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 P54: Fe-Based Superconductors: Quantum Criticality and TopologyFocus Live
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Sponsoring Units: DMP Chair: Paul Malinowski, University of Washington |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P54.00001: Specific Heat and Critical Behavior in BaFe2(As1-xPx)2 Invited Speaker: Camilla Moir With Tc’s below 40 K and evidence of a quantum critical point [1], the iron-based high-temperature superconductor BaFe2(As1-xPx)2 is an appealing system for investigating the behavior underlying superconductivity in high-Tc superconductors. By applying magnetic fields up to 35 T, we are able to suppress superconductivity and reveal the normal state of overdoped BaFe2(As1-xPx)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 high-temperature superconductors. |
Wednesday, March 17, 2021 3:36PM - 4:12PM Live |
P54.00002: Suppression of superconductivity by anisotropic strain near a nematic quantum critical point Invited Speaker: Paul Malinowski In most unconventional and high-temperature superconductors, superconductivity emerges as a nearby symmetry-breaking phase is suppressed by chemical doping or pressure, leading to the belief that the fluctuations associated with the symmetry-breaking phase are beneficial, if not responsible, for the superconducting pairing. A direct test to verify this hypothesis is to observe a decrease of the superconducting critical temperature (Tc) by applying the symmetry-breaking conjugate field that suppresses the dynamic fluctuations of the competing order. Here1, using electrical transport, magnetic susceptibility, and x-ray diffraction measurements, we show that anisotropic strain, the conjugate field of nematicity, reduces the Tc of the iron pnictide superconductor Ba(Fe1-xCox)2As2. For optimally doped samples we show a nearly fivefold reduction of Tc with less than one per cent of strain. This extreme sensitivity disappears as the doping is increased away from optimal. For underdoped samples, Tc becomes zero yielding a metallic ground state. In addition to providing direct evidence of the role played by the nematic fluctuations in the formation of the superconducting state, these results also demonstrate a superconductor to metal transition as a function of a clean tuning parameter in a three-dimensional system. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P54.00003: Dynamical nematic susceptibility from spin excitation anisotropy in quantum critical iron pnictides* Chia-Chuan Liu, Elihu Abrahams, Qimiao Si Quantum criticality in iron pnictides involves both the nematic and antiferromagnetic degrees of freedom[1,2], but the relationship between the two types of fluctuations has yet to be clarified. Here we study this problem in the presence of a small external uniaxial potential, which breaks the C4-symmetry in B1g sector. We establish an identity that connects the spin excitation anisotropy, which is the difference of the dynamical spin susceptibilities at (pi,0) and (0,pi) [3], with the dynamical magnetic susceptibility and static nematic susceptibility. Using this identity, we introduce a scaling procedure to determine the dynamical nematic susceptibility in the quantum critical regime, and illustrate the procedure for the case of the optimally Ni-doped BaFe2As2 [3]. We also provide the theoretical understanding for the so-determined dynamical nematic susceptibility. The implications of our results for the overall physics of the iron-based superconductors are discussed. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P54.00004: Unconventional Dynamical Scaling close to a Nematic Quantum Critical Point Pascal Reiss, David E Graf, Amir-Abbas Haghighirad, Thomas Vojta, Amalia Coldea In the vicinity of quantum critical points, the complex interplay between electronic and structural order can lead to a vast range of highly unconventional phases. Of particular interest is the electronic nematic order, with its predicted long-range interactions mediated through the lattices shear modes. Here, we report an unusual scaling relation of the magnetoresistivity in the iron-based superconductor FeSe0.89S0.11 when tuned to its nematic quantum critical point under hydrostatic pressure. We observe a remarkably sharp crossing over two decades in temperature, which fulfills a power-law scaling relation with diverging critical exponents at low temperatures, in stark contrast to the usual fixed exponent ansatz. We discuss our findings in the context of disconnected static and dynamic quantum fluctuations, a coupling between electronic and phononic modes, and topological changes of the Fermi surface. These lead to the emergence of an atypical non-zero energy scale at the quantum critical point which strongly affects superconductivity. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P54.00005: Prediction of Double-Weyl Nodes in the Iron-based Superconductor CaKFe4As4 Niclas Heinsdorf, Morten Holm Christensen, Mikel Iraola, Shang-Shun Zhang, Turan Birol, Cristian Batista, Rafael Fernandes, Roser Valenti By combining group-theoretical symmetry analysis, low-energy model calculations, and ab-initio simulations, we predict the presence of a pair of magnetic-field induced Weyl nodes close to the Fermi level in the iron-based superconductor CaKFe4As4. Contrary to other topological phenomena proposed in iron-based superconductors, this one originates entirely from the 3d Fe states. As opposed to conventional Weyl fermions, the Weyl states realized in this material carry a topological charge of ±2, making CaKFe4As4 a candiate for a double-Weyl semimetal, when appropriately tuned or doped. The higher-order topological charge is stabilized by the fourfold rotational symmetry of the M-A line, which leads to a quadratic dispersion of the Weyl nodes in the kx-ky plane. We identify the corresponding surface states by comparing the material’s bulk and surface spectra, finding two Fermi arcs that connect the projections of the Weyl nodes. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P54.00006: Nearly quantized conductance plateau of vortex zero mode in an iron-based superconductor Shiyu Zhu, Lingyuan Kong, Lu Cao, Hui Chen, Michal Papaj, Shixuan Du, Yuqing Xing, Wenyao Liu, Dongfei Wang, Chengmin Shen, Fazhi Yang, John Schneeloch, Ruidan Zhong, Genda Gu, Liang Fu, Yu-Yang Zhang, Hong Ding, Hongjun Gao Majorana zero modes (MZMs) are spatially localized, zero-energy fractional quasiparticle with non-Abelian braiding statistics that hold promise for topological quantum computing. Owing to the particle-antiparticle equivalence, MZMs exhibit quantized conductance at low temperature. By using variable-tunnel-coupled scanning tunneling spectroscopy, we studied tunneling conductance of vortex bound states on FeTe0.55Se0.45 superconductors. We report observations of conductance plateaus as a function of tunnel coupling for zero-energy vortex bound states with values close to or even reaching the 2e2/h quantum conductance (where e is the electron charge and h is Plank’s constant). By contrast, no plateaus were observed on either finite energy vortex bound states or in the continuum of electronic states outside the superconducting gap. This behavior of the zero-mode conductance supports the existence of MZMs in FeTe0.55Se0.45. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P54.00007: Spatially dispersing Yu-Shiba-Rusinov states in the unconventional superconductor FeTe0.55Se0.45 Damianos Chatzopoulos, Doohee Cho, Koen Bastiaans, Gorm O Steffensen, Damian Bouwmeester, Alireza Akbari, Genda Gu, Jens Paaske, Brian M Andersen, Milan P. Allan By using scanning tunneling microscopy (STM) we find and characterize dispersive, energy symmetric in-gap states in the iron-based superconductor FeTe0.55Se0.45, a material that exhibits signatures of topological superconductivity, and Majorana bound states at vortex cores or at impurity locations. We use a superconducting STM tip for enhanced energy resolution, which enables us to show that impurity states can be tuned through the Fermi level with varying tip-sample distance. We find that the impurity state is of the Yu-Shiba-Rusinov (YSR) type, and argue that the energy shift is caused by the low superfluid density in FeTe0.55Se0.45, which allows the electric field of the tip to slightly penetrate the sample. We model the newly introduced tip-gating scenario within the single-impurity Anderson model and find good agreement to the experimental data. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P54.00008: Nematic and Antiferromagnetic Quantum Criticality in a Multi-Orbital Hubbard Model for Iron Pnictides Wenjun Hu, Lei Chen, Haoyu Hu, Rong Yu, Hsin-Hua Lai, Luca 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, how to approach it in the multi-orbital Hubbard model has been a long-standing challenge due to the limitation in methods for the intermediate correlations. Here [1] we study this problem within a multi-orbital Hubbard model containing both the Hubbard and Hund’s interactions, by a variational Monte Carlo method based on Jastrow-Slater wave functions that allow for a non-perturbative treatment of the electron correlations. We find strong evidence for the existence of a unique quantum critical point, where both the nematic and (π, 0) antiferromagnetic orders develop, in the bad-metal regime of the phase diagram. A robust signal for superconducting pairing is also found as the system approaches the quantum critical point from the paramagnetic side. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P54.00009: STM study of Quantum phase transition in LiFe1-xCoxAs Daniel Multer, Jiaxin Yin, Songtian Sonia Zhang, Guangyang Dai, Yuanyuan Zhao, Andreas Kreisel, Gennevieve Macam, Xianxin Wu, Hu Miao, Brian M Andersen, Nana Shumiya, Maksim Litskevich, Zijia Cheng, Xian Yang, Tyler Cochran, Guoqing Chang, Ilya Belopolski, Lingyi Xing, Yi Gao, Feng-chuan 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 LiFe1-xCoxAs 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 strong-coupling 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 Born-limit scattering effect in a s±-wave superconductor, unveiling the driving force of the superconductor to metal quantum phase transition. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Live |
P54.00010: Half-integer level shift of vortex bound states in an iron-based superconductor Lingyuan Kong, Shiyu Zhu, Michal Papaj, Hui Chen, Lu Cao, Hiroki Isobe, Yuqing Xing, Wenyao Liu, Dongfei Wang, Peng Fan, Yujie Sun, Shixuan Du, John Schneeloch, Ruidan Zhong, Genda Gu, Liang Fu, Hongjun Gao, Hong Ding Vortices in topological superconductors may host Majorana zero modes (MZMs), which have been proposed as the building blocks of fault-tolerant topological quantum computers. Recently, a new single-material platform with the potential for realizing MZMs has been discovered in iron-based superconductors, without involving hybrid semiconductor–superconductor structures. Here, we report a detailed scanning tunnelling spectroscopy study of a FeTe0.55Se0.45 single crystal and show that this material hosts two distinct classes of vortex. These differ by a half-integer level shift in the energy spectra of the vortex bound states. This level shift is directly tied to the presence or absence of a zero-bias conductance peak and also alters the ratios of higher energy levels from integer to half-odd-integer. Our model calculations fully reproduce the spectra of these two types of vortex bound state, suggesting the presence of regions with and without topological surface states, which coexist within the same crystal. Our findings provide strong evidence for the presence of MZMs in FeTe0.55Se0.45 and establish it as an excellent platform for further studies. |
Wednesday, March 17, 2021 5:48PM - 6:00PM Live |
P54.00011: Quantum oscillations in the high-Tc tetragonal phase of FeSe1-xSx tuned by applied hydrostatic pressure Zachary Zajicek, Pascal Reiss, David E Graf, Amir-Abbas Haghighirad, Amalia Coldea Bulk FeSe suffers a remarkable four-fold enhancement in its critical temperature under hydrostatic pressure. Furthermore, combined chemical and applied pressure in FeSe1-xSx provides important tuning parameters of the phase boundaries between the competing nematic and magnetic phases from which superconductivity emerges. While the isoelectronic substitution leads to the suppression of the nematic order in the vicinity of x~0.170(5), the applied hydrostatic pressure gives additional access to the tetragonal high-pressure phase. We present a quantum oscillations study in magnetic fields up to 45T under applied hydrostatic pressure up to 22kbar and determine the evolution of the Fermi surface and electronic correlations inside the high-Tc tetragonal phase. We detect a monotonous increase in the sizes of the different Fermi surface pockets inside the tetragonal phase, whereas the electronic correlations remain relatively unaffected. |
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