Optical study of the free-carrier response of LaTiO3/SrTiO3 superlattices

Effect of Mn doping and charge transfer on LaTi1-xMnxO3

We report the magnetic and electronic transport properties of Mn-doped LaTi_1-xMn_xO₃(x= 0, 0.1, 0.3, 0.5) as a function of temperature and an applied magnetic field. It was found that the Mn-doped samples show a magnetic transition which is not present in the parent LaTiO₃. The Mn-doped samples showed fluctuations in magnetization at low fields below their Néel transition temperature indicating electronic phase separation in the material. Increased Mn content in the sample strengthens the ferromagnetic-like moment while maintaining G-type antiferromagnetic phase by charge transfer from Mn to Ti and influencing orbital ordering of the Ti³⁺t_2gorbitals. The results are discussed in parallel with transport and bulk magnetization measurements detailing the electronic behavior. An additional context for the mechanism is supported by first-principles density-function theory calculations.

Polarization screening-induced epitaxial growth and interfacial magnetism of BiFeO3/PbTiO3nanoplates

Single-crystal BiFeO₃/PbTiO₃nanoplates have been synthesizedviaa hydrothermal method, where BFO films selectively grew on the negative polar surface of PTO with a saturation thickness of about 18–20 nm and a room-temperature ferromagnetism.

Conducting LaAlO3/SrTiO3 heterointerfaces on atomically-flat substrates prepared by deionized-water

We have investigated how the recently-developed water-leaching method for atomically-flat SrTiO₃ (STO) substrates affects the transport properties of LaAlO₃ (LAO) and STO heterointerfaces. Using pulsed laser deposition at identical growth conditions, we have synthesized epitaxial LAO thin-films on two different STO substrates, which are prepared by water-leaching and buffered hydrofluoric acid (BHF) etching methods. The structural, transport, and optical properties of LAO/STO heterostructures grown on water-leached substrates show the same high-quality as the samples grown on BHF-etched substrates. These results indicate that the water-leaching method can be used to grow complex oxide heterostructures with atomically well-defined heterointerfaces without safety concerns.

Thermopower enhancement by fractional layer control in 2D oxide superlattices

Precise tuning of the 2D carrier density by using fractional δ-doping of d electrons improves the thermoelectric properties of oxide heterostructures. This promising result can be attributed to the anisotropic band structure in the 2D system, indicating that δ-doped oxide superlattices are good candidates for advanced thermoelectrics.

Transparent conducting oxides: a δ-doped superlattice approach

Metallic states appearing at interfaces between dissimilar insulating oxides exhibit intriguing phenomena such as superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using optical spectroscopic measurements and density-functional theory (DFT) simulations, we examine the effect of SrTiO₃ (STO) spacer layer thickness on the optical transparency and carrier distribution in La δ-doped STO superlattices. We experimentally observe that these metallic superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2p and Ti 3d states. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of quantum mechanical wavefunctions between neighboring δ-doped layers. These results highlight the potential for using oxide heterostructures in optoelectronic devices by providing a unique route for creating novel transparent conducting oxides.

High-density two-dimensional small polaron gas in a delta-doped Mott insulator

Heterointerfaces in complex oxide systems open new arenas in which to test models of strongly correlated material, explore the role of dimensionality in metal-insulator-transitions (MITs) and small polaron formation. Close to the quantum critical point Mott MITs depend on band filling controlled by random disordered substitutional doping. Delta-doped Mott insulators are potentially free of random disorder and introduce a new arena in which to explore the effect of electron correlations and dimensionality. Epitaxial films of the prototypical Mott insulator GdTiO₃ are delta-doped by substituting a single (GdO)⁺¹ plane with a monolayer of charge neutral SrO to produce a two-dimensional system with high planar doping density. Unlike metallic SrTiO₃ quantum wells in GdTiO₃ the single SrO delta-doped layer exhibits thermally activated DC and optical conductivity that agree in a quantitative manner with predictions of small polaron transport but with an extremely high two-dimensional density of polarons, ~7 × 10¹⁴ cm⁻².

Layer-by-layer evolution of a two-dimensional electron gas near an oxide interface

We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO(3)/SrTiO(3) interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.

Symmetry lowering in extreme-electron-density perovskite quantum wells

We report on structural distortions in extreme-electron density (∼6×10(14)  cm(-2)) confined quantum wells of SrTiO3 embedded in GdTiO3. Sr-column displacements are measured using high-angle annular dark-field imaging in scanning transmission electron microscopy. Using thick SrTiO3 layers as a reference, orthorhombic-like Sr-site displacements are observed when SrTiO3 quantum wells are thinner than 4 SrO layers, in precise agreement with an experimentally observed metal-to-insulator transition. It is shown that the structural displacements are absent in thicker quantum wells, even in the immediate proximity to the orthorhombic GdTiO3. The results are discussed in the context of strong electron correlations in this system.

Synchrotron radiation-based far-infrared spectroscopic ellipsometer with full Mueller-matrix capability

We developed far-IR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and full-Mueller matrix spectra using rotating retarders, and wire-grid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm(-1). Fourier-transform infrared (FT-IR) spectrometer is used for multi-wavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of θ-2θ angular rotation, χ tilt angle adjustment, and X-Y-Z translation. A LabVIEW-based software controls the motors, sample temperature, and FT-IR spectrometer and also allows to run fully automated experiments with pre-programmed measurement schedules. Data analysis is based on Berreman's 4 × 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability μ ≠ 1. A nonlinear regression of the rotating analyzer ellipsometry and∕or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thin-film samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with μ ≠ 1 are illustrated with experimental results and simulations for TbMnO3 and Dy3Fe5O12 single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO3.

Fractionally δ-doped oxide superlattices for higher carrier mobilities

A two-dimensional (2D) electron gas system in an oxide heterostructure serves as an important playground for novel phenomena. Here, we show that, by using fractional δ-doping to control the interface's composition in La(x)Sr(1-x)TiO(3)/SrTiO(3) artificial oxide superlattices, the filling-controlled 2D insulator-metal transition can be realized. The atomic-scale control of d-electron band filling, which in turn contributes to the tuning of effective mass and density of the charge carriers, is found to be a fascinating route to substantially enhanced carrier mobilities.

Two-dimensional superconducting phase in LaTiO3/SrTiO3 heterostructures induced by high-mobility carrier doping

In this Letter, we show that a superconducting two-dimensional electron gas is formed at the LaTiO3/SrTiO3 interface whose transition temperature can be modulated by a back-gate voltage. The gas consists of two types of carriers: a majority of low-mobility carriers always present, and a few high-mobility ones that can be injected by electrostatic doping. The calculation of the electron spatial distribution in the confinement potential shows that the high-mobility electrons responsible for superconductivity set at the edge of the gas whose extension can be tuned by the field effect.

Optical properties of (SrMnO₃)n/(LaMnO₃)₂n superlattices: an insulator-to-metal transition observed in the absence of disorder

We measure the optical conductivity, σ1(ω), of (SrMnO3)n/(LaMnO3)2n superlattices (SL) for n = 1, 3, 5, and 8 and 10 < T < 400 K. Data show a T-dependent insulator to metal transition (IMT) for n ≤ 3, driven by the softening of a polaronic mid-infrared band. At n = 5 that softening is incomplete, while at the largest-period n = 8 compound the MIR band is independent of T and the SL remains insulating. One can thus first observe the IMT in a Manganite system in the absence of the disorder due to chemical doping. Unsuccessful reconstruction of the SL optical properties from those of the original bulk materials suggests that (SrMnO3)n/(LaMnO3)2n heterostructures give rise to a novel electronic state.

Growth, microstructure, and infrared-ultraviolet optical conductivity of La(0.5)Sr(0.5)CoO(3) nanocrystalline films on silicon substrates by pulsed laser deposition

La(0.5)Sr(0.5)CoO(3) (LSCO) nanocrystalline (nc) films have been directly grown on silicon wafers under different substrate temperatures by pulsed laser deposition. The X-ray diffraction analysis indicate that the films are polycrystalline with the pure perovskite phase at higher substrate temperatures. The columnar growth formation with the nanocrystalline structure in the films has been confirmed by microscopy experiments. Infrared-ultraviolet optical properties of the LSCO films have been investigated with the aid of spectroscopic ellipsometry (SE). Dielectric function in the photon energy range of 1.1-3.1 eV (400-1100 nm) has been extracted by reproducing the experimental data with a Lorentz oscillator model. It is found that the real part is decreased from 4.7 to -0.7 at the near-infrared region with increasing substrate temperature. The optical conductivity shows a different variation trend for the lower and higher growth temperatures, respectively. Note that the films deposited above 650 degrees C exhibit the well-defined metallic phase behavior. The discrepancies could be mainly ascribed to different crystalline structure and surface morphology. The present results may be crucial for future applications of ferromagnetic-based optoelectronic and spin-electronic devices.

Two-dimensional confinement of 3d{1} electrons in LaTiO_{3}/LaAlO{3} multilayers

We report spectroscopic ellipsometry measurements of the anisotropy of the interband transitions parallel and perpendicular to the planes of (LaTiO3)n(LaAlO3)5 multilayers with n=1-3. These provide direct information about the electronic structure of the two-dimensional (2D) 3d{1} state of the Ti ions. In combination with local density approximation, including a Hubbard U calculation, we suggest that 2D confinement in the TiO2 slabs lifts the degeneracy of the t{2g} states leaving only the planar d{xy} orbitals occupied. We outline that these multilayers can serve as a model system for the study of the t{2g} 2D Hubbard model.