Vortex states in R1237, Bi2212, Tl2223 cuprates and type-II superconductors, using Me-μSR analysis

Physica C 235-240 (1994) 2633-2634 North-Holland

PHYSICA

Vortex States in R1237, Bi2212, T12223 cuprates and Type-ll Superconductors, using ME-~tSR analysis. C. Boekema, a T. Her,a V.A. Gubanov,a D.W. Cooke,b M. Leon.b aphysics Department, San Jose State University, San Jose, CA 95192 -0106. bLos Alamos National Laboratory, NM - USA. The Maximum-Entropy (ME) method is applied to analyze the muon-spin-research (I~SR) data of RBa2Cu307 (R1237; R = Er, Gd and Eu), Bi2Sr2CaCu2Ox (Bi2212) and TI2Ba2Ca2Cu3Ox (TI2223) and Pb0.91n0.1 in their vortex states. A comparison of these mixed states indicate that these cuprates appear not to be typical type-II superconductors. 1. I N T R O D U C ' n O N ,

BACKGROUND

AND METHOD

Transverse field (TF) I~SR studies 1,2 on cuprate superconductors are making significant contributions towards fundamental understanding of their vortex states. TFI~SR has been successful in the determination of magnetic penetration depths and the strength of the flux pinning. In most I~SR studies of field distributions, the observed i~-decay time histograms are analyzed by Fourier methods and curve fitting. In principle, the field distribution is given by the real part of the Fourier Transform (FT). For real t~SR data, which are discrete, noisy and truncated after a few l~-Iife times, FT analysis is problematic. Consequently, Gaussian distributions have been reported rather than the distributions with sharp features predicted by canonical Abrikosov theory. An alternative approach is the Maximum Entropy (ME) method. ME has the major advantage of producing in the frequency spectrum only structure for which sufficient statistical evider~ce is present in the time series. We have developed and successfully tested an ME method for TFI~SR data. 2a The predicted i~SR-vortex line shape Ib for a sintered powdered anisotropic type-I! superconductor exhibits a sharp peak occurring slightly below the applied field, with a long tail (up to 150 Oe) at higher fields. Below the sharp peak, over a 50-Oe interval, the field distribution initially falls precipitousiy followed by a steady decline to zero. ME is sensitive to sharp features of the field distribution. 2. R E S U L T S

AND DISCUSSION

We present some significant results of the 0921-4534D4/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSl q 0921-4534(94)01882-0

ME application to TFI~SR data recorded earlier~ for R1237 (R = Er, Gd and Eu), Bi2212, TI2223 and Pbo.91no.1 in their mixed states. The cuprate samples are high-quality, singlephase ceramic polycrystalline samples. For R1237, the Er and Gd ions carry magnetic moments, where Eu does not. Er1237 has been reported to be a 2-dim Ising antiferromagnet with a TN of 0.7 K. Below Tc, for R1237, Bi22,2 and TI2223, asymmetric, non-Gaussian ME transforms are observed. In Fig 1, the ME estimates of the field distribution in the Er1237 vortex state at 4.5 K and 10 K are compared, in Fig 2, the ME transforms for Bi2212 and Ti2223 both at 10K and 5kOe are shown. .

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C. Boekema et aL/Physica C 235-240 (1994) 2633-2634

2634

250 Oe below the main peak and is followed by a long tail till out to 100 Oe. This field tail may be due to the magnetism generated by the Er planes. This "fall-off" is seen for Er1237 at 620 Oe and also for Gd1237 at 680 Oe, indicating this feature is magnetic. The fall-off field scales well with the magnetic moment of the R ion. Below 5 K no fall-off is observed for Eu1237. A low-field tail centered around 630 Oe for Eu1237 at 3.5 K is confirmed when compared to runs at higher temperatures, where this tail disappears. In the vortex field distributions of Bi2212 and TI2223, no lowfield tails are observed at 10K (Fig 2). This remarkable difference with the Iow-T R1237 vortex field distribution is probably caused by vortex-pancake 3 formation in Bi2212 and TI2223. ~.0 •

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Fig 2 The "non-Abrikosov" features seen for R1237 do not show up for ME estimates for Pb0.91n0.1, a typical type-II superconductor, at 3 K, 500 and 800 Oe. 2 The existence of the low-field tail for R1237 at very low temperatures shows that in the mixed state there are regions for which the magnetic field is much lower than predicted by Abrikosov theory. Recall that the muons are probing just below the BaO layers in R1237. The electrons of the neighboring CuO2 planes have managed to substantially screen the applied field for these 'low-field tail' muons. These muons sense competing, possibly random, magnetic fields, enhanced by the onset of 2dim magnetism in the R planes. The temperature dependence of the peak height (PkH) in the Eu1237 field distributions

is anomalous near Tc. PkH corresponds to the spectral density maximum. An 'overshoot' is seen starting well below Tc, peaking at Tc +2 and flattening out at Tc +10. The unusual feature near Tc is also observed for R = Er and Gd, and for Bi2212 and TI2223. This anomaly might be related to the experimental flux melting observations just below Tc, and/or to the observed glassy precursor effects in a 10K region above Tc .za

3. CONCLUSIVEREMARKS We have applied the ME method to analyze the pSR data of R1237 (R = Er, Gd and Eu), Bi2212, TI2223 and Pbo.91no.1 in their vortex states. The obtained magnetic field distributions in the cuprate vortex states are quite different from typical vortex behavior of type-II superconductors. For the cuprate vortex states, we have found asymmetric field distributions. At very low temperatures, the magnetism of the R-layers in R1237 appears to strongly influence the field distribution. Because non-Gauss,an distributions are seen in the ME transforms, Gauss,an curve-fitting to pSR vortex data is only approximate, and s ig n ific a n t vortex features are being overlooked. The low-field tail and the MEintensity anomaly near Tc suggest glassy fea~;ures in the cuprate vortex states. These studies have been supported by Re.~iearch Corporation, Associated Western Universities, and in part by the US Department of Energy. References 1. (a) See the Proceedings 5th and 6th Int Con, on Muon-Spin Rotation, April'90 and June'94, Hyperfine Interactions 6 3 - 6 5 (1990), and in press, and references therein; (b) D.R. Harshman eta/., Phys Rev Lett 66 3313 (1991). 2. (a) S. AIves et al., Phys Rev B49 Rapid t. .~. . ...n. . m m l l -n. -i, r. .:v~w .i4oe n. ~

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therein; (b) R.L. Lichti et ai., Appi Phys Lett 54 2361 (1989). 3. (a) S.L. Lee et aL, Phys Rev Lett 71 3863 (1993); (b) this conference.