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The phase diagram of the kagome metal family \ch{AV3Sb5} (A = K, Rb, Cs) features both superconductivity and charge density wave (CDW) instabilities, which have generated tremendous recent attention. Nonetheless, significant questions remain. In particular, the temperature evolution and demise of the CDW state has not been extensively studied, and little is known about the co-existence of the CDW with superconductivity at low temperatures. We report an x-ray scattering study of \ch{CsV3Sb5} over a broad range of temperatures from 300 K to $\sim$ 2 K, below the onset of its superconductivity at $\textit{T}_\text{c}\sim$ 2.9 K. Order parameter measurements of the $2\times2\times2$ CDW structure show an unusual and extended linear temperature dependence onsetting at $T^*$ $\sim$ 160 K, much higher than the susceptibility anomaly associated with CDW order at $\textit{T}_\text{CDW}=94$ K. This implies strong CDW fluctuations exist to $\sim1.7\times\textit{T}_\text{CDW}$. The CDW order parameter is observed to be constant from $T=16$ K to 2 K, implying that the CDW and superconducting order co-exist below $\textit{T}_\text{c}$, and, at ambient pressure, any possible competition between the two order parameters is manifested at temperatures well below $\textit{T}_\text{c}$, if at all. Anomalies in the temperature dependence in the lattice parameters coincide with $\textit{T}_\text{CDW}$ for $\textit{c}(T)$ and with $T^*$ for $a(T)$.

Transition metal dichalcogenides (TMDC) stand out with their high chemical stability and the possibility to incorporate a wide range of magnetic species between the layers. The behavior of conduction electrons in such materials intercalated by 3d-elements is closely related to their magnetic properties and can be sensitively controlled by external magnetic fields. Here, we study the magnetotransport properties of NbS$_2$ intercalated with Mn, Mn$_{1/4}$NbS$_2$, demonstrating a complex behavior of the magnetoresistance and of the ordinary and anomalous Hall resistivities. Application of pressure as tuning parameter leads to the drastic changes of the magnetotransport properties of Mn$_{1/4}$NbS$_2$ exhibiting large negative magnetoresistance up to $65 \%$ at 7.1 GPa. First-principles electronic structure calculations indicates pressure-induced transition from ferromagnetic to antiferromagnetic state. Theoretical calculations accounting for the finite temperature magnetic properties of Mn$_{1/4}$NbS$_2$ suggest a field-induced metamagnetic ferromagnetic-antiferromagnetic transition as an origin of the large negative magentoresistance. These results inspire the development of materials for spintronic applications based on intercalated TMDC with a well controllable metamagnetic transition.

We report the synthesis and characterization of a new quantum magnet [2-[Bis(2-hydroxybenzyl)aminomethyl]pyridine]Ni(II)-trimer (BHAP-Ni3) in single-crystalline form. Our combined experimental and theoretical investigations reveal an exotic spin state that stabilizes a robust 2/3 magnetization plateau between 7 and 20 T in an external magnetic field. AC-susceptibility measurements show the absence of any magnetic order/glassy state down to 60 mK. The magnetic ground state is disordered and specific-heat measurements reveal the gapped nature of the spin excitations. Most interestingly, our theoretical modeling suggests that the 2/3 magnetization plateau emerges due to the interplay between antiferromagnetic Heisenberg and biquadratic exchange interactions within nearly isolated spin $S$ = 1 triangles.

We report a pressure study on electron doped SrFe$_{2-x}$Co$_x$As$_2$ by electrical-resistivity ($\rho$) and magnetic-susceptibility ($\chi$) experiments. Application of either external pressure or Co substitution rapidly suppresses the spin-density wave ordering of the Fe moments and induces superconductivity in SrFe$_2$As$_2$. At $x=0.2$ the broad superconducting (SC) dome in the $T-p$ phase diagram exhibits its maximum $T_{c,{\rm max}}=20$ K at a pressure of only $p_{\rm max}\approx 0.75$ GPa. In SrFe$_{1.5}$Co$_{0.5}$As$_2$ no superconductivity is observed anymore up to 2.8 GPa. Upon increasing the Co concentration the maximum of the SC dome shifts toward lower pressure accompanied by a decrease in the value of $T_{c,{\rm max}}$. Even though, superconductivity is induced by both tuning methods, Co substitution leads to a much more robust SC state. Our study evidences that in SrFe$_{2-x}$Co$_x$As$_2$ both, the effect of pressure and Co-substitution, have to be considered in order to understand the SC phase-diagram and further attests the close relationship of SrFe$_2$As$_2$ and its sister compound BaFe$_2$As$_2$., Comment: 6 pages, 6 figures

The thermoelectric properties of n type semiconductor, TiNiSn is optimized by partial substitution with metallic, MnNiSb in the half Heusler structure. Herein, we study the transport properties and intrinsic phase separation in the system. The Ti1-xMnxNiSn1-xSbx alloys were prepared by arc-melting and were annealed at temperatures obtained from differential thermal analysis and differential scanning calorimetry results. The phases were characterized using powder X-ray diffraction patterns, energy dispersive X-ray spectroscopy, and differential scanning calorimetry. After annealing the majority phase was TiNiSn with some Ni rich sites and the minority phases was majorly Ti6Sn5, Sn, and MnSn2. Ni rich sites were caused by Frenkel defects, this led to a metal-like behavior of the semiconducting specimens at low temperature. For x up to 0.05 the samples showed an activated conduction, whereas for x>0.05 they showed metallic character. The figure of merit for x=0.05 was increased by 61% (ZT=0.45) in comparison to the pure TiNiSn., Comment: 7 pages and 5 figures. Submitted to Physical Chemistry Chemical Physics - RSC Publishing on 6th of October, 2016

The critical behavior of the cubic helimagnet FeGe was obtained from isothermal magnetization data in very close vicinity of the ordering temperature. A thorough and consistent scaling analysis of these data revealed the critical exponents $\beta=0.368$, $\gamma=1.382$, and $\delta=4.787$. The anomaly in the specific heat associated with the magnetic ordering can be well described by the critical exponent $\alpha=-0.133$. The values of these exponents corroborate that the magnetic phase transition in FeGe belongs to the isotropic 3D-Heisenberg universality class. The specific heat data are well described by ab initio phonon calculations and confirm the localized character of the magnetic moments., Comment: 10 pages, 8 figures

The superconducting pairing state of LaPt$_{4}$Ge$_{12}$ is studied by measuring the magnetic penetration depth $\lambda(T,B)$ and the superfluid density $\rho_s(T)$ using a tunnel-diode-oscillator (TDO)-based method and by transverse field muon-spin rotation ($\mu$SR) spectroscopy. $\lambda(T)$ follows an exponential-type temperature dependence at $T\ll T_{c}$, but its zero-temperature value $\lambda(0)$ increases linearly with magnetic field. Detailed analyses demonstrate that both $\lambda(T)$ and the corresponding $\rho_{s}(T)$, measured in the Meissner state by the TDO method are well described by a two-gap $\gamma$ model with gap sizes of $\Delta_1(0)=1.31k_{B}T_c$ and $\Delta_2(0)=1.80k_{B}T_c$ and a very weak interband coupling. In contrast, $\rho_s(T)$, derived from the $\mu \rm{SR}$ data taken in a small field, can be fitted by a single-gap BCS model with a gap close to $\Delta_2(0)$. We conclude that LaPt$_{4}$Ge$_{12}$ is a marginal two-gap superconductor and the small gap $\Delta_1$ seems to be destroyed by a small magnetic field. In comparison, in PrPt$_4$Ge$_{12}$ the $4f$-electrons may enhance the interband coupling and, therefore, give rise to more robust multiband superconductivity., Comment: 5 pages, 4 figures

Topological insulators are known to their metallic surface states, a result of strong-spin-orbital coupling, that show unique surface transport phenomenon. But these surface transports are buried in presence of metallic bulk conduction. We synthesized very high quality Bi$_2$Te$_2$Se single crystals by modified Bridgman method, that possess high bulk resistivity of $>$20~$\Omega$cm below 20~K, whereas the bulk is mostly inactive and surface transport dominates. Temperature dependence resistivity follows the activation law like a gap semiconductor in temperature range 20-300~K. We designed a special measurement geometry, which aims to extract the surface transport from the bulk. This special geometry is applied to measure the resistance and found that Bi$_2$Te$_2$Se single crystal exhibits a cross over from bulk to surface conduction at 20~K. Simultaneously, the material also shows strong evidence of weak anti-localization in magneto-transport due to the protection against scattering by conducting surface states. This novel simple geometry is an easy route to find the evidence of surface transport in topological insulators, which are the promising materials for future spintronic applications., Comment: 6 Pages, 4 Figures

Noncollinear magnets provide essential ingredients for the next generation memory technology. It is a new prospect for the Heusler materials, already well known due to the diverse range of other fundamental characteristics. Here, we present a combined experimental and theoretical study of novel noncollinear tetragonal Mn2RhSn Heusler material exhibiting unusually strong canting of its magnetic sublattices. It undergoes a spin-reorientation transition, induced by a temperature change and suppressed by an external magnetic field. Because of the presence of Dzyaloshinskii-Moriya exchange and magnetic anisotropy, Mn2RhSn is suggested to be a promising candidate for realizing the Skyrmion state in the Heusler family.

The spin-1/2 alternating Heisenberg chain system Na$_3$Cu$_2$SbO$_6$ features two relevant exchange couplings: $J_{1a}$ within the structural Cu$_2$O$_6$ dimers and $J_{1b}$ between the dimers. Motivated by the controversially discussed nature of $J_{1a}$, we perform extensive density-functional-theory (DFT) calculations, including DFT+$U$ and hybrid functionals. Fits to the experimental magnetic susceptibility using high-temperature series expansions and quantum Monte Carlo simulations yield the optimal parameters $J_{1a}$ = $-$217 K and $J_{1b}$ = 174 K with the alternation ratio $\alpha = J_{1a}/J_{1b} \simeq$ $-$1.25. For the closely related system Na$_2$Cu$_2$TeO$_6$, DFT yields substantially enhanced $J_{1b}$, but weaker $J_{1a}$. The comparative analysis renders the buckling of the chains as the key parameter altering the magnetic coupling regime. Numerical simulation of the dispersion relations of the alternating chain model clarify why both antiferromagnetic and ferrromagnetic $J_{1a}$ can reproduce the experimental magnetic susceptibility data., Comment: published version: 11 pages, 8 figures, 5 tables + Supplemental material

Detailed $^{195}$Pt- and $^{171}$Yb nuclear magnetic resonance (NMR) studies on the heterogeneous mixed valence system YbPtGe$_2$ are reported. The temperature dependence of the $^{195}$Pt-NMR shift $^{195}K(T)$ indicates the opening of an unusual magnetic gap below 200\,K. $^{195}K(T)$ was analyzed by a thermal activation model which yields an isotropic gap $\Delta/k_B \approx 200$\,K. In contrast, the spin-lattice relaxation rate $^{195}$($1/T_1$) does not provide evidence for the gap. Therefore, an intermediate-valence picture is proposed while a Kondo-insulator scenario can be excluded. Moreover, $^{195}$($1/T_1$) follows a simple metallic behavior, similar to the reference compound YPtGe$_2$. A well resolved NMR line with small shift is assigned to divalent $^{171}$Yb. This finding supports the proposed model with two sub-sets of Yb species (di- and trivalent) located on the Yb2 and Yb1 site of the YbPtGe$_2$ lattice., Comment: Submitted in Physical Review B (Rapid Communication)

Antiferromagnetic semiconductors are new alternative materials for spintronic applications and spin valves. In this work, we report a detailed investigation of two antiferromagnetic semiconductors AMnAs (A = Li, LaO), which are isostructural to the well-known LiFeAs and LaOFeAs superconductors. Here we present a comparison between the structural, magnetic, and electronic properties of LiMnAs, LaOMnAs and related materials. Interestingly, both LiMnAs and LaOMnAs show a variation in resistivity with more than five orders of magnitude, making them particularly suitable for use in future electronic devices. From neutron and X-ray diffraction measurements on LiMnAs we have observed a magnetic phase transition corresponding to the Neel temperature of 373.8 K, and a structural transition from the tetragonal to the cubic phase at 768 K. These experimental results are supported by density functional theory (DFT) calculations.

We report measurements of the London penetration depth $\Delta\lambda(T)$ and the electronic specific heat $C_e(T)$ on high-quality single crystals of the filled-skutterudite superconductor PrPt$_4$Ge$_{12}$ ($T_c\simeq$8K). Both quantities show a weak temperature dependence at $T\ll T_c$, following $\Delta\lambda\sim T^n$ ($n\simeq 3.2$) and $C_e/T\sim T^{2.8}$. Such temperature dependences deviate from both conventional s-wave type and nodal superconductivity. A detailed analysis indicates that the superfluid density $\rho_s(T)$, derived from the penetration depth, as well as the electronic specific heat can be consistently described in terms of a two-gap model, providing strong evidence of multiband superconductivity for PrPt$_4$Ge$_{12}$., Comment: 5 pages, 4 figures

We report on a study of the superconducting properties for a series of polycrystalline BaPt(4-x)Au(x)Ge12 filled skutterudite compounds for x = 0, 0.5, 0.75, and 1. Muon spin rotation (muSR) spectroscopy as well as magnetization, specific heat, and electrical resistivity measurements were performed. The magnetic penetration depth \lambda, the coherence length \xi, and the Ginzburg-Landau parameter \kappa are evaluated. The temperature dependence of the superfluid density is well described by an s-wave superconducting gap and this classical scenario is supported by the field-independent \lambda. The gap-to-Tc ratio \Delta/kTc increases with the Au content from 1.70 for x = 0 to 2.1(1) for x = 1. By combining muSR, magnetization, and specific heat data, we find that BaPt(4-x)Au(x)Ge12 compounds are in between the dirty and clean limits with mean free paths of the carriers l ~ \xi. Interestingly, resistivity data for BaPt4Ge12 indicate a much higher upper critical field, which is probably due to defects or impurities close to the surface of the crystallites., Comment: 11 pages, 10 figures

The filled-skutterudite compound CePt4Ge12 is situated close to the border between intermediate-valence of Ce and heavy-fermion behavior. Substitution of Ge by Sb drives the system into a strongly correlated and ultimately upon further increasing the Sb concentration into an antiferromagnetically ordered state. Our experiments evidence a delicate interplay of emerging Kondo physics and the formation of a local 4f moment. An extended non-Fermi-liquid region, which can be understood in the framework of a Kondo-disorder model, is observed. Band-structure calculations support the conclusion that the physical properties are governed by the interplay of electron supply via Sb substitution and the concomitant volume effects., Comment: 5 pages, 3 Figure

A detailed study of the spin dynamics of the S-state ions Gd3+ and Eu2+ in the filled skutterudites La1-xRxPt4Ge12 (R= Gd, Eu) is reported. The spin dynamics is investigated directly by means of Gd3+ and Eu2+ electron spin resonance (ESR), performed at X-band (~ 9.4 GHz) and Q-band (~ 34 GHz) frequencies in the temperature intervals 8

In a joint theoretical and experimental study we investigate the pressure dependence of the Eu valence in EuPd_3B_x (0 <= x <= 1). Density functional band structure calculations are combined with x-ray absorption and x-ray diffraction measurements under hydrostatic pressures up to 30 GPa. It is observed that the heterogenous mixed-valence state of Eu in EuPd_3B_x (x >= 0.2) can be suppressed partially in this pressure range. From the complementary measurements we conclude that the valence change in EuPd_3B_x is mainly driven by the number of additional valence electrons due to the insertion of boron, whereas the volume change is a secondary effect. A similar valence change of Eu in Eu_{1-x}La_xPd_3 is predicted for x >= 0.4, in line with the suggested electron count scenario.

The layered compound EuFe2As2 is an interesting model system to investigate the effects of well defined local Eu2+ 4f states on the itinerant electronic and magnetic properties of the FeAs layers. To address this subject, we investigated the series EuFe2-xCoxAs2 (0.1 <= x <=0.75) by electron spin resonance (ESR) of Eu2+ to probe the spin dynamics of the itinerant subsystem. We relate the results to dc-susceptibility measurements and band structure calculations. As a consequence of the weak coupling between the local and itinerant subsystems we found that the spin relaxation is well understood in terms of the exchange coupling among the local Eu2+ spins. A pronounced field dependence of the Eu2+ spin relaxation demonstrates the direct influence of magnetic fluctuations at the Fe2-xCoxAs2 layers., Comment: 13 pages, 5 figures

We present the first study of codoped iron-arsenide superconductors of the 122 family (Sr/Ba)_(1-x)K_xFe_(2-y)Co_yAs_2 with the purpose to increase the upper critical field H_c2 compared to single doped (Sr/Ba)Fe_2As_2 materials. H_c2 was investigated by measuring the magnetoresistance in high pulsed magnetic fields up to 64 T. We find, that H_c2 extrapolated to T = 0 is indeed enhanced significantly to ~ 90 T for polycrystalline samples of Ba_0.55K_0.45Fe_1.95Co_0.05As_2 compared to ~75 T for Ba_0.55K_0.45Fe_2As_2 and BaFe_1.8Co_0.2As_2 single crystals. Codoping thus is a promising way for the systematic optimization of iron-arsenic based superconductors for magnetic-field and high-current applications., Comment: 7 pages, 5 figures, submitted to Journal of Applied Physics

Zero and longitudinal field muon spin rotation (muSR) experiments were performed on the superconductors PrPt4Ge12 and LaPt4Ge12. In PrPt4Ge12 below Tc a spontaneous magnetization with a temperature variation resembling that of the superfluid density appears. This observation implies time-reversal symmetry (TRS) breaking in PrPt4Ge12 below Tc = 7.9 K. This remarkably high Tc for an anomalous superconductor and the weak and gradual change of Tc and of the related specific heat anomaly upon La substitution in La_(1-x)Pr_xPt_4Ge_(12) suggests that the TRS breaking is due to orbital degrees of freedom of the Cooper pairs., Comment: To appear in Phys. Rev. B. 5 pages, 3 figures

Rare-earth metal filled skutterudites RPt4Ge12 with R=La-Nd, and Eu exhibit a variety of different ground states, e.g., conventional and unconventional superconductivity in LaPt4Ge12 and PrPt4Ge12, respectively, and intermediate valence behavior in CePt4Ge12. In this work we investigate the magnetic state of NdPt4Ge12 and EuPt4Ge12 by specific heat, dc-susceptibility and magnetization. NdPt4Ge12 shows two magnetic phase transitions at T_N1=0.67 K and T_N2=0.58 K, while EuPt4Ge12 displays a complex magnetic phase diagram below the magnetic ordering temperature of 1.78 K. The specific heat indicates that in NdPt4Ge12 the crystalline electric field (CEF) ground state of the Nd3+ is a quartet and that, as expected, in EuPt4Ge12 the Eu2+ state is fully degenerate., Comment: accepted for publication in the proceedings of SCES 2010

In contrast to SrFe2As2, where only the iron possesses a magnetic moment, in EuFe2As2 an additional large, local magnetic moment is carried by Eu2+. Like SrFe2As2, EuFe2As2 exhibits a spin-density wave transition at high temperatures, but in addition the magnetic moments of the Eu2+ order at around 20 K. The interplay of pressure-induced superconductivity and the Eu2+ order leads to a behavior which is reminiscent of re-entrant superconductivity as it was observed, for example, in the ternary Chevrel phases or in the rare-earth nickel borocarbides. Here, we study the delicate interplay of the ordering of the Eu2+ moments and superconductivity in EuFe1.9Co0.1As2, where application of external pressure makes it possible to sensitively tune the ratio of the magnetic (T_C) and the superconducting (T_{c,onset}) critical temperatures. We find that superconductivity disappears once T_C > T_{c,onset}., Comment: 4 pages, 4 figures, submitted to the proceedings of SCES2010

A microscopic magnetic model for the spin-1/2 Heisenberg chain compound CuSe2O5 is developed based on the results of a joint experimental and theoretical study. Magnetic susceptibility and specific heat data give evidence for quasi-1D magnetism with leading antiferromagnetic (AFM) couplings and an AFM ordering temperature of 17 K. For microscopic insight, full-potential DFT calculations within the local density approximation (LDA) were performed. Using the resulting band structure, a consistent set of transfer integrals for an effective one-band tight-binding model was obtained. Electronic correlations were treated on a mean-field level starting from LDA (LSDA+U method) and on a model level (Hubbard model). In excellent agreement of experiment and theory, we find that only two couplings in CuSe2O5 are relevant: the nearest-neighbour intra-chain interaction of 165 K and a non-frustrated inter-chain coupling of 20 K. From a comparison with structurally related systems (Sr2Cu(PO4)2, Bi2CuO4), general implications for a magnetic ordering in presence of inter-chain frustration are made., Comment: 20 pages, 8 figures, 3 tables

We present a joint experimental and computational study of the recently synthesized spin 1/2 system silver-copper-orthovanadate AgCuVO4 [A. M\"oller and J. Jainski, Z. Anorg. Allg. Chem. 634, 1669 (2008)] exhibiting chains of trans corner-sharing [CuO4] plaquettes. The static magnetic susceptibility and specific heat measurements of AgCuVO4 can be described to a good approximation by the Bonner-Fisher spin-chain model with J_intra ~ 330 K. Evidence for a Neel type of order at ~ 2.5 K is obtained from the specific heat and corroborated by ESR studies. To independently obtain a microscopically based magnetic model, density functional electronic structure calculations were performed. In good agreement with the experimental data, we find pronounced one-dimensional magnetic exchange along the corner-sharing chains with small inter-chain couplings. The difference between the experimentally observed and the calculated ordering temperature can be assigned to a sizable inter-chain frustration derived from the calculations., Comment: 8 pages, 10 figures

We present a combined study of the electronic structure of the superconducting skutterudite derivative SrPt4Ge12 by means of X-ray photoelectron spectroscopy and full potential band structure calculations including an analysis of the chemical bonding. We establish that the states at the Fermi level originate predominantly from the Ge 4p electrons and that the Pt 5d shell is effectively full. We find excellent agreement between the measured and the calculated valence band spectra, thereby validating that band structure calculations in combination with photoelectron spectroscopy can provide a solid basis for the modeling of superconductivity in the compounds MPt4Ge12 (M = Sr, Ba, La, Pr) series.

The dehydration of CuCl2*2(H2O) to CuCl2 leads to a dramatic change in magnetic behavior and ground state. Combining density functional electronic structure and model calculations with thermodynamical measurements we reveal the microscopic origin of this unexpected incident -- a crystal water driven switching of the magnetically active orbitals. This switching results in a fundamental change of the coupling regime from a three-dimensional antiferromagnet to a quasi one-dimensional behavior. CuCl2 can be well described as a frustrated J1-J2 Heisenberg chain with ferromagnetic exchange J1 and J2/J1 ~ -1.5 for which a helical ground state is predicted., Comment: 6 pages, 5 figures, 1 table (PRB, accepted)

The magnetism in SrFe2As2 can be suppressed by electron doping through a small substitution of Fe by Co or Ni, giving way to superconductivity. We demonstrate that a massive substitution of Fe by isovalent ruthenium similarly suppresses the magnetic ordering in SrFe2-xRuxAs2 and leads to bulk superconductivity for 0.6 <= x <= 0.8. Magnetization, electrical resistivity, and specific heat data show Tc up to approx 20K. Detailed structural investigations reveal a strong decrease of the lattice parameter ratio c/a with increasing x. DFT band structure calculations are in line with the observation that the magnetic order in SrFe2-xRuxAs2 is only destabilized for large x., Comment: 6 pages, 5 figures, extended and revised version

Using muon-spin rotation, we studied the in-plane (\lambda_ab) and the out of plane (\lambda_c) magnetic field penetration depth in SrFe_1.75Co_0.25As_2 (T_c=13.3 K). Both \lambda_ab(T) and \lambda_c(T) are consistent with the presence of two superconducting gaps with the gap to T_c ratios 2\Delta/k_BT_c=7.2 and 2.7. The penetration depth anisotropy \gamma_\lambda=\lambda_c/\lambda_ab increases from \gamma_\lambda=2.1 at T_c to 2.7 at 1.6 K. The mean internal field in the superconducting state increases with decreasing temperature, just opposite to the diamagnetic response seen in magnetization experiments. This unusual behavior suggests that the external field induces a magnetic order which is maintained throughout the whole sample volume., Comment: 4 pages, 3 figures

The filled skutterudite superconductor PrPt4Ge12 was studied in muon-spin rotation (muSR), specific heat, and electrical resistivity experiments. The continuous increase of the superfluid density with decreasing temperature and the dependence of the magnetic penetration depth \lambda on the magnetic field obtained by means of muSR, as well as the observation of a T^3 dependence of the electronic specific heat establish the presence of point-like nodes in the superconducting energy gap. The energy gap was found to be well described by \Delta =\Delta_0| k_x - ik_y| or \Delta = \Delta_0(1 - k_y^4) functional forms, similar to that obtained for another skutterudite superconductor, PrOs4Sb12. The gap to T_c ratios were estimated to be \Delta_0/k_BT_c = 2.68(5) and 2.29(5), respectively., Comment: 4 pages, 5 figures

In the electron doped compounds SrFe_(2-x)Co_xAs_2 superconductivity with T_c up to 20 K is observed for 0.2 < x < 0.4. Results of structure determination, magnetic susceptibility, electrical resistivity, and specific heat are reported. The observation of bulk superconductivity in all thermodynamic properties -- despite strong disorder in the Fe-As layer -- favors an itinerant picture in contrast to the cuprates and renders a p- or d-wave scenario unlikely. DFT calculations find that the substitution of Fe by Co (x > 0.3) leads to the suppression of the magnetic ordering present in SrFe_2As_2 due to a rigid down-shift of the Fe-3d_(x^2-y^2) related band edge in the density of states., Comment: 5 pages, 3 figures

New germanium-platinum compounds with the filled-skutterudite crystal structure were synthesized. The structure and composition were investigated by X-ray diffraction and microprobe analysis. Magnetic susceptibility, specific heat, and electrical resistivity measurements evidence superconductivity in LaPt4Ge12 and PrPt4Ge12 below 8.3K. The parameters of the normal and superconducting states were established. Strong coupling and a crystal electric field singlet groundstate is found for the Pr compound. Electronic structure calculations show a large density of states at the Fermi level. Similar behavior with lower T_c was observed for SrPt4Ge12 and BaPt4Ge12., Comment: RevTeX, 4 figures, submitted to Physical Review Letters July 12, 2007

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., Comment: Pdf file with fig