Superconductive pairing across electron barriers.
Possible superconductive pairing of two electrons separated by barrier leading to higher transition temperature for superconductors and superconductivity in magnetic metals
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Space applications of superconducting microwave electronics at NASA Lewis Research Center
Since the discovery of high temperature superconductivity in 1987, NASA Lewis Research Center has been involved in efforts to demonstrate its advantages for applications involving microwave electronics in space, especially space communications. The program included thin film fabrication by means of laser ablation. Specific circuitry which was investigated includes microstrip ring resonators at 32 GHz, phase shifters which utilize a superconducting, optically activated switch, an 8x8 32 GHz superconducting microstrip antenna array, and an HTS-ring-resonator stabilized oscillator at 8 GHz. The latter two components are candidates for use in space experiments which are described in other papers. Experimental data on most of the circuits are presented as well as, in some cases, a comparison of their performance with an identical circuit utilizing gold or copper metallization.
Leonard, R. F.↗
Steady-state superconductivity in electronic materials with repulsive interactions
We study the effect of laser driving on a minimal model for a hexagonal two-dimensional material with broken inversion symmetry. Through the application of circularly polarized light and coupling to a thermal free electron bath, the system is driven into a nonequilibrium steady state with asymmetric, nonthermal carrier populations in the two valleys. We show that, in this steady state, interband superconducting correlations between electrons can develop independent of the sign of the electronic interactions. We discuss how our results apply, for example, to transition metal dichalcogenides. Furthermore, this work opens the door to technological applications of superconductivity in a range of materials that were hitherto precluded from it.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Theory of unconventional superconductivity in the 1/4-filled band correlated-electron super-conductors [and previous titles]. Final Report, DOE-BES grant DE-FG02-06ER46315
Determination of the mechanism of correlated-electron superconductivity (SC) has turned out to be the hardest problem in condensed matter physics. After nearly three decades of intense investigation of the high-T c cuprates it now appears that one key idea, viz., - weak doping of Mott-Hubbard semiconductors leads to SC -may not be correct, or is at least incomplete. This conclusion is arrived from recent experiments that have clearly indicated that the pseudogap state in the cuprates, separated by a thermodynamic phase transition from the undoped antiferromagnet, is significantly more complicated than thought before. Buried inside it there is a distinct charge-ordered (CO) state. Understanding the complete set of competing and coexisting phases in the pseudogap state is thus a formidable challenge. It follows that research on other correlated superconductors, which have also been known for a long time, may be able to give much needed fresh insight. It is with this motivation in this project we pursued theoretical research to understand the unconventional SC that is found in an apparently completely separate family of materials, the organic charge-transfer solids (CTS). A unique feature of the CTS is that SC there universally occurs at the carrier concentration $\rho$ of 0.5 per organic molecule, rather than under carrier doping as in the cuprates. Our work in this project focused on the correlated physics of the organic CTS as well as other inorganic materials with the same carrier density, $\rho$=0.5. This project resulted in several theoretical advances in understanding these materials.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Electrical NEP in Hot-Electron Titanium Superconducting Bolometers
We are presenting the current progress on the titanium (Ti) hot-electron transition-edge devices. The ultimate goal of this work is to develop a submillimeter Hot-Electron Direct Detector (HEDD) with the noise equivalent power NEP = 10(sup -1) - 10(sup -20) W/Hz(sup 1/2) for the moderate resolution spectroscopy and Cosmic Microwave Background (CMB) studies on future space telescope (e.g., SPICA, SAFIR, SPECS, CMBPol) with cryogenically cooled (approximately 4-5 K) mirrors. Recently, we have achieved the extremely low thermal conductance (approximately 20 fW/K at 300 mK and approximately 0.1 fW/K at 40 mK) due to the electron-phonon decoupling in Ti nanodevices with niobium (Nb) Andreev contacts. This thermal conductance translates into the "phonon-noise" NEP approximately equal to 3 x 10(sup -21) W/Hz(sup 1/2) at 40 mK and NEP approximately equal to 3 x 10(sup -19) W/Hz(sup 1/2) at 300 mK. These record data indicate the great potential of the hot-electron detector for meeting many application needs. Beside the extremely low phonon-noise NEP, the nanobolometers have a very low electron heat capacitance that makes them promising as detectors of single THz photons. As the next step towards the practical demonstration of the HEDD, we fabricated and tested somewhat larger than in Ref.1 devices (approximately 6 micrometers x 0.35 micrometers x 40 nm) whose critical temperature is well reproduced in the range 300-350 mK. The output electrical noise measured in these devices with a low-noise dc SQUID is dominated by the thermal energy fluctuations (ETF) aka "phonon noise". This indicates the high electrothermal loop gain that effectively suppresses the contributions of the Johnson noise and the amplifier (SQUID) noise. The electrical NEP = 6.7 x 10(sup -18) W/Hz(sup 1/2) derived from these measurements is in good agreement with the predictions based on the thermal conductance data. The very low NEP and the high speed (approximately microns) are a unique combination not found in other detectors.
transition-edge sensor↗
Electron-phonon superconductivity in LaO{sub 0.5}F{sub 0.5}BiSe{sub 2}
We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum and electron–phonon coupling for the newly discovered superconductor LaO{sub 0.5}F{sub 0.5}BiSe{sub 2}. It is confirmed that there is a strong Fermi surface nesting at (π,π,0), which results in unstable phonon branches. Combining the frozen phonon total energy calculations and an anharmonic oscillator model, we find that the quantum fluctuation prevents the appearance of static long–range order. The calculation shows that LaO{sub 0.5}F{sub 0.5}BiSe{sub 2} is highly anisotropic, and same as its cousin LaO{sub 0.5}F{sub 0.5}BiS{sub 2}, this compound is also a conventional electron-phonon coupling induced superconductor.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Emergence of low-energy spin waves in superconducting electron-doped cuprates
In order to fully utilize the technological potential of unconventional superconductors, an enhanced understanding of the superconducting mechanism is necessary. In the best performing superconductors, the cuprates, superconductivity is intimately linked with magnetism, although the details of this coupling remain elusive. Here, we address this gap by studying the electron-doped cuprate Nd 1.85 Ce 0.15 CuO 4−δ that has an antiferromagnetic ground state when synthesized and only becomes superconducting after a reductive annealing process. Using neutron spectroscopy, we show that the as-grown crystal exhibits a large spin pseudogap in the magnetic fluctuation spectrum. Annealing removes defects introduced by the commonly employed synthesis method and significantly reduces the spin pseudogap. While the spin pseudogap in the annealed sample likely arises from superconductivity, in the as-grown sample it results from the absence of long-wavelength spin waves. These results reveal a direct connection between defects, magnetism, and superconductivity, offering new insight into the mechanisms underlying high-temperature superconductivity and guiding the design of improved superconducting materials.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Flux-based three-dimensional electrodynamic modeling approach to superconducting circuits and materials
Modeling the behavior of superconducting electronic circuits containing Josephson junctions is crucial for the design of superconducting information processors and devices. In this paper, we introduce DEC-QED, a computational approach for modeling the electrodynamics of superconducting electronic circuits containing Josephson junctions in arbitrary three-dimensional electromagnetic environments. DEC-QED captures the nonlinear response and induced currents in BCS superconductors and accurately captures phenomena such as the Meissner effect, flux quantization, and Josephson effects. Using a spatial coarse-graining formulation based on discrete exterior calculus (DEC), DEC-QED can accurately simulate transient and long-time dynamics in superconductors. Finally, the expression of the entire electrodynamic problem in terms of the gauge-invariant flux field and charges makes the resulting classical field theory suitable for second quantization.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Measuring Changes in Inductance With Microstrip Resonators
In this work, we measure the frequency dependence of a niobium microstrip resonator as a function of temperature from 1.4 to 8.4 K. In a 2-micrometer-wide half-wave resonator, we find the frequency of resonance changes by a factor of 7 over this temperature range. From the resonant frequencies, we extract inductance per unit length, characteristic impedance, and propagation velocity (group velocity). We discuss how these results relate to superconducting electronics. Over the 2 K to 6 K temperature range where superconducting electronic circuits operate, inductance shows a 19% change and both impedance and propagation velocity show an 11% change.
42 ENGINEERING↗
Simulations and Experimental Plans for a High-Repetition-Rate Field-Enhanced Conduction-Cooled Superconducting RF Electron Source
We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radio-frequency cavity recently demonstrated at Fermilab. In this paper, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.
43 PARTICLE ACCELERATORS↗
Simulations for Field-Enhanced Conduction-Cooled Superconducting RF Electron Source
We present a novel RF design for a field enhanced electron source driven by field emission cathodes. The proposed electron source relies on the enhanced high electric field gradients at the cathode to simultaneously extract and accelerate electrons. The system will be tested in a conduction-cooled superconducting radiofrequency cavity recently demonstrated at Fermilab. In this work, we present electromagnetic and thermal simulations of the setup that support the feasibility of the design.
43 PARTICLE ACCELERATORS↗
Revealing Possible Coherence Limiting Sources in Superconducting Qubit with Advanced Electron Microscopy
Superconducting materials hold great potential for solid-state quantum computing. Their fabrication relies on established semiconductor fabrication techniques, such as thin film deposition and lithography, but the complex processing steps can result in defects at the qubits' interfaces and surfaces that can negatively impact coherence time. To improve superconducting qubit performance, it is essential to understand the structural features, at the atomic scale, that may act as sources of decoherence limiting factor in both the Josephson junction (JJ) and resonators, which are key components of superconducting qubit. This talk will present our recent studies on the microstructures in a 2D-transmon, with an emphasis on the JJ. For this research, a combination of advanced microscopy techniques, including high-resolution (S)TEM imaging, and spectroscopy (EDS and EELS) are used to identify possible coherence-limiting defects or structural features.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Effect of Electron Irradiation on Superconductivity in Single Crystals of Ba ( Fe 1 - x Ru x ) 2 As 2 ( x = 0.24 )
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phonons, thermal conductivity, energy storage (inc↗
Superconducting Hot Electron Bolometer Mixers for Submillimeter Receivers
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mixer mixers heterodyne heterodyne receivers↗
Computational electron–phonon superconductivity: from theoretical physics to material science
The search for room-temperature superconductors is a major challenge in modern physics. The discovery of copper-oxide superconductors in 1986 brought hope but also revealed complex mechanisms that are difficult to analyze and compute. In contrast, the traditional electron–phonon coupling (EPC) mechanism facilitated the practical realization of superconductivity (SC) in metallic hydrogen. Since 2015, the discovery of new hydrogen compounds has shown that EPC can enable room-temperature SC under high pressures, driving extensive research. Advances in computational capabilities, especially exascale computing, now allow for the exploration of millions of materials. This paper reviews newly predicted superconducting systems in 2023–2024, focusing on hydrides, boron–carbon systems, and compounds with nitrogen, carbon, and pure metals. Although many computationally predicted high-T c superconductors were not experimentally confirmed, some low-temperature superconductors were successfully synthesized. This paper provides a review of these developments and future research directions.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Tuning the Pairing Interaction in a d -Wave Superconductor by Paramagnons Injected through Interfaces
Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. In this paper, we fabricate hybrid superlattices consisting of alternating atomic layers of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_{5}$ and antiferromagnetic (AFM) metal ${\mathrm{CeRhIn}}_{5}$, in which the AFM order can be suppressed by applying pressure. We find that the superconducting and AFM states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting properties. An analysis of upper critical fields reveals that, upon suppressing the AFM order by applied pressure, the force binding superconducting electron pairs acquires an extreme strong-coupling nature. Finally, this demonstrates that superconducting pairing can be tuned nontrivially by magnetic fluctuations (paramagnons) injected through the interface.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Atomic-Scale Characterization and Electronic Properties of Gold-Capped Niobium Films for Superconducting Qubits
Niobium thin films are central to superconducting qubits, but their complex native oxides contribute significantly to microwave losses. A promising mitigation strategy is to suppress oxide formation using engineered thin-film encapsulation layers. Recent experiments have shown that ∼10 nm metallic overlayers on Nb capacitor films can improve the energy relaxation time T1 of transmon qubits. Here, we present a comparative study of Au-capped Nb films fabricated in situ by molecular beam epitaxy and ex situ by sequential deposition benchmarked against bare Nb films. Using complementary structural, chemical, and spectroscopic techniques, we correlate the interface quality with superconducting electronic properties of the Au surface. Low-temperature scanning tunneling spectroscopy (STS) provides spatially resolved quasiparticle density-of-states maps. While both Au-capped Nb samples exhibit large areas with uniform, fully gapped density of states, clear differences emerge between the two interfaces. Compared to in situ Nb-Au, ex situ Nb-Au exhibits a reduced induced superconducting gap, broadened coherence peak, and localized in-gap states, consistent with a residual NbxOy layer at the interface. Supported by complementary structural and chemical analyses, these findings demonstrate that STS directly links nanoscale superconducting properties to interface preparation, highlighting the importance of controlled in situ encapsulation for minimizing dissipation and improving quantum coherence.
Makita, Junki [MIT; Temple U.] (ORCID:000000026889↗
Superconducting Microwave Electronics at Lewis Research Center
Over the last three years, NASA Lewis Research Center has investigated the application of newly discovered high temperature superconductors to microwave electronics. Using thin films of YBa2Cu3O7-delta and Tl2Ca2Ba2Cu3Ox deposited on a variety of substrates, including strontium titanate, lanthanum gallate, lanthanum aluminate and magnesium oxide, a number of microwave circuits have been fabricated and evaluated. These include a cavity resonator at 60 GHz, microstrip resonators at 35 GHz, a superconducting antenna array at 35 GHz, a dielectric resonator at 9 GHz, and a microstrip filter at 5 GHz. Performance of some of these circuits as well as suggestions for other applications are reported.
Warner, Joseph D.↗