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

Abstract

We have performed electrical resistivity measurements on single crystal BaFe2As2 under high pressure P up to 16 GPa with a cubic anvil apparatus, and up to 3 GPa with a modified Bridgman anvil cell. The samples were obtained from the same batch, which was grown with a self-flux method. A cubic anvil apparatus provides highly hydrostatic pressure, and a modified Bridgman anvil cell, which contains liquid pressure transmitting medium, provides quasi hydrostatic pressure. For highly hydrostatic pressure, the crystal phase and magnetic transition temperature decreases robustly with P and disappears at around 10 GPa. The superconducting phase appears adjacent to magnetic phase in narrow pressure region between 11 and 14 GPa. The tiny difference of hydrostaticity between the cubic anvil apparatus and modified Bridgman anvil cell induces a drastic effect on the phase diagram of BaFe2As2. This result indicates that small uniaxial stress along c-axis strongly suppresses the structural/antiferromagnetic ordering and stabilizes superconductivity at much lower pressure. . The common feature of the new superconductors is existence of iron pnictide layers, which is analogous to CuO 2 planes of the cuprate superconductors. After extensive investigations, high temperature superconductivity with a transition temperature T c of 56 K was observed in rare earth iron oxypnictides (1-1-1-1 compound) [3,4]. Members of superconducting iron family were observed one after another in oxygen free compounds such as BaFe 2 As 2 having ThCr 2 As 2 type crystal structure (1-2-2 compound) [5,6], LiFeAs (1-1-1 compound) [7] and FeSe (1-1 compound) [8]. New compounds having a thick perovskite layer were also reported recently [9]. Among them, the 1-2-2 compound AEFe 2 As 2 (AE =Ba, Ca and Sr) occupies a singular position, which is particularly suitable for precise physical property measurements, since high quality large single crystals can be grown in the congruent melting condition. The 1-2-2 compounds have a first order antiferromagnetic AF transition on cooling at moderately high temperature, which always accompanied by a phase transition from tetragonal to orthorhombic structure. In the case of BaFe 2 As 2 , the crystal structure/magnetic phase transition occurs at 141 K [10]. The carrier doping of K for Ba, or Co for Fe strongly suppresses the transition in BaFe 2 As 2 . The highest T c of 38 K in the 1-2-2 compounds is obtained in a K doped material located adjacent to AF state [6].Another method to suppress AF ordering is applying pressure P . 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. The simplest reason is sample quality difference. Polycrystalline samples are inhomogeneous and have broader phase transitions than single crystals. Even in single crystals...

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“…2 Among 122-type materials, BaFe 2 As 2 has gained much attention as a parent compound because superconductivity can be induced in this material by various means, i.e., by applying pressure [3][4][5] and by substitutions at the Ba site (by K) 6 , at the Fe site (by Co, Ni, Ru, Rh, Pd) 2,7 and at the As site (by P). 8 An interesting result is that while electron doping by Co, Ni, Rh and Pd at the Fe site in BaFe 2 As 2 induces superconductivity, hole doping by Cr and Mn for Fe does not.…”

Pressure Suppression of Spin-Density-Wave Gap in the Optical Conductivity of SrFe2As2

“…[2][3][4][5][6][7][8][9][10][11][12][13] They exhibit SC with external pressure only, although many other families of Fe based superconductors require some form of chemical doping or deviation from stoichiometry to exhibit SC. 1 In particular, SrFe 2 As 2 (Sr122) and BaFe 2 As 2 (Ba122) show pressure-induced SC at relatively high temperatures of T c =34 K 5,7-9 and 28 K, 4,7,9,11 , respectively.…”

“…Namely, it has been shown that the pressure evolutions of the AFM and SC states strongly depend on the hydrostaticity. 8,11,12 Here, hydrostaticity refers to how isotropic the pressure acting on the sample is. The hydrostaticity in a high pressure experiment depends on the types of pressure cell and pressure transmitting medium used.…”

Pressure-decoupled magnetic and structural transitions of the parent compound of iron-based 122 superconductors BaFe 2 As 2

“…2 and 4). For comparison, the superconducting temperature (T C ) was taken from previous electrical resistivity measurements using hydrostatic medium similar to our experiments (28). Below 3 GPa, the magnetic transition occurs over a narrower temperature region, and the T* curve coincides with the T S by XRD and resistivity measurements as shown in Figs.…”

“…Similar to the chemical doping, applied pressure has been found to cause the same effects on tuning the SDW state to superconductivity (23)(24)(25)(26)(27)(28). Because AFM ordering is strongly coupled with structural distortion, further investigation under controlled pressure temperature (P-T) conditions using in situ probes sensitive to these transitions is needed to unravel the intrinsic interplay between structure and magnetism in the parent compound of iron-based superconductors.…”