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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Stripe Domains and First Order Phase Transition in the Vortex Matter of Anisotropic High-temperature Superconductors

We report the direct imaging of a novel modulated flux striped domain phase in a nearly twin-free YBCO crystal. These domains arise from instabilities in the vortex structure within a narrow region of tilted magnetic fields at small angles from the in-plane direction. By comparing the experimental and theoretically derived vortex phase diagrams we infer that the stripe domains emerge from a first-order phase transition of the vortex structure. The size of domains containing vortices of certain orientations is controlled by the balance between the vortex stray field energy and the positive energy of the domain boundaries. Our results confirm the existence of the kinked vortex chain phase in an anisotropic high temperature superconductor and reveal a sharp transition in the state of this phase resulting in regular vortex domains.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Quantum phase transitions between a class of symmetry protected topological states

The subject of this paper is the phase transition between symmetry protected topological states (SPTs). We consider spatial dimension d and symmetry group G so that the cohomology group, Hd+1(G,U(1)), contains at least one Z2n or Z factor. We show that the phase transition between the trivial SPT and the root states that generate the Z2n or Z groups can be induced on the boundary of a (d+1)-dimensional View the MathML source-symmetric SPT by a View the MathML source symmetry breaking field. Moreover we show these boundary phase transitions can be “transplanted” to d dimensions and realized in lattice models as a function of a tuning parameter. The price one pays is for the critical value of the tuning parameter there is an extra non-local (duality-like) symmetry. In the case where the phase transition is continuous, our theory predicts the presence of unusual (sometimes fractionalized) excitations corresponding to delocalized boundary excitations of the non-trivial SPT on one side of the transition. This theory also predicts other phase transition scenarios including first order transition and transition via an intermediate symmetry breaking phase.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

X-ray nanodiffraction imaging reveals distinct nanoscopic dynamics of an ultrafast phase transition

Significance Phase transitions, the changes between states of matter with distinct electronic, magnetic, or structural properties, are at the center of condensed matter physics and underlie valuable technologies. First-order phase transitions are intrinsically heterogeneous. When driven by ultrashort excitation, nanoscale phase regions evolve rapidly, which has posed a significant experimental challenge to characterize. The newly developed laser-pumped X-ray nanodiffraction imaging technique reported here has simultaneous 100-ps temporal and 25-nm spatial resolutions. This approach reveals pathways of the nanoscale structural rearrangement upon ultrafast optical excitation, different from those transitions under slowly varying parameters. The spatiotemporally resolved structural characterization provides crucial nanoscopic insights into ultrafast phase transitions and opens opportunities for controlling nanoscale phases on ultrafast time scales.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Toluene-induced phase transitions in blue phase liquid crystals

Here, we report phase transitions in blue phase-forming liquid crystals (LCs) that are triggered by exposure to toluene vapours. Specifically, we reveal that room-temperature cholesteric phase mixtures of MLC-2142 and S-811 form blue phases (BP I, II and III) with increasing vapour pressure of toluene. To probe the mechanism underlying this observation, we investigated the phase behaviour of mixtures of BP-forming LCs containing a range of non-volatile aromatic compounds (e.g. pyrene). We interpret our observations to indicate that the principal effect of small aromatic compounds is to decrease the energy penalty associated with the formation of disclination lines in BPs. We also conclude that the absorption of toluene into the BP-forming LCs lowers the energy required for the formation of disclination cores in the BP phase, thus allowing the elastically favoured double-twist cylinders to form at lower temperatures. We further show that BP-forming LCs containing pyrene can be used to detect toluene at concentrations below 200 ppm at room temperature. Overall, these results guide the design of LC-based materials that respond to VOCs at concentrations relevant to occupational settings.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

High pressure structural phase transitions in Dysprosium to 202 GPa

High pressure structural phase transitions in heavy lanthanide metal Dysprosium (Dy) have been studied to 202 GPa (Volume Compression V/V 0 = 0.36) in a diamond anvil cell employing copper as an internal x-ray pressure standard. The previously assigned monoclinic (C2/m) phase above 72 GPa has been reexamined and assigned to an orthorhombic phase with sixteen atoms per cell (oF16) based on structural refinements. The equation of state is presented to 202 GPa and indicates a volume change of 2.3% during the structural phase transition from distorted face-centered cubic (hR24) phase to oF16 phase at 72 GPa. The oF16 phase can be regarded as a pseudo-orthorhombic eight-layered structure with (b/c) ratio decreasing from an ideal value of $\sqrt{3}$ with increasing pressure to 202 GPa. The ultrahigh pressure structural phases of Dy are compared with other members of the lanthanide series.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Asymmetric “melting” and “freezing” kinetics of the magnetostructural phase transition in B2-ordered FeRh epilayers

Synchrotron X-ray diffraction was used to study the phase transformation processes during the magnetostructural transition in a B2-ordered FeRh (001)-oriented epilayer grown on MgO by sputtering. Out-of-plane lattice constant measurements within the hysteretic regime of the transition reveal a microstructure consistent with the coexistence of lattice-expanded and contracted phases in spatially distinct regions. It was found that the phase separation is more pronounced during cooling than heating. Furthermore, whilst lattice-expanded domains that span the height of the film can be undercooled by several kelvins, there is no equivalent superheating. This asymmetry between the cooling and heating processes in FeRh is consistent with the difference in the kinetics of generic freezing and melting transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Pressure-induced structural phase transition in AlN:Mg and AlN:Co nanowires

High-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated by in situ angle dispersive synchrotron X-ray diffraction up to 41.5 GPa and 38.2 GPa, respectively. Their corresponding pressure-induced wurtzite-to-rocksalt phase transitions start at 17.7 GPa and 15.0 GPa and complete at 33.2 GPa and 31.0 GPa, respectively. The phase-transition routes are not affected by the doped ions, while the phase transition pressures are lower than that of pure AlN nanowires. The distinct high-pressure behaviors are ascribed to the doped ions, which reduce the formation energy of cation vacancies and induce Al vacancies defects together with substitution defects, resulting in lattice distortion and affecting structural stability and phase transition pressure. - Graphical abstract: The high-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated by in situ angle dispersive synchrotron X-ray diffraction. - Highlights: • The high-pressure behaviors of AlN:Mg and AlN:Co nanowires have been investigated. • The pressure-induced wurtzite-to-rocksalt phase transitions have been observed. • The phase transition pressures are lower than that of pure AlN nanowires. • The distinct high-pressure behaviors are ascribed to the dopants. • The vacancy defects and substitution defects influence structural stability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Rescuing a Quantum Phase Transition with Quantum Noise

In this work, we show that placing a quantum system in contact with an environment can enhance non-Fermi-liquid correlations, rather than destroy quantum effects, as is typical. The system consists of two quantum dots in series with two leads; the highly resistive leads couple charge flow through the dots to the electromagnetic environment, the source of quantum noise. While the charge transport inhibits a quantum phase transition, the quantum noise reduces charge transport and restores the transition. We find a non-Fermi-liquid intermediate fixed point for all strengths of the noise. For strong noise, it is similar to the intermediate fixed point of the two-impurity Kondo model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Ionic and Optical Properties of Methylammonium Lead Iodide Perovskite across the Tetragonal–Cubic Structural Phase Transition

Abstract Practical hybrid perovskite solar cells (PSCs) must endure temperatures above the tetragonal–cubic structural phase transition of methylammonium lead iodide (MAPbI 3 ). However, the ionic and optical properties of MAPbI 3 in such a temperature range, and particularly, dramatic changes in these properties resulting from a structural phase transition, are not well studied. Herein, we report a striking contrast at approximately 45 °C in the ionic/electrical properties of MAPbI 3 owing to a change of the ion activation energy from 0.7 to 0.5 eV, whereas the optical properties exhibit no particular transition except for the steady increase of the bandgap with temperature. These observations can be explained by the “continuous” nature of perovskite phase transition. We speculate that the critical temperature at which the ionic/electrical properties change, although related to crystal symmetry variation, is not necessarily the same temperature as when tetragonal–cubic structural phase transition occurs.

Hoque, Md Nadim Ferdous↗
DOE OSTI2017

Collapsed tetragonal phase transition in LaRu 2 P 2

Here, the structural properties of LaRu 2 P 2 under external pressure have been studied up to 14 GPa, employing high-energy x-ray diffraction in a diamond-anvil pressure cell. At ambient conditions, LaRu 2 P 2 (I4/ mmm ) has a tetragonal structure with a bulk modulus of B = 105(2) GPa and exhibits superconductivity at T c = 4.1 K. With the application of pressure, LaRu 2 P 2 undergoes a phase transition to a collapsed tetragonal (cT) state with a bulk modulus of B = 175(5) GPa. At the transition, the c-lattice parameter exhibits a sharp decrease with a concurrent increase of the a-lattice parameter. The cT phase transition in LaRu 2 P 2 is consistent with a second-order transition, and was found to be temperature dependent, increasing from P = 3.9(3) GPa at 160 K to P = 4.6(3) GPa at 300 K. In total, our data are consistent with the cT transition being near, but slightly above 2 GPa at 5 K where superconductivity is suppressed. Finally, we compare the effect of physical and chemical pressure in the R Ru 2 P 2 ( R = Y, La–Er, Yb) isostructural series of compounds and find them to be analogous.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗