CsFe_4−δSe₄: A Compound Closely Related to Alkali-Intercalated FeSe Superconductors

Abstract

We report the synthesis and characterizations of a new FeSe-based compound CsFeSe, which is closely related to alkali intercalated FeSe superconductors while exhibits distinct features. It does not undergo phase separation and antiferromagnetic transition. Powder neutron diffractions, electron microscopy and high-angle annular-dark-field images confirm that CsFeSe possesses an ordered Cs arrangement as √2 × √2 superstructure, evidencing a B-centered orthorhombic lattice with a space group of Bmmm. The temperature-dependent powder neutron diffractions indicate no structural and magnetic transition from 320 to 5 K. In contrast to the symmetry-breaking in FeSe, this phase naturally possesses the orthorhombic symmetry even at room temperature. DFT calculations and transport measurements reveal a novel Fermi surface geometry with two electron-like sheets centered on Γ point and intermediate density of states at the Fermi level comparing with the value of FeSe and the superconducting A FeSe.

Unconventional Superconductivity Induced by Suppressing an Iron-Selenium-Based Mott Insulator CsFe4−xSe4

“…The samples are sensitive to air, and thus they are usually vacuum packed and put in the glove box. The synthesis process is similar to that of previous work [18]. The only difference is that in previous work, the quartz tube was sealed with Ar gas, while in our experiments, the quartz tube is sealed under evacuation.…”

“…The iron-selenium (FeSe) based superconductors, which are different from the iron-arsenic based superconductors, have attracted a lot of attention recently because of their intriguing properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The FeSe has the simplest structure, and the critical temperature (T c ) of the bulk samples [1] can be enhanced from about 8 K to 37 K by pressure [2][3][4].…”

“…The normal states of these systems show clear metallic behavior. Recently, a new FeSe based compound CsFe 4−x Se 4 was found [18]; it shows distinct features, although it has a roughly perfect FeSe layer. Unlike the system A x Fe 2−y Se 2 which has a clear phase separation, the CsFe 4−x Se 4 is uniform without the trace of phase separation.…”

“…At room temperature, it forms an orthorhombic lattice structure with a space group of Bmmm; however, bulk FeSe undergoes a structural transition from tetragonal to orthorhombic at around 90 K [2,3,19]. The density functional theory (DFT) calculations on CsFe 4−x Se 4 indicate that it should be a metal with an intermediate density of states (DOS) [18] at the Fermi level compared with the value of FeSe and A x Fe 2−y Se 2 , which is in sharp contrast with the experimentally observed insulatorlike behavior. The insulatorlike behavior in CsFe 4−x Se 4 remains puzzling and elusive.…”

Highly-Tunable Crystal Structure and Physical Properties in FeSe-Based Superconductors

“…Li et al proposed that the existence of Fe vacancies could locally destroy SC [62], and suggested that the K2Fe4Se5 layer may be indispensable to regulate superconductivity in KFe2Se2 by providing charge carriers [63]. More superstructures associated with different Fe-vacancy and alkali-metal-ordering were later revealed later [64][65][66][67]. Phase separation hinders the investigation of the intrinsic property of FeSe-based 122-type superconductors.…”