arXiv subjects

Przemysław Piekarz

Publications and source records attributed to Przemysław Piekarz.

At least 19 recordsLinked to original sources

arXiv2026

Electronic mechanism of sub-100-fs demagnetization induced by a femtosecond light pulse

A quantitative understanding of the processes that trigger light-induced demagnetization on ultrashort timescales is crucial for achieving an ultrafast, radiation-controlled magnetic response in materials. This milestone is essential for developing next-generation magnetic storage devices and ultrafast magnetic switches. In this theoretical study, we investigated demagnetization triggered in a single magnetic domain by light pulses ranging from a few to a few tens of femtoseconds in duration, with photon energies spanning the optical and X-ray regimes, under strongly non-equilibrium conditions. We predicted a loss of magnetization in the sub-100-fs range in all cases, primarily due to the excitation of the electronic system and the subsequent redistribution of electrons within the magneto-sensitive band. The considered timescales were too short for phonon-mediated processes or inter-site Heisenberg exchange processes to contribute significantly. These findings pave the way for highly accurate, radiation-driven magnetization control in magnetic materials at sub-100-femtosecond timescales with potential practical applications.

cond-mat.mtrl-sci↗

arXiv2025

Electronic and magnetic properties of the NdNiO$_2$/SrTiO$_3$ thin films

The hole-doped NdNiO$_2$ layer deposited on the SrTiO$_{3}$ surface exhibits unconventional superconductivity. Here, we present a systematic study of the electronic and magnetic properties of the NdNiO$_2$ superconductor using the density functional theory (DFT). The strong local Coulomb interactions in the Ni($3d)$ and Nd($4f$) states are included within the DFT+$U$ method. The effect of Sr doping on the electronic band structure and density of states was studied for the NdNiO$_2$ thin films deposited on the SrTiO$_3$ (001) surface. The results obtained for the uncapped thin films were compared with the calculations for the NdNiO$_2$ films capped by the SrTiO$_3$ layer. We have found significant changes in the electronic structure and magnetic properties of the thin films compared to the bulk crystal.

cond-mat.mtrl-sci↗

arXiv2025

Lattice dynamics study of electron-correlation-induced charge density wave in antiferromagnetic kagome metal FeGe

Electron-correlation-driven phonon soft modes have been recently reported in the antiferromagnetic kagome FeGe compound and associated with the observed charge density wave (CDW). In this paper, we present a systematic investigation of the CDW origin in the context of the ab initio lattice dynamics study. Performing the group theory analysis of the mentioned soft mode, we found that the stable structure has the Immm symmetry and can be achieved by shifts of Ge atoms. Additionally, we show that the final structure realizes a distorted honeycomb Ge lattice as well as a non-flat kagome-like Fe net. For completeness, we present the electronic properties calculations. From the theoretical STM topography simulation, we indicate that the observed CDW occurs in the deformed honeycomb Ge sublattice.

cond-mat.mtrl-sci↗

arXiv2024

Electronic properties and surface states of RbNi$_2$Se$_2$

Iron-based superconductors, with the ThCr$_{2}$Si$_{2}$-type tetragonal structure (122 family), due to the iron arsenide/selenide layers exhibit several characteristic electronic properties. For example, multiband character mosty associated with the $d$-orbitals of iron and the quasi-two-dimensional (2D) cylindrical Fermi surface. Moreover, external hydrostatic pressure leads to the isostructural phase transition from the tetragonal to collapsed-tetragonal phase. In this paper, in relation to the iron-based superconductors, we discuss the electronic properties of novel 122-family member RbNi$_{2}$Se$_{2}$ [Liu H. et al., Phys. Rev. B 106, 094511 (2022)]. We show that the two Fermi pockets exhibit quasi-2D character. Calculation of the surface spectral function for the (001) surface shows that the surface states are realized independently on the surface termination. Additionally, contrary to the iron-based 122 compounds, RbNi$_{2}$Se$_{2}$ exhibits extraordinary multiple isostructural phase transitions under pressure. Moreover, the Lifshizt transition occurs under external pressure, which results into a strong modification of the shapes of the Fermi pockets.

cond-mat.supr-con↗

arXiv2024

Dirac Dispersions and Fermi Surface Nesting in LaCuSb$_{2}$

LaCuSb$_{2}$ is a superconductor with a transition temperature of about $T_\text{c} = 0.9$K and is a potential platform where Dirac fermions can be experimentally observed. In this paper, we report systematic high-resolution studies of its electronic structure using the angle-resolved photoemission spectroscopy (ARPES) technique supported by the DFT calculation. The Fermi surface consists of four branches, of which the two inner ones are more 3-dimensional and the theoretical calculations reproduce well the experiment. We observe several linear dispersions forming Dirac-like structures. The nodal lines are present in the system along ${\text{M}}$-${\text{A}}$ and ${\text{X}}$-${\text{R}}$ and Dirac crossings along ${\text{X}}$-${\text{R}}$ are observed by ARPES. Finally, the nesting between external Fermi surface pockets, which corresponds to charge density wave (CDW) modulation vector is enhanced in LaCuSb$_{2}$ as compared to LaAgSb$_{2}$, while CDW appears in the latter system.

cond-mat.mtrl-sci↗

arXiv2024

Electronic and dynamical properties of cobalt monogermanide CoGe phases under pressure

We present the pressure dependence of the electronic and dynamical properties of six different CoGe phases: orthorhombic Cmmm, hexagonal P6/mmm and P$\bar{6}$2m, monoclinic C2/m, cubic P2$_{1}$3, and orthorhombic Pnma. Using first-principles DFT calculations and the direct force-constants method, we study the dynamical stability of individual phases under external pressure. We show that the orthorombic Cmmm and hexagonal P6/mmm structures are unstable over a broad pressure range and most pronounced imaginary phonon soft mode in both cases leads to a stable hexagonal P$\bar{6}$2m structure of the lowest ground-state energy of all studied phases at ambient and low (below $\sim 3$ GPa) external pressure. Under these conditions, the cubic P2$_{1}$3 phase has the highest energy, however, together with monoclinic C2/m and orthorombic Pnma it is dynamically stable and all these three structures can potentially coexist as meta-stable phases. Above $\sim 3$ GPa, the cubic P2$_{1}$3 phase becomes the most energetically favorable. Fitting the Birch--Murnaghan equation of state we derive bulk modulus for all mentioned phases, which indicate relatively high resistance of CoGe to compression. Such conclusions are confirmed by band structure calculations. Additionally, we show that electronic bands of the hexagonal P$\bar{6}$2m phase reveal characteristic features of the kagome-like structure, while in the cubic P2$_{1}$3 phase spectrum, one can locate spin-1 and double Weyl fermions. In both cases, the external pressure induces the Lifshitz transition, related to the modification of the Fermi surface topology.

cond-mat.mtrl-sci↗

arXiv2023

Anharmonicity and structural phase transition in the Mott insulator Cu$_2$P$_2$O$_7$

Ab initio investigations of structural, electronic, and dynamical properties of the high-temperature $β$ phase of copper pyrophosphate were performed using density functional theory. The electronic band structure shows the Mott insulating state due to electron correlations in copper ions. By calculating phonon dispersion relations, the soft mode at the A point of the Brillouin zone was revealed, showing the dynamical instability of the $β$ phase at low temperatures. The double-well potential connected with the soft mode is derived and the mechanism of the structural phase transition to the $α$ phase is discussed. The self-consistent phonon calculations based on the temperature-dependent effective potential show the stabilization of the $β$ phase at high temperatures, due to the anharmonic effects. The pronounced temperature dependence and the large line width of the soft mode indicate an essential role of anharmonicity in the structural phase transition.

cond-mat.mtrl-sci↗

arXiv2023

Phononic drumhead surface state in distorted kagome compound RhPb

RhPb was initially recognized as one of a CoSn-like compounds with $P6/mmm$ symmetry, containing an ideal kagome lattice of $d$-block atoms. However, theoretical calculations predict the realization of the phonon soft mode which leads to the kagome lattice distortion and stabilization of the structure with $P\bar{6}2m$ symmetry [A. Ptok et al., Phys. Rev. B 104, 054305 (2021)]. Here, we present the single crystal x-ray diffraction results supporting this prediction. Furthermore, we discuss the main dynamical properties of RhPb with $P\bar{6}2m$ symmetry. The bulk phononic dispersion curves contain several flattened bands, Dirac nodal lines, and triple degenerate Dirac points. As a consequence, the phononic drumhead surface state is realized for the (100) surface, terminated by the zigzag-like edge of Pb honeycomb sublattice.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast demagnetization in bulk nickel induced by X-ray photons tuned to Ni $M_{3}$ and $L_3$ absorption edges

Studies of light-induced demagnetization started with the experiment performed by Beaupaire et al. on nickel. Here, we present theoretical predictions for X-ray induced demagnetization of nickel, with X-ray photon energies tuned to its $M_3$ and $L_3$ absorption edges. We show that the specific feature in the density of states of the d-band of Ni, a sharp peak located just above the Fermi level, strongly influences the change of the predicted magnetic signal, making it stronger than in the previously studied case of cobalt. We believe that this finding will inspire future experiments on magnetic processes in X-ray irradiated nickel.

cond-mat.mtrl-sci↗

arXiv2023

Density functional theory study of experimentally observed Au/Ge interfaces

In recent years, nanostructures with hexagonal polytypes of gold have been synthesised, opening new possibilities in nanoscience and technology. As bulk gold crystallizes in the \textit{fcc} phase, surface effects can play an important role in stabilizing hexagonal gold nanostructures. Here we investigate several hetero-structures with Ge substrate, including the \textit{fcc} and \textit{hcp} phases of gold that have been observed experimentally. We determine and discuss their interfacial energies and optimized atomic arrangements, comparing the theory results with available experimental data. Our calculations for the Au-\textit{fcc}(011)/Ge(001) show how the presence of defects in the interface layer can help to stabilize the atomic pattern consistent with microscopic images. The Au-\textit{hcp}/Ge interface with the significant mismatch between two surfaces reveals large atomic displacements, which might indicate that the (111) germanium substrate is not responsible for the formation of the \textit{hcp} phase of gold. Finally, analyzing the electronic properties, we demonstrate that Au/Ge systems have metallic character but covalent-like bonding states between interfacial Ge and Au atoms are also present.

cond-mat.mtrl-sci↗

arXiv2023

Electronic processes occurring during ultrafast demagnetization of cobalt triggered by X-ray photons tuned to Co L$_3$ resonance

Magnetization dynamics triggered with ultrashort laser pulses has been attracting significant attention, with strong focus on the dynamics excited by VIS/NIR pulses. Only recently, strong magnetic response in solid materials induced by intense X-ray pulses from free-electron lasers (FELs) has been observed. The exact mechanisms that trigger the X-ray induced demagnetization are not yet fully understood. They are subject of on-going experimental and theoretical investigations. Here, we present a theoretical analysis of electronic processes occurring during demagnetization of Co multilayer system irradiated by X-ray pulses tuned to L$_3$-absorption edge of cobalt. We show that, similarly as in the case of X-ray induced demagnetization at M-edge of Co, electronic processes play a predominant role in the demagnetization until the pulse fluence does not exceed the structural damage threshold. The impact of electronic processes can reasonably well explain the available experimental data, without a need to introduce the mechanism of stimulated elastic forward scattering.

cond-mat.mtrl-sci↗

arXiv2023

Influence of $f$ Electrons on the Electronic Band Structure of Rare-Earth Nickelates

Recently, superconductivity was discovered in the infinite layer of hole-doped nickelates NdNiO$_{2}$. Contrary to this, superconductivity in LaNiO$_{2}$ is still under debate. This indicates the crucial role played by the $f$ electrons on the electronic structure and the pairing mechanism of infinite-layer nickelates. Here we discuss the role of the electron correlations on the $f$ electron states and their influence on the electronic structure. We show that the lattice parameters are in good agreement with the experimental values, independent of the chosen parameters within the DFT+$U$ approach. Increasing Coulomb interaction $U$ tends to shift the $f$ states away from the Fermi level. Surprisingly, independently of the position of $f$ states with respect to the Fermi energy, these states play an important role in the electronic band structure, which can be reflected in the modification of the NdNiO$_{2}$ effective models.

cond-mat.str-el↗

arXiv2022

Chiral phonon in the cubic system based on the Laves phase of $A$Bi$_{2}$ ($A=$K, Rb, Cs)

$A$B$_{2}$ ($A=$K, Rb, Cs) compounds crystallize in the cubic Laves phase (symmetry Fd$\bar{3}$m). The geometry of the crystal structure allows the realization of chiral phonons, which are associated with the circulation of atoms around their equilibrium positions. Due to the inversion symmetry and time reversal symmetry, total pseudo-angular momentum (PAM) of the system vanishes. We show that the doping of these system can lead to a new phase with symmetry F$\bar{4}3$m. New systems (KRbBi$_{4}$ and RbCsBi$_{4}$) do not exhibit soft modes (are stable dynamically). Due to the inversion symmetry breaking, realized chiral phonon modes posses a non-zero total PAM. In both type of systems the chiral phonons are realized for the wavectors at the edge of the Brillouine zone. This study explores the possibility of chiral phonon engineering via doping, and predicts two new materials. Discussing the problem opens a new way to study the phonon Hall effect.

cond-mat.mtrl-sci↗

arXiv2022

Electronic band structure and surface states in Dirac semimetal LaAgSb$_{2}$

LaAgSb$_{2}$ is a Dirac semimetal showing charge density wave (CDW) order. Previous ARPES results suggest the existence of the Dirac-cone-like structure in the vicinity of the Fermi level along the $Γ$-M direction [X. Shi et al., Phys. Rev. B 93, 081105(R) (2016)]. This paper is devoted to a complex analysis of the electronic band structure of LaAgSb$_{2}$ by means of angle-resolved photoemission spectroscopy (ARPES) and theoretical calculations within the direct ab initio method as well as tight binding model formulation. To investigate the possible surface states we performed the direct DFT slab calculation and the surface Green function calculation for the (001) surface. The appearance of the surface states, which depends strongly on surface, points to the conclusion that LaSb termination is realized in the cleaved crystals. Moreover, the surface states predicted by our calculations at the $Γ$ and $X$ points are found by ARPES. Nodal lines, which exist along X--R and M-A path due to crystal symmetry, are also observed experimentally. The calculations reveal another nodal lines, which originate from vanishing of spin-orbit coupling and are located at X-M-A-R plane at the Brillouin zone boundary. In addition, we analyze band structure along the $Γ$-M path to verify, whether Dirac surface states can be expected. Their appearance in this region is not confirmed.

cond-mat.mtrl-sci↗

arXiv2021

Dynamical Study of the Origin of the Charge Density Wave in $A$V$_{3}$Sb$_{5}$ ($A=$K, Rb, Cs) Compounds

Systems containing the ideal kagome lattice can exhibit several distinct and novel exotic states of matter. One example of such systems is a recently discovered $A$V$_{3}$Sb$_{5}$ ($A$ = K, Rb, and Cs) family of compounds. Here, the coexistence of the charge density wave (CDW) and superconductivity is observed. In this paper, we study the dynamic properties of the $A$V$_{3}$Sb$_{5}$ systems in context of origin of the CDW phase. We show and discuss the structural phase transition from $P6/mmm$ to $C2/m$ symmetry that are induced by the presence of phonon soft modes. We conclude that the CDW observed in this family of compounds is a consequence of the atom displacement, from the high symmetry position of the kagome net, in low-temperature phase. Additionally, using the numerical {\it ab initio} methods, we discuss the charge distribution on the $A$V$_{3}$Sb$_{5}$ surface. We show that the observed experimental %$4\times 1$ stripe-like modulation of the surface, can be related to surface reconstruction and manifestation of the three dimensional $2 \times 2 \times 2$ bulk CDW. Finally, the consequence of realization of the $C2/m$ structure on the electronic properties are discussed. We show that the electronic band structure reconstruction and the accompanying modification of density of states correspond well to the experimental data.

cond-mat.mtrl-sci↗

arXiv2021

Charge ordering mechanism in silver fluoride AgF$_2$

Using Density Functional Theory, competition between Mott-Hubbard and intervalence charge transfer mechanism of electron localization is revealed in AgF$_2$, an important silver analogue of oxocuprates. We show that at reduced temperature and electron correlations AgF$_2$ becomes metallic and dynamically unstable in respect to CDW phonon modes that promote charge ordering. The CDW instability is closely related to Kohn anomaly and Fermi surface nesting. The long advocated KBrF$_4$ type CDW Ag$^{1+/3+}$F$_2$ structure and its facile transformation to the ground state Ag$^{2+}$F$_2$ phase is explained. Our results point out to intimate interplay between lattice, charge and spin degrees of freedom in seemingly simplistic binary metal fluoride.

cond-mat.str-el↗

arXiv2021

Chiral phonons in honeycomb sublattice of layered CoSn-like compounds

Hexagonal and kagome lattices exhibit extraordinary electronic properties. It is a natural consequence of additional discrete degree of freedom associated with a valley or the occurence of electronic flat-bands. Combination of both types of lattices, observed in CoSn-like compounds, leads not only to the topological electronic behavior, but also to the emergence of chiral phonon modes. Here, we study CoSn-like compounds in the context of realization of chiral phonons. Previous theoretical studies demonstrated that the chiral phonons can be found in ideal two-dimensional hexagonal or kagome lattices. However, it turns out that in the case of CoSn-like systems with the $P6/mmm$ symmetry, the kagome lattice formed by $d$-block element is decorated by the additional $p$-block atom. This results in a two dimensional triangular lattice of atoms with non-equal masses and the absence of chiral phonons in the kagome plane. Contrary to this, the interlayer hexagonal lattice of $p$-block atoms is preserved and allows for the realization of chiral phonons. We discuss properties of these chiral phonons in seven CoSn-like compounds and demonstrate that they do not depend on atomic mass ratio or the presence of intrinsic magnetic order. The chiral phonons of $d$-block atoms can be restored by removing the inversion symmetry. The latter is possible in the crystal structure of CoGe and RhPb with the reduced symmetry ($P\bar{6}2m$) and distorted-kagome-like lattice.

cond-mat.mtrl-sci↗

arXiv2021

Electronic and dynamical properties of CeRh$_{2}$As$_{2}$: Role of Rh$_{2}$As$_{2}$ layers and expected hidden orbital order

Recently discovered heavy fermion CeRh$_{2}$As$_{2}$ compound crystallizes in the nonsymmorphic $P4/nmm$ symmetry, which enables the occurrence of topological protection. Experimental results show that this material exhibits unusual behavior, which is manifested by the appearance of two superconducting phases. In this work, we uncover and discuss a role of Rh$_{2}$As$_{2}$ layers and their impact on the electronic and dynamical properties of the system. The location of Ce atoms between two non-equivalent layers allows for the realization of hidden orbital order. We point out that the electronic band structure around the Fermi level is associated mostly with Ce $4f$ and Rh $4d$ orbitals and suggest the occurrence of the Lifshitz transition induced by the external magnetic field. We discuss also the role played by the $f$--$d$ orbital hybridization in the electronic band structure.

cond-mat.mtrl-sci↗