Some Evidence on Finite Sample Behavior of an Instrumental Variables Estimator of the Linear Quadtratic Inventory Model

We evaluate some aspects of the finite sample distribution of an instrumental variables estimator of a first order condition of the Holt et al. (1960) linear quadratic inventory model. We find that for some but not all empirically relevant data generating processes and sample sizes, asymptotic theory predicts a wide dispersion of parameter estimates, with a substantial finite sample probability of estimates with incorrect signs. For such data generating processes, simulation evidence suggests that different choices of left hand side variables often produce parameter estimates of an opposite sign. More generally, while the asymptotic theory often provides a good approximation to the finite sample distribution, sometimes it does not

Spin-Flipping Half Vortex in a Macroscopic Polariton Spinor Ring Condensate

We report the observation of vorticity in a macroscopic Bose-Einstein condensate of polaritons in a ring geometry. Because it is a spinor condensate, the elementary excitations are "half vortices" in which there is a phase rotation of $\pi$ in connection with a polarization vector rotation of $\pi$ around a closed path. This is clearly seen in the experimental observations of the polarization rotation around the ring. In the ring geometry, a new type of half vortex is allowed in which the handedness of the spin flips from one side of the ring to the other, in addition to the rotation of the linear polarization component; such a state is not allowed in a simply-connected geometry. Theoretical calculation of the energy of this state shows that when many-body interactions are taken into account, it is lower in energy than a simple half vortex. The direction of circulation of the flow around the ring fluctuates randomly between clockwise and counterclockwise from one shot to the next; this corresponds to spontaneous breaking of time-reversal symmetry in the system. These new, macroscopic polariton ring condensates allow for the possibility of direct control of the vorticity of the condensate.Comment: 21 pages, 10 figures, including supplemental information; Proceedings of the National Academy of Sciences (USA) (2015

Dynamic Nuclear Polarization in the Fractional Quantum Hall Regime

We investigate dynamic nuclear polarization in quantum point contacts (QPCs) in the integer and fractional quantum Hall regimes. Following the application of a dc bias, fractional plateaus in the QPC shift symmetrically about half filling of the lowest Landau level, $\nu = 1/2$, suggesting an interpretation in terms of composite fermions. Polarizing and detecting at different filling factors indicates that Zeeman energy is reduced by the induced nuclear polarization. Mapping effects from integer to fractional regimes extends the composite fermion picture to include hyperfine coupling.Physic

Saturation of intersubband transitions in p-doped GaAs/AlGaAs quantum wells

Optical saturation experiments have been performed on hh1-hh2 intersubband transitions in two samples of p-doped GaAs/AlGaAs quantum wells. The transitions had energies of 183 and 160 meV and the measured population relaxation times were 2±1.5 and 0.3±0.1 ps, respectively. Modeling of the quantum wells with a 6×6 k·p method shows that intersubband scattering by LO phonons can account for these relaxation times. The valence bandstructure is typically more complicated than the conduction bandstructure in a quantum well but these measurements show that LO phonons are the dominant intersubband scattering mechanism in both cases

Observation of quantum depletion in a nonequilibrium exciton-polariton condensate

The property of superfluidity, first discovered in liquid 4He, is closely related to Bose-Einstein condensation (BEC) of interacting bosons. However, even at zero temperature, when one would expect the whole bosonic quantum liquid to become condensed, a fraction of it is excited into higher momentum states via interparticle interactions and quantum fluctuations -- the phenomenon of quantum depletion. Quantum depletion of weakly interacting atomic BECs in thermal equilibrium is well understood theoretically but is difficult to measure. This is even more challenging in driven-dissipative systems such as exciton-polariton condensates(photons coupled to electron-hole pairs in a semiconductor), since their nonequilibrium nature is predicted to suppress quantum depletion. Here, we observe quantum depletion of an optically trapped high-density exciton-polariton condensate by directly detecting the spectral branch of elementary excitations populated by this process. Analysis of the population of this branch in momentum space shows that quantum depletion of an exciton-polariton condensate can closely follow or strongly deviate from the equilibrium Bogoliubov theory, depending on the fraction of matter (exciton) in an exciton-polariton. Our results reveal the effects of exciton-polariton interactions beyond the mean-field description and call for a deeper understanding of the relationship between equilibrium and nonequilibrium BECs.Comment: 18 pages, 5 figures, with supplementary informatio

Effect of strain on stripe phases in the Quantum Hall regime

Spontaneous breaking of rotational symmetry and preferential orientation of stripe phases in the quantum Hall regime has attracted considerable experimental and theoretical effort over the last decade. We demonstrate experimentally and theoretically that the direction of high and low resistance of the two-dimensional (2D) hole gas in the quantum Hall regime can be controlled by an external strain. Depending on the sign of the in-plane shear strain, the Hartree-Fock energy of holes or electrons is minimized when the charge density wave (CDW) is oriented along [110] or [1-10] directions. We suggest that shear strains due to internal electric fields in the growth direction are responsible for the observed orientation of CDW in pristine electron and hole samples.Comment: 10 pages, 3 figure