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Contemporary Physics (2015)

SJ Blundell

La2SrCr2O7: Controlling the Tilting Distortions of n = 2 Ruddlesden-Popper Phases through A-Site Cation Order.

Inorganic chemistry 55 (2016) 8951-8960

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

Transverse field muon-spin rotation measurement of the topological anomaly in a thin film of MnSi

PHYSICAL REVIEW B 93 (2016) ARTN 140412

T Lancaster, F Xiao, Z Salman, IO Thomas, SJ Blundell, FL Pratt, SJ Clark, T Prokscha, A Suter, SL Zhang, AA Baker, T Hesjedal

Magnetic ground state of the two isostructual polymeric quantum magnets [Cu(HF2)(pyrazine)(2)]SbF6 and [Co(HF2)(pyrazine)(2)]SbF6 investigated with neutron powder diffraction

PHYSICAL REVIEW B 92 (2015) ARTN 134406

J Brambleby, PA Goddard, RD Johnson, J Liu, D Kaminski, A Ardavan, AJ Steele, SJ Blundell, T Lancaster, P Manuel, PJ Baker, J Singleton, SG Schwalbe, PM Spurgeon, HE Tran, PK Peterson, JF Corbey, JL Manson

Three-terminal graphene single-electron transistor fabricated using feedback-controlled electroburning


P Puczkarski, P Gehring, CS Lau, J Liu, A Ardavan, JH Warner, GAD Briggs, JA Mol

Spectroscopy Methods for Molecular Nanomagnets


ML Baker, SJ Blundell, N Domingo, S Hill

Engineering coherent interactions in molecular nanomagnet dimers


A Ardavan, AM Bowen, A Fernandez, AJ Fielding, D Kaminski, F Moro, CA Muryn, MD Wise, A Ruggi, EJL McInnes, K Severin, GA Timco, CR Timmel, F Tuna, GFS Whitehead, REP Winpenny

Surface acoustic wave devices on bulk ZnO crystals at low temperature


EB Magnusson, BH Williams, R Manenti, M-S Nam, A Nersisyan, MJ Peterer, A Ardavan, PJ Leek

Evidence for magnetic clusters in Ni1-xVx close to the quantum critical concentration


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

Anisotropic local modification of crystal field levels in Pr-based pyrochlores: a muon-induced effect modeled using density functional theory.

Physical review letters 114 (2015) 017602-

FR Foronda, F Lang, JS Möller, T Lancaster, AT Boothroyd, FL Pratt, SR Giblin, D Prabhakaran, SJ Blundell

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound. We demonstrate this effect in some quantum spin ice candidate materials Pr(2)B(2)O(7) (B=Sn, Zr, Hf), where we detect a static distribution of magnetic moments that appears to grow on cooling. Using density functional theory we show how this effect can be explained via a hyperfine enhancement arising from a splitting of the non-Kramers doublet ground states on Pr ions close to the muon, which itself causes a highly anisotropic distortion field. We provide a quantitative relationship between this effect and the measured temperature dependence of the muon relaxation and discuss the relevance of these observations to muon experiments in other magnetic materials.

Robustness of superconductivity to structural disorder in Sr-0.3(NH2)(y)(NH3)(1-y)Fe2Se2

PHYSICAL REVIEW B 92 (2015) ARTN 134517

FR Foronda, S Ghannadzadeh, SJ Sedlmaier, JD Wright, K Burns, SJ Cassidy, PA Goddard, T Lancaster, SJ Clarke, SJ Blundell

Soft chemical control of superconductivity in lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se.

Inorganic chemistry 54 (2015) 1958-1964

H Sun, DN Woodruff, SJ Cassidy, GM Allcroft, SJ Sedlmaier, AL Thompson, PA Bingham, SD Forder, S Cartenet, N Mary, S Ramos, FR Foronda, BH Williams, X Li, SJ Blundell, SJ Clarke

Hydrothermal synthesis is described of layered lithium iron selenide hydroxides Li(1-x)Fe(x)(OH)Fe(1-y)Se (x ∼ 0.2; 0.02 < y < 0.15) with a wide range of iron site vacancy concentrations in the iron selenide layers. This iron vacancy concentration is revealed as the only significant compositional variable and as the key parameter controlling the crystal structure and the electronic properties. Single crystal X-ray diffraction, neutron powder diffraction, and X-ray absorption spectroscopy measurements are used to demonstrate that superconductivity at temperatures as high as 40 K is observed in the hydrothermally synthesized samples when the iron vacancy concentration is low (y < 0.05) and when the iron oxidation state is reduced slightly below +2, while samples with a higher vacancy concentration and a correspondingly higher iron oxidation state are not superconducting. The importance of combining a low iron oxidation state with a low vacancy concentration in the iron selenide layers is emphasized by the demonstration that reductive postsynthetic lithiation of the samples turns on superconductivity with critical temperatures exceeding 40 K by displacing iron atoms from the Li(1-x)Fe(x)(OH) reservoir layer to fill vacancies in the selenide layer.

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

Phys Rev B. Solid State 91 (2015) 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, M Ciomaga Hatnean, DS Keeble, G Balakrishnan

We present the results of transverse field (TF) muon-spin rotation (μ+SR) measurements on Cu2OSeO3, which has a skyrmion-lattice (SL) phase. We measure the response of the TF μ+SR signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale τ>100 ns.

Spin diffusion in the low-dimensional molecular quantum Heisenberg antiferromagnet Cu(pyz)(NO3)(2) detected with implanted muons

PHYSICAL REVIEW B 91 (2015) ARTN 144417

F Xiao, JS Moeller, T Lancaster, RC Williams, FL Pratt, SJ Blundell, D Ceresoli, AM Barton, JL Manson

Magnetostructural relationship in the tetrahedral spin-chain oxide CsCoO2

PHYSICAL REVIEW B 91 (2015) ARTN 024419

NZ Ali, RC Williams, F Xiao, SJ Clark, T Lancaster, SJ Blundell, DV Sheptyakov, M Jansen

A spin-frustrated trinuclear copper complex based on triaminoguanidine with an energetically well-separated degenerate ground state.

Inorganic chemistry 54 (2015) 3432-3438

ET Spielberg, A Gilb, D Plaul, D Geibig, D Hornig, D Schuch, A Buchholz, A Ardavan, W Plass

We present the synthesis and crystal structure of the trinuclear copper complex [Cu3(saltag)(bpy)3]ClO4·3DMF [H5saltag = tris(2-hydroxybenzylidene)triaminoguanidine; bpy = 2,2'-bipyridine]. The complex crystallizes in the trigonal space group R3̅, with all copper ions being crystallographically equivalent. Analysis of the temperature dependence of the magnetic susceptibility shows that the triaminoguanidine ligand mediates very strong antiferromagnetic interactions (JCuCu = -324 cm(-1)). Detailed analysis of the magnetic susceptibility and magnetization data as well as X-band electron spin resonance spectra, all recorded on both powdered samples and single crystals, show indications of neither antisymmetric exchange nor symmetry lowering, thus indicating only a very small splitting of the degenerate S = (1)/2 ground state. These findings are corroborated by density functional theory calculations, which explain both the strong isotropic and negligible antisymmetric exchange interactions.

Supercooled spin liquid state in the frustrated pyrochlore Dy<inf>2</inf>Ti<inf>2</inf>O<inf>7</inf>

Proceedings of the National Academy of Sciences of the United States of America 112 (2015) 8549-8554

ER Kassner, AB Eyvazov, B Pichler, TJS Munsie, HA Dabkowska, GM Luke, JCS Davis

© 2015, National Academy of Sciences. All rights reserved. A "supercooled" liquid develops when a fluid does not crystallize upon cooling below its ordering temperature. Instead, the microscopic relaxation times diverge so rapidly that, upon further cooling, equilibration eventually becomes impossible and glass formation occurs. Classic supercooled liquids exhibit specific identifiers including microscopic relaxation times diverging on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for the dielectric function ε(ω, T), and a general Kohlrausch-Williams-Watts (KWW) form for time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of interest because its frustrated magnetic interactions may, in theory, lead to highly exotic magnetic fluids. However, its true magnetic state at low temperatures has proven very difficult to identify unambiguously. Here, we introduce high-precision, boundary-free magnetization transport techniques based upon toroidal geometries and gain an improved understanding of the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7. We demonstrate a virtually universal HN form for the magnetic susceptibility χ(ω, T), a general KWW form for the real-time magnetic relaxation, and a divergence of the microscopic magnetic relaxation rates with the VTF trajectory. Low-temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled magnetic liquid. One implication is that this translationally invariant lattice of strongly correlated spins may be evolving toward an unprecedented magnetic glass state, perhaps due to many-body localization of spin.

Identifying the 'fingerprint' of antiferromagnetic spin fluctuations in iron pnictide superconductors

Nature Physics 11 (2015) 177-182

MP Allan, K Lee, AW Rost, MH Fischer, F Massee, K Kihou, CH Lee, A Iyo, H Eisaki, TM Chuang, JC Davis, EA Kim

© 2015 Macmillan Publishers Limited. Cooper pairing in the iron-based high-T c superconductors is often conjectured to involve bosonic fluctuations. Among the candidates are antiferromagnetic spin fluctuations and d-orbital fluctuations amplified by phonons. Any such electron-boson interaction should alter the electron's 'self-energy', and then become detectable through consequent modifications in the energy dependence of the electron's momentum and lifetime. Here we introduce a novel theoretical/experimental approach aimed at uniquely identifying the relevant fluctuations of iron-based superconductors by measuring effects of their self-energy. We use innovative quasiparticle interference (QPI) imaging techniques in LiFeAs to reveal strongly momentum-space anisotropic self-energy signatures that are focused along the Fe-Fe (interband scattering) direction, where the spin fluctuations of LiFeAs are concentrated. These effects coincide in energy with perturbations to the density of states N(ω) usually associated with the Cooper pairing interaction. We show that all the measured phenomena comprise the predicted QPI 'fingerprint'of a self-energy due to antiferromagnetic spin fluctuations, thereby distinguishing them as the predominant electron-boson interaction.

Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Cr<inf>x</inf>(Bi<inf>0.1</inf>Sb<inf>0.9</inf>)<inf>2-x</inf>Te<inf>3</inf>

Proceedings of the National Academy of Sciences of the United States of America 112 (2015) 1316-1321

I Lee, CK Kim, J Lee, SJL Billinge, R Zhong, JA Schneeloch, T Liu, T Valla, JM Tranquada, G Gu, JCS Davis

© 2015, National Academy of Sciences. All rights reserved. To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a "Dirac-mass gap" in their spectrum by breaking timereversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92Te3. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship Δ( r ) ∞ n(r ) is confirmed throughout and exhibits an electron-dopant interaction energy J∗= 145 meV•nm2. These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.

Transverse field muon-spin rotation signature of the skyrmion-lattice phase in Cu2OSeO3

PHYSICAL REVIEW B 91 (2015) ARTN 224408

T Lancaster, RC Williams, IO Thomas, F Xiao, FL Pratt, SJ Blundell, JC Loudon, T Hesjedal, SJ Clark, PD Hatton, MC Hatnean, DS Keeble, G Balakrishnan