Quasi Two-dimensional Electron Systems Research Articles

Symmetry breaking at the (111) interfaces of SrTiO3 hosting a two-dimensional electron system

We used x-ray absorption spectroscopy to study the orbital symmetry and the energy band splitting of (111) LaAlO${_3}$/SrTiO${_3}$ and LaAlO${_3}$/EuTiO${_3}$/SrTiO${_3}$ heterostructures, hosting a quasi two-dimensional electron system (q2DES), and of a Ti-terminated (111) SrTiO${_3}$ single crystal, also known to form a q2DES at its surface. We demonstrate that the bulk tetragonal Ti-3d D${_4}$${_h}$ crystal field is turned into trigonal D${_3}$${_d}$ crystal field in all cases. The symmetry adapted a${_1}$${_g}$ and e${^\pi_g}$ orbitals are non-degenerate in energy and their splitting, \Delta, is positive at the bare STO surface but negative in the heterostructures, where the a${_1}$${_g}$ orbital is lowest in energy. These results demonstrate that the interfacial symmetry breaking induced by epitaxial engineering of oxide interfaces has a dramatic effect on their electronic properties, and it can be used to manipulate the ground state of the q2DES.

Interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures

We studied the interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures by photoluminescence and magnetotransport. Photoluminescence contributions from a high-mobility quasi-twodimensional electron system at the interface and GaAs bulk type excitons were analyzed, covering an excitation intensity range of nearly three orders of magnitude. The experiment was compared with selfconsistent band structure calculations that allowed to follow the charge redistribution in the sample. After sample cool down in the dark a parallel conducting quasi-twodimensional channel of low electron mobility appeared in the Al1−xGaxAs-layer, that gained in density up to a saturation value. There was only little interaction between the Al1−xGaxAs and GaAs sides in the persistent regime and a reduction of the depletion charge in the GaAs could largely account for the interface channel density enhancement. Contrary, the negative photoeffect present during continuous illumination strongly depends on the interaction between the interface and the parallel channel, which serves as a temporary sink for excess charges from the GaAs side. When the illumination is switched off the parallel channel acts as a source and the sample essentially returns to the prior illumination state of saturation.

Flat Optical Conductivity in ZrSiS due to Two-Dimensional Dirac Bands.

ZrSiS exhibits a frequency-independent interband conductivity σ(ω)=const(ω)≡σ_{flat} in a broad range from 250 to 2500 cm^{-1} (30-300meV). This makes ZrSiS similar to (quasi-)two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, σ_{flat} is not universal but related to the length of the nodal line in the reciprocal space, k_{0}. Because of spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap Δ, for which we determine an upper bound max(Δ)=30 meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50K.

Temperature dependent symmetry to asymmetry transition in wide quantum wells

Quasi-two dimensional electron systems exhibit peculiar transport effects depending on their density profiles and temperature. A usual two dimensional electron system is assumed to have a δ like density distribution along the crystal growth direction. However, once the confining quantum well is sufficiently large, this situation is changed and the density can no longer be assumed as a δ function. In addition, it is known that the density profile is not a single peaked function, instead can present more than one maxima, depending on the well width. In this work, the electron density distributions in the growth direction considering a variety of wide quantum wells are investigated as a function of temperature. We show that the double peak in the density profile varies from symmetric (similar peak height) to asymmetric while changing the temperature for particular growth parameters. The alternation from symmetric to asymmetric density profiles is known to exhibit intriguing phase transitions and is decisive in defining the properties of the ground state wavefunction in the presence of an external magnetic field, i.e from insulating phases to even denominator fractional quantum Hall states. Here, by solving the temperature and material dependent Schrödinger and Poisson equations self-consistently, we found that such a phase transition may be elaborated by taking into account direct Coulomb interactions together with temperature.

Interaction effects near constriction of a quasi two-dimensional electron system: an exact diagonalization study

• A quasi 2D few-electron system is studied in a closed 3D geometry with a constriction. • In the exact ground-state the majority spin accumulates at the constriction. • A quasi-bound single-electron island is found for odd electron numbers. • In local potential cavities the opposite spin densities are nearly equal to each other. We present a systematic exact diagonalization study of a quasi two-dimensional few-electron system in a closed three-dimensional cylindrical geometry containing a constriction that models a quantum point contact formed within the two-dimensional electron gas. The calculations that are performed using the configuration interaction approach with multicenter Gaussian basis indicate that the tendency of the majority spin to gather in the area of an increased effective potential – e.g. by constriction of the quantum point contact – is distinct already for weak constrictions and low electron numbers. Opposite effects – nearly equal spin up and spin down densities – are obtained for local cavities of the confinement potential. Formation of a quasi-bound single-spin island within the constriction is also discussed.

Ratchet effect induced by linearly polarized near- and mid-infrared radiation in InAs nanowires patterned quasi two-dimensional electron system

In this letter, we studied the polarization dependent current excited by near- and mid-infrared radiations in InAs nanowires patterned quasi two-dimensional structures. We observed a current with a predominated direction along the wires at normal incidence of mid-infrared radiation and attributed it to the ratchet effect. Photogalvanic effect induced by obliquely incident excitation is also observed. Under near-infrared excitation, however, the normal incident ratchet effect becomes insignificant and the linear photogalvanic effect at oblique incidence dominantly contributes to the electric current.

Localized Distributions of Quasi-Two-Dimensional Electronic States near Defects Artificially Created at Graphite Surfaces in Magnetic Fields

We measured the local density of states of a quasi two-dimensional electron system (2DES) near defects, artificially created by Ar-ion sputtering, on surfaces of highly oriented pyrolytic graphite (HOPG) with scanning tunneling spectroscopy (STS) in high magnetic fields. At valley energies of the Landau level spectrum, we found two typical localized distributions of the 2DES depending on the defects. These are new types of distributions which are not observed in the previous STS work at the HOPG surface near a point defect [Y. Niimi, Phys. Rev. Lett. 97, 236804 (2006).10.1103/PhysRevLett.97.236804]. With increasing energy, we observed gradual transformation from the localized distributions to the extended ones as expected for the integer quantum Hall state. We show that the defect potential depth is responsible for the two localized distributions from comparison with theoretical calculations.

STM/STS measurements of quasi two-dimensional electronic states near artificially created defects in magnetic fields

We observed the local density of states (LDOS) of a quasi two-dimensional electron system near defects, artificially created by Ar-ion sputtering, on a surface of highly oriented pyrolytic graphite in magnetic fields with ultra-low temperature scanning tunneling spectroscopy. In differential tunnel conductance images taken at the valley energies in between the adjacent Landau levels, two typical localized distributions of the LDOS were observed depending on the defects. We reveal that the defect potential depth is responsible for the two types of the spatial distributions of LDOS.

Spin-polarized beam splitter for ballistic electrons

It is shown that in ballistic quasi two-dimensional electron systems momentum jumps of spin-polarized electrons appear whenever an increasing in-plane magnetic field depopulates magnetoelectric sub-bands at low temperatures. The system acts as a polarized beam splitter, the filtered spin-polarized electrons being spatially separated from the mainstream electron current. The momentum jumps can be also used to implement quantum logic states or electron switches.

Frequency-dependent acousto-electric interaction in proximity-coupled piezoelectric-semiconductor hybrids

A quantitative analysis of the frequency-dependent acousto-electric interaction between a high-mobility quasi-two dimensional electron system in a semiconductor heterostructure and a surface sound wave propagating on a piezoelectric crystal is presented. We show that for this hybrid system a residual air gap between both partners strongly affects the acousto-electric interaction. Frequency-dependent measurement of the interaction enables us to determine the influence of the air gap on the strength of the interaction and the relevant parameters for the theoretical description. Our analysis might be of importance for the design of future acousto-electric experiments but also devices where strong interaction between the sound wave and the low dimensional electron system is required.

Single-chip fused hybrids for acousto-electric and acousto-optic applications

The combination of the electronic and optical properties of a semiconductor hetero-junction and the acoustic properties of a piezoelectric substrate material yields a new class of very promising hybrids for potential acousto-electric and acousto-optic applications. LiNbO3/GaAs hybrids have been fabricated using the epitaxial lift-off technique resulting in unusually large acousto-electric and acousto-optic interaction between the quasi two-dimensional electron system in the semiconductor and surface acoustic waves on the piezoelectric substrate. Field effect tunability of the interaction at room temperature is demonstrated and possible device applications are discussed. Photoluminescence measurements show the influence of the acousto-electric fields on the optical properties of quantum well structures.

Magnetoresistance oscillations due to intersubband scattering in a two-dimensional electron system

We show that at intermediate temperatures the oscillatory magnetoresistance of the quasi-two dimensional electron system can be dominated by magnetointersubband scattering (MIS). The longitudinal resistance in high carrier concentration Al 0.3Ga 0.7As/GaAs heterojunctions and pseudomorphic Al 0.3Ga 0.7As/In 0.1Ga 0.9As/GaAs quantum wells exhibits magnetoresistance oscillations with fundamental fields significantly different from the Shubnikov-de Haas and magnetophonon resonance fundamental fields. The subband spacing can be calculated from the measured MIS fundamental field, and good agreement is found between measured and calculated subband spacings.

Absence of spin-density excitations in quasi two-dimensional electron systems.

Absence of spin-density excitations in quasi two-dimensional electron systems.

FAST ALGORITHM FOR COMPUTATIONAL STUDY OF DEEP LEVEL PHOTOLUMINESCENCE SPECTRA

An algorithm for fast calculation of theoretical deep level photoluminescence (PL) lineshapes is presented. The algorithm can be used for effective comparison with experimental spectra in cases of bulk photoluminescence. The method was expanded and for the cases of deep level photoluminescence in quasi-two and quasi-one dimensional electron systems in semiconductors.

Lateral p-n junctions and quantum wires formed by quasi two-dimensional electron and hole systems at corrugated GaAs/AlGaAs interfaces

We report the results of modeling lateral p-n junctions and p-n-p quantum wire structures at corrugated GaAs/AlGaAs interfaces, using the surface orientation dependent amphoteric nature of Si doping. We determine the potential landscape and the electron and hole charge densities within a semiclassical Thomas–Fermi screening model, and then solve the two-dimensional Schrödinger equation using finite elements for the quantized electron and hole states at the heterointerfaces. We demonstrate the formation of a one-dimensional electron system confined between two lateral p-n junctions, and discuss the advantages of this structure compared to conventional electrostatic confinement schemes for fabricating quantum wires.

On the Dynamics of Quasi-Twodimensional Correlated Electron Systems in a Strong Transverse Magnetic Field

The quantum dynamics of a correlated quasi-twodimensional (q2d) electron system showing the Quantum Hall Effect has been investigated. Galilei transformations are discussed for the correct description of quantum mechanical states in crossed electric and magnetic fields. Our quantum liquid site model with quasibosons carrying electromagnetic flux quanta uses a spatio-temporary varying configuration of sites {R(t)}. Due to the nontrivial topology of the system and the multiple degeneracy of its ground state non Abelean Berry phases appear in the adiabatic time evolution. The calculation of the effective action functional has been carried out on the basis of a path integral approach. It is shown that the Berry phases entering the Greens function produce a Chern-Simons-like topological action term. Our theory yields a quantized conductivity tensor and, thus, exibita Quantum Hall behaviour. In calculating the local density of states of the inhomogeneous quantum liquid one obtains a remarkable broadening of peaks and a reduction of their heights.

Cyclotron Resonance of Two-Dimensional Electron Systems on III-V Semiconductors

Far-infrared spectroscopy of cyclotron resonance in quasi two-dimensional electron systems on III-V semiconductors is discussed and special emphasis is laid on the consequences of band nonparabolicity and polar nature of these materials on the subband structure and cyclotron masses. Experimental results obtained for metal-oxide-semiconductor structures on InSb and GaAs/Ga1-xAlxAs heterojunctions are explained with the aid of simple pictures. Recent experiments on subband-shifted cyclotron resonance in quasi one-dimensional electron systems are also addressed.

Collective Behavior of Field Induced Spin Density Waves in Bechgaard Salts

Within a model of quasi-two dimensional electron system interacting via on site Coulomb potential we study theoretically the collective behavior of the field induced spin density waves (FISDW) in Bechgaard salts. In a FISDW there are two gapless modes associated with the spin rotation around the external magnetic field and the sliding of the FISDW. These gapless modes cancel exactly the anomaly due to the modification of the quasi-particle spectrum below Tc in the correlation functions like <[S3, S3]>, <[n, n]> and <[jx, jx]> where S3, n and jx are the spin parallel to the external field, the electron density and the electron current parallel to the chain direction. The low frequency behavior of these correlation functions are unchanged at the FISDW transition when the quasi-particle damping is ignored. However, the rotational symmetry of the spin is broken by the dipole energy which gives rise to a small energy gap in the longitudinal spin wave spectrum below Tc, which is accessible to the electron spin resonance. Furthermore the translational invariance of the SDW is in general broken by impurities and/or crystalline defects, which gives rise to the SDW pinning. Then the resulting expression of <[n, n]> and <[jx, jx]> resembles to those in a CDW in quasi-one dimensional conductors. Therefore we predict similar nonlinear conductivity in the presence of electric field and electromechanical effects as found in the CDW systems.

Ferromagnetism of two-dimensional electron systems

The ground state energies of (1) an ideal two-dimensional electron gas, (2) a quasi-two dimensional electron system resembling electrons trapped on a liquid helium surface, (3) a quasi-two dimensional electron system resembling inversion layers of Si(100)SiO 2 system, and (4) an ideal three-dimensional electron gas are obtained by numerically evaluating the sum of “ring diagrams” contributions to the correlation energies for all magnetizations and a wide range of electron densities. The transition from nonmagnetic to ferromagnetic state is abrupt in (1) and (2) and gradual in (3) and (4).