In quantum information science, superconducting quantum circuits, like qubits, have emerged as a useful platform for information processing. Significant progress has been made in extending the coherence time of these qubits, but further advances are required to achieve scalable quantum computing. Coherence time is often limited by loss from two-level systems and excess quasiparticles that arise at surfaces and interfaces as a result of materials’ defects, fabrication processes, and ambient exposure. Our recent efforts to address these loss sources have utilized a range of strategies, including surface encapsulation, substrate preparation methods, modification of the metal film growth, and new processes during fabrication. However, these methods typically address one surface or interface at a time. Here we examine the interplay of different surfaces and interfaces and the knock-on effects that these approaches have on the superconducting device as a whole. This work is supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract No. DE-AC02-07CH11359.
Microcavity exciton-polaritons based on transition metal dichalcogenide monolayers (TMDs) are a promising platform for coherent valleytronics, exhibiting valley-dependent phenomena at room-temperature. Using polarization-dependent transient reflectance, we demonstrate the valley-exclusive nature of the optical Stark effect in WS2 exciton-polaritons. We observe a simultaneous shift of both polariton branches when pump and probe are co-polarized and no appreciable shift when they are cross-polarized, demonstrating a polarization-selective stark shift in exciton-polaritons. This work highlights how the unique features of TMD exciton-polaritons can give rise to new polaritonic phenomena.
In two-dimensional (2D) materials, such as black phosphorus, the hysteresis attributed to surface and interfacial disorder can severely limit applications in electronics. In this work, we characterize the hysteresis in Al2O3-encapsulated black phosphorous samples by studying conductivity switching transients in response to an applied step gate bias. Using the dispersive diffusion model for relaxation in disordered systems, the so-called bimolecular and unimolecular recombination limits were observed in low-disorder pristine and high-disorder oxidized BP samples, respectively. Two different heavy-tail lineshapes ( the algebraic decay and the stretched exponential relaxation ) were clearly distinguished in the low- and high-disorder limits, respectively. The parameterization of these transients allows temperature dependence of the line-fit parameters to be tracked. If interpreted under the continuous time random walk model, the observed temperature dependence of the dispersion parameter beta would result from a disorder-induced tail of localized trap states.
Molybdenum ditelluride (MoTe2), which can exist in a semiconducting prismatic hexagonal (2H) or a metallic distorted octahedral (1T') phases, is one of the very few materials that exhibit metal-semiconductor transition. Temperature-driven 2H – 1T’ phase transition in bulk MoTe2 occurs at high temperatures (above ~900 C) and it is usually accompanied by Te loss. The latter can exacerbate the control over reversibility of the phase transition.
Here, we study effects of high-temperature annealing on phase transition in MoTe2 single crystals. First, MoTe2 were grown in sealed evacuated quartz ampoules from polycrystalline MoTe2 powder in an iodine-assisted chemical vapor transport process at 1000 C. The 2H and 1T’ phases were stabilized by controlling the cooling rate after the growth. In particular, slow cooling at 10 C/h rate yielded the 2H phase whereas the 1T’ phase was stabilized by ice-water quenching. Next, the phase conversion was achieved by annealing MoTe2 single crystals in vacuum-sealed ampoules at 1000 C with or without additional poly-MoTe2 powder followed by fast or slow cooling. Similarly to the CVT growth, slow cooling and quenching consistently produced 2H and 1T’ phases, respectively, regardless of the initial MoTe2 crystal structure.
We will discuss structural and optical properties of the as-grown and phase-converted MoTe2 single crystals using TEM, SEM/EDS, XRD, XPS and Raman. Electrical characteristics of two-terminal devices made from metallic 1T’ and bottom-gated FETs made from 2H exfoliated crystals will also be presented.
Semiconducting single-walled carbon nanotubes (SWNTs) are one of the most intriguing nanomaterials due to
their large aspect ratios, size tunable properties, and dominant many body interactions. While the dynamics of
exciton population relaxation have been well characterized, optical dephasing processes have only been examined
indirectly through steady-state measurements such as single-molecule spectroscopy that can yield highly
variable estimates of the homogeneous linewidth. To bring clarity to these conflicting estimates, a time-domain
measurement of exciton dephasing at an ensemble level is necessary. Using two-pulse photon echo (2PE) spectroscopy,
comparatively long dephasing times approaching 200 fs are extracted for the (6,5) tube species at room
temperature. In this contribution, we extend our previous study of 2PE and pump-probe spectroscopy to low
temperatures to investigate inelastic exciton-exciton scattering. In contrast to the population kinetics observed
upon excitation of the second transition-allowed excitonic state (E22), our one-color pump-probe data instead
shows faster relaxation upon cooling to 60 K when the lowest transition-allowed state (E11) is directly excited for
the (6,5) tube species. Analysis of the kinetics obtained suggests that the observed acceleration of kinetic decay
at low temperature originates from an increasing rate of exciton-exciton annihilation. In order to directly probe
exciton-exciton scattering processes, femtosecond 2PE signal is measured as a function of excitation fluence and
temperature. Consistent with the observed enhancement of exciton-exciton scattering and annihilation at low
temperatures, the dephasing rates show a correlated trend with the temperature dependence of the population
lifetimes extracted from one-color pump-probe measurements.
Optical absorption and transient photobleaching in solutions of surfactant encapsulated and DNA wrapped single-walled carbon nanotubes (SWNTs) are studied. Optical transitions between van Hove singularities are red shifted in solutions of DNA wrapped SWNTs compared with transitions in solutions of sodium dodecyl sulfate (SDS) encapsulated SWNTs. This red shift may be due to changes in the local surrounding dielectric constant and corresponding changes in charge screening. Transient photobleaching at the E11 transition of semiconducting SWNTs is observed in both solutions of SDS encapsulated SWNTs and DNA wrapped SWNTs in response to optical excitation at corresponding E22 transitions, and the saturation of photobleaching at high excitation intensities greater than 500 W cm-2 is studied. It is found that the photobleaching intensity does not saturate as significantly in solutions of DNA wrapped SWNTs as in solutions of SDS isolated SWNTs. Lastly, using degenerate, delayed pump-probe characterization, the temporal relaxation of excited charge carriers is investigated. Measured decays are characterized by both fast and slow processes. The slow decay time constant across the band gap of semiconducting SWNTs is fit to 120 ps for SDS encapsulated SWNTs and 73 ps for DNA wrapped SWNTs.
Recently it has been shown that aqueous solutions of sodium dodecyl sulfate (SDS) encapsulated and polymer wrapped single-walled carbon nanotubes (SWNTs) fluoresce in the near infrared (NIR) in the regime of the E11 van Hove transitions for semiconducting SWNTs. For bundled SWNTs, fluorescence is observed to be quenched along with a shift and broadening of the absorbance spectrum. Here, we study two other commercially available surfactants, BRIJ-97 and Triton-X-100, by analysis of carbon nanotube fluorescence and absorptivity in the NIR. It is found that changing the surfactant alters the corresponding optical properties of the solubilized carbon nanotubes. The NIR absorbance spectra of BRIJ-97 and Triton-X-100 carbon nanotube solutions are also compared with the absorbance spectrum of NaCl destabilized SDS-SWNT solutions. By controlling the amount of NaCl added to an aqueous solution of SDS-SWNTs, the optical absorbance spectrum can be made to match that of BRIJ-97 and Triton-X-100 solutions. Lastly, a correlation is drawn between the amount of shift in the absorbance spectrum and the fluorescence intensity, independent of surfactant used. This shift and decrease in fluorescence intensity may be due to carbon nanotube bundling.
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