arXiv subjects

Matti Lindroos

Publications and source records attributed to Matti Lindroos.

7 recordsLinked to original sources

arXiv2024

Deformation and adiabatic heating of single crystalline and nanocrystalline Ni micropillars at high strain rates

The deformation behavior of single crystal and nanocrystalline nickel were studied using in situ micropillar compression experiments from quasi-static to high strain rates up to 10^3 s-1. Deformation occurred by dislocation slip activity in single crystal nickel whereas extensive grain boundary sliding was observed in nanocrystalline nickel, with a shift towards more inhomogeneous, localized deformation above 1 s-1. The strain rate sensitivity exponent was found to change at higher strain rates for both single crystal and nanocrystalline nickel, while the overall strain rate sensitivity was observed to be of the same value for both. With increasing high strain rate micropillar compression tests being reported, the issue of adiabatic heating in micropillars becomes important. We report crystal plasticity based finite element modeling to estimate the adiabatic heating, spatially resolved within the pillar, at the highest tested strain rates. The simulations predicted a significant temperature rise of up to 200 K in nanocrystalline Ni at the grain boundaries, and 20 K in single crystalline Ni due to strain localization. Transmission Kikuchi Diffraction analysis of nanocrystalline nickel pillar post compression at 10^3 s-1 did not show any grain growth.

cond-mat.mtrl-sci↗

arXiv2021

Multiscale analysis of crystal defect formation in rapid solidification of pure aluminium and aluminium-copper alloys

Rapid solidification leads to unique microstructural features, where a less studied topic is the formation of various crystalline defects, including high dislocation densities, as well as gradients and splitting of the crystalline orientation. As these defects critically affect the material's mechanical properties and performance features, it is important to understand the defect formation mechanisms, and how they depend on the solidification conditions and alloying. To illuminate the formation mechanisms of the rapid solidification induced crystalline defects, we conduct a multiscale modeling analysis consisting of bond-order potential based molecular dynamics (MD), phase field crystal based amplitude expansion (PFC-AE) simulations, and sequentially coupled phase field -- crystal plasticity (PF--CP) simulations. The resulting dislocation densities are quantified and compared to past experiments. The atomistic approaches (MD, PFC) can be used to calibrate continuum level crystal plasticity models, and the framework adds mechanistic insights arising from the multiscale analysis.

cond-mat.mtrl-sci↗

arXiv2015

Resolving unoccupied electronic states with laser ARPES in bismuth-based cuprate superconductors

Angle-resolved photoemission spectroscopy (ARPES) is typically used to study only the occupied electronic band structure of a material. Here we use laser-based ARPES to observe a feature in bismuth-based superconductors that, in contrast, is related to the unoccupied states. Specifically, we observe a dispersive suppression of intensity cutting across the valence band, which, when compared with relativistic one-step calculations, can be traced to two final-state gaps in the bands 6 eV above the Fermi level. This finding opens up possibilities to bring the ultra-high momentum resolution of existing laser-ARPES instruments to the unoccupied electron states. For cases where the final-state gap is not the object of study, we find that its effects can be made to vanish under certain experimental conditions.

cond-mat.supr-con↗

arXiv2010

Reply to Comment on Circular Dichroism in the Angle-Resolved Photoemission Spectrum of the High-Temperature Bi2Sr2CaCu2O8 Superconductor http://arxiv.org/abs/1004.1648

We conclude that arguments of Norman et al. in their Comment do not provide a significant basis for their claim that the geometric mechanism for explaining the observations reported by them is not viable. More generally, our study highlights the importance of assessing structural issues before invoking exotic mechanisms for explaining unusual spectroscopic observations, especially in complex materia.

cond-mat.supr-con↗

arXiv2010

Existence, character and origin of surface-related bands in the high temperature iron pnictide superconductor BaFe_{2-x}Co_{x}As_{2}

Low energy electron diffraction (LEED) experiments, LEED simulations and finite slab density functional calculations are combined to study the cleavage surface of Co doped BaFe_{2-x}Co_{x}As_{2} (x = 0.1, 0.17). We demonstrate that the energy dependence of the LEED data can only be understood from a terminating 1/2 Ba layer accompanied by distortions of the underlying As-Fe_2-As block. As a result, surface related Fe 3d states are present in the electronic structure, which we identify in angle resolved photoemission experiments. The close proximity of the surface-related states to the bulk bands inevitably leads to broadening of the ARPES signals, which excludes the use of the BaFe_{2-x}Co_{x}As_{2} system for accurate determination of self-energies using ARPES.

cond-mat.supr-con↗

arXiv2010

Uniaxial "nematic-like" electronic structure and Fermi surface of untwinned CaFe2As2

Obtaining the electronic structure of the newly discovered iron-based superconductors is the key to understanding the mechanism of their high-temperature superconductivity. We used angle-resolved photoemission spectroscopy (ARPES) to make direct measurements of the electronic structure and Fermi surface (FS) of the untwinned uniaxial state of CaFe2As2, the parent compound of iron-based superconductors. We observed unequal dispersions and FS geometries along the orthogonal Fe-Fe bond directions. More importantly, unidirectional straight and flat FS segments are observed near the zone center, which indicates the existence of a unidirectional nematic charge density wave order, strengthening the case for a quantum electronic liquid crystalline "nematic" phase. Further, the doping dependence extrapolates to a possible quantum critical point of the disappearance of this order in the heavily overdoped regime of these materials.

cond-mat.supr-con↗

arXiv2010

The Structural Influence on the Rashba-type Spin-Splitting in Surface Alloys

The Bi/Ag(111), Pb/Ag(111), and Sb/Ag(111) surface alloys exhibit a two-dimensional band structure with a strongly enhanced Rashba-type spin-splitting, which is in part attributed to the structural asymmetry resulting from an outward relaxation of the alloy atoms. In order to gain further insight into the spin-splitting mechanism, we have experimentally determined the outward relaxation of the alloy atoms in these surface alloys using quantitative low-energy electron diffraction (LEED). The structure plays an important role in the size of the spinsplitting as it dictates the potential landscape, the symmetry as well as the orbital character. Furthermore, we discuss the band ordering of the Pb/Ag(111) surface alloy as well as the reproducible formation of Sb/Ag(111) surface alloys with unfaulted (face-centered cubic) and faulted (hexagonally close-packed) toplayer stacking.

cond-mat.mes-hall↗