Electronic superconductor Pr2CuO4−x Fx: Magnetic correlations at high temperatures (150<T<600 K) according to 19F NMR data

The longitudinal and transverse relaxation rates of ¹⁹F nuclei in Pr₂CuO_4−x F_x (x=0.20) samples are measured at high temperatures (150 K<T<600 K). A feature is found in the temperature dependence of the relaxation rates at temperature T′≅300 K. The magnetic properties of the electronic superconductor Pr₂CuO_4−x F_x as a possible system with a stripe ordering of carriers and spins are discussed. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 4, 328–332 (25 February 1997)     

Superconductivity and calorimetric studies of Pr1.85Ce0.15CuO4+δ under different annealing conditions

Polycrystalline samples of electron-doped Pr_1.85Ce_0.15CuO_4+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 T_cm 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 T_c, superconducting volume fraction, and the superconducting pairing symmetry of Pr_1.85Ce_0.15CuO_4+δ.     

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.     

Nature of the quantum spin correlations through the superconducting-normal phase transition in electron-doped superconducting Pr0.88LaCe0.12CuO4

We use neutron scattering and specific heat measurements to relate the response of the spin fluctuations and static antiferromagnetic (AF) order to the superconductivity in the electron-doped high-transition-temperature superconductor, Pr_.88LaCe_.12CuO_4−δ (PLCCO) (T_c=24 K), as the system is tuned via a magnetic field applied beyond the upper critical field (H_c2) and driven into the normal state. The strength of the collective magnetic excitation commonly termed “resonance” decreases smoothly with increasing field and vanishes in the normal state, paralleling the behavior of the superconducting condensation energy. The suppression of superconductivity is accompanied by a smooth reduction in the very low energy spin fluctuations, and the concomitant emergence of static AF order. Our results suggest an intimate connection between the resonance and the superconducting condensation energy.     

Magnetic order in La2-x Bax CuO4 and La1.6-x Nd0.4 Srx CuO4 for x=0.125

We present μSR experiments on La_2-x Ba_x CuO₄ and La_1.6-x Nd_0.4 Sr_x CuO₄ for x=0.125. Both of these materials order magnetically with T_N\approx30\ K, while a superconducting sample of La_1.4 Nd_0.4 Sr_0.2 CuO₄ showed no evidence for static copper moments. Our results support the conclusion that the so‐called “1/8” anomaly in La_2-x Ba_x CuO₄ is a result of static (pinned) charge segregation. This revised version was published online in August 2006 with corrections to the Cover Date.     

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     

Interplay between the magnetic fluctuations and superconductivity in lanthanum cuprates

We report an analysis of the magnetic fluctuations in superconducting La_2?xSr_xCuO₄ and related lanthanum cuprates that have different symmetry of the low-temperature structure. NMR and ESR investigations revealed a dynamical coexistence of superconductivity and the antiferromagnetic correlations in most of the superconductivity region of the phase diagram. We show that, for all compounds, regardless of their low-temperature symmetry and their superconducting properties, the enhancement of the spin stiffness near 1/8 doping takes place.     

A new superconducting oxide GdBaCuO with Tc (ρ=0) above 92K

A new superconducting oxide Gd_xBa_1−xCuO_3−y with T_c (ρ=0) above 92K has been successfully synthesized by the ceramic technology. The samples with composition of x=0.5, 0.625 and 0.9 are all observed with high T_c superconductivity. The highest T_c for samples measured is observed in Gd_0.5Ba_0.5CuO_3−y which has a resistive midpoint of 95K with a width of 1.5K, and a zero-resistance temperature of 92.5K. The results of a.c. magnetic susceptibility measurement are also reported.     

NMR and NQR studies of spin dynamics and superconductivity in High-T c oxides

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 La_2?xSr_xCuO₄ (LSCO), the heavily-doped Tl₂Ba₂CuO_6+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 χ_O(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 ξ_M is considerably short being presumably (ξ_M/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.     

Relationship between the crystal structure and the reentrant superconducting properties of (Sn1-xErx)Er4Rh6Sn18

Single crystals of (Sn_1-xEr_x)Er₄Rh₆Sn₁₈ are superconducting (T_c = 1.3K) for x 0, are reetrant superconducting (T_c = 1.24K and T_M = 0.34K) for x ～.30 and undergo a singel magnetic transition (T_M=0.68K) for x～.75. Since the occupancy of the [Sn(1)_1-xEr(1)_x]Er(2)₄ sublattice is responsible for the variation of the low-temperature properties, one can make predictions as to new reentrant superconductors in the MRh_xSn_y series (M=RE). This appears to be the first system of reentrant superconductors where stoichiometry within a sublattice controls both magnetic ordering and superconductivity.     

μSR studies of heavy fermion systems

We have performed both zero field and high transverse field measurements at dilution refrigerator temperatures on a number of heavy electron systems, examining the superconducting and magnetic properties of these interesting materials. Among the materials studied to date are UBe₁₃, URu₂Si₂ and U₆Fe. The magnetic field penetration depth in the superconducting state of UBe₁₃ is greater than 10000 Å, as no increase in the transverse field relaxation rate is observed belowT _c . A sharp increase in the precession frequency is seen, starting atT _c . This frequency shift shows little temperature dependence at low temperature; we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we we found no clear evidence for unconventional superconductivity in this material. Zero field measurements in URu₂Si₂ show the weak antiferromagnetic transition at 17.5 K. Finally, we have observed relaxation in high transverse field due to the formation of a flux lattice in U₆Fe, a material where the electron effective mass is rather lighter than in other heavy fermion systems. The relaxation exhibits a sharp onset atT _c=3.9 K, and is flat at low temperatures as expected for a conventional superconductor.     

Advances in single crystal growth and annealing treatment of electron-doped HTSC

High quality electron-doped HTSC single crystals of Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ have been successfully grown by the container-free traveling solvent floating zone technique. The optimally doped Pr_2−xCe_xCuO_4+δ and Nd_2−xCe_xCuO_4+δ crystals have transition temperatures T _c of 25 K and 23.5 K, respectively, with a transition width of less than 1 K. We found a strong dependence of the optimal growth parameters on the Ce content x. We discuss the optimization of the post-growth annealing treatment of the samples, the doping extension of the superconducting dome for both compounds as well as the role of excess oxygen. The absolute oxygen content of the as-grown crystals is determined from thermogravimetric experiments and is found to be ≥ 4.0. This oxygen surplus is nearly completely removed by a post-growth annealing treatment. The reduction process is reversible as demonstrated by magnetization measurements. In as-grown samples the excess oxygen resides on the apical site O(3). This apical oxygen has nearly no doping effect, but rather influences the evolution of superconductivity by inducing additional disorder in the CuO₂ layers. The very high crystal quality of Nd_2−xCe_xCuO_4+δ is particularly manifest in magnetic quantum oscillations observed on several samples at different doping levels. They provide a unique opportunity of studying the Fermi surface and its dependence on the carrier concentration in the bulk of the crystals.     

Charge stripes seen with X-rays in

Superstructure reflections due to the ordering of holes into stripes in have been studied with high energy X-ray diffraction. These reflections have been observed clearly for the first time in a sample which is superconducting at low temperatures ( K). The stripe peaks vanish above 62(5) K whereas the magnetic signal of the stripe ordering which has been seen with neutrons before is already suppressed at K. Our results confirm that the ordering of spins and holes is driven by the charges as it is found in the case of La_1.6-xNd_0.4Sr_xCuO₄ at the doping level of x =0.12. Received 27 April 1999     

Transverse-and zero-field μSR investigation of magnetism and superconductivity in (Y1−xPr x )Ba2Cu3O7

Zero-field muon-spin-rotation (μSR) measurements on (Y₁? _x Pr _x )Ba₂Cu₃O₇ [x=1.0, 0.8, 0.6, and 0.54] show evidence for antiferromagnetic ordering of the Cu moments within the Cu?O planes, with Néel temperatures 285, 220, 35. 30 and 20 K, respectively. Forx=1.0 the local muon magnetic field is ≈16 mT, but decreases to ≈12 mT at 17K, due to additional magnetic ordering. The zero-field data, in conjunction with transport data, allow construction of a complete phase diagram for this system. Transverse-field (1 kOe) μSR data forx=0.2 (T _c =75 K) show that the muon depolarization is determined primarily by the Cu nuclear moments forT>T _c , and by the vortex state forT _c . Fitting the superconducting-state data to a BCS model yields an extrapolated zero-temperature magnetic penetration depth of 2170 Å.     

X-Band ESR and51V NMR study of the Haldane system PbNi2−xMgxV2O8

The temperature and angular dependence of the X-band electron spin resonance (ESR) and⁵¹V nuclear magnetic resonance (NMR) spectra have been measured in a recently discovered Haldenegap system, PbNi_2-xMg_xV₂O₈ (0≤x≤0.24). The angular dependence of the ESR signal suggests that both the spin diffusion as well as the magnetic anisotropy determine the electronic spin correlation functions. However, in doped samples the magnetic anisotropy increasingly dominates the spin dynamics on cooling. The huge broadening of the⁵¹V NMR spectra in doped samples at low temperatures provides evidence for localized magnetic moments in the vicinity of the Mg impurities. Locally distorted structure around each Mg impurity may slightly modify the magnetic interactions and be potentially responsible for the antiferromagnetic ordering (belowT _N≈ 3.5K) in doped compositions.     

吸气溅射Mo1-xSix薄膜的超导性能

对于四个Mo_1-xSi_x薄膜样品进行了超导转变温度、临界温度附近的临界磁场以及在4.2K下的临界电流的测试,并且测量了室温(300K)及低温(8K)下的电阻率,结果显示四个样品具有非晶的特征,其中Mo₇₈Si₂₂薄膜样品有较好的非晶特性。 关键词：     

Floating-zone growth and property characterizations of high-quality La2-хSrхCuO4 superconductor crystals

We have grown underdoped(x=0.11,0.12) and optimally doped(x=0.16) La2 x SrxCuO4 single crystals by the traveling-solvent floating-zone technique.In order to prepare good quality cuprate crystals,we have made much effort to optimize the preparation procedures.For example,we haveadopted the sol-gel route to prepare a highly fine and homogeneous La2xSrxCuO4 precursor powder for fabricating a very dense ceramic feed rod used for the floating-zone growth,and we have also used quite a slow growth rate.The high quality of the grown crystals has been verified by double-crystal x-ray rocking curves,with the full-width-at-half-maximum being only 113-150 arcseconds,which are the best data reported so far for La2xSrxCuO4 crystals.The superconducting critical temperatures of the grown crystals are 30,31 and 38.5 K for x=0.11,0.12 and 0.16 samples,respectively,according to magnetic measurements.     

139La and63Cu-NQR of high-T c oxide superconductors

Nuclear quadrupole resonances (NQR) of¹³⁹La in La_2−xM_xCuO₄ (M=Ba, Sr) and⁶³Cu in YBa₂CU₃O₇ have been investigated with substitution of Cu by magnetic impurities. For La-system, the strong enhancement of the nuclear relaxation rate, 1/T ₁ atT _c ^*≈10 K (suggesting the occurrence of magnetic instability with hole-doping), is suppressed by 3D-antiferromagnetism induced by magnetic impurities. In the superconducting region, fluctuations of Cu moments at low temperatures remain extremely fast as at high temperatures. For Y-system, Fe-ions are substituted for both Cu(1) and Cu(2) sites, But Co-ions are preferentially substituted to Cu1 sites. The peaks in the relaxation rate of Cu indicate the appearance of magnetic ordering without destroying superconductivity for the 0.5%-Fe doped sample.     

Magnetic and transport properties of SmCoAsO1−xFx

A series of SmCoAsO_1−xF_x (with x=0, 0.05, 0.1, and 0.2) samples have been prepared by solid state reactions. X-ray powder diffraction proved that all samples can be indexed as a tetragonal ZrCuSiAs-type structure. A clear shrinkage of the lattice constants a and c with increasing F content indicated that F has been doped into the lattice. The magnetic and transport properties of the samples have been investigated. Parent SmCoAsO compound exhibited complicated magnetism including antiferromagnetism, ferromagnetism, and ferrimagnetism. For the fluorine doped samples, the antiferromagnetic Néel temperatures were almost independent of the F content and metamagnetic transitions were observed below antiferromagnetic Néel temperatures. With increasing F content, high temperature (below 142 K) ferrimagnetic state gradually changed to ferromagnetic state. In the resistivity result, metallic conduction in the region of 2-300 K and Fermi liquid behavior at low temperatures were shown in all samples. Transport properties at applied magnetic fields showed anomalies at low temperatures.     

Local dynamic deformation of the superconducting CuO2 plane in the Nd2 − x Ce x CuO4 + δ compound

The local structure of the electron-doped high-temperature superconductor Nd_{2 ? x} Ce _x CuO_{4 + δ} has been investigated by EXAFS spectroscopy using synchrotron radiation in the temperature range 5–300 K. The radii, coordination numbers, and Debye-Waller factors of the nearest coordination spheres of the copper, neodymium, and cerium local environment have been obtained. The Einstein temperatures, characterizing the strength of Cu-O, Nd-O, and Ce-O interatomic bonds have been determined. The dynamic local deformation of the superconducting CuO₂ plane in the form of vibrations of some part of oxygen ions in a double-well potential due to the difference in the electronic filling of neighboring CuO₄ complexes has been established.