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.     

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.     

Vortex structure and anisotropy of electric resistance in Bi2Sr1.65La0.35CuO6 + δ single crystals

The vortex structure of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals in tilted magnetic fields has been studied by the decoration method. From the observed pattern of vortex chains in the basal plane, the parameter of anisotropy in the superconducting state has been estimated as γ_S = 460 ± 40. The electric resistance of Bi₂Sr_1.65La_0.35CuO_6+δ single crystals has been studied in a broad range of temperatures (T _c < T < 300 K) and magnetic fields (up to 16 T). The ratio of resistivities in the direction perpendicular to the basal plane and in plane near the critical temperature T _c amounted to ρ⊥/ρ‖ = 3.2 × 10⁵. A possible relationship between the anisotropy in the normal and superconducting states is discussed.     

Diamagnetic shielding and Meissner effect in the high Tc superconductor Sr0.2La1.8CuO4

SQUID susceptibility measurements of the high T_c superconductor Sr_0.2La_1.8CuO₄, with a 40 K resistive onset, are reported for fields from 1 μT to 4.5 T (10 mG to 45 kG). Data show an average lower critical field of 30 mT at 5 K, but with nonlinearities appearing as low as 100 μT. Zero field cooling and subsequent application of a field shows a diamagnetic shielding effect reaching 100% in the lowest fields with a 37 K onset and 31 K midpoint. The observed Meissner effect in single phase material exceeds 40% but decreases with oxygen annealing even though T_c increases.     

Relationship between structural phase transition and carrier concentration in La2−x−ySrxRyCuO4 (R = Ce or Tb)

We have studied the relationship between the crystal structure and the carrier concentration in La_2?x?ySr_xCe_yCuO₄ by low-temperature X-ray diffraction method. The analysis for the [1 1 0]_t peak of the tetragonal index confirms that the high-temperature tetragonal phase changes to the low-temperature orthorhombic one in both La_1.89Sr_0.11CuO₄ and La_1.88Sr_0.11Ce_0.01CuO₄. We have also examined the effects of Tb substitution for La-site on the superconductivity and the structure in La_2?x?ySr_xTb_yCuO₄. A dip of the critical temperature T_c(x) due to the 1/8 anomaly and a maximum of T_c(x) at the optimum carrier concentration do not depend on the Tb concentration. This result suggests the possibility that Tb is introduced as the trivalent ion for x = 0.07–0.18.     

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     

Thin film synthesis and the influence of irradiation-induced defects on the superconductivity of La1.8Sr0.2CuO4

Thin superconducting films of La_1.8Sr_0.2CuO₄ have been prepared by magnetron sputter deposition and subsequent temperature treatment. The composition of the films has been determined by Rutherford backscattering and the structure by thin film X-ray diffraction. The onset of superconductivity was about 32 K and the midpoint near 28 K. Defect production in the films by He ion bombardment revealed a drasticT _c reduction with a sensitivity similar to that observed in the Chevrel phases. Oxygen implantation and subsequent annealing led to an enhancement of theT _c onset.     

Experimental observation of Bose condensation in high-temperature superconductors

Emission Mössbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn) and ⁶⁷Ga (⁶⁷Zn) isotopes was used to show that for the superconductors Nd_1.85Ce_0.15CuO₄, La_1.85 Sr_0.15CuO₄, and Tl₂Ba₂ CaCuO₈ in the temperature range T > T _c the temperature dependence of the center of gravity S of the Mössbauer spectrum is determined by the second-order Doppler shift, while in the range T < T _c the Bose condensation of Cooper pairs influences the value of S (here T _c is the superconducting transition temperature). The spatial nonuniformity produced in the electron density by a Bose condensate of Cooper pairs was observed for La_1.85 Sr_0.15CuO₄.     

High temperature superconductivity in (M1−xM′x)2CuO4−δ and related compounds (M = Y,La, Eu,Sm; M′ = Ba,Sr)

Compounds of the form (M_1−xM′_x)₂CuO_4−δ and related compounds where M and M′ are Y, various rare earths from La to Lu, and the alkaline earths Sr and Ba, have been investigated in connection with high temperature superconductivity. High temperature superconductivity is confirmed for the system (La_1−xBa_x)₂CuO_4−δ, (La_1−xSr_x)₂CuO_4−δ and (Y_1−xBa_x)₂CuO_4−δ with superconducting transition temperature T_c onsets of 30 K, 38 K and 90 K, respectively. We have found that the related systems (Eu_1−xBa_x)₂CuO_4−δ and (Sm_1−xBa_x)₂CuO_4−δ also exhibit high temperature superconductivity with T_c onsets of 95 K and 65 K, respectively. The highest T_c onset observed in this investigation was 97 K for a sample with the nominal composition of the spinel structure Y_0.33Ba_0.67Cu₂O_4−δ. Measurements of the specific heat C as a function of temperature T on a La_0.8Sr_0.2CuO_4−δ sample reveal a break in slope in the C/T vs T curve at the T_c midpoint, but no clearly discernable jump in C at T_c. A linear term ≈ λ′T in C was observed at low temperature in the superconducting state.     

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.     

151Eu Mössbauer effect study of T′- and T*-phase superconductors

The (La,Eu(₂ CuO ₄ system shows superconductivity by dopingCe orSr. Carriers inT′-phase doped withCe are electrons and those inT ^*-phase doped withSr are holes. In this work,¹⁵¹ Eu Mössbauer analysis is applied for theT′-phase(La _1?x Eu _x)_2?y Ce _y CuO ₄ and theT ^*-phase(La _1?x Eu _x)_2?y Sr _y CuO ₄ in order to compare the electronic state and the lattice vibration ofEu in these superconductors. In addition, correlations betweenT _c and Mössbauer parameters are examined. The isomer shift of¹⁵¹ Eu is 0.784–0.840 mm/s in theT ^*-phase and 0.689–0.733 mm/s in theT′-phase, which shows that the lanthanide in these superconductors is tri-valent. The Debye temperature of¹⁵¹ Eu is 180–208 K in theT ^*-phase and 160–192 K in theT′-phase. The difference of isomer shift between these two phases is explained by theEu?O distance. For(La _1?x Eu _x)_2?y Ce _y CuO ₄, a light correlation betweenT _c and the Debye temperature is observed, which means the importance of the lattice vibration in high-Tc superconductivity.     

Investigation of impurity effects in La2?xSrxCu1?yMyO4 (M=Ga,Zn) by electronic specific heat

The electronic specific heat C_el was studied at T≤10K on Ga- and Zn-doped La_2−xSr_xCuO₄ (0.16≤x≤0.22). In pure La_2−xSr_xCuO₄ (0.16≤x≤0.22), the C_el at T<T_c contains only a T² component but no T-linear one, which is peculiar to a clean d-wave superconductor. Partial substitution of Ga or Zn for Cu changes the T²-term of C_el into a quite different one described by the sum of a T-linear and a nearly T³ term. The coefficient of the T-linear term, γ, markedly increases with Zn-or Ga-content. The λ/λ_N vs. T_c/T_co relation for Zn-doped samples with x≥0.2 is in good agreement with the theoretical curve for resonant impurity scattering in a d-wave superconductor, while those for Ga-doped samples and for Zn-doped samples with x<0.2 deviate slightly from the theoretical curve. Such a deviation will be discussed in relation to the change in the magnetic properties of the present system caused by impurity-doping.     

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.     

Ultrasonic studies of the high Tc superconductor (La1−xSrx)2CuO4−δ

We synthesized (La_1−xSr_x)₂CuO_4−δ crystals with 0 ≦ x ≦ 0.4 by the sintering method, and obtained several samples with good homogeneity, high onset superconducting transition temperature T_c ≲ 38 K and narrow transition width ΔT_c. For the sample with the maximum T_c among them, ultrasonic measurements were performed. In the curve of the sound velocity change ΔV_s(T) versus temperature T, there was observed an anomalously large decrease of sound velocity as T decreases in the range 150 K < T < 240 K. Furthermore, in the plots of the ultrasonic attenuation coefficient α(T) as a function of T, there were observed a broad and large peak located at T ∼ 100 K and a small peak located at T ∼ 200 K. These results show the existence of an optical mode of the energy ℏω_op/k_B ∼ 100 K and, probably, also of that of ∼ 200 K. On the basis of these experimental results of elastic properties, we suggest the origin of high T_c of this material.     

Upper critical fields of superconducting superconducting rare-earth molybdenum selenides

Upper critical fields, H_c2(T), for superconducting LaMo₆Se₈, La_0.8Eu_0.2Mo₆Se₈, La_0.8Ce_0.2Mo₆Se₈ and PrMo₆Se₈ were measured. For the best material, LaMo₆Se₈, T_c = 11.3 K, (dH_c²/dT)_T=Tc=70 kG/K and and H_c² (4.2 K) = 370 kG. These selenide compounds show promise as very high field superconductors.     

Temperature dependence of (μ−O) paramagnetic shift in highT c superconductor La2−x Sr x CuO4

In order to investigate the electronic structure around oxygen in superconducting La_2–x Sr _x CuO₄ (x=0.10 and 0.14), precise measurements were carried out on the paramagnetic Knight shifts in the spin rotation frequency of the negative muon bound to the oxygen with reference to antiferromagnetic La_2–x Sr _x CuO₄ (x=0.05). Remarkable temperature as well as crystalline-axis dependence was observed for the highT _c case with a divergence character towardsT _c. Possible mechanisms to explain the observed results are given.     

Angular dependence of the upper critical field in Bi2Sr2CuO6+δ

The angular dependence of the upper critical magnetic field was investigated in a wide range of temperatures in very high-quality Bi₂Sr₂CuO_6+δ single crystals with critical temperature T _c (midpoint) ? 9 K in magnetic fields up to 28 T. Although the typical value of the normal state resistivity ratio ρ_c/ρ_ab≈10⁴, the anisotropy ratio H _c2∥ab/H _c2⊥ab of the upper critical fields is much smaller and shows an unexpected temperature dependence. A model based on strong anisotropy and small transparency between superconducting layers is proposed.     

Anisotropy of the heat capacity of a mixed-state superconducting Nd<Subscript>1.85</Subscript>Ce<Subscript>0.15</Subscript>CuO<Subscript>4</Subscript> single crystal for various magnetic field orientations with respect to the crystallographic axes

The anisotropy in the superconducting properties of single-crystal Nd_1.85Ce_0.15CuO₄ was studied from measurements of the heat capacity within the temperature interval 2–40 K in zero magnetic field and in a magnetic field of 8 T. We report on the first observation of heat capacity jumps occurring at the superconducting transition for various magnetic field orientations with respect to the crystallographic axes and on a strong anisotropy of the magnetic contribution to heat capacity in magnetic fields oriented in the a-b plane and perpendicular to it. These measurements yielded the anisotropy in the electronic heat capacity coefficient γ_n(H) and in the superconducting transition temperature T_c(H). The angular dependence of the Sommerfeld coefficient γ_n in the a-b plane observed in a magnetic field of 8 T exhibits four-lobe symmetry and zero gap direction of the order parameter. A comparison of the results obtained on the Nd_1.85Ce_0.15CuO₄ single crystal with the data available for La_1.85Sr_0.15CuO₄ permits one to conclude that the mechanisms of superconductivity in the electron-and hole-doped superconductors are similar.     

Muon spin relaxation study on magnetism in high quality single crystal of a high transition temperature superconductor La2−xSrxCuO4−σ (0.11≤x≤0.14)

The positive muon spin relaxation method is applied to probe magnetic ordering in the superconducting phase of a high quality single crystal of La_2−xSr_xCuO_4−σ (0.11≤x≤0.14). The well characterized crystal ofx=0.11 (T _c=34.5 K) with nearly complete flux exclusion exhibits spin freezing at 8K(T _f) with significant spin fluctuation up to 20 K. The onset of spin fluctuation andT _f decrease against increasingx towardsx=0.15, suggesting an existence of a magnetic phase boundary aroundx whereT _c becomes maximum.     

Field and temperature dependences of the specific heat of the La1.85Sr0.15CuO4 superconductor

This paper reports on the temperature and field dependences of the specific heat of high-quality La_1.85Sr_0.15CuO₄ single crystals carried out at low temperatures in magnetic fields of up to 8 T for two magnetic field orientations, namely, along the [100] and [110] crystallographic axes. The field dependence of the electronic density of states (DOS) was found to be anisotropic for different magnetic field orientations in the a–b plane, with the electronic density being the lowest along the a axis (for H ∥ [100]) and maximum for the field inclined at 45° to the a axis (for H ∥ [110]). Electronic specific heat in a magnetic field was observed to depend linearly on temperature T and nonlinearly on the magnetic field H: C _DOS=bTH ^1/2. In a zero field, the electronic specific heat grows quadratically with temperature as C _DOS=αT ². Estimation of the maximum superconducting gap width from the experimentally determined values of the α coefficient of T ² and of the electronic DOS in the normal state yields Δ ₀=300 K. The observed features indicate that La_1.85Sr_0.15CuO₄ is a superconductor with d symmetry of the order parameter.