Interaction In Two-dimensional Systems Research Articles

Gate-modulated reflectance spectroscopy for detecting excitonic states in two-dimensional semiconductors

We have developed a microspectroscopy technique for measuring gate-modulated reflectance to probe excitonic states in two-dimensional transition metal dichalcogenides. Through the use of gate-modulated spectroscopy, we were able to detect excited states of excitons and trions, which would typically have weak optical signals in traditional reflectance spectroscopy. Using a transfer matrix method for spectral analysis, the binding energy of trion 2s was determined to be 26 meV. As observing the Rydberg series of excitonic states provides fruitful information on the strong Coulomb interaction in two-dimensional systems, gate-modulated spectroscopy can be a versatile tool for understanding underlying many-body physics as well as designing next-generation quantum optoelectronics based on two-dimensional materials.

Aspects of Topological Superconductivity in 2D Systems: Noncollinear Magnetism, Skyrmions, and Higher-order Topology

The review is aimed at highlighting the aspects of topological superconductivity in the absence of spin-orbit interaction in two-dimensional systems with long-range non-collinear spin ordering or magnetic skyrmions. Another purpose is to give a brief introduction to the new concept of topological superconductivity, i.e. higher-order topology in two-dimensional systems including spin-orbit coupled structures. The formation of Majorana modes due to magnetic textures is discussed. The role of effective triplet pairings and odd fermion parity of the ground state wave function in different systems is emphasized. We describe the peculiarities of the magnetic skyrmions, leading to the formation of the Majorana modes and defects on which the modes are localized. The problem of braiding in the two-dimensional systems, especially in higher-order topological superconductors, is considered.

Time-resolved buildup of twisted indirect exchange interaction in two-dimensional systems

We study theoretically the time-domain dynamics of the spin-dependent Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between driven magnetic impurities localized in a spin-orbit-coupled two-dimensional system. Particular attention is given to the influence of the spin-orbit coupling (SOC) on the system's dynamical response to a time-dependent precessional motion of the localized magnetic moment. We show that, via the RKKY mechanism, a flip of the spin $z$ component of one localized moment affects all $x,\phantom{\rule{4pt}{0ex}}y$, and $z$ spin components of the other localized moment. The Friedel oscillations and the transient spin current triggered by the time-varying localized spin depend strongly on the orientation of the spin with respect to the two-dimensional structure. Our results demonstrate how the dynamic interplay between SOC and the RKKY interaction can be used to manipulate and fine-tune the characteristics of impurity spin reversal and to control the generated spin current and the local magnetization.

Antiferromagnetic Chern Insulators in Noncentrosymmetric Systems

We investigate a new class of topological antiferromagnetic (AF) Chern insulators driven by electronic interactions in two-dimensional systems without inversion symmetry. Despite the absence of a net magnetization, AF Chern insulators (AFCI) possess a nonzero Chern number C and exhibit the quantum anomalous Hall effect (QAHE). Their existence is guaranteed by the bifurcation of the boundary line of Weyl points between a quantum spin Hall insulator and a topologically trivial phase with the emergence of AF long-range order. As a concrete example, we study the phase structure of the honeycomb lattice Kane-Mele model as a function of the inversion-breaking ionic potential and the Hubbard interaction. We find an easy z axis C=1 AFCI phase and a spin-flop transition to a topologically trivial xy plane collinear antiferromagnet. We propose experimental realizations of the AFCI and QAHE in correlated electron materials and cold atom systems.

Gate control of the spin mobility through the modification of the spin-orbit interaction in two-dimensional systems

Spin drag measurements were performed in a two-dimensional electron system set close to the crossed spin helix regime and coupled by strong intersubband scattering. In a sample with uncommon combination of long spin lifetime and high charge mobility, the drift transport allows us to determine the spin-orbit field and the spin mobility anisotropies. We used a random walk model to describe the system dynamics and found excellent agreement for the Rashba and Dresselhaus couplings. The proposed two-subband system displays a large tuning lever arm for the Rashba constant with gate voltage, which provides a new path towards a spin transistor. Furthermore, the data shows large spin mobility controlled by the spin-orbit constants setting the field along the direction perpendicular to the drift velocity. This work directly reveals the resistance experienced in the transport of a spin-polarized packet as a function of the strength of anisotropic spin-orbit fields.

Two particle interactions in a confined viscoelastic fluid under shear

Numerical simulations of the interaction between two non-Brownian isotropic particles in a shear flow confined between two sliding plates in two and three dimensions are presented. The suspending fluid is modelled using the Oldroyd-B constitutive equation and so incorporates the effects of viscoelasticity and first normal stress difference, but not shear-thinning. The particles are treated as a separate phase within the simulation with automated adaptive mesh refinement used to increase the resolution near the particle boundaries. The finite element method is used to discretise the spatial domain with a discontinuous Galerkin approximation for polymer stress and is implemented using the deal.II object oriented library. We identify three distinct types of interaction in two-dimensional systems: pass, return and tumble. We also show that the interactions between two spheres in a three-dimensional shear flow is qualitatively similar to the interaction between two circular particles, for particles in the same vorticity plane. For oblique interactions we find a further type of behaviour, similar to the Jeffery orbits of rigid rods.

10.1007/s11448-008-3006-0

The cyclotron excitation spectrum of selectively doped AlGaAs/GaAs quantum wells with a high (up to 2 × 107 cm2/(V s)) mobility of electrons has been studied by means of the Raman scattering. The lines of the Raman scattering by the excitations of D − complexes, the objects in which two electrons localized in a quantum well are coupled to a charged impurity in a barrier, have been detected and identified. Spin-singlet D − complexes have been shown to exist in the entire range of the electron filling factor, from v → 0 to v = 2, owing to the specificity of the Coulomb interaction in two-dimensional systems. The excitation energies of the singlet D − complexes have been studied as functions of the electron density, quantum well width, and magnetic field.

Ground-plane screening of Coulomb interactions in two-dimensional systems: How effectively can one two-dimensional system screen interactions in another

The use of a nearby metallic ground-plane to limit the range of the Coulomb interactions between carriers is a useful approach in studying the physics of two-dimensional (2D) systems. This approach has been used to study Wigner crystallization of electrons on the surface of liquid helium, and most recently, the insulating and metallic states of semiconductor-based two-dimensional systems. In this paper, we perform calculations of the screening effect of one 2D system on another and show that a 2D system is at least as effective as a metal in screening Coulomb interactions. We also show that the recent observation of the reduced effect of the ground-plane when the 2D system is in the metallic regime is due to intralayer screening.

Barrier D − complexes in a high-mobility two-dimensional electron system

The cyclotron excitation spectrum of selectively doped AlGaAs/GaAs quantum wells with a high (up to 2 × 107 cm2/(V s)) mobility of electrons has been studied by means of the Raman scattering. The lines of the Raman scattering by the excitations of D− complexes, the objects in which two electrons localized in a quantum well are coupled to a charged impurity in a barrier, have been detected and identified. Spin-singlet D− complexes have been shown to exist in the entire range of the electron filling factor, from v → 0 to v = 2, owing to the specificity of the Coulomb interaction in two-dimensional systems. The excitation energies of the singlet D− complexes have been studied as functions of the electron density, quantum well width, and magnetic field.

Interlayer interactions in two-dimensional systems: Second-order effects causing ABAB stacking of layers in graphite

The role of second-order perturbations in interlayer interactions of two-dimensional electronic systems is examined. The general features of interlayer interactions are deduced from a simple two-layer model. The preferred motion is discussed specifically for the stacking of layers in graphite, using a transition-density analysis for extended systems. The well-known ABAB stacking of layers in graphite is proposed to be a consequence of orbital interactions between layers rather than of generally accepted van der Waals forces.

Electron-phonon interactions in two-dimensional systems: a microscopic approach

The electron-optical-phonon scattering rates in GaAs/AlAs quantum wells (QW) are calculated on the basis of a fully microscopic description of the phonon spectrum. The results indicate the great importance of confined as well as GaAs-like and AlAs-like interface phonons. The implications of these findings are discussed by using the calculated rates in Monte Carlo simulations of ultrafast carrier relaxation in a 50 AA QW.

Ultrafast field-dependent response of bandedge photogenerated electrons in quantum-well structures

The authors analyze the field-dependent femtosecond dynamics of bandedge photogenerated electrons undergoing parallel transport in quantum-well structures. The intracollisional field effect, breakdown of the Fermi golden rule, quantization of the phonon modes, and transient charge variations are all comprehensively included. The electron heating rates at zero fields are in agreement with experiments. The results are indicative of an adequate description of the electron-phonon interaction in two-dimensional systems. At high fields, an energy overshoot structure in keeping with recently observed data was observed without any real-space transfer. >

Electron-phonon interaction in two-dimensional systems: A microscopic approach

We present a calculation of the electron optical-phonon scattering rates in GaAs/AlAs quantum wells, based on a very accurate microscopic description of the phonon spectra. The results show that — besides the contribution of confined modes — a very large contribution originates from interface phonons of both GaAs-like and AlAs-like character. We then compare our results for phonon displacements and potentials, as well as for scattering rates, with those obtained from several macroscopic phonon models. We are therefore able to provide indications for selecting the model which allows the most appropriate simplified description of vibrations at the wavevectors relevant to the interaction with carriers.

Cyclotron resonance of interacting nondegenerate two-dimensional electrons

The electron-electron interaction in two-dimensional systems subjected to quantizing magnetic fields is shown to cause the qualitatively new mechanisms of electron scattering by impurities and by medium vibrations (phonons, ripplons). With allowance for these mechanisms, the kinetics of the electron liquid and Wigner crystal are considered and the halfwidth and shift of the cyclotron resonance peak are obtained in the explicit form.

Interaction and localisation effects in sputtered Ru and RuO2films

Resistance measurements and magnetoresistance measurements on thin Ru and RuO2 films at low temperatures are reported. The results of these measurements are confronted with recent theories of electron localisation and electron-electron interaction in two-dimensional systems. The measurements on the Ru films can be explained with these theories. It is concluded that the temperature dependence of the resistance of the Ru films is largely due to electron-electron interaction effects. The magnetoresistance of the Ru films consists of a temperature-independent part caused by localisation effects and a temperature-dependent part caused by interaction effects. In the case of RuO2 films a transition from strong to weak localisation is observed as a function of film thickness. All these films, however, fall in a region where theory is not so well developed. If the authors' measurements on the RuO2 films are confronted with existing theories, their measurements can be explained in qualitative terms. The conclusion is that Zeeman effects are extremely important in these films. Finally a saturation of the resistance was observed at extremely low temperature, in Ru as well as in RuO2 films, and an explanation for this is suggested.

Electron localization and interaction in bismuth thin films

We have studied the conductivity of thin bismuth films (thicknessL≈100–200 A) for temperatures from 1.4 to 4.2 K and magnetic fields up to ∼50 kOe. The low-temperature behavior includes an increase in electrical resistance as temperature is lowered and an anomalous positive magnetoresistance. These features can be quantitatively described in terms of a theory accounting for electron localization and interaction in two-dimensional systems. The experimental data for conductivity in a perpendicular magnetic field are analyzed as suggested by the theory of Altshuler, Aronov, Larkin, and Khmel'nitskii and those for a parallel field by the theory of Altshuler and Aronov. The inelastic electron relaxation time at 1.4–4.2 K was found to vary asT−p, wherep=1.3−1.5.

Effect of coexistence of random potential and electron-electron interaction in two-dimensional systems: Wigner glass

A numerical study of two-dimensional electron systems in strong magnetic fields in the presence of both random potential and electron-electron interaction is performed. The interplay of randomness and many-body effects in this quantum system is investigated by varying the relative strength of random potential and Coulcomb interaction. The electron-electron interaction is treated in the self-consistent Hartree-Fock approximation. The formalism covers arbitrary value of the relative strength of random potential and Coulomb interaction, and the results exhibit behaviour ranging from the Wigner crystallisation or the charge density wave with strong Coulomb interaction to the previously reported Anderson localisation with dominant random potential. It is shown that, in the general case when both random potential and electron-electron interaction are present with comparable magnitudes, the ground state of the system consists of an amorphous array of charge density maxima, which may be called a 'Wigner glass' or a 'charge density glass'. A discussion is made of the behaviour of the charge density in the ground state and its dependence on the occupancy of a Landau sub-band.

Random attractive electron-electron interactions in two-dimensional systems

A discussion is given of random attractive electron-electron interactions in two-dimensional systems. The effects on the Si inversion layer of negative U and/or reconstructing centers are explored and suggestions are made as to the role of these and associated states in producing changes in the device characteristics.