Phase Breaking Time Research Articles

INVESTIGATION OF STRUCTURE AND ELECTRICAL TRANSPORT IN PARTIALLY NANOCRYSTALLIZED AMORPHOUS SOFT MAGNETIC ALLOYS

X-ray diffraction patterns of melt-spun Fe-Cu-Nb-Si-B (FINEMET-type) alloys reveal that crystallites of Fe 2 Si and Fe 3 B phases with average sizes of 15(5) and 20(2) nm are present in the surface layer of thickness ≈ 10 Å and these nanocrystallites occupy 5–10% of the total volume. The results of an elaborate analysis of the high-resolution electrical resistivity data taken in a temperature range from 13 K to 300 K and their discussion in the light of existing theories demonstrates that the enhanced electron–electron interaction (EEI), quantum interference (QI) effects, inelastic electron–phonon scattering, coherent electron–magnon (and/or electron-spin fluctuation) scattering are 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 wavefunctions. The physical quantities such as diffusion constant, density of states at the Fermi level and the phase-breaking time, determined for the first time for the alloys in question, exhibit a systematic variation with the copper concentration.

Magneto-transport properties of InAs/AlGaSb open quantum dot structures

We report on the magneto-transport properties of InAs/AlGaSb open quantum dot structures. The open dot structures have been fabricated by using electron beam lithography. The open dot structures are characterized by magnetoresistance measurements at 4.2 K. In addition to the Shubnikov-de Hass oscillations, aperiodic magnetoresistance fluctuations have been observed at low magnetic fields below 1 T. The amplitude of the fluctuations was quite large even at 4.2 K in comparison with the case of the GaAs/AlGaAs system. Electron-wave interference effect and phase-breaking times in InAs mesoscopic structures are discussed from the analyses using correlation functions of magneto-conductance fluctuations.

Simulationapproach to weak localization in inhomogeneous two-dimensionalsystems and diffusive constrictions

The weak localization effect has been studied inmacroscopically inhomogeneous diffusive 2D systems and diffusiveconstrictions. It is shown that such systems can reveal asaturated behaviour of the low-temperature dependence of thatparameter, which is obtained from the standard analysis of thenegative magnetoresistance and is usually identified byexperimentalists with the phase breaking time.

Role of doped layers in the dephasing of two-dimensional electrons in quantum-well structures

The temperature and gate voltage dependences of the phase breaking time are studied experimentally in GaAs/InGaAs heterostructures with single quantum well. It is shown that appearance of states at the Fermi energy in the doped layers leads to a significant decrease of the phase breaking time of the carriers in quantum well and to saturation of the phase breaking time at low temperature.

Dephasing in one-dimensional quantum systems as a result of ensemble averaging

Considering an ensemble of disordered quantum rings, we examine the role of sample-to-sample averaging on the observable phase-breaking of carrier wave functions at zero temperature. We find that inhomogeneous factors associated with electron–impurity interactions lead to dephasing, which can explain the experimental data on temperature dependence of the phase breaking time.

Localization, De-Localization, Phase Breaking and Energy Relaxation in an Array of Quantum Dots

We have studied both the phase-relaxation time and the energy-relaxation time in arrays of quantum dots. The variations of phase-breaking time with respect to temperature and current were related, suggesting a common electron temperature dependence, although these times differ by several orders of magnitude.

Localization, De-Localization, Phase Breaking and Energy Relaxation in an Array of Quantum Dots

We have studied both the phase-relaxation time and the energy-relaxation time in arrays of quantum dots. The variations of phase-breaking time with respect to temperature and current were related, suggesting a common electron temperature dependence, although these times differ by several orders of magnitude.

Phase-breaking time variations with temperature and current in an open quantum dot array

Magnetic characterization of a quantum dot array was carried out to extract the phase-breaking time as a function of temperature and current. These measurements indicate a saturation of the phase-breaking time at low temperature and low current. The phase-breaking time varies as 1/T for temperatures above 1 K, which can be attributed to carrier–carrier scattering processes. Variation of the phase-breaking time with current yields an estimate for the electron temperature.

Magnetotransport spectroscopy of a quantum dot: effects of lead opening and phase coherence

We compare open quantum dot magnetoconductance spectra from experiment and theory in the presence of environmental coupling and attributed broadening. Estimates of the phase-breaking time in experiment, and effective broadening in simulation, are determined independently. In a larger, more open dot, with a significantly shorter phase-breaking time, the observed spectrum is broadened, most noticeably about B=0. The required broadening in simulation is characterized by effective temperatures higher than estimates from experiment; however, without accounting for disorder, which will further broaden the spectrum, the agreement is reasonable.

Charge relaxation and dephasing in Coulomb-coupled conductors

The dephasing time in coupled mesoscopic conductors is caused by the fluctuations of the dipolar charge permitted by the long range Coulomb interaction. We relate the phase breaking time to elementary transport coefficients which describe the dynamics of this dipole: the capacitance, an equilibrium charge relaxation resistance and in the presence of transport through one of the conductors a non-equilibrium charge relaxation resistance. The discussion is illustrated for a quantum point contact in a high magnetic field in proximity to a quantum dot.

Saturation of phase breaking in an open ballistic quantum dot

One of the key parameters for the observation of quantum interference effects in mesoscopic systems is the phase-breaking time τ φ . While there have been many measurements of this important phase-breaking time in diffusive mesoscopic structures, there have been relatively few studies of phase breaking in zero-dimensional structures. Here, a saturation of the phase-breaking time at low temperatures is found, with a decay at higher temperatures generally of the form of T −2/3. Variation with dot size suggests a scaling in which the phase-breaking interaction is described by the total number of electrons N= nA within the dot.

Saturation of Phase Breaking in an Open Ballistic Quantum Dot

Abstract One of the key parameters for the observation of quantum interference effects in mesoscopic systems is the phase-breaking time τ φ . While there have been many measurements of this important phase-breaking time in diffusive mesoscopic structures, there have been relatively few studies of phase breaking in zero-dimensional structures. Here, a saturation of the phase-breaking time at low temperatures is found, with a decay at higher temperatures generally of the form of T −2/3 . Variation with dot size suggests a scaling in which the phase-breaking interaction is described by the total number of electrons N = nA within the dot.

The influence of environmental coupling on phase breaking in open quantum dots

Studies of the phase breaking time in open quantum dots reveal a value of the order of a few tens of picoseconds at low resistances, which increases by as much as one order of magnitude as the quantum point contact leads are narrowed to support just a few modes. In addition, the value of the phase breaking time in the high resistance regime shows considerable device dependent variations, indicative of a sensitivity of phase coherence to device dependent disorder. In order to account for these observations, we suggest an interpretation of phase breaking which invokes the discrete nature of the level spectrum in the open dots and which emphasizes the role of the quantum point contacts in selectively exciting dot eigenstates.

Phase breaking of nonequilibrium electrons in a ballistic quantum dot

A study of phase breaking of equilibrium and nonequilibrium electrons in a ballistic quantum dot is presented. Estimates for the phase breaking time are obtained from the magnitude of the weak-localization peak, observed in the conductance on sweeping either magnetic field or dc bias voltage. These investigations reveal that the phase breaking of equilibrium and nonequilibrium electrons exhibits the same functional dependence on energy. In particular, at sufficiently high energies the phase breaking time is found to vary as an inverse square law of energy, consistent with predictions for electron-electron scattering due to Coulomb interaction in quantum dots. At lower energies, however, a saturation in the phase breaking rate is observed, the onset of which is found to be well correlated to the average level spacing of the dot.

Phase Breaking as a Probe of the Intrinsic Level Spectrum of Open Quantum Dots

We measure the dependence of the phase breaking time on temperature and on dc voltage in ballistic split-gate quantum dots, the lead openings of which are defined by means of quantum point contacts. Estimates for the phase breaking time are obtained by studying the high-field evolution of the reproducible fluctuations, observed in the low-temperature magneto-conductance of the devices, and from the quenching of quantum interference induced by the dc bias. Varying either temperature or dc bias, a saturation of the phase breaking time is observed for sufficiently low values of these parameters. The energy scale at which the saturation onsets is found to be comparable to the average spacing of discrete levels within the quantum dot, indicative of a crossover to zero-dimensional phase breaking. Based on these observations, we suggest that studies of electron phase coherence may provide a powerful probe of the intrinsic eigenspectra of open quantum dots.

Phase breaking of trapped electrons in a gated quantum dot

We have observed conductance fluctuations due to electron interference in a split-gate quantum dot, fabricated in the two-dimensional electron gas of a GaAs/AlxGa1−xAs heterojunction. We have determined the phase breaking time of electrons by two independent analyses, using the correlation field and the amplitude of the fluctuations. Phase breaking times obtained by these distinct approaches are found to differ from each other by a factor of six.

Conductance Fluctuations in a Metallic Wire Interrupted by a Tunnel Junction

The conductance fluctuations of a metallic wire which is interrupted by a small tunnel junction has been explored experimentally. In this system, the bias voltage V, which drops almost completely inside the tunnel barrier, is used to probe the energy dependence of conductance fluctuations due to disorder in the wire. We find that the variance of the fluctuations is directly proportional to V. The experimental data are consistently described by a theoretical model with two phenomenological parameters: the phase breaking time at low temperatures and the diffusion coefficient.

Phase Breaking in a Quantum Billiard

We introduce a theory on phase breaking time, which predicts the temperature dependence of phase breaking time as 1/τϕ∝ kBT and geometrical effects characteristic of ballistic quantum dots. We also compare the result with the experimental data.

Weak Antilocalization in Si δ-Doped InxGa1-xAs Systems

We have investigated the transport properties of a quasi-two-dimensional electron gas in Si δ-doped Inx Ga1-x As/GaAs (001) systems with x=0.2 and 0.3 at T=0.3–3 K. A large electron density of 8.4×1016 m-2 is obtained for the doping density 1017 m-2. We have evaluated the phase breaking time and the spin-orbit scattering time by means of the weak antilocalization effect. The two-dimensional electron-electron interaction in disordered systems is found to be responsible for the dephasing.

Magnetization of a diffusive ring: Beyond the perturbation theory

Average persistent current over a set of diffusive metallic rings with fixed number of electrons is considered. We study the case in which the phase breaking time is much greater than an inverse average interlevel distance. In such a case, many return events for an electron must be taken into account. As a result, one arrives at a nonperturbative problem for a cooperon mode fixed by an external magnetic field. This multi-cooperon problem has been considered previously by Altland et al., [Europhys. Lett. 20, 155 (1992)] and in several following papers within the framework of supersymmetric approach. Such an approach involves very tedious calculations which were performed using a computer algebraic package. Here we solve the problem in question with the help of a replica trick. It is demonstrated that the replica trick in combination with a proper analytical continuation in the replica space allows one to obtain the result in much more explicit way.