The anisotropic resistivity components of YBa2Cu408 have been measured from 0 to 900 K. YBa2Cu408 is an underdoped, genuinely untwinned, and stoichiometric high temperature superconductor. The Cu02 plane resistivity pa exhibits a strong correlation with the spin dynamics, i.e., with Knight shift and spin-lattice relaxation time T\. The resistivity along the b axis (plane plus chain) is consistent with Bloch-Griineisen theory. The cotangent of the Hall angle is quadratic in temperature above the spin gap, and linear below.PACS numbers: 74.72.Bk, 72.15.Qm, 72.15.Gd Since the revolution of high temperature superconductivity (HTSC), initiated by the report of Bednorz and Miiller [1], the unusual normal state transport properties of the HTSC's have been suspected of giving clues to the basic mechanism responsible for superconductivity.Especially the normal state resistivity has revealed an anomalous response [2]: The optimal doped samples as, e.g., YBaiCusOy (1:2:3) have shown a linear behavior in temperature from Tc up to some hundreds degrees Kelvin. In the overdoped regime, the exponent a of pec T" changes from a = l to 2; i.e., a Fermi-liquid behavior becomes dominant. The underdoped samples exhibit a nearly T linear dependence with deviations at low temperatures.In the overdoped region, the basic mechanism responsible for superconductivity is hidden due to the excess carrier density. Hence, as concerns the basic mechanism of HTSC, the underdoped region, which is mainly covered by nonstoichiometric samples, should be explored. Unfortunately, nonstoichiometry is accompanied by distortions, nonhomogeneity, oxygen defects, etc., which, in addition, are modestly screened, due to the low carrier density. As a result, the intrinsic conductivity of the low doped Cu02 plane is not easy to uncover (see also Fig. 1).A fingerprint of underdoped samples is an opening of a spin gap above Tc and a magnetic susceptibility increasing with temperature. Both signatures have been found in YBa2Cu408 (1:2:4) by means of the spin Knight shift [3] and the spin-lattice relaxation time T\ [4]. 1:2:4 has a structure similar to 1:2:3 except the single chains along the b direction are replaced by double chains. YBa2-CU4O8 is a bonus from nature since it is simultaneously stoichiometric, underdoped and, in addition, genuinely untwinned. Because of that, the unvarnished conductivity component of the low doped Cu02 plane can be determined.In this Letter, we present the conductivity components for the a axis representing the CUO2 plane as well as for the b axis containing a plane and a chain contribution. Furthermore, we have measured the Hall effect for the Cu02 plane. The results prove the intimate relationship between dynamic spin susceptibility and normal state transport which seems to be the clue to understanding HTSCs [5].1:2:4 single crystals were grown with a high-pressure flux method using a BaO-CuO-rich flux. The growing parameters were 640 bars O2 at a temperature of 1040°C. For transport measurements, gold pads were sputter deposi...
