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The B20-type cobalt germanide CoGe was investigated by measuring the specific heat, resistivity, and $^{59}$Co nuclear magnetic resonance (NMR). We observed a phase transition at $T_Q=13.7$ K, evidenced by a very narrow peak of the specific heat and sharp changes of the nuclear spin-spin ($T_2^{-1}$) and spin-lattice ($T_1^{-1}$) relaxation rates. The fact that the entropy release is extremely small and the Knight shift is almost independent of temperature down to low temperatures as anticipated in a paramagnetic metal indicates that the $T_Q$ transition is of non-magnetic origin. In addition, we detected a crossover scale $T_0\sim30$ K below which the resistivity and the NMR linewidth increase, and $T_1^{-1}$ is progressively distributed in space, that is, a static and dynamical spatial inhomogeneity develops. While the order parameter for the $T_Q$ transition remains an open question, a group-theoretical analysis suggests that the finite electric quadrupole density arising from the low local site symmetry at cobalt sites could drive the crystal symmetry lowering from the P2$_1$3 symmetry that is commensurate to the R3 symmetry with an incommensurate wavevector, which fairly well accounts for the $T_Q$ transition. The quadrupole-order-driven commensurate-incommensurate phase transition may be another remarkable phenomenon arising from the structural chirality inherent in the noncentrosymmetric B20 family., Comment: 9 pages including appendix; 5 figures; published in Physical Review B

Effects of pressure on the electronic structure, electron-phonon interaction, and superconductivity of the high entropy alloy (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ are studied in the pressure range 0 - 100 GPa. The electronic structure is calculated using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Effects of pressure on the lattice dynamics are simulated using the Debye-Gr\"{u}neisen model and the Gr\"{u}neisen parameter at ambient conditions. In addition, the Debye temperature and Sommerfeld electronic heat capacity coefficient were experimentally determined. The electron-phonon coupling parameter $\lambda$ is calculated using the McMillan-Hopfield parameters and computed within the rigid muffin tin approximation. We find, that the system undergoes the Lifshitz transition, as one of the bands crosses the Fermi level at elevated pressures. The electron-phonon coupling parameter $\lambda$ decreases above 10 GPa. The calculated superconducting $T_c$ increases up to 40 - 50 GPa and, later, is stabilized at the larger value than for the ambient conditions, in agreement with the experimental findings. Our results show that the experimentally observed evolution of $T_c$ with pressure in (TaNb)$_{0.67}$(HfZrTi)$_{0.33}$ can be well explained by the classical electron-phonon mechanism., Comment: 19 pages, 20 figures including Supplemental Material, accepted in PRB

Polycrystalline samples of A$_x$V$_2$Al$_{20}$ (A = Ce, Th, U, Np, Pu; 0.7 =< x =< 1.0) actinide intermetallics were synthesized using the arc-melting method. Crystal structure studies were performed by means of powder x-ray diffraction and the Rietveld refinement method. All studied compounds crystallize in the CeCr$_{2}$Al$_{20}$-type structure (space group Fd-3m, no. 227) with the actinide or Ce atom located in the oversized cage formed by Al atoms. Comparison of the crystallographic results with the reported data for LnV$_{2}$Al$_{20}$ (Ln = lanthanoids) counterparts reveals distinctly different behavior of the lanthanide- and actinide-bearing compounds. This difference is suggested to be caused by fairly localized character of the 4f electrons, whereas itinerant character of the 5f electrons is likely seen for U- and Np-containing phases. Magnetic susceptibility and specific heat measurements did not reveal any magnetic ordering in U$_{0.8}$V$_{2}$Al$_{20}$, Np$_{0.8}$V$_{2}$Al$_{20}$ and Pu$_{0.8}$V$_{2}$Al$_{20}$ down to 2.5 K. A small anomaly in low-temperature specific heat of Ce$_{0.8}$V$_{2}$Al$_{20}$, U$_{0.8}$V$_{2}$Al$_{20}$, and Np$_{0.8}$V$_{2}$Al$_{20}$ is observed, likely arising from a low-energy Einstein mode., Comment: 7 pages, 6 figures

The niobium rich selenide compound Nb5Se4 was synthesized at ambient pressure by high-temperature solid-state reaction in a sealed Ta tube. Resistivity and heat capacity measurements reveal that this compound is superconducting, with a T_c = 1.85K. The electronic contribution to the specific heat {\gamma} and the Debye temperature are found to be 18.1 mJ/mol/K^2 and 298 K respectively. The calculated electron-phonon coupling constant {\lambda}_ep = 0.5 and the {\Delta}C_p/{\gamma}Tc = 1.42 ratio imply that Nb5Se4 is a weak coupling BCS superconductor. The upper critical field and coherence length are found to be 1.44 T and 15.1 nm, respectively.

Polycrystalline HfPd2Al has been synthesized using the arc-melting method and studied under ambient pressure conditions by x-ray diffraction from room temperature up to 450^oC. High pressure x-ray diffraction up to 23 GPa was also performed using Diacell-type membrane diamond anvil cells. The estimated linear thermal expansion coefficient was found to be {\alpha} = 1.40(3)x10^{-5} K^{-1}, and the bulk modulus derived from the fit to the 3rd order Birch-Murnaghan EOS (BMEOS) is B0 = 97(2) GPa. Resistivity studies under applied pressure (p < 7.49 GPa) showed a linear decrease of superconducting critical temperature with increasing pressure and the slope dTc/dp = -0.13(1) K GPa^{-1}. The same behavior is observed for the electron-phonon coupling constant {\lambda_{ep}}(p) that changes from 0.67 to 0.6, estimated for p = 0.05 GPa and 7.49 GPa, respectively. First principles electronic structure and phonon calculation results are presented and used to estimate the magnitude of electron-phonon interaction {\lambda_{ep}} and its evolution with pressure. Theoretical results explain the experimentally observed decrease in Tc due to considerable lattice stiffening., Comment: 10 figures

We report a corrected crystal structure for the CePt2In7 superconductor, refined from single crystal x-ray diffraction data. The corrected crystal structure shows a different Pt-In stacking along the c-direction in this layered material than was previously reported. In addition, all the atomic sites are fully occupied with no evidence of atom site mixing, resolving a discrepancy between the observed high resistivity ratio of the material and the atomic disorder present in the previous structural model The Ce-Pt distance and coordination is typical of that seen in all other reported Ce_nM_mIn_3n+2m compounds. Our band structure calculations based on the correct structure reveal three bands at the Fermi level that are more three dimensional than those previously proposed, and Density functional theory (DFT) calculations show that the new structure has a significantly lower energy., Comment: accepted by JPCM

Resistivity, dc magnetization, and heat capacity measurements are reported for superconducting NaBi. Tc, the electronic contribution to the specific heat {\gamma}, the {\Delta}Cp/{\gamma}Tc ratio, and the Debye temperature are found to be 2.15 K, 3.4 mJmol-1K-2, 0.78, and 140 K respectively. The calculated electron-phonon coupling constant ({\lambda}ep = 0.62) implies that NaBi is a moderately coupled superconductor. The upper critical field and coherence length are found to be 250 Oe and 115 nm, respectively. Electronic structure calculations show NaBi to be a good metal, in agreement with the experiments; the px and py orbitals of Bi dominate the electronic states at the Fermi Energy., Comment: 14 pages, 5 figures

The structural, magnetic, and thermodynamic properties of a new plutonium based compound, Pu2Pt3Si5, are reported. Single crystals produced by a Sn-flux technique have been analyzed showing a ferromagnetic behavior at 58 K. Pu2Pt3Si5 crystallizes in the U2Co3Si5-type orthorhombic Iabm structure (72) with atomic parameters a = 9.9226(2) \AA, b = 11.4436(2) \AA and c = 6.0148(1) \AA. The effective (\mu_eff ~0.74 \mu_B) and saturated (\mu_sat ~0.32 B/Pu) moments as well as the Sommerfeld coefficient (\gamma_e ~2 mJ.mol-1.K-2/Pu) could point towards 5f localization in this material.

Here we present bulk property measurements and electronic structure calculations for PuFeAsO, an actinide analogue of the iron-based rare-earth superconductors RFeAsO. Magnetic susceptibility and heat capacity data suggest the occurrence of an antiferromagnetic transition at TN=50 K. No further anomalies have been observed down to 2 K, the minimum temperature that we have been able to achieve. Structural measurements indicate that PuFeAsO, with its more localized 5f electrons, bears a stronger resemblance to the RFeAsO compounds with larger R ions, than NpFeAsO does., Comment: Accepted for publication in Phys. Rev. B

A neptunium analogue of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T_N = 57 K is suggested by anomalies in the temperature-dependent magnetic susceptibility, electrical resistivity, Hall coefficient, and specific heat curves. Below T_N, powder neutron diffraction measurements reveal an antiferromagnetic structure of the Np sublattice, with an ordered magnetic moment of 1.70(0.07) \mu_B aligned along the crystallographic c-axis. No magnetic order has been observed on the Fe sublattice, setting an upper limit of about 0.3 \mu_B for the ordered magnetic moment on the iron. High resolution x-ray powder diffraction measurements exclude the occurrence of lattice transformations down to 5 K, in sharp contrast to the observation of a tetragonal-to-orthorhombic distortion in the rare-earth analogues, which has been associated with the stabilization of a spin density wave on the iron sublattice. Instead, a significant expansion of the NpFeAsO lattice parameters is observed with decreasing temperature below T_N, corresponding to a relative volume change of about 0.2% and to an invar behavior between 5 and 20 K. First-principle electronic structure calculations based on the local-spin density plus Coulomb interaction and the local density plus Hubbard-I approximations provide results in good agreement with the experimental findings., Comment: Accepted for publication in Phys. Rev. B

Several physical properties of the superconducting Heusler compounds, focusing on two systems (Y, Lu, Sc)Pd2Sn and APd2M, where A=Hf, Zr and M=Al, In, are summarized and compared. The analysis of the data shows the importance of the electron-phonon coupling for superconductivity in this family. We report the superconducting parameters of YPd2Sn, which has the highest Tc among all known Heusler superconductors., Comment: Accepted for publication in Phys. Rev. B

We report extensive measurements on a new compound (Yb0.24Sn0.76)Ru that crystallizes in the cubic CsCl structure. Valence band photoemission and L3 x-ray absorption show no divalent component in the 4f configuration of Yb. Inelastic neutron scattering (INS) indicates that the eight-fold degenerate J-multiplet of Yb3+ is split by the crystalline electric field (CEF) into a {\Gamma}7 doublet ground state and a {\Gamma}8 quartet at an excitation energy 20 meV. The magnetic susceptibility can be fit very well by this CEF scheme under the assumption that a {\Gamma}6 excited state resides at 32 meV; however, the {\Gamma}8/{\Gamma}6 transition expected at 12 meV was not observed in the INS. The resistivity follows a Bloch- Gr\"uneisen law shunted by a parallel resistor, as is typical of systems subject to phonon scattering with no apparent magnetic scattering. All of these properties can be understood as representing simple local moment behavior of the trivalent Yb ion. At 1 K, there is a peak in specific heat that is too broad to represent a magnetic phase transition, consistent with absence of magnetic reflections in neutron diffraction. On the other hand, this peak also is too narrow to represent the Kondo effect in the {\Gamma}7 ground state doublet. On the basis of the field-dependence of the specific heat, we argue that antiferromagnetic shortrange order (possibly co-existing with Kondo physics) occurs at low temperatures. The long-range magnetic order is suppressed because the Yb site occupancy is below the percolation threshold for this disordered compound.

We have searched for time-reversal symmetry breaking fields in the non-centrosymmetric superconductor Mg$_{10}$Ir$_{19}$B$_{16}$ via muon spin relaxation in zero applied field, and we measured the temperature dependence of the superfluid density by muon spin rotation in transverse field to investigate the superconducting pairing symmetry in two polycrystalline samples of signficantly different purities. In the high purity sample, we detected no time-reversal symmetry breaking fields greater than 0.05 G. The superfluid density was also found to be exponentially-flat as T$\to $0, and so can be fit to a single-gap BCS model. In contrast, the lower purity sample showed an increase in the zero-field $\mu$SR relaxation rate below T$_c$ corresponding to a characteristic field strength of 0.6 G. While the temperature-dependence of the superfluid density was also found to be consistent with a single-gap BCS model, the magnitude as T$\to $0 was found to be much lower for a given applied field than in the case of the high purity sample. These findings suggest that the dominant pairing symmetry in high quality Mg$_{10}$Ir$_{19}$B$_{16}$ samples corresponds to the spin-singlet channel, while sample quality drastically affects the superconducting properties of this system., Comment: 6 pages, 5 figures, revised version resubmitted to PRB

The magnetic susceptibilities and specific heats of the crystalline garnet and glass forms of Mn3Al2Si3O12 are reported. This allows a direct comparison of the degree of magnetic frustration of the triangle-based garnet lattice and the structurally disordered solid at the same composition for isotropic spin 5/2 Mn^2+ (3d^5). The results show that the glass phase shows more pronounced signs of magnetic frustration than the crystalline phase. Through comparison of the specific heats of Ca3Al2Si3O12 (grossular) and Mn3Al2Si3O12 (spessartine) garnets, information is provided concerning the anomalous extra specific heat in the latter material.

Superconductivity in LaNiPO is disrupted by small (~5%) amounts of non-stoichiometry on the lanthanum site, even though the electronic contribution to the heat capacity increases with increasing non-stoichiometry. All samples also exhibit specific heat anomalies consistent with the presence of ferromagnetic spin fluctuations (Tsf ~ 14 K). Comparison of layered nickel phosphide and nickel borocarbide superconductors reveals different structure-property correlations in the two families., Comment: Submitted to Phys. Rev. B

We review the properties of Ni-based superconductors which contain Ni2X2 (X=As, P, Bi, Si, Ge, B) planes, a common structural element found also in the recently discovered FeAs superconductors. Strong evidence for the fully gapped nature of the superconducting state has come from field dependent thermal conductivity results on BaNi2As2. Coupled with the lack of magnetism, the majority of evidence suggests that the Ni-based compounds are conventional electron-phonon mediated superconductors. However, the increase in Tc in LaNiAsO with doping is anomalous, and mimics the behavior in LaFeAsO. Furthermore, comparisons of the properties of Ni- and Fe-based systems show many similarities, particularly with regards to structure-property relationships. This suggests a deeper connection between the physics of the FeAs superconductors and the related Ni-based systems which deserves further investigation., Comment: 21 pages, 4 figures, 3 tables; A review to appear in the Physica C special issue on superconducting pnictides

We report 209Bi NMR and NQR measurements on a single crystal of the Kondo insulator U3Bi4Ni3. The 209Bi nuclear spin-lattice relaxation rate ($T_1^{-1}$) shows activated behavior and is well-fit by a spin gap of 220 K. The 209Bi Knight shift (K) exhibits a strong temperature dependence arising from 5f electrons, in which K is negative at high temperatures and increases as the temperature is lowered. Below 50 K, K shows a broad maximum and decreases slightly upon further cooling. Our data provide insight into the evolution of the hyperfine fields in a fully gapped Kondo insulator based on 5f electron hybridization., Comment: 4 pages, 4 figures, submitted to Phys. Rev. B

We report the observation of superconductivity in La3Ni4P4O2 at 2.2 K. The layer stacking in this compound results in an asymmetric distribution of charge reservoir layers around the Ni2P2 planes. The estimated Wilson ratio, Rw ~ 5, indicates the presence of a strongly enhanced normal state susceptibility, but many of the basic superconducting characteristics are conventional. The estimated electronic contribution to the specific heat, gamma ~ 6.2 mJ mol-Ni^-1K^-2, is about 2/3 of that found in layered nickel borocarbide superconductors., Comment: Updated to reflect final version. Added journal reference

We report $^{75}$As Nuclear Magnetic Resonance data in a single crystal of CaFe$_2$As$_2$. The Knight shift, electric field gradient, and spin-lattice relaxation rate are strongly temperature dependent in the paramagnetic state, and change discontinuously at the structural transition temperature, $T_S=T_N=167$ K. Immediately below, the NMR spectra reveal an internal field at the As site associated with the presence of a commensurate magnetic order. These results indicate that the structural and magnetic transitions in CaFe$_2$As$_2$ are first order and strongly coupled, and that the electron density in the FeAs plane is highly sensitive to the out-of-plane structure., Comment: In Fig. 2, we incorrectly assigned AFM lines and quadrupole satellites. Fig.3 and the numbers in the text were corrected accordingly. This mistake has no affect on our main conclusion however

We report the synthesis and physical properties of single crystals of stoichiometric BaNi2As2 that crystalizes in the ThCr2Si2 structure with lattice parameters a = 4.112(4) \AA and c = 11.54(2) \AA. Resistivity and heat capacity show a first order phase transition at T_0 = 130 K with a thermal hysteresis of 7 K. The Hall coefficient is weakly temperature dependent from room temperature to 2 K where it has a value of -4x10^{-10} \Omega-cm/Oe. Resistivity, ac-susceptibility, and heat capacity find evidence for bulk superconductivity at T_c = 0.7 K. The Sommerfeld coefficient at T_c is 11.6 \pm 0.9 mJ/molK^2. The upper critical field is anisotropic with initial slopes of dH_{c2}^{c}/dT = -0.19 T/K and dH_{c2}^{ab}/dT = -0.40 T/K, as determined by resistivity., Comment: Published version: 4 pages, 4 figures

We report an $^{75}$As NMR study of BaFe$_2$As$_2$ in both single crystals and polycrystal forms. We find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K. In contrast to Sn-free samples, we find that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order., Comment: 4 pages, 4 figures ; extensive and significant changes have been made to the previous version, focusing on the effect of impurities on the intrinsic physics

We report pressure-induced superconductivity in a single crystal of CaFe2As2. At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature Tc exceeding 10 K. The rate of Tc suppression with applied magnetic field is -0.7 K/T, giving an extrapolated zero-temperature upper critical field of 10-14T., Comment: Accepted to J. Phys.: Condens. Matter (7 pages, 2 figures)

We report the synthesis and basic physical properties of single crystals of CaFe2As2, an isostructural compound to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystalizes in the ThCr2Si2 structure with lattice parameters a = 3.907(4) A and c = 11.69(2) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T_0 171 K. The magnetic susceptibility is nearly isotropic from 2 K to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 +- 0.3 mJ/molK2, and does not reveal any evidence for the presence of high frequency (> 300 K) optical phonon modes. The Hall coefficient is negative below the transition indicating dominant n-type carriers., Comment: Published version, minor changes

We report the properties of two new isostructural compounds, U3Bi4Ni3 and U3Bi4Rh3. The first of these compounds is non-metallic, and the second is a nearly ferromagnetic metal, both as anticipated from their electron count relative to other U-based members of the larger 3-4-3 family. For U3Bi4Rh3, a logarithmic increase of C/T below 3 K, a resistivity proportional to T^4/3, and the recovery of Fermi-liquid behavior in both properties with applied fields greater than 3T, suggest that U3Bi4Rh3 may be a new example of a material displaying ferromagnetic quantum criticality., Comment: 24 pages, 7 figures, submitted to Physical Review B

The magnetic properties and structure of LixCoO2 for x between 0.5 and 1.0 are reported. Co4+ is found to be high-spin in LixCoO2 for x between 0.94 and 1.0 and low-spin for x between 0.50 and 0.78. Weak antiferromagnetic coupling is observed, increasing in strength as more Co4+ is introduced. At an x value of about 0.65, the temperature-independent contribution to the magnetic susceptibility and the electronic contribution to the specific heat are largest. Neutron diffraction analysis reveals that the lithium oxide layer expands perpendicular to the basal plane and the Li ions displace from their ideal octahedral sites with decreasing x. A comparison of the structures of the NaxCoO2 and LixCoO2 systems reveals that the CoO2 layer changes substantially with alkali content in the former but is relatively rigid in the latter. Further, the CoO6 octahedra in LixCoO2 are less distorted than those in NaxCoO2. We postulate that these structural differences strongly influence the physical properties in the two systems.

Five compounds were investigated for magnetic character and superconductivity, all with non-magnetic nickel and band structures containing flat bands and steep bands. The syntheses and crystal structures, refined by powder X-ray diffraction, are reported for M6Ni16Si7, where M = Mg, Sc, Ti, Nb, and Ta. All compounds form in the Mg6Cu16Si7 structure type. Resistance measurements are also reported on M6Ni16Si7 (M = Mg, Sc, Ti, and Nb) down to 0.3 K, with all four showing metallic conductivity. No superconductivity is observed. Magnetization measurements for all compounds reveal essentially temperature independent paramagnetism, with a tendency toward more enhanced low temperature paramagnetism for the cases of Mg6Ni16Si7 and Sc6Ni16Si7., Comment: 9 pages, 6 figures, 1 table. 2 Authors added. Includes extra figure with accompanying text and references

The intermetallic perovskite MgCNi3 is a superconductor with a Tc=7 K. Substitution of Fe and Ru for Ni decreases Tc monotonically as the doping concentration is increased. Here we report thermopower measurements, S(T), on MgCNi3, MgCNi3-xFex and MgCNi3-xRux. For MgCNi3, the thermopower is negative, - 12.5 mikroV/K, at 300 K. The absolute value of S decreases as x increases in MgCNi3-xFex and MgCNi3-xRux. The sign of S changes from negative to positive at low temperatures for values of x > 0.01. These data show that the carriers in MgCNi3 are electrons, and by increasing x and decreasing temperature, the participation of hole carriers clearly increases. The influence of the magnetic moments of the Fe atoms on the thermopower is not visible., Comment: 14 pages, 4 figures

Specific heat, electrical resistivity, and magnetic susceptibility measurements on a high quality sample of Mg10Ir19B16 provide a self-consistent determination of its superconducting properties. They indicate that Mg10Ir19B16 is a type-II superconductor (Tc=4.45K, k(0)=20), with an electron-phonon coupling constant l_ep=0.66. An analysis of the T-dependent specific heat shows that superconducting properties are dominated by an s-wave gap (D=0.7 meV). Point contact tunneling data provide evidence for multiple superconducting gaps, as expected from strong asymmetric spin-orbit coupling., Comment: Text and figures revised. One author and point contact tunneling data added

Mn3V2O8 is a magnetic system in which S = 5/2 Mn2+ is found in the kagome staircase lattice. Here we report the magnetic phase diagram for temperatures above 2 K and applied magnetic fields below 9 T, characterized by measurements of the magnetization and specific heat with field along the three unique lattice directions. At low applied magnetic fields, the system first orders magnetically below Tm1 ~ 21 K, and then shows a second magnetic phase transition at Tm2 ~ 15 K. In addition, a phase transition that is apparent in specific heat but not seen in magnetization is found for all three applied field orientations, converging towards Tm2 as H -> 0. The magnetic behavior is highly anisotropic, with critical fields for magnetic phase boundaries much higher when the field is applied perpendicular to the Kagome staircase plane than when applied in-plane. The field-temperature (H - T) phase diagrams are quite rich, with 7 distinct phases observed.

The magnetic properties of $\alpha$-NaFeO$_2$ are studied by neutron diffraction and magnetization measurements. An ordered phase with spins aligned along the b$_hex$ axis exists at low temperatures (T < 4 K). At intermediate temperatures (4 K < T < 11 K), the system passes through an incommensurate ordered phase before transforming into a short range ordered state at higher temperatures that persists up to at least 50 K. Although the short range ordering does not persist to room temperature according to neutron diffraction, the magnetic susceptibility does not follow Curie-Weiss behavior, even up to 320 K. This rich magnetic behavior can be understood qualitatively as a competition between different magnetic exchange interactions that are similar in magnitude. The delicate balance between these interactions makes $\alpha$-NaFeO$_2$ a candidate for more detailed theoretical work to understand magnetic behavior in frustrated magnetic systems., Comment: Submitted to Phys Rev B, 7 pages, 8 figures

Mg10Ir19B16, a previously unreported compound in the Mg-Ir-B chemical system, is found to be superconducting at temperatures near 5 K. The fact that the compound exhibits a range of superconducting temperatures between 4 and 5 K suggests that a range of stoichiometries is allowed, though no structural evidence for this is observed. The compound has a large, noncentrosymmetric, body centered cubic unit cell with a = 10.568 Angstrom, displaying a structure type for which no previous superconductors have been reported., Comment: submitted to PRB

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids. They are collective states that arise from intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well-studied examples, with TiSe_2 one of the first CDW-bearing materials known. The competition between CDW and superconducting collective electronic states at low temperatures has long been held and explored, and yet no chemical system has been previously reported where finely controlled chemical tuning allows this competition to be studied in detail. Here we report how, upon controlled intercalation of TiSe_2 with Cu to yield Cu_xTiSe_2, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum T_c of 4.15 K found at x = 0.08. Cu_xTiSe_2 thus provides the first opportunity to study the CDW to Superconductivity transition in detail through an easily-controllable chemical parameter, and will provide new insights into the behavior of correlated electron systems., Comment: Accepted to Nature Physics

The high pressure synthesis, structure and magnetic properties of Ca_25Co_22O_56(OH)_28 are reported. The compound has a misfit structure, consisting of double, square calcium oxide hydroxide rock-salt-like layers between hexagonal CoO_2 layers. The misfit compound crystallizes in the monoclinic space group C2/m, and can be characterized by the coexistence of two subsystems with common a=4.893(5)A, c=8.825(9)A and b=95.745(8) parameters, and different b parameters: b_RS=4.894(5)A, and b_HEX=2.809(3)A, for the rock-salt and hexagonal type planes respectively. The compound shows Curie-Weiss paramagnetism with an antiferromagnetic Weiss temperature of -43K and a reduced Co moment. Substantial deviations from Curie-Weiss behavior are seen below 50K with no indication of magnetic ordering. No superconductivity was observed down to a temperature of 2K., Comment: axis labels Fig.5 revised

The crystal structure of boron doped superconducting MgC_{1-x}{11}^B_{x}Ni_{3}, studied by powder neutron diffraction, is reported. The solubility limit of boron is determined to be approximately x=0.16. The unit cell expands from a = 3.81089(2) to 3.81966(2) Angstroms as x increases from x=0 to x=0.155. Boron ({11}^B) doping decreases Tc with increasing x: from 7.09K (x=0) to 6.44K (x=0.155)., Comment: Submitted to Phys. Rev. B

The hydration phase diagram for sodium cobalt oxyhydrate, Na0.3CoO2*yH2O (y=0, 0.6, 1.3), was determined as a function of relative humidity at 298K. It is found that greater than 75% relative humidity is needed for complete hydration of anhydrous Na0.3CoO2 to the superconducting phase Na0.3CoO2*1.3H2O. Dehydration studies show that a minimum of 43% relative humidity is needed to maintain the stability of the fully hydrated superconducting phase. The intermediate hydrate, Na0.3CoO2*0.6H2O, is stable between 10% and 50% relative humidity on hydration, and 35% to 0% relative humidity on dehydration., Comment: submitted to Mat. Res. Bull., 4 pages, 4 figures

The effect of Carbon isotope substitution on T_c in the intermetallic perovskite superconductor MgCNi_3 is reported. Four independent groups of samples were synthesized and characterized. The average T_c for the Carbon-12 samples was found to be 7.12(2) K and the average T_c for the Carbon-13 samples was found to be 6.82(2) K. The resulting carbon isotope effect coefficient is alfa_C = 0.54(3). This indicates that carbon-based phonons play a critical role in the presence of superconductivity in this compound., Comment: To be published in Phys. Rev. B. 4 pages, 1 figure

The superconductor MgCNi3 has been chemically doped by partial substitution of Ru for Ni in the solid solution MgCNi3-xRux for 0

The observation of superconductivity at 4.3 K in a new crystalline form of Na0.3CoO2*1.3H2O is reported. The new superconductor has three layers of CoO6 octahedra per crystallographic unit cell, in contrast to the previously reported two-layer superconductor. The three-layer cell occurs because the relative orientations of neighboring CoO2 layers are distinctly different from what is seen in the two-layer superconducting phase. This type of structural difference in materials that are otherwise chemically and structurally identical is not possible to attain on the layered copper oxide superconductors. The synthesis and stability of the new phase are described., Comment: 10 pages, 4 figures. Submitted to Physical Review Letters

We present precise measurements of the upper critical field (Hc2) in the recently discovered cobalt oxide superconductor. We have found that the critical field has an unusual temperature dependence; namely, there is an abrupt change of the slope of Hc2(T) in a weak field regime. In order to explain this result we have derived and solved Gor'kov equations on a triangular lattice. Our experimental results may be interpreted in terms of the field-induced transition from singlet to triplet superconductivity., Comment: 6 pages, 5 figures, revtex

Although the microscopic origin of the superconductivity in high Tc copper oxides remains the subject of active inquiry, several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics is the dependence of the superconducting transition temperature on the degree of electronic band filling. Since the discovery of cuprate superconductivity in 1986 (1), the search for other families of superconductors that might help shed light on the superconducting mechanism of the cuprates has been of great interest. The recent report of superconductivity near 4K in the triangular lattice, layered sodium cobalt oxyhydrate, Na0.35CoO2.1.3H2O, is the best indication that superconductors related to the cuprates may be found (2). Here we show that the superconducting transition temperature of this compound displays the same kind of band-filling behavior that is observed in the cuprates. Specifically, that the optimal superconducting Tc occurs in a narrow range of band filling, and decreases for both underdoped and overdoped materials, in dramatic analogy to the phase diagram of the cuprate superconductors. Our results suggest that characterization of the detailed electronic and magnetic behavior of these new materials may help establish which of the many special characteristics of the cuprates is fundamental to their high Tc superconductivity., Comment: revised, publication information added

The layered sodium cobalt oxyhydrate superconductor Na0.3CoO2*1.4H2O is shown through X-ray diffraction and thermogravimetric studies to be one of a series of hydrated phases of Na0.3CoO2. Further, it is shown that the material is exceptionally sensitive to both temperature and humidity near ambient conditions, easily dehydrating to a non-superconducting lower hydrate. The observation of this stable lower hydrate with c=13.8 angstroms implies that the superconductivity turns on in this system between CoO2 layer spacings of 6.9 and 9.9 angstroms at nominally constant chemical doping., Comment: 10 pages and 4 figures