arXiv subjects

Cesare Tresca

Publications and source records attributed to Cesare Tresca.

14 recordsLinked to original sources

arXiv2026

Twistronic control of shift current in multilayer moir\'e system

The bulk photovoltaic effect in non-centrosymmetric materials provides an alternative mechanism for the conversion of light into a current response compared to p-n junctions. Among its various contributions, the shift current is particularly attractive because it is governed by the geometric properties of electronic wavefunctions and can generate large photocurrents in low-dimensional materials. Here, we investigate the evolution of the shift current response in mono-, bi-, and trilayer H-MoS2, as well as in twisted moir\'e bilayers and trilayers. To describe large moir\'e supercells we develop a Slater-Koster tight-binding model parametrized from first-principles calculations. The resulting electronic structures and shift-current responses are compared with density functional theory calculations and Wannier-interpolated results to verify the accuracy of the approach. The model accurately reproduces the electronic structure near the band edges and captures the main spectral features of the shift current conductivity. We show that twisting breaks the crystal symmetry and activates additional conductivity tensor components that are forbidden in untwisted structures, leading to new tunable in-plane photocurrent components. Analysis of the shift distance reveals a direct connection between the twist-induced modification of the electronic wavefunctions and the increase of the nonlinear response. Our results establish the twist angle as an effective parameter for engineering shift current generation in multilayer transition-metal dichalcogenide base systems and demonstrate that tight-binding approaches provide a practical route for exploring nonlinear optical phenomena in large-scale moir\'e materials beyond the limits of conventional first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2026

Antiferromagnetic stripe phase and large-gap insulating ground state of the correlated $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) single atomic layer

The one-third monolayer Sn layer on Si(111) has long been considered a benchmark system for exploring two-dimensional Mott physics, owing to its narrow bandwidth and sizable on-site Coulomb repulsion. Previous experiments suggested the emergence of a low-temperature Mott insulating phase with an energy gap of only a few tens of meV, while theory predicted a possible antiferromagnetic ordering that remained experimentally elusive. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy with first-principles calculations, we reveal that the $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) surface undergoes a transition below 30K into a robust insulating state characterized by a remarkably large gap of about 440 $\pm$ 120 meV at 4K, five to ten times larger than previously reported. Quasiparticle interference imaging uncovers a well-defined $2\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) superstructure, providing direct evidence for a two-dimensional stripe-like antiferromagnetic order. Ab initio calculations reveal that the silicon substrate stabilizes this phase through strong nonlocal tin-tin interactions, highlighting the decisive role of substrate-driven correlations in the $\sqrt{3}\times\sqrt{3}$~R30$^{\circ}$-Sn/Si(111) system.

cond-mat.str-el↗

arXiv2025

Multi-Gap superconductivity in HgS under pressure

Mercury chalcogenides is a class of materials that exhibit diverse structural phases under pressure, hosting exotic physical properties, including topological phases and chiral phonons. In particular, recent experimental results on HgS reports a new superconducting phase at 21 GPa, whose origin is unknown. In this letter we theoretically investigate the pressure-induced structural phase transition in HgS and the emergence of superconductivity in the rock salt phase. Remarkably, we discover that the rock salt phase hosts a two-gap superconducting phase originating from distinct Fermi surfaces. The unusually high critical temperature of 11 K emerges naturally within this multiband scenario, highlighting the role of interband coupling beyond isotropic approximation. These results place HgS among the few systems where multiband superconductivity is observed.

cond-mat.supr-con↗

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↗

arXiv2024

Doping the spin-polarized Graphene minicone on Ni(111)

In the attempt to induce spin-polarized states in graphene, rare-earth deposition on Gr/Co(0001) has been demonstrated to be a successful strategy: the coupling of graphene with the cobalt substrate provides spin-polarized conical-shaped states (mini-cone) and the rare-earth deposition brings these states at the Fermi level. In this manuscript we theoretically explore the feasibility of an analogue approach applied on Gr/Ni(111) doped with rare-earth ions. Even if not well mentioned in the lecture also this system owns a mini-cone, similar to the cobalt case. By testing different rare-earth ions, not only we suggest which one can provide the required doping but we explain the effect behind this proper charge transfer.

cond-mat.mtrl-sci↗

arXiv2023

Unveiling the pairing Symmetry of the superconducting Sn/Si(111) via angle-resolved THz pump spectroscopy

Doping tin surface epitaxially grown on silicon, Sn/Si(111), with boron atoms yields the appearance of a superconducting (SC) phase below $T_c\sim 4-5$K. Even though the pairing mechanism remains unknown, experimental evidence of chiral $d-$wave superconductivity has been recently reported, then ruling out a phonon-mediated pairing. Here we study theoretically the SC phase and symmetries of the doped Sn/Si(111) within a $t-J$ model. We analyze the photo-excitation of the system by intense THz pulses and show that the polarization dependence of the induced current can distinguish between different symmetries of the SC gap, thus providing a novel experimental mean to investigate the spectroscopic features of the Sn/Si(111) across the SC transition.

cond-mat.supr-con↗

arXiv2023

Evidence of Molecular Hydrogen in the N-doped LuH3 System: a Possible Path to Superconductivity?

The discovery of ambient superconductivity would mark an epochal breakthrough long-awaited for over a century, potentially ushering in unprecedented scientific and technological advancements. The recent findings on high-temperature superconducting phases in various hydrides under high pressure have ignited optimism, suggesting that the realization of near-ambient superconductivity might be on the horizon. However, the preparation of hydride samples tends to promote the emergence of various metastable phases, marked by a low level of experimental reproducibility. Identifying these phases through theoretical and computational methods entails formidable challenges, often resulting in controversial outcomes. In this paper, we consider N-doped LuH3 as a prototypical complex hydride: By means of machine-learning-accelerated force-field molecular dynamics, we have identified the formation of H2 molecules stabilized at ambient pressure by nitrogen impurities. Importantly, we demonstrate that this molecular phase plays a pivotal role in the emergence of a dynamically stable, low-temperature, experimental-ambient-pressure superconductivity. The potential to stabilize hydrogen in molecular form through chemical doping opens up a novel avenue for investigating disordered phases in hydrides and their transport properties under near-ambient conditions.

cond-mat.supr-con↗

arXiv2023

Superconductivity induced by gate-driven hydrogen intercalation in the charge-density-wave compound 1T-TiSe2

Hydrogen (H) plays a key role in the near-to-room temperature superconductivity of hydrides at megabar pressures. This suggests that H doping could have similar effects on the electronic and phononic spectra of materials at ambient pressure as well. Here, we demonstrate the non-volatile control of the electronic ground state of titanium diselenide (1T-TiSe$_2$) via ionic liquid gating-driven H intercalation. This protonation induces a superconducting phase, observed together with a charge-density wave through most of the phase diagram, with nearly doping-independent transition temperatures. The H-induced superconducting phase is possibly gapless-like and multi-band in nature, in contrast with those induced in TiSe$_2$ via copper, lithium, and electrostatic doping. This unique behavior is supported by ab initio calculations showing that high concentrations of H dopants induce a full reconstruction of the bandstructure, although with little coupling between electrons and high-frequency H phonons. Our findings provide a promising approach for engineering the ground state of transition metal dichalcogenides and other layered materials via gate-controlled protonation.

cond-mat.supr-con↗

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↗

arXiv2021

Why Mercury is a superconductor

Despite being the oldest known superconductor, solid mercury is mysteriously absent from all current computational databases of superconductors. In this work, we present a critical study of its superconducting properties based on state-of-the-art superconducting density-functional theory. Our calculations reveal numerous anomalies in electronic and lattice properties, which can mostly be handled, with due care, by modern ab-initio techniques. In particular, we highlight an anomalous role of (i) electron-electron correlations on structural properties (ii) spin-orbit coupling on the dynamical stability, and (iii) semicore $d$ levels on the effective Coulomb interaction and, ultimately, the critical temperature.

cond-mat.supr-con↗

arXiv2021

Charge density wave in single-layer Pb/Ge(111) driven by Pb-substrate exchange interaction

Single layer Pb on top of (111) surfaces of group IV semiconductors hosts charge density wave and superconductivity depending on the coverage and on the substrate. These systems are normally considered to be experimental realizations of single band Hubbard models and their properties are mostly investigated using lattice models with frozen structural degrees of freedom, although the reliability of this approximation is unclear. Here, we consider the case of Pb/Ge(111) at 1/3 coverage, for which surface X-ray diffraction and ARPES data are available. By performing first principles calculations, we demonstrate that the non-local exchange between Pb and the substrate drives the system into a $3\times 3$ charge density wave. The electronic structure of this charge ordered phase is mainly determined by two effects: the magnitude of the Pb distortion and the large spin-orbit coupling. Finally, we show that the effect applies also to the $3\times 3$ phase of Pb/Si(111) where the Pb-substrate exchange interaction increases the bandwidth by more than a factor 1.5 with respect to DFT+U, in better agreement with STS data. The delicate interplay between substrate, structural and electronic degrees of freedom invalidates the widespread interpretation available in literature considering these compounds as physical realizations of single band Hubbard models.

cond-mat.str-el↗

arXiv2020

Misfit layer compounds: a platform for heavily-doped two-dimensional transition metal dichalcogenides

Transition metal dichalcogenides (TMDs) display a rich variety of instabilities such as spin and charge orders, Ising superconductivity and topological properties. Their physical properties can be controlled by doping in electric double-layer field-effect transistors (FET). However, for the case of single layer NbSe$_2$, FET doping is limited to $\approx 1\times 10^{14}$ cm$^{-2}$, while a somewhat larger charge injection can be obtained via deposition of K atoms. Here, by performing ARPES, STM, quasiparticle interference measurements, and first principles calculations we show that a misfit compound formed by sandwiching NbSe$_2$ and LaSe layers behaves as a NbSe$_2$ single layer with a rigid doping of $0.55-0.6$ electrons per Nb atom or $\approx 6\times 10^{14}$ cm$^{-2}$. Due to this huge doping, the $3\times3$ charge density wave is replaced by a $2\times2$ order with very short coherence length. As a tremendous number of different misfit compounds can be obtained by sandwiching TMDs layers with rock salt or other layers, our work paves the way to the exploration of heavily doped 2D TMDs over an unprecedented wide range of doping.

cond-mat.mtrl-sci↗

arXiv2019

Charge density wave and spin $1/2$ insulating state in single layer 1T-NbS$_2$

In bulk samples and few layer flakes, the transition metal dichalcogenides NbS$_2$ and NbSe$_2$ assume the H polytype structure with trigonal prismatic coordination of the Nb atom. Recently, however, single and few layers of 1T-NbSe$_2$ with octahedral coordination around the transition metal ion were synthesized. Motivated by these experiments and by using first-principles calculations, we investigate the structural, electronic and dynamical properties of single layer 1T-NbS$_2$. We find that single-layer 1T-NbS$_2$ undergoes a $\sqrt{13}\times\sqrt{13}$ star-of-David charge density wave. Within the generalized gradient approximation, the weak interaction between the stars leads to an ultraflat band at the Fermi level isolated from all other bands. The spin-polarized generalized gradient approximation stabilizes a total spin $1/2$ magnetic state with opening of a $0.15$ eV band gap and a $0.21μ_B$ magnetic moment localized on the central Nb in the star. Within GGA+U, the magnetic moment on the central Nb is enhanced to $0.41μ_{B}$ and a larger gap occurs. Most important, this approximation gives a small energy difference between the 1T and 1H polytypes (only $+0.5$ mRy/Nb), suggesting that the 1T-polytype can be synthesized in a similar way as done for single layer 1T-NbSe$_2$. Finally we compute first and second nearest neighbors magnetic inter-star exchange interactions finding $J_1$=9.5~K and $J_2$=0.4~K ferromagnetic coupling constants.

cond-mat.str-el↗

arXiv2015

Strain effects in monolayer Iron-Chalcogenide superconductors

The successful fabrication of one monolayer FeSe on SrTiO$_{3}$ represented a real breakthrough in searching for high-T$_{c}$ Fe-based superconductors (Ref. 1). Motivated by this important discovery, we studied the effects of tensile strain on one monolayer and bulk iron-chalcogenide superconductors (FeSe and FeTe), showing that it produces important magnetic and electronic changes in the systems. We found that the magnetic ground state of bulk and monolayer FeSe is the block-checkerboard phase, which turns into the collinear stripe phase under in plane tensile strain. FeTe, in both bulk and monolayer phases, shows two magnetic transitions upon increasing the tensile strain: from bicollinear in the ground state to block-checkerboard ending up to the collinear antiferromagnetic phase which could bring it in the superconducting state. Finally, the study of the mechanical properties of both FeSe and FeTe monolayers reveals their enormous tensile strain limits and opens the possibility to grow them on different substrates.

cond-mat.supr-con↗