arXiv subjects

Paweł T. Jochym

Publications and source records attributed to Paweł T. Jochym.

18 recordsLinked to original sources

arXiv2025

Thermal conductivity of 3C-SiC from configuration space sampling

Cubic silicon carbide phonon thermal conductivity has been calculated using anharmonic phonon analysis. The atomic interaction model was built using displacement-force data obtained with the High Efficiency Configuration Space Sampling (HECSS) technique and density functional theory calculated forces. In the new version of HECSS we replaced the Markov chain scheme of Metropolis-Hastings Monte-Carlo with weighting of the final sampling according to the target distribution. This increased the efficiency of the method and allowed to use -- with appropriate weight -- all generated and ab-initio evaluated samples. The quality of the proposed method is confirmed by the accuracy with which the experimental results taken from the literature were reproduced.

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 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

Chiral phonons in binary compounds $A$Bi ($A$ = K, Rb, Cs) with P2$_1$/c structure

Binary compounds $A$Bi ($A$ = K, Rb, Cs) crystallize in P2$_1$/c structure containing both clockwise and anticlockwise chiral chains of Bi atoms. Electronic band structure exhibits the insulating nature of these systems, with the band gap about $0.25$ eV. The presented study of dynamical properties confirm a stability of the system with P2$_1$/c symmetry. The crystal structure contains the quasi-one-dimensional Bi chains, exhibiting four-fold-like rotational ``local'' symmetry. Nevertheless, the system formally posses two-fold rotational symmetry. Independently of the absence of the three-fold (or higher) rotational symmetry axes for the whole crystal, the chiral modes propagate along the Bi atom chains in these systems. We discuss basic properties of these modes in monoatomic chiral chains. We show that the two-fold rotational symmetry axis affects the main properties of the chiral phonons, which are not realized at the high-symmetry points, but along some paths between them in the reciprocal space. In addition, in the doped system, the chiral phonons possess non-zero total angular momentum.

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

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↗

arXiv2017

On reality of dynamical matrix

The symmetry properties of the dynamical matrix are well described in multiple classic textbooks. This short paper revisits the issue to demonstrate alternative form of dynamical matrix which explicitly shows its symmetry and reality in common cases.

cond-mat.other↗

arXiv2017

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 dynamical stability of nanoparticles was investigated for the cuboctahedral and icosahedral symmetries. In cuboctahedra, the structural distortion is observed, which for systems with odd number of Pt layers leads to lowering of the tetragonal symmetry. Significant differences between the vibrational properties of FePt particles and bulk crystal is 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. The heat capacity and entropy of nanoparticles show typical enhancement due to low-energy vibrations at the surface layers.

cond-mat.mes-hall↗

arXiv2015

Soft phonon modes in rutile TiO$_2$

The lattice dynamics of TiO$_2$ in the rutile crystal structure was studied by a combination of thermal diffuse scattering, inelastic x-ray scattering and density functional perturbation theory. We experimentally confirm the existence of an anomalous soft transverse acoustic mode with energy minimum at q = (1/2 1/2 1/4). The phonon energy landscape of this particular branch is reported and compared to the calculation. The harmonic calculation underestimates the phonon energies but despite this the shape of both the energy landscape and the scattering intensities are well reproduced. We find a significant temperature dependence in energy of this transverse acoustic mode over an extended region in reciprocal space which is in line with a substantially anharmonic mode potential-energy surface. The reported low energy branch is quite different from the ferroelectric mode that softens at the Brillouin zone center and may help explain anomalous convergence behavior in calculating TiO$_2$ surface properties.

cond-mat.mtrl-sci↗

arXiv2015

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 DFT molecular dynamics. The investigated systems included multi-shell and disordered nanoparticles of iron and platinum. The study is concerned with icosahedral particles with magic number of atoms (55): iron-terminated Fe$_{43}$Pt$_{12}$, platinum-terminated Fe$_{12}$Pt$_{43}$, and disordered Fe$_{27}$Pt$_{28}$. Additionally, the Fe$_6$Pt$_7$ cluster has been investigated to probe 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 significant tendency of Pt atoms to move to the outside layer of the particles -- even in the platinum deficient cases.

cond-mat.mes-hall↗

arXiv2015

Effect of edge dislocations on structural and electric properties of 4H-SiC

The paper presents a study of two full-core, edge dislocations of opposite Burgers vectors in 4H-SiC, conducted using the first-principles density functional theory methods. We have determined the creation energy of the dislocations as a function of distance between their cores. The radial distribution function has been applied to examine strong impact of the dislocations on the local crystal structure. The analysis of the electronic structure reveals mid-gap levels induced by broken atomic bonds in the dislocation core. The maps of charge distribution and electrostatic potential have been calculated and the significant decrease of the electrostatic barriers in the vicinity of the dislocation cores has been quantified. The obtained results have been discussed in the light of available experimental data.

cond-mat.mtrl-sci↗

arXiv2013

Elastic properties and phase stability of AgBr under pressure

Ab initio calculations have been used to derive the elastic constants, equation of state and free energy of NaCl-, KOH-, and CsCl-type structures of AgBr. The elastic constants have been derived by the stress-strain relation. The shear elastic constants $C_{44}$ has proven to be exceptionally soft. We have found that at temperature T=0 the phase transition from the low pressure NaCl-type to the high-pressure CsCl-type structure goes over a monoclinic KOH-type structure. The free energy comparison indicates that the monoclinic phase ranges from 8 to 35 GPa.

cond-mat.mtrl-sci↗

arXiv2013

TiC lattice dynamics from ab initio calculations

Ab initio calculations and a direct method have been applied to derive the phonon dispersion curves and phonon density of states for the TiC crystal. The results are compared and found to be in a good agreement with the experimental neutron scattering data. The force constants have been determined from the Hellmann-Feynman forces induced by atomic displacements in the 2x2x2 supercell. The calculated phonon density of states suggests that vibrations of Ti atoms form acoustic branches, whereas the motion of C atoms is confined to optic branches. The elastic constants have been found using the deformation method and compared with the results obtained from acoustic phonon slopes.

cond-mat.mtrl-sci↗

arXiv2013

Ab Initio Lattice Dynamics and Elastic Constants of ZrC

Ab initio calculations and a direct method are applied to derive the phonon dispersion relations and phonon density of states for the ZrC crystal. The results are in good agreement with neutron scattering data. The force constants are determined from the Hellmann-Feynman forces induced by atomic displacements in a 2x2x2 supercell. The elastic constants are found using the deformation method and successfully compare with experimental data.

cond-mat.mtrl-sci↗

arXiv2010

{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers

The magnetization distribution, its energetic characterization by the interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt multilayers are investigated using density functional theory combined with the direct method to determine phonon frequencies. It is found that ferromagnetic order between consecutive Fe layers is favoured, with the enhanced magnetic moments at the interface. The bilinear and biquadratic coupling coefficients between Fe layers are shown to saturate fast with increasing thickness of nonmagnetic Pt layers which separate them. The phonon calculations demonstrate a rather strong dependence of partial iron phonon densities of states on the actual position of Fe monolayer in the multilayer structure.

cond-mat.mtrl-sci↗