arXiv subjects

Jose Gonzalez

Publications and source records attributed to Jose Gonzalez.

11 recordsLinked to original sources

arXiv2025

High-temperature superconductivity in flat-band sheared bilayer graphene

We propose a new route to induce flat bands with a strong superconducting instability in graphene bilayers with heteroshear, where the 1D character of the moir\'e leads to stronger correlations than in twisted bilayer graphene. We adopt an exact diagonalization approach, on top of a real-space self-consistent Hartree-Fock approximation, to show how the valley polarization of the flat band of a sheared bilayer drives the condensation of Cooper pairs. A unique feature of the 1D moir\'e is that single-particle states with reverse sign of the valley polarization have complementary charge distributions in the moir\'e supercell. This leads to many-body states where the Coulomb repulsion in a Cooper pair is greatly reduced by placing electrons with opposite spin in different valleys. At small hole-doping of the flat band, the many-body ground states are formed by recursive addition of single-hole states, which allows us to reconstruct a quasi-1D Fermi line in the originally flat band. We show that even (odd) numbers of holes lead consistently to ground states with lower (higher) values of the compressibility. This provides the signature of the condensation of Cooper pairs with emergent quasiparticles above a large energy gap, unveiling a strong-coupling route to high-temperature superconductivity in topological flat-band systems.

cond-mat.mes-hall↗

arXiv2017

Surface and 3D quantum Hall effects from engineering of exceptional points in nodal-line semimetals

We investigate the potential of the surface states of 3D nodal-line semimetals to produce surface and 3D quantized Hall effects in which the electronic currents flow unidirectionally along the surface of the material. This phenomenology is based on the fact that, irrespective of its orientation, a strong magnetic field gives rise to a zero Landau level with huge 2D degeneracy of states in the nodal-line semimetal. When the magnetic field is perpendicular to the plane of the nodal line, we show that the evanescent states within the nodal ring are transmuted into Landau surface states which correspond to exceptional points, i.e. branch points which arise upon extension of the spectrum to complex values of the momentum. These Landau surface states are then topologically protected, as the branch cuts in the complex plane lead to a spectrum with the topology of a torus which cannot be modified by smooth perturbations. When the magnetic field is parallel to the nodal ring, we find instead that the evanescent waves are paired inside the bulk of the semimetal, leading to a 3D quantum Hall effect in which Landau states extended over 2D slices of the 3D bulk form a flat level, but start to get some dispersion as they approach the surface of the material. We show that, for both orientations of the magnetic field, the transverse Hall conductivity is quantized in units of $e^2/h$, with very large values of the Chern number which afford very high conductivities along the surface of the 3D material.

cond-mat.mes-hall↗

arXiv2017

Saliency Fusion in Eigenvector Space with Multi-Channel Pulse Coupled Neural Network

Saliency computation has become a popular research field for many applications due to the useful information provided by saliency maps. For a saliency map, local relations around the salient regions in multi-channel perspective should be taken into consideration by aiming uniformity on the region of interest as an internal approach. And, irrelevant salient regions have to be avoided as much as possible. Most of the works achieve these criteria with external processing modules; however, these can be accomplished during the conspicuity map fusion process. Therefore, in this paper, a new model is proposed for saliency/conspicuity map fusion with two concepts: a) input image transformation relying on the principal component analysis (PCA), and b) saliency conspicuity map fusion with multi-channel pulsed coupled neural network (m-PCNN). Experimental results, which are evaluated by precision, recall, F-measure, and area under curve (AUC), support the reliability of the proposed method by enhancing the saliency computation.

cs.CV↗

arXiv2016

Macroscopic degeneracy of zero-mode rotating surface states in 3D Dirac and Weyl semimetals under radiation

We investigate the development of novel surface states when 3D Dirac or Weyl semimetals are placed under circularly polarized electromagnetic radiation. We find that the hybridization between inverted Floquet bands opens in general a gap, which closes at so-called exceptional points found for complex values of the momentum. This corresponds to the appearance of midgap surface states in the form of evanescent waves decaying from the surface exposed to the radiation. We observe a phenomenon reminiscent of Landau quantization by which the midgap surface states get a large degeneracy proportional to the radiation flux traversing the surface of the semimetal. We show that all these surface states carry angular current, leading to an angular modulation of their charge that rotates with the same frequency of the radiation, which should manifest in the observation of a macroscopic chiral current in the irradiated surface.

cond-mat.mes-hall↗

arXiv2014

Metric of a tidally perturbed spinning black hole

We explicitly construct the metric of a Kerr black hole that is tidally perturbed by the external universe in the slow-motion approximation. This approximation assumes that the external universe changes slowly relative to the rotation rate of the hole, thus allowing the parameterization of the Newman-Penrose scalar $ψ_0$ by time-dependent electric and magnetic tidal tensors. This approximation, however, does not constrain how big the spin of the background hole can be and, in principle, the perturbed metric can model rapidly spinning holes. We first generate a potential by acting with a differential operator on $ψ_0$. From this potential we arrive at the metric perturbation by use of the Chrzanowski procedure in the ingoing radiation gauge. We provide explicit analytic formulae for this metric perturbation in spherical Kerr-Schild coordinates, where the perturbation is finite at the horizon. This perturbation is parametrized by the mass and Kerr spin parameter of the background hole together with the electric and magnetic tidal tensors that describe the time evolution of the perturbation produced by the external universe. In order to take the metric accurate far away from the hole, these tidal tensors should be determined by asymptotically matching this metric to another one valid far from the hole. The tidally perturbed metric constructed here could be useful in initial data constructions to describe the metric near the horizons of a binary system of spinning holes. This perturbed metric could also be used to construct waveforms and study the absorption of mass and angular momentum by a Kerr black hole when external processes generate gravitational radiation.

gr-qc↗

arXiv2012

Non-Abelian gauge potentials in graphene bilayers

We study the effect of spatial modulations in the interlayer hopping of graphene bilayers, such as those that arise upon shearing or twisting. We show that their single-particle physics, characterized by charge accumulation and recurrent formation of zero-energy bands as the pattern period L increases, is governed by a non-Abelian gauge potential arising in the low-energy electronic theory due to the coupling between layers. We show that such gauge-type couplings give rise to a potential that, for certain discrete values of L, spatially confines states at zero energy in particular regions of the Moiré patterns. We also draw the connection between the recurrence of the flat zero-energy bands and the non-Abelian character of the potential.

cond-mat.mes-hall↗

arXiv2011

Cox rings and pseudoeffective cones of projectivized toric vector bundles

We study projectivizations of a special class of toric vector bundles that includes cotangent bundles, whose associated Klyachko filtrations are particularly simple. For these projectivized bundles, we give generators for the cone of effective divisors and a presentation of the Cox ring as a polynomial algebra over the Cox ring of a blowup of a projective space along a sequence of linear subspaces. As applications, we show that the projectivized cotangent bundles of some toric varieties are not Mori dream spaces and give examples of projectivized toric vector bundles whose Cox rings are isomorphic to that of M_{0,n}.

math.AG↗

arXiv2007

Exploring black hole superkicks

Recent calculations of the recoil velocity in black-hole binary mergers have found kick velocities of $\approx2500 $km/s for equal-mass binaries with anti-aligned initial spins in the orbital plane. In general the dynamics of spinning black holes can be extremely complicated and are difficult to analyze and understand. In contrast, the ``superkick'' configuration is an example with a high degree of symmetry that also exhibits exciting physics. We exploit the simplicity of this ``test case'' to study more closely the role of spin in black-hole recoil and find that: the recoil is with good accuracy proportional to the difference between the $(l = 2, m = \pm 2)$ modes of $Ψ_4$, the major contribution to the recoil occurs within $30M$ before and after the merger, and that this is after the time at which a standard post-Newtonian treatment breaks down. We also discuss consequences of the $(l = 2, m = \pm 2)$ asymmetry in the gravitational wave signal for the angular dependence of the SNR and the mismatch of the gravitational wave signals corresponding to the north and south poles.

gr-qc↗

arXiv2007

Binary black holes on a budget: Simulations using workstations

Binary black hole simulations have traditionally been computationally very expensive: current simulations are performed in supercomputers involving dozens if not hundreds of processors, thus systematic studies of the parameter space of binary black hole encounters still seem prohibitive with current technology. Here we show how the multi-layered refinement level code BAM can be used on dual processor workstations to simulate certain binary black hole systems. BAM, based on the moving punctures method, provides grid structures composed of boxes of increasing resolution near the center of the grid. In the case of binaries, the highest resolution boxes are placed around each black hole and they track them in their orbits until the final merger when a single set of levels surrounds the black hole remnant. This is particularly useful when simulating spinning black holes since the gravitational fields gradients are larger. We present simulations of binaries with equal mass black holes with spins parallel to the binary axis and intrinsic magnitude of S/m^2= 0.75. Our results compare favorably to those of previous simulations of this particular system. We show that the moving punctures method produces stable simulations at maximum spatial resolutions up to M/160 and for durations of up to the equivalent of 20 orbital periods.

gr-qc↗

arXiv2007

Head-On collisions of different initial data

We discuss possible origins for discrepancies observed in the radiated energies in head-on collisions of non-spinning binaries starting from Brill-Lindquist and superposed Kerr-Schild data. For this purpose, we discuss the impact of different choices of gauge parameters and a small initial boost of the black holes.

gr-qc↗

arXiv1993

On the Nature of Nonperturbative Effects in Stabilized 2D Quantum Gravity

We remark that the weak coupling regime of the stochastic stabilization of 2D quantum gravity has a unique perturbative vacuum, which does not support instanton configurations. By means of Monte Carlo simulations we show that the nonperturbative vacuum is also confined in one potential well. Nonperturbative effects can be assessed in the loop equation. This can be derived from the Ward identities of the stabilized model and is shown to be modified by nonperturbative terms.

hep-th↗