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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Gate-tunable enhancement of supercurrent in hybrid planar Josephson junctions

Planar Josephson junctions (JJs) have emerged as a promising platform for the realization of topological superconductivity and Majorana zero modes. To obtain robust quasi one-dimensional (1D) topological superconducting states using planar JJs, limiting the number of 1D Andreev bound states’ subbands that can be present, and increasing the size of the topological superconducting gap are two fundamental challenges. It has been suggested that both problems can be addressed by properly designing the interfaces between the JJ’s normal region and the superconducting leads. We fabricated Josephson junctions with periodic hole structures on the superconducting contact leads on InAs heterostructures with epitaxial superconducting Al. By depleting the chemical potential inside the holes region with a top gate, we observed an enhancement of the supercurrent across the junction. The theoretical analysis shows that the enhancement of the JJ’s critical current is achieved when the depletion of the holes is such to optimize the matching of quasiparticles’ wave function at the normal/superconductor interface. Furthermore, these results show how the combination of carefully designed patterns for the Al coverage, and external gates, can be successfully used to tune the density and wave functions’ profiles in the normal region of the JJ, and therefore open an avenue to tune some of the critical properties, such as number of subbands and size of the topological gap, that must be optimized to obtain robust quasi-1D superconducting states supporting Majorana bound states.

Bogoliubov-de Gennes equations↗
DOE OSTI2023

Tunable momentum pair creation of spin excitations in dipolar bilayers

Here we study the temporal growth and spatial propagation of quantum correlations in a two-dimensional bilayer realizing a spin-1/2 quantum XXZ model with couplings mediated by long-range and anisotropic dipolar interactions. Starting with an initial state consisting of spins with opposite magnetization in each of the layers, we predict a dynamic instability that results, at short times, in the creation of correlated pairs of excitations at specific momenta at exponentially fast rates and entanglement between spatially separated modes. The momentum structure of the created pairs can be controlled via the dipolar orientation, the layer separation, or the dipolar couplings. The predicted behavior remains observable at very low filling fractions, making it accessible in state-of-the-art experiments with Rydberg atoms, magnetic atoms, and polar molecule arrays.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2021

Tunable Plasma Linearizer for Compensation of Nonlinear Energy Chirp

The removal of undesired nonlinear time-energy correlation (energy chirp) present in relativistic electron beams is crucial for many accelerator-based scientific applications, such as free-electron lasers, high-energy electron radiography, and MeV ultrafast electron microscopy. Here, we propose and demonstrate that a low-density plasma section can be used as a passive “linearizer” to significantly compensate for the nonlinear energy chirp imprinted on a beam by the curvature of the radio-frequency field in a conventional accelerator. Physically, the passage of the beam through the plasma excites a strong quasi-cosinoidal longitudinal decelerating wakefield that acts to mitigate the beam’s nonlinear energy chirp by superimposing a reverse chirp on the beam. Time-resolved phase-space measurements, combined with high-fidelity three-dimensional particle-in-cell simulations show that the longitudinal phase space of the beam core is almost completely linearized, leading to a fourfold reduction of the beam overall energy spread from 0.148% to 0.036% (FWHM).

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Optimization of Backscatter and Symmetry for Laser Fusion Experiments Using Multiple Tunable Wavelengths

A new additional wavelength-tuning capability has been implemented on the National Ignition Facility (NIF) laser allowing for unprecedented control of crossed-beam energy transfer (CBET) between all groups of beams for better performance of indirect-drive inertial confinement fusion (ICF) ignition experiments. In particular, this advance allows for negation and reversal of flow-induced CBET and the rebalancing of the intensity of different groups of beams within an indirect-drive (ICF) hohlraum. The experiments conducted at the NIF using a 1.1 MJ laser pulse with peak power of 390 TW demonstrate this high level of control through measurements of the precise changes to stimulated Brillouin scattering, typically driven by flow-induced CBET at the end of the pulse. Additionally, this new capability is shown to be able to predictably control gold-wall plasma expansion in the target driven by early-time flow-induced CBET. Estimates of early-time CBET from the wall expansion are shown to be consistent with simulation expectations. This new additional capability to control symmetry and backscatter expands the design space of current experiments and provides extra margin for increased laser energy in near-future experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Transport studies in a gate-tunable three-terminal Josephson junction

Josephson junctions with three or more superconducting leads have been predicted to exhibit topological effects in the presence of few conducting modes within the interstitial normal material. Such behavior, of relevance for topologically protected quantum bits, would lead to specific transport features measured between terminals, with topological phase transitions occurring as a function of phase and voltage bias. Although conventional, two-terminal Josephson junctions have been studied extensively, multiterminal devices have received relatively little attention to date. Motivated in part by the possibility to ultimately observe topological phenomena in multiterminal Josephson devices, as well as their potential for coupling gatemon qubits, here we describe the superconducting features of a top-gated mesoscopic three-terminal Josephson device. The device is based on an InAs two-dimensional electron gas proximitized by epitaxial aluminum. We map out the transport properties of the device as a function of bias currents, top gate voltage, and magnetic field. Finally, we find a very good agreement between the zero-field experimental phase diagram and a resistively and capacitively shunted junction computational model.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Carrier- and strain-tunable intrinsic magnetism in two-dimensional $MAX_3$ transition metal chalcogenides

Here, we present a density functional theory study of the carrier-density and strain dependence of magnetic order in two-dimensional (2D) $MAX_3$ ($\textit{M}$ = V, Cr, Mn, Fe, Co, Ni; $\textit{A}$ = Si, Ge, Sn; and $\textit{X}$ = S, Se, Te) transition metal trichalcogenides. Our ab initio calculations show that this class of compounds includes wide and narrow gap semiconductors, metals, and half-metals, and that most of these compounds are magnetic. Although antiferromagnetic order is most common, ferromagnetism is predicted in $\textit{M}$SiSe 3 for $\textit{M}$ = Mn and Ni; in $\textit{M}$SiTe 3 for $\textit{M}$ = V and Ni; in MnGeSe 3 ; $\textit{M}$GeTe 3 for $\textit{M}$ = Cr, Mn, and Ni; in FeSnS 3 ; and in MSnTe 3 for $\textit{M}$ = V, Mn, and Fe. Among these compounds CrGeTe 3 , VSnTe 3 , and CrSnTe 3 are ferromagnetic semiconductors. Our calculations suggest that the competition between antiferromagnetic and ferromagnetic order can be substantially altered by strain engineering, and in the semiconductor case also by gating. The associated critical temperatures can be enhanced by means of carrier doping and strains.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tunable chiral symmetry breaking in symmetric Weyl materials

Asymmetric Weyl semimetals, which possess an inherently chiral structure, have different energies and dispersion relations for left- and right-handed fermions. They exhibit certain effects not found in symmetric Weyl semimetals, such as the quantized circular photogalvanic effect and the helical magnetic effect. In this work, we derive the conditions required for breaking chiral symmetry by applying an external field in symmetric Weyl semimetals. Here, we explicitly demonstrate that in certain materials with the T d point group, magnetic fields along low symmetry directions break the symmetry between left- and right-handed fermions; the symmetry breaking can be tuned by changing the direction and magnitude of the magnetic field. In some cases, we find an imbalance between the number of type I left- and right-handed Weyl cones (which is compensated by the number of type II cones of each chirality).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tunable fragile topology in Floquet systems

We extend the notion of fragile topology to periodically driven systems. Here we demonstrate driving-induced fragile topology in two different models, namely, the Floquet honeycomb model and the Floquet π-flux square-lattice model. In both cases, we discover a rich phase diagram that includes Floquet fragile topological phases protected by crystalline rotation or mirror symmetries, Floquet Chern insulators, and trivial atomic phases, with distinct boundary features. Remarkably, the transitions between different phases can be feasibly achieved by simply tuning the driving amplitudes, which is a unique feature of driving-enabled topological phenomena. Moreover, corner-localized fractional charges are identified as a “smoking-gun” signal of fragile topology in our systems. Our work paves the way for studying and realizing fragile topology in Floquet systems.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Tunable competing magnetic anisotropies and spin reconfigurations in ferrimagnetic Fe 100– x Gd x alloy films

Here, we report a comprehensive study of the temperature evolution of in-plane (IP) and out-of-plane (OOP) effective magnetic anisotropies in compensated ferrimagnetic Fe 100– x Gd x alloy films by employing direct current magnetometry and radiofrequency (RF) transverse susceptibility (TS) measurements. We suggest that our Fe 100– x Gd x system is chemically inhomogeneous and phase segregates into Fe- and Gd-enriched regions. Our IP and OOP magnetometry results indicate that the system undergoes a temperature-driven transformation from an IP-spin-configuration-dominated state to an OOP-spin-configuration-dominated state below a certain temperature (spin reorientation temperature). A two-step reversal behavior emerges in the OOP $\textit{M(H)}$ loop near compensation, which we attribute to the sequential magnetization reversals of Fe- and Gd-enriched domains. Field-induced spin-flop transitions were also observed near the compensation. Our RF TS measurements indicate that the effective magnetic anisotropy for the OOP configuration dominates over that for the IP configuration below a certain spin reorientation temperature. Both IP and OOP anisotropy fields determined from our TS measurement exhibit a minimum around the compensation temperature, which has been explained in the framework of the Stoner-Wohlfarth model.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Current-induced dynamics and tunable spectra of a magnetic chiral bobber

Chiral bobber (ChB) is an exotic three-dimensional topological texture in chiral magnets. It is composed of a Bloch-point singularity and a skyrmion tube; the incommensurate characteristic length scales of these two components would thus lead to novel dynamics of a ChB. In this work, we report the current-induced motion and the accompanying Hall effect of a ChB. Surprisingly, we find that the Hall angle of a ChB can be tuned in a wide range from -90 ° to positive angles. A further study of spin-wave excitations identified high-frequency resonant modes, which are related to the velocity of ChB and lattice constants. The excited frequency can be two orders of magnitude higher than that of skyrmions. The broad ranges of Hall angles and frequency spectra could boost the development of spintronic devices using topological textures.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Probing the tunability of magnetism with external pressure in metastable Sr 2 NiIrO 6 double perovskite

In Sr 2 NiIrO 6 long-range Ir-Ir antiferromagnetic exchange interactions have been reported to overcome the ferromagnetic Ni-Ir interactions hampering the otherwise expected ferromagnetic behavior. Prompted by this, a combination of x-ray absorption spectroscopy and x-ray diffraction at high pressure is used here to investigate the interplay between the magnetic structure of the Ir sublattice and lattice degrees of freedom. In this work, the compression of Sr 2 NiIrO 6 drives an unexpected nonmonotonic change of the x-ray magnetic circular dichroism (XMCD) spectra: The intensity first decreases in the 0- to 18-GPa range, then shows an increase in the 18- to 30-GPa range and again decreases for higher pressures. The XMCD intensity, a measure of the net magnetization in the Ir sublattice, however, is found to remain very low in the whole pressure range so the observed changes do not correspond with a transition from antiferromagnetic to ferromagnetic or ferrimagnetic order. The evolution of the XMCD is better explained in terms of a weakening/strengthening of the long-range antiferromagnetic (AFM) IrIr interaction between ferromagnetic planes associated with the reduction of the lattice parameters. In particular, a correlation can be established between the evolution of the b/a ratio and the weakening/strengthening of the AFM interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Strain-tunable metamagnetic critical endpoint in Mott insulating rare-earth titanates

Rare-earth titanates are Mott insulators whose magnetic ground state—antiferromagnetic (AFM) or ferromagnetic (FM)—can be tuned by the radius of the rare-earth element. We combine phenomenology and first-principles calculations to shed light on the generic magnetic phase diagram of a chemically substituted titanate on the rare-earth site that interpolates between an AFM and a FM state. Octahedral rotations present in these perovskites cause the AFM order to acquire a small FM component—and vice-versa—removing any multicritical point from the phase diagram. However, for a wide parameter range, a first-order metamagnetic transition line terminating at a critical endpoint survives inside the magnetically ordered phase. Like the liquid-gas transition, a Widom line emerges from the endpoint, characterized by enhanced fluctuations. In contrast to metallic FMs, this metamagnetic transition involves two symmetry-equivalent and insulating canted spin states. Moreover, instead of a magnetic field, we show that uniaxial strain can be used to tune this transition to zero temperature, inducing a quantum critical endpoint.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Tunable artificial topological Hall effects in van der Waals heterointerfaces

The topological Hall effect (THE) originating from a real-space Berry phase is a significant transport signal for chiral spin textures and has been extensively investigated recently due to its potential applications in topological spintronics. Recently, chiral spin textures and THE were realized in heterointerfaces where spatial inversion symmetry is naturally broken. However, multichannel transport in heterointerfaces can mask the intrinsic THE associated with chiral spin textures. We systematically investigate multichannel transport in two different kinds of van der Waals (vdW) heterointerfaces. In ferromagnetic-ferromagnetic (FM-FM) heterointerfaces with two opposite anomalous Hall effects, multichannel transport has mimicked both positive and negative THE without involving any topological charges, which is dubbed as an artificial THE. Moreover, artificial THEs were also observed in FM-metal heterointerfaces consisting of a single FM layer stacked onto a nodal-line semimetal; this was attributed to the presence of multichannel transport as well. Our findings provide an alternative explanation for THE-like features in heterointerfaces. They also reveal exotic multichannel transport properties in vdW heterointerfaces which could facilitate the development of multifunctional nanodevices based on vdW heterointerfaces.

36 MATERIALS SCIENCE↗