Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session M54: Fe-Based Superconductors: Magnetism and Electronic CorrelationsFocus Live
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Sponsoring Units: DMP GMAG Chair: Morten Holm Christensen, University of Minnesota |
Wednesday, March 17, 2021 11:30AM - 12:06PM Live |
M54.00001: Effects of correlations on the electronic structure and pairing in Fe-chalcogenides Invited Speaker: Andreas Kreisel Research on iron based superconducting materials has found evidence that electronic correlations play a significant role in determining the electronic structure and therefore should also influence the superconducting pairing in these systems. Examples are band- and orbital-dependent mass renormalizations and energy shifts, the occurrence of nematic instabilities and orbital order. Here, I present some examples how spin-fluctuations and nonlocal correlation effects determine the electronic structure in Fe based superconductors [1] and lead to orbitally selective decoherence. In terms of the superconducting state, the continuing development of high-resolution experimental probes of superconducting gap structures allows us to quantitatively compare the detailed momentum structure of superconducting pairing. Effects of correlations within the spin-fluctuation pairing approach can change the symmetry of the pairing instability, shift between dominant orbital contributions or modify the momentum structure. |
Wednesday, March 17, 2021 12:06PM - 12:42PM Live |
M54.00002: Effect of hydrostatic pressure and uniaxial stress on the competing phases in iron-based superconductors Invited Speaker: Elena Gati Understanding the phase interplay in iron-based superconductors is considered to be crucial for unravelling the mechanism behind their superconductivity. In this talk, we focus on new insights into the phase relation under pressure [1] of two members, FeSe and BaFe2As2, from specific heat [2] and elastoresistance measurements [3] made under hydrostatic pressures up to ~ 2.5 GPa. |
Wednesday, March 17, 2021 12:42PM - 12:54PM Live |
M54.00003: Suppression of superconducting parameters in magnetic layered superconductors by correlated quasi-two-dimensional magnetic fluctuations Alexei Koshelev Motivated by physics of RbEuFe4As4, we consider a layered magnetic superconductor in which a magnetic transition takes place inside a superconducting state and the exchange interaction between superconducting and magnetic subsystems is weak. We investigate the suppression of the superconducting gap and superfluid density by correlated magnetic fluctuations near the magnetic transition. The behavior is very sensitive to the relation between the magnetic correlation length, ξh, and superconducting coherence length ξs defining the 'scattering' (ξh<ξs) and 'smooth' (ξh>ξs) regimes. We quantified this 'scattering-to-smooth' crossover for the case of quasi-two-dimensional magnetic fluctuations. We find that the corrections to both parameters increase proportionally to ξh until it remains much smaller than ξs. In the opposite limit, when ξh exceeds ξs, both parameters have much weaker dependence on ξh. Moreover, the gap correction may decrease with increasing of ξh close to the magnetic transition. The crossover between the two regimes is unexpectedly broad: the scattering approximation becomes sufficient only when ξh is substantially smaller than ξs. |
Wednesday, March 17, 2021 12:54PM - 1:06PM Live |
M54.00004: Effect of electron irradiation on the magnetic properties of EuFe2(As1−xPx)2 ferromagnetic superconductor Sunil Ghimire, Kyuil Cho, Marcin Konczykowski, olivier cavani, Makariy Tanatar, Tsuyoshi Tamegai, Ruslan Prozorov Radio- frequency AC magnetic susceptibility (10 MHz, 20 mOe AC field) and DC magnetization were measured in single crystals of a ferromagnetic superconductor EuFe2(As1−xPx)2 (x = 0.23) before and after 2.5 MeV electron irradiation with a total fluence of ≈ 2 × 10^19 electrons/cm2. The ferromagnetic transition T_FM ≈ 18 K remains practically unchanged with the applied external DC magnetic field and is unaffected by irradiation, as expected from local moment ferromagnet. The T_SC(H) line starts at ≈ 24 K in the pristine sample in zero DC field for both AC and DC measurements, practically independent of frequency. Remarkably, after the irradiation it moves significantly below the FM transition, to ≈ 16 K in AC measurements and ≈ 12 K in a DC regime in small external field of 20 Oe. We attribute such large difference to appearance of the internal field below FM transition, so that the measurements are now conducted in the mixed vortex state where the onset of diamagnetism is known to be frequency-dependent. Our results suggest that the main coupling between superconductivity and local-moment ferromagnet is via the internal field. |
Wednesday, March 17, 2021 1:06PM - 1:18PM Live |
M54.00005: Magnetic anisotropy from strain-induced dislocations in correlated electron systems Mainak Pal, Laetitia Bettmann, Andreas Kreisel, Peter Hirschfeld Correlated electron systems, particularly iron-based superconductors, are extremely sensitive to strain, which inevitably occurs in the crystal growth process. Built-in strain of this type has been proposed as a possible explanation for experiments where nematic order has been observed at high temperatures corresponding to the nominally tetragonal phase. Here we investigate a simple microscopic model of a strain-induced dislocation in the presence of electronic correlations, which create defect states that can drive magnetic anisotropy of this kind, if spin orbit interaction is present. Such defects can arise, e.g., in Fe-based systems or in Cu-O chains in cuprates. We estimate the contribution of these dislocations to magnetic anisotropy as detected by current torque magnetometry experiments. |
Wednesday, March 17, 2021 1:18PM - 1:30PM Live |
M54.00006: Dynamical Magnetism in Superconducting and Non-Superconducting 11 Iron Chalcogenides Igor Zaliznyak, Yangmu Li, Andrei Savici, Vasile Garlea, Zhijun Xu, Genda Gu, John Tranquada There are profound connections between dynamical magnetism and unconventional superconductivity in Fe-based superconductors. Recently, new aspects associated with their multi-band, multi-orbital nature and the resulting orbital physics came to prominence in the iron chalcogenide family. Neutron scattering experiments revealed signatures of an orbital-selective electron localization and ferro-orbital order in the FeTe parent material. In the superconducting iron chalcogenide materials, the ARPES studies discovered significant temperature- and orbital-dependent correlation effects in the electronic band structure that are indicative of orbital-selective Mott physics. More recently, an orbital-selective Cooper pairing was discovered in the FeSe end member by STM. Here, we present polarized neutron scattering studies of magnetic dynamics in Fe1+yTe1-xSex samples with and without superconductivity. We analyze the evolution of the wave-vector dependence of the dynamical magnetic scattering with temperature and composition in order to understand the nature of magnetic and orbital electronic states arising from the orbital-selective correlation effects. |
Wednesday, March 17, 2021 1:30PM - 1:42PM Live |
M54.00007: Two types of magnetic shape-memory effects in Fe1+yTe Sahana Roessler, Cevriye Koz, Zhaosheng Wang, Yurii Skourski, Mathias Doerr, Deepa Kasinathan, Helge Rosner, Marcus Schmidt, Ulrich Schwarz, Ulrich Roessler, Steffen Wirth The discovery of superconductivity in β-FeSe triggered considerable interest also in the related tellurium compound Fe1+yTe. Here we present a detailed experimental study of Fe1+yTe (y = 0.11 and 0.12) using pulsed magnetic fields up to 60 T. Our studies confirmed two types of magnetic shape memory (MSM) effects in the low temperature antiferromagnetic state of these compounds [1]. Scanning tunneling microscopy measurements at low temperature established a modulated and finely twinned martensitic microstructure. The antiferromagnetism of the monoclinic phase allows for a magnetic–field–induced reorientation of these twin variants by the motion of twin boundaries. At sufficiently high magnetic fields, we observed a second isothermal transformation process. This gives rise to a second MSM effect caused by a phase transition back to the field-polarized tetragonal lattice state [2]. Observation of MSM effects in two known material families [1,3] related to high-Tc superconductors points to a prominent role of electron–phonon coupling arising through the spin–orbit interactions. |
Wednesday, March 17, 2021 1:42PM - 1:54PM Live |
M54.00008: New Investigations of Iron Pnictides Using Zero-Field Nuclear Magnetic Resonance Jaafar Ansari, Igor Mazin, Karen Louise Sauer In Zero-Field Nuclear Magnetic Resonance ZFNMR, the hyperfine field HF and the electric field gradient EFG field dominate the nuclear Hamiltonian, as opposed to traditional NMR where they are simply perturbations. Iron pnictides, in the magnetically ordered state, have strong HF and EFG fields, yielding spectra that intricately depend on the relative size and orientation of the two fields. Previous studies of this effect have overlooked the polarization and Rabi, or nutation, frequency of the ZFNMR signal. It is known however, that signals from a pure EFG system are linearly polarized and that from a Zeeman system are circularly polarized. We present calculations of the signal’s polarization, as well as the Rabi frequency, when HF and EFG are comparable to reveal the additional information afforded by such studies. In conjunction the resonance frequencies, the chief metric used to date, are presented. Iron pnictides are particularly interesting candidates for this study due to the theoretical importance of the transitions, and fluctuations associated with the transitions, from magnetic to nematic to paramagnetic order. Furthermore, ab initio methods are used to calculate EFG and HF fields for iron pnictides. |
Wednesday, March 17, 2021 1:54PM - 2:06PM Live |
M54.00009: STM/S study of surface reconstruction and atomic defects on iron-based superconductors Zhuozhi Ge, Qiang Zou, Mingming Fu, Liurukara D Sanjeewa, Athena S. Sefat, Zheng Gai Surface reconstruction and defects can dramatically modify the properties of iron-based superconductors. Here, we systematically investigated the surface of low-temperature cleaved parent and doped BaFe2As2 superconductors by scanning tunneling microscopy/spectroscopy (STM/S). STM imaging reveals the emergency of a new reconstruction while increasing the temperature from 77K to 165K. The structural and electronic properties of this new surface reconstruction was investigated. We have also induced atomic defects on the surface by tip scanning and the electronic property of those tip-induced defects showed similarity with the reconstructed surface. |
Wednesday, March 17, 2021 2:06PM - 2:18PM Live |
M54.00010: Disorder-Induced Universalily of Hole-doped 122-Type Pnictide Superconductors Phase Diagrams Omar Chmaissem, Ryan Stadel, Keith Taddei, Dmitry Khalyavin, Pascal Manuel, Saul Lapidus, Raymond Osborn, Stephan Rosenkranz Unconventional superconductivity in iron-based pnictides is induced by the suppression of a universal spin density wave ground state (SDW) in which the magnetic moments are AFM aligned in the ab plane. Several hole-doped family members were shown to exhibit reentrance, at different doping, to a C4 magnetic phase with spin reorientation to the out-of-plane direction. The discovery of this phase near superconductivity provided insights into the competing mechanisms underlying their complex phase diagrams but the conditions under which it is stabilized are not well understood. Here, we report the results of a comprehensive study of hole-doped materials as a function of ionic size and dopant concentration. We find that the C4 phase is confined to regions in which the As-Fe-As bond angle is close to that of a perfect tetrahedron and that the TN's of the SDW and C4 phases are linked by a scaling equation consistent with a common origin as itinerant nesting instabilities. Furthermore, we show that all the SDW and C4 transition temperatures collapse onto a universal phase diagram when the hole concentrations are scaled to the C4 dome which represent the amount of disorder. |
Wednesday, March 17, 2021 2:18PM - 2:30PM Live |
M54.00011: Sulfur-induced magnetism in iron-chalcogenide thin films Fuyuki Nabeshima, Yuma Kawai, Naoki Shikama, Yuki Sakishita, andreas suter, Sang Eun Park, Tadashi Adachi, Atsutaka Maeda Magnetic properties of Fe(Se,S) thin films on LaAlO3 were investigated via muon spin rotation/relaxation. A drastic decrease of the initial asymmetry was observed together with the peak structure in the temperature dependence of the relaxation rate of muon spins almost at the same temperature where kink anomalies were observed in the temperature dependent resistivity. With increasing S content, the anomaly temperature increases and the magnetic fluctuations at the lowest temperature was suppressed. These results show that the S substitution induces magnetism at low temperatures in Fe(Se,S) thin films. Although the behaviors of the magnetic and nematic phases in FeSe films towards chemical pressure by S substitution are similar to those for bulk FeSe towards hydrostatic pressure, the behavior of Tc is significantly different between these systems. Our results suggest essential importance of the detailed comparative investigation among physical and chemical pressure effects for bulk and film samples to understand the interplay of the magnetism and the superconductivity in iron chalcogenides. |
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