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Results for “Phase Transition”

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PubMed2005

Structural phase transitions of vortex matter in an optical lattice.

We consider the vortex structure of a rapidly rotating trapped atomic Bose-Einstein condensate in the presence of a corotating periodic optical lattice potential. We observe a rich variety of structural phases which reflect the interplay of the vortex-vortex and vortex-lattice interactions. The lattice structure is very sensitive to the ratio of vortices to pinning sites and we observe structural phase transitions and domain formation as this ratio is varied.

Journal Article↗

PubMed1989

Fluorescence depolarization study of lamellar liquid crystalline to inverted cylindrical micellar phase transition of phosphatidylethanolamine.

The orientational order and rotational dynamics of 2-[3-(diphenyl-hexatrienyl) propanoyl]-3-palmitoyl-L-alpha- phosphatidylcholine (DPH-PC) embedded in dioleoylphosphatidyl-ethanolamine (DOPE) were studied by fluorescence depolarization technique. Upon increasing the temperature, the calculated wobbling diffusion constant D perpendicular of the fluorescent probe was found to decrease at the lamellar (L alpha) to inverted cylindrical (H II) phase transition (10 degrees C). This suggested that the increased gauche rotamers of the alkene chains in the HII phase imposes a constraint in the wobbling motion of the fluorophore. The calculated ratio of order parameter in the L alpha phase to that in the HII phase was 1.7 and different from the theoretical value of 2.0 as predicted from the change in packing symmetry. This result can be explained by a slightly higher local order parameter of the fluorophore or by the fast rotational diffusion motion of the fluorophore around the symmetry axis of the cylindrical tubes in the HII phase.

Colloids↗

PubMed2005

Possible observation of phase separation near a quantum phase transition in doubly connected ultrathin superconducting cylinders of aluminum.

The kinetic energy of superconducting electrons in an ultrathin, doubly connected superconducting cylinder, determined by the applied flux, increases as the cylinder diameter decreases, leading to a destructive regime around half-flux quanta and a superconductor to normal metal quantum phase transition (QPT). Regular steplike features in resistance versus temperature curves taken at fixed flux values were observed near the QPT in ultrathin Al cylinders. It is proposed that these features are most likely resulting from a phase separation near the QPT in which normal regions nucleate in a homogeneous superconducting cylinder.

Journal Article↗

PubMed2001

Irreversible phase transitions driven by an oscillatory parameter in a far-from-equilibrium system.

The dynamic response of a forest-fire model to the harmonic variation of an external parameter is studied by means of numerical simulations. Second-order irreversible phase transitions driven by the harmonic input are reported. The location of such transitions depends on both the amplitude and period of the input signal. By means of epidemic studies the relevant critical exponents can be determined, which allow us to place the reported transitions in the universality class of directed percolation. This conclusion is also supported by a field theoretical calculation.

Journal Article↗

PubMed2003

Quantum phase transition in the Frenkel-Kontorova chain: from pinned instanton glass to sliding phonon gas.

We study analytically and numerically the one-dimensional quantum Frenkel-Kontorova chain in the regime where the classical model is located in the pinned phase characterized by the gaped phonon excitations and devil's staircase. By extensive quantum Monte Carlo simulations, we show that for the effective Planck constant Planck smaller than the critical value Planck(c) the quantum chain is in the pinned instanton glass phase. In this phase, the elementary excitations have two branches: phonons, separated from zero energy by a finite gap, and instantons that have an exponentially small excitation energy. At Planck = Planck(c) the quantum phase transition takes place and for Planck > Planck(c) the pinned instanton glass is transformed into the sliding phonon gas with gapless phonon excitations. This transition is accompanied by the divergence of the spatial correlation length and appearance of sliding modes at Planck > Planck(c).

Journal Article↗

PubMed2002

Interactions and disorder in quantum dots: instabilities and phase transitions.

Using a fermionic renormalization group approach, we analyze a model where the electrons diffusing on a quantum dot interact via Fermi-liquid interactions. Describing the single-particle states by random matrix theory, we find that interactions can induce phase transitions (or crossovers for finite systems) to regimes where fluctuations and collective effects dominate at low energies. Implications for experiments and numerical work on quantum dots are discussed.

Journal Article↗

PubMed2002

Isomorphism and phase transition in triferrocenylboroxine and triferrocenylborazine.

The crystal structures of triferrocenylboroxine, [Fe(3)(C(5)H(5))(3)(C(15)H(12)B(3)O(3))], (I), and triferrocenylborazine, [Fe(3)(C(5)H(5))(3)(C(15)H(15)B(3)N(3))], (II), are isomorphous. At room temperature, the space group is Cmc2(1) and the molecules have crystallographic m symmetry. A reversible phase transition occurs at 283(2)K for (I) and at 263(5)K for (II). In the low-temperature phase, the space group of both compounds is P2(1) and the molecules no longer have internal symmetry. Intermolecular C-H...pi interactions are enhanced in the low-temperature phase.

Journal Article↗

PubMed2026

Nuclear export modulates TDP-43 phase transition and cytoplasmic aggregation.

RNA-binding protein TAR DNA-binding protein 43 (TDP-43) can form liquid-like, nuclear assemblies whose phase behavior may influence its aggregation propensity and neurotoxic activity. The mechanism(s) that modulates the transition of TDP-43 from a liquid to solid phase is poorly defined. Here we combine chemical and genome-wide genetic screenings to identify cellular factors that modulate the phase behavior of an RNA-binding defective TDP-43 mutant that mimics an Amyotrophic Lateral Sclerosis (ALS)-associated variant. Our screens uncover multiple cellular processes including RNA splicing, protein translation, proteostasis imbalance and nuclear export as TDP-43 phase regulators. Importantly, TDP-43 phase transition can be dynamically recapitulated in vitro in a semi-permeabilized cell system, which reveals that the inhibition of nuclear export reshapes the nuclear environment in favor of an RNA-dependent TDP-43 liquid-liquid phase separation (LLPS) state, which mitigates cytoplasmic TDP-43 aggregation. We validated this mechanism in a brain organoid model bearing an ALS-associated mutation, showing that nuclear export deficiency can limit pathogenic phospho-TDP-43 accumulation. These findings establish nuclear export as a key regulator of TDP-43 phase transitions and define a mechanistic framework that links altered nuclear transport and phase dynamics to TDP-43 aggregation potential.

Amyotrophic Lateral Sclerosis (ALS)↗

PubMed1998

Stimulation of cyclin-dependent kinase activity and G1- to S-phase transition in human lymphocytes by the human T-cell leukemia/lymphotropic virus type 1 Tax protein.

The human T-cell leukemia/lymphotropic virus type 1 (HTLV-1) induces a malignant lymphocytic disease. The HTLV-1 transactivator protein, Tax, is believed to be crucial for the development of the disease since it is transforming in vitro and induces tumors in transgenic animals. Although the transcriptional modulation of viral and cellular gene expression by Tax has been analyzed thoroughly, it has remained unclear how the Tax functions act on the cell cycle of primary T cells. To investigate the mechanism of Tax-mediated T-cell stimulation, we transduced primary human cord blood T cells with a conditional, tetracycline repressor-based tax expression system. Permanent Tax expression results in an abnormal proliferation of T cells which closely resemble HTLV-1-infected lymphocytes. Suppression of Tax synthesis stopped lymphocyte growth and caused cell cycle arrest in the G1 phase. Upon reinduction of tax expression, the arrested cells entered the S phase. This showed that Tax has mitogenic activity, which is required for stimulating the G1- to S-phase transition of immortalized lymphocytes. In mammalian cells, the G1-phase progression is controlled by the serial activation of several cyclin-dependent kinases (Cdks), starting with Cdk4 and Cdk6. In the presence of Tax, both Cdk4 and Cdk6 were activated. The suppression of Tax synthesis, however, resulted in a significant reduction of the Cdk4 and Cdk6 activities but did not influence the expression of Cdk4, Cdk6, or cognate D-type cyclin proteins. These data suggest that Tax induces Cdk4 and Cdk6 activity in primary human T lymphocytes; this Cdk activation is likely to account for the mitogenic Tax effect and for the abnormal T-cell proliferation of HTLV-1-infected lymphocytes.

Animals↗

PubMed2006

Phase transitions of methane using molecular dynamics simulations.

Using a short ranged Lennard-Jones interaction and a long ranged electrostatic potential, CH4 under high pressure was modeled. Molecular dynamics simulations on small clusters (108 and 256 molecules) were used to explore the phase diagram. Regarding phase transitions at different temperatures, our numerical findings are consistent with experimental results to a great degree. In addition, the hysteresis effect is displayed in our results.

Journal Article↗

PubMed2003

Critical disorder and phase transitions in random diode arrays.

Random diode arrays represent a new class of nonlinear disordered systems related to the physics of thin-film semiconductor structures and some others. When a disorder strength grows through a certain critical value, they undergo a phase transition from almost uniform to strongly nonuniform random electric potential. A piecewise continuous topography of random potential is predicted.

Journal Article↗

PubMed1977

[Sedimentation rate of erythrocytes as an indicator for phase transitions in the membrane].

Temperature dependent sedimentation measurements with human erythrocytes showed that the sedimentation velocity at 21 degrees C in Krebs-Ringer-solution, pH 7.4, has a peak shaped minimum [1]. In further investigations it could be shown that a phase transition of membrane lipids is the main reason of this peak. Membrane proteins influence the sedimentation behavior of the erythrocytes only by changing the interaction with membrane lipids.

Blood Sedimentation↗

PubMed2004

Phase transition in random walks with long-range correlations.

Motivated by recent results in the theory of correlated sequences, we analyze the dynamics of random walks with long-term memory (binary chains with long-range correlations). In our model, the probability for a unit bit in a binary string depends on the fraction of unities preceding it. We show that the system undergoes a dynamical phase transition from normal diffusion, in which the variance D(L) scales as the string's length L, into a superdiffusion phase ( D(L) approximately Lalpha,alpha>1), when the correlation strength exceeds a critical value. We demonstrate the generality of our results with respect to alternative models, and discuss their applicability to various data, such as coarse-grained DNA sequences, written texts, and financial data.

Journal Article↗

PubMed2003

The preservation of liposomes by raffinose family oligosaccharides during drying is mediated by effects on fusion and lipid phase transitions.

Raffinose family oligosaccharides (RFO) have been implicated as protective agents in the cellular dehydration tolerance, especially of many plant seeds. However, their efficacy in stabilizing membranes during dehydration has never been systematically investigated. We have analyzed the effects of sucrose, raffinose, stachyose, and verbascose on liposome stability during air-drying. With increasing degree of polymerization (DP), the RFO were progressively better able to stabilize liposomes against leakage of aqueous content and against membrane fusion after rehydration. Indeed, there was a very tight linear correlation between fusion and leakage for all RFO. These data indicate that increased protection of liposomes against leakage with increasing DP is due to better protection against fusion. This is in accord with the higher glass transition temperature of the longer chain oligosaccharides. Further evidence for the influence of glass transitions on membrane stability in the dry state was provided by experiments testing the temperature dependence of membrane fusion. During incubation at temperatures up to 95 degrees C for 2 h, fusion increased less with temperature in the presence of higher DP sugars. This indicates that RFO with a higher glass transition temperature are better able to protect dry membranes at elevated temperatures. In addition, Fourier-transform infrared (FTIR) spectroscopy showed a reduction of the gel to liquid-crystalline phase transition temperature of dry liposomes in the presence of all investigated sugars. However, the RFO became slightly less effective with increasing chain length, again pointing to a decisive role for preventing fusion. A direct interaction of the RFO with the lipids was indicated by a strong effect of the sugars on the phosphate asymmetric stretch region of the infrared spectrum.

Desiccation↗

PubMed2002

Dynamic phase transitions in the anisotropic XY spin system in an oscillating magnetic field.

The Ginzburg-Landau model for the anisotropic XY spin system in an oscillating magnetic field below the critical temperature T(c), psi;(r,t)=(T(c)-T)psi-/psi/(2)psi+gammapsi(*)+ nabla (2)psi+h cos(Omegat) is both theoretically and numerically studied. Here psi is the complex order parameter and gamma stands for the real anisotropy parameter. It is numerically shown that the spatially uniform system shows various characteristic oscillations (dynamical phases), depending on the amplitude h and the frequency Omega of the external field. As the control parameter, either h or Omega, is changed, there exist dynamical phase transitions (DPT), separating them. By making use of the mode expansion analysis, we obtain the phase diagrams, which turn out to be in a qualitative agreement with the numerically obtained ones. By carrying out the Landau expansion, the reduced equations of motion near the DPT are derived. Furthermore, taking into account the spatial variation of order parameters, we will derive the analytic expressions for domain walls, which are represented by the Néel and Bloch type walls, depending on the difference of the coexistence of phases.

Journal Article↗

PubMed1983

Gel to liquid-crystalline phase transitions of aqueous dispersions of polyunsaturated mixed-acid phosphatidylcholines.

The thermotropic properties of aqueous dispersions of synthetic mixed-acid polyunsaturated 1,2-diacyl-3-sn-phosphatidylcholines (PC) have been studied by differential scanning calorimetry. The gel to liquid-crystalline phase transition temperature (Tc) of 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC) was -16.2 +/- 1.6 degrees C (means +/- SD, nine dispersions, three separate synthetic preparations); that for a preparation of 1-stearoyl-2-linolenoyl-PC (SLnPC) was -13 degrees C and for a preparation of 1-stearoyl-2-arachidonoyl-PC (SAPC) was -12.6 +/- 1.0 degrees C (three dispersions, one preparation). The temperatures of maximum heat flow (Tmax) for the lipid transitions were -14.4 +/- 1.3, -11, and -10.7 +/- 0.9 degrees C and the enthalpies were 3.3 +/- 1.0, 6.6, and 5.3 +/- 1.8 kcal X mol-1 for SLPC, SLnPC, and SAPC, respectively. The transition temperatures and enthalpies are rationalized on the basis of existing data on the effect of double-bond position on Tc and are interpreted with the use of a statistical mechanical model. The trend of the transition temperatures with the introduction of multiple double bonds reflects opposing effects due to increased molecular area and decreasing degrees of freedom of rotation about carbon-carbon bonds.

Calorimetry, Differential Scanning↗

PubMed2003

Apparent tricritical behavior at a nearly second-order nematic-isotropic phase transition of a cyclic liquid crystalline trimer.

The cyclic liquid crystalline trimer TPB-(c)9(3) was investigated by optical retardation and Fréedericksz techniques within a few tens of millikelvins of the superheating limit of the nearly second-order nematic-isotropic phase transition. Both the optical retardation and the Fréedericksz bend threshold voltage are in good agreement with tricritical behavior for the transition.

Journal Article↗

PubMed2005

Tsallis' q index and Mori's q phase transitions at the edge of chaos.

We uncover the basis for the validity of the Tsallis statistics at the onset of chaos in logistic maps. The dynamics within the critical attractor is found to consist of an infinite family of Mori's q -phase transitions of rapidly decreasing strength, each associated with a discontinuity in Feigenbaum's trajectory scaling function sigma. The value of q at each transition corresponds to the same special value for the entropic index q, such that the resultant sets of q-Lyapunov coefficients are equal to the Tsallis rates of entropy evolution.

Journal Article↗