arXiv subjects

Pengwei Zhao

Publications and source records attributed to Pengwei Zhao.

36 records · Page 2Linked to original sources

arXiv2024

Topological effect on the Anderson transition in chiral symmetry classes

In this Letter, we propose a mechanism of an emergent quasi-localized phase in chiral symmetry classes, where wave function along a spatial direction with weak topology is delocalized but exponentially localized along the other directions. The Anderson transition in 2D chiral symmetry classes is induced by the proliferation of vortex-antivortex pairs of a U(1) phase degree of freedom, while the weak topology endows the pair with the Berry phase. We argue that the Berry phase induces spatial polarizations of the pairs along the topological direction through the quantum interference effect, and the proliferation of the polarized vortex pairs results in the quasi-localized phase.

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arXiv2024

Chiral dynamics in soft triaxial nuclei

The chirality in soft triaxial nuclei is investigated for the first time with the time-dependent and tilted axis cranking covariant density functional theories on a three-dimensional space lattice in a microscopic and self-consistent way. Taking the puzzling chiral nucleus $^{106}$Ag as an example, the experimental energies of the observed nearly degenerate bands are well reproduced without any free parameters beyond the well-defined density functional. A novel chiral mode in soft triaxial nuclei is newly revealed from the microscopic dynamics of the total angular momentum. This opens a new research area for the study of chirality, particularly in relation to soft nuclear shapes.

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arXiv2024

Inelastic neutron scattering on Ce pyrochlores: Signatures of electric monopoles

We revisit the pyrochlore spin liquid materials Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$ by examining the existing experiments. We continue to rely on the special properties of the dipole-octupole nature of the Ce$^{3+}$ moment. The inelastic neutron scattering (INS) measurement in the octupolar U(1) spin liquid selects the (gapped) spinon continuum, and thus has suppressed spectral weights below the energy threshold of two spinon gaps. This measurement, however, includes all other emergent excitations at lower energies in the dipolar U(1) spin liquid, in particular, the gapless photon and the continuum of the electric monopoles. Although the electric monopole continuum is weakly gapped (compared to the larger spinon gap), the energy scale is actually close to the gauge photons, and the spectrum largely overlaps with the photons. Due to the background dual $π$ flux for the electric monopoles, the density of states is enhanced at lower energies, creating peak structures. This can be contrasted with the linearly suppressed spectral weight of the gauge photons at low energies. We propose that the electric monopole continuum should be mostly responsible for the low-energy spectrum in the INS measurement in the dipolar U(1) spin liquid. With these understanding and calculation, we discuss the available experimental results and predict further experiments for Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$.

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arXiv2023

Nuclear chiral rotation within Relativistic Configuration-interaction Density functional theory

The Relativistic Configuration-interaction Density functional (ReCD) theory that combines the advantages of large-scale configuration-interaction shell model and relativistic density functional theory is extended to study nuclear chiral rotation. The energy spectra and transition probabilities of the chiral doublet bands are reproduced satisfactorily without any free parameters. By analyzing the probability amplitudes of the wavefunctions, the significant roles of configuration mixing and four quasiparticle states to the chiral doublets are revealed. The evolution from chiral vibration to static chirality are clearly illustrated by the K plot and azimuthal plot. The present investigation provides both microscopic and quantal descriptions for nuclear chirality for the first time and demonstrates the robustness of chiral geometry against the configuration mixing as well as the four quasiparticle states.

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arXiv2023

Tetrahedral shape of $^{110}$Zr from covariant density functional theory in 3D lattice space

Covariant density functional theory is solved in 3D lattice space by implementing the preconditioned conjugate gradient method with a filtering function (PCG-F). It considerably improves the computational efficiency compared to the previous inverse Hamiltonian method (IHM). This new method is then applied to explore the tetrahedral shape of $^{110}$Zr in the full deformation space. The ground state of $^{110}$Zr is found to have a tetrahedral shape, but the deformations $β_{31}$ and $β_{33}$ greatly soften the potential energy surface. This effect is analysed with the microscopic evolution of the single-particle levels near the Fermi surface driven by the deformation.

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arXiv2023

Accurate relativistic density functional for exchange energy of atomic nuclei

The inclusion of nucleonic exchange energy has been a long-standing challenge for the relativistic density functional theory (RDFT) in nuclear physics. We propose an orbital-dependent relativistic Kohn-Sham density functional theory to incorporate the exchange energy with local Lorentz scalar and vector potentials. The relativistic optimized effective potential equations for the local exchange potentials are derived and solved efficiently. The obtained binding energies and charge radii for nuclei are benchmarked with the results given by the traditional relativistic Hartree-Fock approach, which involves complicated nonlocal potentials. It demonstrates that the present framework is not only accurate but also efficient.

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arXiv2023

Deep-neural-network approach to solving the ab initio nuclear structure problem

Predicting the structure of quantum many-body systems from the first principles of quantum mechanics is a common challenge in physics, chemistry, and material science. Deep machine learning has proven to be a powerful tool for solving condensed matter and chemistry problems, while for atomic nuclei it is still quite challenging because of the complicated nucleon-nucleon interactions, which strongly couple the spatial, spin, and isospin degrees of freedom. By combining essential physics of the nuclear wave functions and the strong expressive power of artificial neural networks, we develop FeynmanNet, a deep-learning variational quantum Monte Carlo approach for \emph{ab initio} nuclear structure. We show that FeynmanNet can provide very accurate solutions of ground-state energies and wave functions for $^4$He, $^6$Li, and even up to $^{16}$O as emerging from the leading-order and next-to-leading-order Hamiltonians of pionless effective field theory. Compared to the conventional diffusion Monte Carlo approaches, which suffer from the severe inherent fermion-sign problem, FeynmanNet reaches such a high accuracy in a variational way and scales polynomially with the number of nucleons. Therefore, it paves the way to a highly accurate and efficient \emph{ab initio} method for predicting nuclear properties based on the realistic interactions between nucleons.

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arXiv2022

Covariant Density Functional Theory with Localized Exchange Terms

A new density-dependent point-coupling covariant density functional PCF-PK1 is proposed, where the exchange terms of the four-fermion terms are local and are taken into account with the Fierz transformation. The coupling constants of the PCF-PK1 functional are determined by empirical saturation properties and ab initio equation of state and proton-neutron Dirac mass splittings for nuclear matter as well as the ground-state properties of selected spherical nuclei. The success of the PCF-PK1 is illustrated with properties of the infinite nuclear matter and finite nuclei including the ground-state properties and the Gamow-Teller resonances. In particular, the PCFPK1 eliminates the spurious shell closures at Z = 58 and Z = 92, which exist commonly in many covariant density functionals without exchange terms. Moreover, the Gamow-Teller resonances are nicely reproduced without any adjustable parameters, and this demonstrates that a self-consistent description for the Gamow-Teller resonances can be achieved with the localized exchange terms in the PCF-PK1. ?

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arXiv2020

Strength of pairing interaction for hyperons in multistrangeness hypernuclei

Pairing correlations play a very important role in atomic nuclei. Although several effective pairing interactions have been used in mean field calculations for nucleons, little is known about effective pairing interactions for hyperons. Based on the quark model, we propose a relationship between effective pairing interactions for hyperons and for nucleons; e.g., for $Λ$s, the strength of the pairing interaction is 4/9 of that for nucleons. A separable pairing force of finite range which has been widely applied to describing pairing correlations in normal nuclei is used to investigate pairing effects in multi-$Λ$ Ca, Sn and Pb hypernuclei.

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arXiv2016

Nuclear chiral and magnetic rotation in covariant density functional theory

Excitations of chiral rotation observed in triaxial nuclei and magnetic and/or antimagnetic rotations seen in near-spherical nuclei have attracted a lot of attention. Unlike conventional rotation in well-deformed or superdeformed nuclei, here the rotational axis is not necessary coinciding with any principal axis of the nuclear density distribution. Thus, tilted axis cranking is mandatory to describe these excitations self-consistently in the framework of covariant density functional theory (CDFT). We will briefly introduce the formalism of tilted axis cranking CDFT and its application for magnetic and antimagnetic rotation phenomena. Configuration-fixed CDFT and its predictions for nuclear chiral configurations and for favorable triaxial deformation parameters are also presented, and the discoveries of the multiple chiral doublets (M\c{hi}D) in 133Ce and 103Rh are discussed.

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arXiv2013

Pseudospin symmetry: Recent progress with supersymmetric quantum mechanics

It is an interesting and open problem to trace the origin of the pseudospin symmetry in nuclear single-particle spectra and its symmetry breaking mechanism in actual nuclei. In this report, we mainly focus on our recent progress on this topic by combining the similarity renormalization group technique, supersymmetric quantum mechanics, and perturbation theory. We found that it is a promising direction to understand the pseudospin symmetry in a quantitative way.

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arXiv2013

Extending the nuclear chart by continuum: from oxygen to titanium

Nuclear masses ranging from O to Ti isotopes are systematically investigated with relativistic continuum Hartree-Bogoliubov (RCHB) theory, which can provide a proper treatment of pairing correlations in the presence of the continuum. From O to Ti isotopes, there are 402 nuclei predicted to be bound by the density functional PC-PK1. For the 234 nuclei with mass measured, the root mean square (rms) deviation is 2.23 MeV. It is found that the proton drip-lines predicted with various mass models are roughly the same and basically agree with the observation. The neutron drip-lines predicted, however, are quite different. Due to the continuum couplings, the neutron drip-line nuclei predicted are extended further neutron-rich than other mass models. By comparison with finite-range droplet model (FRDM), the neutron drip-line nucleus predicted by RCHB theory has respectively 2(O), 10(Ne), 10(Na), 6(Mg), 8(Al), 6(Si), 8(P), 6(S), 14(K), 10(Ca), 10(Sc), and 12(Ti) more neutrons.

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arXiv2013

Pseudospin symmetry in supersymmetric quantum mechanics: II. Spin-orbit effects

Following a previous paper [Haozhao Liang \textit{et al.}, Phys. Rev. C \textbf{87}, 014334 (2013)], we discuss the spin-orbit effects on the pseudospin symmetry (PSS) within the framework of supersymmetric quantum mechanics. By using the perturbation theory, we demonstrate that the perturbative nature of PSS maintains when a substantial spin-orbit potential is included. With the explicit PSS-breaking potential, the spin-orbit effects on the pseudospin-orbit splittings are investigated in a quantitative way.

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arXiv2013

Pseudospin symmetry in supersymmetric quantum mechanics: Schrödinger equations

The origin of pseudospin symmetry (PSS) and its breaking mechanism are explored by combining supersymmetry (SUSY) quantum mechanics, perturbation theory, and the similarity renormalization group (SRG) method. The Schrödinger equation is taken as an example, corresponding to the lowest-order approximation in transforming a Dirac equation into a diagonal form by using the SRG. It is shown that while the spin-symmetry-conserving term appears in the single-particle Hamiltonian $H$, the PSS-conserving term appears naturally in its SUSY partner Hamiltonian $\tilde{H}$. The eigenstates of Hamiltonians $H$ and $\tilde{H}$ are exactly one-to-one identical except for the so-called intruder states. In such a way, the origin of PSS deeply hidden in $H$ can be traced in its SUSY partner Hamiltonian $\tilde{H}$. The perturbative nature of PSS in the present potential without spin-orbit term is demonstrated by the perturbation calculations, and the PSS-breaking term can be regarded as a very small perturbation on the exact PSS limits. A general tendency that the pseudospin-orbit splittings become smaller with increasing single-particle energies can also be interpreted in an explicit way.

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arXiv2012

Localized form of Fock terms in nuclear covariant density functional theory

In most of the successful versions of covariant density functional theory in nuclei, the Fock terms are not included explicitly, which leads to local functionals and forms the basis of their widespread applicability at present. However, it has serious consequences for the description of Gamow-Teller resonances (GTR) and spin-dipole resonances (SDR) which can only be cured by adding further phenomenological parameters. Relativistic Hartree-Fock models do not suffer from these problems. They can successfully describe the GTR and SDR as well as the isovector part of the Dirac effective mass without any additional parameters. However, they are non-local and require considerable numerical efforts. By the zero-range reduction and the Fierz transformation, a new method is proposed to take into account the Fock terms in local functionals, which retains the simplicity of conventional models and provides proper descriptions of the spin-isospin channels and the Dirac masses.

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arXiv2012

Fine structure of charge-exchange spin-dipole excitations in $^{16}$O

The charge-exchange spin-dipole (SD) excitations for both $(p,n)$ and $(n,p)$ channels in $^{16}$O are investigated in the fully self-consistent random phase approximation based on the covariant density functional theory. The fine structure of SD excitations in the most up-to-date $^{16}$O($\vec p, \vec n$)$^{16}$F experiment is excellently reproduced without any readjustment in the functional. The characteristics of SD excitations are understood with the delicate balance between the $σ$- and $ω$-meson fields via the exchange terms. The fine structure of SD excitations for $^{16}$O($n,p$)$^{16}$N channel is predicted for future experiments.

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arXiv2011

Perturbative interpretation of relativistic symmetries in nuclei

Perturbation theory is used systematically to investigate the symmetries of the Dirac Hamiltonian and their breaking in atomic nuclei. Using the perturbation corrections to the single-particle energies and wave functions, the link between the single-particle states in realistic nuclei and their counterparts in the symmetry limits is discussed. It is shown that the limit of S-V=const and relativistic harmonic oscillator (RHO) potentials can be connected to the actual Dirac Hamiltonian by the perturbation method, while the limit of S+V=const cannot, where S and V are the scalar and vector potentials, respectively. This indicates that the realistic system can be treated as a perturbation of spin-symmetric Hamiltonians, and the energy splitting of the pseudospin doublets can be regarded as a result of small perturbation around the Hamiltonian with RHO potentials, where the pseudospin doublets are quasidegenerate.

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arXiv2010

Spin-orbit and orbit-orbit strengths for radioactive neutron-rich doubly magic nucleus $^{132}$Sn in relativistic mean field theory

Relativistic mean field (RMF) theory is applied to investigate the properties of the radioactive neutron-rich doubly magic nucleus $^{132}$Sn and the corresponding isotopes and isotones. The two-neutron and two-proton separation energies are well reproduced by the RMF theory. In particular, the RMF results agree with the experimental single-particle spectrum in $^{132}$Sn as well as the Nilsson spin-orbit parameter $C$ and orbit-orbit parameter $D$ thus extracted, but remarkably differ from the traditional Nilsson parameters. Furthermore, the present results provide a guideline for the isospin dependence of the Nilsson parameters.

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