Using high temperature annealing conditions with a graphite cap covering the C-face of an 8deg off-axis 4H-SiC sample, large and homogeneous single epitaxial graphene layers have been grown. Raman spectroscopy shows evidence of the almost free-standing character of these monolayer graphene sheets, which was confirmed by magneto-transport measurements. We find a moderate p-type doping, high carrier mobility and half integer Quantum Hall effect typical of high quality graphene samples. This opens the way to a fully compatible integration of graphene with SiC devices on the wafers that constitute the standard in today's SiC industry.
We present a study of quasi-free-standing monolayer graphene obtained by intercalation of Au atoms at the interface between the carbon buffer layer (Bu-L) and the silicon-terminated face (0001) of 4H-silicon carbide. Au intercalation is achieved by deposition of atomically thin Au on the Bu-L followed by annealing at 850 °C in an Argon atmosphere. We explore the intercalation of Au and decoupling of the Bu-L into quasi-free-standing monolayer graphene by surface science characterizations and electron transport in top-gated electronic devices. By gate-dependent magnetotransport we find that the…
In this study, we first show that the argon flow during epitaxial graphene growth is an important parameter to control the quality of the buffer and the graphene layer. Atomic force microscopy (AFM) and low-energy electron diffraction (LEED) measurements reveal that the decomposition of the SiC substrate strongly depends on the Ar mass flow rate while pressure and temperature are kept constant. Our data are interpreted by a model based on the competition of the SiC decomposition rate, controlled by the Ar flow, with a uniform graphene buffer layer formation under the equilibrium process at the…
It is essential to explore two-dimensional (2D) material with magnetic ordering in new generation spintronic devices. Particularly, the seeking of room-temperature 2D ferromagnetic (FM) materials is a hot topic of current research. Here, we study magnetism of the Mn-doped and electron-doped SiC monolayer using first-principle calculations. For the Mn-doped SiC monolayer, we find that either electron or hole could mediate the ferromagnetism in the system and the Curie temperature ($T_C$) can be improved by appropriate carrier doping. The codoping strategy is also discussed on improving $T_C$. T…
Using the first-principles calculations based on density functional theory, we systematically investigate the strain-engineering (tensile and compressive strain) electronic, mechanical and transport properties of monolayer penta-SiC$_2$. By applying an in-plane tensile or compressive strain, it is easy to modulate the electronic band structure of monolayer penta-SiC$_2$, which subsequently changes the effective mass of carriers. Furthermore, the obtained electronic properties are predicted to change from indirectly semiconducting to metallic. More interestingly, at room temperature, uniaxial s…
Graphene on silicon carbide (SiC) has proved to be highly successful in Hall conductance quantization for its homogeneity at the centimetre scale. Robust Josephson coupling has been measured in co-planar diffusive Al/monololayer graphene/Al junctions. Graphene on SiC substrates is a concrete candidate to provide scalability of hybrid Josephson graphene/superconductor devices, giving also promise of ballistic propagation.
Growth of epitaxial graphene on the C-face of SiC has been investigated. Using a confinement controlled sublimation (CCS) method, we have achieved well controlled growth and been able to observe propagation of uniform monolayer graphene. Surface patterns uncover two important aspects of the growth, i.e. carbon diffusion and stoichiometric requirement. Moreover, a new "stepdown" growth mode has been discovered. Via this mode, monolayer graphene domains can have an area of hundreds of square micrometers, while, most importantly, step bunching is avoided and the initial uniformly stepped SiC surf…
The results of micro-Raman scattering measurements performed on three different ``graphitic'' materials: micro-structured disks of highly oriented pyrolytic graphite, graphene multi-layers thermally decomposed from carbon terminated surface of 4H-SiC and an exfoliated graphene monolayer are presented. Despite its multi-layer character, most parts of the surface of the graphitized SiC substrates shows a single-component, Lorentzian shape, double resonance Raman feature in striking similarity to the case of a single graphene monolayer. Our observation suggests a very weak electronic coupling bet…
Ca-intercalation has opened a way for superconductivity in graphene on SiC. However, the atomic and electronic structures being critical for superconductivity are still under discussion. We find the essential role of the interface between monolayer graphene and the SiC substrate for superconductivity. In the Ca-intercalation process, at the interface a carbon layer terminating SiC changes to graphene by Ca-termination of SiC (monolayer graphene becomes bilayer) with inducing more carriers than a free-standing model. Then, Ca is intercalated in-between graphene layers, which shows superconducti…
After establishing the fundamental understanding and the high throughput topological characterization of nearly all inorganic three-dimensional materials, the general interest and the demand of functional applications drive the research of topological insulators to the exploration of systems with a more robust topological nature and fewer fabrication challenges. The successful demonstration of the room-temperature quantum spin Hall (QSH) states in bismuthene/SiC(0001), thus, triggers the search of two-dimensional topological systems that are experimentally easy to access and of even larger top…
We developed an easy, upscalable process to prepare lateral spin-valve devices on epitaxially grown monolayer graphene on SiC(0001) and perform nonlocal spin transport measurements. We observe the longest spin relaxation times tau_S in monolayer graphene, while the spin diffusion coefficient D_S is strongly reduced compared to typical results on exfoliated graphene. The increase of tau_S is probably related to the changed substrate, while the cause for the small value of D_S remains an open question.
Monolayer graphene epitaxially grown on SiC(0001) was etched by H-plasma and studied by scanning tunneling microscopy and spectroscopy. The etching created partly hexagonal nanopits of monatomic depth as well as elevated regions with a height of about 0.12 nm which are stable at $T$ = 78 K. The symmetric tunnel spectrum about the Femi energy and the absence of a $6\times6$ corrugation on the elevated regions suggest that in these regions the carbon buffer layer is decoupled from the SiC substrate and quasi-free-standing bilayer graphene appears at originally monolayer graphene on the buffer la…
We employ tip-enhanced infrared near-field microscopy to study the plasmonic properties of epitaxial quasi-free-standing monolayer graphene on silicon carbide. The near-field images reveal propagating graphene plasmons, as well as a strong plasmon reflection at gaps in the graphene layer, which appear at the steps between the SiC terraces. When the step height is around 1.5 nm, which is two orders of magnitude smaller than the plasmon wavelength, the reflection signal reaches 20% of its value at graphene edges, and it approaches 50% for step heights as small as 5 nm. This intriguing observatio…
We propose a table-top method to obtain bilayer quasi-free-standing epitaxial-graphene (QFSEG) on SiC(0001). By applying a microwave annealing in air to a monolayer epitaxial graphene (EG) grown on SiC(0001), the buffer layer is decoupled from the SiC substrate and becomes the second EG layer as confirmed by the low energy electron diffraction, high-resolution transmission electron microscopy, Raman scattering spectroscopy, X-ray photoelectron spectroscopy, and angle-resolved photoelectron spectroscopy. The most likely mechanism of the decoupling is given by the oxidation of the SiC surface, w…
The formation of epitaxial graphene on SiC is monitored in-situ using low-energy electron diffraction (LEED). The possibility of using LEED as an in-situ thickness monitor of the graphene is examined. The ratio of primary diffraction spot intensities for graphene compared to SiC is measured for a series of samples of known graphene thickness (determined using low-energy electron microscopy). It is found that this ratio is effective for determining graphene thicknesses in the range 1 to 3 monolayers. Effects of a distribution of graphene thicknesses on this method of thickness determination are…
Graphene monolayer grown by Si evaporation from the 0001 surface of SiC displays a moiré pattern of corrugation whose structure is ambiguous: different measurements and theoretical studies show either protruding bumps surrounded by valleys, or, reversely, wells surrounded by crests. Here we address the fine structure of monolayer graphene on SiC by means of Density Functional Theory, using a model including the full symmetry of the system and the substrate (1648 atoms) and therefore realistically reproducing the experimental sample. We find that accurate treatment of the vdW interactions betwe…
Magnetotransport measurements on Hall bar devices fabricated on purely monolayer epitaxial graphene on Silicon Carbide (SiC/G) show a very tight spread in carrier concentration and mobility across wafer- size dimensions. In contrast, SiC/G devices containing bilayer graphene domains display variations in their electronic properties linked to the amount of bilayer content.
We present a systematical study via scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED) on the effect of the exposure of Lithium (Li) on graphene on silicon carbide (SiC). We have investigated Li deposition both on epitaxial monolayer graphene and on buffer layer surfaces on the Si-face of SiC. At room temperature, Li immediately intercalates at the interface between the SiC substrate and the buffer layer and transforms the buffer layer into a quasi-free-standing graphene. This conclusion is substantiated by LEED and STM evidence. We show that intercalation occurs th…
The buffer layer has been analysed by combined micro-Raman and micro-transmission experiments. The epitaxial graphene growth on the (0001) Si face of 6H-SiC substrates was tuned to get a mixed surface at the early stage of graphitization with i) bare SiC, ii) buffer layer and iii) in some localized areas small monolayers flakes on top of the buffer layer. These unique samples enabled to measure the Raman spectrum of the buffer layer (close to the Raman spectrum of a carbon layer with a significant percentage of sp3 bonds) and its corresponding relative extinction at 514.5 nm. The Raman spectru…
We investigate the morphology of quasi-free-standing monolayer graphene (QFMLG) formed at several temperatures by hydrogen intercalation and discuss its relationship with transport properties. Features corresponding to incomplete hydrogen intercalation at the graphene-substrate interface are observed by scanning tunneling microscopy on QFMLG formed at 600 and 800°C. They contribute to carrier scattering as charged impurities. Voids in the SiC substrate and wrinkling of graphene appear at 1000°C, and they decrease the carrier mobility significantly.
An in vacuo thermal desorption process has been accomplished to form epitaxial graphene (EG) on 4H- and 6H-SiC substrates using a commercial chemical vapor deposition reactor. Correlation of growth conditions and the morphology and electrical properties of EG are described. Raman spectra of EG on Si-face samples were dominated by monolayer thickness. This approach was used to grow EG on 50 mm SiC wafers that were subsequently fabricated into field effect transistors with fmax of 14 GHz.
The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC(0001) has been studied by means of Low Energy Electron Microscopy (LEEM), microprobe Low Energy Electron Diffraction (\muLEED) and microprobe Angle Resolved Photoemission (\muARPES). We show that the buffer layer of epitaxial graphene on SiC(0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si-C bonds at the interface and intercalates the graphene layer. Accurate control …
Recently the graphene/SiC interface has emerged as a versatile platform for the epitaxy of otherwise unstable, monoelemental, two-dimensional (2D) layers via intercalation. Intrinsically capped into a van der Waals heterostructure with overhead graphene, they compose a new class of quantum materials with striking properties contrasting their parent bulk crystals. Intercalated silver presents a prototypical example where 2D quantum confinement and inversion symmetry breaking entail a metal-to-semiconductor transition. However, little is known about the associated unoccupied states, and control …
Resonantly-enhanced Raman response in graphene-capped bismuthene on SiC
Universität Würzburg（维尔茨堡大学）；Würzburg-Dresden Cluster of Excellence ctd.qmat, Universität Würzburg（维尔茨堡大学 würtzburg-dresden 卓越集群 ctd.qmat）；Sapienza University of Rome（罗马第一大学）；RWTH Aachen University（亚琛工业大学）；Forschungszentrum Jülich（于利希研究中心）；Institut für Theoretische Physik und Astrophysik, Universität Würzburg（维尔茨堡大学理论与天体物理研究所）；TUD Dresden University of Technology（德累斯顿工业大学）
Epitaxial graphene grown on SiC by the confinement controlled sublimation method is reviewed, with an emphasis on multilayer and monolayer epitaxial graphene on the carbon face of 4H-SiC and on directed and selectively grown structures under growth-arresting or growth-enhancing masks. Recent developments in the growth of templated graphene nanostructures are also presented, as exemplified by tens of micron long very well confined and isolated 20-40nm wide graphene ribbons. Scheme for large scale integration of ribbon arrays with Si wafer is also presented.
Intercalation is an established technique for tailoring the electronic structure of epitaxial graphene. Moreover, it enables the synthesis of otherwise unstable two-dimensional (2D) layers of elements with unique physical properties compared to their bulk versions due to interfacial quantum confinement. In this work, we present uniformly Pb-intercalated quasi-freestanding monolayer graphene on SiC, which turns out to be essentially charge neutral with an unprecedented $p$-type carrier density of only $(5.5\pm2.5)\times10^9$ cm$^{-2}$. Probing the low-energy electronic structure throughout the …
We developed a spin transport model for a diffusive channel with coupled localized states that result in an effective increase of spin precession frequencies and a reduction of spin relaxation times in the system. We apply this model to Hanle spin precession measurements obtained on monolayer epitaxial graphene on SiC(0001) (MLEG). Combined with newly performed measurements on quasi-free-standing monolayer epitaxial graphene on SiC(0001) our analysis shows that the different values for the diffusion coefficient measured in charge and spin transport measurements in MLEG and the high values for …
Chalmers University of Technology（查尔姆斯理工大学）；KTH Royal Institute of Technology（瑞典皇家理工学院）；Clemson University（克莱姆森大学）；RISE (Research Institutes of Sweden)（瑞典研究院）；Gothenburg University（哥德堡大学）；Stockholm University（斯德哥尔摩大学）；Nordita（北欧理论天体物理学中心）
2D materials have intriguing quantum phenomena that are distinctively different from their bulk counterparts. Recently, epitaxially synthesized wafer-scale 2D metals, composed of elemental atoms, are attracting attention not only for their potential applications but also for exotic quantum effects such as superconductivity. By mapping momentum-resolved electronic states using time-resolved and angle-resolved photoemission spectroscopy (ARPES), we reveal that monolayer Ag confined between bilayer graphene and SiC is a large gap (> 1 eV) 2D semiconductor, consistent with GW-corrected density fun…
We investigate the low-temperature magneto-transport properties of monolayer epitaxial graphene films formed on the Si-face of semi-insulating 4H-SiC substrates by a high temperature sublimation process. A high-k top-gate on the epitaxial graphene is realized by inserting a fully oxidized nanometer thin aluminum film as a seeding layer, followed by an atomic layer deposition process. At low temperatures, the devices demonstrate a strong field effect by the top gate with an on/off ratio of ~7 and an electron mobility up to ~3250 cm^2/Vs. After the observation of the half-integer quantum Hall ef…
The structure of the SiC(000-1) surface, the C-face of the {0001} SiC surfaces, is studied as a function of temperature and of pressure in a gaseous environment of disilane (Si2H6). Various surface reconstructions are observed, both with and without the presence of an overlying graphene layer (which spontaneously forms at sufficiently high temperatures). Based on cross-sectional scanning transmission electron microscopy measurements, the interface structure that forms in the presence of the graphene is found to contain 1.4 - 1.7 monolayers (ML) of Si, a somewhat counter-intuitive result since,…
We show that in graphene epitaxially grown on SiC the Drude absorption is transformed into a strong terahertz plasmonic peak due to natural nanoscale inhomogeneities, such as substrate terraces and wrinkles. The excitation of the plasmon modifies dramatically the magneto-optical response and in particular the Faraday rotation. This makes graphene a unique playground for plasmon-controlled magneto-optical phenomena thanks to a cyclotron mass 2 orders of magnitude smaller than in conventional plasmonic materials such as noble metals.
We found an intrinsic redistribution of charge arises between epitaxial graphene, which has intrinsically n-type doping, and an undoped substrate. In particular, we studied in detail epitaxial graphene layers thermally elaborated on C-terminated $4H$-$SiC$ ($4H$-$SiC$ ($000{\bar{1}}$)). We have investigated the charge distribution in graphene-substrate systems using Raman spectroscopy. The influence of the substrate plasmons on the longitudinal optical phonons of the $SiC$ substrates has been detected. The associated charge redistribution reveals the formation of a capacitance between the grap…
On the SiC(0001) surface (the silicon face of SiC), epitaxial graphene is obtained by sublimation of Si from the substrate. The graphene film is separated from the bulk by a carbon-rich interface layer (hereafter called the buffer layer) which in part covalently binds to the substrate. Its structural and electronic properties are currently under debate. In the present work we report scanning tunneling microscopy (STM) studies of the buffer layer and of quasi-free-standing monolayer graphene (QFMLG) that is obtained by decoupling the buffer layer from the SiC(0001) substrate by means of hydroge…
We report on quantum-interference measurements in top-gated Hall bars of monolayer graphene epitaxially grown on the Si face of SiC, in which the transition from negative to positive magnetoresistance was achieved varying temperature and charge density. We perform a systematic study of the quantum corrections to the magnetoresistance due to quantum interference of quasiparticles and electron-electron interaction. We analyze the contribution of the different scattering mechanisms affecting the magnetotransport in the $-2.0 \times 10^{10}$ cm$^{-2}$ to $3.75 \times 10^{11}$ cm$^{-2}$ density reg…
We show that Friedel charge oscillation near an interface opens a gap at the Fermi energy for electrons with wave vectors perpendicular to the interface. If the Friedel gaps on two sides of the interface are different, a nonequlibrium effect - shifting of these gaps under bias - leads to asymmetric transport upon reversing the bias polarity. The predicted transport asymmetry is revealed by scanning tunneling potentiometry at monolayer-bilayer interfaces in epitaxial graphene on SiC (0001). This intriguing interfacial transport behavior opens a new avenue towards novel quantum functions such as…
Quasi free standing monolayer graphene (QFMLG) grown on SiC by selective Si evaporation from the Si-rich SiC(0001) face and H intercalation displays irregularities in STM and AFM analysis, appearing as localized features, which we previously identified as vacancies in the H layer coverage [Y Murata, et al. Nano Res, in press, DOI: 10.1007/s12274-017-1697-x]. The size, shape, brightness, location, and concentration of these features, however, are variable, depending on the hydrogenation conditions. In order to shed light on the nature of these features, in this work we perform a systematic Dens…
Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in a Si-rich environment are studied using low-energy electron diffraction (LEED) and low-energy electron microscopy (LEEM). Upon graphitization, an interface with rt(43) x rt(43)-R7.6 degree symmetry is observed by in situ LEED. After oxidation, the interface displays rt(3) x rt(3)-R30 degree symmetry. Electron reflectivity measurements indicate that these interface structures arise from a graphene-like "buffer layer" that forms between the graphene and the SiC, similar to that observed on Si-face Si…
Graphene films prepared by heating the SiC(000-1) surface (the C-face of the {0001} surfaces) in vacuum or in a Si-rich environment are compared. It is found that different interface structures occur for the two situations. The former yields a well known 3x3 reconstructed interface, whereas the latter produces an interface with rt(43)xrt(43)-R\pm7.6 degrees symmetry. This structure is shown to contain a graphene-like layer with properties similar to the 6rt(3)x6rt(3)-R30 degrees "buffer layer" that forms on the Si(0001) surface (the Si-face).
We use angle-resolved photoemission spectroscopy to investigate the electronic structure of bilayer graphene at high n-doping and extreme displacement fields, created by intercalating epitaxial monolayer graphene on silicon carbide with magnesium to form quasi-freestanding bilayer graphene on magnesium-terminated silicon carbide. Angle-resolved photoemission spectroscopy reveals that upon magnesium intercalation, the single massless Dirac band of epitaxial monolayer graphene is transformed into the characteristic massive double-band Dirac spectrum of quasi-freestanding bilayer graphene. Analys…
In this letter we report on transport measurements of epitaxial graphene on SiC(0001) with oxygen adsorption. In a $50\times 50 μ\mathrm{m^2}$ size Hall bar we observe the half-integer quantum Hall effect with a transverse resistance plateau quantized at filling factor around $ν= 2$, an evidence of monolayer graphene. We find low electron concentration of $9\times 10^{11} \textrm{cm}^{-2}$ and we show that a doping of $10^{13}\textrm{cm}^{-2}$ which is characteristic of intrinsic epitaxial graphene can be restored by vacuum annealing. The effect of oxygen adsorption on carrier density is confi…
In search for a new generation of spintronics hardware, material candidates for room temperature quantum spin Hall effect (QSHE) have become a contemporary focus of investigation. Inspired by the original proposal for QSHE in graphene, several heterostructures have been synthesized, aiming at a hexagonal monolayer of heavier group IV elements in order to promote the QSHE bulk gap via increased spin-orbit coupling. So far, however, the monolayer/substrate coupling, which can manifest itself in strain, deformation, and hybridization, has proven to be detrimental to the aspired QSHE conditions fo…
The wavefunction of massless Dirac fermions is a two-component spinor. In graphene, a one-atom-thick film showing two-dimensional Dirac-like electronic excitations, the two-component representation reflects the amplitude of the electron wavefunction on the A and B sublattices. This unique property provides unprecedented opportunities to image the two components of massless Dirac fermions spatially. Here we report atomic resolution imaging of the two-component Dirac-Landau levels in a gapped graphene monolayer by scanning tunnelling microscopy and spectroscopy. A gap of about 20 meV, driven by …
We theoretically discuss the physical origin of the dielectric constants [ε(ω)] and second harmonic generation coefficients [\{chi}(2)(ω)] of the ABA-stacked two-dimensional graphene-like silicon carbide (2D-SiC) with the number of layers up to 5. It is found that the intensities of the pronounced peaks of both ε(ω) and \{chi}(2)(ω) exhibit a clear layer number dependence. For the light polarization parallel to the 2DSiC plane, the monolayer SiC (ML-SiC) and multilayer SiC (MuL-SiC) have very similar pronounced peak positions of ε(ω), which are attributed to the π->π* and σ->σ* transitions. Ho…
Monolayer epitaxial graphene (EG) grown on hexagonal Si-terminated SiC substrates is intrinsically electron-doped (carrier density is about 10^13 cm^(-2)). We demonstrate a clean device fabrication process using a precious-metal protective layer, and show that etching with aqua regia results in p-type (hole) molecular doping of our un-gated, contamination-free EG. Devices fabricated by this simple process can reach a carrier density in the range of 10^10 cm^(-2) to 10^11 cm^(-2) with mobility about 8000 cm^2/V/s or higher. In a moderately doped device with a carrier density n = 2.4 x 10^11 cm^…
Some meta-stable polymorphs of bismuth monolayer (bismuthene) can host topologically nontrivial phases. However, it remains unclear if these polymorphs can become stable through interaction with a substrate, whether their topological properties are preserved, and how to design an optimal substrate to make the topological phase more robust. Using first-principles techniques we demonstrate that bismuthene polymorphs can become stable over silicon carbide (SiC), silicon (Si), silicon dioxide (SiO2) and that the proximity interaction in the heterostructures has a significant effect on the electron…
Non-trivial momentum-space spin texture of electrons can be induced by spin-orbit coupling and underpins various spin transport phenomena, such as current-induced spin polarization and spin Hall effect. In this work, we find a non-trivial spin texture, spin anti-vortex, can appear at certain momenta on the $Γ-\text K$ line in 2D monolayer Pb on top of SiC. Different from spin vortex due to the band degeneracy in the Rashba model, the existence of this spin anti-vortex is guaranteed by Poincare-Hopf theorem and thus topologically stable. Accompanied with this spin anti-vortex, a Lifshtiz transi…
The novel carbon allotrope with particular and unique 2D arrangement of carbon atoms similar to a Cairo pentagonal tiling, with interplay of $sp^{3}$ and $sp^{2}$ hybridized carbon atoms is called of Penta-graphene (PG). Previous theoretical investigations have shown that PG monolayer is mechanically and thermodynamically stable, possessing also a large band gap of $3.25eV$. This new carbon allotrope with unique carbon atom arrangement in a network (non-coplanar pentagons) is the focus of the theoretical investigations in this work. Using the non-equilibrium molecular dynamics simulations with…
We report on magneto-transport measurements on low-density, large-area monolayer epitaxial graphene devices grown on SiC. We show that the zero-energy Landau level (LL) in monolayer graphene, which is predicted to be magnetic field ($B$)-independent, can float up above the Fermi energy at low $B$. This is supported by the temperature ($T$)-driven flow diagram approximated by the semi-circle law as well as the $T$-independent point in the Hall conductivity $σ_{xy}$ near $e^2/h$. Our experimental data are in sharp contrast to conventional understanding of the zeroth LL and metallic-like behavior…
The mobility of graphene is very high because the quantum Hall effects can be observed even at room temperature. Graphene has the potential of the material for novel devices because of this high mobility. But the energy gap of graphene is zero, so graphene can not be applied to semiconductor devices such as transistors, LEDs, etc. In order to control the energy gaps, we propose atomic monolayers which consist of various materials besides carbon atoms. To examine the energy dispersions of atomic monolayers of various materials, we calculated the electronic states of these atomic monolayers usin…
The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers…
