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1. Hydrogen bond symmetrization and high-spin to low-spin transition of ε-FeOOH at the pressure of Earth’s lower mantle

   https://doi.org/10.2138/am-2022-8839  

   Insixiengmay, Leslie; Stixrude, Lars (December 2023, American Mineralogist)

   Abstract We focus on the ferric end-member of phase H: ε-FeOOH using density functional theory at the PBEsol+U level. At 300 K, we find that ε-FeOOH undergoes a hydrogen bond symmetrization at 37 GPa and a sharp high-spin to low-spin transition at 45 GPa. We find excellent agreement with experimental measurements of the equation of state, lattice parameters, atomic positions, vibrational frequencies, and optical properties as related to the band gap, which we find to be finite and small, decreasing with pressure. The hydrogen bond symmetrization transition is neither first-nor second-order, with no discontinuity in volume or any of the elastic moduli. Computed IR and Raman frequencies and intensities show that vibrational spectroscopy may provide the best opportunity for locating the hydrogen bond symmetrization transition experimentally. We find that ε-FeOOH is highly anisotropic in both longitudinal- and shear-wave velocities at all pressures, with the shear wave velocity varying with propagation and polarization direction by as much as 24% at zero pressure and 43% at 46 GPa. The shear and bulk elastic moduli increase by 18% across the high-spin to low-spin transition. 

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2. Rheology of supercooled P–Se glass-forming liquids: From networks to molecules and the emergence of power-law relaxation behavior

   https://doi.org/10.1063/5.0089659  

   Yuan, Bing; Aitken, Bruce G.; Sen, Sabyasachi (June 2022, The Journal of Chemical Physics)

   The effect of the network-to-molecular structural transformation with increasing phosphorus content in P x Se 100− x (30 ≤ x ≤ 67) supercooled liquids on their shear-mechanical response is investigated using oscillatory shear rheometry. While network liquids with 30 ≤ x ≤ 40 are characterized by shear relaxation via a network bond scission/renewal process, a Maxwell scaling of the storage (G′) and loss (G″) shear moduli, and a frequency-independent viscosity at low frequencies, a new relaxation process emerges in liquids with intermediate compositions (45 ≤ x ≤ 50). This process is attributed to an interconversion between network and molecular structural moieties. Predominantly molecular liquids with x ≥ 63, on the other hand, are characterized by a departure from Maxwell behavior as the storage modulus shows a linear frequency scaling G′(ω) ∼ ω over nearly the entire frequency range below the G′–G″ crossover and a nearly constant ratio of G″/G′ in the terminal region. Moreover, the dynamic viscosity of these rather fragile molecular liquids shows significant enhancement over that of network liquids at frequencies below the dynamical onset and does not reach a frequency-independent regime even at frequencies that are four orders of magnitude lower than that of the onset. Such power-law relaxation behavior of the molecular liquids is ascribed to an extremely broad distribution of relaxation timescales with the coexistence of rapid rotational motion of individual molecules and cooperative dynamics of transient molecular clusters, with the latter being significantly slower than the shear relaxation timescale. 

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3. Jamming a terahertz wireless link

   https://doi.org/10.1038/s41467-022-30723-8  

   Shrestha, Rabi; Guerboukha, Hichem; Fang, Zhaoji; Knightly, Edward; Mittleman, Daniel M. (June 2022, Nature Communications)

   Abstract As the demand for bandwidth in wireless communication increases, carrier frequencies will reach the terahertz (THz) regime. One of the common preconceived notions is that, at these high frequencies, signals can radiate with high directivity which inherently provides more secure channels. Here, we describe the first study of the vulnerability of these directional links to jamming, in which we identify several features that are distinct from the usual considerations of jamming at low frequencies. We show that the receiver’s use of an envelope detector provides the jammer with the ability to thwart active attempts to adapt to their attack. In addition, a jammer can exploit the broadband nature of typical receivers to implement a beat jamming attack, which allows them to optimize the efficacy of the interference even if their broadcast is detuned from the frequency of the intended link. Our work quantifies the increasing susceptibility of broadband receivers to jamming, revealing previously unidentified vulnerabilities which must be considered in the development of future wireless systems operating above 100 GHz. 

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4. Field Measurements of Linear and Nonlinear Shear Moduli during Large-Strain Shaking

   Zhang, C; Stokoe, K; Menq, F (June 2019, Proceedings of the 7th International Conference on Earthquake Geotechnical Engineering, (ICEGE 2019))

   : An improvement to the field liquefaction testing method that presently involves one large mobile shaker is under development. The improvement is designed to permit simultane-ously determination of both linear and nonlinear shear moduli of soil during large-strain shaking tests. The improved method requires two mobile shakers. Small-amplitude, high-frequency motions (160 Hz) are generated with a small shaker named Thumper. These motions are su-perimposed on larger-amplitude, lower-frequency motions (25 Hz) gener-ated by a larger shaker named Rattler. By operating the shakers at distinctly different fre-quencies in perpendicular planes, small-strain shear moduli can be determined at multiple times (>6) during each cycle of higher-strain shaking with Rattler. The Spectral-Analysis-of-Body-Waves (SABW) method is implemented to continuously evaluate the small-strain shear moduli. These initial tests show that the soil skeleton can be studied during larger-strain cycling. The goal is to improve the characterization and understanding of soils un-dergoing nonlinear loading processes. 

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5. Joint Probabilistic-Nyquist Pulse Shaping for LDPC-coded 8-PAM Signal in DWDM Data Center Communications

   Han, Xiao; Yang, Migwei; Djordjevic, Ivan B; Yue, Yang; Wang, Qiang; Qu, Zhen; Anderson, Jon (November 2019, Applied sciences)

   M-ary pulse-amplitude modulation (PAM) meets the requirements of data center communication because of its simplicity, but coarse entropy granularity cannot meet the dynamic bandwidth demands, and there is a large capacity gap between uniform formats and the Shannon limit. The dense wavelength division multiplexing (DWDM) system is widely used to increase the channel capacity, but low spectral efficiency of the intensity modulation/direct detection (IM/DD) solution restricts the throughput of the modern DWDM data center networks. Probabilistic shaping distribution is a good candidate to offer us a fine entropy granularity and efficiently reduce the gap to the Shannon limit, and Nyquist pulse shaping is widely used to increase the spectral efficiency. We aim toward the joint usage of probabilistic shaping and Nyquist pulse shaping with low-density parity-check (LDPC) coding to improve the bit error rate (BER) performance of 8-PAM signal transmission. We optimized the code rate of the LDPC code and compared different Nyquist pulse shaping parameters using simulations and experiments. We achieved a 0.43 dB gain using Nyquist pulse shaping, and a 1.1 dB gain using probabilistic shaping, while the joint use of probabilistic shaping and Nyquist pulse shaping achieved a 1.27 dB gain, which offers an excellent improvement without upgrading the transceivers. 

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