Results for "R. J. Cava"

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High entropy alloy superconductors -- status, opportunities and challengesJul 14 2019High entropy alloys (HEAs) are a recently-opened research area in materials science and condensed matter physics. Although 3d-metal-based HEAs have already been the subject of many investigations, studies of HEA superconductors, which tend to be based ... More
Disorder Induced Ferromagnetism in CaRuO3Feb 21 2001The magnetic ground state of perovskite structure CaRuO3 has been enigmatic for decades. Here we show that paramagnetic CaRuO3 can be made ferromagnetic by very small amounts of partial substitution of Ru by Ti. Magnetic hysteresis loops are observed ... More
Carbon isotope effect in superconducting MgCNi_3Oct 20 2004The 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) ... More
A single crystal high-temperature pyrochlore antiferromagnetMay 05 2014We report the magnetic characterization of the frustrated transition metal pyrochlore NaCaCo$_2$F$_7$. This material has high spin Co$^{2+}$ in CoF$_6$ octahedra in a pyrochlore lattice, and disordered non-magnetic Na and Ca on the large-atom sites in ... More
NaCaNi2F7: A frustrated high temperature pyrochlore antiferromagnet with S=1 Ni2+Apr 29 2015NaCaNi$_2$F$_7$ is an insulating, frustrated A$_2$B$_2$F$_7$ pyrochlore with magnetic S = 1 Ni$^{2+}$ on the pyrochlore B site. Non-magnetic Na and Ca are disordered on the A-site. Magnetic susceptibility measurements made on an oriented single crystal, ... More
A single crystal Co2+ fluoride pyrochlore antiferromagnetMar 12 2015We report the crystal growth, crystal structure and basic magnetic properties of a new cobalt-based pyrochlore, NaSrCo$_2$F$_7$. Na and Sr are completely disordered on the non-magnetic large atom A sites, while magnetic S=3/2 Co$^{2+}$ fully occupies ... More
A Spin Glass State in Ba3TiRu2O9Oct 29 2018The magnetic properties of Ba3TiRu2O9 , whose crystal structure is based on stacked triangular planar lattices of MO6 dimers and single MO6 octahedra, are reported. The system is magnetically disturbed by a substantial amount of Ti/Ru chemical disorder. ... More
Magnetic Properties of Lithium-Containing Rare Earth Garnets Li3RE3Te2O12 (RE = Y, Pr, Nd, Sm-Lu)Jun 21 2018The synthesis, structural description, temperature dependent magnetic susceptibilities and field dependent magnetizations of a series of rare earth garnets of the form Li3RE3Te2O12 (RE = Y, La, Pr, Nd, Sm-Lu) are reported. The structure of Li3Dy3Te2O12 ... More
The Effect of Dopants on the Magnetoresistance of WTe2Jun 23 2015Sep 19 2015Elucidating the nature of the large, non-saturating magnetoresistance in WTe2 is a significant step in functionalizing this phenomenon for applications. Here, Mo, Re, and Ta doped WTe2 are compared to determine whether isovalent and aliovalent substitutions ... More
Comparison of the Magnetic properties of Mn3Fe2Si3O12 as a crystalline garnet and as a glassFeb 20 2018The crystalline garnet Mn3Fe2Si3O12 and an amorphous phase of the same nominal composition are synthesized at high pressure. The magnetic properties of the two forms are reported. Both phases order antiferromagnetically. The crystalline phase exhibits ... More
Stoichiometric oxygen content in NaxCoO2Mar 22 2006The crystal structure and oxygen stoichiometry in two-layer Na0.74CoO2 and Na0.38CoO2 at room temperature are analyzed by powder neutron diffraction. Two sets of diffraction data for each sample, taken at different incident neutron wavelengths, lambda=1.1968 ... More
The trimer-based spin liquid candidate Ba4NbIr3O12Nov 04 2018Ba4NbIr3O12, a previously unreported material with a triangular planar geometry of Ir3O12 trimers, is described. Magnetic susceptibility measurements show no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature of -13 K. The material has ... More
Dirty Spin Ice: The Effect of Dilution on Spin Freezing in Dy2Ti2O7May 01 2002We have studied spin freezing in the diluted spin ice compound Dy2-xYxTi2O7 where the non-magnetic Y ions replace the magnetic Dy ions on the frustrated pyrochlore lattice. Magnetic a.c. and d.c. susceptibility data are presented with an analysis of relaxation ... More
Rich magnetic phase diagram in the Kagome-staircase compound Mn3V2O8Apr 12 2007Mn3V2O8 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 ... More
Evidence for Unconventional Superconductivity in the Non-Oxide Perovskite $\mathrm{MgCNi_3}$ from Penetration Depth MeasurementsFeb 21 2003The London penetration depth, $\lambda(T)$, was measured in polycrystalline powders of the non-oxide perovskite superconductor $\mathrm{MgCNi_3}$ by using a sensitive tunnel-diode resonator technique. The penetration depth exhibits distinctly non s-wave ... More
The Stoichiometry of FeSeJul 01 2009Tetragonal iron selenide, FeSe, the layered parent compound of the recently discovered superconducting arsenide family, has previously been shown to be non magnetic and superconducting with a critical temperature near 8 K. There has, however, been a lack ... More
The effect of {11}^B substitution on the superconductivity in MgCNi3Dec 20 2004The 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) ... More
Ferromagnetism in Mn-doped Sb2TeDec 23 2013We report that Sb2Te, a natural superlattice phase consisting of two elemental Sb2 layers interleaved with single Sb2Te3 layers, becomes ferromagnetic at low temperatures on doping with small percentages of Mn. Ferromagnetism appears for Mn concentrations ... More
Hourglass FermionsFeb 17 2016Apr 13 2016Spatial symmetries in crystals are distinguished by whether they preserve the spatial origin. We show how this basic geometric property gives rise to a new topology in band insulators. We study spatial symmetries that translate the origin by a fraction ... More
Superconducting Properties of the K$_{x}$WO$_{3}$ Tetragonal Tungsten Bronze and the Superconducting Phase Diagram of the Tungsten Bronze FamilyFeb 18 2014We report the superconducting properties of the K$_{x}$WO$_{3}$ tetragonal tungsten bronze. The highest superconducting transition temperature ($T_{c}=2.1$K) was obtained for K$_{0.38}$WO$_{3}$. $T_{c}$ decreases linearly with increasing K content. Using ... More
Hydration phase diagram for sodium cobalt oxide Na0.3CoO2*yH2ONov 15 2004The 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 ... More
Anisotropic properties of the layered superconductor Cu0.07TiSe2Nov 13 2006The anisotropic superconducting properties of single crystals of Cu0.07TiSe2 were studied by measurements of magnetization and electrical resistivity. TC is around 3.9 K, and the measured upper critical field (Hc2) values are ~1.25 T and 0.8 T, for applied ... More
The Reactivity of MgB2 with Common Substrate and Electronic MaterialsSep 20 2001The reactivity of MgB2 with powdered forms of common substrate and electronic materials is reported. Reaction temperatures between 600 C and 800 C, encompassing the range commonly employed in thin-film fabrication, were studied. The materials tested for ... More
A Non-Centrosymmetric Superconductor with a Bulk 3D Dirac Cone Gapped by Strong Spin Orbit CouplingOct 31 2013Layered, non-centrosymmetric, heavy element PbTaSe2 is found to be superconducting. We report its electronic properties accompanied by electronic structure calculations. Specific heat, electrical resistivity and magnetic susceptibility measurements indicate ... More
Trimers of MnO6 octahedra and ferrimagnetism of Ba4NbMn3O12Oct 01 2018Ba4NbMn3O12 is reported, synthesized by a solid state method in air. The crystal structure, determined by performing refinements on room temperature powder X-ray diffraction data by the Rietveld method, consists of Mn3O12 trimers in the configuration ... More
Lone Pair Effect, Structural Distortions and Potential for Superconductivity in Tl PerovskitesFeb 07 2013Drawing the analogy to BaBiO3, we investigate via ab-initio electronic structure calculations potential new superconductors of the type ATlX3 with A = Rb, Cs and X = F, Cl, and Br, with a particular emphasis on RbTlCl3. Based on chemical reasoning, supported ... More
Magnetic Behavior of La7Ru3O18Aug 26 2003Rhombohedral La7Ru3O18 can be considered to be nearly geometrically frustrated due to its close structural similarity to the strongly geometrically frustrated compound La4.87Ru2O12. The magnetic ordering of La7Ru3O18 was explored using a combination of ... More
Magnetism in the KBaRE(BO3)2 (RE=Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) series: materials with a triangular rare earth latticeNov 25 2016We report the magnetic properties of compounds in the KBaRE(BO3)2 family (RE= Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), materials with a planar triangular lattice composed of rare earth ions. The samples were analyzed by x-ray diffraction and crystallize in ... More
Superconductivity in WO2.6F0.4 synthesized by reaction of WO3 with TeflonNov 03 2011WO3-xFx (x < 0.45) perovskite-like oxyfluorides were prepared by a chemically reducing fluorination route using the polymer polytetrafluoroethylene (Teflon). The symmetry of the crystal structures of WO3-xFx changes from monoclinic to tetragonal to cubic ... More
The effect of Fe and Ru substitution for Ni on the thermopower in MgCNi_3Jun 14 2007The 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. ... More
Evolution of magnetic fluctuations through the Fe-induced paramagnetic to ferromagnetic transition in Cr$_2$BAug 03 2016In itinerant ferromagnets, the quenched disorder is predicted to dramatically affect the ferromagnetic to paramagnetic quantum phase transition driven by external control parameters at zero temperature. Here we report a study on Fe-doped Cr$_2$B, which, ... More
Kinetically Stable Single Crystals of Perovskite-Phase CsPbI${_3}$May 29 2019We use solid-state methods to synthesize single crystals of perovskite-phase CsPbI${_3}$ (${\gamma}$-CsPbI${_3}$) that are kinetically stable at room temperature. Single crystal X-ray diffraction characterization shows that the compound is orthorhombic ... More
Superconducting Properties of BaBi$_{3}$Jun 11 2014We report the superconducting properties of single crystals of the intermetallic perovskite-related compound BaBi$_{3}$. The superconducting transition temperature ($T_{c}=5.82$~K) was obtained from heat capacity measurements. Using the measured values ... More
Divergent effects of static disorder and hole doping in geometrically frustrated b-CaCr2O4Apr 08 2010The gallium substituted and calcium deficient variants of geometrically frustrated b-CaCr2O4, b-CaCr2-2xGa2xO4 (0.02<= x<= 0.25) and b-Ca1-yCr2O4 (0.075<= y<= 0.15), have been investigated by x-ray powder diffraction, magnetization and specific heat measurements. ... More
Superconductivity in 3R-Ta1-xMxSe2 (M = W, Mo)May 27 2015The 3-layer (3R) polytype of TaSe2-xTex shows a superconducting transition temperature that is between 6 and 17 times higher than that of the two-layer (2H) polytype. The remarkable difference in Tc, although clearly associated with a difference in polytype, ... More
Magnetotransport properties of BaRuO$_3$: Observation of two scattering ratesOct 17 2011Dec 20 2011We report results of low-temperature magnetotransport and Hall measurements on single crystals of four-layered hexagonal (4H) and nine-layered rhombohedral (9R) BaRuO$_3$ that provide insight into the structure-property relationships of BaRuO$_3$ polymorphs. ... More
Optical properties of the perfectly compensated semimetal WTe$_2$Jun 08 2015Sep 22 2015The optical properties of layered tungsten ditelluride have been measured over a wide temperature and frequency range for light polarized in the $a$-$b$ planes. A striking low-frequency plasma edge develops in the reflectance at low temperature where ... More
The emergence of superconductivity in BaNi2(Ge1-xPx)2 at a structural instabilityAug 01 2012The physical properties and structural evolution of the 122-type solid solution BaNi2(Ge1-xPx)2 are reported. The in-plane X-X (X = Ge1-xPx) dimer formation present in the end member BaNi2Ge2, which results in a structural transition to orthorhombic symmetry, ... More
Strong Electron-Phonon Coupling Superconductivity Induced by a Low-lying Phonon in IrGeMar 20 2018The physical properties of the previously reported superconductor IrGe and the Rh1-xIrxGe solid solution are investigated. IrGe has an exceptionally high superconducting transition temperature (Tc = 4.7 K) among the isostructural 1:1 late-metal germanides ... More
Signatures of Fractional Quantum Hall States in Topological InsulatorsAug 23 2011May 13 2012The fractional quantum Hall (FQH) state is a topological state of matter resulting from the many-body effect of interacting electrons and is of vast interest in fundamental physics. The experimental observation of topological surface states (SSs) in three-dimensional ... More
Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructuresJul 05 2014Aug 25 2014We demonstrate the van der Waals epitaxy of the topological insulator compound Bi2Te3 on the ferromagnetic insulator Cr2Ge2Te6. The layers are oriented with (001) of Bi2Te3 parallel to (001) of Cr2Ge2Te6 and (110) of Bi2Te3 parallel to (100) of Cr2Ge2Te6. ... More
Neutron Scattering Study of Crystal Field Energy Levels and Field Dependence of the Magnetic Order in Superconducting HoNi2B2CMay 23 1995Elastic and inelastic neutron scattering measurements have been carried out to investigate the magnetic properties of superconducting (Tc~8K) HoNi2B2C. The inelastic measurements reveal that the lowest two crystal field transitions out of the ground state ... More
Slow spin relaxation in a highly polarized cooperative paramagnetNov 29 2005We report measurements of the ac susceptibility of the cooperative paramagnet Tb2Ti2O7 in a strong magnetic field. Our data show the expected saturation maximum in chi(T) and also an unexpected low frequency dependence (< 1 Hz) of this peak, suggesting ... More
Low Temperature Spin Freezing in Dy2Ti2O7 Spin IceAug 22 2003We report a study of the low temperature bulk magnetic properties of the spin ice compound Dy2Ti2O7 with particular attention to the (T < 4 K) spin freezing transition. While this transition is superficially similar to that in a spin glass, there are ... More
Absence of conventional charge ordering in Na0.5CoO2 from a high resolution neutron diffraction studyJan 07 2006The structure of Na0.5CoO2, the low temperature insulator that separates the magnetic and superconducting regions in the NaxCoO2.yH2O phase diagram, is studied by high resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile ... More
Polarized Temperature Dependent Raman Study of Bi2Te3-Cr2Ge2Te6 heterostructure and the Ferromagnetic Insulator Cr2Ge2Te6Oct 07 2014Cr2Ge2Te6 has been of interest for decades, as it is one of only a few ferromagnetic insulators. Recently, this material has been revisited due to its potential as a substrate for Bi2Te3, a topological insulator. This enables the possibility of studying ... More
Low temperature magnetic structure of the quasi 1-dimensional magnet Ni2SiO4Jul 08 2009Ni2SiO4, Liebenbergite, is an example of a quasi-one-dimensional magnet made up of frustrated corner sharing triangles of Ni2+ (S = 1) ions that propagate parallel to the b axis. Ni2SiO4 is isostructural with olivine, a common mineral of varying composition ... More
Superconductivity in the Niobium-rich compound Nb5Se4Dec 14 2015The 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 ... More
Crystal Structure and Magnetic Properties of the Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14 LangasitesFeb 07 2013We report the structural and magnetic characterizations of Ba3TeCo3P2O14, Pb3TeCo3P2O14, and Pb3TeCo3V2O14, compounds that are based on the mineral Dugganite, which is isostructural to Langasites. The magnetic part of the structure consists of layers ... More
Scalar Chiral Spin-1/2 Order on Kagome Lattices in Nd3Sb3Mg2O14Apr 13 2016We introduce $\mathrm{Nd_{3}Sb_{3}Mg_{2}O_{14}}$ with ideal kagome lattices of neodymium ions in ABC stacking. Thermodynamic measurements show a Curie-Weiss temperature of $\Theta_{CW}=-0.12~$K, a Nd$^{3+}$ spin-1/2 Kramers doublet ground state, and a ... More
Superconductivity and non-metallicity induced by doping the topological insulators Bi2Se3 and Bi2Te3Jun 02 2010We show that by Ca-doping the Bi2Se3 topological insulator, the Fermi level can be fine tuned to fall inside the band gap and therefore suppress the bulk conductivity. Non-metallic Bi2Se3 crystals are obtained. On the other hand, the Bi2Se3 topological ... More
Geometrical Magnetic Frustration in Rare Earth Chalcogenide SpinelsApr 18 2005Apr 21 2005We have characterized the magnetic and structural properties of the CdLn2Se4 (Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all compounds to be normal spinels, possessing a geometrically frustrated sublattice of lanthanide atoms ... More
Electrical Transport Properties of Single Crystal Sr3Ru2O7: The Possible Existence of an Antiferromagnetic Instability at Low TemperaturesFeb 08 2001We report the results of Hall coefficient R_H and magnetoresistance (MR) measurements on single crystalline samples of Sr3Ru2O7 grown by the floating zone method. R_H was found to be positive over the entire temperature range studied (0.3 - 300K). Its ... More
Quantum Spin LiquidsMay 16 2019Spin liquids are quantum phases of matter that exhibit a variety of novel features associated with their topological character. These include various forms of fractionalization - elementary excitations that behave as fractions of an electron. While there ... More
Superconductivity Phase Diagram of Na(x)CoO(2).1.3H(2)OMay 19 2003Sep 03 2003Although 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 ... More
Ising pyrochlore magnets: Low temperature properties, ice rules and beyondFeb 01 1999Mar 22 1999Pyrochlore magnets are candidates for spin-ice behavior. We present theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare earth) supported by magnetothermal measurements on selected systems. By considering long ranged dipole-dipole ... More
Magnetothermodynamics of the Ising Antiferromagnet Dy2Ge2O7Aug 26 2008We report systematic low temperature measurements of the DC magnetization, AC susceptibility, and heat capacity of dysprosium pyrogermanate (Dy2Ge2O7) single crystal and powder samples. Our results confirm that Dy2Ge2O7 is an anisotropic antiferromagnet. ... More
NaSrMn2F7, NaCaFe2F7, and NaSrFe2F7: novel single crystal pyrochlore antiferromagnetsAug 09 2016The crystal structures and magnetic properties of three previously unreported A2B2F7 pyrochlore materials, NaSrMn2F7, NaCaFe2F7, and NaSrFe2F7 are presented. In these compounds, either S=2 Fe2+ or S=5/2 Mn2+ is on the B site, while nonmagnetic Na and ... More
Lattice collapse and the magnetic phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$Sep 10 2009We report that the 122 type Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ solid solution undergoes an anomalous structural transition from the uncollapsed to the collapsed ThCr$_2$Si$_2$ structure at a distinct onset composition near $x=0.5$. Correlated with the structural ... More
The anomalous Hall Effect and magnetoresistance in the layered ferromagnet Fe_{1/4}TaS_2: the inelastic regimeAug 08 2007The large magnetic anisotropy in the layered ferromagnet Fe_{1/4}TaS_2 leads to very sharp reversals of the magnetization $\bf M$ at the coercive field. We have exploited this feature to measure the anomalous Hall effect (AHE), focussing on the AHE conductivity ... More
Bulk Band Gap and Surface State Conduction Observed in Voltage-Tuned Crystals of the Topological Insulator Bi$_2$Se$_3$Mar 19 2010Jun 20 2011We report a transport study of exfoliated few monolayer crystals of topological insulator Bi$_2$Se$_3$ in an electric field effect (EFE) geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier ... More
Local-moment ferromagnetism and unusual magnetic domains in Fe$_{1/4}$TaS$_{2}$ crystalsMay 16 2009Single crystals of Fe$_{1/4}$TaS$_{2}$ have been studied by using magneto-optical (MO) imaging and radio-frequency (rf) magnetic susceptibility, $\chi$. Real time MO images reveal unusual, slow dynamics of dendritic domain formation, the details of which ... More
Magneto-Elastic Coupling in a potential ferromagnetic 2D Atomic CrystalApr 29 2016Cr2Ge2Te6 has been of interest for decades, as it is one of only a few naturally forming ferromagnetic semiconductors. Recently, this material has been revisited due to its potential as a 2 dimensional semiconducting ferromagnet and a substrate to induce ... More
Stoichiometry, Spin Fluctuations, and Superconductivity in LaNiPOApr 27 2009Superconductivity 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 ... More
Crystal structure and electronic structure of CePt2In7Aug 06 2014We 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 ... More
Superconducting NbRuB derived through the fragment formalismFeb 24 2015We employ the chemical fragment formalism to perform a targeted superconductor search in the Nb-Ru-B system, yielding the orthorhombic metal-rich boride NbRuB, which displays BCS-like superconductivity with a Tc = 3.1 K. NbRuB is derived from the chemical ... More
Robust paramagnetism in Bi2-xMxRu2O7 (M=Mn,Fe,Co,Ni,Cu) pyrochloreMay 24 2002We report physical property characterization of Bi2-xMxRu2O7 pyrochlores, including magnetic suseptibility, resistivity, and Seebeck coefficients. The solid solution exists up to x=0.5 for (M=Cu,Ni,Co) and up to x=0.1 for (M=Fe,Mn). None of the doped ... More
Metal - Insulator transition in Fe1.01-xCuxSeMay 12 2009Iron Selenide, Fe1.01Se, the layered parent compound of the recently discovered superconducting arsenide family, has previously been shown to be non magnetic and superconducting with a critical temperature of 8 K. Here we show that copper can be substituted ... More
Physical properties of the noncentrosymmetric superconductor Mg_10Ir_19B_16Jun 14 2007Jan 07 2008Specific 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 ... More
The structures and thermoelectric properties of the infinitely adaptive series (Bi2)m(Bi2Te3)nMar 20 2007The structures and thermoelectric properties of the (Bi2)m(Bi2Te3)n homologous series, derived from stacking hexagonal Bi2 and Bi2Te3 blocks, are reported. The end-members of this series are metallic Bi and semiconducting Bi2Te3; nine members of the series ... More
Carbon Concentration Dependence of the Superconducting Transition Temperature and Structure of MgCxNi3Oct 29 2001The crystal structure of the superconductor MgCxNi3 is reported as a function of carbon concentration determined by powder neutron diffraction. The single-phase perovskite structure was found in only a narrow range of carbon content, 0.88 < x < 1.0. The ... More
Low temperature dynamic freezing and the fragility of ordering in Tb2Sn2O7Nov 26 2012We have probed the low temperature magnetic behavior of the ordered spin ice material Tb2Sn2O7 through ac magnetic susceptibility measurements of both the pure material and samples with small percentages of Ti substituted on the Sn sublattice. We observe ... More
Crystal structure and magnetic properties of Ba$_2R_{2/3}$TeO$_6$ ($R$ = Y, La, Pr, Nd, Sm-Lu) double perovskitesAug 16 2017A new series of cubic double perovskites Ba$_2R_{2/3}$TeO$_6$ ($R$ = Y, La, Pr, Nd, Sm-Lu) was synthesized via solid state reaction. The $R^{3+}$ and Te$^{6+}$ ions are ordered on alternating octahedral sites, with the rare earth sites 2/3 occupied to ... More
NaIrO3 - A pentavalent post-perovskiteNov 23 2010Jan 31 2011Sodium iridium(V) oxide, NaIrO3, was synthesized by a high pressure solid state method and recovered to ambient conditions. It is found to be isostructural with CaIrO3, the much-studied structural analogue of the high-pressure post-perovskite phase of ... More
Spin-flop and antiferromagnetic phases of the ferromagnetic half-twist ladder compounds Ba3Cu3In4O12 and Ba3Cu3Sc4O12Feb 15 2013The title compounds have dominant ferromagnetic (FM) exchange interactions within one-dimensional (1D) half-twist ladders of s =1/2 Cu2^{+} ions and antiferromagnetic(AFM) interactions between ladders, leading to ordered 3D phases at temperatures below ... More
Dominant ferromagnetism in the spin-1/2 half-twist ladder 334 compounds, Ba3Cu3In4O12 and Ba3Cu3Sc4O12Mar 01 2012Mar 27 2012The magnetic properties of polycrystalline samples of Ba3Cu3In4O12 (In-334) and Ba3Cu3Sc4O12 (Sc-334) are reported. Both 334 phases have a structure derived from perovskite, with CuO4 squares interconnected to form half-twist ladders along the c-axis. ... More
Measurement of the topological surface state optical conductance in bulk-insulating Sn-doped Bi$_{1.1}$Sb$_{0.9}$Te$_2$S single crystalsOct 04 2016Topological surface states have been extensively observed via optics in thin films of topological insulators. However, in typical thick single crystals of these materials, bulk states are dominant and it is difficult for optics to verify the existence ... More
High-Field Shubnikov-de Haas Oscillations in the Topological Insulator Bi$_2$Te$_2$SeNov 25 2011Jul 13 2012We report measurements of the surface Shubnikov de Haas oscillations (SdH) on crystals of the topological insulator Bi$_2$Te$_2$Se. In crystals with large bulk resistivity ($\sim$4 $\Omega$cm at 4 K), we observe $\sim$15 surface SdH oscillations (to the ... More
Effect of hydrostatic pressure on transport in the topological insulator Bi$_2$Te$_2$SeOct 05 2011The Hall coefficient $R_H$ and resistivity $\rho$ of the topological insulator Bi$_2$Te$_2$Se display a number of puzzling features vs. temperature $T$. We propose a model that describes well the non-monotonic variation of $R_H(T)$. A key feature of the ... More
beta-Cu3V2O8: Magnetic ordering in a spin-1/2 kagome-staircase latticeNov 25 2002The spin-1/2 Cu2+ ions in beta-Cu3V2O8 occupy the sites of a Kagome-staircase lattice, an anisotropic variant of the Kagome net: buckled layers and imbedded plaquettes of three edge-shared CuO4 squares break the ideal Kagome symmetry. Susceptibility and ... More
Magnetically tunable dielectric materialsApr 13 2009The coupling between localized spins and phonons can lead to shifts in the dielectric constant of insulating materials at magnetic ordering transitions. Studies on isostructural SeCuO3 (ferromagnetic) and TeCuO3 (antiferromagnetic) illustrate how the ... More
Paramagnetic to ferromagnetic phase transition in lightly Fe-doped Cr$_2$BJun 17 2014Cr$_2$B displays temperature independent paramagnetism. We induce ferromagnetism by replacing less than $3\,\%$ of the Cr atoms by Fe. By the lowest Fe doping level made, Curie-Weiss behavior is observed; $\Theta_{CW}$ changes from $-20\,$K for $0.5\,\%$ ... More
Spin entropy as the likely source of enhanced thermopower in $\rm\bf Na_xCo_2O_4Jun 04 2004In an electric field, the flow of electrons in a solid produces an entropy current in addition to the familiar charge current. This Peltier effect underlies all thermoelectric refrigerators. The upsurge in thermoelectric cooling applications has led to ... More
Local phonon mode in thermoelectric Bi2Te2Se from charge neutral antisitesMar 01 2016Local modes caused by defects play a significant role in the thermal transport properties of thermoelectrics. Of particular interest are charge-neutral defects that suppress thermal conductivity, without significantly reducing electrical transport. Here ... More
Influence of structural distortions on the Ir magnetism in Ba2-xSrxYIrO6 double perovskitesJan 06 2016We explore the relative strengths of spin orbit coupling and crystal field splitting in the Ir5+ compounds Ba2-xSrxYIrO6. In the case of strong spin orbit coupling and regular Ir5+ octahedra, one expects a nonmagnetic J = 0 state; in the case of distorted ... More
Intersite 4p-3d hybridization in cobalt oxides: a resonant x-ray emission spectroscopy studyFeb 20 2008We report on strong dipole transitions to 3d orbitals of neighboring Co atoms in the Co 1s x-ray absorption pre-edge. They are revealed by applying high-resolution resonant x-ray emission spectroscopy (RXES) to compounds containing CoO6 clusters. When ... More
Low-lying electronic structure of doped triangular cobaltatesMay 11 2006We report detailed Fermi surface topology and quasiparticle dynamics of the host cobaltate Na0.7CoO2. A direct mapping of the Fermi surface is carried out by angle-resolved photoemission spectroscopy (ARPES). Fermi surface at temperature 16K is a hole-pocket ... More
The Sensitivity of the Magnetic Properties of the ZnCr2O4 and MgCr2O4 Spinels to Non-stoichiometryJan 04 2011We report that small amounts of metal atom non-stoichiometry are possible in the ZnCr2O4 and MgCr2O4 spinels. The non-stoichiometry, though less than 2%, significantly impacts TN and the nature of the magnetic correlations above TN. The Zn1+xCr2-xO4 spinel ... More
Experimental tests of the chiral anomaly magnetoresistance in the Dirac-Weyl semimetals Na$_3$Bi and GdPtBiFeb 05 2018Jun 14 2018In the Dirac/Weyl semimetal, the chiral anomaly appears as an "axial" current arising from charge-pumping between the lowest (chiral) Landau levels of the Weyl nodes, when an electric field is applied parallel to a magnetic field $\bf B$. Evidence for ... More
Cd3As2 is CentrosymmetricDec 29 2013Mar 31 2014This is a revised version of a manuscript that was originally posted here in February of 2014. It has been accepted at the journal Inorganic Chemistry after reviews that included those of two crystallographers who made sure all the t's were crossed and ... More
Stuffed Rare Earth Pyrochlore Solid SolutionsMar 28 2006Synthesis and crystal structures are described for the compounds Ln2(Ti2-xLnx)O7-x/2, where Ln = Tb, Dy, Ho, Er, Tm, Yb, Lu, and x ranges from 0 to 0.67. Rietveld refinements on X-ray powder diffraction data indicate that in Tb and Dy titanate pyrochlores, ... More
Superconductivity in Mg10Ir19B16Aug 26 2006Mg10Ir19B16, 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 ... More
Consequences of breaking time reversal symmetry in LaSb: a resistivity plateau and extreme magnetoresistanceOct 23 2015Time reversal symmetry (TRS) protects the metallic surface modes of topological insulators (TIs). The transport signature of robust metallic surface modes of TIs is a plateau that arrests the exponential divergence of the insulating bulk with decreasing ... More
The Kagome-staircase lattice: Magnetic ordering in Ni3V2O8 and Co3V2O8Nov 25 2002Ni3V2O8 and Co3V2O8 have spin-1 and spin-3/2 magnetic lattices that are a new anisotropic variant of the Kagome net, wherein edge-sharing MO6 octahedra form the rises and rungs of a "Kagome staircase". The anisotropy largely relieves the geometric frustration, ... More
Superconductivity in three-layer Na0.3CoO2*1.3H2OJun 17 2004The 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. ... More
Spin ice behavior in Dy2Sn2-xSbxO7+x/2 and Dy2NbScO7Jul 26 2007We report the magnetic and thermal properties of Dy2Sn2-xSbxO7+x/2, x = 0, 0.25, and 0.5, and Dy2NbScO7. We find evidence for Ising-like single ion ground states in the Dy2Sn2-xSbxO7+x/2 materials. These materials possess nearly the same zero point entropy ... More
Structural Disorder, Octahedral Coordination, and 2-Dimensional Ferromagnetism in Anhydrous AlumsJul 11 2008The crystal structures of the triangular lattice, layered anhydrous alums KCr(SO4)2, RbCr(SO4)2 and KAl(SO4)2 are characterized by X-ray and neutron powder diffraction at temperatures between 1.4 and 773 K. The compounds all crystallize in the space group ... More
Sodium Ion Ordering in NaxCoO2Mar 08 2004The layered sodium cobalt oxide NaxCoO2 is studied by electron diffraction for a wide range of sodium contents, 0.15<x<0.75. An extensive series of ordered Na ion-Na vacancy superlattices is found beyond the simple hexagonal average structure. The most ... More
Why Does Undoped FeSe Become A High Tc Superconductor Under Pressure?Feb 23 2009Apr 03 2009Unlike the parent phases of the iron-arsenide high Tc superconductors, undoped FeSe is not magnetically ordered and exhibits superconductivity with Tc~9K. Equally surprising is the fact that applied pressure dramatically enhances the modest Tc to ~37K. ... More
Possible singlet to triplet pairing transition in NaxCoO2 H2OFeb 19 2004Oct 23 2004We 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) ... More