Many-body Calculations Research Articles

Engineering indefinitely long-lived localization in cavity-QED arrays

By exploiting the nonlinear nature of the Jaynes Cumming's interaction, one can get photon population trapping in cavity-QED arrays. However, the unavoidable dissipative effects in a realistic system would destroy the self-trapped state by continuous photon leakage. To circumvent this issue, we show that a careful engineering of drive, dissipation and Hamiltonian results in achieving indefinitely sustained self-trapping. We show that the intricate interplay between drive, dissipation, and light-matter interaction results in requiring an optimal window of drive strengths in order to achieve such non-trivial steady states. We treat the two-cavity and four-cavity cases using exact open quantum many-body calculations. Additionally, in the semiclassical limit we scale up the system to a long 1-D chain and demonstrate localization de-localization transition in a driven-dissipative system. Although, our analysis is performed keeping cavity-QED systems in mind, our work is applicable to other driven-dissipative systems where nonlinearity plays a defining role.

Relativistic MR-MP Energy Levels for K-like Fe

Abstract Relativistic Multi-Reference Møller-Plesset (MR-MP) many-body perturbation theory calculations are reported for the ground and excited states of Fe viii. Energies are provided for states coming from the configurations (l = s, p, d, and f) with , p ≤ 3, and q ≤ 1. The MR-MP results were compared with the recommended observed values in the CHIANTI-v8 database and with previous ab-initio calculations. The MR-MP calculations reproduce 90% of the available observed values within 3500 cm−1. The previous accurate ab-initio calculations reproduce only 50% of the observed values within 3500 cm−1. The electronic structure of Fe viii is very complex and our MR-MP results show that accurate calculations require the inclusion of not only 3s 2–3d 2, 3p 2–3d 2, and 3s3p–3d 2 promotions but also of all electron correlations via perturbation theory.

Electrically controlled dielectric band gap engineering in a two-dimensional semiconductor

This work proposes and investigates a new physical concept of electrically controlled dielectric band gap engineering in a two-dimensional (2D) semiconductor. The authors show, using first-principles many-body calculations, that the band gap of a 2D semiconductor can be varied by several hundred meV's by varying the doping concentration in a supporting graphene sheet as illustrated in the accompanying image. This provides a method for continuous and dynamical $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0.333em}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ tuning of the band gap. Importantly, the band gap adjustment is achieved via nonlocal Coulomb interactions and does not involve changes in the atomic structure, or even the shape or hybridization pattern, of the wave functions of the 2D semiconductor.

Inelastic electron tunneling spectroscopy for probing strongly correlated many-body systems by scanning tunneling microscopy

We present an extension of the tunneling theory for scanning tunneling microcopy (STM) to include different types of vibrational-electronic couplings responsible for inelastic contributions to the tunnel current in the strong-coupling limit. It allows for a better understanding of more complex scanning tunneling spectra of molecules on a metallic substrate in separating elastic and inelastic contributions. The starting point is the exact solution of the spectral functions for the electronic active local orbitals in the absence of the STM tip. This includes electron-phonon coupling in the coupled system comprising the molecule and the substrate to arbitrary order including the anti-adiabatic strong coupling regime as well as the Kondo effect on a free electron spin of the molecule. The tunneling current is derived in second order of the tunneling matrix element which is expanded in powers of the relevant vibrational displacements. We use the results of an ab-initio calculation for the single-particle electronic properties as an adapted material-specific input for a numerical renormalization group approach for accurately determining the electronic properties of a NTCDA molecule on Ag(111) as a challenging sample system for our theory. Our analysis shows that the mismatch between the ab-initio many-body calculation of the tunnel current in the absence of any electron-phonon coupling to the experiment scanning tunneling spectra can be resolved by including two mechanisms: (i) a strong unconventional Holstein term on the local substrate orbital leads to reduction of the Kondo temperature and (ii) a different electron-vibrational coupling to the tunneling matrix element is responsible for inelastic steps in the $dI/dV$ curve at finite frequencies.

Do we know how to count powers in pionless and pionful effective field theory?

In this article I summarize recent progress in the effective field theory approach to low energy nuclear systems, with a focus on the power counting issue. In the pionless sector, where the power counting is quite well understood at the nucleon–nucleon (NN) level, I discuss some recent developments toward few- and many-body calculations. In the pionful sector, I focus on the actively debated issue of power counting in the NN sector and some recent developments toward a model-independent NN interaction. Finally, the scenario that the power counting might depend on the number of particles is discussed.

Transition energy measurements of the X-ray lines of neon-like europium

We report the measurements of the n = 3 → n = 2 L-shell X-ray transitions of neon-like Eu53+. These include all seven allowed electric dipole (E1) transitions, five electric quadrupole (E2) transitions, and one magnetic quadrupole (M2) transition. The measurements were carried out at the EBIT-I electron beam ion trap facility at Livermore using an X-ray calorimeter. The measured values agree with many-body perturbation theory calculations within the experimental uncertainties. This includes the transitions that involve a 2s electron and that are, therefore, strongly affected by quantum electrodynamical effects.

Kosterlitz-Thouless melting of magnetic order in the triangular quantum Ising material TmMgGaO4

Frustrated magnets hold the promise of material realizations of exotic phases of quantum matter, but direct comparisons of unbiased model calculations with experimental measurements remain very challenging. Here we design and implement a protocol of employing many-body computation methodologies for accurate model calculations—of both equilibrium and dynamical properties—for a frustrated rare-earth magnet TmMgGaO4 (TMGO), which explains the corresponding experimental findings. Our results confirm TMGO is an ideal realization of triangular-lattice Ising model with an intrinsic transverse field. The magnetic order of TMGO is predicted to melt through two successive Kosterlitz–Thouless (KT) phase transitions, with a floating KT phase in between. The dynamical spectra calculated suggest remnant images of a vanishing magnetic stripe order that represent vortex–antivortex pairs, resembling rotons in a superfluid helium film. TMGO therefore constitutes a rare quantum magnet for realizing KT physics, and we further propose experimental detection of its intriguing properties.

Deep learning-enhanced variational Monte Carlo method for quantum many-body physics

Artificial neural networks have been successfully incorporated into variational Monte Carlo method (VMC) to study quantum many-body systems. However, there have been few systematic studies of exploring quantum many-body physics using deep neural networks (DNNs), despite of the tremendous success enjoyed by DNNs in many other areas in recent years. One main challenge of implementing DNN in VMC is the inefficiency of optimizing such networks with large number of parameters. We introduce an importance sampling gradient optimization (ISGO) algorithm, which significantly improves the computational speed of training DNN in VMC. We design an efficient convolutional DNN architecture to compute the ground state of a one-dimensional (1D) SU($N$) spin chain. Our numerical results of the ground-state energies with up to 16 layers of DNN show excellent agreement with the Bethe-Ansatz exact solution. Furthermore, we also calculate the loop correlation function using the wave function obtained. Our work demonstrates the feasibility and advantages of applying DNNs to numerical quantum many-body calculations.

Spin dependent transmission of nickelocene-Cu contacts probed with shot noise

The current $I$ through nickelocene molecules and its noise are measured with a low temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, an abrupt jump to contact and a smooth transition. These data along with conductance spectra ($dI/dV$) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-1/2 state that is Kondo screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of $2eI$ but no influence of the Kondo effect or spin excitations are resolved. The noise can be described in the Landauer picture in terms of spin-polarized transmission of $\approx$35% through two degenerate $d_\pi$-orbitals of the Nickelocene molecule.

Mahan excitons in room-temperature methylammonium lead bromide perovskites

In a seminal paper, Mahan predicted that excitonic bound states can still exist in a semiconductor at electron-hole densities above the insulator-to-metal Mott transition. However, no clear evidence for this exotic quasiparticle, dubbed Mahan exciton, exists to date at room temperature. In this work, we combine ultrafast broadband optical spectroscopy and advanced many-body calculations to reveal that organic-inorganic lead-bromide perovskites host Mahan excitons at room temperature. Persistence of the Wannier exciton peak and the enhancement of the above-bandgap absorption are observed at all achievable photoexcitation densities, well above the Mott density. This is supported by the solution of the semiconductor Bloch equations, which confirms that no sharp transition between the insulating and conductive phase occurs. Our results demonstrate the robustness of the bound states in a regime where exciton dissociation is otherwise expected, and offer promising perspectives in fundamental physics and in room-temperature applications involving high densities of charge carriers.

RELATIVISTIC CALCULATION OF THE HYPERFINE STRUCTURE PARAMETERS FOR COMPLEX ATOMS WITHIN MANY-BODY PERTURBATION THEORY

The hyperfine structure parameters and electric quadrupole moment of the 201Hg mercury isotope the Mn atom are estimated within the relativistic many-body perturbation theory formalism with a correct and effective taking into account the exchange-correlation, relativistic, nuclear and radiative corrections. Analysis of the data shows that an account of the interelectron correlation effects is crucial in the calculation of the hyperfine structure parameters. The fundamental reason of physically reasonable agreement between theory and experiment is connected with the correct taking into account the inter-electron correlation effects, nuclear (due to the finite size of a nucleus), relativistic and radiative corrections. The key difference between the results of the relativistic Hartree-Fock Dirac-Fock and many-body perturbation theory methods calculations is explained by using the different schemes of taking into account the inter-electron correlations as well as nuclear and radiative ones.

Sparse sampling and tensor network representation of two-particle Green's functions

Many-body calculations at the two-particle level require a compact representation of two-particle Green’s functions. In this paper, we introduce a sparse sampling scheme in the Matsubara frequency domain as well as a tensor network representation for two-particle Green’s functions. The sparse sampling is based on the intermediate representation basis and allows an accurate extraction of the generalized susceptibility from a reduced set of Matsubara frequencies. The tensor network representation provides a system independent way to compress the information carried by two-particle Green’s functions. We demonstrate efficiency of the present scheme for calculations of static and dynamic susceptibilities in single- and two-band Hubbard models in the framework of dynamical mean-field theory.

Human-Machine Collaboration in Quantum Many-Body Problems

Machine-learning techniques, in combination with variational wave functions devised by human ingenuity, can greatly improve the precision of quantum many-body calculations.

Transition Dipole Moments of n = 1, 2, and 3 Perovskite Quantum Wells from the Optical Stark Effect and Many-Body Perturbation Theory

Metal halide perovskite quantum wells (PQWs) are quantum and dielectrically confined materials exhibiting strongly bound excitons. The exciton transition dipole moment dictates absorption strength and influences interwell coupling in dipole-mediated energy transfer - a process that influences the performance of PQW optoelectronic devices. Here we use transient reflectance spectroscopy with circularly polarized laser pulses to investigate the optical Stark effect in dimensionally pure single crystals of n = 1, 2 and 3 Ruddlesden-Popper PQWs. From these measurements, we extract in-plane transition dipole moments of 11.1 (± 0.4), 9.6 (± 0.6) and 13.0 (± 0.8) D for n = 1, 2 and 3, respectively. We corroborate our experimental results with density functional and many-body perturbation theory calculations, finding that the nature of bandedge orbitals and exciton wavefunction delocalization depends on the PQW "odd-even" symmetry. This accounts for the non-monotonic relationship between transition dipole moment and PQW dimensionality in the n = 1 - 3 range.

Density-dependent NN interaction from subsubleading chiral 3N forces: Intermediate-range contributions

From the subsubleading chiral three-nucleon force [intermediate-range contributions, published in Phys. Rev. C\,87, 054007 (2013)] a density-dependent NN-interaction $V_\text{med}$ is derived in isospin-symmetric nuclear matter. Following the division of the pertinent 3N-diagrams into two-pion-one-pion exchange topology and ring topology, one evaluates for these all selfclosings and concatenations of nucleon-lines to an in-medium loop. In the case of the $2\pi 1\pi$-exchange topology, the momentum- and $k_f$-dependent potentials associated with the isospin-operators ($1$ and $\vec\tau_1 \!\cdot\! \vec\tau_2$) and five independent spin-structures require at most one numerical integration. For the more challenging (concatenations of the) ring diagrams proportional to $c_{1,2,3,4}$, one ends up with regularized double-integrals $\int_0^\lambda dr\,r \int_0^{\pi/2} d\psi$ from which the $\lambda^2$-divergence has been subtracted and the logarithmic piece $\sim \ln (m_\pi/\lambda)$ is isolated. The derived semi-analytical results are most helpful to implement the subsubleading chiral 3N-forces into nuclear many-body calculations.

Green's function coupled cluster simulation of the near-valence ionizations of DNA-fragments.

Accurate description of the ionization process in DNA is crucial to the understanding of the DNA damage under exposure to ionizing radiation and the exploration of the potential application of DNA strands in nanoelectronics. In this work, by employing our recently developed Green's function coupled-cluster library on supercomputing facilities, we have studied the spectral functions of several guanine-cytosine (G-C) base pair structures ([G-C]n, n = 1-3) for the first time in a relatively broad near-valence regime ([-25.0, -5.0] eV) in the coupled-cluster with singles and doubles level. Our focus is to give a preliminary many-body coupled-cluster understanding and guideline of the vertical ionization energy (VIE), spectral profile, and ionization feature changes of these systems as the system size expands in this near-valence regime. The results show that, as the system size expands, even though the lowest VIEs keep decreasing, the changes of spectral function profile and the relative peak positions get unexpectedly smaller. Further analysis of the ionized states associated with the most intensive peak in the spectral functions reveals non-negligible |2h, 1p⟩'s in the ionized wave functions of the considered G-C base pair systems. The leading |2h, 1p⟩'s associated with the main ionizations from the cytosine part of the G-C base pairs feature a transition from the intra-base-pair cytosine π → π* excitation to the inter-base-pair electron excitation as the size of G-C base pairs expands, which also indicates the minimum quantum region in the many-body calculations of DNA systems.

Absolute cross sections for electron capture and loss of O+ in water molecule

SynopsisAbsolute cross sections for single and double electron loss and single and double electron capture for O+ projectiles colliding with water vapor have been measured. In addition, we have performed many-body clas-sical trajectory Monte Carlo calculations, where the O+ projectile was either one or more particle, depending on the model used. The water target is modeled as two virtual particles. The calculated cross sections are in agree-ment with the experimental data.

Many-body calculations of two-photon, two color matrix elements for attosecond delays

SynopsisInterferometric measurements based on the combination of extreme ultraviolet (XUV) attosecond pulse trains with weak infrared laser fields, have been successfully used to study time-resolved and recently angle-resolved photoemission in various atomic systems. These measurements rely on quantum interference between electron wavepackets created by absorption of XUV radiation, with further absorption or emission of laser photons. Here we present many-body calculations of the full two-photon process.

Electronic and excitonic properties of ultrathin (In,Ga)N layers: the role of alloy and monolayer width fluctuations.

We present an atomistic theoretical analysis of the electronic and excitonic properties of ultrathin, monolayer thick wurtzite (In,Ga)N embedded in GaN. Our microscopic investigation reveals that (i) alloy fluctuations within the monolayer lead to carrier localization effects that dominate the electronic and optical properties of these ultrathin systems and that (ii) excitonic binding energies in these structures exceed the thermal energy at room temperature, enabling excitonic effects to persist even at elevated temperatures. Our theoretical findings are consistent with, and provide an explanation for, literature experimental observations of (i) broad photoluminescence linewidth and (ii) excitonic effects contributing to the radiative recombination process at elevated temperatures. When accounting for small structural inhomogeneities, such as local thickness fluctuations of one monolayer, "indirect" excitons may be found, with electrons and holes independently localized in different spatial positions. This result also provides further arguments for experimentally observed effects such as (i) non-exponential decay curves in time dependent photoluminescence spectra and (ii) the "S"-shape temperature dependence of the photoluminescence peak energies. Overall, our results provide fundamental understanding, on an atomistic level, of the electronic and optical properties of ultrathin, quasi 2D (In,Ga)N monolayers embedded in GaN, and offer guidance for the tailoring of their properties for potential future device applications.

BSHF: A program to solve the Hartree–Fock equations for arbitrary central potentials using a B-spline basis

BSHF solves the Hartree–Fock equations in a B-spline basis for atoms, negatively charged ions, and systems of N electrons in arbitrary central potentials. In the B-spline basis the Hartree–Fock integro-differential equations are reduced to a computationally simpler eigenvalue problem. As well as solving this for the ground-state electronic structure self-consistently, the program can calculate discrete and/or continuum excited states of an additional electron or positron in the field of the frozen-target N-electron ground state. It thus provides an effectively complete orthonormal basis that can be used for higher-order many-body theory calculations. Robust and efficient convergence in the self-consistent iterations is achieved by a number of strategies, including by gradually increasing the strength of the electron–electron interaction by scaling the electron charge from a reduced value to its true value. The functionality and operation of the program is described in a tutorial style example. Program summaryProgram Title: BSHFProgram Files doi:http://dx.doi.org/10.17632/fj3y6c58dy.1Code Ocean Capsule:https://doi.org/10.24433/CO.1226817.v2Licensing provisions: GPLv3Programming language: Fortran 90.External routines/libraries: LAPACK.Nature of problem: Self-consistent solution of electronic structure for atoms and electrons in arbitrary central potentials in the Hartree–Fock approximation.Solution method: A B-spline basis is employed that transforms the Hartree–Fock integro-differential equations to a computationally simpler eigenvalue problem. The eigenvalue problem is solved iteratively until self-consistency is achieved.Unusual or notable features:1. Robust and efficient convergence in the self-consistent iterations is achieved by gradually increasing the value of the electron charge from a reduced value to its true value, i.e., increasing the strength of the electron–electron interaction. In this way all orbitals are calculated simultaneously at every iteration of the self-consistency loop, i.e., without the need to successively fill the occupied shells from the core to valence (as most other Hartree–Fock codes require for convergence).2. In addition to atoms and negative ions, the program solves the Hartree–Fock equations for systems of N electrons confined in an arbitrary central potential specified by the user: thus the program can be used to e.g., calculate the structure of electrons confined in harmonic potentials, which are known to approximate the electron gas.3. In addition to calculating the ground state of the N-electron system, the program calculates the discrete and/or continuum excited states for an additional electron or positron in the field of the ‘frozen’ N-electron target. It thus provides an orthonormal basis that can be used as input for many-body theory calculations.Restrictions: The program solves non-relativistic Hartree–Fock equations for spherically symmetric systems only (in modelling systems other than atoms, e.g., harmonically confined electron gas or jellium-type models of clusters, relativistic effects can often be negligible and a non-relativistic treatment is perfectly suitable). For open-shell atoms the central-field approximation is used, i.e., the potential is angularly averaged.