arXiv subjects

Wei Zhu

Publications and source records attributed to Wei Zhu.

At least 361 records · Page 20Linked to original sources

arXiv2014

Detection of Interlayer Interaction in Few-layer Graphene through Landau Level Spectroscopy

We demonstrate that surface relaxation, which is insignificant in trilayer graphene, starts to manifest in Bernal-stacked tetralayer graphene. Bernal-stacked few-layer graphene has been investigated by analyzing its Landau level spectra through quantum capacitance measurements. We find that in trilayer graphene, the interlayer interaction parameters were similar to that of graphite. However, in tetralayer graphene, the hopping parameters between the bulk and surface bilayers are quite different. This shows a direct evidence for the surface relaxation phenomena. In spite of the fact that the Van der Waals interaction between the carbon layers is thought to be insignificant, we suggest that the interlayer interaction is an important factor in explaining the observed results and the symmetry-breaking effects in graphene sublattice are not negligible.

cond-mat.mtrl-sci↗

arXiv2014

High-Sensitivity Temperature Sensing Using an Implanted Single Nitrogen-Vacancy Center Array in Diamond

We presented a high-sensitivity temperature detection using an implanted single Nitrogen-Vacancy center array in diamond. The high-order Thermal Carr-Purcell-Meiboom-Gill (TCPMG) method was performed on the implanted single nitrogen vacancy (NV) center in diamond in a static magnetic field. We demonstrated that under small detunings for the two driving microwave frequencies, the oscillation frequency of the induced fluorescence of the NV center equals approximately to the average of the detunings of the two driving fields. On basis of the conclusion, the zero-field splitting D for the NV center and the corresponding temperature could be determined. The experiment showed that the coherence time for the high-order TCPMG was effectively extended, particularly up to 108 μs for TCPMG-8, about 14 times of the value 7.7 μs for thermal Ramsey method. This coherence time corresponded to a thermal sensitivity of 10.1 mK/Hz1/2. We also detected the temperature distribution on the surface of a diamond chip in three different circumstances by using the implanted NV center array with the TCPMG-3 method. The experiment implies the feasibility for using implanted NV centers in high-quality diamonds to detect temperatures in biology, chemistry, material science and microelectronic system with high-sensitivity and nanoscale resolution.

quant-ph↗

arXiv2014

Constraining the Oblateness of Kepler Planets

We use Kepler short cadence light curves to constrain the oblateness of planet candidates in the Kepler sample. The transits of rapidly rotating planets that are deformed in shape will lead to distortions in the ingress and egress of their light curves. We report the first tentative detection of an oblate planet outside of the solar system, measuring an oblateness of $0.22 \pm 0.11$ for the 18 $M_J$ mass brown dwarf Kepler 39b (KOI-423.01). We also provide constraints on the oblateness of the planets (candidates) HAT-P-7b, KOI-686.01, and KOI-197.01 to be < 0.067, < 0.251, and < 0.186, respectively. Using the Q'-values from Jupiter and Saturn, we expect tidal synchronization for the spins of HAT-P-7b, KOI-686.01 and KOI-197.01, and for their rotational oblateness signatures to be undetectable in the current data. The potentially large oblateness of KOI-423.01 (Kepler 39b) suggests that the Q'-value of the brown dwarf needs to be two orders of magnitude larger than that of the solar system gas giants to avoid being tidally spun-down.

astro-ph.EP↗

arXiv2014

Multi-mode effects in cavity QED based on a one-dimensional cavity array

We present a microscopic mo del of cavity quantum electro dynamics based on a one-dimensional coupled cavity array (1D CCA), where a sup er cavity (SC) is comp osed by a segment of the 1D CCA with relatively smaller couplings with the outsides. The single-photon scattering problem for the SC empty or with a two-level atom is investigated. We obtain the exact theoretical result on the transmission rate for our system, which predicts the transmission p eaks shall app ear near the eigen-energies. Our numerical results further prove that the SC is a well-defined multi-mo de cavity. When a two-level atom resonant with the SC lo cates at the antino de of the resonant mo de, the transmission sp ectrum shows a clear sign of vacuum Rabi splitting as exp ected. However, when the atom lo cates at the no de of the resonant mo de, we observe a deep valley in the transmission p eak, which can b e explained by the destructive interference of two transmission channels, one is the resonant mo de, while the other is arising from the atom coupling with the non-resonance mo des. The effect of non-resonance mo des on vacuum Rabi splitting is also analyzed.

quant-ph↗

arXiv2014

Emergent Chiral Spin Liquid: Fractional Quantum Hall Effect in a Kagome Heisenberg Model

The fractional quantum Hall effect (FQHE) realized in two-dimensional electron systems under a magnetic field is one of the most remarkable discoveries in condensed matter physics. Interestingly, it has been proposed that FQHE can also emerge in time-reversal invariant spin systems, known as the chiral spin liquid (CSL) characterized by the topological order and the emerging of the fractionalized quasiparticles. A CSL can naturally lead to the exotic superconductivity originating from the condense of anyonic quasiparticles. Although CSL was highly sought after for more than twenty years, it had never been found in a spin isotropic Heisenberg model or related materials. By developing a density-matrix renormalization group based method for adiabatically inserting flux, we discover a FQHE in a spin-$\frac{1}{2}$ isotropic kagome Heisenberg model. We identify this FQHE state as the long-sought CSL with a uniform chiral order spontaneously breaking time reversal symmetry, which is uniquely characterized by the half-integer quantized topological Chern number protected by a robust excitation gap. The CSL is found to be at the neighbor of the previously identified $Z_2$ spin liquid, which may lead to an exotic quantum phase transition between two gapped topological spin liquids.

cond-mat.str-el↗

arXiv2014

Applications of a nonlinear evolution equation I: the parton distributions in the proton

The parton distributions in the proton are evaluated dynamically using a nonlinear QCD evolution equation - the DGLAP equation with twist-4 (the GLR-MQ-ZSR) corrections - starting from a low scale $μ^2$, where the nucleon consists of valence quarks. We find that the negative nonlinear corrections can improve the perturbative stability of the QCD evolution equation at low $Q^2$. Our resulting parton distributions of the proton with four free parameters are compatible with the existing databases. We show that the sea quark distributions exhibit a positive and flattish behavior at small $x$ and low $Q^2$. This approach provides a powerful tool to connect the quark models of the hadron and various non-perturbative effects on them at scale $μ^2$ with the measured structure functions at high scale $Q^2>> μ^2$.

hep-ph↗

arXiv2014

Applications of a nonlinear evolution equation II: the EMC effect

The EMC effect is studied by using the GLR-MQ-ZSR equation with minimum number of free parameters, where the nuclear shadowing effect is a dynamical evolution result of the equation, and nucleon swelling and Fermi motion in the nuclear environment deform the input parton distributions. Parton distributions of both proton and nucleus are predicted in a unified framework. We show that the parton recombination as a higher twist correction plays an essential role in the evolution of parton distributions either of proton or nucleus. We find that the nuclear antishadowing contributes a part of enhancement of the ratio of the structure functions around $x\sim 0.1$, while the other part origins from the deformation of the nuclear valence quark distributions. We point out that the nuclear shadowing and antishadowing effects in the gluon distribution are not stronger than that in the quark distributions.

hep-ph↗

arXiv2014

Empirical Study of Simulated Two-planet Microlensing Event

We undertake the first study of two-planet microlensing models recovered from simulations of microlensing events generated by realistic multi-planet systems in which 292 planetary events including 16 two-planet events were detected from 6690 simulated light curves. We find that when two planets are recovered, their parameters are usually close to those of the two planets in the system most responsible for the perturbations. However, in one of the 16 examples, the apparent mass of both detected planets was more than doubled by the unmodeled influence of a third, massive planet. This fraction is larger than, but statistically consistent with, the roughly 1.5% rate of serious mass errors due to unmodeled planetary companions for the 274 cases from the same simulation in which a single planet is recovered. We conjecture that an analogous effect due to unmodeled stellar companions may occur more frequently. For seven out of 23 cases in which two planets in the system would have been detected separately, only one planet was recovered because the perturbations due to the two planets had similar forms. This is a small fraction (7/274) of all recovered single-planet models, but almost a third of all events that might plausibly have led to two-planet models. Still, in these cases, the recovered planet tends to have parameters similar to one of the two real planets most responsible for the anomaly.

astro-ph.EP↗

arXiv2014

Predictions for Microlensing Planetary Events from Core Accretion Theory

We conduct the first microlensing simulation in the context of planet formation model. The planet population is taken from the Ida & Lin core accretion model for $0.3M_\odot$ stars. With $6690$ microlensing events, we find for a simplified Korea Microlensing Telescopes Network (KMTNet) the fraction of planetary events is $2.9\%$ , out of which $5.5\%$ show multiple-planet signatures. The number of super-Earths, super-Neptunes and super-Jupiters detected are expected to be almost equal. Our simulation shows that high-magnification events and massive planets are favored by planet detections, which is consistent with previous expectation. However, we notice that extremely high-magnification events are less sensitive to planets, which is possibly because the 10 min sampling of KMTNet is not intensive enough to capture the subtle anomalies that occur near the peak. This suggests that while KMTNet observations can be systematically analyzed without reference to any follow-up data, follow-up observations will be essential in extracting the full science potential of very high-magnification events. The uniformly high-cadence observations expected for KMTNet also result in $\sim 55\%$ of all detected planets being non-caustic-crossing, and more low-mass planets even down to Mars-mass being detected via planetary caustics. We also find that the distributions of orbital inclinations and planet mass ratios in multiple-planet events agree with the intrinsic distributions.

astro-ph.EP↗

arXiv2014

Parton recombination effect in polarized parton distributions

Parton recombination corrections to the standard spin-dependent Altarelli-Parisi evolution equation are considered in a nonlinear evolution equation. The properties of this recombination equation and its relation with the spin-averaged form are discussed.

hep-ph↗

arXiv2013

A new modified Altarelli-Parisi evolution equation with parton recombination in proton

The coefficients of the nonlinear terms in a modified Altarelli-Parisi evolution equation with parton recombination are determined in the leading logarithmic ($Q^2$) approximation. The results are valid in the whole $x$ region and contain the translation $GG\to q\bar q$, which is inhibited in the double leading logarithmic approximation. The comparisons of the new evolution equation with the Gribov-Levin-Ryskin equation are presented.

hep-ph↗

arXiv2013

Properties of the gluon recombination functions

The gluon recombination functions in the twist-4 QCD evolution equations are studied at the leading logarithmic approximation using both the covariant and non-covariant methods. We justify that the infrared singularities in the twist-4 coefficient functions impede us to simply separate the QCD evolution kernels from the coefficient functions at the equivalent particle approximation. In particulary, we point out that the gluon recombination functions in the GLR-MQ evolution equation are unavailable. The methods avoiding the IR divergences are discussed, which can be used in the derivations of the evolution kernels and coefficient functions at the higher twist level.

hep-ph↗

arXiv2013

From EMC- and Cronin-effects to signals of quark-gluon plasma

The EMC- and Cronin-effects are explained by a unitarized evolution equation, where the shadowing and antishadowing corrections are dynamically produced by gluon fusions. For this sake, an alternative form of the GLR-MQ-ZRS equation is derived. The resulting integrated and unintegrated gluon distributions in proton and nuclei are used to analyze the contributions of the initial parton distributions to the nuclear suppression factor in heavy ion collisions. A simulation of the fractional energy loss is extracted from the RHIC and LHC data, where the contributions of the nuclear shadowing and antishadowing effects are considered. We find a rapid crossover from week energy loss to strong energy loss at a universal critical energy of gluon jet $E_c\sim 10 GeV$.

hep-ph↗

arXiv2013

Josephson junction on one edge of a two dimensional topological insulator affected by magnetic impurity

Current-phase relation in a Josephson junction formed by putting two s-wave superconductors on the same edge of a two dimensional topological insulator is investigated. We consider the case that the junction length is finite and magnetic impurity exists. The similarity and difference with conventional Josephson junction is discussed. The current is calculated in the semiconductor picture. Both the $2π$- and $4π$-period current-phase relations ($I_{2π}(ϕ), I_{4π}(ϕ)$) are studied. There is a sharp jump at $ϕ=π$ and $ϕ=2π$ for $I_{2π}$ and $I_{4π}$ respectively in the clean junction. For $I_{2π}$, the sharp jump is robust against impurity strength and distribution. However for $I_{4π}$, the impurity makes the jump at $ϕ=2π$ smooth. The critical (maximum) current of $I_{2π}$ is given and we find it will be increased by asymmetrical distribution of impurity.

cond-mat.mes-hall↗

arXiv2012

Are valence quarks rotating?

We suggest to compare the deep inelastic scattering structure functions measured in the unpolarized charged-lepton scattering off a transversely polarized proton and off a longitudinally polarized proton at larger Bjorken variable $x$, one may get a direct evidence concerning the valence quark orbital angular momentum in the proton. This possible rotating effect is estimated by using a proton-ensemble model.

hep-ph↗

arXiv2012

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal phase-transition ferromagnets

The magnetostructural coupling between the structural and the magnetic transition plays a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here, we demonstrate a possibility to obtain a stable magnetostructural coupling in a broad temperature window from 350 to 70 K, showing tunable magnetoresponsive effects, based on simultaneous manipulation of the phase stability and the magnetic structure by suitable chemical substitution of iron in MnNiGe. The resultant MnNiGe:Fe exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite, featuring (i) a large volume increase, (ii) a distinct magnetization change, (iii) small thermal hysteresis and (iv) a giant negative magnetocaloric effect. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

cond-mat.mtrl-sci↗

arXiv2010

Modeling and Simulation of Liquid Crystal Elastomers

We consider a continuum model describing the dynamic behavior of nematic liquid crystal elastomers (LCEs) and implement a numerical scheme to solve the governing equations. In the model, the Helmholtz free energy and Rayleigh dissipation are used, within a Lagrangian framework, to obtain the equations of motion. The free energy consists of both elastic and liquid crystalline contributions, each of which is a function of the material displacement and the orientational order parameter. The model gives dynamics for the material displacement, the scalar order parameter and the nematic director, the latter two of which correspond to the orientational order parameter tensor. Our simulations are carried out by solving the governing equations using an implicit-explicit scheme and the Chebyshev polynomial method. The simulations show that the model can successfully capture the shape changing dynamics of LCEs that have been observed in experiments, and also track the evolution of the order parameter tensor.

cond-mat.mtrl-sci↗

arXiv2010

Particle multiplicities at LHC and deviations from limiting fragmentation

The pseudorapidity density of charged particles produced at LHC collisions are predicted by using two complementary production mechanisms with a set of consistent integrated and unintegrated parton distributions. We discuss the limiting fragmentation hypothesis and its possible violation, and we compare our model with other partonic models.

hep-ph↗