Quantum Interference Of Electrons Research Articles

Persistent current in quantum torus knots

I consider the quantum interference of electrons moving along knotted trajectories under external magnetic field. The induced persistent current is formulated in terms of characteristic parameters that classify the torus knot geometry. The current is found to show a periodic oscillation whose period depends strongly both on the knot's class and the field direction. The shift in the oscillation period caused by geometric distortion is also discussed.

Aharonov-Bohm effect in an electron-hole graphene ring system

Aharonov-Bohm oscillations are observed in a graphene quantum ring with a topgate covering one arm of the ring. As graphene is a gapless semiconductor, this geometry allows to study not only the quantum interference of electrons with electrons or holes with holes, but also the unique situation of quantum interference between electrons and holes. The period and amplitude of the observed Aharonov-Bohm oscillations are independent of the sign of the applied gate voltage showing the equivalence between unipolar and dipolar interferences.

The Aharonov–Bohm effect in graphene rings

This is a review of electronic quantum interference in mesoscopic ring structures based on graphene, with a focus on the interplay between the Aharonov–Bohm effect and the peculiar electronic and transport properties of this material. We first present an overview on recent developments of this topic, both from the experimental as well as the theoretical side. We then review our recent work on signatures of two prominent graphene-specific features in the Aharonov–Bohm conductance oscillations, namely Klein tunneling and specular Andreev reflection. We close with an assessment of experimental and theoretical development in the field and highlight open questions as well as potential directions of the developments in future work.

Quantum description and properties of electrons emitted from pulsed nanotip electron sources

We present a quantum calculation of the electron degeneracy for electron sources. We explore quantum interference of electrons in the temporal and spatial domain and demonstrate how it can be utilized to characterize a pulsed electron source. We estimate effects of Coulomb repulsion on two-electron interference and show that currently available pulsed nanotip electron sources operate in the regime where the quantum nature of electrons can be made dominant.

Quantum Interference of Rashba-Type Spin-Split Surface State Electrons

We studied the quantum interference of electrons in the Bi (p(x), p(y)) orbital-derived j = 1/2 spin-split surface states at Bi/Ag(111)√3 × √3 surfaces of 10 monolayer thick Ag(111) films on Si(111) substrates. Surface electron standing waves were observed clearly at the energy (E) below the intersection of the two spin-split downward dispersing parabola bands (E(x)). The E dependence of the standing wave pattern reveals the dispersion as the average of the two spin-split surface bands due to the interference between |(k + Δ), ↑> and |-(k - Δ), ↑> [or (|(k - Δ), ↓>) and |-(k + Δ), ↓>] states. In contrast, it was impossible to deduce the dispersion from the standing wave pattern at E ≥ E(x) because the surface electron cannot find its backscattered state with the same spin polarization.

Spatial Modulation of Spin Polarization within a Single Co Island

When electrons are confined to nanostructures, quantum interference takes place due to their wave-particle duality. Using spin-polarized scanning tunneling microscopy, we studied the influence of quantum interference of electrons on the spatial distribution of the spin polarization within a single magnetic nanostructure. We find changes in both the magnitude and sign of the spin polarization on a subnanometer scale. We compare our experimental results with ab initio calculations of the spin-resolved local density of states (LDOS). We find that the modulation of the spin-polarization at a given energy can be ascribed to the different magnitudes of spatially modulated majority states and non-modulated minority states contributing to the total LDOS.

Crossed Andreev Reflection and Charge Imbalance in Diffusive Normal-Superconducting-Normal Structures

We formulate a microscopic theory of nonlocal electron transport in three-terminal diffusive normal-superconducting-normal (NSN) structures with arbitrary interface transmissions. At low energies epsilon, we predict strong enhancement of nonlocal spectral conductance g_{12} proportional, variant1/epsilon due to quantum interference of electrons in disordered N terminals. In contrast, nonlocal resistance R_{12} remains smooth at small epsilon and, furthermore, is found to depend neither on parameters of normal-superconducting interfaces nor on those of N terminals. At higher temperatures, R_{12} exhibits a peak caused by the trade-off between charge imbalance and Andreev reflection. Our results are in a good agreement with recent experimental observations and can be used for quantitative analysis of future experiments.

Supramolecular Patterns Controlled by Electron Interference and Direct Intermolecular Interactions

Whereas all 230 three-dimensional space groups occur in organic crystals, out of only 17 plane groups some highly symmetric ones such as p31m have not yet been observed in two-dimensional (2D) crystals of organic molecules. Here a kagome network with p31m symmetry is reported for cobalt phthalocyanine on Cu(111). This unusual structure results from substrate-induced reduction of molecular symmetry and substrate-mediated interaction via quantum interference of surface electrons. These interactions provide additional control over the symmetry of 2D crystals of phthalocyanines and lead to a variety of other symmetries in self-assembled arrays.

Optical Emission from the Interaction of Fast Electrons with Metallic Films Containing a Circular Aperture: A Study of Radiative Decoherence of Fast Electrons

Light emission resulting from the interaction of swift electrons with a distant material is shown to produce an unexpectedly large fraction of decoherence in the moving charges. The decoherence probability diverges for an electron passing through a hole drilled in a perfectly conducting metal film, regardless of the size of the opening. This divergence, which is logarithmic in the ratio of film radius to aperture radius, originates in an infrared catastrophe that differs from other sources of decoherence (e.g., bremsstrahlung radiation). Our results provide new avenues for controlling and assessing the role of coherence during electron acceleration (for example, in transmission electron microscopes) and for exploiting partial quantum interference of fast electrons.

Effects of Electronic Quantum Interference, Photonic-Crystal Cavity, Spontaneous Emission, Longitudinal Field, Surface-Plasmon Modes, and Surface–Plasmon–Polariton for Optical Amplification

Some possibilities for coherent optical amplification of a normally incident and weak radiation field are reviewed based on various physical mechanisms, such as electronic quantum interference induced by a coupling laser field in a three-level system, field enhancement through the cavity confinement of a radiation field in a photonic crystal and field concentration seen in a transmitted near field through a metallic surface grating due to excitation of surface-plasmon-polariton modes. Numerical results are presented and discussed to demonstrate these interesting effects. The modification to the spontaneous emission inside a photonic crystal is also studied. The important role played by a longitudinal field resulting from the absorption by an induced three-dimensional plasma wave inside a doped semiconductor is analyzed using a nonlocal and nonadiabatic model. Moreover, the coupling between two-dimensional plasmons and surface plasmon modes in the nonretardation limit is explored.

Temperature dependence of anisotropic magnetoresistance and atomic rearrangements in ferromagnetic metal break junctions

Recent experiments have found that the anisotropic magnetoresistance AMR of nanometer-scale ferromagnetic contacts at low temperature can be larger than that of bulk samples and can exhibit more complicated variations as a function of sample bias and the angle of an applied magnetic field than in the bulk case. Here, we test a proposal that quantum interference of electrons may explain these results, by measuring the temperature dependence of the AMR signals in nanometer-scale contacts made from Permalloy Ni80Fe20 , Ni, and Co. We find a strong temperature dependence, in quantitative agreement with expectations for a quantuminterference effect. In the course of making these measurements, we also observed that two-level resistance fluctuations as a function of time, associated with reconfigurations of the atomic structure, are present in all of our samples as the temperature is increased above a few tens of Kelvin, and they can be found in some samples even at 4.2 K. The relative energy of the different atomic configurations can be extremely sensitive to the angle of the sample magnetization, so that the conductance in some samples can be made to change abruptly and reproducibly as the angle of an applied magnetic field is rotated.

A phenomenological approach of joint density of states for the determination of bandstructure in the case of a semi-metal studied by FT-STS

The authors show that an accurate determination of the band structure can be achieved by usingFourier transform scanning tunnelling microscopy (FT-STM) techniques in the case of a semi-metallicErSi2 layer grown on a Si(111) substrate. This material provides an ideally confined2D electron and hole gas that is reflected in a complex standing wavepattern at 77 K. The quasi-particles exist over a wide energy range from−800 to+300 meV without mixing with silicon bulk excitations. The Fourier transform ofdI/dV maps have been successfully interpreted using the concept of the joint density ofstates (JDOS), which will be properly introduced. We present here an intuitiveinterpretation of the quasiparticle interference process based on a geometric constructionwhich also allows us to clearly demonstrate that hole–hole and hole–electronquantum interferences dominate over electron–electron quantum interference.

Giant Fluctuations of Coulomb Drag in a Bilayer System

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.

Quantum interference in multiwall carbon nanotubes

Recent low temperature conductance measurements on multiwall carbon nanotubes in perpendicular and parallel magnetic fields are reported. An efficient gating technique allows for considerable tuning of the nanotube doping level. This enables us to study extensively the effect of the nanotube bandstructure on electron quantum interference effects such as weak localization, universal conductance fluctuations and the Aharonov–Bohm effect. We show that the magnetoresistance in the perpendicular magnetic field is strongly suppressed at certain gate voltages Ugate which can be linked with the bottoms of one-dimensional electronic subbands. This assignment allows a detailed comparison of theoretical calculations with the experimental data. In agreement with the theory, a pronounced energy dependence of the elastic mean free path with a strong enhancement close to the charge neutrality point is observed. In the large parallel magnetic field, we observe a superposition of h/2e-periodic Altshuler–Aronov–Spivak oscillations and an additional h/e-periodic contribution to the conductivity. The latter contribution shows a diamond-like pattern in the B − Ugate-plane, which reflects the magnetic field dependence of the density of states of the nanotube's outermost shell.

Switching characteristics of coupled quantum wires with tunable coupling strength

We explore the switching characteristics of coupled quantum wires with a controllable interwire coupling. We perform extensive experimental and theoretical characterization of the coupling region between the wires, evaluating the local barrier that is formed in this region. With strong coupling between the wires, a nonmonotonic switching of their currents is obtained in a magnetic field, suggestive of quantum interference of electrons in their coupling region.

Observation of quantum interference effect in solids

In order to achieve quantum interference of free electrons inside a solid, we have modified the geometry of the solid so that de Broglie waves interfere destructively inside the solid. Quantum interference of de Broglie waves leads to a reduction in the density of possible quantum states of electrons inside the solid and increases the Fermi energy level. This effect was studied theoretically within the limit of the quantum theory of free electrons inside the metal. It has been shown that if a metal surface is modified with patterned indents, the Fermi energy level will increase and consequently the electron work function will decrease. This effect was studied experimentally in both Au and SiO2 thin films of special geometry and structure. Work function reductions of 0.5eV in Au films and 0.2eV in SiO2 films were observed. Comparative measurements of work function were made using the Kelvin probe method based on compensation of internal contact potential difference. Electron emission from the same thin films was studied by two independent research groups using photoelectron emission microscopy.

Quantum Interference of Electrons in a Ring: Tuning of the Geometrical Phase

We calculate the oscillations of the dc conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wave function, accounting for dephasing effects.

Conductance of highly oriented pyrolytic graphite nanocontacts

The conductance-voltage G(V) characteristics for contacts between electrodes of highly oriented pyrolytic graphite were studied in a wide range of G(0), bias voltages, and temperatures using the mechanically controllable break junction technique. Our results show a striking resemblance to corresponding data for conducting multiwall carbon nanotubes. For high-ohmic contacts between a single or a few graphene sheets, zero bias anomalies related to strong e–e Coulomb interactions and quantum interference of electrons is observed.

Quantum interference and long-range adsorbate-adsorbate interactions

Density functional theory and scanning tunneling microscopy are used to resolve the long-range adsorbate interactions between Co adatoms on Cu~111!, caused by the quantum interference of surface-state electrons. Our calculations and experimental results are in very good quantitative agreement. We reveal the effect of the quantum interference of surface state electrons on adatom motion, leading to the self-assembly of onedimensional structures on metal surfaces.

Damping of quantum interference of electrons on helium

The damping of weak localization of electrons on helium is investigated. From previously measured magnetoresistivity line shapes, we are able to separately extract the damping due to electron-electron interaction and helium vapor-atom motion. We verify the theoretical prediction that vapor-atom motion reduces the interference contribution of paths as a squeezed exponential of their duration, rather than as a simple exponential.