arXiv subjects

Tor Pedersen

Publications and source records attributed to Tor Pedersen.

4 recordsLinked to original sources

arXiv2025

Layer controlled orbital selective Mott transition in monolayer nickelate

Dimensionality and electronic correlations are crucial elements of many quantum material properties. An example is the change of the electronic structure accompanied by the loss of quasiparticles when a metal is reduced from three dimensions to a lower dimension, where the Coulomb interaction between carriers becomes poorly screened. Here, using angle-resolved photoemission spectroscopy (ARPES), we report an orbital-selective decoherence of spectral density in the perovskite nickelate LaNiO3 towards the monolayer limit. The spectral weight of the dz2 band vanishes much faster than that of the dx2-y2 band as the thickness of the LaNiO3 layer is decreased to a single unit cell, indicating a stronger correlation effect for the former upon dimensional confinement. Dynamical mean-field theory (DMFT) calculations show an orbital-selective Mott transition largely due to the localization of dz2 electrons along the c axis in the monolayer limit. This orbital-selective correlation effect underpins many macroscopic properties of nickelates, such as metal-to-insulator transition and superconductivity, where most theories are built upon a dx2-y2-dz2 two-band model.

cond-mat.str-el↗

arXiv2022

Spectral Evidence for Unidirectional Charge Density Wave in Detwinned BaNi$_2$As$_2$

The emergence of unconventional superconductivity in proximity to intertwined electronic orders is especially relevant in the case of iron-based superconductors. Such order consists of an electronic nematic order and a spin density wave in these systems. BaNi$_2$As$_2$, like its well-known iron-based analog BaFe$_2$As$_2$, also hosts a symmetry-breaking structural transition that is coupled to a unidirectional charge density wave (CDW), providing a novel platform to study intertwined orders. Here, through a systematic angle-resolved photoemission spectroscopy study combined with a detwinning $B_1g$ uniaxial strain, we identify distinct spectral evidence of band evolution due to the structural transition as well as CDW-induced band folding. In contrast to the nematicity and spin density wave in BaFe$_2$As$_2$, the structural and CDW order parameters in BaNi$_2$As$_2$ are observed to be strongly coupled and do not separate in the presence of uniaxial strain. Our measurements point to a likely lattice origin of the CDW in BaNi$_2$As$_2$.

cond-mat.str-el↗

arXiv2022

A Neural Network Model of a Quasi-Periodic Elliptically Polarizing Undulator in Universal Mode

Machine learning has recently been applied and deployed at several light source facilities in the domain of Accelerator Physics. We introduce an approach based on machine learning to produce a fast-executing model that predicts the polarization and energy of the radiated light produced at an insertion device. This paper demonstrates how a machine learning model can be trained on simulated data and later calibrated to a smaller, limited measured data set, a technique referred to as transfer learning. This result will enable users to efficiently determine the insertion device settings for achieving arbitrary beam characteristics.

physics.acc-ph↗

arXiv2017

Optimization and Testing of a novel Photoacoustic Spectromicroscopy Cell in the Mid-IR Spectrum

We have designed, constructed and optimized a novel cell for micro photoacoustic (microPA) spectroscopy (microPAS) as part of a collaboration between the Canadian Light Source and Natural Resources Canada. The cell allows inspection of micrometric samples under a visible light microscope, together with measurement of mid-infrared absorption spectra. Our goal is to introduce a methodology enabling mid-IR investigation of micrometric samples that are inaccessible by other measurement configurations; this includes opaque samples, as well as those that are prone to scattering or possess irregular topography. We have achieved this objective, building and testing a prototype PA spectromicroscopy cell using several different optical sources.

physics.ins-det↗