Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La(2)CuO(4), the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature. Here, we measure the spin response of isolated layers of La(2)CuO(4) that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La(2)CuO(4), with no evidence for more complex correlations such as resonating valence bond correlations. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.

We measured the momentum dependence of magnetic excitations in the model spin-1=2 2D antiferromagnetic insulator Sr 2 CuO 2 Cl 2 (SCOC). We identify a single-spin-wave feature and a multimagnon continuum, with different polarization dependences. The spin waves display a large (70 meV) dispersion between the zone-boundary points (, 0) and (=2, =2). Employing an extended t-t 0 -t 00 -U one-band Hubbard model, we find significant electronic hopping beyond nearest-neighbor Cu ions, indicative of extended magnetic interactions. The spectral line shape at (, 0) indicates sizable quantum effects in SCOC and probably more generally in the cuprates. DOI: 10.1103/PhysRevLett.105.157006 PACS numbers: 74.72.Cj, 75.30.Ds, 78.70.Ck Magnetism in low-dimensional cuprates remains of considerable interest, in relation both to the fundamental quest to understand strong electron correlation and quantum spin effects in Mott insulators, and to the search for the mechanism of high-T c superconductivity. To lowest order, the undoped cuprate superconductors can be described by the spin 1=2 two-dimensional (2D) square-lattice nearestneighbor (NN) Heisenberg antiferromagnet, which is among the simplest and most studied models in magnetism [1]. The ground state displays classical order, reduced by quantum fluctuations at zero temperature and destroyed by thermal fluctuations at finite temperature. A possible crossover between renormalized classical [2] and quantum critical [3] scaling was tested experimentally in the undoped cuprates Sr 2 CuO 2 Cl 2 (SCOC) [4] and La 2 CuO 4 (LCO) [5], and in the organometallic salt CuðDCOOÞ 2 Á 4D 2 O (CFTD) [6]. However, while the latter shows only nearest-neighbor coupling, high-energy inelastic neutron scattering (INS) data on LCO [7] suggest that further-neighbor magnetic interactions influence the scaling measurements. Frustrated further-neighbor interactions could also bring the undoped cuprates closer to the valence bond liquid proposed as mechanism for superconductivity [8].It is therefore timely to investigate the excitation spectrum of SCOC, as an important model system. Inelastic neutron scattering (INS) measurements of SCOC have been limited to low energies and small momenta around the ordering wave vector [4]. In this Letter we report the full magnetic excitation spectrum measured by resonant inelastic x-ray scattering (RIXS). We discover a surprisingly large dispersion along the magnetic Brillouin zone boundary (ZB). An analysis of the data in terms of an extended Hubbard model yields a quantitative estimation of sizable further-neighbor electronic hopping. The resulting series of longer-ranged magnetic interactions enhance quantum fluctuations, in agreement with the reduced ordered moment. The importance of quantum fluctuations is further revealed by differences in the spectral line shapes at the (À, 0) and (À=2, =2) ZB points.SCOC is an insulating single-layer parent compound of the high-T c superconducting (SC) materials. It is isostructural to the high-temperature tetragonal phase of...

The high-T c cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin wavesmagnons-of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi 2 Sr 2 CaCu 2 O 8 þ d (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle-hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations.

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spins-1/2 is far from complete. The quantum square-lattice Heisenberg antiferromagnet (QSLHAF), for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum. This quantum effect manifests itself for excitations propagating with the specific wave vector (π, 0). We use polarized neutron spectroscopy to fully characterize the magnetic fluctuations in the metal-organic compound CFTD, a known realization of the QSLHAF model. Our experiments reveal an isotropic excitation continuum at the anomaly, which we analyse theoretically using Gutzwiller-projected trial wavefunctions. The excitation continuum is accounted for by the existence of spatially-extended pairs of fractional S=1/2 quasiparticles, 2D analogues of 1D spinons. Away from the anomalous wave vector, these fractional excitations are bound and form conventional magnons. Our results establish the existence of fractional quasiparticles in the high-energy spectrum of a quasi-two-dimensional antiferromagnet, even in the absence of frustration.

We report angle-resolved photoemission (ARPES) measurements, density functional and model tight-binding calculations on Ba 2 IrO 4 (Ba-214), an antiferromagnetic (T N = 230 K) insulator. Ba-214 does not exhibit the rotational distortion of the IrO 6 octahedra that is present in its sister compound Sr 2 IrO 4 (Sr-214), and is therefore an attractive reference material to study the electronic structure of layered iridates. We find that the band structures of Ba-214 and Sr-214 are qualitatively similar, hinting at the predominant role of the spin-orbit interaction in these materials. Temperature-dependent ARPES data show that the energy gap persists well above T N , and favor a Mott over a Slater scenario for this compound.