Quantized transport in two-dimensional spin-ordered structures

We study in detail the transport properties of a model of conducting electrons in the presence of double exchange between localized spins arranged on a 2D Kagome lattice, as introduced by Ohgushi, Murakami and Nagaosa. The relationship between the canting angle of the spin texture θ and the Berry phase field flux per triangular plaquette φ is derived explicitly and we emphasize the similarities between this model and Haldane's honeycomb lattice version of the quantum Hall effect. The quantization of the transverse (Hall) conductivity σ _xy is derived explicitly from the Kubo formula and a direct calculation of the longitudinal conductivity σ _xx shows the existence of a metal–insulator transition as a function of the canting angle θ (or flux density φ). This transition might be linked to that observable in the manganite compounds or in the pyrochlore ones, as the spin ordering changes from ferromagnetic to canted.     

Theoretical aspects of electron transport in modulated structures

In the theory of transport in modulated structures we have studied both transport perpendicular and parallel to the heterojunction interfaces. In perpendicular transport we have investigated models for tunneling through double barriers and find that resonant tunneling and sequential tunneling lead to the same expression for the current as long as the width of the energy distribution of the injected electrons is larger than the width of the resonant level in the diode. We present results for phonon assisted tunneling between two wells in a model which remains valid even when the barrier shrinks and the tunneling probability becomes very high. In parallel transport we show that very satisfactory agreement with extensive measurements of the mobility in modulation doped structures in the whole temperature range from 4 K to 300 K can be obtained if one takes into account the complete quasi-two-dimensional subband structure and all the relevant scattering mechanisms. Having established this we apply this program to systems with more complicated double channel structures, and show how one can tailor the conductivity of a channel in which perpendicular resonant tunneling affects parallel transport.     

Quantized orbits and resonant transport

A tight-binding representation of the kicked Harper model is used to obtain an integrable semiclassical Hamiltonian consisting of "quantized" bands of orbits. New bands appear when renormalized Harper parameters exceed integer multiples of pi/2. Orbits with frequencies commensurate with the kicking frequency are shown to correlate with classical accelerator modes in the original kicked problem. Signatures of this superdiffusive and, in our view, resonant transport are seen in both classical and quantum behavior. An important feature of our analysis is the emergence of a natural scaling relating classical and quantum couplings which is necessary for establishing correspondence.     

Ultrafast coherent electron transport in semiconductor quantum cascade structures

Coherent electron transport is studied in an electrically driven quantum cascade structure. Ultrafast quantum transport from the injector into the upper laser state is investigated by midinfrared pump-probe experiments directly monitoring the femtosecond saturation and subsequent recovery of electrically induced optical gain. We demonstrate for the first time pronounced gain oscillations giving evidence for a coherent electron motion. The coexistence of a long dephasing time of quantum coherence and high Coulomb scattering rates in the injector points to the occurrence of scattering-induced coherence in electron transport.     

Dynamic electron transport theory for multiprobe mesoscopic structures

Using the linear response theory and random phase approximation,we develop a general dynamic electron transport theory for multiprobe mesoscopic structures in an arbitrarily time-dependent external field.In this case,the responses of the dynamic current,charge and internal potential to the external fields can be determined self-consistently.Without loss of generality,charge (current) conservation and gauge invariance under a potential shift are satisfied.As an example,we employ a quantum wire with a single barrier to discuss the response of the internal potential.     

Ab-initio electron transport calculations of carbon based string structures

First-principles calculations show that monatomic strings of carbon have high cohesive energy and axial strength, and exhibit stability even at high temperatures. Because of their flexibility and reactivity, carbon chains are suitable for structural and chemical functionalizations; they also form stable ring, helix, grid, and network structures. Analysis of electronic conductance of various infinite, finite, and doped string structures reveal fundamental and technologically interesting features. Changes in doping and geometry give rise to dramatic variations in conductance. In even-numbered linear chains, strain induces a substantial decrease of conductance. The double covalent bonding of carbon atoms underlies their unusual chemical, mechanical, and transport properties.     

Formation of dissipative structures in crystals during heat and electron transport

The surface restructuring (faceting) of solids subjected to longitudinal electric-field and temperature gradients has been studied experimentally and theoretically. Tungsten crystals and wires preheated with a direct current in vacuum or a hydrogen atmosphere to a temperature higher than half the melting temperature are studied by electron microscopy and metallography. The processes of formation of bulk defects and of a regular surface structure are found to correlate. For the first time, these processes are analyzed in terms of synergetics.     

Imaging metallic multilayer structures through ultrafast optically driven excited electron transport

Received: 20 September 1998 / Revised version: 21 December 1998     

Effects of virtual transitions in a high-frequency field on electron transport in triple-barrier structures

High-frequency conductivity has been calculated for triple-barrier nanostructures at resonance diagonal transitions. It is shown that the neglect of transitions to side nonresonance quasi-levels in a high-frequency electric field leads in some cases to an overestimation of the maximum conductivity by 60%, an increase in the line width of more than 40%, and an increase in the integral conductivity by a factor of almost 2.5. In these cases, the transmission coefficient calculated with allowance made for side satellites is approximately 0.6 rather than 1.     

Thermal effect in phase-periodic electron transport in disordered mesoscopic normal metal/superconductor structures

We report measurements of the temperature dependence of the amplitude of phase-periodic conductance oscillations in disordered normal metal (Ag) structures, attached to a superconducting (Al) wire at two points. The amplitude of oscillations reaches its maximum at temperatureT ^*, when the Thouless energy is of the order ofk _B T. The results are in agreement with recent calculations by Nazarov and Stoof [Phys. Rev. Lett. 76 (1996) 823].     

Quantized charge transport,chiral anomalies,and fractional charge

The phenomenon of quantized charge transport is studied using relativistic field theory. Perhaps surprisingly, the charge outflow per period is sometimes quantized in units of two. The interplay between quantized transport, chiral anomalies, and charge fractionization is clarified. In particular, yet another derivation of the relation between π → 2γ and γ → 3π is given.     

Quantized rosette motion of energetic electron around an atomic row in crystal

Wave functions of energetic electrons bound by an atomic row in a crystal and excitation probability of each level are calculated. Results are in good agreement with experimental ones.     

Quantized electron accumulation states in indium nitride studied by angle-resolved photoemission spectroscopy

Electron accumulation states in InN have been measured using high resolution angle-resolved photoemission spectroscopy (ARPES). The electrons in the accumulation layer have been discovered to reside in quantum well states. ARPES was also used to measure the Fermi surface of these quantum well states, as well as their constant binding energy contours below the Fermi level E(F). The energy of the Fermi level and the size of the Fermi surface for these quantum well states could be controlled by varying the method of surface preparation. This is the first unambiguous observation that electrons in the InN accumulation layer are quantized and the first time the Fermi surface associated with such states has been measured.