Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V23: Superconductors: STM of Fe-based, Cuprates, and others |
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Sponsoring Units: DCMP Room: 322 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V23.00001: STM/S study on the role of Arsenic in Iron-based Superconductivity at Atomic Scale S. H. Pan, J. X. Yin, Zheng Wu, Ang Li, J. H. Wang, X. J. Liang, C. L. Zhang, P. C. Dai, C. -S. Ting, J. P. Hu, Z. Q. Wang, H. P. Hor, G. F. Chen, Hong Ding We use scanning tunneling microscopy /spectroscopy to investigate the role of Arsenic in superconducting Ba0.4K0.6Fe2As2 by directly breaking and repairing the local Fe-As structure. After the up-As-layer peeled away, the tunneling spectrum of the exposed Fe surface reveals a shallow incoherent gap, indicating a severe suppression of superconductivity without As covering. When an As-dimmer is placed on the same Fe surface, a localized topographic feature is formed due to p-d orbital hybridization and the superconducting coherent peaks recover locally with the superconducting gap size exactly the same as the Fe-layer with a complete As-coverage. These observations unravel the Fe-As interactions on an atomic scale and imply its essential roles in the Fe-based superconductivity. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V23.00002: Surface effect of epitaxially grown BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ surface -- Scanning Tunneling Microscopy and Spectroscopy study Sungmin Kim, Sunwouk Yi, Minjun Lee, Hanho Lee, Hoyeon Jeon, Yongchan Yoo, Inhae Zoh, Chao Zhang, Myungchul Oh, Young Kuk The electronic properties of Co-doped BaFe$_{\mathrm{2}}$As$_{\mathrm{2}}$ (BFCA) iron pnictide superconductors were studied using scanning tunneling microscopy and spectroscopy. BFCA samples with superconducting transition temperatures 22-27 K, were grown on SrTiO$_{\mathrm{3}}$(100) by pulsed laser deposition (PLD) growth under ultrahigh vacuum condition. As-grown surfaces revealed $2\times 2$or $2\sqrt 2 \times 2\sqrt 2 $structures as a function of bias voltage at 4.3 K. Missing row structures were also observed on as-grown samples, suggesting many nucleation sites and resultant antiphase boundaries. The I-V and dI/dV spectra were deviated from the bulk spectrum obtained by contact conductance measurement. After removing surface layers by ion beam sputtering, different spectra were observed. The obtained spectra were explained with the surface state effect in BFCA samples as compared with DFT calculations. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V23.00003: Elasto-Scanning Tunneling Microscopy: Visualizing the Coupling of Strain to Electronic Nematicity in NaFeAs Erick Andrade, Ayelete Notis, Lingyi Xing, Xiancheng Wang, Changqing Jin, Abhay Pasupathy Electronic nematicity is a widely observed phenomenon in the pnictide superconductors. In this phenomenon, the electronic structure breaks four-fold rotational symmetry and displays anisotropic behavior that can be observed in several transport and spectroscopic measurements. Understanding the driving force for the nematicity and its relationship to superconductivity remains a key goal in these materials. Motivated by transport measurements that indicate that the nematicity is strongly coupled to crystal strain, we developed a new experimental technique by which tunable uniaxial strain can be applied to a crystal while scanning tunneling microscopy is performed on the crystal surface. The technique allows us to track the same atomically resolved area of the sample as a function of strain. Using this new technique, we measure the response of the local density of states to strain in the tetragonal and orthorhombic phases of the crystal. In the orthorhombic phase, we find that strain can move structural domain walls but does not affect the magnitude of the electronic nematicity. On the other hand, in the tetragonal phase we find that strain controls the magnitude of the electronic nematicity, indicating that the material is in a paranematic state above the structural transition. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V23.00004: Quantum phase transition in Fe1$+$x(Te,Se) induced by Single-atomic Impurities studied by STM/S J. X. Yin, Zheng Wu, Xiong Huang, Z. Y. Ye, Rui Wu, X. J. Liang, H. Q. Mao, Jian Li, C. -S. Ting, J. P. Hu, Z. Q. Wang, P.-H. Hor, Hong Ding, S. H. Pan Previously we discovered a robust zero-energy bound state at an interstitial Fe impurity (IFIs) in Fe1$+$x(Te,Se), which resembles the Majorana mode (Nature Physics 11, 543, (2015)). Here we report our comprehensive study, using scanning tunneling microscopy/spectroscopy technique, of the global effect of IFIs on the ground state of Fe1$+$x(Te,Se) over a wide range of IFI concentration x. Our high resolution tunneling spectroscopy and quasi-particle interference data at very low temperature demonstrate that IFIs do not affect the electron pairing strength, while they cause significant dephasing effect, which eventually drives the ground state of the system from strong-coupling-superconductivity to diffusive-Bose-metal. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V23.00005: Reconciling STS and ARPES data for the correlated superconductor LiFeAs Jongbae Hong, David Abergel The inconsistency between the density of states revealed by scanning tunneling spectroscopy (STS) and that given by angle-resolved photoemission spectroscopy (ARPES) is a substantial problem for understanding the nature of strongly correlated superconductors such as Fe-based LiFeAs and the cuprates. We reveal that the two side peaks commonly appearing in both pnictide and cuprate superconductors are the result of the non-equilibrium behavior associated with singlet cotunneling from the tip to the strongly correlated sample [1]. We accurately reproduce the STS line shape of the Fe-based LiFeAs using a sample density of states which coincides with ARPES data, thereby producing a unified description for these materials. [1] Jongbae Hong and D.S.L. Abergel, arXiv:1411.5532. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V23.00006: Anticorrelation between the parent charge transfer gap and maximum transition temperature in cuprates Wei Ruan, Cheng Hu, Peng Cai, Yingying Peng, Xintong Li, Zhenqi Hao, Xingjiang Zhou, Zheng-Yu Weng, Yayu Wang We use scanning tunneling spectroscopy to measure the electronic structure of the parent Mott insulator of three different types of cuprates. The charge transfer gap size exhibits pronounced variations, and more interestingly it shows an anticorrelation with the maximum superconducting transition temperature achieved at the optimal doping of each cuprate. This result suggests that the Mottness in parent cuprate plays a crucial role in determining the superconducting properties. In particular, reducing the electron correlation strength enhances superconductivity, which is consistent with the pairing mechanism based on the doped Mott insulator picture. [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V23.00007: STM sub-gap structure in cuprates is a consequence of density waves, according to Mean-Field Theory and CDMFT Simon Verret, Jyotirmoy Roy, David S\'en\'echal, A.-M. S. Tremblay Much work has been done to find how the pseudogap is related to charge density waves in cuprates. In scanning tunneling microscopy (STM) measurements, the superconducting gap and pseudogap of cuprates are sometimes accompanied by a small \emph{sub-gap structure} at very low energy. This was documented early in vortex cores studies, and has now been reported at zero field for YBCO.(1) Here, we show that this can be caused by density waves, first through a standard mean-field approach, and then with Cellular Dynamical Mean-Field Theory for the Hubbard model using an exact diagonalization solver. We comment on the implication of these results for the relation between pseudogap and charge order. \\ (1) Jens Bru\'er et al. arXiv:1507.06775 [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V23.00008: Theoretical visualization of charge order in cuprates Peayush Choubey, Wei-Lin Tu, Ting-Kuo Lee, Peter Hirschfeld The anti-phase charge density wave (AP-CDW) state obtained in [1] by solving the renormalized mean-field theory of the t-J model was shown to have a dominant d-form factor for the bond order. However, the local density of states (LDOS) is only defined at the Cu lattice site. In order to compare with scanning tunneling microscopy (STM) experiments [2] in detail, we compute the continuum LDOS in the AP-CDW state at typical STM tip heights using Bi$_{\mathrm{2}}$Sr$_{\mathrm{2}}$Ca$_{\mathrm{2}}$CuO$_{\mathrm{2}}$ Wannier functions obtained from first principles [3]. The resulting spatial patterns compare very well with experimental results, and show the important role of the planar O2p states filtered through the BiO and SrO layers. In addition, we compute the Cu and O sublattice LDOS and Fourier transform it to obtain the amplitudes of s, s' and d-form factors actually reported in an STM experiment [4]. References: \begin{enumerate} \item Wei-Lin Tu and Ting-Kuo Lee, arXiv: 1505.07728. \item Kazuhiro Fujita \textit{et al.}, Proc. Natl. Acad. Sci. 111 30 (2014). \item A. Kreisel \textit{et al.}, Phys. Rev. Lett. 114, 217002 (2015). \item M. H. Hamidian \textit{et al}., arXiv: 1507.07865. \end{enumerate} [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V23.00009: Direct Local Measurement of the Superconducting Energy Gap of Nb doped SrTiO$_{\mathrm{3}}$ Jeonghoon Ha, Guru Khalsa, Fabian Natterer, Hongwoo Baek, William G. Cullen, Young Kuk, Joseph A. Stroscio Strontium titanate (STO) is a perovskite metal oxide insulator that can be electron doped by substitution of Ti or Sr sites with Nb or La, respectively, or by oxygen vacancies. When doped to high electron densities with concentration in the range of 5x10$^{\mathrm{19\thinspace }}$cm$^{\mathrm{-3\thinspace }}$to 2x10$^{\mathrm{20}}$ cm$^{\mathrm{-3}}$, STO becomes superconducting with a transition temperature below 400 mK, at a value highly dependent on the doping concentration. Previous observations were made on bulk crystals or films of doped STO by measuring the transitions in resistivity, magnetic susceptibility or thermal conductivity as a function of temperature or magnetic field. In this work, we use an ultra-low temperature scanning tunneling microscope(STM) to investigate the local electronic structure of the surface of Nb doped STO. The tunneling spectra taken at a sample temperature of $\approx $10 mK reveal a BCS energy gap of $\Delta =$ 40 ueV. Temperature and magnetic field dependent tunneling measurements show a critical temperature of $\approx $250 mK and upper critical field of $\approx $0.07 T. This is the first report of direct measurement of superconducting STO using an STM. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V23.00010: Multiband superconductivity in $2H$-NbSe$_2$ probed by Doppler-modulated scanning tunneling spectroscopy I. Fridman, C. Kloc, C. Petrovic, J. Y. T. Wei Cooper pairing in multiband superconductors can involve carriers from bands having different dimensionalities, and the interband coupling can provide for novel pairing interactions. In addition to MgB$_2$, recent experiments on $2H$-NbSe$_2$ have studied the Fermi surface topology using angle- and temperature-dependent scanning tunneling spectroscopy.[1] We present another novel method for probing multiband pairing: using a field-induced diamagnetic supercurrent, applied along different crystal axes, to perturb the quasiparticle density-of-states spectrum. By measuring the evolution of the quasiparticle spectrum under finite superfluid momentum, we characterize the pairing gaps and gap anisotropies. This approach is demonstrated on $2H$-NbSe$_2$ at 300 mK with a magnetic field of up to 9 T applied in the $ab$-plane.[2] The STM measurements revealed unambiguous evidence for multiband pairing, and evidence for a novel transition of the in-plane vortex lattice. We discuss the characteristics of this transition in light of data from other probes. [1] Y. Noat \textit{et al.}, Phys. Rev. B 92, 134510 (2015). [2] I. Fridman \textit{et al.}, Applied Physics Letters 99, 192505 (2011). [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V23.00011: Coherent long-range magnetic bound states in a superconductor Gerbold Menard, Sebastien Guissart, Christophe Brun, Stephane Pons, Vasily Stolyarov, Francois Debontridder, Matthieu Leclerc, Etienne Janod, Laurent Cario, Dimitri Roditchev, Pascal Simon, Tristan Cren Using low temperature scanning tunneling spectroscopy we accessed to localized states called Shiba states associated to magnetic impurities as well as their spatial dependence [1]. We studied samples of superconducting NbSe2 containing a really small concentration of native magnetic impurities. We observed the appearance of star-shaped structures around individual impurities with a size of the order of the coherence length of the superconductor ($\simeq $10 nm). The fine study of our data revealed an oscillation of the density of states along the star branches. To further analyze our results, we performed tight-binding calculations which reproduced the observed spatial symmetry. A semi-analytical calculation also enabled us to establish a relation between the different spatial scales observed and the physical quantities associated to superconductivity, and to understand the role of dimensionality in such systems. [1] Nature Physics (2015), DOI:10.1038/NPHYS3508 [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V23.00012: Creating nanostructured superconductors on demand by local current annealing Hongwoo Baek, Jeonghoon Ha, Duming Zhang, Bharath Natarajan, Rongwei Hu, Kefeng Wang, Steven Ziemak, Johnpierre Paglione, Young Kuk, Jonathan P. Winterstein, Renu Sharma, Nikolai B. Zhitenev, Joseph A. Stroscio When the effective size of a superconductor becomes comparable to the characteristic length scales, dramatic changes can occur in the superconducting properties that allow various applications in quantum devices. However, challenges remain in controlling the shape and size of specific superconducting materials. Here, we report on a method to create nanostructured superconductors by partial crystallization of the half-Heusler material, YPtBi using scanning tunneling microscopy. Superconducting islands, with diameters in the range of 100 nm, were reproducibly created by local current annealing of disordered YPtBi in the tunneling junction. Tunneling spectra measured on the islands showed non-Bardeen-Cooper-Schreffer behavior with different energy gaps and critical values as a function of spatial position. With increasing magnetic field, conductance maps showed the sequential addition of single vortices in the nanostructure, which fused into a giant vortex ring at 1.25 T. These results demonstrate an interesting method of creating tailored superconductors with complex materials for future applications [1]. [1] H. Baek et al., Phys. Rev. B 92, 094510 (2015). [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V23.00013: Magnetic field dependence of the density of states and tilted vortex lattice in the superconductor $\beta $-Bi$_{\mathrm{2}}$Pd Edwin Herrera-Vasco, Isabel Guillamon, Anton Fente, Jose Galvis, Alexandre Correa, Roberto Luccas, Federico Mompean, Mar Garcia Hernandez, Jean P. Brison, Sebastian Vieira, Hermann Suderow We present very low-temperature scanning tunneling microscopy (STM) experiments on the superconductor $\beta $-Bi$_{\mathrm{2\thinspace }}$Pd. We find a single superconducting gap from the zero-field tunneling conductance. We also find that the hexagonal vortex lattice is locked to the square atomic lattice. The magnetic field dependence of the intervortex tunneling conductance is higher than the one expected in a single-gap superconductor. Such an increase in the intervortex tunneling conductance has been found in superconductors with multiple superconducting gaps. We fit the upper critical field H$_{\mathrm{c2}}$(T) and show that multiband Fermi surface is needed to explain the observed behavior. We propose that $\beta $-Bi$_{\mathrm{2}}$Pd is a single-gap multiband superconductor. We have measured the tilted vortex lattice (TVL) using a three axis superconducting magnet. Our results give first real space imaging of the TVL in a nearly isotropic s-wave BCS superconductor. From a detailed study of the TVL varying polar and azimuthal angles, we find correlations between the square atomic lattice and the TVL. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V23.00014: Josephson STM at mK temperatures: Coupling to the electronic environment Michael Dreyer, Rami Dana, Wan-Ting Liao, Cris Lobb, Fred Wellstood, Bob Anderson Ultra-small Josephson junctions can couple to modes in the electronic environment. This leads to sub-gap peaks in the I(V) curve in addition to the phase diffuse supercurrent. The I(V) curve can - in principle - be explained by P(E) theory [1] which describes the probability of tunneling at energy E. A recent study [2] showed that antenna modes of the STM tips could be responsible for the observed sideband structures. In our case the explanation appears to be less simple. We employ a dual tip STM at a temperature of 30 mK [3]. The I(V) spectra of the two tips show distinct patterns with only one shared mode. While the supercurrent branch for the "inner" tip is visible, it is obscured by a resonance for the “outer” tip. Possible causes and applications to other systems will be discussed. [1] G.-L. Ingold, H. Grabert, U. Eberhardt, PRB 50(1), 395 (1994) [2] Berthold J\"ack, et. al., Appl. Phys. Lett. 106, 013109 (2015) [3] Anita Roychowdhury, et. al., Rev. Sci. Inst., 85, 043706 (2014) [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V23.00015: \textbf{Friedel oscillations and quasiparticle interference: Complementary STM and REXS approach} Pegor Aynajian, András Gyenis, Eduardo da Silva Neto, Feizhou He, Ronny Sutarto, Enrico Schierle, Eugen Weschke, Mariam Kavai, Ryan Baumbach, Joe Thompson, Eric Bauer, Zachary Fisk, Andrea Damascelli, Ali Yazdani The origin of a competing charge ordering in the high-temperature superconducting cuprates has recently been subject to intense experimental and theoretical debates. Dalla Torre et al theoretically proposed that Friedel oscillations can reproduce the experimentally observed scattering peak in the resonant elastic x-ray scattering (REXS) of cuprates [NJP, 17 022001 2015]. Using complementary spectroscopic imaging with the scanning tunneling microscope and REXS on the heavy fermion compound CeMIn$_{5}$ (M$=$Co, Rh) we observe a scattering peak in REXS, similar, yet broader than that found in cuprates. Through temperature and doping dependent experiments, we demonstrate this enhanced peak to originate from the scattering of hybridized heavy $f$-electrons. We discuss the origin of this phenomenon and its relation to Friedel oscillations and charge ordering. [Preview Abstract] |
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