Thermoelectric efficiency at maximum power in low-dimensional systems

Low-dimensional electronic systems in thermoelectrics have the potential to achieve high thermal-to-electric energy conversion efficiency. A key measure of performance is the efficiency when the device is operated under maximum power conditions. Here we study the efficiency at maximum power of three low-dimensional, thermoelectric systems: a zero-dimensional quantum dot (QD) with a Lorentzian transmission resonance of finite width, a one-dimensional (1D) ballistic conductor, and a thermionic (TI) power generator formed by a two-dimensional energy barrier. In all three systems, the efficiency at maximum power is independent of temperature, and in each case a careful tuning of relevant energies is required to achieve maximal performance. We find that quantum dots perform relatively poorly under maximum power conditions, with relatively low efficiency and small power throughput. Ideal one-dimensional conductors offer the highest efficiency at maximum power (36% of the Carnot efficiency). Whether 1D or TI systems achieve the larger maximum power output depends on temperature and area filling factor. These results are also discussed in the context of the traditional figure of merit $ZT$

Short sellers’ accusations against Chinese reverse mergers: Information analytics or guilt by association?

AbstractThis paper studies short sellers’ trading strategies and their effects on the financial market by examining their accusations of fraud against Chinese reverse merger firms (CRMs) in the US. We find that short sellers rely on firms’ fundamental information, especially relative financial indicators, to locate their “prey.” Specifically, they compare a target firm’s financial indicators (e.g., growth and receivables) with both the industry average and the firm’s history. We find no evidence that short sellers accuse CRMs simply because of their reverse merger label. Additionally, we test the accuracy of short sellers’ accusations in the long run and find that accused firms are more likely to delist and less likely to recover from price plunges. Our results also indicate that CRMs’ high exposure to short sellers’ accusations stem from adverse selection problems: firms with high litigation risk are more likely to choose reverse mergers to access the US capital market. Overall, our results support the view that short sellers are sophisticated investors and shed some light on their decision processes

Tunnel spectroscopy of Majorana bound states in topological superconductor-quantum dot Josephson junctions

We theoretically investigate electronic transport through a junction where a quantum dot (QD) is tunnel coupled on both sides to semiconductor nanowires with strong spin-orbit interaction and proximity-induced superconductivity. The results are presented as stability diagrams, i.e., the differential conductance as a function of the bias voltage applied across the junction and the gate voltage used to control the electrostatic potential on the QD. A small applied magnetic field splits and modifies the resonances due to the Zeeman splitting of the QD level. Above a critical field strength, Majorana bound states (MBS) appear at the interfaces between the two superconducting nanowires and the QD, resulting in a qualitative change of the entire stability diagram, suggesting this setup as a promising platform to identify MBS. Our calculations are based on a nonequilibrium Green's function description and is exact when Coulomb interactions on the QD can be neglected. In addition, we develop a simple pictorial view of the involved transport processes, which is equivalent to a description in terms of multiple Andreev reflections, but provides an alternative way to understand the role of the QD level in enhancing transport for certain gate and bias voltages. We believe that this description will be useful in future studies of interacting QDs coupled to superconducting leads (with or without MBS), where it can be used to develop a perturbation expansion in the tunnel coupling.Comment: 12 pages, 6 figures. Published versio

Half-Integer Shapiro Steps in a Short Ballistic InAs Nanowire Josephson Junction

We report on half-integer Shapiro steps observed in an InAs nanowire Josephson junction. We observed the Shapiro steps of the short ballistic InAs nanowire Josephson junction and found anomalous half-integer steps in addition to the conventional integer steps. The half-integer steps disappear as the temperature increases or transmission of the junction decreases. These experimental results agree closely with numerical calculation of the Shapiro response for the skewed current phase relation in a short ballistic Josephson junction

One-pot hydrothermal synthesis of Co(OH)2 nanoflakes on graphene sheets and their fast catalytic oxidation of phenol in liquid phase

A cobalt hydroxide (Co(OH)2) nanoflake-reduced graphene oxide (rGO) hybrid was synthesized by a onepot hydrothermal method using glucose as a reducing agent for graphene oxide (GO) reduction. The structural and surface properties of the material were investigated by scanning and transmission electron microscopies, energy-dispersive X-ray spectrometry, powder X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Catalytic activities of GO, rGO, Co(OH)2 and Co(OH)2–rGOin aqueous phenol degradation using peroxymonosulfate as an oxidant were compared. A synergetic effect on the catalytic activity was found on the Co(OH)2–rGO hybrid. Although rGO has weak catalytic activity, Co(OH)2–rGO hybrid showed a higher catalytic activity than Co(OH)2. The phenol degradation on Co(OH)2–rGO was extremely fast and took around 10 min for 100% phenol removal. The degradation was found to follow the first order kinetics and a mechanism for phenol degradation was presented

Dominant non-local superconducting proximity effect due to electron-electron interaction in a ballistic double nanowire

Cooper pair splitting (CPS) can induce non-local correlation between two normal conductors coupling to a superconductor. CPS into a double one-dimensional electron gas is an appropriate platform for extracting large amount of entangled electron pairs and one of the key ingredients for engineering Majorana Fermions with no magnetic field. Here we study CPS using a Josephson junction of a gate-tunable ballistic InAs double nanowire. The measured switching current into the two nanowires significantly larger than sum of that into the respective nanowires, indicating the inter-wire superconductivity dominant compared to the intra-wire superconductivity. From dependence on the number of propagating channels in the nanowires, the observed CPS is assigned to one-dimensional electron-electron interaction. Our results will pave the way for utilizing one-dimensional electron-electron interaction to reveal physics of high-efficient CPS and engineer Majorana Fermions in double nanowire systems via CPS

Observations of Magnetic Helicity Proxies in Solar Photosphere: Helicity with Solar Cycles

Observations of magnetic helicity transportation through the solar photosphere reflect the interaction of turbulent plasma movements and magnetic fields in the solar dynamo process. In this chapter, we have reviewed the research process of magnetic helicity inferred from the observed solar magnetic fields in the photosphere and also the solar morphological configurations with solar cycles. After introducing some achievements in the study of magnetic helicity, some key points would like to be summarized. The magnetic (current) helicity in the solar surface layer presents a statistical distribution similar to that of the sunspot butterfly diagram, but its maximum value is delayed from the extreme value of the sunspot butterfly diagram and corresponds in the phase with the statistical eruption of solar flares. During the spatial transport of magnetic (current) helicity from the interior of the sun into the interplanetary space at the time-space scale of the solar cycle, it shows the statistical distribution and the fluctuation with the hemispheric sign rule. These show that the current helicity and magnetic helicity transport calculation methods are complementary to each other. We also notice that the study of the inherent relationship between magnetic helicity and the solar cycle still depends on the observed accuracy of the solar magnetic field.Comment: 48 page,17 figure

Platinum single atoms anchored on ultra-thin carbon nitride nanosheets for photoreforming of glucose

Photoreforming of biomass is a fascinating process that harnesses renewable sunlight and biomass to produce hydrogen under ambient conditions, holding a significant promise for future energy sustainability. However, the main challenge lies in developing highly active and stable photocatalysts with high light harvesting efficiency. In this study, we adopted a simple yet effective approach that combines thermal exfoliation and photodeposition to anchor Pt single atoms onto ultra-thin g-C3N4 nanosheets (MCNN). The incorporation of Pt single atoms induced a distinct red-shift in the visible light region, augmenting the solar energy absorption capacity, while the enlarged surface area of g-C3N4 nanosheets improved the mass transfer. Moreover, the enhanced photoelectric properties further contributed to the superior performance of Pt-MCNN-3.0 % in the photoreforming of glucose for hydrogen evolution. Remarkably, Pt-MCNN-3.0 % demonstrated an impressive hydrogen generation rate, approximately 59 times higher than that of MCNN, after a 3 h visible-light irradiation, maintaining a satisfied photo-stability. This work addresses the critical need for design of efficient photocatalysts, bringing us one step closer to realizing the potential of biomass photoreforming as a sustainable and clean energy conversion technology