Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session S41: Superconductivity: Tunneling Phenomena |
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Sponsoring Units: DCMP Chair: Philip Adams, Louisiana State University Room: 388 |
Thursday, March 16, 2017 11:15AM - 11:27AM |
S41.00001: Density of States Modulations in Pseudo Gap of Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ Kyoung Seok Lee, Jae-joon Kim, Sang Hyun Joo, Jung Hoon Yoo, Min Seok Park, Jeong Soo Gwak, Genda Gu, Hiroshi Eisaki, Shin-ichi Uchida, J.C.Séamus Davis, Jinho Lee Pseudo-Gap (PG) phase of the cuprates is believed to hold a key to the understanding of the mechanism of high Tc superconductivity. Recently, QPI, SDW, and PDW modulations were observed in BSCCO and especially CDW phase in PG have been reported. These modulations' detailed origin, however, is still not fully understood let alone the relation to the superconductivity. Our STM study on Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ in PG phase revealed a density of states (DOS) modulation with properties different from conventional CDW or quasiparticle interference (QPI) : particle -- hole asymmetric dispersion and non-locality. In this talk, we will present a quantitative analysis to estimate the correlation length of these density modulations in real space as well as momentum space and their possible origin. We will also discuss the relation of our observation to previously reported CDW and QPI features. [Preview Abstract] |
Thursday, March 16, 2017 11:27AM - 11:39AM |
S41.00002: Electronic structure of the ingredient planes of Bi2Sr2CaCu2O8$+\delta $ and Bi2Sr2CuO6$+\delta $ superconductors XUCUN MA Understanding the mechanism of high transition temperature superconductivity in cuprates has been hindered by the apparent complexity of their multilayered crystal structure. Using a cryogenic scanning tunneling microscopy (STM), we report on layer-by-layer probing of the electronic structures of the ingredient planes (BiO, SrO, CuO2) of Bi2Sr2CaCu2O8$+\delta $ (Bi-2212) and Bi2Sr2CuO6$+\delta $ (Bi-2201) superconductors prepared by argon-ion bombardment and annealing (IBA) technique. We show that the well-known pseudogap (PG) feature observed by STM is inherently a property of the charge reservoir planes and thus irrelevant directly to Cooper pairing. The CuO2 planes are exclusively characterized by a small gap inside the PG. The small gap becomes invisible near Tc, which we identify as the superconducting gap. The results constitute severe constraints on any microscopic model for high Tc superconductivity in cuprates. Contributors: Yan-Feng Lv, Wen-Lin Wang, Hao Ding, Yang Wang, Yong Zhong, Ying Ding, Ruidan Zhong, John Schneeloch, Gen-Da Gu, Lili Wang, Ke He, Shuai-Hua Ji, Lin Zhao, Xing-Jiang Zhou Can-Li Song, and Qi-Kun Xue. [Preview Abstract] |
Thursday, March 16, 2017 11:39AM - 11:51AM |
S41.00003: Detection of s-wave superconductivity on monolayer CuO$_{\mathrm{2}}$ films on Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$. Yang Wang, Yong Zhong, Sha Han, Yanfeng Lv, Wenlin Wang, Ding Zhang, Hao Ding, Yimin Zhang, Lili Wang, Ke He, Canli Song, Xucun Ma, Qikun Xue High temperature superconductivity emerges when the CuO$_{\mathrm{2}}$ layer touches the doped charge reservoir blocks. The redistributed charge carriers at these interfaces condense into coherent Cooper pairs, albeit the exact underlying mechanism is still highly controversial. Targeting at this, we have mimicked the CuO$_{\mathrm{2}}$/charge reservoir interface by depositing the monolayer CuO$_{\mathrm{2}}$ films on optimal doped Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ substrates. Direct investigation on these superconducting CuO$_{\mathrm{2}}$ films, however, yields results in stark contrast with the common recognition. Despite of the well-known V shaped pseudogap, a U shaped gap is identified. This U shaped gap disappears at $T_{C}$ and is indifference to K, Cs and Ag adsorbates, in line with the traditional \textit{s-wave} superconductivity. In view of these results, we propose that superconductivity in cuprates may indeed stem from the modulation doping induced two dimensional hole liquid, which is confined in the CuO$_{\mathrm{2}}$ layers. [Preview Abstract] |
Thursday, March 16, 2017 11:51AM - 12:03PM |
S41.00004: Incommensurate charge ordered states in the $\mathit{t-t^{\prime}-J}$ model Peayush Choubey, Wei-Lin Tu, Ting-Kuo Lee, Peter Hirschfeld We solve the $\mathit{t-t^{\prime}-J}$ model within the Gutzwiller approximation to obtain commensurate and incommensurate charge ordered states, namely anti-phase charge density wave (APCDW) and nodal pair density wave (nPDW), respectively, which have been earlier shown to possess dominant $d$-form factor. We address a recent STM experiment [Hamidian \textit{et al.}, Nat. Phys. 3519 (2015)] on BSCCO-2212 by calculating the spatial pattern and spectrum of local density of states (LDOS) at typical STM tip heights in APCDW and nPDW states using a recently developed Wannier function based approach. With the help of Cu and O sub-lattice LDOS maps, we extract the bias dependence of the intra-unit cell form factors and average spatial phase difference in a manner very similar to the experiment. We find that the LDOS maps exhibit ladder like structures similar what has been observed in the STM studies. Moreover, the bias dependence of form factors and spatial phase difference in the nPDW state agrees very well with the experiment. With these findings, we propose that the nPDW states are good candidates for charge ordered states observed in the superconducting phase of underdoped cuprates. We also study the effects of a single impurity and random disorder in the APCDW and nPDW states [Preview Abstract] |
Thursday, March 16, 2017 12:03PM - 12:15PM |
S41.00005: Fe Impurities in Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ Michael Boyer, Unurbat Erdenemunkh, Brian Koopman, Ling Fu, Kamalesh Chatterjee, William Wise, Genda Gu, Eric Hudson We present our low-temperature scanning tunneling microscopy (STM) measurements on magnetic Fe impurities intentionally doped in Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$CaCu$_{\mathrm{2}}$O$_{\mathrm{8+\delta }}$ (BSCCO). Our high-energy-resolution spectral maps allow us to detail the effects Fe impurities have on the local density of states. Combining the results of our STM measurements with those of previously studied Zn and Ni impurities in BSCCO, we conclude that potential scattering of quasiparticles dominates magnetic scattering even for impurities with large magnetic moments. In addition, we show evidence that impurities act to locally suppress both superconducting and pseudogap states. [Preview Abstract] |
Thursday, March 16, 2017 12:15PM - 12:27PM |
S41.00006: Imaging Superconductivity and Charge Density Waves in 1T-Cu$_x$TiSe$_2$ using Scanning Tunneling Microscopy Thomas Neulinger, Zhenyu Wang, Shichao Yan, Davide Iaia, Vidya Madhavan 1T-Cu$_x$TiSe$_2$ exhibits both charge density wave (CDW) order and superconductivity. The nature of and interplay between these two phases in this material is not yet well-understood. It has been shown that right above the superconducting dome, the CDW undergoes a commensurate/incommensurate transition due to the formation of domain walls, which may play an important role in the superconducting transition. Here, we present scanning tunneling microscopy and spectroscopy measurements investigating the topography and electronic properties of Cu$_{0.08}$TiSe$_2$ with temperature down to 300mK. We find CDW order coexists with s-wave superconductivity, and we study the influence of CDW domain walls on the superconductivity in this material. This work seeks to clarify open questions regarding the phase diagram of this material. [Preview Abstract] |
Thursday, March 16, 2017 12:27PM - 12:39PM |
S41.00007: Imaging the coexistence of superconductivity and antiferromagnetism in Fe$_{1+y}$Te$_{1-x}$Se$_x$ (x=0.1) using spin-polarized scanning tunneling microscopy Haibiao Zhou, Ramakrishna Aluru, Vladimir Tsurkan, Alois Loidl, Joachim Deisenhofer, Peter Wahl Magnetism has been widely thought to play an important role in unconventional superconductivity. In iron chalcogenide Fe$_{1+y}$Te, the bicollinear antiferromagnetim (AFM) can be suppressed by Se doping, and consequently superconductivity appears. Though a competition between the two orders is expected, their relation has never been shown in details. Here, using spin-polarized scanning tunneling microscopy, we explore their relation at the atomic scale in an Fe$_{1+y}$Te$_{1-x}$Se$_x$ ($x$=0.1) single crystal with $T_{\mathrm {C}}$ = 10 K, in a regime of the phase diagram where a spin-glass phase has been detected. We clearly observe the short-range AFM order with domains of a lateral size of $\sim$ 10 nm embedded in a non-magnetic matrix. In addition we observe a superconducting gap with prominent coherent peaks in differential conductance spectroscopy with a gap size $2\Delta\sim$ 4 mV. Surprisingly, no correlation between the superconducting properties (gap size and zero bias conductance) and the local AFM order is observed, while the coherence peaks are weakened by the existence of excess iron atoms. Our observations put constraints on theories that are aimed at explaining the relation between magnetism and unconventional superconductivity. [Preview Abstract] |
Thursday, March 16, 2017 12:39PM - 12:51PM |
S41.00008: Sensing the quantum limit in scanning tunneling spectroscopy Berthold Jaeck, Christian R. Ast, Jacob Senkpiel, Matthias Eltschka, Markus Etzkorn, Joachim Ankerhold, Klaus Kern The tunneling current in scanning tunneling spectroscopy (STS) is typically modeled by a continuous and homogeneous charge flow. If the charging energy of a single-charge quantum sufficiently exceeds the thermal energy, however, the granularity of the current becomes non-negligible. In this quantum limit, the capacitance of the tunnel junction mediates an interaction of the tunneling charges with the surrounding electromagnetic environment and becomes a source of noise itself. Using a scanning tunneling microscope operating at 15\textunderscore mK, we show that we operate in this quantum limit, which determines the ultimate energy resolution in STS. In general, P(E)-theory describes the probability for a tunneling electron to exchange energy with the environment. In the context of STS, the P(E)-function can be regarded as the energy resolution function. We experimentally demonstrate this effect with a superconducting aluminum tip and sample, where it is most pronounced. We expect the P(E)-broadening to be most significant on intrinsically sharp features, such as superconducting coherence peaks, Yu-Shiba-Rusinov states or Kondo peaks, and to become observable at temperatures of 1 K or below. [Preview Abstract] |
Thursday, March 16, 2017 12:51PM - 1:03PM |
S41.00009: Sign Reversal Superconducting Gaps in Electron Fermi Surface Dominated System (Li$_{\mathrm{1-x}}$Fe$_{\mathrm{x}}$OH)FeSe Zengyi Du, Xiong yang, Qiangqiang Gu, Hai Lin, Delong Fang, Guan Du, Jie Xing, Huan Yang, Xiyu Zhu, Hai-Hu Wen In the research of iron based superconductors, the core issue is the pairing mechanism. The early proposed S$+$- manner, although is supported well by experimental data in iron based superconductors with both electron and hole pockets, seems however, facing challenges in some FeSe-derivative superconductors with only the electron-like Fermi surfaces. Our scanning tunneling microscopy measurements on (Li$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$OHFeSe single crystal, which contains only electron pockets on M points, reveal two superconducting gaps and quasiparticle interference (QPI) analysis allows us to rule out the $d$-wave gap and assign the larger (smaller) gap to the outer (inner) hybridized Fermi pockets. We report the evidence of sign reversal between two resolved superconducting gaps. The spectra on Fe-site impurities measured in zinc doped (Li$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$OHFeSe show strong in-gap bound states which are suppressed but hardly shifted with an applied 11T magnetic field. This typical feature is the evidence for sign reversal between two superconducting gaps. And we also get support from the QPI measurement around the single non-magnetic impurity. Our experimental results reveal the sign reversal of the gaps even in the electron Fermi surface dominated system. [Preview Abstract] |
Thursday, March 16, 2017 1:03PM - 1:15PM |
S41.00010: Electroresistance in superconductor-ferroelectric-superconductor junctions Victor Rouco, Anke Sander, Laura Begon-Lours, Sophie Collin, Stephan Fusil, Jacobo Santamaria, Vincent Garcia, Javier E Villegas In tunnel junctions with a ferroelectric barrier, a large resistance variation is observed upon ferroelectric switching, which can be induced by a short voltage pulse. Known as electro-resistance, this phenomenon is connected to the interfacial screening of the electric field generated by the ferroelectric in the junction's electrodes. Usually realized in junctions with normal-metal electrodes, here we experimentally investigate these effects in junctions that combine the ferroelectric BiFeO$_{\mathrm{3}}$ and different types of superconducting electrodes. Using piezo-response force microscopy and electrical measurements, we find an unusual temperature dependence of the electroresistance, which reaches up to 10$^{\mathrm{4}}${\%} .The effects are discussed in terms of the interface charge-carrier depletion produced in the superconducting electrodes... [Preview Abstract] |
Thursday, March 16, 2017 1:15PM - 1:27PM |
S41.00011: Tunneling Spectroscopy of Single Cu-O layer Cuprate Superconductors John Zasadzinski, Mark Warren, Adam Denchfield Tunneling spectroscopy data using a mechanical contact method on single-layer cuprate superconductors are reviewed. Superconductors with relatively high Tc (85-90K) such as Tl2201 reveal spectra quite similar to those found on the two-layer Bi2212 compound including sharp coherence peaks and low zero bias conductance consistent with a d-wave density of states (DOS) with a relatively small scattering rate $\Gamma $. Lower Tc single-layer cuprates such as La214 compounds reveal smaller gaps as expected but with significantly larger $\Gamma $ values. This trend continues with Bi2201 (Tc\textasciitilde 5K) where the spectra are broadened even further. The data suggest that the origin of the lower Tc values originates in a depairing mechanism likely tied to potential scattering in the Cu-O plane, arising from instrinsic disorder. Despite the additional broadening, an above-gap spectral dip is found in all junctions indicating coupling to a bosonic mode such that $\Omega $\textless 2$\Delta $. [Preview Abstract] |
Thursday, March 16, 2017 1:27PM - 1:39PM |
S41.00012: Clocked Single-Spin Source Based On A Spin-Split Superconductor Niklas Dittmann, Janine Splettstoesser, Francesco Giazotto In recent years single-electron turnstiles based on superconducting/normal-metal hybrid nanostructures have been well developed, which allow the manipulation of single electrons at high frequencies. In contrast, the implementation of single-spin sources in solid-state devices is only weakly explored. In our work (\emph{N.~Dittmann, J.~Splettstoesser, F.~Giazotto, \textbf{New J.~Phys.} 18, 083019 (2016)}) we propose a new accurate clocked single-spin source for ac-spintronic applications. The device consists of a superconducting island covered by a ferromagnetic insulator (FI) layer through which it is coupled to superconducting contacts. Single-particle transfer relies on the energy gaps and the island's charging energy, and is enabled by a bias and a time-periodic gate voltage. Accurate spin transfer is achieved by the FI layer which polarizes the island, provides spin-selective tunneling barriers and improves the precision by suppressing Andreev reflection. We analyze realistic material combinations and experimental requirements which allow for a clocked spin current in the MHz regime. [Preview Abstract] |
Thursday, March 16, 2017 1:39PM - 1:51PM |
S41.00013: Superconductivity and tunneling-junctions in epitaxial Nb2N/AlN/GaN heterojunctions Rusen Yan, Yimo Han, Guru Khalsa, Suresh Vishwanath, Scott Katzer, Neeraj Nepal, Brian Downey, David Muller, David Meyer, Grace Xing, Debdeep Jena We have discovered that ultrathin highly crystalline Nb2N layers grown epitaxially (by MBE) on SiC [1] and integrated with AlN and GaN heterostructures [2] are high-quality superconductors with transition temperatures from 9-13 K. The out-of-plane critical magnetic fields are found to be \textasciitilde 14 Tesla range, and the critical current density is 4*1E5 A/cm2 at 5 K. Preliminary in-plane magnetotransport measurements on \textasciitilde 4 nm thin films indicate a significantly high critical magnetic field exceeding 40 T. Since Nb2N superconducting layers can be epitaxially integrated with GaN, AlN, and AlGaN, we also demonstrate Nb2N superconductivity in a layer located beneath an N-polar GaN high-electron-mobility transistor (HEMT) heterostructure that uses a 2DEG channel as a microwave amplifier; such a demonstration illustrates the potential emergence of a new paradigm where an all-epitaxial III-N/Nb2N platform could serve as the basis for microwave qubits to power quantum computation as well as quantum communications. (1) D. S. Katzer et al., Applied Physics Express 8, 085501 (2015); (2) D. J. Meyer et al., IEEE Transactions on Semiconductor Manufacturing, 29, 384 (2016) [Preview Abstract] |
Thursday, March 16, 2017 1:51PM - 2:03PM |
S41.00014: Theory of tunneling conductance of anomalous Rashba metal / spin-singlet superconductor junction Toshiyuki Fukumoto, Katsuhisa Taguchi, Shingo Kobayashi, Yukio Tanaka In superconducting spintronics, the spin configuration is known as one of the key physical properties on the charge transport. Recently, systems dubbed anomalous Rashba metal (ARM), where the Zeeman field and the RSOI coexist, have attracted much attention. In the ARM, the relative direction of the spin and the momentum are locked due to the RSOI, and the electron’s spin degrees of freedom is reduced to be half [1]. Owing to the unique spin configuration of the ARM, it can be expected that we obtain the unique feature of the charge transport. In the presentation, we report the obtained tunneling conductance of two-dimensional ARM / spin-singlet superconductor junctions. As results, it is found that the inner gap conductance is enhanced by the RSOI, and it can be understood from the spin configuration of the ARM [2]. \newline [1] P.~St\v{r}eda \textit{et al.}, Phys. Rev. Lett. \textbf{90}, 256601 (2003). \newline [2] T.~Fukumoto \textit{et al.}, Phys. Rev. B \textbf{92}, 144514 (2015). [Preview Abstract] |
Thursday, March 16, 2017 2:03PM - 2:15PM |
S41.00015: Theory for Spin Selective Andreev Re ection in Vortex Core of Topological Superconductor: Majorana Zero Modes on Spherical Surface and Application to Spin Polarized Scanning Tunneling Microscope Probe Fu-Chun Zhang, Lun-Hui Hu, Chuang Li, Dong-Hui Xu, Yi Zhou Majorana zero modes (MZMs) have been predicted to exist in the topological insulator (TI)/superconductor (SC) heterostructure. Recent spin polarized scanning tunneling microscope(STM) experiment has observed spin-polarization dependence of the zero bias differential tunneling conductance at the center of vortex core. Here we consider a helical electron system described by a Rashba spin orbit coupling Hamiltonian on a spherical surface with a s-wave superconducting pairing due to proximity effect. We examine in-gap excitations of a pair of vortices with one at the north pole and the other at the south pole. While the MZM is not a spin eigenstate, the spin wavefunction of the MZM at the center of the vortex core, r = 0, is parallel to the magnetic field, and the local Andreev reflection of the MZM is spin selective, namely occurs only when the STM tip has the spin polarization parallel to the magnetic field, similar to the case in 1-dimensional nanowire. The total local differential tunneling conductance consists of the normal term proportional to the local density of states and an additional term arising from the Andreev reflection. We apply our theory to examine the recently reported spin-polarized STM experiments and show good agreement with the experiments [Preview Abstract] |
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