Cu,Pu and Fe high Tc superconductors: Spin holes in anti-ferromagnetic clusters form nonmagnetic bipolarons

The new iron based high T_c superconductors with T_c up to 55 K have stirred new interest in this field. It is consensus that the BCS mechanism is not able to explain the high T_c’s. In the following we propose that spin holes in anti-ferromagnetic clusters combine to make nonmagnetic bipolarons, which can condense and lead to superconductivity.     

Rare earth ion size dependence of the Tc of the “RE-123” HTSC within the (d + s)-wave description of their superconducting state

The dependence of the T_c's on the radius of the rare earth ions in the rare earth “123” high temperature superconductors (HTSC) is explained within the framework of a (d + s)-wave model of superconductivity. The appearance of a s-wave symmetry component in the order parameter of a spin fluctuation mediated d-wave superconductor is taken to be due to the orthorhombic distortion which occurs in these HTSC. It is assumed that the structural (orthorhombic) distortion induces a corresponding anisotropy in the pairing interaction, resulting in a nonseparable anisotropic interaction. It is shown that the T_c's are affected by the orthorhombic distortion which in turn is observed to depend on the size of the RE ions.     

Some Features in the behavior of the density of superconducting condensate in superconductors

The correlation between the density ρ_s(T→0) of superconducting condensate and the superconducting transition temperature T _c in underdoped HTSC systems is considered. It is shown that the linear relation between ρ_s(0) and T _c observed in some experiments can easily be interpreted in the framework of the conventional Bardeen-Cooper-Schrieffer (BCS) model without invoking any exotic superconductivity models.     

Nature of pairing in high Tc oxide superconductors

In connection with the unique structural features of the recently discovered high T_c YBa₂Cu₃O₇ and (La,Ba)₂CuO₄ we show that in such a multi-narrow-band system the opposite-spin interband pairs can function concurrently as both antiferromagnetic real pairs in x-space and Cooper pairs in k-space. So, contrary to the usually depairing Coulomb pseudotential μ¹ considered in traditional superconductors, the intra-atomic Coulomb repulsive interactions in a multiband system result in a positive contribution to Cooper pairing and enhance the conventional boson-mediated (electron-phonon and/or electron-exciton) superconductivity.     

An effect of the uniaxial strain on the temperature of Bose–Einstein condensation of the intersite bipolarons

We have studied an effect of uniaxial strain to the temperature of Bose–Einstein condensation of intersite bipolarons within the framework of extended Holstein–Hubbard model. Uniaxial lattice strains are taken into an account by introducing a generalized density–displacement type force for electron–lattice interaction. Associating the superconducting critical temperature T_c with the temperature of Bose–Einstein condensation T_BEC of intersite bipolarons we have calculated strain derivatives of T_BEC and satisfactorily explained the results of the experiments on La-based high-T_c films.     

Novel superconducting phases in copper oxides and iron-oxypnictides: NMR studies

We reexamine the novel phase diagrams of antiferromagnetism (AFM) and high-T_c superconductivity (HTSC) for a disorder-free CuO₂ plane based on an evaluation of local hole density (p) by site-selective Cu-NMR studies on multilayered copper oxides. Multilayered systems provide us with the opportunity to research the characteristics of the disorder-free CuO₂ plane. The site-selective NMR is the best and the only tool used to extract layer-dependent characteristics. Consequently, we have concluded that the uniform mixing of AFM and SC is a general property inherent to a single CuO₂ plane in an underdoped regime of HTSC. The T=0 phase diagram of AFM constructed here is in quantitative agreement with the theories in a strong correlation regime which is unchanged even with mobile holes. This Mott physics plays a vital role for mediating the Cooper pairs to make T_c of HTSC very high. By contrast, we address from extensive NMR studies on electron-doped iron-oxypnictides La1111 compounds that the increase in T_c is not due to the development of AFM spin fluctuations, but because the structural parameters, such as the bond angle α of the FeAs₄ tetrahedron and the a-axis length, approach each optimum value. Based on these results, we propose that a stronger correlation in HTSC than in FeAs-based superconductors may make T_c higher significantly.     

Meissner effect in the underdoped YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>6+<Emphasis Type="Italic">δ</Emphasis></Subscript> HTSC phase

The dependence of the superconducting (Meissner) phase volume V _m on the YBa₂Cu₃O_6+δ doping level was studied. It was found that V _m monotonically decreases as the doping level is lowered and vanishes at the same value of δ ～ 0.3 as T_c does. It was concluded that the T_c decrease and the increase in the pseudogap formation temperature T* as the doping level is lowered are caused by a decrease in the average size of superconducting clusters. This conclusion suggests an extraordinary superconductivity mechanism in HTSC.     

Another approach to mechanism of ferromagnetic superconductor UGe2

We study the ferromagnetic superconductor of UGe₂ applying our previous model [Phys. Rev. B 61 (2000), 4289] for the high transition temperature superconductivity (HTSC). The Coulomb interaction for triplet electron pairs is reduced by a difference of the exchange interaction. In the case of UGe₂ including other heavy fermion superconductors, coexistence of triplet superconductivity and ferromagnetism is possible in the case of our scheme. We also investigate the pressure-dependence of Curie temperature, T_c and superconducting temperature, T_sc.     

A new (Hg,V)-based 1212-type cuprate (Hg,V) Sr2(Y,Ca) Cu2Oz with Tc(onset) up to 110 K

We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg_1?x V _x )Sr₂(Y_1?y Ca _y )Cu₂O _z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT _c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed.     

A new (Hg,V)-based 1212-type cuprate (Hg,V) Sr2(Y,Ca) Cu2Oz with Tc(onset) up to 110 K

We have successfully synthesized a new (Hg, V)-based 1212-type cuprate (Hg_1?x V _x )Sr₂(Y_1?y Ca _y )Cu₂O _z . The electrical resistance measurements showed that some of the as-synthesized materials with proper Hg/V and Y/Ca ratios exhibit weak superconductivity. Oxygen-annealing significantly improved their superconducting behavior, andT _c(onset) up to 110 K was observed. The lattice parameters of (Hg,V)-1212 were found to be in the order ofa=3.8415(1) Å andc=11.8514(6) Å. Substitu ting mercury atoms by vanadium ones results in an important increase of thea parameter and an important decrease of thec parameter compared to the known Hg-based Sr-bearing compounds. The crystal structure of (Hg, V)-1212 was refined by Rietveld refinement against X-ray powder data using the tetragonal symmetry of space groupP4/mmm. The V and Cu valence states and superconductivity in this new (Hg, V)-1212 cuprate are briefly discussed.     

An ARPES view on the high-<Emphasis Type="Italic">T</Emphasis><Subscript>c</Subscript> problem: Phonons <Emphasis Type="Italic">vs.</Emphasis> spin-fluctuations

We review the search for a mediator of high-T _c superconductivity focusing on ARPES experiment. In case of HTSC cuprates, we summarize and discuss a consistent view of electronic interactions that provides natural explanation of both the origin of the pseudogap state and the mechanism for high temperature superconductivity. Within this scenario, the spin-fluctuations play a decisive role in formation of the fermionic excitation spectrum in the normal state and are sufficient to explain the high transition temperatures to the superconducting state while the pseudogap phenomenon is a consequence of a Peierls-type intrinsic instability of electronic system to formation of an incommensurate density wave. On the other hand, a similar analysis being applied to the iron pnictides reveals especially strong electron-phonon coupling that suggests important role of phonons for high-T _c superconductivity in pnictides.     

Coulomb energies and nuclear radii in the energy density formalism

The relation between Coulomb displacement energies,ΔE _c , andΔr=r _n -r _p , the difference between the rms radii of neutrons and protons in nuclei, is investigated within the energy density formalism (EDF). The variational equation, obtained by minimizing the Coulomb plus symmetry energies, is solved assuming the symmetry interaction is a simple functional of the local nuclear matter density. Varying parameters of the model, rather unique relation betweenΔE _c andΔr is obtained (within ±50 keV).ΔE _c isindependent ofr _ex, the rms radius of the excess neutrons distribution. Using the experimental values ofr _p and adjusting the model to reproduce the recent data onΔr (Δr∽~0.05 fm for⁴⁸Ca and²⁰⁸Pb), which are significantly smaller than those obtained from current Hartree-Fock calculations, the calculatedΔE _c agree with the experimental results. Using the value ofΔr～0.05 fm and the experimental values ofr _ex, a small compression (<0.02 fm) of the proton core in the analogue state relative to its parent state emerges, thus contributing an additional electrostatic term to the Coulomb displacement energy. The size of this relative core-compression effect depends on the values assumed forΔr andr _ex, it increases with the decreasing ofΔr and the increasing ofr _ex. IfΔr～0.05 fm the effect is large enough to remove the long standing Coulomb energy anomaly. The main result of the present work is the correlation betweenΔE _c andΔr, suggesting that the difficulties of current Hartree-Fock calculations in reproducing isotope shifts ofr _p , the small value ofr _n ?r _p and the values ofΔE _c may all be different manifestations of some missing residualp n effective interaction.     

Re-entry in ferromagnetic superconductors

The re-entry phenomenon in magnetic superconductors is studied using the generalized Ginzburg-Landau free energy introduced by Blount and Varma. The re-entry temperature T_c2 is simply that temperature at which the magnetization acts as a source of induction strong enough to destroy superconductivity. Above T_c2 ferromagnetism and superconductivity coexist. The structure is an Abrikosov vortex lattice, with ferromagnetic magnetization spreading widely around the vortex cores. Within our approximations, the phase transition at T_c2 is of second order.     

Theory for superconductivity in Pd-H and Pd-D systems

A theory is presented explaining the recently observed superconductivity at 9 °K in Pd-H and at 11 °K in Pd-D as due to the quenching of the spin-fluctuations and as due to the decrease of the Coulomb pseudo-potential μ present in Pd. That one observes for PdD a larger superconducting transition temperatureT _c than for PdH is explained by the larger lattice expansion resulting for adding H to Pd. The observed non-monotonic dependence ofT _c on the Hydrogen concentration is explained by the fact that approximately λ ∝N(0) for smallN(0), whereN(0) is the electronic density of states at the Fermi-energy and where λ denotes the electron-phonon coupling constant. Superconducting transition temperatures are estimated for PtH and RhH.     

Statistical correlations on the BiO surface in Bi2Sr2CaCu2O8−x single crystals

Low temperature scanning tunneling microscope techniques are applied to study statistical properties of the BiO plane in 2212 high-T_c single crystals. The correlation functions of height deviations in x and k -spaces have been numerically obtained. Long range power law correlations have been found and attributed to half lattice cell jumps of known height in the c direction. Analysis of the correlation function in k-space reveals two modulating superstructures in the b direction. The most pronounced structure is the well known superstructure with period a₁ ≈ 27 Å. The other one has period a₂ ≈ 58 Å which is a little more than twice the period of the main superstructure.     

Theory for superconductivity in amorphous transition metals

A theory is presented for superconductivity in amorphous transition metals. It is shown that in contrast to simple metals for transition metals the changes in the phonon spectrum, in the electronic density of states and in the electronic matrix elements which result from strong lattice disorder can enhance as well as decreaseT _c. The numerical results for the superconducting transition temperatureT _c of amorphous 4d-and 5d-transition metals agree well with the experimental results.     

High frequency local modes,superconductivity and anomalous isotope effect in PdH(D) systems

The possible mechanisms of enhanced superconductivity in PdH(D) systems are discussed. It is concluded that quenching of paramagnons due to reduced coulomb pseudopotentialalone cannot account for the highT _c observed in these systems. Possible participation of the high frequency optic phonon band in BCS-type interaction could lead to a highT _c and an opposite isotope effect in these systems. The possibility of the occurrence of some high-q soft modes due to large lattice expansion in these systems to yieldT _c ～9 K requires about 2-fold softing of the effective Pd-Pd force constant in PdH compared to that in metallic Pd.     

Checkerboard patterns and magnetic resonance peak in cuprate superconductors

We investigate a kind of spin-Peierls transition (SP) in high T_c superconductivity. It is found the antiferromagnetic exchange integral of SP corresponds to the magnetic resonance peak. The kind of spin-Peierls transition applied to cuprate superconductors is that without dimerization of lattice ions and with dimerization of localized hole h_Cu attached to the ion. Absence of the magnetic resonance peak in La-Sr-Cu-O results from the dimerized state of localized hole, h_Cu below T_c into tetramerized phase above T_c in SP transition without dimerization of copper-ion. The checkerboard patterns with four unit cell period originate from the SP of electronic part without ion-dimerization and from charge occupation probability of oxygen-atom around Cu.     

NMR and NQR studies of spin dynamics and superconductivity in High-T c oxides

The magnetic and superconducting properties in the high-T _c cuprates have been investigated over a wide hole doping range by⁶³Cu,¹⁷O and²⁰⁵Tl NMR and NQR in the lightly-doped La_2?xSr_xCuO₄ (LSCO), the heavily-doped Tl₂Ba₂CuO_6+y (TBCO) and the Zn-doped YBa₂Cu₃O₇ (YBCO₇). In low doping region, the large antiferromagnetic (AF) spin correlation around the zone boundary (q=Q) causes the Curie-Weiss behavior of⁶³(1/T ₁ T) associated with that of the staggered susceptibility χ_O(T) in LSCO. In the vicinity of the hole content whereT _c has a peak, the AF spin correlation still survives, although the magnetic coherence length ξ_M is considerably short being presumably (ξ_M/a) ～ 1. The further doping destroys progressively the AF spin correlation, which is no longer present is non-superconducting TBCO compounds. These NMR evidences signify that there is an intimate relation between the presence of the AF spin correlation and the onset of the superconductivity. The local collapse of AF spin correlation is a primary cause for the unexpected strong reduction ofT _c in case of the substitution of Zn impurities into the CuO₂ plane. The superconducting properties clarified by NMR experiments cannot be accounted for by the conventional BCS model or other isotropic s-wave models. A d-wave model is applicable in interpreting consistently most of the NMR results, if the finite density of states at the Fermi level is taken into consideration and is associated with the pair breaking effect. There are increasing evidences that the magnetic mechanism for the superconductivity is promising in high-T _c cuprates.     

Self-localized oscillations in high-temperature superconductors

Molecular-dynamic characterization of high-temperature superconductors is discussed. Results obtained show strong anharmonicity of self-localized high-frequency oscillations of individual atoms. The localization of these oscillations near certain defects results in the spatial redistribution of the kinetic energy in the system. By the example of the La-Sr-Cu-O system, it was shown that the presence of the oscillations correlates with the superconductor transition temperature in this compound and causes fluctuations in the phonon and soliton subsystems. This may be a reason for the occurrence of high-temperature (high-T _c ) superconductivity (HTSC). Calculations are compared with experimental data.