Controlling the polarisation correlation of photon pairs from a charge-tuneable quantum dot

Correlation between the rectilinear polarisations of the photons emitted from the biexciton decay in a single quantum dot is investigated in a device which allows the charge-state of the dot to be controlled. Optimising emission from the neutral exciton states maximises the operating efficiency of the biexciton decay. This is important for single dot applications such as a triggered source of entangled photons. As the bias on the device is reduced correlation between the two photons is found to fall dramatically as emission from the negatively charged exciton becomes significant. Lifetime measurements demonstrate that electronic spin-scattering is the likely cause.Comment: 3 figure

Thermoelectric and electrical transport in mesoscopic two-dimensional electron gases

We review some of our recent experimental studies on low-carrier concentration, mesoscopic two-dimensional electron gases (m2DEGs). The m2DEGs show a range of striking characteristics including a complete avoidance of the strongly localised regime even when the electrical resistivity $\rho >> h/e^2$, giant thermoelectric response, and an apparent decoupling of charge and thermoelectric transport. We analyse the results and demonstrate that these observations can be explained based on the assumption that the charge carriers retain phase coherence over the m2DEG dimensions. Intriguingly, this would imply phase coherence on lengthscales of up to 10 $\mu$m and temperature $T$ up to 10 K which is significantly greater than conventionally expected in GaAs-based 2DEGs. Such unprecedentedly large phase coherence lengths open up several possibilities in quantum information and computation schemes.Engineering and Physical Sciences Research Council, Leverhulme TrustThis is the final version of the article. It first appeared from Elsevier via http://dx.doi.org/10.1016/j.crhy.2016.08.01

Inversion of exciton level splitting in quantum dots

The demonstration of degeneracy of exciton spin states is an important step toward the production of entangled photon pairs from the biexciton cascade. We measure the fine structure of exciton and biexciton states for a large number of single InAs quantum dots in a GaAs matrix; the energetic splitting of the horizontally and vertically polarized components of the exciton doublet is shown to decrease as the exciton confinement decreases, crucially passing through zero and changing sign. Thermal annealing is shown to reduce the exciton confinement, thereby increasing the number of dots with splitting close to zero

Anticrossing of spin-split subbands in quasi-one-dimensional wires

In quantum Hall systems, both anticrossings and magnetic phase transitions can occur when opposite-spin Landau levels coincide. Our results indicate that both processes are also possible in quasi-1D quantum wires in an in-plane B field, B-parallel to. Crossings of opposite-spin 1D subbands resemble magnetic phase transitions at zero dc source-drain bias, but display anticrossings at high dc bias. Our data also imply that the well-known 0.7 structure may evolve into a spin-hybridized state in finite dc bias

Acoustic transport of electrons in parallel quantum wires

Over the last few years we have developed a new method to control single-electrons by isolating and moving them through a submicron width channel formed in a GaAs/AlGaAs heterostructure using a surface acoustic wave. The acoustic wave acts to push electrons through the depleted submicron channel in packets each containing an integer number of electrons. Our primary motivation for studying this system has been to develop a new standard of dc current for metrological purposes, but our recent focus has widened to investigate the possibility of single-photon emission. Here we show new experimental results which demonstrate acoustoelectric current flow in adjacent 1D wires. These results have relevance both to the use of the system in a single-photon emission scheme, as well as in the creation of a proposed acoustoelectric quantum computer

Evolution of the second lowest extended state as a function of the effective magnetic field in the fractional quantum hall regime

It has been shown that, at a Landau level filling factor v=1/2, a two-dimensional electron system can be mathematically transformed into a composite fermion system interacting with a Chern-Simons gauge field. At v=1/2, the average of this Chern-Simons gauge field cancels the external magnetic field B-ext so that the effective magnetic field B-eff acting on the composite fermions is zero. Away from v=1/2, the composite fermions experience a net effective magnetic field B-eff. We present the first study of the evolution of the second lowest extended state in a vanishing effective magnetic field in the fractional quantum Hall regime. Our result shows that the evolution of the second lowest extended state has a good linear dependence on the effective magnetic field Beff within the composite fermion picture

Possible evidence of a spontaneous spin polarization in mesoscopic two-dimensional electron systems

We have experimentally studied the nonequilibrium transport in low-density clean two-dimensional (2D) electron systems at mesoscopic length scales. At zero magnetic field (B), a double-peak structure in the nonlinear conductance was observed close to the Fermi energy in the localized regime. From the behavior of these peaks at nonzero B, we could associate them with the opposite spin states of the system, indicating a spontaneous spin polarization at B=0. Detailed temperature and disorder dependence of the structure shows that such a splitting is a ground-state property of low-density 2D systems

Suspended two-dimensional electron gases in In₀.₇₅Ga₀.₂₅As quantum wells

We demonstrate that In0.75Ga0.25As quantum wells can be freely suspended without losing electrical quality when the epitaxial strain-relieving buffer layer is removed. In applied magnetic fields, non-dissipative behavior is observed in the conductivity, and a current induced breakdown of the quantum Hall effect shows a lower critical current in the suspended layers due to efficient thermal isolation compared to the non-suspended-control device. Beyond the critical current, background impurity scattering in the suspended two-dimensional channel regions dominates with stochastic, resonant-like features in the conductivity. This device fabrication scheme offers the potential for thermally isolated devices containing suspension-asymmetry-induced, high spin–orbit coupling strengths with reduced electron–phonon interaction behavior but without introducing high levels of disorder in the processing. This work was funded by EPSRC Grant Nos. EP/K004077/1 and EP/R029075/1, UK. We thank Professor Chris Ford for useful discussions

Sb surface terminated MnSb devices in the niccolite phase

The magneto-electronic properties of ferromagnetic MnSb grown by molecular beam epitaxy can be dominated by the presence of a surface state in the minority spin bandgap when the surface is Sb-terminated. The material resistivity is 120 µΩ.cm at 295 K, and although this is determined by the majority spin population, the anisotropic magnetoresistance, dependent on minority spins, is ∼0.24% for the Sb-terminated devices with Mn-terminated devices showing ∼0.02%. At 295 K, the extraordinary Hall constant is 0.5 Ω/T for the Sb-terminated surface and 1.5 Ω/T for the Mn-terminated surface with the extraordinary Hall constant and anisotropic magnetoresistance behaving with an anomalous temperature dependence between 295 and 1.5 K. The dominant MnSb structural phase on the GaAs (001) orientation is naturally doped p-type with a carrier density ∼1 × 1022 cm−3 determined by the normal Hall effect after the extraordinary Hall effect has saturated at higher fields than ∼2 T. Spintronic device possibilities are discussed, particularly the spin-light emitting diode and magnetic nano-structures. A natural p-type doping in MnSb limits the devices to dominant hole carrier effects although there is compatibility with both III–V and Si–Ge materials for hybrid device possibilities

Detection of anomalous Hall voltages in ultrahigh-mobility two-dimensional hole gases generated by optical spin orientation

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.This work was supported by the EPSRC.This is the author accepted manuscript. The final version was first published by APS at http://journals.aps.org/prb/abstract/10.1103/PhysRevB.91.201406