# Publications

## Comparative study of the magnetic properties of La3Ni2B'O9 for B' = Nb, Ta or Sb

Journal of Solid State Chemistry Elsevier 258 (2017) 825-834

C-M Chin, P Battle, SJ Blundell, EC Hunter, F Lang, M Hendrickx, R Paria Sena, J Hadermann

<p>Polycrystalline samples of La<sub>3</sub>Ni<sub>2</sub>NbO<sub>9</sub> and La<sub>3</sub>Ni<sub>2</sub>TaO<sub>9</sub> have been characterised by X-ray and neutron diffraction, electron microscopy, magnetometry and muon spin relaxation (µSR); the latter technique was also applied to La<sub>3</sub>Ni<sub>2</sub>SbO<sub>9</sub>. On the length scale of a neutron diffraction experiment, the six-coordinate sites of the monoclinic perovskite structure are occupied in a 1:1 ordered manner by Ni and a random ⅓Ni/⅔B’ mixture. Electron microscopy demonstrated that this 1:1 ordering is maintained over microscopic distances, although diffuse scattering indicative of short-range ordering on the mixed site was observed. No magnetic Bragg scattering was observed in neutron diffraction patterns collected from La<sub>3</sub>Ni<sub>2</sub>B’O<sub>9 </sub>(B’ = Nb or Ta) at 5 K although in each case µSR identified the presence of static spins below 30 K. Magnetometry showed that La<sub>3</sub>Ni<sub>2</sub>NbO<sub>9</sub> behaves as a spin glass below 29 K but significant short-range interactions are present in La<sub>3</sub>Ni<sub>2</sub>TaO<sub>9</sub> below 85 K. The contrasting properties of these compounds are discussed in terms of their microstructure.</p>

## Endohedral Metallofullerene as Molecular High Spin Qubit: Diverse Rabi Cycles in Gd2@C79N.

Journal of the American Chemical Society 140 (2018) 1123-1130

Z Hu, B-W Dong, Z Liu, J-J Liu, J Su, C Yu, J Xiong, D-E Shi, Y Wang, B-W Wang, A Ardavan, Z Shi, S-D Jiang, S Gao

An anisotropic high-spin qubit with long coherence time could scale the quantum system up. It has been proposed that Grover's algorithm can be implemented in such systems. Dimetallic aza[80]fullerenes M2@C79N (M = Y or Gd) possess an unpaired electron located between two metal ions, offering an opportunity to manipulate spin(s) protected in the cage for quantum information processing. Herein, we report the crystallographic determination of Gd2@C79N for the first time. This molecular magnet with a collective high-spin ground state (S = 15/2) generated by strong magnetic coupling (JGd-Rad = 350 ± 20 cm-1) has been unambiguously validated by magnetic susceptibility experiments. Gd2@C79N has quantum coherence and diverse Rabi cycles, allowing arbitrary superposition state manipulation between each adjacent level. The phase memory time reaches 5 μs at 5 K by dynamic decoupling. This molecule fulfills the requirements of Grover's searching algorithm proposed by Leuenberger and Loss.

## Low-field spin dynamics of Cr7Ni and Cr7Ni-Cu-Cr7Ni molecular rings as detected by mu SR

PHYSICAL REVIEW B 96 (2017) ARTN 184403

S Sanna, P Arosio, L Bordonali, F Adelnia, M Mariani, E Garlatti, C Baines, A Amato, KPV Sabareesh, G Timco, REP Winpenny, SJ Blundell, A Lascialfari

## 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.

## Room-temperature helimagnetism in FeGe thin films

Scientific Reports Nature Publishing Group 7 (2017) 123

S Zhang, I Stasinopoulos, T Lancaster, F Xiao, A Bauer, F Rucker, AA Baker, AI Figueroa, Z Salman, FL Pratt, SJ Blundell, T Prokscha, A Suter, J Waizner, M Garst, D Grundler, G van der Laan, C Pfleiderer, T Hesjedal

Chiral magnets are promising materials for the realisation of high-density and low-power spintronic memory devices. For these future applications, a key requirement is the synthesis of appropriate materials in the form of thin films ordering well above room temperature. Driven by the Dzyaloshinskii-Moriya interaction, the cubic compound FeGe exhibits helimagnetism with a relatively high transition temperature of 278K in bulk crystals. We demonstrate that this temperature can be enhanced significantly in thin films. Using x-ray spectroscopic and ferromagnetic resonance techniques, we provide unambiguous experimental evidence for long-wavelength helimagnetic order at room temperature and magnetic properties similar to the bulk material. We obtain αintr = 0:0036 ± 0:0003 at 310K for the intrinsic damping parameter. We probe the dynamics of the system by means of muon-spin rotation, indicating that the ground state is reached via a freezing out of slow dynamics. Our work paves the way towards the fabrication of thin films of chiral magnets that host certain spin whirls, so-called skyrmions, at room temperature and potentially offer integrability into modern electronics.

## Crystal structure and magnetic modulation in beta-Ce2O2FeSe2

PHYSICAL REVIEW MATERIALS 1 (2017) ARTN 034403

C-H Wang, CM Ainsworth, SD Champion, GA Stewart, MC Worsdale, T Lancaster, SJ Blundell, HEA Brand, JSO Evans

## Quantum Griffiths Phase Inside the Ferromagnetic Phase of Ni_{1-x}V_{x}.

Physical review letters 118 (2017) 267202-267202

R Wang, A Gebretsadik, S Ubaid-Kassis, A Schroeder, T Vojta, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Möller, K Page

We study by means of bulk and local probes the d-metal alloy Ni_{1-x}V_{x} close to the quantum critical concentration, x_{c}≈11.6%, where the ferromagnetic transition temperature vanishes. The magnetization-field curve in the ferromagnetic phase takes an anomalous power-law form with a nonuniversal exponent that is strongly x dependent and mirrors the behavior in the paramagnetic phase. Muon spin rotation experiments demonstrate inhomogeneous magnetic order and indicate the presence of dynamic fluctuating magnetic clusters. These results provide strong evidence for a quantum Griffiths phase on the ferromagnetic side of the quantum phase transition.

## Local magnetism and spin dynamics of the frustrated honeycomb rhodate Li2RhO3

PHYSICAL REVIEW B 96 (2017) ARTN 094432

P Khuntia, S Manni, FR Foronda, T Lancaster, SJ Blundell, P Gegenwart, M Baenitz

## Strong coupling of microwave photons to antiferromagnetic fluctuations in an organic magnet

Physical Review Letters American Physical Society 119 (2017) 147701-

M Mergenthaler, J Liu, J Le Roy, N Ares, A Thompson, L Bogani, F Luis, S Blundell, T Lancaster, A Ardavan, GAD Briggs, PJ Leek, E Laird

Coupling between a crystal of di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH) radicals and a superconducting microwave resonator is investigated in a circuit quantum electrodynamics (cQED) architecture. The crystal exhibits paramagnetic behavior above 4 K, with antiferromagnetic correlations appearing below this temperature, and we demonstrate strong coupling at base temperature. The magnetic resonance acquires a field angle dependence as the crystal is cooled down, indicating anisotropy of the exchange interactions. These results show that multi-spin modes in organic crystals are suitable for cQED, offering a platform for their coherent manipulation. They also utilize the cQED architecture as a way to probe spin correlations at low temperature.

## Discovery of orbital-selective Cooper pairing in FeSe.

Science (New York, N.Y.) 357 (2017) 75-80

PO Sprau, A Kostin, A Kreisel, AE Böhmer, V Taufour, PC Canfield, S Mukherjee, PJ Hirschfeld, BM Andersen, JCS Davis

The superconductor iron selenide (FeSe) is of intense interest owing to its unusual nonmagnetic nematic state and potential for high-temperature superconductivity. But its Cooper pairing mechanism has not been determined. We used Bogoliubov quasiparticle interference imaging to determine the Fermi surface geometry of the electronic bands surrounding the Γ = (0, 0) and X = (π/aFe, 0) points of FeSe and to measure the corresponding superconducting energy gaps. We show that both gaps are extremely anisotropic but nodeless and that they exhibit gap maxima oriented orthogonally in momentum space. Moreover, by implementing a novel technique, we demonstrate that these gaps have opposite sign with respect to each other. This complex gap configuration reveals the existence of orbital-selective Cooper pairing that, in FeSe, is based preferentially on electrons from the d yz orbitals of the iron atoms.

## Quantum-critical spin dynamics in a Tomonaga-Luttinger liquid studied with muon-spin relaxation

Physical Review B American Physical Society 95 (2017)

JS Möller, T Lancaster, S Blundell, FL Pratt, PJ Baker, F Xiao, RC Williams, W Hayes, MM Turnbull, CP Landee

We demonstrate that quantum-critical spin dynamics can be probed in high magnetic fields using muon-spin relaxation (μ+SR). Our model system is the strong-leg spin ladder bis(2,3-dimethylpyridinium) tetrabromocuprate (DIMPY). In the gapless Tomonaga-Luttinger liquid phase we observe finite-temperature scaling of the μ+SR 1/T1 relaxation rate which allows us to determine the Luttinger parameter K. We discuss the benefits and limitations of local probes compared with inelastic neutron scattering.

## Orbital superconductivity, defects, and pinned nematic fluctuations in the doped iron chalcogenide FeSe0.45Te0.55

Physical Review B 96 (2017)

S Sarkar, J Van Dyke, PO Sprau, F Massee, U Welp, WK Kwok, JCS Davis, DK Morr

© 2017 American Physical Society. We demonstrate that the differential conductance, dI/dV, measured via spectroscopic imaging scanning tunneling microscopy in the doped iron chalcogenide FeSe0.45Te0.55, possesses a series of characteristic features that allow one to extract the orbital structure of the superconducting gaps. This yields nearly isotropic superconducting gaps on the two holelike Fermi surfaces, and a strongly anisotropic gap on the electronlike Fermi surface. Moreover, we show that the pinning of nematic fluctuations by defects can give rise to a dumbbell-like spatial structure of the induced impurity bound states, and explains the related C2 symmetry in the Fourier transformed differential conductance.

## Orbital selective pairing and gap structures of iron-based superconductors

Physical Review B 95 (2017)

A Kreisel, BM Andersen, PO Sprau, A Kostin, JCS Davis, PJ Hirschfeld

© 2017 American Physical Society. We discuss the influence on spin-fluctuation pairing theory of orbital selective strong correlation effects in Fe-based superconductors, particularly Fe chalcogenide systems. We propose that a key ingredient for an improved itinerant pairing theory is orbital selectivity, i.e., incorporating the reduced coherence of quasiparticles occupying specific orbital states. This modifies the usual spin-fluctuation theory via suppression of pair scattering processes involving those less coherent states and results in orbital selective Cooper pairing of electrons in the remaining states. We show that this paradigm yields remarkably good agreement with the experimentally observed anisotropic gap structures in both bulk and monolayer FeSe, as well as LiFeAs, indicating that orbital selective Cooper pairing plays a key role in the more strongly correlated iron-based superconductors.

## The spin resonance clock transition of the endohedral fullerene $^{15}\mathrm{N@C}_{60}$

Physical Review Letters American Physical Society 119 (2017) 140801-

RT Harding, S Zhou, J Zhou, T Lindvall, WK Myers, A Ardavan, GAD Briggs, K Porfyrakis, EA Laird

The endohedral fullerene $^{15}\mathrm{N@C}_{60}$ has narrow electron paramagnetic resonance lines which have been proposed as the basis for a condensed-matter portable atomic clock. We measure the low-frequency spectrum of this molecule, identifying and characterizing a clock transition at which the frequency becomes insensitive to magnetic field. We infer a linewidth at the clock field of 100 kHz. Using experimental data, we are able to place a bound on the clock's projected frequency stability. We discuss ways to improve the frequency stability to be competitive with existing miniature clocks.

## La2SrCr2O7: Controlling the tilting distortions of n = 2 Ruddlesden-Popper phases through A-site cation order

Inorganic Chemistry American Chemical Society 55 (2016) 8951–8960-

R Zhang, BM Abbett, G Read, F Lang, T Lancaster, TT Tran, PS Halasyamani, SJ Blundell, NA Benedek, M Hayward

Structural characterization by neutron diffraction, supported by magnetic, SHG, and μ(+)SR data, reveals that the n = 2 Ruddlesden-Popper phase La2SrCr2O7 adopts a highly unusual structural configuration in which the cooperative rotations of the CrO6 octahedra are out of phase in all three Cartesian directions (ΦΦΦz/ΦΦΦz; a(-)a(-)c(-)/a(-)a(-)c(-)) as described in space group A2/a. First-principles DFT calculations indicate that this unusual structural arrangement can be attributed to coupling between the La/Sr A-site distribution and the rotations of the CrO6 units, which combine to relieve the local deformations of the chromium-oxygen octahedra. This coupling suggests new chemical "handles" by which the rotational distortions or A-site cation order of Ruddlesden-Popper phases can be directed to optimize physical behavior. Low-temperature neutron diffraction data and μ(+)SR data indicate La2SrCr2O7 adopts a G-type antiferromagnetically ordered state below TN ∼ 260 K.

## 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.

## The parent Li(OH)FeSe phase of lithium iron hydroxide selenide superconductors

Inorganic Chemistry American Chemical Society 55 (2016) 9886–9891-

S Clarke, SJ Blundell, F Schild, DN Woodruff, SJ Cassidy, JN Blandy, AL Thompson, CV Topping

Lithiation of hydrothermally synthesized Li1-xFex(OH)Fe1–ySe turns on high temperature superconductivity when iron ions are displaced from the hydroxide layers by reductive lithiation to fill the vacancies in the iron selenide layers. Further lithiation results in reductive iron extrusion from the hydroxide layers which turns off superconductivity again as the stoichiometric composition Li(OH)FeSe is approached. The results demonstrate the twin requirements of stoichiometric FeSe layers and reduction of Fe below the +2 oxidation state as found in several iron selenide superconductors.

## Commensurate 4a0-period charge density modulations throughout the Bi<inf>2</inf>Sr<inf>2</inf>CaCu<inf>2</inf>O<inf>8+x</inf>pseudogap regime

Proceedings of the National Academy of Sciences of the United States of America 113 (2016) 12661-12666

A Mesaros, K Fujita, SD Edkins, MH Hamidian, H Eisaki, SI Uchida, JCS Davis, MJ Lawler, EA Kim

Theories based upon strong real space (r-space) electron-electron interactions have long predicted that unidirectional charge density modulations (CDMs) with four-unit-cell (4a0) periodicity should occur in the hole-doped cuprate Mott insulator (MI). Experimentally, however, increasing the hole density p is reported to cause the conventionally defined wavevector QA of the CDM to evolve continuously as if driven primarily by momentum-space (k-space) effects. Here we introduce phase-resolved electronic structure visualization for determination of the cuprate CDM wavevector. Remarkably, this technique reveals a virtually doping-independent locking of the local CDM wavevector at, Q0,=2π=4a0throughout the underdoped phase diagram of the canonical cuprate Bi2Sr2CaCu2O8. These observations have significant fundamental consequences because they are orthogonal to a k-space (Fermi-surface)-based picture of the cuprate CDMs but are consistent with strong-coupling r-space-based theories. Our findings imply that it is the latter that provides the intrinsic organizational principle for the cuprate CDM state.

## Bimetallic MOFs (H3O)x[Cu(MF6)(pyrazine)2]·(4 - x)H2O (M = V^4+, x = 0; M = Ga^3+, x = 1): co-existence of ordered and disordered quantum spins in the V^4+ system.

Chemical Communications Royal Society of Chemistry 52 (2016) 12653-12656

JL Manson, JA Schlueter, KE Garrett, PA Goddard, T Lancaster, JS Möller, S Blundell, AJ Steele, I Franke, FL Pratt, J Singleton, J Bendix, SH Lapidus, M Uhlarz, O Ayala-Valenzuela, RD McDonald, M Gurak, C Baines

The title compounds are bimetallic MOFs containing [Cu(pyz)2]^2+ square lattices linked by MF6^n- octahedra. In each, only the Cu^2+ spins exhibit long-range magnetic order below 3.5 K (M = V^4+) and 2.6 K (M = Ga^3+). The V^4+ spins remain disordered down to 0.5 K.

## Nanoscale depth-resolved polymer dynamics probed by the implantation of low energy muons

Polymer Elsevier 105 (2016) 516-525

FL Pratt, T Lancaster, PJ Baker, SJ Blundell, T Prokscha, E Morenzoni, A Suter, H Assender

The low energy muon (LEM) technique has been used to probe local changes in the dynamical spectrum of thin film polymer samples taking place as a function of the temperature and the implantation depth below the free surface. The studies have been made on samples of polydimethylsiloxane (PDMS) and polybutadiene (PB) using the transverse magnetic field (TF) configuration and diamagnetic probe muons. In PDMS evidence is found for suppression of the glass transition temperature near the surface, along with significantly modified dynamics in the near-surface region as well as at depths significantly below the surface. For PB the LEM technique reveals well-defined layers of dynamical and spatial inhomogeneity at depths of order 0.1–0.2 μm below the free surface. These inhomogeneous regions may be assigned to nanopores produced by solvent streaming during preparation of spin-cast films. A thermal annealing procedure is shown to significantly reduce the thickness of these inhomogeneous layers. These results demonstrate that using LEM in the TF configuration provides a promising new method for studying surface-modified local dynamics of polymers that is also able to reveal nanostructured buried layers in polymer films.