arXiv subjects

P. Piekarz

Publications and source records attributed to P. Piekarz.

13 recordsLinked to original sources

arXiv2022

Modeling of ultrafast X-ray induced magnetization dynamics in magnetic multilayer systems

In this work, we report on modelling results obtained with our recently developed simulation tool enabling nanoscopic description of electronic processes in X-ray irradiated ferromagnetic materials. With this tool, we have studied the response of Co/Pt multilayer system irradiated by an ultrafast extreme ultraviolet pulse at the M-edge of Co (photon energy $\sim$ 60 eV). It was previously investigated experimentally at the FERMI free-electron-laser facility, using the magnetic small-angle X-ray scattering technique. Our simulations show that the magnetic scattering signal from cobalt decreases on femtosecond timescales due to electronic excitation, relaxation and transport processes both in the cobalt and in the platinum layers, following the trend observed in the experimental data. The confirmation of the predominant role of electronic processes for X-ray induced demagnetization in the regime below the structural damage threshold is a step towards quantitative control and manipulation of X-ray induced magnetic processes on femtosecond timescales.

cond-mat.mtrl-sci↗

arXiv2020

Lattice dynamics of endotaxial silicide nanowires

Self-organized silicide nanowires are considered as main building blocks of future nanoelectronics and have been intensively investigated. In nanostructures, the lattice vibrational waves (phonons) deviate drastically from those in bulk crystals, which gives rise to anomalies in thermodynamic, elastic, electronic, and magnetic properties. Hence, a thorough understanding of the physical properties of these materials requires a comprehensive investigation of the lattice dynamics as a function of the nanowire size. We performed a systematic lattice dynamics study of endotaxial FeSi$_2$ nanowires, forming the metastable, surface-stabilized $α$-phase, which are in-plane embedded into the Si(110) surface. The average widths of the nanowires ranged from 24 to 3 nm, their lengths ranged from several $μ$m to about 100 nm. The Fe-partial phonon density of states, obtained by nuclear inelastic scattering, exhibits a broadening of the spectral features with decreasing nanowire width. The experimental data obtained along and across the nanowires unveiled a pronounced vibrational anisotropy that originates from the specific orientation of the tetragonal $α$-FeSi$_2$ unit cell on the Si(110) surface. The results from first-principles calculations are fully consistent with the experimental data and allow for a comprehensive understanding of the lattice dynamics of endotaxial silicide nanowires.

cond-mat.mtrl-sci↗

arXiv2020

Trimeron-phonon coupling in magnetite

Using density functional theory, we study the lattice dynamical properties of magnetite (Fe$_3$O$_4$) in the high-temperature cubic and low-temperature monoclinic phases. The calculated phonon dispersion curves and phonon density of states are compared with the available experimental data obtained by inelastic neutron, inelastic x-ray, and nuclear inelastic scattering. We find a very good agreement between the theoretical and experimental results for the monoclinic $Cc$ structure revealing the strong coupling between charge-orbital (trimeron) order and specific phonon modes. For the cubic phase, clear discrepancies arise which, remarkably, can be understood assuming that the strong trimeron-phonon coupling can be extended above the Verwey transition, with lattice dynamics influenced by the short-range trimeron order instead of the average cubic structure. Our results establish the validity of trimerons (and trimeron-phonon coupling) in explaining the physics of magnetite much beyond their original formulation.

cond-mat.str-el↗

arXiv2020

Lattice dynamics and polarization-dependent phonon damping in $α$-phase FeSi$_{2}$ nanoislands

We determined the lattice dynamics of metastable, surface-stabilized $α$-phase FeSi$_2$ nanoislands epitaxially grown on the Si(111) surface with average heights and widths ranging from 1.5 to 20 nm and 18 to 72 nm, respectively. The crystallographic orientation, surface morphology and local crystal structure of the nanoislands were investigated by reflection high-energy electron diffraction, atomic force microscopy and X-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS), obtained by nuclear inelastic scattering, exhibits a pronounced damping and broadening of the spectral features with decreasing average island height. First-principles calculations of the polarization-projected Si- and Fe-partial phonon dispersions and PDOS enable the disentanglement of the contribution of the $xy$- and $z$-polarized phonons to the experimental PDOS. Modeling of the experimental data with the theoretical results unveils an enhanced damping of the $z$-polarized phonons for islands with average sizes below 10 nm. This phenomenon is attributed to the fact that the low-energy $z$-polarized phonons couple to the low-energy surface/interface vibrational modes. The thermodynamic and elastic properties obtained from the experimental data show a pronounced size-dependent behavior.

cond-mat.mtrl-sci↗

arXiv2019

Electronic and lattice properties of non-centrosymmetric superconductors ThTSi (T = Co, Ir, Ni, and Pt)

The theoretical studies on the electronic and lattice properties of the series of non-centrosymmetric superconductors ThTSi, where T = Co, Ni, Ir, and Pt are presented. The electronic band structure and crystal parameters were optimized within the density functional theory. The spin-orbit coupling leads to the splitting of the electronic bands and Fermi surfaces, with the stronger effect observed for the compounds with the heavier atoms Ir and Pt. The possible mixing of the spin-singlet and spin-triplet pairing in the superconducting state is discussed. The phonon dispersion relations and phonon density of states were obtained using the direct method. The dispersion curves in ThCoSi and ThIrSi exhibit the low-energy modes along the S-N-S0 line with the tendency for softening and dynamic instability. Additionally, we calculate and analyse the contributions of phonon modes to lattice heat capacity.

cond-mat.mtrl-sci↗

arXiv2019

Ab initio and nuclear inelastic scattering studies of Fe$_3$Si/GaAs heterostructures

The structure and dynamical properties of the Fe$_3$Si/GaAs(001) interface are investigated by density functional theory and nuclear inelastic scattering measurements. The stability of four different atomic configurations of the Fe$_3$Si/GaAs multilayers is analyzed by calculating the formation energies and phonon dispersion curves. The differences in charge density, magnetization, and electronic density of states between the configurations are examined. Our calculations unveil that magnetic moments of the Fe atoms tend to align in a plane parallel to the interface, along the [110] direction of the Fe$_3$Si crystallographic unit cell. In some configurations, the spin polarization of interface layers is larger than that of bulk Fe$_3$Si. The effect of the interface on element-specific and layer-resolved phonon density of states is discussed. The Fe-partial phonon density of states measured for the Fe$_3$Si layer thickness of three monolayers is compared with theoretical results obtained for each interface atomic configuration. The best agreement is found for one of the configurations with a mixed Fe-Si interface layer, which reproduces the anomalous enhancement of the phonon density of states below 10 meV

cond-mat.mtrl-sci↗

arXiv2015

Phonons in ultrathin oxide films - 2D to 3D transition in FeO on Pt(111)

The structural and magnetic properties of ultrathin FeO(111) films on Pt(111) with thicknesses from 1 to 16 monolayers (ML) were studied using the nuclear inelastic scattering (NIS) of synchrotron radiation. Distinct evolution of vibrational characteristics with thickness that is revealed in the phonon density of states (PDOS) witnesses a textbook transition from 2D to 3D lattice dynamics. For the thinnest films of 1 and 2 ML, the low energy part of the PDOS followed a linear dependence in energy that is characteristic for 2-dimensional systems. This dependence gradually transforms with thickness to the bulk ~E-square relation. Density functional theory phonon calculations perfectly reproduced the measured 1 ML PDOS within a simple model of a pseudomorphic FeO/Pt(111) interface. The calculations show that the 2D PDOS character is due to a weak coupling of the FeO film to the Pt(111) substrate. The evolution of the vibrational properties with an increasing thickness is closely related to a transient long range magnetic order and stabilization of an unusual structural phase.

cond-mat.mtrl-sci↗

arXiv2015

Dynamical properties of ordered Fe-Pt alloys

The structure, magnetic properties, and lattice dynamics of ordered Fe-Pt alloys with three stoichiometric compositions, Fe$_3$Pt, FePt and FePt$_3$, have been investigated using the density functional theory. Additionally, the existing experimental data have been complemented by new measurements of the Fe projected phonon density of states performed for the Fe$_3$Pt and FePt$_3$ thin films using the nuclear inelastic scattering technique. The calculated phonon dispersion relations and phonon density of states have been compared with the experimental data. The dispersion curves are very well reproduced by the calculations, although, the softening of the transversal acoustic mode TA$_1$ leads to some discrepancy between the theory and experiment in Fe$_3$Pt. A very goood agreement between the measured spectra and calculations performed for the tetragonal structure derived from the soft mode may signal that the tetragonal phase with the space group $P4/mbm$ plays an important role in the martensitic transformation observed in Fe$_3$Pt. For FePt$_3$, the antiferromagnetic order appearing with decreasing temperature has been also investigated. The studies showed that the phonon density of states of FePt$_3$ very weakly depends on the magnetic configuration.

cond-mat.mtrl-sci↗

arXiv2014

Lattice dynamics of the heavy fermion compound URu$_2$Si$_2$

We report a comprehensive investigation of the lattice dynamics of URu$_2$Si$_2$ as a function of temperature using Raman scattering, optical conductivity and inelastic neutron scattering measurements as well as theoretical {\it ab initio} calculations. The main effects on the optical phonon modes are related to Kondo physics. The B$_{1g}$ ($Γ_3$ symmetry) phonon mode slightly softens below $\sim$100~K, in connection with the previously reported softening of the elastic constant, $C_{11}-C_{12}$, of the same symmetry, both observations suggesting a B$_{1g}$ symmetry-breaking instability in the Kondo regime. Through optical conductivity, we detect clear signatures of strong electron-phonon coupling, with temperature dependent spectral weight and Fano line shape of some phonon modes. Surprisingly, the line shapes of two phonon modes, E$_u$(1) and A$_{2u}$(2), show opposite temperature dependencies. The A$_{2u}$(2) mode loses its Fano shape below 150 K, whereas the E$_u$(1) mode acquires it below 100~K, in the Kondo cross-over regime. This may point out to momentum-dependent Kondo physics. By inelastic neutron scattering measurements, we have drawn the full dispersion of the phonon modes between 300~K and 2~K. No remarkable temperature dependence has been obtained including through the hidden order transition. {\it Ab initio} calculations with the spin-orbit coupling are in good agreement with the data except for a few low energy branches with propagation in the (a,b) plane.

cond-mat.str-el↗

arXiv2010

Comparative study of the electronic structures of Fe3O4 and Fe2SiO4

The electronic properties of two spinels Fe$_3$O$_4$ and Fe$_2$SiO$_4$ are studied by the density functional theory. The local Coulomb repulsion $U$ and the Hund's exchange $J$ between the $3d$ electrons on iron are included. For $U=0$, both spinels are half-metals, with the minority $t_{2g}$ states at the Fermi level. Magnetite remains a metal in a cubic phase even at large values of $U$. The metal-insulator transition is induced by the $X_3$ phonon, which lowers the total energy and stabilizes the charge-orbital ordering. Fe$_2$SiO$_4$ transforms to a Mott insulating state for $U>2$ eV with a gap $Δ_g\sim U$. The antiferromagnetic interactions induce the tetragonal distortion, which releases the geometrical frustration and stabilizes the long-range order. The differences of electronic structures in the high-symmetry cubic phases and the distorted low-symmetry phases of both spinels are discussed.

cond-mat.str-el↗

arXiv2010

Vibrational properties of alpha- and sigma-phase Fe-Cr alloy

Experimental investigation as well as theoretical calculations, of the Fe-partial phonon density-of-states (DOS) for nominally Fe_52.5Cr_47.5 alloy having (a) alpha- and (b) sigma-phase structure were carried out. The former at sector 3-ID of the Advanced Photon Source, using the method of nuclear resonant inelastic X-ray scattering, and the latter with the direct method [K. Parlinski et al., Phys. Rev. Lett. {78, 4063 (1997)]. The characteristic features of phonon DOS, which differentiate one phase from the other, were revealed and successfully reproduced by the theory. Various data pertinent to the dynamics such as Lamb-Mossbauer factor, f, kinetic energy per atom, E_k, and the mean force constant, D, were directly derived from the experiment and the theoretical calculations, while vibrational specific heat at constant volume, C_V, and vibrational entropy, S were calculated using the Fe-partial DOS. Using the values of f and C_V, we determined values for Debye temperatures, T_D. An excellent agreement for some quantities derived from experiment and first-principles theory, like C_V and quite good one for others like D and S was obtained.

cond-mat.mtrl-sci↗

arXiv2008

Order parameters in the Verwey phase transition

The Verwey phase transition in magnetite is analyzed on the basis of the Landau theory. The free energy functional is expanded in a series of components belonging to the primary and secondary order parameters. A low-temperature phase with the monoclinic P2/c symmetry is a result of condensation of two order parameters X_3 and Δ_5 . The temperature dependence of the shear elastic constant C_44 is derived and the mechanism of its softening is discussed.

cond-mat.stat-mech↗

arXiv2006

Mechanism of the Verwey transition in magnetite

By combining {\it ab initio} results for the electronic structure and phonon spectrum with the group theory, we establish the origin of the Verwey transition in Fe$_3$O$_4$. Two primary order parameters with $X_3$ and $Δ_5$ symmetries are identified. They induce the phase transformation from the high-temperature cubic to the low-temperature monoclinic structure. The on-site Coulomb interaction $U$ between 3d electrons at Fe ions plays a crucial role in this transition -- it amplifies the coupling of phonons to conduction electrons and thus opens a gap at the Fermi energy. {\it Published in Phys. Rev. Lett. {\bf 97}, 156402 (2006).}

cond-mat.str-el↗