Central Charge Research Articles

Study on the influence factors of Pt-based catalyst on dehydrogen performance of ammonia borane hydrolysis-Which is more important, geometric effects or electronic effects?

The basic catalytic properties of catalysts are largely determined by their geometric and electronic structures. However, the geometric and electronic interference between the active atoms and their neighbors makes it difficult to determine the extent of the effects of geometric and electronic effects on the catalytic activity. In this paper, Pt-based (Fe, Co, Ni, Cu and Zn) alloy nanoparticles (NPs) were prepared by a one-step reduction method. Through X-ray diffraction (XRD) analysis, high-resolution transmission electron microscopy (HRTEM) analysis and X-ray photoelectron spectroscopy (XPS) analysis, the superior lattice compression effect of Pt3Cu was found. It has great influence on catalytic performance. However, with the increase of Cu content, electron transfer increases, and the effect of electron effect is greater than the effect of lattice compression. Among them, Pt3Cu has the best performance as the content of Pt is high, which is controlled by geometric effects. When the content of Pt is few, the catalytic performance of PtNi3 is the best, showing the results regulated by electronic effects. The d-band center and Bader charge are determined by DFT calculation, and the experimental results are verified. The results show that the position of the d-band center is in a volcanic trend with HER properties. Pt3Cu has a suitable d-band central position, which affects the adsorption and desorption behavior of H2O molecules in the hydrolysis reaction, thus affecting its catalytic performance. These results provide new insights into the relationship between the geometric and electronic effects of catalysts and their catalytic ammonia borane hydrolysis activity.

Dopant-assisted stabilization of negatively charged single nitrogen-vacancy centers in phosphorus-doped diamond at low temperatures

Charge state instabilities have been a bottleneck for the implementation of solid-state spin systems and pose a major challenge to the development of spin-based quantum technologies. Here we investigate the stabilization of negatively charged nitrogen-vacancy (NV−) centers in phosphorus-doped diamond at liquid helium temperatures. Photoionization of phosphorous donors in conjunction with charge diffusion at the nanoscale enhances NV0 to NV− conversion and stabilizes the NV− charge state without the need for an additional repump laser. The phosphorus-assisted stabilization is explored and confirmed both with experiments and our theoretical model. Stable photoluminescence-excitation spectra are obtained for NV− centers created during the growth. The fluorescence is continuously recorded under resonant excitation to real-time monitor the charge state and the ionization and recombination rates are extracted from time traces. We find a linear laser power dependence of the recombination rate as opposed to the conventional quadratic dependence, which is attributed to the photo-ionization of phosphorus atoms.

The Synergetic Effects of Ru and Al Co‐Doping for Improved Cyclability of LiCoO2 at High Voltages

AbstractLiCoO2 is one of the most extensively deployed cathode materials in commercial lithium‐ion batteries. However, it remains a challenge to fulfill the potential specific capacity of LiCoO2 during long‐term cycling, due to the lattice degradations in deep delithiation states. Herein, a dual doping scheme based on Ru and Al is explored, which improves the cyclability of LiCoO2 at high voltages through synergistically reconfiguring electronic structure and stabilizing lattice structure. It is well known that the doped Al will serve as a strong positively charged center and restrain the lattice distortion. The doping of Ru suppresses the occupied O:2p states below Fermi level and stabilizes oxygen redox during cycling. The proposed dual doping strategy improves the accessible energy density and cyclability of LiCoO2 at increased voltages significantly. For example, the dual doped LiCoO2 performs 19% higher energy density at 4.57 V versus Li+/Li than the pristine one. In addition, an initial capacity of 197 mAh g−1 and 86% capacity retention after 100 cycles are achieved from 3.00 to 4.53 V versus Li+/Li. This study sheds a light on the fundamental principles for the development of high‐voltage LiCoO2 and other layered oxide cathode materials with high energy density and excellent cyclability.

Formation of hydroxide anions in amorphous astrophysical ices and recombination with protons: a quantum chemical study

ABSTRACT Quantum chemical cluster calculations employing density functional theory and correlation consistent basis sets reveal the following pathways by which hydroxide anions (OH–) may form in amorphous astrophysical ices: (1) hydroxyl radicals (OH), which may arise in ice via ultraviolet photolysis, can capture electrons; (2) adsorbed hydrogen atoms can capture electrons to form H–, which reacts with water to yield H2 and OH–; (3) NaOH deposited on ice dissociates into Na+ and OH–; (4) NaH deposited on ice dissociates into Na+ and H–; H– then reacts with water to yield H2 and OH– as above. The IR spectrum of ice-bound OH– is presented, based on nine clusters containing up to 31H2O and 1–2 OH– anions. The interaction of OH– in ice with cations is also explored. Prior work shows that when HCO+ is deposited on pure amorphous water clusters, it reacts with H2O to form formic acid (HCOOH) and the hydronium (H3O+). When HCO+ is deposited on a cluster containing OH–, the reaction proceeds in almost the same manner, but the H3O+ and OH– charge centres migrate through the water network toward each other and tend to neutralize one another by forming H2O. This occurred in all but one of seven cases considered; migration occurred even when the oxygen atom attacked by HCO+ is over 10 Å from the oxygen atom in OH–. Cations and anions can interact in ice via pathways not present in the gas phase or incorporated in current models.

Zwitterion Reduces Open‐Circuit Voltage Loss in Wide‐Bandgap Perovskite Solar Cells with 22% Efficiency and Its Application in Tandem Devices

Wide‐bandgap (WBG) perovskite solar cells (PSCs) can be combined with other narrow‐bandgap cells to form tandem solar cells (TSCs), which have excellent commercialization prospects. However, the component of WBG perovskite is complex, and the crystallization process is difficult to control. Various bulk and surface defects in polycrystalline films lead to severe nonradiative recombination of photogenerated carriers, resulting in severe open‐circuit voltage (VOC) loss in WBG PSCs. Herein, a zwitterionic additive is introduced to alleviate the VOC deficit. The sulfinyl group (–SO) in formamidine sulfinic acid (FSA) has a robust interaction with Pb2+, which retards the crystallization and improves the crystal quality in perovskite films. Furthermore, FSA has negatively charged centers (–SO2−) and positively charged centers (–C(NH2)2+), which can passivate the halogen and cation vacancies, respectively. The improved crystalline quality and passivated defects reduce the capture of carriers by defects and facilitate higher VOC. With this strategy, the 1.68 eV bandgap PSCs achieve a champion efficiency of 22.06% and a high VOC of 1.25 V (VOC loss = 0.43 V). Moreover, the 2‐terminal perovskite/silicon TSCs are fabricated with an extraordinary power conversion efficiency of 28.81%.

Optical driving, spin initialization and readout of single SiV− centers in a Fabry-Perot resonator

Large-scale quantum communication networks require quantum repeaters due to the signal attenuation in optical fibers. Ideal quantum repeater nodes efficiently link a quantum memory with photons serving as flying qubits. Color centers in diamond, particularly the negatively charged silicon vacancy center, are promising candidates to establish such nodes. Inefficient connection between the color center’s spin to the optical fiber networks is a major obstacle, that could be resolved by utilizing optical resonators. Here, we couple individual silicon vacancy centers incorporated in a nanodiamond to a hemispherical, stable Fabry-Perot microcavity, achieving Purcell-factors larger than 1. We demonstrate coherent optical driving between ground and excited state with a Rabi-frequency of 330 MHz, all-optical initialization and readout of the electron spin in magnetic fields of up to 3.2 T. Spin initialization within 67 ns with a 80 % fidelity and a lifetime of 350 ns are reached. Our demonstration opens the way to realize quantum repeater applications.

Tensor network variational optimizations for real-time dynamics: Application to the time-evolution of spin liquids

Within the Projected Entangled Pair State (PEPS) tensor network formalism, a simple update (SU) method has been used to investigate the time evolution of a two-dimensional U(1) critical spin-1/2 spin liquid under Hamiltonian quench [Phys. Rev. B 106, 195132 (2022)]. Here we introduce two different variational frameworks to describe the time dynamics of SU(2)-symmetric translationally-invariant PEPS, aiming to improve the accuracy. In one approach, after using a Trotter-Suzuki decomposition of the time evolution operator in term of two-site elementary gates, one considers a single bond embedded in an environment approximated by a Corner Transfer Matrix Renormalization Group (CTMRG). A variational update of the two tensors on the bond is performed under the application of the elementary gate and then, after symmetrization of the site tensors, the environment is updated. In the second approach, a cluster optimization is performed on a finite (periodic) cluster, maximizing the overlap of the exact time-evolved state with a symmetric finite-size PEPS ansatz. Observables are then computed on the infinite lattice contracting the infinite-PEPS (iPEPS) by CTMRG. We show that the variational schemes outperform the SU method and remain accurate over a significant time interval before hitting the entanglement barrier. Studying the spectrum of the transfer matrix, we find that the asymptotic correlations are very well preserved under time evolution, including the critical nature of the singlet correlations, as expected from the Lieb-Robinson (LR) bound theorem. Consistently, the system (asymptotic) boundary is found to be described by the same Conformal Field Theory of central charge c = 1c=1 during time evolution. We also compute the time-evolution of the short distance spin-spin correlations and estimate the LR velocity.

Solving 3d gravity with Virasoro TQFT

We propose a precise reformulation of 3d quantum gravity with negative cosmological constant in terms of a topological quantum field theory based on the quantization of the Teichmüller space of Riemann surfaces that we refer to as “Virasoro TQFT”. This TQFT is similar, but importantly not equivalent, to SL(2, \mathbb{R}ℝ) Chern-Simons theory. This sharpens the folklore that 3d gravity is related to SL(2, \mathbb{R}ℝ) Chern-Simons theory into a precise correspondence, and resolves some well-known issues with this lore at the quantum level. Our proposal is computationally very useful and provides a powerful tool for the further study of 3d gravity. In particular, we explain how together with standard TQFT surgery techniques this leads to a fully algorithmic procedure for the computation of the gravity partition function on a fixed topology exactly in the central charge. Mathematically, the relation leads to many nontrivial conjectures for hyperbolic 3-manifolds, Virasoro conformal blocks and crossing kernels.

The black hole behind the cut

We study the analytic structure of the heavy-heavy-light-light holographic correlators in the supergravity approximation of the AdS3 × S3/CFT2 duality. As an explicit example, we derive the correlator where the heavy operator is a classical microstate of the 5D supersymmetric black hole and its dual geometry interpolates as a function of a continuous parameter between global AdS3 and the extremal BTZ black hole. The simplest perturbation of this interpolating geometry by a light field is described by the Heun equation and we exploit the relation of its connection coefficients to the Liouville CFT to analytically compute the correlator in the two limits, focusing in particular on the black hole regime. In this limit we find that the real poles of the correlator become dense and can be approximated by a cut. We show that, when the charges of the heavy state are in the black hole regime, the discontinuity across the cut has complex poles corresponding to the quasi-normal modes of BTZ. This behaviour is qualitatively similar to what is expected for the large central charge limit of a typical black hole microstate.

Two-dimensional Ising and Potts model with long-range bond disorder: A renormalization group approach

In this paper we provide new analytic results on two-dimensional qq-Potts models (q ≥ 2q≥2) in the presence of bond disorder correlations which decay algebraically with distance with exponent aa. In particular, our results are valid for the long-range bond disordered Ising model (q=2q=2). We implement a renormalization group perturbative approach based on conformal perturbation theory. We extend to the long-range case the RG scheme used in [V. Dotsenko et al., Nucl. Phys. B 455 701-23] for the short-range disorder. Our approach is based on a 22-loop order double expansion in the positive parameters (2-a)(2−a) and (q-2)(q−2). We will show that the Weinrib-Halperin conjecture for the long-range thermal exponent can be violated for a non-Gaussian disorder. We compute the central charges of the long-range fixed points finding a very good agreement with numerical measurements.

High‐Performance and Durable Rotational Triboelectric Nanogenerator Leveraging Soft‐Contact Coplanar Charge Pumping Strategy

AbstractThe triboelectric nanogenerator (TENG) represents a burgeoning approach for harvesting ambient mechanical energy. However, practical applications suffer from the issues of insufficient charge density and material abrasion. Here, a rotational TENG that attains exceptional output performance and durability in mechanical energy harvesting and couples an ingenious coplanar charge pump by soft contact with a suspended electrostatic induction mode is presented. Thanks to the continuous and synchronous charge injection from the central charge pumping unit, the charge density can be up to 74.62 µC m−2, resulting in an outstanding peak power density of 20.72 W m−2. Moreover, the utilization of yielding polyester fur in the charge pumping unit as a tribo‐layer for soft contact decreases the material abrasion and then enhances durability and robustness. And the experimental results show that the TENG can maintain 99.2% electrical output after 100 000 operating cycles. With the optimized design, the prototype can continuously drive four commercial lamps rated 12 W in series. Furthermore, a wireless powering system is demonstrated to power the electronics, demonstrating an extensive application of the TENG. Given the compelling characteristics, this work is expected to provide a sound strategy for improving TENGs’ output and durability in the field of self‐powered sensor systems.

Hidden Critical Points in the Two-Dimensional O(n>2) Model: Exact Numerical Study of a Complex Conformal Field Theory.

The presence of nearby conformal field theories (CFTs) hidden in the complex plane of the tuning parameter was recently proposed as an elegant explanation for the ubiquity of "weakly first-order" transitions in condensed matter and high-energy systems. In this work, we perform an exact microscopic study of such a complex CFT (CCFT) in the two-dimensional O(n) loop model. The well-known absence of symmetry-breaking of the O(n>2) model is understood as arising from the displacement of the nontrivial fixed points into the complex temperature plane. Thanks to a numerical finite-size study of the transfer matrix, we confirm the presence of a CCFT in the complex plane and extract the real and imaginary parts of the central charge and scaling dimensions. By comparing those with the analytic continuation of predictions from Coulomb gas techniques, we determine the range of validity of the analytic continuation to extend up to n_{g}≈12.34, beyond which the CCFT gives way to a gapped state. Finally, we propose a beta function which reproduces the main features of the phase diagram and which suggests an interpretation of the CCFT as a liquid-gas critical point at the end of a first-order transition line.

Modulation of electronic and magnetic properties of graphene-triangular h-BN hybrid through monovacancy: Carbon vacancy-BN size coupling

The effects of monovacancy on the stability, electronic and magnetic properties of the triangular h-BN embedded graphene nanosheet (G) were investigated by density functional theory (DFT) method. The interface is the energetically most favorable site for creating a vacancy in the GBN nanosheet. Defective GBN hybrids are either metallic or exhibit a small band gap energy ranging from 0.01 to 0.23 eV. The dangling bonds induced a localized magnetic moment ranging from 0.02 to 3 μB in the defective hybrid. The magnetic moment induced by the carbon vacancy in the G layout of the N-rich hybrid interestingly increases with island size. This coupling effect can be explained by the local charge imbalance between charge centers of opposite sign. We could not observe similar behavior in B- rich hybrids. Our results showed that the narrow bandgap and induced magnetic moment of the vacancy-defected GBN hybrid could be easily modulated and thus could be potential materials for spintronic /electronic applications.

Ionization of a neutral MOF to disperse and anchor acid for boosting anhydrous proton conductivity

Although incorporation of proton conducting molecules (PCMs) is one of the most effective methods to enhance the anhydrous conductivity, most materials including MOFs are challenged by the leakage and poor dispersion of PCMs, severely decreasing the anhydrous proton conductivity. Herein, we propose a neutral framework ionization strategy to construct positive and negative charge centers for dispersing and anchoring the encapsulated PCMs. The N+ and –SO3– sites in MOF-867-PS are thus constructed through modifying MOF-867 with 1,3-propane sultone (PS). After incorporating trifluoromethanesulfonic acid (CF3SO3H, TFOH) the resultant MOF-867-PS-XTFOH (X is volume of TFOH/μL per 100 mg MOFs) composites disperse and anchor the CF3SO3− and H+ ions through electrostatic interaction to form smooth proton transport pathway, benefiting to the high proton conductivity. Especially, MOF-867-PS-100TFOH exhibits sharply enhanced proton conductivity over 10−3 S cm−1, almost two orders of magnitude higher than pristine MOF-867 loaded with equimolar acid and comparable to the top-ranking anhydrous proton conducting MOF composites. This work is the first post–synthetic modification in MOFs to assemble multiple sites for anchoring the functional molecules, opening an effective path to fabricate the high–performance MOFs for proton conduction or other functionalities.

Equivariant Localization in Supergravity.

We show that supersymmetric supergravity solutions with an R-symmetry Killing vector are equipped with a set of equivariantly closed forms. Various physical observables may be expressed as integrals of these forms, and then evaluated using the Berline-Vergne-Atiyah-Bott fixed point theorem. We illustrate with a variety of holographic examples, including on-shell actions, black hole entropies, central charges, and scaling dimensions of operators. The resulting expressions depend only on topological data and the R-symmetry vector, and hence may be evaluated without solving the supergravity equations.

Asymptotic Symmetry Algebra of Einstein Gravity and Lorentz Generators.

The asymptotic symmetry algebra of four-dimensional Einstein gravity in the asymptotically flat context has been shown recently to be the direct sum of the Poincaré algebra and of an infinite-dimensional Abelian algebra (with central charge) that includes the Bondi-Metzner-Sachs supertranslations. This result, obtained within the Hamiltonian formalism, yields a supertranslation-invariant definition of the Lorentz generators (angular momentum and boosts). Definitions of Lorentz generators free from the "supertranslation ambiguities" have also been proposed recently at null infinity. We prove the equivalence of the two approaches for redefining the charges.

Determining a Lower Limit of Luminosity for the First Satellite Observation of a Reverse Beam Terrestrial Gamma Ray Flash Associated With a Cloud to Ground Lightning Leader

AbstractWe provide an updated analysis of the gamma ray signature of a terrestrial gamma ray flash (TGF) detected by the Fermi Gamma ray Burst Monitor first reported by Pu et al. (2020, https://doi.org/10.1029/2020GL089427). A TGF produced 3 ms prior to a negative cloud‐to‐ground return stroke was close to simultaneous with an isolated low‐frequency radio pulse during the leader’s propagation, with a polarity indicating downward moving negative charge. In previous observations, this “slow” low‐frequency signal has been strongly correlated with upward‐directed (opposite polarity) TGF events (Pu et al., 2019, https://doi.org/10.1029/2019GL082743; Cummer et al., 2011, https://doi.org/10.1029/2011GL048099), leading the authors to conclude that the Fermi gamma ray observation is actually the result of a reverse positron beam generating upward‐directed gamma rays. We investigate the feasibility of this scenario and determine a lower limit on the luminosity of the downward TGF from the perspective of gamma ray timing uncertainties, TGF Monte Carlo simulations, and meteorological analysis of a model storm cell and its possible charge structure altitudes. We determined that the most likely source altitude of the TGF reverse beam was 7.5 km ± 2.6 km, just below an estimated negative charge center at 8 km. At that altitude, the Monte Carlo simulations indicate a lower luminosity limit of 2 × 1018 photons above 1 MeV for the main downward beam of the TGF, making the reverse beam detectable by the Fermi Gamma ray Burst Monitor.

Dualities of adjoint SQCD and supersymmetry enhancement

We propose a new dual description of four-dimensional N\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{N} $$\\end{document} = 1 SU(N) gauge theory with one adjoint (X) and Nf fundamental matters with a superpotential W = TrXp+1. The dual theory consists of the D\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{D} $$\\end{document}p[SU(N)] Argyres-Douglas theory coupled to SU(N) gauge theory and Nf fundamentals with a superpotential deformation. We study renormalization group fixed points of the Argyres-Douglas dual theories with and without superpotential deformations, and identify the conditions for them to be dual to the fixed points of adjoint SQCD. We check our proposal via matching central charges, chiral operators and superconformal indices. We find that when Nf = 2N and p = 2, the dual theory flows to N\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{N} $$\\end{document} = 2 SU(N) superconformal QCD with 2N flavors upon suitable superpotential deformation, exhibiting supersymmetry enhancement.

Exponential Networks, WKB and Topological String

We propose a connection between 3d-5d exponential networks and exact WKB for difference equations associated to five dimensional Seiberg-Witten curves, or equivalently, to quantum mirror curves to toric Calabi-Yau threefolds $X$: the singularities in the Borel planes of local solutions to such difference equations correspond to central charges of 3d-5d BPS KK-modes. It follows that there should be distinguished local solutions of the difference equation in each domain of the complement of the exponential network, and these solutions jump at the walls of the network. We verify and explore this picture in two simple examples of 3d-5d systems, corresponding to taking the toric Calabi-Yau $X$ to be either $\mathbb{C}^3$ or the resolved conifold. We provide the full list of local solutions in each sector of the Borel plane and in each domain of the complement of the exponential network, and find that local solutions in disconnected domains correspond to non-perturbative open topological string amplitudes on $X$ with insertions of branes at different positions of the toric diagram. We also study the Borel summation of the closed refined topological string free energy on $X$ and the corresponding non-perturbative effects, finding that central charges of 5d BPS KK-modes are related to the singularities in the Borel plane.

Singular products and universality in higher-derivative conformal theory

I investigate universality of the two-dimensional higher-derivative conformal theory using the method of singular products. The previous results for the central charge at one loop are confirmed for the quartic and six-derivative actions.