ASTE Simultaneous HCN(4-3) and HCO+(4-3) Observations of the Two Luminous Infrared Galaxies NGC 4418 and Arp 220

We report the results of HCN(J=4-3) and HCO+(J=4-3) observations of two luminous infrared galaxies (LIRGs), NGC 4418 and Arp 220, made using the Atacama Submillimeter Telescope Experiment (ASTE). The ASTE wide-band correlator provided simultaneous observations of HCN(4-3) and HCO+(4-3) lines, and a precise determination of their flux ratios. Both galaxies showed high HCN(4-3) to HCO+(4-3) flux ratios of >2, possibly due to AGN-related phenomena. The J = 4-3 to J = 1-0 transition flux ratios for HCN (HCO+) are similar to those expected for fully thermalized (sub-thermally excited) gas in both sources, in spite of HCN's higher critical density. If we assume collisional excitation and neglect an infrared radiative pumping process, our non-LTE analysis suggests that HCN traces gas with significantly higher density than HCO+. In Arp 220, we separated the double-peaked HCN(4-3) emission into the eastern and western nuclei, based on velocity information. We confirmed that the eastern nucleus showed a higher HCN(4-3) to HCN(1-0) flux ratio, and thus contained a larger amount of highly excited molecular gas than the western nucleus.Comment: 21 pages, 4 figures, accepted for publication in PASJ (Vol.62, No.1, 2010 Feb 25 issue

Interplay of Kondo and superconducting correlations in the nonequilibrium Andreev transport through a quantum dot

Using the modified perturbation theory, we theoretically study the nonequilibrium Andreev transport through a quantum dot coupled to normal and superconducting leads (N-QD-S), which is strongly influenced by the Kondo and superconducting correlations. From the numerical calculation, we find that the renormalized couplings between the leads and the dot in the equilibrium states characterize the peak formation in the nonequilibrium differential conductance. In particular, in the Kondo regime, the enhancement of the Andreev transport via a Kondo resonance occurs in the differential conductance at a finite bias voltage, leading to an anomalous peak whose position is given by the renormalized parameters. In addition to the peak, we show that the energy levels of the Andreev bound states give rise to other peaks in the differential conductance in the strongly correlated N-QD-S system. All these features of the nonequilibrium transport are consistent with those in the recent experimental results [R. S. Deacon {\it et al.}, Phys. Rev. Lett. {\bf 104}, 076805 (2010); Phys. Rev. B {\bf 81}, 12308 (2010)]. We also find that the interplay of the Kondo and superconducting correlations induces an intriguing pinning effect of the Andreev resonances to the Fermi level and its counter position.Comment: 22 pages, 23 figure

Inter-subject neural code converter for visual image representation.

Brain activity patterns differ from person to person, even for an identical stimulus. In functional brain mapping studies, it is important to align brain activity patterns between subjects for group statistical analyses. While anatomical templates are widely used for inter-subject alignment in functional magnetic resonance imaging (fMRI) studies, they are not sufficient to identify the mapping between voxel-level functional responses representing specific mental contents. Recent work has suggested that statistical learning methods could be used to transform individual brain activity patterns into a common space while preserving representational contents. Here, we propose a flexible method for functional alignment, "neural code converter, " which converts one subject's brain activity pattern into another's representing the same content. The neural code converter was designed to learn statistical relationships between fMRI activity patterns of paired subjects obtained while they saw an identical series of stimuli. It predicts the signal intensity of individual voxels of one subject from a pattern of multiple voxels of the other subject. To test this method, we used fMRI activity patterns measured while subjects observed visual images consisting of random and structured patches. We show that fMRI activity patterns for visual images not used for training the converter could be predicted from those of another subject where brain activity was recorded for the same stimuli. This confirms that visual images can be accurately reconstructed from the predicted activity patterns alone. Furthermore, we show that a classifier trained only on predicted fMRI activity patterns could accurately classify measured fMRI activity patterns. These results demonstrate that the neural code converter can translate neural codes between subjects while preserving contents related to visual images. While this method is useful for functional alignment and decoding, it may also provide a basis for brain-to-brain communication using the converted pattern for designing brain stimulation

Dynamics of biexciton localization in AlxGa1-xN mixed crystals under exciton resonant excitation

We report the localization dynamics of biexcitons in AlxGa1−xN mixed crystals under exciton resonant excitation at low temperatures. During a few tens of picoseconds just after intense laser excitation, the photoluminescence (PL) spectral shape obeys an inverse Maxwell–Boltzmann distribution and free biexcitons dominate the PL spectrum. With a further increase in the delay time, the biexciton PL peak energy and edge energy shift to lower energies. These redshift behaviors in AlxGa1−xN mixed crystals are completely different from the behaviors of free biexcitons in GaN crystals. Our observations reveal the rapid transformation dynamics from free to localized biexcitons in band-tail states in AlxGa1−xN mixed crystals

Magnetotransport study of the charged stripes in high-T_c cuprates

We present a study of the in-plane and out-of-plane magnetoresistance (MR) in heavily-underdoped, antiferromagnetic YBa_{2}Cu_{3}O_{6+x}, which reveals a variety of striking features. The in-plane MR demonstrates a "d-wave"-like anisotropy upon rotating the magnetic field H within the ab plane. With decreasing temperature below 20-25 K, the system acquires memory: exposing a crystal to the magnetic field results in a persistent in-plane resistivity anisotropy. The overall features can be explained by assuming that the CuO_2 planes contain a developed array of stripes accommodating the doped holes, and that the MR is associated with the field-induced topological ordering of the stripes.Comment: 4 pages, 5 figures, invited paper at M2S-HTSC-VI, to be published in Physics C (Proceedings of the International Conference on Materials and Mechanisms of Superconductivity, High Temperature Superconductors VI (M2S-HTSC-VI), Houston, Feb 20-25, 2000