# Condensed Matter Seminars

### Abstracts

**Prof. Luigi Sangaletti**

Interface effects at all-oxide epitaxial heterojunction probed by photoemission spectroscopies

Recent advances in the growth of epitaxial oxide thin films have fostered a steady increase of research on perovskite oxide heterojunctions, which are now produced with unprecedented quality. Applications of these ultra-thin interfaces in the field of electronics, photon harvesting, photovoltaics and photocatalysis strongly rely on the capability to master band gap engineering at the nanoscale. X-ray photoemission spectroscopies (XPS) are playing a key role in the investigation of electronic and structural properties of all-oxide heterointerfaces [1]. Core level and valence band XPS can be combined to probe the band gap alignment [2]. The use of tunable light sources allow to change the in-depth sensitivity, with the possibility to profile the band-gap close to the interface and compare the results with bulk electronic states [3].

Furthermore, angle-resolved XPS spectra can probe the local order around the photoemitting atom, but through suitable modeling these data can also be used to track cation interdiffusion across the interfaces [4]. Finally, the spectral weight enhancement obtained by tuning the photon energy [5], has disclosed unexpected possibilities in the study of band dispersion at buried interfaces. Here, the combination of these techniques is focused on perovskite oxide layers (in particular LaAlO3) grown on SrTiO3, as these systems can host a two dimensional electron gas (2DEG) at the interface and display magnetic ordering and superconductivity effects, disclosing possible applications in the next-generation nanoelectronic devices. Electron spectroscopy results add important details to the physics of these systems, displaying a far richer scenario with respect to the bare electronic reconstruction. In particular, origin and signatures of the 2DEG are discussed in connection with cation interdiffusion and surface cation substoichiometry.

- [1] A Giampietri, G Drera, L Sangaletti, Advanced Materials Interfaces, 4 (2017) 1700144
- [2] G Drera, G Salvinelli, A Brinkman, et al. PRB B 87 (2013) 075435
- [3] G Drera, G Salvinelli, F Bondino, et al., PRB B 90 (2014) 035124
- [4] G Salvinelli, G Drera, A Giampietri, L Sangaletti ACS-AMI 7 (2015), 25648
- [5] G Drera, F Banfi, FF Canova, P Borghetti, L Sangaletti, et al. APL 98 (2011), 052907

**Prof. Jie Ma (Shanghai Jiao Tong University)**

Neutron Scattering Study on the Quantum Effect in Ba_{3}CoSb_{2}O_{9}

Ba_{3}CoSb_{2}O_{9} is a spin-1/2 triangular-lattice antiferromagnet and has attracted a lot of attention in the past decade. Since Co2+ ions form the idea triangle, the Dzyaloshinskii-Moriya effect is absent in the highly symmetric hexagonal lattice, and this compound is recognized as an ideal material to study the interplay between frustration, low-dimensionality, and strong quantum fluctuations. A striking quantum phenomena is the transition from an ambient field non-collinear 120∘ spin structure into a collinear up-up-down (uud) state with applied magnetic field, a magnetization plateau at one-third of its saturation value 𝑀 = 𝑀s/3. We applied neutron scattering techniques to study the non-collinear 120∘ (0T), intermediate state (0T< H <9.8T) and the collinear uud states (9.8T< H <16T), and found that although the spin wave theory couldn’t explain the spin-wave with 120∘ structure, the uud phase was simulated quantitatively, which indicated an intrinsic quantum mechanical origin for anomalous zero-field spin dynamics.

**Jian Shen (Fudan University)**

Towards all-in-one spintronics: Manipulating electronic phase separation in complex oxides

In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. Investigation of EPS phenomena is not only important for understanding the strong electronic correlations in these materials, but also very useful for tuning their physical properties. Most work in studying EPS has focused on observing EPS and understanding its formation mechanism. The natural appearance of EPS domains, as expected, is totally random in terms of their size and spatial distribution. In order to control the EPS and thus the physical properties of the complex oxides, in recent years we have developed several methods to control the shape, density, location, size, spatial distribution and even the very existence of the EPS domains in manganites. As a result, we are able to order array the EPS domains and finely tune the corresponding physical properties. It is hoped that these abilities will allow us to soon design spintronic devices by patterning EPS domains in one material. These new spintronic devices do not have chemical interfaces and thus are expected to have high spin transport efficiency.

**Dr. Hao Lin (UT)**

Two-dimensional J_{eff}=1/2 Antiferromagnetism unraveled from interlayer coupling and controlled under external magnetic field

A two-dimensional (2D) lattice formed of IrO_{6} octahedra emerged as a novel playground for some of the most outstanding and challenging problems in condensed matter physics, such as metal-insulator transition and quntum magnetism. A notable example is the confined 2D SrIrO_{3} perovskite layers in iridate Ruddlesden-Popper (RP) phases, in which dimensionality, structure, effective electron-electron correlation and spin-orbit coupling entangle each other and lead to a rich phase diagram. The investigation, unfortunately, has been hindered due to limitation of available bulk compounds with rigid layering structures. Experimentally, epitaxial atomic layering may enable more structural tunabilities and offer remarkable opportunity to fully understand the complex diagram and shed light on the hidden physics.

In this talk, I will show an experimental investigation of the 2D J_{eff} = 1/2 antiferromagnetism by artificially varying the interlayer exchange coupling in superlattices [1]. Both effective electron-electron correlation and magnetic ordering display dimensionality-dependence unobtainable in bulk RP phase. Resonant x-ray magnetic scattering revealed a switchable sign of the interlayer exchange coupling locked to the octahedral rotation pattern. With diminishing interlayer coupling, the results show realization of a 2D antiferromagnet at finite temperatures stabilized by spin anisotropy. The 2D antiferromagnetic order stability shows a dramatic increase under magnetic field effect due to a hidden symmetry. These findings demonstrate a powerful route to discover and realize novel 2D quantum magnets by heteroepitaxial engineering.

[1] L. Hao, D. Meyers, C. Frederick, G. Fabbris, J. Yang, N. Traynor, L. Horak, D. Kriegner, Y. Choi, J.-W. Kim, D. Haskel, P.J. Ryan, M.P.M. Dean, J. Liu, Phys. Rev. Lett. 119, 027204 (2017).

**Prof. Phil Pincus (UT)**

Screening in concentrated electrolyte solutions

Recent surface force experiments suggest that the Debye screening length in aqueous solutions is not monotonic in electrolyte concentration. I shall review the fundamentals of Debye-Huckel theory and discuss some possible scenarios to understand the experimental observations.

**Prof. Steve Johnson**

Probing elementary lattice and magnetic excitations in quasi-one-dimensional cuprates using RIXS

Resonant inelastic x-ray scattering (RIXS) has emerged as a powerful probe of excitations in correlated materials. With continued improvements in instrumentation, this technique is now probing low-energy magnetic and lattice excitations in an ever-growing number of correlated oxides. In this talk, I will present two recent RIXS studies at the oxygen K-edge which examined elementary lattice (phonon) and magnetic (spinon) excitations in the quasi-one-dimensional spin-chain cuprates Li2CuO2 and Sr2CuO3, respectively. Using these examples, I will discuss how the intermediate state dynamics in RIXS plays a critical role in determining the RIXS scattering cross-section, allowing one to probe excitations that are missed by techniques such as neutron scattering.

**Dr. Hidemaro Suwa (UT)**

Enhanced controllability at proximity of hidden SU(2) symmetry in quasi two dimensions

Continuous symmetries cannot be spontaneously broken at finite temperature in pure two-dimensional systems with neighboring interactions. For example, the isotropic Heisenberg spins with the SU(2) symmetry can exhibit a long-range order only at zero temperature. The susceptibility of ordering magnetization exponentially increases toward zero temperature. As a result, small perturbation, such as an interlayer coupling, anisotropy, and external magnetic field, to the isotropic spins in two dimensions is able to trigger enhanced response, which provides a great opportunity for controllable devices.

In this talk, I will first explain the critical phenomena in quasi-two-dimensional systems. The transition or crossover temperature logarithmically increases as a function of a perturbative term, showing an infinite slope. Then I apply this theory to the confined two-dimensional SrIrO3 perovskite layers consisting of IrO6 octahedra, where the low-energy physics is described by Jeff=1/2 antiferromagnets. In spite of the existence of significant spin-orbit coupling which produces the anisotropic and Dzyaloshinskii-Moriya interactions, remarkably, the system has a hidden SU(2) symmetry. Tiny uniform magnetic field drastically increases the crossover temperature to the ordered state. This material shows an excellent figure of merit in the combination of the quasi-two-dimensional physics and the hidden SU(2) symmetry.

**Dr. Fangfei Ming (UT)**

Realization of a hole-doped Mott insulator on a triangular silicon lattice

The physics of doped Mott insulators is at the heart of some of the most exotic physical phenomena in materials research including insulator-metal transitions, colossal magneto-resistance, and high-temperature superconductivity in layered perovskite compounds. These phenomena often emerge as a function of carrier doping and are rooted in the strongly correlated motion of the charge carriers and their coupling to lattice and magnetic excitations of the crystal. Advances in this field would greatly benefit from the availability of new material systems with similar richness of physical phenomena, ideally those that are less complex in structure and composition, and highly ordered. Here we show that such a system can be realized on a silicon platform. Adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a triangular surface lattice with half-filled dangling bond orbitals. Modulation hole-doping of these dangling bonds unveils clear hallmarks of Mott physics, such as spectral weight transfer and the formation of quasi-particle states at the Fermi level, well-defined Fermi contour segments, and a sharp singularity in the density of states. These observations are remarkably similar to those made in complex oxide materials, including high-temperature superconductors, but highly extraordinary within the realm of conventional sp-bonded semiconductor materials. It suggests that exotic quantum matter phases can be housed and engineered on silicon-based materials platforms.