In spin-wave theory 7 , quantum fluctuations describe the occupations of the bosonic modes as perturbations to the Néel state 8 . The ground state can be viewed as a Néel state with finite boson density. The corresponding elementary excitation should therefore be considered as a magnon renormalized by its higher-order expansions, i.e., the magnon-magnon interactions. On an experimental level, it has however been difficult to measure or gauge the strength of quantum fluctuations. As mentioned above, magnon excitations 9 of the Néel state should be influenced by quantum fluctuations. The magnon dispersion is described by ℏω = Z c (k)ϵ k , where ϵ k is the "bare" magnon dispersion set by potential and kinetic energy scales, and Z c ðkÞ = Z 0 c ð1 + f k Þ is the renormalization factor stemming from quantum fluctuations with f k being a momentum dependent function. In the strong coupling limit (U/t → ∞) quantum fluctuations are suppressed implying f k → 0 and Z c (k) ≈ 1.18 is essentially momentum independent 10,11 . As quantum fluctuations grow stronger with gradually moderate values of U/t, the renormalization factor Z c (k) increases and acquires momentum dependence through a non-negligible f k . This limit governed by quantum fluctuations is interesting as it may provide physics beyond the antiferromagnetic Néel state.

Conceptually, this moderate U/t limit is complicated due to a multitude of comparable magnetic exchange interactions. Nearest and nextnearest neighbor exchange interactions are given by J 1 = 4t 2 /U and J 2 = 4t 4 /U 3 , in the projection of the Hubbard onto the Heisenberg model. In the moderate or weak interaction limit, higher-order exchange interaction terms are gaining prominence. The ring-exchange interaction J □ = 80t 4 /U 3 becomes a significant fraction of J 1 ( J □ /J 1 = 20t 2 /U 2 ) and manifests by a magnon zone-boundary dispersion [12][13][14][15][16][17] . In this limit, higher-order hopping integral t 0 , can introduce new magnetic interaction term J 0 = 4t 02 =U that further adds to enhance the zone boundary (ZB) dispersion. Within the Hubbard-Heisenberg model, the zone boundary dispersion, quantum fluctuations, and Z c correlate in the U/t and t 0 =t parameter space. In fact, the renormalization factor Z c gains its momentum dependence from the higher-order exchange interactions.

Enhanced quantum fluctuations may thus introduce new magnetic ground states and with that exotic magnonic quasiparticles 18,19 . It is thus interesting to study materials with significant higher-order exchange couplings. In the cuprates, ACuO 2 with A = Sr, Ca has been studied with electron spectroscopy and resonant inelastic x-ray scattering (RIXS) 17,18,20 due to its large ring exchange interaction. Yet, no experiments have demonstrated the importance of magnon-magnon interactions and quantum fluctuations through direct measurements of a momentum dependent magnon renormalization factor.

Here we provide a RIXS study of SrCuO 2 (SCO) realized in thin-film format, which demonstrates a Mott-insulating nature by electron spectroscopy measurements 3,4 . Analysis of the RIXS spectra led us to derive the single-and bi-magnon dispersions. Starting from an effective Heisenberg representation of the Hubbard model, we show that the observed single-magnon dispersion is inconsistent with a constant Z c for reasonable values of kinetic energy scales. We thus conclude that, in SrCuO 2 , quantum fluctuations are significantly influencing the magnon dispersion. Further, our analysis shows that the observed magnon dispersion is well described when introducing significant momentum dependence to Z c by evaluating the magnon-magnon interactions. This finding is further supported by comparing cuprate compounds with different correlation strengths. Our results thus provide a gauge for quantum fluctuations which are getting increasingly important as U/t is reduced. Possible exotic magnetic ground states, emerging from quantum fluctuations are here explored by classical Monte Carlo calculations.

The crystal field environment around the copper site in SrCuO 2 is shown schematically in Fig. 1a. In contrast to, for example, La 2 CuO 4 (LCO), no apical oxygen is present in SrCuO 2 . Examples of Cu L-edge RIXS spectra, covering magnetic and dd excitations, are shown in Fig. 1b. The absence of the apical oxygen in SrCuO 2 pushes the d z 2 excitation well below the t 2g excitations-as previously established in CaCuO 2 d yz states, respectively. The origin of the additional peak at ~2.06 eV, which has also been observed in CaCuO 2 17,21 , has been attributed to the incoherent component of the d xz /d yz orbital excitations due to the coupling to magnetic excitations 22 .

Magnetic excitations have been recorded systematically along the (h, 0), (h, h), and zone boundary (azimuthal ϕ rotation with a constant in-plane momentum amplitude Q // ) directions with both linear vertical (σ) and linear horizontal (π) incident light polarizations. A singlemagnon excitation manifests clearly in the π channel (see Fig. 1). When switching to σ polarization, the single-magnon is suppressed as expected 23 , and an excitation at higher energy appears. We interpret this as a magnetic continuum that in SrCuO 2 (and CaCuO 2 ) has a structure-sometimes referred to as a bi-magnon excitation [24][25][26] . In what follows, we extract the single-magnon and bi-magnon dispersions along the high symmetry directions.

We analyze the low-energy part of the RIXS spectra by considering four components that include elastic scattering (gray shaded areaenhanced in the σ channel 23 ), single-and bi-magnon (orange shaded area) excitations, and a smoothly varying background (gray dashed line). Elastic scattering is mimicked by a Gaussian function centered at zero energy loss. The energy width is slightly larger than the instrumental resolution due to unresolved phonon modes 27,28 . The singleand bi-magnon excitations are described respectively by a damped harmonic oscillator convoluted with the instrumental resolution and a Gaussian function. Background is modeled by a second-order polynomial. In grazing-exit geometry, RIXS cross section from the magnon (bi-magnon) is generally enhanced when using the π (σ) polarized incident lights-as shown in Fig. 1c, d and Supplementary Fig. 1. We fit globally across the two light polarizations to extract the two magnetic contributions. The resulting single-and bi-magnon dispersions are plotted on top of the inelastic RIXS spectral weight (π polarization) in Fig. 2a-c. Consistent with previous reports on CaCuO 2 17,18 , a large zone boundary dispersion E ZB = _ωð 1 2 ,0Þ À _ωð 1 4 , 1 4 Þ of the single magnon excitation is observed with an essentially non-dispersive section along the (h, h) direction. Away from the Brillouin zone center, the bimagnon dispersion ℏω bm roughly mimics the single magnon dispersion ℏω sm . At the zone boundary position ð 1 4 , 1 4 Þ, ω bm /ω sm ≈ 2. This ratio however varies significantly along the high symmetry directions.

The fitting of the single-and bi-magnon excitations also provides information about spectral weight and quasiparticle lifetime. For most of the Brillouin zone, the energy width of the single-magnon is resolution-limited. However, around (0.5, 0) spectral weight suppression and shorter single-magnon lifetimes are observed consistently with what has previously been reported in La 2 CuO 4 and CaCuO 2 14,17,18 .

The single-magnon dispersion of SrCuO 2 features two peculiar characteristics. A steep zone boundary dispersion is followed by a nondispersive section along the (h, h) direction. Magnon excitations of layered copper-oxides have been discussed via a Heisenberg Hamiltonian derived from the Hubbard model 2 . In the simplest form, the nearest-neighbor exchange interaction J 1 = 4t 2 /U is described through the Coulomb interaction U and nearest-neighbor hopping integral t. The magnon dispersion is, in this limit, isotropic-given by

Early neutron scattering experiments on La 2 CuO 4 12

, however, revealed a zone boundary dispersion indicating the importance of higher-order exchange interaction terms. To account for this zone boundary term, a ring exchange interaction J □ ~t4 /U 3 was included to satisfactorily describe the observed magnon 12,14 . Later more detailed studies 29,30 included higher-order hopping terms, i.e., next, and next-next nearestneighbor hopping integrals t 0 and t″. This extended model, yields a magnon dispersion _ω = Z c ðU,t,t 0 ,t 00 Þϵ k ðU,t,t 0 ,t 00 Þ, where Z c is a momentum dependent quantum renormalization factor and

is the bare magnon dispersion with A k and B k determined by U, t, t 0 , and t″, as described in refs. 29,30. In the U/t → ∞ limit, the magnon-magnon renormalization factor Z c is momentum independent, and ϵ k ðU,t,t 0 ,t 00 Þ has an analytic expression 16,31 . This expression has been used to fit the magnon dispersion of La 2 CuO 4 , with realistic values of U, t, t 0 , and t″. In particular, ratios of t 0 =t ∼ À 0:4 and t″/t ~0.2 are found consistent with density functional theory calculations 32 and angle-resolved photoemission spectroscopy experiments 16,33 . For SrCuO 2 , however, the constant Z c solution does not provide a satisfactory description of the observed single-magnon dispersion (see blue dashed lines in Fig. 3g-l). As La 2 CuO 4 and SrCuO 2 share similar square lattice structures, similar values of t 0 =t are expected. However, an unbiased fit yields unphysical values for the hopping parameters and physical sensible values provide poor fits. We are thus led to reject the initial ansatz

Consequently, we fit _ω = Z c ðU,t,t 0 ,t 00 Þϵ k ðU,t,t 0 ,t 00 Þ in a numerical self-consistent fashion. For La 2 CuO 4 , this methodology confirms that Z c is roughly constant (see Fig. 3a-c and Table 1) with marginal changes to U,t,t 0 ,t 00 (compared to the constant Z c model 16 ). However, for SrCuO 2 , an entirely new solution emerges. Values of t 0 =t and t″/t comparable to La 2 CuO 4 and a smaller U/t now describes the magnon dispersion-see solid lines in Fig. 3 and Table 1. We stress that this new solution describes the observed dispersion using fewer fitting parameters as Z c is now given by U,t,t 0 ,t 00 . The moderate value of U/t implies a magnon-magnon renormalization factor that is strongly momentum dependent (see Fig. 3g-i), i.e., f k in Z c = Z 0 c ð1 + f k Þ is no longer negligible. Our results thus indicate that quantum fluctuations have a significant impact on the magnon dispersion in SrCuO 2 .

In Fig. 4, we plot the constant χ 2 (goodness-of-fit) contour lines that encircle solutions that are within 10% of the minimum of χ 2 . With the currently available data, a rather broad set of parameters describe the magnon dispersion of La 2 CuO 4 . For SrCuO 2 , however, χ 2 has a unique well-defined minimum confined to a narrow region of the parameter space. In Table 1, the fitting values to describe the magnon dispersions for La 2 CuO 4 and SrCuO 2 are listed. For SrCuO 2 , these values represent a minimum in the χ 2 function. For La 2 CuO 4 , we further constrain the solutions by fixing t 0 =t = À 0:4. In Fig. 4, the modeled zone boundary dispersion is plotted as a function of J □ /J 1 and J 0 =J 1 . Within the same parameter space, the Brillouin zone average Z c is shown. Generally, the model displays a correlation between magnon ZB dispersion and Z c . The Heisenberg-Hamiltonian projection from Hubbard model is breaking down in the limit where

Compared to La 2 CuO 4 , SrCuO 2 displays a larger ring exchange coupling J □ . In fact, the fitting parameters obtained for SrCuO 2 are close to the limit where the Heisenberg representation of the Hubbard model breaks down. This limit is characterized by a complete suppression of the staggered magnetization and the imaginary solution of the magnon dispersion (Fig. 4). It has been reported that the absence of apical oxygen in cuprates leads to a decrease in the electronic correlation strength 34,35 , which agrees with the observations here. To The solid lines are corresponding fits using a Hubbard model including higher-order terms (see text). The parameters extracted from the fits are listed in Table 1. Blue dashed lines mark the magnon dispersion obtained assuming a constant Z c = 1.219 16,29 , with U = 2.15 eV, U/t = 6.25, t 0 =t = À 0:4, and t 00 =t 0 = À 0:

5. Error bars indicate one standard deviation. The in-plane momentum amplitude Q // takes 0.461, 0.444, and 0.463 for LCO/STO, LCO/LSAO, and SCO/GSO, respectively. Data on LCO/STO and LCO/LSAO are taken from ref. 16. Top panels (a-c, g-i) display the momentum dependence of the quantum fluctuation factor Z c obtained from fitting with the Hubbard model.

0Þ[%] LCO/STO 408 3.38 0.4 0.5 8.33 1.45 11.6 18.9 LCO/SLAO 474.9 3.83 0.4 0.5 8.06 1.47 12.9 21.1 CCO 17 498.7 3.39 0.42 0.5 6.80 1.89 57.3 40.1 SCO/NGO 425.6 2.66 0.4 0.5 6.25 1.91 64.5 47.3

Nearest, next-nearest neighbor hopping integral t, t 0 , and Coulomb interaction U are obtained through self-consistently fitting the observed single magnon dispersion. Ratio between the next-next nearest hopping integral t″ and t 0 was fixed to literature values 33,54,55 . Resulting electron correlation strength U/t is anti-correlated with the zone boundary dispersion ratio E ZB =Eð 1 2 ,0Þ, average ( Z c ), and

corroborate our findings, we analyzed the magnon dispersion of CaCuO 2 (CCO), which is another infinite-layer cuprate compound with a large ring-exchange coupling 17 . As observed in ref. 17, the one-band Hubbard model with underestimated quantum renormalization fails to describe the magnon dispersion and yields an unrealistically small U (U/t = 4.9). As shown in Fig. 4 and Supplementary Figs. 2 and 3, the magnon dispersions in both compounds are only well described when including substantial quantum corrections generated from magnonmagnon interactions. We thus demonstrate that SrCuO 2 -a moderately correlated Mott insulator-hosts strong quantum fluctuations that can potentially stabilize ground states beyond the AF ordered Néel state. Enhancing further J □ /J 1 or J 0 =J 1 would be of great interest to explore new quantum matter ground states. To gain insight into the possible magnetic ground states when the Néel order breaks down by the increase of t and t 0 , we perform Monte Carlo calculations using a Heisenberg Hamiltonian including the first-, second-, and third-nearest-neighbor, as well as a four-spin ring exchange coupling (see "Methods").

To compare with the experimental results on SrCuO 2 , we fixed U = 2.66 eV and t 00 =t 0 = À 0:5. Incommensurate magnetic orders characterized by a quartet of magnetic Bragg peaks around (0.5, 0.5), i.e., with wave vectors Q M = (0.5 ± δ, 0.5) and (0.5, 0.5 ± δ),

Þ are found in the parameter space between the antiferromagnetic Néel and columnar orders (see Supplementary Fig. 4 for examples of the calculated spin structure factor). We plot in Fig. 4d the distance δ between Q M and the Néel wave vector (0.5, 0.5) as a function of t and t 0 =t. While SrCuO 2 is in the Néel AF ordered state, it is located not far from incommensurate magnetic ordered phases which can be reached by increasing t. Further increase of t 0 =t enhances the next-nearest neighbor coupling and stabilizes the columnar antiferromagnetic order. This tuning can be potentially realized by strain application with different substrates 16,36 . Explorations on possible strain or pressure induced quantum critical behavior would provide more insights into the nature of the magnetic ground states. Note that recent theoretical and numerical works on the two-dimensional Hubbard model have shown that under small doping, the Néel order becomes unstable and replaced by other magnetic orders [37][38][39][40][41] . A spin-charge stripe state with an incommensurate ordering wave vector has also been experimentally established in underdoped cuprates [42][43][44] . We point out that the classical Monte Carlo simulations do not capture the quantum nature of the problem, but they show the parameter space, where the Néel state is expected to break down. When antiferromagnetic order is suppressed by the strong quantum fluctuations-stemming from magnon-magnon interactions-near the phase boundary, superconductivity could be potentially enhanced. It would therefore be of great interest to study how the different magnetic ground states influence superconductivity upon doping. Future theoretical studies including extended dynamical mean-field theory calculations on the spin excitations [45][46][47][48][49] -beyond the scope of the current work-could offer more insights into the relationship between the quantum fluctuations and the degenerate ground states near the critical region. On the experimental front, such quantum effects could also be addressed by comparative RIXS measurements at a higher temperature (T ~0.1J), which call for future exploration.

High-quality SrCuO 2 and La 2 CuO 4 thin films were grown using molecular beam epitaxy (MBE). The SrCuO 2 (~15 nm) film is deposited on a (110) GdScO 3 (GSO) substrate 4,50 , and La 2 CuO 4 films are grown on (001) SrTiO 3 (STO) and LaSrAlO 4 (LSAO) substrates, with a thickness of 7-8 and 18-19 nm, respectively 16 .

Cu L 3 -edge RIXS experiments were carried out at the ADRESS beamline 51,52 , of the Swiss Light Source (SLS) synchrotron at the Paul Scherrer Institut. All data were collected at base temperature (~20 K) under ultrahigh vacuum (UHV) conditions, 10 -9 mbar or better. RIXS spectra were acquired in grazing-exit geometry with both linear horizontal (π) and linear vertical (σ) incident light polarizations with a fixed scattering angle 2θ = 130 ∘ . The two-dimensional nature of the system ensures that the out-of-plane dependence of the magnon dispersion is negligible, as confirmed in a recent RIXS study on CaCuO 2

Fitting of the magnon dispersion was done by calculating selfconsistently Z c and ϵ k in _ω = Z c ðU,t,t 0 ,t 00 Þϵ k ðU,t,t 0 ,t 00 Þ. We used t 00 =t 0 = À 0:5 54 . Fitting parameters U and t were obtained by minimizing χ 2 . The staggered magnetization is calculated by

where N is the number of spin on the square lattice. Our code is available upon request.

Monte Carlo simulations were carried out using the Heisenberg Hamiltonian projected from the Hubbard model, taking into consideration the leading contributions of t 0 and t″:

S i Á S j + J 2 + J 0 À Á X hi,i 0 i S i Á S i 0 + J 3 + J 00 À Á X hi,i 00 i S i Á S i 00 + J & X hi,j,k,li

where

À Á 2 and

The first three sums count respectively nearest, nextnearest, and next-next-nearest neighbor using indices 〈i, j〉, hi,i 0 i and 〈i, i″〉. The last sum counts around the squares following the clockwise direction. Simulations were performed for a temperature T = 0.1 K on a sheet of 50 × 50 unit cells in the (a, b)-plane, i.e., 2500 magnetic sites with classical spin S = 1 2 . For each set of input values, ðU,t,t 0 ,t 00 Þ, the simulation ran for 10 7 Monte Carlo steps with random starting configurations. The Monte Carlo calculation code is available upon request.