arXiv subjects

Michele Pizzochero

Publications and source records attributed to Michele Pizzochero.

33 records · Page 2Linked to original sources

arXiv2022

Electronic excitations and spin interactions in chromium trihalides from embedded many-body wavefunctions

Although chromium trihalides are widely regarded as a promising class of two-dimensional magnets for next-generation devices, an accurate description of their electronic structure and magnetic interactions has proven challenging to achieve. Here, we quantify electronic excitations and spin interactions in Cr$X_3$ ($X=$~Cl, Br, I) using embedded many-body wavefunction calculations and fully generalized spin Hamiltonians. We find that the three trihalides feature comparable $d$-shell excitations, consisting of a high-spin $^4A_2$ $(t^3_{2g}e^0_{g})$ ground state lying 1.5$-$1.7 eV below the first excited state $^4T_2$ ($t^2_{2g}e^1_{g}$). CrCl$_3$ exhibits a single-ion anisotropy $A_{\rm sia} = -0.02$ meV, while the Cr spin-3/2 moments are ferromagnetically coupled through bilinear and biquadratic exchange interactions of $J_1 = -0.97$ meV and $J_2 = -0.05$ meV, respectively. The corresponding values for CrBr$_3$ and CrI$_3$ increase to $A_{\rm sia} = -0.08$ meV and $A_{\rm sia} = -0.12$ meV for the single-ion anisotropy, $J_1 = -1.21$ meV, $J_2 = -0.05$ meV and $J_1 = -1.38$ meV, $J_2 = -0.06$ meV for the exchange couplings, respectively. We find that the overall magnetic anisotropy is defined by the interplay between $A_{\rm sia}$ and $A_{\rm dip}$ due to magnetic dipole-dipole interaction that favors in-plane orientation of magnetic moments in ferromagnetic monolayers and bulk layered magnets. The competition between the two contributions sets CrCl$_3$ and CrI$_3$ as the easy-plane ($A_{\rm sia}+ A_{\rm dip} > 0$) and easy-axis ($A_{\rm sia}+ A_{\rm dip} < 0$) ferromagnets, respectively. The differences between the magnets trace back to the atomic radii of the halogen ligands and the magnitude of spin-orbit coupling. Our findings are in excellent agreement with recent experiments, thus providing reference values for the fundamental interactions in chromium trihalides.

cond-mat.mes-hall↗

arXiv2022

Hydrogen Atoms on Zigzag Graphene Nanoribbons: Chemistry and Magnetism Meet at the Edge

Although the unconventional $π$-magnetism at the zigzag edges of graphene holds promise for a wide array of applications, whether and to what degree it plays a role in their chemistry remains poorly understood. Here, we investigate the addition of a hydrogen atom $-$ the simplest yet the most experimentally relevant adsorbate $-$ to zigzag graphene nanoribbons (ZGNRs). We show that the $π$-magnetism governs the chemistry of ZGNRs, giving rise to a site-dependent reactivity of the carbon atoms and driving the hydrogenation process to the nanoribbon edges. Conversely, the chemisorbed hydrogen atom governs the $π$-magnetism of ZGNRs, acting as a spin-$\frac{1}{2}$ paramagnetic center in the otherwise antiferromagnetic ground state and spin-polarizing the charge carriers at the band extrema. Our findings establish a comprehensive picture of the peculiar interplay between chemistry and magnetism that emerges at the zigzag edges of graphene.

cond-mat.mtrl-sci↗

arXiv2021

Electrically Induced Dirac Fermions in Graphene Nanoribbons

Graphene nanoribbons are widely regarded as promising building blocks for next-generation carbon-based devices. A critical issue to their prospective applications is whether and to what degree their electronic structure can be externally controlled. Here, we combine simple model Hamiltonians with extensive first-principles calculations to investigate the response of armchair graphene nanoribbons to transverse electric fields. Such fields can be achieved either upon laterally gating the nanoribbon or incorporating ambipolar chemical co-dopants along the edges. We reveal that the field induces a semiconductor-to-semimetal transition, with the semimetallic phase featuring zero-energy Dirac fermions that propagate along the armchair edges. The transition occurs at critical fields that scale inversely with the width of the nanoribbons. These findings are universal to group-IV honeycomb lattices, including silicene and germanene nanoribbons, irrespective of the type of edge termination. Overall, our results create new opportunities to electrically engineer Dirac fermions in otherwise semiconducting graphene-like nanoribbons.

cond-mat.mtrl-sci↗

arXiv2021

Edge Disorder in Bottom-Up Zigzag Graphene Nanoribbons: Implications for Magnetism and Quantum Electronic Transport

We unveil the nature of the structural disorder in bottom-up zigzag graphene nanoribbons along with its effect on the magnetism and electronic transport on the basis of scanning probe microscopies and first-principles calculations. We find that edge-missing m-xylene units emerging during the cyclodehydrogenation step of the on-surface synthesis are the most common point defects. These "bite'' defects act as spin-1 paramagnetic centers, severely disrupt the conductance spectrum around the band extrema, and give rise to spin-polarized charge transport. We further show that the electronic conductance across graphene nanoribbons is more sensitive to "bite" defects forming at the zigzag edges than at the armchair ones. Our work establishes a comprehensive understanding of the low-energy electronic properties of disordered bottom-up graphene nanoribbons.

cond-mat.mtrl-sci↗

arXiv2021

Imprinting Tunable $π$-Magnetism in Graphene Nanoribbons via Edge Extensions

Magnetic carbon nanostructures are currently under scrutiny for a wide spectrum of applications. Here, we theoretically investigate armchair graphene nanoribbons patterned with asymmetric edge extensions consisting of laterally fused naphtho groups, as recently fabricated via on-surface synthesis. We show that an individual edge extension acts as a spin-$\frac{1}{2}$ center and develops a sizable spin-polarization of the conductance around the band edges. The Heisenberg exchange coupling between a pair of edge extensions is dictated by the position of the second naphtho group in the carbon backbone, thus enabling ferromagnetic, antiferromagnetic, or non-magnetic states. The periodic arrangement of edge extensions yields full spin-polarization at the band extrema, and the accompanying ferromagnetic ground state can be driven into non-magnetic or antiferromagnetic phases through external stimuli. Overall, our work reveals precise tunability of the $π$-magnetism in graphene nanoribbons induced by naphtho groups, thereby establishing these one-dimensional architectures as suitable platforms for logic spintronics.

cond-mat.mes-hall↗

arXiv2020

Electronic transport across quantum dots in graphene nanoribbons: Toward built-in gap-tunable metal-semiconductor-metal heterojunctions

The success of all-graphene electronics is severely hindered by the challenging realization and subsequent integration of semiconducting channels and metallic contacts. Here, we comprehensively investigate the electronic transport across width-modulated heterojunctions consisting of a graphene quantum dot of varying lengths and widths embedded in a pair of armchair-edged metallic nanoribbons, of the kind recently fabricated via on-surface synthesis. We show that the presence of the quantum dot enables the opening of a width-dependent transport gap, thereby yielding built-in one-dimensional metal-semiconductor-metal junctions. Furthermore, we find that, in the vicinity of the band edges, the conductance is subject to a smooth transition from an antiresonant to a resonant transport regime upon increasing the channel length. These results are rationalized in terms of a competition between quantum-confinement effects and quantum dot-to-lead coupling. Overall, our work establishes graphene quantum dot nanoarchitectures as appealing platforms to seamlessly integrate gap-tunable semiconducting channels and metallic contacts into an individual nanoribbon, hence realizing self-contained carbon-based electronic devices.

cond-mat.mes-hall↗

arXiv2020

Quantum Electronic Transport Across "Bite" Defects in Graphene Nanoribbons

On-surface synthesis has recently emerged as an effective route towards the atomically precise fabrication of graphene nanoribbons of controlled topologies and widths. However, whether and to which degree structural disorder occurs in the resulting samples is a crucial issue for prospective applications that remains to be explored. Here, we experimentally identify missing benzene rings at the edges, which we name "bite" defects, as the most abundant type of disorder in armchair nanoribbons synthesized by the bottom-up approach. First, we address their density and spatial distribution on the basis of scanning tunnelling microscopy and find that they exhibit a strong tendency to aggregate. Next, we explore their effect on the quantum charge transport from first-principles calculations, revealing that such imperfections substantially disrupt the conduction properties at the band edges. Finally, we generalize our theoretical findings to wider nanoribbons in a systematic manner, hence establishing practical guidelines to minimize the detrimental role of such defects on the charge transport. Overall, our work portrays a detailed picture of "bite" defects in bottom-up armchair graphene nanoribbons and assesses their effect on the performance of carbon-based nanoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2020

Structural phase transition and bandgap control through mechanical deformation in layered semiconductors 1T-ZrX2 (X = S, Se)

Applying elastic deformation can tune a material physical properties locally and reversibly. Spatially modulated lattice deformation can create a bandgap gradient, favouring photo-generated charge separation and collection in optoelectronic devices. These advantages are hindered by the maximum elastic strain that a material can withstand before breaking. Nanomaterials derived by exfoliating transition metal dichalcogenides TMDs are an ideal playground for elastic deformation, as they can sustain large elastic strains, up to a few percent. However, exfoliable TMDs with highly strain-tunable properties have proven challenging for researchers to identify. We investigated 1T-ZrS2 and 1T-ZrSe2, exfoliable semiconductors with large bandgaps. Under compressive deformation, both TMDs dramatically change their physical properties. 1T-ZrSe2 undergoes a reversible transformation into an exotic three-dimensional lattice, with a semiconductor-to-metal transition. In ZrS2, the irreversible transformation between two different layered structures is accompanied by a sudden 14 % bandgap reduction. These results establish that Zr-based TMDs are an optimal strain-tunable platform for spatially textured bandgaps, with a strong potential for novel optoelectronic devices and light harvesting.

cond-mat.mtrl-sci↗

arXiv2020

Even-odd conductance effect in graphene nanoribbons induced by edge functionalization with aromatic molecules: Basis for novel chemosensors

We theoretically investigate the electron transport in armchair and zigzag graphene nanoribbons (GNRs) chemically functionalized with p-polyphenyl and polyacene groups of increasing length. Our nearest-neighbor tight-binding calculations indicate that, depending on whether the number of aromatic rings in the functional group is even or odd, the resulting conductance at energies matching the energy levels of the corresponding isolated molecule are either unaffected or reduced by exactly one quantum as compared to the pristine GNR, respectively. Such an even-odd effect is shown to originate from a subtle interplay between the electronic states of the guest molecule that are spatially localized on the binding sites and those of the host nanoribbon. We next generalize our findings by employing more accurate tight-binding Hamiltonians along with density-functional theory calculations, and critically discuss the robustness of the observed physical effects against the level of theory adopted. Our work offers a comprehensive understanding of the influence of aromatic molecules bound to the edge of graphene nanoribbons on their electronic transport properties, an issue which is instrumental to the prospective realization of graphene-based chemosensors.

cond-mat.mes-hall↗

arXiv2020

Inducing Magnetic Phase Transitions in Monolayer CrI$_3$ via Lattice Deformations

Atomically thin films of layered chromium triiodide (CrI$_3$) have recently been regarded as suitable candidates to a wide spectrum of technologically relevant applications, mainly owing to the opportunity they offer to achieve a reversible transition between coexisting in-plane ferro- and out-of-plane antiferro-magnetic orders. However, no routes for inducing such a transition have been designed down to the single-layer limit. Here, we address the magnetic response of monolayer CrI$_3$ to in-plane lattice deformations through a combination of isotropic Heisenberg spin Hamiltonians and first-principles calculations. Depending on the magnitude and orientation of the lattice strain exerted, we unveil a series of direction-dependent parallel-to-antiparallel spins crossovers, which yield the emergence of ferromagnetic, Néel antiferromagnetic, zigzag and stripy antiferromagnetic ground states. Additionally, we identify a critical point in the magnetic phase diagram whereby the exchange couplings vanish and the magnetism is quenched. Our work establishes guidelines for extensively tailoring the spin interactions in monolayer CrI$_3$ via strain engineering, and further expands the magnetically ordered phases which can be hosted in a two-dimensional crystal.

cond-mat.str-el↗

arXiv2020

Atomic-scale defects in the two-dimensional ferromagnet CrI$_3$ from first principles

The family of atomically thin magnets holds great promise for a number of prospective applications in magneto-optoelectronics, with CrI$_3$ arguably being its most prototypical member. However, the formation of defects in this system remains unexplored to date. Here, we investigate native point defects in monolayer CrI$_3$ by means of first-principles calculations. We consider a large set of intrinsic impurities and address their atomic structure, thermodynamic stability, diffusion and aggregation tendencies as well as local magnetic moments. Under thermodynamic equilibrium, the most stable defects are found to be either Cr or I atomic vacancies along with their complexes, depending on the chemical potential conditions. These defects are predicted to be quite mobile at room temperature and to exhibit a strong tendency to agglomerate. In addition, our calculations indicate that the defect-induced deviation from the nominal stoichiometry largely impacts the local magnetic moments, thereby suggesting a marked interplay between magnetism and disorder in CrI$_3$. Overall, this work portrays a comprehensive picture of intrinsic point defects in monolayer CrI$_3$ from a theoretical perspective.

cond-mat.mtrl-sci↗

arXiv2019

Magnetic exchange interactions in monolayer CrI$_3$ from many-body wavefunction calculations

The marked interplay between the crystalline, electronic, and magnetic structure of atomically thin magnets has been regarded as the key feature for designing next-generation magneto-optoelectronic devices. In this respect, a detailed understanding of the microscopic interactions underlying the magnetic responses of these crystals is of primary importance. Here, we combine model Hamiltonians with multi-reference configuration interaction wavefunctions to accurately determine the strength of the spin couplings in the prototypical single-layer magnet CrI$_3$. Our calculations identify the (ferromagnetic) Heisenberg exchange interaction $J = -1.44$ meV as the dominant term, being the inter-site magnetic anisotropies substantially {weaker}. We also find that single-layer CrI$_3$ features an out-of-plane easy axis ensuing from a single-ion anisotropy $A = -0.10$ meV, and predict $g$-tensor in-plane components $g_{xx} = g_{yy} = 1.90$ and out-of-plane component $g_{zz} = 1.92$. In addition, we assess the performance of a dozen widely used density functionals against our accurate correlated wavefunctions {calculations} and available experimental data, thereby establishing reference results for future first-principles investigations. Overall, our findings offer a firm theoretical ground to experimental observations.

cond-mat.str-el↗

arXiv2018

Single-layer $1T'$-MoS$_2$ under electron irradiation from $ab$ $initio$ molecular dynamics

Irradiation with high-energy particles has recently emerged as an effective tool for tailoring the properties of two-dimensional transition metal dichalcogenides. In order to carry out an atomically-precise manipulation of the lattice, a detailed understanding of the beam-induced events occurring at the atomic scale is necessary. Here, we investigate the response of $1T'$-MoS$_2$ to the electron irradiation by $ab$ $initio$ molecular dynamics means. Our simulations suggest that an electron beam with energy smaller than 75 keV does not result in any knock-on damage. The displacement threshold energies are different for the two nonequivalent sulfur atoms in $1T'$-MoS$_2$ and strongly depend on whether the top or bottom chalcogen layer is considered. As a result, a careful tuning of the beam energy can promote the formation of ordered defects in the sample. We further discuss the effect of the electron irradiation in the neighborhood of a defective site, the mobility of the sulfur vacancies created and their tendency to aggregate. Overall, our work provides useful guidelines for the imaging and the defect engineering of $1T'$-MoS$_2$ using electron microscopy.

cond-mat.mtrl-sci↗

arXiv2018

Point defects in the 1$T'$ and 2$H$ phases of single-layer MoS$_2$: A comparative first-principles study

The metastable 1$T'$ phase of layered transition metal dichalcogenides has recently attracted considerable interest due to electronic properties, possible topological electronic phases and catalytic activity. We report a comprehensive theoretical investigation of intrinsic point defects in the 1$T'$ crystalline phase of single-layer molybdenum disulfide (1$T'$-MoS$_2$), and provide comparison to the well-studied semiconducting 2$H$ phase. Based on density functional theory calculations, we explore a large number of configurations of vacancy, adatom and antisite defects and analyse their atomic structure, thermodynamic stability, electronic and magnetic properties. The emerging picture suggests that, under thermodynamic equilibrium, 1$T'$-MoS$_2$ is more prone to hosting lattice imperfections than the 2$H$ phase. More specifically, our findings reveal that the S atoms that are closer to the Mo atomic plane are the most reactive sites. Similarly to the 2$H$ phase, S vacancies and adatoms in 1$T'$-MoS$_2$ are very likely to occur while Mo adatoms and antisites induce local magnetic moments. Contrary to the 2$H$ phase, Mo vacancies in 1$T'$-MoS$_2$ are expected to be an abundant defect due to the structural relaxation that plays a major role in lowering the defect formation energy. Overall, our study predicts that the realization of high-quality flakes of 1$T'$-MoS$_2$ should be carried out under very careful laboratory conditions but at the same time the facile defects introduction can be exploited to tailor physical and chemical properties of this polymorph.

cond-mat.mtrl-sci↗

arXiv2017

Highly-Oriented Atomically Thin Ambipolar MoSe$_2$ Grown by Molecular Beam Epitaxy

Transition metal dichalcogenides (TMDCs), together with other two-dimensional (2D) materials have attracted great interest due to the unique optical and electrical properties of atomically thin layers. In order to fulfill their potential, developing large-area growth and understanding the properties of TMDCs have become crucial. Here, we used molecular beam epitaxy (MBE) to grow atomically thin MoSe$_2$ on GaAs(111)B. No intermediate compounds were detected at the interface of as-grown films. Careful optimization of the growth temperature can result in the growth of highly aligned films with only two possible crystalline orientations due to broken inversion symmetry. As-grown films can be transferred onto insulating substrates allowing their optical and electrical properties to be probed. By using polymer electrolyte gating, we have achieved ambipolar transport in MBE-grown MoSe$_2$. The temperature-dependent transport characteristics can be explained by the 2D variable-range hopping (2D-VRH) model, indicating that the transport is strongly limited by the disorder in the film.

cond-mat.mtrl-sci↗