083 abs 090216abstract

							   Pressure effect of Superconducting Oxypnictide RFeAsO1-xFx (R=La and Sm) and
                                  Related Materials
               *
               H.TakahashiA , Y.TakahashiB, H.OkadaA,Y.KamiharaC,E, M.HiranoD,F, H.HosonoD,E,F,
           A
            College of Humanities and Sciences, Nihon University and JST TRIP, Tokyo 156-8550, Japan
             B
              Graduate School of Integrated Basic Sciences, Nihon University, Tokyo 156-8550, Japan
                      C
                        JST, TRIP, Tokyo Institute of Technology, Yokohama 226-8503 , Japan
            D
              ERATO-SORST, JST, in FRC, Tokyo Institute of Technology, Yokohama 226-8503, Japan
         E
           Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama 226-8503, Japan
                         F
                           FRC, Tokyo Institute of Technology, Yokohama 226-8503, Japan

    The discovery of the iron-based superconducting compound LaFeAsO 1-xF x[1] with T c of 26 K
had a significant impact in the field of condensed matter physics. Right after the discovery of the
superconductivity, applying pressure on optimally doped LaFeAsO 1-xF x rised T c to 43 K[2] and the
pressure-induced superconductivity was discovered in the undoped LaFeAsO[3]. In this way
applying pressure is regarded as an advantageous route to investigate superconductivity. In this
study, the results of electrical resistivity measurements under high pressure for
RFeAsO1-xF x(R=La,Sm) and P-T phase diagrams were presented.
   The electrical resistivity measurements for SmFeAsO 0.93 F 007 reveals that the T c decreases
monotonously from 51 K with applying pressure, which is consistent with the data reported by
other groups [4]. However, it is noted that the rate of decrease of T c was about -6 K/GPa, which is
much larger than ref.4. The undoped LaFeAsO and SmFeAsO show a rapid decrease of electrical
resistivity at around 140 K, where the structural and magnetic transition occurs. These transition
temperature were suppressed by applying pressure, at the rate of -6 K/GPa and -13.7 K/GPa for
SmFeAsO and LaFeAsO, respectively. The pressure-induced superconductivity was observed at 9
GPa and 1.5 GPa for SmFeAsO and LaFeAsO, respectively.
   The LaFeAsO 1-xF x system show the common feature, that is, the Tc ’s increase with increasing
pressure and decreases with further compression though the peak value. The maximum T c ’s under
high pressure are shown in Fig. 1, together with the pressure where the T c shows maximum value.
In many cuprate superconductors, it is known that the superconducting T-P phase diagram is
similar to the T-x (doping) phase diagram. However, the LaFeAsO 1-xF x system does not show the
same behavior. Though the T c of lightly doped compound dose not increase largely with pressure,
highly doped one shows large enhancement of T c under high pressure. It is possible that the
magnetic ordering which is observed for the lightly doped and undoped compound is related to the
small enhancement of the T c for low doping compounds under high pressure. In SmFeAsO 1-xF x, it
is also thought that the magnetic ordering phase is related to the pressure effect on T c .
   Pressure effect for F doped LaFePO and Co doped CaFeAsF with the same crystal structure will
be presented.
                                                                        60
References                                                                    LaFeAsO1-xFx
                                                                                                       (4GPa) (6GPa)
[1] Y.Kamihara et al.: J.Am.Chem. Soc.                                             Tc at 1atm

                                                                        40
128(2006)10012.                                                                    Tc max at HP
                                                                T (K)




                                                                                        (3GPa)
[2] H.Takahashi et al.: Nature 453(2008)376.
                                                                   c




                                                                             (12GPa)
[3] H.Okada et al.: J.Phys.Soc.Jpn 77                                   20
(2008)113712.
                                                                                                superconducting
[4] B. Lorenz et al.: Phys. Rev. B78 (2008)                                                          state
012505.; Y.Takahayashi et al.: J. Am. Chem.                             0
                                                                              0        0.04    0.08     0.12
Soc. 130 (2008) 9242.
                                                                                         F- content (x)

* hiroki@chs.nihon-u.ac.jp                                Fig. 1 T c as a function of F- content (x) for
                                                          LaFeAsO1-xF x.