B.E. Cole

Terahertz response of zero‐dimensional states in resonant tunneling diodes

The photoresponse of zero‐dimensional states in double barrier resonant tunneling diodes has been observed using high intensity THz radiation (photon frequencies ranging from 4 to 10 THz) from a free‐electron laser. The double barrier resonant tunneling diodes have silicon donors in the quantum well which act as individual localized tunneling channels. The high frequency response of these diodes shows additional features due to the presence of these extra channels. The temperature dependence allows us to identify the contribution of the zero‐dimensional states. The absence of wavelength dependence in the observed photoresponse indicates that photoassisted tunneling does not occur under these measurement conditions.

Collective cyclotron modes in high-mobility two-dimensional hole systems in GaAs - (Ga, Al)As heterojunctions: I. Experiments at low magnetic fields and theory

The cyclotron resonance of very high-mobility two-dimensional holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied over the frequency range 30 to 200 GHz. Although the presence of two hole spin subbands in the samples suggests that two cyclotron resonances should be observed, in practice only a single resonance occurs for a wide range of conditions (temperature, field) and sample properties (hole density, mobility). Furthermore, the cyclotron resonance spectra often exhibit a strong temperature dependence. In the case of a single, sharp cyclotron resonance, the resonance field may shift by as much as 20% when warming the sample from 1.4 to 4.2 K. In the case of spectra containing multiple cyclotron resonances, similar changes in temperature shift the resonance positions together to form a single cyclotron absorption. This behaviour is explained in terms of two interacting hole subsystems with different effective masses formed by the two spin subbands. An analytical expression for the contribution to the high-frequency conductivity due to coupled cyclotron motion of the two hole systems is derived and shown to encompass previous theories developed for more restricted ranges of conditions. The expression predicts the complex behaviour of the experimental spectra very well, and enables hole masses, hole - lattice scattering rates and hole - hole scattering rates to be extracted. Comparisons between theory and data also show that a reactive interaction dominates the coupling between the spin subsystems at low temperatures. This is the first of two papers dealing with correlated hole cyclotron resonance; the second shows that the model derived in this work can also be used to treat cyclotron resonance data recorded at very high magnetic fields .

FAR-INFRARED INTERSUBBAND TRANSITIONS IN A TWO-DIMENSIONAL GAAS/(AL,GA)AS HOLE SYSTEM : DIRECT COMPARISON OF EXPERIMENT AND CALCULATION

Measured far‐infrared (FIR) intersubband absorption in two p‐type modulation‐doped GaAs/(Al,Ga)As multiquantum wells is compared with k⋅p modeling of the two‐dimensional heavy‐ and light‐hole (HH, LH) subband structure. Strong absorption for both HH1–HH2 and HH1–LH1 transitions is found. Self‐consistent modeling was performed within a 4×4 k⋅p scheme for heavy and light holes. There is an excellent agreement between the theoretical and experimental FIR intersubband transitions at k∥=0 and at the Fermi wave vector kf.

High magnetic field millimetre and submillimetre spectroscopy of ultra-high mobility 2D hole systems

Abstract Ultra-high mobility holes in (311)A GaAs-(Ga,Al)As heterojunctions are studied by cyclotron resonance (CR) experiments covering almost two orders of magnitude of magnetic field and energy. Features including several distinct CR branches, discontinuities in the CR energy versus field curves, strong non-parabolicity and temperature-dependent effective masses are observed, in reasonable agreement with theoretical predictions.

Saturation absorption studies of intersubband relaxation rates in a p-GaAs/AlGaAs QW using a free electron laser

We present saturation absorption studies of the HH1 --> LH1 intersubband transition in a 70 Angstrom p-type GaAs/AlGaAs multi-quantum well. The subband separation in this sample, with strongly non-parabolic bands, varies from 26 meV at k = 0 to 16 meV at the Fermi wave vector. The absorption behaviour of this transition is studied over a range of four orders of magnitude of intensity of incident THz radiation. The results are analysed within the contexts of a simple two-level model and an instantaneous thermalization model. The results indicate that the lifetime obtained with these models is limited by the pulsewidth, i.e. is greater than 1.7 ps. This places a lower limit on the energy relaxation lime for transitions below the LO-phonon energy

Collective cyclotron modes in high-mobility two-dimensional hole systems in GaAs - (Ga, Al)As heterojunctions: II. Experiments at magnetic fields of up to forty Tesla

The cyclotron resonance of very high-mobility holes in GaAs - (Ga, Al)As heterojunctions grown on (111), (311) and (100) substrates has been studied in high magnetic fields of up to 40 T. As the temperature is increased from to , the cyclotron resonance is found to shift to lower magnetic fields, the size of shift depending on the cyclotron frequency and the substrate orientation. These observations may be explained using the model of interacting hole subsystems developed by Cole et al.

Effective-mass anisotropy in GaAs-(Ga,Al)As two-dimensional hole systems: comparison of theory and very high-field cyclotron resonance experiments

Cyclotron resonance of two-dimensional holes in high-mobility GaAs-(Ga,Al)As heterojunctions with the growth directions (011), (111), (211), (311) and (100) has been measured at magnetic fields of around 35 T, corresponding to Landau level occupancies deep in the ultraquantum limit. A manipulation of the standard four-band Luttinger Hamiltonian has been used to show that the behaviour of the hole ground state is dominated by the leading field-dependent term in a power series expansion for the Landau level dispersion. The experimentally observed trend in measured effective mass with substrate orientation can therefore be qualitatively explained in terms of the variation of bulk hole mass with crystallographic direction.

Effective mass anisotropy and many-body effects in 2D GaAs/(Ga,Al)As hole gases observed in very high magnetic fields: comparison of theory and experiment

Abstract The effective masses of two-dimensional GaAs/(Ga,Al)As hole systems grown with (100), (311)B, (211)B, (111)B and (110) substrate orientations have been measured in magnetic fields of up to 80 T. The measured anisotropy is discussed in terms of bulk effects using a newly developed theory. Low energy (millimetre wave) cyclotron resonance absorptions show a 15% shift to higher fields in the temperature range 1.4 to 4.2 K over two orders of magnitude of energy. Many-body effects are used to discuss this observation.

Cyclotron Resonance Observation of Heavy and Light Spin Subbands in an Ultra-High Mobility 2D Hole Gas in GaAs/(Al,Ga)As: Evidence for Coupled Magnetoplasmons and Many Body Effects

Many-body effects may be conveniently studied in the two dimensional hole system (2DHS) formed in a GaAs/(Al,Ga)As heterostructure. This 2DHS exhibits extreme valence subband non-parabolicity and a large zero-field splitting of the dispersion relationships of the two spin projections1. In consequence, a complex cyclotron resonance (CR) spectrum might be expected2. However, the experimental study of CR in this Communication shows that such a complex spectrum is in fact absent in most cases. This absence is discussed in terms of a collective cyclotron motion due to Coulomb interactions between holes. Additional evidence for such a collective motion effect is provided by the observation of a very strong temperature dependence of the apparent CR position.