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1. Observation of chiral surface excitons in a topological insulator Bi ₂ Se ₃

   https://doi.org/10.1073/pnas.1813514116  

   Kung, H. -H.; Goyal, A. P.; Maslov, D. L.; Wang, X.; Lee, A.; Kemper, A. F.; Cheong, S. -W.; Blumberg, G. (February 2019, Proceedings of the National Academy of Sciences)

   The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin–orbit interaction in solids composed of heavy elements. Here, we study the composite particles—chiral excitons—formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, ${\mathrm{B}\mathrm{i}}_2{\mathrm{S}\mathrm{e}}_3$. Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin–orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin–orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics. 

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2. Thermally generated spin current in the topological insulator Bi ₂ Se ₃

   https://doi.org/10.1126/sciadv.adi4540  

   Jain, Rakshit; Stanley, Max; Bose, Arnab; Richardella, Anthony R.; Zhang, Xiyue S.; Pillsbury, Timothy; Muller, David A.; Samarth, Nitin; Ralph, Daniel C. (December 2023, Science Advances)

   We present measurements of thermally generated transverse spin currents in the topological insulator Bi₂Se₃, thereby completing measurements of interconversions among the full triad of thermal gradients, charge currents, and spin currents. We accomplish this by comparing the spin Nernst magneto-thermopower to the spin Hall magnetoresistance for bilayers of Bi₂Se₃/CoFeB. We find that Bi₂Se₃does generate substantial thermally driven spin currents. A lower bound for the ratio of spin current density to thermal gradient is $\frac{J_s}{{\mathbf{\nabla }}_{\mathit{x}}\mathit{T}}$= (4.9 ± 0.9) × 10⁶ $\left(\frac{\hslash }{2e}\right)\frac{\mathbf{A}{\mathbf{m}}^{-2}}{\mathbf{K}\text{μ}{\mathbf{m}}^{-1}}$, and a lower bound for the magnitude of the spin Nernst ratio is −0.61 ± 0.11. The spin Nernst ratio for Bi₂Se₃is the largest among all materials measured to date, two to three times larger compared to previous measurements for the heavy metals Pt and W. Strong thermally generated spin currents in Bi₂Se₃can be understood via Mott relations to be due to an overall large spin Hall conductivity and its dependence on electron energy. 

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3. T-linear resistivity from magneto-elastic scattering: Application to PdCrO ₂

   https://doi.org/10.1073/pnas.2305609120  

   Mendez-Valderrama, J. F.; Tulipman, Evyatar; Zhakina, Elina; Mackenzie, Andrew P.; Berg, Erez; Chowdhury, Debanjan (September 2023, Proceedings of the National Academy of Sciences)

   An electronic solid with itinerant carriers and localized magnetic moments represents a paradigmatic strongly correlated system. The electrical transport properties associated with the itinerant carriers, as they scatter off these local moments, have been scrutinized across a number of materials. Here, we analyze the transport characteristics associated with ultraclean PdCrO ${}_2$—a quasi-two-dimensional material consisting of alternating layers of itinerant Pd-electrons and Mott-insulating CrO ${}_2$layers—which shows a pronounced regime ofT-linear resistivity over a wide range of intermediate temperatures. By contrasting these observations to the transport properties in a closely related material PdCoO ${}_2$, where the CoO ${}_2$layers are band-insulators, we can rule out the traditional electron–phonon interactions as being responsible for this interesting regime. We propose a previously ignored electron-magneto-elastic interaction between the Pd-electrons, the Cr local moments and an out-of-plane phonon as the main scattering mechanism that leads to the significant enhancement of resistivity and aT-linear regime in PdCrO ${}_2$at temperatures far in excess of the magnetic ordering temperature. We suggest a number of future experiments to confirm this picture in PdCrO ${}_2$as well as other layered metallic/Mott-insulating materials. 

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4. On-chip spin-orbit locking of quantum emitters in 2D materials for chiral emission

   https://doi.org/10.1364/OPTICA.463481  

   Ma, Yichen; Zhao, Haoqi; Liu, Na; Gao, Zihe; Mohajerani, Seyed_Sepehr; Xiao, Licheng; Hone, James; Feng, Liang; Strauf, Stefan (August 2022, Optica)

   Light carries both spin angular momentum (SAM) and orbital angular momentum (OAM), which can be used as potential degrees of freedom for quantum information processing. Quantum emitters are ideal candidates towards on-chip control and manipulation of the full SAM–OAM state space. Here, we show coupling of a spin-polarized quantum emitter in a monolayer ${\mathrm{W}\mathrm{S}\mathrm{e}}_2$with the whispering gallery mode of a ${\mathrm{S}\mathrm{i}}_3{\mathrm{N}}_4$ring resonator. The cavity mode carries a transverse SAM of $\mathrm{\sigma <\#comment/>}=\pm <\#comment/>1$in the evanescent regions, with the sign depending on the orbital power flow direction of the light. By tailoring the cavity–emitter interaction, we couple the intrinsic spin state of the quantum emitter to the SAM and propagation direction of the cavity mode, which leads to spin–orbit locking and subsequent chiral single-photon emission. Furthermore, by engineering how light is scattered from the WGM, we create a high-order Bessel beam which opens up the possibility to generate optical vortex carrying OAM states. 

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5. Dephasing by optical phonons in GaN defect single-photon emitters

   https://doi.org/10.1038/s41598-023-35003-z  

   Geng, Yifei; Luo, Jialun; van Deurzen, Len; Xing, Huili; Jena, Debdeep; Fuchs, Gregory David; Rana, Farhan (December 2023, Scientific Reports)

   Abstract Single-photon defect emitters (SPEs), especially those with magnetically and optically addressable spin states, in technologically mature wide bandgap semiconductors are attractive for realizing integrated platforms for quantum applications. Broadening of the zero phonon line (ZPL) caused by dephasing in solid state SPEs limits the indistinguishability of the emitted photons. Dephasing also limits the use of defect states in quantum information processing, sensing, and metrology. In most defect emitters, such as those in SiC and diamond, interaction with low-energy acoustic phonons determines the temperature dependence of the dephasing rate and the resulting broadening of the ZPL with the temperature obeys a power law. GaN hosts bright and stable single-photon emitters in the 600–700 nm wavelength range with strong ZPLs even at room temperature. In this work, we study the temperature dependence of the ZPL spectra of GaN SPEs integrated with solid immersion lenses with the goal of understanding the relevant dephasing mechanisms. At temperatures below ~ 50 K, the ZPL lineshape is found to be Gaussian and the ZPL linewidth is temperature independent and dominated by spectral diffusion. Above ~ 50 K, the linewidth increases monotonically with the temperature and the lineshape evolves into a Lorentzian. Quite remarkably, the temperature dependence of the linewidth does not follow a power law. We propose a model in which dephasing caused by absorption/emission of optical phonons in an elastic Raman process determines the temperature dependence of the lineshape and the linewidth. Our model explains the temperature dependence of the ZPL linewidth and lineshape in the entire 10–270 K temperature range explored in this work. The ~ 19 meV optical phonon energy extracted by fitting the model to the data matches remarkably well the ~ 18 meV zone center energy of the lowest optical phonon band ($$E_{2}(low)$$ $E_2\left(low\right)$) in GaN. Our work sheds light on the mechanisms responsible for linewidth broadening in GaN SPEs. Since a low energy optical phonon band ($$E_{2}(low)$$ $E_2\left(low\right)$) is a feature of most group III–V nitrides with a wurtzite crystal structure, including hBN and AlN, we expect our proposed mechanism to play an important role in defect emitters in these materials as well. 

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