitution carbon for B in boron cluster network with concentration order of 1% (Yb(B0.99C0.01)12. Neutron scattering measurements of the dynamical magnetic response are reported and analyzed. Zr doping transforms the spin-gap spectral response into a pseudogap shape and makes its temperature evolution much smoother than in the Yb1-xLuxB12 family studied previously. Carbon doping influences on the energy range of ground state excitation spectrum but does not suppress the magnetic gap. The results obtained are discussed in connection with the contribution of different interactions into the formation of the ground state for the Kondo-insulator.

The temperature dependences of the thermal conductivity and magnetization of a PrB6 single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ∼ 7 and ∼4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB6 at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.

Resonant magnetic excitations are widely recognized as hallmarks of unconventional superconductivity in copper oxides, iron pnictides, and heavy-fermion compounds. Numerous model calculations have related these modes to the microscopic properties of the pair wave function, but the mechanisms underlying their formation are still debated. Here we report the discovery of a similar resonant mode in the non-superconducting, antiferromagnetically ordered heavy-fermion metal CeB6. Unlike conventional magnons, the mode is non-dispersive, and its intensity is sharply concentrated around a wave vector separate from those characterizing the antiferromagnetic order. The magnetic intensity distribution rather suggests that the mode is associated with a coexisting order parameter of the unusual antiferro-quadrupolar phase of CeB6, which has long remained "hidden" to the neutron-scattering probes. The mode energy increases continuously below the onset temperature for antiferromagnetism, in parallel to the opening of a nearly isotropic spin gap throughout the Brillouin zone. These attributes bear strong similarity to those of the resonant modes observed in unconventional superconductors below their critical temperatures. This unexpected commonality between the two disparate ground states indicates the dominance of itinerant spin dynamics in the ordered low-temperature phases of CeB6 and throws new light on the interplay between antiferromagnetism, superconductivity, and "hidden" order parameters in correlated-electron materials.

The first ab initio calculations using ultrasoft pseudopotentials within the local-density approximation in superconducting ${\text{ZrB}}_{12}$ are reported, together with an inelastic neutron scattering study of its lattice dynamics. As in previously studied rare-earth dodecaborides, the phonon spectra are mainly determined by vibrations of the rigid boron network whereas Zr atoms vibrations have a quasilocal character and are localized in a narrow energy range around $E\ensuremath{\approx}17\text{ }\text{meV}$. The results consistently indicate that the phonon modes corresponding to Zr vibrations are responsible for a considerable electron-phonon interaction and could take part in the formation of the superconducting state below ${T}_{c}\ensuremath{\approx}6\text{ }\text{K}$ as was suggested previously.

Cavity measurements of a high frequency (60 GHz) electron spin resonance (ESR) have been carried our for a single crystal of ${\text{EuB}}_{6}$ at temperatures 4.2\char21{}50 K in magnetic field up to 7 T, which has been aligned along [001] crystallographic direction. It is found that in the case of homogeneous magnetic field in the sample the ESR spectrum of ${\text{EuB}}_{6}$ is formed by a single line at all temperatures including the ferromagnetic ordering region, whereas the gradient of magnetic field in the samples induces double peak ESR structure at low temperatures. For the quantitative description of the ESR line shape we suggested an analytical approach applicable for the cavity measurements of a metal with an arbitrary value of magnetic permeability including strongly magnetic case and obtained full set of the ESR parameters, namely oscillating magnetization ${M}_{0}$, $g$ factor, and linewidth. Our analysis has explained the visible resonance line shift and revealed the coincidence of the oscillating magnetization defining the resonance amplitude with the static magnetization. The anomalous growth of the linewidth below Curie temperature ${T}_{C}\ensuremath{\approx}15\text{ }\text{K}$ is observed. We argue that ESR in ${\text{EuB}}_{6}$ is not considerably affected by either interaction with the spin polarons or the magnetic phase separation and reflects merely the oscillation of the ${\text{Eu}}^{2+}$ localized magnetic moments, which can be well understood within mean-field approximation.

Some heavy fermion materials show so-called hidden-order phases which are invisible to many characterization techniques and whose microscopic origin remained controversial for decades. Among such hidden-order compounds, CeB6 is of model character due to its simple electronic configuration and crystal structure. Apart from more conventional antiferromagnetism, it shows an elusive phase at low temperatures, which is commonly associated with multipolar order. Here we show that this phase roots in a Fermi surface instability. This conclusion is based on a full 3D tomographic sampling of the electronic structure by angle-resolved photoemission and comparison with inelastic neutron scattering data. The hidden order is mediated by itinerant electrons. Our measurements will serve as a paradigm for the investigation of hidden-order phases in f-electron systems, but also generally for situations where the itinerant electrons drive orbital or spin order., In compounds containing 4f and 5f elements, hidden-order phases exist which are undetectable by many methods, the origins of which are debated. Here, the authors use photoemission and neutron scattering methods to show how such a multipolar-ordered phase emerges due to Fermi surface instability in CeB6.

High-frequency electron spin resonance (ESR) measurements (60-100 GHz) on a high-quality single crystal of EuB 6 have been carried out at temperatures down to 1.8K. The ESR spectrum fine structure has been unambiguously determined for the first time in this experiment. In addition to the single resonance observed at high temperatures T> 100K in previous reports, splitting of the resonance into two broad lines was detected below T 0 ≈ 80 K. A decrease of temperature in the range T< To was found to cause the divergence of these two features. The resonance fields estimated from the experimental data show almost linear frequency dependence. The observed behavior may be explained in terms of the formation and growth of magnetic polarons and provides a first direct experimental confirmation of the magnetic transition scenario recently proposed for EuB 6 by Yu and Min [Phys. Rev. Lett. 94 (2005) 117202].

The ESR measurements of the EuB6 single crystal samples were executed on frequency 9.25 GHz in TE102 rectangular cavity in the temperature range from 15 to 300 K. We used samples of identical form and size, but different crystallografic orientation to estimate their magnetization. At T = 30 ? 40K was observed the magnetic phase separation, which, most likely, is accompanied also by charging separation. The anisotropy magnetization of more intensive magnetic phase (with anti-Kondo interaction) along the different crystallographic directions was found above a temperature of the ferromagnetic transition. We conclude that this result connect with existence the ferronic states and charging separation in the EuB6 single crystal. Estimations of angular distribution of the magnetic moment of ferrons in EuB6 are made.

The angular dependent resistivity and magnetization have been measured on the single crystals of colossal magnetoresistance compound EuB6 at temperatures T axis, the rotation axis is along direction) allows to estimate a field dependence of MRA charcteristic temperature T*[K]≈8+0.4H[kOe], which corresponds to the change of the largest NMR amplitude from H|| (T (T>T*(H)). A maximal amplitude of PMR established in ferromagnetic state below 12K is detected for magnetic field applied close to direction. The observed change in MRA is discussed in terms of crossover from the spin polaronic regime of charge transport in paramagnetic phase to the band conductivity in magnetically ordered state.