Searcharxiv⌕ Search

arXiv subjects

S. P. Li

Publications and source records attributed to S. P. Li.

14 recordsLinked to original sources
arXiv2015

X-ray magnetic circular dichroism study of epitaxial magnetite ultrathin film on MgO (100)

The spin and orbital magnetic moments of the Fe3O4 epitaxial ultrathin film synthesized by plasma assisted simultaneous oxidization on MgO(100) have been studied with X-ray magnetic circular dichroism (XMCD). The ultrathin film retains a rather large total magnetic moment, i.e. (2.7+-0.15) uB/f.u., which is ~ 70% of that for the bulk-like Fe3O4. A significant unquenched orbital moment up to (0.54+-0.05) uB/f.u. was observed, which could come from the symmetry breaking at the Fe3O4/MgO interface. Such sizable orbital moment will add capacities to the Fe3O4-based spintronics devices in the magnetization reversal by the electric field.

cond-mat.mtrl-sci↗
arXiv2015

Spin and orbital moments of nanoscale Fe3O4 epitaxial thin film on MgO/GaAs(100)

Nanoscale Fe3O4 epitaxial thin film has been synthesized on MgO/GaAs(100) spintronic heterostructure, and studied with X-ray magnetic circular dichroism (XMCD). We have observed a total magnetic moment of (3.32 +- 0.1) uB/f.u., retaining 83% of the bulk value. Unquenched orbital moment of (0.47 +- 0.05) uB/f.u. has been confirmed by carefully applying the sum rule. The results offer direct experimental evidence of the bulk-like total magnetic moment and a large orbital moment in the nanoscale fully epitaxial Fe3O4/MgO/GaAs(100) heterostructure, which is significant for spintronics applications.

cond-mat.mtrl-sci↗
arXiv2010

House Price Distributions of Taiwan: A Preliminary Study

The house price distributions of Taiwan are analyzed. The tail of the cumulative distribution function (CDF) follows an approximate power law with an exponent equals to -2.4 while the distribution of the house price per unit area displays a lognormal distribution. Implications of the results are also discussed.

physics.soc-ph↗
arXiv2007

Network Topology of an Experimental Futures Exchange

Many systems of different nature exhibit scale free behaviors. Economic systems with power law distribution in the wealth is one of the examples. To better understand the working behind the complexity, we undertook an empirical study measuring the interactions between market participants. A Web server was setup to administer the exchange of futures contracts whose liquidation prices were coupled to event outcomes. After free registration, participants started trading to compete for the money prizes upon maturity of the futures contracts at the end of the experiment. The evolving `cash' flow network was reconstructed from the transactions between players. We show that the network topology is hierarchical, disassortative and scale-free with a power law exponent of 1.02+-0.09 in the degree distribution. The small-world property emerged early in the experiment while the number of participants was still small. We also show power law distributions of the net incomes and inter-transaction time intervals. Big winners and losers are associated with high degree, high betweenness centrality, low clustering coefficient and low degree-correlation. We identify communities in the network as groups of the like-minded. The distribution of the community sizes is shown to be power-law distributed with an exponent of 1.19+-0.16.

q-fin.ST↗
arXiv2003

Statistical linguistic study of DNA sequences

A new family of compound Poisson distribution functions from statistical linguistic is used to study the n-tuples and nucleotide composition features of DNA sequences. The relative frequency distribution of the 6-tuples and 7- tuples occurrence studies suggest that most of the DNA sequences follow the general shape of the compound Poisson distribution. It is also noted that the $χ$-square test indicated that some of the sequences follow this distribution with a reasonable level of goodness of fit. The compositional segmentation study fits quite well using this new family of distribution functions. Furthermore, the absolute values of the relative frequency come out naturally from the linguistic model without ambiguity. It is suggesting that DNA sequences are not random sequences and they could possibly have subsequence structures.

cond-mat.stat-mech↗
arXiv2003

Guided Simulated Annealing Method for Optimization Problems

Incorporating the concept of order parameter of the mean-field theory into the simulated annealing method, we presented a new optimization algorithm, the guided simulated annealing method. In this method mean-field order parameters are calculated to guide the configuration search for the global minimum. Allowing fluctuations and improvement of mean-field values iteratively, this method successfully identified global minima for several difficult optimization problems. Application of this method to the HP lattice-protein model has found a new lowest energy state for an $N = 100$ sequence that was not found by other methods before. Results for spin glass models are also presented.

cond-mat.stat-mech↗
arXiv2001

Meson propagators in spontaneously broken gauge theories

Contrary to common belief, the longitudinal vector meson and scalar meson propagators have very different forms in spontaneously broken gauge theories. We choose the Abelian-Higgs model as an example and show that the longitudinal vector meson and scalar meson propagators have double poles by an extensive use of the Ward-Takahashi identities.

hep-ph↗
arXiv2001

A guided Monte Carlo method for optimization problems

We introduce a new Monte Carlo method by incorporating a guided distribution function to the conventional Monte Carlo method. In this way, the efficiency of Monte Carlo methods is drastically improved. To further speed up the algorithm, we include two more ingredients into the algorithm. First, we freeze the sub-patterns that have high probability of appearance during the search for optimal solution, resulting in a reduction of the phase space of the problem. Second, we perform the simulation at a temperature which is within the optimal temperature range of the optimization search in our algorithm. We use this algorithm to search for the optimal path of the traveling salesman problem and the ground state energy of the spin glass model and demonstrate that its performance is comparable with more elaborate and heuristic methods.

physics.comp-ph↗
arXiv1999

Is the Standard Model Renormalizable?

In this paper, we study the renormalizability of the Standard Model in the Landau gauge. On the basis of the Ward-Takahashi identities, we derive exact expressions for the physical masses of the W and Z as well as the renormalized coupling constants in the theory. We show that it is impossible to make all these renormalized quantities finite. Thus the quantum theory of the Standard Model with the divergent amplitudes obeying the Ward-Takahashi identities is not renormalizable.

hep-th↗
arXiv1999

The Standard Model in the Alpha gauge is not renormalizable

We study the Ward-Takahashi identities in the standard model with the gauge fixing terms given by (1.1) and (1.2). We find that the isolated singularities of the propagators for the unphysical particles are poles of even order, not the simple poles people have assumed them to be. Furthermore, the position of these poles are ultraviolet divergent. Thus the standard model in the alpha gauge in general, and the Feynman gauge in particular, is not renormalizable. We study also the case with the gauge fixing terms (1.3), and find that the propagators remain non-renormalizable. The only gauge without these difficulties is the Landau gauge. One therefore has to make a distinction between the renormalizability of the Green functions and that of the physical scattering amplitudes.

hep-th↗
arXiv1997

How to treat $γ_5$

We present a method to perform renormalized perturbation calculation in gauge theories with chiral fermions. We find it proper to focus directly on the Ward-Takahashi identities, relegating dimensional regularization into a supplementary and secondary role. We show with the example of the Abelian-Higgs theory how to handle amplitudes involving fermions, particularly how to handle the matrix $γ_5$. As a demonstration of our method of renormalization, we evaluate the radiative corrections of the triangular anomaly in the Abelian-Higgs theory with appropriate number of chiral fermions. This anomaly amplitude is calculated without any regularization and is found to vanish.

hep-th↗
arXiv1996

The radiative corrections of the triangular anomaly

We calculate the triangular anomaly in the next order in the Abelian-Higgs theory with an appropriate number of families of chiral fermions and with Yukawa couplings. The calculation is performed with the method of subtraction aided by the Ward- Takahashi identities. This anomaly amplitude is performed without any regularization and is found to vanish.

hep-th↗
arXiv1996

How to renormalize a quantum gauge field theory with chiral fermions

We propose using the method of subtraction to renormalize quantum gauge theories with chiral fermions and with spontaneous symmetry breaking. The Ward-Takahashi identities derived from the BRST invariance in these theories are complex and rich in content. We demonstrate how to use these identities to determine relationships among renormalization constants of the theory and obtain the subtraction constants needed for the renormalization procedure. We have found it particularly convenient to adopt the Landau gauge throughout the scheme. The method of renormalization by subtraction enables one to calculate physical quantities in the theory in the form of a renormalized perturbation series which is unique and definite. There is no ambiguity in handling the $γ_5$ matrix associated with chiral fermions.

hep-th↗