How ‘pairons’ are revealed in the electronic specific heat of cuprates

Abstract

Understanding the thermodynamic properties of high-Tc cuprate superconductors is a key step to establish a satisfactory theory of these materials. The electronic specific heat is highly unconventional, distinctly non-BCS, with remarkable doping-dependent features extending well beyond Tc. The pairon concept, bound holes in their local antiferromagnetic environment, has successfully described the tunneling and photoemission spectra. In this article, we show that the model explains the distinctive features of the entropy and specific heat throughout the temperature-doping phase diagram. Their interpretation connects unambiguously the pseudogap, existing up to T * , to the superconducting state below Tc. In the underdoped case, the specific heat is dominated by pairon excitations, following Bose statistics, while with increasing doping, both bosonic excitations and fermionic quasiparticles coexist.

Thermodynamics of the pseudogap in cuprates

“…It is also evident in S(T) by the entropy rising to a maximum at x = 0.23 before falling again once the vHs moves into the unoccupied states above E F . This peak should not be confused with the peak expected from normal-state pair dissociation above T c within the so-called "pairon" model [4] of the pseudogap. Such a pair-dissociation peak would result in S(T) curves recovering their bare values at high temperature, which clearly they do not.…”

“…This is crucially important because λ −2 0 is a truly ground-state property. One prominent hypothesis for the pseudogap is that it arises from incoherent pairing above T c [4,28]. But at T = 0, all thermal fluctuations are quenched so such a model would mean the pseudogap is absent at T = 0.…”

“…Many authors have interpreted the distinctive phase behavior of the cuprates in terms of a crossover from a Bose-Einstein condensate (BEC) to a BCS condensate [28,44,45,46,47,48]. In this way, the pseudogap state is viewed as a (partially) paired state which condenses to a coherent paired state at T c [4]. This, of course, explains amongst other things the reduced specific heat jump in the under-doped region because it lacks the pairing contribution which has already reduced the entropy from well above T c [48].…”

“…What perhaps is agreed is that it represents a partial gap in the electronic density of states (DOS) which is asymmetric about E F as evidenced by its dramatic effect on the thermoelectric power in under-doped cuprates [3]. Once, it was also generally agreed to be associated with the under-doped side of the phase diagram but now, researchers often describe a T*(p) line as a line which extends across the entire superconducting phase diagram, falling to zero as T c → 0 at the end of the superconducting dome [4]. It also seems to be associated with a reconstruction of the Fermi surface (FS) from a large hole-like FS on the over-doped side to disconnected hole pockets on the under-doped side [3,5,6] though there remain significant questions in this interpretation especially for Tl 2 Ba 2 CuO 6 [7].…”

Cuprates phase diagram deduced from magnetic susceptibility: what is the `true' pseudogap line?

“…We use the relation : ε i = ∆ i − ∆ p (T, θ), where ∆ p (T, θ) is the standard d-wave gap with a smooth and decreasing temperature dependence as in Ref. [63]. With these considerations, we can write :…”