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D. J. Singh

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

At least 55 records · Page 3Linked to original sources
arXiv2010

Competing magnetic ground states in non-superconducting Ba(Fe1-xCrx)2As2

We present neutron diffraction measurements on single crystal samples of non-superconducting Ba(Fe1-xCrx)2As2 as a function of Cr-doping for 0<=x<=0.47. The average SDW moment is independent of concentration for x<=0.2 and decreases rapidly for x>=0.3. For concentrations in excess of 30% chromium, we find a new G-type antiferromagnetic phase which rapidly becomes the dominant magnetic ground state. Strong magnetism is observed for all concentrations measured and competition between these ordered states and superconductivity naturally explains the absence of superconductivity in the Cr-doped materials.

cond-mat.supr-con↗
arXiv2010

Fermi-surface of underdoped LaFeAsO1-xFx as determined by the Haas-van Alphen-effect

Here, we present a de Haas-van Alphen (dHvA) effect1 study on the newly discovered LaFeAsO1-xFx compounds2,3 in order to unveil the topography of the Fermi surface associated with their antiferromagnetic and superconducting phases, which is essential for understanding their magnetism, pairing symmetry and superconducting mechanism. Calculations 4 and surface-sensitive measurements 5,6,7 provided early guidance, but lead to contradictory results, generating a need for a direct experimental probe of their bulk Fermi surface. In antiferromagnetic LaFeAsO1-xFx 8,9 we observe a complex pattern in the Fourier spectrum of the oscillatory component superimposed onto the magnetic torque signal revealing a reconstructed Fermi surface, whose geometry is not fully described by band structure calculations. Surprisingly, several of the same frequencies, or Fermi surface cross-sectional areas, are also observed in superconducting LaFeAsO1-xFx (with a superconducting transition temperature Tc ~ 15 K). Although one could attribute this to inhomogeneous F doping, the corresponding effective masses are largely enhanced with respect to those of the antiferromagnetic compound. Instead, this implies the microscopic coexistence of superconductivity and antiferromagnetism on the same Fermi surface in the underdoped region of the phase diagram of the LaFeAsO1-xFx series. Thus, the dHvA-effect reveals a more complex Fermi surface topography than that predicted by band structure calculations4 upon which the currently proposed superconducting pairing scenarios10,11,12,13 are based, which could be at the origin of their higher Tcs when compared to their phosphide analogs.

cond-mat.supr-con↗
arXiv2010

Effect of $A$-site size difference on polar behavior in $M$BiScNbO$_6$,($M$$=$Na, K and Rb): Density functional calculations

We investigate the effect of $A$-site size differences in the double perovskites BiScO$_3$-$M$NbO$_3$ ($M$$=$Na, K and Rb) using first-principles calculations. We find that the polarization of these materials is 70$\sim$90 $μ$C/cm$^2$ along the rhombohedral direction. The main contribution to the high polarization comes from large off-centerings of Bi ions, which are strongly enhanced by the suppression of octahedral tilts as the $M$ ion size increases. A high Born effective charge of Nb also contributes to the polarization and this contribution is also enhanced by increasing the $M$ ion size.

cond-mat.mtrl-sci↗
arXiv2010

Electronic Structure of CeFeAsO1-xFx (x=0, 0.11/x=0.12) compounds

We report an extensive study on the intrinsic bulk electronic structure of the high-temperature superconductor CeFeAsO0.89F0.11 and its parent compound CeFeAsO by soft and hard x-ray photoemission, x-ray absorption and soft-x-ray emission spectroscopies. The complementary surface/bulk probing depth, and the elemental and chemical sensitivity of these techniques allows resolving the intrinsic electronic structure of each element and correlating it with the local structure, which has been probed by extended-x-ray absorption fine structure spectroscopy. The measurements indicate a predominant 4f1 (i.e. Ce3+) initial state configuration for Cerium and an effective valence-band-to-4f charge-transfer screening of the core hole. The spectra also reveal the presence of a small Ce f0 initial state configuration, which we assign to the occurrence of an intermediate valence state. The data reveal a reasonably good agreement with the partial density of states as obtained in standard density functional calculations over a large energy range. Implications for the electronic structure of these materials are discussed.

cond-mat.supr-con↗
arXiv2010

Comment on "Unconventional s-wave superconductivity in Fe(Se,Te)"

Hanaguri et al (Science, 8th of January 2010, p. 181) have reported STM measurements on an Fe-based superconductor Fe(Se,Te). After Fourier-transforming their data they found three sets of sharp peaks in the reciprocal space. They interpreted one as a Bragg peak (an umklapp scattering) and two as resulting from quasiparticle interference (QPI) We point out that at least one of these two peaks is also a Bragg peak, corresponding to another reciprocal lattice vector, and therefore is not a QPI peak, and the third peak has similar structure, too sharp for a QPI peak in a nodeless superconductor with an isotropic gap, proposed by Hanaguri et al. We argue that this peak must also be a Bragg peak related to a surface reconstruction doubling the unit cell. We also calculate the QPI pattern, assuming an isotropic gap, and find that indeed it is quite different from the experiment. Finally, we speculate about possible origin of the magnetic field dependence of the Bragg peaks in this compound.

cond-mat.supr-con↗
arXiv2010

Thermopower of SnTe from Boltzmann Transport Calculations

The doping and temperature dependent thermopower of SnTe is calculated from the first principles band structure using Boltzmann transport theory. We find that the $p$-type thermopower is inferior to PbTe consistent with experimental observations, but that the $n$-type thermopower is substantially more favorable.

cond-mat.mtrl-sci↗
arXiv2010

Orbital Symmetry of Ba(Fe1-xCox)2As2 Superconductors Probed with X-ray Absorption Spectroscopy

The orbital symmetries of electron doped iron-arsenide superconductors Ba(Fe1-xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density functional theory.

cond-mat.supr-con↗
arXiv2010

Enhanced Born Charge and Proximity to Ferroelectricity in Thallium Halides

Electronic structure and lattice dynamics calculations on thallium halides show that the Born effective charges in these compounds are more than twice larger than the nominal ionic charges. This is a result of cross-band-gap hybridization between Tl-p and halogen-p states. The large Born charges cause giant splitting between longitudinal and transverse optic phonon modes, bringing the lattice close to ferroelectric instability. Our calculations indeed show spontaneous lattice polarization upon lattice expansion starting at 2%. It is remarkable that the apparently ionic thallium halides with a simple cubic CsCl structure and large differences in electronegativity between cations and anions can be very close to ferroelectricity. This can lead to effective screening of defects and impurities that would otherwise be strong carrier traps and may therefore contribute to the relatively good carrier transport properties in TlBr radiation detectors.

cond-mat.mtrl-sci↗
arXiv2010

Phonons as a probe of the magnetic state in doped and undoped BaFe2As2

We measured phonon frequencies and linewidths in doped and undoped BaFe2As2 single crystals by inelastic x-ray scattering and compared our results with density functional theory (DFT) calculations. In agreement with previous work, the calculated frequencies of some phonons depended on whether the ground state was magnetic or not and, in the former case, whether phonon wavevector was parallel or perpendicular to the magnetic ordering wavevector. The experimental results agreed better with the magnetic calculation than with zero Fe moment calculations, except the peak splitting expected due to magnetic domain twinning was not observed. Furthermore, phonon frequencies were unaffected by the breakdown of the magnetic ground state due to either doping or increased temperature. Based on these results we propose that phonons strongly couple not to the static order, but to high frequency magnetic fluctuations.

cond-mat.supr-con↗
arXiv2010

Polar behavior of double perovskite (Bi,Pb)ZnNbO6 and (Bi,Sr)ZnNbO6: Density functional calculations

The polar behavior of double perovskite (Bi,Pb)ZnNbO$_6$ and (Bi,Sr)ZnNbO$_6$ was investigated using first principles density functional calculations within the local density approximation. These materials have both $A$-site size disorder and ions with sterochemical activity. We found a strong ferroelectric distortion in both materials. The polarization of the Pb material is $>$80 $μ$C/cm$^2$ along the rhombohedral direction and the Sr based material is only slightly inferior. The high polarization results mainly from a large off-centering of Bi and the large Born effective charge of Nb, which yields a large contribution polarization although the Nb off-centers by a smaller amount. Neither of these materials favors a tetragonal state, and therefore solid solutions with PbTiO$_3$ may show morphotoropic phase boundaries.

cond-mat.mtrl-sci↗
arXiv2009

Temperature and Pressure Dependence of the Fe-specific Phonon Density of States in Ba(Fe(1-x)Co(x))2As2

The {57}Fe-specific phonon density of states of Ba(Fe(1-x)Co(x))2As2 single crystals (x=0.0, 0.08) was measured at cryogenic temperatures and at high pressures with nuclear-resonant inelastic x-ray scattering. Measurements were conducted for two different orientations of the single crystals, yielding the orientation-projected {57}Fe-phonon density of states (DOS) for phonon polarizations in-plane and out-of-plane with respect to the basal plane of the crystal structure. In the tetragonal phase at 300 K, a clear stiffening was observed upon doping with Co. Increasing pressure to 4 GPa caused a marked increase of phonon frequencies, with the doped material still stiffer than the parent compound. Upon cooling, both the doped and undoped samples showed a stiffening, and the parent compound exhibited a discontinuity across the magnetic and structural phase transition. These findings are generally compatible with the changes in volume of the system upon doping, increasing pressure, or increasing temperature, but an extra softening of high-energy modes occurs with increasing temperature. First-principles computations of the phonon DOS were performed and showed an overall agreement with the experimental results, but underestimate the Grueneisen parameter. This discrepancy is explained in terms of a magnetic Grueneisen parameter, causing an extra phonon stiffening as magnetism is suppressed under pressure.

cond-mat.supr-con↗
arXiv2009

Electronic Structure, Localization and Spin-State Transition in Cu-substituted FeSe: Fe$_{1-x}$Cu$_x$Se

We report density functional studies of the Fe$_{1-x}$Cu$_x$Se alloy done using supercell and coherent potential approximation methods. Magnetic behavior was investigated using the disordered local moment approach. We find that Cu occurs in a nominal $d^{10}$ configuration and is highly disruptive to the electronic structure of the Fe sheets. This would be consistent with a metal insulator transition due to Anderson localization. We further find a strong cross over from a weak moment itinerant system to a local moment magnet at $x \approx 0.12$. We associate this with the experimentally observed jump near this concentration. Our results are consistent with the characterization of this concentration dependent jump as a transition to a spin-glass.

cond-mat.supr-con↗
arXiv2009

Unconventional electronic reconstruction in undoped (Ba,Sr)Fe$_2$As$_2$ across the spin density wave transition

Through a systematic high resolution angle-resolved photoemission study of the iron pnictide compounds (Ba,Sr)Fe$_2$As$_2$, we show that the electronic structures of these compounds are significantly reconstructed across the spin density wave ordering, which cannot be described by a simple folding scenario of conventional density wave ordering. Moreover, we find that LDA calculations with an incorporated suppressed magnetic moment of 0.5$μ_{\tiny{\textrm{B}}}$ can match well the details in the reconstructed electronic structure, suggesting that the nature of magnetism in the pnictides is more itinerant than local, while the origin of suppressed magnetic moment remains an important issue for future investigations.

cond-mat.supr-con↗
arXiv2009

Electronic correlations in the iron pnictides

In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's kinetic energy leading to remarkable experimental manifestations in optical spectroscopy. The high-Tc superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides puts a spotlight on the relevance of correlation effects in these materials. Here we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. Hallmarks of strong electronic many-body effects reported here are important because the iron pnictides expose a new pathway towards a correlated electron state that does not explicitly involve the Mott transition.

cond-mat.str-el↗
arXiv2009

Spin-Phonon Coupling in Iron Pnictide Superconductors

The magnetic moment in the parent phase of the iron-pnictide superconductors varies with composition even when the nominal charge of iron is unchanged. We propose the spin-lattice coupling due to the magneto-volume effect as the primary origin of this effect, and formulate a Landau theory to describe the dependence of the moment to the Fe-As layer separation. We then compare the superconductive critical temperature of doped iron pnictides to the local moment predicted by the theory, and suggest that the spin-phonon coupling may play a role in the superconductivity of this compound.

cond-mat.supr-con↗
arXiv2009

Electron-phonon superconductivity in non-centrosymmetric LaNiC$_2$: first principles calculations

We report first principles calculations of the electronic structure and electron-phonon coupling in the non-centrosymmetric superconductor LaNiC$_2$. These show that the material is a conventional electron-phonon superconductor with intermediate coupling. There are large contributions to the coupling by two low frequency C non-bond-stretching modes, one of which has strong Kohn anomalies. Since LaNiC$_2$ lacks inversion symmetry, the pairing is of dominant s-wave type with some mixture of p-wave character. This will give exponential decay in the specific heat.

cond-mat.supr-con↗