arXiv subjects

Alessandro Principi

Publications and source records attributed to Alessandro Principi.

At least 37 records · Page 2Linked to original sources

arXiv2025

Broken Lorentz symmetry and violation of the Wiedemann-Franz law in topological insulators

We study a two-dimensional topological insulator in the presence of a static non-uniform gravitational field, which mimics the variations in the temperature distribution. We derive an effective boundary free energy functional for the gravitational field and show that, in contrast to the case of massive Dirac fermions, the addition of a Newtonian mass term significantly modifies the quantum anomalous behavior of the system. A non-zero bulk thermal current appears, which violates the Wiedemann-Franz law. The systematic approach we develop to calculate the contribution of edge states to thermal Hall conductivity and energy magnetization can easily be extended to other models.

cond-mat.mes-hall↗

arXiv2024

Nonconserved Density Accumulations in Orbital Hall Transport: Insights from Linear Response Theory

We present a linear response theory for stationary density accumulations in anomalous transport phenomena, such as the orbital Hall effect, where the transported density is odd under time reversal and the underlying charge is not conserved. Our framework applies to both metals and insulators, topologically trivial or nontrivial, and distinguishes between contributions from bulk and edge states, as well as undergap and dissipative currents. In time-reversal invariant systems, we prove a microscopic reciprocity theorem showing that only dissipative currents at the Fermi level contribute to density accumulation, while undergap currents do not. In contrast, in non-time-reversal invariant systems, non-dissipative density accumulations, such as magnetoelectric polarization, can appear in both the bulk and edges. Importantly, we find that the net density accumulation does not always vanish, pointing to a global non-conservation that implies the existence of a non-vanishing integrated ``net torque'' in addition to a ``distributed torque'', which has zero spatial average. We show that the distributed torque can be absorbed in the divergence of a redefined current that satisfies Onsager reciprocity, while the net torque must be explicitly accounted for. Finally, we apply our theory to two-dimensional models with edge terminations.

cond-mat.mes-hall↗

arXiv2024

Multiphase superconductivity in PdBi2

Unconventional superconductivity, where electron pairing does not involve electron-phonon interactions, is often attributed to magnetic correlations in a material. Well known examples include high-T_c cuprates and uranium-based heavy fermion superconductors. Less explored are unconventional superconductors with strong spin-orbit coupling, where interactions between spin-polarised electrons and external magnetic field can result in multiple superconducting phases and field-induced transitions between them, a rare phenomenon in the superconducting state. Here we report a magnetic-field driven phase transition in β-PdBi2, a layered non-magnetic superconductor. Our tunnelling spectroscopy on thin PdBi2 monocrystals incorporated in planar superconductor-insulator-normal metal junctions reveals a marked discontinuity in the superconducting properties with increasing in-plane field, which is consistent with a transition from conventional (s-wave) to nodal pairing. Our theoretical analysis suggests that this phase transition may arise from spin polarisation and spin-momentum locking caused by locally broken inversion symmetry, with p-wave pairing becoming energetically favourable in high fields. Our findings also reconcile earlier predictions of unconventional multigap superconductivity in β-PdBi2 with previous experiments where only a single s-wave gap could be detected.

cond-mat.supr-con↗

arXiv2024

Extreme electron-hole drag and negative mobility in the Dirac plasma of graphene

Coulomb drag between adjacent electron and hole gases has attracted considerable attention, being studied in various two-dimensional systems, including semiconductor and graphene heterostructures. Here we report measurements of electron-hole drag in the Planckian plasma that develops in monolayer graphene in the vicinity of its Dirac point above liquid-nitrogen temperatures. The frequent electron-hole scattering forces minority carriers to move against the applied electric field due to the drag induced by majority carriers. This unidirectional transport of electrons and holes results in nominally negative mobility for the minority carriers. The electron-hole drag is found to be strongest near-room temperature, despite being notably affected by phonon scattering. Our findings provide better understanding of the transport properties of charge-neutral graphene, reveal limits on its hydrodynamic description and also offer insight into quantum-critical systems in general.

cond-mat.mes-hall↗

arXiv2024

No bulk thermal currents in massive Dirac fermions

We calculate the energy current flowing in the bulk of a (2+1)-dimensional system of massive Dirac fermions and along a (1+1)-dimensional domain wall generated by flipping the sign of the particle mass. We show that, at low temperatures and in the long-wavelenghth limit, the system does not support a bulk thermal Hall current proportional to the temperature gradient. The only such contribution is due to states localised at the domain wall. This puts an end to a controversy existing in the literature and amends previous results obtained via first-order perturbation calculations.

cond-mat.mes-hall↗

arXiv2024

First-order effect of electron-electron interactions on the anomalous Hall conductivity of massive Dirac fermions

We investigate the first-order correction to the anomalous Hall conductivity of 2D massive Dirac fermions arising from electron-electron interactions. In a fully gapped system in the limit of zero temperature, we find that this correction vanishes, confirming the absence of perturbative corrections to the topological Hall conductivity. At finite temperature or chemical potential, we find that the total Hall response decays faster than in the non-interacting case, depending on the strength of electron-electron interactions. These features, which could potentially be observed experimentally, show the importance of two-body interactions for anomalous Hall transport.

cond-mat.mes-hall↗

arXiv2024

Localised Thermal Emission from Topological Interfaces

The control of thermal radiation by shaping its spatial and spectral emission characteristics plays a key role in many areas of science and engineering. Conventional approaches to tailor thermal emission using metamaterials are severely hampered both by the limited spatial resolution of the required sub-wavelength material structures and by the materials' strong absorption in the infrared. Here, we demonstrate a promising new approach based on the concept of topology. By changing a single parameter of a multilayer coating, we control the reflection topology of a surface, with the critical point of zero reflection being topologically protected. As a result, the boundaries between sub-critical and super-critical spatial domains host topological interface states with near-unity thermal emissivity. Our experimental demonstration of this effect shows that topological concepts enable unconventional manipulation of thermal light with promising applications for thermal management, energy harvesting and thermal camouflage.

physics.app-ph↗

arXiv2023

Optical conductivity and damping of plasmons due to electron-electron interaction

We re-visit the issue of plasmon damping due to electron-electron interaction. The plasmon linewidth can related to the imaginary part of the charge susceptibility or, equivalently, to the real part of the optical conductivity, $\mathrm{Re}σ(q,ω)$. Approaching the problem first via a standard semi-classical Boltzmann equation, we show that $\mathrm{Re}σ(q,ω)$ of two-dimensional (2D) electron gas scales as $q^2T^2/ω^4$ for $ω\ll T$, which agrees with the results of Refs. [1] and [2] but disagrees with that of Ref. [3], according to which $\mathrm{Re}σ(q,ω) \propto q^2T^2/ω^2$. To resolve this disagreement, we re-derive $\mathrm{Re}σ(q,ω)$ using the original method of Ref. {mishchenko:2004} for an arbitrary ratio $ω/T$ and show that, while the last term is, indeed, present, it is subleading to the $q^2T^2/ω^4$ term. We give a physical interpretation of both leading and subleading contributions in terms of the shear and bulk viscosities of an electron liquid, respectively. We also calculate $\mathrm{Re}σ(q,ω)$ for a three-dimensional (3D) electron gas and doped monolayer graphene. We find that, with all other parameters being equal, finite temperature has the strongest effect on the plasmon linewidth in graphene, where it scales as $T^4\ln T$ for $ω\ll T$.

cond-mat.str-el↗

arXiv2023

Giant ultra-broadband photoconductivity in twisted graphene heterostructures

The requirements for broadband photodetection are becoming exceedingly demanding in hyperspectral imaging. Whilst intrinsic photoconductor arrays based on mercury cadmium telluride represent the most sensitive and suitable technology, their optical spectrum imposes a narrow spectral range with a sharp absorption edge that cuts their operation to < 25 um. Here, we demonstrate a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2 - 100 um with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. Our work demonstrates a rare instance of an intrinsic infrared-terahertz photoconductor that is complementary metal-oxide-semiconductor compatible and array integratable, and introduces twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability.

cond-mat.mes-hall↗

arXiv2023

Thermopower in hBN/graphene/hBN superlattices

Thermoelectric effects are highly sensitive to the asymmetry in the density of states around the Fermi energy and can be exploited as probes of the electronic structure. We experimentally study thermopower in high-quality monolayer graphene, within heterostructures consisting of complete hBN encapsulation and 1D edge contacts, where the graphene and hBN lattices are aligned. When graphene is aligned to one of the hBN layers, we demonstrate the presence of additional sign reversals in the thermopower as a function of carrier density, directly evidencing the presence of the moiré superlattice. We show that the temperature dependence of the thermopower enables the assessment of the role of built-in strain variation and van Hove singularities and hints at the presence of Umklapp electron-electron scattering processes. As the thermopower peaks around the neutrality point, this allows to probe the energy spectrum degeneracy. Further, when graphene is double-aligned with the top and bottom hBN crystals, the thermopower exhibits features evidencing multiple cloned Dirac points caused by the differential super-moiré lattice. For both cases we evaluate how well the thermopower agrees with Mott's equation. Finally, we show the same superlattice device can exhibit a temperature-driven thermopower reversal from positive to negative and vice versa, by controlling the carrier density. The study of thermopower provides an alternative approach to study the electronic structure of 2D superlattices, whilst offering opportunities to engineer the thermoelectric response on these heterostructures.

cond-mat.mes-hall↗

arXiv2023

Electron cooling in graphene enhanced by plasmon-hydron resonance

Evidence is accumulating for the crucial role of a solid's free electrons in the dynamics of solid-liquid interfaces. Liquids induce electronic polarization and drive electric currents as they flow; electronic excitations, in turn, participate in hydrodynamic friction. Yet, the underlying solid-liquid interactions have been lacking a direct experimental probe. Here, we study the energy transfer across liquid-graphene interfaces using ultrafast spectroscopy. The graphene electrons are heated up quasi-instantaneously by a visible excitation pulse, and the time evolution of the electronic temperature is then monitored with a terahertz pulse. We observe that water accelerates the cooling of the graphene electrons, whereas other polar liquids leave the cooling dynamics largely unaffected. A quantum theory of solid-liquid heat transfer accounts for the water-specific cooling enhancement through a resonance between the graphene surface plasmon mode and the so-called hydrons -- water charge fluctuations --, particularly the water libration modes, that allows for efficient energy transfer. Our results provide direct experimental evidence of a solid-liquid interaction mediated by collective modes and support the theoretically proposed mechanism for quantum friction. They further reveal a particularly large thermal boundary conductance for the water-graphene interface and suggest strategies for enhancing the thermal conductivity in graphene-based nanostructures.

cond-mat.mes-hall↗

arXiv2023

Grassmann phase space dynamics of strongly-correlated fermion

We discuss the numerical implementation of two related representations of fermionic density matrices which have been introduced in Annals of Physics 370, 12 (2016). In both of them, the density matrix is expanded in a basis of Bargmann coherent states with weights given by the two phase space distributions. We derive the equations of motion for the distributions when imaginary time evolution is generated by the Hubbard Hamiltonian. One of them is a Grassmann Fokker-Planck equation that can be re-cast into a remarkably simple Itô form involving solely complex variables. In spite of this simple form, we demonstrate that complications arise in numerically computing the expectation value of any observable. These are due to exponential growth in the matrix elements of the stochastic propagator, delicate numerical sensitivity in performing primitive linear algebra operations, and the re-appearance of a sign problem.

cond-mat.quant-gas↗

arXiv2023

Fractal field-effect transistors: Enhanced photodetection and fractal dependent resonances

We study gated field effect transistors (FETs) with fractal geometries under Dyakonov and Shur asymmetric boundary conditions, where the source and drain span the left and right edges of the device respectively. An AC THz potential difference is applied between source and gate while a static source-drain voltage, rectified by the nonlinearities of FET electrons, is measured. We find, for a recursion depth, n, that resonant peaks in the potential integrated along the drain at $ω_n = 3nω_0$ are amplified while all other are diminished. Additionally, we find the presence of peaks with frequencies dependent on the fractal dimensions of both the Sierpinski carpet and a similar alternative fractal. We then show the advantage of employing the alternative fractal as a superior geometry for the photodetector when compared to the Sierpinski carpet.

cond-mat.mes-hall↗

arXiv2023

Wannier Topology and Quadrupole Moments for a generalized Benalcazar-Bernevig-Hughes Model

We analyze a special separable and chiral-symmetric model with a quantized quadrupole moment, extending the Benalcazar-Bernevig-Hughes model [Science 357, 61 (2017)]. Using nested-Wilson loop formalism, we give an exact expression for Wannier centers, sector polarizations, and quadrupole moments. These are connected to the winding numbers of the constitutive one-dimensional chains. We prove that these winding numbers can characterize the model's Wannier topology as a $\mathbb{Z}\times\mathbb{Z}$ set. These results clearly show that the quantization of the quadrupole moment can arise without additional spatial symmetry (except for translation symmetry) for the bulk. By switching from the Wannier representation to the Bloch representation, we derive an alternative expression for the bulk quadrupole moment and obtain its exact value. Combining the bulk quadrupole and edge polarizations, we analytically calculate the corner charge in a large square system and make the bulk-boundary correspondence explicit in an edge-consistent gauge. Our work reveals the relationship between zero-energy states at the boundary, charge localization, and the bulk quadrupole of the extended model.

cond-mat.str-el↗

arXiv2023

Transition from acoustic plasmon to electronic sound in graphene

Fermi liquids respond differently to perturbations depending on whether their frequency is larger (collisionless regime) or smaller (hydrodynamic regime) than the inter-particle collision rate. This results in a different phase velocity between the collisionless zero sound and hydrodynamic first sound. We performed terahertz photocurrent nanoscopy measurements on graphene devices, with a metallic gate in close proximity to the sample, to probe the dispersion of propagating acoustic plasmons, the counterpart of sound modes in electronic Fermi liquids. We report the observation of a change in the plasmon phase velocity when the excitation frequency approaches the electron-electron collision rate. This first observation of the first sound mode in an electronic Fermi liquid is of fundamental interest and can enable novel terahertz emitter and detection implementations.

cond-mat.mes-hall↗

arXiv2022

Rotating Majorana Zero Modes in a disk geometry

We study the manipulation of Majorana zero modes in a thin disk made from a $p$-wave superconductor in order to understand their use as a building block for topological quantum computers. We analyze the second-order topological corner modes that arise when an in-plane magnetic field is applied, and calculate their dynamical evolution when rotating the magnetic field, with special emphasis on non-adiabatic effects. We characterize the phase transition between high-frequency and near-adiabatic evolution using Floquet analysis. We show that oscillations persist even in the adiabatic phase because of a frequency independent coupling between zero modes and excited states, which we have quantified numerically and analytically. These results show that controlling the rotation frequency can be a simple method to avoid the non-adiabatic errors originated from this coupling and thus increase the robustness of topological quantum computation.

quant-ph↗

arXiv2022

Milliwatt terahertz harmonic generation from topological insulator metamaterials

Achieving efficient, high-power harmonic generation in the terahertz spectral domain has technological applications, for example in sixth generation (6G) communication networks. Massless Dirac fermions possess extremely large terahertz nonlinear susceptibilities and harmonic conversion efficiencies. However, the observed maximum generated harmonic power is limited, because of saturation effects at increasing incident powers, as shown recently for graphene. Here, we demonstrate room-temperature terahertz harmonic generation in a Bi$_2$Se$_3$ topological insulator and topological-insulator-grating metamaterial structures with surface-selective terahertz field enhancement. We obtain a third-harmonic power approaching the milliwatt range for an incident power of 75 mW - an improvement by two orders of magnitude compared to a benchmarked graphene sample. We establish a framework in which this exceptional performance is the result of thermodynamic harmonic generation by the massless topological surface states, benefiting from ultrafast dissipation of electronic heat via surface-bulk Coulomb interactions. These results are an important step towards on-chip terahertz (opto)electronic applications.

physics.optics↗

arXiv2022

On the origin of Abrikosov's quantum linear magnetoresistance

Compensated semimetals with Weyl spectra are predicted to exhibit unsaturated linear growth of their resistivity in quantizing magnetic fields. This so-called quantum linear magnetoresistance was introduced by Abrikosov, but approximations used in the theory remained poorly specified, often causing a confusion about experimental situations in which the analysis is applicable. Here we derive Abrikosov's exact result using an alternative formalism based on diffusion of cyclotron orbits in a random potential. We show that both Weyl spectrum and a disorder smooth on the scale of the magnetic length are essential conditions for the validity of the theory, and the linear magnetoresistance appears in the extreme quantum limit where only the zeroth Landau level is half filled. It is the interplay between the relativistic-like nature of Weyl fermions and the classical dynamics of their cyclotron centers, which leads to the linear magnetoresistance. We also derive an analogous result in two dimensions, which has been missing in the literature and is relevant for numerous graphene-based systems.

cond-mat.mes-hall↗