arXiv subjects

Jorge G. Russo

Publications and source records attributed to Jorge G. Russo.

At least 19 recordsLinked to original sources

arXiv2026

Black holes and causal nonlinear electrodynamics

For generic theories of nonlinear electrodynamics (NLED) we investigate the implications of (a)causality on spherically-symmetric solutions of the Einstein-NLED equations that are asymptotic to a Reissner-Nordström (RN) spacetime. Equal-charge dyonic RN black holes are shown to be exact, but unstable, solutions of (acausal) ``Born-type'' theories. For {\it all causal theories} it is shown that the metric is singular at the centre of symmetry and that it has at most two Killing horizons, implying at most three ``phases": RN-like or S(chwarzschild)-like black holes, and naked timelike singularities. For extreme RN-like black holes, including dyons, we give simple proofs of monotonicity conditions that imply a reduction of mass and entropy due to NLED interactions. We find that causality allows four qualitatively different phase-diagrams. One of the two with finite electromagnetic energy $\mathcal{E}_{\rm em}$ is the previously studied Born-Infeld-type, for which the zero-entropy limit of a ``small-charge" S-like black hole is a naked timelike singularity of mass $M=\mathcal{E}_{\rm em}$; we show that the spacetime geometry of this ``Born particle'' is that of the Bariola-Vilenkin global monopole.

hep-th↗

arXiv2026

Higher-Order Fermion Interactions in Effective Field Theories for Phase Transitions

We investigate the impact of higher-order fermionic deformations in phase transitions analogous to those described by the Bardeen-Cooper-Schrieffer (BCS) theory. Focusing specifically on the 8-fermion interaction, we show that this term can have significant consequences. In certain regions of parameter space, the theory continues to exhibit second-order phase transitions with mean-field critical exponents and the same critical temperature; however, the temperature dependence of the superconducting gap can deviate markedly from conventional BCS behavior. In other regions, the theory exhibits first-order phase transitions. We conclude by discussing potential phenomenological applications of these theories.

cond-mat.supr-con↗

arXiv2026

Shadow curves and quasinormal modes for rotating black holes surrounded by dark matter, radiation and dust

We study the impact of different fluid matter scenarios on the shadow of black holes (BH) and on frequencies of quasinormal modes (QNM) for a black hole subjected to scalar perturbations, with a comparison to the standard Kerr. The analysis of the shadow reveals a dependence on the density parameter of fluid matter $k$, with larger shadow sizes for larger values of $k$ in the dark matter and dust cases, while the shadow size becomes smaller in the radiation case. Notably, dark matter induces more visible deformations compared to radiation or dust, thereby highlighting its distinct imprint on shadow curves. We also find that dark matter reduces the real part of the QNM frequencies and significantly increases the damping time, enhancing the prospects for gravitational wave detection. The variations in spin parameter $a$, density parameter $k$ and multipole number $l$ are investigated. The analysis confirms the significant role of dark matter in modifying the behavior of QNMs, providing a promising avenue for future experiments.

gr-qc↗

arXiv2025

Analytical exploration of the optomechanical attractor diagram and of limit cycles

We analyse the interplay between mechanical and radiation pressure in an optomechanical cavity system. Our study is based on an analytical evaluation of the infinite Bessel summations involved, which previously had led to a numerical exploration of the so-called attractor diagram. The analytical expressions are then suitable for further asymptotic analysis in opposing regimes of the amplitude, which allows for a characterisation of the diagram in terms of elementary functions. Building on this framework, we investigate the emergence and properties of optomechanical limit cycles beyond the constraints of the resolved sideband approximation. By employing a Fokker-Planck formalism originally developed in the context of laser theory and then used in cavity optomechanics, we describe the quantum regime of these limit cycles, offering a more detailed and unified analytical perspective.

quant-ph↗

arXiv2025

Simplified self-dual electrodynamics

We present a new formulation of self-dual nonlinear electrodynamics in which interactions are determined by an auxiliary-field potential, with causality ensuring a unique solution to the auxiliary-field equation. The long-standing problem of an explicit Lagrangian for the generic `analytic' theory is simply solved by restriction to potentials that are even functions of the auxiliary field. In this case the Lagrangian can be linearised in quadratic field-strength scalars by the introduction of an additional pseudoscalar auxiliary field; this generalises, to all analytic self-dual theories, a well-known construction of the Born-Infeld theory.

hep-th↗

arXiv2025

More on phase transitions in ${\cal N}$ = 2 massive gauge theories

We study large $N$ phase transitions in $\mathcal{N}=2$ theories with gauge group $SU(N)$ and massive hypermultiplets in diverse representations. Using supersymmetric localization we identify cases where phase transitions occur. In particular, we consider deformations of UV superconformal fixed points by giving two different masses to the fundamental hypermultiplets, and show that these theories undergo a third-order phase transition at a critical value of the 't Hooft coupling. This provides the first example of a phase transition driven by a mass deformation of a $\mathcal{N}=2$ superconformal field theory. We also comment on integrated correlators in these theories.

hep-th↗

arXiv2025

Causal chiral 2-form electrodynamics

Generic nonlinear theories of chiral 2-form electrodynamics allow superluminal propagation in some stationary homogeneous backgrounds and are therefore acausal. We find a simple parameterisation of the Hamiltonian for causal theories, and we use it to show that the stress-energy tensor satisfies both the Dominant and Strong energy conditions. We also revisit the Perry-Schwarz formulation, clarifying aspects of it and of its relation to the Hamiltonian formulation.

hep-th↗

arXiv2024

Probing Hidden Dimensions via Muon Lifetime Measurements

In the context of Kaluza-Klein theories, the time dilation of charged particles in an external field depends on the charge in a specific way. Experimental tests are proposed to search for extra dimensions using this distinctive feature.

gr-qc↗

arXiv2024

Dualities of Self-Dual Nonlinear Electrodynamics

For any causal nonlinear electrodynamics theory that is "self-dual" (electromagnetic $U(1)$-duality invariant), the Legendre-dual pair of Lagrangian and Hamiltonian densities $\{\mathcal{L},\mathcal{H}\}$ are constructed from functions $\{\ell,h\}$ on ${\bf R}^+$ related to a particle-mechanics Lagrangian and Hamiltonian. We show how a `duality' relating $\ell$ to $h$ implies that $\mathcal{L}$ and $\mathcal{H}$ are related by a simple map between appropriate pairs of variables. We also discuss Born's "Legendre self-duality" and implications of a new "$Φ$-parity" duality. Our results are illustrated with many examples.

hep-th↗

arXiv2024

Strong Magnetic Limit of String Theory

We show that there exists a certain limit in type I and type II superstring theory in the presence of a suitable configuration of magnetic U(1) fields where all string excitations get an infinite mass, except for the neutral massless sector and for the boson and fermion string states lying on the leading Regge trajectory. For a supersymmetric configuration of magnetic fields in internal directions, the resulting theory after the limit is a 3+1 Lorentz invariant supersymmetric theory. Supersymmetry can be broken by introducing extra components of the magnetic field or else by finite temperature. In both cases we compute the one-loop partition function for the type I string model after taking the limit, which turns out to be different from the Yang-Mills result that arises by a direct $α'\to 0$ limit. In the case of finite temperature, no Hagedorn transition appears, in consistency with the reduction of the string spectrum. In type II superstring theory, the analogous limit is constructed by starting with a configuration of Melvin twists in two or more complex planes. The resulting theory contains gravitation plus an infinite number of states of the leading Regge trajectory.

hep-th↗

arXiv2024

Born Again

Born's original 1933 theory of nonlinear electrodynamics (in contrast to the later Born-Infeld theory) is acausal for strong fields. We explore the issue of strong-field causality violation in families of theories containing Born and/or Born-Infeld, and many variants that have been previously proposed in contexts that include cosmology and black hole physics. Many of these variants are acausal and hence unphysical. A notable exception is the modified Born-Infeld theory with ModMax as its conformal weak-field limit.

hep-th↗

arXiv2024

Causal Self-Dual Electrodynamics

Many theories of nonlinear electrodynamics (NLED) that have been proposed in physical contexts involving strong fields are causal for weak fields but acausal for strong fields. We show that for any such theory there is a unique causal and self-dual (electromagnetic duality invariant) theory with the same Lagrangian at zero magnetic field. This follows from a construction of the general causal self-dual NLED, which shows that strong-field causality is implied by weak-field causality for self-dual theories. We illustrate our results with explicit examples.

hep-th↗

arXiv2024

Handbook on string decay

We explain simple semi-classical rules to estimate the lifetime of any given highly-excited quantum state of the string spectrum in flat spacetime. We discuss both the decays by splitting into two massive states and by massless emission. As an application, we study a solution describing a rotating and pulsating ellipse which becomes folded at an instant of time -- the ``squashing ellipse''. This string interpolates between the folded string with maximum angular momentum and the pulsating circular string. We explicitly compute the quantum decay rate for the corresponding quantum state, and verify the basic rules that we propose. Finally, we give a more general (4-parameter) family of closed string solutions representing rotating and pulsating elliptical strings.

hep-th↗

arXiv2024

Causality and Energy Conditions in Nonlinear Electrodynamics

For the general theory of nonlinear electrodynamics (NLED), we prove that causality implies both the Dominant Energy Condition (DEC) and, surprisingly, the Strong Energy Condition (SEC). This has implications for gravitational applications, such as regular black holes supported by NLED matter. For self-dual NLED theories, weak-field causality alone implies both the DEC and SEC, as we illustrate with Born-Infeld and ModMax electrodynamics.

hep-th↗

arXiv2024

Hamiltonian birefringence and Born-Infeld limits

Using Hamiltonian methods, we find six relativistic theories of nonlinear electrodynamics for which plane wave perturbations about a constant uniform background are not birefringent. All have the same conformal strong-field limit to Bialynicki-Birula (BB) electrodynamics, but only four avoid superluminal propagation: Born-Infeld (BI), its non-conformal ``extreme'' limits (electric and magnetic) and the conformal BB limit. The quadratic dispersion relation of BI is shown to degenerate in the extreme limits to a pair of linear relations, which become identical in the BB limit.

hep-th↗

arXiv2024

Landau-Zener transition rates of superconducting qubits and absorption spectrum in quantum dots

New exact formulas are derived for systems involving Landau-Zener transition rates and for absorption spectra in quantum dots. These rectify previous inaccurate approximations utilized in experimental studies. The exact formulas give an explicit expression for the maxima and minima of the transition rate at any oscillating period and reveal a number of striking physical consequences, such as the suppression of oscillations for half-integer values of the detuning parameter and that the periodic dependence on the detuning parameter changes at special values of the driving field amplitude. The fluorescence spectra of quantum dots exhibit similar properties.

quant-ph↗

arXiv2023

New exact solutions in multi-scalar field cosmology

We use the method of the superpotential to derive exact solutions describing inflationary cosmologies in multi-field models. An example that describes a solution that interpolates between two de Sitter universes is described in detail. New analytical solutions for axion-dilaton cosmologies are also presented.

gr-qc↗

arXiv2023

Phantoms and strange attractors in cosmology

We study a cosmological model of gravity coupled to three, self-interacting scalar fields, one of them with negative kinetic term. The theory has cosmological solutions described by three-dimensional quadratic autonomous equations, leading to strange attractors. The associated chaotic cosmologies exhibit highly fluctuating periods of contraction and expansion, alternating with long, steady periods in a de Sitter-like phase.

hep-th↗