Quasiparticle Dispersion Research Articles

Magnetoconductivity of a metal with a closed Fermi surface reconstructed by a biaxial density wave

We investigate quantum dynamics and kinetics of a 2D conductor with closed Fermi surface reconstructed by a biaxial density wave, in which electrons move along a two-dimensional periodic net of semiclassical trajectories coupled by the magnetic breakdown tunnelling under a strong magnetic field. We derive a quasi-particle dispersion law and magnetoconductivity tensor. The quasi-particle spectrum is found to be the alternating series of two-dimensional magnetic energy bands with gaps between them. The longitudinal magnetoconductivity shows giant oscillations with change of magnetic field, while the Hall coefficient changes sign and is absent in a wide range of the magnetic fields in between. Preliminary estimations show that the suggested magnetoconductivity mechanism may be the origin of such behaviour of the Hall coefficient vs. magnetic field, as observed in experiments in materials with analogous topology of the Fermi surface, such as the high-Tc superconducting cuprates.

Revealing the Heavy Quasiparticles in the Heavy-Fermion Superconductor CeCu_{2}Si_{2}.

The superconducting order parameter of the first heavy-fermion superconductor CeCu_{2}Si_{2} is currently under debate. A key ingredient to understand its superconductivity and physical properties is the quasiparticle dispersion and Fermi surface, which remains elusive experimentally. Here, we present measurements from angle-resolved photoemission spectroscopy. Our results emphasize the key role played by the Ce 4f electrons for the low-temperature Fermi surface, highlighting a band-dependent conduction-f electron hybridization. In particular, we find a very heavy quasi-two-dimensional electron band near the bulk X point and moderately heavy three-dimensional hole pockets near the Z point. Comparison with theoretical calculations reveals the strong local correlation in this compound, calling for further theoretical studies. Our results provide the electronic basis to understand the heavy-fermion behavior and superconductivity; implications for the enigmatic superconductivity of this compound are also discussed.

Bandwidth-control orbital-selective delocalization of 4f electrons in epitaxial Ce films

The 4f-electron delocalization plays a key role in the low-temperature properties of rare-earth metals and intermetallics, and it is normally realized by the Kondo coupling between 4f and conduction electrons. Due to the large Coulomb repulsion of 4f electrons, the bandwidth-control Mott-type delocalization, commonly observed in d-electron systems, is difficult in 4f-electron systems and remains elusive in spectroscopic experiments. Here we demonstrate that the bandwidth-control orbital-selective delocalization of 4f electrons can be realized in epitaxial Ce films by thermal annealing, which results in a metastable surface phase with reduced layer spacing. The quasiparticle bands exhibit large dispersion with exclusive 4f character near bar{{{Gamma }}} and extend reasonably far below the Fermi energy, which can be explained from the Mott physics. The experimental quasiparticle dispersion agrees well with density-functional theory calculation and also exhibits unusual temperature dependence, which could arise from the delicate interplay between the bandwidth-control Mott physics and the coexisting Kondo hybridization. Our work opens up the opportunity to study the interaction between two well-known localization-delocalization mechanisms in correlation physics, i.e., Kondo vs Mott, which can be important for a fundamental understanding of 4f-electron systems.

Anisotropic quasiparticle coherence in nematic BaFe2As2 studied with strain-dependent ARPES

The hallmark of nematic order in iron-based superconductors is a resistivity anisotropy but it is unclear to which extent quasiparticle dispersions, lifetimes and coherence contribute. While the lifted degeneracy of the Fe $d_{xz}$ and $d_{yz}$ dispersions has been studied extensively, only little is known about the two other factors. Here, we combine in situ strain tuning with ARPES and study the nematic response of the spectral weight in BaFe$_2$As$_2$. The symmetry analysis of the ARPES spectra demonstrates that the $d_{xz}$ band gains quasiparticle spectral weight compared to the $d_{yz}$ band for negative antisymmetric strain $\Delta \epsilon_{yy}$ suggesting the same response inside the nematic phase. Our results are compatible with a different coherence of the $d_{xz}$ and $d_{yz}$ orbital within a Hund's metal picture. We also discuss the influence of orbital mixing.

Magnon-assisted dynamics of a hole doped in a cuprate superconductor

We calculate the quasiparticle dispersion and spectral weight of the quasiparticle that results when a hole is added to an antiferromagnetically ordered CuO$_2$ plane of a cuprate superconductor. We also calculate the magnon contribution to the quasiparticle spectral function. We start from a multiband model for the cuprates considered previously [Nat. Phys. \textbf{10}, 951 (2014)]. We map this model and the operator for creation of an O hole to an effective one-band generalized $t-J$ model, without free parameters. The effective model is solved using the state of the art self-consistent Born approximation. Our results reproduce all the main features of experiments. They also reproduce qualitatively the dispersion of the multiband model, giving better results for the intensity near wave vector $(\pi,\pi)$, in comparison with the experiments. In contrast to what was claimed in [Nat. Phys. \textbf{10}, 951 (2014)], we find that spin fluctuations play an essential role in the dynamics of the quasiparticle, and hence in both its weight and dispersion.

In Operando Angle‐Resolved Photoemission Spectroscopy with Nanoscale Spatial Resolution: Spatial Mapping of the Electronic Structure of Twisted Bilayer Graphene

To pinpoint the electronic and structural mechanisms that affect intrinsic and extrinsic performance limits of 2D material devices, it is of critical importance to resolve the electronic properties on the mesoscopic length scale of such devices under operating conditions. Herein, angle‐resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) is used to map the quasiparticle electronic structure of a twisted bilayer graphene device. The dispersion and linewidth of the Dirac cones associated with top and bottom graphene layers are determined as a function of spatial position on the device under both static and operating conditions. The analysis reveals that microscopic rotational domains in the two graphene layers establish a range of twist angles from 9.8° to 12.7°. Application of current and electrostatic gating lead to strong electric fields with peak strengths of 0.75 V/μm at the rotational domain boundaries in the device. These proof‐of‐principle results demonstrate the potential of nanoARPES to link mesoscale structural variations with electronic states in operating device conditions and to disentangle such extrinsic factors from the intrinsic quasiparticle dispersion.

Spin-Wave Doppler Shift by Magnon Drag in Magnetic Insulators.

The Doppler shift of the quasiparticle dispersion by charge currents is responsible for the critical supercurrents in superconductors and instabilities of the magnetic ground state of metallic ferromagnets. Here we predict an analogous effect in thin films of magnetic insulators in which microwaves emitted by a proximity stripline generate coherent chiral spin currents that cause a Doppler shift in the magnon dispersion. The spin-wave instability is suppressed by magnon-magnon interactions that limit spin currents to values close to but below the threshold for the instability. The spin current limitations by the backaction of magnon currents on the magnetic order should be considered as design parameters in magnonic devices.

Temperature evolution of quasiparticle dispersion and dynamics in semimetallic 1T−TiTe2 via high-resolution angle-resolved photoemission spectroscopy and ultrafast optical pump-probe spectroscopy

High-resolution angle-resolved photoemission spectroscopy and ultrafast optical pump-probe spectroscopy were used to study semimetallic $1T\ensuremath{-}{\mathrm{TiTe}}_{2}$ quasiparticle dispersion and dynamics. A kink and a flat band, having the same energy scale and temperature-dependent behaviors along the $\overline{\mathrm{\ensuremath{\Gamma}}}\text{\ensuremath{-}}\overline{M}$ direction, were detected. Both manifested at low temperatures but blurred as temperature increased. The kink was formed by an electron-phonon coupling. And the localized flat band might be closely related to an electron-phonon coupling. Ultrafast optical spectroscopy identified multiple distinct time scales in the 10--300 K range. Quantitative analysis of the fastest decay process evidenced a significant lifetime temperature dependence at high temperatures, while this starts to change slowly below $\ensuremath{\sim}100$ K where an anomalous Hall coefficient occurred. At low temperature, a coherent ${A}_{1}g$ phonon mode with a frequency of $\ensuremath{\sim}4.36$ THz was extracted. Frequency temperature dependence suggests that phonon hardening occurs as temperature falls and anharmonic effects can explain it. Frequency fluence dependence indicates that the phonons soften as fluence increases.

Fermi surface and kink structures in hbox {Sr}_{{4}}hbox {Ru}_{{3}}hbox {O}_{{10}} revealed by synchrotron-based ARPES

The low-energy electronic structure, including the Fermi surface topology, of the itinerant metamagnet hbox {Sr}_{{4}}hbox {Ru}_{{3}}hbox {O}_{{10}} is investigated for the first time by synchrotron-based angle-resolved photoemission. Well-defined quasiparticle band dispersions with matrix element dependencies on photon energy or photon polarization are presented. Four bands crossing the Fermi-level, giving rise to four Fermi surface sheets are resolved; and their complete topography, effective mass as well as their electron and hole character are determined. These data reveal the presence of kink structures in the near-Fermi-level band dispersion, with energies ranging from 30 to 69 meV. Together with previously reported Raman spectroscopy and lattice dynamic calculation studies, the data suggest that these kinks originate from strong electron–phonon coupling present in hbox {Sr}_{{4}}hbox {Ru}_{{3}}hbox {O}_{{10}}. Considering that the kink structures of hbox {Sr}_{{4}}hbox {Ru}_{{3}}hbox {O}_{{10}} are similar to those of the other three members of the Ruddlesden Popper structured ruthenates, the possible universality of strong coupling of electrons to oxygen-related phonons in hbox {Sr}_{n+1}hbox {Ru}_{{n}}hbox {O}_{3n+1} compounds is proposed.

Relaxation operator for quasiparticles in a solid

Popular models of the phenomenological relaxation operators that are widely used in the master equation formalism for open condensed-matter systems have significant flaws ranging from limited applicability to violation of fundamental physical principles. We propose a relatively simple universal model of the relaxation operator which is free from these flaws, has a correct static limit, correct direct-current limit in a uniform electric field, includes both interband and intraband transitions, and is valid for an arbitrary dispersion of quasiparticles in a solid. We use the proposed operator to generalize the Lindhard formula and derive explicit expressions for the relaxation operator for Dirac materials with an unconventional energy spectrum of quasiparticles, such as graphene and Weyl semimetals. We compare the linear susceptibility spectra for graphene obtained with different relaxation models and show that the proposed relaxation operator leads to physically meaningful behavior of the susceptibility at low frequencies whereas the existing models become completely invalid.

Direct comparison of ARPES, STM, and quantum oscillation data for band structure determination in Sr2RhO4

Discrepancies in the low-energy quasiparticle dispersion extracted from angle-resolved photoemission, scanning tunneling spectroscopy, and quantum oscillation data are common and have long haunted the field of quantum matter physics. Here, we directly test the consistency of results from these three techniques by comparing data from the correlated metal Sr2RhO4. Using established schemes for the interpretation of the experimental data, we find good agreement for the Fermi surface topography and carrier effective masses. Hence, the apparent absence of such an agreement in other quantum materials, including the cuprates, suggests that the electronic states in these materials are of different, non-Fermi liquid-like nature. Finally, we discuss the potential and challenges in extracting carrier lifetimes from photoemission and quasiparticle interference data.

Emergence of quasiparticles in a doped Mott insulator

How a Mott insulator develops into a weakly coupled metal upon doping is a central question to understanding various emergent correlated phenomena. To analyze this evolution and its connection to the high-Tc cuprates, we study the single-particle spectrum for the doped Hubbard model using cluster perturbation theory on superclusters. Starting from extremely low doping, we identify a heavily renormalized quasiparticle dispersion that immediately develops across the Fermi level, and a weakening polaronic side band at higher binding energy. The quasiparticle spectral weight roughly grows at twice the rate of doping in the low doping regime, but this rate is halved at optimal doping. In the heavily doped regime, we find both strong electron-hole asymmetry and a persistent presence of Mott spectral features. Finally, we discuss the applicability of the single-band Hubbard model to describe the evolution of nodal spectra measured by angle-resolved photoemission spectroscopy (ARPES) on the single-layer cuprate La2−xSrxCuO4 (0 ≤ x ≤ 0.15). This work benchmarks the predictive power of the Hubbard model for electronic properties of high-Tc cuprates.

Dispersion relations and self-localization of quasiparticles in coupled elongated Bose-Einstein condensates

We present a detailed study of the spectrum and dispersion of Bogoliubov quasiparticles in two coupled elongated Bose-Einstein condensates. We develop an analytically solvable model that approximates two infinite homogeneous condensates and compare its predictions to a numerical simulation of a realistic trapped system. While the comparisons show a reasonable agreement between the two models, they also manifest the existence of several anomalous Bogoliubov modes in the spectrum. These modes show degeneracy in both energy and momentum together with self-localization in the coordinate space.

Fermion\u2013boson many-body interplay in a frustrated kagome paramagnet

Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.

Optical and magnetic excitations in the underscreened quasiquartet Kondo lattice

The underscreened Kondo lattice consisting of a single twofold degenerate conduction band and a CEF split 4f-electron quasi-quartet has non-conventional quasiparticle dispersions obtained from the constrained mean-field theory. An additional genuinely heavy band is found in the main hybridization band gap of the upper and lower hybridzed bands whose heavy effective mass is controlled by the CEF splitting. Its presence should profoundly influence the dynamical optical and magnetic response functions. In the former, the onset of the optical conductivity is not the main hybridisation energy but the much lower Kondo energy scale which appears in the direct transitions to the additional heavy band. The dynamical magnetic response is also strongly modified by the in-gap heavy band which can lead to unconventional resonant excitations that may be interpreted as coherent CEF-Kondo lattice magnetic exciton bands. Their instability at low temperature signifies the onset of induced excitonic magnetism in the underscreened Kondo lattice.

Renormalization of electrons in bilayer cuprate superconductors

Cuprate superconductors have a layered structure, and then the physical properties are significantly enriched when two or more two copper-oxide layers are contained in a unit cell. Here the characteristic features of the renormalization of the electrons in the bilayer cuprate superconductors are investigated within the framework of the kinetic-energy driven superconducting mechanism. It is shown that the electron quasiparticle excitation spectrum is split into its bonding and antibonding components due to the presence of the bilayer coupling, with each component that is independent. However, in the underdoped and optimally doped regimes, although the bonding and antibonding electron Fermi surface contours deriving from the bonding and antibonding layers are truncated to form the disconnected bonding and antibonding Fermi arcs, almost all spectral weights in the bonding and antibonding Fermi arcs are reduced to the tips of the bonding and antibonding Fermi arcs, which in this case coincide with the bonding and antibonding hot spots. These hot spots connected by the scattering wave vectors qi construct an octet scattering model, and then the enhancement of the quasiparticle scattering processes with the scattering wave vectors qi is confirmed via the result of the autocorrelation of the electron quasiparticle excitation spectral intensities. Moreover, the peak-dip-hump structure developed in each component of the electron quasiparticle excitation spectrum along the corresponding electron Fermi surface is directly related with the peak structure in the quasiparticle scattering rate except for at around the hot spots, where the peak-dip-hump structure is caused mainly by the pure bilayer coupling. Although the kink in the electron quasiparticle dispersion is present all around the electron Fermi surface, when the momentum moves away from the node to the antinode, the kink energy smoothly decreases, while the dispersion kink becomes more pronounced, and in particular, near the cut close to the antinode, develops into a break separating of the fasting dispersing high-energy part of the electron quasiparticle excitation spectrum from the slower dispersing low-energy part. By comparing with the corresponding results in the single-layer case, the theory also indicates that the characteristic features of the renormalization of the electrons are particularly obvious due to the presence of the bilayer coupling.

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.

Magnetotransport and internodal tunnelling in Weyl semimetals

Internodal dynamics of quasiparticles in Weyl semimetals manifest themselves in hydrodynamic, transport and thermodynamic phenomena and are essential for potential valleytronic applications of these systems. In an external magnetic field, coherent quasiparticle tunnelling between the nodes modifies the quasiparticle dispersion and, in particular, opens gaps in the dispersion of quasiparticles at the zeroth Landau level. We study magnetotransport in a Weyl semimetal taking into account mechanisms of quasiparticle scattering both affected by such gaps and independent of them. We compute the longitudal resistivity of a disordered Weyl semimetal with two nodes in a strong magnetic field microscopically and demonstrate that in a broad range of magnetic fields it has a strong angular dependence $\rho(\eta)\propto C_1+C_2 \cos^2\eta$, where $\eta$ is the angle between the field and the separation between the nodes in momentum space. The first term is determined by the coherent internodal tunnelling and is important only at angles $\eta$ close to $\pi/2$. This contribution depends exponentially on the magnetic field, $\propto \exp\left(-B_0/B\right)$. The second term is weakly dependent on the magnetic field for realistic concentrations of the impurities in a broad interval of fields.

Why the anti-nodal quasiparticle dispersion is so flat in the superconducting cuprates?

The emergence of the coherent quasiparticle peak and the development of the peak-dip-hump structure in the anti-nodal region below T c is the most prominent non-BCS signature of the under-doped high-T c cuprates, in which no coherent quasiparticle can be defined in the anti-nodal region above T c. The peak-dip-hump structure has been commonly interpreted as the result of the coupling of the electron to some Bosonic mode. However, such an electron–Boson coupling picture does not answer the question of why the quasiparticle dispersion is so flat in the anti-nodal region, a behavior totally unexpected for Bogoliubov quasiparticle in a d-wave BCS superconductor. Here we show that the sharp quasiparticle peak in the anti-nodal region should be understood as a new pole in the electron Green’s function generated by the strong coupling of the electron to diffusive spin fluctuation around the antiferromagnetic wave vector Q = (π, π), rather than a nearly free Bogoliubov quasiparticle in a d-wave BCS superconductor. More specifically, we find that the normal self-energy of the electron from the scattering with the diffusive spin fluctuation manifests itself mainly as a level repulsion effect and is responsible for the reduction of both the quasiparticle dispersion and the quasiparticle dissipation rate in the anti-nodal region. We argue that the peak-dip separation in the anti-nodal spectrum should not be interpreted as the energy of the pairing glue.

Exchange-Correlation Effects and the Quasiparticle Properties in a Two-Dimensional Dipolar Fermi Liquid

We investigate the effects of exchange and correlation on the quasiparticle properties such as the self-energy, the many-body effective mass and the renormalization constant in a two-dimensional system of ultracold dipolar fermions with dipole moments aligned in the perpendicular direction to the plane. We use the G0W approximation along with the generalized random phase approximation, where the many-body effects have been incorporated in the effective interaction W through the Hubbard local-field factor. The many-body effective mass and the renormalization constant are reduced with the increase of coupling strength. We also study the effect of dipole-dipole interaction on the single-particle spectral function of the two dimensional dipolar Fermi liquid. We observe composite hole-zero sound excitation which is a bound state of quasiparticles with the collective mode (i.e. zero-sound) at intermediate and high coupling constants. These composite excitations are undamped at small wave vectors. Due to repulsion between quasiparticle and composite excitation resonances, we find a gap-like feature between quasiparticle and composite excitation dispersions at long wavelengths.