Abstract:
On the basis of density functional theory (DFT) calculations thermoelectric properties are derived for Sb-doped skutterudites MPt$_4$Ge$_{12-x}$Sb$_x$ with M=Ba,La,Th. It is predicted that the originally very small absolute values of Seebeck coefficients $|S|$ of the undoped compounds is increased by factors of 10 or more for suitable dopings. The optimal dopings correspond to a "magic" valence electron number for which all electronic states up to a (pseudo)gap are filled. The theoretical findings are corroborated by measurements of $S$ for LaPt$_4$Ge$_{12-x}$Sb$_x$ skutterudites. DFT derived vibrational rattling-like modes for LaPt$_4$Ge$_{12}$ indicate a small value for the lattice thermal conductivity which in combination with a large value of $S^2$ makes the La-based skutterudites appear as promising thermoelectric materials.

Abstract:
The interplay of superconductivity and magnetism is a subject of ongoing interest, stimulated most recently by the discovery of Fe-based superconductivity and the recognition that spin-fluctuations near a magnetic quantum critical point may provide an explanation for the superconductivity and the order parameter. Here we investigate magnetism in the Na filled Fe-based skutterudites using first principles calculations. NaFe4Sb12 is a known ferromagnet near a quantum critical point. We find a ferromagnetic metallic state for this compound driven by a Stoner type instability, consistent with prior work. In accord with prior work, the magnetization is overestimated, as expected for a material near an itinerant ferromagnetic quantum critical point. NaFe4P12 also shows a ferromagnetic instability at the density functional level, but this instability is much weaker than that of NaFe4Sb12, possibly placing it on the paramagnetic side of the quantum critical point. NaFe4As12 shows intermediate behavior. We also present results for skutterudite FeSb3, which is a metastable phase that has been reported in thin film form.

Abstract:
$\rm CePt_3Si$ is a novel ternary compound exhibiting antiferromagnetic order at $T_N \approx 2.2$ K and superconductivity (SC) at $T_c \approx 0.75$ K. Large values of $H_{c2}' \approx -8.5$ T/K and $H_{c2}(0) \approx 5$ T indicate Cooper pairs formed out of heavy quasiparticles. The mass enhancement originates from Kondo interaction with a characteristic temperature $T_K \approx 8$ K. NMR and $\mu$SR measurements evidence coexistence of SC and long range magnetic order on a microscopic scale. Moreover, $\rm CePt_3Si$ is the first heavy fermion SC without an inversion symmetry. This gives rise to a novel type of the NMR relaxation rate $1/T_1$ which is very unique and never reported before for other heavy fermion superconductors. Studies of Si/Ge substitution allow us to establish a phase diagram.

Abstract:
To explore the possibility of quantum phase transitions and even quantum criticality in LaCrSb$_3$ based compounds, we performed measurements under pressure as well as a vanadium substitution study. The Curie temperature of LaCrSb$_3$ was found to be invariant under pressure. Although pressure was not able to suppress the ferromagnetism, chemical substitution was used as another parameter to tune the magnetism. We grew La(V$_x$Cr$_{1-x}$)Sb$_3$ (\textit{x} = 0 -- 1.0) single crystals, and studied the series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for $x \leq 0.22$, and the system manifests a strong anisotropy in its ordered state. The Curie temperature decreases monotonically as the V concentration increases. For $0.42 \leq x \leq 0.73$, the system enters a new magnetic state at low temperatures, and no magnetic ordering above 1.8 K can be observed for $x \geq 0.88$. The effective moment $\mu_{\rm eff}$/Cr varies only slightly as the V concentration increases, from 3.9 $\mu_{\rm B}$ for $x$ = 0 to 2.9 $\mu_{\rm B}$ for $x$ = 0.88. Features related to quantum criticality have not been observed in the La(V$_x$Cr$_{1-x}$)Sb$_3$ system.

Abstract:
Motivated by the presence of competing magnetic interactions in the heavy fermion family Ce$TX_2$ ($T$ = transition metal, $X$ = pnictogen), here we study the novel parent compound CeCd$_{1-\delta}$Sb$_{2}$ by combining magnetization, electrical resistivity, and heat-capacity measurements. Contrary to the antiferromagnetic (AFM) ground state observed in most members of this family, the magnetic properties of our CeCd$_{1-\delta}$Sb$_{2}$ single crystals revealed a ferromagnetic (FM) ordering at $T_{\rm c}$ = 3 K with an unusual soft behavior. By using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian, a systematic analysis of our macroscopic data was obtained. Our fits allowed us to extract a simple but very distinct CEF scheme, as compared to the AFM counterparts. As in the previously studied ferromagnet CeAgSb$_{2}$, a pure $|\pm 1/2 \rangle$ ground state is realized, hinting at a general trend within the ferromagnetic members. We propose a general scenario for the understanding of the magnetism in this family of compounds based on the subtle changes of dimensionality in the crystal structure.

Abstract:
A new ternary intermetallic compound, namely, YbMn2Sb2, has been synthesized and its magnetic and electrical transport properties have been studied in the temperature range of 2 to 300 K. This compound crystallizes in a trigonal, La2O2S type structure (space group P3bm1, No. 164) and is found to be ferromagnetically ordered at room temperature. The magnetism is attributed to the ordering of Mn sublattice. M5 xray absorption spectrum of YbMn2Sb2 obtained at room temperature suggests that the valency of Yb in this compound is close to 2. Electrical resistivity of this compound is metal like and a positive magnetoresistance of 13 percent is observed at 5 K in an applied field of 9T. Key words Rare earth intermetallics and alloys, Magnetic properties, Xray absorption spectroscopy, Electrical transport.

Abstract:
The spin polarization of ferromagnetic alkali-metal iron antimonides KFe4Sb12 and NaFe4Sb12 is studied by point-contact Andreev reflection using superconducting Nb and Pb tips. From these measurements an intrinsic transport spin polarization Pt of 67% and 60% for the K and Na compound, respectively, is inferred which establishes these materials as a new class of highly spin polarized ferromagnets. The results are in accord with band structure calculations within the local spin density approximation (LSDA) that predict nearly 100% spin polarization in the density of states. We discuss the impact of calculated Fermi velocities and spin fluctuations on Pt.

Abstract:
Systematic Raman studies on several cuprates (YBa$_{\rm 2}$Cu$_{\rm 3}$O$_{\rm x}$, YBa$_{\rm 2}$Cu$_{\rm 4}$O$_{\rm 8}$ or Bi$_{\rm 2}$Sr$_{\rm 2}$CaCu$_{\rm 2}$O$_{\rm 8}$) have shown that at optimal doping the compounds are at the edge of lattice instability; once this level is exceeded, by means of doping or applying external hydrostatic pressure, the changes in the transition temperature are accompanied by spectral modifications. There are strong indications that the reduction in T$_{\rm c}$ is correlated with a separation into nanoscale phases, which involve the oxygen atoms of the CuO$_{\rm 2}$ planes. In this work, modifications with doping in the Raman spectra of the La$_{\rm 2-x}$Sr$_{\rm x}$CuO$_{\rm 4}$ compound are presented, which show that spin or charge ordering is coupled with lattice distortions in the whole doping region.

Abstract:
The temperature dependence of the hexagonal lattice parameter $c$ of single crystal $\rm LaCoO_3$ (LCO) with $H=0$ and $800$Oe, as well as LCO bulk powders with $H=0$, was measured using high-resolution x-ray scattering near the transition temperature $T_o\approx 35$K. The change of $c(T)$ is well characterized by a power law in $T-T_o$ for $T>T_o$ and by a temperature independent constant for $T

Abstract:
We study the phase transition from a topological to a normal insulator with concentration $x$ in (Bi$_{1-x}$In$_{x})_2$Se$_3$ and (Bi$_{1-x}$Sb$_{x})_2$Se$_3$ in the Bi$_2$Se$_3$ crystal structure. We carry out first-principles calculations on small supercells, using this information to build Wannierized effective Hamiltonians for a more realistic treatment of disorder. Despite the fact that the spin-orbit coupling (SOC) strength is similar in In and Sb, we find that the critical concentration $x_{\rm c}$ is much smaller in (Bi$_{1-x}$In$_{x})_2$Se$_3$ than in (Bi$_{1-x}$Sb$_{x})_2$Se$_3$. For example, the direct supercell calculations suggest that $x_{\rm c}$ is below 12.5% and above 87.5$%$ for the two alloys respectively. More accurate results are obtained from realistic disordered calculations, where the topological properties of the disordered systems are understood from a statistical point of view. Based on these calculations, $x_c$ is around 17% for (Bi$_{1-x}$In$_{x})_2$Se$_3$, but as high as 78%-83% for (Bi$_{1-x}$Sb$_{x})_2$Se$_3$. In (Bi$_{1-x}$Sb$_{x})_2$Se$_3$, we find that the phase transition is dominated by the decrease of SOC, with a crossover or "critical plateau" observed from around 78$%$ to 83$%$. On the other hand, for (Bi$_{1-x}$In$_{x})_2$Se$_3$, the In 5$s$ orbitals suppress the topological band inversion at low impurity concentration, therefore accelerating the phase transition. In (Bi$_{1-x}$In$_{x})_2$Se$_3$ we also find a tendency of In atoms to segregate.