Underdoped cuprates

Figure 1. Temperature dependence of the resistivity of (a) a polycrystalline sample of the underdoped cuprate Lai.gSro.iCuC>4, taken from [1], and (b) a film of the same composition and thickness 150 nm, taken from [2]. In (b), the measurements correspond to the in-plane direction (parallel to the CuC>2 layers). The temperatures T, Tc and l c correspond, respectively, to the pseudogap opening temperature, to the temperature where the SCF effects become indistinguishable, and to the (inflexion-point) observed normal-superconducting transition temperature.

Conventional CDW and SDW states are ubiqiutous in metals with the commensurate SDW or antiferromagnetic (AF) order being the most common. Compounds with confirmed unconventional density waves sofar are rather scarce and with certainty have only been found in organic metals and perhaps in manimn HF compounds and the pseudogap phase of underdoped cuprates. This may in part be due to the difficulty detecting such hidden order parameters which leave no signature in standard neutron or X-ray diffraction experiments. 

Recently, angle-resolved photoemission spectroscopy (ARPES) for Bi2212 showed that the electronic density of states of underdoped cuprates reveals a normal-state gap-... 

Simon, M.E. and Varma, C.M. (2002) Detection and implications of a time-reversal breaking state in underdoped cuprate. Phys. Rev. Lett, 89, 247003 (4 pages). 

Some time ago, Uemura et al. (1991) proposed a universal Unear relationship Tc = ps between the superconducting transition temperature and the superfluid density ps of charge carriers for a group of superconducting compounds. Although the Uemura law describes underdoped cuprates well, it does not work for optimal or overdoped cuprates. 

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