17 research outputs found
Superglass formation in an atomic BEC with competing long-range interactions
The complex dynamical phases of quantum systems are dictated by atomic
interactions that usually evoke an emergent periodic order. Here, we study a
quantum many-body system with two competing and substantially different
long-range interaction potentials where the dynamical instability towards
density order can give way to a superglass phase, i. e., a superfluid
disordered amorphous solid, which exhibits local density modulations but no
long-range periodic order. We consider a two-dimensional BEC in the
Rydberg-dressing regime coupled to an optical standing wave resonator. The
dynamic pattern formation in this system is governed by the competition between
the two involved interaction potentials: repulsive soft-core interactions
arising due to Rydberg dressing and infinite-range sign changing interactions
induced by the cavity photons. The superglass phase is found when the two
interaction potentials introduce incommensurate length scales. The dynamic
formation of this peculiar phase without any externally added disorder is
driven by quantum fluctuations and can be attributed to frustration induced by
the two competing interaction energies and length scales.Comment: new title, added reference
Characterizing superradiant dynamics in atomic arrays via a cumulant expansion approach
Ordered atomic arrays with subwavelength lattice spacing emit light
collectively. For fully inverted atomic arrays, this results in an initial
burst of radiation and a fast build up of coherences between the atoms at
initial times. Based on a cumulant expansion of the equations of motion, we
derive exact analytical expressions for the emission properties and numerically
analyze the full many-body problem resulting in the collective decay process
for unprecedented system sizes of up to a few hundred atoms. We benchmark the
cumulant expansion approach and show that it correctly captures the cooperative
dynamics resulting in superradiance. For fully inverted arrays, this allows us
to extract the scaling of the superradiant peak with particle number. For
partially excited arrays where no coherences are shared among atoms, we also
determine the critical number of excitations required for the emergence of
superradiance in one- and two-dimensional geometries. In addition, we study the
robustness of superradiance in the case of non-unit filling and position
disorder.Comment: 13 pages, 7 figure
Modifying cooperative decay via disorder in atom arrays
Atomic arrays can exhibit collective light emission when the transition
wavelength exceeds their lattice spacing. Subradiant states take advantage of
this phenomenon to drastically reduce their overall decay rate, allowing for
long-lived states in dissipative open systems. We build on previous work to
investigate whether or not disorder can further decrease the decay rate of a
singly-excited atomic array. More specifically, we consider spatial disorder of
varying strengths in a 1D half waveguide and in 1D, 2D, and 3D atomic arrays in
free space and analyze the effect on the most subradiant modes. While we
confirm that the dilute half waveguide exhibits an analog of Anderson
localization, the dense half waveguide and free space systems can be understood
through the creation of close-packed, few-body subradiant states similar to
those found in the Dicke limit. In general, we find that disorder provides
little advantage in generating darker subradiant states in free space on
average and will often accelerate decay. However, one could potentially change
interatomic spacing within the array to engineer specific subradiant states.Comment: 13 pages, 12 figure
Harnessing quantum emitter rings for efficient energy transport and trapping
Efficient transport and harvesting of excitation energy under low light
conditions is an important process in nature and quantum technologies alike.
Here we formulate a quantum optics perspective to excitation energy transport
in configurations of two-level quantum emitters with a particular emphasis on
efficiency and robustness against disorder. We study a periodic geometry of
emitter rings with subwavelength spacing, where collective electronic states
emerge due to near-field dipole-dipole interactions. The system gives rise to
collective subradiant states that are particularly suited to excitation
transport and are protected from energy disorder and radiative decoherence.
Comparing ring geometries with other configurations shows that that the former
are more efficient in absorbing, transporting, and trapping incident light.
Because our findings are agnostic as to the specific choice of quantum
emitters, they indicate general design principles for quantum technologies with
superior photon transport properties and may elucidate potential mechanisms
resulting in the highly efficient energy transport efficiencies in natural
light-harvesting systems.Comment: 12 pages, 6 figure
A tissue-specific landscape of sense/antisense transcription in the mouse intestine
<p>Abstract</p> <p>Background</p> <p>The intestinal mucosa is characterized by complex metabolic and immunological processes driven highly dynamic gene expression programs. With the advent of next generation sequencing and its utilization for the analysis of the RNA sequence space, the level of detail on the global architecture of the transcriptome reached a new order of magnitude compared to microarrays.</p> <p>Results</p> <p>We report the ultra-deep characterization of the polyadenylated transcriptome in two closely related, yet distinct regions of the mouse intestinal tract (small intestine and colon). We assessed tissue-specific transcriptomal architecture and the presence of novel transcriptionally active regions (nTARs). In the first step, signatures of 20,541 NCBI RefSeq transcripts could be identified in the intestine (74.1% of annotated genes), thereof 16,742 are common in both tissues. Although the majority of reads could be linked to annotated genes, 27,543 nTARs not consistent with current gene annotations in RefSeq or ENSEMBL were identified. By use of a second independent strand-specific RNA-Seq protocol, 20,966 of these nTARs were confirmed, most of them in vicinity of known genes. We further categorized our findings by their relative adjacency to described exonic elements and investigated regional differences of novel transcribed elements in small intestine and colon.</p> <p>Conclusions</p> <p>The current study demonstrates the complexity of an archetypal mammalian intestinal mRNA transcriptome in high resolution and identifies novel transcriptionally active regions at strand-specific, single base resolution. Our analysis for the first time shows a strand-specific comparative picture of nTARs in two tissues and represents a resource for further investigating the transcriptional processes that contribute to tissue identity.</p
Low‐Coordinated Iron(II) Siloxide Complexes – Structural Diversity and Reactivity Towards O2 and Oxygen Atom Transfer Reagents
The coordination chemistry of Fe2+ ions in combination with a monodentate siloxide ligand Ph3SiO− (L) was investigated. Using a Fe/L stoichiometry of 1 : 3 the complex [Na(DME)][FeL3], 2, with the iron center in a trigonal ligand environment was isolated and through decreasing the siloxide amount fraction 2 was shown to form via a unique example of a dinuclear complex, where one of the iron ions has a quasi-trigonal and the other one a tetrahedral coordination sphere, namely [Na(DME)][Fe2L5], 1. If, however, 4 equivalents of L are employed, the tetrasiloxido ferrate(II) anion with a tetrahedral structure is generated, so that the product [Na(DME)]2[FeL4], 3, can be isolated. 2 reacts instantly with O-atom transfer reagents, also at low temperatures, but no reaction intermediate could be identified. From the product mixture the iron(III) siloxide complex [Na(DME)3][FeL4], 4, could isolated by crystallization as the main product. Likewise, the reaction with dioxygen proceeded rather fast and added substrates did not intercept any intermediate upon its formation. However, in the presence of cyclohexene oxidation products were observed. They correspond to the typical radical-chain-derived products of cyclohexene suggesting, that initially a reactive FeOx species is generated that via an H atom abstraction from cyclohexene triggers its autoxidation.Peer Reviewe
Effect of photon propagation on a zero refractive index medium
We present a model describing the transmission of light through atomic media with a vanishing index of refraction. Zero index materials are of particular interest as the infinite phase velocity of light within the material offers the potential to manipulate electromagnetic waves to mediate dipole-dipole interactions over extended distances. We focus on the preparation of zero-index conditions based on atomic coherence using two distinct atomic media as exemplary of generic zero-index materials. We establish a model based on the Maxwell-Bloch equations to describe the propagation of a light pulse through these media. To investigate the sustainability of the zero index under minimal light conditions, we assume single-photon intensity of the propagating pulse. Specifically, we examine whether the spatial phase change of the photon remains zero as it traverses the medium. We employ a finite-element numerical approach to solve the coupled Maxwell-Bloch equations describing the photon propagation. Our results indicate that the presence of a photon within the medium will disrupt the zero-index state, thus disallowing the establishment of enhanced dipole-dipole interactions over large distances
High-Surface-Area Porous Platinum Electrodes for Enhanced Charge Transfer
Cobalt-based electrolytes are highly tunable and have pushed the limits of dye-sensitized solar cells, enabling higher open-circuit voltages and new record efficiencies. However, the performance of these electrolytes and a range of other electrolytes suffer from slow electron transfer at platinum counter electrodes. High surface area platinum would enhance catalysis, but pure platinum structures are too expensive in practice. Here, a material-efficient host-guest architecture is developed that uses an ultrathin layer of platinum deposited upon an electrically conductive scaffold, niobium-doped tin oxide (NTO). This nanostructured composite enhances the counter electrode performance of dye-sensitized solar cells (DSCs) using a (CoBPY3)-B-(II/III) electrolyte with an increased fill factor and power conversion efficiency (11.26%), compared to analogous flat films. The modular strategy is elaborated by integrating a light scattering layer onto the counter electrode to reflect unabsorbed light back to the photoanode to improve the short-circuit current density and power conversion efficiency
Optimized geometries for cooperative photon storage in an impurity coupled to a two-dimensional atomic array
The collective modes of two-dimensional ordered atomic arrays can modify the
radiative environment of embedded atomic impurities. We analyze the role of the
lattice geometry on the impurity's emission linewidth by comparing the
effective impurity decay rate obtained for all non-centered Bravais lattices
and an additional honeycomb lattice. We demonstrate that the lattice geometry
plays a crucial role in determining the effective decay rate for the impurity.
In particular, we find that the minimal effective decay rate appears in
lattices where the number of the impurity's nearest neighbours is maximal and
the number of distinct distances among nearest neighbours is minimal. We
further show that, in the choice between interstitial and substitutional
placement of the impurity, the former always wins by exhibiting a lower decay
rate and longer photon storage. For interstitial placements, we determine the
optimal impurity position in the lattice plane, which is not necessarily found
in the center of the lattice plaquette.Comment: 9 pages, 6 figure
High‐spin square planar iron(II) alkali metal siloxide complexes – influence of the alkali metal and reactivity towards O 2
Complexes [L2Fe][Li(DME)]2, 1(DME), {[L2Fe][Na2(DME)3]}∞, 2(DME) and [L2Fe][K(DME)2]2, 3(DME) were synthesized by deprotonation of LH2 (LH2=O(SiPh2OH)2) with the respective alkali metal tert-butoxides followed by recrystallization from DME. It turned out that upon crossing over from Li+ via Na+ to K+ counterions the structures of the high-spin iron(II) complexes are increasingly distorted from a square planar towards a tetrahedral structure so that 3(DME) represents a borderline case, as indicated by the τ-values. The distortions are also reflected in the Mössbauer spectra through the quadrupole splittings. The compounds behave inert in contact with O atom transfer reagents but react rapidly with dioxygen. The reaction rates are too high to be determined even by stopped-flow measurements quantitatively, but qualitatively it emerged that the rates increase from Li to Na to K. Using NO as an O2 surrogate an NO adduct with an S = 3/2 ground state was isolated where NO is coordinated in an end-on binding mode, formally as a NO− ligand, with a significantly weakened NO bond.Peer Reviewe