Oxygen Isotope Effects on the Superconducting Transition and Magnetic States Within the Phase Diagram ofY1−xPrxBa2Cu3

Abstract

Oxygen isotope (16 O/ 18 O) effects (OIE's) on the superconducting transition (Tc), the spinglass ordering (Tg), and the antiferromagnetic ordering (TN ) temperatures were studied for Y1−xPrxBa2Cu3O 7−δ as a function of Pr content (0.0 ≤ x ≤ 1.0). The OIE on Tc increases with increasing x up to x ≈ 0.55, where superconductivity disappears. For decreasing x the OIE's on TN and Tg increase down to x ≈ 0.7 where antiferromagnetic order and down to x ≈ 0.3 where spin-glass behavior vanish, respectively. The OIE's on Tg and TN are found to have opposite signs as compared to the OIE on Tc. All OIE's are suggested to arise from the isotope dependent mobility (kinetic energy) of the charge carriers.PACS numbers: 74.72. Bk, 74.25.Dw, 76.75.+i High-temperature cuprate superconductors (HTS's) exhibit a rich phase diagram as a function of doping (see e.g. Fig. 2). The undoped parent compounds are characterized by a long range 3D antiferromagnetic (AFM) order which is rapidly destroyed when holes are doped into the CuO 2 planes. The short-range AFM correlations survive, however, well in the superconducting (SC) region of the phase diagram by forming a spin-glass (SG) state. The issues of the interplay of magnetism and superconductivity in HTS's and the nature of doping-induced charge carriers within the antiferromagnetic CuO 2 planes are still controversial. Understanding these fundamental questions can help to clarify the pairing mechanism of high-temperature superconductivity.In conventional superconductors, key experimental evidence for a phonon mediated pairing mechanism was provided by measurements of the isotope effect on the transition temperature T c . In contrast, in HTS's a number of unconventional oxygen-isotope ( 16 O/ 18 O) effects (OIE's) on various physical quantities, including among others the transition temperature T c , the in-plane magnetic penetration depth λ ab (0), the pseudogap temperature T * , and the spin-glass temperature T g , were observed which cannot be explained by standard BCS theory [1,2,3,4,5,6,7,8,9,10,11,12]. For instance, it was found that the OIE's on T c and λ ab (0) are strongly doping dependent [1,2,3,4,5,6,7,8,9,10]. In particular, close to optimal doping the OIE on T c is almost zero [1, 2, 3], while the OIE on λ ab (0) is still substantial [7,8,9,10]. With decreasing doping both OIE's on T c and λ ab (0) increase and, for highly underdoped materials, even exceed the value of the BCS isotope exponent α BCS Tc = 0.5 [2,4,5,7,9,10]. In order to obtain a more global view of cuprate superconductors a detailed study of the isotope dependence of magnetic quantities is needed. So far, to our knowledge only little work has been reported on this subject. This includes experimental studies of the OIE on the antiferromagnetic ordering temperature (T N ) [11] and the OIE on the spin-glass ordering temeprature (T g ) [12], as well as a theoretical investigation of the OIE on T N [13]. Here we report a systematic study of the OIE on T c , T g , and T N in Y 1−x Pr x Ba 2 Cu 3 O 7−δ as a func...

Intrinsic and structural isotope effects in iron-based superconductors

“…5,6 The cuprate high-temperature superconductors (HTS) are characterized by a vanishingly small but positive isotope effect exponent in optimally doped compounds which increases in a monotonic way upon decreasing doping. [7][8][9][10][11][12][13][14] For the optimally doped cuprate HTS the smallest value of the oxygen-isotope exponent α O ≃ 0.02 was obtained for YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ , while it reaches α O ≃ 0.25 for Bi 2 Sr 1.6 La 0.4 CuO 6+δ . [7][8][9]13,14 In addition, it was demonstrated that in underdoped materials α O exceeds substantially the BCS limit α BCS ≡ 0.5.…”

“…[7][8][9][10][11][12][13][14] For the optimally doped cuprate HTS the smallest value of the oxygen-isotope exponent α O ≃ 0.02 was obtained for YBa 2 Cu 3 O 7−δ and Bi 2 Sr 2 Ca 2 Cu 3 O 10+δ , while it reaches α O ≃ 0.25 for Bi 2 Sr 1.6 La 0.4 CuO 6+δ . [7][8][9]13,14 In addition, it was demonstrated that in underdoped materials α O exceeds substantially the BCS limit α BCS ≡ 0.5. 8,10,14 It is important to note here that the values of both, the oxygen and the copper isotope exponents in cuprate HTS are always positive.…”

“…[7][8][9]13,14 In addition, it was demonstrated that in underdoped materials α O exceeds substantially the BCS limit α BCS ≡ 0.5. 8,10,14 It is important to note here that the values of both, the oxygen and the copper isotope exponents in cuprate HTS are always positive. Similar tendencies, with the only few above mentioned exceptions, are realized in a case of conventional phonon mediated superconductors.…”

Evidence for Polaron Formation in High-Temperature Superconducting Cuprates: Experiment and Theory

“…The OIE on the transition temperatures of the various phases observed in all cuprate HTS [superconducting (SC), spin-glass (SG), and antiferromagnetic (AFM)] were investigated for the prototype system of cuprates Y 1 − x Pr x Ba 2 Cu 3 O 7 (see Fig. 3) [7]. All transition temperatures T c , T N , and T g exhibit an OIE which is strongest where the respective phase terminates.…”

“…Even though these observations do not necessarily support phonon mediated superconductivity in the classical sense, further isotope effects on other quantities suggest that lattice involvement is essential to the physics of HTS. These are: isotope effects on T * [5,6], the penetration depth λ [2] [7], the average superconducting energy gap [8], with all of them being interrelated and strongly doping dependent in analogy to the one of T c . In addition, local lattice anomalies are present which drive the system into a dynamically inhomogeneous state emerging from charge-rich and charge-poor, but spin-rich, regions [9][10][11][12].…”