Topological Magnon Band Crossing in Y_{2}Ir_{2}O_{7}.

Topological magnonic materials have attracted much interest because of the potential for dissipationless spintronic applications. Pyrochlore iridates are theoretically regarded as good candidates for designing topological magnon bands. However, experimental identification of topological magnon bands in pyrochlore iridates remains elusive. We explored this possibility in Y_{2}Ir_{2}O_{7} using Raman spectroscopy to measure both the single-magnon excitations and anomalous phonon shifts. From the single-magnon energies and tight-binding model calculations concerning the phonons, we determined the key parameters in the spin Hamiltonian. These confirm that Y_{2}Ir_{2}O_{7} hosts a nontrivial magnon band topology distinct from other pyrochlore iridate compounds. Our work demonstrates that pyrochlore iridates constitute a system in which the magnon band topology can be tailored and that Raman spectroscopy is a powerful technique to explore magnon band topology.

Charge-Spin Correlation in van der Waals Antiferromagnet NiPS_{3}.

Strong charge-spin coupling is found in a layered transition-metal trichalcogenide NiPS_{3}, a van der Waals antiferromagnet, from studies of the electronic structure using several experimental and theoretical tools: spectroscopic ellipsometry, x-ray absorption, photoemission spectroscopy, and density functional calculations. NiPS_{3} displays an anomalous shift in the optical spectral weight at the magnetic ordering temperature, reflecting strong coupling between the electronic and magnetic structures. X-ray absorption, photoemission, and optical spectra support a self-doped ground state in NiPS_{3}. Our work demonstrates that layered transition-metal trichalcogenide magnets are useful candidates for the study of correlated-electron physics in two-dimensional magnetic materials.

Strong Spin-Phonon Coupling Mediated by Single Ion Anisotropy in the All-In-All-Out Pyrochlore Magnet Cd_{2}Os_{2}O_{7}.

Spin-phonon coupling mediated by single ion anisotropy was investigated using optical spectroscopy and first-principles calculations in the all-in-all-out pyrochlore magnet Cd_{2}Os_{2}O_{7}. Clear anomalies were observed in both the phonon frequencies and linewidths at the magnetic ordering temperature. The renormalization of the phonon modes was exceptionally large, signifying the presence of an unconventional magnetoelastic term from large spin-orbit coupling. In addition, the relative phonon frequency shifts show a strong correlation with the modulation of noncubic crystal field by the corresponding lattice distortion. Our observation establishes a new type of spin-phonon coupling through single ion anisotropy, a second-order spin-orbit coupling term, in Cd_{2}Os_{2}O_{7}.

Optical Spectroscopic Studies of the Metal-Insulator Transition Driven by All-In-All-Out Magnetic Ordering in 5d Pyrochlore Cd(2)Os(2)O(7).

We investigated the metal-insulator transition (MIT) driven by all-in-all-out (AIAO) antiferromagnetic ordering in the 5d pyrochlore Cd(2)Os(2)O(7) using optical spectroscopy and first-principles calculations. We showed that the temperature evolution in the band-gap edge and free carrier density were consistent with rigid upward (downward) shifts of electron (hole) bands, similar to the case of Lifshitz transitions. The delicate relationship between the band gap and free carrier density provides experimental evidence for the presence of an AIAO metallic phase, a natural consequence of such MITs. The associated spectral weight change at high energy and first-principles calculations further support the origin of the MIT from the band shift near the Fermi level. Our data consistently support that the MIT induced by AIAO ordering in Cd(2)Os(2)O(7) is not close to a Slater type but instead to a Lifshitz type.

Enhanced tunnelling electroresistance effect due to a ferroelectrically induced phase transition at a magnetic complex oxide interface.

The range of recently discovered phenomena in complex oxide heterostructures, made possible owing to advances in fabrication techniques, promise new functionalities and device concepts. One issue that has received attention is the bistable electrical modulation of conductivity in ferroelectric tunnel junctions (FTJs) in response to a ferroelectric polarization of the tunnelling barrier, a phenomenon known as the tunnelling electroresistance (TER) effect. Ferroelectric tunnel junctions with ferromagnetic electrodes allow ferroelectric control of the tunnelling spin polarization through the magnetoelectric coupling at the ferromagnet/ferroelectric interface. Here we demonstrate a significant enhancement of TER due to a ferroelectrically induced phase transition at a magnetic complex oxide interface. Ferroelectric tunnel junctions consisting of BaTiO3 tunnelling barriers and La(0.7)Sr(0.3)MnO3 electrodes exhibit a TER enhanced by up to ~10,000% by a nanometre-thick La(0.5)Ca(0.5)MnO3 interlayer inserted at one of the interfaces. The observed phenomenon originates from the metal-to-insulator phase transition in La(0.5)Ca(0.5)MnO3, driven by the modulation of carrier density through ferroelectric polarization switching. Electrical, ferroelectric and magnetoresistive measurements combined with first-principles calculations provide evidence for a magnetoelectric origin of the enhanced TER, and indicate the presence of defect-mediated conduction in the FTJs. The effect is robust and may serve as a viable route for electronic and spintronic applications.

Temperature evolution of itinerant ferromagnetism in SrRuO3 probed by optical spectroscopy.

The temperature (T) dependence of the optical conductivity spectra σ(ω) of a single crystal SrRuO(3) thin film is studied over a T range from 5 to 450 K. We observed significant T dependence of the spectral weights of the charge transfer and interband d-d transitions across the ferromagnetic Curie temperature (T(c) ∼ 150 K). Such T dependence was attributed to the increase in the Ru spin moment, which is consistent with the results of density functional theory calculations. T scans of σ(Ω,T) at fixed frequencies Ω reveal a clear T(2) dependence below T(c), demonstrating that the Stoner mechanism is involved in the evolution of the electronic structure. In addition, σ(Ω,T) continues to evolve at temperatures above T(c), indicating that the local spin moment persists in the paramagnetic state. This suggests that SrRuO(3) is an intriguing oxide system with itinerant ferromagnetism.

Forming mechanism of the bipolar resistance switching in double-layer memristive nanodevices.

To initiate resistance switching phenomena, it is usually necessary to apply a strong electric field to a sample. This forming process poses very serious obstacles in real nanodevice applications. In unipolar resistance switching (URS), it is well known that the forming originates from soft dielectric breakdown. However, the forming in bipolar resistance switching (BRS) is poorly understood. In this study, we investigated the forming processes in Pt/Ta2O5/TaOx/Pt and Pt/TaOx/Pt nanodevices, which showed BRS and URS, respectively. By comparing the double- and single-layer systems, we were able to observe differences in the BRS and URS forming processes. Using computer simulations based on an 'interface-modified random circuit breaker network model', we could explain most of our experimental observations. This success suggests that the BRS forming in our Pt/Ta2O5/TaOx/Pt double-layer system can occur via two processes, i.e., polarity-dependent resistance switching in the Ta2O5 layer and soft dielectric breakdown in the TaOx layer. This forming mechanism can be used to improve the performance of BRS devices. For example, we could improve the endurance properties of Pt/Ta2O5/TaOx/Pt cells by using a small forming voltage.

Giant flexoelectric effect in ferroelectric epitaxial thin films.

We report on nanoscale strain gradients in ferroelectric HoMnO(3) epitaxial thin films, resulting in a giant flexoelectric effect. Using grazing-incidence in-plane x-ray diffraction, we measured strain gradients in the films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. The combination of transmission electron microscopy, electrical measurements, and electrostatic calculations showed that flexoelectricity provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.

Scaling theory for unipolar resistance switching.

We investigate a reversible percolation system showing unipolar resistance switching in which percolating paths are created and broken alternately by the application of an electric bias. Owing to the dynamical changes in the percolating paths, different from those in classical percolating paths, a detailed understanding of the structure is demanding and challenging. Here, we develop a scaling theory that can explain the transport properties of these conducting paths; the theory is based on the fractal geometry of a percolating cluster. This theory predicts that two scaling behaviors emerge, depending on the topologies of the conducting paths. We confirm these theoretical predictions experimentally by observing material-independent universal scaling behaviors in unipolar resistance switching.

Two-dimensional confinement of 3d{1} electrons in LaTiO_{3}/LaAlO{3} multilayers.

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n=1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d{1} state of the Ti ions. In combination with local density approximation, including a Hubbard U calculation, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t{2g} states leaving only the planar d{xy} orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t{2g} 2D Hubbard model.

Anomalous anisotropic behaviour of spin-triplet proximity effect in Au/SrRuO3/Sr2RuO4 junctions.

Spin-polarized supercurrents can be generated with magnetic inhomogeneity at a ferromagnet/spin-singlet-superconductor interface. In such systems, complex magnetic inhomogeneity makes it difficult to functionalise the spin-polarized supercurrents. However, spin-polarized supercurrents in ferromagnet/spin-triplet-superconductor junctions can be controlled by the angle between magnetization and spin of Copper pairs (d-vector), that can effectively be utilized in developing of a field of research known as superconducting spintronics. Recently, we found induction of spin-triplet correlation into a ferromagnet SrRuO3 epitaxially deposited on a spin-triplet superconductor Sr2RuO4, without any electronic spin-flip scattering. Here, we present systematic magnetic field dependence of the proximity effect in Au/SrRuO3/Sr2RuO4 junctions. It is found that induced triplet correlations exhibit strongly anisotropic field response. Such behaviour is attributed to the rotation of the d-vector of Sr2RuO4. This anisotropic behaviour is in contrast with the vortex dynamic. Our results will stimulate study of interaction between ferromagnetism and unconventional superconductivity.

Clustering of impurity atoms in Co-doped anatase TiO(2) thin films probed with soft x-ray fluorescence.

The electronic structure of Co-doped anatase TiO(2) epitaxial thin films grown at different partial oxygen pressures is investigated using soft x-ray emission spectroscopy. The resonantly excited Co L(2,3) x-ray emission spectra of ferromagnetic Ti(0.96)Co(0.04)O(2) samples for the oxygen-deficient regime show that the ratio of integral intensities for Co L(2) and L(3) emission lines significantly decreases with respect to nonmagnetic samples in the oxygen-rich regime. This is due to L(2)L(3)M(4,5) Coster-Kronig transitions and suggests that ferromagnetic Ti(0.96)Co(0.04)O(2) samples have n-type charge carriers and Co-Co bonds between substitutional and interstitial Co atoms are present while Co-O bonds are dominant in nonmagnetic Ti(0.96)Co(0.04)O(2) samples in the oxygen-rich regime. Electronic structure calculations show that the presence of free charge carriers and Co segregation play a crucial role in strong ferromagnetism at room temperature in Co-doped TiO(2).

X-ray Absorption Spectroscopy Study of the Effect of Rh doping in Sr2IrO4.

We investigate the effect of Rh doping in Sr2IrO4 using X-ray absorption spectroscopy (XAS). We observed appearance of new electron-addition states with increasing Rh concentration (x in Sr2Ir1-xRhxO4) in accordance with the concept of hole doping. The intensity of the hole-induced state is however weak, suggesting weakness of charge transfer (CT) effect and Mott insulating ground states. Also, Ir Jeff = 1/2 upper Hubbard band shifts to lower energy as x increases up to x = 0.23. Combined with optical spectroscopy, these results suggest a hybridisation-related mechanism, in which Rh doping can weaken the (Ir Jeff = 1/2)-(O 2p) orbital hybridisation in the in-planar Rh-O-Ir bond networks.

Direct penetration of spin-triplet superconductivity into a ferromagnet in Au/SrRuO3/Sr2RuO4 junctions.

Efforts have been ongoing to establish superconducting spintronics utilizing ferromagnet/superconductor heterostructures. Previously reported devices are based on spin-singlet superconductors (SSCs), where the spin degree of freedom is lost. Spin-polarized supercurrent induction in ferromagnetic metals (FMs) is achieved even with SSCs, but only with the aid of interfacial complex magnetic structures, which severely affect information imprinted to the electron spin. Use of spin-triplet superconductors (TSCs) with spin-polarizable Cooper pairs potentially overcomes this difficulty and further leads to novel functionalities. Here, we report spin-triplet superconductivity induction into a FM SrRuO3 from a leading TSC candidate Sr2RuO4, by fabricating microscopic devices using an epitaxial SrRuO3/Sr2RuO4 hybrid. The differential conductance, exhibiting Andreev-reflection features with multiple energy scales up to around half tesla, indicates the penetration of superconductivity over a considerable distance of 15 nm across the SrRuO3 layer without help of interfacial complex magnetism. This demonstrates potential utility of FM/TSC devices for superspintronics.

Stabilization of ferromagnetic ordering in cobaltite double perovskites of La2CoIrO6 and La2CoPtO6.

We investigated the local electronic structure and magnetic properties of the cobaltite double perovskites La2CoIrO6 and La2CoPtO6 using Co L2,3-edge x-ray absorption spectroscopy and x-ray magnetic circular dichroism. Despite similarity in the local electronic structure (Co(2+) high-spin states) as well as in the crystal structure (P2(1)/n), only La2CoIrO6 exhibits substantial orbital and spin magnetic moments of Co(2+), whereas they are much weaker in the case of La2CoPtO6. This composition dependence is consistent with the results of magnetization measurements. The details of the mechanism of ferromagnetic ordering in the Co(2+) sublattice in La2CoIrO6 and the lack thereof in La2CoPtO6 are explained in terms of the orbital hybridization of the Co minority-spin t(2g) state and the Ir/Pt j(eff) = 1/2 state.

Suppression of creep-regime dynamics in epitaxial ferroelectric BiFeO3 films.

Switching dynamics of ferroelectric materials are governed by the response of domain walls to applied electric field. In epitaxial ferroelectric films, thermally-activated 'creep' motion plays a significant role in domain wall dynamics, and accordingly, detailed understanding of the system's switching properties requires that this creep motion be taken into account. Despite this importance, few studies have investigated creep motion in ferroelectric films under ac-driven force. Here, we explore ac hysteretic dynamics in epitaxial BiFeO3 thin films, through ferroelectric hysteresis measurements, and stroboscopic piezoresponse force microscopy. We reveal that identically-fabricated BiFeO3 films on SrRuO3 or La0.67Sr0.33MnO3 bottom electrodes exhibit markedly different switching behaviour, with BiFeO3/SrRuO3 presenting essentially creep-free dynamics. This unprecedented result arises from the distinctive spatial inhomogeneities of the internal fields, these being influenced by the bottom electrode's surface morphology. Our findings further highlight the importance of controlling interface and defect characteristics, to engineer ferroelectric devices with optimised performance.

Study of spin-ordering and spin-reorientation transitions in hexagonal manganites through Raman spectroscopy.

Spin-wave (magnon) scattering, when clearly observed by Raman spectroscopy, can be simple and powerful for studying magnetic phase transitions. In this paper, we present how to observe magnon scattering clearly by Raman spectroscopy, then apply the Raman method to study spin-ordering and spin-reorientation transitions of hexagonal manganite single crystal and thin films and compare directly with the results of magnetization measurements. Our results show that by choosing strong resonance condition and appropriate polarization configuration, magnon scattering can be clearly observed, and the temperature dependence of magnon scattering can be simple and powerful quantity for investigating spin-ordering as well as spin-reorientation transitions. Especially, the Raman method would be very helpful for investigating the weak spin-reorientation transitions by selectively probing the magnons in the Mn(3+) sublattices, while leaving out the strong effects of paramagnetic moments of the rare earth ions.

Composite neuroendocrine and adenocarcinoma of the common bile duct associated with Clonorchis sinensis: a case report.

The biliary tract has neuroendocrine cells of endoderm origin similar to the gastrointestinal tract, however neuroendocrine tumors of the biliary tract are rare. We report a composite glandular-endocrine cell carcinoma of the common bile duct in a 64-year-old Korean man which was associated with Clonorchis sinensis. The patient complained of right upper quadrant abdominal pain. Several parasites of Clonorchis sinensis were removed during the percutaneous transbiliary drainage. Endoscopic retrograde cholangiopancreatography revealed a polypoid mass (3 x 3 cm) with central ulceration in the common bile duct. Pancreaticoduodenectomy was performed. Microscopic examination of the tumor revealed a composite small cell neuroendocrine carcinoma and adenocarcinoma. The small cell carcinoma component showed positive reaction to chromogranin A and neuron-specific enolase and it was located mainly in the deeper portion of the mass. The well-differentiated adenocarcinoma component showed a positive reaction to carcinoembryonic antigen and it was situated in the superficial portion of the mass. Exclusively, the small cell component metastasized to the lymph node. It is suggested that this tumor could arise from a multipotential stem cell and showed neuroendocrine and glandular differentiation and that Clonorchis sinensis could be a predisposing factor, as in cholangiocarcinoma.

Thymoma with pseudosarcomatous stroma.

Thymoma with psuedosarcomatous stroma is a recently described, rare variant of thymomas that are characterized by having a biphasic histologic pattern which consists of both an epithelial and a spindle cell stromal component. So far only 11 cases having similar histologic findings have been reported worldwide. At this time we report a case of this rare variant of thymoma which occurred in a 53-year-old Korean man. This previously healthy patient presented with coughing and an anterior mediastinal mass was then detected radiographically. Mediastinal exploration revealed a 9 x 8 x 8 cm-sized well- encapsulated, ovoid, cystic mass. Histological examination showed a biphasic neoplasm composed of anastomosing nests of epithelial cells and somewhat cellular stromal spindle cells that had advanced degenerative changes. Immunohistochemical staining using the antibodies for cytokeratins, EMA, e-cadherin, and p75NGFR showed a strong expression of these markers in the epithelial component but no expression in the spindle stromal cells. The epithelial tumor cells showed no reactivity to CD5 and L26 and a high proportion of the infiltrated lymphocytes were the cortical type that expressed CD99 and terminal deoxynucleotidyl transferase. Ultrastructural examinations revealed tonofilaments in the spindle cells. Follow up has been done for 5 years after the surgical excision and the patient has been free of disease during that period. Similar to previous reports, this patient had a benign clinical course that was unassociated with myasthenia gravis which appears to be a characteristic of this histologic variant of thymoma. However, our case also showed advanced degenerative features and we could demonstrate ultrastructural evidence of epithelial differentiation in the stromal spindle cells that were not mentioned in the previously reported cases. Based on the results of our studies, we suggest this entity is a distinct type of organotypic thymoma that shows cortical differentiation and abundant cellular stroma.

Teratoid hepatoblastoma: multidirectional differentiation of stem cell of the liver.

Hepatoblastoma is the most common malignant hepatic neoplasm of childhood, showing a wide spectrum of epithelial and mesenchymal components. Teratoid hepatoblastoma, which reveals multiple lines of tissue differentiation such as mucinous epithelium, melanin pigment, endocrine differentiation, glial and mesenchymal components, has rarely been observed. We report a case of teratoid hepatoblastoma in a 22-month-old girl. She had been diagnosed with hepatoblastoma through percutaneous needle biopsy of the liver and treated with 10 chemotherapy cycles of epirubicin, VP-16 and cisplatin and with hepatic artery embolization. After 10 months, an extended left lobectomy was performed. Grossly, a multinodular, partly well-demarcated, solid mass (7 x 5 cm) with dense fibrosis and focal cystic change occupied almost the entire specimen. There was extensive necrosis due to preoperative treatment. Microscopically, the tumor showed multiple lines of differentiation, which was composed of embryonal, fetal hepatocytes and mesenchymal elements with numerous foci of osteoid. There were also other components showing endodermal, neural, melanocytic and endocrine differentiation. These teratoid components were considered relatively resistant to preoperative chemotherapy, in contrast to extensive necrosis of both embryonal and fetal hepatocytes. These teratoid features of hepatoblastoma are considered to be a multidirectional differentiation of the small epithelial cells or stem cells of the tumor.