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1.
J Phys Chem Lett ; 13(33): 7861-7869, 2022 Aug 25.
Artigo em Inglês | MEDLINE | ID: mdl-35977384

RESUMO

To use efficiently the magnetic functionalities emerging at the surfaces or interfaces of novel lanthanides-based materials, there is a need for complementary methods to probe the atomic-layer resolved magnetic properties. Here, we show that 4f photoelectron spectroscopy is highly sensitive to the collective orientation of 4f magnetic moments and, thus, a powerful tool for characterizing the related properties. To demonstrate this, we present the results of systematic study of a family of layered crystalline 4f-materials, which are crystallized in the body-centered tetragonal ThCr2Si2 structure. Analysis of 4f spectra indicates that the 4f moments at the surface experience a strong reorientation with respect to the bulk, caused by changes of the crystal-electric field. The presented database of the computed 4f spectra for all trivalent rare-earth ions in their different MJ states will facilitate the estimation of the orientation of the 4f magnetic moments in the layered 4f-systems for efficient control of their magnetic properties.

2.
ACS Nano ; 16(3): 3573-3581, 2022 Mar 22.
Artigo em Inglês | MEDLINE | ID: mdl-35156797

RESUMO

The f-driven temperature scales at the surfaces of strongly correlated materials have increasingly come into the focus of research efforts. Here, we unveil the emergence of a two-dimensional Ce Kondo lattice, which couples ferromagnetically to the ordered Co lattice below the P-terminated surface of the antiferromagnet CeCo2P2. In its bulk, Ce is passive and behaves tetravalently. However, because of symmetry breaking and an effective magnetic field caused by an uncompensated ferromagnetic Co layer, the Ce 4f states become partially occupied and spin-polarized near the surface. The momentum-resolved photoemission measurements indicate a strong admixture of the Ce 4f states to the itinerant bands near the Fermi level including surface states that are split by exchange interaction with Co. The temperature-dependent measurements reveal strong changes of the 4f intensity at the Fermi level in accordance with the Kondo scenario. Our findings show how rich and diverse the f-driven properties can be at the surface of materials without f-physics in the bulk.

3.
Nat Commun ; 12(1): 6837, 2021 Nov 25.
Artigo em Inglês | MEDLINE | ID: mdl-34824213

RESUMO

In the standard model of charge density wave (CDW) transitions, the displacement along a single phonon mode lowers the total electronic energy by creating a gap at the Fermi level, making the CDW a metal-insulator transition. Here, using scanning tunneling microscopy and spectroscopy and ab initio calculations, we show that VS2 realizes a CDW which stands out of this standard model. There is a full CDW gap residing in the unoccupied states of monolayer VS2. At the Fermi level, the CDW induces a topological metal-metal (Lifshitz) transition. Non-linear coupling of transverse and longitudinal phonons is essential for the formation of the CDW and the full gap above the Fermi level. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to coexisting charge and spin density waves in the ground state.

4.
Sci Adv ; 5(10): eaax2805, 2019 10.
Artigo em Inglês | MEDLINE | ID: mdl-31620556

RESUMO

Fabrication of biomimetic materials and scaffolds is usually a micro- or even nanoscale process; however, most testing and all manufacturing require larger-scale synthesis of nanoscale features. Here, we propose the utilization of naturally prefabricated three-dimensional (3D) spongin scaffolds that preserve molecular detail across centimeter-scale samples. The fine-scale structure of this collagenous resource is stable at temperatures of up to 1200°C and can produce up to 4 × 10-cm-large 3D microfibrous and nanoporous turbostratic graphite. Our findings highlight the fact that this turbostratic graphite is exceptional at preserving the nanostructural features typical for triple-helix collagen. The resulting carbon sponge resembles the shape and unique microarchitecture of the original spongin scaffold. Copper electroplating of the obtained composite leads to a hybrid material with excellent catalytic performance with respect to the reduction of p-nitrophenol in both freshwater and marine environments.


Assuntos
Biomimética , Colágeno/química , Carbono/química , Catálise , Colágeno/ultraestrutura , Cobre/química , Análise Espectral , Tecidos Suporte/química
5.
Phys Rev Lett ; 123(2): 027001, 2019 Jul 12.
Artigo em Inglês | MEDLINE | ID: mdl-31386544

RESUMO

We provide a novel experimental method to quantitatively estimate the electron-phonon coupling and its momentum dependence from resonant inelastic x-ray scattering (RIXS) spectra based on the detuning of the incident photon energy away from an absorption resonance. We apply it to the cuprate parent compound NdBa_{2}Cu_{3}O_{6} and find that the electronic coupling to the oxygen half-breathing phonon branch is strongest at the Brillouin zone boundary, where it amounts to ∼0.17 eV, in agreement with previous studies. In principle, this method is applicable to any absorption resonance suitable for RIXS measurements and will help to define the contribution of lattice vibrations to the peculiar properties of quantum materials.

6.
J Phys Condens Matter ; 31(30): 305602, 2019 Jul 31.
Artigo em Inglês | MEDLINE | ID: mdl-30933934

RESUMO

We present a detailed study of the temperature evolution of the crystal structure, specific heat, magnetic susceptibility and resistivity of single crystals of the paradigmatic valence-fluctuating compound [Formula: see text]. A comparison to stable-valent isostructural compounds [Formula: see text] (with Eu3+), and [Formula: see text], (with Eu2+) reveals an anomalously large thermal expansion indicative of the lattice softening associated to valence fluctuations. A marked broad peak at temperatures around 65-75 K is observed in specific heat, susceptibility and the derivative of resistivity, as thermal energy becomes large enough to excite Eu into a divalent state, which localizes one f electron and increases scattering of conduction electrons. In addition, the intermediate valence at low temperatures manifests in a moderately renormalized electron mass, with enhanced values of the Sommerfeld coefficient in the specific heat and a Fermi-liquid-like dependence of resistivity at low temperatures. The high residual magnetic susceptibility is mainly ascribed to a Van Vleck contribution. Although the intermediate/fluctuating valence duality is to some extent represented in the interconfiguration fluctuation model commonly used to analyze data on valence-fluctuating systems, we show that this model cannot describe the different physical properties of [Formula: see text] with a single set of parameters.

7.
J Phys Chem Lett ; 10(5): 911-917, 2019 Mar 07.
Artigo em Inglês | MEDLINE | ID: mdl-30717591

RESUMO

We investigate the magnetic and electronic properties of europium cyclooctatetraene (EuCot) nanowires by means of low-temperature X-ray magnetic circular dichroism (XMCD) and scanning tunneling microscopy (STM) and spectroscopy (STS). The EuCot nanowires are prepared in situ on a graphene surface. STS measurements identify EuCot as an insulator with a minority band gap of 2.3 eV. By means of Eu M5,4 edge XMCD, orbital and spin magnetic moments of (-0.1 ± 0.3)µB and (+7.0 ± 0.6)µB, respectively, were determined. Field-dependent measurements of the XMCD signal at the Eu M5 edge show hysteresis for grazing X-ray incidence at 5 K, thus confirming EuCot as a ferromagnetic material. Our density functional theory calculations reproduce the experimentally observed minority band gap. Modeling the experimental results theoretically, we find that the effective interatomic exchange interaction between Eu atoms is on the order of millielectronvolts, that magnetocrystalline anisotropy energy is roughly half as big, and that dipolar energy is approximately ten times lower.

8.
Nano Lett ; 17(2): 811-820, 2017 02 08.
Artigo em Inglês | MEDLINE | ID: mdl-28032768

RESUMO

Finding ways to create and control the spin-dependent properties of two-dimensional electron states (2DESs) is a major challenge for the elaboration of novel spin-based devices. Spin-orbit and exchange-magnetic interactions (SOI and EMI) are two fundamental mechanisms that enable access to the tunability of spin-dependent properties of carriers. The silicon surface of HoRh2Si2 appears to be a unique model system, where concurrent SOI and EMI can be visualized and controlled by varying the temperature. The beauty and simplicity of this system lie in the 4f moments, which act as a multiple tuning instrument on the 2DESs, as the 4f projections parallel and perpendicular to the surface order at essentially different temperatures. Here we show that the SOI locks the spins of the 2DESs exclusively in the surface plane when the 4f moments are disordered: the Rashba-Bychkov effect. When the temperature is gradually lowered and the system experiences magnetic order, the rising EMI progressively competes with the SOI leading to a fundamental change in the spin-dependent properties of the 2DESs. The spins rotate and reorient toward the out-of-plane Ho 4f moments. Our findings show that the direction of the spins and the spin-splitting of the two-dimensional electrons at the surface can be manipulated in a controlled way by using only one parameter: the temperature.

9.
ACS Nano ; 10(10): 9353-9360, 2016 Oct 25.
Artigo em Inglês | MEDLINE | ID: mdl-27726335

RESUMO

We investigate the electronic and magnetic properties of TbPc2 single ion magnets adsorbed on a graphene/Ni(111) substrate, by density functional theory (DFT), ab initio complete active space self-consistent field calculations, and X-ray magnetic circular dichroism (XMCD) experiments. Despite the presence of the graphene decoupling layer, a sizable antiferromagnetic coupling between Tb and Ni is observed in the XMCD experiments. The molecule-surface interaction is rationalized by the DFT analysis and is found to follow a relay-like communication pathway, where the radical spin on the organic Pc ligands mediates the interaction between Tb ion and Ni substrate spins. A model Hamiltonian which explicitly takes into account the presence of the spin radical is then developed, and the different magnetic interactions at play are assessed by first-principle calculations and by comparing the calculated magnetization curves with XMCD data. The relay-like mechanism is at the heart of the process through which the spin information contained in the Tb ion is sensed and exploited in carbon-based molecular spintronics devices.

10.
Inorg Chem ; 53(17): 8854-8, 2014 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-25110811

RESUMO

The possible hybridization between Pr 4f and O 2p states in Pr(0.50)Sr(0.50)CoO3 at low temperatures was investigated by different techniques. First, using neutron diffraction we observed a strong contraction of some Pr-O bonds across the magnetostructural transition at T(S) ∼ 120 K. In contrast to the Pr-O bond contraction in Pr(0.50)Sr(0.50)CoO3, this transition is not accompanied by the appearance of Pr(4+) at low temperatures, as revealed by X-ray absorption spectroscopy at Pr edges. Despite the fact that a Pr valence change is not the mechanism that drives this transition, we point out an active participation of Pr ions across T(S). Moreover, Co L(2,3)-edge and O K edge X-ray absorption spectra did not reveal any spin-state variation and showed the stability of the average formal valence of cobalt ions. The large density of empty t(2g) symmetry states in the studied thermal range does not suggest the occurrence of Co(3+) in a pure low-spin state. The overall metallic behavior agrees with our findings. We propose a mixture of Co(3+) ions in the intermediate-spin or high-spin configuration together with Co(4+) ions in a low- or intermediate-spin state.

11.
Beilstein J Nanotechnol ; 4: 320-4, 2013.
Artigo em Inglês | MEDLINE | ID: mdl-23766956

RESUMO

The magnetic and electronic properties of single-molecule magnets are studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We study the magnetic coupling of ultrathin Co and Ni films that are epitaxially grown onto a Cu(100) substrate, to an in situ deposited submonolayer of TbPc2 molecules. Because of the element specificity of the X-ray absorption spectroscopy we are able to individually determine the field dependence of the magnetization of the Tb ions and the Ni or Co film. On both substrates the TbPc2 molecules couple antiferromagnetically to the ferromagnetic films, which is possibly due to a superexchange interaction via the phthalocyanine ligand that contacts the magnetic surface.

12.
Int J Biol Macromol ; 51(1-2): 129-37, 2012.
Artigo em Inglês | MEDLINE | ID: mdl-22546360

RESUMO

Until now, there is a lack of knowledge about the presence of chitin in numerous representatives of corals (Cnidaria). However, investigations concerning the chitin-based skeletal organization in different coral taxa are significant from biochemical, structural, developmental, ecological and evolutionary points of view. In this paper, we present a thorough screening for the presence of chitin within the skeletal formations of a poorly investigated Mediterranean black coral, Parantipathes larix (Esper, 1792), as a typical representative of the Schizopathidae family. Using a wide array variety of techniques ((13)C solid state NMR, Fourier transform infrared (FTIR), Raman, NEXAFS, Morgan-Elson assay and Calcofluor White Staining), we unambiguously show for the first time that chitin is an important component within the skeletal stalks as well as pinnules of this coral.


Assuntos
Antozoários/química , Quitina/química , Animais , Antozoários/ultraestrutura , Quitina/isolamento & purificação , Quitinases/metabolismo
13.
J Phys Chem B ; 115(10): 2401-7, 2011 Mar 17.
Artigo em Inglês | MEDLINE | ID: mdl-21338139

RESUMO

Strong chemical interaction between bacterial surface protein layers and calcium atoms deposited in situ on top was revealed by means of photoemission spectroscopy. The interaction appears to mainly happen at the oxygen site of the peptide bonds and involves a large charge transfer from Ca 4s states into the peptide backbone. Chemical kinetics of this reaction was characterized using time-dependent valence band photoemission, and the reaction rate constant was determined.


Assuntos
Proteínas de Bactérias/química , Cálcio/química , Peptídeos/química , Bacillus , Proteínas de Bactérias/metabolismo , Cálcio/metabolismo , Cinética , Modelos Moleculares , Conformação Molecular , Espectroscopia Fotoeletrônica
14.
Nat Chem ; 2(12): 1084-8, 2010 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-21107374

RESUMO

The minerals involved in the formation of metazoan skeletons principally comprise glassy silica, calcium phosphate or carbonate. Because of their ancient heritage, glass sponges (Hexactinellida) may shed light on fundamental questions such as molecular evolution, the unique chemistry and formation of the first skeletal silica-based structures, and the origin of multicellular animals. We have studied anchoring spicules from the metre-long stalk of the glass rope sponge (Hyalonema sieboldi; Porifera, Class Hexactinellida), which are remarkable for their size, durability, flexibility and optical properties. Using slow-alkali etching of biosilica, we isolated the organic fraction, which was revealed to be dominated by a hydroxylated fibrillar collagen that contains an unusual [Gly-3Hyp-4Hyp] motif. We speculate that this motif is predisposed for silica precipitation, and provides a novel template for biosilicification in nature.


Assuntos
Colágeno/química , Poríferos/química , Dióxido de Silício/química , Motivos de Aminoácidos , Sequência de Aminoácidos , Animais , Evolução Molecular , Hidroxilação , Nanopartículas/química , Nanopartículas/ultraestrutura
15.
J Phys Chem B ; 114(29): 9645-52, 2010 Jul 29.
Artigo em Inglês | MEDLINE | ID: mdl-20608694

RESUMO

The electronic structure of genomic DNA has been comprehensively characterized by synchrotron-based X-ray absorption and X-ray photoelectron spectroscopy. Both unoccupied and occupied states close to the Fermi level have been unveiled and attributed to particular sites within the DNA structure. A semiconductor-like electronic structure with a band gap of approximately 2.6 eV has been found at which the pi and pi* orbitals of the nucleobase stack make major contributions to the highest occupied and lowest unoccupied molecular orbitals, respectively, in agreement with previous theoretical predictions.


Assuntos
DNA/química , Genoma , Espectroscopia Fotoeletrônica , Teoria Quântica
16.
Nano Lett ; 10(3): 992-5, 2010 Mar 10.
Artigo em Inglês | MEDLINE | ID: mdl-20141155

RESUMO

The outstanding properties of graphene, a single graphite layer, render it a top candidate for substituting silicon in future electronic devices. The so far exploited synthesis approaches, however, require conditions typically achieved in specialized laboratories and result in graphene sheets whose electronic properties are often altered by interactions with substrate materials. The development of graphene-based technologies requires an economical fabrication method compatible with mass production. Here we demonstrate for the fist time the feasibility of graphene synthesis on commercially available cubic SiC/Si substrates of >300 mm in diameter, which result in graphene flakes electronically decoupled from the substrate. After optimization of the preparation procedure, the proposed synthesis method can represent a further big step toward graphene-based electronic technologies.


Assuntos
Compostos Inorgânicos de Carbono/química , Cristalização/métodos , Eletrônica/instrumentação , Grafite/química , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Compostos de Silício/química , Silício/química , Condutividade Elétrica , Desenho de Equipamento , Análise de Falha de Equipamento , Substâncias Macromoleculares/química , Teste de Materiais , Conformação Molecular , Nanotecnologia/métodos , Tamanho da Partícula , Propriedades de Superfície
17.
J Phys Chem B ; 111(48): 13491-8, 2007 Dec 06.
Artigo em Inglês | MEDLINE | ID: mdl-17997540

RESUMO

The electronic structure of individual sheets of the bacterial surface protein layer (S layer) of Bacillus sphaericus NCTC 9602 was studied using a photoemission electron microscope (PEEM) operating in near-edge X-ray absorption fine structure spectroscopy mode. The laterally resolved measurements performed at the C 1s, N 1s, and O 1s thresholds on fresh samples revealed characteristic differences compared to the laterally integrated data, where substrate contributions were taken along with the protein signals. During the PEEM experiments an irradiation-induced increase of the C-C bond density at the cost of the densities of the C-O and C-N bonds related to a rearrangement of the contributing atoms of the S layer deposited onto a Si substrate was observed. The critical irradiation doses for the C-O and C-N bond breaking and formation of the new C-C bonds were derived.


Assuntos
Proteínas de Bactérias/análise , Microscopia Eletrônica/métodos , Proteínas de Bactérias/efeitos da radiação , Raios X
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