Decrease of upper critical field with underdoping in cuprate superconductors

Abstract

It is still unclear why the transition temperature T c of cuprate superconductors falls with underdoping. The doping dependence of the critical magnetic field H c2 is directly relevant to this question, but different estimates of H c2 are in sharp contradiction. We resolve this contradiction by tracking the characteristic field scale of superconducting fluctuations as a function of doping, via measurements of the Nernst effect in La 1.8−x Eu 0.2 Sr x CuO 4 . H c2 is found to fall with underdoping, with a minimum where stripe order is strong. The same non-monotonic behaviour is observed in the archetypal cuprate superconductor YBa 2 Cu 3 O y . We conclude that competing states such as stripe order weaken superconductivity and cause both H c2 and T c to fall as cuprates become underdoped.T wo paradigms have been proposed to account for the dome-like region of superconductivity in the temperaturedoping phase diagram of cuprate superconductors 1 . In the first, the amplitude of the superconducting order parameter grows monotonically as the doping p is reduced, but its phase is increasingly disordered 2 , causing T c to fall at low p. The signature of this scenario is strong phase fluctuations and a superconducting gap above T c in the underdoped regime. In the second paradigm, the fall of T c at low p is due to the onset of a state that competes with superconductivity. The signature of this scenario is a small superconducting gap and a small H c2 in the underdoped regime.Whether strong phase fluctuations or a decrease in the pairing gap is causing T c to fall in underdoped cuprates is currently an open question. Different interpretations of photoemission data disagree on the evolution of the pairing gap 3-6 and different estimates of the upper critical field H c2 are in sharp contradiction 7,8 . The Nernst signal observed above T c in underdoped cuprates has been attributed to superconducting fluctuations 8-10 , and because it persists up to temperatures several times T c , it was deemed incompatible with the standard Gaussian fluctuations of the superconducting order parameter. It was attributed instead to vortex-like excitations in a phase-fluctuating superconductor 9,10 with a non-zero pairing amplitude above T c . The critical field H c2 deduced from the Nernst data on cuprates such as Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) was reported to increase with underdoping 8 , even though T c falls. As shown in Fig. 1a, this is in striking contrast to the rapid drop in H c2 deduced from a Gaussian analysis of fluctuations in the magneto-conductivity of YBa 2 Cu 3 O y (YBCO; ref. 7). Nernst effect in Eu-LSCOHere we re-examine the Nernst effect in cuprates with a study of La 1.8−x Eu 0.2 Sr x CuO 4 (Eu-LSCO), an underdoped cuprate in which the ratio of superconducting (N sc ) to quasiparticle (N qp ) contributions to the Nernst signal N is exceptionally large-at

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“…[The ρ(T) and magnetoresistance data, presented in the Supporting Information, show superconducting fluctuations up to about 10-15 K above T c , in line with the estimates from NMR (34), terahertz spectroscopy (35), Nernst effect (36), and magnetization measurements (37).] The observed ρ H (T) dependence is striking (Fig.…”

Low anisotropy of the upper critical field in a strongly anisotropic layered cuprateBi2.15Sr1.9CuO6+δ:Evidence for a paramagnetically limited superconductivity

“…Development of high magnetic field techniques in recent years has lead to a significant progress in studies of H c2 in high-T c superconductors [13,14,18,19]. But the problem of disentanglement of superconducting and PG magnetic responses remains.…”

“…The high T c in combination with a strong coupling leads to a large superconducting energy gap ∆ ∼ 20 − 50 meV [6][7][8][9][10][11] and H c2 (0) ∼ 10 2 T [12][13][14][15][16][17][18][19]. Such strong fields may alter the ground state of the material.…”

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“…21 Figs. 18 and 19 show how the behavior of N sc in the hole-doped cuprate Eu-LSCO is qualitatively identical to that of electron-doped PCCO, as a function of field and temperature, respectively.…”