arXiv subjects

A. T. S. Wee

Publications and source records attributed to A. T. S. Wee.

12 recordsLinked to original sources

arXiv2023

Direct Determination of Spin-Splitting Energy in Magnetic Graphene by Landau Fan Shifts

Spin-polarized two-dimensional materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, we demonstrate the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3Fe5O12, as confirmed by its X-ray magnetic circular dichroism. The spin-splitting energies are directly measured and visualized by the shift in their landau fan diagram mapped by analyzing the measured subnikov-de-Haas oscillations as a function of applied electric fields, showing consistent fit with our first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by cooling fields. Our methods and results are applicable to other two-dimensional (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.

cond-mat.mtrl-sci↗

arXiv2022

Observation of perfect diamagnetism and interfacial effect on the electronic structures in Nd0.8Sr0.2NiO2 superconducting infinite layers

Nickel-based complex oxides have served as a playground for decades in the quest for a copper-oxide analog of the high-temperature superconductivity. They may provide clues towards understanding the mechanism and an alternative route for high-temperature superconductors. The recent discovery of superconductivity in the infinite-layer nickelate thin films has fulfilled this pursuit. However, material synthesis remains challenging, direct demonstration of perfect diamagnetism is still missing, and understanding of the role of the interface and bulk to the superconducting properties is still lacking. Here, we show high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and demonstrate the interface and strain effects on the electrical, magnetic and optical properties. Perfect diamagnetism is achieved, confirming the occurrence of superconductivity in the films. Unlike the thick films in which the normal-state Hall-coefficient changes signs as the temperature decreases, the Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a thickness-driven band structure modification. Moreover, X-ray absorption spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer nickelates, and the hybridization is enhanced as the thickness decreases. Consistent with band structure calculations on the nickelate/SrTiO3 heterostructure, the interface and strain effect induce a dominating electron-like band in the ultrathin film, thus causing the sign-change of the Hall-coefficient.

cond-mat.supr-con↗

arXiv2021

Room-temperature colossal magnetoresistance in terraced single-layer graphene

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 terminated SrTiO3, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~1012 cm-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

cond-mat.mes-hall↗

arXiv2019

Electronic correlation determining correlated plasmons in Sb-doped Bi$_2$Se$_3$

Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magneto resistance and high temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy and Hall measurements all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas those in the bulk states increase, and that the newly-discovered correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining new correlated plasmons and opens a new path in engineering plasmonic-based topologically-insulating devices.

cond-mat.mtrl-sci↗

arXiv2018

Oxygen Electromigration and Energy Band Reconstruction Induced by Electrolyte Field Effect at Oxide Interfaces

Electrolyte gating is a powerful means for tuning the carrier density and exploring the resultant modulation of novel properties on solid surfaces. However, the mechanism, especially its effect on the oxygen migration and electrostatic charging at the oxide heterostructures, is still unclear. Here we explore the electrolyte gating on oxygen-deficient interfaces between SrTiO3 (STO) crystals and LaAlO3 (LAO) overlayer through the measurements of electrical transport, X-ray absorption spectroscopy (XAS) and photoluminescence (PL) spectra. We found that oxygen vacancies (Ovac) were filled selectively and irreversibly after gating due to oxygen electromigration at the amorphous LAO/STO interface, resulting in a reconstruction of its interfacial band structure. Because of the filling of Ovac, the amorphous interface also showed an enhanced electron mobility and quantum oscillation of the conductance. Further, the filling effect could be controlled by the degree of the crystallinity of the LAO overlayer by varying the growth temperatures. Our results reveal the different effects induced by electrolyte gating, providing further clues to understand the mechanism of electrolyte gating on buried interfaces and also opening a new avenue for constructing high-mobility oxide interfaces.

cond-mat.str-el↗

arXiv2012

Cationic vacancy induced room-temperature ferromagnetism in transparent conducting anatase Ti_{1-x}Ta_xO_2 (x~0.05) thin films

We report room-temperature ferromagnetism in highly conducting transparent anatase Ti1-xTaxO2 (x~0.05) thin films grown by pulsed laser deposition on LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), x-ray diffraction (XRD), proton induced x-ray emission (PIXE), x-ray absorption spectroscopy (XAS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) indicated negligible magnetic contaminants in the films. The presence of ferromagnetism with concomitant large carrier densities was determined by a combination of superconducting quantum interference device (SQUID) magnetometry, electrical transport measurements, soft x-ray magnetic circular dichroism (SXMCD), XAS, and optical magnetic circular dichroism (OMCD) and was supported by first-principle calculations. SXMCD and XAS measurements revealed a 90% contribution to ferromagnetism from the Ti ions and a 10% contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites though carrier activation was only 50% at 5% Ta concentration implying compensation by cationic defects. The role of Ti vacancy and Ti3+ was studied via XAS and x-ray photoemission spectroscopy (XPS) respectively. It was found that in films with strong ferromagnetism, the Ti vacancy signal was strong while Ti3+ signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localised magnetic moments, Ti vacancy sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to ferromagnetism in wide-band-gap semiconducting oxides without any magnetic elements.

cond-mat.str-el↗

arXiv2011

Direct observation of room temperature high-energy resonant excitonic effects in graphene

Using a combination of ultraviolet-vacuum ultraviolet reflectivity and spectroscopic ellipsometry, we observe a resonant exciton at an unusually high energy of 6.3eV in epitaxial graphene. Surprisingly, the resonant exciton occurs at room temperature and for a very large number of graphene layers $N$$\approx$75, thus suggesting a poor screening in graphene. The optical conductivity ($σ_1$) of resonant exciton scales linearly with number of graphene layer (up to \emph{at least} 8 layers) implying quantum character of electrons in graphene. Furthermore, a prominent excitation at 5.4eV, which is a mixture of interband transitions from $π$ to $π^{*}$ at the M point and a $π$ plasmonic excitation, is observed. In contrast, for graphite the resonant exciton is not observable but strong interband transitions are seen instead. Supported by theoretical calculations, for $N \leq$ 28 the $σ_1$ is dominated by the resonant exciton, while for $N >$ 28 it is a mixture between exitonic and interband transitions. The latter is characteristic for graphite, indicating a crossover in the electronic structure. Our study shows that important elementary excitations in graphene occur at high binding energies and elucidate the differences in the way electrons interact in graphene and graphite.

cond-mat.str-el↗

arXiv2010

Experimental observation of the crystallization of a paired holon state

A new excitation is observed at 201 meV in the doped-hole ladder cuprate Sr$_{14}$Cu$_{24}$O$_{41}$, using ultraviolet resonance Raman scattering with incident light at 3.7 eV polarized along the direction of the rungs. The excitation is found to be of charge nature, with a temperature independent excitation energy, and can be understood via an intra-ladder pair-breaking process. The intensity tracks closely the order parameter of the charge density wave in the ladder (CDW$_L$), but persists above the CDW$_L$ transition temperature ($T_{CDW_L}$), indicating a strong local pairing above $T_{CDW_L}$. The 201 meV excitation vanishes in La$_{6}$Ca$_{8}$Cu$_{24}$O$_{41+δ}$, and La$_{5}$Ca$_{9}$Cu$_{24}$O$_{41}$ which are samples with no holes in the ladders. Our results suggest that the doped holes in the ladder are composite bosons consisting of paired holons that order below $T_{CDW}$.

cond-mat.str-el↗

arXiv2008

Raman spectroscopy of epitaxial graphene on a SiC substrate

The fabrication of epitaxial graphene (EG) on SiC substrate by annealing has attracted a lot of interest as it may speed up the application of graphene for future electronic devices. The interaction of EG and the SiC substrate is critical to its electronic and physical properties. In this work, Raman spectroscopy was used to study the structure of EG and its interaction with SiC substrate. All the Raman bands of EG blue shift from that of bulk graphite and graphene made by micromechanical cleavage, which was attributed to the compressive strain induced by the substrate. A model containing 13 x 13 honeycomb lattice cells of graphene on carbon nanomesh was constructed to explain the origin of strain. The lattice mismatch between graphene layer and substrate causes the compressive stress of 2.27 GPa on graphene. We also demonstrate that the electronic structures of EG grown on Si and C terminated SiC substrates are quite different. Our experimental results shed light on the interaction between graphene and SiC substrate that are critical to the future applications of EG.

cond-mat.mtrl-sci↗

arXiv2002

Possible complete miscibility of $(BN)_x(C_2)_{1-x}$ alloys

The stabilities of $(BN)_x(C_2)_{1-x}$ alloys and related superlattices are investigated by ab initio pseudopotential calculations. We find that the $(BN)_1/(C_2)_1$ superlattices in (111) orientations have the lowest formation energy among many short-range ordered $BNC_2$ structures due to the smallest number of B-C and C-N bonds. Based on the calculated formation energies at several compositions and for various ordered structures and assuming thermodynamic equilibrium, the solid solution phase diagram of $(BN)_x(C_2)_{1-x}$ alloys is constructed. We find that the complete miscibility of $(BN)_x(C_2)_{1-x}$ alloys is possible, which is in contrast with previous theoretical predictions but in agreement with experimental reports.

cond-mat.mtrl-sci↗

arXiv2002

Structural and electronic properties of Al nanowires: an ab initio pseudopotential study

The stability and electronic structure of a single monatomic Al wire has been studied using the ab initio pseudopotential method. The Al wire undergoes two structural rearrangements under compression, i.e., zigzag configurations at angles of $140^o$ and $60^o$. The evolution of electronic structures of the Al chain as a function of structural phase transition has been investigated. The relationship between electronic structure and geometric stability is also discussed. The 2p bands in the Al nanowire are shown to play a critical role in its stability. The effects of density functionals (GGA and LDA) on cohesive energy and bond length of Al nanostructures (dimmer, chains, and monolayers) are also examined. The link between low dimensional 0D structure (dimmer) to high dimensional 3D bulk Al is estimated. An example of optimized tip-suspended finite atomic chain is presented to bridge the gap between hypothetical infinite chains and experimental finite chains.

cond-mat.mtrl-sci↗

arXiv2002

Trends in bonding configuration at SiC/III-V semiconductor interfaces

The structural and electronic properties of interfaces between beta-SiC and III-V semiconductors are studied by first-principles calculations. Favorable bonding configurations are found to form between Si-V and C-III (model A) for BN, AlN, AlP, AlAs, GaN, GaP, GaAs, InN, InP, InAs and InSb, and Si-III and C-V (model B) for BP, BAs, BSb, AlSb and GaSb. The relationship between formation energy difference and lattice constant difference as well as charge distribution for these two models is found. The origin of bonding configurations can be explained in terms of the ionicity of III-V semiconductors, electrostatic effect, charge distribution and band-structure component.

cond-mat.mtrl-sci↗