arXiv subjects

Mengen Wang

Publications and source records attributed to Mengen Wang.

7 recordsLinked to original sources

arXiv2026

Machine learning insights into band gap properties in halide-based perovskites

Halide perovskites show great promise for applications in optoelectronic devices. The lead-free perovskites are attracting increasing interest due to their low toxicity and motivate the exploration of alternative compositions and structures, including A$_2$BX$_6$, A$_2$BB$^\prime$X$_6$, A$_3$B$_2$X$_9$, and A$_4$BX$_6$. Accurate predictions of a wide range of band gap energies are important for designing new materials. It is also desired to generate a direct relationship between the structural and elemental descriptors and the band gap energies. In this work, we develop machine learning models to predict band gap energies across various types of halide perovskites based on atomic and structural properties. Algorithms including ensemble tree-based methods, random forest regression (RFR), gradient boosted regression trees (GBRT), and extreme gradient boosting (XGB) showed strong predictive accuracy. We also analyzed feature importance to identify key descriptors, including B-site and X-site elemental properties, as well as the number of A- and B-site atoms, as primary factors influencing band gap energies. These results improve our understanding of the ML models and provide guidance for designing new halide perovskite materials.

cond-mat.mtrl-sci↗

arXiv2026

A defect in diamond with millisecond-scale spin relaxation time at room temperature

Spin defects in diamond are promising platforms for quantum sensing. The longest electron spin relaxation times ($T_1$) at room temperature for solid-state defects are observed in nitrogen vacancy centers in diamond, which can reach 6.67 ms, and substitutional nitrogen ("P1 centers") in diamond, which exhibit a $T_1$ of 2 ms. No other solid-state defect has exhibited millisecond-scale spin relaxation times at room temperature thus far. Here, we characterize the spin properties of the WAR5 defect in diamond with pulsed electron spin resonance. The observed $T_1$ is one of the longest for solid-state spin defects: 0.97(27) ms at room temperature and 14.38(19) min at 4 K. The observed coherence time ($T_2$) is 246(7) $\mu$s, which can be extended to 6.49(34) ms at 4 K with dynamical decoupling. Furthermore, we demonstrate optical spin polarization with a range of wavelengths from 405 nm to 500 nm and propose potential zero-phonon line candidates.

cond-mat.mtrl-sci↗

arXiv2024

Defect formation in CsSnI$_3$ from Density Functional Theory and Machine Learning

Sn-based perovskites as low-toxic materials are actively studied for optoelectronic applications. However, their performance is limited by $p$-type self-doping, which can be suppressed by substitutional doping on the cation sites. In this study, we combine density functional theory (DFT) calculations with machine learning (ML) to develop a predictive model and identify the key descriptors affecting formation energy and charge transition levels of the substitutional dopants in CsSnI$_{3}$. Our DFT calculations create a dataset of formation energies and charge transition levels and show that Y, Sc, Al, Nb, Ba, and Sr are effective dopants that pin the fermi level higher in the band gap, suppressing the $p$-type self-doping. We explore ML algorithms and propose training the random forest regression model to predict the defect formation properties. This work shows the predictive capability of combining DFT with machine learning and provides insights into the important features that determine the defect formation energetics.

cond-mat.mtrl-sci↗

arXiv2024

Density Functional Theory Study of Surface Stability and Phase Diagram of Orthorhombic CsPbI3

CsPbI3 has been recognized as a promising candidate for optoelectronic device applications. To further improve the efficiency of the devices, it is imperative to better understand the surface properties of CsPbI3, which affect charge carrier transport and defect formation properties. In this study, we perform density functional theory calculations to explore the stability of the (001), (110), and (100) surfaces of orthorhombic CsPbI3, considering different stoichiometries and surface reconstructions. Our results show that, under the chemical potentials confined by the thermodynamically stable region of bulk CsPbI3, the CsI-terminated surfaces of (001) and (110) and the stoichiometric surface of (100) are stable. Among these three surfaces, the CsI-terminated (110) surface has the lowest surface energy and no mid-gap states, which benefits the transport properties of the material.

cond-mat.mtrl-sci↗

arXiv2024

Surface phase diagram of CsSnI$_3$ from first-principles calculations

CsSnI$_3$ is widely studied as an environmentally friendly Pb-free perovskite material for optoelectronic device applications. To further improve material and device performance, it is important to understand the surface structures of CsSnI$_3$. We generate surface structures with various stoichiometries, perform density functional theory calculations to create phase diagrams of the CsSnI$_3$ (001), (110), and (100) surfaces, and determine the most stable surfaces under a wide range of Cs, Sn, and I chemical potentials. Under I-rich conditions, surfaces with Cs vacancies are stable, which lead to partially occupied surface states above the valence band maximum. Under I-poor conditions, we find the stoichiometric (100) surface to be stable under a wide region of the phase diagram, which does not have any surface states and can contribute to long charge carrier lifetimes. Consequently, the I-poor (Sn-rich) conditions will be more beneficial to improve the device performance.

cond-mat.mtrl-sci↗

arXiv2022

Coherent control of a nuclear spin via interactions with a rare-earth ion in the solid-state

Individually addressed Er$^{3+}$ ions in solid-state hosts are promising resources for quantum repeaters, because of their direct emission in the telecom band and compatibility with silicon photonic devices. While the Er$^{3+}$ electron spin provides a spin-photon interface, ancilla nuclear spins could enable multi-qubit registers with longer storage times. In this work, we demonstrate coherent coupling between the electron spin of a single Er$^{3+}$ ion and a single $I=1/2$ nuclear spin in the solid-state host crystal, which is a fortuitously located proton ($^1$H). We control the nuclear spin using dynamical decoupling sequences applied to the electron spin, implementing one- and two-qubit gate operations. Crucially, the nuclear spin coherence time exceeds the electron coherence time by several orders of magnitude, because of its smaller magnetic moment. These results provide a path towards combining long-lived nuclear spin quantum registers with telecom-wavelength emitters for long-distance quantum repeaters.

quant-ph↗

arXiv2022

Role of carbon and hydrogen in limiting $n$-type doping of monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$

We use hybrid density functional calculations to assess n-type doping in monoclinic (Al$_x$Ga$_{1-x}$)$_2$O$_3$ alloys. We focus on Si, the most promising donor dopant, and study the structural properties, formation energies and charge-state transition levels of its various configurations. We also explore the impact of C and H, which are common impurities in metal-organic chemical vapor deposition (MOCVD). In Ga$_2$O$_3$, Si$_{Ga}$ is an effective shallow donor, but in Al$_2O_3$ Si$_{Al}$ acts as a DX center with a (+/-) transition level in the band gap. Interstitial H acts as a shallow donor in Ga$_2$O$_3$, but behaves as a compensating acceptor in n-type Al$_2O_3$. Interpolation indicates that Si is an effective donor in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ up to 70% Al, but it can be compensated by H already at 1% Al. We also assess the diffusivity of H and study complex formation. Si$_{cation}$-H complexes have relatively low binding energies. Substitutional C on a cation site acts as a shallow donor in Ga$_2$O$_3$, but can be stable in a negative charge state in (Al$_x$Ga$_{1-x}$)$_2$O$_3$ when x>5%. Substitutional C on an O site (C$_O$) always acts as an acceptor in n-type (Al$_x$Ga$_{1-x}$)$_2$O$_3$, but will incorporate only under relatively O-poor conditions. C$_O$-H complexes can actually incorporate more easily, explaining observations of C-related compensation in Ga$_2$O$_3$ grown by MOCVD. We also investigate C$_{cation}$-H complexes, finding they have high binding energies and act as compensating acceptors when x>56%; otherwise the H just passivates the unintentional C donors. C-H complex formation explains why MOCVD grown Ga$_2$O$_3$ can exhibit record-low free-carrier concentrations, in spite of the unavoidable incorporation of C. Our study highlights that, while Si is a suitable shallow donor in ALGO alloys, control of unintentional impurities is essential to avoid compensation.

cond-mat.mtrl-sci↗