arXiv subjects

Amit Kumar Shah

Publications and source records attributed to Amit Kumar Shah.

5 recordsLinked to original sources

arXiv2026

Temperature-doping phase diagram and endurance in Ce-doped HfO2

The structural and ferroelectric properties of epitaxial Hf1-xCexO2 (CHO) thin films in the ultrathin regime are investigated as a function of Ce concentration (5% <= x <= 20%) and temperature. A temperature-doping phase diagram is established for 10 nm films, showing a systematic evolution from the ferroelectric orthorhombic phase to tetragonal and cubic phases with increasing Ce content. The orthorhombic-tetragonal transition temperature decreases from ~800{\deg}C at x = 5% to $~300{\deg}C$ at x = 15%, indicating strong stabilization of higher-symmetry phases with doping. Consistently, the remanent polarization decreases from ~15 to $3.8 {\mu}C/cm2$ as x increases from 5% to 20%. In contrast, the endurance improves significantly, with higher Ce concentrations exhibiting markedly enhanced cycling stability up to 108 cycles. The opposing trends of polarization and endurance are correlated with reduced orthorhombic distortion, suggesting that fatigue mitigation in Ce-doped HfO2 is linked to structural evolution. These results provide a framework for optimizing composition and reliability in ultrathin ferroelectric HfO2 devices.

cond-mat.mtrl-sci↗

arXiv2025

Strong tunability of epitaxial relationship and reconstruction at improper ferroelectric interface

The atomic structures at epitaxial film-substrate interfaces determine scalability of thin films and can result in new phenomena. However, it is challenging to control the interfacial structures since they are decided by the most stable atomic bonding. In this work, we report strong tunability of the epitaxial interface of improper ferroelectric hexagonal ferrites deposited on spinel ferrites. The selection of two interface types, related by a 90 deg rotation of in-plane epitaxial relations and featured by disordered and hybridized reconstructions respectively, can be achieved by growth conditions, stacking sequences, and spinel compositions. While the disordered type suppresses the primary K3 structure distortion and ferroelectricity in hexagonal ferrites, the hybridized type is more coherent with the distortion with minimal suppression. This tunable interfacial structure provides critical insight on controlling interfacial clamping and may offer a solution for the long-standing problem of practical critical thickness in improper ferroelectrics.

cond-mat.mtrl-sci↗

arXiv2025

Antiferroelectricity with metastable polar state from Kittel model

We have revisited the Kittel model that describes antiferroelectricity (AFE) in terms of two sublattices of spontaneous polarization with antiparallel couplings. By constructing the comprehensive phase diagram including the antiferroelectric, ferroelectric, and paraelectric phases in the parameter space, we identified an AFE phase with stable antipolar states and metastable polar state (SAMP) due to the weak coupling between sublattices. We found that the metastability of the polar state in SAMP phase could lead to apparent remanent polarization, depending on the measurement timescale. This explains the observed ferroelectric behavior of orthorhombic hafnia, which is predicted to be antipolar by density functional theories.

cond-mat.mtrl-sci↗

arXiv2024

Improper flexoelectricity in hexagonal rare-earth ferrites

Flexoelectricity is a universal effect that generates electric polarization due to broken inversion symmetry caused by local strain gradient. The large strain gradient at nanoscale makes flexo-electric effects, especially in nanoscopic ferroelectric materials, promising in sensors, actuator, energy harvesting, and memory applications. In this work, we studied flexoelectricity in hexagonal ferrites h-YbFeO3, an improper ferroelectric expected to have weak piezoelectricity and low sensitivity to depolarization field, which are advantageous for studying flexoelectric effects. We show that in h-YbFeO3 epitaxial thin films, strain gradient on the order of 10^6 m-1 occurs near grain boundaries and edge dislocation, which has a significant impact on the non-polar K3 structural distortion that induces spontaneous polarization. The phenomenological model based on the Landau theory of improper ferroelectricity suggests an indirect flexoelectric effect on the order of 10 nC/m in h-YbFeO3, which is substantially larger than the expectation from Kogan mechanism. These results reveal a novel microscopic mechanism of coupling between strain gradient and polarization mediated by structural distortion, which we call improper flexoelectricity.

cond-mat.mtrl-sci↗

arXiv2024

Antiferroelectric Hafnia Down to the 2D Limit

Antiferroelectricity is a material property characterized by alternating electric dipoles spontaneously ordered in antiparallel directions. Antiferroelectrics are promising for energy storage, solid-state cooling, and memory technologies; however, these materials are scarce, and their scalability remains largely unexplored. In this work, we demonstrate that single-crystalline hafnia, a lead-free CMOS-compatible material, exhibits antiferroelectricity under compressive-strain conditions. We observe antiparallel sublattice polarization and stable double-hysteresis in single-crystalline (111)-oriented epitaxial La-doped hafnia films grown on yttrium-stabilized zirconia and show that the antipolar orthorhombic phase of hafnia adheres to the Kittel model of antiferroelectricity. Notably, compressive strain strengthens the antiferroelectric order in thinner La-doped hafnia films, achieving an unprecedented 850 C ordering temperature in the two-dimensional limit, highlighting hafnia's potential for advanced antiferroelectric devices.

cond-mat.mtrl-sci↗