1,721,001 research outputs found
Electron-phonon superconductivity in APt3P (A=Sr, Ca, La) compounds: From weak to strong coupling
No full textVibrational spectrum and electron-phonon coupling of doped solid picene from first principles
No full textPressure-tunable structural instabilities in single-layer-trilayer LaNiO
No full textLayered nickelates are believed to exhibit superconductivity similar to that found in the cuprates. However, the precise crystal structure of the superconducting phase of the layered nickelates has not been fully clarified. Here, I use first principles calculations to study the pressure dependence of the structural instabilities in the single-layer-trilayer LaNiO, which is one member of the layered nickelates family that also shows signatures of superconductivity. I find a nearly dispersionless nondegenerate phonon branch in the parent phase that is unstable along the Brillouin zone edge at all investigated pressures up to 30 GPa. Calculations show additional doubly-degenerate instabilities along the edge at lower pressures. I used group-theoretical analysis to identify the distinct low-symmetry distortions possible due to these instabilities and generated them using the eigenvectors of the unstable modes. Structural relaxations show that the lowest energy structures at 0 and 10 GPa involve condensation of both the nondegenerate and doubly-degenerate instabilities, which is in contrast to the experimental refinements that involve condensation of only the doubly-degenerate branch. I also find that structural distortions are energetically favorable at 20 GPa, contrary to the experiments that do not observe any distortions of the parent structure at high pressures
Possible structural quantum criticality tuned by rare-earth ion substitution in infinite-layer nickelates
No full textI show the infinite-layer rare-earth nickelates are near a structural quantum critical point by mapping the energetics of their structural instabilities using first priniciples calculations. I first confirm previous results that show a phonon instability in the phase leading to the structure for NiO with = Sm--Lu. I then study the non-spin-polarized phonon dispersions of the phase and find that they exhibit rare-earth size dependent instabilities at the and points for materials with = Eu--Lu. Group-theoretical analysis was used to enumerate all the isotropy subroups due to these instablities, and the distorted structures corresponding to their order parameters were generated using the eigenvectors of the unstable phonons. These structures were then fully relaxed by minimizing both the atomic forces and lattice stresses. I was able to stabilize only five out of the twelve possible distortions. The isotropy subgroup with the order parameter shows noticeable energy gain relative to other distortions for the compounds with late rare-earth ions. However, the order parameter of the lowest-energy phase switches first to and then to as the size of the rare-earth ion is progressively increased. Additionally, several distorted structures lie close in energy for the early members of this series. These features of the structural energetics persist even when antiferromagnetism is allowed. Such a competition between different order parameters that can be tuned by rare-earth ion substitution suggests that any structural transition that could arise from the phonon instabilities present in these materials can be suppressed to 0 K
Hexagonal-to-base-centered-orthorhombic charge density wave order in kagome metals KVSb, RbVSb, and CsVSb
No full textInternational audienceI search for the ground state structures of the kagome metals KVSb, RbVSb, and CsVSb using first principles calculations. Group-theoretical analysis shows that there are seventeen different distortions that are possible due to the phonon instabilities at the and points in the Brilouin zone of the parent phase of these materials. I generated these structures for the three compounds and performed full structural relaxations that minimize the atomic forces and lattice stresses. I find that the phase with the order parameter has the lowest energy among these possibilities in all three compounds. However, the exhibits a dynamical instability at its point, which corresponds to the point in the parent phase. Condensation of this instability leads to a base-centered orthorhombic structure with the space group and order parameter
Order-by-disorder charge density wave condensation at in kagome metal ScVSn
Get PDFThe recent discovery of a charge density wave order at the wave vector
in the kagome metal ScVSn has
created a mystery because subsequent theoretical and experimental studies show
a dominant phonon instability instead at another wave vector
. In this paper, I use first principles
total energy calculations to map out the landscape of the structural
distortions due to the unstable phonon modes at ,
, and present in this material. In agreement
with previous results, I find that the distortions due to the instability
cause the largest gain in energy relative to the parent structure, followed in
order by the and instabilities. However, only two distinct structure
occur due to this instability, which are separated by 6 meV/f.u. The
instability at results in three distinct structures separated in energy by
5 meV/f.u. In contrast, six different distorted structures are stabilized due
to the instability at , and they all lie within 2 meV/f.u.\ of each other.
Hence, despite a lower energy gain, the condensation at could be favorable
due to a larger entropy gain associated with the fluctuations within a manifold
with larger multiplicity via the order-by-disorder mechanism.Comment: Fix a typo; 6 pages, 2 figures, 2 tables; crystal structure
information are given in ancillary file
Possible structural quantum criticality tuned by rare-earth ion substitution in infinite-layer nickelates
No full textI show the infinite-layer rare-earth nickelates are near a structural quantum critical point by mapping the energetics of their structural instabilities using first priniciples calculations. I first confirm previous results that show a phonon instability in the phase leading to the structure for NiO with = Sm--Lu. I then study the non-spin-polarized phonon dispersions of the phase and find that they exhibit rare-earth size dependent instabilities at the and points for materials with = Eu--Lu. Group-theoretical analysis was used to enumerate all the isotropy subroups due to these instablities, and the distorted structures corresponding to their order parameters were generated using the eigenvectors of the unstable phonons. These structures were then fully relaxed by minimizing both the atomic forces and lattice stresses. I was able to stabilize only five out of the twelve possible distortions. The isotropy subgroup with the order parameter shows noticeable energy gain relative to other distortions for the compounds with late rare-earth ions. However, the order parameter of the lowest-energy phase switches first to and then to as the size of the rare-earth ion is progressively increased. Additionally, several distorted structures lie close in energy for the early members of this series. These features of the structural energetics persist even when antiferromagnetism is allowed. Such a competition between different order parameters that can be tuned by rare-earth ion substitution suggests that any structural transition that could arise from the phonon instabilities present in these materials can be suppressed to 0 K
Interplay between structure and chemistry of materials and their physical properties
Get PDFFirst principles calculations provide a powerful tool for sorting out the interplay of chemical composition and structure with the physical properties of materials. In this dissertation, I discuss the physical properties and their microscopic basis within this framework for following illustrative examples. (i) The Zintl phase hydrides, where I find H is anionic and the formation of covalent sp2 bonds in the Al/Ga/Al-Si planes, which is a highly unusual bonding configuration for these elements. (ii) PbTe, which shows strong coupling between the longitudinal acoustic and transverse optic modes that may explain its low thermal conductivity. (iii) The double perovskites BiPbZnNbO6 and BiSrZnNbO6, where introducing size disorder at A-site prevents the BO6 octahedra from tiling and enhances the polar behavior. (iv) FeSe, which shares the salient electronic and magnetic features of other Fe superconductors and cannot be described as a conventional electron phonon superconductor. (v) NbFe2, which is near a magnetic quantum critical point and shows strong competition between various magnetic orderings that may explain its unusual non-Fermi liquid behavior at very low temperatures. (vi) The nickel analogues of Fe superconductors LaNiPO and BaNi2As2, where I show that superconductivity is of conventional electron-phonon type in contrast to the Fe-based superconductors. (vii) Noncentrosymmetric LaNiC2, which I find is a conventional electron-phonon superconductor with intermediate coupling
First principles study of thermal conductivity of InO in relation to AlO, GaO, and KTaO
No full textI use first principles calculations to investigate the thermal conductivity of -InO and compare the results with that of -AlO, -GaO, and KTaO. The calculated thermal conductivity of -InO agrees well with the experimental data obtain recently, which found that the low-temperature thermal conductivity in this material can reach values above 1000 W/mK. I find that the calculated thermal conductivity of -GaO is larger than that of -InO at all temperatures, which implies that -GaO should also exhibit high values of thermal conductivity at low temperatures. The thermal conductivity of KTaO calculated ignoring the temperature-dependent phonon softening of low-frequency modes give high-temperature values similar that of -GaO. However, the calculated thermal conductivity of KTaO does not increase as steeply as that of the binary compounds at low temperatures, which results in KTaO having the lowest low-temperature thermal conductivity despite having acoustic phonon velocities larger than that of -GaO and -InO. I attribute this to the fact that the acoustic phonon velocities at low frequencies in KTaO is less uniformly distributed because its acoustic phonon branches are more dispersive compared to the binary oxides, which causes enhanced momentum loss even during the normal phonon-phonon scattering processes. I also calculate thermal diffusivity using the theoretically obtained thermal conductivity and heat capacity and find that all four materials exhibit the expected behavior at high temperatures. Additionally, the calculated ratio of the average phonon scattering time to Planckian time is larger than the lower bound of 1 that has been observed empirically in numerous other materials
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