Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B40: Surfaces, Interfaces, and Thin Films: Electronic and Magnetic Properties |
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Sponsoring Units: DCMP Chair: Michael Horn von Hoegen, Universitaet Duisburg-Essen Room: 349 |
Monday, March 18, 2013 11:15AM - 11:27AM |
B40.00001: OAM and spin structure of Cu(111) and Au(111) surface state bands Beomyoung Kim, Panjin Kim, Wonsig Jung, Yeongkwan Kim, Yoonyoung Koh, Changyoung Kim, Masashi Arita, Kenya Shimada, Hirofumi Namatame, Masaki Taniguchi, Choong H. Kim, Jaejun Yu We performed angle-resolved photoemission studies on Cu(111) and Au(111) surface states with circularly polarized light to investigate local orbital angular momentum (OAM) structures. Existence of OAM is confirmed, as predicted, to exist in systems with an inversion symmetry breaking. Cu(111) surface state bands are found to have chiral OAM in spite of very small spin-orbit coupling, consistent with the theoretical prediction. As for Au(111), we observe split bands for which OAM for the inner and outer bands are parallel, unlike the Bi2Se3 case. We also performed first-principles calculations and the results are found to be consistent with experimental results. Moreover, the majority of OAM is found to have $d$orbital origin while a small contribution comes from $p$orbitals. An effective Hamiltonian that incorporates the role of OAM is derived and is used to extract the spin and OAM structures. We discuss the evolution of angular momentum structures from a pure OAM system to a strongly spin-orbit-entangled state. [Preview Abstract] |
Monday, March 18, 2013 11:27AM - 11:39AM |
B40.00002: Electronic properties of precious-metal coated W tips in STM: Role of spin-orbit coupling T. Yamashita, T. Akiyama, K. Nakamura, T. Ito, S.H. Rhim, A.J. Freeman Scanning tunneling microscopy (STM) has proved a versatile tool invigorating many physics at an atomic scale, where chemical identity and shape of the probe tip greatly affect resolution and sensitivity. There have been many efforts to functionalize STM tips: coating W tips with organic molecules and {\em 3d} transition metals, which facilitate the selective imaging with enhanced tunneling current. In this work, we model W(110) tips coated by precious metals such as Au, Ag, and Pt, in which large spin-orbit coupling significantly influences the electronic structure of the STM probe. Furthermore, we argue that this spin-orbit coupling can be used as a spin detecting STM probe without additional bias switching. The stability of the W(110) apex atom for each metal coating is also discussed. [Preview Abstract] |
Monday, March 18, 2013 11:39AM - 11:51AM |
B40.00003: Ion bombardment of Ni(110) studied with inverse photoemission, LEED, and simulations Benjamin Young, Jim Warner, David Heskett Inverse Photoemission Spectroscopy (IPES) performed on clean Ni(110) reveals an unoccupied electronic surface state $\sim$2eV above the Fermi level at the \={Y} point of the surface Brillouin Zone. Ion bombardment (sputtering) of the sample creates vacancies and adatoms, which reduce the intensity of the representative state peak in IPES spectra. While the intensity of this IPES peak decreases with sputtering, well-defined diffraction spots in the surface LEED pattern give way to more diffuse ones with higher background intensity. Quantization of these permits analysis of their intensity profiles. Results of these techniques are presented for various sputtering conditions with 1keV Ne$\ ^{+}$ and compared to previous results for 500eV Ar$\ ^{+}$ on the same sample. Finally, we connect sputtering trends in the IPES and LEED data to Monte Carlo simulations of the sputtering process. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B40.00004: Measurement of the Spectral Distribution of Low Energy Electrons Emitted as a Result of M$_{2,3}$VV Auger Transitions in Cu(100) and the N$_{2,3}$VV transition in Ag Prasad Joglekar, Suman Satyal, Karthik Shastry, Steven Hulbert, Alexander Weiss Auger Photoelectron Coincidence Spectroscopy (APECS) was used to investigate the physics of electron emission in the Low Energy Tail (LET) of the MVV and NVV Auger spectra obtained from Cu(100) and Ag(100) surfaces, respectively. A beam of 200eV photons (180 eV in the case of Ag) was used to probe the Cu (Ag) sample. Two Cylindrical Mirror Analyzers (CMAs) were used to select the energy of electrons emitted from the sample. Auger electrons were detected in coincidence with the 3p $_{3/2}$ photoemission peak in the case Cu and the 4p photoemission peak in the case of Ag. A set of coincidence measurements were made with the fixed analyzer set at a series of energies between the core and the valence band in order to obtain an estimate of the background due to the inelastic scattering of the valence band electrons. This background was then subtracted yielding a spectrum consisting only of electrons emitted as a result of the Auger transition process. [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B40.00005: Transient Exciton at Ag(111) Surface Cong Wang, Xuefeng Cui, Adam John Argondizzo, Sean Garrett-Roe, Hrovje Petek We investigate the surface states on Ag(111) by means of multi-photon photoemission using ultrashort laser pulses. The angle-resolved photoemission spectra at the non-resonant range are consistent with the well-known structures of Shockley states and image potential states. But when we tune the wavelength to the resonant range by two photon, the spectra is dominated by a non-dispersive feature, which should correspond to a localized state, and we assign it to transient exciton. Then we do time-resolved measurements and take Fourier Transformation with respect to the delay-axis. The dominant response of the Ag(111) sample is the driving frequency, which is unexpected because there is no one-photon resonant transition in the excitation scheme. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B40.00006: Phonon spectra on ultrathin Pb films with scanning tunneling spectroscopy Hyoungdo Nam, Chih-Kang Shih After Blatt and Thomson's prediction [Phys. Rev. Lett. 10, 332 (1963)], several groups have reported the quantum size effect on transition temperature(Tc) as a function of thicknesses of atomically flat ultrathin Pb film. In those cases, Tc oscillation related to film thickness was attributed to oscillation of the density of states (DOS) near the Fermi energy. However, the Tc oscillation amplitude is much smaller than that derived from the DOS oscillation. One therefore would ask: What is the role of electron-phonon interaction? Also as reported by Qin, et. al. [Science 324, 1314 (2009)], when the film is only 2ML thick, the pseduomorphically strained film has lower Tc than the unstrained one, suggesting that interfacial phonons may play a role. To answer to above question, we perform layer-dependent scanning tunneling spectroscopy of Pb films on Si(111) at 2.3 K to observe the phonon related features in the tunneling spectra. Detailed analysis of thickness dependence of photon spectra will be reported. [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B40.00007: An STM and STS study on Iridium modified Si(111) Surface Nuri Oncel, Dylan Nicholls The structure of Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface have been investigated with the help of scanning tunneling microscopy/spectroscopy and low energy electron diffraction. We propose a model based on the experimental data. The model defines a unit cell containing one surface substitutional iridium atom centered under six silicon ad-atoms. Once the sample is annealed at 1200 $^{\circ}$C, a low density lattice gas of these ring clusters forms on top of an impurity stabilized $'1\times 1'$ domains. These ring clusters and $'1\times 1'$ domains co-exist with $7\times 7$ domains of clean Si(111) surface. The local density of states graphs measured on Si(111) $\sqrt 7 \times \sqrt 7 \,R\,19.1^{0}-Ir$ reconstructed surface contains an asymmetric peak at the edge of the valence band suggesting that there is a surface state exhibiting a Rashba type spin-orbit coupling. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B40.00008: Surface Electronic Excitations of Quantum Confined Mg Films on Si(111) Ao Teng, Krzysztof Kempa, Xiaoguang Li, Mustafa Ozer, Saban Hus, Paul Snijders, Geunseop Lee, Hanno Weitering We have investigated surface electronic excitations at atomically-smooth ultrathin Mg(0001) films on a Si(111)-7x7 substrate using high-resolution electron energy loss spectroscopy. The monopole and multipole surface plasmons of bulk Mg have their counterparts in the thin film regime. The dispersion of the monopole mode, as well as the relative intensity of the multipole mode, exhibit interesting thickness dependencies that are directly associated with quantum size effects in the Mg films. Additionally, we present the first clear observation of a photo-threshold excitation not seen at the surface of bulk Mg. Its intensity is also thickness dependent and anti-correlates with the multipole mode intensity. The results can be modeled with an effective jellium model in which the local Wigner-Seitz radius follows the thickness-dependent variation of the ground-state charge density at the surface. The results are a clear manifestation of quantum-size phenomena in the collective plasmon response of ultrathin metal films. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:03PM |
B40.00009: Oscillation of conductivity in layer-by-layer growth of Bi thin film phase Yasunori Fujikawa, Eiji Saitoh Thin film growth of Bi and related compounds has been attracted much attention because of their exotic properties originating in the large spin-orbit interaction of Bi. Growth of its simple substance is known to result in the formation of a thin-film phase in the initial stage, which is taken over by the bulk growth when the coverage exceeds several monolayers (ML). [1] With typical growth conditions, this transition takes place before the completion of the thin-film layer, which tends to agglomerate to form 4-ML thick islands, making it difficult to measure the intrinsic property of the thin-film phase. In this work, Bi growth on Si(111)-7x7 has been performed in a multi-probe VT-STM system, which provides wide-ranging opportunity of kinetic control and in-situ transport measurement during the thin film growth. By tuning the kinetic condition of the growth, it becomes possible to grow the thin-film phase uniformly covering the substrate in layer-by-layer mode. In-situ transport measurement has been performed during the layer-by-layer growth of the Bi thin-film phase, distinguishing the conductivity of each growth unit. It oscillates with a period of 2 ML, which reflects the atomic structure of the thin-film phase. [1] Nagao et al., Phys. Rev. Lett. 93, 105501 (2004). [Preview Abstract] |
Monday, March 18, 2013 1:03PM - 1:15PM |
B40.00010: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 1:15PM - 1:27PM |
B40.00011: Measurement of bandgap and doping effects in ultrathin MoS2 layers using scanning tunneling spectroscopy. Chih-Pin Lu, Guohong Li, Ivan Skachko, Eva Andrei Molybdenum disulfide MoS$_{2}$, a semiconductor in the layered transition-metal dichalcogenide family of materials which is composed of weakly interacting layers held together by van der Waals interactions, offers an attractive possibility as a field effect transistor in low-power switching devices. We studied ultrathin MoS2 samples, ranging from single to several layers in thickness, that were extracted by mechanical exfoliation from the bulk material. Using a device geometry which allows varying the carrier density by gating across a 300nm insulating layer of SiO$_{2}$, together with low temperature Scanning Tunneling Microscopy and Spectroscopy, we investigated the bandgap and its dependence on doping and number of layers. For few layer samples we observe a well resolved atomic structure and a band gap of $\sim$ 1.1eV which is a little small than bulk band gap of 1.2eV. In addition we observe that electron doping shifts the Fermi energy into the conduction band. In single layer samples the measured bandgap is about $\sim$ 1.8eV in agreement with photoluminescence measurements and can change by backgate voltage. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B40.00012: Probing the Effects of Interface Band Structure Using Ballistic Electron Emission Microscopy Robert Balsano, Vincent LaBella Ballistic electron emission microscopy (BEEM) is a scanning tunneling microscopy (STM) technique that can measure transport of hot electrons through materials and interfaces with high spatial and energetic resolution. Using this technique an attenuation length for electrons in the film can be extracted from the relationship between film thickness and the number of hot electrons transmitted through the film. The behavior of the attenuation lengths of carriers with energies just above the Schottky barrier height is indicative of the interface band structure. BEEM requires an additional contact to ground the metal base layer of a metal semiconductor junction. Performing BEEM in situ with the sample fabrication greatly increases the through put for these types of measurements. This presentation will detail our data on electron transport through metals and across different interfaces and also highlight our work to develop a special silicon substrate that has the extra contact and oxide hard mask built in to enable in situ BEEM without modifications to the STM. [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B40.00013: Interface States in the metal-CdSe interfaces Michelle Tomasik, Jeffrey Grossman, Varadharajan Srinivasan CdSe, a potential material for hybrid solar cells, has a well known reconstruction at the surface which removes the surface states. Using Density Functional Theory (DFT) we explore what happens to the now-removed surface states when CdSe is interfaced with two different metals, Al and Au. We compare and contrast this with the interfaces of a pristine unreconstructed CdSe surface with the two metals. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B40.00014: Calculated Stability and Band Offsets for Compensated and Abrupt Polar Si/Zn(S,Se) (111) Interfaces David Foster, Guenter Schneider Heterovalent semiconductor interfaces, particularly in the non-symmetrizable (111) and (0001) directions, present computational challenges that must be addressed in order to predict properties such as band offsets and interface energies. We perform first principles GGA+$U$ calculations of interface energies and band offsets for the nominally polar interfaces Si/Zn(S,Se) (111). Such wide-gap/narrow-gap heterostructures have been proposed as a possible means for altering the relaxation channel branching ratios for the decay of high energy photoelectrons (blue to UV) in favor of impact ionization (two carrier pairs from one photon). Examining configurations with one and two substitutional defect layers, we find the expected trend that compensated interfaces typically have lower energies than abrupt interfaces. The valence band offset ($-0.8 \pm 0.1$ eV) for the lowest energy abrupt Si/ZnS interfaces agrees well with the experimentally determined value of $-0.7$ eV. We examine methods to address the ambiguities that arise from both finite size induced inter-interface charge transfer and the non-symmetrizability of (111) oriented supercells. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B40.00015: Atom probe characterization of an AlN interlayer within HEMT structures grown by molecular beam epitaxy and metal-organic chemical vapor deposition Baishakhi Mazumder, Stephen W. Kaun, Jing Lu, Stacia Keller, Umesh K. Mishra, James S. Speck An AlN interlayer is introduced in a conventional AlGaN/GaN HEMT to enhance the density and mobility of the two dimensional electron gas (2DEG). MBE and MOCVD are two competitive and proven techniques to grow high quality AlN, but a chemical characterization technique is desired to investigate the purity of the AlN interlayer. Amongst nanoanalyzing techniques, atom probe tomography (APT) is unique for its spatial resolution and 3-D compositional images (\textless\ 0.2nm) with analytical sensitivity (10appm). In this work, plasma assisted MBE(PAMBE) and MOCVD techniques were employed to grow AlGaN/AlN/GaN heterostructures. Detailed compositional data from atom probe shows that a pure AlN layer was grown by PAMBE. From Hall measurements, the carrier density (sheet resistance) was found to be 1.65 $\times$ 10$^{13}$cm$^{-3}$(188$\Omega $/sq). The MOCVD structures do not form a pure AlN layer but that of Al$_{0.45}$Ga$_{0.55}$N layer. The carrier density was found to be 1.15 $\times$ 10$^{13}$cm$^{-3}$ (425 $\Omega $/sq). This work showed that MBE technique is more suitable than MOCVD for growing pure AlN interlayers and that APT can provide valuable nano scale information for further optimization of growth structures, thereby improving device performance. [Preview Abstract] |
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