More Like this

1. Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2

   https://doi.org/10.1038/s41467-021-25499-2  

   Lin, Kai-Qiang; Ong, Chin Shen; Bange, Sebastian; Faria Junior, Paulo E.; Peng, Bo; Ziegler, Jonas D.; Zipfel, Jonas; Bäuml, Christian; Paradiso, Nicola; Watanabe, Kenji; et al (December 2021, Nature Communications)

   Abstract Monolayer transition-metal dichalcogenides (TMDCs) show a wealth of exciton physics. Here, we report the existence of a new excitonic species, the high-lying exciton (HX), in single-layer WSe 2 with an energy of ~3.4 eV, almost twice the band-edge A-exciton energy, with a linewidth as narrow as 5.8 meV. The HX is populated through momentum-selective optical excitation in the K -valleys and is identified in upconverted photoluminescence (UPL) in the UV spectral region. Strong electron-phonon coupling results in a cascaded phonon progression with equidistant peaks in the luminescence spectrum, resolvable to ninth order. Ab initio GW -BSE calculations with full electron-hole correlations explain HX formation and unmask the admixture of upper conduction-band states to this complex many-body excitation. These calculations suggest that the HX is comprised of electrons of negative mass. The coincidence of such high-lying excitonic species at around twice the energy of band-edge excitons rationalizes the excitonic quantum-interference phenomenon recently discovered in optical second-harmonic generation (SHG) and explains the efficient Auger-like annihilation of band-edge excitons. 

   more » « less
2. Theory and Ab Initio Calculation of Optically Excited States—Recent Advances in 2D Materials

   https://doi.org/10.1002/adma.201904306  

   Xie, Kaichen; Li, Xiaosong; Cao, Ting (December 2019, Advanced Materials)

   Abstract Recent studies of the optical properties of 2D materials have reported unique phenomena and features that are absent in conventional bulk semiconductors. Many of these interesting properties, such as enhanced light‐matter coupling, gate‐tunable photoluminescence, and unusual excitonic optical selection rules arise from the nature of the two‐ and multi‐particle excited states such as strongly bound Wannier excitons and charged excitons. The theory, modeling, and ab initio calculations of these optically excited states in 2D materials are reviewed. Several analytical and ab initio approaches are introduced. These methods are compared with each other, revealing their relative strength and limitations. Recent works that apply these methods to a variety of 2D materials and material‐defect systems are then highlighted. Understanding of the optically excited states in these systems is relevant not only for fundamental scientific research of electronic excitations and correlations, but also plays an important role in the future development of quantum information science and nano‐photonics. 

   more » « less
3. Phonon screening and dissociation of excitons at finite temperatures from first principles

   https://doi.org/10.1073/pnas.2403434121  

   Alvertis, Antonios M; Haber, Jonah B; Li, Zhenglu; Coveney, Christopher_J N; Louie, Steven G; Filip, Marina R; Neaton, Jeffrey B (July 2024, Proceedings of the National Academy of Sciences)

   The properties of excitons, or correlated electron–hole pairs, are of paramount importance to optoelectronic applications of materials. A central component of exciton physics is the electron–hole interaction, which is commonly treated as screened solely by electrons within a material. However, nuclear motion can screen this Coulomb interaction as well, with several recent studies developing model approaches for approximating the phonon screening of excitonic properties. While these model approaches tend to improve agreement with experiment, they rely on several approximations that restrict their applicability to a wide range of materials, and thus far they have neglected the effect of finite temperatures. Here, we develop a fully first-principles, parameter-free approach to compute the temperature-dependent effects of phonon screening within the ab initio $\mathit{GW}$-Bethe–Salpeter equation framework. We recover previously proposed models of phonon screening as well-defined limits of our general framework, and discuss their validity by comparing them against our first-principles results. We develop an efficient computational workflow and apply it to a diverse set of semiconductors, specifically AlN, CdS, GaN, MgO, and ${\text{SrTiO}}_3$. We demonstrate under different physical scenarios how excitons may be screened by multiple polar optical or acoustic phonons, how their binding energies can exhibit strong temperature dependence, and the ultrafast timescales on which they dissociate into free electron–hole pairs. 

   more » « less
4. Intrinsic donor-bound excitons in ultraclean monolayer semiconductors

   https://doi.org/10.1038/s41467-021-21158-8  

   Rivera, Pasqual; He, Minhao; Kim, Bumho; Liu, Song; Rubio-Verdú, Carmen; Moon, Hyowon; Mennel, Lukas; Rhodes, Daniel A.; Yu, Hongyi; Taniguchi, Takashi; et al (February 2021, Nature Communications)

   Abstract The monolayer transition metal dichalcogenides are an emergent semiconductor platform exhibiting rich excitonic physics with coupled spin-valley degree of freedom and optical addressability. Here, we report a new series of low energy excitonic emission lines in the photoluminescence spectrum of ultraclean monolayer WSe₂. These excitonic satellites are composed of three major peaks with energy separations matching known phonons, and appear only with electron doping. They possess homogenous spatial and spectral distribution, strong power saturation, and anomalously long population (>6 µs) and polarization lifetimes (>100 ns). Resonant excitation of the free inter- and intravalley bright trions leads to opposite optical orientation of the satellites, while excitation of the free dark trion resonance suppresses the satellitesʼ photoluminescence. Defect-controlled crystal synthesis and scanning tunneling microscopy measurements provide corroboration that these features are dark excitons bound to dilute donors, along with associated phonon replicas. Our work opens opportunities to engineer homogenous single emitters and explore collective quantum optical phenomena using intrinsic donor-bound excitons in ultraclean 2D semiconductors. 

   more » « less
5. Layer-resolved many-electron interactions in delafossite PdCoO2 from standing-wave photoemission spectroscopy

   https://doi.org/10.1038/s42005-021-00643-y  

   Lu, Qiyang; Martins, Henrique; Kahk, Juhan Matthias; Rimal, Gaurab; Oh, Seongshik; Vishik, Inna; Brahlek, Matthew; Chueh, William C.; Lischner, Johannes; Nemsak, Slavomir (December 2021, Communications Physics)

   Abstract When a three-dimensional material is constructed by stacking different two-dimensional layers into an ordered structure, new and unique physical properties can emerge. An example is the delafossite PdCoO 2 , which consists of alternating layers of metallic Pd and Mott-insulating CoO 2 sheets. To understand the nature of the electronic coupling between the layers that gives rise to the unique properties of PdCoO 2 , we revealed its layer-resolved electronic structure combining standing-wave X-ray photoemission spectroscopy and ab initio many-body calculations. Experimentally, we have decomposed the measured VB spectrum into contributions from Pd and CoO 2 layers. Computationally, we find that many-body interactions in Pd and CoO 2 layers are highly different. Holes in the CoO 2 layer interact strongly with charge-transfer excitons in the same layer, whereas holes in the Pd layer couple to plasmons in the Pd layer. Interestingly, we find that holes in states hybridized across both layers couple to both types of excitations (charge-transfer excitons or plasmons), with the intensity of photoemission satellites being proportional to the projection of the state onto a given layer. This establishes satellites as a sensitive probe for inter-layer hybridization. These findings pave the way towards a better understanding of complex many-electron interactions in layered quantum materials. 

   more » « less