16 research outputs found
Anharmonicity and structural phase transition in the Mott insulator CuPO
Ab initio investigations of structural, electronic, and dynamical properties
of the high-temperature 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 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 phase is
discussed. The self-consistent phonon calculations based on the
temperature-dependent effective potential show the stabilization of the
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
Influence of anharmonicity on the negative thermal expansion of
The lattice vibrational properties of (gray tin) were investigated experimentally by temperature-dependent x-ray diffraction and theoretically by density functional theory calculations. Similar to the other elements of group IV, exhibits a lattice anomaly at low temperatures and negative thermal expansion, with a minimum at and a magnitude three times larger than in Si. The influence of anharmonic effects up to fourth-order potential terms on the phonon dispersion relations, the lattice parameters, and the thermal expansion coefficient have been tested. The performed analysis gives an excellent agreement with experiment when quartic potential terms are included in the theory. We point out that negative thermal expansion in is not driven by the anharmonicity of the interatomic potential. This resolves the long-standing puzzle in the thermal behavior of
Structure and elastic properties of Mg(OH) from density functional theory
The structure, lattice dynamics and mechanical properties of the magnesium
hydroxide have been investigated with static density functional theory
calculations as well as \it {ab initio} molecular dynamics. The hypothesis of a
superstructure existing in the lattice formed by the hydrogen atoms has been
tested. The elastic constants of the material have been calculated with static
deformations approach and are in fair agreement with the experimental data. The
hydrogen subsystem structure exhibits signs of disordered behaviour while
maintaining correlations between angular positions of neighbouring atoms. We
establish that the essential angular correlations between hydrogen positions
are maintained to the temperature of at least 150 K and show that they are well
described by a physically motivated probabilistic model. The rotational degree
of freedom appears to be decoupled from the lattice directions above 30K
Phonon confinement and interface lattice dynamics of ultrathin high-rare earth sesquioxide films: the case of Eu₂O₃ on YSZ(001)
The spatial confinement of atoms at surfaces and interfaces significantly alters the lattice dynamics of thin films, heterostructures and multilayers. Ultrathin films with high dielectric constants (high-k) are of paramount interest for applications as gate layers in current and future integrated circuits. Here we report a lattice dynamics study of high-k EuO films with thicknesses of 21.3, 2.2, 1.3, and 0.8 nm deposited on YSZ(001). The Eu-partial phonon density of states (PDOS), obtained from nuclear inelastic scattering, exhibits broadening of the phonon peaks accompanied by up to a four-fold enhancement of the number of low-energy states compared to the ab initio calculated PDOS of a perfect EuO crystal. Our analysis demonstrates that while the former effect reflects the reduced phonon lifetimes observed in thin films due to scattering from lattice defects, the latter phenomenon arises from an ultrathin EuO layer formed between the thin EuO film and the YSZ(001) substrate. Thus, our work uncovers another potential source of vibrational anomalies in thin films and multilayers, which has to be cautiously considered
Phononic drumhead surface state in the distorted kagome compound RhPb
RhPb was initially recognized as one of 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 P2m 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 P2m symmetry, i.e. phonon dispersions and surface Green's functions using the modern theoretical methods based on density functional theory. 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 zigzaglike edge of Pb honeycomb sublattice
Phononic drumhead surface state in distorted kagome compound RhPb
RhPb was initially recognized as one of a CoSn-like compounds with
symmetry, containing an ideal kagome lattice of -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
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
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.Comment: 10 pages, 7 figure
Lattice Dynamics and Structural Phase Transitions in
Using the density functional theory, we study the structural and lattice dynamical properties of europium sesquioxide () in the cubic, trigonal, and monoclinic phases. The obtained lattice parameters and energies of the Raman modes show a good agreement with the available experimental data. The Eu-partial phonon density of states calculated for the cubic structure is compared with the nuclear inelastic scattering data obtained from a 20 nm thick film deposited on a YSZ substrate. A small shift of the experimental spectrum to higher energies results from a compressive strain induced by the substrate. On the basis of lattice and phonon properties, we analyze the mechanisms of structural transitions between different phases of
Electronic and dynamical properties of : role of layers and expected orbital order
Recently discovered heavy fermion CeRhAs compound crystallizes in
the nonsymmorphic symmetry, which allows unexpected behavior
associated with 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
RhAs 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 with
orbital electrons and suggest the occurrence of the magnetic Lifshitz
transition induced by the external magnetic field. In contrast, the Ce
electrons only weakly influence the system properties.Comment: 7 pages, 6 figures; Supplemental Material: 6 pages, 5 figure