There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

Redox‐Based Resistive Switching Memories – Nanoionic Mechanisms, Prospects, and Challenges

Polarity discontinuities at the interfaces between different crystalline materials (heterointerfaces) can lead to nontrivial local atomic and electronic structure, owing to the presence of dangling bonds and incomplete atomic coordinations. These discontinuities often arise in naturally layered oxide structures, such as the superconducting copper oxides and ferroelectric titanates, as well as in artificial thin film oxide heterostructures such as manganite tunnel junctions. If polarity discontinuities can be atomically controlled, unusual charge states that are inaccessible in bulk materials could be realized. Here we have examined a model interface between two insulating perovskite oxides--LaAlO3 and SrTiO3--in which we control the termination layer at the interface on an atomic scale. In the simple ionic limit, this interface presents an extra half electron or hole per two-dimensional unit cell, depending on the structure of the interface. The hole-doped interface is found to be insulating, whereas the electron-doped interface is conducting, with extremely high carrier mobility exceeding 10,000 cm2 V(-1) s(-1). At low temperature, dramatic magnetoresistance oscillations periodic with the inverse magnetic field are observed, indicating quantum transport. These results present a broad opportunity to tailor low-dimensional charge states by atomically engineered oxide heteroepitaxy.

A high-mobility electron gas at the LaAlO3/SrTiO3 heterointerface

Nanocrystals (NCs) discussed in this Review are tiny crystals of metals, semiconductors, and magnetic material consisting of hundreds to a few thousand atoms each. Their size ranges from 2-3 to about 20 nm. What is special about this size regime that placed NCs among the hottest research topics of the last decades? The quantum mechanical coupling * To whom correspondence should be addressed. E-mail: dvtalapin@uchicago.edu. † The University of Chicago. ‡ Argonne National Lab. Chem. Rev. 2010, 110, 389–458 389

/pdf/prospects-of-colloidal-nanocrystals-for-electronic-and-3dxpyhl3r7.pdf

Prospects of Colloidal Nanocrystals for Electronic and Optoelectronic Applications

We investigated temperature-dependent optical conductivity spectra σ(ω) of Nd1−xSrxMnO3 (x=0.40, 0.50, 0.54, and 0.65) single crystals. At room temperature, mid-infrared σ(ω) are nearly the same for all compositions and spectral weights seem to depend only on the doping concentration. At low temperature, however, σ(ω) are quite different and spectral weights seem to strongly depend on the orbital/spin patterns, i.e., F (x=0.40), CE (x=0.50), A (x=0.54), and C (x=0.65)-type. We will discuss the strong doping dependence of the mid-infrared optical conductivity spectra in terms of the orbital pattern dependent Jahn–Teller polaron hopping.

Mid-infrared optical conductivity spectra of Nd1−xSrxMnO3: orbital pattern dependent polaron hopping

Using optical spectroscopy, we investigated the electrodynamic properties of Nb:SrTiO3/SrTiO3 superlattices. In these superlattices, a large enhancement of the Seebeck coefficient (S) has been reported with decreasing Nb:SrTiO3 layer thickness [refer to H. Ohta et al., Nature Mater. 6, 129 (2007)]. By analyzing the optical spectra, we found that the polaron plays an important role in determining the electrodynamic properties of the superlattices. With decreasing Nb:SrTiO3 layer thickness from eleven to one unit cell, we observed a threefold enhancement of the polaron effective mass and relaxation time. Such increases were attributed to a dimensional crossover of polaron from 3D to quasi-2D. Moreover, the modified nature of the polaron at low dimensions enhanced the thermoelectric properties of the oxide superlattice, by simultaneously increasing S and preventing the decrease of carrier mobility. Our results indicate that strong electron-phonon coupling can provide an alternative pathway in searching efficient thermoelectric materials.

Dimensional Crossover of the Polaron Dynamics in Thermoelectric Nb:SrTiO$_{3}$/SrTiO$_{3}$ Superlattices

Jeong Rae Kim,1, 2, ∗ Byungmin Sohn,1, 2, 3, ∗ Hyeong Jun Lee,4, ∗ Sangmin Lee,5 Eun Kyo Ko,1, 2 Sungsoo Hahn,1, 2 Sangjae Lee,1 Younsik Kim,1, 2 Donghan Kim,1, 2 Hong Joon Kim,1, 2 Youngdo Kim,1, 2 Jaeseok Son,1, 2 Charles H. Ahn,3, 6 Frederick J. Walker,3 Ara Go,7 Miyoung Kim,5 Choong H. Kim,1, 2, † Changyoung Kim,1, 2, ‡ and Tae Won Noh1, 2, § Center for Correlated Electron Systems, Institute for Basic Science, Seoul 08826, Korea Department of Physics and Astronomy, Seoul National University, Seoul 08826, Korea Department of Applied Physics, Yale University, New Haven, Connecticut 06520, USA Center for Theoretical Physics of Complex Systems, Institute for Basic Science (IBS), Daejeon 34126, Republic of Korea Department of Materials Science and Engineering and Research Institute of Advanced Materials, Seoul National University, Seoul 08826, Korea Department of Physics, Yale University, New Haven, Connecticut 06520, USA Department of Physics, Chonnam National University, Gwangju 61186, Republic of Korea (Dated: March 9, 2022)

Heteroepitaxial control of Fermi liquid, Hund metal, and Mott insulator phases in the single-atomic-layer limit

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low-density. Since the low-temperature mobilities determined by dc-transport in the LaAlO3/SrTiO3 heterostructures are much higher than those extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

Ruddlesden–Popper oxides (A2BO4) have attracted significant attention regarding their potential application in novel electronic and energy devices. However, practical uses of A2BO4 thin films have been limited by extended defects such as out‐of‐phase boundaries (OPBs). OPBs disrupt the layered structure of A2BO4, which restricts functionality. OPBs are ubiquitous in A2BO4 thin films but inhomogeneous interfaces make them difficult to suppress. Here, OPBs in A2BO4 thin films are suppressed using a novel method to control the substrate surface termination. To demonstrate the technique, epitaxial thin films of cuprate superconductor La2‐xSrxCuO4 (x = 0.15) are grown on surface‐reconstructed LaSrAlO4 substrates, which are terminated with self‐limited perovskite double layers. To date, La2‐xSrxCuO4 thin films are grown on LaSrAlO4 substrates with mixed‐termination and exhibit multiple interfacial structures resulting in many OPBs. In contrast, La2‐xSrxCuO4 thin films grown on surface‐reconstructed LaSrAlO4 substrates energetically favor only one interfacial structure, thus inhibiting OPB formation. OPB‐suppressed La2‐xSrxCuO4 thin films exhibit significantly enhanced superconducting properties compared with OPB‐containing La2‐xSrxCuO4 thin films. Defect engineering in A2BO4 thin films will allow for the elimination of various types of defects in other complex oxides and facilitate next‐generation quantum device applications.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/smtd.202200880

Tae Won Noh

Papers

Mid-infrared optical conductivity spectra of Nd1−xSrxMnO3: orbital pattern dependent polaron hopping

Dimensional Crossover of the Polaron Dynamics in Thermoelectric Nb:SrTiO$_{3}$/SrTiO$_{3}$ Superlattices

Heteroepitaxial control of Fermi liquid, Hund metal, and Mott insulator phases in the single-atomic-layer limit

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

Defect Engineering in A2BO4 Thin Films via Surface‐Reconstructed LaSrAlO4 Substrates