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1.
High-temperature superconductivity has been discovered in La2−xBaxCuO4 [J.G. Bednorz, K.A. Müller, Z. Phys. B 64 (1986) 189. [1]], a compound that derives from the undoped La2CuO4 crystallizing in the perovskite T-structure. In this structure oxygen octahedra surround the copper ions. It is common knowledge that charge carriers induced by doping in such an undoped antiferromagnetic Mott-insulator lead to high-temperature superconductivity [V.J. Emery, Phys. Rev. Lett. 58 (1987) 2794; C.M. Varma, S. Schmitt-Rink, E. Abrahams, Solid State Commun. 62 (1987) 681; E. Dagotto, Rev. Mod. Phys. 66 (1994) 763. [2], [3] and [4]]. The undoped material La2CuO4 is also the basis of the electron-doped cuprate superconductors [Y. Tokura, H. Takagi, S. Uchida, Nature (London) 337 (1989) 345. [5]] of the form La2−xCexCuO4+y [M. Naito, M. Hepp, Jpn. J. Appl. Phys. 39 (2000) L485; A. Sawa, M. Kawasaki, H. Takagi, Y. Tokura, Phys. Rev. B 66 (2002) 014531. [6] and [7]] which, however, crystallize in the so-called T′-structure, i.e. without apical oxygen above or below the copper ions of the CuO2-plane. It is well known that for La2−xCexCuO4+y the undoped T′-structure parent compound cannot be prepared due to the structural phase transition back into the T-structure occurring around x∼0.05. Here, we report that if La is substituted by RE=Y, Lu, Sm, Eu, Gd, or Tb, which have smaller ionic radii but have the same valence as La, nominally undoped La2−xRExCuO4 can be synthesized by molecular beam epitaxy in the T′-structure. The second important result is that all these new T′-compounds are superconductors with fairly high critical temperatures up to 21 K. For this new class of cuprates La2−xRExCuO4, which forms the T′-parent compounds of the La-based electron doped cuprates, we have not been able to obtain the Mott-insulating ground state for small x before the structural phase transition into the T-structure takes place.  相似文献   

2.
《Physics letters. A》1987,126(1):61-66
We consider the effect of the doping parameter x and the oxygen nonstoichiometry parameter y (in La2−xBaxCuO4+y and in YBa2Cu3O7−y) on the short range structure in high Tc cuprates and argue that both x and y tend to create finite Cu-O structures (FCuOS) e.g., chainlets, in these materials. This is shown to be consistent with the details of their structures, optical properties, Hall effect data etc. The electronic excitations between the confined states of these FCuOS provide a plausible pairing mechanism for superconductivity in the 2-D CuO planes present in these materials.  相似文献   

3.
We have successfully synthesized polycrystalline bulk samples of La2CuO4 with the Nd2CuO4-type (T′) structure at low temperatures. First of all, La2CuO4 with the K2NiF4-type (T) structure was prepared by the conventional solid-state reaction. Secondly, La2CuO3.5+δ with the Sr2CuO3-type structure was obtained by heating mixed powder of T-La2CuO4 and CaH2 at 175 °C in an evacuated Pyrex tube. Thirdly, T′-La2CuO4+δ was prepared by heating La2CuO3.5+δ at 250 °C in air. Finally, T′-La2CuO4+δ was reduced using CaH2 to remove excess oxygen. The powder X-ray diffraction pattern has revealed that the product is of the single phase of T′-La2CuO4, but no superconductivity has been observed at T > 2 K in the magnetic susceptibility measurement. The residual excess oxygen may suppress the appearance of superconductivity.  相似文献   

4.
The oxygen hyperstoichiometry of K2NiF4-type La2Ni0.9Fe0.1O4+δ, studied by thermogravimetric analysis and coulometric titration in the oxygen partial pressure range 6×10−5-0.7 atm at 923-1223 K, is considerably higher than that of undoped lanthanum nickelate. The p(O2)-T-δ diagram of iron-doped lanthanum nickelate can be adequately described by introducing point-defect interaction energy in the concentration-dependent part of defect chemical potentials and accounting for the site-exclusion effects. The critical factors affecting the equilibrium oxygen incorporation process include coulombic repulsion of interstitial anions, trapping of the p-type electronic charge carriers by iron, and interaction between Fe3+ and holes localized on nickel cations. Due to low chemical expansion of La2Ni0.9Fe0.1O4+δ lattice, the thermodynamic functions governing oxygen intercalation, site-blocking factors and hole mobility are all independent of the defect concentrations. The predominant 3+ state of iron cations under oxidizing conditions was confirmed by the Mössbauer spectroscopy. The stability of La2NiO4-based phase in reducing atmospheres is essentially unaffected by doping.  相似文献   

5.
We have studied doping evolution of the temperature dependent local Cu-O displacements in the La2-x Sr x CuO4 superconductor by polarized Cu K-edge extended X-ray absorption fine structure (EXAFS) measurements. While temperature dependent Debye-Waller factor of the Cu-O bonds, measuring the local Cu-O displacements, shows an anomalous increase at low temperature for the underdoped single crystals, we do not find such a dependence for the case of the overdoped system. The results, which are discussed in the light of recent angle resolved photoemission measurements, provide an evidence for some important correlation between the doping dependent electron-lattice interaction, the charge inhomogeneities and the local Cu-O displacements in the copper oxide superconductors.Received: 9 July 2003, Published online: 19 November 2003PACS: 74.72.Dn La-based cuprates - 61.10.Ht X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc. - 74.81.-g Inhomogeneous superconductors and superconducting systems  相似文献   

6.
The electronic structure of T*-type compounds (Bi0.5Sr1.5)(Y2-xCex)Cu2Oy has been studied by X-ray photoelectron spectroscopy. From analysis of the results, it was found that cation deficiency in Ce4+ introduced holes to conduction planes in the T* phase as in the p-type infinite layer compounds. Some Cu ions in the sample without cation-deficiency contained two-oxygen coordination, which resulted in the incompleteness of the CuO2 sheets. Bi ions in the T* phase cuprates take exact valence of 3+, Pb ions cannot replace them due to the too large an ionic radius of Pb2+/Pb4+.  相似文献   

7.
Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La1.25Sr0.75MnCoO6, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co2+/Co3+ and Mn3+/Mn4+, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at TC=217 K and a frequency-dependent transition at Tg<100 K. The high frequency dependence of Tg is indicative of the cluster-glass behavior of La1.25Sr0.75MnCoO6 (7 5 0) at T<TC within the ferromagnetic state.  相似文献   

8.
The spectral density (SD) in the ARPES spectra of antiferromagnetic (AFM) dielectrics Sr2CuO2Cl2 and Ca2CuO2Cl2 along the principal symmetry directions of the Brillouin zone was studied by the generalized tight binding method. At the valence band top of these undoped cuprates in the AFM state, there is a pseudogap of magnetic nature with E s(k)~0–0.4 eV between a virtual level and the valence band proper. The observed similarity of dispersion along the Γ-M and X-Y directions can be explained by the proximity of the 3 B 1g triplet and the Zhang-Rice singlet levels. The value of parity of the polarized ARPES spectra at the Γ, M, and X points calculated for the AFM phase of undoped cuprates with an allowance for the partial contributions is even. The conditions favoring observation of the partial contributions in polarized ARPES spectra are indicated. Due to the spin fluctuations, the virtual level acquires dispersion and possesses a small spectral weight. Probably, this level cannot be resolved on the background of the main quasi-particle peak as a result of the damping effects.  相似文献   

9.
The magnetic and superconducting properties in the high-T c cuprates have been investigated over a wide hole doping range by63Cu,17O and205Tl NMR and NQR in the lightly-doped La2?xSrxCuO4 (LSCO), the heavily-doped Tl2Ba2CuO6+y (TBCO) and the Zn-doped YBa2Cu3O7 (YBCO7). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of63(1/T 1 T) associated with that of the staggered susceptibility χO(T) in LSCO. In the vicinity of the hole content whereT c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξM is considerably short being presumably (ξM/a) ~ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction ofT c in case of the substitution of Zn impurities into the CuO2 plane. The superconducting properties clarified by NMR experiments cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T c cuprates.  相似文献   

10.
The results on the nonlinear longitudinal response (second order uniform dynamic susceptibility χ2(ω)) of two moderately doped La2CuO4+x single crystals in the temperature region of the phase separation of extra oxygen are reported. Large 2D-fragments (I≈7·103 Å) of stoichiometric La2CuO4 are shown to be responsible for the signal in sample withT ps>-T N≈243 K. The signal of the crystal withT ps<T N≈272 K is due to oxygen-rich phase. Its peculiarities are related either to change and destruction of magnetic ordering in the course of the phase separation or to the ordering of the extra oxygen.  相似文献   

11.
The conductivity and dielectric permittivity spectra of single-crystalline La1.87Sr0.13CuO4 are directly measured with the electric field polarized perpendicular to the CuO planes (Ec) covering the frequency range 10-40 cm−1 and temperatures 5-300 K. We observe in the superconducting state a well pronounced excitation with strongly temperature dependent parameters. We suggest that the excitation is caused by the transverse Josephson plasma mode that appears due to the different strengths of Josephson coupling between the superconducting charge stripes in the neighboring and next-nearest neighboring copper-oxygen planes of La1.87Sr0.13CuO4. A strongly enhanced low-frequency (below 15 cm−1) absorption is seen in the superconducting state that is assigned to delocalized quasiparticles of as yet unknown origin.  相似文献   

12.
Copper 2P core level X-ray photoelectron spectra on several high Tc superconductors La1.85Ba0.15CuO4, Y1.2Ba0.8CuO4−δ and Y0.4B0.6CuO4−δ oxides have been measured. The results indicate the coexistence of Cu2+ and Cu3+ in these compounds.  相似文献   

13.
Experimental investigations of phase separation in high-Tc cuprates are reviewed with special emphasis on recent results obtained on doped La2CuO4+x single crystals. Particular attention is paid to the magnetic properties. The experiments give very strong evidence that magnetic polarons are responsible for macroscopic phase separation observed in La2CuO4+x for x>0.  相似文献   

14.
Polycrystalline samples of electron-doped Pr1.85Ce0.15CuO4+δ have been prepared under different annealing conditions and investigated by means of X-ray-diffraction, oxygen content analysis, electrical resistivity, magnetic susceptibility and low temperature specific heat measurements. X-ray-diffraction patterns show that samples contain a single T′ phase. The superconducting transition temperatures Tcm taken with the onset of diamagnetism in magnetic-susceptibility measurements are 20 and 19.5 K for sample annealed in flowing Ar gas and in vacuum (∼10−3 torr), respectively. The data of the samples, which are annealed in flowing Ar gas, show clear evidence for an αT2 term at zero magnetic field in superconducting electronic specific heat, and are consistent with d-wave superconductivity. However, this behavior is not observed in the other sample, which is annealed in vacuum. These results indicate that different heat treatments affect the oxygen content, homogeneity, superconducting transition temperature Tc, superconducting volume fraction, and the superconducting pairing symmetry of Pr1.85Ce0.15CuO4+δ.  相似文献   

15.
The optical conductivity of CuO2 (copper-oxygen) planes in p- and n-type cuprates thin films at various doping levels is deduced from highly accurate reflectivity data. The temperature dependence of the real part σ1 (ω) of this optical conductivity and the corresponding spectral weight allow to track the opening of a partial gap in the normal state of n-type Pr2−xCexCuO4 (PCCO) but not of p-type Bi2Sr2CaCu2O8+δ (BSCCO) cuprates. This is a clear difference between these two families of cuprates, which we briefly discuss. In BSCCO, the change of the electronic kinetic energy Ekin—deduced from the spectral weight—at the superconducting transition is found to cross over from a conventional BCS behavior (increase of Ekin below Tc) to an unconventional behavior (decrease of Ekin below Tc) as the free carrier density decreases. This behavior appears to be linked to the energy scale over which spectral weight is lost and goes into the superfluid condensate, hence may be related to Mott physics.  相似文献   

16.
We prepared thin films of T′-La2CuO4, which usually crystallizes in the T structure, by MBE, and investigated their properties while systematically changing the post-reduction conditions with a view towards obtaining superconductivity along the lines of the parent compound superconductors we have recently reported (O. Matsumoto et al., Phys. Rev. B 79 (2009) 100508(R)). The results indicate that the optimal reduction window is very narrow, near which metallic conductivity is obtained down to 50 K. The resistivity of the T′-La2CuO4 films is in the range of 10−2–10−3 Ω cm, which is several orders of magnitude lower than that of the counterpart T-La2CuO4, the implication of which is briefly discussed from the viewpoint of the difference in electronic structure induced by different oxygen coordination.  相似文献   

17.
Using ALL-MBE technique, we have synthesized different heterostructures consisting of an insulator La2CuO4 (I) and a metal La1.56Sr0.44CuO4 (M) layer neither of which is superconducting by itself. The M-I bilayers were superconducting with a critical temperature Tc≈30-36 K. This highly robust phenomenon is confined within 1-2 nm from the interface and is primarily caused by the redistribution of doped holes across the interface. In this paper, we present a comprehensive study of the interface superconductivity by a range of experimental techniques including transport measurements of superconducting properties.  相似文献   

18.
The investigation of the manganites La2/3−xPrxSr1/3MnO3, La2/3Sr1/3−xCaxMnO3 and La2/3+xCa1/3−2xAgxMnO3, which all exhibit Mn3+:Mn4+=2, shows that it is possible to reach high magnetoresistance at room temperature, up to 21% under 1.2 T. These materials are compared to La5/6Ag1/6MnO3 which corresponds to the same Mn3+:Mn4+ ratio and exhibits a magnetoresistance of 25% in this field. An interesting feature deals with the value of the insulator-metal transition temperature TIM, often higher than TC, especially for Ag-based compounds. It is suggested that the latter results either from a better oxygenation of the surface of the grains or from a migration of silver toward the surface.  相似文献   

19.
The study of the structural and magnetic phase diagram of the manganites La1−xAgxMnO3 shows similarity with the La1−xSrxMnO3 series, involving a metallic ferromagnetic domain at relatively high temperature (≈300 K). The Ag-system differs from the Sr-one by a much smaller homogeneity range (x≤1/6) and the absence of charge ordering. But the most important feature of the Ag-manganites deals with the exceptionally high magnetoresistance (−25%) at room temperature under 1.2 T, that appears for the composition x=1/6. The latter is interpreted as the coincidence of the optimal double exchange condition (Mn3+:Mn4+=2) with Tmax=300 K (maximum of the ρ(T) curve in zero field).  相似文献   

20.
The significance of heterovalent, substitutional disorder for the distribution of charge carriers in La2−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 La2−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.  相似文献   

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