Microscopic phase separation in the overdoped region of high-Tc cuprate superconductors

Abstract

We propose a phenomenological model for high-Tc superconductors (HTSC) assuming: (1) a microscopic phase separation between superconducting and normal-metal areas in the overdoped region; and (2) existence of a homogeneous superconducting phase only below the pseudo-gap T * line, which shows a sharp reduction towards T * ∼ 0 at a mildly overdoped critical concentration xc. This model explains anomalous doping and temperature dependences of ns/m * (superconducting carrier density / effective mass) observed in several overdoped HTSC systems. We point out an analogy to superfluid 4 He/ 3 He films, and discuss an energetic origin of microscopic phase separation. 74.25.Dw, Since the discovery of high-T c cuprate superconductors (HTSC), accumulated studies have revealed unusual phenomena in the underdoped (UD) region, such as, correlations between T c and n s /m * (superconducting carrier density / effective mass) at T → 0 shown by measurements of the magnetic field penetration depth λ [1], and the pseudo-gap phenomena [2]. These results stimulated development of various theories / models for condensation, including Bose-Einstein (BE) to BCS crossover [3,4], phase fluctuations [5,6], XY-model [7] and BE condensation [8]. These models [3][4][5][6][7][8] assume the existence of pre-formed charge pairs above T c . Different pictures, however, assume single charge above T c based on the resonating-valence-bond concepts [9,10].Several anomalous results have been found also in the overdoped (OD) region: µSR studies on Tl 2 Ba 2 CuO 6+δ (Tl2201) [11,12] revealed that n s /m * at T → 0 decreases with increasing hole doping. This tendency has been observed subsequently in thin film La 2−x Sr x CuO 4 (LSCO) [13] and bulk YBa 2 Cu 3 O y (YBCO) [14] systems in the OD region. The "coherence peak" in ARPES spectra [15] also follows this behavior of n s /m * in the OD region. Meanwhile, Tallon and Loram [16] noticed a sharp reduction of the pseudo-gap T * line in the temperature T vs. hole-concentration x phase diagram heading towards T * ∼ 0 at a critical concentration x c ∼ 0.19 holes/Cu which lies in the mildly OD region. In view of the existence of superconductivity in x > x c where the T * line does not exist, Tallon and Loram [16] have advocated a view point that the pseudogap phenomena is not representing pre-cursor superconducting phenomena, but should be ascribed to a tendency towards a ground state competing with superconductivity.We have, however, suggested another possible view point in which T * is ascribed to a signature of a gradual pre-formation of pairs [4] while the anomaly in the OD region to a phase separation [4,11,17,18]. In the present paper, resorting to a model-calculation of n s /m * , comparison with experimental results, a crude estimate of competing electrostatic and condensation/pairing energies, and analogy to 4 He/ 3 He films, we demonstrate that our picture with microscopic phase separation between superconducting and normal-metal regions can quantitatively account for several anomalous results in ...

Electronic specific heat and low-energy quasiparticle excitations in the superconducting state ofLa2−xSrxCuO4Wen1, Liu2, Zhou3 et al. 2004Phys. Rev. B613440View full textAdd to dashboardCite

“…In underdoped Bi − 2212 single crystals, scanning-tunnelingmicroscopic (STM) measurement indeed reveal a mixture of superconducting regions with sharp quasiparticle coherent peaks on the tunnelling spectrum, and the nonsuperconducting regions with pseudogap-like tunneling spectrum 50 . In the overdoped side, the tiny normal cores as proposed in the Swiss cheese model 51 , or the mesosscopic normal regions suggested by Fukuzumi et al 52 and Wen et al 53 will contribute a residual term γ 0 which does not show an apparent increase with the field. As proposed by Fukuzumi et al 52 that the dome-like electronic phase diagram may be formed by the mixture of three phases: anti-ferromagnetic phase in the extremely underdoped region, a d-wave superconducting region with the robust superconductivity near the optimal doping point and a non-superconducting Fermi liquid in the overdoped region.…”

Condensation, excitation, pairing, and superfluid density in high-Tc superconductors: the magnetic resonance mode as a roton analogue and a possible spin-mediated pairing

“…It is also possible that p c substantially varies from system to system. In our view, the phase separation in the overdoped region occurs to save condensation and pairing energies at a cost of (screened) Coulomb energy necessary for disproportionation of charge density [79]. Without taking into account the possibility of phase separation, the authors of refs.…”

“…The inset illustrates the proposal of microscopic phase separation between superconducting and normal metal regions in the overdoped region [79]. (b) The region of the Nernst effect, shown in the T -x phase diagram for LSCO [83].…”

Global Phase Diagram of the High-Tc Cuprates

“…In cuprate system, the muon spin relaxation rate σ(T → 0) is proportional to n s /m * where n s is the superfluid density and m * is the effective mass of holepairs 82 . To explain the deviation from the linear relation between σ and doping in the overdoped regime, Uemura proposed that some of the doped holes do not form pairs and are phase separated from the SC hole-pairs, even at zero temperature 82 . A similar phase separation picture was proposed by Uchida 83 .…”