Bulletin of the American Physical Society
2024 APS March Meeting
Monday–Friday, March 4–8, 2024; Minneapolis & Virtual
Session S01: FeSe: S Substitution and Spectroscopic ProbesFocus

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Sponsoring Units: DMP Chair: Joshua Ballard, Zyvex Labs Room: L100A 
Thursday, March 7, 2024 8:00AM  8:36AM 
S01.00001: Unconventional Signatures of a Quantum Griffiths Phase in the Vicinity of a Quenched Nematic Quantum Critical Point Invited Speaker: Pascal Reiss Upon suppression of a continuous electronic phase transition towards 0K, for example through hydrostatic pressure as an external tuning parameter, a quantum critical point (QCP) occurs in many different material classes. These points constitute a fascinating research area as the diverging quantum fluctuations and electronic correlations could be linked to many unconventional and exciting quantum phases, including the emergence of highT_{c} superconductivity. Nevertheless, even more exotic phase may be expected if the symmetry of the electronic order parameter allows for a bilinear coupling to the structural degrees of freedom. The electronic nematic order, ubiquitous in the ironbased superconductors, is one such candidate, where longrange interactions can be mediated through the lattices shear modes. 
Thursday, March 7, 2024 8:36AM  8:48AM 
S01.00002: Tuning nematic electronic phases, electronic correlations and shifting the dxy hole band via isoelectronic substitution in FeSe_{1x}S_{x} and FeSe_{1x}Te_{x} Amalia I Coldea Electronic nematic phase of ironchalcogenides superconductors can be finely tuned via isoelectronic substitution and their electronic behaviour can be explored in detail via angledependent photoemission spectroscopy and quantum oscillations [1,2]. In this talk, I will compare angleresolved photoemission studies probing the nematic electronic phase of FeSe1xSx versus FeSe_{1x}Te_{x} [3,4,5]. I will discuss the evolution of the electronic bands, the quasiparticle effective masses as well as the sensitivity of the d_{xy} hole band to the chalcogen height. We find that the dxy hole band shifts significantly with increasing the chalcogen height and it could be involved in promoting an additional pairing channel and enhance the density of states to stabilize the second superconducting dome in FeSe_{1−x}Te_{x} systems [5]. This is in contrast to FeSe_{1x}S_{x} series, where the d_{xy} band does not shift and there is no enhancement in superconductivity outside the nematic phase, despite both series displaying large nematic susceptibility at the nematic end point. 
Thursday, March 7, 2024 8:48AM  9:00AM 
S01.00003: Nonmeanfieldlike gap formation in the ultranodal pairing state of tetragonal FeSe_{1x}S_{x} Shigeru Kasahara, Kazuto Akiba, Yuya Kitanishi, Kaoru Tanaka, Hideaki Fujii, Tatsuo C Kobayashi, Takumi Kihara The isovalently substituted FeSe_{1x}S_{x} superconductors have attracted significant interest due to various exotic properties associated with the intertwining of nematicity, magnetism, and unconventional superconductivity [1]. Of particular interest is the abrupt change in the superconducting gap function that occurs at the nematic critical point at x_{c} ~ 0.17 [26], above which the formation of an ultranodal pairing state with a putative Bogolubov Fermi surface has been proposed [510]. Although the emergence of the Bogoluvov Fermi surface appears to reasonably explain the huge residual density of states observed in the tetragonal FeSe_{1x}S_{x}, the nature of the exotic pairing state remains largely elusive, including the links between the ultranodal state and the colossal fluctuationlike behavior reported in the previous experiments [11]. Here, using the highquality single crystals of FeSe_{1x}S_{x} and the precise measurements of thermodynamic and charge transport properties, we discuss the gap formation of the ultranodal pairing state, which deviates from the meanfieldlike behavior. In addition, we report the evolution of the superconducting gap in FeSe_{1x}S_{x} under high pressures, where striking enhancement of T_{c} is found in the nonmagnetic tetragonal regime [12]. The results provide fresh insights into our understanding of the ultranodal pairing state. 
Thursday, March 7, 2024 9:00AM  9:12AM 
S01.00004: Unconventional Superconductivity near a Nematic Instability in a MultiOrbital system Kazi Ranjibul Islam, Andrey V Chubukov We analyze superconductivity in a multiorbital fermionic system near the onset of a nematic order, using doped FeSe as an example. We 
Thursday, March 7, 2024 9:12AM  9:24AM 
S01.00005: Twofold rotational symmetric Bogoliubov Fermi surface in tetragonal Fe(Se,S Yifu Cao, Chandan Setty, Laura Fanfarillo, Andreas Kreisel, Peter J Hirschfeld The intriguing iron based superconductor FeSe exhibits rich phenomena at low temperatures upon chemical pressure, including time reversal symmetry breaking, nematic criticality and topological phases. For sufficiently large Ssubstitution, the system recovers C4 symmetry in its normal state, and at the same time acquires a nonzero density of states at zero energy at zero temperature in its superconducting state, consistent with the existence of a socalled Bogoliubov Fermi surface (BFS). Recent angleresolved photoemission (ARPES) experiments has found more direct evidence for BFSs in such materials, but despite the tetragonal normal state, the BFSs seems to be curiously only twofold symmetric. In this work we search for a microscopic model that can support the coexistence of singlet pairing with other orders, including interband nonunitary triplet pairing with magnetization, and discuss several candidates that indeed stabilize ground states with BFSs. We show that with proper choice of the coupling strength of the various orders in our model, spontaneous breaking of C4 rotational symmetry is realized at low temperatures. This feature resembles the findings of recent ARPES experiments. 
Thursday, March 7, 2024 9:24AM  9:36AM 
S01.00006: Decoding Chemical Inhomogeneity in Iron Chalcogenides: Insight from SelfOrganizing Map Analysis of STM/S Data Pedram Tavadze, Qiang Zou, Basu D Oli, Joseph A Benigno, Lian Li Chemical pressure from the isovalent substitution in the epitaxial film iron chalcogenide can effectively tune their properties. However, such substitution during epitaxial growth inherently leads to chemical inhomogeneity, making the determination of alloy composition and substitutional sites challenging. To address this issue, we have employed twostep machine learning solution using Kmean and SVD [1,2] to analyze scanning tunneling microscopy/spectroscopy (STM/S) data. In this study, we introduce a different approach using selforganizing map (SOM), a type of artificial neural network, to discern the Se/S ratio in superconducting single layer FeSe_{1x}S_{x} alloys. Similar to previous methodologies, this unsupervised competitive learning method can determine the Se/S ratio effectively. However, this SOMbased approach offers an improved approach to interpret nonlinear STM data, while eliminating the need to prespecify the number of clusters. 
Thursday, March 7, 2024 9:36AM  9:48AM 
S01.00007: Visualizing superconductivity mediated by nematic fluctuations in the Febased superconductor FeSe_{1x}S_{x}: Part 1 Kirsty Scott, Pranab Kumar Nag, Vanuildo S. de Carvahlo, Journey K Byland, Xinze Yang, Morgan Walker, Aaron G Greenberg, Peter Klavins, Eduardo Miranda, Adrian Gozar, Valentin Taufour, Rafael M Fernandes, Eduardo H Da Silva Neto The FeSe_{1x}S_{x }system presents an ideal platform for studies of intertwined nematicity and superconductivity, a topic of ongoing interest in Febased superconductors, due to the lack of magnetic ground state at ambient pressure. In the parent compound FeSe, the superconductivity, mediated by spin fluctuations, coexists with a nematic phase. Ssubstitution (x) in FeSe_{1x}S_{x} suppresses the nematic phase at the quantum critical point (QCP), x_{c}~0.17, where nematic fluctuations become largest. The pairing mechanism in the x>x_{c} regime is undetermined, presenting an ideal test for theoreticallypredicted superconductivity mediated by nematic fluctuations. 
Thursday, March 7, 2024 9:48AM  10:00AM 
S01.00008: Visualizing superconductivity mediated by nematic fluctuations in the Febased superconductor FeSe_{1x}S_{x}: Part 2 Pranab Kumar Nag, Kirsty Scott, Vanuildo S. de Carvalho, Journey K Byland, Xinze Yang, Morgan Walker, Aaron G Greenberg, Peter Klavins, Eduardo Miranda, Adrian Gozar, Valentin Taufour, Rafael M Fernandes, Eduardo H Da Silva Neto The FeSe_{1x}S_{x} system presents an ideal platform for studies of intertwined nematicity and superconductivity (SC), a topic of ongoing interest in Febased superconductors, due to its lack of magnetic ground state at ambient pressure. In the parent compound FeSe, the SC, mediated by spin fluctuations, coexists with a nematic phase. Ssubstitution (x) in FeSe_{1x}S_{x} suppresses the nematic phase at the quantum critical point (QCP), x_{c}~0.17, where nematic fluctuations become largest. The pairing mechanism in the x>x_{c} regime is undetermined, presenting an ideal test for theoreticallypredicted SC mediated by nematic fluctuations. 
Thursday, March 7, 2024 10:00AM  10:12AM 
S01.00009: Qualitative Analysis of Quasiparticle Interference Using Feynman Parametrization Xinze Yang, Alexander F Kemper, Adrian Gozar, Eduardo H Da Silva Neto Quasiparticle interference (QPI) is observed in Scanning Tunneling Spectroscopy (STS) where an impurity causes oscillating patterns of the local density of states (LDOS) [1].Green's function methods commonly used to simulate QPI [24] often show disagreement with experiments, especially when computing interband scattering. Motivated by our own STS data [5] on Febased superconductors we developed a new scheme to help experimentalists to qualitatively analyze the calculated QPI. It relies on Feynman parametrization (FP), a technique widely used in quantum field theory for loop evaluation. We show that the interband QPI problem can be represented by a collection of new 'intermediate bands' possessing relatively easier geometries. We discuss two example applications. First, the QPI between two quadratic bands, where IBA predicts DOS divergences at momentum values that are confirmed by straight numerical calculations. Second, Bogoliubov QPI of FeSexS1x, where IBA provides a qualitative explanation for the particlehole asymmetric Intensity in the calculation and experiments. 
Thursday, March 7, 2024 10:12AM  10:24AM 
S01.00010: Study on nematic superconductivity in tetragonal Fe(Se, S) using magnetic torque measurements Reona Kondo, Kota Ishihara, Kohei Matsuura, Supeng Liu, Yuta Mizukami, Kenichiro Hashimoto, Takasada Shibauchi The ironbased superconductor FeSe undergoes a nematic transition at 90 K accompanied by a structural transition from orthorhombic to tetragonal structure. Also, this nematic transition temperature is suppressed as the amount of Ssubstitutions increases in FeSe_{1x}S_{x}, and it becomes zero at x = 0.17 (nematic quantum critical point). Recently, in a SC phase outside this nematic phase (x > 0.17), a large residual quasiparticle density of states appears and anomalously suppressed superfluid density has been found in the zerotemperature limit [13]. These are the characteristics of theoretically suggested new gap structure for superconductivity with broken timereversal symmetry, in which the gap closes in twodimensional planes (Bogoliubov Fermi surfaces, BFSs) [4].

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