Quantum Interference Of Electrons Research Articles

Magneto-conductance fluctuations in a V-grooved GaAs quantum-wire

We study the magneto-resistance of a V-grooved GaAs quantum-wire field-effect transistor (QWR-FET) realized by selective growth of metalorganic vapor phase epitaxy. At low temperatures, the magneto-resistance of the wire shows reproducible fluctuations resulting from the quantum interference of electrons in the wire. The gate-voltage and temperature dependence of the conductance fluctuations are studied, and the latter reveal evidence for the robust phase coherence of electrons in the V-grooved QWR.

Experimental observation of quantum corrections to electrical resistivity in nanocrystalline soft magnetic alloys

X-ray diffraction patterns of nanocrystalline Fe-Cu-Nb-Si-B (FINEMET) alloys reveal that bcc α-Fe/α-FeSi crystallites with the average grain size of 20(5) nm are dispersed in amorphous matrix. Enhanced electron—electron interaction (EEI) and quantum interference (QI) effects as well as electron-magnon (and/or electron-spin fluctuation) scattering turn out to be the main mechanisms that govern the temperature dependence of resistivity. Of all the inelastic scattering processes, inelastic electron-phonon scattering is the most effective mechanism to destroy phase coherence of electron wave functions. The diffusion constant, density of states at the Fermi level and the inelastic scattering time have been estimated, for the first time, for the alloys in question

Quantum interference of conduction electrons in carbon nanotubes

We have given a multiple-cycle quantum interference model and obtained magnetoresistance (MR) expression in the framework of weak localization. The MR expression based upon finite phase-breaking length L ϕ can well explain several experimental results about distinct negative magnetoresistance of carbon nanotubes. The higher-order oscillation peaks with magnetic flux existing in the MR can also be explained consistently by our model. And a new method to measure the phase-breaking length L ϕ of the carbon nanotubes has been proposed.

Quantum interference and supercurrent in multiple-barrier proximity structures

We analyze an interplay between the proximity effect and quantum interference of electrons in hybrid structures superconductor-normal metal-superconductor which contain several insulating barriers. We demonstrate that the dc Josephson current in these structures may change qualitatively due to quantum interference of electrons scattered at different interfaces. In junctions with few conducting channels mesoscopic fluctuations of the supercurrent are significant and its amplitude can be strongly enhanced due to resonant effects. In the many channel limit averaging over the scattering phase effectively suppresses interference effects for systems with {\it two} insulating barriers. In that case a standard quasiclassical approach describing scattering at interfaces by means of Zaitsev boundary conditions allows to reproduce the correct results. However, in systems with {\it three} or more barriers the latter approach fails even in the many channel limit. In such systems interference effects remain important in this limit as well. For short junctions these effects result in additional suppression of the Josephson critical current indicating the tendency of the system towards localization. For relatively long junctions interference effects may -- on the contrary -- enhance the supercurrent with respect to the case of independent barriers.

One-dimensional energy dispersion of single-walled carbon nanotubes by resonant electron scattering.

We characterized the energy band dispersion near the Fermi level in single-walled carbon nanotubes using low-temperature scanning tunneling microscopy. Analysis of energy-dependent standing wave oscillations, which result from quantum interference of electrons resonantly scattered by defects, yields a linear energy dispersion near E(F), and indicates the importance of parity in scattering for armchair single-walled carbon nanotubes. Additionally, these data provide values of the tight-binding overlap integral and Fermi wave vector, in good agreement with previous work, but indicate that the electron coherence length is substantially shortened.

Kinetics of exciton photoluminescence in type-II semiconductor superlattices

The exciton decay rate at a rough interface in type-II semiconductor superlattices is investigated. It is shown that the possibility of recombination of the indirect excitons at a plane interface essentially affects the kinetics of the exciton photoluminescence at a rough interface. This is the result of the quantum interference of electrons scattered from the plane interface and at the roughnesses. Expressions that relate the parameters of the luminescence kinetics with the statistical characteristics of the rough interface are obtained. The mean height and length of the roughnesses in GaAs/AlAs superlattices are estimated from the experimental data.

Large magnetoresistance in SrFeO 2.95

A large magnetoresistance effect in a Fe-based perovskite has been observed. A typical magnetoresistance (MR) is 15.5% for SrFeO 2.95 at 5 K, and also a high field induced MR is noted at a temperature well above the antiferromagnetic transition temperature. Magnetization and transport measurements show that two transitions occur at temperatures around 107 and 65 K, respectively. The upper transition appears to correspond to the antiferromagnetic transition. The second transition, at low temperature, shows a resistivity minimum which coincides with the weak anomaly in the χ( T) curve. The upturn in ρ below 65 K arises from a T 1/2 dependence of the conductivity. This upturn in ρ at low temperatures is a manifestation of electron–electron interaction and quantum interference effects in electronic conduction, such as weak localization in this helical magnetic system.

GaN-Based Quantum-Effect Electron Devices Using Quantum Interference of Hot Electron Waves

We theoretically discuss GaN-based quantum-effect electron devices using quantum interference of hot electron waves in the conduction band of the large bandgap collector barrier. In this type of device, the quantum interference of electrons plays a main role in the operation principle and originates the interesting device functions. The device consists of AlN/GaN/AlN double-barrier resonant tunneling (RT) emitter and AlN collector barrier. Double barrier RT emitter provides electrons with narrow energy distribution into the conduction band of the collector barrier, and the modulation of the transmission probability of the hot electron waves occurs due to the quantum interference effect. Multiple negative differential resistance (NDR) with relatively high collector current density is expected from the theoretical results, suggesting new possibilities for a variety of novel device applications.

Thermopower of atomic-size metallic contacts

The thermopower and conductance of atomic-size metallic contacts have been simultaneously measured using a mechanically controllable break junction. For contacts approaching atomic dimensions, abrupt steps in the thermopower are observed which coincide with jumps in the conductance. The measured thermopower for a large number of atomic-size contacts is randomly distributed around the value for large contacts and can be either positive or negative in sign. However, it is suppressed at the quantum value of the conductance G_0 = 2e^2/h. We derive an expression that describes these results in terms of quantum interference of electrons backscattered in the banks.

Magneto-electric Aharonov–Bohm effect in metal rings

The quantum-mechanical phase of the wavefunction of an electron can be changed by electromagnetic potentials, as was predicted by Aharonov and Bohm1 in 1959. Experiments on propagating electron waves in vacuum have revealed both the magnetic2,3,4 and electrostatic5 Aharonov–Bohm effect. Surprisingly, the magnetic effect was also observed in micrometre-sized metal rings6,7,8, demonstrating that electrons keep their phase coherence in such samples despite their diffusive motion. The search for the electrostatic contribution to the electron phase in these metal rings9,10 was hindered by the high conductivity of metal, which makes it difficult to apply a well defined voltage difference across the ring. Here we report measurements of quantum interference of electrons in metal rings that are interrupted by two small tunnel junctions. In these systems, a well defined voltage difference between the two parts of the ring can beapplied. Using these rings we simultaneously explore the influence of magnetic and electrostatic potentials on the Aharonov–Bohm quantum-interference effect, and we demonstrate that these two potentials play interchangeable roles.

Photoluminescence investigation of the carrier confining properties of multiquantum barriers

A comparative luminescence study of two Ga/sub 0.52/In/sub 0.48/P-(Al/sub 0.5/Ga/sub 0.5/)/sub 0.52/In/sub 0.48/P single-quantum-well (SQW) samples with bulk and multiquantum barrier (MQB) barriers is presented. When excess carriers are only created in the quantum wells (QW's) of the samples by resonant excitation using a dye laser, the luminescence efficiency of both samples as a function of temperature is found to be essentially identical. We find, therefore, no evidence for any enhancement in the confining potential of the MQB sample over the bulk barrier sample. From Arrhenius plots of the integrated luminescence intensity, it is found that carrier loss from the QW is dominated by a nonradiative loss mechanism with an activation energy considerably smaller than that expected from direct thermal loss of electrons and holes into the barriers. We suggest that the improved device characteristics reported for lasers containing MQB's is due to effects other than the quantum interference of electrons.

Quantum interference of electrons transmitted throughout a double-barrier resonant tunnelling structure under a perpendicular magnetic field

Recently, we have discovered a fine oscillatory structure (FOS) of the current flowing through resonant tunnelling devices consisting basically of two very thin AlAs barriers separated by a GaAs quantum well. In this work we have found two striking properties of FOS in a magnetic field applied normally to the current flow: (i) a phase shift of the FOS on the bias-voltage scale which is proportional to the squared induction of the magnetic field; (ii) rapid oscillatory-type fall-off in FOS amplitude with increasing magnetic field. We demonstrate, using a simple quantitative model, that both properties can be consistently explained in terms of quantum interference between electrons which escaped from the quantum well and those partly reflected at a potential step on the collector side of the device. The physical conditions necessary for the appearance of FOS are discussed.

Room-temperature observation of multiple negative differential resistance in a metal (CoSi 2)/insulator (CaF 2) quantum interference transistor structure

We report the room-temperature observation of multiple negative differential resistance (NDR) due to quantum interference of hot electrons in a metal (CoSi 2)/insulator (CaF 2) heterstructure. The device for the observation consists of a metal/insulator resonant tunneling emitter for electrons with monochromatic energy and an insulator collector barrier for the quantum interference. The effect of the parasitic collector resistance was reduced by making the device area small with electron-beam lithography, and also leakage current around the device was suppressed. These improvements led to the room-temperature observation of multiple negative differential resistance.

Dephasing and selection rules of interband and intraband polarizations in superlattices

We study the coherent dynamics of interband and intraband polarizations of Bloch oscillations in a GaAs/Al0.7Ga0.3 As semiconductor superlattice. Time-resolved measurements of both polarizations are simultaneously performed by combination of four-wave mixing and an electro-optic detection scheme in one experimental setup. This combination allows us to gain a much more comprehensive picture of the dynamics of a multilevel system. Distinctive differences in the oscillation frequencies and the dephasing of both polarizations are found. They arise from the fact that excitonic quantum beats are observed in four-wave mixing, whereas a quantum interference of electrons in continuum states is detected in the electro-optic signal. As a consequence, the polarizations are governed by different selection rules that explain the pronounced differences in the oscillation frequencies found for specific experimental conditions.

Phase-controlled currents in semiconductors.

The direction of emission of photoexcited electrons in semiconductors is controlled by adjusting the relative phase difference between a midinfrared radiation and its second harmonic. This is achieved by using quantum interference of electrons produced with one- and two-photon bound-to-free intersubband transitions in AlGaAs/GaAs quantum well superlattices.

Enhanced conductance in superconductor semiconductor junctions at zero voltage.

An enhancement of the zero-voltage conductance of a niobium-silicon junction is found at very low temperatures. This enhancement is a result of quantum interference of electrons, near the superconductor semiconductor interface, which results in an increase of Andreev reflection probability.

Observation of X-band electron quantum interference and transport through virtual states in AlxGa1−xAs/AlAs heterostructures

The X-band conduction electron quantum interference and its transport through Γ virtual state were observed in the AlxGa1−xAs/AlAs/AlxGa1−xAs/AlAs/AlxGa1−xAs heterostructures with x=0.42, 0.45, and 0.5. Due to the existence of the deep donors, the carriers in AlxGa1−xAs electrodes were frozen out at low temperature, and the current-voltage (I-V) characteristics could only be measured under illumination. For x=0.45 and 0.5, the interference patterns were clearly observed in the differential conductance (dI/dV) versus voltage plot at 8.2 K and they disappear at about 15 K for x=0.45 and 25 K for x=0.5. For x=0.42, however, the interference pattern is seen only in the d2I/dV2 vs V characteristics under low carrier concentration. The observation of this quantum interference effect leads to the conclusions that the scattering rate for X-band electrons in AlxGa1−xAs was 1.13×1012 s−1 at 25 K for x=0.5 and 8.75×1011 s−1 at 15 K for x=0.45, respectively. For x=0.42, the scattering events at 8.2 K were dominated by electron–electron and X–Γ intervalley scattering.

Correlation energy of conductance fluctuations in ballistic silver point contacts.

We have studied conductance fluctuations of microfabricated silver point contacts, which operate in the ballistic electron-transport regime. The correlation energy E c of the conductance fluctuations has been determined from the voltage correlation of the fluctuations, and from the temperature and modulation voltage dependence of the fluctuation amplitude. For all the devices investigated E c ≃1 meV. This value is in good agreement with a model where the conductance fluctuations are due to quantum interference of electrons backscattered by a few impurities at a distance of the order of the elastic mean free path from the constriction

Novel charge integrating pulsed I(V) technique: A measurement of Fowler–Nordheim currents through thin SiO2 films

The design, characterization, and applications of a novel charge integrating pulsed current-voltage I(V) measurement are described. Tunneling transport through thin metal-oxide-semiconductor capacitors is measured over ten orders of magnitude of current. Short pulse widths (<1 μs) allow electrical characterization of these films under high current densities without significant charge injection. A study of the quantum interference of electrons during Fowler–Nordheim (FN) conduction is used to illustrate the measurement.

Quantum interference of electrons in a field-controlled double-quantum-wire interferometer.

Quantum interference effects are observed in a GaAs/Al x Ga 1-x As interferometer which consists of double quantum wires and triple Schottky gates. Both wires are straight, stacked in the growth direction, and coupled via a thin barrier. The electron wave function is controlled by tuning the gate voltages in such a way that an interferometer is constructed in the straight wires. A quasiballistic electron transport was confirmed, and oscillations of resistance with variation of the voltage of the central gate are clearly observed at 1.8 K