Attractive Dipole-dipole Interactions Research Articles

Two-Dimensional Quantum Droplets in Binary Dipolar Bose-Bose Mixture

We study two-dimensional (2D) isotropic quantum droplets (QDs) in dipolar binary Bose–Einstein condensates (BECs). The QDs are supported by the competition between the 2D form of the Lee-Huang-Yang (LHY) term and the isotropic dipole-dipole interactions (DDIs). Moreover, the DDIs in the 2D plane can be tuned to be either repulsive or attractive. Before that, QDs in dipolar BECs were often explored in three-dimensional (3D) systems, with competition between the attractive DDIs and the repulsive LHY term. Unlike the 3D system, the LHY term of the 2D binary system behaves in a logarithmic form, which can feature both attraction and repulsion. In this case, the QDs can be produced regardless of the interactions (attraction, repulsion, or zero) that the mean-field effect represents. In this paper, we model the aforementioned QDs via the 2D binary dipolar BECs with the competition between isotropic DDIs and the logarithmic LHY term. Their characteristic parameters (the peak density, IP, chemical potential, μ, and effective area, Aeff) using both numerical and theoretical methods are discussed. The centripetal collision and oblique collision between moving QDs are also studied.

Enhanced interactions of interlayer excitons in free-standing heterobilayers.

Strong, long-range dipole-dipole interactions between interlayer excitons (IXs) can lead to new multiparticle correlation regimes1,2, which drive the system into distinct quantum and classical phases2-5, including dipolar liquids, crystals and superfluids. Both repulsive and attractive dipole-dipole interactions have been theoretically predicted between IXs in a semiconductor bilayer2,6-8, but only repulsive interactions have been reported experimentally so far3,9-16. This study investigated free-standing, twisted (51°, 53°, 45°) tungsten diselenide/tungsten disulfide (WSe2/WS2) heterobilayers, in which we observed a transition in the nature of dipolar interactions among IXs, from repulsive to attractive. This was caused by quantum-exchange-correlation effects, leading to the appearance of a robust interlayer biexciton phase (formed by two IXs), which has been theoretically predicted6-8 but never observedbefore in experiments. The reduced dielectric screening in a free-standing heterobilayer not only resulted in a much higher formation efficiency of IXs, but also led to strongly enhanced dipole-dipole interactions, which enabled us to observe the many-body correlations of pristine IXs at the two-dimensional quantum limit. In addition, we firstly observed several emission peaks from moiré-trapped IXs at room temperature in a well-aligned, free-standing WSe2/WS2 heterobilayer. Our findings open avenues for exploring new quantum phases with potential for applications in non-linear optics.

Intercalation of 3X-pyridine with X = F, Cl, Br, I, in 2D ferrous nitroprusside. Thermal induced spin transition in Fe(3F-pyridine)2[Fe(CN)5NO

The series of 2D solids, Fe(3XPy)2[Fe(CN)5NO] with 3X ​= ​3-Fluoro, 3-Chloro, 3-Bromo, 3-Iodo and Py ​= ​Pyridine, was obtained by intercalation of the organic molecule (3XPy) between neighboring layers of 2D ferrous nitroprusside. The organic ligand was found coordinated to the axial positions for the iron atom linked at the equatorial N ends of the CN groups in the nitroprusside building block, [Fe(CN)5NO]. The axial CN and NO ligands remain unlinked and contribute to the solid stability through attractive dipole-dipole interactions. The formed hybrid solids preserve the 2D nature because no chemical bond is established between neighboring organic molecules in the interlayer region. For X ​= ​F, a thermal induced spin transition was observed, but not for the remaining derivatives. The nature of such behavior is discussed according to the recorded XRD, Raman and Mössbauer data and computational calculations. The spin crossover transition in Fe(3FPy)2[Fe(CN)5NO] is an incomplete process, a sample fraction of about 50% remains in paramagnetic state, even at 5 ​K. The spin transition is accompanied of an atypical small structural change, corresponding to a variation of 0.7% for the unit cell volume. Such behavior finds explanation in both, the bonding properties of the N end of the CN ligands in the nitroprusside pseudo-octahedral block, and in the intermolecular interactions in the interlayer region.

Magnetic resonance and magnetization studies of Fe implanted TlInS2 and TlGaSe2 crystals

In this article, the results of investigations of the structure and magnetization of iron implanted TlInS2 and TlGaSe2 layered ferroelectric—semiconductor crystals have been reported. SEM studies revealed that a high—fluency implantation with Fe ions brings to metal nanoparticle formation in the near-surface irradiated region of both samples. Ferromagnetic Resonance (FMR) investigations exhibited an out-of-plane uniaxial magnetic anisotropy of Fe-implanted surface layer of the crystals. A soft ferromagnetic state in the Fe-implanted surface layer of the crystals has been observed from the magnetization measurements in a wide temperature interval. An unexpected magnetization versus temperature behaviour has been revealed by field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements. It has been found that the temperature dependencies of the ZFC magnetization of both samples are dominated on FC—curve in the temperature region of ∼145–393 K and ∼58–370 K for Fe—implanted TlGaSe2 and TlInS2, respectively. The observed phenomenon has been discussed taking into account various interaction mechanisms between magnetic nanoparticles, including attractive magnetic dipole-dipole interaction and repulsive electrostatic and interparticle exchange coupling interactions.

Constraints of neutron star on new interaction ofspin-dependent axial-vector coupling

It is predicted in many theories beyond the standard model that the new interaction relevant to spin is existent. The exchange of an axial vector particle will result in attractive dipole-dipole interaction which can be viewed as an effective magnetic potential that looks quite different from those expected from electromagnetism. In this work, we demonstrate that, instead of the laboratory spin source, stringent constraints can be set on these attractive spin-spin interactions from polarized nuclear matters within neutron stars which have extremely strong magnetic fields (up to 10<sup>15</sup> G in some cases). By considering such an exotic interaction within the framework of relativistic mean field model, we find that the stability of infinite nuclear matter can be influenced significantly when the ratio of coupling strength to boson mass become larger than <inline-formula><tex-math id="M4">\begin{document}$g_{\rm A}^2/m_{\rm Z'}^2 \sim {\cal O}(10 \, {\rm GeV^{-2}})$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20190477_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20190477_M4.png"/></alternatives></inline-formula>. Furthermore, based on the curvature matrix approach, when <inline-formula><tex-math id="M5">\begin{document}$g_{\rm A}^2/m_{\rm Z'}^2 > 130 \, {\rm GeV^{-2}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20190477_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="18-20190477_M5.png"/></alternatives></inline-formula>, phase transition inside low-density nuclear matter will no longer take place before the pressure of nuclear matter becomes zero, which forbids core-crust transition at the inner edge separating the liquid core from the solid crust of neutron stars. Thus bare neutron stars without any crusts are predicted. However, observations of pulsar glitches, i.e., the occasional disruptions of the extremely regular pulsations from magnetized, rotating neutron stars, imply the existence of crusts inside these dense objects. This in turn constrains the strength of the exotic interaction. In fact, in the case of dipole-dipole force on a length scale between µm to cm, the highest value of these constraints can be 8 orders of magnitude higher than those from existing laboratory results.

Association of κ-carrageenan subjected to deep alkaline hydrolysis.

κ-carrageenan is a linear sulfated anionic gelling polysaccharide obtained from red seaweed algae by an alkaline hydrolysis. We applied static and dynamic light scattering (DLS), capillary viscometry, FT-IR, and electrophoretic DLS to gain insight into the effect of deep alkaline hydrolysis (95 °C, pH = 10 during Th = 60, 75, and 90 min) on κ-carrageenan macromolecules in a coil conformation. As DLS of coil-like κ-carrageenans is usually complicated by spurious permanent aggregates, the alkaline hydrolysis of κ-carrageenans has not been studied by DLS. By applying a double-dialysis procedure (first against water, then against 0.1 M NaCl), we succeeded in obtaining dilute solutions of coil-like κ-carrageenans with unimodal DLS distributions. Association that started with increasing concentration was attributed to attractive dipole-dipole interactions between ion-pairs formed by condensed counter-ions in the vicinity of carboxyl groups of the reducing ends. The second type of stickers resulted from the compositional heterogeneity of κ-carrageenan, namely, from the traces of ι-carrageenan blocks which amount increased with hydrolysis. With increasing Th we observed both the progressive decrease of molecular mass and the growth of association. Lowering the temperature and increasing concentration during lyophilization strengthened the intermolecular junctions, leading to the formation of permanent aggregates.

Vortex patterns and the critical rotational frequency in rotating dipolar Bose-Einstein condensates

Based on the two-dimensional mean-field equations for pancake-shaped dipolar Bose-Einstein condensates in a rotating frame with both attractive and repulsive dipole-dipole interaction (DDI) as well as arbitrary polarization angle, we study the profiles of the single vortex state and show how the critical rotational frequency change with the s-wave contact interaction strengths, DDI strengths and the polarization angles. In addition, we find numerically that at the `magic angle' $\vartheta=\arccos(\sqrt{3}/3)$, the critical rotational frequency is almost independent of the DDI strength. By numerically solving the dipolar GPE at high rotational speed, we identify different patterns of vortex lattices which strongly depend on the polarization direction. As a result, we undergo a study of vortex lattice structures for the whole regime of polarization direction and find evidence that the vortex lattice orientation tends to be aligned with the direction of the dipoles.

Quantum phases of constrained dipolar bosons in coupled one-dimensional optical lattices

We investigate a system of two- and three-body constrained dipolar bosons in a pair of one-dimensional optical lattices coupled to each other by the non-local dipole-dipole interactions. Assuming attractive dipole-dipole interactions, we obtain the ground state phase diagram of the system by employing the cluster mean-field theory. The competition between the repulsive on-site and attractive nearest-neighbor interactions between the chains yields three kinds of superfluids; namely the trimer superfluid, pair superfluid and the usual single particle superfluid along with the insulating Mott phase at the commensurate density. Besides, we also realize simultaneous existence of Mott insulator and superfluid phases for the two- and three-body constrained bosons, respectively. We also analyze the stability of these quantum phases in the presence of a harmonic trap potential.

Trimeric Assembly of Dendritic Light-Harvesting Antenna with Two Kinds of Porphyrin Cores.

A trimeric assembly of light-harvesting antennas was prepared using a copper-catalyzed Hüisgen 1,3-dipolar cycloaddition reaction between a dendrimer having a zinc diethynyldiphenylporphyrin core (ZnDEDPP) with two azide terminals and two equivalents of dendrimers having a zinc tetraphenylporphyrin core (ZnTPP) with one ethynyl terminal. The absorptions of the trimer appear in a longer-wavelength region compared to monomeric references in toluene; however, there is almost no shift in wavelength in 1,1,2,2-tetrachloroethane (TCE). Fluorescence spectra of the trimer show that the singlet energy transfer from ZnTPP to ZnDEDPP takes place more effectively in toluene than in TCE. These absorption and fluorescence studies are compatible with solvent-dependent conformation; the extended forms of the trimers are favored by solvation in polar TCE, and the folded conformation is stabilized by the attractive van der Waals and dipole-dipole interactions between the dendritic chains in nonpolar toluene.

Aggregation of cyclic polypeptoids bearing zwitterionic end-groups with attractive dipole-dipole and solvophobic interactions: a study by small-angle neutron scattering and molecular dynamics simulation.

Aggregation behavior of cyclic polypeptoids bearing zwitterionic end-groups in methanol has been studied using a combination of experimental and simulation techniques. The data from SANS and cryo-TEM indicate that the solution contains small clusters of these cyclic polypeptoids, ranging from a single polypeptoid chain to small oligomers, while the linear counterpart shows no cluster formation. Atomistic molecular dynamics simulations reveal that the driving force for this clustering behavior is due to the interplay between the effective repulsion due to the solvation of the dipoles formed by the charged end-groups in each polypeptoid chain and the attractive forces due to dipole-dipole interactions and the solvophobic effect.

Like Charges Attract?

Using multiscale first-principles calculations, we show that two interacting negatively charged B12I9(-) monoanions not only attract, in defiance of the Coulomb's law, but also the energy barrier at 400 K is small enough that these two moieties combine to form a stable B24I18(2-) moiety. Ab initio molecular dynamics simulations further confirm its stability up to 1500 K. Studies of other B12X9(-) (X = Br, Cl, F, H, Au, CN) show that while all of these B24X18(2-) moieties are stable against dissociation, the energy barrier, with the exception of B24Au18(2-), is large so as to hinder their experimental observation. Our results explain the recent experimental observation of the "spontaneous" formation of B24I18(2-) in an ion trap. A simple model based upon electrostatics shows that this unusual behavior is due to competition between the attractive dipole-dipole interaction caused by the aspherical shape of the particle and the repulsive interaction between the like charges.

Bright solitons in a two-dimensional spin-orbit-coupled dipolar Bose-Einstein condensate

We study a two-dimensional spin-orbit-coupled dipolar Bose-Einstein condensate with repulsive contact interactions by both the variational method and the imaginary time evolution of the Gross-Pitaevskii equation. The dipoles are completely polarized along one direction in the 2D plane so as to provide an effective attractive dipole-dipole interaction. We find two types of solitons as the ground states arising from such attractive interactions: a plane wave soliton with a spatially varying phase and a stripe soliton with a spatially oscillating density for each component. Both types of solitons possess smaller size and higher anisotropy than the soliton without spin-orbit coupling. Finally, we discuss the properties of moving solitons, which are nontrivial because of the violation of Galilean invariance.

All-optical measurement of Rydberg-state lifetimes

We have developed an all-optical method for measuring the lifetimes of $nS$ and $nD$ Rydberg states and demonstrate its capabilities with measurements on a dilute cloud of ultracold $^{87}\mathrm{Rb}$ atoms in a cryogenic environment. The method is based on the time-resolved observation of resonant light absorption by ground-state atoms and selective transfer of Rydberg atoms into the ground state at varying delay times in order to reconstruct Rydberg decay curves. Our measurements of the $^{87}\mathrm{Rb}$ $30{S}_{1/2}$ state indicate an increase of the lifetime at lowered environment temperatures, as expected due to decreased blackbody radiation. For the $38{D}_{5/2}$ state with an attractive dipole-dipole interaction, ionization and lifetime reduction due to collisional effects are observed.

Synthesis and higher-order structure of linear dendrimeric assemblies.

A series of linear covalent assemblies of dendrimers with conjugated backbones were synthesized by means of an iterative divergent and convergent method in which copper-catalyzed acetylene-azide cycloaddition reactions were used to form flexible linkages. UV-vis and fluorescence spectra indicate that dendrimeric assemblies have a strong propensity to adopt folded conformations in which conjugated backbones adopt planar conformations. The folded conformation is stabilized by strong attractive van der Waals and dipole-dipole interactions between the dendrimers.

Square tiling by square macrocycles at the liquid/solid interface: co-crystallisation with one- or two-dimensional order.

We have systematically investigated the self-assembled monolayers of seven bimolecular mixtures of square-shaped pyridinophanes and cyclophanes bearing alkoxy or alkoxycarbonyl substituents in the presence of the tropylium ion as a marker of pyridinophanes at liquid/graphite interfaces by means of scanning tunnelling microscopy (STM). The purpose of this work was to elucidate the mixing behaviour of these macrocycles highlighting the formation of one- or two-dimensionally ordered square tilings consisting of alternating alignments of different macrocycles as a result of attractive dipole-dipole or hydrogen-bonding interactions; four co-crystals differing in the dimensionality of the ordering of pyridinophane and cyclophane were observed. The different modes of interaction between the functional groups (ether or carbonyl group) in the side-chains of the pyridinophanes and cyclophanes lead to the formation of co-crystals with dimensionally different orderings of the two macrocycles. These observations revealed that a slight modification of the molecular structure may dramatically change the mixing behaviour and structures of the co-crystals.

Trapping and chaining self-assembly of colloidal polystyrene particles over a floating electrode by using combined induced-charge electroosmosis and attractive dipole-dipole interactions.

We propose a novel low-frequency strategy to trap 10 μm colloidal polystyrene (PS) particles of small buoyancy velocity on the surface of a floating electrode, on the basis of combined induced-charge electroosmotic (ICEO) flow and dipole-dipole chaining phenomenon. For field frequencies of 5-50 Hz, much lower than the reciprocal RC time scale, double-layer polarization makes electric field lines pass around the 'insulating' surface of the ideally polarizable floating electrode. Once the long-range ICEO convective micro-vortexes transport particles quickly from the bulk fluid to the electrode surface, neighbouring particles aligned along the local horizontal electric field attract one another by attractive dipolar interactions, and form arrays of particle chains that are almost parallel with the applied electric field. Most importantly, this low-frequency trapping method takes advantage of the dielectrophoretic (DEP) particle-particle interaction to enhance the downward buoyancy force of this dipolar chaining assembly structure, in order to overcome the upward ICEO fluidic drag and realize stable particle trapping around the flow stagnation region. For the sake of comparison, the field frequency is further raised far above the DC limit. At the intermediate frequencies of 200 Hz-2 kHz, this trapping method fails to work, since the normal electric field component emanates from the conducting electrode surface. Besides, at high field frequencies (>3 kHz), particles can be once again effectively trapped at the electrode center, though with a compact (3 kHz) or disordered (10 kHz) 2D packing state on the electrode surface and mainly governed by the short-range negative DEP force field, resulting in requiring a much longer trapping time. To gain a better interpretation of the various particle behaviours observed in experiments, we develop a theoretical framework that takes into account both Maxwell-Wagner interfacial charge relaxation at the particle/electrolyte interface and the field-induced double-layer polarization at the electrode/electrolyte interface, and apply it to quantify the particle-particle electrokinetic interactions. With this simple geometrical configuration of a floating electrode, our results provide a new way to realize trapping of colloidal particles with a small buoyancy velocity under the combined action of ICEO flow and an attractive dipole-dipole interaction.

The CH-π interactions of methyl ethers as a model for carbohydrate-N-heteroarene interactions.

CH-π interactions have been cited as an important contributor to carbohydrate recognition. To determine whether N-heterocycles form stronger CH-π interactions, the interactions of methyl ether groups with heterocyclic and nonheterocyclic aromatic surfaces were studied. Both experimental and computational experiments found that N-heterocyclic aromatic surfaces formed stronger interactions. This enhancement was attributed to attractive dipole-dipole interactions between the methyl ether C-O bond and the N-heterocyclic aromatic dipole.

Controllable Binding of Polar Molecules and Metastability of One-Dimensional Gases with Attractive Dipole Forces

We explore one-dimensional samples of ultracold polar molecules with attractive dipole-dipole interactions and show the existence of a repulsive barrier caused by a strong quadrupole interaction between molecules. This barrier can stabilize a gas of ultracold KRb molecules and even lead to long-range wells supporting bound states between the molecules. The properties of these wells can be controlled by external electric fields, allowing the formation of long polymerlike chains of KRb and studies of quantum phase transitions by varying the effective interaction between molecules. We discuss the generalization of those results to other systems.

Collapse of Highly Charged Polyelectrolytes Triggered by Attractive Dipole−Dipole and Correlation-Induced Electrostatic Interactions

In the first part of the paper, we study the collapse of flexible highly charged polyelectrolyte chains induced by attractive dipole-dipole interactions. The latter emerge due to the formation of dipoles between the chain monomers and counterions condensed on the polyelectrolyte from solution. Using the statistics of slightly perturbed Gaussian polymers, we obtain the scaling relations for the chain dimensions as a function of polyelectrolyte linear charge density in the limit of compacting chains. The results are in good agreement with the outcomes of recent molecular dynamics simulations of the collapse of flexible polyelectrolytes in the presence of explicit counterions. In the second part, we analyze the results of molecular dynamics simulations for the complex formation by two highly charged polyelectrolyte chains carrying opposite charges. We use the scaling arguments based on the picture of complexation of electrostatic blobs in order to rationalize the size of the complexes of two polyelectrolyte chains in the collapsed state. Similar scaling relation for the complex size was recently obtained in computer simulations of complexation of diblock polyampholytes and was described theoretically on the basis of similar electrostatic blob concepts. We also analyze the density of complexes formed and polyelectrolyte linear charge densities required for the onset onto the collapse as a function of interaction strength between the monomers. In both parts of the paper, we overview the scaling relationships obtained for similar systems with alternating charges from other theoretical approaches.

Theory of polyzwitterion conformations

Conformational characteristics of polyzwitterionic molecules in aqueous solutions are investigated using the variational method. Analytical relations are derived for the radius of gyration of a single polyzwitterionic chain as a function of the chain length, electrostatic interaction strength, added salt concentration, dipole moment, and degree of ionization of the zwitterionic monomers. In the absence of the small ions (counterions and coions) near the polyzwitterionic chain, attractive dipole-dipole interactions are shown to induce a collapse of the polyzwitterionic chain. However, in the presence of the small ions, the radius of gyration is shown to be an interplay of the screening of the electrostatic interactions and the counterion adsorption on the zwitterionic sites. In addition to the well-known Debye–Hückel screening of the charge-charge interactions, screening of the charge-dipole, and dipole-dipole interactions are found to play important roles in determining the size of the chain. Functional forms for the screened charge-dipole and dipole-dipole interaction potentials are presented. Furthermore, counterion adsorption on the zwitterionic monomers is predicted to be asymmetric depending on the nature of the added salt and the zwitterionic groups. Qualitative remarks regarding the solubility of these molecules in aqueous solutions along with the classical “antipolyelectrolyte” effect (increase in the solubility in water with the addition of salt) are presented.