Selected papers

Vibrational properties and stability of FePt nanoalloys

The structural and dynamical properties of FePt nanoparticles were studied within the density functional theory. The effect of size and chemical composition on the dynamical stability of nanoparticles was investigated for the cuboctahedral and icosahedral symmetries. In cuboctahedra, a structural distortion is observed, which for systems with odd number of Pt layers leads to lowering of the tetragonal symmetry (see the figure). The soft mode, which lowers the symmetry is presented in this animation of the vibrations of atoms in the Fe24Pt31 nanoparticle. Significant differences between the vibrational properties of FePt particles and bulk crystals are observed, but similarly to the FePt crystal, cuboctahedral particles exhibit a strong anisotropy of atomic vibrations. The icosahedral particles with perfect shell geometry are unstable due to enlarged distances between Fe atoms. They can be stabilized by removing a central atom or replacing it by a smaller one.

Phys. Rev. B 95, 134303 (2017), arXiv:1704.04056

Ab initio study of the unconventional superconductivity in CeCoIn5 and FeSe

The electronic structure and the shape of the Fermi surface are known to be of fundamental importance for the superconducting instability in real materials. We have demonstrated that such an instability may be explored by the static Cooper pair susceptibility renormalized by pairing interaction and present an efficient method of its evaluation using Wannier orbitals derived from the ab initio calculation. As an example, this approach is used to search for an unconventional superconducting phase of the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) type in a heavy-fermion compound CeCoIn5 and an iron-based superconductor FeSe. The Cooper pair susceptibility calculated at each band crossing the Fermi level in FeSe for momenta at the xy, yz, and xz planes and at the magnetic field H = 20 T is presented in the figure. The results suggest that the FFLO superconducting phase occurs at finite magnetic field in both materials.

New. J. Phys. 9, 0633039 (2017), arXiv:1710.01988, Video abstract

Magnetic Lifshitz transition

We have studied the Lifshitz transition induced by applied external magnetic field in iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic band structure of iron-based superconductors and thus the obtained results can explain the experimental observations in FeSe and Co-doped BaFe2As2 compounds.

Anomalous lattice dynamics in EuSi2 nanoislands

We have performed a systematic lattice dynamics study of EuSi2 films and nanoislands by in situ nuclear inelastic scattering on Eu151 and ab initio theory. Comparison of the calculated Eu-partial phonon density of states (DOS) (convoluted with a Gaussian function, FWHM=1.2 meV) and the DOS of the bulk crystal (sample A), reveals a remarkable agreement between the experiment and theory (see the figure). Complete understanding of the DOS of the thin film (sample B) is achieved by combining the calculated DOS of the surface and bulk EuSi2. The Eu-partial phonon density of states of the nanoislands (samples C and D) exhibits anomalous excess of phonon states at low and high energies, not present in the bulk and at the EuSi2(001) surface. We have demonstrated that atomic vibrations along the island-substrate interface give rise to phonon states both at low and high energies, while atomic vibrations across the island-island interface result in localized high-energy phonon modes.

Phys. Rev. Lett. 117, 276101 (2016)

Dynamics and stability of icosahedral Fe–Pt nanoparticles

The structure, dynamics and stability of Fe–Pt nanoparticles have been investigated using DFT-based techniques: total energy calculations and molecular dynamics. The investigated systems included multi-shell and disordered nanoparticles of iron and platinum. The study concerns icosahedral particles with the magic number of atoms (55): iron-terminated Fe43Pt12, platinum-terminated Fe12Pt43, and disordered Fe27Pt28. Additionally, the Fe6Pt7 cluster has been investigated to probe the behaviour of extremely small Fe–Pt particles.

Molecular dynamics simulations have been performed for a few temperatures between T = 150–1000 K. The calculations revealed high structural instability of the Fe-terminated nanoparticles and a strong stabilising effect of the Pt-termination in the shell-type icosahedral particles. The platinum termination prevented disordering of the particle even at T = 1000 K indicating very high melting temperatures of these Fe–Pt icosahedral structures. The analysis of evolution of the radial distribution function has shown a significant tendency of Pt atoms to move to the outside layer of the particles – even in the platinum deficient cases.

This research has been supported by the COST Action MP0903: "Nanoalloys as advanced materials: from structure to properties and applications" and the article is part of themed collection: Recent advances in the chemical physics of nanoalloys.

Phys. Chem. Chem. Phys. 17, 28096 (2015)

Electronic and dielectric properties of FeO

Wustite (FeO) has challenged experimental and theoretical condensed matter physics for over sixty years. In particular, the extent of its nonstoichiometry and its defect structure have been the subject of numerous experimental studies and are still a matter of discussion and a source of controversy. Generally, these studies indicate that the crystal lattice of FeO is modified by the presence of Fe(3+) cations and vacant Fe(2+) sites. The crystal structure and charge distribution in FeO are very complex as the defects are likely to coalesce and arrange into clusters, which in turn can aggregate into larger defect structures. This work investigates the role of strong electron correlations on Fe atoms and the high concentration of cation vacancies in modifying the electronic and dielectric properties of wustite. As we have shown, both of them influence substantially the electronic properties of this compound and have opposite effect on its band structure. While the local electron interactions in the Fe 3d states are responsible for the opening of the insulating gap, the Fe vacancies reduce its magnitude.

The mechanism of gap reduction remains the same, irrespectively of the type of incorporated defects. Either monovacancies or vacancy clusters induce the band of empty Fe(3+) states inside the gap of the perfect FeO crystal. The width of this band increases monotonically with the increased concentration of cation vacancies reducing the distance from the top of valence states, so effectively diminishing the band gap. We discuss the effects observed in the optical properties of wustite and explained them by the presence of defects. Our studies also explain the anomalous broadening of infrared spectra, demonstrating a strong effect of cation vacancies on phonons in wustite.

Phys. Rev. B 91, 195111 (2015)

Structures of Late Transition Metal Monoxides

Transition metal monoxides i.e. chemical compounds of the formula MO are often found in nature and they have been used as early as in Neolithic period for pottery - and later glassware - colouring. These important materials exhibit a great variety of physical properties such as diverse electronic and thermal conductivity, magnetic or optical features, and they find numerous daily life applications. Nearly twenty MOs have been prepared to this day and even more wait to be synthesized. It has long been known that most of them crystallize in the prototypic rock salt (NaCl) structure – either undistorted or only slightly distorted due to magnetic interactions operating at low temperatures. This important cubic structure is frequently found for inorganic compounds, especially ionic ones. But five oxides: those of copper, silver, palladium, platinum and mercury, do not fit the family since they adopt much more complex lower - symmetry structures. Still, do they have anything in common with the cubic ones?

Read more ... Phys. Rev. Lett. 113, 025505

Short-Range Correlations in Magnetite above the Verwey Temperature

Magnetite is the first magnetic material discovered and utilized by mankind in Ancient Greece, yet it still attracts attention due to its puzzling properties. This is largely due to the quest for a full and coherent understanding of the Verwey transition that occurs at TV=124  K and is associated with a drop of electric conductivity and a complex structural phase transition. A recent detailed analysis of the structure, based on single crystal diffraction, suggests that the electron localization pattern contains linear three-Fe-site units, the so-called trimerons. Here, we show that whatever the electron localization pattern is, it partially survives up to room temperature as short-range correlations in the high-temperature cubic phase, easily discernible by diffuse scattering. Additionally, ab initio electronic structure calculations reveal that characteristic features in these diffuse scattering patterns can be correlated with the Fermi surface topology.

Phys. Rev. X 4, 011040

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