A specific heat and magnetization study of highT c ceramic superconductors

From the temperature dependence of the specific heat of the semiconductor La₂CuO₄ and the high temperature superconductors La_1.8Sr_0.2CuO₄ (T _c =37.2 K) and YBa_1.9K_0.1Cu₃O_6.9 (T _c =91.5 K) in the range 1.5–30 K, a strong similarity of the lowfrequency part of their phonon density of states with a peak around 10 meV could be inferred. In the case of La_1.8Sr_0.2CuO₄ the thermodynamical critical field belowT _c has been determined and using the Rutger's formula and the BCS model, a Sommerfeld coefficient γ=9 mJ·mol^?1 K^?1 was obtained, which, taking into account recent results of band structure calculations leads to an electron-phonon enhancement factor γ=1.3, value compatible withT _c =36 K when using McMillan's formula forT _c . A systematic study of the magnetization offered evidence for strong flux trapping effects at higher fields and for Meissner shielding by superconducting Josephson currents in fields below 6 mT at 4.2 K.     

Andreev bound states in high temperature superconductors

Andreev bound states at the surface of superconductors are expected for any pair potential showing a sign change in different k-directions with their spectral weight depending on the relative orientation of the surface and the pair potential. We report on the observation of Andreev bound states in high temperature superconductors (HTS) employing tunneling spectroscopy on bicrystal grain boundary Josephson junctions (GBJs). The tunneling spectra were studied as a function of temperature and applied magnetic field. The tunneling spectra of GBJ formed by YBa₂Cu₃O (YBCO), Bi₂Sr₂CaCu₂O(BSCCO), and La_1.85Sr_0.15CuO₄ (LSCO) show a pronounced zero bias conductance peak that can be interpreted in terms of Andreev bound states at zero energy that are expected at the surface of HTS having a d-wave symmetry of the order parameter. In contrast, for the most likely s-wave HTS Nd_1.85Ce_0.15CuO_4-y (NCCO) no zero bias conductance peak was observed. Applying a magnetic field results in a shift of spectral weight from zero to finite energy. This shift is found to depend nonlinearly on the applied magnetic field. Further consequences of the Andreev bound states are discussed and experimental evidence for anomalous Meissner currents is presented. Received: 17 February 1998 / Revised: 27 April 1998 / Accepted: 23 June 1998     

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.     

μSR study of internal magnetic fields in superconducting La1.9Sr0.1CuO4 in the mK region

Muon-spin-rotation (μSR) experiments were performed on superconducting La_1.90Sr_0.10CuO₄ in zero external field. Below 2 K, a fast depolarization of the muon signal is observed, indicating that internal magnetic fields are present at the muon site. The average magnitude of the magnetic field in La_1.90Sr_0.10CuO₄ is in the order of 3 mT and thus approximately 14 times smaller than the corresponding field in antiferromagnetic La₂CuO₄. The present data support theoretical models of superconductivity in which magnetic correlations are responsible for pairing.     

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     

Superconductivity at 40 K in La1.8Sr0.2CuO4

High-T _c superconductivity withT _c onsets up to 42 K (midpoint: 37 K, zero resistance: 34 K) is observed in X-rays homogeneous single phase La_1.8Sr_0.2CuO₄. The quarternary compounds La_2–x Ba _x CuO₄ and La_2–x Sr _x CuO₄ (0x0.3 for Ba and 0x<1 for Sr, depending on the heating conditions) are mixed-valence oxygen defect oxides, characterized by the presence of Cu²⁺ and Cu³⁺ simultaneously. These oxides have a tetragonal symmetry (space group:I 4/mmm) similar to that of K₂NiF₄. We report the synthesis, characterization, and superconducting properties of various high-temperature superconducting La–Ba–Cu–O and La–Sr–Cu–O compounds. Through the substitution of Sr for Ba in these oxygen defect compounds an increasing superconducting transition temperature onset from 28 K to 35 K for La_1.8Sr_0.1Ba_0.1CuO₄ was observed. A positive initial pressure coefficient ofdTc/dp=290 (mK/kbar) has been found for La_1.8Sr_0.2CuO₄ with a magnetic susceptibility change consistent with the 100% diamagnetic expectation value.     

Anomalous thermal expansion of high-temperature superconductors Bi2Sr2−x LaxCuO6 at low temperatures

The low-temperature thermal expansion of Bi₂Sr_2?x La_xCuO₆ single crystals with different doping levels is measured. The effect of a magnetic field on the thermal expansion of Bi₂Sr_2?x La_xCuO₆ is studied. Anomalous (negative) thermal expansion whose characteristics depend on the doping level of the sample is revealed at T ≤ 10–20 K. It is found that these characteristics are significantly affected by moderately strong magnetic fields (2–4 T). The experimental results can be interpreted within a model relating the anomalous thermal expansion in high-temperature superconductors at low temperatures and the strong effect of the magnetic field on this expansion to the change in the stabilizing role of the charge-density waves in the oxygen sublattice.     

119Sn and57Fe Mössbauer study of La2−x Sr x CuO4

Mössbauer absorption measurements were carried out for¹¹⁹Sn and⁵⁷Fe impurities in La₂CuO₄, La_1.85Sr_0.15CuO₄ and La_1.6Sr_0.4CuO₄. For all cases,⁵⁷Fe spectra at 4.2 K show magnetic hyperfine splittings. On the other hand, a magnetic hyperfine field at Sn nucleus is observed only in the case of La_1.85Sr_0.15CuO₄. The results indicate that the Cu magnetic spins are collectively fluctuating in the superconducting sample.     

Effect of annealing conditions on high temperature superconductivity in La2−xSrxCuO4

Abstract

Besides investigating a number of oxides of the series La_2?xM_xCuO₄ (M=Ba, Pb, Sr), we have, in this investigation emphasised on the studies of the effect of annealing conditions on superconducting properties of composition La_1.8Sr_0.2CuO₄. The effect is remarkable and is di'scuss'ed in the light of X-ray diffraction results. These samples, studied after a gap of two months, showed interesting changes which has been discussed in the present paper.     

Electronic phase separation in a low-temperature tetragonal phase of lanthanum-strontium cuprates according to 139La NQR data

A nonuniform electron density distribution is observed in La_{1−x− y} Nd_ySr_xCuO₄ and La_1−x−y Eu_ySr_xCuO₄, and long-lived magnetic fluctuations in these compounds are studied. The dynamics of the magnetic fluctuations depends strongly on the magnetic properties of the rare-earth ions, which stabilize the low-temperature tetragonal phase. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 5, 344–349 (10 March 1998)     

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)     

Spin-lattice relaxation and Knight shift on protons in the hydrogen-doped superconducting system H0.2La1.8Sr0.2CuO4

Measurements of the spin-lattice relaxation rate and Knight shift on protons in hydrogen-doped superconducting H_0.2La_1.8Sr_0.2CuO₄ samples are performed in the temperature range 4.2–300 K. An anomalous behavior of the spin-lattice relaxation rate is observed at low temperatures T∼20 K. A model is constructed that explains the appearance of carrier-depleted regions in the bulk of the semiconductor on the basis of the formation of a charged defect (proton). Pis’ma Zh. éksp. Teor. Fiz. 63, No. 7, 533–538 (10 April 1996)     

Introduction of bismuth into the highT c superconductor La2−x Sr x CuO4−y

Novel high-T _c superconductors La_2–x Bi _x Sr _x–x CuO_4–y have been isolated and studied. The influence of the experimental conditions of synthesis on the superconductivity has been especially studied for the composition La_1.7Bi_0.1Sr_0.2CuO_4–y ; it is shown thatT _c decreases as oxygen is introduced into the matrix, likely due to the partial oxidation of Bi(III) to Bi(V). The highT _c value observed for one sample (T _c 42 K), as compared withT _c 38 K of La_1.8Sr_0.2CuO_4–y suggests that bismuth is also a potential candidate for improving the superconducting properties in these oxides.     

Structural investigation of high-tc superconductors under high pressure

Abstract

The results of X-ray investigations of Tl₂Ba₂Ca₂Cu₃O₁₀ show a preferential c-axis decrease under high pressure, leading to a bulk modulus of B = 1370 kbar at 300 K. The results are compared to previous investigations of La₂CuO₄, La_1.8Sr_0.2CuO₄ and Pr₂CuO₄, that show a similar linear compressibility in a- and b- axis direction. The linear compressibility in c-axis direction is dominated by the degeneration of the Cu-O octohedra and by the compressibility of intercalated layers.     

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)     

Effect of strontium substitution on the structural and magnetic properties of La1.8Sr0.2MMnO6 (M = Ni,Co)-layered manganites

Sr-substituted perovskites, La_1.8Sr_0.2MMnO₆ (M = Ni, Co), were synthesized using the solid-state reaction technique to present a systematic study on their morphological, structural and magnetic properties. The average grain size of the as-prepared La_1.8Sr_0.2NiMnO₆ samples are in the range of 0.2–0.7 µm and those for La_1.8Sr_0.2CoMnO₆ manganites are 0.1–2.8 μm, which is significantly less than that of unsubstituted La₂NiMnO₆ (LNMO) and La₂CoMnO₆ (LCMO) manganites. The XPS analysis enlightened about phase purity, binding energy and oxygen vacancy of La_1.8Sr_0.2MMnO₆ manganites. The Sr-substituted LNMO has revealed a sharp ferromagnetic to paramagnetic phase transition at 160 ± 2 K, which is about 120 K less than that of parent LNMO. The Sr-substituted LCMO exhibited such a transition at 220 ± 2 K, which is 8 K less than that of parent LCMO. The temperature-dependent magnetization measurements suggest that the effect of Sr on the transition temperature in LNMO is more significant than that of LCMO.     

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La_1.85Sr_0.15CuO₄ are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated. Zh. éksp. Teor. Fiz. 112, 1386–1395 (October 1997)     

Magnetism in electron-doped copper oxides

We report muon spin relaxation/rotation measurements on sintered powder samples of Nd_2−x Ce _x CuO_4−y and a large single crystal of Nd₂CuO_4−y . We find an electronic phase diagram which is quite similar to that of hole-doped superconductors such as La_2−x Sr _x CuO_4−y , although the doping of electrons into the system is less efficient in destroying the static moments on the copper spins. Static magnetic order in Nd₂CuO_4−y appears below about 250 K, and two spin reorientations are seen atT=75 K andT=35 K. Measurements of the magnetic field penetration depth have been unsuccessful due to the rare-earth paramagnetism of these materials.     

Structure and superconductivity of La1.8Sr0.2CuO4 and multi-phase Y1.2Ba0.8CuO4 under high pressure

Superconducting La_1.8Sr_0.2CuO₄ has been investigated under pressure. Its resistiveT _c -midpoint was found to be 37 K at ambient pressure. It inductively determinedT _c is 33 K monotonically shifting up to 38.4 K at 19.2 kbar. The initial linear increase is 0.295 K/kbar. X-ray investigations in a diamond anvil revealed no phase transformation of the K₂NiF₄ structure up to about 50 kbar at 300 K, 60 K and 15 K. The bulk modulus of La_1.8Sr_0.2CuO₄ was 1070 kbar at 300 K and 1500 kbar at 15 K. Thec/a ratio decreases and shows pronounced differences between room temperature and low temperature data under pressure. Multi-phase Y–Ba–Cu-Oxide with resistiveT _c -onset of 90 K was investigated under pressure, too. The inductive transition starts at 70 K and is completed at 20 K. The superconducting volume fraction was estimated to be about 2% at 10 K.     

Low-energy excitations and stripes in superconducting cuprate La1.87Sr0.13CuO4

The conductivity and dielectric permittivity spectra of single-crystalline La_1.87Sr_0.13CuO₄ are directly measured with the electric field polarized perpendicular to the CuO planes (E∥c) 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 La_1.87Sr_0.13CuO₄. 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.