Competitive cooling with capacitors: Current large-scale cooling devices use vapor compression refrigeration. The efficiency of air conditioners has been optimized, but they can be noisy and rely on problematic greenhouse gases. Two groups now present designs for electrocaloric cooling using lead scandium tantalate capacitors that change temperature under an electric field. Y. Wang et al. obtained a very large heat flux using only solid materials and a cooling fan to remove heat from their device. Torello et al. used fluids for heat transfer, leading to a very large temperature difference between the hot side and the cold side. The new designs demonstrate the potential for devices that might be competitive with vapor compression–based appliances with further optimization. Science , this issue p. 129 , p. 125

Competitive cooling with capacitors: Current large-scale cooling devices use vapor compression refrigeration. The efficiency of air conditioners has been optimized, but they can be noisy and rely on problematic greenhouse gases. Two groups now present designs for electrocaloric cooling using lead scandium tantalate capacitors that change temperature under an electric field. Y. Wang et al. obtained a very large heat flux using only solid materials and a cooling fan to remove heat from their device. Torello et al. used fluids for heat transfer, leading to a very large temperature difference between the hot side and the cold side. The new designs demonstrate the potential for devices that might be competitive with vapor compression–based appliances with further optimization. Science , this issue p. 129 , p. 125

Cooling through solid-state electrocaloric materials is an attractive replacement for vapor compression. Despite recent efforts, devices that are potentially commercially competitive have not been developed. We present an electrocaloric cooler with a maximum temperature span of 20.9 kelvin and a maximum cooling power of 2.1 watts under the moderate applied electric field of 10 volts per micrometer without any observed breakdown. Moreover, the maximum coefficient of performance, even taking into account energy expended on fluid pumping, reaches 54% of Carnot’s efficiency as long as energy is properly recovered. We believe that this demonstration shows electrocaloric cooling to be a very promising alternative to vapor compression cooling.

Ferroaxial materials that exhibit spontaneous ordering of a rotational structural distortion with an axial vector symmetry have gained growing interest, motivated by recent extensive studies on ferroic materials. As in conventional ferroics (e.g., ferroelectrics and ferromagnetics), domain states will be present in the ferroaxial materials. However, the observation of ferroaxial domains is non-trivial due to the nature of the order parameter, which is invariant under both time-reversal and space-inversion operations. Here we propose that NiTiO₃ is an order-disorder type ferroaxial material, and spatially resolve its ferroaxial domains by using linear electrogyration effect: optical rotation in proportion to an applied electric field. To detect small signals of electrogyration (order of 10⁻⁵ deg V⁻¹), we adopt a recently developed difference image-sensing technique. Furthermore, the ferroaxial domains are confirmed on nano-scale spatial resolution with a combined use of scanning transmission electron microscopy and convergent-beam electron diffraction. Our success of the domain visualization will promote the study of ferroaxial materials as a new ferroic state of matter.

The ferroaxial order, which is characterized by a rotational structural distortion in a crystal, has been recently proposed as one of ferroic orders. Though the domain formation is a characteristic feature in ferroic materials, there has been little study done concerning that for the ferroaxial order. Here, we investigate ferroaxial domains that are formed through a ferroaxial transition in two representative ferroaxial materials, ${\mathrm{NiTiO}}_3$ and $\mathrm{RbFe}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2$. We spatially resolve their domain structures using an optical method based on electric-field-induced optical rotation, that is, electrogyration (EG). In ${\mathrm{NiTiO}}_3$, multidomains are constructed when crystals undergo a ferroaxial transition and the domain size depends on the cooling rate around the transition temperature. Furthermore, the ferroaxial domain structure obtained by the EG measurement is well matched with that by scanning x-ray diffraction. $\mathrm{RbFe}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2$ also exhibits multidomain states in which domain patterns are different each time a crystal undergoes a ferroaxial transition. In addition, the temperature dependence of the EG signal well obeys that of the order parameter of a first-order phase transition. These results ensure the effectiveness of the EG effect to elucidate the nature of ferroaxial order.