arXiv subjects

Zhongqing Wu

Publications and source records attributed to Zhongqing Wu.

15 recordsLinked to original sources

arXiv2026

Sparse Channel Acquisition in Aging Fluid Antenna Systems: A Reliability-Calibrated Posterior Reconstruction Approach

Fluid antenna systems (FASs) exploit port-domain diversity within a compact aperture, but their gain can be lost if the receiver has to scan too many ports before data transmission. This paper studies sparse channel acquisition for aging FAS receivers, where the available information has mixed reliability: current observations are fresh but sparse, whereas historical channel state information (CSI) is dense but stale due to Doppler-induced temporal decorrelation. We formulate the problem from a communication perspective by linking aperture-field reconstruction to port-selection rate, outage, probing budget, and online inference latency. We then cast the acquisition task as reliability-calibrated posterior reconstruction, in which the sparse current observations serve as hard measurement evidence and historical CSI is reused only as soft temporal side information. The resulting reliability-controlled conditional diffusion posterior (DCP) framework conditions the denoising process on the sparse current observations, the observation mask, and the historical CSI, and refines the reverse trajectory through observation consistency, mismatch-aware temporal gating, spatial trust weighting, progressive temporal activation, and warm initialization. Analysis shows how aging and reconstruction error induce rate distortion and rate regret. Simulations across probing budgets, signal-to-noise ratios (SNRs), temporal correlations, observation patterns, ablation settings, and latency tests show consistent reconstruction gains with millisecond-level online inference latency, indicating that stale CSI can be beneficial when its strength, timing, and spatial support are explicitly controlled.

eess.SP↗

arXiv2024

Beamforming Design for Secure MC-NOMA Empowered ISAC Systems with an Active Eve

As the integrated sensing and communication(ISAC) technology emerges as a promising component of sixth generation (6G), the study of its physical layer security has become a key concern for researchers. Specifically, in this work, we focus on the security issues over a multi-carrier (MC)-non-orthogonal multiple access (NOMA) assisted ISAC system, considering imperfect channel state information (CSI) of an active Eve and graded confidentiality demands for users. To this end, the subcarrier allocation, the information, and artificial noise beamforming are designed to maximize the minimum communication rate, while ensuring diverse confidentiality and sensing performance demands. An effective security strategy is devised via the Lagrangian dual transformation and successive convex approximation methods. Simulations confirm the validity and robustness of the proposed scheme in terms of the security performance.

cs.IT↗

arXiv2022

Elastic anisotropy of lizardite at subduction zone conditions

Subduction zones transport water into Earth's deep interior through slab subduction. Serpentine minerals, the primary hydration product of ultramafic peridotite, are abundant in most subduction zones. Characterization of their high-temperature elasticity, particularly their anisotropy, will help us better estimate the extent of mantle serpentinization and the Earth's deep water cycle. Lizardite, the low-temperature polymorph of serpentine, is stable under the P-T conditions of cold subduction slabs (< 260°C at 2 GPa), and its high-temperature elasticity remains unknown. Here we report ab initio elasticity and acoustic wave velocities of lizardite at P-T conditions of subduction zones. Our static results agree with previous studies. Its high-temperature velocities are much higher than previous experimental-based lizardite estimates with chrysotile but closer to antigorite velocities. The elastic anisotropy of lizardite is much larger than that of antigorite and could better account for the observed large shear-wave splitting in some cold slabs such as Tonga.

physics.geo-ph↗

arXiv2021

cij: A Python code for quasiharmonic thermoelasticity

The Wu-Wentzcovitch semi-analytical method (SAM) is a concise and predictive formalism to calculate the high-pressure and high-temperature (high-PT) thermoelastic tensor (Cij) of crystalline materials. This method has been successfully applied to materials across different crystal systems in conjunction with ab initio calculations of static elastic coefficients and phonon frequencies. Such results have offered first-hand insights into the composition and structure of the Earth's mantle. Here we introduce the cij package, a Python implementation of the SAM-Cij formalism. It enables a thermoelasticity calculation to be initiated from a single command and fully configurable from a calculation settings file to work with solids within any crystalline system. These features allow SAM-Cij calculations to work on a personal computer and to be easily integrated as a part of high-throughput workflows. Here we show the performance of this code for three minerals from different crystal systems at their relevant PTs: diopside (monoclinic), akimotoite (trigonal), and bridgmanite (orthorhombic).

physics.comp-ph↗

arXiv2020

Anomalous thermal properties and spin crossover of ferromagnesite (Mg,Fe)CO3

Ferromagnesite (Mg,Fe)CO3, also referred to as magnesiosiderite at high iron concentration, is a solid solution of magnesite (MgCO3) and siderite (FeCO3). Ferromagnesite is believed to enter the Earth's lower mantle via subduction and is considered a major carbon carrier in the Earth's lower mantle, playing a key role in the Earth's deep carbon cycle. Experiments have shown that ferromagnesite undergoes a pressure-induced spin crossover, accompanied by volume and elastic anomalies, in the lower-mantle pressure range. In this work, we investigate thermal properties of (Mg,Fe)CO3 using first-principles calculations. We show that nearly all thermal properties of ferromagnesite are drastically altered by iron spin crossover, including anomalous reduction of volume, anomalous softening of bulk modulus, and anomalous increases of thermal expansion, heat capacity, and Guneisen parameter. Remarkably, the anomaly of heat capacity remains prominent (up to 40%) at high temperature without smearing out, which suggests that iron spin crossover may significantly affect the thermal properties of subducting slabs and the Earth's deep carbon cycle.

cond-mat.mtrl-sci↗

arXiv2016

Shear softening of Earth's inner core indicated by its high Poisson's ratio and elastic anisotropy

Earth's inner core exhibits an unusually high Poisson's ratio and noticeable elastic anisotropy. The mechanisms responsible for these features are critical for understanding the evolution of the Earth but remain unclear. This study indicates that once the correct formula for the shear modulus is used, shear softening can simultaneously explain the high Poisson's ratio and strong anisotropy of the inner core. Body-centred-cubic (bcc) iron shows shear instability at the pressures found in the inner-core and can be dynamically stabilized by temperature and light elements. It is very likely that some combinations of light elements stabilize the bcc iron alloy under inner-core conditions. Such a bcc phase would exhibit significant shear softening and match the geophysical constraints of the inner core. Identifying which light elements and what concentrations of these elements stabilize the bcc phase will provide critical information on the light elements of the inner core.

physics.geo-ph↗

arXiv2016

Influence of the iron spin crossover in ferropericlase on the lower mantle geotherm

The iron spin crossover in ferropericlase introduces anomalies in its thermodynamics and thermoelastic properties. Here we investigate how these anomalies can affect the lower mantle geotherm. The effect is examined in mantle aggregates consisting of mixtures of bridgmanite, ferropericlase, and CaSiO$_3$ perovskite, with different Mg/Si ratios varying from harzburgitic to perovskitic (Mg/Si$\sim$1.5 to 0.8). We find that the anomalies introduced by the spin crossover increase the isentropic gradient and thus the geotherm proportionally to the amount of ferropericlase. The geotherms can be as much as $\sim$ 200K hotter than the conventional adiabatic geotherm at deep lower mantle conditions. Aggregate elastic moduli and seismic velocities are also sensitive to the spin crossover and the geotherm, which impacts analyses of lower mantle velocities and composition.

physics.geo-ph↗

arXiv2015

Thermoelasticity of Fe2+-bearing bridgmanite

We present LDA+U calculations of high temperature elastic properties of bridgmanite with composition (Mg$_{(1-x)}$Fe$_{x}^{2+}$)SiO$_3$ for $0\le{x}\le0.125$. Results of elastic moduli and acoustic velocities for the Mg-end member (x=0) agree very well with the latest high pressure and high temperature experimental measurements. In the iron-bearing system, we focus particularly on the change in thermoelastic parameters across the state change that occurs in ferrous iron above $\sim$30 GPa, often attributed to a high-spin (HS) to intermediate spin (IS) crossover but explained by first principles calculations as a lateral displacement of substitutional iron in the perovskite cage. We show that the measured effect of this change on the equation of state of this system can be explained by the lateral displacement of substitutional iron, not by the HS to IS crossover. The calculated elastic properties of (Mg$_{0.875}$Fe$_{0.125}^{2+}$)SiO$_3$ along an adiabatic mantle geotherm, somewhat overestimate longitudinal velocities but produce densities and shear velocities quite consistent with Preliminary Reference Earth Model data throughout most of the lower mantle.

cond-mat.mtrl-sci↗

arXiv2008

An efficient method for calculating thermoelastic properties

First-principles quasi-harmonic calculations play a very important role in mineral physics because they can accurately predict the structure and thermodynamic properties of materials at pressure and temperature conditions that are still challenging for experiments. It also enables calculations of thermoelastic properties by obtaining the second-order derivatives of the free energies with respect to strain. However, these are exceedingly demanding computations requiring thousands of large jobs running on 101 processors each. Here we introduce a simpler approach that requires only calculations of static elastic constants and phonon density of states for unstrained configurations. This approach decreases the computational time by more than one order of magnitude. We show results on MgO and forsterite that are in very good agreement with previous first-principles results and experimental data.

cond-mat.mtrl-sci↗

arXiv2008

Lattice Dynamics and Thermal Equation of State of Platinum

Platinum is widely used as a pressure calibration standard. However, the established thermal EOS has uncertainties, especially in the high $P$-$T$ range. We use density functional theory to calculate the thermal equation of state of platinum, up to 550 GPa and 5000 K. The static lattice energy is computed by using the LAPW method, with LDA, PBE, and the recently proposed WC functional. The electronic thermal free energy is evaluated using the Mermin functional. The vibrational part is computed within the quasi-harmonic approximation using density functional perturbation theory and pseudopotentials. Special attention is paid to the influence of the electronic temperature to the phonon frequencies. We find that in overall LDA results agree best with the experiments. Based on the DFT calculations and the established experimental data, we develop a consistent thermal EOS of platinum as a reference for pressure calibration.

cond-mat.mtrl-sci↗

arXiv2006

An efficient method to calculate the anharmonicity free energy

The anharmonicity resulted from the intrinsic phonon interaction is neglected by quasiharmonic approximation. Although the intensive researches about anharmonicity have been done, up to now the free energy contributed by the anharmonicity is still difficult to calculate. Here we put forward a new method that can well include the anharmonicity. We introduce the implicit temperature dependence of effective frequency by volume modification. The quasiharmonic approximation becomes a special case in our method corresponding to non volume modification. Although our method is simple and only a constant need to determine, the anharmonicity is well included. Thermodynamic properties of MgO predicted with our method are excellent consistent with the experiment results at very wide temperature range. We also believe that our method will be helpful to reveal the characteristic of anharmonicity and intrinsic phonon interaction.

cond-mat.stat-mech↗

arXiv2004

Unusual vortex structure in ultrathin Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_3$ films

Using a first-principles-based approach, we determine the ferroelectric pattern in PbZr$_{0.5}$Ti$_{0.5}$O$_3$ ultrathin film. It is found that vortex stripes are formed in the system and they are responsible for the 180$^\circ$ domains observed. When a local external field is exerted, the vortex stripe transforms into the vortex loop structure, which leads to the formation of a smaller domain with the polarization antiparallel to the field in the center of the field region. This may provide a convenient way to manipulate nanodomains in thin films.

cond-mat.mtrl-sci↗

arXiv2004

Ferroelectricity in Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ thin films: critical thickness and 180$^\circ$ stripe domains

The ferroelectric properties of disorder Pb(Zr$_{0.5}$Ti$_{0.5}$)O$_{3}$ thin films are investigated with Monte Carlo simulations on the basis of a first-principles-derived Hamitonian. It is found that there exists a critical thickness of about three unit cells ($\sim$12 Å) below which the ferroelectricity disappears under the condition that the in-plane polarizations are suppressed by sufficient clamping effect. Above the critical thickness, periodic $180^{\circ}$ stripe domains with out-of-plane polarizations are formed in the systems in order to minimize the energy of the depolarizaing field. The stripe period increases with increasing film thickness. The microscopic mechanism responsible for these phenomena is discussed.

cond-mat.mtrl-sci↗

arXiv2003

Huge enhancement of electronmechanical responses in compositionally modulated PZT

Monte Carlo simulations based on a first-principles-derived Hamiltonian are conducted to study the properties of PZT alloys compositionally modulated along the [100] pseudocubic direction near the morphotropic phase boundary (MPB). It is shown that compositional modulation causes the polarization to continuously rotate away from the modulation direction, resulting in the unusual triclinic and C-type monoclinic ground states and huge enhancement of electromechanical responses (the peak of piezoelectric coefficient is as high as 30000 pC/N). The orientation dependence of dipole-dipole interaction in modulated structure is revealed as the microscopic mechanism to be responsible for these anomalies.

cond-mat.mtrl-sci↗

arXiv2003

Charge fluctuations in cuprate superconductors

The effect of the lattice periodic potential on superconductivity which was ignored by BCS theory has been investigated. According to the effective mass approximation of band theory, the effect of lattice periodic potential can be embodied in the effective mass of the Ginzburg-Landau (GL) equations. The effective mass has s special property. It can be negative. Negative effective mass leads to many unusual phenomena. The superconducting order parameter shows the period distribution. Its modulate wavevector is proportional to the condensed carrier density, which explains the linear relation between the magnetic peaks displacement epsilon and x for La2-xSrxCuO4. The superconducting phase is always local and separated originally and evolves into global superconducting phase at the certain pairs concentration, which explains why the cuprate superconductors must be insulator at low doped. The doped concentrations of insulator to superconductor transition for La2-xSrxCuO4 is consistent with the experiment results. The relation of the superconducting gap (SG) and the pseudogap (PG) was discussed. Spin density wave state is obtained when the order parameter is homogenous distribution.

cond-mat.supr-con↗