arXiv subjects

Hiromichi Ohta

Publications and source records attributed to Hiromichi Ohta.

39 records · Page 3Linked to original sources

arXiv2009

Electric-Field Modulation of Thermopower for the KTaO3 Field Effect Transistors

We show herein fabrication and field-modulated thermopower for KTaO3 single-crystal based field-effect transistors (FETs). The KTaO3 FET exhibits field effect mobility of ~8 cm2/Vs, which is ~4 times larger than that of SrTiO3 FETs. The thermopower of the KTaO3 FET decreased from 600 to 220 microV/K by the application of gate electric field up to 1.5 MV/cm, ~400 microV/K below that of an SrTiO3 FET, clearly reflecting the smaller carrier effective mass of KTaO3.

cond-mat.str-el↗

arXiv2009

Enhanced effective mass in doped SrTiO3 and related perovskites

The effective mass is one of the main factors determining the Seebeck coefficient and electronic conductivity. Nb-doping increases the effective mass because of two reasons, lattice constants increase and electronic effects. In this ab-initio study the effective mass is estimated from the curvature of electronic bands and it could be clarified that the deformation of SrTiO3 crystals has a significant influence on bandgap and effective DOS and band mass, which are both in excellent agreement to experimental data. However, the electronic effect due to the e2g- band flattening near the Gamma-point due to Nb-doping up to 0.2 at% is the main factor for the increase of effective mass. Doping of La shows a linear decrease of the effective mass; this is explained by the different surrounding of A- and B-site. Substitution of other elements like Ba on the A-site and V on the B-site in SrTiO3 were also found to increase the effective mass.

cond-mat.str-el↗

arXiv2008

Effective mass calculations of SrTiO3-based superlattices for thermoelectric applications lead to new layer design

The effective mass is one of the main factors for enlarging the Seebeck coefficient and electronic conductivity of SrTiO3-based thermoelectric materials [1,2]. The goal of this paper is to clarify, how superlattices can change the effective mass and other features of the bandstructure. The natural Ruddlesden-Popper phase (SrTiO3)n(SrO)m with n=2, m=1 the situation changes, because the TiO6-octahedrons are slightly extended, due to diluted density of the SrO layer. Another effect is the deformed electron density, which leads to reduced effective mass perpendicular to the layer, but enlarged parallel to the plane [3]. The average value of the effective mass over this anisotropy of the 2-dimensional electron gas (2DEG) for pure Ruddlesden-Popper phases is smaller, but can increase beyond the value of pure Pervoskite for certain doping elements. In the same way, artificial superlattices (SrTiO3)x/(SrTi1-z(Nb)zO3)y were examined. When fine nanostructures (n=2, m=3 or n=3, m=2) are present the effective mass increases, when the structure becomes coarser (n=4, m=1) smaller values are determined.

cond-mat.mtrl-sci↗