arXiv subjects

L. Cheng

Publications and source records attributed to L. Cheng.

At least 37 records · Page 2Linked to original sources

arXiv2015

Graphdiyne: a two-dimensional thermoelectric material with high figure of merit

As a new carbon allotrope, the recently fabricated graphdiyne has attracted much attention due to its interesting two-dimensional character. Here we demonstrate by multiscale computations that, unlike graphene, graphdiyne has a natural band gap, and simultaneously possess high electrical conductivity, large Seebeck coefficient, and low thermal conductivity. At a carrier concentration of 2.74*10^11/cm^2 for holes and 1.62*10^11/cm^2 for electrons, the room temperature ZT value of graphdiyne can be optimized to 3.0 and 4.8, respectively, which makes it an ideal system to realize the concept of "phonon-glass and electron-crystal" in the thermoelectric community.

cond-mat.mes-hall↗

arXiv2015

Enhanced thermoelectric performance of carbon nanotubes at elevated temperature

The electronic and transport properties of (10, 0) single-walled carbon nanotube are studied by performing the first-principles calculations and semi-classical Boltzmann theory. It is found that the (10, 0) tube exhibits considerably large Seebeck coefficient and electrical conductivity which is highly desirable for good thermoelectric materials. Together with the lattice thermal conductivity predicted by non-equilibrium molecular dynamics simulations, the room temperature ZT value of (10, 0) tube is estimated to be 0.15 for p-type carriers. Moreover, the ZT value exhibits strong temperature dependence and can be reached to 0.77 at 1000 K. Such ZT value can be further enhanced to as high as 1.7 by isotope substitution and chemisorptions of hydrogen on the tube.

cond-mat.mes-hall↗

arXiv2014

Correlated Source Coded Sequences with Compromised Channel and Source Symbols using Shannon's Cipher System

Correlated sources are present in communication systems where protocols ensure that there is some predetermined information for sources to transmit. Here, two correlated sources across a channel with eavesdroppers are investigated, and conditions for perfect secrecy when some channel information and some source data symbols (the predetermined information) have been wiretapped are determined. The adversary in this situation has access to more information than if a link is wiretapped only and can thus determine more about a particular source. This scenario caters for an application where the eavesdropper has access to some preexisting information. We provide bounds for the channel and key rates for this scenario. Further, we provide a method to reduce the key lengths required for perfect secrecy.

cs.IT↗

arXiv2014

Radiation fields by intermediate-age stellar populations with binaries as ionizing sources of H II regions

Radiation fields emitted by O, B-type stars or young stellar populations (SPs) are generally considered as significant central ionizing sources (CISs) of classic H II regions. In our previous studies, we show that the inclusion of binary interactions in stellar population synthesis models can significantly increase the ultraviolet spectrum hardness and the number of ionizing photons of intermediate-age (IA, 7<~log(t/yr)<~8) SPs. In this work, we present photoionization models of H II regions ionized by radiation fields emitted by IA SPs, and show that radiation fields of IA SPs, including binary systems, are in theory possible candidates of significant CISs of classic H II regions. When radiation fields of IA SPs comprising binary systems are used as the CISs of classic H II regions, the theoretical strengths of a number of lines (such as [O III]4959', [S II]6716', etc.), weaker than observations, are raised; the border /selection-criterion lines, between star-forming galaxies and AGNs in the diagnostic diagrams (for example, [N II]6583/Ha~[O III]5007/Hb), move into the region occupied originally by AGNs; and He II 1640 line, observed in Lyman break and high-redshift gravitationally lensed galaxies, also can be produced.

astro-ph.SR↗

arXiv2014

Theoretical study of the thermoelectric properties of SiGe nanotubes

The thermoelectric properties of two typical SiGe nanotubes are investigated using a combination of density functional theory, Boltzmann transport theory, and molecular dynamics simulations. Unlike carbon nanotubes, these SiGe nanotubes tend to have gear-like geometry, and both the (6, 6) and (10, 0) tubes are semiconducting with direct band gaps. The calculated Seebeck coefficients as well as the relaxation time of these SiGe nanotubes are significantly larger than those of bulk thermoelectric materials. Together with smaller lattice thermal conductivity caused by phonon boundary and alloy scattering, these SiGe nanotubes can exhibit very good thermoelectric performance. Moreover, there are strong chirality and temperature dependence of the ZT values, which can be optimized to 4.9 at room temperature and further enhanced to 5.4 at 400 K for the armchair (6, 6) tube.

cond-mat.mtrl-sci↗

arXiv2014

Phosphorene nanoribbon as a promising candidate for thermoelectric applications

In this work, the electronic properties of phosphorene nanoribbons with different width and edge configurations are studied by using density functional theory. It is found that the armchair phosphorene nanoribbons are semiconducting while the zigzag nanoribbons are metallic. The band gaps of armchair nanoribbons decrease monotonically with increasing ribbon width. By passivating the edge phosphorus atoms with hydrogen, the zigzag series also become semiconducting, while the armchair series exhibit a larger band gap than their pristine counterpart. The electronic transport properties of these phosphorene nanoribbons are then investigated using Boltzmann theory and relaxation time approximation. We find that all the semiconducting nanoribbons exhibit very large values of Seebeck coefficient and can be further enhanced by hydrogen passivation at the edge. Taking armchair nanoribbon with width N=7 as an example, we calculate the lattice thermal conductivity with the help of phonon Boltzmann transport equation. Due to significantly enhanced Seebeck coefficient and decreased thermal conductivity, the phosphorene nanoribbon exhibit a very high figure of merit (ZT value) of 4.0 at room temperature, which suggests its appealing thermoelectric applications.

cond-mat.mes-hall↗

arXiv2014

MoS2 nanoribbons as promising thermoelectric materials

The thermoelectric properties of MoS2 armchair nanoribbons with different width are studied by using first-principles calculations and Boltzmann transport theory, where the relaxation time is predicted from deformation potential theory. Due to the dangling bonds at the armchair edge, there is obvious structure reconstruction of the nanoribbons which plays an important role in governing the electronic and transport properties. The investigated armchair nanoribbons are found to be semiconducting with indirect gaps, which exhibit interesting width-dependent oscillation behavior. The smaller gap of nanoribbon with width N = 4 leads to a much larger electrical conductivity at 300 K, which outweighs the relatively larger electronic thermal conductivity when compared with those of N = 5, 6. As a results, the room temperature ZT values can be optimized to 2.7 (p-type) and 2.0 (n-type), which significantly exceed the performance of most laboratory results reported in the literature.

cond-mat.mes-hall↗

arXiv2014

Effects of van der Waals interactions and quasiparticle corrections on the electronic and transport properties of Bi2Te3

We present a theoretical study of the structural, electronic and transport properties of bulk Bi2Te3 within density functional theory taking into account the van der Waals interactions (vdW) and the quasiparticle self-energy corrections. It is found that the optB86b-vdW functional can well reproduce the experimental lattice constants and interlayer distances for Bi2Te3. Based on the fully optimized structure, the band structure of Bi2Te3 is obtained from first-principles calculations with the GW approximation and the Wannier function interpolation method. The global band extrema are found to be off the high-symmetry lines, and the real energy band calculated is in good agreement with that measured experimentally. In combination with the Boltzmann theory, the GW calculations also give accurate prediction of the transport properties, and the calculated thermoelectric coefficients of Bi2Te3 almost coincide with the experimental data.

cond-mat.mtrl-sci↗

arXiv2011

YBCO-buffered NdBCO film with higher thermal stability in seeding REBCO Growth

In this work, we report a strengthened superheating effect caused by a buffering YBa2Cu3Oy (Y123 or YBCO) layer in the Nd1+xBa2-xCu3O7-y (Nd123 or NdBCO) thin film with MgO substrate (i.e., NdBCO/YBCO/MgO thin film). In the cold-seeding melt-textured (MT) growth, the NdBCO/YBCO/MgO film presented an even higher superheating level, about 20 °C higher than that of non-buffered NdBCO film (i.e., NdBCO/MgO film). Using this NdBCO/YBCO/MgO film as seeds and undergoing a maximum processing temperature (Tmax) up to 1120 °C, we succeeded in growing various RE1+xBa2-xCu3O7-y (REBCO, RE=rare elements) bulk superconductors, including Gd1+xBa2-xCu3O7-y (GdBCO), Sm1+xBa2-xCu3O7-y (SmBCO) and NdBCO that have high peritectic temperatures (Tp). The pole figure (X-Ray ϕ-scan) measurement reveals that the NdBCO/YBCO/MgO film has better in-plane alignment than the NdBCO/MgO film, indicating that the induced intermediate layer improves the crystallinity of the NdBCO film, which could be the main origin of the enhanced thermal stability. In short, possessing higher thermal stability and enduring a higher Tmax in the MT process, the NdBCO/YBCO/MgO film is beneficial to the growth of bulk superconductors in two aspects: (1) broad application for high-Tp REBCO materials; (2) effective suppression against heterogeneous nucleation, which is of great assistance in growing large and high-performance REBCO crystals.

cond-mat.supr-con↗

arXiv2010

Multi-seeded melt growth (MSMG) of bulk Y-Ba-Cu-O using thin-film seeds

Y-Ba-Cu-O (YBCO) and Sm-Ba-Cu-O (SmBCO) thin films have been used for the first time as heterogeneous seeds to multi-seed successfully the melt growth of bulk YBCO in a multi-seeded melt growth (MSMG) process. The use of thin film seeds, which may be prepared with highly controlled orientation (i.e. with a well-defined a-b plane and precisely known a-direction), is based on their superheating properties and reduces significantly contamination of the bulk sample by the seed material. A variety of grain boundaries were obtained by varying the angle between the seeds. Microstructural studies indicate that the extent of residual melt deposited at the grain boundary decreases with increasing grain boundary contact angle. It is established that the growth front proceeds continuously at the (110)/(110) grain boundary without trapping liquid, which leads to the formation of a clean grain boundary.

cond-mat.supr-con↗

arXiv2009

"Listening" to the spin noise of conduction electrons in bulk n:GaAs

We report a comprehensive study of stochastic electron spin fluctuations -- spin noise -- in lightly doped ($n$-type) bulk GaAs, which are measured using sensitive optical magnetometry based on off-resonant Faraday rotation. Frequency spectra of electron spin noise are studied as a function of electron density, magnetic field, temperature, probe laser wavelength and intensity, and interaction volume. Electron spin lifetimes $τ_s$ are inferred from the width of the spin noise spectra, and are compared with direct measurements of $τ_s$ using conventional Hanle effect methods. Both methods reveal a strong and similar dependence of $τ_s$ on the wavelength and intensity of the probe laser, highlighting the undesired influence of sub-bandgap absorption effects on the nominally `non-perturbative' spin noise measurements. As a function of temperature, the spin noise power increases approximately linearly from 1.5 K to 30 K, as expected for degenerate electrons obeying Fermi-Dirac statistics, but with an additional zero-temperature offset. Finally, as the cross-sectional area of the probe laser shrinks and fewer electrons are probed, the measured Faraday rotation fluctuations due to electron spin noise are shown to increase, as expected.

cond-mat.mes-hall↗

arXiv2008

Viscous Effects on Elliptic Flow and Shock Waves

Fast thermalization and a strong buildup of elliptic flow of QCD matter as found at RHIC are understood as the consequence of perturbative QCD (pQCD) interactions within the 3+1 dimensional parton cascade BAMPS. The main contributions stem from pQCD bremsstrahlung $2 \leftrightarrow 3 $ processes. By comparing to Au+Au data of the flow parameter $v_2$ as a function of participation number the shear viscosity to entropy ratio is dynamically extracted, which lies in the range of 0.08 and 0.2, depending on the chosen coupling constant and freeze out condition. Furthermore, first simulations on the temporal propagation of dissipative shock waves are given. The cascade can either simulate true ideal shocks as well as initially diluted, truely viscous shocks, depending on the employed cross sections or mean free path, respectively.

hep-ph↗

arXiv2006

Low relaxation rate in a low-Z alloy of iron

The longest relaxation time and sharpest frequency content in ferromagnetic precession is determined by the intrinsic (Gilbert) relaxation rate \emph{$G$}. For many years, pure iron (Fe) has had the lowest known value of $G=\textrm{57 Mhz}$ for all pure ferromagnetic metals or binary alloys. We show that an epitaxial iron alloy with vanadium (V) possesses values of $G$ which are significantly reduced, to 35$\pm$5 Mhz at 27% V. The result can be understood as the role of spin-orbit coupling in generating relaxation, reduced through the atomic number $Z$.

cond-mat.mtrl-sci↗

arXiv2006

Accretion Disk Temperatures and Continuum Colors in QSOs

Accretion disks around supermassive black holes are widely believed to be the dominant source of the optical-ultraviolet continuum in many classes of active galactic nuclei (AGN). We study here the relationship between the continuum colors of AGN and the characteristic accretion disk temperature (T_max). Based on NLTE models of accrection disks in AGN computed as described by Hubeny et al. (2000), we find that continuum intensity ratios for several pairs of wavelengths between 1350 and 5100 A should show a trend of bluer colors for higher T_max, notwithstanding random disk inclinations. We compare this theoretical expectation with observed colors of QSOs in the Sloan Digital Sky Survey,deriving black hole mass and thence T_max from the width of the Mg II broad emission line. The observed colors generally do not show the expected trend and in some cases show a reverse trend of redder colors with increasing T_max. The cause of this discrepancy does not appear to be dust reddening or galaxy contamination but may relate to the accretion rate, as the offset objects are accreting above ~30 % of the Eddington limit. The derived disk temperature depends primarily on line width, with little or no dependence on luminosity.

astro-ph↗

arXiv2006

Low Ghz loss in sputtered epitaxial Fe

We show that sputtered, pure epitaxial iron films can have high-frequency loss as low as, or lower than, any known metallic ferromagnetic heterostructure. Minimum 34 Ghz ferromagnetic resonance (FMR) linewidths of 41 Oe are demonstrated, some ~ 5-10 % lower than the previous minimum reported for molecular beam epitaxially (MBE) deposited Fe. Intrinsic and extrinsic damping have been separated over 0-40 Ghz, giving a lower bound for intrinsic LL(G) relaxation rates of lambda or G = 85 MHz (alpha = 0.0027) and extrinsic 50 Mhz. Swept frequency measurements indicate the potential for integrated frequency domain devices with Q>100 at 30-40 Ghz.

cond-mat.mtrl-sci↗

arXiv2005

Kondo resonances and anomalous gate dependence of electronic conduction in single-molecule transistors

We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.

cond-mat.mes-hall↗

arXiv2004

Inelastic electron tunneling via molecular vibrations in single-molecule transistors

In single-molecule transistors, we observe inelastic cotunneling features that correspond energetically to vibrational excitations of the molecule, as determined by Raman and infrared spectroscopy. This is a form of inelastic electron tunneling spectroscopy of single molecules, with the transistor geometry allowing in-situ tuning of the electronic states via a gate electrode. The vibrational features shift and change shape as the electronic levels are tuned near resonance, indicating significant modification of the vibrational states. When the molecule contains an unpaired electron, we also observe vibrational satellite features around the Kondo resonance.

cond-mat.mes-hall↗

arXiv2004

Precessional dynamics of elemental moments in a ferromagnetic alloy

We demonstrate an element-specific measurement of magnetization precession in a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19. Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short magnetic field pulses, is used to measure free magnetization oscillations up to 2.6 GHz with elemental specificity and a rotational resolution of < 2 deg. Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin film are found to precess together at all frequencies, coupled in phase within instrumental resolution of 90 ps.

cond-mat.mtrl-sci↗