Microscopic Calculations Research Articles

Suppression of intervalley exchange coupling in the presence of momentum-dark states in transition metal dichalcogenides

Monolayers of transition metal dichalcogenides are promising materials for valleytronic applications, since they possess two individually addressable excitonic transitions at the non-equivalent $K$ and $K'$ points with different spins, selectively excitable with light of opposite circular polarization. Here, it is of crucial importance to understand the elementary processes determining the lifetime of these optically injected valley excitons. In this study, we perform microscopic calculations based on a Heisenberg equation of motion formalism to investigate the efficiency of the intervalley coupling in the presence (W based TMDCs) and absence (Mo based TMDCs) of energetically low lying momentum-dark exciton states. While we predict a valley exciton lifetime on the order of some hundreds of fs in the absence of low lying momentum-dark states we demonstrate a strong quenching of the valley lifetime in the presence of such states.

Core swelling in spherical nuclei: An indication of the saturation of nuclear density

Background: Nuclear radius is one of the most important and basic properties of atomic nuclei and its evolution is closely related to the saturation of the nuclear density in the internal region but the systematics of the nuclear radii for the neutron-rich unstable nuclei is not well known. Purpose: Motivated by the recent interaction cross section measurement which indicates the 48Ca core swelling in the neutron-rich Ca isotopes, we explore the mechanism of the enhancement of the neutron and proton radii for spherical nuclei. Methods: Microscopic Hartree-Fock calculations with three sets of Skyrme-type effective interactions are performed for the neutron-rich Ca, Ni and Sn isotopes. The total reaction cross sections for the Ca isotopes are evaluated with the Glauber model to compare them with the recent cross section data. Results: We obtain good agreement with the measured cross sections and charge radii. The neutron and proton radii of the various "core" configurations are extracted from the full Hartree-Fock calculation and discuss the core swelling mechanism. Conclusions: The core swelling phenomena occur depending on the properties of the occupying valence single-neutron states to minimize the energy loss that comes from the saturation of the densities in the internal region, which appears to be prominent in light nuclei such as Ca isotopes.

Enhanced dynamics in fusion of neutron-rich oxygen nuclei at above-barrier energies

Above-barrier fusion cross-sections for an isotopic chain of oxygen isotopes with A=16-19 incident on a $^{12}$C target are presented. Experimental data are compared with both static and dynamical microscopic calculations. These calculations are unable to explain the $\sim$37\% increase in the average above-barrier fusion cross-section observed for $^{19}$O as compared to $\beta$-stable oxygen isotopes. This result suggests that for neutron-rich nuclei existing time-dependent Hartree-Fock calculations underpredict the role of dynamics at near-barrier energies. High-quality measurement of above-barrier fusion for an isotopic chain of increasingly neutron-rich nuclei provides an effective means to probe this fusion dynamics.

Ambiguity of applying the Wildermuth-Tang rule to estimate the quasibound states of α particles in α emitters

The Wildermuth-Tang (WT) prescription is used to verify the Bohr-Sommerfeld (BS) quantization condition in the $\ensuremath{\alpha}$-decay problem. It gives the global quantum number that relates the number of nodes of the quasibound radial wave function of the $\ensuremath{\alpha}$-daughter system to the shell model and Pauli exclusion principle. Here we examine the applicability of the WT rule in the $\ensuremath{\alpha}$-decay microscopic calculations that start with solving the stationary Schr\"odinger wave equation for different types of the interaction potentials. We found that applying the BS quantization condition along with the WT prescription for the potentials that have no internal pocket yields a large number of nodes in the radial wave function compared to the potentials characterized with an automatic physical internal pocket, which likely produce nodeless or at most a two-node interior wave function. This gives confidence in the latter type of the potentials that efficaciously simulates the Pauli principle by considering the change in the intrinsic kinetic energy. However, it is possible to reproduce the observed half-life data using the potentials that have no automatic internal pocket with applying the BS quantization condition with quantum numbers which are significantly less than that obtained from the WT rule, upon properly normalizing the potential.

Beta Decay Rates Evaluated Within the Gross Theory Using Experimental Data for the Gamow-Teller Resonance Parameter

We improve the results for the β−-decay rates obtained within the context of the Gross Theory of Beta Decay (GTBD) by using new values of the parameter σN related to the standard deviation for the Gamow-Teller resonance: we include experimental data when they are available and, if not, we adopt the value of the nearest neighbor previous (that is, with less mass) that has experimental data. We evaluate the β−-decay rates using a Gaussian energy distribution function with the axial-vector weak coupling constant |gA| = 1, considering updated experimental mass defects and also an improved approximation for the Fermi function. Our sample consists of 94 nuclei with mass in the range 46 < A < 70, all of them decaying by means of allowed transitions, which are of interest in the pre-supernova phase and have experimental data in terrestrial conditions available in the Letter of Nuclides. We compare our result with those obtained within the same GTBD but using values of σN adjusted in Possidonio (Braz. J. Phys. 48:485, 2018) by minimizing the χ2-function, and also with systematic microscopic calculations. We have shown that the substitution of the adjusted parameters by experimental data in GTBD improves the results for β−-decay rates by 6.4%.

Microstructured flexible capacitive sensor with high sensitivity based on carbon fiber-filled conductive silicon rubber

Flexible sensors with high sensitivity and stability have been tremendously attractive in recent years. In this study, flexible capacitive sensors (FCSs) with microstructured-surface were fabricated, where electrode layers (e-layers) and dielectric layers (d-layers) of the FCSs were sprayed with carbon fiber (CF) filled-polydimethylsiloxane (PDMS) and pure PDMS on substrate with surface roughness, respectively. Sensitivity of the fabricated FCSs to compressive load was evaluated. Influence of the surface microstructure of d- and e-layers on the sensitivity was studied through in-situ microscopic observations and theoretical capacitance calculations. Results showed that the roughness of the surface microstructure did have a correlation with the sensitivity. Excellent sensing performance, including a high sensitivity (∼0.82 kPa−1), ultrafast response (∼0.2 s) and relaxation (∼0.3 s) time, ultralow detection limit (∼1.2 Pa), and excellent stability (104 loading/unloading cycles) over a wide pressure range (0−50 kPa) appeared on the FCS with its surface microstructure size of 48.6 μm in roughness. The high sensitivity of the fabricated FCS was dominantly resulted from the closing of air gap between e- and d-layers. A 3 × 3 FCS array capable of distinguishing space pressure distribution and a mouse glove capable of inputting computer information were finally fabricated which demonstrated a prospect of the FCS in micro-load response and human-machine interaction applications.

Transition properties of low-lying states inSi28probed via inelastic proton andαscattering

${0}^{+}, {1}^{\ensuremath{-}}, {2}^{+}$, and ${3}^{\ensuremath{-}}$ excitations of $^{28}\mathrm{Si}$ are investigated via proton and $\ensuremath{\alpha}$ inelastic scattering off $^{28}\mathrm{Si}$. The structure calculation of $^{28}\mathrm{Si}$ is performed with the energy variation after total angular momentum and parity projections in the framework of antisymmetrized molecular dynamics (AMD). As a result of the AMD calculation, the oblate ground and prolate bands, ${0}^{+}$ and ${3}^{\ensuremath{-}}$ excitations, and the ${1}^{\ensuremath{-}}$ and ${3}^{\ensuremath{-}}$ states of the ${K}^{\ensuremath{\pi}}={0}^{\ensuremath{-}}$ band are obtained. Using the matter and transition densities of $^{28}\mathrm{Si}$ obtained by AMD, microscopic coupled-channels calculations of proton and $\ensuremath{\alpha}$ scattering off $^{28}\mathrm{Si}$ are performed. The proton-$^{28}\mathrm{Si}$ potentials in the reaction calculation are microscopically derived by folding the Melbourne $g$-matrix $NN$ interaction with the AMD densities of $^{28}\mathrm{Si}$. The $\ensuremath{\alpha}\ensuremath{-}^{28}\mathrm{Si}$ potentials are obtained by folding the nucleon-$^{28}\mathrm{Si}$ potentials with an $\ensuremath{\alpha}$ density. The calculation reasonably reproduces the observed elastic and inelastic cross sections of proton and $\ensuremath{\alpha}$ scattering. Transition properties are discussed by combining the reaction analysis of proton and $\ensuremath{\alpha}$ scattering and structure features such as transition strengths and form factors. The isoscalar monopole and dipole transitions are focused.

Crystalline nodal topological superconductivity and Bogolyubov Fermi surfaces in monolayer NbSe2

We present a microscopic calculation of the phase diagram of the Ising superconductor NbSe$_{2}$ in presence of both in-plane magnetic field and Rashba spin-orbit coupling (SOC). Repulsive interactions lead to two distinct instabilities, in singlet- and triplet- interaction channels. While we recover the previously predicted nodal topological superconducting state in the absence of Rashba SOC at large magnetic field with six pairs of nodes along \(\Gamma\)-\(M\) lines, a finite Rashba SOC breaks the symmetry that protects these nodes and therefore generally lifts them, resulting in a topologically trivial phase. There is an exception when the field is applied along one of the three $\Gamma$-$K$ lines, however. In that case, a single mirror symmetry remains that can protect two pairs of nodes out of the original six, resulting in a \emph{crystalline} topological superconducting phase. Depending on the Cooper pairs' center-of-mass momentum, this superconducting state displays either Bogolyubov Fermi surfaces or point nodes. Moreover, a chiral topological superconducting phase with Chern number of 6 is realized in the regime of large Rashba SOC and dominant triplet interactions, spontaneously breaking time-reversal symmetry.

Mixed higher-order anisotropic flow and nonlinear response coefficients of charged particles in mathrm {PbPb} collisions at sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 2.76 and 5.02,text {TeV}

Anisotropies in the initial energy density distribution of the quark-gluon plasma created in high energy heavy ion collisions lead to anisotropies in the azimuthal distributions of the final-state particles known as collective anisotropic flow. Fourier harmonic decomposition is used to quantify these anisotropies. The higher-order harmonics can be induced by the same order anisotropies (linear response) or by the combined influence of several lower order anisotropies (nonlinear response) in the initial state. The mixed higher-order anisotropic flow and nonlinear response coefficients of charged particles are measured as functions of transverse momentum and centrality in mathrm {PbPb} collisions at nucleon-nucleon center-of-mass energies sqrt{smash [b]{s_{_{mathrm {NN}}}}} = 2.76 and 5.02,text {TeV} with the CMS detector. The results are compared with viscous hydrodynamic calculations using several different initial conditions, as well as microscopic transport model calculations. None of the models provides a simultaneous description of the mixed higher-order flow harmonics and nonlinear response coefficients.

Ionization probability of sputtered indium atoms under impact of slow highly charged ions

In order to investigate the different role of kinetic and potential projectile energy for secondary ion formation, the authors have measured the ionization probability of indium atoms sputtered from a clean indium surface under irradiation with rare gas (Xeq+) ions of different charge states q at the same kinetic impact energy of 20 keV. In this energy range, the kinetic energy of the projectile is predominantly deposited via nuclear stopping, leading to a collision-dominated sputtering process. The authors find that the ionization probability increases significantly if a highly charged ion is used as a projectile, where the ionization energy becomes comparable to or even exceeds the kinetic energy, indicating that a higher level of electronic substrate excitation induced by the potential energy stored in the projectile can boost the secondary ion formation process. This experimental result is discussed in terms of microscopic model calculations describing the secondary ion formation process. At the same time, the authors observe a significant change of the emission velocity distribution of the sputtered particles, leading to a pronounced low-energy contribution at higher projectile charge states. It is shown that this “potential sputtering” contribution strongly depends on surface chemistry even under conditions where the surface is dynamically cleaned by interleaved 5 keV Ar+ ion bombardment.

Energy transport between critical one-dimensional systems with different central charges

Energy transport can reveal information about interacting many-body systems beyond other transport probes. In particular, in one dimension it has been shown that the energy current is directly proportional to the central charge, thus revealing information about the degrees of freedom of critical systems. In this paper, we explicitly verify this result in two cases for translationally invariant systems based on explicit microscopic calculations. More importantly, we generalize the result to nontranslation invariant setups and use this to study a composite system of two subsystems possessing different central charges. We find a bottleneck effect, meaning the smaller central charge limits the energy transport.

Drastic enhancement of the thermal Hall angle in a d -wave superconductor

A drastic enhancement of the thermal Hall angle in $d$-wave superconductors was observed experimentally in a cuprate superconductor and in CeCoIn$_5$ at low temperatures and very weak magnetic field [Phys. Rev. Lett. $\bf 86$, 890 (2001); Phys. Rev. B $\bf 72$, 214515 (2005)]. However, to the best of our knowledge, its microscopic calculation has not been performed yet. To study this microscopically, we derive the thermal Hall coefficient in extreme type-II superconductors with an isolated pinned vortex based on the augmented quasiclassical equations of superconductivity with the Lorentz force. Using it, we can confirm that the quasiparticle relaxation time and the thermal Hall angle are enhanced in $d$-wave superconductors without impurities of the resonant scattering because quasiparticles around the gap nodes which become dominant near zero temperature are restricted to the momentum in a specific orientation. This enhancement of the thermal Hall angle may also be observed in other nodal superconductors with large magnetic-penetration depth.

Pairing effects on neutron elastic scattering at low-energies

For the first time, a realistic microscopic calculation for low-energy neutron-nucleus elastic scattering off open-shell nuclei is carried out within the framework of particle-vibration coupling (PVC). In this study, the pairing correlations of the ground state are taken into account. The dependence of the angular distributions on the pairing gaps have been discussed.

Early Stages of Precipitation in γ' Phase of a Ni–Al–Ti Model Alloy: Phase-Field and First-Principles Study

The early stages of precipitation process of the γ' phase of a Ni–Al–Ti alloy are investigated by microscopic phase-field and first-principles calculations. The simulated results indicate that a pre-precipitate with L10 structure appears before the L12 ordered phase, and then this metastable phase gradually transforms to L12 ordered phase; finally, the precipitated phase is composed of γ' ordered phase and γ matrix phase. The occupation probabilities of Al, Ni, and Ti atoms also illustrate the formation of the L10 phase and its situ conversion to L12 ordered phase constituted by a complicated compound Ni3(AlTi). Through the analyses of order parameter and occupation probability, the precipitation mechanism of γ' phase is drawn as a combination of congruent ordering and destabilization decomposition. Meanwhile, we also find that the growth and coarsening of the γ' phase occur via mixed mechanisms of Ostwald ripening and coalescence coarsening of neighboring precipitates. Moreover, the first-principles method is applied to calculate the thermodynamic parameters and validate further the appearance of the metastable phase and the site preference of Ti atom, which offers an explanation for atomic occupancy characteristics in the precipitate.

Unmixing Symmetries.

The low-lying spectra of atomic nuclei display diverse behaviors, for example, rotational bands, which can be described phenomenologically by simple symmetry groups such as spatial SU(3). This leads to the idea of dynamical symmetry, where the Hamiltonian commutes with the Casimir operator(s) of a group, and is block diagonal in subspaces defined by the group's irreducible representations or irreps. Detailed microscopic calculations, however, show these symmetries are in fact often strongly mixed and the wave function fragmented across many irreps. More commonly, the fragmentation across members of a band are similar, which is called a quasidynamical symmetry. In this Letter I explicitly, albeit numerically, construct unitary transformations from a quasidynamical symmetry to a dynamical symmetry, adapting the similarity renormalization group (SRG) in order to transform away the symmetry-mixing parts of the Hamiltonian. The standard SRG produces unsatisfactory results, forcing the induced dynamical symmetry to be dominated by high-weight irreps irrespective of the original decomposition. Using spectral distribution theory to rederive and diagnose standard SRG, I introduce a new form of SRG. The new SRG transforms a quasidynamical symmetry to a dynamical symmetry, that is, unmixes the mixed symmetries, with intuitively more appealing results.

Resummation for the field-theoretical derivation of the negative magnetoresistance

We show detailed derivation of the electric conductivity of quark matter at finite temperature and density under a magnetic field. We especially focus on the longitudinal electric conductivity along the magnetic direction and establish the field-theoretical description of the negative magnetoresistance as observed in chiral materials. With increasing magnetic field our microscopic calculation leads to changing behavior from approximately quadratic to asymptotically linear dependence of the electric conductivity, while the magnetic dependence is quadratic in the conventional relaxation time approximation. The presented formulation founds a firm basis for the physical interpretation of the negative magnetoresistance in terms of the particle and the hydrodynamic contributions, as well as it offers general methodology applicable for various transport coefficients.

A flow cytometry-based analysis to establish a cell cycle synchronization protocol for Saccharum spp.

Modern sugarcane is an unusually complex heteroploid crop, and its genome comprises two or three subgenomes. To reduce the complexity of sugarcane genome research, the ploidy level and number of chromosomes can be reduced using flow chromosome sorting. However, a cell cycle synchronization (CCS) protocol for Saccharum spp. is needed that maximizes the accumulation of metaphase chromosomes. For flow cytometry analysis in this study, we optimized the lysis buffer, hydroxyurea(HU) concentration, HU treatment time and recovery time for sugarcane. We determined the mitotic index by microscopic observation and calculation. We found that WPB buffer was superior to other buffers for preparation of sugarcane nuclei suspensions. The optimal HU treatment was 2 mM for 18 h at 25 °C, 28 °C and 30 °C. Higher recovery treatment temperatures were associated with shorter recovery times (3.5 h, 2.5 h and 1.5 h at 25 °C, 28 °C and 30 °C, respectively). The optimal conditions for treatment with the inhibitor of microtubule polymerization, amiprophos-methyl (APM), were 2.5 μM for 3 h at 25 °C, 28 °C and 30 °C. Meanwhile, preliminary screening of CCS protocols for Badila were used for some main species of genus Saccharum at 25 °C, 28 °C and 30 °C, which showed that the average mitotic index decreased from 25 °C to 30 °C. The optimal sugarcane CCS protocol that yielded a mitotic index of >50% in sugarcane root tips was: 2 mM HU for 18 h, 0.1 X Hoagland’s Solution without HU for 3.5 h, and 2.5 μM APM for 3.0 h at 25 °C. The CCS protocol defined in this study should accelerate the development of genomic research and cytobiology research in sugarcane.

α decay to a doubly magic core in the quartetting wave function approach

We present a microscopic calculation of $\alpha$-cluster formation in heavy nuclei $^{104}$Te ($\alpha$+$^{100}$Sn), $^{212}$Po ($\alpha$+$^{208}$Pb) and their neighbors $^{102}$Sn, $^{102}$Te, $^{210}$Pb and $^{210}$Po by using the quartetting wave function approach. Improving the local density approximation, the shell structure of the core nucleus is considered, and the center-of-mass (c.o.m.) effective potential for the quartet is obtained self-consistently from the shell model wavefunctions. The $\alpha$-cluster formation and decay probabilities are obtained by solving the bound-state of the c.o.m. motion of the quartet and the scattering state of the formed $\alpha$-cluster in the Gurvitz approach. Striking shell effects on the $\alpha$-cluster formation probabilities are analyzed for magic numbers 50, 82 and 126. The computed $\alpha$-decay half-lives of these special nuclei are compared with the newest experimental data.

Quasifission Dynamics in Microscopic Theories

In the search for superheavy elements quasifission reactions represent one of the reaction pathways that curtail the formation of an evaporation residue. In addition to its importance in these searches quasifission is also an interesting dynamic process that could assist our understanding of many-body dynamical shell effects and energy dissipation thus forming a gateway between deep-inelastic reactions and fission. This manuscript gives a summary of recent progress in microscopic calculations of quasifission employing time-dependent Hartree-Fock (TDHF) theory and its extensions.

Electric-dipole transitions in 6Li with a fully microscopic six-body calculation

Exploring new excitation modes and the role of the nuclear clustering has been of great interest. An interesting speculation was made in the recent photoabsorption measurement of ^66Li that implied the importance of the nuclear clustering. To understand the excitation mechanism of ^66Li, we perform a fully microscopic six-body calculation on the electric-dipole (E1E1) transitions and discuss how ^66Li is excited by the E1E1 field as a function of the excitation energy. We show the various cluster components in the six-body wave functions and discuss the role of the nuclear clustering in the E1E1 excitations of ^66Li.