Physical Review B 75,13 (2007) 132506;
Superconductors with low superfluid density are often dominated by phase fluctuations of the order parameter. In this regime, their physics may be described by XY models. The transition temperature T-c of such models is of the same order as the zero-temperature phase stiffness (helicity modulus), a long-wavelength property of the system: T-c=A Upsilon(0). However, the constant A is a nonuniversal number, depending on dimensionality and the degree of inhomogeneity. In this Brief Report, we discuss strategies for maximizing A for two-dimensional XY models; that is, how to maximize the transition temperature with respect to the zero-temperature, long-wavelength properties. We find that a framework type of inhomogeneity can increase the transition temperature significantly. For comparison, we present similar results for Ising models.
Physics, Condensed Matter
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