arXiv subjects

Yuan Ping Feng

Publications and source records attributed to Yuan Ping Feng.

64 records · Page 4Linked to original sources

arXiv2011

Adsorbate and defect effects on electronic and transport properties of gold nanotubes

First-principles calculations have been performed to study the effects of adsorbates (CO molecules and O atoms) and defects on electronic structures and transport properties of Au nanotubes (Au(5, 3) and Au(5, 5)). For CO adsorption, various adsorption sites of CO on the Au tubes were considered. The vibrational frequency of the CO molecule was found to be very different for two nearly degenerate stable adsorption configurations of Au(5, 3), implying the possibility of distinguishing these two configurations via measuring the vibrational frequency of CO in experiments. After CO adsorption, the conductance of Au(5, 3) decreases by 0.9G0 and the conductance of Au(5, 5) decreases by approximately 0.5G0. For O-adsorbed Au tubes, O atoms strongly interact with Au tubes, leading to around 2G0 of drop in conductance for both Au tubes. These results may have implications for Au-tube-based chemical sensing. When a monovacancy defect is present, we found that, for both tubes, the conductance decreases by around 1G0. Another type of defect arising from the adhesion of one Au atom is also considered. For this case, it is found that, for the Au(5, 3) tube, the defect decreases the conductance by nearly 1G0, whereas for Au(5, 5), the decrease in conductance is only 0.3G0.

cond-mat.mes-hall↗

arXiv2011

Strain effects on work functions of pristine and potassium-decorated carbon nanotubes

Strain dependence of electronic structures and work functions of both pristine and potassium doped (5,5) (armchair) and (9,0) (zigzag) carbon nanotubes (CNTs) has been thoroughly studied using first-principles calculations based on density functional theory (DFT). We found that for pristine cases, the uniaxial strain has strong effects on work functions of CNTs, and the responses of work functions of CNT (5,5) and (9,0) to the strain are distinctly different. When the strain changes from -10% to +10%, the work function of the CNT (5,5) increases monotonically from 3.95 eV to 4.57 eV, and the work function of the (9,0) varies between 4.27 eV and 5.24 eV in a complicated manner. When coated with potassium, for both CNTs, work functions can be lowered down by more than 2.0 eV, and the strain dependence of work functions changes drastically. Our studies suggested that the combination of chemical coating and tuning of strain may be a powerful tool for controlling work functions of CNTs, which in turn will be useful in future design of CNT-based electronic and field-emitting devices.

cond-mat.mtrl-sci↗

arXiv2011

Bandgap engineering of zigzag graphene nanoribbons by manipulating edge states via defective boundaries

One of severe limits of graphene nanoribbons (GNRs) in future applications is that zigzag GNRs (ZGNRs) are gapless, so cannot be used in field effect transistors (FETs). In this paper, using tight-binding approach and first principles method, we derived and proved a general edge (boundary) condition for the opening of a significant bandgap in ZGNRs with defective edge structures. The proposed semiconducting GNRs have some interesting properties including the one that they can be embedded and integrated in a large piece of graphene without the need of completely cutting them out. We also demonstrated a new type of high-performance all-ZGNR FET.

cond-mat.mes-hall↗

arXiv2011

Differential Conductance Anomaly in Superconducting Quantum Point Contacts

We present in this letter a theoretical analysis of the current-voltage (I-V) characteristics of a hybrid normal-superconducting device consisting of a quantum dot and two electrodes that can be either normal or superconducting. We show that voltage drops at two different contacts that have been regarded unimportant in literature play essential roles in the Andreev tunneling process when at least one of electrodes is superconducting. A differential-conductance-anomaly caused by the aforementioned voltage drops is predicted. We also propose a new spectroscopy method to measure the energy levels of a quantum dot as well as voltage drops at contacts between the quantum dot and the two leads. Our findings have potential applications for the next generation of electronic devices at nanoscale.

cond-mat.supr-con↗

arXiv2011

Electron tunneling through hybrid superconducting-normal quantum point contacts under microwave radiation

We present a theoretical analysis for photon-assisted electron tunneling through a hybrid superconducting-normal quantum point contact (QPC) consisting of a superconducting lead (S), a normal two-level quantum dot (N), and a normal lead (N). Using single particle non-equilibrium Green's function formalism, we incorporate Floquet basis in the Nambu space and solve the Green's function with finite matrix truncation to obtain the transport properties numerically. Based on the proposed method, we studied the effects of photon-induced single-level oscillations and quantum transition between two levels on the current-voltage (I-V) characteristics of a superconducting QPC. For the single level case, the main dc resonance in the I-V curve remains unchanged regardless of the frequency and amplitude of the radiation, and a series of secondary resonances due to multi-photon processes are present. When the transition between two levels ($ε_{1}$, $ε_{2}$) is taken into account and level oscillations are neglected, photons only have significant effects at Rabi resonance when $\hbarω=(ε_{2}-ε_{1})$. At Rabi resonance, the main dc resonance splits into two, and the separation between them is determined by the coupling strength of the two levels.

cond-mat.supr-con↗

arXiv2011

Two-dimensional Graphene Superlattice Made with Partial Hydrogenation

Electronic properties of two-dimensional graphene superlattice made with partial hydrogenation were thoroughly studied via Density Functional Tight Binding approach (DFTB) which incorporates the tight-binding method into the density functional formalism. The two-dimensional (2-d) pattern of hydrogen atoms on graphene was found to have great effects on electronic structures of graphene superlattice. In particular, the edges of the 2-d pattern, armchair or zigzag, are essential for the energy band gap opening, and the energy band gap sensitively depends on the shape, size, and the 2-d periodicity of the pattern. Based on these findings, we suggested that the 2-d graphene superlattice may be used in fabricating quantum dots and 2-d heterojunctions on graphene without the need for cutting or etching.

cond-mat.mes-hall↗

arXiv2011

Design of a low band gap oxide ferroelectric: Bi$_6$Ti$_4$O$_{17}$

A strategy for obtaining low band gap oxide ferroelectrics based on charge imbalance is described and illustrated by first principles studies of the hypothetical compound Bi$_6$Ti$_4$O$_{17}$, which is an alternate stacking of the ferroelectric Bi$_4$Ti$_3$O$_{12}$. We find that this compound is ferroelectric, similar to Bi$_4$Ti$_3$O$_{12}$ although with a reduced polarization. Importantly, calculations of the electronic structure with the recently developed functional of Tran and Blaha yield a much reduced band gap of 1.83 eV for this material compared to Bi$_4$Ti$_3$O$_{12}$. Therefore, Bi$_6$Ti$_4$O$_{17}$ is predicted to be a low band gap ferroelectric material.

cond-mat.mtrl-sci↗

arXiv2011

Theory of high energy optical conductivity and the role of oxygens in manganites

Recent experimental study reveals the optical conductivity of La$_{1-x}$Ca$_x$MnO$_3$ over a wide range of energy and the occurrence of spectral weight transfer as the system transforms from paramagnetic insulating to ferromagnetic metallic phase [Rusydi {\it et al.}, Phys. Rev. B {\bf 78}, 125110 (2008)]. We propose a model and calculation within the Dynamical Mean Field Theory to explain this phenomenon. We find the role of oxygens in mediating the hopping of electrons between manganeses as the key that determines the structures of the optical conductivity. In addition, by parametrizing the hopping integrals through magnetization, our result suggests a possible scenario that explains the occurrence of spectral weight transfer, in which the ferromagnatic ordering increases the rate of electron transfer from O$_{2p}$ orbitals to upper Mn$_{e_g}$ orbitals while simultaneously decreasing the rate of electron transfer from O$_{2p}$ orbitals to lower Mn$_{e_g}$orbitals, as temperature is varied across the ferromagnetic transition. With this scenario, our optical conductivity calculation shows very good quantitative agreement with the experimental data.

cond-mat.mtrl-sci↗

arXiv2008

Uniaxial strain on graphene: Raman spectroscopy study and bandgap opening

Graphene was deposited on a transparent and flexible substrate and tensile strain up to ~0.8% was loaded by stretching the substrate in one direction. Raman spectra of strained graphene show significant redshifts of 2D and G band (-27.8 cm^-1 and -14.2 cm^-1per 1% strain, respectively), because of the elongation of the carbon-carbon bonds. This indicates that uniaxial strain has been successfully applied on graphene. We also proposed that by applying uniaxial strain on graphene, tunable bandgap at K point can be realized. First principle calculations predicted a bandgap opening of ~300 meV for graphene under 1% uniaxial tensile strain. The strained graphene provides an alternative way to experimentally tune the bandgap of graphene, which would be more efficient and more controllable than other methods that are used to open bandgap in graphene. Moreover, our results suggest that the flexible substrate is ready for such strain process and Raman spectroscopy can be used as an ultra-sensitive method to determine the strain.

cond-mat.mtrl-sci↗

arXiv2006

First-principles calculation of the thermodynamics of In$_x$Ga$_{1-x}$N alloys: Effect of lattice vibrations

The thermodynamics properties of the wurtzite and zinc-blende \InGaN alloys are calculated using first-principles density-functional calculations. Special quasi-random structures are used to describe the disordered alloys, for $x= 1/4, 1/2$, and 3/4. The effect of lattice vibrations on the phase diagram, commonly omitted from semiconductor alloy phase diagram calculations, are included through first-principles calculations of phonon spectra. Inclusion of lattice vibrations leads to a large reduction in the order-disorder critical temperature ($\sim 29$% and $\sim 26$% for the wurtzite and zinc-blende structures, respectively) and changes the shape of the solubility and spinodal curve through changes in the entropies of the competing phases. Neglect of such effect produces significant errors in the phase diagrams of complex ordered semiconductor compounds. The critical temperature for phase separation is 1654 K (1771 K) for the wurtzite (zinc-blende) structures. The predicted phase diagrams are in agreement with experimental measurements on MOCVD \InGaN\ films.

cond-mat.mtrl-sci↗