Publications


Mn(dca) 2(o-phen) {dca = dicyanamide; o-phen = 1,10-phenanthroline}: Long-range magnetic order in a low-dimensional Mn-dca polymer

Polyhedron (2012)

JL Manson, CM Brown, Q Huang, JA Schlueter, T Lancaster, SJ Blundell, J Singleton, JW Lynn, FL Pratt


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


Three-dimensional Heisenberg spin-glass behavior in SrFe <inf>0.90</inf>Co <inf>0.10</inf>O <inf>3.0</inf>

Physical Review B - Condensed Matter and Materials Physics 86 (2012)

J Lago, SJ Blundell, A Eguia, M Jansen, T Rojo

The series SrFe 1-xCo xO 3 evolves from spiral antiferromagnetic order to long-range ferromagnetism on increasing Co doping. In the Fe-rich region below the onset of ferromagnetism (x ≤ 0.20), there exists a number of disordered magnetic ground states. Here we present a detailed study of the composition SrFe 0.90Co 0.10O 3, which we find to display quasicanonical spin-glass behavior. The analysis of its freezing transition by means of muon spin relaxation (μSR) spectroscopy and ac and dc susceptibility suggests that the system constitutes a new experimental realization of the three-dimensional Heisenberg spin-glass model with weak random anisotropy. The derived critical exponents are consistent with predictions by Kawamura's chiral driven freezing scenario with a bimodal exchange distribution. © 2012 American Physical Society.


[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.


Ag(nic)2 (nic = nicotinate): a spin-canted quasi-2D antiferromagnet composed of square-planar S = 1/2 Ag(II) ions.

Inorg Chem 51 (2012) 1989-1991

JL Manson, TJ Woods, SH Lapidus, PW Stephens, HI Southerland, VS Zapf, J Singleton, PA Goddard, T Lancaster, AJ Steele, SJ Blundell

Square-planar S = 1/2 Ag(II) ions in polymeric Ag(nic)(2) are linked by bridging nic monoanions to yield 2D corrugated sheets. Long-range magnetic order occurs below T(N) = 11.8(2) K due to interlayer couplings that are estimated to be about 30 times weaker than the intralayer exchange interaction.


Proximal magnetometry of monolayers of magnetic moments

12TH INTERNATIONAL CONFERENCE ON MUON SPIN ROTATION, RELAXATION AND RESONANCE (MUSR2011) 30 (2012) 168-173

Z Salman, SJ Blundell


Importance of halogen···halogen contacts for the structural and magnetic properties of CuX2(pyrazine-N,N′-dioxide)(H2O)2 (X = Cl and Br).

Inorg Chem 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, M-H Whangbo, T Lancaster, AJ Steele, I Franke, JD Wright, SJ Blundell, FL Pratt, J deGeorge, MM Turnbull, CP Landee

The structural and magnetic properties of the newly crystallized CuX(2)(pyzO)(H(2)O)(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(2)O)(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 CuCl(2)(pyzO)(H(2)O)(2) is magnetically ordered with T(N) = 1.06(1) K, while the signature for long-range magnetic order in CuBr(2)(pyzO)(H(2)O)(2) was much less definitive down to 0.26 K. The results for the CuX(2)(pyzO)(H(2)O)(2) complexes are compared to the related CuX(2)(pyrazine) materials.


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


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.


Spectroscopic Imaging STM Studies of Electronic Structure in Both the Superconducting and Pseudogap Phases of Underdoped Cuprates

in Conductor-Insulator Quantum Phase Transitions, 9780199592593 (2012)

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

© Vladimir Dobrosavljevic, Nandini Trivedi & James M. Valles, Jr., 2012. All rights reserved. A motivation for the development of atomically resolved spectroscopic imaging STM (SISTM) has been to study the broken symmetries in the electronic structure of cuprate high temperature superconductors. Both the d-wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates exhibit two distinct classes of electronic states when studied using SI-STM. The class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d-wave superconductor. These signature are detected below a lower energy scale |E| = D0 and only upon a momentum space (k-space) arc which terminates near the lines connecting k = ±(p/a0, 0) to k = ±(0.,p/a0). This 'nodal' arc shrinks continuously with decreasing hole density. In both the dSC and PG phases, the only broken symmetries detected in the |E|≤D0 states are those of a d-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale |E|~.D1 can be associated with the 'antinodal' states near k = ±(p/a0,0) and k = ±(0.,p/a0). These states break the expected 90°-rotational (C4) symmetry of electronic structure within CuO2 unit cells, at least down to 180°-rotational (C2) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell C4 symmetry breaking coexists at |E|~.D1 with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). Empirically, the characteristic wavevector Q of the latter is determined by the k-space points where Bogoliubov quasiparticle interference terminates and therefore evolves continuously with doping. The properties of these two classes of |E|~.D1 states are indistinguishable in the dSC and PG phases. To explain these two regimes of k-space that are distinguished by the symmetries of their electronic states and their energy scales |E|~D1 and |E|≤D0, and to understand their relationship to the electronic phase diagram and the mechanism of high- Tc superconductivity, represent key challenges for cuprate studies.


Visualization of the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator

Nature Physics 8 (2012) 534-538

Y Kohsaka, T Hanaguri, M Azuma, M Takano, JC Davis, H Takagi

Superconductivity emerges from the cuprate antiferromagnetic Mott state with hole doping. The resulting electronic structure is not understood, although changes in the state of oxygen atoms seem paramount. Hole doping first destroys the Mott state, yielding a weak insulator where electrons localize only at low temperatures without a full energy gap. At higher doping levels, the ' pseudogap', a weakly conducting state with an anisotropic energy gap and intra-unit-cell breaking of 90 ° rotational (C 4v) symmetry, appears. However, a direct visualization of the emergence of these phenomena with increasing hole density has never been achieved. Here we report atomic-scale imaging of electronic structure evolution from the weak insulator through the emergence of the pseudogap to the superconducting state in Ca 2-x Na x CuO 2 Cl 2 . The spectral signature of the pseudogap emerges at the lowest doping level from a weakly insulating but C 4v -symmetric matrix exhibiting a distinct spectral shape. At slightly higher hole density, nanoscale regions exhibiting pseudogap spectra and 180 ° rotational (C 2v ) symmetry form unidirectional clusters within the C 4v -symmetric matrix. Thus, hole doping proceeds by the appearance of nanoscale clusters of localized holes within which the broken-symmetry pseudogap state is stabilized. A fundamentally two-component electronic structure then exists in Ca 2-x Na x CuO 2 Cl 2 until the C 2v -symmetric clusters touch at higher doping levels, and the long-range superconductivity appears. © 2012 Macmillan Publishers Limited. All rights reserved.


Picometer registration of zinc impurity states in Bi <inf>2</inf>Sr <inf>2</inf>CaCu <inf>2</inf>O <inf>8+δ</inf> for phase determination in intra-unit-cell Fourier transform STM

New Journal of Physics 14 (2012)

MH Hamidian, IA Firmo, K Fujita, S Mukhopadhyay, JW Orenstein, H Eisaki, S Uchida, MJ Lawler, EA Kim, JC Davis

Direct visualization of electronic-structure symmetry within each crystalline unit cell is a new technique for complex electronic matter research (Lawler et al 2010 Nature 466 347-51, Schmidt et al 2011 New J. Phys. 13 065014, Fujita K et al 2012 J. Phys. Soc. Japan 81 011005). By studying the Bragg peaks in Fourier transforms of electronic structure images and particularly by resolving both the real and imaginary components of the Bragg amplitudes, distinct types of intra-unit-cell symmetry breaking can be studied. However, establishing the precise symmetry point of each unit cell in real space is crucial in defining the phase for such a Bragg-peak Fourier analysis. Exemplary of this challenge is the high-temperature superconductor Bi 2Sr 2CaCu 2O 8+δ for which the surface Bi atom locations are observable, while it is the invisible Cu atoms that define the relevant CuO 2 unit-cell symmetry point. Here we demonstrate, by imaging with picometer precision the electronic impurity states at individual Zn atoms substituted at Cu sites, that the phase established using the Bi lattice produces a ∼2%(2π) error relative to the actual Cu lattice. Such a phase assignment error would not diminish reliability in the determination of intra-unit-cell rotational symmetry breaking at the CuO 2 plane (Lawler et al 2010 Nature 466 347-51, Schmidt et al 2011 New J. Phys. 13 065014, Fujita K et al 2012 J. Phys. Soc. Japan 81 011005). Moreover, this type of impurity atom substitution at the relevant symmetry site can be of general utility in phase determination for the Bragg-peak Fourier analysis of intra-unit-cell symmetry. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.


Anisotropic energy gaps of iron-based superconductivity from intraband quasiparticle interference in LiFeAs

Science 336 (2012) 563-567

MP Allan, AW Rost, AP Mackenzie, Y Xie, JC Davis, K Kihou, CH Lee, A Iyo, H Eisaki, TM Chuang

If strong electron-electron interactions between neighboring Fe atoms mediate the Cooper pairing in iron-pnictide superconductors, then specific and distinct anisotropic superconducting energy gaps Δi(k →) should appear on the different electronic bands i. Here, we introduce intraband Bogoliubov quasiparticle scattering interference (QPI) techniques for determination of Δi(k→) in such materials, focusing on lithium iron arsenide (LiFeAs). We identify the three hole-like bands assigned previously as γ, α2, and α1, and we determine the anisotropy, magnitude, and relative orientations of their Δi(k→): These measurements will advance quantitative theoretical analysis of the mechanism of Cooper pairing in iron-based superconductivity.


Spectroscopic imaging STM studies of broken electronic symmetries in underdoped cuprates

Physica B: Condensed Matter 407 (2012) 1859-1863

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

We use spectroscopic imaging scanning tunneling microscopy (SI-STM) to visualize the spatial symmetries of the electronic states that occur at the pseudogap energy scale in underdoped cuprates. We find evidence for the local intra-unit-cell electronic nematicity - by which we mean the disordered breaking of C 4v symmetry within each CuO 2 unit cell [1]. We also find that the coexisting incommensurate (smectic) electronic modulations couple to the intra-unit-cell nematicity through their 2π topological defects [2]. © 2012 Elsevier B.V. All rights reserved.


Spectroscopic imaging scanning tunneling microscopy studies of electronic structure in the superconducting and pseudogap phases of cuprate high-T <inf>c</inf> superconductors

Journal of the Physical Society of Japan 81 (2012)

K Fujita, AR Schmidt, EA Kim, MJ Lawler, DH Lee, C Davis, H Eisaki, SI Uchida

One of the key motivations for the development of atomically resolved spectroscopic imaging scanning tunneling microscopy (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the d-wave superconducting (dSC) and the pseudogap (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d-wave superconductor. These are detected below a lower energy scale |E| =Δ 0 and only upon a momentum space (k-space) arc which terminates near the lines connecting k - ±(π/a 00)toπ/ a 00 Below optimal doping, this "nodal" arc shrinks continuously with decreasing hole density. In both the dSC and PG phases, the only broken symmetries detected in the |E| ≤Δ 0 states are those of a d-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale |E|∼Δ 1 which is associated conventionally with the "antinodal" states near k=±(0π. a/0).; and k=±(0π. a/0). We find that these states break the expected 90-rotational (C 4) symmetry of electronic structure within CuO 2 unit cells, at least down to 180°-rotational (C 2) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell C 4 symmetry breaking coexists at |E| ∼Δ 1 with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). The characteristic wavevector Q of the latter is determined, empirically, by the k-space points where Bogoliubov quasiparticle interference terminates, and therefore evolves continuously with doping. The properties of these two classes of |E| ∼Δ 1 states are indistinguishable in the dSC and PG phases. To explain this segregation of k-space into the two regimes distinguished by the symmetries of their electronic states and their energy scales |E| ∼Δ 1 and |E|≤Δ 0, and to understand how this impacts the electronic phase diagram and the mechanism of high-T c superconductivity, represents one of a key challenges for cuprate studies. © 2012 The Physical Society of Japan.


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.


BF <inf>4</inf> (py = pyridine): Evidence for spin exchange along strongly distorted F⋯H⋯F <sup>-</sup> bridges in a one-dimensional polymeric chain

Inorganic Chemistry 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, MH Whangbo, C Baines

Figure Persented: [Ni(HF 2)(3-Clpy) 4]BF 4 (py = pyridine) is a simple one-dimensional (1D) coordination polymer composed of compressed NiN 4F 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. © 2012 American Chemical Society.

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