Quasiparticle Features Research Articles

Minimal one-dimensional model of bad metal behavior from fast particle-hole scattering

A strongly interacting plasma of linearly dispersing electron and hole excitations in two spatial dimensions (2D), also known as a Dirac fluid, can be captured by relativistic hydrodynamics and shares many universal features with other quantum critical systems. We propose a one-dimensional (1D) model to capture key aspects of the 2D Dirac fluid while including lattice effects and being amenable to non-perturbative computation. When interactions are added to the Dirac-like 1D dispersion without opening a gap, we show that this kind of irrelevant interaction is able to preserve Fermi-liquid-like quasi-particle features while relaxing a zero-momentum charge current via collisions between particle-hole excitations, leading to resistivity that is linear in temperature via a mechanism previously discussed for large-diameter metallic carbon nanotubes. We further provide a microscopic lattice model and obtain numerical results via density-matrix renormalization group (DMRG) simulations, which support the above physical picture. The limits on such fast relaxation at strong coupling are of considerable interest because of the ubiquity of bad metals in experiments.

Kondo effect in defect-bound quantum dots coupled to NbSe2

We report the fabrication of a van der Waals tunneling device hosting a defect-bound quantum dot coupled to NbSe$_2$. We find that upon application of magnetic field, the device exhibits a zero-bias conductance peak. The peak, which splits at higher fields, is associated with a Kondo effect. At the same time, the junction retains conventional quasiparticle tunneling features at finite bias. Such coexistence of a superconducting gap and a Kondo effect are unusual, and are explained by noting the two-gap nature of the superconducting state of NbSe$_2$, where a magnetic field suppresses the low energy gap associated with the Se band. Our data shows that van der Waals architectures, and defect-bound dots in them, can serve as a novel and effective platform for investigating the interplay of Kondo screening and superconducting pairing in unconventional superconductors.

Ultracold Bose mixtures with spin-dependent fermion-mediated interactions

We develop a functional integral formulation for binary Bose-Einstein condensates coupled to polarized fermions. We find that spin-dependent fermion-mediated interactions have dramatic effects on the properties of the binary condensates. The quasiparticle spectrum features two branches. The upper branch, which is of density nature, gets modified by the induced interactions, while the lower branch, which is of spin nature, is left intact. The ground-state phase diagram consists of stable region of miscible phases and unstable region toward phase separation. In the stable region, it is further classified by the damping of excitations of the upper branch. We show that it is possible to find region of well-defined, long-lived quasiparticle excitations by tuning relevant parameters, such as boson-fermion mass ratio, boson-fermion number density ratio, and interspecies interactions between bosons as well. We explore the effects of quantum fluctuation due to the effective potential on the binary condensates. It turns out that both the density structure factor and spin density structure factor fulfill the Feynman relation, except that the latter is immune to the fermion-mediated interactions.

Two-dimensional optical excitations in the mixed-valence Cs2Au2I6 fully inorganic double perovskite

The first dark exciton wavefunction square modulus in Cs2Au2I6.

Stabilization of metallic phase in V2O3 thin film

The tailoring and understanding of the metal-insulator transitions (MITs) in vanadium sesquioxide, V2O3, is of major interest for both applications and fundamental physics. V2O3 has been characterized by MIT and concurrent structural transition at ∼155 K; however, the nature of the MIT has remained more elusive. We investigated the MIT and the electronic structure (in metallic phase) of the pulsed laser deposition grown strained vanadium sesquioxide thin films on Si. The strained thin films synthesized here show the suppression (by ∼23 K) of the MIT to lower temperatures, whilst the structural transition temperature decreases only by ∼10 K. Our results systematically confirm that albeit the structural changes are crucial in V2O3, electronic transition seems to be of Mott-Hubbard type. Stabilization of the metallic phase in the strained V2O3 thin film has been manifested from resistivity data and observations of the increased crystal field and quasiparticle features.

Spectral functions of the uniform electron gas via coupled-cluster theory and comparison to theGWand related approximations

We use, for the first time, ab initio coupled-cluster theory to compute the spectral function of the uniform electron gas at a Wigner-Seitz radius of $r_\mathrm{s}=4$. The coupled-cluster approximations we employ go significantly beyond the diagrammatic content of state-of-the-art $GW$ theory. We compare our calculations extensively to $GW$ and $GW$-plus-cumulant theory, illustrating the strengths and weaknesses of these methods in capturing the quasiparticle and satellite features of the electron gas. Our accurate calculations further allow us to address the long-standing debate over the occupied bandwidth of metallic sodium. Our findings indicate that the future application of coupled-cluster theory to condensed phase material spectra is highly promising.

A Weyl Fermion semimetal with surface Fermi arcs in the transition metal monopnictide TaAs class.

Weyl fermions are massless chiral fermions that play an important role in quantum field theory but have never been observed as fundamental particles. A Weyl semimetal is an unusual crystal that hosts Weyl fermions as quasiparticle excitations and features Fermi arcs on its surface. Such a semimetal not only provides a condensed matter realization of the anomalies in quantum field theories but also demonstrates the topological classification beyond the gapped topological insulators. Here, we identify a topological Weyl semimetal state in the transition metal monopnictide materials class. Our first-principles calculations on TaAs reveal its bulk Weyl fermion cones and surface Fermi arcs. Our results show that in the TaAs-type materials the Weyl semimetal state does not depend on fine-tuning of chemical composition or magnetic order, which opens the door for the experimental realization of Weyl semimetals and Fermi arc surface states in real materials.

Mean-field RVB ground states of lattice models of CuNCN

Recently we proposed to describe the fascinating physics of copper carbodiimide, CuNCN, with help of the anisotropic triangular antiferromagnetic Heisenberg model with the parameters Ja and Jab extending along the a, and a ± b lattice directions and a new frustrated Heisenberg antiferromagnetic model with exchange parameters Jc, Ja, and Jac, extending along the c, a, and a ± c (c-a-ca model) directions assuming the resonating valence bond (RVB) type of the corresponding phases. Here we discuss possible RVB ground states of these models in the mean-field approximation and show that in either case it is a two-dimensional RVB state. The difference between the models is that in the ground state of the triangular model the quasiparticle spectrum features a finite (although exponentially small) energy gap for arbitrary weak Jab whereas that of the c-a-ca model shows two pseudogaps and a linear dependence of the quasiparticle density of states in the low-energy range.

Resonant Soft-X-Ray Emission as a Bulk Probe of Correlated Electron Behavior in MetallicSrxCa1−xVO3

The evolution of electron correlation in SrxCa1-xVO3 has been studied using a combination of bulk-sensitive resonant soft x-ray emission spectroscopy, surface-sensitive photoemission spectroscopy, and ab initio band structure calculations. We show that the effect of electron correlation is enhanced at the surface. Strong incoherent Hubbard subbands are found to lie ∼20% closer in energy to the coherent quasiparticle features in surface-sensitive photoemission spectroscopy measurements compared with those from bulk-sensitive resonant soft x-ray emission spectroscopy, and a ∼10% narrowing of the overall bandwidth at the surface is also observed.

Quasiparticles and anomalous temperature dependence of the low-lying states in the colossal magnetoresistant oxideLa2−2xSr1+2xMn2O7(x=0.36)from angle-resolved photoemission

After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials ${\mathrm{La}}_{2\ensuremath{-}2x}{\mathrm{Sr}}_{1+2x}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ using angle-resolved photoemission or scanning probe microscopy. Here, we report temperature-dependent, angle-resolved photoemission data from single crystals with a doping level of $x=0.36$. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature ${T}_{C}$. The data show unprecedented sharp spectral features, enabling the unambiguous identification of clear, resolution-limited quasiparticle features from the bilayer split $3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$-derived Fermi surfaces both at the zone-face and zone diagonal ${k}_{F}$ locations. The data show that these low temperature Fermi surfaces describe closed shapes in ${k}_{\ensuremath{\parallel}}$, centered at the $(\ensuremath{\pi}∕a,\ensuremath{\pi}∕a)$ points in the two dimensional Brillouin zone, and are not open and arclike in nature. The second important result concerns the temperature dependence of the electronic states. The spectra display strong incoherent intensity at high binding energies and a very strong temperature dependence, both characteristics reminiscent of polaronic systems. However, the clear and strong quasiparticle peaks at low temperatures are difficult to place within a polaronic scenario. A careful analysis of the temperature-dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in $k$ space indicates that the coherent quasiparticle weight disappears for temperatures significantly above ${T}_{C}$ and that the $k$ dependence of the $T$-induced changes in the spectra invalidates an interpretation of these data in terms of the superposition of a ``universal'' metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.

Low-energy electronic structure of thehigh-Tc cuprates La2−xSrxCuO4 studied by angle-resolved photoemission spectroscopy

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of thehigh-Tc cuprates La2−xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. Wehave revealed a systematic doping evolution of the band dispersions and (underlying)Fermi surfaces, pseudogap and quasi-particle features under the influence of strongelectron–electron interaction and electron–phonon interaction. The unusual transport andthermodynamic properties are explained by taking into account the pseudogap opening andthe Fermi arc formation, due to which the carrier number decreases as the doped holeconcentration decreases.

Femtosecond formation of collective modes due to mean-field fluctuations

Starting from a quantum kinetic equation including the mean field and a conserving relaxation-time approximation we derive an analytical formula which describes the time dependence of the dielectric function in a plasma created by a short intense laser pulse. This formula reproduces universal features of the formation of collective modes seen in recent experimental data of femtosecond spectroscopy. The presented formula offers a tremendous simplification for the description of the formation of quasiparticle features in interacting systems. Numerical demanding treatments can now be focused on effects beyond these gross features found here to be describable analytically.

Neutrino collective excitations in the standard model at high temperature

Neutrino collective excitations are studied in the Standard Model at high temperatures below the symmetry breaking scale. Two parameters determine the properties of the collective excitations: a mass scale $m_\nu=gT/4$ which determines the \emph{chirally symmetric} gaps in the spectrum and $\Delta=M^2_W(T)/2m_\nu T$. The spectrum consists of left handed negative helicity quasiparticles, left handed positive helicity quasiholes and their respective antiparticles. For $\Delta < \Delta_c = 1.275...$ there are two gapped quasiparticle branches and one gapless and two gapped quasihole branches, all but the higher gapped quasiparticle branches terminate at end points. For $\Delta_c < \Delta < \pi/2$ the quasiparticle spectrum features a pitchfork bifurcation and for $\Delta >\pi/2$ the collective modes are gapless quasiparticles with dispersion relation below the light cone for $k\ll m_\nu$ approaching the free field limit for $k\gg m_\nu$ with a rapid crossover between the soft non-perturbative to the hard perturbative regimes for $k\sim m_\nu$.The \emph{decay} of the vector bosons leads to a \emph{width} of the collective excitations of order $g^2$ which is explicitly obtained in the limits $k =0$ and $k\gg m_\nu \Delta$. At high temperature this damping rate is shown to be competitive with or larger than the collisional damping rate of order $G^2_F$ for a wide range of neutrino energy.

Comparison of Intrinsic Josephson and SIS Tunneling Spectroscopy of&amp;lt;tex&amp;gt;$rm Bi_2rm Sr_2rm CaCu_2rm O_8+delta$&amp;lt;/tex&amp;gt;

Tunneling spectroscopy measurements are reported on optimally-doped and overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals. A novel point contact method is used to obtain superconductor-insulator-normal metal (SIN) and SIS break junctions as well as intrinsic Josephson junctions (IJJ) from nanoscale crystals. Three junction types are obtained on the same crystal to compare the quasiparticle peaks and higher bias dip/hump structures which have also been found in other surface probes such as scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy. However, our IJJ quasiparticle spectra consistently reveal very sharp conductance peaks and no higher bias dip structures. The IJJ conductance peak voltage divided by the number of junctions in the stack consistently leads to a significant underestimate of $\Delta$ when compared to the single junction values. The comparison of the three methods suggests that the markedly different characteristics of IJJ are a consequence of nonequilibrium effects and are not intrinsic quasiparticle features.

Quasiparticle anisotropy and pseudogap formation: a functional renormalization group study

Using the one-loop functional renormalization group technique we evaluate the self-energy in the weak-coupling regime of the 2D t – t ′ Hubbard model. At van Hove (vH) band fillings and at low temperatures the quasiparticle weight along the Fermi surface (FS) continuously vanishes on approaching the ( π , 0 ) point where the quasiparticle concept is invalid. Away from vH band fillings the quasiparticle peak is formed inside an anisotropic pseudogap and the self-energy has the conventional Fermi-liquid characteristics near the Fermi level. The spectral weight of the quasiparticle features is reduced on parts of the FS between the near vicinity of hot spots and the FS points closest to ( π , 0 ) and ( 0 , π ) .

Electron-Phonon Interaction Close to a Mott Transition

The effect of Holstein electron-phonon interaction on a Hubbard model close to a Mott-Hubbard transition at half filling is investigated by means of dynamical mean-field theory. We observe a reduction of the effective mass that we interpret in terms of a reduced effective repulsion. When the repulsion is rescaled to take into account this effect, the quasiparticle low-energy features are unaffected by the electron-phonon interaction. Phonon features are only observed within the high-energy Hubbard bands. The lack of electron-phonon fingerprints in the quasiparticle physics can be explained interpreting the quasiparticle motion in terms of rare fast processes.

Single-particle spectral function of the Holstein-Hubbard bipolaron

The one-electron spectral function of the Holstein-Hubbard bipolaron in one dimension is studied using cluster perturbation theory together with the Lanczos method. In contrast to other approaches, this allows one to calculate the spectrum at continuous wave vectors and thereby to investigate the dispersion and the spectral weight of quasiparticle features. The formation of polarons and bipolarons, and their manifestation in the spectral properties of the system, is studied for the cases of intermediate and large phonon frequencies, with and without Coulomb repulsion. A good agreement is found with the most accurate calculations of the bipolaron band dispersion available. Pronounced deviations of the bipolaron band structure from a simple tight-binding band are observed, which can be attributed to next-nearest-neighbor hopping processes.

Interplay between single-particle and collective features in the boson fermion model

We study the interplay between the single particle and fermion pair features in the boson fermion model, both above and below the transition temperature $T_{c}$, using the flow equation method. Upon lowering the temperature the single particle fermionic spectral function: (a) gradually develops a depletion of the low energy states (pseudogap) for $T^{*}>T>T_{c}$ and a true superconducting gap for $T<T_{c}$, (b) exhibits a considerable transfer of spectral weight between the incoherent background and the narrow coherent peak(s) signifying long-lived quasi-particle features. The cooperon spectral function consists of a delta function peak, centered at the renormalized boson energy $\omega=\tilde{E}_{\bf q}$, and a surrounding incoherent background which is spread over a wide energy range. When the temperature approaches $T_{c}$ from above this peak for ${\bf q}={\bf 0}$ moves to $\omega=0$, so that the static pair susceptibility diverges (Thouless criterion for the broken symmetry phase transition). Upon decreasing the temperature below $T_{c}$ the cooperon peak becomes the collective (Goldstone) mode $E_{\bf q} \propto |{\bf q}|$ in the small momentum region and simultaneously splits off from the incoherent background states which are expelled to the high energy sector $|\omega| \geq 2 \Delta_{sc}(T)$. We discuss the smooth evolution of these features upon approaching $T_{c}$ from above and consider its feedback on the single particle spectrum where a gradual formation of damped Bogoliubov modes (above $T_{c}$) is observed.

Quasiparticle anisotropy and pseudogap formation from the weak-coupling renormalization group point of view.

Using the one-loop functional renormalization group technique, we evaluate the self-energy in the weak-coupling regime of the 2D t-t(') Hubbard model. At van Hove (vH) band fillings and at low temperatures, the quasiparticle weight along the Fermi surface (FS) continuously vanishes on approaching the (pi,0) point where the quasiparticle concept is invalid. Away from vH band fillings the quasiparticle peak is formed inside an anisotropic pseudogap and the self-energy has the conventional Fermi-liquid characteristics near the Fermi level. The spectral weight of the quasiparticle features is reduced on parts of the FS between the near vicinity of hot spots and the FS points closest to (pi,0) and (0,pi).

Strong electron–electron and electron–phonon interactions in one- and two-dimensional solids

We discuss the spectral signatures of strong electronic correlations and electron–phonon interactions in two representative low-dimensional systems. In the 2D compound 1T-TaSe 2 a charge-density-wave (CDW) induces a narrowing of the conduction band, which triggers a surface metal–insulator (M–I) transition and the disappearance of the quasiparticle (QP) features. In the 1D blue bronze K 0.3MoO 3 strong interaction with the lattice lead to pseudogapped polaronic lineshapes, and the large mass renormalization masks the signatures of electronic correlations.