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Search Results: 1 - 10 of 143732 matches for " Yu-Chang Chen "
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Effects of Isotope Substitution on Local Heating and Inelastic current in Hydrogen Molecular Junctions
Yu-Chang Chen
Physics , 2008, DOI: 10.1103/PhysRevB.78.233310
Abstract: Using first principle approaches, we investigate the effects of isotope substitution on the inelastic features in the hydrogen molecular junction. We observe thatlocal heating and inelastic current have significant isotope-substitution effects. Due to the contact characters, the energies of excited molecular vibrationsare inverse proportional to the square root of the mass. The heavier the molecule, the smaller the onset bias. In the $H_{2}$ and $D_{2}$ junctions, the heavier molecule has a smaller magnitude of electron-vibration interaction. Consequently, there is a crossing in the local temperature around $80 K$. In the HD junction, the electron-vibration interaction is enhanced by asymmetric distribution in mass. It leads to the largest discontinuity in the differential conductance and the most prominent heating in the HD junction. We predict that the junction instability is relevant to isotope substitution. The HD junction has the smallest breakdown voltage compared with the $H_{2}$ and $D_{2}$ junction.
Effect of electron-phonon scattering on shot noise in nanoscale junctions
Yu-Chang Chen,Massimiliano Di Ventra
Physics , 2005, DOI: 10.1103/PhysRevLett.95.166802
Abstract: We investigate the effect of electron-phonon inelastic scattering on shot noise in nanoscale junctions in the regime of quasi-ballistic transport. We predict that when the local temperature of the junction is larger than its lowest vibrational mode energy $eV_c$, the inelastic contribution to shot noise (conductance) increases (decreases) with bias as $V$ ($\sqrt{V}$). The corresponding Fano factor thus increases as $\sqrt{V}$. We also show that the inelastic contribution to the Fano factor saturates with increasing thermal current exchanged between the junction and the bulk electrodes to a value which, for $V>>V_c$, is independent of bias. A measurement of shot noise may thus provide information about the local temperature and heat dissipation in nanoscale conductors.
Shot Noise in Nanoscale Conductors From First Principles
Yu-Chang Chen,Massimiliano Di Ventra
Physics , 2002, DOI: 10.1103/PhysRevB.67.153304
Abstract: We describe a field-theoretic approach to calculate quantum shot noise in nanoscale conductors from first principles. Our starting point is the second-quantization field operator to calculate shot noise in terms of single quasi-particle wavefunctions obtained self-consistently within density functional theory. The approach is valid in both linear and nonlinear response and is particularly suitable in studying shot noise in atomic-scale conductors. As an example we study shot noise in Si atomic wires between metal electrodes. We find that shot noise is strongly nonlinear as a function of bias and it is enhanced for one- and two-Si wires due to the large contribution from the metal electrodes. For longer wires it shows an oscillatory behavior for even and odd number of atoms with opposite trend with respect to the conductance, indicating that current fluctuations persist with increasing wire length.
Information Security Management for Microsoft Windows Enterprise Environments
Yu-Chang Chang,Tzong-Chen Wu
Research Journal of Applied Sciences, Engineering and Technology , 2013,
Abstract: In this study, we introduce how these new tools improved the information security of users’ operating system and assisted enterprises or organizations to comply with ISMS and ISO standards; this study also used case studies to explain what improvement and advantages that these tools brought to users’ information security of these enterprises or organizations.
Seebeck coefficient of thermoelectric moleculat junction: First-principles calculations
Yu-Shen Liu,Yu-Chang Chen
Physics , 2008, DOI: 10.1103/PhysRevB.79.193101
Abstract: A first-principles approach is presented for the thermoelectricity in molecular junctions formed by a single molecule contact. The study investigates the Seebeck coefficient considering the source-drain electrodes with distinct temperatures and chemical potentials in a three-terminal geometry junction. We compare the Seebeck coefficient in the amino-substituted and unsubstituted butanethiol junction and observe interesting thermoelectric properties in the amino-substituted junction. Due to the novel states around the Fermi levels introduced by the amino-substitution, the Seebeck coefficient could be easily modulated by using gate voltages and biases. When the temperature in one of the electrodes is fixed, the Seebeck coefficient varies significantly with the temperature in the other electrode, and such dependence could be modulated by varying the gate voltages. As the biases increase, richer features in the Seebeck coefficient are observed, which are closely related to the transmission functions in the vicinity of the left and right Fermi levels.
Do Thermoelectric Materials in Nanojunctions Display Material Property or Junction Property?
Yu-Chang Chen,Yu-Shen Liu
Physics , 2009,
Abstract: The miniaturization of thermoelectric nanojunctions raises a fundamental question: do the thermoelectric quantities of the bridging materials in nanojunctions remain to display material properties or show junction properties? In order to answer this question, we investigate the Seebeck coefficient $S$ and the thermoelectric figure of merit $ZT$ especially in relation to the length characteristics of the junctions from the first-principles approaches. For $S$, the metallic atomic chains reveal strong length characteristics related to strong hybridization in the electronic structures between the atoms and electrodes, while the insulating molecular wires display strong material properties due to the cancelation of exponential scalings in the DOSs. For $ZT$, the atomic wires remain to show strong junction properties. However, the length chrematistics of the insulation molecular wires depend on a characteristic temperature $T_{0}= \sqrt{\beta/\gamma(l)}$ around 10K. When $T \ll T_{0}$, where the electron transport dominates the thermal current, the molecular junctions remain to show material properties. When $T \gg T_{0}$, where the phonon transport dominates the thermal current, the molecular junctions display junction properties.
Counting Statistics in Nanoscale Junctions
Yu-Shen Liu,Yu-Chang Chen
Physics , 2010, DOI: 10.1103/PhysRevB.83.035401
Abstract: We present first-principles calculations for moments of the current up to the third order in atomic-scale junctions. The quantum correlations of the current are calculated using the current operator in terms of the wave functions obtained self-consistently within the static density-functional theory. We investigate the relationships of the conductance, the second, and the third moment of the current for carbon atom chains of various lengths bridging two metal electrodes in the linear and nonlinear regimes. The conductance, the second-, and the third-order Fano factors exhibit odd-even oscillation with the number of carbon atoms due to the full and half filled {\pi}* orbital near the Fermi levels. The third-order Fano factor and the conductance are positively correlated.
Alkanethiol-Based Single-Molecule Transistors
Chun-Lan Ma,Diu Nghiem,Yu-Chang Chen
Physics , 2008, DOI: 10.1063/1.3043438
Abstract: The transport properties of unsubstituted and amino-substituted butanethiol molecules sandwiched between Au electrodes are investigated by using first-principles approaches. New states are observed around the Fermi levels when -NH2 is substituted for -H in the bridging butanethiol. The amino-substituted states lead to a sharp increase of the current which is credited to the resonant tunneling in the junction. We observe a novel conductance peak at VSD = 0.1 V and negative differential resistance (NDR) in a certain range of source-drain bias. In addition to the I-VSD characteristics, we also investigate the current as a function of gate voltage (I-VG) and find that, for a fixed source-drain bias (VSD = 0.01 V), the gate voltage can modulate the conductance by up to 30 times in the amino-substituted butanethiol junction. These I-VG characteristics suggest that the amino-substituted butanethiol molecular junction may be a promising candidate for field-effect transistors.
Theory of local heating in nanoscale conductors
Yu-Chang Chen,Michael Zwolak,Missimiliano Di Ventra
Physics , 2003, DOI: 10.1021/nl0348544
Abstract: We report first-principles calculations of local heating in nanoscale junctions formed by a single molecule and a gold point contact. Due to a larger heat dissipation, the single molecule heats up less than the gold point contact. We also find, at zero temperature, a threshold bias $V_{onset}$ of about 6 mV and 11 mV for the molecule and the point contact, respectively, is required to excite the smallest vibrational mode and generate heat. The latter estimate is in very good agreement with recent experimental results on the same system. At a given external bias $V$ below $V_{onset}$, heating becomes noticeable when the background temperature is on the order of $\sim e(V_{onset}-V)/k_{B}$. Above $V_{onset}$, local heating increases dramatically with increasing bias but is also considerably suppressed by thermal dissipation into the electrodes. The results provide a microscopic picture of current-induced heat generation in atomic-scale structures.
Current Streamline Flow on Current-induced Effects in Highly Asymmetric Molecular Junctions
Bailey C. Hsu,Allen Tseng,Yu-Chang Chen
Physics , 2011,
Abstract: From first-principles approaches, we illustrate that the current-induced forces and the selection rule for inelastic effects are highly relevant to the current density in an asymmetric molecular junction. The curved flow of current streamline around the asymmetric molecule may induce a net torque, which tends to rotate the benzene molecule, similar to the way a stream of water rotates a waterwheel. Thus, the Pt/benzene junction offers a practical system in the exploration of the possibility of atomic-scale motors. We also enumerate examples to show that the use of selection rule can lead to misjudgement of the importance of normal modes in the inelastic profiles when the detailed information about the current density is not considered.
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