76
|
DiCarlo L, Lynch HJ, Johnson AC, Childress LI, Crockett K, Marcus CM, Hanson MP, Gossard AC. Differential charge sensing and charge delocalization in a tunable double quantum dot. PHYSICAL REVIEW LETTERS 2004; 92:226801. [PMID: 15245249 DOI: 10.1103/physrevlett.92.226801] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/12/2003] [Indexed: 05/24/2023]
Abstract
We report measurements of a tunable double quantum dot, operating in the quantum regime, with integrated local charge sensors. The spatial resolution of the sensors allows the charge distribution within the double dot system to be resolved at fixed total charge. We use this readout scheme to investigate charge delocalization as a function of temperature and strength of tunnel coupling, demonstrating that local charge sensing can be used to accurately determine the interdot coupling in the absence of transport.
Collapse
|
77
|
Gossard AC, Hindermann DK, Robin MB, Kuebler NA, Geballe TH. Nuclear Magnetic Resonance and Superconductivity in the Clathrate Salt[Ag7O8]+HF2-. J Am Chem Soc 2004. [DOI: 10.1021/ja01002a600] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
|
78
|
Craig NJ, Taylor JM, Lester EA, Marcus CM, Hanson MP, Gossard AC. Tunable Nonlocal Spin Control in a Coupled-Quantum Dot System. Science 2004; 304:565-7. [PMID: 15044752 DOI: 10.1126/science.1095452] [Citation(s) in RCA: 303] [Impact Index Per Article: 15.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
The effective interaction between magnetic impurities in metals that can lead to various magnetic ground states often competes with a tendency for electrons near impurities to screen the local moment (known as the Kondo effect). The simplest system exhibiting the richness of this competition, the two-impurity Kondo system, was realized experimentally in the form of two quantum dots coupled through an open conducting region. We demonstrate nonlocal spin control by suppressing and splitting Kondo resonances in one quantum dot by changing the electron number and coupling of the other dot. The results suggest an approach to nonlocal spin control that may be relevant to quantum information processing.
Collapse
|
79
|
Butov LV, Levitov LS, Mintsev AV, Simons BD, Gossard AC, Chemla DS. Formation mechanism and low-temperature instability of exciton rings. PHYSICAL REVIEW LETTERS 2004; 92:117404. [PMID: 15089167 DOI: 10.1103/physrevlett.92.117404] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/06/2003] [Indexed: 05/24/2023]
Abstract
The macroscopic rings observed in the photoluminescence patterns of excitons in coupled quantum wells are explained by a mechanism of carrier imbalance, transport, and recombination. The rings originate from the spatial separation of p and n carriers, and occur at the interface of the p and n domains, where excitons are generated. We explore the states of excitons in the ring over a range of temperatures down to 380 mK and report a transition of the ring into a periodic array of aggregates, a new low-temperature ordered exciton state.
Collapse
|
80
|
Abstract
Degenerate exciton systems have been produced in quasi-two-dimensional confined areas in semiconductor coupled quantum well structures. We observed contractions of clouds containing tens of thousands of excitons within areas as small as (10 micron)2 near 10 kelvin. The spatial and energy distributions of optically active excitons were determined by measuring photoluminescence as a function of temperature and laser excitation and were used as thermodynamic quantities to construct the phase diagram of the exciton system, which demonstrates the existence of distinct phases. Understanding the formation mechanisms of these degenerate exciton systems can open new opportunities for the realization of Bose-Einstein condensation in the solid state.
Collapse
|
81
|
Kato Y, Myers RC, Gossard AC, Awschalom DD. Coherent spin manipulation without magnetic fields in strained semiconductors. Nature 2004; 427:50-3. [PMID: 14702080 DOI: 10.1038/nature02202] [Citation(s) in RCA: 410] [Impact Index Per Article: 20.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2003] [Accepted: 11/03/2003] [Indexed: 11/09/2022]
Abstract
A consequence of relativity is that in the presence of an electric field, the spin and momentum states of an electron can be coupled; this is known as spin-orbit coupling. Such an interaction opens a pathway to the manipulation of electron spins within non-magnetic semiconductors, in the absence of applied magnetic fields. This interaction has implications for spin-based quantum information processing and spintronics, forming the basis of various device proposals. For example, the concept of spin field-effect transistors is based on spin precession due to the spin-orbit coupling. Most studies, however, focus on non-spin-selective electrical measurements in quantum structures. Here we report the direct measurement of coherent electron spin precession in zero magnetic field as the electrons drift in response to an applied electric field. We use ultrafast optical techniques to spatiotemporally resolve spin dynamics in strained gallium arsenide and indium gallium arsenide epitaxial layers. Unexpectedly, we observe spin splitting in these simple structures arising from strain in the semiconductor films. The observed effect provides a flexible approach for enabling electrical control over electron spins using strain engineering. Moreover, we exploit this strain-induced field to electrically drive spin resonance with Rabi frequencies of up to approximately 30 MHz.
Collapse
|
82
|
Poggio M, Steeves GM, Myers RC, Kato Y, Gossard AC, Awschalom DD. Local manipulation of nuclear spin in a semiconductor quantum well. PHYSICAL REVIEW LETTERS 2003; 91:207602. [PMID: 14683395 DOI: 10.1103/physrevlett.91.207602] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/31/2003] [Indexed: 05/24/2023]
Abstract
The shaping of nuclear spin polarization profiles and the induction of nuclear resonances are demonstrated within a parabolic quantum well using an externally applied gate voltage. Voltage control of the electron and hole wave functions results in nanometer-scale sheets of polarized nuclei positioned along the growth direction of the well. Applying rf voltages across the gates induces resonant spin transitions of selected isotopes. This depolarizing effect depends strongly on the separation of electrons and holes, suggesting that a highly localized mechanism accounts for the observed behavior.
Collapse
|
83
|
Potok RM, Folk JA, Marcus CM, Umansky V, Hanson M, Gossard AC. Spin and polarized current from Coulomb blockaded quantum dots. PHYSICAL REVIEW LETTERS 2003; 91:016802. [PMID: 12906563 DOI: 10.1103/physrevlett.91.016802] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/02/2003] [Indexed: 05/24/2023]
Abstract
We report measurements of spin transitions for GaAs quantum dots in the Coulomb blockade regime and compare ground and excited state transport spectroscopy to direct measurements of the spin polarization of emitted current. Transport spectroscopy reveals both spin-increasing and spin-decreasing transitions, as well as higher-spin ground states, and allows g factors to be measured down to a single electron. The spin of emitted current in the Coulomb blockade regime, measured using spin-sensitive electron focusing, is found to be polarized along the direction of the applied magnetic field regardless of the ground state spin transition.
Collapse
|
84
|
Miller JB, Zumbühl DM, Marcus CM, Lyanda-Geller YB, Goldhaber-Gordon D, Campman K, Gossard AC. Gate-controlled spin-orbit quantum interference effects in lateral transport. PHYSICAL REVIEW LETTERS 2003; 90:076807. [PMID: 12633263 DOI: 10.1103/physrevlett.90.076807] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/19/2002] [Indexed: 05/24/2023]
Abstract
In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.
Collapse
|
85
|
Kato Y, Myers RC, Driscoll DC, Gossard AC, Levy J, Awschalom DD. Gigahertz electron spin manipulation using voltage-controlled g-tensor modulation. Science 2003; 299:1201-4. [PMID: 12543982 DOI: 10.1126/science.1080880] [Citation(s) in RCA: 60] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
We present a scheme that enables gigahertz-bandwidth three-dimensional control of electron spins in a semiconductor heterostructure with the use of a single voltage signal. Microwave modulation of the Landé g tensor produces frequency-modulated electron spin precession. Driving at the Larmor frequency results in g-tensor modulation resonance, which is functionally equivalent to electron spin resonance but without the use of time-dependent magnetic fields. These results provide proof of the concept that quantum spin information can be locally manipulated with the use of high-speed electrical circuits.
Collapse
|
86
|
Brown ER, Bacher A, Driscoll D, Hanson M, Kadow C, Gossard AC. Evidence for a strong surface-plasmon resonance on ErAs nanoparticles in GaAs. PHYSICAL REVIEW LETTERS 2003; 90:077403. [PMID: 12633271 DOI: 10.1103/physrevlett.90.077403] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/01/2002] [Indexed: 05/24/2023]
Abstract
Room-temperature attenuation measurements are made between lambda=0.8 and 10.0 microm on three GaAs epitaxial samples containing layers of ErAs nanoparticles. An asymmetric attenuation peak is observed around 2.5 microm that increases in strength with ErAs density, and is modeled well by a Maxwell-Garnett formulation and semiclassical transport theory. The nanoparticles are assigned a distribution function of oblate spheroids having a minimum volume corresponding to a 1.0-nm sphere. This is consistent with the self-organizing tendency of ErAs in GaAs, and explains the sharp attenuation peak as a spherical-particle surface-plasmon (i.e., Fröhlich) resonance.
Collapse
|
87
|
Zumbühl DM, Miller JB, Marcus CM, Campman K, Gossard AC. Spin-orbit coupling, antilocalization, and parallel magnetic fields in quantum dots. PHYSICAL REVIEW LETTERS 2002; 89:276803. [PMID: 12513231 DOI: 10.1103/physrevlett.89.276803] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/22/2002] [Indexed: 05/24/2023]
Abstract
We investigate antilocalization due to spin-orbit coupling in ballistic GaAs quantum dots. Antilocalization that is prominent in large dots is suppressed in small dots, as anticipated theoretically. Parallel magnetic fields suppress both antilocalization and also, at larger fields, weak localization, consistent with random matrix theory results once orbital coupling of the parallel field is included. In situ control of spin-orbit coupling in dots is demonstrated as a gate-controlled crossover from weak localization to antilocalization.
Collapse
|
88
|
Gossard AC, Jaccarino V. Boron Nuclear Magnetic Resonance in Rare Earth Intermetallic Compounds. ACTA ACUST UNITED AC 2002. [DOI: 10.1088/0370-1328/80/4/309] [Citation(s) in RCA: 20] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
|
89
|
Abstract
There is a rich variety of quantum liquids -- such as superconductors, liquid helium and atom Bose-Einstein condensates -- that exhibit macroscopic coherence in the form of ordered arrays of vortices. Experimental observation of a macroscopically ordered electronic state in semiconductors has, however, remained a challenging and relatively unexplored problem. A promising approach for the realization of such a state is to use excitons, bound pairs of electrons and holes that can form in semiconductor systems. At low densities, excitons are Bose-particles, and at low temperatures, of the order of a few kelvin, excitons can form a quantum liquid -- that is, a statistically degenerate Bose gas or even a Bose-Einstein condensate. Here we report photoluminescence measurements of a quasi-two-dimensional exciton gas in GaAs/AlGaAs coupled quantum wells and the observation of a macroscopically ordered exciton state. Our spatially resolved measurements reveal fragmentation of the ring-shaped emission pattern into circular structures that form periodic arrays over lengths up to 1 mm.
Collapse
|
90
|
Fromer NA, Lai CE, Chemla DS, Perakis IE, Driscoll D, Gossard AC. Dynamics of inter-landau-level excitations of a two-dimensional electron gas in the quantum Hall regime. PHYSICAL REVIEW LETTERS 2002; 89:067401. [PMID: 12190609 DOI: 10.1103/physrevlett.89.067401] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/14/2002] [Indexed: 05/23/2023]
Abstract
The femtosecond inter-Landau-level dynamics of a two-dimensional electron gas in a large magnetic field is investigated by degenerate four-wave mixing on modulation doped quantum wells. We observe a large transfer of oscillator strength to the lowest Landau level, and unusual dynamics due to Coulomb correlation. We interpret the effects using a model based on shakeup of the electron gas.
Collapse
|
91
|
Butov LV, Lai CW, Ivanov AL, Gossard AC, Chemla DS. Towards Bose-Einstein condensation of excitons in potential traps. Nature 2002; 417:47-52. [PMID: 11986661 DOI: 10.1038/417047a] [Citation(s) in RCA: 104] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
An exciton is an electron-hole bound pair in a semiconductor. In the low-density limit, it is a composite Bose quasi-particle, akin to the hydrogen atom. Just as in dilute atomic gases, reducing the temperature or increasing the exciton density increases the occupation numbers of the low-energy states leading to quantum degeneracy and eventually to Bose-Einstein condensation (BEC). Because the exciton mass is small--even smaller than the free electron mass--exciton BEC should occur at temperatures of about 1 K, many orders of magnitude higher than for atoms. However, it is in practice difficult to reach BEC conditions, as the temperature of excitons can considerably exceed that of the semiconductor lattice. The search for exciton BEC has concentrated on long-lived excitons: the exciton lifetime against electron-hole recombination therefore should exceed the characteristic timescale for the cooling of initially hot photo-generated excitons. Until now, all experiments on atom condensation were performed on atomic gases confined in the potential traps. Inspired by these experiments, and using specially designed semiconductor nanostructures, we have collected quasi-two-dimensional excitons in an in-plane potential trap. Our photoluminescence measurements show that the quasi-two-dimensional excitons indeed condense at the bottom of the traps, giving rise to a statistically degenerate Bose gas.
Collapse
|
92
|
Salis G, Kato Y, Ensslin K, Driscoll DC, Gossard AC, Awschalom DD. Electrical control of spin coherence in semiconductor nanostructures. Nature 2001; 414:619-22. [PMID: 11740554 DOI: 10.1038/414619a] [Citation(s) in RCA: 59] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
The processing of quantum information based on the electron spin degree of freedom requires fast and coherent manipulation of local spins. One approach is to provide spatially selective tuning of the spin splitting--which depends on the g-factor--by using magnetic fields, but this requires their precise control at reduced length scales. Alternative proposals employ electrical gating and spin engineering in semiconductor heterostructures involving materials with different g-factors. Here we show that spin coherence can be controlled in a specially designed AlxGa1-xAs quantum well in which the Al concentration x is gradually varied across the structure. Application of an electric field leads to a displacement of the electron wavefunction within the quantum well, and because the electron g-factor varies strongly with x, the spin splitting is therefore also changed. Using time-resolved optical techniques, we demonstrate gate-voltage-mediated control of coherent spin precession over a 13-GHz frequency range in a fixed magnetic field of 6 T, including complete suppression of precession, reversal of the sign of g, and operation up to room temperature.
Collapse
|
93
|
Butov LV, Lai CW, Chemla DS, Lozovik YE, Campman KL, Gossard AC. Observation of magnetically induced effective-mass enhancement of quasi-2D excitons. PHYSICAL REVIEW LETTERS 2001; 87:216804. [PMID: 11736364 DOI: 10.1103/physrevlett.87.216804] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/07/2001] [Indexed: 05/23/2023]
Abstract
We present the first measurements of the dispersion relation of a quasi-2D magnetoexciton. We demonstrate that the magnetoexciton effective mass is determined by the coupling between the center-of-mass motion and internal structure and becomes overwhelmingly larger than the sum of the electron and hole masses in high magnetic fields.
Collapse
|
94
|
Kawakami RK, Kato Y, Hanson M, Malajovich I, Stephens JM, Johnston-Halperin E, Salis G, Gossard AC, Awschalom DD. Ferromagnetic imprinting of nuclear spins in semiconductors. Science 2001; 294:131-4. [PMID: 11588255 DOI: 10.1126/science.1063186] [Citation(s) in RCA: 104] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
We examine how a ferromagnetic layer affects the coherent electron spin dynamics in a neighboring gallium arsenide semiconductor. Ultrafast optical pump-probe measurements reveal that the spin dynamics are unexpectedly dominated by hyperpolarized nuclear spins that align along the ferromagnet's magnetization. We find evidence that photoexcited carriers acquire spin-polarization from the ferromagnet, and dynamically polarize these nuclear spins. The resulting hyperfine fields are as high as 9000 gauss in small external fields (less than 1000 gauss), enabling ferromagnetic control of local electron spin coherence.
Collapse
|
95
|
Williams JB, Sherwin MS, Maranowski KD, Gossard AC. Dissipation of intersubband plasmons in wide quantum wells. PHYSICAL REVIEW LETTERS 2001; 87:037401. [PMID: 11461588 DOI: 10.1103/physrevlett.87.037401] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/13/2000] [Indexed: 05/23/2023]
Abstract
This Letter reports detailed measurements of the dissipation times tau(d) of approximately 10 meV intersubband (ISB) plasmons, and of the (single-particle) transport lifetimes tau(mu), in a remotely doped 40 nm GaAs quantum well. Introduced here as the time for ISB plasmons to dissipate into other modes of the electron gas, tau(d) is deduced from the homogeneous ISB absorption linewidth, measured as a function of sheet concentration and perpendicular dc electric field. Modeling in this and the next Letter [C. A. Ullrich and G. Vignale, Phys. Rev. Lett. 87, 037402 (2001)] indicates that scattering from rough interfaces dominates tau(d), while scattering from ionized impurities dominates tau(mu).
Collapse
|
96
|
Kycia JB, Chen J, Therrien R, Kurdak C, Campman KL, Gossard AC, Clarke J. Effects of dissipation on a superconducting single electron transistor. PHYSICAL REVIEW LETTERS 2001; 87:017002. [PMID: 11461486 DOI: 10.1103/physrevlett.87.017002] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/14/2000] [Indexed: 05/23/2023]
Abstract
We measure the effect of dissipation on the minimum zero-bias conductance, G(min)0, of a superconducting single electron transistor (sSET) capacitively coupled to a two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. Depleting the 2DEG with a back gate voltage decreases the dissipation experienced by the sSET in situ. We find that G(min)0 increases as the dissipation is increased or the temperature is reduced; the functional forms of these dependences are compared with the model of Wilhelm et al. in which the leads coupled to the sSET are represented by lossy transmission lines.
Collapse
|
97
|
Butov LV, Ivanov AL, Imamoglu A, Littlewood PB, Shashkin AA, Dolgopolov VT, Campman KL, Gossard AC. Stimulated scattering of indirect excitons in coupled quantum wells: signature of a degenerate Bose-gas of excitons. PHYSICAL REVIEW LETTERS 2001; 86:5608-5611. [PMID: 11415313 DOI: 10.1103/physrevlett.86.5608] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/19/2001] [Indexed: 05/23/2023]
Abstract
We observe and analyze strongly nonlinear photoluminescence kinetics of indirect excitons in GaAs/AlGaAs coupled quantum wells at low bath temperatures, > or = 50 mK. The long recombination lifetime of indirect excitons promotes accumulation of these Bose particles in the lowest energy states and allows the photoexcited excitons to cool down to temperatures where the dilute 2D gas of indirect excitons becomes statistically degenerate. Our main result--a strong enhancement of the exciton scattering rate to the low-energy states with increasing concentration of the indirect excitons--reveals bosonic stimulation of exciton scattering, which is a signature of a degenerate Bose-gas of excitons.
Collapse
|
98
|
Harris JG, Knobel R, Maranowski KD, Gossard AC, Samarth N, Awschalom DD. Magnetization measurements of magnetic two-dimensional electron gases. PHYSICAL REVIEW LETTERS 2001; 86:4644-4647. [PMID: 11384304 DOI: 10.1103/physrevlett.86.4644] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/22/2000] [Indexed: 05/23/2023]
Abstract
We directly measure the magnetization of both the conduction electrons and Mn2+ ions in (Zn,Cd,Mn)Se two-dimensional electron gases (2DEGs) by integrating them into ultrasensitive micromechanical magnetometers. The interplay between spin and orbital energy in these magnetic 2DEGs causes Landau level degeneracies at the Fermi energy. These Landau level crossings result in novel features in the de Haas-van Alphen oscillations, which are quantitatively reproduced by a simple model.
Collapse
|
99
|
Topinka MA, LeRoy BJ, Westervelt RM, Shaw SE, Fleischmann R, Heller EJ, Maranowski KD, Gossard AC. Coherent branched flow in a two-dimensional electron gas. Nature 2001; 410:183-6. [PMID: 11242072 DOI: 10.1038/35065553] [Citation(s) in RCA: 113] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
Semiconductor nanostructures based on two-dimensional electron gases (2DEGs) could form the basis of future devices for sensing, information processing and quantum computation. Although electron transport in 2DEG nanostructures has been well studied, and many remarkable phenomena have already been discovered (for example, weak localization, quantum chaos, universal conductance fluctuations), fundamental aspects of the electron flow through these structures have so far not been clarified. However, it has recently become possible to image current directly through 2DEG devices using scanning probe microscope techniques. Here, we use such a technique to observe electron flow through a narrow constriction in a 2DEG-a quantum point contact. The images show that the electron flow from the point contact forms narrow, branching strands instead of smoothly spreading fans. Our theoretical study of this flow indicates that this branching of current flux is due to focusing of the electron paths by ripples in the background potential. The strands are decorated by interference fringes separated by half the Fermi wavelength, indicating the persistence of quantum mechanical phase coherence in the electron flow. These findings may have important implications for a better understanding of electron transport in 2DEGs and for the design of future nanostructure devices.
Collapse
|
100
|
Eom J, Cho H, Kang W, Campman KL, Gossard AC, Bichler M, Wegscheider W. Quantum hall ferromagnetism in a two-dimensional electron system. Science 2000; 289:2320-3. [PMID: 11009411 DOI: 10.1126/science.289.5488.2320] [Citation(s) in RCA: 64] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/02/2022]
Abstract
Experiments on a nearly spin degenerate two-dimensional electron system reveals unusual hysteretic and relaxational transport in the fractional quantum Hall effect regime. The transition between the spin-polarized (with fill fraction nu = 1/3) and spin-unpolarized (nu = 2/5) states is accompanied by a complicated series of hysteresis loops reminiscent of a classical ferromagnet. In correlation with the hysteresis, magnetoresistance can either grow or decay logarithmically in time with remarkable persistence and does not saturate. In contrast to the established models of relaxation, the relaxation rate exhibits an anomalous divergence as temperature is reduced. These results indicate the presence of novel two-dimensional ferromagnetism with a complicated magnetic domain dynamic.
Collapse
|