Publications


Coherent spin control by electrical manipulation of the magnetic anisotropy

Physical Review Letters 110 (2013)

RE George, JP Edwards, A Ardavan

High-spin paramagnetic manganese defects in polar piezoelectric zinc oxide exhibit a simple, almost axial anisotropy and phase coherence times of the order of a millisecond at low temperatures. The anisotropy energy is tunable using an externally applied electric field. This can be used to control electrically the phase of spin superpositions and to drive spin transitions with resonant microwave electric fields. © 2013 American Physical Society.


Quantifying magnetic exchange in doubly-bridged Cu-X(2)-Cu (X = F, Cl, Br) chains enabled by solid state synthesis of CuF(2)(pyrazine).

Chemical communications (Cambridge, England) 49 (2013) 3558-3560

SH Lapidus, JL Manson, J Liu, MJ Smith, P Goddard, J Bendix, CV Topping, J Singleton, C Dunmars, JF Mitchell, JA Schlueter

Solid state techniques involving pressure and temperature have been used to synthesize the fluoride member of the CuX(2)(pyrazine) (X = F, Cl, Br) family of coordination polymers that cannot be crystallized by solution methods. CuF(2)(pyrazine) exhibits unique trans doubly-bridged Cu-F(2)-Cu chains that provide an opportunity to quantify magnetic superexchange in an isostructural Cu-X(2)-Cu series.


Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives

PHYSICAL REVIEW B 88 (2013) ARTN 060401

JD Wright, MJ Pitcher, W Trevelyan-Thomas, T Lancaster, PJ Baker, FL Pratt, SJ Clarke, SJ Blundell


Another dimension: Investigations of molecular magnetism using muon-spin relaxation

Physica Scripta 88 (2013)

T Lancaster, SJ Blundell, FL Pratt

We review examples of muon-spin relaxation measurements on molecule-based magnetic coordination polymers, classified by their magnetic dimensionality. These include the one-dimensional s = 1/2 spin chain Cu(pyz)(NO 3)2 and the two-dimensional s = 1/2 layered material [Cu(HF2)(pyz)2]BF4. We also describe some of the more exotic ground states that may become accessible in the future given the ability to tune the interaction strengths of our materials through crystal engineering. © 2013 The Royal Swedish Academy of Sciences.


Playing quantum hide-and-seek with the muon: Localizing muon stopping sites

Physica Scripta 88 (2013)

JS Möller, P Bonfà, D Ceresoli, F Bernardini, SJ Blundell, T Lancaster, R De Renzi, N Marzari, I Watanabe, S Sulaiman, MI Mohamed-Ibrahim

The most fundamental limitations of a muon-spin relaxation experiment can be the lack of knowledge of the implantation site of the muon and the uncertainty about the muon's perturbation of its host. Here we review some of the work done on the 'muon site problem' in the solid state and highlight some recent applications of electronic structure calculations that have successfully characterized the quantum states of muons in a number of insulating compounds containing fluorine, in a number of pnictide superconductors, and in ZnO. © 2013 The Royal Swedish Academy of Sciences.


Superconducting fluctuations in organic molecular metals enhanced by Mott criticality

(2013)

M Nam, C Meziere, B Batail, A Ardavan


AC magnetic measurement of LiFeAs at pressures up to 5.2 GPa: The relation between T<inf>c</inf>and the structural parameters

Journal of the Korean Physical Society 63 (2013) 445-447

S Yamaguchi, N Yamaguchi, M Mito, H Deguchi, PJ Baker, SJ Blundell, MJ Pitcher, DR Parker, SJ Clarke

The pressure effects on the 111-type Fe-based superconductor LiFeAs were investigated through AC susceptibility measurements and X-ray diffraction experiments, and revealed a correlation between the superconducting transition temperature (Tc) and the As-Fe-As bond angle (α) rather than the height of As from the Fe layers (hAs). As the pressure was increased, Tcof 17 K at P = 0 GPa decreased down to 10 K at P = 5.2 GPa. According to a previous report from an X-ray diffraction experiment, α changes from 101.5° at 0 GPa to 97.8° at 17 GPa. The obtained change in Tcis consistent with Lee et al.'s plot of Tcas a function of α, and from this result, we conclude that Tcwill fall to zero at around α = 98°. © 2013 The Korean Physical Society.


Magnetic transition and spin dynamics in the triangular Heisenberg antiferromagnet $α$-KCrO$_2$

Phys. Rev. B American Physical Society 88 (2013) 180401-180401

F Xiao, T Lancaster, PJ Baker, FL Pratt, SJ Blundell, JS Möller, NZ Ali, M Jansen


Enhancement of the superconducting transition temperature of FeSe by intercalation of a molecular spacer layer.

Nat Mater 12 (2013) 15-19

M Burrard-Lucas, DG Free, SJ Sedlmaier, JD Wright, SJ Cassidy, Y Hara, AJ Corkett, T Lancaster, PJ Baker, SJ Blundell, SJ Clarke

The discovery of high-temperature superconductivity in a layered iron arsenide has led to an intensive search to optimize the superconducting properties of iron-based superconductors by changing the chemical composition of the spacer layer between adjacent anionic iron arsenide layers. Superconductivity has been found in iron arsenides with cationic spacer layers consisting of metal ions (for example, Li(+), Na(+), K(+), Ba(2+)) or PbO- or perovskite-type oxide layers, and also in Fe(1.01)Se (ref. 8) with neutral layers similar in structure to those found in the iron arsenides and no spacer layer. Here we demonstrate the synthesis of Li(x)(NH(2))(y)(NH(3))(1-y)Fe(2)Se(2) (x~0.6; y~0.2), with lithium ions, lithium amide and ammonia acting as the spacer layer between FeSe layers, which exhibits superconductivity at 43(1) K, higher than in any FeSe-derived compound reported so far. We have determined the crystal structure using neutron powder diffraction and used magnetometry and muon-spin rotation data to determine the superconducting properties. This new synthetic route opens up the possibility of further exploitation of related molecular intercalations in this and other systems to greatly optimize the superconducting properties in this family.


Correlated Electrons in Quantum Matter

CONTEMPORARY PHYSICS 54 (2013) 117-118

SJ Blundell


A two-step approach to the synthesis of N@C<inf>60</inf> fullerene dimers for molecular qubits

Chemical Science 4 (2013) 2971-2975

SR Plant, M Jevric, JJL Morton, A Ardavan, AN Khlobystov, GAD Briggs, K Porfyrakis

We report the two-step synthesis of a highly soluble fullerene dimer, both for short reaction times and at the microscale. We apply this reaction scheme to starting materials that contain 15N@C60 and 14N@C60, and we demonstrate how, if applied to highly pure N@C60 in the future, this scheme may be used to produce (14N@C60)2 or (15N@C60)2 dimers in one step, and crucially 14N@C60-15N@C60 dimers in a second step. Such dimers represent isolated electron spin pairs that may be used to demonstrate entanglement between the spins. Additionally, CW EPR spectroscopy of the 15N@C60-C60 dimer in the solid state reveals permanent zero-field splitting (D = 14.6 MHz and E = 0.56 MHz). © 2013 Royal Society of Chemistry.


Influence of HF2- geometry on magnetic interactions elucidated from polymorphs of the metal-organic framework [Ni(HF2)(pyz)2]PF6 (pyz = pyrazine).

Dalton Trans 41 (2012) 7235-7243

JL Manson, KE Carreiro, SH Lapidus, PW Stephens, PA Goddard, RE Del Sesto, J Bendix, S Ghannadzadeh, I Franke, J Singleton, T Lancaster, JS Möller, PJ Baker, FL Pratt, SJ Blundell, J Kang, C Lee, M-H Whangbo

A tetragonal polymorph of [Ni(HF(2))(pyz)(2)]PF(6) (designated β) is isomorphic to its SbF(6)-congener at 295 K and features linear Ni-FHF-Ni pillars. Enhancements in the spin exchange (J(FHF) = 7.7 K), Néel temperature (T(N) = 7 K), and critical field (B(c) = 24 T) were found relative to monoclinic α-PF(6). DFT reveals that the HF(2)(-) bridges are significantly better mediators of magnetic exchange than pyz (J(pyz)), where J(FHF) ≈ 3J(pyz), thus leading to quasi-1D behavior. Spin density resides on all atoms of the HF(2)(-) bridge whereas N-donor atoms of the pyz ring bear most of the density.


Monopoles, magnetricity, and the stray field from spin ice.

Phys Rev Lett 108 (2012) 147601-

SJ Blundell

An analysis is presented of the behavior of muons in the low-temperature state in spin ice. It is shown in detail how the behavior observed in some previous muon experiments on spin ice in a weak transverse field may result from the macroscopic stray field of magnetized spin ice. A model is presented which allows these macroscopic field effects to be simulated and the results agree with experiment. The persistent spin dynamics at low temperature originate from the sample and could be a muon-induced implantation effect that is operative in out-of-equilibrium systems with long relaxation times.


Recent Topics of Organic Superconductors

JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 81 (2012) ARTN 011004

A Ardavan, S Brown, S Kagoshima, K Kanoda, K Kuroki, H Mori, M Ogata, S Uji, J Wosnitza


Data formats and analysis codes - new software for mu SR

12TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE (MUSR2011) 30 (2012) 20-25

S Cottrell, F Pratt, A Hillier, P King, F Akeroyd, AJ Markvardsen, N Draper, Y Yao, S Blundell


Dimensionality selection in a molecule-based magnet.

Phys Rev Lett 108 (2012) 077208-

PA Goddard, JL Manson, J Singleton, I Franke, T Lancaster, AJ Steele, SJ Blundell, C Baines, FL Pratt, RD McDonald, OE Ayala-Valenzuela, JF Corbey, HI Southerland, P Sengupta, JA Schlueter

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S=1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbor coupling strength.


Chemical Engineering of Molecular Qubits

Phys. Rev. Lett. American Physical Society 108 (2012) 107204-107204

CJ Wedge, GA Timco, ET Spielberg, RE George, F Tuna, S Rigby, EJL McInnes, REP Winpenny, SJ Blundell, A Ardavan


A Bayesian approach to magnetic moment determination using mu SR

12TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE (MUSR2011) 30 (2012) 113-116

SJ Blundell, AJ Steele, T Lancaster, JD Wright, FL Pratt


Persistent dynamics in the S=1/2 quasi-one-dimensional chain compound Rb 4Cu(MoO 4) 3 probed with muon-spin relaxation

Physical Review B - Condensed Matter and Materials Physics 85 (2012)

T Lancaster, PJ Baker, FL Pratt, SJ Blundell, W Hayes, D Prabhakaran

We report the results of muon-spin relaxation measurements on the low-dimensional antiferromagnet Rb 4Cu(MoO 4) 3. No long-range magnetic order is observed down to 50 mK implying a ratio T N/J<0.005 (where J is the principal exchange strength along the spin chains) and an effective ratio of interchain to intrachain exchange of |J ⊥/J|<2×10 -3, making the material an excellent realization of a one-dimensional quantum Heisenberg antiferromagnet. We probe the persistent spin excitations at low temperatures and find that ballistic spin transport dominates the excitations detected below 0.3 K. © 2012 American Physical Society.


Gradual destruction of magnetism in the superconducting family NaFe1-xCoxAs

PHYSICAL REVIEW B 85 (2012) ARTN 054503

JD Wright, T Lancaster, I Franke, AJ Steele, JS Moeller, MJ Pitcher, AJ Corkett, DR Parker, DG Free, FL Pratt, PJ Baker, SJ Clarke, SJ Blundell

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