An international research team has succeeded in gaining an in-depth insight into an unusual phenomenon. The researchers succeeded for the first time in directly measuring the spin of electrons in a material that exhibits the quantum spin Hall effect, which was theoretically predicted in 2004 and first observed in 2007.
Astonishingly, the spin currents flow without any external stimulus as a result of the internal structure of the material. The flow of information is loss-free, even for slight irregularities. This paves the way towards fault-tolerant quantum computers and towards a source of spin currents.
The spin is a quantum-mechanical property of elementary particles and as a rule it occurs in two variations. This is what makes it suitable for use as a binary information carrier. In hard disk drives, for example, spins are already being used to store digital information.
In 2007, physicists from Germany and the USA observed a new phenomenon that could make it possible to transport and electrically manipulate information in future storage media almost loss-free – the quantum spin Hall effect. The discovery was hailed by the journal Science as one of the ten most important scientific breakthroughs of 2007.
The first study that succeeded in directly observing the spin of flowing particles was published February 13 in Science by an international research team, which included Dr. Gustav Bihlmayer from Forschungszentrum Jülich, member of the Helmholtz Association. Until now, the quantum spin Hall effect could only be indirectly proven.
"We were able to show for the first time that two spin currents flow in opposite directions in the edge region of an alloy of bismuth and antimony. An external energy supply is not required; losses cannot occur," explained Dr. Gustav Bihlmayer from the Jülich Institute of Solid State Research. The causes of this astonishing phenomenon are interactions within the material. Of particular interest to materials scientists is the fact that imperfections in the material do not impair the spin currents.
"This means that materials known as topological materials have spin currents that can be manipulated electrically and are therefore suitable for use as spin sources.
They could even pave the way towards fault-tolerant quantum computers," said Bihlmayer. "Our process will make it possible to test the suitability of materials for this purpose in the future."
The current study makes use of theoretical calculations and photoelectron spectroscopy. The photons in a synchrotron beam cause electrons to be emitted from the material surface. The energy and momentum distribution, as well as the spin of the particles, can be used to derive concrete information on the occurrence of the quantum spin Hall effect. Previous methods were based on measurements of the conductivity in the materials at variable voltages.
Spins for Data Processing
Spins are a hot topic in research. Physicists and nanoelectricians have high hopes for what is known as spin electronics. Spin electronics does not just exploit the electric charge of electrons and nuclei but also their spin, and should therefore lead to the development of new approaches for the processing and coding of information in information processing.
Faster, smaller and more energy-efficient computers could thus become a reality, as could completely new components capable of performing a number of different functions such as storage, logic and communication. One of the most prominent ideas is that of the quantum computer. For spin-electronic concepts, scientists conducting basic research are desperately searching for new materials and phenomena that will make it possible to control both spin orientation and spin flow.
Electron Spin Rotated With Electric Field
Researchers at the Delft University of Technology's Kavli Institute of Nanoscience and the Foundation for Fundamental Research on Matter (FOM) have succeeded in controlling the spin of a single electron merely by using electric fields. This clears the way for a much simpler realization of the building blocks of a (future) super-fast quantum computer.
Controlling the spin of a single electron is essential if this spin is to be used as the building block of a future quantum computer. An electron not only has a charge but, because of its spin, also behaves as a tiny magnet. In a magnetic field, the spin can point in the same direction as the field or in the opposite direction, but the laws of quantum mechanics also allow the spin to exist in both states simultaneously.
As a result, the spin of an electron is a very promising building block for the yet-to-be-developed quantum computer; a computer that, for certain applications, is far more powerful than a conventional computer.
At first glance it is surprising that the spin can be rotated by an electric field. However, we know from the Theory of Relativity that a moving electron can 'feel' an electric field as though it were a magnetic field. Researchers Katja Nowack and Dr. Frank Koppens therefore forced an electron to move through a rapidly-changing electric field.
Working in collaboration with Prof. Yuli V. Nazarov, theoretical researcher at the Kavli Institute of Nanoscience Delft, they showed that it was indeed possible to turn the spin of the electron by doing so.
The advantage of controlling spin with electric fields rather than magnetic fields is that the former are easy to generate. It will also be easier to control various spins independently from one another - a requirement for building a quantum computer - using electric fields. The team, led by Dr. Lieven Vandersypen, is now going to apply this technique to a number of electrons.
The scientists published their work in Science Express on 1 November, 2007.
Researchers Predict A New State Of Matter In Semiconductors
Conventional matter exists in three familiar forms-solid, liquid and gas. But under special circumstances, quantum theory predicts exotic states of matter, such as superconductors in which electrons flow with no resistance and Bose-Einstein condensates in which atoms move as a collective whole. Now, in the Dec. 15 issue of the journal Science, three Stanford physicists theorize a new state of matter that may pave the way for electronic devices that dissipate less energy and generate less heat.
''Searching for new states of matter has become the holy grail of condensed matter physics, just as the quest for new elements dominated chemistry and the pursuit of new subatomic particles dominates particle physics,'' says physics Professor Shoucheng Zhang, who also holds courtesy appointments in the Applied Physics and Electrical Engineering departments.
With graduate student Taylor Hughes and former graduate student and current Princeton University postdoctoral fellow Andrei Bernevig, Zhang proposed the existence of the so-called ''quantum spin Hall state,'' which has extraordinary properties. The U.S. Department of Energy and National Science Foundation funded their work.
Say 'Cheese'
To understand the quantum spin Hall state, it's key to first understand the related quantum Hall state. Imagining a cheese sandwich will help. Swap semiconductor sheets for the bread, and turn the cheese into an electron gas. Instead of sticking your cheese sandwich in the fridge, place your semiconductor-electron gas concoction in an environment where it's way colder (below 1 degree Kelvin). Apply an intense magnetic field of several Teslas-more than 10,000 times greater than Earth's magnetic field.
''In such a state, the electrical current does not flow through the two-dimensional sheet, but is confined at the edges,'' Zhang explains. ''The current at a given edge flows
without dissipation, and only in one direction; it cannot be scattered backward by impurities.''
In essence, current flows only around the bread crusts. ''This property gives rise to the remarkable observation of quantized Hall voltage measured in the direction perpendicular to the current flow.'' In contrast, in conventional electronics, currents flow in the same direction as applied voltage, and the resistance can take arbitrary or nonquantized values. That means greater energy dissipation.
So that's the recipe for creating the quantum Hall effect, which Zhang calls ''one of the most profound phenomena in physics.'' The stuff of dreams, the quantum Hall effect was the basis of Nobel Prizes in 1985 and 1998.
A New State
Physicists often use math to convert complex physics concepts into terms of shape, or topology. It makes it easier to describe the extraordinary properties of different states of matter. ''If one performs smooth distortions of the donut, one can never get rid of the hole in its center and transform it into a sphere,'' Zhang says. ''Similarly, the electronic state of the quantum Hall effect is topologically distinct from that of any conventional semiconductor states.''
As cool and exotic as the quantum Hall state is, it has a serious drawback, Zhang notes. ''Unfortunately, the quantum Hall effect can only be realized under high magnetic field and low temperature, and cannot, therefore, be used for semiconductor devices operating under ambient conditions.''
In their report in Science, the three Stanford researchers proposed that a new state, called the quantum spin Hall effect, could be realized without applying an external magnetic field. They stacked and skewed alternating layers of mercury telluride and cadmium telluride. Just as in a slightly skewed stack of checkerboards, where red squares are bordered by black squares and vice versa, the material made a crystal lattice structure similar to that of the silicon or gallium arsenide of semiconductors.
The researchers say that by controlling the thickness of wells in the mercury telluride, the result will be a quantum phase transition into a new state that is distinct from that of conventional semiconductor states.
Conventional semiconductors are insulators at low temperatures. That means the resistance of the material is so high that no current can flow. But insulators can be turned into conductors-materials with some resistance, but not enough to stop current from flowing-using n-type doping, which adds electrons to the material, or p-type doping, which removes electrons to leave behind holes.
But matter in the quantum spin Hall state can carry electric currents without any doping, Zhang says. Just like with the quantum Hall effect, electrical current flows only at the edges of the sample.
What's more, the quantum spin Hall state would display an ''extraordinary'' property, Zhang says. On any given edge, electrons oriented with their spins aligned pointing up would flow in one direction, while the electrons oriented with their spins aligned pointing down would flow in the opposite direction. Because impurities usually do not flip the spin orientation, they cannot easily scatter the electrons into the backward direction, thus leading to far less energy dissipation or heat generation compared to conventional semiconductors. Basically, the quantum spin Hall effect has most of the desirable features of the quantum Hall effect, but without the cost of applying a huge magnetic field to a device, Zhang says.
''Similar to the quantum Hall effect, the quantum spin Hall effect is also topologically distinct from any conventional semiconductors,'' says Zhang. ''In this precise mathematical sense, the quantum spin Hall effect is a topologically distinct new state of matter.''
Putting Theory to the Test
Since quantum wells in mercury telluride/cadmium telluride sheets can be readily fabricated, it is possible to experimentally test the theoretical predictions of Zhang, Bernevig and Hughes. A research group at the University of Würzburg in Germany, under the direction of Professor Laurens Molenkamp, is currently doing this.
If the theory pans out, the quantum spin Hall effect may eventually inspire room-temperature devices with new capabilities. Zhang notes the potential for getting around a well-known roadblock of the electronics industry, the dictum saying the number of transistors fitting on a computer chip will double every 18 months: ''Transistors built based on the quantum spin Hall effect are expected to dissipate far less heat compared to conventional transistors, thus paving the way for extending Moore's law.''
In fact, hoping to turn Zhang's vision into a commercial reality, the Microelectronics Advanced Research Corporation, a consortium of leading U.S. semiconductor companies, has started to fund his research on the quantum spin Hall effect.
Scientists Control The Spin Of Semiconductor Quantum Dot Shell States
Scientists at the Naval Research Laboratory (NRL) have recently demonstrated the ability to control the spin population of the individual quantum shell states of self-assembled indium arsenide (InAs) quantum dots (QDs). These results are significant in the understanding of QD behavior and scientists' ability to utilize QDs in active devices or for information processing.
The scientists, from NRL's Materials Science and Technology Division, used a spin-polarized bias current from an iron (Fe) thin film contact and determined the strength of the interaction between spin-polarized electrons in the s, p and d shells.
Semiconductor QDs are nanoscale circular disks of one semiconducting material, typically 3 nm high by 30 nm in diameter, embedded within layers of a second material. Figure 1 shows such a structure, with an atomic force microscope image of the uncovered QDs in figure 2. Semiconductor QDs are attractive for a variety of quantum information processing, electronic and spintronic applications. In spintronic applications, the electron's spin rather than charge is used to store and process information. The International Technology Roadmap for Semiconductors has identified the electron's spin as a new state variable which should be explored as an alternative to the electron's charge for use beyond standard CMOS technology. The QD electronic structure exhibits the s,p,d,f shells characteristic of single atoms, so they are often referred to as "artificial atoms."
The NRL researchers monitor the shell population and spin polarization by measuring the polarized light emitted as a function of the bias current from the Fe contact. In contrast with previous work, they resolve features in the electroluminescence (EL) spectra associated with the individual quantum levels (s-, p-, d-, and f- shells). As the bias current is increased, the shell states fill, and the EL from the QDs exhibits peaks characteristic of the shell energies, as labeled in figure 3.
Intershell exchange strongly modifies the optical polarization observed from that expected for simple models of shell occupation. From a detailed analysis of the EL spectra, the NRL researchers were able to obtain the first experimental measure of the exchange energies between electrons in the s- and p-shells, and between electrons in the p- and d-shells. These energies describe the degree of interaction between these quantum levels.
A complete description of this work can be found in Physical Review Letters (28 November 2008).
Nanotechnology: Quantum Computer May Be Closer With Extended Quantum Lifetime Of Electrons
Physicists in the USA and at the London Centre for Nanotechnology have found a way to extend the quantum lifetime of electrons by more than 5,000 per cent, as reported recently in Physical Review Letters. Electrons exhibit a property called 'spin' and work like tiny magnets which can point up, down or a quantum superposition of both.
The state of the spin can be used to store information and so by extending their life the research provides a significant step towards building a usable quantum computer.
"Silicon has dominated the computing industry for decades," says Dr Gavin Morley, lead author of the paper. "The most sensitive way to see the quantum behaviour of electrons held in silicon chips uses electrical currents. Unfortunately, the problem has always been that these currents damage the quantum features under study, degrading their usefulness."
Marshall Stoneham, Professor of Physics at UCL (University College London), commented: "Getting the answer from a quantum computation isn't easy. This new work takes us closer to solving the problem by showing how we might read out the state of electron spins in a silicon-based quantum computer."
To achieve the record quantum lifetime the team used a magnetic field twenty-five times stronger than those used in previous experiments. This powerful field also provided an additional advantage in the quest for practical quantum computing: it put the electron spins into a convenient starting state by aligning them all in one direction.
For more information, see the paper published in Physical Review Letters, November 14 2008, by G. W. Morley (London Center for Nanotechnology), D. R. McCamey (University of Utah), H. A. Seipel (University of Utah), L.-C. Brunel (National High Magnetic field Laboratory), J. van Tol (National High Magnetic field Laboratory) and C. Boehme (University of Utah).
Fuente: http://www.sciencedaily.com/releases/2009/02/090212141248.htm, http://www.sciencedaily.com/releases/2007/11/071101144942.htm, http://www.sciencedaily.com/releases/2006/12/061215091036.htm, http://www.sciencedaily.com/releases/2009/02/090205120403.htm, http://www.sciencedaily.com/releases/2008/11/081114081220.htm
Nombre: Rodriguez B. Joiver I.
Asignatura: CRF
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