Effects of quantum coupling on the performance of metal-oxide-semiconductor field transistors

Based on the analysis of the three-dimensional Schrödinger equation, the effects of quantum coupling between the transverse and the longitudinal components of channel electron motion on the performance of ballistic MOSFETs have been theoretically investigated by self-consistently solving the coupled Schrödinger-Poisson equations with the finite-difference method. The results show that the quantum coupling between the transverse and the longitudinal components of the electron motion can largely affect device performance. It suggests that the quantum coupling effect should be considered for the performance of a ballistic MOSFET due to the high injection velocity of the channel electron.     

Charge transport in a spin-polarized 2D electron system in silicon

The temperature dependences of the conductivity σ(T) of a strongly interacting 2D electron system in silicon have been analyzed both in zero magnetic field and in a spin-polarizing magnetic field of 14.2 T that is parallel to the sample plane. The measurements were carried out in a wide temperature range of 1.4–9 K in the ballistic regime of electron-electron interaction, i.e., for Tτ > 1. In zero magnetic field, the data obtained for σ(T) are quantitatively described by the theory of interaction corrections. In the fully spin-polarized state, the measured σ(T) dependences are not linear and even nonmonotonic in the same temperature range, where the dependences σ(T) are monotonic in the absence of the field. Nevertheless, the low-temperature parts of the experimental σ(T) dependences are linear and qualitatively consistent with the calculated quantum corrections.     

Interplay of Ehrenfest and dephasing times in ballistic conductors

Quantum interference corrections in ballistic conductors require a minimal time: the Ehrenfest time. In this Letter, we investigate the fate of the interference corrections to quantum transport in bulk ballistic conductors if the Ehrenfest time and the dephasing time are comparable.     

Universal quantum computation through control of spin-orbit coupling

We propose a method for quantum computation which uses control of spin-orbit coupling in a linear array of single electron quantum dots. Quantum gates are carried out by pulsing the exchange interaction between neighboring electron spins, including the anisotropic corrections due to spin-orbit coupling. Control over these corrections, even if limited, is sufficient for universal quantum computation over qubits encoded into pairs of electron spins. The number of voltage pulses required to carry out either single-qubit rotations or controlled-Not gates scales as the inverse of a dimensionless measure of the degree of control of spin-orbit coupling.     

Integral and local density of states of InAs quantum dots in GaAs/AlGaAs heterostructure observed by ballistic electron emission spectroscopy near one-electron ground state

Density of states is studied by a ballistic electron emission microscopy/spectroscopy on self-assembled InAs quantum dots embedded in GaAs/AlGaAs heterostructure prepared by metal–organic vapor phase epitaxy. An example of integral quantum dot density of states which is proportional to superposition of a derivative of ballistic current–voltage characteristics measured at every pixel (1.05 nm×1.05 nm) of quantum dot is presented. For the two lowest observed energy levels of quantum dot (the maxima in density of states) the density of states is mapped and correlated with the shape of quantum dot. It was found that prepared quantum dots have a few peaks on their flatter top and a split of the lowest energy level can be observed. This effect can be explained by inhomogeneous (nonuniform) stress distribution in the examined quantum dot.     

Charge storage and electron/light emission properties of silicon nanocrystals

Monodispersed silicon nanocrystals show novel electrical and optical characteristics of silicon quantum dots, such as single-electron tunneling, ballistic electron transport, visible photoluminescence and high-efficiency electron emission.Single-electron memory effects have been studied using a short-channel MOSFET incorporating Si quantum dots as a floating gate. Surface nitridation of Si nanocrystal memory nodes extends the charge-retention time significantly. Single-electron storage in individual Si dots has been evaluated by Kelvin probe force microscopy.Photoluminescence and electron emission are observed for surface-oxidized silicon nanocrystals. Efficiency of the no-phonon-assisted transition increases with decreasing core Si size. Electron emission efficiency as high as 5% has been achieved for the Si-nanocrystal-based cold electron emitter devices. The non-Maxwellian energy distribution of emitted electrons suggests that the mechanism of electron emission is due to ballistic transport through arrays of surface-oxidized Si nanocrystals. Combined with the ballistic electron emission, the quasi-direct light emission properties can be used for developing Si-based lasers.     

Ballistic conductance in kane type semiconductor quantum wire

The energy spectrum, ballistic conductance of an electron on the surface of a Kane type semiconductor hollow cylinder has been calculated by using the Kane equation with an additional term that takes into account the spin-orbit (SO) interaction. This term, known as Rashba term, occurs for asymmetric quantum wells, where two directions on the normal n are physically nonequivalent. If Rashba spin-orbital interaction is incorporated into energy spectrum, it leads to the emergence of new extrema. We obtained electron energy spectrum, which depends on the sign of the effective spin orbital constant. The energy spectrum of electrons has two branches when the magnetic field does not exist. One of these branches has only one minimum while the other branch has one maximum around k = 0 and two minima. The external magnetic field can control these extrema which occur in the event transport. The results were used to obtain the ballistic conductance at finite temperature of the Kane type hollow cylinder. It has been found that the presence of additional local extremum points in the subband of the electronic spectrum leads to a nonmonotonic dependence of the ballistic conductance of the system on the chemical potential. The g-factor of electrons was observed to depend on Rashba parameter in a linear manner. The effect of finite temperature smears out the sharp steps in the zero-temperature conductance.     

Ballistic transport in PbTe-based nanostructures

We describe our study of ballistic transport in nanostructures of lead telluride, PbTe. Submicron devices have been fabricated by electron beam lithography and chemical etching of 50 nm wide PbTe single quantum wells embedded between Pb_0.92Eu_0.08Te barriers grown by MBE on BaF₂. The electron concentration in the devices was tuned by the gate voltage applied across an interfacial p–n junction. The most important observation was zero-magnetic field conductance quantization (in multiplies of 2e²/h) in narrow constrictions of dimensions comparable to electron mean free path calculated from transport mobility. This indicates considerable relaxation of requirements for quantum ballistic transport in comparison with other materials. We argue that the huge static dielectric constant of PbTe (₀=1350 at 4.2 K) leads to suppression of the long-range Coulomb potentials of charged impurities and, thus, provides favorable conditions for the conductance quantization.     

Gate-controlled electron–electron interactions in an In0.53Ga0.47As/InP quantum well structure

We study the parabolic negative magnetoresistivity in a gated In_0.53Ga_0.47As/InP quantum well structure where the scattering potential is predominantly long range. This magnetoresistivity is caused by the electron–electron interactions and is fitted to estimate the interaction corrections to the Drude conductivity. These corrections are smaller than the prediction of a recent theory [I.V. Gornyi, A.D. Mirlin, Phys. Rev. Lett. 90 (2003) 076801], and can be quantitatively described by Altshuler’s theory.     

Ballistic phonon studies in the lowest Landau level

We report time-resolved studies of ballistic phonon absorption in the fractional quantum Hall regime at Landau level filling factors of and . The technique used can resolve the interaction of the two-dimensional electron system with LA and TA phonons and has been used to measure the temperature variation of the heat capacity of a single layer of electrons at . The energy gaps at have also been measured and found to be in good agreement with theory. The roles of compressible and incompressible regions in the phonon absorption process are discussed. Angle resolved measurements at are also in good agreement with theory.     

Diffusion and ballistic contributions of electron-electron interaction to the conductivity in an Al0.26Ga0.74N/AlN/GaN heterostructure

The results of an experimental study of quantum correction of electron-electron interaction (EEI) to the conductivity of two-dimensional electron gas (2DEG) in an undoped heterostructure are reported. A small but significant decrease of the Hall slope with the increase of temperature was discovered. This is not due to the increase of electron concentration as temperature increases but to the EEI effect. Both diffusion and ballistic contributions of EEI to the conductivity of 2DEG were observed. As the temperature increases, the negative diffusion EEI correction to the conductivity increases in an absolute value while the ballistic EEI correction reduces to a renormalization of the transport mobility.     

Logarithmic corrections to $${\mathcal{N} = 2}$$ black hole entropy: an infrared window into the microstates

Logarithmic corrections to the extremal black hole entropy can be computed purely in terms of the low energy data—the spectrum of massless fields and their interaction. The demand of reproducing these corrections provides a strong constraint on any microscopic theory of quantum gravity that attempts to explain the black hole entropy. Using quantum entropy function formalism we compute logarithmic corrections to the entropy of half BPS black holes in N=2{{\mathcal N}=2} supersymmetric string theories. Our results allow us to test various proposals for the measure in the OSV formula, and we find agreement with the measure proposed by Denef and Moore if we assume their result to be valid at weak topological string coupling. Our analysis also gives the logarithmic corrections to the entropy of extremal Reissner–Nordstrom black holes in ordinary Einstein–Maxwell theory.     

Electron self-energy in nonlocal field theory

In the framework of quantum electrodynamics with the nonlocal interaction it is shown that the correspondence principle holds in the problem on the self-mass of an electron and a particle with arbitrary spin. It appears that the second order of perturbation theory in the limit ? → 0 gives just the classical expression for the electron self-energy, and all higher order corrections are zero.     

Ballistic transport and quantum interference in InSb nanowire devices

An experimental realization of a ballistic superconductor proximitized semiconductor nanowire device is a necessary step towards engineering topological quantum electronics. Here, we report on ballistic transport in In Sb nanowires grown by molecular-beam epitaxy contacted by superconductor electrodes. At an elevated temperature, clear conductance plateaus are observed at zero magnetic field and in agreement with calculations based on the Landauer formula. At lower temperature, we have observed characteristic Fabry–Pérot patterns which confirm the ballistic nature of charge transport.Furthermore, the magnetoconductance measurements in the ballistic regime reveal a periodic variation related to the Fabry–Pérot oscillations. The result can be reasonably explained by taking into account the impact of magnetic field on the phase of ballistic electron's wave function, which is further verified by our simulation. Our results pave the way for better understanding of the quantum interference effects on the transport properties of In Sb nanowires in the ballistic regime as well as developing of novel device for topological quantum computations.     

Nonlinear magnetoconductance of a classical ballistic system

We study nonlinear transport through a classical ballistic system accounting for the Coulomb interaction between electrons. The joint effect of the applied bias V and magnetic field H on the electron trajectories results in a component of the nonlinear current I(V,H) which lacks the H-->-H symmetry: deltaI=alpha(cl)V(2)H. At zero temperature the magnitude of alpha(cl) is of the same order as that arising from the quantum interference mechanism. At higher temperatures the classical mechanism is expected to dominate due to its relatively weak temperature dependence.     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.     

Weak localization in mesoscopic hole transport: berry phases and classical correlations

We consider phase-coherent transport through ballistic and diffusive two-dimensional hole systems based on the Kohn-Luttinger Hamiltonian. We show that intrinsic heavy-hole-light-hole coupling gives rise to clear-cut signatures of an associated Berry phase in the weak localization which renders the magnetoconductance profile distinctly different from electron transport. Nonuniversal classical correlations determine the strength of these Berry phase effects and the effective symmetry class, leading even to antilocalization-type features for circular quantum dots and Aharonov-Bohm rings in the absence of additional spin-orbit interaction. Our semiclassical predictions are confirmed by numerical calculations.     

Correlated electron current and temperature dependence of the conductance of a quantum point contact

We investigate finite temperature corrections to the Landauer formula due to electron–electron interaction within the quantum point contact. When the Fermi level is close to the barrier height, the conducting wavefunctions become peaked on the barrier, enhancing the electron–electron interaction. At the same time, away from the contact the interaction is strongly suppressed by screening. To describe electron transport we formulate and solve a kinetic equation for the density matrix of electrons. The correction to the conductance G is negative and strongly enhanced in the region 0.5 × 2e²/h ≤ G ≤ 1.0 × 2e²/h. Our results for conductance agree with the so-called “0.7 structure” observed in experiments.     

2D-1D coupling in cleaved edge overgrowth

We study the scattering properties of an interface between a one-dimensional (1D) wire and a two-dimensional (2D) electron gas. Experiments were conducted in the highly controlled geometry provided by molecular bean epitaxy overgrowth onto the cleaved edge of a high quality GaAs /AlGaAs quantum well. Such structures allow for the creation of variable length 1D-2D coupling sections. We find ballistic 1D electron transport through these interaction regions with a mean free path as long as 6 &mgr;m. Our results explain the origin of the puzzling nonuniversal conductance quantization observed previously in such 1D wires.     

Transit-time frequency quantum beats and THz negative conductance in nanoscale semiconductor heterostructures

A quantum-mechanical calculation of the THz negative conductance of hot ballistic electrons in nanoscale semiconductor heterostructures has been performed. It is shown that quantum beats of the ballistic electron wave function induce an ac current in the structure, which can do negative work on the ac field.