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At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Pyroelectric response and temperature-induced α-β phase transitions in α -In 2 Se 3 and other α -III 2 VI 3 (III = Al, Ga, In; VI = S, Se) monolayers

The pyroelectric response of materials is a promising phenomenon that is used in many applications, but two-dimensional (2D) pyroelectricity has not received due attention. Here, in this study, we report ab initio molecular dynamics simulations to investigate the pyroelectric effect in α-III 2 VI 3 (III = Al, Ga, In; VI = S, Se) monolayers, an emerging class of 2D pyroelectric materials. The in-plane and out-of-plane spontaneous polarizations are found to be robust at room temperature. We demonstrate large pyroelectric response in these materials that is associated with temperature-induced ferroelectric-to-paraelectric phase transitions. Identifying the high-temperature nonpolar phase as the β-phase, we demonstrate strong displacive character in the α-β phase transition, which is achieved through relative displacements of the top three and bottom two atomic sublayers in opposite directions. The present work unravels and highlights the primary pyroelectric effect and the α-β phase transitions that are responsible for the promising pyroelectricity in α-III 2 VI 3 monolayers.

36 MATERIALS SCIENCE↗
NASA NTRSDate not supplied

Phase Transitions and Melting in the Venusian Basaltic Crust: Implications for Crustal Recycling

Understanding phase transitions and melting in the Venusian crust and the associated changes in density are critical to constrain crustal thickness, recycling, and remelting processes. For example, average surface conditions on Venus of 92 bars and 460 °C correspond to low-grade metamorphic conditions on Earth equivalent to hornfels- and greenschist-facies. Further, the primary igneous mineralogy in the Venusian crust should be replaced by metamorphic minerals over time and potentially melt, as temperatures and pressures increase with depth along geothermal gradients. Here, we use Perple_X (Connolly, 2005), a Gibbs free energy minimization program to calculates table phase equilibria over a range of pressure and temperature conditions for whole-rock compositions of a dry basalt, alkali basalt, and peridotite. Further, we extract mineral abundancies along five geotherms of 5, 10, 15, 20 and 25 °C/km and calculate the rock density after the extraction of melt. On the coldest geotherm of 5 °C/km, melting would start at ~ 197 km and ~121 km in the basalt and alkali basalt, respectively, and therefore at depths that significantly exceed the estimated thickness of the Venusian crust (8-25 km). A density cross-over, where the crustal density would exceed that of the mantle could potentially induce delamination and is estimated to occur at ~ 40 km for both compositions. On the 10 °C/km geotherm, melting of the basalt starts at 65 km and melt extraction only causes a gradual densification due the formation of initially lower melt proportions. In contrast, the alkali basalt starts to melt at a depth of ~ 57 km and the subsequent densification of the residual composition could trigger delamination at ~ 60km depth. For hotter geotherms (i.e., 25 °C/km gradient), melting would occur at shallower depths of 21 – 23 km. However, the basalt will not get significantly denser than the mantle while the residuum of the alkali basalt would reach a density cross-over at 30 km. Hence, differences in Venusian crustal compositions can significantly influence the thickness of the crust on individual geotherms. Our results demonstrate that phase transitions and melting are strongly dependent on the assumed geotherm and could, combined with geodynamic models, further constrain crustal parameters.

Julia Semprich↗
DOE OSTI2022

Quantum phase transitions in a model Hamiltonian exhibiting entangled simultaneous fermion-pair and exciton condensations

Quantum states of a novel Bose-Einstein condensate, in which both fermion-pair and exciton condensations are simultaneously present, have recently been realized theoretically in a model Hamiltonian system. Here, in this study, we identify quantum phase transitions in that model between fermion-pair and exciton condensations based on a geometric analysis of the convex set of ground-state two-particle reduced density matrices (2-RDMs). The 2-RDM set provides a finite representation of the infinite parameter space of Hamiltonians that readily reveals a fermion-pair condensate phase and two distinct exciton condensate phases, as well as the emergence of first- and second-order phase transitions as the particle number of the system is increased. The set, furthermore, shows that the fermion-exciton condensate (FEC) lies along the second-order phase transition between the exciton and fermion-pair condensate phases. The detailed information about the exciton and fermion-pair phases, the forces behind these phases, as well as their associated transitions provides additional insight into the formation of the FEC condensate, which we anticipate will prove useful in its experimental realization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Superconducting scanning tunneling microscopy tips in a magnetic field: Geometry-controlled order of the phase transition

The properties of geometrically confined superconductors significantly differ from their bulk counterparts. Here, we demonstrate the geometrical impact for superconducting scanning tunneling microscopy (STM) tips, where the confinement ranges from the atomic to the mesoscopic scale. To this end, we compare the experimentally determined magnetic field dependence for several vanadium tips to microscopic calculations based on the Usadel equation. For our theoretical model of a superconducting cone, we find a direct correlation between the geometry and the order of the superconducting phase transition. Increasing the opening angle of the cone changes the phase transition from first to second order. Comparing our experimental findings to the theory reveals first and second order quantum phase transitions in the vanadium STM tips. In addition, the theory also explains experimentally observed broadening effects by the specific tip geometry.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1991

Late-time cosmological phase transitions

It is shown that the potential galaxy formation and large scale structure problems of objects existing at high redshifts (Z approx. greater than 5), structures existing on scales of 100 M pc as well as velocity flows on such scales, and minimal microwave anisotropies ((Delta)T/T) (approx. less than 10(exp -5)) can be solved if the seeds needed to generate structure form in a vacuum phase transition after decoupling. It is argued that the basic physics of such a phase transition is no more exotic than that utilized in the more traditional GUT scale phase transitions, and that, just as in the GUT case, significant random Gaussian fluctuations and/or topological defects can form. Scale lengths of approx. 100 M pc for large scale structure as well as approx. 1 M pc for galaxy formation occur naturally. Possible support for new physics that might be associated with such a late-time transition comes from the preliminary results of the SAGE solar neutrino experiment, implying neutrino flavor mixing with values similar to those required for a late-time transition. It is also noted that a see-saw model for the neutrino masses might also imply a tau neutrino mass that is an ideal hot dark matter candidate. However, in general either hot or cold dark matter can be consistent with a late-time transition.

Schramm, David N.↗
DOE OSTI2018

Dark Conductivity and Photoconductivity of Nonaqueous Liposomes: a New Method for Measuring the Phase-Transition Temperatures of Lipid Membranes

A new method is developed to measure the phase-transition temperatures in artificial phospholipid membranes. This method is based on studying the temperature dependence of dark conductivity and photoconductivity in a symmetric cell with current-conducting indium–tin oxide (ITO) electrodes. Internal electron photoemission into a thin liposome layer is induced by visible and near IR light from the ITO electrodes. This method is applied to study the lyotropic phases in 1,2-dipalmitoyl-rac-glycero-3-phosphocholine (DPPC) and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) with ethylene glycol (EG) and glycerol (G). The results of time-of-flight measurements are used to calculate the carrier mobilities in liposome vesicles. The measurement results are compared with the results obtained by dc conductometry. We are the first to detect the effect of a positive temperature coefficient of resistivity in a liquid-crystal phase. The proposed method makes it possible to detect the phase transitions in lyotropic liquid-crystal systems and, hence, can be used to create biocompatible drug carriers based on thermosensitive liposomes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Structure family and polymorphous phase transition in the compounds with soft sublattice: Cu{sub 2}Se as an example

Quite a few interesting but controversial phenomena, such as simple chemical composition but complex structures, well-defined high-temperature cubic structure but intriguing phase transition, coexist in Cu{sub 2}Se, originating from the relatively rigid Se framework and “soft” Cu sublattice. However, the electrical transport properties are almost uninfluenced by such complex substructures, which make Cu{sub 2}Se a promising high-performance thermoelectric compound with extremely low thermal conductivity and good power factor. Our work reveals that the crystal structure of Cu{sub 2}Se at the temperature below the phase-transition point (∼400 K) should have a group of candidate structures that all contain a Se-dominated face-centered-cubic-like layered framework but nearly random site occupancy of atoms from the “soft” Cu sublattice. The energy differences among those structures are very low, implying the coexistence of various structures and thus an intrinsic structure complexity with a Se-based framework. Detailed analyses indicate that observed structures should be a random stacking of those representative structure units. The transition energy barriers between each two of those structures are estimated to be zero, leading to a polymorphous phase transition of Cu{sub 2}Se at increasing temperature. Those are all consistent with experimental observations.

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

Baryogenesis at a lepton-number-breaking phase transition

We study a scenario in which the baryon asymmetry of the universe arises from a cosmological phase transition where lepton-number is spontaneously broken. If the phase transition is first order, a lepton-number asymmetry can arise at the bubble wall, through dynamics similar to electroweak baryogenesis, but involving right-handed neutrinos. In addition to the usual neutrinoless double beta decay in nuclear experiments, the model may be probed through a variety of “baryogenesis by-products”, which include a stochastic background of gravitational waves created by the colliding bubbles. Depending on the model, other aspects may include a network of topological defects that produce their own gravitational waves, additional contribution to dark radiation, and a light pseudo-Goldstone boson (majoron) as dark matter candidate.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Ferromagnetic domain behavior and phase transition in bilayer manganites investigated at the nanoscale

Understanding the underlying mechanism and phenomenology of colossal magnetoresistance in manganites has largely focused on atomic and nanoscale physics such as double exchange, phase separation, and charge order. Here in this article, we consider a more macroscopic view of manganite materials physics, reporting on the ferromagnetic domain behavior in a bilayer manganite sample with a nominal composition of La 2-2x Sr 1+2x Mn 2 O 7 with x = 0:38, studied using in-situ Lorentz transmission electron microscopy. The role of magnetocrystalline anisotropy on the structure of domain walls was elucidated. On cooling, magnetic domain contrast was seen to appear first at the Curie temperature within the a - b plane. With further reduction in temperature, the change in area fraction of magnetic domains was used to estimate the critical exponent describing the ferromagntic phase transition. Lastly, the ferromagnetic phase transition was accompanied by a distinctive nanoscale granular contrast close to the Curie temperature, which we infer to be related to the presence of ferromagnetic nanoclusters in a paramagnetic matrix, which has not yet been reported in bilayer manganites.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Comparing the anomalous Hall effect and the magneto-optical Kerr effect through antiferromagnetic phase transitions in Mn 3 Sn

In the non-collinear antiferromagnet Mn 3 Sn, we compare simultaneous measurements of the anomalous Hall effect (AHE) and the magneto-optical Kerr effect (MOKE) through two magnetic phase transitions: the high-temperature paramagnetic/antiferromagnetic (AF) phase transition at the Néel temperature ( T N ≈ 420 K) and a lower-temperature incommensurate magnetic ordering at T 1 ≈ 270 K. While both the AHE and MOKE are sensitive to the same underlying symmetries of the AF non-collinear spin order, we find that the transition temperatures measured by these two techniques unexpectedly differ by approximately 10 K. Moreover, the applied magnetic field at which the AF order reverses is significantly larger when measured by MOKE than when measured by AHE. Furthermore these results point to a difference between the bulk and surface magnetic properties of Mn 3 Sn.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Surface charge sensing by altering the phase transition in VO{sub 2}

Detection of surface charges has various applications in medicine, electronics, biotechnology, etc. The source of surface charge induction may range from simple charge-polarized molecules like water to complicated proteins. It was recently discovered that surface charge accumulation can alter the temperature at which VO{sub 2} undergoes a Mott transition. Here, we deposited polar molecules onto the surface of two-terminal thin-film VO{sub 2} lateral devices and monitored the joule-heating-driven Mott transition, or conductance switching. We observed that the power required to induce the conductance switching reduced upon treatment with polar molecules and, using in-situ blackbody-emission direct measurement of local temperature, we show that this reduction in power was accompanied by reduction in the Mott transition temperature. Further evidence suggested that this effect has specificity to the nature of the species used to induce surface charges. Using x-ray absorption spectroscopy, we also show that there is no detectable change in oxidation state of vanadium or structural phase in the bulk of the 40 nm VO{sub 2} thin-film even as the phase transition temperature is reduced by up to 20 K by the polar molecules. The ability to alter the phase transition parameters by depositing polar molecules suggests a potential application in sensing surface charges of different origins and this set of results also highlights interesting aspects of the phase transition in VO{sub 2}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Tuning Spin-Orbit Torques Across the Phase Transition in VO 2 /NiFe Heterostructure

The emergence of spin-orbit torques as a promising approach to energy-efficient magnetic switching has generated large interest in material systems with easily and fully tunable spin-orbit torques. We report current-induced spin-orbit torques in VO 2 /NiFe heterostructures are investigated using spin-torque ferromagnetic resonance, where the VO 2 layer undergoes a prominent insulator-metal transition. A roughly twofold increase in the Gilbert damping parameter, α, with temperature is attributed to the change in the VO 2 /NiFe interface spin absorption across the VO 2 phase transition. More remarkably, a large modulation (±100%) and a sign change of the current-induced spin-orbit torque across the VO 2 phase transition suggest two competing spin-orbit torque generating mechanisms. The bulk spin Hall effect in metallic VO 2 , corroborated by the first-principles calculation of the spin Hall conductivity σ SH ≈ -10 4 ($\frac{\hbar}{e}$) Ω -1 m -1 , is verified as the main source of the spin-orbit torque in the metallic phase. The self-induced/anomalous torque in NiFe, with opposite sign and a similar magnitude to the bulk spin Hall effect in metallic VO 2 , can be the other competing mechanism that dominates as temperature decreases. For applications, the strong tunability of the torque strength and direction opens a new route to tailor spin-orbit torques of materials that undergo phase transitions for new device functionalities.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrafast Martensitic Phase Transition Driven by Intense Terahertz Pulses

We report on an ultrafast nonequilibrium phase transition with a strikingly long-lived martensitic anomaly driven by above-threshold single-cycle terahertz pulses with a peak field of more than 1 MV/cm. A nonthermal, terahertz-induced depletion of low-frequency conductivity in Nb 3 Sn indicates increased gap splitting of high-energy 12 bands by removal of their degeneracies, which induces the martensitic phase above their equilibrium transition temperature. In contrast, optical pumping leads to a Γ 12 gap thermal melting. Such light-induced nonequilibrium martensitic phase exhibits a substantially enhanced critical temperature up to ~100 K, i.e., more than twice the equilibrium temperature, and can be stabilized beyond technologically relevant, nanosecond time scales. Together with first-principle simulations, we identify a compelling terahertz tuning mechanism of structural order via Γ 12 phonons to achieve the ultrafast phase transition to a metastable electronic state out of equilibrium at high temperatures far exceeding those for equilibrium states.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Kinetic frustration by limited bond availability controls the LAT protein condensation phase transition on membranes

LAT is a membrane-linked scaffold protein that undergoes a phase transition to form a two-dimensional protein condensate on the membrane during T cell activation. Governed by tyrosine phosphorylation, LAT recruits various proteins that ultimately enable condensation through a percolation network of discrete and selective protein-protein interactions. Here, we describe detailed kinetic measurements of the phase transition, along with coarse-grained model simulations, that reveal that LAT condensation is kinetically frustrated by the availability of bonds to form the network. Unlike typical miscibility transitions in which compact domains may coexist at equilibrium, the LAT condensates are dynamically arrested in extended states, kinetically trapped out of equilibrium. Modeling identifies the structural basis for this kinetic arrest as the formation of spindle arrangements, favored by limited multivalent binding interactions along the flexible, intrinsically disordered LAT protein. These results reveal how local factors controlling the kinetics of LAT condensation enable formation of different, stable condensates, which may ultimately coexist within the cell.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Phase transition systematics in BiVO 4 by means of high-pressure–high-temperature Raman experiments

We report here high-pressure–high-temperature Raman experiments performed on BiVO4. We characterized the fergusonite and scheelite phases (powder and single crystal samples) and the zircon polymorph (nanopowder). The experimental results are supported by ab initio calculations, which, in addition, provide the vibrational patterns. The temperature and pressure behavior of the fergusonite lattice modes reflects the distortions associated with the ferroelastic instability. The linear coefficients of the zircon phase are in sharp contrast to the behavior observed in the fergusonite phase. The boundary of the fergusonite-to-scheelite second-order phase transition is given by T F–Sch (K) = –166(8) P (GPa) + 528(5). The zircon-to-scheelite, irreversible, first-order phase transition takes place at T Z–Sch (K) = –107(8) P(GPa) + 690(10). We found evidence of additional structural changes around 15.7 GPa, which in the downstroke were found to be not reversible. We analyzed the anharmonic contribution to the wave-number shift in fergusonite using an order parameter. As a result, the introduction of a critical temperature depending both on temperature and pressure allows for a description of the results of all the experiments in a unified way.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Floquet band engineering and topological phase transitions in 1T’ transition metal dichalcogenides

Using ab initio tight-binding approaches, we investigate Floquet band engineering of the 1T' phase of transition metal dichalcogenides (MX 2 , M = W, Mo and X = Te, Se, S) monolayers under the irradiation with circularly polarized light. Furthermore, our first principles calculations demonstrate that light can induce important transitions in the topological phases of this emerging materials family. For example, upon irradiation, Te-based MX2 undergoes a phase transition from quantum spin Hall (QSH) semimetal to time-reversal symmetry broken QSH insulator with a nontrivial band gap of up to 92.5 meV. On the other hand, Se- and S-based MX 2 undergoes the topological phase transition from the QSH effect to the quantum anomalous Hall effect and into trivial phases with increasing light intensity. From a general perspective, this theoretical work brings further insight into non-equilibrium topological systems.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Structural Phase Transitions of NbO 2 : Bulk versus Surface

The metal to insulator transition of NbO 2 has been predicted to be a result of a structural phase transition (SPT) governed by Peierls physics. However, direct observation of the SPT using experimental techniques is still restricted by the extremely high transition temperature (810 °C) and the proclivity for NbO 2 to oxidize into Nb 2 O 5 above 400 °C when exposed to air. In this work, we address these issues and employ temperature-dependent X-ray spectroscopy to describe the SPT of NbO 2 from the bulk to surface. Temperature-dependent extended X-ray absorption fine structure spectroscopy (T-EXAFS) reveals a gradual weakening of the bulk Nb dimers over a large temperature range, which is indicative of a second-order Peierls mechanism. From these measurements, we determine the critical dimer distance to be 2.77 Å. Our T-EXAFS observations are supported by density functional theory of the phonon dispersion and the electronic density of states of NbO 2 , which conclude that the dimerization is responsible for the insulating phase. The dimerization does not extend to the topmost layers, where an oxygen rich surface reconstruction is preferred irrespective of temperature even in extremely reducing environments; changes in the low-energy electron diffraction patterns are attributed to oxygen concentration and are independent of the underlying bulk phase transitions of NbO 2 .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Reconfigurable Vanadium Dioxide Nanomembranes and Microtubes with Controllable Phase Transition Temperatures

Here, two additional structural forms, free-standing nanomembranes and microtubes, are reported and added to the vanadium dioxide (VO 2 ) material family. Free-standing VO 2 nanomembranes were fabricated by precisely thinning as-grown VO 2 thin films and etching away the sacrificial layer underneath. VO 2 microtubes with a range of controllable diameters were rolled-up from the VO 2 nanomembranes. When a VO 2 nanomembrane is rolled-up into a microtubular structure, a significant compressive strain is generated and accommodated therein, which decreases the phase transition temperature of the VO 2 material. The magnitude of the compressive strain is determined by the curvature of the VO 2 microtube, which can be rationally and accurately designed by controlling the tube diameter during the rolling-up fabrication process. The VO 2 microtube rolling-up process presents a novel way to controllably tune the phase transition temperature of VO 2 materials over a wide range toward practical applications. Furthermore, the rolling-up process is reversible. A VO 2 microtube can be transformed back into a nanomembrane by introducing an external strain. Because of its tunable phase transition temperature and reversible shape transformation, the VO 2 nanomembrane-microtube structure is promising for device applications. As an example application, a tubular microactuator device with low driving energy but large displacement is demonstrated at various triggering temperatures.

77 NANOSCIENCE AND NANOTECHNOLOGY↗