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DOE OSTI2022

Electron-phonon coupling strength from ab initio frozen-phonon approach

In this work, we propose a fast method for high-throughput screening of potential superconducting materials. The method is based on calculating metallic screening of zone-center phonon modes, which provides an accurate estimate for the electron-phonon coupling strength. This method is complementary to the recently proposed Rigid Muffin Tin (RMT) method, which amounts to integrating the electronphonon coupling over the entire Brillouin zone (as opposed to the zone center), but in a relatively inferior approximation. We illustrate the use of this method by applying it to MgB 2 , where the high-temperature superconductivity is known to be driven largely by the zone-center modes, and compare it to a sister compound AlB 2 . We further illustrate the usage of this descriptor by screening a large number of binary hydrides, for which accurate first-principle calculations of electron-phonon coupling have been recently published. Together with the RMT descriptor, this method opens a way to perform initial high-throughput screening in search of conventional superconductors via machine learning or data mining.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Magic Gap Ratio for Optimally Robust Fermionic Condensation and Its Implications for High- $T_c$ Superconductivity

Bardeen-Schrieffer-Cooper (BCS) and Bose-Einstein condensation (BEC) occur at opposite limits of a continuum of pairing interaction strength between fermions. A crossover between these limits is readily observed in a cold atomic Fermi gas. Whether it occurs in other systems such as the high temperature superconducting cuprates has remained an open question. We uncover here unambiguous evidence for a BCS-BEC crossover in the cuprates by identifying a universal magic gap ratio 2Δ/k B T c ≈ 6.5 (where Δ is the pairing gap and T c is the transition temperature) at which paired fermion condensates become optimally robust. At this gap ratio, corresponding to the unitary point in a cold atomic Fermi gas, the measured condensate fraction N0 and the height of the jump δγ(T c ) in the coefficient γ of the fermionic specific heat at T c are strongly peaked. In the cuprates, δγ(T c ) is peaked at this gap ratio when Δ corresponds to the antinodal spectroscopic gap, thus reinforcing its interpretation as the pairing gap. We find the peak in δγ(T c ) also to coincide with a normal state maximum in γ, which is indicative of a pairing fluctuation pseudogap above T c .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Scanning SQUID characterization of extremely overdoped La 2-x Sr x CuO 4

Recently, advances in film synthesis methods have enabled a study of extremely overdoped La 2 - x Sr x Cu O 4 . This has revealed a surprising behavior of the superfluid density as a function of doping and temperature, the explanation of which is vividly debated. One popular class of models posits electronic phase separation, where the superconducting phase fraction decreases with doping, while some competing phase (e.g., ferromagnetic) progressively takes over. A problem with this scenario is that all the way up to the dome edge the superconducting transition remains sharp, according to mutual inductance measurements. However, the physically relevant scale is the Pearl penetration depth Λ P , and this technique probes the sample on a length scale L that is much larger than Λ P . In the present paper, we use local scanning superconducting quantum interference device (SQUID) measurements that probe the susceptibility of the sample on the scale L << Λ P . Our SQUID maps show uniform landscapes of susceptibility and excellent overall agreement of the local penetration depth data with the bulk measurements. Overall, these results contribute an important piece to the puzzle of how high-temperature superconductivity vanishes on the overdoped side of the cuprate phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

Capping layer influence and isotropic in-plane upper critical field of the superconductivity at the FeSe / SrTiO 3 interface

Understanding the superconductivity at the interface of FeSe/SrTiO 3 is a problem of great contemporary interest due to the significant increase in critical temperature (T c ) compared to that of bulk FeSe, as well as the possibility of an unconventional pairing mechanism and topological superconductivity. We report a study of the influence of a capping layer on superconductivity in thin films of FeSe grown on SrTiO 3 using molecular beam epitaxy. We used in vacuo four-probe electrical resistance measurements and ex situ magnetotransport measurements to examine the effect of three capping layers that provide distinct charge transfer into FeSe: insulating FeTe, nonmetallic Te, and metallic Zr. Our results show that FeTe provides an optimal cap that barely influences the inherent T c found in pristine FeSe/SrTiO 3 , while the transfer of holes from a nonmetallic Te cap completely suppresses superconductivity and leads to insulating behavior. Finally, we used ex situ magnetoresistance measurements in FeTe capped FeSe films to extract the angular dependence of the in-plane upper critical magnetic field. Our observations reveal an almost isotropic in-plane upper critical field, providing insight into the symmetry and pairing mechanism of high-temperature superconductivity in FeSe.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Strong Correlation Between Superconductivity and Ferromagnetism in an Fe-Chalcogenide Superconductor

The interplay among topology, superconductivity, and magnetism promises to bring a plethora of exotic and unintuitive behaviors in emergent quantum materials. The family of Fe-chalcogenide superconductors FeTe x Se 1–x are directly relevant in this context due to their intrinsic topological band structure, high-temperature superconductivity, and unconventional pairing symmetry. Despite enormous promise and expectation, the local magnetic properties of FeTe x Se 1–x remain largely unexplored, which prevents a comprehensive understanding of their underlying material properties. Exploiting nitrogen vacancy (NV) centers in diamond, here we report nanoscale quantum sensing and imaging of magnetic flux generated by exfoliated FeTe x Se 1–x flakes, demonstrating strong correlation between superconductivity and ferromagnetism in FeTe x Se 1–x . The coexistence of superconductivity and ferromagnetism in an established topological superconductor opens up new opportunities for exploring exotic spin and charge transport phenomena in quantum materials. The demonstrated coupling between NV centers and FeTe x Se 1–x may also find applications in developing hybrid architectures for next-generation, solid-state-based quantum information technologies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2025

Nanoconfinement of High Hydrogen-to-Metal Ratio Lanthanum Hydrides in Functionalized Carbon Hosts

Metal hydrides with a high hydrogen content are important for materials-based hydrogen storage and high-temperature superconductivity. Nanoconfinement of metal hydrides in porous hosts is a promising strategy to tune the thermodynamic stability and control the hydrogen-to-metal ratio. However, lanthanum hydride (LaH x ) nanoconfinement in porous materials has been limited due to the challenges associated with isolating and stabilizing nanoparticles of La or La-hydrides. Here we successfully demonstrated the chemical reduction of La(III) salts to La(0), and subsequent infiltration into pure CMK-3 and nitrogen-doped CMK-3 (NCMK-3) porous carbons. Transmission electron microscopy measurements revealed a uniform distribution of LaH x species within the carbon hosts, while X-ray absorption and photoelectron spectroscopy provided detailed information about the local chemical environment. Sieverts measurements indicate that LaH x @NCMK-3 could desorb up to 0.75 wt % hydrogen, which is higher than non-nitrogen-functionalized CMK-3 (0.43 wt % H). Density Functional Theory and ab initio molecular dynamics calculations indicate that host–guest interaction energies are favorable for porous carbon with nitrogen defects, which is supported by experimental evidence. Moreover, high-pressure synchrotron X-ray diffraction measurements were conducted using a diamond anvil cell up to 60 GPa and reveal that the nitrogen-functionalized nanoporous carbon host favors the formation of higher H:La ratios in the presence of ammonia borane compared to the pure CMK-3 host. This approach could serve as a suitable platform for developing nanoscale superconducting materials at lower pressures and temperatures compared to bulk.

Shivanna, Mohana↗

DOE OSTI2020

RbB 3 Si 3 : An Alkali Metal Borosilicide that is Metastable and Superconducting at 1 atm

The stability, electronic structure and potential superconductivity in AB 3 Si 3 (A = Na, K, Rb, and Cs) compounds that assume the clathrate-based sodalite structure whose frameworks consists of covalent B-Si bonds is investigated via first-principles calculations. This structure type has recently been predicted in a number of high temperature superconducting hydrides, but these are only stable under megabar pressures. Herein, we predict a novel superconducting phase, RbB 3 Si 3 , that could be synthesized under pressures that are a factor of ten smaller, ~10 GPa, and quenched to atmospheric conditions. Electron-phonon coupling calculations predict that RbB 3 Si 3 possesses a superconducting critical temperature, T c , of 14 K at 1 atm. Here, the dynamic stability of RbB 3 Si 3 and CsB 3 Si 3 at ambient pressure can be explained by considering the chemical pressure exerted on the B-Si framework that is caused by the size effect of the alkali metal atom.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Quench Propagation On Rare-Earth Barium Copper Oxide (REBCO) Tape

Rare Earth Barium Copper Oxide (REBCO) is a high‑temperature superconducting material that has zero electrical resistance. A quench occurs when a section of the superconductor suddenly transitions to a resistive state after exceeding its critical current. This localized loss of superconductivity generates a voltage rise, serving as an essential signal for quench detection and protection. While capable of carrying extremely high currents, REBCO is mechanically brittle and have slow quench propagation which makes voltage change difficult to detect.

Ju, Han [Northwestern U.]↗

DOE OSTI2024

Evidence of Dirac Quantum Spin Liquid in YbZn 2⁢ GaO 5

The emergence of a quantum spin liquid (QSL), a state of matter that can result when electron spins are highly correlated but do not become ordered, has been the subject of a considerable body of research in condensed matter physics [1,2]. Spin liquid states have been proposed as hosts for high-temperature superconductivity [3] and can host topological properties with potential applications in quantum information science [4]. The excitations of most quantum spin liquids are not conventional spin waves but rather quasiparticles known as spinons, whose existence is well established experimentally only in one-dimensional systems; the unambiguous experimental realization of QSL behavior in higher dimensions remains challenging. Here, in this work, we investigate the novel compound $YbZn_{2}GaO_{5}$, which hosts an ideal triangular lattice of effective spin-1/2 moments with no detectable inherent chemical disorder. Thermodynamic and inelastic neutron scattering measurements performed on high-quality single crystal samples of $YbZn_{2}GaO_{5}$ exclude the possibility of long-range magnetic ordering down to 0.06 K, demonstrate a quadratic power law for the specific heat and reveal a continuum of magnetic excitations in parts of the Brillouin zone. Both low-temperature thermodynamics and inelastic neutron scattering spectra suggest that $YbZn_{2}GaO_{5}$ is a U(1) Dirac QSL with spinon excitations concentrated at certain points in the Brillouin zone. We advanced these results by performing additional specific heat measurements under finite fields, further confirming the theoretical expectations for a Dirac QSL on the triangular lattice of $YbZn_{2}GaO_{5}$.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

DOE OSTI2024

High Current Density Cables for Simpler HTS Magnets in Fusion Energy Systems

High current density cables are needed for the engineering design of potentially low cost, simpler geometry high temperature superconducting (HTS) magnets in the promising magnetic configurations as a fusion pilot plant (FPP) option. Significant technology maturation efforts are underway by privately funded startups with the goal to demonstrate mature HTS magnet technology. Test results, however, indicate critical engineering issues remain to be addressed to meet performance goals, and demonstrate HTS coil operation repeatability and reliability. To this end, exploring and enabling multiple viable conductor and cable options is vital. Partnering with a private fusion startup and manufacturers of superconducting strands and cables, Princeton Plasma Physics Laboratory (PPPL) is exploring and seeking to de-risk the aggressive high field approach presently targeted by others. Our main objective is to develop, test and calibrate novel high current density cables for a broad deployment of affordable and reliable coils using Bi-2212 conductors. If successful, such a project will provide technical feasibility for promising FPP configurations including spherical tokamaks (ST) and compact stellarators. Here, we aim at the state-of-the-art Bi-2212 cable technologies toward a current density of 100 A/mm 2 at 16 T and 4.2 K–10 K operation for low cost, simpler geometry toroidal field (TF) coils for compact stellarators developed by fusion startup companies on a timeline consistent with the FPP initiatives and beyond.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2025

Symmetrization of Strong Hydrogen Bond under High Pressure in Bihydroxide-Ion-Containing NaCu 2 (SO 4 ) 2 ·H 3 O 2 Revealed by Experimental Charge Density, Single-Crystal Electron Diffraction, and Neutron Diffraction Studies

In minerals and inorganic compounds, strong hydrogen bonding can lead to the formation of complex ionic species such as the H 3 O 2 – bihydroxide anion and Zundel cation H 5 O 2 + . We studied [NaCu 2 (SO 4 ) 2 ·H 3 O 2 ] natrochalcite, which contains bihydroxide anions and undergoes hydrogen bond symmetrization at the lowest pressure reported so far among inorganic compounds. Hydrogen bond symmetrization leads to changes in the bulk modulus, seismic wave velocities, and proton mobility and plays a primary role in high-temperature superconductivity, but its characteristics are not well understood due to a lack of systematic studies and limitations of experimental methods sensitive to this subtle change. In this work, we applied experimental charge density analysis based on in situ single-crystal X-ray diffraction data, along with the single-crystal neutron and electron diffraction experiments, to probe the behavior of hydrogen atoms during the hydrogen bond symmetrization process under high-pressure conditions. On the way to the symmetrical H-bonding, natrochalcite undergoes a series of complex redistributions of electron density, which we trace with multipole refinement and detailed analysis of changes in the Laplacian of electron density values. Additionally, we deconvoluted the equation of state (volume of the unit cell vs pressure relation) into the atomic equation of states describing dependencies of atomic charges or volumes vs pressure.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Critical role of magnetic moments in the lattice dynamics of YBa 2 Cu 3 O 6

The role of lattice dynamics in unconventional high-temperature superconductivity is still vigorously debated. Theoretical insights into this problem have long been prevented by the absence of an accurate first-principles description of the combined electronic, magnetic, and lattice degrees of freedom. Utilizing the recently constructed r 2 SCAN density functional that stabilizes the antiferromagnetic (AFM) state of the pristine oxide YBa 2 ⁢Cu 3 ⁢O 6 , we faithfully reproduce the experimental dispersion of key phonon modes. Here we further find significant magnetoelastic coupling in numerous high-energy Cu-O bond stretching optical branches, where the AFM results improve over the soft nonmagnetic phonon bands.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2022

Quantum phase transition from superconducting to insulating-like state in a pressurized cuprate superconductor

Copper oxide superconductors continue to fascinate the communities of condensed matter physics and material sciences because they host the highest ambient-pressure superconducting transition temperature and unconventional electronic behaviour that are not fully explained. Searching for universal links between the superconducting state and its normal metallic state is believed to be an effective approach to elucidate the underlying mechanism of superconductivity. One of the common expectations for copper oxide superconductors is that a metallic phase will appear after the superconductivity is entirely suppressed by chemical doping or the application of a magnetic field9. Here we report the first observation of a quantum phase transition from a superconducting state to an insulating-like state as a function of pressure in Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) superconductors with two CuO 2 planes in a unit cell for doping below, at and above a level that achieves the highest transition temperature. We also find the same phenomenon in related compounds with a single CuO 2 plane as well as three CuO 2 planes in a unit cell. This apparently universal phenomenon poses a challenge for achieving a unified understanding of the mechanism of high-temperature superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Searching Materials Space for Hydride Superconductors at Ambient Pressure

Abstract A machine‐learning‐assisted approach is employed to search for superconducting hydrides under ambient pressure within an extensive dataset comprising over 150 000 compounds. The investigation yields ≈50 systems with transition temperatures surpassing 20 K, and some even reaching above 70 K. These compounds have very different crystal structures, with different dimensionality, chemical composition, stoichiometry, and arrangement of the hydrogens. Interestingly, most of these systems display slight thermodynamic instability, implying that their synthesis will re quire conditions beyond ambient equilibrium. Moreover, a consistent chemical composition is found in the majority of these systems, which combines alkali or alkali‐earth elements with noble metals. This observation suggests a promising avenue for future experimental investigations into high‐temperature superconductivity within hydrides at ambient pressure.

Cerqueira, Tiago F. T.↗

DOE OSTI2020

Superconducting air core inductor systems and methods

Provided is a low-weight, high-efficiency inductor design for use with or in electrical power equipment, such as inverters. A toroidal power inductor includes a support structure comprising an outer shell, an inner shell, and one or more coolant channels formed therebetween, a plurality of conductors wrapped around and supported by an exterior surface of the outer shell, and an interior cavity substantially enclosed by the inner shell of the toroidal support structure. The plurality of conductors are configured to provide an inductance for the toroidal power inductor, and the one or more coolant channels are distributed beneath the exterior surface of the outer shell to cool the plurality of conductors. An air-core power inductor may implement the conductors using high-temperature superconducting (HTS) tapes cooled by cryogenic fluid flowing within the coolant channels.

42 ENGINEERING↗

DOE OSTI2025

Ab initio quantum many-body description of superconducting trends in the cuprates

Using a systematic ab initio quantum many-body approach that goes beyond low-energy models, we directly compute the superconducting pairing order and estimate the pairing gap of several doped cuprate materials and structures within a purely electronic picture. We find that we can correctly capture two well-known trends: the pressure effect, where the pairing order and gap increase with intra-layer pressure, and the layer effect, where the pairing order and gap vary with the number of copper-oxygen layers. From these calculations, we observe that the strength of superexchange and the covalency at optimal doping are the best descriptors for these trends. Our microscopic analysis further identifies that strong short-range spin fluctuations and multi-orbital charge fluctuations drive the development of the pairing order. Our work illustrates the possibility of a material-specific ab initio understanding of unconventional high-temperature superconducting materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2026

Double Aperture HTS Dipole Magnet Model for the Persistent Current Operation

Particle accelerators use many different types of magnets. There is an interest in replacing rather low-field conventional resistive magnets with high-temperature superconducting (HTS) magnets. The main goal is to reduce operational expenses, which is critical during the continuously increasing cost of electricity. At Fermilab, several HTS accelerator magnets were built and successfully tested. The paper described a variant of the double-aperture dipole magnet that could be suitable for future Lepton Colliders. A single short-circuited HTS coil of 3.8 kA generates a homogeneous magnetic field of 57 mT in both magnet gaps of 84 mm. The short model magnet was designed, built, and successfully tested at liquid nitrogen temperature. This magnet works in a persistent current mode, which provides additional cost savings when used in particle storage rings operated at a stable magnetic field. The results of design, fabrication, and tests are presented and discussed. The magnet demonstrated a high magnetic field stability in both apertures, efficiency of correction coils, and primary powering circuit.

Kashikhin, Vladimir [Fermilab] (ORCID:000000017249↗

DOE OSTI2026

Unified Description of Cuprate Superconductors by Fractionalized Electrons Emerging from Integrated Analyses of Photoemission Spectra and Quasiparticle Interference

Electronic structure of high-temperature superconducting cuprates is studied by analyzing experimental data independently obtained from two complementary spectroscopies: one, quasiparticle interference (QPI) measured by scanning-tunneling microscopy, and the other, angle-resolved photoemission spectroscopy (ARPES). We combine these two sets of data in a unified theoretical analysis. Through explicit calculations of experimentally measurable quantities, we show that a simple two-component fermion model (TCFM) representing electron fractionalization succeeds in reproducing various detailed features of these experimental data: ARPES and QPI data are concomitantly reproduced by the TCFM in full energy and momentum spaces. The measured QPI pattern reveals a signature characteristic of the TCFM, distinct from the conventional single-component prediction, supporting the validity of the electron fractionalization in the cuprates. The integrated analysis also solves the puzzles of ARPES and QPI data that are seemingly inconsistent with each other. The overall success of the TCFM offers a comprehensive understanding of the electronic structure of the cuprates, in particular, the unoccupied side of the spectra, of which momentum-resolved structure has long been unexplored experimentally. We further predict that a characteristic QPI pattern should appear in the unoccupied high-energy part if the fractionalization is at work. We propose that integrated-spectroscopy analyses offer a promising way to explore challenging issues of strongly correlated electron systems.

Sakai, Shiro [Sophia University; RIKEN Center for ↗