Preformed Pairs Research Articles

Exposure of conjugative plasmid carrying Escherichia coli biofilms to male-specific bacteriophages

Escherichia coli carrying a natural conjugative F-plasmid generates F-pili mating pairs, which is important for early biofilm formation. In this study, we investigated the effect of male-specific filamentous single stranded DNA bacteriophage (f1) and RNA bacteriophage (MS2) on the formation of biofilms by E. coli carrying a natural conjugative F-plasmid. We showed that the early biofilm formation was completely inhibited by addition of the f1 phage, but not the MS2 phage. This suggests that the tip of F-pili is the specific attachment site for mating pairs formation and the side of F-pili has a non-obligatory role during biofilm formation. The inhibitory effect of the f1 phage was dependent on the time of addition during the biofilm formation. No inhibitory effect was observed when the f1 phages were added to the mature biofilms. This resistant mechanism of the mature biofilms could be attributed to the biofilm-specific phenotypes representing that the F-pili mating pairs were already formed and then the curli production commenced during the biofilm maturation. The pre-formed mating pairs seemed to resist the f1 phages. Altogether, our results indicate a close relationship between the presence of conjugative plasmid and male-specific bacteriophages within sessile biofilm communities, as well as the possibility of using the male-specific bacteriophages to control biofilm formation.

Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction

Within a gauge approach to the t–J model, we propose a new, non-BCS mechanism of superconductivity for underdoped cuprates. The gluing force of the superconducting mechanism is an attraction between spin vortices on two different Néel sublattices, centered around the empty sites described in terms of fermionic holons. The spin fluctuations are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Through gauge interaction the spin vortex attraction induces the formation of spin-singlet (RVB) spinon pairs with a lowering of the spinon gap. Lowering the temperature, the approach exhibits two crossover temperatures: at the higher crossover a finite density of incoherent holon pairs are formed leading to a reduction of the hole spectral weight, while at the lower crossover a finite density of incoherent spinon RVB pairs are also formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic vortices appear in the plasma phase. Finally, at a even lower temperature the hole pairs become coherent, the magnetic vortices becoming dilute and superconductivity appears. The superconducting mechanism is not of BCS-type since it involves a gain in kinetic energy (for spinons) coming from the spin interactions.

Doped high-Tccuprate superconductors elucidated in the light of zeros and poles of the electronic Green’s function

We study electronic structure of hole- and electron-doped Mott insulators in the two-dimensional Hubbard model to reach a unified picture for the normal state of cuprate high-Tc superconductors. By using a cluster extension of the dynamical mean-field theory, we demonstrate that structure of coexisting zeros and poles of the single-particle Green's function holds the key to understand Mott physics in the underdoped region. We show evidence for the emergence of non-Fermi-liquid phase caused by the topological quantum phase transition of Fermi surface by analyzing low-energy charge dynamics. The spectra calculated in a wide range of energy and momentum reproduce various anomalous properties observed in experiments for the high-Tc cuprates. Our results reveal that the pseudogap in hole-doped cuprates has a d-wave-like structure only below the Fermi level, while it retains non-d-wave structure with a fully opened gap above the Fermi energy even in the nodal direction due to a zero surface extending over the entire Brillouin zone. In addition to the non-d-wave pseudogap, the present comprehensive identifications of the spectral asymmetry as to the Fermi energy, the Fermi arc, and the back-bending behavior of the dispersion, waterfall, and low-energy kink, in agreement with the experimental anomalies of the cuprates, do not support that these originate from (the precursors of) symmetry breakings such as the preformed pairing and the d-density wave fluctuations, but support that they are direct consequences of the proximity to the Mott insulator. Several possible experiments are further proposed to prove or disprove our zero mechanism.

Resonant ultrasound and high temperature superconductivity.

The first experimental test of the 1957 Bardeen–Cooper–Schreiffer theory of superconductivity was a measurement of ultrasonic attenuation. Since then, ultrasound has been sporadically used as a probe of the fundamental physics of superconductivity. One primary difficulty in the application of ultrasonic techniques to high temperature superconductivity has been dissipation due to twin grain boundaries in the crystals. With the availability of detwinned yttrium‐barium‐copper‐oxide superconducting single crystals, resonant ultrasound can now make precision measurements that use the fundamental connection between sound speeds and thermodynamics to explore questions relating to the nature of the so‐called pseudogap state, the precursor state to superconductivity, in which preformed Cooper pairs and a strong superconducting interaction have been proposed. Because the simplistic connection from superconductivity to the bulk modulus is of the order of the square of the ratio of the superconducting transition temp...

BEC-BCS crossover and the liquid-gas phase transition in hot and dense nuclear matter

The effect of nucleon-nucleon correlations in symmetric nuclear matter at finite temperature is studied beyond BCS theory. Starting from a Hartree-Fock description of nuclear matter with the Gogny effective interaction, we add correlations corresponding to the formation of preformed pairs and scattering states above the superfluid critical temperature within the in-medium T-matrix approach, which is analogous to the Nozieres-Schmitt-Rink theory. We calculate the critical temperature for a BEC superfluid of deuterons, of a BCS superfluid of nucleons, and in the crossover between these limits. The effect of the correlations on thermodynamic properties (equation of state, energy, entropy) and the liquid-gas phase transition is discussed. Our results show that nucleon-nucleon correlations beyond BCS play an important role for the properties of nuclear matter, especially in the low-density region.

To pair or not to pair?

Is the mysterious pseudogap in the copper oxide superconductors a signature of preformed pairs or a competing ordered state? Measurements of broken symmetries suggest that the pseudogap cannot originate from superconductivity alone.

Particle-hole mixing driven by the superconducting fluctuations

Development of the STM and ARPES spectroscopy enabled to reach the resolution sufficient for probing the particle-hole entanglement in superconducting materials, even above the critical temperature Tc. On a quantitative level one can characterize such entanglement in terms of the Bogoliubov angle which determines to what extent the particles and holes constitute the effective quasiparticles. In classical superconductors, where the phase transition is related to formation of the Cooper pairs almost simultaneously accompanied by onset of their long-range phase coherence, the Bogoliubov angle is slanted (due to finite particle-hole mixing) all the way up to Tc. In the high temperature superconductors and in superfluid ultracold fermion atoms near the Feshbach resonance the situation is different because the preformed pairs can exist above Tc albeit loosing coherence due to the strong quantum fluctuations. We discuss a generic temperature dependence of the Bogoliubov angle in such pseudogap state indicating a novel, non-BCS behavior. For analysis we use the two-component model describing the pairs coexisting with single fermions and study selfconsistently their feedback effects by the similarity transformation originating from the renormalization group approach.

Evidence for Two Competing Order Parameters in Underdoped Cuprate Superconductors from a Model Analysis of Fermi-Arc Effects

Preformed pairs above T{c} and the two-gap scenarios are two main proposals for describing the low-doping pseudogap phase of high-T{c} cuprates. Very recent angle-resolved photoemission experiments have shown features which were interpreted as evidence for preformed pairs. Here it is shown that those results can be explained also in the context of the two-gap scenario if self-energy effects are considered. The discussion is based on the d charge-density wave theory or the flux phase of the t-J model.

UNCONVENTIONAL LOW-ENERGY EXCITATIONS OF CUPRATE SUPERCONDUCTORS

Recent development in the physics of high-temperature cuprate superconductivity is reviewed, with special emphasis on the phenomena of unconventional and non-universal low-energy excitations of hole- and electron-type cuprate superconductors and the possible physical origin. A phenomenology based on coexisting competing orders with cuprate superconductivity in the ground state appears to provide a consistent account for a wide range of experimental findings, including the presence (absence) of pseudogaps and Fermi arcs above the superconducting transition Tc in hole-type (electron-type) cuprate superconductors and the novel conductance modulations below Tc, particularly in the vortex state. Moreover, the competing order scenario is compatible with the possibility of pre-formed Cooper pairs and significant phase fluctuations in cuprate superconductors. The physical implications of the unified phenomenology and remaining open issues for the microscopic mechanism of cuprate superconductivity are discussed.

Mechanism of Pseudogap Detected by Electronic Raman Scattering: Phase Fluctuation or Hidden Order?

We study the electronic Raman scattering in the cuprates to distinguish the two possible scenarios of the pseudo-gap normal state. In one scenario, the pseudogap is assumed to be caused by phase fluctuations of the preformed Cooper pairs. We find that pair-breaking peaks appear in both the B1g and B2g Raman channels, and they are smeared and tend to shift to the same energy with the increasing strength of phase fluctuations. Thus both channels reflect the same pairing energy scale, irrespectively of the doping level. In another scenario, the pseudogap is assumed to be caused by a hidden order that competes with the superconducting order. As an example, we assume that the hidden order is the d-density-wave (DDW) order. We find analytically and numerically that in the DDW normal state there is no Raman peak in the B2g channel in a tight-binding model up to the second nearest-neighbor hopping, while the Raman peak in the B1g channel reflects the energy gap caused by the DDW order. This behavior is in agreement with experiments in the pseudogap normal state. To gain further insights, we also calculate the Raman spectra in the DDW+SC state. We study the doping and temperature dependence of the peak energy in both channels and find a two-gap behavior, which is in agreement with recent Raman experiments. Therefore, our results shed light on the hidden order scenario for the pseudogap.

DISORDERED SUPERCONDUCTORS: A SIMPLE MODEL MANIFESTING PSEUDOGAP AND BCS-BEC CROSSOVER

The effect of random on-site disorder on s-wave (BCS) superconductors described by a two dimensional negative-U Hubbard model is studied using Bogoliubov-de Gennes (BdG) method. The mean field pairing amplitudes become spatially inhomogeneous at large values of disorder where the system breaks up into superconducting islands with large pairing amplitudes, separated by insulating strips. The amplitude fluctuations are correctly accounted for via BdG calculations, however it misses phase fluctuations which are inherent to low dimensions. The phase fluctuations affect superconducting properties strongly, and the effect is more pronounced in the limit of large disorder. We provide a close estimate of the actual transition temperature, Tcby incorporating phase fluctuations about the inhomogeneous BdG state. This is obtained by relating the jump in renormalized Ds(obtained from a self consistent Harmonic approximation on a phase-only Hamiltonian) at the Kosterlitz-Thouless (KT) transition and the KT transition temperature, TKTobtained from Ds(T) = (2/π)TKTas temperature tends to TKTfrom below. This yields opening of a large region sandwiched between [Formula: see text] (obtained by the vanishing of [Formula: see text]) and TKTwhere there is no phase coherence between the pairs, however amplitude correlations continue to exist, reminiscent of a pseudogap phase in high-Tcsuperconductors which is marked by short ranged preformed pairs without any definite phase relation between them. Further, the appearance of the superconducting islands from a homogeneous phase indicates the evolution of a system consisting of large and overlapping pairs to one that contains short and tightly bound pairs - a scenario termed as BCS-BEC crossover. We have investigated in details the crossover phenomenon as a function of disorder and confirmed it's existence at small values of electron concentration, however, absent at larger densities.

BCS-to-BEC evolution details

Sá de Melo replies: I thank D. M. Eagles for his comments. My statement concerning the evolution from Bardeen-Cooper-Schrieffer to Bose–Einstein condensation superfluids was about clarity and not who was the first to propose the idea. Although I appreciate Eagles’s work, I still think that Anthony Leggett’s papers 1 are the clearest presentation on the topic up to 1980.In his very interesting book written in 1964, John Blatt describes the BEC theory of superconductivity and its relation to the BCS theory. 2 As he recounts, the possibility of pairing without superconductivity and Bose condensation of electron pairs at a lower temperature was suggested as early as 1946 by Richard Ogg Jr. In 1954 and subsequent years, Max Schafroth developed a firm theoretical framework for such pairing, but it was not supported by experimental evidence: No preformed pairs were found, and the BEC temperature was much too high for known superconductors. Blatt’s book thoroughly documents a clear connection between a variational, fixed-number ground state comprising antisymmetrized products of pair wavefunctions and the BCS equations that Eagles used in his 1969 paper. Eagles noticed that the BCS and number equations at zero temperature could be used to describe the evolution from BCS to BEC superconductivity in low-carrier-density thin films or bulk materials for fixed (and sufficiently strong) interaction and changing carrier density. However, Blatt’s main interest was in metals with high carrier densities. If one reads the full statement in the box entitled “Feshbach resonances” on page 47 of my article, it is clear that I am talking about changing interactions for fixed density and not changing density for fixed interactions. I do not think it is currently possible in any superconductor to control the interactions between fermions. Thus my statement is accurate.In many instances the change in carrier concentration in thin films or three-dimensional (bulk) systems is made via chemical doping, which may introduce disorder or distortions in the crystal structure and thus change the system in more than the desired way. I have serious doubts that ceramic samples of zirconium-doped strontium titanate described in Eagles’s letter indeed show the BCS-to-BEC evolution; the references provided do not offer clear evidence of such a phenomenon.Since cuprate superconductors are known to be quasi-2D and have d-wave pairing symmetry, the evolution from BCS to BEC superconductivity for sufficiently strong fixed interactions and changing carrier density would be accompanied by a topological quantum phase transition 3 and would not be a crossover as in the s-wave case. However, as much as I might want such a transition to be found for the cuprates, current experimental evidence shows that their normal state and superconducting properties are more complicated than simple BCS-to-BEC theories would predict.The beauty of the observation of the BCS-to-BEC evolution in ultracold atoms is that many of the materials difficulties encountered in standard condensed matter can be avoided and the tuning of the interaction for fixed low density can be controlled. However, one should keep an open mind that a similar realization may appear elsewhere. For instance, if carrier density could be controlled and changed continuously via electrostatic doping in good-quality thin films or at the surface of bulk low-carrier-density superconductors like some titanates or cuprates, then there is a good chance of observing the BCS-to-BEC evolution as a function of carrier density as envisioned by Eagles in 1969.REFERENCESSection:ChooseTop of pageREFERENCES << REFERENCES 1. A. J. Leggett, in Modern Trends in the Theory of Condensed Matter: Proceedings of the XVI Karpacz Winter School of Theoretical Physics, February 19-March 3, 1979, Karpacz, Poland, A. Pekalski, J. Przystawa, eds., Springer, New York (1980) Google Scholar A. J. Leggett, J. Phys. Colloq. 41, C7–19 (1980). https://doi.org/10.1051/jphyscol:1980704 , Google ScholarCrossref2. J. M. Blatt, Theory of Superconductivity, Academic Press, New York (1964). Google Scholar3. L. S. Borkowski, C. A. R. Sá de Melo, http://arxiv.org/abs/cond-mat/9810370v1 , Google Scholar R. D. Duncan, C. A. R. Sá de Melo, Phys. Rev. B 62, 9675 (2000). https://doi.org/10.1103/PhysRevB.62.9675 , Google ScholarCrossref, ISI© 2009 American Institute of Physics.

HTSC cuprate phase diagram using a modified Boson–Fermion–Gossamer model describing competing orders, a quantum critical point and possible resonance complex

AbstractThere has been considerable effort expended toward understanding high temperature superconductors (HTSC), and more specifically the cuprate phase diagram as a function of doping level. Yet, the only agreement seems to be that HTSC is an example of a strongly correlated material where Coulomb repulsion plays a major role. This manuscript proposes a model based on a Feshbach resonance pairing mechanism and competing orders. An initial BCS‐type superconductivity at high doping is suppressed in the two particle channel by a localized preformed pair (PP) (Nozieres and Schmitt‐Rink, J Low Temp Phys, 1985, 59, 980) (circular density wave) creating a quantum critical point. As doping continues to diminish, the PP then participates in a Feshbach resonance complex that creates a new electron (hole) pair that delocalizes and constitutes HTSC and the characteristic dome (Squire and March, Int J Quantum Chem, 2007, 107, 3013; 2008, 108, 2819). The resonant nature of the new pair contributes to its short coherence length. The model we propose also suggests an explanation (and necessity) for an experimentally observed correlated lattice that could restrict energy dissipation to enable the resonant Cooper pair to move over several correlation lengths, or essentially free. The PP density wave is responsible for the pseudogap as it appears as a “localized superconductor” since its density of states and quasiparticle spectrum are similar to those of a superconductor (Peierls–Fröhlich theory), but with no phase coherence between the PP. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009

Breakdown of the Bardeen–Cooper–Schrieffer ground state at a quantum phase transition

Advances in solid-state and atomic physics are exposing the hidden relationships between conventional and exotic states of quantum matter. Prominent examples include the discovery of exotic superconductivity proximate to conventional spin and charge order, and the crossover from long-range phase order to preformed pairs achieved in gases of cold fermions and inferred for copper oxide superconductors. The unifying theme is that incompatible ground states can be connected by quantum phase transitions. Quantum fluctuations about the transition are manifestations of the competition between qualitatively distinct organizing principles, such as a long-wavelength density wave and a short-coherence-length condensate. They may even give rise to 'protected' phases, like fluctuation-mediated superconductivity that survives only in the vicinity of an antiferromagnetic quantum critical point. However, few model systems that demonstrate continuous quantum phase transitions have been identified, and the complex nature of many systems of interest hinders efforts to more fully understand correlations and fluctuations near a zero-temperature instability. Here we report the suppression of magnetism by hydrostatic pressure in elemental chromium, a simple cubic metal that demonstrates a subtle form of itinerant antiferromagnetism formally equivalent to the Bardeen-Cooper-Schrieffer (BCS) state in conventional superconductors. By directly measuring the associated charge order in a diamond anvil cell at low temperatures, we find a phase transition at pressures of approximately 10 GPa driven by fluctuations that destroy the BCS-like state but preserve the strong magnetic interaction between itinerant electrons and holes. Chromium is unique among stoichiometric magnetic metals studied so far in that the quantum phase transition is continuous, allowing experimental access to the quantum singularity and a direct probe of the competition between conventional and exotic order in a theoretically tractable material.

Formation of cation–radical anion pairs derived from carboxybenzophenone–tetrabutylammonium salts. Pulse radiolysis studies

Pulse radiolysis of acetonitrile solutions of tetra-n-butyl ammonium salts of 2- and 4-carboxybenzophenones [BP-COO−···N+(C4H9)4] were performed in order to generate directly the reduced forms of the benzophenone moieties within pre-formed ion pairs. In earlier studies on photochemical electron transfer reactions, ion pairs containing a tetraalkyl ammonium cation and a benzophenone radical anion were formed in an electron transfer to the triplet BP from a quencher consisting of a tetraalkyl ammonium salt of (phenylthio)acetic acid. In the current work, the [BP•−COO−···N+(C4H9)4] ion pairs were formed by direct reduction of the salts without the complication of a third moiety, i.e., the (phenylthio)acetic anion. The spectra and kinetic parameters of the radiolytically-reduced salts were compared to the behavior of reduced forms of the 2- and 4-COOH substituted benzophenones. The results from the pulse radiolysis and photochemistry were compared and explained in terms of the different structures of the ion pairs.

Superconductivity close to the Mott state: From condensed-matter systems to superfluidity in optical lattices

Since the discovery of high-temperature superconductivity in 1986 by Bednorz and Müller, great efforts have been devoted to finding out how and why it works. From the d-wave symmetry of the order parameter, the importance of antiferromagnetic fluctuations, and the presence of a mysterious pseudogap phase close to the Mott state, one can conclude that high- T c superconductors are clearly distinguishable from the well-understood BCS superconductors. The d-wave superconducting state can be understood through a Gutzwiller-type projected BCS wavefunction. In this review article, we revisit the Hubbard model at half-filling and focus on the emergence of exotic superconductivity with d-wave symmetry in the vicinity of the Mott state, starting from ladder systems and then studying the dimensional crossovers to higher dimensions. This allows to confirm that short-range antiferromagnetic fluctuations can mediate superconductivity with d-wave symmetry. Ladders are also nice prototype systems allowing to demonstrate the truncation of the Fermi surface and the emergence of a Resonating Valence Bond (RVB) state with preformed pairs in the vicinity of the Mott state. In two dimensions, a similar scenario emerges from renormalization group arguments. We also discuss theoretical predictions for the d-wave superconducting phase as well as the pseudogap phase, and address the crossover to the overdoped regime. Finally, cold atomic systems with tunable parameters also provide a complementary insight into this outstanding problem.

Polarized Superfluidity in the Attractive Hubbard Model with Population Imbalance

We study a two-component Fermi system with attractive interactions and different populations of the two species in a cubic lattice. For an intermediate coupling, we find a uniformly polarized superfluid which is stable down to very low temperatures. The momentum distribution of this phase closely resembles that of the Sarma phase, characterized by two Fermi surfaces. This phase is shown to be stabilized by a potential energy gain, as in a BCS superfluid, in contrast with the unpolarized Bose-Einstein condensate which is stabilized by kinetic energy. We present general arguments suggesting that preformed pairs in the unpolarized superfluid favor the stabilization of a polarized superfluid phase.

Emergence of preformed Cooper pairs from the doped Mott insulating state in Bi2Sr2CaCu2O8+δ

Superconductors are characterized by an energy gap that represents the energy needed to break the pairs of electrons (Cooper pairs) apart. At temperatures considerably above those associated with superconductivity, the high-transition-temperature copper oxides have an additional 'pseudogap'. It has been unclear whether this represents preformed pairs of electrons that have not achieved the coherence necessary for superconductivity, or whether it reflects some alternative ground state that competes with superconductivity. Paired electrons should display particle-hole symmetry with respect to the Fermi level (the energy of the highest occupied level in the electronic system), but competing states need not show such symmetry. Here we report a photoemission study of the underdoped copper oxide Bi(2)Sr(2)CaCu(2)O(8+delta) that shows the opening of a symmetric gap only in the anti-nodal region, contrary to the expectation that pairing would take place in the nodal region. It is therefore evident that the pseudogap does reflect the formation of preformed pairs of electrons and that the pairing occurs only in well-defined directions of the underlying lattice.

Spin-gapped incoherent metal with preformed pairing in the doped antiferromagnetic Mott insulator

We investigate how the antiferromagnetic Mott insulator evolves into the $d$-wave BCS superconductor through hole doping. Allowing spin fluctuations in the strong-coupling approach, we find a spin-gapped incoherent metal with preformed pairing as an intermediate phase between the antiferromagnetic Mott insulator and $d$-wave superconductor. This non-Fermi-liquid metal is identified with an infrared stable fixed point in the spin-decomposition gauge theory, analogous to the spin liquid insulator in the slave-boson gauge theory. We consider the single particle spectrum and dynamical spin susceptibility in the anomalous metallic phase, and discuss physical implications.

ABSENCE OF BCS CONDENSATION OF ATOMS IN A REPULSIVE FERMI GAS OF ATOMS

Bose–Einstein condensation of molecules and BCS condensation of Cooper pairs of atoms in an ultra-cold Fermi gas of atoms with a repulsive two-body interaction are studied by using the path integral representation of the grand partition function. From the self-consistent equations obtained in the present work for the order parameters, we have found that BCS condensation of atoms cannot occur in such a Fermi gas of atoms and that the condensate observed experimentally is composed of condensed molecules or possibly of preformed Cooper pairs. To substantiate our conclusions from the self-consistent equations for the order parameters, the effective atom–atom interaction mediated through molecules has also been computed with the Foldy–Wouthuysen transformation and has been found to be repulsive, which implies a net repulsive two-body interaction between atoms and hence the absence of BCS condensation of atoms.