Aharonov-Bohm Type Oscillations Research Articles

Modification of Landau Levels in a 2D Ring Due to Rotation Effects and Edge States

AbstractThe properties of a 2D quantum ring under rotating and external magnetic field effects are investigated. The Landau levels and their inertial effects on them are initially analyzed. Among the results obtained, it is emphasized that the rotation lifted the degeneracy of Landau levels. The second part deals with the electronic confinement in a 2D ring modeled by a hard wall potential. The eigenstates are described by Landau states as long as they are not too close to the ring edges. On the other hand, near the ring edges, the energies increase monotonically. These states are known as edge states. Edge states have a significant role in the physical properties of the ring. Thus, the Fermi energy and magnetization are analyzed. In the specific case of magnetization, two approaches are considered. In the first approach, an analytical result for magnetization is obtained but without considering rotation. Numerical results show the de Haas‐Van Alphen (dHvA) oscillations. In the second approach, rotating effects are considered. In addition to the dHvA oscillations, the Aharonov–Bohm‐type (AB) oscillations are verified, which are associated with the presence of edge states. The effects of rotation on the results are discussed and it is found that rotation is responsible for inducing AB oscillations.

Flux periodic oscillations and phase-coherent transport in GeTe nanowire-based devices

Despite the fact that GeTe is known to be a very interesting material for applications in thermoelectrics and for phase-change memories, the knowledge on its low-temperature transport properties is only limited. We report on phase-coherent phenomena in the magnetotransport of GeTe nanowires. From universal conductance fluctuations measured on GeTe nanowires with Au contacts, a phase-coherence length of about 280 nm at 0.5 K is determined. The distinct phase-coherence is confirmed by the observation of Aharonov–Bohm type oscillations for parallel magnetic fields. We interpret the occurrence of these magnetic flux-periodic oscillations by the formation of a tubular hole accumulation layer. For Nb/GeTe-nanowire/Nb Josephson junctions we obtained a critical current of 0.2 μA at 0.4 K. By applying a perpendicular magnetic field the critical current decreases monotonously with increasing field, whereas in a parallel field the critical current oscillates with a period of the magnetic flux quantum confirming the presence of a tubular hole channel.

Quantum Transport in Topological Surface States of Selectively Grown Bi2Te3 Nanoribbons

AbstractQuasi‐1D nanowires of topological insulators are candidate structures in superconductor hybrid architectures for Majorana fermion based quantum computation schemes. Here, selectively grown Bi2Te3 topological insulator nanoribbons at cryogenic temperatures are investigated. The nanoribbons are defined in deep‐etched Si3N4/SiO2 nano‐trenches on a silicon (111) substrate followed by a selective area growth process via molecular beam epitaxy. The selective area growth is beneficial to the device quality, as no subsequent fabrication needs to be performed to shape the nanoribbons. In the diffusive transport regime of these unintentionally n‐doped Bi2Te3 topological insulator nanoribbons, electron trajectories are identified by analyzing angle dependent universal conductance fluctuation spectra. When the sample is tilted from a perpendicular to a parallel magnetic field orientation, these high frequent conductance modulations merge with low frequent Aharonov–Bohm type oscillations originating from the topologically protected surface states along the nanoribbon perimeter. For 500 nm wide Hall bars low frequent Shubnikov–de Haas oscillations are identified in a perpendicular magnetic field orientation. These reveal a topological, high‐mobility, 2D transport channel, partially decoupled from the bulk of the material.

Effects of disorder on magnetotransport oscillations in a two-dimensional electron gas terminated by superconductors

The coupling of superconductivity in a two-dimensional electron gas (2DEG) generates a number of magnetotransport oscillations. For instance, an Aharonov–Bohm-type oscillation at intermediate magnetic fields and an Altshuler–Aronov–Spivak-like oscillation around zero magnetic field appear under the circumstance of the coexistence of Andreev and normal reflections from the interface between the normal-conductor and the superconductor. The presence and the characteristics of such magnetotransport oscillations are investigated in this work by carrying out fully quantum-mechanical simulations. The significant role of the quantum interference is thereby demonstrated. It is also shown how the oscillations are affected by the presence of a potential disorder and finite excitation biases. Shubnikow–de Haas oscillations of the 2DEG are not always suppressed by the disorder under the influence of superconductivity, making their distinction from the Aharonov–Bohm-type oscillation possibly ambiguous.

Transport Studies on GaAs/AlGaAs Two-Dimensional Electron Systems Modulated by Triangular Array of Antidots**Supported by the National Basic Research Program of China under Grant Nos 2015CB921101 and 2014CB920904, and the Strategic Priority Research Program B of Chinese Academy of Sciences under Grant No XDB07010200.

Triangular antidot lattices of various periods and aspect ratios are fabricated on high mobility GaAs/AlGaAs two-dimensional electron systems (2DESs), and are characterized by magneto-transport measurements at low temperatures down to 300 mK. Commensurability peaks are generally observed in the magneto-resistivity ρxx, and remarkable similarity between dρxy/dB and ρxx is found. In samples of relatively large aspect ratio d/a, the Aharonov–Bohm-type oscillations are clearly observed in both ρxx and ρxy, as well as the quenching of the Hall resistivity ρxy in the vicinity of B = 0. These observations evince the good quality of our samples, and attest to the adequate preparation for fabricating antidot lattices of a reduced period to realize artificial graphene from GaAs/AlGaAs 2DESs.

Kondo-induced electric polarization modulated by magnetic flux through a triangular triple quantum dot

The Kondo effect plays an important role in emergence of electric polarization in a triangular triple-quantum-dot system, where one of the three dots is point-contacted with a single lead, and a magnetic flux penetrates through the triangular loop. The Kondo-induced electric polarization exhibits an Aharonov-Bohm type oscillation as a function of the magnetic flux. Our theoretical study shows various oscillation patterns associated with the field-dependent mixing of twofold orbitally degenerate ground states and their sensitivity to the point contact.

Electron gas high-frequency conductivity on the surface of a nanotube with superlattice in magnetic field

Kubo formula was obtained for conductivity tensor of electron gas on the surface of nanotube with superlattice in magnetic field. The high-frequency conductivity tensor components were calculated for quantum and quasiclassical cases. Electromagnetic wave Landau damping areas in the tube were determined. The conductivity tensor components show Aharonov–Bohm type oscillations and de Haas–van Alphen ones. When Fermi energy exceeds the miniband width, beatings are observed in the plot of conductivity vs. the tube parameters. Otherwise, the beatings are absent.

Aharonov-Bohm type oscillations in the system of two tunnel point-contacts in the presence of single scatterer: determination of the depth of the buried impurity

The conductance of a system containing two tunnel point-contacts and a single subsurface scatterer in the presence of a magnetic field is investigated theoretically. A general formula for the dependence of the conductance on the distance between contacts, the defect position, and the magnetic field is obtained. It is shown that in the presence of a magnetic field the conductance undergoes Aharonov-Bohm type oscillations. We find a simple relation between the period of the oscillations and the depth of the subsurface impurity. On the basis of this fact a new and easy method of determination the depth of the buried impurity is proposed.

Observing Altshuler–Aronov–Spivak Oscillation in a Hexagonal Antidot Array of Monolayer Graphene

We show that hexagonal antidot lattices of monolayer graphene exhibited the Altshuler–Aronov–Spivak (AAS) effect in low field magnetoresistance. In higher magnetic fields, Aharonov–Bohm-type oscillations were visible. The phase of AAS oscillation indicated that the chirality effect of graphene is suppressed because of inter-valley scattering due to boundary scatterings.

Heat capacity of an electron gas at the surface of a nanotube with its superlattice in a magnetic field

The effect of modulating potential at the surface of a nanotube in a longitudinal magnetic field on heat capacity of a degenerate and nondegenerate electron gas is considered. The heat capacity is represented by monotonic and oscillating terms. Heat capacity of a degenerate electron gas exhibits de Haas-van Alphen type oscillations, dependent on density of electrons, and Aharonov-Bohm type oscillation dependent on the intensity of the magnetic field going through the nanotube cross section.

Resistance Fluctuations and Aharonov–Bohm-Type Oscillations in Antidot Arrays in the Quantum Hall Regime

Resistance fluctuation phenomenon in antidot lattices in the quantum Hall regime are studied. Magnetoresistance of finite antidot array systems in the quantum Hall plateau transition regime exhibits two types of oscillatory effect. One is the aperiodic resistance fluctuations (RFs) and the other is the Aharonov-Bohm (AB)-type oscillations. Their dependences on the magnetic field and the gate voltage are quite distinct. While the aperiodic RFs are attributed to the complex evolution of the conducting network of compressible channels, the AB-type oscillations are interpreted in terms of edge states formed around individual antidots. The self-consistent screening effect is important for the both phenomenon, whereas, the single electron charging effect plays a minor role in the present case.

Theoretical studies of spin-dependent electrical transport through carbon nanotubes

Spin-dependent coherent quantum transport through carbon nanotubes (CNT) is studied theoretically within a tight-binding model and the Green function partitioning technique. End-contacted metal/nanotube/metal systems are modelled and next studied in the magnetic context, i.e. either with ferromagnetic electrodes or in external magnetic fields. The former case shows that quite a substantial giant magnetoresistance (GMR) effect occurs (±20%) for disorder-free CNTs. Anderson-disorder averaged GMR, in turn, is positive and reduced down to several per cent in the vicinity of the charge neutrality point. In parallel magnetic fields, characteristic Aharonov–Bohm-type oscillations are revealed with pronounced features due to a combined effect of: length-to-perimeter ratio, unintentional electrode-induced doping, Zeeman splitting and energy-level broadening. In particular, a CNT is predicted to lose its ability to serve as a magneto-electrical switch when its length and perimeter become comparable. In the case of perpendicular geometry, there are conductance oscillations approaching asymptotically the upper theoretical limit to the conductance, 4e2/h. Moreover in the ballistic transport regime, initially the conductance increases only slightly with the magnetic field or remains nearly constant because spin up- and spin down-contributions to the total magnetoresistance partially compensate each other.

Aharonov–Bohm-type effects in different arrays of antidots

We have investigated Aharonov–Bohm-type oscillation in the quantum Hall plateau transition region in three types of square arrays of antidots; a large ( 50 × 160 antidots) array, a small ( 5 × 10 antidots) array, and the sample with antidots placed only near the side edges. The temperature dependence of the amplitude confirmed that the oscillation originates from the fine structure in the density of single particle states circumnavigating around each antidot. In addition, we have also observed Altshuler–Aronov–Spivak oscillation near zero magnetic field in square arrays of antidots.

Atomic-scale structure and formation of self-assembled In(Ga)As quantum rings

We present an atomic-scale analysis of the indium distribution of self-assembled (In,Ga)As quantum rings (QRs), which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20 nm, which is consistent with the observed electronic radius of QR structures. Based on the structural information from the X-STM measurements, we calculate the magnetization as a function of the applied magnetic field. We conclude that, although the real QR shape differs strongly from an idealized circular-symmetric open ring structure, Aharonov–Bohm-type oscillations in the magnetization can be expected.

Commensurability effects in hexagonal antidot lattices with large antidot diameters

The observation of a novel type of commensurability resonance in two-dimensional, hexagonal antidot lattices is reported. These resonances have a classical character and occur at magnetic fields above the resonance that corresponds to the cyclotron motion around a single antidot. The resonances are visible only for antidots with effective diameters larger than 50 % of the lattice constant. Simulations reveal that they originate from quasi-stable electron trajectories that bounce between three neighboring antidots. This interpretation is backed by the observation of large-period Aharonov-Bohm type oscillations at low temperatures.

AFM-defined antidot lattices with top- and back-gate tunability

Antidot lattices with a period of 150 nm were defined by AFM lithography on a wafer with a back-gate electrode and later covered with a TiAu top-gate. This layout enabled us to study the electronic properties of the lattice at different electron densities and as a function of top- and back-gate voltage at constant Fermi energy. Commensurability peaks and Aharonov–Bohm-type oscillations were observed in the magnetoresistance and characterized under continuously varying conditions. The small lattice constant brings the classical commensurability oscillations to magnetic fields B>1.5 T , where quantum effects become important.

Chaotic transport in antidot lattices

A review is given of quantum effects and roles of chaos in transport in antidot lattices mainly from a theoretical point of view. The topics include diffusive orbits combined with a magnetic focusing effect as the origin of the commensurability peaks, semiclassical quantization of periodic orbits as the origin of the Aharonov-Bohm type oscillation superimposed, and the importance of inherent disorder in the antidot potential itself.

Aharonov–Bohm type oscillation in antidot lattices

The resistance is calculated for square antidot lattices in magnetic fields. Aharonov–Bohm type oscillation closely related to quantized energy levels associated with a periodic orbit encircling an antidot has a maximum amplitude around d/ a=0.5–0.6 and almost disappears for d/ a∼0.7. For d/ a∼0.8 another type of oscillation appears, whose period corresponds exactly to the flux quantum per unit cell. A possible scenario explaining this peculiar behavior is discussed.

Quantum transport in antidot lattices

A review is given of quantum transport phenomena in antidot lattices mainly from a theoretical point of view. The topics include theoretical study performed for the purpose of understanding the origin of the commensurability peaks and its relation to the chaotic electron motion, the Aharonov-Bohm type oscillation and its relation to semiclassical quantization of periodic orbits, the Altshuler-Aronov-Spivak oscillation and roles of disorder in antidot potential itself, and an S matrix formalism in which antidot lattices are replaced by an array of quantum-wire junctions.

Fluxoid quantization in quantum dots with Landau level edge states

The conductance of a GaAs/GaAlAs quantum dot with only one spin-degenerate Landau level populated exhibits Aharonov-Bohm type oscillations when plotted as a function of magnetic field. The conductance oscillations are due to resonant transmission through the Landau level edge states. A closely related oscillatory behaviour may be studied when the quantum dot is influenced by an external gate voltage, which changes the carrier density as well as the area of the quantum dot. The oscillation period in both magnetic field and gate voltage is analysed using a model where the fluxoid, i.e. the line integral of the canonical momentum rather than just the enclosed magnetic flux, is quantized.