Topics in Quantum Magnetism, Nanoelectronics and Superconductivity

Trustees Of Boston University, Boston

Investigators

Abstract

This award supports theoretical research and education in condensed matter physics centered on three related areas: quantum magnetism, nanoelectronics and superconductivity. Although a classical view of magnetism is often successful, it fails badly in certain cases, in particular for quasi-one-dimensional (1D) systems where the atomic spins interact much more strongly along chains in a crystal than between chains. From a classical viewpoint, at zero temperature the atomic spins would align in fixed directions ("up "and "down "). This often doesn't occur in low dimensional systems. Instead, strongly quantum mechanical groundstates occur, in which the spins are in collective linear superpositions of up and down. In recent years this has become a very active experimental field, with numerous examples of quasi-one-dimensional antiferromagnets being synthesized and studied by increasingly refined techniques. This is motivated in part by the connection of these materials, and some of the phenomena that occur in them, with high-temperature superconductivity and spintronics. The PI will continue developing fundamental theory and useful phenomenology for understanding current experiments on various quasi-one-dimensional and quasi-two-dimensional magnetic insulators. As electronic components continue to miniaturize, a limit approaches where the largely classical views of memory elements, transistors, etc. break down and quantum mechanics plays a crucial role. In particular, remarkable quantum phenomena have been recently observed in "single electron transistors "or quantum dots, where the number of electrons on the dot can be varied in single steps. Such nano-engineered devices can exhibit behavior previously studied in atomic impurities doped into metals, with the quantum dot playing the role of a single atomic spin. Such a spin gets "screened " by an electron from the metal (or the leads connected to the quantum dot). It has been claimed that this screening electron is spread out over a very large distance, of order .1-1 microns. This large length scale has never been verified experimentally and has been a source of theoretical confusion. Quantum dots provide unique opportunities to finally observe this Kondo screening cloud. The PI will develop a theoretical understanding of this screening cloud and aims to propose realistic devices and experiments whereby it could be measured. The high-temperature superconductors hold out the promise of important technological applications and, at the same time, raise very difficult fundamental science issues. The will address several theoretical issues in this field. In particular, by collaborating with experts on large scale numerical simulations, he intends to study the possibility of holes arranging themselves into narrow "stripes "separated y insulating antiferromagnetic regions in some of these materials (and in some models used to study them). How generally this occurs, for what reasons and whether it hinders or helps superconductivity are important open questions in the field. %%% This award supports theoretical research and education in condensed matter physics centered on three related areas: quantum magnetism, nanoelectronics and superconductivity. Although a classical view of magnetism is often successful, it fails badly in certain cases, in particular for quasi-one-dimensional (1D) systems where the atomic spins interact much more strongly along chains in a crystal than between chains. From a classical viewpoint, at zero temperature the atomic spins would align in fixed directions (say, "up "and "down "). This often doesn't occur in low dimensional systems. Instead, strongly quantum mechanical groundstates occur, in which the spins are in collective linear superpositions of up and down. In recent years this has become a very active experimental field, with numerous examples of quasi-one-dimensional antiferromagnets being synthesized and studied by increasingly refined techniques. This is motivated in part by the connection of these materials, and some of the phenomena that occur in them, with high-temperature superconductivity and spintronics. The PI will continue developing fundamental theory and useful phenomenology for understanding current experiments on various quasi-one-dimensional and quasi-two-dimensional magnetic insulators. As electronic components continue to miniaturize, a limit approaches where the largely classical views of memory elements, transistors, etc. break down and quantum mechanics plays a crucial role. In particular, remarkable quantum phenomena have been recently observed in "single electron transistors "or quantum dots, where the number of electrons on the dot can be varied in single steps. Such nano-engineered devices can exhibit behavior previously studied in atomic impurities doped into metals, with the quantum dot playing the role of a single atomic spin. Such a spin gets "screened " by an electron from the metal (or the leads connected to the quantum dot). It has been claimed that this screening electron is spread out over a very large distance, of order .1-1 microns. This large length scale has never been verified experimentally and has been a source of theoretical confusion. Quantum dots provide unique opportunities to finally observe this Kondo screening cloud. The PI will develop a theoretical understanding of this screening cloud and aims to propose realistic devices and experiments whereby it could be measured. The high-temperature superconductors hold out the promise of important technological applications and, at the same time, raise very difficult fundamental science issues. The will address several theoretical issues in this field. In particular, by collaborating with experts on large scale numerical simulations, he intends to study the possibility of holes arranging themselves into narrow "stripes "separated by insulating antiferromagnetic regions in some of these materials (and in some models used to study them). How generally this occurs, for what reasons and whether it hinders or helps superconductivity are important open questions in the field. ***