arXiv subjects

Nicolás Grandi

Publications and source records attributed to Nicolás Grandi.

At least 19 recordsLinked to original sources

arXiv2026

On the robustness of the chiral soliton lattice and the propagation of test quarks

In this paper, we demonstrate the robustness of the chiral soliton lattice (ChSL) against a specific class of operators: the higher-order corrections in the 't Hooft large-$N_c$ expansion within the generalized Skyrme model coupled to Maxwell theory. By considering a suitable Ansatz adapted to describe topological solitons at finite baryon density in a constant magnetic field, the generalized Skyrme model coupled to Maxwell theory is reduced to the effective Lagrangian of the ChSL phase, which describes a lattice of domain-walls made of hadrons. One of the key points in this construction is the fact that even when the usual topological charge density vanishes, the presence of the Callan-Witten term in the topological charge density allows for a non-vanishing baryon number. In the present approach, rather than assuming a prescribed external magnetic field (as is usually assumed for the ChSL), we solve the fully coupled Skyrme-Maxwell field equations. Finally, we show how our formulation allows us to study the interaction of the ChSL with test quarks. In particular, we derive the exact analytical spectrum of the Dirac equation in the high-density limit, providing a microscopic characterization of the fermionic excitations within the inhomogeneous hadronic background provided by the ChSL. The comparison of the spectrum of the Dirac operator in the ChSL with that of the standard Dirac operator in a constant magnetic field reveals the fundamental role of both the quark-Skyrmion coupling and the hadronic profile in opening a gap and shifting the spectrum.

hep-th↗

arXiv2026

Bumpy horizons from non-linear sigma models

We show that black hole horizons with non-constant curvature can be constructed in any even dimension, when the source of the Einstein equations is given by the stress-energy tensor of a non-linear sigma model of a particular class

hep-th↗

arXiv2026

A smooth road to bumpy horizons: shaping black holes with non-linear sigma models, from supergravity to higher dimensions

We construct new families of solutions for General Relativity coupled to a general class of non-linear sigma models, some of which can be embedded in supergravity. The solutions include neutral, charged and magnetized black holes with bumpy horizons, bumpy stars, and anisotropic cosmologies in $d\geq 4$ dimensions, as well as black strings and black $p$-branes. We also present a family of time-dependent solutions in $2+1$-dimensions. The construction relies on a set of first-order Bogomol'nyi-Prasad-Sommerfield relations for the coset scalars, that were recently exploited for the construction of bumpy black holes on the non-linear sigma model with homogenous target $SU(2)/U(1)$ in 2601.22914 [hep-th].

hep-th↗

arXiv2026

Viscoelastic and thermal response on nuclear pasta states at finite baryon density

We compute the viscoelastic and thermal response of non-homogeneous hadronic condensates at finite baryon density representing crystals of baryonic tubes and layers. We describe them using analytic solutions of the Skyrme model in $3+1$ dimensions as ground states for a perturbative approach. At low enough temperatures, the lowest-energy excitations are described by a free massless scalar field theory in $1+1$ dimensions. We apply the Green-Kubo formulas to such excitations to obtain the elasticity tensor and other response coefficients. The analytic results of our computations are compared with available results on the nuclear pasta phase.

hep-th↗

arXiv2025

Hurst exponent and planetary rings

We test on Saturn's rings whether the Hurst exponent is a "robust" observable, in the sense that it returns consistent values for different observations of the same object. We calculate the exponent of the series corresponding to the optical depth as a function of the radius, as obtained from the grayscale value of Cassini pictures, as well as from stellar occultation data from Voyager missions.

astro-ph.EP↗

arXiv2025

Dynamical fractalization of ring systems

We develop an effective model to describe the dynamics of a system of particle moving in circular configurations around a central mass, by considering the continuum limit of the angular distribution, to obtain the stable configurations for different initial parameters. We compute the resulting fractal dimension and compare it with that of a statistical Cantor ring system.

astro-ph.EP↗

arXiv2025

Existence of a supersymmetric extension of the gauged non-linear $Σ$-model and the transition from integrability to chaos

The non-linear $Σ$-Model minimally coupled with Maxwell theory in $3+1$ dimensions possesses a topologically non-trivial sector characterized by ``lasagna''-like configurations. We demonstrate that, when a specific quantization condition is met, the associated second-order field equations admit a first-order Bogomol'nyi-Prasad-Sommerfield system. This discloses a hidden $(1+1)$-dimensional supersymmetry with $\mathcal{N}=2$ supercharges. We examine the supersymmetric imprint on the time-dependent regime, with particular emphasis on the transition from integrability to chaos.

hep-th↗

arXiv2025

Holographic explorations of regular black holes in pure gravity

It has been recently established in \textit{Phys.Lett.B 861 (2025) 139260} that regular, asymptotically flat black holes exist in pure gravity theories in dimension greater than four. We extend this result to asymptotically Anti-de Sitter spacetimes and perform bottom-up holographic explorations: Hawking-Page phase transitions, quasinormal modes of black holes providing decay times of excitations in the finite temperature conformal field theory and U-shaped string probes leading to the quark-antiquark potential in the dual field theory. The gravitational action contains a bared, negative cosmological constant and an Einstein-Hilbert term, supplemented by an infinite series of higher-curvature terms of the family known as Quasi-topological (QT) Lagrangians, leading to a lapse function in terms of infinite series which can be resumed to an analytic function for different coupling choices. The QT terms are defined as those satisfying a Birkhoff's theorem and possessing a holographic c-function, and exist at all orders in the curvature, for arbitrary dimension greater than four. Besides presenting the general formulae, we explore the physics of three specific cases leading to Hayward, Dymnikova and Bardeen-like regular black holes. We also found the field redefinition that permits recasting the $α'^3 W^4$ correction of Type IIB supergravity on $\mathcal{A}_5\times S^5$ as a series of QT terms. Before finishing, beyond holography, we also provide an extended thermodynamic setup, that via variations of the dimensionful $α$ and $L$, allow obtaining the Euler relation between finite thermodynamical quantities for the black hole of the theory.

hep-th↗

arXiv2025

(Quasi-)normal modes of rotating black holes and new solitons in Einstein-Gauss-Bonnet

In this paper, we analyze the scalar field (quasi-)normal modes of recently derived rotating black holes within the framework of Einstein-Gauss-Bonnet theory at the Chern-Simons point in five dimensions. We also examine the mode spectrum of these probes on new static gravitational solitons. These solitons, featuring a regular center, are constructed from static black holes with gravitational hair via a double analytic continuation. By imposing ingoing boundary conditions at the horizons of rotating black holes, ensuring regularity at the soliton centers, and imposing Dirichlet boundary conditions at infinity, we obtain numerical spectra for the rotating black holes and solitons. For static black holes, we demonstrate analytically that the imaginary part of the mode frequencies is negative. Our analysis of the massless Klein-Gordon equation on five-dimensional geometries reveals an infinite family of gaped, massive three-dimensional Klein-Gordon fields, despite the presence of a non-compact extended direction. For the static solitons, the frequencies are real and non-equispaced, whereas in the rotating black holes, counter-rotating modes are absorbed more quickly, and the imaginary part of the co-rotating modes approaches zero as extremality is approached. Additionally, we show that both the rotating black holes and solitons can be equipped with non-trivial torsion, leading to a novel branch of solutions.

hep-th↗

arXiv2024

Fermionic correlators on the holographic neutron star

We investigate the fermionic perturbations of the asymptotically AdS background known as holographic neutron star, which represents a highly degenerate state of strongly coupled fermions on a sphere at finite temperature. We calculate the two-point correlator of a fermionic operator and obtain its scaling properties as we approach the critical region of the phase diagram.

hep-th↗

arXiv2024

Probing holographic flat bands at finite density

Flat band electronic systems exhibit a rich landscape of correlation-driven phases. Motivated by these developments, in this paper, we explicitly include the effects of the chemical potential in a holographic model featuring approximately flat bands. In particular, we explore the phase diagram of this holographic flat band system as a function of the chemical potential. We find that at low temperatures and densities, the system features a nematic phase, transitioning into the Lifshitz phase as the chemical potential or temperature increases. To further characterize the ensuing phases, we investigate the optical conductivity and find that this observable shows strong anisotropies in the nematic phase.

hep-th↗

arXiv2023

Solitonic self-sustained charge and energy transport on the superconducting cylinder

We present an exact time-dependent solution for a charged scalar field on a two-dimensional cylinder, that can be interpreted as representing a long-standing excitation on a $s$-wave superconducting state, which propagates along a nanotube constructed out of twisted bilayer graphene. The solution has a topological charge characterized by an integer number, which counts the winding of the Higgs phase winds around the cylinder. The resulting electric current generates its own electromagnetic field in a self-consistent way, without the need of any external fields to keep it alive.

hep-th↗

arXiv2022

Holographic superconductivity in Einsteinian Cubic Gravity

We study the condensation of a charged scalar field in a $(3+1)$-dimensional asymptotically AdS background in the context of Einsteinian cubic gravity, featuring a holographic superconductor with higher curvature corrections corresponding to a CFT with a non-vanishing value of the stress tensor three-point function $t_4$. As it was previously noticed for higher dimensional Gauss-Bonnet theory, we observe that the critical temperature of the superconducting phase transition is lowered as the higher curvature coupling grows.

hep-th↗

arXiv2021

Engineering holographic flat fermionic bands

In electronic systems with flat bands, such as twisted bilayer graphene, interaction effects govern the structure of the phase diagram. In this paper, we show that a strongly interacting system featuring fermionic flat bands can be engineered using the holographic duality. In particular, we find that in the holographic nematic phase, two bulk Dirac cones separated in momentum space at low temperature, approach each other as the temperature increases. They eventually collide at a critical temperature yielding a flattened band with a quadratic dispersion. On the other hand, in the symmetric (Lifshitz) phase, this quadratic dispersion relation holds for any finite temperature. We therefore obtain a first holographic, strong-coupling realization of a topological phase transition where two Berry monopoles of charge one merge into a single one with charge two, which may be relevant for two- and three-dimensional topological semimetals.

hep-th↗

arXiv2021

Scalar correlators and normal modes in holographic neutron stars

We study a scalar field in the background of a holographic neutron star at finite temperature and analyze its asymptotic behavior to compute the two-point correlator at the boundary. From the normal modes of the field we determine the resonances and decay constants of the boundary field theory. We show that the correlator is dominated by the normal modes in the stable regions of phase space of the neutron star, becoming a power law as we move into the unstable zones.

hep-th↗

arXiv2021

Pomeranchuk instabilities in holographic metals

We develop a method to detect instabilities leading to nematic phases in strongly coupled metallic systems. We do so by adapting the well-known Pomeranchuk technique to a weakly coupled system of fermions in a curved asymptotically AdS bulk. The resulting unstable modes are interpreted as corresponding to instabilities on the dual strongly coupled holographic metal. We apply our technique to a relativistic $3+1$-dimensional bulk with generic quartic fermionic couplings, and explore the phase diagram at zero temperature for finite values of the fermion mass and chemical potential, varying the couplings. We find a wide region of parameters where the system is stable, which is simply connected and localized around the origin of coupling space.

hep-th↗

arXiv2021

Diving inside a hairy black hole

We investigate the interior of the Einstein-Gauss-Bonnet charged black-hole with scalar hair. We find a variety of dynamical epochs, with the particular important feature that the Cauchy horizon is not present. This makes the violation of the no-hair theorem a possible tool to understand how might the strong cosmic censorship conjecture work.

gr-qc↗

arXiv2021

Towards holographic flat bands

Motivated by the phenomenology in the condensed-matter flat-band Dirac systems, we here construct a holographic model that imprints the symmetry breaking pattern of a rather simple Dirac fermion model at zero chemical potential.In the bulk we explicitly include the backreaction to the corresponding Lifshitz geometry and compute the dynamical critical exponent. Most importantly, we find that such a geometry is unstable towards a nematic phase, exhibiting an anomalous Hall effect and featuring a Drude-like shift of its spectral weight. Our findings should motivate further studies of the quantum phases emerging from such holographic models.

hep-th↗