arXiv subjects

Sergio Pezzini

Publications and source records attributed to Sergio Pezzini.

At least 19 recordsLinked to original sources

arXiv2026

Direct observation of flat bands in near-magic-angle twisted bilayer CVD graphene

Advances in chemical vapor deposition (CVD) growth have driven graphene crystal quality to unprecedented levels, yet it is still unknown whether this route can realize the fragile flat-band and correlated states of the magic-angle (MA) twisted bilayer graphene (TBG). Here, we report on the experimental observation by room-temperature nano-angle-resolved photoemission spectroscopy (nano-ARPES) of flat bands in a TBG sample close to the MA, assembled via a grow-and-stack protocol based on low-pressure CVD of graphene on copper. Our study indicates electronic bands fully comparable to those measured in exfoliation-based samples and determines the size of the largest near-MA domain to be compatible with electronic transport experiments, motivating further experiments on flat band physics in CVD-graphene.

cond-mat.mes-hall↗

arXiv2026

Lifshitz transitions and isospin polarization in twist-decoupled monolayer-bilayer graphene

Bernal-stacked bilayer graphene (BLG) hosts correlated electronic phases tied to low-energy Lifshitz transitions at saddle points in its valence band. To access this regime, ultralow charge disorder and control over a vertical electric field are simultaneously required. Here, we employ a twist-decoupled monolayer (MLG) to bias a proximal BLG in the absence of an external displacement field (D). We thereby reveal three-fold degenerate quantum Hall states at D = 0, with multiple transitions driven by doping, magnetic and electric field. Spontaneous broken symmetry in the vicinity of the valence band edge is signaled by the emergence of quantum oscillations with anomalous frequencies and large quasiparticle mass. These results indicate that electronic interactions in BLG are preserved in presence of an atomically close MLG, while showcasing the potential of CVD-grown graphene multilayers for the exploration of correlated phases of matter.

cond-mat.mes-hall↗

arXiv2026

Scanning Gate Microscopy Modulation of Supercurrent in Graphene Josephson Junctions

Graphene Josephson junctions represent an excellent platform for quantum technologies, thanks to the combination of high carrier mobility, ballistic transport, and large gate-tunable critical currents, preserved even under quantizing magnetic fields. Investigating the spatial distribution of supercurrent flow could be crucial for elucidating transport mechanisms and advancing the engineering of these devices. In this work, we employ a Scanning Gate Microscope to investigate supercurrent transport in hBN-encapsulated graphene Josephson junctions contacted by Niobium leads. We study the supercurrent modulation as a function of the applied tip voltage bias and tip-to-sample distance, and provide a complete characterization of the tip-induced modulation. Our experimental results are quantitatively consistent with numerical simulations and pave the way towards local mapping and manipulation of gate-tunable superconducting phenomena with unprecedented spatial resolution.

cond-mat.mes-hall↗

arXiv2026

Graphene Josephson diodes from inherent asymmetric disorder

Josephson diodes are non-reciprocal superconducting devices characterized by different switching currents depending on the current flow direction. They recently attracted considerable theoretical and experimental attention, in view of their possible application as rectifying elements in the field of superconducting electronics, and as probes to investigate symmetry breaking mechanisms in mesoscopic systems. In this work, we show that graphene Josephson junctions provide rectification of supercurrent with an efficiency exceeding 20%. The effect appears applying a mT out-of-plane magnetic field and is enhanced close to the nodes of the Fraunhofer interference pattern. Our theoretical model identifies long-range scattering potentials in the junction as the symmetry-breaking mechanism, which yields supercurrent rectification in highly transparent junctions. While graphene stands as an ultra-clean transmission medium, our work shows that unavoidable residual disorder in a clean two-dimensional system is sufficient to promote this effect. Tailoring of the inversion (mirror) symmetry breaking could be obtained via proper design of external gates.

cond-mat.supr-con↗

arXiv2025

Phason-driven temperature-dependent transport in moiré graphene

The electronic and vibrational properties of 2D materials are dramatically altered by the formation of a moiré superlattice. The lowest-energy phonon modes of the superlattice are two acoustic branches (called phasons) that describe the sliding motion of one layer with respect to the other. Considering their low-energy dispersion and damping, these modes may act as a significant source of scattering for electrons in moiré materials. Here, we investigate temperature-dependent electrical transport in minimally twisted bilayer graphene, a moiré system developing multiple weakly-dispersive electronic bands and a reconstructed lattice structure. We measure a linear-in-temperature resistivity across the band manyfold above $T\sim{10}$ K, preceded by a quadratic temperature dependence. While the linear-in-temperature resistivity is up to two orders of magnitude larger than in monolayer graphene, it is reduced (approximately by a factor of three) with respect to magic-angle twisted bilayer graphene. Moreover, it is modulated by the recursive band filling, with minima located close to the full filling of each band. Comparing our results with a semiclassical transport calculation, we show that the experimental trends are compatible with scattering processes mediated by longitudinal phasons, which dominate the resistivity over the contribution from conventional acoustic phonons of the monolayer. Our findings highlight the close relation between vibrational modes unique to moiré materials and carrier transport therein.

cond-mat.mes-hall↗

arXiv2025

Vortex Pinning in Niobium covered by a thin polycrystalline Gold

Owing to its superconducting properties, Niobium (Nb) is an excellent candidate material for superconducting electronics and applications in quantum technology. Here we perform scanning tunneling microscopy and spectroscopy experiments on Nb films covered by a thin gold (Au) film. We investigate the minigap structure of the proximitized region and provide evidence for a highly transparent interface between Nb and Au, beneficial for device applications. Imaging of Abrikosov vortices in presence of a perpendicular magnetic field is reported. The data show vortex pinning by the granular structure of the polycrystalline Au film. Our results show robust and homogeneous superconducting properties of thin Nb film in the presence of a gold capping layer. The Au film not only protects the Nb from surface oxidation but also preserves its excellent superconducting properties.

cond-mat.supr-con↗

arXiv2025

Quasi-$Φ_0$-periodic supercurrent at quantum Hall transitions

The combination of superconductivity and quantum Hall (QH) effect is regarded as a key milestone in advancing topological quantum computation in solid-state systems. Recent quantum interference studies suggest that QH edge states can effectively mediate a supercurrent across high-quality graphene weak links. In this work we report the observation of a supercurrent associated with transitions between adjacent QH plateaus, where transport paths develop within the compressible two-dimensional bulk. We employ a back-gated graphene Josephson junction, comprising high-mobility CVD-grown graphene encapsulated in hexagonal Boron Nitride (hBN) and contacted by Nb leads. Superconducting pockets are detected persisting beyond the QH onset, up to 2.4 T, hence approaching the upper critical field of the Nb contacts. We observe an approximate $Φ_0=h/2e$ periodicity of the QH-supercurrent as a function of the magnetic field, indicating superconducting interference in a proximitized percolative phase. These results provide a promising experimental platform to investigate the transport regime of percolative supercurrents, leveraging the flexibility of van der Waals devices.

cond-mat.mes-hall↗

arXiv2024

Built-in Bernal gap in large-angle-twisted monolayer-bilayer graphene

Atomically thin materials offer multiple opportunities for layer-by-layer control of their electronic properties. While monolayer graphene (MLG) is a zero-gap system, Bernal-stacked bilayer graphene (BLG) acquires a finite band gap when the symmetry between the layers' potential energy is broken, usually, via a displacement electric field applied in double-gate devices. Here, we introduce a twistronic stack comprising both MLG and BLG, synthesized via chemical vapor deposition, showing a Bernal gap in the absence of external fields. Although a large ($\sim30^{\circ}$) twist angle decouples the MLG and BLG electronic bands near Fermi level, proximity-induced energy shifts in the outermost layers result in a built-in asymmetry, which requires a displacement field of $0.14$ V/nm to be compensated. The latter corresponds to a $\sim10$ meV intrinsic BLG gap, a value confirmed by our thermal-activation measurements. The present results highlight the role of structural asymmetry and encapsulating environment, expanding the engineering toolbox for monolithically-grown graphene multilayers.

cond-mat.mes-hall↗

arXiv2023

Growth and applications of two-dimensional single crystals

Two-dimensional (2D) materials have received extensive research attentions over the past two decades due to their intriguing physical properties (such as the ultrahigh mobility and strong light-matter interaction at atomic thickness) and a broad range of potential applications (especially in the fields of electronics and optoelectronics). The growth of single-crystal 2D materials is the prerequisite to realize 2D-based high-performance applications. In this review, we aim to provide an in-depth analysis of the state-of-the-art technology for the growth and applications of 2D materials, with particular emphasis on single crystals. We first summarize the major growth strategies for monolayer 2D single crystals. Following that, we discuss the growth of multilayer single crystals, including the control of thickness, stacking sequence, and heterostructure composition. Then we highlight the exploration of 2D single crystals in electronic and optoelectronic devices. Finally, a perspective is given to outline the research opportunities and the remaining challenges in this field.

cond-mat.mtrl-sci↗

arXiv2023

Scalable High-Mobility Graphene/hBN Heterostructures

Graphene-hexagonal boron nitride (hBN) scalable heterostructures are pivotal for the development of graphene-based high-tech applications. In this work, we demonstrate the realization of high-quality graphene-hBN heterostructures entirely obtained with scalable approaches. hBN continuous films were grown via ion beam-assisted physical vapor deposition directly on commercially available $SiO_2/Si$ and used as receiving substrates for graphene single-crystal matrixes grown by chemical vapor deposition on copper. The structural, chemical, and electronic properties of the heterostructure were investigated by atomic force microscopy, Raman spectroscopy, and electrical transport measurements. We demonstrate graphene carrier mobilities exceeding $10,000 cm^2/Vs$ in ambient conditions, 30% higher than those directly measured on $SiO_{2}/Si$. We prove the scalability of our approach by measuring more than 100 transfer length method devices over a centimeter scale, which present an average carrier mobility of $7500 \pm 850 cm^{2}/Vs$. The reported high-quality all-scalable heterostructures are of relevance for the development of graphene-based high-performing electronic and optoelectronic applications.

physics.app-ph↗

arXiv2023

Phonon-mediated room-temperature quantum Hall transport in graphene

The quantum Hall (QH) effect in two-dimensional electron systems (2DESs) is conventionally observed at liquid-helium temperatures, where lattice vibrations are strongly suppressed and bulk carrier scattering is dominated by disorder. However, due to large Landau level (LL) separation (~2000 K at B = 30 T), graphene can support the QH effect up to room temperature (RT), concomitant with a non-negligible population of acoustic phonons with a wave-vector commensurate to the inverse electronic magnetic length. Here, we demonstrate that graphene encapsulated in hexagonal boron nitride (hBN) realizes a novel transport regime, where dissipation in the QH phase is governed predominantly by electron-phonon scattering. Investigating thermally-activated transport at filling factor 2 up to RT in an ensemble of back-gated devices, we show that the high B-field behaviour correlates with their zero B-field transport mobility. By this means, we extend the well-accepted notion of phonon-limited resistivity in ultra-clean graphene to a hitherto unexplored high-field realm.

cond-mat.mes-hall↗

arXiv2022

Moiré-Induced Transport in CVD-Based Small-Angle Twisted Bilayer Graphene

To realize the applicative potential of 2D twistronic devices, scalable synthesis and assembly techniques need to meet stringent requirements in terms of interface cleanness and twist-angle homogeneity. Here, we show that small-angle twisted bilayer graphene assembled from separated CVD-grown graphene single-crystals can ensure high-quality transport properties, determined by a device-scale-uniform moireé potential. Via low-temperature dual-gated magnetotransport, we demonstrate the hallmarks of a $2.4^\circ$ -twisted superlattice, including tunable regimes of interlayer coupling, reduced Fermi velocity, large interlayer capacitance, and density-independent Brown-Zak oscillations. The observation of these moiré-induced electrical transport features establishes CVD-based twisted bilayer graphene as an alternative to 'tear-and-stack' exfoliated flakes for fundamental studies, while serving as a proof-of-concept for future large-scale assembly.

cond-mat.mes-hall↗

arXiv2022

Parallel transport and layer-resolved thermodynamic measurements in twisted bilayer graphene

We employ dual-gated 30°-twisted bilayer graphene to demonstrate simultaneous ultra-high mobility and conductivity (up to 40 mS at room temperature), unattainable in a single-layer of graphene. We find quantitative agreement with a simple phenomenology of parallel conduction between two pristine graphene sheets, with a gate-controlled carrier distribution. Based on the parallel transport mechanism, we then introduce a method for in situ measurements of the chemical potential of the two layers. This twist-enabled approach, neither requiring a dielectric spacer, nor separate contacting, has the potential to greatly simplify the measurement of thermodynamic quantities in graphene-based systems of high current interest.

cond-mat.mes-hall↗

arXiv2021

Wafer-scale integration of graphene-based photonic devices

Graphene and related materials can lead to disruptive advances in next generation photonics and optoelectronics. The challenge is to devise growth, transfer and fabrication protocols providing high (>5,000 cm2 V-1 s-1) mobility devices with reliable performance at the wafer scale. Here, we present a flow for the integration of graphene in photonics circuits. This relies on chemical vapour deposition (CVD) of single layer graphene (SLG) matrices comprising up to ~12000 individual single crystals (SCs), grown to match the geometrical configuration of the devices in the photonic circuit. This is followed by a transfer approach which guarantees coverage over ~80% of the device area, and integrity for up to 150 mm wafers, with room temperature mobility ~5000 cm2 V-1 s-1. We use this process flow to demonstrate double SLG electro-absorption modulators with modulation efficiency ~0.25, 0.45, 0.75, 1 dB V-1 for device lengths ~30, 60, 90, 120 μm. The data rate is up to 20 Gbps. Encapsulation with single-layer hBN is used to protected SLG during plasma-enhanced CVD of Si3N4, ensuring reproducible device performance. Our full process flow (from growth to device fabrication) enables the commercial implementation of graphene-based photonic devices.

physics.app-ph↗

arXiv2020

Ultrafast, Zero-Bias, Graphene Photodetectors with Polymeric Gate Dielectric on Passive Photonic Waveguides

We report compact, scalable, high-performance, waveguide integrated graphene-based photodetectors (GPDs) for telecom and datacom applications, not affected by dark current. To exploit the photothermoelectric (PTE) effect, our devices rely on a graphene-polymer-graphene stack with static top split gates. The polymeric dielectric, poly(vinyl alcohol) (PVA), allows us to preserve graphene quality and to generate a controllable p-n junction. Both graphene layers are fabricated using aligned single-crystal graphene arrays grown by chemical vapor deposition. The use of PVA yields a low charge inhomogeneity 8 x 10$^{10}$ $cm^{-2}$ at the charge neutrality point, and a large Seebeck coefficient 140 $μ$V K$^{-1}$, enhancing the PTE effect. Our devices are the fastest GPDs operating with zero dark current, showing a flat frequency response up to 67 GHz without roll-off. This performance is achieved on a passive, low-cost, photonic platform, and does not rely on nanoscale plasmonic structures. This, combined with scalability and ease of integration, makes our GPDs a promising building block for next-generation optical communication devices.

physics.app-ph↗

arXiv2020

High-quality electrical transport using scalable CVD graphene

Producing and manipulating graphene on fab-compatible scale, while maintaining its remarkable carrier mobility, is key to finalize its technological application. We show that a large-scale approach (chemical vapor deposition on Cu followed by polymer-mediated semi-dry transfer) yields single-layer graphene crystals fully comparable, in terms of electronic transport, to micro-mechanically exfoliated flakes. hBN is used to encapsulate the graphene crystals $-$ without taking part to their detachment from the growth catalyst $-$ and study their intrinsic properties in field-effect devices. At room temperature, the electron-phonon coupling sets the mobility to $\sim1.3 \times10^5$ cm$^2$V$^{-1}$s$^{-1}$ at $\sim10^{11}$ cm$^{-2}$ concentration. At cryogenic temperatures, the mobility ($ > 6\times10^5$ cm$^2$V$^{-1}$s$^{-1}$ at $\sim10^{11}$ cm$^{-2}$) is limited by the devices' physical edges, and charge fluctuations $ < 7\times10^9$ cm$^{-2}$ are detected. Under perpendicular magnetic fields, we observe early onset of Landau quantization ($B\sim50$ mT) and signatures of electronic correlation, including the fractional quantum Hall effect.

cond-mat.mes-hall↗

arXiv2020

Synthesis of large-area rhombohedral few-layer graphene by chemical vapor deposition on copper

Rhombohedral-stacked few-layer graphene (FLG) has been receiving an ever-increasing attention owing to its peculiar electronic properties that could lead to enticing phenomena such as superconductivity and magnetic ordering. Up to now, experimental studies on such material have been mainly limited by the difficulty in isolating it in thickness exceeding 3 atomic layers with device-compatible size. In this work, rhombohedral graphene with thickness up to 9 layers and areas up to ~50 micrometers square is grown via chemical vapor deposition (CVD) on suspended Cu foils and transferred onto target substrates via etch-free delamination. The domains of rhombohedral FLG are identified by Raman spectroscopy and are found to alternate with domains of Bernal-stacked FLG within the same crystal in a stripe-like configuration. A combined analysis of micro-Raman mapping, atomic force microscopy and optical microscopy indicates that the formation of rhombohedral-stacked FLG is strongly correlated to the copper substrate morphology. Cu step bunching results in bending of FLG and interlayer displacement along preferential crystallographic orientations, as determined experimentally by electron microscopy, thus inducing the stripe-like domains. The growth and transfer of rhombohedral FLG with the reported thickness and size shall facilitate the observation of predicted unconventional physics and ultimately add to its technological relevance.

cond-mat.mtrl-sci↗

arXiv2020

30$^\circ$-twisted bilayer graphene quasicrystals from chemical vapor deposition

The artificial stacking of atomically thin crystals suffers from intrinsic limitations in terms of control and reproducibility of the relative orientation of exfoliated flakes. This drawback is particularly severe when the properties of the system critically depend on the twist angle, as in the case of the dodecagonal quasicrystal formed by two graphene layers rotated by 30$^\circ$. Here we show that large-area 30$^\circ$-rotated bilayer graphene can be grown deterministically by chemical vapor deposition on Cu, eliminating the need of artificial assembly. The quasicrystals are easily transferred to arbitrary substrates and integrated in high-quality hBN-encapsulated heterostructures, which we process into dual-gated devices exhibiting carrier mobility up to $10^5$ cm$^2$/Vs. From low-temperature magnetotransport, we find that the graphene quasicrystals effectively behave as uncoupled graphene layers, showing 8-fold degenerate quantum Hall states: this result indicates that the Dirac cones replica detected by previous photo-emission experiments do not contribute to the electrical transport.

cond-mat.mes-hall↗