# Publications by Franziska Kirschner

## Spin Jahn-Teller antiferromagnetism in CoTi$_2$O$_5$

Physical Review B American Physical Society 99 (2019) 064403-

F Kirschner, R Johnson, F Lang, DD Khalyavin, P Manuel, T Lancaster, D Prabhakaran, S Blundell

We have used neutron powder diffraction to solve the magnetic structure of orthorhombic CoTi$_2$O$_5$, showing that the long-range ordered state below 26 K identified in our muon-spin rotation experiments is antiferromagnetic with propagation vector ${\bf k}=(\pm \frac{1}{2}, \frac{1}{2}, 0)$ and moment of 2.72(1)$\mu_{\rm B}$ per Co$^{2+}$ ion. This long range magnetic order is incompatible with the experimentally determined crystal structure because the imposed symmetry completely frustrates the exchange coupling. We conclude that the magnetic transition must therefore be associated with a spin Jahn-Teller effect which lowers the structural symmetry and thereby relieves the frustration. These results show that CoTi$_2$O$_5$ is a highly unusual low symmetry material exhibiting a purely spin-driven lattice distortion critical to the establishment of an ordered magnetic ground state.

## Static and fluctuating magnetic moments in the ferroelectric metal LiOsO3

JPS Conference Proceedings Physical Society of Japan 日本物理学会 21 (2018) 1-6

F Kirschner, F Lang, FL Pratt, T Lancaster, Y Shi, Y Guo, A Boothroyd, S Blundell

LiOsO3 is the first example of a new class of material called a ferroelectric metal. We performed zero-field and longitudinal-field μSR, along with a combination of electronic structure and dipole field calculations, to determine the magneticground state of LiOsO3. We find that the sample contains both static Li nuclear moments and dynamic Os electronic moments. Below ≈0.7 K, the fluctuations of the Os moments slow down, though remain dynamic down to 0.08 K. We expect this could result in a frozen-out, disordered ground state at even lower temperatures.

## Nodal multigap superconductivity in KCa2Fe4As4F2

Physical Review B American Physical Society 97 (2018) 1-6

M Smidman, F Kirschner, DT Adroja, AD Hillier, F Lang, Z-C Zhang, G-H Cao, S Blundell

We find evidence that the newly discovered Fe-based superconductor KCa2Fe4As4F2 (Tc = 33.36(7) K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation (μSR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband s-wave model, yet are well described by two-gap models with line nodes on either one or both of the gaps. Meanwhile the zero-field μSR results indicate a lack of time reversal symmetry breaking in the superconducting state, but suggest the presence of magnetic fluctuations. These results demonstrate a different route for realizing nodal superconductivity in iron-based superconductors. Here the gap structure is drastically altered upon replacing one of the spacer layers, indicating the need to understand how the pairing state is tuned by changes of the asymmetry between the pnictogens located either side of the Fe planes.

## LaSr3NiRuO4H4: a 4d transition-metal oxide-hydride containing metal hydride sheets

Angewandte Chemie International Edition Wiley 57 (2018) 5025-5028

L Jin, M Lane, D Zeng, F Kirschner, F Lang, P Manuel, S Blundell, J McGrady, M Hayward

We report the synthesis of the first 4d transition metal oxide-hydride, LaSr3NiRuO4H4, prepared via topochemical anion exchange. Neutron diffraction data show that the hydride ions occupy the equatorial anion sites in the host lattice and as a result the Ru and Ni cations are located in a plane containing only hydride ligands - a unique structural feature with obvious parallels to the CuO2 sheets present in the superconducting cuprates. DFT calculations confirm the presence of S = ½ Ni1+ and S = 0, Ru2+ centres, but neutron diffraction and μSR data show no evidence for long-range magnetic order between the Ni centres down to 1.8 K. The observed weak inter-cation magnetic coupling can be attributed to poor overlap between Ni 3d_(z^2 ) and H 1s in the super-exchange pathways.

## Observation of a crossover from nodal to gapped superconductivity in LuxZr1-xB12

Physical Review B American Physical Society 98 (2018) 094505-

FKK Kirschner, NE Sluchanko, VB Filipov, FL Pratt, C Baines, NY Shitsevalova, SJ Blundell

We have determined the superconducting and magnetic properties of four samples of LuxZr1-xB12 (x=0.04, 0.07, 0.17, and 0.8) using muon spin rotation (μSR) and magnetometry measurements. We observed a strong magnetic signal in both the μSR and magnetometry data in one sample (x=0.07), likely caused by the formation of static moments of size ≈1μB due to a clustering effect of the Lu3+ ions. In all other samples, we find only a small magnetic signal in the μSR data thought to originate from boron nuclei in the B12 cages. The superconductivity is found to evolve with x, with a decrease in x resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find evidence for the formation of nodes in the superconducting gap for x≤0.17, providing a potential new example of an s-to-s+d-wave crossover in a superconductor.

## Two-gap superconductivity with line nodes in CsCa2Fe4As4F2

Physical Review B American Physical Society 97 (2018) 060506(R)-

F Kirschner, DT Adroja, Z-W Wang, F Lang, M Smidman, PJ Baker, G-H Cao, SJ Blundell

We report the results of a muon-spin rotation ($\mu$SR) experiment to determine the superconducting ground state of the iron-based superconductor $CsCa_2Fe_4As_4F_2$ with $T_{\rm c} \approx 28.3\,K$. This compound is related to the fully-gapped superconductor $CaCsFe_4As_4$, but here the Ca-containing spacer layer is replaced with one containing $Ca_2F_2$. The temperature evolution of the penetration depth strongly suggests the presence of line nodes and is best modelled by a system consisting of both an $s-$ and a $d-$wave gap. We also find a potentially magnetic phase which appears below $\approx 10\,K$ but does not appear to compete with the superconductivity. This compound contains the largest alkali atom in this family of superconductors and our results yield a value for the in-plane penetration depth of $\lambda_{ab}(T=0)=423(5)\,nm$.

## Proposal for the detection of magnetic monopoles in spin ice via nanoscale magnetometry

Physical Review B American Physical Society 97 (2018) 140402(R)-

FKK Kirschner, F Flicker, A Yacoby, NY Yao, S Blundell

We present a proposal for applying nanoscale magnetometry to the search for magnetic monopoles in the spin ice materials holmium and dysprosium titanate. Employing Monte Carlo simulations of the dipolar spin ice model, we find that when cooled to below 1.5K these materials exhibit a sufficiently low monopole density to enable the direct observation of magnetic fields from individual monopoles. At these temperatures we demonstrate that noise spectroscopy can capture the intrinsic fluctuations associated with monopole dynamics, allowing one to isolate the qualitative e↵ects associated with both the Coulomb interaction between monopoles and the topological constraints implied by Dirac strings. We describe in detail three di↵erent nanoscale magnetometry platforms (muon spin rotation, nitrogen vacancy defects, and nanoSQUID arrays) that can be used to detect monopoles in these experiments, and analyze the advantages of each.

## Multigap superconductivity in RbCa2Fe4As4F2 investigated using μSR measurements

Journal of the Physical Society of Japan Physical Society of Japan 87 (2018) 124705

DT Adroja, FKK Kirschner, F Lang, M Smidman, AD Hillier, Z-C Wang, G-H Cao, GBG Stenning, S Blundell

The superconducting properties of the recently discovered high-Tc superconductor RbCa2Fe4As4F2 with double Fe2As2 layers and Tc ≈ 30 K have been investigated using magnetization, heat capacity, transverse-field (TF) and zero-field (ZF) muon-spin rotation/relaxation (μSR) measurements. Low field magnetization and heat capacity (Cp) measurements reveal an onset of bulk superconductivity with Tc ∼ 30.0(4) K, where there is a jump at Tc of ΔCp/Tc = 94.6 (mJ/mole-K2). The temperature dependence of the magnetic penetration depth obtained from transverse-field (TF) μSR is better described by the two-gap s + s and s + d models than the single gap s- or d-wave scenarios. The presence of two superconducting gaps in RbCa2Fe4As4F2 is consistent with the multigap superconductivity observed in other Fe-based superconductors, including ACa2Fe4As4F2 (A = K and Cs). From the TF-μSR study, an in-plane magnetic penetration depth of λab(0) = 231.5(3) nm, a superconducting carrier density ns = 1.29(1) × 1027 m−3, and an effective mass of the carriers of m* = 2.45(1)me are estimated. Zero-field μSR measurements do not reveal clear signs of time reversal symmetry breaking below Tc, but the temperature dependence of the relaxation of the asymmetry between 150 and 1.2 K may indicate the presence of spin-fluctuations.

## Coexistence of magnetism and superconductivity in separate layers of the iron-based superconductor

Physical Review B American Physical Society 95 (2017) 134419-

CV Topping, FKK Kirschner, S Blundell, PJ Baker, DN Woodruff, F Schild, H Sun, SJ Clarke

The magnetic properties attributed to the hydroxide layer of Li1-xFex(OH)Fe1-ySe have been elucidated by the study of superconducting and nonsuperconducting members of this family of compounds. Both ac magnetometry and muon spin relaxation measurements of nonsuperconductors find a magnetic state existing below ≈10 K which exhibits slow relaxation of magnetization. This magnetic state is accompanied by a low-temperature heat capacity anomaly present in both superconducting and nonsuperconducting variants suggesting that the magnetism persists into the superconducting state. The estimated value of magnetic moment present within the hydroxide layer supports a picture of a glassy magnetic state, probably comprising clusters of iron ions of varying cluster sizes distributed within the lithium hydroxide layer.

## Robustness of superconductivity to competing magnetic phases in tetragonal FeS

Physical Review B American Physical Society 94 (2016)

FKK Kirschner, F Lang, CV Topping, PJ Baker, FL Pratt, SE Wright, DN Woodruff, SJ Clarke, S Blundell

We have determined the superconducting and magnetic properties of a hydrothermally synthesized powder sample of tetragonal FeS using muon spin rotation (μSR). The superconducting properties are entirely consistent with those of a recently published study, showing fully gapped behavior and giving a penetration depth of λab=204(3) nm. However, our zero-fieldμSR data are rather different and indicate the presence of a small, nonsuperconducting magnetic phase within the sample. These results highlight that sample-to-sample variations in magnetism can arise in hydrothermally prepared phases, but interestingly the superconducting behavior is remarkably insensitive to these variations.