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We analyze pseudogap phenomena widely observed in the underdoped cuprates. We assume the existence of a strong d-wave pairing force competing with antiferromagnetic(AFM) fluctuations and the formation of flat and damped dispersion around…
We present a self-consistent approach to deal with the pairing-fluctuation effects in quasi-two-dimensional superconducting systems. Besides the Cooper pairs in the Bose-Einstein condensate, there are pairs occupying the excited states,…
Unveiling the nature of the pseudogap and its relation to both superconductivity and antiferromagnetic Mott insulators, the pairing mechanism, and a non-Fermi liquid phase is a key issue for understanding high temperature superconductivity…
High-temperature copper oxide superconductors (cuprates) display unconventional physics when they are lightly doped whereas the standard theory of metals prevails in the opposite regime. For example, the thermoelectric power, that is the…
The mutual interaction between Cooper pairs is proposed as a mechanism for the superconducting state. Above $T_c$, pre-existing but fluctuating Cooper pairs give rise to the unconventional {\it pseudogap} (PG) state, well-characterized by…
We review our angle-resolved photoemission spectroscopy (ARPES) results on different layered oxide superconductors, and on their insulating parent compounds. The low energy excitations are discussed with emphasis on some of the most recent…
We explore the effects of various kinds of random disorder on the quasiparticle density of states of two-dimensional d-wave superconductors using an exact real-space method, incorporating realistic details known about the cuprates. Random…
The phase diagram of the cuprate superconductors continues to pose formidable scientific challenges. While these materials are typically viewed as doped Mott insulators, it is well known that they are Fermi liquids at high hole-dopant…
We address the question of why strongly correlated d-wave superconductors, such as the cuprates, prove to be surprisingly robust against the introduction of non-magnetic impurities. We show that, very generally, both the pair-breaking and…
Besides superconductivity, copper-oxide high temperature superconductors are susceptible to other types of ordering. We use scanning tunneling microscopy and resonant elastic x-ray scattering measurements to establish the formation of…
The evolution of Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$ from Mott insulator to superconductor was studied using angle-resolved photoemission spectroscopy. By measuring both the excitations near the Fermi energy as well as non-bonding states,…
Besides the mechanism responsible for high critical temperature superconductivity, the grand unresolved issue of the cuprates is the occurrence of a strange metallic state above the so-called pseudogap temperature $T^*$. Even though such…
We discuss the physics of the high temperature superconductivity in hole doped copper oxide ceramics in the pseudogap region. Starting from an effective reduced Hamiltonian relevant to the dynamics of holes injected into the copper oxide…
Recently developed numerical methods have enabled the explicit construction of the superconducting state of the Hubbard model of strongly correlated electrons in parameter regimes where the model also exhibits a pseudogap and a Mott…
The transport experiments reveal that the low-temperature resistivity in the normal-state of cuprate superconductors is quadratic in temperature (T-quadratic) in the underdoped pseudogap phase, while it is linear in temperature (T-linear)…
Although the microscopic origin of the superconductivity in high Tc copper oxides remains the subject of active inquiry, several of their electronic characteristics are well established as universal to all the known materials, forming the…
We have studied the density of states of the normal state of the electron-doped superconductor Pr$_{2-x}$Ce$_x$CuO$_4$ at low temperatures and in magnetic fields up to 31 Tesla using point contact spectroscopy on single crystals. Our data…
High-temperature superconducting cuprates are distinguished by an enigmatic pseudogap which opens near optimal doping where the superconducting transition temperature is highest. Key questions concern its origin and whether it is essential…
I present a microscopic theory I proposed recently to describe high-T_c superconductivity in cuprates. I show that coherent pairing states consisting of extended singlet Cooper pairs and triplet $\pi$ pairs can manifest both the Mott…
Normal state orbital diamagnetism of charged bosons quantitatively accounts for recent high-resolution magnetometery results near and above the resistive critical temperature Tc of superconducting cuprates. Our parameter-free descriptions…