arXiv subjects

Alessandro Principi

Publications and source records attributed to Alessandro Principi.

At least 19 recordsLinked to original sources

arXiv2026

Topological phases of a generalised tripartite Haldane model

We present a generalised tripartite Haldane model with a complex nearest neighbour hopping parameter. The total magnetic flux through the primitive unit cell, which consists of three hexagonal unit cells is zero, and thus the model has the structure of a loop-current model. We calculate topological phase diagrams of the Chern numbers as a function of the phases of the nearest and next-nearest neighbour hopping parameters for a fixed ratio of the magnitude of the hopping parameters. We show that, unlike the Haldane model, the topological phase diagram of this model is very complex, but some aspects can still be dealt with analytically. Furthermore, the Chern numbers of the topological phases are as large as $7$ in absolute value. The analysis is supported by explicit expressions for energy bands crossing at high symmetry points in the Brillouin zone, which show as linear phase boundaries in the phase diagram. We show that such lines explain many features in the topological phase diagrams. Finally, we analyse a few representative examples of the nature of level crossings away from high symmetry and their evolution with model parameters.

cond-mat.mes-hall↗

arXiv2026

Graphene-based Photodetector with Engineered Hot Carrier Cooling Dynamics

Graphene has emerged as a promising material for integration into silicon photonics, owing to its ultrafast and broadband photoresponse without the need for an external bias voltage. This photoresponse relies on the photo-thermoelectric effect created by hot carriers. A key factor underlying the performance of graphene photodetectors is the cooling dynamics of these hot carriers. In this work, we engineer these dynamics in a WSe2-graphene-WSe2 waveguide-integrated photodetector. In particular, by introducing proximity screening by a nearby graphite layer to this structure, we prolong the hot-carrier cooling time, leading to an enhanced photoresponse. We characterize the cooling dynamics under continuous-wave laser excitation by employing a photomixing technique, revealing an increase in the cooling time by up to a factor of four. Direct photoresponse measurements show that the internal photoresponsivity improves by approximately 50%. Together, these results demonstrate the potential of proximity screening to enhance the performance of graphene-based photodetectors on an integrated photonics platform.

cond-mat.mes-hall↗

arXiv2026

Valley-Controlled Viscosity of Two-Dimensional Dirac Fluids

Motivated by recent experiments in weakly hybridized small-angle twisted bilayer graphene, we investigate how valley imbalance affects the viscosity of two-dimensional Dirac fluids. We show that shifting the two low-energy Dirac cones relative to one another provides a direct knob to control the viscosity of the electron fluid. As the splitting is increased, the system passes through distinct transport regimes associated with valley depletion, charge-neutrality crossover, and the onset of electron-hole scattering, producing a pronounced nonmonotonic response. To place this result in context, we also analyze the viscosity in monolayer graphene (MLG) and two-dimensional electron gas (2DEG). We show that, due to the strong dependence of its inertial mass density on temperature, the kinematic viscosity of MLG is a monotonically decreasing function of temperature. Our results identify valley control as a route to tuning hydrodynamic transport in Dirac materials and clarify the interplay between band structure, scattering phase space, and screening in setting the viscous response.

cond-mat.mes-hall↗

arXiv2026

Operator Space Transport and the Emergence of Boundary Time Crystals

Boundary time crystals (BTCs) are prominent examples of continuous time crystals in collective spin systems governed by Lindbladian evolution. To date, their analysis has mostly relied on semiclassical and numerical approaches. Here, we develop a fully quantum-compatible framework to classify collective spin dynamics and show that BTC behavior emerges from the absence of non-trivial weak symmetries of the Liouvillian. To this end, we introduce an irreducible tensor representation of operator space, in which the Lindbladian dynamics maps onto a non-Hermitian hopping problem. Within this picture, the dynamics corresponds to the transport of operator weight across tensor sectors. This mapping allows an identification of distinct dynamical regimes, including collective precession, pure relaxation, and the BTC phase, within a single unified framework. We show that BTCs arise from non-reciprocal transport in operator space, which delocalizes Liouvillian eigenmodes across multiple tensor sectors. This non-reciprocal transport provides a microscopic mechanism for the insensitivity to initial conditions of BTC oscillations. More broadly, our results establish operator space transport as a perspective for understanding dissipative many-body dynamics and highlights connections to non-Hermitian phenomena.

quant-ph↗

arXiv2026

Dielectric control of ultrafast carrier dynamics and transport in graphene

Understanding the ultrafast dynamics of photoexcited charges in graphene is essential, as the microscopic mechanisms underlying these dynamics determine many of graphene's optical, optothermal, and optoelectronic properties. These are crucial properties for many functionalities and devices enabled by graphene, such as high-speed photodectors. Therefore, beyond scientific understanding, it is highly desirable to control ultrafast carrier dynamics for practical applications. Here, we establish this control by engineering the dielectric environment of graphene, thereby regulating both heating and cooling dynamics without altering the Fermi energy, optical power, or ambient temperature. By combining optical pump-terahertz probe experiments with theoretical calculations, we show that dielectric screening suppresses carrier-carrier interactions and slows the dynamics. In particular, reduced carrier-carrier scattering delays the formation of a quasi-equilibrium hot electron distribution, thus slowing carrier heating. It also slows carrier cooling because re-thermalization after optical-phonon emission depends on the same interactions. The enhanced screening further reduces the energy of electron-hole puddles, thereby increasing charge mobility and the Seebeck coefficient. This ability to externally control internal graphene dynamics and transport properties enables the optimization of device performance, such as the sensitivity of photodetectors for data communication and wireless communication applications.

cond-mat.mes-hall↗

arXiv2026

The role of polarization field terms in a model for a cavity quantum material

Constructing models for cavity quantum materials requires a careful treatment of the light-matter coupling. In general, one must specify matrix elements constructed from the material wavefunctions, which are often unknown in a tight-binding framework. The Peierls substitution is frequently used to avoid introducing these additional parameters in the multi-center dipole (or Peierls) gauge, under the assumption that contributions from intraband and interband dipole moments can be neglected. We present the derivation of the Peierls gauge description, including these dipole moment terms, in the passive view of canonical transformations. We construct a toy model for a multi-band system with two sites, which we couple to a uniform field in the Coulomb, dipole, and Peierls gauges. We find that all polarization field terms are required to describe multi-band coupling in the Peierls gauge. The Peierls substitution can only be justified under restriction to a single band in one dimension, provided one also ignores self-polarization corrections arising from bands outside the retained subspace. However, these corrections are frequently non-negligible. More generally, the Coulomb, dipole, and Peierls gauges define distinct partitions of the composite system into the light and matter subsystems. We illustrate the implications of this subsystem relativity for observables such as the photon number and on the performance of orbital truncations in each gauge.

cond-mat.mes-hall↗

arXiv2026

Anomalous thermoelectric Hall response of interacting 2D Dirac fermions

We study the anomalous thermoelectric Hall response of two-dimensional massive Dirac fermions to first order in the electron-electron interaction. We compute both the Nernst response to a Luttinger-type gravitational potential and the particle magnetization, the latter being required to remove spurious non-transport contributions. We show that, for arbitrary interactions, the magnetization is described by a remarkably simple formula. Surprisingly, and contrary to expectations, subtracting the magnetization currents does not make the thermoelectric Hall coefficient vanish in the zero-temperature limit. We attribute this to violation of locality on the smallest length scales, which is inevitable in a quantized field theory, that happens to manifest itself in infrared physics.

cond-mat.mes-hall↗

arXiv2026

Non-Fermi-liquid behaviour of electrons coupled to gauge phonons

We identify overdamped gauge phonons as a new microscopic route to non-Fermi-liquid behaviour in Dirac materials. These phonons couple to electronic currents rather than densities, thereby realising a lattice analogue of transverse gauge-field mechanisms without requiring proximity to a quantum critical point. By computing the electronic self-energy with a phonon propagator dressed by electron-phonon interactions, we show that the low-energy behaviour is controlled by the orbital susceptibility chi and a dimensionless damping parameter alpha. In the overdamped regime, alpha >> 1, quasiparticles display strong deviations from Fermi-liquid theory. For chi > 0, Fermi-liquid behaviour persists only in a parametrically narrow infrared window before crossing over to non-Fermi-liquid scaling. For chi < 0, the Fermi-liquid regime is replaced by marginal-Fermi-liquid behaviour at the lowest energies, followed by a crossover to non-Fermi-liquid scaling. These results establish strain-induced gauge phonons as a promising source of anomalous metallic behaviour in systems such as twisted bilayer graphene.

cond-mat.str-el↗

arXiv2026

Topological Boundary Time Crystal Oscillations

Boundary time crystals (BTCs) break time-translation symmetry and exhibit long-lived, robust oscillations insensitive to initial conditions. We show that collective spin BTCs can admit emergent topological winding numbers in operator space. Expanding the density operator in a spherical tensor basis, we map the Lindblad dynamics onto an effective local hopping problem, where collective degrees of freedom label sites of an emergent two-dimensional operator space lattice and identify topological obstructions that enforce the delocalization of operator modes on the lattice. The resulting spectral delocalization provides a natural explanation for the robust oscillatory dynamics observed in BTCs. When combined with non-reciprocal transport of operator weight across operator space, this mechanism moreover also leads to the universality of long-time dynamics across a broad class of initial states. Our results frame BTC dynamics as a form of topologically constrained operator space transport and suggest a close connection to non-Hermitian skin-effects.

quant-ph↗

arXiv2026

Enhanced Terahertz Photoresponse via Acoustic Plasmon Cavity Resonances in Scalable Graphene

Precise control and nanoscale confinement of terahertz (THz) fields are essential requirements for emerging applications in photonics, quantum technologies, wireless communications, and sensing. Here, we demonstrate a polaritonic cavity enhanced THz photoresponse in an antenna coupled device based on chemical vapor deposited (CVD) monolayer graphene. The dipole antenna lobes simultaneously serve as two gate electrodes, concentrate the impinging THz field, and efficiently launch acoustic graphene plasmons (AGPs), which drive a strong photo-thermoelectric (PTE) signal. Between 6 and 90 K, the photovoltage exhibits pronounced peaks, modulating the PTE response by up to 40\%, that we attribute to AGPs forming a Fabry P\'erot THz cavity in the full or half graphene channel. Combined full wave and transport thermal simulations accurately reproduce the gate controlled plasmon wavelength, spatial absorption profile, and the resulting nonuniform electron heating responsible for the PTE response. The lateral and vertical maximum confinement factors of the AGP wavelength relative to the incident wavelength are 165 and 4000, respectively, for frequencies from 1.83 to 2.52 THz. These results demonstrate that wafer scalable CVD graphene, without hBN encapsulation, can host coherent AGP resonances and exhibit an efficient polaritonic enhanced photoresponse under appropriate gating, antenna coupling, and AGP cavity design, opening a route to scalable, polarization and frequency selective, liquid nitrogen cooled, and low power consumption THz detection platforms based on plasmon thermoelectric transduction.

cond-mat.mes-hall↗

arXiv2025

Analytical solution of boundary time crystals via the superspin basis

Boundary time crystals (BTCs) in dissipative collective spin systems have been extensively studied using numerical, mean-field, and perturbative approaches. However, an explicit Liouvillian description governing the long-time dynamics deep within the time crystal phase has remained elusive. Here, we derive an effective Liouvillian that analytically captures the extreme BTC regime, where dissipation is parametrically weak and oscillatory order is maximally robust. By introducing a superspin representation of Liouville space, we obtain closed-form expressions for the Liouvillian eigenvalues to first order in the dissipation strength, providing direct access to decay rates, oscillation frequencies, and their thermodynamic scaling. Applying this framework to the canonical BTC model we analytically recover spontaneous breaking of continuous time-translation symmetry and persistent oscillations in the thermodynamic limit. In contrast, we show that other dissipative spin models exhibit only single-frequency oscillatory dynamics and therefore do not support genuine BTC phases. Our results establish a controlled analytical framework for the long-time dynamics in the extreme BTC regime.

quant-ph↗

arXiv2025

A new scale anomaly in Dirac matter

The dynamics of Dirac semimetals is modeled at low energies by the massless Dirac Hamiltonian with the Fermi velocity replacing the velocity of light. The classical action is scale invariant. In 3D materials, Coulomb interactions induce a conformal anomaly associated to the charge renormalization already known in quantum field theory. In this work, we describe a new conformal anomaly induced by the running of the Fermi velocity that applies to Dirac semimetals in two and three dimensions. The case of graphene is particularly interesting. We analyze the anomaly and explore its thermodynamic and hydrodynamic consequences. The anomaly modifies the propagation speed of hydrodynamic sound waves, alters the thermodynamic equation of state, and induces a non-vanishing bulk viscosity proportional to the beta function of the Fermi velocity.

hep-th↗

arXiv2025

Spin and Charge Transport Mediated by Fractionalized Excitations

We study charge and spin transport in systems composed of itinerant electrons and localized magnetic moments of a Kitaev quantum spin liquid (QSL) phase. For example, $\alpha-{\rm RuCl}_3$ either intrinsically doped or in proximity to graphene. Our analysis reveals distinct temperature-dependent transport behaviors due to the QSL gap and the Majorana excitation spectrum, which are greatly enhanced when the velocities of itinerant-electrons and QSL excitations become comparable. These transport characteristics could potentially be employed to reveal the fractionalized excitations of the spin liquid.

cond-mat.mes-hall↗

arXiv2024

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

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 \beta-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 \beta-PdBi2 with previous experiments where only a single s-wave gap could be detected.

cond-mat.supr-con↗

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

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↗