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


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

[Ni(HF2)(3-Clpy)4]BF4 (py = pyridine): evidence for spin exchange along strongly distorted F···H···F- bridges in a one-dimensional polymeric chain.

Inorg Chem 51 (2012) 7520-7528

JL Manson, AG Baldwin, BL Scott, J Bendix, RE Del Sesto, PA Goddard, Y Kohama, HE Tran, S Ghannadzadeh, J Singleton, T Lancaster, JS Möller, SJ Blundell, FL Pratt, VS Zapf, J Kang, C Lee, M-H Whangbo, C Baines

[Ni(HF(2))(3-Clpy)(4)]BF(4) (py = pyridine) is a simple one-dimensional (1D) coordination polymer composed of compressed NiN(4)F(2) octahedra that form chains with bridging HF(2)(-) ligands. In spite of significant distortion of the HF(2)(-) bridge, a quasi-1D antiferromagnetic (AFM) behavior was observed with J(FHF) = 4.86 K.

Proximal magnetometry of monolayers of magnetic moments


Z Salman, SJ Blundell

Chemistry at the nanoscale: synthesis of an N@C60-N@C60 endohedral fullerene dimer.

Angew Chem Int Ed Engl 51 (2012) 3587-3590

BJ Farrington, M Jevric, GA Rance, A Ardavan, AN Khlobystov, GAD Briggs, K Porfyrakis

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.

Magnetism in crown-ether-substituted nitronyl nitroxide derivatives and their metal complexes

Physica Status Solidi (C) Current Topics in Solid State Physics 9 (2012) 1205-1207

T Sugano, SJ Blundell, W Hayes, H Tajima, H Mori

Rare-earth-metal (M = Nd, Gd and Dy) complexes of the organic monoradicals, 15-crown-5-phenyl nitronyl nitroxide 1 and 15-crown-5- or 18-crown-6-phenyl iminonitroxide 2, were synthesized. We present here magnetic properties of the metal-free radicals and the metal complexes of these organic monoradicals studied using SQUID megnetometry. Almost of all the rare-earth-metal complexes show two-sublattice behavior in a wide range of antiferromagnetic (AFM) intrasublattice and ferromagnetic (FM) intersublattice interactions, Γ and λ, with |CΓ | and |Cλ | ≅ 10 to 100 K (C is the Curie constant) over the temperature range between 1.8 and 300 K, while the metal-free radicals exhibit only weak AFM intermolecular interactions, less than the Weiss constant θ = -1 K. The magnitude of interactions observed in the metal complexes of each radical increase in order from Gd to Dy to Nd complexes. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Importance of halogen ··· halogen contacts for the structural and magnetic properties of CuX <inf>2</inf>(pyrazine-N,N'-dioxide) (H <inf>2</inf>O) <inf>2</inf> (X = Cl and Br)

Inorganic Chemistry 51 (2012) 2121-2129

JA Schlueter, H Park, GJ Halder, WR Armand, C Dunmars, KW Chapman, JL Manson, J Singleton, R McDonald, A Plonczak, J Kang, C Lee, MH Whangbo, T Lancaster, AJ Steele, I Franke, JD Wright, SJ Blundell, FL Pratt, J De George, MM Turnbull, CP Landee

The structural and magnetic properties of the newly crystallized CuX 2(pyzO)(H 2O) 2 (X = Cl, Br; pyzO = pyrazine-N,N'-dioxide) coordination polymers are reported. These isostructural compounds crystallize in the monoclinic space group C2/c with, at 150 K, a = 17.0515(7) Å, b = 5.5560(2) Å, c = 10.4254(5) Å, β = 115.400(2)°, and V = 892.21(7) Å 3 for X = Cl and a = 17.3457(8) Å, b = 5.6766(3) Å, c = 10.6979(5) Å, β = 115.593(2)°, and V = 950.01(8) Å 3 for X = Br. Their crystal structure is characterized by one-dimensional chains of Cu 2+ ions linked through bidentate pyzO ligands. These chains are joined together through OH···O hydrogen bonds between the water ligands and pyzO oxygen atoms and Cu-X⋯X-Cu contacts. Bulk magnetic susceptibility measurements at ambient pressure show a broad maximum at 7 (Cl) and 28 K (Br) that is indicative of short-range magnetic correlations. The dominant spin exchange is the Cu-X···X-Cu supersuperexchange because the magnetic orbital of the Cu 2+ ion is contained in the CuX 2(H 2O) 2 plane and the X··· X contact distances are short. The magnetic data were fitted to a Heisenberg 1D uniform antiferromagnetic chain model with J 1D/k B = -11.1(1) (Cl) and -45.9(1) K (Br). Magnetization saturates at fields of 16.1(3) (Cl) and 66.7(5) T (Br), from which J 1D is determined to be -11.5(2) (Cl) and -46.4(5) K (Br). For the Br analog the pressure dependence of the magnetic susceptibility indicates a gradual increase in the magnitude of J 1D/k B up to -51.2 K at 0.84 GPa, suggesting a shortening of the Br···Br contact distance under pressure. At higher pressure X-ray powder diffraction data indicates a structural phase transition at ∼3.5 GPa. Muon-spin relaxation measurements indicate that CuCl2(pyzO)(H 2O) 2 is magnetically ordered with T N = 1.06(1) K, while the signature for long-range magnetic order in CuBr2(pyzO)(H 2O) 2 was much less definitive down to 0.26 K. The results for the CuX 2(pyzO)(H 2O) 2 complexes are compared to the related CuX 2(pyrazine) materials. © 2012 American Chemical Society.

Interplay of rotational, relaxational, and shear dynamics in solid <sup>4</sup>He

Science 332 (2011) 821-824

EJ Pratt, B Hunt, V Gadagkar, M Yamashita, MJ Graf, AV Balatsky, JC Davis

Using a high-sensitivity torsional oscillator (TO) technique, we mapped the rotational and relaxational dynamics of solid helium-4 (4He) throughout the parameter range of the proposed supersolidity. We found evidence that the same microscopic excitations controlling the torsional oscillator motions are generated independently by thermal and mechanical stimulation. Moreover, a measure for the relaxation times of these excitations diverges smoothly without any indication for a critical temperature or critical velocity of a supersolid transition. Finally, we demonstrated that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the solid's shear modulus. This implies that the rotational responses of solid 4He attributed to supersolidity are associated with generation of the same microscopic excitations as those produced by direct shear strain.

Superconductors: What they're good for

New Scientist 212 (2011) 6-7

S Blundell

Already used in applications as diverse as body scanning and discovering the origin of mass, superconductors hold promise for even greater technologies. © 2011 Reed Business Information Ltd, England.

Superconductors: Cold beginnings

New Scientist 212 (2011)

S Blundell

It took more than half a century to figure out how superconductivity might work and how to make it useful. © 2011 Reed Business Information Ltd, England.

Critical behavior in the inhomogeneous ferromagnet SrFe <inf>0.80</inf>Co<inf>0.20</inf>O<inf>3.0</inf>

Physical Review B - Condensed Matter and Materials Physics 83 (2011)

J Lago, MJ Rosseinsky, SJ Blundell, PD Battle, M Diaz, I Uriarte, T Rojo

A detailed muon spin relaxation (μSR) and magnetization study is presented of the paramagnetic-to-ferromagnetic transition in the compound SrFe0.80Co0.20O3. The critical exponents derived from the static critical analysis are close to the theoretical predictions for the Heisenberg model in three dimensions. However, a small drift toward mean-field values is interpreted as arising from the presence of long-range dipolar interactions between the Fe(IV) centers. The evolution of spin dynamics across the transition determined from the μSR study is consistent with this interpretation. μSR and magnetization data also provide evidence of an inhomogeneous magnetic state both above and below Tc, placing this system in line with other double-exchange materials such as manganites and cobaltites where spontaneous electronic and magnetic phase separation appears conspicuous and explains much of the encountered experimental phenomenology. © 2011 American Physical Society.

Photochemical stability of N@C-60 and its pyrrolidine derivatives


G Liu, AN Khlobystov, A Ardavan, GAD Briggs, K Porfyrakis

Probing magnetic order in LiMPO<inf>4</inf> (M= Ni, Co, Fe) and lithium diffusion in Li<inf>x</inf>FePO<inf>4</inf>

Physical Review B - Condensed Matter and Materials Physics 84 (2011)

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

Muon spin relaxation measurements are reported on three members of the LixMPO4 series. The magnetic properties of stoichiometric samples with M= Ni, Co, Fe were investigated at low temperature. In LiNiPO 4 we observe different forms of the muon decay asymmetry in the commensurate and incommensurate antiferromagnetic phases, accompanied by a change in the temperature dependence of the muon oscillation frequency. In LiCoPO4 the form of the muon decay asymmetry indicates that the correlation between layers decreases as the Néel temperature is approached from below. LiFePO4 shows more conventional behavior, typical for a three-dimensional antiferromagnet. Measurements on Li xFePO4 with x=0.8,0.9, and 1 show evidence for lithium diffusion below ∼250 K and muon diffusion dominating the form of the relaxation at higher temperature. The thermally activated form of the observed hopping rate suggests an activation barrier for lithium diffusion of ∼100 meV and a diffusion constant of DLi∼10-10to10 -9 cm2 s-1 at room temperature. © 2011 American Physical Society.

Topological defects coupling smectic modulations to intra-unit-cell nematicity in cuprates

Science 333 (2011) 426-430

A Mesaros, K Fujita, H Eisaki, S Uchida, JC Davis, S Sachdev, J Zaanen, MJ Lawler, EA Kim

We study the coexisting smectic modulations and intra-unit-cell nematicity in the pseudogap states of underdoped Bi 2Sr 2CaCu 2O 8+δ. By visualizing their spatial components separately, we identified 2π topological defects throughout the phase-fluctuating smectic states. Imaging the locations of large numbers of these topological defects simultaneously with the fluctuations in the intra-unit-cell nematicity revealed strong empirical evidence for a coupling between them. From these observations, we propose a Ginzburg-Landau functional describing this coupling and demonstrate how it can explain the coexistence of the smectic and intra-unit-cell broken symmetries and also correctly predict their interplay at the atomic scale. This theoretical perspective can lead to unraveling the complexities of the phase diagram of cuprate high-critical- temperature superconductors.

Superconductivity and fluctuating magnetism in quasi-two-dimensional κ-(BEDT-TTF) 2Cu[N(CN) 2]Br probed with implanted muons

Physical Review B - Condensed Matter and Materials Physics 83 (2011)

T Lancaster, SJ Blundell, FL Pratt, JA Schlueter

A muon-spin relaxation (μ+SR) investigation is presented for the molecular superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br. Evidence is found for low-temperature phase separation throughout the bulk of the material, with only a fraction of the sample showing a superconducting signal, even for slow cooling. Rapid cooling reduces the superconducting fraction still further. For the superconducting phase, the in-plane penetration depth is measured to be λ||=0.47(1)μm, and evidence is seen for a vortex decoupling transition in applied fields above 40 mT. The magnetic fluctuations in the normal state produce a precipitous drop in relaxation rate above 100 K, and we discuss the possible causes for the unusual relaxation that we observe for T>Tc. © 2011 American Physical Society.

Magnetic phase transitions in transition-metal complexes with triazole derivatives

Polyhedron 30 (2011) 3202-3205

T Sugano, M Kurmoo, SJ Blundell, W Hayes, S Vilminot

Three divalent transition-metal (Co, Ni and Cu) complexes with the organic anion, 1,2,4-triazolato (tr), as a ligand molecule were prepared by means of hydrothermal syntheses and their magnetic properties were investigated by SQUID magnetometry. The Co(tr)2 and Cu(tr)2 complexes exhibit long range ordering below 8 and 30 K, respectively, while the Ni(tr)2 complex does not show any magnetic phase transition down to 1.8 K. The magnetization isotherms of Co(tr)2 and Cu(tr)2 measured at 2.0 K show hysteresis loops with the coercive fields of 0.5 and 4.7 kOe, respectively. At temperatures higher than about 50 K, the temperature dependence of the magnetic susceptibility of Co(tr)2, Ni(tr)2 and Cu(tr)2 follows the Curie-Weiss law with the Curie constants of 2.95, 0.945 and 0.420 emu K mol-1 and the Weiss temperatures of -62, -74 and -97 K, respectively. These results suggest that the magnetically ordered phases observed in Co(tr)2 and Cu(tr)2 at low temperatures come from antiferromagnetic interactions resulting in canted arrangements of magnetic moments of the transition-metal cations. We discuss here the magnetic interactions in these transition-metal complexes by referring the results of the magnetization measurements. © 2011 Elsevier B.V. All rights reserved.

[Cu(HF<inf>2</inf>)<inf>2</inf>(pyz)]<inf>n</inf>: A rectangular antiferromagnetic lattice with a spin exchange path made up of two different FHF<sup>-</sup> bridges

Angewandte Chemie - International Edition 50 (2011) 1573-1576

JL Manson, ML Warter, JA Schlueter, T Lancaster, AJ Steele, SJ Blundell, FL Pratt, J Singleton, RD McDonald, C Lee, MH Whangbo, A Plonczak

The two-dimensional antiferromagnet [Cu(HF2)2(pyz)] n (pyz=pyrazine) has a rectangular lattice (see picture) displaying two types of FHF- bridging modes. The spin exchange through Cu-(FHF)2-Cu is about 90% stronger than through Cu-pyz-Cu, and the 1,1-coordinated FHF-, which is close in nature to F - ⋯ HF, is largely responsible for the exchange. C black, Cu red, F green, H cyan. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Electronic structure of the cuprate superconducting and pseudogap phases from spectroscopic imaging STM

New Journal of Physics 13 (2011)

AR Schmidt, K Fujita, EA Kim, MJ Lawler, H Eisaki, S Uchida, DH Lee, JC Davis

We survey the use of spectroscopic imaging scanning tunneling microscopy (SI-STM) to probe the electronic structure of underdoped cuprates. Two distinct classes of electronic states are observed in both the d-wave superconducting (dSC) and the pseudogap (PG) phases. The first class consists of the dispersive Bogoliubov quasiparticle excitations of a homogeneous d-wave superconductor, existing below a lower energy scale E = Δ0. We find that the Bogoliubov quasiparticle interference (QPI) signatures of delocalized Cooper pairing are restricted to a k-space arc, which terminates near the lines connecting k = ±(π/a0, 0) to k = ±(0, π/a 0). This arc shrinks continuously with decreasing hole density such that Luttinger's theorem could be satisfied if it represents the front side of a hole-pocket that is bounded behind by the lines between k = ±(π/a0, 0) and k = ±(0, π/a0). In both phases, the only broken symmetries detected for the \E\ < Δ0 states are those of a d-wave superconductor. The second class of states occurs proximate to the PG energy scale E = Δ1. Here the non-dispersive electronic structure breaks the expected 90°-rotational symmetry of electronic structure within each unit cell, at least down to 180°-rotational symmetry. This electronic symmetry breaking was first detected as an electronic inequivalence at the two oxygen sites within each unit cell by using a measure of nematic (C2) symmetry. Incommensurate non-dispersive conductance modulations, locally breaking both rotational and translational symmetries, coexist with this intra-unit-cell electronic symmetry breaking at E = Δ1. Their characteristic wavevector Q is determined by the k-space points where Bogoliubov QPI terminates and therefore changes continuously with doping. The distinct broken electronic symmetry states (intraunit-cell and finite Q) coexisting at E ∼ Δ1 are found to be indistinguishable in the dSC and PG phases. The next challenge for SI-STM studies is to determine the relationship of the E ∼ Δ1 broken symmetry electronic states with the PG phase, and with the E < Δ0 states associated with Cooper pairing. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

The statistical mechanics of community assembly and species distribution

New Phytologist (2011)

CK Kelly, SJ Blundell, MG Bowler, GA Fox, PH Harvey, MR Lomas, F Ian Woodward