Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E41: Time-Resolved and Static Photoemission in Superconductors |
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Sponsoring Units: DCMP Chair: Alexander Kemper, North Carolina State University Room: 388 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E41.00001: Coherent Excitations Induced by Pumping a Mott System Yao Wang, Martin Claassen, Brian Moritz, Thomas Devereaux Time-domain or non-equilibrium dynamics of correlated materials has attracted attention due to the possibility to characterize, tune and create complex ordered states. To understand how single and multi-particle excitations develop in a strongly-correlated systems during a pulsed pump, we perform a time-resolved exact-diagonalization study on a single-band Hubbard model. Starting from the Mott insulator at half-filling, we observe the suppression of antiferromagnetism and development of low-energy charge excitations through a series of Floquet-like sidebands. By correlating with the numerically evaluated single-particle spectra in non-equilibrium, the transient dynamics of multi-particle excitations can be attributed to the interplay between virtual Floquet sidebands and change of density of states due to the existence of strong correlations. The autocorrelation of this time-dependent spectral function reflects that it is the resonance of floquet states and upper Hubbard band that causes the remnant change of charge and spin excitations. This nonperturbative, nonequilibrium and nonstatic study reveals the underlying physical process while correlated electrons are pumped in experiment and provide the opportunity of designing nonequilibrium state of matter by a short pulsed laser. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E41.00002: Measuring particle-hole asymmetry in Bi2212 with time-resolved spectroscopy \newline Tristan Miller, Wentao Zhang, Hiroshi Eisaki, Alessandra Lanzara One of the central questions in high-temperature superconductivity is the nature of the pseudogap state and its relation to superconductivity. Competing explanations of the pseudogap raise basic questions about its structure. The superconducting gap is known to have particle-hole symmetry, but it is not yet known whether the pseudogap shares this property. Here, we introduce a sensitive way to measure particle-hole symmetry in high-temperature superconductor Bi2212 by pumping it with laser pulses and measuring the ultrafast response of the chemical potential. By comparing samples with different doping, we discover new clues into the structure of the pseudogap. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E41.00003: Theory of Laser-Controlled Competing Superconducting and Charge Orders Michael Sentef, Akiyuki Tokuno, Antoine Georges, Corinna Kollath We investigate the nonequilibrium dynamics of competing coexisting superconducting (SC) and charge-density wave (CDW) orders in an attractive Hubbard model. A time-periodic laser field A(t) lifts the SC-CDW degeneracy, since the CDW couples linearly to the field ($A$), whereas SC couples in second order ($A^2$) due to gauge invariance. This leads to a striking resonance: When the photon energy is red-detuned compared to the equilibrium single-particle energy gap, CDW is enhanced and SC is suppressed, while this behavior is reversed for blue detuning. Both orders oscillate with an emergent slow frequency, which is controlled by the small amplitude of a third induced order, namely $\eta$ pairing, given by the commutator of the two primary orders. The \textit{induced} $\eta$ pairing is shown to control the enhancement and suppression of the dominant orders. Finally, we demonstrate that light-induced superconductivity is possible starting from a predominantly CDW initial state. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E41.00004: Quasiparticle dynamics in cuprate superconductors: probing the superconducting condensate Fabio Boschini, E. H. da Silva Neto, E. Razzoli, M. Zonno, G. Levy, M. Michiardi, B. Zwartsenberg, P. Nigge, R. P. Day, A. K. Mills, J. Wen, J. Schneeloch, Z. Xu, G. Gu, D. J. Jones, A. Damascelli High-T$_{C}$ superconductivity is a long-standing open problem in the modern physics. Thus, the development of novel techniques to approach the problem from yet unexplored points of view is crucial and highly desirable. Time- and angle-resolved photoemission spectroscopy (TR-ARPES) can provide new insights on long sought-after dynamical properties in High-T$_{C}$ superconductors [1,2]. Here we will present a TR-ARPES study on underdoped Bi2201 and Bi2212 cuprate superconductors where we are able to track the temporal evolution of the superconducting condensate. In particular, we will show how the quasiparticle lifetime is modified upon the optical excitation, and also how the temporal evolution of the different terms contributing to the quasiparticle lifetime can be disentangled. Finally, the study of the momentum- and energy-dependence of the relaxation processes allows us to find a direct connection between recombination dynamics and ultrafast evolution of the superconducting condensate. [1] C. L. Smallwood et al. Science 336, 1137 (2012) [2] C. Piovera et al. Phys. Rev. B 91, 224509 (2015) [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E41.00005: Momentum-resolved coherent phonon oscillations in cuprate superconductors. Shuolong Yang, Jonathan Sobota, Yu He, Dominik Leuenberger, Hadas Soifer, Patrick Kirchmann, Zhi-Xun Shen Using time- and angle-resolved photoemission spectroscopy, we resolve coherent phonon modes on a cuprate superconductor Bi2212. Upon optical excitation, the electronic band dispersion near the node oscillates with a dominant mode at 3.94 THz. The amplitude of this mode weakly increases as a function of momentum away from the node. Comparing with phonon frequencies extracted from Raman spectroscopy and electron energy loss spectroscopy, we identify these coherent modes as A1g phonons involving the motion of Cu-O planes. Our study provides a new route to characterize the momentum-resolved electron-phonon coupling strength, which is essential to disentangle the competing interactions in cuprate superconductors. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E41.00006: Higgs Oscillations of Non-equilibrium Superconductors Nathan Cheng, Nikolaj Bittner, Mona Berciu, Dirk Manske In superconductors studied under equilibrium conditions, the Higgs mode is often overdamped by the continuum of excitations in the superconductor, or not directly coupled with experimental observables except in special cases of superconductors with fortuitous properties. However, under non-equilibrium conditions, such as immediately following a strong light pulse, collective Higgs oscillations are induced as a result of changes to the “Mexican hat” potential – regardless of the superconductor type. These experiments employ ultrafast pump-probe spectroscopy, which stretch and shrink the free energy “Mexican hat” potential causing oscillations. This amounts to introducing excitations that disrupt the condensate and break electron pairs. If the time scale of the pump is faster than the intrinsic response time of the condensate and the energy from the light is sufficiently small to not entirely destroy the condensate paired state, this process can induce oscillations in the collective modes of the condensate about a new induced equilibrium. This talk will describe a density matrix formalism approach to solving the non-equilibrium effects of a pump-probe pulse on the superconducting condensate and the effects of different superconducting symmetry on the response. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E41.00007: Character of charge carriers in a low-carrier superconductor Ag$_{5}$Pb$_{2}$O$_{6}$ revealed by core-level photoemission Soobin Sinn, Kyung Dong Lee, Choong Jae Won, Tamio Oguchi, Ji Seop Oh, Hyang Keun Yoo, Cheng-Tai Kuo, Moonsup Han, Young Jun Chang, Namjung Hur, Byeong-Gyu Park, Changyoung Kim, Hyeong-Do Kim, Tae Won Noh Identifying an element-specific orbital character of electrons near the Fermi level is important to understand electronic properties of a metallic solid, as manifested in high-$T_{c}$ superconducting cuprates in which doped-hole and --electron characters are different from each other. Photoemission spectroscopy (PES) may serve as an ideal tool for that purpose by varying photon energies. However, in the case of a low-carrier superconductor Ag$_{5}$Pb$_{2}$O$_{6}$, distinguishing between Ag 5$s$ and Pb 6$s$ conduction bands is difficult by the technique. Here, we present another method utilizing core-level PES. The Pb 4$f$ spectrum exhibits a quite asymmetric line shape unlike the Ag 3$d$ spectrum. The asymmetry can be attributed to electron-hole pair excitations across the Fermi level in the Pb 6$s$ band affected by strong local attractive Coulomb potential due to the Pb 4$f$ hole. However, the Pb 4$f$ spectrum is not explained by the well-known Doniach-\v{S}unji\'{c} line shape for a simple metal. By employing Pb 6$s$ PDOS from first-principle calculations, the line shape is successfully generated, thus confirming that the main charge-carrier character in Ag$_{5}$Pb$_{2}$O$_{6}$ is Pb 6$s$ electrons. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E41.00008: NaFe$_{0.56}$Cu$_{0.44}$As: A pnictide insulating phase induced by on-site Coulomb interaction Christian Matt, Nan Xu, Baiqing Lyu, Junzhang Ma, Federiko Bisti, Jihwey Park, Tian Shang, Chongde Cao, Yu Song, Andriy Nevidomskyy, Pengcheng Dai, Nicholas Plumb, Milan Radovic, Joel Mesot, Ming Shi In the studies of iron-pnictides, a key question is whether their bad-metal state from which the superconductivity emerges lies in close proximity with a magnetically ordered insulating phase. Recently it was found that at low temperatures, the heavily Cu-doped NaFe$_{1-x}$Cu$_{x}$As iron-pnictide is an insulator AFM order, similar to the parent compound of cuprates but distinct from all other iron-pnictides. Using angle-resolved photoemission spectroscopy, we determined the electronic structure of NaFe$_{0.56}$Cu$_{0.44}$As and identified that its ground state is a narrow-gap insulator. Combining the experimental results with DFT$+$U calculations reveals that the on-site Coulombic and Hund's coupling energies play crucial roles in formation of the band gap about the chemical potential. We propose that at finite temperatures charge carriers are thermally excited from the Cu-As-like valence band into the conduction band, which is of Fe 3d-like character. With increasing temperature, the number of electrons in the conduction band becomes larger and the hopping energy between Fe sites increases, and finally the long-range AFM order is destroyed at T \textgreater T$_{N}$. Our study provides a basis for investigating the evolution of the electronic structure of a Mott insulator transforming into a bad metallic phase, and eventually forming a superconducting state in iron-pnictides. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E41.00009: Investigation of FeSe nematic phase Soonsang Huh, Jeongjin Seo, Yoonyoung Koh, Yeongkwan Kim, Changyoung Kim The nematic phase is believed to play an important role in iron based superconductors(IBSC). It has been argued that nematic phase is related to magnetic spin fluctuation or/and orbital degree of freedom, but it is not yet understood. FeSe, known to have orbital order without long range magnetic order, is a unique system to study the nematic phase To investigate the electronic structure of FeSe nematic phase, we performed angle resolved photoemission spectroscopy (ARPES) and X-ray linear dichroism(XLD) experiments on fully detwined sample by using piezo. We report Fermi surface topology of nematic phase and its orbital character. With XLD experiment, our results suggest Ferro orbital order exists in FeSe. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E41.00010: Electronic structure of square planar nickelates revisited: relationship to cuprates Antia S. Botana, Victor Pardo, Warren E. Pickett, Michael R. Norman Based on ab initio calculations, a close connection between the Ruddlesden Popper layered nickelates and high temperature superconducting cuprates is established. The electronic structure of La$_4$Ni$_3$O$_8$ and La$_3$Ni$_2$O$_6$ is similar to that of cuprates not only in terms of filling of $d$-levels (close to $d^9$) but also because they show Ni$^{1+}$(S=1/2)/Ni$^{2+}$(S=0) stripe ordering. The Ni$^{2+}$ ions are in a low-spin configuration (S=0) yielding an antiferromagnetic arrangement of Ni$^{1+}$ (pseudo Cu$^{2+}$) S=1/2 ions like the long-sought spin-1/2 antiferromagnetic insulator analog of the cuprate parent materials. The analogy extends further with the main contribution to the bands near the Fermi energy coming from hybridized Ni-$d_{x^2-y^2}$ and O-$p$ states [1]. These results bring renewed justification that layered nickelates of this type are cuprate analog systems that are promising for studying the interplay between structure, magnetism, and superconductivity. [1] A. S. Botana, V. Pardo, W. E. Pickett and M. R. Norman. Phys. Rev. B 94, 081105(R)(2016) [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E41.00011: Effect of elastic strain on the band gaps, band alignment, and electronic structure of epitaxial ASn$O_{3}$ (A = Ca, Sr, and Ba) films and heterostructures revealed through in situ photoemission, spectroscopic ellipsometry, and density functional theory John Baniecki, Takashi Yamazaki, Hiroyuki Aso, Yoshihiko Imanaka, Dan Ricinschi The alkaline earth stannates ASn$O_{3}$ (A = Ba, Sr, and Ca) are emerging as important materials. Band gaps in stannates are remarkably dependent on volumetric strain with a decrease in volumetric strain of 3 percent in SrSn$O_{3}$ resulting in an increase in the band gap of 0.35 eV. However, little understanding of volumetric strain dependence on the valence band (VB) electronic structure and band alignments between stannates and other oxides exits. In this talk we will examine the effect of elastic strain on the band gaps, band alignment, and electronic structure of stannate films and heterostructures through in situ photoemission, spectroscopic ellipsometry, scanning transmission electron microscopy with geometric phase analysis, and density functional theory. CaSn$O_{3}$ (CSO), SrSn$O_{3}$ (SSO) and La-doped BaSn$O_{3}$ (BLSO) thin films were grown by pulsed laser epitaxy with strain control via epitaxial buffer layers. While the VB electronic structure is strain dependent VB offsets do not vary significantly with strain, which resulted in ascribing most of the difference in band alignment to the conduction band (CB) edge. Significantly, strain-induced tuning of CB offset differences are as large as 0.6 eV for SSO and may provide a pathway to enhance stannate-based devices. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E41.00012: Proximity-induced superconductivity in the topological surface state of thin Bi$_{2}$Se$_{3}$ films on Nb probed by ARPES D. Floetotto, Y. Ota, Y. Bai, C. Zhang, K. Okazaki, A. Tsuduki, T. Hashimoto, S. Watanabe, C.-T. Chen, J. N. Eckstein, S. Shin, T.-C. Chiang Topological superconductors (TSCs) are of great interest, since these systems could support novel electronic states such as Majorana fermions. A promising approach to realize TSCs is the preparation of artificial heterostructures involving a superconductor (SC) and a topological insulator (TI) in which the superconductivity is induced into the spin- and momentum-locked topological surface state by proximity coupling. By using angle-resolved photoemission spectroscopy, we have mapped the electronic band structure and determined the proximity-induced superconducting gaps for the simple case of prototypical TI Bi$_{2}$Se$_{3}$ on elemental s-wave SC Nb as functions of temperature and the thickness of single crystalline Bi$_{2}$Se$_{3}$ films. For both the bulk and the topological surface states coherence peaks and leading edge shifts of similar magnitude emerge at the Fermi level in the thickness rage of 4-10 QL. The study is an important step towards a comprehensive understanding of helical Cooper pairing in Dirac surface states and the optimization of artificial topological superconductors. Our smart, cleavage-based sample preparation technique is also applicable to other TI/SC heterostructures. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E41.00013: Electronic Structure of ZrTe3 from Angle-Resolved Photoemission Spectroscopy Shoupeng LYU, Li Yu, Jianwei Huang, Yuxiao Zhang, Cheng Hu, Ying Ding, Cong Li, Gudong Liu, Lin Zhao, Chengtian Lin, Chuangtian Chen, Zuyan Xu, Xingjiang Zhou ZrTe$_{3}$ has both quasi-one dimensional and quasi-two-dimensional structural components in its crystal structure. It shows not only charge density wave (CDW), but also deserves studying the coexistence or competition between different broken-symmetry ground states. In this talk, we will present electronic study on ZrTe$_{3\, }$by angle-resolved Time-of-Flight electron analyzer that can cover two-dimensional momentum space simultaneously with high energy (\textasciitilde 1meV) and momentum resolution, Bilayer splitting feature is clearly resolved and its momentum and temperature dependence is investigated. We will discuss on the CDW ground state and the normal state quasi-particle formation in ZrTe$_{3}$ based on our measurements. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E41.00014: High-resolution angle-resolved photoemission study of electronic structure and charge-density wave formation in HoTe3. Guodong Liu, Chenlu Wang, Yan Zhang, Bingfeng Hu, Daixiang Mou, Li Yu, Lin Zhao, Xingjiang Zhou, Nanlin Wang, Chuangtian Chen, Zuyan Xu We performed high-resolution angle-resolved photoemission spectroscopy (ARPES) measurement on high quality crystal of HoTe3, an intriguing quasi-two-dimensional rare-earth-element tritelluride charge-density-wave (CDW) compound. The main features of the electronic structure in this compound are established by employing a quasi-CW laser (7eV) and a helium discharging lamp (21.22 eV) as excitation light sources. It reveals many bands back folded according to the CDW periodicity and two incommensurate CDW gaps created by perpendicular Fermi surface (FS) nesting vectors. A large gap is found to open in well nested regions of the Fermi surface sheets, whereas other Fermi surface sections with poor nesting remain ungapped. In particular, some peculiar features are identified by using our ultra-high resolution and bulk sensitive laser-ARPES. [Preview Abstract] |
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