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DOE OSTI2025

Listening for Neutrinos with Quantum Acoustics

Many low-threshold superconducting detectors rely on athermal phonon sensing, in which meV-scale phonons from a DM scatter are sensed via their ability to break Cooper pairs in superconducting films. While detectors based on such pairbreaking sensors are highly advantageous and scalable, their sensor threshold is ultimately limited by the energy needed to break a Cooper pair, 2Δ. We present a novel detector architecture for single phonon detection at the O(10μeV) scale, the qubit-coupled hBAR, that does not share this limitation. This architecture, composed of a superconducting qubit piezoelectrically coupled to a high-overtone bulk acoustic resonator, was originally developed in the context of the quantum acoustics community and has a sensor threshold ultimately limited by thermal noise. Such ultra-low-threshold detectors have a wide range of fundamental science applications, including reactor monitoring via neutrinos.

Baxter, Daniel Stephen [Fermilab] (ORCID:000000030↗

DOE OSTI2021

Asymmetric Fraunhofer spectra in a topological insulator-based Josephson junction

Josephson junctions with topological insulators as their weak link (S–TI–S junctions) are predicted to host Majorana fermions, which are key to creating qubits for topologically protected quantum computing. But the details of the S–TI–S current-phase relation and its interplay with magnetic fields are not well understood. We fabricate a Bi 2 Se 3 junction with NbTi leads and measure the Fraunhofer patterns of the junction with applied in-plane fields. We observe that asymmetric Fraunhofer patterns appear in the resistance maps of B z vs B x,y , with aperiodic node spacings. These asymmetric patterns appear even at zero parallel field and for temperatures up to 1 K. The anomalous features are compared to asymmetric Fraunhofer patterns expected for finite Cooper pair momentum shifts as well as geometric effects. We show that the geometric effects can dominate, independent of in-plane field magnitude. Furthermore, these results are important for differentiating geometrical phase shifts from those caused by Cooper pair momentum shifting, Majorana mode signatures, or other unconventional superconducting behavior.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Cavity-QED Quantum Simulator of Dynamical Phases of a Bardeen-Cooper-Schrieffer Superconductor

Herein we propose to simulate dynamical phases of a BCS superconductor using an ensemble of cold atoms trapped in an optical cavity. Effective Cooper pairs are encoded via the internal states of the atoms, and attractive interactions are realized via the exchange of virtual photons between atoms coupled to a common cavity mode. Control of the interaction strength combined with a tunable dispersion relation of the effective Cooper pairs allows exploration of the full dynamical phase diagram of the BCS model as a function of system parameters and the prepared initial state. Our proposal paves the way for the study of the nonequilibrium features of quantum magnetism and superconductivity by harnessing atom-light interactions in cold atomic gases.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2025

Superconducting coherence peak in near-field radiative heat transfer

Enhancement and peaks in near-field radiative heat transfer (NFRHT) typically arise due to surface phonon-polaritons, plasmon-polaritons, and electromagnetic (EM) modes in structured materials. However, the role of material quantum coherence in enhancing near-field radiative heat transfer remains unexplored. Here, we unravel that NFRHT in superconductor-ferromagnetic systems displays a unique peak at the superconducting phase transition that originates from the quantum coherence of Bogoliubov quasiparticles in superconductors. Our theory takes into account evanescent EM radiation emanating from fluctuating currents related to Cooper pairs and Bogoliubov quasiparticles in stark contrast to the current-current correlations induced by free electrons in conventional materials. Our proposed NFRHT configuration exploits ferromagnetic resonance at frequencies deep inside the superconducting band gap to isolate this superconducting coherence peak. Furthermore, we reveal that Cooper pairs and Bogoliubov quasiparticles have opposite effects on near-field thermal radiation and isolate their effects on many-body radiative heat transfer near superconductors. As a result, our proposed phenomenon can have applications for developing thermal isolators and heat sinks in superconducting circuits.

Dipole approximation↗

DOE OSTI2024

Correlating aluminum layer deposition rates, Josephson junction microstructure, and superconducting qubits’ performance

An ultra-thin aluminum oxide layer is a key component for Josephson junctions (JJ) in superconducting quantum bits (qubits). This layer serves as a barrier layer for Cooper pairs tunneling between the superconducting electrodes and significantly influences the overall performance of the junction. In this study, we investigate the impact of aluminum deposition rates on the microstructure and chemical variation of the aluminum oxide layer, as well as the device's yields and qubits’ lifetimes. Surprisingly, although the oxide layer roughness and thickness variation improve noticeably with an increasing Al deposition rate from 0.5 Å/s to 5 Å/s, the qubit's coherence time is almost unchanged. We attribute this to the fact that Cooper pairs tunnel only through the thinnest region of the barrier. Additionally, we revealed a stress-induced grain boundary sliding short-circuit failure mechanism in the JJ. Our discovery provides a vital understanding of oxide microstructure and JJ functionality, which are critical for improving the coherence time of superconducting qubits.

Coherence time↗

DOE OSTI2020

A density-functional theory study of the Al/AlOx/Al tunnel junction

The aluminum oxide tunnel junction is a key component of the majority of superconducting quantum devices. For high-quality, reproducible, and scalably manufacturable qubits, the ability to fabricate Josephson junctions (JJs) with a targeted critical current and high uniformity is essential. In this study, we use first-principles modeling to assess fundamental aspects of the atomic structure of both amorphous and crystalline aluminum oxide tunnel junctions and relate the structure to predicted performance metrics. We use modified ab initio molecular dynamics to develop realistic models of the tunnel junction, from which interface roughness and local thickness fluctuations are analyzed in an unbiased manner by training a neural network to identify the boundary between metal and oxide. We show that the effective thickness of the insulating part of the junction can be different from the apparent physical thickness. We calculate the rate of Cooper pair tunneling for the atomically resolved electrostatic potential using direct numerical solution in 3D, which shows a channeling effect that impacts the junction critical current. The predicted critical current is a useful JJ design parameter that can be accessed from the ab initio calculations without fitting parameters. To assess the limits of uniformity and fabrication choices (e.g., oxidation vs epitaxy), we compare the amorphous junctions to crystalline models, which show order of magnitude more efficient tunneling compared to the amorphous case, underlining the connection between atomistic structure and Cooper pair tunneling efficiency. Further, this work provides a foundation for ab initio materials design and evaluation to help accelerate future development of improved tunnel junctions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Quantum information processing with majorana bound states in superconducting circuits

In a weak link of two s-wave superconductors (SCs) coupled via a time-reversal-invariant (TRI) topological superconducting (TSC) island, a Josephson current can flow due to Cooper pairs tunneling in and out of spatially separated Majorana Kramers pairs (MKPs), which are doublets of Majorana bound states (MBSs). The sign of the resulting Josephson current is fixed by the joint parity of the four Majorana bound states that make up the MKPs on the TSC island. This parity-controlled Josephson effect can be used as a read-out mechanism for the joint parity in Majorana-based quantum computing. For a TSC island with four terminals, the SC leads can address a Majorana superconducting qubit (MSQ) formed by the charge ground states of the TSC island's terminals. Cooper pair splitting enables single-qubit operations, qubit read-out, as well as two-qubit entangling gates. Hence, TSC islands between SC leads may provide an alternative approach to superconducting quantum computation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Simulating Dynamical Phases of Chiral p + i p Superconductors with a Trapped ion Magnet

Two-dimensional p + ip superconductors and superfluids are systems that feature chiral behavior emerging from the Cooper pairing of electrons or neutral fermionic atoms with nonzero angular momentum. Their realization has been a longstanding goal because they offer great potential utility for quantum computation and memory. However, they have so far eluded experimental observation both in solid-state systems as well as in ultracold quantum gases. Here, we propose to leverage the tremendous control offered by rotating two-dimensional trapped-ion crystals in a Penning trap to simulate the dynamical phases of two-dimensional p + ip superfluids. This is accomplished by mapping the presence or absence of a Cooper pair into an effective spin-1/2 system encoded in the ions’ electronic levels. We show how to infer the topological properties of the dynamical phases, and discuss the role of beyond mean-field corrections. More broadly, our work opens the door to use trapped-ion systems to explore exotic models of topological superconductivity and also paves the way to generate and manipulate skyrmionic spin textures in these platforms.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2020

Orbital selectivity of layer-resolved tunneling in the iron-based superconductor Ba 0.6 K 0.4 Fe 2 As 2

Here, we use scanning tunneling microscopy/spectroscopy to elucidate the Cooper pairing of the iron pnictide superconductor Ba 0.6 K 0.4 Fe 2 As 2 . By a cold-cleaving technique, we obtain atomically resolved termination surfaces with different layer identities. Remarkably, we observe that the low-energy tunneling spectrum related to superconductivity has an unprecedented dependence on the layer identity. By cross referencing with the angle-revolved photoemission results and the tunneling data of LiFeAs, we find that tunneling on each termination surface probes superconductivity through selecting distinct Fe-3$\textit{d}$ orbitals. These findings imply the real-space orbital features of the Cooper pairing in the iron pnictide superconductors, and propose a general concept that, for complex multiorbital material, tunneling on different terminating layers can feature orbital selectivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Electronic nature of the pseudogap in electron-doped Sr 2 IrO 4

In high-temperature ($T_c$) cuprate superconductors, many exotic phenomena are rooted in the enigmatic pseudogap state, which has been interpreted as consisting of preformed Cooper pairs or competing orders or a combination thereof. Observation of pseudogap phenomenologically in electron-doped Sr 2 IrO 4 —the 5d electron counterpart of the cuprates, has spurred intense interest in the strontium iridates as a testbed for exploring the exotic physics of the cuprates. Here, we examine the pseudogap state of electron-doped Sr 2 IrO 4 by angle-resolved photoemission spectroscopy (ARPES) and parallel theoretical modeling. Our analysis demonstrates that the pseudogap state of Sr 2 IrO 4 appears without breaking the particle–hole symmetry or inducing spectral broadening which are telltale signatures of competing orders in the cuprates. We find quasiparticle dispersion and its temperature dependence in the pseudogap state of Sr 2 IrO 4 to point to an electronic order with a zero scattering wave vector and limited correlation length. Particle–hole symmetric preformed Cooper pairs are discussed as a viable mechanism for such an electronic order. The potential roles of incommensurate density waves are also discussed.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Revealing symmetry-broken superconducting configurations by density functional theory

A coherent theory for the superconductivity of both conventional and unconventional superconductors is currently lacking. Here we show that superconductivity arises from the formation of a symmetry-broken superconducting configuration (SCC) due to atomic perturbation of the normal conducting configuration (NCC). This electron–phonon interaction creates straight one-dimensional tunnels (SODTs) for charge density of electrons and/or holes as revealed by the calculations based on density functional theory (DFT). The SODTs act as resistance-free superhighways and are correlated to the Cooper pairs in the Bardeen–Cooper–Schrieffer (BCS) theory. The formation of SODTs implies that the electron–phonon interaction in the BCS theory can be represented by the difference in charge densities between SCC and NCC predicted by DFT. The present work highlights that in conventional superconductors, SODTs are embedded within the bulk materials and are easily destroyed by phonon vibrations, resulting in a low critical superconducting temperature (T C ). Conversely, in unconventional superconductors such as YBa 2 Cu 3 O 7 (YBCO 7 ), SODTs are protected by a layered pontoon structure with very weak bonding to the bulk materials, maintaining SODTs’ stability at higher temperatures and leading to a much higher T C . The present approach is validated for 14 conventional superconductors of 18 pure elements and MgB 2 examined in this work, including the presently predicted superconductivity in Cu, Ag, Au, Sb, Bi, and MgB 2 at 0 K and 0 GPa, and one unconventional superconductor of YBCO 7 . Our discovery indicates that DFT can be a practical tool for predicting superconductors, enabling a systematic search for new superconducting materials in the future.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Critical magnetic fields and electron pairing in magic-angle twisted bilayer graphene

The velocities of the quasiparticles that form Cooper pairs in a superconductor are revealed by the upper critical magnetic field. Here we use this property to assess superconductivity in magic-angle twisted bilayer graphene (MATBG), which has been observed over a range of moiré band-filling, twist angle, and screening environment conditions. An average Fermi velocity can be defined as $v$$^{*}_{F}$ ≡ k B T c ℓ c /ℏ, where T c and ℓ c are the critical temperature and magnetic length, respectively. An advantage of this definition is that $v$$^{*}_{F}$ can be directly extracted from the existing experimental data. Mean-field theory calculations of upper critical fields in model superconductors are consistent with the expectation that Fermi velocities defined in this way are nearly independent of the strength of pairing interaction. Moreover, for fixed strength pairing interaction, minima in $v$$^{*}_{F}$ as a function of band filling are coincident with maxima in T c , as expected from the McMillan formula. Since no association between T c maxima and $v$$^{*}_{F}$ minima is present in MATBG experimental data, we argue that the pairing interaction in MATBG is strongly filling-factor dependent. Furthermore, any theory of MATBG superconductivity must explain this dependence, which is apparently primarily responsible for the observed superconducting domes.

2-dimensional systems↗

DOE OSTI2023

Evidence for Dirac flat band superconductivity enabled by quantum geometry

In a flat band superconductor, the charge carriers’ group velocity v F is extremely slow. Superconductivity therein is particularly intriguing, being related to the long-standing mysteries of high-temperature superconductors and heavy-fermion systems. Yet the emergence of superconductivity in flat bands would appear paradoxical, as a small v F in the conventional Bardeen–Cooper–Schrieffer theory implies vanishing coherence length, superfluid stiffness and critical current. Here, using twisted bilayer graphene, we explore the profound effect of vanishingly small velocity in a superconducting Dirac flat band system. Using Schwinger-limited non-linear transport studies we demonstrate an extremely slow normal state drift velocity v n ≈ 1,000 m s –1 for filling fraction ν between -1/2 and -3/4 of the moiré superlattice. In the superconducting state, the same velocity limit constitutes a new limiting mechanism for the critical current, analogous to a relativistic superfluid. Importantly, our measurement of superfluid stiffness, which controls the superconductor’s electrodynamic response, shows that it is not dominated by the kinetic energy but instead by the interaction-driven superconducting gap, consistent with recent theories on a quantum geometric contribution. Here we find evidence for small Cooper pairs, characteristic of the Bardeen–Cooper–Schrieffer to Bose–Einstein condensation crossover, with an unprecedented ratio of the superconducting transition temperature to the Fermi temperature exceeding unity and discuss how this arises for ultra-strong coupling superconductivity in ultra-flat Dirac bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Imaging the energy gap modulations of the cuprate pair-density-wave state

The defining characteristic of Cooper pairs with finite center-of-mass momentum is a spatially modulating superconducting energy gap Δ(r). Recently, this concept has been generalized to the pair density wave (PDW) state predicted to exist in cuprates. Although the signature of a cuprate PDW has been detected in Cooperpair tunneling, the distinctive signature in single-electron tunneling of a periodic Δ(r) modulation has never been observed. Here, using a new approach, we discover strong Δ(r) modulations in Bi 2 Sr 2 CaCu 2 O 8+δ that have eight-unit-cell periodicity or wavevectors Q ≈ (2π/a0)(1/8, 0) and Q ≈ (2π/a0)(0, 1/8). Simultaneous imaging of the localdensity- of-states N(r, E) reveals electronic modulations with wavevectors Q and 2Q, as anticipated when the PDW coexists with superconductivity. Finally, by visualizing the topological defects in these N(r, E) density waves at 2Q, we discover them to be concentrated in areas where the PDW spatial phase changes by !, as predicted by the theory of half-vortices in a PDW state. Overall, this is a compelling demonstration, from multiple single-electron signatures, of a PDW state coexisting with superconductivity in the canonical cuprate Bi 2 Sr 2 CaCu 2 O 8+δ .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Non-equilibrium anti-Stokes Raman spectroscopy for investigating Higgs modes in superconductors

Abstract Even before its role in electroweak symmetry breaking, the Anderson-Higgs mechanism was introduced to explain the Meissner effect in superconductors. Spontaneous symmetry-breaking yields massless phase modes representing the low-energy excitations of the Mexican-Hat potential. Only in superconductors the phase mode is shifted towards higher energies owing to the gauge field of the charged condensate. This results in a low-energy excitation spectrum governed by the Higgs mode. Consequently, the Bardeen-Cooper-Schrieffer-like Meissner effect signifies a macroscopic quantum condensate in which a photon acquires mass, representing a one-to-one analogy to high-energy physics. We report on an innovative spectroscopic technique to study symmetries and energies of the Higgs modes in the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8 after a soft quench of the Mexican-Hat potential. Population inversion induced by an initial laser pulse leads to an additional anti-Stokes Raman-scattering signal, which is consistent with polarization-dependent Higgs modes. Within Ginzburg-Landau theory, the Higgs-mode energy is connected to the Cooper-pair coherence length. Within a Bardeen-Cooper-Schrieffer weak-coupling model we develop a quantitative and coherent description of single-particle and two-particle channels. This opens the avenue for Higgs Spectroscopy in quantum condensates and provides a unique pathway to control and explore Higgs physics.

Science & Technology - Other Topics↗

DOE OSTI2022

Distinguishing finite-momentum superconducting pairing states with two-electron photoemission spectroscopy

Here, we show theoretically that double photoemission (2e-ARPES) may be used to identify the pairing state in superconductors in which the Cooper pairs have a nonzero center-of-mass momentum, q cm . We theoretically evaluate the 2e-ARPES counting rate P (2) for the cases of a d x 2 -y 2 -wave superconductor, a pair-density-wave (PDW) phase, and a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase. We show that P (2) provides direct insight into the center-of-mass momentum and spin state of the superconducting condensate, and thus can distinguish between these three different superconducting pairing states. In addition, P (2) can be used to map out the momentum dependence of the superconducting order parameter. Our results identify 2e-ARPES as an ideal tool for identifying and probing q cm ≠ 0 superconducting pairing states in superconductors.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Pseudospin Paramagnons and the Superconducting Dome in Magic Angle Twisted Bilayer Graphene

We present a theory of superconductivity in twisted bilayer graphene in which attraction is generated between electrons on the same honeycomb sublattice when the system is close to a sublattice polarization instability. The resulting Cooper pairs are spin-polarized valley singlets. Because the sublattice polarizability is mainly contributed by interband fluctuations, superconductivity occurs over a wide range of filling fraction. It is suppressed by (i) applying a sublattice polarizing field (generated by an aligned BN substrate) or (ii) changing moiré band filling to favor valley polarization. The enhanced intrasublattice attraction close to sublattice polarization instability is analogous to enhanced like-spin attraction in liquid 3 He near the melting curve and the enhanced valley-singlet repulsion close to valley-polarization instabilities is analogous to enhanced spin-singlet repulsion in metals that are close to a ferromagnetic instability. Here, we comment on the relationship between our pseudospin paramagnon model and the rich phenomenology of superconductivity in twisted bilayer and multilayer graphene.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Disentangling the role of bond lengths and orbital symmetries in controlling $T_c$ of optimally doped YBa 2 Cu 3 O 7

Optimally doped YBa 2 Cu 3 O 7 (YBCO) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, but the issue and the detailed mechanism are far from settled. In the present work, we employ a state-of-the-art first-principles ability to compute both the low- and high-energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic x-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of barium-apical oxygen hybridization, interlayer coupling, and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive to enhanced Fermi surface nesting, which increases spin fluctuations and drives up $T_c$. However, when the structure is fully relaxed, electrons flow to the $d_{z^2}$ orbital as a consequence of a shortened Ba-O bond, which is detrimental for superconductivity.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗