A Monoclinic Semiorganic Molecular Crystal GUHP for Terahertz Photonics and Optoelectronics

In this paper we describe the properties of the crystal of guanylurea hydrogen phosphate (NH2)2CNHCO(NH2)H2PO3 (GUHP) and propose its application in terahertz photonics and optoelectronics. GUHP crystal has a wide window of transparency and a high optical threshold in the visible and IR spectral regions and narrow absorption bands in the terahertz frequency range. The possibility of application of the crystal under study for the conversion of femtosecond laser radiation from visible an IR to terahertz range was demonstrated. It was shown that dispersion properties of the crystal allow the generation of narrow band terahertz radiation, whose spectral properties are determined by conditions close to phase matching. The properties of the generated terahertz radiation under various temperatures suggest the possibility of phonon mechanism of enhancement for nonlinear susceptibility of the second order.

Materials I: High-Tc Copper Oxides

This chapter focuses on the cuprates, which are uniquely interesting superconducting compounds due to their high Tc, peculiar properties, and potential for applications. The history of the discovery of this very unusual class of superconductors is described, together with the properties and key theoretical concepts that can be used to understand their superconducting and normal behaviours. This chapter contains a description of some very key aspects of these materials: their very unusual phase diagram, where doping takes the compounds from antiferromagnetic insulators to high-temperature superconductors and finally to metallic conductivity; their very anomalous upper critical field Hc2; the symmetry of their order parameter; and the unusual isotope effect on Tc and penetration depth. There are two main approaches to the issue of the origin of high Tc in the cuprates: the phonon mechanism, with the strong impact of the polaronic effect, and a mechanism based on strong correlation effects.

Mechanisms

This chapter introduces the general concepts of pair correlation, and superconductivity as a strong non-adiabatic phenomenon. Phonon, electronic, and magnetic mechanisms play the major roles; each of them can serve as the origin of the superconducting state. Speaking of the phonon mechanism, it is essential that an explicit expression for Tc depend on the intensity of the electron–phonon interaction. In principle, the electron–phonon mechanism can provide high values of Tc, up to room temperature. Pioneering work by Little, who introduced the electronic mechanism, is described. The mechanism has been analysed for one-dimensional, two-dimensional, and three-dimensional systems. The plasmon mechanism can play a role for layered materials. The superconducting state can be provided by magnetic degrees of freedom. The band limit with spin fluctuations and the regime of strong electron correlations are described. The Hubbard Hamiltonian and the t − J model are the ingredients of the approach.