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Second quantization of a covariant relativistic spacetime string in Steuckelberg–Horwitz–Piron theory
Michael Suleymanov, Lawrence Horwitz, Asher Yahalom
Front. Phys. . 2017, 12 (3 ): 121103-.
https://doi.org/10.1007/s11467-017-0666-x
A relativistic 4D string is described in the framework of the covariant quantum theory first introduced by Stueckelberg [Helv. Phys. Acta 14, 588 (1941)], and further developed by Horwitz and Piron [Helv. Phys. Acta 46, 316 (1973)], and discussed at length in the book of Horwitz [Relativistic Quantum Mechanics, Springer (2015)]. We describe the space-time string using the solutions of relativistic harmonic oscillator [J. Math. Phys. 30, 66 (1989)]. We first study the problem of the discrete string, both classically and quantum mechanically, and then turn to a study of the continuum limit, which contains a basically new formalism for the quantization of an extended system. The mass and energy spectrum are derived. Some comparison is made with known string models.
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Role of disorder in determining the vibrational properties of mass-spring networks
Yunhuan Nie, Hua Tong, Jun Liu, Mengjie Zu, Ning Xu
Front. Phys. . 2017, 12 (3 ): 126301-.
https://doi.org/10.1007/s11467-017-0668-8
By introducing four fundamental types of disorders into a two-dimensional triangular lattice separately, we determine the role of each type of disorder in the vibration of the resulting mass-spring networks. We are concerned mainly with the origin of the boson peak and the connection between the boson peak and the transverse Ioffe–Regel limit. For all types of disorders, we observe the emergence of the boson peak and Ioffe–Regel limits. With increasing disorder, the boson peak frequency ωBP , transverse Ioffe–Regel frequency ω I R T , and longitudinal Ioffe–Regel frequency ω I R L all decrease. We find that there are two ways for the boson peak to form: developing from and coexisting with (but remaining independent of) the transverse van Hove singularity without and with local coordination number fluctuation. In the presence of a single type of disorder, ω I R T ≥ ω B R , and ω I R T ≈ ω B P only when the disorder is sufficiently strong and causes spatial fluctuation of the local coordination number. Moreover, if there is no positional disorder, ω I R T ≈ ω I R L . Therefore, the argument that the boson peak is equivalent to the transverse Ioffe–Regel limit is not general. Our results suggest that both local coordination number and positional disorder are necessary for the argument to hold, which is actually the case for most disordered solids such as marginally jammed solids and structural glasses. We further combine two types of disorders to cause disorder in both the local coordination number and lattice site position. The density of vibrational states of the resulting networks resembles that of marginally jammed solids well. However, the relation between the boson peak and the transverse Ioffe–Regel limit is still indefinite and condition-dependent. Therefore, the interplay between different types of disorders is complicated, and more in-depth studies are required to sort it out.
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Quantum transport in topological semimetals under magnetic fields
Hai-Zhou Lu,Shun-Qing Shen
Front. Phys. . 2017, 12 (3 ): 127201-.
https://doi.org/10.1007/s11467-016-0609-y
Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topological semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.
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Topological nodal line semimetals predicted from first-principles calculations
Rui Yu,Zhong Fang,Xi Dai,Hongming Weng
Front. Phys. . 2017, 12 (3 ): 127202-.
https://doi.org/10.1007/s11467-016-0630-1
Topological semimetals are newly discovered states of quantum matter, which have extended the concept of topological states from insulators to metals and attracted great research interest in recent years. In general, there are three kinds of topological semimetals, namely Dirac semimetals, Weyl semimetals, and nodal line semimetals. Nodal line semimetals can be considered as precursor states for other topological states. For example, starting from such nodal line states, the nodal line structure might evolve into Weyl points, convert into Dirac points, or become a topological insulator by introducing the spin–orbit coupling (SOC) or mass term. In this review paper, we introduce theoretical materials that show the nodal line semimetal state, including the all-carbon Mackay–Terrones crystal (MTC), anti-perovskite Cu3PdN, pressed black phosphorus, and the CaP3 family of materials, and we present the design principles for obtaining such novel states of matter.
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Carrier balance and linear magnetoresistance in type-II Weyl semimetal WTe2
Xing-Chen Pan,Yiming Pan,Juan Jiang,Huakun Zuo,Huimei Liu,Xuliang Chen,Zhongxia Wei,Shuai Zhang,Zhihe Wang,Xiangang Wan,Zhaorong Yang,Donglai Feng,Zhengcai Xia,Liang Li,Fengqi Song,Baigeng Wang,Yuheng Zhang,Guanghou Wang
Front. Phys. . 2017, 12 (3 ): 127203-.
https://doi.org/10.1007/s11467-016-0629-7
Unsaturated magnetoresistance (MR) has been reported in type-II Weyl semimetal WTe2 , manifested as a perfect compensation of opposite carriers. We report linear MR (LMR) in WTe2 crystals, the onset of which was identified by constructing the MR mobility spectra for weak fields. The LMR further increased and became dominant for fields stronger than 20 T, while the parabolic MR gradually decayed. The LMR was also observed in high-pressure conditions.
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Negative magnetoresistance in Weyl semimetals NbAs and NbP: Intrinsic chiral anomaly and extrinsic effects
Yupeng Li,Zhen Wang,Pengshan Li,Xiaojun Yang,Zhixuan Shen,Feng Sheng,Xiaodong Li,Yunhao Lu,Yi Zheng,Zhu-An Xu
Front. Phys. . 2017, 12 (3 ): 127205-.
https://doi.org/10.1007/s11467-016-0636-8
Chiral anomaly-induced negative magnetoresistance (NMR) has been widely used as critical transport evidence for the existence of Weyl fermions in topological semimetals. In this mini-review, we discuss the general observation of NMR phenomena in non-centrosymmetric NbP and NbAs. We show that NMR can arise from the intrinsic chiral anomaly of Weyl fermions and/or extrinsic effects, such as the superimposition of Hall signals; field-dependent inhomogeneous current flow in the bulk, i.e., current jetting; and weak localization (WL) of coexistent trivial carriers. The WL-controlled NMR is heavily dependent on sample quality and is characterized by a pronounced crossover from positive to negative MR growth at elevated temperatures, resulting from the competition between the phase coherence time and the spin-orbital scattering constant of the bulk trivial pockets. Thus, the correlation between the NMR and the chiral anomaly need to be scrutinized without the support of complimentary techniques. Because of the lifting of spin degeneracy, the spin orientations of Weyl fermions are either parallel or antiparallel to the momentum, which is a unique physical property known as helicity. The conservation of helicity provides strong protection for the transport of Weyl fermions, which can only be effectively scattered by magnetic impurities. Chemical doping with magnetic and non-magnetic impurities is thus more convincing than the NMR method for detecting the existence of Weyl fermions.
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Two-carrier transport in SrMnBi2 thin films
Xiao Yan,Cheng Zhang,Shan-Shan Liu,Yan-Wen Liu,David Wei Zhang,Fa-Xian Xiu,Peng Zhou
Front. Phys. . 2017, 12 (3 ): 127209-.
https://doi.org/10.1007/s11467-017-0663-0
Monocrystalline SrMnBi2 thin films were grown by molecular beam epitaxy (MBE), and their transport properties were investigated. A high and unsaturated linear magnetoresistance (MR) was observed, which exhibited a transition from a semi-classical weak-field B 2 dependence to a high-field linear dependence. An unusual nonlinear Hall resistance was also observed because of the anisotropic Dirac fermions. The two-carrier model was adopted to analyze the unusual Hall resistance quantitatively. The fitting results yielded carrier densities and mobilities of 3.75×1014 cm−2 and 850 cm2 ·V−1 s−1 , respectively, for holes, and 1.468×1013 cm−2 , 4118 cm2 ·V−1 ·s−1 , respectively, for electrons, with a hole-dominant conduction at 2.5 K. Hence, an effective mobility can be achieved, which is in reasonable agreement with the effective hole mobility of 1800 cm2 ·V−1 ·s−1 , extracted from the MR. Further, the angle-dependent MR, proportional to cosθ , where θ is the angle between the external magnetic field and the perpendicular orientation of the sample plane, also implies a high anisotropy of the Fermi surface. Our results about SrMnBi2 thin films, as one of a new class of AEMnBi2 and AEMnSb2 (AE= Ca, Sr, Ba, Yb, Eu) materials, suggest that they have a lot of exotic transport properties to be investigated, and that their high mobility might facilitate electronic device applications.
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Spin in the extended electron model
Thomas Pope,Werner Hofer
Front. Phys. . 2017, 12 (3 ): 128503-.
https://doi.org/10.1007/s11467-017-0669-7
It has been found that a model of extended electrons is more suited to describe theoretical simulations and experimental results obtained via scanning tunnelling microscopes, but while the dynamic properties are easily incorporated, magnetic properties, and in particular electron spin properties pose a problem due to their conceived isotropy in the absence of measurement. The spin of an electron reacts with a magnetic field and thus has the properties of a vector. However, electron spin is also isotropic, suggesting that it does not have the properties of a vector. This central conflict in the description of an electron’s spin, we believe, is the root of many of the paradoxical properties measured and postulated for quantum spin particles. Exploiting a model in which the electron spin is described consistently in real three-dimensional space – an extended electron model – we demonstrate that spin may be described by a vector and still maintain its isotropy. In this framework, we re-evaluate the Stern–Gerlach experiments, the Einstein–Podolsky–Rosen experiments, and the effect of consecutive measurements and find in all cases a fairly intuitive explanation.
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Ordered quasi-two-dimensional structure of nanoparticles in semiflexible ring polymer brushes under compression
Yunfeng Hua,Zhenyu Deng,Yangwei Jiang,Linxi Zhang
Front. Phys. . 2017, 12 (3 ): 128701-.
https://doi.org/10.1007/s11467-017-0665-y
Molecular dynamics simulations of a coarse-grained bead-spring model of ring polymer brushes under compression are presented. Flexible polymer brushes are always disordered during compression, whereas semiflexible polymer brushes tend to be ordered under sufficiently strong compression. Further, the polymer monomer density of the semiflexible polymer brush is very high near the brush surface, inducing a peak value of the free energy near the surface. Therefore, when nanoparticles are compressed in semiflexible ring polymer brushes, they tend to exhibit a closely packed single-layer structure between the brush surface and the impenetrable wall, and a quasi-two-dimensional ordered structure near the brush surface is formed under strong compression. These findings provide a new approach to designing responsive applications.
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Regulation of 1, 4, 5-triphosphate receptor channel gating dynamics by mutant presenilin in Alzheimer’s disease cells
Fang Wei,Xiang Li,Meichun Cai,Yanping Liu,Peter Jung,Jianwei Shuai
Front. Phys. . 2017, 12 (3 ): 128702-.
https://doi.org/10.1007/s11467-017-0670-1
In neurons of patients with Alzheimer’s disease, the intracellular Ca2+ concentration is increased by its release from the endoplasmic reticulum via the inositol 1, 4, 5-triphosphate receptor (IP3 R). In this paper, we discuss the IP3 R gating dynamics in familial Alzheimer’s disease (FAD) cells induced with mutation PS1. By fitting the parameters of an IP3 R channel model to experimental data of the open probability, the mean open time and the mean closed time of IP3 R channels, in control cells and FAD mutant cells, we suggest that the interaction of presenilin mutation PS1 with IP3 R channels leads the decrease in the unbinding rates of IP3 and the activating Ca2+ from IP3 Rs. As a result, the increased affinities of IP3 and activating Ca2+ for IP3 R channels induce the increase in the Ca2+ signal in FAD mutant cells. Specifically, the PS1 mutation decreases the IP3 dissociation rate of IP3 R channels significantly in FAD mutant cells. Our results suggest possible novel targets for FAD therapeutic intervention.
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Exponential distance distribution of connected neurons in simulations of two-dimensional in vitro neural network development
Zhi-Song Lv,Chen-Ping Zhu,Pei Nie,Jing Zhao,Hui-Jie Yang,Yan-Jun Wang,Chin-Kun Hu
Front. Phys. . 2017, 12 (3 ): 128902-.
https://doi.org/10.1007/s11467-017-0602-0
The distribution of the geometric distances of connected neurons is a practical factor underlying neural networks in the brain. It can affect the brain’s dynamic properties at the ground level. Karbowski derived a power-law decay distribution that has not yet been verified by experiment. In this work, we check its validity using simulations with a phenomenological model. Based on the in vitro twodimensional development of neural networks in culture vessels by Ito, we match the synapse number saturation time to obtain suitable parameters for the development process, then determine the distribution of distances between connected neurons under such conditions. Our simulations obtain a clear exponential distribution instead of a power-law one, which indicates that Karbowski’s conclusion is invalid, at least for the case of in vitro neural network development in two-dimensional culture vessels.
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Diversity of chimera-like patterns from a model of 2D arrays of neurons with nonlocal coupling
Chang-Hai Tian, Xi-Yun Zhang, Zhen-Hua Wang, Zong-Hua Liu
Front. Phys. . 2017, 12 (3 ): 128904-.
https://doi.org/10.1007/s11467-017-0656-z
Chimera states have been studied in 1D arrays, and a variety of different chimera states have been found using different models. Research has recently been extended to 2D arrays but only to phase models of them. Here, we extend it to a nonphase model of 2D arrays of neurons and focus on the influence of nonlocal coupling. Using extensive numerical simulations, we find, surprisingly, that this system can show most types of previously observed chimera states, in contrast to previous models, where only one or a few types of chimera states can be observed in each model. We also find that this model can show some special chimera-like patterns such as gridding and multicolumn patterns, which were previously observed only in phase models. Further, we present an effective approach, i.e., removing some of the coupling links, to generate heterogeneous coupling, which results in diverse chimera-like patterns and even induces transformations from one chimera-like pattern to another.
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21 articles