Quasiparticle Self-energy Research Articles

Fate of Quasiparticles at High Temperature in the Correlated Metal Sr_{2}RuO_{4}.

We study the temperature evolution of quasiparticles in the correlated metal Sr_{2}RuO_{4}. Our angle resolved photoemission data show that quasiparticles persist up to temperatures above 200K, far beyond the Fermi liquid regime. Extracting the quasiparticle self-energy, we demonstrate that the quasiparticle residue Z increases with increasing temperature. Quasiparticles eventually disappear on approaching the bad metal state of Sr_{2}RuO_{4} not by losing weight but via excessive broadening from super-Planckian scattering. We further show that the Fermi surface of Sr_{2}RuO_{4}-defined as the loci where the spectral function peaks-deflates with increasing temperature. These findings are in semiquantitative agreement with dynamical mean field theory calculations.

Extracting spinon self-energies from two-dimensional coherent spectroscopy

Two-dimensional coherent spectroscopy (2DCS) is a nonlinear spectroscopy technique capable of identifying whether apparent continua in linear response are made out of multiplets of sharp deconfined quasiparticles. This makes it a potent tool for experimental identification of fractionalized phases. Previous discussions have focused on limits where the quasiparticles in question are infinitely long lived. In this paper we discuss 2DCS in the regime where the fractionalized quasiparticles can themselves decay. We introduce a powerful path-integral-based approach, whereby the computation of nonlinear susceptibilities reduces to an efficient exercise in diagrammatic perturbation theory. We apply this method to compute the 2DCS response of the one-dimensional transverse field Ising model, in the presence of integrability-breaking perturbations. We discuss aspects of the self-energy of the fractionalized quasiparticles that may be extracted via 2DCS, such as the momentum-dependent decay rate.

Efficient full frequency GW for metals using a multipole approach for the dielectric screening

The properties of metallic systems with important and structured excitations at low energies, such as Cu, are challenging to describe with simple models like the plasmon pole approximation (PPA), and more accurate and sometimes prohibitive full frequency approaches are usually required. In this paper we propose a numerical approach to $GW$ calculations on metals that takes into account the frequency dependence of the screening via the multipole approximation (MPA), an accurate and efficient alternative to current full-frequency methods that was recently developed and validated for semiconductors and overcomes several limitations of PPA. We now demonstrate that MPA can be successfully extended to metallic systems by optimizing the frequency sampling for this class of materials and introducing a simple method to include the $\mathbf{q}\to 0$ limit of the intra-band contributions. The good agreement between MPA and full frequency results for the calculations of quasi-particle energies, polarizability, self-energy and spectral functions in different metallic systems confirms the accuracy and computational efficiency of the method. Finally, we discuss the physical interpretation of the MPA poles through a comparison with experimental electron energy loss spectra for Cu.

High-temperature self-energy corrections to x-ray absorption spectra

Effects of finite-temperature quasiparticle self-energy corrections on x-ray absorption spectra are investigated within the finite-temperature quasiparticle local density $GW$ approximation up to temperatures $T$ of order the Fermi temperature. To facilitate the calculations, we parametrize the quasiparticle self-energy using low-order polynomial fits. We show that temperature-driven decrease in the electron lifetime substantially broadens the spectra in the near-edge region with increasing $T$. However, the quasiparticle shift is most strongly modified near the onset of plasmon excitations.

Far-from-Equilibrium Electron-Phonon Interactions in Optically Excited Graphene.

Comprehending far-from-equilibrium many-body interactions is one of the major goals of current ultrafast condensed matter physics research. Here, a particularly interesting but barely understood situation occurs during a strong optical excitation, where the electron and phonon systems can be significantly perturbed and the quasiparticle distributions cannot be described with equilibrium functions. In this work, we use time- and angle-resolved photoelectron spectroscopy to study such far-from-equilibrium many-body interactions for the prototypical material graphene. In accordance with theoretical simulations, we find remarkable transient renormalizations of the quasiparticle self-energy caused by the photoinduced nonequilibrium conditions. These observations can be understood by ultrafast scatterings between nonequilibrium electrons and strongly coupled optical phonons, which signify the crucial role of ultrafast nonequilibrium dynamics on many-body interactions. Our results advance the understanding of many-body physics in extreme conditions, which is important for any endeavor to optically manipulate or create non-equilibrium states of matter.

Correlation-driven electron-hole asymmetry in graphene field effect devices

Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in twisted graphene and moiré heterostructures has spurred interest into whether it stems from single-particle effects or from correlations, which are core to the emergence of intriguing phases in moiré systems. Here, we report an effective way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field-effect devices. As the chemical potential moves from the hole to the electron-doped side, we see an increased strength of electronic correlations manifested by an increase in the band velocity and inverse quasiparticle lifetime. These results suggest that electronic correlations intrinsically drive the electron-hole asymmetry in graphene and by leveraging this asymmetry can provide alternative avenues to generate exotic phases in twisted moiré heterostructures.

Early deconfinement of asymptotically conformal color-superconducting quark matter in neutron stars

We present a relativistic density functional approach to color superconducting quark matter that mimics quark confinement by a fast growth of the quasiparticle selfenergy in the confining region. The approach is shown to be equivalent to a chiral model of quark matter with medium dependent couplings. While the (pseudo)scalar sector of the model is fitted to the vacuum phenomenology of quantum chromodynamics, the strength of interaction in the vector and diquark channels is varied in order to provide the best agreement with the observational constraints on the mass-radius relation and tidal deformability of neutron stars modelled with our approach. In order to recover the conformal behavior of quark matter at asymptotically high densities we introduce a medium dependence of the vector and diquark couplings motivated by the non-perturbative gluon exchange. Our analysis signals that the onset of deconfinement to color superconducting quark matter is likely to occur in neutron stars with masses below 1.0 M⊙.

Momentum-resolved view of highly tunable many-body effects in a graphene/hBN field-effect device

Integrating the carrier tunability of a functional two-dimensional material electronic device with a direct probe of energy- and momentum-resolved electronic excitations is essential to gain insights on how many-body interactions are influenced during device operation. Here, we use micro-focused angle-resolved photoemission in order to analyze many-body interactions in back-gated graphene supported on hexagonal boron nitride. By extracting the doping-dependent quasiparticle dispersion and self-energy, we observe how these interactions renormalize the Dirac cone and impact the electron mobility of our device. Our results are not only limited to a finite energy range around the Fermi level, as in electron transport measurements, but describe interactions on a much wider energy scale, extending beyond the regime of hot carrier excitations.

Extended Drude Model Analysis of the Optical Spectra of Correlated Electron Systems in a d-Wave Superconducting State

Correlation information in strongly correlated electron systems can be obtained using an extended Drude model. An interesting method related to the extended Drude model analysis of superconducting optical data was proposed recently, and it has attracted attention from researchers. This method aims to extract the optical self-energy of quasiparticles (or residual unpaired electrons) from measured optical data in the superconducting state. However, this residual optical self-energy is a partial optical self-energy. The interpretation and significance of this partial optical self-energy is unclear. We investigate this method using a reverse process with simple electron-boson spectral density functions. With our obtained results, we conclude that the residual (or partial) optical self-energy is difficult to interpret because it contains unphysical features, in particular, a negative optical effective mass. The present study clarifies the extended Drude analysis method for superconducting optical data.

Temperature induced shift of electronic band structure in Fe(Se,Te)

FeSe, the simplest iron-based superconductor, reveals a variety of puzzling properties and features that could hold a key for the pairing mechanism in this family of superconductors. In particular, it’s complex electronic band structure differs essentially from the DFT calculated one in a specially way called the “red-blue shift” and, in addition, drifts anomalously with temperature. Here we study this band structure evolution in Fe(Se,Te) crystals and reveal essential downdrift of all the hole-like bands with increasing temperature, which is opposite to the “red-blue shift” expectation. We show that this drift cannot be described by temperature-dependent contribution to quasiparticle self-energy within the Fermi-liquid concept but could result in charge redistribution between the bulk and topological surface states. If such a scenario is confirmed, one can tune the topologically non-trivial bands near the Fermi level with temperature, potentially allowing temperature-induced crossover between different Fermi surface topologies.

Anomalous Conductance Oscillations in the Hybridization Gap of InAs/GaSb Quantum Wells.

We observe the magnetic oscillation of electric conductance in the two-dimensional InAs/GaSb quantum spin Hall insulator. Its insulating bulk origin is unambiguously demonstrated by the antiphase oscillations of the conductance and the resistance. Characteristically, the in-gap oscillation frequency is higher than the Shubnikov-de Haas oscillation close to the conduction band edge in the metallic regime. The temperature dependence shows both thermal activation and smearing effects, which cannot be described by the Lifshitz-Kosevich theory. A two-band Bernevig-Hughes-Zhang model with a large quasiparticle self-energy in the insulating regime is proposed to capture the main properties of the in-gap oscillations.

Variational Approach to Many-Body Problems Incorporating Many-Body Effects at Finite Temperature

We develop a variational approach at finite temperature that incorporates many-body correlation self-consistently. The grand potential is constructed in terms of Green's function expressed by the variational parameters. We apply this formalism to weakly interacting Bose-Einstein condensates to incorporate the dynamical 3/2-body processes, which are considered important in the dynamical properties. The processes lower the free energy below the mean-field Hartree--Fock--Bogoliubov's value in the same way as a previous zero-temperature formalism. From our numerical results, the pair creation or annihilation processes neglected in the Popov--Shohno approximation are enhanced, particularly in the long wavelength region, owing to the many-body effects. Because the 3/2-body correlations give a finite contribution to the self-energy of quasiparticles, they may change the microscopic properties qualitatively, even in the weak-coupling region.

Coulomb-hole and screened exchange in the electron self-energy at finite temperature

We present a finite temperature generalization of the COHSEX approximation to the quasiparticle electron self-energy, with explicit comparisons of the static and dynamic screened exchange (SEX) and Coulomb-hole (COH) contributions for the homogeneous electron gas. Consistent with the zero-temperature behavior, we show that the static SEX approximation agrees well with the fully dynamic SEX at all temperatures and densities. In contrast, dynamic corrections to the COH approximation are always significant, especially for high energy unoccupied states. Notably the COHSEX quasiparticle self-energy is complex valued at finite temperature even at the Fermi level.

Accurate Determination of the Quasiparticle and Scaling Properties Surrounding the Quantum Critical Point of Disordered Three-Dimensional Dirac Semimetals.

Exploiting the enabling power of the Lanczos method in momentum space, we determine accurately the quasiparticle and scaling properties of disordered three-dimensional Dirac semimetals surrounding the quantum critical point separating the semimetal and diffusive metal regimes. We unveil that the imaginary part of the quasiparticle self-energy obeys a common power law before, at, and after the quantum phase transition, but the power law is nonuniversal, whose exponent is dependent on the disorder strength. More intriguingly, whereas a common power law is also found for the real part of the self-energy before and after the phase transition, a distinctly different behavior is identified at the critical point, characterized by the existence of a nonanalytic logarithmic singularity. This nonanalytical correction serves as the very basis for the unusual power-law behaviors of the quasiparticles and many other physical properties surrounding the quantum critical point. Our approach also allows the ready and reliable determination of the scaling properties of the correlation length and dynamical exponents. We further show that the central findings are valid for both uncorrelated and correlated disorder distributions and should be directly comparable with future experimental observations.

Impurity-Induced Smearing of the Spin Resonance Peak in Fe-Based Superconductors

The spin resonance peak in the iron-based superconductors is observed in inelastic neutron scattering experiments and agrees well with predicted results for the extended s-wave ($s_\pm$) gap symmetry. On the basis of four-band and three-orbital tight binding models we study the effect of nonmagnetic disorder on the resonance peak. Spin susceptibility is calculated in the random phase approximation with the renormalization of the quasiparticle self-energy due to the impurity scattering in the static Born approximation. We find that the spin resonance becomes broader with the increase of disorder and its energy shifts to higher frequencies. For the same amount of disorder the spin response in the $s_\pm$ state is still distinct from that of the $s_{++}$ state.

Quasiparticle band gap of organic-inorganic hybrid perovskites: Crystal structure, spin-orbit coupling, and self-energy effects

The quasiparticle band gap is one of the most important materials properties for photovoltaic applications. Often the band gap of a photovoltaic material is determined (and can be controlled) by various factors, complicating predictive materials optimization. An in-depth understanding of how these factors affect the size of the gap will provide valuable guidance for new materials discovery. Here we report a comprehensive investigation on the band gap formation mechanism in organic-inorganic hybrid perovskites by decoupling various contributing factors which ultimately determine their electronic structure and quasiparticle band gap. Major factors, namely, quasiparticle self-energy, spin-orbit coupling, and structural distortions due to the presence of organic molecules, and their influences on the quasiparticle band structure of organic-inorganic hybrid perovskites are illustrated. We find that although methylammonium cations do not contribute directly to the electronic states near band edges, they play an important role in defining the band gap by introducing structural distortions and controlling the overall lattice constants. The spin-orbit coupling effects drastically reduce the electron and hole effective masses in these systems, which is beneficial for high carrier mobilities and small exciton binding energies.

Inelastic carrier lifetime in a coupled graphene/electron-phonon system: Role of plasmon-phonon coupling

We calculate the inelastic scattering rates and the hot electron inelastic mean free paths for both monolayer and bilayer graphene on a polar substrate. We study the quasiparticle self-energy by taking into account both electron-electron and electron-surface optical (SO) phonon interactions. In this calculation the leading order dynamic screening approximation (G$_0$W approximation) is used to obtain the quasiparticle self-energy by treating electrons and phonons on an equal footing. We find that the strong coupling between the SO phonon and plasmon leads to a new decay channel for the quasiparticle through the emission of the coupled mode, and gives rise to an abrupt increase in the scattering rate, which is absent in the uncoupled system. In monolayer graphene a single jump in the scattering rate occurs, arising from the emission of the low energy branch of the coupled plasmon-phonon modes. In bilayer graphene the emission of both low and high energy branches of the coupled modes contributes to the scattering rate and gives rise to two abrupt changes in the scattering rate. The jumps in the scattering rate can be potentially used in the hot electron device such as switching devices and oscillators.

Fractional power-law behavior and its origin in iron-chalcogenide and ruthenate superconductors: Insights from first-principles calculations

We perform realistic first-principles calculations of iron chalcogenides and ruthenate-based materials to identify experimental signatures of Hund's-coupling-induced correlations in these systems. We find that FeTe and K${}_{x}$Fe${}_{2\ensuremath{-}y}$Se${}_{2}$ display unusual orbital-dependent fractional power-law behavior in their quasiparticle self-energy and optical conductivity, a phenomenon first identified in SrRuO${}_{3}$. Strong incoherence in the paramagnetic state of these materials results in electronic states hidden to angle-resolved photoemission spectroscopy which reemerge at low temperatures. We identify the effective low-energy Hamiltonian describing these systems and show that these anomalies are not controlled by the proximity to a quantum critical point but result from coexistence of fast quantum mechanical orbital fluctuations and slow spin fluctuations.

Electronic structure and the local electroneutrality level of SiC polytypes from quasiparticle calculations within the GW approximation

The most important interband transitions and the local charge neutrality level (CNL) in silicon carbide polytypes 3C-SiC and nH-SiC (n = 2–8) are calculated using the GW approximation for the self energy of quasiparticles. The calculated values of band gap Eg for various polytypes fall in the range 2.38 eV (3C-SiC)-3.33 eV (2H-SiC) and are very close to the experimental data (2.42–3.33 eV). The quasiparticle corrections to Eg determined by DFT-LDA calculations (about 1.1 eV) are almost independent of the crystal structure of a polytype. The positions of CNL in various polytypes are found to be almost the same, and the change in CNL correlates weakly with the change in Eg, which increases with the hexagonality of SiC. The calculated value of CNL varies from 1.74 eV in polytype 3C-SiC to 1.81 eV in 4H-SiC.

Optical self-energy in graphene due to correlations

In highly correlated systems one can define an optical self-energy in analogy to its quasiparticle (QP) self-energy counterpart. This quantity provides useful information on the nature of the excitations involved in inelastic scattering processes. Here we calculate the self-energy of the intraband optical transitions in graphene originating in the electron–electron interaction (EEI) as well as electron–phonon interaction (EPI). Although optics involves an average over all momenta (k) of the charge carriers, the structure in the optical self-energy is nevertheless found to mirror mainly that of the corresponding quasiparticles for k equal to or near the Fermi momentum kF. Consequently, plasmaronic structures which are associated with momenta near the Dirac point at k = 0 are not important in the intraband optical response. While the structure of the electron–phonon interaction (EPI) reflects the sharp peaks of the phonon density of states, the excitation spectrum associated with the electron–electron interaction is in comparison structureless and flat and extends over an energy range which scales linearly with the value of the chemical potential. We introduce a method whereby detailed quantitative information on such excitation spectra can be extracted from optical data. Modulations seen on the edge of the interband optical conductivity as it rises towards its universal background value are traced to structure in the quasiparticle self-energies around kF of the lower Dirac cone associated with the occupied states.