Transport In Mesoscopic Structures Research Articles

Distributions of electron waiting times in quantum-coherent conductors

The distribution of electron waiting times is useful to characterize quantum transport in mesoscopic structures. Here we consider a generic quantum-coherent conductor consisting of a mesoscopic scatterer in a two-terminal setup. We extend earlier results for single-channel conductors to setups with several (possibly spin-degenerate) conduction channels and we discuss the effect of a finite electronic temperature. We present detailed investigations of the electron waiting times in a quantum point contact as well as in two mesoscopic interferometers with energy-dependent transmissions: a Fabry-P\'erot interferometer and a Mach-Zehnder interferometer. We show that the waiting time distributions allow us to determine characteristic features of the scatterers, for instance the number of resonant levels in the Fabry-P\'erot interferometer that contribute to the electronic transport.

Single particle sources and quantum heat fluctuations

The miniaturisation of electronic devices has been a well-known trend in engineering over almost 50 years. The technological advancement in the field can now provide an astonishing control of charge transport in mesoscopic structures. Single particle pumping, namely the control in time and space of the flow of an arbitrarily small number of electrons or holes, has been realised in various kind of structure with, in some cases, very high accuracies. The first half of the manuscript provides a brief overview of different experimental realisations of single particle sources. Though these devices allow to minimise charge fluctuations in the charge current, because of Heisenberg’s uncertainty principle, the emitted particles are characterised by energy fluctuations. The consequences of it are of great relevance and presented in the second part of the paper.

Mechanically controlled spin-selective transport

A device enabling mechanically-controlled spin and electric transport in mesoscopic structures is proposed. It is based on the transfer of electrons through weak links formed by suspended nanowires, on which the charge carriers experience a strong Rashba spin-orbit interaction that twists their spins. It is demonstrated that when the weak link bridges two magnetically-polarised electrodes, a significant spintro-voltaic effect takes place. Then, by monitoring the generated voltage one is able to measure electronic spins accumulated in the electrodes, induced e.g., by circularly-polarised light, or alternatively, the amount of spin twisting. Mechanically-tuning the device by bending the nanowire allows one to achieve full control over the spin orientations of the charge carriers.

Nonlinear mesoscopic transport in a strongly cooperative electron system: The La0.67Ca0.33MnO3microbridge

We investigate the electrical transport in mesoscopic structures of La${}_{0.7}$Ca${}_{0.3}$MnO${}_{3}$ in the regime of the metal-insulator transition by fabricating microbridges from strained and unstrained thin films. We measure current-voltage characteristics as function of temperature and in high magnetic fields. For strained films we find nonlinear effects in the steep part of the transition characterized by a differential resistance with a strong peak around zero applied current, and saturation at higher currents after a resistance drop of up to 60$%$. We propose that this nonlinear behavior is associated with the melting of the insulating state by injecting charge carriers, signaling the occurrence of an intervening phase that involves the formation of short-range polaron correlations.

Electrical and thermal conductance quantization in nanostructures

In the paper problems of electron transport in mesoscopic structures and nanostructures are considered. The electrical conductance of nanowires was measured in a simple experimental system. Investigations have been performed in air at room temperature measuring the conductance between two vibrating metal wires with standard oscilloscope. Conductance quantization in units of G0 = 2e2/h = (12.9 kΩ)-1 up to five quanta of conductance has been observed for nanowires formed in many metals. The explanation of this universal phenomena is the formation of a nanometer-sized wire (nanowire) between macroscopic metallic contacts which induced, due to theory proposed by Landauer, the quantization of conductance. Thermal problems in nanowires are also discussed in the paper.

Description of unconventional electronic transport in mesoscopic structures

Inspired by a fair amount of experimental and theoretical works describing nonlinear transport in hybrid mesoscopic structures, we study a confined state coupled to different types of noninteracting leads. It is shown that specific properties of the electron bath, here simulated by the presence of van Hove singularities in the spectral density of the reservoirs, determine sharp resonances in the differential conductance at finite applied voltages. For both magnetic and nonmagnetic single impurities, the two-channel network exhibits a resonant negative differential conductance behavior in a region of parameters which is not expected for ideal electrodes. This result may have important ramifications in probing resonant tunneling experiments.

A Quantum Many-Body Density Matrix Model for Sub-Femtosecond Transport in Mesoscopic Structures

Theoretical model for transient transport in mesoscopic structures was presented, and the approach with numerical simulation of a resonant-tunneling diode (RTD) was illustrated. A second-order master equation for the evolution of the many-body reduced density matrix of the active region was being introduced. This contains full quantum-mechanical information about the processes in the current-limiting active region. The master equation is derived directly form the Liouville equation for the active region+contacts, by using the partial-trace-free approach, and it incorporates the memory terms that describe the information exchange between the contacts and the active region. In this way, the state of the contacts is accounted for, but does not explicitly enter the calculation, allowing us to compute the full many-body reduced density matrix for the active region. The model accounts for the presence of the bias by assuming a special form of the forcing term in the master equation. The numerical simulation shows that incorporation of the memory terms is crucial for description of charging/discharging in the well, and obtaining the proper transient behavior. Electron-electron interaction between the active region and the contacts, as well as phonon scattering (within the relaxation-time approximation), have been taken into account.

Quantum waveguide theory for hole transport in mesoscopic structures

A quantum waveguide theory is proposed for hole transport in the mesoscopic structures, including the band mixing effect. We found that due to the interference between the ‘light’ hole and ‘heavy’ wave, the transmission and reflection coefficients oscillate more irregularly as a function of incident wave vector geometry parameters. Furthermore conversion between the heavy hole and light hole states occurs at the intersection.

Quantum transport and memory effects in mesoscopic structures

A modern theory of electronic transport in mesoscopic structures needs to non-perturbatively treat the active region of the device as an open subsystem of a larger, interacting closed system, and be able to successfully capture transient processes, as well as a far-from-equilibrium steady state. We present a basis for generalization of the powerful non-equilibrium Green's functions formalism for treatment of open systems. During transient processes and far from equilibrium, we analyze the magnitude and importance of the memory terms, which appear in the two-time correlation functions and are the signatures of the system's openness.

Numerical Acceleration of Three-Dimensional Quantum Transport Method Using a Seven-Diagonal Pre-Conditioner

The open-boundary planar supercell stack method treats three-dimensional quantum transport in mesoscopic structures in a numerically stable and efficient manner. The method formulates quantum mechanical scattering problems for the supercell geometry as sparse linear systems, which can be solved by iterative methods. Recent improvement in the solution algorithm using a seven-diagonal pre-conditioner has resulted in over two orders of magnitude of numerical acceleration, bringing more flexibility in the range of problems that can be solved. We demonstrate the effectiveness of the seven-diagonal pre-conditioning algorithm by applying it to the studies of interface roughness in double barrier resonant tunneling structures and tunneling characteristics of n + poly-Si/SiO2/p-Si with ultra-thin oxides undergoing dielectric breakdown.

Time-Dependent Transport in Mesoscopic Structures

A discussion of recent work on time-dependent transport inmesoscopic structures is presented. The discussion emphasizesthe use of time-dependent transport to gain information on thecharge distribution and its collective dynamics. We discussthe RC-time of mesoscopic capacitors, the dynamic conductanceof quantum point contacts and dynamic weak localizationeffects in chaotic cavities. We review work on adiabaticquantum pumping and photon-assisted transport, and concludewith a list which demonstrates the wide range of problemswhich are of interest.

CDW transport in mesoscopic structures of submicroand nanometer scale

We have studied the CDW transport in different mesoscopic structures with size variation from the micro to the nanometer scale. The structures of three types, all based on thin NbSe 3 crystals, were investigated: 1) periodic system of antidots (holes) with size and spacing variation from micron to the submicron scale, 2) randomly distributed columnar defects (CDs) of various concentration with diameter of 16 nm, 3) short bridges across the conducting bc-planes with the length of 30-50 nm. We report here a number of new features of the CDW transport in mesostructures, in particular, non-linear properties of submicron network, the experimental evidence of quantum interference effect of sliding CDW on CDs threaded by magnetic flux, zero bias conductance peak in the a * -axis transport.

Electron transport in mesoscopic structures

Recent advances in electron beam lithography have allowed to prepare high-quality metal structures with submicrometer dimensions. At low temperatures the size of these structures can become smaller than the coherence length over which the electrons maintain their phase memory. Therefore, a mesoscopic regime can be reached, where the quantum interference between conduction electron waves becomes important. The interference effects can be conveniently tuned by applying a perpendicular magnetic field. Using a lift-off technique we have prepared various submicrometer Al and Au structures. In the superconducting Al loops the Ginzburg-Landau coherence length, which diverges near T c, corresponds to the relevant characteristic length scale. Due to their mesoscopic sample size the Al loops behave as superconducting quantum interference devices (SQUIDs), even when no artificial Josephson junctions are present in the branches of the loop. The mesoscopic confinement of the superconducting wave function causes in addition anomalous Little-Parks oscillations of T c when a magnetic field is applied. We have been able to further reduce the linewidth of the mesoscopic sample by using the tip of a scanning tunneling microscope (STM) for local exposure of a very thin Langmuir-Blodgett layer used as an electron sensitive resist.

Numerical simulation of electron transport in mesoscopic structures with weak dissipation.

A numerical method for simulating transient through steady-state electron transport in multidimensional mesoscopic structures in the presence of weak electron-phonon interactions is presented. This method allows both visualization and quantitative analysis of such electron-phonon coupling processes in these structures. To allow simulation of quantum interference effects within multidimensional structures, a previously developed method for simulating dissipationless transport based on the time-dependent Schr\odinger equation is used as a starting point. Then, to allow simulation of the effects of electron-phonon coupling yet retain numerical tractability, a limited number of discretized harmonic oscillator degrees of freedom are added to those of the electron. Coupling between the electron and oscillator degrees of freedom is via Monte Carlo sampled potential functions that are obtained from the true electron-phonon coupling potentials. The method is exact to first order in the coupling and models some higher order coupling processes as well. Example simulations are performed for both real emission of polar-optical phonons and self-energy processes in prototypical mesoscopic structures.

A further examination of optical analogy of quantum transport in mesoscopic structures

We show that the transmittance of light through waveguides is analogous to the conductance of electrons through mesoscopic wires only when a quasi-monochromatic light source is used. We also show that when casting the Maxwell equations into a Schrödinger-type equation in an optical waveguide with a spatially varying dielectric function ϵ(ω, r ) (the optical of a mesoscopic electron conductor with disorders), a velocity-like, non-hermitian term in the effective Hamiltonian prevails. The latter distinguishes the optical case from the electron case, in addition to the vector nature of electromagnetic waves.

Electrical transport in mesoscopic structures

Electrical transport in mesoscopic structures