Long-range Pairing Correlations Research Articles

Understanding the Re-entrant Phase Transition in a Non-magnetic Scheelite.

The stereochemical activity of lone pair electrons plays a central role in determining the structural and electronic properties of both chemically simple materials such as H2O, as well as more complex condensed phases such as photocatalysts or thermoelectrics. TlReO4 is a rare example of a non-magnetic material exhibiting a re-entrant phase transition and emphanitic behavior in the long-range structure. Here, we describe the role of the Tl+ 6s2 lone pair electrons in these unusual phase transitions and illustrate its tunability by chemical doping, which has broad implications for functional materials containing lone pair bearing cations. First-principles density functional calculations clearly show the contribution of the Tl+ 6s2 in the valence band region. Local structure analysis, via neutron total scattering, revealed that changes in the long-range structure of TlReO4 occur due to changes in the correlation length of the Tl+ lone pairs. This has a significant effect on the anion interactions, with long-range ordered lone pairs creating a more densely packed structure. This resulted in a trade-off between anionic repulsions and lone pair correlations that lead to symmetry lowering upon heating in the long-range structure, whereby lattice expansion was necessary for the Tl+ lone pairs to become highly correlated. Similarly, introducing lattice expansion through chemical pressure allowed long-range lone pair correlations to occur over a wider temperature range, demonstrating a method for tuning the energy landscape of lone pair containing functional materials.

Photomolecular High-Temperature Superconductivity

Superconductivity in organic conductors is often tuned by the application of chemical or external pressure. With this type of tuning, orbital overlaps and electronic bandwidths are manipulated, whilst the properties of the molecular building blocks remain virtually unperturbed.Here, we show that the excitation of local molecular vibrations in the charge-transfer salt $\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br$ induces a colossal increase in carrier mobility and the opening of a superconducting-like optical gap. Both features track the density of quasi-particles of the equilibrium metal, and can be achieved up to a characteristic coherence temperature $T^* \approxeq 50 K$, far higher than the equilibrium transition temperature $T_C = 12.5 K$. Notably, the large optical gap achieved by photo-excitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. First-principle calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photo-molecular superconductivity.

Interplay between nematic fluctuation and superconductivity in a two-orbital Hubbard model: a quantum Monte Carlo study

To understand the interplay between nematic fluctuation and superconductivity in iron-based superconductors, we performed a systematic study of the realistic two-orbital Hubbard model at intermedium correlation regimes by using the constrained-path quantum Monte Carlo method. Our numerical results showed that the on-site nematic interaction induces a strong enhancement of nematic fluctuations at various momentums, especially at (). Simultaneously, it was found that the on-site nematic interaction suppresses the antiferromagnetic order and long-range electron pairing correlations for dominant pairing channels. Our findings suggest that on-site nematic fluctuation seems to compete with superconductivity in iron-based superconductors.

Long-range superharmonic Josephson current and spin-triplet pairing correlations in a junction with ferromagnetic bilayers.

The long-range spin-triplet supercurrent transport is an interesting phenomenon in the superconductor/ferromagnet () heterostructure containing noncollinear magnetic domains. Here we study the long-range superharmonic Josephson current in asymmetric junctions. It is demonstrated that this current is induced by spin-triplet pairs − or + in the thick layer. The magnetic rotation of the particularly thin layer will not only modulate the amplitude of the superharmonic current but also realise the conversion between − and + . Moreover, the critical current shows an oscillatory dependence on thickness and exchange field in the layer. These effect can be used for engineering cryoelectronic devices manipulating the superharmonic current. In contrast, the critical current declines monotonically with increasing exchange field of the layer, and if the layer is converted into half-metal, the long-range supercurrent is prohibited but still exists within the entire region. This phenomenon contradicts the conventional wisdom and indicates the occurrence of spin and charge separation in present junction, which could lead to useful spintronics devices.

Ground-state pairing correlations in the S4 symmetric microscopic model for iron-based superconductors

We present the ground-state pairing correlations in the S4 symmetric microscopic model for iron-based superconductors, computed with the constrained-path Monte Carlo method. For various electron fillings and interaction strengths, we find that the sxy pairing dominates over other pairing correlations and is positive when the pair separation exceeds several lattice constants, both for iron-pnictides and iron-chalcogenides. These ground-state properties, especially the long-range part pairing correlations reconfirm the previous finite-temperature results published in Phys. Rev. Lett., 110 (2013) 107002. We further our study by including the nearest-neighbour interaction V and it is found that the sxy pairing correlation is slightly suppressed by increasing V.

Extinction of polarized light in ferrofluids with different magnetic particle concentrations

The magnetic field intensity and nanoparticle concentration dependence of the polarized light extinction in a ferrofluid made of magnetite particles stabilized with technical grade oleic acid dispersed in transformer oil was experimentally investigated. The magnetically induced optical anisotropy, i.e. the dichroism divided by concentration, was found to decrease with increasing sample concentration from 2% to 8%. The magnetically induced change in the optical extinction of light polarized at 54.74° with respect to the magnetic field direction was found to be positive for the less concentrated sample (2%) and negative for the samples with 4% and 8% magnetic nanoparticle concentrations, the more negative the higher the concentration and field intensity. Based on the theoretically proven fact [11] that the particle orientation mechanism has no effect on the extinction of light polarized at 54.74° with respect to the field direction, we analyzed the experimental findings in the frames of the agglomeration and long-range pair correlations theories for the magnetically induced optical anisotropy in ferrofluids. We developed a theoretical model in the approximation of single scattering for the optical extinction coefficient of a ferrofluid with magnetically induced particle agglomeration. The model predicts the existence of a polarization independent component of the optical extinction coefficient that is experimentally measurable at 54.74° polarization angle. The change in the optical extinction of light polarized at 54.74° is positive if only the formation of straight n-particle chains is considered and may become negative in the hypothesis that the longer chains degenerate to more isotropic structures (polymer-like coils, globules or bundles of chains). The model for the influence on the light absorption of the long-range pair correlations, published elsewhere, predicts that the change in the optical extinction of light polarized at 54.74° is always negative, the more negative the higher the magnetic field intensity and particle concentration.

Critical current behavior in Josephson junctions with the weak ferromagnet PdNi

We have studied the variation of critical current in Superconductor/Ferromagnet/Superconductor (S/F/S) Josephson Junctions as a function of ferromagnet thickness using a weakly-ferromagnetic alloy Pd82Ni12. Measurements were performed for the thickness range 32 to 100 nm, over which the critical current density decreases by five orders of magnitude. The critical current density oscillates with a period of 12.9 +/- 0.3 nm, and decays over a characteristic length of 8.0 +/- 0.5 nm. There is no evidence of a crossover to a slower decay, which might indicate the presence of long-range spin-triplet pair correlations. We discuss possible reasons for their absence, including the possibility of strong spin flip scattering in PdNi.

Symmetries of Pairing Correlations in Superconductor–Ferromagnet Nanostructures

Using selection rules imposed by the Pauli principle, we classify pairing correlations according to their symmetry properties with respect to spin, momentum, and energy. We observe that inhomogeneity always leads to mixing of even- and odd-energy pairing components. We investigate the superconducting pairing correlations present near interfaces between superconductors and ferromagnets, with focus on clean systems consisting of singlet superconductors and either weak or half-metallic ferromagnets. Spin-active scattering in the interface region induces all of the possible symmetry components. In particular, the long-range equal-spin pairing correlations have odd-frequency s-wave and even-frequency p-wave components of comparable magnitudes. We also analyze the Josephson current through a half-metal. We find analytic expressions and a universality in the temperature dependence of the critical current in the tunneling limit.

Ring-exchange mechanism for triplet superconductivity in a two-chain Hubbard model: Possible relevance to Bechgaard salts

The density-matrix renormalization group method is used to study the ground state of the two-chain zigzag-bond Hubbard model at quarter filling. We show that, with a proper choice of the signs of hopping integrals, the ring exchange mechanism yields ferromagnetic spin correlations between interchain neighboring sites, and produces the attractive interaction between electrons as well as the long-range pair correlations in the spin-triplet channel, thereby leading the system to triplet superconductivity. We argue that this novel mechanism may have possible relevance to observed superconductivity in Bechgaard salts.

High-light-intensity photoassociation in a Bose-Einstein condensate

We investigate theoretically the molecular yield in photoassociation of Bose-Einstein condensed sodium atoms for light intensities of the order of and above those applied in a recent experiment. Our results show that the rate at which ground-state molecules may be formed saturates at high light intensities whereas the loss rate of condensate atoms does not. This is caused by the opposing roles of the short- and long-range pair correlations present near resonance under the influence of the laser and is crucial for the development of efficient photoassociation procedures in a condensate.

The pairing correlations in the phased t – U Hubbard model

In this paper we have studied the phased t–U Hubbard model. Using the constrained path Monte Carlo method, we investigated the effects of phase factor on pairing correlation functions in the ground state and we found that the long-range correlations are dependent on the choice of phase factor and for some special values of phase factor there exist long-range pairing correlations only in the strong coupling region.

The giant dipole resonance and pair correlations in nuclei

Abstract A calculation of the energy of the giant dipole resonance is carried out using the wave function proposed by de-Shalit and Feshbach. The expectation value of the energy is shown to be determined by the differences in the mean square distances between neutron-neutron, proton-proton and neutron-proton pairs. The experimentally observed A − 1 3 dependence of the energy on nucleon number is shown to be a consequence of long-range pair correlations which vary as r −4 (as is the case for the ideal Fermi gas). An estimate based on the bulk Fermi gas correlation function, and neglecting Coulomb and other contributions, gives an energy of about twice the experimental value.

Semi-microscopic calculation of the giant dipole resonances in deformed nuclei

Within the framework of the semi-microscopic approach a method is proposed for calculation of the giant dipole resonance in deformed doubly-even nuclei. For the residual interaction the long-range dipole-dipole forces and pairing correlations are used. Calculations were carried out in the random-phase approximation, without anharmonicity. The results of calculations for a number of nuclei from 152Sm to 238U are consistent with the experimental data.

Long-range pair correlations in dilute gases

It is shown that there exist nonequilibrium pair correlations which are not described by the Boltzmann transport equation but which persist even in a dilute gas.

A new dilute gas transport phenomenon

A new transport phenomenon is deduced by arguments based on the existence of nonequilibrium long-range pair correlations which persist even in a dilute gas.

Molecular Distribution and Elasticity in Crystals

The well-known fluctuation-compressibility theorem for fluids is generalized to crystalline solids. It is pointed out that in anisotropic media, fluctuating local strains induce long-range pair correlations. The resulting anomalous contributions to the theorem's pair-correlation-function integral are related to certain components of an elastic tensor for the crystal. Detailed knowledge of particle-singlet and pair equilibrium distributions alone thus provides constraints on, but does not wholly determine, the solid's elastic constants.