Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session H10: Fe-based Superconductors IVFocus
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Sponsoring Units: DMP DCOMP Chair: Matthias Ikeda Room: BCEC 151B |
Tuesday, March 5, 2019 2:30PM - 3:06PM |
H10.00001: Pairing mechanisms in iron-based superconductors: variations on the s+/- theme Invited Speaker: Peter Hirschfeld I will review some of the recent developments in the theory of superconductivity |
Tuesday, March 5, 2019 3:06PM - 3:18PM |
H10.00002: Fermi surface pockets in iron pnictides revised by screened exchange dynamical mean-field theory Steffen Backes, Ambroise van Roekeghem, Silke Biermann Until today our understanding of iron based superconductors is hampered by a persistent systematic discrepancy between theory and experiment: the size of the Fermi surface pockets consistently turn out too large in theoretical calculations[1]. Here we show that this puzzle can be solved when taking into account the effect of nonlocal exchange beyond the Density Functional Theory plus dynamical mean-field theory(DFT+DMFT) approach. As it turns out, the nonlocal exchange properties in DFT+DMFT are rather poorly described, leading to an overestimation of the size of Fermi surface pockets. We show that an approximative treatment within screened exchange dynamical mean-field theory gives rise to momentum dependent Fermi surface modifications leading to a significant improvement with experiment. Additionally, we observe a Fermi surface modification in paramagnetic BaFe2As2, where the Fe 3dxy hole pocket in the Brillouin zone center vanishes completely. Reinterpreting existing angle-resolved photoemission (ARPES) experiments, we propose that indeed the Fe 3dxy states are located below the Fermi level, in contrast to previous belief[2]. |
Tuesday, March 5, 2019 3:18PM - 3:30PM |
H10.00003: Doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopant Shiyong Tan, Xingyu Ji, Xinchun Lai Both superconductivity and structural transition/orbital order are found to be suppressed by Co doping in FeSe single crystal. We have grown FeSe thin films on STO substrates using MBE, and studied the doping dependence of the electronic structure and superconductivity of FeSe thin films by Cobalt dopant. For single layer FeSe, The electron pockets at M point are enlarged when doped with cobalt, and the Tc is slightly increased. When the 2.5 UC, 3.25 UC and 4 UC FeSe sample are electron doped by cobalt, the nematic order is suppressed and superconductivity can be detected. For the multi-layer FeSe thin films, the electronic structure turns out to be alike of single layer FeSe when they are effectively Co doped. But it is not superconducting at the largest doping level, which indicates that the interface effect plays an important role for the high Tc in 1UC FeSe/STO system. |
Tuesday, March 5, 2019 3:30PM - 3:42PM |
H10.00004: Phase diagram of compressed RbEuFe4As4 and CsEuFe4As4: Coexistence of superconductivity and magnetism Daniel Jackson, Derrick VanGennep, Wenli Bi, Dongzhou Zhang, Philipp Materne, Yi Liu, Guang-Han Cao, Samuel T Weir, Yogesh Kumar Vohra, James Hamlin RbEuFe4As4 and CsEuFe4As4 are members of the recently discovered 1144 superconducting family, they exhibit the unusual combination of superconductivity (Tc ~ 35 K) and ferromagnetism (Tm ~ 15 K). We performed a series of measurements under compression including x-ray diffraction, magnetic susceptibility, and electrical resistivity. Compression monotonically suppresses Tc and enhances Tm. Near 7 GPa, Tc onset and Tm become comparable, at higher compression evidence of bulk superconductivity gradually disappears. Room-temperature x-ray diffraction measurements provide evidence of a transition from tetragonal to a half-collapse-tetragonal structure near 10 GPa for RbEuFe4As4 and near 12 GPa for CsEuFe4A4. The cross-over from ferromagnetic superconductor (Tc > Tm), to superconducting ferromagnet (Tm > Tc) occurs before the structural transition, suggesting that RbEuFe4As4 and CsEuFe4As4 are an ideal system for studying the interplay between superconductivity and magnetism. |
Tuesday, March 5, 2019 3:42PM - 3:54PM |
H10.00005: Specific heat under pressure on selected iron-pnictide superconductors Elena Gati, Gil Drachuk, Li Xiang, Anna Boehmer, Sergey Budko, Paul Canfield The phase diagrams of iron-pnictide superconductors manifest a complex interplay of superconductivity, magnetism and structure which can typically be tuned by external tuning parameters, such as pressure. In this contribution, we report a study of the specific heat under pressure on selected iron-pnictide superconductors, such as FeSe, as well as pure and Co-doped BaFe2As2. We will compare pressure-dependent phase diagrams, as well as provide further characterization of the superconducting properties. The latter will, in particular, include a discussion of the evolution of the specific heat jump sizes at the superconducting transition with pressure. |
Tuesday, March 5, 2019 3:54PM - 4:06PM |
H10.00006: Specific heat evidence for rotational symmetry breaking in RbFe2As2 Ohei Tanaka, Yuta Mizukami, Kousuke Ishida, Masaya Tsujii, Shigeyuki Ishida, Akira Iyo, Hiroshi Eisaki, Thomas Wolf, Kai Grube, Hilbert loehneysen, Takasada Shibauchi RbFe2As2 has the 3d5.5 electronic configuration, which corresponds to the most heavily hole-doped system among 122-type iron-based superconductors. In this system, a large mass enhancement has been observed, and the importance of the vicinity to the Mott insulator phase at 3d5 configuration has been suggested [1]. Recently, we have observed nematic fluctuations along the Fe-As direction, 45° tilted from the nematic direction usually found in other iron-based superconductors. Although the two-fold symmetry along the Fe-As direction has also been suggested from STM measurements [2], the bulk evidence for nematic ordering has not been reported. Here, we perform field-angle-resolved specific heat measurements in RbFe2As2 single crystals by using a high-resolution long-relaxation calorimeter. We choose tiny crystals of about 10 μg to avoid cancellation effect of multiple nematic domains. We find clear two-fold oscillations as a function of in-plane field angle near the superconducting transition. This provides bulk evidence for long-range nematic ordering along the Fe-As direction in RbFe2As2. |
Tuesday, March 5, 2019 4:06PM - 4:18PM |
H10.00007: Specific heat in strongly hole-doped Iron-based superconductors Dmitry Chichinadze, Andrey Chubukov In our work we compute the specific heat C(T) in an Fe-based superconductor with only hole pockets. We use a three orbital/three pocket model with two smaller hole pockets made out of dxz and dyz orbitals and a larger pocket made out of dxy orbital. We use the experimental fact that the mass of dxy fermion is much heavier than that of dxz/dyz fermions as an input. We argue that the heavy dxy band contributes most to the specific heat in the normal state, but the superconducting gap on the dxy pocket is parametrically smaller than that on dxz/dyz pockets. We also argue that in this situation the jump of C(T) at Tc is determined by dxz/dyz fermions, and the ratio (Cs - Cn)/Cn is a fraction of that in a one-band BCS theory. Below Tc, C(T) remains relatively flat down to some T*, below which C(T) rapidly drops. This behavior is consistent with the data for KFe2As2 and related materials. We claim that the data on C(T) can be reproduced without assuming that the quasiparticle residue Z on dxy band is small. We further argue that the very existence of a finite T* < Tc favors s+- gap structure over d-wave, because in the latter case T* would vanish. |
Tuesday, March 5, 2019 4:18PM - 4:30PM |
H10.00008: Coexistence of superconductivity and magnetism in CaK(Fe1-xNix)4As4 as probed by 57Fe Mössbauer spectroscopy Sergey Budko, Vladimir G. Kogan, Ruslan Prozorov, William Meier, Mingyu Xu, Paul Canfield Temperature dependent 57Fe Mössbauer spectroscopy and specific heat measurements for CaK(Fe1-xNix)4As4 with x = 0, 0.017, 0.033, and 0.049 are presented. No magnetic hyperfine field (i.e. no static magnetic order) was detected down to 5.5 K for x = 0 and 0.017 in agreement with the absence of any additional feature below superconducting transition temperature, Tc, in the specific heat data. The evolution of magnetic hyperfine field with temperature was studied for x = 0.033 and 0.049. The long-range magnetic order in these two compounds coexists with superconductivity. The magnetic hyperfine field, Bhf, (ordered magnetic moment) below Tc in CaK(Fe0.967Ni0.033)4As4 is continuously suppressed with the developing superconducting order parameter. The Bhf(T) data for CaK(Fe0.967Ni0.033)4As4, and CaK(Fe0.951Ni0.049)4As4 can be described by Machida's model for coexistence of itinerant spin density wave magnetism and superconductivity [K. Machida, J. Phys. Soc. Jpn. 50, 2195 (1981)]. |
Tuesday, March 5, 2019 4:30PM - 4:42PM |
H10.00009: Nuclear magnetic resonance techniques for in-situ strain measurements using surface coils Charles Snider, Erick Garcia, Johanna Palmstrom, Ian R Fisher, Vesna F Mitrovic Widespread efforts have been made to study the role of nematic order in the high temperature superconductivity of iron-based pnictides. This includes the increasing use of strain to explore the anisotropies in electronic states and the nematic susceptibility. Nuclear magnetic resonance (NMR) is the ideal probe to explore such properties because it is sensitive to spin and charge degrees of freedom while allowing the measurement of nematic properties in the superconducting state. It has been shown that strain can be fully transmitted to samples up to approximately 100 μm thick using piezoelectric stacks [J.H. Chu et al. Science 2012], but there are practical limitations in the application of such techniques in NMR. The nontrivial issue of probing such small samples can be overcome by the novel use of surface coils that enhance the signal to noise ratio [W. Liu et al. Rev. Sci. Instrum. 2017] and allow for in-situ sample rotations and controllable application of strain. In this talk, we demonstrate how to simultaneously implement such techniques in NMR experiments. |
Tuesday, March 5, 2019 4:42PM - 4:54PM |
H10.00010: Switching effect in metal-superconductor-metal Fe(Te0.7Se0.3) nanojunction Yun Ling, Andrew Steely, Abin Joshy, Zhiqiang Mao, Jiang Wei We report our transport study on the metal-superconductor-metal (MSM) device based on Fe(Te0.7Se0.3) nanoflakes. An unusual switching behavior in I-V characteristics for flakes with a thickness below 10nm has been observed. Furthermore, we discovered that such switching behavior could be controlled by an external magnetic field and bias voltage. Given the traditional understanding that superconductivity in Fe(Te0.7Se0.3) manifests as the collective effect from clusters of nanoscale superconducting domains, we proposed a self-heating model to explain such switching behavior. Our work demonstrates that nanoscale junction between superconducting and normal metal naturally exists in thin Fe(Te0.7Se0.3) nanoflakes, which provides an opportunity to study transport behavior MSM junction in nanoscale. |
Tuesday, March 5, 2019 4:54PM - 5:06PM |
H10.00011: Nodeless superconductivity with broken time reversal symmetry Huiqiu Yuan In unconventional superconductors, additional symmetries, e.g., rotational symmetry and time reveral symmetry (TRS), may be broken below the superconducting transition temperature. In the past, evidence for TRS breaking was observed in a few nodal superconductors with a possible triplet pairing state. Recently, there are a growing number of compounds which behave like a simple s-wave BCS superconductor, but break TRS below T_c. How to reconcile these seemingly contradictory feature is currently an interesting problem. In this presentation, we will present our recent results on a few Re- and La-based nodelss superconductors with broken TRS at T_c, including a few new examples. We will try to summrize their common features and discuss the possible pairing states and origin of the TRS breaking. |
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