arXiv subjects

Francesco M. D. Pellegrino

Publications and source records attributed to Francesco M. D. Pellegrino.

18 recordsLinked to original sources

arXiv2026

Angular Momentum Transfer in Magnetic Weyl Semimetal Spheres

In this work, we investigate the problem of electromagnetic angular momentum within the framework of topological materials, described by axion electrodynamics. Specifically, we calculate the electromagnetic angular momentum for a spherical sample of magnetic Weyl semimetal in the presence of a point charge. We then analyze how, by moving the point charge quasi-statically, the angular momentum stored in the electromagnetic field can be converted into mechanical angular momentum of the sphere. Finally, we derive analytical expressions for the resulting angular velocity and angular displacement, demonstrating that the effect is enhanced in the Weyl semimetal compared with a topological insulator.

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arXiv2026

Interplay between photon condensation and electron-electron interactions in molecular systems

We investigate a minimal molecular model consisting of square planar plaquettes hosting multiple electrons, whose dynamics is governed by a tight-binding Hamiltonian supplemented by on-site Hubbard repulsion. By coupling this system to a spatially nonuniform cavity mode, we analyze the emergence of a magnetostatic instability, namely photon condensation, originating from the paramagnetic Van Vleck mechanism. The global behavior of the system is analyzed for different electronic filling factors, and we find that, except for the special cases of half-filling and single electron, where the transition, if it occurs, is necessarily a second order phase transition, the global system may also undergo a first order transition because of the action of the electron-electron interaction. The polaritonic excitation energies are analyzed, providing clear spectroscopic signatures of the magnetostatic instability and of its order.

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arXiv2025

Localized surface plasmons in a Weyl semimetal nanosphere

In this study, we investigate the localized surface plasmon modes of a sub-wavelength spherical nanoparticle composed of a Weyl semimetal, taking into account the axion modification of electrodynamics. We derive analytical solutions for dipole and quadrupole normal modes by employing the quasistatic approximation. The axion term leads to modified Fröhlich conditions, resulting in multiple non-degenerate plasmonic resonances with distinct polarization dependencies. In contrast to isotropic conventional metals, the magnetoelectric properties of Weyl semimetals enable an incident electromagnetic field, with the electric field transverse to the surface of the sphere, to excite a localized surface plasmon.

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arXiv2025

Current phase relation in a planar graphene Josephson junction with spin-orbit coupling

We study a graphene Josephson junction where the inner graphene layer is subjected to spin-orbit coupling by proximity effect. This could be achieved, for example, by growing the graphene layer on top of a transition metal dichalcogenide, such as WS$_2$. Here, we focus on the ballistic, wide, and short junction limits and study the effects of the spin-orbit interaction on the supercurrent. In particular, we analyze the current phase relation using an analytical approach based on the continuum model. We find combinations of types of spin-orbit coupling that significantly suppress the supercurrent by opening a gap in the graphene band structure. At the same time, other combinations enhance it, acting as an effective spin-valley resolved chemical potential. Moreover, we find that a strong Rashba spin-orbit coupling leads to a junction with a highly voltage tunable harmonic content.

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arXiv2025

Local analysis of a single impurity on a graphene Josephson Junction

In this work, we investigate the local effects of a single short-range impurity on the electron system of a short ballistic graphene Josephson Junction. Within the Dirac-Bogoliubov-De Gennes approach, we systematically analyze the local density states, whose subgap energy dependence enables us to distinguish between elastic and inelastic scattering processes and identify the magnetic nature of the impurity. Furthermore, we observe that the spatial dependence of the local density of states is a sensitive probe of the microscopic processes resulting in subgap impurity-induced bound states. The Fourier analysis evidences the wavevectors related to the momenta of the high transmissive channels in ballistic graphene.

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arXiv2024

Photon condensation, Van Vleck paramagnetism, and chiral cavities

We introduce a gauge-invariant model of planar, square molecules coupled to a quantized spatially-varying cavity electromagnetic vector potential A(r). Specifically, we choose a temporally chiral cavity hosting a uniform magnetic field B, as this is the simplest instance in which a transverse spatially-varying A(r) is at play. We show that when the molecules are in the Van Vleck paramagnetic regime, an equilibrium quantum phase transition to a photon condensate state occurs.

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arXiv2023

Second spectrum of charge carrier density fluctuations in graphene due to trapping/detrapping processes

We investigate the second spectrum of charge carrier density fluctuations in graphene within the McWorther model, where noise is induced by electron traps in the substrate. Within this simple picture, we obtain a closed-form expression including both Gaussian and non-Gaussian fluctuations. We show that a very extended distribution of switching rates of the electron traps in the substrate leads to a carrier density power spectrum with a non-trivial structure on the scale of the measurement bandwidth. This explains the appearance of a $1/f$ component in the Gaussian part of the second spectrum, which adds up to the expected frequency-independent term. Finally, we find that the non-Gaussian part of the second spectrum can become quantitatively relevant by approaching extremely low temperatures.

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arXiv2022

Effect of dilute impurities on short graphene Josephson junctions

We study the effect of a dilute homogeneous spatial distribution of non-magnetic impurities on the equilibrium supercurrent sustained by a ballistic graphene Josephson junction in the short junction limit. Within the Dirac-Bogoliubov-de Gennes approach and modeling impurities by the Anderson model we derive the supercurrent and its equilibrium power spectrum. We find a modification of the current-phase relation with a reduction of the skewness induced by disorder, and a nonmonotonic temperature dependence of the critical current. The potentialities of the supercurrent power spectrum for accurate spectroscopy of the hybridized Andreev bound states-impurities spectrum are highlighted. In the low temperature limit, the supercurrent zero frequency thermal noise directly probes the spectral function at the Fermi energy.

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arXiv2020

Charge carrier density noise in graphene: effect of localized/delocalized traps

Graphene-based devices show $1/f$ low-frequency noise in several electronic transport properties, such as mobility and charge carrier concentration. The recent outburst of experimental studies on graphene-based devices integrated into circuit quantum electrodynamics systems has rekindled the interest in low-frequency charge noise. We investigate charge carrier density noise in graphene within the McWorther model where noise is induced by electron traps in the substrate. We focus on the large doping regime and introduce a simple modelization of the effect of localized/delocalized traps in terms of single/double spin occupancy of trap states. We find that in both cases the charge carrier spectrum of graphene obeys the $1/f^α$ power-law behavior where $α$ is very close to the unity, and for each case we evaluate the deviation $β=1-α$. The amplitude of the noise is found to depend on the trap energy distribution and on temperature. Single/double spin occupancy of trap states influences the temperature dependence of noise amplitude only to second order.

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arXiv2020

Low-frequency critical current noise in graphene Josephson junctions in the open-circuit gate voltage limit

We investigate critical current noise in short ballistic graphene Josephson junctions in the open-circuit gate-voltage limit within the McWorther model. We find flicker noise in a wide frequency range and discuss the temperature dependence of the noise amplitude as a function of the doping level. At the charge neutrality point we find a singular temperature dependence $T^{-3}$, strikingly different from the linear dependence expected for short ballistic graphene Josephson junctions under fixed gate voltage.

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arXiv2019

1/f critical current noise in short ballistic graphene Josephson junctions

Short ballistic graphene Josephson junctions sustain superconducting current with a non-sinusoidal current-phase relation up to a critical current threshold. The current-phase relation, arising from proximitized superconductivity, is gate-voltage tunable and exhibits peculiar skewness observed in high quality graphene superconductors heterostructures with clean interfaces. These properties make graphene Josephson junctions promising sensitive quantum probes of microscopic fluctuations underlying transport in two-dimensions. We show that the power spectrum of the critical current fluctuations has a characteristic $1/f$ dependence on frequency, $f$, probing two points and higher correlations of carrier density fluctuations of the graphene channel induced by carrier traps in the nearby substrate. Tunability with the Fermi level, close to and far from the charge neutrality point, and temperature dependence of the noise amplitude are clear fingerprints of the underlying material-inherent processes. Our results suggest a roadmap for the analysis of decoherence sources in the implementation of coherent devices by hybrid nanostructures.

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arXiv2019

Magnetic hallmarks of viscous electron flow in graphene

We propose a protocol to identify spatial hallmarks of viscous electron flow in graphene and other two-dimensional viscous electron fluids. We predict that the profile of the magnetic field generated by hydrodynamic electron currents flowing in confined geometries displays unambiguous features linked to whirlpools and backflow near current injectors. We also show that the same profile sheds light on the nature of the boundary conditions describing friction exerted on the electron fluid by the edges of the sample. Our predictions are within reach of vector magnetometry based on nitrogen-vacancy centers embedded in a diamond slab mounted onto a graphene layer.

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arXiv2018

Quantum non-local theory of topological Fermi arc plasmons in Weyl semimetals

The surface of a Weyl semimetal (WSM) displays Fermi arcs, i.e. disjoint segments of a two-dimensional Fermi contour. We present a quantum-mechanical non-local theory of chiral Fermi arc plasmons in WSMs with broken time-reversal symmetry. These are collective excitations constructed from topological Fermi arc and bulk electron states and arising from electron-electron interactions, which are treated in the realm of the random phase approximation. Our theory includes quantum effects associated with the penetration of the Fermi arc surface states into the bulk and dissipation, which is intrinsically non-local in nature and arises from decay processes mainly involving bulk electron-hole pair excitations.

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arXiv2017

Non-local transport and the Hall viscosity of 2D hydrodynamic electron liquids

In a fluid subject to a magnetic field the viscous stress tensor has a dissipationless antisymmetric component controlled by the so-called Hall viscosity. We here propose an all-electrical scheme that allows a determination of the Hall viscosity of a two-dimensional electron liquid in a solid-state device.

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arXiv2016

Modulated phases of graphene quantum Hall polariton fluids

There is growing experimental interest in coupling cavity photons to the cyclotron resonance excitations of electron liquids in high-mobility semiconductor quantum wells or graphene sheets. These media offer unique platforms to carry out fundamental studies of exciton-polariton condensation and cavity quantum electrodynamics in a regime in which electron-electron interactions are expected to play a pivotal role. Focusing on graphene, we present a theoretical study of the impact of electron-electron interactions on a quantum Hall polariton fluid, that is a fluid of magneto-excitons resonantly coupled to cavity photons. We show that electron-electron interactions are responsible for an instability of graphene integer quantum Hall polariton fluids towards a modulated phase. We demonstrate that this phase can be detected by measuring the collective excitation spectra, which soften at a characteristic wave vector of the order of the inverse magnetic length.

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arXiv2016

Electron hydrodynamics dilemma: whirlpools or no whirlpools

In highly viscous electron systems such as, for example, high quality graphene above liquid nitrogen temperature, a linear response to applied electric current becomes essentially nonlocal, which can give rise to a number of new and counterintuitive phenomena including negative nonlocal resistance and current whirlpools. It has also been shown that, although both effects originate from high electron viscosity, a negative voltage drop does not principally require current backflow. In this work, we study the role of geometry on viscous flow and show that confinement effects and relative positions of injector and collector contacts play a pivotal role in the occurrence of whirlpools. Certain geometries may exhibit backflow at arbitrarily small values of the electron viscosity, whereas others require a specific threshold value for whirlpools to emerge.

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arXiv2015

Helicons in Weyl semimetals

Helicons are transverse electromagnetic waves propagating in three-dimensional (3D) electron systems subject to a static magnetic field. We present a theory of helicons propagating through a 3D Weyl semimetal. Our approach relies on the evaluation of the optical conductivity tensor from semiclassical Boltzmann transport theory, with the inclusion of certain Berry curvature corrections that have been neglected in the earlier literature (such as the one due to the orbital magnetic moment). We demonstrate that the axion term characterizing the electromagnetic response of Weyl semimetals dramatically alters the helicon dispersion with respect to that in nontopological metals. We also discuss axion-related anomalies that appear in the plasmon dispersion relation.

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arXiv2014

Spin-resolved optical conductivity of two-dimensional group-VIB transition-metal dichalcogenides

We present an ab-initio study of the spin-resolved optical conductivity of two-dimensional (2D) group-VIB transition-metal dichalcogenides (TMDs). We carry out fully-relativistic density-functional-theory calculations combined with maximally localized Wannier functions to obtain band manifolds at extremely high resolutions and focus on the photo-response of 2D TMDs to circularly-polarized light in a wide frequency range. We present extensive numerical results for monolayer TMDs involving molybdenum and tungsten combined with sulphur and selenium. Our numerical approach allows us to locate with a high degree of accuracy the positions of the points in the Brillouin zone that are responsible for van Hove singularities in the optical response. Surprisingly, some of the saddle points do not occur exactly along high-symmetry directions in the Brillouin zone, although they happen to be in their close proximity.

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