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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,621 records · Page 90
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Itinerant electrons, local moments, and magnetic correlations in the pnictide superconductors CeFeAsO₁₋ x F x and Sr(Fe₁₋ x Co x )₂As₂

A direct and element-specific measurement of the local Fe spin moment has been provided by analyzing the Fe 3s core level photoemission spectra in the parent and optimally doped CeFeAsO₁₋ x F x (x = 0, 0.11) and Sr(Fe₁₋ x Co x )2As2 (x = 0, 0.10) pnictides. The rapid time scales of the photoemission process allowed the detection of large local spin moments fluctuating on a 10⁻¹⁵ s time scale in the paramagnetic, antiferromagnetic, and superconducting phases, indicative of the occurrence of ubiquitous strong Hund's magnetic correlations. The magnitude of the spin moment is found to vary significantly among different families, 1.3μ B in CeFeAsO and 2.1μB in SrFe₂As₂. Surprisingly, the spin moment is found to decrease considerably in the optimally doped samples, 0.9μ B in CeFeAsO₀.₈₉F₀.₁₁ and 1.3μ B in Sr(Fe₀.₉Co₀.₁)₂As₂. The strong variation of the spin moment against doping and material type indicates that the spin moments and the motion of itinerant electrons are influenced reciprocally in a self-consistent fashion, reflecting the strong competition between the antiferromagnetic superexchange interaction among the spin moments and the kinetic energy gain of the itinerant electrons in the presence of a strong Hund's coupling. By describing the evolution of the magnetic correlations concomitant with the appearance of superconductivity, these results constitute a fundamental step toward attaining a correct description of the microscopic mechanisms shaping the electronic properties in the pnictides, including magnetism and high-temperature superconductivity.

Vilmercati, Paolo↗
DOE OSTI2012

Ab Initio Molecular Dynamics Simulations of Low-Energy Recoil Events in ThO2, CeO2, and ZrO2

Ab initio molecular dynamics simulations of low-energy recoil events in ThO2, CeO2, and ZrO2 have been carried out to determine the threshold displacement energies, resulting defect configurations, dynamics of defect generation, and role of charge transfer during the process. The results reveal that, in most cases, these fluorite structure oxides exhibit a similar response to low-energy recoils. A variety of different defect configurations are created, consisting mainly of vacancies and interstitials. Charge transfer occurs during the dynamic displacement process. Local charge redistribution leads to cation and O vacancies being negatively and positively charged, respectively. Likewise, due to charge redistribution, the cation and O interstitials are less positively and negatively charged, respectively, than the ions on lattice sites in perfect MO2.

Environmental Molecular Sciences Laboratory↗
DOE OSTI2015

Layer-dependent electronic structure of an atomically heavy two-dimensional dichalcogenide

Here we report angle-resolved photoemission spectroscopic measurements of the evolution of the thickness-dependent electronic band structure of the atomically heavy two-dimensional layered dichalcogenide, tungsten diselenide ( WS e 2 ) . Our data, taken on mechanically exfoliated WS e 2 single crystals, provide direct evidence for shifting of the valence-band maximum from Γ ¯ (multilayer WS e 2 ) to K ¯ (single-layer WS e 2 ). Further, our measurements also set a lower bound on the energy of the direct band gap and provide direct measurement of the hole effective mass.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Antiferromagnetic and nematic phase transitions in BaF e 2 ( A s 1 - x P x ) 2 studied by ac microcalorimetry and SQUID magnetometry

In this work, we study the antiferromagnetic (AFM) and structural phase transitions in single-crystal BaF e 2 ( A s 1 – x P x ) 2 ( x = 0 , 0.3 ) at temperatures T N and T S , respectively, by high-resolution ac microcalorimetry and SQUID magnetometry. The specific heat measurements of both as-grown and annealed BaF e 2 A s 2 display a sharp peak at the AFM-structural transitions. A kink in the entropy of annealed BaF e 2 A s 2 gives evidence for splitting of the two transitions by approximately 0.5 K. No additional features could be identified in the specific heat of both BaF e 2 A s 2 and BaF e 2 ( A s 0.7 P 0.3 ) 2 in the temperature regions around T * > T S where torque measurements had revealed the “true” nematic phase transition, even though the Ginzburg-Landau model used to fit the magnetic torque data indicates that the expected thermal anomaly should be easily observable with our experimental resolution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of disorder on superconductivity in the presence of spin-density wave order

The majority of unconventional superconductors has close proximity to a magnetic phase. In many cases, the magnetic phase coexists with superconductivity in some fraction of the phase diagram. The response of these two competing phases to disorder can be used as a tool to gain a better understanding of these complex systems. Here I consider the effect of disorder on a multiband superconductor appropriate for the ferro-pnictide superconductors. I consider both interband and intraband scattering for a two-band model consisting of a hole pocket and an electron pocket. The scattering from pointlike impurities is treated within the self-consistent Born approximation. I calculate the effect of disorder on the transition temperature to the superconducting state. Finally, the influence of impurity scattering on the low-energy excitation spectrum in the superconducting state is also studied for different kinds of gap structures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Spin glass in semiconducting KFe 1.05 Ag 0.88 Te 2 single crystals

We report discovery of KFe 1.05 Ag 0.88 Te 2 single crystals with semiconducting spin glass ground state. Composition and structure analysis suggest nearly stoichiometric I4/mmm space group but allow for the existence of vacancies, absent in long range semiconducting antiferromagnet KFe 1.05 Ag 0.88 Te 2 . The subtle change in stoichometry in Fe/Ag sublattice changes magnetic ground state but not conductivity, giving further insight into the semiconducting gap mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Sr 2 Ir 1 - x Rh x O 4 ( x < 0.5 ) : An inhomogeneous j eff = 1 2 Hubbard system

In a combined experimental and theoretical study, we investigate the properties of Sr 2 Ir 1 - x Rh x O 4 . From the branching ratios of the L -edge isotropic x-ray absorption spectra, we determine that the spin-orbit coupling is remarkably independent of x for both iridium and rhodium sites. DFT + U calculations show that the doping is close to isoelectronic and introduces impurity bands of predominantly rhodium character close to the lower Hubbard band. Overlap of these two bands leads to metallic behavior. Since the low-energy states for x < 0.5 have predominantly j eff = 1 /2 character, we suggest that the electronic properties of this material can be described by an inhomogeneous Hubbard model, where the on-site energies change due to local variations in the spin-orbit interaction strength combined with additional changes in binding energy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Mott localization in a pure stripe antiferromagnet Rb 1 - δ Fe 1.5 - σ S 2

A combination of neutron diffraction and angle-resolved photoemission spectroscopy measurements on a pure antiferromagnetic stripe ${\mathrm{Rb}}_{1{-}{\delta}}{\mathrm{Fe}}_{1.5{-}{\sigma}}{\mathrm{S}}_{2}$ is reported here. A neutron diffraction experiment on a powder sample shows that a $98$% volume fraction of the sample is in the antiferromagnetic stripe phase with rhombic iron vacancy order and a refined composition of ${\mathrm{Rb}}_{0.66}{\mathrm{Fe}}_{1.36}{\mathrm{S}}_{2}$, and that only $2$% of the sample is in the block antiferromagnetic phase with $\sqrt{5}\times{}\sqrt{5}$ iron vacancy order. Furthermore, a neutron diffraction experiment on a single crystal shows that there is only a single phase with the stripe antiferromagnetic order with the refined composition of ${\mathrm{Rb}}_{0.78}{\mathrm{Fe}}_{1.35}{\mathrm{S}}_{2}$, while the phase with block antiferromagnetic order is absent. Angle-resolved photoemission spectroscopy measurements on the same crystal with the pure stripe phase reveal that the electronic structure is gapped at the Fermi level with a gap larger than 0.325 eV. The data collectively demonstrate that the extra $10$% iron vacancies in addition to the rhombic iron vacancy order effectively impede the formation of the block antiferromagnetic phase; the data also suggest that the stripe antiferromagnetic phase with rhombic iron vacancy order is a Mott insulator.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Magnetic field stabilized electron-hole liquid in indirect-band-gap A l x G a 1 - x As

An electron-hole liquid (EHL), a condensed liquidlike phase of free electrons and holes in a semiconductor, presents a unique system for exploring quantum many-body phenomena. And while the behavior of EHLs is generally understood, less attention has been devoted to systematically varying the onset of their formation and resulting properties. Here, we report on an experimental approach to tune the conditions of formation and characteristics using a combination of low excitation densities and high magnetic fields up to 90 T. Demonstration of this approach was carried out in indirect-band-gap A l 0.387 G a 0.613 As . EHL droplets can be nucleated from one of two multiexciton complex states depending on the applied excitation density. Furthermore, the excitation density influences the carrier density of the EHL at high magnetic fields, where filling of successive Landau levels can be controlled. The ability to manipulate the formation pathway, temperature, and carrier density of the EHL phase under otherwise fixed experimental conditions makes our approach a powerful tool for studying condensed carrier phases in further detail.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Photoinduced ultrafast charge-order melting: Charge-order inversion and nonthermal effects

The effect of photoexcitation is studied for a system with checkerboard charge order induced by displacements of ligands around a metal site. The motion of the ligands is treated classically and the electronic charges are simplified to two-level molecular bond charges. The calculations are done for a checkerboard charge-ordered system with about 100 000 ligand oscillators coupled to a fixed-temperature bath. The initial photoexcitation is followed by a rapid decrease in the charge-order parameter within 50–100 femtoseconds while leaving the correlation length almost unchanged. Depending on the fluence, a complete melting of the charge order occurs in less than a picosecond. While for low fluences, the system returns to its original state, for full melting, it recovers to its broken-symmetry state leading to an inversion of the charge order. For small long-range interactions, recovery can be slow due to domain formation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ultrafast terahertz-field-driven ionic response in ferroelectric BaTiO 3

The dynamical processes associated with electric field manipulation of the polarization in a ferroelectric remain largely unknown but fundamentally determine the speed and functionality of ferroelectric materials and devices. Here in this paper we apply subpicosecond duration, single-cycle terahertz pulses as an ultrafast electric field bias to prototypical BaTiO 3 ferroelectric thin films with the atomic-scale response probed by femtosecond x-ray-scattering techniques. We show that electric fields applied perpendicular to the ferroelectric polarization drive large-amplitude displacements of the titanium atoms along the ferroelectric polarization axis, comparable to that of the built-in displacements associated with the intrinsic polarization and incoherent across unit cells. This effect is associated with a dynamic rotation of the ferroelectric polarization switching on and then off on picosecond time scales. These transient polarization modulations are followed by long-lived vibrational heating effects driven by resonant excitation of the ferroelectric soft mode, as reflected in changes in the c-axis tetragonality. The ultrafast structural characterization described here enables a direct comparison with first-principles-based molecular-dynamics simulations, with good agreement obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tuning the electronic and the crystalline structure of LaBi by pressure: From extreme magnetoresistance to superconductivity

Here, extreme magnetoresistance (XMR) in topological semimetals is a recent discovery which attracts attention due to its robust appearance in a growing number of materials. To search for a relation between XMR and superconductivity, we study the effect of pressure on LaBi. By increasing pressure, we observe the disappearance of XMR followed by the appearance of superconductivity at P ≈ 3.5 GPa. We find a region of coexistence between superconductivity and XMR in LaBi in contrast to other superconducting XMR materials. The suppression of XMR is correlated with increasing zero-field resistance instead of decreasing in-field resistance. At higher pressures, P ≈ 11 GPa, we find a structural transition from the face-centered cubic lattice to a primitive tetragonal lattice, in agreement with theoretical predictions. The relationship between extreme magnetoresistance, superconductivity, and structural transition in LaBi is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Multiple superconducting states induced by pressure in Mo 3 Sb 7

Tuning competing ordering mechanisms with hydrostatic pressure in the 4 d intermetallic compound Mo 3 Sb 7 reveals an intricate interplay of structure, magnetism, and superconductivity. Synchrotron x-ray diffraction and magnetic susceptibility measurements, both employing diamond anvil cell technologies, link a first-order structural phase transition to a doubling of the superconducting transition temperature. In contrast to the spin-dimer picture for Mo 3 Sb 7 , we deduce from x-ray absorption near-edge structure and dc magnetization measurements at ambient pressure that Mo 3 Sb 7 should possess only very small, itinerant magnetic moments. As a result, the pressure evolution of the superconducting transition temperature strongly suggests its enhancement is due to a difference in the phonon density-of-states with changed crystal symmetry.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗