Spin-polarized transport in graphene nanoribbon superlattices

By the Green’s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice(ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.     

Quantum interference and ballistic transmission in nanotube electron waveguides

The electron transport properties of well-contacted individual single-walled carbon nanotubes are investigated in the ballistic regime. Phase coherent transport and electron interference manifest as conductance fluctuations as a function of Fermi energy. Resonance with standing waves in finite-length tubes and localized states due to imperfections are observed for various Fermi energies. Two units of quantum conductance 2G(0) = 4e(2)/h are measured for the first time, corresponding to the maximum conductance limit for ballistic transport in two channels of a nanotube.     

Quantum interference in carbon-nanotube electron resonators

A new mechanism is proposed to explain the slow conductance fluctuations in the conductance-gate voltage plot observed in the nanotube electron resonators. It is found that the slow conductance fluctuation is an intrinsic quantum interference phenomenon and exists in all metallic nanotube resonators except zigzag ones. Analytical expressions for both slow and rapid oscillation periods of the conductance fluctuations have been derived, which are well consistent with the existing experiments. It is predicted that the ratio of the slow oscillation period to the rapid one is independent of the gate-voltage efficiency, and determined only by the nanotube length used in experiments.     

Quantum adiabatic transport in the presence of a finite temperature electrodynamic environment

We investigate the influence of electromagnetic fluctuations on quantum transport in a two-dimensional electron gas. We calculate the conductance of a quantum point contact under the influence of transport and gate-voltage fluctuations at finite temperature, using a generalized Landauer-Büttiker approach. The fluctuations are described by a suitable bath of bosons. In contrast to fluctuations of the gate-voltage, transport voltage fluctuations can completely block the electron transport at T = 0. This blockade is lifted as a result of finite temperature of the electrons in the Fermi reservoirs and also of the coupled bosons. In a typical experiment, these two temperatures need not to be the same. We show that the temperature of the coupled bosons limits the accuracy of the conductance quantization of a quantum point contact to a few percent.     

Fano resonance in graphene anti-dot and periodic graphene anti-dot arrays

We study the electron transport properties of graphene anti-dot and periodic graphene anti-dot arrays using the nonequilibrium Green?s function method and Landauer–Büttiker formula. Fano resonant peaks are observed in the vicinity of Fermi energy, because discrete states coexist with continuum energy states. These peaks move closer to Fermi energy with increasing the width of anti-dots, but move away from the Fermi energy with increasing the length of anti-dots. When N periodic anti-dots exist in the longitude direction, a rapid fluctuation appears in the conductance with varying resonance peaks, which is mainly from the local resonances created by quasibound state. When P periodic anti-dots exist in the transverse direction, P-fold resonant splitting peaks are observed around the Fermi energy, owing to the symmetric and antisymmetric superposition of quasibound states.     

Mesoscopic conductance fluctuations in graphene

We study fluctuations of the conductance of micron-sized graphene devices as a function of the Fermi energy and magnetic field. The fluctuations are studied in combination with analysis of weak localization which is determined by the same scattering mechanisms. It is shown that the variance of conductance fluctuations depends not only on inelastic scattering that controls dephasing but also on elastic scattering. In particular, contrary to its effect on weak localization, strong intervalley scattering suppresses conductance fluctuations in graphene. The correlation energy, however, is independent of the details of elastic scattering and can be used to determine the electron temperature of graphene structures.     

Pseudogap formation in copper oxide superconductors with inclusion of spin and charge fluctuations

The influence of spin and charge fluctuations on the pseudogap formation in cuprate superconductors has been studied using the diagram technique for Hubbard operators. It has been shown that the joint inclusion of the spin and charge fluctuations leads to the formation of “shadow” bands with a strong modulation of the spectral intensity and to a decrease in the density of electronic states at the Fermi level.     

开口悬挂端对单壁碳纳米管电子输运特性的影响

采用紧束缚模型研究了悬挂端对单壁碳纳米管电子输运特性的影响.结果表明：有限长悬挂端开口碳纳米管的电导在费米能级附近作周期性振荡.椅型(armchair)碳纳米管的振荡同时具有快、慢两个准周期，而锯齿型(zigzag)碳纳米管的振荡仅有一个周期；碳纳米管电导在费米能级附近的振荡周期随着悬挂端的增长而减小.研究还发现：有限长悬挂端开口碳纳米管的平均电导随探针与碳纳米管间耦合强度的增加而增大，其大小约为无限长悬挂端开口碳纳米管平均电导的两倍. 关键词： 输运特性 碳纳米管 紧束缚模型     

Subband structure of a two-dimensional electron gas formed at the polar surface of the strong spin-orbit perovskite KTaO3

We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO3. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, our measurements provide a direct upper bound for the potential Rashba spin splitting of only Δk(parallel)}~0.02 ?(-1) at the Fermi level. The polar nature of the KTaO3(100) surface appears to help mediate the formation of the 2DEG as compared to nonpolar SrTiO3(100).     

InAlN/GaN异质结二维电子气波函数的变分法研究

使用变分法推导了InAlN/GaN异质结二维电子气波函数和基态能级的解析表达式,并讨论了InAlN/GaN异质结结构参数对二维电子气电学特性的影响.在假设二维电子气来源于表面态的前提下,使用了一个包含两个变分参数的尝试波函数推导电子总能量期望值,并通过寻找能量期望极小值确定变分参数.计算结果显示,二维电子气面密度随InAlN厚度的增大而增大,且理论结果与实验结果一致.二维电子气面密度增大抬高了基态能级与费米能级,并保持二者之差增大以容纳更多电子.InAlN/GaN界面处的极化强度失配随着In组分增大而减弱,二维电子气面密度随之减小,并导致基态能级与费米能级减小.所建立的模型能够解释InAlN/GaN异质结二维电子气的部分电学行为,并为电子输运与光学跃迁的研究提供了解析表达式.     

Non-Fermi-liquid behavior of compressible electron states on the lowest Landau level

Experiments show that at even denominator fractions (EDF) (7p = 1=2;3=4;3=2,...) the two-dimensional electron gas (2DEG) in a strong magnetic field becomes compressible, has no energy gap, and demonstrates the presence of an ostensible Fermi surface (FS). Since this phenomenon results from a minimization of the interaction, rather than the kinetic energy, the EDF states might well exhibit deviations from a conventional Fermi liquid (FL). We show that impurity scattering and its interference with electronelectron and electron-phonon interactions provide examples of intrinsically non-Fermi-liquid (NFL) transport at EDFs.     

Influence of electromagnetic fluctuations on the resonant tunneling of electrons

We have considered the influence of electromagnetic fluctuations on electron tunneling via one non-degenerate resonant level, the problem that is relevant for electron transport through quantum dots in the Coulomb blockade regime. We show that the overall effect of the fluctuations depends on whether the electron bands in external electrodes are empty or filled. In the empty band case, depending on the relation between the tunneling rate Γ and characteristic frequency Ω of the fluctuations, the field either simply shifts the conductance peak (for rapid tunneling, Γ Ω) or broadens it (for Γ Ω). In the latter case, the system can be in three different regimes for different values of the coupling g between electrons and the field. Increasing interaction strength in the region g < 1 leads to gradual suppression of the conductance peak at the bare energy of the resonant level ε₀, while at g 1 it leads to the formation of a new peak of width at the energy ε₀ + E_cis a charging energy. For intermediate values of g the conductance is non-vanishing in the entire energy range from ε₀ to ε₀ + E_c. For filled bands the problem is essentially multi-electron in character. One consequence of this is that, in contrast to the situation with the empty band, the fluctuations of the resonant level do not suppress conductance at resonance for g < 1. At g> 1 a Coulomb gap appears in the position of the resonant level as a function of its bare energy which leads to suppression of conductance.     

Design of electron wave filters in monolayer graphene with velocity modulations

We compare the transport properties of electrons in monolayer graphene by modulating the Fermi velocity inside the barrier. A critical transmission angle is found only when the Fermi velocity in the barriers is larger than the one outside the barriers. It is shown that the transmission exhibits periodicity with the incident angle below the critical transmission angle, and attenuates exponentially in the opposite situation. For both situations, peak splitting occurs in the transmission as the number of the velocity barriers increases, and the characteristics of the transmission suggest an interesting application of an excellent band-pass filter. The dependence of the conductance on the Fermi energy through an identical velocity- modulation structure differs wildly with different Fermi velocities of the barrier. The counterpart of the peak splitting is the sharp oscillations in the conductance profile. Furthermore, some oscillations for the multiple barriers are so sharp that the structure may be used as an excellent sensor.     

A two-dimensional electron gas as a sensitive detector to observe the charge carrier dynamics of self-assembled QDs

The carrier tunneling dynamics of self-assembled InAs quantum dots (QD) is studied using a time-resolved conductance measurement of a nearby two-dimensional electron gas (2DEG). The investigated heterostructures consist of a layer of QDs with different coupling strengths to a 2DEG, adjusted by different thicknesses of the spacer layers. We demonstrate a strong influence of charged QDs on the conductance of the 2DEG, even for very weak coupling between the QD layer and the 2D system, where standard capacitance (C)–voltage (V) spectroscopy is unsuitable to investigate the electronic structure of these QDs.     

Quantum interference effects in semiconductor–superconductor microjunctions

We investigate the quantum interference effects in quantum dots of a two-dimensional electron gas attached to a superconductor. When the dot size is comparable to the Fermi wavelength of an electron, transmission resonance shows up in the conductance as distinct peaks and dips. The coupling of electron-like and hole-like excitations by the Andreev reflection leads to a rich variety of behavior of the resonance, in particular, against the bias voltage. Enlarging the dot size, the transmission resonance evolves into conductance fluctuations. The low-magnetic-field conductance fluctuations are shown to be remarkably geometry-specific in comparison to those in the normal counterparts.     

Plasmons in the modulated and confined 2DEG: A Raman scattering study

Lateral wire arrays have been fabricated from a single modulation-doped GaAs quantum well employing reactive ion etching. Depending on the etch depth, the two-dimensional electron gas (2DEG) in the well acquires different degrees of modulation up to complete confinement. The different regimes are identified by their unique photoluminescence and Raman spectra. Deep-etched wires show plasmon resonances down to a width of 100nm.     

Side-gate modulation of critical current in mesoscopic Josephson junction

We study the normal state conductance and the Josephson current in a superconductor–2DEG–superconductor structure where the size/shape of the 2DEG-region can be modified by an additional side-gate electrode. The considered transport properties follow from the retarded Green function which we compute by employing a tight-binding-like representation of the Hamiltonian in the 2DEG region. Our model studies offer a qualitative demonstration of the recently observed effects caused by side-gate modulation.     

Electronic transport in mesoscopic superconductor/2D electron gas junctions in strong magnetic field

The hybrid superconductor/2D electron gas (S/2DEG) structures based on InGaAs-InP hetero-junctions with a high-mobility 2D electron gas and superconducting NbN electrodes have been investigated. The electronic transport and current-voltage characteristics of S/2DEG/normal metal (S/2DEG/N) structures in strong perpendicular magnetic fields have been studied. Oscillations in the magnetoresistance of S/2DEG/N structures have been found in strong magnetic fields. It is shown that at bias voltages lower than the superconducting gap the amplitude of oscillations in S/2DEG/N structures significantly exceeds the oscillation amplitude in the reference N/2DEG/N samples. The experimental results can be explained within the quasiclassical theory of magnetotransport in S/2DEG structures developed by N.M. Chtchelkatchev and I.S. Burmistrov (Phys. Rev. B, 2007, vol. 75, 214 510).     

Transport properties of a 2DEG in the presence of tilted magnetic field: the influence of a 1D modulation

The effect of 1D periodic modulation on the transport and thermodynamic properties of a non-interacting two-dimensional electron gas (2DEG) is investigated. The Hamiltonian used also includes a tilted magnetic field, Rashba, and Dresselhauss spin-orbit interactions. The 1D periodic modulation introduces non-quantized regions on Hall conductivity and a non-zero diffusive conductivity. A method to estimate the modulation periodicity is given using the periodicity of Weiss oscillations on the diffusive conductivity.     

Transport properties and typical electron orbits in 2DEG under a local gradient magnetic field

Local current information in nanodevices is very helpful for good understanding the fundamental physical phenomena. In this work, the electron motion in 2DEG under a local gradient magnetic field is studied by the recursive non-equilibrium green's function method. The considered structure is a sandwiched configuration, lead-device-lead. For clearly observing the electron trajectories, we use the wide and narrow leads to contact the device respectively. The results show that in wide lead case, the quantized conductance plateaus with oscillations are found. These oscillations might be related to the resonant transmission in longitudinal electronic states. We also observe several typical snake electron orbits when the corresponding incident energy locates at the odd conductance plateaus, because there is exactly one opened transport channel around the horizontal middle line of device. When we shrink the width of leads and let electrons inject only from the lower half of device, the conductance suppression is very significant due to the energy level broadening in center device. In this case, we can find clear S-shape and 8-shape electron orbits at a very wide range of energy. These results can help us to understand the basic physics in 2D electron systems and may guide the design of future photoelectric device.