Dimensionality of superconductivity and vortex dynamics in the infinite-layer cuprateSr0.9M0.1CuO2

Abstract

The high magnetic-field phase diagram of the electron-doped infinite layer high-temperature superconducting ͑high-T c ͒ compound Sr 0.9 La 0.1 CuO 2 was probed by means of penetration depth and magnetization measurements in pulsed fields to 60 T. An anisotropy ratio of 8 was detected for the upper critical fields with H parallel ͑H c2 ab ͒ and perpendicular ͑H c2 c ͒ to the CuO 2 planes, with H c2 ab extrapolating to near the Pauli paramagnetic limit of 160 T. The longer superconducting coherence length than the lattice constant along the c axis indicates that the orbital degrees of freedom of the pairing wave function are three dimensional. By contrast, low-field magnetization and specific heat measurements of Sr 0.9 Gd 0.1 CuO 2 indicate a coexistence of bulk s-wave superconductivity with large moment Gd paramagnetism close to the CuO 2 planes, suggesting a strong confinement of the spin degrees of freedom of the Cooper pair to the CuO 2 planes. The region of the magnetic field-temperature phase diagram between H c2 ab and the irreversibility line in the magnetization, H irr ab , in Sr 0.9 La 0.1 CuO 2 is anomalously large for an electron-doped high-T c cuprate. The large reversible region even approaching zero temperature rules out thermal depinning scenarios. The temperature dependence of H irr ab also differs fundamentally from those predicted for the quenched-disorder-induced vortex phase transitions for H ʈ c at low temperatures. Thus, our finding of a strongly suppressed H irr ab relative to H c2 ab at low temperatures is suggestive of the existence of additional quantum fluctuations, possibly due to a magnetic-field-induced competing order such as the spin-density wave ͑SDW͒.

Unconventional Low-Energy Excitations of Cuprate Superconductors

“…[4,5,9,66]. Moreover, strong quantum fluctuations are expected due to proximity to quantum criticality [9,10,85,86] Macroscopically, the presence of COs and strong quantum fluctuations naturally lead to weakened superconducting stiffness upon increasing T and magnetic field H, [9,85,87,88] which may be responsible for the extreme type-II nature and the novel vortex dynamics of cuprate superconductors, [85,86,89,90,91,92,93,94,95] as schematically illustrated in Figs. 4(a) and 4(b) for the magnetic field (H) vs. temperature (T ) vortex phase diagrams of cuprate superconductors for H ĉ-axis and H ⊥ĉ-axis, respectively.…”

“…The dashed curve represents the phase boundary modified by magnetic field misalignment and stacking faults that tend to smear the modulations of the S/IS to VL phase boundary and reduce the phase boundary into being monotonically dependent on H. (c) For H ab, assuming dominating quantum fluctuations associated with the proximity to quantum criticality and competing orders. [9,10,85,86] superconductivity. Above all, the one-gap model cannot provide natural explanations for all of the non-universal and unconventional low-energy excitations (e.g.…”

“…Empirically, the important signature of fractionalization and spin-charge separation associated with the one-gap model has never been realized. Moreover, small zero-field quantum fluctuations in the ground state [81,82] and strong magnetic field-induced quantum fluctuations of most cuprate superconductors [85,86] are inconsistent with the notion of a spin liquid. Above all, the necessary condition of a pseudogap phase and strong phase fluctuations asserted by the pre-formed pair scenario is completely inapplicable to electron-type cuprate superconductors.…”

“…Moreover, external variables such as temperature (T ) and applied magnetic field (H) can vary the interplay of SC and CO, such as inducing or enhancing the CO [56,69] at the price of more rapid suppression of SC, thereby leading to weakened superconducting stiffness and strong thermal and field-induced fluctuations. [85,86,89,90] On the other hand, the quasi two-dimensional nature of the cuprates can also lead to quantum criticality in the limit of decoupling of CuO 2 planes. Indeed, recent studies have demonstrated experimental evidence from macroscopic magnetization measurements for field-induced quantum fluctuations among a wide variety of cuprate superconductors with different microscopic variables such as the doping level (δ) of holes or electrons, and the number of CuO 2 layers per unit cell (n).…”

Competing orders and the doping and momentum dependent quasiparticle excitations in cuprate superconductors

“…This PG has only been observed in hole-type cuprates and is found to correlate with the onset of the Nernst effect [1,11,12]. Our recent studies of the quasiparticle spectral density function and the DOS of various optimally doped cuprates [13] have revealed that the occurrence (absence) of the low-energy PG in hole-(electron-) type cuprate superconductors and the dichotomy in quasiparticle coherence [9,14] can be quantitatively understood as the result of a ground state consisting of coexisting competing orders (COs) and SC [15,16,[18][19][20][21][22] with the presence of finite quantum fluctuations [6,23,24]. The objective of this work is to expand on our previous work [13] to further investigate how the coexistence of COs and SC may influence the doping and momentum dependence of the low-energy quasiparticle excitations in different cuprates.…”