Questioning the cuprate paradigm -- absence of superfluid density loss in several overdoped cuprates I

TL;DR

This study challenges the conventional view that superfluid density decreases with overdoping in cuprates, showing instead that it remains constant or increases, suggesting all carriers contribute to superconductivity.

Contribution

It provides evidence that superfluid density does not diminish with overdoping in several cuprates, contradicting previous assumptions and proposing a more conventional understanding of the phenomenon.

Findings

Superfluid density remains undiminished with overdoping in most cuprates.

Superfluid density per CuO_2 plaquette increases with doping, aligning with Hall number.

Tl_2Ba_2CuO_6 is an exception to this behavior.

Abstract

It is long established that overdoped cuprate superconductors experience a loss of superfluid density (SFD) with increasing doping, p, along with the decline in T_c. Such behavior is unconventional and suggests a depletion of the condensate by increasing pairbreaking or the growth of a second non-pairing channel. This led to a recent suggestion that the condensate arises from an incoherent charge channel which progressively gives way with overdoping to a second, coherent non-pairing channel. Contra these ideas, we report analysis of the field-dependent electronic specific heat of several cuprates from which we find no apparent loss of SFD with overdoping. The SFD per CuO_2 plaquette is found to rise progressively with overdoping from p towards (1+p), undiminished and much the same as the Hall number, thus implying that all available carriers contribute to the condensate. We suggest this could be the underlying intrinsic behavior for all cuprates. Our samples include (Y,Ca)Ba_2Cu_3O_{7-\delta}, Bi_2Sr_2CaCu_2O_{8+\delta}, La_{2-x}Sr_xCuO_4 and Tl_2Ba_2CuO_6, with the latter being the only exception. Our results signal a possible return to a more conventional picture.