Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session A61: FeBased Superconductors  Nematicity IFocus

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Sponsoring Units: DMP DCMP DCOMP Chair: Ulrich Welp, Argonne Natl Lab Room: Mile High Ballroom 4B 
Monday, March 2, 2020 8:00AM  8:36AM 
A61.00001: Specific Heat and Critical Behavior in BaFe_{2}(As_{1x}P_{x})_{2} Invited Speaker: Camilla Moir With T_{c}’s below 40 K and evidence of a quantum critical point [1], the ironbased hightemperature superconductor BaFe_{2}(As_{1x}P_{x})_{2} is an appealing system for investigating the behavior underlying superconductivity in highT_{c} superconductors. By applying magnetic fields up to 35 T, we are able to suppress superconductivity and reveal the normal state of overdoped BaFe_{2}(As_{1x}P_{x})_{2}. We observe √H behavior indicating a nodal superconducting gap, saturation of the heat capacity at a magnetic field corresponding to the onset of the normal state, and enhancement of the quasiparticle mass sum as calculated from electronic specific heat coefficient as optimal doping is approached [1]. Our comparison of specific heat as a function of magnetic field to specific heat as a function of temperature, as well as other measurements, forms a consistent treatment of specific heat measurements in hightemperature superconductors. 
Monday, March 2, 2020 8:36AM  8:48AM 
A61.00002: Quantum critical nematic fluctuations and spin excitation anisotropy in the iron pnictides* ChiaChuan Liu, Elihu Abrahams, Qimiao Si Quantum criticality in iron pnictides involves nematic and antiferromagnetic degrees of freedom in a concurrent way [1,2], but the relationship between the two types of fluctuations has yet to be clarified. To elucidate the nematic correlations, we study the spin excitation anisotropy defined by the difference between the dynamical spin susceptibilities at (pi,0) and (0,pi) [3]. We start from an effective GinzburgLandau theory for both the Isingnematic and antiferromagnetic fluctuations in the presence of a small external uniaxial potential, which breaks the C4symmetry in B1g channel, and establish an identity that connects the spin excitation anisotropy with the dynamical magnetic susceptibility and static nematic susceptibility. Using this identity, we show that the dynamical nematic susceptibility in the quantum critical regime can be determined, and we illustrate it by considering the case of the optimally Nidoped BaFe2As2 [3]. 
Monday, March 2, 2020 8:48AM  9:00AM 
A61.00003: Quantum phase transition of correlated ironbased superconductivity in LiFe_{1x}Co_{x}As Nana Shumiya, Jiaxin Yin, Songtian Sonia Zhang, Guangyang Dai, Yuanyuan Zhao, Andreas Kreisel, Gennevieve Macam, Brian M. Andersen, Fengchuan Chuang, Hsin Lin, Ziqiang Wang, Changqing Jin, Yunkyu Bang, Zahid Hasan We use scanning tunneling microscopy (STM) to image the electronic impact of Co atoms on the ground state of the LiFe_{1x}Co_{x}As system. We observe that impurities progressively suppress the global superconducting gap and introduce low energy states near the gap edge, with the superconductivity remaining in the strongcoupling limit. Unexpectedly, the fully opened gap evolves into a nodal state before the Cooper pair coherence is fully destroyed. Our systematic theoretical analysis shows that these new observations can be quantitatively understood by the nonmagnetic Bornlimit scattering effect in a s±wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition. 
Monday, March 2, 2020 9:00AM  9:12AM 
A61.00004: Nematic and Antiferromagnetic Quantum Criticality in a MultiOrbital Hubbard Model for Iron Pnictides Lei Chen, Wenjun Hu, Haoyu Hu, Rong Yu, HsinHua Lai, Luca Fausto Tocchio, Federico Becca, Qimiao Si The extent to which quantum criticality drives the physics of iron pnictides is a central question in the field. While the issue had been addressed by effective field theories [1,2], how to approach it in the multiorbital Hubbard model has been a longstanding challenge due to the limitation in methods for intermediate correlations. Here [3] we study this problem within a multiorbital Hubbard model containing both the Hubbard and Hund’s interactions, by a variational Monte Carlo method based on JastrowSlater wave functions that allow for a nonperturbative treatment of the electron correlations. We find strong evidence for the existence of a unique quantum critical point, where both nematic and (π, 0) antiferromagnetic orders develop, in the badmetal regime of the phase diagram. A robust signal for unconventional superconducting pairing is also found as the system approaches the quantum critical point from the paramagnetic side. 
Monday, March 2, 2020 9:12AM  9:24AM 
A61.00005: Fluctuating orders induced nonFermiliquid behavior near quantum critical point in ironbased superconductors Rong Li, ZhenSu She The nonFermiliquid behavior near quantum critical point (QCP) in ironbased superconductors (IBSCs) is a matter of considerable debate. Recently, we have proposed a novel transport theory quantifying scattering by multiorder fluctuations, yielding a new resistivity model, i.e., ρ=ρ_{a}+(ρ_{b}_{2}+α^{2}T^{2}+β^{2}B^{2})^{1/2}(see also another contributed talk, She and Li, "A symmetrybreaking analysis for nonFermiliquids induced by order fluctuations in correlated electron systems"). For IBSCs near QCP, the theory quantitatively explains the widely observed lowT plateau as a result of scattering by antiferromagnetic (AFM) fluctuations, and the unusual linear T and B scaling at high T and B induced by thermal and magneticvortex fluctuations. Furthermore, the scaling transition from T^{2} to T under increasing T and doping is explained by the increase of vortex fluctuations relative to AFM fluctuations. We present supporting evidence by comparing the predictions with data of dozens of samples. It thus forms a novel framework to clarify the difference between IBSCs and cuprates, successfully quantifying the feature of weaker AFM fluctuations in LiFe_{1x}Co_{x}As, and stronger vortex fluctuations in BaFe_{2}(As_{1x}P_{x})_{2} based on the link between macroscopic resistivity and microscopic fluctuating orders. 
Monday, March 2, 2020 9:24AM  9:36AM 
A61.00006: Manifestation of the multiband nature in the BCSBEC crossover of FeSe_{1x}S_{x} Takahiro Hashimoto, Yuichi Ota, Akihiro Tsuzuki, Tsubaki Nagashima, Akiko Fukushima, Shigeru Kasahara, Yuji Matsuda, Kohei Matsuura, Yuta Mizukami, Takasada Shibauchi, Shik Shin, Kozo Okazaki The crossover from the superconductivity of BardeenCooperShrieffer (BCS) regime to BoseEinstein condensation (BEC) regime holds the key to understanding the nature of pairing and condensation of fermions [1]. In electron systems in solids, however, it is generally difficult to control the paring strength of the electrons, and complete evidence of the BCSBEC crossover has been elusive so far. Here, we provide the first example of complete evidence for the BCSBEC crossover in an ironbased superconductor FeSe_{1x}S_{x} from laserexcited angleresolved photoemission spectroscopy, and propose a multiband mechanism for BCSBEC crossover in this system. 
Monday, March 2, 2020 9:36AM  9:48AM 
A61.00007: Unusual BCSBEC crossover in FeSe_{1x}S_{x} superconductors Yuta Mizukami, Masahiro Haze, Ohei Tanaka, Kohei Matsuura, Daiki Sano, Jakob Böker, Ilya Eremin, Shigeru Kasahara, Yuji Matsuda, Takasada Shibauchi The BCSBEC crossover from strongly overlapping Cooper pairs to nonoverlapping 
Monday, March 2, 2020 9:48AM  10:00AM 
A61.00008: FeSe as a Polymorphous Network Zhi Wang, Xingang Zhao, Simon J L Billinge, Alex Zunger The observed electronic structure of FeSe has lower apparent symmetry than the one that would be suggested by its macroscopic crystallographic structure. It has been argued that such nematicity must be electronic symmetry lowering, driven by strong correlations, rather than a local structural symmetry lowering, the latter being judged on the basis of global structural probes to be too small. Standard structure predictions use small unit cells that cannot accommodate structural symmetry lowering. Using a predictive first principles minimization of the internal total energy without restricting it to a small unit cell reveals that the lowest energy configuration whose average macroscopic symmetry is tetragonal consists, in fact, of a distribution of different local lowsymmetries. This polymorphous network explains the PDF pattern in both the local and longrange regions without a fit. When used as input to electronic structure calculations, the predicted polymorphous structure reveals electronic symmetry breaking that is unique to this unusual compound. 
Monday, March 2, 2020 10:00AM  10:12AM 
A61.00009: Theoretical study on the ironbased twoleg ladder tellurides Yang Zhang, Lingfang Lin, Adriana Moreo, Shuai Dong, Elbio Dagotto The recent discovery of superconductivity in the twoleg ladder compounds BaFe_{2}X_{3} (X=S, Se) started the novel field of quasionedimensional ironbased superconductors. Here, we predict that the previously barely explored ladder compound RbFe_{2}Te_{3} should be magnetic with a CXtype arrangement[1]. Moreover, at n = 6.0 our DFT phase diagrams (with/without lattice tetramerization) reveal that the stable magnetic states could be either a 2x2 magnetic Blocktype, as for X=Se, or a previously never observed before CYtype state. In the Tebased studies, electrons are more localized than in S, implying that the degree of electronic correlation is enhanced for the Te case. This potential relevance of strong correlation in n=6 Te123 ladders could also induce exotic phenomena [2], such as Blocktype order, the orbital selective Mott physics, and superconductivity under high pressure. Our overarching conclusion is that experimental studies of iron ladder tellurides are worth pursuing. 
Monday, March 2, 2020 10:12AM  10:24AM 
A61.00010: Substitution and doping in iron pnictides Stefan Schuppler, Peter Nagel, MengJie Huang, Robert Eder, Thomas Wolf, Michael Merz For insight into the compositiondependent electronic structure of iron pnictides, we performed a systematic study of spatial structure and electronic states by xray diffraction and xray absorption. A large number of compositions in the (Ba,Sr)(Fe,TM)_{2}(As,P)_{2} family of compounds was investigated, covering the substitution of Ba by Sr; of Fe by transition metals (TM); and of As by P. Our observations on doping effects upon such substitutions include “reluctant” doping (charge carriers are only partially transferred away from the substituent) or “sitedecoupled” doping (transferred charge carriers affect either Fe sites or As sites but not both). Here, we focus on isovalent substitutions. Our findings suggest that Indirect, structural effects of substitution appear to be more important for magnetism and superconductivity in iron pnictides than the direct, chargecarrier doping effects. 
Monday, March 2, 2020 10:24AM  10:36AM 
A61.00011: Isovalent S and Te substitution effect on superconductivity in FeSe thin films Fuyuki Nabeshima, Tomoya Ishikawa, Naoki Shikama, Yuki Sakishita, Sota Nakamura, Hodaka Kurokawa, Atsutaka Maeda We investigated chemical pressure effects from positive to negative (S and Te substitution) and inplane strain effect from tensile to compressive on physical properties in FeSe thin films. Both S and Te substitution suppresses the structural transition temperature. The behavior of superconducting transition temperature, Tc, is completely different between S and Te substitution at the composition when the structural transition disappears. Tc increases drastically for Te substitution at the ortho.tetra. boundary, while Tc shows monotonic decrease in S substituted samples. These results demonstrate that the relationship between the nematicity and the superconductivity is not universal in FeSe[1]. A magnetotransport study revealed a positive correlation between carrier densities and Tc in our films[2,3]. Our results suggest that the structural transition affects the electronic structure differently between Fe(Se,S) and Fe(Se,Te) and that this is the direct cause of the difference in the Tc behaviors at the ortho.tetra. boundary[3]. 
Monday, March 2, 2020 10:36AM  10:48AM 
A61.00012: Resistivity and Magnetic Susceptibility Under Pressure (0–2.0 GPa)
of Fe_{1+ε}Te_{0.5}Se_{0.5 }and R_{1x}Ce_{x}NiO_{3 }[R = (La,Y), Pr]* Zachary P. Kuklinski, Gregorio Ponti, Quinn D. B. Timmers, Rabia Husain, John Markert We report measurements of the ac magnetic susceptibility of the ironbased mixedchalcogenide superconductor Fe_{1+ε}Te_{0.5}Se_{0.5}, and of the electrical resistivity of doped nickeloxide compounds R_{1x}Ce_{x}NiO_{3}, with R = Pr or (La,Y), over the pressure range 0–20 kbar (0–2.0 GPa). Our ironbased material is optimized (ε ≈ 0.07) for bulk superconductivity, exhibiting an ambientpressure superconducting transition at T_{c }≈ 14 K, well above the transition of bulk FeSe (T_{c }= 9 K). With applied pressure, we observe an immense, nearly linear increase of T_{c}, with dT_{c }/dP = +7.0±0.5 K/GPa. We have prepared bulk “113” doped nickelates using highoxygenpressure (150–200 bar), hightemperature (T ≈ 1000°C) synthesis. For undoped PrNiO_{3}, the metal insulator transition temperature decreases with pressure, with reported dT_{MI}/dP ≈ –42 to –76 K/GPa [1,2]. For Pr_{1x}Ce_{x}NiO_{3} with x ≈ 0.0–0.2, we are exploring variations in dT_{MI}/dP. We consider both steric (e.g., bandbroadening, bondstraightening), electrondoping, and holefilling contributions to this behavior. 
Monday, March 2, 2020 10:48AM  11:00AM 
A61.00013: Unconventional quantum criticality in a twodimensional fermionboson coupled system Jianqiao Liu, Ryuichi Shindou We study a twodimensional twoband Fermi system with finite Fermi surfaces that couples with a $phi^4$ action of O(3) vector boson field with dynamical exponent $z=1$ through Yukawatype coupling. By using RG analysis, we show that quantum criticality associated with an ordering of the O(3) vector field is controlled by a new saddlepoint fixed point instead of the WilsonFisher fixed point. The new fixed point has a finite fermionboson coupling and the boson velocity is strongly renormalized by the Yukawa coupling. We discuss possible superconducting instability near such a quantum critical point. 
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