Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G61: Fe-Based Superconductors - Vortex Properties / Magnetic SuperconductorsFocus
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Sponsoring Units: DMP DCMP DCOMP Chair: Milan Allan, Leiden University Room: Mile High Ballroom 4B |
Tuesday, March 3, 2020 11:15AM - 11:51AM |
G61.00001: Transport properties and flux pinning analysis of high-performance FeAs122 superconducting wires Invited Speaker: Yanwei Ma Iron-based superconductors (IBS), especially 122 type, are very promising candidates for high-field applications because of its ultra-high Hc2 > 70 T at 20 K, low anisotropy (g < 2 for 122), and ease of fabrication. In recent years, tremendous progress has been made on the critical current density (Jc) of the 122-type IBS wires based on a powder-in-tube technique. Encouraging breakthroughs were made, including a high transport Jc exceeding the practical level of 105 A cm−2 (at 4.2 K, 10 T), the first 100 meter-class wire and the first performance test of a 30 mm IBS inserted coil under a 24 T background field. More recently, the highest transport Jc value has achieved 0.15 MA/cm2 (Ic = 437 A) at 4.2 K and 10 T in densified and textured 122 tapes made by hot pressing. The transport Jc measured at 4.2 K under high magnetic field of 27 T is still on the level of 55 kA/cm2. Herein we compared the Tc and Jc distributions of the K-doped FeAs122 tapes by a calorimetric method. We found that hot-pressing provides a better environment for a complete chemical reaction and a more homogenous dopant distribution, which are beneficial to the global current of a superconductor. We further study the vortex dynamics of the hot-pressed high-Jc tapes. We found that the magnetization relaxation rate below 10 K shows a temperature insensitive plateau with a value comparable to that of low temperature superconductors. Moreover, the relaxation rate below 20 K tends to saturate with the increasing field which is beneficial for high field application. We also highlight some remarkable advances relevant to practical applications, including mechanical strain properties, copper sheaths, multifilamentary fabrication, and superconducting joints. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G61.00002: Superconducting phase diagram of Ni-doped magnetically ordered superconductor RbEu(Fe1-xNix)4As4 for x ≤ 0.04 in pulsed fields up to 65 T Matthew Smylie, Kristin Willa, Yilmaz Simsek, Jinke Bao, Hendrik Hebbeker, Wai-Kwong Kwok, Duck Young Chung, Mercouri Kanatzidis, John Singleton, Fedor Balakirev, Ulrich Welp The superconducting phase diagram of single-crystal RbEu(Fe1-xNix)4As4 for x = 0.02, 0.03, 0.04 has been measured in pulsed magnetic fields up to 65 T using a proximity diode oscillator technique. Doping lowers Tc and slightly lowers the superconducting anisotropy, but does not affect the Eu magnetic order. Upon doping, the curvature of phase boundaries for both H || ab and H || c remains remarkably consistent, indicating that the suppression of Tc is doping driven, not disorder driven. As in the parent compound, Pauli limiting dominates Hc2ab, whereas orbital limiting dominates Hc2c. The resultant high Maki parameter α for H|| ab is consistent with a possible FFLO state at low temperatures, but none is seen in the accessible field range. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G61.00003: Modification of the Upper Critical Field of RbEuFe4As4 due to Eu-Magnetism Ulrich Welp, Alexei Koshelev, Roland Willa, Clement Burns, Matthew S. Cook, Jinke Bao, Duck Young Chung, Mercouri Kanatzidis, Wai-Kwong Kwok
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Tuesday, March 3, 2020 12:15PM - 12:27PM |
G61.00004: Angular dependent studies of vortex pinning and dynamics in RbEuFe4As4 single crystals with columnar defects Ivan Nekrashevich, Matthew Smylie, Ulrich Welp, Wai-Kwong Kwok, Alexei Koshelev, Jin-Ke Bao, Duck Young Chung, Mercouri Kanatzidis, Leonardo Civale We performed studies of vortex pinning and dynamics in RbEuFe4As4 single crystals with different densities and orientations of aligned columnar defects (CDs) introduced by heavy ion irradiation, as a function of temperature (T) and magnetic field strength (H) and orientation (Θ). Pristine crystals have a superconducting transition temperature Tc~37K and a magnetic ordering transition of the Eu at Tm~15K, neither of which is changed by the irradiations. We developed a model to account for the modifications to the vortex critical state produced by the magnetic moments. Irradiated crystals exhibit large critical current densities (Jc) and a lock-in phase (where vortices remain fully trapped into the CDs even if H is tilted away from them), which is the fingerprint of correlated pinning. For large CDs densities (e.g. matching field BΦ ~ 10T) the lock-in phase spans an unusually large angular range and extends to fields above 2T. This allows us to investigate the angular variations of Jc and the creep rates associated with the expansion of half-loops and double-kinks inside the lock-in region, due to the changes in the pinning energy arising from the –B●H/4π term in the free energy. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G61.00005: Fermi Surface Folding and Surface Reconstruction in an Iron-Based Superconductor CaKFe4As4 Yongqing Cai, Huan Yang, Tao Xie, Cong Li, Yu Xu, Dingsong Wu, Guodong Liu, Huiqian Luo, H.-J. Gao, Lin Zhao, Xingjiang Zhou High resolution angle resolved photoemission spectroscopy and scanning tunneling microscopy were carried on the CaKFe4As4 to study its electronic structure. CaKFe4As4 consists of parent compound CaFe2As2 and heavily hole doped KFe2As2 but with a high TC of 35K. We found three hole-like pockets at the Brillouin zone center and two electron-like pockets around the Brillouin zone corner. We also discovered a large hole pocket around the Brillouin zone corner which has the same size as the outmost hole pocket at the Brillouin center. Using scanning tunneling microscopy, we determine that there exists × reconstruction in CaKFe4As4, which will lead to the folding of the hole pocket from Brillouin zone center to Brillouin zone corner. The implication of these observations related to other iron-based superconductors will be discussed. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G61.00006: Scattering studies of the nature of long-range magnetic order coexisting with superconductivity in single-crystal RbEuFe4As4 Zahirul Islam, Omar Chmaissem, Ulrich Welp, Jong Woo Kim, Huibo Cao, Alexei Koshelev, Kristin Willa, Matthew Smylie, Zhu Diao, Andreas Rydh, Jinke Bao, Duck Young Chung, Mercouri Kanatzidis, Wai-Kwong Kwok, Stephan Rosenkranz We present magnetic long-range order studied by x-ray scattering and magnetic neutron diffraction (MND) on a single crystal RbEuFe4As4 (RbEu1144) suerpconductor. In tetragonal RbEu1144 superconductivity (SC) appears at ~37 K followed by an antiferromagnetic (AFM) transition at TN ~15 K. Our diffraction studies revealed a long-range, fully coherent, and 3D ordered magnetic phase characterized by a single wavevector, q=(0,0,1/4), which indicates a 4-unit-cell magnetic periodicity along the 4-fold c axis. High-resolution x-ray scattering did not find any detectable structural distortions (e.g. orthorhombic) at low temperatures. Element-selective x-ray resonant scattering (XRS) and MND measurements indicated a helical magnetic order of full Eu2+ moments locked into the ab plane, without the presence of any ferromagnetism (FM). The use of charge-magnetic interference phenomena in XRS for measuring hysteresis and determining trapped magnetic field value is presented. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G61.00007: Mechanism of helical interlayer magnetic structure in RbEuFe4As4 Alexei Koshelev Motivated by the discovery of helical magnetic structure in RbEuFe4As4 [1], we study interlayer ordering of magnetic moments in a material composed of spatially-separated superconducting and ferromagnetic layers [2]. We consider the interplay between the normal and superconducting indirect exchange interaction mediated by tunneling between the conducting layers. The normal interlayer interaction can be related with the 2D density of states of an isolated layer. For shallow bands, such interaction is ferromagnetic and short-range. On the other hand, the superconducting contribution always gives antiferromagnetic interaction and extends over several layers if the interlayer hopping energy exceeds the superconducting gap. The frustration between the normal and superconducting parts may lead to a spiral magnetic configuration. The 90ο angle between the neighboring-layers moments observed in RbEuFe4As4 is caused by four-fold in-plane anisotropy. This model explains a physical origin of the magnetic spiral in RbEuFe4As4. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G61.00008: Competition between spin-vortex crystal and superconductivity in Ni-doped CaKFe4As4 Andreas Kreyssig, John Wilde, Anna E. Böhmer, Wei Tian, William Meier, Bing Li, Benjamin G. Ueland, Mingyu Xu, Sergey L. Bud'ko, Paul C. Canfield, Robert McQueeney, Alan I. Goldman CaKFe4As4 is a member of the iron arsenide superconductors in which partial substitution of Ni for Fe shifts the ground state from superconducting to antiferromagnetically (AFM) ordered. Employing neutron diffraction we determined that the AFM order is a non-collinear, commensurate structure with a hedgehog spin-vortex crystal (SVC) arrangement in the Fe planes and a simple AFM stacking perpendicular to them. The long-range SVC order coexists with superconductivity, however, similar to the doped 122 compounds with collinear stripe AFM, the ordered magnetic moment is gradually suppressed as the superconducting order parameter develops. Almost complete suppression of the AFM order is observed in a sample close to the critical Ni concentration, resulting in a back-bending of the AFM-transition-temperature phase line as a function of concentration. This supports the notion that both collinear and non-collinear magnetism as well as superconductivity compete for the same Fermi-surface nested electrons in the iron arsenide superconductors. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G61.00009: Magnetic susceptibility of realistic superconducting samples Ruslan Prozorov, Makariy A Tanatar, Naufer M Nusran, Kyuil Cho, Kamal Joshi, Sunil Ghimire Finite element numerical exploration of the Meissner-London equations was performed for realistic three-dimensional (3D) superconducting samples of non-ellipsoidal shape and, also, having internal textures. The goal was to establish the connection between the effective finite-size magnetic susceptibility, geometric parameters of the model and the intrinsic parameters such as London penetration depth. Theoretical results are compared with the experiments where the total magnetic susceptibility was measured using tunnel-diode resonator and local first critical field was measured using NV-centers in diamond optical magnetometry. The observed differences are attributed to the specific morphological features of iron-based superconductors and high-Tc cuprates. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G61.00010: Cooperative response of the magnetic and superconducting subsystems in magnetic superconductor RbEuFe4As4 Vitalii Vlasko-Vlasov, Ulrich Welp, Alexei Koshelev, Matthew Smylie, Jinke Bao, Duck Young Chung, Mercouri Kanatzidis, Wai-Kwong Kwok We present magneto-optics and magnetization based studies of the magnetic flux evolution in single crystals of the magnetic superconductor RbEuFe4As4 during field cooling and warming, and magnetic field cycling at temperatures above and below the magnetic transition point, Tm. The vortex patterns emerging at T below and ~ Tm on different crystal facets in the fields of different orientations, reveal a peculiar magnetization process, where the magnetic subsystem serves as an internal magnetic flux pump while the superconducting subsystem controls the entry of magnetic flux quanta into the bulk. The interplay of magnetic susceptibility amplifying the magnetic induction and vortex pinning attenuating the magnetic flux entry, results in a field and temperature dependent critical state that mimics a paramagnetic Meissner effect. The observed anisotropic vortex dynamics is associated with a peculiar fine scale Eu-spin helical structure modified by the vortex field and resulting in a nontrivial spatial current distribution, which yields a self-consistent inhomogeneous enhancement of the sample magnetization. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G61.00011: Temperature dependent resistivity in hole-doped Ba1−xKxFe2As2 subject to electron irradiation Makariy Tanatar, Yong Liu, T. A. Lograsso, Ruslan Prozorov, Marcin Konczykowski, Olivie Cavani Temperature dependence of in-plane and inter-plane electrical resistivity was studied in single crystals of hole doped iron based superconductor Ba1−xKxFe2As2 (0≤x≤1). Low temperature (20 K) irradiation with relativistic 2.5 MeV electrons was used to control residual resistivity of the samples. Matthiessen rule is found to be universally violated for all compositions and for both current flow directions. This is in stark contrast with electron doped Ba(Fe1−xCox)2As2 [1] and iso-electron substituted BaFe2(As1−xPx)2 [2]. |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G61.00012: Strain effect on the superconductivity of FeSe0.5Te0.5 thin films induced by mica substrate Ye Gao, Chenguang Mei, Miao Meng, Xi Zhang, Yonggang Zhao Iron-based superconductors have attracted much attention because of their promising prospects in both applied and fundamental research. Iron chalcogenide superconductors, which have the simplest structure, play an important role in this field. It has been shown that pressure or strain has remarkable influence on their superconducting transition temperatures (TC) [1, 2]. To further explore the strain effect, we grow epitaxial FeSe0.5Te0.5 (FST) thin films on muscovite mica substrates by pulsed laser deposition, because different strains can be induced by bending mica owing to the flexibility of mica after mechanical exfoliation [3]. It is shown that TC value of FST increases under compressive strain while decreases under tensile strain. It is also indicated that the relative orientations of electrical current and strain are important in the change of transport property. The results are discussed by considering different contributions. This work is helpful to understand superconducting mechanism of iron chalcogenide superconductors, and shows a great potential for novel applications with flexible superconductors. |
Tuesday, March 3, 2020 2:03PM - 2:15PM |
G61.00013: Spontaneous Nernst Effect in an Iron-based Topological Superconductor Fe1+yTe1-xSex Lu Chen, Ziji Xiang, Colin B Tinsman, Genda Gu, Lu Li We report on a study of Nernst effect in an iron-based superconductor Fe1+yTe1-xSex. We observe a zero-field Nernst effect that appears around the superconducting transition temperature Tc. This spontaneous Nernst signal does not follow the temperature and field dependence of the thermal power and occur only in the temperature range near Tc where the superconducting fluctuation is strong and vortex liquid is robust. This intrinsic spontaneous Nernst signal indicates the violation of time reversal symmetry (TRS) in the superconducting state. The TRS in Fe1+yTe1-xSex may be broken by pinning flux introduced by the interstitial iron impurity. To further understand the role of excess Fe atoms, we study a series of Fe1+yTe1-xSex single crystals that have different Tc and different levels of excess Fe concentrations and demonstrate how they affect the spontaneous Nernst effect signal. |
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