Cooperative domain type interlayer sp3sp^3-bond formation in graphite

Using the classical molecular dynamics and the semiempirical Brenner's potential, we theoretically study the interlayer sigma bond formation, as cooperative and nonlinear phenomena induced by visible light excitations of a graphite crystal. We have found several cases, wherein the excitations of certain lattice sites result in new interlayer bonds even at non-excited sites. We have also found that, a new interlayer bond is easier to be formed around a bond, if it is already existing. As many more sites are going to be excited, the number of interlayer bonds increases nonlinearly with the number of excited sites. This nonlinearity shows 1.7 power of the total number of excited sites, corresponding to about three- or four-photon process.Comment: 7 pages, 8 figure

Enhanced coherent dynamics near a transition between neutral quantum-paraelectric and ionic ferroelectric phases in the quantum Blume-Emery-Griffiths model

Nonequilibrium dynamics are studied near the quantum phase transition point in the one-dimensional quantum Blume-Emery-Griffiths model. Its pseudo-spin component $S^z$ represents an electric polarization, and $(S^z)^2$ corresponds to ionicity, in mixed-stack charge-transfer complexes that exhibit a transition between neutral quantum-paraelectric and ionic ferroelectric (or antiferroelectric) phases. The time-dependent Schr\"odinger equation is solved for the exact many-body wave function in the quantum paraelectric phase. After impact force is introduced on a polarization locally in space and time, polarizations and ionicity coherently oscillate. The oscillation amplitudes are large near the quantum phase transition point. The energy supplied by the impact flows linearly into these oscillations, so that the nonequilibrium behavior is uncooperative.Comment: 6 pages, 4 figures, accepted for publication in Phys. Rev.

Numerical Study of Photo-Induced Dynamics in Double-Exchange Model

Photo-induced spin and charge dynamics in double-exchange model are numerically studied. The Lanczos method and the density-matrix renormalization-group method are applied to one-dimensional finite-size clusters. By photon irradiation in a charge ordered (CO) insulator associated with antiferromagnetic (AFM) correlation, both the CO and AFM correlations collapse rapidly, and appearances of new peaks inside of an insulating gap are observed in the optical spectra and the one-particle excitation spectra. Time evolutions of the spin correlation and the in-gap state are correlated with each other, and are governed by the transfer integral of conduction electrons. Results are interpreted by the charge kink/anti-kink picture and their effective motions which depend on the localized spin correlation. Pump-photon density dependence of spin and charge dynamics are also studied. Roles of spin degree of freedom are remarkable in a case of weak photon density. Implications of the numerical results for the pump-probe experiments in perovskite manganites are discussed.Comment: 16 pages, 16 figure

Novel theoretical approach in photoemission spectroscopy: application to isotope effect and boron-doped diamond

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.Comment: 6 pages, 4 figures, Procs. of LEHTSC 2007, submitted to J. Phys.: Conf. Se

Introduction: The expanded conception of security and institutions

This is the author accepted manuscript. The final version is available from CUP via the DOI in this recordIntroduction Security is a dynamic, context-dependent concept that is inevitably shaped by social conditions and practices. The socio-political perception of security threats influences our security policies relevant to political decisions about the design of social institutions specifically addressing those security concerns. Security is traditionally understood to be physical protection of national territory and its population from the destructive effects of warfare through military means. Social institutions including but not limited to national governing institutions, inter-governmental institutions and the military are all devices developed through human history to collectively address traditional security threats. Security is often considered to be an antithesis of the rule of law and civil liberty, justifying violation of rules and the restriction of freedom. However, the development of international law and the institutionalisation of international public authorities have contributed to the increased normalcy or containment of extra-legal responses to security threats. For example, the Charter of the United Nations (‘UN Charter’) provides institutionalised mechanisms as the means of regulating the behaviour of sovereign states and conflict among them. The nuclear non-proliferation regime establishes mechanisms for preventing the proliferation of nuclear weapons and facilitating the development of peaceful nuclear energy technology by institutionalising the asymmetric obligations between designated nuclear-weapon states and other non-nuclear-weapon states. Yet, towards the end of the Cold War the concept of security began to expand, which subsequently led to the proliferation of contemporary security issues such as economic security, environmental security, energy and resource security, health security and bio-security. The conception of security also took a dramatic turn following the 2001 terrorist attacks on New York and Washington, blurring the traditional boundaries between international security and national security threats. Those changes in the conception of security world-wide have tested the potential of existing institutions, such as the World Trade Organization (WTO), the World Health Organization (WHO), the International Maritime Organization (IMO), the Organization for Security and Co-operation in Europe (OSCE), the European Union (EU) and the Association of Southeast Asian Nations (ASEAN), to assume a new role in the changing security paradigms, both at international and domestic levels

Real-space observation of current-driven domain wall motion in submicron magnetic wires

Spintronic devices, whose operation is based on the motion of a magnetic domain wall (DW), have been proposed recently. If a DW could be driven directly by flowing an electric current instead of a magnetic field, the performance and functions of such device would be drastically improved. Here we report real-space observation of the current-driven DW motion by using a well-defined single DW in a micro-fabricated magnetic wire with submicron width. Magnetic force microscopy (MFM) visualizes that a single DW introduced in the wire is displaced back and forth by positive and negative pulsed-current, respectively. We can control the DW position in the wire by tuning the intensity, the duration and the polarity of the pulsed-current. It is, thus, demonstrated that spintronic device operation by the current-driven DW motion is possible.Comment: Accepted and published in PR

Photoinduced magnetic bound state in itinerant correlated electron system with spin-state degree of freedom

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photo-irradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.Comment: 15 pages, 16 figure

Orbital Compass Model as an Itinerant Electron System

Two-dimensional orbital compass model is studied as an interacting itinerant electron model. A Hubbard-type tight-binding model, from which the orbital compass model is derived in the strong coupling limit, is identified. This model is analyzed by the random-phase approximation (RPA) and the self-consistent RPA methods from the weak coupling. Anisotropy for the orbital fluctuation in the momentum space is qualitatively changed by the on-site Coulomb interaction. This result is explained by the fact that the dominant fluctuation is changed from the intra-band nesting to the inter-band one by increasing the interaction.Comment: 7 pages, 8 figure

Photogenerated Carriers in SrTiO3 Probed by Mid-Infrared Absorption

Infrared absorption spectra of SrTiO$_3$ have been measured under above-band-gap photoexcitations to study the properties of photogenerated carriers, which should play important roles in previously reported photoinduced phenomena in SrTiO$_3$. A broad absorption band appears over the entire mid-infrared region under photoexcitation. Detailed energy, temperature, and excitation power dependences of the photoinduced absorption are reported. This photo-induced absorption is attributed to the intragap excitations of the photogenerated carriers. The data show the existence of a high density of in-gap states for the photocarriers, which extends over a wide energy range starting from the conduction and valence band edges.Comment: 5 pages, 5 figures, submitted to J. Phys. Soc. Jp

What is the true charge transfer gap in parent insulating cuprates?

A large body of experimental data point towards a charge transfer instability of parent insulating cuprates to be their unique property. We argue that the true charge transfer gap in these compounds is as small as 0.4-0.5\,eV rather than 1.5-2.0\,eV as usually derived from the optical gap measurements. In fact we deal with a competition of the conventional (3d$^9$) ground state and a charge transfer (CT) state with formation of electron-hole dimers which evolves under doping to an unconventional bosonic system. Our conjecture does provide an unified standpoint on the main experimental findings for parent cuprates including linear and nonlinear optical, Raman, photoemission, photoabsorption, and transport properties anyhow related with the CT excitations. In addition we suggest a scenario for the evolution of the CuO$_2$ planes in the CT unstable cuprates under a nonisovalent doping.Comment: 13 pages, 5 figures, submitted to PR