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Clock frequency estimation under spontaneous emission
Xi-Zhou Qin (秦锡洲), Jia-Hao Huang (黄嘉豪), Hong-Hua Zhong (钟宏华), Chaohong Lee (李朝红)

Frontiers of Physics. 2018, 13 (1 ): 130302-.
https://doi.org/10.1007/s11467-017-0706-6
We investigate the quantum dynamics of a driven two-level system under spontaneous emission and its application in clock frequency estimation. By using the Lindblad equation to describe the system, we analytically obtain its exact solutions, which show three different regimes: Rabi oscillation, damped oscillation, and overdamped decay. From the analytical solutions, we explore how the spontaneous emission affects the clock frequency estimation. We find that under a moderate spontaneous emission rate, the transition frequency can still be inferred from the Rabi oscillation. Our results enable potential practical applications in frequency measurement and quantum control under decoherence.

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Two-dimensional matter-wave solitons and vortices in competing cubic-quintic nonlinear lattices
Xuzhen Gao, Jianhua Zeng

Frontiers of Physics. 2018, 13 (1 ): 130501-.
https://doi.org/10.1007/s11467-017-0697-3
The nonlinear lattice — a new and nonlinear class of periodic potentials — was recently introduced to generate various nonlinear localized modes. Several attempts failed to stabilize two-dimensional (2D) solitons against their intrinsic critical collapse in Kerr media. Here, we provide a possibility for supporting 2D matter-wave solitons and vortices in an extended setting — the cubic and quintic model — by introducing another nonlinear lattice whose period is controllable and can be different from its cubic counterpart, to its quintic nonlinearity, therefore making a fully “nonlinear quasi-crystal”.

A variational approximation based on Gaussian ansatz is developed for the fundamental solitons and in particular, their stability exactly follows the inverted Vakhitov–Kolokolov stability criterion, whereas the vortex solitons are only studied by means of numerical methods. Stability regions for two types of localized mode — the fundamental and vortex solitons — are provided. A noteworthy feature of the localized solutions is that the vortex solitons are stable only when the period of the quintic nonlinear lattice is the same as the cubic one or when the quintic nonlinearity is constant, while the stable fundamental solitons can be created under looser conditions. Our physical setting (cubic-quintic model) is in the framework of the Gross–Pitaevskii equation or nonlinear Schrödinger equation, the predicted localized modes thus may be implemented in Bose–Einstein condensates and nonlinear optical media with tunable cubic and quintic nonlinearities.

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PEG-PLGA electrospun nanofibrous membranes loaded with Au@Fe2O3 nanoparticles for drug delivery applications
Salvatore Spadaro, Marco Santoro, Francesco Barreca, Angela Scala, Simona Grimato, Fortunato Neri, Enza Fazio

Frontiers of Physics. 2018, 13 (1 ): 136201-.
https://doi.org/10.1007/s11467-017-0703-9
A PEGylated-PLGA random nanofibrous membrane loaded with gold and iron oxide nanoparticles and with silibinin was prepared by electrospinning deposition. The nanofibrous membrane can be remotely controlled and activated by a laser light or magnetic field to release biological agents on demand. The nanosystems were characterized using scanning electron microscopy, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and thermogravimetric analyses. The drug loading efficiency and drug content percentages were determined by UV-vis optical absorption spectroscopy. The nanofibrous membrane irradiated by a relatively low-intensity laser or stimulated by a magnetic field showed sustained silibinin release for at least 60 h, without the burst effect. The proposed low-cost electrospinning procedure is capable of assembling, via a one-step procedure, a stimuli-responsive drug-loaded nanosystem with metallic nanoparticles to be externally activated for controlled drug delivery.

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Topological superradiant state in Fermi gases with cavity induced spin–orbit coupling
Dongyang Yu, Jian-Song Pan, Xiong-Jun Liu, Wei Zhang, Wei Yi

Frontiers of Physics. 2018, 13 (1 ): 136701-.
https://doi.org/10.1007/s11467-017-0695-5
Coherently driven atomic gases inside optical cavities hold great promise for generating rich dynamics and exotic states of matter. It was shown recently that an exotic topological superradiant state exists in a two-component degenerate Fermi gas coupled to a cavity, where local order parameters coexist with global topological invariants. In this work, we characterize in detail various properties of this exotic state, focusing on the feedback interactions between the atoms and the cavity field. In particular, we demonstrate that cavity-induced interband coupling plays a crucial role in inducing the topological phase transition between the conventional and topological superradiant states. We analyze the interesting signatures in the cavity field left by the closing and reopening of the atomic bulk gap across the topological phase boundary and discuss the robustness of the topological superradiant state by investigating the steady-state phase diagram under various conditions. Furthermore, we consider the interaction effect and discuss the interplay between the pairing order in atomic ensembles and the superradiance of the cavity mode. Our work provides many valuable insights into the unique cavity– atom hybrid system under study and is helpful for future experimental exploration of the topological superradiant state.

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Structural properties of water confined by phospholipid membranes
Fausto Martelli, Hsin-Yu Ko, Carles Calero Borallo, Giancarlo Franzese

Frontiers of Physics. 2018, 13 (1 ): 136801-.
https://doi.org/10.1007/s11467-017-0704-8
Biological membranes are essential for cell life and hydration. Water provides the driving force for the assembly and stability of many cell components. Here, we study the structural properties of water in a phospholipid membrane. We characterize the local structures, inspecting the intermediate range order (IRO) and adopting a sensitive local order metric recently proposed by Martelli et al . that measures and grades the degree of overlap of the local environment with the structures of perfect ice. Close to the membrane, water acquires a high IRO and changes its dynamical properties; i.e., its translational and rotational degrees of freedom slow in a region that extends over ≃ 1 nm from the membrane interface. Surprisingly, we show that at distances as far as ≃ 2:5 nm from the interface, although the bulk-like dynamics are recovered, the IRO of water is still slightly higher than that in the bulk under the same thermodynamic conditions. Therefore, the water-membrane interface has a structural effect at ambient conditions that propagates further than the often-invoked 1-nm length scale. Consequently, this should be considered when analyzing experimental data of water confined by membranes and could help us to understand the role of water in biological systems.

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Transport evidence of 3D topological nodal-line semimetal phase in ZrSiS
Junran Zhang, Ming Gao, Jinglei Zhang, Xuefeng Wang, Xiaoqian Zhang, Minhao Zhang, Wei Niu, Rong Zhang, Yongbing Xu

Frontiers of Physics. 2018, 13 (1 ): 137201-.
https://doi.org/10.1007/s11467-017-0705-7
Topological nodal-line semimetal is a new emerging material, which is viewed as a three-dimensional (3D) analog of graphene with the conduction and valence bands crossing at Dirac nodes, resulting in a range of exotic transport properties. Herein, we report on the direct quantum transport evidence of the 3D topological nodal-line semimetal phase of ZrSiS with angular-dependent magnetoresistance (MR) and the combined de Hass-van Alphen (dHvA) and Shubnikov-de Hass (SdH) oscillations. Through fitting by a two-band model, the MR results demonstrate high topological nodal-line fermion densities of approximately 6×10^{21} cm^{−3} and a perfect electron/hole compensation ratio of 0.94, which is consistent with the semi-classical expression fitting of Hall conductance Gxy and the theoretical calculation. Both the SdH and dHvA oscillations provide clear evidence of 3D topological nodal-line semimetal characteristic.

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High-temperature dynamic behavior in bulk liquid water: A molecular dynamics simulation study using the OPC and TIP4P-Ew potentials
Andrea Gabrieli, Marco Sant, Saeed Izadi, Parviz Seifpanahi Shabane, Alexey V. Onufriev, Giuseppe B. Suffritti

Frontiers of Physics. 2018, 13 (1 ): 138203-.
https://doi.org/10.1007/s11467-017-0693-7
Classical molecular dynamics simulations were performed to study the high-temperature (above 300 K) dynamic behavior of bulk water, specifically the behavior of the diffusion coefficient, hydrogen bond, and nearest-neighbor lifetimes. Two water potentials were compared: the recently proposed “globally optimal” point charge (OPC) model and the well-known TIP4P-Ew model. By considering the Arrhenius plots of the computed inverse diffusion coefficient and rotational relaxation constants, a crossover from Vogel–Fulcher–Tammann behavior to a linear trend with increasing temperature was detected atT *≈309 and T *≈285 K for the OPC and TIP4P-Ew models, respectively. Experimentally, the crossover point was previously observed atT *≈315±5 K. We also verified that for the coefficient of thermal expansion α _{P} (T , P ), the isobaric α _{P} (T ) curves cross at about the same T * as in the experiment. The lifetimes of water hydrogen bonds and of the nearest neighbors were evaluated and were found to cross nearT *, where the lifetimes are about 1 ps. For T <T *, hydrogen bonds persist longer than nearest neighbors, suggesting that the hydrogen bonding network dominates the water structure at T <T *, whereas for T >T *, water behaves more like a simple liquid. The fact that T * falls within the biologically relevant temperature range is a strong motivation for further analysis of the phenomenon and its possible consequences for biomolecular systems.

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Dynamics of supercooled confined water measured by deep inelastic neutron scattering
Vincenzo De Michele, Giovanni Romanelli, Antonio Cupane

Frontiers of Physics. 2018, 13 (1 ): 138205-.
https://doi.org/10.1007/s11467-017-0699-1
In this paper, we present the results of deep inelastic neutron scattering (DINS) measurements on supercooled water confined within the pores (average pore diameter ~20 Å) of a disordered hydrophilic silica matrix obtained through hydrolysis and polycondensation of the alkoxide precursor Tetra-Methyl- Ortho-Silicate via the sol-gel method. Experiments were performed at two temperatures (250 K and 210 K, i.e., before and after the putative liquid–liquid transition of supercooled confined water) on a “wet” sample with hydrationh ~40% w/w, which is high enough to have water-filled pores but low enough to avoid water crystallization. A virtually “dry” sample ath ~7% was also investigated to measure the contribution of the silica matrix to the neutron scattering signal. As is well known, DINS measurements allow the determination of the mean kinetic energy and the momentum distribution of the hydrogen atoms in the system and therefore, allow researchers to probe the local structure of supercooled confined water. The main result obtained is that at 210 K the hydrogen mean kinetic energy is equal or even slightly higher than at 250 K. This is at odds with the predictions of a semiempirical harmonic model recently proposed to describe the temperature dependence of the kinetic energy of hydrogen in water. This is a new and very interesting result, which suggests that at 210 K, the water hydrogens experience a stiffer intermolecular potential than at 250 K. This is in agreement with the liquid–liquid transition hypothesis.

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Dynamical properties of water in living cells
Irina Piazza, Antonio Cupane, Emmanuel L. Barbier, Claire Rome, Nora Collomb, Jacques Ollivier, Miguel A. Gonzalez, Francesca Natali

Frontiers of Physics. 2018, 13 (1 ): 138301-.
https://doi.org/10.1007/s11467-017-0731-5
With the aim of studying the effect of water dynamics on the properties of biological systems, in this paper, we present a quasi-elastic neutron scattering study on three different types of living cells, differing both in their morphological and tumor properties. The measured scattering signal, which essentially originates from hydrogen atoms present in the investigated systems, has been analyzed using a global fitting strategy using an optimized theoretical model that considers various classes of hydrogen atoms and allows disentangling diffusive and rotational motions. The approach has been carefully validated by checking the reliability of the calculation of parameters and their 99% confidence intervals. We demonstrate that quasi-elastic neutron scattering is a suitable experimental technique to characterize the dynamics of intracellular water in the angstrom/picosecond space/time scale and to investigate the effect of water dynamics on cellular biodiversity.

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