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1. Coherent momentum control of forbidden excitons

   https://doi.org/10.1038/s41467-022-34740-5  

   Ma, Xuezhi ; Kudtarkar, Kaushik ; Chen, Yixin ; Cunha, Preston ; Ma, Yuan ; Watanabe, Kenji ; Taniguchi, Takashi ; Qian, Xiaofeng ; Hipwell, M. Cynthia ; Wong, Zi Jing ; et al ( November 2022 , Nature Communications)

   A double-edged sword in two-dimensional material science and technology is optically forbidden dark exciton. On the one hand, it is fascinating for condensed matter physics, quantum information processing, and optoelectronics due to its long lifetime. On the other hand, it is notorious for being optically inaccessible from both excitation and detection standpoints. Here, we provide an efficient and low-loss solution to the dilemma by reintroducing photonics bound states in the continuum (BICs) to manipulate dark excitons in the momentum space. In a monolayer tungsten diselenide under normal incidence, we demonstrated a giant enhancement (~1400) for dark excitons enabled by transverse magnetic BICs with intrinsic out-of-plane electric fields. By further employing widely tunable Friedrich-Wintgen BICs, we demonstrated highly directional emission from the dark excitons with a divergence angle of merely 7°. We found that the directional emission is coherent at room temperature, unambiguously shown in polarization analyses and interference measurements. Therefore, the BICs reintroduced as a momentum-space photonic environment could be an intriguing platform to reshape and redefine light-matter interactions in nearby quantum materials, such as low-dimensional materials, otherwise challenging or even impossible to achieve.
2. Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

   https://doi.org/10.1038/s41598-019-44602-8  

   Shivaram, B. S. ; Holleis, Ludwig ; Ulrich, V. W. ; Singleton, John ; Jaime, Marcelo ( June 2019 , Scientific Reports)

   Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt₃with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin phases. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these phase transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which forθ = 90° (B||c-axis) and vicinity abruptly become very strong in the 24.8–30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T forθ = 0° but moves to higher fields as 1/cosθ. This merge, suggesting a tricritical behavior, occurs atθ ≈ 51° from the ab-plane. This is an unique off-symmetry angle where the length change along the c-axis is precisely zero due to the anisotropic nature of MS in UPt₃formore »all magnetic field values.

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3. Optomechanical crystal with bound states in the continuum

   https://doi.org/10.1038/s41467-022-30965-6  

   Liu, Shengyan ; Tong, Hao ; Fang, Kejie ( June 2022 , Nature Communications)

   Chipscale micro- and nano-optomechanical systems, hinging on the intangible radiation-pressure force, have shown their unique strength in sensing, signal transduction, and exploration of quantum physics with mechanical resonators. Optomechanical crystals, as one of the leading device platforms, enable simultaneous molding of the band structure of optical photons and microwave phonons with strong optomechanical coupling. Here, we demonstrate a new breed of optomechanical crystals in two-dimensional slab-on-substrate structures empowered by mechanical bound states in the continuum (BICs) at 8 GHz. We show symmetry-induced BIC emergence with optomechanical couplings up tog/2π≈ 2.5 MHz per unit cell, on par with low-dimensional optomechanical crystals. Our work paves the way towards exploration of photon-phonon interaction beyond suspended microcavities, which might lead to new applications of optomechanics from phonon sensing to quantum transduction.
4. Symmetry controlled photo-selection and charge separation in butadiyne-bridged donor–bridge–acceptor compounds

   https://doi.org/10.1039/D0CP01235A  

   Li, Xiao ; Valdiviezo, Jesús ; Banziger, Susannah D. ; Zhang, Peng ; Ren, Tong ; Beratan, David N. ; Rubtsov, Igor V. ( May 2020 , Physical Chemistry Chemical Physics)

   Electron transfer (ET) in donor–bridge–acceptor (DBA) compounds depends strongly on the structural and electronic properties of the bridge. Among the bridges that support donor–acceptor conjugation, alkyne bridges have attractive and unique properties: they are compact, possess linear structure permitting access to high symmetry DBA molecules, and allow torsional motion of D and A, especially for longer bridges. We report conformation dependent electron transfer dynamics in a set of novel DBA compounds featuring butadiyne (C4) bridge, N -isopropyl-1,8-napthalimide (NAP) acceptors, and donors that span a range of reduction potentials (trimethyl silane (Si-C4-NAP), phenyl (Ph-C4-NAP), and dimethyl aniline (D-C4-NAP)). Transient mid-IR absorption spectra of the CC bridge stretching modes, transient spectra in the visible range, and TD-DFT calculations were used to decipher the ET mechanisms. We found that the electronic excited state energies and, especially, the transition dipoles (S 0 → S n ) depend strongly on the dihedral angle ( θ ) between D and A and the frontier orbital symmetry, offering an opportunity to photo-select particular excited states with specific ranges of dihedral angles by exciting at chosen wavelengths. For example, excitation of D-C4-NAP at 400 nm predominantly prepares an S 1 excited state in the planar conformations ( θmore »∼ 0) but selects an S 2 state with θ ∼ 90°, indicating the dominant role of the molecular symmetry in the photophysics. Moreover, the symmetry of the frontier orbitals of such DBA compounds not only defines the photo-selection outcome, but also determines the rate of the S 2 → S 1 charge separation reaction. Unprecedented variation of the S 2 –S 1 electronic coupling with θ by over four orders of magnitude results in slow ET at θ ca. 0° and 90° but extremely fast ET at θ of 20–60°. The unique features of high-symmetry alkyne bridged DBA structures enable frequency dependent ET rate selection and make this family of compounds promising targets for the vibrational excitation control of ET kinetics.« less
5. Chiral and counter-propagating Majorana fermions in a p-wave superconductor

   https://doi.org/10.1088/1367-2630/ab5cad  

   Hu, Haiping ; Satija, Indubala I. ; Zhao, Erhai ( December 2019 , New Journal of Physics)

   Chiral and helical Majorana fermions are two archetypal edge excitations in two-dimensional topological superconductors. They emerge from systems of different Altland–Zirnbauer symmetries and characterized by$\mathbb{Z}$and${\mathbb{Z}}_2$topological invariants respectively. It seems improbable to tune a pair of co-propagating chiral edge modes to counter-propagate in a single system without symmetry breaking. Here, we explore the peculiar behaviors of Majorana edge modes in topological superconductors with an additional ‘mirror’ symmetry which changes the bulk topological invariant to$\mathbb{Z}\oplus \mathbb{Z}$type. A theoretical toy model describing the proximity structure of a Chern insulator and ap_x-wave superconductor is proposed and solved analytically to illustrate a direct transition between two topologically nontrivial phases. The weak pairing phase has two chiral Majorana edge modes, while the strong pairing phase is characterized by mirror-graded Chern number and hosts a pair of counter-propagating Majorana fermions protected by the mirror symmetry. The edge theory is worked out in detail, and implications to braiding of Majorana fermions are discussed.