Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W69: Fe-based Superconductors: FeSe Monolayer, 1144 MaterialsFocus Recordings Available
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Sponsoring Units: DMP DCMP DCOMP Chair: Yongjin Shin, University of California, Santa Barbara Room: Hyatt Regency Hotel -Jackson Park A |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W69.00001: Criteria for achieving superconducting monolayer FeSe films Invited Speaker: Ke Zou Monolayer FeSe on SrTiO3 substrate has an enhanced superconducting transition temperature Tc than bulk FeSe. The criteria for achieving the enhanced superconductivity, specifically, the film stoichiometry and the interfacial lattice and electronic structures, will be discussed. The superconducting FeSe films can only be grown by molecular beam epitaxy with hours’ post-growth annealing. We adjust the film stoichiometry by depositing additional amounts of Fe atoms. The monolayers become superconducting after the Fe deposition without annealing and show similar superconducting transition temperatures in transport measurements. The annealing process in essence removes Fe vacancies and the additional Fe deposition serves as a more efficient way. By a complementary analysis of scanning transmission electron microscopy and in situ surface X-ray diffraction, we resolve the atomic-scale structure of the FeSe/SrTiO3 interface with a double TiO2 layer presented. ab initio calculations reveal a stronger tendency for electrons to transfer from an oxygen-deficient SrTiO3 surface to FeSe when the double TiO2 layer is present. The interfacial charge transfer together with the stoichiometry of FeSe film has been well-accepted as necessary criteria for achieving the enhanced superconductivity. |
Thursday, March 17, 2022 3:36PM - 3:48PM |
W69.00002: Role of Doping and Disorder in the Fermi Surface Reconstruction of FeSe / SrTiO3 Hunter Sims, Alexander Kellerhouse, Tom Berlijn Electron-doping FeSe stabilizes its superconducting state, raising Tc from around 8 K in the bulk to above 30 K. Monolayer (ML) FeSe on SrTiO3 (and other transition-metal-oxides) shows a similar or greater enhancement to Tc, accompanied by the vanishing of a Γ-centered hole pocket in the electronic band structure obtained from ARPES. Earlier work has proposed that O vacancies or excess Ti at the FeSe/STO interface could be responsible for this doping. We present calculations based on density functional theory that model the electronic structure of FeSe in the presence of disordered charge-doping impurities in the underlying Ti-O layer. This Wannier-based method allows one to disentangle the underlying cause of the doping from specifics of a given substrate-ML interface. ML FeSe behaves similarly on TiO2 and LaTiO3 as well as on substrates not containing Ti-Ox layers (e.g. FeSe/LaFeO3). We study disordered defects (interstitial cations and/or oxygen vacancies) at the FeSe / STO interface and determine their effect on the averaged-out Fermi surfaces measured by ARPES, providing direct insight into the connection between the atomic structure of this interface and the observed electronic properties that coincide with the superconducting state. |
Thursday, March 17, 2022 3:48PM - 4:00PM |
W69.00003: Theory of BCS-to-BEC Crossover in FeSe Gautam Rai, Stephan W Haas, Stefan Kettemann The BCS-BEC crossover has been challenging to realize in condensed matter systems. Recently, there has been experimental evidence for such a transition in a FeSe monolayer on a trilayer graphene substrate. There are two crucial ingredients in this observation: 1) Pairing interactions induced by spin fluctuations can condense Cooper pairs even in incipient bands. 2) The Fermi level of the FeSe monolayer is controlled by the local work function of the substrate which depends on how the graphene layers are stacked. At the interface between ABA and ABC stacked graphene, the chemical potential in the FeSe monolayer crosses the hole band edge, such that the hole carriers in the FeSe monolayer are in a BCS state on one side and in a BEC state on the other. In this talk, I will describe a minimal two-band model that explains these observations and discuss how the parameters of the model must be chosen to make contact with the real world. Furthermore, I will treat the problem using a real-space Bogoliubov-de Gennes approach to also explain interface effects. |
Thursday, March 17, 2022 4:00PM - 4:12PM |
W69.00004: Modulation doping of the FeSe monolayer on SrTiO3 Fengmiao Li, Ilya Elfimov, George A Sawatzky The FeSe monolayer on STO has a higher superconducting Tc than its bulk counterpart and exhibits interesting characteristics such as missing hole pockets at the BZ center and replica bands in photoemission experiments. It remains ambiguous whether interactions possibly involving the STO substrate behind those experimental observations contribute to the Tc increase. One consensus reached to date is the modulation doping of FeSe by STO is critical for the enhancement. Monitoring the Tc change with a variable carrier density would provide valuable information for the electron pairing itself and the significance of interlayer interactions. However, varying the carrier density via the modulation doping method is challenging, and many groups have reported similar doping levels. We have developed the theory underlying the charge transfer at the FeSe/STO interface and found the key factor limiting the FeSe doping level. This information provides an experimental pathway to accommodate a variable carrier density in the FeSe monolayer via modulation doping. |
Thursday, March 17, 2022 4:12PM - 4:24PM |
W69.00005: Ultrafast Melting of Superconductivity in an Fe-Based Superconductor Daniel Nevola, Nader Zaki, Genda Gu, Qiang Li, Peter D Johnson Optically pumping systems out of equilibrium provides a unique interpretation of quantum states of matter due to their ability to disentangle certain degrees of freedom, often yielding information inaccessible from equilibrium measurements. Here, we employ time-and-angle resolved photoemission spectroscopy (trARPES) on an iron-based superconductor to directly observe the destruction of superconducting order on ultrafast timescales. Close examination of these ultrafast changes provides vital clues to the pairing strength as debated in the BCS-BEC crossover. Our results expose the wealth of information that can be learned from probing the nonequilibrium changes to the band structure in the iron-based superconductors. |
Thursday, March 17, 2022 4:24PM - 4:36PM |
W69.00006: Ultrafast quasiparticle dynamics in Co-doped BaFe2As2 Sergiy Lysenko, Alexander Bartenev, Ki-Tae Eom, Jong-Hoon Kang, Roman Kolodka, Eric E Hellstrom, Chang-Beom Eom, Armando Rua The understanding of excited state dynamics in iron-based superconductors is the subject of great interest in theoretical and experimental condensed matter physics. Here we report on time-domain studies of light-induced quasiparticle dynamics in Co-doped BaFe2As2 superconducting epitaxial thin films at different excitation regimes. The evolution of transient optical signal in superconducting and non-superconducting states reveals several regions with noticeably different electron-phonon relaxation. The most distinct dynamics associated with multi-stage quasiparticle relaxation rise up on a sub-30 ps time scale. The sub-500 fs process is associated with the instantaneous formation of the nonequilibrium state of quasiparticles. It is followed by the energy exchange between this photoexcited state and lattice with electron thermalization within 3 ps. The characteristic time of this process grows with excitation and is associated with the evolution of quasiparticles and, possibly, hidden phases. New mechanisms seem to underpin the photoinduced relaxation dynamics of Co-doped BaFe2As2 in low- and high-excitation regimes. |
Thursday, March 17, 2022 4:36PM - 4:48PM |
W69.00007: Dispersionless orbital excitations in (Li,Fe)OHFeSe superconductors Qian Xiao, Wenliang Zhang, Teguh C Asmara, Dong Li, Qizhi Li, Shilong Zhang, Xiaoli Dong, Yao Wang, Cheng-Chien Chen, Thorsten Schmitt, Yingying Peng The origin of superconductivity in Fe-based superconductors remains unclear. Although spin fluctuation plays an important role in Fe-based superconductors, other factors, such as orbital fluctuation, are also in play [1,2]. The intercalated iron selenide (Li,Fe)OHFeSe (FeSe11111) is a single-phase bulk superconductor, which can reach a high Tc of 42K [3,4]. Intriguingly, it shows remarkably similar electronic behaviors as those in monolayer FeSe with the Tc up to 65K [5], providing a bulk counterpart to explore the origin of high Tc in iron selenides. Here we performed an extensive study on FeSe11111 by using resonant inelastic x-ray scattering (RIXS) at Fe L3-edge [6]. We have observed four Raman-like features at ∼ 0.1 eV, ∼ 0.3 eV, ∼ 0.7 eV, ∼ 2.5 eV, which are dispersionless versus momentum transfer. Moreover, these excitations show different temperature behaviors. Using atomic multiplet calculations we identify the excitations at ∼ 0.3 and 0.7 eV as local eg - eg and eg - t2g orbital excitations involving spin degrees of freedom. Our results uncover the excitations in FeSe11111 and provide an important insight to understand the superconductivity in iron selenides. |
Thursday, March 17, 2022 4:48PM - 5:00PM |
W69.00008: Quantum Monte Carlo Simulations of High-Temperature FeSe Superconductors with no Sign Problem Jose P Rodriguez, Ernesto Delvillar Electron doping of a hidden spin-density wave (hSDW) state that exists over isotropic d+ = dxz + idyz and d- = dxz - idyz orbitals can lead to electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone, but with no Fermi surface pockets at the center of the two-iron Brillouin zone[1]. The hSDW is a Mott insulator that results from nesting of bare electron-type and hole-type Fermi surfaces, respectively, at the center and at the corner of the one-iron Brillouin zone[2]. A recent Eliashberg theory analysis of a spin-fermion model based on the latter nested Fermi surfaces recovers the Lifshitz transition to electron-type Fermi surface pockets at the corner of the two-iron Brillouin zone, at weak electron doping[3]. The Eliashberg theory also finds an instability to S-wave Cooper pairing, with sign changes between the electron Fermi surface pockets and faint hole pockets that are also located at the corner of the two-iron Brillouin zone. We study the same two-orbital spin-fermion model by carrying out determinant quantum Monte Carlo simulations that are free of the fermion sign problem[4]. The theoretical prediction of a Lifshitz transition to Fermi surface pockets at the corner of the Brillouin zone will be tested, while the competition between hSDW order, S-wave Cooper pairing, and D-wave Cooper paring will be examined. |
Thursday, March 17, 2022 5:00PM - 5:12PM |
W69.00009: Superconductivity and magnetism in CaK(Fe1-xMnx)4As4 system Mingyu Xu, Li Xiang, Elena Gati, William Meier, Juan Schmidt, Sergey L Budko, Paul C Canfield Substitution of Fe with Ni or Co in CaKFe4As4 suppresses superconductivity and stabilizes an unusual hedgehog spin-vortex crystal magnetic structure [1-4]. Similarly, as Mn is substituted in CaKFe4As4, superconductivity is suppressed and magnetic order appears. Given that it was not possible to induce superconductivity in Ba(Fe1-xMnx)2As2, we can now study the CaK(Fe1-xMnx)4As4 system so as to gain insight into how the often more local moment like Mn affects superconductivity in these Fe-based superconducting systems. The T-x phase diagram of CaK(Fe1-xMnx)4As4 determined by resistance, magnetization and heat capacity will be presented and compared with the Ni or Co substituted systems. The superconducting critical field data as well as BNC scaling [5] will be also discussed. In addition Pressure-temperature phase diagram determined by resistance under hydrostatic pressure up to 5.15 GPa will also be presented and discussed. |
Thursday, March 17, 2022 5:12PM - 5:24PM |
W69.00010: Magnetic field dependence of antiferromagnetism deep within a superconducting state in RbEuFe4As4 exploiting a charge-magnetic interference inherent in x-ray resonance Zahir Islam, Ulrich Welp, Jong Woo Kim, Jinke Bao, Duck Young Chung, Mercouri G Kanatzidis, Wai-Kwong Kwok We present an application of a unique methodology to probe magnetic long-range order which is difficult to study using bulk magnetization when masked by a large diamagnetic signal from a superconducting (SC) state as in RbEuFe4As4 (RbEu1144) superconductor. In RbEu1144 SC state appears at ~37 K followed by an antiferromagnetic (AFM) transition at TN ~15 K characterized by a 4-unit-cell periodicity of Eu moments along c axis. We exploit an interference of charge and magnetic scattering of x-rays on resonance to observe field suppression of this AFM order as ferromagnetism is induced in the Eu sublattice under an increasing magnetic field applied along the basal plane. This method is free from detrimental effects of any diamagnetic signals from SC state. Due to a high magnetic contrast, we were able to observe hysteretic dichotomy of AFM and FM and determine trapped flux. Future potential to use this technique to image bulk distribution of vortex pinning using a diffraction contrast x-ray microscopy will be discussed. |
Thursday, March 17, 2022 5:24PM - 5:36PM |
W69.00011: Spin waves in layered superconductors with helical magnetic structure: the case of RbEuFe4As4 Alexei E Koshelev We evaluated the spin-wave spectrum and dynamic susceptibility in a layered superconductor with helical interlayer magnetic structure [1]. We especially focus on the structure in which the moments rotate 90 degrees from layer to layer realized in the iron pnictide RbEuFe4As4. While in nonmagnetic superconductors low-frequency magnetic field decays on the distance of the order of the London penetration depth, spin waves mediate its propagation to much larger distances limited by external dissipation mechanisms. The spin-wave spectrum in superconductors is strongly renormalized due to the long-range electromagnetic interactions between the oscillating magnetic moments. This leads to strong enhancement of the frequency of the mode coupled with uniform field and this enhancement exists only within a narrow range of the c-axis wave vectors of the order of the inverse London penetration depth. The key feature of materials like RbEuFe4As4 is that this uniform mode corresponds to the maximum frequency of the spin-wave spectrum with respect to the c-axis wave vector. As a consequence, the high-frequency surface resistance acquires a very distinct asymmetric feature spreading between the bare and renormalized frequencies. For parameters of RbEuFe4As4, this feature is expected in the range 10-20 GHz. |
Thursday, March 17, 2022 5:36PM - 5:48PM |
W69.00012: Superconductivity coexisting with ferromagnetic order in RbEuFe4As4 studied by scanning SQUID microscopy Huiyuan Man, Yusuke Iguchi, Jinke Bao, Duck Young Chung, Mercouri G Kanatzidis, Kathryn Moler The superconductivity coexisting with ferromagnetic order has intrigued scientists for decades. RbEuFe4As4 is one of these cases with the superconductivity in the FeAs layers and ferromagnetic order in the Eu layers. Systematic spatially resolved studies are lacking but necessary to investigate the superconductivity, ferromagnetic order, and the relation between them. We used scanning SQUID microscopy to image the in situ diamagnetic and ferromagnetic responses of RbEuFe4As4 with micrometer scale resolution in both magnetometry mode and susceptometry mode. We observed homogeneous superconductivity with Tc ~ 37 K, and unexpected and inhomogeneous double ferromagnetic transitions with Tm ~ 15 K upon cooling down. The vortices coexist with the magnetic domains below Tm, but are not rearranged in the temperature cycles through Tm. The observed spatial feature of suppressed superfluid density matches well with the inhomogeneous ferromagnetic domain structure close to Tm, which is consistent with the scenario that the ferromagnetic spin fluctuations strongly suppress the superconductivity. In our study, the ferromagnetic order in RbEuFe4As4 is not strong enough to rearrange the vortices, but the superconductivity is strongly suppressed by the magnetic fluctuations. Our results will stimulate future investigations into the physical phenomena arising from the superconducting and magnetic subsystems by scanning probes. |
Thursday, March 17, 2022 5:48PM - 6:00PM |
W69.00013: Superconducting phase diagram of the magnetically ordered superconductor RbEuFe4As4 in pulsed fields up to 86 T Matthew P Smylie, Alexei E Koshelev, Jinke Bao, Wai-Kwong Kwok, Duck Young Chung, Mercouri G Kanatzidis, Vincent Oliviero, Maxime Massoudzadegan, David Vignolles, Maxime Leroux, Ulrich Welp The superconducting phase diagram of single-crystal RbEuFe4As4 (Tc ~ 36.5 K), which has long-range Eu magnetic order at Tm ~ 15 K, has been measured in pulsed magnetic fields up to 86 T using a proximity diode oscillator technique. Upon decreasing temperature, the anisotropy Hc2ab/Hc2c approaches unity at approximately 5 K, with pronounced curvature observed in Hc2-line, especially in the in-plane Hc2ab(T). A two-band theoretical model for a Fermi surface in the form of warped cylinders yields high Maki parameters for both H // ab and H // c, indicating significant Pauli paramagnetic limiting effects for both orientations. However, no upturn at low temperature / high field characteristic of a transition into an FFLO state is observed. |
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