Session JJ02: V: Doped FeSe Superconductors and Nickelate

Interface superconductivity in FeSe thin films on SrTiO3 grown by the PLD technique

Iron chalcogenide superconductor, FeSe, has attracted much attention because monolayer films on SrTiO₃ (STO) grown by molecular beam epitaxy (MBE) exhibit interface superconductivity with significantly enhanced superconducting transition temperature (T_c) from 9 K to 40-65 K. Further T_c enhancement is expected to be achieved through fabricating various heterostructures with other oxide materials. In this context, Pulsed laser deposition (PLD) is a promising growth technique rather than MBE.

Here, we report the successful realization of interface superconductivity in FeSe/STO using PLD. We found that using atomically flat STO substrates is crucial in realizing interface superconductivity in FeSe/STO by PLD. All films showed superconductivity with onset T_c much higher than that of bulk FeSe, which cannot be explained by the strain effects of the same material established so far. Furthermore, the grown films showed large anisotropy of the upper critical fields with H_c2^H//ab/H_c2^H//c larger than 11. These values are higher than those of intercalated FeSe compounds which show large anisotropy induced by the expansion of distance between FeSe layers. These results indicate that superconductivity in the PLD-grown FeSe/STO is confined at the interface between FeSe and STO.   

Quantum and topological phases in iron chalcogenides

Iron chalcogenide FeTexSe1-x offers a remarkable playground in case studying correlated physics and macroscopic quantum states. However, due to its rich physics and multi-orbital nature, it is challenging to disentangle the intricate interrelation between different phases and figure out the main controlling factors of its superconductivity and normal state. In this talk, we present examples that go beyond the traditional chemical substitution experimental method and tackle the problem from several new angles: 1. we demonstrate the compositional control for potential topological superconductivity by combining microscale spatial-resolved spectroscopy and transport measurements, based on which we construct detailed quantum and topological phase diagrams over 2,000 data sets [1]; 2. we apply ionic liquid gating to simplify the complex effect of chemical substitution and pin down the power-law index of the recently discovered scaling relationship [2] between the slope of T-linear resistivity and superconducting transition temperature; 3. by altering film thicknesses and surface conditions, we find evidence for interconnected quantum percolation in the normal and superconducting states.   

Orbital selective Mott phase and spin nematicity in Ni-substituted FeTe0.65Se0.35 single crystals

The normal state in iron chalcogenides is metallic but highly unusual, with orbital and spin degrees of freedom partially itinerant or localized depending on temperature, a consequence of multi-orbital character and Hund's interaction. The unusual phenomena include, between others, the orbital-selective Mott phase (OSMP), documented by angular resolved photoemission (ARPES) [1], and C4 symmetry breaking of in-plane angular magnetoresistance (AMR) [2].

We study these phenomena using magnetization and magnetotransport measurements (resistivity, Hall effect, AMR) in Fe_1-yNi_yTe_0.65Se_0.35 single crystals (0xy orbital. We show that electron-dominated transport, observed at low T at large y, is replaced by hole-dominated transport at T > 180 K, suggesting direct link with the appearance of the d_z2 hole pockets at X points of the Brillouin zone in the OSMP phase, as recently reported [1]. The AMR shows the C₄ rotational symmetry of in-plane magnetocrystalline anisotropy at small y, replaced by C₂ symmetry at intermediate y, indicating development of spin nematicity induced by Ni doping, likely contributing to the suppression of superconductivity.

[1] J. Huang, et al., Commun. Phys. 5, 29 (2022).

Synthesis and physical properties of infinite-layer nickelate crystals

Infinite-layer (IL) nickelates are an emerging family of superconductors whose synthesis in thin film form is an established process by now, whereas the growth of their bulk counterparts remains a formidable challenge. In a previous study, we achieved the reduction of perovskite La_1-xCa_xNiO₃ single-crystals grown by a flux method under high external pressure to the IL phase La_1-xCa_xNiO₂ [1]. The typical lateral dimension of these crystals was 150 μm. As an advanced approach, we recently accomplished the reduction of millimeter-sized LaNiO₃ crystals obtained by optical floating zone growth under high oxygen gas pressure to the IL phase LaNiO₂ [2]. We will present our characterization of the crystalline, magnetic, and electronic properties of the LaNiO₂ crystals. Furthermore, we will provide an outlook on the realization of millimeter-sized PrNiO₂ crystals, as well as crystals with hole- and electron doping.

    

Phase manipulation and Majorana zero modes in iron-based superconductor

Recent experiments reported the evidence of Majorana mode trapped by vortex, crystalline domain wall and magnetic impurity in some iron-based superconductors. For the latter two cases, the underlying physics is not settled down, and the debate is still present. We argue that the phase of superconducting pairing can be manipulated by the ferromagnetism spontaneously formed in the crystalline domain wall and the natively arising in the magnetic impurity. Such phase manipulation plays a crucial role to induce the Majorana mode. Following this line, we show that our theory can be used to understand the experimental signatures about the Majornana mode induced by crystalline domain wall in Fe(Te,Se) and by magnetic impurity in Fe(Te,Se), LiFeAs, monolayer Fe(Te,Se)/SrTiO3 and other iron-based SC platform, and can explain the controversial observations and confusing features of the zero modes from different experiments. We also discuss the some interesting features of the Majorana mode beyond the current experiments and beyond the conventional understandings.