@incollection{statphys23_1048, title = {Crossing problems and thresholds in percolation and the Potts model}, address = {Genova, Italy}, author = {R.M. Ziff and C.M. Scullard and P. Kleban and J.J.H. Simmons}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=1048}, abstract = {Various crossing problems in percolation and on Fortuin-Kasteleyn clusters of the Potts and Ising models is discussed. For certain boundary conditions in the Potts case, a perfectly dual system can be made so that the crossing probability for a square system at the critical threshold is exactly 1/2, as it is for simple independent bond percolation at the threshold. Likewise, a new scaled probability parameter can be constructed to give symmetric results for the crossing, just like percolation. The crossing probability for rectangular systems is shown to agree with theory [Gruzberg]. For horizontal-but-not-vertical crossing, the asymptotic decay of the crossing probability is found numerically -- here, there are no theoretical results. Another advance in percolation is also presented: new lattices where the thresholds are known exactly, namely the martini lattice and its relatives [work with C. Scullard] and generalizations of the bow-tie lattice of Wierman. Self-dual systems of these lattices are constructed, and the duality of the crossing probability leads to the new exact thresholds. References: 1. R. M. Ziff and C. R. Scullard, J. Phys. A 39, 15083 (2006) 2. J. C. Wierman, J. Phys. A 17, 1525 (1984) 3. Ilya A. Gruzberg, Stochastic geometry of critical curves, Schramm-Loewner evolutions, and conformal field theory, arXiv:math-ph/0607046}, biburl = {http://www.bibsonomy.org/bibtex/27860c06f9225825a2cb9e69e37541ec1/statphys23}, keywords = {thresholds model potts topic-2 universality percolation statphys23 ising} } @incollection{statphys23_1017, title = {Universality and the temperature-dependent zero-bias conductance of nanodevices}, address = {Genova, Italy}, author = {L.N. Oliveira and A.C. Seridonio and M. Yoshida}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=1017}, abstract = {Late in the 1980's, theoretical analyses came to the conclusion that the Anderson model for dilute magnetic alloys should describe the low-temperature transport properties of nanodevices. A decade later, the development of the 'single-electron transistor', a quantum dot bridging two otherwise independent two-dimensional electron gases, ractified those predictions. In particular, the universal curve $g_{univ}(T)$ for the thermal dependence of the impurity contribution to the resistivity of a dilute magnetic alloy was shown to reproduce quantitatively the measured conductances through the quantum dot. More recently, a more complex nanostructure known as the 'T-shaped' or 'side-coupled' device, which couples a quantum dot to a quantum wire and the latter to two leads, was developed. When the current through the device was measured as a function of the gate voltage applied to the dot, instead of the flat plateaus observed in single-electron transistors, Fano antiresonances emerged, showing that (i) the currents through the wire and through the dot interfere; and (ii) the thermal dependence of the conductance is qualitatively different from $g_{univ}(T)$. Our work focuses the side-coupled device. We will show that, even under conditions that maximize interference, the thermal dependence of the conductance can always be mapped onto $g_{univ}(T)$. The mapping is linear, with coefficients that depend on the low-temperature phase shift $\delta$ introduced in the quantum wire by the coupling to the dot. In the realm of the Anderson model, one of the coefficients is determined by a simple expression involving the phase-shift $\delta$ and the Fano parameter $q$, which measures the amplitude for conduction through the dot relative to the amplitude for conduction through the wire. The second coefficient, by contrast, is constant. As an illustration, we will report a numerical renormalization-group computation of the temperature-dependent conductance for the Anderson model of the side-coupled device for various ratios between the currents through the dot and the wire (i.~e., various $q$'s) and show that, in all cases, our expression maps the resulting conductances onto $g_{univ}(T)$. In addition, we will show that the mapping provides a straightforward procedure simplifying the interpretation of experimental curves. Applied to the measurements recently published by Sato et al. [Phys. Rev. Lett. 95, 066801 (2005)], that procedure yields conductance curves in excellent agreement with the experimental results and provides first-principle justification for the authors' phenomenological interpretation of their data. Work supported by the FAPESP, CNPq, and IBEM (Brazil).}, biburl = {http://www.bibsonomy.org/bibtex/2c005f048c122b0699658b047632cc9ba/statphys23}, keywords = {nanodevices conductance wires quantum topic-8 universality statphys23 dots} } @incollection{statphys23_0864, title = {Scaling and universality in real elections}, address = {Genova, Italy}, author = {S. Fortunato and C. Castellano}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=864}, abstract = {Human societies exhibit many examples of nontrivial collective behavior generated by the interaction of a large number of individuals. To characterize these emergent phenomena and understand their origin it is crucial to investigate statistically the macroscopic patterns, with the aim to uncover regularities that may open the way to a physical modelling of social dynamics. Elections are a prominent example of social collective phenomena, and a quantitative statistical analysis has begun. Here we show that, in proportional elections, the distribution of the number of votes received by candidates is a universal scaling function, identical in different countries and years. A simple dynamical model for the behaviour of voters, based on the spreading of word of mouth, reproduces the universal distribution. This finding reveals the existence in the voting process of a general microscopic dynamics that does not depend on the historical, political and/or economical context where voters operate.}, biburl = {http://www.bibsonomy.org/bibtex/26bb775b94201c2aa49ae7142fe817373/statphys23}, keywords = {universality topic-11 elections statphys23 scaling} } @incollection{statphys23_0816, title = {Nonequilibrium phase transitions into absorbing states}, address = {Genova, Italy}, author = {H. Park}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=816}, abstract = {Systems with trapped (absorbing) states may exhibit a nonequilibrium phase transition from a noise-free inactive phase into an ever-lasting active phase. In this talk, I will briefly review the absorbing critical phenomena and universality classes, and discuss over some controversial issues like pair contact process with diffusion.}, biburl = {http://www.bibsonomy.org/bibtex/29f7093a1f5b0a51cd60987d3820de9ab/statphys23}, keywords = {nonequilibrium transitions topic-3 universality states class statphys23 absorbing phase} } @incollection{statphys23_0763, title = {Critical exponents and amplitude ratios of the three dimensional XY universality class}, address = {Genova, Italy}, author = {M. Campostrini and M. Hasenbusch and A. Pelissetto and E. Vicari}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=763}, abstract = {We improve the theoretical estimates of the critical exponents for the three-dimensional XY universality class, which apply to the superfluid transition in $^4$He along the $\lambda$-line $T_\lambda(P)$ of its phase diagram. We obtain the estimates $\alpha=-0.0151(3)$, $\nu=0.6717(1)$, $\eta=0.0381(2)$, $\gamma=1.3178(2)$, $\beta=0.3486(1)$, and $\delta=4.780(1)$. These results are obtained by a finite-size scaling analyses of high-statistics Monte Carlo simulations up to lattice size $L=128$ and resummations of 22nd order high-temperature expansions of two lattice models with suppressed leading scaling corrections. We note that our result for the specific-heat exponent $\alpha$ disagrees with the most recent experimental estimate $\alpha=-0.0127(3)$ at the superfluid transition of $^4$He in microgravity environment. Furthermore, we have simulated one of the two models in the low and the high temperature phase for reduced temperatures down to $|T-T_c|/T_c \approx 0.0017$ on lattices of a size up to $350^3$. Our results for the internal energy and the specific heat, combined with our accurate estimate of $T_c$ and data for the internal energy and the specific heat at $T_c$, lead to the estimate $R_{\alpha} = (1-A_+/A_-)/\alpha = 4.01(5)$, where $A_{\pm}$ is the amplitude of the specific heat in the high and the low temperature phase, respectively. Also here, the most recent experimental result $R_{\alpha} = 4.154(22)$ is not fully consistent with our value.}, biburl = {http://www.bibsonomy.org/bibtex/2e7c2d7587e739d93ccb8c3ed2a90534e/statphys23}, keywords = {lambda-transition series lattice class universality simulation xy temperature models improved topic-2 high montecarlo statphys23} } @incollection{statphys23_0664, title = {Spreading transitions and universality classes in CMLs}, address = {Genova, Italy}, author = {N. Gupte and Z. Jabeen}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=664}, abstract = {The phase diagram of the coupled sine circle map lattice shows spatio-temporal intermittency of two distinct types: spatio-temporal intermittency of the directed percolation class, and spatial intermittency which does not belong to this class. These two types of behaviour are seen to be special cases of the spreading and non-spreading regimes seen in the system, with the two regimes being separated by an infection line. The coupled map lattice can be mapped on to an equivalent cellular automaton which shows a transition from a probabilistic cellular automaton (PCA) to a deterministic cellular automaton (DCA) at the infection line. Thus the existence of the DP and non-DP universality classes in the same system is reflected in the PCA to DCA transition. We also provide pointers to the dynamical reasons for this transition.}, biburl = {http://www.bibsonomy.org/bibtex/2d8233074e2d2d6b7f8e81c21dbab2c7d/statphys23}, keywords = {cellular classes lattice universality topic-5 directed automata map coupled percolation statphys23 deterministic probabilistic} } @incollection{statphys23_0642, title = {Dynamics of a tagged particle in the 1D asymmetric simple exclusion process}, address = {Genova, Italy}, author = {T. Imamura and T. Sasamoto}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=642}, abstract = {The asymmetric simple exclusion process(ASEP) has been playing an important role in the understanding of non-equilibrium systems. Recently the non-stationary properties of the ASEP are vigorously discussed. In this presentation, we report the results on the position fluctuation of a particular particle (`tagged particle') in the 1D totally asymmetric simple exclusion process (TASEP). So far the fluctuation properties are discussed for one time case. On the other hand, we consider the multi-time position fluctuations of a tagged particle in the step initial condition. Based on the combinatorial techniques (RSK correspondence, Schur process, etc), we represent the distribution function as a Fredholm determinant with a suitable kernel for which the scaling limit can be analyzed exactly. We show that in the scaling limit, the fluctuations are equivalent to those of the largest eigenvalue in the Hermitian multimatrix model in random matrix theory. The effect of the defect particles on the position fluctuation is also discussed. ref. math-ph/0702009}, biburl = {http://www.bibsonomy.org/bibtex/24de7baf84551c2231937ab902144b47d/statphys23}, keywords = {exclusion simple random universality class kpz tracy-widom distribution process asymmeric matrices topic-3 airy statphys23} } @incollection{statphys23_0590, title = {Simulational study on dynamical universality}, address = {Genova, Italy}, author = {Y. Murase and T. Shimada and N. Ito}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=590}, abstract = {Dynamical universality classes of bcc and fcc Ising models, and triangular and honeycomb three states Potts models are numerically studied with the nonequilibrium relaxation analysis[1]. The local exponents $Ľlambda$ ($=Ľfrac{Ľbeta}{zĽnu}$) of the three dimensional Ising model are 0.251(4) (bcc), 0.257(7) (fcc) and these values are identical with that of simple cubic lattice within the error bars. Here $Ľbeta$, $Ľnu$ and $z$ are the critical exponents of the order parameter, correlation length and dynamical critical exponent, respectively. The dynamical critical exponents of the two dimensional Potts models are 2.188(4) (sq), 2.202(14) (tri), 2.198(4) (hc). These values are consistent with the previous work[2] and the dynamical universality is confirmed. In addition, the Ising model with alternating coupling constant on square lattice (model 1) and the Ising model with frustration on square lattice (model 2) is also studied (see Figure). Model 1 has two positive alternating coupling constant $J_{1},J_{2}$ and they are alternately configured. Model 2 has two coupling constans $J$ and $-J$ and anti-ferro bonds are distributed at regular intervals with the ratio of 1/16. Again the dynamical universality is confirmed in these models and this suggests that the modification to the bond strength without randomness do not affect the dynamical critical universality. 1) N. Ito, Physica A, 196, 591 (1993)\\ 2) L. Schulke and B. Zheng, Phys. Lett. A, 204, 295 (1995)\\ 3) F.-G. Wang and C.-K. Hu, Phys. Rev. E, 56, 2310 (1997)}, biburl = {http://www.bibsonomy.org/bibtex/2f75f13f966991266b7340c01d20222c9/statphys23}, keywords = {nonequilibrium model potts topic-2 three dynamical universality states method statphys23 relaxation ising} } @incollection{statphys23_0537, title = {Some aspects of universality in spin glasses}, address = {Genova, Italy}, author = {T. Jorg and J. Lukic and E. Marinari and O.C. Martin and F. Ricci-Tersenghi}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=537}, abstract = {We study universality in the spin glass (SG) transition of the two- and three-dimensional Edwards-Anderson (EA) model by means of large scale Monte Carlo (MC) simulations. We compare the finite-size scaling functions of the correlation length and the SG susceptibility for different disorder distributions. We find that our data for the three-dimensional EA model is in very good agreement with universal critical behavior. For the two-dimensional EA model, that has the SG transition at zero temperature, the analysis is more involved due to the presence of large finite-size corrections for discrete coupling distributions. A careful analysis of the data for different coupling distributions, however, reveals that also the critical behavior of the two-dimensional EA model is universal with a power-law divergence of the correlation length. This result stands in clear contrast with results from zero temperature ground state calculations where the model with bimodal couplings is supposed to have an exponential divergence while the model with Gaussian couplings is supposed to have a power-law divergence of the correlation length. This and the fact that the model with bimodal couplings has an extensive number of ground state pairs while the model with Gaussian couplings has just one pair of ground states needs further explanation. We illustrate in the framework of the Migdal-Kadanoff Renormalization Group scheme on hierarchical lattices how such a discrepancy between the properties seen at zero and at finite temperature can arise due to presence of an additional unphysical fixed point (FP) at zero temperature in the case of discrete coupling distributions. We discuss implications of this phenomenon for optimizition methods working at zero temperature and how in certain circumstances the problems related to this unphysical FP can be avoided.}, biburl = {http://www.bibsonomy.org/bibtex/2d31aa43435aa311762a54b9d441f8649/statphys23}, keywords = {glasses transitions universality topic-9 spin optimization simulations statphys23 montecarlo phase} } @incollection{statphys23_0528, title = {Study on elastic-interaction-mediated order-disorder phase transition as a new universality class}, address = {Genova, Italy}, author = {S. Miyashita and Y. Konishi and M. Nishino}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=528}, abstract = {The type of order-disorder phase transition of local Ising-type variables depends on the range of the interaction. In the case of short-range interaction the universality class has been well understood. The case of dipole-dipole interaction which is a kind of long range has been also studied extensively. Recently, the property of the spin-crossover phase transition has been studied in detail.[1,2] In order to explain the nature of cooperative property, models of Ising type variables with interaction have been introduced, and the structure of ordering processes has been well explained. However, the most probable origin of the interaction between the spin states, i.e., the high spin and low spin, is the elastic interaction among distortions of the lattice due to the change of the volume of the molecules.[3,4] We study the critical nature of the elastic-interaction-mediated phase transition. There, the positions of spin change continuously and the size of each spin changes between two stable values with different energy and degeneracy. This model shows a kind of order-disorder phase transition, but the critical property is not known and we expect a new type of universality class. We will estimate the critical properties of the model through the temperature and pressure dependences. [5] 1) S. Miyashita, Y. Konishi, H. Tokoro, M. Nishino, K. Boukheddaden and F. Varret, Prog. Theor. Phys. 114, 719-735 (2005).\\ 2) Y. Konishi, H. Tokoro, M. Nishino and S. Miyashita, J. Phys. Soc. J. 75, 114603 (2006).\\ 3) M. Nishino, K. Boukheddaden, Y. Konishi, and S. Miyashita, unpublished.\\ 4) Y. Konishi, M. Nishino and S. Miyashita, in preparation.\\ 5) S. Miyashita,Y. Konishi, and M. Nishino, in preparation.}, biburl = {http://www.bibsonomy.org/bibtex/2522c7ad68302379ccba2ba0071ed8e64/statphys23}, keywords = {crosover topic-2 universality transition spin class interaction statphys23 elastic phase} } @incollection{statphys23_0482, title = {Percolation transitions in complex networks}, address = {Genova, Italy}, author = {J.D. Noh}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=482}, abstract = {We introduce a model for complex networks with tunable degree-degree correlation, and investigate the nature of the percolation transition. With the degree distribution fixed to the Poisson distribution, we study numerically the percolation transition in the model networks with positive, neutral, and negative correlations. The results show that those networks with neutral and negative correlations display the percolation transitions in the same universality class. On the other hand, the networks with positive correlations are found to exhibit the percolation transition in a distinct universality class. The scaling bahaviors are presented in detail.}, biburl = {http://www.bibsonomy.org/bibtex/27c606caf97073378f04256728034bdfc/statphys23}, keywords = {complex degree universality topic-11 percolation correlation statphys23 network} } @incollection{statphys23_0481, title = {Moment Ratios for the Pair Contact Process with Diffusion}, address = {Genova, Italy}, author = {M.M. De Oliveira and R. Dickman}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=481}, abstract = {We study the continuous absorbing-state phase transition in the one-dimensional pair contact process with diffusion (PCPD). The PCPD is a reaction-diffusion process which involves two competing reactions: $2A\to 3A$ (fission) and $2A \to \emptyset$ (annihilation), where the particles must be first neighbors in order to react. In addition, individual particles can diffuse by the lattice. While this model has attracted a considerable attention in last few years , it is not yet clear how to classify its critical behaviour. At present there are two principal schools of thought: in one, the PCPD belongs to a novel universality class distinct from DP, with a unique set of critical exponents, or possibly continuously varying exponents due to a marginal perturbation. The opposing school holds that the PCPD should be attracted to a DP fixed-point after a huge crossover time. A recent review on these and other scenarios can be found in [Henkel and Hinrichsen, J. Phys. A: Math. Gen. {\bf 37}, R117 (2004)]. In this work we study the PCPD via quasistationary (QS) simulations [de Oliveira and Dickman, Phys. Rev. E, {\bf 71} 016129 (2005)], focussing on the order parameter and its moments. In previous studies [Dickman and de Menezes, Phys. Rev. E, {\bf 66} 045101(R) (2002)], the critical point moment ratios of the order parameter showed anomalous behavior, growing with system size rather than taking universal values, as expected. Using the QS simulation method we determine the moments of the order parameter up to fourth order at the critical point, in systems of up to 40960 sites. Due to strong finite-size effects, the ratios converge only for large system sizes. Moment ratios and associated order-parameter histograms are compared with those of directed percolation. We also report an improved estimate for the nondiffusive pair contact process.}, biburl = {http://www.bibsonomy.org/bibtex/2a398a5e00d39379443b52c0db8a051e7/statphys23}, keywords = {processes pair state quasistationary universality absorbing contact process transitions topic-3 reaction-diffusion statphys23 phase} } @incollection{statphys23_0401, title = {Critical properties of the two-dimensional contact process in heterogeneous environments}, address = {Genova, Italy}, author = {S.V. Fallert and Y.M. Kim and C.J. Neugebauer and S.N. Taraskin}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=401}, abstract = {The contact process (CP) has been thoroughly studied in homogeneous environments in the past. Recently, interest has increasingly turned towards its behaviour in heterogeneous and disordered systems. In this study, the critical behaviour of the CP on heterogeneous periodic $2d$-lattices is investigated. The analysis is carried out via two routes: analytical and numerical. Analytically, an approximate expression for the phase-separation lines around the homogeneous critical point is suggested guided by the structure of the Liouville operator which governs the time evolution of the CP. The locus of critical points thus obtained is supported by extensive Monte Carlo simulations and compared with the mean-field results for a range of binary lattices characterized by different unit cells. Numerically calculated values of the dynamical scaling exponents $\eta$, $\delta$ and $z$ are found to coincide with the values established for the homogeneous case thus confirming that the CP in all studied heterogeneous environments belongs to the directed percolation universality class.}, biburl = {http://www.bibsonomy.org/bibtex/2be57e3dd6ea7c50c2b9461bccce15e93/statphys23}, keywords = {process topic-3 directed universality percolation statphys23 montecarlo simulation contact} } @incollection{statphys23_0372, title = {Insulator-Metal-Insulator Transition in Two Dimensional Disordered Electronic Systems with Flat Bands}, address = {Genova, Italy}, author = {S. Nishino and K. Yakubo}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=372}, abstract = {We find a disorder-induced insulator-metal-insulator reentrant transition in a novel two-dimensional noninteracting electron system which has highly-degenerated eigenstates unless introducing disorders. First, we propose a tight-binding Hamiltonian on a square lattice decorated in a specific manner with spin-orbit interactions described by the Ando model. We show that the system has a dispersionless band, which is called flat band, at zero energy in its band structure. Next, we introduce disorders into on-site potentials of the Hamiltonian. Due to the diagonal disorder, the flat band becomes a finite width single band. Since the system belongs to the symplectic universality class because of spin-orbit interactions breaking the spin-rotational symmetry, the metal-insulator transition can be expected in this band. In order to examine the transition in this system, we employed the nearest-neighbor level spacing statistics which is a powerful tool for studying the Anderson transition. We numerically diagonalize the Hamiltonian and calculate the distribution function of nearest-neighbor level spacings $P(s)$ which becomes the Wigner and the Poissonian distributions for metallic and insulating states, respectively. For a fixed nonzero energy, $P(s)$ is found to be close to the Poissonian distribution for weak and very strong disorders, while $P(s)$ becomes the Wigner surmise for intermediate disorders. This behavior of $P(s)$ implies the existence of the insulator-metal-insulator reentrant transition as increasing the strength of disorder. We carry out the finite-size scaling analysis of the level statistics to determine the precise position of the transition. It is found that the insulator-metal-insulator transition exists over a wide energy region in our two-dimensional disordered systems.}, biburl = {http://www.bibsonomy.org/bibtex/2e02297e1ef0f1b509cf850adebc17205/statphys23}, keywords = {band localization flat topic-8 anderson universality transition spin-orbit class interaction statphys23} } @incollection{statphys23_0142, title = {Exact evidence for a weak-universal critical behaviour of the hybrid Ising-Heisenberg model with two- and four-spin interactions}, address = {Genova, Italy}, author = {J. Strecka and L. Canova and M. Jascur}, booktitle = {Abstract Book of the XXIII IUPAP International Conference on Statistical Physics}, editor = {Luciano Pietronero and Vittorio Loreto and Stefano Zapperi}, month = {9-13 July}, year = 2007, url = {http://st23.statphys23.org/webservices/abstract/preview_pop.php?ID_PAPER=142}, abstract = {The exact mapping relationship between the hybrid Ising-Heisenberg model with two-spin Heisenberg and four-spin Ising interactions and its corresponding zero-field (symmetric) eight-vertex model, respectively, is used to study an interesting critical behaviour of the model under investigation that satisfies the weak universality hypothesis. In particular, it is shown that the critical exponents of the studied Ising-Heisenberg model basically depend on a ratio between the two-spin Heisenberg and four-spin Ising interactions, as well as, the strength of exchange anisotropy that allows to control a regime of the pairwise Heisenberg interaction between easy-axis and easy-plane type. In the limit of the isotropic Heisenberg coupling, an exact evidence is found for a peculiar quantum phase transition with diverging critical exponents $\alpha \to -\infty$, $\beta \to \infty$, $\gamma \to \infty$, etc.}, biburl = {http://www.bibsonomy.org/bibtex/2b49a8683da14dd360b39dc569bd9dccd/statphys23}, keywords = {topic-1 critical phenomena model results eight-vertex exact universality weak statphys23 ising-heisenberg} }