Effective Attractive Interaction Research Articles

Superfluid pairing in a mixture of a spin-polarized Fermi gas and a dipolar condensate

We consider a mixture of a spin-polarized Fermi gas and a dipolar Bose-Einstein condensate in which s-wave scattering between fermions and the quasiparticles of the dipolar condensate can result in an effective attractive Fermi-Fermi interaction anisotropic in nature and tunable by the dipolar interaction. We show that such an interaction can significantly increase the prospect of realizing a superfluid with a gap parameter characterized with a coherent superposition of all odd partial waves. We formulate, in the spirit of the Hartree-Fock-Bogoliubov mean-field approach, a theory which allows us to estimate the critical temperature when the anisotropic Fock potential is taken into consideration and to determine the system parameters that optimize the critical temperature at which such a superfluid emerges before the system begins to phase separate.

Imbalanced ultracold Fermi gas in the weakly repulsive regime: Renormalization-group approach forp-wave superfluidity

We theoretically study a possible new pairing mechanism for a two-dimensional population imbalanced Fermi gas with short-range repulsive interactions which can be realized on the upper branch of a Feshbach resonance. We use a well-controlled renormalization group approach, which allows an unbiased study of the instabilities of imbalanced Fermi liquid without assumption of a broken symmetry and gives a numerical calculation of the transition temperature from microscopic parameters. Our results show a leading superfluid instability in the p-wave channel for the majority species. The corresponding mechanism is that there are effective attractive interactions for the majority species, induced by the particle-hole susceptibility of the minority species, where the mismatch of the Fermi surfaces of the two species plays an important role. We also propose an experimental protocol for detecting the p-wave superfluidity and discuss the corresponding experimental signatures.

Theory and hierarchical calculations of the structure and energetics of [0001] tilt grain boundaries in graphene

Several experiments have revealed the presence of grain boundaries in graphene that may change its electronic and elastic properties. Here, we present a general theory for the structure of [0001] tilt grain boundaries in graphene based on the coincidence site lattice (CSL) theory. We show that the CSL theory uniquely classifies the grain boundaries in terms of the misorientation angle $\ensuremath{\theta}$ and periodicity $d$ using two grain-boundary indices $(m,n)$, similar to the nanotube indices. The structure and formation energy of a large set of grain boundaries generated by the CSL theory for ${0}^{\ensuremath{\circ}}<\ensuremath{\theta}<{60}^{\ensuremath{\circ}}$ (up to 15 608 atoms) were optimized by a hierarchical methodology and validated by density functional calculations. We find that low-energy grain boundaries in graphene can be identified as dislocation arrays. The dislocations form hillocks like those observed by scanning tunneling microscopy in graphene grown on Ir(111) for small $\ensuremath{\theta}$ that flatten out at larger misorientation angles. We find that, in contrast to three-dimensional materials, the strain created by the grain boundary can be released via out-of-plane distortions that lead to an effective attractive interaction between dislocation cores. Therefore, the dependence on $\ensuremath{\theta}$ of the formation energy parallels that of the out-of-plane distortions, with a secondary minimum at $\ensuremath{\theta}=32.{2}^{\ensuremath{\circ}}$ where the grain boundary is made of a flat zigzag array of only 5 and 7 rings. For $\ensuremath{\theta}>32.{2}^{\ensuremath{\circ}}$, other nonhexagonal rings are also possible. We discuss the importance of these findings for the interpretation of recent experimental results.

Phase transitions and spatially ordered counterion association in ionic-lipid membranes: A statistical model

We propose a statistical model to account for the gel-fluid anomalous phase transitions in charged bilayer- or lamellae-forming ionic lipids. The model Hamiltonian comprises effective attractive interactions to describe neutral-lipid membranes as well as the effect of electrostatic repulsions of the discrete ionic charges on the lipid headgroups. The latter can be counterion dissociated (charged) or counterion associated (neutral), while the lipid acyl chains may be in gel (low-temperature or high-lateral-pressure) or fluid (high-temperature or low-lateral-pressure) states. The system is modeled as a lattice gas with two distinct particle types--each one associated, respectively, with the polar-headgroup and the acyl-chain states--which can be mapped onto an Ashkin-Teller model with the inclusion of cubic terms. The model displays a rich thermodynamic behavior in terms of the chemical potential of counterions (related to added salt concentration) and lateral pressure. In particular, we show the existence of semidissociated thermodynamic phases related to the onset of charge order in the system. This type of order stems from spatially ordered counterion association to the lipid headgroups, in which charged and neutral lipids alternate in a checkerboard-like order. Within the mean-field approximation, we predict that the acyl-chain order-disorder transition is discontinuous, with the first-order line ending at a critical point, as in the neutral case. Moreover, the charge order gives rise to continuous transitions, with the associated second-order lines joining the aforementioned first-order line at critical end points. We explore the thermodynamic behavior of some physical quantities, like the specific heat at constant lateral pressure and the degree of ionization, associated with the fraction of charged lipid headgroups.

Entropy effects in self-assembling mechanisms: Also a view from the information theory

At first sight, formation of self-assembled structures such as micelles, bilayers, DNA–cationic bilayers complexes, and the enzyme–substrate complexation in biological catalytic reactions look as totally different phenomena. However, common in all of them is the effective attractive interactions among the self-assembled molecular components that are required to occur. In general, the direct interactions among the components of an assembled structure are not sufficiently attractive to justify self-assembling by itself. Furthermore, in some cases these direct interactions are repulsive as in the case of like charged particles. In this paper, by means of theory and simulation, we consider the effective interactions of the following simple systems: 1) two like-charged parallel rods immersed in an electrolyte solution, 2) two like-charged parallel rods confined between two parallel plates, and 3) a sphere with an open cavity and a spherical macroparticle, both immersed in a hard sphere fluid. In the first two systems we show that effective attractive interaction among the components of a self-assembled structure may only occur when they are in high electrolyte volume fractions. In the lock–key system (3rd case), it is demonstrated that the perfect match between the spherical cavity and macroparticle is the most favorable condition for their self-assembly to occur. All these are examples of attraction driven by the need of the systems to optimize the available volume. That is, self-assembling is driven by the maximum entropy principle, or maximum missing information, as in information theory.

Mean-Field Study of Charge, Spin, and Orbital Orderings in Triangular-Lattice CompoundsANiO2(A= Na, Li, Ag)

We present our theoretical results on the ground states in layered triangular-lattice compounds ANiO2 (A=Na, Li, Ag). To describe the interplay between charge, spin, orbital, and lattice degrees of freedom in these materials, we study a doubly-degenerate Hubbard model with electron-phonon couplings by the Hartree-Fock approximation combined with the adiabatic approximation. In a weakly-correlated region, we find a metallic state accompanied by \sqroot3x\sqroot3 charge ordering. On the other hand, we obtain an insulating phase with spin-ferro and orbital-ferro ordering in a wide range from intermediate to strong correlation. These phases share many characteristics with the low-temperature states of AgNiO2 and NaNiO2, respectively. The charge-ordered metallic phase is stabilized by a compromise between Coulomb repulsions and effective attractive interactions originating from the breathing-type electronphonon coupling as well as the Hund's-rule coupling. The spin-orbital-ordered insulating phase is stabilized by the cooperative effect of electron correlations and the Jahn-Teller coupling, while the Hund's-rule coupling also plays a role in the competition with other orbital-ordered phases. The results suggest a unified way of understanding a variety of low-temperature phases in ANiO2. We also discuss a keen competition among different spin-orbital-ordered phases in relation to a puzzling behavior observed in LiNiO2.

Solvent-free coarse-grained lipid model for large-scale simulations

A coarse-grained molecular model, which consists of a spherical particle and an orientation vector, is proposed to simulate lipid membrane on a large length scale. The solvent is implicitly represented by an effective attractive interaction between particles. A bilayer structure is formed by orientation-dependent (tilt and bending) potentials. In this model, the membrane properties (bending rigidity, line tension of membrane edge, area compression modulus, lateral diffusion coefficient, and flip-flop rate) can be varied over broad ranges. The stability of the bilayer membrane is investigated via droplet-vesicle transition. The rupture of the bilayer and worm-like micelle formation can be induced by an increase in the spontaneous curvature of the monolayer membrane.

Particle-localized ground state of atom-molecule Bose-Einstein condensates in a double-well potential

We study the effect of atom-molecule internal tunneling on the ground state of atom-molecule Bose-Einstein condensates in a double-well potential. In the absence of internal tunneling between atomic and molecular states, the ground state is symmetric, which has equal-particle populations in two wells. From the linear stability analysis, we show that the symmetric stationary state becomes dynamically unstable at a certain value of the atom-molecule internal tunneling strength. Above the critical value of the internal tunneling strength, the ground state bifurcates to the particle-localized ground states. The origin of this transition can be attributed to the effective attractive interatomic interaction induced by the atom-molecule internal tunneling. This effective interaction is similar to that familiar in the context of BCS-BEC crossover in a Fermi gas with Feshbach resonance. Furthermore, we point out the possibility of reentrant transition in the case of the large detuning between the atomic and molecular states.

SrTiO3: From Quantum Paraelectric to Superconducting

SrTiO3 exhibits unusually complex ground states which reach from insulating quantum paraelectric to poor metallic behavior depending on doping and/or oxygen stoichiometry where superconductivity might appear. Upon Nb doping two-gap superconductivity is induced and with this the first two-gap superconductor realized long after its theoretical prediction. Here we concentrate on this state and show that Nb doped SrTiO3 is the most unconventional multi-band superconductor of known multi-band superconductors since the smaller of the two superconducting gaps deviates substantially from a BCS temperature dependence. We attribute these deviations to two cooperating effects, namely an extreme anisotropy in the frequency dependent effective attractive interactions, involving one very soft mode, and an almost vanishing interband interaction. As a consequence, the superfluid density of Nb doped SrTiO3 is predicted to exhibit an inflection point close to the superconducting transition temperature Tc and not—as seen in other multi-band superconductors—close to T = 0 K.

Superfluid transition temperature in a dilute Bose–Fermi mixture with imbalanced two-component Fermi gas

We study s- and p-wave pairing of population imbalanced Fermi atoms in a dilute Bose–Fermi mixture. s-wave pairing takes place only between atoms of different spins, while p-wave Cooper pair is formed by atoms in the same spin component. We calculate the transition temperature of this system in the presence of all interactions between two-spin states. The effective interaction between two-spin states is the sum of the direct one g FF , the one arising from polarization of the bosonic medium g FBF and that due to polarization of the fermionic medium itself g FFF . The p-wave channel can exist due to polarization of the bosonic medium with the effective attractive interaction and we estimate of the transition temperature increase due to this effect.

Impedance spectroscopy of H and OH adsorption on stepped single-crystal platinum electrodes in alkaline and acidic media

The adsorption kinetics of hydrogen and hydroxyl on Pt(111) and stepped Pt[n(111) × (110)] electrodes with n = 29, 9, and 4 in acidic and alkaline electrolytes have been studied using impedance spectroscopy. We found a potential dependent charge transfer resistance, R(ct), (∼30 Ω cm(2) to ∼1 kΩ cm(2)) for hydrogen underpotential deposition in alkaline media (0.05 M NaOH), whereas in acidic media (0.025 M HClO(4)) R(ct) was too low to be determined accurately. Assuming simple (mean field) isotherms to fit our data, we obtain repulsive interactions between H(upd) at the (111) terrace and effective attractive interactions at the steps. The adsorption of OH on (111) terraces is fast in both acidic and alkaline media.

Interplay of the single particle and Josephson Cooper pair tunneling on supercurrent across the superconducting quantum dot junction

Abstract In the present paper the interplay of the single particle and Josephson Cooper pair tunneling on the Josephson supercurrent across the Superconducting Quantum dot junction has been investigated. For this purpose, we have used the renormalized Anderson model that includes the single particle coupling between the superconducting leads and quantum dot states, attractive BCS effective interaction in the superconducting leads responsible for pairing and Josephson Cooper pair tunneling responsible for the Cooper pair tunneling through such Superconducting Quantum dot junction. We have employed Green's function and the Ambegaokar–Baratoff formalism to analyse the Josephson supercurrent across such junction. It has been pointed out that the Josephson supercurrent across the superconducting Quantum dot junction depends on the competitive role of the single particle tunneling, the energy of the level on the dot with respect to binding energy of the Cooper pairs and also on the Josephson Cooper pair tunneling.

Mixtures of correlated bosons and fermions: Dynamical mean‐field theory for normal and condensed phases

AbstractWe derive a dynamical mean‐field theory for mixtures of interacting bosons and fermions on a lattice (BF‐DMFT). The BF‐DMFT is a comprehensive, thermodynamically consistent framework for the theoretical investigation of Bose‐Fermi mixtures and is applicable for arbitrary values of the coupling parameters and temperatures. It becomes exact in the limit of high spatial dimensions d or coordination number Z of the lattice. In particular, the BF‐DMFT treats normal and condensed bosons on equal footing and thus includes the effects caused by their dynamic coupling. Using the BF‐DMFT we investigate two different interaction models of correlated lattice bosons and fermions, one where all particles are spinless (model I) and one where fermions carry a spin one‐half (model II). In model I the local, repulsive interaction between bosons and fermions can give rise to an attractive effective interaction between the bosons. In model II it can also lead to an attraction between the fermions.

Study of the Josephson supercurrent through nanoscopic superconducting-quantum dot tunnel junction

The present work deals with the theoretical analysis of the Josephson supercurrent through a quantum dot sandwiched between superconducting leads having s-wave symmetry of the superconducting order parameter. Such a junction is called superconducting-quantum dot (S–QD–S) junction. For this purpose, we have considered a renormalized Anderson model that includes the single-particle coupling of the superconducting leads with the quantum dots level and an attractive BCS-type effective interaction in superconducting leads responsible for s-wave pairing. We employed the Green's function technique to obtain superconducting order parameter within the BCS framework and Ambegaoker Baratoff formalism to analyze the Josephson current through such S–QD–S junction. It has been pointed out that Josephson supercurrent through such a junction is dominated by the attractive pairing interaction in the leads, energy of the level on the dot with respect to Fermi energy and also on the coupling parameter of the two in an essential way. On the basis of numerical analysis we have compared the theoretical results of Josephson supercurrent with the recent transport experiment performed on such superconducting-quantum dot junctions and existing theoretical analysis.

Water adsorption on the stoichiometric and reduced CeO2(111) surface: a first-principles investigation

We present a density functional theory investigation of the interaction between water and cerium oxide surfaces, considering both the stoichiometric and the reduced surfaces. We study the atomic structure and energetics of various configurations of water adsorption (for a water coverage of 0.25 ML) and account for the effect of temperature and pressure of the environment, containing both oxygen and water vapor, employing the ab initio atomistic thermodynamics approach. Through our investigation, we obtain the phase diagram of the water-ceria system, which enables us to discuss the stability of various surface structures as a function of the ambient conditions. For the stoichiometric surface, we find that the most stable configuration for water is when it is bonded at the cerium site, involving two O-H bonds of hydrogen and oxygen atoms at the surface. If oxygen vacancies are introduced at the surface, which is predicted under more reducing conditions, the binding energy of water is stronger, indicating an effective attractive interaction between water molecules and oxygen vacancies. Water dissociation, and the associated activation energies, are studied, and the role of oxygen vacancies is found to be crucial to stabilize the dissociated fragments. We present a detailed analysis of the stability of the water-ceria system as a function of the ambient conditions, and focus on two important surface processes: water adsorption/desorption on the stoichiometric surface and oxygen vacancy formation in the presence of water vapor. A study of the vibrational contribution to the free energy allows us to estimate the effect of this term on the stability range of adsorbed water.

P-wave superfluid and phase separation in atomic Bose-Fermi mixtures

We consider a system of repulsively interacting Bose-Fermi mixtures of spin polarized uniform atomic gases at zero temperature. We examine possible realization of p-wave superfluidity of fermions due to an effective attractive interaction via density fluctuations of Bose-Einstein condensate within mean-field approximation. We find the ground state of the system by direct energy comparison of p-wave superfluid and phase-separated states, and suggest an occurrence of the p-wave superfluid for a strong boson-fermion interaction regime. We study some signatures in the p-wave superfluid phase, such as anisotropic energy gap and quasi-particle energy in the axial state, that have not been observed in spin unpolarized superfluid of atomic fermions. We also show that a Cooper pair is a tightly bound state like a diatomic molecule in the strong boson-fermion coupling regime and suggest an observable indication of the p-wave superfluid in the real experiment.

Are there stable long-range ordered Fe1−xCrx compounds?

The heat of formation of Fe–Cr alloys undergoes an anomalous change of sign at small Cr concentrations. This observation raises the question as to whether there are intermetallic phases present in this composition range. Here, we report the discovery of several long-range ordered structures that represent ground state phases at 0K. In particular, we have identified a structure at 3.7% Cr with an embedding energy which is 49meV∕Cr atom below the solid solution. This implies that there is an effective long-range attractive interaction between Cr atoms. We propose that the structures found in this study complete the low temperature–low Cr region of the phase diagram.

Charge Fluctuations and Superconductivity in an Extended Periodic Anderson Model: A Weak Coupling Theory

An extended periodic Anderson model, a model where repulsive interaction between an f -electron and a conduction electron is considered in addition to repulsive interaction between f -electrons, on a square lattice is studied with the fluctuation exchange approximation. It is found that charge susceptibilities at q =(π,π) are enhanced as the f -electron level ε f becomes shallow, implying the tendency towards a charge density wave instability. The effective attractive interaction leading to superconductivity is then calculated, and is shown to depend on ε f nonmonotonically.

Madelung energy of the valence-skipping compoundBaBiO3

Several elements show valence skip fluctuation, for instance, Tl forms the compounds in valence states $+1$ and $+3$ and Bi forms in $+3$ and $+5$ states. This kind of fluctuation gives rise to a negative effective attractive interaction and the Kondo effect. In the compounds of valence-skipping elements, the carrier doping will induce superconductivity with high critical temperature. For example, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}\mathrm{Bi}{\mathrm{O}}_{3}$ shows high ${T}_{c}$ which is unlikely from the conventional electron-phonon mechanism. The reason for missing of some valence states in such valence skip compounds remains a mystery. We have performed the evaluation of the Madelung potential for $\mathrm{Ba}\mathrm{Bi}{\mathrm{O}}_{3}$ and have shown that the charge-ordered state is stabilized if we take into account the polarization of the oxygen charge. We argue that the effective Coulomb interaction energy $U$ may be negative evaluating the local excitation energy.

Excitonic effects in a time-dependent density functional theory

Excited state properties of one-dimensional molecular materials are dominated by many-body interactions resulting in strongly bound confined excitons. These effects cannot be neglected or treated as a small perturbation and should be appropriately accounted for by electronic structure methodologies. We use adiabatic time-dependent density functional theory to investigate the electronic structure of one-dimensional organic semiconductors, conjugated polymers. Various commonly used functionals are applied to calculate the lowest singlet and triplet state energies and oscillator strengths of the poly(phenylenevinylene) and ladder-type (poly)(para-phenylene) oligomers. Local density approximations and gradient-corrected functionals cannot describe bound excitonic states due to lack of an effective attractive Coulomb interaction between photoexcited electrons and holes. In contrast, hybrid density functionals, which include long-range nonlocal and nonadiabatic corrections in a form of a fraction of Hartree-Fock exchange, are able to reproduce the excitonic effects. The resulting finite exciton sizes are strongly dependent on the amount of the orbital exchange included in the functional.