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Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer cuprate Sr_{0.9}La_{0.1}CuO_2 reveal characteristics that counter a number of common phenomena in the hole-doped (p-type) cuprates. The optimally doped Sr_{0.9}La_{0.1}CuO_2 with T_c = 43 K exhibits a momentum-independent superconducting gap \Delta = 13.0 +- 1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu-sites also differs fundamentally from that of the p-type cuprates with d_{x^2-y^2}-wave pairing symmetry., Comment: 4 pages, 3 figures. Published in Physical Review Letter. Corresponding author: Nai-Chang Yeh (e-mail: ncyeh@caltech.edu)

Quasiparticle tunneling spectra of the electron-doped (n-type) infinite-layer cuprate Sr0.9La0.1CuO2 reveal characteristics that counter a number of common phenomena in the hole-doped (p-type) cuprates. The optimally doped Sr0.9La0.1CuO2 with T-c = 43 K exhibits a momentum-independent superconducting gap Delta = 13.0 +/- 1.0 meV that substantially exceeds the BCS value, and the spectral characteristics indicate insignificant quasiparticle damping by spin fluctuations and the absence of pseudogap. The response to quantum impurities in the Cu sites also differs fundamentally from that of the p-type cuprates with d(x2-y2)-wave pairing symmetry.

A new method for spatially-resolved, spectroscopic investigation of subsurface interface structure has been developed. The method, Ballistic Electron Emission Microscopy (BEEM), is based on Scanning Tunneling Microscopy (STM) techniques. BEEM combines STM vacuum tunneling with unique ballistic electron spectroscopy capabilities. BEEM enables, for the first time, direct imaging of subsurface interface electronic properties with nanometer spatial resolution. STM topographic images of surface structure and BEEM images of subsurface properties are obtained simultaneously. BEEM capabilities are demonstrated by investigation of important metal-semiconductor interfaces.

Synthetic polynucleotides have been shown to exhibit antiviral properties in a variety of biological systems. This thesis describes the synthesis and properties of some novel and potentially active compounds. Poly N[6]-hydroxyadenylate and polyisoxanthylate, both potential interferon inducers, were prepared by the enzymatic polymerisation of their corresponding nucleoside 5'-diphosphates. N6 -hydroxyadenosine 5'-diphosphate was obtained from 6-thioinosine 5'-diphosphate by a simple substitution reaction. Poly N6 -hydroxyadenylate shows the properties of a highly stacked, single-stranded polynucleotide and forms a hydrogen-bonded complex with polyxanthylate. Polyisoxanthylate, which displays some unusual physical characteristics, forms a doublestranded complex with polyadenylate. The inhibitory potential of a polynucleotide might be increased by introduction of a sulphur substituent. 6-Thioinosine 5'-diphosphate alone will not act as a substrate for polynucleotide phosphorylase but can be incorporated into a copolymer with inosine 5'-diphosphate. Compared with polyinosinate, the thiolated copolymer exhibits a loss of secondary structure and also a loss of in vivo antiviral activity in mice. However, as the 6-thioinosine content increases, the polynucleotides show a greater degree of inhibition against a viral reverse transcriptase in vitro. Poly 4-thiouridylate was conveniently prepared by thiolation of polycytidylate with liquid hydrogen sulphide. A combination of enzymatic and chemical phosphorylation procedures furnished the 5'-diphosphates of 2,4-dithiouridine and 2-thiocytidine. These were incorporated into polynucleotides by the action of polynucleotide phosphorylase. The thiolated polynucleotides exhibit excellent inhibitory potency in an in vitro reverse transcriptase assay. The initial deoxynucleotide sequence of the reverse transcript from the genome of three distinct tumour viruses is d(AATGAA). The synthesis of this conserved, 'strategic' sequence and closely related structures was carried out. The presence of this hexadeoxynucleotide in a reconstituted tumour virus assay system stimulated the transcription reaction. The significance of this finding and the potential for the design of antiviral agents is discussed.