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DOE OSTI2015

Hysteresis phenomena of the two dimensional electron gas density in lattice-matched InAlN/GaN heterostructures

High-temperature transport properties in high-mobility lattice-matched InAlN/GaN heterostructures have been investigated. An interesting hysteresis phenomenon of the two dimensional electron gas (2DEG) density is observed in the temperature-dependent Hall measurements. After high-temperature thermal cycles treatment, the reduction of the 2DEG density is observed, which is more serious in thinner InAlN barrier samples. This reduction can then be recovered by light illumination. We attribute these behaviors to the shallow trap states with energy level above the Fermi level in the GaN buffer layer. The electrons in the 2DEG are thermal-excited when temperature is increased and then trapped by these shallow trap states in the buffer layer, resulting in the reduction and hysteresis phenomenon of their density. Three trap states are observed in the GaN buffer layer and C{sub Ga} may be one of the candidates responsible for the observed behaviors. Our results provide an alternative approach to assess the quality of InAlN/GaN heterostructures for applications in high-temperature electronic devices.

36 MATERIALS SCIENCE↗

DOE OSTI2016

Spectroscopic analysis of the NIR emission in Tm implanted Al{sub x}Ga{sub 1-x}N layers

Al{sub x}Ga{sub 1-x}N samples, with different AlN molar fractions, x = 0, 0.15, 0.77, and 1, grown by halide vapor phase epitaxy were implanted with Tm ions. Photoluminescence (PL) measurements revealed that after thermal annealing all the samples exhibit intraionic Tm{sup 3+} luminescence. In samples with x > 0, the low temperature emission is dominated by the lines that appear in the near infrared (NIR) spectral region, corresponding to the overlapped {sup 1}G{sub 4} → {sup 3}H{sub 5} and {sup 3}H{sub 4} → {sup 3}H{sub 6} multiplet transitions. A detailed spectroscopic analysis of NIR emission of the thulium implanted and annealed Al{sub x}Ga{sub 1-x}N layers is presented by using temperature dependent steady-state PL, room temperature PL excitation, and time resolved PL. The results indicate that the excitonic features sensitive to the alloy disorder are involved in the excitation population processes of the Tm{sup 3+} luminescence and the highest thermal stability for the NIR emission occurs for the AlN:Tm sample.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2016

Anti-site mixing and magnetic properties of Fe 3 Co 3 Nb 2 studied via neutron powder diffraction

Here, we studied the crystal structure and magnetic properties of the rare-earth-free intermetallic compound Fe 3 Co 3 Nb 2 , which has recently been demonstrated to have potentially high magnetic anisotropy, using temperature-dependent neutron powder diffraction. Furthermore, the temperature dependence of the diffraction spectra reveals a magnetic transition between 300 and 400 K, in agreement with the magnetometry measurements. According to the structural refinement of the paramagnetic state and the substantial magnetic contribution to the diffuse scattering in the ferromagnetic state, the Fe/Co anti-site mixing is so strong that the site occupation for Fe and Co is almost random. The projection of the magnetic moments turned out to be non-zero along the c axis and in the a–b plane of Fe 3 Co 3 Nb 2 , most likely because of the exchange interactions between the randomly orientated nanograins in the samples. These findings suggest that future studies on the magnetism of Fe 3 Co 3 Nb 2 need to take the Fe/Co anti-site mixing into account, and the exchange interactions need to be suppressed to obtain large remanence and coercivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2023

Effects of Co substitution on the structural and magnetic properties of Sr ( Ni 1 - x Co x ) 2 P 2

Although SrNi 2 P 2 adopts the common ThCr 2 Si 2 structure for T ≥ 325 K, being in an uncollapsed tetragonal state rather than a collapsed tetragonal version, it is a special case for the ThCr 2 Si 2 class: on cooling below 325 K it adopts a one-third collapsed orthorhombic phase where one out of every three P rows bonds across the Sr layers. On the other hand, SrCo 2 P 2 only exhibits the uncollapsed ThCr 2 Si 2 structure from room temperature down to 1.8 K. Regardless of their low-temperature structures, neither SrNi 2 P 2 nor SrCo 2 P 2 manifests magnetic transitions down to 50 mK and 2 K, respectively. In this work we report the effects of Co substitution in Sr(Ni 1 - x Co x ) 2 P 2 , which allows for tuning the transition between the one-third collapsed and the uncollapsed structure. We find a rapid decrease of the one-third collapsed structural transition temperature with increasing Co fraction, until reaching full suppression for x ≥ 0.1. Substitution levels in the range 0.11 ≤ x ≤ 0.58 show no signs of any transition down to 1.8 K in the magnetization or resistance measurements in the range 1.8 ≤ T ≤ 300 K. However, different magnetically ordered states emerge for x ≥ 0.65, and disappear for x ≥ 0.99, recovering the known paramagnetic properties of the parent compound SrCo 2 P 2 . Further, these results are summarized in a phase diagram, built upon the characterization by energy dispersive x-ray spectroscopy, x-ray diffraction, temperature-dependent resistance, and field- and temperature-dependent magnetization measurements done on single crystals with different Co fraction. Both the magnetic and structural properties are compared to other systems with ThCr 2 Si 2 structure that exhibit magnetic ordering and collapsed tetragonal transitions. The magnetic ordering and moment formation are well described by Takahashi's spin fluctuation theory of itinerant electron magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2016

Superconducting and normal state properties of the systems La 1 - x M x Pt 4 Ge 12 ( M = Ce , Th )

Electrical resistivity, magnetization, and specific heat measurements were performed on polycrystalline samples of the filled-skutterudite systems La 1-x M x Pt 4 Ge 12 (M = Ce, Th). Superconductivity in LaPt4Ge12 was quickly suppressed with Ce substitution and no evidence for superconductivity was found down to 1.1 K for x > 0.2. Temperature-dependent specific heat data at low temperatures for La 1-x Ce x Pt 4 Ge 12 show a change from power-law to exponential behavior, which may be an indication for multi-band superconductivity in LaPt 4 Ge 12 . A similar crossover was observed in the Pr 1-x Ce x Pt 4 Ge 12 system. However, the suppression rates of the superconducting transition temperatures, T c (x), in the two systems are quite disparate, indicating a difference in the nature of superconductivity, which is conventional in LaPt 4 Ge 12 and unconventional in PrPt 4 Ge 12 . In comparison, a nearly linear and smooth evolution of T c with increasing Th was observed in the La 1-x Th x Pt 4 Ge 12 system, with no change of the superconducting energy gap in the temperature dependence of the specific heat, suggesting similar types of superconductivity in both the LaPt 4 Ge 12 and ThPt 4 Ge 12 compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

NASA NTRS2000

The Density-Potential (N(sub e)-V(sub s/c)) Relation in the High-Latitude Prenoon Ionosphere

Sounding of the Cleft Ion Fountain Energization Region (SCIFER) Thermal Electron Capped Hemisphere Spectrometer (TECHS) data are used to study the plasma density, spacecraft potential relation (N(sub e)-V(sub s/c)) in the prenoon topside auroral and cleft ionosphere during a period of low solar activity. The SCIFER TECHS data show a power-law electron temperature dependence on ambient density across the transition from positive to negative spacecraft potential values in the high-latitude ionosphere. The illuminated ionospheric N(sub e)-V(sub s/c) relation is numerically modeled by imposing equilibrium of current flowing to/from the conductive surfaces of the payload. This modeling demonstrates a strong dependence of spacecraft potential on electron temperature across a wide range of densities. This electron temperature dependence is especially significant in the negative spacecraft potential regime. SCIFER TECHS observations of the N(sub e)-V(sub s/c) relation are used to extend the previous magnetospheric observation-model results to the higher density ionospheric regime.

Adrian, M. L.↗

DOE OSTI2016

Dynamic Simulation with on the fly Doppler Broadening treatment in RMC

In conventional transient analysis, thermal hydraulics calculations are generally coupled to deterministic neutronics solvers. Due to the limited accuracy of deterministic methods and the rapid development of computer power, Monte Carlo neutronics calculations have received increasing attention nowadays. One of challenges for Monte Carlo methods coupled to thermal hydraulics calculations is the temperature dependent cross sections. Taking the detailed temperature distribution feedback into account, each nuclide may be in a large range of temperatures, pre-generated libraries at every temperature point are no longer applicable owing to unaffordable memory footprint. Different solutions have been proposed to deal with this problem. TMS, accounting for Doppler broadening effect by the explicit treatment of thermal motion, was used to model fast transients in Serpent 2. Stochastic mixing, using two sets of libraries for each nuclide and mixing them to get the correct temperature, was used in transient analysis by Sjenitzer and Hoogenboom. Besides, there are other on the fly Doppler broadening methods being developed, including series expansion of cross sections, multipole representation, different integration algorithms based on Doppler broadening equation. In this paper, on the fly Doppler broadening treatment based on integration algorithms was applied to transient calculation in Reactor Monte Carlo code RMC . In the following section, the integration algorithms implemented and dynamic simulation method in RMC is briefly introduced. Finally, a pure physical transient fuel assembly model was chosen for numerical validation. (authors)

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗

DOE OSTI2016

Viscous effects on the Rayleigh-Taylor instability with background temperature gradient

Here we studied the growth rate of the compressible Rayleigh-Taylor instability in the presence of a background temperature gradient, Θ, using a normal mode analysis. The effect of Θ variation is examined for three interface types corresponding to the combinations of the viscous properties of the fluids (inviscid-inviscid, viscous-viscous, and viscous-inviscid) at different Atwood numbers, At, and when at least one of the fluids' viscosity is non-zero, as a function of the Grashof number. For the general case, the resulting ordinary differential equations are solved numerically; however, dispersion relations for the growth rate are presented for several limiting cases. An analytical solution is found for the inviscid-inviscid interface and the corresponding dispersion equation for the growth rate is obtained in the limit of large Θ. For the viscous-inviscid case, a dispersion relation is derived in the incompressible limit and Θ=0. Compared to Θ=0 case, the role of Θ<0 (hotter light fluid) is destabilizing and becomes stabilizing when Θ>0 (colder light fluid). The most pronounced effect of Θ ≠ 0 is found at low At and/or at large perturbation wavelengths relative to the domain size for all interface types. On the other hand, at small perturbation wavelengths relative to the domain size, the growth rate for the Θ<0 case exceeds the infinite domain incompressible constant density result. The results are applied to two practical examples, using sets of parameters relevant to Inertial Confinement Fusion coasting stage and solar corona plumes. The role of viscosity on the growth rate reduction is discussed together with highlighting the range of wavenumbers most affected by viscosity. The viscous effects further increase in the presence of background temperature gradient, when the viscosity is temperature dependent.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2023

Interplay of molecular dynamics and radiative decay of a TADF emitter in a glass-forming liquid

Herein we investigate the role of molecular dynamics in the luminescent properties of a prototypical thermally activated delayed fluorescence (TADF) emitter, NAI-DMAC, in solution using a combination of temperature dependent time-resolved photoluminescence and absorption spectroscopies. We use a glass forming liquid, 2-methylfuran, to introduce an abrupt change in the temperature dependent diffusion dynamics of the solvent and examine the influence this has on the emission intensity of NAI-DMAC molecules. Comparison of experiment with first principles molecular dynamics simulations reveals that the emission intensity of NAI-DMAC molecules follows the temperature-dependent self-diffusion dynamics of the solvent. A marked reduction of emission intensity is observed as the temperature decreases toward the glass transition because the rate at which NAI-DMAC molecules can access emissive molecular conformations is greatly reduced. Below the glass transition, the diffusion dynamics of the solvent changes more slowly with temperature, which causes the emission intensity to decrease more slowly as well. The combination of experiment and computation suggests a pathway by which TADF emitters may transiently access a distribution of conformational states and avoid the need for an average conformation that strikes a balance between lower singlet–triplet energy splittings versus higher emission probabilities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2014

Low temperature crystal structure and magnetic properties of RAl{sub 2}

Low temperature crystal structure and magnetic properties of RAl{sub 2} (R = Pr and Nd) have been studied using temperature dependent powder x-ray diffraction, magnetization, and heat capacity measurements. Unlike PrAl{sub 2}, NdAl{sub 2} retains cubic MgCu{sub 2}-type structure from room temperature down to 5 K, which is also confirmed from first principles electronic structure calculations. The magnetization measurements show both PrAl{sub 2} and NdAl{sub 2} order ferromagnetically at T{sub C} = 32 K and 77 K, respectively. However, the magnetization measurements show the former is a hard ferromagnet compared to the latter which is a soft ferromagnetic material. The magnetic entropy change obtained from heat capacity measurements at ΔH = 30 kOe for PrAl{sub 2} and NdAl{sub 2} are 3.15 J mol{sup −1} K{sup −1} and 1.18 J mol{sup −1} K{sup −1}, respectively.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2013

“Multi-temperature” method for high-pressure sorption measurements on moist shales

A simple and effective experimental approach has been developed and tested to study the temperature dependence of high-pressure methane sorption in moist organic-rich shales. This method, denoted as “multi-temperature” (short “multi-T”) method, enables measuring multiple isotherms at varying temperatures in a single run. The measurement of individual sorption isotherms at different temperatures takes place in a closed system ensuring that the moisture content remains constant. The multi-T method was successfully tested for methane sorption on an organic-rich shale sample. Excess sorption isotherms for methane were measured at pressures of up to 25 MPa and at temperatures of 318.1 K, 338.1 K, and 348.1 K on dry and moisture-equilibrated samples. The measured isotherms were parameterized with a 3-parameter Langmuir-based excess sorption function, from which thermodynamic sorption parameters (enthalpy and entropy of adsorption) were obtained. Using these, we show that by taking explicitly into account water vapor as molecular species in the gas phase with temperature-dependent water vapor pressure during the experiment, more meaningful results are obtained with respect to thermodynamical considerations. The proposed method can be applied to any adsorbent system (coals, shales, industrial adsorbents) and any supercritical gas (e.g., CH{sub 4}, CO{sub 2}) and is particularly suitable for sorption measurements using the manometric (volumetric) method.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2015

Influence of buffer-layer construction and substrate orientation on the electron mobilities in metamorphic In{sup 0.70}Al{sup 0.30}As/In{sup 0.76}Ga{sup 0.24}As/In{sup 0.70}Al{sup 0.30}As structures on GaAs substrates

The influence of construction of the buffer layer and misorientation of the substrate on the electrical properties of In{sup 0.70}Al{sup 0.30}As/In{sup 0.76}Ga{sup 0.24}As/In{sup 0.70}Al{sup 0.30}As quantum wells on a GaAs substrate is studied. The temperature dependences (in the temperature range of 4.2 K < T < 300 K) and field dependences (in magnetic fields as high as 6 T) of the sample resistances are measured. Anisotropy of the resistances in different crystallographic directions is detected; this anisotropy depends on the substrate orientation and construction of the metamorphic buffer layer. In addition, the Hall effect and the Shubnikov–de Haas effect are studied. The Shubnikov–de Haas effect is used to determine the mobilities of electrons separately in several occupied dimensionally quantized subbands in different crystallographic directions. The calculated anisotropy of mobilities is in agreement with experimental data on the anisotropy of the resistances.

36 MATERIALS SCIENCE↗

NASA NTRS1974

Drift mobility of holes in phenanthrene single crystals

The temperature dependence of drift mobilities of holes in single crystals of phenanthrene was measured in the range from 203 to 353 K in three crystallographic directions. Below the anomaly temperature of 72 C, the mobility temperature dependences are consistent with the Munn and Siebrand slow-phonon hopping process in the b direction and the Munn and Siebrand slow-phonon coherent mode in the a and c prime directions. The drift mobility temperature dependences in crystals that have been cooled through the anomaly temperature in the presence of illumination and an electric field are consistent with the model of Spielberg et al. (1971), in which the hindered vibration of the 4,5 hydrogens introduces a new degree of freedom above 72 C.

Sonnonstine, T. J.↗

DOE OSTI2011

Structure and properties of ternary manganese nitride Mn{sub 3}CuN{sub y} thin films fabricated by facing target magnetron sputtering

Graphical abstract: This figure reveals the dependence of the texture coefficient TC{sub (111)} and TC{sub (200)} of the Mn{sub 3}CuN{sub y} thin films on T{sub sub}. These results indicate that the preferred orientation of the Mn{sub 3}CuN{sub y} thin films dramatically changed from (1 1 1) to (2 0 0) with the increase of T{sub sub}. In one word, the orientation of Mn{sub 3}CuN{sub y} thin film can be tailored by adjusting the magnetron sputtering processing parameters and substrate. It is the first time to report the deposition of Mn{sub 3}CuN{sub y} thin films on single crystal Si (1 0 0) at various substrate temperatures (T{sub sub}) by facing target magnetron sputtering. The influences of substrate temperature on crystal structure and morphology are discussed. The temperature dependences of magnetization, lattice parameters and resistivity for the resulting film are systematically investigated, which shows several different physical properties from those of previous work. Highlights: {yields} Variation of the substrate temperatures adjusted the preferred orientation from (1 1 1) to (2 0 0) of the Mn{sub 3}CuN{sub y} thin films. {yields} There exists a magnetic ordering transition at 225 K from paramagnetic (PM) to ferrimagnetic (FI), which is different than previous results. {yields} It shows a positive thermal expansion behavior with average linear thermal expansion coefficient 2.49 x 10{sup -5} K{sup -1}. However there was no crystal phase transition occurred around the magnetic transition in the film. {yields} The temperature dependence of resistivity displayed a semiconductor-like behavior, which is quite different from the bulk sample. {yields} We believe that our findings may provide an important role to further utilize the potential of antiperovskite manganese nitride film. -- Abstract: Deposition of Mn{sub 3}CuN{sub y} thin films on single crystal Si (1 0 0) at various substrate temperatures (T{sub sub}) by facing target magnetron sputtering is reported. The crystal structure and composition were characterized by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results confirmed that the crystalline antiperovskite Mn{sub 3}CuN{sub y} thin film with (2 0 0) highly preferred texture had been obtained at T{sub sub} = 180 {sup o}C. Furthermore, for the resulting Mn{sub 3}CuN{sub y} thin film, it showed different properties compared with the bulk counterpart. There was a paramagnetic to ferrimagnetic transition at 225 K with decreasing temperature. The change of the lattice constant with temperature presented positive thermal expansion behavior and no structural transition was observed. The average linear thermal expansion coefficient ({alpha}) is 2.49 x 10{sup -5} K{sup -1} from 123 K to 298 K. More interestingly, the temperature dependence of resistivity displayed a semiconductor-like behavior, i.e. an obvious monotonous decrease of resistivity with increasing temperature.

36 MATERIALS SCIENCE↗

DOE OSTI2015

Electronic transition above room temperature in CaMn{sub 7}O{sub 12} films

We report on the electronic phase transition in CaMn{sub 7}O{sub 12} quadruple perovskite films synthesized by oxide molecular beam epitaxy on SrLaAlO{sub 4} and La{sub 0.3}Sr{sub 0.7}Al{sub 0.65}Ta{sub 0.35}O{sub 3} substrates. We use x-ray diffraction and transmission electron microscopy to confirm that the CaMn{sub 7}O{sub 12} phase has been realized. Temperature dependent resistivity measurements reveal a signature of a charge ordering phase transition at ≈425 K, consistent with bulk CaMn{sub 7}O{sub 12}. The transition temperature is found to be relatively invariant to changes in the cation stoichiometry. Density functional theory calculations reveal the changes in atomic and electronic structure induced by the charge ordering transition.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2019

A 17 O paramagnetic NMR study of Sm 2 O 3 , Eu 2 O 3 , and Sm/Eu-substituted CeO 2

We report that paramagnetic solid-state NMR of lanthanide ( Ln ) containing materials can be challenging due to the high electron spin states possible for the Ln ƒ electrons, which result in large paramagnetic shifts, and these difficulties are compounded for 17 O due to the low natural abundance and quadrupolar character. In this work, we present examples of 17 O NMR experiments for lanthanide oxides and strategies to overcome these difficulties. In particular, we record and assign the 17 O NMR spectra of monoclinic Sm 2 O 3 and Eu 2 O 3 for the first time, as well as performing density functional theory (DFT) calculations to gain further insight into the spectra. The temperature dependence of the Sm 3+ and Eu 3+ magnetic susceptibilities are investigated by measuring the 17 O shift of the cubic sesquioxides over a wide temperature range, which reveal non-Curie temperature dependence due to the presence of low-lying electronic states. This behaviour is reproduced by calculating the electron spin as a function of temperature, yielding shifts which agree well with the experimental values. Using the understanding of the magnetic behaviour gained from the sesquioxides, we then explore the local oxygen environments in 15 at% Sm- and Eu-substituted CeO 2 , with the 17 O NMR spectrum exhibiting signals due to environments with zero, one and two nearest neighbour Ln ions, as well as further splitting due to oxygen vacancies. Finally, we extract an activation energy for oxygen vacancy motion in these systems of 0.35 ± 0.02 eV from the Arrhenius temperature dependence of the 17 O T1 relaxation constants, which is found to be independent of the Ln ion within error. Finally, the relation of this activation energy to literature values for oxygen diffusion in Ln -substituted CeO 2 is discussed to infer mechanistic information which can be applied to further develop these materials as solid-state oxide-ion conductors.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Correcting thermal-emission-induced detector saturation in infrared spectroscopy

We found that temperature-dependent infrared spectroscopy measurements (i.e., reflectance or transmittance) using a Fourier-transform spectrometer can have substantial errors, especially for elevated sample temperatures and collection using an objective lens. These errors can arise as a result of partial detector saturation due to thermal emission from the measured sample reaching the detector, resulting in nonphysical apparent reduction of reflectance or transmittance with increasing sample temperature. Here, we demonstrate that these temperature-dependent errors can be corrected by implementing several levels of optical attenuation that enable convergence testing of the measured reflectance or transmittance as the thermal-emission signal is reduced, or by applying correction factors that can be inferred by looking at the spectral regions where the sample is not expected to have a substantial temperature dependence.

47 OTHER INSTRUMENTATION↗

DOE OSTI2017

Temperature-Independent Nuclear Quantum Effects on the Structure of Water

Nuclear quantum effects (NQEs) have a significant influence on the hydrogen bonds in water and aqueous solutions and have thus been the topic of extensive studies. However, the microscopic origin and the corresponding temperature dependence of NQEs have been elusive and still remain the subject of ongoing discussion. Previous x-ray scattering investigations indicate that NQEs on the structure of water exhibit significant temperature dependence. Here, by performing wide-angle x-ray scattering of H 2 O and D 2 O droplets at temperatures from 275 K down to 240 K, we determine the temperature dependence of NQEs on the structure of water down to the deeply supercooled regime. The data reveal that the magnitude of NQEs on the structure of water is temperature independent, as the structure factor of D 2 O is similar to H 2 O if the temperature is shifted by a constant 5 K, valid from ambient conditions to the deeply supercooled regime. Analysis of the accelerated growth of tetrahedral structures in supercooled H 2 O and D 2 O also shows similar behavior with a clear 5 K shift. The results indicate a constant compensation between NQEs delocalizing the proton in the librational motion away from the bond and in the OH stretch vibrational modes along the bond. In conclusion, this is consistent with the fact that only the vibrational ground state is populated at ambient and supercooled conditions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗