共查询到20条相似文献，搜索用时 46 毫秒
1.
An electron in a solid, that is, bound to or nearly localized on the specific atomic site, has three attributes: charge, spin, and orbital. The orbital represents the shape of the electron cloud in solid. In transition-metal oxides with anisotropic-shaped d-orbital electrons, the Coulomb interaction between the electrons (strong electron correlation effect) is of importance for understanding their metal-insulator transitions and properties such as high-temperature superconductivity and colossal magnetoresistance. The orbital degree of freedom occasionally plays an important role in these phenomena, and its correlation and/or order-disorder transition causes a variety of phenomena through strong coupling with charge, spin, and lattice dynamics. An overview is given here on this "orbital physics," which will be a key concept for the science and technology of correlated electrons. 相似文献
2.
We observed negative Coulomb drag for parallel coupled quantum wires, in which electrons flow in the opposite directions between the wires. This only occurred under the conditions of strong correlation in the wires, that is, low density, high magnetic field, and low temperature, and cannot be addressed by a standard theory of momentum transfer. We propose a Coulomb drag model in which formation of a Wigner crystal state in the drag wire and a particle-like state in the drive wire is taken into account. 相似文献
3.
Li L Checkelsky JG Hor YS Uher C Hebard AF Cava RJ Ong NP 《Science (New York, N.Y.)》2008,321(5888):547-550
The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy. 相似文献
4.
Price AS Savchenko AK Narozhny BN Allison G Ritchie DA 《Science (New York, N.Y.)》2007,316(5821):99-102
The Coulomb drag in a system of two parallel layers is the result of electron-electron interaction between the layers. We have observed reproducible fluctuations of the drag, both as a function of magnetic field and electron concentration, which are a manifestation of quantum interference of electrons in the layers. At low temperatures the fluctuations exceed the average drag, giving rise to random changes of the sign of the drag. The fluctuations are found to be much larger than previously expected, and we propose a model that explains their enhancement by considering fluctuations of local electron properties. 相似文献
5.
Auslaender OM Yacoby A de Picciotto R Baldwin KW Pfeiffer LN West KW 《Science (New York, N.Y.)》2002,295(5556):825-828
The collective excitation spectrum of interacting electrons in one dimension has been measured by controlling the energy and momentum of electrons tunneling between two closely spaced, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. The excitation spectrum deviates from the noninteracting spectrum, attesting to the importance of Coulomb interactions. An observed 30% enhancement of the excitation velocity relative to noninteracting electrons with the same density, a parameter determined experimentally, is consistent with theories on interacting electrons in one dimension. In short wires, 6 and 2 micrometers long, finite size effects, resulting from the breaking of translational invariance, are observed. 相似文献
6.
Coherent short-pulse laser excitation has been used to control the approximate energy and relative proximity of two valence electrons within the same alkaline-earth atom, thereby providing insight into the dynamical evolution of a three-body Coulomb system. Our time-domain experiments enable direct experimental study of the electron dynamics at the classical limit of a two-electron atom. As an example, we look at the mechanism of autoionization for one two-electron configuration class and find that the doubly excited atom decays through a single violent electron-electron collision rather than a gradual exchange of energy between the electrons. 相似文献
7.
Nellist PD Chisholm MF Dellby N Krivanek OL Murfitt MF Szilagyi ZS Lupini AR Borisevich A Sides WH Pennycook SJ 《Science (New York, N.Y.)》2004,305(5691):1741
Despite the use of electrons with wavelengths of just a few picometers, spatial resolution in a transmission electron microscope (TEM) has been limited by spherical aberration to typically around 0.15 nanometer. Individual atomic columns in a crystalline lattice can therefore only be imaged for a few low-order orientations, limiting the range of defects that can be imaged at atomic resolution. The recent development of spherical aberration correctors for transmission electron microscopy allows this limit to be overcome. We present direct images from an aberration-corrected scanning TEM that resolve a lattice in which the atomic columns are separated by less than 0.1 nanometer. 相似文献
8.
An atomic force microscope was used to study single-electron motion in nanotube quantum dots. By applying a voltage to the microscope tip, the number of electrons occupying the quantum dot could be changed, causing Coulomb oscillations in the nanotube conductance. Spatial maps of these oscillations were used to locate individual dots and to study the electrostatic coupling between the dot and the tip. The electrostatic forces associated with single electrons hopping on and off the quantum dot were also measured. These forces changed the amplitude, frequency, and quality factor of the cantilever oscillation, demonstrating how single-electron motion can interact with a mechanical oscillator. 相似文献
9.
Most of our knowledge of the structures of free molecules comes from indirect observations that focus on other properties (for example, energy or mass spectra) that depend on structure. Recently, a new method has been developed that instead yields geometrical images of individual molecules. This method takes advantage of the large Coulomb repulsion of the nuclei within molecules rapidly stripped of their electrons. The first experiments with this new technique have already revealed structural images of several important species, such as positively charged methane ions. 相似文献
10.
N Ge CM Wong RL Lingle JD McNeill KJ Gaffney CB Harris 《Science (New York, N.Y.)》1998,279(5348):202-205
The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes. 相似文献
11.
Heathman S Haire RG Le Bihan T Lindbaum A Idiri M Normile P Li S Ahuja R Johansson B Lander GH 《Science (New York, N.Y.)》2005,309(5731):110-113
Curium lies at the center of the actinide series and has a half-filled shell with seven 5f electrons spatially residing inside its radon core. As a function of pressure, curium exhibits five different crystallographic phases up to 100 gigapascals, of which all but one are also found in the preceding element, americium. We describe here a structure in curium, Cm III, with monoclinic symmetry, space group C2/c, found at intermediate pressures (between 37 and 56 gigapascals). Ab initio electronic structure calculations agree with the observed sequence of structures and establish that it is the spin polarization of curium's 5f electrons that stabilizes Cm III. The results reveal that curium is one of a few elements that has a lattice structure stabilized by magnetism. 相似文献
12.
Chen G Bonadeo NH Steel DG Gammon D Katzer DS Park D Sham LJ 《Science (New York, N.Y.)》2000,289(5486):1906-1909
Optically induced entanglement is identified by the spectrum of the phase-sensitive homodyne-detected coherent nonlinear optical response in a single gallium arsenide quantum dot. The electron-hole entanglement involves two magneto-excitonic states differing in transition energy and polarization. The strong coupling needed for entanglement is provided through the Coulomb interaction involving the electrons and holes. The result presents a first step toward the optical realization of quantum logic operations using two or more quantum dots. 相似文献
13.
Qazilbash MM Brehm M Chae BG Ho PC Andreev GO Kim BJ Yun SJ Balatsky AV Maple MB Keilmann F Kim HT Basov DN 《Science (New York, N.Y.)》2007,318(5857):1750-1753
Electrons in correlated insulators are prevented from conducting by Coulomb repulsion between them. When an insulator-to-metal transition is induced in a correlated insulator by doping or heating, the resulting conducting state can be radically different from that characterized by free electrons in conventional metals. We report on the electronic properties of a prototypical correlated insulator vanadium dioxide in which the metallic state can be induced by increasing temperature. Scanning near-field infrared microscopy allows us to directly image nanoscale metallic puddles that appear at the onset of the insulator-to-metal transition. In combination with far-field infrared spectroscopy, the data reveal the Mott transition with divergent quasi-particle mass in the metallic puddles. The experimental approach used sets the stage for investigations of charge dynamics on the nanoscale in other inhomogeneous correlated electron systems. 相似文献
14.
Fan H Yang K Boye DM Sigmon T Malloy KJ Xu H López GP Brinker CJ 《Science (New York, N.Y.)》2004,304(5670):567-571
We report the synthesis of a new nanocrystal (NC) mesophase through self-assembly of water-soluble NC micelles with soluble silica. The mesophase comprises gold nanocrystals arranged within a silica matrix in a face-centered cubic lattice with cell dimensions that are adjustable through control of the nanocrystal diameter and/or the alkane chain lengths of the primary alkanethiol stabilizing ligands or the surrounding secondary surfactants. Under kinetically controlled silica polymerization conditions, evaporation drives self-assembly of NC micelles into ordered NC/silica thin-film mesophases during spin coating. The intermediate NC micelles are water soluble and of interest for biolabeling. Initial experiments on a metal-insulator-metal capacitor fabricated with an ordered three-dimensional gold nanocrystal/silica array as the "insulator" demonstrated collective Coulomb blockade behavior below 100 kelvin and established the current-voltage scaling relationship for a well-defined three-dimensional array of Coulomb islands. 相似文献
15.
Delahaye J Hassel J Lindell R Sillanpää M Paalanen M Seppä H Hakonen P 《Science (New York, N.Y.)》2003,299(5609):1045-1048
We used the band structure of a mesoscopic Josephson junction to construct low-noise amplifiers. By taking advantage of the quantum dynamics of a Josephson junction, i.e., the interplay of interlevel transitions and the Coulomb blockade of Cooper pairs, we created transistor-like devices, Bloch oscillating transistors, with considerable current gain and high-input impedance. In these transistors, the correlated supercurrent of Cooper pairs is controlled by a small base current made up of single electrons. Our devices reached current and power gains on the order of 30 and 5, respectively. The noise temperature was estimated to be around 1 kelvin, but noise temperatures of less than 0.1 kelvin can be realistically achieved. These devices provide quantum-electronic building blocks that will be useful at low temperatures in low-noise circuit applications with an intermediate impedance level. 相似文献
16.
Much current experimental research on transport in molecular junctions focuses on finite voltages, where substantial polarization-induced nonlinearities may result in technologically relevant device-type responses. Because molecules have strong polarization responses to changing charge state or external field, molecules isolated between electrodes can show strongly nonlinear current-voltage responses. For small applied voltages (up to approximately 0.3 volt), weak interaction between transporting electrons and molecular vibrations provides the basis for inelastic electron tunneling spectroscopy. At higher voltages and for certain time scale regimes, strong coupling effects occur, including Coulomb blockade, negative differential resistance, dynamical switching and switching noise, current hysteresis, heating, and chemical reactions. We discuss a general picture for such phenomena that arise from charging, strong correlation, and polarization (electronic and vibrational) effects in the molecule and at the interface. 相似文献
17.
Dai X Savrasov SY Kotliar G Migliori A Ledbetter H Abrahams E 《Science (New York, N.Y.)》2003,300(5621):953-955
We constructed computer-based simulations of the lattice dynamical properties of plutonium using an electronic structure method, which incorporates correlation effects among the f-shell electrons and calculates phonon spectra at arbitrary wavelengths. Our predicted spectrum for the face-centered cubic delta phase agrees well with experiments in the elastic limit and explains unusually large shear anisotropy of this material. The spectrum of the body-centered cubic phase shows an instability at zero temperature over a broad region of the wave vectors, indicating that this phase is highly anharmonic and can be stabilized at high temperatures by its phonon entropy. 相似文献
18.
Hot electrons in semiconductors lose their energy very quickly (within picoseconds) to lattice vibrations. Slowing this energy loss could prove useful for more efficient photovoltaic or infrared devices. With their well-separated electronic states, quantum dots should display slow relaxation, but other mechanisms have made it difficult to observe. We report slow intraband relaxation (>1 nanosecond) in colloidal quantum dots. The small cadmium selenide (CdSe) dots, with an intraband energy separation of approximately 0.25 electron volts, are capped by an epitaxial zinc selenide (ZnSe) shell. The shell is terminated by a CdSe passivating layer to remove electron traps and is covered by ligands of low infrared absorbance (alkane thiols) at the intraband energy. We found that relaxation is markedly slowed with increasing ZnSe shell thickness. 相似文献
19.
A quantum critical point transforms the behavior of electrons so strongly that new phases of matter can emerge. The interactions at play are known to fall outside the scope of the standard model of metals, but a fundamental question remains: Is the basic concept of a quasiparticle-a fermion with renormalized mass-still valid in such systems? The Wiedemann-Franz law, which states that the ratio of heat and charge conductivities in a metal is a universal constant in the limit of zero temperature, is a robust consequence of Fermi-Dirac statistics. We report a violation of this law in the heavy-fermion metal CeCoIn5 when tuned to its quantum critical point, depending on the direction of electron motion relative to the crystal lattice, which points to an anisotropic destruction of the Fermi surface. 相似文献
20.
Incremental single-electron charging of size-quantized states has been observed in the well in submicrometer double-barrier resonant tunneling devices. In order to distinguish between the effects of size quantization and the single-electron charging, the heterostructure material was grown asymmetrical so that one barrier is substantially less transparent than the other. In the voltage polarity such that the emitter barrier is more transparent than the collector barrier, electrons accumulate in the well; incremental electron occupation of the well is accompanied by Coulomb blockade, which leads to sharp steps of the tunneling current. In the opposite voltage polarity the emitter barrier is less transparent than the collector barrier and the tunneling current reflects resonant tunneling through size-quantized well states. 相似文献