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This paper assesses intersubband transitions in the 1 to 10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular quantum wells in the two perpendicular inplane directions, with high-resolution images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects introduced by the epitaxial process. Low-temperature intersubband absorption from 1.5 to 9 THz is demonstrated, covering part of the 7 to 10 THz band forbidden to GaAs-based technologies.

This paper assesses nonpolar m- and a-plane GaN/Al(Ga)N multi-quantum-wells grown on bulk GaN for intersubband optoelectronics in the short- and mid-wavelength infrared ranges. The characterization results are compared to those for reference samples grown on the polar c-plane, and are verified by self-consistent Schr\"odinger-Poisson calculations. The best results in terms of mosaicity, surface roughness, photoluminescence linewidth and intensity, as well as intersubband absorption are obtained from m-plane structures, which display room-temperature intersubband absorption in the range from 1.5 to 2.9 um. Based on these results, a series of m-plane GaN/AlGaN multi-quantum-wells were designed to determine the accessible spectral range in the mid-infrared. These samples exhibit tunable room-temperature intersubband absorption from 4.0 to 5.8 um, the long-wavelength limit being set by the absorption associated with the second order of the Reststrahlen band in the GaN substrates.

The order parameter of a quantum-coherent many-body system can include a phase degree of freedom, which, in the presence of an electromagnetic field, depends on the choice of gauge. Because of the relationship between the phase gradient and the velocity, time-of-flight measurements reveal this gradient. Here, we make such measurements using initially trapped Bose-Einstein condensates (BECs) subject to an artificial magnetic field. Vortices are nucleated in the BEC for artificial field strengths above a critical value, which represents a structural phase transition. By comparing to superfluid-hydrodynamic and Gross-Pitaevskii calculations, we confirmed that the transition from the vortex-free state gives rise to a shear in the released BEC's spatial distribution, representing a macroscopic method to measure this transition, distinct from direct measurements of vortex entry. Shear is also affected by an artificial electric field accompanying the artificial magnetic field turn-off, which depends on the details of the physical mechanism creating the artificial fields, and implies a natural choice of gauge. Measurements of this kind offer opportunities for studying phase in less-well-understood quantum gas systems., Comment: 5 pages, 4 figures + 2 pages supplementary data

GaN/AlN nanowire heterostructures can display photoluminescence (PL) decay times on the order of microseconds that persist up to room temperature. Doping the GaN nanodisk insertions with Ge can reduce these PL decay times by two orders of magnitude. These phenomena are explained by the three-dimensional electric field distribution within the GaN nanodisks, which has an axial component in the range of a few MV/cm associated to the spontaneous and piezoelectric polarization, and a radial piezoelectric contribution associated to the shear components of the lattice strain. At low dopant concentrations, a large electron-hole separation in both the axial and radial directions is present. The relatively weak radial electric fields, which are about one order of magnitude smaller than the axial fields, are rapidly screened by doping. This bidirectional screening leads to a radial and axial centralization of the hole underneath the electron, and consequently, to large decreases in PL decay times, in addition to luminescence blue shifts.

Spin-orbit coupling (SOC) is an essential ingredient in topological materials, conventional and quantum-gas based alike.~Engineered spin-orbit coupling in ultracold atom systems --unique in their experimental control and measurement opportunities-- provides a major opportunity to investigate and understand topological phenomena.~Here we experimentally demonstrate and theoretically analyze a technique for controlling SOC in a two component Bose-Einstein condensate using amplitude-modulated Raman coupling., Comment: 5 pages, 4 figues

Electronic properties like current flow are generally independent of the electron's spin angular momentum, an internal degree of freedom present in quantum particles. The spin Hall effects (SHEs), first proposed 40 years ago, are an unusual class of phenomena where flowing particles experience orthogonally directed spin-dependent Lorentz-like forces, analogous to the conventional Lorentz force for the Hall effect, but opposite in sign for two spin states. Such spin Hall effects have been observed for electrons flowing in spin-orbit coupled materials such as GaAs or InGaAs and for laser light traversing dielectric junctions. Here we observe the spin Hall effect in a quantum-degenerate Bose gas, and use the resulting spin-dependent Lorentz forces to realize a cold-atom spin transistor. By engineering a spatially inhomogeneous spin-orbit coupling field for our quantum gas, we explicitly introduce and measure the requisite spin-dependent Lorentz forces, in excellent agreement with our calculations. This atomtronic circuit element behaves as a new type of velocity-insensitive adiabatic spin-selector, with potential application in devices such as magnetic or inertial sensors. In addition, such techniques --- for both creating and measuring the SHE --- are clear prerequisites for engineering topological insulators and detecting their associated quantized spin Hall effects in quantum gases. As implemented, our system realized a laser-actuated analog to the Datta-Das spin transistor., Comment: Accepted version

Ultracold gases of interacting spin-orbit coupled fermions are predicted to display exotic phenomena such as topological superfluidity and its associated Majorana fermions. Here, we experimentally demonstrate a route to strongly-interacting single-component atomic Fermi gases by combining an s-wave Feshbach resonance (giving strong interactions) and spin-orbit coupling (creating an effective p-wave channel). We identify the Feshbach resonance by its associated atomic loss feature and show that, in agreement with our single-channel scattering model, this feature is preserved and shifted as a function of the spin-orbit coupling parameters., Comment: 5 pages, 4 figures

Zitterbewegung, a force-free trembling motion first predicted for relativistic fermions like electrons, was an unexpected consequence of the Dirac equation's unification of quantum mechanics and special relativity. Though the oscillatory motion's large frequency and small amplitude have precluded its measurement with electrons, zitterbewegung is observable via quantum simulation. We engineered an environment for 87Rb Bose-Einstein condensates where the constituent atoms behaved like relativistic particles subject to the one-dimensional Dirac equation. With direct imaging, we observed the sub-micrometer trembling motion of these clouds, demonstrating the utility of neutral ultracold quantum gases for simulating Dirac particles., Comment: v2: add journal reference, update to published version (11 pages, 5 figures)

Artificial gauge fields open new possibilities to realize quantum many-body systems with ultracold atoms, by engineering Hamiltonians usually associated with electronic systems. In the presence of a periodic potential, artificial gauge fields may bring ultracold atoms closer to the quantum Hall regime. Here, we describe a one-dimensional lattice derived purely from effective Zeeman-shifts resulting from a combination of Raman coupling and radiofrequency magnetic fields. In this lattice, the tunneling matrix element is generally complex. We control both the amplitude and the phase of this tunneling parameter, experimentally realizing the Peierls substitution for ultracold neutral atoms., Comment: 6 pages, 5 figures

Measurement techniques based upon the Hall effect are invaluable tools in condensed matter physics. When an electric current flows perpendicular to a magnetic field, a Hall voltage develops in the direction transverse to both the current and the field. In semiconductors, this behaviour is routinely used to measure the density and charge of the current carriers (electrons in conduction bands or holes in valence bands) -- internal properties of the system that are not accessible from measurements of the conventional resistance. For strongly interacting electron systems, whose behaviour can be very different from the free electron gas, the Hall effect's sensitivity to internal properties makes it a powerful tool; indeed, the quantum Hall effects are named after the tool by which they are most distinctly measured instead of the physics from which the phenomena originate. Here we report the first observation of a Hall effect in an ultracold gas of neutral atoms, revealed by measuring a Bose-Einstein condensate's transport properties perpendicular to a synthetic magnetic field. Our observations in this vortex-free superfluid are in good agreement with hydrodynamic predictions, demonstrating that the system's global irrotationality influences this superfluid Hall signal., Comment: 5 pages, 4 figures, plus 3 pages supplementary information

Interactions between particles can be strongly altered by their environment. We demonstrate a technique for modifying interactions between ultracold atoms by dressing the bare atomic states with light, creating an effective interaction of vastly increased range that scatters states of finite relative angular momentum at collision energies where only s-wave scattering would normally be expected. We collided two optically dressed neutral atomic Bose-Einstein condensates with equal, and opposite, momenta and observed that the usual s-wave distribution of scattered atoms was altered by the appearance of d- and g-wave contributions. This technique is expected to enable quantum simulation of exotic systems, including those predicted to support Majorana fermions.

We study the order parameter of a quasi-2D gas of ultracold atoms trapped in an optical potential in the presence of controllable disorder. Our results show that disorder drives phase fluctuations without significantly affecting the amplitude of the quasi-condensate order parameter. This is evidence that disorder can drive phase fluctuations in 2D systems, relevant to the phase-fluctuation mechanism for the superconductor-to-insulator phase transition (SIT) in disordered 2D superconductors.

We experimentally investigate the role of localization on the adiabaticity of loading a Bose-Einstein condensate into a one-dimensional optical potential comprised of a shallow primary lattice plus one or two perturbing lattice(s) of incommensurate period. We find that even a very weak perturbation causes dramatic changes in the momentum distribution and makes adiabatic loading of the combined lattice much more difficult than for a single period lattice. We interpret our results using a band structure model and the one-dimensional Gross-Pitaevskii equation., Comment: 4 pages, 3 figures; v2: figures improved (particularly fig 3), some refs. added, clarifications in discussion, fixed typos

This handbook represents a first attempt to provide library practitioners with practical guidance and tools for measuring the quality of library service. Excerpts from a wide range of literature provide a creative approach to understanding the needs of potential users. A variety of actual instruments for measuring the effectiveness of service and for determining community needs are included, along with specific recommendations for action taken from completed studies. (Author/PF)