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 J47: Superconductivity: Tunneling Phenomena (single particle tunneling and STM)Live
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Sponsoring Units: DCMP Chair: James Hamlin, University of Florida |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J47.00001: Spectroscopic Evidence for the Direct Involvement of Local Moments in the Pairing Process of the Heavy-Fermion Superconductor CeCoIn5 Keshav Shrestha, Shengzhi Zhang, Laura H H Greene, You Lai, Ryan Baumbach, Kalyan Sasmal, M Brian Maple, Wan Kyu Park The electron pairing in heavy-fermion superconductors has been widely considered to be mediated by antiferromagnetic spin fluctuations. Nonetheless, the exact origin of the neutron spin resonance in CeCoIn5 remains controversial and the corresponding tunneling signature is missing, contrary to other unconventional superconductors including the cuprate and Fe-based superconductors. Here we report results from planar tunneling spectroscopy performed along three major crystallographic orientations of CeCoIn5 [1]. Our detailed and reproducible conductance spectra provide strong evidence for: i) the existence of preformed pairs well above Tc; and ii) the direct involvement of localized f-electron moments in the pairing process, suggesting that the local physics manifested via the Kondo resonance plays a key role. We will discuss the underlying microscopic mechanism in terms of a proposed theoretical model [2]. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J47.00002: Superconductor Energy-Gap Symmetry Determination using Multi-Atom Quasiparticle Interference Imaging Rahul Sharma, Andreas Kreisel, Miguel Antonio Sulangi, Jakob Böker, Andrey Kostin, Milan Allan, Hiroshi Eisaki, Anna Boehmer, Paul C. Canfield, Ilya Eremin, James C Davis, Peter Hirschfeld, Peter Oliver Sprau A detailed knowledge of symmetry of superconducting gap Δα(k), for all momenta k on Fermi surface of each band α is required for a complete understanding of complex superconductors. Numerous techniques exist for determining |Δα(k)| but few to measure the sign of Δα(k). Recently, a new technique was introduced where energy- and phase-resolved Fourier analysis of quasiparticle interference images identifies whether the wavevectors connect Δα(k) of same-sign or opposite-sign. But the reliance on precise single impurity atom location for phase accuracy renders this technically difficult. We introduce a generalized technique which can be used with multiple impurities. We show its validity by comparing results of our technique with FeSe where s± Δα(k) symmetry is established. To exemplify utility, we apply our technique on LiFeAs and find that scattering interference between the hole- and electron-pockets indicates opposite sign. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J47.00003: Atomic-Scale Visualization of the Cooper-Pair Density Wave State in NbSe2 Xiaolong Liu, Yi Xue Chong, Rahul Sharma, J.C. Séamus Davis Transition-metal dichalcogenides (TMDs) have emerged as a rich platform hosting novel states of quantum matter. One such is the Cooper-pair density wave (PDW) state in which the electron-pair density spatially modulates at one or more wavevectors. Using atomic-resolution scanned Josephson-tunneling microscopy, we visualize the electron-pair density and discover a PDW state in the canonical TMD superconductor NbSe2. We show that the PDW shares the wavevectors of the preexisting charge density wave (CDW) and is directly coupled to the background superconductivity as evidenced by their mutual decay into a superconducting vortex core. However, simultaneous PDW and CDW imaging reveals them to be spatially distinct at atomic-scale due to a global spatial phase difference |δΦΙ≈2π/3 (one unit cell) between the two states. Given the abundance of TMDs sustaining both CDW and superconductivity, detection and visualization of PDW in NbSe2 presages abundant new PDW physics. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J47.00004: A strongly inhomogeneous superfluid in an iron-based superconductor Doohee Cho, Koen Bastiaans, Damianos Chatzopoulos, Genda Gu, Milan Allan Among the mysteries surrounding unconventional superconductors is the possibility of spatial variations in their superfluid density. We use Josephson scanning tunneling microscopy (JSTM) to reveal a strongly inhomogeneous superfluid in an iron-based superconductor. JSTM is based on the phenomenon of Josephson tunneling; Cooper-pairs tunnel through the vacuum barrier between two superconductors. One can extract the superconducting order parameter from the current-voltage spectra. Here we will present our JSTM results on FeTe0.55Se0.45 acquired with a Pb-coated tip [1]. By simultaneously measuring the topographic and electronic properties, we find that the inhomogeneity in the superfluid density is not caused by structural disorder or inter-band scattering, and is not correlated with the variations in the pair-breaking gap. Importantly, we reveal a spatial correlation between the inhomogeneous order parameter and the quasiparticle strength defined as the height of the coherence peak. This relation has been observed in copper oxide superconductors on the macroscopic scale. When repeated at different temperatures, our technique could help to elucidate what mechanisms limit the critical temperature in unconventional superconductors. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J47.00005: Visualizing the Cuprate Pair Density Wave State Zengyi Du, Hui Li, Sanghyun Joo, Elizabeth P. Donoway, Jinho Lee, James C Davis, Genda Gu, Peter David Johnson, Kazuhiro Fujita Recently wide attention has been focused on the pair density wave (PDW) state in the cuprate high temperature superconductors. By implementing a novel superconducting tip technology in the spectroscopic imaging-scanning tunneling microscopy (SI-STM), we have observed a modulating gap with 8-unit-cell periodicity or at wavevectors Q=2π/a0(1/8,0) and 2π/a0(0,1/8) in the single particle tunneling on nearly optimally doped Bi2Sr2CaCu2O8+δ. This is the first spectroscopic evidence for a PDW to exist in the cuprates. We also observed induced density wave modulations in the local density-of-states N(r, E) at both Q and 2Q, which further supports the existence of the PDW that is intertwined with the spatially uniform d-wave superconductivity. By mapping density wave spatial phases, PDW spatial phases tend to change by π surrounding 2π topological defects found in the induced N(r,E) modulations at 2Q. This suggests a presence of the half vortex in the PDW state. In this talk, we will present a canonical signature of the PDW states in the cuprate. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J47.00006: Local electronic structure of highly doped cuprate superconductors Willem Tromp, Irene Battisti, Koen Bastiaans, Damianos Chatzopoulos, Steef Smit, Yingkai Huang, Erik Van Heumen, Mark Golden, Milan Allan Superconductivity in the unconventional, high-Tc cuprate superconductors emerges with sufficient hole-doping of the parent antiferromagnetic Mott insulating state. Upon further doping the superconductivity is eventually suppressed before fully breaking down. In this overdoped regime the superconducting state is thought to emerge from a more conventional Fermi liquid. The suppression of superconductivity would then follow a more BCS-like picture, with gap closure due to a vanishing pairing interaction. Here, we map the electronic structure and the superconducting gap of the single layer cuprate (Pb,Bi)2Sr2CuO6+x using Spectroscoping Imaging Scanning Tunneling Microscopy and show how they evolve as Tc approaches zero in the overdoped regime. We find a highly heterogeneous electronic structure, with spectra with a gap and without a gap coexisting in close proximity at sufficiently high doping. Despite this strong inhomogeneity we still find a QPI signal consistent with full hole-like Fermi surfaces well-matched with ARPES results on the same samples. In this talk, I will discuss the implications of our data for the overdoped cuprates. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J47.00007: Imaging doubled shot noise in a Josephson scanning tunneling microscope Koen Bastiaans, Doohee Cho, Damianos Chatzopoulos, Maarten Leeuwenhoek, Corne Koks, Milan Allan In this talk I will show how we have imaged the current noise with atomic resolution in a Josephson scanning tunneling microscope [1]. By measuring the current noise as a function of applied bias, we reveal the step from single-electron tunneling above the superconducting gap energy to double-electron charge transfer below the gap energy when Andreev processes become dominant. By spatially resolving the noise on a superconducting Pb(111) surface we demonstrate that this doubling of the noise occurs homogeneously on the sample surface, also on impurity locations, indicating that indeed the charge pairing is not influenced by disruptions in the superconductor smaller than the superconducting coherence length. In addition, I will show how we have used this spectroscopic noise STM technique on a disordered film of TiN to discover Cooper pairing without a spectral gap in a disordered superconductor above Tc. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J47.00008: Tunneling dynamics between superconducting bound states at the atomic limit Haonan Huang, Ciprian Padurariu, Jacob Senkpiel, Robert Drost, Alfredo Levy Yeyati, Juan Carlos Cuevas, Björn Kubala, Joachim Ankerhold, Klaus Kern, Christian Ast A Yu-Shiba-Rusinov (YSR) state is a pair of protected in-gap levels generated by magnetic atoms on a superconductor. We are able to introduce YSR state of desired properties to the apex of a superconducting vanadium tip of a scanning tunneling microscope (STM) with a base temperature of 10mK, and then use this novel tip to tunnel into intrinsic YSR states on superconducting vanadium sample, thus studying the transport between YSR states, the Shiba-Shiba tunneling. As a realization of the fundamental tunneling process between two single levels, Shiba-Shiba tunneling features unusual sharp current peaks in the I(V) spectrum. By varying the conductance and tracing the evolution of the intensity of such peaks, we demonstrate a transition from sequential tunneling to emergent coherent tunneling from which we can extract the lifetime of YSR states directly. This paves the road for characterizing and manipulating coupled superconducting bound states, such as Andreev levels and Majorana bound states using STM. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J47.00009: Giant anisotropic magneto-resistance in an Ising superconductor-ferromagnetic insulator tunnel junction Kaifei Kang, Shengwei Jiang, Helmuth Berger, Laszlo Forro, Kenji Watanabe, Takashi Taniguchi, Jie Shan, Kin Fai Mak Superconductor-magnet interfaces built on van der Waals’ materials are promising hybrid systems to realize unconventional and topological superconductivity. Novel transport and topological phenomena have been predicted but remain largely unexplored experimental-wise. We will discuss the observation of giant anisotropic magneto-resistance (AMR) up to ~ 100% in an Ising superconductor-ferromagnetic insulator planar tunnel junction formed by 2D NbSe2 and CrBr<span style="font-size:10.8333px">3</span>. By examining the evolution of the tunneling spectrum under an in-plane magnetic field, we reveal that the giant AMR is originated from interfacial magnetic exchange coupling and spin-dependent quasiparticle scattering. We have also observed an intriguing magnetic hysteresis in the superconducting gap and the quasiparticle scattering rate. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J47.00010: Particle-Hole Asymmetry of Subgap Conductances in Superconductors Without Quasiparticle Poisoning Fnu Setiawan, Jay Sau One universal consequence of the mean-field theory of superconductivity is a precise particle-hole symmetry (PHS) of conductances into any subgap states. However, subgap conductances are often observed experimentally to be particle-hole asymmetric. The breaking of this PHS has mostly been attributed to quasiparticle poisoning (QP), a process which destroys the coherence of SC-based qubits. Here, we provide a generic mechanism for the PHS breaking without involving QP, namely the coupling of the subgap states to bosonic modes. Using the rate equation and Keldysh formalism, we show that such coupling gives rise to a particle-hole asymmetry in subgap conductances both in the weak and strong tunnel-coupling regimes. Our results therefore show that the PHS breaking of subgap conductances in SCs could be a signature of significant modification of the conductance spectra by coupling to low-frequency bosonic modes such as phonons instead of QP. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J47.00011: Visualizing the unusual spectral weight shift in DyBa2Cu3O7-δ thin film Hui Li, Zengyi Du, Zebin Wu, Daniel Putzky, Asish Kumar Kundu, Shize Yang, Sanghyun Joo, Jinho Lee, Yimei Zhu, Gennady Logvenov, Bernhard Keimer, Kazuhiro Fujita, Tonica Valla, Ivan Bozovic, Ilya K Drozdov As a member of Rare-earth barium copper oxides (Re-123) high temperature superconductors, DyBa2Cu3O7-δ (DBCO) may help understanding the material specifics in the electronic structures of cuprates, but not many studies have been reported yet. Here we report a spectroscopic imaging scanning tunneling microscopy (SI-STM) study of DyBa2Cu3O7-δ thin film, which is synthesized by oxide molecular beam epitaxy (MBE). A mutual conductance measurement shows that the critical temperature Tc is around 78K. In the SI-STM measurements, we have observed an unusual spectral weight transfer in the local density of states spectra, simultaneously forming multiple ring-like patterns in the local density of states maps. By changing the resistance between tip and sample, we also found that such spectral weight transfer showed a junction resistance dependence. In this talk, we will discuss these unusual phenomena and possible implications. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J47.00012: Theory of tunnelling conductance of d+ip-wave superconductor Yukio Tanaka, Yuhi Takabatake, Shu-Ichiro Suzuki The physics of zero energy Andreev bound states (ZESABS) in high Tc cuprate has been studied up to now [1-2]. One of the interesting issue is the instability of ZESABS against time reversal symmetry breaking [3]. Recently, d+ip pairing state has been proposed at the (110) surface [4]. We theoretically study the tunneling conductance of the d+ip-wave superconductor based on the quasiclassical Eilenberger theory. We demonstrate that the zero-bias conductance peak is robust against the spin-triplet p-wave surfacesubdominant order, whereas it is fragile against the spin-singlet s-wave one. Comparing our numerical results and the experimental results, we conclude the surface induced spin-triplet p-wave surface subdominant order is feasible. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J47.00013: Nonequilibrium interplay between Andreev bound states and Kondo effect Jiangbo He, Dong Pan, Guang Yang, Mingli Liu, Jianghua Ying, Zhaozheng Lyu, Jie Fan, Xiunian Jing, Guangtong Liu, Bo Lu, Dong E Liu, Jianhua Zhao, Li Lu, Fanming Qu Andreev bound states (ABSs) were explored as an equilibrium behavior employing tunneling spectroscopy. Under nonequilibrium, theorists did predict that the local density of states (DOS) of the ABSs split and broaden. We investigated an InAsSb quantum dot coupled to two superconductors to explore the nonequilibrium dynamics of ABSs and their interplay with Kondo effect. Through tunneling spectroscopy, we demonstrated a full picture of the nonequilibrium dynamic splitting and broadening of ABSs, which interplays with the nonequilibrium Kondo DOS. Since the observed features might mimic those in the normal superconductor-topological superconductor or Majorana junctions, our discovery could provide a relevant comparison to benchmark the Majorana signal in future experiments. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J47.00014: Periodicity transition of Coulomb oscillation in hybrid superconductor-semiconductor (Pb-InSb) system Jiayu Shi Because of the Cooper pair transport, the oscillation of charge electron of superconducting ground state is always 2e- periodic in an island with quantized electrons. The transition from 2e- to 1e- periodicity has been explored in different superconductor-semiconductor system when an external magnetic field or bias voltage is applied. But it hasn't been reported in the hybrid Pb-InSb structure yet. Comparing with other superconductor like Aluminum, Lead has a larger energy gap which is up to ~2.7meV and potentially leads to a larger topological gap. We observed an even-odd split at finite bias voltage, indicating the hard superconducting gap. Pb-InSb-Pb Josephson junction also shows the hard superconducting gap in the tunneling limit. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Not Participating |
J47.00015: Measurement of Heavy-Fermion Cooper Pair Symmetry from Multi-Atom Quasiparticle Interference in CeCoIn5 Weijiong Chen, Rahul Sharma, Shuqiu Wang, Andreas Kreisel, Cedomir Petrovic, Tien-Ming Chuang, Milan P. Allan, Peter Hirschfeld, James C Davis In heavy fermion superconductors, the orbital symmetry of the Cooper-pair is encoded in the momentum-space symmetry of the superconductor's energy-gap function Δ(k). However, while the magnitude |Δ(k)| can be determined by a variety of techniques, the sign of Δ(k) and thus its symmetry have never been directly measured for any heavy-fermion superconductor. Recently, a new technique dubbed MAHAEM based on phase-resolved superconducting quasiparticle interference patterns at multiple impurity atoms, has proven highly effective Δ(k) determination. Here we introduce MAHAEM for heavy fermion superconductor CeCoIn5. The analysis demonstrates that the orbital symmetry of the Cooper-pair wavefunction is dx2-y2. Such capabilities to directly measure the Δ(k) of any heavy fermion superconductor presage significant advances, especially for establishing the Δ(k) of putative topological superconductors. |
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