Conformal Regime Research Articles

Infrared Renormalons versus Operator Product Expansions in Supersymmetric and Related Gauge Theories.

We use the connection between infrared (IR) renormalons and condensates in the operator product expansion for correlation functions to make predictions concerning the structure of singularities in the Borel plane for the perturbative series in quantum field theories with different levels of supersymmetry. The same conspiracy can be used for establishing the absence of condensates or IR renormalons in gauge theories with an IR conformal regime or gauge theories in the Higgs phase. The absence of the renormalon-induced factorial divergence implies that instanton contributions (where present) must be well defined. We show that the conventional bubble-chain method for detecting renormalon-induced factorial divergences in these theories is not sufficient.

Plectoneme tip bubbles: coupled denaturation and writhing in supercoiled DNA.

We predict a novel conformational regime for DNA, where denaturation bubbles form at the tips of plectonemes, and study its properties using coarse-grained simulations. For negative supercoiling, this regime lies between bubble-dominated and plectoneme-dominated phases, and explains the broad transition between the two observed in experiment. Tip bubbles cause localisation of plectonemes within thermodynamically weaker AT-rich sequences, and can greatly suppress plectoneme diffusion by a pinning mechanism. They occur for supercoiling densities and forces that are typically encountered for DNA in vivo, and may be exploited for biological control of genomic processes.

Bile salts at the air–water interface: Adsorption and desorption

Bile salts (BS) are bio-surfactants which constitute a vital component in the process of fat digestion. Despite the importance of the interfacial properties in their biological role, these have been scarcely studied in the literature. In this work, we present the adsorption–desorption profiles of two BS (NaTC and NaGDC) including dilatational rheology. Findings from this study reveal very different surface properties of NaTC and NaGDC which originate from different complexation properties relevant to the digestion process. Dynamic adsorption curves show higher adsorption rates for NaTC and suggest the existence of various conformational regimes in contrast to NaGDC which presents only one conformational regime. This is corroborated by analysis of the adsorption isotherms and more in detail by the rheological behaviour. Accordingly, the dilatational response at 1Hz displays two maxima of the dilatational modulus for NaTC as a function of bulk concentration, in contrast to NaGDC which displays only one maximum. The desorption profiles reveal that NaTC adopts an irreversibly adsorbed form at high surface coverage whereas NaGDC fully desorbs from the surface within the whole range of concentrations used. Analysis of the adsorption–desorption profiles provides new insight into the surface properties of BS, suggesting a surface complexation of NaTC. This knowledge can be useful since through interfacial engineering we might control the extent of lipolysis providing the basis for the rational design of food products with tailored digestibility.

On the stability of D7–[formula omitted] probes in near-conformal backgrounds

We investigate the perturbative stability of a nonsupersymmetric D7–D7¯ brane embedding in a particular class of type IIB backgrounds. These backgrounds are the gravitational duals of certain strongly-coupled gauge theories, that exhibit a nearly conformal regime (known as a walking regime). Previous studies in the literature have led to conflicting results as to whether the spectrum of fluctuations around the flavor D7–D7¯ embedding has a tachyonic mode or not. Here we reconsider the problem with a new analytical method and recover the previously obtained numerical results. We also point out that the earlier treatments relied on a coordinate system, in which it was not possible to take into account fluctuations of the point of confluence of the D7–D7¯ branes. Using an improved coordinate system, we confirm the presence of a normalizable tachyonic mode in this model, in agreement with the numerical calculations. Finally, we comment on the possibility of turning on worldvolume flux to stabilize the probe branes configuration.

Electron backscatter diffraction analysis on the microstructures of electrolytic Cu deposition in the through hole filling process: Butterfly deposition mode

Electrolytic Cu deposits in a through hole (TH) structure are a critical reliability issue for the high density interconnection (HDI) technologies of three-dimensional packaging. In this study, we investigated the butterfly deposition of electrolytic Cu fillings via optical microscopy (OM) and field-emission scanning electron microscopy (FE-SEM) in combination with electron backscatter diffraction (EBSD). The Cu TH filling proceeds through three distinct regimes relative to the plating time (t): (1) the conformal deposition regime (t=30–40min), (2) fast deposition regime (t=40–55min), and (3) final deposition regime (t=55–110min). The EBSD analyses showed that the Cu grains were predominantly oriented with [111]‖TD (TD: transverse direction) in the conformal and fast deposition regimes; however, the [111]‖RD (RD: rolling direction) orientation became dominant in the final deposition regime. Additionally, a large number of high angle grain boundaries (HAGBs) with strong coincidence site lattices (CSLs) existed at Σ3 (60° rotation at 〈111〉) and Σ9 (38.9° rotation at 〈101〉) in the Cu fillings. The microstructural/crystallographic evolutions of the butterfly and other deposition modes (i.e., dogbonding) were compared.

Thermal Properties of Particle Brush Materials: Effect of Polymer Graft Architecture on the Glass Transition Temperature in Polymer‐Grafted Colloidal Systems

SummaryThe governing parameters controlling the glass transition temperature of polymer‐grafted particle systems are analyzed for the particular case of polystyrene (PS)‐grafted silica colloids in the dense grafting limit. At a given degree of polymerization of surface‐grafted chains the glass transition temperature is found to increase as compared to linear chain polymers of equivalent degree of polymerization. The difference in the glass transition temperature between polymer‐grafted particle systems and their respective linear polymer analogs increases with decreasing degree of polymerization of surface‐grafted chains and levels off at similar plateau values for particle brushes of distinct particle core size. The trend toward increased glass transition temperature is interpreted as a consequence of the increased steric hindrance in polymer‐grafted particles that counteracts the relaxation of surface‐grafted polymer chains. The increase in glass transition temperature is shown to be approximately consistent with the chain conformational regimes that are predicted on the basis of a Daoud‐Cotton type scaling model.

Adsorption Kinetics of Asphaltenes at the Oil–Water Interface and Nanoaggregation in the Bulk

Asphaltenes constitute high molecular weight constituents of crude oils that are insoluble in n-heptane and soluble in toluene. They contribute to the stabilization of the water-in-oil emulsions formed during crude oil recovery and hinder drop-drop coalescence. As a result, asphaltenes unfavorably impact water-oil separation processes and consequently oil production rates. In view of this there is a need to better understand the physicochemical effects of asphaltenes at water-oil interfaces. This study elucidates aspects of these effects based on new data on the interfacial tension in such systems from pendant drop experiments, supported by results from nuclear magnetic resonance (NMR) and dynamic light scattering (DLS) studies. The pendant drop experiments using different asphaltene concentrations (mass fractions) and solvent viscosities indicate that the interfacial tension reduction kinetics at short times are controlled by bulk diffusion of the fraction of asphaltenes present as monomer. At low mass fractions much of the asphaltenes appear to be present as monomers, but at mass fractions greater than about 80 ppm they appear to aggregate into larger structures, a finding consistent with the NMR and DLS results. At longer times interfacial tension reduction kinetics are slower and no longer diffusion controlled. To investigate the controlling mechanisms at this later stage the pendant drop experiment was made to function in a fashion similar to a Langmuir trough with interfacial tension being measured during expansion of a droplet aged in various conditions. The interfacial tension was observed to depend on surface coverage and not on time. All observations indicate the later stage transition is to an adsorption barrier-controlled regime rather than to a conformational relaxation regime.

Automated Prediction of Protein Association Rate Constants

The association rate constants (k(a)) of proteins with other proteins or other macromolecular targets are a fundamental biophysical property. Observed rate constants span over ten orders of magnitude, from 1 to 10(10) M(-1)s(-1). Protein association can be rate limited either by the diffusional approach of the subunits to form a transient complex, with near-native separation and orientation but without short-range native interactions, or by the subsequent conformational rearrangement to form the native complex. Our transient-complex theory showed promise in predicting k(a) in the diffusion-limited regime. Here, we develop it into a web server called TransComp (http://pipe.sc.fsu.edu/transcomp/) and report on the server's accuracy and robustness based on applications to over 100 protein complexes. We expect this server to be a valuable tool for systems biology applications and for kinetic characterization of protein-protein and protein-nucleic acid association in general.

Holographic fermions in external magnetic fields

We study the Fermi-level structure of $2+1$-dimensional strongly interacting electron systems in external magnetic field using the gague/gravity duality correspondence. The gravity dual of a finite density fermion system is a Dirac field in the background of the dyonic AdS-Reissner-Nordstr\"om black hole. In the probe limit, the magnetic system can be reduced to the nonmagnetic one, with Landau-quantized momenta and rescaled thermodynamical variables. We find that at strong enough magnetic fields, the Fermi surface vanishes and the quasiparticle is lost either through a crossover to conformal regime or through a phase transition to an unstable Fermi surface. In the latter case, the vanishing Fermi velocity at the critical magnetic field triggers the non-Fermi-liquid regime with unstable quasiparticles and a change in transport properties of the system. We associate it with a metal--''strange-metal'' phase transition. Next, we compute the DC Hall and longitudinal conductivities using the gravity-dressed fermion propagators. For dual fermions with a large charge, many different Fermi surfaces contribute and the Hall conductivity is quantized as expected for integer quantum Hall effect (QHE). At strong magnetic fields, as additional Fermi surfaces open up, new plateaus typical for the fractional QHE appear. The somewhat irregular pattern in the length of fractional QHE plateaus resembles the outcomes of experiments on thin graphite in a strong magnetic field. Finally, motivated by the absence of the sign problem in holography, we suggest a lattice approach to the AdS calculations of finite density systems.

AdS/QCD, LIGHT-FRONT HOLOGRAPHY, AND THE NONPERTURBATIVE RUNNING COUPLING

The combination of Anti-de Sitter space (AdS) methods with light-front (LF) holography provides a remarkably accurate first approximation for the spectra and wavefunctions of meson and baryon light-quark bound states. The resulting bound-state Hamiltonian equation of motion in QCD leads to relativistic light-front wave equations in terms of an invariant impact variable ζ which measures the separation of the quark and gluonic constituents within the hadron at equal light-front time. These equations of motion in physical space-time are equivalent to the equations of motion which describe the propagation of spin-J modes in anti–de Sitter (AdS) space. The eigenvalues give the hadronic spectrum, and the eigenmodes represent the probability distributions of the hadronic constituents at a given scale. A positive-sign confining dilaton background modifying AdS space gives a very good account of meson and baryon spectroscopy and form factors. The light-front holographic mapping of this model also leads to a non-perturbative effective coupling [Formula: see text] which agrees with the effective charge defined by the Bjorken sum rule and lattice simulations. It displays a transition from perturbative to nonperturbative conformal regimes at a momentum scale ~ 1 GeV. The resulting β-function appears to capture the essential characteristics of the full β-function of QCD, thus giving further support to the application of the gauge/gravity duality to the confining dynamics of strongly coupled QCD.

Nonperturbative QCD coupling and itsβfunction from light-front holography

The light-front holographic mapping of classical gravity in anti-de Sitter space, modified by a positive-sign dilaton background, leads to a nonperturbative effective coupling ${\ensuremath{\alpha}}_{s}^{\mathrm{AdS}}({Q}^{2})$. It agrees with hadron physics data extracted from different observables, such as the effective charge defined by the Bjorken sum rule, as well as with the predictions of models with built-in confinement and lattice simulations. It also displays a transition from perturbative to nonperturbative conformal regimes at a momentum scale $\ensuremath{\sim}1\text{ }\text{ }\mathrm{GeV}$. The resulting $\ensuremath{\beta}$ function appears to capture the essential characteristics of the full $\ensuremath{\beta}$ function of QCD, thus giving further support to the application of the gauge/gravity duality to the confining dynamics of strongly coupled QCD. Commensurate scale relations relate observables to each other without scheme or scale ambiguity. In this paper we extrapolate these relations to the nonperturbative domain, thus extending the range of predictions based on ${\ensuremath{\alpha}}_{s}^{\mathrm{AdS}}({Q}^{2})$.

Adsorption of star polymers: computer simulations

The behaviour of star polymers adsorbed on smooth surfaces is studied using coarse-grained bead-spring models and Langevin dynamics simulations. The conformational properties of a single adsorbed star polymer in good-solvent conditions are considered as functions of the functionality (number of arms) f, the number of monomers per arm N, and the monomer-surface interaction energy es. Four conformational regimes are identified: a linear-polymer regime; a two-dimensional star polymer regime; a sombrero regime; and a colloidal regime. The latter three correspond to regimes predicted theoretically by Halperin and Joanny [J. Phys. II (France), 1991, 1, 623–636]. Solvent effects are explored by dialing in effective attractions between the monomer beads; with decreasing solvent quality, the star polymers adopt more compact, globular structures. Good-solvent to bad-solvent quenches at finite surface coverages are considered; these correspond to established experimental protocols for adsorbing and then drying polymer sub-monolayers on surfaces. The structure of the polymer film is surveyed as a function of surface coverage, f, N, and es, in good-solvent and bad-solvent conditions. The simulated post-quench structures are in good qualitative agreement with those observed in atomic-force microscopy measurements, while the simulated pre-quench structures shed light on the microscopic mechanisms of film formation. This study draws together much of what is known about surface-adsorbed star polymers from theory, simulation, and experiment.

Enantiodiscrimination of Flexible Cyclic Solutes Using NMR Spectroscopy in Polypeptide Chiral Mesophases: Investigation ofcis-Decalin and THF

The conformational dynamics and orientational behavior of two model cyclic molecules, cis-decalin (cis-dec) and tetrahydrofurane (THF), dissolved in weakly ordering, polypeptidic chiral liquid crystals (CLCs) are theoretically discussed and experimentally investigated using deuterium and carbon-13 NMR spectroscopies. The analysis of enantiomeric and enantiotopic discriminations in these compounds is shown to depend on the rate of conformational exchange regime, slow or fast. The slow exchange regime is illustrated through the case of cis-dec at low temperature (243 K). We show that the deuterium NMR spectra in this regime can be qualitatively and quantitatively interpreted by restricting the conformational pathway of cis-dec to two enantiomeric conformers of C(2)-symmetry. The orientational order parameters of these interconverting enantiomers are calculated by matching the (2)H quadrupolar splittings with calculated conformer structures. The fast exchange regime is investigated through the examples of cis-dec at high temperature (356 K) and THF at room temperature (300 K). The (2)H NMR spectra above the coalescence temperature are analyzed by introducing the concept of "average molecular structure". This fictitious structure allows easily identifying NMR equivalences of solutes dissolved in CLC. However, it cannot be applied to determine consistent orientational order parameters. This study emphasizes that enantiotopic discriminations observed for flexible molecules in the fast exchange regime can be quantitatively interpreted only by considering the orientational order of each conformer.

N=2cascade revisited and the enhançon bearings

Supergravity backgrounds with varying fluxes generated by fractional branes at nonisolated Calabi-Yau singularities had escaped a precise dual field theory interpretation so far. In the present work, considering the prototypical example of such models, the $\mathbb{C}\ifmmode\times\else\texttimes\fi{}{\mathbb{C}}^{2}/{\mathbb{Z}}_{2}$ orbifold, we propose a solution for this problem, and show that the known cascading solution corresponds to a vacuum on the Coulomb branch of the corresponding quiver gauge theory involving a sequence of strong coupling transitions reminiscent of the baryonic root of $\mathcal{N}=2$ supersymmetric quantum chromodynamics . We also find a slight modification of this cascading vacuum which upon mass deformation is expected to flow to the Klebanov-Strassler cascade. Finally, we discuss an infinite class of vacua on the Coulomb branch whose renormalization group flows include infinitely coupled conformal regimes, and explain their gravitational manifestation in terms of new geometric structures that we dub enhan\ifmmode \mbox{\c{c}}\else \c{c}\fi{}on bearings. Repulson-free backgrounds dual to all the vacua we analyze are explicitly provided.

On The Pomeron at Large 't Hooft Coupling

We begin the process of unitarizing the Pomeron at large 't Hooft coupling. We do so first in the conformal regime, which applies to good accuracy to a number of real and toy problems in QCD. We rewrite the conformal Pomeron in the $J$-plane and transverse position space, and then work out the eikonal approximation to multiple Pomeron exchange. This is done in the context of a more general treatment of the complex $J$-plane and the geometric consequences of conformal invariance. The methods required are direct generalizations of our previous work on single Pomeron exchange and on multiple graviton exchange in AdS space, and should form a starting point for other investigations. We consider unitarity and saturation in the conformal regime, noting elastic and absorptive effects, and exploring where different processes dominate. Our methods extend to confining theories and we briefly consider the Pomeron kernel in this context. Though there is important model dependence that requires detailed consideration, the eikonal approximation indicates that the Froissart bound is generically both satisfied and saturated.

Parameterization and Conformational Sampling Effects in Pharmacophore Multiplet Searching

Pharmacophore patterns in ligands can be effectively characterized in terms of their constituent pharmacophore multiplets. Bitsets (fingerprints) encoding which particular multiplets are found in a given ligand have been and continue to be used as molecular descriptors in a range of molecular modeling applications, from ligand alignment and diversity analysis to pharmacophore-based flexible searching. Being able to create, store, and manipulate multiplets in compressed form - as bitmaps - has made it possible to integrate them into high-throughput technologies. A number of key parameters affect how well multiplets perform, including the granularity of edge length binning; how different multiplets are weighted in creating hypotheses from multiple ligands; and the number of bits that should be included in a pharmacophore hypothesis. The similarity metric employed for bitmap comparisons also affects search performance, as does the conformational sampling regime used for characterizing flexible molecules. In this report we explore the effect of parameter variation on within- and between-class similarity across seven different pharmacological classes and introduce a new measure of molecular similarity - the asymmetric stochastic cosine - uniquely suited to searching a database for matches to query hypotheses deduced from multiple ligands. Surprisingly, it turns out that the most discriminating bitmaps are obtained using relatively few conformers. The extreme discrimination power seen for single conformers, however, seems to reflect consistent effects of 2D connectivity on the 3D structure obtained. Conformational sampling by systematic search reinforces such circumstantial discrimination and should be avoided. The potential for systematic bias becomes clear when the behavior of otherwise similar conformational ensembles created by local energy minimization or by random sampling is considered. Consolidating information from multiple known actives or establishing single "bioactive" conformations a priori are safer ways to improve discrimination in pharmacophoric multiplet searching.

Enhanced radiation response through directed molecular targeting approaches.

In both the pre- and post-human genome sequencing eras, there has been an increase in the understanding of the molecular mechanisms influencing cellular sensitivity to DNA damaging agents such as ionizing radiation. Out of this work have arisen many cellular factors that could be specifically targeted, at the molecular level, to alter the functionality of a single protein or pathway involved in the response to radiation damage as a means to increase cell killing following radiation treatment. As such, there are many promising new combination radio-gene therapy approaches being developed and assessed in pre-clinical and clinical studies for several different malignancies. Combination of such modalities aims to increase the therapeutic index, giving rise to increased tumor cell killing with a simultaneous reduction in normal cell toxicity. Restricted delivery and/or targeting modalities combined with conformal radiotherapy regimes could provide significant local control of tumors, impeding their development into metastatic disease, which poses a greater challenge for palliative and curative treatments. This review will summarize current and novel gene therapy strategies that are being developed aimed at enhancing the effects of radiotherapy through the use of directed molecular targeting approaches.

Specific features of a magnetic impurity in a correlated electron model with Ising anisotropy

We present the Bethe ansatz solution of a magnetic hybridization impurity in a correlated electron host with Ising-like magnetic anisotropy. On the one hand, the strong correlations between host electrons produce a mixed-valence behavior of the localized electrons. On the other hand, the spin-gap of the low-lying unbound electron states caused by the ``easy-axis'' magnetic anisotropy suppresses the standard Kondo effect. For small enough magnetic fields the magnetic susceptibility of the impurity is zero. The usual Kondo logarithms, characteristic of SU(2)-symmetric systems with gapless spin-carrying excitations, are replaced by square root singularities caused by the closing of the spin gap at a critical field. Similar behavior is predicted for the free edges of an open chain, which again is very different from the isotropic situation. Using the low-lying excitations in the conformal regime we calculate asymptotics of correlation functions.

Adsorption of Sulfobetaine Polyampholyte on Silica Surfaces from Aqueous Salt Solutions

Adsorption of poly(N-3-sulfopropyl-N-methacrylooxyethyl-N,N-dimethyl ammonium betaine) (polysulfobetaine abbreviated as PSB), which is a neutral- and zwitterion-type polyampholyte, onto a silica surface from aqueous NaCl solutions was studied by adsorbance measurement and ellipsometry. Adsorbance A increases with decrease of added salt concentration Cs0, while the thickness of adsorbed layer L decreases with the decrease of Cs0. Comparison of L with the radius of gyration of PSB in NaCl solutions reveals the existence of collapsed (pancake)-, normal (fence)-, and elongated (pole)-like conformational regimes of PSB at the surfaces depending on the salt concentration. The salt distribution between the adsorbed layer and the bulk solution was estimated, and it was found that at low Cs0 positive ad(b)sorption of NaCl occurs while high Cs0 results in negative absorption of NaCl. The distribution of NaCl among adsorbed layers and bulk solution could be explained by Debye−Hückel theory, which takes into account the electrostatic attractive interaction among salt ions and zwitterion groups on PSB chain. It was also found that the polymeric osmotic and deformation pressure of adsorbed layer Δπ is related to the thickness of adsorbed layer L as Δπ ≈ L-9/4.

Effective action for membrane dynamics in DLCQ M theory on a two-torus

The effective action for the membrane dynamics on the background geometry of the $N$-sector DLCQ $M$ theory compactified on a two-torus is computed via supergravity. We compare it to the effective action obtained from the matrix theory, i.e., the (2+1)-dimensional supersymmetric Yang-Mills (SYM) theory, including the one-loop perturbative and full non-perturbative instanton effects. Consistent with the DLCQ prescription of $M$ theory {\em a la} Susskind, we find the precise agreement for the finite $N$-sector (off-conformal regime), as well as for the large $N$ limit (conformal regime), providing us with a concrete example of the correspondence between the matrix theory and the DLCQ $M$ theory. Non-perturbative instanton effects in the SYM theory conspire to yield the eleven-dimensionally covariant effective action.