Mean-field approximation for the chiral soliton in a chiral phase transition

In the mean-field approximation we study the chiral soliton within the linear sigma model in a thermal vacuum. The chiral soliton equations with different boundary conditions are solved at finite temperatures and densities. The solitons are discussed before and after the chiral restoration. We find that the system has soliton solutions even after the chiral restoration and they are very different from those before the chiral restoration, which indicates that the quarks are still bounded after the chiral restoration.Comment: 9 pages, 11 figures. Some sentences and words have been correcte

Oblique Klein tunneling in 8-Pmmn borophene p-n junctions

The 8-\textit{Pmmn} borophene is one kind of new elemental monolayer, which hosts anisotropic and tilted massless Dirac fermions (MDF). The planar \textit{p-n} junction (PNJ) structure as the basic component of various novel devices based on the monolayer material has attracted increasing attention. Here, we analytically study the transport properties of anisotropic and tilted MDF across 8-\textit{Pmmn} borophene PNJ. Similar to the isotropic MDF across graphene junctions, perfect transmission exists but its direction departures the normal direction of borophene PNJ induced by the anisotropy and tilt, i.e., oblique Klein tunneling. The oblique Klein tunneling does not depend on the doping levels in \textit{N} and \textit{P} regions of PNJ as the normal Klein tunneling but depends on the junction direction. Furthermore, we analytically derive the special junction direction for the maximal difference between perfect transmission direction and the normal direction of PNJ and clearly distinguish the respective contribution of anisotropy and tilt underlying the oblique Klein tunneling. In light of the rapid advances of experimental technologies, we expect the oblique Klein tunneling to be observable in the near future.Comment: 7 pages, 3 figure

Space-filling curves of self-similar sets (I): Iterated function systems with order structure

This paper is the first paper of three papers in a series, which intend to provide a systematic treatment for the space-filling curves of self-similar sets. In the present paper, we introduce a notion of \emph{linear graph-directed IFS} (linear GIFS in short). We show that to construct a space-filling curve of a self-similar set, it is amount to explore its linear GIFS structures. Some other notions, such as chain condition, path-on-lattice IFS, and visualizations of space-filling curves are also concerned. In sequential papers \cite{Dai15} and \cite{RZ14}, we obtain a universal algorithm to construct space-filling curves of self-similar sets of finite type, that is, as soon as the IFS is given, the computer will do everything automatically. Our study extends almost all the known results on space-filling curves.Comment: 24 pages, 14 figure

Space-filling curves of self-similar sets (III): Skeletons

Skeleton is a new notion designed for constructing space-filling curves of self-similar sets. It is shown in [Dai, Rao and Zhang, Space-filling curves of self-similar sets (II): Edge-to-trail substitution rule,https://doi.org/10.1088/1361-6544/ab1275] that for a connected self-similar set, space-filling curves can be constructed provided that it possesses a skeleton. In this paper, we give a criterion of existence of skeletons by using the so-called neighbor graph of a self-similar set. In particular, we show that a connected self-similar set satisfying the finite type condition always possesses skeletons: an algorithm is obtained here

Perfect transmission at oblique incidence by trigonal warping in graphene P-N junctions

We develop an analytical mode-matching technique for the tight-binding model to describe electron transport across graphene P-N junctions. This method shares the simplicity of the conventional mode-matching technique for the low-energy continuum model and the accuracy of the tight-binding model over a wide range of energies. It further reveals an interesting phenomenon on a sharp P-N junction: the disappearance of the well-known Klein tunneling (i.e., perfect transmission) at normal incidence and the appearance of perfect transmission at oblique incidence due to trigonal warping at energies beyond the linear Dirac regime. We show that this phenomenon arises from the conservation of a generalized pseudospin in the tight-binding model. We expect this effect to be experimentally observable in graphene and other Dirac fermions systems, such as the surface of three-dimensional topological insulators.Comment: 11 pages, 11 figure

The baryon mass calculation in the chiral soliton model at finite temperature and density

In the mean-field approximation, we have studied the soliton which is embedded in a thermal medium within the chiral soliton model. The energy of the soliton or the baryon mass in the thermal medium has been carefully evaluated, in which we emphasize that the thermal effective potential in the soliton energy should be properly treated in order to derive a finite and well-defined baryon mass out of the thermal background. The result of the baryon mass at finite temperatures and densities in chiral soliton model are clearly presented.Comment: 7 pages, 4 figures, 1 tabl

Velocity-determined anisotropic behaviors of RKKY interaction in 8-\textit{Pmmn} borophene

As a new two-dimensional Dirac material, 8-\textit{Pmmn} borophene hosts novel anisotropic and tilted massless Dirac fermions (MDFs) and has attracted increasing interest. However, the potential application of 8-\textit{Pmmn} borophene in spin fields has not been explored. Here, we study the long-range RKKY interaction mediated by anisotropic and tilted MDFs in magnetically-doped 8-\textit{Pmmn} borophene. To this aim, we carefully analyze the unique real-space propagation of anisotropic and tilted MDFs with noncolinear momenta and group velocities. As a result, we analytically demonstrate the anisotropic behaviors of long-range RKKY interaction, which have no dependence on the Fermi level but are velocity-determined, i.e., the anisotropy degrees of oscillation period and envelop amplitude are determined by the anisotropic and tilted velocities. The velocity-determined RKKY interaction favors to fully determine the characteristic velocities of anisotropic and tilted MDFs through its measurement, and has high tunability by engineering velocities shedding light on the application of 8-\textit{Pmmn} borophene in spin fields.Comment: 8 pages, 5 figure

Deep Learning for Sequential Recommendation: Algorithms, Influential Factors, and Evaluations

In the field of sequential recommendation, deep learning (DL)-based methods have received a lot of attention in the past few years and surpassed traditional models such as Markov chain-based and factorization-based ones. However, there is little systematic study on DL-based methods, especially regarding to how to design an effective DL model for sequential recommendation. In this view, this survey focuses on DL-based sequential recommender systems by taking the aforementioned issues into consideration. Specifically,we illustrate the concept of sequential recommendation, propose a categorization of existing algorithms in terms of three types of behavioral sequence, summarize the key factors affecting the performance of DL-based models, and conduct corresponding evaluations to demonstrate the effects of these factors. We conclude this survey by systematically outlining future directions and challenges in this field.Comment: 36 pages, 17 figures, 6 tables, 104 reference

Space-filling curves of self-similar sets (II): Edge-to-trail substitution rule

It is well-known that the constructions of space-filling curves depend on certain substitution rules. For a given self-similar set, finding such rules is somehow mysterious, and it is the main concern of the present paper. Our first idea is to introduce the notion of skeleton for a self-similar set. Then, from a skeleton, we construct several graphs, define edge-to-trail substitution rules, and explore conditions ensuring the rules lead to space-filling curves. Thirdly, we summarize the classical constructions of the space-filling curves into two classes: the traveling-trail class and the positive Euler-tour class. Finally, we propose a general Euler-tour method, using which we show that if a self-similar set satisfies the open set condition and possesses a skeleton, then space-filling curves can be constructed. Especially, all connected self-similar sets of finite type fall into this class. Our study actually provides an algorithm to construct space-filling curves of self-similar sets.Comment: 44 pages, 16 figure

Helical damping and anomalous critical non-Hermitian skin effect

Non-Hermitian skin effect and critical skin effect are unique features of non-Hermitian systems. In this Letter, we study an open system with its dynamics of single-particle correlation function effectively dominated by a non-Hermitian damping matrix, which exhibits $\mathbb{Z}_2$ skin effect, and uncover the existence of a novel phenomenon of helical damping. When adding perturbations that break anomalous time reversal symmetry to the system, the critical skin effect occurs, which causes the disappearance of the helical damping in the thermodynamic limit although it can exist in small size systems. We also demonstrate the existence of anomalous critical skin effect when we couple two identical systems with $\mathbb{Z}_2$ skin effect. With the help of non-Bloch band theory, we unveil that the change of generalized Brillouin zone equation is the necessary condition of critical skin effect.Comment: 7+5 pages, 4+5 figure