Search

Crystalline materials with broken inversion symmetry can exhibit a spontaneous electric polarization, which originates from a microscopic electric dipole moment. Long-range polar or anti-polar order of such permanent dipoles gives rise to ferroelectricity or antiferroelectricity, respectively. However, the recently discovered antiferroelectrics of fluorite structure (HfO2 and ZrO2) are different: A non-polar phase transforms into a polar phase by spontaneous inversion symmetry breaking upon the application of an electric field. Here, we show that this structural transition in antiferroelectric ZrO2 gives rise to a negative capacitance, which is promising for overcoming the fundamental limits of energy efficiency in electronics. Our findings provide insight into the thermodynamically forbidden region of the antiferroelectric transition in ZrO2 and extend the concept of negative capacitance beyond ferroelectricity. This shows that negative capacitance is a more general phenomenon than previously thought and can be expected in a much broader range of materials exhibiting structural phase transitions.

The continued miniaturization of nanoelectronic devices approaches its fundamental physical limits due to power dissipation. Negative capacitance field-effect transistors using ferroelectric gate insulators are promising to overcome these limits, which would allow further device scaling. However, the microscopic details of negative capacitance are not well understood so far, since mainly Landau based mean-field theories are used to model these phenomena. Here we use an educational and simplified approach to better understand the basic microscopic origin of ferroelectric negative capacitance. Our “toy” model shows that negative capacitance originates from the thermodynamic instability of the ferroelectric polarization and is bounded by the saturation of microscopic dipole polarizability. This shows that negative capacitance is strongly connected to the origin of ferroelectricity itself. Furthermore, our microscopic model results in the same qualitative behavior as mean-field Landau based approaches.

The Landau theory of phase transitions predicts the presence of a negative capacitance in ferroelectric materials based on a mean-field approach. While recent experimental results confirm this prediction, the microscopic origin of negative capacitance in ferroelectrics is often debated. This study provides a simple, physical explanation of the negative capacitance phenomenon—i.e., ‘S’-shaped polarization vs. electric field curve—without having to invoke the Landau phenomenology. The discussion is inspired by pedagogical models of ferroelectricity as often presented in classic text-books such as the Feynman lectures on Physics and the Introduction of Solid State Physics by Charles Kittel, which are routinely used to describe the quintessential ferroelectric phenomena such as the Curie-Weiss law and the emergence of spontaneous polarization below the Curie temperature. The model presented herein is overly simplified and ignores many of the complex interactions in real ferroelectrics; however, this model reveals an important insight: The polarization catastrophe phenomenon that is required to describe the onset of ferroelectricity naturally leads to the thermodynamic instability that is negative capacitance. Considering the interaction of electric dipoles and saturation of the dipole moments at large local electric fields we derive the full ‘S’-curve relating the ferroelectric polarization and the electric field, in qualitative agreement with Landau theory.

Electroencephalography (EEG)-based Brain Computer Interface has been used for image and voice familiarity detection in earlier works. It is known that efficient feature extraction is the key to a good classification accuracy in BCI systems. EEG discriminative features between familiar and unfamiliar names of people, presented in text format as visual stimuli, have not been reported in literature. Detection of familiar and unfamiliar names can play a distinct role in psychology, clinical assessment of cognitive impairment, lie detection, investigations and interrogations. In this paper, the subject's familiarity to names of people was studied based on EEG signals obtained while the subjects were exposed to familiar and unfamiliar names as a visual stimulus in the form of text. The discriminative features were found to be subject-specific. Using three subject-specific features, an average classification accuracy of 84.37% for familiar vs. unfamiliar names was achieved among 8 subjects.

A ferroelectric material in a ferroelectric–dielectric heterostructure can provide a charge boost, as often discussed in the context of negative capacitance, and, in doing so, can reduce the power dissipation in field-effect transistor technology. However, there is an ongoing debate on whether the charge boost in such a heterostructure is a transient phenomenon or a steady state one. In this Letter, we use the positive-up-negative-down (PUND) measurement technique on a ferroelectric–dielectric (FE–DE) capacitor to show that the charge boost is a steady-state effect—i.e., the charge boost remains intact after the initial transient effects subside and all the voltages in the system reach constant values and steady states. We also demonstrate differential charge boost in steady state using a staircase voltage pulse measurement technique. An experimentally calibrated multi-domain SPICE model of an FE–DE stack is used to accurately simulate the PUND and staircase voltage hopping methods.