R esearch Highlights

R eesearch Highlights
NMR Measurements ofProtein using a High Tc
Superconducting Coil

Discovery of New Fe-based
Superconductor FeTe1-x Sx

Magnet Development Group,
Superconducting Materials Center

Nano Frontier Materials Group,
Superconducting Materials Center

Group Leader

Yoshikazu Mizuguchi

material with a structure that resembles the FeAs layer. We
focused on FeSe because it has the simplest crystal structure
in Fe-based superconductor, but does not contain arsenic.
When the lattice is compressed by applying pressure at
approximately 40,000atm, we discovered that the Tc of FeSe
increases to 37K, that is the highest class of Tc among thebinary superconductors. In addition, our attention was also
drawn to FeTe, which does not display a superconducting
transition even though its crystal structure resembles that ofFeSe. When we replaced the Te site with 20% S, which has asmall ion radius, we made the new discovery that FeTe0.8S0.2
displays superconductivity when the lattice is slightly compressed.
One of the attractive features of Fe-based superconductor is the diversity of superconducting layers. In addition tothe FeAs, FeP, and FeSe layers which were discovered previously, we found that the FeTe layer also shows superconductivity. In the future, it may be possible to realize even higher
superconducting transition temperatures than those to date if
superconducting layers with new structures and new combinations of elements can be discovered. Thus, research on Febased superconductor truly has great potential.

As4O La

Ba(a)LaFeAsO (LaFePO)(b)BaFe2As2As

SeFeFeLionElectrical resistivity (mΩcm)

FeTc

3.5

= 10 K

2.5

FeTe0.8S0.208

International 2009. Vol.7 No.4†††* NOESY: Nuclear Overhauser Effect Spectroscopy(a)(b)-10Bronze-reinforced
Bi-2223 conductor

1.5121zeroTc

0.5

FeTe0.92=8K5101545679102011Absolute temperature (K)Fig.2 Temperature dependence of electrical resistivity in FeTe0.8S0.2. A zero resistance state wasobserved at temperatures of approximately 8Kand below.

38Layer-wound coil

1212 11 10 9500MHz NMR magnet

Fig.1 Crystal structures of various Fe-based superconductors. The
respective Tc are (a) 55K, (b) 38K, and (c) 18K. The Tc of(d) FeSe is 13K, but this increases to 37K under pressure.

††excellent stability was used, fluctuations occurred in the field
due to noise and ambient temperature change. Furthermore, aphenomenon in which the central magnetic field gradually
increased due to magnetization of the Bi-2223 coil was alsoobserved. However, it was possible to solve these problems
by detecting the changes in the magnetic field and applying
compensation techniques. Fig. (b) shows a result obtainedwith the developed NMR spectrometer by the NOESY* technique of an aqueous solution sample of lysozyme. Although
NOESY is the most important measurement for obtaining the3-dimensional structure of proteins, good signals cannot beobtained if the field stability and homogeneity of the NMR
spectrometer are poor. The result shown in the figure is comparable to the results obtained with NMR spectrometers
operating in the persistent current mode using low Tc superconducting coils.
In this research, we successfully demonstrated theapplication of a bismuth-based superconducting material
discovered at NIMS to an NMR magnet, which is considered
to require the most difficult magnetic field conditions. We
plan to develop an “above 1GHz” NMR spectrometer, which
will be a first step toward ultra-high field NMR spectrometers and can demonstrate the usefulness of HTS. Applying
the developed technology to MRI (magnetic resonance imaging) and other measurement instruments is also expected.
This research was supported by SENTAN, JST.

20(d)FeSe (FeTe)†30(c)LiFeAs

An NMR (nuclear magnetic resonance) spectrometer isa powerful tool for analyzing the structures of organic compounds. NIMS improved the Nb3Sn superconductors, leading
to the development of 920MHz (21.6T) and 930MHz (21.8T)
NMR magnets. Because the Nb 3 Sn conductor is alreadybeing used near its critical magnet field, any large increase in
the magnetic field would be difficult. As high Tc superconductors (HTS) exhibit large critical current even in fields
exceeding 30T, we attempted to apply an HTS coil to an
NMR magnet.
In conventional NMR magnets, conductors with asquare or round profile are wound in an aligned manner without spacing (layer-winding), and the magnet is operated in
the persistent current mode in order to maintain the spatialhomogeneity and temporal stability of the magnetic field. Incase of Bi-2223 conductors, layer-winding is difficult
because of its tape shape. Furthermore, due to the resistanceof the conductor itself and the joints between conductors, themagnetic field is greatly decreased in the persistent current
mode, making high quality NMR measurement impossible.
We used bronze-reinforced Bi-2223 conductors. (Fig.
(a)) This not only improves the mechanical strength of theconductor, but also increases the thickness of the conductor,
enabling layer-winding. The wound coil was replaced withthe Nb3Sn innermost coil of a conventional NMR magnet,
and was operated at 11.7T (corresponding to 500MHz) using
a power supply with high stability 20 times greater than thatof the conventional type. Even when the power supply withGroup Leader

Tsukasa Kiyoshi, Hideaki Maeda, Mamoru Hamada, Masami Hosono

H (ppm)AsFeRIKEN,† Kobe Steel, Ltd.,†† JOEL, Ltd.†††1In February 2008, a group under Prof. Hideo Hosono atTokyo Institute of Technology discovered that an Fe-As compound, LaFeAsO, displays a superconducting transition at anabsolute temperature of 26K. In spite of the fact that thecompound contains iron, which had been considered disadvantageous for superconductivity because it is magnetic, thismaterial attracted worldwide attention due its relatively highsuperconducting transition temperature (Tc) and spurred aboom in research on Fe-based superconductor. Immediately
after this discovery, it was found that SmFeAsO, in which La
is replaced with Sm, displays an even higher Tc of 55K. As aresult, high expectations have been placed on the Fe-basedmaterials as new high temperature superconductors succeeding the copper oxide-based materials.
The crystal structures of the main Fe-based superconductors discovered to date are summarized in Fig. 1. Here,(a), (b), and (c) have a common FeAs layer (superconductinglayer), and superconductivity occurs in this layer. The Tc ofthese three materials differs greatly, as the Tc of (a) is 55K,
that of (b) is 38K, and that of (c) is 18K, showing that Tc isstrongly dependent on the crystal structure between the FeAslayers.
The FeSe in Fig. 1 (d) is a Tc = 13K superconducting

Yoshihiko Takano

Highly-stabilizedpower supply

8761543H (ppm)210-1

Result of NOESY measurement of lysozyme

Fig. NMR magnet containing a high Tc superconducting coil and an example of NMR measurement.

International 2009. Vol.7 No.409