Normal-State Gap, Supercarrier Mass Anisotropy and Isotope Effects in La2−xSrxCuO4

Magnetization measurements have been performed on the oxygen-isotopeexchanged samples (¹⁶O and ¹⁸O) of the one-layer cuprate superconductorsLa_2?x Sr _x CuO₄ (0.06≤x≤0.20). We find that there is an oxygen-isotopeeffect on the effective supercarrier mass m**, which is huge for x=0.06,and reduced to a smaller value for x=0.15. We also find that there coexistsmall (bi)polarons and free carriers for x≥0.09, but only small (bi)polaronsare present for x≤0.09. The normal-state gap is related to the bipolaronbinding energy which is inversely proportional to x for 0.05≤x≤0.15. Theobserved isotope effects, supercarrier mass anisotropy, normal-state gap,and mid-infrared spectra for x≤0.09 can be quantitatively explained by the small(bi)polaron theory of superconductivity proposed by Alexandrov, Kabanov, andMott (AKM).     

Finite-Wave-Vector Phonon Coupling to Degenerate Electronic States in La2?xSrxCuO4

We consider finite-k electron–phonon coupling appropriate for a superconductor with a very short coherence length and local intersite pairs. From group theoretical analysis we find that k 0 phonons can couple to degenerate electronic states in La_{2– x} Sr _x CuO₄ and a Jahn-Teller-like distortion is found to be operative for E _g- and E _u-symmetry O states. Experimentally observed anomalies in inelastic neutron scattering, ESR, and EXAFS are found to be consistent with such an interaction of the ₁ symmetry -point mode with the planar O P _x and P _y states. The proposed interaction is found to naturally facilitate symmetry breaking associated with pairing and/or stripe formation.     

Microscopic Charge Inhomogeneities in Underdoped La2?xSrxCuO4: Local Structural Evidence

We present local structural evidence for the existence of charge inhomogeneities at low temperature in underdoped and optimally doped La_2–x Sr _x CuO₄. The inhomogeneities disappear for x 0.2. The evidence for the charge inhomogeneities comes from an anomalous increase in the inplane Cu–O bond length distribution in the underdoped samples as well as evidence for CuO₆ octahedral tilt inhomogeneities in the intermediate range structure. Preliminary analysis of the temperature dependence of this phenomenon indicates that the inhomogeneities set in at temperatures in the range 60 K < T _co < 130 K, which depends on doping.     

Phonon Vibration and Conduction of La2 ? xSrxCuO4 ? δ and La1.85 ? xSr0.15 + xCu1 ? xCoxO4 ? δ

Polycrystalline samples La_{2 – x} Sr _x CuO_{4 –} (0.06 x 0.5) and La_{1.85 – x} Sr_{0.15 + x} Cu_{1 – x} Co _x O_{4 –} (0 x 1) were synthesized by a conventional solid state reaction method. The structure, phonon vibration, and conduction were investigated by means of XRD, infrared (IR) spectra, and resistance. It is found that the increase of itinerant hole carriers could mask the in-plane stretching vibration mode (689 cm^–1). The softening of the phonon vibration mode gives an index of the weakening of hybridization. The distortion of CuO₆ octahedron and the microstructural inhomogeneity induced by Co doping lead to the widening of the IR absorption peaks.     

A new route to synthesize La1?xSrxMnO3

Self-Propagating High-temperature Synthesis (SHS) was used to producecomplex oxides (La_1–x Sr _x MnO₃ with x = 0, 0.1 and 0.2), which are used as the cathode in solid oxide fuel cells (SOFCs). Thermodynamic predications and experiments show that La_1–x Sr _x MnO₃ can be prepared via SHS under moderate conditions from a mixture of La₂O₃ + SrO₂ + Mn, using either gaseous oxygen or solid NaClO₄ as the oxidant. Partial melting at the high combustion temperature increased product homogeneity. The electrical conductivity of the product was 180 S·cm^–1 at 1000°C in air, matching that of sample made by other synthesis processes. SHS enables a more economical production of La_1–x Sr _x MnO₃ than existing commercial processes.     

Synthesis, Structure, and Superconductivity of (La1.5Pb0.5?xSrx)CuOz

The substitution of strontium for lead in the material (La_1.5Pb_0.5–x Sr _x )CuO _z , x = 0–0.15 has been carried out. A stable and reproducible single phased superconducting materials can be obtained inside an evacuated quartz tube. The X-ray diffraction pattern shows that the superconducting phase can be indexed on the basis of an orthorhombic symmetry (F _mmm) for x = 0 and on the basis of tetragonal symmetry (I _4/mmm) for x > 0. The transition temperature T _c increases as the strontium substitution parameter x increases. We observed the maximal T _c around x = 0.15 with 38 K with fairly large Meissner volume fraction of 38% (FC).     

Isotope Studies of the CMR Compounds La1?xCaxMnO3+δ

Oxygen isotope effect studies of the ferromagnetic Curie temperature T _c of La_1–x Ca _x MnO₃ are presented. The isotope exponent ₀=–lnT _c/lnm ₀ changes from 0.4 to 0.14 in the range 0.2<x<0.43. The isotope exponent decreases strongly with increasing tolerance factor, or decreasing lattice distortion. Above Tc the conductivity is characteristic of small polarons. Raman scattering shows a prominent peak at 230 cm–1. The peak width could be related to site-dependent Jahn–Teller distortions above T _c, becoming significantly smaller at and below T _c. IR reflectivity data show a much larger zero frequency IR conductivity than dc conductivity. The IR peaks are independent of temperature between 150 and 295 K.     

The Superconducting and Normal-State Transport Properties in Pure, Pr-Doped, and Ca-Doped NdBa2Cu3O7?y

We report detailed studies of the superconducting and normal-state transport properties in NdBa₂Cu₃O_7–y , Nd_0.9Ca_0.1Ba₂Cu₃O_7–y , and Nd_0.8Pr_0.2Ba₂Cu₃O_7–y , systems. We found a linear temperature dependence of resistivity and the Hall number n _H above T _c. For a given sample n _H, which is inconsistent with any simple transport model in science (1/n). However, and n _H at a fixed temperature for these three samples follow a relation 1/n _H. This is evidence that n _H could scale with actual mobile carrier concentration. The unit cell volume V is a linear function of n _H. The T _c –V–n _H diagram indicates that there exist an optimal concentration n _H and an optimal V for T _c in these systems.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

Specific Heat of YBa2Cu3O7??δ and (La2?xSrx)CuO4: Evidence for d-Wave Pairing

New low-temperature specific-heat data for YBa₂Cu₃O_7–, 00.2, and (La_1.85Sr_0.15)CuO₄ in magnetic fields (H) to 9 T are reported. They show features predicted for the lines of nodes in the energy gap that are expected for d-wave pairing; for H=0, a T ² term; for H0 an H ^1/2 T term with a crossover to a more complicated T dependence at higher T and lower H. The results are compared with recent measurements in other laboratories.     

Spin Gap Effects on Bulk R1+xba2?x Cu3O7?δ (R=Eu or Nd and 0≤x≤0.4)

Spin gap effects on the underdoping states of the bulk system of R_1+x Ba_2–x Cu₃O_7– (R = Eu or Nd and 0 x 0.4) were investigated through transport property measurements. The underdoping states were achieved by, alternatively substituting R³⁺ for Ba²⁺ ions in the system rather than adjusting the oxygen deficiency. The excess R³⁺ ions were to occupy the Ba sites of the crystalline lattice as revealed from Rietveld analysis for powder X-ray diffraction. The underdoped materials were observed to first undergo spin pairing transition in the temperature range well above T _c, and come across with superconducting transition at T _c. The increasing feature observed for spin gap temperature and the decreasing one for T _c, as the concentration of holes decreases, are in qualitatively good agreement with theoretical predictions from the mean-field RVB model.     

Potassium ion conducting K2 ? 2 x Fe2 ? x P x O4 solid electrolytes

Potassium ion conducting solid electrolytes based on potassium monoferrite have been prepared by substituting pentavalent phosphorus cations for Fe³⁺. The highest conductivity of K_{2 − 2x} Fe_{2 − x} P _x O₄ is achieved in the range x = 0.05−0.10: 7.1 × 10⁻³ S/cm at 300°C and 1.6 × 10⁻¹ S/cm at 700°C. The rise in the conductivity of KFeO₂ upon phosphorus doping is due to the formation of potassium vacancies. Original Russian Text ? E.I. Burmakin, G.Sh. Shekhtman, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 8, pp. 995–998.     

Polaron Absorption and the Insulator to Metal Transition in Nd2?xCexCuO4?y Cuprates

The optical conductivity of Nd_2–x Ce_xCuO_4–y single crystals with x ranging from 0 to 0.122 has been measured at differenttemperatures. The polaron band observed at 1100 cm–1 in Nd₂CuO_4–y shifts to lower frequencies when the Ce concentration xapproaches 0.125, namely, the critical value for the insulator to metal transition. Thisunexpected behavior indicates a progressive delocalization of polarons in theCu–O plane for increasing x and may explain the anomalous Drudebehavior reported for the carriers in the metallic phase of superconducting cuprates.     

Magnetic order and evolution of the electronic state aroundx=0.12 in La2?xBaxCuO4 and La2?xSrxCuO4

Muon spin rotation ( ⁺SR) measurement provides clear evidence of the antiferromagnetic order of Cu moments below 35 K for La_2–x Ba _x CuO₄ and below 15 K for La_2–x Sr _x CuO₄ in the narrow range ofx where the high-T _c superconductivity (SC) is suppressed remarkably. The results suggest that the change of the electronic state coupled with the lattice instability is relevant to the local suppression of SC and freezing of spin fluctuations of the Cu moment.     

Magnetic and dielectric properties of Mg1+x Mnx, Fe2?2x, O4 ferrite system

Ferrites with the general formula Mg_1+xMn_xFe_2–2xO₄(where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9) were prepared by the standard ceramic technique and studied by means of X-ray diffraction, magnetization, a.c. susceptibility and dielectric constant measurements. The X-ray analysis confirmed the single-phase formation of the samples. The lattice parameter is found to increase up to x = 0.3 and thereafter it decreases as x increases. The cation distribution has been studied by X-ray analysis and magnetization. Magnetization results exhibit collinear ferrimagnetic structure for x 0.3 and thereafter structure changes into non-collinear for x > 0.3. Curie temperature (T_C) obtained from a.c. susceptibility data decreases with increasing x. The dielectric constant (), loss tangent (tan ) show strong frequency dependence.     

Experimental determination of the spatial distribution of electron defects in La2 ? xSrxCuO4 and Nd2 ? xCexCuO4 crystal lattices

The data of M?ssbauer emission spectroscopy on ⁶⁷Cu(⁶⁷Zn) and ⁶⁷Ga(⁶⁷Zn) isotopes show that holes appearing as a result of the Sr²⁺ substitution for La³⁺ in the La_{2 − x} Sr _x CuO₄ crystal lattice are localized predominantly at oxygen atoms occurring in the same atomic plane as the copper atoms. In contrast, electrons appearing as a result of the Ce⁴⁺ substitution for Nd³⁺ in the Nd_{2 − x} Ce _x CuO₄ crystal lattice are localized in the copper sublattice. These results are consistent with the model assuming that a mechanism responsible for the high-temperature superconductivity in La_{2 − x} Sr _x CuO₄ and Nd_{2 − x} Ce _x CuO₄ crystal lattices is based on the interaction of electrons with two-site two-electron centers possessing negative correlation energies (negative-U centers).     

M?ssbauer Spectroscopy Determination of Iron Foreign Phases in?the Superconducting Systems; RAsFeO1?x, RAsFeO1?xFx, and?Sr1?xKxFe2As2

The recently discovered superconducting—spin density wave materials, containing Fe and As, have raised huge interest. However, most materials prepared to date, suffer from a varying degree of content of foreign Fe–As phases, Fe₂As, FeAs₂, and FeAs, which can lead to wrong conclusions concerning the properties of these materials. We show here that M?ssbauer spectroscopy is able to determine quite easily the relative content of the foreign phases. This procedure is demonstrated by a study of seven samples of superconducting or spin density wave materials, prepared in three different laboratories.      

Electronic structure and chemical bonds in the magnetic semiconductors Mn x Cd1 ? x GeAs2 and Mn x Zn1 ? x GeAs2

The electronic structure of the magnetic semiconductors Mn _x Cd_{1 − x} GeAs₂ and Mn _x Zn_{1 − x} GeAs₂, where x = 0, 0.0625, and 0.125, is calculated by the method of electron density functional in the plane-wave basis. Features of participation of Mn3d-electrons in the chemical bond are revealed. It is found that their role in the chemical bond with group-V element depends on the group-II element and the concentration of the magnetic dopant. Original Russian Text ? V.G. Yarzhemskii, S.V. Murashov, V.I. Nefedov, E.N. Muraviev, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, no. 11, pp. 1300–1306.     

Electronic Properties and Superconductivity of Electron-Doped La1?x/2Pr1?x/2CexCuO4

La_1−x/2Pr_1−x/2Ce _x CuO _y (LPCCO) samples with doping level up to 20% have been synthesized and their electronic and superconductive properties are studied. X-ray diffraction revealed that the LPCCO system is a pure phase system with a T’214 structure. Magnetic and resistive measurements show that the system is superconducting at 0.08≤x<0.20 with T _cmax=25 K at x=0.12. An anti-ferromagnetism transition was also observed in under-doped PLCCO (x=0.05) with the window width of the anti-ferromagnetism phase being significantly narrower than that NCCO and PCCO systems. The behavior of the Ce doping is compared with Sr doping in this system, and the mechanism is discussed.