Photoinduced insulator-metal transition in correlated electrons -- a Floquet analysis with the dynamical mean-field theory

In order to investigate photoinduced insulator-metal transitions observed in correlated electron systems, we propose a new theoretical method, where we combine a Floquet-matrix method for AC-driven systems with the dynamical mean-field theory. The method can treat nonequilibrium steady states exactly beyond the linear-response regime. We have applied the method to the Falicov-Kimball model coupled to AC electric fields, and numerically obtained the spectral function, the nonequilibrium distribution function and the current-voltage characteristic. The results show that intense AC fields indeed drive Mott-like insulating states into photoinduced metallic states in a nonlinear way.Comment: 4 pages, 3 figures, Proceedings of LT2

Photoinduced insulator-metal transition and nonlinear optical response of correlated electrons -- a DMFT analysis

We investigate a photoinduced insulator-metal transition in the Falicov-Kimball model with the dynamical mean-field theory (DMFT) extended to nonequilibrium systems in periodic modulations in time. When the photon energy of the pump light is $\Omega \approx U$ ($U$: the interaction strength), a Drude-like peak is found to grow in the optical conductivity spectrum, which is an evidence that the system is driven into a metallic state. During the transition, the band gap does not collapse, whereas the distribution function exhibits a non-monotonic behaviour away from the Fermi distribution. This indicates that the transition cannot be accounted for by heating effects, but creation of photo-carriers is responsible.Comment: 3pages, 1 figure, Proceedings of Yamada Conference LXIII on Photo-induced Phase Transitions and Cooperative Phenomena (PIPT 2008

A Simple Model for Magnetization Ratios in Doped Nanocrystals

Recent experiments on Mn-doped ZnS nanocrystals have shown unusual magnetization properties. We describe a nearest-neighbor Heisenberg exchange model for calculating the magnetization ratios of these antiferromagnetically doped crystals, in which the dopant atoms are distributed inhomogeneously within the nanocrystal. This simple inhomogeneous doping model is capable of reproducing the experimental results, and suggests that interior dopant atoms are localized within the crystal.Comment: 8 pages, 1 figure, 2 tables. Submitted to J. Appl. Phy

Water vapor on supergiants. The 12 micron TEXES spectra of mu Cephei

Several recent papers have argued for warm, semi-detached, molecular layers surrounding red giant and supergiant stars, a concept known as a MOLsphere. Spectroscopic and interferometric analyses have often corroborated this general picture. Here, we present high-resolution spectroscopic data of pure rotational lines of water vapor at 12 microns for the supergiant mu Cephei. This star has often been used to test the concept of molecular layers around supergiants. Given the prediction of an isothermal, optically thick water-vapor layer in Local Thermodynamic Equilibrium around the star (MOLsphere), we expected the 12 micron lines to be in emission or at least in absorption but filled in by emission from the molecular layer around the star. Our data, however, show the contrary; we find definite absorption. Thus, our data do not easily fit into the suggested isothermal MOLsphere scenario. The 12 micron lines, therefore, put new, strong constraints on the MOLsphere concept and on the nature of water seen in signatures across the spectra of early M supergiants. We also find that the absorption is even stronger than that calculated from a standard, spherically symmetric model photosphere without any surrounding layers. A cool model photosphere, representing cool outer layers is, however, able to reproduce the lines, but this model does not account for water vapor emission at 6 microns. Thus, a unified model for water vapor on mu Cephei appears to be lacking. It does seem necessary to model the underlying photospheres of these supergiants in their whole complexity. The strong water vapor lines clearly reveal inadequacies of classical model atmospheres.Comment: Accepted for publication in the Astrophysical Journa

Nonequilibrium Steady State of Photoexcited Correlated Electrons in the Presence of Dissipation

We present a framework to determine nonequilibrium steady states in strongly correlated electron systems in the presence of dissipation. This is demonstrated for a correlated electron (Falicov-Kimball) model attached to a heat bath and irradiated by an intense pump light, for which an exact solution is obtained with the Floquet method combined with the nonequilibrium dynamical mean-field theory. On top of a Drude-like peak indicative of photometallization as observed in recent pump-probe experiments, new nonequilibrium phenomena are predicted to emerge, where the optical conductivity exhibits dip and kink structures around the frequency of the pump light, a midgap absorption arising from photoinduced Floquet subbands, and a negative attenuation (gain) due to a population inversion.Comment: 5 pages, 3 figures, published versio

Dust in the Photospheric Environment: Unified Cloudy Models of M, L, and T Dwarfs

We address the problem of how dust forms and how it could be sustained in the static photospheres of cool dwarfs for a long time. In the cool and dense gas, dust forms easily at the condensation temperature, T_cond, and the dust can be in detailed balance with the ambient gas so long as it remains smaller than the critical radius, r_cr. However, dust will grow larger and segregate from the gas when it will be larger than r_cr somewhere at the lower temperature, which we refer to as the critical temperature, T_cr. Then, the large dust grains will precipitate below the photosphere and only the small dust grains in the region of T_cr < T < T_cond can be sustained in the photosphere. Thus a dust cloud is formed. Incorporating the dust cloud, non-grey model photo- spheres in radiative-convective equilibrium are extended to T_eff as low as 800K. Observed colors and spectra of cool dwarfs can consistently be accounted for by a single grid of our cloudy models. This fact in turn can be regarded as supporting evidence for our basic assumption on the cloud formation.Comment: 50 pages with 14 postscript figures, to be published in Astrophys.

Search for magnetic monopoles using proportional counters filled with helium gas

Slow magnetic monopoles in cosmic rays have been searched at sea level with the detector which consists of seven layers of proportional counters filled with a mixture of He + 20% CH4. The velocities and the energy losses of the incident particles are measured. The upper limit of flux for the monopoles in the velocity range of 1 x 0.001 Beta 4 x 0.001 is 2.78 x 10 to the minus 12th power square centimeters sr sec of 90% confidence level

Infrared Spectra and Visibilities as Probes of the Outer Atmospheres of Red Supergiant Stars

In the light of the recent results of the stellar interferometry, we examine the nature of the extra molecular layer outside the photosphere of red super- giant stars, so far studied mostly with the use of the infrared spectra. Although the visibility data are more direct probes of the spatial structure of the outer atmosphere, it is essential that they are analyzed in combination with the spectral data of a wide spectral coverage. In the case of the M2 supergiant mu Cephei, several sets of data, both spectra and visibilities, strongly suggested the presence of an extra-molecular layer, and its basic parameters are estimated to be: excitation temperature T_ex = 1600 K, column densities of CO and H2O N_col = 3.0d+20/cm2, and inner radius R_in = 2.0R*. The result shows reasonable agreement with the one based on the infrared spectra alone, and this may be because the infrared spectra already include some information on the spatial structure of the outer atmosphere. It is important, however, that the model inferred from the spectra is now fully supported with the recent visibility data. In the case of the M2 supergiant alpha Orionis, the infrared spectra and visibilities show a consistent picture in that its molecular layer is closer to the photosphere (R_in = 1.3R*) with higher gas temperature (T_ex = 2250 K) and lower gas column density (N_col = 1.0d+20/cm2), compared with that of mu Cephei. Some controversy on the interpretation of the mid infrared data of alpha Orionis can be reconciled.Comment: 47 pages, 14 Postscript figures, to be published in the Astrophysical Journa

Characterization of local deformation and fracture behavior in ferrite plus martensite dual-phase steels having different grain sizes

42nd Risø International Symposium on Materials Science: Microstructural variability: Processing, analysis, mechanisms and properties 5–9 September 2022, Department of Civil and Mechanical Engineering, Technical University of Denmark, DenmarkLow carbon dual-phase (DP) steels composed of soft ferrite and hard martensite have been widely used in the automotive industry due to their good strength-ductility balance and large strain hardening ability. DP steels have a wide variation in mechanical properties depending on several microstructural features such as grain size, phase fraction and distribution. Among them, the grain refinement of DP steels is known to be an effective option for enhancing mechanical performance in strength and ductility (especially post-uniform elongation). However, the exact reason for the significant improvement of post-uniform elongation by grain refinement has not been fully understood. It is considered that the characterization of local deformation behavior and micro-void formation/growth behavior in connection with microstructures is an essential approach for understanding the enhanced post-uniform elongation realized in the fine-grained DP specimen. In the present study, we prepared two kinds of DP specimens with mean ferrite grain sizes of 14.9 μm (coarse-grained DP) and 7.1 μm (fine-grained DP), and carefully investigated local strain distribution of tensile specimen and micro-void formation/growth behavior using digital image correlation (DIC) analysis and SEM observations. The fine-grained DP specimen exhibited a gradual strain localization after necking and had sufficient strain capacity that could endure against fracture. The fine-grained DP structure had a great number of micro-voids in the necked region, but almost all the micro-voids maintained a very small size, which was contrasted with the case of coarse-grained DP specimen containing very large-sized micro-voids. Such a significant difference in micro-void size/number characters between two kinds of DP specimens would be one possible reason for exhibiting greatly different post-uniform elongation behavior