quasi-2D Electron Research Articles

Hot Carrier Nanowire Transistors at the Ballistic Limit.

We demonstrate experimentally nonequilibrium transport in unipolar quasi-1D hot electron devices reaching the ballistic limit at room temperature. The devices are realized with heterostructure engineering in nanowires to obtain dopant- and dislocation-free 1D-epitaxy and flexible bandgap engineering. We show experimentally the control of hot electron injection with a graded conduction band profile and the subsequent filtering of hot and relaxed electrons with rectangular energy barriers. The number of electrons passing the barrier depends exponentially on the transport length with a mean-free path of 200-260 nm, and the electrons reach the ballistic transport regime for the shortest devices with 70% of the electrons flying freely through the base electrode and the barrier reflections limiting the transport to the collector.

Coexistence of near- EF Flat Band and Van Hove Singularity in a Two-Phase Superconductor

Quantum many-body systems, particularly, the ones with large near-EF density states, are well known for exhibiting rich phase diagrams as a result of enhanced electron correlations. The recently discovered locally noncentrosymmetric heavy fermion superconductor CeRh2As2 has stimulated extensive attention due to its unusual H−T phase diagram consisting of two-phase superconductivity, antiferromagnetic order, and possible quadrupole-density wave orders. However, the critical near-EF electronic structure remains experimentally elusive. Here, we provide this key information by combining soft-x-ray and vacuum ultraviolet (VUV) angle-resolved-photoemission-spectroscopy measurements and atom-resolved density-functional-theory (DFT)+U calculations. With bulk-sensitive soft x ray, we reveal quasi-2D hole and electron pockets near the EF. On the other hand, under VUV light, the Ce flat bands are resolved with the c−f hybridization persisting up to well above the Kondo temperature. Most importantly, we observe a symmetry-protected fourfold Van Hove singularity (VHS) coexisting with the Ce 4f5/21 flat bands at the X point, which, to the best of our knowledge, has never been reported before. Such a rare coexistence is expected to lead to a large density of states at the zone edge, a large upper critical field of the odd-parity phase, as well as spin and/or charge instabilities with a vector of (1/2, 1/2, 0). Uniquely, it will also result in a new type of f-VHS hybridization that alters the order and fine electronic structure of the VHS and flat bands. Our findings provide not only key insights into the nature of multiple phases in CeRh2As2 but also open up new prospects for exploring the novelties of many-body systems with f-VHS hybridization. Published by the American Physical Society 2024

Observation of Anomalous Planar Hall Effect Induced by One-Dimensional Weak Antilocalization.

The confinement of electrons in one-dimensional (1D) space highlights the prominence of the role of electron interactions or correlations, leading to a variety of fascinating physical phenomena. The quasi-1D electron states can exhibit a unique spin texture under spin-orbit interaction (SOI) and thus could generate a robust spin current by forbidden electron backscattering. Direct detection of such 1D spin or SOI information, however, is challenging due to complicated techniques. Here, we identify an anomalous planar Hall effect (APHE) in the magnetotransport of quasi-1D van der Waals (vdW) topological materials as exemplified by Bi4Br4, which arises from the quantum interference correction of 1D weak antilocalization (WAL) to the ordinary planar Hall effect and demonstrates a deviation from the usual sine and cosine curves. The occurrence of 1D WAL is correlated to the line-shape Fermi surface and persistent spin texture of (100) topological surface states of Bi4Br4, as revealed by both our angle-resolved photoemission spectroscopy and first-principles calculations. By generalizing the observation of APHE to other non-vdW bulk materials, this work provides a possible characteristic of magnetotransport for identifying the spin/SOI information and quantum interference behavior of 1D states in 3D topological material.

Low-frequency noise behaviors of quasi-two-dimensional electron systems based on complex oxide heterostructures

We report the low-frequency noise behaviors in quasi-two-dimensional (quasi-2D) electron systems based on complex oxide heterostructures. First, the surface 2D electron gas (2DEG) on SrTiO3 (STO) exhibits the 1/fα-type current power spectral density (PSD) with α∼1.39. The non-unity exponent α indicates the discrepancy between the depth distributions of electrons and oxygen vacancies in the STO substrate. Second, the amorphous LaAlO3/KTaO3 (LAO/KTO) interface, another quasi-2D electron system, shows Lorentzian components of PSD at a high-frequency region around 1 kHz, implying that the amorphous overlayer can provide additional shallow charge-trapping sites to the quasi-2D electrons in the crystalline KTO substrate. Lastly, ultrathin SrRuO3 (SRO) film grown on STO substrate exhibits the Lorentzian components of PSD at a low-frequency region around 200 Hz. The slight suppression of the fast charge trapping is attributed to the intrinsic band bending at the interface between film and substrate. These results will provide a guideline for understanding the defect-induced charge trapping and the relevant electron dynamics in the quasi-2D electron systems as well as the oxide-based electronic materials in general.

Coupling and interface effects in MoS2/WSe2 van der Waals nanostructure

In this work we report the effects of the interface coupling, the effective mass, the Hartree and exchange–correlation potential on the total energy in quasi-2D MoS2/WSe2 van der Waals nanostructure. Analytical and numerical solutions of the total energy as a function of carrier density and effective mass are determined without self-consistent calculation. The calculation carefully indicates how the quasi-2D electron gas arises from the interface coupling between MoS2 and WSe2 layers. Moreover, the results showed that the width of the wave function and the length scale between the two materials’ interface decrease with increasing electron density while the corresponding ground state quantisation energy in the z-direction increases considerably. Furthermore, in MoS2 layer with density n s = 4 × 1012 cm−2 this additional separation is approximately 2 nm and 〈z〉 = 1 nm. Due to the disparity in effective mass of electrons and holes in MoS2, the majority of the excess energy is absorbed as kinetic energy by electrons. The comparison with available experimental and DFT calculation indicates that the present work reproduces properly known results for MoS2/WSe2 interface.

A semiclassical approach to the magnetotransport in quasi-1D electron systems

The issue of the magnetotransport in any quasi one-dimensional (quasi-1D) electron system has not hoarded so much attention as the magnetotransport in two-dimensional (2D) system. At most, at the beginning of the realization of those systems, some experimental studies and phenomenological models were developed. However, it is an interesting subject that can throw light on the physical mechanisms determining the transport properties of low-dimensional electron systems. In our previous paper, Hidalgo (Eur Phys J Plus 137:1–-14, 2022), we described in detail a semiclassical global approach to the quantum Hall and Shubnikov-de Haas phenomena in a 2D system for both, the integer and fractional quantum Hall effects (IQHE and FQHE), and not only in semiconductors quantum wells but also in graphene. Here, we focus on the magnetotransport in a quasi-1D electron system following also a semiclassical approach, i.e., taking into consideration the Landau-type density of states for such system and its implication in the conductivity.

Band structure ofn- andp-doped core-shell nanowires

We investigate the electronic band structure of modulation-doped GaAs/AlGaAs core-shell nanowires for both n- and p-doping. We developed an 8-band Burt-Foreman k.p Hamiltonian approach to describe coupled conduction and valence bands in heterostructured nanowires of arbitrary composition, growth directions, and doping. Coulomb interactions with the electron/hole gas are taken into account within a mean-field self-consistent approach. We map the ensuing multi-band envelope function and Poisson equations to optimized, non-uniform real-space grids by the finite element method. Self-consistent charge density, single-particle subbands, density of states and absorption spectra are obtained at different doping regimes. For n-doped samples, the large restructuring of the electron gas for increasing doping results in the formation of quasi-1D electron channels at the core-shell interface. Strong heavy-hole/light-hole coupling of hole states leads to non parabolic dispersions with mass inversion, similarly to planar structures, which persist at large dopings, giving rise to direct LH and indirect HH gaps. In p-doped samples the hole gas forms an almost isotropic, ring-like cloud for a large range of doping. Here, as a result of the increasing localization, HH and LH states uncouple, and mass inversion takes place at a threshold density. A similar evolution is obtained at fixed doping as a function of temperature. We show that signatures of the evolution of the band structure can be singled out in the anisotropy of linearly polarized optical absorption.

Sub-angstrom noninvasive imaging of atomic arrangement in 2D hybrid perovskites.

Noninvasive imaging of the atomic arrangement in two-dimensional (2D) Ruddlesden-Popper hybrid perovskites (RPPs) is challenging because of the insulating nature and softness of the organic layers. Here, we demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized scanning tunneling microscopy (STM) and noncontact atomic force microscopy (ncAFM) corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable nonperturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. Moreover, such a joint technique also allows for the atomic-scale imaging of the electrostatic potential variation across the twin-domain walls, revealing alternating quasi-1D electron and hole channels at neighboring twin boundaries, which may influence in-plane exciton transport and dissociation.

Competing Superior Electronic Structure and Complex Defect Chemistry in Quasi-One-Dimensional Antimony Chalcogenide Photovoltaic Absorbers

Antimony chalcogenides Sb2Se3 and Sb2S3 are an emerging class of quasi-one-dimensional van der Waals bonded materials with rapid progress and great promise for low-cost, high-efficiency, and scalable thin-film photovoltaics. Here using first-principles theoretical calculations, we show that antimony chalcogenides Sb2Se3 and Sb2S3 possess competing superior electronic structure and complex defect chemistry, limiting their photovoltaic performance. The high optical absorption, large Born effective charge, and high dielectric constant lead to low exciton binding energy and small hole effective mass, which ultimately benefit the photovoltaic performance. In contrast, quasi-1D antimony chalcogenides exhibit complex defect chemistry. The cation–anion antisites are the dominating native defects in the Sb rich condition, while in the Sb poor condition the anion–cation antisites are dominant. In both cases, the anion vacancies are also easy to form. These native defects introduce midgap transition levels which act as electron–hole recombination centers, limit the dopability, and lower the open circuit voltage. As a result, Sb2Se3 is a p-type semiconductor in Se rich condition, while Sb2S3 is an intrinsic semiconductor without significant free carrier density. Additionally, due to the unique quasi-1D structures and strong electron–lattice coupling, antimony chalcogenides are able to accommodate larger lattice relaxation than conventional 3D bulk crystals, resulting in negative-U behavior in their defect structures. Therefore, the competing superior electronic structure and complex defect chemistry suggest that, while antimony chalcogenides are excellent photoabsorbers, effort shall be made to suppress the formation of the detrimental native defects to further improve their open-circuit voltage, electronic transport, and, thus, power conversion efficiency.

Coexistence of Photoelectric Conversion and Storage in van der Waals Heterojunctions.

Van der Waals (vdW) heterojunctions, based on two-dimensional (2D) materials, have great potential for the development of ecofriendly and high-efficiency nanodevices, which shows valuable applications as photovoltaic cells, photodetectors, etc. However, the coexistence of photoelectric conversion and storage in a single device has not been achieved until now. Here, we demonstrate a simple strategy to construct a vdW p-n junction between a WSe_{2} layer and quasi-2D electron gas. After an optical illumination, the device stores the light-generated carriers for up to seven days, and then releases a very large photocurrent of 2.9mA with bias voltage applied in darkness; this is referred to as chargeable photoconductivity (CPC), which completely differs from any previously observed photoelectric phenomenon. In normal photoconductivity, the recombination of electron-hole pairs occurs at the end of their lifetime; in contrast, infinite-lifetime photocarriers can be generated and stored in CPC devices without recombination. The photoelectric conversion and storage are completely self-excited during the charging process. The ratio between currents in full- and empty-photocarrier states below the critical temperature reaches as high as 10^{9}, with an external quantum efficiency of 93.8% during optical charging. A theoretical model developed to explain the mechanism of this effect is in good agreement with the experimental data. This work paves a path toward the high-efficiency devices for photoelectric conversion and storage.

Collinear orbital antiferromagnetic order and magnetoelectricity in quasi-two-dimensional itinerant-electron paramagnets, ferromagnets, and antiferromagnets

We develop a comprehensive theory for magnetoelectricity in magnetically ordered quasi-2D systems whereby in thermal equilibrium an electric field can induce a magnetization $m$ and a magnetic field can induce a polarization. This effect requires that both space-inversion and time-reversal symmetry are broken. Antiferromagnetic (AFM) order plays a central role in this theory. We define a N\'eel operator $\tau$ such that a nonzero expectation value $\langle \tau \rangle$ signals AFM order, in the same way $m$ signals ferromagnetic (FM) order. While $m$ is even under space inversion and odd under time reversal, $\tau$ describes a toroidal moment that is odd under both symmetries. Thus $m$ and $\langle \tau \rangle$ quantify complementary aspects of magnetic order in solids. In quasi-2D systems FM order can be attributed to dipolar equilibrium currents that give rise to $m$. In the same way, AFM order arises from quadrupolar currents that generate the moment $\langle \tau \rangle$. The electric-field-induced magnetization can then be attributed to the electric manipulation of the quadrupolar currents. We develop a $k \cdot p$ envelope-function theory for AFM diamond structures that allows us to derive explicit expressions for the operator $\tau$. Considering FM zincblende and AFM diamond, we derive quantitative expressions for the magnetoelectric responses due to electric and magnetic fields that reveal explicitly the inherent duality of these responses required by thermodynamics. Magnetoelectricity is found to be small in realistic calculations for quasi-2D electron systems. The magnetoelectric response of quasi-2D hole systems turns out to be sizable, however, with moderate electric fields being able to induce a magnetic moment of one Bohr magneton per charge carrier. Our theory provides a broad framework for the manipulation of magnetic order by means of external fields.

Internal screening and dielectric engineering in magic-angle twisted bilayer graphene

Magic-angle twisted bilayer graphene (MA-tBLG) has appeared as a tunable testing ground to investigate the conspiracy of electronic interactions, band structure, and lattice degrees of freedom to yield exotic quantum many-body ground states in a two-dimensional Dirac material framework. While the impact of external parameters such as doping or magnetic field can be conveniently modified and analyzed, the all-surface nature of the quasi-2D electron gas combined with its intricate internal properties pose a challenging task to characterize the quintessential nature of the different insulating and superconducting states found in experiments. We analyze the interplay of internal screening and dielectric environment on the intrinsic electronic interaction profile of MA-tBLG. We find that interlayer coupling generically enhances the internal screening. The influence of the dielectric environment on the effective interaction strength depends decisively on the electronic state of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric environment, e.g. by varying the capping layer composition and thickness, as a promising pursuit to provide further evidence for resolving the hidden nature of the quantum many-body states in MA-tBLG.

Many-Body Quantum Dynamics by the Reduced Density Matrix Based on Time-Dependent Density-Functional Theory.

We evaluate the density matrix of an arbitrary quantum mechanical system in terms of the quantities pertinent to the solution of the time-dependent density functional theory (TDDFT) problem. Our theory utilizes the adiabatic connection perturbation method of Görling and Levy, from which the expansion of the many-body density matrix in powers of the coupling constant λ naturally arises. We then find the reduced density matrix ρ_{λ}(r,r^{'},t), which, by construction, has the λ independent diagonal elements ρ_{λ}(r,r,t)=n(r,t), n(r,t) being the particle density. The off-diagonal elements of ρ_{λ}(r,r^{'},t) contribute importantly to the processes unaccessible via the density, directly or by the use of the known TDDFT functionals. Of those, we consider the momentum-resolved photoemission, doing this to the first order in λ, i.e., on the level of the exact exchange theory. In illustrative calculations of photoemission from the quasi-2D electron gas and isolated atoms, we find quantitatively strong and conceptually far-reaching differences with the independent-particle Fermi's golden rule formula.

How adsorbates alter the metallic behavior of quasi-1D electron systems of the Si(5 5 3)-Au surface

The plasmonic signals of quasi-1D electron systems are a clear and direct measure of their metallic behavior. Due to the finite size of such systems in reality, plasmonic signals from a gold-induced superstructure on Si(5 5 3) can be studied with infrared spectroscopy. The infrared spectroscopic features have turned out to be extremely sensitive to adsorbates. Even without geometrical changes of the surface superstructure, the effects of doping, of the adsorbate induced electronic surface scattering, and of the electronic polarizability changes on top of the substrate surface give rise to measurable changes of the plasmonic signal. Especially strong changes of the plasmonic signal have been observed for gold, oxygen, and hydrogen exposure. The plasmonic resonance gradually disappears under these exposures, indicating the transion to an insulating behavior, which is in accordance with published results obtained from other experimental methods. For C70 and, as shown here for the first time, TAPP-Br, the plasmonic signal almost retains its original intensity even up to coverages of many monolayers. For C70, the changes of the spectral shape, e.g. of electronic damping and of the resonance position, were also found to be marginal. On the other hand, TAPP-Br adsorption shifts the plasmonic resonance to higher frequencies and strongly increases the electronic damping. Given the dispersion relation for plasmonic resonances of 1D electron systems, the findings for TAPP-Br indicate a push-back effect and therefore stronger confinement of the free charge carriers in the quasi-one-dimensonal channel due to the coverage by the flat TAPP-Br molecules. On the gold-doped Si(5 5 3)-Au surface TAPP-Br acts as counter dopant and increases the plasmonic signal.

On the Heat Capacity of a Quasi-Two-Dimensional Electron Gas

Numerical and analytical results of the investigation of the thermodynamic properties of a quasi-2D electron gas are presented. The density of states, the temperature derivative of the chemical potential, and the heat capacity of the gas at the resonance points and away from it are analyzed. It is shown that, in the dependence of the heat capacity on the chemical potential, there are additional steps at the resonance points. The width of the additional steps increases with temperature. With the increase in temperature, when the main steps are practically blurred, there are still visible marks from the additional steps.

Investigation of the Dyakonov–Shur instability for THz wave generation based on the Boltzmann transport equation

We compute the growth rate of the Dyakonov–Shur plasma instability of a quasi-2D electron gas by numerical simulations based on the Boltzmann transport equation. We present a method to solve the characteristic equation by a Newton approach to find the two Vlasov modes. A deterministic solver using Fourier harmonics in the 2D k-space is applied to carrier transport in the framework of the Dyakonov–Shur model. We investigate the suppression of the plasma growth rate due to the Pauli principle and temperature.

Crossover between collective and independent-particle excitations in quasi-2D electron gas with one filled subband

While it has been recently demonstrated that, for quasi-2D electron gas (Q2DEG) with one filled miniband, the dynamic exchange $f_x$ and Hartree $f_H$ kernels cancel each other in the low-density regime $r_s\rightarrow \infty$ (by half and completely, for the spin-neutral and fully spin-polarized cases, respectively), here we analytically show that the same happens at arbitrary densities at short distances. This motivates us to study the confinement dependence of the excitations in Q2DEG. Our calculations unambiguously confirm that, at strong confinements, the time-dependent exact exchange excitation energies approach the single-particle Kohn-Sham ones for the spin-polarized case, while the same, but less pronounced, tendency is observed for spin-neutral Q2DEG.

Shubnikov-de Haas-like Quantum Oscillations in Artificial One-Dimensional LaAlO_{3}/SrTiO_{3} Electron Channels.

The widely reported magnetoresistance oscillations in LaAlO_{3}/SrTiO_{3} heterostructures have invariably been attributed to the Shubnikov-de Haas (SdH) effect, despite a pronounced inconsistency with low-field Hall resistance measurements. Here we report SdH-like resistance oscillations in quasi-1D electron waveguides created at the LaAlO_{3}/SrTiO_{3} interface by conductive atomic force microscopy lithography. These oscillations can be directly attributed to magnetic depopulation of magnetoelectric subbands. Our results suggest that the SdH oscillations in 2D SrTiO_{3}-based systems may originate from naturally forming quasi-1D channels.

Growth and Electrostatic/chemical Properties of Metal/LaAlO3/SrTiO3 Heterostructures.

The quasi 2D electron system (q2DES) that forms at the interface between LaAlO3 (LAO) and SrTiO3 (STO) has attracted much attention from the oxide electronics community. One of its hallmark features is the existence of a critical LAO thickness of 4 unit-cells (uc) for interfacial conductivity to emerge. Although electrostatic mechanisms have been proposed in the past to describe the existence of this critical thickness, the importance of chemical defects has been recently accentuated. Here, we describe the growth of metal/LAO/STO heterostructures in an ultra-high vacuum (UHV) cluster system combining pulsed laser deposition (to grow the LAO), magnetron sputtering (to grow the metal) and X-ray photoelectron spectroscopy (XPS). We study step by step the formation and evolution of the q2DES and the chemical interactions that occur between the metal and the LAO/STO. Additionally, magnetotransport experiments elucidate on the transport and electronic properties of the q2DES. This systematic work not only demonstrates a way to study the electrostatic and chemical interplay between the q2DES and its environment, but also unlocks the possibility to couple multifunctional capping layers with the rich physics observed in two-dimensional electron systems, allowing the fabrication of new types of devices.

Bistable transport properties of a quasi-one-dimensional Wigner solid on liquid helium under continuous driving

We investigate low-frequency fluctuations in the transport characteristics of a quasi-1D Wigner solid (WS) moving along a liquid helium substrate in response to a sinusoidal driving voltage. The fluctuations occur between distinct transport modes in which the decoupling of the WS from ripplonic polarons (or 'dimple lattice', DL) formed on the helium surface does, or does not, occur during each ac cycle. We demonstrate that a Gaussian-like distribution in the decoupling threshold force gives rise to this bistability, as the low-frequency switching occurs when the probability of decoupling during each ac cycle is small but finite. We attribute the distribution in the decoupling threshold force to the range of structural configurations allowed for the quasi-1D electron lattice, which influences the strength of the WS-DL coupling. Hence, the switching rate between the ac transport modes is extremely sensitive to the microscopic properties of the electron solid.