Transition from ultrafast laser photo-electron emission to space charge limited current in a 1D gap

A one-dimensional (1D) model has been constructed to study the transition of the time-dependent ultrafast laser photo-electron emission from a flat metallic surface to the space charge limited (SCL) current, including the effect of non-equilibrium laser heating on metals at the ultrafast time scale. At a high laser field, it is found that the space charge effect cannot be ignored and the SCL current emission is reached at a lower value predicted by a short pulse SCL current model that assumed a time-independent emission process. The threshold of the laser field to reach the SCL regime is determined over a wide range of operating parameters. The calculated results agree well with particle-in-cell (PIC) simulation. It is found that the space charge effect is more important for materials with lower work function like tungsten (4.4 eV) as compared to gold (5.4 eV). However for a flat surface, both materials will reach the space charge limited regime at the sufficiently high laser field such as $>$ 5 GV/m with a laser pulse length of tens to one hundred femtoseconds.Comment: 12 pages, 3 figures, printed in {\itshape J. Phys. D: Appl. Phys.

Risk, Return and Dividends

We characterize the joint dynamics of dividends, expected returns, stochastic volatility, and prices. In particular, with a given dividend process, one of the processes of the expected return, the stock volatility, or the price-dividend ratio fully determines the other two. For example, together with dividends, the stock volatility process fully determines the dynamics of the expected return and the price-dividend ratio. By parameterizing one or more of expected returns, volatility, or prices, common empirical specifications place strong, and sometimes counter-factual, restrictions on the dynamics of the other variables. Our relations are useful for understanding the risk-return trade-off, as well as characterizing the predictability of stock returns.

How to Discount Cashflows with Time-Varying Expected Returns

While many studies document that the market risk premium is predictable and that betas are not constant, the dividend discount model ignores time-varying risk premiums and betas. We develop a model to consistently value cashflows with changing risk-free rates, predictable risk premiums and conditional betas in the context of a conditional CAPM. Practical valuation is accomplished with an analytic term structure of discount rates, with different discount rates applied to expected cashflows at different horizons. Using constant discount rates can produce large mis-valuations, which, in portfolio data, are mostly driven at short horizons by market risk premiums and at long horizons by time-variation in risk-free rates and factor loadings.

Hybrid Analog-Digital Precoding for Interference Exploitation

We study the multi-user massive multiple-input-single-output (MISO) and focus on the downlink systems where the base station (BS) employs hybrid analog-digital precoding with low-cost 1-bit digital-to-analog converters (DACs). In this paper, we propose a hybrid downlink transmission scheme where the analog precoder is formed based on the SVD decomposition. In the digital domain, instead of designing a linear transmit precoding matrix, we directly design the transmit signals by exploiting the concept of constructive interference. The optimization problem is then formulated based on the geometry of the modulation constellations and is shown to be non-convex. We relax the above optimization and show that the relaxed optimization can be transformed into a linear programming that can be efficiently solved. Numerical results validate the superiority of the proposed scheme for the hybrid massive MIMO downlink systems.Comment: 5 pages, EUSIPCO 201

Fast radio bursts and their gamma-ray or radio afterglows as Kerr-Newman black hole binaries

Fast radio bursts (FRBs) are radio transients lasting only about a few milliseconds. They seem to occur at cosmological distances. We propose that these events can be originated in the collapse of the magnetosphere of Kerr-Newman black holes (KNBHs). We show that the closed orbits of charged particles in the magnetosphere of these objects are unstable. After examining the dependencies on the specific charge of the particle and the spin and charge of the KNBH, we conclude that the resulting timescale and radiation mechanism fit well with the extant observations of FRBs. Furthermore, we argue that the merger of a KNBH binary is one of the plausible central engines for potential gamma-ray or radio afterglow following a certain FRBs, and can also account for gravitational wave (GW) events like GW 150914. Our model leads to predictions that can be tested by combined multi-wavelength electromagnetic and GW observations.Comment: 6 pages, 4 figures, accepted for publication in Ap

Why Stocks May Disappoint

Recently much progress has been made in developing optimal portfolio choice models accomodating time-varying opportunity sets, but unless investors are unreasonably risk averse, optimal holdings include unreasonably large equity positions. One reason is that most studies assume investors behave as expected utility maximizers with power utility. In this article, we provide a formal treatment of both static and dynamic portfolio choice using the Disappointment Aversion preferences of Gul (1991). While different from the Kahneman-Tversky (1979) loss aversion utility, these preferences imply asymmetric aversion to gains versus losses and are consistent with the tendency of some people to like lottery-type gambles but dislike stock in-vestments. By calibrating a number of data generating processes to actual US data on stock and bond returns, we find very reasonable portfolios for moderately disappointment averse investors with utility functions exhibiting low curvature. Disappointment aversion preferences affect intertemporal hedging demands and the state dependence of asset allocation in such a way as to not be replicable by standard expected utility functions with higher curvature. Furthermore, it is easy to reconcile the large equity premium observed in the data with disappointment aversion utility of low curvature and reasonable disappointment aversion.

In situ High-Pressure Studies of Hydrogen Storage Materials by Vibrational Spectroscopy

Hydrogen has been considered as the most promising alternative source of energy as the world seeks a replacement for the conventional fossil fuel energy source. However, the storage of hydrogen still poses as a big challenge. As a result, development and investigation of hydrogen storage materials have received increasing attention. Here we report studies of two promising hydrogen storage materials, NaNH2 and NH3BH3, under high pressure by Raman and IR spectroscopy. First, sodium amide (NaNH2) was investigated at room temperature and pressures up to 15 GPa. Starting with an orthorhombic crystal structure at ambient pressure, sodium amide was found to transform to two new phases upon compression as evidenced by changes of characteristic Raman and IR modes as well as by examining the pressure dependences of these modes. Raman and IR measurements on NaNH2 collectively provided consistent information about the structural evolution of NaNH2 under compression. Upon decompression, all Raman and IR modes were completely recovered indicating the reversibility of the pressure-induced transformations in the entire pressure region. The Raman and IR spectroscopic data together allowed for the analysis of possible structures of the new high-pressure phases of NaNH2. Another potential hydrogen storage material, ammonia borane (NH3BH3), was investigated at simultaneous high pressures (up to 15 GPa) in a diamond anvil cell and low temperatures (down to 80 K) using a cryostat by Raman spectroscopy in situ. Upon cooling from room temperature to 220 K at near ambient pressure, an expected phase transformation from 74TM to Pmn2\ was observed. Then the sample was compressed to 15 GPa isothermally at 180 K. Three pressure induced structural transformations were observed as evidenced by the change of Raman profile as well as the pressure dependen­ ce of the major Raman modes. The decompression and warming-up experiments suggest these P-T-induced transformations are reversible. These observations, together with fact­ or group analysis, allowed us to examine the possible structures of the new high pressure phases and the nature of phase transitions. Raman measurements from multiple runs covering various P-T paths, when combined with previously established room-tempera­ ture and high-pressure data, enabled the update of the P-T phase diagram of ammonia borane in the pressure region of 0-15 GPa and the temperature region of 80-350 K

Searching for bulk motions in the ICM of massive, merging clusters with Chandra CCD data

We search for bulk motions in the intracluster medium (ICM) of massive clusters showing evidence of an ongoing or recent major merger with spatially resolved spectroscopy in {\sl Chandra} CCD data. We identify a sample of 6 merging clusters with $>$150 ks {\sl Chandra} exposure in the redshift range $0.1 < z < 0.3$. By performing X-ray spectral analysis of projected ICM regions selected according to their surface brightness, we obtain the projected redshift maps for all of these clusters. After performing a robust analysis of the statistical and systematic uncertainties in the measured X-ray redshift $z_{\rm X}$, we check whether or not the global $z_{\rm X}$ distribution differs from that expected when the ICM is at rest. We find evidence of significant bulk motions at more than 3$\sigma$ in A2142 and A115, and less than 2$\sigma$ in A2034 and A520. Focusing on single regions, we identify significant localized velocity differences in all of the merging clusters. We also perform the same analysis on two relaxed clusters with no signatures of recent mergers, finding no signs of bulk motions, as expected. Our results indicate that deep {\sl Chandra} CCD data enable us to identify the presence of bulk motions at the level of $v_{\rm BM} >$ 1000\ ${\rm km\ s^{-1}}$ in the ICM of massive merging clusters at $0.1<z<0.3$. Although the CCD spectral resolution is not sufficient for a detailed analysis of the ICM dynamics, {\sl Chandra} CCD data constitute a key diagnostic tool complementing X-ray bolometers on board future X-ray missions