Publications

Physical Review Letters, Vol. 124, Iss. 7, Feb. 21, 2020

Magnetic Semimetals and Quantized Anomalous Hall Effect in EuB6

Authors: Simin Nie, Yan Sun, Fritz B. Prinz, Zhijun Wang, Hongming Weng, Zhong Fang, and Xi Dai

Exploration of the novel relationship between magnetic order and topological semimetals has received enormous interest in a wide range of both fundamental and applied research. Here we predict that “soft” ferromagnetic material EuB6 can achieve multiple topological semimetal phases by simply tuning the direction of the magnetic moment. Explicitly, EuB6 is a topological nodal-line semimetal when the moment is aligned along the [001] direction, and it evolves into a Weyl semimetal with three pairs of Weyl points by rotating the moment to the [111] direction. Interestingly, we identify a composite semimetal phase featuring the coexistence of a nodal line and Weyl points with the moment in the [110] direction. Topological surface states and anomalous Hall conductivity, which are sensitive to the magnetic order, have been computed and are expected to be experimentally observable. Large-Chern-number quantum anomalous Hall effect can be realized in its [111]-oriented quantum-well structures.

Physical Review B, Vol. 99, 035125, Jan. 14, 2019

Topological Nodal-line Semimetals in Ferromagnetic Rare-earth-metal Monohalides

AUTHORS:  SIMIN NIE, HONGMING WENG AND FRITZ B. PRINZ

The topological nodal-line semimetals (TNLSs) found so far are extremely limited to nonmagnetic materials and most of them are spinless. Here, the authors find from first-principles calculations and an effective model analysis that the single-layer rare-earth monohalides La$X$ and single-layer Gd$X$ (where $X$ is Cl or Br) are ideal 2D Weyl semimetals and large-gap 2D quantum anomalous Hall insulators (QAHIs), respectively. Moreover, 3D La$X$ and 3D Gd$X$ are TNLSs and 3D weak QAHIs, respectively. The nodal lines in 3D La$X$ are robust against strong spin-orbit coupling, providing a novel platform toward exploring the exotic properties in nodal-line fermions.

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Physical Review B, Vol. 87, Iss. 12-15, September 2018

Topological Phases in the TaSe3 Compound

AUTHORS:  SIMIN NIE, LINGYI XING, RONGYING JIN, WEIWEI XIE, ZHIJUN WANG AND FRITZ B. PRINZ

Based on first-principles calculations, we show that stoichiometric TaSe3, synthesized in space group P21/m, belongs to a three-dimensional strong topological insulator (TI) phase with Z2 invariants (1;100). The calculated surface spectrum shows clearly a single Dirac cone on surfaces, with helical spin texture at a constant-energy contour. To check the stability of the topological phase, strain effects have been systematically investigated, showing that many topological phases survive in a wide range of the strains along both the a and c axes, such as strong TI, weak TI, and Dirac semimetal phases. TaSe3 provides us an ideal platform for experimental study of topological phase transitions. More interestingly, since superconductivity in TaSe3 has been reported for a long time, the coexistence of topological phases and a superconducting phase suggests that TaSe3 is a realistic system to study the interplay between topological and superconducting phases in the future.

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Nature Catalysis, July 30, 2018

Extending the Limits of Pt/C Catalysis with Passivation-gas-incorporated Atomic Layer Deposition

AUTHORS:  SHICHENG XU, YONGMIN KIM, JOONSUK PARK, DREW HIGGINS, SHIH-JIA SHEN, PETER SCHINDLER, DICKSON THIAN, J. PROVINE, JAN TORGERSEN, TANJA GRAF, THOMAS D. SCHLADT, MARAT ORAZOV, BERNARD HAOCHIH LIU, THOMAS F. JARAMILLO AND FRITZ B. PRINZ

Controlling the morphology of noble metal nanoparticles during surface depositions is strongly influenced by precursor–substrate and precursor–deposit interactions. Depositions can be improved through a variety of means, including tailoring the surface energy of a substrate to improve precursor wettability, or by modifying the surface energy of the deposits themselves. Here, we show that carbon monoxide can be used as a passivation gas during atomic layer deposition to modify the surface energy of already deposited Pt nanoparticles to assist direct deposition onto a carbon catalyst support. The passivation process promotes two-dimensional growth leading to Pt nanoparticles with suppressed thicknesses and a more than 40% improvement in Pt surface-to-volume ratio. This approach to synthesizing nanoparticulate Pt/C catalysts achieved high Pt mass activities for the oxygen reduction reaction, along with excellent stability likely facilitated by strong catalyst–support interactions afforded by this synthetic technique.

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Proceedings of the National Academy of Sciences, Vol. 114, No. 40, Oct. 2017

Topological Semimetal in Honeycomb Lattice LnSI

AUTHORS:  SIMIN NIE, GANG XU, FRITZ B. PRINZ, AND SHOU-CHENG ZHANG

Recognized as elementary particles in the standard model, Weyl fermions in condensed matter have received growing attention. However, most of the previously reported Weyl semimetals exhibit rather complicated electronic structures that, in turn, may have raised questions regarding the underlying physics. Here, we report promising topological phases that can be realized in specific honeycomb lattices, including ideal Weyl semimetal structures, 3D strong topological insulators, and nodal-line semimetal configurations. In particular, we highlight a semimetal featuring both Weyl nodes and nodal lines. Guided by this model, we showed that GdSI, the long-perceived ideal Weyl semimetal, has two pairs of Weyl nodes residing at the Fermi level and that LuSI (YSI) is a 3D strong topological insulator with the right-handed helical surface states. Our work provides a mechanism to study topological semimetals and proposes a platform for exploring the physics of Weyl semimetals as well as related device designs.

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Physical Review B 93, 115`51 Mar 31 2016

Benchmarking Density Functional Perturbation Theory to enable High-throughput Screening of Materials for Dielectric Constant and Refractive Index

AUTHORS:  IOANNIS PETOUSIS, WEI CHEN, GEOFFROY HAUTIER, TANJA GRAF, THOMAS D. SCHLADT, KRISTIN A. PERSSON, AND FRITZ B. PRINZ

In this paper, we demonstrate a high-throughput density functional perturbation theory (DFPT) methodology capable of screening compounds for their dielectric properties. The electronic and ionic dielectric tensors are calculated for 88 compounds, where the eigenvalues of the total dielectric tensors are compared with single crystal and polycrystalline experimental values reported in the literature. We find that GGA/PBE has a smaller mean average deviation from experiments (MARD=16.2%) when compared to LDA. The prediction accuracy of DFPT is lowest for compounds that exhibit complex structural relaxation effects (e.g., octahedra rotation in perovskites) and/or strong anharmonicity. Despite some discrepancies between DFPT results and reported experimental values, the high-throughput methodology is found to be useful in identifying interesting compounds by ranking. This is demonstrated by the high Spearman correlation factor (ρ=0.92). Finally, we demonstrate that DFPT provides a good estimate for the refractive index of a compound without calculating the frequency dependence of the dielectric matrix (MARD=5.7%).

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Science, Vol 354, Issue 6315, Nov 25, 2016

Direct and Continuous Strain Control of Catalysts with Tunable Battery Electrode Materials

AUTHORS: HAOTIAN WANG, SHICHENG XU, CHARLOTTE TSAI, YUZHANG LI, CHONG LIU, JIE ZHAO, YAYUAN LIU, HONGYUAN YUAN, FRANK ABILD-PEDERSON, FRITZ. B. PRINZ, JENS K. NORSKOV, YI CUI

We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.

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The Journal of Physical Chemistry C, 120 (15), pp. 8148-8156, 2016

Atomically Flat Silicon Oxide Monolayer Generated by Remote Plasma

AUTHORS: DICKSON THIAN, YONAS T. YEMANE, MANCA LOGAR, SHICHENG XU, PETER SCHINDLER, MARTIN M. WINTERKORN, J. PROVINE, FRITZ B. PRINZ

We demonstrate stable, atomically smooth monolayer oxidation of Si(111) using a remote plasma. Scanning tunneling microscopy (STM) confirms the atomically flat nature of the oxidized surface, while cross-sectional transmission electron microscopy (TEM) proves the monolayer to bilayer oxide thickness. Fourier transform infrared spectroscopy (FTIR) and atomic layer deposition (ALD) indicate oxygen is incorporated onto the silicon surface in the form of Si–O–Si and Si–OH bonds. The incorporation of Si–OH bonds is inferred by using TiCl₄, a highly specific ALD precursor, for TiO₂ ALD. This plasma technique provides precise control of the surface chemistry and yields abrupt yet stable SiO/Si interfaces. It enables production of atomically flat, ALD-active silicon surfaces that could serve as a well-defined platform for investigation of various surface chemistries via STM. Using this substrate, we present the first ever STM observations of ALD TiO₂ on silicon oxide.

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