Pressure Effect on the superconducting properties of LaO_{1-x}F_{x}FeAs(x=0.11) superconductor

Diamagnetic susceptibility measurements under high hydrostatic pressure (up to 1.03 GPa) were carried out on the newly discovered Fe-based superconductor LaO_{1-x}F_{x}FeAs(x=0.11). The transition temperature T_c, defined as the point at the maximum slope of superconducting transition, was enhanced almost linearly by hydrostatic pressure, yielding a dT_c/dP of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated. It is suggested that pressure plays an important role on pushing low T_c superconducting phase toward the main (optimal) superconducting phase.


Introduction
The newly discovered LaO 1-x F x FeAs superconductor with critical temperature T C of 26 K has stimulated extensive interest on the layered rare-earth metal oxypnictide family which is free of copper, aiming at pursuing higher T C or uncovering new evidence on its mechanism [1][2][3][4][5][6][7][8]. The fact that T C varies, via lattice site substitution, from 4K to 26 K especially the very recently reported 43K [8] further implies a possible rich superconducting phase diagram of this new layered superconductor family of ZrCuSiAs type structure. Besides element substitution, high pressure is often served as an effective parameter to raise T C by varying carrier concentration through compressing lattice. In addition, high pressure experiments, by changing the interaction between particles, often play very important role on providing new information on the paring interaction and testing proposed theory [9]. Here we report the pressure effect on the newly discovered LaO 1-x F x FeAs superconductor through magnetic susceptibility measurements. The transition temperature T C was defined as the point where maximum slope of superconducting transition achieves, i.e., the peak of differential diamagnetic susceptibility. T C was found to increase almost linearly upon hydrostatic pressure increasing, showing a dT C /dP value of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicate that the underlying superconducting state is complicated, at 1 least two superconducting transitions coexist. The lower T C is dramatically enhanced by pressure while the higher one shows little pressure dependence. The role of pressure is hence discussed based on its different effect on the existing superconducting transitions.

Experimental details
The F-doped LaO 1-x F x FeAs polycrystalline sample was synthesized by a two-step method. during the measurements through a commercial functional program. All the data reported here were corrected for the demagnetization factor [10].

Results and discussion:
The sample's powder XRD pattern shown in Fig.1a indicated a well indexed tetragonal structure (space group P4/mmn) with a = 4.030(1) Å and b = 8.706(2) Å except for weak impurities peaks of LaOF and FeAs 2 . A SEM image of our sample is presented in Fig. 1b, showing the layered structure feature with the grain size of about 10 micrometers. Fig.2 a is the temperature-dependence of resistivity. A sharp transition starts at 26.3K and zero resistivity is reached when temperature is lowered to 22.3K. The temperature-dependent diamagnetic susceptibility χ measured under 1 Oe after zero-field-cooling is shown in Fig.2 b.
The onset T C of 23K is consistent with the resistivity results, while the 100% magnetic shielding signal evidences bulk superconductivity. All these results indicate the good quality of our sample.
Differential diamagnetic susceptibility was focused to monitor any tiny change in order to investigate the pressure effect on the superconducting properties of this newly discovered superconductor. Shown in Fig. 3 is the differential diamagnetic susceptibility dχ/dT measured at 1 Oe under hydrostatic pressure. T C , defined as the peak of dχ/dT, showed obvious pressure dependence. With pressure increasing from ambient up to 1.03 GPa, Tc raised almost linearly from 20.3K to 21.5 K, yielding a dT C /dP value of about 1.2 K/GPa (the inset of Fig. 3). To our surprise, the onset T C showed no obvious change when pressure increased up to 1.03 GPa. In contrast, the superconducting transition, illustrated by the dχ/dT peak in Fig. 3, was narrowed with increasing pressure. In other words, it seems that pressure (below 1.03 GPa) will first improve the superconducting correlation network before interfering the main superconducting phase characterized by onset T C of 23 K.
In order to clarify the pressure effect on both T C and superconducting transition width, the pressure dependence of dχ/dT measured at 10 Oe was also investigated, as plotted in Fig.   4. Although the magnetic field of 10 Oe is rather low, the dχ/dT curves were largely broadened and extra features occurred, implying a complicated superconducting state. Two peaks are evident, the temperature of higher peak T PH is around 19.2K while the lower one  Fig.4, generates a nearly linear pressure dependence with a large slope, i.e., dT PL /dP, of about 4 K/GPa (inset of Fig.4). In other words, low T C phase is more sensitive to pressure, compared with the high T C phase, which is also the main (optimal) superconducting phase with onset T C of 23K as illustrated in Fig.2.
Clearly, the onset T C of optimal superconducting phase showed little pressure dependence, implying that a pressure up to 1.03 GPa made little effect, by either varying carrier concentration or adjusting inner-and intra-plane interaction via lattice compressing, on the structural frame that bears the optimal superconducting phase. However, notice the fact that the pressure-induced improvement of superconducting transition is obvious as presented above, we still expect the potential of higher T C triggered by pressure above 1.03 GPa. In addition, the fact that superconducting transition is very sensitive to applied field implies that pressure effect on such as lattice defect (especially in La 2 (O,F) 2 layer) and grain boundaries as well as other possible weak links might also be considerable.

Conclusion:
In summary, we have studied the pressure dependence of diamagnetic susceptibilities of the newly discovered Fe-based superconductor LaO 1-x F x FeAs(x=0.11). The transition temperature T C , manifested by the peak of dχ/dT, increased linearly upon hydrostatic pressure, yielding a dTc/dP value of about 1.2 K/GPa. Differential diamagnetic susceptibility curves indicated that the underlying superconducting state is complicated. Pressure was found to play a key role on pushing low T C superconducting phase toward the main (optimal) superconducting phase. Higher T C is expected via applying pressure larger than 1.03 GPa.