Twisted Configurations Research Articles

Isospin-0 ππ s-wave scattering length from twisted mass lattice QCD

We present results for the isospin-0 $\pi\pi$ s-wave scattering length calculated with Osterwalder-Seiler valence quarks on Wilson twisted mass gauge configurations. We use three $N_f = 2$ ensembles with unitary (valence) pion mass at its physical value (250$\sim$MeV), at 240$\sim$MeV (320$\sim$MeV) and at 330$\sim$MeV (400$\sim$MeV), respectively. By using the stochastic Laplacian Heaviside quark smearing method, all quark propagation diagrams contributing to the isospin-0 $\pi\pi$ correlation function are computed with sufficient precision. The chiral extrapolation is performed to obtain the scattering length at the physical pion mass. Our result $M_\pi a^\mathrm{I=0}_0 = 0.198(9)(6)$ agrees reasonably well with various experimental measurements and theoretical predictions. Since we only use one lattice spacing, certain systematics uncertainties, especially those arising from unitary breaking, are not controlled in our result.

Numerical Investigation of Thermal Performance Factor for different Twisted Tape Configurations

In this article an attempt has been made to study the heat transfer variations for different twisted tape geometry. Twisted tapes of different pitch and split ratios arranged in alternate clock and counter clockwise for fully developed laminar flow were used to study heat enhancement. The numerical analysis was carried out in different Reynolds number ranging between 400-1000 for a different pitch ratio (y = p/w, 3.2, 6.5 and 13) and split ratio (S=L/n, n=2, 3, 4), every split arranged in a counter clockwise with preceding one. The investigation revealed that decrement in pitch ratio and increment in split ratio enhances the heat transfer rate. Thermal Performance factor (TPF) is also calculated for different tape configurations and its variation is also notified in the work.

Spherical nematic shells with a prolate ellipsoidal core.

Liquid crystal shells have attracted considerable attention in recent years. In such systems, a combination of confinement and curvature generates topological defect structures that do not exist in the bulk. Past studies, however, have largely focused on perfectly spherical shells, and little attention has been devoted to the impact of core geometry on the configuration and arrangement of topological defects. In this work, a microfluidic glass capillary device is used to encapsulate spherical and prolate ellipsoidal particles in nematic liquid crystal (LC) droplets dispersed in aqueous media. Our experimental studies show that, when trapped inside a radial LC droplet, spherical particles with both homeotropic and planar anchoring are highly localized at the droplet's center. While the radial configuration of the LC droplets is not altered by a homeotropic particle, polystyrene particles with strong planar anchoring disturb the radial ordering, leading to a twisted structure. Experiments indicate that off-center particle positions can also arise, in which defects are displaced towards the vicinity of the droplet's surface. In contrast, when prolate ellipsoidal particles are encapsulated in a thick radial LC shell, the minimum free energy corresponds to configurations where the particle is positioned at the droplet center. In this case, defects arise at the two ends of the prolate ellipsoid, where the curvature of the particle is maximal, leading to the formation of peculiar hybrid and twisted structures.

Nontrivial center dominance in high temperature QCD

We investigate the properties of quarks and gluons above the chiral phase transition temperature [Formula: see text], using the renormalization group (RG) improved gauge action and the Wilson quark action with two degenerate quarks mainly on a [Formula: see text] lattice. In the one-loop perturbation theory, the thermal ensemble is dominated by the gauge configurations with effectively [Formula: see text] center twisted boundary conditions, making the thermal expectation value of the spatial Polyakov loop take a nontrivial [Formula: see text] center. This is in agreement with our lattice simulation of high temperature quantum chromodynamics (QCD). We further observe that the temporal propagator of massless quarks at extremely high temperature [Formula: see text] remarkably agrees with the temporal propagator of free quarks with the [Formula: see text] twisted boundary condition for [Formula: see text], but differs from that with the [Formula: see text] trivial boundary condition. As we increase the mass of quarks [Formula: see text], we find that the thermal ensemble continues to be dominated by the [Formula: see text] twisted gauge field configurations as long as [Formula: see text] and above that the [Formula: see text] trivial configurations come in. The transition is similar to what we found in the departure from the conformal region in the zero-temperature many-flavor conformal QCD on a finite lattice by increasing the mass of quarks.

Axial charges of hyperons and charmed baryons usingNf=2+1+1twisted mass fermions

The axial couplings of the low lying baryons are evaluated using a total of five ensembles of dynamical twisted mass fermion gauge configurations. The simulations are performed using the Iwasaki gauge action and two degenerate flavors of light quarks, and a strange and a charm quark fixed to approximately their physical values at two values of the coupling constant. The lattice spacings, determined using the nucleon mass, are $a=0.082$ fm and $a=0.065$ fm and the simulations cover a pion mass in the range of about 210 MeV to 430 MeV. We study the dependence of the axial couplings on the pion mass in the range of about 210 MeV to 430 MeV as well as the $SU(3)$ breaking effects as we decrease the light quark mass towards its physical value.

Two-photon decays of $$\eta _c$$ η c from lattice QCD

We present an exploratory lattice study for the two-photon decay of $\eta_c$ using $N_f=2$ twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration. Two different lattice spacings of $a=0.067$fm and $a=0.085$fm are used in the study, both of which are of physical size of 2$fm$. The decay widths are found to be $1.025(5)$KeV for the coarser lattice and $1.062(5)$KeV for the finer lattice respectively where the errors are purely statistical. A naive extrapolation towards the continuum limit yields $\Gamma\simeq 1.122(14)$KeV which is smaller than the previous quenched result and most of the current experimental results. Possible reasons are discussed.

Lattice study of(D¯1D*)±near-threshold scattering

In this exploratory lattice study, low-energy near threshold scattering of the $(\bar{D}_1 D^{*})^\pm$ meson system is analyzed using lattice QCD with $N_f=2$ twisted mass fermion configurations. Both s-wave ($J^P=0^-$) and p-wave ($J^P=1^+$) channels are investigated. It is found that the interaction between the two charmed mesons is attractive near the threshold in both channels. This calculation provides some hints in the searching of resonances or bound states around the threshold of $(\bar{D}_1 D^{*})^\pm$ system.

Zig-zag twins and helical phase transformations.

We demonstrate the large bending deformation induced by an array of permanent magnets (applied field ∼0.02 T) designed to minimize poles in the bent state of the crystal. Planar cantilevers of NiMnGa (5M modulated martensite) ferromagnetic shape memory alloy deform into an arched shape according to theory, with a zig-zag microstructure that complies with the kinematic and magnetic compatibility between adjacent twin variants. A general theory of bent and twisted states is given, applicable to both twinning and austenite/martensite transformations. Some of these configurations achieve order-of-magnitude amplification of rotation and axial strain. We investigate also atomistic analogues of these bent and twisted configurations with perfect interfaces between phases. These mechanisms of large deformation, induced by small magnetic fields or temperature changes, have potential application to the development of new actuation technologies for micro-robotic systems.

HTS CroCo: A Stacked HTS Conductor Optimized for High Currents and Long-Length Production

In order to build high-current cables from second-generation high-temperature superconductor (HTS) rare-earth barium-copper-oxide (REBCO) tapes, numerous approaches were studied, such as conductor on round core, Roebel cable, and several versions of twisted stacked-tape cable types. Based on the work of Takayasu et al. and Uglietti et al., we developed and tested a modified type of stacked HTS tape arrangement optimized for high engineering critical current density. Key aspects were the implementation of a simple and reliable continuous fabrication routine for production of application-relevant lengths of twisted conductors in the range of 100 m and above and the subsequent enveloping by a seamless copper tube. Several samples of this HTS Cross-Conductor (HTS CroCo) were prepared successfully, both partially and fully equipped with REBCO tapes in untwisted and twisted configurations. The critical current of the samples was measured at T = 77 K and self-field conditions. The measurements showed the expected critical currents calculated from the individual tape values if taking into account the enhanced self-field in the tape arrangement. Furthermore, one untwisted partially REBCO-equipped sample was tested in the FBI facility at KIT at T = 4.2 K and in magnetic fields up to B = 12 T, showing good performance with no degradation even at high Lorentz forces. In addition, the mechanical performance of this sample was studied under tensile loads. No degradation could be observed, and the strain dependence was equal to that of single REBCO tapes. Due to the combination of excellent mechanical and electrical performance, the HTS CroCo is a promising candidate as a strand for long-length high-current cables, for example, with I c (4.2 K, self-field) ≈ 30 kA for power transmission or with I c (4.5 K, 14 T) ≈ 8 kA as a strand for high-current cables targeting large high-field magnets.

Low-energy scattering of the(D*D¯*)±system and the resonancelike structureZc(4025)

In this paper, low-energy scattering of the $(D^{*}\bar{D}^{*})^\pm$ meson system is studied within L\"uscher's finite-size formalism using $N_{f}=2$ twisted mass gauge field configurations. With three different pion mass values, the $s$-wave threshold scattering parameters, namely the scattering length $a_0$ and the effective range $r_0$, are extracted in $J^P=1^+$ channel. Our results indicate that, in this particular channel, the interaction between the two vector charmed mesons is weakly repulsive in nature hence do not support the possibility of a shallow bound state for the two mesons, at least for the pion mass values being studied. This study provides some useful information on the nature of the newly discovered resonance-like structure $Z_c(4025)$ observed in various experiments.

Orientation-Dependent London-van der Waals Interaction Energy between Macroscopic Bodies.

The aim of this work is to derive formulas for numerical calculations of the orientation-dependent London-van der Waals (vdW) interaction energy (V(A)) between two rectangular bodies with arbitrary dimensions, arranged at arbitrary relative angles (θ) and separations in twisted and coplanar rotational modes. The formulation is made using a simple volume-element-integration method in the framework of the microscopic approach, in which V(A) is the sum of the local vdW energy (Vp) between body 1 and each thin plate constituting body 2. Examples of the calculation results are the following: (1) The θ values that give maximal and minimal values of V(A) depend on their shapes and relative positions. (2) As the bodies come close to each other, the variations of V(A) with θ and thus vdW dispersion torques generated are drastically intensified. (3) Upon increasing the length of crossing rods in twisted configurations, the V(A) values become constant beyond a critical length (depending on θ and separation), where the length effect on V(A) disappears. (4) The distribution curves of Vp show that the region in body 2 which interacts effectively with body 1 (i.e., the effective interaction region) is more sharply localized in the vicinity of the surface (closest to body 1) as the separation is decreased.

Mass spectra of Ds meson from Nf = 2 twisted mass lattice QCD

In this paper, we study the mass spectra of Ds mesons, such as Ds, [Formula: see text] and Ds1. In the simulation, we use Nf = 2 twisted mass lattice QCD gauge configurations generated by the European Twisted Mass Collaboration (ETMC). The simulation is performed on lattices with lattice spacings a = 0.067 fm and lattice size 323×64, and a = 0.085 fm and lattice size 243×48, respectively. Our numerical results for the mass spectra of Ds mesons reached impressive agreement with the experimental data.

Baryon spectrum withNf=2+1+1twisted mass fermions

The masses of the low-lying baryons are evaluated using a total of ten ensembles of dynamical twisted mass fermion gauge configurations. The simulations are performed using two degenerate flavors of light quarks, and a strange and a charm quark fixed to approximately their physical values. The light sea quarks correspond to pseudo scalar masses in the range of about 210 to 430 MeV. We use the Iwasaki improved gluonic action at three values of the coupling constant corresponding to lattice spacing $a=0.094$, 0.082 and 0.065 fm determined from the nucleon mass. We check for both finite volume and cutoff effects on the baryon masses. We examine the issue of isospin symmetry breaking for the octet and decuplet baryons and its dependence on the lattice spacing. We show that in the continuum limit isospin breaking is consistent with zero, as expected. We performed a chiral extrapolation of the forty baryon masses using SU(2) $\ensuremath{\chi}\mathrm{PT}$. After taking the continuum limit and extrapolating to the physical pion mass our results are in good agreement with experiment. We provide predictions for the mass of the doubly charmed ${\mathrm{\ensuremath{\Xi}}}_{cc}^{*}$, as well as of the doubly and triply charmed $\mathrm{\ensuremath{\Omega}}$s that have not yet been determined experimentally.

Constraints on $${\rm I}\beta$$ I β cellulose twist from DFT calculations of $$^{13}\hbox {C}$$ 13 C NMR chemical shifts

We investigate theoretically the NMR response of twisted configurations of $${\rm I}\beta$$ cellulose in the tg conformation. These finite helical angle structures were constructed by a mathematical deformation of zero-angle configurations obtained via the periodic density functional energy minimizations with dispersion corrections (DFT-D2). Subsequent calculations of the $${^{13}\hbox {C}}$$ nuclear magnetic resonance chemical shifts $$({\delta}^{13} \hbox {C})$$ were compared with experimental findings by Erata et al. (Cellul Commun 4:128–131, 1997) and Kono et al. (Macromolecules 36:5131–5138, 2003). We determine the sensitivity of the NMR chemical shifts to helical deformation of the microfibril and find that a substantial range of helical angle, ±2 degrees/nm, is consistent with current experimental observations, with a most probable angle of ∼0.2 degree/nm. Through exhaustive combinatorial provisional assignments, we also demonstrate that there are different choices of the chemical shift $$({\delta}^{13} \hbox {C})$$ assignments which are consistent with the experiments, including ones with lower deviations than existing identifications.

Low-energy scattering of the(DD¯*)±system and the resonance-like structureZc(3900)

In this exploratory lattice study, low-energy scattering of the $(D{\overline{D}}^{*}{)}^{\ifmmode\pm\else\textpm\fi{}}$ meson system is analyzed using lattice QCD with ${N}_{f}=2$ twisted mass fermion configurations with three pion mass values. The calculation is performed within single-channel L\"uscher's finite-size formalism. The threshold scattering parameters, namely the scattering length ${a}_{0}$ and the effective range ${r}_{0}$, for the $s$-wave scattering in ${J}^{P}={1}^{+}$ channel are extracted. For the cases in our study, the interaction between the two charmed mesons is weakly repulsive. Our lattice results therefore do not support the possibility of a shallow bound state for the two mesons for the pion mass values we studied. This calculation provides some useful information on the nature of the newly discovered resonance-like structure ${Z}_{c}(3900)$ by various experimental groups.

Nucleon excited states inNf=2lattice QCD

We investigate the excited states of the nucleon using ${N}_{f}=2$ twisted mass gauge configurations with pion masses in the range of about 270 to 450 MeV and one ensemble of ${N}_{f}=2$ Clover fermions at almost physical pion mass. We use two different sets of variational bases and study the resulting generalized eigenvalue problem. We present results for the two lowest positive and negative parity states.

Natural torsion in chiral single-wall carbon nanotubes

Optimizations performed with two geometrical, molecular mechanical and density functional models show that the configuration of single-wall carbon nanotubes is slightly twisted with respect to that predicted by conventional techniques. Geometrical models are used to explain the effect by breaking of local symmetry of the equally distant neighbors in graphene in the course of rolling, which is minimized by torsion. The torsion angle is small, and decreases with nanotube diameter, being less than 1° Å−1 for ultrathin tubes. Therefore this effect can be hard to observe in experiments. Still, we single out features in diffraction patterns which can distinguish between twisted and non-twisted configurations. In addition, torsion makes a blue shift in the radial breathing mode, which for thin tubes can be seen by Raman measurement. The effect is slightly strengthened at room temperature.

Laser-directed hierarchical assembly of liquid crystal defects and control of optical phase singularities

Topological defect lines are ubiquitous and important in a wide variety of fascinating phenomena and theories in many fields ranging from materials science to early-universe cosmology, and to engineering of laser beams. However, they are typically hard to control in a reliable manner. Here we describe facile erasable “optical drawing” of self-assembled defect clusters in liquid crystals. These quadrupolar defect clusters, stabilized by the medium's chirality and the tendency to form twisted configurations, are shaped into arbitrary two-dimensional patterns, including reconfigurable phase gratings capable of generating and controlling optical phase singularities in laser beams. Our findings bridge the studies of defects in condensed matter physics and optics and may enable applications in data storage, singular optics, displays, electro-optic devices, diffraction gratings, as well as in both optically- and electrically-addressed pixel-free spatial light modulators.

Absorption and Fluorescence of PRODAN in Phospholipid Bilayers: A Combined Quantum Mechanics and Classical Molecular Dynamics Study

Absorption and fluorescence spectra of PRODAN (6-propionyl-2-dimethylaminonaphthalene) were studied by means of the time-dependent density functional theory and the algebraic diagrammatic construction method. The influence of environment, a phosphatidylcholine lipid bilayer and water, was taken into account employing a combination of quantum chemical calculations with empirical force-field molecular dynamics simulations. Additionally, experimental absorption and emission spectra of PRODAN were measured in cyclohexane, water, and lipid vesicles. Both planar and twisted configurations of the first excited state of PRODAN were taken into account. The twisted structure is stabilized in both water and a lipid bilayer, and should be considered as an emitting state in polar environments. Orientation of the excited dye in the lipid bilayer significantly depends on configuration. In the bilayer, the fluorescence spectrum can be regarded as a combination of emission from both planar and twisted structures.

Radiative transitions in charmonium fromNf=2twisted mass lattice QCD

We present a study for charmonium radiative transitions: $J/\psi\rightarrow\eta_c\gamma$, $\chi_{c0}\rightarrow J/\Psi\gamma$ and $h_c\rightarrow\eta_c\gamma$ using $N_f=2$ twisted mass lattice QCD gauge configurations. The single-quark vector form factors for $\eta_c$ and $\chi_{c0}$ are also determined. The simulation is performed at a lattice spacing of $a= 0.06666$ fm and the lattice size is $32^3\times 64$. After extrapolation of lattice data at nonzero $Q^2$ to 0, we compare our results with previous quenched lattice results and the available experimental values.