Self-energy Effects Research Articles

Relativistic calculations of M-shell photoionization and X-ray production cross-sections for Hg at 5.96keV excitation energy

Abstract In this work we calculate photoionization and X-ray production cross-sections (XPCS) of M-shell vacancies in Hg at an incident photon energy of 5.96 keV (low energy X-rays of 55Fe radioactive source) using the Dirac–Fock method. Calculations are performed in single configuration approach with the Breit interaction and some vacuum polarization corrections included in the self-consistent method. Higher-order retardation corrections, self-energy and other vacuum polarization effects were also included as perturbations. Fluorescence and Coster–Kronig yields necessary to derive XPCS were obtained in a previous work using the exact same approach.

Dynamical spin response in cuprate superconductors from low-energy to high-energy

Within the framework of the kinetic energy driven superconducting mechanism, the dynamical spin response of cuprate superconductors is studied from low-energy to high-energy. The spin self-energy is evaluated explicitly in terms of the collective charge carrier modes in the particle–hole and particle–particle channels, and employed to calculate the dynamical spin structure factor. Our results show the existence of damped but well-defined dispersive spin excitations in the whole doping phase diagram . In particular, the low-energy spin excitations in the superconducting-state have an hour-glass-shaped dispersion, with commensurate resonance that appears in the superconducting-state only , while the low-energy incommensurate spin fluctuations can persist into the normal-state. The high-energy spin excitations in the superconducting-state on the other hand retain roughly constant energy as a function of doping, with spectral weights and dispersion relations comparable to those in the corresponding normal-state. The theory also shows that the unusual magnetic correlations in cuprate superconductors can be ascribed purely to the spin self-energy effects which arise directly from the charge carrier–spin interaction in the kinetic energy of the system. • We provide an explanation to dynamical spin response in cuprate superconductors. • Low-energy spin excitations have an hour-glass-shaped dispersion. • Commensurate resonance appears in the superconducting-state only. • High-energy spin excitations are similar to those found in the parent compound.

Ultrafast quenching of electron-boson interaction and superconducting gap in a cuprate superconductor.

Ultrafast spectroscopy is an emerging technique with great promise in the study of quantum materials, as it makes it possible to track similarities and correlations that are not evident near equilibrium. Thus far, however, the way in which these processes modify the electron self-energy--a fundamental quantity describing many-body interactions in a material--has been little discussed. Here we use time- and angle-resolved photoemission to directly measure the ultrafast response of self-energy to near-infrared photoexcitation in high-temperature cuprate superconductor. Below the critical temperature of the superconductor, ultrafast excitations trigger a synchronous decrease of electron self-energy and superconducting gap, culminating in a saturation in the weakening of electron-boson coupling when the superconducting gap is fully quenched. In contrast, electron-boson coupling is unresponsive to ultrafast excitations above the critical temperature of the superconductor and in the metallic state of a related material. These findings open a new pathway for studying transient self-energy and correlation effects in solids.

Impurity cyclotron resonance of anomalous Dirac electrons in graphene

We have investigated a new feature of impurity cyclotron resonances common to various localized potentials of graphene. A localized potential can interact with a magnetic field in an unexpected way in graphene. It can lead to formation of anomalous boundstates that have a sharp peak with a width R in the probability density inside the potential and a broad peak of size magnetic length ℓ outside the potential. We investigate optical matrix elements of anomalous states and find that they are unusually small and depend sensitively on the magnetic field. The effect of many-body interactions on their optical conductivity is investigated using a self-consistent time-dependent Hartree–Fock approach. For a completely filled Landau level we find that an excited electron-hole pair, originating from the optical transition between two anomalous impurity states, is nearly uncorrelated with other electron-hole pairs, although it displays substantial exchange self-energy effects. This absence of correlation is a consequence of a small vertex correction in comparison to the difference between renormalized transition energies computed within the one electron-hole pair approximation. However, an excited electron-hole pair originating from the optical transition between a normal and an anomalous impurity state can be substantially correlated with other electron-hole states with a significant optical strength.

Near mass degeneracy in the scalar meson sector: Implications forB(s)0*andB(s)1′mesons

The empirical observation of near degeneracy of scalar mesons above 1 GeV, namely, the mass of the strange-flavor scalar meson is similar to that of the non-strange one, is at variance with the naive expectation of the quark model. Qualitatively, the approximate mass degeneracy can be understood as a consequence of self-energy effects due to strong coupled channels which will push down the mass of the heavy scalar meson in the strange sector more than that in the non-strange partner. However, it works in the conventional model without heavy quark expansion, but not in the approach of heavy meson chiral perturbation theory as mass degeneracy and the physical masses of $D_{s0}^*$ and $D_0^*$ cannot be accounted for simultaneously. In the heavy quark limit, near mass degeneracy observed in the scalar charm sector will imply the same phenomenon in the $B$ system. We have the prediction $M_{B_0^*}\approx M_{B_{s0}^*}\approx 5715\,{\rm MeV}+\delta\Delta_S$ based on heavy quark symmetry and the leading-order QCD correction, where $\delta\Delta_S$ arises from $1/m_Q$ corrections. A crude estimate indicates that $\delta\Delta_S$ is of order $-35$ MeV or less. We stress that the closeness of $B_{s0}^*$ and $B_0^*$ masses implied by heavy quark symmetry is not spoiled by $1/m_Q$ or QCD corrections. The mass-shift effect on $K_0^*(1430)$ is discussed.

Self-energy effects in cuprates and the dome-shaped behavior of the superconducting critical temperature

Hole doped cuprates show a superconducting critical temperature $T_c$ which follows an universal dome-shaped behavior as function of doping. It is believed that the origin of superconductivity in cuprates is entangled with the physics of the pseudogap phase. An open discussion is whether the source of superconductivity is the same that causes the pseudogap properties. The $t$-$J$ model treated in large-N expansion shows $d$-wave superconductivity triggered by non-retarded interactions, and an instability of the paramagnetic state to a flux phase or $d$-wave charge density wave ($d$-CDW) state. In this paper we show that self-energy effects near $d$-CDW instability may lead to a dome-shaped behavior of $T_c$. In addition, it is also shown that these self-energy contributions may describe several properties observed in the pseudogap phase. In this picture, although fluctuations responsible for the pseudogap properties leads to a dome-shaped behavior, they are not involved in pairing which is mainly non-retarded.

Many-body effects in semiconducting single-wall silicon nanotubes

The electronic and optical properties of semiconducting silicon nanotubes (SiNTs) are studied by means of the many-body Green’s function method, i.e., GW approximation and Bethe–Salpeter equation. In these studied structures, i.e., (4,4), (6,6) and (10,0) SiNTs, self-energy effects are enhanced giving rise to large quasi-particle (QP) band gaps due to the confinement effect. The strong electron−electron (e−e) correlations broaden the band gaps of the studied SiNTs from 0.65, 0.28 and 0.05 eV at DFT level to 1.9, 1.22 and 0.79 eV at GW level. The Coulomb electron−hole (e−h) interactions significantly modify optical absorption properties obtained at noninteracting-particle level with the formation of bound excitons with considerable binding energies (of the order of 1 eV) assigned: the binding energies of the armchair (4,4), (6,6) and zigzag (10,0) SiNTs are 0.92, 1.1 and 0.6 eV, respectively. Results in this work are useful for understanding the physics and applications in silicon-based nanoscale device components.

Optical Spectrum ofMoS2: Many-Body Effects and Diversity of Exciton States

We present first-principles calculations of the optical response of monolayer molybdenum disulfide employing the GW-Bethe-Salpeter equation (GW-BSE) approach including self-energy, excitonic, and electron-phonon effects. We show that monolayer MoS2 possesses a large and diverse number of strongly bound excitonic states with novel k-space characteristics that were not previously seen experimentally or theoretically. The absorption spectrum is shown to be dominated by excitonic states with a binding energy close to 1 eV and by strong electron-phonon broadening in the visible to ultraviolet range. Our results explain recent experimental measurements and resolve inconsistencies between previous GW-BSE calculations.

Reluctance of a neutral nanoparticle to enter a charged pore

We consider the translocation of a neutral (uncharged) nanoparticle through a pore in a thin membrane with constant surface charge density. If the concomitant Debye screening layer is sufficiently thin, the resulting forces experienced by the particle on its way through the pore are negligible. But when the Debye length becomes comparable to the pore diameter, the particle encounters a quite significant potential barrier while approaching and entering the pore, and symmetrically upon exiting the pore. The main reason is an increasing pressure, which acts on the particle when it intrudes into the counter ion cloud of the Debye screening layer. In case the polarizability of the particle is different (usually smaller) than that of the ambient fluid, a second, much smaller contribution to the potential barrier is due to self-energy effects. Our numerical treatment of the problem is complemented by analytical approximations for sufficiently long cylindrical particles and pores, which agree very well with the numerics.

Doping dependence of correlation effects in K1 − x Fe2 − y Se2 superconductors: LDA’ + DMFT investigation

We present detailed LDA'+DMFT investigation of doping dependence of correlation effects in novel K{1-x}Fe{2-y}Se2 superconductor. Calculations were performed at four different hole doping levels, starting from hypothetical stoichiometric composition with total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons towards the case of 26.52, which corresponds to chemical composition K{0.76}Fe{1.72}Se2 studied in recent ARPES experiments. In general case the increase of hole doping leads to quasiparticle bands in wide energy window +/-2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation induced compression of Fe-3d LDA' bandwidths stays almost the same and of the order of ~1.3 for all hole concentrations. However close to the Fermi level situation is more complicated. Here in the energy interval from -1.0 eV to 0.4 eV the bare Fe-3d LDA' bands are compressed by significantly larger renormalization factors up to 5 with hole doping increase, while the value of Coulomb interaction remains the same. This fact manifests the increase of correlation effects with hole doping in K{1-x}Fe{2-y}Se2 system. Moreover in contrast to typical pnictides K{1-x}Fe{2-y}Se2 does not have well defined quasiparticle bands on the Fermi leves but pseudogap like "dark" region instead. We also find that with the growth of hole doping Fe-3d orbitals of various symmetries are affected by correlations in a different way in different parts of Brillouin zone. To illustrate this we determine quasiparticle mass renormalization factors and energy shifts, which transform the bare Fe-3d LDA' bands of various symmetries into LDA'+DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy dependent self-energy effects and can be used to analyze the available ARPES data.

Strong many-body effects in silicene-based structures

Silicene, which is the silicon equivalent of carbon-based graphene and shares some unique properties with graphene, has been attracting more and more attention since its successful synthesis. Using Green's function perturbation theory, many-body effects in silicene, hydrogenated silicene (silicane), fluorinated silicene (fluorosilicene), as well as armchair silicene nanoribbons (ASiNRs) are studied. Optical resonances in silicene have been aroused by excitonic effects: The $\ensuremath{\pi}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ excitonic resonance at 1.23 eV is contributed by the characteristic dispersion of Dirac fermions, while the one at 3.75 eV is due to the $\ensuremath{\sigma}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{*}$ transition. Hydrogenation or fluorination of silicene removes the conductivity at the Dirac point and causes band-gap opening. In addition to the remarkable self-energy effects, optical absorption properties of silicane, fluorosilicene, and ASiNRs are dominated by strong excitonic effects with formation of bound excitons with considerable binding energies.

Ab Initio Electronic Gaps of Ge Nanodots: The Role of Self-Energy Effects

Nanostructuring of a material leads to enormous effects on its excited state properties. This study, through the application of different state-of-the-art ab initio theoretical tools, investigates the effect of size on the electronic gap of germanium nanocrystals highlighting similarities and differences with respect to equivalent silicon nanostructures. We performed both GW and ΔSCF calculations for the determination of their electronic structure. While it is known that ΔSCF corrections to the Kohn–Sham gap vanish for extended systems, the two approaches were expected to be equivalent in the limit of small clusters. However, it has been recently found that for hydrogenated Si clusters the ΔSCF gaps are systematically smaller than the GW ones, while the opposite is true for Ag clusters. In this work we find that the GW gaps are larger than the ΔSCF ones for all the Ge dots, with the exception of the smallest one. Such crossing between the ΔSCF and the GW gap values was not expected and has never been obse...

Self-energy effects in electronic Raman spectra of doped cuprates due to magnetic fluctuations

We present results for magnetic excitations in doped copper oxides using the random phase approximation and itinerant electrons. In the [1,0] direction the observed excitations resemble dispersive quasi-particles both in the normal and superconducting state similar as in recent resonant inelastic X-ray scattering (RIXS) experiments. In the [1,1] direction the excitations form, except for the critical region near the antiferromagnetic wave vector ${\bf Q}=(\pi,\pi)$, only very broad continua. Using the obtained spin propagators we calculate electron self-energies and their effects on electronic Raman spectra. We show that the recently observed additional peak at about twice the pair breaking in B$_{1g}$ symmetry below T$_c$ in HgBa$_2$CuO$_{4+\delta}$ can be explained as a self-energy effect where a broken Cooper pair and a magnetic excitation appear as final states. The absence of this peak in B$_{2g}$ symmetry, which probes mainly electrons near the nodal direction, is explained by their small self-energies compared to those in the antinodal direction.

Derivation of static low-energy effective models by anab initiodownfolding method without double counting of Coulomb correlations: Application to SrVO3, FeSe, and FeTe

Derivation of low-energy effective models by a partial trace summation of the electronic degrees of freedom far away from the Fermi level, called downfolding, is reexamined. We propose an improved formalism free from the double counting of electron correlation in the low-energy degrees of freedom. In this approach, the exchange-correlation energy in the local-density approximation (LDA) is replaced with the $GW$ self-energy; herewith its low-energy part associated with the double counting is subtracted. Moreover, in our formalism, the frequency dependence of the effective parameter is renormalized into the static one. We apply the formalism to SrVO${}_{3}$ as well as to two iron-based superconductors, FeSe and FeTe. The resultant bandwidths of the effective models are nearly the same as those of the previous downfolding formalism because of striking cancellations between an increase arising from the exclusion of the low-energy correlation and a shrinking arising from the renormalization of the frequency dependence. In the nondegenerate multiband materials such as FeSe and FeTe, the momentum dependent self-energy effects yield substantial modifications of the band structures and relative shifts of orbital-energy levels of the effective models, which may explain the stability of the bicollinear antiferromagnetic phase in FeTe as well as the experimental absence of the antiferromagnetic phase in FeSe.

Optical response of stoichiometric and congruent lithium niobate from first-principles calculations

The frequency-dependent dielectric function and the second-order polarizability tensor of ferroelectric ${\mathrm{LiNbO}}_{3}$ are calculated from first principles. The calculations are based on the electronic structure obtained from density-functional theory. The subsequent application of the $GW$ approximation to account for quasiparticle effects and the solution of the Bethe-Salpeter equation for the stoichiometric material yield a dielectric function that slightly overestimates the absorption onset and the oscillator strength in comparison with experimental measurements. Calculations at the level of the independent-particle approximation indicate that these deficiencies are, at least, partially related to the neglect of intrinsic defects typical for the congruent material. The second-order polarizability calculated within the independent-particle approximation predicts strong nonlinear coefficients for photon energies above 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear optical response leads to a better agreement with experiments. The intrinsic defects of congruent samples reduce the optical nonlinearities, in particular, for the 21 and 31 tensor components, further improving the agreement between experiments and theory.

Real Space Green's Function Approach to Resonant Inelastic X-Ray Scattering and HERFD XAS

We present a theory of resonant inelastic x-ray scattering (RIXS) and high energy resolution fluorescence detected (HERFD) x-ray absorption spectra (XAS) based on the real space multiple scattering Green's function formalism. The Kramers-Heisenberg formula is transformed into an expression involving Single particle Green's functions. Simplifying assumptions give the RIXS cross section in terms of a convolution of an effective absorption spectrum with the x-ray emission spectrum. Given the RIXS cross section, calculations of the HERFD XAS are also possible. Self-energy effects are calculated using an efficient many-pole model. Further many-body effects can be treated via a convolution with an effective spectral function. The approach is implemented as an extension of the FEFF9 code. Calculated results are compared with experimental RIXS of WO2.

Effects of momentum-dependent self-energy in the electronic structure of correlated materials

We study how the $\mathbf{k}$ dependence in the self-energy affects the quasiparticle band structure and one-particle spectral functions. It is known that, in electron-gas-like materials, the self-energy depends significantly on $\mathbf{k}$ and there is a strong cancellation between the $\mathbf{k}$ dependence and the energy dependence of the self-energy. Analysis of the $GW$ self-energy reveals that, even in correlated materials with narrow bands, such as ${\mathrm{SrVO}}_{3}$, the self-energy significantly depends on $\mathbf{k}$. When the nonlocal effect is neglected, the quasiparticle band structure is over-renormalized, yielding too large mass enhancement compared to the case of k-dependent self-energy. The present result suggests that partial cancellation between the frequency dependence and the k dependence in the self-energy is important when discussing the quasiparticle band structure of correlated materials.

Approximate recalculation of the [formula omitted] contribution to the self-energy effect on hydrogenic states with a multipole expansion

A contribution of virtual electron states with large wave numbers to the self-energy of an electron bound in the weak Coulomb field is analyzed in the context of the evaluation method suggested in the previous paper. The contribution is of the order α(Zα)5. The same value for this contribution is found here as the one found in the previous calculations using different evaluation methods. When we add the remaining terms of the order α(Zα)5 to the calculation of the self-energy effect in hydrogen-like ions presented in the previous paper we find a very good agreement with numerical evaluations. The relative difference between present and numerical evaluations ranges from 2 parts in 106 for Z=1 up to 6 parts in 104 for Z=10.

Linear and nonlinear optical response of LiNbO3 calculated from first principles

The dielectric function and second-harmonic generation spectrum of ferroelectric LiNbO(3) are calculated from first principles. The calculations are based on the electronic structure obtained within density functional theory. The use of the GW approach to account for quasiparticle effects and the subsequent solution of the Bethe-Salpeter equation lead to a dielectric function in excellent agreement with measured data. The second harmonic generation spectrum calculated within the independent (quasi) particle approximation predicts strong nonlinear coefficients for photon energies above about 1.5 eV. The comparison with measured data suggests that the inclusion of self-energy effects in the nonlinear response improves the agreement with experiment.

Pole expansion of self-energy and interaction effect for topological insulators

We study effect of interactions on time-reversal-invariant topological insulators. Their topological indices are expressed by interacting Green's functions. Under the local self-energy approximation, we connect topological index and surface states of an interacting system to an auxiliary noninteracting system, whose Hamiltonian is related to the pole-expansions of the local self-energy. This finding greatly simplifies the calculation of interacting topological indices and gives an noninteracting pictorial description of interaction driven topological phase transitions. Our results also bridge studies of the correlated topological insulating materials with the practical dynamical-mean-field-theory calculations.