The LISA Time-Delay Interferometry Zero-Signal Solution. I: Geometrical Properties

Time-Delay Interferometry (TDI) is the data processing technique needed for generating interferometric combinations of data measured by the multiple Doppler readouts available onboard the three LISA spacecraft. Within the space of all possible interferometric combinations TDI can generate, we have derived a specific combination that has zero-response to the gravitational wave signal, and called it the {\it Zero-Signal Solution} (ZSS). This is a two-parameter family of linear combinations of the generators of the TDI space, and its response to a gravitational wave becomes null when these two parameters coincide with the values of the angles of the source location in the sky. Remarkably, the ZSS does not rely on any assumptions about the gravitational waveform, and in fact it works for waveforms of any kind. Our approach is analogous to the data analysis method introduced by G\"ursel & Tinto in the context of networks of Earth-based, wide-band, interferometric gravitational wave detectors observing in coincidence a gravitational wave burst. The ZSS should be regarded as an application of the G\"ursel & Tinto method to the LISA data.Comment: 29 pages, 17 Figure

Coherent exciton transport in semiconductors

We review the topic of Bose-Einstein condensation of excitons in semiconductors, focusing on the signatures of the macroscopic order of the exciton condensate.Comment: Some references were updated with respect to the published version. appears as Chapter 19 in Novel Superfluids Volume 2, edited by K. H. Bennemann and J. B. Ketterson, International Series of Monographs on Physics no. 157, pages 423-474 (Oxford University Press, Oxford, 2014

On the soft limit of closed string amplitudes with massive states

We extend our analysis of the soft behaviour of string amplitudes with massive insertions to closed strings at tree level (sphere). Relying on our previous results for open strings on the disk and on KLT formulae we check universality of the soft behaviour for gravitons to sub-leading order for superstring amplitudes and show how this gets modified for bosonic strings. At sub-sub-leading order we argue in favour of universality for superstrings on the basis of OPE of the vertex operators and gauge invariance for the soft graviton. The results are illustrated by explicit examples of 4-point amplitudes with one massive insertion in any dimension, including D=4, where use of the helicity spinor formalism drastically simplifies the expressions. As a by-product of our analysis we confirm that the `single valued projection' holds for massive amplitudes, too. We briefly comment on the soft behaviour of the anti-symmetric tensor and on loop corrections.Comment: 18+7 pages; added some important references and corrected some typo

Thermoelectric properties of junctions between metal and strongly correlated semiconductor

We propose a junction of metal and rare-earth compound semiconductor as the basis for a possible efficient low-temperature thermoelectric device. If an overlayer of rare earth atoms differing from the bulk is placed at the interface, very high values of the figure of merit ZT can be reached at low temperature. This is due to sharp variation of the transmission coefficient of carriers across the junction at a narrow energy range, which is intrinsically linked to the localized character of the overlayer f-orbital.Comment: RevTeX 3.0, 4 pages, 3 postscript figures. To be published in Applied Physics Letter

Predictions of short-term rates and the expectations hypothesis of the term structure of interest rates

Despite its important role in monetary policy and finance, the expectations hypothesis (EH) of the term structure of interest rates has received virtually no empirical support. The empirical failure of the EH was attributed to a variety of econometric biases associated with the single-equation models used to test it; however, none account for it. This paper analyzes the EH by focusing on its fundamental tenet - the predictability of the short-term rate. This is done by comparing h-month ahead forecasts for the 1- and 3-month Treasury yields implied by the EH with the forecasts from random-walk, Diebold and Lei (2006), and Duffee (2002) models. The evidence suggests that the failure of the EH is likely a consequence of market participants’ inability to predict the short-term rate. JEL Classification: E40, E52expectations theory, random walk, time-varying risk premium

Predictions of short-term rates and the expectations hypothesis

Despite its role in monetary policy and finance, the expectations hypothesis (EH) of the term structure of interest rates has received virtually no empirical support. The empirical failure of the EH was attributed to a variety of econometric biases associated with the single-equation models most often used to test it, although no bias seems to account for the extent and magnitude of the failure. This paper analyzes the EH by focusing on the predictability of the short-term rate. This is done by comparing h-month ahead forecasts for the 1- and 3-month Treasury bill yields implied by the EH with the forecasts from random-walk, Diebold and Li’s (2006), and Duffee’s (2002) models. The evidence suggests that the failure of the EH is likely a consequence of market participants’ inability to adequately predict the short-term rate, in that none of these models out-performs a simple random walk model in recursive, real time experiments. Using standard methods that take into account the additional uncertainty caused by the need to estimate model parameters, the null hypothesis of equal predictive accuracy of each models relative to the random walk alternative is never rejected.Rational expectations (Economic theory) ; Interest rates

Advanced Diagnostics of Position Sensors for the Actuation Systems of High-Speed Tilting Trains

Trains tilting permits a train to travel at a high speed while maintaining an acceptable passenger ride quality with respect to the lateral acceleration, and the consequent lateral force, received by the passengers when the train travels on a curved track at a speed in excess of the balance speed built into the curve geometry. The tilting of a train carbody is performed by a control and actuation system which operates as a closed servoloop accepting the commands from the train control system, generating the torque necessary to tilt the carbody with respect to the bogie and measuring the tilt angle to close the control loop. Measurement of the tilt angle of each train vehicle is performed by two sensors located in the front and rear part of the vehicle. Since a correct tilt angle measurement is vital for the system operation and for ensuring a safe ride, in case of discrepancy between the signals of the two tilt angle sensors of any vehicle, the tilting operation is disabled and the train speed is reduced. An innovative tilt angle sensors health management system is herein presented that makes intelligent use of all available information to allow detection of malfunctioning of an individual tilt angle sensor, thereby enabling a continued operation of the tilting system and a high speed travel after a sensor failure occurs