The understanding of the interplay of electron correlations and randomness in solids is enhanced by demonstrating that particle-hole ( p−h) symmetry plays a crucial role in determining the effects... Show moreThe understanding of the interplay of electron correlations and randomness in solids is enhanced by demonstrating that particle-hole ( p−h) symmetry plays a crucial role in determining the effects of disorder on the transport and thermodynamic properties of the half-filled Hubbard Hamiltonian. We show that the low-temperature conductivity decreases with increasing disorder when p−h symmetry is preserved, and shows the opposite behavior, i.e., conductivity increases with increasing disorder, when p−h symmetry is broken. The Mott insulating gap is insensitive to weak disorder when there is p−h symmetry, whereas in its absence the gap diminishes with increasing disorder. Show less
An optical single-molecule study is reported of a quickly frozen solution of 2.3,8.9-dibenzanthanthrene (DBATT) in n-tetradecane at 1.4 K. The orientation has been measured of several hundreds of... Show moreAn optical single-molecule study is reported of a quickly frozen solution of 2.3,8.9-dibenzanthanthrene (DBATT) in n-tetradecane at 1.4 K. The orientation has been measured of several hundreds of DBATT molecules within a confocal detection volume of similar to 10 mum(3) as a function of their resonance frequency in the range of the two 0-0 bands in the fluorescence-excitation spectrum. Each band is found to correspond to a distinct distribution of orientations of DBATT molecules. A particular resonance frequency within a band is not correlated with a specific molecular orientation. (C) 2001 Elsevier Science B.V. All rights reserved. Show less