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Results for “superconducting phase transition”

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 613 records · Page 34
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Orbital correlations in ultrathin films of late transition metals

We develop a two-orbital Hubbard model of electron correlations in ultrathin (111)-oriented fcc films of late transition metals such as Co and Ni. Our model indicates that the Mott-Hund’s interaction results in ferromagnetic nearest-neighbor orbital correlations. Frustration associated with the mismatch between orbital and crystal symmetries prevents orbital ordering, resulting in the orbital liquid state. This state can be manifested in phenomena involving spin-orbit coupling, such as magnetic anisotropy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Towards exact finite temperature electronic structure in solids and molecules (Final Technical Report)

This report describes the University of Iowa portion of a project that is now continuing at Michigan State University. We are developing novel methods, algorithms, and software to enable simulations of molecules and materials at high temperature. This is a key challenge in chemistry and materials science. By refining an approach called Density Matrix Quantum Monte Carlo (DMQMC), we developed faster and more accurate ways to conduct these simulations. These advances will help us understand how temperature affects the behavior of electrons, chemical bonds, and phase transitions in solids and molecules. These breakthroughs are especially important for applications where light and heat drive chemical reactions, superconductivity, and materials used in energy and sensing. In addition, this project involved the development of the open-source HANDE-QMC software package, supporting the broader community in benchmarking and developing finite-temperature electronic structure methods.

36 MATERIALS SCIENCE↗
NASA NTRS1996

Preparation and Properties of High-T(sub c) Bi-Pb-Sr-Ca-Cu-O Thick Film Superconductors on YSZ Substrates

An evaluation of four firing profiles was performed to determine the optimum processing conditions for producing high-T(sub c) Bi-Pb-Sr-Ca-Cu-O thick films on yttria-stabilized zirconia substrates. Using these four profiles, the effects of sintering temperatures of 830-850 C and soak times of 0.5 to 12 hours were examined. In this study, T-c, zero values of 100 K were obtained using a firing profile in which the films were sintered for 1.5 to 2 hours at 840 to 845 C and then quenched to room temperature. X-ray diffraction analyses of these specimens confirmed the presence of the high-T(sub c) phase. Films which were similarly fired and furnace cooled from the peak processing temperature exhibited a two-step superconductive transition to zero resistance, with T-c,zero values ranging from 85 to 92 K. The other firing profiles evaluated in this investigation yielded specimens which either exhibited critical transition temperatures below 90 K or did not exhibit a superconductive transition above 77 K.

Hooker, Matthew W.↗
DOE OSTI2020

Universal temperature dependence of the London penetration depth in κ - ( ET ) 2 X superconductors

High-precision radio-frequency magnetic susceptibility measurements were performed on single crystals of fully deuterated $κ-(ET)_2Cu[N(CN)_2]\,Br$, hereafter designated as $κ-(ET)_2Cu[N(CN)_2]\,Br$. This material phase separates into superconducting and antiferromagnetic regions, the degree of which depends strongly upon the cooling rate. Here, we show that the screening fraction $η_{sc}$ varies logarithmically with the cooling rate over nearly five decades. The average size of superconducting regions is estimated to vary from 5 to $40μm$, depending upon cooling rate, consistent with previous infrared microscopy measurements. In the region $T≲T_c/3$, the effective magnetic penetration depth exhibits power-law behavior $λ(T)-λ(0)~T^n$ with $n=1.6$, independent of the cooling rate. Changes in cooling rate and the consequent phase separation evidently do not introduce the kind of disorder that would alter the exponent $n$ in a $d$-wave superconductor. The exponent remains close to $n=$ 1.5, reported in single crystals of $κ-(ET)_2Cu[N(CN)_2]\,Br$ and $κ-(ET)_2Cu(NCS)_2$ [A. Carrington et al., Phys. Rev. Lett 83, 4172 (1999)]. The transition temperature fell linearly with $1-η_{sc}$. Measurements were also made on $κ-(ET)_2Cu[N(CN)_2]\,Cl$ of normal isotopic abundance in which a very small amount of superconducting phase $η_{sc}≈10^{-4}$ developed, presumably through the strain-induced sample mounting. This material showed a power-law exponent of $n=1.64$, independent of the cooling rate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Purely electronic insulator-metal transition in rutile VO 2

Volatile resistive switching in neuromorphic computing can be tuned by external stimuli such as temperature or electric-field. However, this type of switching is generally coupled to structural changes, resulting in slower reaction speed and higher energy consumption when incorporated into an electronic device. The vanadium dioxide (VO 2 ), which has near room temperature metal-insulator transition (MIT), is an archetypical volatile resistive switching system. Here, we demonstrate an isostructural MIT in an ultrathin VO 2 film capped with a photoconductive cadmium sulfide (CdS) layer. Transmission electron microscopy, resistivity experiments, and first-principles calculations show that the hole carriers induced by CdS photovoltaic effect are driving the MIT in rutile VO 2 . The insulating-rutile VO 2 phase has been proved and can remain stable for hours. Our finding provides a new approach to produce purely electronically driven MIT in VO 2 , and widens its applications in fast-response, low-energy neuromorphic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Band-Selective Spin-Charge Separation across the Charge Density Wave Transition in Quasi-1D NbSe 3

Non-Fermi liquid (non-FL) phase is a pivotal enigma in understanding intriguing quantum phases in strongly correlated systems, such as high-temperature superconductivity. Tomonaga-Luttinger liquid (TLL) theory, designed for one-dimensional (1D) systems, serves as one of the microscopic frameworks that elucidates non-FL behavior. Despite its theoretical concreteness, comprehensive experimental verification has remained incomplete. In particular, addressing the persistence of the TLL nature within ordered phases, such as charge density wave (CDW), has posed a significant challenge. We report the observation of TLL characteristics across the CDW transitions in a quasi-1D material NbSe 3 using angle-resolved photoemission spectroscopy. Spin-charge separation, a disentanglement of spin and charge degrees of freedom of electrons, is clearly observed within the band, accompanied by the suppression of spectral weight following a power-law behavior with anticipated temperature scaling. Surprisingly, the spin-charge separation persists even below the CDW transition temperatures, indicating the validity of the TLL nature in the CDW phase. In conclusion, our findings offer a unique opportunity to explore the interplay between the TLL and ordered phases, which could be connected to the interplay between non-FL and ordered phases in strongly correlated systems.

1-dimensional systems↗
DOE OSTI2021

Metal-insulator transition in transition metal dichalcogenide heterobilayer moiré superlattices

Moiré superlattices formed in two-dimensional semiconductor heterobilayers provide a new realization of Hubbard model physics in which the number of electrons per effective atom can be tuned at will. Here we report on an exact diagonalization study of the electronic properties of half-filled narrow moiré bands in which correlation strengths are varied by changing twist angles or interaction strengths. We construct a phase diagram for the bilayer, identifying where the metal-insulator phase transition occurs, estimating the sizes of the charge gaps in the insulating phase, and commenting on the nature of the transition and the importance of subdominant interaction parameters.

2-dimensional systems↗
NASA NTRS1983

Experiments with phase transitions at very high pressure

Diamond cells were constructed for use to 1 Mbar. A refrigerator for cooling diamond cells was adapted for studies between 15 and 300 K. A cryostat for superconductivity studies between 1.5 to 300 K was constructed. Optical equipment was constructed for fluorescence, transmission, and reflectance studies. X-ray equipment was adapted for use with diamond cells. Experimental techniques were developed for X-ray diffraction studies using synchrotron radiation. AC susceptibility techniques were developed for detecting superconducting transitions. The following materials were studied: compressed solidified gases (Xe, Ar), semiconductors (Ge, Si, GaAs), superconductors (Nb3Ge, Nb3Si, Nb3As, CuCl), molecular crystals (I).

Spain, I. L.↗
DOE OSTI2022

Rare-earth monopnictides: Family of antiferromagnets hosting magnetic Fermi arcs

We report since the discovery of topological insulators a great deal of research effort has been devoted to magnetic topological materials, in which nontrivial spin properties can be controlled by magnetic fields, culminating in a wealth of fundamental phenomena and possible applications. The main focus was on ferromagnetic materials that can host Weyl fermions and therefore spin-textured Fermi arcs. The recent discovery of Fermi arcs and new magnetic band splitting in the antiferromagnet (AFM) NdBi has opened up new avenues for exploration. Here we show that these uncharted effects are not restricted to this specific compound, but also emerge in CeBi and NdSb when they undergo paramagnetic to AFM transition. Our data show that the Fermi arcs in NdSb have twofold symmetry, leading to strong anisotropy that may enhance effects of spin textures on transport properties. Our findings thus demonstrate that the RBi and RSb series are materials that host magnetic Fermi arcs and may be a potential platform for modern spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Pressure Induced Topological Quantum Phase Transition in Weyl Semimetal T d -MoTe 2

In this paper, we report the temperature and pressure (p max ≃1.5 GPa) evolution of the crystal structure of the Weyl semimetal T d -MoTe 2 by combination of neutron diffraction and the X-ray total scattering experiments. We find that the fundamental non-centrosymmetric structure T d is fully suppressed and transforms into a centrosymmertic 1T' structure at a critical pressure of p cr ~ 1.2–1.4 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting (SC) critical temperature T c , the superfluid density, and the SC gap all change smoothly and continuously across p cr and no sudden effects are seen concomitantly with the suppression of the T d structure. This implies that the T c , and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s +- to a trivial s ++ state, which we discuss in this work. Our systematic characterizations of the structure and SC properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Spin-orbital liquids and insulator-metal transitions on the pyrochlore lattice

The two orbital Hubbard model, with the electrons additionally coupled to a complex magnetic background, arises in the pyrochlore molybdates. The background involves local moments Hund's coupled to the electrons, driving double exchange ferromagnetism, and antiferromagnetic (AF) tendency arising from competing superexchange. The key scales include the Hubbard repulsion and the superexchange, both of which can be tuned in these materials. They control the phase transition from a ferromagnetic metal to a spin glass metal and then to a spin glass (Mott) insulator. We provide a comprehensive description of the ground state of this model using an unrestricted Hartree-Fock scheme implemented via a simulated annealing procedure and establish the metal-insulator transition line for varying Hubbard interaction and superexchange. The electrons see an effective disorder, due to orbital frustration, already in the ferromagnetic phase. The disorder is further enhanced by antiferromagnetic coupling and the resulting magnetic disorder. As a result, increasing AF coupling shifts the metal-insulator transition to lower Hubbard interaction and gives it an additional "Anderson" character. In conclusion, we provide detailed results on the magnetic and orbital correlations, the density of states, and the optical conductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Effect of inversion asymmetry on the superconducting and exciton condensates of bilayer graphene

Inversion asymmetry in bilayer graphene can be tuned by the displacement field. As a result, the band dispersion in biased bilayer graphene acquires flatband regions near the Dirac points along with a nontrivial band geometry. We analyze the effect of inversion asymmetry on the critical temperature and superfluid stiffness of the superconducting state of AB-stacked graphene bilayer and the exciton condensate in double layers formed by two AB-stacked graphene bilayers. We find that the geometric superfluid stiffness in bilayer graphene superconductors is negligible due to the small superconducting gap. Furthermore, since the geometric superfluid stiffness is maximized for a constant order parameter, it can be neglected in biased bilayer graphene superconductors with any pairing symmetry. In contrast, the displacement field enhances the geometric superfluid stiffness in exciton condensates. It is most prominent at low densities and high displacement fields. Here, a consequence of the geometric superfluid stiffness is a modest enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature in bilayer graphene’s exciton condensate.

BKT transition↗
DOE OSTI2021

Measurements of nonequilibrium interatomic forces using time-domain x-ray scattering

Here, we demonstrate an experimental approach to determining the excited-state interatomic forces using femtosecond x-ray pulses from an x-ray free-electron laser. We determine experimentally the excited-state interatomic forces that connect photoexcited carriers to the nonequilibrium lattice dynamics in the prototypical Peierls-distorted material, bismuth. The forces are obtained by a constrained least-squares fit of a pairwise interatomic force model to the excited-state phonon dispersion relation as measured by the time- and momentum-resolved x-ray diffuse scattering. We find that photoexcited carriers weaken predominantly the nearest-neighbor forces, which drives the measured softening of the transverse acoustic modes throughout the Brillouin zone as well as the zone-center A 1g optical mode. This demonstrates a bond-selective approach to measuring electron-phonon coupling relevant to a broad range of photoinduced phase transitions and transient light-driven states in quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Acoustic phonon dispersion of α – RuCl 3

Acoustic phonons have recently been posited as playing an integral role in explaining the halfquantized thermal Hall effect in α-RuCl 3 . Therefore, we present much needed inelastic x-ray scattering measurements of its acoustic phonon dispersion, along with calculations using the frozen-phonon method. We also discuss a temperature study which conclusively shows a first-order structural transition to a non-C2/m space group at low temperature. Together these results are an important backbone for future theoretical and experimental studies of α-RuCl 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Evidence for an odd-parity nematic phase above the charge-density-wave transition in a kagome metal

The search for quantum states arising from the interplay between correlation, frustration and topology is a central topic for condensed-matter physics. Recently discovered non-magnetic kagome metals AV 3 Sb 5 (A = K, Cs or Rb) with charge-density-wave and superconducting instabilities may host such exotic states. Here we report evidence that an odd-parity electronic nematic state emerges at a higher temperature than the charge density wave in CsV 3 Sb 5 . Our torque measurements reveal a two-fold in-plane magnetic anisotropy that breaks the crystal rotational symmetry. Moreover, in the temperature range between the formation of the charge density wave and a nematic state, rotating an external magnetic field in a conical fashion yields a distinct first-order phase transition, indicating time-reversal symmetry breaking. These results provide thermodynamic evidence for the emergence of an odd-parity nematic order. In addition, elastoresistance shows no discernible anomalies near the onset of nematicity, consistent with the odd-parity order. In conclusion, these findings suggest that an exotic loop current state precedes the charge-density-wave transition in CsV 3 Sb 5 .

36 MATERIALS SCIENCE↗
DOE OSTI2022

Complex structure due to As bonding and interplay with electronic structure in superconducting BaNi 2 As 2

BaNi 2 As 2 is a superconductor chemically related to the Fe-based superconductors, with a complex and poorly understood structural phase transition. We show, based on first-principles calculations, that, in fact, there are two distinct competing structures. These structures are different from the electronic, transport, and bonding points of view but are close in energy. These arise due to complex As bonding patterns and drive distortions of the Ni layers. This is supported by photoemission experiments. This is very distinct from views of the distortion as being primarily driven by electrons at the Fermi surface as in a classical charge density wave, or by correlated electron physics associated with the Ni d electrons and the Ni d orbitals. The structural distortion, although connected with As does lead to an interplay of electronic and structural behavior including induced anisotropic electronic transport. Here, the local bonding nature of the instabilities and the competition between distortions is discussed in the context of the complex behavior observed in BaNi 2 As 2 samples.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Metastable piezoelectric group-IV monochalcogenide monolayers with a buckled honeycomb structure

Multiple two-dimensional materials are being naïvely termed stable on the grounds of displaying phonon dispersions with no negative frequencies and of not collapsing on molecular dynamics calculations at fixed volume. But, if these phases do not possess the smallest possible structural energy, how does one understand and establish their actual meta stability? To answer this question, twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure—belonging to symmetry group P3m1—displaying an out-of-plane intrinsic electric polarization are shown to be metastable by three independent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy Pnm2 1 (or Pmmn for PbS, PbSe, and PbTe) phase to estimate energy barriers E B that must be overcome during such structural transformation. Using the curvature of the local minima and E B as inputs to Kramers escape formula, large escape times are found, implying the structural metastability of the buckled honeycomb phase (with the exception of PbS and PbSe, these phases display escape times ranging from 700 years to multiple times the age of the universe and can be considered “stable” for practical purposes in that relative sense). The second demonstration is provided by phonon dispersion relations that include the effect of long-range Coulomb forces and display no negative vibrational modes. The third and final demonstration of structural metastability is furnished by room-temperature ab initio molecular dynamics for selected compounds. Here, the magnitude of the electronic band gap evolves with chemical composition. Different from other binary two-dimensional compounds such as transition metal dichalcogenide monolayers and hexagonal boron nitride monolayers which only develop an in-plane piezoelectric response, the twelve group-IV monochalcogenide monolayers with a buckled honeycomb structure also display out-of-plane piezoelectric properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗