Investigations into the nature of magnetism in transition-metal-phthalocyanines

In this dissertation, the nature of magnetism in transition-metal-doped phthalocyanines (TMPc, TM = Co, Ni, Cu, V, and Mn, chemical formula: C 32H16N8TM) is investigated by measuring their magnetization (M) and ac susceptibilities in magnetic fields up to 90 kOe and covering the wide temperature range of 0.5 K to 300 K. These low temperatures and high magnetic field measurements combined with detailed analysis of the data have provided new information about the nature of magnetism of TMPc. The powder samples used here were structurally characterized using x-ray diffraction and scanning electron microscopy.;TMPc are a class of organic semiconductors with potential applications in spintronics and they are planar molecules with the TM atom at the center bound to four N atoms and forming a linear chain along the monoclinic b-axis. Because of this symmetry, the ground states of TMPc often violate the Hunds\u27 rules; e.g. the S = 3/2 state for d5 Mn(II) in beta-MnPc, S = 1/2 state for the d7 Co(II) in beta-CoPc, and S = 0 state for the d8 Ni(II) in NiPc. The magnetic properties of TMPc are also affected by the stack angle delta between the orientation of the molecular plane and the b-axis, delta being 65° (45°) for alpha (beta) phase.;For both alpha-CuPc and beta-CuPc, M vs. T data fit well with the Bonner-Fisher model for S = 1/2 antiferromagnetic (AFM) Heisenberg linear chain yielding the Cu2+-Cu2+ exchange constant Jex/kB = - 1.7 K (- 0.2 K) for alpha-CuPc(beta-CuPc). This order of magnitude difference in Jex/kB is likely due to the smaller Cu2+-Cu2+ distance in alpha-phase than in beta-phase. The M vs. H at 2 K and 5 K data are interpreted including the effect of the exchange coupling.;For beta-CoPc, the predicted peak in chi vs. T data by Bonner-Fisher model was observed at Tmax = 1.9 K for the first time yielding the Heisenberg exchange constant Jex/kB = - 1.5 K between the Co2+-Co2+ ions. The isothermal data of M vs. H at 1 K shows good agreement with the Bonner-Fisher model; M vs. H at 5 K data is interpreted by including the effect of the exchange interaction.;For VOPc (phase II), the analysis of the M vs. T data shows VO 2+ electronic state with spin S = 1/2 and theta = - 0.1 indicates a very weak AFM exchange coupling between neighboring VO2+ ions which implies the near-perfect paramagnetism of VOPc. This conclusion is also supported by the analysis of the M vs. H data at several temperatures. The diamagnetic nature of beta-NiPc corresponding to the S = 0 state for the 3d8 Ni(II) is confirmed by measurements and analysis of the magnetization with temperature-independent susceptibility chiD = - 3.38 x 10-7 emu/gOe. The observed ferromagnetic component in the beta-NiPc powder sample is identified with Fe impurity.;For beta-MnPc with S = 3/2, detailed analysis of the dc and ac susceptibilities leads to the conclusion of the absence of 3-dimensional long range magnetic ordering in this system contrary to previous reports. This conclusion is supported by the Arrott plots and lack of a peak in the ac susceptibilities and specific heat near the quoted TC ≈ 8.6 K. Instead, beta-MnPc is best described as an Ising-like chain magnet with Arrhenius relaxation of the magnetization governed by an intra-layer ferromagnetic exchange constant Jex/kB = 2.6 K and the single ion anisotropy energy constant |D|/kB = 8.3 K. The absence of 3D long-range ordering is consistent with the measured |D| being \u3eJex

Coherent Multidimensional Spectroscopy: Development of Efficient Data Acquisition and Analyses of Quantum Dot 2D Spectra

Coherent multidimensional spectroscopy (CMDS) is the most complete nonlinear optical technique based on the interaction of multiple short laser pulses with matter. It has grown to play a significant role in studies of optoelectronic materials and pigment protein complexes. In this work we apply CMDS to investigate electronic excitations and their dynamics in semiconductor nanocrystals, so called quantum dots (QDs). We also apply modern sparsed sampling algorthms to speed up the data collection in the multidimensional experiments. We test the new methods on CMDS experiments on photosynthetic light harvesting complexes. In CMDS spectral information is spread in multiple, typically two, dimensions. Such 2D spectra depend on time providing information about dynamics of the processes, both coherent and incoherent. The 2D view enables disentangling spectral features and dynamics which otherwise would be left “behind” the simple 1D spectra and thereby not visible in simpler low-dimensional representation. In this thesis, we focus on the dynamics and data acquisition of CMDS from five aspects: the excited state dynamics in CdSe QDs at 77 K studied via two-color 2D spectroscopy; the quantum beating of CdSe QDs using 2D electronic spectroscopy (2DES); quantum size effects of CdSe QDs in CMDS; electron-phonon coupling in perovskite nanocrystals, and sparse sampling in the CMDS implemented via the theory of compressed sensing. Specifically, we characterize the details of the relaxation dynamics of CdSe QDs through two-color 2D Spectroscopy. The beating 2D signal was analysed in terms of coherent LO-phonons of CdSe QDs. Besides, the correlation between the excited state energies and the size of CdSe QDs due to the quantum confinement is also analyzed at 77 K via the 2DES. The phonon coupling with excitons and free carriers in perovskite nanocrystals is characterized. Finally, the compressed sensing theory is used to reconstruct the fluorescence detected 2D spectra of the photosynthetic LH2 complexes

East meets west? Determinants of Chinese firms\u27 response to pressures towards international corporate governance standards

The diffusion of corporate governance standards globally has received special attention from researchers in an increasingly globalized economy. This topic is particularly significant in emerging economies as they encounter both economic pressures to adopt international governance standards and pressures to conform to local institutional resistance to change in governance. Drawing on multi-theoretical perspectives including agency theory, resource dependence theory and institutional theory, this study examines the role of CEO and board characteristics, ownership structure, prior firm performance, and firm’s selection of accounting standards and auditing firms in determining Chinese publicly listed firms’ responses to pressures to adopt international governance standards. This study finds that (1) Chinese publicly listed firms with better prior performance measured by ROA are more likely to be early adopters of international governance model; (2) in general, the antecedents of CEO and board characteristics are not significant predictors of firms’ adoption of international governance standards; (3) direct (ownership) and indirect links to Chinese government play significant roles in shaping firms’ governance standards and practice; and (4) firms’ ownership structure particularly proportion of tradable shares and presence of foreign ownership are significant predictors of firms’ corporate governance orientation, while ownership concentration is not. This research enriches the bodies of international corporate governance literature and contributes to institutional change literature by empirically testing how firms facing similar political pressure, functional pressure, and social pressure (Oliver, 1992) produce heterogeneous strategic responses in an emerging context. It also contributes practically to the development of government business policy and effective management of firm strategies in China

Pairwise Quantum Correlations for Superpositions of Dicke States

Pairwise correlation is really an important property for multi-qubit states. For the two-qubit X states extracted from Dicke states and their superposition states, we obtain a compact expression of the quantum discord by numerical check. We then apply the expression to discuss the quantum correlation of the reduced two-qubit states of Dicke states and their superpositions, and the results are compared with those obtained by entanglement of formation, which is a quantum entanglement measure.Comment: 17pages, 8 figures,slightly improved and slightly extended version,and added some reference

Optimal measurements to access classical correlations of two-qubit states

We analyze the optimal measurements accessing classical correlations in arbitrary two-qubit states. Two-qubit states can be transformed into the canonical forms via local unitary operations. For the canonical forms, we investigate the probability distribution of the optimal measurements. The probability distribution of the optimal measurement is found to be centralized in the vicinity of a specific von Neumann measurement, which we call the maximal-correlation-direction measurement (MCDM). We prove that for the states with zero-discord and maximally mixed marginals, the MCDM is the very optimal measurement. Furthermore, we give an upper bound of quantum discord based on the MCDM, and investigate its performance for approximating the quantum discord.Comment: 8 pages, 3 figures, version accepted by Phys. Rev.

Review of Financial Pressure

Different from foreign capital markets, china’s domestic capital markets are special, which also determines that the research on financial pressure starts from the reform of state-owned enterprises, and draws lessons from the relevant theories of financial risk and financial fraud, thus gradually forming a more diversified research results

Spatio-temporal Joint Modelling on Moderate and Extreme Air Pollution in Spain

Very unhealthy air quality is consistently connected with numerous diseases. Appropriate extreme analysis and accurate predictions are in rising demand for exploring potential linked causes and for providing suggestions for the environmental agency in public policy strategy. This paper aims to model the spatial and temporal pattern of both moderate and extremely poor PM10 concentrations (of daily mean) collected from 342 representative monitors distributed throughout mainland Spain from 2017 to 2021. We firstly propose and compare a series of Bayesian hierarchical generalized extreme models of annual maxima PM10 concentrations, including both the fixed effect of altitude, temperature, precipitation, vapour pressure and population density, as well as the spatio-temporal random effect with the Stochastic Partial Differential Equation (SPDE) approach and a lag-one dynamic auto-regressive component (AR(1)). Under WAIC, DIC and other criteria, the best model is selected with good predictive ability based on the first four-year data (2017--2020) for training and the last-year data (2021) for testing. We bring the structure of the best model to establish the joint Bayesian model of annual mean and annual maxima PM10 concentrations and provide evidence that certain predictors (precipitation, vapour pressure and population density) influence comparably while the other predictors (altitude and temperature) impact reversely in the different scaled PM10 concentrations. The findings are applied to identify the hot-spot regions with poor air quality using excursion functions specified at the grid level. It suggests that the community of Madrid and some sites in northwestern and southern Spain are likely to be exposed to severe air pollution, simultaneously exceeding the warning risk threshold