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.


$μ$SR study of magnetic order in the organic quasi-one-dimensional ferromagnet F4BImNN

Phys. Rev. B American Physical Society 88 (2013) 064423-064423

SJ Blundell, JS Möller, T Lancaster, PJ Baker, FL Pratt, G Seber, PM Lahti


Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch

Phys. Rev. B American Physical Society 87 (2013) 241102-241102

S Ghannadzadeh, JS Möller, PA Goddard, T Lancaster, F Xiao, SJ Blundell, A Maisuradze, R Khasanov, JL Manson, SW Tozer, D Graf, JA Schlueter


Low-field superconducting phase of (TMTSF)2ClO4.

Phys Rev Lett 110 (2013) 107005-

FL Pratt, T Lancaster, SJ Blundell, C Baines

The low-field phase of the organic superconductor (TMTSF)(2)ClO(4) is studied by muon-spin rotation. The zero temperature limit of the magnetic penetration depth within the TMTSF layers is obtained to be λ(ab)(0) = 0.86(2) μm. Temperature dependence of the muon-spin relaxation shows no indication of gap nodes on the Fermi surface nor of any spontaneous fields due to time-reversal-symmetry breaking. The weight of evidence suggests that the symmetry of this low-field phase is odd-frequency p-wave singlet, a novel example of odd-frequency pairing in a bulk superconductor.


Antiferromagnetic ordering through a hydrogen-bonded network in the molecular solid CuF2(H2O)2(3-chloropyridine).

Chem Commun (Camb) 49 (2013) 499-501

SH Lapidus, JL Manson, H Park, AJ Clement, S Ghannadzadeh, P Goddard, T Lancaster, JS Möller, SJ Blundell, MTF Telling, J Kang, M-H Whangbo, JA Schlueter

CuF(2)(H(2)O)(2)(3-chloropyridine) possesses a five-coordinate Cu(2+) center with a slightly distorted trigonal bypyramidal coordination geometry. Strong intermolecular F···H-O hydrogen bonds enable the formation of 2D layers and provide the primary magnetic exchange path that leads to the stabilization of long-range antiferromagnetic (AFM) order below T(N) = 2.1 K.


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 (T c) and the As-Fe-As bond angle (α) rather than the height of As from the Fe layers (h As). As the pressure was increased, T c of 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 T c is consistent with Lee et al.'s plot of T c as a function of α, and from this result, we conclude that T c will fall to zero at around α = 98°. © 2013 The Korean Physical Society.


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

Nature Materials 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.


Quantum states of muons in fluorides

Phys. Rev. B American Physical Society 87 (2013) 121108-121108

JS Möller, D Ceresoli, T Lancaster, N Marzari, SJ Blundell


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.


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

Polyhedron 52 (2013) 679-688

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

The crystal structure, phonon density-of-states, and magnetic properties of Mn(dca)2(o-phen) (dca = dicyanamide, N(CN)2-; o-phen = 1,10-phenanthroline) have been studied using several methods including magnetization, muon-spin relaxation, and neutron and X-ray scattering. From X-ray crystallography the structure was shown to consist of octahedral MnN 6 sites that are connected via four μ1,5-bridging dca ligands while the two remaining cis-positions are occupied by N-donors of the o-phen ligand. Two Mn2+ ions are bridged together by two dca anions to afford [Mn2(dca)2(o-phen)2]2+ "dimers." These dimers are linked in 2D via μ1,5-dca ligands to afford a polymeric structure whereas the o-phen ligands act as spacers. Bulk magnetic susceptibility data show a characteristic broad maximum at 2.65 K while neutron scattering and muon-spin relaxation data establish TN = 1.85 K. For T ≪ TN, the magnetization, as obtained in pulsed-fields, saturates at 5.75 T and reaches a moment typical of an isotropic S = 5/2 ion. The magnetic structure was determined and found to consist of antiferromagnetically ordered Mn2+ moments oriented in the ac-plane with no spin-canting being evident. The magnetic space group symmetry for the Mn2+ moments is P21′/c with each Mn 2+ ion carrying a magnetic moment of 4.7(1) μB, in good agreement with the value of 5.0 μB expected for an isotropic, high-spin S = 5/2 ion. The fact that LRO is observed in Mn(dca) 2(o-phen) is unusual among low-dimensional dicyanamide complexes especially since only μ1,5-dca modes are involved.


Imaging Cooper pairing of heavy fermions in CeCoIn <inf>5</inf>

Nature Physics 9 (2013) 468-473

MP Allan, F Massee, DK Morr, J Van Dyke, AW Rost, AP Mackenzie, C Petrovic, JC Davis

The Cooper pairing mechanism of heavy fermionsuperconductors, long thought to be due to spin fluctuations, has not yet been determined. It is the momentum space (k-space) structure of the superconducting energy gap Δ(k) that encodes specifics of this pairing mechanism. However, because the energy scales are so low, it has not been possible to directly measure Δ(k) for any heavy fermion superconductor. Bogoliubov quasiparticle interference imaging, a proven technique for measuring the energy gaps of superconductors with high critical temperatures, has recently been proposed as a new method to measure Δ(k) in heavy fermion superconductors, specifically CeCoIn 5 Ref.). By implementing this method, we detect a superconducting energy gap whose nodes are oriented along kâ̂¥(±1,±1)π/a 0 directions. Moreover, for the first time in any heavy fermion superconductor, we determine the detailed structure of its multiband energy gaps Δ i (k). For CeCoIn 5, this information includes: the complex band structure and Fermi surface of the hybridized heavy bands, the fact that largest magnitude Δ(k) opens on a high- k band so that the primary gap nodes occur at unforeseen k-space locations, and that the Bogoliubov quasiparticle interference patterns are most consistent with d x 2 -y 2 gap symmetry. Such quantitative knowledge of both the heavy band-structure and superconducting gap-structure will be critical in identifying the microscopic pairing mechanism of heavy fermion superconductivity. © 2013 Macmillan Publishers Limited. All rights reserved.


Anisotropic impurity states, quasiparticle scattering and nematic transport in underdoped Ca(Fe <inf>1-x</inf> Co <inf>x</inf> ) <inf>2</inf> As <inf>2</inf>

Nature Physics 9 (2013) 220-224

MP Allan, TM Chuang, F Massee, Y Xie, N Ni, SL Bud'Ko, GS Boebinger, Q Wang, DS Dessau, PC Canfield, MS Golden, JC Davis

Iron-based high-temperature superconductivity develops when the 'parent' antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms. But their impact on atomic-scale electronic structure, although in theory rather complex, is unknown experimentally. What is known is that a strong transport anisotropy with its resistivity maximum along the crystal b axis, develops with increasing concentration of dopant atoms; this 'nematicity'vanishes when the parent phase disappears near the maximum superconducting T c. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom and the transport nematicity has therefore become a pivotal focus of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe 1-x Co x ) 2 As 2 generates a dense population of identical anisotropic impurity states. Each is ∼ 8 Fe-Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b axis. These data provide direct support for the recent proposals that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density and for the occurrence of the highest resistivity along the b axis. Copyright © 2013 Macmillan Publishers Limited. All rights reserved.


Correlated Electrons in Quantum Matter

CONTEMPORARY PHYSICS 54 (2013) 117-118

SJ Blundell


Weak magnetic transitions in pyrochlore Bi$_2$Ir$_2$O$_7$

Phys. Rev. B American Physical Society 87 (2013) 180409-180409

PJ Baker, JS Möller, FL Pratt, W Hayes, SJ Blundell, T Lancaster, TF Qi, G Cao


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.


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.


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.


Generalized rotational susceptibility studies of solid <sup>4</sup>He

Journal of Low Temperature Physics 169 (2012) 180-196

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

Using a novel SQUID-based torsional oscillator (TO) technique to achieve increased sensitivity and dynamic range, we studied TO's containing solid 4He. Below ∼250 mK, the TO resonance frequency f increases and its dissipation D passes through a maximum as first reported by Kim and Chan. To achieve unbiased analysis of such 4He rotational dynamics, we implemented a new approach based upon the generalized rotational susceptibility χ 4He-1(ω, T ). Upon cooling, we found that equilibration times within f (T) and D(T) exhibit a complex synchronized ultraslow evolution toward equilibrium indicative of glassy freezing of crystal disorder conformations which strongly influence the rotational dynamics. We explored a more specific χ 4He-1(ω, τ(T)) with τ(T) representing a relaxation rate for inertially active microscopic excitations. In such models, the characteristic temperature T* at which df/dT and D pass simultaneously through a maximum occurs when the TO angular frequency ω and the relaxation rate are matched: ωτ(T*) = 1. Then, by introducing the free inertial decay (FID) technique to solid 4He TO studies, we carried out a comprehensive map of f(T,V) and D(T,V) where V is the maximum TO rim velocity. These data indicated that the same microscopic excitations controlling the TO motions are generated independently by thermal and mechanical stimulation of the crystal. Moreover, a measure for their relaxation times τ(T,V) diverges smoothly everywhere without exhibiting a critical temperature or velocity, as expected in ωτ = 1 models. Finally, following the observations of Day and Beamish, we showed that the combined temperature-velocity dependence of the TO response is indistinguishable from the combined temperature-strain dependence of the 4He shear modulus. Together, these observations imply that ultra-slow equilibration of crystal disorder conformations controls the rotational dynamics and, for any given disorder conformation, the anomalous rotational responses of solid 4He are associated with generation of the same microscopic excitations as those produced by direct shear strain. © Springer Science+Business Media, LLC 2012.


Resonant soft X-ray scattering, stripe order, and the electron spectral function in cuprates

Physica C: Superconductivity and its Applications 481 (2012) 15-22

P Abbamonte, E Demler, JC Séamus Davis, JC Campuzano

We review the current state of efforts to use resonant soft X-ray scattering (RSXS), which is an elastic, momentum-resolved, valence band probe of strongly correlated electron systems, to study stripe-like phenomena in copper-oxide superconductors and related materials. We review the historical progress including RSXS studies of Wigner crystallization in spin ladder materials, stripe order in 214-phase nickelates, 214-phase cuprates, and other systems. One of the major outstanding issues in RSXS concerns its relationship to more established valence band probes, namely angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM). These techniques are widely understood as measuring a one-electron spectral function, yet a relationship between RSXS and a spectral function has so far been unclear. Using physical arguments that apply at the oxygen K edge, we show that RSXS measures the square modulus of an advanced version of the Green's function measured with STM. This indicates that, despite being a momentum space probe, RSXS is more closely related to STM than to ARPES techniques. Finally, we close with some discussion of the most promising future directions for RSXS. We will argue that the most promising area lies in high magnetic field studies, particularly of edge states in strongly correlated heterostructures, and the vortex state in superconducting cuprates, where RSXS may clarify the anomalous periodicities observed in recent quantum oscillation experiments. © 2012 Elsevier B.V. All rights reserved.

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