Long-range Dipolar Interactions Research Articles

Fluid of hard spheres with dipolar-like patch interaction and effect of adding an isotropic adhesion

We compare two fluid models of spherical molecules with anisotropic, purely surface interactions. Both models admit an analytical solution of the molecular Ornstein–Zernike integral equation, within the Percus–Yevick approximation plus orientational linearization. In the first model, the molecular surface corresponds to a unique non-uniform patch, with a potential obtained by truncating a long-ranged dipolar interaction exactly at the contact distance between two hard sphere particles. In the second model, a further isotropic adhesion is added to the intermolecular potential. The study is focused on the local orientational ordering. Differences and similarities with respect to hard spheres with full long-ranged dipolar forces are analysed in detail. The effect of the competition between anisotropic patch interaction and isotropic adhesion is investigated through the pair correlation function as well as via two novel anisotropic order parameters.

Anisotropic superfluidity in the two-species polar Fermi gas

We study the superfluid pairing in a two-species gas of heteronuclear fermionic molecules with equal density. The interplay of the isotropic s-wave interaction and anisotropic long-range dipolar interaction reveals rich physics. We find that the single-particle momentum distribution has a characteristic ellipsoidal shape that can be reasonably represented by a deformation parameter $\alpha$ defined similarly to the normal phase. Interesting momentum-dependent features of the order parameter are identified. We calculate the critical temperatures of both the singlet and triplet superfluid, suggesting a possible pairing symmetry transition by tuning the s-wave or dipolar interaction strength.

Critical behavior near the Lifshitz point inSn2P2(S1 − xSex)6 ferroelectric semiconductors from thermal diffusivity measurements

The thermal diffusivity of the ferroelectric familySn2P2(SexS1 − x)6 (0 ≤ x ≤ 1) has been measured by a high-resolution ac photopyroelectric technique, using single crystals,with the aim of studying the evolution of the ferroelectric transition with Se doping. Itschange from second order character to first order while passing the Lifshitz point (x≈0.28) has been evaluated, as well as the splitting of the transition at high Se concentrations.The critical behavior of the ferroelectric transition in terms of the different universalityclasses and their underlying physical dominant effects (tricriticality, long-range dipoleinteractions, Lifshitz point) has been discussed using thermal diffusivity measurements inthe very close vicinity of the critical temperature. This study reveals that for Seconcentrations around the Lifshitz point, long-range dipole interactions do not play asignificant role and that the critical parameters are close to those predicted for the Lifshitzuniversality class.

Irreversibility, reversibility, and thermal equilibrium in domain patterns of Fe films with perpendicular magnetization

We investigate the details of pattern formation and transitions between different modulated phases in ultra-thin Fe films on Cu(001). At high temperature, the transitions between the uniform saturated state, the bubble state and the striped state are completely reversible, while at low temperature the bubble phase is avoided. The observed non-equilibrium behavior can be qualitatively explained by considering the intrinsic energy barriers appearing in the system due to the competition between the short-ranged exchange and the long-ranged dipolar interactions. Our experiments suggest that the height of these energy barriers is related to the domain size and is therefore strongly temperature dependent.

Center motions of nonoverlapping condensates coupled by long-range dipolar interaction in bilayer and multilayer stacks

We investigate the effect of anisotropic and long-range dipole-dipole interaction (DDI) on the center motions of nonoverlapping Bose-Einstein condensates (BEC) in bilayer and multilayer stacks. In the bilayer, it is shown analytically that while DDI plays no role in the in-phase modes of center motions of condensates, out-of-phase mode frequency ($\omega_o$) depends crucially on the strength of DDI ($a_d$). At the small-$a_d$ limit, $\omega_o^2(a_d)-\omega_o^2(0)\propto a_d$. In the multilayer stack, transverse modes associated with center motions of coupled condensates are found to be optical phonon like. At the long-wavelength limit, phonon velocity is proportional to $\sqrt a_d$.

Magnetic resonance line shifts in the quantum gas of atomic hydrogen

We present an experimental study of the quantum gas of atomic hydrogen in a strong magnetic field (4.6 T). The gas is compressed to densities of ∼1018 cm−3 at temperatures of 0.2–0.5 K. We observed shifts of the electron spin resonance (ESR) lines caused by the exchange interaction in atomic collisions (clock shift) and the long-range dipolar interactions. The clock shift was found to be vanishingly small in a doubly polarized gas, in contrast to a mixture of two hyperfine states. The difference is explained by properly including quantum statistical effects in the treatment of atomic collisions. From our data, we extract the difference between the triplet and singlet s-wave scattering lengths, at−as=60(10) pm, in agreement with existing theories. At densities above 1017 cm−3, weakly resolved structures appeared in the ESR spectra, interpreted in terms of electron spin waves. We consider exchange and dipolar interactions to possibly induce the spin waves.

A simulation study on the magnetic ordering in an artificial geometrically frustrated lattice

A Monte Carlo simulation on the magnetic ordering of the artificial geometrically frustrated square lattice shows that the system exhibits the spin ice state and the disorder state for strong and weak dipolar interactions. We demonstrate that the long-range dipolar interactions are significant for the short-range spin ice state.

Multiple coherent states semiclassical initial value representation spectra calculations of lateral interactions for CO on Cu(100)

Lateral interactions between carbon monoxide molecules adsorbed on a copper Cu(100) surface are investigated via semiclassical initial value representation (SC-IVR) molecular dynamics. A previous analytical potential is extended to include long-range dipole interactions between coadsorbed molecules and preliminary classical simulations were performed to tune the potential parameters. Then, the spectra for several coadsorbed molecules are calculated using the multiple coherent states approximation of the time-averaging representation of the SC-IVR propagator. Results show strong resonances between coadsorbed molecules as observed by past experiments. Resonances turn into dephasing when isotopical substitutions are performed.

Model for flexural phonon dispersion in graphite and graphene

A simple model for flexural phonons in graphite (and graphene, corresponding to the limiting case of infinite distance between carbon planes) is proposed, in which the local dipolar moment is assumed to be proportional to the curvature of the carbon sheets. Explicit expressions for dispersion curves with full account for the long-range dipolar interaction forces are obtained and fitted to the experimental data using a single adjustable parameter of the model. The parameter is expected to depend on the ground-state configuration of molecular $\ensuremath{\pi}$ orbitals, the same both for graphite and for graphene. At decreasing carbon sheet separation (high pressures) the phonon spectrum displays instability, corresponding to the graphite to diamond transition. Being explicitly based on the local dipolar moments, the proposed simple model may prove useful for considering electron-phonon interaction.

Negative and positive ion trapping by isotopic molecules in cryocrystals in case of solid parahydrogen containing electrons and H6+ radical cations

We performed electron spin resonance studies of trapped electrons and H(6) (+) radical cations produced by radiolysis of solid parahydrogen (p-H(2)), p-H(2)-ortho-D(2) (o-D(2)), and p-H(2)-HD mixtures. Yields of trapped electrons, H(6) (+) radical cations, and its isotopic analogs H(6-n)D(n) (+) (4>or=n>or=1) increased with increasing o-D(2) and HD concentrations in solid p-H(2). Electrons were found trapped near an o-D(2) or an HD in solid p-H(2) due to the long-range charge-induced dipole and quadrupole interactions between electrons and isotopic hydrogen molecules. H(6) (+) radical cations diffuse in solid p-H(2) by repetition of H(6) (+)+H(2)-->H(2)+H(6) (+) and are trapped by ortho-D(2) or HD to form H(6-n)D(n) (+) (4>or=n>or=1) as isotope condensation reactions. Decay behaviors of these cations by the repetition, isotope condensation, and geminate recombination between electrons and H(6-n)D(n) (+) (4>or=n>or=0) were reproduced by determining the corresponding reaction rate constants k(1), k(2), and k(3). Values of 0.045 and 0.0015 L mol(-1) min(-1) were obtained for k(1) (H(6) (+)+D(2)-->H(2)+H(4)D(2) (+)) and k(2) (H(4)D(2) (+)+D(2)-->H(2)+H(2)D(4) (+)), respectively, and the value was quasinull for k(3) (H(2)D(4) (+)+D(2)-->H(2)+D(6) (+)). These rate constants suggest that hole mobility drastically decreased in the repetition reaction when H(6) (+) radical cations acting as hole carriers formed H(4)D(2) (+) or H(2)D(4) (+). HD and D(2) molecules, therefore, act as electron and hole acceptors in irradiated solid p-H(2)-o-D(2) and p-H(2)-HD mixtures.

Distance Determination in Proteins inside Xenopus laevis Oocytes by Double Electron−Electron Resonance Experiments

DEER (double electron-electron resonance) enables the observation of long-range dipole interactions (1.5-8 nm) between electron spin centers and has become a unique method for structural analysis of site-directed spin-labeled (SDSL) proteins. The method was applied to proteins inside eukaryotic cells, Xenopus laevis oocytes. DEER measurements of the oocytes, into which SDSL-ubiquitin derivates were injected, gave rise to interpretable signals and allowed us to perform in situ analyses of the interspin distances of the proteins.

H−Tphase diagram of the two-dimensional Ising model with exchange and dipolar interactions

We explore the equilibrium properties of a two-dimensional Ising spin model with short-range exchange and long-range dipolar interactions as a function of the applied magnetic field H. The model is studied through extensive Monte Carlo simulations that show the existence of many modulated phases with long range orientational order for a wide range of fields. These phases are characterized by different wave vectors that change discontinuously with the magnetic field. We provide numerical evidence supporting the existence of first order transitions between these phases. At higher fields our results suggest a Kosterliz-Thouless scenario for the transition from a bubble to a ferromagnetic phase.

Dipolar collisions of polar molecules in the quantum regime

Ultracold polar molecules offer the possibility of exploring quantum gases with interparticle interactions that are strong, long-range and spatially anisotropic. This is in stark contrast to the much studied dilute gases of ultracold atoms, which have isotropic and extremely short-range (or 'contact') interactions. Furthermore, the large electric dipole moment of polar molecules can be tuned using an external electric field; this has a range of applications such as the control of ultracold chemical reactions, the design of a platform for quantum information processing and the realization of novel quantum many-body systems. Despite intense experimental efforts aimed at observing the influence of dipoles on ultracold molecules, only recently have sufficiently high densities been achieved. Here we report the experimental observation of dipolar collisions in an ultracold molecular gas prepared close to quantum degeneracy. For modest values of an applied electric field, we observe a pronounced increase in the loss rate of fermionic potassium-rubidium molecules due to ultracold chemical reactions. We find that the loss rate has a steep power-law dependence on the induced electric dipole moment, and we show that this dependence can be understood in a relatively simple model based on quantum threshold laws for the scattering of fermionic polar molecules. In addition, we directly observe the spatial anisotropy of the dipolar interaction through measurements of the thermodynamics of the dipolar gas. These results demonstrate how the long-range dipolar interaction can be used for electric-field control of chemical reaction rates in an ultracold gas of polar molecules. Furthermore, the large loss rates in an applied electric field suggest that creating a long-lived ensemble of ultracold polar molecules may require confinement in a two-dimensional trap geometry to suppress the influence of the attractive, 'head-to-tail', dipolar interactions.

Phase diagram of magnetization reversal processes in nanorings

We investigate magnetization reversal processes of magnetic nanorings. Exploring extensive parameter sets using Monte Carlo technique combined with scaling method, we constructed magnetization switching phase diagrams for such structures. The dominant switching mechanisms are found to strongly depend on the relative spatial dimensions of the rings, i.e., height, width, and radius. Furthermore, they are shown to depend on a characteristic exchange length scale, which is determined by the competing short-range ferromagnetic and long-range dipolar interactions in these systems. Detailed simulations allow us to identify three switching mechanisms, which should be useful for building high-density storage systems.

Transition from spin ice to spin liquid——evolvement of structure and magnetic properties

The polycrystal samples of Ho2-xTbxTi2O7（x=0，05，1，15，2） were prepared by solid reaction method. The result of X-ray diffraction （XRD） and dc susceptibility measurement from 2 K to 300 K both indicate that as the content of Tb3+，i.e. x，increases，the system crosses over from Ho2Ti2O7spin ice to Tb2Ti2O7 spin liquid，the lattice constant of cubic pyrochlore increases，while the Curie constant and effective magnetic moment decrease. Especially，the Curie-Weiss temperature decreases from positive to negative with the decrease of x，implying the transition from ferromagnetic Ho2Ti2O7 to anti-ferromagnetic Tb2Ti2O7. The detailed analysis shows that as x in Ho2-xTbxTi2O7 increases，the intensity of long-range dipolar interaction Dnn decreases. Being ferromagnetic or anti-ferromagnetic，and the change in intensity of long-range dipolar interaction, may be the two crucial factors influencing on the system to form spin ice state or cooperative paramagnetic state，i.e. the spin liquid state.

10.1007/s11490-008-3022-6

A theoretical study of vortex-lattice formation in atomic Bose-Einstein condensates confined by a rotating elliptical trap is presented. For the conventional case of purely s-wave interatomic interactions, this is done through a consideration of both hydrodynamic equations and time-dependent simulations of the Gross-Pitaevskii equation. We discriminate three distinct, experimentally testable regimes of instability: ripple, interbranch, and catastrophic. Additionally, we generalize the classical hydrodynamical approach to include long-range dipolar interactions, showing how the static solutions and their stability in the rotating frame are significantly altered. This enables us to examine the routes towards unstable dynamics, which, in analogy to conventional condensates, may lead to vortex-lattice formation.

Phase transitions and autocorrelation times in two-dimensional Ising model with dipole interactions

The two-dimensional Ising model with nearest-neighbor ferromagnetic and long-range dipolar interactions exhibits a rich phase diagram. The presence of the dipolar interaction changes the ferromagnetic ground state expected for the pure Ising model to a series of striped phases as a function of the interaction strengths. Monte Carlo simulations and histogram reweighting techniques applied to multiple histograms are performed to identify the critical temperatures for the phase transitions taking place for stripes of width h = 2 on square lattices. In particular, we aim to study the intermediate nematic phase, which is observed for large lattice sizes only. For these lattice sizes, we calculate the critical temperatures for the striped-nematic and nematic-tetragonal transitions, critical exponents, and the bulk free-energy barrier associated with the coexisting phases. We also evaluate the long-term correlations in our time series near the finite-size critical points by studying the integrated autocorrelation time τ as a function of the lattice size. This allows us to infer how severe the critical slowing down for this system with long-range interaction and nearby thermodynamic phase transitions is.

Devil’s staircases and supersolids in a one-dimensional dipolar Bose gas

We consider a single-component gas of dipolar bosons confined in a one-dimensional optical lattice, where the dipoles are aligned such that the long-ranged dipolar interactions are maximally repulsive. In the limit of zero intersite hopping and sufficiently large on-site interaction, the phase diagram is a complete devil's staircase for filling fractions between 0 and 1, wherein every commensurate state at a rational filling is stable over a finite interval in chemical potential. We perturb away from this limit in two experimentally motivated directions involving the addition of hopping and a reduction in the on-site interaction. The addition of hopping alone yields a phase diagram, which we compute in perturbation theory in the hopping, where the commensurate Mott phases now compete with the superfluid. Further softening of the on-site interaction yields alternative commensurate states with double occupancies which can form a staircase of their own, as well as one-dimensional ``supersolids'' which simultaneously exhibit discrete broken symmetries and superfluidity.

Ferronematic Ground State of the Dilute Dipolar Fermi Gas

It is shown that a homogeneous two-component Fermi gas with (long-range) dipolar and short-range isotropic interactions has a ferronematic phase for suitable values of the dipolar and short-range coupling constants. The ferronematic phase is characterized by having a nonzero magnetization and long-range orientational uniaxial order. The Fermi surface of the spin-up (-down) component is elongated (compressed) along the direction of the magnetization.

Variational approach for Bose–Einstein condensates in strongly disordered traps

Recently, Nattermann and Pokrovsky (2008 Phys. Rev. Lett. 100 060402) have proposed a scaling approach for studying Bose–Einstein condensates in strongly disordered traps. In this paper we implement their scaling argument in the framework of the variational method for solving the time-dependent Gross–Pitaevskii equation. We consider atomic gases with both short-range s-wave interaction and long-range anisotropic dipolar interaction. The theory is addressed to the regime of strong disorder and weak interactions where the physics is dominated by the collective pinning due to the disorder. The phenomenon of condensate fragmentation in dipolar gases is also analysed.