Characteristics of transmission of electrons in a quantum wire tangentially attached to a superconductor ring threaded by magnetic flux

We present numerical investigations of the transmission properties of electrons in a normal quantum wire tangentially attached to a superconductor ring threaded by magnetic flux. A point scatterer with a δ -function potential is placed at node to model scattering effect. We find that the transmission characteristics of electrons in this structure strongly depend on the normal or superconducting state of the ring. The transmission probability as a function of the energy of incident electrons, in the case of a superconductor ring threaded by one quantum magnetic flux, emerges one deep dip, imposed upon the first broad bump in spectrum. This intrinsic conductance dip originates from the superconductor state of the ring. When increasing the magnetic flux from one quantum magnetic flux to two, the spectrum shifts toward higher energy region in the whole. This conductance dip accordingly shifts and appears in the second bump. In the presence of a point-scatterer at the node, the spectrum is substantially modified. Based on the condition of the formation of the standing wave functions in the ring and the broken of the time-reserve symmetry of Schr?dinger equation after switching magnetic flux, the characteristics of transmission of electrons in this structure can be well understood. Received 6 November 2001     

Electronic transport through a periodic array of quantum-dot rings

Using the tight-binding approximation and the transfer matrix method, this paper studies the electronic transport properties through a periodic array of quantum-dot (QD) rings threaded by a magnetic flux. It demonstrates that the even--odd parity of the QD number in a single ring and the number of the QD rings in the array play a crucial role in the electron transmission. For a single QD ring, the resonance and antiresonance transmission depend not only on the applied magnetic flux but also on the difference between the number of QDs on the two arms of the ring. For an array of QD rings, the transmission properties are related not only to the even--odd parity of the number $N_{0}$ of QDs in the single ring but also to the even--odd parity of the ring number $N$ in the array. When the incident electron energy is aligned with the site energy, for the array of $N$ rings with $N_{0}={\rm odd}$ the antiresonance transmission cannot occur but the resonance transmission may occur and the transmission spectrum has $N$ resonance peaks ($N-1$ resonance peaks) in a period for $N={\rm odd}$ (for $N={\rm even}$). For the array of $N$ rings with $N_{0}={\rm even}$ the transmission properties depend on the flux threading the ring and the QD number on one arm of the ring. These results may be helpful in designing QD devices.     

Aharonov–Bohm effect in Kane-type semiconductor quantum wire

In the present study, the Aharonov–Bohm effect for bound states in Kane-type quantum wire is analysed. It is demonstrated that the wave function and the energy spectra of carriers depend on the magnetic flux. The energy and the g factor of electrons are of a periodic function of the magnetic flux. The statistical property of the orbital magnetism of electrons at low temperatures and strong magnetic fields is presented. It is shown that the magnetisation of electron oscillations depends on the magnetic flux.     

“Aharonov-Bohm antiferromagnetism” and compensation points in the lattice of quantum rings

We investigate the magnetic properties of the lattice of non-interacting quantum rings using the 2D rotator model. The exact analytic expressions for the free energy as well as for the magnetization and magnetic susceptibility are found and analyzed. It is shown that such a system can be considered as a system with antiferromagnetic-like properties. We have shown also that all observable quantities in this case (free energy, entropy, magnetization) are periodic functions of the magnetic flux through the ring's area (as well known, such a behavior is typical for the Aharonov-Bohm effect). For the lattice of quantum rings with two different geometric parameters we investigate the ordinary compensation points (“temperature compensation points”, i.e. points at which the magnetization vanishes at fixed values of the magnetic field strength). It is shown that the positions of compensation points in the temperature scale are very sensitive to small changes in the magnetic field strength.     

Spin-polarized transmission through a mesoscopic ring with a magnetic impurity

Based on one-dimensional quantum waveguide theory we study the symmetry of the spin-polarized transmission through an Aharonov–Bohm ring with a magnetic impurity, in which the spin-exchange interaction between an incident electron and the magnetic impurity leads to spin–flip scattering. It shows that for some special Fermi energies, both spin-up and spin-down transmission coefficients are symmetric under the flux reversal in the spin–flip scattering process and the spin-polarized conductance also is symmetric. In above case, AB oscillations of spin-down transmission and reflection are perfectly identical. The effect of the exchange interaction strength and Fermi wave vector on transmission behavior of spin-state electrons is examined.     

Persistent current and transmission probability in the Aharonov-Bohm ring with an embedded quantum dot

Persistent current and transmission probability in the Aharonov-Bohm (AB) ring with an embedded quantum dot (QD) are studied using the technique of the scattering matrix. For the first time, we find that the persistent current can arise in the absence of magnetic flux in the ring with an embedded QD. The persistent current and the transmission probability are sensitive to the lead-ring coupling and the short-range potential barrier. It is shown that increasing the lead-ring coupling or the short-range potential barrier causes the suppression of the persistent current and the increasing resonance width of the transmission probability. The effect of the potential barrier on the number of the transmission peaks is also investigated. The dependence of the persistent current and the transmission probability on the magnetic flux exhibits a periodic property with period of the flux quantum.     

Contradiction between the results of observations of resistance and critical current quantum oscillations in asymmetric superconducting rings

Magnetic field dependences of critical current, resistance, and rectified voltage of asymmetric (half circles of different widths) and symmetrical (half circles of equal widths) aluminum rings close to the super-conducting transition were measured. All these dependences are periodic magnetic field functions with periods corresponding to the flux quantum in the ring. The periodic dependences of critical current measured in opposite directions were found to be close to each other for symmetrical rings and shifted with respect to each other by half the flux quantum in asymmetric rings with ratios between half circle widths of from 1.25 to 2. This shift of the dependences by a quarter of the flux quantum as the ring becomes asymmetric makes critical current anisotropic, which explains the effect of alternating current rectification observed for asymmetric rings. Shifts of the extrema of the periodic dependences of critical current by a quarter of the flux quantum directly contradict the results obtained by measuring asymmetric ring resistance oscillations, whose extrema are, as for symmetrical rings, observed at magnetic fluxes equal to an integer and a half of flux quanta.     

三臂环中的持续电流

利用量子波导理论研究三臂环中的持续电流.结果表明,输运电流存在时,不含磁场且上、下臂等长的三臂环中仍可以有持续电流出现,而且上臂和下臂中的持续电流是相同的.三臂环的各臂长不等时,三个臂中的持续电流各不相同.我们还发现,即使三臂环和单环的上、下臂比值一样,两个环中的持续电流也明显不同.     

Finite size effects on the optical transitions in quantum rings under a magnetic field

We present a theoretical study of the energy spectrum of single electron and hole states in quantum dots of annular geometry under a high magnetic field along the ring axis in the frame of uncorrelated electron-hole theory. We predict the periodic disappearance of the optical emission of the electron-hole pair as the magnetic field increases, as a consequence of the finite height of the barriers. The model has been applied to semiconductor rings of various internal and external radii, giving as limiting cases the disk and antidot.     

Time evolution of initially localized states in two-dimensional quantum dot arrays in a magnetic field

The evolution of non-stationary localized states |Ψ(t=0) is investigated in two-dimensional tight binding systems of N potential wells with and without a homogeneous field perpendicular to the plane. Most results are presented in analytical form, what is almost imperative if the patterns are as complex as for rings in a magnetic field, where the qualitatively different features arise depending on rational or irrational numbers. The systems considered comprise finite linear chains (N=2,3), finite rings (N=3–6), infinite chains, finite rings (N=3–6) in a magnetic field, and rings with leads attached to each ring site. The position of the particle at time t is described by the projection of the wave function P_m(t)=|m|Ψ(t)|² onto the localized basis function at site m. For finite chains and rings with N=3,4,6 the time evolution is periodic, whereas it is non-periodic for N=5 and N greater then 6. Rings in a magnetic field show a rich spectrum of different features depending on N and the number of flux quanta through the ring, including periodic oscillation and rotation of the charge as well as non-periodic charge fluctuations.     

开口磁环的周期聚焦系统仿真分析与实验

当前行波管周期永磁聚焦系统的波端口位置处的磁环通常采用单向开口磁环。在波端口位置引入波导阻抗调谐支节的基础上,提出了两种不同的双向开口磁环结构。利用三维电磁仿真软件Opera-3D,分析了双开口磁环的中心轴线附近的磁场,并据此进一步介绍了带双开口磁环周期永磁聚焦系统的设计方法。为了验证带双开口磁环的周期永磁聚焦系统应用的可行性,设计和测试了一套E波段折叠波导行波管电子光学系统。在行波管试验中,电子枪发射电流83 mA,带双开口磁环的周期永磁聚焦系统聚焦的电子束流通率达到99%。     

Quantum transport in coupled resonators enclosed synthetic magnetic flux

Quantum transport properties are instrumental to understanding quantum coherent transport processes. Potential applications of quantum transport are widespread, in areas ranging from quantum information science to quantum engineering, and not restricted to quantum state transfer, control and manipulation. Here, we study light transport in a ring array of coupled resonators enclosed synthetic magnetic flux. The ring configuration, with an arbitrary number of resonators embedded, forms a two-arm Aharonov–Bohm interferometer. The influence of magnetic flux on light transport is investigated. Tuning the magnetic flux can lead to resonant transmission, while half-integer magnetic flux quantum leads to completely destructive interference and transmission zeros in an interferometer with two equal arms.     

Electronic properties of a quantum ring perturbed with a quantum well in the presence of perpendicular magnetic flux

Here, we study the effects of the number of sites, quantum ring radius and potential well depth on the energy levels, persistent current, magnetic susceptibility and density of states (DOS) of a quantum ring with a quantum well within its circumstance in a magnetic flux perpendicular to its plane. We show that, for small radius quantum ring systems, there are periodic local gaps along the magnetic flux axis in the DOS plots and along the axis ‘energy’. For large radius quantum ring systems, a uniform gap along the energy axis exists and along the phi axis nothing changes. In quantum rings with a quantum well in their circumstance, by using the large confining potential, we can create uniform gaps in the Energy–phi plane. The energy eigenvalues, persistent current and magnetic susceptibility decrease by increasing the confining potential. A quantum ring even with a very small confining potential in its circumstance can sensibly decrease the persistent current and magnetic susceptibility, although it may do not change the energy eigenvalues and DOS maximum considerably. Thus, by using the abovementioned parameters, we are able to tune the DOS, persistent current, magnetic susceptibility and energy levels, desirably.     

Possibility of persistent voltage observation in a system of asymmetric superconducting rings

The possibility of observing persistent voltage in superconducting rings of different arm widths is experimentally investigated. It was previously found that switching of the arms between superconducting and normal states by an AC current induces DC voltage oscillation in the magnetic field with a period corresponding to the flux quantum inside the ring. We used systems with a large number of asymmetric rings connected in series to investigate the possibility of observing this quantum phenomenon near the superconducting transition, where thermal fluctuations lead to switching of ring segments without an external influence and the persistent current is much smaller than in the superconducting state.     

Superconducting state evolution with applied magnetic flux in mesoscopic rings

The magnetic flux dependence of the vortex state for small mesoscopic superconducting rings surrounded by a medium is investigated by the phenomenological Ginzburg-Landau theory. The influences of the ring size and the surface superconductivity on the free energy and total supercurrent are studied. For narrow rings, the persistent current evolves towards a periodic behaviour with magnetic flux. The complete paramagnetic or diamagnetic state, corresponding to positive or negative current flowing in the whole ring, can occur. A remarkable intermittent superconducting behaviour for the ground-state transition is found when the strength of surface-suppressed superconductivity is enlarged or the ring size is decreased. Consequently, a pure superconducting state with positive total current can be obtained.     

铁磁/半导体/铁磁结构的双量子环自旋输运的特性

研究了与铁磁/半导体/铁磁结构相关的双量子环自旋输运的规律,研究结果表明：总磁通为零条件下,铁磁电极磁化方向反平行时,双量子环与单量子环相比提高了自旋电子透射概率的平均值.铁磁电极磁化方向平行时,双量子环对提高自旋向下电子平均透射概率的效果更明显;双量子环受到Rashba自旋轨道耦合作用影响时,自旋电子的平均透射概率明显高于单量子环,即使再加上外加磁场的影响,透射概率较高这一特征依然存在;双量子环所含的δ势垒具有阻碍自旋电子输运的作用,随δ势垒强度Z的增大透射概率 关键词： 双量子环 Rashba自旋轨道耦合 透射概率 δ势垒')" href="#">δ势垒     

二维色散和各向异性磁化等离子体光子晶体色散特性研究

采用平面波展开法和时域有限差分法研究了二维色散和各向异性磁化等离子体光子晶体的色散特性.当波矢在周期平面时,由于外加磁场的作用使TE模的色散曲线出现两个不同区域的平带,改变磁场的大小不但可以控制平带的位置,而且可以控制光子带隙的位置和大小.增大背景材料的介电常数,可以形成全方向光子带隙,随着背景材料介电常数的增加,带隙的中心位置降低但带隙宽度增加.当波矢偏离周期平面时,色散曲线不再分为TE和TM模,随着非周期平面波矢的增加,带隙位置上移,带隙宽度先增加随后基本保持不变.     

The electron transport characters in a nanostructure with the periodic magnetic-electric barriers

This paper detailedly studies the transmission probability, the spin polarization and the conductance of the ballistic electron in a nanostrueture with the periodic magnetic-electric barriers These observable quantities are found to be strongly dependent not only on the magnetic configuration, the incident electron energy and the incident wave vector, but also on the number of the periodic magnetic-electric barriers The transmission coefficient and the spin polarization show a periodic pattern with the increase of the separation between two adjacent magnetic fields, and the resonance splitting increases as the number of periods increases. Surprisingly, it is found that a polarization can be achieved by spin-dependent resonant tunnelling in this structure, although the average magnetic field of the structure is zero.     

The electron transport characters in a nanostructure with the periodic magnetic-electric barriers

This paper detailedly studies the transmission probability, the spin polarization and the conductance of the ballistic electron in a nanostructure with the periodic magnetic-electric barriers. These observable quantities are found to be strongly dependent not only on the magnetic configuration, the incident electron energy and the incident wave vector, but also on the number of the periodic magnetic-electric barriers. The transmission coefficient and the spin polarization show a periodic pattern with the increase of the separation between two adjacent magnetic fields, and the resonance splitting increases as the number of periods increases. Surprisingly, it is found that a polarization can be achieved by spin-dependent resonant tunnelling in this structure, although the average magnetic field of the structure is zero.     

多结超导环在外磁场中的行为

分析和计算了双结超导环在无偏置电流时的磁通、环流、自由能与外磁场的关系.发现双结环与单结环在磁场中的行为有所不同.当两个结的Ic相等时，双结环在1＜β=2πLIcΦ0＜2时，若无偏置电流，总磁通Φ、环流I与外磁通Φe的关系仍然是非回滞的曲线.仅当β≥2时，曲线才出现回滞.另外双结环有两支解，并且每支解的周期为2Φ0. 关键词： 双结环 双支解 2Φ0周期 回滞