Dipolar Systems Research Articles

Crystal size dependence of dipolar ferromagnetic order between Mn6 molecular nanomagnets

We study how crystal size influences magnetic ordering in arrays of molecular nanomagnets coupled by dipolar interactions. Compressed fluid techniques have been applied to synthesize crystals of ${\mathrm{Mn}}_{6}$ molecules (spin $S=12$) with sizes ranging from $28\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ down to 220 nm. The onset of ferromagnetic order and the spin thermalization rates have been studied by means of ac susceptibility measurements. We find that the ordered phase remains ferromagnetic, as in the bulk, but the critical temperature ${T}_{\mathrm{c}}$ decreases with crystal size. Simple magnetostatic energy calculations, supported by Monte Carlo simulations, account for the observed drop in ${T}_{\mathrm{c}}$ in terms of the minimum attainable energy for finite-sized magnetic domains limited by the crystal boundaries. Frequency-dependent susceptibility measurements give access to the spin dynamics. Although magnetic relaxation remains dominated by individual spin flips, the onset of magnetic order leads to very long spin thermalization time scales. The results show that size influences the magnetism of dipolar systems with as many as ${10}^{11}$ spins and are relevant for the interpretation of quantum simulations performed on finite lattices.

Anisotropy-mediated reentrant localization

We consider a 2d dipolar system, d=2d=2, with the generalized dipole-dipole interaction \sim r^{-a}∼r−a, and the power aa controlled experimentally in trapped-ion or Rydberg-atom systems via their interaction with cavity modes. We focus on the dilute dipolar excitation case when the problem can be effectively considered as single-particle with the interaction providing long-range dipolar-like hopping. We show that the spatially homogeneous tilt \betaβ of the dipoles giving rise to the anisotropic dipole exchange leads to the non-trivial reentrant localization beyond the locator expansion, a<da<d, unlike the models with random dipole orientation. The Anderson transitions are found to occur at the finite values of the tilt parameter \beta = aβ=a, 0<a<d0<a<d, and \beta = a/(a-d/2)β=a/(a−d/2), d/2<a<dd/2<a<d, showing the robustness of the localization at small and large anisotropy values. Both exact analytical methods and extensive numerical calculations show power-law localized eigenstates in the bulk of the spectrum, obeying recently discovered duality a↔2d-aa↔2d−a of their spatial decay rate, on the localized side of the transition, a>a_{AT}a>aAT. This localization emerges due to the presence of the ergodic extended states at either spectral edge, which constitute a zero fraction of states in the thermodynamic limit, decaying though extremely slowly with the system size.

Determination of the barrier height of a crystalline dipolar molecular rotor directly from temperature‐dependent dielectric spectra

AbstractAn explicit relation between half‐width at half‐maximum ΔT in a dissipation factor of dipolar molecular rotors in a solid phase and rotational barrier height Ub was derived within the Debye relaxation model. The formula for Ub is simply expressed as . Here, T* is the temperature at the peak position in the dissipation factor, and k is the Boltzmann constant. The formula is an alternative to the familiar Arrhenius equation, from which Ub has so far been estimated in various dielectric experiments. The distinction of the proposed formula is the determination of Ub from a single peak in a dissipation factor induced by an electric field, while the Arrhenius equation needs data for the magnitudes of the dissipation factor and temperatures at several peaks. By applying the proposed formula to dipole molecular rotors (ROT‐2F) in a single crystalline and polycrystalline, for which Ub values were reported (Horansky et al., Phys. Rev. B 74, 054306 [2006]), the calculated values of Ub were quantitatively reproduced. This indicates the validity of the proposed formula, and the formula is expected to be applied to the analysis of dielectric spectra of 2D dipolar molecular rotor systems.

Tris(4'-Nitrobiphenyl)amine─An Octupolar Chromophore with High Two-Photon Absorption Cross-Section and Its Application for Uncaging of Calcium Ions in the Near-Infrared Region.

Compounds with high two-photon absorption (2PA) performance in the near-infrared region have attracted great attention because of their application in the material and biological science. In this study, we have developed a simple and novel octupolar chromophore, tris(4'-nitrobiphenyl)amine 1, with three nitro peripheral groups attached to a triphenylamine core via biphenyl linkers. A mono-branched analogue 2 has also been prepared to investigate the effects of octupolar and dipolar systems on photophysical and 2PA behaviors. Compound 1, despite having a much simpler structure than the previous three-branched scaffolds, exhibits comparable σ2 values, reaching 1330 GM at 730 nm and 900 GM at 820 nm in toluene. Combined with an outstanding σ2/MW ratio (2.2 GM g-1 mol) and a high fluorescence quantum yield (0.51), 1 displays potential as a promising two-photon (2P) probe for bioimaging. Subsequently, the ethylene glycol tetraacetic acid-substituted derivatives featuring octupolar (3 and 5) or dipolar (4 and 6) character have been synthesized and their one-photon (1P) and 2P photochemical reactions have been examined. Finally, 1P- and 2P-triggered uncaging of Ca2+ from these calcium chelators has been confirmed.

Pair localization in dipolar systems with tunable positional disorder

Strongly interacting quantum systems subject to quenched disorder exhibit intriguing phenomena such as glassiness and many-body localization. Theoretical studies have mainly focused on disorder in the form of random potentials, while many experimental realizations naturally feature disorder in the interparticle interactions. Inspired by cold Rydberg gases, where such disorder can be engineered using the dipole blockade effect,we study a Heisenberg XXZ spin model where the disorder is exclusively due to random spin-spin couplings, arising from power-law interactions between randomly positioned spins. Using established spectral and eigenstate properties and entanglement entropy, we show that this system exhibits a localization crossover and identify strongly interacting pairs as emergent local conserved quantities in the system, leading to an intuitive physical picture consistent with our numerical results.

Exact ground state energy of a system with an arbitrary number of dipoles at the sites of a regular one-dimensional crystal lattice

We consider a finite classical system of electric dipoles localized at the sites of a regular one-dimensional crystal lattice. It is shown that the ground state energy of such a system can be calculated exactly for an arbitrary number of dipoles. The expression for the ground state energy can be given in terms of special functions such as Riemann zeta functions and polygamma functions for any arbitrary number of dipoles, and reduces to the correct result in the thermodynamic limit as the number of dipoles tends to infinity. Simpler, but very accurate, approximate expressions for the ground state energy are also introduced.

Universal Properties of Anisotropic Dipolar Bosons in Two Dimensions

The energy of ultra-dilute quantum many-body systems is known to exhibit a universal dependence on the gas parameter x=n a_0^dx=na0d, with nn the density, dd the dimensionality of the space (d=1,2,3d=1,2,3) and a_0a0 the ss-wave scattering length. The universal regime typically extends up to x\approx 0.001x≈0.001, while at larger values specific details of the interaction start to be relevant and different model potentials lead to different results. Dipolar systems are peculiar in this regard since the anisotropy of the interaction makes a_0a0 depend on the polarization angle \alphaα, so different combinations of n and \alphaα can lead to the same value of the gas parameter x. In this work we analyze the scaling properties of dipolar bosons in two dimensions as a function of the density and polarization dependent scattering length up to very large gas parameter values. Using Quantum Monte Carlo (QMC) methods we study the energy and the main structural and coherence properties of the ground state of a gas of dipolar bosons by varying the density and scattering length for a fixed gas parameter. We find that the dipolar interaction shows relevant scaling laws up to unusually large values of xx that hold almost to the boundaries in the phase diagram where a transition to a stripe phase takes place.

Can DLVO theory be applied to MOF in different dielectric solvents?

ZIF-8 is widely used as a porous crystal material in the field of colloids. Its interfacial interactions are crucial to the performance of colloidal particles. In this paper, the Hamaker constant of ZIF-8 (3.45 × 10−20 J) was calculated for the first time by contact angle measurement parameters. The non-retarded Hamaker constant of ZIF-8 (4.66 × 10−21 J) was calculated using the Lifshitz's theory. Its dispersion in the dipolar (water) and monopolar (toluene) solvent systems was discussed using DLVO theory. The results showed that the energy potential barrier reached +183.9 KBT in the water system. However, the particles tended to be unstable in the toluene system, which did not correspond to the experimental phenomena we observed. Using the XDLVO theory, we took the Lewis acid-base fraction as an object of investigation, and the results showed that the hydrophobic attraction force between particles was 102 mJ/m2 in the dipolar solvent (water). In the monopolar system, the steric repulsion was linked to the Lewis acid-base fraction of the particles. XDLVO theory can better describe the dispersion of ZIF-8 particles in different solvents. In future work, it is necessary to consider the Lewis acid-base surface energy due to crystal defects, especially for the interfacial interaction between solvent and particles during the preparation of colloidal suspensions.

Behavior of local quantum Fisher information and local quantum uncertainty in dipolar spin system with DM and KSEA interactions

In this paper, a two-spin-1/2 particle coupled via dipolar interaction with the interplay of antisymmetric Dzyaloshinskii–Moriya and symmetric Kaplan–Shekhtman–Entin–Wohlman–Aharony interactions is considered at thermal equilibrium. The local quantum uncertainty and local quantum Fisher information are investigated as the pairwise nonclassical correlations quantifiers. The effects of the different system parameters on the quantified quantum correlation are analyzed in detail. Our results showed that the two studied thermal quantum correlation measures have similar qualitative behavior as well as quantitative sometimes. Also, it is shown that the quantumness of correlations is lost as the temperature rises sufficiently. Nevertheless, the antisymmetric and symmetric contributions of spin-orbit coupling considerably improve the amount of quantum correlations. Our analysis emphasized also that the local quantum uncertainty is bounded by local quantum Fisher information.

Analysis of Shape Change of Droplet in Dipolar Bose–Hubbard Model

The long-range and anisotropic nature of the dipolar interaction provides the so-called supersolid phases in Bose-Einstein condensates (BECs) in an optical lattice. However, in a certain area of dipole interaction parameters, BECs can form into a droplet. In this paper, in order to qualitatively understand the droplet formations, we propose a toy model that allows us to estimate the size and shape of droplets in dipolar Bose-Hubbard system in the optical lattice. We compare results of the toy model with numerical solutions of the mean-field calculation.

A semi-analytical solution of a system of dipoles placed along a ring tending to an exact solution at low temperatures

A semi-analytical solution of a system of dipoles placed along a ring tending to an exact solution at low temperatures

Formation of couples of topological defects in one-dimensional magnetic dipole systems

Formation of couples of topological defects in one-dimensional magnetic dipole systems

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below T = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(T). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε')-1 down to T ≈ 0.3 K and comparing it with the data obtained for conventional quantum paraelectrics, like SrTiO3, KTaO3, we discovered clear signatures of a quantum-critical behavior of the interacting water molecular dipoles: Between T = 6 and 14 K, the reciprocal permittivity follows a quadratic temperature dependence and displays a shallow minimum below 3 K. This is the first observation of "dielectric fingerprints" of quantum-critical phenomena in a paraelectric system of coupled point electric dipoles.

On the attractive inverse-square potential in the induced electric dipole system under the influence of the harmonic oscillator

We obtain the analytical solutions to the Schrödinger equation for the attractive inverse-square potential in an induced electric dipole moment system under the influence of the harmonic oscillator. We show that bound states can exist when the electric field configuration brings a cut-off point that imposes a forbidden region for the neutral particle. Then, by dealing with s-waves, we obtain the energy eigenvalues in the strong electric field regime and for small values of the angular frequency of the harmonic oscillator. Further, we extend our discussion about the energy eigenvalues beyond the s-waves.

Benzotriazole-based multidonor-acceptor systems as attractive two-photon absorption dye platforms

Pyrazine-decorated benzotriazole cores allow the orthogonal combination of two dipolar systems within a single molecule. A series of this type of derivatives was synthesized and their photophysical features were studied. The properties of these compounds showed remarkable differences in function depending on the substitution in the pyrazine ring of the benzotriazole core. Furthermore, the two-photon absorption property (2PA) was studied to determine the structure-properties relationship for the reported compounds. The best dye achieved a cross-section of 1532 GM, which was higher than values previously obtained for similar D-π-A-π-D benzotriazole derivatives. TD-DFT calculations, which supported the experimental observations, indicating the interaction between the two dipolar systems was responsible for enhancing the 2 PA properties and favoring bathochromic shifts.

Comment on: ‘A doubly anharmonic oscillator in an induced electric dipole system’

We argue that recent conclusions about a doubly anharmonic oscillator appear to be wrong. The radial eigenvalue equation is conditionally solvable and the authors derived some analytical eigenvalues for states with polynomial solutions. The allowed discrete values of the cyclotron frequency or intensity of the uniform magnetic field are an artefact of the approach used to obtain such particular solutions. There are bound states when the angular velocity of the rotating reference frame is null, contrary to what the authors claimed.

Much of a Muchness: On the Origins of Two- and Three-Photon Absorption Activity of Dipolar Y-Shaped Chromophores.

The molecular origin of two- (2PA) and three-photon absorption (3PA) activity in three experimentally studied chromophores, prototypical dipolar systems, is investigated. To that end, a generalized few-state model (GFSM) formula is derived for the 3PA transition strength for nonhermitian theories and employed at the coupled-cluster level of theory. Using various computational techniques such as molecular dynamics, linear and quadratic response theories, and GFSM, an in-depth analysis of various optical channels involved in 2PA and 3PA processes is presented. It is found that the four-state model involving the second and third excited singlet states as intermediates is the smallest model among all considered few-state approximations that produces 2PA and 3PA transition strengths (for S0 → S1 transition) close to the reference results. By analyzing various optical channels appearing in these models and involved in studied multiphoton processes, we found that the 2PA and 3PA activities in all the three chromophores are dominated and hence controlled by the dipole moment of the final excited state. The similar origins of the 2PA and the 3PA in these prototypical dipolar chromophores suggest transferability of structure–property relations from the 2PA to the 3PA domain.

Choosing Bad versus Worse: Predictions of Two-Photon-Absorption Strengths Based on Popular Density Functional Approximations.

We present a benchmark study of density functional approximation (DFA) performances in predicting the two-photon-absorption strengths in π-conjugated molecules containing electron-donating/-accepting moieties. A set of 48 organic molecules is chosen for this purpose, for which the two-photon-absorption (2PA) parameters are evaluated using different DFAs, including BLYP, PBE, B3LYP, PBE0, CAM-B3LYP, LC-BLYP, and optimally tuned LC-BLYP. Minnesota functionals and ωB97X-D are also used, applying the two-state approximation, for a subset of molecules. The efficient resolution-of-identity implementation of the coupled-cluster CC2 model (RI-CC2) is used as a reference for the assessment of the DFAs. Two-state models within the framework of both DFAs and RI-CC2 are used to gain a deeper insight into the performance of different DFAs. Our results give a clear picture of the performance of the density functionals in describing the two-photon activity in dipolar π-conjugated systems. The results show that global hybrids are best suited to reproduce the absolute values of 2PA strengths of donor–acceptor molecules. The range-separated functionals CAM-B3LYP and optimally tuned LC-BLYP, however, show the highest linear correlations with the reference RI-CC2 results. Hence, we recommend the latter DFAs for structure–property studies across large series of dipolar compounds.

Thermal radiation in dipolar many-body systems

The framework of fluctuational electrodynamics for dipolar many-body systems is one of the working horse for theoretical studies of thermal radiation at the nanoscale which includes dissipation and retardation in a naturally way. Based on this framework I will discuss near-field thermal radiation in non-reciprocal and topological many-body systems. The appearance of the Hall and non-reciprocal diode effect for thermal radiation illustrates nicely the interesting physics in such systems as well as the edge mode dominated heat transfer in topological Su-Schrieffer-Heeger chains and a honeycomb lattices of plasmonic nanoparticles. In the latter, the theory allows for quantifying the effciency of the edge-mode dominated heat transfer as function of the dissipation. Finally, I will present how the theoretical framework can be generalized to study far-field thermal emission of many-body systems close to an environment like a substrate, for instance. This theory might be particularly interesting for modelling thermal imaging microscopes.

Dynamical effective field model for interacting ferrofluids: I. Derivations for homogeneous, inhomogeneous, and polydisperse cases

Quite recently I have proposed a nonperturbative dynamical effective field model (DEFM) to quantitatively describe the dynamics of interacting ferrofluids. Its predictions compare very well with the results from Brownian dynamics simulations. In this paper I put the DEFM on firm theoretical ground by deriving it within the framework of dynamical density functional theory, taking into account nonadiabatic effects. The DEFM is generalized to inhomogeneous finite-size samples for which the macroscopic and mesoscopic scale separation is nontrivial due to the presence of long-range dipole–dipole interactions. The demagnetizing field naturally emerges from microscopic considerations and is consistently accounted for. The resulting mesoscopic dynamics only involves macroscopically local quantities such as local magnetization and Maxwell field. Nevertheless, the local demagnetizing field essentially couples to magnetization at distant macroscopic locations. Thus, a two-scale parallel algorithm, involving information transfer between different macroscopic locations, can be applied to fully solve the dynamics in an inhomogeneous sample. I also derive the DEFM for polydisperse ferrofluids, in which different species can be strongly coupled to each other dynamically. I discuss the underlying assumptions in obtaining a thermodynamically consistent polydisperse magnetization relaxation equation, which is of the same generic form as that for monodisperse ferrofluids. The theoretical advances presented in this paper are important for both qualitative understanding and quantitative modeling of the dynamics of ferrofluids and other dipolar systems.