NO scattering from metallic and insulating surfaces represent contrasting benchmark systems for understanding energy transfer at gas-surface interface. Strikingly different behaviors of highly vibrationally excited NO scattered from Au(111) and LiF(001) were observed and attributed to disparate electronic structures between metals and insulators. Here, we reveal an alternative mechanical origin of this discrepancy by comparative molecular dynamics simulations with globally accurate adiabatic neural network potentials of both systems. We find that highly-vibrating NO can reach for the high dissociation barrier on Au(111), by which vibrational energy can largely transfer to translation/rotation and further dissipate into substrate phonons. This mechanical energy transfer channel is forbidden in the purely repulsive NO/LiF(001) system or for low-vibrating NO on Au(111), where molecular vibration is barely coupled to other degrees of freedom. Our results emphasize that the initial state and potential energy landscape concurrently influence the mechanical energy transfer dynamics of gas-surface scattering.
We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi$_2$Se$_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two…
We present a constrained density functional perturbation theory scheme for the calculation of structural and harmonic vibrational properties of insulators in the presence of an excited and thermalized electron-hole plasma. The method is ideal to tame ultrafast light induced structural transitions in the regime where the photocarriers thermalize faster than the lattice, the electron-hole recombination time is longer than the phonon period and the photocarrier concentration is large enough to be approximated by an electron-hole plasma. The complete derivation presented here includes total energy…
We report the structural, elastic and vibrational properties of five ionic-molecular solid oxidizers MNO$_3$ (M = Li, Na, K) and MClO$_3$ (M = Na, K). By treating long range electron-correlation effects, dispersion corrected method leads to more accurate predictions of structural properties and phase stability of KNO$_3$ polymorphs. The obtained elastic moduli show soft nature of these materials and are consistent with Ultrasonic Pulse Echo measurements. We made a complete assignment of vibrational modes which are in good accord with available experimental results. From calculated IR and Raman…
Classification of Metal - Insulator Transitions: Relating characteristic Properties to Quantum Chemical Bonding Descriptors
RWTH Aachen University（亚琛工业大学）；John A. Paulson School of Engineering and Applied Sciences, Harvard University（哈佛大学约翰·A·保尔森工程与应用科学学院）；School of Advanced Materials JNCASR（JNCASR高级材料学院）；Forschungszentrum Jülich GmbH（于利希研究中心有限公司）
Pressure-induced metal-insulator and spin-state transition in low-valence layered nickelates
从头计算预测，在零温度下的金属-绝缘体转变发生在La $_4$ Ni $_3$ O $_8$在中等压力下作为压力诱导的自旋态转变的结果。已经观察到在105 K在环境压力下从低温高自旋状态到高温低自旋状态的自旋状态转变随着压力的施加而向较低温度转移。从我们的计算中，我们发现，较小的晶胞体积有利于金属低自旋态，它变得更稳定，在5 GPa。类似的物理现象应该发生在相关化合物La $_3$ Ni $_2$ O $_6$中，但在不同的能量尺度上，这可能解释了为什么在这种材料中没有观察到跃迁。
The unique surface edge states make topological insulators a primary focus among different applications. In this article, we synthesized a large single crystal of Niobium(Nb)-doped Bi2Se3 topological insulator (TI) with a formula Nb0.25Bi2Se3. The single crystal has characterized by using various techniques such as Powder X-ray Diffractometer (PXRD), DC magnetization measurements, Raman, and Ultrafast transient absorption spectroscopy (TRUS). There are (00l) reflections in the PXRD, and Superconductivity ingrown crystal is evident from clearly visible diamagnetic transition at 2.5K in both FC …
We present a theory based on first-principles calculations explaining (i) why the tunability of spin polarizations of photoelectrons from Bi$_2$Se$_3$ (111) depends on the band index and Bloch wavevector of the surface state and (ii) why such tunability is absent in the case of isosymmetric Au (111). The results provide not only an explanation for the recent, puzzling experimental observations but also a guide toward making highly-tunable spin-polarized electron sources from topological insulators.
While CaFeO$_3$ exhibits upon cooling a metal-insulator transition linked to charge ordering, SrFeO$_3$ and BaFeO$_3$ keep metallic behaviors down to very low temperatures. Moreover, alkaline-earth ferrites do not seem prone to orbital ordering in spite of the d$^4$ formal occupancy of Fe$^{4+}$. Here, from first-principles simulations, we show that the metal-insulator transition of CaFeO$_3$ is structurally triggered by oxygen rotation motions as in rare-earth nickelates. This not only further clarifies why SrFeO$_3$ and BaFeO$_3$ remain metallic but allows us to predict that an insulating ch…
Images of uniform and upright nanowires are fascinating, but often, they are quite puzzling, when epitaxial templating from the substrate is clearly absent. Here, we reveal the physics underlying one such hidden growth guidance mechanism through a specific example - the case of ZnO nanowires grown on silicon oxide and glass. We show how electric fields exerted by the insulating substrate may be manipulated through the surface charge to define the orientation and polarity of the nanowires. Surface charge is ubiquitous on the surfaces of semiconductors and insulators, and as a result, substrate …
The element specificity of soft X-ray spectroscopy makes it an ideal tool for analyzing the microscopic origin of ultrafast dynamics induced by localized optical excitation in metal-insulator heterostructures. Using [Fe/MgO]$_n$ as a model system, we perform ultraviolet pump/soft X-ray probe experiments, which are sensitive to all constituents of these heterostructures, to probe both electronic and lattice excitations. Complementary ultrafast electron diffraction experiments independently analyze the lattice dynamics of the Fe constituent, and together with ab initio calculations yield compreh…
While the phase diagrams of the one- and multi-orbital Hubbard model have been well studied, the physics of real Mott insulators is often much richer, material dependent, and poorly understood. In the prototype Mott insulator V$_{2}$O$_{3}$, chemical pressure was initially believed to explain why the paramagnetic-metal to antiferromagnetic-insulator transition temperature is lowered by Ti doping while Cr doping strengthens correlations, eventually rendering the high-temperature phase paramagnetic insulating. However, this scenario has been recently shown both experimentally and theoretically t…
Raman spectroscopy is a widely used, powerful, and nondestructive tool for studying the vibrational properties of bulk and low-dimensional materials. Raman spectra can be simulated using first-principles methods, but due to the high computational cost calculations are usually limited only to fairly small unit cells, which makes it difficult to carry out simulations for alloys and defects. Here, we develop an efficient method for simulating Raman spectra of alloys, benchmark it against full density-functional theory calculations, and apply it to several alloys of two-dimensional transition meta…
The main scientific activity in the field of topological insulators (TIs) consists of determining their electronic structure by means of magneto-transport and electron spectroscopy with a view to devices based on topological transport. There is however a caveat in this approach. There are systematic experimental discrepancies on the electronic structure of the most pristine surfaces of TI single crystals as determined by Shubnikov de Haas (SdH) oscillations and by Angle Resolved PhotoElectron Spectroscopy (ARPES). We identify intense ultraviolet illumination -that is inherent to an ARPES exper…
Symmetry breaking transforms strong to normal correlation and false metals to true insulators
Renewable and Sustainable Energy Institute, University of Colorado（科罗拉多大学可再生与可持续能源研究所）；Department of Physics and Engineering Physics, Tulane University（杜兰大学物理与工程物理系）
