Actively Tuned and Spatially Trapped Polaritons

We report active tuning of the polariton resonance of quantum well excitons in a semiconductor microcavity using applied stress. Starting with the quantum well exciton energy higher than the cavity photon mode, we use stress to reduce the exciton energy and bring it into resonance with the photon mode. At the point of zero detuning, line narrowing and strong increase of the photoluminescence are seen. By the same means, we create an in-plane harmonic potential for the polaritons, which allows trapping, potentially making Bose-Einstein condensation of polaritons analogous to trapped atoms possible. We demonstrate drift of the polaritons into this trap.Comment: 10 pages, 5 figure

A spin foam model for pure gauge theory coupled to quantum gravity

We propose a spin foam model for pure gauge fields coupled to Riemannian quantum gravity in four dimensions. The model is formulated for the triangulation of a four-manifold which is given merely combinatorially. The Riemannian Barrett--Crane model provides the gravity sector of our model and dynamically assigns geometric data to the given combinatorial triangulation. The gauge theory sector is a lattice gauge theory living on the same triangulation and obtains from the gravity sector the geometric information which is required to calculate the Yang--Mills action. The model is designed so that one obtains a continuum approximation of the gauge theory sector at an effective level, similarly to the continuum limit of lattice gauge theory, when the typical length scale of gravity is much smaller than the Yang--Mills scale.Comment: 18 pages, LaTeX, 1 figure, v2: details clarified, references adde

Single electron capacitance spectroscopy of vertical quantum dots using a single electron transistor

We have incorporated an aluminum single electron transistor (SET) directly on top of a vertical quantum dot, enabling the use of the SET as an electrometer that is extremely responsive to the motion of charge into and out of the dot. Charge induced on the SET central island from single electron additions to the dot modulates the SET output, and we describe two methods for demodulation that permit quantitative extraction of the quantum dot capacitance signal. The two methods produce closely similar results for the determined single electron capacitance peaks.Comment: Submitted to Applied Physics Letters (reformatted to fit correctly on a page

Anisotropic Fermi Contour of (001) GaAs Holes in Parallel Magnetic Fields

We report a severe, spin-dependent, Fermi contour anisotropy induced by parallel magnetic field in a high-mobility (001) GaAs two-dimensional hole system. Employing commensurability oscillations created by a unidirectional, surface-strain-induced, periodic potential modulation, we directly probe the anisotropy of the two spin subband Fermi contours. Their areas are obtained from the Fourier transform of the Shubnikov-de Haas oscillations. Our findings are in semi-quantitative agreement with the results of parameter-free calculations of the energy bands.Comment: 4 pages, 4 figure

Area dependence of interlayer tunneling in strongly correlated bilayer two-dimensional electron systems at ν_T = 1

The area and perimeter dependence of the Josephson-like interlayer tunneling signature of the coherent ν_T = 1 quantum Hall phase in bilayer two-dimensional electron systems is examined. Electrostatic top gates of various sizes and shapes are used to locally define distinct ν_T = 1 regions in the same sample. Near the phase boundary with the incoherent ν_T = 1 state at large layer separation, our results demonstrate that the tunneling conductance in the coherent phase is closely proportional to the total area of the tunneling region. This implies that tunneling at ν_T = 1 is a bulk phenomenon in this regime

Enhancement of the ν=5/2\nu = 5/2 Fractional Quantum Hall State in a Small In-Plane Magnetic Field

Using a 50-nm width, ultra-clean GaAs/AlGaAs quantum well, we have studied the Landau level filling factor $\nu = 5/2$ fractional quantum Hall effect in a perpendicular magnetic field $B \sim$ 1.7 T and determined its dependence on tilted magnetic fields. Contrary to all previous results, the 5/2 resistance minimum and the Hall plateau are found to strengthen continuously under an increasing tilt angle $0 < \theta < 25^\circ$ (corresponding to an in-plane magnetic field 0 $<$ $B_\parallel$ $< 0.8$ T). In the same range of $\theta$ the activation gaps of both the 7/3 and the 8/3 states are found to increase with tilt. The 5/2 state transforms into a compressible Fermi liquid upon tilt angle $\theta > 60^\circ$, and the composite fermion series [2+$p/(2p\pm1)$], $p =$ 1, 2 can be identified. Based on our results, we discuss the relevance of a Skyrmion spin texture at $\nu = 5/2$ associated with small Zeeman energy in wide quantum wells, as proposed by W$\acute{\text o}$js $et$ $al$., Phys. Rev. Lett. 104, 086801 (2010).Comment: 5+ pages, 3 figures, accepted for by Phy. Rev. Let

Reentrant nu = 1 quantum Hall state in a two-dimensional hole system

We report the observation of a reentrant quantum Hall state at the Landau level filling factor nu = 1 in a two-dimensional hole system confined to a 35-nm-wide (001) GaAs quantum well. The reentrant behavior is characterized by a weakening and eventual collapse of the nu = 1 quantum Hall state in the presence of a parallel magnetic field component B||, followed by a strengthening and reemergence as B|| is further increased. The robustness of the nu = 1 quantum Hall state during the transition depends strongly on the charge distribution symmetry of the quantum well, while the magnitude of B|| needed to invoke the transition increases with the total density of the system