Outbursts on young stars are usually interpreted as accretion bursts caused by instabilities in the disk or the star-disk connection. However, some protostellar outbursts may not fit into this framework. In this paper, we analyze optical and near-infrared spectra and photometry to characterize the 2015 outburst of the probable young star ASASSN-15qi. The ̃3.5 mag brightening in the V band was sudden, with an unresolved rise time of less than one day. The outburst decayed exponentially by 1 mag for 6 days and then gradually back to the pre-outburst level after 200 days. The outburst is dominated by emission from ̃10,000 K gas. An explosive release of energy accelerated matter from the star in all directions, seen in a spectacular cool, spherical wind with a maximum velocity of 1000 km s-1. The wind and hot gas both disappeared as the outburst faded and the source returned to its quiescent F-star spectrum. Nebulosity near the star brightened with a delay of 10-20 days. Fluorescent excitation of H2 is detected in emission from vibrational levels as high as v = 11, also with a possible time delay in flux increase. The mid-infrared spectral energy distribution does not indicate the presence of warm dust emission, though the optical photospheric absorption and CO overtone emission could be related to a gaseous disk. Archival photometry reveals a prior outburst in 1976. Although we speculate about possible causes for this outburst, none of the explanations are compelling.

Single crystalline vanadium dioxide (VO2) nanobeams offer an ideal material basis for exploring the widely observed insulator-metal transition in strongly correlated materials. Here, we investigate nonequilibrium carrier dynamics and electronic structure in single crystalline VO2 nanobeam devices using scanning photocurrent microscopy in the vicinity of their phase transition. We extracted a Schottky barrier height of similar to 0.3 eV between the metal and the insulator phases of VO2, providing direct evidence of the nearly symmetric band gap opening upon phase transition. We also observed unusually long photocurrent decay lengths in the insulator phase, indicating unexpectedly long minority carrier lifetimes on the order of microseconds, consistent with the nature of carrier recombination between two d-subbands of VO2.

We present a public catalog of transients from the Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS), a magnitude-limited (m<19 mag in either the g or r filter) survey for extragalactic transients in the ZTF public stream. We introduce cuts on survey coverage, sky visibility around peak light, and other properties unconnected to the nature of the transient, and show that the resulting statistical sample is spectroscopically 97% complete at <18 mag, 93% complete at <18.5 mag, and 75% complete at <19 mag. We summarize the fundamental properties of this population, identifying distinct duration-luminosity correlations in a variety of supernova (SN) classes and associating the majority of fast optical transients with well-established spectroscopic SN types (primarily SN Ibn and II/IIb). We measure the Type Ia SN and core-collapse (CC) SN rates and luminosity functions, which show good consistency with recent work. About 7% of CC SNe explode in very low-luminosity galaxies (M_i > -16 mag), 10% in red-sequence galaxies, and 1% in massive ellipticals. We find no significant difference in the luminosity or color distributions between the host galaxies of Type II and Type Ib/c supernovae, suggesting that line-driven wind stripping does not play a major role in the loss of the hydrogen envelope from their progenitors. Future large-scale classification efforts with ZTF and other wide-area surveys will provide high-quality measurements of the rates, properties, and environments of all known types of optical transients and limits on the existence of theoretically predicted but as of yet unobserved explosions.

Abstract We present the results of a pressure‐dependent photoluminescence (PL) study on CuGaSe 2 films grown on GaAs substrate by metalorganic vapor phase epitaxy. The low‐temperature PL spectra of the CuGaSe 2 samples measured at atmospheric pressure are dominated by one near band edge exciton luminescence line and two strong and relatively broad emissions associated with donor acceptor pairs (DAP) transitions. All the observed luminescence emission lines shift toward higher energy with increasing pressure at the same rate. The nearly identical pressure coefficients of the two DAP emissions as compared to that of the exciton emission confirm the suggestion that the recombination processes associated with the DAPs involve one shallow effective‐mass donor and two different acceptor species with different binding energies and related to two different native defects. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We have modeled the field and space charge distributions in back-gate and top-gate nanowire field effect transistors by solving the three-dimensional Poisson's equation numerically. It is found that the geometry of the gate oxide, the semiconductivity of the nanowire, and the finite length of the device profoundly affect both the total amount and the spatial distribution of induced charges in the nanowire, in stark contrast to the commonly accepted picture where metallic dielectric properties and infinite length are assumed for the nanowire and the specific geometry of the gate oxide is neglected. We provide a comprehensive set of numerical correction factors to the analytical capacitance formulas, as well as to numerical calculations that neglect the semiconductivity and finite length of the nanowire, that are frequently used for quantifying carrier transport in nanowire field effect transistors.

GaN 1 − x As x alloys grown across the composition range by low temperature molecular beam epitaxy have great technological potential for photovoltaic applications owing to their strong absorption coefficient and wide tunability of band gap and band edges. We found that amorphous GaN1−xAsx alloys that are formed for the compositions x, in the range of x∼0.3–0.7 are stable up to 700 °C. This is surprising since growth of GaN1−xAsx above 400 °C results in phase segregation. At annealing temperatures higher than 700 °C the alloy phase segregates into GaAs:N and GaN:As. The relative size of the nanocrystals depends on the initial film composition and annealing conditions.

Charge transfer, surface/interface, defect states, and internal fields strongly influence carrier statics and dynamics in semiconductor nanowires. These effects are usually probed using spatially resolved scanning current techniques, where charge carriers are driven to move by diffusion force due to a density gradient, drift force due to internal fields, and thermoelectric force due to a temperature gradient. However, in the analysis of experimental data, analytical formulas are usually used which are based on the assumption that a single component of these forces dominates the carrier dynamics. In this work we show that this simplification is generally not justified even in the simplest configurations, and the scanning microscopy data need to be analyzed with caution. We performed a comprehensive numerical modeling of the electrothermal dynamics of free charge carriers in the scanning photocurrent microscopy configuration. The simulation allows us to reveal and predict important, surprising effects that are previously not recognized, and assess the limitation as well as potential of these scanning current techniques in nanowire characterization.

Miniaturization of the steam engine to the microscale is hampered by severe technical challenges. Microscale mechanical motion is typically actuated with other mechanisms ranging from electrostatic interaction, thermal expansion, and piezoelectricity to more exotic types including shape memory, electrochemical reaction, and thermal responsivity of polymers. These mechanisms typically offer either large-amplitude or high-speed actuation, but not both. In this work we demonstrate the working principle of a microscale solid engine (μSE) based on the phase transition of VO2 at 68 °C with large transformation strain (up to 2%), analogous to the steam engine invoking large volume change in a liquid-vapor phase transition. Compared to polycrystal thin films, single-crystal VO2 nanobeam-based bimorphs deliver higher performance of actuation both with high amplitude (greater than bimorph length) and at high speed (greater than 4 kHz in air and greater than 60 Hz in water). The energy efficiency of the devices is calculated to be equivalent to thermoelectrics with figure of merit ZT = 2 at the working temperatures, and much higher than other bimorph actuators. The bimorph μSE can be easily scaled down to the nanoscale, and operates with high stability in near-room-temperature, ambient, or aqueous conditions. On the basis of the μSE, we demonstrate a macroscopic smart composite of VO2 bimorphs embedded in a polymer, producing high-amplitude actuation at the millimeter scale.

In this paper, we present a comprehensive, correlative study of the structural, transport, optical and thermoelectric properties of high-quality VO2 thin films across its metal-insulator phase transition. Detailed x-ray diffraction study shows that it's textured polycrystalline along [010]M1, with in-plane lattice orienting along three equivalent crystallographic directions. Across the metal-insulator transition, the conductivity increases by more than 3 orders of magnitude with a value of 3.8 × 103 S/cm in the metallic phase. This increase is almost entirely accounted for by a change in electron density, while the electron mobility changes only slightly between the two phases, yet shows strong domain boundary scattering when the two phases coexist. Electron effective mass was determined to be ∼65m0 in the insulating phase. From the optical and infrared reflection spectra in the metallic phase, we obtained the plasma edge of VO2, from which the electron effective mass was determined to be ∼23m0. The bandgap of VO2 was determined from optical absorption to be 0.70 ± 0.05 eV at room temperature and rapidly shrinks before the phase transition occurs. In the temperature range where metallic and insulating phases coexist, the Seebeck coefficient was found to be significantly lower than that predicted by a linear combination of volumetric contributions from the insulating and metallic domains, indicating abnormal thermoelectric effect at the metal/insulator domain walls in such two-dimensional domain structure.

We report on temperature-dependent charge and magneto transport of chemically doped MoS2, p-type molybdenum disulfide degenerately doped with niobium (MoS2:Nb). The temperature dependence of the electrical resistivity is characterized by a power law, ρ(T) ∼ T−0.25, which indicates that the system resides within the critical regime of the metal-insulator (M-I) transition. By applying high magnetic field (∼7 T), we observed a 20% increase in the resistivity at 2 K. The positive magnetoresistance shows that charge transport in this system is governed by the Mott-like three-dimensional variable range hopping (VRH) at low temperatures. According to relationship between magnetic-field and temperature dependencies of VRH resistivity, we extracted a characteristic localization length of 19.8 nm for MoS2:Nb on the insulating side of the M-I transition.

Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN) or 0.2 μm (6.2 eV for AlN). The continuous range of bandgap energies now spans the near infrared, raising the possibility of new applications for group-III nitrides. In this article we present a detailed review of the physical properties of InN and related group III-nitride semiconductors. The electronic structure, carrier dynamics, optical transitions, defect physics, doping disparity, surface effects, and phonon structure will be discussed in the context of the InN bandgap re-evaluation. We will then describe the progress, perspectives, and challenges in the developments of new electronic and optoelectronic devices based on InGaN alloys. Advances in characterization and understanding of InN and InGaN nanostructures will also be reviewed in comparison to their thin film counterparts.

Giant physical responses were discovered, in numerous systems, when two phases coexist; for example, near a phase transition. An intermetallic FeRh system undergoes a first-order antiferromagnetic to ferromagnetic transition above room temperature and shows two-phase coexistence near the transition. We have investigated the effect of an electric field to FeRh/PMN-PT heterostructures and report 8% change in the electrical resistivity of FeRh films. Such a 'giant' electroresistance (GER) response is striking in metallic systems, in which external electric fields are screened, and thus only weakly influence the carrier concentrations and mobilities. We show that our FeRh films comprise coexisting ferromagnetic and antiferromagnetic phases with different resistivities and the origin of the GER effect is the strain-mediated change in their relative proportions. Finally, the observed behaviour is reminiscent of colossal magnetoresistance in perovskite manganites and illustrates the role of mixed-phase coexistence in achieving large changes in physical properties with low-energy external perturbation.

A near-field powermeter that directly quantifies light absorption and heat transfer in single nanowires is demonstrated. The mechanism is based on the metal-insulator transition in single-crystal VO₂ microbeams, where the domain wall exhibits distinct optical contrast between the two phases. The powermeter is contactless and optically readable, allowing quick determination of optical absorbance, thermal conductivity, and contact thermal resistance of single nanostructures.

Abstract The bandgap range of InGaN extends from the near‐IR (InN, 0.65 eV) to the ultraviolet. To exploit this wide tuning range in light generation and conversion applications, pn junctions are required. The large electron affinity of InN (5.8 eV) leads to preferential formation of native donor defects, resulting in excess electron concentration in the bulk and at surfaces and interfaces. This creates difficulties for p‐type doping and/or measuring of the bulk p‐type activity. Capacitance–voltage measurements, which deplete the n‐type surface inversion layer, have been used to show that Mg is an active acceptor in InN and In x Ga 1– x N for 0.2 < x < 1.0, i.e. over the entire composition range. Mg acceptors can be compensated by irradiation‐induced native donors. Thermopower measurements were used to provide definitive evidence that Mg‐doped InN has mobile holes between 200 K and 300 K. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)