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.
The room temperature electronic transport properties of 100 nm-thick thermoelectric Bi0.8Sb0.2 films, sputter-deposited onto quartz substrates and post-annealed in an ex-situ furnace, systematically correlate with the overall microstructural quality, improving with increasing annealing temperature until close to the melting point for the alloy composition. The optimized films have high crystalline quality with ∼99% of the grains oriented with the trigonal axis perpendicular to the substrate surface. Film resistivities and Seebeck coefficients are accurately measured by preventing deleterious surface oxide formation via a SiN capping layer and using Nd-doped Al for contacts. The resulting values are similar to single crystals and significantly better than previous reports from films and polycrystalline bulk alloys.
The transport properties of ceramic materials strongly depend on oxygen activity, which is tuned by changing the partial oxygen pressure (pO2) prior to and during measurement. Within, we describe an instrument for highly stable measurements of Seebeck coefficient and electrical resistivity at temperatures up to 1300 K with controlled oxygen partial pressure. An all platinum construction is used to avoid potential materials instabilities that can cause measurement drift. Two independent heaters are employed to establish a small temperature gradient for Seebeck measurements, while keeping the average temperature constant and avoiding errors associated with pO2-induced drifts in thermocouple readings. Oxygen equilibrium is monitored using both an O2 sensor and the transient behavior of the resistance as a proxy. A pO2 range of 10-25–100 atm can be established with appropriate gas mixtures. Seebeck measurements were calibrated against a high purity platinum wire, Pt/Pt–Rh thermocouple wire, and a Bi2Te3 Seebeck coefficient Standard Reference Material. To demonstrate the utility of this instrument for oxide materials we present measurements as a function of pO2 on a 1 % Nb-doped SrTiO3 single crystal, and show systematic changes in properties consistent with oxygen vacancy defect chemistry. Thus, an approximately 11% increase in power factor over a pO2 range of 10-19–10-8 atm at 973 K for the donor-doped single crystals is observed.
Sachet, Edward; Shelton, Christopher T.; Harris, Joshua S.; Gaddy, Benjamin E.; Irving, Douglas L.; Curtarolo, Stefano; Donovan, Brian F.; Hopkins, Patrick E.; Sharma, Peter A.; Sharma, Peter A.; Ihlefeld, Jon F.; Franzen, Stefan; Maria, Jon P.
The interest in plasmonic technologies surrounds many emergent optoelectronic applications, such as plasmon lasers, transistors, sensors and information storage. Although plasmonic materials for ultraviolet-visible and near-infrared wavelengths have been found, the mid-infrared range remains a challenge to address: few known systems can achieve subwavelength optical confinement with low loss in this range. With a combination of experiments and ab initio modelling, here we demonstrate an extreme peak of electron mobility in Dy-doped CdO that is achieved through accurate 'defect equilibrium engineering'. In so doing, we create a tunable plasmon host that satisfies the criteria for mid-infrared spectrum plasmonics, and overcomes the losses seen in conventional plasmonic materials. In particular, extrinsic doping pins the CdO Fermi level above the conduction band minimum and it increases the formation energy of native oxygen vacancies, thus reducing their populations by several orders of magnitude. The substitutional lattice strain induced by Dy doping is sufficiently small, allowing mobility values around 500 cm2 V-1 s-1 for carrier densities above 1020 cm-3. Our work shows that CdO:Dy is a model system for intrinsic and extrinsic manipulation of defects affecting electrical, optical and thermal properties, that oxide conductors are ideal candidates for plasmonic devices and that the defect engineering approach for property optimization is generally applicable to other conducting metal oxides.
In this study, we demonstrate chemical doping of a topological insulator Bi2Se3 using ion implantation. Ion beam-induced structural damage was characterized using grazing incidence X-ray diffraction and transmission electron microscopy. Ion damage was reversed using a simple thermal annealing step. Carrier-type conversion was achieved using ion implantation followed by an activation anneal in Bi2Se3 thin films. These two sets of experiments establish the feasibility of ion implantation for chemical modification of Bi2Se3, a prototypical topological insulator. Ion implantation can, in principle, be used for any topological insulator. The direct implantation of dopants should allow better control over carrier concentrations for the purposes of achieving low bulk conductivity. Ion implantation also enables the fabrication of inhomogeneously doped structures, which in turn should make possible new types of device designs.
