R.B. Dunford

A low-temperature insulating phase at for 2D holes in high-mobility heterostructures with Landau level degeneracy

Magneto-transport measurements of the 2D hole system (2DHS) in p-type Si-Si1-xGex heterostructures identify the integer quantum Hall effect (IQHE) at dominantly odd-integer filling factors v and two low-temperature insulating phases (IPs) at v = 1.5 and v less than or similar to 0.5, with re-entrance to the quantum Hall effect at v = 1. The temperature dependence, current-voltage characteristics, and tilted field and illumination responses of the IP at v = 1.5 indicate that the important physics is associated with an energy degeneracy of adjacent Landau levels of opposite spin, which provides a basis for consideration of an intrinsic, many-body origin.

Australian national pulsed magnet laboratory for condensed matter physics research

Abstract The National Pulsed Magnet Laboratory (NPML) provides access to high pulsed and steady magnetic fields, with particular emphasis on combining high fields with temperatures down to the millikelvin regime and a range of physical probes for condensed matter studies. NPML provides both short pulse (≈ 10 ms) fields to 60 T and long duration (1 s) fields to 30 T.

Time-resolved magneto-photoluminescence measurements of the integer QHE, fractional QHE and extreme quantum limit

Low-temperature, time-resolved magneto-photoluminescence (TRPL) measurements of 2D electron-free-va-lence-hole recombination have been carried out in the IQHE, FQHE and magnetically induced electron solid regimes of a GaAs/AlGaAs single heterojunction. Clear differences, not present at elevated temperatures, are found in the measured PL decay time (~ 1 ns) for components of multiple-peak structure observed beyond v = 15 used previously to construct a phase diagram associated with the Wigner solid. The new TRPL results and earlier PL data, taken together, are consistent with a mechanism in which the spectral feature associated with crystallisation derives from the trapping of excitons within a few Bohr radii (z separation) of the heterointerface and their subsequent (x,y) localisation by the presence of a 2D electron system with a degree of spatial order at this interface.

The extreme quantum regime of 2D electron and hole systems

Abstract The ground state of a 2D electron system (2DES) in a high magnetic field (Laughlin liquid versus Wigner solid) is probed by low temperature measurements of the intrinsic optical spectra of GaAs single heterojunctions (SHJs) unperturbed by the presence of deliberately introduced impurity layers. The objective is to understand the relationship of the signature observed in natural recombination optics to the quantum Hall effect (QHE) and magnetically-induced Wigner solid (MIWS) regions of the ground state phase diagram. Once understood, the photoluminescence (PL) signature then offers the potential to elicit quantitative information on the nature of the ground states. Recombination dynamics provide essential information on the nature of optical processes and time-resolved PL (TRPL) data are presented which identify the importance of excitonic recombination mechanisms in standard SHJ samples at high magnetic fields. A coherent model is put forward which accounts for the observed PL and TRPL signature throughout the magnetic field range-integer, fractional QHE and MIWS regimes. Our results are consistent with a mechanism in which the spectral feature associated with crystallisation derives from the trapping of excitons within a few Bohr radii ( z separation) of the heterointerface and their subsequent ( x , y ) localisation by the presence of a 2D electron lattice at this interface. The development of an insulated, optically-transparent top gate architecture that extends the PL studies in the MIWS regime to examine the additional perturbation of well-defined patterns in the 2DES, located up to 700 nm below the semiconductor surface in our low density high quality samples, is described. A summary is also given of preliminary electrical transport data for the 2D hole system in p-type GaAs heterojunctions, in intense pulsed magnetic fields (to 52 T) at low temperatures (0.3 K), which examine competition of fractional QHE ground states and an insulating phase in the extreme quantum limit that has been associated with a hole solid.

High-field photoluminescence in GaAs single heterojunctions: Mapping of an optically determined phase boundary correlated with the electron liquid-solid transition

Abstract Bandgap magneto-photoluminescence is used to study the optical signature of a high-quality, low-density GaAs-AlGaAs heterojunction in the integer QHE, fractional QHE and Wigner regimes. In the extreme quantum limit an intense, new spectral feature is observed to emerge for v 1 5 , the temperature dependence of which is used to define two characteristic temperatures, Tc1 and Tc2. The boundary formed by the lower mapping, Tc1, in the (B, T)-plane correlates well with the electron liquid-solid transition established by other techniques.

Low temperature magnetotransport of 2D electron and hole systems in high-mobility SiSi1 − xGex heterostructures

Low-temperature magnetotransport measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n- and p-type modulation-doped SiSi1 − xGex heterostructures (respectively) have been extended to high magnetic fields (50 T) and low temperatures (30 mK). For the high-mobility 2DES in n-Si, a two-valley system, signatures of the fractional quantum Hall effect (FQHE) in the region v < 1 (one valley occupied, lowest spin state) usually observed in GaAsAlGaAs are replicated out to v = 2/5 at B ≈ 48T. For 1 < v < 2, however, (both valleys occupied, lowest spin state), prominent FQHE states such as v = 5/3 are absent, indicative of the importance of valley occupation. For the 2DHS in p-Si1 − xGex, in addition to the QHE, two low-temperature insulating phases (IP) are identified at v = 1.5 and v ≲ 0.5 (B ≈ 30T), with re-entrance to the QHE regime at v = 1 (B ≈ 15T). The IP centred at v = 1.5 has the characteristics of a Hall insulator but is unanticipated by the global phase diagram for 2D systems. The important physics associated with the IP is related to an energy degeneracy of adjacent Landau levels of opposite spin.

Transport studies of the extreme quantum regime in 2D hole gas systems in pulsed magnetic fields

Abstract Intense, pulsed magnetic field studies of 2D hole gas systems at low temperatures examine competition of fractional QHE ground states with an insulating phase in the region ν

Magneto-photoluminescence studies of a 2D electron system: Signatures of the fractional quantum Hall effect and Wigner solid

Abstract The anticipated transition from a liquid to a magnetically-induced Wigner solid (MIWS) ground state in a two-dimensional electron system is investigated by bandgap photoluminescence (electron-valence band hole recombination) in two GaAs single heterojunctions of different densities and intersubband spacings. For Landau level filling factors ν 1 5 the photoluminescence spectra are dominated by a new line, the temperature dependence of which is used to map a phase boundary which is situated close to the boundary associated with a pinned MIWS phase mapped by a variety of other techniques. A weak peak, split off to lower energy from the main PL emission, may also be of relevance to the MIWS. Comparison is made with optical data that probes this transition by electron-neutral acceptor recombination in Be δ-doped samples. In our higher density sample (∼ 10 11 cm −2 ), a temperature-dependent laser-induced 2D electron density depletion effect observed at high magnetic fields and low temperatures is investigated systematically. It has also been possible to study 2D electron to acceptor-bound hole recombination in this sample associated with residual, low concentration carbon impurity levels as a further probe of the QHE regime. Additionally, a definitive optical signature of the ν= 1 3 FQHE hierarchy is reported.

Low-temperature transport and photoconductivity response of high mobility Si-SiGe heterostructures in strong magnetic fields

Low-temperature (mK) magneto-transport and photoconductivity measurements of 2D electron and hole systems (2DES, 2DHS) in high quality n- and p-type modulation-doped Si-SiGe heterostructures (respectively) have been extended to high magnetic fields. For the high mobility 2DES in n-Si, a two valley system, signatures of the fractional quantum Hall effect (FQHE) in the region v < 1 (one valley occupied) usually observed in GaAs are replicated out to v = 2/5(B ∼ 40 T). For 1 < v < 2 however (two valleys occupied) prominent FQHE states are absent. For the 2DHS in p-SiGe, in addition to the QHE two low temperature insulating phases (IP) are identified at v = 1.5 and v < 1/2 (B ∼ 30 T). The IP at v = 1.5 has the characteristics of a Hall insulator and our measurements indicate that the important physics is related to an unusual degeneracy of adjacent Landau levels (LL) of opposite spin. Illuminating the p-SiGe samples results in a lifting of the LL degeneracy together with a quenching of the IP at v = 1.5. Magneto-photoconductivity measurements of these samples closely resemble dρ xx /dB and provide an additional probe of this indirect bandgap system. The incorporation of a dilution refrigerator in pulsed magnets with CuAg conductor for extended use at 60 T is described.

A tunable ballistic electron cavity exhibiting geometry-induced weak localisation

We report the use of a lateral surface gate stripe bisecting a relatively large, high mobility ballistic electron cavity to examine geometry-induced weak localisation effects. Illumination and positive bias applied to the stripe gate are used to produce a controllable trajectory scrambling region within the cavity.