Two mechanisms of pseudogap formation in Bi-2201: Ev- idence from the c-axis magnetoresistance

– Measurements of the c-axis resistivity and magnetoresistance have been used to investigate the pseudogap (PG) behavior in Bi2+zSr2−x−zLaxCuOy (Bi-2201) crystals at various hole densities. While the PG opening temperature T ∗ increases with decreasing hole doping, the magnetic-field sensitivity of the PG is found to have a very different trend: it appears at lower temperatures in more underdoped samples and vanishes in non-superconducting samples. These data suggest that besides the field-insensitive pseudogap emerging at T , a distinct one is formed above Tc as a precursor to superconductivity. Introduction. – In high-Tc cuprates, the electronic density of states (DOS) near the Fermi energy has been demonstrated to decrease gradually with decreasing temperature, resulting in the pseudogap (PG) formation [1–4]. This PG, which has been found to progressively “destroy” the Fermi surface (FS) [4,5], is on one hand a challenge to the conventional view of the FS itself, while on the other hand it allows one to reconcile the small number of carriers that participate in the charge transport in underdoped cuprates with the large FS observed by photoemission [1, 5]. Although the existence of the PG has been documented by many experiments [1], its nature and, particularly, its relation to the superconductivity (SC) remain far from being clear. For example, according to photoemission and surface-tunneling studies [2–4], the PG evolves smoothly into the SC gap below Tc, which implies a precursor-pairing origin of the PG; in contrast, recent observations of a distinct SC gap that coexists with the PG below Tc and tends to close at Tc [6,7] suggest that the PG might have nothing to do with superconductivity [8,9]. Apparently, a crucial test for the origin of the PG would be its sensitivity to the magnetic field; however, studies of the magnetic-field dependence of the PG have reported surprisingly controversial results [10–14]. While the electronic DOS in cuprates has mostly been studied by photoemission [3–5] or surface-tunneling [2, 15] spectroscopies, the extremely anisotropic nature of the Bi-based cuprates, where the crystal structure itself forms a stack of tunnel junctions, offers a possibility of intrinsic tunneling spectroscopy [6,7,16]. In fact, it has been shown that the c-axis transport in the Bi-based cuprates is governed by the tunneling between CuO2 planes and, accordingly,