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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 253 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Scanning tunneling microscopy of ferrocenecarboxylic acid assemblies on Ag(111): a comparison to Au(111)

Scanning tunneling microscopy was used to investigate the arrangement of ferrocenecarboxylic acid (FcCOOH) monolayers on the Ag(111) surface. Four distinct structures were observed, none of which had previously been observed on other surfaces. Structural analysis indicates that these assemblies are primarily composed of dimers. VASP calculations support the molecular assignments of the monolayer structures, while ESI-MS experiments confirm that dimers are the predominant species in solution. Of particular note, the cyclic pentamers and aperiodic packing observed for FcCOOH on Au(111) were not observed, despite (1) the similarity of the Ag(111) and Au(111) surfaces in reactivity, flatness, and lattice constant, and (2) prior explanation of the Au(111) monolayer in terms of molecule–molecule interactions alone. It is clear that while the surface does not have a template structure, it has a significant influence on which structures are formed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Electrode-molecule energy level offsets in a gold-benzene diamine-gold single molecule tunnel junction

One means for describing electron transport across single molecule tunnel junctions (MTJs) is to use density functional theory (DFT) in conjunction with a nonequilibrium Green’s function formalism. This description relies on interpreting solutions to the Kohn–Sham (KS) equations used to solve the DFT problem as quasiparticle (QP) states. Many practical DFT implementations suffer from electron self-interaction errors and an inability to treat charge image potentials for molecules near metal surfaces. For MTJs, the overall effect of these errors is typically manifested as an overestimation of electronic currents. Correcting KS energies for self-interaction and image potential errors results in MTJ current–voltage characteristics in close agreement with measured currents. An alternative transport approach foregoes a QP picture and solves for a many-electron wavefunction on the MTJ subject to open system boundary conditions. It is demonstrated that this many-electron method provides similar results to the corrected QP picture for electronic current. The analysis of these two distinct approaches is related through corrections to a junction’s electronic structure beyond the KS energies for the case of a benzene diamine molecule bonded between two gold electrodes.

Szepieniec, Mark S.↗
DOE OSTI2023

Scanning tunneling microscopy and spectroscopy characterization of Nb films for quantum applications

Niobium thin films are key components of superconducting microwave resonators. Interest in these devices has increased dramatically because of their application in quantum systems. Despite tremendous effort to improve their performance, loss mechanisms are still not well understood. Nb/substrate and Nb/air interfaces are likely culprits in contributing to decoherence and ultimately limiting the performance of superconducting devices. Here, we investigate the Nb/substrate interface by studying the effect of hydrogen-passivated H:Si(111) substrates on the local superconducting properties of ∼40 nm thick Nb films compared to Nb films grown on typical Si(001) substrates. Specifically, low-temperature scanning tunneling microscopy and spectroscopy are employed to compare nanoscale material properties. Further, the atomically flat monohydride H:Si(111) substrates are found to yield a smoother and less defective interface with the Nb film. Correspondingly, the Nb films grown on H:Si(111) substrates present more uniform superconducting properties and exhibit less quasiparticle broadening.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Atom-resolved imaging with a silicon tip integrated into an on-chip scanning tunneling microscope

Limited throughput is a shortcoming of the Scanning Tunneling Microscope (STM), particularly when used for atomically precise lithography. To address this issue, we have developed an on-chip STM based on Microelectromechanical-Systems (MEMS) technology. The device reported here has one degree of freedom, replacing the Z axis in a conventional STM. The small footprint of the on-chip STM provides a great opportunity to increase STM throughput by incorporating a number of on-chip STMs in an array to realize parallel STM. The tip methodology adopted for the on-chip STM presented here, which is a batch-fabricated Si tip, makes our design conducive to this goal. In this work, we investigate the capability of this on-chip STM with an integrated Si tip for STM imaging. We integrate the on-chip STM into a commercial ultrahigh-vacuum STM system and perform imaging with atomic resolution on par with conventional STMs but at higher scan speeds due to the higher sensitivity of the MEMS actuator relative to a piezotube. The results attest that it is possible to achieve a parallel and high-throughput STM platform, which is a fully batch-fabricated MEMS STM nanopositioner capable of performing atomic-resolution STM imaging.

Instruments & Instrumentation↗
DOE OSTI2024

Wavefront distortion correction in scanning tunneling microscope image

Here, we report an algorithm to identify and correct distorted wavefronts in atomic resolution scanning tunneling microscope images. This algorithm can be used to correct nonlinear in-plane distortions without prior knowledge of the physical scanning parameters, the characteristics of the piezoelectric actuator, or individual atom positions. The 2D image is first defined as a sum of sinusoidal plane waves, where a nonlinear distortion renders a curve for an otherwise ideal linear wavefront. Using the Fourier transforms of local areas of the image, the algorithm generates a wavefront vector field. The identified wavefronts are subsequently linearized for each plane wave without changing lattice orders, giving rise to distortion corrections. Our algorithm is complementary to conventional post-processing algorithms that require prior detection of real space features, which can also be used to correct nonlinear distortions in 2D images acquired by other microscopy techniques.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Quick-connect scanning tunneling microscope head with nested piezoelectric coarse walkers

To meet changing research demands, new scanning tunneling microscope (STM) features must constantly evolve. We describe the design, development, and performance of a modular plug-in STM, which is compact and stable. The STM head is equipped with a quick-connect socket that is matched to a universal connector plug, enabling it to be transferred between systems. This head can be introduced into a vacuum system via a load-lock and transferred to various sites equipped with the connector plug, permitting multi-site STM operation. Its design allows for reliable operation in a variety of experimental conditions, including a broad temperature range, ultra-high vacuum, high magnetic fields, and closed-cycle pulse-tube cooling. The STM’s compact size is achieved by a novel nested piezoelectric coarse walker design, which allows for large orthogonal travel in the X, Y, and Z directions, ideal for studying both bulk and thin film samples ranging in size from mm to μm. Its stability and noise tolerance are demonstrated by achieving atomic resolution under ambient conditions on a laboratory desktop with no vibrational or acoustic isolation. The operation of the nested coarse walkers is demonstrated by successful navigation to a μm-sized 2D sample.

Instruments & Instrumentation↗
DOE OSTI2024

Cryogen-free modular scanning tunneling microscope operating at 4-K in high magnetic field on a compact ultra-high vacuum platform

One of the daunting challenges in modern low temperature scanning tunneling microscopy (STM) is the difficulty of combining atomic resolution with cryogen-free cooling. Further functionality needs, such as ultra-high vacuum (UHV), high magnetic field (HF), and compatibility with μm-sized samples, pose additional challenges to an already ambitious build. We present the design, construction, and performance of a cryogen-free, UHV, low temperature, and high magnetic field system for modular STM operation. An internal vibration isolator reduces vibrations in this system, allowing for atomic resolution STM imaging while maintaining a low base temperature of ∼4 K and magnetic fields up to 9 T. Samples and tips can be conditioned in situ utilizing a heating stage, an ion sputtering gun, an e-beam evaporator, a tip treater, and sample exfoliation. In situ sample and tip exchange and alignment are performed in a connected UHV room temperature stage with optical access. Multisite operation without breaking vacuum is enabled by a unique quick-connect STM head design. A novel low-profile vertical transfer mechanism permits transferring the STM between room temperature and the low temperature cryostat.

Instruments & Instrumentation↗
DOE OSTI2026

On the role of symmetry in quenching OH tunneling in 2,6-dimethylphenol

Here, we model the spectroscopy of the two methyl torsional degrees of freedom coupled to the OH torsional motion in 2,6-dimethylphenol. Recent gas-phase rotational transitions [Welsh et al ., J. Phys. Chem. Lett. 17 , 3749–3758 (2026)] have been interpreted to be the result of certain symmetry states of the methyl groups leading to the quenching of the OH torsional motion. Both a three-dimensional model and an adiabatic model, in which the OH torsion is treated as the slow mode, are developed to test these conjectures. Good agreement is found between the exact and approximate adiabatic models. The adiabatic model is developed to interpret and elucidate the key couplings leading to the quenching of the tunneling splitting.

Sibert, III, Edwin L. [Univ. of Wisconsin, Madison↗
DOE OSTI2021

Impact of direct exchange coupling via the insulator on the magnetic tunnel junction based molecular spintronics devices with competing molecule induced inter-electrode coupling

The magnetic tunnel junction (MTJ) based molecular spintronics device (MTJMSD) approach is suitable for mass production. This approach provides solutions to fabrication difficulties related to reliably connecting molecular device elements to the ferromagnets (FMs). To producing MTJMSD, the molecular channels are bridged across the insulator of an MTJ testbed with exposed side edges. In an MTJMSD, two FMs are simultaneously connected by an insulator film and the molecular channels along the exposed sides. In our prior experimental studies, we observed that molecules could produce strong coupling between ferromagnets in the presence of the competing coupling via an insulator. In this paper, our Monte Carlo Simulation (MCS) was used to study the impact of coupling variation via insulator (a.k.a. Ji) on the magnetic properties of an MTJMSD. We studied the effect of Ji while varying the molecule induced antiferromagnetic exchange coupling. The ferromagnetic or antiferromagnetic nature and magnitude of Ji determined the resultant effect. Antiferromagnetic Ji enhanced the pre-existing antiferromagnetic molecular coupling effect. Ferromagnetic Ji competed with the opposite nature of antiferromagnetic molecular coupling. Our MCS may help to understand the impact of insulator thickness and defects on the molecular spintronics device performance and design process.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Monte Carlo simulation to study the effect of molecular spin state on the spatio-temporal evolution of equilibrium magnetic properties of magnetic tunnel junction based molecular spintronics devices

With a variable spin state, paramagnetic molecules can affect the impact of magnetic exchange coupling strength between two ferromagnetic electrodes. Our magnetic tunnel junction based molecular spintronics devices (MTJMSD) were successful in connecting paramagnetic single molecular magnet (SMM) between two ferromagnetic electrodes. Isolated SMM exhibited a wide range of spin states. However, it was extremely challenging to identify the SMM spin state when connected to the ferromagnetic electrodes. Our prior experimental and Monte Carlo Simulations (MCS) studies showed that paramagnetic molecules produced unprecedented strong antiferromagnetic coupling between two ferromagnets at room temperature. The overall antiferromagnetic coupling occurred when a paramagnetic SMM made antiferromagnetic coupling to the first electrode and ferromagnetic coupling to the second ferromagnetic electrode. This paper studies the impact of variable molecular spin states of the SMMs, producing strong antiferromagnetic coupling between the ferromagnetic electrodes of MTJMSD. The MTJMSD used in this study was represented by an 11 x 50 x 50 Ising model, with 11 being the thickness of the MTJMSD and 5 x 10 x 50 being each electrode’s size. We employed a continuous MCS algorithm to investigate SMM’s spin state’s impact as a function of molecular exchange coupling strength and thermal energy.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Magnetic force microscopy revealing long-range room temperature stable molecule bridge-induced magnetic ordering on magnetic tunnel junction (MTJ) pillars

Magnetic tunnel junctions (MTJs) can integrate novel single molecular device elements to overcome long-standing fabrication challenges, thus unlocking their novel potential. This study employs magnetic force microscopy (MFM) to demonstrate that organometallic molecules, when placed between two ferromagnetic electrodes along cross-junction shaped MTJ edges, dramatically altered the magnetic properties of the electrodes, affecting areas several hundred microns in size around the molecular junction vicinity at room temperature. These findings are supported by magnetic resonance and magnetometer studies on ∼7000 MTJ pillars. MFM on the pillar sample showed an almost complete disappearance of the magnetic contrast. The spatial magnetic image suggests that molecular channels significantly impacted the spin density of states in the ferromagnetic electrodes. This advancement in MTJ-based molecular devices paves the way for a new generation of commercially viable logic and memory devices controlled by molecular quantum states at near-room temperatures.

Tyagi, Pawan (ORCID:0000000275411344)↗
DOE OSTI2023

Temporal and spatial resolution of distal protein motions that activate hydrogen tunneling in soybean lipoxygenase

The enzyme soybean lipoxygenase (SLO) provides a prototype for deep tunneling mechanisms in hydrogen transfer catalysis. This work combines room temperature X-ray studies with extended hydrogen–deuterium exchange experiments to define a catalytically-linked, radiating cone of aliphatic side chains that connects an active site iron center of SLO to the protein–solvent interface. Employing eight variants of SLO that have been appended with a fluorescent probe at the identified surface loop, nanosecond fluorescence Stokes shifts have been measured. We report a remarkable identity of the energies of activation ( E a ) for the Stokes shifts decay rates and the millisecond C–H bond cleavage step that is restricted to side chain mutants within an identified thermal network. These findings implicate a direct coupling of distal protein motions surrounding the exposed fluorescent probe to active site motions controlling catalysis. While the role of dynamics in enzyme function has been predominantly attributed to a distributed protein conformational landscape, the presented data implicate a thermally initiated, cooperative protein reorganization that occurs on a timescale faster than nanosecond and represents the enthalpic barrier to the reaction of SLO.

Science & Technology - Other Topics↗
DOE OSTI2024

Scanning tunneling microscopy study of epitaxial Fe 3 GeTe 2 monolayers on Bi 2 Te 3

Abstract Introducing magnetism to the surface state of topological insulators, such as Bi 2 Te 3 , can lead to a variety of interesting phenomena. We use scanning tunneling microscopy (STM) to study a single quintuple layer (QL) of the van der Waals magnet Fe 3 GeTe 2 (FGT) that is grown on Bi 2 Te 3 via molecular beam epitaxy. STM topographic images show that the FGT grows as free-standing islands on Bi 2 Te 3 and outwards from Bi 2 Te 3 steps. Atomic resolution imaging shows triangular lattices of 390 ± 10 pm for FGT and 430 ± 10 pm for Bi 2 Te 3 , consistent with the respective bulk crystals. A moiré pattern is observed on FGT regions with a periodicity of 4.3 ± 0.4 nm that can be attributed solely to this lattice mismatch and thus indicates zero rotational misalignment. While most of the surface is covered by a single QL of the FGT, there are small double QL regions, as well as regions with distinct chemical terminations due to an incomplete QL. The most common partial QL surface termination is the FeGe layer, in which the top two atomic layers are missing. This termination has a distinctive electronic structure and a 3 x 3 R 30 ∘ reconstruction overlaid on the moiré pattern in STM images. Magnetic circular dichroism measurements confirm these thin FGT films are ferromagnetic withT C ∼190 K.

Materials Science↗
DOE OSTI2024

Giant resistance switch in twisted transition metal dichalcogenide tunnel junctions

Resistance switching in multilayer structures are typically based on materials possessing ferroic orders. Here, in this work, we predict an extremely large resistance switching based on the relative spin–orbit splitting in twisted transition metal dichalcogenide (TMD) monolayers tunnel junctions. Because of the valence band spin splitting which depends on the valley index in the Brillouin zone, the perpendicular electronic transport through the junction depends on the relative reciprocal space overlap of the spin-dependent Fermi surfaces of both layers, which can be tuned by twisting one layer. Our quantum transport calculations reveal a switching resistance larger than 10 6 % when the relative alignment of TMDs goes from 0° to 60° and when the angle is kept fixed at 60° and the Fermi level is varied. By creating vacancies, we evaluate how inter-valley scattering affects the efficiency and find that the resistance switching remains large (10 4 %) for typical values of vacancy concentration. Not only should this resistance switching be observed at room temperature due to the large spin splitting, but our results also show how twist angle engineering and control of van der Waals heterostructures could be used for next-generation memory and electronic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Excitation and tunneling spectra of a fractional quantum Hall system in the thin-cylinder limit

Here, the excitations of fractional quantum Hall effect (FQHE) states have been largely inaccessible to experimental probes until recently. New electron scanning tunneling microscopy (STM) results from Hu et al. [Nat. Phys. 21, 716 (2025)] show promise in detecting and identifying these excited states via the local density of states (LDOS) spectrum. On a torus, there exists a mapping from the lowest Landau level states to a 1D lattice with a Hamiltonian that features dipole moment conservation. In this work, we apply perturbation theory starting from the thin-cylinder limit (𝐿 𝑥 → ∞, 𝐿 𝑦 < 𝑙 𝐵 for torus dimensions 𝐿 𝑥 and 𝐿 𝑦 and magnetic length 𝑙 𝐵 ) to obtain an analytical approach to the low-lying neutral and charged excitations of the 𝜈 = 1/3 FQHE state. Notably, in the thin cylinder, we can systematically enumerate all the low-lying excitations by the patterns of “dipoles” formed by the electron occupation pattern on the 1D lattice. We find that the thin-cylinder limit predicts a significant dispersion of the low-lying neutral excitations but sharpness of the LDOS spectra, which measure charged excitations. We also discuss connections between our work and several different approaches to the FQHE STM spectra, including those using the composite fermion theory. Numerical exact diagonalization beyond the thin-cylinder limit suggests that the energies of charged excitations remain largely confined to a narrow range of energies, which in experiments might appear as a single peak.

Adhidewata, Jyesta M. [University of California, B↗
DOE OSTI2023

Theoretical Analysis of Resonant Tunneling Enhanced Field Emission

In this paper, we develop an exact analytical quantum theory for field emission from surfaces with a nearby quantum well, by solving the one-dimensional time-independent Schrödinger equation. The quantum well, which may be introduced by ions, atoms, nanoparticles, etc., is simplified as a square potential well with depth H, width d, and distance to the surface L. The theory is used to analyze the effects of the quantum well (d, H, and L), the cathode properties (work function W and Fermi energy E F ), and dc field F. It is found that the quantum well can lead to resonant tunneling enhanced field emission up to several orders of magnitude larger than that from bare cathode surfaces. In the meantime, the electron-emission-energy spectrum is significantly narrowed. The strong enhancement region is bounded by the conditions eFL + H ≥ W + C and eFL ≤ W, with e being the elementary charge (positive) and C a constant dependent on dc field F. It is also found that the linear shift of resonance peaks in the electron-emission-energy spectrum with dc field F follows ε p =ε p⁢0 –e⁢FL, with ε p⁢0 being approximately the eigenenergies for electrons confined in a square potential well without a dc field. Finally, the theory provides insights for the design of high-efficiency field emitters, which can produce a high current and highly collimated electron beams.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Slow spin dynamics and quantum tunneling of magnetization in the dipolar antiferromagnet DyScO 3

Here, we present a comprehensive study of static and dynamic magnetic properties in the Ising-like dipolar antiferromagnet (AFM) DyScO 3 by means of DC and AC magnetization measurements supported by classical Monte Carlo calculations. Our AC-susceptibility data show that the magnetic dynamics exhibit a clear crossover from an Arrhenius-like regime to quantum tunneling of magnetization (QTM) at T*=10K. Below T N =3.2K, DyScO 3 orders in an antiferromagnetic GxAy-type magnetic structure and the magnetization dynamics slow down to the minute timescale. The low-temperature magnetization curves exhibit complex hysteretic behavior, which depends strongly on the magnetic field sweep rate. We demonstrate that the low-field anomalies on the magnetization curve are related to the metamagnetic transition, while the hysteresis at higher fields is induced by a strong magnetocaloric effect. Our theoretical calculations, which take into account dipolar interaction between Dy 3+ moments, reproduce essential features of the magnetic behavior of DyScO 3 . We demonstrate that DyScO 3 represents a rare example of an inorganic compound, which exhibits QTM at a single-ion level and magnetic order due to classical dipolar interaction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Persistent Josephson tunneling between Bi 2 Sr 2 CaCu 2 O 8 + x flakes twisted by 45 ° across the superconducting dome

There is a heated debate on the Josephson effect in twisted Bi 2 Sr 2 CaCu 2 O 8+x flakes. Recent experimental results suggest the presence of either anomalously isotropic pairing or exotic d+id-wave pairing, in addition to the commonly believed d-wave one. Here, we address this controversy by fabricating ultraclean junctions with uncompromised crystalline quality and stoichiometry at the junction interfaces. In the optimally doped regime, we obtain prominent Josephson coupling (2-4 mV) in multiple junctions with the twist angle of 45°, in sharp contrast to a recent report that shows two orders of magnitude suppression around 45° from the value at 0°. We further extend this study to the previously unexplored overdoped regime and observe pronounced Josephson tunneling at 45° together with Josephson diode effect up to 50 K. In conclusion, our work helps establish the persistent presence of an isotropic pairing component across the entire superconducting phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗