arXiv subjects

Alessandro De Martino

Publications and source records attributed to Alessandro De Martino.

At least 19 recordsLinked to original sources

arXiv2026

Channel-selective magnetic filtering in a nodal-line semimetal

We study quantum transport through a magnetic barrier in a nodal-line semimetal. When the Fermi energy lies near the nodal ring, the Fermi surface has toroidal geometry. Each cross-section in a plane parallel to the nodal ring consists of two concentric contours, inner and outer, carrying distinct transport channels. We show that a magnetic barrier resolves these two channels: because the contours enclose different momentum-space areas, they accommodate the field-induced transverse-momentum shift unequally, and the inner channel is cut off at a weaker barrier strength than the outer. Using a two-band effective Hamiltonian and a wave-function matching approach, we obtain closed-form, channel-resolved transmission amplitudes. Over a finite window of barrier strength the inner contour is fully blocked while the outer still transmits, so the barrier acts as a channel-selective filter. This sequential quenching shapes the two-terminal conductance, which decreases with barrier strength as the two channels close in turn and terminates once the outer channel is cut off, providing experimentally accessible fingerprints of the toroidal Fermi surface of a nodal-line semimetal.

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arXiv2026

Toroidal Fermi-surface geometry and phonon-limited transport in nodal-line semimetals

Nodal-line semimetals (NLSs) can display unconventional quasiparticle dynamics and charge transport properties due to their extended band degeneracy and the peculiar geometry of their Fermi surface. We consider electron-acoustic phonon scattering as the dominant relaxation mechanism and compute the quasiparticle decay rate and dc conductivity by solving the linearized semiclassical Boltzmann equation in a minimal model of a doped circular NLS. We find that the toroidal geometry of the Fermi surface gives rise to two parametrically distinct Bloch-Grüneisen temperatures, associated with momentum transfers along the poloidal and toroidal directions, respectively. As a result, an intermediate temperature window opens between these two scales, in which the decay rate follows $Γ\propto T^2$, while the conductivity follows $σ\propto T^{-2}$. We also obtain the low- and high-temperature asymptotic behaviors, and discuss implications for ARPES and transport measurements in candidate NLS materials.

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arXiv2025

Angular-momentum-selective nanofocusing with Weyl semimetals

We investigate the theory of surface plasmon polaritons on a magnetic Weyl semimetal conical tip. We show that the axion term in the effective electrodynamics modifies the surface plasmon polariton dispersion relation and allows all modes with a given sign of the orbital angular momentum to be focused at the end of the tip. This is in contrast with normal metals, in which only one mode can reach the end. We discuss how this orbital angular momentum nanofocusing expands the potential of technologies that use this degree of freedom.

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arXiv2025

Nonreciprocal Weyl semimetal waveguide

We study a cylindrical plasmonic waveguide consisting of a magnetic Weyl semimetal embedded in a dielectric medium. We determine the dispersion relation of the surface plasmon polaritons and show how it depends on the plasma frequency, the radius of the semimetal and the separation between the nodes. We show that the band structure, which modifies the electrodynamics in the medium, manifests itself through a pronounced asymmetry in the dispersion curves and a giant splitting in the group velocity, with the orbital angular momentum as a control parameter for the direction of propagation.

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arXiv2024

Quantum description of Fermi arcs in Weyl semimetals in a magnetic field

For a Weyl semimetal (WSM) in a magnetic field, a semiclassical description of the Fermi-arc surface state dynamics is usually employed for explaining various unconventional magnetotransport phenomena, e.g., Weyl orbits, the three-dimensional Quantum Hall Effect, and the high transmission through twisted WSM interfaces. For a half-space geometry, we determine the low-energy quantum eigenstates for a four-band model of a WSM in a magnetic field perpendicular to the surface. The eigenstates correspond to in- and out-going chiral Landau level (LL) states, propagating (anti-)parallel to the field direction near different Weyl nodes, which are coupled by evanescent surface-state contributions generated by all other LLs. These replace the Fermi arc in a magnetic field. Computing the phase shift accumulated between in- and out-going chiral LL states, we compare our quantum-mechanical results to semiclassical predictions. We find quantitative agreement between both approaches.

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arXiv2024

Interfaces of nodal-line semimetals: drum states, transport and refraction

We study transport through interfaces in topological nodal-line semimetals, focusing on two geometries: a single interface between two large samples, one nodal-line semimetal and one metal, and an infinite nodal-line semimetal slab in between two metallic regions. We investigate the dependence of the spectra on the boundary conditions, showing how they affect the surface states and the band dispersion. We find a set of drum states, arising from the hybridization of the drumhead states on opposite surfaces at finite slab width, and describe their signatures in the transport properties of a clean sample. Finally, we compute the electronic trajectories in the ballistic regime and show that there is a series of resonant angles that ensure perfect transmission. We also show how the current density profile acquires an inhomogeneous distribution in the radial direction.

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arXiv2024

Dispersive Drumhead States in Nodal-Line Semimetal Junctions

We consider a smooth interface between a topological nodal-line semimetal and a topologically trivial insulator (e.g., the vacuum) or another semimetal with a nodal ring of different radius. Using a low-energy effective Hamiltonian including only the two crossing bands, we show that these junctions accommodate a two-dimensional zero-energy level and a set of two-dimensional dispersive bands, corresponding to states localized at the interface. We characterize the spectrum, identifying the parameter ranges in which these states are present, and highlight the role of the nodal radius and the smoothness of the interface. We also suggest material-independent ways to detect and identify these states, using optical conductivity and infrared absorption spectroscopy in magnetic field.

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arXiv2023

Chiral spin channels in curved graphene $pn$ junctions

We show that the chiral modes in circular graphene $pn$ junctions provide an advantage for spin manipulation via spin-orbit coupling compared to semiconductor platforms. We derive the effective Hamiltonian for the spin dynamics of the junction's zero modes and calculate their quantum phases. We find a sweet spot in parameter space where the spin is fully in-plane and radially polarized for a given junction polarity. This represents a shortcut to singular spin configurations that would otherwise require spin-orbit coupling strengths beyond experimental reach.

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arXiv2023

Two-channel Kondo problem in coupled interacting helical liquids

We study the two-channel Kondo problem in the context of two interacting helical liquids coupled to a spin-$\frac12$ magnetic impurity. We show that the interactions between the two helical liquids significantly affect the phase diagram and other observable properties. Using a multichannel Luttinger liquid formalism, we analyze both the Toulouse limit, where an exact solution is available, and the weak coupling limit, which can be studied via a perturbative renormalization group (RG) approach. We recover the results for the `decoupled' limit (interactions between the helical liquids switched off) and point out deviations from the known results due to this coupling. The model under study is mapped to a model of two effectively decoupled helical liquids coupled to an impurity. The perturbative RG study shows that each of these channels can flow to either a Ferromagnetic (FM) or an Anti-Ferromagnetic (AFM) fixed point. We obtain the phase diagram of the coupled system as a function of the system parameters. The observable consequences of the interaction between the two channels are captured using linear response theory. We compute the negative correction to the conductance due to the Kondo scattering processes and show how it scales with the temperature as a function of inter-channel interaction.

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arXiv2022

Transport, refraction and interface arcs in junctions of Weyl semimetals

We study the low-energy single-electron transport across a junction of two magnetic Weyl semimetals, in which the anisotropy axes are tilted one respect to the other. Using a two-band model with a potential step, we compute the transmission factor for normal and Klein tunneling and the refraction properties of the interface as a function of the tilt angle. We show that the interface acts as a beam splitter, separating electrons with different chiralities. We also characterize interface states, only appearing for finite tilt angle, which connect the projection of the Fermi surfaces on the two sides of the junction, and we discuss transport effects due to their interplay with Fermi arcs.

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arXiv2022

Phonon-limited Transport and Fermi Arc Lifetime in Weyl Semimetals

Weyl semimetals harbor topological Fermi-arc surface states which determine the nontrivial charge current response to external fields. We here study the quasiparticle decay rate of Fermi arc states arising from their coupling to acoustic phonons, as well as the phonon-limited conductivity tensor for a clean Weyl semimetal slab. Using the phonon modes for an isotropic elastic continuum with a deformation potential coupling to electrons, we determine the temperature dependence of the quasiparticle decay rate, both near and far away from the arc termination points. By solving the coupled Boltzmann equations for the bulk and arc state distribution functions in the slab geometry, we show how the linear response conductivity depends on key parameters such as the temperature, the chemical potential, the geometric shape of the Fermi arcs, or the slab width. The chiral nature of Fermi arc states causes an enhancement of the longitudinal conductivity along the chiral direction at low temperatures, together with a 1/T^2 scaling regime at intermediate temperatures without counterpart for the conductivity along the perpendicular direction.

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arXiv2021

Phonon-induced magnetoresistivity of Weyl semimetal nanowires

We study longitudinal magnetotransport in disorder-free cylindrical Weyl semimetal nanowires. Our theory includes a magnetic flux $Φ$ piercing the nanowire and captures the finite curvature of the Fermi arc in the surface Brillouin zone through a boundary angle $α$. Electron backscattering by acoustic phonons via the deformation potential causes a finite resistivity which we evaluate by means of the semiclassical Boltzmann approach. We find that low-energy transport is dominated by surface states, where transport observables are highly sensitive to the angle $α$ and to Aharonov-Bohm phases due to $Φ$. A generic subband dispersion relation allows for either one or two pairs of Fermi points. In the latter case, intra-node backscattering is possible and implies a parametrically larger resistivity than for a single Fermi point pair. As a consequence, large and abrupt resistivity changes take place across the transition points separating parameter regions with a different number of Fermi point pairs in a given subband.

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arXiv2020

Volkov-Pankratov states in topological graphene nanoribbons

In topological systems, a modulation in the gap onset near interfaces can lead to the appearance of massive edge states, as were first described by Volkov and Pankratov. In this work, we study graphene nanoribbons in the presence of intrinsic spin-orbit coupling smoothly modulated near the system edges. We show that this space modulation leads to the appearance of Volkov-Pankratov states, in addition to the topologically protected ones. We obtain this result by means of two complementary methods, one based on the effective low-energy Dirac equation description and the other on a fully numerical tight-binding approach, finding excellent agreement between the two. We then show how transport measurements might reveal the presence of Volkov-Pankratov states, and discuss possible graphene-like structures in which such states might be observed.

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arXiv2020

Phase diagram and phonon-induced backscattering in topological insulator nanowires

We present an effective low-energy theory of electron-phonon coupling effects for clean cylindrical topological insulator nanowires. Acoustic phonons are modelled by isotropic elastic continuum theory with stress-free boundary conditions. We take into account the deformation potential coupling between phonons and helical surface Dirac fermions, and also include electron-electron interactions within the bosonization approach. For half-integer values of the magnetic flux $Φ_B$ along the wire, the low-energy theory admits an exact solution since a topological protection mechanism then rules out phonon-induced $2k_F$-backscattering processes. We determine the zero-temperature phase diagram and identify a regime dominated by superconducting pairing of surface states. As example, we consider the phase diagram of HgTe nanowires. We also determine the phonon-induced electrical resistivity, where we find a quadratic dependence on the flux deviation $δΦ_B$ from the nearest half-integer value.

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arXiv2019

Spin-orbit interaction and snake states in graphene $p$-$n$ junctions

We study a model of a $p$-$n$ junction in single-layer graphene in the presence of a perpendicular magnetic field and spin-orbit interactions. By solving the relevant quantum-mechanical problem for a potential step, we determine the exact spectrum of spin-resolved dispersive Landau levels. Close to zero energy, we find a pair of linearly dispersing zero modes, which possess a wave-vector-dependent spin polarization and can be regarded as quantum analogous of spinful snake states. We show that the Rashba spin-orbit interaction, in particular, produces a wave vector shift between the dispersions of these modes with observable interference effects. These effects can in principle provide a way to detect the presence of Rashba spin-orbit interaction and measure its strength. Our results suggest that a graphene $p$-$n$ junction in the presence of strong spin-orbit interaction could be used as a building block in a spin field-effect transistor.

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arXiv2019

Superconductivity from piezoelectric interactions in Weyl semimetals

We present an analytical low-energy theory of piezoelectric electron-phonon interactions in undoped Weyl semimetals, taking into account also Coulomb interactions. We show that piezoelectric interactions generate a long-range attractive potential between Weyl fermions. This potential comes with a characteristic angular anisotropy. From the one-loop renormalization group approach and a mean-field analysis, we predict that superconducting phases with either conventional s-wave singlet pairing or nodal-line triplet pairing could be realized for sufficiently strong piezoelectric coupling. For small couplings, we show that the quasi-particle decay rate exhibits a linear temperature dependence where the prefactor vanishes only in a logarithmic manner as the quasi-particle energy approaches the Weyl point. For practical estimates, we consider the Weyl semimetal TaAs.

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arXiv2011

Magnetic superlattice and finite-energy Dirac points in graphene

We study the band structure of graphene's Dirac-Weyl quasi-particles in a one-dimensional magnetic superlattice formed by a periodic sequence of alternating magnetic barriers. The spectrum and the nature of the states strongly depend on the conserved longitudinal momentum and on the barrier width. At the center of the superlattice Brillouin zone we find new Dirac points at finite energies where the dispersion is highly anisotropic, in contrast to the dispersion close to the neutrality point which remains isotropic. This finding suggests the possibility of collimating Dirac-Weyl quasi-particles by tuning the doping.

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arXiv2009

Low-energy theory and RKKY interaction for interacting quantum wires with Rashba spin-orbit coupling

We present the effective low-energy theory for interacting 1D quantum wires subject to Rashba spin-orbit coupling. Under a one-loop renormalization group scheme including all allowed interaction processes for not too weak Rashba coupling, we show that electron-electron backscattering is an irrelevant perturbation. Therefore no gap arises and electronic transport is described by a modified Luttinger liquid theory. As an application of the theory, we discuss the RKKY interaction between two magnetic impurities. Interactions are shown to induce a slower power-law decay of the RKKY range function than the usual 1D noninteracting $\cos(2k_F x)/|x|$ law. Moreover, in the noninteracting Rashba wire, the spin-orbit coupling causes a twisted (anisotropic) range function with several different spatial oscillation periods. In the interacting case, we show that one special oscillation period leads to the slowest decay, and therefore dominates the RKKY interaction for large separation.

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