Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

Point-Contact Spectroscopy in Bulk Samples of Electron-Doped Cuprate Superconductors

Point-contact spectroscopy was performed on bulk samples of electron-doped high temperature superconductor Nd2-xCexCuO4-δ. The samples were characterized using X-ray diffraction and scanning electron microscopy equipped with a wavelength-dispersive spectrometer and an electron backscatter diffraction detector. Samples with Ce content x = 0.15 showed the absence of spurious phases and randomly oriented grains, most of which had dimensions of approximately 220 µm2. The low-bias spectra in the tunneling regime, i.e., high-transparency interface, exhibited a gap feature at about ±5 meV and no zero-bias conductance, despite the random oriented grains investigated within our bulk samples, consistent with most of the literature data on oriented samples. High-bias conductance was also measured in order to obtain information on the properties of the barrier. A V-shape was observed in some cases, instead of the parabolic behavior expected for tunnel junctions.

Quantum Monte Carlo study of the Hubbard model with next-nearest-neighbor hopping t': pairing and magnetism

Using the finite-temperature determinant quantum Monte Carlo (DQMC) algorithm, we study the pairing symmetries of the Hubbard Hamiltonian with next-nearest-neighbor (NNN) hopping t' on square lattices. By varying the value of t', we find that the d-wave pairing is suppressed by the onset of t', while the p + ip-wave pairing tends to emerge for low electron density and t' around -0.7. Together with the calculation of the anti-ferromagnetic and ferromagnetic spin correlation function, we explore the relationship between anti-ferromagnetic order and the d-wave pairing symmetry, and the relationship between ferromagnetic order and the p + ip-wave pairing symmetry. Our results may be useful for the exploration of the mechanism of the electron pairing symmetries, and for the realization of the exotic p + ip-wave superconductivity.

Hole pocket-driven superconductivity and its universal features in the electron-doped cuprates

After three decades of intensive research attention, the emergence of superconductivity in cuprates remains an unsolved puzzle. One major challenge has been to arrive at a satisfactory understanding of the unusual metallic "normal state" from which the superconducting state emerges upon cooling. A second challenge has been to achieve a unified understanding of hole- and electron-doped compounds. Here, we report detailed magnetoresistance measurements for the archetypal electron-doped cuprate Nd_2-x Ce _x CuO_4+δ that, in combination with previous data, provide crucial links between the normal and superconducting states and between the electron- and hole-doped parts of the phase diagram. The characteristics of the normal state (magnetoresistance, quantum oscillations, and Hall coefficient) and those of the superconducting state (superfluid density and upper critical field) consistently indicate two-band (electron and hole) features and point to hole pocket-driven superconductivity in these nominally electron-doped materials. We show that the approximate Uemura scaling between the superconducting transition temperature and the superfluid density found for hole-doped cuprates also holds for the small hole component of the superfluid density in electron-doped cuprates.

Fermi-surface-free superconductivity in underdoped (Bi,Pb)(Sr,La)2CuO(6+δ) (Bi2201)

Fermi-surface-free superconductivity arises when the superconducting order pulls down spectral weight from a band that is completely above the Fermi energy in the normal state. We show that this can arise in hole-doped cuprates when a competing order causes a reconstruction of the Fermi surface. The change in Fermi surface topology is accompanied by a characteristic rise in the spectral weight. Our results support the presence of a trisected superconducting dome, and suggest that superconductivity is responsible for stabilizing the (π,π) magnetic order at higher doping.

Nodeless superconducting phase arising from a strong (π, π) antiferromagnetic phase in the infinite-layer electron-doped Sr(1-x)La(x)CuO2 compound

The asymmetry between electron and hole doping remains one of the central issues in high-temperature cuprate superconductivity, but our understanding of the electron-doped cuprates has been hampered by apparent discrepancies between the only two known families: Re(2-x)Ce(x)CuO4 and A(1-x)La(x)CuO2. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially stabilized Sr(1-x)La(x)CuO2 thin films synthesized by oxide molecular-beam epitaxy. Our results reveal a strong coupling between electrons and (π, π) antiferromagnetism that induces a Fermi surface reconstruction which pushes the nodal states below the Fermi level. This removes the hole pocket near (π/2, π/2), realizing nodeless superconductivity without requiring a change in the symmetry of the order parameter and providing a universal understanding of all electron-doped cuprates.

Specific heat evidence for bulk s-wave gap symmetry of optimally electron-doped Pr(1.85)Ce(0.15)CuO(4-y) superconductors

We reanalyze specific heat data for optimally electron-doped Pr(1.85)Ce(0.15)CuO(4-y) superconductors. The magnetic field dependence of the electronic specific heat in the vortex state does not support d-wave gap symmetry but agrees quantitatively with an s-wave theory. Furthermore, the field dependence at a finite temperature almost coincides with that for a conventional s-wave superconductor, Vi(3)Si. The present work provides bulk evidence for a nodeless s-wave gap symmetry in optimally electron-doped cuprates.

Evolution of the Fermi surface of the electron-doped high-temperature superconductor Nd(2-x)Ce(x)CuO(4) revealed by Shubnikov-de Haas oscillations

We report on the direct probing of the Fermi surface in the bulk of the electron-doped superconductor Nd(2-x)Ce(x)CuO(4) at different doping levels by means of magnetoresistance quantum oscillations. Our data reveal a sharp qualitative change in the Fermi surface topology, due to translational symmetry breaking in the electronic system which occurs at a critical doping level significantly exceeding the optimal doping. This result implies that the (pi/a, pi/a) ordering, known to exist at low doping levels, survives up to the overdoped superconducting regime.