Showing papers on "Quantum tunnelling published in 2003"

Quantum Tunneling of Magnetization and Related Phenomena in Molecular Materials

Abstract: Molecules comprising a large number of coupled paramagnetic centers are attracting much interest because they may show properties which are intermediate between those of simple paramagnets and classical bulk magnets and provide unambiguous evidence of quantum size effects in magnets. To date, two cluster families, usually referred to as Mn12 and Fe8, have been used to test theories. However, it is reasonable to predict that other classes of molecules will be discovered which have similar or superior properties. To do this it is necessary that synthetic chemists have a good understanding of the correlation between the structure and properties of the molecules, for this it is necessary that concepts such as quantum tunneling, quantum coherence, quantum oscillations are understood. The goal of this article is to review the fundamental concepts needed to understand quantum size effects in molecular magnets and to critically report what has been done in the field to date.

Single-electron transistor of a single organic molecule with access to several redox states

Abstract: A combination of classical Coulomb charging, electronic level spacings, spin, and vibrational modes determines the single-electron transfer reactions through nanoscale systems connected to external electrodes by tunnelling barriers1. Coulomb charging effects have been shown to dominate such transport in semiconductor quantum dots2, metallic3 and semiconducting4 nanoparticles, carbon nanotubes5,6, and single molecules7,8,9. Recently, transport has been shown to be also influenced by spin—through the Kondo effect—for both nanotubes10 and single molecules8,9, as well as by vibrational fine structure7,11. Here we describe a single-electron transistor where the electronic levels of a single π-conjugated molecule in several distinct charged states control the transport properties. The molecular electronic levels extracted from the single-electron-transistor measurements are strongly perturbed compared to those of the molecule in solution, leading to a very significant reduction of the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital. We suggest, and verify by simple model calculations, that this surprising effect could be caused by image charges generated in the source and drain electrodes resulting in a strong localization of the charges on the molecule.

Quantum mechanical methods for enzyme kinetics

Abstract: ▪ Abstract This review discusses methods for the incorporation of quantum mechanical effects into enzyme kinetics simulations in which the enzyme is an explicit part of the model. We emphasize three aspects: (a) use of quantum mechanical electronic structure methods such as molecular orbital theory and density functional theory, usually in conjunction with molecular mechanics; (b) treating vibrational motions quantum mechanically, either in an instantaneous harmonic approximation, or by path integrals, or by a three-dimensional wave function coupled to classical nuclear motion; (c) incorporation of multidimensional tunneling approximations into reaction rate calculations.

Spin-dependent tunnelling in magnetic tunnel junctions

Abstract: The phenomenon of electron tunnelling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT) in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunnelling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random-access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. In this review article we present an overview of this field of research. We discuss various factors that control the spin polarization and magnetoresistance in MTJs. Starting from early experiments on SDT and their interpretation, we consider thereafter recent experiments and models which highlight the role of the electronic structure of the ferromagnets, the insulating layer, and the ferromagnet/insulator interfaces. We also discuss the role of disorder in the barrier and in the ferromagnetic electrodes and their influence on TMR.

Quantum Theory of Tunneling

Abstract: A Brief History of Quantum Tunneling Some Basic Questions Concerning Quantum Tunneling Simple Solvable Problems Time-Dependence of the Wave Function in One-Dimensional Tunneling Semiclassical Approximations Generalization of the Bohr - Sommerfeld Quantization Rule and its Application to Quantum Tunneling Gamow's Theory, Complex Eigenvalues, and the Wave Function of a Decaying State Tunneling in Symmetric and Asymmetric Local Potentials and Tunneling in Nonlocal and Quasi-Solvable Barriers Classical Descriptions of Tunneling Tunneling in Time-Dependent Barriers Decay Width and Scattering Theory The Method of Variable Reflection Amplitude Applied to Solve Multichannel Tunneling Problems Path Integral and Its Semi-Classical Approximation in Quantum Tunneling Heisenberg's Equations of Motion for Tunneling Wigner Distribution Function in Quantum Tunneling Decay Widths of Siegert States, Complex Scaling and Dilatation Transformation Multidimensional Quantum Tunneling Group and Signal Velocities Time-Delay, Reflection Time Operator and Minimum Tunneling Time More about Tunneling Time Tunneling of a System with Internal Degrees of Freedom Motion of a Particle in a Waveguide with Variable Cross Section and in a Space Bounded by a Dumbbell-Shaped Object Relativistic Formulation of Quantum Tunneling Inverse Problems of Quantum Tunneling Some Examples of Quantum Tunneling in Atomic and Molecular Physics Some Examples from Condensed Matter Physics Alpha Decay

Real-time detection of electron tunnelling in a quantum dot

Abstract: Nanostructures in which strong (Coulomb) interactions exist between electrons are predicted to exhibit temporal electronic correlations1. Although there is ample experimental evidence that such correlations exist2, electron dynamics in engineered nanostructures have been observed directly only on long timescales3. The faster dynamics associated with electrical currents or charge fluctuations4 are usually inferred from direct (or quasi-direct) current measurements. Recently, interest in electron dynamics has risen, in part owing to the realization that additional information about electronic interactions can be found in the shot noise5 or higher statistical moments6,7 of a direct current. Furthermore, interest in quantum computation has stimulated investigation of quantum bit (qubit) readout techniques8,9, which for many condensed-matter systems ultimately reduces to single-shot measurements of individual electronic charges. Here we report real-time observation of individual electron tunnelling events in a quantum dot using an integrated radio-frequency single-electron transistor10,11. We use electron counting to measure directly the quantum dot's tunnelling rate and the occupational probabilities of its charge state. Our results provide evidence in favour of long (10 µs or more) inelastic scattering times in nearly isolated dots.

Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier

Abstract: Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Entangled quantum state of magnetic dipoles

Abstract: Free magnetic moments usually manifest themselves in Curie laws, where weak external magnetic fields produce magnetizations that vary as the reciprocal of the temperature (1/T). For a variety of materials that do not display static magnetism, including doped semiconductors and certain rare-earth intermetallics, the 1/T law is replaced by a power law T^-α with α < 1. Here we show that a much simpler material system—namely, the insulating magnetic salt LiHo_xY_(1-x)F_4—can also display such a power law. Moreover, by comparing the results of numerical simulations of this system with susceptibility and specific-heat data, we show that both energy-level splitting and quantum entanglement are crucial to describing its behaviour. The second of these quantum mechanical effects—entanglement, where the wavefunction of a system with several degrees of freedom cannot be written as a product of wavefunctions for each degree of freedom—becomes visible for remarkably small tunnelling terms, and is activated well before tunnelling has visible effects on the spectrum. This finding is significant because it shows that entanglement, rather than energy-level redistribution, can underlie the magnetic behaviour of a simple insulating quantum spin system.

Selectivity in vibrationally mediated single-molecule chemistry.

Abstract: The selective excitation of molecular vibrations provides a means to directly influence the speed and outcome of chemical reactions. Such mode-selective chemistry has traditionally used laser pulses to prepare reactants in specific vibrational states to enhance reactivity or modify the distribution of product species. Inelastic tunnelling electrons may also excite molecular vibrations and have been used to that effect on adsorbed molecules, to cleave individual chemical bonds and induce molecular motion or dissociation. Here we demonstrate that inelastic tunnelling electrons can be tuned to induce selectively either the translation or desorption of individual ammonia molecules on a Cu(100) surface. We are able to select a particular reaction pathway by adjusting the electronic tunnelling current and energy during the reaction induction such that we activate either the stretching vibration of ammonia or the inversion of its pyramidal structure. Our results illustrate the ability of the scanning tunnelling microscope to probe single-molecule events in the limit of very low yield and very low power irradiation, which should allow the investigation of reaction pathways not readily amenable to study by more conventional approaches.

Spin-polarized scanning tunnelling microscopy

Abstract: The recent experimental progress in spin-polarized scanning tunnelling microscopy (SP-STM)—a magnetically sensitive imaging technique with ultra-high resolution—is reviewed. The basics of spin-polarized electron tunnelling are introduced as they have been investigated in planar tunnel junctions for different electrode materials, i.e. superconductors, optically excited GaAs, and ferromagnets. It is shown that ferromagnets and antiferromagnets are suitable tip materials for the realization of SP-STM. Possible tip designs and modes of operations are discussed for both classes of materials. The results of recent spatially resolved measurements as performed with different magnetic probe tips and using different modes of operation are reviewed and discussed in terms of applicability to surfaces, thin films, and nanoparticles. The limits of spatial resolution, and the impact of an external magnetic field on the imaging process are debated.

Chiral Luttinger liquids at the fractional quantum Hall edge

Abstract: The edges of quantum Hall fluids behave as one-dimensional conductors. This article reviews electron transport into these edge states, covering both the theory based on the chiral Luttinger liquid and the experimental findings using electron tunneling as the probe. The first part of the review presents a basic description of this theory, including a derivation of the density of states, to provide a framework and language for discussing the experimental observations. The signature of the chiral Luttinger liquid is a power-law behavior for the density of states and the tunneling conductances. Experimentally, two techniques have been applied to study the tunneling conductance, using a gated point contact between two quantum Hall edges, or using a cleaved-edge barrier between an edge and a normal conductor. The point-contact method exhibits resonant tunneling, which appears to show some aspects of the Luttinger liquid, and the cleaved-edge method has yielded clear power-law dependences in the off-resonance conductances. Power-law behavior over many orders of magnitude is observed, confirming the Luttinger-liquid character of the edge states. However, the power-law exponents, while in agreement with finite-size numerical calculations, can differ from the universal values predicted by the Chern-Simon field theory. This disagreement is still not well understood. The review concludes with a brief survey of other one-dimensional conductors that have been studied to look for characteristics of the nonchiral Tomonaga-Luttinger liquid.

Analysis of the Transport Process Providing Spin Injection through an Fe/AlGaAs Schottky Barrier

Abstract: Electron spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single step tunneling is the dominant transport mechanism. The current-voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like longitudinal-optical phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Electron transport in quantum dots

Abstract: 1 Interactions, Spins and the Kondo Effect in Quantum-Dot Systems.- 1 Introduction.- 2 Atom-Like Properties of Electrons Confined in a Quantum Dot.- 3 Tunable Spin States with Magnetic Field.- 4 Spin Blockade in Single Electron Tunneling.- 5 Energy Relaxation with and Without Spin-Flip.- 6 The Kondo Effect in Quantum Dots.- 7 Summary.- 2 Microwave Spectroscopy on Single and Coupled Quantum Dots.- 1 Introduction.- 2 Aspects of Fabrication.- 3 Measurement Techniques.- 4 Coherent Modes in Quantum Dots.- 5 Photon Assisted Tunneling in Quantum Dots.- 6 Dynamic Response of Single Quantum Dots.- 7 The On-Chip Spectrometer.- 8 Non-Linear Transmission-Lines for Probing Single Dots.- 9 Summary.- 3 Nano-Spintronics with Lateral Quantum Dots.- 1 Introduction.- 2 Theoretical Framework.- 3 Experimental Devices and Techniques.- 4 Spin-Polarized Injection and Detection.- 5 Coulomb and Spin Blockade Spectrum.- 6 The First Few Electrons.- 7 The ? = 2 Regime.- 8 The Spin Flip Regime.- 9 Negative Differential Resistance Achieved by Spin Blockade.- 10 Conclusions.- 4 Novel Phenomena in Small Individual and Coupled Quantum Dots.- 1 Introduction.- 2 Models of Single and Double Quantum Dot Systems.- 3 Non-Gaussian Distribution of Coulomb Blockade Peak Heights in Individual Quantum Dots: Porter-Thomas Distribution of Resonance Widths.- 4 Spin and Pairing Effects in Ultra-Small Dots.- 5 Coupling between Two Dots and Leads-Coherent Many-Body Kondo States.- 6 Other Ultra-Small Devices and Phenomena.- 5 Classical and Quantum Transport in Antidot Arrays.- 1 Introduction.- 2 Antidot Arrays.- 3 Early Experiments and Pinball Model.- 4 Chaotic Dynamics in Antidot Lattices.- 5 Quantum Effects in Antidot Arrays.- 6 Random Antidot Arrays.- 7 Finite Antidot Lattices.- 8 InAs Based Arrays.- 9 Other Experiments.- 6 On the Influence of Resonant States on Ballistic Transport in Open Quantum Dots: Spectroscopy and Tunneling in the Presence of Multiple Conducting Channels.- 1 Introduction.- 2 Some Comments about Semiclassical Theories and their Underlying Assumptions.- 3 The Method of Calculation Used Primarily in this Work: A Fully Quantum Mechanical Treatment.- 4 Conductance Resonances in Open Dots.- 5 The Correspondence Between Conductance Resonances in Open Dots and Closed Dot Eigenstates.- 6 The Effect of Finite Temperature and Ensemble Averaging.- 7 Direct Comparisons of Theory with Experiment.- 8 An Alternate Semiclassical Interpretation of Transport in Open Quantum Dots: Dynamical Tunneling.- 9 Summary.- 10 Acknowledgment.- 7 A Review of Fractal Conductance Fluctuations in Ballistic Semiconductor Devices.- 1 Introduction.- 2 The Semiconductor Sinai Billiard: Can Chaos be Controlled with the "Flick of a Switch?".- 3 The Experimental Observation of Exact Self-Affinity.- 4 The Interpretation of Exact Self-Affinity.- 5 The Observation of Statistical Self-Affinity.- 6 The Classical to Quantum Transition: How do Fractals "Disappear?".- 7 The Role Played by the Billiard Walls.- 8 Conclusions.- 8 Electron Ratchets-Nonlinear Transport in Semiconductor Dot and Antidot Structures.- 1 Introduction.- 2 Non-Linear Rectification in the Quantum Regime.- 3 Nonlinear Transport in Antidot Structures.- 4 Outlook.- 9 Single-Photon Detection with Quantum Dots in the Far-Infrared/Submillimeter-Wave Range.- 1 Introduction.- 2 Fundamental Characteristics of the SET.- 3 Designing a Single-Photon Detector.- 4 Detection in Magnetic Fields.- 5 Detection in the Absence of Magnetic Field.- 6 Detector Performance.- 7 Conclusion.- 10 Quantum-Dot Cellular Automata.- 1 Introduction.- 2 The Quantum-Dot Cellular Automata Paradigm.- 3 Experimental Demonstrations of QCA: Metal-Dot Systems.- 4 Molecular QCA.- 5 Architecture for QCA.- 6 Magnetic QCA.- 11 Carbon Nanotubes for Nanoscale Spin-Electronics.- 1 Introduction.- 2 Spin Transport in Carbon Nanotubes.- 3 Conclusions.

Delay Time and the Hartman Effect in Quantum Tunneling

Abstract: A general relation between the group delay and the dwell time is derived for quantum tunneling. It is shown that the group delay is equal to the dwell time plus a self-interference delay. The Hartman effect in quantum tunneling is explained on the basis of saturation of the integrated probability density (or number of particles) under the barrier.

Carbon Tunneling from a Single Quantum State

Abstract: We observed ring expansion of 1-methylcyclobutylfluorocarbene at 8 kelvin, a reaction that involves carbon tunneling. The measured rate constants were 4.0 × 10 −6 per second in nitrogen and 4 × 10 −5 per second in argon. Calculations indicated that at this temperature the reaction proceeds from a single quantum state of the reactant so that the computed rate constant has achieved a temperature-independent limit. According to calculations, the tunneling contribution to the rate is 152 orders of magnitude greater than the contribution from passage over the barrier. We discuss environmental effects of the solid-state inert-gas matrix on the reaction rate.

High tunnel magnetoresistance in epitaxial Fe/MgO/Fe tunnel junctions

Abstract: We report on spin-polarized tunneling in fully epitaxial Fe/MgO/Fe/Co tunnel junctions. By increasing the thickness of the insulating layer (tMgO), we have strongly enhanced the tunnel magnetoresistance. Values up to ∼100% at 80 K (∼67% at room temperature) have been observed with tMgO=2.5 nm. This tunnel magnetoresistance ratio, which is much larger than the one predicted by the Julliere’s model, can be understood in the framework of ab initio calculations.

Quantum Theory of Tunneling

Abstract: A Brief History of Quantum Tunneling Some Basic Questions Concerning Quantum Tunneling Simple Solvable Problems Time-Dependence of the Wave Function in One-Dimensional Tunneling Semiclassical Approximations Generalization of the Bohr - Sommerfeld Quantization Rule and its Application to Quantum Tunneling Gamow's Theory, Complex Eigenvalues, and the Wave Function of a Decaying State Tunneling in Symmetric and Asymmetric Local Potentials and Tunneling in Nonlocal and Quasi-Solvable Barriers Classical Descriptions of Tunneling Tunneling in Time-Dependent Barriers Decay Width and Scattering Theory The Method of Variable Reflection Amplitude Applied to Solve Multichannel Tunneling Problems Path Integral and Its Semi-Classical Approximation in Quantum Tunneling Heisenberg's Equations of Motion for Tunneling Wigner Distribution Function in Quantum Tunneling Decay Widths of Siegert States, Complex Scaling and Dilatation Transformation Multidimensional Quantum Tunneling Group and Signal Velocities Time-Delay, Reflection Time Operator and Minimum Tunneling Time More about Tunneling Time Tunneling of a System with Internal Degrees of Freedom Motion of a Particle in a Waveguide with Variable Cross Section and in a Space Bounded by a Dumbbell-Shaped Object Relativistic Formulation of Quantum Tunneling Inverse Problems of Quantum Tunneling Some Examples of Quantum Tunneling in Atomic and Molecular Physics Some Examples from Condensed Matter Physics Alpha Decay.

SU(4) Fermi Liquid State and Spin Filtering in a Double Quantum Dot System

Abstract: We study a symmetrical double quantum dot (DD) system with strong capacitive interdot coupling using renormalization group methods. The dots are attached to separate leads, and there can be a weak tunneling between them. In the regime where there is a single electron on the DD the low-energy behavior is characterized by an SU(4)-symmetric Fermi liquid theory with entangled spin and charge Kondo correlations and a phase shift pi/4. Application of an external magnetic field gives rise to a large magnetoconductance and a crossover to a purely charge Kondo state in the charge sector with SU(2) symmetry. In a four-lead setup we find perfectly spin-polarized transmission.

Spin-dependent tunneling through a symmetric semiconductor barrier

Abstract: The problem of electron tunneling through a symmetric semiconductor barrier based on zinc-blende-structure material is studied. The ${k}^{3}$ Dresselhaus terms in the effective Hamiltonian of bulk semiconductor of the barrier are shown to result in a dependence of the tunneling transmission on the spin orientation. The difference of the transmission probabilities for opposite spin orientations can achieve several percents for the reasonable width of the barriers.

Quantum dynamics of a single vortex.

Abstract: Vortices occur naturally in a wide range of gases and fluids, from macroscopic to microscopic scales. In Bose-Einstein condensates of dilute atomic gases, superfluid helium and superconductors, the existence of vortices is a consequence of the quantum nature of the system. Quantized vortices of supercurrent are generated by magnetic flux penetrating the material, and play a key role in determining the material properties and the performance of superconductor-based devices. At high temperatures the dynamics of such vortices are essentially classical, while at low temperatures previous experiments have suggested collective quantum dynamics. However, the question of whether vortex tunnelling occurs at low temperatures has been addressed only for large collections of vortices. Here we study the quantum dynamics of an individual vortex in a superconducting Josephson junction. By measuring the statistics of the vortex escape from a controllable pinning potential, we demonstrate the existence of quantized levels of the vortex energy within the trapping potential well and quantum tunnelling of the vortex through the pinning barrier.

Mesoscopic electronics in solid state nanostructures

Abstract: 1. Introduction. 1.1 Preliminary remarks. 1.2 Mesoscopic transport. 1.2.1 Ballistic transport. 1.2.2 The quantum Hall effect and Shubnikov -- de Haas oscillations. 1.2.3 Size quantization. 1.2.4 Phase coherence. 1.2.5 Single electron tunnelling and quantum dots. 1.2.6 Superlattices. 1.2.7 Samples and experimental techniques. 2 An Update of Solid State Physics. 2.1 Crystal structures. 2.2 Electronic energy bands. 2.3 Occupation of energy bands. 2.3.1 The electronic density of states. 2.3.2 Occupation probability and chemical potential. 2.3.3 Intrinsic carrier concentration. 2.4 Envelope wave functions. 2.5 Doping. 2.6 Diffusive transport and the Boltzmann equation. 2.6.1 The Boltzmann equation. 2.6.2 The conductance predicted by the simplified Boltzmann equation. 2.6.3 The magneto--resistivity tensor. 2.7 Scattering mechanisms. 2.8 Screening. 3 Surfaces, Interfaces, and Layered Devices. 3.1 Electronic surface states. 3.1.1 Surface states in one dimension. 3.1.2 Surfaces of 3--dimensional crystals. 3.1.3 Band bending and Fermi level pinning. 3.2 Semiconductor--metal interfaces. 3.2.1 Band alignment and Schottky barriers. 3.2.2 Ohmic contacts. 3.3 Semiconductor heterointerfaces. 3.4 Field effect transistors and quantum wells. 3.4.1 The silicon metal--oxide--semiconductor FET (Si--MOSFET). 3.4.2 The Ga[Al]As high electron mobility transistor (GaAs--HEMT). 3.4.3 Other types of layered devices. 3.4.4 Quantum confined carriers in comparison to bulk carriers. 4 Experimental Techniques. 4.1 Sample fabrication. 4.1.1 Single crystal growth. 4.1.2 Growth of layered structures. 4.1.3 Lateral patterning. 4.1.4 Metallization. 4.1.5 Bonding. 4.2 Elements of cryogenics. 4.2.1 Properties of liquid helium. 4.2.2 Helium cryostats. 4.3 Electronic measurements on nanostructures. 4.3.1 Sample holders. 4.3.2 Application and detection of electronic signals. 5 Important Quantities in Mesoscopic Transport. 6 Magnetotransport Properties of Quantum Films. 6.1 Landau quantization. 6.1.1 2DEGs in perpendicular magnetic fields. 6.1.2 The chemical potential in strong magnetic fields. 6.2 The quantum Hall effect. 6.2.1 Phenomenology. 6.2.2 Origin of the integer quantum Hall effect. 6.2.3 The quantum Hall effect and three dimensions. 6.3 Elementary analysis of Shubnikov--de Haas oscillations. 6.4 Some examples of magnetotransport experiments. 6.4.1 Quasi--two--dimensional electron gases. 6.4.2 Mapping of the probability density. 6.4.3 Displacement of the quantum Hall plateaux. 6.5 Parallel magnetic fields. 7 QuantumWires and Quantum Point Contacts. 7.1 Diffusive quantum wires. 7.1.1 Basic properties. 7.1.2 Boundary scattering. 7.2 Ballistic quantum wires. 7.2.1 Phenomenology. 7.2.2 Conductance quantization in QPCs. 7.2.3 Magnetic field effects. 7.2.4 The "0.7 structure". 7.2.5 Four--probe measurements on ballistic quantum wires. 7.3 The Landauer--B uttiker formalism. 7.3.1 Edge states. 7.3.2 Edge channels. 7.4 Further examples of quantum wires. 7.4.1 Conductance quantization in conventional metals. 7.4.2 Carbon nanotubes. 7.5 Quantum point contact circuits. 7.5.1 Non--ohmic behavior of collinear QPCs. 7.5.2 QPCs in parallel. 7.6 Concluding remarks. 8. Electronic Phase Coherence. 8.1 The Aharonov--Bohm effect in mesoscopic conductors. 8.2 Weak localization. 8.3 Universal conductance fluctuations. 8.4 Phase coherence in ballistic 2DEGs. 8.5 Resonant tunnelling and S -- matrices. 9 Singe Electron Tunnelling. 9.1 The principle of Coulomb blockade. 9.2 Basic single electron tunnelling circuits. 9.2.1 Coulomb blockade at the double barrier. 9.2.2 Current--voltage characteristics: the Coulomb staircase. 9.2.3 The SET transistor. 9.3 SET circuits with many islands the single electron pump. 10 Quantum Dots. 10.1 Phenomenology of quantum dots. 10.2 The constant interaction model. 10.3 Beyond the constant interaction model. 10.4 Shape of conductance resonances and current--voltage characteristics. 10.5 Other types of quantum dots. 11 Mesoscopic Superlattices. 11.1 One--dimensional superlattices. 11.2 Two--dimensional superlattices. A SI and cgs Units. Appendices. B Correlation and Convolution. B.1 Fourier transformation. B.2 Convolutions. B.3 Correlation functions. C Capacitance Matrix and Electrostatic Energy. D The Transfer Hamiltonian. E Solutions to Selected Exercises. References. Index.

Tunneling and optical spectroscopy of semiconductor nanocrystals.

Abstract: ▪ Abstract The use of a combination of tunneling and optical spectroscopy to investigate the size and shape-dependent level structure and single-electron charging phenomena in semiconductor nanocrystals is reviewed. The artificial atom character of semiconductor nanocrystal quantum dots is manifested in both the discrete level structure and in the charging multiplicity of the single-electron tunneling data, revealing s and p atomic-like states. Such states can be directly imaged using scanning tunneling microscopy, providing the extent and symmetry of the envelope wavefunctions. A detailed description of the effect of the tunneling geometry on the single-electron tunneling spectra is presented. Correlation of the optical and tunneling data allows for the assignment of the level spectrum. The generality of this powerful combination is further demonstrated in the study of quantum rods that manifest the transition from zero-dimensional quantum dots to one-dimensional quantum wires.

Resonant inversion of tunneling magnetoresistance.

Abstract: Resonant tunneling via localized states in the barrier can invert magnetoresistance in magnetic tunnel junctions. Experiments performed on electrodeposited Ni/NiO/Co nanojunctions of area smaller than 0.01 microm(2) show that both positive and negative values of magnetoresistance are possible. Calculations based on Landauer-Buttiker theory explain this behavior in terms of disorder-driven statistical variations in magnetoresistance with a finite probability of inversion due to resonant tunneling.

Electrical detection of spin accumulation in a p-type GaAs quantum well.

Abstract: We report on experiments in which a spin-polarized current is injected from a GaMnAs ferromagnetic electrode into a GaAs layer through an AlAs barrier. The resulting spin polarization in GaAs is detected by measuring how the tunneling current, to a second GaMnAs ferromagnetic electrode, depends on the orientation of its magnetization. Our results can be accounted for by sequential tunneling with the nonrelaxed spin splitting of the chemical potential, that is, spin accumulation, in GaAs. We discuss the conditions on the hole spin relaxation time in GaAs that are required to obtain the large effects we observe.

Mechanism of the reverse gate leakage in AlGaN/GaN high electron mobility transistors

Abstract: The off-state gate current in AlGaN/GaN high electron mobility transistors is shown to arise from two parallel gate to substrate tunneling paths: a direct path, and a path via deep traps, which are distributed throughout the AlGaN layer and spread over an energy band. A model to calculate this current is given, which shows that trap-assisted tunneling dominates below T∼500 K, and direct tunneling (thermionic field emission) dominates at higher temperatures. A model fit to experimental results yields the following fabrication process sensitive parameters: trap concentration of ∼1013–1015 cm−3, and trap bandwidth of ∼50%–70% of the barrier height located 0.4–0.55 V below the conduction band edge.

Tunneling Current in Polycrystalline Organic Thin‐Film Transistors

Abstract: Several recent papers have demonstrated that charge-carrier mobility in organic field-effect transistors made of vacuum-evaporated films may become temperature-independent at low temperature. To account for this behavior, we developed a model based on the polycrystalline nature of these films, where charge transport is mostly limited by grain boundaries. The free-carrier density in the intergrain regions is controlled by traps, which leads to the formation of back-to-back Schottky barriers at each side of the grain boundaries. The height and width of these barriers is estimated from solving Poisson’s equation using the graded-channel approximation. It is shown that in most cases the barrier width is negligibly small as compared to the physical size of the grain boundaries. In the high-temperature regime, the conducting channel can be simply described by grains and grain boundaries connected in series, so that the overall resistance reduces to that of the grain boundaries. At low temperatures, tunneling through the barrier becomes predominant, leading to temperature-independent mobility. A complete two-dimensional model for charge tunneling through the barriers is developed. A quantitative check of the model is made by least-squares fitting of the gate voltage-dependent current measured on an octithiophene transistor at low temperature, which gives a reasonable determination of the trap density and size of the grain boundaries.

Non-magnetic semiconductor spin transistor

Abstract: We propose a spin transistor using only non-magnetic materials that exploits the characteristics of bulk inversion asymmetry (BIA) in (110) symmetric quantum wells. We show that extremely large spin splittings due to BIA are possible in (110) InAs/GaSb/AlSb heterostructures, which together with the enhanced spin decay times in (110) quantum wells demonstrates the potential for exploitation of BIA effects in semiconductor spintronics devices. Spin injection and detection is achieved using spin-dependent resonant interband tunneling and spin transistor action is realized through control of the electron spin lifetime in an InAs lateral transport channel using an applied electric field (Rashba effect). This device may also be used as a spin valve, or a magnetic field sensor. The electronic structure and spin relaxation times for the spin transistor proposed here are calculated using a nonperturbative 14-band k.p nanostructure model.

Tunneling broadening of vibrational sidebands in molecular transistors

Abstract: Transport through molecular quantum dots coupled to a single vibration mode is studied in the case with strong coupling to the leads. We use an expansion in the correlation between electrons on the molecule and electrons in the leads, and show that the tunneling broadening is strongly suppressed by the combination of the Pauli principle and the quantization of the oscillator. As a consequence the first Frank-Condon step is sharper than the higher order ones, and its width, when compared to the bare tunneling strength, is reduced by the overlap between the ground states of the displaced and nondisplaced oscillators.

Nature of "superluminal" barrier tunneling.

Abstract: We show that the distortionless tunneling of electromagnetic pulses through a barrier is a quasistatic process in which the slowly varying envelope of the incident pulse modulates the amplitude of a standing wave. For pulses longer than the barrier width, the barrier acts as a lumped element with respect to the pulse envelope. The envelopes of the transmitted and reflected fields can adiabatically follow the incident pulse with only a small delay that originates from energy storage. The theory presented here provides a physical explanation of the tunneling process and resolves the mystery of apparent superluminality.