Suppression of Magnetic Order by Pressure in BaFe₂As₂

Abstract

1)has accelerated further investigations of the related superconductors. [2][3][4] The common feature of these compounds is the possession of an FeAs layer, which is analogous to the CuO 2 plane in high-superconducting-transition-temperature (high-T c ) cuprates. Soon after the intensive investigations of the oxypnictides, the oxygen-free iron pnictide BaFe 2 As 2 5)was proposed as the next-candidate parent material of superconductors with high T c . The crystal structure of this pnictide is the ThCr 2 Si 2 -type structure and possesses a similar FeAs layer to that realized in LaFeAsO. Moreover, this material exhibits spin density wave (SDW) anomaly at T SDW ¼ 140 K (BaFe 2 As 2 ). 5,6) It is important to note that the compound exhibits structural phase transition from tetragonal (I4=mmm) to orthorhombic (Fmmm) as well as SDW anomaly. 5,6) The most striking feature of this compound is that the SDW anomaly disappears and superconductivity is induced by hole doping, for example, K substitution for Ba. 7) In addition, the pressure P-induced superconductivity in BaFe 2 As 2 has recently been reported by the magnetic susceptibility measurements at pressures of up to about 6 GPa. 8) Although it should be revealed by additional experimental approaches whether this superconductivity is intrinsic, it is clear that the investigation of this compound under pressure is important. To understand the relationship between the SDW instability and expected superconductivity in BaFe 2 As 2 , we performed resistivity and zero-externalfield (ZF)75 As nuclear-magnetic-resonance (NMR) measurements of BaFe 2 As 2 under high pressure.In our previous letter, we reported the 75 As-NMR spectra of BaFe 2 As 2 at various temperatures and the temperature T dependence of the spin-lattice relaxation rate 1=T 1 of 75 As under ambient pressure.9) 75 As-NMR spectra clearly revealed that magnetic transition occurs at around 131 K in our samples, which corresponds to the emergence of spin density wave. The T dependence of the internal magnetic field H int suggested that the transition is likely of the first order. We also deduced from the difference in linewidth between the highly and partially oriented powders that the H int at the As site is parallel to the c-axis. The steep decrease in 1=T 1 below T SDW was observed to be due to the gap formation of the SDW at a certain part of the Fermi surface. 1=T 1 below approximately 100 K was nearly proportional to T, which indicates the relaxation due to the remaining conduction electron at the Fermi level even below T SDW . In our previous paper, we also reported a ZF 75 As-NMR spectrum at 1.5 K.9,10) The ZF 75 As-NMR spectrum clearly revealed that the magnetically ordered state of this compound is commensurate. Furthermore, the center line of the ZF 75 As-NMR spectrum is a good measure of the H int at the As site.Polycrystalline BaFe 2 As 2 samples were synthesized by a high-temperature and high-pressure method. They were confirmed to be nearly single phase by X-ray diffraction analysis. Electrical resis...

Appearance of pressure-induced superconductivity inBaFe2As2under hydrostatic conditions and its extremely high sensitivity to uniaxial stress

“…The change of the transition is robust against pressure, diminishing gradually and disappearing above 10 GPa. Our present result below 8 GPa is similar to that obtained in a single crystal with a cubic anvil apparatus by Matsubayashi et al [15], and is also consistent with that previously obtained in polycrystalline samples [12]. An expanded view of resistivity above 10 GPa at low temperatures is shown in Fig.…”

“…Duncan et al have studied the difference of hydrostaticity in BaFe 2 As 2 using a piston cylinder cell, an alumina anvil cell with Daphne oil and a Bridgman anvil cell with solid steatite as pressure transmitting medium [21]. They reported that uniaxial stress strongly suppresses structural/AF ordering and induces superconductivity at lower pressure.We have studied difference of hydrostaticity in very highly hydrostatic pressure range since the confusing results appear in this region [11,12,[15][16][17]21]. We performed resistivity measurements using self-flux grown single crystals of BaFe 2 As 2 from the same batch up to 16 GPa with a cubic anvil apparatus and compared the results with those obtained with a Bridgman anvil cell containing liquid pressure transmitting medium.…”

“…This has a great advantage over chemical doping since the ground state of the system can be tuned without introducing any disorder. Hence, many high pressure measurements have been performed in iron pnictides [11][12][13][14][15][16][17][18][19][20][21]. However, contradicting results were reported in the 1-2-2 compound up to now.…”

“…We have studied difference of hydrostaticity in very highly hydrostatic pressure range since the confusing results appear in this region [11,12,[15][16][17]21]. We performed resistivity measurements using self-flux grown single crystals of BaFe 2 As 2 from the same batch up to 16 GPa with a cubic anvil apparatus and compared the results with those obtained with a Bridgman anvil cell containing liquid pressure transmitting medium.…”

Pressure-induced antiferromagnetic bulk superconductor EuFe2As2

“…There were some reports that the 122 parent compounds AFe 2 As 2 (A = Ca, Sr, Ba, and Eu) could be tuned to SC by the application of high pressure P [2,3,4,5,6,7,8,9]. However, most of these reports were based only on resistivity measurements and hence could not establish the bulk nature of P-induced SC.…”