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J. H. Wei

Publications and source records attributed to J. H. Wei.

11 recordsLinked to original sources
arXiv2011

The Difficulty of Gate Control in Molecular Transistors

The electrostatic gating effects on molecular transistors are investigated using the density functional theory (DFT) combined with the nonequilibrium Green's function (NEGF) method. When molecular energy levels are away from the Fermi energy they can be linearly shifted by the gate voltage, which is consistent with recent experimental observations [Nature 462, 1039 (2009)]. However, when they move near to the Fermi energy (turn-on process), the shifts become extremely small and almost independent of the gate voltage. The fact that the conductance may be beyond the gate control in the "ON" state will challenge the implementation of molecular transistors.

cond-mat.mes-hall↗
arXiv2011

Linear response theory for quantum open systems

Basing on the theory of Feynman's influence functional and its hierarchical equations of motion, we develop a linear response theory for quantum open systems. Our theory provides an effective way to calculate dynamical observables of a quantum open system at its steady-state, which can be applied to various fields of non-equilibrium condensed matter physics.

cond-mat.mes-hall↗
arXiv2010

Gapped Ferromagnetic Graphene Nanoribbons

We theoretically design a graphene-based all-organic ferromagnetic semiconductor by terminating zigzag graphene nanoribbons (ZGNRs) with organic magnets. A large spin-split gap with 100% spin polarized density of states near the Fermi energy is obtained, which is of potential application in spin transistors. The interplays among electron, spin and lattice degrees of freedom are studied using the first-principles calculations combined with fundamental model analysis. All of the calculations consistently demonstrate that although no d electrons existing, the antiferromagnetic π-πexchange together with the strong spin-lattice interactions between organic magnets and ZGNRs make the ground state ferromagnetic. The fundamental physics makes it possible to optimally select the organic magnets towards practical applications.

cond-mat.mtrl-sci↗
arXiv2009

Raman phonons of $α$-FeTe and Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$ single crystals

The polarized Raman scattering spectra of nonsuperconducting $α$-FeTe and of the newly discovered, As-free superconductor Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$ are measured at room temperature on single crystals. The phonon modes are assigned by combining symmetry analysis with first-principles calculations. In the parent compound $α$-FeTe, the A$_{1g}$ mode of the Te atom and the B$_{1g}$ mode of the Fe atom are observed clearly, while in superconducting Fe$_{1.03}$Se$_{0.3}$Te$_{0.7}$, only a softened Fe B$_{1g}$ mode can be seen. No electron-phonon coupling feature can be distinguished in the spectra of the two samples. By contrast, the spectra of the superconducting system show a slight enhancement below 300$cm^{-1}$, which may be of electronic origin.

cond-mat.supr-con↗
arXiv2009

Spin-Peierls-like transition in AFe$_2$As$_2$(A=Ba, Sr)

From first-principles density functional theory calculations combined with varying temperature Raman experiments, we show that AFe$_2$As$_2$ (A=Ba, Sr), the parent compound of the FeAs based superconductors of the new structural family, undergoes a spin-Peierls-like phase transition at low temperature. The coupling between the phonons and frustrated spins is proved to be the main cause of the structural transition from the tetragonal to orthorhombic phase. These results well explain the magnetic and structural phase transitions in AFe$_2$As$_2$(A=Ba, Sr) recently observed by neutron scattering.

cond-mat.supr-con↗
arXiv2008

Raman spectra in iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs

Raman spectra have been measured on iron-based quaternary CeO$_{1-x}$F$_x$FeAs and LaO$_{1-x}$F$_x$FeAs with varying fluorine doping at room temperatures. A group analysis has been made to clarify the optical modes. Based on the first principle calculations, the observed phonon modes can be assigned accordingly. In LaO$_{1-x}$F$_x$FeAs, the E$_g$ and A$_{1g}$ modes related to the vibrations of La, are suppressed with increasing F doping. However F doping only has a small effect on the E$_g$ and A$_{1g}$ modes of Fe and As. The Raman modes of La and As are absent in rare-earth substituted CeO$_{1-x}$F$_x$FeAs, and the E$_g$ mode of oxygen, corresponding to the in-plane vibration of oxygen, moves to around 450 cm$^{-1}$ and shows a very sharp peak. Electronic scattering background is low and electron-phonon coupling is not evident for the observed phonon modes. Three features are found above 500 cm$^{-1}$, which may be associated with multi-phonon process. Nevertheless it is also possible that they are related to magnetic fluctuations or interband transitions of d orbitals considering their energies.

cond-mat.supr-con↗
arXiv2007

Bias-induced insulator-metal transition in organic electronics

We investigate the bias-induced insulator-metal transition in organic electronics devices, on the basis of the Su-Schrieffer-Heeger model combined with the non-equilibrium Green's function formalism. The insulator-metal transition is explained with the energy levels crossover that eliminates the Peierls phase and delocalizes the electron states near the threshold voltage. This may account for the experimental observations on the devices that exhibit intrinsic bistable conductance switching with large on-off ratio.

cond-mat.mtrl-sci↗
arXiv2006

Conductance switching, hysteresis, and magnetoresistance in organic semiconductors

The controllability of charge transport through an organic molecular spin-valve system is theoretically investigated on the basis of a Su-Schrieffer-Heeger model combined with the non-equilibrium Green's function formalism. We show how the formation of polaron in the organic sub-structure leads to a hysteretic conductance switching, via sweeping either the bias voltage or the electrochemical potential. We further obtain an exponential dependence of the magnetoresistance as a function of the applied bias voltage. The implications of calculated results in relation to experiments and device applications are addressed and commented.

cond-mat.mtrl-sci↗
arXiv2005

Charge-transfer polaron induced negative differential resistance and giant magnetoresistance in organic spintronics: A Su-Schrieffer-Heeger model study

Combining the Su-Schrieffer-Heeger model and the non-equilibrium Green's function formalism, we investigate the negative differential resistance effect in organic spintronics at low temperature and interprete it with a self-doping picture. A giant negative magnetoresistance exceeding 300% is theoretically predicted as the results of the negative differential resistance effects.

cond-mat.mtrl-sci↗
arXiv2005

Trapping and hopping of bipolarons in DNA: Su-Schrieffer-Heeger model calculations

With the Su-Schrieffer-Heeger model involving the effects of solvent polarization and external electric field, we show that bipolaron maybe more stable than two polarons when a dication induced into a DNA stack. Under the high electric field, the dication can move quite a long distance through the DNA by a series of hopping process, partially losing its configuration instantaneously due to the nonadiabatic effects.

cond-mat.soft↗