The emission of positronium negative ions from Cs deposited W(100) surfaces has been studied. A dramatic change in the emission efficiency was observed upon coating a W(100) surface with Cs. The conversion efficiency (the fraction of incident slow positrons yielding the ions) of the W(100) target with a 2.2 × 10 14 atoms cm −2 Cs coverage, measured over a time interval of 3 × 10 3 s immediately after deposition, was found to be 1.25%, which is two orders of magnitude higher than that obtained for the clean, uncoated W(100) surface and 45 times greater than the highest efficiency reported thus far.
We report the results of ab initio energy-band calculations for the ternary intermetallic compound GdMn 6 Ge 6 . The ferrimagnetic arrangement of Gd and Mn magnetic sublattices is reproduced as well as the average moment per Mn atom. The comparatively low strength of the Gd-Mn effective exchange coupling and the occurrence of a weak orbital polarization (3%) at the manganese sites are further results of this analysis. The numerical results are compared with prior magnetization and hyperfine-field analyses.
We construct a kink solution on a non-BPS D-brane using Berkovits' formulation of superstring field theory in the level truncation scheme. The tension of the kink reproduces 95% of the expected BPS D-brane tension. We also find a lump-like solution which is interpreted as a kink-antikink pair, and investigate some of its properties. These results may be considered as successful tests of Berkovits' superstring field theory combined with the modified level truncation scheme.
We extend our previous work on coherent paired states associated with the Lie group SU(1, 1). Whereas the earlier states were defined with respect to a single type of canonical boson or (linear) quantum harmonic oscillator, the new states are defined in terms of two distinct types of bosons or oscillators. The new coherent states may again, on the one hand, be viewed as ordinary (Glauber) coherent states in the two-boson Hilbert space spanned by arbitrary numbers of two distinct Bogoliubov quasiparticles associated with the original bosons via a generalised Bogloiubov transformation. Alternatively, expressed wholly in terms of the original bosons these new coherent states are reached from the ordinary coherent states via a unitary (pairing) transformation which is shown to be associated with the entire so-called discrete series of representations of the group SU(1, 1). As an important illustration of the use of these states and transformations, we study in detail a rather general class of quantum Lagrangians which includes the damped linear harmonic oscillator. We thereby illustrate their possible usefulness in applications to quantum many-body or field-theoretic processes involving fluctuation-dissipation phenomena in general. t Present address: perturbative vacuum IO), which is the vacuum for a-type bosons ( a ( 0 ) = 0) is not the real one. Instead there exists a lower vacuum, lo),,, which corresponds to that for b-type bosons. This is defined by the relation blO)b = 0, or equivalently as lo), = U210). Now, although the algebra S U ( 1 , l ) arose as outlined above from a general discussion of pairing, it is important to realise that the pairing operators d o form only a special representation of the algebra. In other words, the Casimir operator for the algebra takes only a particular value. It is natural, therefore, now to proceed by exploring other representations of the SU( 1 , l ) group, and in this paper we accordingly investigate the so-called discrete series of representations. We show that, whereas our previous representation could be associated with quantum states which could be realised by the ordinary harmonic oscillator (for which the operators a and at may be thought of as the usual ladder operators), this is no longer the case for the discrete series of representations. Instead, we show how they may very profitably be associated with the damped harmonic oscillator, which may in turn itself be described with the help of not one but two commuting bosonic operators a, and a,. We show how to construct for this case the analogue V, of the previous unitary operator U,, and show how it correspondingly leads to a canonical transformation between the original operators a , and a, and two new operators b, and b,. We refer to this transformation as a generalised Bogoliubov transformation.Again, the operator V, is used to construct new coherent states, which turn out once more to be ordinary coherent states with respect to the transformed operators b , and b,, but which have extremely interesting properties ...
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