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At least 541 records · Page 30
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Observation of nearly identical superconducting transition temperatures in the pressurized Weyl semimetals MIrTe 4 (M=Nb and Ta)

In this work, we report the observation of pressure-induced superconductivity in type-II Weyl semimetal (WSM) candidate NbIrTe 4 and the evolution of its Hall coefficient (R H ), magnetoresistance (MR), and lattice with increasing pressure to ~ 63 GPa. These results provide a significant opportunity to investigate the universal high-pressure behavior of ternary WSMs, including the sister compound TaIrTe 4 that has been known through our previous studies. We find that the pressure-tuned evolution from the WSM to the superconducting (SC) state in these two compounds exhibit the same trend, i.e., a pressure-induced SC state emerges from the matrix of the non-superconducting WSM state at ~ 27 GPa, and then the WSM state and the SC state coexist up to 40 GPa. Above this pressure, an identical high-pressure behavior, characterized by almost the same value of R H and MR in its normal state and the same value of T c in its SC state, appears in both compounds. Our results not only reveal the evolution from the WSM state to the SC state, but also demonstrate that NbIrTe 4 and TaIrTe 4 can make the same contribution to the normal and SC states that inhabit in the high-pressure phase, although these two compounds have dramatically different band structure at ambient pressure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Strongly correlated itinerant magnetism near superconductivity in NiTa 4 Se 8

Metallic ferromagnets with strongly interacting electrons often exhibit remarkable electronic phases such as ferromagnetic superconductivity, complex spin textures, and nontrivial topology. Here, in this report, we discuss the synthesis of a layered magnetic metal NiTa 4 Se 8 (or Ni 1/4 TaSe 2 ) with a Curie temperature of 58 Kelvin. Magnetization data and density functional theory calculations indicate that the nickel atoms host uniaxial ferromagnetic order of about 0.7µ B per atom, while an even smaller moment is generated in the itinerant tantalum conduction electrons. Strong correlations are evident in flat bands near the Fermi level, a high heat capacity coefficient, and a high Kadowaki-Woods ratio. Density functional theory calculations suggest that electron and hole Fermi surfaces in the ferromagnetic phase are associated with opposite spin polarization. When the system is diluted of magnetic ions, the samples become superconducting below about 2 Kelvin. We discuss possible mechanisms for superconductivity in this family.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Superconducting properties and gap structure of the topological superconductor candidate Ti 3 Sb

Here, we present a study of the superconducting properties of the candidate topological superconductor Ti 3 Sb. Electrical transport measurements show zero resistance with a T c,onset of ≈ 5.9 K with a transition width ΔT c ≈ 0.6 K. The superconducting phase boundaries as derived from magnetotransport and magnetic susceptibility measurements agree well. We estimate an upper critical field B c2 (0) ≈ 4.5 T. A Ginzburg-Landau (GL) analysis yields values of the coherence length and penetration depth of ξ = 6.2 nm and λ = 340 nm, respectively, and a GL parameter κ ≈ 55, indicating extreme type-II behavior. Furthermore, we observe a step height in the specific heat $\frac{Δ C_e}{γ T_c}$ ≈ 1.61 ± 0.24 (with T c = 4.8 ± 0.4 K), a value larger than the Bardeen-Cooper-Schrieffer value of 1.43, suggesting modest coupling. Measurements of the temperature dependence of the London penetration depth via the tunnel-diode oscillator technique down to ≈ 450 mK show a full superconducting gap, consistent with a conventional s-wave gap structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Higher-order nodal hinge states in doped superconducting topological insulator

Doped strong topological insulators are one of the most promising candidates to realize a fully gapped three-dimensional topological superconductor (TSC). In this Letter, we revisit this system and reveal a possibility for higher-order topology which was previously missed. We find that over a finite range of doping, the Fu-Berg superconducting pairing can give rise to both Majorana surface states and nodal hinge states. Interestingly, we observe the coexistence of surface and hinge modes in the superconducting state only when there are both bulk and surface Fermi surfaces in the normal state. Furthermore, we find that the hinge modes can appear for normal states consisting of doped strong or weak topological insulators. In summary, this work may allow for the discovery of superconducting hinge modes in a well explored class of materials, i.e., doped strong or weak topological insulators.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Electronic and magnetic structures that hinder the superconducting state in the collapsed phase of $\mathrm{SrCr_2}$ $\mathrm{As_2}$

The lack of superconductivity in the family of materials $AB_2$ $\mathrm{As_2}$ ($A$ = $\mathrm{Sr, Ba}$ and $B$ = $\mathrm{Cr, Mn}$) under doping or pressure is an intriguing issue, considering the resemblance of these materials to their cousin materials and superconductors $\mathrm{(Ba, Sr) Fe_2}$ $\mathrm{As_2}$. In this context, the suppression of magnetism together with the presence of electron and hole pockets in the Brillouin zone seem to be fundamental ingredients to explain superconductivity. In this paper, we report a tetragonal to collapsed-tetragonal phase transition in $\mathrm{SrCr_2}$ $\mathrm{As_2}$ under high pressure with the appearance of hole and electron pockets in the Brillouin zone. In this collapsed-tetragonal phase, a residual local magnetic moment in the Cr-ion site and a finite global magnetic energy are derived. Importantly, this scenario would suggest why superconductivity is prevented in this compound. Our observations were obtained from the analysis of synchrotron x-ray diffraction measurements of polycrystalline samples of $\mathrm{SrCr_2}$ $\mathrm{As_2}$, and using first-principles simulations, under pressures $P$ in the range $\mathrm{1.4}$ $\mathrm{GPa}$ < $P$ < $\mathrm{20 GPa}$ at room temperature.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Possible topological superconductivity in the topological crystalline insulator $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$

Superconductivity in topological insulators is expected to show very unconventional features such as a $p+ip$ order parameter, Majorana fermions, etc. However, intrinsic superconductivity has been observed in a very limited number of materials in which the pairing symmetry is still a matter of debate. Here, we study the topological crystalline insulator (TCI) $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$, for which a peculiar insulator to superconductor transition was previously reported near the gap inversion transition, where the system is nearly a three-dimensional Dirac semimetal. Both the existence of superconductivity near the three-dimensional Dirac semimetal and the occurrence of an insulator to superconductor transition in an isotropic material are highly unusual. We suggest that the observed phenomena are related to the intrinsic instability of a three-dimensional Dirac semimetal state in $\mathrm{(Pb_{1-x} Sn_x)_{1-y}In_yTe}$ and “flattening” of the bulk valence and conduction bands as they acquire a Mexican-hat-like dispersion on the inverted side of the phase diagram. Importantly, this favors the pairing instability if the chemical potential is pinned to these flat regions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Topological nodal point superconductivity in checkerboard magnet-superconductor hybrid systems

Here, we demonstrate that checkerboard magnet-superconductor hybrid (MSH) systems possess a rich phase diagram exhibiting both strong topological superconducting (STSC) and topological nodal point superconducting (TNPSC) phases. We show that TNPSC phases exist both for ferromagnetic and antiferromagnetic systems, yielding a plethora of qualitatively different edge mode structures. Checkerboard MSH systems also facilitate the emergence of STSC phases, which can be induced even in the limit of vanishing magnetization. Our results provide a new path for the quantum engineering of topological superconducting phases using atomic manipulation techniques.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Fermi surface topology and magnetotransport properties of superconducting Pd 3 Bi 2 Se 2

Pd 3 Bi 2 Se 2 is a rare realization of a superconducting metal with a non-zero topological invariant. Here, in this study, we report the growth of high-quality single crystals of layered Pd 3 Bi 2 Se 2 with a superconducting transition at T c ≈ 0.80 K and upper critical fields of ~10 mT and ~5 mT for the in plane and out-of-plane directions, respectively. Our density functional theory (DFT) calculations reveal three pairs of doubly degenerate bands crossing the Fermi level all displaying clear three dimensional dispersion consistent with the overall low electronic anisotropy (<2). The multiband electronic nature of Pd 3 Bi 2 Se 2 is evident in magneto-transport measurements, yielding a sign changing Hall resistivity at low temperatures. The magnetoresistance is non-saturating and follows Kohler’s scaling rule. We interpret the magneto-transport data in terms of open orbits that are revealed in the DFT calculated Fermi surface. de Haas-van Alphen (dHvA) oscillation measurements using torque magnetometry on single crystals yield four frequencies for out-of-plane fields: F α = (150 ± 26) T, F β = (293 ± 10) T, F γ = (375 ± 20) T and F η = (1017 ± 12) T, with the low frequency dominating the spectrum. Through the measurement of angular dependent dHvA oscillations and DFT calculations we identify the F α frequency with an approximately ellipsoidal electron pocket centered on the L 2 point of the Brillouin zone. Lifshitz-Kosevich analysis of the dHvA oscillations reveals a small cyclotron effective mass m* = (0.11 ± 0.02)m 0 and a nontrivial Berry phase for the dominant orbit. The presence of nontrivial topology in a bulk superconductor positions Pd 3 Bi 2 Se 2 as a potential candidate for exploring topological superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Lithium's low-temperature phase transitions: Insights into quantum lattice dynamics and superconductivity

The large lattice dynamics of lithium, driven by its low atomic mass, results in energetically similar structures and significant isotope effects under pressure, posing challenges to current theoretical models. Above 20 GPa and at low temperatures, lithium's electronic properties deviate from simple metallic behavior, with superconductivity emerging in a complex, pressure-dependent manner, alongside an unusual isotope effect. The structural phases of 7 Li reported under these conditions are inconsistent across studies, and the structures of 6 Li remain unexamined. These gaps limit our understanding of the effects of pressure on lithium's electronic properties and the role of quantum lattice effects on its structural behavior under pressure. Here, we integrate experimental and theoretical approaches to investigate the low-temperature structural phase boundaries in lithium isotopes. We map the structural phase diagram of 7 Li from 5 to 55 GPa and 15–75 K, identifying the sequence fcc → ℎ⁡R⁢1 → cI16. A pronounced isotope effect is observed, with 6 Li shifting the fcc → ℎ⁡R⁢1 phase boundary to lower pressures at 15 K. Density functional theory calculations further clarify how these structural changes affect superconducting properties, particularly emphasizing the role of the fcc → ℎ⁡R⁢1 transition in lithium's superconductivity. Furthermore, our findings offer insights into the unique behavior of lithium isotopes under pressure.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Superconductivity in the lightly doped Hubbard model on the cylindrical honeycomb lattice

Here, we have performed large-scale density-matrix renormalization group studies of the lightly doped Hubbard model on the honeycomb lattice on long three- and four-leg cylinders. We find that the ground state of the system upon lightly doping is consistent with that of a superconducting state with coexisting quasi-long-range superconducting and charge density wave orders. Both the superconducting and charge density wave correlations decay as a power law at long distances with corresponding exponents 𝐾 𝑠⁢𝑐 < 2 and 𝐾 𝑐 < 2. On the contrary, the spin-spin and single-particle correlations decay exponentially, although with relatively long correlation lengths.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Topological Superconductivity in the Doped Chiral Spin Liquid on the Triangular Lattice

It has been established that the undoped, i.e., δ = 0 , system has a CSL ground state in the parameter region 0.32 ≤ J χ / J ≤ 0.56 . Upon light doping, we find that the ground state of the system is consistent with a Luther-Emery liquid with power-law superconducting and charge-density-wave correlations but short-range spin-spin correlations. In particular, the superconducting correlations, whose pairing symmetry is consistent with d ± i d wave, are dominant at all hole doping concentrations. Overall, our results provide direct evidences that doping the CSL on the triangular lattice can naturally give rise to topological superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Competing Zero-Field Chern Insulators in Superconducting Twisted Bilayer Graphene

The discovery of magic angle twisted bilayer graphene has unveiled a rich variety of superconducting, magnetic, and topologically nontrivial phases. Here, we show that the zero-field states at odd integer filling factors in h-BN nonaligned devices are consistent with symmetry broken Chern insulators, as is evidenced by the observation of the anomalous Hall effect near moiré cell filling factor ν = +1. The corresponding Chern insulator has a Chern number C = ±1 and a relatively high Curie temperature of T c ≈ 4.5 K. In a perpendicular magnetic field above B > 0.5 T we observe a transition of the ν = +1 Chern insulator from Chern number C = ±1 to C = 3, characterized by a quantized Hall plateau with R yx = h/3e 2 . These observations demonstrate that interaction-induced symmetry breaking leads to zero-field ground states that include almost degenerate and closely competing Chern insulators, and that states with larger Chern numbers couple most strongly to the B field. In addition, the device reveals strong superconducting phases with critical temperatures of up to T c ≈ 3.5 K. By providing the first demonstration of a system that allows gate-induced transitions between magnetic and superconducting phases, our observations mark a major milestone in the creation of a new generation of quantum electronics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Evolution of 1 / f Flux Noise in Superconducting Qubits with Weak Magnetic Fields

The microscopic description of 1/f magnetic flux noise in superconducting circuits has remained an open question for several decades despite extensive experimental and theoretical investigation. Recent progress in superconducting devices for quantum information has highlighted the need to mitigate sources of qubit decoherence, driving a renewed interest in understanding the underlying noise mechanism(s). Though a consensus has emerged attributing flux noise to surface spins, their identity and interaction mechanisms remain unclear, prompting further study. Here, we apply weak in-plane magnetic fields to a capacitively shunted flux qubit (where the Zeeman splitting of surface spins lies below the device temperature) and study the flux-noise-limited qubit dephasing, revealing previously unexplored trends that may shed light on the dynamics behind the emergent 1/f noise. Notably, we observe an enhancement (suppression) of the spin-echo (Ramsey) pure-dephasing time in fields up to B = 100 G. With direct noise spectroscopy, we further observe a transition from a 1/f to approximately Lorentzian frequency dependence below 10 Hz and a reduction of the noise above 1 MHz with increasing magnetic field. Here, we suggest that these trends are qualitatively consistent with an increase of spin cluster sizes with magnetic field. These results should help to inform a complete microscopic theory of 1/f flux noise in superconducting circuits.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Homogeneous superconducting gap in Dy Ba 2 Cu 3 O 7 - δ synthesized by oxide molecular beam epitaxy

Much of what is known about high-temperature cuprate superconductors stems from studies based on two surface analytical tools, angle-resolved photoemission spectroscopy (ARPES) and spectroscopic imaging scanning tunneling microscopy (SI-STM). A question of general interest is whether and when the surface properties probed by ARPES and SI-STM are representative of the intrinsic properties of bulk materials. We find this question is prominent in thin films of a rarely studied cuprate Dy Ba 2 Cu 3 O 7 - δ DBCO). We synthesize DBCO films by oxide molecular beam epitaxy and study them by in situ ARPES and SI-STM. Both ARPES and SI-STM show that the surface DBCO layer is different from the bulk of the film—it is heavily underdoped, while the doping level in the bulk is close to optimal doping evidenced by bulk-sensitive mutual inductance measurements. ARPES shows the typical electronic structure of a heavily underdoped CU O 2 plane and two sets of one-dimensional bands originating from the CuO chains with one of them gapped. SI-STM reveals two different energy scales in the local density of states, with one (at ~ 18 meV ) corresponding to the superconductivity and the other one (at ~ 90 meV ) to the pseudogap. Finally, while the pseudogap shows large variations over the length scale of a few nanometers, the superconducting gap is very homogeneous. This indicates that the pseudogap and superconductivity are of different origins.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Nodeless kagome superconductivity in LaRu 3 Si 2

We report muon spin rotation (μSR) experiments together with first-principles calculations on microscopic properties of superconductivity in the kagome superconductor LaRu 3 Si 2 with T c ≃ 7K. Below Tc, μSR reveals type-II superconductivity with a single s-wave gap, which is robust against hydrostatic pressure up to 2 GPa. We find that the calculated normal state band structure features a kagome flat band, and Dirac as well as van Hove points formed by the Ru-dz 2 orbitals near the Fermi level. We also find that electron-phonon coupling alone can only reproduce a small fraction of T c from calculations, which suggests other factors in enhancing T c such as the correlation effect from the kagome flat band, the van Hove point on the kagome lattice, and the high density of states from narrow kagome bands. Our experiments and calculations taken together point to nodeless moderate coupling kagome superconductivity in LaRu 3 Si 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Superconducting phase of Ti x O y thin films grown by molecular beam epitaxy

Here we investigate the complex relationship between the growth conditions and the structural and transport properties of Ti x O y thin films grown by molecular beam epitaxy. Transport properties ranging from metallicity to superconductivity and insulating states are stabilized by effectively tuning the O/Ti ratio via the Ti flux rate and the O partial pressure P Ox for films grown on (0001)-Al 2 O 3 substrates at 850° C. A cubic c-TiO 1±δ buffer layer is formed for low O/Ti ratios, while a corundum cr-Ti 2 O 3 layer is formed under higher-oxidizing conditions. Metallicity is observed for c-TiO 1-δ buffer layers. The superconducting γ -Ti 3 O 5 Magnéli phase is found to nucleate on a c-TiO 1-δ buffer for intermediate POx conditions, and an insulator-superconducting transition is observed at 4.5 K (T$^{onset}_{C}$ = 6K) for 85 nm thick films. Strain relaxation of γ -Ti 3 O 5 occurs with increasing film thickness and correlates with a thickness-dependent increase in T C observed for Ti x O y thin films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Identifying topological superconductivity in two-dimensional transition-metal dichalcogenides

Here we study the superconducting pairing instabilities and gap functions for prototypical two-dimensional (2D) transition-metal dichalcogenides (TMDCs) WS 2 , MoTe 2 , and MoS 2 in the 2H phase under both hole and electron doping at 10 K. Our first-principles quantum many-body Green's function approach allows us to treat the full d and p manifold of orbitals with strong spin-orbit coupling, yielding pairing predictions with material-specific detail. The resulting gap functions exhibit a variety of mixed-parity superconducting states, including s, p, d, f, d ± id, and p ± ip pairing modes. In particular, we predict 3% and 4% hole-doped WS 2 to be a chiral p ± ip topological superconductor. For 1% hole-doped MoS 2 , we find a competition between three doubly degenerate chiral and nonchiral instabilities. Overall, the relative pairing strengths are found to follow the Fermi-surface topology, due to nesting between the Fermi-surface sheets. Finally, we discuss our predictions in relation to available experimental data and classify the topology of the predicted superconducting pairing symmetries.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Kondo enabled transmutation between spinons and superconducting vortices: Origin of magnetic memory in 4 H b − TaS 2

Recent experiments [Persky , ] demonstrate a magnetic memory effect in 4 H b − TaS 2 above its superconducting transition temperature, where Abriokosov vortices are spontaneously generated by lowering the temperature at zero magnetic field after field training the normal state. Motivated by the experiment, we propose the chiral quantum spin liquid (QSL) stabilized in the constituent layers of 4 H b − TaS 2 as a mechanism. We model 4 H b − TaS 2 as coupled layers of the chiral QSL and the superconductor. Through the Kondo coupling between the localized moments and conduction electrons, there is mutual transmutation between spinons and vortices during the thermal-cycling process, which yields the magnetic memory effect observed in experiments. We also propose a mechanism to stabilize chiral and nematic superconductivity in 4 H b − TaS 2 through the Kondo coupling of conduction electrons to the chiral QSL. Our picture suggests 4 H b − TaS 2 as an exciting platform to explore the interplay between QSL and superconductivity through the Kondo effect. Published by the American Physical Society 2024

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗