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Results for “surface phase transitions”

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 OSTI2024

Controllable strain-driven topological phase transition and dominant surface-state transport in HfTe5

Abstract The fine-tuning of topologically protected states in quantum materials holds great promise for novel electronic devices. However, there are limited methods that allow for the controlled and efficient modulation of the crystal lattice while simultaneously monitoring the changes in the electronic structure within a single sample. Here, we apply significant and controllable strain to high-quality HfTe 5 samples and perform electrical transport measurements to reveal the topological phase transition from a weak topological insulator phase to a strong topological insulator phase. After applying high strain to HfTe 5 and converting it into a strong topological insulator, we found that the resistivity of the sample increased by 190,500% and that the electronic transport was dominated by the topological surface states at cryogenic temperatures. Our results demonstrate the suitability of HfTe 5 as a material for engineering topological properties, with the potential to generalize this approach to study topological phase transitions in van der Waals materials and heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Observation of band splitting and magnetically induced band structure reconstruction in TbTi 3 ⁢Bi 4

The magnetic kagome materials are a promising platform to study the interplay between magnetism, topology, and correlated electronic phenomena. Among these materials, the 𝑅⁢Ti 3 ⁢Bi 4 family received a great deal of attention recently because of its chemical versatility and wide range of magnetic properties. Here, we use angle-resolved photoemission spectroscopy measurements and density functional theory calculations to investigate the electronic structure of TbTi 3 ⁢Bi 4 in paramagnetic and antiferromagnetic phases. Our experimental results show the presence of unidirectional band splitting of unknown nature in both phases. In addition, we observed a complex reconstruction of the band structure in the antiferromagnetic phase. Furthermore, some aspects of this reconstruction are consistent with effects of additional periodicity introduced by the magnetic ordering vector, while the nature of several other features remains unknown.

Angle-resolved photoemission spectroscopy↗
DOE OSTI2015

Unconventional transformation of spin Dirac phase across a topological quantum phase transition

The topology of a topological material can be encoded in its surface states. These surface states can only be removed by a bulk topological quantum phase transition into a trivial phase. Here we use photoemission spectroscopy to image the formation of protected surface states in a topological insulator as we chemically tune the system through a topological transition. Surprisingly, we discover an exotic spin-momentum locked, gapped surface state in the trivial phase that shares many important properties with the actual topological surface state in anticipation of the change of topology. Using a spin-resolved measurement, we show that apart from a surface bandgap these states develop spin textures similar to the topological surface states well before the transition. Our results provide a general paradigm for understanding how surface states in topological phases arise from a quantum phase transition and are suggestive for the future realization of Weyl arcs, condensed matter supersymmetry and other fascinating phenomena in the vicinity of a quantum criticality.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

In-situ electron microscopy mapping of an order-disorder transition in a superionic conductor

Solid-solid phase transitions are processes ripe for the discovery of correlated atomic motion in crystals. Here, we monitor an order-disorder transition in real-time in nanoparticles of the super-ionic solid, Cu 2-x Se. The use of in-situ high-resolution transmission electron microscopy allows the spatiotemporal evolution of the phase transition within a single nanoparticle to be monitored at the atomic level. The high spatial resolution reveals that cation disorder is nucleated at low co-ordination, high energy sites of the nanoparticle where cationic vacancy layers intersect with surface facets. Time-dependent evolution of the reciprocal lattice of individual nanoparticles shows that the initiation of cation disorder is accompanied by a ~3% compression of the anionic lattice, establishing a correlation between these two structural features of the lattice. The spatiotemporal insights gained here advance understanding of order-disorder transitions, ionic structure and transport, and the role of nanoparticle surfaces in phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Dynamic non-reciprocal meta-surfaces with arbitrary phase reconfigurability based on photonic transition in meta-atoms

We introduce a distinct class of dynamic non-reciprocal meta-surfaces with arbitrary phase-reconfigurability. This meta-surface consists of an array of meta-atoms, each of which is subject to temporal refractive index modulation, which induces photonic transitions between the states of the meta-atom. We show that arbitrary phase profile for the outgoing wave can be achieved by controlling the phase of the modulation at each meta-atom. Moreover, such dynamic meta-surfaces exhibit non-reciprocal response without the need for magneto-optical effects. The use of photonic transition significantly enhances the tunability and the possible functionalities of meta-surfaces.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

A two-component protein condensate of the EGFR cytoplasmic tail and Grb2 regulates Ras activation by SOS at the membrane

We reconstitute a phosphotyrosine-mediated protein condensation phase transition of the ~200 residue cytoplasmic tail of the epidermal growth factor receptor (EGFR) and the adaptor protein, Grb2, on a membrane surface. The phase transition depends on phosphorylation of the EGFR tail, which recruits Grb2, and crosslinking through a Grb2-Grb2 binding interface. The Grb2 Y160 residue plays a structurally critical role in the Grb2-Grb2 interaction, and phosphorylation or mutation of Y160 prevents EGFR:Grb2 condensation. By extending the reconstitution experiment to include the guanine nucleotide exchange factor, SOS, and its substrate Ras, we further find that the condensation state of the EGFR tail controls the ability of SOS, recruited via Grb2, to activate Ras. These results identify an EGFR:Grb2 protein condensation phase transition as a regulator of signal propagation from EGFR to the MAPK pathway.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2023

Band structure, superconductivity, and polytypism in AuSn 4

The orthorhombic compound AuSn 4 is compositionally similar to the Dirac node arc semimetal PtSn 4 . AuSn 4 is, contrary to PtSn 4 , superconducting with a critical temperature of T c = 2.35 K. Recent measurements present indications for quasi-two-dimensional superconducting behavior in AuSn 4 . Here we present measurements of the superconducting density of states and the band structure of AuSn 4 through scanning tunneling microscopy and angular resolved photoemission spectroscopy (ARPES). The superconducting gap values in different portions of the Fermi surface are spread around Δ 0 = 0.4 meV, which is close to but somewhat larger than Δ = 1.76k B T c expected from BCS theory. We observe superconducting features in the tunneling conductance at the surface up to temperatures about 20% larger than bulk T c . The band structure calculated with density functional theory follows well the results of ARPES. The crystal structure presents two possible stackings of Sn layers, giving two nearly degenerate polytypes. This makes AuSn 4 a rather unique case with a three-dimensional electronic band structure but properties ressembling those of low-dimensional layered compounds.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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 OSTI2014

Surface effects and phase stability in metal oxides nanoparticles under visible irradiation

The light induced phase transformation between stable phases of metal oxides nanoparticles is analyzed. The surrounding atmosphere as well as the defect density at the surface play a fundamental role. It has been found that in oxygen poor chamber atmosphere the phase transformation is favored, while the phase transition cannot be achieved if the defects at the surface are properly passivated. The phase transition is activated by intragap irradiation, able to activate the F- center at the surface connected to oxygen vacancies, and promoting the activation of the surface and the nucleation of neighboring crystallites. The phase transition was studied in Titanium oxide (TiO{sub 2}) and in Iron oxide (Fe{sub 2}O{sub 3}): Maghemite is subjected to a phase transformation to α−Fe{sub 2}O{sub 3} (hematite), Anatase nanoparticles converts to Rutile. The general mechanism of the phase transition and, more in general, the possibility to optically control the surface activity of metal oxides is discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Ferro-ionic states and domains morphology in Hf x Zr 1–x O 2 nanoparticles

Unique polar properties of nanoscale hafnia-zirconia oxides (Hf x Zr 1–x O 2 ) are of great interest for condensed matter physics, nanophysics, and advanced applications. These properties are connected (at least partially) to the ionic–electronic and electrochemical phenomena at the surface, interfaces, and/or internal grain boundaries. Here, we calculated the phase diagrams, dielectric permittivity, spontaneous polar, and antipolar ordering, as well as the domain structure morphology in Hf x Zr 1–x O 2 nanoparticles covered by ionic–electronic charge originating from surface electrochemical adsorption. We revealed that the ferro-ionic coupling supports the polar long-range order in nanoscale Hf x Zr 1–x O 2 , induces, and/or enlarges the stability region of the labyrinthine domains toward smaller sizes and smaller environmental dielectric constant at low concentrations of the surface ions. The ferro-ionic coupling causes the transition to the single-domain ferro-ionic state at high concentrations of the surface ions. We predict that the labyrinthine domain states, being multiple-degenerated, may significantly affect the emergence of the negative differential capacitance state in the nanograined/nanocrystalline Hf x Zr 1–x O 2 films.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Surface Energies of LaMnO 3 High-Index Surfaces Obtained from Density-Functional Theory

Here, the first six cubic LaMnO 3 high-index surfaces, (210), (211), (221), (310), (311), and (320), are examined for the first time. The unrelaxed and relaxed surface energies of the surfaces are computed as a function of surface model thickness using unrelaxed and relaxed surface models, respectively. The (210), (320), (211), and (221) surfaces are found to be more stable than the low-index (011) and (111) surfaces. Helping to explain this result, the surface terminations of the (210), (320), (211), or (221) surface are seen to exhibit a rotational relaxation of the MnO 6 or oxygen octahedra extending from the surface into the bulk. By contrast, the relaxed surface energies of the (310) and (311) surfaces are concluded to be difficult to determine due to a structural transformation or phase change in the surface models of the surfaces away from cubic LaMnO 3 . The finding of a phase change is important. It indicates that a phase change in the surface models of a surface for another material could occur if the surface is relatively open and a different phase of the material is stable at low temperature. When modeling such a surface, two steps are suggested to be taken.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Phase Pattern Formation in Grain Boundaries

Here, in this Letter, we derive conditions that predict the existence of two-phase periodic-pattern grain boundary structures that are stable against coarsening. While previous research has established that elastic effects can lead to phase pattern formation on crystal surfaces, the possibility of stable grain boundary structures composed of alternating grain boundary phases has not been previously analyzed. Our theory identifies the specific combination of grain boundary and materials properties that enable the emergence of patterned grain boundary states and shows that the dislocation content of grain boundary phase junctions, absent in surface phenomena, weakens the stability of the patterned structures. The predictions of the theory are tested using a model copper grain boundary that exhibits multiple phases and two-phase pattern formation. We discuss how, similarly to surfaces, elastic effects associated with grain boundary phase junctions have profound implications for how grain boundary phases transform.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Consequences of aging on ferromagnetic amorphous Fe 75 Si 10 B 15 microwires for advanced inductive applications

Amorphous ferromagnetic microwires fabricated by water-quenching have been assessed to show favorable properties for next-generation high-frequency electric machines as compared to those of conventional amorphous magnetic alloys in sheet or ribbon form. Here, water-quenched amorphous Fe 75 Si 10 B 15 microwires were subjected to a range of aging times of up to 5 years in the air at room temperature. While both newer and aged microwires are X-ray amorphous, the aged microwires do exhibit a slight degree of crystallinity, a lower initial susceptibility and a reduced enthalpy change for full devitrification. These small differences (typically < 5%) are attributed to the formation of minor surface oxidation products such as iron oxides and SiO 2 which modify the surface strain state. The resultant stress couples to magnetostriction to promote the formation of radial magnetic domains that impair the magnetic reversal process. These results demonstrate that while amorphous ferromagnetic microwires are essentially stable against aging in the air, consideration of their surface state will be significant for advanced applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Hard, transparent, sp3-containing 2D phase formed from few-layer graphene under compression

Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp 3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO 2 /Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp 3 -containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4e6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to ≈ 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO 2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure (≈ 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Overall, theoretical calculations and experimental data indicate a novel, surface to- bulk phase transition mechanism that gives hint of diamondene formation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2021

Integrated atomic force microscopy and x-ray irradiation for in situ characterization of radiation-induced processes

Understanding radiation-induced chemical and physical transformations at material interfaces is important across diverse fields, but experimental approaches are often limited to either ex situ observations or in situ electron microscopy or synchrotron-based methods in which cases the radiation type and dose are inextricably tied to the imaging basis itself. In this work we overcome this limitation by demonstrating integration of an X-ray source with an atomic force microscope to directly monitor radiolytically-driven interfacial chemistry at the nanoscale. We illustrate the value of in situ observations by examining effects of radiolysis on material adhesion forces in aqueous solution, as well as examining the production of alkali nitrates at the interface between an alkali halide crystal surface and air. For the examined salt-air interface, direct visualization under flexible experimental conditions greatly extends prior observations by enabling the transformation process to be followed comprehensively from source-to-sink with mass balance quantitation. Overall, our novel rad-AFM opens doors into understanding the dynamics of radiolytically-driven mass transfer and surface alteration at the nanoscale in real-time.

47 OTHER INSTRUMENTATION↗
DOE OSTI2023

Theory for anomalous terahertz emission in striped cuprate superconductors

Recent experiments in the doped cuprates La 2−𝑥 ⁢Ba 𝑥⁢ CuO 4 have revealed the emission of anomalous terahertz radiation after impulsive optical excitation. Here, we theoretically investigate the nonlinear electrodynamics of such striped superconductors and explore the origin of the observed radiation. We argue that photoexcitation is converted into a photocurrent by a second-order optical nonlinearity, which is activated by the breaking of inversion symmetry in certain stripe configurations. We point out the importance of including umklapp photocurrents modulated at the stripe periodicity itself, which impulsively drive surface Josephson plasmons and lead to a resonant structure of outgoing radiation, consistent with the experiments. We speculate on the utility of the proposed mechanism in the context of generating tunable terahertz radiation.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Martensitic Transformation and Magnetocaloric Effect in High-Pressure Synthesized Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} Alloys

The polycrystalline Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} ferromagnetic alloy was synthesized by high-pressure method. The Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} ribbons were prepared by melt-spinning; then, the ribbons were pressed to the bulk samples under the 6-GPa high-pressure and heated to 1173 K for 30 min. The surface morphology, phase transition, and magnetocaloric effect (MCE) were investigated in the Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} bulk sample. Structural analysis and magnetic measurements studies of the bulk samples revealed the martensitic transformation at room temperature with a low thermal hysteresis of magnetization, and the nanocrystalline was observed by atomic force microscope measurements. The Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} bulk sample exhibits the directionality, and the magnetic properties have the large magnetization and low coercivity when the sample direction of the applied pressure was perpendicular to the direction of the applied magnetic field. The maximum entropy changes (∆S{sub M}) of − 4.8 J/kg K have been obtained at 330 K under the field of 30 kOe in the Ni{sub 53}Mn{sub 23.5}Ga{sub 23.5} bulk sample.

36 MATERIALS SCIENCE↗