Hajime Takayama

Bio: Hajime Takayama is an academic researcher from Hokkaido University. The author has contributed to research in topics: Spin glass & Monte Carlo method. The author has an hindex of 11, co-authored 25 publications receiving 460 citations.

Magnetic surface anisotropy of transition metals

Abstract: A model calculation is presented for the computation of the magnetic surface anisotropy ${K}_{s}$ for transition metals Special reference is made to Ni It is demonstrated that the changes at the surface in the effective atomic potential of the different $d$ orbitals are very important in determining ${K}_{s}$ Furthermore it is shown that one must include all $d$ orbitals in the calculations The problem is studied by successively treating the weak-hopping limit, the one-layer problem, and the surface of a semi-infinite crystal The sign and the order of magnitude of the magnetic surface anisotropy which we find for Ni agree with the experimental results on Ni alloys A comparison is made to earlier work on the subject

Solitons in a One-Dimensional Bose System with the Repulsive Delta-Function Interaction

Abstract: Lieb's type II excitation, derived exactly by means of a Bethe ansatz, in a one-dimensional Bose system with the repulsive delta-function interaction is identified with the soliton mode, which is a solution of the Pitaevskii-Gross (or nonlinear Schrodinger) equation for the corresponding Bose field.

Dynamical Properties of Quasi-One-Dimensional Conductors: A Phase Hamiltonian Approach

Abstract: Since the realization of quasi-one-dimensional conductors, there have been extensive investigations into the electronic properties of other such low-dimensional systems, both experimentally and theoretically [1–4]. Above all, systems having a Peierls transition [5] have been investigated in detail. The novelty of this Peierls—Frohlich (PF) [6] state lies in the novel possibility of the transport phenomenon being associated with the collective degree of freedom, i.e. the charge density wave (CDW) [7]. In a CDW, electrons follow the periodic lattice distortion adiabatically, resulting in a periodic spatial variation of the self-consistent charge density. Lee, Rice and Anderson [8] have shown that the low-lying excitation of CDW is due to the sliding motion associated with the lattice distortion, which is described by the phase of the complex order parameter, i.e. the periodic lattice distortion. This phase is related to the choice of the origin of coordinates and then to the translational symmetry of the system. Hence this sliding motion, or sliding conductivity, is sensitive to the impurity scattering and the Umklapp scattering [8]. These scattering mechanisms result in impurity and commensurability pinning, respectively, whose various interesting properties have been revealed since that time. In these investigations the phase Hamiltonian, which is the effective Hamiltonian to describe the motion of the phase and is derived from the full Hamiltonian with electron-phonon interactions, has proved very useful [8–12]. Recently anomalous properties typically observed in NbSe3 have also been discussed in a similar context [13, 14] and by Monceau in Part II of this volume.

Monte Carlo Study of a Triangular Ising Lattice. II. Occurrence of the Kosterlitz-Thouless-Like Phase Transition

Abstract: A two-dimensional triangular Ising lattice with the nearest-neighbor antiferromagnetic interaction J 1 and the next-nearest-neighbor ferromagnetic interaction - J 2 is studied by the Monte Carlo simulation. It is shown that the system with a finite J 2 undergoes the successive phase transition, paramagnetic→Kosterlitz-Thouless (KT)-like→ferrimagnetic phases, as temperature decreases. The sub-lattice switching phenomenon observed in our previous work is shown to be an intrinsic property inherent to the present system which has an XY character. The marginal exponent of the order-parameter correlation function at the KT-like transition point is seen to increase from 1/4 as the ratio J 2 / J 1 decreases, namely, it reveals a deviation from the universal behavior of a genuine XY model.

Many-Body Physics with Ultracold Gases

Abstract: This paper reviews recent experimental and theoretical progress concerning many-body phenomena in dilute, ultracold gases. It focuses on effects beyond standard weak-coupling descriptions, such as the Mott-Hubbard transition in optical lattices, strongly interacting gases in one and two dimensions, or lowest-Landau-level physics in quasi-two-dimensional gases in fast rotation. Strong correlations in fermionic gases are discussed in optical lattices or near-Feshbach resonances in the BCS-BEC crossover.

Spin glasses: Experimental facts, theoretical concepts, and open questions

Abstract: This review summarizes recent developments in the theory of spin glasses, as well as pertinent experimental data. The most characteristic properties of spin glass systems are described, and related phenomena in other glassy systems (dielectric and orientational glasses) are mentioned. The Edwards-Anderson model of spin glasses and its treatment within the replica method and mean-field theory are outlined, and concepts such as "frustration," "broken replica symmetry," "broken ergodicity," etc., are discussed. The dynamic approach to describing the spin glass transition is emphasized. Monte Carlo simulations of spin glasses and the insight gained by them are described. Other topics discussed include site-disorder models, phenomenological theories for the frozen phase and its excitations, phase diagrams in which spin glass order and ferromagnetism or antiferromagnetism compete, the Ne\'el model of superparamagnetism and related approaches, and possible connections between spin glasses and other topics in the theory of disordered condensed-matter systems.

Organic conductors and superconductors

Abstract: This review attempts to present the most salient developments of research on organic conductors and superconductors during the past 10 years. A theoretical introduction treats instabilities of quasi-one-dimensional electron systems and associated precursor effects which are relevant to the experimental results on organic conductors. We then describe the characterization of quasi-one-dimensional organic conductors by their transport, optical and magnetic properties. Finally, two sections are devoted to the experimental investigation of the low temperature instabilities: lattice instability in TTF-TCNQ and related compounds, superconducting or antiferromagnetic instabilities in the (TMTSF) 2 X series. The importance of one-dimensional fluctuations is emphasized in both lattice and superconducting instabilities.

The superoxide-generating oxidase of phagocytic cells

Abstract: Professional phagocytes (neutrophils, eosinophils, monocytes and macrophages) possess an enzymatic complex, the NADPH oxidase, which is able to catalyze the one-electron reduction of molecular oxygen to superoxide, O 2 - . The NADPH oxidase is dormant in non-activated phagocytes. It is suddenly activated upon exposure of phagocytes to the appropriate stimuli and thereby contributes to the microbicidal activity of these cells. Oxidase activation in phagocytes involves the assembly, in the plasma membrane, of membrane-bound and cytosolic components of the oxidase complex, which were disassembled in the resting state. One of the membrane-bound components in resting phagocytes has been identified as a low-potential b-type cytochrome, a heterodimer composed of two subunits of 22-kDa and 91-kDa. The link between NADPH and cytochrome b is probably a flavoprotein whose subcellular localization in resting phagocytes remains to be determined. Genetic defects in the cytochrome b subunits and in the cytosolic factors have been shown to be the molecular basis of chronic granulomatous disease, a group of inherited disorders in the host defense, characterized by severe, recurrent bacterial and fungal infections in which phagocytic cells fail to generate O 2 - upon stimulation. The present review is focused on recent data concerning the signaling pathway which leads to oxidase activation, including specific receptors, the production of second messengers, the organization of the oxidase complex and the molecular defects responsible for granulomatous disease.

Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications

Abstract: Spin electronics is a rapidly expanding field stimulated by a strong synergy between breakthrough basic research discoveries and industrial applications in the fields of magnetic recording, magnetic field sensors, nonvolatile memories [magnetic random access memories (MRAM) and especially spin-transfer-torque MRAM (STT-MRAM)]. In addition to the discovery of several physical phenomena (giant magnetoresistance, tunnel magnetoresistance, spin-transfer torque, spin-orbit torque, spin Hall effect, spin Seebeck effect, etc.), outstanding progress has been made on the growth and nanopatterning of magnetic multilayered films and nanostructures in which these phenomena are observed. Magnetic anisotropy is usually observed in materials that have large spin-orbit interactions. However, in 2002 perpendicular magnetic anisotropy (PMA) was discovered to exist at magnetic metal/oxide interfaces [for instance Co(Fe)/alumina]. Surprisingly, this PMA is observed in systems where spin-orbit interactions are quite weak, but its amplitude is remarkably large—comparable to that measured at Co/Pt interfaces, a reference for large interfacial anisotropy (anisotropy ∼1.4 erg/cm2 = 1.4 mJ/m2). Actually, this PMA was found to be very common at magnetic metal/oxide interfaces since it has been observed with a large variety of amorphous or crystalline oxides, including AlOx, MgO, TaOx, HfOx, etc. This PMA is thought to be the result of electronic hybridization between the oxygen and the magnetic transition metal orbit across the interface, a hypothesis supported by ab initio calculations. Interest in this phenomenon was sparked in 2010 when it was demonstrated that the PMA at magnetic transition metal/oxide interfaces could be used to build out-of-plane magnetized magnetic tunnel junctions for STT-MRAM cells. In these systems, the PMA at the CoFeB/MgO interface can be used to simultaneously obtain good memory retention, thanks to the large PMA amplitude, and a low write current, thanks to a relatively weak Gilbert damping. These two requirements for memories tend to be difficult to reconcile since they rely on the same spin-orbit coupling. PMA-based approaches have now become ubiquitous in the designs for perpendicular STT-MRAM, and major microelectronics companies are actively working on their development with the first goal of addressing embedded FLASH and static random access memory-type of applications. Scalability of STT-MRAM devices based on this interfacial PMA is expected to soon exceed the 20-nm nodes. Several very active new fields of research also rely on interfacial PMA at magnetic metal/oxide interfaces, including spin-orbit torques associated with Rashba or spin Hall effects, record high speed domain wall propagation in buffer/magnetic metal/oxide-based magnetic wires, and voltage-based control of anisotropy. This review deals with PMA at magnetic metal/oxide interfaces from its discovery, by examining the diversity of systems in which it has been observed and the physicochemical methods through which the key roles played by the electronic hybridization at the metal/oxide interface were elucidated. The physical origins of the phenomenon are also covered and how these are supported by ab initio calculations is dealt with. Finally, some examples of applications of this interfacial PMA in STT-MRAM are listed along with the various emerging research topics taking advantage of this PMA.