CONDENSED-MATTER PHYSICS:Single-Electron Spin Measurement Heralds Deeper Look at Atoms
Erik Stokstad
After 8 years of effort, physicists in California have pulled off a technical tour de force: detecting the spin of a single electron inside a glassy chip of silica. The feat marks a significant step toward seeing individual atoms inside a material--a prerequisite for building a microscope that could map the three-dimensional structure of molecules--and may prove critical for so-called spintronic devices, including some kinds of quantum computers. The new success at imaging a single spin "is a physics breakthrough," says John Sidles of the University of Washington, Seattle.
Researchers can already visualize individual atoms with scanning tunneling and atomic force microscopes, but only on the surface of a sample. They can peer inside materials with magnetic resonance imaging--aligning the spin (a quantum-mechanical property that's the essence of magnetism) of protons in a sample's hydrogen atoms, zapping them with radio waves, and using an induction coil to track the changing magnetic fields as the energized protons return to their equilibrium spin states. The resulting images are useful for doctors, but they are a far cry from atomic resolution: At least a quadrillion protons must respond for a pixel to light up.
Spin detector. Wiggling cantilever, tracked by laser, can spot coil-driven changes in an electron's magnetic orientation.
CREDIT: D. RUGAR ET AL., NATURE 430, 329 (2004)Researchers could sharpen the picture by detecting a proton's spin directly, via magnetic forces, rather than through voltages induced in a coil. That's a tough challenge. These forces, measured in attonewtons, are extremely weak. To make the job easier, a group led by physicist Daniel Rugar of the IBM Almaden Research Center in San Jose, California, set out in 1996 to detect the spin of a single electron, which has a magnetic moment 600 times stronger than a proton's.
The team made a flexible cantilever, just 85 micrometers long and 100 nanometers thin, and added a tiny but powerful magnet at the tip. Applying a high-frequency magnetic field, they manipulated the spin of the electron so that it would resonate with the magnetic field around the cantilever tip. Then they set the cantilever wiggling. If the tip was hovering above an electron with a detectable spin, the resonance repeatedly flipped the spin of the electron, giving the cantilever a slight boost each time. The regular nudging revealed the spin amid the noise created by much stronger electrostatic and van der Waals forces, the researchers report this week in Nature.
It's a slow process right now: Scanning a 170-nanometer stretch of the sample took several weeks. The cantilever's resolution--about 25 nanometers--isn't atomic-scale yet, but the technique can spot an electron as deep as 100 nanometers, or about 400 atomic layers below the surface.
"It's quite an impressive achievement," says physicist Chris Hammel of Ohio State University, Columbus. Unlike other methods of detecting single spins, he says, the new technique has the advantage of working on many kinds of materials. Rugar's team is improving the resolution by cooling the system and outfitting the cantilever with a stronger magnet. That should increase resolution and speed enough to enable 2D and 3D scans, Rugar says.
From: http://www.sciencemag.org/cgi/content/full/305/5682/322a
2008/04/28
Quantum spin probe able to measure spin states at individual atoms; could have application in quantum computing
Quantum spin probe able to measure spin states at individual atoms; could have application in quantum computing 20 June 2001
By Robert Sanders, Media Relations
Image of the electron cloud around a nickle impurity in a high-temperature superconductor, obtained with a scanning tunneling microscope. The central spot is the nickle atom, surrounded by a cloverleaf pattern whose fourfold symmetry is indicative of the underlying d-wave nature of the high-temperature superconducting state.PHOTO CREDIT: Seamus Davis/UC BerkeleyBerkeley - University of California, Berkeley, scientists probing the quantum weirdness of high-temperature superconductors have built a scanning tunneling microscope that can measure for the first time the quantum spin of an electronic state of a single atom, in this case an impurity atom embedded in the superconductor.
Until now, scientists have had to trap isolated atoms and zap them with a laser to measure their spin state.
The technique already has improved understanding of high temperature superconductors, which, 15 years after their discovery, are still an enigma. It also can help probe the spin states of atoms in metals and semiconductors, as well as in new materials such as carbon nanotubes or strontium ruthenate superconductors.
The researchers, however, are most excited about its potential application in quantum computers.
The results are reported in the June 21 issue of Nature by J. C. S嶧mus Davis, professor of physics at UC Berkeley and a researcher in the Materials Sciences Division of Lawrence Berkeley National Laboratory, and his collaborators at Boston University, Tokyo University and the National Institute of Standards and Technology (NIST).
In atoms or nuclei, spin is analogous to counterclockwise rotation, like a top, and the energy of a state depends on whether the spin axis points up or down. Spin interactions can have important consequences - the pairing of two electrons of opposite spin, a Cooper pair, in a low-temperature superconductor is the source of its zero electrical resistance and other unique properties.
A two-state system - spin up or spin down - is also what scientists look for in the "qubit" of a quantum computer. The two spin states are analogous to the ones and zeros of the binary alphabet used by today's computers, from palm pilots to Cray supercomputers.
Scientists predict that quantum computers taking advantage of two-level quantum states like this will be able to perform calculations far faster than conventional transistor-based computers, and in the process shrink the size of computers immensely.
"One of the holy grails of solid state physics is to write and store information in just one atom," Davis said. "We now have the ability to separate the spin-up state from the spin-down state at a single atom. This could lead to new tools for using spins for quantum computing."
"This is a fantastic new atomic-scale quantum probe of atoms in the solid state," said theoretical chemist K. Birgitta Whaley, professor of chemistry at UC Berkeley. Whaley is leading an effort to create a new center at UC Berkeley to study quantum computing, and Davis is part of the team.
His co-authors on the Nature paper are former UC Berkeley postdoctoral associate Eric W. Hudson, PhD, now a National Research Council Fellow at NIST in Gaithersburg, Md.; Shuheng Pan, a professor at Boston University; Shin-ichi Uchida, professor, and Hiroshi Eisaki, PhD, of the University of Tokyo's Department of Superconductivity; and graduate student Kristine M. Lang and postdoctoral associate Vidya Madhavan of UC Berkeley. Eisaki currently is on sabbatical at Stanford University.
In a quantum computer, each one or zero is called a qubit or quantum bit, and various systems have been proposed to represent these qubits. They range from the up and down spins of atomic nuclei manipulated with a nuclear magnetic resonance machine, to ions trapped in a cryogenic vacuum chamber, their states flipped with pulses of radio waves.
The disadvantage of these systems is that the devices used to read or write the qubits are large and cumbersome and not scalable to large numbers of qubits, Davis said.
Scanning tunneling microscopes (STM), however, were designed to probe materials on an atomic scale by way of a microscopic tip that hovers over the surface and maps the intensity of the electron clouds. Qubits stored as closely packed impurity atoms in a solid crystal could be read out by an STM much like a recording head reads and writes magnetic bits on a CD. Davis notes that the scanning tunneling microscope should work just as well at measuring spin states around impurities in semiconductors and metals.
"Bruce Kane of the University of Maryland has proposed using phosphorus atom impurities in a silicon semiconductor as qubits of a quantum computer," Whaley said. "The problem has been, how do you manipulate the spins of single atoms? This is a great way to do that."
Davis's group discovered the power of their STM while studying the effect of single-atom impurities in pure samples of a copper oxide high-temperature superconductor (Bi2Sr2CaCu2O8+d) - dubbed BSCCO (BIS-ko) because it is composed of bismuth, strontium, calcium, copper and oxygen. The researchers' goal is to find out why these composite materials are superconductors at relatively high temperatures - 85 degrees above absolute zero (-307蚌) instead of 4 degrees above zero (-452蚌) typical of low-temperature superconductors - and perhaps find a recipe for making better ones.
Superconductors carry electricity without energy loss, and thus are attractive components of electrical transmission lines, motors or high-powered magnets.
Last year the UC Berkeley group used its STM to make the first images of electron clouds around a zinc atom impurity in a BSCCO superconductor. The researchers next placed a magnetic atom, nickel, into the regular crystal structure of the superconductor to see what more it would reveal about the unknown quantum processes at work in high-temperature superconductors. Because the superconductivity of high-temperature superconductors is thought to involve interactions among the magnetic moments of the individual atoms, theoreticians have predicted that nickel should disrupt the superconductivity less than a non-magnetic atom such as zinc.
In mapping the electron clouds with the STM, they found, in fact, that nickel left the superconductivity undamaged, in contrast to the nearby disruption caused by zinc. Because of this, the scientists were able to observe quantum phenomena around the nickel impurity that they did not see with zinc.
In high-temperature superconductors, many electrons fly about as unpaired quasiparticles - basically broken Cooper pairs, which are the form in which electrons travel in conventional low-temperature superconductors. These quasiparticles scatter off an impurity and form a small electron cloud that hovers around the impurity like a cloud around a mountain peak.
Around a zinc impurity, Davis and his team saw a simple quasiparticle cloud. Around nickel, however, they saw structure in the cloud created by the magnetic moment of the nickel atom. It was as if two different kinds of clouds could hug the impurity, one - a spin-up electron cloud - at a higher energy than the other, spin-down electron cloud. They could tell the difference by its effect on the electrical current flowing through the STM tip.
They observed another quantum phenomenon, too. Because of the ambiguities of quantum mechanics, quasiparticles in a superconductor are actually a 50/50 mixture of particle and non-particle, which is called a hole. The scientists verified this with the STM, detecting both particles and holes around the nickel impurity.
The presence of holes as well as particles proves that nickel does not impair the superconductivity, and it strongly supports the theory that high-temperature superconductors work through magnetic interactions among the atoms.
Theoretician Alexander V. Balatsky of Los Alamos National Laboratory, who predicted the spin splitting by a magnetic atom impurity, was amazed at how closely the experiment fit his predictions, which initially were met with skepticism.
"Our prediction was made long before the experimental capability to tunnel into a very clean system with very few impurities existed," he said. "It shows that some aspects of our theory work and gives us confidence that we have predictive power for these enigmatic materials."
From: http://berkeley.edu/news/media/releases/2001/06/20_physc.html
By Robert Sanders, Media Relations
Image of the electron cloud around a nickle impurity in a high-temperature superconductor, obtained with a scanning tunneling microscope. The central spot is the nickle atom, surrounded by a cloverleaf pattern whose fourfold symmetry is indicative of the underlying d-wave nature of the high-temperature superconducting state.PHOTO CREDIT: Seamus Davis/UC BerkeleyBerkeley - University of California, Berkeley, scientists probing the quantum weirdness of high-temperature superconductors have built a scanning tunneling microscope that can measure for the first time the quantum spin of an electronic state of a single atom, in this case an impurity atom embedded in the superconductor.
Until now, scientists have had to trap isolated atoms and zap them with a laser to measure their spin state.
The technique already has improved understanding of high temperature superconductors, which, 15 years after their discovery, are still an enigma. It also can help probe the spin states of atoms in metals and semiconductors, as well as in new materials such as carbon nanotubes or strontium ruthenate superconductors.
The researchers, however, are most excited about its potential application in quantum computers.
The results are reported in the June 21 issue of Nature by J. C. S嶧mus Davis, professor of physics at UC Berkeley and a researcher in the Materials Sciences Division of Lawrence Berkeley National Laboratory, and his collaborators at Boston University, Tokyo University and the National Institute of Standards and Technology (NIST).
In atoms or nuclei, spin is analogous to counterclockwise rotation, like a top, and the energy of a state depends on whether the spin axis points up or down. Spin interactions can have important consequences - the pairing of two electrons of opposite spin, a Cooper pair, in a low-temperature superconductor is the source of its zero electrical resistance and other unique properties.
A two-state system - spin up or spin down - is also what scientists look for in the "qubit" of a quantum computer. The two spin states are analogous to the ones and zeros of the binary alphabet used by today's computers, from palm pilots to Cray supercomputers.
Scientists predict that quantum computers taking advantage of two-level quantum states like this will be able to perform calculations far faster than conventional transistor-based computers, and in the process shrink the size of computers immensely.
"One of the holy grails of solid state physics is to write and store information in just one atom," Davis said. "We now have the ability to separate the spin-up state from the spin-down state at a single atom. This could lead to new tools for using spins for quantum computing."
"This is a fantastic new atomic-scale quantum probe of atoms in the solid state," said theoretical chemist K. Birgitta Whaley, professor of chemistry at UC Berkeley. Whaley is leading an effort to create a new center at UC Berkeley to study quantum computing, and Davis is part of the team.
His co-authors on the Nature paper are former UC Berkeley postdoctoral associate Eric W. Hudson, PhD, now a National Research Council Fellow at NIST in Gaithersburg, Md.; Shuheng Pan, a professor at Boston University; Shin-ichi Uchida, professor, and Hiroshi Eisaki, PhD, of the University of Tokyo's Department of Superconductivity; and graduate student Kristine M. Lang and postdoctoral associate Vidya Madhavan of UC Berkeley. Eisaki currently is on sabbatical at Stanford University.
In a quantum computer, each one or zero is called a qubit or quantum bit, and various systems have been proposed to represent these qubits. They range from the up and down spins of atomic nuclei manipulated with a nuclear magnetic resonance machine, to ions trapped in a cryogenic vacuum chamber, their states flipped with pulses of radio waves.
The disadvantage of these systems is that the devices used to read or write the qubits are large and cumbersome and not scalable to large numbers of qubits, Davis said.
Scanning tunneling microscopes (STM), however, were designed to probe materials on an atomic scale by way of a microscopic tip that hovers over the surface and maps the intensity of the electron clouds. Qubits stored as closely packed impurity atoms in a solid crystal could be read out by an STM much like a recording head reads and writes magnetic bits on a CD. Davis notes that the scanning tunneling microscope should work just as well at measuring spin states around impurities in semiconductors and metals.
"Bruce Kane of the University of Maryland has proposed using phosphorus atom impurities in a silicon semiconductor as qubits of a quantum computer," Whaley said. "The problem has been, how do you manipulate the spins of single atoms? This is a great way to do that."
Davis's group discovered the power of their STM while studying the effect of single-atom impurities in pure samples of a copper oxide high-temperature superconductor (Bi2Sr2CaCu2O8+d) - dubbed BSCCO (BIS-ko) because it is composed of bismuth, strontium, calcium, copper and oxygen. The researchers' goal is to find out why these composite materials are superconductors at relatively high temperatures - 85 degrees above absolute zero (-307蚌) instead of 4 degrees above zero (-452蚌) typical of low-temperature superconductors - and perhaps find a recipe for making better ones.
Superconductors carry electricity without energy loss, and thus are attractive components of electrical transmission lines, motors or high-powered magnets.
Last year the UC Berkeley group used its STM to make the first images of electron clouds around a zinc atom impurity in a BSCCO superconductor. The researchers next placed a magnetic atom, nickel, into the regular crystal structure of the superconductor to see what more it would reveal about the unknown quantum processes at work in high-temperature superconductors. Because the superconductivity of high-temperature superconductors is thought to involve interactions among the magnetic moments of the individual atoms, theoreticians have predicted that nickel should disrupt the superconductivity less than a non-magnetic atom such as zinc.
In mapping the electron clouds with the STM, they found, in fact, that nickel left the superconductivity undamaged, in contrast to the nearby disruption caused by zinc. Because of this, the scientists were able to observe quantum phenomena around the nickel impurity that they did not see with zinc.
In high-temperature superconductors, many electrons fly about as unpaired quasiparticles - basically broken Cooper pairs, which are the form in which electrons travel in conventional low-temperature superconductors. These quasiparticles scatter off an impurity and form a small electron cloud that hovers around the impurity like a cloud around a mountain peak.
Around a zinc impurity, Davis and his team saw a simple quasiparticle cloud. Around nickel, however, they saw structure in the cloud created by the magnetic moment of the nickel atom. It was as if two different kinds of clouds could hug the impurity, one - a spin-up electron cloud - at a higher energy than the other, spin-down electron cloud. They could tell the difference by its effect on the electrical current flowing through the STM tip.
They observed another quantum phenomenon, too. Because of the ambiguities of quantum mechanics, quasiparticles in a superconductor are actually a 50/50 mixture of particle and non-particle, which is called a hole. The scientists verified this with the STM, detecting both particles and holes around the nickel impurity.
The presence of holes as well as particles proves that nickel does not impair the superconductivity, and it strongly supports the theory that high-temperature superconductors work through magnetic interactions among the atoms.
Theoretician Alexander V. Balatsky of Los Alamos National Laboratory, who predicted the spin splitting by a magnetic atom impurity, was amazed at how closely the experiment fit his predictions, which initially were met with skepticism.
"Our prediction was made long before the experimental capability to tunnel into a very clean system with very few impurities existed," he said. "It shows that some aspects of our theory work and gives us confidence that we have predictive power for these enigmatic materials."
From: http://berkeley.edu/news/media/releases/2001/06/20_physc.html
How to Create a Spin Current
PHYSICS:How to Create a Spin CurrentPrashant Sharma*
We usually think of a current as a flow of particles, such as the flow of electrons in a charge current generated by a battery. However, besides its charge, the electron also carries a spin, whose projection along the spin axis can point up or down. Conventional electronic devices ignore this property of the electron, but new devices are now being built that rely on the spin (1, 2). Such devices should have faster switching times and lower power consumption than conventional devices, mainly because spins can be manipulated faster and at lower energy cost than charges can.
All currently available spin-based devices are memory devices that use the spin to store information. Spin-based electronic (spintronic) devices such as transistors (2) require spin currents, just as conventional electronic devices require charge currents. Unfortunately, it is very difficult to generate and transport a spin current.
To understand what is meant by a spin current, consider an electron current that flows through a channel and contains only up-spin polarized electrons. Add to this a similar current in which all electrons are down-spin polarized and flow in the opposite direction. The result is a current of spins only; there is no net particle transfer across any cross section of the channel.
A spin current differs from a charge current in two important ways. First, it is invariant under time reversal: If the clock ran backward, spin current would flow in the same direction. Second, spin current is associated with a flow of angular momentum, which is a vector quantity. This feature allows quantum information to be sent across semiconducting structures, just as quantum optics involves distribution of information across optical networks via polarization states of the photon.
Most methods currently under investigation use ferromagnets to inject a spin current into a nonmagnetic material. However, this process is inefficient. For practical applications, the generation and detection of spin currents should not require strong magnetic fields and interfaces between semiconductors and ferromagnets.
Creating a spin current through spin pumping I. A single channel of electrons is formed in a 2D electron system through electrostatic confinement. When out-of-phase ac voltages are applied to the two gates, the channel is perturbed, resulting in a dc electron current. If one of the gates is replaced by an oscillating magnetic field, a spin current is pumped. One method that meets these requirements is spin pumping, which involves the scattering of electrons off a small region (a quantum cavity). In the cavity, electrons with different spins take dissimilar paths and therefore scatter differently off the cavity walls. Through modulating the shape of the cavity periodically in time, a constant spin current can be generated.
The theory of pumping electrons without changing their spin state is based on the idea (3) that one can create a traveling wave potential for electrons to ride on. At any moment, the electrons sit in the minima of the wave and move along with it. The efficiency of this mechanism depends on the depth of the traveling wave: The deeper the potential, the greater the chance that the electrons remain trapped and are transported along with the wave.
One practical way to create such a traveling wave is to periodically modulate the transmission of the electron flow at two points in space (4). This can be done by applying two metal gates on a channel (see the first figure) or on a quantum cavity (second figure, top panel). Experimentally, these structures are created in a two-dimensional (2D) layer of electrons (an electron gas), which forms in semiconducting heterostructures typically made from GaAs and AlGaAs. Switkes et al. have shown experimentally that a quantum cavity can indeed be modulated in time (5).
These ideas have recently been generalized to create spin currents. The earliest proposal (6) was to create a traveling wave for up spin-polarized electrons that is opposite in direction to that for down spin-polarized electrons. Creation of such spin-selective traveling waves requires the use of some externally controllable mechanism to break spin-rotation symmetry--that is, to define a unique spin axis in space. This can be achieved by replacing one of the metal gates in the first figure with an external, time-dependent magnetic field. To obtain efficient spin transport, one must increase the depth of the minima in the traveling waves by restricting the electrons to a long narrow channel--a quantum wire (7)--such that they repel each other.
Creating a spin current through spin pumping II. (Top) A quantum cavity is perturbed through out-of-phase ac voltages applied to the two gates. Electrons entering the cavity scatter off the cavity walls several times before leaving. (Bottom) In a sufficiently large cavity, pumping leads to a spin current with a tunable direction of spin polarization. Both in-plane (green arrow) and out-of-plane (red arrow) polarizations are possible. However, the picture of a single traveling wave is inadequate for describing pumping in a finite cavity, because the electron follows a complicated path before exiting the cavity. The direction of the current is therefore determined by the details of the scattering in the cavity (8). This sensitivity to the electron's path in the cavity is an essential feature of mesoscopic semiconductor devices, which can be up to several tens of micrometers in size.
In a theoretical proposal (9) for spin pumping, this dependence of the current through the cavity on externally controllable parameters is used to generate a spin current. The proposal is to modulate the shape of a quantum cavity through the use of two magnetic fields. A strong magnetic field applied in the plane of the cavity (see the second figure, top panel) couples only to the spin of the electrons. For such a strong in-plane magnetic field, there are more up-spin electrons (whose spins are aligned with the magnetic field) than down-spin ones inside the cavity. As a result, the pumped current is spin-polarized along the direction of the strong magnetic field and in the cavity plane; that is, it is a mixed charge and spin current.
To achieve only a spin current, a second, weak magnetic field is added. The field is weak enough not to affect the spin of an electron, but the Lorentz force exerted by this field affects the spatial motion of the charged electrons. One would therefore expect (on average) the up-spin charge current to flow in one direction while the down-spin charge current flows in the opposite direction. Spin currents have recently been produced experimentally with such a device (10).
An alternative to using magnetic fields in the cavity has also been proposed (11). It is based on the fact that the spin state of an electron moving inside a semiconductor is not independent of its momentum state. Because the quantum cavity is formed in a semiconductor--typically a GaAs/AlGaAs heterostructure--the spin of the electron is coupled to its motion inside the cavity. Because of this spin-orbit coupling, the direction of the spin of an electron follows the electron's motion. As a result, the direction of the spin polarization coming out of the cavity depends on the details of the scattering in the cavity (12). By increasing the number of times an electron scatters off the cavity walls, the in-plane spin projection of the electron can be made small, and its spin can be made to point in a direction perpendicular to the plane of the electron gas (see the second figure).
This approach should allow spin currents to be pumped through a quantum cavity without an in-plane magnetic field. Because of spin-orbit coupling, the outgoing current will be spin-polarized. By either applying a weak perpendicular magnetic field [as in the experiments in (10)] or by inducing small changes in the density of electrons in the cavity, a pure spin current can be obtained. In this method of pumping, the direction of polarization of the spin current can be changed from in-plane to out-of-plane by altering the shape and size of the quantum cavity (second figure, bottom panel). The approach has not yet been realized experimentally.
Spin pumping in mesoscopic systems allows the spin-polarization direction of currents to be manipulated without the use of strong magnetic fields and ferromagnets. However, some experimental challenges remain before this method is ready for use in actual spintronic devices. One difficulty lies in efficiently detecting spin currents whose polarization direction is arbitrary.
Spin currents polarized in the plane of the 2D electron system have been detected electrically (10). Out-of-plane polarization in a 2D electron system may be detected (11) via the spin Hall effect. Because of this effect, an electron with its spin polarized perpendicular to its momentum is deflected in a direction orthogonal to both its momentum and its spin. Reversing the direction of either the momentum or the spin polarization reverses the direction of deflection. As a result, a spin current with an out-of-plane polarization generates a transverse electric field in a material that shows the spin Hall effect. Recently, the spin Hall effect has been observed for the first time in a semiconducting GaAs/InGaAs heterostructure (13).
We have yet to create a spin pump that can generate spin currents with any chosen direction of spin polarization. Nonetheless, recent experimental and theoretical advances give hope that devices relying on spin currents will soon be realized.
References
S. A. Wolf et al., Science 294, [1488] (2001).
I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004) [APS].
D. J. Thouless, Phys. Rev. B 27, 6083 (1983) [APS].
B. L. Altshuler, L. I. Glazman, Science 283, [1864] (1999).
M. Switkes et al., Science 283, [1905] (1999).
P. Sharma, C. Chamon, Phys. Rev. Lett. 87, 096401 (2001) [APS].
T. Giamarchi, Quantum Physics in One Dimension (Clarendon, Oxford, 2004) [publisher's information].
P. W. Brouwer, Phys. Rev. B 58, 10135 (1998) [APS].
E. R. Mucciolo, C. Chamon, C. Marcus, Phys. Rev. Lett. 89, 146802 (2002) [APS].
S. K. Watson, R. M. Potok, C. M. Marcus, V. Umansky, Phys. Rev. Lett. 91, 258301 (2003) [APS].
P. Sharma, P. W. Brouwer, Phys. Rev. Lett. 91, 166801 (2003) [APS].
I. L. Aleiner, V. I. Fa'lko, Phys. Rev. Lett. 87, 256801 (2001) [APS].
Y. K. Kato, R. C. Myers, A. C. Gossard, D. D. Awschalom, Science 306, [1910] (2004).
10.1126/science.1099388
From: http://www.sciencemag.org/cgi/content/full/307/5709/531
We usually think of a current as a flow of particles, such as the flow of electrons in a charge current generated by a battery. However, besides its charge, the electron also carries a spin, whose projection along the spin axis can point up or down. Conventional electronic devices ignore this property of the electron, but new devices are now being built that rely on the spin (1, 2). Such devices should have faster switching times and lower power consumption than conventional devices, mainly because spins can be manipulated faster and at lower energy cost than charges can.
All currently available spin-based devices are memory devices that use the spin to store information. Spin-based electronic (spintronic) devices such as transistors (2) require spin currents, just as conventional electronic devices require charge currents. Unfortunately, it is very difficult to generate and transport a spin current.
To understand what is meant by a spin current, consider an electron current that flows through a channel and contains only up-spin polarized electrons. Add to this a similar current in which all electrons are down-spin polarized and flow in the opposite direction. The result is a current of spins only; there is no net particle transfer across any cross section of the channel.
A spin current differs from a charge current in two important ways. First, it is invariant under time reversal: If the clock ran backward, spin current would flow in the same direction. Second, spin current is associated with a flow of angular momentum, which is a vector quantity. This feature allows quantum information to be sent across semiconducting structures, just as quantum optics involves distribution of information across optical networks via polarization states of the photon.
Most methods currently under investigation use ferromagnets to inject a spin current into a nonmagnetic material. However, this process is inefficient. For practical applications, the generation and detection of spin currents should not require strong magnetic fields and interfaces between semiconductors and ferromagnets.
Creating a spin current through spin pumping I. A single channel of electrons is formed in a 2D electron system through electrostatic confinement. When out-of-phase ac voltages are applied to the two gates, the channel is perturbed, resulting in a dc electron current. If one of the gates is replaced by an oscillating magnetic field, a spin current is pumped. One method that meets these requirements is spin pumping, which involves the scattering of electrons off a small region (a quantum cavity). In the cavity, electrons with different spins take dissimilar paths and therefore scatter differently off the cavity walls. Through modulating the shape of the cavity periodically in time, a constant spin current can be generated.
The theory of pumping electrons without changing their spin state is based on the idea (3) that one can create a traveling wave potential for electrons to ride on. At any moment, the electrons sit in the minima of the wave and move along with it. The efficiency of this mechanism depends on the depth of the traveling wave: The deeper the potential, the greater the chance that the electrons remain trapped and are transported along with the wave.
One practical way to create such a traveling wave is to periodically modulate the transmission of the electron flow at two points in space (4). This can be done by applying two metal gates on a channel (see the first figure) or on a quantum cavity (second figure, top panel). Experimentally, these structures are created in a two-dimensional (2D) layer of electrons (an electron gas), which forms in semiconducting heterostructures typically made from GaAs and AlGaAs. Switkes et al. have shown experimentally that a quantum cavity can indeed be modulated in time (5).
These ideas have recently been generalized to create spin currents. The earliest proposal (6) was to create a traveling wave for up spin-polarized electrons that is opposite in direction to that for down spin-polarized electrons. Creation of such spin-selective traveling waves requires the use of some externally controllable mechanism to break spin-rotation symmetry--that is, to define a unique spin axis in space. This can be achieved by replacing one of the metal gates in the first figure with an external, time-dependent magnetic field. To obtain efficient spin transport, one must increase the depth of the minima in the traveling waves by restricting the electrons to a long narrow channel--a quantum wire (7)--such that they repel each other.
Creating a spin current through spin pumping II. (Top) A quantum cavity is perturbed through out-of-phase ac voltages applied to the two gates. Electrons entering the cavity scatter off the cavity walls several times before leaving. (Bottom) In a sufficiently large cavity, pumping leads to a spin current with a tunable direction of spin polarization. Both in-plane (green arrow) and out-of-plane (red arrow) polarizations are possible. However, the picture of a single traveling wave is inadequate for describing pumping in a finite cavity, because the electron follows a complicated path before exiting the cavity. The direction of the current is therefore determined by the details of the scattering in the cavity (8). This sensitivity to the electron's path in the cavity is an essential feature of mesoscopic semiconductor devices, which can be up to several tens of micrometers in size.
In a theoretical proposal (9) for spin pumping, this dependence of the current through the cavity on externally controllable parameters is used to generate a spin current. The proposal is to modulate the shape of a quantum cavity through the use of two magnetic fields. A strong magnetic field applied in the plane of the cavity (see the second figure, top panel) couples only to the spin of the electrons. For such a strong in-plane magnetic field, there are more up-spin electrons (whose spins are aligned with the magnetic field) than down-spin ones inside the cavity. As a result, the pumped current is spin-polarized along the direction of the strong magnetic field and in the cavity plane; that is, it is a mixed charge and spin current.
To achieve only a spin current, a second, weak magnetic field is added. The field is weak enough not to affect the spin of an electron, but the Lorentz force exerted by this field affects the spatial motion of the charged electrons. One would therefore expect (on average) the up-spin charge current to flow in one direction while the down-spin charge current flows in the opposite direction. Spin currents have recently been produced experimentally with such a device (10).
An alternative to using magnetic fields in the cavity has also been proposed (11). It is based on the fact that the spin state of an electron moving inside a semiconductor is not independent of its momentum state. Because the quantum cavity is formed in a semiconductor--typically a GaAs/AlGaAs heterostructure--the spin of the electron is coupled to its motion inside the cavity. Because of this spin-orbit coupling, the direction of the spin of an electron follows the electron's motion. As a result, the direction of the spin polarization coming out of the cavity depends on the details of the scattering in the cavity (12). By increasing the number of times an electron scatters off the cavity walls, the in-plane spin projection of the electron can be made small, and its spin can be made to point in a direction perpendicular to the plane of the electron gas (see the second figure).
This approach should allow spin currents to be pumped through a quantum cavity without an in-plane magnetic field. Because of spin-orbit coupling, the outgoing current will be spin-polarized. By either applying a weak perpendicular magnetic field [as in the experiments in (10)] or by inducing small changes in the density of electrons in the cavity, a pure spin current can be obtained. In this method of pumping, the direction of polarization of the spin current can be changed from in-plane to out-of-plane by altering the shape and size of the quantum cavity (second figure, bottom panel). The approach has not yet been realized experimentally.
Spin pumping in mesoscopic systems allows the spin-polarization direction of currents to be manipulated without the use of strong magnetic fields and ferromagnets. However, some experimental challenges remain before this method is ready for use in actual spintronic devices. One difficulty lies in efficiently detecting spin currents whose polarization direction is arbitrary.
Spin currents polarized in the plane of the 2D electron system have been detected electrically (10). Out-of-plane polarization in a 2D electron system may be detected (11) via the spin Hall effect. Because of this effect, an electron with its spin polarized perpendicular to its momentum is deflected in a direction orthogonal to both its momentum and its spin. Reversing the direction of either the momentum or the spin polarization reverses the direction of deflection. As a result, a spin current with an out-of-plane polarization generates a transverse electric field in a material that shows the spin Hall effect. Recently, the spin Hall effect has been observed for the first time in a semiconducting GaAs/InGaAs heterostructure (13).
We have yet to create a spin pump that can generate spin currents with any chosen direction of spin polarization. Nonetheless, recent experimental and theoretical advances give hope that devices relying on spin currents will soon be realized.
References
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I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004) [APS].
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10.1126/science.1099388
From: http://www.sciencemag.org/cgi/content/full/307/5709/531
Spintronics may save Moore's Law
Stanford University and three other California schools have formed a joint effort to advance research of spintronics, a technology that one day could lead to computers that begin working as soon as the power comes on.
The project, called the Western Institute of Nanoelectronics (WIN), will have its administrative headquarters at UCLA Henry Samueli School of Engineering and Applied Science, one of the four member institutions. Scientific and technical work will also be dispersed over the campuses of Stanford, the University of California at Berkeley and UC Santa Barbara.
WIN is being established with grants of $18.2 million to be dispersed over four years, largely from semiconductor companies with an interest in breakthroughs in spintronics, which holds promise in minimizing power consumption for next-generation consumer electronics. Chipmaker Intel granted the project $2 million, along with $10 million in equipment. The Nanoelectronics Research Initiative, a grant funded by computer companies IBM, Texas Instruments, Advanced Micro Devices and Intel, among others, provided $2.38 million.
The group expects the participating universities to spend more than $200 million in infrastructure and personnel support for the project over those four years.
Researchers say that chipmakers in the coming years will likely hit a barrier in Moore's Law that could prevent chip designers from gaining performance by shrinking their chips, the engine behind the exponential growth in computer power for more than three decades.
"Simply put, today's devices, which are based on complementary metal oxide semiconductor standards, can't get much smaller and still function properly and effectively. That's where spintronics comes in," said UCLA engineering professor Kang Wang, who will act as director of the institute.
Spintronics uses the spin of an electron to carry digital information. Until now and for years to come, data-processing technology has relied on charge-based devices, ranging from vacuum tubes to million-transistor microchips. Conventional electronic devices move these electric charges around, ignoring the spin that piggybacks on each electron. The study of spintronics intends to use that extra spin, turning those electrons into one smooth reactive chain of motion.
The metaphorical name of the technology derives from way electrons are said to spin.
From: http://www.news.com/2100-1008_3-6048228.html
The project, called the Western Institute of Nanoelectronics (WIN), will have its administrative headquarters at UCLA Henry Samueli School of Engineering and Applied Science, one of the four member institutions. Scientific and technical work will also be dispersed over the campuses of Stanford, the University of California at Berkeley and UC Santa Barbara.
WIN is being established with grants of $18.2 million to be dispersed over four years, largely from semiconductor companies with an interest in breakthroughs in spintronics, which holds promise in minimizing power consumption for next-generation consumer electronics. Chipmaker Intel granted the project $2 million, along with $10 million in equipment. The Nanoelectronics Research Initiative, a grant funded by computer companies IBM, Texas Instruments, Advanced Micro Devices and Intel, among others, provided $2.38 million.
The group expects the participating universities to spend more than $200 million in infrastructure and personnel support for the project over those four years.
Researchers say that chipmakers in the coming years will likely hit a barrier in Moore's Law that could prevent chip designers from gaining performance by shrinking their chips, the engine behind the exponential growth in computer power for more than three decades.
"Simply put, today's devices, which are based on complementary metal oxide semiconductor standards, can't get much smaller and still function properly and effectively. That's where spintronics comes in," said UCLA engineering professor Kang Wang, who will act as director of the institute.
Spintronics uses the spin of an electron to carry digital information. Until now and for years to come, data-processing technology has relied on charge-based devices, ranging from vacuum tubes to million-transistor microchips. Conventional electronic devices move these electric charges around, ignoring the spin that piggybacks on each electron. The study of spintronics intends to use that extra spin, turning those electrons into one smooth reactive chain of motion.
The metaphorical name of the technology derives from way electrons are said to spin.
From: http://www.news.com/2100-1008_3-6048228.html
2008/04/14
Einstein 1, Quantum Gravity 0
SPACETIME:Einstein 1, Quantum Gravity 0 Adrian Cho*
For 5 years, physicists have hoped that a flaw in Einstein's special theory of relativity might reveal that space and time aren't smooth at the smallest scale, but fuzzy and foaming. Now, that tantalizing prospect has vanished in a puff of gamma rays. Two independent measurements of cosmic gamma rays show that Einstein was right after all--and that current plans to detect the foam are doomed. "The results rule out these possibilities on empirical grounds," says Floyd Stecker, a theoretical astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The frothiness of space and time is predicted by many theories that attempt to meld Einstein's theory of gravity and quantum mechanics. Physicists hoped to detect it by finding a hole in Einstein's dictum that it is meaningless to say an object is moving or stationary relative to the universe, a principle known as Lorentz invariance. One consequence of the principle is that all particles of light, or photons, travel through empty space at the same speed regardless of how much energy they pack.
In recent years, however, various quantum gravity theories have suggested that Lorentz invariance might not hold. In that case, a photon's speed would vary with its energy, so that light of different wavelengths would travel at slightly different rates. That would make intuitive sense, says Giovanni Amelino-Camelia, a theoretical physicist at the University of Rome, La Sapienza. After all, when light flows through water or air, its speed depends on its energy; perhaps foamy spacetime has the same effect.
Researchers might spot the tiny differences in high-energy light that had traveled far enough for faster photons to pull ahead of slower ones. In 1998, Amelino-Camelia and colleagues suggested that astronomers scrutinize gamma ray bursts--enormous extragalactic explosions that last only seconds--for evidence that rays of different energy reach Earth at different times. Such data will be collected by NASA's Gamma-ray Large Area Space Telescope (GLAST).
But 2 years before the launch of GLAST, Stecker and others have shown that Lorentz invariance holds firm. Stecker and colleagues studied gamma rays from the hearts of the galaxies Markarian 421 and Markarian 501, some 450 million light-years from Earth. En route the rays pass through a thin haze of infrared photons that fill intergalactic space. If Lorentz invariance were violated, the gamma rays would zip right through the haze. According to special relativity, however, the highest energy gamma rays should collide with the infrared photons to make electron-antielectron pairs. This process should soak up gamma rays above a well-defined cutoff energy--just what the researchers observed, Stecker reports in a paper to be published in the journal Astroparticle Physics.
Gamma rays from the Crab Nebula also bear out Einstein's theory, gravitation theorist Ted Jacobson and colleagues at the University of Maryland, College Park, report in this week's issue of Nature. The rays come from extremely energetic electrons spiraling in the magnetic fields inside the gargantuan cloud of gas. If Lorentz invariance were violated, the electrons would slam up against a virtual speed limit slower than the speed of light. From the energy of the gamma rays, however, Jacobson and colleagues deduced that the electrons were traveling within a 10-billion-billionth of the speed of light--even stronger evidence that Einstein was right.
A loophole in special relativity "would have been great," Jacobson says. "We're desperate for some observational input into quantum gravity." But the new results are just that, says Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "I think it's great," Smolin says, "because it means that physically plausible hypotheses are being confronted with experimental data."
The results sink several quantum gravity theories, but not string theory, which assumes that every particle is a little loop of "superstring," or a leading alternative called loop quantum gravity (Science, 8 November 2002, p. 1166). Because neither requires violations of Lorentz invariance, both theories remain viable for now--and quantum gravity remains undetectable.
Originated from: http://www.sciencemag.org/cgi/content/full/301/5637/1169a?etoc
For 5 years, physicists have hoped that a flaw in Einstein's special theory of relativity might reveal that space and time aren't smooth at the smallest scale, but fuzzy and foaming. Now, that tantalizing prospect has vanished in a puff of gamma rays. Two independent measurements of cosmic gamma rays show that Einstein was right after all--and that current plans to detect the foam are doomed. "The results rule out these possibilities on empirical grounds," says Floyd Stecker, a theoretical astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The frothiness of space and time is predicted by many theories that attempt to meld Einstein's theory of gravity and quantum mechanics. Physicists hoped to detect it by finding a hole in Einstein's dictum that it is meaningless to say an object is moving or stationary relative to the universe, a principle known as Lorentz invariance. One consequence of the principle is that all particles of light, or photons, travel through empty space at the same speed regardless of how much energy they pack.
In recent years, however, various quantum gravity theories have suggested that Lorentz invariance might not hold. In that case, a photon's speed would vary with its energy, so that light of different wavelengths would travel at slightly different rates. That would make intuitive sense, says Giovanni Amelino-Camelia, a theoretical physicist at the University of Rome, La Sapienza. After all, when light flows through water or air, its speed depends on its energy; perhaps foamy spacetime has the same effect.
Researchers might spot the tiny differences in high-energy light that had traveled far enough for faster photons to pull ahead of slower ones. In 1998, Amelino-Camelia and colleagues suggested that astronomers scrutinize gamma ray bursts--enormous extragalactic explosions that last only seconds--for evidence that rays of different energy reach Earth at different times. Such data will be collected by NASA's Gamma-ray Large Area Space Telescope (GLAST).
But 2 years before the launch of GLAST, Stecker and others have shown that Lorentz invariance holds firm. Stecker and colleagues studied gamma rays from the hearts of the galaxies Markarian 421 and Markarian 501, some 450 million light-years from Earth. En route the rays pass through a thin haze of infrared photons that fill intergalactic space. If Lorentz invariance were violated, the gamma rays would zip right through the haze. According to special relativity, however, the highest energy gamma rays should collide with the infrared photons to make electron-antielectron pairs. This process should soak up gamma rays above a well-defined cutoff energy--just what the researchers observed, Stecker reports in a paper to be published in the journal Astroparticle Physics.
Gamma rays from the Crab Nebula also bear out Einstein's theory, gravitation theorist Ted Jacobson and colleagues at the University of Maryland, College Park, report in this week's issue of Nature. The rays come from extremely energetic electrons spiraling in the magnetic fields inside the gargantuan cloud of gas. If Lorentz invariance were violated, the electrons would slam up against a virtual speed limit slower than the speed of light. From the energy of the gamma rays, however, Jacobson and colleagues deduced that the electrons were traveling within a 10-billion-billionth of the speed of light--even stronger evidence that Einstein was right.
A loophole in special relativity "would have been great," Jacobson says. "We're desperate for some observational input into quantum gravity." But the new results are just that, says Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. "I think it's great," Smolin says, "because it means that physically plausible hypotheses are being confronted with experimental data."
The results sink several quantum gravity theories, but not string theory, which assumes that every particle is a little loop of "superstring," or a leading alternative called loop quantum gravity (Science, 8 November 2002, p. 1166). Because neither requires violations of Lorentz invariance, both theories remain viable for now--and quantum gravity remains undetectable.
Originated from: http://www.sciencemag.org/cgi/content/full/301/5637/1169a?etoc
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