Magnetic order in LaEuSrCuO studied [2pt] by Fe Mössbauer spectroscopy

⁵⁷Fe M?ssbauer effect studies of La_1.65Eu_0.20Sr_0.15CuO₄ doped with 0.5 at% ⁵⁷Fe performed in the temperature region 300 K > T > 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La_2-xSr_xCuO₄. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La_2-xSr_xCuO₄ is independent of the RE moment. The direction of the ⁵⁷Fe magnetic moment in the magnetically ordered state is within the CuO₂ plane, while it has been found to be parallel to the c-axis in Nd doped La_2-xSr_xCuO₄. Received: 23 June 1998 / Accepted: 14 July 1998     

Origin of incommensurate spin modulation in La<Subscript>2−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

A model for explaining incommensurate spin modulation observed in La_2?xSr_xCuO₄ is proposed without assuming stripe formation. In this model, all features of the observed spin textures in the entire doping range are associated only with the geometrical relations for the square lattice and with the competition between various forms of ordering of Sr ions.     

Magnetic order in La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125

We present μSR experiments on La_2-x Ba_x CuO₄ and La_1.6-x Nd_0.4 Sr_x CuO₄ for x=0.125. Both of these materials order magnetically with T_N\approx30\ K, while a superconducting sample of La_1.4 Nd_0.4 Sr_0.2 CuO₄ showed no evidence for static copper moments. Our results support the conclusion that the so‐called “1/8” anomaly in La_2-x Ba_x CuO₄ is a result of static (pinned) charge segregation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Floating-zone growth and property characterizations of high-quality La2-хSrхCuO4 superconductor crystals

We have grown underdoped(x=0.11,0.12) and optimally doped(x=0.16) La2 x SrxCuO4 single crystals by the traveling-solvent floating-zone technique.In order to prepare good quality cuprate crystals,we have made much effort to optimize the preparation procedures.For example,we haveadopted the sol-gel route to prepare a highly fine and homogeneous La2xSrxCuO4 precursor powder for fabricating a very dense ceramic feed rod used for the floating-zone growth,and we have also used quite a slow growth rate.The high quality of the grown crystals has been verified by double-crystal x-ray rocking curves,with the full-width-at-half-maximum being only 113-150 arcseconds,which are the best data reported so far for La2xSrxCuO4 crystals.The superconducting critical temperatures of the grown crystals are 30,31 and 38.5 K for x=0.11,0.12 and 0.16 samples,respectively,according to magnetic measurements.     

Charge ordering,superconductivity, and stripes in doped La<Subscript>2</Subscript>CuO<Subscript>4</Subscript>

The special features of the phase diagrams of La_2?xSr_xCuO₄ are considered in terms of the high-temperature superconductivity model according to which the mechanism responsible for the anomalous properties of these compounds is the interaction of electrons with diatomic negative U-centers. A microstructural model that assumes the coexistence of domains with different types of dopant ion ordering is suggested for La_2?xSr_xCuO₄. According to this model, the main characteristics of the experimental phase diagrams of La_2?xSr_xCuO₄ only reflect square lattice geometric relations and competition between different dopant ordering types. Close agreement between the calculated and experimental “superconducting” and “magnetic” phase diagrams is an important argument in favor of the suggested high-temperature superconductivity model.     

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and ¹³⁹LaNQR relaxation rates in La_2–xSr_xCuO₄ for Sr content around 2 and 3 percent are discussed in terms of spin and lattice excitations and of the related ordering processes. It is argued how the phase diagram of La_2–xSr_xCuO₄ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previously thought, with a transition to a quasi-long range ordered state at K, as indicated by neutron scattering data. On the other hand, the ¹³⁹LaNQR spectra are compatible with a transition to an AF phase around K, in agreement with the phase diagram commonly accepted in literature. In this case the peaks in NQR and anelastic relaxation rates around 150 K and 80 K respectively in La_1.98Sr_0.02CuO₄, yield the first evidence of freezing process involving simultaneously lattice and spin excitations, possibly corresponding to motion of charged stripes. Received 18 May 2000 and Received in final form 11 July 2000     

Nernst effect of stripe ordering La<Subscript>1.8−x</Subscript>Eu<Subscript>0.2</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Subscript>

We investigate the transport properties of La_1.8−xEu_0.2Sr_xCuO₄ (x = 0.04, 0.08, 0.125, 0.15, 0.2) with a special focus on the Nernst effect in the normal state. Various anomalous features are present in the data. For x = 0.125 and 0.15 a kink-like anomaly is present in the vicinity of the onset of charge stripe order in the LTT phase, suggestive of enhanced positive quasiparticle Nernst response in the stripe ordered phase. At higher temperature, all doping levels except x = 0.2 exhibit a further kink anomaly in the LTO phase which cannot unambiguously be related to stripe order. Moreover, a direct comparison between the Nernst coefficients of stripe ordering La_1.8−xEu_0.2Sr_xCuO₄ and superconducting La_2−xSr_xCuO₄ at the doping levels x = 0.125 and x = 0.15 reveals only weak differences. Our findings make high demands on any scenario interpreting the Nernst response in hole-doped cuprates.     

Muon spin rotation studies of local magnetism in magnetic and superconducting systems based on the highT c copper oxide structures

In the present paper we will review studies of local magnetism by the muon spin rotation technique in two classes of High-T_C superconductors, namely La_2-xSr_xCuO₄ and YBa₂Cu₃O_x. In addition we present some new results for the existence of local magnetism in hydrogen doped YBa₂Cu₃O₇.     

Effect of heat treatment on electronic phase in underdoped La2-xSrxCuO4 single crystal

Superconducting La1.937Sr0.063CuO4 crystals grown by the travelling-solvent floating-zone technique were thermally treated under various temperatures and oxygen pressures for moderately adjusting the oxygen content. The response of intrinsic electronic property of the crystals to the change of hole density in La2-xSrxCuO4 in the vicinity of the magic doping of x= 1/16 （= 0.0625） is studied in detail by magnetic measurements under various fields up to 1 T. It is found that when the superconducting critical temperature （Tc） increases with the oxygen content, there appears also a new subtle electronic state that can be detected from the differential curves of diamagnetic susceptibility dx/dT of the crystal sample. In contrast with the intrinsic state, the new subtle electronic state is very fragile under the magnetic fields. Our results indicate that a moderate change in oxygen doping does not significantly modify the intrinsic electronic state originally existing at the magic doping level.     

Magnetic interaction in Mg, Ti, Nb doped manganites

An effect of Mn substitution with Me=Mg²⁺, Ti⁴⁺, Nb⁵⁺ in manganites has been investigated by preparing La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃ series. It was established that substitution of manganese with magnesium up to x = 0.16 leads to a collapse of a long-range ferromagnetic order whereas La_0.7Sr_0.3(Mn ^{3 +} _0.85Nb ^{5 +} _0.15)O₃ is ferromagnet with T _C = 123 K and exhibits a large magnetoresistance below Curie point despite an absence of four-valent manganese. Hypothetical magnetic phase diagrams are constructed for La_0.7Sr_0.3(Mn_1-xMe_x)O₃ and La_1-xSr_x(Mn_{1 - x/2}Nb_x/2)O₃. Our results show that Mn³⁺-O-Mn³⁺ exchange interaction is ferromagnetic in the orbitally disordered manganites as well as an increase of Mn⁴⁺ content above 50% from a total amount of manganese ions leads to formation of a spin glass state due to a competition between antiferromagnetic Mn⁴⁺-O-Mn⁴⁺ and ferromagnetic Mn³⁺-O-Mn⁴⁺(Mn³⁺) superexchange interactions. Received 24 January 2002 Published online 9 July 2002     

Theory for the Gaussian Component of the 63Cu Nuclear Spin-Echo Decay Rate 1/T 2G in La2−x Sr x CuO4

We report calculations of the Gaussian component of the ⁶³Cu nuclear spin-echo decay rate 1/T _2G employing the theory for spin susceptibility as derived within the t-J model starting from carrier-free La₂CuO₄ and right up to optimally doped superconducting layered copper oxide La_2?x Sr _x CuO₄. The theory reproduces the temperature and doping behavior of the experimental data for 1/T _2G. A quantitative agreement with experimental 1/T _2G data in doped La_2?x Sr _x CuO₄ compounds is obtained with account of both the spin–spin and “fermion”–“fermion” correlations.     

Theory of the electronic structure and spin susceptibility of La2−x SrxCuO4

We solve the problem of the effect of strong electron correlations on the homogeneous spin susceptibility of current carriers in CuO₂ planes. We show that the dependence of the spin susceptibility χ(T) of high-T _c superconductors of the La_2−x Sr_xCuO₄ type on temperature and the doping index x can be explained fairly well by the two-band model suggested earlier (the singlet-correlated oxygen band plus the lower Hubbard band of copper). The model has features in common with the phenomenological t-J model but cannot be reduced to the latter completely. In contrast to the t-J model, the density of states of the oxygen holes has a peak near the bottom of the band. It is the presence of this peak together with the non-Fermi-liquid properties that explain the unusual behavior of the spin susceptibility of La_2−x Sr_xCuO₄. Zh. éksp. Teor. Fiz. 112, 1763–1777 (November 1997)     

Growth mechanism, microstructure and transport properties of Sr1−x La x CuO2 (x=0.10−0.15) thin films

Sr_1−x La _x CuO₂ (x=0.10−0.15) thin films with an infinite-layer type structure were grown on BaTiO₃ buffered (001) SrTiO₃ substrates by pulsed laser deposition (PLD). The evolution of the growth front was monitored, in-situ, by high-pressure reflection high-energy electron diffraction (RHEED), while the surface morphology was analyzed by means of atomic force microscopy (AFM), ex-situ. X-ray diffraction (XRD) was used to determine the evolution of the film structure with deposition and cooling parameters, as well as to study the type and level of epitaxial strain in the Sr_1−x La _x CuO₂ films. The RHEED data showed that the Sr_1−x La _x CuO₂ films grow on BaTiO₃/SrTiO₃ following a 2D or Stranski-Krastanov mechanism, depending on the La doping level. The transition point (critical thickness d _c) from layer-by-layer like (2D) to island (3D) growth depends on the film stoichiometry: decreasing the La doping concentration x from 0.15 to 0.10, the critical thickness d _c increases from ∼45 nm to ∼75 nm. In order to induce superconductivity, the Sr_1−x La _x CuO₂ films were cooled down under reduction conditions. The as-deposited films showed semiconducting or metallic behavior, the resistivity decreasing with increasing La concentration. Post-deposition vacuum annealing resulted in a superconducting transition onset (but no zero resistance down to 4.2 K) only for some of the x=0.15 Sr_1−x La _x CuO₂ films.     

Observation of a sharp lambda peak in the third harmonic voltage response of high-T superconductor thin films

In this paper, we report on the sharp peak observed in the third harmonic voltage response generated by a bias sinusoidal current applied to several strips patterned in a YBa₂Cu₃O _{7 - δ} thin film and in two La_2-xSr_xCuO₄ thin films, when the temperature is close to the normal-superconductor transition. The lambda-shaped temperature dependence of the third harmonic signal on the current characteristics is studied. Several physical mechanisms of third harmonic generation are examined. Received 13 November 2002 / Received in final form 21 February 2003 Published online 7 May 2003     

Charge stripes seen with X-rays in

Superstructure reflections due to the ordering of holes into stripes in have been studied with high energy X-ray diffraction. These reflections have been observed clearly for the first time in a sample which is superconducting at low temperatures ( K). The stripe peaks vanish above 62(5) K whereas the magnetic signal of the stripe ordering which has been seen with neutrons before is already suppressed at K. Our results confirm that the ordering of spins and holes is driven by the charges as it is found in the case of La_1.6-xNd_0.4Sr_xCuO₄ at the doping level of x =0.12. Received 27 April 1999     

O1s and Mn2p NEXAFS on single-layered La1-xSr1+xMnO4: crystal field effect versus orbital coupling mechanism

O1s and Mn2p near-edge X-ray absorption spectroscopy on La_1-xSr_1+xMnO₄ (0 ≤x ≤0.5) single crystals shows that Sr doping does not only provide holes to the system but also induces a continuous transfer of electrons from out-of-plane d_3z2-r2 to in-plane d_3x2-r2/d_3y2-r2 orbitals. Furthermore, a non-vanishing electron occupation of in-plane d_x2-y2 and out-of-plane d_3z2-r2 orbitals is observed up to relatively high doping contents. These findings demonstrate that the energy difference between all these orbital types has to be very small and manifest that the orbital degree of freedom is determined not just by crystal field effects but also by orbital coupling. Moreover, the doping-dependent transfer of spectral weight observed in the current data identifies La_1-xSr_1+xMnO₄ as a charge-transfer insulator.     

The two-dimensional t-t'-U model as a minimal model for cuprate materials

The addition to the Hubbard Hamiltonian of a t' diagonal hopping term, which is considered to be material dependent for high-T _c cuprate superconductors, is generally suggested to obtain a model capable to describe the physics of high-T _c cuprate materials. In this line of thinking, the two-dimensional t-t'-U model has been studied by means of the Composite Operator Method, which allows to determine the dynamics in a fully self-consistent way by use of symmetry requirements, as the ones coming from the Pauli principle. At first, some local quantities have been calculated to be compared with quantum Monte Carlo data. Then, the structure of the energy bands, the shape of the Fermi surface and the position of the van Hove singularity have been computed as functions of the model parameters and studied by the light of the available experimental data. The results of our study show that there exists two sets of parameters that allows the model to describe the relevant features of the 1-layer compounds Nd_2-xCe_xCuO₄ and La_2-xSr_xCuO₄. On the other hand, for the 2-layer compound YBa₂Cu₃O _{7 - δ} is not possible to find a reasonable set of parameters which could reproduce the position of the van Hove singularity as predicted by ARPES experiments. Hence, it results questionable the existence of an unique model that could properly describe the variety of cuprate superconductors, as the two-dimensional t-t'-U model was thought to be. Received 29 March 2000 and Received in final form 10 August 2000     

Modeling of a structural phase transition in La2−x SrxCuO4

The structural, vibrational, and elastic properties of La₂CuO₄ are calculated using a model for calculating the energy of the crystal based on interionic potentials with the multiparticle Jahn-Teller contribution included explicitly. The microscopic reasons for the structural instability of the La₂CuO₄ lattice relative to rotations of the oxygen octahedra are investigated. A structural phase transition from the orthorhombic phase (space group D _2h ¹⁸ ) into the tetragonal phase (space group D _4h ¹⁷ ) under hydrostatic compression of an La_2−x Sr_xCuO₄ crystal is modeled. The (P,x) structural phase diagram for La_2−x Sr_xCuO₄ is constructed. Fiz. Tverd. Tela (St. Petersburg) 41, 1096–1102 (June 1999)     

Variations of ionic binding energies and the distribution of charge carriers in orthorhombic La2−x Sr x CuO4

The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La_2−x Sr _x CuO₄ has been investigated. Disorder is shown to cause strong variations of binding energies of the ions ranging to some eV for Sr contentsx=0.1. Balancing the energy for a hole transport, Cu³⁺+O²⁻→Cu²⁺+O⁻, and taking binding energy variations into account, the process is realized to become possible without consuming energy for a subset Θ for allx Cu³⁺ in one formula unit of La_2−x Sr _x CuO₄. The functions Θ(x) are presented for hole transports to apex and in-plane oxygens, respectively. The delocalization of charge carriers is interpreted to be caused by valency disorder on metal lattice sites.