arXiv subjects

Luca Moreschini

Publications and source records attributed to Luca Moreschini.

At least 19 recordsLinked to original sources

arXiv2026

Layer-Controlled Intermolecular Coupling and Many-Body Effects in C$_{60}$ Films

Crystalline C$_{60}$ is a molecular solid whose electronic properties emerge from the interplay of intermolecular hopping, electron correlations, and electron-vibration coupling. Unlike moir$\rm\acute{e}$ van der Waals heterostructures, where interaction strength is commonly tuned by twist angle, molecular materials offer a complementary route in which layer number, molecular orientation, and substrate registry provide experimentally accessible control parameters. Here we present a systematic thickness-dependent angle-resolved photoemission study of C$_{60}$ films, spanning the monolayer to the bulk limit. The HOMO-derived band exhibits a non-monotonic evolution: the intermediate-thickness film shows larger bandwidth, reduced effective mass, and pronounced gap-like and sub-band features. The experimental trends, together with Holstein-model simulations, point to strengthened effective intermolecular electronic coupling and enhanced electron-phonon-induced spectral renormalization in the intermediate-thickness regime. These results identify a dimensional crossover in C$_{60}$ films and establish layer number as an effective knob for engineering electronic structure and many-body interactions in molecular thin films.

cond-mat.mtrl-sci↗

arXiv2026

Spin-Valley Locking in 2H-TaS2 and Its Co-Intercalated Counterpart: Roles of Surface Domains and Co Intercalation

Tuning and probing spin-valley coupling is key to understanding correlated ground states in 2$\it{H}$-TaS$_2$. Its magnetically intercalated analogue, Co$_{1/3}$TaS$_2$, introduces additional degrees of freedom, including modified interlayer coupling and magnetism, to modulate spin-valley physics. Surface-sensitive probes like ARPES are essential for accessing surface spin texture, yet previous studies on 2$\it{H}$-TMDs have reported conflicting results regarding spin-polarized bands, leaving open whether these discrepancies are intrinsic or extrinsic. Here we performed spatially resolved spin-ARPES measurements on 2$\it{H}$-TaS$_2$ and Co$_{1/3}$TaS$_2$. Our results reveal robust spin-valley locking on both compounds. Importantly, Co intercalation enhances interlayer hybridization and introduces magnetism while preserving the TaS$_2$-derived spin texture. We further observe a spatial reversal of the out-of-plane spin polarization, which we attribute to different surface domains. This effect complicates quantifying spin textures and may underlie prior inconsistent observations. Our findings provide microscopic insight into how interlayer interactions and surface domains together govern spin-valley phenomena in layered TMDs.

cond-mat.mtrl-sci↗

arXiv2026

Discovery of Van Hove Singularities: Electronic Fingerprints of 3Q Magnetic Order in a van der Waals Quantum Magnet

Magnetically intercalated transition metal dichalcogenides are emerging as a rich platform for exploring exotic quantum states in van der Waals magnets. Among them, CoxTaS2 has attracted intense interest following the recent discovery of a distinctive 3Q magnetic ground state and a pronounced topological Hall effect below a critical doping of x=1/3, both intimately tied to cobalt concentration. To date, direct signatures of this enigmatic 3Q magnetic order in the electronic structure remain elusive. Here we report a comprehensive doping dependent angle resolved photoemission spectroscopy study that unveils these long-sought fingerprints. Our data reveal an unexpected "inverse Mexican hat" dispersion along the K-M-K direction, accompanied by two van Hove singularities. These features are consistent with theoretical predictions for a 3Q magnetic order near three-quarters band filling on a cobalt triangular lattice. These results provide evidence of 3Q magnetic order in the electronic structure, establishing TMD van der Waals magnets as tunable materials to explore the interplay between magnetism and topology.

cond-mat.mtrl-sci↗

arXiv2025

Buried unstrained germanium channels: a lattice-matched platform for quantum technology

Strained Ge ($\epsilon$-Ge) and strained Si ($\epsilon$-Si) buried quantum wells have enabled advanced spin-qubit quantum processors. However, in the absence of suitable lattice-matched substrates, $\epsilon$-Ge and $\epsilon$-Si are deposited on defective, metamorphic SiGe substrates, which may impact device performance and scaling. Here an alternative platform is introduced, based on the heterojunction between unstrained Ge and a lattice-matched strained SiGe ($\epsilon$-SiGe) barrier, eliminating the need for metamorphic buffers altogether. In a structure with a 52-nm-thick $\epsilon$-SiGe barrier, a low-disorder two-dimensional hole gas is demonstrated with a high-mobility of 1.33$\times$10$^5$ cm$^2$/Vs and a low percolation density of 1.4(1)$\times$10$^1$$^0$ cm$^-$$^2$. Quantum transport shows that holes confined in the buried unstrained Ge channel have a strong density-dependent in-plane effective mass and out-of-plane $g$-factor, pointing to a significant heavy-hole$-$light-hole mixing in agreement with theory. Measurements of Zeeman spin-split levels in quantum point contacts further highlight this character, showing a two-fold larger in-plane $g$-factor in Ge than in $\epsilon$-Ge. The prospect of strong spin-orbit interaction, isotopic purification, and of hosting superconducting pairing correlations make this platform appealing for fast quantum hardware and hybrid quantum systems.

cond-mat.mes-hall↗

arXiv2024

Ultrafast creation of a light induced semimetallic state in strongly excited 1T-TiSe$_2$

Screening, a ubiquitous phenomenon associated with the shielding of electric fields by surrounding charges, has been widely adopted as a means to modify a material's properties. While so far most studies have relied on static changes of screening through doping or gating, here we demonstrate that screening can also drive the onset of distinct quantum states on the ultrafast timescale. By using time and angle-resolved photoemission spectroscopy we show that intense optical excitation can drive 1T-TiSe$_2$, a prototypical charge density wave material, almost instantly from a gapped into a semimetallic state. By systematically comparing changes in bandstructure over time and excitation strength with theoretical calculations we find that the appearance of this state is likely caused by a dramatic reduction of the screening length. In summary, this work showcases how optical excitation enables the screening driven design of a non-equilibrium semimetallic phase in TiSe$_2$, possibly providing a general pathway into highly screened phases in other strongly correlated materials.

cond-mat.mes-hall↗

arXiv2023

New three-dimensional dispersion in the type-II Dirac semimetals PtTe$_2$ and PdTe$_2$ revealed through Angle Resolved Photoemission Spectroscopy

PtTe$_2$ and PdTe$_2$ are among the first transition metal dichalcogenides that were predicted to host type-II Dirac fermions, exotic particles prohibited in free space. These materials are layered and air-stable, which makes them top candidates for technological applications that take advantage of their anisotropic magnetotransport properties. Here, we provide a detailed characterization of the electronic structure of PtTe$_2$ and PdTe$_2$ using Angle Resolved Photoemission Spectroscopy (ARPES) and Density Functional Theory (DFT) calculations, unveiling a new three-dimensional dispersion in these materials. Through the use of circularly polarized light, we report a different behavior of such dispersion in PdTe$_2$ compared to PtTe$_2$, that we relate to a symmetry analysis of the dipole matrix element. Such analysis reveals a link between the observed circular dichroism and the different momentum-dependent terms in the dispersion of these two compounds, despite their close similarity in crystal structure. Additionally, our data shows a clear difference in the circular dichroic signal for the type-II Dirac cones characteristic of these materials, compared to their topologically protected surface states. Our work provides a useful reference for the ARPES characterization of other transition metal dichalcogenides with topological properties and illustrates the use of circular dichroism as a guide to identify the topological character of two otherwise equivalent band dispersions, and to recognize different attributes in the band structure of similar materials.

cond-mat.mes-hall↗

arXiv2023

Linearly dispersive bands at the onset of correlations in K$_x$C$_{60}$ films

Molecular crystals are a flexible platform to induce novel electronic phases. Due to the weak forces between molecules, intermolecular distances can be varied over relatively larger ranges than interatomic distances in atomic crystals. On the other hand, the hopping terms are generally small, which results in narrow bands, strong correlations and heavy electrons. Here, by growing K$_x$C$_{60}$ fullerides on hexagonal layered Bi$_2$Se$_3$, we show that upon doping the series undergoes a Mott transition from a molecular insulator to a correlated metal, and an in-gap state evolves into highly dispersive Dirac-like fermions at half filling, where superconductivity occurs. This picture challenges the commonly accepted description of the low energy quasiparticles as appearing from a gradual electron doping of the conduction states, and suggests an intriguing parallel with the more famous family of the cuprate superconductors. More in general, it indicates that molecular crystals offer a viable route to engineer electron-electron interactions.

cond-mat.mes-hall↗

arXiv2022

Antiferromagnetic metal phase in an electron-doped rare-earth nickelate

Long viewed as passive elements, antiferromagnetic materials have emerged as promising candidates for spintronic devices due to their insensitivity to external fields and potential for high-speed switching. Recent work exploiting spin and orbital effects has identified ways to electrically control and probe the spins in metallic antiferromagnets, especially in noncollinear or noncentrosymmetric spin structures. The rare earth nickelate NdNiO3 is known to be a noncollinear antiferromagnet where the onset of antiferromagnetic ordering is concomitant with a transition to an insulating state. Here, we find that for low electron doping, the magnetic order on the nickel site is preserved while electronically a new metallic phase is induced. We show that this metallic phase has a Fermi surface that is mostly gapped by an electronic reconstruction driven by the bond disproportionation. Furthermore, we demonstrate the ability to write to and read from the spin structure via a large zero-field planar Hall effect. Our results expand the already rich phase diagram of the rare-earth nickelates and may enable spintronics applications in this family of correlated oxides.

cond-mat.str-el↗

arXiv2021

Enhanced tunability of two-dimensional electron gas on SrTiO3 through heterostructuring

Two-dimensional electron gases (2DEGs) on the SrTiO3 (STO) surface or in STO-based heterostructures have exhibited many intriguing phenomena, which are strongly dependent on the 2DEG-carrier density. We report that the tunability of the 2DEG-carrier density is significantly enhanced by adding a monolayer LaTiO3 (LTO) onto the STO. Ultraviolet (UV) irradiation induced maximum carrier density of the 2DEG in LTO/STO is increased by a factor of ~4 times, compared to that of the bare STO. By oxygen gas exposure, it becomes 10 times smaller than that of the bare STO. This enhanced tunability is attributed to the drastic surface property change of a polar LTO layer by UV irradiation and O2 exposure. This indicates that the 2DEG controllability in LTO/STO is more reliable than that on the bare STO driven by defects, such an oxygen vacancy.

cond-mat.str-el↗

arXiv2021

In situ investigation of conducting interface formation in LaAlO3/SrTiO3 heterostructure

The high-mobility conducting interface (CI) between LaAlO_{3}(LAO) and SrTiO_{3}(STO) has revealed many fascinating phenomena, including exotic magnetism and superconductivity. But, the formation mechanism of the CI has not been conclusively explained. Here, using in situ angle-resolved photoemission spectroscopy, we elucidated the mechanisms for the CI formation. In as-grown samples, we observed a built-in potential (V_{bi}) proportional to the polar LAO thickness starting from the first unit cell (UC) with CI formation appearing above 3 UCs. However, we found that the V bi is removed by synchrotron ultraviolet (UV)-irradiation; The built-in potential is recovered by oxygen gas (O_{2}(g))-exposure. Furthermore, after UV-irradiation, the CI appears even below 3UC of LAO. Our results demonstrate not only the V_{bi}-driven CI formation in asgrown LAO/STO, but also a new route to control of the interface state by UV lithographic patterning or other surface modification.

cond-mat.str-el↗

arXiv2019

Tunable Electronic Structure in Gallium Chalcogenide van der Waals Compounds

Transition metal monochalcogenides comprise a class of two-dimensional materials with electronic band gaps that are highly sensitive to material thickness and chemical composition. Here, we explore the tunability of the electronic excitation spectrum in GaSe using angle-resolved photoemission spectroscopy. The electronic structure of the material is modified by $\textit{in-situ}$ potassium deposition as well as by forming GaS$_{x}$Se$_{1-x}$ alloy compounds. We find that potassium decouples the top-most tetra-layer of the GaSe unit cell, leading to a substantial change of the dispersion around the valence band maximum (VBM). The observed band dispersion of a single tetralayer is consistent with a transition from the direct gap character of the bulk to the indirect gap character expected for monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from AB to $\text{AA}^{\prime}$ stacking. Alloying also results in a rigid energy shift of the VBM towards higher binding energies which correlates with a blue shift in the luminescence. The increase of the band gap upon sulfur alloying does not appear to change the dispersion or character of the VBM appreciably, implying that it is possible to engineer the gap of these materials while maintaining their salient electronic properties.

cond-mat.mes-hall↗

arXiv2018

Atomic-layer-resolved composition and electronic structure of the cuprate Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ from soft x-ray standing-wave photoemission

A major remaining challenge in the superconducting cuprates is the unambiguous differentiation of the composition and electronic structure of the CuO$_2$ layers and those of the intermediate layers. The large c axis for these materials permits employing soft x-ray (930.3 eV) standing wave (SW) excitation in photoemission that yields atomic layer-by-atomic layer depth resolution of these properties. Applying SW photoemission to Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$ yields the depth distribution of atomic composition and the layer-resolved densities of states. We detect significant Ca presence in the SrO layers and oxygen bonding to three different cations. The layer-resolved valence electronic structure is found to be strongly influenced by the supermodulation structure--as determined by comparison to DFT calculations, by Ca-Sr intermixing, and by the Cu 3d-3d Coulomb interaction, further clarifying the complex interactions in this prototypical cuprate. Measurements of this type for other quasi-two-dimensional materials with large-c represent a promising future direction.

cond-mat.supr-con↗

arXiv2017

Universal Mechanism of Band-Gap Engineering in Transition-Metal Dichalcogenides

Two-dimensional (2D) van-der-Waals semiconductors have emerged as a class of materials with promising device characteristics owing to the intrinsic bandgap. For realistic applications, the ideal is to modify the bandgap in a controlled manner by a mechanism that can be generally applied to this class of materials. Here, we report the observation of a universally tunable bandgap in the family of bulk 2H transition metal dichalcogenides (TMDs) by in situ surface doping of Rb atoms. A series of angle-resolved photoemission spectra unexceptionally shows that the bandgap of TMDs at the zone corners is modulated in the range of 0.8 ~ 2.0 eV, which covers a wide spectral range from visible to near infrared, with a tendency from indirect to direct bandgap. A key clue to understand the mechanism of this bandgap engineering is provided by the spectroscopic signature of symmetry breaking and resultant spin splitting, which can be explained by the formation of 2D electric dipole layers within the surface bilayer of TMDs. Our results establish the surface Stark effect as a universal mechanism of bandgap engineering based on the strong 2D nature of van-der-Waals semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

A Novel Quasi-One-Dimensional Topological Insulator in Bismuth Iodide $β$-Bi$_4$I$_4$

Recent progress in the field of topological states of matter(1,2) has largely been initiated by the discovery of bismuth and antimony chalcogenide bulk topological insulators (TIs)(3-6), followed by closely related ternary compounds(7-16) and predictions of several weak TIs(17-19). However, both the conceptual richness of Z$_2$ classification of TIs as well as their structural and compositional diversity are far from being fully exploited. Here, a new Z$_2$ topological insulator is theoretically predicted and experimentally confirmed in the $β$-phase of quasi-one-dimensional bismuth iodide Bi$_4$I$_4$. The electronic structure of $β$-Bi$_4$I$_4$, characterized by Z$_2$ invariants (1;110), is in proximity of both the weak TI phase (0;001) and the trivial insulator phase (0;000). Our angle-resolved photoemission spectroscopy measurements on the (001) surface reveal a highly anisotropic band-crossing feature located at the point of the surface Brillouin zone and showing no dispersion with the photon energy, thus being fully consistent with the theoretical prediction.

cond-mat.mtrl-sci↗

arXiv2015

Latent Instabilities in Metallic LaNiO3 Films by Strain Control of Fermi-Surface Topology

Strain control is one of the most promising avenues to search for new emergent phenomena in transition-metal-oxide films. Here, we investigate the strain-induced changes of electronic structures in strongly correlated LaNiO3 (LNO) films, using angle-resolved photoemission spectroscopy and the dynamical mean-field theory. The strongly renormalized eg-orbital bands are systematically rearranged by misfit strain to change its fermiology. As tensile strain increases, the hole pocket centered at the A point elongates along the kz-axis and seems to become open, thus changing Fermi-surface (FS) topology from three- to quasi-two-dimensional. Concomitantly, the FS shape becomes flattened to enhance FS nesting. A FS superstructure with Q1 = (1/2,1/2,1/2) appears in all LNO films, while a tensile-strained LNO film has an additional Q2 = (1/4,1/4,1/4) modulation, indicating that some instabilities are present in metallic LNO films. Charge disproportionation and spin-density-wave fluctuations observed in other nickelates might be their most probable origins.

cond-mat.str-el↗

arXiv2013

Dimensional crossover of the electronic structure in LaNiO3 ultrathin films: Orbital reconstruction, Fermi surface nesting, and the origin of the metal-insulator transition

Dimensionality control in the LaNiO3 (LNO) heterostructure has attracted attention due to its two-dimensional (2D) electronic structure was predicted to have an orbital ordered insulating ground state, analogous to that of the parent compound of high-Tc cuprate superconductors [P. Hansmann et al., Phys. Rev. Lett. 103, 016401 (2009)]. Here, we directly measured the electronic structure of LNO ultrathin films using in situ angle-resolved photoemission spectroscopy (ARPES). We recognized the dimensional crossover of the electronic structure around 3-unit cells (UC)-thick LNO film and observed the orbital reconstruction. However, complete orbital ordering was not achieved. Instead, we observed that the Fermi surface nesting effect became strong in the 2D LNO ultrathin film. These results indicated that the orbital reconstruction should be described by taking into account the strong nesting effect to search for the novel phenomena, such as superconductivity in 2D LNO heterostructure. In addition, the APRES spectra showed that the Fermi surface existed down to a 1-UC-thick film, which showed insulating behavior in transport measurements. We suggested that the metal-insulator transition in the transport properties may originate from Anderson localization.

cond-mat.str-el↗

arXiv2012

Small scale rotational disorder observed in epitaxial graphene on SiC(0001)

Interest in the use of graphene in electronic devices has motivated an explosion in the study of this remarkable material. The simple, linear Dirac cone band structure offers a unique possibility to investigate its finer details by angle-resolved photoelectron spectroscopy (ARPES). Indeed, ARPES has been performed on graphene grown on metal substrates but electronic applications require an insulating substrate. Epitaxial graphene grown by the thermal decomposition of silicon carbide (SiC) is an ideal candidate for this due to the large scale, uniform graphene layers produced. The experimental spectral function of epitaxial graphene on SiC has been extensively studied. However, until now the cause of an anisotropy in the spectral width of the Fermi surface has not been determined. In the current work we show, by comparison of the spectral function to a semi-empirical model, that the anisotropy is due to small scale rotational disorder ($\sim\pm$ 0.15$^{\circ}$) of graphene domains in graphene grown on SiC(0001) samples. In addition to the direct benefit in the understanding of graphene's electronic structure this work suggests a mechanism to explain similar variations in related ARPES data.

cond-mat.mes-hall↗

arXiv2011

Electronic structure of graphene on single crystal copper substrates

The electronic structure of graphene on Cu(111) and Cu(100) single crystals is investigated using low energy electron microscopy, low energy electron diffraction and angle resolved photoemission spectroscopy. On both substrates the graphene is rotationally disordered and interactions between the graphene and substrate lead to a shift in the Dirac crossing of $\sim$ -0.3 eV and the opening of a $\sim$ 250 meV gap. Exposure of the samples to air resulted in intercalation of oxygen under the graphene on Cu(100), which formed a ($\sqrt{2} \times 2\sqrt{2}$)R45$^{\rm o}$ superstructure. The effect of this intercalation on the graphene $π$ bands is to increase the offset of the Dirac crossing ($\sim$ -0.6 eV) and enlarge the gap ($\sim$ 350 meV). No such effect is observed for the graphene on Cu(111) sample, with the surface state at $Γ$ not showing the gap associated with a surface superstructure. The graphene film is found to protect the surface state from air exposure, with no change in the effective mass observed.

cond-mat.mes-hall↗