news digest ♦ Novel Devices
substantially. The result is cutting-edge silicon germanium devices such as the IHP Microelectronics 800 GHz transistor. Such designs combine SiGe›s extremely high performance with silicon›s traditional advantages - low cost, high yield, smaller size and high levels of integration and manufacturability - making silicon with added germanium highly competitive with the other materials. Cressler and his team demonstrated the 800 GHz transistor speed at 4.3 Kelvins (-268oC). This transistor has a breakdown voltage of 1.7 V, a value which is adequate for most intended applications.
The 800 GHz transistor was manufactured using IHP’s 130 nm BiCMOS process, which has a cost advantage compared with today’s highly-scaled CMOS technologies. This 130 nm SiGe BiCMOS process is offered by IHP in a multi-project wafer foundry service. The Georgia Tech team used liquid helium to achieve the extremely low cryogenic temperatures of 4.3 Kelvins in achieving the observed 798 GHz speeds. «When we tested the IHP 800 GHz transistor at room temperature during our evaluation, it operated at 417 GHz,» Cressler notes. «At that speed, it›s already faster than 98 percent of all the transistors available right now.» This work is described in detail in the paper, «A 0.8 THz fMAX SiGe HBT Operating at 4.3 K” by P.S. Chakraborty et alin IEEE Electron Device Letters, 35 (2), p 151 - 153. DOI: 10.1109/LED.2013.2295214
Helical spin order in GaAs quantum wires
Researchers have examined the electron and nuclear spin order in gallium arsenide nanowires at temperatures of 0.1 kelvin
Physicists at the University of Basel have observed a spontaneous magnetic order of electron and nuclear spins in a GaAs quantum wire at temperatures of 0.1 kelvin.
In the past, this was possible only at much lower temperatures, typically in the microkelvin range. The coupling of nuclei and electrons creates a new state of matter whereby a nuclear spin order arises at a much higher temperature. The results are consistent with a theoretical model developed in Basel a few years ago, as reported by the researchers in the scientific journal Physical Review Letters.
Helical order: The spins of the electrons and nuclei (red arrows) take the form of a helix rotating along the axis of the quantum wire. The blue ribbon is a guide to the eye for the helix. (Illustration: B. Braunecker, P. Simon, and D. Loss, Phys. Rev. B 80, 165119 (2009))
The researchers, led by r Dominik Zumbühl a professor at the University of Basel’s Department of Physics, used quantum wires made from GaAs. These are one- dimensional structures in which the electrons can move in only one spatial direction.
At temperatures above 10 kelvin, the quantum wires exhibited universal, quantised conductance, suggesting that the electron spins were not ordered.
However, when the researchers used liquid helium to cool the wires to a temperature below 100 millikelvin (0.1 kelvin), the electronic measurements showed a drop in conductance by a factor of two, which would suggest a collective orientation of the electron spin. This state also remained constant when the researchers cooled the sample to even lower temperatures, down to 10 millikelvin.
Electron-nuclear spin coupling
The results are exceptional because this is the first time that nuclear spin order has been measured at temperatures as high as 0.1 kelvin. Previously, spontaneous nuclear spin order was observed only at much lower temperatures, typically below 1 microkelvin; i.e. five orders of magnitude lower in temperature.
The reason why nuclear spin order is possible already at 0.1 kelvin is that the nuclei of the gallium and arsenic atoms in these quantum wires couple to the electrons, which themselves act back on the nuclear spins, which again interact with the electrons, and so on.
This feedback mechanism strongly amplifies the interaction between the magnetic moments, thus creating the combined nuclear and electron spin magnetism. This order is further stabilised by the fact that the electrons in such quantum wires have strong mutual interactions, bumping into each other like railcars on a single track.
Helical electron and nuclear spin order
Interestingly, in the ordered state, the spins of the electrons and nuclei do not all point in the same direction. Instead, they take the form of a helix rotating along the quantum wire. This helical arrangement is predicted by a theoretical model described by Professor Daniel Loss and collaborators at the University of Basel in 2009. According to this model, the conductance drops by a factor of two in the presence of a nuclear spin helix. All other existing theories are incompatible with the data
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www.compoundsemiconductor.net March 2014
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