What can we learn from comparison between cuprates and He films ? : phase separation and fluctuating superfluidity

In the underdoped, overdoped, Zn-doped or stripe-forming regions of high-$T_{c}$ cuprate superconductors (HTSC), the superfluid density $n_{s}/m^{*}$ at $T\to 0$ shows universal correlations with $T_{c}$. Similar strong correlations exist between 2-dimensional superfluid density and superfluid transition temperature in thin films of $^{4}$He in non-porous or porous media, and $^{4}$He/$^{3}$He film adsorbed on porous media. Based on analogy between HTSC and He film systems, we propose a model for cuprates where: (1) the overdoped region is characterized by a phase separation similar to $^{4}$He/$^{3}$He; and (2) pair (boson) formation and fluctuating superconductivity occur at separate temperatures above $T_{c}$ in the underdoped region.Comment: 8 pages, 5 figures. Invited paper presented at the third international conference on stripes and high-Tc superconductivity (STRIPE-2000), Sept. 25-30th, 2000, Rome, Italy. To be published in the International Journal of Modern Physics

Cooper pairs as low-energy excitations in the normal state

We discuss the normal state of a fermionic system in an idealized PSEUDOGAP REGIME, $k_B T_c < k_B T << |\Delta| << E_F$. Stable Cooper pairs induce a pseudogap of width $|\Delta|$ in the fermion energy spectrum. Near two dimensions, we find a Bose-like condensation temperature in this predominantly fermionic system.Comment: 2 pages, LaTeX, espcrc2.sty file included. An outline of a presentation at the Beijing conference M2S-HTSC-V. To be published in Physica

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

We propose a phenomenological model for high-$T_{c}$ 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 $T^{*}\sim 0$ at a mildly overdoped critical concentration $x_{c}$. This model explains anomalous doping and temperature dependences of $n_{s}/m^{*}$ (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.Comment: 5 pages, 4 figure

Muon spin relaxation studies of incommensurate magnetism and superconductivity in stage-4 La2_{2}CuO4.11_{4.11} and La1.88_{1.88}Sr0.12_{0.12}CuO4_{4}

This paper reports muon spin relaxation (MuSR) measurements of two single crystals of the title high-Tc cuprate systems where static incommensurate magnetism and superconductivity coexist. By zero-field MuSR measurements and subsequent analyses with simulations, we show that (1) the maximum ordered Cu moment size (0.36 Bohr magneton) and local spin structure are identical to those in prototypical stripe spin systems with the 1/8 hole concentration; (2) the static magnetism is confined to less than a half of the volume of the sample, and (3) regions with static magnetism form nano-scale islands with the size comparable to the in-plane superconducting coherence length. By transverse-field MuSR measurements, we show that Tc of these systems is related to the superfluid density, in the same way as observed in cuprate systems without static magnetism. We discuss a heuristic model involving percolation of these nanoscale islands with static magnetism as a possible picture to reconcile heterogeneity found by the present MuSR study and long-range spin correlations found by neutron scattering.Comment: 19 pages, 15 figures, submitted to Phys. Rev. B. E-mail: [email protected]

Bose-Einstein condensation in multilayers

The critical BEC temperature $T_{c}$ of a non interacting boson gas in a layered structure like those of cuprate superconductors is shown to have a minimum $T_{c,m}$, at a characteristic separation between planes $a_{m}$. It is shown that for $a<a_{m}$, $T_{c}$ increases monotonically back up to the ideal Bose gas $T_{0}$ suggesting that a reduction in the separation between planes, as happens when one increases the pressure in a cuprate, leads to an increase in the critical temperature. For finite plane separation and penetrability the specific heat as a function of temperature shows two novel crests connected by a ridge in addition to the well-known BEC peak at $T_{c}$ associated with the 3D behavior of the gas. For completely impenetrable planes the model reduces to many disconnected infinite slabs for which just one hump survives becoming a peak only when the slab widths are infinite.Comment: Four pages, four figure

Thermodynamics of Crossover from Weak- to Strong-Coupling Superconductivity

In this paper we study an evolution of low-temperature thermodynamical quantities for an electron gas with a $\delta$-function attraction as the system crosses over from weak-coupling (BCS-type) to strong-coupling (Bose-type) superconductivity in three and two dimensions.Comment: Replaced with journal version. Insignificant presentation changes. Links to related papers are also available at the author home page http://www.teorfys.uu.se/PEOPLE/egor

Superfluid density of high-Tc cuprate systems: implication on condensation mechanisms, heterogeneity and phase diagram

Extensive muon spin relaxation measurements have been performed to determine the magnetic field penetration depth in high-Tc cuprate superconductors with simple hole doping, Zn-doping, overdoping, and formation of static SDW nano islands. System dependence of $n_{s}/m^{*}$ (superconducting carrier density / effective mass) reveals universal correlations between Tc and $n_{s}/m^{*}$ in all these cases with / without perturbation. Evidence for spontaneous and microscopic phase separation was obtained in the cases with strong perturbation, i.e., Zn-doping. overdoping and SDW nano-islands. The length scale of this heterogeneity is shown to be comparable to the in-plance coherence length. We discuss implications of these results on condensation mechanisms of HTSC systems, resorting to an analogy with He films, on regular and porous media, reminding essential features of Bose-Einstein, BCS and Kosterlitz-Thouless condensation/transition in 2-d and 3-d systems, and comparing models of BE-BCS crossover and phase fluctuations. We propose a new phase diagram for HTSC systems based on distinction between pair formation and superconducting phase fluctuations in the pseudogap region and spontaneous phase separation in the overdoped region. We also remind anomaly in BEDT and A3C60 systems similar to that in overdoped cuprates, seen in the evolution from superconducting to metallic ground state.Comment: 21 pages, 18 figures, invited papter presented at the HTSC Workshop, Williamsburg, Virginia, June 7-8, 2002, to appear in Solid State Communications (Special Issue edited by A.J. Millis, S. Uchida, Y.J. Uemura): contact [email protected]

Heat transport of electron-doped Cobaltates

Within the t-J model, the heat transport of electron-doped cobaltates is studied based on the fermion-spin theory. It is shown that the temperature dependent thermal conductivity is characterized by the low temperature peak located at a finite temperature. The thermal conductivity increases monotonously with increasing temperature at low temperatures T $<$ 0.1$J$, and then decreases with increasing temperature for higher temperatures T $>$ 0.1$J$, in qualitative agreement with experimental result observed from Na$_{x}$CoO$_{2}$ .Comment: 4 pages, 1 fig, corrected typos, accepted for publication in Commun. Theor. Phy

Kinetic energy driven superconductivity in doped cuprates

Within the t-J model, the mechanism of superconductivity in doped cuprates is studied based on the partial charge-spin separation fermion-spin theory. It is shown that dressed holons interact occurring directly through the kinetic energy by exchanging dressed spinon excitations, leading to a net attractive force between dressed holons, then the electron Cooper pairs originating from the dressed holon pairing state are due to the charge-spin recombination, and their condensation reveals the superconducting ground-state. The electron superconducting transition temperature is determined by the dressed holon pair transition temperature, and is proportional to the concentration of doped holes in the underdoped regime. With the common form of the electron Cooper pair, we also show that there is a coexistence of the electron Cooper pair and antiferromagnetic short-range correlation, and hence the antiferromagnetic short-range fluctuation can persist into the superconducting state. Our results are qualitatively consistent with experiments.Comment: 6 pages, Revtex, two figures are included, corrected typo

Kinetic energy driven superconductivity in the electron doped cobaltate Nax_{x}CoO2⋅y_{2}\cdot yH2_{2}O

Within the charge-spin separation fermion-spin theory, we have shown that the mechanism of superconductivity in the electron doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O is ascribed to its kinetic energy. The dressed fermions interact occurring directly through the kinetic energy by exchanging magnetic excitations. This interaction leads to a net attractive force between dressed fermions, then the electron Cooper pairs originating from the dressed fermion pairing state are due to the charge-spin recombination, and their condensation reveals the superconducting ground state. The superconducting transition temperature is identical to the dressed fermion pair transition temperature, and is suppressed to a lower temperature due to the strong magnetic frustration. The optimal superconducting transition temperature occurs in the electron doping concentration $\delta\approx 0.29$, and then decreases for both underdoped and overdoped regimes, in qualitative agreement with the experimental results.Comment: 6 pages, 2 figs, corrected typos, accepted for publication in Commun. Theor. Phy