Table of contents

LEHTSC2007 was held at AIST in Tsukuba, Japan, from 31 October to 3 November, and was attended by 110 scientists from twelve countries. In late October, the autumn leaves of the beautiful ginkgo trees change colour from verdant green to bright yellow, as always. For its part, the conference set out to search for the `Genome information', addressing the greatest unsolved mystery in current condensed matter physics, that is, the pairing mechanism of high temperature superconductivity (HTSC).

Although there has been much effort and tremendous accumulated knowledge since the discovery of HTSC by Bednorz and Müller in 1986, we have failed to reach an unified view on the microscopic mechanism of HTSC. The fact that no mechanism has yet been accepted as an universal model may indicate that we may still be lacking `some important ingredients'. The search for the `missing ingredients' is now being accelerated as experimental techniques have improved to become state-of-the-art and overwhelmingly detailed information is readily attainable. Our attitude of cooperative communication is even more needed. Unfortunately, researchers with different ideas often tend to `phase-separate', and cross participation in scientific meetings is low. A complex phenomena like HTSC, often show different `shapes and colours' which can easily lead to misunderstanding if we narrow our view. This issue is expressed in the subtitle, `Spin, phonon, or the third way?' meaning that the `missing ingredients' (electron–lattice interaction) which has long been neglected must be properly taken into account to reach an understanding of the true mechanism.

As Chairman of the Organizing Committee, I would like to express my greatest appreciation to all invited speakers, oral and poster contributors as their works confirmed that we are indeed taking the third way. In total there were 53 invited and contributed oral talks and 25 poster presentations. All lectures were followed by active comments and discussions. Starting from well-organized reviews of HTSC, the conference developed into detailed lectures. The atmosphere of the conference was open and casual so that everyone, including young scientists who will take on future responsibility, could have the opportunity to participate in constructive discussions. An interactive-mode voluntarymeeting was held before closing the last session which focused on important issues left for future discussions. The conference made us confident that our voyage will certainly continue in the future.

In this conference, we took the open access policy. We recorded all talks in the conference as a multimedia, which will be open at the conference web site. The electronic proceeding (Journal of Physics: Conference Series) is an open access journal. By linking multimedia talks and electronic proceeding, scientific contents of LEHTSC2007 are accessible to all researchers around the world. We are optimistic that following conferences will be equally successful and contribute to shortening the time for the discovery of HTSC mechanism. Lastly, I would like to introduce some words from a philosopher that explains the process of finding the truth in the HTSC research.

All truth passes through three stages. First, it is ridiculed. Second, it is violently opposed. Third, it is accepted as self-evident. —Arthur Schopenhauer

Acknowledgements We would like to express our greatest thanks to Kazunobu Tanaka in providing financial support for this conference. The efforts of the local organizing committee are appreciated which persisted for almost a year. Particular thanks are due to Misako Tsutsui for her outstanding work in website design and technical editing of the proceedings. AIST is thanked for the use of the conference rooms and related facilities. We appreciate backup support by The Physical Society of Japan and The Japan Society of Applied Physics. For multimedia recording, we wish to thank Media Site KK for their support and cooperation. We also appreciate exhibitors of Cryoportal 2007 for promoting the conference and financial support. The efforts of Atsuyuki Fukano, Zhyu Sun and Changjing Zhang for arranging the conference site and audio-visual equipment are appreciated. Also thanks are due to Masayo Fujita for maintaining the secretariat and Masako Oyanagi for coordinating social events. Lastly we wish to express our sincere thanks to the editorial team of IOP Publishing.

Hiroyuki Oyanagi Chairman of LEHTSC2007

Recently, it was proposed that phonons are specularly reflected below ~0.5 K in ordinary single-crystal samples of high-T_c cuprates, and that the low-temperature thermal conductivity k should be analyzed by fitting the data up to ~0.5 K with the formula k = aT + bT^α where α is a fitting parameter. Such an analysis yields a result different from that obtained from the conventional analysis in which the fitting is restricted to a region below ~0.15 K, where k has been found to obey k = aT + bT³ very well, which means that phonons are in the boundary scattering regime without specular reflection. Here we show that the proposed new analysis is most likely flawed, because the specular phonon reflection means that the phonon mean free path l gets longer than the mean sample width, while the estimated l is actually much shorter than the mean sample width above ~0.15 K.

In this paper, I introduce recent our results on superconductivity fluctuation measurement of high-T_c cuprate both for the hole-doped La_2-xSr_xCuO₄ (LSCO) and the electron-doped La_2-xCe_xCuO₄ (LCCO). In hole doped LSCO, the universality class was found to change twice as a function of doping, starting from the 2D-XY, changing to the 3D XY and another 2D 'unknown' behavior. The results favors theoretical interpretations of the phase diagram of high-T_c cuprate which assume the existence of an additional hidden quantum critical point around at the optimum doping. In electron doped cuprate, the superconductivity fluctuation is 3D XY for all samples with different hole doping, which is in sharp contrast to the hole doped cuprate. Thus, the asymmetry of the phase diagram between the hole doped and the electron doped materials is another important key factor to judge the applicability of various theories on high-T_c superconductivity.

Under finite magnetic fields, the scaling relation was valid only for weak fields, and for higher fields, aspects as vortices appeared. However, even at low temperatures, just above the first order phase transition, vortex picture alone cannot describe the data satisfactory. Thus, we need a unified theory for the description of a large superconductivity fluctuation under finite magnetic fields for high-T_c cuprates.

Finally, I showed that our novel technique of fabricating high-T_c Josephson bridge junction using a small island of Fe was turned out to be very promising.

Our recent experimental works on the inhomogeneity of superconductivity in both underdoped and overdoped regimes of La_2-xSr_xCuO₄ are reviewed. In order to estimate the superconducting (SC) volume fraction in a sample, bulk-sensitive magnetic-susceptibility and specific-heat measurements have been carried out, using La_2-xSr_xCuO₄ single crystals of good quality with x = 0.05−0.3. As a result, it has been found that the SC volume fraction is not 100% in both underdoped and overdoped regimes, suggesting the occurrence of a phase separation into SC and non-SC regions. Moreover, strong vortex-pinning effects have been observed in moderate magnetic fields in the overdoped regime, suggesting that the phase separation in the overdoped regime is not macroscopic but microscopic.

Precise zero-field (ZF) and longitudinal-field (LF) muon spin relaxation (μSR) measurements have been carried out on La_2-xSr_xCuO₄ in the underdoped regime of 0.024 ≤ x ≤ 0.15 to investigate the dynamics of local fields in the normal state. ZF-μSR time spectra show the Gaussian shape, indicating that muon spins depolarize mainly by nuclear dipole fields which are randomly distributed at the muon site. A slight deviation of the shape of the ZF-μSR time spectrum from the Gaussian shape has been observed with decreasing temperature. This deviation is due to a change of the dynamic depolarization rate of the muon-spin polarization. From LF-μSR measurements, it has been found that a small and quasi-static local field exists at the muon site even in the normal state around 100 K. The change of the muon-spin depolarization rate observed in ZF is due to a change of the magnitude of the small and quasi-static local field at the muon site.

On the basis of a rigorous constraint we show that the doped cuprates either exhibit electronic phase-separation, or balance right at the phase-separation threshold. We suggest that the pseudo-gap phase can be identified with the state that is nano-scale phase-separated in magnetic and non-magnetic phases with different charge carrier concentrations. Long-range Coulomb interaction should drive the nano-scale phase-separated state into a self-organized ordered phase, even though such ordering is most likely frustrated by chemical disorder. Identifying this nano-scale self-organization should clarify a number of confusing issues related to the origin of the superconductivity in the cuprates.

We performed neutron scattering measurements on the overdoped sample of electron-doped superconductor Pr_0.83LaCe_0.17CuO₄ (T_c~16 K) to clarify an overall spin excitation spectrum. Clear commensurate low-energy spin fluctuations were observed at the (π, π) reciprocal position below ~10 meV. With increasing energy transfer, the width of commensurate peak drastically broadens and the intensity rapidly decreases. However, magnetic scattering intensities at high-energy region around 100 meV were confirmed to exist. These results are quantitatively same with those obtained for optimally-doped Pr_0.89LaCe_0.11CuO₄, while the broadening of peak-width against energy transfer is more marked and the intensities at high-energy region are weaker in the present overdoped sample. Detailed comparison of excitation spectra with the optimally-doped sysmte is given.

To date, there has been no evidence for the macroscopic structural phase transition to the low temperature tetragonal structure (LTT) with a space group P4₂/ncm in high-T_C cuprate of rare earth-free La_2-xSr_xCuO₄ (LSCO). By investigating Cu-NMR on single crystals, we have found that spatially incoherent LTT structure emerges below 50 K in the sample with x = 0.12. This incoherent structure is considered to play a key role for the slight depression of the superconductivity around x = 1/8.

On the basis of STM/STS, break-junction tunneling and electronic Raman scattering experiments on Bi₂Sr₂CaCu₂O_8+δ reported so far, we suggest that the static, electronic charge order is associated with inhomogeneous electronic states on antinodal parts of the Fermi surface that are outside the Fermi-arc around the node and responsible for the pseudogap, and coexists with the homogeneous superconductivity caused by the pairing of coherent quasiparticles on the Fermi-arc, the so-called 'Fermi-arc superconductivity', in the real space, although the two electronic orders or the corresponding energy gaps compete with each other in the k-space.

Gradual evolution of two phase coexistence between dynamical and static regimes in cuprates is first investigated in the real space by making use of available neutron scattering, NMR and μSR data. Analyses of the Hall effect and the ARPES spectra reveals the presence of two groups of charge carriers in LSCO. The T-dependent component is due to the thermal activation of bound electron-hole structures seen near antinodal points in the Brillouin zone, thus introducing the two component physics also for the momentum representation. Interpretation of so-called 'van Hove bands' undergoes drastic changes. Importance of the findings for pseudo-gap physics is stressed. Relation to some recent STM and STS results is discussed.

The electronic state of high-temperature cuprates is investigated on the basis of the two-dimensional Hubbard model. We investigate the three-band Hubbard model (which is sometimes called the d-p model) as well as the single-band Hubbard model. In particular, the three-band Hubbard model with d and p orbitals in the CuO₂ plane probably captures the physics of high-temperature superconductivity. Numerical studies of the two-dimensional d-p and Hubbard models have shown that they exhibit antiferromagnetism, stripes, d-wave pairing, coexistent antiferromagnetism with superconductivity, and arc spectra. They are basic phenomena seen in the high-temperature cuprates.

'Valence skip' means that an element leaps a particular valence state in compounds. For example, Bi forms the compounds in +3 and +5 valence states, and T1 forms in +1 and +3 valence states. In BaBiO₃, the average valence of Bi is +4, but the system spontaneously brakes its crystal symmetry and forms the charge density wave (CDW) state like (Bi³⁺,Bi⁵⁺). This means that the effective electron-electron interaction is attractive, contrary to the well-known on-site Coulomb repulsive interaction. In the valence skip compounds, it is expected that some carrier doping can induce superconductivity with high-T_c. For example, (Ba,K) BiO₃ shows high-T_c that is not plausible merely from the convectional electron-phonon mechanism. T1-doped PbTe is also considered as the superconducting compound that T1 valence fluctuates and enhances T_c. We estimated the formation energy of the CDW in BaBiO₃, T1S and T1Se by evaluating the Madelung energy, in order to show the CDW state is more stable than the uniform charge state, and the valence skip is actually realized.

The interplay between electron-phonon interaction and strong electronic correlation is analyzed by means of Dynamical Mean Field Theory. Suppressing all antiferromagnetic correlations, the electron-phonon interaction is found to be strongly suppressed by Coulomb repulsion. In particular, close to the Mott transition at half-filling the electron-phonon interaction has very little effect on quasiparticles: In fact it is possible to describe the low-energy physics in terms of an effective Hubbard model with a slightly renormalized repulsive coupling. The situation changes completely if antiferromagnetic correlations are included: Electron-phonon interaction has a strong effect on the electronic self-energy even for large values of the Coulomb repulsion. Phonon-induced modifications of electronic properties like, e.g. photoemission spectra, are therefore expected to be more pronounced in underdoped cuprates where antiferromagnetic correlations are stronger than in overdoped.

Quantized massive gauge fields have been introduced as collective modes, which might contain effects of spin fluctuations, charge fluctuations, and phonons, around doped holes from the viewpoint of adjoining interactions. One has discussed the relationship among anomalus properties in high-Tc cuprates from the standpoint of the effect of quantized massive gauge fields around doped holes and the restoration of spontaneous symmetry breaking. It is suggested strongly that the quantized massive gauge fields are mediating Cooper pairing in high-Tc cuprates.

This short paper summarizes the results we presented at the LEHTSC2007 conference. Recent doping and temperature dependence of angle-resolved photoemission data of underdoped superconducting cuprate Bi₂Sr₂CaCu₂O₈ (Bi2212) have revealed the presence of two energy scales exhibiting distinct behaviours [1,2]. One, which dominates the antinodal region, increases with underdoping and does not show obvious temperature dependence across Tc. This is a behaviour known for more than a decade and considered as the general gap behaviour in the underdoped regime. The other, which dominates the near-nodal regime, does not increase with less doping and opens near Tc via a BCS-like temperature dependence. This is a behaviour not previously observed in the single particle spectra. We propose a momentum space picture of these two energy scales or energy gaps that could resolve the seemingly contradictory gap measurements among different experimental techniques. Our results have also further constrained the theory for high-Tc superconducting cuprates.

We have investigated the electronic structure of the optimally doped trilayer cuprate superconductor Bi₂Sr₂Ca₂Cu₃O_10+δ by angle-resolved photoemission spectroscopy. We have found that the d-wave like superconducting gap opens below T_c and the pseudogap in the antinodal region persists at slightly above T_c, consistent with the previous report. The energy position of the kink in the quasi-particle dispersion was nearly isotropic below T_c, but became momentum dependent above T_c, consistent with the scenario that the antinodal states are coupled to the B_1g phonon mode and the nodal states to the half-breathing mode.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of as-grown Nd_1.85Ce_0.15CuO₄ (ag-NCCO) and annealed Nd_1.85Ce_0.15CuO₄ (an-NCCO) samples in order to elucidate how the superconductivity is induced by the annealing process in Nd_2-xCe_xCuO₄. In the ARPES spectra of ag-NCCO, a gap opens in the nodal region due to the effects of antiferromagnetism. This nodal gap is closed in an-NCCO, indicating that the effects of antiferromagnetism become weaker by annealing, similar to the previous ARPES results on Pr_1.85Ce_0.15CuO₄. However, while in as-grown Pr_1.85Ce_0.15CuO₄ the gap opens on the entire Fermi surface, in ag-NCCO the gap does not open around (π, 0).

A new path-integral theory is developed to calculate the photoemission spectra (PES) of correlated many-electron systems. The application to the study on Bi₂Sr₂CaCu₂O₈ (Bi2212) and boron-doped diamond (BDD) is discussed in details. It is found that the isotopic shift in the angle-resolved photoemission spectra of Bi2212 is due to the off-diagonal quadratic electron-phonon (e-ph) coupling, whereas the presence of electron-electron repulsion partially suppresses this effect. For the BDD, a semiconductor-metal phase transition, which is induced by increasing the e-ph coupling and dopant concentration, is reproduced by our theory. Additionally, the presence of Fermi edge and phonon step-like structure in PES is found to be due to a co-existence of itinerant and localized electronic states in BDD.

We have performed x-ray photoemission spectroscopy (XPS) measurements on the high-T_c superconductor La_2-xSr_xCuO₄ (LSCO) at various temperatures. From the shifts of core-level spectra, we have deduced the temperature dependence of the chemical potential in this system. In the underdoped region, the chemical potential shifts upwards with increasing temperature, whereas it shows an opposite shift in the overdoped region. These results qualitatively agree with the predictions of the t-J and tight-binding model calculations as well as thermodynamic quantities but a large quantitative discrepancy exists.

Various unconventional isotope effect experiments on high-temperature cuprate superconductors are reported together with observations of multi-component order parameters. The experiments are interpreted in terms of multi-band superconductivity where polaron formation takes place which leads to a renormalization of the single particle energies. Experiments and theory comply consistently with each other.

We have studied the effect that variations of the local microstrain have on the polaronic correlation function obtained from the exact diagonalization of a simple model Hamiltonian. Although the correlation function exhibits a monotonic behavior as a function of the hole hopping parameter, and thus microstrain, its isotopic shift exhibits a maximum for specific values of t. This result suggests that if there is a coupling between polaronic objects and free carriers, coexisting in high-T_c cuprate superconductors, then this will manifest not only in the superconducting transition, but also on the properties directly related to the polaron formation in these materials.

In this paper we investigate the optical spectra of one hole in the Holstein-t-J model. We employ a dynamical mean-field theory which becomes exact in the limit of infinite connectivity. This allow us to investigate the local (incoherent) features which are related to the internal structure of the polaron, disregarding coherent motion associated with the Drude-like peak. We show that magnetic and electron-phonon interactions sustain each other in establishing polaronic regime. Polaron formation is reflected in a peculiar mid-infrared (MIR) band which is however notably different in the case of a lattice or magnetic origin. The dependence of σ(ω) on the electron-phonon coupling constant λ, on the exchange interaction J and on temperature T is investigated. We compare our results with experimental data in Nd_2-xCe_xCuO₄ showing that the doping and temperature dependences of the optical conductivity in this compounds is naturally reproduced by a spin/lattice polaronic model.

The present Raman scattering experiment in La_2-xSr_xCuO₄ disclosed that the low-energy conductive electronic states at (π/2, π/2) in the underdoped phase are composed of dual electronic states of the central resonant peak and the electron-phonon coupled polaron states. Both states develop at the lowest-energy end of the B_2g electron - two-magnon coupled high-energy peak. The narrow resonant peak is enhanced at the insulator-metal transition, but it is insensitive to the superconducting transition indicating no relation to the superconductivity. On the other hand the polaron peak changes into the superconducting coherent peak (SCP) exactly below T_c. The gap energy determined from the SCP is almost constant from the optimum doping to near the insulator-metal transition, indicating that the gap symmetry is s or d(xy), because the Fermi arc with the large electronic density of states shrinks into the nodal direction of the d(x² - y²) gap. In the overdoped phase the low-energy conductive electronic states are created at (π, 0) by decreasing the peak energy of the B_1g electron - two-magnon coupled states. The gap symmetry in the overdoped phase is consistent with d(x² - y²).

The doping and temperature dependence of the Raman spectra of high-quality La_2-xSr_x^16,18CuO₄ polycrystalline compounds with x = 0.0−0.3 have been investigated. The spectral characteristics of the soft phonon that appears in the (zz) polarization spectra of the LTO phase show an abnormal behaviour close to Néel transition temperature for x = 0.0 (in the temperature range 285–305K) and for x = 0.015 (in the range 77–100K), indicating an apparent coupling of the lattice to the spin ordering. The isotopic shift of the soft mode deviates from the mass harmonic law at all temperatures and this deviation increases substantially for T>175K towards room temperature. In the other scattering polarization (xx or yy), certain new bands at ~150 cm^-1, ~280 cm^-1, and ~370 cm^-1 appear mainly at low temperatures and in the superconducting doping region 0.03<x<0.27. From the study of the effects of the oxygen isotopic substitution, the possible reduction of symmetry, the doping, and the temperature dependence, the new modes have been assigned to specific IR phonons, which are activated in the Raman spectra from the local breaking of the inversion symmetry.

Micro-Raman measurements under hydrostatic pressures up to ~6 GPa have been carried out on high-quality La_2-xSr_xCuO₄ polycrystalline compounds with Sr concentration up to x = 0.24. The zz scattering polarization has been investigated at room temperature, where two strong modes due to La/Sr and the apex oxygen atoms are observed. The energy of the strong modes increases almost linearly with pressure for the Sr concentrations studied. Modifications in the increment rate dω/dp have been detected depending on the amount of doping. Finally, measurements at low temperatures denote that the application of pressure minimizes the distortions of the CuO₆ octahedra.

Recently a series of experiments in Scanning Tunneling Spectroscopy, angle-resolved photoemission spectroscopy (ARPES) and isotope effects have provided a great deal of information about the nature of the superconducting state, even thought many questions are still open. In this paper we present a discussion of the various theoretical frameworks in the light of these new experiments. Strong correlations started as the possible path to an entirely novel physics but, in many recent versions, they are reduced to the modest role of just raising the coupling in a BCS gap equation with phonon mediators. We argue instead that a conceptual generalization of the Migdal-Eliashberg (ME) theory is necessary and we discuss the consequences arising from the breakdown of the adiabatic assumption. The Nonadiabatic Theory of Superconductivity represents a complex generalization of the ME theory which is natural and, in many respects, unavoidable. In fact Migdal's theorem is strongly violated in these materials for a variety of reasons. The quantification of these effects is technically complicated and it is rather model dependent. In this framework strong correlations can have an important role in locating the system in a favourable range of parameters (forward scattering). We present a critical discussion of these developments in the light of the recent experiments and we also propose some crucial tests.

The phonon effect of the in-plane oxygen breathing vibration in the high-Tc cuprates is investigated by the numerical diagonalization of an extended t-J-Holstein model, including the modulation of t. As a result, it is found that if the modulation of t due to the phonon is sufficiently large, the breathing mode possibly stabilizes the superconductivity.

We briefly summarize a perspective on high-temperature oxide superconductors (HTC) as systems of intrinsic, functional, and connected spatial scales, with an associated set of temporal scales. We emphasize the importance of local bonding constraints, leading to a framework of coexisting short- and long-range fields, as the source of multiscale systems in HTC and many other strongly correlated materials.

One of key challenges in high-T_c superconductivity research is to identify a predominant mechanism controlling superconductivity at atomic scale. Using the spectroscopic imaging scanning tunnelling microscopy technique[1] we find a ~9±1% sinusoidal variation in local energy gap Δ triggered by the crystal 'supermodulation' revealing a strong non-random out-of-plane effect on superconductivity[2], substitutional isotope effect on the d²I/dV² spectrum reveals 6% reduction of mode energy Ω indicating the involvement of the lattice vibrations that couple strongly to the quasiparticle states of Bi₂Sr₂CaCu₂O_8+δ [3]. We also find that the heterogeneous electronic states coupling to the lattice only occur above a certain energy which separates the spatially homogeneous and heterogeneous excitations[4].

We show that nanoscale superstructures observed in STM experiments in high temperature superconductor (HTSC) such as cuprates in superconducting state [1] are well described by a model of two-dimensional charged boson gas(2DBG). The bosons are located on top of a uniform, structureless, jellium neutralizing background. This model is the most fundamental and clean quantum system where different properties of the formation of the superconducting and insulating states mcan be studied by changing only one parameter -the boson density n₀. At high densities the ground state is always superconducting, that is associated with the Bose-Einstein condensation of the charged bosons which form a superfluid state. Whereas at very low densities bosons localize into a Wigner crystal. Here we show that a dilute amount of impurities with opposite charge to bosons alter dramatically the properties of the system. Any charged impurity induces density oscillations, similar to Friedel oscillations in Fermi liquids. When the density decreases the amplitude of these charge density wave(CDW) oscillations increases. At some critical density there arises the CDW instability. As the result around each impurity a Coulomb bubble(CB) is formed. At such a CB there arises an orthogonality catastrophe associated with the formation of localized states inside CB orthogonal to the fluid of free bosons. The creation of localized states may be accompanied by a formation of local lattice distortions. The phenomenon is very similar to the conventional self-trapping or a formation of electronic strings[2]. The CDW instability arises due to over-screening of the Coulomb interaction. The phenomenon of CB formation is very general. They can be created and become localized around any potential induced by phonons or by individual impurities or both. As the result the quantum system separates into two phases: localized CBs distributed randomly and superfluid bosons forming a homogeneous state. When the density decreases after the formation of CBs the area covered by them increases while the superfluid area decreases. Also with decreasing density the number of Coulomb bubbles grows in a step-like manner. At each such step more remote impurities outside the superconducting plane take part in trapping of CBs and reduce significantly the superfluid density. The insulating state emerges either via creation of an infinite percolating cluster of CBs or due to the Wigner crystallization of the bosonic phase.

The extended two-story house model which is now called the Kamimura-Suwa (K-S) model has clarified how the interplay of Mott physics and Jahn-Teller physics plays an important role in determining the superconducting as well as metallic state of underdoped cuprates. In this paper it is first pointed out for underdoped cuprates that Mott physics leads to the existence of local antiferromagnetic order constructed from the localized spins while that the anti-Jahn-Teller effect as a central issue of Jahn-Teller physics leads to the existence of two kinds of orbitals parallel and perpendicular to a CuO₂ plane whose states have nearly the same energy. As a result of the interplay of both physics the K-S model has shown that the exchange interactions between the spins of a localized hole and of a carrier hole play an important role in producing the coexistence of superconductivity and antiferromagnetism in underdoped cuprates. The appearance of d-wave superconductivity even in the phonon-involved mechanism is also shown to be due to the interplay of Jahn-Teller physics and Mott Physics. Brief review of these facts as well as the K-S model is given in this paper. More outstanding result in this paper is that the origin of pseudogap in the deeply underdoped regime has been clarified. In this paper it is shown theoretically for the first time that the so-called T* pseudogap observed in ARPES, STM and tunneling experiments below T_c in underdoped cuprates corresponds to the real transition of photo-excited electrons from the occupied states in the originally conduction band below the superconducting gap to a free-electron state above the vacuum level. Thus we conclude that the T* pseudogap in the underdoped cuprates which increases with decreasing the hole concentration is not 'pseudo', but a real gap which exists even below T_c.

We first present our molecular orbital cluster calculation results that indicate the presence of strong interaction between doped holes and the underlying lattice in the cuprate. It is also indicated that the naive Zhang-Rice singlet picture for the local electronic state needs to be revised: the doped hole is mainly concentrated in a single bridging oxygen, and the bridging oxygen p orbital and two nearby copper d_x²-y² orbitals form a valence bond type bonding; this state is stabilized by local lattice distortions and the doped hole becomes a small polaron.

Next, we consider the current generation mechanism compatible with the above small polaron formation; actually, we present a new current generation mechanism that is effective even if all the doped holes become self-trapped small polarons. The current generation here reverses a long-standing belief that at half-filing the effective Hamiltonian for the U >> t Hubbard model is the Heisenberg model, thus, possible low energy excitations are those include spin degrees of freedom only. The new current generation mechanism yields the Fermi arc seen in the ARPES simulation; it also suggests that the phase of the superconductivity order parameter in underdoped cuprates arises without Cooper pair formation; i.e., the London current formula with effective charge 2e is realized without the Cooper pair formation.

The electric current here is generated by a fictitious magnetic field induced by spin vortices: because the presence of spin vortices causes a sign ambiguity of wave functions for electron hoping motion, a phase factor that ensures the single-valuedness of the wave function appears; and this phase factor gives rise to a vector potential for the fictitious magnetic field. The strong hole-lattice interaction stabilizes spin vortices by pinning each center at a polaron occupied site; thus, it will also stabilize the spin vortex induced current. Based on the new current generation mechanism, possibility of an artificial superconducting nano wire is presented. It is made in the Cu-O plane, and consists of pining centers of nonmagnetic atoms arranged in two parallel lines; here the nonmagnetic atoms play the role of lattice small polarons; and the region between the two arrays of vortex centers is a superconducting wire where a thermodynamically stable current flows.

We report our recent progress of deriving the low-energy effective one-band Hamiltonians for the prototypical cuprate superconductor Ca₂CuO₂Cl₂, based on a newly developed first-principles Wannier-states approach that takes into account large on-site Coulomb repulsion. The apical atom p_z state is found to affect the general properties of the low-energy hole state, namely the Zhang-Rice singlet, via additional intra-sublattice hoppings, nearest-neighbor 'super-repulsion,' and other microscopic many-body processes.

The values of superconductive transition temperatures T_c in conventional crystalline metallic superconductors are widely scattered, depend on the details of the Fermi surface (especially the density of states N(E_F)), and are typically below 1 K. By far the highest T_c 's (~100 K) are found in the cuprates, and there T_c is apparently insensitive to the geometry of the Fermi surface, while it depends strongly on dopant density and configuration. For the cuprates there is a smooth master curve T_c^max(<R>) for the highest T_c's as a function of the average number <R> of Pauling resonating bonds/atom in the parent (undoped) insulator. The existence of such an <R>-dependent optimized T_c is suggested by analogies with the microscopic theory of optimized molecular glass networks (such as window glass), but it remains to derive a rigorous quantum theory of the origin of T_c^max(<R>).

Numerous indications point to the multiband nature of the superconductivity in doped cuprates. The electronic structure of these compounds evolves with doping. Based on experimental findings, a simple model for the description of cuprate superconductor characteristics on the doping is developed. The leading interband pairing channel between an itinerant band and defect states created by doping is postulated. Bare gaps between them are supposed and to be closed by extended doping. Band overlap conditions determine the phase diagram critical points. The supercarrier density varies with the disposition of the chemical potential and bands. Nodal and antinodal momentum regions are distinguished. In the case of hole doping the relevant regime lies near the top of the oxygen band. Illustrative calculations have been made using a mean-field multiband Hamlitonian. Manifestations of various gaps and pairing strength characteristics on the doping level are discussed. A nonmonotonic dependence of the critical coherence length is calculated. The coherence of noncritical mode is also investigated. The supercarrier effective mass diminishes moderately out of underdoping. The penetration depth curve is of the observed type. The isotope effect on supercarrier density is predicted. In the case of electron doping the events concentrate near the bottom of the upper Hubbard band. The gross features of the phase diagram with two pseudogaps remind the hole doping case. The pairing strength and the phase coherence develop simultaneously. The agreement of the results of our model calculations with experimental findings suggests that multiband pairing is an essential aspect of cuprate superconductivity.

Quasiparticle states around a nonmagnetic impurity in inhomogeneous superconducting states observed in high-T_c cuprates are studied using a two-dimensional t-J type model. The spatial dependences of the superconducting order parameters are determined within the Bogoliubov-de Gennes theory based on the Gutzwiller approximation. We show that a peak structure is found in the local density of states around the impurity in the large gap regions of the inhomogeneous superconducting states, which has not been experimentally observed yet. Our results suggest that a competing order is realized in the large gap regions in high-T_c cuprates.

The additional long range order parameter that competes with the high T_c superconductivity long range order is identified as an electronic crystal of pseudo Jahn-Teller polarons beyond the critical value of the electron lattice interaction. We show that the region of quantum critical fluctuations in the two variables phase diagram of cuprates: the doping δ and the chemical pressure (i.e., the tolerance factor, or the average ionic radius of A-site cations) can be measured via the microstrain of the Cu-O length in the CuO₂ lattice. The fluctuating order in the proximity of the microstrain quantum critical point that competes with the superconducting long range order is the polaron electronic crystalline phase called a Wigner polaron crystal and the variation of the spin gap energy as a function of microstrain provides a strong experimental support for this proposal.

Undoped cuprates have long been considered to be antiferromagnetic Mott insulators (charge-transfer insulators). In this article we report that one group of cuprates, namely RE₂CuO₄ with the Nd₂CuO₄ (T') structure, become superconducting without doping. Our discovery was achieved by metal-organic decomposition, an inexpensive and easy-to-implement thin-film process. The highest T_c of undoped T'-RE₂CuO₄ is over 30 K, substantially higher than their 'electron-doped' analogs. Remarkably, Gd₂CuO₄, even the derivatives of which have not shown superconductivity so far, gets superconducting with T_c^onset as high as 20 K. Our discovery contradicts with the past results supporting undoped cuprates to be generally insulating. The clue to understanding the sharp contrast between the past and our results is impurity oxygen (O_ap) at the apical site. The implication of our discovery is briefly discussed.

In contrast to conventionl superconductivity where phonons lead to the formation of Cooper pairs, in high-tempratures superconductivity (HTSC), the role of electron-phonon coupling has long been neglected. The in-plane Cu-O bonds in HTSC cuprates show unconventionalbroadening at low temperature as carier are doped. Here we focus on high-quality polarized x-ray absorption spectroscopy (XAS)data for a model HTSC system, (La,Sr)₂CuO₄ (LSCO). Thin film signal crystal samples were prepared by state-of-the-arts MBE, precisely controlling compositions. High quality data was obtained by use of a segmented x-ray detector. the inplane CuO radial distribution function (RDF)in LSCO (x = 0.15)shows broadening as temprature is lowered, which shows a sharp drop at the critical temprature which is followed by a gradual increase (disorder). Comparing the data with resistivity, we find a remarkeble coinsidence between the sharpening and the onset supercondectivity. Since the sharpening of RDF is interpreated as correleted motion of oxyzen atoms (phase coherence due to supercondectivity), the result demonstrate that the super conducting state directly relates to the unconventionl oxyzen displacements in a bond stretching mode. The result will be discussed in relation to the local models of distortion of the diffrent nature (metallic vs. insulating) that is strongly influnced by strain.

Heterostructures and superlattices consisting of the half-metal ferromagnet La_0.67Ca_0.33MnO₃ (LCMO) and the superconductor YBa₂Cu₃O₇ (YBCO) stacked along the c-axis of the superconductor were fabricated by pulsed laser deposition techniques. Apart from their structural characterization by x-ray diffractometry and high resolution TEM, the magnetic as well as electronic interaction at the interfaces and between the layers is studied by a variety of techniques ranging from transport measurements to magnetic, neutron diffraction and XMCD analysis. It turns out that these interaction effects occurring at several different length scales. At a short length scale of ~ 3 nm, charge transfer and/or orbital reconstruction as revealed by resonant x-ray absorption and XMCD measurements are dominant, the length scale of ~ 10 nm is the regime of self-injection of spin-polarized quasiparticles and at a scale in the 100 nm range and above magnetic interactions are present affecting the flux-line lattice of the superconductor, the domain structure of the magnetic layer as well as the interlayer coupling. The different and controversially discussed ferromagnet/superconducting interactions in YBCO/LCMO hybrids are analyzed and combined to a universal picture.

Since 1997 we systematically perform Direct ARPES ( = DARPES) on in-situ grown, non-cleaved, ultra-thin (<25nm) cuprate films. Specifically, we probe low energy electronic structure and properties of high-T_c films under different degree of epitaxial (compressive vs tensile) strain. In overdoped in-plane compressed La_2-xSr_xCuO₄ (LSCO) thin films we double T_c from 20K to 40K, yet the Fermi surface (FS) remains essentially 2-dimensional (2D). In contrast, tensile strained films show 3-dimensional (3D) dispersion, while T_c is drastically reduced. It seems that the in-plane compressive strain tends to push the apical oxygen far away from the CuO₂ plane, enhances the 2D character of the dispersion and increases T_c, while the tensile strain seems to act exactly in the opposite direction and the resulting dispersion is 3D. We have the FS topology for both cases. As the actual lattice of cuprates is 'Napoleon-cake' -like i.e. rigid CuO₂ planes alternate with softer 'reservoir' (that strains distort differently) our results tend to rule out 2D rigid lattice mean field models. Finally, we briefly discuss recent successful determination of the FS topology from the observed wavevector quantization by DARPES in cuprate films thinner than 18 units cells (<24nm). Such an approach is of broader interest as it can be extended to other similar confined (ultra-thin) functional oxide systems.

We have grown (001)-oriented thin films of Gd_2-xCe_xCuO₄with cerium composition 0 < x < 0.2 by state-of-the-art reactive molecular beam epitaxy and characterized them by x-ray diffraction and transport measurements. A systematical change in the c-axis length upon cerium doping indicates that single-phase films were obtained for the whole doping range. Based on a log p_o2 - 1/T phase diagram in combination with reflection high energy electron diffraction (RHEED), phase stability has been determined in order to achieve optimized reduction conditions. Gd_2-xCe_xCuO₄ thin films even after controlled reductions treatment did not show superconductivity in the whole range of Ce concentration studied. However, the room temperature resistivity of optimally reduced Gd_2-xCe_xCuO₄ thin films shows a minimum at around x_Ce = 0.16. Our results on the growth and characterization of Gd_2-xCe_xCuO₄ thin films on (100) SrTiO3 substrates are described in detail.

We present a theoretical analysis of a triplet superconductor–ferromagnet–triplet superconductor (TFT) junction. We describe in detail the construction of the quasiclassical scattering Green's function and provide a derivation of the general expression for the Josephson current between two triplet superconductors. Specializing to the TFT junction, we compare and contrast the dependence of the Andreev bound state energies and the current upon the scattering at the ferromagnetic barrier, for the two cases of aligned and mis-aligned d-vectors in the triplet superconductors. We also discuss the unconventional temperature-dependence of the current through the junction.

We have investigated the switching dynamics of current biased REBa₂Cu₃O_y (RE123; RE = Eu, Er) intrinsic Josephson junctions (IJJs) with typical dimensions of ~1 × 1 × 0.1 μm³. The heavily under-doped Re123 IJJs exhibited clear multiple-branches with hysteresis structure. The critical current density, J_c, of both IJJs was 15 - 17 kA/cm² at 4.2 K which was about ten times higher than that of Bi₂Sr₂CaCuO_y (Bi2212) IJJs. From switching current measurements from the V = 0 state to the V ≠ 0 state of those IJJs, switching current distributions, P(I), were found to be significantly broader than that expected from the escape model of a single Josephson junction. This is in contrast with the fact that P(I) of Bi2212 IJJs have been well explained by the model. The P(I) of Eu123 IJJs were found to be independent of temperature below 1.4 K, which indicated the crossover temperature to the quantum regime from the thermally activated regime.

An easy and inexpensive method to fabricate mesa-type intrinsic Josephson junctions (IJJs) is presented. After fabricating a cross-whisker junction (CWJ) of Bi₂Sr₂CaCu₂O_8+δ, the CWJ was milled by argon ion beam. No special mask is necessary because the upper whisker plays a role of mask in the milling process. The milled CWJ exhibits typical multi-branch structures in current-voltage characteristics, similar to the one seen in mesa-type and s-shaped small IJJs of Bi₂Sr₂CaCu₂O_8+δ.

Materials may show anisotropic properties on different crystal directions and this is also the case of High Temperature Superconductors (HTS). To take advantage of the materials anisotropy one concept of interest is 'orientation engineering' in thin films. This can be realized through the control of the film-substrate lattice relationship. Some examples in this regard are presented in this work and through comparative analysis we try to evaluate the viability of this approach and of he entcountered problems. It is expected that in the future this approach will generate new nano composite materials with new properties and effects leading to development of new devices with new or improved functionality.

Multilayered cuprates having crystallographically inequivalent CuO₂ planes are worthful materials to realize the mechanism of high-Tc superconductivity as well as a stage to ascertain new phenomena. The material synthesis of HgBa₂Ca_n-1Cu_nO_y multilayered cuprates by employing high-pressure synthesis technique is demonstrated. A starting mixture with a composition ratio of Hg:Ba:Ca:Cu:O = 1:2:n-1:n:2n+2+δ was synthesized by mixing the precursor of Ba₂Ca₂Cu₃O_y (y~7.5) with an appropriate amount of HgO, Ca₂CuO₃, CuO and Cu₂O (or Cu). The oxygen content of the mixture was adjusted incisively by ascertaining a ratio of CuO, Cu₂O (or Cu) to obtain samples. Almost single-phase samples up to n = 6 were synthesized. The X-ray diffraction analysis of the grain-aligned sample with nominal compositions of n ⩾ 7 revealed that the sample simultaneously comprises from n = 6 up to 16 phases.

The superconductivity in apical fluorine system of Ba₂Ca₃Cu₄O_10-yF_y: the fourth member of Ba₂Ca_n-1Cu_nO_2n(O,F)₂ homologous series is significant to study as it deals with the novel phenomenon of superconductivity like co-existence of superconductivity and antiferromagnetism, in-equivalent and non-similar charge distribution in the CuO₂ planes in a unit cell etc. The polycrystalline samples of Ba₂Ca₃Cu₄O₈(O,F)₂ (F-0234) were synthesized under high pressure and showed sharp superconducting transitions temperature at 105 K. From the DA magnetization hysteresis loops, we determined the intragrain critical current density (J_c) and irreversibility field (B_irr) using Bean's critical state model. The irreversibility line(IL) of F-0234 is higher than that for optimally-doped Bi-2212. However, IL much lower than that of Cu-1212 (Y-123) although the spacing of F-0234 (7.3 Å) is much thinner than that of Cu-1212.

The variation of local lattice displacements of the in-plane Cu-O bond upon Cu-site impurity doping in La_2-xSr_xCuO₄ superconductor is investigated by Cu K-edge EXAFS studies. In Mn doping case, the analysis of the temperature dependence of Debye-Waller factor indicates that the local lattice displacements of the in-plane Cu-O bond exhibits similar behavior in all La_1.875−1.5xSr_0.125+1.5xCu_1-xMn_xO₄ (0⩽x⩽0.15) samples. This may reflect that the doping of Mn does not alter the local lattice environments in the CuO₂ plane and thus the Mn impurity does not destroy the superconducting transition in a similar way with other ionic doping cases. The local lattice displacements at room temperature upon ionic doping are compared, which indicates that Fe doping leads to largest local displacements while Ni doping is intermediate. The possible interplay between the local lattice displacements and the normal-state resistivity is compared. These results indicate that the local lattice displacements largely affect the superconducting properties in La_2-xSr_xCuO₄ superconductor.