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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 1,225 records · Page 68
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Tuning the charge density wave quantum critical point and the appearance of superconductivity in Ti Se 2

The transition metal dichalcogenide TiSe 2 is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cu-intercalated Cu 0.025 TiSe 2 is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivity-temperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of Cu 0.025 TiSe 2 is 3.51 kOe, 16 times larger than that of pristine TiSe 2 , and its temperature dependence is linear, indicating that SC of TiSe 2 is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in TiSe 2 is strongly correlated with CDW quantum criticality.

2-dimensional systems↗
DOE OSTI2023

First-principles derivation of magnetic interactions in the triangular quantum spin liquid candidates KYb C h 2 ( C h = S , Se , Te ) and A YbSe 2 ( A = Na , Rb )

The AYb Ch 2 (A=alkali metal, Ch=chalcogen) delafossites are a family of crystals which have a triangular lattice of effective S=1/2 moments on their Yb atoms. They hold great promise for the realization of the triangular quantum spin liquid state because of their defect-minimized growth and the ability to interchange chemical constituencies among the family. Here we use ab initio computations to evaluate the exchange couplings of four realized (and one theoretical) rhombohedral delafossite structures and examine the influence of chemical substitution on promoting the development of a quantum spin liquid state. We find good agreement with experiment regarding the antiferromagnetic nearest-neighbor exchange J 1 , but our calculations underestimate J 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

The Impact of Deposition Temperature on Sodium Fluoride Recrystallization in Cu(In,Ga)Se 2 Solar Cells

Deposited CIGS layers typically undergo a NaF post deposition treatment (PDT) to improve the electrical properties of the deposited films. Here, these NaF annealing processes can also be used to recrystallize CIGS films. To determine the effect of NaF as a fluxing agent, CIGS was deposited at varying substrate temperatures. In-situ annealing with NaF was then done to determine how the initial grain structure energy impacts recrystallization. Gallium appears to be a significant deterrent to recrystallization; the most crystallized film experienced a phase segregation between CIS and gallium containing compounds. By investigating these phase separations more, it might be feasible that a process of rapidly depositing CIGS at extremely low temperatures could create films of sufficient quality to be economically viable.

14 SOLAR ENERGY↗
DOE OSTI2021

Vapor Treatment and In-situ Recrystallization by Copper Chloride on Cu(In,Ga)Se 2 Thin Film

Deposition of CIGS semiconductor thin films was performed at low temperature and high rate by three-stage coevaporation process on molybdenum coated glass substrate. Here, a vapor treatment was done in between the second and third stage by flashing CuCl 2 for 5 mins at 400 °C. A large change in morphology and crystal structure was observed after the treatment. XRD and SEM showed that small grains transformed into large grains. A smoother Ga profile was observed by SIMS measurements for the treated films as compared to as-deposited films. Furthermore, the Na profile was also modified in the recrystallized samples, with a lower content after recrystallization.

14 SOLAR ENERGY↗
DOE OSTI2015

Temperature effect on zinc oxysulfide-Zn(O,S) films synthesized by atomic layer deposition for Cu(In,Ga)Se{sub 2} solar cells

Thin films of Zn(O,S) were deposited by atomic layer deposition from diethylzinc, water (H{sub 2}O), and hydrogen sulfide (H{sub 2}S). First, a study on the influence of the H{sub 2}S/(H{sub 2}O+H{sub 2}S) pulse ratio from pure ZnO to pure ZnS was performed at deposition temperature T{sub dep}=120 and 200 °C. Zn(O,S) films had higher S content than expected, and this effect was stronger at T{sub dep}=200 °C. Then, Zn(O,S) films have been synthesized over the range of temperature 120–220 °C at the constant H{sub 2}S/(H{sub 2}O+H{sub 2}S) pulse ratio of 9%. For T{sub dep}<180 °C, high and almost constant S content has been measured in the films. The significant increase of the S/(O+S) atomic ratio for T{sub dep}>180 °C confirmed that exchange reactions occurred between the Zn(O,S) growing films and H{sub 2}S. The grazing incidence x-ray diffraction patterns showed Zn(O,S) films with hexagonal wurtzite structures and with an optimum crystallization for temperatures T{sub dep}=160–180 °C. Indeed, in this temperature range, well crystallized and large grains were obtained which was in good correlation with the film morphology determined by scanning electron microscope; and Hall effect measurements revealed low resistivities, high carrier concentrations (>10{sup 19} cm{sup −3}), and low mobilities. From these results, the authors propose the existence of a temperature range where the properties undergo significant changes while the atomic composition remains constant.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Growth of topological insulator Bi 2 Se 3 particles on GaAs via droplet epitaxy

The discovery of topological insulators (TIs) and their unique electronic properties has motivated research into a variety of applications, including quantum computing. It has been proposed that TI surface states will be energetically discretized in a quantum dot nanoparticle. These discretized states could then be used as basis states for a qubit that is more resistant to decoherence. In this work, prototypical TI Bi2Se3 nanoparticles are grown on GaAs (001) using the droplet epitaxy technique, and we demonstrate the control of nanoparticle height, area, and density by changing the duration of bismuth deposition and substrate temperature. Within the growth window studied, nanoparticles ranged from 5 to 15 nm in height with an 8–18 nm equivalent circular radius, and the density could be relatively well controlled by changing the substrate temperature and bismuth deposition time.

Materials Science↗