Temperature Dependence of $\sqrt{2} \times \sqrt{2}$ Phase in Superconducting K0.8Fe1.6Se2 Single Crystal

Abstract

We report the structural phase transition properties of the newly discovered K 0.8 Fe 1.6 Se 2 superconductor (T c = 31.8 K) using synchrotron single crystal x-ray diffraction. The basic structure of the sample at room temperature is found to be tetragonal ThCr 2 Si 2 -type, modulated by a vacancy ordering induced superlattice structure together with a coexisting minority phase having a 2 2 × ordering. At 520 K, the reflections corresponding to the 2 2 × phase merge with the parent tetragonal phase. The super-lattice peaks corresponding to the vacancy ordering disappears at 580 K, indicating an order-disorder phase transition at this temperature.

Functional Nanoscale Phase Separation and Intertwined Order in Quantum Complex Materials

“…NPS shows that intertwined and interlocked nanoscale lattice structures form heterostructures at the atomic limit [13]. This induces the emergence of novel functionalities, such as high-temperature superconductivity, in complex quantum materials as organics [13][14][15][16][17][18][19][20], doped perovskites [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39], iron-based superconductors [40][41][42][43][44], chargedensity-wave materials [45][46][47][48][49][50][51][52], manganites showing colossal magnetoresistance [53][54][55][56][57], doped diborides [58][59]…”

“…In fact, looking at the nanoscale, we have observed different nanoregions with different lattice structures, showing different electronic and magnetic ordered phases, with different topologies that influence the Fermi surfaces, the pseudogap energy, and even the superconducting critical temperature. We would like to underline archetypal cases of intrinsic phase separation in A x Fe 2−y Se 2 , which are originated by the coexistence of an insulating magnetic phase and a paramagnetic metallic phase with an in-plane compressed and expanded lattice [40][41][42][43][44]. A second clear case of phase separation has been found in YBa 2 Cu 3 O 6+y , with y < 0.5, where superconductivity (y > 0.33) percolates in a network of oxygen-ordered nanoregions (y = 0.5), interspersed with oxygen-depleted domains (y = 0) [37].…”

Nanoscale Phase Separation of Incommensurate and Quasi-Commensurate Spin Stripes in Low Temperature Spin Glass of La2−xSrxNiO4

“…The layered La 2−x Sr x NiO 4+y nickelate is an archetypal doped Mott insulator exhibiting the universal features of the inhomogeneous spatial distribution of spin stripes phases in complex quantum materials [36][37][38], as lightly doped La 2−x Sr x CuO 4 [39,40], and photoinduced stripes in La 2 CuO 4+y [41]. The nanoscale phase separation [42,43] can be generated by oxygen interstitial ordering [24,25,[44][45][46][47], CDW [24,25] strain driven superstripes in cuprates [21,22,[31][32][33], in iron-based superconductors [48,49], and diborides [50][51][52][53].…”

Understanding Doping, Vacancy, Lattice Stability, and Superconductivity in KxFe2−ySe2

“…The results in ref . revealed that the obtained SC phases differ only in K contents, K 0.3 Fe 2 Se 2 with a T c = 44 K and K 0.6 Fe 2 Se 2 with a T c = 30 K. Superconductivity with T c of 30 K and 43 K was also observed in K x Fe 2− y Se 2 samples with √2 × √2 × 1 superstructure due to K vacancy ordering, but the phases for different T c have not been specified . In addition, K‐intercalated iron selenide with excess Fe was also proposed to have T c of 44 K …”