Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr$_2$EuCu$_{2}$O$_{8}$

B. C. Chang; C. Y. Yang; H. C. Ku; J. C. Ho; C. B. Tsai; Y. Y. Chen; D. C. Ling

doi:10.1016/j.physb.2007.10.350

paper

Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr $_{2}$ EuCu $_{2}$ O $_{8}$

arXiv:cond-mat/0702040 · doi:10.1016/j.physb.2007.10.350

Abstract

Similar to the optimal-doped, weak-ferromagnetic (WFM induced by canted antiferromagnetism, T $_{C u r i e}$ = 131 K) and superconducting (T $_{c}$ = 56 K) RuSr $_{2}$ GdCu $_{2}$ O $_{8}$ , the underdoped RuSr $_{2}$ EuCu $_{2}$ O $_{8}$ (T $_{C u r i e}$ = 133 K, T $_{c}$ = 36 K) also exhibited a spontaneous vortex state (SVS) between 16 K and 36 K. The low field ( $\pm$ 20 G) superconducting hysteresis loop indicates a weak and narrow Meissner state region of average lower critical field B $_{c 1}^{a v e}$ (T) = B $_{c 1}^{a v e}$ (0)[1 - (T/T $_{S V S}$ ) $^{2}$ ], with B $_{c 1}^{a v e}$ (0) = 7 G and T $_{S V S}$ = 16 K. The vortex melting transition (T $_{m e l t in g}$ = 21 K) below T $_{c}$ obtained from the broad resistivity drop and the onset of diamagnetic signal indicates a vortex liquid region due to the coexistence and interplay between superconductivity and WFM order. No visible jump in specific heat was observed near T $_{c}$ for Eu- and Gd-compound. This is not surprising, since the electronic specific heat is easily overshadowed by the large phonon and weak-ferromagnetic contributions. Furthermore, a broad resistivity transition due to low vortex melting temperature would also lead to a correspondingly reduced height of any specific heat jump. Finally, with the baseline from the nonmagnetic Eu-compound, specific heat data analysis confirms the magnetic entropy associated with antiferromagnetic ordering of Gd $^{3 +}$ (J = S = 7/2) at 2.5 K to be close to $N_{A} k$ ln8 as expected.

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Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr2​EuCu2​O8​

Abstract

Anomalous physical properties of underdoped weak-ferromagnetic superconductor RuSr $_{2}$ EuCu $_{2}$ O $_{8}$