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Journal Articles

Alpha-ray detection with a MgB$$_{2}$$ transition edge sensor

Okayasu, Satoru; Katagiri, Masaki; Hojo, Kiichi; Morii, Yukio; Miki, Shigenobu*; Shimakage, Hisashi*; Wang, Z.*; Ishida, Takekazu*

Physica C, 468(15-20), p.1998 - 2000, 2008/09

 Times Cited Count:0 Percentile:0(Physics, Applied)

We have been investigating for neutron detection with the MgB $$_{2}$$ transition edge sensor (TES). For the purpose, we have been developing a low noise measurement system for neutron detection. To confirm the usefulness of our detecting system, alpha-ray detection from an $$^{241}$$Am source was achieved. To improve the sensitivity, short meander samples with 35 $$mu$$m length and 1 $$mu$$m width is used. Samples are fabricated by the Kansai group. Superconducting transition temperature Tc is 23.3 K. The transition width is sharp enough less than 1 K. The detection was achieved under a constant current condition, and the resistivity changes at the sample due to the alpha-ray irradiation were detected. The bias current is 6 $$mu$$A.

Journal Articles

Shell structures of vortices in superconducting disks of an amorphous MoGe film

Kokubo, Nobuhito*; Okayasu, Satoru; Shinozaki, Bunju*

Physica C, 468(15-20), p.1316 - 1318, 2008/09

 Times Cited Count:6 Percentile:31.88(Physics, Applied)

We have investigated spatial distributions of vortices induced in small superconducting disks of amorphous MoGe. Scanning SQUID microcopy allows us to observe directly magnetic distributions near the surface of the disks. Results on disks with diameter 0.05 mm demonstrate that quasi-symmetric distributions imposed by the circle geometry appear for vorticity up to 5. Meanwhile, for vorticity more than 6, vortices form an unique shell structure, in which one vortex is located in the center and others form a circle around. The critical vorticity for this structural transition is consistent with results on Nb disks imaged by the magnetic decoration experiments.

Journal Articles

Possible mechanism of charge stripe formation based on the ring exchange interaction

Sakai, Toru

Physica C, 468(15-20), p.1163 - 1165, 2008/09

 Times Cited Count:0 Percentile:0(Physics, Applied)

The charge stripes observed in the high-Tc cuprate superconductors have attracted a great interest in the field of superconductivity for a long time. A lot of clear evidences were detected by the neutron scattering and the STM etc. In our previous works, we proposed a possible mechanism of the charge stripe formation based on the ring exchange interaction which is originated from the strong electron correlation. The numerical exact diagonalization study of the t-J model with the ring exchange gave a reasonable phase diagram of the vertical charge stripe in the work. However, some transition metal oxides did not exhibit only the vertical charge stripe but also the diagonal one. In the present study, we investigate two-types of charge stripes and present a new phase diagram including the both charge stripe phases.

Journal Articles

Vortex core structure in strongly correlated superfluidity

Yamada, Susumu; Okumura, Masahiko; Machida, Masahiko; Ohashi, Yoji*; Matsumoto, Hideki*

Physica C, 468(15-20), p.1237 - 1240, 2008/09

 Times Cited Count:1 Percentile:6.77(Physics, Applied)

In order to study the vortex core electronic structure in high-T$$_{c}$$ superconductors, we examine 1-D Hubbard model with trap potential by using the density matrix renormalization group method. Instead of directly treating the vortex, the approach mimics the carrier density depression inside the vortex core via the trap potential and exactly calculate strong correlation effects on the depressed region. Consequently, we find that the Mott state emerges in the central region and the metallic edge surrounds the Mott region. Furthermore, when adding spin imbalance, the calculations reveal that a local antiferromagnetic order covers the Mott state region, and moreover, the antiferromagnetic order modulates with a long periodicity. We expect that these results closely relate to the vortex core electronic structure while their calculation results can be directly compared with atomic Fermi gases loaded on an optical lattice.

Journal Articles

Simulation studies for the vortex depinning dynamics around a columnar defect in superconductors

Nakai, Noriyuki; Hayashi, Nobuhiko; Machida, Masahiko

Physica C, 468(15-20), p.1270 - 1273, 2008/09

 Times Cited Count:5 Percentile:27.8(Physics, Applied)

Journal Articles

Hole localization in strongly correlated and disordered systems; DMRG studies for 1-D and 3-leg ladder random Hubbard models

Okumura, Masahiko; Yamada, Susumu; Taniguchi, Nobuhiko*; Machida, Masahiko

Physica C, 468(15-20), p.1241 - 1244, 2008/09

 Times Cited Count:0 Percentile:0(Physics, Applied)

We study one-dimensional and 3-leg ladder repulsive Hubbard models with random potential by using the density-matrix renormalization group method. When holes are slightly doped to the half-filling, the local Mott phase called "Mott plateau" and the local dip structure in density profile called "hole localized valley" are well-separately observable in strong on-site repulsive U=t and strong randomness for both models. We suggest that these separated structures qualitatively capture locally inhomogeneous characters seen in heavily under-doped High-Tc superconductors.

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