μ− SR studies of oxide-superconductorsSR studies of oxide-superconductors

The negative muon spin rotation method (μ⁻ SR) has been applied to studies of electronic states at oxygen sites of oxide superconductors YBa₂Cu₃O₇, Nd_2−x Ce _x CuO_4−δ (x=0.15, oxygen reduced), LiTi₂O₄ and related oxide-insulators La₂CuO_4−δ, CuO, Cu₂O. The paramagnetic shifts of μ⁻ trapped at oxygen nuclei in these polycrystalline powder samples have been measured at 300 K. All the measured shifts are positive. In copper-oxides the paramagnetic shifts are of the order 10⁻³, while in LiTi₂O₄ is very small (8.4±3.34×10⁻⁵). In YBa₂Cu₃O₇, a fast μ⁻ spin relaxation timeT ₂ ^* (∼ 200 ns) has been observed; the reason for this is unknown and further studies are now in progress.     

Anomaly in the upper critical magnetic field common to YBa2Cu3O7−δ , HoBa2Cu3O7−δ , and Nd1.85Ce0.15CuO4−δ irradiated by helium ions

The high-temperature superconductors YBa₂Cu₃O_7−δ , HoBa₂Cu₃O_7−δ , and Nd_1.85Ce_0.15CuO_4−δ are found to possess a common anomaly in the variation of the upper critical magnetic field owing to irradiation by helium ions. While the resistivity ρ increases by many times, the increase in H _c2 from scattering by radiation defects typical of ordinary superconductors does not occur. In terms of the ordinary mechanism, the anomaly may be explained by a significant reduction in the conduction electron density resulting from a loss of oxygen, which causes a significant rise in ρ with a small change in the scattering. Fiz. Tverd. Tela (St. Petersburg) 41, 1372–1376 (August 1999)     

Diffusion and surface energy barrier for labile oxygen in YBa2Cu3O7−δ

A study is made of the isotope exchange kinetics of tracer oxygen atoms between the gas phase and the CuO_1−δ plane in the YBa₂Cu₃O_7−δ crystal under isochronous and isothermal annealing. The parameters of the surface energy barrier for oxygen atoms are found. Fiz. Tverd. Tela (St. Petersburg) 39, 42–48 (January 1997)     

Photoelectron spectra of thin films of the high-T c superconductor YBa2Cu3O7−δ

The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra of the valence band of the compound YBa₂Cu₃O_7−δ for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa₂Cu₃O₆ films is observed to increase relative to YBa₂Cu₃O₇ films. Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997)     

Stripe motion in CuO2 planes of Y1−x Pr x Ba2Cu3O7 as observed from the Cu(2) quadrupole resonance

The NQR spectra of copper in the compounds YBa₂Cu₃O₇, TmBa₂Cu₃O₇, and Y_0.9Pr_0.1Ba₂Cu₃O₇ at temperatures of 4.2–200 K are investigated by a pulsed NQR technique at frequencies of 28–33 MHz. Quantitative analysis of the spectra shows that the shape of the “plane” Cu(2) spectra is well described by using a model of 1D correlations of the charge and spin distribution in the CuO₂ planes (stripe correlations). In the undoped superconductors the charge-spin stripe structure moves fast in the CuO₂ planes, but doping the YBa₂Cu₃O₇ lattice with praseodymium slows this motion down. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 782–787 (25 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Superconductor/ferroelectric epitaxial heterostructures for tunable microwave devices

The formation of epitaxial heterostructures including YBa₂Cu₃O_7−δ and SrTiO₃ thin films on R-plane sapphire substrates has been studied. The thin CeO₂ layer and the CeO₂/SrBi₂Nb₂O₉ epitaxial bilayer served as buffers. The temperature dependence of the dielectric permittivity ɛ of the SrTiO₃ film behaved in essentially different ways when incorporated in the YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ and in Ag/SrTiO₃/YBa₂Cu₃O_7−δ capacitor structures. An external bias U _b=±2.5 V applied to a plane-parallel YBa₂Cu₃O_7−δ /SrTiO₃/YBa₂Cu₃O_7−δ capacitor structure (T<100 K) reduced the ɛ of the SrTiO₃ film to one half its original value. Fiz. Tverd. Tela (St. Petersburg) 39, 222–227 (February 1997)     

Epitaxial combination of NdBa2Cu3O7−δ /SrTiO3: growth characteristics, structure, and parameters

We have studied the growth characteristics, structure, and parameters of the epitaxial heterostructures (001)NdBa₂Cu₃O_7−δ /(100)SrTiO₃/(001)NdBa₂Cu₃O_7−δ grown by laser ablation on a (100)LaAlO₃ substrate with a thin (∼2 nm) YBa₂Cu₃O_7−δ intermediate layer. The use of an YBa₂Cu₃O_7−δ intermediate layer promotes layered growth of the (200 nm) NdBa₂Cu₃O_7−δ layer, whose free-surface roughness is 4–5 nm. The resistance of the NdBa₂Cu₃O_7−δ layers began to fall off abruptly at T=92 K, and at T≈87 K it vanished completely. The critical current density in the NdBa₂Cu₃O_7−δ layers at T=76 K exceeded 10⁶ cm² A/cm². The dielectric constant of the (400 nm) SrTiO₃ layer sandwiched between the NdBa₂Cu₃O_7−δ epitaxial layers grew by roughly threefold as the temperature was lowered in the interval 300–4.2 K. When a bias voltage of ±2.5V was applied to the NdBa₂Cu₃O_7−δ electrodes, the relative dielectric constant of the (400 nm) SrTiO₃ intermediate layer fell from 1150 to 400 (T=32 K, f=100 kHz). The conductivity of the SrTiO₃ intermediate layer in the direction perpendicular to the substrate plane increased with temperature and the electric field strength. Fiz. Tverd. Tela (St. Petersburg) 41, 395–403 (March 1999)     

Shape of the Cu(2) NQR spectra in YBa2Cu3O7, TmBa2Cu3O7 and TmBa2Cu4O8

We present a study of shape of the Cu(2) NQR spectra in YBa₂Cu₃O₇, TmBa₂Cum₃O₇, and TmBa₂Cu₄O₈ compounds at temperatures of 4.2–300 K. The results of the quantitative analysis lead us to conclude that the shape of the Cu(2) NQR spectra in all the samples studied can be described in the framework of the “motional narrowing” model, which implies that the Cu(2) nucleus possesses two different NQR frequencies between which it can rapidly jump. The difference in frequencies seems to be related to the charge-stripe correlations in CuO₂ planes resulting in a dynamical modulation of the electric field gradients at the Cu(2) nuclei. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 9, 594–598 (10 November 1997)     

Luminescence spectra and crystal structure of high-temperature superconductors

A study is reported of the effect of temperature (4.2 K≤T≤150 K), the content of weakly bonded oxygen [6.1≤(7?δ)≤6.9], illumination [E _exc=3.4 eV, (Φt)_max～1.5 ×10²⁰ photon cm^?2, and substitution (Cu → Ag, Cr, Fe, Mn) on the luminescence spectra of the high-T _c superconductor YBa₂Cu₃O_7?δ. Only two bands with E _lum～2.4 and ～2.8 eV were observed in the luminescence spectra in all these cases. A clearly pronounced correlation between the electronic and structural changes in YBa₂Cu₃O_7?δ, caused by the influence of temperature, illumination, doping with oxygen or metal ions, and the spectral parameters (peak position E _lum, width Δλ _lum, and intensity I _lum of the luminescence bands), has been established. It is shown that luminescence spectra of HTSCs can be employed as a fairly reliable optical probe to study the electronic processes occurring in these substances, in particular, the electron (hole) transfer between the CuO₂ plane and the CuO_1?δ chain plane serving as a charge reservoir.     

Photoinduced transformation of luminescence centers in C60 crystals at high pressure

The influence of laser irradiation on the photoluminescence spectra of perfect C₆₀ crystals in the orientationally disordered phase is investigated. It is shown that irradiation of the crystals with low-power light for short durations at T=200 K produces radical changes in the luminescence spectrum. The pressure dependences of the spectral bands of the phototransformed and initial (without irradiation) spectra differ significantly, indicating a photoinduced structural transformation of the X centers responsible for the luminescence of C₆₀. The phototransformed C₆₀ crystals are stable against further exposure to light irradiation and pressure. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 12, 784–788 (25 December 1999) Deceased.     

Synthesis and electrical properties of Ln<Subscript>2</Subscript>CuO<Subscript>4+</Subscript><Subscript><Emphasis Type="Italic">δ</Emphasis></Subscript> (Ln: Nd or La) mixed conductor sputter deposited coatings

Nd₂CuO₄ and La₂CuO₄ are potential candidates as cathode material for intermediate temperature-solid oxide fuel cells. Nd–Cu and La–Cu oxides were co-sputtered on rotating substrates from metallic La, Nd and Cu targets in the presence of a reactive argon–oxygen gas mixture. Structural and chemical features of these films have been determined by X-ray diffraction and energy-dispersive spectroscopy. Their electrical resistivity was measured using the four-point probe method. As-deposited Nd–Cu based coatings are amorphous and, after annealing, crystallise in K₂NiF_{4+ δ} -type structure for Nd/Cu atomic ratio of 2, with more or less Nd₂O₃ or CuO, depending on whether Nd or Cu is in excess, respectively. As-deposited La–Cu based coatings are also amorphous and crystallise in La₂CuO₄ for La/Cu >2 or in LaCuO_3±δ perovskite-type structure when Cu is in excess. The electrical measurements show a clear relation between resistivity and structure of the coatings. After annealing, crystallised neodymium-based coatings show higher resistivity than lanthanum-based ones. Finally, LaCuO_3±δ exhibit higher resistivity than La₂CuO₄. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

The influence of interplane oxygen redistribution on the magnetization of fine-grained HTSC YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The magnetization of the fine-grained high-temperature superconductor (HTSC) YBa₂Cu₃O _y is experimentally investigated at T < T _c . A distinctive feature of this material is the increased oxygen content in CuO_δ planes. The magnetization decrease with an increase in δ is revealed. This correlation indicates that during interplane oxygen redistribution, which is characteristic of fine-grained samples, the oxygen content in the chain planes increases due to its reduction in the superconducting CuO₂ planes.     

Two-dimensional quantum interference contributions to the magnetoresistance of Nd2−x CexCuO4−δ single crystals

Two-dimensional (2D) weak localization effects at low temperatures T=0.2–4.2 K are investigated in a nonsuperconducting sample Nd_1.88Ce_0.12CuO_4−δ and in the normal state of a superconducting sample Nd_1.82Ce_0.18CuO_4−δ for B>B _c2≃3 T. The phase coherence time τφ (≃5×10⁻¹¹ s at 1.9 K) and the effective thickness d of a conducting CuO₂ layer (≃1.5 Å) are estimated by fitting the expressions from the 2D weak localization theory to the magnetoresistivity data for the normal-to-plane and in-plane magnetic fields. The estimated value of the parameter d ensures the condition of strong carrier confinement and justifies a model of almost decoupled 2D metallic sheets for the Nd_2−x Ce_xCuO_4−δ single crystals. Pis’ma Zh. éksp. Teor. Fiz. 70, No. 2, 93–99 (25 July 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Influence of interatomic forces on the lattice dynamics of YBa2Cu3O7 (molecular dynamics method)

The method of molecular dynamics is used to study the dynamic properties of the high-temperature superconductor YBa₂Cu₃O₇. It is shown in the system La_2−x Sr_xCuO₄ that, due to the presence of strong anharmonism and a substantial difference in the interatomic forces, local “hot” regions arise around the Ba⁴⁺ charge defects in the CuO₂ and CuO planes, where the mean kinetic energy of the vibrations of the oxygen atoms reaches ∼0.5 eV. All other types of defects: oxygen and copper vacancies, substitutional atoms, and charge defects, do not lead to such effects. Fiz. Tverd. Tela (St. Petersburg) 41, 1729–1733 (October 1999)     

Polar Jahn-Teller centers and the anomalous isotope effect in copper-oxygen high-T c superconductors

We examine the isotope effect in copper-oxygen high-T _c superconductors using the model of polar Jahn-Teller centers, a variant of the local-boson model. The isotope-effect exponents for oxygen and copper depend on the structure of Jahn-Teller centers and its variation due to boson motion. A number of YBa₂Cu₃O_7−δ -based compounds are used to illustrate the feasibility of quantitatively describing the experimentally observed values of α _Cu and α _O and their relationship to T _c. Zh. éksp. Teor. Fiz. 111, 644–653 (February 1997)     

Valence states of copper atoms in cuprates determined from the change in the electron-capture constant of 64Cu

The change in the ⁶⁴Cu electron-capture constant for the compounds Cu₂O, YBa₂Cu₃O₇, YBa₂Cu₃O₆, Nd₂CuO₄, Nd_1.8Ce_0.2CuO₄, La₂CuO₄, La_1.7Sr_0.3CuO₄, Tl₂Ba₂CaCu₂O₈, and Bi₂Sr₂CaCu₂O₈ has been measured. It is concluded that the copper in YBa₂Cu₃O₇, Nd₂CuO₄, La₂CuO₄, La_1.7Sr_0.3CuO₄, Tl₂Ba₂CaCu₂O₈, and Bi₂Sr₂CaCu₂O₈ is exclusively divalent, whereas in Cu₂O, YBa₂Cu₃O₆, and Nd_1.8Ce_0.2CuO₄ univalent copper is present. Fiz. Tverd. Tela (St. Petersburg) 39, 1793–1794 (October 1998)     

Polar-center phase nuclei in He+-irradiated CuO single crystals

Irradiation of various single-crystal CuO faces [ac,bc,(110)] with 4.6-MeV He⁺ ions has been found to result in reduction of CuO to Cu₂O and Cu on the irradiated and unirradiated sides, lifting of forbiddenness from optical transitions in the [CuO₄]⁷⁻ electron center in the 0.7–0.95-eV energy range, a change in dichroism near the bands corresponding to transitions in the hole centers, [CuO₄]⁵⁻, and electron centers, [CuO₄]⁷⁻, as well as in a resonant increase of absorption at 0.95–1.30 eV with an unusual polarization dependence. The results of He⁺ irradiation of CuO single crystals are discussed in terms of a model of the nucleation of the phase of polar (electron and hole) centers in copper-oxygen systems. Fiz. Tverd. Tela (St. Petersburg) 40, 419–424 (March 1998)     

On the splitting of the lower band of charged elementary excitations of a two-dimensional antiferromagnet

A spin-polaron approach is developed on the basis of a generalized Kondo lattice model for a two-dimensional antiferromagnet (CuO₂ plane model). This approach makes it possible to describe the splitting of polaron bands of carriers both in the presence of long-range magnetic order (T=0) and for a finite spin-correlation length (T≠0) and also can account for the strong deviation from the Luttinger theorem and the shadow-bands effect at both zero and finite temperatures. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 386–391 (10 September 1998)     

Effect of nominal copper concentration in YBa2Cu3±x O7−δ on the superconducting transition

Variation of the copper concentration in the nominal composition YBa₂Cu_3±x O_7−δ markedly influences the superconducting transition. The transition occurs at the highest temperature with lower width, whenx = +0·25. Communication No. 82 from Materials Research Laboratory.     

Microphase separation and magnetic Jahn-Teller polarons in LaSrAl1−xCuxO4−δ

An EPR study of the solid solutions LaSrAl_1−x Cu_xO₄, which are isostructural to La₂CuO₄, shows that microphase separation of the structure occurs already at small copper concentrations (x≈0.01). A phase enriched with Cu appears along with a phase of La₂AlO₄, which contains isolated CuO₆ centers. It is established that the nature of the states and the deformations of the CuO₆ centers is determined by internal Jahn-Teller factors. When x⩽0.1, EPR signals are detected for new dynamic centers, which are identified as CuO₆ Jahn-Teller centers with a hole delocalized among the four in-plane oxygen ions (the total spin S=1). When 0.1⩽x<1, local CuO ₆ ⁻ hole centers transform into magnetic Jahn-Teller polarons, which include five or more CuO₆ fragments, in the copper phase of the structure. Their transformations and the conditions for observing them are discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 622–628 (April 1998)