Introduction to Spintronics

and Spin Quantum Computation

Spintronics, or spin electronics, refers to the study of the role played by electron (and more generally nuclear) spin in solid state physics, and possible devices that specifically exploit spin properties instead of or in addition to charge degrees of freedom. For example, spin relaxation and spin transport in metals and semiconductors are of fundamental research interest not only for being basic solid state physics issues, but also for the already demonstrated potential these phenomena have in electronic technology (some short reviews). The prototype device that is already in use in industry as a read head and a memory-storage cell is the giant-magnetoresistive (GMR) sandwich structure which consists of alternating ferromagnetic and nonmagnetic metal layers. Depending on the relative orientation of the magnetizations in the magnetic layers, the device resistance changes from small (parallel magnetizations) to large (antiparallel magnetizations). This change in resistance (also called magnetoresistance) is used to sense changes in magnetic fields. Recent efforts in GMR technology have also involved magnetic tunnel junction devices where the tunneling current depends on spin orientations of the electrodes.

Current efforts in designing and manufacturing spintronic devices involve two different approaches. The first is perfecting the existing GMR-based technology by either developing new materials with larger spin polarization of electrons or making improvements or variations in the existing devices that allow for better spin filtering. The second effort, which is more radical, focuses on finding novel ways of both generation and utilization of spin-polarized currents. These include investigation of spin transport in semiconductors and looking for ways in which semiconductors can function as spin polarizers and spin valves. The importance of this effort lies in the fact that the existing metal-based devices do not amplify signals (although they are successful switches or valves), whereas semiconductor based spintronic devices could in principle provide amplification and serve, in general, as multi-functional devices. Perhaps even more importantly, it would be much easier for semiconductor-based devices to be integrated with traditional semiconductor technology.

While there are clear advantages for introducing semiconductors in novel spintronic applications, many basic questions pertaining to combining semiconductors with other materials to produce a viable spintronic technology remain open. For example, whether placing a semiconductor in contact with another material would impede spin transport across the interface is far from well-understood. In the past, one of the strategies to advance understanding of spin transport in hybrid semiconductor structures was to directly borrow knowledge obtained from studies of more traditional magnetic materials. However, there is also an alternative approach involving the direct investigation of spin transport in all-semiconductor device geometries. In such a scenario a combination of optical manipulation (for example, shining circularly polarized light to create net spin polarization) and material inhomogeneities (e.g. by suitable doping as in the recently discovered Ga1-xMnxAs type ferromagnetic materials where Mn impurities act as dopants) could be employed to tailor spin transport properties.

In addition to the near-term studies of various spin transistors and spin transport properties of semiconductors, a long-term and ambitious subfield of spintronics is the application of electron and nuclear spins to quantum information processing and quantum computation (for more information on quantum computation, check out the following site). It has long been pointed out that quantum mechanics may provide great advantages over classical physics in physical computation. However, the real boom started after the advent of Shor's factorization algorithm and quantum error correction schemes. Among the many quantum computer hardwares that were proposed are the ones based on electron and nuclear spins. Obviously, the spins of electrons and spin-1/2 nuclei provide perfect candidates for quantum bits (qubits) as their Hilbert spaces are generally well-defined and their decoherence relatively slow.

At the University of Maryland, the theoretical study of spintronics is part of the theory group led by Professor Sankar Das Sarma, which presently includes 6 postdoctoral research fellows and 1 doctoral graduate students. Currently the spintronics research effort in our group focuses on the following directions:

  • Creation of spin polarization through optical or magnetic injection
  • Spin polarized Transport through semiconductor/superconductor interfaces
  • Spin relaxation in metals and semiconductors
  • Spin-based devices such as pn junctions and amplifiers
  • Spin-based quantum computation and electron entanglement in semiconductors

Recent publications of Das Sarma's group that are related to spintronics can be found in the Publications section of this website. Our research is supported by the United States Office of Naval Research, the United States Army Research Office, the National Science Foundation, DARPA, ARDA, LPS, and the University of Maryland. Professor Das Sarma and his research group are a part of the Condensed Matter Theory Group at the physics department of the University of Maryland, and are also affiliated with the Center for Superconductivity Research and National Science Foundation Materials Research Science and Engineering Center (NSF- MRSEC) at Maryland.

There are several experimental groups at the University of Maryland that are involved in the study of various aspects of spintronics. In particular, the group of Professor Dennis Drew studies how to use NSOM (Near Field Scanning Optical Microscopy) to detect electrons in semiconductor quantum dots; the group of Professor Richard Webb studies how to detect electron spins using transport experiments, whether electron spin entanglement can be measured using noise correlation measurements, and whether electron spins trapped in gated quantum dots can be used as qubits; the group of Dr Bruce Kane studies the feasibility of using phosphorous donor nuclear spins in Si for the purpose of quantum computing, and in particular whether SET (Single Electron Transistor) can be effectively used as a single electron spin detector.