11,509 research outputs found
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
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
Superfluidity and Quantum Melting of para-Hydrogen clusters
Structural and superfluid properties of para-Hydrogen clusters of size up to
N=40 molecules, are studied at low temperature (0.5 K < T < 4 K) by Path
Integral Monte Carlo simulations. The superfluid fraction displays an
interesting, non-monotonic behavior for 22 < N < 30. We interpret this
dependence in terms of variations with N of the cluster structure.
Superfluidity is observed at low T in clusters of as many as 27 molecules; in
the temperature range considered here, quantum melting is observed in some
clusters, which freeze at high temperature
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