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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 1,639 records · Page 91
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Thickness dependence of magnetotransport properties of tungsten ditelluride

Here we investigate the electronic structure of tungsten ditelluride (WTe 2 ) flakes with different thicknesses in magnetotransport studies. The temperature-dependent resistance and magnetoresistance (MR) measurements both confirm the breaking of carrier balance induced by thickness reduction, which suppresses the “turn-on” behavior and large positive MR. The Shubnikov-de-Haas oscillation studies further confirm the thickness-dependent change of electronic structure of WTe 2 and reveal a possible temperature-sensitive electronic structure change. Finally, we report the thickness-dependent anisotropy of the Fermi surface, which reveals that multilayer WTe 2 is an electronic 3D material and the anisotropy decreases as thickness decreases.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Efficient time-dependent orbital-free density functional theory: Semilocal adiabatic response

Orbital-free density functional theory and its time-dependent extension are efficient ab initio methods for calculating the electronic structure and dynamics of large systems. Through the calculation of the optical spectra of selected clusters, we reach three important conclusions: (1) The quality of the spectra is strongly affected by the quality of the corresponding ground-state electron density; (2) the adiabatic part of the electronic response to external perturbations can be safely evaluated at the semilocal level; and (3) the nonadiabatic, current-dependent part of the time-dependent Pauli potential is key to recover correct spectral envelopes.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Field orientation dependent magnetic phases in the Weyl semimetal Co 3 ⁢Sn 2 ⁢S 2

Magnetism plays a key role in the emergence of topological phenomena in the Weyl semimetal Co 3 ⁢Sn 2 ⁢S 2 , which exhibits ferromagnetic interactions along the c-axis of the crystal and antiferromagnetic (AFM) interactions within the ab plane. Extensive studies on the temperature dependence of the magnetism with the magnetic field along the c-axis have uncovered a number of magnetic phases. Currently, the nature and origins of the reported magnetic phases are under debate. Here, in this work, we report on magnetic field orientation effects on the magnetism in Co 3 ⁢Sn 2 ⁢S 2 . The shape of the hysteresis loop of the Hall resistance at a fixed temperature is found to change from rectangular to bow tie-like as the magnetic field is tilted from the c-axis toward the ab plane, resembling that reported for magnetic fields along the c-axis as the temperature approaches the Curie temperature from below. Unlike their temperature-dependent counterparts, the newly observed bow tie-like hysteresis loops show exchange bias. Our results showcase the contribution of the in-plane AFM interactions to the magnetism in Co 3 ⁢Sn 2⁢ S 2 and demonstrate a new way to tune its magnetic phases. They also shed light on the temperature-dependent magnetic phases occurring in the magnetic field along the c-axis of the crystal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Physically motivated analytical expression for the temperature dependence of the zero-field splitting of the nitrogen-vacancy center in diamond

The temperature dependence of the zero-field splitting (ZFS) between the | $m_s$ = 0 $\rangle$ and | $m_s$ = ±1 $\rangle$ levels of the nitrogen-vacancy (NV) center's electronic ground-state spin triplet can be used as a robust nanoscale thermometer in a broad range of environments. However, despite numerous measurements of this dependence in different temperature ranges, to our knowledge no analytical expression has been put forward that captures the scaling of the ZFS of the NV center across all relevant temperatures. Here we present a simple, analytical, and physically motivated expression for the temperature dependence of the NV center's ZFS that matches all experimental observations, in which the ZFS shifts in proportion to the occupation numbers of two representative phonon modes. In contrast to prior models our expression does not diverge outside the regions of fitting. In conclusion, we show that our model quantitatively matches experimental measurements of the ZFS from 15 to 500 K in single NV centers in ultrapure bulk diamond, and we compare our model and measurements to prior models and experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Coupling-dependent antiferromagnetic-ferromagnetic ordering in a pinwheel artificial spin ice

Nanopatterned magnetic thin films offer a platform for exploration of tailored magnetic properties such as emergent long-range order. A prominent example is artificial spin ice (ASI), where an arrangement of nanoscale magnetic elements, acting as macrospins, interact via their dipolar fields. In this study, we discuss the transition from antiferromagnetic (AFM) to ferromagnetic (FM) long-range order in a square lattice ASI as the magnetic elements are gradually rotated through 45⁢° to a “pinwheel” configuration. The AFM-FM transition is observed experimentally using synchrotron radiation x-ray spectromicroscopy and occurs for a certain rotation angle of the nanomagnets, dependent on the dipolar coupling strength determined by the separation of the magnets in the lattice. Large-scale magnetic dipole simulations show that the point-dipole approximation fails to capture the correct AFM-FM transition angle. However, excellent agreement with experimental data is obtained using a dumbbell-dipole model, which better reflects the actual dipolar fields of the magnets. This model also explains the coupling dependence of the transition angle, another feature not captured by the point-dipole model. Our findings resolve a discrepancy between measurement and theory in previous work on “pinwheel” ASIs and establish the coupling dependence of the AFM-FM transition. The revised dipole model, with a more accurate representation of the stray field, offers more precise control of magnetic order in artificial spin systems.

Artificial spin ice↗
DOE OSTI2010

Slow heavy dimers in a two-dimensional degenerate electron gas: Calculation of orientation-dependent retarding forces

We present a theoretical study on the orientation-dependent retarding force experienced by slow dimers moving at arbitrary alignment with the direction of their velocity in a two dimensional degenerate electron gas. The influence of the individual constituents of a dimer on independent electrons is modeled by effective, s-type phase shifts {eta}. Analytical results are derived by considering the effects of interference and multiple scattering. The orientation-dependent expressions reveal the dependence of stopping power on the classical geometry and these effects of quantum dynamics. A brief account on strongly related problems of resistivity caused by imperfections in the many-body system and pure dephasing of elastically scattered electrons is made, as well. The physically motivated case of {eta}(k{sub F})={+-}({pi}/2) for the leading phase shift is explicitly discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Exotic paired phases in ladders with spin-dependent hopping

Fermions in two dimensions, when subject to anisotropic spin-dependent hopping, can potentially give rise to unusual paired states in unpolarized mixtures that can behave as non-Fermi liquids. One possibility is a fully paired state with a gap for fermion excitations in which the Cooper pairs remain uncondensed. Such a ''Cooper-pair Bose-metal'' phase would be expected to have a singular Bose surface in momentum space. As demonstrated in the context of two-dimensional bosons hopping with a frustrating ring-exchange interaction, an analogous Bose-metal phase has a set of quasi-one-dimensional descendant states when put on a ladder geometry. Here we present a density matrix renormalization group study of the attractive Hubbard model with spin-dependent hopping on a two-leg ladder geometry. In our setup, one spin species moves preferentially along the leg direction, while the other does so along the rung direction. We find compelling evidence for the existence of a novel Cooper-pair Bose-metal phase in a region of the phase diagram at intermediate coupling. We further explore the phase diagram of this model as a function of hopping anisotropy, density, and interaction strength, finding a conventional superfluid phase as well as a phase of paired Cooper pairs with d-wave symmetry, similar to the one found in models of hard-core bosons with ring exchange. We argue that simulating this model with cold Fermi gases on spin-dependent optical lattices is a promising direction for realizing exotic quantum states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2011

Systematics of the temperature-dependent interplane resistivity in Ba(Fe 1-x M x )₂As₂ (M=Co, Rh, Ni, and Pd)

Temperature-dependent interplane resistivity ρ c (T) was measured systematically as a function of transition-metal substitution in the iron-arsenide superconductors Ba(Fe 1-x M x )₂As₂, M=Ni, Pd, Rh. The data are compared with the behavior found in Ba(Fe 1-x Co x )₂As₂, revealing resistive signatures of pseudogap. In all compounds we find resistivity crossover at a characteristic pseudogap temperature T* from nonmetallic to metallic temperature dependence on cooling. Suppression of T* proceeds very similarly in cases of Ni and Pd doping and much faster than in similar cases of Co and Rh doping. In cases of Co and Rh doping an additional minimum in the temperature-dependent ρ c emerges for high dopings, when superconductivity is completely suppressed. These features are consistent with the existence of a charge gap covering part of the Fermi surface. The part of the Fermi surface affected by this gap is notably larger for Ni- and Pd-doped compositions than in Co- and Rh-doped compounds.

Tanatar, M. A.↗
DOE OSTI2011

Thickness Dependence of Magnetic Relaxation and E-J Characteristics in Superconducting (Gd-Y)-Ba-Cu-O Films with Strong Vortex Pinning

The dependence of the critical current density Jc on temperature, magnetic field, and film thickness has been investigated in (Gd-Y)BaCu-oxide materials of 0.7, 1.4, and 2.8 m thickness. Generally, the Jc decreases with film thickness at investigated temperatures and magnetic fields. The nature and strength of the pinning centers for vortices have been identified through angular and temperature measurements, respectively. These films do not exhibit c-axis correlated vortex pinning, but do have correlated defects oriented near the ab-planes. For all film thicknesses studied, strong pinning dominates at most temperatures. The vortex dynamics were investigated through magnetic relaxation studies in the temperature range of 5 77 K in 1 T and 3 T applied magnetic fields, H || surface-normal. The creep rate S is thickness dependent at high temperatures, implying that the pinning energy is also thickness dependent. Maley analyses of the relaxation data show an inverse power law variation for the effective pinning energy Ueff ~ (J0/J) . Finally, the electric field-current density (E-J) characteristics were determined over a wide range of dissipation by combining experimental results from transport, swept field magnetometry (VSM), and Superconducting Quantum Interference Device (SQUID) magnetometry. We develop a self-consistent model of the combined experimental results, leading to an estimation of the critical current density Jc0(T) in the absence of flux creep.

42 ENGINEERING↗
DOE OSTI2013

CoFe alloy as middle layer for strong spin dependent quantum well resonant tunneling in double barrier magnetic tunnel junctions

We report the spin-dependent quantum well resonant tunneling effect in CoFe/MgO/CoFe/MgO/CoFeB (CoFe) double barrier magnetic tunnel junctions. The dI/dV spectra reveal clear resonant peaks for the parallel magnetization configurations, which can be matched to quantum well resonances obtained from calculation. The differential TMR exhibits an oscillatory behavior with a sign change due to the formation of the spin-dependent QW states in the middle CoFe layer. Also, we observe pronounced TMR enhancement at resonant voltages at room temperature, suggesting that it is very promising to achieve high TMR using the spin-dependent QW resonant tunneling effect.

Liu, R. S.↗
DOE OSTI2014

Comprehensive scenario for single-crystal growth and doping dependence of resistivity and anisotropic upper critical fields in (Ba1-xKx)Fe2As2 (0.22 <= X <= 1)

Large high-quality single crystals of hole-doped iron-based superconductor (Ba1-xKx)Fe2As2 were grown over a broad composition range 0.22 <= x <= 1 by inverted temperature gradient method. We found that high soaking temperature, fast cooling rate, and an adjusted temperature window of the growth are necessary to obtain single crystals of heavily K-doped crystals (0.65 <= x <= 0.92) with narrow compositional distributions as revealed by sharp superconducting transitions in magnetization measurements and close to 100% superconducting volume fraction. The crystals were extensively characterized by x-ray and compositional analysis, revealing monotonic evolution of the c-axis crystal lattice parameter with K substitution. Quantitative measurements of the temperature-dependent in-plane resistivity rho(T) found doping-independent, constant within error bars, resistivity at room temperature, rho(300 K), in sharp contrast with the significant doping dependence in electron and isovalent substituted BaFe2As2 based compositions. The shape of the temperature-dependent resistivity, rho(T), shows systematic doping-evolution, being close to T-2 in overdoped and revealing significant contribution of the T-linear component at optimum doping. The slope of the upper critical field, d H-c2/dT, scales linearly with T-c for both H parallel to c, H-c2,(c), and H parallel to ab, H-c2,H-ab. The anisotropy of the upper critical field. equivalent to Hc(2,ab)/H-c2,H-c determined near zero-field T-c increases from similar to 2 to 4-5 with increasing K doping level from optimal x similar to 0.4 to strongly overdoped x = 1.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Frequency-dependent dielectric function of semiconductors with application to physisorption

Furthermore, the dielectric function is one of the most important quantities that describes the electrical and optical properties of solids. Accurate modeling of the frequency-dependent dielectric function has great significance in the study of the long-range van der Waals (vdW) interaction for solids and adsorption. In this work we calculate the frequency-dependent dielectric functions of semiconductors and insulators using the GW method with and without exciton effects, as well as efficient semilocal density functional theory (DFT), and compare these calculations with a model frequency-dependent dielectric function. We find that for semiconductors with moderate band gaps, the model dielectric functions, GW values, and DFT calculations all agree well with each other. However, for insulators with strong exciton effects, the model dielectric functions have a better agreement with accurate GW values than the DFT calculations, particularly in high-frequency region. To understand this, we repeat the DFT calculations with scissors correction, by shifting the DFT Kohn-Sham energy levels to match the experimental band gap. We find that scissors correction only moderately improves the DFT dielectric function in the low-frequency region. Based on the dielectric functions calculated with different methods, we make a comparative study by applying these dielectric functions to calculate the vdW coefficients (C3 and C5) for adsorption of rare-gas atoms on a variety of surfaces. We find that the vdW coefficients obtained with the nearly free electron gas-based model dielectric function agree quite well with those obtained from the GW dielectric function, in particular for adsorption on semiconductors, leading to an overall error of less than 7% for C 3 and 5% for C 5 . This demonstrates the reliability of the model dielectric function for the study of physisorption.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Time-dependent nonequilibrium soft x-ray response during a spin crossover

Here, the rapid development of high-brilliance pulsed X-ray sources with femtosecond time resolution has created a need for a better theoretical understanding of the time-dependent soft-X-ray response of dissipative many-body quantum systems. It is demonstrated how soft-X-ray spectroscopies, such as X-ray absorption and resonant inelastic X-ray scattering at transition-metal L-edges, can provide insight into intersystem crossings, such as a spin crossover. The photoinduced doublet-to-quartet spin crossover on cobalt in Fe-Co Prussian blue analogues is used as an example to demonstrate how the X-ray response is affected by the dissipative nonequilibrium dynamics. The time-dependent soft-X-ray spectra provide a wealth of information that reflect the changes in the nonequilibrium initial state via continuously changing spectral lineshapes that cannot be decomposed into initial photoexcited and final metastable spectra, strong broadenings, a collapse of clear selection rules during the intersystem crossing, strong fluctuations in the isotropic branching ratio in X-ray absorption, and crystal-field collapse/oscillations and strongly time-dependent anti-Stokes processes in RIXS.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Magnetic field dependence of nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr 1 / 3 Nb S 2

We present a comprehensive study of the magnetization dynamics and phase evolution in the chiral helimagnet Cr 1/3 NbS 2 , which realizes a chiral soliton lattice (CSL). The magnetic field dependence of the ac magnetic response is analyzed for the first five harmonic components, M nω (H ) (n = 1–5), using a phase sensitive measurement over a frequency range, f = 11–10 000 Hz. At a critical field, the modulated CSL continuously evolves from a helicity-rich to a ferromagnetic domain-rich structure, where the crossover is revealed by the onset of an anomalous nonlinear magnetic response that coincides with extremely slow dynamics. The behavior is indicative of the formation of a spatially coherent array of large ferromagnetic domains, which relax on macroscopic time scales. The frequency dependence of the ac magnetic loss displays an asymmetric distribution of relaxation times across the highly nonlinear CSL regime, which shift to shorter time scales with increasing temperature. We experimentally resolve the tricritical point at T TCP in a temperature regime above the ferromagnetic Curie temperature which separates the linear and nonlinear magnetic regimes of the CSL at the phase transition. A comprehensive phase diagram is constructed which summarizes the features of the field and temperature dependence of the magnetic crossovers and phase transitions in Cr 1/3 NbS 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2019

Doping dependence of the magnitude of fluctuating spin moments in the normal state of the pnictide superconductor Sr ( F e 1 - x C o x ) 2 A s 2 inferred from photoemission spectroscopy

We report systematic temperature- and doping-dependent measurements of the Fe3s core-level photoemission spectra in the normal state of superconducting Sr(Fe 1- x Co x ) 2 As 2 . The analysis of the Fe3s spectrum provides an element-specific determination of the mean value of the magnitude of the Fe spin moment measured on the fast (10 -16 -10 -15 s) timescale of the photoemission process. The data reveal the ubiquitous presence in the normal state of Fe spin moments with magnitude fluctuating on short timescales. The data reveal a significant reduction of the magnitude of the effective Fe spin moment on going from the parent to the optimal doped compound. The doping dependence of the magnitude of the spin moment at higher doping level is less clear, being either constant, or even nonmonotonic, depending on temperature. This phenomenology indicates the importance of the interaction between spin and itinerant degrees of freedom in shaping the properties of the normal state. These findings reaffirm the complexity of the normal state of 122 Fe-pnictides, which are typically viewed as the least correlated of the high-temperature unconventional superconductors.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Nuclear temperature and its dependence on the source neutron-proton asymmetry deduced using the Albergo thermometer

Albergo thermometers with double isotope, isotone and isobar yield ratio pairs with one proton or/and neutron difference are investigated. Without any extra sequential decay correction, a real temperature value of 4.9 ± 0.5 MeV is deduced from the yields of the experimentally reconstructed primary hot intermediate mass fragments (IMFs) from 64 Zn + 112 Sn collisions at 40 MeV/nucleon using the Albergo thermometer for the first time. An experimental sequential decay correction from the apparent temperatures to the real ones for twelve other reaction systems with different neutron-proton (N/Z) asymmetries in the same experiment, 70 Zn, 64 Ni on 112,124 Sn, 58,64 Ni, 197 Au, 232 Th at 40 MeV/nucleon, is performed using an empirical correction factor approach of Tsang et al. with the deduced 4.9 MeV temperature value. The dependence of nuclear temperature on the source N/Z asymmetry is further investigated using these deduced real source temperature values from the present thirteen systems. It is found that the deduced real source temperatures at the present source N/Z range show a rather weak dependence on the source N/Z asymmetry. In conclusion, by comparison between our previous results and those from other independent experiments, a consistent description for the N/Z asymmetry dependence of nuclear temperature is addressed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Excitation energy dependence of prompt fission γ-ray emission from 241 Pu*

Prompt fission γ rays (PFGs) resulting from the 240 Pu(d,pf) reaction have been measured as a function of fissioning nucleus excitation energy E x at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range 5.75 < E x < 8.25 MeV. The physical implications of this result are discussed. The experimental results are compared to simulations conducted using the computational fission model FREYA. We find that FREYA reproduces the experimental PFG characteristics within 8% deviation across the E x range studied. Furthermore, previous excitation energy-dependent PFG measurements conducted below the second-chance fission threshold have large uncertainties, but are generally in agreement with our results within a 2σ confidence interval. However, both a published parametrization of the PFG energy dependence and the most recent PFG evaluation included in ENDF/B-VIII.0 were found to poorly describe the PFG excitation-energy dependence observed in this and previous experiments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

Model study of the energy dependence of the correlation between anisotropic flow and the mean transverse momentum in Au + Au collisions

A hybrid model that employs the hadron-string transport model UrQMD and the (3+1)-dimensional relativistic viscous hydrodynamic code vHLLE, is used to investigate the beam energy dependence of the correlation coefficient ρ(v$^{2}_{2}$,[p T ]) between the average transverse momentum [p T ] of hadrons emitted in an event and the square of the anisotropic flow coefficient v$^{2}_{2}$. For Au+Au collisions, the model predicts characteristic patterns for the energy and event-shape dependence of the variances for [p T ] and v$^{2}_{n}$ [Var([p T ] and Var(v$^{2}_{2}$)], and the covariance of v$^{2}_{n}$ and [p T ][cov(v$^{2}_{2}$,[p T ])], consistent with the attenuation effects of the specific shear viscosity η/s. Here in contrast, ρ(v$^{2}_{2}$,[p T ]) is predicted to be insensitive to the beam energy but sensitive to the initial-state geometry of the collisions. These observations suggest that a precise set of measurements for Var([p T ]), Var(v$^{2}_{2}$), cov(v$^{2}_{2}$,[p T ]) and ρ(v$^{2}_{2}$,[p T ]) as a function of beam energy and event shape, could serve to constrain better the eccentricities and their fluctuations over the entire span of beam energies, and consequently aid precision extraction of the temperature and baryon chemical-potential dependence of η/s from the wealth of Au+Au data obtained in the RHIC beam energy scan.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗