Observation of Cooper pairs in high-temperature superconductors by 67Cu(67Zn) Mössbauer spectroscopy

A Mössbauer emission spectroscopy study on the ⁶⁷Cu(⁶⁷Zn) isotope showed that the superconducting transition in the Nd_1.85Ce_0.15CuO₄, La_1.85Sr_0.15CuO₄, and Tl₂Ba₂CaCu₂O₈ compounds is accompanied by an electron density redistribution in the crystal, which is considered evidence of Cooper-pair Bose condensation.     

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.     

Experimental observation of localization-delocalization of Cooper pairs in Nd1.85Ce0.15CuO4

Localization—delocalization of cooper pairs on ⁶⁷Zn impurity centers in the copper sublattice in the high-temperature superconductor Nd_1.85Ce_0.15CuO₄ was observed by emission Mössbauer spectroscopy on ⁶⁷Cu (⁶⁷Zn).     

Electron density variation at copper sites at the superconducting transition in cuprates

The temperature dependence of the center of gravity S of the Mössbauer spectrum produced by ⁶⁷Zn²⁺ impurity ions occupying copper and yttrium sites in YBa₂Cu₃O_6.6, YBa₂Cu₃O_6.9, YBa₂Cu₄O₈, Bi₂Sr₂CaCu₂O₈, HgBa₂CuO₄, and HgBa₂CaCu₂O₆ at temperatures above the superconducting transition point T_c was shown to be dominated by the second-order Doppler shift. For T<T_c, the quantity S is affected by the band mechanism associated with the formation of Cooper pairs and their Bose condensation. The variation of electron density at a metal site of the crystal was found to be related to the superconducting transition temperature. The variation of electron density created by the Cooper pair Bose condensate in compounds with two structurally inequivalent copper positions was shown to be different for these copper sites. The experimental dependence of the fraction of superconducting electrons on temperature agrees with the analogous dependence following from BCS theory for all the sites studied.     

Magnetic behaviour in the semiconducting and superconducting regions of57Fe-doped La2−xSrxCuO4−σ

From Mössbauer and resistivity measurements on⁵⁷Fe-doped La_2?xSr_xCuO_4?σ samples, we have determined the phase diagram of the system for 0<x<0.35 and 1.5 K<T<295 K. Forx<~0.15, a magnetic ordering is detected at low temperature within the Cu sublattice within both the semiconducting and the superconducting phases. Forx>0.15, a Fe-induced spin-glass state is observed. Recent data on oriented powder samples are discussed.     

Orientation of the main axis of the electric field gradient at the iron nucleus in some iron doped copper oxide compounds

We have performed⁵⁷Fe Mössbauer measurements on aligned iron-doped crystallites of La_1.85Sr_0.15CuO_4?y and Pb₂Sr₂R_0.50Cu₃O_8?y where R=Eu or Ho. From the intensity ratio of the two lines of the quadrupole doublets we deduce that the main axis of the electric field gradient does not coincide with thec axis of the lattice. This surprising result indicates that at the iron probe, substituted in the Cu site of the CuO₂ plane, the local electric charge symmetry is different from the lattice symmetry. We discuss this result in relation with the electronic configuration nearby this Cu site. The Pb based samples without Ca are magnetic at room temperature. Therefore in these samples Ca plays the same role as Sr in (La, Sr)₂CuO_4?y .     

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.     

Mössbauer studies of57Co-doped layered perovskites

Mössbauer spectra of⁵⁷Co-doped polycrystalline or single-crystal samples of layered perovskites La₂Mo₄ (M=Cu, Co, Ni) and R₂CuO₄ (R=Nd, Eu, Gd) were recorded at room temperature and below. Of the samples studied, only La₂CuO₄ shows a widely separated doublet at room temperature and a single clearly resolved sextet well belowT _N.     

Specific heat of the electronic superconductor Pr1.85Ce0.15CuO4

We have investigated the specific heat of the ceramic Pr_1.85Ce_0.15CuO₄ in the temperature range 2–800K in magnetic fields up to 8 T. We have determined the magnitude of the specific-heat discontinuity at the superconducting transition ΔC/T _c and estimate the coefficient of the electronic specific heat. In the temperature interval 5–800K we have separated out the phonon contribution to the specific heat, determined the temperature dependence of the characteristic Debye temperature Θ, and calculated the mean frequencies (moments) of the phonon spectrum. We compare the parameter values obtained in this way with data for the compound La_1.85Sr_0.15CuO₄ having similar crystal structure, but hole conductivity. The magnitude of the specific-heat discontinuity and consequently the effective electron mass in the “electronic” superconductor Pr_1.85Ce_0.15CuO₄ is several times smaller than in the “hole” superconductor La_1.85Sr_0.15CuO₄. The phonon spectrum in Pr_1.85Ce_0.15CuO₄ in the low-energy region is somewhat “more rigid,” and in the high-energy region somewhat “softer” than in La_1.85Sr_0.15CuO₄. Fiz. Tverd. Tela (St. Petersburg) 41, 1154–1159 (July 1999)     

Variation of electronic density in the superconducting phase transition in Nb<Subscript>3</Subscript>Al

The variation of electronic density in the superconducting phase transition in the classical superconductor Nb₃Al with critical temperature T_c=18.6 K was studied using ⁷³Ge emission Mössbauer spectroscopy. A comparison of the results obtained and the data available for the ⁶⁷Zn isotope in the lattices of high-temperature superconductors revealed a correlation between the electronic density variation at the Mössbauer probe nuclei sites and the value of T_c. This correlation is assumed to be related to the dependence of the electronic density variation on the standard correlation length.     

Antiferromagnetism in57Co-doped La2CuO4−y studied by Mössbauer spectroscopy

Mössbauer effect studies of⁵⁷Co-doped La₂CuO_4?y were performed at temperatures between 4.2 K and room temperature. These confirm the antiferromagnetic ordering of these compounds below room temperature. Temperature dependence of the quadrupole splitting shows that the hyperfine field is at an angle with thec-axis.     

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     

Mössbauer study of super and semi-conducting samples of57Fe-doped (La2−y Sr y ) CuO4−δ

From Mössbauer and resistivity measurements on (La_2?y Sr _y ) CuO_4?δ samples doped with 5/1000⁵⁷Fe, three successive phases are observed depending on they value: a semiconducting antiferromagnetic phase fory<0.02; a semiconducting phase with two-dimensional spin-glass Cu magnetic order for 0.02<y<0.07; and a superconducting phase fory>0.07. The Mössbauer data show that a Cu spin-glass order survives in the superconducting phase up toy～0.15. Fory>0.15, a different type of magnetic ordering is observed on⁵⁷Fe, which is clearly governed by the iron impurities interacting through the Cu sublattice.     

151Eu Mössbauer studies in the high Tc superconducting oxide materials

¹⁵¹Eu Mössbauer studies have been made across the respective superconductive transition temperatures in two compounds, La_1.7Eu_0.1Sr_0.2CuO_4?y and EuBa₂Cu₃O₇, having zero resistance state at 25K and 85K respectively. The¹⁵¹Eu isomer shift data establish that Eu ions are in trivalent state at all temperatures. The Mössbauer f-factor does not show any significant anomaly across the T_C.     

Mössbauer study of 1/8 anomaly in La2−x Ba x CuO4

The measurements of Mössbauer effect, magnetic susceptibility and muon spin relaxation have been carried out for the high-T _c superconductor La_2?x Ba _x CuO₄. The intensity of Mössbauer doublet spectrum of the sample of x～1/8 begins to decrease rapidly at a certain temperature T _m, which we define as a magnetic transition temperature T _Möss. This temperature almost agrees with T _μSR determined from muon spin relaxation. The quadrupole doublet disappears at low temperature below T _m but a clearly splitted spectrum is not observed even at 4.2 K, which indicates a peculiar magnetic state with a wide distribution of internal magnetic field. Around x～1/8, the superconducting critical temperature T _c and T _m are competed each other. In conclusion, superconductivity disappears around 1/8 hole concentration and a peculiar magnetic state such as spin density wave appears.     

Relaxation of magnetic clusters in Sr and Fe doped cobaltate perovskites

Mössbauer study of La_1???x Sr _x ⁵⁷Fe_0.025Co_0.975O₃ (0.15 ≤?x?≤ 0.25) perovskites below their magnetic ordering temperature shows cooperative electronic and magnetic phase separation for all compositions investigated. It is shown that the increase of the Sr concentration results in a transition from superparamagnetism to long range ferromagnetism of Co clusters between x = 0.18 and 0.2. In-field Mössbauer measurements revealed the existence of frustrated magnetic interactions at x = 0.22.     

57Fe and119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4

⁵⁷Fe- and¹¹⁹Sn Mössbauer effect studies on 0.5 at %⁵⁷Fe (or¹¹⁹Sn) dopednon-superconducting La_1.25Nd_0.6Sr_0.15CuO₄ in the temperature region 300 K T4.2 K reveal an onset of local magnetic ordering occurring atT _m 32 K for both⁵⁷Fe—as well as¹¹⁹Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field ofB _thf 11.0(5)T at the¹¹⁹Sn nuclei. Since such a large field is neither present in antiferromagnetic La₂CuO₄, nor in superconducting La_1.85Sr_0.15CuO₄ or overdoped La_2–xSr_xCuO₄ (x=0.4), the occurrence of such local spin correlations seems to be a signature of the non-superconducting low temperature tetragonal phase of La_1.25Nd_0.6Sr_0.15CuO₄.     

A search for extra lattice distortions associated with the superconductivity and the charge inhomogeneities in the high-Tc La1.85Sr0.15CuO4 system

Precise measurements of the temperature-dependent lattice parameters in a single crystal of La_1.85Sr_0.15CuO₄ are made by X-ray bond method and analyzed comparing the normal thermal expansion term, proportional to T⁴. A spontaneous lattice contraction of about 2.3×10⁻⁵ Å was found in the ab-plane, while along the c-axis a small expansion of about 1.5×10⁻⁵ Å was observed below the superconducting transition temperature of the system, showing some correlation between the structure and the high-T_c superconductivity. A careful search has been made for existence of any extra lattice distortion above the T_c, that might be associated with the stripe inhomogeneities in the title system, however, within the experimental sensitivity we could not find such distortions.     

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.     

Microhardness of the compounds La2CuO4 and La1.85Sr0.15CuO4 at temperatures from 81 to 292 K

The microhardness of La₂CuO₄ and La_1.85Sr_0.15CuO₄ crystals is measured in the temperature range 81–292 K, and the influence of various factors on it is investigated. The thermal activation parameters of the plastic deformation process in the vicinity of the indenter impression are estimated. The change of the phase state of the compound La_1.85Sr_0.15CuO₄ at the temperature T ₀=180 K and the appearance of domain (twin) boundaries formed in the ferroelastic tetraortho transition are not seen in the temperature dependence of the microhardness. The results of the study are compared with previously published results for YBa₂CuO_7−x crystals. Fiz. Tverd. Tela (St. Petersburg) 40, 493–497 (March 1998)