arXiv subjects

Iolanda Di Bernardo

Publications and source records attributed to Iolanda Di Bernardo.

14 recordsLinked to original sources

arXiv2026

The Hidden Cost of Alloying: Disorder-Driven Transport Collapse in TMDs

Alloying in two dimensional semiconductors is widely used to tune bandgaps, yet its implications for charge and energy transport remain poorly understood. Here, we investigate MoS2xSe2(1-x) alloys as a model system to study the interplay between composition, thickness, and disorder. Optical transitions and valence band dispersions evolve continuously with both stoichiometry and number of layers, with negligible bandgap bowing and a composition dependent attenuation of thickness driven renormalization. In contrast, time resolved spatial mapping of photoexcited carriers reveals a pronounced and asymmetric collapse of carrier diffusivity at intermediate compositions, which cannot be accounted for by changes in effective mass or band alignment, and instead emerges from strong real space fluctuations in the local energetic landscape generated by random chalcogen substitution. Microscopic simulations reproduce the experimental trends and show that the character of disorder depends critically on the direction of alloying, producing either scattering barriers or deep trapping sites. Together, these results demonstrate that transport in TMD alloys is governed by disorder physics, overlooked by conventional optical and photoemission probes at equilibrium. Our findings establish transport as a stringent metric of electronic quality and high-light intrinsic limitations in the usage of TMD alloys for layered semiconductor devices.

cond-mat.mtrl-sci↗

arXiv2026

Bottom-up realization of a type-II organic-TMD heterointerface: Pentacene on monolayer WS2

Stacked van der Waals heterostructures based on transition metal dichalcogenides (TMDs) exhibit a rich variety of exotic interfacial phenomena. Substituting one component with an organic semiconductor (OSC) enables the design of hybrid heterostructures with tunable functionalities for optoelectronic, photovoltaic, and spintronic applications. In this work, exploiting scanning tunneling spectroscopy (STS), photoemission orbital tomography (POT) and G0W0 electronic structure calculations, we experimentally and theoretically demonstrate the self-assembly of an ordered single layer of pentacene (5A) above monolayer WS2, exhibiting a type-II (staggered) band alignment in the hybrid 5A/WS2 interface. Central to this result is the synthesis of extended, atomically flat WS2 - an essential prerequisite for a highly ordered and electronically homogeneous OSC/TMD interface - which can only be reliably achieved via bottom-up growth, most notably molecular beam epitaxy (MBE). We realize this by leveraging Au(111) as an atomically clean and conductive sample for epitaxial growth - a necessary requirement for reliable and comparable STS/POT characterizations. The high quality of the synthesized heterostructure, together with its type-II band alignment, establishes pentacene/WS2 as a model system for orbital-resolved studies of charge transfer, energy-level renormalization, and non-equilibrium interfacial processes in hybrid organic-inorganic-2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Excitonic order in quantum materials: fingerprints, platforms and opportunities

The exciton insulator (EI) is a unique many-body ground state of condensed, spontaneously formed excitons (electron-hole pairs) in equilibrium, distinct from conventional band or Mott insulators. Originally proposed over half a century ago, the concept has recently gained renewed experimental traction thanks to advances in spectroscopic resolution, ultrafast probes, and materials synthesis. In this Review, we outline the essential theoretical ingredients underpinning excitonic order and discuss how dimensionality, disorder and screening affect stability. We then examine the diverse experimental fingerprints of the excitonic state, with central focus on strategies to disentangle excitonic order from competing phases such as charge density waves, Mott insulating states, and hybridization-driven insulators, particularly in systems where non-trivial band topology plays a role. We survey the rapidly expanding family of candidate materials, from layered chalcogenides and correlated rare-earth compounds to artificial excitonic platforms and optically driven non-equilibrium condensates. Finally, we discuss the key challenges and emerging opportunities in the field, identifying the theoretical and experimental frontiers that promise to shape the next decade of research.

cond-mat.str-el↗

arXiv2026

Engineering of SnO2-Graphene Oxide Nano-Heterojunctions for Selective Room-temperature Chemical Sensing and Optoelectronic Devices

The development of high-performing sensing materials, able to detect ppb-trace concentrations of volatile organic compounds at low temperatures, is required for the development of next-generation miniaturized wireless sensors. Here, we present the engineering of selective room-temperature chemical sensors, comprising highly porous tin dioxide (SnO2) - graphene oxide (GO) nano-heterojunction layouts. The optoelectronic and chemical properties of these highly porous (above 90%) p-n heterojunctions were systematically investigated in terms of composition and morphologies. Optimized SnO2-GO layouts demonstrate significant potential as both visible-blind photodetectors and as selective room-temperature chemical sensors. Notably, a low GO content results in an excellent UV light responsivity (400A x W-1), with short rise and decay times, and room-temperature high chemical sensitivity with selective detection of volatile organic compounds such as ethanol down to 100~ppb. In contrast, a high concentration of GO drastically decreases the room-temperature response to ethanol and results in good selectivity to ethylbenzene. The feasibility of tuning the chemical selectivity of the sensor response by engineering the relative amount of GO and SnO2 is a promising feature that may guide the future development of miniaturized solid-state gas sensors. Furthermore, the excellent optoelectronic properties of these SnO2-GO nano-heterojunctions may find applications in various other areas such as optoelectronic devices and (photo)electrocatalysis.

cond-mat.mtrl-sci↗

arXiv2026

Electronic structure of Graphene/Co interfaces

Photoemission, from core levels and valence band, and low-energy electron diffraction (LEED) have been employed to investigate the electronic and structural properties of novel graphene-ferromagnetic (G-FM) systems,obtained by intercalation of one mono-layer (1ML) and several layers (4ML) of Co on G grown on Ir(111). Upon intercalation of 1ML of Co, the Co lattice is resized to match the Ir-Ir lattice parameter, resulting in a mismatched G/Co/Ir(111) system. The intercalation of further Co layers leads to a relaxation of the Co lattice and a progressive formation of a commensurate G layer lying on top. We show the C 1s line shape and the band structure of G in the two artificial phases, mismatched and commensurate G/Co, through a comparison with the electronic structure of G grown directly on a Co thick film. Our results show that while the G valence band mainly reflects the hybridization with the d states of Co, regardless of the structural phase, the C 1s line shape is very sensitive to the rumpling of the G layer and the coordination of carbon atoms with the underlying Co. Even in the commensurate (1x1) G/Co phase, where graphene is in register with the Co film, from the angular dependence of the C 1s core level we infer the presence of a double component, due to in-equivalent adsorption sites of carbon sub-lattices.

cond-mat.mtrl-sci↗

arXiv2025

Observation of charge density wave excitonic order parameter in topological insulator monolayer WTe2

Strong electron-hole interactions in a semimetal or narrow-gap semiconductor may drive a ground state of condensed excitons. Monolayer WTe2 has been proposed as a host material for such an exciton condensate, but the order parameter - the key signature of a macroscopic quantum-coherent condensate - has not been observed. Here we use Fourier-transform scanning tunnelling spectroscopy (FT-STS) to study quasi-particle interference (QPI) and periodic modulations of the local density of states (LDOS) in monolayer WTe2. In WTe2 on graphene, in which the carrier density can be varied via back-gating, FT-STS shows QPI features in the 2D bulk bands, confirming the interacting nature of the bandgap in neutral WTe2 and the semi-metallic nature of highly n- and p-doped WTe2. We observe additional non-dispersive spatial modulations in the LDOS imprinted on the topological edge mode of neutral WTe2 on metallic substrates (graphene and graphite), which we interpret as the interaction of the topological edge mode with the expected charge density wave order parameter of the excitonic condensate in WTe2 at low interaction strength due to screening by the metallic substrates.

cond-mat.str-el↗

arXiv2025

Emergent Magnetic Structures at the 2D Limit of the Altermagnet MnTe

MnTe has recently emerged as a canonical altermagnet, a newly identified class of magnetism characterized by compensated antiferromagnetic order coexisting with spin-split electronic bands, traditionally considered exclusive to ferromagnets. However, the extent to which altermagnetism persists as altermagnets are thinned to the two-dimensional (2D) limit remains unexplored. Here, we investigate the magnetic behaviour of 2D MnTe, specifically atomically-thin monolayers (MLs) and bilayers (BLs) grown on graphene/Ir(111) substrate, by combining experimental scanning tunneling microscopy, x-ray photoelectron spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism with density functional theory calculations. We find that while ML and BL MnTe adopt atomic structures with symmetries incompatible with altermagnetism, they exhibit intriguing magnetic phases: the BL forms a highly-robust layered antiferromagnet with in-plane spin anisotropy, whereas the ML exhibits a spin-glass-like behavior below its freezing temperature, a phenomenon not previously observed in an atomically thin material. These findings highlight how reduced dimensionality can promote the emergence of unusual magnetic structures distinct from those of their three-dimensional counterparts, providing new insights into low-dimensional magnetism.

cond-mat.mtrl-sci↗

arXiv2024

Reversal of charge transfer doping on the negative electronic compressibility surface of MoS2

The strong electron-electron interaction in transition metal dichalcogenides (TMDs) gives rise to phenomena such as strong exciton and trion binding and excitonic condensation, as well as large negative exchange and correlation contributions to the electron energies, resulting in negative electronic compressibility. Here we use angle-resolved photoemission spectroscopy to demonstrate a striking effect of negative electronic compressibility in semiconducting TMD MoS2 on the charge transfer to and from a partial overlayer of monolayer semimetallic WTe2. We track the changes in binding energy of the valence bands of both WTe2 and MoS2 as a function of surface transfer doping with donor (K) and acceptor (F4-TCNQ) species. Donor doping increases the binding energy of the MoS2 valence band, as expected, while counterintuitively reducing the binding energy of the WTe2 valence bands and core levels. The inverse effect is observed for acceptor doping, where a typical reduction in the MoS2 binding energies is accopanied by an unexpected increase in those of WTe2. The observations imply a reversal of the expected charge transfer; donor (acceptor) deposition decreases (increases) the carrier density in the WTe2 adlayer. The charge transfer reversal is a direct consequence of the negative electronic compressibility of the MoS2 surface layer, for which addition (subtraction) of charge leads to attraction (repulsion) of further charge from neighbouring layers. These findings highlight the importance of many-body interactions for the electrons in transition metal dichalcogenides and underscore the potential for exploring strongly correlated quantum states in two-dimensional semiconductors.

cond-mat.str-el↗

arXiv2023

Defects, band bending and ionization rings in MoS2

Chalcogen vacancies in transition metal dichalcogenides are widely acknowledged as both donor dopants and as a source of disorder. The electronic structure of sulphur vacancies in MoS2 however is still controversial, with discrepancies in the literature pertaining to the origin of the in-gap features observed via scanning tunneling spectroscopy (STS) on single sulphur vacancies. Here we use a combination of scanning tunnelling microscopy (STM) and STS to study embedded sulphur vacancies in bulk MoS2 crystals. We observe spectroscopic features dispersing in real space and in energy, which we interpret as tip position- and bias-dependent ionization of the sulphur vacancy donor due to tip induced band bending (TIBB). The observations indicate that care must be taken in interpreting defect spectra as reflecting in-gap density of states, and may explain discrepancies in the literature.

cond-mat.mtrl-sci↗

arXiv2023

Imaging the breakdown and restoration of topological protection in magnetic topological insulator MnBi$_2$Te$_4$

Quantum anomalous Hall (QAH) insulators transport charge without resistance along topologically protected chiral one-dimensional edge states. Yet, in magnetic topological insulators (MTI) to date, topological protection is far from robust, with the zero-magnetic field QAH effect only realised at temperatures an order of magnitude below the Néel temperature TN, though small magnetic fields can stabilize QAH effect. Understanding why topological protection breaks down is therefore essential to realising QAH effect at higher temperatures. Here we use a scanning tunnelling microscope to directly map the size of the exchange gap (Eg,ex) and its spatial fluctuation in the QAH insulator 5-layer MnBi$_2$Te$_4$. We observe long-range fluctuations of Eg,ex with values ranging between 0 (gapless) and 70 meV, uncorrelated to individual point defects. We directly image the breakdown of topological protection, showing that the chiral edge state, the hallmark signature of a QAH insulator, hybridizes with extended gapless metallic regions in the bulk. Finally, we unambiguously demonstrate that the gapless regions originate in magnetic disorder, by demonstrating that a small magnetic field restores Eg,ex in these regions, explaining the recovery of topological protection in magnetic fields. Our results indicate that overcoming magnetic disorder is key to exploiting the unique properties of QAH insulators.

cond-mat.mtrl-sci↗

arXiv2023

Metastable polymorphic phases in monolayer TaTe2

Polymorphic phases and collective phenomena - such as charge density waves (CDWs) - in transition metal dichalcogenides (TMDs) dictate the physical and electronic properties of the material. Most TMDs naturally occur in a single given phase, but the fine-tuning of growth conditions via methods like molecular beam epitaxy (MBE) allows to unlock otherwise inaccessible polymorphic structures. Exploring and understanding the morphological and electronic properties of new phases of TMDs is an essential step to enable their exploitation in technological applications. Here, we use scanning tunneling microscopy to map MBE-grown monolayer TaTe2. We report the first observation of the 1H polymorphic phase, coexisting with the 1T, and demonstrate that their relative coverage can be controlled by adjusting synthesis parameters. Several super-periodic structures, compatible with CDWs, are observed to coexist on the 1T phase. Finally, we provide theoretical insight on the delicate balance between Te...Te and Ta-Ta interactions that dictates the stability of the different phases. Our findings demonstrate that TaTe2 is an ideal platform to investigate competing interactions, and indicate that accurate tuning of growth conditions is key to accessing metastable states in TMDs.

cond-mat.mtrl-sci↗

arXiv2021

Formation of a stable surface oxide in MnBi$_2$Te$_4$ thin films

Understanding the air-stability of MnBi$_2$Te$_4$ thin films is crucial for the development and long-term operation of electronic devices based around magnetic topological insulators. In the present work, we study MnBi$_2$Te$_4$ thin films upon exposure to atmosphere using a combination of synchrotron-based photoelectron spectroscopy, room temperature electrical transport and atomic force microscopy to determine the oxidation process. After 2 days air exposure a 2 nm thick oxide passivates the surface, corresponding to oxidation of only the top two surface layers, with the underlying layers preserved. This protective oxide layer results in samples that still exhibit metallic conduction even after several days air exposure. Furthermore, the work function decreases from 4.4 eV for pristine MnBi$_2$Te$_4$ to 4.0 eV after the formation of the oxide, along with only a small shift in the core levels indicating minimal doping as a result of air exposure. With the oxide confined to the top surface layers, and the underlying layers preserved, it may be possible to explore new avenues in how to handle, prepare and passivate future MnBi$_2$Te$_4$ devices.

cond-mat.mtrl-sci↗

arXiv2021

Crossover from 2D ferromagnetic insulator to wide bandgap quantum anomalous Hall insulator in ultra-thin MnBi2Te4

Intrinsic magnetic topological insulators offer low disorder and large magnetic bandgaps for robust magnetic topological phases operating at higher temperatures. By controlling the layer thickness, emergent phenomena such as the Quantum Anomalous Hall (QAH) effect and axion insulator phases have been realised. These observations occur at temperatures significantly lower than the Neel temperature of bulk MnBi2Te4, and measurement of the magnetic energy gap at the Dirac point in ultra-thin MnBi2Te4 has yet to be achieved. Critical to achieving the promise of this system is a direct measurement of the layer-dependent energy gap and verifying whether the gap is magnetic in the QAH phase. Here we utilise temperature dependent angle-resolved photoemission spectroscopy to study epitaxial ultra-thin MnBi2Te4. We directly observe a layer dependent crossover from a 2D ferromagnetic insulator with a bandgap greater than 780 meV in one septuple layer (1 SL) to a QAH insulator with a large energy gap (>100 meV) at 8 K in 3 and 5 SL MnBi2Te4. The QAH gap is confirmed to be magnetic in origin, as it abruptly diminishes with increasing temperature above 8 K. The direct observation of a large magnetic energy gap in the QAH phase of few-SL MnBi2Te4 is promising for further increasing the operating temperature of QAH materials.

cond-mat.mtrl-sci↗

arXiv2020

Overcoming Boltzmann's Tyranny in a Transistor via the Topological Quantum Field Effect

The sub-threshold swing is the fundamental critical parameter determining the operation of a transistor in low-power applications such as switches. It determines the fraction of dissipation due to the gate capacitance used for turning the device on and off, and in a conventional transistor it is limited by Boltzmann's tyranny to kTln(10)/q, or 60 mV per decade. Here, we demonstrate that the sub-threshold swing of a topological transistor, in which conduction is enabled by a topological phase transition via electric field switching, can be sizably reduced in a non-interacting system by modulating the Rashba spin-orbit interaction via a top-gate electric field. We refer to this as the Topological Quantum Field Effect and to the transistor as a Topological Quantum Field Effect transistor (TQFET). By developing a general theoretical framework for quantum spin Hall materials with honeycomb lattices we explicitly show that the Rashba interaction can reduce the sub-threshold swing by more than 25% compared to Boltzmann's limit in currently available materials, but without any fundamental lower bound, a discovery that can guide future materials design and steer the engineering of topological quantum devices.

cond-mat.mes-hall↗