Quasi-optical SIS mixers with normal metal tuning structures

We recently reported (1996) a quasi-optical SIS mixer which used Nb/Al-oxide/Nb tunnel junctions and a normal-metal (Al) tuning circuit to achieve an uncorrected receiver noise temperature of 840 K (DSB) at 1042 GHz. Here we present results on several different device designs, which together cover the 300-1200 GHz frequency range. The mixers utilize an antireflection-coated silicon hyper-hemispherical lens, a twin-slot antenna, and a two-junction tuning circuit. The broad-band frequency response was measured using Fourier transform spectrometry (FTS), and is in good agreement with model calculations. Heterodyne tests were carried out from 400 GHz up to 1040 GHz, and these measurements agree well with the FTS results and with calculations based on Tucker's theory (1985)

Extremal dyonic black holes in D=4 Gauss-Bonnet gravity

We investigate extremal dyon black holes in the Einstein-Maxwell-dilaton (EMD) theory with higher curvature corrections in the form of the Gauss-Bonnet density coupled to the dilaton. In the same theory without the Gauss-Bonnet term the extremal dyon solutions exist only for discrete values of the dilaton coupling constant $a$. We show that the Gauss-Bonnet term acts as a dyon hair tonic enlarging the allowed values of $a$ to continuous domains in the plane $(a, q_m)$ the second parameter being the magnetic charge. In the limit of the vanishing curvature coupling (a large magnetic charge) the dyon solutions obtained tend to the Reissner-Nordstr\"om solution but not to the extremal dyons of the EMD theory. Both solutions have the same values of the horizon radius as a function of charges. The entropy of new dyonic black holes interpolates between the Bekenstein-Hawking value in the limit of the large magnetic charge (equivalent to the vanishing Gauss-Bonnet coupling) and twice this value for the vanishing magnetic charge. Although an expression for the entropy can be obtained analytically using purely local near-horizon solutions, its interpretation as the black hole entropy is legitimate only once the global black hole solution is known to exist, and we obtain numerically the corresponding conditions on the parameters. Thus, a purely local analysis is insufficient to fully understand the entropy of the curvature corrected black holes. We also find dyon solutions which are not asymptotically flat, but approach the linear dilaton background at infinity. They describe magnetic black holes on the electric linear dilaton background.Comment: 19 pages, 3 figures, revtex