arXiv subjects

D. J. Singh

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

At least 73 records · Page 4Linked to original sources

arXiv2009

Spin-Phonon Coupling and Superconductivity in Fe Pnictides

We propose a theory of strong spin-lattice and spin-phonon coupling in iron pnictides and discuss its implications on superconductivity. Strong magneto-volume effect in iron compounds has long been known as the Invar effect. Fe pnictides also exhibit this effect, reflected in particular to the atomic volume of Fe defined by the nearest neighbor atoms. Through the phenomenological Landau theory, developed on the basis of the calculations by the density functional theory (DFT) and the experimental results, we quantify the strength of the spin-lattice interaction as it relates to the Stoner criterion for the onset of magnetism. We suggest that the coupling between electrons and phonons through the spin channel may be sufficiently strong to be an important part of the superconductivity mechanism in Fe pnictides.

cond-mat.supr-con↗

arXiv2009

Evidence for Three-Dimensionality in the Fermi Surface Topology of Layered Electron Doped Ba(Fe1-XCox)2As2 Iron Superconductors

The electronic structure of electron doped iron-arsenide superconductors Ba(Fe1- xCox)2As2 has been measured with Angle Resolved Photoemission Spectroscopy. The data reveal a marked photon energy dependence of points in momentum space where the bands cross the Fermi energy, a distinctive and direct signature of three-dimensionality in the Fermi surface topology. By providing a unique example of high temperature superconductivity hosted in layered compounds with three-dimensional electronic structure, these findings suggest that the iron-arsenides are unique materials, quite different from the cuprates high temperature superconductors.

cond-mat.supr-con↗

arXiv2009

Electronic structure of Ba(Fe,Ru)2As2 and Sr(Fe,Ir)2As2 alloys

The electronic structures of Ba(Fe,Ru)$_2$As$_2$ and Sr(Fe,Ir)$_2$As$_2$ are investigated using density functional calculations. We find that these systems behave as coherent alloys from the electronic structure point of view. In particular, the isoelectronic substitution of Fe by Ru does not provide doping, but rather suppresses the spin density wave characteristic of the pure Fe compound by a reduction in the Stoner enhancement and an increase in the band width due hybridization involving Ru. The electronic structure near the Fermi level otherwise remains quite similar to that of BaFe$_{2}$As$_{2}$. The behavior of the Ir alloy is similar, except that in this case there is additional electron doping.

cond-mat.supr-con↗

arXiv2009

Properties of KCo$_2$As$_2$ and Alloys with Fe and Ru: Density Functional Calculations

Electronic structure calculations are presented for KCo$_2$As$_2$ and alloys with KFe$_2$As$_2$ and KRu$_2$As$_2$. These materials show electronic structures characteristic of coherent alloys, with a similar Fermi surface structure to that of the Fe-based superconductors, when the $d$ electron count is near six per transition metal. However, they are less magnetic than the corresponding Fe compounds. These results are discussed in relation to superconductivity.

cond-mat.supr-con↗

arXiv2009

Low-temperature magneto-thermal transport investigation of a Ni-based superconductor BaNi2As2: Evidence for fully gapped superconductivity

We have performed low-temperature specific heat and thermal conductivity measurements of the Ni-based superconductor BaNi$_2$As$_2$ ($T_\mathrm{c}$ = 0.7 K) in magnetic field. In zero field, thermal conductivity shows $T$-linear behavior in the normal state and exhibits a BCS-like exponential decrease below $T_\mathrm{c}$. The field dependence of the residual thermal conductivity extrapolated to zero temperature is indicative of a fully gapped superconductor. This conclusion is supported by the analysis of the specific heat data, which are well fit by the BCS temperature dependence from $T_\mathrm{c}$ down to the lowest temperature of 0.1 K.

cond-mat.supr-con↗

arXiv2009

Itinerant antiferromagnetism in BaCr$_2$As$_2$

We report single crystal synthesis, specific heat and resistivity measurements and electronic structure calculations for BaCr$_2$As$_2$. This material is a metal with itinerant antiferromagnetism, similar to the parent phases of Fe-based high temperature superconductors, but differs in magnetic order. Comparison of bare band structure density of states and the low temperature specific heat implies a mass renormalization of $\sim$ 2. BaCr$_2$As$_2$ shows stronger transition metal - pnictogen covalency than the Fe compounds, and in this respect is more similar to BaMn$_2$As$_2$. This provides an explanation for the observation that Ni and Co doping is effective in the Fe-based superconductors, but Cr or Mn doping is not.

cond-mat.supr-con↗

arXiv2009

Electronic Structure of the BaFe$_2$As$_2$ Family of Iron Pnictides

We use high resolution angle-resolved photoemission spectroscopy to study the band structure and Fermi surface topology of the BaFe$_2$As$_2$ iron pnictides. We observe two electron bands and two hole bands near the X-point, $(π,π)$ of the Brillouin zone, in the paramagnetic state for different doping levels, including electron-doped Ba(Co$_{0.06}$Fe$_{0.94}$)$_2$As$_2$, undoped BaFe$_2$As$_2$, and hole-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$. Among these four bands, only the electron bands cross the Fermi level, forming two electron pockets around X, while the hole bands approach but never reach the Fermi level. We show that the band structure of the BaFe$_2$As$_2$ family matches reasonably well with the prediction of LDA calculations after a momentum-dependent shift and renormalization. Our finding resolves a number of inconsistencies regarding the electronic structure of pnictides.

cond-mat.supr-con↗

arXiv2009

ARPES studies of the electronic structure of LaOFe(P,As)

We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around (pi,pi). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds.

cond-mat.supr-con↗

arXiv2009

Electronic structure of BaCu$_2$As$_2$ and SrCu$_2$As$_2$: sp-band metals

The electronic structures of ThCr$_2$Si$_2$ structure BaCu$_2$As$_2$ and SrCu$_2$As$_2$ are investigated using density functional calculations. The Cu $d$ orbitals are located at 3 eV and higher binding energy, and are therefore chemically inert with little contribution near the Fermi energy. These materials are moderate density of states, sp-band metals with large Fermi surfaces and low anisotropy.

cond-mat.supr-con↗

arXiv2009

Density Functional Study of the Over-Doped Iron Chalcogenide: TlFe$_{2}$Se$_{2}$ with ThCr$_{2}$Si$_{2}$ structure

We report density functional calculations of electronic structure and magnetic properties of ternary iron chalcogenide TlFe$_{2}$Se$_{2}$, which occurs in the ThCr$_{2}$Si$_{2}$ structure and discuss the results in relation to the iron-based superconductors. The ground state is antiferromagnetic with checkerboard order and Fe moment $\sim$ 1.90 $μ$B. There is strong magnetoelastic coupling similar to the Fe-based superconductors, reflected in a sensitivity of the Se position to magnetism. Tl is monovalent in this compound, providing heavy electron-doping of 0.5 additional carriers per Fe relative to the parent compounds of the Fe-based superconductors. Other than the change in electron count, the electronic structure is rather similar to those materials. In particular, the Fermi surface is closely related to those of the Fe-based superconductors, except that the electron cylinders are larger, and the hole sections are suppressed. This removes the tendency towards a spin density wave.

cond-mat.supr-con↗

arXiv2009

Electronic Structure of Fe-Based Superconductors

The electronic structure of the Fe-based superconductors is discussed, mainly from the point of view of first principles calculations in relation to experimental data. Comparisons and contrasts with cuprates are made. The problem of reconciling experiments indicating an $s$ symmetry gap with experiments indicating line nodes is discussed and a possible resolution is given.

cond-mat.supr-con↗

arXiv2009

Electronic structure and Magnetism in BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$

We study the properties of ThCr$_2$Si$_2$ structure BaMn$_2$As$_2$ and BaMn$_2$Sb$_2$ using density functional calculations of the electronic and magnetic as well experimental measurements on single crystal samples of BaMn$_2$As$_2$. These materials are local moment magnets with moderate band gap antiferromagnetic semiconducting ground states. The electronic structures show substantial Mn - pnictogen hybridization, which stabilizes an intermediate spin configuration for the nominally $d^5$ Mn. The results are discussed in the context of possible thermoelectric applications and the relationship with the corresponding iron / cobalt / nickel compounds Ba(Fe,Co,Ni)$_2$As$_2$.

cond-mat.mtrl-sci↗

arXiv2008

Renormalized behavior and proximity to a magnetic quantum critical point in BaCo$_2$As$_2$

We report synthesis and single crystal measurements of magnetic, transport and thermal properties of single crystalline BaCo$_2$As$_2$ as well as first principles calculations of the electronic structure and magnetic behavior. These results show that BaCo$_2$As$_2$ is a highly renormalized paramagnet in proximity to a quantum critical point, presumably of ferromagnetic character and that BaFeNiAs$_2$ behaves similarly. These results are discussed in relation to the properties of Ba(Fe,Co)$_2$As$_2$ and Ba(Fe,Ni)$_2$As$_2$, which are superconducting for low Co and Ni concentrations.

cond-mat.supr-con↗

arXiv2008

Evidence for Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO(0.89)F(0.11) Iron-Oxypnictides

The electronic structure in the normal state of CeFeAsO0.89F0.11 oxypnictide superconductors has been investigated with x-ray absorption and photoemission spectroscopy. All the data exhibit signatures of Fe d-electron itinerancy. Exchange multiplets appearing in the Fe 3s core level indicate the presence of itinerant spin fluctuations. These findings suggest that the underlying physics and the origin of superconductivity in these materials are likely to be quite different from those of the cuprate high-temperature superconductors. These materials provide opportunities for elucidating the role of magnetic fluctuations in high-temperature superconductivity.

cond-mat.supr-con↗

arXiv2008

C-axis Phonons in Fe-As Based Superconductors Investigated by Inelastic X-ray Scattering

In Fe-As based superconductors magnetism and superconductivity show strong sensitivity to the lattice, suggesting a possibility of unconventional electron-phonon coupling. We investigated c-axis polarized phonons in doped and undoped BaFe2As2 by inelastic X-ray scattering. Phonon peak positions and linewidths did not vary significantly as a function of doping either by Co or by K. The linewidth of the Fe vibration shows unusual wavevector-dependence, which may be due to hybridization with in-plane modes. Comparison with the density functional theory shows significant difference for the energy of the As-As vibrations (Raman active at the zone center) but not for the Fe-As vibrations (infrared-active at the zone center). This behavior cannot be explained by a 10% softening of Fe-As interaction strength as proposed previously for in-plane polarized vibrations.

cond-mat.supr-con↗

arXiv2008

Possible Superconductivity in Fe-Sb Based Materials: Density Functional Study of LiFeSb

We investigate the electronic and other properties of the hypothetical compound LiFeSb in relation to superconducting LiFeAs and FeSe using density functional calculations. The results show that LiFeSb in the LiFeAs structure would be dynamically stable in the sense of having no unstable phonon modes, and would have very similar electronic and magnetic properties to the layered Fe based superconductors. Importantly, a very similar structure for the Fermi surface and a spin density wave related to but stronger than that in the corresponding As compound is found. These results are indicative of possible superconductivity analogous to the Fe-As based compounds if the spin density wave can be suppressed by doping or other means. Prospects for synthesizing this material in pure form or in solid solution with FeTe are discussed.

cond-mat.supr-con↗

arXiv2008

Density Functional Study of Excess Fe in Fe$_{1+x}$Te: Magnetism and Doping

The electronic and magnetic properties of the excess Fe in iron telluride Fe$_{(1+x)}$Te are investigated by density functional calculations. We find that the excess Fe occurs with valence near Fe$^{+}$, and therefore provides electron doping with approximately one carrier per Fe, and furthermore that the excess Fe is strongly magnetic. Thus it will provide local moments that interact with the plane Fe magnetism, and these are expected to persist in phases where the magnetism of the planes is destroyed for example by pressure or doping. These results are discussed in the context of superconductivity.

cond-mat.supr-con↗

arXiv2008

Electronic Structure, Magnetism and Superconductivity of Layered Iron Compounds

The layered iron superconductors are discussed using electronic structure calculations. The four families of compounds discovered so far, including Fe(Se,Te) have closely related electronic structures. The Fermi surface consists of disconnected hole and electron cylinders and additional hole sections that depend on the specific material. This places the materials in proximity to itinerant magnetism, both due to the high density of states and due to nesting. Comparison of density functional results and experiment provides strong evidence for itinerant spin fluctuations, which are discussed in relation to superconductivity. It is proposed that the intermediate phase between the structural transition and the SDW transition in the oxy-pnictides is a nematic phase.

cond-mat.supr-con↗