In this talk I introduce ‘alterferroics’ as a new class of quantum materials, integrating the recently established concept of ‘altermagnetism’ with our with other ferroic orders like ferroelectricity and ferroelasticity, and the more exotic superconductivity and topology. I discuss our recent results on predicting new materials in this class, and new routes to controlling coupled and coexisting orders in these materials with spin-momentum locking. Finally, I give a perspective on how such higher-order complex order parameters can couple to experimental probes, with some initial results on our experimental work achieving this.

Interface asymmetry has been a fertile ground in the field of condensed matter physics, leading to numerous notable phenomena. Despite this, its intrinsic role in electromechanical coupling has remained elusive. In this study, we reveal a new phenomenon known as the 'interface piezoelectric effect' which emerges due to the inherent inversion symmetry breaking at heterostructure interfaces. We will elucidate the underlying mechanism and describe the methods employed for its characterization. A distinctive aspect of this interface-induced effect is its applicability to materials of any symmetry, including centrosymmetric semiconductors. This contrasts with traditional piezoelectric effects, which are confined to non-centrosymmetric insulators. Consequently, our findings allow for the exploration of piezoelectric effect in a broader spectrum of established semiconductors, enhancing their potential applications across various technological domains. Furthermore, through deliberate engineering of the interface polar symmetry, we have induced exotic electromechanical coupling phenomena. These phenomena mirror the electrical equivalent of a negative Poisson’s ratio. Dubbed the 'auxetic piezoelectric effect,' it exhibits the same sign of the longitudinal (d33) and the transverse (d31, d32) piezoelectric coefficients, resulting in simultaneous contraction or expansion in all dimensions when subjected to an external electrical stimulus. These discoveries not only challenge existing paradigms in piezoelectric materials science but also open up new avenues for the development of next-generation electromechanical devices, potentially transforming a myriad of applications from sensors to energy harvesting systems.

The phase diagram of ice is complex and contains many phases, but the most common (frozen water at ambient pressure, also known as Ih ice) is a non-polar material despite individual water molecules being polar. Consequently, ice is not piezoelectric and cannot generate electricity under pressure. On the other hand, it can in theory be flexoelectric because the coupling between polarization and strain gradient is universal. Therefore, ice may polarize under bending or any inhomogeneous deformation. In this work, we report the flexoelectricity of ice and its profound consequences. By fabricating bendable ice capacitors and measuring the bending-induced displacement current, we have quantified the flexoelectric coefficient of ice and find it to be in the same range (1~10 nC/m) as dielectric ceramics such as SrTiO3, TiO2, or PbZrO3. Combining the measured flexoelectric coefficient, our theoretical simulations demonstrate that the flexoelectricity of ice can account for important ice-electrification processes occurring in nature, such as the genesis of lightning in storm clouds. In addition, the sensitivity of flexoelectricity to surface boundary conditions has revealed the presence of a ferroelectric phase that appears around 163 K confined within the surface “skin layer” of bulk ice. This is further evidenced by the observed classical butterfly loop of the effect of the pre-poling field on the flexoelectric polarization. Our ab initio calculations reveal that this surface phase transition originates from the reduced free energy of proton-ordered iceXI due to metal-ice interfacial interactions.