μ+SR studies on superconducting YBa2(Cu1−xFex)3Oy system

Spin-glass like magnetic ordering of iron moments was observed in both orthorhombic and tetragonal YBa₂(Cu_1−xFe_x)₃O_y (x=0.08) by μ⁺SR measurements. In a “Tetra” sample, all the muons sense the superconducting transition at 60 K and magnetic ordering at around 15 K, while in an “Ortho” sample they reveal that two magnetically different parts exist in the sample: about 40% of the sample is superconducting withT _c ≈90K and the remaining part is magnetic withT _M≈33K. These phenomena can be explained in terms of clustering of the Fe atoms in the “Ortho” sample.     

The superconducting properties of YBa2(Cu1−xMx)3O7 for M=Zn and Ni

Transverse and zero-field μSR measurements were made on YBa₂(Cu_1−xNi_x)₃O_7−y withx=0.1 and 0.2, and YBa₂(Cu_1−x Zn _x )₃O_7−y withx=0.03, 0.06, 0.1, and 0.16, wherey≈0.1. Since doping may lead to magnetic ordering this was searched for with both zero and transverse field μSR, but no evidence was found over the temperature range studied: 10–100 K. However, depolarization rates as functions of temperature were obtained, and the low temperature values of these are σ=3.2 μs⁻¹.1.6μs⁻¹, and 1 μs⁻¹ forx=0.01, and 0.2 Ni, respectively, and σ=0.8 μs⁻¹, 0.75 μs⁻¹, 0.65 μs⁻¹, and 0.4 μs⁻¹ forx=0.03, 0.06, 0.1, and 0.16 Zn, respectively. Estimates for the effect of decreasing electron concentration for Zn are made, but these alone do not account for the drop in σ. Estimates for the effect of scattering on λ and hence σ are made. The reduction in σ for Ni dopant is in surprisingly good agreement with these estimates. For Zn the order of magnitude is correct, but the relative lack of further change in σ after the effect of the first 0.03 addition seems to imply a saturation of the effect of scattering.     

Combination of muon spin relaxation and neutron scattering studies on magnetism

We combine the results from muon spin relaxation (μSR) and neutron scattering measurements performed on the same specimen (or system) of magnetic materials. The example on a spin glassCuMn (5%) shows that the two techniques have complementary time windows for studying dynamic spin fluctuations. In combining the results, one should note that muons and neutrons probe dynamic phenomena with different wavevectors. The results on antiferromagnetic La₂CuO_4−y illustrate the difference in the spatial range of static spin correlations reflected in the μSR precession frequency and the neutron Bragg peak intensity. With the examples of CeCu_2.1Si₂, YBa₂Cu₃O_x and Bi₂Sr₂YCu₂O_8+y , we point out that μSR is a superb tool for discovering static magnetic order while neutron scattering is the unique method to determine the spin structure. We emphasize that it is very fruitful to perform μSR and neutron experiments on the same specimen and to compare and combine the results for the better understanding of magnetism of various system.     

Zero-field μSR study of ErBa2Cu3Ox for various oxygen contentx

Results from zero-field μSR experiments are presented, performed on the high-T _c compounds ErBa₂Cu₃O_x for various oxygen contentx with 6≤x≤7 at temperatures between 40 mK and 300 K. The aim was to study the magnetic ordering behavior of the Cu moments and the Er moments, and its interplay with superconductivity.     

Superconductivity, Seebeck coefficient, and band structure transformation in Y1−x CaxBa2Cu3−x CoxOy (x=0–0.3)

The temperature dependences of the resistivity and of the Seebeck coefficient S is studied in three series of Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples (x=0–0.3) differing in oxygen content. It was found that the critical temperature decreases for y≈7.0, and S(T=300 K) increases with doping, whereas oxygen deficiency results in a nonmonotonic variation of these quantities with increasing x. The band structure parameters have been determined from an analysis of the S(T) relations using a phenomenological theory of electron transport. It was found that an increase in x results in a gradual increase in band asymmetry, which is caused by calcium-induced creation of additional states in the band responsible for conduction in the normal phase. An analysis has shown that high impurity concentrations in oxygen-deficient Y_1−x Ca_xBa₂Cu_3−x Co_xO_y samples bring about an additional ordering of the structure, which may be caused by formation of a cobalt superlattice. It has also been shown that, in the case of Ca and Co codoping, the dependence of critical temperature on effective conduction-band width coincides with the universal correlation relation observed in the YBa₂Cu₃O_y system with single substitutions in various lattice sites. Fiz. Tverd. Tela (St. Petersburg) 41, 1363–1371 (August 1999)     

Comparative analysis of the effect of La and Co on the superconductivity and energyband spectrum of YBa2Cu3Oy for different oxygen contents

Temperature dependences of the resistivity and Seebeck coefficient of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y samples (x=0–0.25) have been measured under maximum sample saturation with oxygen, as well as following their anneal in an oxygen-deficient atmosphere. The T _c (x) dependences for as-prepared samples were found to pass through a maximum at x=0.05, which persists after annealing for Y(Ba_1−xLa_x)₂Cu₃O_y and disappears for YBa₂Cu_3−x Co_xO_y. A phenomenological model of the band spectrum in normal phase has been used to determine the parameters of the conduction band and of the carrier system, and to analyze their variation with the dopant type and content, as well as with annealing. Despite the differences observed in the T _c (x) dependence, the critical temperatures for all the sample series studied were found to correlate with the conduction-band effective width. The mechanism of the effect of impurities on the band-structure parameters and the reasons for the different influence of annealing on the properties of Y(Ba_1−x La_x)₂Cu₃O_y and YBa₂Cu_3−x Co_xO_y are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 389–394 (March 1997)     

Structural studies and T c dependence in La2−x Dy x Ca y Ba2Cu4+y O z type mixed oxide superconductors

A new series of mixed oxide superconductors with the stoichiometric composition La_2−x Dy _x Ca _y Ba₂Cu_4+y O _z (x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La_2−x (Y/Er) _x Ca _y Ba₂Cu_4+y O _z series, show a strong dependence of T _c on hole concentration (p _sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant displacement of La onto Ba site. Superconductivity studies show that maximum T _c is obtained for x=0.5, y=1.0 sample (T _c ∼ 75 K), for La_1.5Dy_0.5Ca₁Ba₂Cu₅O _z (La-2125).     

Magnetic ordering of Fe atoms in YBa2(Cu1−x Fe x )3O3 in superand semiconducting states

Magnetic behavior of Fe atoms in YBa₂(Cu_1−x Fe _x )₃O _y system in superconducting (SC) and non-SC states is studied. In SC region one magnetic ordering point T_m1 is observed for all Fe atoms in Cu1 sites: T_m1=10 K and 16 K for x=0.05 and x=0.10, respectively. In the non-SC state for x=0.05 Cu1 and Cu2 sublattices are magnetically independent and two magnetic ordering points are found: T_m1=20 K and T_m2=405K. For x=0.10 a strong magnetic interaction between Cu1 and Cu2 sublattices appears and only one ordering point T_m2=435 K is observed.     

Superconductivity and magnetic ordering in YBa2(Cu1−x Fe x )3O7

Transverse-and zero-field μSR measurements have been made for YBa₂(Cu_1−x Fe _x )₃O₇ withx=0.04, 0.08 and 0.12. The temperature range studied was from approximately 7.5 K to 100 K. The onset of magnetic ordering commences at about 7.5 K forx=0.04, 10 K forx=0.08 and 20 K forx=0.12. The Gaussian depolarization parameter, σ ofG _x (t) = exp(−σ² t ²/2), is depressed by a factor of about 0.6 forx=0.04, but for thex=0.08 sample σ is depressed by a factor of 10 and increasing suppression is seen as the temperature is lowered below 45 K. This decrease in σ is interpreted in terms of decreasing electronic mean free paths.     

A muon spin rotation study of hydrogen in YBa2Cu3O y

Superconducting YBa₂Cu₃O _y samples were doped with hydrogen and investigated by the muon spin rotation technique. For hydrogen concentrations above a thresholdx, which depends on the oxygen stoichiometry, we find a well defined precession signal in zero external field. This is a clear indication of magnetic ordering in these samples. In samples with a smaller hydrogen content no magnetic ordering was found. For these samples however, the depolarization rate σ(T→0) as measured in a transverse external field depends strongly on the hydrogen content. Our data are consistent with the assumption that hydrogen acts as an electron donor, filling the hole states in YBa₂Cu₃O _y .     

μSR studies on CeAl2

μSR studies on REAl₂ type compounds have so far given rather inconclusive results since no μSR frequency has been observed in the ordered magnetic states. Therefore, the results from the paramagnetic region [1,2] have been interpreted without detailed knowledge of the muon site or the mobility of the muons. In the present study of a single crystal sample of CeAl₂ we investigated in some detail the paramagnetic temperature range including the transition region to magnetic ordering around 3.6 K. The ordered magnetic state is antiferromagnetic with a modulated structure [3], and the absence of a spontaneous μSR precession signal belowT _N is therefore not unexpected.     

Effect of Pr on electron band spectrum of YBa2Cu3Oy: study by thermopower data analysis

The temperature dependences of the thermopower, S(T) of Y_1−xPr_xBa₂Cu₃O_y (0≤x≤0.6) have been measured and analyzed within a phenomenological model of electron transport in the case of a narrow conductive band. The band spectrum parameters have been estimated by a quantitative comparison of the experimental and calculated S(T) curves. It was revealed that an increase of the Pr content results in remarkable broadening of the conductive band and a strong localization of the states. This can be considered as the main reason for T_c depression. The effect of Pr on the band filling is very slight that makes it possible to conclude a valence of Pr to be close to 3+. The Pr influence on superconducting properties of YBa₂Cu₃O_y cannot be described in the terms of hole-filling effect.     

Superconductivity and magnetic order in CaLaBaCu3O z

X-ray diffraction, thermogravimetric analysis, magnetization and Mossbauer studies of a⁵⁷Fe probe, in CaLaBa(Cu_1−x Fe _x )₃O _z withz=7, 6.5 andx=0.00 to 0.05 have been performed. Forz=7,T _c drops from 74 K forx=0.00 to 35 K forx=0.05. In CaLaBa(Cu_1−x Fe _x )₃O _z withz=7, 10% of the iron is magnetically ordered withH _eff=530 K kOe andT _N=400 K, even though the sample is superconducting. In the oxygen poor, non-superconducting samples (z=6.5) 20% of the iron is magnetically ordered withH _eff=340 kOe andT _N=340 K. Since the iron reflects the magnetic order of the Cu(2) ions, this may show that perhaps two inequivalent Cu(2) sites exist in CaLaBa(Cu_1−x Fe _x )₃O₇, ones of which is magnetically ordered. The experimental observations may be interpreted in terms of the special crystal structure which can allow superconductivity and magnetism to coexist, probably in separate Cu(2) planes.     

NMR and other magnetic resonance techniques in unstable magnets

This paper reviews and compares the use of nuclear magnetic resonance (NMR) and related hyperfine techniques [muon spin rotation (μSR) and, to a lesser extent, other methods] in the study of 4f and 5f magnetism in “unstable magnets”, i.e., intermediate-valent and heavy-fermion materials. In both NMR and μSR the features of interest are the spectral shape, the frequency shiftK (Knight shift in metals) and the spin-lattice relaxation rate 1/T ₁. For temperatures below the characteristic or “Kondo” temperatureT ₀ these experiments given evidence for (1) modification of the transferred hyperfine field [nonlinearK(χ)]. (2) spin fluctuations with a characteristic fluctuation rate ∼k _B T ₀/h, (3) strong energy-gap anisotropy (zeros of the gap along lines on the Fermi surface) in heavy-fermion superconductors, (4) spin-singlet Cooper pairing from the change in muon Knight shift in superconducting UBe₁₃, and (5) very weak static magnetism (10⁻¹–10⁻³ μ_B/f atom) in CeAl₃, CeCu₂Si₂, U_1−x Th _x Be₁₃ (x=0.033), and UPt₃. There is some controversy concerning the interpretation of 1/T ₁ well aboveT ₀ in UBe₁₃; the situation is reviewed.     

120 K superconductivity and the zero-field absorption in the BiCaSrCu2Ox system

Onset transition temperature (123±1) K was observed in the BiCaSrCu₂O_x bulk materials prepared by means of conventional ceramic technique. The resistance and AC susceptibility measurements show that there are two highT _c phases. EPR measurement shows that belowT _c (R=0), an intense line at zero field was observed. It is either associated with the glass feature or the spin triplet transition of Cu²⁺–Cu²⁺ pair via exchange interaction.The Project Supported by National Natural Science Foundation of China     

Mössbauer absorption in some high Tc superconductors:170Yb in YbBa2Cu3O7−x,166Er in ErBa2Cu3O7−x,and57Fe in YBa2Cu3O7−x

For the high T_c compound YbBa₂Cu₃O_7−x, Mossbauer absorption measurements show that magnetic ordering occurs within the Yb³⁺ sublattice at 0.35K (μ_sat=1.7μ_B). For ErBa₂Cu₃O_7−x the Er³⁺ sublattice orders at 0.7K (μ_sat=4μ_B). Measurements on⁵⁷Fe diluted into YBa₂Cu₃O_7−x shows that no ordered magnetic moments exist within the Cu sublattices down to at least 4.2K. Instituto de Fisica, Porto-Alegre, Brasil (supported by CNPq)     

Cu NMR/NQR Studies on Magnetism in Impurity/Hole-Doped Spin-Ladder Compounds

Cu nuclear magnetic resonance (NMR) spectra of impurities (Zn, Ni, and La)-doped spin-1/2 Heisenberg ladder compounds SrCu₂O₃ (Sr123) are broadened with Curie-like temperature (T) dependence. The spectra have been successfully fit by using a quasi-one-dimensional (Q1D) staggered polarization (SP) model. Such a SP has also revealed in Cu NMR measurements of Sr_14–x Ca _x Cu₂₄O₄₁ (Cax) with hole-doped ladders. The origin of possible 3D antiferromagnetic (AF) long-range ordering in (Zn and Ni)-doped Sr123 and Cax around x=12 at low T is considered to be similar. Once unpaired spins S ₀'s are induced and 3D interlayer interaction occurs, the localized spins couple in the whole system.     

Unusual Pr antiferromagnetic ordering in high-Tc cuprates

The anomalous Pr antiferromagnetic order with high Néel temperature T_N(Pr) are reported throughout the whole Pr_1+xBa_2−xCu₃O_7−y or 1212-type Cu(Ba_2−xPr_x)PrCu₂O_7−y system (−0.2<x<1; −0.4<y<1), where three distinct crystal structures were observed: orthorhombic 1212-chain O(I) (space group Pmmm), tetragonal T (P4/mmm), and orthorhombic O(II) (Cmmm). Systematic variation of T_N(Pr) in this system as well as in other 1212 and 2212 cuprates M_nA₂PrCu₂O₇ (n=1, 2, M=Hg, Tl, Pb/Cu, Bi, Nb, Cu; A=Sr, Ba, Ba/Pr) was discussed through the correlation of T_N(Pr) with Pr–O bond length. The importance of quasi-two-dimensional Pr–O–Pr superexchange magnetic coupling through strong wave function overlap between the overextended Pr-4f orbital with eight O-2p_π orbitals in the adjacent CuO₂ bi-layer is discussed. No superconductivity was observed in the present study.     

Positive muon sites and possibility of anyons in the YBa2Cu3O x system

Possible positive muon sites in YBa₂Cu₃O _x were determined from the observedμ ⁺ hyperfine fields in antiferromagnetically ordered YBa₂Cu₃O _x and GdBa₂Cu₃O₇. After determining theμ ⁺ sites, the possibility of anyons or chiral spin ordering in the superconducting YBa₂Cu₃O₇ is discussed. Positive muon implanted in YBa₂Cu₃O₇ feel static magnetic fields of average 1.4G, which are explicable in terms of nuclear magnetic dipolar fields. Non observation of static local magnetic fields of electronic origin (the upper limit is the order of 0.1 G) means that anyons or chiral spin ordering might not exist in superconducting YBa₂Cu₃O₇.     

Spin gap systems: What can be measured by muon spin relaxation?

Muon spin relaxation (μSR) and nuclear magnetic resonance (NMR) are powerful probes of magnetism, which have been extensively applied to studies of spin gap systems. Comparison of results obtained with the two techniques gives complementary results, as each is sensitive to different aspects of spin gap magnetism. We discuss recent μSR measurements of the spin ladder compounds Sr _n?1Cu _n+1O_2n , pure and doped Haldane materials (Y_2?x Ca _x )Ba(Ni_1?y Mg _y )O₅, and doped spin Peierls compounds (Cu_1?x Zn _x )(Ge_1?y Si _y )O₃.