Cuprate La2CuO4 Research Articles

New Ideas for Understanding the Structure and Magnetism in AgF2 : Prediction of Ferroelasticity.

In the search for new high‐temperature superconductors, it has been proposed that there are strong similarities between the fluoroargentate AgF2 and the cuprate La2CuO4. We explored the origin of the possible layered structure of AgF2 by studying its parent high‐symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF2. Our findings first stress the large differences between CuF2 and AgF2. Indeed, the parent structure of AgF2 is found to be cubic, naturally devoid of any layering, even though Ag2+ ions occupy trigonal sites that, nevertheless, allow the existence of a Jahn‐Teller effect. The observed Pbca orthorhombic phase is found when the system is cooperatively distorted by a local E⊗e trigonal Jahn‐Teller effect around the silver sites that creates both geometrical and magnetic layering. While the distortion implies that two Ag2+−F− bonds increase their distance by 15 % and become softer, our simulations indicate that covalent bonding and interlayer electron hopping is strong, unlike the situation in cuprate superconductors, and that, in fact, exfoliation of individual planes might be a harder task than previously suggested. As a salient feature, these results prove that the actual magnetic structure in AgF2 is a direct consequence of vibronic contributions involved in the Jahn‐Teller effect. Finally, our findings show that, due to the multiple minima intrinsic to the Jahn‐Teller energy surface, the system is ferroelastic, a property that is strongly coupled to magnetism in this argentate.

Manganite/Cuprate Superlattice as Artificial Reentrant Spin Glass

Emerging physical phenomena at the unit‐cell‐controlled interfaces of transition‐metal oxides have attracted lots of interest because of the rich physics and application opportunities. This work reports a reentrant spin glass behavior with strong magnetic memory effect discovered in oxide heterostructures composed of ultrathin manganite La0.7Sr0.3MnO3 (LSMO) and cuprate La2CuO4 (LCO) layers. These heterostructures are featured with enhanced ferromagnetism before entering the spin glass state: a Curie temperature of 246 K is observed in the superlattice with six‐unit‐cell LSMO layers, while the reference LSMO film with the same thickness shows much weaker magnetism. Furthermore, an insulator–metal transition emerges at the Curie temperature, and below the freezing temperature the superlattices can be considered as a glassy ferromagnetic insulator. These experimental results are closely related to the interfacial spin reconstruction revealed by the first‐principles calculations, and the dependence of the reentrant spin glass behavior on the LSMO layer thickness is in line with the general phase diagram of a spin system derived from the infinite‐range SK model. The results of this work underscore the manganite/cuprate superlattices as a versatile platform of creating artificial materials with tailored interfacial spin coupling and physical properties.

Competing Striped Structures in La2CuO4+y

High temperature superconductivity emerges in unique materials, like cuprates, that belong to the class of heterostructures at atomic limit, made of a superlattice of superconducting atomic layers intercalated by spacer layers. The physical properties of a strongly correlated electronic system, emerge from the competition between different phases with a resulting inhomogeneity from nanoscale to micron scale. Here, we focus on the spatial arrangements of two types of structural defects in the cuprate La2CuO4+y : (i) the local lattice distortions in the CuO2 active layers and (ii) the lattice distortions around the charged chemical dopants in the spacer layers. We use a new advanced microscopy method: scanning nano X-ray diffraction (nXRD). We show here that local lattice distortions form incommensurate nanoscale ripples spatially anticorrelated with puddles of self-organized chemical dopants in the spacer layers.

Synthesis of CuO, La2O3, and La2CuO4 by the Thermal-Decomposition of Oxalates Precursors Using a New Method

A new procedure to prepare oxides materials from oxalates complexes precursors is presented. Detailed description of the method is reported, illustrated with examples of the preparation of oxides. A solid-solid reaction between the oxalic acid and nitrate salt of metal leads to the formation of the corresponding oxalate. These oxalates are used as precursors and are heated to form the corresponding oxides by thermal-decomposition. This article relates the first examples of oxides prepared by this new method: the synthesis of the copper oxide CuO, the hexagonal phase of lanthanum oxide La2O3, and the lanthanum cuprate La2CuO4. All the samples are characterized by SEM and XRD. Thermal analysis TGA and DTA show the expected thermo-decomposition behavior for both copper and lanthanum oxalates. This study reveals also the formation mechanism of the cuprate La2CuO4 from the used precursor.

Transitions Between Lanthanum Cuprates: Crystal Structures of T′, Orthorhombic, and K2NiF4‐type La2CuO4

AbstractThrough low‐temperature synthesis in CsOH flux, lanthanum cuprate La2CuO4 can be obtained in a metastable form, the so‐called T′ modification (tetragonal, I4/mmm, no. 139, a = 400.95(2) pm, c = 1254.08(7) pm). When heated, this T′ phase transforms into a K2NiF4‐type modification, whose crystal structure was now refined from X‐ray powder data (tetragonal, I4/mmm, no. 139, a = 383.29(3) pm, c = 1331.3(2) pm at T = 1073 K). The well‐known orthorhombic phase (s.g. Cmce, no. 64, a = 536.14(3) pm, b = 1315.53(8) pm, c = 540.20(3) pm) – usually obtained via conventional solid state synthesis – was observed to form upon cooling from the K2NiF4‐type modification. High‐temperature powder diffractometry allowed crystal structure refinements for all of the three phases.

Energy and symmetry of dd excitations in undoped layered cuprates measured by Cu L3 resonant inelastic x-ray scattering

We measured the high-resolution Cu L3 edge resonant inelastic x-ray scattering (RIXS) of undoped cuprates La2CuO4, Sr2CuO2Cl2, CaCuO2 and NdBa2Cu3O6. The dominant spectral features were assigned to dd excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used these to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3d states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the single-ion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of dd excitation energies carries important consequences for the physics of high-Tc superconductors. On the one hand, we found that the minimum energy of orbital excitation is always ⩾1.4 eV, i.e. well above the mid-infrared spectral range, which leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of dd excitations on the superconducting gap in cuprates.

Dispersion of Magnetic Excitations in the CuprateLa2CuO4andCaCuO2Compounds Measured Using Resonant X-Ray Scattering

By resonant inelastic x-ray scattering in the soft x-ray regime we probe the dynamical multiple-spin correlations in the antiferromagnetic cuprates La2CuO4 and CaCuO2. High resolution measurements at the copper L3 edge allow the clear observation of dispersing bimagnon excitations. Theory based on the ultrashort core-hole lifetime expansion fits the data on these coherent spin excitations without free parameters.

Dzyaloshinsky-Moriya antisymmetric exchange coupling in cuprates: Oxygen effects

We revisit a problem of Dzyaloshinsky-Moriya antisymmetric exchange coupling for a single bond in cuprates specifying the local spin-orbital contributions to Dzyaloshinsky vector focusing on the oxygen term. The Dzyaloshinsky vector and respective weak ferromagnetic moment is shown to be a superposition of comparable and, sometimes, competing local Cu and O contributions. The intermediate oxygen $^{17}$O Knight shift is shown to be an effective tool to inspect the effects of Dzyaloshinsky-Moriya coupling in an external magnetic field. We predict the effect of $strong$ oxygen weak antiferromagnetism in edge-shared CuO$_2$ chains due to uncompensated oxygen Dzyaloshinsky vectors. Finally, we revisit the effects of symmetric spin anisotropy, in particular, those directly induced by Dzyaloshinsky-Moriya coupling.

Complex Phase Separation in Oxygen-Doped Cuprates La2CuO4+ y Superconductors

The multiple-phase separation with coexistence of two different superconducting phases in the phase diagram of oxygen-doped La2CuO4+y cuprates is interpreted with a modified Van der Waals scheme. The model includes the tendency of intercalated oxygen ions to form clusters at preferential densities. We obtain the coexistence and competition of two different superconducting phases with different Tc and interstitial oxygen content y, in agreement with experiments.

Analysis of resonant inelastic x-ray scattering inLa2CuO4

We provide a semiquantitative explanation of a recent experiment on the resonant inelastic x-ray scattering (RIXS) in insulating cuprate La2CuO4 (Y.J. Kim et al., Phys. Rev. Lett. 89, 177003 (2002).). We show theoretically that there are three characteristic peaks in RIXS spectra, two of which are attributed to the charge transfer excitation and are reasonably assigned to those observed experimentally. The lowest energy peak has a relatively large dispersion ($\sim 0.8$ eV) and is suppressed near the zone corner (\pi, \pi), in agreement with the experiment. We stress that electron correlation is an essential factor for explaining the overall energy-momentum dependence of the RIXS spectra consistently.

Anomalies of the low-temperature specific heat of the cuprates La2CuO4 and La2−x MxCuO4 (M = Sr, Ba)

The low-temperature specific heat of the cuprates La2CuO4, La2−x MxCuO4 (M = Sr, Ba) in the temperature interval 2–45 K has been investigated by the technique of pulsed differential calorimetry. It is found that the coefficient of the residual linear term in the specific heat remains constant in the entire experimental temperature interval. It is shown that La atoms play a special role in the formation of the anomaly, associated with the specific nature of the interaction of these atoms with their environment, in the acoustic region of the phonon spectrum of these objects near 6 meV.

The role of La atoms in the formation of a low-temperature heat-capacity anomaly in cuprates La2CuO4,La2−xMxCuO4(M= Sr, Ba) and Nd2−xLaxCuO4

The low-temperature heat capacity of cuprates(La2CuO4,La2−xMxCuO4(M = Sr, Ba) and Nd2−xLaxCuO4) is studied in the temperature range 2–45 K by using pulsed differential calorimetry. It is found that the coefficient of the linear term of heat capacity remains unchanged over the entire temperature interval under investigation. The special role of La atoms in the formation of the anomaly in the acoustic region of the phonon spectrum of these compounds near 6 meV is demonstrated. This anomaly is connected with peculiarities in the interaction of these atoms with the environment.

A nonlinear spin-wave theory of quasi-2D quantum Heisenberg antiferromagnets

Based on the Hartree-Fock approximation the author proposes a nonlinear spin-wave theory of anisotropic 3D (or quasi-2D) quantum Heisenberg antiferromagnets, which reduces to the known spin-wave theories of isotropy when the anisotropy parameter delta takes special values. In the Hartree-Fock approximation the Dyson-transformed Hamiltonian is equivalent to the Holstein-Primakoff-transformed Hamiltonian truncated up to quartic operator terms. The spin-wave lifetime is obtained in the first-order approximation. For very small delta , the Neel temperature TN is much smaller than the coupling constant J, in contrast with TN approximately J in the 3D isotropic case, so that the nonlinear anisotropic spin-wave theory is suitable for a description of the ordering phase as well as the paramagnetic phase (up to J) of layer-like antiferromagnets. Applied to the antiferromagnetism of the cuprate La2CuO4 the quasi-2D nonlinear spin-wave theory describes quite satisfactorily the existing experimental data of the Neel transition temperature, the correlation length above the Neel temperature, and staggered magnetization of the material if J=1034 K and the anisotropy parameter is set to be 4*10-5.