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DOE OSTI2024

Band Structure and Charge Ordering of Dirac Semimetal EuAl 4 at Low Temperatures

In this article, EuAl 4 is proposed to host a topological Hall state. This material also undergoes four consecutive antiferromagnetic (AFM) transitions upon cooling below T N1 = 15.4 K in the presence of charge density wave (CDW) order that sets in below T CDW = 140 K. We use angle-resolved photoemission spectroscopy and density-functional-theory calculations to study how magnetic ordering affects the electronic properties in EuAl 4 . Here, we found changes in the band structure upon each of the four consecutive AFM transitions including band splitting, renormalizations, and appearance of new bands forming additional Fermi sheets. In addition we also found significant enhancement of the quasiparticles’ lifetime due to suppression of spin flip scattering, similar to what was previously reported for ferromagnetic EuCd 2 As 2 . Surprisingly, we observe that most significant changes in electronic properties occur not at T N1 , but instead at the AFM3 to AFM4 transition, which coincides with the largest drop in resistivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2020

Massive Suppression of Proximity Pairing in Topological ( Bi 1 - x Sb x ) 2 Te 3 Films on Niobium

Interfacing bulk conducting topological Bi 2 Se 3 films with s-wave superconductors initiates strong superconducting order in the nontrivial surface states. Yet, bulk insulating topological (Bi 1-x Sb x ) 2 Te 3 films on bulk Nb instead exhibit a giant attenuation of surface superconductivity, even for films only two-layers thick. This massive suppression of proximity pairing is shown by ultrahigh-resolution band mappings and by contrasting quantified superconducting gaps with those of heavily n-doped topological Bi 2 Se 3 /Nb. The results underscore the limitations of using superconducting proximity effects to realize topological superconductivity in nearly intrinsic systems.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

DOE OSTI2022

In situ Study on the Evolution of Atomic and Electronic Structure of LaTiO 3 /SrTiO 3 System

As with other transition-metal oxide interfaces, LaTiO 3 /SrTiO 3 interfaces exhibit interesting electronic properties, such as a two-dimensional electron gas. While the available Mott states and multiple pathways to metallicity in LaTiO 3 can lead to a variety of applications in oxide electronics, the origin of the different properties remains uncertain. Herein, utilizing real-time, in situ x-ray diffraction at the synchrotron, robust but thin LaTiO 3 /SrTiO 3 heterostructures were fabricated using oxide molecular beam epitaxy. A thickness of six unit cells was determined to be critical, at which point the LaTiO 3 /SrTiO 3 heterostructures begin to sustain sharp, well-defined interfaces. The layers of LaTiO 3 and SrTiO 3 were sufficiently thin to benefit from the finite escape length of electrons at resonant soft x-ray photoemission, and the heterostructures were transferred in vacuo after growth to another chamber for synchrotron-based x-ray photoemission spectroscopy studies. With the x-ray energy tuned to the Ti-2p resonance, the LaTiO 3 /SrTiO 3 interfaces could be probed with enough penetration depth and selectivity. It is shown that all of the heterointerfaces exhibit the two-dimensional electron gas.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Anomalously strong near-neighbor attraction in doped 1D cuprate chains

In the cuprates, one-dimensional (1D) chain compounds provide a distinctive opportunity to understand the microscopic physics, owing to the availability of reliable theories. However, progress has been limited by the challenge of controllably doping these materials. We report the synthesis and spectroscopic analysis of the 1D cuprate Ba 2-x Sr x CuO 3+δ over a wide range of hole doping. Our angle-resolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong near-neighbor attraction, which may arise from coupling to phonons, quantitatively explains experiments for all accessible doping levels. Furthermore, considering structural and quantum chemistry similarities among cuprates, this attraction may play a similarly important role in high-temperature cuprate superconductors.

1D Chain↗

DOE OSTI2020

Solvent Hold Tank Sample Results for MCU-19-366- 367-368 (April 2019), MCU-19-469-470-471 (May 2019), MCU-19-524-525-526 (June 2019): Quarterly Report

A trend summary of three sets of Solvent Hold Tank (SHT) monthly samples MCU-19-366-367-368 (April), MCU-19-469-470-471 (May), and MCU-19-524-525-526 (June) are reported. Most of the conclusions are based on the June SHT sample (MCU-19-524-525-526). Analyses of the June SHT sample (MCU-19-524- 525-526) indicated that the Modifier (Cs-7SB) and the Extractant (MaxCalix) concentrations were below their nominal recommended concentrations (169,000 mg/L and 46,900 mg/L respectively) by 4% and 2% respectively but consistent with previously observed levels during normal runs. The Suppressor (N,N’,N”– tris(3,7-dimethyloctyl)guanidine or TiDG) concentration has decreased since the March 2019 measurement to 793 mg/L, which is above the minimum recommended concentration (479 mg/L). The Semi-Volatile Organic Analysis (SVOA) did not detect any impurities. However, the Fourier Transform Hydrogen Nuclear Magnetic Resonance (FT-HNMR) analysis detected presence of phthalates at a few ppm. The impurity concentration was highest in the April sample and consistently detected in the May and June samples (but always registered as a few ppm or less). Another impurity observed in the samples was mercury. Based on the June SHT sample, up to 25 ± 5 micrograms of mercury per mL of solvent was detected (as the average of the Direct Mercury Analysis (DMA) and X-Ray Fluorescence (XRF) methods). The higher mercury concentration in the solvent (as determined in the last three-monthly samples) is possibly due to the higher mercury concentration in Salt Batches 9 and 10. The gamma concentration (~2.90E4 dpm/mL) measured in the June 2019 SHT samples was consistent with previous values observed when the Modular Caustic-Side Solvent Extraction Unit (MCU) was operating continuously (for example, between December 2016 and January 2017). Statistical analysis of the different analytical methods detected a bias between the FT-HNMR and the High Performance Liquid Chromatography (HPLC) methods when measuring the MaxCalix and Modifier concentrations; the FT-HNMR gave a higher concentration possibly due to peak overlapping in the measured data. A bias was detected when measuring total mercury by the DMA method versus the XRF method with the XRF method reported higher mercury concentrations. The laboratory will continue to monitor the quality of the solvent for any new impurities or degradation of the solvent components.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2020

Solvent Hold Tank Sample Results for MCU-19-557- 558-559 (July 2019), MCU-19-560-561-562 (August 2019), MCU-19-566-567-568 (September 2019) (Quarterly Report)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2020

Solvent Hold Tank Sample Results for MCU-19-569-571 (December 2019), MCU-20-1-28 (January 2020), and MCU-20-29-30-31 (June 2020) (Quarterly Report)

A trend summary of three Solvent Hold Tank (SHT) monthly samples MCU-19-569-570-571 (December 2019), MCU-20-1-28 (January 2020), and MCU-20-29-30-31 (June 2020) are reported. Most of the conclusions are based on the June 2020 SHT sample (MCU-20-29-30-31). Analyses of the June 2020 SHT sample indicated that the Modifier (Cs-7SB) and the Extractant (MaxCalix) concentrations were above their nominal recommended concentrations (169,000 mg/L and 46,900 mg/L respectively) by 6% and 7% respectively. The Suppressor (N,N’,N”–tris(3,7-dimethyloctyl)guanidine or TiDG) concentration has remained at 833 ± 16 mg/L, but it is above the minimum recommended concentration (479 mg/L). The Semi-Volatile Organic Analysis (SVOA) and FT-HNMR did not detect any organic impurities. Another impurity observed in the samples was mercury. Based on the June 2020 SHT sample, up to 23 ± 5 micrograms of mercury per gram of solvent (or 19 ± 4 mg/L) was detected. The mercury concentration steadily decreased from 34.8 ug/g solvent (or 21.9 mg/L) in the December 2019 SHT sample to 34.8 ug/g solvent (or 19 mg/L) in the June 2020 SHT sample. The gamma concentration (~3.33E4 dpm/mL) measured in the June 2020 SHT samples was consistent with previous values observed when MCU was idle (for example, between February 2017 and August 2017). If additional SHT samples become available, the laboratory will continue to monitor the quality of the solvent for any new impurities or degradation of the solvent components.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2021

Solvent Hold Tank Sample Results for MCU-20-32-33-34 (Quarterly Report, September 2020)

In late FY13, MCU implemented the Next Generation Solvent (NGS) flow sheet. Facility personnel added a non-radioactive, NGS “cocktail” containing the new Extractant (MaxCalix) and a new Suppressor (TiDG) to the SHT heel to implement the NGS flow sheet. The resulting “blend” solvent (“NGS blend solvent”) is essentially NGS with residual amounts of calix[4]arene-bis(tert-octylbenzo-crown-6) (BOBCalixC6) and trioctylamine (TOA). For process monitoring, SHT samples are sent to Savannah River National Laboratory (SRNL) to examine solvent composition changes over time. With the exception of Isopar™ L which is regularly added to the SHT due to its high vapor pressure, this report shows the cumulative chemical composition data, including impurities like mercury, of the September 2020 (MCU-20-32-33-34) solvent SHT sample. A summary report of the September SHT solvent sample was issued earlier. This report examines the cumulative results from these and several past monthly reports. These samples are intended to verify that the solvent is within the specified composition range. A baseline “scratch” solvent - a scratch solvent is a preparation of all 6 solvent components (this includes the old extractant BoBCalix and the old suppressor TrioctylAmine that remained when solvent was converted to Next Generation Solvent [NGS]) of the composition that approximates the blend of cocktail and heel solvent - was prepared in the lab (September 2018) and used for comparison and evaluation. The results from the analyses are presented in this document. This report is the last characterization analysis of the SHT solvent. MCU entered a lay-up state and about 128 gallons of the SHT solvent were pumped into 55 gallons drums for future disposal.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2025

Time-dependent phenomena in correlated materials

Understanding time-dependent processes and light-matter interaction in strongly correlated materials, and the interplay between electronic, orbital, vibrational, and spin degrees of freedom, is a cornerstone of condensed matter. These mechanisms can be proven by measuring the response of the systems to time-dependent perturbations. The corresponding time scales are dictated by the way light couples to the different excitations, and how these excitations exchange energy and momentum. Our research advances our understanding of these processes, and the interpretation of different equilibrium and time-resolved spectroscopies. Our project encompasses two main themes: (i) developing and refining computational techniques to study non-equilibrium spectroscopies including non-perturbative effects and (ii) applications to non-equilibrium phenomena. We have developed a new computational approach that works directly in the time domain: by including all the degrees of freedom involved in the scattering process (e.g. electrons, photons, neutrons), we solve the time dependent problem: a faithful numerical simulation of the experiment. By measuring the energy and momentum of the outgoing particles, we can extract information about the energy and momentum absorbed by the system. Prior to our work, people attempting to model and calculate non-equilibrium spectral functions relied on a description of the scattering cross section based on a formulation in the frequency domain, a treatment that is extremely cumbersome and complex. Our technique works in and out of equilibrium and can reproduce spectra by several spectroscopic techniques, such as time-resolved photoemission, neutron scattering, Raman, X-ray spectroscopies (RIXS, Auger, XAS, XMCD), and, by not relying on analytical approximations, yields results that reveal novel overlooked transient mechanisms. These tools provide sorely needed intuition for understanding the phenomenology of strongly correlated materials and will help experimentalists in identifying signatures of relevant excitations in pump-probe experiments, such as those conducted in DOE supported facilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2024

Uniaxial stress effect on the electronic structure of quantum materials

Uniaxial stress has proven to be a powerful experimental tuning parameter for effectively controlling lattice, charge, orbital, and spin degrees of freedom in quantum materials. In addition, its ability to manipulate the symmetry of materials has garnered significant attention. Recent technical progress to combine uniaxial stress cells with quantum oscillation and angle-resolved photoemission techniques allowed to study the electronic structure as function of uniaxial stress. This review provides an overview on experimental advancements in methods and examines studies on diverse quantum materials, encompassing the semimetal WTe 2 , the unconventional superconductor Sr 2 RuO 4 , Fe-based superconductors, and topological materials.

36 MATERIALS SCIENCE↗