High-temperature superconductivity in doped antiferromagnets

In the context of an effective model for doped antiferromagnets, whereby the charge carriers are treated as hard-core bosons, we demonstrate that the ground state energy close to half-filling is an even periodic function of the external magnetic flux threading the square lattice in an Aharonov-Bohm geometry. The period is equal to the flux quantum $\Phi_{0}=2\pi\hbar c/q$ entering the Peierls phase factor of the hopping matrix elements. Thus flux quantization and a concomitant finite value of superfluid weight D_{s} occur along with metallic antiferromagnetism. We argue that the charge q in the associated flux quantum might be set equal to 2e. The superconducting transition temperature T_{c} is related to D_{s} linearly, in accordance to the generic Kosterlitz-Thouless type of transition in a two-dimensional system, signalling the coherence of the phase fluctuations of the condensate. The calculated dependence of T_{c} on hole concentration is qualitatively similar to that observed in the high-temperature superconducting cuprates.Comment: 5 pages, REVTEX file (2 Postscript figures). Proc. of 1st Euroconference on "Anomalous Complex Superconductors". To appear in Physica C (1999

Flux quantization and superfluid weight in doped antiferromagnets

Doped antiferromagnets, described by a t-t'-J model and a suitable 1/N expansion, exhibit a metallic phase-modulated antiferromagnetic ground state close to half-filling. Here we demonstrate that the energy of latter state is an even periodic function of the external magnetic flux threading the square lattice in an Aharonov-Bohm geometry. The period is equal to the flux quantum $\Phi_{0}=2\pi\hbar c/q$ entering the Peierls phase factor of the hopping matrix elements. Thus flux quantization and a concomitant finite value of superfluid weight D_s occur along with metallic antiferromagnetism. We argue that in the context of the present effective model, whereby carriers are treated as hard-core bosons, the charge q in the associated flux quantum might be set equal to 2e. Finally, the superconducting transition temperature T_c is related to D_s linearly, in accordance to the generic Kosterlitz-Thouless type of transition in a two-dimensional system, signaling the coherence of the phase fluctuations of the condensate. The calculated dependence of T_c on hole concentration is qualitatively similar to that observed in the high-temperature superconducting cuprates.Comment: 5 pages, 2 figures, to be published in J. Phys. Condens. Matte

Drude weight and total optical weight in a t-t'-J model

We study the Drude weight D and the total optical weight K for a t-t'-J model on a square lattice that exhibits a metallic phase-modulated antiferromagnetic ground state close to half-filling. Within a suitable 1/N expansion that includes leading quantum-fluctuation effects, D and K are found to increase linearly with small hole doping away from the Mott metal-insulator transition point at half-filling. The slow zero-sound velocity near the latter transition identifies with the velocity of the lower-energy branch of the twofold excitation spectrum. At higher doping values, D and K eventually saturate and then start to decrease. These features are in qualitative agreement with optical conductivity measurements in doped antiferromagnets.Comment: 7 pages, REVTEX file (3 Postscript figures). To appear in J. Phys.: Condens. Mattte

Monte Carlo study of the superfluid weight in doped antiferromagnets

The phase fluctuations of the condensate in doped antiferromagnets, described by a t-t'-J model and a suitable 1/N expansion, provide a mechanism for a Kosterlitz-Thouless (KT) type of transition to a superconducting state below T_{c}. In this paper, we present a Monte Carlo study of the corresponding superfluid weight D_{s}(T) in the classical (large-N) limit, as a function of temperature and doping. Consistent with generic experimental trends, D_{s}(T) exhibits a T-linear decrease at low temperatures, with the magnitude of the slope D_{s}'(0) increasing upon doping. Finite-size scaling in the underdoped regime predicts values for the dimensionless ratio A=k_{B}T_{c}/D_{s}(0) of order unity, with A=0.4435(5) in the half-filled-band limit, thus confirming D_{s}(0) as the fundamental energy scale determining T_{c}. Our Monte Carlo results for D_{s}(T)/D_{s}(0) vs k_{B}T/D_{s}(0), at 10% hole doping, are found to be in reasonable agreement with recent measurements on La_{2-x}Sr_{x}CuO_{4}, with x=0.10, throughout the temperature range below the theoretical KT transition temperature T_{c}.Comment: 9 pages, REVTEX file (4 Postscript figures). To appear in Phys. Rev.