arXiv subjects

Shun-Li Shang

Publications and source records attributed to Shun-Li Shang.

45 records · Page 3Linked to original sources

arXiv2021

DFTTK: Density Functional Theory ToolKit for High-throughput Lattice Dynamics Calculations

In this work, we present a software package in Python for high-throughput first-principles calculations of thermodynamic properties at finite temperatures, which we refer to as DFTTK (Density Functional Theory Tool Kit). DFTTK is based on the atomate package and integrates our experiences in the last decades on the development of theoretical methods and computational software. It includes task submissions on all major operating systems and task execution on high-performance computing environments. The distribution of the DFTTK package comes with examples of calculations of phonon density of states, heat capacity, entropy, enthalpy, and free energy under the quasi-harmonic phonon scheme for the stoichiometric phases of Al, Ni, Al3Ni, AlNi, AlNi3, Al3Ni4, and Al3Ni5, and the fcc solution phases treated using the special quasirandom structures at the compositions of Al3Ni, AlNi, and AlNi3.

cond-mat.mtrl-sci↗

arXiv2020

Adsorption-Controlled Growth of Ga2O3 by Suboxide Molecular-Beam Epitaxy

This paper introduces a growth method---suboxide molecular-beam epitaxy (S-MBE)---which enables the growth of Ga2O3 and related materials at growth rates exceeding 1 micrometer per hours with excellent crystallinity in an adsorptioncontrolled regime. Using a Ga + Ga2O3 mixture with an oxygen mole fraction of x(O) = 0.4 as an MBE source, we overcome kinetic limits that had previously hampered the adsorption-controlled growth of Ga2O3 by MBE. We present growth rates up to 1.6 micrometer per hour for Ga2O3--Al2O3 heterostructures with unprecedented crystalline quality and also at unparalleled low growth temperature for this level of perfection. We combine thermodynamic knowledge of how to create molecular-beams of targeted suboxides with a kinetic model developed for the S-MBE of III-VI compounds to identify appropriate growth conditions. Using S-MBE we demonstrate the growth of phase-pure, smooth, and high-purity homoepitaxial Ga2O3 films that are thicker than 4 micrometer. With the high growth rate of S-MBE we anticipate a significant improvement to vertical Ga2O3-based devices. We describe and demonstrate how this growth method can be applied to a wide-range of oxides. S-MBE rivals leading synthesis methods currently used for the production of Ga2O3-based devices.

cond-mat.mtrl-sci↗

arXiv2020

A Thermochemical Database from High-throughput First-Principles Calculations and Its Application to Analyzing Phase Evolution in AM-fabricated IN718

A comprehensive thermochemical database is constructed based on high-throughput first-principles phonon calculations of over 3000 atomic structures in Ni, Fe, and Co alloys involving a total of 26 elements including Al, B, C, Cr, Cu, Hf, La, Mn, Mo, N, Nb, O, P, Re, Ru, S, Si, Ta, Ti, V, W, Y, and Zr, providing thermochemical data largely unavailable from existing experiments. The database can be employed to predict the equilibrium phase compositions and fractions at a given temperature and an overall chemical composition directly from first-principles by minimizing the chemical potential. It is applied to the additively manufactured nickel-based IN718 superalloy to analyze the phase evolution with temperature. In particular, we successfully predicted the formation of L1$_0$-FeNi, $γ'$-Ni$_3$(Fe,Al), $α$-Cr, $γ$-Ni$_3$(Nb,Mo), $γ''$-Ni$_3$Nb , and $η$-Ni$_3$Ti at low temperatures, $γ'$-Ni$_3$Al, $δ$-Ni$_3$Nb, $γ''$-Ni$_3$Nb, $α$-Cr, and $γ$-Ni(Fe,Cr,Mo) at intermediate temperatures, and $δ$-Ni$_3$Nb and $γ$-Ni(Fe,Cr,Mo) at high temperatures in IN718. These predictions are validated by EDS mapping of compositional distributions and corresponding identifications of phase distributions. The database is expected to be a valuable source for future thermodynamic analysis and microstructure prediction of alloys involving the 26 elements.

cond-mat.mtrl-sci↗

arXiv2020

Route to in situ synthesis of epitaxial Pr2Ir2O7 thin films guided by thermodynamic calculations

In situ growth of pyrochlore iridate thin films has been a long-standing challenge due to the low reactivity of Ir at low temperatures and the vaporization of volatile gas species such as IrO3(g) and IrO2(g) at high temperatures and high oxygen partial pressures. To address this challenge, we combine thermodynamic analysis of the Pr-Ir-O2 system with experimental results from the conventional physical vapor deposition (PVD) technique of co-sputtering. Our results indicate that only high growth temperatures yield films with crystallinity sufficient for utilizing and tailoring the desired topological electronic properties. Thermodynamic calculations indicate that high deposition temperatures and high partial pressures of gas species O2(g) and IrO3(g), are required to stabilize Pr2Ir2O7. We further find that the gas species partial pressure requirements are beyond that achievable by any conventional PVD technique. We experimentally show that conventional PVD growth parameters produce exclusively Pr3IrO7, which conclusion we reproduce with theoretical calculations. Our findings provide solid evidence that in situ synthesis of Pr2Ir2O7 thin films is fettered by the inability to grow with oxygen partial pressure on the order of 10 Torr, a limitation inherent to the PVD process. Thus, we suggest high-pressure techniques, in particular chemical vapor deposition (CVD), as a route to synthesis of Pr2Ir2O7, as this can support thin film deposition under the high pressure needed for in situ stabilization of Pr2Ir2O7.

cond-mat.mtrl-sci↗

arXiv2020

An ab initio molecular dynamics exploration of associates in Ba-Bi liquid with strong ordering trends

Fictive associates are widely used to describe and model liquid phases with strong ordering trends. However, little evidence is known about the assumed associates in most cases. In the present work, an ab initio molecular dynamics (AIMD) study is employed to investigate the characters of the Ba-Bi liquid, in which associates have been assumed in existing thermodynamic modeling. It is found that in the Ba rich melt, the Bi atoms are almost completely surrounded by Ba atoms. The Bi-centered coordination polyhedrons are strongly associated to crystalline structures of Ba5Bi3 and Ba4Bi3 with a longer lifetime than other polyhedrons during the AIMD simulations. In addition, these Bi-centered polyhedrons in Ba rich melt connect with each other through vertex, edge, face, and/or bipyramid sharing to form medium range orders (MRO). In the Bi rich melt, the Ba-centered polyhedrons also form MROs, but they are both structurally and compositionally diverse with a shorter lifetime. These findings from AIMD study provide evidences that there exist a strongly ordering Ba4Bi3 associate and a weakly ordering BaBi3 associate in the Ba-Bi liquid. The predicted enthalpy of mixing in the liquid agrees well with the results by the CALPHAD modeling in the literature.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous Phonon-mode Dependence in Polarized Raman Spectroscopy of Topological Weyl Semimetal TaP

Topological Weyl semimetals (WSMs) have attracted widespread interests due to the chiral Weyl fermions and surface Fermi arcs that enable unique optical and transport phenomena. In this work, we present angle-resolved Raman spectroscopy of TaP, a prototypical noncentrosymmetric WSM, for five excitation wavelengths ranging from 364 to 785 nm. The Raman active modes, $A_1$, $B_1^1$, and $B_1^2$ modes, exhibit two main unique features beyond the conventional Raman theory. First, the relative intensities of Raman active modes change as a function of the excitation wavelength. Second, angle-resolved polarized Raman spectra show systematic deviation from the Raman tensor theory. In particular, the $B_1^1$ mode is absent for 633 nm excitation, whereas the $B_1^2$ mode shows an unusual two-fold symmetry instead of a four-fold symmetry for 488, 532, and 633 nm excitations. These unconventional phenomena are attributed to the interference effect in the Raman process owing to the existence of multiple carrier pockets with almost the same energy but different symmetries.

cond-mat.mtrl-sci↗

arXiv2019

Realization of Epitaxial Thin Films of the Topological Crystalline Insulator Sr$_3$SnO

Topological materials are derived from the interplay between symmetry and topology. Advances in topological band theories have led to the prediction that the antiperovskite oxide Sr$_3$SnO is a topological crystalline insulator, a new electronic phase of matter where the conductivity in its (001) crystallographic planes is protected by crystallographic point group symmetries. Realization of this material, however, is challenging. Guided by thermodynamic calculations we design and implement a deposition approach to achieve the adsorption-controlled growth of epitaxial Sr$_3$SnO single-crystal films by molecular-beam epitaxy (MBE). In-situ transport and angle-resolved photoemission spectroscopy measurements reveal the metallic and non-trivial topological nature of the as-grown samples. Compared with conventional MBE, the synthesis route used results in superior sample quality and is readily adapted to other topological systems with antiperovskite structures. The successful realization of thin films of topological crystalline insulators opens opportunities to manipulate topological states by tuning symmetries via epitaxial strain and heterostructuring.

cond-mat.mtrl-sci↗

arXiv2017

First-Principles Thermodynamic Theory of Seebeck Coefficients

Thermoelectric effects, measured by the Seebeck coefficients, refer to the phenomena in which a temperature difference or gradient imposed across a thermoelectric material induces an electrical potential difference or gradient, and vice versa, enabling the direct conversion of thermal and electric energies. All existing understanding and first-principles calculations of Seebeck coefficients have been based on the Boltzmann kinetic transport theory. Here we demonstrate that the Seebeck coefficient is a well-defined thermodynamic quantity that can be determined from the change in the chemical potential of electrons induced by the temperature change and thus can be efficiently computed solely based on the electronic density of states through first-principles calculations at different temperatures. The proposed approach is demonstrated using the prototype PbTe and SnSe thermoelectric materials. The proposed thermodynamic approach dramatically simplifies the calculations of Seebeck coefficients, making it possible to search for high performance thermoelectric materials using high-throughput first-principles calculations.

cond-mat.mtrl-sci↗

arXiv2017

A First-principles approach to predict Seebeck coefficients: Application to La3-xTe4

Theoretical descriptions of the Seebeck coefficient in terms of the differential electrical conductivity given by Cutler and Mott is the foundation of later works in terms of transmission function from the thermoelectric transport theory. On the other hand, recent studies in the literature have shown the relation between the Seebeck coefficient and chemical potential of electrons. In this work, this relation is rigorously derived from fundamental thermodynamics, and an formalism for the parameter-free calculation of the Seebeck coefficient based on the electronic density-of-states from first-principles calculations is presented. Numerical results are given using the n-type La3-xTe4 thermoelectric material as the prototype. With the rigid band approximation, the calculated temperature dependences of the Seebeck coefficients of La3-xTe4 as a function of carrier concentration show excellent agreements with experimental data.

cond-mat.mtrl-sci↗