arXiv subjects

Amador García-Fuente

Publications and source records attributed to Amador García-Fuente.

12 recordsLinked to original sources

arXiv2026

Geometrical Confinement of Magnetic Domains and Domain Walls on Two-Dimensional Cylindrical-Shaped Ferromagnetic Surfaces

The rolling of two-dimensional (2D) van der Waals (vdW) magnetic monolayers into cylindrical architectures provides an ideal platform for confining magnetic domain walls (DWs) without edge-pinning effects. Here, we present a combined analytical and numerical investigation of DW confinement and interaction in ferromagnetic (FM) nanotubes. Using an analytical Heisenberg model on a 2D rectangular lattice, we show that transversal curvature introduces an azimuthal exchange energy penalty that intrinsically compresses the DW width. This model demonstrates excellent quantitative agreement with atomistic Landau-Lifshitz-Gilbert (LLG) spin dynamics simulations. By extending our study to realistic CrSBr (CSB) nanotubes, we model a bound state of two interacting DWs arising from a balance between long-range dipolar attraction and short-range exchange repulsion. Micromagnetic simulations reveal a non-monotonic evolution of the inter-wall distance with the tube perimeter. These findings highlight vdW magnetic nanotubes as robust platforms for stable domain confinement in quasi-one-dimensional spintronic architectures.

cond-mat.mtrl-sci↗

arXiv2026

Proximity-Induced Spin Reorientation in Monolayer CrI$_3$ on Hexagonal WTe$_2$

Magnetic anisotropy controls the orientation and thermal stability of two-dimensional magnetic order. Predicting proximity-induced changes in anisotropy requires linking the electronic structure to microscopic magnetic interactions and finite-temperature behavior. Here we study monolayer CrI$_3$ on hexagonal WTe$_2$ using a first-principles-to-finite-temperature workflow centered on relativistic spin-Hamiltonian mapping. We find that WTe$_2$ reorients the CrI$_3$ magnetization from out-of-plane to in-plane and substantially enhances the magnetic ordering scale within the extracted spin models. Analysis of the extracted spin Hamiltonians shows that the reorientation is driven by a substrate-induced change in the balance between symmetric anisotropic exchange and onsite anisotropy. We establish a transferable workflow for proximity-controlled magnetism in two-dimensional van der Waals heterostructures.

cond-mat.mtrl-sci↗

arXiv2026

Strain induced magnetic phase transitions in Fe3GeTe2 monolayer

We investigate the magnetic properties of a monolayer of Fe3GeTe2 as a function of the lattice constant by combining first-principles calculations with atomistic spin dynamics simulations. The calculated magnetic exchange interactions reveal a competition between ferromagnetic and antiferromagnetic couplings, with the latter being significantly strengthened under compressive strain. Stochastic Landau-Lifshitz-Gilbert simulations reveal a substantial decrease in the Curie temperature with decreasing lattice constant, and predict a transition of the magnetic ground state from a ferromagnetic configuration to a conical spin-spiral state. We introduce a simple spin-model which explains the stabilization of the spiral phase due to competing exchange interactions. We found multiple magnetic phase transitions involving ferromagnetic, conical spin-spiral, and planar Neel states, depending on both the lattice constant and the temperature. The absence of Dzyaloshinskii-Moriya interactions is found to significantly reduce the Neel temperature, while leaving the Curie temperature largely unaffected. Our findings reveal the importance of lattice distortions in controlling complex magnetic phases and their evolution with temperature.

cond-mat.mtrl-sci↗

arXiv2025

Graphene-based quantum heterospin graphs

We investigate from first principles a variety of low-dimensional open quantum spin systems based on magnetic nanographene structures that contain spin-1/2 and spin-1 triangulenes and/or olympicenes. These graphene nanostructures behave as localized spins and can be effectively described by a quantum bilinear-biquadratic Heisenberg Hamiltonian, for which we will compute the energy spectrum and the quantum numbers associated with the low-energy eigenstates. We propose the experimental realization of antiferromagnetic alternating spin chains using these graphene nanostructures, which result in ferrimagnetic systems whose ground state spin and degeneracy depend on the length of the chain. We identify a double degeneracy in the total spin quantum number $S$ of the first excited state in three-leg spin graphs (3-LSGs) and other heterospin nanostructures, which depends on both the number of sites and the spin species, and originates from the swapping transformation symmetry of the Hamiltonian. Numerical simulations indicate that this degeneracy remains largely robust for $N=7$ spin-1 3-LSGs under realistic perturbations present in experimental conditions.

cond-mat.mes-hall↗

arXiv2025

Solution of the mean-field Hubbard model of graphene rectangulenes

We present a complete analytical solution of the mean-field Hubbard model of undoped and doped graphene rectangulenes. These are non-chiral ribbons of arbitrary length and width, whose dimensions range from simple short acene molecules all the way up to the bulk limit. We rewrite the Hubbard model in the basis of bulk and edge non-interacting eigen-states, and provide explicit expressions for the Coulomb matrix elements. We present a general mean-field decoupling of the Hamiltonian, and discuss in detail the paramagnetic, ferromagnetic and antiferromagnetic mean-field solutions. We calculate the eigen-energies, occupations, spin densities and addition energies of rectangulenes with lengths and widths ranging from a nanometer to several hundreds of them. We rewrite the exact mean-field tight-binding Hamiltonian back in the site-occupation basis, that can be used to model electronic, thermo-electric, transport and optical properties of experimental-size graphene flakes.

cond-mat.str-el↗

arXiv2024

Tuning magnetic exchange interactions in 2D magnets: the case of CrGeX$_3$ (X = Se, Te) and Janus Cr$_2$Ge$_2$(Se,Te)$_3$ monolayers

We present a computational study to explore the potential of different experimental approaches to tune the magnetic interactions in two-dimensional van der Waals magnets. We selected CrGeSe$_3$, CrGeTe$_3$, and Janus Cr$_2$Ge$_2$(Se,Te)$_3$ monolayers as case studies and calculated the full exchange tensors among all relevant atomic pairs and analyze their dependence on different external parameters, such as biaxial and uniaxial strain, as well as gate voltage. We pay special attention to interactions that emerge or vanish due to changes of the symmetry of the system. We find that biaxial and uniaxial strains significantly modify isotropic exchange couplings, which can lead to a transition from a ferromagnetic to an antiferromagnetic phase, while a gate voltage induces Dzyaloshinskii-Moriya interactions, forming a vortex pattern whose chirality is determined by the sign of the electric field. The electric dipole moment of the Janus material is large, raising the possibility of multiferroic behaviour. The polarizability is similar for the three compounds.

cond-mat.mes-hall↗

arXiv2024

Surface reconstruction limited magnetism of the nodal loop semimetal Ca$_3$P$_2$

Nodal loop semimetals are topological materials with drumhead surface states characterized by reduced kinetic energy. If the Fermi energy of such a system is near these nondispersive states interaction among charge carriers substantially impacts their electronic structure. The emergence of magnetism in these surface states is one of the possible consequences. Ca3P2 an already synthesized material possesses a remarkably large nodal loop which is situated exactly at the Fermi level of the bulk system. In the present work, we investigate how surface magnetism is impacted by surface reconstruction and lattice termination in finite slabs in this material. We show that a slight deviation from the stoichiometric occupation of Ca sites results in the stabilization of magnetic patterns.

cond-mat.mtrl-sci↗

arXiv2023

Relativistic magnetic interactions from non-orthogonal basis sets

We propose a method to determine the magnetic exchange interaction and on-site anisotropy tensors of extended Heisenberg spin models from density functional theory including relativistic effects. The method is based on the Liechtenstein-Katsnelson-Antropov-Gubanov torque formalism, whereby energy variations upon infinitesimal rotations are performed. We assume that the Kohn-Sham Hamiltonian is expanded in a non-orthogonal basis set of pseudo-atomic orbitals. We define local operators that are both hermitian and satisfy relevant sum rules. We demonstrate that in the presence of spin-orbit coupling a correct mapping from the density functional total energy to a spin model that relies on the rotation of the exchange field part of the Hamiltonian can not be accounted for by transforming the full Hamiltonian. We derive a set of sum rules that pose stringent validity tests on any specific calculation. We showcase the flexibility and accuracy of the method by computing the exchange and anisotropy tensors of both well-studied magnetic nanostructures and of recently synthesized two-dimensional magnets. Specifically, we benchmark our approach against the established Korringa-Kohn-Rostoker Green's function method and show that they agree well. Finally, we demonstrate how the application of biaxial strain on the two-dimensional magnet T-CrTe2 can trigger a magnetic phase transition.

cond-mat.mtrl-sci↗

arXiv2023

Electrically driven singlet-triplet transition in triangulene spin-1 chains

Recently, graphene triangulene chains have been synthesized and their magnetic response has been analyzed by STM methods by Mishra and coworkers (Nature 598, 287 (2021)). Motivated by this study, we determine the exchange bilinear and biquadratic constants of the triangulene chains by calculating two-spin rotations in the spirit of the magnetic force theorem. We then analyze open-ended, odd-numbered chains, whose edge states pair up forming a triplet ground state. We propose three experimental approaches that enable us to trigger and control a singlet-triplet spin transition. Two of these methods are based on applying a mechanical distortion to the chain. We finally show that the transition can be controlled efficiently by the application of an electric field.

cond-mat.mes-hall↗

arXiv2022

Relevance of Shockley states on the electrical and thermoelectric response of gold-based single-molecule junctions

Noble metals break preferably exposing (111)-oriented surfaces, that host Shockley type surface states (SSs). Nevertheless, the relevance of SSs on the electrical properties of gold-based molecular junctions has not been explored in detail yet. Here, we present ab initio simulations that show how the gold (111) SS, that lies approximately 0.5 eV below the Fermi energy, is key to determining correctly the electrical and thermoelectric response of the above junctions. We show how the ability to shift in a controlled way the energy position of gold SS enables us to tune the electrical and thermoelectric response of gold molecular junctions. We also show that gold's SS appears in our simulations only if the 5d orbitals are included explicitly in the valence shell. To illustrate this behaviour, we discuss in detail Benzenediamine (BDA) and Benzenedicarbonitrile (BDCN) gold (111) junctions.

cond-mat.mes-hall↗

arXiv2018

Mechanically Controlled Quantum Interference in Graphene Break Junctions

The ability to detect and distinguish quantum interference signatures is important for both fundamental research and for the realization of devices including electron resonators, interferometers and interference-based spin filters. Consistent with the principles of subwavelength optics, the wave nature of electrons can give rise to various types of interference effects, such as Fabry-Pérot resonances, Fano resonances and the Aharonov-Bohm effect. Quantum-interference conductance oscillations have indeed been predicted for multiwall carbon nanotube shuttles and telescopes, and arise from atomic-scale displacements between the inner and outer tubes. Previous theoretical work on graphene bilayers indicates that these systems may display similar interference features as a function of the relative position of the two sheets. Experimental verification is, however, still lacking. Graphene nanoconstrictions represent an ideal model system to study quantum transport phenomena due to the electronic coherence and the transverse confinement of the carriers. Here, we demonstrate the fabrication of bowtie-shaped nanoconstrictions with mechanically controlled break junctions (MCBJs) made from a single layer of graphene. We find that their electrical conductance displays pronounced oscillations at room temperature, with amplitudes that modulate over an order of magnitude as a function of sub-nanometer displacements. Surprisingly, the oscillations exhibit a period larger than the graphene lattice constant. Charge-transport calculations show that the periodicity originates from a combination of quantum-interference and lattice-commensuration effects of two graphene layers that slide across each other. Our results provide direct experimental observation of Fabry-Pérot-like interference of electron waves that are partially reflected/transmitted at the edges of the graphene bilayer overlap region.

cond-mat.mes-hall↗

arXiv2018

Spin-State dependent Conductance Switching in Single Molecule-Graphene Junctions

Spin-crossover (SCO) molecules are versatile magnetic switches with applications in molecular electronics and spintronics. Downscaling devices to the single-molecule level remains, however, a challenging task since the switching mechanism in bulk is mediated by cooperative intermolecular interactions. Here, we report on electron transport through individual Fe-SCO molecules coupled to few-layer graphene electrodes \textit{via} $π- π$ stacking. We observe a distinct bistability in the conductance of the molecule and a careful comparison with density functional theory (DFT) calculations allows to associate the bistability with a SCO-induced orbital reconfiguration of the molecule. We find long spin-state lifetimes that are caused by the specific coordination of the magnetic core and the absence of intermolecular interactions according to our calculations. In contrast with bulk samples, the SCO transition is not triggered by temperature but induced by small perturbations in the molecule at any temperature. We propose plausible mechanisms that could trigger the SCO at the single-molecule level.

cond-mat.mes-hall↗