Wide Parabolic Wells Research Articles

Valence band tail states in disordered superlattices embedded in wide parabolic AlGaAs well

Optical properties of intentionally disordered multiple quantum well (QW) system embedded in a wide AlGaAs parabolic well were investigated by photoluminescence (PL) measurements as functions of the laser excitation power and the temperature. The characterization of the carriers localized in the individual wells was allowed due to the artificial disorder that caused spectral separation of the photoluminescence lines emitted by different wells. We observed that the photoluminescence peak intensity from each quantum well shifted to high energy as the excitation power was increased. This blue-shift is associated with the filling of localized states in the valence band tail. We also found that the dependence of the peak intensity on the temperature is very sensitive to the excitation power. The temperature dependence of the photoluminescence peak energy from each QW was well fitted using a model that takes into account the thermal redistribution of the localized carriers. Our results demonstrate that the band tails in the studied structures are caused by alloy potential fluctuations and the band tail states dominate the emission from the peripheral wells.

Magneto-optical probe of quantum Hall states in a wide parabolic well modulated by random potential

Polarized photoluminescence from weakly coupled random multiple well quasi-three-dimensional electron system is studied in the regime of the integer quantum Hall effect. Two quantum Hall ferromagnetic ground states assigned to the uncorrelated miniband quantum Hall state and to the spontaneous interwell phase coherent dimer quantum Hall state are observed. Photoluminescence associated with these states exhibits features caused by finite-size skyrmions: dramatic reduction of the electron spin polarization when the magnetic field is increased past the filling factor $\ensuremath{\nu}=1$. The effective skyrmion size is larger than in two-dimensional electron systems.

Magnetotransport in a wide parabolic well superimposed with a superlattice

The electron properties of artificially disordered superlattices embedded in a wide AlGaAs parabolic well were investigated in a strong magnetic field. We demonstrated that in the extreme quantum limit the interlayer disorder results in formation of a new correlated phase. A nearly uniform electron distribution over the superlattice wells was found in a weak magnetic field. However, a nonuniform phase with partially localized electrons, representing well-developed fractional quantum Hall effect features, was observed in high magnetic field (at the filling factor ν < 1). A distinct magnetic field–induced transition separates these two phases.

Magnetic-field-induced transition in a wide parabolic well superimposed with a superlattice

We study a $Al_{x}Ga_{x-1}As$ parabolic quantum wells (PQW) with $GaAs/Al_{x}Ga_{x-1}As$ square superlattice. The magnetotransport in PQW with intentionally disordered short-period superlattice reveals a surprising transition from electrons distribution over whole parabolic well to independent-layer states with unequal density. The transition occurs in the perpendicular magnetic field at Landau filling factor $\nu\approx3$ and is signaled by the appearance of the strong and developing fractional quantum Hall (FQH) states and by the enhanced slope of the Hall resistance. We attribute the transition to the possible electron localization in the x-y plane inside the lateral wells, and formation of the FQH states in the central well of the superlattice, driven by electron-electron interaction.

A study of disorder effects in random (AlxGa1−xAs)n(AlyGa1−yAs)m superlattices embedded in a wide parabolic potential

A photoluminescence (PL) study of the individual electron states localized in a random potential is performed in artificially disordered superlattices embedded in a wide parabolic well. The valence band bowing of the parabolic potential provides a variation of the emission energies which splits the optical transitions corresponding to different wells within the random potential. The blueshift of the PL lines emitted by individual random wells, observed with increasing disorder strength, is demonstrated. The variation of temperature and magnetic field allowed for the behavior of the electrons localized in individual wells of the random potential to be distinguished.

Enhanced Hall slope in wideAlxGax−1Asparabolic wells

We report measurements of the Hall effect in $1000--4000\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ wide ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{x\ensuremath{-}1}\mathrm{As}$ parabolic wells with quasi-two-dimensional electrons and holes in a perpendicular magnetic field. Above a critical magnetic field $Bg3T$, the Hall resistance for wide parabolic wells is found to be enhanced when the temperature decreases. We attribute this enhanced Hall slope to a carrier density effect. The Hartree and exchange-correlation terms produce a strong variation of the potential-well shape. The width of the electronic and hole slabs shrinks as the magnetic field is increased, which leads to the redistribution of the charge between the well and impurity layer.

Spin valve effect and hall resistance in a wide parabolic well

We report observation of the Hall slope change in wide AlcGa1-cAs parabolic wells in the presence of a quasi-parallel magnetic field. Above the critical magnetic field B > 4T, the Hall resistance becomes temperature dependent and can be described by equation Rxy/cosQ ~ (B-B0)/ens, where ns is the electron density, B0=2-2.6 T and Q is the angle between magnetic field and the normal to the well plane. The effect strongly depends on the electron density; it is observed only in parabolic wells, which are almost completely filled by electrons. We attribute the Hall slope change to the unusual behaviour of the effective g factor in such parabolic well, which depends on the Al composition and changes the sign along the well width.

Spin Polarization by Tilted Magnetic Field in Wide Ga1−xAlxAs Parabolic Quantum Wells

Wide parabolic wells can be created by properly controlling the Al content during the growth of successive Ga1−xAlxAs thin layers. Under a tilted magnetic field these systems present interesting transport properties, which are associated to their composition dependent g-factor. We present an exact solution for the eigenstates of an electron gas inside such a quantum well. We calculate the renormalized cyclotron frequencies as functions of the angle of tilt, as well as the density of states, and the Fermi level. We discuss the conditions for the existence of spin-polarized charge.

Anomalous Hall effect in a wide parabolic well

We report observation of the anomalous Hall effect in a wide Ga1–xAlxAs parabolic wells in quasi-parallel magnetic field. We found that the slope of the Hall resistance at high magnetic field B is two time larger than the slope at low B and depends on the temperature. The anomalous Hall effect can be attributed to the unusual spin properties in parabolic wells, for example, the effective g-factor in these structures changes the sign along well width. It may suppress the motion of electrons in the crossed electric and magnetic fields. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spontaneous symmetry breaking and exotic quantum orders in integer quantum Hall systems under a tilted magnetic field

We use the microscopic Hartree-Fock approximation to investigate various quantum phase transitions associated with possible spontaneous symmetry breaking induced by a tilted magnetic field in the integral quantum Hall regime of wide parabolic wells and zero width double well (bilayer) systems. We propose a general class of variational wavefunctions that describe several types of parity, spin, and translational symmetry breaking, including spin and charge density wave phases. Zero temperature quantum phase diagrams for these systems are calculated in the parameter regime of experimental interest. We discuss the symmetry properties of our predicted quantum phase diagrams and give a unified picture of these novel many-body phases. A conceptually new aspect of our theory is the predicted possibility for the spontaneous breaking of parity symmetry, which indicates a ``ferroelectric'' quantum order in integer quantum Hall systems and has not been considered in the literature before.

Commensurability oscillations in wide parabolic well in the presence of an in-plane magnetic field

The commensurability oscillations in antidot lattices with different periods are studied in the tilted magnetic field in wide parabolic quantum well with five occupied subbands. We find that the commensurability peaks are continuously transformed to new magnetoresistance oscillations, when magnetic field is directed parallel to the well plane. The positions of the new peaks correspond to Rc/We≈2.3 and 1, where Rc is the cyclotron radius, and We is the quantum well width. This result is consistent with recent quantum-mechanical calculations of the magnetoresistance in the presence of boundary-roughness scattering.

Quantum Hall effect in a wide parabolic well

We examine the activated conductivity for Landau levels at filling factors N=1,2 in a parabolic AlxGa1−xAs well down to 30mK. We assume that the possible mechanism of transport is a hopping conductivity in the middle between overlapping levels. These results demonstrate that the plateau at N=1,2 in quantum Hall effect are due to the energy gaps between electric subbands.

Quantum hall effect in a wide parabolic quantum well

We investigate the activated conductivity for Landau levels at lling factors v =1,2 in a parabolic Al xGa1-x As well down to 30 mK. We obtain a very small activation energy ~ 0.02 meV, which is almost 1-2 orders of magnitude smaller than the expected values for energy gaps in a wide quantum well. The resistivity minima vanish completely when a parallel component of magnetic field is applied. The collapse of the energy gaps in the wide parabolic well occurs due to the new electron correlated state, probably the charge density wave state predicted for a uniform wide electron slab in high magnetic field.

Electron cooling times in PbTe Landau quantized wires and dots

We present measurements of electron cooling times in `quasi' quantum wires and dots as a function of the z-confinement. The states were created by application of a magnetic field along the growth direction of bulk material, a wide parabolic well and a narrow quantum well, based on the IV-VI material PbTe. These three systems thus provide a progression of successively 1D-like, weakly 0D-like and 0D-like density of states. The measurements were all made above the LO-phonon energy, and the increasing confinement is represented in the LO-phonon cooling times observed, which become successively longer.

Magnetooscillations in a wide parabolic well in the presence of an in-plane magnetic field

The magnetoresistance oscillations in a wide parabolic quantum well are investigated at low temperatures in a tilted magnetic field. Magnetooscillations in the presence on an in-plane magnetic field cannot be explained by the diamagnetic Shubnikov–de Haas (Sd H) effect. Three-dimensional character of the electron motion can be responsible for this behaviour.

Landau-level lifetimes in PbTe nipi superlattices, PbTe/PbEuTe and InAs/AlSb quantum wells

Landau-level lifetimes are determined from saturation cyclotron resonance (CR) in wide parabolic wells, quantum wells and bulk PbTe–Pb 1− x Eu x Te systems. These narrow gap structures exhibit strong band non-parabolicity necessary to terminate the normally equi-spaced Landau-level ladder. It was not possible to saturate the bulk sample, but short lifetimes, of between 1.5 and 8 ps, were obtained for the wide parabolic well and the quantum well, respectively, utilising a multi-level rate equation model. We also report the first pump–probe cyclotron resonance result in an InAs–AlSb quantum structure. The pump–probe experiment provides a direct determination of the lifetime, giving τ=40 ps in this InAs–AlSb sample. This shows good agreement with an 8×8 k · p calculation.

Charge Carrier Transport in Artificially Structured Two-Dimensional Semiconductor Systems

General aspects of the electronic transport in two-dimensional and quasi-three-dimensional semiconductor systems are discussed. Contributions of dicherent scattering processes to the total electron and hole mobilities in various types of modulation doped heterostructures are calculated. It is shown that in a wide temperature range phonon scattering is the principal scattering mechanism limiting electron and hole mobilities in high quality AlGaAs/GaAs modulation doped heterostructures. Scattering from rough walls in wide parabolic wells is briefly reviewed.

Magnetocapacitance spectroscopy of multilayer electron systems in wide parabolic wells

We present a technique to detect quantum capacitance in the presence of strong density profile effects. We demonstrate our technique by performing capacitance-voltage subband spectroscopy on a system of coupled electron layers in a heterostructure which is the superposition of a wide parabolic well and multiple square wells. Events inducing Fermi level variations are detected as features in the derivative profile dC/dVg as the electron gas width is reduced by an applied gate voltage. Quantum features are differentiated from density profile effects by considering differences in derivative profiles dC/dVg for increasing magnetic field perpendicular to the electron layers. Results are compared with theory.

Low-temperature carrier distributions in wide quantum wells of different shapes from capacitance-voltage measurements

The carrier distributions in modulation-doped wide graded quantum wells that a measurement of the capacitance C between a surface gate and an ohmic contact to the carriers as a function of the applied bias V would yield are calculated. These capacitance-voltage (C-V) distributions are found to agree inexactly, but closely, with the calculated true carrier distributions. Density modulation features, induced by superlattices or by abrupt changes in the curvature of band-gap grading, are strikingly reproduced. Electron distributions extracted from actual measurements on a wide parabolic well and on a parabolic well with superimposed superlattice are in good agreement with theory. For the case of the parabolic well, the occupancy of a finite number of subbands is manifested as structure in the C-V distributions. This technique is relevant to the measurement of carrier distributions in any wide carrier system with more than one electric subband occupied.

Interacting intersubband excitations in parabolic semiconductor quantum wells.

We calculate the finite wave-vector intersubband collective excitation spectra in wide parabolic wells at low two-dimensional electron densities where only the lowest quantum subband is occupied by electrons. We use a self-consistent time-dependent local-density approximation to calculate the linear response of the system, comparing our density-functional results with the noninteracting time-dependent Hartree approximation to estimate the magnitudes of exchange-correlation corrections to the collectivemode dispersion. We predict a qualitatively new phenomenon at low electron densities where, in the presence of exchange-correlation effects, it becomes possible for the collective charge-density excitation (i.e., the intersubband plasmon mode) to lie below the intersubband quasiparticle continuum