Superconducting gap function of Iron-Pnictide Co-doped BaFe₂As₂

Since the discovery of superconductivity in iron oxypnictide , its superconductivity with high transition temperature Tc has attracted great attention because this system bears some analogy to high-Tc cuprates. However, in contrast to high-Tc cuprates, the fully gapped state has been reported by specific heat, penetration depth, angle resolve photoemission measurements and so on. What is intriguing in this system is that theoretically the antiferromagnetic fluctuations with Q = (π, 0) is expected to induce the sign-reversing s-wave superconductivity with fully opened gap, which is called "s±-wave state". Including the possibility of the s±-wave state, however, the nature of the superconductivity has not been clarified yet.

Yo Machida and co-workers have performed thermal conductivity measurements on the iron oxypnictide Ba(Fe_1−xCo_x)₂As₂ (x = 0.07) in order to clarify the gap structure. They found a sizable residual T-linear term of the thermal conductivity, possibly due to the impurity-induced in-gap state. In addition, significant peak anomalies were observed in both thermal conductivity and thermal Hall conductivity originating from the prominent enhancement of the quasi-particle (QP) mean-free path below Tc. The field dependence of the delocalized QP density of states is apparently different from that of nodal gap excitation. These observations all point to the fully gapped sign-reversing s-wave state.

• ・ Authors: Yo Machida, Kosuke Tomokuni, Takayuki Isono, Koichi Izawa, Yasuyuki Nakajima, and Tsuyoshi Tamegai
• ・ Title of original paper: Possible Sign-Reversing s-Wave Superconductivity in Co-Doped BaFe₂As₂ Proved by Thermal Transport Measurements.
• ・ Journal, volume, pages and year: J. Phys. Soc. Jpn. 78, 073705 (2009).
• ・ Digital Object Identifier (DOI): 10.1143/JPSJ.78.073705
• ・ Affiliations: Department of Physics, Tokyo Institute of Technology,
• ・ Department website: http://izawa.ap.titech.ac.jp/

Figure caption: Temperature dependence of thermal conductivity κ_xx. The inset: κ_xx /T vs T² plot under zero field.

Ultrasonic mini-motors using lead-free piezoelectric ceramics

Miniaturised ultrasonic motors (USMs) driven with piezoelectric ceramics could soon be used to operate the zoom lenses of small digital cameras in mobile phones. They are superior to other motors in the mini-motor area because their efficiency is not affected by their size, but most USMs in the market are driven with ceramics called lead zirconate titanates (PZTs), which contain more than 60 % lead by weight.

In order to protect the environment, it is hoped that the lead-based ceramics in USMs could be replaced by lead-free ceramics. However, the piezoelectric properties of lead-free ceramics may not be sufficient to drive certain systems, in particular the traditional, so-called longitude mode (d₃₃) applications.

Now, Takaaki Tsurumi and colleagues at Tokyo Institute of Technology have successfully made ultrasonic motors driven with lead-free piezoelectric ceramics, by using a different mode called the shear mode (d₁₅).

The motors were fabricated by a three-step process. Firstly, the researchers designed the shear mode USMs, using finite element analytical software for modeling and simulation to decide on the optimum design. Then, they studied the crystallography of lead-free ceramics in order to learn how to enhance the shear mode and choose the ceramics with the best properties. Finally they built and tested the shear mode USMs.

The researchers successfully made three kinds of USMs driven with lead-free ceramics using the shear mode, and achieved a highest speed of 486 revolutions per minute. Their work reveals both academic and practical insights into the shear mode of lead-free piezoceramics. It provides a possible basis for actual application of lead-free piezoceramics as replacements for lead-based PZT ceramics, thereby helping to protect the environment.

Authors: E. Li, H. Kakemoto, T. Hoshina & T. Tsurumi.
Japanese Journal of Applied Physics 48, (2009)
Graduate school of science and engineering
Department website: http://www.cim.ceram.titech.ac.jp/
Title of original paper: A Shear-Mode Ultrasonic Motor Using Potassium Sodium Nibate-Based Ceramics with High Mechanical Quality Factor.
Digital Object Identifier (DOI): 10.1143/JJAP.48.09KD11

Figure caption: A miniature ultrasonic motor (USM) built using lead-free ceramics.

Estimating the future global population at risk of river flooding

River flooding is one of the major environmental hazards that are anticipated to become more frequent in future, because anthropogenically induced climate change is expected to intensify the global water cycle. Currently, 20 to 300 million people per year are affected by floods that threaten both social security and sustainable development. However, to date there has been no adequate estimate of the future global population that might be at risk of flooding.

Now, Shinjiro Kanae in the Department of Mechanical and Environmental Informatics at Tokyo Institute of Technology and his colleague Yukiko Hirabayashi have used a global-scale river discharge simulation to make the first estimate of future populations at risk of flooding. The researchers used their simulation to detect extreme conditions of river discharge, and applied the results alongside a distributed population dataset.

The results indicate that in the case of 3 ℃ warming above the 1980-1999 temperature average, approximately 300 million people could be at risk, even in years of relatively low flooding. This number corresponds to the number of people affected by a devastating flood every year at present. If the temperature increase is greater than 3 ℃, the flood-affected population would likely be even larger. Therefore, the famous '2-degree' tipping point turns out to be a reasonable guideline for continental river flooding.

Y. Hirabayashi & S. Kanae
Hydrological Research Letters 3, 6-9 (2009)
Department of Mechanical and Environmental Informatics
Department website: http://www.mei.titech.ac.jp/index-e.html
Title of original paper: First estimate of the future global population at risk of flooding
Digital Object Identifier (DOI): 10.3178/hrl.3.6

Figure caption: The predicted population affected by floods versus the change in global mean surface air temperature in the 21st century. For reference, the variability range of the actual flood-affected population in the past, the variability range of the flood-affected population with climate model outputs in 1901–2000, and the population fixed at the year 2000 value are shown at the left.