arXiv subjects

D. Angom

Publications and source records attributed to D. Angom.

At least 19 recordsLinked to original sources

arXiv2025

Probing transition rates, nuclear moments and electric dipole polarizability in nobelium using multireference FSRCC and PRCC theories

We employ an all-particle multireference Fock-space relativistic coupled-cluster (FSRCC) theory to compute the ionization potential, excitation energy, transition rate and hyperfine structure constants associated with $7s^2\;^{1}S_{0}\rightarrow 7s7p\;^{3}P_{1}$ and $7s^2\;^{1}S_{0}\rightarrow 7s7p\;^1P_{1}$ transitions in nobelium (No). Using our state-of-the-art calculations in conjunction with available experimental data \cite{raeder-18}, we extract the values of nuclear magnetic dipole ($\mu$) and electric quadrupole ($Q$) moments for $^{253}$No. Further, information on nuclear deformation in even-mass isotopes is extracted from the isotope shift calculations. Moreover, we employ a perturbed relativistic coupled-cluster (PRCC) theory to compute the ground state electric dipole polarizability of No. In addition, to assess the accuracy of our calculations, we compute the ionization potential and dipole polarizability of lighter homolog ytterbium (Yb). To account for strong relativistic and quantum electrodynamical (QED) effects in No, we incorporate the corrections from Breit interaction, vacuum polarization and self-energy in our calculations. The contributions from triple excitations in coupled-cluster is accounted perturbatively. Our calculations reveal a significant contribution of $\approx$10\% from the perturbative triples to the transition rate of $7s^2\;^1S_{0}\rightarrow 7s7p\;^3P_{1}$ transition. The largest cumulative contribution from Breit+QED is observed to be $\approx$4\%, to the magnetic dipole hyperfine structure constant of $7s7p\;^1P_{1}$ state. Our study provides a comprehensive understanding of atomic and nuclear properties of nobelium with valuable insights into the electron correlation and relativistic effects in superheavy elements.

physics.atom-ph↗

arXiv2024

Fock-space perturbed relativistic coupled-cluster calculations of electric dipole polarizability and nuclear spin-dependent parity non-conservation in Cs

We implement the Fock-space perturbed relativistic coupled-cluster theory to compute the electric dipole polarizability of ground and low lying excited states, and nuclear spin-dependent parity violating (NSD-PNC) transition amplitudes in Cs. Moreover, to check the accuracy of the wavefunctions used in the calculations, we compute the excitation energies, E1 transition amplitudes and magnetic dipole hyperfine constants for ground and low lying excited states. To improve the accuracy of the computed properties, we have incorporated the corrections from the relativistic and QED effects in our calculations. The contributions from triple excitations are accounted perturbatively. Our results on excitation energies, E1 transition amplitudes and hyperfine constants are in good agreement with the available experimental results. Our polarizability results using FS-PRCC theory match well with the experimental values. The values of parity-violating transition amplitudes from our calculations are, in general, on the lower side of the previous values. From the detail analysis of electron correlations, we find that the corrections from the Breit interaction and QED effects are important to get accurate results of NSD-PNC amplitudes in Cs. The largest cumulative contribution from the Breit and QED corrections is found to be $\approx$ 3.2\% of the total value. The upper bound on the theoretical uncertainty in our calculated NSD-PNC amplitudes is estimated to be about 1\%.

physics.atom-ph↗

arXiv2024

Fock-space relativistic coupled-cluster calculations of clock transition properties in Pb$^{2+}$

We have implemented an all-particle multireference Fock-space relativistic coupled-cluster theory to probe $6s^2{\;^1}S_{0} - 6s6p{\;^3P^o_{0}}$ clock transition in an even isotope of Pb$^{2+}$. We have computed, excitation energy for several low lying states, E1 and M1 transition amplitudes, and the lifetime of the clock state. Moreover, we have also calculated the ground state dipole polarizability using perturbed relativistic coupled-cluster theory. To improve the accuracy of results, we incorporated the corrections from the relativistic and QED effects in all our calculations. The contributions from triple excitations are accounted perturbatively. Our computed excitation energies are in excellent agreement with the experimental values for all the states. Our result for lifetime, $9.76\times10^{6}$ s, of clock state is $\approx$ 8.5\% larger than the previous value using CI+MBPT [Phys. Rev. Lett. {\bf 127}, 013201 (2021)]. Based on our analysis, we find that the contributions from the {\em valence-valence} correlations arising from higher energy configurations and the corrections from the perturbative triples and QED effects are essential to get accurate clock transition properties in Pb$^{2+}$. Our computed value of dipole polarizability is in good agreement with the available theoretical and experimental data.

physics.atom-ph↗

arXiv2023

Quench dynamics across the MI-SF quantum phase transition with cluster mean field theory

In this work, we study the quench dynamics of quantum phases of ultracold neutral bosons trapped in optical lattices. We investigate the validity of the Kibble-Zurek (KZ) scaling laws with the single-site Gutzwiller mean-field (SGMF) and cluster Gutzwiller mean-field (CGMF) theory. With CGMF, we note the evolution of the dynamical wavefunction in the ``impulse" regime of the Kibble-Zurek mechanism. We obtain the power law scalings for the crossover time and defect density with the quench rate predicted by KZ scaling laws. The critical exponents obtained from dynamics are close to their equilibrium values. Furthermore, it is observed that the obtained dynamical critical exponent $z$ improves towards the equilibrium value with increasing cluster sizes in CGMF.

cond-mat.quant-gas↗

arXiv2023

Exact diagonalization using hierarchical wave functions and calculation of topological entanglement entropy

In this work we describe a new technique for numerical exact diagonalization. The method is particularly suitable for cold bosonic atoms in optical lattices, in which multiple atoms can occupy a lattice site. We describe the use of the method for Bose-Hubbard model of a two-dimensional square lattice system as an example; however, the method is general and can be applied to other lattice models and can be adapted to three-dimensional systems. The proposed numerical technique focuses in detail on how to construct the basis states as a hierarchy of wave functions. Starting from single-site Fock states, we construct the basis set in terms of row states and multirow states. This simplifies the application of constraints and calculation of the Hamiltonian matrix. The approach simplifies the calculation of the reduced density matrices, and this has applications in characterizing the topological entanglement of the state. Each step of the method can be parallelized to accelerate the computation. As a case study, we discuss the computation of the spatial bipartite entanglement entropy in the correlated $\nu = 1/2$ fractional quantum Hall state.

cond-mat.quant-gas↗

arXiv2021

Fine-grained domain counting and percolation analysis in 2D lattice systems with linked-lists

We present a fine-grained approach to identify clusters and perform percolation analysis in a 2D lattice system. In our approach, we develop an algorithm based on the linked-list data structure whereby the members of a cluster are nodes of a path. This path is mapped to a linked-list. This approach facilitates unique cluster labeling in a lattice with a single scan. We use the algorithm to determine the critical exponent in the quench dynamics from the Mott insulator to the superfluid phase of bosons in 2D square optical lattices. The results obtained are consistent with the Kibble-Zurek mechanism. We also employ the algorithm to compute the correlation length using definitions based on percolation theory and use it to identify the quantum critical point of the Bose Glass to superfluid transition in the disordered 2D square optical lattices. In addition, we compute the critical exponent $\nu$ which quantify the divergence of the correlation length $\xi$ across the phase transition and the fractal dimension of the hulls of the superfluid clusters.

cond-mat.quant-gas↗

arXiv2021

Fock-space perturbed relativistic coupled-cluster theory for electric dipole polarizability of one-valence atomic systems: Application to Al and In

We have developed a Fock-space relativistic coupled-cluster theory based method for the calculation of electric dipole polarizability of one-valence atoms and ions. We employ this method to compute the ground-state and spin-orbit coupled excited state electric dipole polarizability of Al and In. To check the quality of many-electron wavefunctions, we also compute the excitation energies of some low-lying states of Al and In. The effects of Breit interaction and QED corrections from the Uehling potential and the self-energy are included to improve the accuracy of $\alpha$ further. Our recommended value of ground-state $\alpha$ for both atoms are in good agreement with the previous theoretical results. From our computations, we find that more than 65\% of contributions come from the dipolar mixing of $3p$($5p$) with $3d$($5d$) and $4s$($6s$)-electrons for Al(In). The largest Breit and QED contributions are found to be 1.3\% and 0.6\%, respectively.

physics.atom-ph↗

arXiv2021

Quantum phases of dipolar bosons in multilayer optical lattice

We consider a minimal model to investigate the quantum phases of hardcore, polarized dipolar atoms confined in multilayer optical lattices. The model is a variant of the extended Bose-Hubbard model, which incorporates intralayer repulsion and interlayer attraction between the atoms in nearest-neighbour sites. We study the phases of this model emerging from the competition between the attractive interlayer interaction and the interlayer hopping. Our results from the analytical and cluster-Gutzwiller mean-field theories reveal that multimer formation occurs in the regime of weak intra and interlayer hopping due to the attractive interaction. In addition, intralayer isotropic repulsive interaction results in the checkerboard ordering of the multimers. This leads to an incompressible checkerboard multimer phase at half-filling. At higher interlayer hopping, the multimers are destabilized to form resonating valence-bond like states. Furthermore, we discuss the effects of thermal fluctuations on the quantum phases of the system.

cond-mat.quant-gas↗

arXiv2021

Quantum quench dynamics of tilted dipolar bosons in 2D optical lattices

We investigate the quench dynamics of the dipolar bosons in two dimensional optical lattice of square geometry using the time dependent Gutzwiller method. The system exhibits different density orders like the checkerboard and the striped pattern, depending upon the polarization angle of the dipoles. We quench the hopping parameter across the striped density wave (SDW) to striped supersolid (SSS) phase transition, and obtain the scaling laws for the correlation length and topological vortex density, as function of the quench rate. The results are reminiscent of the Kibble-Zurek mechanism (KZM). We also investigate the dynamics from the striped supersolid phase to the checkerboard supersolid phase, obtained by quenching the dipole tilt angle $\theta$. This is a first order structural quantum phase transition, and we study the non-equilibrium dynamics from the perspective of the KZM. In particular, we find the number of the domains with checkerboard order follows a power law scaling with the quench rate. This indicates the applicability of the KZM to this first order quantum phase transition.

cond-mat.quant-gas↗

arXiv2021

RCC calculation of electric dipole polarizability and correlation energy of Cn, Nh$^+$ and Og: Correlation effects from lighter to superheavy elements

We employ a fully relativistic coupled-cluster theory to calculate the ground-state electric dipole polarizability and electron correlation energy of superheavy elements Cn, Nh$^+$ and Og. To assess the trend of electron correlation as function of $Z$, we also calculate the correlation energies for three lighter homologs--Zn, Cd and Hg; Ga$^+$, In$^+$ and Tl$^+$; Kr, Xe and Rn--for each superheavy elements. The relativistic effects and quantum electrodynamical corrections are included using the Dirac-Coulomb-Breit Hamiltonian with the corrections from the Uehling potential and the self-energy. The effects of triple excitations are considered perturbatively in the theory. Furthermore, large bases are used to test the convergence of results. Our recommended values of polarizability are in good agreement with previous theoretical results for all SHEs. From our calculations we find that the dominant contribution to polarizability is from the valence electrons in all superheavy elements. Except for Cn and Og, we observe a decreasing contribution from lighter to superheavy elements from the Breit interaction. For the corrections from the vacuum polarization and self-energy, we observe a trend of increasing contributions with $Z$. From energy calculations, we find that the second-order many-body perturbation theory overestimates the electron correlation energy for all the elements considered in this work.

physics.atom-ph↗

arXiv2020

Phases and collective modes of bosons in a triangular lattice at finite temperature: A cluster mean field study

Motivated by the realization of Bose-Einstein condensates (BEC) in non-cubic lattices, in this work we study the phases and collective excitation of bosons with nearest neighbor interaction in a triangular lattice at finite temperature, using mean field (MF) and cluster mean field (CMF) theory. We compute the finite temperature phase diagram both for hardcore and softcore bosons, as well analyze the effect of correlation arising due to lattice frustration and interaction systematically using CMF method. A semi-analytic estimate of the transition temperatures between different phases are derived within the framework of MF Landau theory, particularly for hardcore bosons. Apart from the usual phases such as density waves (DW) and superfluid (SF), we also characterize different supersolids (SS). These phases and their transitions at finite temperature are identified from the collective modes. The low lying excitations, particularly Goldstone and Higgs modes of the supersolid can be detected in the ongoing cold atom experiments.

cond-mat.quant-gas↗

arXiv2020

Segregated quantum phases of dipolar bosonic mixtures in two-dimensional optical lattices

We identify the quantum phases in a binary mixture of dipolar bosons in two-dimensional optical lattices. Our study is motivated by the recent experimental realization of binary dipolar condensate mixtures of Er-Dy [Phys. Rev. Lett. 121, 213601 (2018)]. We model the system by using the extended two-species Bose-Hubbard model and calculate the ground-state phase diagrams by using mean-field theory. For selected cases we also obtain analytical phase boundaries by using the site-decoupled mean-field theory. For comparison we also examine the phase diagram of two-species Bose-Hubbard model. Our results show that the quantum phases with the long-range intraspecies interaction phase separate with no phase ordering. The introduction of the long-range interspecies interaction modifies the quantum phases of the system. It leads to the emergence of phase-separated quantum phases with phase ordering. The transition from the phase-separated quantum phases without phase ordering to phase ordered ones breaks the inversion symmetry.

cond-mat.quant-gas↗

arXiv2020

Fock-space relativistic coupled-cluster calculation of hyperfine induced $\bf {^1S_0 \rightarrow {^3P^o_0}}$ clock transition in Al$^+$

We have developed an all-particle Fock-space relativistic coupled-cluster method to calculate the properties of two-valence atoms and ions. Using the method we compute the properties associated with hyperfine induced $^1S_0 - ^3P^o_0$ clock transition in Al$^+$. Our result of the $^3P^o_0$ metastable state life time, $20.20 \pm 0.91$ s, is in excellent agreement with the experimental value, $20.60 \pm 1.4$ s [Phys. Rev. Lett. {\bf 98}, 220801 (2007)]. Our studies show that the contributions from the triple excitations, and the corrections from the Breit interaction and QED effects are essential to obtain accurate clock properties in Al$^+$.

physics.atom-ph↗

arXiv2019

Quantum Hall states for $\alpha = 1/3$ in optical lattices

We examine the quantum Hall (QH) states of the optical lattices with square geometry using Bose-Hubbard model (BHM) in presence of artificial gauge field. In particular, we focus on the QH states for the flux value of $\alpha = 1/3$. For this, we use cluster Gutzwiller mean-field (CGMF) theory with cluster sizes of $3\times 2$ and $3\times 3$. We obtain QH states at fillings $\nu = 1/2, 1, 3/2, 2, 5/2$ with the cluster size $3\times 2$ and $\nu = 1/3, 2/3, 1, 4/3, 5/3, 2, 7/3, 8/3$ with $3\times 3$ cluster. Our results show that the geometry of the QH states are sensitive to the cluster sizes. For all the values of $\nu$, the competing superfluid (SF) state is the ground state and QH state is the metastable state.

cond-mat.quant-gas↗

arXiv2019

Quantum phases of tilted dipolar bosons in two-dimensional optical lattice

We consider a minimal model to describe the quantum phases of ultracold dipolar bosons in two-dimensional (2D) square optical lattices. The model is a variation of the extended Bose-Hubbard model and apt to study the quantum phases arising from the variation in the tilt angle $\theta$ of the dipolar bosons. At low tilt angles $0^{\circ}\leqslant\theta\apprle25^{\circ}$, the ground state of the system are phases with checkerboard order, which could be either checkerboard supersolid or checkerboard density wave. For high tilt angles $55^{\circ}\apprge\theta\apprge35^{\circ}$, phases with striped order of supersolid or density wave are preferred. In the intermediate domain $25^{\circ}\apprle\theta\apprle35^{\circ}$ an emulsion or SF phase intervenes the transition between the checkerboard and striped phases. The attractive interaction dominates for $\theta\apprge55^{\circ}$, which renders the system unstable and there is a density collapse. For our studies we use Gutzwiller mean-field theory to obtain the quantum phases and the phase boundaries. In addition, we calculate the phase boundaries between an incompressible and a compressible phase of the system by considering second order perturbation analysis of the mean-field theory. The analytical results, where applicable, are in excellent agreement with the numerical results.

cond-mat.quant-gas↗

arXiv2019

Dynamics of the creation of a rotating Bose-Einstein condensate by two-photon Raman transition using Laguerre-Gaussian pulse

We examine the dynamics associated with the creation of a vortex in a Bose-Einstein condensate (BEC), from another nonrotating BEC using two-photon Raman transition with Gaussian (G) and Laguerre-Gaussian (LG) laser pulses. In particular, we consider BEC of Rb atoms at their hyperfine ground states confined in a quasi two dimensional harmonic trap. Optical dipole potentials created by G and LG laser pulses modify the harmonic trap in such a way that density profiles of the condensates during the Raman transition process depend on the sign of the generated vortex. We investigate the role played by the Raman coupling parameter manifested through dimensionless peak Rabi frequency and intercomponent interaction on the dynamics of the population transfer process and on the final population of the rotating condensate. During the Raman transition process, the two BECs tend to have larger overlap with each other for stronger intercomponent interaction strength.

cond-mat.quant-gas↗

arXiv2019

Supersolid phase of the extended Bose-Hubbard model with an artificial gauge field

We examine the zero and finite temperature phase diagrams of soft-core bosons of the extended Bose-Hubbard model on a square optical lattice. To study various quantum phases and their transitions we employ single-site and cluster Gutzwiller mean-field theory. We have observed that the Mott insulator phase vanishes above a critical value of nearest-neighbour interaction and the supersolid phase occupies a larger region in the phase diagram. We show that the presence of artificial gauge field enlarges the domain of supersolid phase. The finite temperature destroys the crystalline structure of the supersolid phase and thereby favours normal fluid to superfluid phase transition. The presence of an envelope harmonic potential demonstrates coexistence of different phases and at $z~k_{B}T\geqslant V$, thermal energy comparable and higher to the long-range interaction energy, the supersolidity of the system is destroyed.

cond-mat.quant-gas↗

arXiv2019

Electric dipole polarizability of group-IIIA ions using PRCC: Large correlation effects from nonlinear terms

We compute the ground-state electric dipole polarizability of group-IIIA ions using the perturbed relativistic coupled-cluster (PRCC) theory. To account for the relativistic effects and QED corrections, we use the Dirac-Coulomb-Breit Hamiltonian with the corrections from the Uehling potential and the self-energy. The effects of triple excitations are considered perturbatively in the PRCC. Our PRCC results for $\alpha$ are good in agreement with the previous theoretical results for all the ions. From our computations we find that the nonlinear terms in PRCC have significant contributions and must be included to obtain the accurate value of $\alpha$ for group-IIIA ions. For the correction from the Breit interaction, we find that it is largest for Al$^+$ and decreases as we go towards the heavier ions. The corrections from the vacuum polarization and the self-energy increase from lighter to heavier ions.

physics.atom-ph↗