arXiv subjects

Guo-Dong Zhao

Publications and source records attributed to Guo-Dong Zhao.

11 recordsLinked to original sources

arXiv2026

Probing (sub)nanoscale ferrons in an electron microscope

Ferrons are collective excitations of polarization fluctuations that can enable terahertz communications and quantum transduction due to long propagation lengths. Although ferrons have been experimentally demonstrated in van der Waals ferroelectrics and relaxor ferroelectrics, there is no direct (sub)nanoscale experimental evidence of ferrons in three-dimensional ferroelectrics. Here, we detect two types of ferrons, Higgs and pseudo-Goldstone, at the (sub)nanoscale in lead titanate by measuring vibrational signals due to polarization fluctuations. By harnessing momentum transfer in electron energy loss spectroscopy (EELS), we directly distinguish between soft-phonons and ferrons. We observe that the Higgs mode originates from the soft optical phonon parallel to the polar axis, whereas the pseudo-Goldstone mode originates from the soft optical phonon perpendicular to the polarization axis. Together with Landau theory, Raman spectroscopy measurements, and EELS, we observe that Higgs group velocities, in the bulk limit, are eight times greater than those of out-of-plane soft phonons and the pseudo-Goldstone ferrons have group velocities six times greater than in-plane soft phonons due to long-range dipole interactions. We further show that the domain size confinement effects lead to doubling of the respective bulk ferron group velocities, reaching up to approximately 15 km per second (almost 15 times higher than the out-of-plane soft phonon modes). Overall, this study opens a pathway to the detection of ferrons in three-dimensional ferroelectrics with domain engineering as a promising avenue for terahertz communication and transduction.

cond-mat.mtrl-sci↗

arXiv2026

Emergent spin-resolved electronic density waves from strong $d$-wave altermagnetism and pseudogap phenomena from phason fluctuations

Metallic $d$-wave altermagnets provide a unique setting in which strong spin-momentum locking can qualitatively reshape electronic instabilities. Here we develop a self-consistent microscopic theory beyond the mean-field approximation for density-wave order in $d$-wave altermagnetic metals. The altermagnetic band structure with strong spin-momentum locking reconstructs the Fermi surface into mutually orthogonal spin-selective quasi-1D sectors, whose strong nesting drives density-wave instabilities in the respective spin sectors. The resulting spin-resolved density-wave orders nevertheless share a common ordering wave vector and give rise to a distinct class of stripe states, accompanied by pronounced gap openings on the nested Fermi sheets. The relative phase between the two spin sectors controls the character of the collective order, allowing charge density wave, spin density wave, and mixed density-wave orders to emerge within the same ordered manifold. As temperature increases from zero, thermal excitation of the emergent phason mode induces strong phase fluctuations that destroy long-range density-wave order at $T_c$, while the single-particle gap remains finite, giving rise to a robust pseudogap regime that persists up to a higher temperature $T_g$. The theory reveals a density-wave mechanism unique to metallic $d$-wave altermagnets, rooted in strong spin-selective Fermi-surface reconstruction, and establishes a fluctuation-driven route to pseudogap phenomena governed by phason dynamics. Applied to the metallic $d$-wave altermagnet KV$_2$Se$_2$O, our simulations quantitatively reproduce the key spectroscopic features reported in recent experiments, providing a unified microscopic understanding of the underlying phenomena.

cond-mat.str-el↗

arXiv2026

Anomalous Piezoelectricity from Polarization-Dependent Electrostriction in Wurtzites

The piezoelectric coefficient is a third-rank tensor connecting the strain or stress with the electric field or polarization, whereas the electrostriction coefficient is a fourth-rank tensor relating the strain to the square of electric polarization. The electrostriction tensor components in the current literature are often treated as constants independent of polarization, resulting in piezoelectric tensor components that are linearly proportional to polarization and the dielectric susceptibility tensor. Here, we study the electrostriction and piezoelectricity in strongly polar wurtzites, including AlN, Al$_{1-x}$Sc$_x$N, Al$_{1-x}$B$_x$N, GaN, and ZnO. We discover that electrostriction and the elastic modulus in wurtzites are both strongly polarization-dependent, and the piezoelectric coefficient is highly nonlinear with respect to polarization, including the anomalous possibility that decreasing polarization increases the electromechanical strain response. These unusual dependencies of electrostriction and piezoelectric effects on polarization arise from the evolution of a layered reference nonpolar structure toward a tetrahedrally coordinated wurtzite network structure as the polarization increases. The findings have important implications in understanding the thermodynamics of the general class of wurtzite ferroelectrics and in manipulating their piezoelectric and ferroelectric behaviors.

cond-mat.mtrl-sci↗

arXiv2026

Chemical heterogeneity at conducting ferroelectric domain walls

Natural interfaces in ferroic oxides have developed into versatile playgrounds for studying electronic correlation effects in 2D systems. The microscopic origin of the emergent local electronic properties is often debated, however, as quantitative atomic-scale characterization remains challenging. A prime example is enhanced conductivity at ferroelectric domain walls, attributed to mechanisms ranging from local band gap reduction to point defect accumulations. Here, we resolve the microscopic mechanisms for domain wall conduction in the ferroelectric model system BiFeO3, by combining transport measurements with atom probe tomography to quantify the local chemical composition and correlate it with the electrical properties. Significant chemical variations along the walls are observed, demonstrating an outstanding chemical flexibility at domain walls, which manifest in spatially varying physical properties. The results give a unifying explanation for the diverse electronic behavior observed and establish the fundamental notion that multiple conduction mechanisms can coexist within individual domain walls.

cond-mat.mtrl-sci↗

arXiv2025

First-principles prediction of altermagnetism in transition metal graphite intercalation compounds

We report the emergence of altermagnetism, a magnetic phase characterized by the coexistence of compensated spin ordering and momentum-dependent spin splitting, in graphite intercalation compounds (GICs), a prototypical material system long investigated for its tunable electronic and structural properties. Through first-principles calculations, we demonstrate that vanadium-intercalated stage-1 graphite compounds, exhibit inherent altermagnetic properties. The hexagonal crystal system and antiferromagnetic ordering of V atoms generate a magnetic space group that enforces alternating spin polarization in momentum space while maintaining zero net magnetization. The calculated band structure reveals robust altermagnetic signatures: along the high-symmetry direction, we observe a pronounced spin splitting of ~270 meV with alternating spin polarization. Crucially, the spin splitting exhibits minimal sensitivity to spin-orbit coupling (SOC) effect, highlighting the dominance of exchange interactions over relativistic effects. From Monte Carlo simulations, we predict a magnetic transition temperature ($T_m$ ) of ~228 K, indicating stable magnetic ordering above liquid nitrogen temperatures. The combination of symmetry-protected spin textures, SOC-independent splitting, and elevated $T_m$ temperature makes V-GICs as a promising candidate for spintronic applications, particularly for zero-field spin-polarized current generation and topologically robust spin transport. As the first demonstration of carbon-based alternating magnetic systems, this work offers a design paradigm for engineering spin-polarized quantum states governed by crystalline symmetry constraints.

cond-mat.mtrl-sci↗

arXiv2025

Thermodynamic stabilization and electronic effects of oxygen vacancies at BiFeO$_3$ neutral ferroelectric domain walls

Enhanced conductivity at ferroelectric domain walls in BiFeO$_3$ has been widely observed, yet the microscopic origins of this effect, including electronic contributions from domain-wall defects, are incompletely understood at the atomistic level. Here, we carry out first-principles simulations to quantify the thermodynamic stability and electronic impact of oxygen vacancies at charge-neutral 71$^\circ$, 109$^\circ$, and 180$^\circ$ domain walls of BiFeO$_3$. We find that vacancies are energetically favored at domain walls by up to 0.3 eV compared to the bulk, leading to orders-of-magnitude increase in vacancy equilibrium concentration. The corresponding formation energy landscapes are discontinued and explained by local bond weakening. The vacancies induce localized electronic intragap states corresponding to small polarons, which promote thermally activated n-type conduction in the low-current regime, and their tendency to aggregate facilitate Schottky emission in the high-current regime. Our results provide a quantitative foundation for interpreting domain-wall conduction, offer guidance for defect engineering in ferroelectrics, and provide important information to phase-field simulations of defect-domain wall interactions in a ferroelectric domain structure.

cond-mat.mtrl-sci↗

arXiv2025

Flexocoupling-induced phonons and ferrons in van der Waals ferroelectrics

The contribution of flexoelectric coupling to the long-range order parameter fluctuations in ferroics can be critically important to the ferron dispersion and related polar, pyroelectric and electrocaloric properties. Here we calculate analytically the dispersion relations of soft optic and acoustic flexocoupling-induced phonons and ferrons by incorporating the flexoelectric coupling, damping, and higher elastic gradients in the Landau-Ginzburg-Devonshire free energy functional using the van der Waals uniaxial ferrielectric CuInP2S6 as an example. We analyze the changes in the flexocoupling-induced phonon and ferron spectra arising from the appearance of spatially modulated phases induced by the flexoelectric coupling. We show that the free energy landscape of CuInP2S6 determines the specific features of its phonon spectra and ferron dispersion. We also discuss the contributions of optic and acoustic flexocoupling-induced ferrons to the pyroelectric and electrocaloric responses of CuInP2S6 at low temperatures.

cond-mat.mtrl-sci↗

arXiv2024

Realization of Landau-Zener Rabi Oscillations on optical lattice clock

Manipulating quantum states is at the heart of quantum information processing and quantum metrology. Landau-Zener Rabi oscillation (LZRO), which arises from a quantum two-level system swept repeatedly across the avoided crossing point in the time domain, has been suggested for widespread use in manipulating quantum states. Cold atom is one of the most prominent platforms for quantum computing and precision measurement. However, LZRO has never been observed in cold atoms due to its stringent requirements. By compensating for the linear drift of the clock laser and optimizing experimental parameters, we successfully measured LZRO on the strontium atomic optical clock platform under both fast and slow passage limits within $4$ to $6$ driving periods. Compared to previous results on other platforms, the duration of the plateau is $10^4$ times longer in the optical lattice clock. The experimental data also suggest that destructive Landau-Zener interference can effectively suppress dephasing effects in the optical lattice clock, paving the way for manipulating quantum states against various environmental effects in cold atomic systems.

quant-ph↗

arXiv2024

Lifetime Determination of the $5s5p$ ${}^{3}P^{\rm{o}}_{0}$ Metastable State in ${}^{87}$Sr from the Electric Dipole Matrix Element

We report a measurement of the radiative lifetime of the $5s5p \; {}^{\rm{3}}P^{\rm{o}}_{\rm{0}}$ metastable state in ${}^{87}$Sr, which is coupled to the 5$s^{\rm{2}} \;$ ${}^{\rm{1}}S_{\rm{0}}$ ground state via a hyperfine-induced electric dipole transition. The radiative lifetime is determined to be 151.4(48) s, in good agreement with theoretical results. Our approach relies on accurate measurements of laser intensity and free-space Rabi frequency, enabling lifetime measurements of any excited state and particularly suitable for long-lived states.

physics.atom-ph↗

arXiv2023

Polarization description of successive ferroelectric switching in hafnia

Intertwined ionic conduction and ferroelectric (FE) switching in HfO2 lead to extensive focuses. To describe its fundamental phenomena, we present a free-energy model describing the potential of ferroelectrics with successive FE switching paths, and extend the domain model of ionic conduction to ferroelectric domains. Associate theoretical analyses and first-principles calculations suggest a nesting-domain pattern with opposite piezoelectric loops during the nucleation-and-growth process in displacive FE-HfO2. A collective oxygen ion conduction mechanism is also proposed with a field-dependent ionic conductivity following the Merz's law. We conclude that the ionic conductibility is concomitant with the ferroelectricity in HfO2, and it may provide a new venue for pursuing low temperature fast oxide-ion conductors and artificial synapses.

cond-mat.mtrl-sci↗

arXiv2022

Symmetry of ferroelectric switching and domain walls in hafnium dioxide

Hafnium dioxide (HfO2) is a promising ferroelectric (FE) material for achieving high-density nonvolatile memory and neuromorphic computing, due to its compatibility with the mainstream integrated circuit technology and the surprisingly enhanced ferroelectricity by reduced thickness. The FE switching dynamics is essential to the device performance, but the complexity of HfO2 atomic structure causes unknown of various FE switching paths and domain wall configurations. Here, we demonstrate that its low-barrier paths and domain walls can be comprehensively found and understood from a perspective of topological symmetry. By discussing pseudo-chirality and equivalent transformation relations in crystal with first principles and lattice modes, we classify and analyze 4 low-barrier FE switching paths and 93 irreducible topology domain wall configurations in HfO2. Anisotropic switching mechanism is found based on the mobility investigation for 12 types of 180° side domain walls. This methodology is expected to be generally applicable to displacive ferroelectrics with low unit cell point group symmetries, and lay a foundation for mechanism study of the switching dynamics.

cond-mat.mtrl-sci↗