Mesoscopic Physics

Douglas Natelson is interested in nanoscale physics, particularly with the electrical and magnetic properties of systems on the single-nanometer scale. Particular research projects include:

• Single molecule transistors and atomic-scale junctions--

  His group is able to fabricate field-effect transistors, each with an active region consisting of an individual nanometer-scale molecule. These devices can exhibit remarkable phenomena, including correlated many-body states, and also have significant potential as sensors. Atomic-scale metal junctions may be made by related methods, and can also show surprising properties.

• Organic semiconductor devices--
  

  Students also study electronic transport in micro and nanoscale devices incorporating organic semiconductor materials. Much of the basic physics of these systems remains poorly understood, and by studying devices on the scale of, e.g., single polymer chains, the situation may become more clear.

• Quantum coherence in normal and magnetic metals--
  

  Quantum interference effects in electrical conduction remain a powerful probe of underlying physics, particularly how the quantum world leads to classical behavior. Studies include coherence in novel nanostructures and ferromagnetic metals, where magnetic order can strongly affect coherence.

C₆₀ single molecule transistor: Conductance vs. bias/gate voltage

The group of Junichiro Kono is working in the field of optical spectroscopy of semiconductor nanostrucures and device structures. They use a variety of spectroscopic methods, ranging from ultrafast/nonlinear spectroscopy, near-field spectroscopy to Fourier-transform infrared spectroscopy, to investigate charge and spin dynamics/states in various types of quantum wells (2D), wires (1D) and dots (0D). Three of their current efforts are summaried here:

• Extreme nonlinear optical processes--
  

  They study the interaction of quantum-confined carriers with intense laser fields. In particular, they study laser-induced phenomena that cannot be understood by treating the laser field as a small perturbation. These so-called strong-field / non-perturbative phenoma have been extensively studied in laser-driven atoms but not in solids due to the unavoidable sample damage. The Kono group uses intense mid-infrared (3-20 microns) radiation from an optical parametric amplifier (in their lab at Rice) and far-infrared (20-70 microns) radiation from a free-electron laser (at the Stanford FEL Center; see also the last paragraph in the Terahertz Radiation section). The phenomena being investigated include: Autler-Townes effect in a quantum well, electromagneticallyinduced transparency, dynamical Franz-Keldysh effect, and highorder harmonic generation.

• Search for exciton condensation--
  

  In this project they are investigating the optical and infrared properties of spatially-separated electron-hole systems in strong magnetic fields. The objective is to observe exciton condensation, or other excitonic groud states, predicted to occur in such systems. This research will provide new insight into the unusual nature of interacting many-particle systems, but equally importantly, the realization of an electron-hole ground state in semiconductors would lead to the development of novel semiconductor devices. They use a 10-tesla superconducting magnet (in their lab at Rice) and a 200-tesla destructive pulse magnet (in the Megagauss Lab at the University of Tokyo).

• Spin dynamics and devices--
  

  This project explores the optical and terahertz dynamics of spins in bulk and quantum confined dilute magnetic III-V semiconductors. The objectives are: i) to investigate the origin of ferromagnetism in these materials, ii) to understand how the ferromagnetism manifests itself in the optical properties and affects and influences the degree of spin polarization achievable in these systems and, iii) to understand how we can control and implement the spins to produce novel magneto-optical devices for spin-enhanced semiconductor technologies (or "spintronics"). Furthermore, they design, fabricate, and characterize highly innovative quantum semiconductor nanostructures based on magnetic III-V semiconductors for revolutionary optoelectronics and quantum information processing and communication.