arXiv subjects

Se Young Park

Publications and source records attributed to Se Young Park.

22 records · Page 2Linked to original sources

arXiv2015

Metal-insulator transitions in GdTiO3/SrTiO3 superlattices

The density functional plus U method is used to obtain the electronic structure, lattice relaxation and metal-insulator phase diagram of superlattices consisting of $m$ layers of Gadolinium Titanate (GdTiO$_{3}$) alternating with $n$ layers of Strontium Titanate (SrTiO$_{3}$). Metallic phases occur when the number of SrTiO$_3$ layers is large or the interaction $U$ is small. In metallic phases, the mobile electrons are found in the SrTiO$_3$ layers, with near-interface electrons occupying $xy$-derived bands, while away from the interface the majority of electrons reside in $xz/yz$ bands. As the thickness of the SrTiO$_3$ layers decreases or the on-site interaction U increases a metal-insulator transition occurs. Two different insulating states are found. When the number of SrTiO$_{3}$ layers is larger than one, we find an insulating state with two sublattice charge and orbital ordering and associated Ti-O bond length disproportionations. When the number of SrTiO$_{3}$ units per layer is one, a different insulating phase occurs driven by orbital ordering within the quasi one-dimensional $xz/yz$ bonding bands connecting Ti atoms across the SrO layer. In this phase there is no sublattice charge ordering or bond disproportionation. The critical U for the single-layer insulator is $\sim$ 2.5 eV, much less than critical U $\sim$ 3.5 eV required to drive the metal-insulator transition when the number of SrTiO$_3$ is larger than one. Inconsistencies between the calculation and the experiment suggest that many-body correlations may be important. A local inversion symmetry breaking around Ti atoms suggests the possibility of in-plane ferroelectric polarization in the insulating phase.

cond-mat.str-el↗

arXiv2013

Charge density distribution and optical response of the LaAlO3/SrTiO3 interface

We present calculations of the charge density profile, subband occupancy and ellipsometry spectra of the electron gas at the LaAlO3/SrTiO3 interface. The calculations employ self-consistent Hartree and random phase approximations, a tight binding parametrization of the band structure and a model for the optical phonon of SrTiO3. The dependence of the spatial structure and occupancy of subbands on the magnitude of the polarization charge at the interface and the dielectric function is determined. The interface-confined subbands may be labelled by the symmetry (xy, xz, or yz) of the Ti d-orbitals from which they mainly derive. The xy-derived band nearest the interface contains the major proportion of the electronic charge, but a large number of more distant, slightly occupied xy-derived bands are also found. Depending on the magnitude of the polarization charge, zero, one, or two xz/yz derived subbands are found. When present, these xz/yz bands give the dominant contribution to the long-distance tail of the interface charge. The response to applied ac electric fields polarized parallel and perpendicular to the interface is calculated and the results are presented in terms of ellipsometry angles. Two features are found: a dip in the spectrum near the LO feature of the STO phonon and a peak at the higher energy. We show that the form and magnitude of the dip is related to the Drude response of carriers moving in the plane of the interface while the peak arises from the plasmon excitation of the xz and yz electrons. The relation of the features of the subband occupancies and the in-plane conductivities is given.

cond-mat.str-el↗

arXiv2009

Dominant role of local-moment interactions in the magnetism in iron pnictides : comparative study of arsenides and antimonides from first-principles

The magnetic properties of various iron pnictides are investigated using first-principles pseudopotential calculations. We consider three different families, LaFePnO, BaFe$_2$Pn$_2$, and LiFePn with Pn=As and Sb, and find that the Fe local spin moment and the stability of the stripe-type antiferromagnetic phase increases from As to Sb for all of the three families, with a partial gap formed at the Fermi energy. In the meanwhile, the Fermi-surface nesting is found to be enhanced from Pn=As to Sb for LaFePnO, but not for BaFe$_2$Pn$_2$ and LiFePn. These results indicate that it is not the Fermi surface nesting but the local moment interaction that determines the stability of the magnetic phase in these materials, and that the partial gap is an induced feature by a specific magnetic order.

cond-mat.supr-con↗

arXiv2008

Enhanced spin density wave in LaOFeSb

We predict atomic, electronic, and magnetic structures of a hypothetical compound LaOFeSb by first-principles density-functional calculations. It is shown that LaOFeSb prefers a stripe-type antiferromagnetic phase (i.e., spin density wave (SDW) phase) to the non-magnetic (NM) phase, with a larger Fe spin moment and greater SDW-NM energy difference than those of LaOFeAs. The SDW phase is found to favor the orthorhombic structure while the tetragonal structure is more stable in the NM phase. In the NM-phase LaOFeSb, the electronic bandwidth near the Fermi energy is reduced compared with LaOFeAs, indicating smaller orbital overlap between Fe $d$ states and subsequently enhanced intra-atomic exchange coupling. The calculated Fermi surface in the NM phase consists of three hole and two electron sheets, and shows increased nesting between two hole and two electron sheets compared with LaOFeAs. Monotonous changes found in our calculated material properties of LaOFePn (Pn=P, As, and Sb), along with reported superconducting properties of doped LaOFeP and LaOFeAs, suggest that doped LaOFeSb may have a higher superconducting transition temperature.

cond-mat.supr-con↗