POSSIBLE SECOND ORDER IN STRONGLYPRASEODYMIUM-ANTIDOPED SUPERCONDUCTING CUPRATES

V. SANDU1, P. GYAWALI1, B. J. TAYLOR3, M. B. MAPLE3, C. C. ALMASAN2

1 National Institute of Material Physics, Bucharest–Magurele, P.O. Box MG-7, Romania,e-mail: vsandu@infim.ro

2 Department of Physics, Kent State University, Kent, OH-44242, USA3 Department of Physics, University of California at San Diego, La Jolla, CA-92093-5004, USA

Received September 26, 2005

A reentrant irreversibility was detected in magnetization measurements onY0.47Pr0.53Ba2Cu3O7 single crystals in the vicinity of the superconducting transitiontemperature (Tc = 7 K). The transition to the second hysteresis loop occurs after themelting temperature which appears to be a first order transition. The temperaturerange of this irreversibility extends up to very high temperature, of order of 200 K.The hysteresis is also visible in magnetization vs temperature measurements where itis superposed on the expected paramagnetic signal. Generally, this double hystereticregime was reported in low dimensional systems and has been associated with thepinning of a certain density waves, spin or charge.

Key words: Y0.47Pr0.53Ba2Cu3O7, magnetization, fluctuation, density waves.

1. INTRODUCTION

The strange physics of the underdoped cuprates is an extremely debatedtopic still waiting for elucidation.

It is known that some cuprates become superconducting by doping theparent Mott insulator with holes. Cuprates insulators display antiferromagneticorder, therefore, it is expected that remnants of this order survive in thesuperconducting state at low doping level or when the system is driven in thevicinity of the quantum phase transition by an external magnetic field.

The presence of these antiferromagnetic correlations in superconductingstate have been reported in neutron scattering [1–7] and μSR experiments [8, 9].Scanning tunneling spectroscopy revealed the presence of spin density waves(SDW) associated with charge density waves (CDW) [10–12]. Josephson plasmaresonance experiments were interpreted as the spontaneous formation of dropletsof superconducting and antiferromagnetic phases [13], while Raman experimentproved the existence of a field enhanced spectral weight of spin excitation [14].

Paper presented at the National Conference of Physics, 13–17 September, 2005,Bucharest, Romania.

Rom. Journ. Phys., Vol. 51, Nos. 5–6 , P. 623–630, Bucharest, 2006

624 V. Sandu et al. 2

Various theoretical attempts tried to elucidate the physics of this coexistingand competing orders (see the review of Sachdev [15] and references therein)seen as strong fluctuation of an ordered state, in our case the antiferromagneticorder, beyond its equilibrium state. The fluctuations are enhanced in cuprates byseveral specific features like low dimensionality and reduced carrier density.Therefore, the competition of different order extends far away the phasetransition lines.

Another example is the transition superconductor–“normal” metal, where“normal” state is the resistive state above critical temperature known aspseudogap. In this case, numerous experiments were interpreted in terms of theexistence of strong fluctuations of the phase of the superconducting orderparameter [16–20] above critical temperature. However, in this latter case, othermodels invoked the presence of d-density waves, already detected in underdopedcuprates in superconducting state, as responsible for pseudogap [21, 22].

2. EXPERIMENTAL STUDY

Fig. 1. – Temperature T de-pendence of the ac-magneticsusceptibilities χ′ (lower panel)and χ″ (upper panel) for a single crystal of Y0.47Pr0.53Ba2Cu3O7.

3 Strongly praseodymium-antidoped superconducting cuprates 625

Single crystals of Y0.47Pr0.53Ba2Cu3O7 were submitted to complex magneticinvestigations: magnetization M vs. magnetic field H and dc- and ac-magnetizationvs. temperature T. These materials are characterized by a very low charge carrierdensity, which locates them close to the Mott antiferromagnetic–superconductingphase transition line. The substitution of Pr for Y produces a strong depletion ofthe charge carriers responsible for superconductivity due to their localization onthe p-f hybridization band [23].

The measurements were performed with Quantum Design MPMS SQUIDmagnetometer. Each run was performed after the sample, mounted on a Teflonsupport, has been warmed up to temperatures several times higher than Tc andcooled down to the temperature for investigation in zero-magnetic field. TheTeflon support was submitted to the same investigations as the sample in orderto subtract the background. The ac measurements were carried out at drivenamplitude of 3.8 Oe and frequency of 1 Hz.

The critical temperature Tc, as obtained from ac-susceptibility χ′ vs. Tmeasurements at zero dc-field, gave Tc = 7.0 K (Fig. 1), which was 6 K lowerthan the critical temperature obtained from resistive measurements.

3. RESULTS AND DISCUSSIONS

Fig. 2 shows the magnetization M vs. H loops in the field range 0–2 T at5 K, a temperature lower than the zero-field critical temperature (7 K). Thedemagnetization factor N, as determined from the slope at very low field almostequals unity. The hysteresis loops are asymmetric with an emphasized firstmagnetization peak.

At fields higher than the irreversible field Hirr, the sample presents afluctuating regime which ends up at a temperature Ts where the slope of themagnetization jumps crosses over to a paramagnetic behavior with a linearrelationship between field and magnetization. The jump in magnetization slopeat Ts might be attributed to a melting transition. It becomes unambiguous athigher temperatures, above 5.9 K, when it looks like a jump in magnetization,typical for a first order transition. The most surprising effect occurring above6.3 K is that magnetization above the jump is no more irreversible.

It is salient in Fig. 3 that after the closing of the hysteresis loop at Hirr, itfollows a field range with fluctuating magnetization which transforms in a newhysteretic loop at higher fields even before the jump. The magnetization lookslike a double hysteretic curve with the new hysteresis enhanced after the jump ascan be seen in the marked area on Fig. 3. With increasing T, both theirreversibility field and the opening of the new hysteresis shift, as expected, tolower fields. However, the latter field decreases faster with increasing temperature,

626 V. Sandu et al. 4

Fig. 2. – Field H dependence of the magnetization M at 5.0 K for asingle crystal of Y0.47Pr0.53Ba2Cu3O7. The arrows and the correspond-ing labels, Hirr and Hs, mark the irreversibility field and the transitionto paramagnetic state, respectively. Inset: zoom in of the irreversible

range of the magnetization loop.

Fig. 3. – Field H dependence of the magnetization M at 6.5 K for asingle crystal of Y0.47Pr0.53Ba2Cu3O7. The arrows and the correspond-ing labels, Hirr and Hs, mark the irreversibility field and the jump inmagnetization, respectively. Marked is the new hysteresis loop that

opens after the magnetization jump.

5 Strongly praseodymium-antidoped superconducting cuprates 627

hence, at 6.7 K, the new developed loop overlaps over the superconductingirreversibility and the magnetization displays again a single loop (Fig. 4).

A second unusual effect is the nonvanishing of he hysteresis loop at andabove the critical temperature. The second irreversibility is salient up totemperatures as high as 200 K (Fig. 5). Differently from the superconductingstate, there is no remanence above Tc, hence, the loop closes at zero field.Moreover, the magnetization is positive for the whole range of the applied field.

This irreversibility occurring high above critical temperature is also visiblein magnetization vs temperature plots (Fig. 6). The hysteresis is visible startingfrom 100 Oe and is conspicuous up to 500 Oe. However, the temperatureextension is different for different fields with a maximum around 300 Oe.

The second irreversibility cannot be attributed to superconductivity at suchhigh temperature. Pinning revival once the system overcomes the melting line isnot physically acceptable and it is not realistic to introduce a kind of hightemperature pinning as speculate Panagopoulos et al. [24].

The most probable source of these phenomena must be searched in thefluctuational state occurring in strongly underdoped superconducting cuprateswhich are nearby the Mott antiferromagnetic state. In these cases, even in theresistive state, the fluctuations of both antiferromagnetic and superconductingorder are strong enough to extend far above critical temperature.

Fig. 4. – Field H dependence of the magnetization M at 6.7 K for asingle crystal of Y0.47Pr0.53Ba2Cu3O7. The arrows and the correspond-

ing label, Hirr marks the putative position of the irreversibility field.

628 V. Sandu et al. 6

Fig. 5. – Field H dependence of the magnetization M at 24, 32, and 40 Kfor a single crystal of Y0.47Pr0.53Ba2Cu3O7.

Fig. 6. – Temperature T dependence of the magnetization M at of a singlecrystal of Y0.47Pr0.53Ba2Cu3O7 at an applied field of 110 Oe.

7 Strongly praseodymium-antidoped superconducting cuprates 629

The superconducting fluctuations concern the phase of the superconductingorder parameter at low charge doping, whereas the magnetic fluctuations appearin form of spin density waves, most probable d-density waves. Generally, thesetwo orders compete, even within fluctuational regime. However, the spinmodulation (density waves) has to be emphasized above Tc because the constraintof phase order imposed by the superconductivity is relaxed. Therefore, the spacemodulation of the spin and\or charge replaces the superconducting coherence[25]. This kind of fluctuations is privileged by the low dimensionality of thecuprates. The irreversibility is the result of the infinite number of metastable stateof density waves due to the presence of impurities. Another explanation shouldbe the development of a kind of glassy state prior to the paramagnetic transition.

In conclusion, we have found that strongly underdoped Y0.47Pr0.53Ba2Cu3O7

displays a second magnetic hysteresis loop below zero-field critical temperaturewhich transforms in a single hysteresis at temperatures above Tc. We speculatethat this hysteresis is due to the development of antiferromagnetic fluctuations inthe presence of impurities.

Acknowledgments. This research was supported by the National Science Foundation underGrant No. DMR-0406471 at KSU, the US Department of Energy under Grant No. DEFG03-86ER-45230 at UCSD, and the Romanian Ministry of Education and Research in the frameworkof the Research for Excelence Program.

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