arXiv subjects

Mao-Hua Du

Publications and source records attributed to Mao-Hua Du.

30 records · Page 2Linked to original sources

arXiv2018

Antisite pairs suppress the thermal conductivity of BAs

BAs was predicted to have an unusually high thermal conductivity at room temperature of 2000$\,$Wm$^{-1}$$\,$K$^{-1}$, comparable to that of diamond. However, the experimentally measured thermal conductivity of BAs single crystals is an order of magnitude lower. To identify the origin of this large inconsistency, we investigated the lattice structure and potential defects in BAs single crystals at atomic scale using aberration-corrected scanning transmission electron microscopy (STEM). Rather than finding a large concentration As vacancies ($V_\mathrm{As}$), as widely thought to dominate the thermal resistance in BAs crystals, our STEM results showed enhanced intensity of some B columns and reduced intensity of some As columns, suggesting the presence of antisite defects with As$_\mathrm{B}$ (As-atom on B site) and B$_\mathrm{As}$ (B-atom on As site) with significant concentrations. Further calculations show that the antisite pair with As$_\mathrm{B}$ next to B$_\mathrm{As}$ is preferred energetically among the different types of point defects investigated, and confirm that such defects lower the thermal conductivity for BAs. Using a concentration of 6.6$\pm$3$\times$10$^{20}$$\,$cm$^{-3}$ for the antisite pairs estimated from STEM images, thermal conductivity is estimated to be 65-100$\,$Wm$^{-1}$$\,$K$^{-1}$, in reasonable agreement with our measured value. Our study suggests that As$_\mathrm{B}$-B$_\mathrm{As}$ antisite pairs are the primary lattice defects suppressing thermal conductivity of BAs. Possible approaches are proposed for growth of high quality crystals or films with high thermal conductivity.

cond-mat.mtrl-sci↗

arXiv2016

Design of ternary alkaline-earth metal Sn(II) oxides with potential good p-type conductivity

Oxides with good p-type conductivity have been long sought after to achieve high performance all-oxide optoelectronic devices. Divalent Sn(II) based oxides are promising candidates because of their rather dispersive upper valence bands caused by the Sn-5s/O-2p anti-bonding hybridization. There are so far few known Sn(II) oxides being p-type conductive suitable for device applications. Here, we present via first-principles global optimization structure searches a material design study for a hitherto unexplored Sn(II)-based system, ternary alkaline-earth metal Sn(II) oxides in the stoichiometry of MSn2O3 (M = Mg, Ca, Sr, Ba). We identify two stable compounds of SrSn2O3 and BaSn2O3, which can be stabilized by Sn-rich conditions in phase stability diagrams. Their structures follow the Zintl behaviour and consist of basic structural motifs of SnO3 tetrahedra. Unexpectedly they show distinct electronic properties with band gaps ranging from 1.90 (BaSn2O3) to 3.15 (SrSn2O3) eV, and hole effective masses ranging from 0.87 (BaSn2O3) to above 6.0 (SrSn2O3) m0. Further exploration of metastable phases indicates a wide tunability of electronic properties controlled by the details of the bonding between the basic structural motifs. This suggests further exploration of alkaline-earth metal Sn(II) oxides for potential applications requiring good p-type conductivity such as transparent conductors and photovoltaic absorbers.

cond-mat.mtrl-sci↗

arXiv2016

Scintillation Properties and Electronic Structure of Intrinsic and Extrinsic Mixed Elpasolites Cs2NaRBr3I3 (R = La, Y)

Scintillators attract wide research interest for their distinct applications in radiation detection. Elpasolite halides are among the most promising scintillators due to their high structural symmetry and good scintillation performance. Better understanding of their underlying scintillation mechanism opens up new possibilities in scintillator development. In this work, we employ a variety of experimental techniques to study the two mixed-anion elpasolites Cs2NaRBr3I3 (R = La, Y). The emission of intrinsic Cs2NaRBr3I3 with a light yield ranging from 20,000 to 40,000 ph/MeV is dominant by self-trapped exciton emission. Partial substitution of R with Ce introduces a competing emission, the Ce3+ 5d to 4f radiative transition. Ab initio calculations were performed to investigate the electronic structures as well as the binding energies of polarons in Cs2NaRBr6. The calculated large self-trapped exciton binding energies are consistent with the observed high light yield due to self-trapped exciton emission. The unique electronic structure of halide elpasolites as calculated enhances the STE stability and the STE emission. The highly tunable scintillation properties of mixed-anion elpasolites underscore the role of their complex scintillation mechanism. Our study provides guidance for the design of new elpasolites scintillators with exceptional energy resolution and light yield desirable for applications.

cond-mat.mtrl-sci↗

arXiv2015

Ba_{2}TeO as an optoelectronic material: First-principles study

The band structure, optical and defects properties of Ba_{2}TeO are systematically investigated using density functional theory with a view to understanding its potential as an optoelectronic or trans- parent conducting material. Ba_{2}TeO crystallizes with tetragonal structure (space group P4/nmm) and with a 2.93 eV optical band gap 1 . We find relatively modest band masses for both electrons and holes suggesting applications. Optical properties show a infrared-red absorption when doped. This could potentially be useful for combining wavelength filtering and transparent conducting functions. Furthermore, our defect calculations show that Ba_{2}TeO is intrinsically p-type conducting under Ba-poor condition. However, the spontaneous formation of the donor defects may constrain the p-type transport properties and would need to be addressed to enable applications.

cond-mat.mtrl-sci↗

arXiv2014

Connecting thermoelectric performance and topological-insulator behavior: Bi$_2$Te$_3$ and Bi$_{2}$Te$_{2}$Se from first principles

Thermoelectric performance is of interest for numerous applications such as waste heat recovery and solid state energy conversion, and will be seen to be closely connected to topological insulator behavior. In this context we here report first principles transport and defect calculations for Bi$_{2}$Te$_{2}$Se in relation to Bi$_{2}$Te$_{3}$. The two compounds are found to contain remarkably different electronic structures in spite of being isostructural and isoelectronic. We discuss these results in terms of the topological insulator characteristics of these compounds.

cond-mat.mtrl-sci↗

arXiv2013

Potential thermoelectric performance of hole-doped Cu2O

High thermoelectric performance in oxides requires stable conductive materials that have suitable band structures. Here we show based on an analysis of the thermopower and related properties using first-principles calculations and Boltzmann transport theory that hole doped Cu2O may be such a material. We find that hole-doped Cu2O has a high thermopower of above 200 microV/K even with doping levels as high as 5.5x10 20 cm-3 at 500 K, mainly attributed to the heavy valence bands of Cu2O. This is reminiscent of the cobaltate family of high performance oxide thermoelectrics and implies that hole-doped Cu2O could be an excellent thermoelectric material if suitably doped.

cond-mat.mtrl-sci↗

arXiv2012

Energy Transport and Scintillation of Cerium Doped Elpasolite Cs2LiYCl6: Hybrid Density Functional Calculations

Elpasolites are a large family of halides which have recently attracted considerable interest for their potential applications in room-temperature radiation detection. Cs2LiYCl6 is one of the most widely studied elpasolite scintillators. In this paper, we will show hybrid density functional calculations on electronic structure, energetics of small electron and hole polarons and self-trapped excitons, and the excitation of luminescence centers (Ce impurities) in Cs2LiYCl6. The results provide important understanding in energy transport and scintillation mechanisms in Cs2LiYCl6 and rare-earth elpasolites in general.

cond-mat.mtrl-sci↗

arXiv2012

What Causes High Resistivity in CdTe

CdTe can be made semi-insulating by shallow donor doping. This is routinely done to obtain high resistivity in CdTe-based radiation detectors. However, it is widely believed that the high resistivity in CdTe is due to the Fermi level pinning by native deep donors. The model based on shallow donor compensation of native acceptors was dismissed based on the assumption that it is practically impossible to control the shallow donor doping level so precisely that the free carrier density can be brought below the desired value suitable for radiation detection applications. In this paper, we present our calculations on carrier statistics and energetics of shallow donors and native defects in CdTe. Our results show that the shallow donor can be used to reliably obtain high resistivity in CdTe. Since radiation detection applications require both high resistivity and good carrier transport, one should generally use shallow donors and shallow acceptors for carrier compensation and avoid deep centers that are effective carrier traps.

cond-mat.mtrl-sci↗

arXiv2009

First principles study of the graphene/Ru(0001) interface

Annealing the Ru metal that typically contains residual carbon impurities offers a facile way to grow graphene on Ru(0001) at the macroscopic scale. Two superstructures of the graphene/Ru(0001) interface with periodicities of 3.0-nm and 2.7-nm, respectively, have been previously observed by scanning tunneling microscopy. Using first-principles density functional theory, we optimized the observed superstructures and found interfacial C-Ru bonding of C atoms atop Ru atoms for both superstructures, which causes the graphene sheet to buckle and form periodic humps of ~1.7 A in height within the graphene sheet. The flat region of the graphene sheet, which is 2.2-2.3 A above the top Ru layer and has more C atoms occupying the atop sites, interacts more strongly with the substrate than does the hump region. We found that interfacial adhesion is much stronger for the 3.0-nm superstructure than for the 2.7-nm superstructure, suggesting that the former is the thermodynamically more stable phase. We explained the 3.0-nm superstructure's stability in terms of the interplay between C-Ru bonding and lattice matching.

cond-mat.mtrl-sci↗

arXiv2008

Possible Superconductivity in Fe-Sb Based Materials: Density Functional Study of LiFeSb

We investigate the electronic and other properties of the hypothetical compound LiFeSb in relation to superconducting LiFeAs and FeSe using density functional calculations. The results show that LiFeSb in the LiFeAs structure would be dynamically stable in the sense of having no unstable phonon modes, and would have very similar electronic and magnetic properties to the layered Fe based superconductors. Importantly, a very similar structure for the Fermi surface and a spin density wave related to but stronger than that in the corresponding As compound is found. These results are indicative of possible superconductivity analogous to the Fe-As based compounds if the spin density wave can be suppressed by doping or other means. Prospects for synthesizing this material in pure form or in solid solution with FeTe are discussed.

cond-mat.supr-con↗

arXiv2008

Density Functional Study of Excess Fe in Fe$_{1+x}$Te: Magnetism and Doping

The electronic and magnetic properties of the excess Fe in iron telluride Fe$_{(1+x)}$Te are investigated by density functional calculations. We find that the excess Fe occurs with valence near Fe$^{+}$, and therefore provides electron doping with approximately one carrier per Fe, and furthermore that the excess Fe is strongly magnetic. Thus it will provide local moments that interact with the plane Fe magnetism, and these are expected to persist in phases where the magnetism of the planes is destroyed for example by pressure or doping. These results are discussed in the context of superconductivity.

cond-mat.supr-con↗

arXiv2008

Density functional study of FeS, FeSe and FeTe: Electronic structure, magnetism, phonons and superconductivity

We report density functional calculations of the electronic structure, Fermi surface, phonon spectrum, magnetism and electron-phonon coupling for the superconducting phase FeSe, as well as the related compounds FeS and FeTe. We find that the Fermi surface structure of these compounds is very similar to that of the Fe-As based superconductors, with cylindrical electron sections at the zone corner, cylindrical hole surface sections, and depending on the compound, other small hole sections at the zone center. As in the Fe-As based materials, these surfaces are separated by a 2D nesting vector at ($π$,$π$). The density of states, nesting and Fermi surface size increase going from FeSe to FeTe. Both FeSe and FeTe show spin density wave ground states, while FeS is close to an instability. In a scenario where superconductivity is mediated by spin fluctuations at the SDW nesting vector, the strongest superconductor in this series would be doped FeTe.

cond-mat.supr-con↗