Welcome to the Frandsen Group!

About the Group

We are an experimental condensed matter physics group focused on investigating the structure and magnetism of fascinating--and often technologically promising--materials, such as superconductors, strongly correlated electron systems, multiferroics, magnetocalorics, molten salts for nuclear reactors, and more. We use beams of neutrons, x-rays, and muons produced at large-scale accelerator facilities to probe the atomic and magnetic correlations in these materials, together with advanced computational modeling to gain quantitative insight into the spatial arrangement of atoms and spins in a given material. Specific techniques include atomic and magnetic pair distribution function (PDF) analysis of neutron/x-ray total scattering data and muon spin relaxation/rotation (μSR). Interested and motivated undergraduate and prospective graduate students are encouraged to reach out to learn more about our research and find opportunities to participate.

Research Projects

Thermoelectrics, Magnetocalorics, and Multiferroics--Oh My!

This project focuses on the connection between the local atomic and magnetic structure and the energy-relevant properties of magnetocaloric, thermoelectric, and multiferroic materials. Magnetocaloric materials exhibit large temperature changes with the application and removal of a magnetic field, offering promising applications in solid-state refrigeration and waste heat harvesting. Thermoelectric materials experience an electrical voltage when subjected to a temperature gradient or vice versa, also providing novel routes for energy-efficient cooling and waste heat harvesting. Multiferroic materials show cross-order coupling between electric polarization and magnetic order, potentially enabling unique functionalities for energy transformation, information science, and signal processing. We are using combined atomic and magnetic pair distribution function analysis, together with muon spin spectroscopy, to establish the local atomic and magnetic structure of representative compounds for these material classes and better understand the origin of their outstanding properties. In the process, we are developing new experimental and computational methods for magnetic pair distribution function analysis, which will be widely applicable to many other materials, as well. Funding: US Department of Energy, Early Career program.

Superconductors, Geometrically Frustrated Magnets, Magnetic Nanoparticles, and More

We maintain broad interest and involvement in structural studies of numerous material systems where knowledge of the local atomic and magnetic structure can add value. We have ongoing projects on iron-based superconductors, geometrically frustrated triangular lattice antiferromagnets, magnetic nanoparticles, Mott insulator systems, high-entropy alloys and oxides, and more. We are always open to collaborations on interesting material systems.

Molten Salts For Improved Nuclear Reactors

Molten salt reactors (MSRs) are a promising nuclear reactor design concept in which molten ionic salts function as the coolant and/or fuel source in the reactor. MSRs have many potential advantages over standard designs in commercial use today, including greatly enhanced safety/security and the ability to produce critical medical radioisotopes in addition to vast amounts of carbon-free electricity. To make MSRs a reality, it is necessary to understand and predict the behavior of the salts in operating conditions. Gaining a detailed knowledge of the local structure of the molten salts on the atomic scale is an essential step in this direction, since the local interactions between constituent atoms determine the macroscopic properties. In this project, we use cutting-edge neutron and x-ray total scattering and computational modeling techniques to establish the structure of relevant molten salts. We work closely with collaborators in BYU Chemical Engineering. Funding: US Department of Energy, Nuclear Energy University Program (pending).

Selected Publications

Control of dopant crystallinity in electrochemically treated cuprate thin films

By B. Frandsen (et al.)

Abstract:

We present a methodology based on ex situ (postgrowth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide La2CuO_4+y grown epitaxially on substrates of isostructural LaSrAlO₄. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and we describe the resulting methodology as a platform to be explored in thin films of other transition-metal oxides.

Incommensurate Magnetism Near Quantum Criticality in CeNiAsO

By Benjamin A. Frandsen (et al.)

Abstract: We report the discovery of incommensurate magnetism near quantum criticality in CeNiAsO through neutron scattering and zero field muon spin rotation. For T < TN1 = 8.7(3) K, a second order phase transition yields an incommensurate spin density with a wave vector k = (0.44(4),0,0). For
T < TN2 = 7.6(3) K, we find coplanar commensurate order with a moment of 0.37(5) μB, reduced to 30% of the saturation moment of the |±1/2⟩ Kramers doublet ground state, which we establish through inelastic neutron scattering. Muon spin rotation in CeNiAs1−x PxO shows the commensurate order only exists for x ≤ 0.1 so we infer the transition at xc = 0.4(1) is between an incommensurate longitudinal spin density wave and a paramagnetic Fermi liquid.

Probing the quantum phase transition in Mott insulator BaCoS₂ tuned by pressure and Ni substitution

By B. A. Frandsen (et al.)

Abstract: We present a muon spin relaxation study of the Mott transition in BaCoS2 using two independent control parameters: (i) pressure p to tune the electronic bandwidth and (ii) Ni substitution x on the Co site to tune the band filling. For both tuning parameters, the antiferromagnetic insulating state first transitions to an antiferromagnetic metal and finally to a paramagnetic metal without undergoing any structural phase transition. BaCoS2 under pressure displays minimal change in the ordered magnetic moment Sord until it collapses abruptly upon entering the antiferromagnetic metallic state at pcr∼1.3GPa. In contrast, Sord in the Ni-doped system Ba(Co1−xNix)S2 steadily decreases with increasing x until the antiferromagnetic metallic region is reached at xcr∼0.22. In both cases, significant phase separation between regions with static magnetic order and paramagnetic/nonmagnetic regions develops when approaching pcr or xcr, and the antiferromagnetic metallic state is characterized by weak, random, static magnetism in a small volume fraction. No dynamical critical behavior is observed near the transition for either tuning parameter. These results demonstrate that the quantum evolution of both the bandwidth- and filling-controlled metal-insulator transition at zero temperature proceeds as a first-order transition. This behavior is common to magnetic Mott transitions in RNiO3 and V2O3, which are accompanied by structural transitions without the formation of an antiferromagnetic metal phase.

Ba(Zn,Co)₂As₂: A diluted ferromagnetic semiconductor with n-type carriers and isostructural to 122 iron-based superconductors

By Benjamin A. Frandsen (et al.)

Abstract:

We report the successful synthesis of a 122 diluted ferromagnetic semiconductor with n-type carriers, Ba(Zn,Co)₂As₂. Magnetization measurements show that the ferromagnetic transition occurs up to TC ∼ 45 K. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation, a volume-sensitive magnetic probe, we have also confirmed that the ferromagnetism in Ba(Zn,Co)₂As₂ is intrinsic and the internal field is static.

Widespread orthorhombic fluctuations in the (Sr,Na)Fe₂As₂ family of superconductors

By Benjamin A. Frandsen (et al.)

Abstract: We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr_1−xNa_xFe₂As₂. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels (x ≳ 0.48, near optimal superconductivity) and high temperatures (above room temperature for x = 0, decreasing to 150 K for x = 0.48), with a typical length scale of 1–3 nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.

Gradual enhancement of stripe-type antiferromagnetism in the spin-ladder material BaFe₂S₃ under pressure

By Benjamin A. Frandsen (et al.)

Abstract: We report pressure-dependent neutron diffraction and muon spin relaxation/rotation measurements combined with first-principles calculations to investigate the structural, magnetic, and electronic properties of BaFe₂S₃ under pressure. The experimental results reveal a gradual enhancement of the stripe-type ordering temperature with increasing pressure up to 2.6 GPa and no observable change in the size of the ordered moment. The ab initio calculations suggest that the magnetism is highly sensitive to the Fe-S bond lengths and angles, clarifying discrepancies with previously published results. In contrast to our experimental observations, the calculations predict a monotonic reduction of the ordered moment with pressure. We suggest that the robustness of the stripe-type antiferromagnetism is due to strong electron correlations not fully considered in the calculations.