arXiv subjects

Giorgio Frangi

Publications and source records attributed to Giorgio Frangi.

5 recordsLinked to original sources

arXiv2025

The probe limit in MHD and its implications for magnetic transport

Many phenomenological and effective field-theoretical (EFT) applications of magnetohydrodynamics (MHD) in the presence of a background magnetic field employ a simplifying assumption whereby the electromagnetic and the energy-momentum fluctuations decouple. In studies of magnetic transport, for example in magnetic diffusion, the conservation of energy and momentum is then neglected. In this paper, we investigate the details and the consistency of this so-called $\textit{probe limit}$ in different parametric regimes of MHD plasmas. In the first part of the paper, our discussion explores the hydrodynamic (higher-form) theory of MHD. In the second part, we then explicitly test the probe limit by using a microscopic holographic (AdS/CFT) model of a strongly coupled plasma. In the process, we develop the holographic Schwinger-Keldysh EFT prescription for describing the bulk 2-form fields and their dual 1-form symmetries. Moreover, we find evidence of a phase transition at low temperatures and show that magnetic Hall transport can emerge as a consequence of background charge density that breaks the charge conjugation symmetry of the state. Finally, we discuss the implications for magnetic transport, with a particular view towards the dynamics of dense nuclear matter in neutron stars.

hep-th↗

arXiv2025

Wilsonian renormalisation group and thermal field theory in the Schwinger-Keldysh closed-time-path formalism

We study perturbative Wilsonian renormalisation group (RG) for the scalar $\phi^4$ theory at finite temperature to one loop order in the Schwinger-Keldysh closed-time-path (CTP) formalism. By explicitly integrating out the UV modes, we show how effective interactions coupling the two branches of the CTP contour arise. This provides a microscopic derivation of the type of effective CTP actions used in the literature. While the full effective theory has no critical points, we instead proceed to study critical properties of the RG flow in a reduced space of complex couplings that characterise certain time-ordered correlators and show the existence of two novel fixed points in $d=4$ spacetime dimensions. We relate one to the Wilson-Fisher fixed point known to exist only in $4-\epsilon$ dimensions, and the other to the standard Gaussian fixed point.

hep-th↗

arXiv2024

Quantum origin of Ohm's reciprocity relation and its violation: conductivity as inverse resistivity

Conventional wisdom teaches us that the electrical conductivity in a material is the inverse of its resistivity. In this work, we show that when both of these transport coefficients are defined in linear response through the Kubo formulae as two-point correlators of conserved currents in quantum field theory, this Ohm's reciprocity relation is generically violated in theories with dynamical electromagnetism. We then elucidate how in certain special limits (e.g., in the DC limit in the presence of thermal effects, in certain 2+1$d$ conformal theories, and in holographic supersymmetric theories) the reciprocity relation is reinstated as an emergent property of conductive and resistive transport. We also show that if the response of a material is measured with respect to the total electric field that includes quantum corrections, then the reciprocity relation is satisfied by definition. However, in that case, the transport coefficients are given by the photon vacuum polarisation and not the correlators of conserved currents that dominate the hydrodynamic macroscopic late-time transport.

hep-th↗

arXiv2024

Geometrisation of Ohm's reciprocity relation in a holographic plasma

It has been recently pointed out that the familiar reciprocity relation between the conductivity $\sigma$ and resistivity $\rho$, which I refer to as $\textit{Ohm's reciprocity relation}$, should not be expected to hold in all possible settings, but is rather a property that may (or may not) emerge as a consequence of specific features, or in certain limits of interest, of a given theory. In this work I prove an analogous statement: $\rho= \sigma^{-1}$, across two different classes of holographic theories related by a generalisation of the electric-magnetic duality in the $D=4+1$ bulk. In terms of the dual hydrodynamic theories, this statement is shown to imply the suppression of any contributions to the transport coefficients from dynamical electromagnetic fields, present in only one of the two theories. This makes the two theories, as far as late-time linear electric transport is concerned, equivalent. I then confirm these findings by considering one specific model and run numerical simulations in different settings.

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arXiv2022

Holographic Supersolids

A supersolid is a system that presents long-range order and shear rigidity as a solid but which also supports a non-dissipative superflow as a superfluid. From an effective perspective, supersolids are identified with phases of matter that break spontaneously translational invariance together with a global $U(1)$ symmetry. By using this symmetry prescription, we build a holographic bottom-up model for supersolids and we start the investigation of its thermodynamic and mechanical properties. More precisely, we analyze the behaviour of the critical temperature, the condensate, the shear modulus and the viscosity across all the phase diagram. Finally, we successfully compare our results with a simple Ginzburg-Landau model for supersolids deriving some universal physical correlations between the observables mentioned above.

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