Aharonov-Casher phase and persistent current in a polyacetylene ring

We investigate a polyacetylene ring in an axially symmetric, static electric field with a modified SSH Hamiltonian of a polyacetylene chain. An effective gauge potential of the single electron Hamiltonian due to spin-field interaction is obtained and it results in a Fr\"{o}hlich's type of superconductivity equivalent to the effect of travelling lattice wave. The total energy as well as the persistent current density are shown to be a periodic function of the flux of the gauge field embraced by the polyacetylene ring.Comment: 12 pages, 5 figure

Does the 2D Hubbard Model Really Show d-Wave Superconductivity?

Some issues concerning the question if the two-dimensional Hubbard model really show d-wave superconductivity are briefly discussed.Comment: Revtex, no figure

KINEMATICS OF UPPER LIMB AND TRUNK IN TENNIS PLAYERS USING SINGLE HANDED BACKHAND STROKES

INTRODUCTION: Tennis injuries caused by improper drives or overuse of the upper limb are very common in tennis. They may be prevented by using correct movement patterns. The purpose of this study was to analyze the threedimensional kinematics of the upper limb and trunk associated with performances of the single-handed backhand tennis stroke. METHODS: Six male national representatives performed tennis single-handed backhand drives in the laboratory. Sixteen reflective markers were placed on the subjects to represent the movements of the upper limb and trunk. They included one each on processus xiphoideus, incisura jugularis, 7th cervical vertebra, acromion, medialis and lateralis epicondylus, ulnar and radial styloid process, 2nd and 4th metacarpal heads, two anterior superior illiac spine and posterior superior illiac spine, and a triad markers on the upper limb. The markers’ positions were collected with the ExpertVision motion analysis system (Motion Analysis Corp., Santa Rosa, CA, USA) in order to estimate the joint movements of the shoulder, elbow, and wrist joints and the trunk. Ten trials were sampled for each subject. Eulerian angles were used to represent joint kinematics. RESULTS: The joint movements of the upper limbs were consistent while performing the tennis single-handed backhand stroke. The angular velocities of the clockwise rotation of the trunk and ulnar deviation of the wrist reached the maximum values at the middle acceleration phase and then decreased. In addition, the angular velocities of the shoulder external rotation, elbow flexion and wrist extension reached maximum values in the instant subsequent to impact. The results showed that the major movements of the shoulder joint were abduction/adduction (73.6±11.5°), flexion/extension (45.7±20.2°), and internal/external rotation (46.3±13.7°). The major movements of the elbow joint were pronation/supination (71.2±20.8°), and flexion/extension (35.3±14.4°). The results provide basic guidelines for tennis training and tennis evaluation. It is also helpful for the physician and therapist to assist in the diagnosis of sports injuries and to plan clinical treatment. CONCLUSIONS: In the acceleration phase, the trunk moves with the racket to increase angular momentum for the preparation of the impact. During this period, the movement of the shoulder is small. When the maximum angular velocities of the shoulder external rotation, elbow flexion and wrist extension occur in the instant prior to impact, they then immediately decrease. In this way, the hyperextension of the wrist joint, the cause of tennis elbow, may be prevented. The stability of the elbow joint and forearm is maintained by the contraction of the biceps and triceps

Fabrication and transport critical currents of multifilamentary MgB2/Fe wires and tapes

Multifilamentary MgB2/Fe wires and tapes with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. After annealing, we measured transport jc values up to 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T in a MgB2/Fe square wire with 7 filaments fabricated by two-axial rolling, and up to 5 * 104 A/cm2 at 4.2 K in 1 T in a MgB2/Fe tape with 7 filaments. For higher currents these multifilamentary wires and tapes quenched due to insufficient thermal stability of filaments. Both the processing routes and deformation methods were found to be important factors for fabricating multifilamentary MgB2 wires and tapes with high transport jc values.Comment: 13 pages, 7 figure

Cryptanalysis of the Hillery-Buzek-Berthiaume quantum secret-sharing protocol

The participant attack is the most serious threat for quantum secret-sharing protocols. We present a method to analyze the security of quantum secret-sharing protocols against this kind of attack taking the scheme of Hillery, Buzek, and Berthiaume (HBB) [Phys. Rev. A 59 1829 (1999)] as an example. By distinguishing between two mixed states, we derive the necessary and sufficient conditions under which a dishonest participant can attain all the information without introducing any error, which shows that the HBB protocol is insecure against dishonest participants. It is easy to verify that the attack scheme of Karlsson, Koashi, and Imoto [Phys. Rev. A 59, 162 (1999)] is a special example of our results. To demonstrate our results further, we construct an explicit attack scheme according to the necessary and sufficient conditions. Our work completes the security analysis of the HBB protocol, and the method presented may be useful for the analysis of other similar protocols.Comment: Revtex, 7 pages, 3 figures; Introduction modifie

Feedback first: the surprisingly weak effects of magnetic fields, viscosity, conduction, and metal diffusion on galaxy formation

Using high-resolution simulations with explicit treatment of stellar feedback physics based on the FIRE (Feedback in Realistic Environments) project, we study how galaxy formation and the interstellar medium (ISM) are affected by magnetic fields, anisotropic Spitzer-Braginskii conduction and viscosity, and sub-grid metal diffusion from unresolved turbulence. We consider controlled simulations of isolated (non-cosmological) galaxies but also a limited set of cosmological "zoom-in" simulations. Although simulations have shown significant effects from these physics with weak or absent stellar feedback, the effects are much weaker than those of stellar feedback when the latter is modeled explicitly. The additional physics have no systematic effect on galactic star formation rates (SFRs) . In contrast, removing stellar feedback leads to SFRs being over-predicted by factors of $\sim 10 -100$. Without feedback, neither galactic winds nor volume filling hot-phase gas exist, and discs tend to runaway collapse to ultra-thin scale-heights with unphysically dense clumps congregating at the galactic center. With stellar feedback, a multi-phase, turbulent medium with galactic fountains and winds is established. At currently achievable resolutions and for the investigated halo mass range $10^{10}-10^{13} M_{\odot}$, the additional physics investigated here (MHD, conduction, viscosity, metal diffusion) have only weak ($\sim10\%$-level) effects on regulating SFR and altering the balance of phases, outflows, or the energy in ISM turbulence, consistent with simple equipartition arguments. We conclude that galactic star formation and the ISM are primarily governed by a combination of turbulence, gravitational instabilities, and feedback. We add the caveat that AGN feedback is not included in the present work

Gossamer Superconductivity near Antiferromagnetic Mott Insulator in Layered Organic Conductors

Layered organic superconductors are on the verge of the Mott insulator. We use Gutzwiller variational method to study a Hubbard model including a spin exchange coupling term. The ground state is found to be a Gossamer superconductor at small on-site Coulomb repulsion U and an antiferromagnetic Mott insulator at large U, separated by a first order phase transition. Our theory is qualitatively consistent with major experiments reported in organic superconductors.Comment: 5 pages, 3 figure

A New Experimental Technique for Applying Impulse Tension Loading

This paper deals with a new experimental technique for applying impulse tension loads. Briefly, the technique is based on the use of pulsed-magnetic-driven tension loading. Electromagnetic forming (EMF) can be quite effective in increasing the forming limits of metal sheets, such as aluminium and magnesium alloys. Yet, why the forming limit is increased is still an open question. One reason for this is the difficulty to let forming proceed on a certain influence monotonically: the main phenomena causing this increase in formability are considered to due to “body force” effect, inertia effect, changes in strain rate sensitivity. In this study, an impulse tension loading setup is presented. “Body force” effect and strain rate, which are known to be the two key factors leading to higher formability, can now be separated freely by our designed device. Reproducible and adjustable loading rate (80s-1~3267s-1) can be achieved by adjusting the discharge voltage and capacitance. The relation between the discharge voltage and strain rate was obtained with the help of finite element calculations and high-camera measurement results. The results of an exploratory experiment carried out on the designed device are presented for aluminum alloy AA5052 sheet. It shows that this technique could be used to study the dynamic response of sheets

Exploiting Cognitive Structure for Adaptive Learning

Adaptive learning, also known as adaptive teaching, relies on learning path recommendation, which sequentially recommends personalized learning items (e.g., lectures, exercises) to satisfy the unique needs of each learner. Although it is well known that modeling the cognitive structure including knowledge level of learners and knowledge structure (e.g., the prerequisite relations) of learning items is important for learning path recommendation, existing methods for adaptive learning often separately focus on either knowledge levels of learners or knowledge structure of learning items. To fully exploit the multifaceted cognitive structure for learning path recommendation, we propose a Cognitive Structure Enhanced framework for Adaptive Learning, named CSEAL. By viewing path recommendation as a Markov Decision Process and applying an actor-critic algorithm, CSEAL can sequentially identify the right learning items to different learners. Specifically, we first utilize a recurrent neural network to trace the evolving knowledge levels of learners at each learning step. Then, we design a navigation algorithm on the knowledge structure to ensure the logicality of learning paths, which reduces the search space in the decision process. Finally, the actor-critic algorithm is used to determine what to learn next and whose parameters are dynamically updated along the learning path. Extensive experiments on real-world data demonstrate the effectiveness and robustness of CSEAL.Comment: Accepted by KDD 2019 Research Track. In Proceedings of the 25th ACM SIGKDD International Conference on Knowledge Discovery & Data Mining (KDD'19