High-mobility Two-dimensional Electron System Research Articles

High precision, low excitation capacitance measurement methods from 10 mK to room temperature.

Capacitance measurement is a useful technique in studying quantum devices, as it directly probes the local particle charging properties, i.e., the system compressibility. Here, we report one approach that can measure capacitance from mK to room temperature with excellent accuracy. Our experiments show that such a high-precision technique is able to reveal delicate and essential properties of high-mobility two-dimensional electron systems.

Terahertz plasma edge engineering in semiconductor membranes with a two-dimensional electron layer

We have studied the terahertz response of a dielectric membrane with a high-mobility two-dimensional electron system (2DES) on one side. The developed device exhibits electromagnetic transparency above the plasma frequency that is found to be determined by the 2DES density and membrane thickness. Significantly, we demonstrate the opaque low-frequency region to be highly tunable. Our results can find applications in the area of active terahertz devices.

Contactless transport method of two-dimensional electron system studies

We introduce two contactless measurement methods at extremely low temperature: capacitances and surface acoustic waves. Both methods can be used to study the physical properties of the quantum system through the interaction between electrons and high frequency electric field. We first present preliminary results of high-mobility two-dimensional electron systems studied by a high-precision capacitance measurement method at extremely low temperature. Our setup can resolve < 0.05% variation of a < 1 pF capacitance at 10 mK–300 K and 0–14 T. Second, we also study two-dimensional electron systems using surface acoustic waves. We can use 0.1 nW excitation and obtain < 10<sup>–5</sup> sensitivity. These measurement methods may be widely applied to the study of two-dimensional systems, especially the materials without high quality contacts.

Negative nonlocal vicinity resistance of viscous flow in a two-dimensional electron system

We perform a quantum transport study of Hall viscous liquid in multiterminal narrow Hallbar devices of a high-mobility two-dimensional electron system (2DES) in GaAs/AlGaAs heterostructure. In the nonlocal transport measurements of vicinity geometry under magnetic fields $(B)$, we observe that the absolute negative magnetoresistance $({R}_{xx})$ value $({R}_{\text{min}}<0)$ and the corresponding magnetic field $({B}_{\text{min}})$ are both inversely proportional to the adjacent Hallbar arm distance $(L)$. The occurrence of negative resistance is dependent on the characteristic lengths of the devices and the electron flow direction under $B$-fields. The minimal resistance ${R}_{\text{min}}$ occurs when the cyclotron radius ${R}_{C}$ approximates $L$. Multiples of high-order ($n\mathrm{th}$-order) ${R}_{\text{min},n}$'s persist from low to high magnetic fields in a large sample size of $L=5\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$. Our experimental study reveals the transport behaviors in the vicinity regime, where the negative resistances depict viscous electronic flows in the high-mobility 2DES. In addition, the negative and high-order minimal resistances expand to a magnetic field of several kGs.

Interplay of spin\u2013orbit coupling and Coulomb interaction in ZnO-based electron system

Spin–orbit coupling (SOC) is pivotal for various fundamental spin-dependent phenomena in solids and their technological applications. In semiconductors, these phenomena have been so far studied in relatively weak electron–electron interaction regimes, where the single electron picture holds. However, SOC can profoundly compete against Coulomb interaction, which could lead to the emergence of unconventional electronic phases. Since SOC depends on the electric field in the crystal including contributions of itinerant electrons, electron–electron interactions can modify this coupling. Here we demonstrate the emergence of the SOC effect in a high-mobility two-dimensional electron system in a simple band structure MgZnO/ZnO semiconductor. This electron system also features strong electron–electron interaction effects. By changing the carrier density with Mg-content, we tune the SOC strength and achieve its interplay with electron–electron interaction. These systems pave a way to emergent spintronic phenomena in strong electron correlation regimes and to the formation of quasiparticles with the electron spin strongly coupled to the density.

Hydrodynamic and Ballistic Transport over Large Length Scales in GaAs/AlGaAs.

We study hydrodynamic and ballistic transport regimes through nonlocal resistance measurements and high-resolution kinetic simulations in a mesoscopic structure on a high-mobility two-dimensional electron system in a GaAs/AlGaAs heterostructure. We evince the existence of collective transport phenomena in both regimes and demonstrate that negative nonlocal resistances and current vortices are not exclusive to only the hydrodynamic regime. The combined experiments and simulations highlight the importance of device design, measurement schemes, and one-to-one modeling of experimental devices to demarcate various transport regimes.

High harmonics of the cyclotron resonance in microwave transmission of a high-mobility two-dimensional electron system

We report an observation of magnetooscillations of the microwave power transmitted through the high mobility two-dimensional electron system hosted by a GaAs quantum well. The oscillations reflect an enhanced absorption of radiation at high harmonics of the cyclotron resonance and follow simultaneously measured microwave-induced resistance oscillations (MIRO) in the dc transport. While the relative amplitude (up to 1%) of the transmittance oscillations appears to be small, they represent a significant (>50%) modulation of the absorption coefficient. The analysis of obtained results demonstrates that the low-B decay, magnitude, and polarization dependence of the transmittance oscillations accurately follow the theory describing photon-assisted scattering between distant disorder-broadened Landau levels. The extracted sample parameters reasonably well describe the concurrently measured MIRO. Our results provide an insight into the MIRO polarization immunity problem and pave the way to probe diverse high-frequency transport properties of high-mobility systems using precise transmission measurements.

A tunable plasmonic resonator using kinetic 2D inductance and patch capacitance

We have studied the microwave response of a high-mobility two-dimensional electron system (2DES) contacted by two side electrodes. Using kinetic inductance of the 2DES and inter-electrode capacitance, we have constructed a subwavelength 2D plasmonic resonator. We have shown that the resonant frequency of this circuit can be controlled by 2D electron density, external magnetic field, or size of the electrodes. This opens up possibilities for using arrays of plasmonic circuits as tunable components in different frequency ranges.

Working principles of doping-well structures for high-mobility two-dimensional electron systems

Suppressing electron scattering is essential to achieve high-mobility two-dimensional electron systems (2DESs) that are clean enough to probe exotic interaction-driven phenomena. In heterostructures it is common practice to utilize modulation doping, where the ionized dopants are physically separated from the 2DES channel. The doping-well structure augments modulation doping by providing additional screening for all types of charged impurities in the vicinity of the 2DES, which is necessary to achieve record-breaking samples. Despite its prevalence in the design of ultrahigh-mobility 2DESs, the working principles of the doping-well structure have not been reported. Here we elaborate on the mechanics of electron transfer from doping wells to the 2DES, focusing on GaAs/AlGaAs samples grown by molecular beam epitaxy. Based on this understanding we demonstrate how structural parameters in the doping well can be varied to tune the properties of the 2DES.

Phase transitions at ν = 5∕2 in ZnO-based heterostructures

The ability to control the orbital degrees of freedom in high mobility two-dimensional electron systems could open the door to tuning novel phase transitions between strongly correlated states of matter. In these proceedings, we present magnetotransport data of MgZnO/ZnO-based two-dimensional electron systems to study the phase transitions at Landau filling ν = 5∕2. A rich cascade of phase transitions is resolved as we modify the polarization of carriers between the N = 1 ↓ and 0 ↑ levels by rotating the sample within a magnetic field. These include a gapped fractional quantum Hall state in close proximity to compressible and anisotropic phases, providing evidence for a delicate competition of a manifold of correlated states that are present at fractional fillings, and whose nature is yet to be fully understood. Note: Contribution to proceedings of Physics at the Nanoscale 2018, in dedication to Professor Tapash Chakraborty.

Two-Dimensional Quantum Transport in Free-Standing InSb Nanosheets.

Low-dimensional narrow band gap III-V compound semiconductors, such as InAs and InSb, have attracted much attention as one of promising platforms for studying Majorana zero modes and non-Abelian statistics relevant for topological quantum computation. So far, most of experimental studies were performed on hybrid devices based on one-dimensional semiconductor nanowires. In order to build complex topological circuits toward scalable quantum computing, exploring high-mobility two-dimensional (2D) III-V compound electron system with strong spin-orbit coupling is highly desirable. Here, we study quantum transport in high-mobility InSb nanosheet grown by molecular-beam epitaxy. The observations of Shubnikov-de Hass oscillations and quantum Hall states, together with the angular dependence of magnetotransport measurements, provide the evidence for the 2D nature of electronic states in InSb nanosheet. The presence of strong spin-orbit coupling in the InSb nanosheet is verified by the low-field magnetotransport measurements, characterized by weak antilocalization effect. Finally, we demonstrate the realization of high-quality InSb nanosheet-superconductor junctions with transparent interface. Our results not only advance the study of 2D quantum transport but also open up opportunities for developing hybrid topological devices based on 2D semiconducting nanosheets with strong spin-orbit coupling.

Andreev Reflection at the Interface with an Oxide in the Quantum Hall Regime

Quantum Hall/superconductor junctions have been an attractive topic as the two macroscopically quantum states join at the interface. Despite longstanding efforts, however, experimental understanding of this system has not been settled yet. One of the reasons is that most semiconductors hosting high-mobility two-dimensional electron systems (2DES) usually form Schottky barriers at the metal contacts, preventing efficient proximity between the quantum Hall edge states and Cooper pairs. Only recently have relatively transparent 2DES/superconductor junctions been investigated in graphene. In this study, we propose another material system for investigating 2DES/superconductor junctions, that is ZnO-based heterostrcuture. Due to the ionic nature of ZnO, a Schottky barrier is not effectively formed at the contact with a superconductor MoGe, as evidenced by the appearance of Andreev reflection at low temperatures. With applying magnetic field, while clear quantum Hall effect is observed for ZnO 2DES, conductance across the junction oscillates with the filling factor of the quantum Hall states. We find that Andreev reflection is suppressed in the well developed quantum Hall regimes, which we interpret as a result of equal probabilities of normal and Andreev reflections as a result of multiple Andreev reflection at the 2DES/superconductor interface.

High-temperature observation and power modulation of radiation-induced resistance oscillations in the terahertz band

We report on a theoretical work on magnetotransport under terahertz radiation with high-mobility two-dimensional electron systems focussing on the radiation power and temperature dependence. On the one hand, we study the interaction between the obtained radiation-induced magnetoresistance oscillations (RIRO) and the Shubnikov-de Haas oscillations (SdHO) from the power dependence standpoint. We obtain strong modulation of the SdHO at sufficient terahertz radiation power. On the other hand from the temperature dependence standpoint we obtain an important result: the range of temperature where RIRO are observed can be largely extended by using terahertz radiation. In the terahertz region we still obtain RIRO up to 20 K at a radiation frequency of 400 GHz. Since an increasing T gives rise in turn to more disorder in the sample, we would expect also the observation of RIRO with terahertz radiation when using low-mobility (high-disorder) samples at low T.

Hanle spin precession in a two-dimensional electron system

We investigate the nonlocal Hanle effect in high mobility two-dimensional electron systems using (Ga,Mn)As/GaAs spin Esaki diodes as spin selective contacts. Spin signals in these systems can be strongly affected by dynamic nuclear polarization, which mimics long spin-relaxation times extracted from the measured Hanle curves. Here, we introduce a method which largely suppresses these effects by using an ac injection-detection setup. This allows us to extract from the measurements realistic spin lifetimes on the order of single nanoseconds. As the detection of Hanle signals is also strongly affected by offset signals we discuss the magnetic field dependence of these background voltages observed in lateral nonlocal spin injection devices. We show how the strength of the background magnetoresistance can be minimized by choosing a proper device geometry.

Observation of anomalous Hall effect in a non-magnetic two-dimensional electron system

Anomalous Hall effect, a manifestation of Hall effect occurring in systems without time-reversal symmetry, has been mostly observed in ferromagnetically ordered materials. However, its realization in high-mobility two-dimensional electron system remains elusive, as the incorporation of magnetic moments deteriorates the device performance compared to non-doped structure. Here we observe systematic emergence of anomalous Hall effect in various MgZnO/ZnO heterostructures that exhibit quantum Hall effect. At low temperatures, our nominally non-magnetic heterostructures display an anomalous Hall effect response similar to that of a clean ferromagnetic metal, while keeping a large anomalous Hall effect angle θAHE≈20°. Such a behaviour is consistent with Giovannini–Kondo model in which the anomalous Hall effect arises from the skew scattering of electrons by localized paramagnetic centres. Our study unveils a new aspect of many-body interactions in two-dimensional electron systems and shows how the anomalous Hall effect can emerge in a non-magnetic system.

Terahertz-Induced Magnetoresistance Oscillations in High-Mobility 2D Electron Systems Under Bichromatic and Multichromatic Excitation

In this work, we investigated the magnetotransport under terahertz radiation in high-mobility two-dimensional electron systems, focusing on irradiation by bichromatic and multichromatic terahertz sources. We observed strong modulation of the Shubnikov-de Haas oscillations at sufficient terahertz radiation power. We determined that the origin of the modulation was the interference between the average distance advanced by the scattered electrons between irradiated Landau states and the available initial density of states at a certain magnetic field. In the case of multifrequency illumination, we found that with the appropriate frequencies, the irradiated magnetoresistance could reach an almost zero-resistance state regime even at moderate radiation power.

Magnetofermionic condensate in two dimensions.

Coherent condensate states of particles obeying either Bose or Fermi statistics are in the focus of interest in modern physics. Here we report on condensation of collective excitations with Bose statistics, cyclotron magnetoexcitons, in a high-mobility two-dimensional electron system in a magnetic field. At low temperatures, the dense non-equilibrium ensemble of long-lived triplet magnetoexcitons exhibits both a drastic reduction in the viscosity and a steep enhancement in the response to the external electromagnetic field. The observed effects are related to formation of a super-absorbing state interacting coherently with the electromagnetic field. Simultaneously, the electrons below the Fermi level form a super-emitting state. The effects are explicable from the viewpoint of a coherent condensate phase in a non-equilibrium system of two-dimensional fermions with a fully quantized energy spectrum. The condensation occurs in the space of vectors of magnetic translations, a property providing a completely new landscape for future physical investigations.

2D surprises at the surface of 3D materials: Confined electron systems in transition metal oxides

The scope of this article is to review the state-of-the-art in the field of confined electron systems generated at the bare surfaces of transition metal oxides (TMOs). This scientific field is a prime example of a domain where two-dimensional physics and photoemission-based spectroscopic techniques have together set up the development of the story. The discovery of a high-mobility two-dimensional electron system (2DES) at interfaces of transition metal oxides has attracted an immense scientific interest due to new opportunities opened in the emerging field of oxide electronics. The subsequent paradigm shift from interfaces to the bare surfaces of TMOs made the confined electron system accessible to surface-sensitive spectroscopic techniques and this new era is the focus of the present article. We describe how results by means of Angle-Resolved Photoemission Spectroscopy (ARPES) establish the presence of confined electron carriers at the bare surface of SrTiO3(100), which exhibit complex physics phenomena such as orbital ordering, electron–phonon interactions and spin splitting. The key element behind the 2DES generation is oxygen vacancies. Moreover, we review the experimental evidence on the generation of 2DESs on surfaces with different orientation, as well as on different TMO substrates. The electronic structure of the confined electron system responds to such changes, thereby providing external means for engineering its properties. Finally, we identify new directions for future research by introducing a device-friendly fabrication protocol for the generation of 2DESs on TMO surfaces.

Polarization-dependent plasmonic photocurrents in two-dimensional electron systems

Plasmonic polarization dependent photocurrents in a homogeneous two-dimensional electron system are studied. Those effects are completely different from the photon drag and electronic photogalvanic effects as well as from the plasmonic ratchet effect in a density modulated two-dimensional electron system. Linear and helicity-dependent contributions to the photocurrent are found. The linear contribution can be interpreted as caused by the longitudinal and transverse plasmon drag effect. The helicity-dependent contribution originates from the non-linear electron convection and changes its sign with reversing the plasmonic field helicity. It is shown that the helicity-dependent component of the photocurrent can exceed the linear one by several orders of magnitude in high-mobility two-dimensional electron systems. The results open possibilities for all-electronic detection of the radiation polarization states by exciting the plasmonic photocurrents in two-dimensional electron systems.

Structured back gates for high-mobility two-dimensional electron systems using oxygen ion implantation

We present a reliable method to obtain patterned back gates compatible with high mobility molecular beam epitaxy via local oxygen ion implantation that suppresses the conductivity of an 80 nm thick silicon doped GaAs epilayer. Our technique was optimized to circumvent several constraints of other gating and implantation methods. The ion-implanted surface remains atomically flat which allows unperturbed epitaxial overgrowth. We demonstrate the practical application of this gating technique by using magneto-transport spectroscopy on a two-dimensional electron system (2DES) with a mobility exceeding 20 × 106 cm2/V s. The back gate was spatially separated from the Ohmic contacts of the 2DES, thus minimizing the probability for electrical shorts or leakage and permitting simple contacting schemes.