Doublon-holon pairing mechanism via exchange interaction in two-dimensional cuprate Mott insulators.

Coupling of charge and spin degrees of freedom is a critical feature of correlated electron oxides, as represented by the spin-related mechanism of a Cooper pair under high-T c superconductivity. A doublon-holon pair generated on an antiferromagnetic spin background is also predicted to attract each other via the spin-spin interaction J, similar to a Cooper pair, while its evidence is difficult to obtain experimentally. Here, we investigate such an excitonic effect by electroreflectance spectroscopy using terahertz electric field pulses in undoped cuprates: Nd2CuO4, Sr2CuO2Cl2, and La2CuO4. Analyses of the spectral changes of reflectivity under electric fields reveal that the splitting of odd-parity and even-parity excitons, a measure of doublon-holon binding energy, increases with J. This trend is reproduced by t-J-type model calculations, providing strong evidence of the spin-related doublon-holon pairing. Agreement with the calculations supports the s-wave symmetry of the doublon-holon pair in contrast to the d-wave Cooper pair in doped cuprates.

Cluster-Based Haldane State in an Edge-Shared Tetrahedral Spin-Cluster Chain: Fedotovite K_{2}Cu_{3}O(SO_{4})_{3}.

Fedotovite K_{2}Cu_{3}O(SO_{4})_{3} is a candidate of new quantum spin systems, in which the edge-shared tetrahedral (EST) spin clusters consisting of Cu^{2+} are connected by weak intercluster couplings forming a one-dimensional array. Comprehensive experimental studies by magnetic susceptibility, magnetization, heat capacity, and inelastic neutron scattering measurements reveal the presence of an effective S=1 Haldane state below T≅4 K. Rigorous theoretical studies provide an insight into the magnetic state of K_{2}Cu_{3}O(SO_{4})_{3}: an EST cluster makes a triplet in the ground state and a one-dimensional chain of the EST induces a cluster-based Haldane state. We predict that the cluster-based Haldane state emerges whenever the number of tetrahedra in the EST is even.

Recovery After Critical Illness Polyneuropathy in a Patient With Orthotopic Liver Transplantation: A Case Report.

After liver transplantation, some patients show neuromuscular abnormalities. A 43-year-old man with liver cirrhosis due to hepatitis C virus underwent living-donor liver transplantation. He developed severe neuromuscular dysfunction after sepsis, and acute respiratory distress syndrome. After the inflammatory reaction gradually improved, we observed bilateral weakness of the extremities and foot drop. Electrophysiological studies indicated primary axonal degeneration of peripheral motor and sensory fibers without inflammation. Critical illness polyneuropathy was diagnosed. During follow-up, complaints gradually recovered with rehabilitation by approximately 1 year later. Based on this case, we suggest that paralysis should be evaluated for critical illness polyneuropathy in patients with unexplained muscle weakness.

Point nodes persisting far beyond Tc in Bi2212.

In contrast to a complex feature of antinodal state, suffering from competing orders, the pairing gap of cuprates is obtained in the nodal region, which therefore holds the key to the superconducting mechanism. One of the biggest question is whether the point nodal state as a hallmark of d-wave pairing collapses at Tc like the BCS-type superconductors, or it instead survives above Tc turning into the preformed pair state. A difficulty in this issue comes from the small magnitude of the nodal gap, which has been preventing experimentalists from solving it. Here we use a laser ARPES capable of ultrahigh-energy resolution, and detect the point nodes surviving far beyond Tc in Bi2212. By tracking the temperature evolution of spectra, we reveal that the superconductivity occurs when the pair-breaking rate is suppressed smaller than the single-particle scattering rate on cooling, which governs the value of Tc in cuprates.

Kondo resonance from p-wave hybridization in graphene.

The p-wave hybridization in graphene present a distinct class of Kondo problem in pseudogap Fermi systems with bath density of states (DOS) ρ0(ε) ∝ |ε|. The peculiar geometry of substitutional and hollow-site ad-atoms, and effectively the vacancies allow for a p-wave form of momentum dependence in the hybridization of the associated local orbital with the Dirac fermions of the graphene host which results in a different picture than the s-wave momentum independent hybridization. For the p-wave hybridization function, away from the Dirac point we find closed-form formulae for the Kondo temperature TK which in contrast to the s-wave case is non-zero for any value of hybridization strength V of the single impurity Anderson model (SIAM). At the Dirac point where the DOS vanishes, we find a conceivably small value of Vmin above which the Kondo screening takes place even in the presence of particle-hole symmetry. We also show that the non-Lorentzian line shape of the local spectrum arising from anomalous hybridization function leads to much larger TK in vacant graphene compared to a metallic host with similar bandwidth and SIAM parameters.

High-energy spin and charge excitations in electron-doped copper oxide superconductors.

The evolution of electronic (spin and charge) excitations upon carrier doping is an extremely important issue in superconducting layered cuprates and the knowledge of its asymmetry between electron- and hole-dopings is still fragmentary. Here we combine X-ray and neutron inelastic scattering measurements to track the doping dependence of both spin and charge excitations in electron-doped materials. Copper L3 resonant inelastic X-ray scattering spectra show that magnetic excitations shift to higher energy upon doping. Their dispersion becomes steeper near the magnetic zone centre and they deeply mix with charge excitations, indicating that electrons acquire a highly itinerant character in the doped metallic state. Moreover, above the magnetic excitations, an additional dispersing feature is observed near the Γ-point, and we ascribe it to particle-hole charge excitations. These properties are in stark contrast with the more localized spin excitations (paramagnons) recently observed in hole-doped compounds even at high doping levels.

Persistent spin excitations in doped antiferromagnets revealed by resonant inelastic light scattering.

How coherent quasiparticles emerge by doping quantum antiferromagnets is a key question in correlated electron systems, whose resolution is needed to elucidate the phase diagram of copper oxides. Recent resonant inelastic X-ray scattering (RIXS) experiments in hole-doped cuprates have purported to measure high-energy collective spin excitations that persist well into the overdoped regime and bear a striking resemblance to those found in the parent compound, challenging the perception that spin excitations should weaken with doping and have a diminishing effect on superconductivity. Here we show that RIXS at the Cu L3-edge indeed provides access to the spin dynamical structure factor once one considers the full influence of light polarization. Further we demonstrate that high-energy spin excitations do not correlate with the doping dependence of Tc, while low-energy excitations depend sensitively on doping and show ferromagnetic correlations. This suggests that high-energy spin excitations are marginal to pairing in cuprate superconductors.

Quantum dynamics of a driven correlated system coupled to phonons.

Nonequilibrium interplay between charge, spin, and lattice degrees of freedom on a square lattice is studied for a single charge carrier doped in the t-J-Holstein model. In the presence of a static electric field we calculate the quasistationary state. With increasing electron-phonon (e-ph) coupling the carrier mobility decreases; however, we find increased steady state current due to e-ph coupling in the regime of negative differential resistance. We explore the distribution of absorbed energy between the spin and the phonon subsystem. For model parameters as relevant for cuprates, the majority of the gained energy flows into the spin subsystem.

Bipolaron in the t-J model coupled to longitudinal and transverse quantum lattice vibrations.

We explore the influence of two different polarizations of quantum oxygen vibrations on the spacial symmetry of the bound magnetic bipolaron in the context of the t-J model by using exact diagonalization within a limited functional space. Linear as well as quadratic electron-phonon coupling to transverse polarization stabilize d-wave symmetry. The existence of a magnetic background is essential for the formation of a d-wave bipolaron state. With increasing linear electron-phonon coupling to longitudinal polarization the symmetry of a d-wave bipolaron state changes to a p wave. Bipolaron develops a large anisotropic effective mass.

Modeling the antiferromagnetic phase in iron pnictides: weakly ordered state.

We examine electronic states of antiferromagnetic phase in iron pnictides by mean-field calculations of the optical conductivity. We find that a five-band model exhibiting a small magnetic moment, inconsistent with the first-principles calculations, reproduces well the excitation spectra characterized by a multipeak structure emerging below the Néel temperature at low energy, together with an almost temperature-independent structure at high energy. Investigating the interlayer magnetoresistance for this model, we also predict its characteristic field dependence reflecting the Fermi surface.

Temperature and doping dependence of the high-energy kink in cuprates.

It is shown that spectral functions within the extended t-J model, evaluated using the finite-temperature diagonalization of small clusters, exhibit the high-energy kink in single-particle dispersion consistent with recent angle-resolved photoemission results on hole-doped cuprates. The kink and waterfall-like features persist up to large doping and to temperatures beyond J; hence, the origin can be generally attributed to strong correlations and incoherent hole propagation at large binding energies. In contrast, our analysis predicts that electron-doped cuprates do not exhibit these phenomena in photoemission.

Late flare-up of nephrotic lupus nephritis transforming histological pattern and autoantibody reactions.

A 15-year-old boy developed a nephrotic syndrome. At that time, autoantibodies related to systemic lupus erythematosus (SLE) had been persistently negative, even on repeated evaluation. C1q was normal, but C4, C3 and CH50 were low. Renal biopsy revealed membranous lupus nephritis (LN) based on the new classification of glomerulonephritis in SLE [Weenig et al. 2004]. We did not establish our diagnosis of SLE on the criteria of the American Rheumatism Association (ARA). The patient showed complete remission ofnephrotic syndrome treated with prednisolone and cyclophosphamide. Thereafter, he had no proteinuria and clinical evidence of SLE for 22 years. At the age of 37, however, he developed facial discoid eruption, proteinuria in the nephrotic range, hypocomplementemia and positive reaction to autoantibodies of SLE. Light microscopic findings of renal biopsy indicated mesangial LN, which showed "full-house" immunofluorescence and mesangial dense deposits associated with diffuse epithelial cell foot process effacement in electron microscopy. Steroid therapy was very effective. This case initially showed autoantibody-negative and hypocomplementemic LN with membranous type, and transformed to SLE with mesangial LN after a long interval.

Momentum dependence of charge excitations in the electron-doped superconductor Nd1.85 Ce0.15 CuO4: a resonant inelastic x-ray scattering study.

We report a resonant inelastic x-ray scattering (RIXS) study of charge excitations in the electron-doped high-T(c) superconductor Nd1.85 Ce0.15 CuO4. The intraband and interband excitations across the Fermi energy are separated for the first time by tuning the experimental conditions properly to measure charge excitations at low energy. A dispersion relation with q-dependent width emerges clearly in the intraband excitation, while the intensity of the interband excitation is concentrated around 2 eV near the zone center. The experimental results are consistent with theoretical calculation of the RIXS spectra based on the Hubbard model.

Mott gap excitations in twin-free YBa2Cu3O7-delta (Tc=93 K) studied by resonant inelastic x-ray scattering.

Mott gap excitations in the optimally doped high-T(c) superconductor YBa(2)Cu(3)O(7-delta) (T(c)=93 K) have been studied by the resonant inelastic x-ray scattering method. Anisotropic spectra in the ab plane are observed in a twin-free crystal. The excitation from the one-dimensional CuO chain is enhanced at 2 eV near the zone boundary of the b* direction, while the excitation from the CuO2 plane is broad at 1.5-4 eV and almost independent of the momentum transfer. Theoretical calculations based on the one-dimensional and two-dimensional Hubbard model reproduces the observed spectra when different values of the on-site Coulomb energy are assumed. The Mott gap of the CuO chain site is found to be much smaller than that of the CuO2 plane site.

Resonant two-magnon Raman scattering and photoexcited States in two-dimensional mott insulators.

We investigate the resonant two-magnon Raman scattering in two-dimensional (2D) Mott insulators by using a half-filled 2D Hubbard model in the strong coupling limit. By performing numerical diagonalization calculations for small clusters, we find that the Raman intensity is enhanced when the incoming photon energy is not near the optical absorption edge but well above it, being consistent with experimental data. The absence of resonance near the gap edge is associated with the presence of background spins, while photoexcited states for resonance are found to be characterized by the charge degree of freedom. The resonance mechanism is different from those proposed previously.

Large third-order optical nonlinearity of Cu-O chains investigated by third-harmonic generation spectroscopy.

Spectra of the third-order nonlinear susceptibility chi((3)) have been investigated for one-dimensional Mott insulators, Sr(2)CuO(3) and Ca(2)CuO(3), by applying the third-harmonic generation (THG) spectroscopy on their single-crystalline thin films. The three-photon resonance to the lowest charge-transfer (CT) state with odd parity strongly enhances chi((3)), which is of the order of 10(-9) esu. The two-photon resonant structure unravels the even-CT state, located close to the odd-CT state. Two types of chi((3)) spectra obtained from THG and the electroreflectance measurements are explained based on the concept of spin-charge separation.

Development of a cryopreservation procedure employing a freezer bag for pancreatic islets using a newly developed cryoprotectant.

One of the most important requirements for success in clinical islet transplantation is the use of a large number of viable donor islets. To achieve this, the ability to cryopreserve islets and to establish an islet bank are critical. Previously, we developed a two-step cryopreservation procedure with freezing tubes utilizing low and high concentrations of dimethyl sulfoxide (DMSO) and using a fully automated cryomachine for human pancreatic islets and porcine islet-like cell clusters (ICCs). Based on these experiments, we developed a simple and efficient cryopreservation procedure of a freezer bag for isolated islets using a fully automated computer-controlled cryomachine with a newly developed cryoprotectant consisting of ethylene glycol (EG) instead of DMSO for decreasing injury of the islets by freezing. A 250 ml Cryocyte blood freezer bag and our newly developed cryoprotectant containing ethylene glycol (EG) were used in the freezing procedure. The islets were frozen by a fully automated computer-controlled cryomachine (GE 9,000) with our original program of slow cooling. Nucleation occurred at -8 degrees C, and the frozen islets were stored at -196 degrees C in a liquid nitrogen tank. The frozen-stored islets were subsequently rapidly thawed in a 37 degrees C water bath and cultured before viability testing. In vitro function, the stimulation index of insulin release during the static incubation test for rat islets cryopreserved in a freezer bag vs. nonfrozen islets as control, was 2.13 +/- 0.42 and 2.02 +/- 0.38 (94.8% compared with control), respectively (n = 5, p = NS). The islet recovery compared with the nonfrozen control group was 85% (n = 5) in insulin content. When 1000 rat islets cryopreserved in a freezer bag were transplanted into the renal capsule of diabetic athymic mice, all the mice became normoglycemic within 7 days from transplantation. Before nephrectomy, the intravenous glucose torelance test (IVGTT) was performed. The fractional decay constant of the glucose level (K value) of the frozen-thawed group was 0.42 +/- 0.06%/min. A histological study of renal subcapsular grafts demonstrated the morphological integrity of the islets. These results demonstrate the utility of our cryopreservation procedure of a freezer bag for isolated islets using a fully automated computer-controlled cryomachine with a newly developed cryoprotectant for the maintenance of viability and function of frozen-stored islets both in culture and after transplantation. Cryopreservation using freezer bags with the new cryoprotectant is an effective and simple method for making an islet bank for clinical trials of islet transplantation.

Electronic structure of mott insulators studied by inelastic X-ray scattering

The electronic structure of Mott insulators continues to be a major unsolved problem in physics despite more than 50 years of research. Well-developed momentum-resolved spectroscopies such as photoemission or neutron scattering cannot probe the full Mott gap. High-resolution resonant inelastic x-ray scattering revealed dispersive charge excitations across the Mott gap in a high-critical temperature parent cuprate (Ca(2)CuO(2)Cl(2)), shedding light on the anisotropy of the Mott gap. These charge excitations across the Mott gap can be described within the framework of the Hubbard model.