On the basis of successful first-principles predictions of new functional ferroelectric materials, a number of new ferroelectrics have been experimentally discovered. Using trilinear coupling of two types of octahedron rotation, hybrid improper ferroelectricity has been theoretically predicted in ordered perovskites and the Ruddlesden-Popper compounds (Ca3Ti2O7, Ca3Mn2O7 and (Ca/Sr/Ba)3(Sn/Zr/Ge)2O7). However, the ferroelectricity of these compounds has never been experimentally confirmed and even their polar nature has been under debate. Here we provide the first experimental demonstration of room-temperature switchable polarization in bulk crystals of Ca3Ti2O7, as well as Sr-doped Ca3Ti2O7. Furthermore, (Ca, Sr)3Ti2O7 is found to exhibit an intriguing ferroelectric domain structure resulting from orthorhombic twins and (switchable) planar polarization. The planar domain structure accompanies abundant charged domain walls with conducting head-to-head and insulating tail-to-tail configurations, which exhibit a conduction difference of two orders of magnitude. These discoveries provide new research opportunities, not only for new stable ferroelectrics of Ruddlesden-Popper compounds, but also for meandering conducting domain walls formed by planar polarization.

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We discovered that perovskite (Ba,La)SnO 3 can have excellent carrier mobility even though its band gap is large. The Hall mobility of Ba 0.98 La 0.02 SnO 3 crystals with the n-type carrier concentration of ∼8-10×10 19 cm -3 is found to be ∼103 cm 2 V -1 s -1 at room temperature, and the precise measurement of the band gap Δ of a BaSnO 3 crystal shows Δ=4.05 eV, which is significantly larger than those of other transparent conductive oxides. The high mobility with a wide band gap indicates that (Ba,La)SnO 3 is a promising candidate for transparent conductor applications and also epitaxial all-perovskite multilayer devices. a) Author to whom correspondence should be addressed. Electronic mail: sangc@physics.rutgers.edu 2 Transparent conducting oxides (TCOs), exhibiting the contraindicative properties of optical transparency in the visible region and high DC electrical conductivity, are widely utilized as optical window electrodes in photovoltaic devices, liquid crystal displays and solar energy conversion devices. [1][2][3] There have been significant attempts to find alternative TCO materials to replace indium tin oxide due to its soaring price, and new TCOs such as doped ZnO and SnO 2 have been, in fact, already utilized. On the other hand, most of TCOs that have been investigated so far are associated with band gaps significantly less than 4 eV, so they do not transmit ultraviolet (UV) light.For example, the band gaps of SrTiO 3 (STO), ZnO, In 2 O 3 , and SnO 2 are 3.25, 3.3, 2.9, and 3.6 eV, respectively. 4-7 Thus, the discovery of TCOs with band gaps > 3.6 eV is central to enhance the efficiency of, for example, solar energy harvesting.Furthermore, the epitaxial all-perovskite multilayer heterostructures based on STO have recently attracted much attention due to the multiplicity of advantageous physical properties of perovskites. These all-perovskite multilayer heterostructures have great potentials for innovative micro-and nanoelectronic devices. 8,9 However, the mobility of doped STO is low at room temperature (RT) (~11 cm 2 V -1 s -1 ), 10 and the instability of oxygen content in doped oxides such as oxygen-deficient STO is often a detrimental issue inducing fatigue and degradation. 11 Therefore, the critical matter of the development of all-perovskite multilayer devices is finding new perovskite materials exhibiting high carrier mobility near RT and good oxygen stability.Herein, we report that La-doped perovskite BaSnO 3 (BSO) exhibits high carrier mobility with a large band gap, and thus is a promising candidate for transparent conductor applications with possible use in epitaxial all-perovskite multilayer devices.Alkaline earth stannates, with the formula of ASnO 3 (A=Ca, Sr and Ba), are widely used in the electronic industry for their optimal dielectric and gas-sensing properties. 12-14 BSO has an ideal cubic perovskite structure, and is an insulator with a valence band derived from orbitals of π-symmetry (mainly of oxygen 2p-character) and a conduction band with dominant Sn 5s-character. 15 T...

, superconductivity [6][7][8][9] , and discommensurations [10][11][12][13][14][15] . Intercalation of other transition metal ions between the MC 2 layers gives rise to distinct superstructures, leading to significant changes in crystallographic structures and physical properties. Fe-intercalated TaS 2 shows highly anisotropic ferromagnetism at low temperatures [16][17][18][19][20] . . Magnetic hysteresis curves of the crystals were obtained using a Quantum DesignMagnetic Property Measurement System, and the real Fe compositions were estimated from the saturation magnetic moments in the magnetic hysteresis curves with the assumption that each Supplementary Information, section S1). The distinct feature between the 2a×2a and √3a×√3a superstructures in Fe x TaS 2 is the different stacking sequence of the 2D supercells along the c-axis. Specifically, the 2a×2a superstructure consists of identically stacked 2D supercells (i.e., AA-type stacking), while the √3a×√3a superstructure contains shifted 2D supercells with AB-type stacking, as shown in Fig. 1b and Fig. 1f, -5 -respectively. These different stacking sequences result in the centrosymmetric P6 3 /mmc and noncentrosymmetric and chiral P6 3 22 space groups for the 2a×2a and √3a×√3a superstructures, respectively.We found complicated configurations of antiphase domains in the dark-field images of There is an extinction rule for the dark-field images of antiphase boundaries in the 2a×2a superstructure. For example, the antiphase boundary between the BB-type and CC-type antiphase domains appears in the S1=( /2 00) (Fig 2a) and S2=(0 1/2 0) (Fig. 2b) dark-field images, but disappears in the S3=(1/2 /2 0) dark-field image of Fig. 2c (see also Supplementary Information, section S3). Each antiphase boundary becomes invisible in a dark-field image taken using one out of three superlattice spots (namely, S1, S2, or S3), when no antiphase shift at the boundary exists along a certain superlattice modulation wave vector. This absence of antiphase shifts at the antiphase boundary leads to the extinction rule for the antiphase boundaries in superlattice dark-field images. This rule is summarized in Fig. 3, showing the local structures near boundaries between two antiphase domains. The boundaries are highlighted with yellow bands, and the three directions of the superlattice modulation wave vectors are denoted by S1, S2, and S3, respectively, as shown in Fig. 1a. The red, yellow, blue, and green circles correspond to -6 -AA-, BB-, CC-, and DD-type superstructures, respectively, which are associated with four possible origins of the 2a×2a Fe superstructure. It is evident that the superlattice modulation along only one out of three equivalent crystallographic directions does not show any antiphase shift; this is indicated by light green dashed lines (along the S1 direction), light blue dashed lines (along the S2 direction), and pink dashed lines (along the S3 direction). For example, the antiphase boundary between BB-type and CC-type (or AA-type and DD-type) antiphase domains ha...

Spectroscopic refractive indices of monoclinic single crystal and ceramic lutetium oxyorthosilicate from 200 to 850nm J. Appl. Phys. 112, 063524 (2012) Valence band offset at Al2O3/In0.17Al0.83N interface formed by atomic layer deposition Appl. Phys. Lett. 101, 122110 (2012) Electronic structure and optical properties of β-FeSi2(100)/Si(001) interface at high pressure Appl. Phys. Lett. 101, 111909 (2012) First-principles prediction of a new class of photovoltaic materials: I-III-IV2-V4 phosphides J. Appl. Phys. 112, 053102 (2012) Electronic, structural, and elastic properties of metal nitrides XN (X = Sc, Y): A first principle study Optical properties of insulating BaSnO 3 (BSO) and conducting Ba 0.97 La 0.03 SnO 3 (BLSO) single crystals were studied at room temperature in a wide spectral range between 0.01 and 5.9 eV by means of spectroscopic ellipsometry. The far-infrared spectra of the optical phonons in BSO and BLSO were complemented by polarized Raman scattering measurements in BSO. The electronic band structure and the optical response (dielectric function) were calculated using density functional theory, which allowed for the interpretation of the main spectroscopic features such as optical phonons and electronic interband transitions. To reconcile the observed experimental spectra with the theory, a departure from the ideal perovskite structure on the local scale was proposed for BSO.