AVS2016 Session SP+2D+AS+NS+SS-MoA: Probing Topological States And Superconductivity

Monday, November 7, 2016 1:40 PM in Room 104A
Monday Afternoon

Time Period MoA Sessions | Abstract Timeline | Topic SP Sessions | Time Periods | Topics | AVS2016 Schedule

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1:40 PM Invited SP+2D+AS+NS+SS-MoA-1 Tuning Dirac States by Strain in Topological Insulators
Lian Li (University of Wisconsin-Milwaukee)

Topological insulators (TIs) are distinguished by their metallic boundary states populated by massless Dirac fermions and bulk topological Z2 index. Changes in the band topology induced by external variables such as strain, electrical field, and composition thus provide a means to tune the boundary states. As a large spin-orbit coupling is necessary to produce an inverted band gap, most TIs discovered to date are narrow gap semiconductors consisting of heavy elements. These materials typically exhibit layered crystal structure with anisotropic bonding characteristic: strong covalent bonding in-plane and weak van der Waals (vdW) bonding out-of-plane, which has been predicted to facilitate effective strain engineering of their bulk band topology.

In this talk, I will first give an overview of the opportunities and challenges in the epitaxial growth of layered TIs. Using the prototypical 3D TI Bi2Se3 as an example, I will show that the characteristic anisotropic bonding facilitates a spiral growth mode on virtually any substrates by molecular beam epitaxy. The coalescence of these spirals results in a high density of grain boundaries that consist of alternating edge dislocation pairs, leading to periodic in-plane stretching and compression. Using scanning tunneling spectroscopy, I will show that this local strain field strongly modifies the Dirac surface states, where in-plane compression expands the vdW gap and destroys the Dirac states.

Next, I will show our recent work on the strain engineering of Dirac edge states of epitaxial Bi bilayer films grown on three different substrates: the (111) surface of 3D TIs Bi2Se3, Sb2Te3, and Bi2Te3. Using scanning tunneling microscopy/spectroscopy, I will show that for moderately strained (<6%) single Bi bilayer on Sb2Te3 and Bi2Te3, edge states are observed; while on highly compressed single Bi bilayer on Bi2Se3 (>8%), edge states are suppressed. These findings, supported by density functional theory calculations, demonstrate the uniform control of edge states in 2D topological insulators by strain.

2:20 PM SP+2D+AS+NS+SS-MoA-3 Detection of Current Induced Spin Polarization in Topological Insulators via Four-Probe Spectroscopy
Saban Hus (Oak Ridge National Laboratory); Yong Chen (Purdue University); An-Ping Li (Oak Ridge National Laboratory)

Charge currents carried by the nontrivial surface states of topological insulators (TIs) exhibit a net spin polarization due to spin-momentum locking. Electrical detection of such a spin polarization is crucial for technological applications. However, in 3D TI materials the existence of a bulk conduction channel makes it difficult to quantify the density and the spin polarization of the current carried by the surface states. Here we report in-situ, spin sensitive four-probe spectroscopy measurements on Bi2Te2Se single crystals. A ferromagnetic probe detects the net spin accumulation on the surface states while a set of four-probe spectroscopy measurement is used for a quantitative separation of 2D and 3D conduction. We also examine the effect of surface doping by residual gas molecules on the current induced spin polarization. Even though, the additional carriers by dopants enhance the 2D conductance in TIs they reduce the net spin polarization of current carried by topological surface states.

This research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.
2:40 PM SP+2D+AS+NS+SS-MoA-4 Switching Handedness of Chiral Solitons Under Z4 Topology
Tae-Hwan Kim (Pohang University of Science and Technology, Republic of Korea); Sangmo Cheon, HanWoong Yeom (Institute for Basic Science (IBS), Republic of Korea)

Chirality is a ubiquitous and interesting property of asymmetry in many fields ranging from biology, chemistry to physics. Because of its topologically distinct nature, such chiral objects in condensed matter are often topologically excited states, which are protected by system’s topology and can be used to carry information robustly against external perturbations. For instance, nanoscale magnetic skyrmions, spatially localized chiral spin texture with particle-like properties in ferromagnets, have been investigated intensively as topological information carriers for next generation spintronic devices. However, logic operations using topological excitations such as skyrmions are only conceptually proposed. On the other hand, chiral solitons are recently discovered as the topologically protected edge states of one-dimensional Z4 topological insulators [1,2], which can be exploited as topological information carriers in electronic system. In this talk, I show experimentally and directly that switching between solitons with different chirality is possible by merging them with achiral solitons [3]. I will also show that this chiral switching corresponds to the realization of topological addition of the Z4 topological number or chirality. With their distinct topologically protected chirality, chiral solitons could uniquely be applied for robust multilevel information storage and logic operation by storing, carrying, and switching three differently topological bits of information.

[1] T.-H. Kim and H. W. Yeom, Phys. Rev. Lett. 109, 246802 (2012).

[2] S. Cheon, T.-H. Kim, S.-H. Lee, and H. W. Yeom, Science 350, 182 (2015).

[3] T.-H. Kim, S. Cheon, and H. W. Yeom, submitted (2016).

3:00 PM Invited SP+2D+AS+NS+SS-MoA-5 Spectroscopic-imaging STM Studies on Dirac-Landau Levels in the Topological Surface State
Tetsuo Hanaguri (RIKEN Center for Emergent Matter Science, Japan)

We show that spectroscopic-imaging scanning tunneling microscopy (SI-STM) is a powerful tool to investigate unique electronic features of massless Dirac electrons in a magnetic field.

In contrast to the conventional massive electron that is described by a single-component wave function, the massless counterpart demands the two-component wave function. In the case of the surface state of topological insulators, these two components are associated with the spin degrees of freedom, thereby governing the magnetic properties. Thus, it is highly desirable for spintronics applications to elucidate where and how the two-component nature emerges. We found that the two-component nature manifests itself in the internal structures of Landau orbits. We visualized the local density-of-states (LDOS) distributions associated with the Landau orbits in the topological surface state of Bi2Se3 using SI-STM. In the presence of the potential variation, Landau orbits drift along the equipotential lines, forming ring-like patterns in the LDOS images. The observed internal structures of the rings are qualitatively different from those of conventional massive electrons but are well reproduced by the calculation based on a two-component model Dirac Hamiltonian. Our model further predicts non-trivial energy-dependent spin-magnetization textures around the potential minimum. This is originated from the interplay between the two components and may provide a clue to manipulate spins in the topological surface state.

In addition to the Landau orbits, we succeeded in observing the Zeeman shift of the lowest Landau level from which precise g factor of the massless Dirac electron can be estimated. We performed experiments on two topological insulators, Bi2Se3 and Sb2Te2Se, and determined the surface g factors of them to be 18 and -6, respectively. Such remarkable material dependence suggests that the Zeeman effect is tunable by controlling the chemical composition, providing a new knob in manipulating the spins in the topological surface state.

3:40 PM BREAK
4:00 PM Invited SP+2D+AS+NS+SS-MoA-8 The Rashba and Quantum Size Effects in Ultrathin Bi films
Toru Hirahara (Tokyo Institue of Technology, Japan)

Precise characterization of physical properties in nanometer-scale materials is interesting not only in terms of low-dimensional physics but also in application to devices. Due to the reduced dimensionality and symmetry, these systems possess various interesting properties that cannot be found in the bulk. In this presentation, focusing on epitaxial ultrathin bismuth films formed on a silicon substrate, we introduce an intriguing interplay of the quantum size and Rashba effects in reciprocal space. Utilizing spin- and angle-resolved photoemission spectroscopy, we observed clear Rashba-split nature of the surface-state bands in these Bi films. However, the band dispersion did not follow the simple Rashba picture and the spin-splitting was lost where they overlapped with the bulk projection. From first-principles calculations, this was explained as a change in the nature of the band-splitting into an even-odd splitting induced by the quantum size effect [1]. Furthermore, we show that the interplay of the quantum size effect and the presence of the surface state induces a complicated change in the Fermi level position of the bulk states in bismuth, which is critical in discussing the surface-state contribution in the film properties [2,3].

[1] T. Hirahara, Journal of Electron Spectroscopy and Related Phenomena 201, 98 (2015).

[2] T. Hirahara, T. Shirai, T. Hajiri, M. Matsunami, K. Tanaka, S. Kimura, S. Hasegawa, and K. Kobayashi, Physical Review Letters 115, 106803 (2015).

[3] M. Aitani, T. Hirahara, S. Ichinokura, M. Hanaduka, D. Shin, and S. Hasegawa, Physical Review Letters 113, 206802 (2014).

4:40 PM SP+2D+AS+NS+SS-MoA-10 Understanding the Microscopic Effects of Annealing in Ba(Fe1-xCox)2As2 Superconductor
Qiang Zou, Zhiming Wu, Qiang Zheng, Shivani RajputƗ, David Parker, Athena Sefat, Zheng Gai (Oak Ridge National Laboratory)

By hole or electron doping of the parent iron-based BaFe2As2 compound, the high-transition temperature superconductivity emerges from the suppression of the antiferromagnetic order. 1 It was widely reported that thermal-annealing significantly improves some superconducting characteristics in Ba(Fe1−xCox )2As2, including Tc 2 The microscopic origin of such effect is still an open question. To make a connection between the global and the microscopic behavior of the materials, we did a comparison measurement on the pair of well-characterized x, we call ‘ as-grown’ vs ‘annealed’ Ba(Fe1-xCox)2As2 crystals, and using low temperature scanning tunneling microscopy and spectroscopy (STM/S). The superconducting gap maps deducted from the dI/dV maps were compared. The gap width distribution of the as-grown sample are obviously narrower than that of the annealed one. The coherent peak position also shifted to higher value for the annealed sample. The corresponding reduced-gaps of 2Δ/kbTc1 are about 2.3 and 5.4 for the as-grown and annealed crystals, respectively. The difference of the reduced-gaps indicates that the pairing strength of the annealed crystal is stronger than the as-grown one.

This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division . A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Reference

1. Stewart, G. R. (2011). “Superconductivity in iron compounds”, Reviews of Modern Physics, 83(4), 1589.

2. Gofryk, K., et al. "Effect of annealing on the specific heat of Ba(Fe1−xCox)2As2" Physical Review B, 83(6), 064513

5:00 PM SP+2D+AS+NS+SS-MoA-11 Annealing Effect on the Properties of Superconducting Parent BaFe2As2 Crystal
Shivani Rajput, Qiang Zou, Athena Sefat, Zheng Gai (Oak Ridge National Laboratory)

Understanding of electronic interactions in a parent phase of a superconducting crystal is crucial in determining the mechanism behind high Tc superconductivity. Bulk measurements show that annealing of parent BaFe2As2 crystal at 700 0C for 30 days causes a 5 K shift in magnetic transition temperature (TN) compared to as grown crystal. To understand the effect of annealing and details of magnetic phase transition, we investigate as-grown and annealed BaFe2As2 crystals at atomic scale using a variable temperature scanning tunneling microscopy/ spectroscopy at various temperature points across TN. Tunneling spectroscopy exhibit a ~ 0.53 eV gap type feature above TN, while V-shape dI/dV spectra below TN. The dI/dV mapping measurements show that as-grown BaFe2As2 crystals are electronically inhomogeneous, and averaging the differential conductance spectra over a large area does not truly represent the electronic properties of the sample at local scale, whereas annealed sample is comparatively electronically homogeneous.

This research was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Division. A portion of this research was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility.

Time Period MoA Sessions | Abstract Timeline | Topic SP Sessions | Time Periods | Topics | AVS2016 Schedule