arXiv subjects

Fabio Taddei

Publications and source records attributed to Fabio Taddei.

At least 19 recordsLinked to original sources

arXiv2026

Antidot superlattices in two-dimensional topological insulators

We investigate the electronic properties of a two-dimensional topological insulator patterned with an array of circular holes. The band structures of the topological insulator superlattices are calculated using the Bernevig-Hughes-Zhang model, discretized with the finite element method. The band topology is studied using the Fukui-Hatsugai-Suzuki method, exploiting the fact that topological charge is conserved when a band gap closes and reopens. We find that when the holes are close to each other and the edge states overlap significantly, the material loses its topological character and becomes a trivial insulator. The results show that patterning topological insulators changes their properties, and this can be achieved with a feature size compatible with current lithographic techniques. This shows the potential to incorporate patterned topological insulators in future electronics.

cond-mat.mes-hall↗

arXiv2026

Simulating the Haldane model in ultra-clean GaAs heterostructures

The Haldane model represents the minimal lattice-based realization of a Chern insulator, exhibiting a quantized Hall conductance in the absence of Landau levels. Despite its conceptual elegance, the implementation in crystalline solids of the requisite pattern of Peierls phases breaking time-reversal symmetry remains experimentally demanding. In this work, we theoretically investigate the possibility to simulate the Haldane model in ultra-clean GaAs/AlGaAs heterostructures. Our proposal relies on recent experiments in which a high-mobility two-dimensional electron gas is subject to a gate-defined honeycomb electrostatic potential and a laterally periodic magnetic field generated by patterned ferromagnetic structures. The combined electrostatic and magnetic superlattices furnish a viable route to emulate the topological properties of the Haldane model.

cond-mat.mes-hall↗

arXiv2026

Subgap Linear Thermoelectricity in Superconducting Quantum Hall Systems

We show that an integer quantum Hall setup proximized by superconductors can exhibit subgap thermoelectric effects in the linear-response regime when triplet superconducting correlations are present. We devise a minimal setup that enables a nonzero Seebeck effect mediated by Andreev processes and predict that the corresponding Seebeck coefficient can reach values on the order of $k_B/e$ in the middle of the quantum Hall plateau. We analytically show that both triplet correlations and spin polarization are essential for the emergence of the thermoelectric effect, which arises despite the linear band dispersion of the edge states. We characterize the dependence of the thermoelectric response on the Hamiltonian parameters and the system's temperature regime.

cond-mat.mes-hall↗

arXiv2026

Persistent currents, whirlpools, and local Chern markers in twisted TMD Chern insulators

Recent materials advances have made it possible to fabricate twisted transition metal dichalcogenide homobilayers. These systems have been shown to host integer and fractional Chern insulating states. Because of spontaneous time reversal symmetry breaking, their ground state harbors intriguing spin-polarized currents with whirlpools on the moir\'e length scale that can be measured by scanning probe methods. We first provide a quantitative analysis of these persistent currents and then show that the maximum of the amplitude of the current density in the bulk of the sample is an accurate tracker of topological order. We conclude by calculating how the quantization of the Hall conductance is affected by finite-size effects.

cond-mat.mes-hall↗

arXiv2026

Current precision in interacting hybrid Normal-Superconducting systems

We study Andreev-mediated transport and current fluctuations in interacting normal-superconducting quantum-dot systems. Using a generalized master equation based on real-time diagrammatics and full counting statistics, we compute the steady-state current, zero-frequency noise, and rate of entropy production in the large superconducting-gap limit. We show how Coulomb interactions modify Andreev-mediated transport by renormalizing resonant conditions and suppressing superconducting coherence, leading to a pronounced reduction of current precision even when average currents are only weakly affected. These effects are particularly evident at high temperatures, where conventional Coulomb-blockade features are thermally smeared while fluctuation properties remain highly sensitive. By analyzing thermodynamic uncertainty relations, we demonstrate that violations of the quantum bound present in the noninteracting regime are progressively reduced and eventually suppressed as interactions increase, whereas the recently proposed hybrid bound remains satisfied. Our results clarify how Coulomb interactions, and nonequilibrium fluctuations jointly determine transport properties in hybrid superconducting devices, and establish current precision as a robust benchmark for interacting Andreev transport beyond the noninteracting limit.

cond-mat.mes-hall↗

arXiv2025

Enhanced Thermoelectricity in Nanowires with inhomogeneous Helical states

Semiconductor nanowires (NWs) with strong Rashba spin-orbit coupling (RSOC), when exposed to a suitably applied Zeeman field, exhibit one-dimensional helical channels with a spin orientation locked to the propagation direction within the magnetic energy gap. Here, by adopting a scattering-matrix approach applied to a tight-binding model of the NW, we demonstrate that the thermoelectric (TE) properties can be widely controlled by tuning the misalignment angle $\phi$ between the spin-orbit directions of two NW segments. In particular, when the RSOC vectors are antiparallel (Dirac paradox configuration) we predict a significant violation of the Wiedemann-Franz law, and a strong enhancement of the Seebeck coefficient and the $ZT$ figure of merit. We also show that the Zeeman gap determines the optimal energy window for doping and temperatures. These results suggest that controlling the spin-orbit field direction, which can be achieved with suitably applied wrap gates, is a promising alternative for tuning and optimizing the TE response in quantum-coherent semiconducting NW devices.

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arXiv2025

Enhanced thermoelectric effects in a driven one-dimensional system

We investigate the thermoelectric properties of a one-dimensional quantum system in the presence of an external driving. We employ Floquet scattering theory to calculate linear-response stationary thermoelectric figures of merit in a single-channel conductor subjected to a periodically varying delta-like potential barrier. We also include a step barrier in one of the leads as a model of a nanoscale inhomogeneous semiconducting system. In the absence of a step barrier, we found that external driving can significantly enhance the Seebeck coefficient, particularly at low temperatures, with a relative increase of up to 200% at high frequencies compared to the static case. In the presence of a step barrier, we found that the thermoelectric Onsager coefficient for the driven case is also enhanced compared to the static case, with a significant photon-assisted effect at low temperatures when the chemical potential is within the semiconductor's gap. Our results demonstrate that external driving can be used to tune and enhance the thermoelectric capabilities of low-electron-density nanodevices.

cond-mat.mes-hall↗

arXiv2025

Quantum thermodynamic uncertainty relation and macroscopic superconducting coherence

Stability and efficiency are mutually exclusive in a thermodynamic process, e.g. in a thermal machine. Any effort to reduce the fluctuations of a certain output quantity is necessarily accompanied by an increase of entropy production, therefore lowering its efficiency. This interplay is beautifully captured by the so called Thermodynamic Uncertainty Relations (TURs) which set a lower bound on the relative uncertainty of a current for a given rate of entropy production. Their status in hybrid normal-superconducting (N-S) devices has remained unsettled. We show that, in the subgap regime, departures from the normal quantum TUR are governed by {\it macroscopic} superconducting coherence quantified by the pair amplitude, and that introducing a dephasing probe suppresses this coherence and restores the bound. We further derive a hybrid quantum TUR that is general for two-terminal N-S junctions in the Andreev regime: the inequality is never violated, is saturated only at vanishing current, and is related to the normal quantum bound under the replacement (e to 2e). For N-S quantum dot and Cooper-pair-splitter systems we compute current and noise and show that deviations from the normal bound track the pair amplitude on the central region. The results establish a direct link between superconducting macroscopic coherence and nonequilibrium fluctuations and supply a general bound for the Andreev regime.

cond-mat.mes-hall↗

arXiv2025

High-performance Andreev interferometer-based electronic coolers

In this paper, we propose an electronic refrigerator based on a ballistic Andreev interferometer that allows to reach a maximum cooling power per channel up to five orders of magnitude larger than that of the conventional normal metal-insulator-superconductor cooler. This effect is achieved by exploiting the destructive interference that occurs when the superconducting phase difference equals $\pi$. This results in a strongly suppressed charge current below the superconducting gap, while still allowing the extraction of excitations above the gap, leading to a cooler with enhanced performance. Interestingly, we find that such a large cooling power per channel enables the achievement of an electronic temperature close to the theoretical lower bound. Additionally, we derive an approximate expression for this bound in the regime of low bath temperatures. Finally, we propose potential implementations of the ballistic Andreev interferometer cooler using semiconductors, graphene, and topological insulators.

cond-mat.mes-hall↗

arXiv2025

Formal Integration of Electron Scattering Processes via Separation of Dynamical and Geometric Contributions

By decoupling the geometric from the dynamical contributions in the scattering processes, we develop a method to compute the scattering matrix of electrons in a one-dimensional coherent conductor connected to two electrodes. In particular, we demonstrate that, in the high-energy regime, the transmission matrix converges to the Berry operator of the system. We showcase the method through several examples featuring different in-plane magnetic field profiles. Notably, our results reveal the possibility of achieving near-perfect spin-flip transmission, highlighting potential applications in spintronics.

quant-ph↗

arXiv2024

$\Delta T$-noise in Multiterminal Hybrid Systems

The study of charge current fluctuations (noise) can give useful insights into the properties of nanoscale systems. In this work, the peculiar properties of noise in multiterminal hybrid normal-superconducting systems are explored in the thermal out-of-equilibrium regime, i.e., when temperature biases are present ($\Delta T$-noise). Using the Landauer-B\"uttiker approach, we identify two contributions: background noise and excess noise, analyzing them when both electrical and thermal biases are applied. When temperature biases are present, and superconducting terminals are grounded, we find that the first contribution depends not only on the electrical conductance, as the Johnson-Nyquist at equilibrium, but also on a quantity strictly related to the heat conductance. This is our first main result. On the other hand, the second contribution shows, as expected, additional terms originating from the partitioning of currents into different transport channels, including the ones associated with Andreev reflection processes. However, noise induced by the temperature differences unveil also interference terms that cannot be present either in voltage bias or in the absence of any Andreev processes. Finally, we apply the results obtained to two different specific physical situations. The first is a generic three-terminal normal-superconductor-normal system, and the second is a device based on spin-resolved co-propagating chiral channels in the integer quantum Hall regime with a superconducting region. In these example setups, we investigate mainly the shot-noise regimes, when high-voltage or high-temperature biases are applied. We find remarkable differences between the two limits, which ultimately show the different nature of electrically and thermally induced charge current fluctuations.

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arXiv2024

Nonlocal thermoelectricity in quantum wires as a signature of Bogoliubov-Fermi points

We study nonlocal thermoelectricity in a superconducting wire subject to spin-orbit coupling and a magnetic field with a relative orientation $\theta$ between them. We calculate the current flowing in a normal probe attached to the bulk of a superconducting wire, as a result of a temperature difference applied at the ends of the wire. We focus on the linear response regime, corresponding to a small temperature bias. We find that the nonlocal thermoelectric response is strongly dependent on the angle $\theta$ and occurs in ranges which correspond to the emergence of Bogoliubov Fermi points in the energy spectrum of the superconducting wire.

cond-mat.mes-hall↗

arXiv2024

Heat-charge separation in a hybrid superconducting quantum Hall setup

Separating heat from charge in a material is an extremely challenging task since they are transported by the very same carriers, i.e. electrons or holes. In this Letter we show that such separation can reach 100% efficiency in a hybrid superconducting quantum Hall setup, provided that the quantum Hall system is tuned to integer filling factor. We present microscopic calculations for a three-terminal setup to illustrate our idea.

cond-mat.mes-hall↗

arXiv2024

Topological nano-switches in higher-order topological insulators

We consider multi-terminal transport through a flake of rectangular shape of a two-dimensional topological insulator in the presence of an in-plane magnetic field. This system has been shown to be a second-order topological insulator, thus exhibiting corner states at its boundaries. The position of the corner states and their decay length can be controlled by the direction of the magnetic field. In the leads we assume that the magnetic field is absent and therefore we have helical one-dimensional propagating states characteristic of the spin-Hall effect. Using a low-energy effective Hamiltonian we show analytically that, in a two-terminal setup, transport can be turned on and off by a rotation of the in-plane magnetic field. Similarly, in a three terminal configuration, the in-plane magnetic field can be used to turn on and off the transmission between neighbouring contacts, thus realising a directional switch. Analytical calculations are supplemented by a numerical finite-difference method. For small values of the Fermi energy and field strength, the analytical results agree exceptionally well with the numerics. The effect of disorder is also addressed in the numerical approach. We find that the switching functionality is remarkably robust to the presence of strong disorder stemming from the topological nature of the states contributing to the electron transport.

cond-mat.mes-hall↗

arXiv2023

Giant chirality-induced spin polarization in twisted transition metal dichalcogenides

Chirality-induced spin selectivity (CISS) is an effect that has recently attracted a great deal of attention in chiral chemistry and that remains to be understood. In the CISS effect, electrons passing through chiral molecules acquire a large degree of spin polarization. In this work we study the case of atomically-thin chiral crystals created by van der Waals assembly. We show that this effect can be spectacularly large in systems containing just two monolayers, provided they are spin-orbit coupled. Its origin stems from the combined effects of structural chirality and spin-flipping spin-orbit coupling. We present detailed calculations for twisted homobilayer transition metal dichalcogenides, showing that the chirality-induced spin polarization can be giant, e.g. easily exceeding $50\%$ for ${\rm MoTe}_2$. Our results clearly indicate that twisted quantum materials can operate as a fully tunable platform for the study and control of the CISS effect in condensed matter physics and chiral chemistry.

cond-mat.mes-hall↗

arXiv2023

Corner states of two-dimensional second-order topological insulators with a chiral symmetry and broken time reversal and charge conjugation

Two-dimensional second-order topological insulators are characterized by the presence of topologically protected zero-energy bound states localized at the corners of a flake. In this paper we theoretically study the occurrence and features of such corner states inside flakes in the shape of a convex polygon. We consider two different models, both in Cartan class IIIA, the first obeying inversion symmetry and the other obeying a combined $π/4$ rotation symmetry and time-reversal symmetry ($\hat{C}_4^z\hat{T}$). By using an analytical effective model of an edge corresponding to a massive Dirac fermion, we determine the presence of a corner state between two given edges by studying the sign of their induced masses and derive general rules for flakes in the shape of a convex polygon. In particular, we find that the number of corner states in a flake is always two in the first model, while in the second model there are either 0, 2 or 4. To corroborate our findings, we focus on flakes of specific shapes (a triangle and a square) and use a numerical finite-difference approach to determine the features of the corner states in terms of their probability density. In the case of a triangular flake, we can change the position of corner states by rotating the flake in the first model, while in the second model we can also change their number. Remarkably, when the induced mass of an edge is zero the corresponding corner state becomes delocalized along the edge. In the case of a square flake and the model with $\hat{C}_4^z\hat{T}$ symmetry, there is an orientation of the flake with respect to the crystal axes, for which the corner states extend along the whole perimeter of the square.

cond-mat.mes-hall↗

arXiv2023

Thermodynamic uncertainty relations for systems with broken time reversal symmetry: the case of superconducting hybrid systems

We derive bounds to the thermodynamic uncertainty relations in the linear-response regime for steady-state transport in two-terminal systems when time reversal symmetry is broken. We find that such bounds are different for charge and heat currents and depend on the details of the system, through the Onsager coefficients, and on the ratio between applied voltage and temperature difference. As a function of such a ratio, the bounds can take any positive values. The bounds are then calculated for a hybrid coherent superconducting system using the scattering approach, and the concrete case of an Andreev interferometer is explored. Interestingly, we find that the bound on the charge current is always smaller than 2 when the system operates as a heat engine, while the bound on the heat current is always larger than 2 when the system operates as a refrigerator.

cond-mat.mes-hall↗

arXiv2023

Topological Josephson Junctions in the Integer Quantum Hall Regime

Robust and tunable topological Josephson junctions (TJJs) are highly desirable platforms for investigating the anomalous Josephson effect and topological quantum computation applications. Experimental demonstrations have been done in hybrid superconducting-two dimensional topological insulator (2DTI) platforms, sensitive to magnetic disorder and interactions with phonons and other electrons. In this work, we propose a robust and electrostatically tunable TJJ by combining the physics of the integer quantum Hall (IQH) regime and of superconductors. We provide analytical insights about the corresponding Andreev bound state spectrum, the Josephson current and the anomalous current. We demonstrate the existence of protected zero-energy crossings, that can be controlled through electrostatic external gates. This electrostatic tunability has a direct advantage to compensate for non-ideal interfaces and undesirable reflections that may occur in any realistic samples. TJJs in the IQH regime could be realized in graphene and other 2D materials. They are of particular relevance towards scalable and robust Andreev-qubit platforms, and also for efficient phase batteries.

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