Browsing by Author "Zhang, Tingting"

Now showing 1 - 3 of 3

Robust circular polarization of indirect Q-K transitions in bilayer 3R-W S2
(2019-10-23) Du, Luojun; Zhang, Qian; Zhang, Tingting; Jia, Zhiyan; Liang, Jing; Liu, Gui Bin; Yang, Rong; Shi, Dongxia; Xiang, Jianyong; Liu, Kaihui; Sun, Zhipei; Yao, Yugui; Zhang, Qingming; Zhang, Guangyu
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
Valley-contrasting Berry curvature and orbital magnetic moment have led to highly selective circular polarization of direct excitons at the K valleys in transition-metal dichalcogenides. In addition to K valleys, Q valleys, another critical point in the conduction band, also possess well-defined but distinct magnetic moment. Being akin to the direct excitons at K valleys, indirect excitons associated with Q (K) valleys in the conduction (valence) band could allow circular polarization in principle. Here, we report an experimental observation of the circular polarization of indirect Q-K transitions in noncentrosymmetric bilayer 3R-WS2. In stark contrast to the circular polarization of direct excitons which depolarizes with increasing lattice temperature, the circular polarization of indirect Q-K excitons is extremely robust and independent on the temperature. Such robust circular polarization can be understood as follows: The spin-orbit coupling in the Q valley is much stronger than that in the K point of the conduction band, significantly suppressing the temperature induced valley depolarization. Our results open up opportunities for exotic valleytronics and quantum information processing applications.
Transmit Energy Focusing for Parameter Estimation in Slow-time Transmit Beamspace L-shaped MIMO Radar
(2024-11-06) Zhang, Tingting; Vorobyov, Sergiy A.; Xu, Feng
A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
We present a novel slow-time transmit beamspace (TB) multiple-input multiple-output (MIMO) technique for L-shaped array radar with uniform linear subarrays to estimate target parameters including 2-dimensional (2-D) directions of arrival (DOA) and unambiguous velocity. Doppler division multiple access (DDMA) approach, as a type of slow-time waveform achieving waveform orthogonality across multiple pulses within a coherent processing interval, disperses the transmit energy over the entire spatial region, suffering from beam-shape loss. Moreover, Doppler spectrum division, which is necessary for transmit channel separation prior to parameter estimation, leads to the loss of crucial information for velocity disambiguation. To optimize transmit energy distribution, slow-time TB technique is proposed to focus the energy within a desired spatial region. Unlike DDMA approach, slow-time TB technique divides the entire Doppler spectrum into more subbands than the number of transmit antenna elements to narrow down the beam mainlobe intervals between adjacent beams formed by DDMA modulation vectors. As a result, more beams are incorporated into the region of interest, and slow-time TB radar can direct transmit energy to the region of interest by properly selecting the DDMA modulation vectors whose beams are directed there. To resolve velocity ambiguity, tensor signal modeling, by storing measurements in a tensor without Doppler spectrum division, is used. Parameter estimation is then addressed using canonical polyadic decomposition (CPD), and the performance of slow-time TB L-shaped MIMO radar is shown to be improved as compared to DDMA MIMO techniques. Simulations are conducted to validate the proposed method.
Transmit Energy Focusing For Parameter Estimation in Transmit Beamspace Slow-Time MIMO Radar
(2023) Zhang, Tingting; Xu, Feng; Vorobyov, Sergiy A.
A4 Artikkeli konferenssijulkaisussa
Recently, Parallel Factor-Direct (PARAFAC-Direct) method has been proposed for parameter estimation including velocity disambiguation for Doppler Division Multiple Access (DDMA) Multiple-Input Multiple-Output (MIMO) radar. However, DDMA MIMO radar spreads the overall transmit energy into the entire spatial region, and therefore, suffers from beam-shape loss that can limit the performance of PARAFAC-Direct method. To solve this problem, a Transmit Beamspace (TB) Slow-Time MIMO (ST-MIMO) approach is proposed that focuses the transmit energy within a desired spatial region. Unlike traditional DDMA MIMO radars, the Doppler spectrum is divided into more subbands than the number of transmit elements to reduce the mainlobe intervals between adjacent beams formed by DDMA modulation vectors. Then, the TB ST-MIMO beam set can be directed to the spatial region of interest via a proper selection of DDMA modulation vectors. Parameter estimation performance of TB ST-MIMO is improved as compared to conventional DDMA MIMO techniques. Simulations are conducted to validate the proposed method.