Incommensurate smectic phase in close proximity to the high-Tc superconductor FeSe/SrTiO3

Electronic stripe patterns near the fermi level of tetragonal Fe(Se,S)

FeSe1-xSx remains one of the most enigmatic systems of Fe-based superconductors. While much is known about the orthorhombic parent compound, FeSe, the tetragonal samples, FeSe1-xSx with x > 0.17, remain relatively unexplored. Here, we provide an in-depth investigation of the electronic states of tetragonal FeSe0.81S0.19, using scanning tunneling microscopy and spectroscopy (STM/S) measurements, supported by angle-resolved photoemission spectroscopy (ARPES) and theoretical modeling. We analyze modulations of the local density of states (LDOS) near and away from Fe vacancy defects separately and identify quasiparticle interference (QPI) signals originating from multiple regions of the Brillouin zone, including the bands at the zone corners. We also observe that QPI signals coexist with a much stronger LDOS modulation for states near the Fermi level whose period is independent of energy. Our measurements further reveal that this strong pattern appears in the STS measurements as short range stripe patterns that are locally two-fold symmetric. Since these stripe patterns coexist with four-fold symmetric QPI around Fe-vacancies, the origin of their local two-fold symmetry must be distinct from that of nematic states in orthorhombic samples. We explore several aspects related to the stripes, such as the role of S and Fe-vacancy defects, and whether they can be explained by QPI. We consider the possibility that the observed stripe patterns may represent incipient charge order correlations, similar to those observed in the cuprates.

Spin-Resolved Imaging of Antiferromagnetic Order in Fe4 Se5 Ultrathin Films on SrTiO3

Unraveling the magnetic order in iron chalcogenides and pnictides at atomic scale is pivotal for understanding their unconventional superconducting pairing mechanism, but is experimentally challenging. Here, by utilizing spin-polarized scanning tunneling microscopy, real-space spin contrasts are successfully resolved to exhibit atomically unidirectional stripes in Fe₄ Se₅ ultrathin films, the plausible closely related compound of bulk FeSe with ordered Fe-vacancies, which are grown by molecular beam epitaxy. As is substantiated by the first-principles electronic structure calculations, the spin contrast originates from a pair-checkerboard antiferromagnetic ground state with in-plane magnetization, which is modulated by a spin-lattice coupling. These measurements further identify three types of nanoscale antiferromagnetic domains with distinguishable spin contrasts, which are subject to thermal fluctuations into short-ranged patches at elevated temperatures. This work provides promising opportunities in understanding the emergent magnetic order and the electronic phase diagram for FeSe-derived superconductors.

Suppression and Revival of Superconducting Phase Coherence in Monolayer FeSe/SrTiO3

Monolayer FeSe grown on SrTiO₃ (FeSe/STO) is an interfacial high-temperature superconductor distinctively different from bulk FeSe. However, the superconducting phase coherence of the interface is challenging to probe due to its fragility in the atmosphere. Here, we perform in situ mutual inductance under ultrahigh vacuum on FeSe/STO in combination with band mapping by angle-resolved photoemission spectroscopy. We find that even though the monolayer shows a gap-closing temperature above 50 K, no diamagnetism is visible down to 5 K. This is the case for few-layer FeSe/STO until it exceeds a critical number of five layers, where diamagnetism suddenly appears. The suppression of diamagnetism in the monolayer is also lifted by depositing a top FeTe layer. However, T_c and superfluid density both decrease with thicker FeTe, suggesting unconventional electron pairing and phase coherence competition. Our observation may be understood by a scenario in which the interfacial superconducting phase coherence is highly anisotropic.

Mechanism of exotic density-wave and beyond-Migdal unconventional superconductivity in kagome metal AV3Sb5 (A = K, Rb, Cs)

Exotic quantum phase transitions in metals, such as the electronic nematic state, have been discovered one after another and found to be universal now. The emergence of unconventional density-wave (DW) order in frustrated kagome metal AV3Sb5 and its interplay with exotic superconductivity attract increasing attention. We find that the DW in kagome metal is the bond order, because the sizable intersite attraction is caused by the quantum interference among paramagnons. This mechanism is important in kagome metals because the geometrical frustration prohibits the freezing of paramagnons. In addition, we uncover that moderate bond-order fluctuations mediate sizable pairing glue, and this mechanism gives rise to both singlet s-wave and triplet p-wave superconductivity. Furthermore, characteristic pressure-induced phase transitions in CsV3Cb5 are naturally understood by the present theory. Thus, both the exotic density wave and the superconductivity in geometrically frustrated kagome metals are explained by the quantum interference mechanism.

Research Progress of FeSe-based Superconductors Containing Ammonia/Organic Molecules Intercalation

As an important part of Fe-based superconductors, FeSe-based superconductors have become a hot field in condensed matter physics. The exploration and preparation of such superconducting materials form the basis of studying their physical properties. With the help of various alkali/alkaline-earth/rare-earth metals, different kinds of ammonia/organic molecules have been intercalated into the FeSe layer to form a large number of FeSe-based superconductors with diverse structures and different layer spacing. Metal cations can effectively provide carriers to the superconducting FeSe layer, thus significantly increasing the superconducting transition temperature. The orientation of organic molecules often plays an important role in structural modification and can be used to fine-tune superconductivity. This review introduces the crystal structures and superconducting properties of several typical FeSe-based superconductors containing ammonia/organic molecules intercalation discovered in recent years, and the effects of FeSe layer spacing and superconducting transition temperature are briefly summarized.

Visualizing Spatial Evolution of Electron-Correlated Interface in Two-Dimensional Heterostructures

Microscopically visualizing the evolution of electronic structures at the interface between two electron-correlated domains shows fundamental importance in both material science and physics. Here, we report scanning tunneling microscopy and spectroscopy studies of the interfacial electronic structures evolution in a phase-engineered monolayer NbSe₂ heterostructure. The H-NbSe₂ metallic state penetrates the Mott insulating T-NbSe₂ at the H/T phase interface, with a prominent 2D charge density wave (CDW) proximity effect. Moreover, an insulating Mott gap collapse with the disappearance of the upper Hubbard band is detected at the electronic phase transition region. Theoretical calculations reveal that such insulating Mott gap collapse can be attributed to the electron doping effect induced by the interface. Our findings promote a microscopical understanding of the interactions between different electron-correlated systems and provide an effective method for controlling the Mott insulating states with phase engineering.

Using Genetic Marginal Effects to Study Gene-Environment Interactions with GWAS Data

Gene-environment interactions (GxE) play a central role in the theoretical relationship between genetic factors and complex traits. While genome wide GxE studies of human behaviors remain underutilized, in part due to methodological limitations, existing GxE research in model organisms emphasizes the importance of interpreting genetic associations within environmental contexts. In this paper, we present a framework for conducting an analysis of GxE using raw data from genome wide association studies (GWAS) and applying the techniques to analyze gene-by-age interactions for alcohol use frequency. To illustrate the effectiveness of this procedure, we calculate genetic marginal effects from a GxE GWAS analysis for an ordinal measure of alcohol use frequency from the UK Biobank dataset, treating the respondent's age as the continuous moderating environment. The genetic marginal effects clarify the interpretation of the GxE associations and provide a direct and clear understanding of how the genetic associations vary across age (the environment). To highlight the advantages of our proposed methods for presenting GxE GWAS results, we compare the interpretation of marginal genetic effects with an interpretation that focuses narrowly on the significance of the interaction coefficients. The results imply that the genetic associations with alcohol use frequency vary considerably across ages, a conclusion that may not be obvious from the raw regression or interaction coefficients. GxE GWAS is less powerful than the standard "main effect" GWAS approach, and therefore require larger samples to detect significant moderated associations. Fortunately, the necessary sample sizes for a successful application of GxE GWAS can rely on the existing and on-going development of consortia and large-scale population-based studies.