Direct observation of orbital hybridisation in a cuprate superconductor.

The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates) materials remains heavily debated. Effective low-energy single-band models of the copper-oxygen orbitals are widely used because there exists no strong experimental evidence supporting multi-band structures. Here, we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital ([Formula: see text] and [Formula: see text]) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity.

Introduction

Identifying the factors that limit the transition temperature Tc of high-temperature cuprate superconductivity is a crucial step towards revealing the design principles underlying the pairing mechanism[1]. It may also provide an explanation for the dramatic variation of Tc across the known single-layer compounds[2]. Although superconductivity is certainly promoted within the copper-oxide layers, the apical oxygen position may play an important role in defining the transition temperature[3-7]. The CuO6 octahedron lifts the degeneracy of the nine copper 3d-electrons and generates fully occupied t2g and 3/4-filled eg states[8]. With increasing apical oxygen distance dA to the CuO2 plane, the eg states split to create a 1/2-filled band. The distance dA thus defines whether single or two-band models are most appropriate to describe the low-energy band structure. It has also been predicted that dA influences Tc in at least two different ways. First, the distance dA controls the charge transfer gap between the oxygen and copper site which, in turn, suppresses superconductivity[5,9]. Second, Fermi-level hybridisation, depending on dA, reduces the pairing strength[6,10]. Experimental evidence, however, points in opposite directions. Generally, single-layer materials with larger dA have indeed a larger Tc[2]. However, scanning tunneling microscopy (STM) studies of Bi-based cuprates suggest an anti-correlation between dA and Tc[11].

In the quest to disentangle these causal relation between dA and Tc, it is imperative to experimentally reveal the orbital character of the cuprate band structure. The comparably short apical oxygen distance dA makes La2−SrCuO4 (LSCO) an ideal candidate for such a study. Experimentally, however, it is challenging to determine the orbital character of the states near the Fermi energy (EF). In fact, the band has never been identified directly by angle-resolved photoelectron spectroscopy (ARPES) experiments. A large majority of ARPES studies have focused on the pseudogap, superconducting gap and quasiparticle self-energy properties in near vicinity to the Fermi level[12]. An exception to this trend are studies of the so-called waterfall structure[13-17] that lead to the observation of band structures below the band[14,16]. However, the origin and hence orbital character of these bands was never addressed. Resonant inelastic X-ray scattering has been used to probe excitations between orbital d-levels. In this fashion, insight about the position of , d, d and d states with respect to has been obtained[18]. Although difficult to disentangle, it has been argued that for LSCO the level is found above d, d and d[19,20]. To date, a comprehensive study of the momentum dependence is missing and therefore the coupling between the and bands has not been revealed. X-ray absorption spectroscopy (XAS) experiments, sensitive to the unoccupied states, concluded only marginal hybridisation of and states in LSCO[21]. Therefore, the role of hybridisation remains ambiguous[22].

Here we provide direct ultraviolet and soft-X-ray ARPES measurements of the band in La-based single-layer compounds. The band is located about 1 eV below the Fermi level at the Brillouin zone (BZ) corners. From these corners, the band is dispersing downwards along the nodal and anti-nodal directions, consistent with density functional theory (DFT) calculations. The experimental and DFT band structure, including only and orbitals, is parametrised using a two-orbital tight-binding model[23]. The presence of the band close to the Fermi level allows to describe the Fermi surface topology for all single-layer compounds (including HgBa2CuO4+ and Tl2Ba2CuO6+) with similar hopping parameters for the orbital. This unification of electronic parameters implies that the main difference between single-layer cuprates originates from the hybridisation between and orbitals. The significantly increased hybridisation in La-based cuprates pushes the van-Hove singularity close to the Fermi level. This explains why the Fermi surface differs from other single-layer compounds. We directly quantify the orbital hybridisation that plays a sabotaging role for superconductivity.

Results

Material choices

Different dopings of LSCO spanning from x = 0.12 to 0.23 in addition to an overdoped compound of La1.8−Eu0.2SrCuO4 with x = 0.21 have been studied. These compounds represent different crystal structures: low-temperature orthorhombic, low-temperature tetragonal and the high-temperature tetragonal. Our results are very similar across all crystal structures and dopings (Supplementary Fig. 1). To keep the comparison to band structure calculations simple, this paper focuses on results obtained in the tetragonal phase of overdoped LSCO with x = 0.23.

Electronic band structure

A raw ARPES energy distribution map (EDM), along the nodal direction, is displayed in Fig. 1a. Near EF, the widely studied nodal quasiparticle dispersion with predominately character is observed[12]. This band reveals the previously reported electron-like Fermi surface of LSCO, x = 0.23[24,25] (Fig. 1b), the universal nodal Fermi velocity vF ≈ 1.5 eVÅ[26] and a band dispersion kink around 70 meV[26]. The main observation reported here is the second band dispersion at ~1 eV below the Fermi level EF (Figs. 1 and 2) and a hybridisation gap splitting the two (Fig. 3). This second band—visible in both raw momentum distribution curves (MDC) and constant energy maps—disperses downwards away from the BZ corners. Since a pronounced k dependence is observed for this band structure (Figs. 2 and 4) a trivial surface state can be excluded. Subtracting a background intensity profile (Supplementary Fig. 2) is a standard method that enhances visualisation of this second band structure. In fact, using soft X-rays (160–600 eV), at least two additional bands (β and γ) are found below the dominated band crossing the Fermi level. Here, focus is set entirely on the β band dispersion closest to the dominated band. This band is clearly observed at the BZ corners (Figs. 1–3). The complete in-plane (k, k) and out-of-plane (k) band dispersion is presented in Fig. 4.

Fig. 1

ARPES spectra showing eg-bands of overdoped La2−SrCuO4x = 0.23. a Raw ARPES energy distribution map (EDM) along cut 1 as indicated in (c). Dashed green line indicates the position of MDC displayed on top by turquoise circles. A linear background has been subtracted from the MDC which is fitted (blue line) by four Lorentzians (red lines). b–e Constant binding energy maps at EF (b) and at higher binding energies (c–e) as indicated. The photoemission intensity, shown in false colour scale, is integrated over ± 10 meV. Black (red) lines indicate the position of bands. The curve thickness in b, e is scaled to the contribution of the orbital. Semitransparent lines are guides to the eye. f, g EDMs along cut 1 recorded with and light, f sensitive to the low-energy and d/d bands and g the and d-derived bands. All data have been recorded with hν = 160 eV

Fig. 2

Comparison of observed and calculated band structure. a–d Background subtracted (see Methods section) soft-X-ray ARPES EDMs recorded on La2−SrCuO4, x = 0.23 along in-plane high-symmetry directions for k = 0 and k = π/c′ as indicated in g. White lines represent the two-orbital ( and ) tight-binding model as described in the text. The line width in b, d indicates the orbital weight of the orbital. e, f Corresponding in-plane DFT band structure at k = 0 and k = π/c′, calculated for La2CuO4 (see Methods section). The colour code indicates the orbital character of the bands. Around the anti-nodal points (X or R), strong hybridisation of and orbitals is found. In contrast, symmetry prevents any hybridisation along the nodal lines (Γ–M or Z–A). g Sketch of the 3D BZ of LSCO with high symmetry lines and points as indicated

Fig. 3

Avoided band crossing. Left panel: ultraviolet ARPES data recorded along the ant-inodal direction using 160 eV linear horizontal polarised photons. Solid white lines are the same tight-binding model as shown in Fig. 2. Right panel: tight-binding model of the and bands along the anti-nodal direction. Grey lines are the model prediction in absence of inter-orbital hopping (t = 0) between and . In this case, the bands are crossing near the Γ-point. This degeneracy is lifted once a finite inter-orbital hopping parameter is considered. For solid black lines t = −210 meV and other hopping parameters have been adjusted accordingly. Inset indicates the Fermi surface (green line) and the Γ − X cut directions. Coloured background displays the amplitude of the hybridisation term Ψ(k) that vanishes on the nodal lines

Fig. 4

Three-dimensional band dispersion. a k dispersion recorded along the diagonal (π, π) direction of the and bands (along grey plane in b). Whereas the band displays no k dependence beyond matrix element effects, the band displays a discernible k dispersion. The iso-energy map below the cube has binding energy E − EF = −1.3 eV. White lines represent the tight-binding model. b, c Tight-binding representation of the Fermi surface (α band) and iso-energy surface (−1.3 eV) of the β band. The colour code indicates the k-dependent orbital hybridisation. The orbital hybridisation at EF is largest in the anti-nodal region of the k = π/c′ plane where the admixture at k amounts to ~1/3

Orbital band characters

To gain insight into the orbital character of these bands, a comparison with a DFT band structure calculation (see Methods section) of La2CuO4 is shown in Fig. 2. The eg states ( and ) are generally found above the t2g bands (d, d and d). The overall agreement between the experiment and the DFT calculation (Supplementary Fig. 3) thus suggests that the two bands nearest to the Fermi level are composed predominately of and orbitals. This conclusion can also be reached by pure experimental arguments. Photoemission matrix element selection rules contain information about the orbital band character. They can be probed in a particular experimental setup where a mirror-plane is defined by the incident light and the electron analyser slit[12]. With respect to this plane the electromagnetic light field has odd (even) parity for () polarisation (Supplementary Fig. 4). Orienting the mirror plane along the nodal direction (cut 1 in Fig. 1), the and d () orbitals have even (odd) parity. For a final-state with even parity, selection rules[12] dictate that the and d-derived bands should appear (vanish) in the () polarisation channel and vice versa for . Due to their orientation in real-space, the d and d orbitals are not expected to show a strict switching behaviour along the nodal direction[27]. As shown in Fig. 1f, g, two bands (α and γ) appear with -polarised light while for -polarised light bands β and γ′ are observed. Band α which crosses EF is assigned to while band γ has to originate from d/d orbitals as and d-derived states are fully suppressed for -polarised light. In the EDM, recorded with -polarised light, band (β) at ~1 eV binding energy and again a band (γ′) at ~1.6 eV is observed. From the orbital shape, a smaller k dispersion is expected for and d-derived bands than for those from orbitals. As the β band exhibits a significant k dispersion (Fig. 4), much larger than observed for the band, we conclude that it is of character. The γ′ band which is very close to the γ band is therefore of d character. Interestingly, this -derived band has stronger in-plane than out-of-plane dispersion, suggesting that there is a significant hopping to in-plane p and p oxygen orbitals. Therefore the assumption that the states are probed uniquely through the apical oxygen p orbital[21] has to be taken with caution.

Discussion

Most minimal models aiming to describe the cuprate physics start with an approximately half-filled single band on a two-dimensional square lattice. Experimentally, different band structures have been observed across single-layer cuprate compounds. The Fermi surface topology of LSCO is, for example, less rounded compared to (Bi,Pb)2(Sr,La)2CuO6+ (Bi2201), Tl2Ba2CuO6+ (Tl2201) and HgBa2CuO4+ (Hg1201). Within a single-band tight-binding model the rounded Fermi surface shape of the single-layer compounds Hg1201 and Tl2201 is described by setting [6], where t, and are nearest neighbour (NN), next–nearest neighbour (NNN) and next-next–nearest neighbour (NNNN) hopping parameters (Table 1 and Supplementary Fig. 4). For LSCO with more flat Fermi surface sections, significantly lower values of r have been reported. For example, for overdoped La1.78Sr0.22CuO4, r ~ 0.2 was found[24,25]. The single-band premise thus leads to varying hopping parameters across the cuprate families, stimulating the empirical observation that roughly scales with [2]. This, however, is in direct contrast to t–J models that predict the opposite correlation[28,29]. Thus the single-band structure applied broadly to all single-layer cuprates lead to conclusions that challenge conventional theoretical approaches.

Table 1

Tight-binding parameters for single-layer cuprate materials

Compound Doping p	LSCO 0.22	Hg1201 0.16	Tl2201 0.26	LSCO 0.23
Tight binding parameters in units of tα = −1.21 eV
−μ	0.88	1.27	1.35	0.96
−tα′	0.13	0.47	0.42	0.32
tα′′	0.065	0.02	0.02	0.0
t αβ	0	0	0	0.175
t β	–	–	–	0.062
tβ′	–	–	–	0.017
t βz	–	–	–	0.017
−tβz′	–	–	–	0.0017
Ref.	[24]	[39,40]	[41,42]	This work

Comparison of tight-binding hopping parameters obtained from single-orbital and two-orbital models. Once a coupling t between the and band is introduced for La2−SrCuO4, the hopping parameters become comparable to those of Hg1201 and Tl2201

The observation of the band calls for a re-evaluation of the electronic structure in La-based cuprates using a two-orbital tight-binding model (see Methods section). Crucially, there is a hybridisation term between the and orbitals, where t is a hopping parameter that characterises the strength of orbital hybridisation. In principle, one may attempt to describe the two observed bands independently by taking t = 0. However, the problem then returns to the single-band description with the above mentioned contradictions. Furthermore, t = 0 implies a band crossing in the anti-nodal direction that is not observed experimentally (Fig. 3). In fact, from the avoided band crossing one can directly estimate t ≈ −200 meV. As dictated by the different eigenvalues of the orbitals under mirror symmetry, the hybridisation term Ψ(k) vanishes on the nodal lines k = ±k (see inset of Fig. 3). Hence the pure and orbital band character is expected along these nodal lines. The hybridisation Ψ(k) is largest in the anti-nodal region, pushing the van-Hove singularity of the upper band close to the Fermi energy and in case of overdoped LSCO across the Fermi level.

In addition to the hybridisation parameter t and the chemical potential μ, six free parameters enter the tight-binding model that yields the entire band structure (white lines in Figs. 2 and 4). Nearest and next-nearest in-plane hopping parameters between (t, ) and orbitals are introduced to capture the Fermi surface topology and in-plane band dispersion (Supplementary Fig. 4). The k dispersion is described by nearest and next-nearest out-of-plane hoppings (t, ) of the orbital. The four hopping parameters and the chemical potential μ are determined from the experimental band structure along the nodal direction where Ψ(k) = 0. Furthermore, the α and β band dispersion in the anti-nodal region and the Fermi surface topology provide the parameters t, and t. Our analysis reveals a finite band coupling t = −0.21 eV resulting in a strong anti-nodal orbital hybridisation (Fig. 2 and Table 1). Compared to the single-band parametrisation[24] a significantly larger value r ~ −0.32 is found and hence a unification of ratios for all single-layer compounds is achieved.

Finally, we discuss the implication of orbital hybridisation for superconductivity and pseudogap physics. First, we notice that a pronounced pseudogap is found in the anti-nodal region of La1.8−Eu0.2SrCuO4 with x = 0.21—consistent with transport experiments[30] (Supplementary Fig. 5). The fact that t of La1.59Eu0.2Sr0.21CuO4 is similar to t of LSCO suggests that the pseudogap is not suppressed by the hybridisation. To this end, a comparison to the 1/4-filled eg system Eu2−SrNiO4 with x = 1.1 is interesting[31,32]. This material has the same two-orbital band structure with protection against hybridisation along the nodal lines. Both the and bands are crossing the Fermi level, producing two Fermi surface sheets[31]. Despite an even stronger admixture of the derived band a d-wave-like pseudogap has been reported[32]. The pseudogap physics thus seems to be unaffected by the orbital hybridisation.

It has been argued that orbital hybridisation—of the kind reported here—is unfavourable for superconducting pairing[6,10]. It thus provides an explanation for the varying across single-layer cuprate materials. Although other mechanisms, controlled by the apical oxygen distance, (e.g. variation of the copper–oxygen charge transfer gap[4]) are not excluded our results demonstrate that orbital hybridisation exists and is an important control parameter for superconductivity.

Methods

Sample characterisation

High-quality single crystals of LSCO, x = 0.12, 0.23, and La1.8−Eu0.2SrCuO4, x = 0.21, were grown by the floating-zone technique. The samples were characterised by SQUID magnetisation[33] to determine superconducting transition temperatures (Tc = 27, 24 and 14 K). For the crystal structure, the experimental lattice parameters are a = b = 3.78 Å and c = 2c′ = 13.2 Å[34].

ARPES experiments

Ultraviolet and soft-X-ray ARPES experiments were carried out at the SIS[43] and ADRESS[44] beam-lines at the Swiss Light Source and at the I05 beamline at Diamond Light Source. Samples were pre-aligned ex situ using a X-ray LAUE instrument and cleaved in situ—at base temperature (10–20 K) and ultra high vacuum (≤5 × 10−11 mbar)—employing a top-post technique or cleaving device[35]. Ultraviolet (soft X-ray[36]) ARPES spectra were recorded using a SCIENTA R4000 (SPECS PHOIBOS-150) electron analyser with horizontal (vertical) slit setting. All data was recorded at the cleaving temperature 10–20 K. To visualise the -dominated band, we subtracted in Fig. 1f, g and Figs. 2–4 the background that was obtained by taking the minimum intensity of the MDC at each binding energy.

Tight-binding model

A two-orbital tight-binding model Hamiltonian with symmetry-allowed hopping terms is employed to isolate and characterise the extent of orbital hybridisation of the observed band structure[23]. For compactness of the momentum-space Hamiltonian matrix representation, we introduce the vectorswhere κ, κ1 and κ2 take values ±1 as defined by sums in the Hamiltonian and ⊤ denotes vector transposition.

Neglecting the electron spin (spin–orbit coupling is not considered) the momentum-space tight-binding Hamiltonian, (k), at a particular momentum k = (k, k, k) is then given byin the basis , where the operator c annihilates an electron with momentum k in an eg-orbital d, with α ∈ {x2 − y2, z2}. The diagonal matrix entries are given byandwhich describe the intra-orbital hopping for and orbitals, respectively. The inter-orbital nearest-neighbour hopping term is given by

In the above, μ determines the chemical potential. The hopping parameters t, and characterise NN, NNN and NNNN intra-orbital in-plane hopping between orbitals. t and characterise NN and NNN intra-orbital in-plane hopping between orbitals, while t and characterise NN and NNN intra-orbital out-of-plane hopping between orbitals, respectively (Supplementary Fig. 3). Finally, the hopping parameter t characterises NN inter-orbital in-plane hopping. Note that in our model, intraorbital hopping terms described by the vectors (Eq. (1)) are neglected as these are expected to be weak compared to those of the orbital. This is due to the fact that the inter-plane hopping is mostly mediated by hopping between apical oxygen p orbitals, which in turn only hybridise with the orbitals, not with the orbitals. Such an argument highlights that the tight-binding model is not written in atomic orbital degrees of freedom, but in Wannier orbitals, which are formed from the Cu d orbitals and the ligand oxygen p orbitals. As follows from symmetry considerations and is discussed in ref. [10], the Cu orbital together with the apical oxygen p orbital forms a Wannier orbital with symmetry, while the Cu orbital together with the four neighbouring p orbitals of the in-plane oxygen forms a Wannier orbital with symmetry. One should thus think of this tight-binding model as written in terms of these Wannier orbitals, thus implicitly containing superexchange hopping via the ligand oxygen p orbitals. Additionally we stress that all hopping parameters effectively include the oxygen orbitals. Diagonalising Hamiltonian (2), we find two bandsand make the following observations: along the k = ±k lines, Ψ(k) vanishes and hence no orbital mixing appears in the nodal directions. The reason for this absence of mixing lies in the different mirror eigenvalues of the two orbitals involved. Hence it is not an artifact of the finite range of hopping processes included in our model. The parameters of the tight-binding model are determined by fitting the experimental band structure and are provided in Table 1.

DFT calculations

DFT calculations were performed for La2CuO4 in the tetragonal space group I4/mmm, No. 139, found in the overdoped regime of LSCO using the WIEN2K package[37]. Atomic positions are those inferred from neutron diffraction measurements[34] for x = 0.225. In the calculation, the Kohn–Sham equation is solved self-consistently by using a full-potential linear augmented plane wave (LAPW) method. The self consistent field calculation converged properly for a uniform k-space grid in the irreducible BZ. The exchange-correlation term is treated within the generalised gradient approximation in the parametrisation of Perdew, Burke and Enzerhof[38]. The plane wave cutoff condition was set to RKmax = 7 where R is the radius of the smallest LAPW sphere (i.e. 1.63 times the Bohr radius) and Kmax denotes the plane wave cutoff.

Data availability

All experimental data are available upon request to the corresponding authors.