arXiv subjects

Paolo Moras

Publications and source records attributed to Paolo Moras.

At least 19 recordsLinked to original sources

arXiv2026

Charge-to-spin conversion in epitaxial and polycrystalline Bi and Bi/Ag layers

Bi is predicted to be an efficient generator of spin-orbit torques (SOTs), with charge-to-spin conversion efficiency comparable to those of prototypical heavy metals, such as Ta, W, and Pt. However, experimental reports provide widely scattered interconversion efficiencies, while the origin of the large conversion signal in Bi/Ag bilayers remains controversial. Here, we investigate charge-to-spin conversion in epitaxial and polycrystalline Bi-based magnetic heterostructures by measuring the damping-like SOT using magneto-optic Kerr effect magnetometry, complemented by structural and spectroscopic analyses and harmonic Hall resistance measurements. We show that inserting an Ag spacer between Bi(001) and metallic ferromagnets (FeCo or Ni) enhances the SOT efficiency by more than one order of magnitude, reaching an effective spin Hall conductivity of approximately $2 \times 10^5 (\hbar/2e)$ S/m, in excellent agreement with theoretical expectations for bulk Bi. This enhancement can be consistently explained by the preservation of the structural and chemical integrity of Bi, otherwise compromised by the direct deposition of a ferromagnetic overlayer, rather than by Rashba spin-orbit coupling at the Bi/Ag interface. Comparative studies across epitaxial, polycrystalline, and intentionally surface-oxidized Bi films, beyond oxygen doses known to destroy Bi(001) surface states, reveal that structural disorder has a negative impact on the SOT efficiency and indicate a dominant bulk contribution to spin-current generation in Bi/Ag heterostructures, yielding an effective Bi spin Hall angle of approximately 1. By establishing a direct correlation between atomic-scale integrity and charge-to-spin conversion, this study provides design principles to improve the reliability of Bi-based SOT devices and offers a robust framework for interpreting spin-charge interconversion in Bi and Bi/Ag systems.

cond-mat.mtrl-sci↗

arXiv2026

Spin polarization enhancement in a single-layer Bi(1-x)Sb(x) alloy on Ag(111) via isovalent substitution

Co-adsorption of Bi and Sb on Ag(111) at room temperature yields a single-layer Bi(1-x)Sb(x) alloy with a rectangular 3xsqrt(3) structure containing four atoms per unit cell (2/3 ML total coverage) and lacking long-range chemical order. We present an electronic structure study of this system combining angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT) calculations. To investigate the effect of inversion symmetry breaking induced by substituting a heavier atom (Bi) with a lighter isoelectronic one (Sb) within a fixed crystallographic framework, we focused on a Bi-rich composition. ARPES measurements reveal four surface-state bands, in good agreement with DFT calculations based on a rectangular four-atom overlayer unit cell. DFT calculations further show that Sb incorporation induces both in-plane and out-of-plane asymmetries in the electronic potential, leading to sizable spin splitting and spin polarization of the overlayer bands. Although these effects are partially reduced by interaction with the substrate, they remain significant. Our work illustrates, through a concrete model system, a general principle: incorporating a lighter isovalent element can significantly enhance spin polarization, potentially offering a useful design guideline for understanding and engineering Rashba-related systems.

cond-mat.mtrl-sci↗

arXiv2026

Emergent surface resonance from charge density wave symmetry breaking in TiSe2

Surface confined electronic states provide a fertile ground for discovering emergent phenomena that have no counterpart in the bulk, offering new routes to manipulate correlations, symmetry breaking, and dimensionality at the atomic scale. Here, we show that charge density wave (CDW) symmetry breaking can yield a surface states in 1T-TiSe2. Micro angle resolved photoemission spectroscopy resolves a sharp, two dimensional surface resonant state (SRS) that emerges within the CDW reconstructed low energy spectrum. The SRS exhibits notable temperature dependence and its spectral weight collapses around 160 K, while CDW transition temperature TCDW is commonly reported as 202 K. Slab DFT+U calculations reproduce a surface localized resonance when CDW folding brings valence and conduction states into near degeneracy, suggesting a correlation tuned, surface selective origin. These results point to a form of correlation-tuned surface resonance in a layered CDW compound and suggest a framework for engineering low dimensional quantum states in van der Waals materials via symmetry breaking and electronic structure tuning.

cond-mat.mtrl-sci↗

arXiv2026

Plasmon Engineering in Intercalated 2H-TaS$_2$

Plasmons in low dimensional materials provide a powerful platform for nanoscale control of light matter interactions, yet strategies to tailor their coherence and dissipation remain limited. Here, we demonstrate that transition metal intercalation offers a fundamentally distinct route to engineer plasmonic response in layered materials. By combining high-resolution core-level photoemission spectroscopy with first-principles calculations, we show that Fe and Co intercalation in 2H-TaS2 does not act as conventional electron doping, but instead reshapes the low energy electronic structure through orbital hybridization and structural reconstruction. This process introduces a dense continuum of low energy excitations that efficiently damp and ultimately suppress the plasmon mode. First principle calculations of the energy loss function reveal a transition from a well defined collective excitation to an overdamped response, signaling the breakdown of coherent charge dynamics. Our results establish intercalation as a chemically controlled pathway to tune plasmon losses and dielectric response in quantum van der Waals materials, providing a new design principle for plasmonic and optoelectronic functionalities at the nanoscale.

cond-mat.mtrl-sci↗

arXiv2026

Emergent 3D Fermiology and Magnetism in an Intercalated Van der Waals System

Intercalation of magnetic atoms into van der Waals materials provides a versatile platform for tailoring unconventional magnetic properties. However, its impact on electronic dimensionality and exchange mechanisms remains poorly understood. Using Fe-intercalated TaS$_2$ as a model system, we combine X-ray absorption and resonant inelastic scattering with angle-resolved photoemission and first-principles calculations to reveal that intercalation reshapes the host electronic structure. We identify a spin-polarized intercalant-host hybridized band with pronounced out-of-plane dispersion crossing the Fermi level, providing an itinerant channel for interlayer magnetic exchange. This mechanism explains the breakdown of a purely atomic picture and establishes a direct link between lattice geometry, electronic dispersion, and magnetic order. Our findings demonstrate that intercalant-induced itinerancy enables tunable interlayer coupling in otherwise layered magnets, offering a general microscopic framework for engineering magnetic dimensionality in a broad class of intercalated vdW materials.

cond-mat.mtrl-sci↗

arXiv2025

The electronic structure of a doped Mott-Hubbard surface

The Sn/Si(111)-({\sqrt}3{\times}{\sqrt}3)R30{\deg} surface, a 2D Mott insulator, has long been predicted and then found experimetally to metallize and even turn superconducting upon boron doping. In order to clarify the structural, spectroscopic and theoretical details of that evolution, here we present ARPES data supplementing morphology and scanning tunneling measurements. These combined experimental results are compared with predictions from a variety of electronic structure approaches, mostly density functional DFT+U, but not neglecting Mott-Hubbard models, both ordered and disordered. These theoretical pictures address different spectroscopic aspects, including the 2D Fermi surface, the Hubbard bands, etc. While no single picture account for all observations at once,the emergent hypothesis compatible with all data is that metallization arises from sub-subsurface boron doping, additional to the main standard subsurface boron geometry, that would leave the surface insulating. These results advance the indispensable frame for the further understanding of this fascinating system.

cond-mat.str-el↗

arXiv2025

Mastering the growth of antimonene on Bi2Se3: strategies and insights

Antimonene, the two-dimensional phase of antimony, appears in two distinct allotropes when epitaxially grown on Bi2Se3: the puckered asymmetric washboard ({\alpha}) and buckled honeycomb ({\beta}) bilayer structures. As-deposited antimony films exhibit varying proportions of single {\alpha} and {\beta} structures. We identify the conditions necessary for ordered, pure-phase growth of single to triple {\beta}-antimonene bilayers. Additionally, we determine their electronic structure, work function, and characteristic core-level binding energies, offering an explanation for the relatively large chemical shifts observed among the different phases. This study not only establishes a protocol for achieving a single {\beta} phase of antimonene but also provides key signatures for distinguishing between the different allotropes using standard spectroscopic and microscopic techniques.

cond-mat.mtrl-sci↗

arXiv2024

Giant Rashba-Splitting of One-Dimensional Metallic States in Bi Dimer Lines on InAs(100)

Bismuth produces different types of ordered superstructures on the InAs(100) surface, depending on the growth procedure and coverage. The (2x1) phase forms at completion of a Bi monolayer and consists of a uniformly oriented array of parallel lines of Bi dimers. Scanning tunneling and core level spectroscopies demonstrate its metallic character, in contrast with the semiconducting properties expected on the basis of the electron counting principle. The weak electronic coupling among neighboring lines gives rise to quasi one-dimensional Bi-derived bands with open contours at the Fermi level. Spin- and angle-resolved photoelectron spectroscopy reveals a giant Rashba splitting of these bands, in good agreement with ab-initio electronic structure calculations. The very high density of the dimer lines, the metallic and quasi one-dimensional band dispersion and the Rashba-like spin texture make the Bi/InAs(100)-(2x1) phase an intriguing system, where novel transport regimes can be studied.

cond-mat.mtrl-sci↗

arXiv2023

Unidirectional Nano-Modulated Binding and Electron Scattering in Epitaxial Borophene

A complex interplay between the crystal structure and electron behavior within borophene renders this material an intriguing 2D system with many of its electronic properties still undiscovered. Experimental insight into those properties is additionally hampered by the limited capabilities of the established synthesis methods, which in turn inhibits the realization of potential borophene applications. In this multi-method study, photoemission spectroscopies and scanning probe techniques complemented by theoretical calculations have been used to investigate the electronic characteristics of a high-coverage, single-layer borophene on Ir(111) substrate. Our results show that the binding of borophene to Ir(111) exhibits pronounced one-dimensional modulation and transforms borophene into a nano-grating. The scattering of photoelectrons from this structural grating gives rise to the replication of electronic bands. In addition, the binding modulation is reflected in the chemical reactivity of borophene and gives rise to its inhomogeneous aging effect. Such aging is easily reset by dissolving boron atoms in iridium at high temperature followed by their reassembly into a fresh atomically-thin borophene mesh. Besides proving electron-grating capabilities of the boron monolayer, our data provides a comprehensive insight into the electronic properties of epitaxial borophene which is vital for further examination of other boron systems of reduced dimensionality.

cond-mat.mes-hall↗

arXiv2023

Spin Selective Evolution of Zhang-Rice State in Binary Transition Metal Oxide

The Zhang-Rice (ZR) state is a strongly hybridized bound state formed by the transition metal and oxygen atoms. The spin-fluctuations within the ZR state are known to play an important role in high-$T_\mathrm{c}$ superconductivity in cuprates. Here, we employ a combination of angle-resolved photoemission spectroscopy (ARPES), X-ray photoemission spectroscopy (XPS), and {\it ab initio} embedded dynamical mean-field theory (eDMFT) to investigate the influence of magnetic ordering on the spectral characteristics of the valence band and Mn 2$p$ core-level in MnO (001) ultrathin films. Our results demonstrate that a complex spin-selective evolution of Mn 3$d$$-$O 2$p$ hybridization develops due to the long-range antiferromagnetic (AFM) ordering. This hybridization significantly alters the spectral shape and weight of the ZR state. Specifically, in the AFM phase, we observed the sharpening of the ZR state and band folding with the periodicity of the AFM unit cell of MnO(001). We also demonstrated a strong connection between the spectral evolution of the ZR state and the non-local screening channels of the photoexcited core holes. Further, our detailed temperature-dependent study reveals the presence of short-range antiferromagnetic correlations that exist at much higher temperatures than $T_\mathrm{N}$. Such comprehensive studies showing the evolution of the ZR state across the magnetic transitions and its implication to the core-hole screening have never been reported in any 3$d$ binary transition metal oxides.

cond-mat.str-el↗

arXiv2023

Interplay between magnetic order and electronic band structure in ultrathin GdGe$_2$ metalloxene films

Dimensionality can strongly influence the magnetic structure of solid systems. Here, we predict theoretically and confirm experimentally that the antiferromagnetic (AFM) ground state of bulk gadolinium germanide metalloxene, which has a quasi-layered defective GdGe$_2$ structure, is preserved in the ultrathin film limit. \textit{Ab initio} calculations demonstrate that ultrathin GdGe$_2$ films present in-plane intra-layer ferromagnetic coupling and AFM inter-layer coupling in the ground state. Angle-resolved photoemission spectroscopy finds the AFM-induced band splitting expected for the 2 and 3 GdGe$_2$ trilayer (TL) films, which disappear above the Néel temperature. The comparative analysis of isostructural ultrathin DyGe$_2$ and GdSi$_2$ films confirms the magnetic origin of the observed band splitting. These findings are in contrast with the recent report of ferromagnetism in ultrathin metalloxene films, which we ascribe to the presence of uncompensated magnetic moments.

cond-mat.mtrl-sci↗

arXiv2023

Synthesis of a mesoscale ordered 2D-conjugated polymer with semiconducting properties

2D materials with high charge carrier mobility and tunable electronic band gaps have attracted intense research effort for their potential use as active components in nanoelectronics. 2D-conjugated polymers (2DCP) constitute a promising sub-class due to the fact that the electronic band structure can be manipulated by varying the molecular building blocks, while at the same time preserving the key features of 2D materials such as Dirac cones and high charge mobility. The major challenge for their use in technological applications is to fabricate mesoscale ordered 2DCP networks since current synthetic routes yield only small domains with a high density of defects. Here we demonstrate the synthesis of a mesoscale ordered 2DCP with semiconducting properties and Dirac cone structures via Ullmann coupling on Au(111). This material has been obtained by combining rigid azatriangulene precursors and a hot dosing approach which favours molecular diffusion and reduces the formation of voids in the network. These results open opportunities for the synthesis of 2DCP Dirac cone materials and their integration into devices.

cond-mat.mtrl-sci↗

arXiv2023

Electronic structure of the Ge/Si(105) hetero-interface

Thin Ge layers deposited on Si(105) form a stable single-domain film structure with large terraces and rebonded-step surface termination, thus realizing an extended and ordered Ge/Si planar hetero-junction. At the coverage of four Ge monolayers angle-resolved photoemission spectroscopy reveals the presence of two-dimensional surface and film bands displaying energy-momentum dispersion compatible with the 5x4 periodicity of the system. The good agreement between experiment and first-principles electronic structure calculations confirms the validity of the rebonded-step structural model. The direct observation of surface features within 1 eV below the valence band maximum corroborates previously reported analysis of the electronic and optical behavior of the Ge/Si hetero-interface.

cond-mat.mtrl-sci↗

arXiv2023

Silicene on Ag(111): an honeycomb lattice without Dirac bands

The discovery of (4x4) silicene formation on Ag(111) raised the question on whether silicene maintains its Dirac fermion character, similar to graphene, on a supporting substrate. Previous photoemission studies indicated that the π-band forms Dirac cones near the Fermi energy, while theoretical investigations found it shifted at deeper binding energy. By means of angle-resolved photoemission spectroscopy and density functional theory calculations we show instead that the π-symmetry states lose their local character and the Dirac cone fades out. The formation of an interface state of free-electron-like Ag origin is found to account for spectral features that were theoretically and experimentally attributed to silicene bands of π-character.

cond-mat.mtrl-sci↗

arXiv2023

Coexistence of multiple silicene phases in silicon grown on Ag(111)

Silicene, the silicon equivalent of graphene, is attracting increasing scientific and technological attention in view of the exploitation of its exotic electronic properties. This novel material has been theoretically predicted to exist as a free-standing layer in a low-buckled, stable form, and can be synthesized by the deposition of Si on appropriate crystalline substrates. By employing low-energy electron diffraction and microscopy, we have studied the growth of Si on Ag(111) and observed a rich variety of rotationally non-equivalent silicene structures. Our results highlight a very complex formation diagram, reflecting the coexistence of different and nearly degenerate silicene phases, whose relative abundance can be controlled by varying the Si coverage and growth temperature. At variance with other studies, we find that the formation of single-phase silicene monolayers cannot be achieved on Ag(111).

cond-mat.mtrl-sci↗

arXiv2023

Harnessing the magnetic proximity effect: induced spin polarization in Ni/Si interfaces

The investigation of the properties of metal-semiconductor interfaces has gained significant attention due to the unique features that emerge from the combination of both metal and semiconductor attributes. In this report, the magnetic properties of Ni/Si interfaces utilizing X-ray magnetic circular dichroism (XMCD) spectroscopy at the Ni and Si edges have been studied. This approach allows to distinguish unambiguously the local magnetism on Ni and Si via individual core-level excitations. Two samples with different semiconductor dopings were investigated using both total electron yield (TEY) and reflectivity configurations. The experimental results uncovered magnetization at equilibrium in both the metallic layer and in the proximal layer of the semiconductor substrate, implying the presence of induced spin polarization in Si at equilibrium, possibly arising from the depletion layer region. These results hold significant value in the field of spintronics, as similar systems have been demonstrated to generate spin injection through optical medium, opening a new pathway for next generation nonvolatile high speed devices.

cond-mat.mtrl-sci↗

arXiv2023

Defect engineering over anisotropic brookite towards substrate-specific photo-oxidation of alcohols

Generally adopted design strategies for enhancing the photocatalytic activity are aimed at tuning properties such as the visible light response, the exposed crystal facets, and the nanocrystal shape. Here, we present a different approach for designing efficient photocatalysts displaying a substrate-specific reactivity upon defect engineering. The defective anisotropic brookite TiO2 photocatalyst functionalized with Pt nanocrystals are tested for alcohol photoreforming showing up to an 11-fold increase in methanol oxidation rate, compared to the unreduced one, whilst presenting much lower ethanol or isopropanol specific oxidation rates. We demonstrate that the alcohol oxidation and hydrogen evolution reactions are tightly related, and when the substrate-specific alcohol oxidation ability is increased, the hydrogen evolution is significantly boosted. The reduced anisotropic brookite shows up to twenty-six-fold higher specific photoactivity with respect to anatase and brookite with isotropic nanocrystals, reflecting the different type of defective catalytic sites formed depending on the TiO2 polymorph and its crystal shape. Advanced in-situ characterizations and theoretical investigations reveal that controlled engineering over oxygen vacancies and lattice strain produces large electron polarons hosting the substrate-specific active sites for alcohol photo-oxidation.

cond-mat.mtrl-sci↗

arXiv2022

Can the "shadow" of graphene band clarify its flatness?

Graphene band renormalization at the proximity of the van Hove singularity (VHS) has been investigated by angle-resolved photoemission spectroscopy (ARPES) on the Li-doped quasi-freestanding graphene on the cobalt (0001) surface. The absence of graphene band hybridization with the substrate, the doping contribution well represented by a rigid energy shift and the excellent electron-electron interaction screening ensured by the metallic substrate offer a privileged point of view for such investigation. A clear ARPES signal is detected along the M point of the graphene Brillouin zone, giving rise to an apparent flattened band. By simulating the graphene spectral function from the density functional theory calculated bands, we demonstrate that the photoemission signal along the M point originates from the "shadow" of the spectral function of the unoccupied band above the Fermi level. Such interpretation put forward the absence of any additional strong correlation effects at the VHS proximity, reconciling the mean field description of the graphene band structure even in the highly doped scenario.

cond-mat.str-el↗