Quasiparticle Peak Research Articles

Quasiparticle dispersions and lifetimes in the normal state of the BCS-BEC crossover

We compute the spectral density in the normal phase of an interacting homogenous Fermi gas using a T-matrix approximation. We fit the quasiparticle peaks of the spectral density to BCS-like dispersion relations, and extract estimates of a "pseudo-gap" energy scale and an effective Fermi-wavevector as a function of interaction strength. We find that the effective Fermi-wavevector of the quasiparticles vanishes when the inverse scattering length exceeds some positive threshold. We also find that near unitarity the quasiparticle lifetimes, estimated from the widths of the peaks in the spectral density, approach values on the order of the inverse Fermi-energy. These results are consistent with the "breakdown of Fermi liquid theory" observed in recent experiments.

Novel P-T Phase Diagram of the Multiorbital Mott Insulator Sr2VO4.

The electrical and optical properties of the Mott insulator Sr2VO4 are investigated under high pressure on a phase pure polycrystalline sample. The system undergoes a pressure-driven insulator to metal transition (IMT) with a crossover between 20 and 24 GPa. The effect of pressure on the thermally driven electronic changes resulting from spin-orbital ordering transitions is studied. A multiorbital analysis of the low frequency optical conductivity spectra suggests a bandwidth-controlled and orbital selective nature of the Mott IMT transition. Dramatic enhancement of the low energy spectral weight in the high pressure correlated metallic phase is explained in terms of the formation of a quasiparticle peak in the spectral function of the narrow and degenerate d(yz,zx) orbitals. Our results overall establish a novel electronic phase diagram of tetragonal Sr2VO4.

Comparative analysis of models for theα−γphase transition in cerium: A DFT+DMFT study using Wannier orbitals

We clarify the orbital mechanism of the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ transition in cerium. First we built an $sdf$ Wannier orbital basis to describe the electronic structure of cerium. Second, we use this basis to study the relative role of several orbital hoppings upon compression of cerium. Third, we use DFT + DMFT calculations to quantify the impact of these hoppings on electronic structure. Our conclusion is that upon compression of $\ensuremath{\gamma}$ cerium, the change of hybridization is due to both interatomic $ff$ and $fd$ hopping integrals. In particular, neglecting $ff$ hoppings leads to an important renormalization of both the hybridization and the quasiparticle peak. Thus, neither the Kondo volume collapse nor the Mott transition model are sufficient to describe the isostructural transition in cerium.

Bands, resonances, edge singularities and excitons in core level spectroscopy investigated within the dynamical mean-field theory

Using a recently developed impurity solver we exemplify how dynamical mean-field theory captures band excitations, resonances, edge singularities and excitons in core level x-ray absorption spectroscopy (XAS) and core level photo electron spectroscopy (cPES) on metals, correlated metals and Mott insulators. Comparing XAS at different values of the core-valence interaction shows how the quasiparticle peak in the absence of core-valence interactions evolves into a resonance of similar shape, but different origin. Whereas XAS is rather insensitive to the metal insulator transition, cPES can be used, due to nonlocal screening, to measure the amount of local charge fluctuation.

Fate of quasiparticles in the superconducting state

Quasiparticle properties in the superconducting state are masked by the superfluid and are not directly accessible to infrared spectroscopy. We show how one can use a Kramers--Kronig transformation to separate the quasiparticle from superfluid response and extract intrinsic quasiparticle properties in the superconducting state. We also address the issue of a narrow quasiparticle peak observed in microwave measurements, and demonstrate how it can be combined with infrared measurements to obtain unified picture of electrodynamic properties of cuprate superconductors.

Quasi-particle spectrum in trilayer graphene: Role of onsite coulomb interaction and interlayer coupling

Stacking dependent quasi-particle spectrum and density of states (DOS) in trilayer (ABC-, ABA- and AAA-stacked) graphene are analyzed using mean-field Green׳s function equations of motion method. Interlayer coupling (t1) is found to be responsible for the splitting of quasi-particle peaks in each stacking order. Coulomb interaction suppresses the trilayer splitting and generates a finite gap at Fermi level in ABC- while a tiny gap in ABA-stacked trilayer graphene. Influence of t⊥ is prominent for AAA-stacking as compared to ABC- and ABA-stacking orders. The theoretically obtained quasi-particle energies and DOS has been viewed in terms of recent angle resolved photoemission spectroscopic (ARPES) and scanning tunneling microscopic (STM) data available on these systems.

Many-body Bose systems and the hard-sphere model: dynamic properties from the weak to the strong interaction regime

We obtain ab-initio estimations of the dynamic structure factor, S(q,ω), of Bose gases at zero temperature. More precisely, we use the Genetic Inversion via Falsification of Theories (GIFT) algorithm to perform analytic continuations of imaginary time correlation functions computed via an exact Path Integral projector method. Using the hard-sphere potential to model the two-body interactions between the atoms, we compute S(q,ω) changing the gas parameter from the dilute regime (na3 = 10−4) up to the density corresponding to superfluid 4He at equilibrium (na3 = 0.2138). With increasing density, we observe the emergence of a broad multiphonon contribution accompanying the quasiparticle peak and a crossover of the dispersion of elementary excitations from a Bogoliubov-like spectrum to a phonon-maxon- roton curve. Apart from the low wave vector region, for na3 = 0.2138 the energy-momentum dispersion relation and the static density response function, χ(q), turns out to be in good agreement with the superfluid 4He experimental data at equilibrium density.

Weakly damped acoustic plasmon mode in transition metal dichalcogenides with Zeeman splitting

We analyze the effect of a strong Zeeman field on the spectrum of collective excitations of monolayer transition metal dichalcogenides. The combination of the Dresselhaus type spin orbit coupling and an external Zeeman field result in the lifting of the valley degeneracy in the valence band of these crystals. We show that this lifting of the valley degeneracy manifests in the appearance of an additional plasmon mode with linear in wavenumber dispersion along with the standard square root in wavenumber mode. Despite this novel mode being subject to the Landau damping, it corresponds to a well defined quasiparticle peak in the spectral function of the electron gas.

Electronic excitation spectra of the five-orbital Anderson impurity model: From the atomic limit to itinerant atomic magnetism

We study the competition of Coulomb interaction and hybridization effects in the five-orbital Anderson impurity model by means of continuous time quantum Monte Carlo, exact diagonalization, and Hartree Fock calculations. The dependence of the electronic excitation spectra and thermodynamic ground state properties on hybridization strength and the form of the Coulomb interaction is systematically investigated for impurity occupation number $N \approx 6$. With increasing hybridization strength, a Kondo resonance emerges, broadens and merges with some of the upper and lower Hubbard peaks. Concomitantly, there is an increase of charge fluctuations at the impurity site. In contrast to the single orbital model, some atomic multiplet peaks and exchange split satellites persist despite strong charge fluctuations. We find that Hund's coupling leads to a state that may be characterized as an itinerant single atom magnet. As the filling is increased, the magnetic moment decreases, but the spin freezing phenomenon persists up to $N\approx 8$. When the hybridization is weak, the positions of atomic ionization peaks are rather sensitive to shifts of the impurity on-site energies. This allows to distinguish atomic ionization peaks from quasiparticle peaks or satellites in the electronic excitation spectra. On the methodological side we show that a comparison between the spectra obtained from Monte Carlo and exact diagonalization calculations is possible if the charge fluctuations are properly adjusted.

Electronic spectrum of trilayer graphene

Present work deals with the analysis of the single particle electronic spectral function in trilayer (ABC-, ABA- and AAA-stacked) graphene. Tight binding Hamiltonian containing intralayer nearest-neighbor and next-nearest neighbor hopping along-with the interlayer coupling parameter within two triangular sub-lattice approach for trilayer graphene has been employed. The expression of single particle spectral functions A(kw) is obtained within mean-field Green’s function equations of motion approach. Spectral function at Γ, M and K points of the Brillouin zone has been numerically computed. It is pointed out that the nature of electronic states at different points of Brillouin zone is found to be influenced by stacking order and Coulomb interactions. At Γ and M points, a trilayer splitting is predicted while at K point a bilayer splitting effect is observed due to crossing of two bands (at K point). Interlayer coupling ( $$ t_{ \bot } $$ ) is found to be responsible for the splitting of quasi-particle peaks at each point of Brillouin zone. The influence of $$ t_{ \bot } $$ in trilayer graphene is prominent for AAA-stacking compared to ABC- and ABA-stacking. On the other hand, onsite Coulomb interaction reduces the trilayer splitting effect into bilayer splitting at Γ and M points of Brillouin zone and bilayer splitting into single peak spectral function at K point with a shifting of the peak away from Fermi level.

Evolution from incoherent to coherent electronic states and its implications for superconductivity inFeTe1−xSex

We have performed systematic angle-resolved photoemission spectroscopy (ARPES) of iron-chalcogenide superconductor FeTe1-xSex to elucidate the electronic states relevant to the superconductivity. While the Fermi-surface shape is nearly independent of x, we found that the ARPES spectral line shape shows prominent x dependence. A broad ARPES spectrum characterized by a small quasiparticle weight at x = 0, indicative of incoherent electronic states, becomes progressively sharper with increasing x, and a well-defined quasiparticle peak appears around x = 0.45 where bulk superconductivity is realized. The present result suggests the evolution from incoherent to coherent electronic states and its close relationship to the emergence of superconductivity.

Vector meson quasinormal modes in a finite-temperature AdS/QCD model

We study the spectrum of vector mesons in a finite temperature plasma. The plasma is holographically described by a black hole AdS/QCD model. We compute the boundary retarded Green's function using AdS/CFT prescriptions. The corresponding thermal spectral functions show quasiparticle peaks at low temperatures. Then we calculate the quasinormal modes of vector mesons in the soft-wall black hole geometry and analyse their temperature and momentum dependences.

Multiple satellites in materials with complex plasmon spectra: From graphite to graphene

The photoemission spectrum of graphite is still debated. To help resolve this issue, we present photoemission measurements at high photon energy and analyze the results using a Green's function approach that takes into account the full complexity of the loss spectrum. Our measured data show multiple satellite replicas. We demonstrate that these satellites are of intrinsic origin, enhanced by extrinsic losses. The dominating satellite is due to the $\ensuremath{\pi}+\ensuremath{\sigma}$ plasmon of graphite, whereas the $\ensuremath{\pi}$ plasmon creates a tail on the high-binding energy side of the quasiparticle peak. The interplay between the two plasmons leads to energy shifts, broadening, and additional peaks in the satellite spectrum. We also predict the spectral changes in the transition from graphite towards graphene.

Low-temperature evolution of the spectral weight of a spin-up carrier moving in a ferromagnetic background

We derive the lowest-temperature correction to the self-energy of a spin-up particle injected in a ferromagnetic background. The background is modeled with both Heisenberg and Ising Hamiltonians so that differences due to gapless vs. gapped magnons can be understood. Beside the expected thermal broadening of the quasiparticle peak as it becomes a resonance inside a continuum, we also find that spectral weight is transferred to regions lying outside this continuum. We explain the origin of this spectral weight transfer and its low-temperature evolution.

Dynamical structure factor of the triangular-lattice antiferromagnet

We elucidate the role of magnon interaction and spontaneous decays in the spin dynamics of the triangular-lattice Heisenberg antiferromagnet by calculating its dynamical structure factor within the spin-wave theory. Explicit theoretical results for neutron-scattering intensity are provided for spins S=1/2 and S=3/2. The dynamical structure factor exhibits unconventional features such as quasiparticle peaks broadened by decays, non-Lorentzian lineshapes, and significant spectral weight redistribution to the two-magnon continuum. This rich excitation spectrum illustrates the complexity of the triangular-lattice antiferromagnet and provides distinctive qualitative and quantitative fingerprints for experimental observation of decay-induced magnon dynamics.

Phonon structure in dispersion curves and density of states of massive Dirac fermions

Dirac fermions exist in many solid state systems including graphene, silicene and other two dimensional membranes such as are found in group VI dichalcogenides, as well as on the surface of some insulators where such states are protected by topology. Coupling of those fermions to phonons introduces new structures in their dispersion curves and, in the case of massive Dirac fermions, can shift and modify the gap. We show how these changes present in angular-resolved photoemission spectroscopy of the dressed charge carrier dispersion curves and scanning tunneling microscopy measurements of their density of states. In particular we focus on the region around the band gap. In this region the charge carrier spectral density no longer consists of a dominant quasiparticle peak and a smaller incoherent phonon related background. The quasiparticle picture has broken down and this leads to important modification in both dispersion curves and density of states.

Empirical relationship between x-ray photoemission spectra and electrical conductivity in a colossal magnetoresistive manganite La1−xSrxMnO3

By using laboratory x-ray photoemission spectroscopy (XPS) and hard x-ray photoemission spectroscopy (HX-PES) at a synchrotron facility, we report an empirical semi-quantitative relationship between the valence/core-level x-ray photoemission spectral weight and electrical conductivity in La1−xSrxMnO3 as a function of x. In the Mn 2p3∕2 HX-PES spectra, we observed the shoulder structure due to the Mn3+ well-screened state. However, the intensity at x = 0.8 was too small to explain its higher electrical conductivity than x = 0.0, which confirms our recent analysis on the Mn 2p3∕2 XPS spectra. The near-Fermi level XPS spectral weight was found to be a measure of the variation of electrical conductivity with x in spite of a far lower energy resolution compared with the energy scale of the quasiparticle (coherent) peak because of the concurrent change of the coherent and incoherent spectral weight.

Formation of orbital-selective electron states in LaTiO3/SrTiO3superlattices

The interface electronic structure of correlated LaTiO${}_{3}$/SrTiO${}_{3}$ superlattices is investigated by means of the charge self-consistent combination of the local density approximation (LDA) to density functional theory with dynamical mean-field theory. Utilizing a pseudopotential technique together with a continuous-time quantum Monte Carlo approach, the resulting complex multiorbital electronic states are addressed in a coherent fashion beyond static mean field. General structural relaxations are taken into account on the LDA level and cooperate with the driving forces from strong electronic correlations. This alliance leads to a Ti($3{d}_{xy}$) dominated low-energy quasiparticle peak and a lower Hubbard band in line with photoemission studies. Furthermore correlation effects close to the band-insulating bulk SrTiO${}_{3}$ limit as well as the Mott-insulating bulk LaTiO${}_{3}$ limit are studied via realistic single-layer embeddings.

Nonlocal excitations and 1/8 singularity in cuprates

Momentum-dependent excitation spectra of the two-dimensional Hubbard model on a square lattice have been investigated at zero temperature on the basis of the full self-consistent projection operator method in order to clarify nonlocal effects of electron correlations on the spectra. Intersite antiferromagnetic correlations are found to cause shadow bands and enhance the Mott-Hubbard splittings near the half-filling. Furthermore, nonlocal excitations are shown to move the critical doping concentration δ h *, at which the singular quasiparticle peak is located just on the Fermi level, from δ h * = 0.153 (the single-site value) to δ h * = 0.123. The latter suggests the occurrence of an instability such as the stripe at δ h * = 1/8.

Excitation Spectra of Transition-Metal Atoms on the Ag (100) Surface Controlled by Hund’s Exchange

We report photoemission experiments revealing the valence electron spectral function of Mn, Fe, Co, and Ni atoms on the Ag (100) surface. The series of spectra shows splittings of higher energy features which decrease with the filling of the 3d shell and a highly nonmonotonic evolution of spectral weight near the Fermi edge. First principles calculations demonstrate that two manifestations of Hund's exchange J are responsible for this evolution. First, there is a monotonic reduction of the effective exchange splittings with increasing filling of the 3d shell. Second, the amount of charge fluctuations and, thus, the weight of quasiparticle peaks at the Fermi level varies nonmonotonically through this 3d series due to a distinct occupancy dependence of effective charging energies U(eff).