Publications


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


Surface acoustic wave devices on bulk ZnO crystals at low temperature

APPLIED PHYSICS LETTERS 106 (2015) ARTN 063509

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


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


Strontium vanadium oxide-hydrides: "square-planar" two-electron phases.

Angewandte Chemie (International ed. in English) 53 (2014) 7556-7559

F Denis Romero, A Leach, JS Möller, F Foronda, SJ Blundell, MA Hayward

A series of strontium vanadium oxide-hydride phases prepared by utilizing a low-temperature synthesis strategy in which oxide ions in Sr(n+1)V(n)O(3n+1) (n=∞, 1, 2) phases are topochemically replaced by hydride ions to form SrVO2H, Sr2VO3H, and Sr3V2O5H2, respectively. These new phases contain sheets or chains of apex-linked V(3+)O4 squares stacked with SrH layers/chains, such that the n=∞ member, SrVO2H, can be considered to be analogous to "infinite-layer" phases, such as Sr(1-x)Ca(x)CuO2 (the parent phase of the high-T(c) cuprate superconductors), but with a d(2) electron count. All three oxide-hydride phases exhibit strong antiferromagnetic coupling, with SrVO2H exhibiting an antiferromagnetic ordering temperature, T(N)>300 K. The strong antiferromagnetic couplings are surprising given they appear to arise from π-type magnetic exchange.


Spectroscopy methods for molecular nanomagnets

Structure and Bonding 164 (2014) 231-292

ML Baker, SJ Blundell, N Domingo, S Hill

© 2015 Springer-Verlag Berlin Heidelberg. This chapter provides a detailed overview of some of the primary spectroscopic methods that have contributed to the current understanding of molecular nanomagnets (MNs). These include: electron paramagnetic resonance (EPR); optical spectroscopy, including magnetic and X-ray magnetic circular dichroism (MCD/XMCD); inelastic neutron scattering (INS); and muon spin rotation (μ +SR). For each technique, a historical survey of the most important discoveries is provided, up to and including the most recent developments. Each section gives an introduction to the theoretical principles underpinning the techniques, as well as a description of experimental requirements and protocols. A common theme among the described spectroscopies is the fact that state-of-the-art measurements typically have to be performed at major research facilities such as synchrotrons (terahertz EPR and XMCD), high magnetic field laboratories (EPR), and accelerator facilities or reactors (INS and μ +SR). Details of such facilities are given where appropriate. Forefront issues that are addressed in the chapter include: the fundamental properties of both mono- and poly-nuclear single-molecule magnets (SMMs); the deployment of MNs in quantum information processing applications; the addressing of individual magnetic molecules on surfaces or in devices; the probing of spin dynamics in MNs using EPR, INS, and μ +SR; and studies of long-range magnetic ordering in MN crystals. An extensive list of references is provided. The chapter is intended for physicists, chemists, and materials scientists, particularly junior researchers who are just starting work in the field.


Coherent electron spin manipulation in a dilute oriented ensemble of molecular nanomagnets: pulsed EPR on doped single crystals.

Chem Commun (Camb) 50 (2014) 91-93

F Moro, D Kaminski, F Tuna, GFS Whitehead, GA Timco, D Collison, REP Winpenny, A Ardavan, EJL McInnes

Doping a Cr7Zn molecular nanomagnet into a diamagnetic and isostructural host allows pulsed X-band EPR on single crystals, including relaxation and nutation experiments on the S = 3/2 ground state.


The effect of the demagnetizing field in cylindrical samples in high transverse field mu+SR experiments

13TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE 551 (2014)

F Lang, SJ Blundell


Probing the magnetic phases in the Ni-V alloy close to the disordered ferromagnetic quantum critical point with mu SR

13TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE 551 (2014)

A Schroeder, R Wang, PJ Baker, FL Pratt, SJ Blundell, T Lancaster, I Franke, JS Moeller


Quantum spin coherence in halogen-modified Cr7Ni molecular nanomagnets

PHYSICAL REVIEW B 90 (2014) ARTN 184419

D Kaminski, AL Webber, CJ Wedge, J Liu, GA Timco, IJ Vitorica-Yrezabal, EJL McInnes, REP Winpenny, A Ardavan


Lattice-site-specific spin dynamics in double perovskite Sr2CoOsO6.

Physical review letters 112 (2014) 147202-

B Yan, AK Paul, S Kanungo, M Reehuis, A Hoser, DM Többens, W Schnelle, RC Williams, T Lancaster, F Xiao, JS Möller, SJ Blundell, W Hayes, C Felser, M Jansen

Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr2CoOsO6. Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature TN1=108  K, cobalt (3d7, S=3/2) and osmium (5d2, S=1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below TN2=67  K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T≈5  K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr2CoOsO6 distinct from previously reported double-perovskite compounds.


Local magnetism and spin correlations in the geometrically frustrated cluster magnet LiZn <inf>2</inf> Mo<inf>3</inf> O<inf>8</inf>

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

JP Sheckelton, FR Foronda, L Pan, C Moir, RD McDonald, T Lancaster, PJ Baker, NP Armitage, T Imai, SJ Blundell, TM McQueen

LiZn2Mo3O8 has been proposed to contain S=Mo3O13 magnetic clusters arranged on a triangular lattice with antiferromagnetic nearest-neighbor interactions. Here, microwave and terahertz electron spin resonance, Li7 nuclear magnetic resonance, and muon spin rotation spectroscopies are used to characterize the local magnetic properties of LiZn2Mo3O8. These results show the magnetism in LiZn2Mo3O8 arises from a single isotropic S=electron per cluster and that there is no static long-range magnetic ordering down to T = 0.07 K. Further, there is evidence of gapless spin excitations with spin fluctuations slowing down as the temperature is lowered. These data indicate strong spin correlations, which, together with previous data, suggest a low-temperature resonating valence-bond state in LiZn2Mo3O8. © 2014 American Physical Society.


Upper critical field of NaFe1-xCoxAs superconductors

PHYSICAL REVIEW B 89 (2014) ARTN 054502

S Ghannadzadeh, JD Wright, FR Foronda, SJ Blundell, SJ Clarke, PA Goddard


Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator la 5 / 3 Sr 1 / 3 CoO 4

Physical Review B - Condensed Matter and Materials Physics 89 (2014)

T Lancaster, SR Giblin, G Allodi, S Bordignon, M Mazzani, R De Renzi, PG Freeman, PJ Baker, FL Pratt, P Babkevich, SJ Blundell, AT Boothroyd, JS Möller, D Prabhakaran

We investigate the magnetic ordering and dynamics of the stripe phase of La5/3Sr1/3CoO4, a material shown to have an hourglass magnetic excitation spectrum. A combination of muon-spin relaxation, nuclear magnetic resonance, and magnetic susceptibility measurements strongly suggest that the physics is determined by a partially disordered configuration of charge and spin stripes whose frustrated magnetic degrees of freedom are dynamic at high temperature and which undergo an ordering transition around 35 K with coexisting dynamics that freeze out in a glassy manner as the temperature is further reduced. © 2014 American Physical Society.


Controlling Magnetic Order and Quantum Disorder in Molecule-Based Magnets

PHYSICAL REVIEW LETTERS 112 (2014) ARTN 207201

T Lancaster, PA Goddard, SJ Blundell, FR Foronda, S Ghannadzadeh, JS Moeller, PJ Baker, FL Pratt, C Baines, L Huang, J Wosnitza, RD McDonald, KA Modic, J Singleton, CV Topping, TAW Beale, F Xiao, JA Schlueter, AM Barton, RD Cabrera, KE Carreiro, HE Tran, JL Manson


Dipolar ordering in a molecular nanomagnet detected using muon spin relaxation

PHYSICAL REVIEW B 89 (2014) ARTN 144420

FL Pratt, E Micotti, P Carretta, A Lascialfari, P Arosio, T Lancaster, SJ Blundell, AK Powell


Direct evidence for a magnetic f-electron-mediated pairing mechanism of heavy-fermion superconductivity in CeCoIn5

Proceedings of the National Academy of Sciences of the United States of America 111 (2014) 11663-11667

JS Van Dyke, F Massee, MP Allan, JCS Davis, C Petrovic, DK Morr

To identify the microscopic mechanism of heavy-fermion Cooper pairing is an unresolved challenge in quantum matter studies; it may also relate closely to finding the pairing mechanism of hightemperature superconductivity. Magnetically mediated Cooper pairing has long been the conjectured basis of heavy-fermion superconductivity but no direct verification of this hypothesis was achievable. Here, we use a novel approach based on precision measurements of the heavy-fermion band structure using quasiparticle interference imaging to reveal quantitatively the momentum space (k-space) structure of the f-electron magnetic interactions of CeCoIn5. Then, by solving the superconducting gap equations on the two heavy-fermion bands Eα,βk with these magnetic interactions as mediators of the Cooper pairing, we derive a series of quantitative predictions about the superconductive state. The agreement found between these diverse predictions and the measured characteristics of superconducting CeCoIn5 then provides direct evidence that the heavy-fermion Cooper pairing is indeed mediated by f-electron magnetism.


Direct phase-sensitive identification of a d-form factor density wave in underdoped cuprates

Proceedings of the National Academy of Sciences of the United States of America 111 (2014)

K Fujita, MH Hamidian, SD Edkins, CK Kim, Y Kohsaka, M Azuma, M Takano, H Takagi, H Eisaki, SI Uchida, A Allais, MJ Lawler, EA Kim, S Sachdev, JC Séamus Davis

The identity of the fundamental broken symmetry (if any) in the underdoped cuprates is unresolved. However, evidence has been accumulating that this state may be an unconventional density wave. Here we carry out site-specific measurements within each CuO2 unit cell, segregating the results into three separate electronic structure images containing only the Cu sites [Cu(r)] and only the x/y axis O sites [Ox (r) and Oy( r)]. Phase-resolved Fourier analysis reveals directly that the modulations in the Ox(r) and Oy(r) sublattice images consistently exhibit a relative phase of π. We confirm this discovery on two highly distinct cuprate compounds, ruling out tunnel matrix-element and materials-specific systematics. These observations demonstrate by direct sublattice phaseresolved visualization that the density wave found in underdoped cuprates consists of modulations of the intraunit-cell states that exhibit a predominantly d-symmetry form factor.


Simultaneous transitions in cuprate momentum-space topology and electronic symmetry breaking

Science 344 (2014) 612-616

K Fujita, CK Kim, I Lee, J Lee, MH Hamidian, IA Firmo, S Mukhopadhyay, H Eisaki, S Uchida, MJ Lawler, EA Kim, JC Davis

The existence of electronic symmetry breaking in the underdoped cuprates and its disappearance with increased hole density p are now widely reported. However, the relation between this transition and the momentum-space (k →-space) electronic structure underpinning the superconductivity has not yet been established. Here, we visualize the Q→ = 0 (intra-unit-cell) and Q→ ≠ 0 (density-wave) broken-symmetry states, simultaneously with the coherent k→-space topology, for Bi2Sr2CaCu2O8+δ samples spanning the phase diagram 0.06 ≤ p ≤ 0.23. We show that the electronic symmetry-breaking tendencies weaken with increasing p and disappear close to a critical doping pc = 0.19. Concomitantly, the coherent k →-space topology undergoes an abrupt transition, from arcs to closed contours, at the same pc. These data reveal that the k →-space topology transformation in cuprates is linked intimately with the disappearance of the electronic symmetry breaking at a concealed critical point.


Iron-Based superconductors: Enigmatic nematic

Nature Physics 10 (2014) 184-185

JC Davis, PJ Hirschfeld

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