Magnetic anticrossing of 1D subbands in ballistic double quantum wires

We study the low-temperature in-plane magnetoconductance of vertically coupled double quantum wires. Using a novel flip-chip technique, the wires are defined by two pairs of mutually aligned split gates on opposite sides of a  ≤ 1 micron thick AlGaAs/GaAs double quantum well heterostructure. We observe quantized conductance steps due to each quantum well and demonstrate independent control of each 1D wire. A broad dip in the magnetoconductance at  ∼ 6 T is observed when a magnetic field is applied perpendicular to both the current and growth directions. This conductance dip is observed only when 1D subbands are populated in both the top and bottom constrictions. This data is consistent with a counting model whereby the number of subbands crossing the Fermi level changes with field due to the formation of an anticrossing in each pair of 1D subbands.     

Shape-dependence of magnetotransport through quantum dots

Magnetotransport through quantum dot structures is investigated numerically via a scattering matrix technique. The results for two typical structures show that the magnetoconductance is strongly dependent on the quantum dot geometry. For the symmetric quantum dot structure, it is found that the magnetoconductance profiles exhibit irregular structures and the magnetic field plays a similar role to that of disorder in the electron transport. For the T-shaped quantum dot structure, the oscillations in the conductance are found to be completely suppressed and the quantized conductance plateaus are recovered in a strong magnetic field, which is attributed to the asymmetry of the structure geometry with respect to the right- and left-moving edge states.     

Magnetoconductance of carbon nanotube p-n junctions

The magnetoconductance of p-n junctions formed in clean single wall carbon nanotubes is studied in the geometry where a magnetic field is along the tube axis. For long junctions the low temperature magnetoconductance is anomalously large; the relative change in the conductance becomes of order unity even when the flux through the tube is much smaller than the flux quantum. The magnetoconductance is negative for metallic tubes. For semiconducting and small gap tubes the magnetoconductance is nonmonotonic: positive at small and negative at large fields. The identified magnetoconductance mechanism is relevant to magnetotransport in undoped metallic and small gap tubes in the presence of a long range disorder potential.     

Effect of band structure on quantum interference in multiwall carbon nanotubes

We report conductance measurements on multiwall carbon nanotubes in a perpendicular magnetic field. A gate electrode with large capacitance is used to considerably vary the nanotube Fermi level. This enables us to search for signatures of the unique electronic band structure of the nanotubes in the regime of diffusive quantum transport. We find an unusual quenching of the magnetoconductance and the zero bias anomaly in the differential conductance at certain gate voltages, which can be linked to the onset of quasi-one-dimensional subbands.     

The invisible Majorana bound state at the helical edge

The presence of a Majorana bound state in condensed matter systems is often associated to a zero bias peak in conductance measurements. Here, we analyze a system were this paradigm is violated. A Majorana bound state is always present at the interface between a quantum spin Hall system that is magnetically gapped and a quantum spin Hall system gapped by proximity induced s-wave superconductivity. However, the linear conductance could be either zero or non-zero and quantized depending on the energy and length scales of the barriers. The transition between the two values is reminiscent of the topological phase transition in proximitized spin–orbit coupled quantum wires in the presence of an applied magnetic field. We interpret the behavior of the conductance in terms of scattering states at both zero and non-zero energy.     

Experimental investigation of the breakdown of the Onsager-Casimir relations

We use magnetoconductance fluctuation measurements of phase-coherent semiconductor billiards to quantify the contributions to the nonlinear electric conductance that are asymmetric under reversal of magnetic field. We find that the average asymmetric contribution is linear in magnetic field (for magnetic flux much larger than 1 flux quantum) and that its magnitude depends on billiard geometry. In addition, we find an unexpected asymmetry in the power spectrum of the magnetoconductance with respect to reversal of magnetic field and bias voltage.     

Electronic transport through a Majorana bound state coupled to a T-shaped quantum-dot system with Coulomb interaction

Using the Green’s function technique, we respectively investigate the electron transport properties of two spin components through the system of a T-shaped double quantum dot structure coupled to a Majorana bound state, in which only one quantum dot is connected with two metallic leads. We explore the interplay between the Fano effect and the MBSs for different dot-MBS coupling strength λ, dot-dot coupling strength t, and MBS-MBS coupling strength ε_M in the noninteracting case. Then the Coulomb interaction and magnetic field effect on the conductance spectra are investigated. Our results indicate that G_↓(ω) is not affected by the Majorana bound states, but a “0.5” conductance signature occurs in the vicinities of Fermi level of G_↑(ω). This robust property persists for a wide range of dot-dot coupling strength and dot-MBS coupling strength, but it can be destroyed by Coulomb interaction in quantum dots. By adjusting the size and direction of magnetic field around the quantum dots, the “0.5” conductance signature damaged by U can be restored. At last, the spin magnetic moments of two dots by applying external magnetic field are also predicted.     

自旋对磁量子反点能谱和磁电导的影响

利用分区求解单粒子薛定谔方程的方法,研究了电子自旋对磁量子反点中电子能级和磁电导的影响.结果表明,电子自旋与非均匀磁场的相互作用使电子能级发生劈裂,其特征与均匀磁场中电子自旋引起的劈裂显著不同,磁量子反点中能级的劈裂与角动量量子数密切相关.电子共振隧穿到磁边缘态,导致了磁电导随磁场的非周期性振荡.考虑电子自旋与不考虑电子自旋相比,磁电导谱中谷的数目增多且深度减小. 关键词：     

Kondo effect for electron transport through an artificial quantum dot

We have calculated the transport properties of electron through an artificial quantum dot by using the numerical renormalization group technique in this paper. We obtain the conductance for the system of a quantum dot which is embedded in a one-dimensional chain in zero and finite temperature cases. The external magnetic field gives rise to a negative magnetoconductance in the zero temperature case. It increases as the external magnetic field increases. We obtain the relation between the coupling coefficient and conductance. If the interaction is big enough to prevent conduction electrons from tunnelling through the dot, the dispersion effect is dominant in this case. In the Kondo temperature regime, we obtain the conductivity of a quantum dot system with Kondo correlation.     

De Haas-van Alphen oscillations in the quasi-two-dimensional organic metal (BEDO-TTF)5[CsHg(SCN)4]2

The magnetization oscillations in the quasi-two-dimensional organic metal (BEDO-TTF)₅[CsHg(SCN)₄]₂ are thoroughly investigated over a wide range of magnetic field directions at different temperatures down to 0.4 K. The results obtained are in good agreement with the shape and sizes of the Fermi surface calculated from the x-ray diffraction data. Apart from the fundamental frequencies, the combination frequencies are found in the magnetization oscillation spectrum. It is demonstrated that these combination frequencies are governed by the motion of charge carriers along the real closed orbits inside the network of magnetic breakdown orbits formed under the action of the magnetic field. It is uniquely established that the combination frequencies previously revealed in the magnetoresistance oscillation spectrum of the same metal are associated with the quantum interference effect. The angular dependences of the oscillation amplitude exhibit minima, which are explained by the spin splitting of the Landau levels.     

Frequency dependent Shubnikov-De Haas oscillations in Si-MOS-FET under high magnetic fields

The quantum oscillations in the AC magnetoconductance was studied for inversion layers in Si-MOS-FETs under high magnetic fields at 1.5 K. Each peak in the Shubnikov-de Haas oscillation was found to be remarkably sharpened with increasing frequency up to 50 MHz. It was also found that the width of the gate voltage for non-conductive region (σ_xx=0) increases with increasing frequency.     

Transport properties of a double quantum dot molecule in the presence of impurity effects

In this Letter, we studied the electronic transport through a parallel-coupled double quantum dot (DQD) molecule including impurity effects at zero temperature. The linear conductance can be calculated by using the Green's function method. An obvious Fano resonance arising from the impurity state in the quantum dot is observed for the symmetric dot-lead coupling structure in the absence of the magnetic flux through the quantum device. When the magnetic flux is presented, two groups of conductance peaks appear in the linear conductance spectra. Each group is decomposed into one Breit-Wigner and one Fano resonances. Tuning the system parameters, we can control effectively the shapes of these conductance peaks. The Aharonov-Bohm (AB) oscillation for the magnetic flux is also studied. The oscillation period of the linear conductance with π, 2π or 4π may be observed by tuning the interdot tunneling coupling or the dot-impurity coupling strengths.     

Magnetoresistance of the quasi-one-dimensional conductor NbSe<Subscript>3</Subscript> with a charge density wave in magnetic fields of different orientations

The magnetoresistance of NbSe₃ single crystals is measured as a function of magnetic field for different field orientations. A possibility of studying the Fermi surface by using the dependence of the Shubnikov-de Haas oscillation phase on the rotation angle of the magnetic field is demonstrated. The results of the study show that the quasi-momenta of carriers that are not condensed in the charge density wave are predominantly oriented along the conducting chains.     

Controllable spin-dependent transport in silicene superlattice

Based on the transfer-matrix method, we theoretically investigate the spin-dependent transport properties in magnetic silicene superlattice in the presence of extrinsic Rashba spin–orbit interaction (RSOI). It is found that the spin transmission probability and spin conductivities can be efficiently controlled by the number of magnetic barriers. As the number of magnetic barriers increases, spin conductivities strongly decrease, and reduce to zero in the large on-site potential difference between A and B sublattices (Δ_z) region. The results indicate that a magnetic silicene superlattice exhibits a remarkable wavevector-dependent spin filtering effect. Also, the magnetoresistance (MR) ratio exhibits an oscillatory behavior with the Fermi energy. The MR ratio can be tuned by the Fermi energy, number of magnetic barriers and extrinsic RSOI. Specifically, in the presence of magnetic field the spin polarization can be observed, and increases by increasing the magnetic field.     

Effect of magnetic field on Mott's variable-range hopping parameters in multiwall carbon nanotube mat

We report the temperature and magnetic field dependence of the conductivity of multiwall carbon nanotube mat in the temperature range 1.4-150 K and in magnetic fields up to 10 T. It is observed that charge transport in this system is governed by Mott’s variable-range hopping of three-dimensional type in the higher temperature range and two-dimensional type in the lower temperature range. Mott’s various parameters, such as localization length, hopping length, hopping energy and density of states at the Fermi level are deduced from the variable-range hopping fit. The resistance of the sample decreases with the magnetic field applied in the direction of tube axis of the nanotubes. The magnetic field gives rise to delocalization of states with the well-known consequence of a decrease in Mott’s T0 parameter in variable-range hopping. The application of magnetic field lowers the crossover temperature at which three-dimensional variable-range hopping turns to two-dimensional variable-range hopping. The conductivity on the lower temperature side is governed by the weak localization giving rise to positive magnetoconductance. Finally, a magnetic field-temperature diagram is proposed showing different regions for different kinds of transport mechanism.     

Time-dependent electron transport in HgTe/CdTe quantum wells

Based on the Floquet theory and Keldysh's nonequilibrium Green's function methods, we study the electron transport through the HgTe/CdTe quantum wells (QWs) irradiated by a monochromatic laser field. We find that when the laser field is applied, the edge states are split into a series of sidebands. When the Fermi level lies among these sidebands, the quantized plateau of the conductance is destroyed. Instead, the conductance versus the radiation frequency exhibits the successive oscillation peaks corresponding to the resonant tunneling through the sidebands of the edge states. The resonant interaction between the quasiparticles and the radiation field opens the gaps in the crossing region of the sidebands, which can be tuned by the radiation strength and frequency. This leads to the shift of the oscillation peaks in the conductance. We also show that the amplitudes of the oscillation peaks in the conductance are governed by the radiation strength and frequency.     

Quantum-chaotic scattering effects in semiconductor microstructures

We show that classical chaotic scattering has experimentally measurable consequences for the quantum conductance of semiconductor microstructures. These include the existence of conductance fluctuations-a sensitivity of the conductance to either Fermi energy or magnetic field-and weak-localization-a change in the average conductance upon applying a magnetic field. We develop a semiclassical theory and present numerical results for these two effects in which we model the microstructures by billiards attached to leads. We find that the difference between chaotic and regular classical scattering produces a qualitative difference in the fluctuation spectrum and weak-localization lineshape of chaotic and nonchaotic structures. While the semiclassical theory within the diagonal approximation accounts well for the weak-localization lineshape and for the spectrum of the fluctuations, we uncover a surprising failure of the semiclassical diagonal-approximation theory in describing the magnitude of these quantum transport effects.     

Transport properties of Dirac electrons in graphene based double velocity-barrier structures in electric and magnetic fields

Using transfer matrix method, transport properties in graphene based double velocity-barrier structures under magnetic and electric fields are numerically studied. It is found that velocity barriers for the velocity ratio (the Fermi velocity inside the barrier to that outside the barrier) less than one (or for the velocity ratio greater than one) have properties similar to electrostatic wells (or barriers). The velocity barriers for the velocity ratio greater than one significantly enlarge the resonant tunneling region of electrostatic barriers. In the presence of magnetic field, the plateau width of the Fano factor with a Poissonian value shortens (or broadens) for the case of the velocity ratio less than one (or greater than one). When the Fermi energy is equal to the electrostatic barrier height, for different values of the velocity ratio, both the conductivities and the Fano factors remain fixed.     

Aharonov-Bohm conductance modulation in ballistic carbon nanotubes

We report on magnetoconductance experiments in ballistic multiwalled carbon nanotubes threaded by magnetic fields as large as 55 T. In the high temperature regime (100 K), giant modulations of the conductance, mediated by the Fermi level location, are unveiled. The experimental data are consistently analyzed in terms of the field-dependent density of states of the external shell that modulates the injection properties at the electrode-nanotube interface, and the resulting linear conductance. This is the first unambiguous experimental evidence of Aharonov-Bohm effect in clean multiwalled carbon nanotubes.     

Electron scattering by a ring-shaped barrier in magnetic fields

We investigate the transport phenomena through a region containing a ring-shaped barrier in a quasi-one-dimensional quantum wire in magnetic fields. The calculated magnetoconductance curve shows a periodic dip structure, which is superimposed upon by another quasi-periodic dip structure. The current distributions for resonant states and the magnetoconductance features are well explained on the basis of the magnetic field dependence of the eigenvalue in the two-dimensional system.