In the underdoped pseudogap regime of cuprate superconductors, the normal state is commonly probed by applying a magnetic field (H). However, the nature of the H-induced resistive state has been the subject of a long-term debate, and clear evidence for a zero-temperature (T = 0) H-tuned superconductor-insulator transition (SIT) has proved elusive. Here we report magnetoresistance measurements in underdoped La 2−x Sr x CuO 4 , providing striking evidence for quantum critical behavior of the resistivity -the signature of a H-driven SIT. The transition is not direct: it is accompanied by the emergence of an intermediate state, which is a superconductor only at T = 0. Our finding of a two-stage H-driven SIT goes beyond the conventional scenario in which a single quantum critical point separates the superconductor and the insulator in the presence of a perpendicular H. Similar two-stage H-driven SIT, in which both disorder and quantum phase fluctuations play an important role, may also be expected in other copper-oxide high-temperature superconductors.The SIT is an example of a quantum phase transition (QPT): a continuous phase transition that occurs at T = 0, controlled by some parameter of the Hamiltonian of the system, such as doping or the external magnetic field 1 . A QPT can affect the behavior of the system up to surprisingly high temperatures. In fact, many unusual properties of various strongly correlated materials have been attributed to the proximity of quantum critical points (QCPs). An experimental signature of a QPT at nonzero T is the observation of scaling behavior with relevant parameters in describing the data. Although the SIT has been studied extensively 2 , even in conventional superconductors many questions remain about the perpendicular-field-driven SIT in two-dimensional (2D) or quasi-2D systems 3 . In high-T c cuprates (T c -transition temperature), which have a quasi-2D nature, early magnetoresistance (MR) experiments showed the suppression of superconductivity with high H, revealing the insulating behavior 4-6 and hinting at an underlying H-field-tuned SIT 7 . However,
New Mn(II) complexes containing 5-(2-pyridyl)tetrazole, 5-(3-cyano-4-pyridyl)tetrazole or 5-(4-pyridyl)tetrazole ligands are described. The complexes are prepared by reaction of the corresponding cyanopyridines with sodium azide in the presence of Mn(II) salts. All the complexes have been characterized by X-ray crystallography, which reveals that 5-(pyridyl)tetrazole ligands can coordinate to Mn through either type of nitrogen atom in the tetrazole residue or via the pyridyl group. In the solid state, extended 2D and 3D structures are produced through networks of hydrogen bonding (involving water molecules and the tetrazolate residue). Acidification of the complexes produces the corresponding free 5-(pyridyl)-1H-tetrazole.
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