We present the anisotropic optical conductivity of MgB2 between 0.1 and 3.7 eV at room temperature obtained on single crystals of different purity by the spectroscopic ellipsometry and reflectance measurements. The bare (unscreened) plasma frequency ωp is almost isotropic and equal to 6.3 eV, which contrasts some earlier reports of a very small value of ωp. The data suggests that the σ-bands are characterized by a stronger electron-phonon coupling λtr but smaller impurity scattering γimp, compared to the π-bands. The optical response along the boron planes is marked by an intense interband transition at 2.6 eV, due to which the reflectivity plasma edges along the a-and c-axes are shifted with respect to each other. As a result, the sample spectacularly changes color from a blueish-silver to the yellow as the polarization is rotated from the in-plane direction towards the c-axis. The optical spectra are in good agreement with the published ab initio calculations. The remaining discrepancies can be explained by the relative shift of σ-bands and π-bands by about 0.2 eV compared to the theoretical band structure, in agreement with the de Haas-van Alphen experiments. The widths of the Drude and the interband peaks are both very sensitive to the sample purity.

We present specific-heat data for Nb 3 Sn, a well-known technically applied superconductor with a critical temperature T c Х 18 K, in the temperature range from 1.2 to 200 K in zero magnetic field, and from 1.5 to 22 K in fields H ഛ 16 T. The particularly dense and homogeneous polycrystalline sample used for this study is characterized in detail. We determine the bulk upper critical field H c2 ͑T͒ from specific-heat data, and the Sommerfeld constant ␥ from the entropy S͑T͒. We investigate in detail a low-temperature anomaly already noticed in previous investigations in zero field, and find that this feature can be quantitatively ascribed to the presence of a second superconducting gap 2⌬ S ͑0͒Х0.8k B T c , in addition to the main one 2⌬ L ͑0͒Х4.9k B T c . The signature of this minor gap, which affects 7.5% of the electronic density-of-states, vanishes in high fields.

Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu4Sn6 employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the a − a plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their origin.Correlated materials with gapped or pseudo-gapped ground states continue to be of great interest. The gap in the electronic density of states (DOS) either opens gradually with decreasing temperature, as the pseudogap of high-temperature superconductors [1], or emerges at a continuous or first order phase transition [2][3][4]. In heavy fermion compounds [5] -systems in which f and conduction electrons strongly interact -a narrow hybridization gap is known to emerge gradually [6][7][8][9]. Generically, the Fermi energy is situated in one of the hybridized bands and a metallic heavy fermion ground state arises. Only for special cases the Fermi energy lies within the gap and the ground state is Kondo insulating. Metallic heavy fermion systems have been intensively investigated over the past decades and are now, at least away from quantum criticality [10], well understood [11] within the framework of Landau Fermi liquid theory. Hence, a very few parameters, most notably the effective mass, allow us to describe thermodynamic and transport properties at the lowest temperatures. In comparison, the physics of Kondo insulators has proven to be much less tractable. This is at least in part due to the fact that the gapped ground state inhibits a characterization via the above properties. Many experimental efforts have therefore focussed on the determination of the gap width from temperature dependencies, which has frequently led to conflicting results, in particular for anisotropic Kondo insulators such as CeNiSn [12]. Here the strongly anisotropic transport and magnetic properties have been interpreted phenomenologically on the basis of a V-shaped DOS [13] or by invoking a hybridization gap with nodes [14][15][16] or extrinsic effects such as impurities, off stoichiometry or strain [17,18]. To advance the field it appears mandatory to model a number of carefully chosen materials ab initio, taking all essential ingredients into account.Here we investigate a new material, CeRu 4 Sn 6 , which due to its tetragonal crystal structure is simpler than the previously studied orthorhombic materials. In a combined experimental and theoretical effort we provide direct spectroscopic evidence for the development of an anisotropic pseudogap: While weak metallicity prevails in the optical conductivity along the c axis, insulatorlike behavior without a Drude peak is observed in the a − a plane. We trace this back to a correlated band structure whic...

We have investigated the optical conductivity of the prominent valence-fluctuating compounds EuIr(2)Si(2) and EuNi(2)P(2) in the infrared energy range to get new insights into the electronic properties of valence-fluctuating systems. For both compounds, we observe upon cooling the formation of a renormalized Drude response, a partial suppression of the optical conductivity below 100 meV, and the appearance of a midinfrared peak at 0.15 eV for EuIr(2)Si(2) and 0.13 eV for EuNi(2)P(2). Most remarkably, our results show a strong similarity with the optical spectra reported for many Ce- or Yb-based heavy-fermion metals and intermediate valence systems, although the phase diagrams and the temperature dependence of the valence differ strongly between Eu systems and Ce- or Yb-based systems. This suggests that the hybridization between 4f and conduction electrons, which is responsible for the properties of Ce and Yb systems, plays an important role in valence-fluctuating Eu systems.

We report the discovery of a new spin glass ground state in the transition metal monosilicides with the B20 crystallographic structure. Magnetic, transport, neutron and muon investigation of the solid solution Mn1−xCoxSi have revealed a new dome in the phase diagram with evidence of antiferromagnetic interactions. For Mn rich compounds, a sharp decrease of the Curie temperature is observed upon Co doping and neutron elastic scattering shows that helimagnetic order of MnSi persists up to x = 0.05 with a shortening of the helix period. For higher Co (0.05 < x < 0.90) concentrations, the Curie-Weiss temperature changes sign and the system enters a spin glass state upon cooling (Tg = 9 K for xCo = 0.50), due to chemical disorder. In this doping range, a minimum appears in the resistivity, attributed to scattering of conduction electron by localized magnetic moments.

The optical spectrum of the cubic helimagnetic metal FeGe has been investigated in the frequency range from 0.01 to 3.1 eV for different temperatures from 30 to 296 K. The optical conductivity shows the evolution of a low-energy ͑0.22 eV͒ interband transition and the development of a narrow free-carrier response with a strong energy and temperature dependence. The frequency-dependent effective mass and scattering rate derived from the optical data indicate the formation of dressed quasiparticles with a mass renormalization factor of 5. Similar to FeSi the spectral weight in FeGe is not recovered over a broad frequency range, an effect usually attributed to the influence of the on-site Coulomb interaction. DOI: 10.1103/PhysRevB.75.155114 PACS number͑s͒: 78.20.Ϫe, 71.27.ϩa, 75.10.Lp, 75.50.Bb Cubic FeGe is a good metal at low temperature, which undergoes a transition to helimagnetic order 1 at T C = 280 K with the magnetic moment at the iron sites of 1 B . The helix changes its orientation in a temperature interval T 2 ±20 K and shows pronounced temperature hysteresis 2 between 211 and 245 K. This material crystallizes in the B20 structure and the cubic space group P2 1 3 lacking a center of symmetry which is responsible for this long-range order. The isoelectronic compound FeSi has the same crystal structure. It has a large magnetic susceptibility at room temperature, which vanishes as the temperature approaches zero due to a small ͑70 meV͒ semiconductor gap at E F . A continuous series FeSi 1−x Ge x can be formed, where the metal insulator transition 3 occurs for x Ϸ 0.25. Theoretical models, which have been proposed to explain this behavior, invoke disorder, 4 narrow bands, and different ways of incorporating electron correlations.5-8 The temperature-dependent disappearance of the gap has been explained as a result of a correlation gap using a two-band Hubbard model, 9,10 and excellent agreement was obtained with optical data, 10-12 but it has been shown that vibrational disorder, if sufficiently strong, also closes the gap. 13 Anisimov et al. 14 have predicted a magnetic-field-driven semiconductor to metal transition in FeSi 1−x Ge x , and argued that the difference in electronic structure between FeSi and FeGe in essence consists of a rigid relative shift of the majority and minority-spin bands for the latter material. According to this model the optical spectra at low energies are expected to be the superposition of a Drude peak and an interband transition across an energy range corresponding to the forementioned relative shift of the majority and minority bands. Experimentally relatively little is known about the electronic structure of FeGe, for example, no optical data have been published.Here, we report optical measurements on a cubic FeGe single crystal at different temperatures. The real and imaginary parts of the dielectric function were derived from the reflectivity and ellipsometry measurements. Optical spectra of FeGe reveal the presence of an important interband transition at 0.22 eV and unusu...

We performed a study of the structural and physical properties of YbPtGe 2 . This compound is a multivalent charge-ordered system presenting an unusual spin pseudogap below 200 K. The crystal structure of YbPtGe 2 is refined from single-crystal and powder high-resolution synchrotron x-ray diffraction data at different temperatures. Analysis of the structural features of YbPtGe 2 , together with a combined study of Yb L III x-ray absorption spectroscopy, magnetic susceptibility χ (T ), thermopower S(T ), and 171 Yb and 195 Pt NMR indicate half of the Yb atoms to be in an intermediate valence state with an electronic configuration close to 4f 13 (Yb 3+ ), while for the remaining Yb atoms the 4f 14 (Yb 2+ ) configuration with almost no valence fluctuations is most likely. A drastic drop of the magnetic susceptibility and a decrease of the isotropic shift 195 K iso (T ) with decreasing temperature in the temperature range of 50-200 K evidence the opening of a spin pseudogap with an activation energy of /k B ∼ 200 K. Surprisingly, transport properties do not show clear evidence for the opening of a charge gap, thus excluding a standard Kondo-insulator scenario. Possible origins for this unusual electronic (valence) behavior are discussed.

We have measured the resistivity, optical conductivity, and magnetic susceptibility of LaSb2 to search for clues as to the cause of the extraordinarily large linear magnetoresistance and to explore the properties of the superconducting state. We find no evidence in the optical conductivity for the formation of a charge density wave state above 20 K despite the highly layered crystal structure. In addition, only small changes to the optical reflectivity with magnetic field are observed indicating that the MR is due to scattering rate, not charge density, variations with field. Although a superconducting ground state was previously reported below a critical temperature of 0.4 K, we observe, at ambient pressure, a fragile superconducting transition with an onset at 2.5 K. In crystalline samples, we find a high degree of variability with a minority of samples displaying a full Meissner fraction below 0.2 K and fluctuations apparent up to 2.5 K. The application of pressure stabilizes the superconducting transition and reduces the anisotropy of the superconducting phase.