Striped superconductors: How spin, charge and superconducting orders intertwine in the cuprates

Recent transport experiments in the original cuprate high temperature superconductor, La_2-xBa_xCuO₄ have revealed a remarkable sequence of transitions and crossovers that give rise to a form of dynamical dimensional reduction, in which a bulk crystal becomes essentially superconducting in two directions while it remains poorly metallic in the third. We identify these phenomena as arising from a distinct new superconducting state, the 'striped superconductor', in which the superconducting order is spatially modulated, so that its volume average value is zero. Here, in addition to outlining the salient experimental findings, we sketch the order parameter theory of the state, stressing some of the ways in which a striped superconductor differs fundamentally from an ordinary (uniform) superconductor, especially concerning its response to quenched randomness. We also present the results of density matrix renormalization group calculations on a model of interacting electrons in which sign oscillations of the superconducting order are established. Finally, we speculate concerning the relevance of this state to experiments in other cuprates, including recent optical studies of La _2-xSr_xCuO₄ in a magnetic field, neutron scattering experiments in underdoped YBa₂Cu₃O _6+x and a host of anomalies seen in STM and ARPES studies Of Bi ₂Sr₂CaCu₂O_8+δ.