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1.
Nat Commun ; 15(1): 4854, 2024 Jun 06.
Article in English | MEDLINE | ID: mdl-38844776

ABSTRACT

The kagome lattice is an exciting solid state physics platform for the emergence of nontrivial quantum states driven by electronic correlations: topological effects, unconventional superconductivity, charge and spin density waves, and unusual magnetic states such as quantum spin liquids. While kagome lattices have been realized in complex multi-atomic bulk compounds, here we demonstrate from first-principles a process that we dub kagomerization, in which we fabricate a two-dimensional kagome lattice in monolayers of transition metals utilizing an hexagonal boron nitride (h-BN) overlayer. Surprisingly, h-BN induces a large rearrangement of the transition metal atoms supported on a fcc(111) heavy-metal surface. This reconstruction is found to be rather generic for this type of heterostructures and has a profound impact on the underlying magnetic properties, ultimately stabilizing various topological magnetic solitons such as skyrmions and bimerons. Our findings call for a reconsideration of h-BN as merely a passive capping layer, showing its potential for not only reconstructing the atomic structure of the underlying material, e.g. through the kagomerization of magnetic films, but also enabling electronic and magnetic phases that are highly sought for the next generation of device technologies.

2.
J Phys Condens Matter ; 34(45)2022 Sep 13.
Article in English | MEDLINE | ID: mdl-36055232

ABSTRACT

The discovery of two-dimensional (2D) van der Waals magnets opened unprecedented opportunities for the fundamental exploration of magnetism in quantum materials and the realization of next generation spintronic devices. Here, based on a multiscale modelling approach that combines first-principles calculations and a Heisenberg model supplied with ab-initio parameters, we report a strong magnetoelastic coupling in a free-standing monolayer of CrTe2. We demonstrate that different crystal structures of a single CrTe2give rise to non-collinear magnetism through magnetic frustration and emergence of the Dzyaloshinskii-Moriya interaction. Utilizing atomistic spin dynamics, we perform a detailed investigation of the complex magnetic properties pertaining to this 2D material impacted by the presence of various types of structural distortions akin to charge density waves.

3.
J Phys Condens Matter ; 34(38)2022 Jul 22.
Article in English | MEDLINE | ID: mdl-35835084

ABSTRACT

Individual nuclear spin states can have very long lifetimes and could be useful as qubits. Progress in this direction was achieved on MgO/Ag(001) via detection of the hyperfine interaction (HFI) of Fe, Ti and Cu adatoms using scanning tunneling microscopy. Previously, we systematically quantified from first-principles the HFI for the whole series of 3d transition adatoms (Sc-Cu) deposited on various ultra-thin insulators, establishing the trends of the computed HFI with respect to the filling of the magnetic s- and d-orbitals of the adatoms and on the bonding with the substrate. Here we explore the case of dimers by investigating the correlation between the HFI and the magnetic state of free standing Fe dimers, single Fe adatoms and dimers deposited on a bilayer of MgO(001). We find that the magnitude of the HFI can be controlled by switching the magnetic state of the dimers. For short Fe-Fe distances, the antiferromagnetic state enhances the HFI with respect to that of the ferromagnetic state. By increasing the distance between the magnetic atoms, a transition toward the opposite behavior is observed. Furthermore, we demonstrate the ability to substantially modify the HFI by atomic control of the location of the adatoms on the substrate. Our results establish the limits of applicability of the usual hyperfine hamiltonian and we propose an extension based on multiple scattering processes.

4.
J Phys Condens Matter ; 34(28)2022 May 12.
Article in English | MEDLINE | ID: mdl-35453127

ABSTRACT

We propose a systematic and sequential expansion of the Landau-Lifshitz-Gilbert equation utilizing the dependence of the Gilbert damping tensor on the angle between magnetic moments, which arises from multi-body scattering processes. The tensor consists of a damping-like term and a correction to the gyromagnetic ratio. Based on electronic structure theory, both terms are shown to depend on e.g. the scalar, anisotropic, vector-chiral and scalar-chiral products of magnetic moments:ei⋅ej, (nij⋅ei)(nij⋅ej),nij⋅ (ei×ej),(ei⋅ej)2,ei⋅ (ej×ek) …, where some terms are subjected to the spin-orbit fieldnijin first and second order. We explore the magnitude of the different contributions using both the Alexander-Anderson model and time-dependent density functional theory in magnetic adatoms and dimers deposited on Au(111) surface.

5.
Acta Med Port ; 35(6): 425-432, 2022 Jun 01.
Article in English | MEDLINE | ID: mdl-35171094

ABSTRACT

INTRODUCTION: The SARS-CoV-2 infection has been associated with the acute onset of mental and behavioural symptoms and psychiatric disorders. The aim of this study was to assess the prevalence of the different neuropsychiatric diagnoses in hospitalized patients with SARS-CoV-2 infection assessed by Liaison Psychiatry. MATERIAL AND METHODS: We performed a cross-sectional study in a hospital near Lisbon, Portugal. We reviewed the electronic health records from all inpatients with a positive SARS-CoV-2 RT-PCR test that were assessed by the Liaison Psychiatry Unit (LPU) between February and December 2020. We reviewed relevant sociodemographic and clinical data, including 15 neuropsychiatric symptoms. The prevalence of psychiatric disorders was our main outcome. We also explored differences between two groups: patients with delirium (delirium group) and patients without delirium (no delirium group). RESULTS: We included 46 cases [Age: median = 67 years; interquartile range (IQR) = 24)], with 60.9% male individuals. Delirium was the most frequent diagnosis in our sample (43.5%), followed by major depressive disorder (21.7%). Patients with delirium were more likely to suffer from COVID-19 symptoms (delirium: 19/20, 95%; no delirium: 14/26, 53.8%; p = 0.02), and to have a longer time interval between a positive SARS-CoV-2 RT-PCR test and an evaluation by the LPU (delirium: median = 16.5 days, IQR = 16; no delirium: median = 8 days, IQR = 16.3; p = 0.045). Agitation (52.2%) and cognitive symptoms (47.8%) were the most reported neuropsychiatric symptoms. CONCLUSION: We found a high prevalence of delirium in our sample. This finding is in line with recent literature concerning hospitalized COVID-19 patients The higher frequency of COVID-19 symptoms found in the delirium group suggests a possible association between symptomatic SARS-CoV-2 infection and delirium onset.


Introdução: A infecção por SARS-CoV-2 tem sido associada ao desenvolvimento agudo de sintomas mentais e comportamentais e perturbações psiquiátricas. O objetivo deste estudo foi determinar a prevalência de diferentes diagnósticos neuropsiquiátricos em doentes hospitalizados com infeção SARS-CoV-2 avaliados pela Psiquiatria de Ligação. Material e Métodos: Realizámos um estudo transversal num hospital da região de Lisboa, em Portugal. Revimos os processos clínicos dos pacientes internados com um resultado RT-PCR positivo para SARS-CoV-2 avaliados pela Unidade de Psiquiatria de Ligação (UPL) entre fevereiro e dezembro de 2020. Incluímos dados sociodemográficos e clínicos, incluindo quinze sintomas neuropsiquiátricos. A incidência de diferentes diagnósticos psiquiátricos foi o nosso outcome primário. Explorámos também diferenças entre dois grupos: doentes com delirium e doentes sem delirium. Resultados: Incluímos 46 casos [Idade: mediana = 67 anos; amplitude interquartil (AIQ) = 24)], a maioria do sexo masculino (60,9%). Delirium foi o diagnóstico mais frequente na nossa amostra (43,5%), seguido de perturbação depressiva major (21,7%). Doentes com delirium tiveram uma prevalência maior de sintomas de COVID-19 (delirium: 19/20, 95%; sem delirium: 14/26, 53,8%; p = 0,02), bem como um intervalo de tempo mais longo entre um teste RT-PCR SARS-CoV-2 positivo e observação pela UPL (delirium: mediana = 16,5, AIQ = 16; sem delirium: mediana = 8, AIQ = 16,3; p = 0,045). Agitação (52,2%) e sintomas cognitivos (47,8%) foram os sintomas neuropsiquiátricos mais relatados. Conclusão: Foi encontrada na nossa amostra uma elevada prevalência de delirium. Este resultado está de acordo com literatura recente relativamente a doentes internados com COVID-19. A maior frequência de sintomas COVID-19 no grupo com delirium sugere uma possível associação entre infecção sintomática por SARS-CoV-2 e o desenvolvimento desta síndrome.


Subject(s)
COVID-19 , Depressive Disorder, Major , Psychiatry , Humans , Male , Aged , Female , SARS-CoV-2 , COVID-19/diagnosis , COVID-19/epidemiology , Cross-Sectional Studies , Prevalence
6.
Nat Commun ; 11(1): 4707, 2020 Sep 18.
Article in English | MEDLINE | ID: mdl-32948776

ABSTRACT

Chains of magnetic atoms with either strong spin-orbit coupling or spiral magnetic order which are proximity-coupled to superconducting substrates can host topologically non-trivial Majorana bound states. The experimental signature of these states consists of spectral weight at the Fermi energy which is spatially localized near the ends of the chain. However, topologically trivial Yu-Shiba-Rusinov in-gap states localized near the ends of the chain can lead to similar spectra. Here, we explore a protocol to disentangle these contributions by artificially augmenting a candidate Majorana spin chain with orbitally-compatible nonmagnetic atoms. Combining scanning tunneling spectroscopy with ab-initio and tight-binding calculations, we realize a sharp spatial transition between the proximity-coupled spiral magnetic order and the non-magnetic superconducting wire termination, with persistent zero-energy spectral weight localized at either end of the magnetic spiral. Our findings open a new path towards the control of the spatial position of in-gap end states, trivial or Majorana, via different chain terminations, and the realization of designer Majorana chain networks for demonstrating topological quantum computation.

7.
J Phys Condens Matter ; 32(42): 425801, 2020 May 27.
Article in English | MEDLINE | ID: mdl-32460267

ABSTRACT

The chiral spin textures of a two-dimensional (2D) triangular system, where both antiferromagnetic (AF) Heisenberg exchange and chiral Dzyaloshinsky-Moriya interactions co-exist, are investigated numerically with an optimized quantum Monte Carlo method based on mean-field theory. We find that: helical, skyrmionic and vortical AF crystals can be formed when an external magnetic field is applied perpendicular to the 2D monolayer; the sizes of these skyrmions and vortices change abruptly at several critical points of the external magnetic field; each of these AF crystals can be decomposed into three periodical ferromagnetic sublattices. The quantum ingredient implemented into the theoretical framework helps to track the existence of AF skyrmion lattices down to low temperatures.

8.
J Phys Condens Matter ; 31(48): 485801, 2019 Dec 04.
Article in English | MEDLINE | ID: mdl-31382246

ABSTRACT

B20 compounds are the playground for various non-trivial magnetic textures such as skyrmions, which are topologically protected states. Recent measurements on B20-MnGe indicate no clear consensus on its magnetic behavior, which is characterized by the presence of either spin-spirals or three-dimensional objects interpreted to be a cubic lattice of hedgehogs and anti-hedgehogs. Utilizing a massively parallel linear scaling all-electron density functional algorithm, we find from full first-principles simulations on cells containing thousands of atoms that upon increase of the compound volume, the state with lowest energy switches across different magnetic phases: ferromagnetic, spin-spiral, hedgehog and monopole.

9.
Nat Commun ; 10(1): 2565, 2019 06 12.
Article in English | MEDLINE | ID: mdl-31189872

ABSTRACT

Spins of single atoms adsorbed on substrates are promising building blocks for spintronics and quantum computation schemes. To process spin information and for increased magnetic stability, these spins have to be coupled to arrays. For a single atom, a high symmetry of the environment increases its spin stability. However, little is known about the role of the symmetry of the magnetic couplings in the arrays. Here, we study arrays of atomic spins coupled via Ruderman-Kittel-Kasuya-Yosida interaction, focusing on Dzyaloshinskii-Moriya and symmetric anisotropic exchange. We show that the high spin stability of a trimer can be remotely detected by a nearby atom, and how the Dzyaloshinskii-Moriya interaction leads to its destabilization. Adding more nearby atoms further destabilizes the trimer, due to a non-local effective transverse anisotropy originating in the symmetric anisotropic exchange. This transverse anisotropy can be quenched for highly symmetric structures, where the spin lifetime of the array increases drastically.

10.
J Phys Condens Matter ; 31(25): 255802, 2019 Jun 26.
Article in English | MEDLINE | ID: mdl-30897560

ABSTRACT

Relaxation effects are of primary importance in the description of magnetic excitations, leading to a myriad of methods addressing the phenomenological damping parameters. In this work, we consider several well-established forms of calculating the intrinsic Gilbert damping within a unified theoretical framework, mapping out their connections and the approximations required to derive each formula. This scheme enables a direct comparison of the different methods on the same footing and a consistent evaluation of their range of validity. Most methods lead to very similar results for the bulk ferromagnets Fe, Co and Ni, due to the low spin-orbit interaction (SOI) strength and the absence of the spin pumping mechanism. The effects of inhomogeneities, temperature and other sources of finite electronic lifetime are often accounted for by an empirical broadening of the electronic energy levels. We show that the contribution to the damping introduced by this broadening is additive, and so can be extracted by comparing the results of the calculations performed with and without SOI. Starting from simulated ferromagnetic resonance spectra based on the underlying electronic structure, we unambiguously demonstrate that the damping parameter obtained within the constant broadening approximation diverges for three-dimensional bulk magnets in the clean limit, while it remains finite for monolayers. Our work puts into perspective the several methods available to describe and compute the Gilbert damping, building a solid foundation for future investigations of magnetic relaxation effects in any kind of material.

11.
J Phys Condens Matter ; 30(34): 343002, 2018 Aug 30.
Article in English | MEDLINE | ID: mdl-30020083

ABSTRACT

Single adatoms offer an exceptional playground for studying magnetism and its associated dynamics at the atomic scale. Here we review recent results on single adatoms deposited on metallic substrates, based on time-dependent density functional theory. First we analyze quantum zero-point spin-fluctuations (ZPSF) as calculated from the fluctuation-dissipation theorem, and show how they affect the magnetic stability by modifying the magnetic anisotropy energy. We also assess the impact of ZPSF in the limit of small hybridization to the substrate characteristic of semi-insulating substrates, connecting to recent experimental investigations where magnetic stability of a single adatom was achieved for the first time. Secondly, we inspect further the dynamics of single adatoms by considering the longitudinal and transverse spin-relaxation processes, whose time-scales are analyzed and related to the underlying electronic structure of both the adatom and the substrate. Thirdly, we analyze spin-fluctuation modes of paramagnetic adatoms, i.e. adatoms where the Stoner criterion for magnetism is almost fulfilled. Interestingly, such modes can develop well-defined peaks in the meV range, their main characteristics being determined by two fundamental electronic properties, namely the Stoner parameter and the density of states at the Fermi level. Furthermore, simulated inelastic scanning tunneling spectroscopy curves reveal that these spin-fluctuation modes can be triggered by tunneling electrons, opening up potential applications also for paramagnetic adatoms. Lastly, an overview of the outstanding issues and future directions is given.

12.
Phys Rev Lett ; 119(1): 017203, 2017 Jul 07.
Article in English | MEDLINE | ID: mdl-28731747

ABSTRACT

We predict the existence of paramagnetic spin excitations (PSE) in nonmagnetic single adatoms. Our calculations demonstrate that PSE develop a well-defined structure in the meV region when the adatom's Stoner criterion for magnetism is close to the critical point. We further reveal a subtle tunability and enhancement of PSE by external magnetic fields. Finally, we show how PSE can be detected as moving steps in the dI/dV signal of inelastic scanning tunneling spectroscopy, opening a potential route for experimentally accessing electronic properties of nonmagnetic adatoms, such as the Stoner parameter.

13.
Sci Rep ; 7(1): 3686, 2017 06 16.
Article in English | MEDLINE | ID: mdl-28623297

ABSTRACT

Spin-orbit-related effects offer a highly promising route for reading and writing information in magnetic units of future devices. These phenomena rely not only on the static magnetization orientation but also on its dynamics to achieve fast switchings that can reach the THz range. In this work, we consider Co/Pt and Fe/W bilayers to show that accounting for the phase difference between different processes is crucial to the correct description of the dynamical currents. By tuning each system towards its ferromagnetic resonance, we reveal that dynamical spin Hall angles can non-trivially change sign and be boosted by over 500%, reaching giant values. We demonstrate that charge and spin pumping mechanisms can greatly magnify or dwindle the currents flowing through the system, influencing all kinds of magnetoresistive and Hall effects, thus impacting also dc and second harmonic experimental measurements.

14.
Nat Commun ; 7: 13613, 2016 12 20.
Article in English | MEDLINE | ID: mdl-27995909

ABSTRACT

When electrons are driven through unconventional magnetic structures, such as skyrmions, they experience emergent electromagnetic fields that originate several Hall effects. Independently, ground-state emergent magnetic fields can also lead to orbital magnetism, even without the spin-orbit interaction. The close parallel between the geometric theories of the Hall effects and of the orbital magnetization raises the question: does a skyrmion display topological orbital magnetism? Here we first address the smallest systems with nonvanishing emergent magnetic field, trimers, characterizing the orbital magnetic properties from first-principles. Armed with this understanding, we study the orbital magnetism of skyrmions and demonstrate that the contribution driven by the emergent magnetic field is topological. This means that the topological contribution to the orbital moment does not change under continuous deformations of the magnetic structure. Furthermore, we use it to propose a new experimental protocol for the identification of topological magnetic structures, by soft X-ray spectroscopy.

15.
Nano Lett ; 16(7): 4305-11, 2016 07 13.
Article in English | MEDLINE | ID: mdl-27248465

ABSTRACT

Stabilizing the magnetic signal of single adatoms is a crucial step toward their successful usage in widespread technological applications such as high-density magnetic data storage devices. The quantum mechanical nature of these tiny objects, however, introduces intrinsic zero-point spin-fluctuations that tend to destabilize the local magnetic moment of interest by dwindling the magnetic anisotropy potential barrier even at absolute zero temperature. Here, we elucidate the origins and quantify the effect of the fundamental ingredients determining the magnitude of the fluctuations, namely, the (i) local magnetic moment, (ii) spin-orbit coupling, and (iii) electron-hole Stoner excitations. Based on a systematic first-principles study of 3d and 4d adatoms, we demonstrate that the transverse contribution of the fluctuations is comparable in size to the magnetic moment itself, leading to a remarkable ≳50% reduction of the magnetic anisotropy energy. Our analysis gives rise to a comprehensible diagram relating the fluctuation magnitude to characteristic features of adatoms, providing practical guidelines for designing magnetically stable nanomagnets with minimal quantum fluctuations.

16.
J Med Entomol ; 42(1): 48-56, 2005 Jan.
Article in English | MEDLINE | ID: mdl-15691008

ABSTRACT

Two Trypanosoma cruzi-derived cloning vectors, pTREX-n and pBs:CalB1/CUB01, were used to drive the expression of green fluorescent protein (GFP) and DsRed in Trypanosoma rangeli Tejera, 1920, and Trypanosoma cruzi Chagas, 1909, isolates, respectively. Regardless of the species, group, or strain, parasites harboring the transfected constructs as either episomes or stable chromosomal integrations showed high-level expression of fluorescent proteins. Tagged flagellates of both species were used to experimentally infect Rhodnius prolixus Stal, 1953. In infected bugs, single or mixed infections of T. cruzi and T. rangeli displayed the typical cycle of each species, with no apparent interspecies interactions. In addition, infection of kidney monkey cells (LLC-MK2) with GFP-T. cruzi showed that the parasite retained its fluorescent tag while carrying out its life cycle within cultured cells. The use of GFP-tagged parasites as a tool for biological studies in experimental hosts is discussed, as is the application of this method for copopulation studies of same-host parasites.


Subject(s)
Gene Expression , Green Fluorescent Proteins/genetics , Luminescent Proteins/genetics , Trypanosoma/genetics , Animals , Cell Line , Cells, Cultured , Genetic Vectors , Haplorhini , Kidney , Life Cycle Stages , Rhodnius/parasitology , Transfection , Trypanosoma/growth & development , Trypanosoma cruzi/genetics , Trypanosoma cruzi/growth & development , Red Fluorescent Protein
17.
Clin Diagn Lab Immunol ; 10(5): 775-9, 2003 Sep.
Article in English | MEDLINE | ID: mdl-12965903

ABSTRACT

We used the species specificity and repetitious nature of subtelomeric kinetoplastida sequences to generate a duplex PCR assay for the simultaneous detection of Trypanosoma cruzi and Trypanosoma rangeli in experimentally and naturally infected triatomine (Reduviid) bugs and in infected human subjects. The assay was species specific and was capable of detecting 1/20th of T. cruzi and 1/4th of T. rangeli cell equivalents without complementary hybridization. In addition, the PCR-based assay was robust enough for direct application to difficult biological samples such as Reduviid feces or guts and was capable of recognizing all T. cruzi and T. rangeli strains and lineages. Because the assay primers amplify entirely different target sequences, no reaction interference was observed, facilitating future adaptation of this assay to an automated format.


Subject(s)
Polymerase Chain Reaction/methods , Trypanosoma/immunology , Trypanosomiasis/diagnosis , Animals , DNA Primers , Humans , Reduviidae/parasitology , Sensitivity and Specificity , Species Specificity , Trypanosoma/isolation & purification , Trypanosomiasis/immunology
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