High-Pressure Single-Crystal Neutron Scattering Study of Magnetic and Fe Vacancy Orders in (Tl,Rb) ₂ Fe ₄ Se ₅ Superconductor

Abstract

The magnetic and iron vacancy orders in superconducting (Tl,Rb)2Fe4Se5 single-crystals were investigated using a high-pressure neutron diffraction technique. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous disappearance at the critical pressure Pc =8.3 GPa. Combined with previously determined Pc for superconductivity, our phase diagram under pressure reveals the concurrence of the block AFM order, the √ 5 × √ 5 iron vacancy order and superconductivity for the 245 superconductor. A synthesis of current experimental data in a coherent physical picture is attempted.PACS numbers: 74.62. Fj, 25.40.Dn, 74.25.Ha, The recently discovered metal-intercalated iron selenide superconductors A 2 Fe 4 Se 5 (A=K, Cs, Tl-K, Rb, Tl-Rb) (245) compounds, with T c ∼ 30 K, have attracted much interest [1,2].A high transitiontemperature (T N ≈ 470-560 K) and large magnetic moment (3.3µ B /Fe) block antiferromagnetic (AFM) order exists in the superconducting samples [3][4][5]. And magnetic order-parameter experiences an anomaly when T c is approached [4,5]. The superconductors crystallize with a highly ordered √ 5 × √ 5 superstructure, in which the Fe1 site of the I4/m structure is only a few percent occupied and the Fe2 site fully occupied [4,6]. The nonsuperconducting samples at low-T also crystallize in the I4/m structure, but both Fe sites are fractionally occupied [7,8], since the numbers of the Fe vacancies in the samples and the vacant sites in the √ 5 × √ 5 pattern are mismatched. The partially ordered √ 5 × √ 5 vacancy order becomes one of three competing phases for temperature below the room temperature up to ∼ 500 K, namely, these samples are phase-separated and in the miscibility gap at ambient condition [8,9].Close to the miscibility gap, it is not surprising that the nonstoichiometric 245 superconductors often contain several phases of different space-group symmetry. It has been a complex and controversial issue to determine the sample composition of the superconductors. The KFe 1.5 Se 2 (234) of the orthorhombic Fe vacancy order has been proposed as the parent compound [10]. However, this phase is not even the ground state for KFe 1.5 Se 2 , and a partially ordered √ 5 × √ 5 vacancy superlattice is more stable at low temperature [8]. The KFe 2 Se 2 (122) of I4/mmm symmetry has also been proposed as the superconducting phase [11]. But its existence in films grown by molecular beam epitaxy method likely requires charge transfer with the substrate, and there is no trace of its existence in bulk superconducting samples [4,12]. Detected in the 245 superconductors is the alkaline metal deficient A x Fe 2 Se 2 (x ∼ 0.3-0.6) phase embedded in √ 5 × √ 5 iron vacancy ordered superstructure [12][13][14], forming various microstructure patterns in plane [15,16] High pressure adds an additional dimension to the complex composition phase-diagram of 245 superconductors [8], offering a "clea...

Superconductivity in pressurized Rb0.8Fe2−ySe2−xTex

“…Upon increasing pressure, the hump is suppressed significantly, same as that seen in the pressurized K-245 and Tl(Rb)-245 superconductors [30,32]. At pressure ∼8.4 GPa, we found that the resistance hump becomes almost featureless, which is signed the destruction of the long-ranged AFM order [34]. Zooming in the plot of R-T curve in the low temperature range, the pressureinduced decrease in Tc is shown more clearly ( figure 1(b)).…”

“…Our results reveal that the Tc of the SC-I phase in the A-245 superconductors is very sensitive to the local lattice distortion from Te doping. It is noteworthy that neither the SC-I nor SC-II phases is observed in the sample of x = 0.4, at the doping level of which the long-ranged AFM order at ambient pressure is fully suppressed [30,34,36,37] (upper right panel of figure 4). No observable sign exists of the SC-II phase, even when the paramagnetic semiconducting sample (x = 0.4) is pressurized into a metallic state (figures 3(d) and (e) and 4).…”

“…Neutron diffraction studies on A-245 superconductors have confirmed that the AFM long-ranged order is associated with the superlattice structure [34]. To clarify the role of the superlattice in stabilizing the superconductivity of the A-245 superconductors, we performed high-pressure synchrotron x-ray diffraction measurements at SSRF for Rb 0.8 Fe 2−y Se 2−x Te x (x = 0, 0.19, 0.28 and 0.4) samples.…”

“…Previous studies found that applying external pressure on the A 0.8 Fe 1.7 Se 2 (A = K, Rb and Cs) superconductors can fully suppress the superconductivity of the ambient-pressure superconducting phase (SC-I) [30][31][32][33] and induce a new superconducting phase (SC-II) at higher pressure [33]. Experimental evidences have exhibited that the pressure-induced SC-II phase is probably driven by a quantum critical phase transition in which the AFM phase in the pressurized sample undergoes a transition from an AFM state to a paramagnetic (PM) state [30,34]. These results suggest that the superconductivity of the SC-II phase may be closely related to the AFM fluctuations [35,36].…”

Robust antiferromagnetism preventing superconductivity in pressurized (Ba0.61K0.39)Mn2Bi2

“…Comparing with the evolution of AFM order with pressure in Fe-based pnictides4344 and chalcogenides4546 whose AFM order can be destroyed at modest pressure below ~10 GPa, the AFM order in the K-doped BaMn 2 Bi 2 exhibits insensitive to pressure. These results demonstrates that the Mn's d 5 electrons in the sample investigated are much more localized than Fe's d 6 electrons in the Fe-based pnictides and Fe-based chalcogenides, which leads to the K-doped BaMn 2 Bi 2 belongs to a strong Hund's AFM metal with a hybridization between localized spin electrons and itinerant electrons.…”