Result of the Month(ROM)
The Result of the Month (ROM) section is an institution with a long history. Our goal is to present remarkable and interesting measurements and/or papers done by our customers with Scienta Omicron systems and instruments to the scientific community.
Correlating Josephson Supercurrents and Shiba States in Quantum Spins Unconventionally Coupled to Superconductors
Result of the Month, April 2021
Here, Paolo Sessi et al. use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, they reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, they detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy.
Inelastic Background Modelling Applied to Hard X-ray Photoelectron Spectroscopy of Deeply Buried Layers
Result of the Month, March 2021
In this work, Ben Spencer et al investigate layers of an organic LED material buried deeply below up to 200 nm of organic material using HAXPES (hv > 9 keV). To gain information from deep below the surface using photoelectron spectroscopy, they model the inelastic background originating from Ir 3d photoelectrons at ~ 2000 eV binding energy. The methodology is applied to both synchrotron and Scienta Omicron HAXPES Lab system, where data taken at Diamond Light Source also helped to benchmark the HAXPES Lab system.
One-Dimensional Confinement and Width-Dependent Bandgap Formation in Epitaxial Graphene Nanoribbons
Result of the Month, February 2021
In this work, Hrag Karakachian et al. grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. ARPES and STS measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. ARPES measurements were carried out at the Bloch beamline of the MAX IV synchrotron facility in Lund, Sweden. High-resolution energy-momentum cuts were measured using a high performance deflector-based DA30 hemispherical analyser from Scienta Omicron. The STM/STS measurements were performed in UHV (p < 2 × 10−11 mbar) at 80 K using a Scienta Omicron LT-STM system.
Valley Interference and Spin Exchange at the Atomic Scale in Silicon
Result of the Month, January 2021
Researchers from the ARC Centre for Quantum Computation and Communication Technology (CQC2T) working with Silicon Quantum Computing (SQC) have situated the ‘sweet spot’ for positioning qubits in silicon to scale up atom-based quantum processors.
Creating quantum bits, or qubits, by exactly putting phosphorus atoms in silicon is a world-leading approach in the growth of a silicon quantum computer. This method has been pioneered by CQC2T Director Prof. Michelle Simmons.
In their research, published in Nature Communications, precision placement has proven to be essential for developing robust interactions—or coupling—between qubits.
The team found that there is a special angle, or sweet spot, within a particular plane of the silicon crystal where the interaction between the qubits is most resilient to a valley interference effect.
This was located using scanning tunnelling microscope (STM) lithography techniques developed at UNSW, to observe the atomic-scale details of the interactions between the coupled atom qubits, including the valley interference between the atoms and the envelope anisotropy.
Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES
Result of the Month, December 2020
In their paper, A J H Jones & S Ulstrup et al. explore engineering properties of quantum materials by placing a singularity of the density of states near the Fermi energy. Using ebeam lithography for structuring and formation of electrical contacts, a device consisting of twisted bilayer graphene (twBG) on a stack of hBN and graphite was formed. The device was wire bonded to a chip package and introduced to the nanoARPES branch of the I05 beamline at the Diamond light source.
The measurements for this paper were acquired at the I05 beamline at the Diamond light source where a Fresnel zone plate focused 60 eV light down to a 690 nm spot on the sample. A piezo electric stage scanned the sample at 250 nm increments relative to the spot while a Scienta Omicron DA30-L acquired the photoemission spectra at each position (E, k). The DA30-L deflector mode was used to obtain (E, kx, ky)-dependent photoemission intensity.
Interfacial Polarons in van der Waals Heterojunction of Monolayer SnSe2 on SrTiO3 (001)
Result of the Month, November 2020
Interfacial polarons have been demonstrated to play important roles in heterostructures containing polar substrates. However, most of polarons found so far are diffusive large polarons; the discovery and investigation of small polarons at interfaces are scarce. In this ACS Nano Letters, Aidi Zhao and Bing Wang et al report the emergence of interfacial polarons in monolayer SnSe2 epitaxially grown on Nb-doped SrTiO3 (STO) surface using ARPES and STM. The STM measurements were performed using LT STM and ARPES measurements were performed with 21.21eV He Iα rays using a DA30-L – 08 energy analyser, both from Scienta Omicron.
Symmetry Breakdown of 4,4″-Diamino-p-Terphenyl on a Cu(111) Surface by Lattice Mismatch
Result of the Month, October 2020
In a symmetric molecule with identical functional groups, selective activation of only one site is challenging. In this Nature Communications Paper, Ebeling D et al show that 4,4″-diamino-p-terphenyl adsorbs asymmetrically to a metal surface, leading to a change in binding affinity of one of its amine groups. The measurements were performed with a commercial combined Low Temperature AFM/STM (Scienta Omicron). All STM/AFM images were acquired at 5 K under ultra-high vacuum (base pressure < 1.0 × 10–10 mbar).
Flat Bands in Twisted Bilayer Transition Metal Dichalcogenides
Result of the Month, September 2020
In this Nature Physics (2020), Zhang, Z, Wang, Y, Watanabe, K et al show the existence of a flat band in the electronic structure of 3° and 57.5° twisted bilayer WSe2 via Scanning Tunnelling Microscopy (STM) and Scanning Tunnelling Spectroscopy (STS). The featured figure shows the STM topography of the sample, both a graphene/hBN moiré pattern. The STM/STS measurements were performed in the ultrahigh-vacuum LT-STM from Scienta Omicron operating at 4.6 K.
Tomonaga–Luttinger Liquid in the Edge Channels of a Quantum Spin Hall Insulator
Result of the Month, July 2020
In this study, Stühler, R., Reis, F., Müller, T. et al. explore the impact of electronic correlations on highly localized edge states of the unique quantum spin Hall material bismuthene on SiC(0001). Exploiting the advantage of having an accessible monolayer substrate system, they use STM/STS to visualize the close-to-perfect one-dimensional confinement of the edge channels and scrutinize their suppressed density of states at the Fermi level. Based on the observed spectral behavior and its universal scaling with energy and temperature, they demonstrate the correspondence with a (helical) Tomonaga–Luttinger liquid. The STM/STS measurements have been performed with a low-temperature STM from Scienta Omicron under UHV conditions.
Effects of Nitridation on SiC/SiO2 Structures Studied by Hard X-ray Photoelectron Spectroscopy
Result of the Month, June 2020
This research paper investigates device-relevant stacks of SiC and SiO2 using energy-dependent X-ray photoelectron spectroscopy and combines results from both laboratory and synchrotron HAXPES systems to form a complete picture of the depth distribution and chemical states of nitrogen.
This paper contributes to investigating the main challenge that the semiconductor industry is tackling now, the quality of the interface between SiC and its native dielectric SiO2, which currently limits further improvement of device performance.
New Progress in the Basic Research for Topological Quantum Computing: Zero-Energy Bound States in the High-Temperature Superconductors at Two-Dimensional Limit
Result of the Month, May 2020
In this work, Prof. Jian Wang Group at Peking University detected novel Majorana zero-energy bound state (ZEBS) resembling the characteristics of Majorana zero modes (MZMs) in interstitial Fe adatoms deposited on the high-temperature superconducting thin films at two-dimensional limit.
This research was enabled by a Lab10 MBE and multiprobe LT STM from Scienta Omicron.
Light Induced Non-Volatile Switching of Superconductivity in Single Layer FeSe on SrTiO3 Substrate
Result of the Month, April 2020
In this study, Ming Yang et al. show that, in FeSe/SrTiO3 heterostructures, the superconducting transition temperature in FeSe monolayer can be effectively raised by the interband photoexcitations in the SrTiO3 substrate. This research was enabled by a Materials Innovation Platform (MIP) from Scienta Omicron combining two MBE chambers, VT STM, LT STM, and a ARPES Lab including a DA30L-8000 and VUV5k.
High Speed Dynamic and Operando Experiments: Thermal Reduction of Titanium Oxides
Result of the Month, March 2020
The thermal reduction of Titanium oxides was observed in-situ by time-resolved XPS using a new type of multi peak monitoring allowing to observe several XPS lines in a timely manner (figure 1&2). The color-coded maps show multiple XPS spectra over time where the x-axis represents the energy axis and y-axis the time. Intensity is shown from dark red (low count rate) to blue (high count rate). The oxygen reduction occurred within ~ 1 minute by applying a rapid temperature ramp and was found to be a step-wise process with an initial transition from 4+ to suboxide and the metallic state of Titanium at intermediate temperatures between 600 K and 725 K before further reduction of the 3+ oxidation state at temperatures > 725 K.
Controlled Growth of Transition Metal Dichalcogenide Monolayers using Knudsen-Type Effusion Cells for the Precursors
Result of the Month, February 2020
In this work, Antony George & Andrey Turchanin et al present a simple method for controlling the precursor flow rates using the Knudsen-type effusion cells. They characterised the grown MoS2 and WS2 monolayers by optical, atomic force and transmission electron microscopies as well as by X-ray photoelectron, Raman and photoluminescence spectroscopies, and by electrical transport measurements showing their high optical and electronic quality based on the single crystalline nature. XPS was performed in a ultra-high vacuum Multiprobe System from Scienta Omicron using a monochromatized x-ray source and an electron analyser Argus CU with a spectral energy resolution of 0.6 eV.
Selective Triplet Exciton Formation in a Single Molecule
Result of the Month, January 2020
In this work, Kimura K. et al report a single-molecule investigation of electroluminescence using a scanning tunneling microscope and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. All experiments were conducted using a Scienta Omicron LT STM operating at 4.7 K under ultrahigh vacuum.
Electronic Band Dispersion Determination in Azimuthally Disordered Transition-Metal Dichalcogenide Monolayers
Result of the Month, December 2019
In this work, N Koch et al. demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide monolayers with 2H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. They exemplify this by analysing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. The spectra for an azimuthally disordered WSe2 monolayer/highly oriented pyrolytic graphite (HOPG) sample were measured using Scienta Omicron ARPES Lab equipped with a DA30-L analyser and a VUV5k He source.
A Time- and Angle-Resolved Photoemission Spectroscopy with Probe Photon Energy up to 6.7 eV
Result of the Month, November 2019
In this Review, Prof. Wentao Zhang et al. introduce the development of an advanced trARPES system with probe photon energy up to 6.7 eV. The system is based on a Yb-based laser, KBe2BO3F2 (KBBF) and BBO nonlinear optical crystals, and a hemispherical electron analyser DA30-L 8000 from Scienta Omicron.
Evaluation of Sn-Doped Indium Oxide Film and Interface Properties on a-Si Formed by Reactive Plasma Deposition
Result of the Month, October 2019
Aiming to improve the performance of heterojunction Si solar cells, we evaluated the Sn-doped indium oxide (ITO) / a-Si structure using conventional and hard X-ray photoelectron spectroscopy (XPS, HAXPES), and the cause of the solar cell performance Identified deterioration. HAXPES allows non-destructive evaluation of the SiOx layer at the ITO / a-Si interface. The formation of SiOx at the ITO / a-Si interface increases the contact resistance, which can be reduced by post-deposition annealing (PDA). In addition, PDA facilitated Fermi level evaluation, ITO component precipitation in the a-Si layer, and increased interface roughness. Before the PDA, diffusion of a-Si Sn atoms was observed. In addition, PDA confirmed that Si atoms diffused into ITO. These reactions at the ITO / a-Si interface may be part of the deterioration factor of Si solar cells.
Multiple Topological States in Iron-Based Superconductors
Result of the Month, September 2019
In this work, Peng Zhang & Shik Shin et al. identified topological insulator and Dirac semimetal states near the Fermi energy in different iron-based superconducting compounds. More specifically, they 1) observe that TI bands reminiscent of Fe(Te,Se) exist in Li(Fe,Co)As; 2) predict and observe topological Dirac semimetal (TDS) bands in Li(Fe,Co)As and Fe(Te,Se); and 3) discuss phase diagram of these superconducting topological state4s as a function of doping. The ARPES measurements were performed with R4000 electron analyzer and spin-resolved ARPES measurements were carried out with DA30-L analyser – both from Scienta Omicron.