Large Energy Variation Research Articles

Uncovering the mechanisms of efficient upconversion in two-dimensional perovskites with anti-Stokes shift up to 220 meV.

Phonon-assisted photon upconversion holds great potential for numerous applications, e.g., optical refrigeration. However, traditional semiconductors face energy gain limitations due to thermal energy, typically achieving only ~25 milli-electron volts at room temperature. Here, we demonstrate that quasi-two-dimensional perovskites, with a soft hybrid organic-inorganic lattice, can efficiently upconvert photons with an anti-Stokes shift exceeding 200 milli-electron volts. By using microscopic transient absorption measurements and density functional theory calculations, we explicate that the giant energy gain stems from strong lattice fluctuation leading to a picosecond timescale transient band energy renormalization with a large energy variation of around ±180 milli-electron volts at room temperature. The motion of organic molecules drives the deformation of inorganic framework, providing energy and local states necessary for efficient upconversion within a time constant of around 1 ps. These results establish a deep understanding of perovskite-based photon upconversion and offer previously unknown insights into the development of various upconversion applications.

Ideal quadratic fermionic point state with multiple band degeneracy

As the study of topological states witnessed rapid progress and fast development, the current research has been expanded from conventional linear dispersion into high order conditions. In comparison with the linear type, high order topological elements feature multiple exotic properties, such as large topological charge, peculiar Berry phase, and chiral surface states, which could lead to the finding of other new physics. In this study, we present a realistic material candidate hosting an ideal quadratic fermionic point state with multiple band degeneracy. Based on first principles calculation and effective model argument, the origin of the quadratic order is analyzed, and its dispersion conditions are examined. Due to the clean band structure, prominent surface states are observed and they exhibit both large energy variation and broad spatial distribution. These beneficial features are very advantageous for future experimental investigations. This work can significantly enhance the research on high order topological states and, in particular, the proposed material can provide an effective platform for quadratic fermionic point states.

An ideal Weyl nodal ring with a large drumhead surface state in the orthorhombic compound TiS2.

Topological metals or semimetals have attracted great research attention and interest in condensed matter physics and chemistry due to their exotic properties. Different from the conventional topological insulators, topological metals or semimetals are characterized by distinct topological surface states, such as a Fermi arc or a drumhead surface state, which are often used in experiments to verify the corresponding topological properties. However, the current study in this field is strongly limited in the experimental characterization because of the extreme lack of perfect material candidates with a clean band structure and clear surface states. In this work, based on theoretical calculations, we propose a new topological semimetal TiS2, which has an orthorhombic structure and exhibits excellent stability. Calculated electronic band structures reveal that there is a single Weyl nodal ring in the ky = 0 plane. A detailed symmetry analysis is provided and the corresponding surface state is calculated, which exhibits both a large energy variation of 1.5 eV and wide space distribution without and with the spin orbit coupling effect. Besides, the surface states are well separated from the bulk state. These ideal features together make TiS2 a promising nodal line semimetal for experimental investigation. In combination with the other two isostructural compounds TiSe2 and TiTe2 with similar properties, their further experimental synthesis and characterization can be highly expected and the corresponding study for the topological nodal line state can thus be greatly facilitated.

Research on Improved Algorithm of Frame Detection for COFDM High-Definition Wireless Transmission System and FPGA Implementation

COFDM is one of the more advanced modulation technologies. COFDM wireless image transmission technology is adopted on high-speed mobile UAVs, which can meet the requirements of long-range aerial photography and monitoring. Frame detection is the first step in receiving data. The accuracy of the detection data directly affects the reliability and robustness of the reception performance. Through the research of traditional energy detection algorithm, a delay correlation algorithm based on 802.11a frame structure is adopted, which overcomes the shortcomings of large energy variation and rapid decrease of threshold under low SNR. It also achieves good frame detection performance and can be implemented on an FPGA. Combined with the simulation results of ISE, the frame detection algorithm is suitable for COFDM high-definition wireless image transmission system.

Unexpectedly large energy variations from dopant interactions in ferroelectric HfO2 from high-throughput ab initio calculations

Insight into the origin of process-related properties like small-scale inhomogeneities is key for material optimization. Here, we analyze DFT calculations of randomly doped HfO2 structures with Si, La, and VO and relate them to the kind of production process. Total energies of the relevant ferroelectric Pbc21 phase are compared with the competing crystallographic phases under the influence of the arising local inhomogeneities in a coarse-grained approach. The interaction among dopants adds to the statistical effect from the random positioning of the dopants. In anneals after atomic layer or chemical solution deposition processes, which are short compared to ceramic process tempering, the large energy variations remain because the dopants do not diffuse. Since the energy difference is the criterion for the phase stability, the large variation suggests the possibility of nanoregions and diffuse phase transitions because these local doping effects may move the system over the paraelectric-ferroelectric phase boundary.

The sound of avalanches: from a global to a local perspective.

We present an experimental study aiming to understand the local origin of the acoustic bursts recorded globally during a two-dimensional shear experiment of a granular medium. In order to explain the power-law distribution of energy of these events (also called avalanches), the analysis focuses on their acoustic frequencies. Experiments with single grains have also been performed, trying to separate the contributions related to collision between neighbours from shear movements between grains. Preliminary results indicate that both frictional sliding and collision-like mechanisms are involved in the origin of the acoustic bursts. The evolution of the structure of the medium around a large acoustic burst is analysed both with high resolution and high-speed imaging. The energy of the acoustic bursts seems related to single and well-localised events with very large variations in energy. However, these local reorganisation processes may eventually bring the global relaxation of the system, which is also captured by a force drop in the resistance to shear.

Optical hyperpolarization ofC13nuclear spins in nanodiamond ensembles

Here we propose and analyse in detail protocols that can achieve rapid hyperpolarization of 13C nuclear spins in randomly oriented ensembles of nanodiamonds at room temperature. Our protocols exploit a combination of optical polarization of electron spins in nitrogen-vacancy centers and the transfer of this polarization to 13C nuclei by means of microwave control to overcome the severe challenges that are posed by the random orientation of the nanodiamonds and their nitrogen-vacancy centers. Specifically, these random orientations result in exceedingly large energy variations of the electron spin levels that render the polarization and coherent control of the nitrogen-vacancy center electron spins as well as the control of their coherent interaction with the surrounding 13C nuclear spins highly inefficient. We address these challenges by a combination of an off-resonant microwave double resonance scheme in conjunction with a realization of the integrated solid effect which, together with adiabatic rotations of external magnetic fields or rotations of nanodiamonds, leads to a protocol that achieves high levels of hyperpolarization of the entire nuclear-spin bath in a randomly oriented ensemble of nanodiamonds even at room temperature. This hyperpolarization together with the long nuclear spin polarization lifetimes in nanodiamonds and the relatively high density of 13C nuclei has the potential to result in a major signal enhancement in 13C nuclear magnetic resonance imaging and suggests functionalized and hyperpolarized nanodiamonds as a unique probe for molecular imaging both in vitro and in vivo.

Adaptive local frequency

The frequency characteristics of seismic data play an important role in seismic data processing and interpretation. Many methods have been used to extract a dominant frequency profile. Among these, the instantaneous frequency has been widely used due to its fast calculation and superior vertical resolution. Its incapability of handling noisy data has been rectified by the weighted average method or the generalized Hilbert transform. Several other robust algorithms involve relatively complicated computations, such as sine fitting, wavelet- or spectrum-based filtering, and empirical mode decomposition. Compared to these methods, the local frequency algorithm has the advantage because it handles noisy data or even partially missing data in an elegant and systematic manner. Even though, in cases in which the seismic data energy varies greatly and true amplitude processing is required, the local frequency approach may lose the vertical resolution in weak regions due to the globally controlled smoothing scheme. I have extended the local frequency algorithm with a localized data-adaptive shaping scalar. This data adaptive scheme adds flexibility while preserving all the advantages of the local frequency algorithm. It is capable of removing unrealistic spiky and negative frequencies while keeping the high resolution of the frequency profile. Therefore, it is suitable for records with large energy variations such as true amplitude poststack volumes or prestack shot gathers.

Information kinetics—an extension

We present an operator formalism for the recently developed kinetic information theory, construct Poisson brackets between the Liouville L and information Ĩ operators in μ space, proposing its quantum version. Making use of the universal quantum of time, the Planck time τp, a pseudo-energy-time uncertainty relation is constructed. It suggests that tiny amounts of information production may cause large variations in energy. The Hubble time τH sets an upper bound on information in the universe.

Emittance increase due to transient beam loading

A stretcher/storage ring is filled with a 2.1 μs beam pulse delivered by a 700 MeV linear electron accelerator . The filling time of the accelerator waveguide structures is 1.3 μs. The large energy variations during the filling time due to transient beam loading are compensated for by a time delay of the RF power of some accelerator stations. In this way, the final energy spread can be minimized but along the accelerator large energy variations still exist. As a consequence the dispersive property of the accelerator mainly due to misalignments causes a substantial increase of emittance during the filling time. This effect showed to be dependent on the location of accelerator stations of which the RF power is delayed in time and on the beam focussing along the accelerator. A recipe has been developed to minimize the emittance increase. For this purpose computer simulations with a variety of alignment errors have been carried out for different beam focussing situations. Also timing imperfections (jitter, risetime) have been taken into account.

Theoretical and experimental determination of phantom scatter factors for photon fields with different radial energy variation

The output factor used for monitor unit determination in radiotherapy can be divided into two factors: the head scatter factor and the phantom scatter factor. Theoretical and experimental phantom scatter factors have been compared for different beam qualities between 4 MV and 50 MV and field sizes from to . The theoretical data were obtained through a convolution method based on Monte Carlo calculated energy spectra and dose kernels. The calculations have been performed both for accelerators with a rather large energy variation within the field and for accelerators with a constant energy distribution in the field. Deviations between theoretical and experimental data were found to be less than 1%.

Stiffness and energy conservation in molecular dynamics: An improved integrator

AbstractMolecular dynamics is the integration of a set of coupled differential equations describing the motion of atoms over time. These equations exhibit the unfortunate property of stiffness, that is, terms of the equations (the forces on the atoms) are defined on several scales—ranging from tens of kcal/mol/Å to thousands of kcal/mol/Å. Additional nonconservative and stiff effects occur when a distance cutoff is used for the electrostatics and nonbonded potentials. Because the first derivative at the cutoff is essentially infinite, small variations in positions will cause large variations in energy and violate conservation of energy. The effects are demonstrated in a small system of 125 isolated water molecules. It is possible to greatly reduce and nearly eliminate the stiff integration effects with an improved integrator. The nonconservative effects of the distance cutoff cannot be removed by changing the integrator. © John Wiley & Sons, Inc.

Base-stacking interactions in double-helical DNA structures: experiment versus theory.

Atom-atom potential energy calculations have been undertaken for deriving stacking energies in double-helical structures. A comparison between the energy patterns of A- and B-type double-helical fragments determined by single-crystal X-ray diffraction methods versus idealized uniform models based on fibre diffraction data shows that the van der Waals stacking energy is largely sensitive to local changes in the relative orientation of adjacent base pairs. The sequence-dependent conformational variability observed in the high-resolution structures appears to be a consequence of the equipartitioning of the stacking energy along the double helix. The large energy variations expected for a uniform structure are dampened considerably in the observed structures by means of local changes in conformational features such as helix rotation and roll angles between base pairs.

Time variations in the spatial scales and intensities of ocean circulation in the vicinity of the Cape of Good Hope

A spatial structure function analysis has been carried out on the physical oceanologic data collected during a series of regular monthly cruises from 1962 to 1967 in the vicinity of the Cape of Good Hope. The distribution of energy on the spatial spectrum shows that during most months there is a predilection for all disturbances to be smaller than 150 km. The area is subject to large variations in energy, but these variations involve the whole measured spectrum showing no preference for any specific spatial scales. No significant periodicity in the monthly energy variations is found. DOI: 10.1111/j.2153-3490.1977.tb00747.x

Structure of theη→3πAmplitude

Difficulties with the usual phenomenological theory of the $C$-conserving part of $\ensuremath{\eta}\ensuremath{\rightarrow}3\ensuremath{\pi}$, with $|\ensuremath{\Delta}I|=1$ and dominance by the nearly constant symmetric amplitude, are summarized. It is discussed why the difficulties are indicative of the presence of a significant asymmetric part in the amplitude. An attempt is made to find the possible reductions in the theoretical branching ratio $R=\frac{\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}3{\ensuremath{\pi}}^{0})}{\ensuremath{\Gamma}(\ensuremath{\eta}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\pi}}^{0})}$ as a result of including some large asymmetric parts. We found that a branching ratio of the order $R\ensuremath{\approx}1.1$ is the lower limit of what can be reasonably achieved in these theories. If the experimental value should turn out to be $R\ensuremath{\simeq}0.5$, it will be extremely difficult to reconcile with the theory even when large energy variations in the decay amplitude are allowed for; and it is nearly certain that a part with $|\ensuremath{\Delta}I|>2$ is present in $\ensuremath{\eta}\ensuremath{\rightarrow}3\ensuremath{\pi}$ in that case.