arXiv subjects

D. Soriano

Publications and source records attributed to D. Soriano.

13 recordsLinked to original sources

arXiv2022

Domain wall formation and magnon localization in twisted chromium trihalides

The rise of twistronics has revolutionized the field of condensed matter physics, and more specifically the future applications of two-dimensional materials. At small twist angles, the microscopic world becomes strongly correlated, and unexpected physical phenomena such as superconductivity emerge. For magnetic layers, stacking plays a crucial role in the magnetic exchange coupling between the layers leading to non-trivial spin configurations and flat spin-wave dispersion when twisted. In this work, we give a short overview of the most recent theoretical and experimental works reporting the effect of twist angles on two-dimensional magnets. Besides, we discuss the effect of the twist angle and the local antiferromagnetic interlayer exchange coupling on the formation of antiferromagnetic domains in chromium trihalides. Finally, we show some preliminary results on the effect of the stacking and the twist angle on the spin-wave dispersion of bilayer CrI$_3$

cond-mat.mes-hall↗

arXiv2021

Spin-orbit correlations and exchange-bias control in twisted Janus dichalcogenide multilayers

Janus dichalcogenide multilayers provide a paradigmatic platform to engineer electronic phenomena dominated by spin-orbit coupling. Their unique spin-orbit effects stem from local mirror symmetry breaking in each layer, which induces a colossal Rashba spin-orbit effect in comparison with the conventional dichalcogenide counterparts. Here we put forward twisted dichalcogenide bilayers as a simple platform to realize spin-orbit correlated states. We demonstrate the emergence of flat bands featuring strong spin-momentum locking and the emergence of non-collinear symmetry broken states when interactions are included. We further show that the symmetry broken states can be controlled by means of a magnetic substrate, strongly impacting the non-collinear magnetic texture of the moire unit cell. Our results put forward twisted Janus multilayers as a powerful platform to explore spin-orbit correlated physics, and highlighting the versatility of magnetic substrates to control unconventional moire magnetism.

cond-mat.mes-hall↗

arXiv2021

Environmental Screening and Ligand-Field Effects to Magnetism in CrI$_3$ Monolayer

We present a detailed study on the microscopic origin of magnetism in suspended and dielectrically embedded CrI$_3$ monolayer. To this end, we down-fold two distinct minimal generalized Hubbard models with different orbital basis sets from \emph{ab initio} calculations using the constrained random phase approximation. Within mean-field approximation, we show that these models are capable of describing the formation of localized magnetic moments in CrI$_3$ and of reproducing electronic properties of full \emph{ab initio} calculations. We utilize the magnetic force theorem to study microscopic magnetic exchange channels between the different orbital manifolds. We find a multi-orbital super-exchange mechanism as the origin of magnetism in CrI$_3$ resulting from a detailed interplay between effective ferro- and anti-ferromagnetic Cr-Cr $d$ coupling channels, which is decisively affected by the ligand (I) $p$ orbitals. We show how environmental screening such as resulting from encapsulation with hexagonal boron nitride (hBN) of the CrI$_3$ monolayer affects the Coulomb interaction in the film and how this successively controls its magnetic properties. Driven by a non-monotonic interplay between nearest and next-nearest neighbour exchange interactions we find the magnon dispersion and the Curie temperature to be non-trivially affected by the environmental dielectric screening.

cond-mat.str-el↗

arXiv2020

Exchange-bias controlled correlations in magnetically encapsulated twisted van der Waals dichalcogenides

Twisted van der Waals materials have become a paradigmatic platform to realize exotic correlated states of matter. Here, we show that a twisted dichalcogenide bilayer (WSe$_2$) encapsulated between a magnetic van der Waals material (CrBr$_3$) features flat bands with tunable valley and spin flavors. We demonstrate that, when electron-electron interactions are included, spin-ferromagnetic and valley-ferromagnetic states emerge in the flat bands, stemming from the interplay between correlations, intrinsic spin-orbit coupling and exchange proximity effects. We show that the specific symmetry broken state is controlled by the relative alignment of the magnetization of the encapsulation, demonstrating the emergence of correlated states controlled by exchange bias. Our results put forward a new van der Waals heterostructure where symmetry broken states emerge from a genuine interplay between twist engineering, spin-orbit coupling and exchange proximity, providing a powerful starting point to explore exotic collective states of matter.

cond-mat.mes-hall↗

arXiv2020

Magnetic Two-Dimensional Chromium Trihalides: A Theoretical Perspective

The discovery of ferromagnetic order in monolayer 2D crystals has opened a new venue in the field of two dimensional (2D) materials. 2D magnets are not only interesting on their own, but their integration in van der Waals heterostructures allows for the observation of new and exotic effects in the ultrathin limit. The family of Chromium trihalides, CrI$_3$, CrBr$_3$ and CrCl$_3$, is, so far, the most studied among magnetic 2D crystals. In this mini-review, we provide a perspective of the state of the art of the theoretical understanding of magnetic 2D trihalides, most of which will also be relevant for other 2D magnets, such as vanadium trihalides. We discuss both the well-established facts, such as the origin of the magnetic moment and magnetic anisotropy and address as well open issues such as the nature of the anisotropic spin couplings and the magnitude of the magnon gap. Recent theoretical predictions on Moir\' e magnets and magnetic skyrmions are also discussed. Finally, we give some prospects about the future interest of these materials and possible device applications.

cond-mat.mes-hall↗

arXiv2019

Magnetic polaron and antiferro-ferromagnetic transition in doped bilayer CrI$_3$

Gate-induced magnetic switching in bilayer CrI$_3$ has opened new ways for the design of novel low-power magnetic memories based on van der Waals heterostructures. The proposed switching mechanism seems to be fully dominated by electrostatic doping. Here we explain, by first-principle calculations, the ferromagnetic transition in doped bilayer CrI$_3$. For the case of a very small electron doping, our calculations predict the formation of magnetic polarons ("ferrons", "fluctuons") where the electron is self-locked in a ferromagnetic droplet in an antiferromagnetic insulating matrix. The self-trapping of holes is impossible, at least, within our approximation.

cond-mat.mes-hall↗

arXiv2019

Interplay between interlayer exchange and stacking in CrI$_3$ bilayers

We address the interplay between stacking and interlayer exchange for ferromagnetically ordered CrI$_3$, both for bilayers and bulk. Whereas bulk CrI$_3$ is ferromagnetic, both magneto-optical and transport experiments show that interlayer exchange for CrI$_3$ bilayers is antiferromagnetic. Bulk CrI$_3$ is known to assume two crystal structures, rhombohedral and monoclinic, that differ mostly in the stacking between monolayers. Below 210-220 Kelvin, bulk CrI$_3$ orders in a rhombohedral phase. Our density functional theory calculations show a very strong dependence of interlayer exchange and stacking. Specifically, the ground states of both bulk and free-standing CrI$_3$ bilayers are ferromagnetic for the rhombohedral phase. In contrast, the energy difference between both configurations is more than one order of magnitude smaller for the monoclinic phase, and eventually becomes antiferromagnetic when either positive strain or on-site Hubbard interactions ($U \geq 3$) are considered. We also explore the interplay between interlayer hybrydization and stacking, using a Wannier basis, and between interlayer hybrydization and relative magnetic alignment for CrI$_3$ bilayers, that helps to account for the very large tunnel magnetoresistance obvserved in recent experiments.

cond-mat.mes-hall↗

arXiv2018

Van der Waals spin valves

We propose spin valves where a 2D non-magnetic conductor is intercalated between two ferromagnetic insulating layers. In this setup, the relative orientation of the magnetizations of the insulating layers can have a strong impact on the in-plane conductivity of the 2D conductor. We first show this for a graphene bilayer, described with a tight-binding model, placed between two ferromagnetic insulators. In the anti-parallel configuration, a band gap opens at the Dirac point, whereas in the parallel configuration, the graphene bilayer remains conducting. We then compute the electronic structure of graphene bilayer placed between two monolayers of the ferromagnetic insulator CrI$_3$, using density functional theory. Consistent with the model, we find that a gap opens at the Dirac point only in the antiparallel configuration.

cond-mat.mes-hall↗

arXiv2012

Spin-filtered edge states in graphene

Spin orbit coupling changes graphene, in principle, into a two-dimensional topological insulator, also known as quantum spin Hall insulator. One of the expected consequences is the existence of spin-filtered edge states that carry dissipationless spin currents and undergo no back-scattering in the presence of non-magnetic disorder, leading to quantization of conductance. Whereas, due to the small size of spin orbit coupling in graphene, the experimental observation of these remarkable predictions is unlikely, the theoretical understanding of these spin-filtered states is shedding light on the electronic properties of edge states in other two-dimensional quantum spin Hall insulators. Here we review the effect of a variety of perturbations, like curvature, disorder, edge reconstruction, edge crystallographic orientation, and Coulomb interactions on the electronic properties of these spin filtered states.

cond-mat.mes-hall↗

arXiv2012

Interplay between sublattice and spin symmetry breaking in graphene

We study the effect of sublattice symmetry breaking on the electronic, magnetic and transport properties of two dimensional graphene as well as zigzag terminated one and zero dimensional graphene nanostructures. The systems are described with the Hubbard model within the collinear mean field approximation. We prove that for the non-interacting bipartite lattice with unequal number of atoms in each sublattice midgap states still exist in the presence of a staggered on-site potential $\pm Δ/2$. We compute the phase diagram of both 2D and 1D graphene with zigzag edges, at half-filling, defined by the normalized interaction strength $U/t$ and $Δ/t$, where $t$ is the first neighbor hopping. In the case of 2D we find that the system is always insulating and, we find the $U_c(Δ)$ curve above which the system goes antiferromagnetic. In 1D we find that the system undergoes a phase transition from non-magnetic insulator for $U<U_C(Δ)$ to a phase with ferromagnetic edge order and antiferromagnetic inter-edge coupling. The conduction properties of the magnetic phase depend on $Δ$ and can be insulating, conducting and even half-metallic, yet the total magnetic moment in the system is zero. We compute the transport properties of a heterojunction with two non-magnetic graphene ribbon electrodes connected to a finite length armchair ribbon and we find a strong spin filter effect.

cond-mat.mes-hall↗

arXiv2010

Spontaneous persistent currents in magnetically ordered graphene ribbons

We present a new mechanism for dissipationless persistent charge current. Two dimensional topological insulators hold dissipationless spin currents in their edges so that, for a given spin orientation, a net charge current flows which is exactly compensated by the counter-flow of the opposite spin. Here we show that ferromagnetic order in the edge upgrades the spin currents into persistent charge currents, without applied fields. For that matter, we study an interacting graphene zigzag ribbon with spin-orbit coupling. We find three electronic phases with magnetic edges that carry currents reaching 0.4nA, comparable to persistent currents in metallic rings, for the small spin orbit coupling in graphene. One of the phases is a valley half-metal.

cond-mat.mes-hall↗

arXiv2010

Hydrogenated Graphene Nanoribbons for Spintronics

We show how hydrogenation of graphene nanoribbons at small concentrations can open new venues towards carbon-based spintronics applications regardless of any especific edge termination or passivation of the nanoribbons. Density functional theory calculations show that an adsorbed H atom induces a spin density on the surrounding $π$ orbitals whose symmetry and degree of localization depends on the distance to the edges of the nanoribbon. As expected for graphene-based systems, these induced magnetic moments interact ferromagnetically or antiferromagnetically depending on the relative adsorption graphene sublattice, but the magnitude of the interactions are found to strongly vary with the position of the H atoms relative to the edges. We also calculate, with the help of the Hubbard model, the transport properties of hydrogenated armchair semiconducting graphene nanoribbons in the diluted regime and show how the exchange coupling between H atoms can be exploited in the design of novel magnetoresistive devices.

cond-mat.mes-hall↗

arXiv2007

Localized basis sets for unbound electrons in nanoelectronics

It is shown how unbound electron wave functions can be expanded in a suitably chosen localized basis sets for any desired range of energies. In particular, we focus on the use of gaussian basis sets, commonly used in first-principles codes. The possible usefulness of these basis sets in a first-principles description of field emission or scanning tunneling microscopy at large bias is illustrated by studying a simpler related phenomenon: The lifetime of an electron in a H atom subjected to a strong electric field.

cond-mat.mes-hall↗