Manipulating quantum states through light-matter interactions has been actively pursued in two-dimensional (2D) materials research. Significant progress has been made towards the optical control of the valley degrees of freedom in semiconducting monolayer transition-metal dichalcogenides (TMD), based on doubly degenerate excitons from their two distinct valleys in reciprocal space. Here, we introduce a novel kind of optically controllable doubly degenerate exciton states that come from a single valley, dubbed as single-valley exciton doublet (SVXD) states. They are unique in that their constituent holes originate from the same valence band, making possible the direct optical control of the spin structure of the excited constituent electrons. Combining ab initio GW plus Bethe-Salpeter equation (GW-BSE) calculations and a newly developed theoretical analysis method, we demonstrate such novel SVXD in substrate-supported monolayer bismuthene -- which has been successfully grown using molecular beam epitaxy. In each of the two distinct valleys in the Brillouin zone, strong spin-orbit coupling and $C_{3v}$ symmetry lead to a pair of degenerate 1s exciton states (the SVXD states) with opposite spin configurations. Any coherent linear combinations of the SVXD in a single valley can be excited by light with a specific polarization, enabling full manipulation of their internal spin configurations. In particular, a controllable net spin magnetization can be generated through light excitation. Our findings open new routes to control quantum degrees of freedom, paving the way for applications in spintronics and quantum information science.
Interlayer valley excitons in bilayer MoS2 feature concurrently large oscillator strength and long lifetime, and hence represent an advantageous scenario for valleytronic applications. However, control of valley pseudospin of interlayer excitons in pristine bilayer MoS2, which lies at the heart of valleytronics, has remained elusive. Here we report the observation of highly circularly polarized photoluminescence from interlayer excitons of bilayer MoS2 with both optical and magnetic addressability. Under excitation of circularly polarized light near exciton resonance, interlayer excitons of bi…
Monolayers of transition metal dichalcogenides are ideal materials to control both spin and valley degrees of freedom either electrically or optically. Nevertheless, optical excitation mostly generates excitons species with inherently short lifetime and spin/valley relaxation time. Here we demonstrate a very efficient spin/valley optical pumping of resident electrons in n-doped WSe2 and WS2 monolayers. We observe that, using a continuous wave laser and appropriate doping and excitation densities, negative trion doublet lines exhibit circular polarization of opposite sign and the photoluminesce…
Single-layer transition metal dichalcogenides (TMDs) provide a promising material system to explore the electron's valley degree of freedom as a quantum information carrier. The valley degree of freedom in single-layer TMDs can be directly accessed by means of optical excitation. The rapid valley relaxation of optically excited electron-hole pairs (excitons) through the long-range electron-hole exchange interaction, however, has been a major roadblock. Theoretically such a valley relaxation does not occur for the recently discovered dark excitons, suggesting a potential route for long valley l…
We report on nanosecond long, gate-dependent valley lifetimes of free charge carriers in monolayer WSe$_2$, unambiguously identified by the combination of time-resolved Kerr rotation and electrical transport measurements. While the valley polarization increases when tuning the Fermi level into the conduction or valence band, there is a strong decrease of the respective valley lifetime consistent with both electron-phonon and spin-orbit scattering. The longest lifetimes are seen for spin-polarized bound excitons in the band gap region. We explain our findings via two distinct, Fermi level-depen…
As hosts for tightly-bound electron-hole pairs carrying quantized angular momentum, atomically-thin semiconductors of transition metal dichalcogenides provide an appealing platform for optically addressing the valley degree of freedom. In particular, the valleytronic properties of neutral and charged excitons in these systems have been widely investigated. Meanwhile, correlated quantum states involving more particles are so far elusive and controversial. Here we present experimental evidence for valleytronic four-particle biexcitons and five-particle exciton-trions in high-quality monolayer tu…
Heterobilayers consisting of MoSe$_2$ and WSe$_2$ monolayers can host optically bright interlayer excitons with intriguing properties such as ultralong lifetimes and pronounced circular polarization of their photoluminescence due to valley polarization, which can be induced by circularly polarized excitation or applied magnetic fields. Here, we report on the observation of an intrinsic valley-magnetophonon resonance for localized interlayer excitons promoted by invervalley hole scattering. It leads to a resonant increase of the photoluminescence polarization degree at the same field of 24.2 Te…
Optically addressable spins in materials are important platforms for quantum technologies, such as repeaters and sensors. Identification of such systems in two-dimensional (2d) layered materials offers advantages over their bulk counterparts, as their reduced dimensionality enables more feasible on-chip integration into devices. Here, we report optically detected magnetic resonance (ODMR) from previously identified carbon-related defects in 2d hexagonal boron nitride (hBN). We show that single-defect ODMR contrast can be as strong as 6% and displays a magnetic-field dependence with both positi…
We investigate with polarized microphotoluminescence the optical pumping of the valley degree of freedom in charge-tunable MoS2 monolayers encapsulated with hexagonal boron nitride at cryogenic temperatures. We report a large steady state valley polarization of the different excitonic complexes following circularly-polarized laser excitation 25 meV above the neutral exciton transition. For the first time in this material we reveal efficient valley pumping of positively-charged trions, which were so far elusive in non-encapsulated monolayers due to defect and laser-induced large electron doping…
Hidden valley dynamics behind vanishing circular polarization in moiré excitons
National Institute for Materials Science（物质材料研究机构）；Graduate School of Chemical Sciences and Engineering, Hokkaido University（北海道大学化学理工学研究科）；University of Tsukuba（筑波大学）
The moire potential, induced by stacking two monolayer semiconductors with slightly different lattice mismatches, acts as periodic quantum confinement for optically generated excitons, resulting in spatially ordered zero-dimensional quantum systems. However, there are limitations to exploring intrinsic optical properties of moire excitons due to ensemble averaged and broadened emissions from many peaks caused by the inhomogeneity of the moire potential. In this study, we proposed a microfabrication technique based on focused Ga+ ion beams, which enables us to control the number of peaks origin…
In monolayer transition-metal dichalcogenide semiconductors, many-body correlations can manifest in optical spectra when photoexcited electron-hole pairs (excitons) are introduced into a 2D Fermi sea of mobile carriers. At low carrier densities, the formation of positively and negatively charged excitons ($X^\pm$) is well documented. However, in WSe$_2$ monolayers, an additional absorption resonance, often called $X^{-\prime}$, emerges at high electron density. Its origin is not understood. Here we investigate the $X^{-\prime}$ state via polarized absorption spectroscopy of electrostatically-g…
Excitations of impurity complexes in semiconductors can not only provide a route to fill the terahertz gap in optical technologies, but can also connect local quantum bits to scale up solid-state quantum-computing devices. However, taking into account both the interactions among electrons/holes, and the host band structures, is challenging. Here we combine first-principles band-structure calculations with quantum-chemistry methodology to evaluate the ground and excited states of a pair of phosphorous donors in silicon within s single framework. We use a broken-symmetry Hartree-Fock approach, f…
Beginning with the 'conventional' two-dimensional (2D) quantum wells (QWs) based on III-V and II-VI semiconductors in the 1970s, to the recent atomically-thin sheets of van der Waals materials such as 2D semiconducting transition metal dichalcogenides (TMDCs) and 2D magnets, the research in 2D materials is continuously evolving and providing new challenges. Magnetooptical spectroscopy has played a significant role in this area of research, both from fundamental physics and technological perspectives. A major challenge in 2D semiconductors such as TMDCs is to understand their spin-valley-resolv…
Motivated by the recently synthesized layered material MoSi2N4, we investigated excitonic response of quasiparticle of monolayer MoSi2N4 by using G0W0 and Bethe-Salpeter equation (BSE) calculations. With a dually sandwiched structure consisting of a central MoN2 layer analogue of 2H-MoS2 capped with silicon-nitrogen (SiN) honeycomb outer layers, MoSi2N4 possesses frontier orbitals confined at the central MoN2 layer with similar sub-valley at K-point as 2H-MoS2. The valley splitting (~130 meV) due to the spin-orbital coupling (SOC) gives rise to a doublet in the spectrum. Excitons in MoSi2N4 sh…
Optical control of conductivity type and valley polarization via persistent photoconductivity in (Pb,Sn)Se quantum wells
International Research Centre MagTop, Institute of Physics, Polish Academy of Sciences（波兰科学院物理研究所磁学国际研究中心）；Laboratoire de Physique de l’Ecole normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université（巴黎高等师范学院物理实验室，ENS，PSL大学，CNRS，索邦大学）；National Technical University ”KhPI”（国立技术大学（哈尔科夫理工学院））；Laboratoire National des Champs Magnétiques Intenses, CNRS, LNCMI, Université Grenoble Alpes, Univ. Toulouse, INSA Toulouse, EMFL（强磁场国家实验室，CNRS，LNCMI，格勒诺布尔阿尔卑斯大学，图卢兹大学，图卢兹国立理工学院，EMFL）；Department of Physics, Chungnam National University（忠南国立大学物理系）；Institute for Sciences of the Universe, Chungnam National University（忠南国立大学宇宙科学研究所）
We develop a model of the coherent exciton spin-valley dynamics in two-dimensional transition metal dichalcogenides under elastic strain. The strain splits the exciton radiative doublet in linearly polarized states. Consequently, it induces an effective magnetic field acting on the exciton pseudospin and causes its precession. As a result, under circularly polarized excitation, the circular polarization of excitons oscillates with time, also a time-oscillating linear polarization appears. We study competition of the strain-induced effective magnetic field with the field caused by the exciton l…
Monolayer transition metal dichalcogenides are a promising platform to investigate many-body interactions of excitonic complexes. In monolayer tungsten diselenide, the ground-state exciton is dark (spin-indirect), and the valley degeneracy allows low-energy dark momentum-indirect excitons to form. Interactions between the dark exciton species and the optically accessible bright exciton (X) are likely to play significant roles in determining the optical properties of X at high power, as well as limiting the ultimate exciton densities that can be achieved, yet so far little is known about these …
Two-dimensional excitons formed in quantum materials such as monolayer transition-metal dichalcogenides and their strong light-matter interaction have attracted unrivalled attention by the research community due to their extraordinarily large oscillator strength as well as binding energy, and the inherent spin-valley locking. Semiconducting few-layer and monolayer materials with their sharp optical resonances such as WSe2 have been extensively studied and envisioned for applications in the weak as well as strong light-matter coupling regimes, for effective nano-laser operation with various dif…
The optical properties of atomically thin transition metal dichalcogenides (TMDCs) are dominated by Coulomb bound quasi-particles, such as excitons, trions, and biexcitons. Due to the number and density of possible states, attributing different spectral peaks to the specific origin can be difficult. In particular, there has been much conjecture around the presence, binding energy and/or nature of biexcitons in these materials. In this work, we remove any ambiguity in identifying and separating the optically excited biexciton in monolayer WS2 using two-quantum multidimensional coherent spectros…
