Interband pairing as the origin of the sublattice dichotomy in monolayer FeSe/SrTiO_3

TL;DR

This work investigates the origin of the sublattice dichotomy seen in monolayer FeSe/SrTiO$_3$ by identifying interband pairing as a key mechanism. Using symmetry considerations, a $B_{2u}$ perturbation is shown to be required to break all Fe-sublattice exchange symmetries, which can arise either from normal-state order or pairing-order mixing. Through a low-energy $k\cdot p$ model and a simple two-band BdG analysis, the authors show that interband pairing with the same sign together with intraband pairing of opposite signs yields the observed sublattice-dichotomy in the density of states, while other configurations do not. The results provide a unified framework for understanding the superconductivity of monolayer FeSe/SrTiO$_3$ and highlight interband pairing as essential for interpreting the spectroscopic dichotomy and related spectra.

Abstract

Sublattice dichotomy in monolayer FeSe/SrTiO$_3$, signaling the breaking of symmetries exchanging the two Fe sublattices, has recently been reported. We propose that interband pairing serves as the origin of this dichotomy, regardless of whether the symmetry is broken in the normal state or in the pairing state. If symmetry breaking occurs in the normal state, the Fermi surfaces are sublattice-polarized, and the intersublattice d-wave pairing naturally acts as interband pairing, reproducing the observed dichotomy in the spectra. Alternatively, if symmetry breaking takes place in the pairing state, it manifests as the coexistence of intraband and interband pairing, with the constraint that interband pairings share the same sign while intraband pairings carry opposite signs. In both cases, interband pairing is indispensable, establishing it as a key ingredient for understanding superconductivity in monolayer FeSe/SrTiO$_3$.

Figures (3)

• Figure 1: (a) (Left) Coordinate systems used in this paper. (Right) Top view of monolayer FeSe. Fe atoms are depicted with solid circles in black and gray, corresponding to the two Fe sublattices. Se atoms above and below the Fe plane are represented by upward-pointing and downward-pointing filled triangles, respectively. Fe$_\mathrm{A}$, Fe$_\mathrm{B}$, Se$^+$ and Se$^-$ are the notations for each type of atom. A single unit cell is indicated by the shaded square region. Symmetries exchanging the two Fe sublattices, $\{I|\boldsymbol{\tau_0}\}$, $\{C_{4z}|\boldsymbol{\tau_0}\}$, $\{M_Y|\boldsymbol{\tau_0}\}$, $\{M_z|\boldsymbol{\tau_0}\}$, and $\{C_{2x}|\boldsymbol{\tau_0}\}$ where $\boldsymbol{\tau_0} = (1/2,1/2)$, are depicted in blue, with paired "R" symbols visualizing the symmetry action---each pair represents the original object and its image under the corresponding operation. The dark blue "R" is above the Fe plane, and the light blue one is below. The name of each symmetry is indicated nearbyvafek_PhysRevB.88.134510. (b) Schematic of the observed tunneling spectra on two sublattices. $V_i$ and $V_o$ are the inner and outer gaps. (c) Schematic of interband pairing. (d) Density of states (DOS) projected onto each band for a two-band superconductor with interband pairingPhysRevB.110.094517.
• Figure 2: (a) Fermi surfaces without symmetry-breaking terms. Parameters are taken as $\epsilon_1=-95.00$, $\epsilon_3=-60.00$, $\frac{1}{2m_1}=-2.40$, $\frac{1}{2m_3}=20.58$, $a_1=38.06$, $a_3=-48.56$, $v=39.90$, $p_1=-0.63$, $p_2=-1.87$ in unit of meV to mimic the experimental Fermi surface results ding2024sublattice. (b) Fermi surfaces with symmetry-breaking terms. Parameters in $h'_M(\mathbf{k})$ are taken as $v_1=v_2=v_3=v_4=1.00$ meV. The color in (a) and (b) represents the sublattice weight. (c) Schematic illustration of nodeless $d$-wave pairing in real space. The pairing is positive (red) in the $X-$direction and negative (blue) in the $Y-$direction. (d) Calculated DOS with the combination of normal state symmetry breaking Eq. \ref{['eq:perturbation_band']} and nodeless $d$-wave pairing Eq. \ref{['eq:b2g']}, where $\Delta_d = 12.20$ meV.
• Figure 3: Calculated DOSs with pairing combinations of (a) Eq. \ref{['eq:b2u']} with \ref{['eq:a1g2']}, (b) \ref{['eq:b2u']} with \ref{['eq:a1g0']}. Parameters are (a) $\Delta_{b2u}=15$ meV, $\Delta_{a1g}=10$ meV, (b) $\Delta_{b2u}=18$ meV, $\Delta_{a1g}=-0.5$ meV. The inset in (a) is the schematic illustration of the pairing combination of Eq. \ref{['eq:b2u']} with \ref{['eq:a1g2']} in real space. The intrasublattice pairing is positive (red) on $\mathrm{Fe_A}$–$\mathrm{Fe_A}$ bonds and negative (blue) on $\mathrm{Fe_B}$–$\mathrm{Fe_B}$ bonds, while all intersublattice pairings share the same sign (brown).