Anomalous diamagnetic transitions in Ba2Cu3O4Cl2 and Sr2Cu3O4Cl2 antiferromagnets

The magnetic properties of Ba₂Cu₃O₄Cl₂ and Sr₂Cu₃O₄Cl₂ oxychlorides have been studied. A spontaneous manifestation of diamagnetism has been found at quasi-zero and low fields. The temperature of these transitions coincides with the temperature of the antiferromagnetic ordering in Ba₂Cu₃O₄Cl₂ (T _NI ≈ 337 K) and Sr₂Cu₃O₄Cl₂ (T _NI ≈ 386 K). The recorded diamagnetic signal is ?10^?4 emu. It is comparable with the percentage of the superconducting phase in the sample. It is supposed that the arising superconductivity is due to the formation of the superconducting clusters with the direct overlap of the copper d orbitals owing to the frustration processes.     

High energy spin dynamics of La2CuO4 and La1.9Sr0.1CuO4

The magnetic excitations in La₂CuO₄ and La_1.9Sr_0.1CuO₄ were studied by inelastic neutron scattering up to energies of 76 meV. For the pure sample, the results forE15 meV can be well described by conventional spin wave theory with a spin wave velocity c=0.89±0.07 eVÅ. For lower energies, the observed intensities were somewhat higher than expected from spin wave theory and did not follow the Bose-Einstein factor. For the doped sample, the linewidth in constant-E scans at smallE shows a very short correlation length of =7.5 Å only, which is considerably below the value expected from the concentration dependence reported by other authors. An increase of the linewidth for largeE indicates a reduced spin-wave stiffness when compared to the undoped material. However, the use of a spin-wave picture may not be appropriate as the standard expression for damped spin waves in a paramagnetic material is in serious conflict with experiment.     

Correlated spin dynamics in 2-D quantum Heisenberg antiferromagnets from NMR relaxation in copper formiate tetradeuterate

Proton nuclear magnetic resonance (NMR) T₁ in a single crystal of copper formiate tetradeuterate is used to study the correlated Cu²⁺ spin dynamics and to derive the temperature behavior of the in-plane magnetic correlation length. The results are compared with the predictions of recent theoretical models for the spin dynamics in planar quantum Heisenberg antiferromagnets in a wide temperature range (from the Néel temperature up to a reduced temperatureT/J ～ 1.4, withJ in-plane exchange integral). In particular, it is shown that, in contrast to the predictions of the nonlinear σ model, no crossover to a quantum critical regime occurs and that the experimental findings are well reproduced by deriving the NMR relaxation rate in the framework of the standard mode-mode coupling theory.     

Orbital selection of the double [CuO2] layer compound Ca3Cu2O4Cl2

We present a first-principles investigation on the dynamics and mechanism of the oxidation reaction between water molecules and the reduced PuO_2(110) surface using ab initio molecular dynamics(AIMD) simulations in combination with density functional theory(DFT) + U calculations. We find a dominating dissociation preference of water molecules for the vacancy defect sites on the PuO_2(110) surface, irrespective of the water or vacancy coverage. Due to hybridizations between the frontier orbitals of water molecule and the electronic states of the vacancy vicinity, partial water dissociation at the vacancy sites is exothermic and barrierless. The dissociation product, an OH group, further hydrogenates the PuO_2(110) surface by occupying the vacancy site.We also observe surface vacancy diffusion induced by the interactions between the water molecules and the surface oxygen atom in the proximity of the defect sites.     

Impurity induced spin texture in quantum critical 2D antiferromagnets

We describe the uniform and staggered magnetization distributions around a vacancy in a quantum critical two-dimensional S=1/2 antiferromagnet. The distributions are delocalized across the entire sample with a universal functional form arising from an uncompensated Berry phase. The numerical results, obtained using quantum Monte Carlo simulations of the Heisenberg model on bilayer lattices with up to approximately equal to 10(5) spins, are in good agreement with the proposed scaling structure. We determine the exponent eta' = 0.40+/-0.02, which governs both the staggered and uniform magnetic structure away from the impurity and also controls the impurity spin dynamics.     

Critical temperatures of the anisotropic Heisenberg antiferromagnets

The first eight terms of the high temperature series expansions for the mean-square fluctuations of the magnetization variables are derived for the spin $\text{?}\text{1}\text{2}$ anisotropic Heisenberg antiferromagnets. The antiferromagnetic critical temperatures are determined for the s.c. and b.c.c. lattices.     

Nonlinear magnetic susceptibility and microwave spin dynamics of R2CuO4 quasi-2D antiferromagnets (R=Eu, Pr, Gd)

An experimental study of nonlinear ac magnetic susceptibility and microwave spin dynamics of R₂CuO₄ quasi-2D Heisenberg antiferromagnets (R=Eu, Pr, Gd) has been carried out. The data obtained can be accounted for if one assumes the existence of a random-field (RF) state in the Eu₂CuO₄ and Pr₂CuO₄ tetragonal crystals within the 77⩽T⩽350 K range covered. If this is so, 3D antiferromagnetic order persists only within limited regions, while in CuO₂ layers there are 2D Heisenberg antiferromagnetic spin fluctuations with large correlation lengths. In the Gd₂CuO₄ crystal there exists, besides uniform 3D antiferromagnetic long-range order with weak ferromagnetism, an admixture of an RF-type state. Fiz. Tverd. Tela (St. Petersburg) 41, 1437–1444 (August 1999)     

Neutron scattering study of the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr2CuO2Cl2

We have carried out a neutron scattering investigation of the static structure factorS(q _2D ) (q _2D is the in-plane wave vector) in the two-dimensional spinS=1/2 square-lattice Heisenberg antiferromagnet Sr₂CuO₂Cl₂. For the spin correlation length we find quantitative agreement with Monte Carlo results over a wide range of temperature. The combined Sr₂CuO₂Cl₂-Monte Carlo data, which cover the length scale from 1 to 200 lattice constants, are predicted without adjustable parameteres by renormalized classical theory for the quantum nonlinear sigma model. For the structure factor peakS(0), on the other hand, we findS(0) ² for the reduced temperature range 0.16<T/2 _s <0.36, whereas current theories predict that at low temperaturesS(0)T ^{2 2}. This discrepancy has important implications for the interpretation of many derivative quantities such as NMR relaxation rates. In the ordered phase, we have measured the temperature dependence of the out-of-plane spin-wave gap. Its low-temperature value of 5.0 meV corresponds to an XY anisotropyJ _XY /J=1.4×10^–4. From measurements of the sublattice mangetization we obtain =0.22±0.01 for the order parameter exponent. This may either reflect tricricality as in La₂CuO₄, or it may indicate finite-size two-dimensional XY behavior as suggested by Bramwell and Holdsworth. As in theS=1 system K₂NiF₄, the gap energy in Sr₂CuO₂Cl₂ scales linearly with the order parameter up to the Néel temperature. We also reanalyze static structure factor data for K₂NiF₄ using the exact low temperature result for the correlation length of Hasenfratz and Niedermayer and including the Ising anisotropy explicitly. Excellent agreement between experiment and theory is obtained for the correlation length, albeit with the spin-stiffness _s reduced by 20% from the spin-wave value. As in Sr₂CuO₂Cl₂ we find thatS(0) ² for the reduced temperature range 0.22<T/2 _s <0.47.     

Theory of a single Oxygen hole propagating in Sr2CuO2Cl2: the spin of the quasiparticles in CuO2 planes

Recent photoemission experiments have measured E vs. k for a single hole propagating in antiferromagnetically aligned Sr₂CuO₂Cl₂. Comparisons with (i) the t - t′ - J model, for which the carrier is a spinless vacancy, and (ii) a strong-coupling version of the three-band Emery model, for which the carrier is a S = 1/2 hole moving on the Oxygen sublattice, have demonstrated that if one wishes to describe the quasiparticle throughout the entire first Brillouin zone the three-band model is superior. Here we present a new variational wave function for a single Oxygen hole in the three-band model: it utilizes a classical representation of the antiferromagnetically ordered Cuspin background but explicitly includes the quantum fluctuations of the lowest energy doublet of the Cu-O-Cu bond containing the Oxygen hole. We find that this wave function leads to a quasiparticle dispersion for physical exchange and hopping parameters that is in excellent agreement with the measured ARPES data. We also obtain the average spin of the Oxygen hole, and thus deduce that this spin is only quenched to zero at certain wave vectors, helping to explain the inadequacy of the t - t′ - J model to match the experimentally observed dispersion relation everywhere in the first Brillouin zone.     

μ + SR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sitesSR studies on Sr2CuO2Cl2, La2CuO4 and Nd2CuO4: 2-d magnetism,local fields and muon sites

We compare the temperature dependence of the zero-field muon spin precession frequency observed in Sr₂CuO₂Cl₂ and La₂CuO₄ below the Néel temperature with a 2-d Heisenberg model with an additional small anisotropy. Good agreement of the observed results with this model reflects the highly 2 dimensional magnetic properties of these cuprates. The measured local fields are compared with a simple calculation of dipolar fields in the crystal to search for muon site locations. Our results provide evidence that the muon sites are related to the oxygen atoms in these compounds.