We have analyzed various characteristic temperatures and energies of hole-doped high-Tc cuprates as a function of a dimensionless hole-doping concentration (pu). Entirely based on the experimental grounds we construct a unified electronic phase diagram (UEPD), where three characteristic temperatures (T * 's) and their corresponding energies (E * 's) converge as pu increases in the underdoped regime. T * 's and E * 's merge together with the Tc curve and 3.5kBTc curve at pu ∼ 1.1 in the overdoped regime, respectively. They finally go to zero at pu ∼ 1.3. The UEPD follows an asymmetric half-dome-shaped Tc curve in which Tc appears at pu ∼ 0.4, reaches a maximum at pu ∼ 1, and rapidly goes to zero at pu ∼ 1.3. The asymmetric half-dome-shaped Tc curve is at odds with the well-known symmetric superconducting dome for La2−xSrxCuO4 (SrD-La214), in which two characteristic temperatures and energies converge as pu increases and merge together at pu ∼ 1.6, where Tc goes to zero. The UEPD clearly shows that pseudogap phase precedes and coexists with high temperature superconductivity in the underdoped and overdoped regimes, respectively. It is also clearly seen that the upper limit of high-Tc cuprate physics ends at a hole concentration that equals to 1.3 times the optimal doping concentration for almost all high-Tc cuprate materials, and 1.6 times the optimal doping concentration for the SrD-La214. Our analysis strongly suggests that pseudogap is a precursor of high-Tc superconductivity, the observed quantum critical point inside the superconducting dome may be related to the end point of UEPD, and the normal state of the underdoped and overdoped high temperature superconductors cannot be regarded as a conventional Fermi liquid phase.

We have measured thermoelectric power (TEP) as a function of hole concentration per CuO 2 layer P pl in Y 1−x Ca x Ba 2 Cu 3 O 6 ͑P pl = x /2͒ with no oxygen in the Cu-O chain layer. The room-temperature TEP as a function of P pl , S 290 ͑P pl ͒, of Y 1−x Ca x Ba 2 Cu 3 O 6 behaves identically to that of La 2−z Sr z CuO 4 ͑P pl = z͒. We argue that S 290 ͑P pl ͒ represents a measure of the intrinsic equilibrium electronic states of doped holes and, therefore, can be used as a common scale for the carrier concentrations of layered cuprates. We shows that the P pl determined by this new universal scale is consistent with both hole concentration microscopically determined by NQR and the hole concentration macroscopically determined by the formal valency of Cu. We find two characteristic scaling temperatures, T S * and T S2 * , in the TEP versus temperature curves that change systematically with doping. Based on the universal scale, we uncover a universal phase diagram in which almost all the experimentally determined pseudogap temperatures as a function of P pl fall on two common curves; lower pseudogap temperature defined by the T S * versus P pl curve and upper pseudogap temperature defined by the T S2 * versus P pl curve. We find that while pseudogaps are intrinsic properties of doped holes of a single CuO 2 layer for all high-T c cuprates, T c depends on the number of layers, therefore, the inter layer coupling, in each individual system.

We argue that in cuprate physics there are two types, hole content per CuO 2 plane (P pl ) and the corresponding hole content per unit volume (P 3D ), of holedoping concentrations for addressing physical properties that are two-dimensional (2D) and three-dimensional (3D) in nature, respectively. We find that superconducting transition temperature (T c ) varies systematically with P 3D as a superconducting "dome" with a universal optimal hole-doping concentration P opt. 3D = 1.6 × 10 21 cm −3 for single-layer high temperature superconductors. We suggest that P opt.3D determines the upper bound of the electronic energy of underdoped single-layer high-T c cuprates.

The effect of hydrostatic pressure up to 2 GPa on the superconductiong transitions in La 2−x Sr x CuO 4+δ is investigated. The ambient and high pressure properties of two series of samples with x=0 and x=0.015 and 0< δ <0.1 are characterized and compared by ac-susceptibility measurements. At ambient pressure both sets of samples fit into the same phase diagram as a function of the total hole concentration, n h . For n h < 0.085 there is a single superconducting transition (T c ≈ 30K) with an unusually large pressure coefficient, dT (30) c /dp ≈ 10 K/GP a. At higher hole density (n h > 0.085) a second superconducting transition (T c ≈ 15K) follows the first transition upon cooling and the pressure shift of this transition is negative, dTAt the boundary as the hole density is close to 0.085 the phase separation can be induced by pressure. The results are explained in terms of a strong correlation of the interstitial oxygen with the hole system in the CuO-planes.Pressure, applied at ambient temperature, causes a redistribution of holes.The mobile oxygen dopants follow and enhance T c as well as the tendency to phase 1 separation. If pressure is changed at low temperature (< 100K) the effects on T c and phase separations are greatly diminished because the interstitial oxygen becomes immobile at low T. Our results indicate that the dopant effects are important. Dopants and holes should be treated as a single globally correlated state. When thermodynamic euqilibrium is approached in the oxygen-doped samples, we find that there is an intrinsic tendency of electronic phase separation of doped holes into two distinct superconducting states.