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Results for “Superconducting 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 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Fragile Topology and Flat-Band Superconductivity in the Strong-Coupling Regime

In flat bands, superconductivity can lead to surprising transport effects. The superfluid “mobility”, in the form of the superfluid weight D s , does not draw from the curvature of the band but has a purely band-geometric origin. In a mean-field description, a nonzero Chern number or fragile topology sets a lower bound for D s , which, via the Berezinskii-Kosterlitz-Thouless mechanism, might explain the relatively high superconducting transition temperature measured in magic-angle twisted bilayer graphene (MATBG). For fragile topology, relevant for the bilayer system, the fate of this bound for finite temperature and beyond the mean-field approximation remained, however, unclear. Here, we numerically use exact Monte Carlo simulations to study an attractive Hubbard model in flat bands with topological properties akin to those of MATBG. We find a superconducting phase transition with a critical temperature that scales linearly with the interaction strength. Then, we investigate the robustness of the superconducting state to the addition of trivial bands that may or may not trivialize the fragile topology. Our results substantiate the validity of the topological bound beyond the mean-field regime and further stress the importance of fragile topology for flat-band superconductivity.

2-dimensional systems↗
DOE OSTI2025

Anomalous lattice relaxation dynamics in optimally doped La 2−𝑥⁢ Sr 𝑥 ⁢CuO 4

The atomic lattice plays a critical role in the emergence of high-𝑇 𝑐 superconductivity in cuprates. While the dynamics associated with electron-lattice coupling typically unfold on picosecond-to-femtosecond timescales, we present an x-ray photon correlation spectroscopy investigation on an optimally doped La-based cuprate that reveals a strong response of kilosecond-scale lattice relaxation dynamics to the superconducting state. Notably, an anomaly emerges around 𝑇 𝑐 : upon cooling into the superconducting state, the average atomic relaxation lifetime decreases, i.e., dynamics accelerate. This indicates a significant change in the local disorder-induced strain field dynamics at the superconducting transition, highlighting a remarkable coupling between superconductivity and the lattice on quasistatic timescales.

Petsch, A. N. [SLAC National Accelerator Laborator↗
DOE OSTI2020

Interfacial Spin-Orbit Coupling: A Platform for Superconducting Spintronics

Spin-orbit coupling (SOC) is a key interaction in spintronics, allowing electrical control of spin or magnetization and, vice versa, magnetic control of electrical current. However, recent advances have revealed much broader implications of SOC that is also central to the design of topological states with potential applications from low-energy dissipation and faster magnetization switching to high tolerance of disorder. SOC and the resulting emergent interfacial spin-orbit fields are simply realized in junctions through structural inversion asymmetry, while the anisotropy in magnetoresistance (MR) allows their experimental detection. Surprisingly, we demonstrate that an all-epitaxial ferromagnet/MgO/metal junction with a single ferromagnetic region and only negligible MR anisotropy undergoes a remarkable transformation below the superconducting transition temperature of the metal. The superconducting junction has a MR anisotropy 3 orders of magnitude higher and could enable novel applications in superconducting spintronics. In contrast to common realizations of MR effects that require a finite applied magnetic field, our system is designed to have two stable zero-field states with mutually orthogonal magnetizations: in plane and out of plane. This bistable magnetic anisotropy allows us to rule out orbital and vortex effects due to an applied magnetic field and identify the SOC origin of the observed MR. Such MR reaches approximately 20% without an applied magnetic field and could be further increased for large magnetic fields that support vortices. Our findings call for a revisit of the role of SOC, even when it seems negligible in the normal state, and suggest an alternative platform for superconducting spintronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Artificial High-Transition Temperature Superconductors

In this work, we have used the well-understood quantum Hall (QH) stripes in high quality two-dimensional electron gases to mimic charge stripes in high transition temperature (Tc) superconductors. The science question we want to address is “Can QH stripes mimic high Tc superconductor stripes and provide a controlled experimental setup to pin-down the role of stripes in high Tc superconductivity?”. We have observed anomalous superconducting transition like behavior in GaAs double quantum well systems (DQWs) when each quantum well (QW) is tuned to the charge stripe states but with different Landau level fillings. Furthermore, we have shown that the transition like behavior is sharper in the DQWs when the two QWs are more strongly coupled. Our results suggest, for the first time, experimental evidence of the paired charge stripes model, which might lead to room-temperature superconductors that have enormously wide applications in computing, energy, and transportation industries. Advancing the science of high transition temperature superconductivity will have a profound impact in advancing energy technologies, ranging from the next generation microchips, new energy transfer grid to public transportation, and thus is important to nation’s energy security and relevant across the landscape of many mission spaces. Sandia has been a leader in materials science research and development. The proposed research takes advantage of Sandia’s state-ofthe-art MBE facilities at the Center for Integrated Nanotechnologies (CINT) and utilizes Sandia’s extensive advanced materials characterization resources. We envision a significant impact on the nation’s energy research and security challenges by investing in this research.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tuning superconductivity of Bi 2 Sr 2 CaCu 2 O 8+x by fluoride ion intercalation with LaF 3 gate dielectric

Here, we demonstrate fluorine intercalation into Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO) flakes, as thick as 100 nm, by back-gating a single crystalline LaF 3 substrate. Element-sensitive analysis not only confirms the presence of fluorine in BSCCO after gating but also reveals lateral diffusion of fluorine in BSCCO. Transport measurements further demonstrate the hole-doping effect of fluoride ions in BSCCO through effective modulation of the superconducting transitions. Our work introduces a distinct type of ions (F - ) that can be intercalated into materials via solid state back-gating. It broadens the toolbox of ion back-gating and is useful for addressing exotic phenomena in the heavily hole doped regime.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Effects of anisotropy and disorder on the superconducting properties of niobium

We report results for the superconducting transition temperature and anisotropic energy gap for pure niobium based on Eliashberg’s equations and electron and phonon band structures computed from density functional theory. The electronic band structure is used to construct the Fermi surface and calculate the Fermi velocity at each point on the Fermi surface. The phonon bands are in excellent agreement with inelastic neutron scattering data. The corresponding phonon density of states and electron–phonon coupling define the electron–phonon spectral function, α 2 F(p, p'; ω), and the corresponding electron–phonon pairing interaction, which is the basis for computing the superconducting properties. The electron–phonon spectral function is in good agreement with existing tunneling spectroscopy data except for the spectral weight of the longitudinal phonon peak at ℏω LO = 23 meV. We obtain an electron–phonon coupling constant of λ = 1.057, renormalized Coulomb interaction μ* = 0.218, and transition temperature T c = 9.33 K. The corresponding strong-coupling gap at T = 0 is modestly enhanced, Δ 0 = 1.55 meV, compared to the weak-coupling BCS value $Δ^{\text {wc}}_{0}$ = 1.78 k B T c = 1.43 meV. The superconducting gap function exhibits substantial anisotropy on the Fermi surface. We analyze the distribution of gap anisotropy and compute the suppression of the superconducting transition temperature using a self-consistent T-matrix theory for quasiparticle-impurity scattering to describe niobium doped with non-magnetic impurities. We compare these results with experimental results on niobium SRF cavities doped with nitrogen impurities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Hund's coupling assisted orbital-selective superconductivity in Ba1-xKxFe2As2

While the superconducting transition temperature of hole-doped Ba_{1-x}K_{x}Fe_{2}As_{2} decreases past optimal doping, superconductivity does not completely disappear even for the fully doped KFe_{2}As_{2} compound. In fact, superconductivity is robust through a Lifshitz transition where electron bands become hole-like around the zone corner at around x=0.7, thus challenging the conventional understanding of superconductivity in iron-based systems. High-resolution angle-resolved photoemission spectroscopy is used to investigate the superconducting gap structure, as well as the normal state electronic structure, around optimal doping and across the Lifshitz transition. Our findings reveal a largely orbital-dependent superconducting gap structure, where the more strongly correlated d_{xy} band has a vanishing superconducting gap at higher doping, aligning with the Hund's metal behavior observed in the normal state. Notably, the superconducting gap on the d_{xy} band disappears before the Lifshitz transition, suggesting that the Fermi surface topology may play a secondary role. We discuss how these results point to orbital-selective superconducting pairing and how strong correlations via Hund's coupling may shape superconducting gap structures in iron-based and other multiorbital superconductors.

FOS: Physical sciences↗
DOE OSTI2025

Epitaxial aluminum layer on antimonide heterostructures for exploring Josephson junction effects

In this study, we present results of our recent work of epitaxially-grown aluminum (epi-Al) on antimonide heterostructures, where the epi-Al thin film is grown at either room temperature or below zero °C. A sharp superconducting transition at T ∼ 1.3 K is observed in these epi-Al films. We further show that supercurrent states are realized in Josephson junctions fabricated in the epi-Al/antimonide heterostructures with mobility μ ∼ 1.0 × 10 6 cm 2 /Vs. These results clearly demonstrate we have achieved growing high-quality epi-Al/antimonide heterostructures, a promising platform for the exploration of Josephson junction effects for quantum information science and microelectronics applications.

Antimonide heterostructures↗
DOE OSTI2020

Polar Nanodomains in a Ferroelectric Superconductor

The mechanisms by which itinerant carriers compete with polar crystal distortions is a key unresolved issue for polar superconductors, which offer new routes to unconventional Cooper pairing. Strained, doped SrTiO 3 films undergo successive ferroelectric and superconducting transitions, making them ideal candidates to elucidate the nature of this competition. Here, we reveal these interactions using scanning transmission electron microscopy studies of the evolution of polar nanodomains as a function of doping. These nanodomains are a precursor to the ferroelectric phase and a measure of long-range Coulomb interactions. With increasing doping, the magnitude of the polar displacements, the nanodomain size, and the Curie temperature are systematically suppressed. In addition, we show that disorder caused by the dopant atoms themselves presents a second contribution to the destabilization of the ferroelectric state. Furthermore, the results provide evidence for two distinct mechanisms that suppress the polar transition with doping in a ferroelectric superconductor.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Superconductivity Observed in Tantalum Polyhydride at High Pressure

We report experimental discovery of tantalum polyhydride superconductor. It was synthesized under high-pressure and high-temperature conditions using diamond anvil cell combined with in situ high-pressure laser heating techniques. The superconductivity was investigated via resistance measurements at pressures. The highest superconducting transition temperature T c was found to be ∼ 30 K at 197 GPa in the sample that was synthesized at the same pressure with ∼ 2000 K heating. The transitions are shifted to low temperature upon applying magnetic fields that support the superconductivity nature. The upper critical field at zero temperature μ 0 H c2 (0) of the superconducting phase is estimated to be ∼ 20 T that corresponds to Ginzburg–Landau coherent length ∼ 40 Å. Our results suggest that the superconductivity may arise from I 4 ¯ 3 d phase of TaH 3 . It is, for the first time to our best knowledge, experimental realization of superconducting hydrides for the VB group of transition metals.

Physics↗
DOE OSTI2021

Nanoscale Superconducting States in the Fe-Based Filamentary Superconductor of Pr-Doped CaFe 2 As 2

The low-temperature scanning tunneling microscope and spectroscopy (STM/STS) are used to visualize superconducting states in the cleaved single crystal of 9% praseodymium-doped CaFe 2 As 2 (Pr-Ca122) with Tc ≈ 30 K. The spectroscopy shows strong spatial variations in the density of states (DOS), and the superconducting map constructed from spectroscopy discloses a localized superconducting phase, as small as a single unit cell. The comparison of the spectra taken at 4.2 K and 22 K (below vs. close to the bulk superconducting transition temperature) from the exact same area confirms the superconducting behavior. Nanoscale superconducting states have been found near Pr dopants, which can be identified using dI/dV conductance maps at +300 mV. There is no correlation of the local superconductivity to the surface reconstruction domain and surface defects, which reflects its intrinsic bulk behavior. We, therefore, suggest that the local strain of Pr dopants is competing with defects induced local magnetic moments; this competition is responsible for the local superconducting states observed in this Fe-based filamentary superconductor.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Superconductivity and phase diagrams of CaK(Fe 1- x Mn x ) 4 As 4 single crystals

In order to study the effects of Mn substitution on the superconducting and magnetic ground state of CaKFe 4 As 4 (T c = 35 K), members of the CaK (Fe 1–x Mn x ) 4 As 4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by elemental analysis, thermodynamic, and transport measurements. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as x is increased from 0 to 0.036. For x values greater than 0.016, signatures of a magnetic transition can be detected in both thermodynamic and transport measurements in which kinklike features allow for the determination of the transition temperature T* that increases as Mn substitution increases. Here, a temperature-composition (T–x) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature T* appearing near 26 K for x ~ 0.017 and rising slowly up to 33 K for x ~ 0.036. In addition to the creation of the T–x phase diagram for CaK(Fe 1–x Mn x ) 4 As 4 , specific-heat data are used to track the jump in specific heat at T c ; the CaK(Fe 1–x Mn x ) 4 As 4 data do not follow the scaling of ΔC p with $T$$^{3}_{c}$ as many of the other Fe-based superconducting systems do. These data suggest that, as magnetic pair breaking is present, the jump in C p for a given T c is reduced. Elastoresistivity coefficients 2m 66 and m 11 – m 12 as a function of temperature are also measured. 2m 66 and m 11 – m 12 are qualitatively similar to CaK(Fe 1–x Ni x ) 4 As 4 . This may indicate that the magnetic order in Mn-substituted system may be still the same as CaK(Fe 1–x Mn x ) 4 As 4 . Superconductivity of CaK(Fe 1–x Mn x ) 4 As 4 is also studied as a function of magnetic field. A clear change in H' c2 (T)/T c , where H' c2 (T) is dH' c2 (T)/dT, at x ~ 0.015 is observed and probably is related to change of the Fermi surface due to magnetic order. Coherence lengths and the London penetration depths are also calculated based on H c1 and H c2 data. Coherence lengths as the function of x also show the changes near x = 0.015, again consistent with Fermi-surface changes associated with the magnetic ordering seen for higher-x values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Superconductivity and magnetic and transport properties of single-crystalline CaK ( Fe 1 – x Cr x ) 4 As 4

Members of the CaK (Fe 1–x Cr x ) 4 As 4 series have been synthesized by high-temperature solution growth in single-crystalline form and characterized by x-ray diffraction, elemental analysis, and magnetic and transport measurements. The effects of Cr substitution on the superconducting and magnetic ground states of CaKFe 4 As 4 (T c = 35 K) have been studied. These measurements show that the superconducting transition temperature decreases monotonically and is finally suppressed below 1.8 K as x is increased from 0 to 0.038. For x-values greater than 0.012, signatures of a magnetic transition can be detected in magnetic measurements with the associated features in the transport measurements becoming detectable for x ≥ 0.038. The magnetic transition temperature increases in a roughly linear manner as Cr substitution increases. A temperature-composition (T – x) phase diagram is constructed, revealing a half-dome of superconductivity with the magnetic transition temperature, T*, appearing near 22 K for x ~ 0.017 and rising slowly up to 60 K for x ~ 0.077. The T – x phase diagrams for CaK (Fe 1–x T x ) 4 As 4 for T = Cr and Mn are essentially the same despite the nominally different band filling; this is in marked contrast to T = Co and Ni series for which the T – x diagrams scale by a factor of 2, consistent with the different changes in band filling Co and Ni would produce when replacing Fe. Superconductivity of CaK (Fe 1–x Cr x ) 4 As 4 is also studied as a function of magnetic field. A clear change in $H^{'}_{c2}$ (T)/T c , where $H^{'}_{c2}$ (T) is dH c2 (T)/dT, at x ~ 0.012 is observed and probably is related to a change of the Fermi surface due to magnetic order. Further, coherence length and the London penetration depths are also calculated based on H c1 and H c2 data. Both of them as a function of x show changes near x = 0.012, again consistent with Fermi surface changes associated with the magnetic ordering seen for higher x-values.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Correlation between thickness dependent nanoscale structural chemistry and superconducting properties of ultrathin epitaxial NbN films

NbN-based superconductors have attracted interest for superconducting circuits, quantum computation and high frequency devices. The superconducting properties of NbN films are predominately reliant on the microstructure, therefore, an atomic-level understanding of the structure-chemistry is required to achieve high quality epitaxial NbN films. Here, in this study, the thickness-dependent superconducting properties of NbN films (5, 10, and 50 nm) within NbN/AlN/Al 2 O 3 heterostructures are investigated. NbN and AlN were epitaxially grown by an industrial scale physical vapor deposition technique. The role of nanoscale chemistry on the ultrathin NbN superconducting film is investigated at the atomic level for the first time via atom probe tomography, providing three-dimensional atomic distribution, chemical homogeneity, effect of impurities, specially, in secondary phase formations and interfacial abruptness. The NbN film with 5 nm of thickness demonstrates a superconducting transition temperature of 11.2 K as compared to 50 nm NbN films with a transition temperature of 15.3 K. These thickness dependent variation of superconducting properties are associated with the chemical inhomogeneity in terms of in-plane N:Nb distribution, secondary phase formations and NbN/AlN interfacial abruptness as a function of the NbN films thicknesses. The analysis depicts the interplay between the surface/interface and the superconducting properties of ultrathin NbN films. These results provide insights on material design and growth that will enable the development of optimized superconducting NbN films for quantum devices.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Enhanced superconductivity and ferroelectric quantum criticality in plastically deformed strontium titanate

The properties of quantum materials are commonly tuned using experimental variables such as pressure, magnetic field and doping. Here we explore a different approach using irreversible, plastic deformation of single crystals. We show that compressive plastic deformation induces low-dimensional superconductivity well above the superconducting transition temperature (Tc) of undeformed SrTiO3, with evidence of possible superconducting correlations at temperatures two orders of magnitude above the bulk Tc. The enhanced superconductivity is correlated with the appearance of self-organized dislocation structures, as revealed by diffuse neutron and X-ray scattering. We also observe deformation-induced signatures of quantum-critical ferroelectric fluctuations and inhomogeneous ferroelectric order using Raman scattering. Our results suggest that strain surrounding the self-organized dislocation structures induces local ferroelectricity and quantum-critical dynamics that strongly influence Tc, consistent with a theory of superconductivity enhanced by soft polar fluctuations. Our results demonstrate the potential of plastic deformation and dislocation engineering for the manipulation of electronic properties of quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Novel Valence Transition in Elemental Metal Europium around 80 GPa

We report valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160 GPa. We provide compelling evidence for a valence transition around 80 GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase ($Pnma$). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Quasiparticle Spectroscopy, Transport, and Magnetic Properties of Nb Films Used in Superconducting Qubits

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, the London penetration depth - based quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. Here we study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films have a superconducting transition temperature of T c = 9.35 K and a fairly clean superconducting gap. The estimated superfluid density is enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb and indicate outstanding quality. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat dissipation (to a substrate), leading to thermomagnetic instabilities. This may present a challenge for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Quasiparticle spectroscopy, transport, and magnetic properties of Nb films used in superconducting transmon qubits

Niobium thin films on silicon substrate used in the fabrication of superconducting qubits have been characterized using scanning and transmission electron microscopy, electrical transport, magnetization, quasiparticle spectroscopy, and real-space real-time magneto-optical imaging. We study niobium films to provide an example of a comprehensive analytical set that may benefit superconducting circuits such as those used in quantum computers. The films show outstanding superconducting transition temperature of $T_{c}=9.35$ K and a fairly clean superconducting gap, along with superfluid density enhanced at intermediate temperatures. These observations are consistent with the recent theory of anisotropic strong-coupling superconductivity in Nb. However, the response to the magnetic field is complicated, exhibiting significantly irreversible behavior and insufficient heat conductance leading to thermo-magnetic instabilities. These may present an issue for further improvement of transmon quantum coherence. Possible mitigation strategies are discussed.

Joshi, Kamal R.↗