Abstract: Abstract We report thermodynamic and neutron diffraction measurements on the magnetic ordering properties of the honeycomb lattice magnet YbCl3. We find YbCl3 exhibits a Néel type long-range magnetic order at the wavevector (0, 0, 0) below TN = 600 mK. This magnetic order is associated with a small sharp peak in heat capacity and most magnetic entropy release occurs above the magnetic ordering temperature. The magnetic moment lies in-plane, parallel to the monoclinic a-axis, whose magnitude mYb = 0.86(3) μB is considerably smaller than the expected fully ordered moment of 2.24 μB for the doublet crystal-field ground state. The magnetic ordering moment gradually increases with increasing magnetic field perpendicular to the ab-plane, reaching a maximum value of 1.6(2) μB at 4 T, before it is completely suppressed above ∼ 9 T. These results indicate the presence of strong quantum fluctuations in YbCl3. PubDate: 2020-12-31

Abstract: Abstract The present study carries out systematic thermodynamics analysis of Grain Boundary (GB) segregation and relaxation in Nano-Grained (NG) polycrystalline alloys. GB segregation and relaxation is an internal process towards thermodynamic equilibrium, which occurs naturally in NG alloys without any applied loads, causes deformation and generates internal stresses. The analysis comprehensively investigates the multiple coupling effects among chemical concentrations and mechanical stresses in GBs and grains. A hybrid approach of eigenstress and eigenstrain is developed herein to solve the multiple coupling problem. The analysis results indicate that the GB stress and grain stress induced by GB segregation and relaxation can be extremely high in NG alloys, reaching the GPa level, which play an important role in the thermal stability of NG alloys, especially via the coupling terms between stress and concentration. The present theoretic analysis proposes a novel criterion of thermal stability for NG alloys, which is determined by the difference in molar free energy between a NG alloy and its reference single crystal with the same nominal chemical composition. If the difference at a temperature is negative or zero, the NG alloy is thermal stable at that temperature, otherwise unstable. PubDate: 2020-12-30

Abstract: Abstract Magnetic skyrmion tubes and bobbers are two types of different nanoscale spin configurations that can coexist in nanostructures of chiral magnets. They are then proposed to be utilized as binary bits to build racetrack memory devices. The ability to manipulate the two magnetic objects controllably by current in nanostructures is the prerequisite to realize the device. Here, we demonstrate by numerical simulations that a magnetic bobber and a skyrmion tube can be transformed to each other using spin-polarized current in nanostripes with stepped shape. We also show such stepped nanostructures can be readily applied as the write head for the skyrmion-bobber-based racetrack memory. PubDate: 2020-12-30

Abstract: Abstract In classical machine learning, a set of weak classifiers can be adaptively combined for improving the overall performance, a technique called adaptive boosting (or AdaBoost). However, constructing a combined classifier for a large data set is typically resource consuming. Here we propose a quantum extension of AdaBoost, demonstrating a quantum algorithm that can output the optimal strong classifier with a quadratic speedup in the number of queries of the weak classifiers. Our results also include a generalization of the standard AdaBoost to the cases where the output of each classifier may be probabilistic. We prove that the query complexity of the non-deterministic classifiers is the same as those of deterministic classifiers, which may be of independent interest to the classical machine-learning community. Additionally, once the optimal classifier is determined by our quantum algorithm, no quantum resources are further required. This fact may lead to applications on near term quantum devices. PubDate: 2020-12-30

Abstract: Abstract The meridional circulation of the Sun, which is observed to be poleward at the surface, should have a return flow at some depth. Since large-scale flows like the differential rotation and the meridional circulation are driven by turbulent stresses in the convection zone, these flows are expected to remain confined within this zone. Current observational (based on helioseismology) and theoretical (based on dynamo theory) evidences point towards an equatorward return flow of the meridional circulation at the bottom of the convection zone. Assuming the mean values of various quantities averaged over turbulence to be axisymmetric, we study the large-scale flows in solar-like stars on the basis of a 2D mean field theory. Turbulent stresses in a rotating star can transport angular momentum, setting up a differential rotation. The meridional circulation arises from a slight imbalance between two terms which try to drive it in opposite directions: a thermal wind term (arising out of the higher efficiency of convective heat transport in the polar regions) and a centrifugal term (arising out of the differential rotation). To make these terms comparable, the poles of the Sun should be slightly hotter than the equator. We discuss the important role played by the meridional circulation in the flux transport dynamo model. The poloidal field generated by the Babcock-Leighton process at the surface is advected poleward, whereas the toroidal field produced at the bottom of the convection zone is advected equatorward. The fluctuations in the meridional circulation (with coherence time of about 30–40 yr) help in explaining many aspects of the irregularities in the solar cycle. Finally, we discuss how the Lorentz force of the dynamo-generated magnetic field can cause periodic variations in the large-scale flows with the solar cycle. PubDate: 2020-12-30

Abstract: Abstract In this work, we systematically studied the magnetic and transport properties of EuAg4As2 single crystals. It was found that the two antiferromagnetic transitions (TN1 = 10 K and TN2=15 K) were driven to lower temperatures by an applied magnetic field. Below TN1, two successive metamagnetic transitions were observed when a magnetic field was applied in the ab plane (H//ab-plane). For both H//ab and H//c, EuAg4As2 showed a positive, unexpectedly large magnetoresistance (up to 202%) in lower magnetic fields below TN1, and a large negative magnetoresistance (up to −78%) at high fields/intermediate temperatures, thus presenting potential applications in magnetic sensors. Finally, the magnetic phase diagrams of EuAg4As2 were constructed for both H//ab and H//c using the resistivity and magnetisation data. PubDate: 2020-12-29

Abstract: Abstract Stimulated adiabatic passage has been extensively studied to achieve robust and selective population transfer in quantum systems. Recently, the quantum-classic analogy has been rapidly developing and can be considered responsible for the implementation of the adiabatic transfer of sound energy in cavity chain systems. In this article, we investigate the adiabatic transfer of sound energy between two topological end states in the Su-Schrieffer-Heeger (SSH) cavity chain, which can be considered to be the acoustic analog of the quantum chirped-pulse excitation. The topological adiabatic passage in SSH cavity chain has two categories. When the single-cavity resonance frequencies on the sublattices A and B in the SSH cavity chain do not switch their spectrum positions, the topologically protected adiabatic evolution results in the returning passage of the sound excited in one end cavity. When a level crossing with single-cavity resonance frequencies on the sublattices A and B exhibits switch in the frequency spectrum, acoustic energy is observed to be topologically pumped between the two end cavities of the SSH chain. PubDate: 2020-12-25

Abstract: Abstract Surface instability of compliant film/substrate bilayers has raised considerable interests due to its broad applications such as wrinkle-driven surface renewal and antifouling, shape-morphing for camouflaging skins, and micro/nano-scale surface patterning control. However, it is still a challenge to precisely predict and continuously trace secondary bifurcation transitions in the nonlinear post-buckling region. Here, we develop lattice models to precisely capture the nonlinear morphology evolution with multiple mode transitions that occur in the film/substrate systems. Based on our models, we reveal an intricate post-buckling phenomenon involving successive flat-wrinkle-doubling-quadrupling-fold bifurcations. Pre-stretch and pre-compression of the substrate, as well as bilayer modulus ratio, can alter surface morphology of film/substrate bilayers. With high substrate pre-tension, hierarchical wrinkles emerge in the bilayer with a low modulus ratio, while a wrinkle-to-ridge transition occurs with a high modulus ratio. Besides, with moderate substrate pre-compression, the bilayer eventually evolves into a period-tripling mode. Phase diagrams based on neo-Hookean and Arruda-Boyce constitutions are drawn to characterize the influences of different factors and to provide an overall view of ultimate pattern formation. Fundamental understanding and quantitative prediction of the nonlinear morphological transitions of soft bilayer materials hold potential for multifunctional surface regulation. PubDate: 2020-12-03

Abstract: Abstract The three-body problem can be traced back to Newton in 1687, but it is still an open question today. Note that only a few periodic orbits of three-body systems were found in 300 years after Newton mentioned this famous problem. Although triple systems are common in astronomy, practically all observed periodic triple systems are hierarchical (similar to the Sun, Earth and Moon). It has traditionally been believed that non-hierarchical triple systems would be unstable and thus should disintegrate into a stable binary system and a single star, and consequently stable periodic orbits of non-hierarchical triple systems have been expected to be rather scarce. However, we report here one family of 135445 periodic orbits of non-hierarchical triple systems with unequal masses; 13315 among them are stable. Compared with the narrow mass range (only 10−5) in which stable “Figure-eight” periodic orbits of three-body systems exist, our newly found stable periodic orbits have fairly large mass region. We find that many of these numerically found stable non-hierarchical periodic orbits have mass ratios close to those of hierarchical triple systems that have been measured with astronomical observations. This implies that these stable periodic orbits of non-hierarchical triple systems with distinctly unequal masses quite possibly can be observed in practice. Our investigation also suggests that there should exist an infinite number of stable periodic orbits of non-hierarchical triple systems with distinctly unequal masses. Note that our approach has general meaning: in a similar way, every known family of periodic orbits of three-body systems with two or three equal masses can be used as a starting point to generate thousands of new periodic orbits of triple systems with distinctly unequal masses. PubDate: 2020-12-03

Abstract: Abstract We report the crystal structures and physical properties of trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8. Measurements of magnetization and electrical resistivity display contrasting behaviors in the two compounds. Nd4Ni3O10 shows a paramagnetic metallic behavior with a metal-to-metal phase transition (T*) at about 162 K, as revealed by both magnetic susceptibility and resistivity. Further magnetoresistance and Hall coefficient results show a negative magnetoresistance at low temperatures and the carrier type of Nd4Ni3O10 is dominated by hole-type charge carriers. The significant enhancement of Hall coefficient and resistivity below T* suggests that effective charge carrier density decreases when cooling through the transition temperature. In contrast, Nd4Ni3O8 shows an insulating behavior. In addition, this compound shows a paramagnetic behavior with the similar magnetic moment as that of Nd4Ni3O10 derived from the Curie-Weiss fitting. This may suggest that the magnetic moments in both systems are contributed by Nd3+ ions. By applying pressures up to about 49 GPa, the insulating behavior is still present and becomes even stronger under a high pressure. Our results suggest that the different Ni configurations (Ni1+/2+ or Ni2+/3+) and the changes of coordination environment of Ni sites may account for the contrasting behaviors in trilayer nickelates Nd4Ni3O10 and Nd4Ni3O8. PubDate: 2020-12-02

Abstract: Abstract Lithium niobate on insulator (LNOI) provides a platform for the fundamental physics investigations and practical applications of integrated photonics. However, as an indispensable building block of integrated photonics, lasers are in short supply. In this paper, erbium-doped LNOI laser in the 1550-nm band was demonstrated in microdisk cavities with high quality factors fabricated in batches by UV exposure, inductively coupled plasma reactive ion etching, and chemomechanical polishing. The threshold and conversion efficiency of the erbium-doped LNOI microdisk laser were measured to be lower than 1 mW and 6.5×10−5%, respectively. This work will benefit the development of integrated photonics based on LNOI. PubDate: 2020-12-02

Abstract: Abstract We propose a refined version of trans-Planckian censorship conjecture (TCC), which could be elaborated from the strong scalar weak gravity conjecture combined with some entropy bounds. In particular, no fine-tuning on the inflation model-building is required in the refined TCC, and it automatically passes the tests from those stringy examples that support the original TCC. Furthermore, our refined TCC could be consistent with hilltop eternal inflation. PubDate: 2020-11-26

Abstract: Abstract We make a full classification of scalar monomials built of the Riemann curvature tensor up to the quadratic order and of the covariant derivatives of the scalar field up to the third order. From the point of view of the effective field theory, the third or even higher order covariant derivatives of the scalar field are of the same importance as the higher curvature terms, and thus should be taken into account. Moreover, the higher curvature terms and the higher order derivatives of the scalar field are complementary to each other, of which novel ghostfree combinations may exist. We make a systematic classification of all the possible monomials, according to the numbers of the Riemann tensor and the higher derivatives of the scalar field in each monomial. A complete basis of monomials at each order is derived, of which the linear combinations may yield novel ghostfree Lagrangians. We also develop diagrammatic representations for the monomials, which may help to simplify the analysis. PubDate: 2020-11-24

Abstract: Abstract The energy level separation between the edge states in topological insulator quantum dots lies in the terahertz (THz) range. Quantum confinement ensures the nonuniformity of the energy level separation near the Dirac point. Based on these features, we propose that a topological insulator quantum dot array can be operated as an electrically pumped continuous-wave THz laser. The proposed device can operate at room temperature, with power exceeding 10 mW and quantum efficiency reaching ∼50%. This study may promote the usage of topological insulator quantum dots as an important source of THz radiation. PubDate: 2020-11-23