Is bimodality a sufficient condition for a first-order phase transition existence?

Here we present two explicit counterexamples to the widely spread beliefs about an exclusive role of bimodality as the first-order phase transition signal. On the basis of an exactly solvable statistical model generalizing the statistical multifragmentation model of nuclei, we demonstrate that the bimodal distributions can naturally appear both in infinite and in finite systems without a phase transition. In the first counterexample a bimodal distribution appears in an infinite system at the supercritical temperatures due to the negative values of the surface tension coefficient. In the second counterexample we explicitly demonstrate that a bimodal fragment distribution appears in a finite volume analog of a gaseous phase. In contrast to the statistical multifragmentation model, the developed statistical model corresponds to the compressible nuclear liquid with the tricritical endpoint located at one third of the normal nuclear density. The suggested parameterization of the liquid phase equation of state is consistent with the L. Van Hove axioms of statistical mechanics and it does not lead to an appearance of the nonmonotonic isotherms in the macroscopic mixed phase region which are typical for the classical models of the Van der Waals type. Peculiarly, such a way to account for the nuclear liquid compressibility automatically leads to an appearance of an additional state that in many respects resembles the physical antinuclear matter.     

Is the collapse a phase transition?

It is shown that the process of the collapse during quantum measurements is a consequence of the quantum symmetry built in the formalism: there exists a nontrivial group of symmetry under which all possible states are invariant after the measurement of the given observable. On the basis of this fact, the postulate of collapse is derived with purely group theoretical considerations. These are analogous to the ones used in Landau's problem of phase transitions. Some further analogies of the processes of the collapse and the phase transitions are discussed.     

Metal–insulator transition and colossal magnetoresistance: relevance of electron–lattice coupling and electronic phase separation

In this article, we review the insulator–metal transition and the colossal magnetoresistance effect in manganites. The relevance of electron–lattice coupling and the resulting Jahn–Teller polaron is elaborated. The general features of electronic phase separation, which results from disorder and strain effects, are discussed along with electron–lattice coupling effects. Although a comprehensive theory is still lacking that can account for all the intricate features of manganite physics, electronic-phase separation and electron–lattice coupling appear to capture the essence of the colossal magnetoresistance effect in manganites.     

Is there contextuality for a single qubit?

Cabello and Nakamura have shown [A. Cabello, Phys. Rev. Lett. 90, 190401 (2003)10.1103/PhysRevLett.90.190401] that the Kochen-Specker theorem can be applied to two-dimensional systems if one uses positive operator-valued measures (POVM). We show that the contextuality in their models is not of the Kochen-Specker type, but it is rather a result of not keeping track of the whole system on which the measurement is performed. This is connected to the fact that there is no one-to-one correspondence between the POVM elements and projectors on the extended Hilbert space, and the same POVM element has to originate from two different projectors when used in Cabello-Nakamura models. Moreover, we propose a hidden-variable formulation of the above models.     

Is d* a candidate for a hexaquark-dominated exotic state?

We confirm our previous prediction of a d* state with I(JP) = 0(3+) [Phys. Rev. C 60, 045203(1999)] and report for the first time based on a microscopic calculation that d* has about 2/3 hidden color(CC)configurations and thus is a hexaquark-dominated exotic state. By performing a more elaborate dynamical coupledchannels investigation of the △△-CC system within the framework of the resonating group method(RGM) in a chiral quark model, we find that the d* state has a mass of about 2.38–2.42 Ge V, a root-mean-square radius(RMS) of0.76–0.88 fm, and a CC fraction of 66%–68%. The last may cause a rather narrow width for the d* which, together with the quantum numbers and our calculated mass, is consistent with the newly observed resonance-like structure(M ≈2380 Me V, Γ≈70 Me V) in double-pionic fusion reactions reported by the WASA-at-COSY Collaboration.     

Enhancement of the colossal magnetoresistance in Eu1 − x Ca x B6

The transport and magnetic properties of single crystal samples of substitutional solid solutions Eu_{1 ? x} Ca _x B₆ (0 ≤ x ≤ 0.26) have been studied at temperatures 1.8–300 K in magnetic fields up to 80 kOe. It has been shown that an increase in the calcium concentration results in the suppression of the charge transport accompanied by an increase in the amplitude of the colossal magnetoresistance (CMR) up to the value (ρ(0) ? ρ(H))/ρ(H) ≈ 7 × 10⁵ detected for x = 0.26 at liquid-helium temperature in a field of 80 kOe. The transition from the hole-like conductivity to the electron-like conductivity has been observed in the Eu_0.74Ca_0.26B₆ solid solution in the CMR regime at T < 40 K. The Hall mobility values μH = 200?350 cm²/(V s) estimated for charge carriers in the strongly disordered matrix of the Eu_0.74Ca_0.26B₆ solid solution are comparable with the charge carrier mobility μH = 400?600 cm²/(V s) for the undoped EuB₆ compound. The anomalous behavior of the transport and magnetic parameters of the Eu_{1 ? x} Ca _x B₆ solid solutions is discussed in terms of a metal-insulator transition predicted within the double exchange model for this system with low carrier density.     

Is d a candidate for a hexaquark-dominated exotic state?

We confirm our previous prediction of a d^* state with I(J^P)=0(3⁺) [Phys. Rev. C 60, 045203 (1999)] and report for the first time based on a microscopic calculation that d^* has about 2/3 hidden color (CC) configurations and thus is a hexaquark-dominated exotic state. By performing a more elaborate dynamical coupled-channels investigation of the ΔΔ-CC system within the framework of the resonating group method (RGM) in a chiral quark model, we find that the d^* state has a mass of about 2.38--2.42 GeV, a root-mean-square radius (RMS) of 0.76--0.88 fm, and a CC fraction of 66%--68%. The last may cause a rather narrow width for the d^* which, together with the quantum numbers and our calculated mass, is consistent with the newly observed resonance-like structure (M≈ 2380 MeV, Γ≈ 70 MeV) in double-pionic fusion reactions reported by the WASA-at-COSY Collaboration.     

Magnetism of colossal magnetoresistance material Fe1−x Cd x Cr2S4

The magnetism of the colossal magnetoresistance material FeCr₂S₄ was studied through substitution by nonmagnetic element Cd. With the increase of Cd content, the high-field magnetization measured at 5 K increases, demonstrating a ferromagnetic Cr–Cr interaction in CdCr₂S₄. As comparing with the anti-ferromagnetic material ZnCr₂S₄, it is deduced that the ferromagnetic interaction in CdCr₂S₄ is favored by its larger Cr–Cr distance. On the other hand, due to Cd substitution, the low-field magnetization irreversibility between zero-field-cooling and field-cooling magnetization weakens with the increase of Cd content and disappears at last when x = 1 in the Fe_1?x Cd _x Cr₂S₄ (0 ≤ x ≤ 1.0) system. By taking account of the single-ion anisotropy of the ferrous ion on the tetrahedral site, the picture of irreversible magnetic behavior induced by magnetic anisotropy is examined.     

Does sex induce a phase transition?

We discovered a dynamic phase transition induced by sexual reproduction. The dynamics is a pure Darwinian rule applied to diploid bit-strings with both fundamental ingredients to drive Darwin's evolution: (1) random mutations and crossings which act in the sense of increasing the entropy (or diversity); and (2) selection which acts in the opposite sense by limiting the entropy explosion. Selection wins this competition if mutations performed at birth are few enough, and thus the wild genotype dominates the steady-state population. By slowly increasing the average number m of mutations, however, the population suddenly undergoes a mutational degradation precisely at a transition point m_c. Above this point, the “bad” alleles (represented by 1-bits) spread over the genetic pool of the population, overcoming the selection pressure. Individuals become selectively alike, and evolution stops. Only below this point, m < m_c, evolutionary life is possible. The finite-size-scaling behaviour of this transition is exhibited for large enough “chromosome” lengths L, through lengthy computer simulations. One important and surprising observation is the L-independence of the transition curves, for large L. They are also independent on the population size. Another is that m_c is near unity, i.e. life cannot be stable with much more than one mutation per diploid genome, independent of the chromosome length, in agreement with reality. One possible consequence is that an eventual evolutionary jump towards larger L enabling the storage of more genetic information would demand an improved DNA copying machinery in order to keep the same total number of mutations per offspring.     

Is Causality a Necessary Tool for Understanding Our Universe,or Is It a Part of the Problem?

In this paper, the concept of causality in physics is discussed. Causality is a necessary tool for the understanding of almost all physical phenomena. However, taking it as a fundamental principle may lead us to wrong conclusions, particularly in cosmology. Here, three very well-known problems—the Einstein–Podolsky–Rosen paradox, the accelerating expansion and the asymmetry of time—are discussed from this perspective. In particular, the implications of causality are compared to those of an alternative approach, where we instead take the probability space of all possible developments as the starting point.     

Is theS-meson a baryonium state?

If theS-meson is assumed to be a baryonium state composed of an isospin one diquark and antidiquark, it will be produced in \(\bar pp\) reactions as a mixture ofI=0 andI=1 baryonium states. The experimentally observed large ratio of the cross sections of the reactions \(\bar pp \to S \to \bar pp\) and \(\bar pp \to S^0 \to \bar nn\) is then explained on basis of quark additivity and conservation of isospin in thes-channel. The model predicts: \(\sigma (\bar pp \to S^0 \to \bar pp):\sigma (\bar pp \to S^0 \to \bar nn):\sigma (\bar pn \to S^ - \to \bar pn) = 25:1:16\) .     

Strong electron–phonon interaction and colossal magnetoresistance in EuTiO<Subscript>3</Subscript>

At low temperatures, EuTiO₃ system has very large resistivities and exhibits colossal magnetoresistance. Based on a first principle calculation and the dynamical mean-field theory for small polaron we have calculated the transport properties of EuTiO₃. It is found that due to electron–phonon interaction the conduction band may form a tiny polaronic subband which is close to the Fermi level. The tiny subband is responsible for the large resistivity. Besides, EuTiO₃ is a weak antiferromagnetic material and its magnetization would slightly shift the subband via exchange interaction between conduction electrons and magnetic atoms. Since the subband is close to the Fermi level, a slight shift of its position gives colossal magnetoresistance.     

Is there a Problem with our Hamiltonians for Quantum Nonlinear Optical Processes?

The models we use, habitually, to describe quantum nonlinear optical processes have been remarkably successful, yet, with few exceptions, they each contain a mathematical flaw. We present this flaw, show how it can be fixed, and, in the process, suggest why we can continue to use our favored Hamiltonians.