The elastic constants c11, c12 and c44 of U, Np, Pu, and Am telluride single crystals have been obtained by Brillouin scattering from the measured sound velocities in prominent crystallographic directions. In case of the U chalcogenides also by ultrasound techniques. The Cauchy pressure, the Poisson ratio, the anisotropy ratio, the bulk modulus and the Debye temperature have been derived from these data. U and Pu telluride have a negative c12, implying intermediate valence. AmTe has a very low bulk modulus, but a positive c12. It is extremely soft, with a very low sound velocity. For AmTe also the LO and TO phonon frequencies could be determined. The elastic data point to a divalent Am state. The optical reflectivity of the light actinide tellurides (and sometimes of all chalcogenides) has been measured between UV and infrared wavelengths, in the case of AmTe down to the far infrared. The plasma edge of the free carriers has been determined, which yields the ratio of n/m*. Together with the γ value of the specific heat and magnetic data a consistent proposal for the electronic structure of the light actinide chalcogenides can be given. Thus, the Pu chalcogenides are intermediate valent and represent the high-pressure phase of the corresponding Sm chalcogenides. AmTe, as judged by the electronic, optic and magnetic properties seems to be in the 5f7 configuration, i.e. divalent Am, but with a narrow, half-filled (24 meV) wide 5f band, about 0.1 eV below the bottom of the 6d conduction band. We thus propose a new kind of unhybridized heavy fermion. Also AmTe seems to represent a high-pressure phase of EuTe.

Electronic and elastic properties of the light actinide tellurides

Publisher Summary This chapter describes semiconductor clathrates. The word “clathrate” derives from the Latin “clathratus” meaning “furnished with a lattice.” It is currently used in chemistry to describe a particular type of compound, usually a polyatomic compound, in which one component forms a cage structure imprisoning the other. The crystalline complexes of water, H 2 O, with simple molecules such as chlorine, Cl 2 , have been known to form clathrate compounds for more than a century. The chapter discusses the two common forms of ice clathrates, which are formed when the water is cooled and agitated in the presence of a sufficient concentration of the guest atoms. Type-I silicon (Si) and germanium (Ge) clathrates are metallic, whereas type-II clathrates maintain the semiconducting properties of Si and Ge for low alkaline metal concentrations. The semiconductivity diminishes as the metal concentration increases. A clathrate material is one in which the voids in the host lattice are present in the absence of a guest atom or molecule. These are called “true clathrates.” One good example in the field of thermoelectrics are the semiconducting skutterudites based on CoAs 3 . The chapter outlines several methods employed in forming Si, Ge, and tin (Sn) clathrates and in mixed-crystal clathrates containing two or more of these elements.

Chapter 6 Semiconductor clathrates: A phonon glass electron crystal material with potential for thermoelectric applications

Hybridization between conduction electrons and the strongly interacting f-electrons in rare earth or actinide compounds may result in new states of matter. Depending on the exact location of the concomitant hybridization gap with respect to the Fermi energy, a heavy fermion or an insulating ground state ensues. To study this entanglement locally, we conducted scanning tunneling microscopy and spectroscopy (STS) measurements on the “Kondo insulator” SmB6. The vast majority of surface areas investigated were reconstructed, but infrequently, patches of varying sizes of nonreconstructed Sm- or B-terminated surfaces also were found. On the smallest patches, clear indications for the hybridization gap with logarithmic temperature dependence (as expected for a Kondo system) and for intermultiplet transitions were observed. On nonreconstructed surface areas large enough for coherent cotunneling, we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure, not proving but leaving open the possibility of the existence of a topologically protected surface state.

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Hybridization gap and Fano resonance in SmB6

Experimental study of the Kondo insulator SmB6 provides an alternative route to realize a Fermi surface in the absence of a conventional Fermi liquid.

/pdf/fermi-surface-in-the-absence-of-a-fermi-liquid-in-the-kondo-4g1mqr6ckf.pdf

Fermi surface in the absence of a Fermi liquid in the Kondo insulator SmB 6

Current theories of superfluidity are based on the idea of a coherent quantum state with topologically protected quantized circulation. When this topological protection is absent, as in the case of ^{3}He-A, the coherent quantum state no longer supports persistent superflow. Here, we argue that the loss of topological protection in a superconductor gives rise to an insulating ground state. We specifically introduce the concept of a Skyrme insulator to describe the coherent dielectric state that results from the topological failure of superflow carried by a complex-vector order parameter. We apply this idea to the case of SmB_{6}, arguing that the observation of a diamagnetic Fermi surface within an insulating bulk can be understood as a realization of this state. Our theory enables us to understand the linear specific heat of SmB_{6} in terms of a neutral Majorana Fermi sea and leads us to predict that in low fields of order a Gauss, SmB_{6} will develop a Meissner effect.

https://link.aps.org/accepted/10.1103/PhysRevLett.119.057603

Skyrme Insulators: Insulators at the Brink of Superconductivity

The paramagnetic response of mixed-valence samarium hexaboride has been studied by inelastic neutron scattering using a low-absorption double-isotope single crystal of 154Sm11 B6. Measurements were performed for energy transfers 0<or=h(cross) omega <or=50 meV at temperatures ranging from 2 to 100 K. Two main contributions were observed in the spectra: a broad intermultiplet transition of single-ion type at h(cross) omega approximately=36 meV, and a narrow low-energy excitation centred at h(cross) omega approximately=14 meV, which is strongly anisotropic and temperature dependent and exhibits a weak but visible dispersion. The latter excitation appears to be an inherent feature of the mixed-valence state due to f-electron hybridization effects, which denotes the formation of a local bound state around the Sm sites.

https://iopscience.iop.org/article/10.1088/0953-8984/7/2/007/pdf

Magnetic excitation spectrum of mixed-valence SmB6 studied by neutron scattering on a single crystal

All acoustic and the lowest optic branches in three main symmetry directions have been determined by inelastic neutron scattering at room temperature using a double isotope monocrystal 154Sm11B6. In comparison with the integer valence system LaB6 the phonons are softer, and the LA branches exhibit remarkable anomalies. These features are interpreted on the basis of a specific coupling of the lattice vibrations with the dipole (f-d) and monopole (f-f) excitations of the Sm shells, the latter being characteristic for the mixed valence semiconductor. A weak broad signal has been observed in the energy gap between acoustic and optic modes, which can be ascribed to an additional mode appearing due to the nonadiabatic interaction of phonons with valence fluctuations.

http://iopscience.iop.org/article/10.1209/0295-5075/10/5/012/pdf

Lattice dynamics of intermediate valence semiconductor SmB6

Inelastic-neutron-scattering spectra were measured on the polycrystalline double isotopic sample 154Sm 11B6 in a wide range of the energy transfer. The intermultiplet transitions corresponding to f6 and f5 configurations of the intermediate-valent state of the Sm ion, as well as a low-energy excitation with unusual momentum transfer and temperature dependences have been observed in the magnetic part of the scattering function.

https://iopscience.iop.org/article/10.1209/0295-5075/23/5/008/pdf

E.S. Konovalova

Papers

Magnetic excitation spectrum of mixed-valence SmB6 studied by neutron scattering on a single crystal

Lattice dynamics of intermediate valence semiconductor SmB6

Neutron Scattering Study of the Intermediate-Valent Ground State in SmB6

Properties of the in-gap states in SmB6

High pressure studies of cerium hexaboride