Destruction of the gap in the intermediate valence compound SmB6 by neutron irradiation

The electrical resistivity ? of a SmB₆ single crystal has been measured during and after a fast neutron irradiation performed at 21 K : for the first time, it is shown that a very low defect concentration destroys the low temperature divergence of ? in this compound. These results are considered as an evidence of the existence of an intrinsic gap in pure SmB₆, and of its destruction by point defects. On the contrary, the irradiation has increased the resistivity value at 300 K, confirming the metallic character of SmB₆ at this temperature. The recovery of defects at 300 K is very weak.     

Pulsed magnetic field study of the spin gap in intermediate valence compound SmB6

In this work, we report the behavior of electrical resistivity of SmB₆ at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at T_C~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB₆ proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.     

Raman scattering in intermediate valent SmB6

We have measured the Raman spectra of intermediate valent (IV) SmB₆ and compared it to LaB₆ and EuB₆. Beside the three high energy Raman active phonons we found additional features in the spectra. Most prominent is a peak at 172 cm^-1 for SmB₆, at 214 cm^-1 for LaB₆ and at ～220 cm^-1 for EuB₆. The spectra are analysed in terms of defect induced phonon excitations yielding a weighted phonon density of states. The softening of the line in IV SmB₆ compared to the other hexaborides is unusual since even in the trivalent MB₆ compounds the Raman active phonon modes normally are at lower frequencies than in semiconducting, divalent samples. This phonon softening is explained in analogy with the phonon anomalies in other IV compounds like Sm_1?xY_xS and TmSe.     

Point-contact spectroscopy on SmB6, TmSe,LaB6 and LaSe

We measured the dynamic resistance dU/dI (U) and its derivative d²U/dI²(U) of point contacts with intermediate valent SmB⁶ and TmSe and their integer valent reference compounds LaB₆ and LaSe at liquid helium temperatures. While dU/dI(U) for point contacts with LaB₆ and LaSe did not show any anomalous structures, for SmB₆ and TmSe a strong resistance peak at zero voltage was observed with a characteristic width of ～ 5 mV for SmB₆ and ～ 2.5 mV for TmSe.     

ESCA study of electronic structure of SmB6

The electronic structure of SmB₆ was investigated by X-ray photoelectron spectroscopy (XPS or ESCA). The spectrum of each core level of Sm in SmB₆ consists of three peaks. Such a splitting was not observed in Sm₂O₃, SmF₃, and SmB₄. The analysis of the results shows that three kinds of Sm ions — Sm²⁺(4f⁶), Sm²⁺(4f⁵5d^0.86s^0.2), and another Sm²⁺ — are present in SmB₆. The last kind of Sm ions is probably localized in a surface region.     

Optic phonon anomalies and f-d hybridization in SmS and SmB6

The optic phonons of semiconducting SmS and of (homogeneously) mixed-valent SmS and SmB₆ have been investigated by means of Raman scattering. The dominant electron - phonon interaction upon resonantly exciting the 4f shell is revealed as a coupling of the strongly localized 4f hole at the Sm site to full symmetric (A_{j g}) displacements of the nearest neighbour anions i.e. mainly to LO(L) phonons. Semiconducting and mixed-valent, metallic SmS show a “softening” of the LO(L) phonon frequencies with respect to the other divalent and trivalent rare earth sulfides, respectively, going in parallel with a “softening” of the bulk modulus. The f-d hybridization is considered as the common origin of these phonon frequency renormalizations.     

Configurational mixing and 4f-photoemission lineshapes in SmB6

Single crystal surfaces of the interconfiguration fluctuation material SmB₆ have been studied by X-ray photoemission spectroscopy. Comparison with data for LaB₆ allows a detailed analysis of the Sm 4f-spectrum. It yields a Sm²⁺/Sm³⁺ valence ratio of 0.4, a Coulomb correlation correlation energy oif 7 eV, and a 1 eV broadening of the Sm³⁺ 4f spectrum     

Azimuthal anisotropy in low-energy ion scattering from SmB6 (001)

A remarkable azimuthal anisotropy has been observed in the scattering of He⁺ ions of an incident energy of 1 KeV from the SmB₆(001) surface. The anisotropy can be interpreted on the basis of “shadowing” effects and clearly reflects the structure of the surface.     

Structures and phase transition in Rb2NaHoF6

The phase transition at T_c=172(2) K and high as well as the low temperature structures at 205 K and 17 K were studied with a low temperature Guinier diffractometer and camera. A second order phase transition (Γ-condensation of the T_1g mode) lowers the symmetry from Fm3m at T>;T_c to I4/m at T<T_c. In the low temperature phase the octahedra are tilted (5.2 degrees at 17 K) and slightly stretched. The increase of the tilt angle and the linear increase of the tetragonal distortion suggests a mean field power law. This classical behaviour would agree with the assumption of a Jahn Teller splitting of the Ho³⁺ electronic state.     

Nuclear spin relaxation,hybridization, and low-temperature 4f spin fluctuations in intermediate-valent SmB6

¹¹B spin-lattice relaxation measurements have been carried out in SmB₆ samples with large low-temperature resistivities. Above 15 K the relaxation is activated, with approximately the same gap (≈6 meV) as found previously in transport and optical measurements. 4f spin fluctuations apparently dominate the relaxation, so that these results give strong evidence for 4f-conduction band hybridization at the gap edge. An anomalous peak in the relaxation rate was observed at ≈5 K, which is tentatively attributed to fluctuations of “remagnetized” Sm³⁺ ions near Sm-site vacancies.     

Phase transition in (NH4)3FeF6

By use of Mössbauer spectroscopy we have found that the transition from tetragonal to cubic structure in (NH₄)₃FeF₆ takes place at 263 K. The phase transition exhibits a hysteresis of 0.5 K. The experimental data indicate that the tetragonal deformation found at low temperatures diminishes gradually as the transition temperature is approached. The spectra are influenced by electronic relaxation.     

The valence band structure of metal hexaborides: An ESCA study of CaB6, and YB6

Two typical hexaborides CaB₆ and YB₆ have been studied by X-ray photoelectron spectroscopy (XPS or ESCA). The ESCA spectra of these compounds in the energy band region can be explained successfully by the calculation of Longuet-Higgins and Roberts rather than those of other authors. This means that covalent bonding between the boron and metal atoms in the hexaborides is weak. Therefore, we can regard divalent-metal hexaborides such as CaB₆ as “ionic crystals” which can be expressed approximately as M²⁺(B₆)^2?. At the same time, trivalent-metal hexaborides such as YB₆ are thought to be metals in which the conduction electron concentration is one per unit cell.     

化合物HoMn6Sn6的磁晶各向异性及自旋重取向相变研究

采用交换相互作用的分子场理论模型对金属间化合物HoMn₆Sn₆的自旋重取向相变进行了研究. 从理论上计算了HoMn₆Sn₆的易磁化方向以及Ho和Mn离子磁矩与c轴夹角随温度的变化. 基于单离子模型计算了Ho离子的一阶和二阶磁晶各向异性常数K_1R和K_2R随温度的变化. 研究表明，为了很好描述该化合物的自旋重取向相变，必须考虑Ho离子的四阶晶场项及相应的二阶磁晶各向异性常数K_2R，K_2R与K_1R和Mn离子磁晶各向异性常数K_1t之间的相互竞争是导致HoMn₆Sn₆自旋重取向相变的重要因素. 关键词： 稀土-过渡族金属间化合物 自旋重取向 磁晶各向异性     

First-order phase transition in the graphite compound LiC6

Elastic neutron scattering and scanning calorimetry reveal a weakly first-order “melting” of the Li layers in the graphite intercalation compound LiC₆ at T₀ = 715.4 K, in excellent agreement with recent experiment with recent experiments by Robinson and Salamon. The first-order nature of the transition also agrees with a prediction by Bak and Domany derived from Landau expansions, but the detailed behavior of the fluctuation-induced power-law precursor differs from recent Monte Carlo simulations of the 3D?3q Potts model.     

Ab initio SCF investigation of the core and inner valence electron binding and relaxation energies of the CH4, C2 H2 and C2 H6 molecules

Ab initio ΔSCF values of the electron binding and relaxation energies are presented for the four deepest molecular orbitals of methane, acetylene and the two symmetry forms of ethane. Special attention is paid to the inner valence molecular region with regard to the symmetry-constrained Hartree—Fock calculations. The symmetry-constrained effect for the 1t₂ outer valence orbital of methane has also been investigated. The phenomena due to the broken symmetry are discussed on the basis of the configuration-interaction procedure.     

Continuous-wave CO2 laser spectroscopy of SF6, WF6, and UF6

The linear absorption of CO₂ laser radiation in SF₆, WF₆, and UF₆ has been measured by using optoacoustic detection techniques. Absolute absorption coefficients per Torr as low as 1 × 10^?7 cm^?1 Torr^?1 in a 2-cm active path length could be measured by taking advantage of calibration measurements performed with SF₆.     

Pressure-induced valence instability in Sm4Bi3

Sm₄Bi₃ has the anti-Th₃P₄ (cubic) structure with three Sm²⁺ ions and one Sm³⁺ ion. The compound undergoes a first-order, isostructural transition at about 26 kbar. The resistance abruptly decreases by a factor of three and the volume by about 10%. The Sm²⁺ ions undergo a change towards the 3⁺ state in the transition and the material goes from semimetallic to a metallic state. The behavior of the resistance anomaly suggests termination of the phase boundary at a critical point.     

Photoluminescence in the mixed valence compound Eu3S4

We have investigated the 5d-4f interband and the intra-4f photoluminescence of Eu₃S₄. An anomalous shift of the interband luminescence can be explained by temperature dependent valence fluctuations. The ferromagnetic ordering of Eu₃S₄ is also verified.     

Soft mode and structural phase transition in the cubic elpasolite Cs2NaNdCl6

Inelastic neutron scattering measurements have been performed on the elpasolite Cs₂NaNdCl₆ in the cubic phase. It is found that the cubic-to-tetragonal phase transition at 137 K is driven by a zone-center soft phonon mode. The mode is of symmetry Λ^′₁₅ and corresponds to a rotation of the Cl₆ octahedra. The characteristics of the phase transition are similar to those observed in some compounds of the perovskite and antifluorite structures.