Granular metals (GMs), consisting of metal nanoparticles separated by an insulating matrix, frequently serve as a platform for fundamental electron transport studies. However, few technologically mature devices incorporating GMs have been realized, in large part because intrinsic defects (e.g., electron trapping sites and metal/insulator interfacial defects) frequently impede electron transport, particularly in GMs that do not contain noble metals. Here, we demonstrate that such defects can be minimized in molybdenum-silicon nitride (Mo-SiNx) GMs via optimization of the sputter deposition atmosphere. For Mo-SiNx GMs deposited in a mixed Ar/N2 environment, x-ray photoemission spectroscopy shows a 40%-60% reduction of interfacial Mo-silicide defects compared to Mo-SiNx GMs sputtered in a pure Ar environment. Electron transport measurements confirm the reduced defect density; the dc conductivity improved (decreased) by 104-105 and the activation energy for variable-range hopping increased 10×. Since GMs are disordered materials, the GM nanostructure should, theoretically, support a universal power law (UPL) response; in practice, that response is generally overwhelmed by resistive (defective) transport. Here, the defect-minimized Mo-SiNx GMs display a superlinear UPL response, which we quantify as the ratio of the conductivity at 1 MHz to that at dc, Δ σ ω . Remarkably, these GMs display a Δ σ ω up to 107, a three-orders-of-magnitude improved response than previously reported for GMs. By enabling high-performance electric transport with a non-noble metal GM, this work represents an important step toward both new fundamental UPL research and scalable, mature GM device applications.
Skyrmions and antiskyrmions are nanoscale swirling textures of magnetic moments formed by chiral interactions between atomic spins in magnetic noncentrosymmetric materials and multilayer films with broken inversion symmetry. These quasiparticles are of interest for use as information carriers in next-generation, low-energy spintronic applications. To develop skyrmion-based memory and logic, we must understand skyrmion-defect interactions with two main goals—determining how skyrmions navigate intrinsic material defects and determining how to engineer disorder for optimal device operation. Here, we introduce a tunable means of creating a skyrmion-antiskyrmion system by engineering the disorder landscape in FeGe using ion irradiation. Specifically, we irradiate epitaxial B20-phase FeGe films with 2.8 MeV Au4+ ions at varying fluences, inducing amorphous regions within the crystalline matrix. Using low-temperature electrical transport and magnetization measurements, we observe a strong topological Hall effect with a double-peak feature that serves as a signature of skyrmions and antiskyrmions. These results are a step towards the development of information storage devices that use skyrmions and antiskyrmions as storage bits, and our system may serve as a testbed for theoretically predicted phenomena in skyrmion-antiskyrmion crystals.
Gold-germanium (Au xGe 1 - x) solid solutions have been demonstrated as highly sensitive thin film thermometers for cryogenic applications. However, little is known regarding the performance of the films for thicknesses less than 100 nm. In response, we report on the resistivity and temperature coefficient of resistance (TCR) for sputtered films with thicknesses ranging from 10 to 100 nm and annealed at temperatures from 22 to 200 °C. The analysis is focused upon composition x = 0.17, which demonstrates a strong temperature sensitivity over a broad range. The thinnest films are found to provide an enhancement in TCR, which approaches 20% K - 1 at 10 K. Furthermore, reduced anneal temperatures are required to crystallize the Ge matrix and achieve a maximum TCR for films of reduced thickness. These features favor the application of ultra-thin films as high-sensitivity, on-device thermometers in micro- and nanolectromechanical systems.
The fabrication of long-lived electrical contacts to thermoelectric Bi2Te3-based modules is a challenging problem due to chemical incompatibilities and rapid diffusion rates. Previously, technical guidance from SAND report 2015-7203 selected electroplated Au as the preferred method for fabrication of long-lived contacts because of concerns that the grain structure of sputtered/physical vapor deposited (PVD) Au contacts can evolve during aging. We have re-evaluated PVD Au contacts and show that they are appropriate for long-life service. We measure grain size and morphology at different aging times under accelerated temperature gradient conditions, and we show that the PVD Au contacts are stable and remain relatively unchanged. The PVD Au fabricated here is not subject to the deterioration observed in the previous report.
Two-dimensional (2D) metal-boride-derived nanostructures have been a focus of intense research for the past decade, with an emphasis on new synthetic approaches, as well as on the exploration of possible applications in next-generation advanced materials and devices. Their unusual mechanical, electronic, optical, and chemical properties, arising from low dimensionality, present a new paradigm to the science of metal borides that has traditionally focused on their bulk properties. This Perspective discusses the current state of research on metal-boride-derived 2D nanostructures, highlights challenges that must be overcome, and identifies future opportunities to fully utilize their potential.
The surfaces of textured polycrystalline N-type bismuth telluride and P-type antimony telluride materials were investigated using ex situ photoelectron emission microscopy (PEEM). PEEM enabled imaging of the work function for different oxidation times due to exposure to air across sample surfaces. The spatially averaged work function was also tracked as a function of air exposure time. N-type bismuth telluride showed an increase in the work function around grain boundaries relative to grain interiors during the early stages of air exposure-driven oxidation. At longer time exposure to air, the surface became homogenous after a ∼5 nm-thick oxide formed. X-ray photoemission spectroscopy was used to correlate changes in PEEM imaging in real space and work function evolution to the progressive growth of an oxide layer. The observed work function contrast is consistent with the pinning of electronic surface states due to the defects at a grain boundary.
Coherent manipulation of quantum states is at the core of quantum information science (QIS). Many state-of-the-art quantum systems rely on microwave fields for quantum operations. As such, the microwave electromagnetic fields serve as the ideal "quantum bus" to integrate different types of QIS systems into a hybrid quantum system. Superconducting metamaterials are artificial materials consisting of arrays of superconducting resonant microstructures with sizes much smaller than the microwave wavelengths of interest. Superconducting metamaterials are a strong candidate medium for the microwave quantum bus, because the effective impedance, field distributions, and frequency response can all be controlled by engineering the microstructures, electrical bias, and magnetic flux while maintaining extremely low loss. In this project, we investigate the fundamental unit of a superconducting metamaterial - a resonator with physical dimensions much smaller than the microwave wavelengths - using NbTiN as the working superconductor, whose high operating temperatures and magnetic fields are desirable attributes for compatibility with a wide variety of quantum systems. We first studied the properties of sputtered NbTiN thin films by correlating the film thickness with the normal state resistivity, superconducting transition temperature, and resonances of transmission line resonators made from these films. We developed a process flow and designed a coplanar waveguide platform for studying small resonators. The platform significantly shortens the turnaround times of the resonator fabrication and testing cycles. Several resonators with different designs were fabricated and tested at 4 Kelvin. Resonances were observed in some resonator testers. Potential paths for improvements and future directions are discussed.
The room temperature electronic transport properties of 1 μm thick Bi0.4Sb1.6Te3 (BST) films correlate with overall microstructural quality. Films with homogeneous composition are deposited onto fused silica substrates, capped with SiN to stop both oxidation and Te loss, and postannealed to temperatures ranging from 200 to 450 °C. BST grain sizes and (00l) orientations improve dramatically with annealing to 375 °C, with smaller increases to 450 °C. Tiny few-nanometer-sized voids in the as-deposited film grain boundaries coalesce into larger void sizes up to 300 nm with annealing to 350 °C; the smallest voids continue coalescing with annealing to 450 °C. These voids are decorated with few-nanometer-sized Sb clusters that increase in number with increasing annealing temperatures, reducing the Sb content of the remaining BST film matrix. Resistivity decreases linearly with increasing temperature over the entire range studied, consistent with improving crystalline quality. The Seebeck coefficient also improves with crystalline quality to 350 °C, above which void coalescence and reduced Sb content from the BST matrix correlate with a decrease in the Seebeck coefficient. Yet, a plateau exists for an optimal power factor between 350 and 450 °C, implying thermal stability to higher temperatures than previously reported.
In the past decade, basic physics, chemistry, and materials science research on topological quantum materials - and their potential use to implement reliable quantum computers - has rapidly expanded to become a major endeavor. A pivotal goal of this research has been to realize materials hosting Majorana quasiparticles, thereby making topological quantum computing a technological reality. While this goal remains elusive, recent data-mining studies, performed using topological quantum chemistry methodologies, have identified thousands of potential topological materials - some, and perhaps many, with potential for hosting Majoranas. We write this Review for advanced materials researchers who are interested in joining this expanding search, but who are not currently specialists in topology. The first half of the Review addresses, in readily understood terms, three main areas associated with topological sciences: (1) a description of topological quantum materials and how they enable quantum computing; (2) an explanation of Majorana quasiparticles, the important topologically endowed properties, and how it arises quantum mechanically; and (3) a description of the basic classes of topological materials where Majoranas might be found. The second half of the Review details selected materials systems where intense research efforts are underway to demonstrate nontrivial topological phenomena in the search for Majoranas. Specific materials reviewed include the groups II-V semiconductors (Cd3As2), the layered chalcogenides (MX2, ZrTe5), and the rare-earth pyrochlore iridates (A2Ir2O7, A = Eu, Pr). In each case, we describe crystallographic structures, bulk phase diagrams, materials synthesis methods (bulk, thin film, and/or nanowire forms), methods used to characterize topological phenomena, and potential evidence for the existence of Majorana quasiparticles.
Here we present the development of a Zeptocalorimeter. The motivation for designing and implementing such a device is driven, ultimately, by its anticipated exceptional sensitivity (10-21 J/K, at 2K). Such a device would be highly valuable in detecting minute quantities of mass for threat detection, studying fundamental phonon physics, and detecting energetic dissipation events at the attojoule level. To date, the most sensitive calorimeter demonstrated in the literature at 2K has been developed by the Roukes group at Caltech, where they achieved an addendum heat capacity of 10-15 J/K with a 1/1000 sensitivity to external stimuli. To obtain such a low value of heat capacity requires a very small thermal mass, and thus, one of the greatest challenges in this project is the fabrication of this device, which requires numerous precision nanofabrication techniques. Furthermore, the heat capacity measurement of this device, as performed from room temperature to cryogenic temperatures, is equally challenging, as the transient signals used to determine the platform's thermal time constant require careful attention to the mitigation of feedthrough capacitance and delicate amplifier offsets. In this report we describe in detail the fabrication process flow for developing the calorimeter, including the layout and device design for obtaining a single lumped RC thermal resistance and capacitance, so that the device can be used for quantitative measurements of nanoscale materials with a suitable thermal link. The measurement method and experimental setup are also given, where we explain the heater and thermometer calibration methods, the thermal resistance measurements, the transient measurements, and lastly the cryogenic setup with intermediate frequency cabling and the thermal sinking of those lines.