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Impact ionization across the conduction-band-edge discontinuity of quantum-well heterostructures
Impact ionization across the band-edge discontinuity of quantum-well heterostructures is studied theoretically. A heterolayer structure of alternating Al(x)Ga(1-x)As and GaAs layers is considered where the GaAs layers are heavily doped with donors. Thus a large number of electrons is confined to the quantum-well region. Incident electrons are heated up by applied electric fields and collide with the electrons confined in the well regions. Both the ionization rate as a function of the incident energy, and average ionization rates are computed. Device applications of such multiple quantum-well structures and the possibility of a complete analog to the conventional photomultiplier are discussed.
Nanosecond Structure of Radical Pair Intermediates from High-Frequency Quantum Oscillations: Insight into the Q A •– to Q B Electron Transfer Step in Purple Bacterial Photosynthesis
We demonstrate the validity of our approach to deduce, from the anisotropy of quantum oscillations, the geometry of short-lived radical pair intermediates in photosynthesis. A global fit of a two-dimensional W-band (94 GHz) electron paramagnetic resonance (EPR) experiment provides the same global minimum values for the geometry of the A-side radical pair P 700 •+ A 1A •− in photosystem I (PSI) as observed in a previous Q-band (34 GHz) EPR study, yet with a significantly increased convergence rate of 62%. This demonstrates that the global fit yields the correct radical pair geometry even at Q-band frequencies. With this information, we revisit our previous Q-band study of the cofactor arrangement of P 865 •+ Q A •− , the stabilized charge-separated state in purple bacterial reaction centers (RCs). Analysis of calculated two-dimensional data sets of P 865 •+ Q A •− reveals that the quantum oscillation technique is unaffected by a mirror ambiguity in disordered solids and thus can provide unambiguous solutions for all five Euler angles of the radical pair geometry. This enables us to elucidate the Q A •− to Q B electron transfer step in purple bacterial photosynthesis, the subject of controversial discussions for more than 25 years. Our results show that this electron transfer step involves a gating mechanism requiring a 60° rotation of the headgroup of Q A •− in its binding pocket.
Quantum-inspired genetic algorithm for designing planar multilayer photonic structure
Not Available
Micrometre-scale single-crystalline borophene on a square-lattice Cu(100) surface
Borophene, a crystalline monolayer boron sheet, is a new two-dimensional (2D) quantum material, predicted to feature tunable structure, intriguing physics and to find applications in flexible electronics, energy storage and catalysis. Nanoscale borophene flakes have been synthesized on noble-metal surfaces, but for device fabrication, one needs large single-crystal domains. In this work, we report the synthesis of borophene on a square lattice Cu(100) surface and show that incommensurate coordinations could reduce the borophene-substrate interactions and alter the borophene structures in interesting ways. Micrometer-scale single-crystal domains can form as isolated faceted islands or merge together to achieve full monolayer coverage. We have discovered a new crystal structure of borophene, with ten boron atoms and two hexagonal vacancies in the unit cell. First-principle calculations indicate that charge transfer rather than covalent bonding binds 2D boron to the copper surface, and confirm its integrity and uniformity The electronic band structure features multiple anisotropic tilted Dirac cones, heralding emergent quantum fermions.
Structure, strain, and composition profiling of InAs/GaAs(211)B quantum dot superlattices
The morphology, nanostructure, and strain properties of InAs quantum dots (QDs) grown on GaAs(211)B, uncapped or buried, are explored by transmission electron microscopy and related quantitative techniques. Besides the built-in piezoelectric field, other differences of (211) growth compared to (100)-oriented growth are discussed in terms of the (211) surface non-singularity, leading to anisotropic shape of the QDs and local chemical inhomogeneity of the wetting layer. The shape of the uncapped QDs was precisely defined as truncated pyramidal, elongated along the 〈111〉 direction, and bounded by the (110), (100), and (213) facets. Local strain measurements showed that large surface QDs were almost unstrained due to plastic relaxation, exhibiting small residual elastic strain at the interface that gradually diminished toward their apex. Conversely, buried QDs were pseudomorphically grown on GaAs. By postulating a plane stress state, we have established a systematic increase of the local strain from the base toward the apex region of the QDs. Using Vegard's law, their chemical composition profiles were calculated, revealing an indium content gradient along the growth direction and compositional variants among different QDs. Photoluminescence measurements showed variations in emission energy between the QDs and consistency with a graded In-content, which complied with the quantitative strain analysis.
Growth and structure of In{sub 0.5}Ga{sub 0.5}Sb quantum dots on GaP(001)
Stranski-Krastanov (SK) growth of In{sub 0.5}Ga{sub 0.5}Sb quantum dots (QDs) on GaP(001) by metalorganic vapor phase epitaxy is demonstrated. A thin GaAs interlayer prior to QD deposition enables QD nucleation. The impact of a short Sb-flush before supplying InGaSb is investigated. QD growth gets partially suppressed for GaAs interlayer thicknesses below 6 monolayers. QD densities vary from 5 × 10{sup 9} to 2 × 10{sup 11} cm{sup −2} depending on material deposition and Sb-flush time. When In{sub 0.5}Ga{sub 0.5}Sb growth is carried out without Sb-flush, the QD density is generally decreased, and up to 60% larger QDs are obtained.
Error-mitigated nonorthogonal quantum eigensolver via shadow tomography
We present a shadow-tomography-enhanced nonorthogonal quantum eigensolver (NOQE) for more efficient and accurate electronic structure calculations on near-term quantum devices. By integrating shadow tomography into the NOQE, the measurement cost scales linearly rather than quadratically with the number of reference states, while also reducing the required qubits and circuit depth by half. This approach enables extraction of all matrix elements via randomized measurements and classical postprocessing. We analyze its sample complexity and show that, for small systems, it remains constant in the high-precision regime, while for larger systems, it scales linearly with the system size. We further apply shadow-based error mitigation to suppress noise-induced bias without increasing quantum resources. Demonstrations on the hydrogen molecule in the strongly correlated regime achieve chemical accuracy under realistic noise, showing that our method is both resource-efficient and noise-resilient for practical quantum chemistry simulations in the near term.
QM/MM Simulation (B3LYP) of the RNase A Cleavage-Transesterification Reaction Supports a Triester AN+DN Associative Mechanism with an O2´ H Internal Proton Transfer
The mechanism of the backbone cleavage transesterification step of the RNase A enzyme remains controversial even after 60 years of study. We report QM/MM free energy calculations for two optimized reaction paths based on an analysis of all structural data and identified by a search for reaction coordinates using a reliable quantum chemistry method (B3LYP), equilibrated structural optimizations, and free energy estimations. Both paths are initiated by nucleophilic attack of the ribose O2? oxygen on the neighboring diester phosphate bond and both reach the same product state (PS) (a O3??O2? cyclic phosphate and a O5? hydroxyl terminated fragment). Path 1, resembles the widely accepted dianionic transition state (TS) general acid (His 119)/base (His 12) classical mechanism. However, this path has a barrier (25 kcal/mol) higher than that of the rate limiting hydrolysis step and a very loose TS. In Path 2, the proton initially coordinating the O2? migrates to the non-bridging O1P in the initial reaction path rather than directly to the general base resulting in a triester (substrate as base) AN+DN mechanism with a monoanionic weakly stable intermediate. The structures in the transition region are associative with low barriers (TS1 10, TS2 7.5 kcal/mol). The Path 2 mechanism is consistent with the many results from enzyme and buffer catalyzed and uncatalyzed analog reactions and leads to a PS consistent with the reactive state for the following hydrolysis step. The differences between the consistently estimated barriers in Path 1 and 2 lead to a 1011 difference in rate strongly supporting the less accepted triester mechanism.
Site-controlled InGaN/GaN single-photon-emitting diode
We report single-photon emission from electrically driven site-controlled InGaN/GaN quantum dots. The device is fabricated from a planar light-emitting diode structure containing a single InGaN quantum well, using a top-down approach. The location, dimension, and height of each single-photon-emitting diode are controlled lithographically, providing great flexibility for chip-scale integration.
Magneto-optical response of InAs lens-shaped self-assembled quantum dots
In this work, we demonstrate a realistic modeling of the electronic structure for InAs self-assembled quantum dots and investigate the magneto-optical response, i.e., Zeeman splitting and transition rates between electron and hole levels.
Engineering Small HOMO–LUMO Gaps in Polycyclic Aromatic Hydrocarbons with Topologically Protected States
Topological phases in laterally confined low-dimensional nanographenes have emerged as versatile design tools that can imbue otherwise unremarkable materials with exotic band structures ranging from topological semiconductors and quantum dots to intrinsically metallic bands. The periodic boundary conditions that define the topology of a given lattice have thus far prevented the translation of this technology to the quasi-zero-dimensional (0D) domain of small molecular structures. Here, we describe the synthesis of a polycyclic aromatic hydrocarbon (PAH) featuring two localized zero modes (ZMs) formed by the topological junction interface between a trivial and nontrivial phase within a single molecule. First-principles density functional theory calculations predict a strong hybridization between adjacent ZMs that gives rise to an exceptionally small HOMO–LUMO gap. Scanning tunneling microscopy and spectroscopy corroborate the molecular structure of 9/7/9-double quantum dots and reveal an experimental quasiparticle gap of 0.16 eV, corresponding to a carbon-based small molecule long-wavelength infrared (LWIR) absorber.
Development of One and Two Current Transition Amplitudes with Two Body Final States
One of the biggest challenges facing modern day nuclear physics is understanding the structure of states within the hadronic spectrum of Quantum Chromodynamics (QCD). We have shown this spectrum to be quite rich with hundreds of allowed states; however, if we wish to better grasp the dynamics of QCD, then it is necessary that we start investigating the internal structure of these states. This may be done by probing these states with external currents, but this requires a description of the intersection of QCD with the electroweak sector. This work acts as a stepping stone in this direction. Working to all orders in the strong interaction and to leading order in electroweak effects we present novel formalisms for transition amplitudes with one and two currents with two hadrons in the final state, namely the 2 + J ? 2 and 1 + J ? 2 + J amplitudes. Our results are valid for spinless hadrons with any number of open channels between two- and three-particle threshold, place no restrictions on the Lorentz structure or quantum numbers of the current, and are independent of any underlying model. We also show how this formalism may be used to rigorously access elastic form factors of resonances which has proven challenging to do experimentally. Finally, we present a toy calculation of elastic resonance form factors using the 2 + J ? 2 formalism in which we stress the importance of treating resonances rigorously as poles in the complex plane.
Strong coupling and polariton lasing in Te based microcavities embedding (Cd,Zn)Te quantum wells
We report on properties of an optical microcavity based on (Cd,Zn,Mg)Te layers and embedding (Cd,Zn)Te quantum wells. The key point of the structure design is the lattice matching of the whole structure to MgTe, which eliminates the internal strain and allows one to embed an arbitrary number of unstrained quantum wells in the microcavity. We evidence the strong light-matter coupling regime already for the structure containing a single quantum well. Embedding four unstrained quantum wells results in further enhancement of the exciton-photon coupling and the polariton lasing in the strong coupling regime.
Some implications of signature-change in cosmological models of loop quantum gravity
Signature change at high density has been obtained as a possible consequence of deformed space-time structures in models of loop quantum gravity. This article provides a conceptual discussion of implications for cosmological scenarios, based on an application of mathematical results for mixed-type partial differential equations (the Tricomi problem). While the effective equations from which signature change has been derived are shown to be locally regular and therefore reliable, the underlying theory of loop quantum gravity may face several global problems in its semiclassical solutions.
Origin of the Q{sup 2}-dependence of the DIS structure functions
We consider in detail Q{sup 2}-dependence of the DIS structure functions. Very often this dependence is claimed to be originated by the Q{sup 2}-dependence of the QCD coupling. This leads to the small-x asymptotics of the structure functions with Q{sup 2}-dependent intercepts. We demonstrate that the DGLAP parametrization α{sub s} = α{sub s}(Q{sup 2}) is an approximation valid in the region of large x (where 2pq can be approximated by Q{sup 2}) only, providing the factorization scale is also large. Outside this region, the DGLAP parametrization fails, so α{sub s} should be replaced by an effective coupling which is independent of Q{sup 2} at small x. As a consequence, intercepts of the structure functions are independent of Q{sup 2}. Nevertheless, the small-x asymptotics of the structure functions explicitly depend on Q{sup 2}, even when the coupling does not depend on it. We also consider the structure functions at small Q{sup 2} and give a comment on power-Q{sup 2} corrections to the structure functions at large and small Q{sup 2}.
Photoconductivity of structures based on the SnO{sub 2} porous matrix coupled with core-shell CdSe/CdS quantum dots
Embedding of quantum dots into porous oxide matrixes is a perspective technique for photosensitization of a structure. We show that the sensitization efficiency may be increased by the use of core-shell quantum dots. It is demonstrated that the photoresponse amplitude in a SnO{sub 2} porous matrix with CdSe/CdS quantum dots depends non-monotonously on the number of atomic layers in a shell. The best results are obtained for SnO{sub 2} matrixes coupled with the quantum dots with three atomic layers of a shell. Mechanisms responsible for the structure sensitization are discussed.
Strain-graded quantum dots with spectrally pure, stable and polarized emission
Structural deformation modifies the bandgap, exciton fine structure and phonon energy of semiconductors, providing an additional knob to control their optical properties. The impact can be exploited in colloidal semiconductor quantum dots (QDs), wherein structural stresses can be imposed in three dimensions while defect formation is suppressed by controlling surface growth kinetics. Yet, the control over the structural deformation of QDs free from optically active defects has not been reached. Here, we demonstrate strain-graded CdSe-ZnSe core-shell QDs with compositionally abrupt interface by the coherent pseudomorphic heteroepitaxy. Resulting QDs tolerate mutual elastic deformation of varying magnitudes at the interface with high structural fidelity, allowing for spectrally stable and pure emission of photons at accelerated rates with near unity luminescence efficiency. We capitalize on the asymmetric strain effect together with the quantum confinement effect to expand emission envelope of QDs spanning the entire visible region and exemplify their use in photonic applications.