The announcement of graphene as the topic of this year’s Nobel in Physics has been rapidly followed by more announcements of novel properties of graphene that will pave the way for exciting new applications, such as the two discoveries that are the focus of this week’s posts. The focus with both discoveries is on exciting near-term applications, but these sorts of advances will also help, at least indirectly, to pave the road towards advanced nanotechnology. Advances in electronics and spintronics will mean cheaper, more powerful computers, which will advance both computational nanotechnology and the development of artificial general intelligence. Further, a vigorous incremental nanotechnology industry will provide a fertile infrastructure for funding efforts to develop advanced nanotechnology. With this preface, “Paving the way for graphene-based spin computers“:
Physicists at the University of California, Riverside have taken an important step forward in developing a “spin computer” by successfully achieving “tunneling spin injection” into graphene.
An electron can be polarized to have a directional orientation, called “spin.” This spin comes in two forms – electrons are said to be either “spin up” or “spin down” – and allows for more data storage than is possible with current electronics.
Spin computers, when developed, would utilize the electron’s spin state to store and process vast amounts of information while using less energy, generating less heat and performing much faster than conventional computers in use today.
Tunneling spin injection is a term used to describe conductivity through an insulator. Graphene, brought into the limelight by this year’s Nobel Prize in physics, is a single-atom-thick sheet of carbon atoms arrayed in a honeycomb pattern. Extremely strong and flexible, it is a good conductor of electricity and capable of resisting heat.
“Graphene has among the best spin transport characteristics of any material at room temperature,” explained Roland Kawakami, an associate professor of physics and astronomy, who led the research team, “which makes it a promising candidate for use in spin computers. But electrical spin injection from a ferromagnetic electrode into graphene is inefficient. An even greater concern is that the observed spin lifetimes are thousands of times shorter than expected theoretically. We would like longer spin lifetimes because the longer the lifetime, the more computational operations you can do.”
To address these problems, in the lab Kawakami and colleagues inserted a nanometer-thick insulating layer, known as a “tunnel barrier,” in between the ferromagnetic electrode and the graphene layer. They found that the spin injection efficiency increased dramatically.
“We found a 30-fold increase in the efficiency of how spins were being injected by quantum tunneling across the insulator and into graphene,” said Kawakami, who is also a member of UC Riverside’s Center for Nanoscale Science and Engineering. “Equally interesting is that the insulator was operating like a one-way valve, allowing electron flow in one direction – from the electrode to graphene – but not the other. The insulator helps to keep the injected spin inside the graphene, which is what leads to high spin injection efficiency. This counterintuitive result is the first demonstration of tunneling spin injection into graphene. We now have world record values for spin injection efficiency into graphene.” …
A crucial element of this advance was the use of molecular beam epitaxy to layer titanium atoms on top the graphene so that the magnesium oxide tunnel barrier can be inserted without forming clumps.
Journal Reference (courtesy of ScienceDaily):
Wei Han, K. Pi, K. McCreary, Yan Li, Jared Wong, A. Swartz, R. Kawakami. Tunneling Spin Injection into Single Layer Graphene. Physical Review Letters, 2010; 105 (16) DOI: 10.1103/PhysRevLett.105.167202
The full text PDF is available at the arxiv preprint repository: http://arxiv.org/pdf/1003.2669