Publications

Subtle metastability of the layered magnetic topological insulator MnBi2Te4 from weak interactions

Abstract Layered quantum materials can host interesting properties, including magnetic and topological, for which enormous computational predictions have been done. Their thermodynamic stability is much less visited computationally, which however determines the existence of materials and can be used to guide experimental synthesis. MnBi 2 Te 4 is one of such layered quantum materials that was predicted to be an intrinsic antiferromagnetic topological insulator, and later experimentally realized but in a thermodynamically metastable state. Here, using a combined first-principles-based approach that considers lattice, charge, and spin degrees of freedom, we investigate the metastability of MnBi 2 Te 4 by calculating the Helmholtz free energy for the reaction Bi 2 Te 3 + MnTe → MnBi 2 Te 4 . We identify a temperature range (~500–873 K) in which the compound is stable with respect to the competing binary phases, consistent with experimental observation. We validate the predictions by comparing the calculated specific heats contributed from different degrees of freedom with experimental results. Our findings indicate that the degrees of freedom responsible for the van der Waals interaction, lattice vibration, magnetic coupling, and nontrivial band topology in MnBi 2 Te 4 not only enable emergent phenomena but also play a crucial role in determining its thermodynamic stability. This conclusion lays the foundation for the future computational material synthesis of novel layered systems.

(Invited) SCAN Meta-GGA: An Accurate, Efficient, and Physically Sound Density Functional for Materials Discovery and Design

The accuracy and computational efficiency of the widely used Kohn-Sham density functional theory (DFT) are limited by the approximation to its exchange-correlation energy Exc. The earliest local density approximation (LDA) overestimates the strengths of all bonds near equilibrium (even the vdW bonds). By adding the electron density gradient to model Exc, generalized gradient approximations (GGAs) generally soften the bonds to give robust and overall more accurate descriptions, except for the vdW interaction which is largely lost. Further improvement for covalent, ionic, and hydrogen bonds can be obtained by the computationally more expensive hybrid GGAs, which mix GGAs with the nonlocal exact exchange. Meta-GGAs are still semilocal in computation and thus efficient. Compared to GGAs, they add the kinetic energy density that enables them to recognize and accordingly treat different bonds, which no LDA or GGA can [2]. In this talk, I will present an advance in DFT, the recently developed non-empirical strongly constrained and appropriately normed (SCAN) meta-GGA [1]. SCAN predicts accurate geometries and energies of diversely-bonded molecules and materials (including covalent, metallic, ionic, hydrogen, and van der Waals bonds), significantly improving over its predecessors, the GGAs that dominate materials computation, at comparable efficiency [2]. SCAN’s excellent performance on formation enthalpies and ground structure predictions, critical for materials discovery and design, will be highlighted [3]. I will further explain how SCAN was constructed [1], why it can improve over GGAs [2], and where it should fail [4]. At the end, efforts to improve SCAN via nonlocal corrections will be discussed. [1] J. Sun, A. Ruzsinszky, and J.P. Perdew, Strongly constrained and appropriately normed semilocal density functional, PRL 115 , 036402 (2015). [2] J. Sun, R.C. Remsing, Y. Zhang, Z. Sun, A. Ruzsinszky, H. Peng, Z. Yang, A. Paul, U. Waghmare, X. Wu, M.L. Klein, and J.P. Perdew, Accurate First-principles structures and energies of diversely-bonded systems from an efficient density functional, Nat. Chem. 8 , 831 (2016). [3] Y. Zhang, D.A. Kitchaev, J. Yang, T. Chen, S.T. Dacek, R.A. Sarmiento-Perez, M.A.L. Margues, H. Peng, G. Ceder, J.P. Perdew, and J. Sun, Efficient first-principles prediction of solid stability: Towards chemical accuracy, NPJ Computational Materials 4 , 9 (2018). [4] H. Peng, Z. Yang, J.P. Perdew, and J. Sun, Versatile van der Waals density functional based on a meta-generalized gradient approximation, PRX 6 , 041005 (2016).

A new pseudopotential for simple metals

The gradient approximation to the exchange-correlation energy functional: A generalization that works

Solar Water Oxidation by Composite Catalyst/α-Fe₂O₃ Photoanodes

Electrodeposition of an amorphous cobalt catalyst layer over a high-surface-area alpha-Fe(2)O(3) photoanode causes a more than 350 mV cathodic shift in the onset potential for photoelectrochemical water oxidation using this anode and simulated solar irradiation. The catalyst layer is shown to deposit conformally onto the mesostructured alpha-Fe(2)O(3), leading to a large contact area at the interface between the two halves of the composite photoanode. Photoelectrochemical measurements show that the photocurrent generated from this composite photoanode still derives from alpha-Fe(2)O(3) excitation but is now accessible at an external bias several hundred millivolts below what is typically required for alpha-Fe(2)O(3) photoanodes alone, indicating a reduced external bias would be needed to drive overall water splitting. These results demonstrate modification of this prototypical photoanode material with a conformal layer of a competent electrocatalyst to separate the tasks of photon absorption and redox catalysis, a strategy that may have important and general ramifications for solar photoelectrochemical hydrogen generation.

The exchange-correlation energy of a metallic surface

Exchange and correlation in open systems of fluctuating electron number

While the exact total energy of a separated open system varies linearly as a function of average electron number between adjacent integers, the energy predicted by semilocal density-functional approximations is concave up and the exact-exchange-only or Hartree-Fock energy is concave down. As a result, semilocal density functionals fail for separated open systems of fluctuating electron number, as in stretched molecular ions $\mathrm{A}_{2}{}^{+}$ and in solid transition-metal oxides. We develop an exact-exchange theory and an exchange-hole sum rule that explain these failures and we propose a way to correct them via a local hybrid functional.

Structural phase transitions in Na, Mg and Al crystals: dominant role of the valence in local pseudopotential theory

Planar-surface charge densities and energies beyond the local-density approximation

We present a self-consistent calculation of the ground-state properties of simple metallic planar surfaces using density-functional theory with nonlocal exchange-correlation effects included. Our calculational scheme closely follows the classic work of Lang and Kohn for the jellium model, except that the exchange-correlation energy and potential within the local-density approximation (LDA) are replaced by the corresponding nonlocal functionals of Langreth and Mehl (LM). We also include the discrete-lattice effects, following the variational scheme of Perdew and Monnier. The physical properties considered include the one-electron effective potential, charge-density profile, surface energy, and work function. For each of the most densely packed surfaces of seven simple metals with fcc or bcc structure, we find that, when compared with results in the LDA, the Friedel oscillation in the electron density near the surface is systematically depressed as a result of positive LM contributions to the effective potential at places where the LDA density oscillation is peaked. We find a systematic increase in surface energies, with bigger increases for higher-density metals. Increases are also found in the work functions. Our results and those from other density-functional calculations are compared with results from variational treatments of the ground-state wave function, and with experiment. We also comment upon the Fermi hypernetted-chain jellium surface energies and work functions.

Enhancing the accuracy of interior-scaled Perdew-Zunger self-interaction correction

Erratum: Correlation energy of the uniform electron gas from an interpolation between high- and low-density limits [Phys. Rev. B 81, 085123 (2010)]

Received 4 January 2018DOI:https://doi.org/10.1103/PhysRevB.98.079903©2018 American Physical SocietyPhysics Subject Headings (PhySH)TechniquesDensity functional theory developmentCondensed Matter, Materials & Applied Physics

Erratum: “Subtlety of TiO2 phase stability: Reliability of the density functional theory predictions and persistence of the self-interaction error” [J. Chem. Phys. 150, 014105 (2019)]

First Page

Exchange-correlation energy functional based on the Airy-gas reference system

In recent work, generalized gradient approximations (GGAs) have been constructed from the energy density of the Airy gas for exchange but not for correlation. We report the random-phase approximation (RPA) conventional correlation energy density of the Airy gas, the simplest edge electron gas, in which the auxiliary noninteracting electrons experience a linear potential. By fitting the Airy-gas RPA exchange-correlation energy density and making an accurate short-range correction to RPA, we propose a simple beyond RPA GGA density functional (``$\text{ARPA}+$'') for the exchange-correlation energy. Our functional, tested for jellium surfaces, atoms, molecules, and solids, improves mildly over the local spin-density approximation for atomization energies and lattice constants without much worsening the already good surface exchange-correlation energies.

Erratum: Dominant density parameters and local pseudopotentials for simple metals

25th Anniversary Article: Semiconductor Nanowires – Synthesis, Characterization, and Applications

Semiconductor nanowires (NWs) have been studied extensively for over two decades for their novel electronic, photonic, thermal, electrochemical and mechanical properties. This comprehensive review article summarizes major advances in the synthesis, characterization, and application of these materials in the past decade. Developments in the understanding of the fundamental principles of “bottom‐up” growth mechanisms are presented, with an emphasis on rational control of the morphology, stoichiometry, and crystal structure of the materials. This is followed by a discussion of the application of nanowires in i) electronic, ii) sensor, iii) photonic, iv) thermoelectric, v) photovoltaic, vi) photoelectrochemical, vii) battery, viii) mechanical, and ix) biological applications. Throughout the discussion, a detailed explanation of the unique properties associated with the one‐dimensional nanowire geometry will be presented, and the benefits of these properties for the various applications will be highlighted. The review concludes with a brief perspective on future research directions, and remaining barriers which must be overcome for the successful commercial application of these technologies.

Erratum: Pair-distribution function and its coupling-constant average for the spin-polarized electron gas [Phys. Rev. B<b>46</b>, 12 947 (1992)]

Pair-distribution function and its coupling

The general-purpose SCAN meta-GGA for ferroelectric materials

Constraint-based wave vector and frequency dependent exchange-correlation kernel of the uniform electron gas

According to time-dependent density functional theory, the exact exchange-correlation kernel ${f}_{xc}(n,q,\ensuremath{\omega})$ for wave vector q and frequency $\ensuremath{\omega}$ determines not only the ground-state energy but also the excited-state energies/lifetimes and time-dependent linear density response of an electron gas of uniform density $n=3/(4\ensuremath{\pi}{r}_{s}^{3})$. Here we propose a parametrization of this function based upon the satisfaction of exact constraints. For the static $(\ensuremath{\omega}=0)$ limit, we modify the model of Constantin and Pitarke to recover at small q the known second-order gradient expansion, and to correct its approach to the large q limit. For all $\ensuremath{\omega}$ at $q=0$, we use the model of Gross, Kohn, and Iwamoto. A Cauchy integral extends this model to complex $\ensuremath{\omega}$. Scaling relations are identified. We then combine these ingredients, damping out the $\ensuremath{\omega}$ dependence at large q. Away from $q=0$ and $\ensuremath{\omega}=0$, the correlation contribution to the kernel becomes dominant over exchange, even at ${r}_{s}=4$. The resulting correlation energies for $1\ensuremath{\le}{r}_{s}\ensuremath{\le}10$ from integration over imaginary $\ensuremath{\omega}$ are essentially exact. The plasmon pole of the density response function is found by analytic continuation of ${f}_{xc}$ to $\ensuremath{\omega}$ just below the real axis, and the resulting plasmon lifetime at ${r}_{s}=$ 4 is found for $q&lt;{k}_{F}$. A static charge-density wave is found for ${r}_{s}&gt;69$, and shown to be associated with softening of the plasmon mode.