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. 

Signature of Kondo Hybridisation with an Orbital-Selective Mott Phase in 4d Ca2−xSrxRuO4

Result of the Month March 2023

The heavy fermion state with Kondo-hybridisation (KH), usually manifested in f-electron systems with lanthanide or actinide elements, was recently discovered in several 3d transition metal compounds without f-electrons. However, KH has not yet been observed in 4d/5d transition metal compounds, since more extended 4d/5d orbitals do not usually form flat bands that supply localised electrons appropriate for Kondo pairing. Here, we report a substitution- and temperature-dependent angle-resolved photoemission study on 4d Ca2−xSrxRuO4, which shows the signature of KH. We observed a spectral weight transfer in the γ-band, reminiscent of an orbital-selective Mott phase (OSMP). The Mott localised γ-band induces the KH with an itinerant β-band, resulting in spectral weight suppression around the Fermi level. Our work demonstrates the evolution of the OSMP with possible KH among 4d electrons, and thereby expands the material boundary of Kondo physics to 4d multi-orbital systems.

ARPES measurements: were performed at Seoul National University (SNU) and at the MERLIN beamline (BL) 4.0.3 of the Advanced Light Source, Lawrence Berkeley National Laboratory using Scienta Omicron DA30 (SNU) and R8000 (BL 4.0.3) electron analysers with energy resolutions of 10 and 15 meV, respectively.

Robustness of Bilayer Hexagonal Ice against Surface Symmetry and Corrugation

Result of the Month February 2023

Two-dimensional (2D) bilayer hexagonal ice (BHI) is regarded as the first intrinsic 2D ice crystal. However, the robustness of such a structure or its derivatives against surface symmetry and corrugation is still unclear. Here, we report the formation of 2D BHI on gold surfaces with 1D corrugation, using noncontact atomic force microscopy. The hexagonal arrangement of the first wetting layer was visualized on the Au(110)-1 × 2 surface. Upon depositing more water molecules, the first layer would rearrange and shrink, resulting in the formation of buckled BHI. Such a buckled BHI is hydrophobic despite the appearance of dangling OH, due to the strong interlayer bonding. Furthermore, the BHI is also stable on the Au(100)-5 × 28 surface. This work reveals the unexpected generality of the BHI on corrugated surfaces with nonhexagonal symmetry, thus shedding new light on the microscopic understandings of the low dimensional ice formation on solid surfaces or under confinement.

STM and AFM experiments: All the experiments were carried out with an ultra-high vacuum Scienta Omicron POLAR-STM/AFM combined system operated at 4.8 K using a qPlus sensor.

Atomic-Scale Thermopower in Charge Density Wave States

Result of the Month January 2023

Here, we report on the atomic-scale investigation of thermopower and phonon puddles in the CDW states of 1T-TaS2 using SThEM. In previous studies, large thermopower has been observed at the device scale in the commensurate CDW phase in 1T-TaS2, which is promising for space applications with radiation hardness. In our study, the Star-of-David (SoD) clusters of tantalum and sulfur atoms exhibit drastic and counterintuitive variations in thermopower (4 mV) over a distance of 1 nm. Moreover, in contrast with former scanning tunneling microscopy (STM) studies, the spatial variation of thermopower in 1T-TaS2 exhibits a broken three-fold rotational symmetry, which can be explained by the unique interlayer coupling in the layered material. The localized electrons with large effective mass, which generate the Mott insulating gap in the commensurate CDW phase, contribute to the atomic-scale thermopower features in 1T-TaS2. In addition, local heat transfer rates with a spatial range shorter than the conventional mean free path of phonons, called phonon puddles in our study, were probed for the first time in 1T-TaS2 with SThEM. The phonon puddles reflect the propagation and scattering of phonons in the subsurface structures of 1T-TaS2, which determines local heat dissipation in the CDW phase.  All STM measurements were performed with a chemically etched tungsten tip using a Scienta Omicron VT-SPM system under a base pressure of 10−11 torr.

Revealing the Order Parameter Dynamics of 1T-TiSe2 Following Optical Excitation 

Result of the Month (ROM), December 2022

In this work, A Lanzara et al. use time and angle resolved photoelectron spectroscopy (ARPES) to optically perturb the charge-density phase in 1T-TiSe2 and follow the recovery of its order parameter as a function of energy, momentum and excitation density. Time-resolved ARPES measurements were conducted at the Lawrence-Berkeley National Laboratory with 22.3 eV extreme-ultraviolet (XUV) femtosecond pulses. Photoelectrons are detected with a hemispherical electron analyzer (Scienta Omicron R4000). 

Surface Chemistry on a Polarizable Surface: Coupling of CO with KTaO3(001)

Result of the month November 2022

In this work, Ulrike Diebold & Martin Setvin et al. use carbon monoxide to probe whether and how ferroelectric lattice distortions couple to adsorbates. Combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) is used to identify the adsorption configurations of CO, temperature-programmed desorption (TPD) determines the desorption kinetics and energies, and density functional theory (DFT) calculations provide insights into the coupling with ferroelectric lattice distortions. The AFM/STM experiments were performed at T = 4.8 K in an ultrahigh vacuum (UHV) chamber with a base pressure below 2 × 10−9 Pa, equipped with a Scienta Omicron q-Plus LT head and a custom-design cryogenic preamplifier.

Dry pick-and-flip assembly of van der Waals heterostructures for microfocus angle-resolved photoemission spectroscopy

Result of the month October 2022

In this work, Satoru Masubuchi, Masato Sakano, Kyoko Ishizaka, and Tomoki Machida et al. present a dry pick-and-flip assembly technique for the ARPES of van der Waals heterostructures by employing the differences in the glass transition temperatures of a polymer composed of Elvacite2552C acrylic resin and 1-ethyl-3-methylimidazolium ionic liquid (IL). The laser-based µ-ARPES measurement was performed by using a combination of the hemispherical analyzer (DA30-L, Scienta Omicron) and a 6.42 eV laser, at the Department of Applied Physics, The University of Tokyo.

Real-Space Investigation of the Multiple Halogen Bonds by Ultrahigh-Resolution Scanning Probe Microscopy

Result of the month September 2022

The chemical bond is of central interest in chemistry, and it is of significance to study the nature of intermolecular bonds in real-space. Herein, non-contact atomic force microscopy (nc-AFM) and low-temperature scanning tunneling microscopy (LT-STM) are employed to acquire real-space atomic information of molecular clusters, i.e., monomer, dimer, trimer, tetramer, formed on Au(111). The formation of the various molecular clusters is due to the diversity of halogen bonds, and AFM images clearly show bright line features at the specific sites where halogen bond forms. DFT calculation also suggest the formation of three distinct halogen bonds among the molecular clusters, which originate from the noncovalent interactions of Br-atoms with the positive potential H-atoms, neutral potential Br-atoms and negative potential N-atoms, respectively. This work shows that nc-AFM and BR-STM can directly investigate halogen bonds in real-space and can be used to unambiguously discriminate their bonding features. This work demonstrates the potential use of this technique to study other non-covalent intermolecular bonds and to understand complex supramolecular assemblies at the sub-molecular level.

Dual topological states in the layered titanium-based oxypnictide superconductor BaTi2Sb2O

Result of the Month (ROM), August 2022

In this work, Z Huang, Y F Guo, G Li & Dawei Shen et al. predict and then unveil the coexistence of topological Dirac semimetal and topological insulator states in the vicinity of Fermi energy (EF) in the titanium-based oxypnictide superconductor BaTi2Sb2O using angle-resolved photoemission spectroscopy (ARPES). Further spin-resolved measurements confirm its spin-helical surface states around EF, which are topologically protected and give an opportunity for realization of Majorana zero modes and Majorana flat bands in one material. The high-resolution and spin-resolved ARPES measurements were performed using a Scienta Omicron DA30-L electron analyser at Shanghai Synchrotron Radiation Facility (SSRF).

Sub-Angstrom Noninvasive Imaging of Atomic Arrangement in 2D Hybrid Perovskites

Result of the Month (ROM), July 2022

In this work, Mykola Telychko, Shayan Edalatmanesh, Kai Leng & , Jiong Lu et al. demonstrate a sub-angstrom resolution imaging of both soft organic layers and inorganic framework in a prototypical 2D lead-halide RPP crystal via combined tip-functionalized STM and ncAFM corroborated by theoretical simulations. STM measurements unveil the atomic reconstruction of the inorganic lead-halide lattice and overall twin-domain composition of the RPP crystal, while ncAFM measurements with a CO-tip enable nonperturbative visualization of the cooperative reordering of surface organic cations driven by their hydrogen bonding interactions with the inorganic lattice. The STM and ncAFM experiments were performed under ultrahigh vacuum conditions at 4.4 K using a Scienta Omicron LT STM/AFM System.

The State of Zinc in Methanol Synthesis over a Zn/ZnO/Cu(211) Model Catalyst

Result of the Month (ROM), June 2022

In the latest study, Peter Amann et al. have for the first time been able to study the surface of a copper-zinc catalyst when carbon dioxide is reduced to methanol. They used XPS at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO2/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO2 methanol synthesis. Using a well-focused and low-divergence x-ray beam from beamline P22 at the Petra III synchrotron radiation facility they probed the surface under extreme grazing incidence conditions using the BAR XPS endstation, developed by Stockholm University and commercialized by Scienta Omicron.

Atomic Insight into the Interfacial Effect on the Molecular Solvation

Result of the Month (ROM), May 2022

In this work, Duanyun Cao & Jing Guo et al. probe the microsolvation of methanol in water on copper surfaces, using qPlus-based nc-AFM with a CO-terminated tip at 4.8 K. The configurations of water and methanol molecules in the complexes, including the detailed OH directionalities and H-bonding networks could be identified unambiguously by the height-dependent AFM images in combination with theoretical simulations. They demonstrate the excellent cooperativity between the water and methanol molecules through hydrogen-bonding in the formation of ordered clusters on both Cu(110) and Cu(111) surfaces, which are composed of pentagonal and hexagonal rings. All the experiments were carried out with an ultra-high vacuum Scienta Omicron POLAR-STM/AFM combined system operated at 4.8 K using a qPlus sensor equipped with a W tip.

CO-Terminated Tip for Intramolecular Resolution with Scienta Omicron Infinity SPM

Result of the Month, April 2022

Sylvain Clair, Christian Loppacher & Laurent Nony have performed tip functionalization with a single CO molecule to achieve intramolecular resolution with the Closed Cycle Infinity SPM from Scienta Omicron operating at 9 K. The measurements have been carried out in constant height mode and the corresponding frequency shift images clearly resolve the internal carbon skeleton of the porphyrinoid molecule on Au(111). This measurement is the very first qPlus result with a CO terminated tip and sub-molecular resolution with a closed cycle SPM!

Visualizing Band Selective Enhancement of Quasiparticle Lifetime in a Metallic Ferromagnet

Result of the Month, March 2022

In this work, Na Hyun, Yun Wu & Adam Kaminski et al. demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Their detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications. The data were acquired using a tunable VUV laser ARPES system that consists of Scienta Omicron DA30-L electron analyzer.

Voltage-Induced Bistability of Single Spin-Crossover Molecules in a Two-Dimensional Monolayer

Result of the Month, February 2022

In this work, Amandine Bellec, Yongfeng Tong, Massine Kelaï et al, report the voltage-induced switching of spin states in spin-crossover molecules. The measurements have been carried out on self-assembled 2D networks of SCO molecules on Au(111) and Cu(111) by scanning tunneling microscopy (STM) at low temperature. All the measurements have been carried out on a low temperature scanning tunneling microscope (LT STM, Scienta Omicron) operating at 4.6 K.

Band-Selective Holstein Polaron in Luttinger Liquid Material A0.3MoO3 (A = K, Rb)

Result of the Month, January 2022

In this work, Y. L. Chen & L. X. Yang et al. study molybdenum blue bronze A0.3MoO3 (A = K, Rb) using laser-based angle-resolved photoemission spectroscopy (laser-based ARPES). Their experiment suggests that the normal phase of A0.3MoO3 is a prototypical Luttinger liquid, from which the charge-density-wave emerges with decreasing temperature. Prominently, they observe strong renormalizations of band dispersions, which are recognized as the spectral function of Holstein polaron derived from band-selective electron-phonon coupling in the system. Laser-based ARPES measurements were performed using DA30-L analyzers from Scienta Omicron.

Constructing Covalent Organic Nanoarchitectures Molecule by Molecule via Scanning Probe Manipulation

Result of the Month, December 2021

Constructing low-dimensional covalent assemblies with tailored size and connectivity is challenging yet often key for applications in molecular electronics where optical and electronic properties of the quantum materials are highly structure dependent. In this work, André Schirmeisen, Daniel Ebeling, Qigang Zhong et al. present a versatile approach for building such structures block by block on bilayer sodium chloride (NaCl) films on Cu(111) with the tip of an atomic force microscope, while tracking the structural changes with single-bond resolution. All the STM and AFM measurements were performed at ~5.2 K under ultra-high-vacuum conditions (base pressure <1.0 × 10−10 mbar) using a commercial STM/AFM system (Scienta Omicron).

Role of Alkali Cations in Stabilizing Mixed-Cation Perovskites to Thermal Stress and Moisture Conditions

Result of the Month, November 2021

In this work, Suresh Maniyarasu, Andrew G Thomas, Wendy R Flavell, Ben F Spencer et al. investigate the chemical composition of two different mixed-cation perovskite compositions using laboratory based Ga Kα (9.25 keV) HAXPES. Chemical composition over 45 nm of sampling depth was quantified using angle-dependent HAXPES measurements. This technique allows more accurate quantification of the alkali metal-cation distribution than is possible using conventional XPS. The elemental distribution and bulk composition of the multiple-cation perovskite materials were studied through angle-dependent HAXPES measurements with a constant energy X-ray source. The high-throughput HAXPES Lab (Scienta Omicron) is located in the Henry Royce Institute, University of Manchester.

Bandwidth-Control Orbital-Selective Delocalization of 4f Electrons in Epitaxial Ce Films

Result of the Month, October 2021

In this paper, Yang Liu, Yi-feng Yang, Yi Wu et al. present direct spectroscopic evidence for the bandwidth-control OSD of 4f electrons in epitaxial Ce films, by combining high-quality thin film growth by MBE and in situ measurements from ARPES. More specifically, they uncovered a hitherto unreported metastable phase of Ce, where sharp dispersive quasiparticle bands with pure 4f character can be observed near EF, accompanied by an appreciable dispersion of the lower Hubbard band. Ce films were grown on top of epitaxial graphene layers at a temperature of approx. 100 °C and at a rate of 2 Å/min using a high-temperature effusion cell of a Scienta Omicron Lab10 MBE. For ARPES measurements samples were transferred in UHV from the MBE chamber to a connected Scienta Omicron ARPES Lab equipped with a DA30-L analyser and a VUV5k He lamp. 

Energy Funnelling within Multichromophore Architectures Monitored with Subnanometre Resolution

Result of the Month, September 2021

In this work, Guillaume Schull, Fabrice Scheurer et al. have used luminescence induced by scanning tunnelling microscopy to probe model multichromophoric structures assembled on a surface. Mimicking strategies developed by photosynthetic systems, individual molecules were used as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. As it relies on organic chromophores as the elementary components, this approach constitutes a powerful model to address fundamental physical processes at play in natural light-harvesting complexes. The STM data were acquired with a LT STM (4.5 K) Scienta Omicron set-up that operated in a ultrahigh vacuum, and was configured to detect the light emitted at the tip–sample junction.

A Time-Domain Phase Diagram of Metastable States in a Charge Ordered Quantum Material

Result of the Month, August 2021

In this work, by using time-resolved optical techniques and femtosecond-pulse-excited STM, Dragan Mihailovic, Jan Ravnik, Michele Diego et al. track the evolution of the metastable states in a material that has been wide recent interest, the quasi-two-dimensional dichalcogenide 1T-TaS2. They map out its temporal phase diagram using the photon density and temperature as control parameters on timescales ranging from 10—12 to 103 s. The introduction of the time-domain axis in the phase diagram enables them to follow the evolution of metastable emergent states created by different phase transition mechanisms on different timescale. For STM experiments they use a Scienta Omicron LT STM (LT Nanoprobe Lab) with optical access.

Identifications and Electronic Characterizations of Four Cyclodehydrogenation Products of H2TPP Molecules on Au(111)

Result of the Month, July 2021

C–H bond activation and dehydrogenative coupling reactions have always been significant approaches to construct microscopic nanostructures on surfaces. By using scanning tunneling microscopy/spectroscopy (STM/STS) and non-contact atomic force microscopy (nc-AFM) combined with density functional theory (DFT), we systematically characterized the atomically precise topographies and electronic properties of H2TPP cyclodehydrogenation products on Au(111). Through surface-assisted thermal excitation, four types of cyclodehydrogenation products were obtained and clearly resolved in the nc-AFM images.

Laboratory Based Hard X-ray Photoelectron Spectroscopy for Buried Interface Analysis of Microelectronic Components

Result of the Month, June 2021

In this work, Nick Barrett et al. have carried out a benchmarking of the Scienta Omicron HAXPES Lab with respect to a synchrotron radiation HAXPES set-up for analysis of buried interfaces for microelectronics applications. The synchrotron HAXPES experiments were done at the BL15XU beamline at JASRI (SPring-8) using 8 keV photons and a Scienta Omicron R4000 analyzer. The HAXPES Lab measurements were done using the 9.25 keV Ga Kα1 source and Scienta Omicron EW4000 analyzer.

Photodriven Transient Picosecond Top‐Layer Semiconductor to Metal Phase‐Transition in p‐Doped Molybdenum Disulfide

Result of the Month, May 2021

In this work, Nomi L A N Sorgenfrei, Erika Giangrisostomi, Raphael M Jay et al. establish how visible light itself can create a transient metallic top layer on bulk crystalline p‐doped 2H‐MoS2. Electron–hole pairs created by optical excitation separate in the surface band bending region of p‐doped semiconducting 2H‐MoS2. This causes a transient accumulation of electrons in the surface region, driving the top‐layer within several picoseconds from the p‐doped semiconducting 2H‐MoS2 into a sheet of metallic 1T‐MoS2 at a remarkably low optical fluence threshold. This mechanism has significant implications on how optically illuminated MoS2 surfaces behave and will positively change the catalytic properties for water splitting. This study was enabled due to the very high transmission delivered by Wide Angle ARTOF spectrometer from Scienta Omicron and was performed at the FEMTOSPEX end station Helmholtz-Zentrum Berlin.

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.