arXiv subjects

Reza Ghanbari

Publications and source records attributed to Reza Ghanbari.

6 recordsLinked to original sources

arXiv2026

Defect-engineered scaling of lead-free ferroelectrics with ultralow-voltage switching

Scaling ferroelectrics to nanometer thicknesses remains a central challenge for low-power, nonvolatile electronics, as leakage currents increasingly dominate with reduced dimensions. Alkali-based, lead-free ferroelectrics offer an environmentally sustainable alternative to lead-based systems, yet their scaling is severely limited by leakage arising from volatile alkali constituents. Here, we show that this intrinsic limitation can be transformed into an advantageous degree of freedom through defect engineering. By precisely modulating alkali deficiency during thin-film synthesis, we engineer clustered defect complexes that function as deep trap states, strongly suppressing leakage and enabling robust ferroelectric operation in ultrathin films down to the sub-10 nm regime at voltages below 100 mV. Our results establish defect-enabled scaling as a viable pathway for advancing environmentally benign ferroelectrics toward ultra-low-power, non-volatile electronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Deterministic fabrication of large-area, high-crystallinity oxide moire superlattices

Oxide twistronics extends moire engineering beyond van der Waals materials, offering a promising platform for accessing emergent interfacial phenomena arising from the strong coupling of lattice, charge, and orbital degrees of freedom in complex oxides. However, deterministic fabrication of high-crystallinity oxide moire superlattices over large lateral dimensions remains challenging due to the three-dimensional bonding network of oxides. Here, we demonstrate a scalable, generalized fabrication strategy that enables the formation of high-crystallinity oxide moire superlattices with clean, chemically bonded interfaces and precisely controlled twist angles down to nominal values of 0.1 degree, achieving sub-degree twist-angle accuracy across large contiguous lateral dimensions approaching the millimeter scale. Using NaNbO3 as a model system, we show that the resulting interlayer coupling drives pronounced structural reconstruction that modifies both the phase structure and ferroelectric domain configuration. Synchrotron-based X-ray 3D reciprocal space mapping reveals the emergence of a single-phase state in twisted bilayers, in contrast to the mixed-phase structure observed in single-layer membranes prior to twist assembly. The structural signatures are further consistent with gradual lattice rotation distributed along the thickness direction that may accommodate interfacial shear strain, distinct from reconstruction observed in van der Waals moire systems, which primarily occurs through in-plane stacking rearrangement. This collective lattice response is correlated with twist-dependent nanoscale electromechanical modulations observed by piezoresponse force microscopy. These results establish a scalable materials platform for oxide twistronics and open new pathways towards integrating twist-engineered complex oxides into practical, macroscale device architectures.

cond-mat.mtrl-sci↗

arXiv2026

Establishing Atomic Coherence in Twisted Oxide Membranes Containing Volatile Elements

Twisted oxide membranes represent a promising platform for exploring moire physics and emergent quantum phenomena. However, the presence of amorphous interfacial dead layers in conventional oxide heterostructures impedes coherent coupling and suppresses moire-induced interactions. While high-temperature thermal treatments can facilitate interfacial bonding, additional care is needed for materials containing volatile elements, where elevated temperatures may cause elemental loss. This study demonstrates the realization of atomically coherent, chemically bonded interface in twisted NaNbO3 heterostructures through controlled oxygen-annealing treatment. Atomic-resolution imaging and spectroscopy reveal ordered perovskite registry accompanied by systematic lattice contraction and modified electronic structure at the twisted interface, providing signatures of chemical reconstruction rather than physical adhesion. This reconstructed interface mediates highly asymmetric strain propagation in which the bottom membrane remains nearly relaxed while the top membrane accommodates substantial shear strain, thereby establishing a strain gradient that enables long-range electromechanical coupling throughout the twisted oxide membranes. By resolving the nature of the reconstructed interface, these findings establish a robust pathway for achieving coherent and strain-tunable oxide moire superlattices, opening pathways to engineer emergent ferroic and electronic functionalities.

cond-mat.mtrl-sci↗

arXiv2025

Multiscale transitional flow in anisotropic nanoparticle suspensions revealed by time-resolved x-ray scatter microscopy

Complex fluids transition from laminar to transitory flow above a critical control parameter, akin to their Newtonian counterparts. In a continuum mechanics sense, fluid elements follow the ensuing complex trajectories, giving rise to secondary flows in terms of macroscopic vortices and patterns thereof. However, if we replace idealized fluid elements with actual anisotropic nanoparticles, would their trajectories still reveal the same spatiotemporal behavior as the macroscopic flow field? This question is fundamental for complex fluids, where fully developed turbulence is suppressed by high viscosities and where understanding particle-flow coupling is central to transport, processing, and structure formation. To address this question, we develop small-angle x-ray scatter microscopy of unprecedented temporal resolution combined with polarized light imaging, thereby bridging seven orders of magnitude in lengtscales. Proof-of-principle is demonstrated on a classical stability problem, Taylor-Couette flow, of platelet-like graphene oxide nanoparticle and rod-like cellulose nanocrystal suspensions. The analysis shows hitherto hidden, markedly different multiscale dynamics underlying flow stability; while the platelet-like particles follow the wavy motion of the macroscopic, secondary flow field, the rod-like particles exhibit high-frequency motion that is uncorrelated with the vortex instabilities.

physics.flu-dyn↗

arXiv2025

Low-loss far-infrared surface phonon polaritons in suspended SrTiO3 nanomembranes

Phonon polaritons (PhPs), excitations arising from the coupling of light with lattice vibrations, enable light confinement and local field enhancement, which is essential for various photonic and thermal applications. To date, PhPs with high confinement and low loss have been mainly observed in the mid-infrared regime and mostly in manually exfoliated flakes of van der Waals (vdW) materials. In this work, we demonstrate the existence of low-loss, thickness-tunable phonon polaritons in the far-infrared regime within transferable free-standing SrTiO3 membranes synthesized through a scalable approach, achieving high figures of merit, which are comparable to the previous record values from the vdW materials. Leveraging atomic precision in thickness control, large dimensions, and compatibility with mature oxide electronics, functional oxide membranes present a promising large-scale two-dimensional (2D) platform alternative to vdW materials for on-chip polaritonic technologies in the far-infrared regime.

physics.optics↗

arXiv2023

Painting Taylor vortices with cellulose nanocrystals: supercritical spectral dynamics

We study the flow stability and spatio-temporal spectral dynamics of cellulose nanocrystal (CNC) suspensions in a custom Taylor-Couette flow cell using the intrinsic shear induced birefringence and liquid crystalline properties of CNC suspensions for flow visualizations for the first time. The analysis is performed at constant ramped speed inputs of the independently rotating cylinders for several cases ranging from only inner or outer rotating cylinders to three counter-rotation cases. All CNC suspensions have measurable elastic and shear thinning, both increasing with CNC concentration. We show that the flow patterns recorded are essentially Newtonian-like, with non-Newtonian effects ranging from a decrease in wavenumbers to altering the critical parameters for the onset of instability modes. Outer cylinder rotation flow cases are stable for all concentrations whereas inner cylinder rotation flow cases transition to axisymmetric and azimuthally periodic secondary flows. However, unstable counter-rotation cases become unstable to asymmetric spiral modes. With increasing CNC concentration a counter-rotation case was found where azimuthally periodic wavy patterns transition to asymmetric spiral modes. In contrast to polymeric solutions of similar low to moderate elasticity and shear thinning, the shear-thinning region of CNC suspensions is expected to lead to the breakdown of the chiral nematic phase, whose elastic constants constitute the dominant structural elasticity mechanism. Thus, we interpret the Taylor-Couette stability of the CNC suspensions as dominated by their shear-thinning character due to the expected loss of elasticity in nonlinear flow conditions.

physics.flu-dyn↗