Tracing out the Berry curvature dipole and multipoles in second harmonic Hall responses of time-reversal symmetric insulators

Various nonlinear characteristics of solid states, such as the circular photogalvanic effect of time-reversal symmetric insulators, the quantized photogalvanic effect of Weyl semimetals, and the nonlinear Hall effect of time-reversal symmetric metals, have been associated with the Berry curvature dipole (BCD). Here, we explore the question of whether the Berry curvature dipole and multipoles of time-reversal symmetric insulators can be traced in the nonlinear optical responses. We performed real-time time-dependent density functional theory calculations and examined the second harmonic generation susceptibility tensors. The two-band term of the susceptibility tensor is sharply proportional to the interband BCD, dominating over the Hall response once the cancellation effect of the multiple reflection symmetries is lifted. We suggest that the nonlinear Hall component of the second-harmonic spectra of insulators can also be utilized as an effective tool to extract the band structure geometry through Berry curvature dipole and possibly multipoles.Comment: main text: 21 pages with 3 figures; supplementary material: 11 pages with 3 figure

Ruthenium anchored on carbon nanotube electrocatalyst for hydrogen production with enhanced Faradaic efficiency

Developing efficient and stable electrocatalysts is crucial for the electrochemical production of pure and clean hydrogen. For practical applications, an economical and facile method of producing catalysts for the hydrogen evolution reaction (HER) is essential. Here, we report ruthenium (Ru) nanoparticles uniformly deposited on multi-walled carbon nanotubes (MWCNTs) as an efficient HER catalyst. The catalyst exhibits the small overpotentials of 13 and 17 mV at a current density of 10 mA cm(-2) in 0.5M aq. H2SO4 and 1.0M aq. KOH, respectively, surpassing the commercial Pt/C (16 mV and 33 mV). Moreover, the catalyst has excellent stability in both media, showing almost "zeroloss" during cycling. In a real device, the catalyst produces 15.4% more hydrogen per power consumed, and shows a higher Faradaic efficiency (92.28%) than the benchmark Pt/C (85.97%). Density functional theory calculations suggest that Ru-C bonding is the most plausible active site for the HER

Nonlinear electromagnetic responses of insulators: real-time dynamics and perturbation theories

Department of Physicsclos

Resonant amplification of the inverse Faraday effect magnetization dynamics of time reversal symmetric insulators

All-optical helicity-dependent manipulations of magnetism have attracted broad attention in the context of ultrafast control of magnetic units. Here, we investigate the spin dynamics in time reversal symmetric insulators induced by strong circularly polarized light. We perform real-time time-dependent density functional theory calculations together with model Hamiltonian analyses for MoS2 and WS2 monolayers, which are exemplary spin-orbit-coupled time reversal symmetric insulators. We trace the evolution of dynamical spin states, starting from the Kramers-paired electronic ground state, and find that the induced magnetization exhibits a sharp resonance peak when the applied light frequency is close to half the spin-flipping energy gap. The resonance condition is secondarily affected by the field strength and the pulse width. We suggest that low-energy time reversal broken excitations of insulators can be pursued with a sharp frequency selection as another class of ultrafast phenomena

Identification of the Mott Insulating Charge Density Wave State in 1T-TaS2

We investigate the low-temperature charge density wave (CDW) state of bulk TaS2 with a fully self-consistent density-functional theory with the Hubbard U potential, over which the controversy has remained unresolved regarding the out-of-plane metallic band. By examining the innate structure of the Hubbard U potential, we reveal that the conventional use of atomic-orbital basis could seriously misevaluate the electron correlation in the CDW state. By adopting a generalized basis, covering the whole David star, we successfully reproduce the Mott insulating nature with the layer-by-layer antiferromagnetic order. Similar consideration should be applied for description of the electron correlation in molecular solid

Velocity-Gauge Real-Time Time-Dependent Density Functional Tight-Binding for Large-Scale Condensed Matter Systems.

We present a new velocity-gauge real-time, time-dependent density functional tight-binding (VG-rtTDDFTB) implementation in the open-source DFTB+ software package (https://dftbplus.org) for probing electronic excitations in large, condensed matter systems. Our VG-rtTDDFTB approach enables real-time electron dynamics simulations of large, periodic, condensed matter systems containing thousands of atoms with a favorable computational scaling as a function of system size. We provide computational details and benchmark calculations to demonstrate its accuracy and computational parallelizability on a variety of large material systems. As a representative example, we calculate laser-induced electron dynamics in a 512-atom amorphous silicon supercell to highlight the large periodic systems that can be examined with our implementation. Taken together, our VG-rtTDDFTB approach enables new electron dynamics simulations of complex systems that require large periodic supercells, such as crystal defects, complex surfaces, nanowires, and amorphous materials

Second harmonic Hall responses of insulators as a probe of Berry curvature dipole

Diverse nonlinear optical responses of metallic band states have been characterized in terms of the Berry curvature dipole (BCD) or other multipole structures of Berry curvature. Here, we find that the second harmonic optical responses of insulators to sub-bandgap light are also delicately associated with the interband BCD. We performed real-time time-dependent density functional theory calculations and theoretically analyzed the second harmonic generation susceptibility tensors. The two-band term of the second-order susceptibility is precisely proportional to the interband BCD, which is particularly significant for low-symmetric systems with a small bandgap. We show that higher-order responses to nonperturbative strong fields can be associated with higher poles of Berry curvature. We suggest that the consequences of symmetry lowering can be detected by nonlinear optical responses through adjustments of the dipole or other multipole structures of the Berry curvature texture

Bifunctional sulfur-doped cobalt phosphide electrocatalyst outperforms all-noble-metal electrocatalysts in alkaline electrolyzer for overall water splitting

Sulfur-doped CoP (S:CoP) nanoparticles are synthesized as a noble metal-free electrocatalyst via a novel and eco-friendly thiourea-phosphate-assisted solvothermal route. When used as a bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions from water splitting in an alkaline solution, the electrode exhibits excellent activity and stability outperforming noble mental-based Pt/C, IrO2, and reported non-noble metal-based electrocatalysts. Density functional theory calculations indicate that the excellent performance is attributable to the improved charge-transfer characteristics of the S:CoP nanoparticles owing to their modified electronic structure. It also increases the number of exposed active sites especially on the conductive substrates. A bifunctional S:CoP catalyst-based alkaline electrolyzer for overall water splitting exhibits a stable current density of 100 mA/cm(2) at an overvoltage of 0.55 V during a long-term operation; this performance is superior to that obtained from all-noble metal electrolyzer with a Pt/C cathode and an IrO2 anode

Hexaazatriphenylene-Based Two-Dimensional Conductive Covalent Organic Framework with Anisotropic Charge Transfer

The development of covalent organic frameworks (COFs) with efficient charge transport is of immense interest for applications in optoelectronic devices. To enhance COF charge transport properties, electroactive building blocks and dopants can be used to induce extended conduction channels. However, understanding their intricate interplay remains challenging. We designed and synthesized a tailor-made COF structure with electroactive hexaazatriphenylene (HAT) core units and planar dioxin (D) linkages, denoted as HD-COF. With the support of theoretical calculations, we found that the HAT units in the HD-COF induce strong, eclipsed & pi;-& pi; stacking. The unique stacking of HAT units and the weak in-plane conjugation of dioxin linkages leads to efficient anisotropic charge transport. We fabricated HD-COF films to minimize the grain boundary effect of bulk COFs, which resulted in enhanced conductivity. As a result, the HD-COF films showed an electrical conductivity as high as 1.25 S cm-1 after doping with tris(4-bromophenyl)ammoniumyl hexachloroantimonate. Electroactive hexaazatriphenylene (HAT)-dioxin linked covalent organic framework, denoted as HD-COF, was designed, and synthesized. The unique & pi;-& pi; stacking of HAT units and the weak in-plane conjugation of dioxin linkages in the HD-COF structure led to efficient anisotropic charge transport. In situ formed HD-COF film demonstrated enhanced electric conductivity as high as 1.25 S cm-1 after doping with magic blue.+imag

Coordination Polymers for High-Capacity Li-Ion Batteries: Metal-Dependent Solid-State Reversibility

Electrode materials exploiting multielectron-transfer processes are essential components for large-scale energy storage systems. Organic-based electrode materials undergoing distinct molecular redox transformations can intrinsically circumvent the structural instability issue of conventional inorganic-based host materials associated with lattice volume expansion and pulverization. Yet, the fundamental mechanistic understanding of metal-organic coordination polymers toward the reversible electrochemical processes is still lacking. Herein, we demonstrate that metal-dependent spatial proximity and binding affinity play a critical role in the reversible redox processes, as verified by combined 13C solid-state NMR, X-ray absorption spectroscopy, and transmission electron microscopy. During the electrochemical lithiation, in situ generated metallic nanoparticles dispersed in the organic matrix generate electrically conductive paths, synergistically aiding subsequent multielectron transfer to ??-conjugated ligands. Comprehensive screening on 3d-metal-organic coordination polymers leads to a high-capacity electrode material, cobalt-2,5-thiophenedicarboxylate, which delivers a stable specific capacity of ???1100 mA h g-1 after 100 cycles