Superconducting topological systems formed by a strong 3D topological insulator (TI) in proximity to a conventional s-wave superconductor (SC) have been intensely studied, as they may host Majorana zero modes. However, there are limited experimental realizations of TI-SC systems in which robust superconducting pairing is induced on the surface states of the TI and a topological superconducting state is established. Here, we fabricate a TI-SC system by depositing, via a focused ion beam, tungsten (W) nanoscale clusters on the surface of TI Bi0.91Sb0.09. We find that the resulting heterostructure supports phase-slip lines (PSLs) that act as effective Josephson junctions (JJs). We probe the response of the system to microwave radiation. We find that for some ac frequencies, and powers, the resulting Shapiro steps' structure of the voltage-current characteristic exhibits a missing first step and an unexpectedly wide second Shapiro step. The theoretical analysis of the measurements shows that the unusual Shapiro response arises from the interplay between a static JJ and a dynamic one and allows us to identify the conditions under which the missing first step can be attributed to the topological nature of the JJs formed by the PSLs. Our results suggest an approach to induce superconductivity in a TI, a route to realizing highly transparent topological JJs, and show how the response of superconducting systems to microwave radiation can be used to infer the dynamics of PSLs. Highly transparent topological junctions are promising candidates to realize vector field sensors with very high sensitivity. In addition, due to the nontrivial Berry phase of the TI's surface states such junctions can be in a topological state which is ideal to create topologically protected qubits.

Superconducting topological systems formed by a strong 3D topological insulator (TI) in proximity to a conventional $s$-wave superconductor (SC) have been intensely studied as they may host Majorana zero modes. However, there are limited experimental realizations of TI-SC systems in which robust superconducting pairing is induced on the surface states of the TI and a topological superconducting state is established. Here, we fabricate a novel TI-SC system by depositing, via focused ion beam, tungsten (W) nanoscale clusters on the surface of TI Bi$_{0.91}$Sb$_{0.09}$. We find that the resulting heterostructure supports phase-slip lines that act as effective Josephson junctions. We probe the response of the system to microwave radiation. We find that for some ac frequencies, and powers, the resulting Shapiro steps' structure of the voltage-current characteristic exhibits a missing first step and an unexpectedly wide second Shapiro step. The theoretical analysis of the measurements shows that the unusual Shapiro response arises from the interplay between a static Josephson junction and a dynamic one, and allows us to identify the conditions under which the missing first step can be attributed to the topological nature of the Josephson junctions formed by the phase-slip lines. Our results suggest a new approach to induce superconductivity in a TI, a novel route to realizing highly-transparent topological Josephson junctions, and show how the response of superconducting systems to microwave radiation can be used to infer the dynamics of phase-slip lines.

In this work, we investigate the viability of the Hydride-Dehydride (HDH) process coupled with plasma spheroidization for producing U-6 wt.% Nb powders as an alternative method to atomization. The HDH process produces irregularly shaped powder with sub-micron U-Nb segregation and phase heterogeneity, and the subsequent spheroidization process re-homogenizes both the U-Nb composition and phase that is similar to atomized U-6 wt.% Nb powders. The surface oxide layer of the spheroidized powder is thinner than atomized U-6 wt.% Nb powders, ranging from 5 to 15 nm and display an isotropic microstructure, absent of large dendrites across the sphere as seen in atomized U-6 wt.% Nb. HDH also offers the advantage that oversized and undersized powders can be reprocessed. Combined, we demonstrate that HDH coupled with plasma spheroidization is a low-waste, alternative production route for spherical U6 wt.% Nb powders.(c) 2023 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/4.0/).

We report on fabrication and characterization of layered, tungsten doped, spherical about 2 mm diameter microcrystalline diamond ablator shells for inertial confinement fusion (ICF) experiments at the National Ignition Facility. As previously reported, diamond ICF ablator shells can be fabricated by chemical vapor deposition (CVD) on solid spherical silicon mandrels using an ellipsoidal microwave plasma reactor. In the present work, we further developed these ablator shells by embedding a W -doped diamond layer sandwiched between two undoped diamond regions. W incorporation in diamond was achieved by adding tungsten hexacarbonyl to the CH 4 /H 2 CVD feed gas. We observe that the W doping concentration decreases with increasing deposition rate which, in turn, is controlled by adjusting the total gas pressure. Cross sectional microstructural analysis reveals sharp interfaces between doped and undoped regions of the diamond shell and uniform W distribution with concentrations up to about 0.3 at.%. At higher W concentrations (>0.3 at.%) formation of tungsten carbide precipitates is observed. Using a 3‐shock 1.6 MJ laser pulse, the targets described in this work produced the first laser driven implosion to break the 1 × 10 16 neutron yield barrier, followed by experiments (described in future publications) with similar targets and slightly more laser energy producing yields as high as 4 × 10 17 .

As theoretically hypothesized for several decades in group IV transition metals, we have discovered a dynamically stabilized body-centered cubic (bcc) intermediate state in Zr under uniaxial loading at sub-nanosecond timescales. Under ultrafast shock wave compression, rather than the transformation from α-Zr to the more disordered hex-3 equilibrium ω-Zr phase, in its place we find the formation of a previously unobserved nonequilibrium bcc metastable intermediate. We probe the compression-induced phase transition pathway in zirconium using time-resolved sub-picosecond x-ray diffraction analysis at the Linac Coherent Light Source. We also present molecular dynamics simulations using a potential derived from first-principles methods which independently predict this intermediate phase under ultrafast shock conditions. In contrast with experiments on longer timescale (> 10 ns) where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time dependence in the kinetics of phase transformations.

To date, the primary focus of metal additive manufacturing (AM) research has been the development of strategies for fabricating complex architectures, reducing internal stress and optimizing microstructure. Traditional Al alloys have presented further challenges in this effort due to solidification cracking and complex laser coupling dynamics. To overcome these limitations, identification of novel alloys that exploit the rapid solidification conditions inherent in laser-based AM is required. In this work, laser-induced melting of an Al-8Ce-10Mg alloy is revealed to generate a nanoscale microstructure which results in improved hardness, tensile strength, and mitigated solidification cracking (e.g., hot tearing) in single laser tracks in as-cast material and laser powder bed fusion (LPBF)-fabricated components. In situ X-ray imaging shows the nanostructure arises from laser-induced melting of intermetallic particles embedded into the alloy during casting and then rapid resolidification of the molten material in similar to 400 mu s. The formed Ce-rich nanostructures are highly resistant to thermal coarsening at 300 degrees C, as confirmed by microscopy and retention of tensile properties. These results pave the way for development of AM-specific Al alloys that possess the ability to form mechanically favorable nanostructures in fabricated components due to the rapid cooling inherent in LPBF. (C) 2021 Elsevier Ltd. All rights reserved.

Laser-induced damage threshold of transparent conductors, such as Indium Tin Oxide (ITO), is limited by their high optical absorption due to free carriers. However, the effective absorption of a transparent conductor thin film can be reduced by an order of magnitude, without changing the electrical characteristics of the film, when placed in a low electric field section of a multilayer coating. A Fabry-Perot thin film interference filter has both high transmittance and low electric field positions, so it is an ideal thin film structure for this application. Although Fabry-Perot interference filters are not known as particularly high laser-induced damage resistant coatings due to their resonant characteristics, a laser-induced damage threshold (LIDT) improvement of up to 8× was observed with this technique compared to single layer ITO coatings fabricated using either radio frequency magnetron sputtering or electron-beam deposition. Additionally, an approximately 4× LIDT improvement for a Fabry-Perot interference filter has been observed by the addition of ITO into the multilayer structure.

A key characteristic of excitable ion channels is the ability to interact with neighboring pores so that localized environmental stimuli can initiate complex large-scale responses, such as an action potential propagating along a neuron. In the case of a nerve signal, this is accomplished through local changes in the membrane voltage and ion concentration inducing nearby pores to change conductance states. The replication of this behavior in synthetic nanopore platforms has the potential to enable new responsive materials useful for biomimetic componentry, drug delivery and filtration technologies. While gating has been demonstrated in synthetic nanopores, previous fabrication methods make it difficult to incorporate such pores into more complex systems due to lack of control over key attributes such as pore location. Here, we fabricate two neighboring nanochannels with controlled spacing such that the transport through one pore can induce a change in conductance of a neighboring pore which has been chemically modified to be ion concentration responsive. The two-pore system is made using focused ion beam nanomachining followed by ion-beam-controlled deposition of SiOx around one pore entrance. Chemistry selective for SiOx is then used to graft single stranded DNA only to the regions where the local deposition was performed providing a final platform that enables local control over the location, size, shape and surface chemistry of each pore in the system. The pore-pore interaction is measured through a step-like increase of the total current passing through the SiNx membrane vs time following a change in applied voltage. We explore the effects of voltage and bulk ionic concentration on the way the inter-pore signal behaves. This successful demonstration of a signal passed from one man-made pore to another is an important step towards the integration of nanopore technology into more complex multi-component systems.

•Al2O3 can be conformally coated onto organic powders at reduced temperatures <115 °C.•XPS of coating surface identifies ultra-thin hydroxide rich aluminum oxide.•Surface modification can significantly reduce inter-particle cohesion and improve powder flow.

Excitable ion channels play an essential role in biology by regulating ion transport across cell membranes. A key characteristic of these nanoscale pores is the ability to respond to environmental stimuli such as changes in the local membrane voltage or ion concentration. The replication of this behavior in synthetic nanopore platforms has the potential to enable new responsive materials useful in applications such as biomimetic componentry, drug delivery and filtration technologies. While voltage gating has been demonstrated before in synthetic nanopores, previous fabrication methods make it difficult to incorporate such pores into more complex systems due to lack of control over key attributes such as pore location. Here, we present the fabrication of a voltage and ion concentration responsive nanochannel with local control over the pore location, size, shape and surface chemistry. This is achieved using focused ion beam (FIB) to nanomachine a pore in a SiNx membrane followed by ion-beam-controlled deposition of SiOx around the pore entrance. Chemistry selective for SiOx is then used to graft single stranded DNA (ssDNA) only to the regions where the local deposition was performed. Recorded voltage-sensitive transport is complex and dictated by both the charge and conformational changes of ssDNA chains in confinement. For small enough pore diameters, interplay between the applied voltage and local ion concentrations at the pore opening results in three pore states, each with different conductance behavior. Hysteresis was also observed, which is attributed to steric barriers to ssDNA conformational changes in a confined space. This successful demonstration of biomimetic nanopores with local control of multiple chemical and geometric properties is an important step towards the integration of nanopore technology into more complex multi-component systems.

In order to probe the mechanism of solid-solid phase transformations, we have applied ultrafast shock wave compression (120 picosecond duration) and ultrashort (130 femtosecond) x-ray diffraction at the Linac Coherent Light Source (LCLS) to probe the compression-induced phase transition pathway in zirconium. Surprisingly, rather than transform from alpha-Zr to the more disordered hex-3 equilibrium omega-Zr phase, in its place we find the formation of a non-equilibrium body-centered cubic (bcc) metastable intermediate. Theoretically hypothesized for several decades, this bcc intermediate state has now been found to be dynamically stabilized under uniaxial loading at sub-nanosecond timescales. Molecular dynamics simulations of shock-wave propagation in zirconium predict this transformation via the dynamical intermediate state. In contrast with longer timescale experiments where the phase diagram alone is an adequate predictor of the crystalline structure of a material, our recent study highlights the importance of metastability and time-dependence in the kinetics of phase transformation at extreme conditions.

Inducing superconductivity in a topological insulator can lead to novel quantum effects. However, experimental approaches to turn a topological insulator into a superconductor are limited. Here, we report on superconductivity in topological insulator Bi-0.91 Sb-0.09 induced via focused ion-beam deposition of a Pt thin film. The superconducting phase exhibits a Berezinski-Kosterlitz-Thouless transition, demonstrative of its two-dimensional character. From the in-plane upper critical field measurements, we estimate the superconducting thickness to be similar to 17 nm for a 5.5-mu m- thick sample. Our results provide evidence that the interface superconductivity could originate from the surface states of Bi-0.91 Sb-0.09

Focused ion beam (FIB) coupled with scanning electron microscopy (SEM) investigations were performed on pristine and altered areas of two uraninite samples in order to better understand their 3-dimensional mineralogical and chemical nature, and their impact on trace element abundances obtained by laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) analyses. Trace element contents determined by both LA- and solution mode (SM)–ICP–MS analyses together with FIB results identify Pb as the optimal internal standard for LA–ICP–MS analyses of uraninite since it is a major chemical constituent, part of the structure, and similar in abundance to the trace elements of interest.

The ability to process near-net shape structures is of particular interest in applications using rare or expensive materials. Laser powder bed fusion additive manufacturing can meet this challenge, yet also introduces significant processing and materials challenges toward achieving structural integrity. Broadly addressed aspects affecting powder bed fusion structures include surface roughness, residual stresses, and porosity. Here, we focus on materials-specific challenges—namely, the influence of multiple pass reheating during processing and post-processing homogenization on phase transformations and the role of impurities and oxides on grain morphology and mechanical behavior—with a view of adopting laser powder bed fusion processing for the shape memory alloy, uranium-6 wt.% niobium. Our observations to date on the factors influencing material behavior and post-processing requirements and recommendations for process improvement are discussed herein.

We report results from ultrafast compression experiments conducted on β-HMX single crystals. Results consist of nominally 12 picosecond time-resolved wave profile data, (ultrafast time domain interferometry –TDI measurements), that were analyzed to determine high-velocity wave speeds as a function of piston velocity. TDI results are used to validate calculations of anisotropic stress-strain behavior of shocked loaded energetic materials. Our previous results derived using a 350 ps duration compression drive revealed anisotropic elastic wave response in single crystal β-HMX from (110) and (010) impact planes. Here we present results using a 1.05 ns duration compression drive with a 950 ps interferometry window to extend knowledge of the anisotropic dynamic response of β-HMX within eight microns of the initial impact plane. We observe two distinct wave profiles from (010) and three wave profiles from (010) impact planes. The (110) impact plane wave speeds typically exceed (010) impact plane wave speeds at t...

We report two examples of initiated chemical vapor deposition (iCVD) polymers being developed for use in laser targets for high peak-power laser systems. First, we show that iCVD poly(divinylbenzene) is more photo-oxidatively stable than the plasma polymers currently used in laser targets. Thick layers (10–12μm) of this highly crosslinked polymer can be deposited with near-zero intrinsic film stress. Second, we show that iCVD epoxy polymers can be crosslinked after deposition to form thin adhesive layers for assembling precision laser targets. The bondlines can be made as thin as ~1μm, approximately a factor of 2 thinner than achievable using viscous resin-based adhesives. These bonds can withstand downstream coining and stamping processes.

Inertial confinement fusion targets are complex systems designed to allow fine control of temperature and pressure for making precise spherical ice layers of hydrogen isotopes at cryogenic temperatures. We discuss the various technical considerations for a maximum leak rate based on heat load considerations. This maximum flow rate turns out to be $5\times 10^{-6}$ standard cc per second, which can be caused by an orifice less than half a micron in diameter. This makes the identification of the location and resolution of the leak a significant challenge. To illustrate this, we showcase one example of a peculiar failure mode that appeared suddenly but persisted whereby target production yield was severely lowered. Identification of the leak source and the root cause requires very careful analysis of multiple thermomechanical aspects to ensure that the end solution is indeed the right remedy and is robust.

We have used a 45 mu J laser pulse to accelerate the free surface of fine-grained tantalum films up to peak velocities of similar to 1.2 km s(-1). The films had thicknesses of similar to 1-2 mu m and in-plane grain widths of similar to 75-150 nm. Using ultrafast interferometry, we have measured the time history of the velocity of the surface at different spatial positions across the accelerated region. The initial part of the histories (assumed to correspond to the "elastic precursor" observed previously) exhibited measured strain rates of similar to 0.6 to similar to 3.2 x 10(9) s(-1) and stresses of similar to 4 to similar to 22GPa. Importantly, we find that elastic amplitudes exhibit little variation with strain rate for a constant peak surface velocity, even though, via covariation of the strain rate with peak surface velocity, they vary with strain rate. Furthermore, by comparison with data obtained at lower strain rates, we find that amplitudes are much better predicted by peak velocities rather than by either strain rate or sample thickness. Published by AIP Publishing.