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

Condensation Energy and High Tc Superconductivity

From an analysis of the specific heat of one of the cuprate superconductors it is shown, that even if a large part of the experimental specific heat associated with the superconducting phase transition is due to fluctuations, this part must be counted when one tries to extract the condensation energy from the data. Previous work by Chakravarty, Kee and Abrahams, where the fluctuation part was subtracted, has resulted in an incorrect estimation of the condensation energy.Comment: 4 pages, 5 encapsulated Postscript figures, uses ReVTeX.st

Condensation energy in strongly coupled superconductors

We consider the condensation energy in superconductors where the pairing is electronic in origin and is mediated by a collective bosonic mode. We use magnetically-mediated superconductivity as an example, and show that for large spin-fermion couplings, the physics is qualitatively different from the BCS theory as the condensation energy results from the feedback on spin excitations, while the electronic contribution to the condensation energy is positive due to an ``undressing'' feedback on the fermions. The same feedback effect accounts for the gain of the kinetic energy at strong couplings.Comment: 4 pages, revtex 4, 3 eps figure

The electronic specific heat in the pairing pseudogap regime

When pairing correlations in a quasi two dimensional electron system induce a pseudogap in the single particle density of states, the specific heat must also contain a sizeable pair contribution. The theoretically calculated specific heat for such a system is compared to the experimental results of Loram and his collaborators for underdoped YBa_2Cu_3O_{6+x} and La_{2-x}Sr_{x}CuO_4 samples. The size and doping dependence of the extracted pseudogap energy scale for both materials is comparable to the values obtained from a variety of other experiments.Comment: 4 pages, 5 eps figure

Specific Heat of the 2D Hubbard Model

Quantum Monte Carlo results for the specific heat c of the two dimensional Hubbard model are presented. At half-filling it was observed that $c \sim T^2$ at very low temperatures. Two distinct features were also identified: a low temperature peak related to the spin degrees of freedom and a higher temperature broad peak related to the charge degrees of freedom. Away from half-filling the spin induced feature slowly disappears as a function of hole doping while the charge feature moves to lower temperature. A comparison with experimental results for the high temperature cuprates is discussed.Comment: 6 pages, RevTex, 11 figures embedded in the text, Submitted to Phys. Rev.

Thermodynamic properties of the d-density wave order in cuprates

We solve a popular effective Hamiltonian of competing $d$-density wave and d-wave superconductivity orders self-consistently at the mean-field level for a wide range of doping and temperature. The theory predicts a temperature dependence of the $d$-density wave order parameter seemingly inconsistent with the neutron scattering and $\mu$SR experiments of the cuprates. We further calculate thermodynamic quantities, such as chemical potential, entropy and specific heat. Their distinct features can be used to test the existence of the $d$-density wave order in cuprates.Comment: changed to 4 pages and 4 figures. More reference added. Accepted by Phys. Rev.

On the pseudogap and doping-dependent magnetic properties of La2-xSrxCu1-yZnyO4

The effects of planar hole content, p (= x), on the uniform (q = 0) magnetic susceptibility, c(T), of La2-xSrxCu1-yZnyO4 were investigated over a wide range of Sr (x) and Zn (y) contents. A strongly p-dependent Zn-induced magnetic behavior was observed. The apparent Zn-induced magnetic moment is larger in underdoped La2-xSrxCu1-yZnyO4 and it decreases quite sharply around p ~ 0.19. It does not change much for further overdoping. This indicates a possible role of the pseudogap on the Zn induced magnetic behavior, as there is growing evidence that pseudogap vanishes quite abruptly at p ~ 0.19.Comment: Submitted to Physica C (Proceedings of the M2S-HTSC-VIII Conference

Optical sum rule violation, superfluid weight and condensation energy in the cuprates

The model of hole superconductivity predicts that the superfluid weight in the zero-frequency $\delta$-function in the optical conductivity has an anomalous contribution from high frequencies, due to lowering of the system's kinetic energy upon entering the superconducting state. The lowering of kinetic energy, mainly in-plane in origin, accounts for both the condensation energy of the superconductor as well as an increased potential energy due to larger Coulomb repulsion in the paired state. It leads to an apparent violation of the conductivity sum rule, which in the clean limit we predict to be substantially larger for in-plane than for c-axis conductivity. However, because cuprates are in the dirty limit for c-axis transport, the sum rule violation is found to be greatly enhanced in the c-direction. The model predicts the sum rule violation to be largest in the underdoped regime and to decrease with doping, more rapidly in the c-direction that in the plane. So far, experiments have detected sum rule violation in c-axis transport in several cuprates, as well as a decrease and disappearance of this violation for increasing doping, but no violation in-plane. We explore the predictions of the model for a wide range of parameters, both in the absence and in the presence of disorder, and the relation with current experimental knowledge.Comment: submitted to Phys.Rev.

Changes in Optical Conductivity due to Readjustments in Electronic Density of States

Within the model of elastic impurity scattering, we study how changes in the energy dependence of the electronic density of states (EDOS) $N(\epsilon)$ around the Fermi energy $\epsilon_F$ are reflected in the frequency-dependent optical conductivity $\sigma(\omega)$. While conserving the total number of states in $N(\epsilon)$ we compute the induced changes in $\sigma(\omega)$ as a function of $\omega$ and in the corresponding optical scattering rate $1/\tau_{\rm op}(\omega)$. These quantities mirror some aspects of the EDOS changes but the relationship is not direct. Conservation of optical oscillator strength is found not to hold, and there is no sum rule on the optical scattering rate although one does hold for the quasiparticle scattering. Temperature as well as increases in impurity scattering lead to additional changes in optical properties not seen in the constant EDOS case. These effects have their origin in an averaging of the EDOS around the Fermi energy $\epsilon_F$ on an energy scale set by the impurity scattering.Comment: 13 pages, 7 figure