Superconducting Pairing Mechanism Research Articles

Extending Universal Nodal Excitations Optimizes Superconductivity in Bi 2 Sr 2 CaCu 2 O 8+δ

Understanding the mechanism by which d wave superconductivity in the cuprates emerges and is optimized by doping the Mott insulator is one of the major outstanding problems in condensed-matter physics. Our high-resolution scanning tunneling microscopy measurements of the high-transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+delta show that samples with different Tc values in the low doping regime follow a remarkably universal d wave low-energy excitation spectrum, indicating a doping-independent nodal gap. We demonstrate that Tc instead correlates with the fraction of the Fermi surface over which the samples exhibit the universal spectrum. Optimal Tc is achieved when all parts of the Fermi surface follow this universal behavior. Increasing the temperature above Tc turns the universal spectrum into an arc of gapless excitations, whereas overdoping breaks down the universal nodal behavior.

Thermodynamical stability of odd-frequency superconducting state

Odd-frequency pairing mechanism of superconductivity has been investigated for several decades. Nevertheless, its properties, including the thermodynamic stability, have remained unclear. In particular, it has been argued that the odd-frequency state is thermodynamically unstable, has an unphysical (anti-)Meissner effect, and thus cannot exist as a homogeneous equilibrium phase. We argue that this conclusion is incorrect because it implicitly relies on the inappropriate assumption that the odd-frequency superconductor can be described by an effective Hamiltonian that breaks the particle conservation symmetry. We demonstrate that the odd-frequency state can be properly described within the functional-integral approach using nonlocal-in-time effective action. Within the saddle-point approximation, we find that this phase is thermodynamically stable, exhibits ordinary Meissner effect, and therefore can be realized as an equilibrium homogenous state of matter.

Coexistence and interplay of superconductivity and ferromagnetism in URhGe

As ferromagnetism and superconductivity are usually considered to be antagonistic, the discovery of theircoexistence in UGe2, URhGe, UIr and UCoGe has attracted a lot of interest. The mechanism to explain such astate has, however, not yet been fully elucidated. In these compounds superconductivitymay be unconventional: Cooper pairs could be formed by electrons with parallel spins andmagnetic fluctuations might be involved in the pairing mechanism. URhGe becomesferromagnetic below a Curie temperature of 9.5 K, with a spontaneous moment aligned to thec-axis. For temperatures below 260 mK and fields lower than 2 T, superconductivity was firstobserved in 2001. Recently, we discovered a second pocket of superconductivity. Thisnew pocket of superconductivity appears at higher fields applied close to theb-axis, enveloping a sudden magnetic moment rotation transition atHR = 12 T. Detailed studies of the field induced metamagnetic transition and superconductivity arepresented. The possibility that magnetic fluctuations emerging from a quantumcritical point provide the pairing mechanism for superconductivity is discussed.

Orbital fluctuation mechanism for superconductivity in iron-based compounds

We propose orbital fluctuations in a multiband ground state as the superconducting pairing mechanism in the new iron-based materials. We develop a general SU(4) theoretical framework for studying a two-orbital model and discuss a number of scenarios that may be operational within this orbital fluctuation paradigm. The orbital and spin symmetry of the superconducting order parameter is argued to be highly nonuniversal and dependent on the details of the underlying band structure. We introduce a minimal two-orbital model for the Fe-pnictides characterized by nondegenerate orbitals that strongly mix with each other. They correspond to the iron ``${d}_{xy}$'' orbital and to an effective combination of ``${d}_{zx}$'' and ``${d}_{zy}$,'' respectively. Using this effective model we perform random-phase-approximation calculations of susceptibilities and effective pairing interactions. We find that spin and orbital fluctuations are, generally, strongly coupled and identify the parameters that control this coupling as well as the relative strength of various channels.

Superconductivity at 41 K and Its Competition with Spin-Density-Wave Instability in LayeredCeO1−xFxFeAs

A series of layered CeO1-xFxFeAs compounds with x=0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_{c} strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.

Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4±δ

High-transition-temperature (high-T(c)) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4+/-delta (NCCO) shows bulk superconductivity above x approximately 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x approximately 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals.

Evidence for Charge Kondo Effect in Superconducting Tl-Doped PbTe

We report results of low-temperature thermodynamic and transport measurements of Pb1-xTlxTe single crystals for Tl concentrations up to the solubility limit of approximately x=1.5%. For all doped samples, we observe a low-temperature resistivity upturn that scales in magnitude with the Tl concentration. The temperature and field dependence of this upturn are consistent with a charge Kondo effect involving degenerate Tl valence states differing by two electrons, with a characteristic Kondo temperature T(K) approximately 6 K. The observation of such an effect supports an electronic pairing mechanism for superconductivity in this material and may account for the anomalously high T(c) values.

Direct superconducting pairing: a quantum statistical finite-temperature analysis

On the basis of quantum statistical considerations, we suggest a direct (i.e. first-order) superconducting pairing mechanism which does not necessarily involve second-order elements such as the electron–phonon coupling or magnetic interactions submitted by spin fluctuations. We employ four-center four-electron clusters with different topologies to demonstrate that Cooper pair formation can be accompanied by an attenuation of the destabilizing influence of the Pauli antisymmetry principle (PAP). The interatomic hoppings of unpaired electrons are controlled by PAP constraints while the moves of Cooper pairs are not. The quantum statistics of Cooper pairs combines fermionic (fe) and bosonic (b) features. With respect to their diagonal (i.e. density–density) behaviour Cooper pairs obey a fermionic statistics but with respect to their off-diagonal (i.e. hopping) behaviour they follow a bosonic statistics. We derive the energy of the four-site models in unpaired and Cooper-paired configurations in thermal equilibrium. The transition temperature T c from an unpaired to a paired configuration is evaluated by calculating the free energy A of the different states. The free energy A is studied under special consideration of those entropic features which have to be traced back to the degeneracy of the different electronic configurations. We touch upon the possibility of an unusual phenomenon, i.e. the formation of a paired a state, which exists only at high temperatures. The charge and spin degrees of freedom of the model clusters considered are analyzed with the help of calculated charge and spin fluctuations. The two-parameter Hubbard Hamiltonian is used to derive the present finite-temperature data.

Possible pairing mechanisms ofPuCoGa5superconductor

We examine possible pairing mechanisms of superconductivity in PuCoGa$_5$ based on spin-fluctuations or phonons as mediating bosons. We consider experimental data of specific heat C(T) and resistivity $\rho(T)$ as input to determine a consistent scattering boson with the superconducting transition temperature of 18.5K in PuCoGa$_5$. Irrespective to the type of boson, the characteristic boson frequency is found to be $\sim 150 K$ from the resistivity fitting. The spin fluctuation model is most consistent with the experimental resistivity, successfully explaining the anomalous temperature dependence ($\sim \frac{T^2}{150 K +T}$) at low temperatures as well as the saturation behavior at high temperatures. Assuming that the pairing state is non s-wave, the large residual resistivity $\rho_{imp} \sim 20 \mu \Omega cm \sim 120 K$ suggests that an ideally pure sample of PuCoGa$_5$ would have a maximum T$_c$ of 39 K.

The effect of pressure on the phase diagram of the magnetic field-induced superconducting state of λ -(BETS)2FeCl4

We report the effect of a small hydrostatic pressure p ≅ 1.4 kBar on the temperature-magnetic field phase diagram of the organic conductor λ-(BETS) 2 FeCl 4 . At zero field a first-order superconducting (T c ≅ 5.6 K) to antiferromagnetic insulator transition (T N ≅ 4 K) is observed. The close proximity between both phases is suggestive of an unconventional superconducting pairing mechanism. Furthermore, the transition temperature towards the field-induced superconducting state (FISC), as well as its concomitant upper critical field, is found to increase by a factor of ∼ 33 %. The phase diagram of the FISC state for both p = 1 Bar and 1.4 kBar are well described in terms of the Jaccarino-Peter effect if the quasiparticle nature of the charge carriers is taken into account.

Signatures of valence fluctuations inCeCu2Si2under high pressure

Simultaneous resistivity and a.c.-specific heat measurements have been performed under pressure on single crystalline CeCu2Si2 to over 6 GPa in a hydrostatic helium pressure medium. A series of anomalies were observed around the pressure coinciding with a maximum in the superconducting critical temperature, $T_c^{max}$. These anomalies can be linked with an abrupt change of the Ce valence, and suggest a second quantum critical point at a pressure $P_v \simeq 4.5$ GPa, where critical valence fluctuations provide the superconducting pairing mechanism, as opposed to spin fluctuations at ambient pressure. Such a valence instability, and associated superconductivity, is predicted by an extended Anderson lattice model with Coulomb repulsion between the conduction and f-electrons. We explain the T-linear resistivity found at $P_v$ in this picture, while other anomalies found around $P_v$ can be qualitatively understood using the same model.

The Mott-Hubbard insulating state and orbital degeneracy in the superconducting C60(3-) fulleride family.

Electron correlation controls the properties of important materials such as superconducting and magnetoresistive transition metal oxides and heavy fermion systems. The role of correlation in driving metal-to-insulator transitions assumes further importance because many superconducting materials are located close to such transitions. The nature of the insulating ground state often reveals the dominant interactions in the superconductor, as shown by the importance of the properties of La2CuO4 in understanding the high-temperature-superconducting cuprates. The A3C60 alkali metal fullerides are superconducting systems in which the role of correlation in both the normal state and the superconducting pairing mechanism is controversial, because no magnetic insulator comparable to the superconducting materials has been identified. We describe the first example of a cubic C60(3-) system with degenerate orbitals that adopts the Mott-Hubbard insulating localized electron ground state. Electron repulsion is identified as the interaction that is suppressed on the transition to metallic and superconducting behaviour in the fullerides. This observation is combined with ab initio calculations to demonstrate that it is the orbital degeneracy that allows the superconducting cubic A3C60 fullerides to remain metallic while provoking electron localization in systems with lower symmetry.

Superconductivity in magnetically orderedCeTe1.82

We report the discovery of pressure-induced superconductivity in a semimetallic magnetic material ${\mathrm{CeTe}}_{1.82}.$ The superconducting transition temperature ${T}_{c}=2.7\mathrm{K}$ (well below the magnetic ordering temperatures) under pressure $(>2\mathrm{kbar})$ is remarkably high, considering the relatively low carrier density due to a charge-density-wave transition associated with lattice modulation. The mixed magnetic structure of antiferromagnetism coexisting with ferromagnetism can provide a clue for this high ${T}_{c}.$ We discuss a possible theoretical model for the superconducting pairing mechanism.

Structurally tuned superconductivity in heavy-fermion CeMIn 5 (MCo, Ir, Rh)

We discuss systematic trends in the high-temperature physical properties of the heavy Fermion superconductors (HFS) CeCoIn 5 (T c =2.3 K , γ=300 mJ/ mol K 2 ), CeIrIn 5 (T c =0.4 K , γ=750 mJ/ mol K 2 ), and CeRhIn 5 (P c =16 kbar , T c =2.1 K , γ=400 mJ/ mol K 2 ) in terms of crystalline-electrical-field effects(CEF). We suggest the possibility that the interplay between the symmetry of the CEF ground-state (or low- T CEF scheme of levels) and the f–s hybridization could generate spin fluctuations relevant to the superconducting pairing mechanism in these materials. This hypothesis may provide insight into the fact that some crystal structures appear to favor superconductivity. Further, CeMIn 5 (MCo, Ir, Rh) appear to be structural relatives of the cubic heavy Fermion superconductor CeIn 3, but with much higher T c's. We argue that structural layering inherent in the tetragonal CeMIn 5 crystal structure determines the magnetic and electronic anisotropy responsible for the enhanced T c's. We also describe similarities and differences between these compounds and the high- T c cuprates.

Charge–spin–orbital coupling in CuO

The electron pairing mechanism for superconductivity in cuprates is generally attributed to magnetic interactions because superconductivity occurs on the verge of frustrated low-dimensional antiferromagnetic order in Cu–O planes or chains. CuO is the simplest copper oxide that may be the fundamental one not just because till now only compounds containing the Cu–O–Cu bond have produced the high- T c superconductivity but also because recently we found that charge stripes exist in this simplest compound. Here we report strong charge–spin–orbital coupling in CuO, which shows that CuO is a reference system for studying the electron correlation in cuprates.

A possible mechanism of superconductivity in high- Tc cuprates

This paper derives a generic T c formula by using the long-range phase coherence condition in quantum phase fluctuation of the order parameter. Taking the two-local-spin-mediated interaction (TLSMI) proposed by Liu and Chen [Phys. Rev. B 58 (1998) 8812] as a Cooper pair potential, and the T c formula, this paper explains five basic experimental facts in high- T c cuprates. The aim of this paper is to show that TLSMI is a possible pairing mechanism of superconductivity in high- T c cuprates.

Basic considerations on broken-symmetry states of organic superconductors

In this brief account, we shall review the different mechanisms giving rise to itinerant and localized antiferromagnetism in quasi-one-dimensional organic conductors: the Bechgaard salts and their sulfur analogs. We will then focus on the problem of spin correlations and their impact on the pairing mechanism for organic superconductivity.

Indirect-exchange coupling as a basic pairing mechanism of high-temperature superconductivity: Quantitative analysis of TC( x) in electron-doped Nd 1+ xBa 2− xCu 3O y and YBa 2Cu 3O 7− x

A quantitative analysis of critical temperatures T C( x) for the cuprate Nd 1+ x Ba 2− x Cu 3O y and the cuprate YBa 2Cu 3O 7− x is carried through on the basis of an earlier proposed, first-principle, indirect-exchange mechanism of pairing between conduction electrons (quasiparticles) mediated by closed-shell oxygen anions. We conjecture a two-component (two-phase) description for these systems, one component stable at low doping, the other at higher doping content. It is found that T C( x), including the two-plateau structure, is accurately reproduced for both systems. Doping dependence of the quasiparticle Fermi-vector length appears to determine T C( x) of the Nd-compound to a high degree of accuracy. On the other hand, in YBa 2Cu 3O 7− x the development of T C with oxygen deficiency x depends sensitively also on the density of oxygen anions as Cooper-pair mediators. A detailed analysis of these separate effects is given. In conformity with earlier applications of the same formalism, the present results strongly indicate indirect-exchange pairing as a basic mechanism in high- T C cuprate superconductivity.

Electrons and nuclei of C 6H 6 and C 6D 6; a combined Feynman path integral – ab initio approach

We have linked an ab initio approach of the Hartree–Fock (HF) type to the Feynman path integral quantum Monte Carlo (PIMC) formalism in order to study C 6H 6 and C 6D 6 under consideration of the quantum character of the nuclei and electrons. The combination of the statistical Monte Carlo approach with an electronic Hamiltonian offers the possibility to study the influence of the quantum and classical (=thermal) nuclear degrees of freedom on electronic expectation values. The PIMC technique has been used to derive the finite-temperature properties of the nuclei of both π rings. We discuss the temperature ( T) dependence of the energy of the C 6H 6 and C 6D 6 nuclei, their spatial delocalization properties as well as the radial and angular distribution functions. The nuclear configurations generated by the PIMC formalism have been used as input for ab initio HF calculations. Electronic expectation values have been derived as ensemble averages over 6000 different nuclear configurations which are populated in thermal equilibrium. As a result of the large quantum effects of the C 6H 6 and C 6D 6 nuclei, we derive ensemble averaged electronic expectation values which differ sizeable from the corresponding single-configuration quantities at the energy minimum. This difference is mainly caused by nuclear quantum effects; thermal degrees of freedom are of minor importance only. The electronic origin of the potential energy part of the total vibrational energy is emphasized. It is largely determined by the raise in the electron–core attraction under the influence of the spatial uncertainty of the nuclei. The all-quantum approach yields a temperature and isotope dependence of bare electronic quantities already in the framework of the Born–Oppenheimer approximation (BOA). The principal findings of the all-quantum study have been used to reconsider certain solid state problems. We mention theoretical difficulties to reproduce Compton profiles and consider metal–insulator transitions of the Mott type as well as superconductivity. On the basis of the present all-quantum results we have to emphasize, that there is no unambiguous justification to adopt an observed isotope shift in the superconducting transition temperature T c as an indicator of an electron–phonon-coupled superconducting pairing mechanism.

Jahn-Teller distortion and correlation effects in fullerene molecule

Electron-correlation effects in fullerene molecule are investigated in the framework of the Hubbard-Peierls model using the Gutzwiller approximate method. The influence of these effects on lattice distortions and pairing is studied. It is shown that the strongest pair-binding is realized for non-interacting case. Electron-electron interactions suppress the tendency to Jahn-Teller distortion and destroy the pairing. Our results provide strong evidence for the importance of Jahn-Teller distortion in the pairing mechanism of superconductivity in alkali-doped C 60.