Spinning into the Future
By Professor Min Ouyang

Spin, as an intrinsic quantum mechanical property of the electron, has been a very active and fast moving research subject in condensed matter physics. For example, the origin of material magnetism lies in the spin and orbital motions of electrons and how spins of electrons interact with one another. In addition to its fundamental science significance, the manipulation and utilization of the electron and nuclear spin degree of freedom heralds an exciting and rapidly developing technology of “Spintronics,” which integrates two traditional branches of physics, magnetism and electronics.

Figure 1. Chemically synthesized zero- dimensional quantum dots (Up) and one- dimensional quantum wires (Bottom Left) showing single crystalline atomic lattice structure (Bottom Right)

In the mainstream of current microelectronic industry, the spin degree of freedom of the electrons has been completely ignored. Information processing is mainly based on the different families of electrical carriers which are distinguished by their different effective charges. However, with the rapid progresses in the miniaturization of semiconductor electronic devices through the top down and bottom up methodologies of the field of nano- and micro- technology, the functional device feature size becomes smaller and smaller (<50 nanometers) and will inevitably approach the regime where the quantum mechanics dominate. Spin, however, provides unique opportunities to store and manipulate phase coherence over length and time scales much larger than is typically possible in charge-based devices.

One of our group’s major focuses has been on experimentally exploring spin and spin dynamic properties in nanometer scale condensed matter systems (dubbed “NanoSpintronics”), in which electrons are quantum confined in two- or three- dimensions to the length scale of the order or smaller than their characteristic quantum wavelengths (for example, electron Bohr radius). The simplest of these nanostructures are zero- dimensional quantum dots/shells and one-dimensional quantum wires, in which the quantum confinement is in three- or two- dimensions, respectively. From the device point of view, quantum dots and wires represent the smallest building blocks and interconnect components for larger scale functional devices. From the fundamental science point of view, many exotic physics can be expected from these low dimensional structures compared with their higher dimensional counterparts. For example, Fermi liquid picture widely used for three- (and two-) dimensional structures will break and a new theory – Luttinger liquid theory – will have to be taken to describe low dimensional interacting electron systems. In Luttinger liquid theory the elementary excitations in low dimensional systems are not quasi- particles with charge and spin but collective charge and spin density fluctuations with bosonic character, so called spinons and holons. More dramatic effects, such as spin and charge separation, has been predicted in such low dimensional systems.

Figure 2. (Up) Cartoon showing spin coherence couplings between two quantum dots linked through organic molecules; (Bottom) Femto- second optical spectroscopy experimentally reveal that spins start out in large linked quantum dots first jump and inhibit both dots simultaneously (two peaks) before setting in the smaller dots.

The field of “NanoSpintronics” is at an exciting stage since major fundamental problems are still being addressed by experiment and theory. From the experimental aspect, breakthroughs in this field will require both the development of new low dimensional material system with tailored spin properties and the advance in experimental techniques with the capability of probing spin and spin dynamics in single nanostructures. Therefore, it represents an interdisciplinary pursuit that includes physics, chemistry, material science and engineering.

We are currently interested in developing rational chemical synthetic methodologies (wet and dry chemical routes) for making hybrid spin based organic and inorganic quantum dots, quantum shells and quantum wires in a control manner. We focus on nanostructures with tailored electronic, magnetic and optical properties that clearly reflect the dimensionality. Figure 1 showed some typical zero- and one- dimensional structures we have synthesized in lab.

These as-synthesized nanostructures will not only represent promising building blocks for the future integration of functional Spintronic devices but also provide an ideal system for investigating fundamental spin-spin/spin-charge interactions at low dimension. In our research group we also focus on investigating fundamental basis for spin-charge interactions and spin transport within these nanostructured systems with new experimental techniques such as femtosecond optical spectroscopy, magnetotransport and low temperature scanning probe microscopy. One example is demonstrated in Figure 2. In order to use quantum dot for building up future quantum computer based on their spin states, a “must” for any type of such device scheme is to establish spin based information communication channels from dot to dot which has turned out to be extremely challenging. We recently hurdled such an obstacle by a simple chemistry based scheme. We first linked two different sized quantum dots successively with organic conjugated molecules between them (Figure 2, Up). The interconnected organic molecules served as not only physical links but also the quantum information bus shuttling information between quantum dots. To observe such phenomena we used a pair of ultrashort- pulse lasers and watched electron spin coherent states hopped over the molecular bridge between dots by monitoring their characteristic spin precession features (such as Lande g-factors in bottom figure of Figure 2). This work was believed to be “an advance that could revolutionize the field of quantum computing” (R.F. Service, Science 301, 580), and imagination of future large-scale spintronic devices built up from such hybrid quantum dot-molecular units is shown in the cover from an artist point of view (cover image courtesy from David Awschalom at UCSB)

For more information about Dr. Ouyang's research, please click here.