SignificanceShift current is one of the bulk photovoltaic phenomena in the materials without inversion symmetry, originating from the geometric Berry phase of the constituting electron bands. This concept of photocurrent generation based on the real-space shift of the electron cloud on the short timescale of optical transition is distinct from that of conventional p–n junction photovoltaics, where the carriers are driven by the built-in Coulomb potential. We experimentally demonstrate for a ferroelectric polar semiconductor how the subpicosecond charge swing on the relevant chemical bond changes its dynamics while scanning the excitation photon energy across the bandgap. On the interband photoexcitation above the bandgap, a finite net charge flow is produced along the electrically polar direction.

Spontaneous photocurrent, termed shift current, can flow in noncentrosymmetric bulk crystals due to topological nature of constituting electronic bands. The shift current with less-dissipative character may have remarkable advantages over the conventional drift photocurrent driven by built-in potential or external electric field. We revisit the generation and transport of shift current in a prototypical ferroelectric semiconductor SbSI near its band gap energy. It is revealed that the switchable shift current is steadily generated by photoexcitation down to low temperature, appears over a distance of millimeter range in a highly insulating bulk without noticeable decay, and largely exceeds polarization charge in the sample, reflecting its Berry phase origin.

Because the functions of polar materials are governed primarily by their polarization response to external stimuli, the majority of studies have focused on controlling polar lattice distortions. In some perovskite oxides, polar distortions coexist with nonpolar tilts and rotations of oxygen octahedra. The interplay between nonpolar and polar instabilities appears to play a crucial role, raising the question of how to design materials by exploiting their coupling. Here, we introduce the concept of ‘polarization twist’, which offers enhanced control over piezoelectric responses in polar materials. Our experimental and theoretical studies provide direct evidence that a ferrielectric perovskite exhibits a large piezoelectric response because of extended polar distortion, accompanied by nonpolar octahedral rotations, as if twisted polarization relaxes under electric fields. The concept underlying the polarization twist opens new possibilities for developing alternative materials in bulk and thin-film forms.

We have investigated the crystal structure of (Bi1/2Na1/2)TiO3–7%BaTiO3 (BNT–7%BT) by high-resolution neutron powder diffraction (NPD) and high-energy synchrotron radiation X-ray diffraction (SR-XRD) analyses. The NPD study revealed that the BNT–7%BT crystals have a single-phase tetragonal structure with P4b m symmetry. The crystal structure refined by the Rietveld method was found to be similar to the ferrielectric P4b m phase reported for BNT at a high temperature of 673 K. The SR-XRD analyses for single crystals of BNT–7%BT demonstrated that the P4b m phase remains as a stable phase in the crystals even after a high electric field is applied for poling, which is different from the structural analysis of ceramics by Ma et al. [Phys. Rev. Lett. 109 (2012) 107602].

Abstract:The features of the crystal structures and spontaneous polarization (P s ) under an electric field (E) have been reviewed for (1 − x)(Bi 0.5 Na 0.5 )TiO 3 -xBaTiO 3 (BNT-BT). In-situ measurements of high-resolution synchrotron radiation X-ray diffraction (SR-XRD) under electric fields show that single crystals with x = 0 (BNT) and 5% have a monoclinic distortion in space group Cc at 25 °C. The SR-XRD study combined with density functional theory (DFT) calculations demonstrates that BNT-5%BT exhibits a rotation of P s in the monoclinic a-c plane by 2° under an E of 70 kV/cm along the <001> pseudo-cubic direction, which is much larger than BNT.