Ground-state factorization and quantum phase transition in dimerized spin chains

We study the occurrence of ground-state factorization in dimerized $XY$ spin chains in a transverse field. Together with the usual ferromagnetic and antiferromagnetic regimes, a third case emerges, with no analogous in translationally-invariant systems, consisting of an antiferromagnetic Ne\'{e}l-type ground state where pairs of spins represent the unitary cell. Then, we calculate the exact solution of the model and show that the factorizing field represent an accidental degeneracy point of the Hamiltonian. Finally, we extend the study of the existence of ground-state factorization to a more general class of models

A nonlinear model dynamics for closed-system, constrained, maximal-entropy-generation relaxation by energy redistribution

We discuss a nonlinear model for the relaxation by energy redistribution within an isolated, closed system composed of non-interacting identical particles with energy levels e_i with i=1,2,...,N. The time-dependent occupation probabilities p_i(t) are assumed to obey the nonlinear rate equations tau dp_i/dt=-p_i ln p_i+ alpha(t)p_i-beta(t)e_ip_i where alpha(t) and beta(t) are functionals of the p_i(t)'s that maintain invariant the mean energy E=sum_i e_ip_i(t) and the normalization condition 1=sum_i p_i(t). The entropy S(t)=-k sum_i p_i(t) ln p_i(t) is a non-decreasing function of time until the initially nonzero occupation probabilities reach a Boltzmann-like canonical distribution over the occupied energy eigenstates. Initially zero occupation probabilities, instead, remain zero at all times. The solutions p_i(t) of the rate equations are unique and well-defined for arbitrary initial conditions p_i(0) and for all times. Existence and uniqueness both forward and backward in time allows the reconstruction of the primordial lowest entropy state. The time evolution is at all times along the local direction of steepest entropy ascent or, equivalently, of maximal entropy generation. These rate equations have the same mathematical structure and basic features of the nonlinear dynamical equation proposed in a series of papers ended with G.P.Beretta, Found.Phys., 17, 365 (1987) and recently rediscovered in S. Gheorghiu-Svirschevski, Phys.Rev.A, 63, 022105 and 054102 (2001). Numerical results illustrate the features of the dynamics and the differences with the rate equations recently considered for the same problem in M.Lemanska and Z.Jaeger, Physica D, 170, 72 (2002).Comment: 11 pages, 7 eps figures (psfrag use removed), uses subeqn, minor revisions, accepted for Physical Review

The Role of Monumental Trees in Defining Local Identity and in Tourism. A case study in the Marches region

The paper presents an explorative research about the relict Monumental Trees in the Marches region, with the aim to describe and give some interpretations about the problematic relationship between economic activities and the landscape. After the definition of Monumental Tree, we describe its multiple meanings, in religion, mystics, education, environment, and even in tourism. The paper gives the inventory of the resource in the Marches, and reports results of Key Informants\u2019 perceptions about the necessity to conserve Monumental Trees in the framework of the Regional Landscape and Environment Plan and the role of Monumental Trees in defining the local \u2018territorial identity\u2019

Il ruolo degli immigrati nell\u2019integrazione d\u2019uso del territorio urbano e rurale. Il caso della Sicilia Sud-orientale.

Nella Sicilia Sud-orientale, dove la presenza di immigrati attivi nelle produzioni orticole in serra \ue8 rilevante e indispensabile, gli assetti produttivi e residenziali si sono modificati nel tempo per la necessit\ue0 di dare alloggio non fortuito ai lavoratori immigrati e alle loro famiglie. La presenza di immigrati attivi in agricoltura \ue8 significativa sia per il settore primario sia per la qualit\ue0 urbanistica dei centri urbani e delle campagne. La ricerca \ue8 stata svolta col metodo qualitativo, attraverso una dozzina di interviste Face-to-Face a interlocutori privilegiati, attivi nei settori produttivi, nelle professioni, nell\u2019amministrazione pubblica e nella comunit\ue0 di immigrati. L\u2019analisi dei dati rilevati permette di affermare che la presenza degli immigrati \ue8 sufficientemente tollerata dai residenti, che volentieri cedono in affitto abitazioni prevalentemente localizzate nei centri urbani ma in cattivo stato di manutenzione. Le parti abbandonate o degradate dei centri urbani sono rivitalizzate dalla presenza di immigrati che animano il mercato degli affitti anche per immobili \u201cfatiscenti\u201d, non usati dagli italiani. Gli imprenditori agricoli concedono in uso costruzioni rurali, a volte come integrazione salariale, e allo scopo di esercitare un controllo diretto sulla forza lavoro. Dopo il primo ambientamento, durante il quale sono disponibili anche ad abitare in residenze molto degradate, i lavoratori immigrati preferiscono le abitazioni urbane, intorno alle quali ricostruiscono la presenza di piccoli commerci specializzati rispetto alle loro preferenze (macellerie, drogherie). L\u2019azione degli enti locali \ue8 ritenuta insufficiente ed \ue8 supplita dall\u2019azione di associazioni di volontariato e di parrocchie, anche per mancanza di fondi pubblici da destinare sia all\u2019accoglienza sia alle politiche di edilizia popolare. Nonostante la disponibilit\ue0 ad accettare la presenza di comunit\ue0 estranee alla cultura locale, la maggior parte dei residenti intervistati ritiene che le amministrazioni di ogni livello dovrebbero sentire maggiormente il compito di formulare politiche abitative destinate ad alleviare il disagio abitativo e di convivenza tra comunit\ue0 diverse

Double dot chain as a macroscopic quantum bit

We consider an array of N quantum dot pairs interacting via Coulomb interaction between adjacent dots and hopping inside each pair. We show that at the first order in the ratio of hopping and interaction amplitudes, the array maps in an effective two level system with energy separation becoming exponentially small in the macroscopic (large N) limit. Decoherence at zero temperature is studied in the limit of weak coupling with phonons. In this case the macroscopic limit is robust with respect to decoherence. Some possible applications in quantum information processing are discussed.Comment: Phys. Rev. A (in press

Kramers polarization in strongly correlated carbon nanotube quantum dots

Ferromagnetic contacts put in proximity with carbon nanotubes induce spin and orbital polarizations. These polarizations affect dramatically the Kondo correlations occurring in quantum dots formed in a carbon nanotube, inducing effective fields in both spin and orbital sectors. As a consequence, the carbon nanotube quantum dot spectral density shows a four-fold split SU(4) Kondo resonance. Furthermore, the presence of spin-orbit interactions leads to the occurrence of an additional polarization among time-reversal electronic states (polarization in the time-reversal symmetry or Kramers sector). Here, we estimate the magnitude for the Kramer polarization in realistic carbon nanotube samples and find that its contribution is comparable to the spin and orbital polarizations. The Kramers polarization generates a new type of effective field that affects only the time-reversal electronic states. We report new splittings of the Kondo resonance in the dot spectral density which can be understood only if Kramers polarization is taken into account. Importantly, we predict that the existence of Kramers polarization can be experimentally detected by performing nonlinear differential conductance measurements. We also find that, due to the high symmetry required to build SU(4) Kondo correlations, its restoration by applying an external field is not possible in contrast to the compensated SU(2) Kondo state observed in conventional quantum dots.Comment: 8 pages, 4figure

From Surviving to Thriving: Top Tips to Help Newer Chairs

This session shares concrete strategies for helping department chairs navigate common problems more effectively and efficiently. With attention to managing email, handling complaints, constructing schedules, building relationships, and working with upper administration, the presenters offer practical tips to help newer chairs succeed

Kinetic and Structural Analysis of the Mg2+ -binding Site of the Guanine Nucleotide-binding Protein p21 H-ras.

The coordination and binding of the Mg2+ ion in the nucleotide−binding site of p21 have been investigated using site−directed mutagenesis, kinetic methods, and phosphorous NMR. Mg2+ in the p21.nucleotide.Mg2+ complex appears to be in fast equilibrium with the solvent. The dissociation constant between Mg2+ and the p21.GDP complex was determined to be 2.8 microM. It decreases 30− or 16−fold on substituting Ser−17 or Asp−57 with alanine, respectively, whereas the T35A mutation has no effect. All three mutations influence the dissociation constants and the association and dissociation rate constants of the interaction between guanine nucleotides and p21, but to a different degree. We conclude that Thr−35 is only complexed to Mg2+ in the GTP conformation and both Asp−57 and Ser−17 appear to be critical for both GDP and GTP binding. 31P NMR spectra of the GDP and Gpp(NH)p (guanosine−5'−(beta,gamma−imido)triphosphate) complexes of mutated p21 show a remarkable perturbation of the guanine nucleotide− binding site compared to wild−type protein. The mutant proteins show reduced GTPase rates, which are not stimulated by the GTPase−activating protein GAP. p21(S17A) has been reported to function just as p21(S17N) as a dominant negative inhibitor of normal p21. We find that it inhibits oncogenic p21−induced survival of primary neuron