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

Patterns of coexisting superconducting and particle-hole condensates

We have studied systematically the influence of particle-hole symmetric and asymmetric kinetic terms on the ordered phases that we may observe competing or coexisting in a tetragonal system. We show that there are precise patterns of triplets of ordered phases that are accessible (i.e. it is impossible to observe two of them without the third one). We found a systematic way to predict these patterns of states and tested it by identifying at least 16 different patterns of three order parameters that necessarily coexist in the presence of the kinetic terms. We show that there are two types of general equations governing the competition of all these triplets of order parameters and we provide them.Comment: Published versio

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.

Proximity Effect and Josephson Coupling in the SO(5) Theory of High-Tc Superconductivity

We consider proximity effect coupling in Superconducting/Antiferromagnetic/Superconducting (S-A-S) sandwiches using the recently developed SO(5) effective theory of high temperature superconductivity. We find that, for narrow junctions, the A region acts like a strong superconductor, and that there is a critical junction thickness which depends on the effective SO(5) coupling constants and on the phase difference across the junction, at which the A region undergoes a Freedericksz-like transition to a state which is intermediate between superconductor and antiferromagnet. For thick junctions, the current-phase relation is sinusoidal, as in standard S-N-S and S-I-S junctions, but for thin junctions it shows a sharp break in slope at the Freedericksz point.Comment: 4 pages, LATEX, 5 eps fig

Dimensionless fragility analysis of seismic acceleration demands through low-order building models

This paper deals with the estimation of fragility functions for acceleration-sensitive components of buildings subjected to earthquake action. It considers ideally coherent pulses as well as real non-pulselike ground-motion records applied to continuous building models formed by a flexural beam and a shear beam in tandem. The study advances the idea of acceleration-based dimensionless fragility functions and describes the process of their formulation. It demonstrates that the mean period of the Fourier Spectrum, Tm , is associated with the least dispersion in the predicted dimensionless mean demand. Likewise, peak ground acceleration, PGA-, and peak ground velocity, PGV-based length scales are found to be almost equally appropriate for obtaining efficient ‘universal’ descriptions of maximum floor accelerations. Finally, this work also shows that fragility functions formulated in terms of dimensionless Π-terms have a superior performance in comparison with those based on conventional non-dimensionless terms (like peak or spectral acceleration values). This improved efficiency is more evident for buildings dominated by global flexural type lateral deformation over the whole intensity range and for large peak floor acceleration levels in structures with shear-governed behaviour. The suggested dimensionless fragility functions can offer a ‘universal’ description of the fragility of acceleration-sensitive components and constitute an efficient tool for a rapid seismic assessment of building contents in structures behaving at, or close to, yielding which form the biggest share in large (regional) building stock evaluations

Universal Spin-Flip Transition in Itinerant Antiferromagnets

We report a universal spin flip (SF) transition as a function of temperature in spin-density-wave (SDW) systems. At low temperatures the antiferromagnetic (AFM) polarization is parallel to the applied field and above a critical temperature the AFM polarization {\it flips} perpendicular to the field. This transition occurs in {\it any} SDW system and may be considered as a qualitative probe of the itinerant character of AFM in a given material. Our SF transition resolves the longstanding puzzle of the SF transition observed in cromium and may be at the origin of the equally puzzling SDW-I to SDW-II transition in Bechgaard salts for which we make experimental predictions

Coexistence of SDW, d-wave singlet and staggered π\pi-triplet superconductivity

We have studied the competition and coexistence of staggered triplet SC with d-wave singlet SC and SDW in the mean-field approximation. Detailed numerical studies demonstrate that particle-hole asymmetry mixes these states and therefore they are simultaneously present. Even more interesting were the results of our study of the influence of a uniform magnetic field. We observe novel transitions that show the characteristics of Fulde-Ferrel phases, yet they concern transitions to different combinations of the above orders. For example, above a given field, in a particle-hole symmetric system we observe a transition from d-wave singlet SC to a state in which d-wave singlet SC coexists with staggered triplet SC and SDW. We believe our results may provide, among others, a direct explanation to recent puzzles about the Fulde Ferrel like states that are apparently observed in CeCoIn5.Comment: To be published in J. Phys. Cond. Ma

Density Matrix Renormalization Group Applied to the Ground State of the XY-Spin-Peierls System

We use the density matrix renormalization group (DMRG) to map out the ground state of a XY-spin chain coupled to dispersionless phonons of frequency $\omega$. We confirm the existence of a critical spin-phonon coupling $_c\propto \omega ^{0.7}$ for the onset of the spin gap bearing the signature of a Kosterlitz-Thouless transition. We also observe a classical-quantum crossover when the spin-Peierls gap $\Delta$ is of order $$. In the classical regime, $\Delta >\omega$, the mean-field parameters are strongly renormalized by non-adiabatic corrections. This is the first application of the DMRG to phonons.Comment: 10 pages, 5 figures. To be published in PR

Spin-density wave versus superconducting fluctuations for quasi-one-dimensional electrons in two chains of Tomonaga-Luttinger liquids

We study possible states at low temperatures by applying the renormalization-group method to two chains of Tomonaga-Luttinger liquids with both repulsive intrachain interactions and interchain hopping. As the energy decreases below the hopping energy, three distinct regions I, III, and II appear successively depending on properties of fluctuations. The crossover from the spin-density wave (SDW) state to superconducting (SC) state takes place in region III where there are the excitation gaps of transverse charge and spin fluctuations. The competition between SDW and SC states in region III is crucial to understanding the phase diagram in the quasi-one-dimensional organic conductors.Comment: 11 pages, Revtex format, 1 figure, to be published in Phys. Rev.