Searcharxiv⌕ Search

arXiv subjects

J. P. Rodriguez

Publications and source records attributed to J. P. Rodriguez.

At least 37 records · Page 2Linked to original sources
arXiv2006

Collective pinning of imperfect vortex lattices by material line defects in extreme type-II superconductors

The critical current density shown by a superconductor at the extreme type-II limit is predicted to follow an inverse square-root power law with external magnetic field if the vortex lattice is weakly pinned by material line defects. It acquires an additional inverse dependence with thickness along the line direction once pinning of the interstitial vortex lines by material point defects is included. Moderate quantitative agreement with the critical current density shown by second-generation wires of high-temperature superconductors in kG magnetic fields is achieved at liquid-nitrogen temperature.

cond-mat.supr-con↗
arXiv2005

Macroscopic Phase Coherence of Defective Vortex Lattices in Two Dimensions

The superfluid density is calculated theoretically for incompressible vortex lattices in two dimensions that have isolated dislocations quenched in by a random arrangement of pinned vortices. The latter are assumed to be sparse and to be fixed to material defects. It is shown that the pinned vortices act to confine a single dislocation of the vortex lattice along its glide plane. Plastic creep of the two-dimensional vortex lattice is thereby impeded, and macroscopic phase coherence results at low temperature in the limit of a dilute concentration of quenched-in dislocations.

cond-mat.supr-con↗
arXiv2004

Collective pinning of the vortex lattice by columnar defects in layered superconductors

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated XY model in the presence of random columnar pins. A small fraction of pinned vortex lines is assumed. Thermally induced plastic creep of the vortex lattice within isolated layers results in an intermediate Bose glass phase that exhibits weak superconductivity across layers in the limit of weak Josephson coupling. The correlation volume of the vortex lattice is estimated in the strongly-coupled Bose-glass regime at lower temperature. In the absence of additional point pins, no peak effect in the critical current density is predicted to occur on this basis as a function of the Josephson coupling. Also, the phase transition observed recently inside of the vortex-liquid phase of high-temperature superconductors pierced by sparse columnar defects is argued to be a sign of dimensional cross-over.

cond-mat.supr-con↗
arXiv2004

Defective Vortex Lattices in Layered Superconductors with Both Point and Correlated Pins

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding XY model in the presence of random pinning centers. Sufficiently weak Josephson coupling between adjacent layers results in an entangled vortex solid that exhibits weak superconductivity across layers. The corresponding vortex liquid state shows an inverted specific heat anomaly that is a precursor to the weak superconductor.

cond-mat.supr-con↗
arXiv2004

Defective Vortex Lattices in Layered Superconductors with Point Pins at the Extreme Type-II Limit

The mixed phase of layered superconductors with no magnetic screening is studied through a partial duality analysis of the corresponding frustrated XY model in the presence of weak random point pins. Isolated layers exhibit a defective vortex lattice at low temperature that is phase coherent. Sufficiently weak Josephson coupling between adjacent layers results in an entangled vortex solid that exhibits weak superconductivity across layers. The corresponding vortex liquid state shows an inverted specific heat anomaly that we propose accounts for that seen in YBCO. A three-dimensional vortex lattice with dislocations occurs at stronger coupling. This crossover sheds light on the apparent discrepancy concerning the observation of a vortex-glass phase in recent Monte Carlo simulations of the same XY model.

cond-mat.supr-con↗
arXiv2004

Sublimated Decoupling of the Vortex Lattice in Extremely Type-II Layered Superconductors

The question of whether layer decoupling and vortex-lattice melting occur seperately or not in the mixed phase of pristine layered superconductors in the extreme type-II limit is studied through a partial duality analysis of the layered XY model with uniform frustration. We find that both transitions occur simultaneously if the normal/superconducting transition of the vortex lattice in an isolated layer is first-order and if a sufficient degree of layer anisotropy exists. We also find that a cross-over to a highly entangled vortex lattice phase with relatively low phase rigidity across layers does not occur in practice under any circumstances at temperatures below the two-dimensional vortex-lattice melting transition.

cond-mat.supr-con↗
arXiv2003

Optimum pinning of the vortex lattice in extremely type-II layered superconductors

The two-dimensional (2D) vortex lattice in the extreme type-II limit is studied by Monte Carlo simulation of the corresponding 2D Coulomb gas, with identical pins placed at sites coinciding with the zero-temperature triangular vortex lattice. At weak pinning we find evidence for 2D melting into an intermediate hexatic phase. The strong pinning regime shows a Kosterlitz-Thouless transition, driven by interstitial vortex/anti-vortex excitations. A stack of such identical layers with a weak Josephson coupling models a layered superconductor with a triangular arrangement of columnar pins at the matching field. A partial duality analysis finds that layer decoupling of the flux-line lattice does not occur at weak pinning for temperatures below 2D melting.

cond-mat.supr-con↗
arXiv2001

Degradation of Phase Coherence by Defects in a Two-Dimensional Vortex Lattice

The thermodynamic nature of two-dimensional vortex matter is studied theoretically through a duality analysis of the XY model over the square lattice with low uniform frustration. A phase-coherent vortex lattice state is found at low temperature if rigid translations are prohibited. It shows a non-zero phase rigidity that is degraded exclusively by the creation of dislocation pairs. The unbinding of such pairs causes the vortex lattice to simultaneously lose phase coherence and to melt at a continuous (Kosterlitz-Thouless) phase transition. General phase auto-correlation functions are also computed, and these results are used to argue for the existence of a continuous melting transition of vortex matter in layered superconductors.

cond-mat.supr-con↗
arXiv2001

Nature of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors

The uniformly frustrated layered XY model is analyzed in its Villain form. A decouple pancake vortex liquid phase is identified. It is bounded by both first-order and second-order decoupling lines in the magnetic field versus temperature plane. These transitions, respectively, can account for the flux-lattice melting and for the flux-lattice depinning observed in the mixed phase of clean high-temperature superconductors.

cond-mat.supr-con↗
arXiv2001

Theory of Decoupling in the Mixed Phase of Extremely Type-II Layered Superconductors

The mixed phase of extremely type-II layered superconductors in perpendicular magnetic field is studied theoretically via the layered XY model with uniform frustration. A partial duality analysis is carried out in the weak-coupling limit. It consistently accounts for both intra-layer (pancake) and inter-layer (Josephson) vortex excitations. The main conclusion reached is that dislocations of the two-dimensional (2D) vortex lattices within layers drive a unique second-order melting transition at high perpendicular fields between a low-temperature superconducting phase that displays a Josephson effect and a high-temperature ``normal'' phase that displays no Josephson effect. The former state is best described by weakly coupled 2D vortex lattices, while the latter state is best characterized by a decoupled vortex liquid. It is further argued on the basis of the duality analysis that the second-order melting transition converts itself into a first-order one as the perpendicular field is lowered and approaches the dimensional cross-over scale. The resulting critical endpoint potentially accounts for the same phenomenon that is observed in the mixed phase of clean high-temperature superconductors.

cond-mat.supr-con↗
arXiv2001

Decoupling of the S=1/2 antiferromagnetic zig-zag ladder with anisotropy

The spin-1/2 antiferromagnetic zig-zag ladder is studied by exact diagonalization of small systems in the regime of weak inter-chain coupling. A gapless phase with quasi long-range spiral correlations has been predicted to occur in this regime if easy-plane (XY) anisotropy is present. We find in general that the finite zig-zag ladder shows three phases: a gapless collinear phase, a dimer phase and a spiral phase. We study the level crossings of the spectrum,the dimer correlation function, the structure factor and the spin stiffness within these phases, as well as at the transition points. As the inter-chain coupling decreases we observe a transition in the anisotropic XY case from a phase with a gap to a gapless phase that is best described by two decoupled antiferromagnetic chains. The isotropic and the anisotropic XY cases are found to be qualitatively the same, however, in the regime of weak inter-chain coupling for the small systems studied here. We attribute this to a finite-size effect in the isotropic zig-zag case that results from exponentially diverging antiferromagnetic correlations in the weak-coupling limit.

cond-mat.str-el↗
arXiv2000

Wigner Crystal State for the Edge Electrons in the Quantum Hall Effect at Filling $ν= 2$

The electronic excitations at the edges of a Hall bar not much wider than a few magnetic lengths are studied theoretically at filling $ν= 2$. Both mean-field theory and Luttinger liquid theory techniques are employed for the case of a null Zeeman energy splitting. The first calculation yields a stable spin-density wave state along the bar, while the second one predicts dominant Wigner-crystal correlations along the edges of the bar. We propose an antiferromagnetic Wigner-crystal groundstate for the edge electrons that reconciles the two results. A net Zeeman splitting is found to produce canting of the antiferromagnetic order.

cond-mat.mes-hall↗
arXiv2000

Theory for Decoupling in High-T_c Superconductors from an Analysis of the Layered XY Model with Frustration

The nature of decoupling in the mixed phase of extremely type-II layered superconductors is studied theoretically through a duality transformation of the layered XY model with frustration. In the limit of weak coupling, we generally find that the Josephson effect is absent if and only if the phase correlations within isolated layers are short range. In the case specific to uniform frustration, we notably identify a decoupled pancake vortex liquid phase that is bounded by first-order and second-order decoupling lines in the magnetic field vs. temperature plane. These transitions potentially account for the flux-lattice melting and for the flux-lattice depinning that is observed in clean high-temperature superconductors.

cond-mat.supr-con↗
arXiv1999

Superconductivuty versus Tunneling in a Doped Antiferromagnetic Ladder

The low-energy charge excitations of a doped antiferromagnetic ladder are modeled by a system of interacting spinless fermions that live on the same ladder. A relatively large spin gap is assumed to ``freeze out'' all spin fluctuations. We find that the formation of rung hole pairs coincides with the opening of a single-particle gap for charge excitations along chains and with the absence of coherent tunneling in between chains. We also find that such hole pairs condense into either a crystalline or superconducting state as a function of the binding energy.

cond-mat.str-el↗
arXiv1998

Metallic Surface Reconstruction Driven by Frustrated Antiferromagnetism

A magnetic origin for the honeycomb reconstruction of metallic surfaces with three-fold symmetry like Pb/Ge (111) is proposed. Assuming that the groundstate is an antiferromagnetic insulator over the triangular lattice of adatom sites (Pb), we demonstrate that the former is simultaneously unstable to canting and to a structural distortion if the surface is soft enough. We therefore predict a net magnetization over the reconstructed surface at sufficiently low temperature.

cond-mat.str-el↗
arXiv1998

Critical Behavior of Layered Superconducting Films in Parallel Magnetic Field

The equilibrium magnetization for layered superconducting films that experience a nonzero component, $H_{\parallel}$, of magnetic field applied parallel to the layers is computed at temperatures and at perpendicular field components in the vicinity of the decoupling transition. A fermion analogy is exploited for this purpose, whereby it is found that the parallel magnetization shows an anomalous $H_{\parallel}^{-1}$ tail at high fields due to entropic fluctuations of the (parallel) lattice of Josephson vortices. A collective pinning effect is also identified for $c$-axis transport limited by a single planar defect oriented parallel to both $c$ and to the applied magnetic field.

cond-mat.supr-con↗
arXiv1998

Skyrmion Liquid Phase of the Quantum Ferromagnet in Two Dimensions

The two-dimensional quantum ferromagnet filled with a liquid of skyrmions is studied theoretically in the context of the quantum Hall effect near electronic filling factor $ν= 1$. A cross-over between the classical ferromagnetic phase at $ν=1$ and a classical paramagnetic phase at $ν\neq 1$ is obtained, which is consistent with recent Knight-shift measurements. A new collective mode associated with the skyrmion liquid that is of the cyclotron type is also identified.

cond-mat.mes-hall↗