Phase Relaxation Time Research Articles

Threshold temperature of superfluorescence generation in CuCl quantum dots under resonant excitation of excitons and resonant two-photon excitation of biexcitons

We studied the threshold temperature of superfluorescence (SF) generation with regard to biexcitons in CuCl quantum dots (QDs) under resonant two-photon excitation of biexcitons and resonant excitation of excitons to demonstrate the influence of initial population densities in the QDs on SF generation. As a result, the threshold temperature under the resonant excitation of excitons was higher than that under the two-photon excitation of biexcitons. This indicates that the high density of excited dots facilitates the rapid establishment of coherence among the dots, overcoming disadvantages of incomplete population inversion and formation process of biexcitons. We performed a theoretical calculation of the time profiles of the biexcitonic emission based on semiconductor luminescence equations. The experimentally obtained temperature dependence of the time profiles was qualitatively reproduced by calculating their dependence on the dephasing rate. In addition, we estimated the temperature dependence of the phase relaxation time of the biexcitons in the CuCl QDs by analyzing the temperature dependence of SF.

Liquid‐Ice Mass Partitioning Across the Edge of Mixed‐Phase Cumulus Clouds

AbstractOne of the major factors controlling the phase partitioning in mixed‐phase cloud is entrainment mixing, but it is still poorly understood. In this study, the liquid‐ice mass partitioning across the edge of shallow to moderately deep cumulus clouds are analyzed using airborne measurements. The results show the concentration and water content of both liquid and ice decrease toward the cloud edge. However, the liquid mass fraction remains similar across the cloud. The mechanism responsible for the phase partitioning is that extreme inhomogeneous entrainment‐mixing dominates. This is evident as both the droplet and ice sizes remain similar with changing concentrations. The comparison between the time scale of turbulent mixing and the phase relaxation time of water also suggests the turbulence strength is too low to homogenize the cloud. The findings from this study improve our understanding on the role of entrainment in phase partitioning, and are useful in evaluating model simulations.

Investigation on the organic–inorganic hybrid crystal [NH2(CH3)2]2CuBr4: structure, phase transition, thermal property, structural geometry, and dynamics

Understanding the physical properties of the organic–inorganic hybrid [NH2(CH3)2]2CuBr4 is essential to expand its applications. The single [NH2(CH3)2]2CuBr4 crystals were grown and their comprehensive properties were investigated. The crystals had a monoclinic structure with the space group P21/n and lattice constants of a = 8.8651 (5) Å, b = 11.9938 (6) Å, c = 13.3559 (7) Å, and β = 91.322°. The transition temperature from phase I to phase II was determined to be 388 K. Variations in the 1H nuclear magnetic resonance chemical shifts of NH2 and 14N NMR chemical shifts according to the temperature changes in the cation were attributed to vibrations of NH2 groups at their localization sites. The 1H and 13C spin–lattice relaxation times (T1ρ) in phase II changed significantly with temperature, indicating that these values are governed by molecular motion. The T1ρ values were much longer in phase I than in phase II, which means energy transfer was difficult. Finally, the activation energies for phases I and II were considered. According to the basic mechanism of [NH2(CH3)2]2CuBr4 crystals, organic–inorganic materials may have potential applications in various fields.

Terahertz radiation by quantum interference of excitons in a one-dimensional Mott insulator

Nearly monocyclic terahertz waves are used for investigating elementary excitations and for controlling electronic states in solids. They are usually generated via second-order optical nonlinearity by injecting a femtosecond laser pulse into a nonlinear optical crystal. In this framework, however, it is difficult to control phase and frequency of terahertz waves. Here, we show that in a one-dimensional Mott insulator of a nickel-bromine chain compound a terahertz wave is generated with high efficiency via strong electron modulations due to quantum interference between odd-parity and even-parity excitons produced by two-color femtosecond pulses. Using this method, one can control all of the phase, frequency, and amplitude of terahertz waves by adjusting the creation-time difference of two excitons with attosecond accuracy. This approach enables to evaluate the phase-relaxation time of excitons under strong electron correlations in Mott insulators. Moreover, phase- and frequency-controlled terahertz pulses are beneficial for coherent electronic-state controls with nearly monocyclic terahertz waves.

Quantification of spatially localized MRS by a novel deep learning approach without spectral fitting.

To propose a novel end-to-end deep learning model to quantify absolute metabolite concentrations from in vivo J-point resolved spectroscopy (JPRESS) without using spectral fitting. A novel encoder-decoder-style neural network was created, which was trained to predict metabolite concentrations and individual component signals concurrently from 3T JPRESS data in the time domain. The training data set contained 100 000 samples created by spin-density simulations using experimentally used RF pulses. Concentrations, phase, frequencies, linewidths, and T2 relaxation times in the training data set were varied over a large range with uniform distributions. Random synthesized noise and extraneous signals were added to the data set. Two thousand validation samples were created similarly to the training data set but with mean concentrations close to in vivo values. An in vivo test was conducted with 20 samples acquired from the human brain. Both validation and in vivo test results showed that the proposed model successfully predicted metabolite concentrations as well as individual metabolite signals without involving spectral fitting, while extraneous peaks or unregistered signals were filtered out. Compared with the short-TE spectral fitting by LCModel, the proposed method had the advantage that the undesired correlations between the estimated concentrations and noise levels and between metabolites were eliminated or substantially reduced. The proposed method provides a working deep learning model that directly maps in vivo JPRESS data to metabolite concentrations. Because spectral fitting is not used, the trained model does not depend on the assumptions associated with parameter tuning when applied to in vivo data.

Relaxation echo

The possibility of generation of a microwave echo-signal due to irreversible phase relaxation in a system of equidistant triplets with a cascade scheme of magnetic-dipole transitions is predicted. The disappearance of relaxation entails the disappearance of this echo-signal. The mechanism of this effect lies in the destructive interference of quantum transitions, which radiate in anti-phase with the same eigen-frequencies. The difference in phase relaxation times for these transitions leads to incomplete damping of the resulting coherence. As a result an echo-signal is generated.

Ultrasonic Spin Echo Caused by Irreversible Phase Relaxation

The possibility of the generation of a primary ultrasonic echo in the system of equidistant Zeeman triplets owing to irreversible phase relaxation has been predicted. The disappearance of phase relaxation results in the disappearance of an echo signal. This effect is due to the destructive interference of two allowed quantum transitions emitting in antiphase. The difference between the phase relaxation times at these transitions leads to an incomplete quenching of resulting coherence, leading to generation of the echo signal.

Ultrasonic Spin Echo Caused by Irreversible Phase Relaxation

The possibility of the generation of a primary ultrasonic echo in the system of equidistant Zeeman triplets owing to irreversible phase relaxation has been predicted. The disappearance of phase relaxation results in the disappearance of an echo signal. This effect is due to the destructive interference of two allowed quantum transitions emitting in antiphase. The difference between the phase relaxation times at these transitions leads to an incomplete quenching of resulting coherence, leading to generation of the echo signal.

Superconducting properties of Cu0.5Tl0.5(BaCa)(CaMg)1.5Cu4−yCdyO12−δ (y=0,1,2,3) samples

In order to investigate the significance of electron–phonons interaction in the origin of high [Formula: see text] superconductivity, we synthesized Cd-doped Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples and studied the superconducting properties. Onset temperature of the superconductivity Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples suppress with the increased Cd doping ratio in the final. The phonon modes are associated to the vibrations of apical and planar oxygen whereas the CuO2 planar oxygen modes are observed to harden with increasing Cd ratio in sample doping. The phase relaxation time is suppressed for the superconducting charge carriers in Cd-doped samples. The energy amount required to break the Cooper pairs apart increases. Dielectric measurements of Cu[Formula: see text]Tl[Formula: see text](BaCa)(CaMg)[Formula: see text]Cu[Formula: see text]CdyO[Formula: see text] ([Formula: see text], 1, 2, 3) samples have been done.

Структура и особенности автомодуляции сверхизлучательных состояний в асимметричном резонаторе Фабри-Перо

The dependence of the structure and stability of strongly asymmetric stationary states of a superradiant laser with a slightly asymmetric low-Q Fabry-Perot cavity on its length, reflection factors of mirrors, and pumping level is studied. The states are related to a self-consistent inhomogeneous half-wavelength population inversion grating. The possibility of the existence of two dynamic phase transitions from a stationary (monochromatic) state of this type to a nonstationary one is established: 1) a dissipative superradiant transition to a regime with a quasi-continuous lasing spectrum (in a weakly asymmetric cavity) and 2) a self-modulation transition to a regime with a discrete lasing spectrum. It is shown that the latter can be caused by excitation of both polariton and electromagnetic laser modes due to resonant Rabi oscillations of active centers with a sufficiently long phase relaxation time.

Low-temperature magnetoresistance of functionalized multiwall carbon nanotubes

Temperature and magnetic field dependencies of resistance for functionalized multiwall carbon nanotubes (MWCNTs) have been studied. The measurements were carried out in the temperature range T = 4.2–200 K. It is shown that in magnetic fields up to B = 9 T, the conductivity behavior for the functionalized MWCNTs sample can be described in terms of charge carriers weak localization and interaction phenomena. We show that the contribution to the functionalized MWCNTs conductivity due to the weak localization effect exceeds the quantum correction due to the effect of the charge carriers interaction for all the temperatures and in the entire range of the applied magnetic fields except for the magnetic fields above B = 6.5 T at T = 5 K. Within the terms of the specified models, we estimate the value of the Fermi energy and determine the explicit form of the temperature dependence of the phase relaxation time for the wave function. We show that for the functionalized MWCNTs sample, the phase relaxation time for the wave function has a less pronounced temperature dependence, and its Fermi energy is more shifted to the valence band compared to non-functionalized MWCNTs. The charge carriers’ interaction constant at different temperatures can also be estimated from our experiments.

Low-temperature motion of the scandium bimetal in endofullerene Sc2@C80(CH2Ph).

The spin decoherence of the scandium bimetal in Sc2@C80(CH2Ph) is studied at low temperatures (20-120 K) by the electron spin echo technique. The correlation between the magnetic quantum number m of the total spin state of the scandium nuclei j and the decay rates is established. For the total spin j = 5, a decrease of the phase relaxation time by a factor of two is observed by changing the transition from m = -1 to m = +1 and m = -3 at 120 K. The observed results are rationalized in the framework of the rotational diffusion of the endohedral fragment in the fullerene cage. It is found that the characteristic rotation time is of the order of a microsecond at 100 K and increases at lower temperatures.

Microphysical timescales and local supersaturation balance at a warm cloud top boundary

Recent results have shown that there is an acceleration in the spread of the size distribution of droplet populations in the region bordering the cloud and undersaturated ambient. We have analyzed the supersaturation balance in this region, which is typically a highly intermittent shearless turbulent mixing layer, under a condition where there is no mean updraft. We have investigated the evolution of the cloud–clear air interface and of the droplets therein via direct numerical simulations. We have compared horizontal averages of the phase relaxation, evaporation, reaction, and condensation times within the cloud–clear air interface for the size distributions of the initial monodispersed and polydisperse droplets. For the monodisperse population, a clustering of the values of the reaction, phase, and evaporation times, that is around 20–30 s, is observed in the central area of the mixing layer, just before the location where the maximum value of the supersaturation turbulent flux occurs. This clustering of values is similar for the polydisperse population but also includes the condensation time. The mismatch between the time derivative of the supersaturation and the condensation term in the interfacial mixing layer is correlated with the planar covariance of the horizontal longitudinal velocity derivatives of the carrier air flow and the supersaturation field, thus suggesting that a quasi-linear relationship may exist between these quantities.

Optimization of gain region in mid-IR ( ≈ 5 μm) QCL.

Non-equilibrium Green's function (NEGF) formalism is used to optimize the gain region of a quantum cascade laser (QCL) tailored to emit radiation at ∼5 µm wavelength, originally designed by Evans et al. [Appl. Phys. Lett., 88,051105(2006)10.1063/1.2171476]. The optimization strategy uses electron-photon selfenergies to find characteristics of devices under the "operating conditions," i.e., interacting with the laser field. These conditions can be quite different from the one when the device is in no-lasing state and the unsaturated gain is being optimized. The saturation caused by the optical field can push the structure from strong to weak coupling conditions, what changes laser parameters in a non-linear manner. Moreover, the NEGF method does not require any phenomenological parameters (such as, e.g., the phase relaxation times), so the quantities dependent on these parameters are determined solely on physical grounds. The use of the above procedure for the structure under investigation shows that the increase of the quantum efficiency by 24% and the output power by 83% in comparison to the original design can be achieved when the widths of injection and extraction barriers are changed to their optimal values.

Deformations, relaxation, and broken symmetries in liquids, solids, and glasses: A unified topological field theory.

We combine hydrodynamic and field theoretic methods to develop a general theory of phonons as Goldstone bosons in crystals, glasses, and liquids based on nonaffine displacements and the consequent Goldstone phase relaxation. We relate the conservation, or lack thereof, of specific higher-form currents with properties of the underlying deformation field-nonaffinity-which dictates how molecules move under an applied stress or deformation. In particular, the single-valuedness of the deformation field is associated with conservation of higher-form charges that count the number of topological defects. Our formalism predicts, from first principles, the presence of propagating shear waves above a critical wave vector in liquids, thus giving a formal derivation of the phenomenon in terms of fundamental symmetries. The same picture provides also a theoretical explanation of the corresponding "positive sound dispersion" phenomenon for longitudinal sound. Importantly, accordingly to our theory, the main collective relaxation timescale of a liquid or a glass (known as the α relaxation for the latter) is given by the phase relaxation time, which is not necessarily related to the Maxwell time. Finally, we build a nonequilibrium effective action using the in-in formalism defined on the Schwinger-Keldysh contour, that further supports the emerging picture. In summary, our work suggests that the fundamental difference between solids, fluids, and glasses has to be identified with the associated generalized higher-form global symmetries and their topological structure, and that the Burgers vector for the displacement fields serves as a suitable topological order parameter distinguishing the solid (ordered) phase and the amorphous ones (fluids, glasses).

Low-temperature magnetoresistance of multi-walled carbon nanotubes with perfect structure

The magnetoresistance of multi-walled carbon nanotubes is studied in the temperature range 4.2–200 K and magnetic fields up to 9 T. The magnetoresistance is negative in the whole temperature range. For small magnetic fields and low temperatures, the dependence of the relative conductivity on the magnetic field is quadratic. However, as the magnetic field increases, it becomes logarithmic, which may be described by weak localization and charge carriers’ interaction models. We show that the addition to conductivity due to the charge carriers’ weak localization significantly exceeds the addition due to the effect of the charge carriers’ interaction. The Fermi energy and the charge carriers’ interaction constant were estimated in terms of these models using the experimental data on the magnetoresistance field and temperature dependences. Also, we determined the exact form for the temperature dependence of the phase relaxation time of the charge carriers’ wave function.

Loading during Midstance of Gait Is Associated with Magnetic Resonance Imaging of Cartilage Composition Following Anterior Cruciate Ligament Reconstruction.

ObjectiveA complex association exists between aberrant gait biomechanics and posttraumatic knee osteoarthritis (PTOA) development. Previous research has primarily focused on the link between peak loading during the loading phase of stance and joint tissue changes following anterior cruciate ligament reconstruction (ACLR). However, the associations between loading and cartilage composition at other portions of stance, including midstance and late stance, is unclear. The objective of this study was to explore associations between vertical ground reaction force (vGRF) at each 1% increment of stance phase and tibiofemoral articular cartilage magnetic resonance imaging (MRI) T1ρ relaxation times following ACLR.DesignTwenty-three individuals (47.82% female, 22.1 ±4.1 years old) with unilateral ACLR participated in a gait assessment and T1ρ MRI collection at 12.25 ± 0.61 months post-ACLR. T1ρ relaxation times were calculated for the articular cartilage of the weightbearing medial and lateral femoral (MFC, LFC) and tibial (MTC, LTC) condyles. Separate bivariate, Pearson product moment correlation coefficients (r) were used to estimate strength of associations between T1ρ MRI relaxation times in the medial and lateral tibiofemoral articular cartilage with vGRF across the entire stance phase.ResultsGreater vGRF during midstance (46%-56% of stance phase) was associated with greater T1ρ MRI relaxation times in the MFC (r ranging between 0.43 and 0.46).ConclusionsBiomechanical gait profiles that include greater vGRF during midstance are associated with MRI estimates of lesser proteoglycan density in the MFC. Inability to unload the ACLR limb during midstance may be linked to joint tissue changes associated with PTOA development.

Vertical Variations of Cloud Microphysical Relationships in Marine Stratocumulus Clouds Observed During the ACE‐ENA Campaign

AbstractThis study examines the vertical variations of cloud microphysical relationships and their implications to cloud microphysical processes in marine stratocumulus clouds using in‐situ aircraft observations during the Aerosol and Cloud Experiments in Eastern North Atlantic (ACE‐ENA) field campaign. A new diagram with a coordinate system based on cloud droplet liquid water content (Lc) and phase relaxation time scale is proposed to investigate mixing mechanisms. This new diagram analysis shows that the inhomogeneous mixing trait is dominant near the cloud top, but homogeneous mixing trait is stronger at lower altitudes. The relevant scale parameters (i.e., transition length scale and transition scale number) also indicate a high likelihood of inhomogeneous mixing. The relationship between Lc and standard deviation of droplet radius (σR) clearly shows the vertical transition: the correlation between Lc and σR is positive at lower cloud altitudes, but it becomes negative as altitude increases. Such a vertical transition is consistent with the vertical circulation mixing, modulating the cloud microphysical relationships to suggest homogeneous mixing at a significant depth from the cloud top.

First-Principles-Based Quantum Transport Simulations of Interfacial Point Defect Effects on InAs Nanowire Tunnel FETs

The effects of a single point defect on InAs gate-all-around nanowire tunnel FETs (NW TFETs) are investigated. We considered two kinds of interfacial defects, the arsenic dangling bond (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DB</sub> ) and the arsenic antisite (As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> ). The critical physics related to the point defects, which are the charge trapping (CT), the trap-assisted tunneling, and their interplay, are rigorously captured using the full quantum transport model with the physical defect Hamiltonian obtained from the density functional theory calculations. We found that both defects on the NW cause the bandgap states, which crucially affects the TFETs performances. Through nonequilibrium Green’s function method self-consistently coupled with Poisson’s equation, the characteristics of the point defects under finite bias conditions are analyzed. We found that the CT is strong in As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> , and thus the defect level, referenced to the valence band edge, is significantly changed by the gate bias. The strong CT leads to the screening of the gate field, which makes the defect level of As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> effectively pinned near the drain Fermi level. The radial distributions of the density of states (DOS) and the phase relaxation time are analyzed. As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> shows the strongly localized DOS at the defect center, while the DOS of As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DB</sub> is relatively delocalized due to the coupling with the valence bands. The impact of the defects on the TFET performances is examined as the gate length is scaled down. It is shown that As <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">In</sub> acts as a limiting factor for scaling down of the TFETs.

Composites ofABSwithSEBS‐g‐MAand copper microparticles modified by mussel‐bioinspired polydopamine: A comparative rheological study

AbstractCharacterizing and understanding composite polymeric systems' rheological behavior is essential to tune their melt flow in polymer processing machinery. Also, rheological tests are suitable tools to evaluate the microstructure and interface adhesion of polymeric composites, which are relevant parameters on the composite performance in their applications. Poly(acrylonitrile‐co‐butadiene‐co‐styrene) (ABS) and its composites are essential materials used to manufacture consumer goods necessary to maintain the quality of life of human beings. In this work, we report on polymer systems based on ABS and poly(styrene‐block‐ethylene‐co‐butylene‐block‐styrene)‐graft‐maleic anhydride (SEBS‐g‐MA) as polymer matrix containing dispersed copper microparticles (ABS:SEBS‐g‐MA:Cu). Polydopamine, a bioinspired polymer, was applied as a filler‐matrix interface compatibilizer of the composites. The linear viscoelastic rheological measurements evidence a shear‐thinning behavior for the composites, and the polydopamine coating affects the polymer phase's relaxation time. The cohesive energy density (Ec) and SEM micrographs indicate good adhesion between copper and ABS:SEBS‐g‐MA due to polydopamine interfacial adherence.