arXiv subjects

K. Shima

Publications and source records attributed to K. Shima.

14 recordsLinked to original sources

arXiv2023

Cathodoluminescence spectroscopy of monolayer hexagonal boron nitride

Cathodoluminescence (CL) spectroscopy is a powerful technique for studying emission properties of optoelectronic materials because CL is free from excitable bandgap limits and from ambiguous signals due to simple light scattering and resonant Raman scattering potentially involved in the photoluminescence (PL) spectra. However, direct CL measurements of atomically thin two-dimensional materials, such as transition metal dichalcogenides and hexagonal boron nitride (hBN), have been difficult due to the small excitation volume that interacts with high-energy electron beams (e-beams). Herein, distinct CL signals from a monolayer hBN, namely mBN, epitaxial film grown on a highly oriented pyrolytic graphite substrate are shown by using a home-made CL system capable of large-area and surface-sensitive excitation by an e-beam. The spatially resolved CL spectra at 13 K exhibited a predominant 5.5-eV emission band, which has been ascribed to originate from multilayered aggregates of hBN, markedly at thicker areas formed on the step edges of the substrate. Conversely, a faint peak at 6.04 eV was routinely observed from atomically flat areas. Since the energy agreed with the PL peak of 6.05 eV at 10 K that has been assigned as being due to the recombination of phonon-assisted direct excitons of mBN by Elias et al. [Nat. Commun. 10, 2639 (2019)], the CL peak at 6.04 eV is attributed to originate from the mBN epilayer. The CL results support the transition from indirect bandgap in bulk hBN to direct bandgap in mBN, in analogy with molybdenum disulfide. The results also encourage to elucidate emission properties of other low-dimensional materials with reduced excitation volumes by using the present CL configuration.

cond-mat.mtrl-sci↗

arXiv2021

Gravitational Fragmentation of Extremely Metal-poor Circumstellar Discs

We study the gravitational fragmentation of circumstellar discs accreting extremely metal-poor ($Z \leq 10^{-3}$ Zsun) gas, performing a suite of three-dimensional hydrodynamic simulations using the adaptive mesh refinement code Enzo. We systematically follow the long-term evolution for 2000 years after the first protostar's birth, for the cases of $Z = 0$, $10^{-5}$, $10^{-4}$, and $10^{-3}$ Zsun. We show that evolution of number of self-gravitating clumps qualitatively changes with $Z$. Vigorous fragmentation induced by dust cooling occurs in the metal-poor cases, temporarily providing about 10 self-gravitating clumps at $Z = 10^{-5}$ and $10^{-4}$ Zsun. However, we also show that the fragmentation is a very sporadic process; after an early episode of the fragmentation, the number of clumps continuously decreases as they merge away in these cases. The vigorous fragmentation tends to occur later with the higher $Z$, reflecting that the dust-induced fragmentation is most efficient at the lower density. At $Z = 10^{-3}$ Zsun, as a result, the clump number stays smallest until the disc fragmentation starts in a late stage. We also show that the clump mass distribution also depends on the metallicity. A single or binary clump substantially more massive than the others appear only at $Z = 10^{-3}$ Zsun, whereas they are more evenly distributed in mass at the lower metallicities. We suggest that the disc fragmentation should provide the stellar multiple systems, but their properties drastically change with a tiny amount of metals.

astro-ph.GA↗

arXiv2015

Isolating signatures of major cloud-cloud collisions II: The lifetimes of broad bridge features

We investigate the longevity of broad bridge features in position-velocity diagrams that appear as a result of cloud-cloud collisions. Broad bridges will have a finite lifetime due to the action of feedback, conversion of gas into stars and the timescale of the collision. We make a series of analytic arguments with which to estimate these lifetimes. Our simple analytic arguments suggest that for collisions between clouds larger than R~10 pc the lifetime of the broad bridge is more likely to be determined by the lifetime of the collision rather than the radiative or wind feedback disruption timescale. However for smaller clouds feedback becomes much more effective. This is because the radiative feedback timescale scales with the ionising flux Nly as R^{7/4}Nly^{-1/4} so a reduction in cloud size requires a relatively large decrease in ionising photons to maintain a given timescale. We find that our analytic arguments are consistent with new synthetic observations of numerical simulations of cloud-cloud collisions (including star formation and radiative feedback). We also argue that if the number of observable broad bridges remains ~ constant, then the disruption timescale must be roughly equivalent to the collision rate. If this is the case our analytic arguments also provide collision rate estimates, which we find are readily consistent with previous theoretical models at the scales they consider (clouds larger than about 10 pc) but are much higher for smaller clouds.

astro-ph.GA↗

arXiv2015

Isolating signatures of major cloud-cloud collisions using position-velocity diagrams

Collisions between giant molecular clouds are a potential mechanism for triggering the formation of massive stars, or even super star clusters. The trouble is identifying this process observationally and distinguishing it from other mechanisms. We produce synthetic position-velocity diagrams from models of: cloud-cloud collisions, non-interacting clouds along the line of sight, clouds with internal radiative feedback and a more complex cloud evolving in a galactic disc, to try and identify unique signatures of collision. We find that a broad bridge feature connecting two intensity peaks, spatially correlated but separated in velocity, is a signature of a high velocity cloud-cloud collision. We show that the broad bridge feature is resilient to the effects of radiative feedback, at least to around 2.5Myr after the formation of the first massive (ionising) star. However for a head on 10km/s collision we find that this will only be observable from 20-30 per cent of viewing angles. Such broad-bridge features have been identified towards M20, a very young region of massive star formation that was concluded to be a site of cloud-cloud collision by Torii et al (2011), and also towards star formation in the outer Milky Way by Izumi et al (2014).

astro-ph.GA↗

arXiv2009

On vacuum structures of N = 2 LSUSY QED equivalent to N = 2 NLSUSY model

The vacuum structure of N = 2 linear supersymmetry (LSUSY) invariant QED, which is equivalent to N = 2 nonlinear supersymmetry (NLSUSY) model, is studied explicitly in two dimensional space-time (d = 2). Two different isometries SO(1,3) and SO(3,1) appear for the vacuum field configuration corresponding to the various parameter regions. Two different field configurations of SO(3,1) isometry describe the two different physical vacua, i.e. one breaks spontaneously both U(1) and SUSY and the other breaks spontaneously SUSY alone.

hep-th↗

arXiv2007

Low energy particle physics and cosmology of nonlinear supersymmetric general relativity

We show a low energy physical meaning of nonlinear supersymmetric general relativity (NLSUSY GR) in asymptotic Riemann-flat space-time by studying the vacuum structure of N = 2 linear supersymmetry (LSUSY) invariant QED, which is equivalent to N = 2 NLSUSY model, in two dimensional space-time. Two different vacuum field configurations of SO(3,1) isometry describe the two different physical vacua, i.e. one breaks spontaneously both U(1) and SUSY and the other breaks spontaneously SUSY alone, where the latter elucidates the mysterious relations between the cosmology and the (low energy) particle physics and gives a new insight into the origin of mass.

hep-th↗

arXiv2004

New Einstein-Hilbert-type Action and Superon-Graviton Model(SGM) of Nature

A nonlinear supersymmetric(NLSUSY) Einstein-Hilbert(EH)-type new action for unity of nature is obtained by performing the Einstein gravity analogue geomtrical arguments in high symmetry spacetime inspired by NLSUSY. The new action is unstable and breaks down spontaneously into E-H action with matter in ordinary Riemann spacetime. All elementary particles except graviton are composed of the fundamental fermion "superon" of Nambu-Goldstone(NG) fermion of NLSUSY and regarded as the eigenstates of SO(10) super-Poincar\'e (SP) algebra, called superon-graviton model(SGM) of nature. Some phenomenological implications for the low energy particle physics and the cosmology are discussed. The linearization of NLSUSY including N=1 SGM action is attempted explicitly to obtain the linear SUSY local field theory, which is equivalent and renormalizable.

hep-th↗

arXiv2003

Toward linearization of nonlinear supersymmetric Einstein-Hilbert-type action of superon-graviton model(SGM)

We attempt the linearization of the nonlinear supersymmetry encoded in the Einstein-Hilbert-type action {\it superon-graviton model(SGM)} describing the nonlinear supersymmetric gravitational interaction of Nambu-Goldstone fermion {\it superon}. We discuss the linearizaton procedure in detail by the heuristic arguments referring to supergravity, in which particular attentions are paid to the local Lorentz invariance in the minimal interaction. We show explicitly up to the leading order that 80(bosons)+80(fermions) may be the minimal off-shell supermultiplet of the linearized theory.

hep-th↗

arXiv2003

On linearization of superon-graviton model(SGM)

Referring to the supermultiplet of N=1 supergravity (SUGRA) the linearization of N=1 SGM action describing the nonlinear supersymmetric (NL SUSY) gravitational interaction of superon (Nambu-Goldstone fermion) is attempted. The field contents of on-shell SUGRA supermultiplet are realized as the composites, though they have new SUSY transformations which closes on super-Poincaré algebra. Particular attentions are paid to the local Lorentz invariance.

hep-th↗

arXiv2003

Linearizing superon-graviton model(SGM)

We attempt the linearization of N=1 SGM action describing the nonlinear supersymmetric gravitational interaction of superon(Nambu-Goldstone fermion). We find that 80+80 field contents may give the off-shell supermultiplet of the supergravity-like linearized theory and they are realized explicitly up to $O(ψ^2)$ as the composites, though they have modified SUSY transformations which closes on super-Poincaré algebra. Particular attentions are paid to the local Lorentz invariance in the minimal interaction.

hep-th↗

arXiv2002

Superon-graviton model and supersymmetric structure of spacetime and matter

A new Einstein-Hilbert type (SGM) action describing gravitational interaction of Nambu-Goldstone(N-G) fermion of nonlinear supersymmetry(NL SUSY) is obtained by performing the Einstein gravity analogue geometrical arguments in high symmetric four dimensional (SGM) spacetime. All elementary particles except graviton are regarded as the composite eigenstates of SO(10) super-Poincaré algebra(SPA) composed of the fundamental N-G fermion ``superons'' of NL SUSY. Some phenomenological implications of the composite picture of SGM, the linearlization of SGM and N = 2 Volkov-Akulov model are discussed.

hep-th↗

arXiv2002

Linearizing N = 2 Nonlinear Supersymmetry

We investigate for the N = 2 supersymmetry (SUSY) a relation between a vector supermultiplet of the linear SUSY and the Volkov-Akulov model of the nonlinear SUSY. We express component fields of the vector supermultiplet in terms of Nambu-Goldstone fermion fields at the leading orders in a SUSY invariant way, and show the vector nature of the U(1) gauge field explicitly. A relation of the actions for the two models is also discussed briefly.

hep-th↗

arXiv2002

Linearization of N = 2 Nonlinear Supersymmetry and Spontaneous Supersymmetry Breaking Parameters

We show explicitly by the heuristic and practical arguments that for $N = 2$ supersymmetry (SUSY) a SUSY invariant relation between component fields of a vector supermultiplet of linear SUSY and Nambu-Goldstone fermions of the Volkov-Akulov model of nonlinear SUSY is written by using only three arbitrary dimensionless parameters, which can be recasted as the vacuum expectation values of auxiliary fields in the vector supermultiplet.

hep-th↗

arXiv2001

On Linearization of N=1 Nonlinear Supersymmetry

The N=1 Volkov-Akulov model of nonlinear supersymmetry is explicitly related to a vector supermultiplet model with a Fayet-Iliopoulos D term of linear supersymmetry. The physical significance of the results is discussed briefly.

hep-th↗