Direct observation of nodeless superconductivity and phonon modes in electron-doped copper oxide Sr1-xNdxCuO2

Optical Absorption in Tilted Geometries as an Indirect Measurement of Longitudinal Plasma Waves in Layered Cuprates

Electromagnetic waves propagating in a layered superconductor with arbitrary momentum, with respect to the main crystallographic directions, exhibit an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity when light propagates at small tilting angles relative to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the c-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell's equations with a full quantum derivation of the plasma modes based on modeling the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS).

Angular dependence of the upper critical induction of cleans- anddx2-y2-wave superconductors with self-consistent ellipsoidal effective mass and Zeeman anisotropies

We employ the Schrödinger-Dirac method generalized to an ellipsoidal effective mass anisotropy in order to treat the spin and orbital effective mass anisotropies self consistently, which is important when Pauli-limiting effects on the upper critical field characteristic of singlet superconductivity are present. By employing the Klemm-Clem transformations to map the equations of motion into isotropic form, we then calculate the upper critical magnetic inductionBc2(θ,ϕ,T)at arbitrary directions and temperaturesTfor isotropics-wave and for anisotropicdx2-y2-wave superconducting order parameters. As for anisotropics-wave superconductors,Bc2is largest in the direction of the lowest effective mass, and is proportional to the universal orientation factorα(θ,ϕ). However, fordx2-y2-wave pairing and vanishing planar effective mass anisotropy,Bc2(π/2,ϕ,T)exhibits a four-fold azimuthal pattern withC₄symmetry the maxima of which are along the crystal axes just below the transition temperatureT_c, but these maxima are rotated byπ/4about thezaxis asTis lowered to 0. However fordx2-y2-wave pairing with weak planar effective mass anisotropy,Bc2(π/2,ϕ,T)exhibits a two-fold pattern withC₂symmetry for allT ⩽ Tc, which also rotates byπ/4about thezaxis asTis lowered to 0. These low planar effective mass anisotropy cases provide a new method to distinguishs-wave anddx2-y2-wave pairing symmetries in clean unconventional superconductors.