Criticality of O(N) symmetric models in the presence of discrete gauge symmetries

We investigate the critical properties of the three-dimensional (3D) antiferromagnetic RP(N-1}) model, which is characterized by a global O(N) symmetry and a discrete Z_2 gauge symmetry. We perform a field-theoretical analysis using the Landau-Ginzburg-Wilson (LGW) approach and a numerical Monte Carlo study. The LGW field-theoretical results are obtained by high-order perturbative analyses of the renormalization-group (RG) flow of the most general Phi^4 theory with the same global symmetry as the model, assuming a gauge-invariant order-parameter field. For N=4 no stable fixed point is found, implying that any transition must necessarily be of first order. This is contradicted by the numerical results that provide strong evidence for a continuous transition. This suggests that gauge modes are not always irrelevant, as assumed by the LGW approach, but they may play an important role to determine the actual critical dynamics at the phase transition of O(N) symmetric models with a discrete Z_2 gauge symmetry.Comment: 12 pages. arXiv admin note: text overlap with arXiv:1502.07599, arXiv:1706.0436

Landau-Ginzburg-Wilson approach to critical phenomena in the presence of gauge symmetries

We critically reconsider the Landau-Ginzburg-Wilson (LGW) approach to critical phenomena in the presence of gauge symmetries. In the LGW framework, to obtain the universal features of a continuous transition, one identifies the order parameter Phi and considers the corresponding most general Phi4 field theory that has the same symmetries as the original model. In the presence of gauge symmetries, one usually considers a gauge-invariant order parameter and a LGW field theory that is invariant under the global symmetries of the original model. We show that this approach, in which the gauge dynamics is effectively integrated out, may sometimes lead to erroneous conclusions on the nature of the critical behavior. As an explicit example, we show that the above-described LGW approach generally fails for the three-dimensional ferromagnetic and antiferromagnetic CP(N-1) models, which are invariant under global U(N) and local U(1) transformations. We point out possible implications for the finite-temperature chiral transition of nuclear matter.Comment: 7 pages, few changes, refs adde

Plastic ridge formation in a compressed thin amorphous film

We demonstrate that surface morphogenesis in compressed thin films may result from spatially correlated plastic activity. A soft glassy film strongly adhering to a smooth and rigid substrate and subjected to uniaxial compression, indeed, does not undergo any global elastic pattern-forming instability, but responds plastically via localized burst events that self-organize, leading to the emergence of a series of parallel ridges transverse to the compression axis. This phenomenon has been completely overlooked, but results from common features of the plastic response of glasses, hence should be highly generic for compressed glassy thin films

Conceptual Design of the Steam Generators for the EU DEMO WCLL Reactor

In the framework of the EUROfusion Horizon Europe Programme, ENEA and its linked third parties are in charge of the conceptual design of the steam generators belonging to EU DEMO WCLL Breeding Blanket Primary Heat Transfer Systems (BB PHTSs). In particular, in 2021, design activities and supporting numerical simulations were carried out in order to achieve a feasible and robust preliminary concept design of the Once Through Steam Generators (OTSGs), selected as reference technology for the DEMO Balance of Plant at the end of the Horizon 2020 Programme. The design of these components is very challenging. In fact, the steam generators have to deliver the thermal power removed from the two principal blanket subsystems, i.e., the First Wall (FW) and the Breeding Zone (BZ), to the Power Conversion System (PCS) for its conversion into electricity, operating under plasma pulsed regime and staying in dwell period at a very low power level (decay power). Consequently, the OTSG stability and control represent a key point for these systems' operability and the success of a DEMO BoP configuration with direct coupling between the BB PHTS and the PCS. In this paper, the authors reported and critically discussed the FW and BZ steam generators' thermal-hydraulic and mechanical design, the developed 3D CAD models, as well as the main results of the stability analyses and the control strategy to be adopted

Colon Cancer Stem Cells Dictate Tumor Growth and Resist Cell Death by Production of Interleukin-4

A novel paradigm in tumor biology suggests that cancer growth is driven by stem-like cells within a tumor. Here, we describe the identification and characterization of such cells from colon carcinomas using the stem cell marker CD133 that accounts around 2% of the cells in human colon cancer. The CD133+ cells grow in vitro as undifferentiated tumor spheroids, and they are both necessary and sufficient to initiate tumor growth in immunodeficient mice. Xenografts resemble the original human tumor maintaining the rare subpopulation of tumorigenic CD133+ cells. Further analysis revealed that the CD133+ cells produce and utilize IL-4 to protect themselves from apoptosis. Consistently, treatment with IL-4Rα antagonist or anti-IL-4 neutralizing antibody strongly enhances the antitumor efficacy of standard chemotherapeutic drugs through selective sensitization of CD133+ cells. Our data suggest that colon tumor growth is dictated by stem-like cells that are treatment resistant due to the autocrine production of IL-4

Mutual information, vibrational scaling, short-time dynamical heterogeneity and secondary relaxation in coarse-grained polymer systems

Understanding the extraordinary viscous slow-down that accompanies glass formation is one of the major open challenges in condensed matter physics. On approaching the glass transition from the high fluidity regime, a particle spends increasing time within the cage formed by the first neighbors where it rattles with amplitude . Its average escape time, i.e. the structural relaxation time, increases from a few picoseconds in the low-viscosity regime up to thousands of seconds at the glass transition. The transition from a liquid to a glass is accompanied by the progressive appearance of dynamical heterogeneity: it is observed the growth of transient spatial fluctuations in the local dynamical behavior. Experimentally, dynamical heterogeneity manifests itself in the relaxation spectra, measured through mechanical or dielectric probes, showing a very broad range of relaxation times and a strongly non-exponential behavior. This suggests the existence of wide distributions of relaxation rates. Despite the huge difference in time scales between the rattling motion and the relaxation, several studies addressed the fast rattling process within the cage to understand the slow relaxation dynamics. Within this context, several correlations between the amplitude of the rattling motion and the structural relaxation time have been found in a large variety of systems. This correlation is summarized in the form of a universal master curve. An analytical derivation for this relation, in the framework of the Hall-Wolynes model, relies on the wide distribution of relaxation time, which is a manifestation of the dynamical heterogeneity. This Thesis work has two main purposes, both central in the field of research of the glass transition physics by means of coarse-grained molecular dynamics simulations: i) achieving a deeper understanding of the connection between fast dynamics and slow relaxation ii) gaining further insight on the role of the dynamical heterogeneity in such a connection. Chapters 1 and 2 are the introductory ones. The first one gives a quick presentation of the general context of this research. The second chapter is dedicated to a brief introduction to Information Theory, as in some of the works presented in this thesis, Mutual Information is employed as a more refined and sensitive measure of correlation. Chapter 3 and 4 are dedicated to the study of displacement-displacement correlation in a simple molecular liquid by means of mutual information. The research is motivated by, and as a follow-up to, previous studies based on displacements correlation function in the light of novel investigation carried out on atomic liquids by employing mutual information. Chapter 3 focuses on the mutual information correlation length. A comparison with both the results obtained in atomic liquids and the ones resulting from simple correlation function is carried out to determine whether mutual information could improve the analysis of correlated motion. Chapter 4 extends the previous investigation with a closer look at dynamical heterogeneity. Two particle fractions, with different mobilities and relaxations, are identified on the basis of mutual information related properties. The two fraction scalings, in the form of the aforementioned universal relation between relaxation and rattling amplitude, are investigated. The spatial and structural properties of these two fractions are studied as well. In Chapter 5 and 6 a slightly more complex system is taken into consideration: a liquid of 25-mers with bending potential and a nearly fixed bond length shorter than the zero of the Lennard-Jones interaction. Such peculiar features give rise to the emergence of a faster secondary relaxation process. Chapter 5 investigates whether the presence of a secondary relaxation could interfere with the universal correlation between vibrational dynamics and primary relaxation. An analysis of the performance of secondary relaxation probe functions, including mutual information, is also carried out. Chapter 6 focuses on the role of the secondary relaxation on the system dynamical heterogeneity as sensed by the non-Gaussian parameter. A microscopical explanation of the phenomenon, as well as its impact on other standard observables, is also given

Critical behavior of the three-dimensional antiferromagnetic RP^{N-1} model

Phase transitions are interesting phenomena, which are found in many field of research of modern physics. Their study is crucial for understanding a large variety of phenomena. Landau was the first to propose a general framework able to provide a semi-quantitative explanation of the critical phenomena. His theory is based on the assumption that a phase transition is always associated with the growth of a certain quantity called order parameter, a measure of the system transition towards the low-temperature ordered phase. According to Landau, once an order parameter is identi- fied, the symmetry of the system is sufficient to provide an effective theory allowing to obtain a semi-quantitative interpretation of the critical phenomenon. The modern approach to phase transitions is still based on this theory, even though it has been modified and completed over time. Nowadays Landau-Ginsburg-Wilson (LGW) theory represents a powerful technique able to accurately classify a large variety of phase transitions. As the Landau theory, the LGW approach requires the identification of an order parameter. Then, one proceeds constructing an effective field theory whose action is obtained considering the most general fourth-order polynomial in the order parame- ter consistent with the original symmetry of the system. Hence, the universal critical behavior is inferred studying the renormalization group flow of such a theory. In systems characterized by a gauge symmetry, the LGW approach starts by considering a gauge invariant order parameter implicitly assuming that gauge degrees of freedom do not couple with critical modes. This choice is supported by the fact that, in any spontaneous symmetry breaking phenomenon, gauge symmetry is preserved. However, there are some cases in which this assumption is found lacking. Recent works suggested the hypothesis that, in systems characterized by a gauge symmetry, critical modes might not always be entirely represented by a gauge invariant order parameter. In order to investigate the correctness of the LGW theory predictions in models featuring a local gauge symmetry, in this thesis, we considered the RP^(N−1) model. It is a lattice model characterized by a global O(N) symmetry, a local Z_2 gauge symmetry. We performed an analysis, based on LGW approach, of the antiferromagnetic RP^(N−1) model. We constructed our LGW Hamiltonian with a staggered gauge-invariant order parameter. According to this analysis, RP^(N−1) models undergo a continuous phase transition only for N ≤ 3. In particular, we verified that for N = 2 the system undergoes a continuous transition that belongs to the O(2) vector model universality class. In the same way, for N = 3 it undergoes a continuous phase transition that belongs to the O(5) universality class with a dynamical enlargement of the symmetry at the critical point. Monte Carlo simulations confirm this result. Instead, for N ≥ 4, we did not find any stable fixed point. This suggests that any phase transition for N ≥ 4 should be of first order. In order to test this result, we performed a Monte Carlo analysis of the RP^3 model. We found a phase transition at the inverse tem- perature β ≈ 6.77. Using a finite-size scaling analysis, we obtained an estimate for the critical exponent of the correlation length ν ≈ 0.66. This value is significantly distant from the limit one ν = 1/3 that would have indicated a first order phase transition. In order to reinforce this statement, we performed an analysis of the energy distribution near the critical point and found no trace of double peaks. Therefore, we are led to the conclusion that the antiferromagnetic RP^3 model undergoes a continuous phase transition. This result is in contrast with our field theory analysis based on the LGW theory. This proves that a gauge invariant order parameter might not always capture all the relevant critical modes questioning, as a consequence, the validity of the traditional LGW approach when gauge symmetries are present

Mutual Information in Molecular and Macromolecular Systems

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari–Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling