Dislocation induced energy dissipation in a tunable trilayer graphene resonator

In crystalline materials, the creation and modulation of dislocations are often associated with plastic deformation and energy dissipation. Here we report a study on the energy dissipation of a trilayer graphene ribbon resonator. The vibration of the ribbon generates cyclic mechanical loading to the graphene ribbon, during which mechanical energy is dissipated as heat. Measuring the quality factor of the graphene resonator provides a way to evaluate the energy dissipation. The graphene ribbon is integrated with silicon micro actuators, allowing its in-plane tension to be finely tuned. As we gradually increased the tension, we observed, in addition to the well-known resonance frequency increase, a large change in the energy dissipation. We propose that the dominating energy dissipation mechanism shifts over three regions. With small applied tension, the graphene is in elastic region, and the major energy dissipation is through graphene edge folding; as the tension increases, dislocations start to develop in the sample to gradually dominate the energy dissipation; finally, at large enough tension, graphene layers become decoupled and start to slide and cause friction, which induces the more severe energy dissipation. The generation and modulation of dislocations are modeled by molecular dynamics calculation and a method to count the energy loss is proposed and compared to the experiment.

X(3872) Production in Relativistic Heavy-Ion Collisions

Heavy ion collisions provide a unique opportunity to study the nature of X(3872) compared with electron-positron and proton-proton (antiproton) collisions. With the abundant charm pairs produced in heavy-ion collisions, the production of multicharm hadrons and molecules can be enhanced by the combination of charm and anticharm quarks in the medium. We investigate the centrality and momentum dependence of X(3872) in heavy-ion collisions via the Langevin equation and instant coalescence model (LICM). When X(3872) is treated as a compact tetraquark state, the tetraquarks are produced via the coalescence of heavy and light quarks near the quantum chromodynamic (QCD) phase transition due to the restoration of the heavy quark potential at $Trightarrow T_c$. In the molecular scenario, loosely bound X(3872) is produced via the coalescence of $D^0$-$bar D^{*0}$ mesons in a hadronic medium after kinetic freeze-out. The phase space distributions of the charm quarks and D mesons in a bulk medium are studied with the Langevin equation, while the coalescence probability between constituent particles is controlled by the Wigner function, which encodes the internal structure of the formed particle. First, we employ the LICM to explain both $D^0$ and $J/psi$ production as a benchmark. Then, we give predictions regarding X(3872) production. We find that the total yield of tetraquark is several times larger than the molecular production in Pb-Pb collisions. Although the geometric size of the molecule is huge, the coalescence probability is small due to strict constraints on the relative momentum between $D^0$ and $bar D^{*0}$ in the molecular Wigner function, which significantly suppresses the molecular yield.

Topological charge and spin pumping in a semiconductor nanowire

The adiabatic topological pumping is proposed by periodically modulating a semiconductor nanowire double-quantum-dot chain. We demonstrate that the quantized charge transport can be achieved by a nontrivial modulation of the quantum-dot well and barrier potentials. When the quantum-dot well potential is replaced by a time-dependent staggered magnetic field, the topological spin pumping can be realized by periodically modulating the barrier potentials and magnetic field. We also demonstrate that in the presence of Rashba spin-orbit interaction, the double-quantum-dot chain can be used to implement the topological spin pumping. However, the pumped spin in this case can have a quantization axis other than the applied magnetic field direction. Moreover, we show that all the adiabatic topological pumping are manifested by the presence of gapless edge states traversing the band gap as a function of time.

The weak decays of $Xi^{()}_{c}toXi$ in the light-front quark model

Without contamination from the final state interactions, the calculation of the branching ratios of semileptonic decays $Xi^{()}_{c}toXi+e^+ u_e$ may provide us more information about the inner structure of charmed baryons. Moreover, by studying those processes, one can better determine the form factors of $Xi_ctoXi$ which can be further applied to relevant estimates. In this work, we use the light-front quark model to carry out the computations where the three-body vertex functions for $Xi_c$ and $Xi$ are employed. To fit the new data of the Belle II, we re-adjust the model parameters and obtain $beta_{s[sq]}=1.07$ GeV which is 2.9 times larger than $beta_{sbar s}=0.366$ GeV. This value may imply that the $ss$ pair in $Xi$ constitutes a more compact subsystem. Furthermore, we also investigate the non-leptonic decays of $Xi^{()}_cto Xi$ which will be experimentally measured soon, so our model would be tested by consistency with the new data.

Topological states and interplay between spin-orbit and Zeeman interactions in a spinful Su-Schrieffer-Heeger nanowire

The interplay between the spin-orbit and Zeeman interactions acting on a spinful Su-Schrieffer-Heeger model is studied based on an InAs nanowire subjected to a periodic gate potential along the axial direction. It is shown that a nontrivial topological phase can be achieved by regulating the confining-potential configuration. In the absence of the Zeeman field, we prove that the topology of the chain is not affected by the Rashba spin-orbit interaction due to the persisting chiral symmetry. The energies of the edge modes can be manipulated by varying the magnitude and direction of the external magnetic field. Remarkably, the joint effect of the two spin-related interactions leads to novel edge states that appear in the gap formed by the anti-crossing of the bands of a finite spinful dimerized chain, and can be merged into the bulk states by tilting the magnetic-field direction.

Tetraquark state $X(6900)$ and the interaction between diquark and antidiquark

Recently LHCb declared a new structure $X(6900)$ in the final state di-$J/psi$ which is popularly regarded as a $cc$-$bar cbar c$ tetraquark state. %popularly. Within the Bethe-Salpeter (B-S) frame we study the possible $cc$-$bar cbar c$ bound states and the interaction between diquark ($cc$) and antidiquark ($bar cbar c$). In this work $cc$ ($bar cbar c$) is treated as a color anti-triplet (triplet) axial-vector so the quantum numbers of $cc$-$bar cbar c$ bound state are $0^+$, $1^+$ and $2^+$. Learning from the interaction in meson case and using the effective coupling we suggest the interaction kernel for diquark and antidiquark system. Then we deduce the B-S equations for different quantum numbers. Solving these equations numerically we find the spectra of some excited states can be close to the mass of $X(6900)$ when we assign appropriate values for parameter $kappa$ introduced in the interaction (kernel).We also briefly calculate the spectra of $bb$-$bar bbar b$ bound states. Future measurement of $bb$-$bar bbar b$ state will help us to determine the exact form of effective interaction.

Spatio-Temporal Data Mining for Aviation Delay Prediction

To accommodate the unprecedented increase of commercial airlines over the next ten years, the Next Generation Air Transportation System (NextGen) has been implemented in the USA that records large-scale Air Traffic Management (ATM) data to make air travel safer, more efficient, and more economical. A key role of collaborative decision making for air traffic scheduling and airspace resource management is the accurate prediction of flight delay. There has been a lot of attempts to apply data-driven methods such as machine learning to forecast flight delay situation using air traffic data of departures and arrivals. However, most of them omit en-route spatial information of airlines and temporal correlation between serial flights which results in inaccuracy prediction. In this paper, we present a novel aviation delay prediction system based on stacked Long Short-Term Memory (LSTM) networks for commercial flights. The system learns from historical trajectories from automatic dependent surveillance-broadcast (ADS-B) messages and uses the correlative geolocations to collect indispensable features such as climatic elements, air traffic, airspace, and human factors data along posterior routes. These features are integrated and then are fed into our proposed regression model. The latent spatio-temporal patterns of data are abstracted and learned in the LSTM architecture. Compared with previous schemes, our approach is demonstrated to be more robust and accurate for large hub airports.

Gross-Neveu Heisenberg criticality: dynamical generation of quantum spin Hall masses

We consider fermions on a honeycomb lattice supplemented by a spin invariant interaction that dynamically generates a quantum spin Hall insulator. This lattice model provides an instance of Gross-Neveu Heisenberg criticality, as realized for example by the Hubbard model on the honeycomb lattice. Using auxiliary field quantum Monte Carlo simulations we show that we can compute with unprecedented precision susceptibilities of the order parameter. In O(N) Gross-Neveu transitions, the anomalous dimension of the bosonic mode grows as a function of N such that in the large-N limit it is of particular importance to consider susceptibilities rather than equal time correlations so as to minimize contributions from the background. For the N=3 case, we obtain $1/ u=1.11(4)$, $eta_{phi}=0.80(9)$, and $eta_{psi}=0.29(2)$ for respectively the correlation length exponent, bosonic and fermionic anomalous dimensions.

Threshold effects as the origin of $Z_{cs}(4000)$, $Z_{cs}(4220)$ and $X(4700)$ observed in $B^+to J/psi phi K^+$

We investigate the $B^+to J/psi phi K^+$ decay via various rescattering diagrams. Without introducing genuine exotic resonances, it is shown that the $Z_{cs}(4000)$, $Z_{cs}(4220)$ and $X(4700)$ reported by the LHCb collaboration can be simulated by the $J/psi K^{*+}$, $psi^prime K^+$ and $psi^prime phi$ threshold cusps, respectively. These cusps are enhanced by some nearby triangle singularities. The $X(4685)$ with $J^P=1^+$ cannot be well simulated by the threshold effects in our model, which implies that it may be a genuine resonance.

Highly tunable quadruple quantum dot in a narrow bandgap semiconductor InAs nanowire

Quantum dots (QDs) made from semiconductors are among the most promising platforms for the developments of quantum computing and simulation chips, and have advantages over other platforms in high density integration and in compatibility to the standard semiconductor chip fabrication technology. However, development of a highly tunable semiconductor multiple QD system still remains as a major challenge. Here, we demonstrate realization of a highly tunable linear quadruple QD (QQD) in a narrow bandgap semiconductor InAs nanowire with fine finger gate technique. The QQD is studied by electron transport measurements in the linear response regime. Characteristic two-dimensional charge stability diagrams containing four groups of resonant current lines of different slopes are found for the QQD. It is shown that these current lines can be individually assigned as arising from resonant electron transport through the energy levels of different QDs. Benefited from the excellent gate tunability, we also demonstrate tuning of the QQD to regimes where the energy levels of two QDs, three QDs and all the four QDs are energetically on resonance, respectively, with the fermi level of source and drain contacts. A capacitance network model is developed for the linear QQD and the simulated charge stability diagrams based on the model show good agreements with the experiments. Our work presents a solid experimental evidence that narrow bandgap semiconductor nanowires multiple QDs could be used as a versatile platform to achieve integrated qubits for quantum computing and to perform quantum simulations for complex many-body systems.