First investigation of superconducting phase diagram in the simplest iron-basedsuperconductor-Pressure induced superconductor-metal phase transition in FeSe0.5Te0.5<Content>
The research group of Dr. K. Horigane and Prof. K. Yamada in WPI has revealed thesuperconducting phase diagram of the simplest iron-based superconductor including thepressure-induced metallic phase for the first time.
The research result is published in Journal of Physical Society of Japan (JPSJ) in June
2009. This work was selected as a paper of Editor’s Choice in JPSJ.
<Background>
Since the discovery of iron-based high-temperature superconductor LaFeAs(O1-xFx), alot of iron-based superconductors have been made to search for new superconducting
materials and the recent observations of superconducting transition temperature, Tc, areraised up to 55K in LaFeAs(O1-xFx). Tc of iron-based superconductor is as high as thatof cuprate superconductor, therefore extensive attentions have been paid to elucidate themechanism of superconductivity. Recently, superconductivity was discovered in FeSewith Tc=8K. The crystal structure of this system is composed of a stack of FeSe
conduction layers, which is the simplest crystal structure in iron-based superconductor(Fig.1-(a)). Moreover, Tc was dramatically enhanced in FeSe under high pressure. Fromthese characteristic properties, FeSe is considered to be a good candidate to enhance theTc and understand the mechanism of superconductivity. However, several problems
make difficult to research in this system.
(1) It is difficult to make a single phase FeSe sample.
(2) The crystal structure is changed from superconducting α-FeSe phase to nonsuperconducting hexagonal phase.
(3) Zero resistivity cannot be observed by using diamond anvil cell (DAC)<Research>
The research group of Tohoku University has revealed the superconducting phase
diagram of the simplest iron-based superconductor including the pressure-induced
metallic phase for the first time in Te substituted FeSe0.5Te0.5. By using cubic anvil cell,
we have also succeeded in observing the zero resistivity with precise pressure. Figure
1-(b) shows the temperature dependence of the resistivity of FeSe0.5Te0.5 at variouspressures up to 7.5GPa. Tconset(Tcoffset) was enhanced from 13.5K(12.5K) to26.2K(21.5K) at 2GPa. In a comparison with the previous reports on the FeSe1-xTexsystem, the present Tcoffset is the highest value for this system. In FeSe0.5Te0.5, theα-phase can be easily formed and ⊿Tc(Tconset-Tcoffset) is almost constant value above
2GPa, indicating structural transition by applying pressure is suppressed in this system.
Moreover, it is clear that Tc is connected smoothly between FeSe and FeSe0.5Te0.5 andshow a general curve (Fig.2). Therefore, our result is the first observation of a phasetransition from the superconducting to metallic phase on the P-Tc phase diagram in theFeSe system.
<Future aspect>
Our researches open the new research field in iron-based superconducting systems. By
comparing the difficulties of metallic phase and researching structural parameters, we
expected to elucidate the mechanism of superconductivity<Contact>
Research Associate Kazumasa Horigane
WPI-AIMR, Tohoku University
Tel:022-215-2037
E-mail: khorigane@imr.tohoku.ac.jp

Professor Kazuyoshi Yamada
WPI-AIMR, Tohoku University
Tel:022-215-2037
E-mail: kyamada@imr.tohoku.ac.jp

<Figure>(a)(b)
3.0
Resistivity [mΩcm]

2.0

Resistivity [mΩcm]

2.5
2.014GPa
1.5
1.00.51.5

1.0GPa1.3
1.6
2.02.53.24.57.5

1.00.50.050 100 150 200 250
Temperature [K]5101520253035

Temperature [K]Figure １：(a) Crystal structure of FeSe (Fe : blue, Se: yellow). (b) Superconducting transition in
FeSe0.5Te0.5 under high pressure. Tconset was enhanced from 13.5K to 26K at P=2GPa. Inset show themetallic phase at P=14GPa.

1.2FeSe0.5Te0.5

1FeSeFigure ２：Pressure dependenceof Tc. Normalized pressure Pn isrepresented by the equation Pn =(P-P*)/(Pc-P*), where P* andPc are pressures with maximum
Tc and Tc=0, respectively.

0.6

cT /Tmaxc

0.8

0.4
0.2
0-0.5

00.5
Pn1