99 research outputs found
Towards Molecule Generation with Heterogeneous States via Reinforcement Learning
De novo molecular design and generation are frequently prescribed in the field of chemistry and biology, for it plays a critical role in maintaining the prosperity of the chemical industry and benefiting the drug discovery. Nowadays, many significant problems in this field are based on the philosophy of designing molecular structures towards specific desired properties. This research is very meaningful in both medical and AI fields, which can benefits novel drug discovery for some diseases. However, It remains a challenging task due to the large size of chemical space. In recent years, reinforcement learning-based methods leverage graphs to represent molecules and generate molecules as a decision making process. However, this vanilla graph representation may neglect the intrinsic context information with molecules and limits the generation performance accordingly. In this paper, we propose to augment the original graph states with the SMILES context vectors. As a result, SMILES representations are easily processed by a simple language model such that the general semantic features of a molecule can be extracted; and the graph representations perform better in handling the topology relationship of each atom. Moreover, we propose a framework that combines supervised learning and reinforcement learning algorithm to take a solid consideration of these two heterogeneous state representations of a molecule, which can fuse the information from both of them and extract more comprehensive features so that more sophisticated decisions can be made by the policy network. Our model also introduces two attention mechanisms, i.e., action-attention, and graph-attention, to further improve the performance. We conduct our experiments on a practical dataset, ZINC, and the experiment results demonstrate that our framework can outperform other baselines in the learning performance of molecule generation and chemical property optimization
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Essays in Ownership Structure and Corporate Governance
This dissertation delves into ownership structure and corporate governance. The first chapter investigates the causal link between business group affiliation and new firms' profitability. To overcome selection issues related to group affiliation, I focus on ownership changes at least two levels away in the ownership chain that lead to a change in group affiliation. I provide evidence suggesting that these "unintentional" changes are likely exogenous. I find that business group affiliation leads to a 12% increase in new firms' profitability during the first six years. I further present evidence consistent with two channels. First, new firms quickly increase revenues and expand market shares after joining business groups, possibly leveraging on groups' marketing networks. Second, group affiliation triggers a higher ratio of top manager turnover and leads to more experienced top managers and more productive employees. It is possible that business groups provide a talent pool of managers and better monitor new firms' labor force. Results suggest that business groups parallel the role of venture capital firms in sponsoring new firms in economies with concentrated equity ownership.
The second chapter examines the impact of input and product market competition on private benefits of control (PBC), as measured by the voting premia between shares with differential voting rights. The main findings are three. First, increases in the intensity of competition lead to lower estimates of PBC. Second, competition significantly reduces the dispersion in the voting premia, affecting especially the top of the PBC distribution. Third, competition effects are particularly prominent in weak-rule-of-law countries, in manufacturing industries and in less-profitable firms. Overall, the results show that competition leads to a meaningful reduction in the level and dispersion of PBC.
The third chapter directly examines the correlation between insider trading and executive compensation at the firm level. Using panel data on US firms from 1992 to 2011, we find that 1% decrease in cash compensation leads to a 21.7 percentage points increase in 6-month buy-and-hold excess returns, as well as a large increase in trading profits. These results indicate that insiders are using insider trading as a substitute to cash compensation, and keeping the total direct compensation level less volatile than previous research relied on. This effect is robust to exogenous shock to insider trading return, such as Sarbanes-Oxley Act of 2002. The result suggests the importance to take into account of insider trading profit in context of executive compensation
Processing conditions and mechanisms for the plasma defect-engineering of bulk oxygen-deficient zirconia
In recent years, the utilisation of oxygen-deficient zirconia (ZrO2-α), commonly referred to as black zirconia, has garnered considerable attention due to its potential applications for solid oxide fuel cells (SOFCs), gas sensors, biomedical implant materials, and photocatalysis. However, current methods employed to manufacture ZrO2-α exhibit noticeable limitations regarding their scalability, environmental sustainability, and cost-effectiveness. Our recent work has successfully demonstrated the feasibility for bulk conversion of conventional white zirconia into oxygen-deficient black zirconia through direct current (DC) plasma treatment (i.e. plasma blackening). This study elucidates the conditions for plasma blackening and provides a unique mechanism for the bulk transformation of zirconia. A systematic investigation of different plasma technologies (DC, active-screen plasma), treatment configurations (contact conditions, cathode material, and cathode potential), and treatment parameters (voltage, temperature, duration) uncover the crucial variables that influence the feasibility and rate of the reduction process. The reduction of zirconia is shown to initiate from localised contacting points at the cathode-facing surface and grow, with a hemispherical shape, towards the anode-facing surface. A series of development stages are proposed for the process, namely: bulk oxygen vacancy conductance, surface activation, oxygen vacancy generation and a moving cathode front. The findings of this study provide insights into the underlying mechanisms involved in the bulk-reduction of zirconia and help to pave the way towards future scalable and cost-effective generation of oxygen-deficient zirconia
Replay Attack Detection Based on Parity Space Method for Cyber-Physical Systems
The replay attack detection problem is studied from a new perspective based
on parity space method in this paper. The proposed detection methods have the
ability to distinguish system fault and replay attack, handle both input and
output data replay, maintain certain control performance, and can be
implemented conveniently and efficiently. First, the replay attack effect on
the residual is derived and analyzed. The residual change induced by replay
attack is characterized explicitly and the detection performance analysis based
on two different test statistics are given. Second, based on the replay attack
effect characterization, targeted passive and active design for detection
performance enhancement are proposed. Regarding the passive design, four
optimization schemes regarding different cost functions are proposed with
optimal parity matrix solutions, and the unified solution to the passive
optimization schemes is obtained; the active design is enabled by a marginally
stable filter so as to enlarge the replay attack effect on the residual for
detection. Simulations and comparison studies are given to show the
effectiveness of the proposed methods
Quantum Discord for Investigating Quantum Correlations without Entanglement in Solids
Quantum systems unfold diversified correlations which have no classical
counterparts. These quantum correlations have various different facets. Quantum
entanglement, as the most well known measure of quantum correlations, plays
essential roles in quantum information processing. However, it has recently
been pointed out that quantum entanglement cannot describe all the
nonclassicality in the correlations. Thus the study of quantum correlations in
separable states attracts widely attentions. Herein, we experimentally
investigate the quantum correlations of separable thermal states in terms of
quantum discord. The sudden change of quantum discord is observed, which
captures ambiguously the critical point associated with the behavior of
Hamiltonian. Our results display the potential applications of quantum
correlations in studying the fundamental properties of quantum system, such as
quantum criticality of non-zero temperature.Comment: 4 pages, 4 figure
Plasma defect-engineering of bulk oxygen-deficient zirconia
Oxygen-deficient zirconia (ZrO2-x) has recently emerged as a promising material for light absorption and photocatalytic applications. However, the economic and environmentally friendly manufacture of bulk ZrO2-x remains challenging and has limited widespread adoption. In this study, we present a novel low-pressure (300 Pa) plasma treatment (H2 gas at 500 °C for 5 h) capable of producing fully-dense bulk ZrO2-x without significant structural modifications. EPR (electron paramagnetic resonance) and XPS (X-ray photoelectron spectroscopy) characterisation of the plasma treated zirconia indicate the formation of Zr3+ ions and F2+ (V0) centres. The increase of oxygen vacancies is also supported by the greater exothermic heat flow and relative mass gain observed through TGA (thermogravimetric analysis) and DSC (differential scanning calorimetry) analyses. Diffuse reflectance spectroscopy (DRS) reveals a substantial enhancement in light absorption, with an average increase of 66.2 % and >65 % absolute absorption across the entire spectrum (200–3000 nm). XPS and DRS measurements suggest significant reduction in both direct (from 4.84 to 2.61 eV) and indirect (from 3.19 to 1.45 eV) bandgap transition. By effectively enhancing the light absorption capability, reducing bandgap transitions, and maintaining the structural integrity of zirconia, low-pressure plasma treatments offer a promising and scalable approach for the environmentally friendly production of next-generation ZrO2-x materials
Minute-cadence Observations of the LAMOST Fields with the TMTS: III. Statistic Study of the Flare Stars from the First Two Years
Tsinghua University-Ma Huateng Telescopes for Survey (TMTS) aims to detect
fast-evolving transients in the Universe, which has led to the discovery of
thousands of short-period variables and eclipsing binaries since 2020. In this
paper, we present the observed properties of 125 flare stars identified by the
TMTS within the first two years, with an attempt to constrain their eruption
physics. As expected, most of these flares were recorded in late-type red stars
with > 2.0 mag, however, the flares associated with
bluer stars tend to be on average more energetic and have broader profiles. The
peak flux (F_peak) of the flare is found to depend strongly on the equivalent
duration (ED) of the energy release, i.e., , which is consistent with results derived from the Kepler
and Evryscope samples. This relation is likely related to the magnetic loop
emission, while -- for the more popular non-thermal electron heating model -- a
specific time evolution may be required to generate this relation. We notice
that flares produced by hotter stars have a flatter relation compared to that from cooler stars. This is related to the
statistical discrepancy in light-curve shape of flare events with different
colors. In spectra from LAMOST, we find that flare stars have apparently
stronger H alpha emission than inactive stars, especially at the low
temperature end, suggesting that chromospheric activity plays an important role
in producing flares. On the other hand, the subclass having frequent flares are
found to show H alpha emission of similar strength in their spectra to that
recorded with only a single flare but similar effective temperature, implying
that the chromospheric activity may not be the only trigger for eruptions.Comment: 17 pages, 15 figures, 2 tables, refereed version. For associated data
files, see https://cdsarc.cds.unistra.fr/viz-bin/cat/J/MNRAS/523/219
The Sterility of Allotriploid Fish and Fertility of Female Autotriploid Fish
Based on the formation of an autotetraploid fish line (4nAUT, 4n = 200; F2–F11) derived from the distant hybridization of female Carassius auratus red var. (RCC, 2n = 100) × male Megalobrama amblycephala (BSB, 2n = 48), we produced autotriploid hybrids (3nAUT) by crossing females of RCC with males of 4nAUT and allotriploid hybrids (3nALT) by crossing females of Cyprinus carpio (CC, 2n = 100) with males of 4nAUT. The aim of this study was to comparatively investigate the reproductive characteristics of 3nALT and 3nAUT. We investigated morphological traits, chromosomal numbers, DNA content and gonadal development in 3nAUT and 3nALT. The results indicated both 3nAUT and 3nALT possessed 150 chromosomes and were triploid hybrids. The females and males of 3nALT and males of 3nAUT had abnormal gonadal development and could not generate mature eggs or sperm, but the females of 3nAUT had normal gonadal development and generated mature eggs at 2 years old. The females of 3nAUT generated different sizes of eggs, which fertilized with haploid sperm from RCC and formed viable diploid, triploid, and tetraploid offspring. The formation of these two kinds of triploid hybrids provides an ideal model for studying the reproductive traits of triploid hybrids, which is of great value in animal genetics and reproductive biology
Properties and Asteroseismological analysis of a new ZZ ceti discovered by TMTS
Tsinghua university-Ma Huateng Telescope for Survey (TMTS) aims to discover
rapidly evolving transients by monitoring the northern sky. The TMTS catalog is
cross-matched with the white dwarf (WD) catalog of Gaia EDR3, and light curves
of more than a thousand WD candidates are obtained so far. Among them, the WD
TMTS J23450729+5813146 (hereafter J2345) is one interesting common source.
Based on the light curves from the TMTS and follow-up photometric observations,
periods of 967.113 s, 973.734 s, 881.525 s, 843.458 s, 806.916 s and 678.273 s
are identified. In addition, the TESS observations suggest a 3.39 h period but
this can be attributed to the rotation of a comoving M dwarf located within 3".
The spectroscopic observation indicates that this WD is DA type with Teff =
11778+/-617K,log g = 8.38+/-0.31,mass=0.84+/-0.20Msun and age=0.704+/-0.377
Gyrs. Asteroseismological analysis reveals a global best-fit solution of Teff
=12110+/-10K and mass=0.760+/-0.005Msun,consistent with the spectral fitting
results, and Oxygen and Carbon abundances in the core center are 0.73 and 0.27,
respectively. The distance derived from the intrinsic luminosity given by
asteroseismology is 93 parsec, which is in agreement with the distance of 98
parsec from Gaia DR3. Additionally, kinematic study shows that this WD is
likely a thick disk star. The mass of its zero-age main-sequence mass is
estimated to be 3.08 Msun and has a main-sequence plus cooling age of roughly
900 Myrs.Comment: 10 pages, 10 figures, accepted for publication in MNRA
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