A.A. Zakharov

Superconductivity and phase separation in La2CuO4+x single crystals

Abstract We report a phase separation study on high-quality single crystals of La 2 CuO 4+ x for two concentrations x =0.03 and x =0.04. The excess oxygen was loaded into the crystals at high oxygen pressure p =3 kbar and temperature T =650–700° C . The X-ray diffraction study of these crystals shows the presence of a phase separation process for the x =0.04 sample over the temperature range 230 T x =0.03 sample within the same temperature region. The superconducting transition is observed for both crystals, through the detected T c ≈ K value for the x =0.03 sample is much lower than usually reported in the La 2 CuO 4+ x system (∼30 K), which is additional evidence that here superconductivity is not connected with the new phase formation. Possible reasons for such a behavior and phase diagram of La 2 CuO 4+ x are discussed.

Neutron diffraction study of phase separation in La2CuO4+δ single crystals

Abstract Single crystals of La 2 CuO 4+δ with δ = 0.03 and δ = 0.04 have been studied in the temperature region 10 K ≤ T ≤ 293 K using a high resolution neutron diffractometer. In both crystals, the concentration of excess oxygen is inside the miscibility gap. However, the La 2 CuO 4.03 crystal does not undergo a phase separation — homogeneous oxygen distribution was preserved over the entire temperature region studied, while in the La 2 CuO 4.04 crystal, the phase separation phenomenon, connected with excess oxygen diffusion was observed. Analysis of the diffraction peak width as a function of interplane spacing allowed us to determine the dimensions of the coherent regions of coexisting phases: 975 ± 22 A in the direction of the tetragonal axis of the high temperature phase, and (1460 ± 160) A in the perpendicular direction. Our data provide direct evidence that both the oxygen-rich and oxygen-poor regions have the same Cmca space symmetry.

Electron transitions at 0.1–0.6 eV and DC-conductivity in the semiconducting phase of La2CuO4+x single crystals

Abstract Infrared reflection spectra R(ν) and DC-conductivity σDC in thesemiconducting antiferromagnetic phase of La2CuO4+X single crystals have been investigated. A set of 110, 156, 240 and 550 meV electron transitions active in E ⊥ C polarization has been revealed. As a result of the study of the temperature dependence of R(ν) and σDC, the oxygen content x being varied, a model of spin-polaron complexes has been proposed. This model allows one to qualitatively explain the properties of electron transitions and DC-conductivity.

Spin-glass ordering in non phase separated La2CuO4.03 studied by μSR

Abstract Electron spin-freezing at Tf = 8 K has been detected by muon spin rotation μSR in a superconducting (Tc = 12 K) single crystal of La2CuO4+y ( y ⋍ 0.03) . The sample belongs the series of La2CuO4 single crystals, which after being doped with extra oxygen, do not reveal any macroscopic structural phase separation for y ≤ 0.03, verified by X-ray and neutron diffraction. At T = 2.5 K the crystal fraction with frozen electronic moments amounts to more than 65% of the crystal volume. The shape and parameters of the muon spin polarization function are typical for the Cu2+spin-glass-like state.

Infrared spectra of optical phonons in semiconducting La2CuO4+x single crystals

Abstract Far-infrared reflectivity spectra R ( v ) in polarization E ⊥ C and DC-conductivity σ DC have been studied in a semiconducting phase of La 2 CuO 4+ x single crystals with various oxygen contents. The spectrum of infrared-active optical phonons has been obtained for stoichiometric La 2 CuO 4 . It has been shown that the phonon spectrum should be analyzed in a framework of the orthorhombic crystal structure at T ≤300 K. Extra lines have been found in the phonon spectrum upon doping the crystals with oxygen. The results of studying the temperature dependence of R ( v ) and σ DC allow one to interpret these features as vibrational modes, which are active in the Raman scattering spectrum and displayed in the IR spectrum due to their interaction with self-localized holes.

Structure of non-phase-separated La2CuO4.03 studied by single-crystal neutron diffraction

Abstract The structure of a La 2 CuO 4+ y single crystal with the excess oxygen content y =0.03 has been studied by means of neutron diffraction at T =13 K. Although the extra oxygen content of y =0.03 is inside the miscibility gap of La 2 CuO 4+ y (0.01 y Cmca symmetry. It was confirmed that the extra oxygen, O4, is located close to the site (1/4, 1/4, 1/4). The structural parameters for all atoms lay between those previously determined for the oxygen-poor and oxygen-rich phases representing the edges of the miscibility gap, y =0.01 and 0.06, respectively. Our data allow us to conclude that (1) the phase-separated and non-phase-separated La 2 CuO 4+ y compounds are structurally identical, and (2) the crystal structure of La 2 CuO 4+ y changes monotonically when the extra oxygen content changes from 0 to 0.05.

Mesoscopic phase separation in La 2 CuO 4.02 : A 139 La NQR study

In crystals of La2CuO4.02 oxygen diffusion can be limited to such small length scales, that the resulting phase separation is invisible for neutrons. Decomposition of the 139La NQR spectra shows the existence of three different regions, of which one orders antiferromagnetically below 17K concomitantly with the onset of a weak superconductivity in the crystal. These regions are compared to the macroscopic phases seen previously in the title compound and the cluster-glass and striped phases reported for the underdoped Sr-doped cuprates.

Mesoscopic phase separation in La2CuO4.02 - a 139La NQR study

In crystals of La2CuO4.02 oxygen diffusion can be limited to such small length scales, that the resulting phase separation is invisible for neutrons. Decomposition of the 139La NQR spectra shows the existence of three different regions, of which one orders antiferromagnetically below 17K concomitantly with the onset of a weak superconductivity in the crystal. These regions are compared to the macroscopic phases seen previously in the title compound and the cluster-glass and striped phases reported for the underdoped Sr-doped cuprates.

Phase separation in La2CuO4+y single crystals studied by μSR and neutron diffraction

Abstract μSR—muon spin rotation—and neutron diffraction measurements have been performed with the superconducting La2CuO4+y having an excess oxygen concentration inside the miscibility gap. The crystals belong to the series of high quality single crystals of La2CuO4, which after being doped with extra oxygen, do not reveal any macroscopic structural phase separation for y≤0.03, verified by X-ray and neutron diffraction. The crystal with y = 0.03 undergoes a transition to a short-range ordered magnetic state of spin-glass type. The crystal with y ⋍ 0.02 becomes antiferromagnetic (AFM) below 15K, according to μSR data. However, neutron diffraction do not reveal long range AFM order, implying that coherent regions of AFM phase are too small to be detected as a Bragg reflection. In addition to magnetic ordering the crystals reveal superconductivity with Tc close to TN,f. Presence of magnetic and superconducting transitions in structurally homogeneous system allows the speculation in terms of microscopic or electronic phase separation providing small hole-rich and hole-poor regions, possessing SC and magnetic order respectively.

Single-crystal diffraction study of phase separation in La2CuO4+δ☆

Abstract Single crystals of La 2 CuO 4+δ with δ = 0.04 and δ = 0.04 have been studied in the temperature range 10 K ⩽ T ⩽ 293 K using high-resolution neutron diffractometer. In both crystals, the concentration of excess oxygen is inside the miscibility gap. However, the La 2 CuO 4.03 crystal does not undergo a phase separation - homogeneous oxygen distribution was preserved over the entire temperature region studied - while in the La 2 CuO 4.04 crystal, the phase separation phenomenon, connected with excess oxygen diffusion was observed. Our data provide direct evidence that both the oxygen-rich and oxygen-poor regions have the same Cmca space symmetry. The dimensions of the coherent regions of coexisting phases are (975 ± 22) A in the (0 1 0) direction, and (1460 ± 160) A in the perpendicular direction.