Publications


[Cu(HF(2))(2)(pyrazine)](n): A Rectangular Antiferromagnetic Lattice with a Spin Exchange Path Made Up of Two Different FHF(-) Bridges.

Angew Chem Int Ed Engl (2011)

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


Quantum interference between photo-excited states in a solid-state Mott insulator

Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference: 2010 Laser Science to Photonic Applications, CLEO/QELS 2010 (2010)

S Wall, D Brida, SR Clark, HP Ehrke, D Jaksch, A Ardavan, S Bonora, H Uemura, Y Takahashi, T Hasegawa, H Okamoto, G Cerullo, A Cavalleri

By exciting with sub-10-fs 1.6-μm pulses the quasi-one-dimensional Mott insulator ETF 2 TCNQ, we observe prompt collapse of the Mott gap modulated by 24-THz oscillations of the gap, which are assigned to quantum interference between holon-doublon excitations. © 2010 Optical Society of America.


Relaxation of muon spins in molecular nanomagnets

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

T Lancaster, SJ Blundell, FL Pratt, I Franke, AJ Steele, PJ Baker, Z Salman, C Baines, I Watanabe, S Carretta, GA Timco, REP Winpenny

We address the cause of the unusual muon spin-relaxation (μ+ SR) results on molecular nanomagnets (MNMs). Through measurements on protonated and deuterated samples of the MNMs Cr7 Mn (S=1) and Cr8 (S=0), we show that the muon spin for S≠0 MNMs is relaxed via dynamic fluctuations of the electronic spins. A freezing out of dynamic processes occurs on cooling and at low temperatures the muon spins are relaxed by the electronic spins which themselves are dephased by incoherent nuclear-field fluctuations. We observe a transition to a state of static magnetic order of the MNM electronic spins in Cr7Mn below 2 K. © 2010 The American Physical Society.


Electron paramagnetic resonance investigation of purified catalyst-free single-walled carbon nanotubes

ACS Nano 4 (2010) 7708-7716

M Zaka, Y Ito, H Wang, W Yan, A Robertson, YA Wu, MH Rümmeli, D Staunton, T Hashimoto, JJL Morton, A Ardavan, GAD Briggs, JH Warner

Electron paramagnetic resonance of single-walled carbon nanotubes (SWCNTs) has been bedevilled by the presence of paramagnetic impurities. To address this, SWCNTs produced by laser ablation with a nonmagnetic PtRhRe catalyst were purified through a multiple step centrifugation process in order to remove amorphous carbon and catalyst impurities. Centrifugation of a SWCNT solution resulted in sedimentation of carbon nanotube bundles containing clusters of catalyst particles, while isolated nanotubes with reduced catalyst particle content remained in the supernatant. Further ultracentrifugation resulted in highly purified SWCNT samples with a narrow diameter distribution and almost no detectable catalyst particles. Electron paramagnetic resonance (EPR) signals were detected only for samples which contained catalyst particles, with the ultracentrifuged SWCNTs showing no EPR signal at X-band (9.4 GHz) and fields < 0.4 T. © 2010 American Chemical Society.


Concepts in Thermal Physics

, 2010

SJ Blundell, KM Blundell

© Stephen J. Blundell and Katherine M. Blundell 2010. All rights reserved. An understanding of thermal physics is crucial to much of modern physics, chemistry, and engineering. This book provides a modern introduction to the main principles that are foundational to thermal physics, thermodynamics, and statistical mechanics. The key concepts are carefully presented in a clear way, and new ideas are illustrated with worked examples as well as a description of the historical background to their discovery. Applications are presented to subjects as diverse as stellar astrophysics, information and communication theory, condensed matter physics, and climate change. Each chapter concludes with detailed exercises. This second edition of the text maintains the structure and style of the first edition but extends its coverage of thermodynamics and statistical mechanics to include several new topics, including osmosis, diffusion problems, Bayes theorem, radiative transfer, the Ising model, and Monte Carlo methods. New examples and exercises have been added throughout.


Spin fluctuations and orbital ordering in quasi-one-dimensional α-Cu(dca)<inf>2</inf>(pyz) {dca = dicyanamide = N(CN)<inf>2</inf><sup>-</sup>; pyz = pyrazine}, a molecular analogue of KCuF<inf>3</inf>

Polyhedron 29 (2010) 514-520

JL Manson, T Lancaster, SJ Blundell, Y Qiu, J Singleton, P Sengupta, FL Pratt, J Kang, C Lee, MH Whangbo

The magnetic properties of α-Cu(dca)2(pyz) were examined by magnetic susceptibility, magnetization, inelastic neutron scattering (INS), muon-spin relaxation (μSR) measurements and by first-principles density functional theoretical (DFT) calculations and quantum Monte Carlo (QMC) simulations. The χ versus T curve shows a broad maximum at 3.5 K, and the data between 2 and 300 K is well described by an S = 1/2 Heisenberg uniform chain model with g = 2.152(1) and J/kB = -5.4(1) K. μSR measurements, conducted down to 0.02 K and as a function of longitudinal magnetic field, show no oscillations in the muon asymmetry function A(t). This evidence, together with the lack of spin wave formation as gleaned from INS data, suggests that no long-range magnetic order takes place in α-Cu(dca)2(pyz) down to the lowest measured temperatures. Electronic structure calculations further show that the spin exchange is significant only along the Cu-pyz-Cu chains, such that α-Cu(dca)2(pyz) can be described by a Heisenberg antiferromagnetic chain model. Further support for this comes from the M versus B curve, which is strongly concave owing to the reduced spin dimensionality. α-Cu(dca)2(pyz) is a molecular analogue of KCuF3 owing to dx2 - y2 orbital ordering where nearest-neighbor magnetic orbital planes of the Cu2+ sites are orthogonal in the planes perpendicular to the Cu-pyz-Cu chains. © 2009 Elsevier Ltd.


Quantum interference between photo-excited states in a solid-state mott insulator

Optics InfoBase Conference Papers (2010)

S Wall, D Brida, SR Clark, HP Ehrke, D Jaksch, A Ardavan, S Bonora, H Uemura, Y Takahashi, T Hasegawa, H Okamoto, G Cerullo, A Cavalleri

By exciting with sub-10-fs 1.6-μm pulses the quasi-one-dimensional Mott insulator ETF 2 TCNQ, we observe prompt collapse of the Mott gap modulated by 24-THz oscillations of the gap, which are assigned to quantum interference between holon-doublon excitations. © 2010 Optical Society of America.


Magnetic field sensors using 13-spin cat states

PHYSICAL REVIEW A 82 (2010) ARTN 022330

S Simmons, JA Jones, SD Karlen, A Ardavan, JJL Morton


Giant magnetic hardness in the synthetic mineral ferrimagnet K <inf>2</inf>Co<sup>II</sup><inf>3</inf>(OH)<inf>2</inf>(SO<inf>4</inf>) <inf>3</inf>(H2O)<inf>2</inf>

Chemistry of Materials 22 (2010) 4090-4095

S Vilminot, PJ Baker, SJ Blundell, T Sugano, G André, M Kurmoo

Wepresent the synthesis, single-crystal X-ray (173 K) and powder neutron (2-30 K) structures and its thermal, optical and magnetic properties of K 2CoII3(OH)2(SO4) 2(H2O)2. It is a ferrimagnet (TC= 29.7 K) constructed of Co3(OH)2 diamond chains connected by sulfate and it displays hysteresis loops ranging from being soft with nearly zero coercivity between 29 and 10 K to very hard reaching coercive field exceeding 70 kOe at 1.8 K. This dramatic change is associated with the changes in domain shape due to the strong exchange anisotropy. Considerable frequency dependence of the acsusceptibilities is observed in the ordered state.Measurements on a single crystal have established the magnetic axes to be a-axis (easy), b-axis (intermediate), and c-axis (hard). © 2010 American Chemical Society.


Phase transition in the localized ferromagnet EuO probed by μsR

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

SJ Blundell, T Lancaster, FL Pratt, PJ Baker, W Hayes, JP Ansermet, A Comment

We report results of muon-spin-rotation measurements performed on the ferromagnetic semiconductor EuO, which is one of the best approximations to a localized ferromagnet. We argue that implanted muons are sensitive to the internal field primarily through a combination of hyperfine and Lorentz fields. The temperature dependences of the internal field and the relaxation rate have been measured and are compared with previous theoretical predictions. © 2010 The American Physical Society.


Local magnetism and magnetoelectric effect in HoMnO3 studied with muon-spin relaxation

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

HJ Lewtas, T Lancaster, PJ Baker, SJ Blundell, D Prabhakaran, FL Pratt

We present the results of muon-spin relaxation (μ+ SR) measurements on the hexagonal manganite HoMnO3. Features in the temperature-dependent relaxation rate λ correlate with the magnetic transitions at 76, 38, and 34 K. The highest temperature transition, associated with the ordering of Mn3+ moments has the largest effect on λ. The application of a static electric field of E=5× 106 Vm-1 below T=50K causes a small reduction in λ which is suggestive of coupling between ferroelectric and magnetic domain walls in the ordered state of the material. © 2010 The American Physical Society.


Exchange interactions of spin-active metallofullerenes in solid-state carbon networks

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

M Zaka, JH Warner, Y Ito, JJL Morton, MH Rümmeli, T Pichler, A Ardavan, H Shinohara, GAD Briggs

The electron paramagnetic resonance (EPR) of spin-active metallofullerenes (MFs) La@ C82 and Sc@ C82 diluted in solid-state C60 crystalline matrices with molar concentrations varying from 0.4% to 100% are investigated. For dilute concentrations, the hyperfine structure of the MFs is resolved, and as the concentration increases exchange narrowing is observed leading to a single peak in the EPR. Sc@ C82 MFs are inserted into single-walled carbon nanotubes to form peapods with concentrations of 10% and 0.1%, diluted with C60. For the case of peapods containing 10% Sc@ C82 a strong narrow peak is observed in X -band CW EPR, but not pulsed measurements. Peapods containing Ce@ C82 MFs are prepared and these also show similar CW EPR to the Sc@ C82, indicating the peak arises from charge transfer with the SWNT. © 2010 The American Physical Society.


Compositional control of the superconducting properties of LiFeAs.

J Am Chem Soc 132 (2010) 10467-10476

MJ Pitcher, T Lancaster, JD Wright, I Franke, AJ Steele, PJ Baker, FL Pratt, WT Thomas, DR Parker, SJ Blundell, SJ Clarke

The response of the superconducting state and crystal structure of LiFeAs to chemical substitutions on both the Li and the Fe sites has been probed using high-resolution X-ray and neutron diffraction measurements, magnetometry, and muon-spin rotation spectroscopy. The superconductivity is extremely sensitive to composition: Li-deficient materials (Li(1-y)Fe(1+y)As with Fe substituting for Li) show a very rapid suppression of the superconducting state, which is destroyed when y exceeds 0.02, echoing the behavior of the Fe(1+y)Se system. Substitution of Fe by small amounts of Co or Ni results in monotonic lowering of the superconducting transition temperature, T(c), and the superfluid stiffness, rho(s), as the electron count increases. T(c) is lowered monotonically at a rate of 10 K per 0.1 electrons added per formula unit irrespective of whether the dopant is Co and Ni, and at higher doping levels superconductivity is completely suppressed. These results and the demonstration that the superfluid stiffness in these LiFeAs-derived compounds is higher than in all of the iron pnictide materials underlines the unique position that LiFeAs occupies in this class.


Control of the competition between a magnetic phase and a superconducting phase in cobalt-doped and nickel-doped NaFeAs using electron count

Physical Review Letters 104 (2010)

DR Parker, MJP Smith, T Lancaster, AJ Steele, I Franke, PJ Baker, FL Pratt, MJ Pitcher, SJ Blundell, SJ Clarke

Using a combination of neutron, muon, and synchrotron techniques we show how the magnetic state in NaFeAs can be tuned into superconductivity by replacing Fe by either Co or Ni. The electron count is the dominant factor, since Ni doping has double the effect of Co doping for the same doping level. We follow the structural, magnetic, and superconducting properties as a function of doping to show how the superconducting state evolves, concluding that the addition of 0.1 electrons per Fe atom is sufficient to traverse the superconducting domain, and that magnetic order coexists with superconductivity at doping levels less than 0.025 electrons per Fe atom. © 2010 The American Physical Society.


Muon-spin relaxation and heat capacity measurements on the magnetoelectric and multiferroic pyroxenes LiFeSi<inf>2</inf>O<inf>6</inf> and NaFeSi <inf>2</inf>O<inf>6</inf>

Physical Review B - Condensed Matter and Materials Physics 81 (2010)

PJ Baker, HJ Lewtas, SJ Blundell, T Lancaster, I Franke, W Hayes, FL Pratt, L Bohatý, P Becker

We present the results of muon-spin relaxation and heat capacity measurements on LiFeSi2O6 and NaFeSi2O 6. In synthetic samples of both compounds we see a single muon oscillation frequency consistent with commensurate magnetic structures below TN. In contrast, for a specimen of naturally occurring NaFeSi2O 6, in which multiferroicity has been observed, a rapid Gaussian depolarization of the muon polarization is observed instead, showing that the magnetic structure in this case is more complex. Heat capacity measurements reproduce the phase diagrams previously derived from other techniques and demonstrate that the main contribution to the magnetic entropy is associated with the buildup of correlations in the quasi-one-dimensional Fe3+ chains. © 2010 The American Physical Society.


Magnetic field sensors using 13-spin cat states

Physical Review A - Atomic, Molecular, and Optical Physics 82 (2010)

S Simmons, JA Jones, SD Karlen, A Ardavan, JJL Morton

Measurement devices could benefit from entangled correlations to yield a measurement sensitivity approaching the physical Heisenberg limit. Building upon previous magnetometric work using pseudoentangled spin states in solution-state NMR, we present two conceptual advancements to better prepare and interpret the pseudoentanglement resource. We apply these to a 13-spin cat state to measure the local magnetic field with a 12.2 sensitivity increase over an equivalent number of isolated spins. © 2010 The American Physical Society.


Electron spin coherence in metallofullerenes: Y, Sc, and La@C<inf>82</inf>

Physical Review B - Condensed Matter and Materials Physics 82 (2010)

RM Brown, Y Ito, JH Warner, A Ardavan, H Shinohara, GAD Briggs, JJL Morton

Endohedral fullerenes encapsulating a spin-active atom or ion within a carbon cage offer a route to self-assembled arrays such as spin chains. In the case of metallofullerenes the charge transfer between the atom and the fullerene cage has been thought to limit the electron spin phase coherence time (T 2) to the order of a few microseconds. We study electron spin relaxation in several species of metallofullerene as a function of temperature and solvent environment, yielding a maximum T2 in deuterated o-terphenyl greater than 200 μs for Y, Sc, and La@C82. The mechanisms governing relaxation (T1, T2) arise from metal-cage vibrational modes, spin-orbit coupling and the nuclear spin environment. The T2 times are over 2 orders of magnitude longer than previously reported and consequently make metallofullerenes of interest in areas such as spin labeling, spintronics, and quantum computing. © 2010 The American Physical Society.


Controlling intermolecular spin interactions of La@C<inf>82</inf> in empty fullerene matrices

Physical Chemistry Chemical Physics 12 (2010) 1618-1623

Y Ito, JH Warner, R Brown, M Zaka, R Pfeiffer, T Aono, N Izumi, H Okimoto, JJL Morton, A Ardavan, H Shinohara, H Kuzmany, H Peterlik, GAD Briggs

The ESR properties and crystal structures of solid-state La@C82 in empty fullerene matrices were investigated by changing the concentration of La@C82 and the species of an empty fullerene matrix: C60, C70, C78(C2v(3)), C82(C2) and C84(D2d(4)). The rotational correlation time of La@C82 molecules tended to be shorter when La@C82 is dispersed in larger fullerene matrices because large C2n molecules provide more space for La@C82 molecules for rotating. La@C 82 dispersed in a hcp-C82 matrix showed the narrowest hyperfine structure (hfs) due to the ordered nature of La@C82 molecules in the C82 crystal. On the other hand, in a C60 matrix, La@C82 molecules formed clusters because of the large different solubility, which leads to the ESR spectra being broad sloping features due to strong dipole-dipole and exchange interactions. © the Owner Societies 2010.


Storage of multiple coherent microwave excitations in an electron spin ensemble.

Phys Rev Lett 105 (2010) 140503-

H Wu, RE George, JH Wesenberg, K Mølmer, DI Schuster, RJ Schoelkopf, KM Itoh, A Ardavan, JJL Morton, GAD Briggs

Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize a coherent memory using a pulsed magnetic field gradient and demonstrate the storage and retrieval of up to 100 weak 10 GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds.


Electron paramagnetic resonance investigation of purified catalyst-free single-walled carbon nanotubes.

ACS Nano 4 (2010) 7708-7716

M Zaka, Y Ito, H Wang, W Yan, A Robertson, YA Wu, MH Rümmeli, D Staunton, T Hashimoto, JJL Morton, A Ardavan, GAD Briggs, JH Warner

Electron paramagnetic resonance of single-walled carbon nanotubes (SWCNTs) has been bedevilled by the presence of paramagnetic impurities. To address this, SWCNTs produced by laser ablation with a nonmagnetic PtRhRe catalyst were purified through a multiple step centrifugation process in order to remove amorphous carbon and catalyst impurities. Centrifugation of a SWCNT solution resulted in sedimentation of carbon nanotube bundles containing clusters of catalyst particles, while isolated nanotubes with reduced catalyst particle content remained in the supernatant. Further ultracentrifugation resulted in highly purified SWCNT samples with a narrow diameter distribution and almost no detectable catalyst particles. Electron paramagnetic resonance (EPR) signals were detected only for samples which contained catalyst particles, with the ultracentrifuged SWCNTs showing no EPR signal at X-band (9.4 GHz) and fields < 0.4 T.

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