Magneto-coulomb oscillations in GaAs-AlGaAs quantum dot structures

We report on experiments of the magnetotransport properties of GaAs-AlGaAs lateral quantum dots. At high magnetic fields for a 1 μm square dot structure, current flow occurred via edge states and, with the point contacts adjusted to allow transmission of one or more edge states, a strong backscattering resonance followed by short period oscillations were observed in the magnetoresistance, as B increased. At higher fields for a 2 μm dot, we observe a rapid rise in the magnetoresistance associated with the depopulation of the point contacts and the isolation of the dot from the leads. At still higher fields there occur periodic oscillations whose period was two orders of magnitude larger than would result from interference, or Aharonov-Bohm type effects.We analyze these phenomena using self-consistent electronic structure calculations for our devices. In particular, we show that the evolution of the terrace like structure of the potential profile profoundly affects the single particle spectrum within the dot when several Landau levels are occupied. For the large dot device, we expect that in the high field regime with the dot isolated from the leads, only a single Landau level is occupied in both the dot and the 2DEG region. In this regime, tunneling into and out of the dot is regulated by charging effects. We have introduced a "magneto-Coulomb oscillations" explanation of the periodic resonances that are observed.     

Influence of the coulomb blockade effect on heat transfer in a one-dimensional system of spinless electrons

An analysis is made of some characteristics of the low-temperature thermal conductivity of a ballistic quantum dot, attributed to the influence of long-range Coulomb interaction in the geometric capacitance approximation. It is shown that at fairly low temperatures the thermal conductivity K exhibits Coulomb oscillations as a function of the electrostatic potential of the quantum dot. At the maximum of the Coulomb peak we find K ∝ T whereas at the minimum K ∝ T ³. The dependence K(T) is essentially nonmonotonic at temperatures corresponding to the characteristic spacing between the size-quantization levels in the quantum dot.     

Interference of ballistic electrons in an open quantum dot at high temperatures

The Aharonov-Bohm-type oscillations in the ballistic magnetoconductivity of an open quantum dot ～1 μm in size created in the GaAs/AlGaAs heterostructure by local anode oxidation have been studied. The measurements have been performed at temperatures of 4.2 and 1.5 K, which are high enough to expect a considerable suppression of the oscillations with period h/e (in magnetic flux units). The magnetoconductivity oscillations with a period less than the quantum h/e with respect to the magnetic field are observed at 1.5 K. The explanation is proposed on the basis of the interference of the electrons moving along the time-reversed paths inscribed into the quantum dot that have their initial and final points at one of the contacts, i.e., corresponding to a period of h/2e.     

Terahertz photoresponse of quantum wires in magnetic fields

We have investigated the terahertz photoresponse of quantum wires in high magnetic fields, employing intense far-infrared (FIR) radiation from the UCSB Free-Electron Lasers. Both GaAs-based and InAs-based quantum wires, with widths ranging from 50 nm to 1 μm, were studied. At high FIR power we observed Shubnikov–de Haas type oscillations in photoresponse versus magnetic field,B, resulting from non-resonant electronic heating the oscillations were much more pronounced than those in resistance versusB. At low FIR power we observed resonant peaks due to magnetoplasmon excitations, whose strength shows strong polarization-dependence and whose energy extrapolates to a finite value at zeroB. These results provide a powerful tool for characterizing 1D electronic states in quantum wires.     

Coulomb blockade in a lateral triangular small quantum dot

An AlGaAs/GaAs lateral quantum dot of triangular shape with a characteristic size L<100 nm (the smallest size for dots of this type) containing less than ten electrons was studied theoretically and experimentally. Single-electron oscillations of the conductance G of this dot were measured at G<e ₂/h. When going from G?e ₂/h to G≈0.5e ₂/h, a decrease was found not only in the amplitude but also in the period of oscillations. A calculation of the electrostatics demonstrated that this effect is due to a change in the dot size produced by control voltages.     

Coulomb blockade and the thermopower of a suspended quantum dot

On the basis of the 2D electron gas in an AlGaAs/GaAs membrane separated from a wafer, a one-electron transistor is created that operates on the Coulomb blockade effect—a two-barrier structure with a quantum dot. The separation of the sample from the wafer, which has a large dielectric constant, leads to a sharp decrease in the total capacity C of the quantum dot and, as a result, to high charge energy E _C = e ²/C and critical temperature T _C = E _C/k _B ≈ 40 K. The dependence of the conductance of the quantum dot on the driving and gate voltages includes a rhombic structure characteristic of the Coulomb blockade effect. The phonon-drag thermopower is found in this system. This thermopower exhibits an anomalous alternating dependence on the gate voltage and intensity of the phonon flux. Possible mechanisms are proposed for explaining the indicated anomalies in the thermopower.     

Coulomb oscillations in partially open quantum dots

We report tunneling measurements of the Coulomb blockade in a single quantum dot at zero magnetic field and dilution refrigerator temperatures with weak tunneling from the dot to one lead (the ‘closed’ lead, conductanceG_closed) and strong tunneling to the other lead (the ‘open’ lead, conductanceG_open). We observe suppression of the Coulomb oscillations withG_open≈2e²/h, and then see the oscillations return forG_open>2e²/h. The oscillations show a strikingly lower threshold temperature atG_open≈2e²/hthan for greater or lesserG_open.     

Binding energy of a hydrogenic impurity in a 2D circular quantum dot

The binding energies of hydrogenic impurity states with an impurity atom located at the center of a two-dimensional circular quantum dot confined by an infinite barrier potential are studied as a function of the dot radius and of the screening parameter in the potential. Accurate binding energies are obtained for the 1s, 2s and 2p states by numerical integration of the 2D Schrödinger equation. The binding energies are found to increase with a decrease in the dot radius, and decrease with an increase in the value of the screening parameter q_sin all cases. Further the levels become unbound at a finite value of the dot radius.     

Time-dependent photon correlations for incoherently pumped quantum dot strongly coupled to the cavity mode

The time dependence of correlations between the photons emitted from a microcavity with an embedded quantum dot under incoherent pumping is studied theoretically. Analytic expressions for the second-order correlation function g ⁽²⁾(t) are presented in strong and weak coupling regimes. The qualitative difference between the incoherent and coherent pumping schemes in the strong coupling case is revealed: under incoherent pumping, the correlation function demonstrates pronounced Rabi oscillations, but in the resonant pumping case, these oscillations are suppressed. At high incoherent pumping, the correlations decay monoexponentially. The decay time nonmonotonically depends on the pumping value and has a maximum corresponding to the self-quenching transition.     

Coulomb oscillations of the ballistic conductance in a quasi-one-dimensional quantum dot

It is demonstrated that localized states of an open quasi-one-dimensional quantum dot can be charged by the Coulomb blockade mechanism. A new effect—Coulomb oscillations of the ballistic conductance—is observed because of the high sensitivity of the ballistic current to single-electron variations of the self-consistent potential of the dot. The model proposed explains experimental results [C.-T. Liang, M.Y. Simmons, C. G. Smith, et al., Phys. Rev. Lett. 81, 3507 (1998)].     

Resonant magnetotunneling through individual self-assembled InAs quantum dots

A single-barrier GaAs/AlAs/GaAs heterostructure, with self-assembled In-based quantum dots incorporated in the AlAs tunnel barrier, exhibits a series of resonant peaks in the low temperature current–voltage characteristics. We argue that each peak arises fromsingle-electrontunneling through thediscrete zero-dimensionalstate of anindividualInAs dot. We use the tunneling for fine probing of the local density of states in the emitter-accumulation layer. Landau-quantized states are resolved at magnetic field B∥ as low as 0.2 T. Spin-splitting of the dot electron states has been observed forB⊥I.     

Mesoscopic spin-flip transport through a hybrid system with a single molecular dot system applied with ac magnetic fields

We have investigated the current for the system of vibrating quantum dot irradiated with a rotating magnetic field and an oscillating magnetic field by nonequilibrium Green's function. The rotating magnetic field rotates with the angular frequency ωr around the z-axis with the tilt angle ?, and the time-oscillating magnetic field is located in the z-axis with the angular frequency ω. Different behaviors have been shown in the presence of electron-phonon interaction (EPI) which plays a significant role in the transport. The current displays asymmetric behavior as the source-drain bias eV=0, novel side peaks or shoulders can be found due to the phonon absorption and emission procedure, and the negative differential resistance becomes stronger as the parameter g increases. Furthermore, the strong EPI also destroys the quasiperiodic oscillations of current in the region μ₀B₁>2.5Δ. The electron transport properties are also significantly influenced by the linewidth function Γ.     

Rabi oscillations in two-level semiconductor systems

Rabi oscillations in coherent optical excitations in bulk GaAs and quantum dot two-level systems may be converted into deterministic photocurrents, with the impurities or dots providing the tag for each qubit. Here we perform a theoretical analysis of the damping of Rabi oscillations in two-level semiconductor systems. Present calculations, through optical Bloch equations on excitonic two-level In_xGa_1−xAs quantum-dot systems, are found in good agreement with the corresponding experimental data. Calculated results indicate that the nature underlying the dephasing mechanism associated to the damping of the measured Rabi oscillations, which has previously remained as an open question, may be associated with a field-dependent recombination rate related to the inhomogeneous broadening of the excitonic lines in the In_xGa_1−xAs two-level QD system.     

Evidence for charging effects in an open dot at zero magnetic field

We have measured the low-temperature transport properties of an open quantum dot formed in a clean one-dimensional channel. At zero magnetic field, continuous and periodic oscillations superimposed upon ballistic conductance steps are observed when the conductance through the dot G exceeds 2e²/h. We ascribe the observed conductance oscillations to evidence for charging effects in an open dot. This is supported by the evolution of the oscillating features for G>2e²/h as a function of both temperature and barrier transparency.     

Quantum modulation effect in a graphene-based magnetic tunnel junction

The electronic (quantum) transport in a NG/F_B/FG tunnel junction (where NG, F_B and FG are a normal graphene layer, a ferromagnetic barrier connected to a gate and a ferromagnetic graphene layer, respectively) is investigated. The motions of the electrons in the graphene layers are taken to be governed by the Dirac Equation. Parallel (P) and antiparallel alignment (AP) of the magnetizations in the barrier and in the ferromagnetic graphene are considered. Our work focuses on the oscillation of the electrical conductance (Gq), of the spin conductance (Gs) and of the tunneling magneto resistance (TMR) of this magnetic tunnel junction. We find that, the quantum modulation due to the effect of the exchange field in F_B will be seen in the plots the conductance and of the TMR as functions of the thickness of ferromagnetic barrier (L). The period of two multiplied sinusoidal terms of the modulation are seen to be controlled by varying the gate potential and the exchange field of the F_B layer. The phenomenon, a quantum beating, is built up with two oscillating spin conductance components which have different periods of oscillation related to the splitting of Dirac's energies in the F_B region. The amplitudes of oscillations of Gq, Gs and TMR are not seen to decrease as the thickness increases. The decaying behaviors seen in the conventional transport through an insulator do not appear.     

On the possibility of the dynamic self-polarization of nuclear spins in a quantum dot

The possibility of self-polarization of nuclear spins predicted by M.I. D’yakonov and V.I. Perel’ (JETP Lett. 16, 398 (1972)) has been investigated in the case of the electric current passing through a single quantum dot. The mechanisms of nuclear spin relaxation in the quantum dot leading to the polarization and depolarization of the nuclei are discussed. To make the nuclear polarization possible, it has been proposed to increase the nuclear polarization rate via the interaction of an electron localized in the quantum dot with electromagnetic oscillations in an electric circuit, whose proper frequency is tuned to a resonance with the Zeeman splitting of an electron level in the quantum dot.     

Proton-positron oscillations

The possibility that baryonic and leptonic matter in nature can oscillate into one another is considered. Consistent with electric charge conservation this leads to proton-positron (or anti-proton-electron) oscillations that conserve baryon (B) minus lepton (L) number. Other equally likely oscillations are neutron-neutrino oscillations that conserve (B+L)-number and neutron-antineutrino oscillations that conserve (B?L) number. Simple theories like the SU(5) and SO(10) predict the transition probability of such modes to be ? 10^?60.     

Nonlinear magnetotransport in a high-mobility quasi-two-dimensional electron system

The influence of a dc electric current I _dc on the low-temperature magnetotransport of high-mobility electrons in a GaAs double quantum well with two occupied size-quantization levels has been studied. The oscillations of the resistance ρ _xx , which are periodic in the inverse magnetic field, have been shown to appear in the quasitwo-dimensional system under consideration at a temperature of T = 4.2 K in magnetic fields B > 0.1 T; the oscillations are caused by isoenergetic resonance transitions of the electrons between the Landau levels of different subbands. The inversion of the oscillations with an increase in I _dc has been discovered. It has been found that the observed effect is due to the electron transport in a nonlinear regime.     

Magnetotransport of the low-carrier density one-dimensional S=1/2 antiferromagnet Yb4As3

The transport properties of the semimetallic quasi-one-dimensional S=1/2 antiferromagnet Yb₄As₃ have been studied by performing low-temperature (T≥0.02 K) and high magneticfield (B≤60 T) measurements of the electrical resistivity ρ(T, B). For T ≿ 2 K a ‘heavy-fermion’-like behavior Δρ(T)=AT ² with huge and nearly field-independent coefficient A ≈ 3 μΩ cm/K² is observed, whereas at lower temperatures ρ(T) deviates from this behavior and slightly increases to the lowest T. In B>0 and T ≾ 6 K the resistivity shows an anomalous magnetic-history dependence together with an unusual relaxation behavior. In the isothermal resistivity Shubnikov-de Haas (SdH) oscillations, arising from a low-density system of mobile As-4p holes, with a frequency of 25 T have been recorded. From the T- and B-dependence of the SdH oscillations an effective carrier mass of (0.275±0.005)m ₀ and a charge-carrier mean-free path of 215 ? are determined. Furthermore, in B≥15 T, the system is near the quantum limit and spin-splitting effects are observed.     

Generation of dislocation loops in strained quantum dots embedded in a heterolayer

The generation of prismatic dislocation loops in strained quantum dots is investigated. The quantum dots are embedded in a film-substrate heterostructure with mechanical stresses caused by the difference between the lattice parameters of the film (heterolayer) and the substrate. The intrinsic plastic strain ?_m of a quantum dot arises from the misfit between the lattice parameters of the materials of the quantum dot and the surrounding matrix. The interface between the heterolayer and the substrate is characterized by a misfit parameter f. The critical radius of a quantum dot R _c at which the generation of a dislocation loop in the quantum dot becomes energetically favorable is analyzed as a function of the intrinsic plastic strain ?_m and the misfit parameter f.