Spin-independent v-representability of Wigner crystal oscillations in one-dimensional Hubbard chains: The role of spin-charge separation

Electrons in one-dimension display the unusual property of separating their spin and charge into two independent entities: The first, which derive from uncharged spin-1/2 electrons, can travel at different velocities when compared with the second, built from charged spinless electrons. Predicted theoretically in the early sixties, the spin-charge separation has attracted renewed attention since the first evidences of experimental observation, with usual mentions as a possible explanation for high-temperature superconductivity. In one-dimensional (1D) model systems, the spin-charge separation leads the frequencies of Friedel oscillations to suffer a 2k_F -- 4k_F crossover, mainly when dealing with strong correlations, where they are referred to as Wigner crystal oscillations. In non-magnetized systems, the current density functionals which are applied to the 1D Hubbard model are not seen to reproduce this crossover, referring to a more fundamental question: Are the Wigner crystal oscillations in 1D systems non-interacting v-representable? Or, is there a spin-independent Kohn-Sham potential which is able to yield spin-charge separation? Finding an appropriate answer to both questions is our main task here. By means of exact and DMRG solutions, as well as, a new approach of exchange-correlation potential, we show the answer to be positive. Specifically, the v-representable 4k_F oscillations emerge from attractive interactions mediated by positively charged spinless holes -- the holons -- as an additional contribution to the repulsive on-site Hubbard interaction

Information entropy of classical versus explosive percolation

We study the Shannon entropy of the cluster size distribution in classical as well as explosive percolation, in order to estimate the uncertainty in the sizes of randomly chosen clusters. At the critical point the cluster size distribution is a power-law, i.e. there are clusters of all sizes, so one expects the information entropy to attain a maximum. As expected, our results show that the entropy attains a maximum at this point for classical percolation. Surprisingly, for explosive percolation the maximum entropy does not match the critical point. Moreover, we show that it is possible determine the critical point without using the conventional order parameter, just analysing the entropy's derivatives.Comment: 6 pages, 6 figure

Web applications security and vulnerability analysis financial web applications security audit – a case study

Information security can no longer be neglected in any area. It is a concern to everyone and every organization. This is particularly important in the finance sector, not only because the financial amounts involved but also clients and organization’s private and sensitive information. As a way to test security in infrastructures, networks, deployed web applications and many other assets, organizations have been performing penetration testing which simulates an attacker’s behavior in a controlled environment in order to identify its vulnerabilities. This article focus on the analysis of the results of security audits conducted on several financial web applications from one institution with aid of automatic tools in order to assess their web applications security level. To help in security matters, many organizations build security frameworks for vulnerability assessment, security assessment, threat modeling, penetration testing, risk management and many more. As for penetration testing, organizations such as OWASP provide vulnerability and security information, a testing methodology, risk analysis and penetration testing tools.info:eu-repo/semantics/publishedVersio

Web security in the finance sector: Analysing the security of financial web applications: A case study

Nowadays, information security is an increasing concern in institutions and organizations. This concern is even greater in the finance sector, not only because the financial amount involved but also clients and organization’s private and sensitive information. As a way to test security in infrastructures, networks, deployed web applications and many other assets, organizations have been performing penetration testing (pentest) which simulates an attacker’s behavior in a controlled environment in order to identify its vulnerabilities. This article focusses on the analysis of the results of security audits conducted on several financial web applications from one institution with aid of automatic tools in order to assess their web applications security level. To help in security matters, many organizations build security frameworks for vulnerability assessment, security assessment, threat modeling, penetration testing, risk management and many more. As for penetration testing, organizations such as OWASP provide vulnerability and security information, a testing methodology, risk analysis and penetration testing tools.info:eu-repo/semantics/acceptedVersio

Aperiodic quantum XXZ chains: Renormalization-group results

We report a comprehensive investigation of the low-energy properties of antiferromagnetic quantum XXZ spin chains with aperiodic couplings. We use an adaptation of the Ma-Dasgupta-Hu renormalization-group method to obtain analytical and numerical results for the low-temperature thermodynamics and the ground-state correlations of chains with couplings following several two-letter aperiodic sequences, including the quasiperiodic Fibonacci and other precious-mean sequences, as well as sequences inducing strong geometrical fluctuations. For a given aperiodic sequence, we argue that in the easy-plane anisotropy regime, intermediate between the XX and Heisenberg limits, the general scaling form of the thermodynamic properties is essentially given by the exactly-known XX behavior, providing a classification of the effects of aperiodicity on XXZ chains. We also discuss the nature of the ground-state structures, and their comparison with the random-singlet phase, characteristic of random-bond chains.Comment: Minor corrections; published versio