Research

Spintronics

Spintronics is a branch of physics and nanoscience concerned with the storage and transfer of information by means of electron spins in addition to electron charge as in conventional electronics. The birth of spintronics can be traced to basic research on nanoscale ferromagnet/normal-metal multilayers in late 1980s, as recognized by the Nobel Prize in Physics 2007 awarded for the discovery of giant magnetoresistance (GMR). The GMR phenomenon also exemplifies one of the fastest transfers of basic research into applications where in less then ten years since its discovery it has revolutionized information storage technologies by enabling 100 times increase in hard disk storage capacity.

Unlike early non-coherent spintronics phenomena (such as GMR), the major themes of the second-generation spintronics are exploiting quantum-coherent spin states where spin component persists in the direction transverse to external or effective internal magnetic fields. Examples of such phenomena in metal spintronics are spin transfer torque (spin current of large enough density injected into a ferromagnetic layer either switches its magnetization from one static configuration to another or generates a dynamical situation with steady-state precessing magnetization) and spin pumping (moving magnetization generates pure spin current with no applied bias voltage and accompanying net charge current). In semiconductor spintronics recent second-generation experiments have succed in: transporting coherent precessing spins across 100 micron thick silicon wafers; detecting direct and inverse spin Hall effect; and manipulating localized coherent spins as building blocks of futuristic solid-state-based quantum computers. A major teme of second-generation spintronics is exploration of various quantum phenomena that can be exploited to generate pure spin currents as a situation with no net charge current.

Our spintronics projects are focused on:

• spin pumping and related time-dependent spin-transport phenomena,
• spin-transfer torque,
• spin Hall effect.

For a popular introduction see Spintronics page at the Department of Physics & Astronomy Website, while more technical details are available from our Publications page.

Graphene Nanoelectronics

Semiconductor technology has taken us a long way by making devices of ever smaller size. But eventually, fundamental physical barriers will pose a huge challenge for further shrinking of present silicon-based electronic devices, such as quantum and coherence effects (e.g., quantum tunneling of carriers through the gate insulator and through the body-to-drain junction of field-effect transistors is a highly undesirable effect), high electric fields creating avalanche dielectric breakdowns, and non-uniformity of dopant atoms and the relevance of single atom defects. The most important limitation is set by the power dissipation through various leakage mechanisms that are especially dangerous for minimal field-effect transistor (FET) dimensions and oxide thickness.

The aim of nanoelectronics is to process, transmit and store information by taking advantage of properties of matter that are distinctly different from macroscopic properties. In the context of electrical conduction, the key quantity that characterizes nanoscale systems is the current density (current pre unit area), which is typically orders of magnitude larger than those found in mesoscopic and macroscopic systems.

Our exploration of graphene-based nanoelectronic devices is focused on:

• first-principles computation of transport properties of graphene nanoelectronic devices composed of thousands of atoms,
• molecular electronic devices with graphene nanoribbons as electrodes,
• noise in graphene devices,
• magnetism in zigzag graphene nanoribbons,
• graphene-based spintronic devices.

For a popular introduction see Graphene Nanoelectronics page at the Department of Physics & Astronomy Website, while more technical details are available from our Publications page.

Nanoscale Thermoelectrics

Thermoelectrics transform temperature gradients into electric voltage and vice versa. Although a plethora of widespread applications has been envisioned, such as generation of electricity from waste heat to improve vehicle fuel efficiency or solid-state Peltier coolers for electronic circuits, their usage is presently limited by their small efficiency. Thus, careful tradeoffs are required to optimize the dimensionless figure of merit $ZT=S^{2}GT/K$ quantifying the maximum efficiency of a thermoelectric cycle conversion because $ZT$ contains unfavorable combination of the thermopower $S$, average temperature $T$, electrical conductance $G$ and total thermal conductance $K=Kel+K_{ph}$ which has contributions from both electrons $K_{el}$ and phonons $K_{ph}$. The devices with $ZT>1$ are regarded as good thermoelectrics, but $ZT>3$ is required to compete with conventional generators.

The major experimental efforts to increase ZT have been directed toward suppressing the lattice thermal conductivity Kph using either complex (through disorder in the unit cell) bulk materials or bulk nanostructured materials. A complementary approach engineers electronic density of states to obtain a sharp singularity near the Fermi energy which can enhance the power factor $S^{2}G$ (e.g., as in PbTe doped with Tl reaching $ZT\sim 1.5$ at 775 K).

In recent years, there has been a growing experimental and theoretical interest to explore nanowires and devices where a single molecule is attached to metallic or semiconducting electrodes for thermoelectric applications. In such devices, the dimensionality reduction (e.g., rough silicon nanowires can act as efficient thermoelectric materials although bulk silicon is not) and possible strong electronic correlations can make it possible to increase $S$ concurrently with diminishing $K_{ph}$ while keeping the nanodevice disorder-free.

For a popular introduction see Nanoscale Thermoelectrics page at the Department of Physics & Astronomy Website, while more technical details are available from our Publications page.