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At least 1,171 records · Page 65
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Twisted bilayer graphene. V. Exact analytic many-body excitations in Coulomb Hamiltonians: Charge gap, Goldstone modes, and absence of Cooper pairing

We find exact analytic expressions for the energies and wave functions of the charged and neutral excitations above the exact ground states (at rational filling per unit cell) of projected Coulomb Hamiltonians in twisted bilayer graphene. Our exact expressions are valid for any form of the Coulomb interaction and any form of A A and A B / B A tunneling. The single charge excitation energy is a convolution of the Coulomb potential with a quantum geometric tensor of the TBG bands. The neutral excitations are (high-symmetry group) magnons, and their dispersion is analytically calculated in terms of the form factors of the active bands in TBG. The two-charge excitation energy and wave functions are also obtained, and a sufficient condition on the graphene eigenstates for obtaining a Cooper pair from Coulomb interactions is obtained. For the actual TBG bands at the first magic angle, we can analytically show that the Cooper pair binding energy is zero in all such projected Coulomb models, implying that either phonons and/or nonzero kinetic energy are needed for superconductivity. Since Vafek and Kang [Phys. Rev. Lett. 125, 257602 (2020)] showed that the kinetic energy bounds on the superexchange energy are less 10 -3 in Coulomb units, the phonon mechanism becomes then very likely. If nonetheless the superconductivity is due to kinetic terms which render the bands nonflat, one prediction of our theory is that the highest T c would not occur at the highest DOS.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Superconductivity-enhanced spin pumping: Role of Andreev resonances

Here we describe a simple hybrid superconductor–ferromagnetic-insulator structure manifesting spin-resolved Andreev bound states in which dynamic magnetization is employed to probe spin related physics. We show that, at low bias and below T c , the transfer of spin angular momentum pumped by an externally driven ferromagnetic insulator is greatly affected by the formation of spin-resolved Andreev bound states. Our results indicate that these bound states capture the essential physics of condensate-facilitated spin flow. For finite thicknesses of the superconducting layer, comparable to the coherence length, resonant Andreev bound states render highly transmitting subgap spin transport channels. We point out that the resonant enhancement of the subgap transport channels establishes a prototype Fabry-Pérot resonator for spin pumping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Thermal and thermoelectric properties of an antiferromagnetic topological insulator MnBi 2 Te 4

The discovery of an intrinsic antiferromagnetic topological insulator (AFMTI) in MnBi 2 Te 4 has attracted intense attention, most of which lies in its electrical properties. Here in this paper, we report electronic, thermal, and thermoelectric transport studies of this newly found AFMTI. The temperature and magnetic field dependence of its resistivity, thermal conductivity, and Seebeck coefficient indicate strong coupling between charge, lattice, and spin degrees of freedom in this system. Furthermore, MnBi 2 Te 4 exhibits a large anomalous Nernst signal, which is associated with nonzero Berry curvature of the field-induced canted antiferromagnetic state.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Stacking faults and alternate crystal structures for the shape-memory alloy NiTi

Here, we use ab initio calculations to study the role of stacking faults in connecting the high-temperature B2 and the theoretically predicted low-temperature B33 NiTi phases. In contrast with prior work, we describe the B 2 → B 33 phase transformation in terms of alternate bilayer shifts by 1 2 [100] on the ( 011 ) B 2 plane, obtaining a viable pathway; the same mechanism could also work with the B19 parent phase. We then examine B33-like structures built from alternate stacking sequences of B19 bilayers, constructed to have monoclinic tilt angles close to the experimentally reported NiTi B19' martensite, and find four low-energy stacking-fault variants with energies 5.8–8.5 meV/atom above the calculated B19' martensite structure, suggesting that such structures might appear as a part of the NiTi martensite phase at low temperatures. Investigating further the occurrence of specific coordinated planar shifts in NiTi systems, we report a dynamically stable NiTi B27 phase and find that it is only 1.2 meV/atom above the calculated B33 ground-state structure, thus having a potential to also play a role in NiTi martensitic phase transformation.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Quantum oscillations and transport properties of layered single-crystal SrCu 4 ⁢As 2

Here, we report a systematic investigation of the physical properties and Fermi-surface topology of layered single-crystal SrCu 4⁢ As 2 using electrical transport, magnetotransport, and quantum-oscillation experiments plus band-structure calculations. The temperature-dependent electrical resistivity reveals a hysteretic phase transition at 𝑇 𝑃 =59 K, most likely associated with a structural change. Hall resistivity data suggest a marked change in the average hole density resulting from the latter phase transition near 𝑇 𝑃 . A large, linear, and nonsaturating magnetoresistance is observed at low temperatures in SrCu 4 ⁢As 2 , likely attributable to the multipocket Fermi surface. Quantum-oscillation data measured in magnetic fields of up to 60 T show several oscillation frequencies exhibiting low effective masses, indicating the presence of Dirac-like band dispersion in SrCu 4 ⁢As 2 , as suggested by the band structure calculations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

CABLE Big Idea RDD&D Workshop (Workshop Summary Report)

The U.S. Department of Energy’s (DOE’s) Advanced Manufacturing Office (AMO) held the CABLE Big Idea RDD&D Workshop April 7–9, 2021. The virtual workshop brought together approximately 250 leading scientific and technical experts to gather information on the state of the art in conductivity-enhanced materials and their applications. These stakeholders included scientists, engineers, manufacturers, materials experts, utility companies, and other entities within the conductor material and electrical product manufacturing supply chains. The two main goals of the workshop were to 1) start building and strengthening a research ecosystem around conductivity-enhanced materials and 2) inform AMO’s future portfolio of research, development, demonstration, and deployment (RDD&D) investments and other program activities in the area of conductivity-enhanced materials. CABLE—or Conductivity-enhanced materials for Affordable, Breakthrough Leapfrog Electric and thermal applications—was was established as an Office of Energy Efficiency and Renewable Energy initiative as a result of a competitive internal process to identify and prioritize potentially high-impact research topics. Since then, conductivity-enhanced materials have been identified as an important element of the shift to an electrified and decarbonized industry sector, and CABLE remains a Big Idea. The CABLE effort is led by AMO and supported by eight other offices within DOE. The first major effort under CABLE was the development of several subtopics for DOE’s Small Business Innovation Research/Small Business Technology Transfer (SBIR/STTR) programs in 2020. Another major activity was the launch of the CABLE Conductor Manufacturing Prize in March 2021.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2023

Collective Energy Transport of Excitons in Two-dimensional Materials

Electronic thermal transport and thermoelectric measurements are pursued in this project to obtain unique insights into the unusual collective energy transport behaviors in two-dimensional (2D) heterostructures. Electronic thermal transport and thermoelectric measurements are essential techniques for characterizing bulk superconductors by probing the heat-carrying quasi-particles. Thermoelectric and electronic thermal transport measurements are expanded in this work beyond bulk systems to probe interactions among electrons, holes, and phonons in 2D heterostructures. In one experiment, a microbridge platform is advanced to demonstrate field-effect resistive-thermometry measurements of the electronic thermal conductivity of graphene heterostructures. Together with first principles theoretical calculations and analytical models, the experimental results suggest that tunable electron coupling with flexural phonons provides a knob to control quantum matters in graphene heterostructures with broken reflection symmetry. In another experiment, the Seebeck coefficient (S) is measured to probe interlayer interactions in electron-hole bilayers that are predicted to give rise to the emergence of a variety of correlated states. As a measure of the entropy, the measured S reveals the signature of electric injection of interlayer excitons in transition metal dichalcogenide (TMD) structures.

36 MATERIALS SCIENCE↗
NASA NTRS1990

High pressure superconducting radial magnetic bearing

In a conventional radial magnetic bearing, the rotor (soft magnetic material) can only have attraction force from one of the electromagnets in the stator. The stator electromagnets consist of small copper windings with a soft magnetic material iron yoke. The maximum pressure obtainable is about 200 N/sq cm. It is shown that replacing the stator copper winding by a superconducting winding in the above configuration can increase the pressure to about 1000 N/sq cm. It is also shown that replacing the iron in the rotor by a group of superconducting windings in persistent mode and using a group of saddle coils in the stator can produce a pressure in excess of 2000 N/sq cm.

Eyssa, Y. M.↗
DOE OSTI2022

Metal to Wigner-Mott insulator transition in two-leg ladders

Here we study theoretically the quantum phase transition from a metal to a Wigner-Mott insulator at fractional commensurate filling on a two-leg ladder. We show that a continuous transition out of a symmetry-preserving Luttinger liquid metal is possible where the onset of insulating behavior is accompanied by the breaking of the lattice translation symmetry. At fillings $v$=1/$m$ per spin per unit cell, we find that the spin degrees of freedom also acquire a gap at the Wigner-Mott transition for odd integer $m$. In contrast for even integer $m$, the spin sector remains gapless and the resulting insulator is a ladder analog of the two-dimensional spinon surface state. In both cases, a charge neutral spinless mode remains gapless across the Wigner-Mott transition. We discuss physical properties of these transitions and comment on insights obtained for thinking about continuous Wigner-Mott transitions in two-dimensional systems which are being studied in moire materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Physically motivated analytical expression for the temperature dependence of the zero-field splitting of the nitrogen-vacancy center in diamond

The temperature dependence of the zero-field splitting (ZFS) between the | $m_s$ = 0 $\rangle$ and | $m_s$ = ±1 $\rangle$ levels of the nitrogen-vacancy (NV) center's electronic ground-state spin triplet can be used as a robust nanoscale thermometer in a broad range of environments. However, despite numerous measurements of this dependence in different temperature ranges, to our knowledge no analytical expression has been put forward that captures the scaling of the ZFS of the NV center across all relevant temperatures. Here we present a simple, analytical, and physically motivated expression for the temperature dependence of the NV center's ZFS that matches all experimental observations, in which the ZFS shifts in proportion to the occupation numbers of two representative phonon modes. In contrast to prior models our expression does not diverge outside the regions of fitting. In conclusion, we show that our model quantitatively matches experimental measurements of the ZFS from 15 to 500 K in single NV centers in ultrapure bulk diamond, and we compare our model and measurements to prior models and experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Effects of Grain Size and Interstitial Content on Recrystallization in Nb after Cold Rolling

A fully recrystallized microstructure with a fine grain size can improve the performance of superconducting radio-frequency (SRF) cavities produced from high-purity Nb (ASTM B393–18 Type 5 Nb) while providing mechanical strength. Recrystallization depends on impurity content, initial microstructure, deformation state, and annealing conditions. To better understand how fine-grained, fully recrystallized microstructures may be produced, the recrystallization behaviors of Type 5 and Type 2 Nb materials were studied. Type 5 Nb specimens were produced with both fine and coarse initial grain sizes. All were cold rolled and then annealed under vacuum for one hour to determine the rolling reductions and temperatures required for recrystallization. The recrystallized fraction exceeded 95% in fine-grained Type 5 Nb rolled to a 30% or greater thickness reduction and then annealed at 800 °C or higher. The coarse grained Type 5 Nb required greater rolling reductions to produce any recrystallized grains at 800 °C. The higher interstitial content of the Type 2 Nb required a temperature of 1000 °C or higher to reach a recrystallized fraction greater than 95% in specimens rolled to a 60% reduction. Recrystallization is more easily achieved during annealing at a given temperature for a set time by increasing rolling reduction, decreasing interstitial content, and decreasing initial grain size prior to cold rolling. In conclusion, to avoid grain growth following recrystallization, which increases grain size, the minimum time and temperature necessary for complete recrystallization should be applied.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Nb3Sn Coating of Twin Axis Cavity for SRF Applications

The twin axis cavity with two identical accelerating beams has been proposed for Energy recovery linac (ERL) applications. Nb3Sn is a superconducting material with a higher critical temperature and a higher critical field as compared to Nb, which promises a lower operating cost due to higher quality factors. Two niobium twin axis cavities fabricated at JLab and were proposed to be coated with Nb3Sn. Due to their more complex geometry, the typical coating process used for basic elliptical cavities needs to be improved to coat these cavities. This devel-opment advances the current coating system at JLab for coating complex cavities. Two twin axis cavities were coated recently for the first time. This contribution dis-cusses initial results from coating of twin axis cavities, RF testing and witness sample analysis with an overview of the current challenges towards high performance Nb3Sn coated twin axis cavities.

Tiskumara, J.↗
DOE OSTI2023

Hybrid-Magnon Quantum Devices: Strategies and Approaches

Hybrid magnonics offers novel paths towards quantum information by utilizing the unique advantages of magnon excitations. We review two strategies of hybrid magnonics for advancing quantum computing, i.e., multi-magnon entanglement and nonreciprocal magnon transduction. We also present our efforts on low-damping magnetic materials, short-wavelength magnon transduction, and nanowire superconducting circuits.

36 MATERIALS SCIENCE↗
NASA NTRS1995

Proceedings of the 4th International Conference and Exhibition: World Congress on Superconductivity, volume 1

The papers presented at the 4th International Conference Exhibition: World Congress on Superconductivity held at the Marriott Orlando World Center, Orlando, Florida, are contained in this document and encompass the research, technology, applications, funding, political, and social aspects of superconductivity. Specifically, the areas covered included: high-temperature materials; thin films; C-60 based superconductors; persistent magnetic fields and shielding; fabrication methodology; space applications; physical applications; performance characterization; device applications; weak link effects and flux motion; accelerator technology; superconductivity energy; storage; future research and development directions; medical applications; granular superconductors; wire fabrication technology; computer applications; technical and commercial challenges, and power and energy applications.

Krishen, Kumar↗
DOE OSTI2022

Resonantly Enhanced Electromigration Forces for Adsorbates on Graphene

We investigate the electromigration forces for weakly bonded adsorbates on graphene by using density-functional based calculations. We find that the nature of electromigration forces on an adsorbate critically depends on the energy level alignment between the adsorbate state and the Fermi level of the graphene. For a resonant adsorbate, whose frontier orbitals lie close to the Fermi level, the electromigration force is dominated by the electron wind force that is strongly enhanced along the electron flow direction, irrespective of the sign of the adsorbate charge. For a nonresonant adsorbate, the electromigration force is essentially the direct force that depends on the adsorbate charge. We also show that the magnitude of electromigration forces can be continuously tunable through electrostatic gating for resonant adsorbates. Our results provide new insight for understanding and controlling how nanoscale objects behave in or on host materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Assessing spin-density wave formation in La 3 Ni 2 O 7 from electronic structure calculations

Here, we employ correlated density-functional theory methods (DFT + Hubbard U) to investigate the spin-density wave state of the bilayer Ruddlesden-Popper (RP) nickelate La 3 Ni 2 O 7 which becomes superconducting under pressure. We predict that the ground state of this bilayer RP material has traits of both the double spin-stripe and the single spin-charge stripe phases proposed in the literature as it corresponds to in-plane up/up′/down/down′ diagonal stripes with up/down being high spin (formally Ni 2+ : d 8 ), and up′ /down′ being low spin (formally Ni 3+ : d 7 ). The main feature of this solution (that is insulating even at U = 0) is the dominant role of the d x 2 −y 2 bands around the Fermi level, which would become doped with the introduction of electrons via oxygen vacancies. In spite of the similarity with cuprates in terms of the dominant role of d x 2 −y 2 bands, some differences are apparent in the magnetic ground state of La 3 Ni 2 O 7 : the antiferromagnetic out-of-plane coupling within the bilayer (linked to the d z 2 orbitals forming a spin-singlet-like configuration) is found to be the dominant one while in-plane interactions are reduced due to the stripe order of the ground state. With pressure, this striped magnetic ground state remains similar in nature but the increase in bandwidth quickly transitions La 3 Ni 2 O 7 into a metallic state with all the activity close to the Fermi level involving, to a large extent, d x 2 −y 2 orbitals. This is reminiscent of the cuprates and may provide key insights into how superconductivity arises in this material under pressure.

LaBollita, Harrison↗
DOE OSTI2026

Theoretical Study of High Harmonic Generation in Monolayer NbSe 2

High harmonic generation (HHG) is a powerful probe of electron dynamics on attosecond to femtosecond time scales and has been successfully used to detect electronic and structural changes in solid-state quantum materials, including transition-metal dichalcogenides (TMDs). Among TMDs, bulk NbSe 2 exhibits charge density wave (CDW) order below 33 K and becomes superconducting below 7.3 K. Monolayer NbSe 2 also has superconducting and CDW behavior and is therefore interesting as a material whose different structural and electronic properties could be probed via HHG. Here, we predict the HHG response of the pristine 2H and CDW phases of monolayer NbSe 2 using real-time time-dependent density functional theory under the application of a simulated laser pulse excitation. We find that due to the lack of inversion symmetry in both monolayer phases, it is possible to excite even harmonics and that the even harmonics appear as the transverse components of the current response under excitations polarized along the zigzag direction of the monolayer, while odd harmonics arise from the longitudinal current response in all excitation directions. This suggests that the even and odd harmonic responses can be controlled via the polarization of the probing field, opening an avenue for potentially useful applications in optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Symmetry-breaking polymorphous descriptions for correlated materials without interelectronic U

Correlated materials with open-shell d- and f-ions having degenerate band edge states show a rich variety of interesting properties ranging from metal-insulator transition to unconventional superconductivity. The textbook view for the electronic structure of these materials is that mean-field approaches are inappropriate, as the interelectronic interaction U is required to open a band gap between the occupied and unoccupied degenerate states while retaining symmetry. We show that the latter scenario often defining what Mott insulators are, is in fact not needed for the 3d binary oxides MnO, FeO, CoO, and NiO. The mean-field-like band theory can indeed lift such degeneracies in the binaries when nontrivial unit cell representations (polymorphous networks) are allowed to break symmetries, in conjunction with a recently developed non-empirical exchange and correlation density-functional without an on-site interelectronic interaction U. We explain how density-functional theory (DFT) in the polymorphous representation achieves band gap opening in correlated materials through a separate mechanism to the Mott-Hubbard approach. Here, we show the method predicts magnetic moments and gaps for the four binary monoxides in both the antiferromagnetic and paramagnetic phases, offering an effective alternative to symmetry-conserving approaches for studying a range of functionalities in open d- and f-shell complex materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗