Quasiparticle Description of Angle-Resolved Photoemission Spectroscopy for SrCuO2

TL;DR

The paper questions the necessity of spin-charge separation to explain ARPES in SrCuO2 and presents a parameter-free, first-principles approach based on a self-consistent QS$G\hat{W}$ framework with ladder corrections and the Bethe-Salpeter equation. Spin disorder and long-range antiferromagnetic correlations are shown to produce spinon-like and holon-like ARPES features within a conventional quasiparticle picture, while SrCuO2 is revealed to be a highly anisotropic 2D system with non-negligible interchain coupling. The optical conductivity and excitonic spectra are well described by intersite $d$-$d$ and $dp$ transitions in a multi-orbital ligand-field framework, with Zhang-Rice–like excitons emerging from a multisite description rather than a localized ligand-field model. Overall, the work provides a parameter-free, first-principles bridge between 1D and 2D cuprates and highlights the importance of multisite exciton physics in understanding their electronic structure.

Abstract

SrCuO2 has long been considered a near-archetypal realization of a quasi one dimensional (1D) system of interacting electrons with short-range interactions. Within this framework, experimental observations - interpreted through the lens of the 1D Hubbard model-suggest that electron and hole excitations decay into two types of (unphysical) collective bosonic modes: spinons, which carry the spin degree of freedom, and holons, which carry the charge degree of freedom. This model, known as spin-charge separation, is most directly evidenced by angle-resolved photoemission spectroscopy (ARPES), where a photo-induced hole decays into a continuum of these excitations. Here we present an alternative perspective grounded in first-principles, self-consistent, and parameter-free many-body perturbation theory. In this revised quasiparticle framework, ARPES can be understood as a one-body effect arising from mild disorder in a long-range antiferromagnetic ground state. the emergence of the so-called spinon branch arises naturally from spin disorder, the anomalous line widths are accurately captured, and we provide a compelling explanation for the spectral weight observed at the non-magnetic zone boundary. This reinterpretation provides a unified explanation for key experimental signatures previously attributed to spin-charge separation, including features observed in optical conductivity. Additionally, we show that SrCuO2 exhibits a nontrivial interchain coupling that significantly influences both its one-particle and two-particle spectral functions. By comparing the spectral features of SrCuO2 with those of La2CuO4, we argue that SrCuO2 shares notable similarities with the two-dimensional cuprates - both being rooted in a common CuO4 plaquette-based molecular orbital framework.

Figures (4)

• Figure 1: (a) SrCuO2 in the minimal (16 atom) unit cell. Two unit cells are shown. Cu, O, and Sr almost lie a single plane. (b) corresponding (minimal) Brillouin zone with C2/$m$ symmetry. X is a zone-boundary point along the chain, at (0,0,1/2), i.e. the midpoint of the reciprocal lattice vector along x. Q, which corresponds to (5/16,5/16,1/2), is where the valence band maximum falls, according to QS$G\hat{W}$ theory. (c) Molecular fragment for SrCuO2 and La2CuO4. The O atoms form a nearly perfect square. For both systems, Cu $d_{x^2-y^2}$ and O $p_{x}$ combine to form strong $\sigma$ bonds and antibonds in the chain, depicted by the bar. Cu$^{(1)}$ is not present in La2CuO4; instead Cu$^{(2)}$ appears on both x and y axes. In SrCuO2 Cu$^{(1)}$ is present but weakly bonds to Cu$^{(0)}$ and Cu$^{(2)}$ on the chain, as explained in the text. (d) Noninteracting energy bands of SrCuO2 within the quasiparticle self-consistent $G\hat{W}$ approximation, for the unit cell of panel (a). The zero defines the Fermi level $E_{F}$, placed at midgap. Colors depict orbital weights: blue, red, green project onto Cu $d_{x^2-y^2}$, $t_{2g}$, $d_{z^2}$ character, respectively. Black depicts Sr content. O content is depicted by bleaching out the colors. States between $E_{F}-4$ eV and $E_{F}+2$ eV are composed of roughly equal admixtures of Cu $3d$ and O $2p$ character, while states above $E_{F}+2$ eV consist of mostly Sr character (La character in La2CuO4). (e) Interacting band structure calculated from the dynamical QS$G\hat{W}$ self-energy $\Sigma(q,\omega)$, in the window ($-$4,2.5) eV. (f) Noninteracting energy band structure of La2CuO4, with the same color scheme as panel (d). (g) Corresponding interacting band structure of La2CuO4.
• Figure 2: Spectral functions for selected spin orderings of a 64-atom superlattice, unfolded to the minimal Brillouin zone, on the $\Gamma$-X line. The bandgap is given for each panel. (a) a trivial supercell of the mBZ (equivalent to bands of Fig. \ref{['fig:Fig1']}(c)). (b) spins in alternate planes along z are spin flipped (standard cell) (c) Permutation of (b) where a pair of adjacent Cu marked by the arrows are spin flipped. (d) Configurationally averaged spectral functions from 20 random spin configurations.
• Figure 3: Optical properties of SrCuO2. (a): conductivity $\sigma(\omega)$. Ellipsometry data from Ref. Hilgers09 (green); $\sigma^\mathrm{BSE}(\omega)$ for the ground state spin configuration (black); $\sigma^\mathrm{BSE}(\omega)$ for the minimal cell (dotted); configurationally averaged $\sigma^\mathrm{BSE}(\omega)$ for disordered spins (dashed). Main peak at 2.00 eV (g.s.), 2.02 eV (ellipsometry), 1.77 eV (minimal), 2.29 eV (disordered). Also shown are RIXS spectra at $q$=0 from Ref. Kim04 (light grey hexagons). (b): joint density-of-states. Total (black); projection onto Cu($d_{x^2+y^2}$) + O components for both electrons and holes (red); difference between red and black (green); subtracting Cu($d_{x^2+y^2}$) + Cu($d_{xy}$) + O components from total (blue). (c): Selected excitons for the minimal unit cell. Top panels: Energy band structure of the minimal unit cell (Fig. \ref{['fig:Fig1']}), projecting the distribution of each of the three lowest-energy excitons into band contributions (orange). The two most strongly bound excitons (1.64 eV and 1.78 eV) are composed from the highest valence frontier orbital ($v_1$) and the first or second frontier orbital ($c_1$ and $c_2$). Also shown are excitons that make bright peaks at 2.09 eV and 4.29 eV. Middle panels: Real-space structure of the electron part of the exciton for a hole centered at Cu. Atoms are denoted by white, orange and purple circles; the exciton constant-value contour is depicted in sky blue. Bottom panels: Pie chart resolving the excitons' Mulliken decomposition into atom-atom pairs, distinguishing on-site from inter-site contributions. Superscript '+' and '$-$' refer to 'electron' and 'hole.' The Cu-Cu onsite portion is small or negligible.
• Figure 4: Optical properties of La2CuO4. (a): QS$G\hat{W}$ band structure of Fig. \ref{['fig:Fig1']}(f), projecting the distribution of the pair of lowest-energy excitons into band contributions, as for SrCuO2 in Fig.\ref{['fig:optics']} (orange). (b): Real-space structure of the electron part of the pair of excitons at 1.55 eV for a hole centered at Cu, projected onto in the xy plane. Along z the exciton is confined to a single plane (not shown). Atoms are denoted by spheres: black (Cu), blue (O) and white (La). The exciton constant-value contour is depicted in sky blue. (c): Pie chart resolving the excitons' Mulliken decomposition into atom-atom pairs, distinguishing on-site from inter-site contributions. The intersite portion dominates, while the onsite portion is small or negligible. (d): Optical conductivity $\sigma^\mathrm{BSE}(\omega)$ compared to an ellipsometry measurement, Ref. Baldini20.