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

Local Orthorhombicity in the Magnetic C₄ Phase of the Hole-Doped Iron-Arsenide Superconductor Sr_1−xNa_xFe₂As₂

By Benjamin A. Frandsen (et al.)

Abstract: We report on temperature-dependent pair distribution function measurements of
Sr
1
−
x
Na
x
Fe
2
As
2
, an iron-based superconductor system that contains a magnetic phase with reentrant tetragonal symmetry, known as the magnetic
C
4
phase. Quantitative refinements indicate that the instantaneous local structure in the
C
4
phase comprises fluctuating orthorhombic regions with a length scale of
∼
2

nm
, despite the tetragonal symmetry of the average static structure. Additionally, local orthorhombic fluctuations exist on a similar length scale at temperatures well into the paramagnetic tetragonal phase. These results highlight the exceptionally large nematic susceptibility of iron-based superconductors and have significant implications for the magnetic
C
4
phase and the neighboring
C
2
and superconducting phases.

Signatures of the topological s⁺⁻ superconducting order parameter in the type-II Weyl semimetal T_d-MoTe₂

By B. A. Frandsen (et al.)

Abstract: In its orthorhombic T d polymorph, MoTe2 is a type-II Weyl semimetal, where the Weyl fermions emerge at the boundary between electron and hole pockets. Non-saturating magnetoresistance and superconductivity were also observed in T d-MoTe2. Understanding the superconductivity in T d-MoTe2, which was proposed to be topologically non-trivial, is of eminent interest. Here, we report high-pressure muon-spin rotation experiments probing the temperature-dependent magnetic penetration depth in T d-MoTe2. A substantial increase of the superfluid density and a linear scaling with the superconducting critical temperature T c is observed under pressure. Moreover, the superconducting order parameter in T d-MoTe2 is determined to have 2-gap s-wave symmetry. We also exclude time-reversal symmetry breaking in the superconducting state with zero-field μSR experiments. Considering the strong suppression of T c in MoTe2 by disorder, we suggest that topologically non-trivial s +− state is more likely to be realized in MoTe2 than the topologically trivial s ++ state.

Pressure tuning of structure, superconductivity, and novel magnetic order in the Ce-underdoped electron-doped cuprate T′−Pr_1.3−xLa_0.7Ce_xCuO₄ (x=0.1)

By B. A. Frandsen (et al.)

Abstract: High-pressure neutron powder diffraction, muon-spin rotation, and magnetization studies of the structural, magnetic, and the superconducting properties of the Ce-underdoped superconducting (SC) electron-doped cuprate system with the
Nd
2
CuO
4
(the so-called
T
′
) structure
T
′
−
Pr
1.3
−
x
La
0.7
Ce
x
CuO
4
with
x
=
0.1
are reported. A strong reduction of the in-plane and out-of-plane lattice constants is observed under pressure. However, no indication of any pressure-induced phase transition from
T
′
to the
K
2
NiF
4
(the so-called T) structure is observed up to the maximum applied pressure of
p
= 11 GPa. Large and nonlinear increase of the short-range magnetic order temperature
T
so
in
T
′
−
Pr
1.3
−
x
La
0.7
Ce
x
CuO
4
(
x
=
0.1
) was observed under pressure. Simultaneous pressure causes a nonlinear decrease of the SC transition temperature
T
c
. All these experiments establish the short-range magnetic order as an intrinsic and competing phase in SC
T
′
−
Pr
1.3
−
x
La
0.7
Ce
x
CuO
4
(
x
=
0.1
). The observed pressure effects may be interpreted in terms of the improved nesting conditions through the reduction of the in-plane and out-of-plane lattice constants upon hydrostatic pressure.

Uniaxial pressure effect on the magnetic ordered moment and transition temperatures in BaFe_2−xT_xAs₂ (T=Co,Ni)

By Benjamin A. Frandsen (et al.)

Abstract: We use neutron diffraction and muon spin relaxation to study the effect of in-plane uniaxial pressure on the antiferromagnetic (AF) orthorhombic phase in
BaFe
2
As
2
and its Co- and Ni-substituted members near optimal superconductivity. In the low-temperature AF ordered state, uniaxial pressure necessary to detwin the orthorhombic crystals also increases the magnetic ordered moment, reaching an 11% increase under 40 MPa for
BaFe
1.9
Co
0.1
As
2
, and a 15% increase for
BaFe
1.915
Ni
0.085
As
2
. We also observe an increase of the AF ordering temperature (
T
N
) of about 0.25 K/MPa in all compounds, consistent with density functional theory calculations that reveal better Fermi surface nesting for itinerant electrons under uniaxial pressure. The doping dependence of the magnetic ordered moment is captured by combining dynamical mean field theory with density functional theory, suggesting that the pressure-induced moment increase near optimal superconductivity is closely related to quantum fluctuations and the nearby electronic nematic phase.

New Fluoride-arsenide Diluted Magnetic Semiconductor (Ba,K)F(Zn,Mn)As with Independent Spin and Charge Doping

By Benjamin Frandsen (et al.)

Abstract: We report the discovery of a new fluoride-arsenide bulk diluted magnetic semiconductor (Ba,K)F(Zn,Mn)As with the tetragonal ZrCuSiAs-type structure which is identical to that of the “1111” iron-based superconductors. The joint hole doping via (Ba,K) substitution & spin doping via (Zn,Mn) substitution results in ferromagnetic order with Curie temperature up to 30 K and demonstrates that the ferromagnetic interactions between the localized spins are mediated by the carriers. Muon spin relaxation measurements confirm the intrinsic nature of the long range magnetic order in the entire volume in the ferromagnetic phase. This is the first time that a diluted magnetic semiconductor with decoupled spin and charge doping is achieved in a fluoride compound. Comparing to the isostructure oxide counterpart of LaOZnSb, the fluoride DMS (Ba,K)F(Zn,Mn)As shows much improved semiconductive behavior that would be benefit for further application developments.

μSR and magnetometry study of superconducting 5% Pt-doped IrTe₂

By B. A. Frandsen (et al.)

Abstract: We present magnetometry and muon spin rotation
(
μ
SR
)
measurements of the superconducting dichalcogenide
Ir
0.95
Pt
0.05
Te
2
. From both sets of measurements, we calculate the penetration depth and thence superfluid density as a function of temperature. The temperature dependence of the superfluid densities from both sets of data indicate fully gapped superconductivity that can be fit to a conventional
s
-wave model and yield fitting parameters consistent with a BCS weak coupling superconductor. We therefore see no evidence for exotic superconductivity in
Ir
0.95
Pt
0.05
Te
2
.