587 research outputs found
Approaching the adiabatic timescale with machine-learning
The control and manipulation of quantum systems without excitation is
challenging, due to the complexities in fully modeling such systems accurately
and the difficulties in controlling these inherently fragile systems
experimentally. For example, while protocols to decompress Bose-Einstein
condensates (BEC) faster than the adiabatic timescale (without excitation or
loss) have been well developed theoretically, experimental implementations of
these protocols have yet to reach speeds faster than the adiabatic timescale.
In this work, we experimentally demonstrate an alternative approach based on a
machine learning algorithm which makes progress towards this goal. The
algorithm is given control of the coupled decompression and transport of a
metastable helium condensate, with its performance determined after each
experimental iteration by measuring the excitations of the resultant BEC. After
each iteration the algorithm adjusts its internal model of the system to create
an improved control output for the next iteration. Given sufficient control
over the decompression, the algorithm converges to a novel solution that sets
the current speed record in relation to the adiabatic timescale, beating out
other experimental realizations based on theoretical approaches. This method
presents a feasible approach for implementing fast state preparations or
transformations in other quantum systems, without requiring a solution to a
theoretical model of the system. Implications for fundamental physics and
cooling are discussed.Comment: 7 pages main text, 2 pages supporting informatio
Production of a highly degenerate Fermi gas of metastable helium-3 atoms
We report on the achievement of quantum degeneracy in both components of a
Bose-Fermi mixture of metastable helium atoms, He* and He*. Degeneracy
is achieved via Doppler cooling and forced evaporation for He*, and
sympathetically cooling He* with He*. We discuss our simplified
implementation, along with the high versatility of our system. This technique
is able to produce a degenerate Fermi gas with a minimum reduced temperature of
, consisting of He* atoms. Due to the high
internal energy of both isotopes single atom detection is possible, opening the
possibility of a large number of experiments into Bose-Fermi mixtures.Comment: 13 pages, 8 figure
Multiwavelength Transit Observations of the Candidate Disintegrating Planetesimals Orbiting WD 1145+017
We present multiwavelength, multi-telescope, ground-based follow-up
photometry of the white dwarf WD 1145+017, that has recently been suggested to
be orbited by up to six or more, short-period, low-mass, disintegrating
planetesimals. We detect 9 significant dips in flux of between 10% and 30% of
the stellar flux from our ground-based photometry. We observe transits deeper
than 10% on average every ~3.6 hr in our photometry. This suggests that WD
1145+017 is indeed being orbited by multiple, short-period objects. Through
fits to the multiple asymmetric transits that we observe, we confirm that the
transit egress timescale is usually longer than the ingress timescale, and that
the transit duration is longer than expected for a solid body at these short
periods, all suggesting that these objects have cometary tails streaming behind
them. The precise orbital periods of the planetesimals in this system are
unclear from the transit-times, but at least one object, and likely more, have
orbital periods of ~4.5 hours. We are otherwise unable to confirm the specific
periods that have been reported, bringing into question the long-term stability
of these periods. Our high precision photometry also displays low amplitude
variations suggesting that dusty material is consistently passing in front of
the white dwarf, either from discarded material from these disintegrating
planetesimals or from the detected dusty debris disk. For the significant
transits we observe, we compare the transit depths in the V- and R-bands of our
multiwavelength photometry, and find no significant difference; therefore, for
likely compositions the radius of single-size particles in the cometary tails
streaming behind the planetesimals in this system must be ~0.15 microns or
larger, or ~0.06 microns or smaller, with 2-sigma confidence.Comment: 16 pages, 12 figures, submitted to ApJ on October 8th, 201
CZTSe solar cells prepared by co-evaporation of multilayer Cu–Sn/Cu,Zn,Sn,Se/ZnSe/Cu,Zn,Sn,Se stacks
Upscaling of perovskite solar modules: The synergy of fully evaporated layer fabrication and all‐laser‐scribed interconnections
Given the outstanding progress in research over the past decade, perovskite photovoltaics (PV) is about to step up from laboratory prototypes to commercial products. For this to happen, realizing scalable processes to allow the technology to transition from solar cells to modules is pivotal. This work presents all-evaporated perovskite PV modules with all thin films coated by established vacuum deposition processes. A common 532-nm nanosecond laser source is employed to realize all three interconnection lines of the solar modules. The resulting module interconnections exhibit low series resistance and a small total lateral extension down to 160 μm. In comparison with interconnection fabrication approaches utilizing multiple scribing tools, the process complexity is reduced while the obtained geometrical fill factor of 96% is comparable with established inorganic thin-film PV technologies. The all-evaporated perovskite minimodules demonstrate power conversion efficiencies of 18.0% and 16.6% on aperture areas of 4 and 51 cm, respectively. Most importantly, the all-evaporated minimodules exhibit only minimal upscaling losses as low as 3.1% per decade of upscaled area, at the same time being the most efficient perovskite PV minimodules based on an all-evaporated layer stack sequence
Fine‐Tuning the Photophysics of Donor‐Acceptor (D‐A 3 ) Thermally Activated Delayed Fluorescence Emitters Using Isomerisation
Here two D–A3 regioisomers, comprising three dibenzothiophene-S,S-dioxide acceptor units attached to a central triazatruxene core, are studied. Both molecules show thermally activated delayed fluorescence (TADF), however, the efficiency of the TADF mechanism is strongly affected by the D–A substitution position. The meta- substituted emitter (1 b) shows a slightly higher-lying singlet charge transfer state and a lower-lying triplet state than that observed in the para- substituted emitter (1 a), resulting in a larger singlet–triplet splitting (ΔEST) of 0.28 eV compared to only 0.01 eV found in 1 a. As expected, this ΔEST difference strongly impacts the reverse intersystem crossing (rISC) rates and the para- isomer 1 a exhibits a much faster delayed fluorescence emission. Calculations show that the triplet energy difference between the two isomers is due to steric hindrance variances along the donor–acceptor rotation axis in these molecules: as 1 b is less restricted, rotation of its acceptor unit leads to a lower T1 energy, further away from the region of high density of states (the region where larger vibronic coupling is found, favouring rISC). Therefore, our results show how the substitution pattern has a marked effect on triplet state energies and character, verifying the key structural designs for highly efficient TADF materials
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Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates
Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.Chemistry and Chemical Biolog
An up-conversion luminophore with high quantum yield and brightness based on BaF:Yb,Er single crystals
Up-conversion (UC) of near-infrared radiation to visible light has received much attention because of its use in the conversion of solar radiation, luminescence thermometry, biosensing, and anti-counterfeiting applications. However, the main issue hindering the successful utilization of UC is the relatively low quantum efficiency of the process. In order to design new UC systems with high quantum yield (ϕ) values, we synthesized two series of co-doped BaF single crystals with nominal concentrations of Yb (2–15 mol%)/Er (2 mol%) as well as Yb (3 mol%)/Er (2–15 mol%). The highest ϕ value of 10.0% was demonstrated for the BaF:Er (2 mol%) and Yb (3 mol%) sample under 490 W cm of 976 nm excitation. To study the natural limit of UC efficiency, quantum yield values upon direct excitation (ϕ) of the S (ϕ ≤ 26%) levels were measured. Comparison of experimental values of quantum yields to the ones obtained using Judd–Ofelt theory reveals strong quenching of the S state for all investigated compositions. In addition, we observed an unusually strong contribution of the Er:4I excited state to both UC and down-shifting luminescent processes. This contribution becomes possible due to the very low maximum phonon energy of BaF crystals (240 cm)
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