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D. J. Singh

Publications and source records attributed to D. J. Singh.

At least 37 records · Page 2Linked to original sources
arXiv2015

Electronic structure and weak itinerant magnetism in metallic Y2Ni7

We report a density functional study of the electronic structure and magnetism of Y2Ni7. The results show itinerant magnetism very similar to that in the weak itinerant ferromagnet Ni3Al. The electropositive Y atoms in Y2Ni7 serves to donate charge to the Ni host mostly in the form of $s$ electrons. The non-spin-polarized state shows a high density of states at the Fermi level, N(EF) due to flat bands. This leads to the ferromagnetic instability. However, there are also several much more dispersive bands crossing E(F), which should promote the conductivity. Spin fluctuation effects appear to be comparable to or weaker than Ni3Al, based on comparison with experimental data. Y2Ni7 provides a uniaxial analogue to cubic Ni3Al for studying weak itinerant ferromagnetism, suggesting detailed measurements of its low temperature physical properties and spin fluctuations, as well experiments under pressure.

cond-mat.mtrl-sci↗
arXiv2015

Synthesis of monoclinic IrTe2 under high pressure and its physical properties

In a pressure-temperature (P-T) diagram for synthesizing IrTe2 compounds, the well-studied trigonal (H) phase with the CdI2-type structure is stable at low pressures. The superconducting cubic (C) phase can be synthesized under higher temperatures and pressures. A rhombohedral phase with the crystal structure similar to the C phase can be made at ambient pressure; but the phase contains a high concentration of Ir deficiency. In this paper, we report that a rarely studied monoclinic (M) phase can be stabilized in narrow ranges of pressure and temperature in this P-T diagram. The peculiar crystal structure of the M-IrTe2 eliminates the tendency to form Ir-Ir dimers found in the H phase. The M phase has been fully characterized by structural determination and measurements of electrical resistivity, thermoelectric power, DC magnetization, and specific heat. These physical properties have been compared with those in the H and C phases of Ir1-xTe2. Moreover, magnetic and transport properties and specific heat of the M-IrTe2 can be fully justified by calculations with the density-functional theory presented in this paper.

cond-mat.mtrl-sci↗
arXiv2015

Magnetic order and electronic structure of 5d3 double perovskite Sr2ScOsO6

The magnetic susceptibility, crystal and magnetic structures, and electronic structure of double perovskite Sr2ScOsO6 are reported. Using both neutron and x-ray powder diffraction we find that the crystal structure is monoclinic P21/n from 3.5 to 300 K. Magnetization measurements indicate an antiferromagnetic transition at TN=92K, one of the highest transition temperatures of any double perovskite hosting only one magnetic ion. Type I antiferromagnetic order is determined by neutron powder diffraction, with an Os moment of only 1.6(1) muB, close to half the spin-only value for a crystal field split 5d electron state with t2g^3 ground state. Density functional calculations show that this reduction is largely the result of strong Os-O hybridization, with spin-orbit coupling responsible for only a ~0.1 muB reduction in the moment.

cond-mat.str-el↗
arXiv2015

Spectroscopic Evidence for Strong Quantum Spin Fluctuations with Itinerant Character in YFe2Ge2

We report x-ray absorption and photoemission spectroscopy of the electronic structure in the normal state of metallic YFe2Ge2. The data reveal evidence for large fluctuating spin moments on the Fe sites, as indicated by exchange multiplets appearing in the Fe 3s core level photoemission spectra, even though the compound does not show magnetic order. The magnitude of the multiplet splitting is comparable to that observed in the normal state of the Fe-pnictide superconductors. This shows a connection between YFe2Ge2 and the Fe-based superconductors even though it contains neither pnictogens nor chalcogens. The implication is that the chemical range of compounds showing at least one of the characteristic magnetic signatures of the Fe-based superconductors is broader than previously thought.

cond-mat.supr-con↗
arXiv2014

Multiband Te $p$ Based Superconductivity of Ta$_4$Pd$_3$Te$_{16}$

Ta$_4$Pd$_3$Te$_{16}$ is a recently discovered superconductor that has been suggested to be an unconventional superconductor near magnetism. We report electronic structure calculations showing that in spite of the layered crystal structure the material is an anisotropic three dimensional metal. The Fermi surface contains prominent one and two dimensional features, including nested 1D sheets, a 2D cylindrical section and a 3D sheet. The electronic states that make up the Fermi surface are mostly derived from Te $p$ states, with small Ta $d$ and Pd $d$ contributions. This places the compound far from magnetic instabilities. The results are discussed in terms of multiband superconductivity.

cond-mat.supr-con↗
arXiv2014

Complex magnetism and strong electronic correlations in Ce2PdGe3

Single crystal x-ray diffraction, magnetic susceptibility, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound Ce$_{2}$PdGe$_{3}$, which forms in the space group $P4_{2}/mmc$ (\#131) $-$ a relative of the $α$-ThSi$_2$-type structure. Measurements reveal a two-part antiferromagnetic phase transition at $T_{\rm{N,1}}$ $=$ 10.7 K and $T_{\rm{N,2}}$ $=$ 9.6 K and subsequent ferromagnetic ordering near $T_{\rm{C}}$ $\approx$ 2.25 K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat $γ$ $\approx$ 50 mJ/mol-Ce-K$^2$ (extrapolated to $T$ $=$ 0 for $T$ $<$ $T_{\rm{C}}$). Electronic structure calculations suggest that there is significant f-electron weight in the density of states near the Fermi energy and that the Fermi surface includes sheets with distinct nesting vectors. Chemical/structural disorder also plays an important role, as evidenced by results from single crystal x-ray diffraction, the width of the peaks in the heat capacity at $T_{\rm{N}}$ and $T_{\rm{C}}$, and the small residual resistivity ratio RRR $=$ $ρ_{\rm{300K}}$/$ρ_{\rm{0}}$ $=$ 1.8.

cond-mat.str-el↗
arXiv2014

2DEGs at perovskite interfaces between KTaO3 or KNbO3 and stannates

We report density functional studies of electron rich interfaces between KTaO3 or KNbO3 and CaSnO3 or ZnSnO3 and in particular the nature of the interfacial electron gasses that can be formed. We find that depending on the details these may occur on either the transition metal or stannate sides of the interface and in the later case can be shifted away from the interface by ferroelectricity. We also present calculations for bulk KNbO3, KTaO3, CaSnO3, BaSnO3 and ZnSnO3, showing the different transport and optical properties that may be expected on the two sides of such interfaces. The results suggest that these interfaces may display a wide range of behaviors depending on conditions, and in particular the interplay with ferroelectricity suggests that electrical control of these properties may be possible.

cond-mat.mtrl-sci↗
arXiv2013

Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase-space for three-phonon scattering processes, rather than just the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optical ferroelectric mode.

cond-mat.mtrl-sci↗
arXiv2012

Electronic structure, disconnected Fermi surfaces and antiferromagnetism in the layered pnictide superconductor Na$_x$Ba$_{1-x}$Ti$_2$Sb$_2$O

We report electronic structure calculations for BaTi$_2$Sb$_2$O and discuss the results in relation to the observed superconductivity of this material when hole doped with Na. The Fermi surface shows several sheets. These include a nested nearly 2D cylinder. There are also two sheets, which are three dimensional, in spite of the layered crystal structure. A magnetic instability associated with Fermi surface nesting is found. A sign-changing s-wave state, different from the one in the Fe-based superconductors, is predicted within a scenario of spin-fluctuation mediated superconductivity.

cond-mat.supr-con↗
arXiv2012

A direct probe of the variability of Coulomb correlation in Fe-pnictide superconductors

We use core-valence-valence (CVV) Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final state spectra to density functional theory calculations of the single particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping, x, in Ba(Fe1 xCox)2As2. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter "U" used in model and first principles calculations based on dynamical mean field theory, and establish an upper bound for its effective value. Our results impose stringent constraints on model based phase diagrams

cond-mat.supr-con↗
arXiv2012

Thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2

The thermoelectric transport properties of CaMg2Bi2, EuMg2Bi2, and YbMg2Bi2 were characterized between 2 and 650K. As synthesized, the polycrystalline samples are found to have lower p-type carrier concentrations than single-crystalline samples of the same empirical formula. These low carrier concentration samples possess the highest mobilities yet reported for materials with the CaAl2Si2 structure type, with a mobility of ~740cm$^2$/V/s observed in EuMg2Bi2 at 50K. Despite decreases in the Seebeck coefficient (α) and electrical resistivity (ρ) with increasing temperature, the power factor (α^2/ρ) increases for all temperatures examined. This behavior suggests a strong asymmetry in the conduction of electrons and holes. The highest figure of merit (zT) is observed in YbMg2Bi2, with zT approaching 0.4 at 600K for two samples with carrier densities of approximately 2x10^{18}cm^{-3} and 8x10^{18}cm^{-3} at room temperature. Refinements of neutron powder diffraction data yield similar behavior for the structures of CaMg2Bi2 and YbMg2Bi2, with smooth lattice expansion and relative expansion in $c$ being ~35% larger than relative expansion in $a$ at 973K. First principles calculations reveal an increasing band gap as Bi is replaced by Sb then As, and subsequent Boltzmann transport calculations predict an increase in αfor a given $n$ associated with an increased effective mass as the gap opens. The magnitude and temperature dependence of αsuggests higher zT is likely to be achieved at larger carrier concentrations, roughly an order of magnitude higher than those in the current polycrystalline samples, which is also expected from the detailed calculations.

cond-mat.mtrl-sci↗
arXiv2011

Comment on "Uncovering the Complex Behavior of Hydrogen in Cu2O"

In a recent Letter Scanlon and Watson reported their first principles results on hydrogen in Cu2O. Their main conclusions are: (1) an interstitial H in Cu2O prefers to occupy the tetrahedral site, which is coordinated with four Cu cations, in all three charge states (+1, neutral, and -1); (2) H will bind with a Cu vacancy and form an electrically active H-VCu defect complex, which is amphoteric with (+/0) and (0/-)transition levels at Ev + 0.1 and Ev + 1.1 eV, respectively. However, these two conclusions contradict two generally observed behaviors of H in oxides: (i) cationic H usually binds with an O atom, forming a single O-H bond, while the anionic H usually binds with cations with multi-coordination; (ii) H usually passivates cation vacancies in oxides. In this Comment, we explicitly show that with charge state +1, H prefers to bind with a single O anion rather than with four Cu cations and that H-VCu does not induce any defect levels inside the band gap.

cond-mat.mtrl-sci↗
arXiv2011

Quasi-one-dimensional magnons in an intermetallic marcasite

We present inelastic neutron scattering measurements and first principles calculations examining the intermetallic marcasite CrSb2. The observed spin wave dispersion implies that the magnetic interactions are strongly one-dimensional with antiferromagnetic chains parallel to the crystalline c-axis. Such low-dimensional excitations are unexpected in a semiconducting intermetallic system. Moreover this material may be further interesting in that the magnetic anisotropy may enhance thermoelectric properties along particular crystallographic directions.

cond-mat.str-el↗
arXiv2011

Spin Ice: Magnetic Excitations without Monopole Signatures using Muon Spin Rotation

Theory predicts the low-temperature magnetic excitations in spin ices consist of deconfined magnetic charges, or monopoles. A recent transverse-field (TF) muon spin rotation (muSR) experiment [S T Bramwell et al, Nature 461, 956 (2009)] reports results claiming to be consistent with the temperature and magnetic field dependence anticipated for monopole nucleation - the so-called second Wien effect. We demonstrate via a new series of muSR experiments in Dy_2Ti_2O_7 that such an effect is not observable in a TF muSR experiment. Rather, as found in many highly frustrated magnetic materials, we observe spin fluctuations which become temperature independent at low temperatures, behavior which dominates over any possible signature of thermally nucleated monopole excitations.

cond-mat.str-el↗
arXiv2011

Three dimensional magnetism and coupling to conduction electrons in PdCrO2

We report density functional calculations addressing the electronic structure and magnetic properties of delafossite PdCrO2. We find substantial magnetic interactions in the c-axis direction as well as beyond first nearest neighbors in-plane, so that PdCrO2 is a 3D frustrated antiferromagnet. We also find substantial coupling between the Cr moments and the Pd derived conduction electrons.

cond-mat.str-el↗
arXiv2011

Giant Anharmonic Phonon Scattering in PbTe

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic (TO) mode and the longitudinal acoustic (LA) modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying LA phonons. The LA-TO anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

cond-mat.mtrl-sci↗
arXiv2011

Fully gapped superconductivity in Ni-pnictide superconductors BaNi2As2 and SrNi2P2

We have performed low-temperature specific heat $C$ and thermal conductivity $κ$ measurements on the Ni-pnictide superconductors BaNi$_2$As$_2$ ($T_\mathrm{c}$=0.7 K and SrNi$_2$P$_2$ ($T_\mathrm{c}$=1.4 K). The temperature dependences $C(T)$ and $κ(T)$ of the two compounds are similar to the results of a number of s-wave superconductors. Furthermore, the concave field responses of the residual $κ$ for BaNi$_2$As$_2$ rules out the presence of nodes on the Fermi surfaces. We postulate that fully gapped superconductivity could be universal for Ni-pnictide superconductors. Specific heat data on Ba$_{0.6}$La$_{0.4}$Ni$_2$As$_2$ shows a mild suppression of $T_\mathrm{c}$ and $H_\mathrm{c2}$ relative to BaNi$_2$As$_2$.

cond-mat.supr-con↗