Low-Temperature Quantum Critical Behaviour of Systems with Transverse Ising-like Intrinsic Dynamics

The low-temperature properties and crossover phenomena of $d$-dimensional transverse Ising-like systems within the influence domain of the quantum critical point are investigated solving the appropriate one-loop renormalization group equations. The phase diagram is obtained near and at $d=3$ and several sets of critical exponents are determined which describe different responses of a system to quantum fluctuations according to the way of approaching the quantum critical point. The results are in remarkable agreement with experiments for a wide variety of compounds exhibiting a quantum phase transition, as the ferroelectric oxides and other displacive systems.Comment: 36 pages, 2 figures, accepted in Physica

Quantum tricriticality in transverse Ising-like systems

The quantum tricriticality of d-dimensional transverse Ising-like systems is studied by means of a perturbative renormalization group approach focusing on static susceptibility. This allows us to obtain the phase diagram for 3<d<4, with a clear location of the critical lines ending in the conventional quantum critical points and in the quantum tricritical one, and of the tricritical line for temperature T \geq 0. We determine also the critical and the tricritical shift exponents close to the corresponding ground state instabilities. Remarkably, we find a tricritical shift exponent identical to that found in the conventional quantum criticality and, by approaching the quantum tricritical point increasing the non-thermal control parameter r, a crossover of the quantum critical shift exponents from the conventional value \phi = 1/(d-1) to the new one \phi = 1/2(d-1). Besides, the projection in the (r,T)-plane of the phase boundary ending in the quantum tricritical point and crossovers in the quantum tricritical region appear quite similar to those found close to an usual quantum critical point. Another feature of experimental interest is that the amplitude of the Wilsonian classical critical region around this peculiar critical line is sensibly smaller than that expected in the quantum critical scenario. This suggests that the quantum tricriticality is essentially governed by mean-field critical exponents, renormalized by the shift exponent \phi = 1/2(d-1) in the quantum tricritical region.Comment: 9 pages, 2 figures; to be published on EPJ

Field-Induced Quantum Criticality of Systems with Ferromagnetically Coupled Structural Spin Units

The field-induced quantum criticality of compounds with ferromagnetically coupled structural spin units (as dimers and ladders) is explored by applying Wilson's renormalization group framework to an appropriate effective action. We determine the low-temperature phase boundary and the behavior of relevant quantities decreasing the temperature with the applied magnetic field fixed at its quantum critical point value. In this context, a plausible interpretation of some recent experimental results is also suggested.Comment: to be published in Physics Letters

Thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet with long-range interactions via the spectral density method

The thermodynamic properties of a classical d-dimensional spin-S Heisenberg ferromagnet, with long-range interactions decaying as $r^{-p}$ and in the presence of an external magnetic field, is investigated by means of the spectral density method in the framework of classical statistical mechanics. We find that long-range order exists at finite temperature for $d<p<2d$ with $d\leq 2$ and for $p>d$ with $d>2$, consistently with known theorems. Besides, the related critical temperature is determined and a study of the critical properties is performed.Comment: 27 pages, 2 figures, Submitted to Physica

Low-temperature behaviour of the transverse Ising model in the influence domain of the quantum critical point

The critical properties of the d-dimensional transverse Ising model in the influence domain of its quantum critical point are studied solving the one-loop renormalization group equations around and at d = 3. Different critical regimes occur involving crossover behaviours which can be described in terms of effective exponents, consistently with the conventional crossover theory

Volumi antichi di Fisica nella Biblioteca Storica della Facoltà di Ingegneria alla "Federico II''

La Facoltà di Ingegneria dell'Università di Napoli "Federico II" fu fondata da Gioacchino Napoleone Murat Re delle Due Sicilie con decreto del 4 agosto 1811 come "Scuola d'Applicazione de' Ponti e Strade" sulle orme di quella francese nata nel 1747. Ebbe padrini di eccezione come il molisano Vincenzo Cuoco, illuminato autore di progetti legislativi di riordinamento della pubblica istruzione con forte attenzione alle discipline fisiche e matematiche; il generale Campredon, limpido sostenitore dei principi educativi del Locke e del Rousseau; l’energico Direttore Generale e storico Pietro Colletta. Nell'occasione del suo 2000 anniversario. la Facoltà ha dato corso al Progetto Libri An¬tichi ovvero un lavoro di catalogazione ragionata dei volumi contenuti nella sua biblioteca storica. Fra questi, numerosi sono i libri di interesse per la fisica generale. Ne presentiamo qui alcuni fra i più notevoli. non solo per il contenuto tecnico ma anche per il loro interesse storico in quanto, attraverso le personalità degli autori, riecheggiano in essi ambientazioni ed eventi cui inevitabilmente le discipline scientifiche ed i loro operatori dell' epoca erano fortemente legati