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Register a new userWhy the superfluid density tracks $T_{rm c}$ in cuprate superconductors?
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Added by
Evandro V. L. de Mello
Publication date
2020
fields
Physics
and research's language is
English
Authors
E. V. L. de Mello
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One of the first finding concerning the superconducting (SC) density $n_{rm sc}$ in cuprates was their small magnitudes that revealed the importance of phase fluctuations. More recently, measurements in a variety of overdoped cuprates indicate that it is also much smaller than expected from BCS theories and falls smoothly to zero as doping is increased. We explain these observations by an electronic phase separation theory with a Ginzburg-Landau potential $V_{rm GL}$ that produces alternating charge domains whose fluctuations lead to localized SC order parameters that are connected by Josephson coupling $E_{rm J}$. The average ${left <E_{rm J}( p,T)right>}$ is proportional to the local superfluid phase stiffness $rho_{rm sc} propto n_{rm sc}$. The fraction of condensed carriers decreases in the overdoped region due to the weakening of $V_{rm GL}$. The results agreed with $rho_{rm sc}(p)$ vs. $T_{rm c}(p)$ and the Drude-like peak measurements.
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To fathom the mechanism of high-temperature ($T_{rm c}$) superconductivity, the dynamical vertex approximation (D$Gamma$A) is evoked for the two-dimensional repulsive Hubbard model. After showing that our results well reproduce the cuprate phase diagram with a reasonable $T_{rm c}$ and dome structure, we keep track of the scattering processes that primarily affect $T_{rm c}$. We find that local particle-particle diagrams significantly screen the bare interaction at low frequencies, which in turn suppresses antiferromagnetic spin fluctuations and hence the pairing interaction. Thus we identify dynamical vertex corrections as one of the main oppressors of $T_{rm c}$, which may provide a hint toward higher $T_{rm c}$s.
A universal scaling relation, $rho_s propto sigma(T_c)times T_c$ has been reported by Homes $et$ $al$. (Nature (London) {bf 430}, 539 (2004)) where $rho_s$ is the superfluid density and $sigma(T)$ is the DC conductivity. The relation was shown to apply to both c-axis and in-plane dynamics for high-$T_c$ superconductors as well as to the more conventional superconductors Nb and Pb, suggesting common physics in these systems. We show quantitatively that the scaling behavior has several possible origins including, marginal Fermi-liquid behavior, Josephson coupling, dirty-limit superconductivity and unitary impurity scattering for a d-wave order parameter. However, the relation breaks down seriously in overdoped cuprates, and possibly even at lower doping.
We calculate superfluid density for a dirty d-wave superconductor. The effects of impurity scattering are treated within the self-consistent t-matrix approximation, in weak-coupling BCS theory. Working from a realistic tight-binding parameterization of the Fermi surface, we find a superfluid density that is both correlated with T_c and linear in temperature, in good correspondence with recent experiments on overdoped La2-xSrxCuO4.
Signatures of strong coupling effects in cuprate high-$T_{c}$ superconductors have been authenticated through a variety of spectroscopic probes. However, the microscopic nature of relevant excitations has not been agreed upon. Here we report on magneto-optical studies of the CuO$_{2}$ plane carrier dynamics in a prototypical high-$T_{c}$ superconductor YBa$%_{2} $Cu$_{3}$O$_{y}$ (YBCO). Infrared data are directly compared with earlier inelastic neutron scattering results by Dai textit{et al}. [Nature (London) textbf{406}, 965 (2000)] revealing a characteristic depression of the magnetic resonance in H $parallel $ textit{c} field less than 7 T. This analysis has allowed us to critically assess the role of magnetic degrees of freedom in producing strong coupling effects for YBCO system.
We propose a weakly coupled two-band model with $d_{x^2-y^2}$ pairing symmetry to account for the anomalous temperature dependence of superfluid density $rho_s$ in electron-doped cuprate superconductors. This model gives a unified explanation to the presence of a upward curvature in $rho_s$ near $T_c$ and a weak temperature dependence of $rho_s$ in low temperatures. Our work resolves a discrepancy in the interpretation of different experimental measurements and suggests that the pairing in electron-doped cuprates has predominately $d_{x^2-y^2}$ symmetry in the whole doping range.
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