Quasiparticle Approximation Research Articles

All-out band structure and band offset ab initio predictions for AlN/GaN and AlP/GaP interfaces

We studied the electronic properties of the AlP/GaP and AlN/GaN interfaces and their cubic constituents. The work is developed using approximate quasiparticle approach LDA-1/2 method. First, we provide a detailed calculation of bulk AlN, AlP, GaN, and GaP. In addition to accurate results for fundamental band gap, we also obtained accurate electronic transition levels along valence and conduction bands profiles, compared to experiments and theoretical results using GW and hybrid functionals methods. For the conduction (valence) band offset we obtained 0.27 eV (0.43 eV) for AlP/GaP and 1.47 eV (0.36 eV) for AlN/GaN interfaces, in good agreement with experimental results.

Single-Layer Group-III Monochalcogenide Photocatalysts for Water Splitting

The recent synthesis of single-layer GaS and GaSe opens the question of stability for other single-layer group-III monochalcogenides (MX, M = Ga and In, X = S, Se, and Te) and how the dimension reduction affects the properties of these materials. Using a first-principles design approach, we determine that the single-layer group-III monochalcogenides exhibit low formation energies and are suitable for photocatalytic water splitting. First, density-functional calculations using a van der Waals functional reveal that the monochalcogenides have formation energies similar to that of single-layer MoS2, implying the ease of mechanically extracting single-layer monochalcogenides from their layered bulk counterparts. Next, calculations using a hybrid density functional and the quasiparticle many-body G0W0 approximation determine the conduction and valence band edge positions. Comparing the band edge positions with the redox potentials of water shows that single-layer monochalcogenides are potential photocatalysts ...

Systematic approach toΔL=1processes in thermal leptogenesis

In this work we study the contribution to leptogenesis from Delta L=1 decay and scattering processes mediated by the Higgs with quarks in the initial and final states using the formalism of non-equilibrium quantum field theory. Starting from fundamental equations for correlators of the quantum fields we derive quantum-corrected Boltzmann and rate equations for the total lepton asymmetry improved in that they include quantum-statistical effects and medium corrections to the quasiparticle properties. To compute the collision term we take into account one- and two-loop contributions to the lepton self-energy and use the extended quasiparticle approximation for the Higgs two-point function. The resulting CP-violating and washout reaction densities are numerically compared to the conventional ones.

Perturbative non-equilibrium thermal field theory to all orders in gradient expansion

We present a new perturbative formulation of non-equilibrium thermal field theory, based upon non-homogeneous free propagators and time-dependent vertices. The resulting time-dependent diagrammatic perturbation series are free of pinch singularities without the need for quasi-particle approximation or effective resummation of finite widths. After arriving at a physically meaningful definition of particle number densities, we derive master time evolution equations for statistical distribution functions, which are valid to all orders in perturbation theory and to all orders in a gradient expansion. For a scalar model, we perform a perturbative loopwise truncation of these evolution equations, whilst still capturing fast transient behaviour, which is found to be dominated by energy-violating processes, leading to the non-Markovian evolution of memory effects.

Bosonization from Quasiparticle Representation and Effects of Nucleon-Number Conservation

We discuss microscopic ground of the IBM from the quasiparticle viewpoint, paying attention to the effects of nucleon-number conservation. The OAI mapping is revisited, showing for a single-j system how the number conservation effects are taken into account. The number-conserving quasiparticle approximation exploited by Li is applied to incorporate the number conservation effects. The significance of the number conservation is confirmed via the tests in the Ginocchio model and in a 2j system. An effective boson number is evaluated based on the number-conserving quasiparticle method. It is shown that the nucleon-number conservation effects are well incorporated in the IBM, even for systems with non-degenerate single-particle orbits.

Ab-initio study of Mg-doped InN(0001) surface

We study the incorporation of Mg atoms into the InN(0001) surface. Energies and atomic geometries are described within density functional theory, while the electronic structure is investigated by an approximate quasiparticle method that yields a gap value of 0.7 eV for bulk InN. The formation of substitutional Mg is energetically favored in the surface layer. The surface electronic structure is less influenced by Mg-derived states. The Fermi level is pinned by In-derived surface states. With increasing depth of Mg beneath the surface the Fermi-level position moves toward the valence band top, suggesting formation of holes and, hence, p-doping of Mg in bulk-like layers.

Mott transition of excitons in GaAs-GaAlAs quantum wells

We investigate the breakup of bound electron–hole pairs, known as Mott transition of excitons, in GaAs-GaAlAs quantum wells with increasing excitation, comparing two different theoretical approaches. Firstly, a thermodynamic approach is used to investigate the ionization equilibrium between electrons, holes and excitons, where the abrupt jump of the degree of ionization from 0 to 1 indicates the Mott density. It is extended to a self-consistent quasi-particle approximation (QPA) for the carrier properties, including dynamical screening of the Coulomb interaction between carriers. Secondly, a spectral approach based on the semiconductor Bloch equations within linear optical response is used, considering the quasi-particle (QP) properties of carriers and the dynamical screening between electron–hole pairs. While the first is effectively a one-particle approach, in the second the whole two-particle spectrum is analyzed. Within the thermodynamic approach, a simple criterion for the Mott transition can be given: namely, if the sum of chemical potentials of carriers, reflecting the effective shrinkage of the band edge, crosses the exciton energy with increasing excitation. We demonstrate that this simple picture cannot be maintained in the two-particle approach. Here, a compact quantity, which describes the behavior of the band edge, does not exist. In fact, the behavior of the single states in the spectrum is generated by the interplay of dynamical screening in the interband self-energy and the effective interaction of the electron–hole pairs. Moreover, the band edge cannot be clearly resolved, since it is merged with excited exciton states (e.g. 2s state), which show up only for densities far below the Mott density. Instead of a Mott density, only a density range can be given, where the Mott transition appears. We demonstrate that a small damping as a prerequisite for the validation of the extended QPA in the thermodynamic approach breaks down, analyzing (i) the dephasing processes with increasing excitation, (ii) the strong increase of the excitonic linewidth and (iii) comparing with the lifetime of carriers in the QP description.

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.

Energetics and approximate quasiparticle electronic structure of low-index surfaces of SnO2

The geometry and energetics of the unreconstructed tin- and oxygen-terminated (100), (010), and (110) surfaces, the tin-terminated (111) surface, and the stoichiometric (001) surface of rutile-SnO${}_{2}$ are investigated. Total energies and relaxed atomic geometries are calculated within density functional theory using the local density approximation (LDA). We conclude from these results that the (110) and (100) surfaces are most stable. Their termination depends on the experimental situation: While under oxygen-rich preparation conditions the oxygen termination is preferred, reduced surfaces are more likely to occur in the oxygen-poor limit. In addition, electronic band structures and densities of states are calculated using a recently developed approximate quasiparticle approach, the LDA-$\frac{1}{2}$ method. Except for the SnO-terminated (110) surface, all other faces are found to be insulating and O- or Sn-derived surface states appear in the projected bulk fundamental gap. While the surface barrier heights vary by more than 2 eV with orientation and termination, the ionization energies tend to the smallest values for the energetically favored surfaces.

First and second harmonic generation of the XAl2Se4 (X=Zn,Cd,Hg) defect chalcopyrite compounds

The chemical bonding of the ZnAl 2 Se 4 , CdAl 2 Se 4 and HgAl 2 Se 4 defect chalcopyrites has been studied in the framework of the quantum theory of atoms in molecules (AIM). The GW quasi-particle approximation is used to correct the DFT-underestimation of energy gap, and as a consequence the linear and nonlinear optical properties are significantly enhanced. The second harmonic generation (SHG) displays certain dependence with the ionicity degree decrease through the dependency of the SHG on the band gap. The occurrence of the AIM saddle point is characterized and some clarifying features in relationship with the density topology are exposed, which enable to understand the relation with the second harmonic generation effect.

Effect of correlations on electronic structure and transport across (001) Fe/MgO/Fe junctions

We developed a parametrization of transmission probability that reliably captures essential elements of the tunneling process in magnetic tunnel junctions. The electronic structure of the Fe/MgO/Fe junction is calculated within the quasiparticle self-consistent GW approximation and used to evaluate transmission probability across (001) Fe/MgO interfaces. The transmission has a peak at $+0.12$ V, in excellent agreement with recent differential conductance measurements for electrodes with antiparallel spin. These findings confirm that the observed current-voltage characteristics are intrinsic to well-defined (001) Fe/MgO interfaces, in contrast to previous predictions based on the local spin-density approximation, and also that many-body effects are important to realistically describe electron transport across well-defined metal-insulator interfaces.

Electronic structure of Pu and Am metals by self-consistent relativisticGWmethod

We present the results of calculations for Pu and Am performed using an implementation of a self-consistent fully relativistic $GW$ method. The key feature of our scheme is to evaluate polarizability and self-energy in real space and Matsubara's time. We compare our $GW$ results with the calculations using local density and quasiparticle approximations and also with scalar-relativistic calculations. We highlight the importance of both relativistic effects and effects of self-consistency in $GW$ calculation for Am and Pu. We also have found that $GW$ enhances the hybridization between $5f$ and $6d$ states in Pu, suggesting that the physics of Pu should not be understood based only on $5f$ electrons.

Analysis of dilepton production inAu+Aucollisions atsNN=200GeV within the parton-hadron-string dynamics transport approach

We address dilepton production in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s{}_{NN}}=200$ GeV by employing the parton-hadron-string dynamics (PHSD) off-shell transport approach. Within the PHSD, one goes beyond the quasiparticle approximation by solving generalized transport equations on the basis of the off-shell Kadanoff-Baym equations for the Green's functions in the phase-space representation. The approach consistently describes the full evolution of a relativistic heavy-ion collision, from the initial hard scatterings and string formation, through the dynamical deconfinement phase transition to the quark-gluon plasma (QGP) as well as hadronization, to the subsequent interactions in the hadronic phase. With partons described in the PHSD by the dynamical quasiparticle model (DQPM)---matched to reproduce lattice QCD results in thermodynamic equilibrium---we calculate, in particular, the dilepton radiation from partonic interactions through the reactions $q\overline{q}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}$, $q\overline{q}\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}+g$, and $qg\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}q$ ($\overline{q}g\ensuremath{\rightarrow}{\ensuremath{\gamma}}^{*}\overline{q}$) in the early stage of relativistic heavy-ion collisions. By comparing our results to the data from the PHENIX Collaboration, we study the relative importance of different dilepton production mechanisms and point out the regions in phase space where partonic channels are dominant. Furthermore, explicit predictions are presented for dileptons within the acceptance of the STAR detector system and compared to the preliminary data.

Flavour-coherent propagators and Feynman rules: covariant cQPA formulation

We present a simplified and generalized derivation of the flavour-coherent propagators and Feynman rules for the fermionic kinetic theory based on coherent quasiparticle approximation (cQPA) [1–7]. The new formulation immediately reveals the composite nature of the cQPA Wightman function as a product of two spectral functions and an effective two-point interaction vertex, which contains all quantum statistical and coherence information. We extend our previous work to the case of nonzero dispersive self-energy, which leads to a broader range of applications. By this scheme, we derive flavoured kinetic equations for local 2-point functions $$ S_{\text{k}}^{{ <, > }} $$ (t, t), which are reminiscent of the equations of motion for the density matrix. We emphasize that in our approach all the interaction terms are derived from first principles of nonequilibrium quantum field theory.

Flavoured quantum Boltzmann equations from cQPA

We develop a Boltzmann-type quantum transport theory for interacting fermion and scalar fields including both flavour and particle-antiparticle mixing. Our formalism is based on the coherent quasiparticle approximation (cQPA) for the 2-point correlation functions, whose extended phase-space structure contains new spectral shells for flavour- and particle-antiparticle coherence. We derive explicit cQPA propagators and Feynman rules for the transport theory. In particular the nontrivial Wightman functions can be written as composite operators $\sim {\cal A} F {\cal A}$, which generalize the usual Kadanoff-Baym ansatz. Our numerical results show that particle-antiparticle coherence can strongly influence CP-violating flavour mixing even for relatively slowly-varying backgrounds. Thus, unlike recently suggested, these correlations cannot be neglected when studying asymmetry generation due to time-varying mass transition, for example in electroweak-type baryogenesis models. Finally, we show that the cQPA coherence solutions are directly related to squeezed states in the more familiar operator formalism.

Dielectric function spectra and critical-point energies of Cu2ZnSnSe4 from 0.5 to 9.0 eV

We present dielectric function ɛ = ɛ1 + iɛ2 spectra and critical-point energies of Cu2ZnSnSe4 determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu2ZnSnSe4 thin film. Ellipsometric data are analyzed by the multilayer model and the ɛ spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the ɛ spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified.

Leading-order infrared quantum chromodynamics in Coulomb gauge

A truncation scheme for the Dyson-Schwinger equations of quantum chromodynamics in Coulomb gauge within the first order formalism is presented. The truncation is based on an Ansatz for the Coulomb kernel occurring in the action. Results at leading loop order and in the infrared are discussed for both the Yang-Mills and quark sectors. It is found that the resulting equations for the static gluon and quark propagators agree with those derived in a quasi-particle approximation to the canonical Hamiltonian approach. Moreover, a connection to the heavy quark limit is established. The equations are analyzed numerically and it is seen that in both the gluonic and quark sectors, a nontrivial dynamical infrared mass scale emerges.

Lattice dynamics and elastic properties of the4felectron system: CeN

The electronic structure, structural stability, and lattice dynamics of cerium mononitride are investigated using ab initiodensity-functional methods involving an effective potential derived from the generalized gradient approximation and without special treatment for the $4f$ states. The $4f$ states are hence allowed to hop from site to site, without an on-site Hubbard $U$, and contribute to the bonding, in a picture often referred to as itinerant. It is argued that this picture is appropriate for CeN at low temperatures, while the anomalous thermal expansion observed at elevated temperatures indicates entropy-driven localization of the Ce $f$ electrons, similar to the behavior of elemental cerium. The elastic constants are predicted from the total energy variation of strained crystals and are found to be large, typical for nitrides. The phonon dispersions are calculated showing no soft modes, and the Gr\"uneisen parameter behaves smoothly. The electronic structure is also calculated using the quasiparticle self-consistent GW approximation (where G denotes the Green's function and W denotes the screened interaction). The Fermi surface of CeN is dominated by large egg-shaped electron sheets centered on the $X$ points, which stem from the $p$-$f$ mixing around the $X$ point. In contrast, assuming localized $f$ electrons leads to a semimetallic picture with small band overlaps around $X$.

Quasiparticle band structure of Zn-IV-N2compounds

Electronic energy-band structures of the Zn-IV-N${}_{2}$ compounds, with IV equal to Si, Ge, and Sn calculated in the quasiparticle self-consistent GW approximation and using the full-potential linearized muffin-tin orbital approach, are presented. A comparison is made with local-density approximation results. The bands near the gap are fitted to an effective Kohn-Luttinger-type Hamiltonian appropriate for the orthorhombic symmetry, and conduction-band effective masses are presented. Exciton binding energies and zero-point motion corrections to the gaps are estimated. While ZnSiN${}_{2}$ is found to be an indirect gap semiconductor, ZnGeN${}_{2}$ and ZnSnN${}_{2}$ are direct gap semiconductors. The gaps range from the orange-red to deep UV. The valence-band maximum is split in three levels of different symmetry, even in the absence of spin-orbit coupling, and should show transitions to the conduction band, each for a separate polarization. Spin-orbit effects are found to be surprisingly small, indicating almost exact compensation of the N-$2p$ and Zn-$3d$ contributions.

Selfconsistent evaluation of charm and charmonium in the quark-gluon plasma

A selfconsistent calculation of heavy-quark (HQ) and quarkonium properties in the quark-gluon plasma (QGP) is conducted to quantify flavor transport and color screening in the medium. The main tool is a thermodynamic T-matrix approach to compute HQ and quarkonium spectral functions in both scattering and bound-state regimes. The T-matrix, in turn, is employed to calculate HQ selfenergies which are implemented into spectral functions beyond the quasiparticle approximation. Charmonium spectral functions are used to evaluate Eulcidean-time correlation functions, which are compared to results from thermal lattice QCD. The comparisons are performed in various hadronic channels, including zero-mode contributions consistently accounting for finite charm-quark width effects. The zero modes are closely related to the charm-quark number susceptibility, which is also compared to existing lattice ‘data’. Both the susceptibility and the heavy-light quark T-matrix are applied to calculate the thermal charm-quark relaxation rate, or, equivalently, the charm diffusion constant in the QGP. Implications of our findings in the HQ sector for the viscosity-to-entropy-density ratio of the QGP are briefly discussed.