Publications


Persistence to high temperatures of interlayer coherence in an organic superconductor.

Phys Rev Lett 99 (2007) 027004-

J Singleton, PA Goddard, A Ardavan, AI Coldea, SJ Blundell, RD McDonald, S Tozer, JA Schlueter

The interlayer magnetoresistance rho(zz) of the organic metal kappa-(BEDT-TTF)(2)Cu(NCS)(2) is studied in fields of up to 45 T and at temperatures T from 0.5 to 30 K. The peak in rho(zz) seen in in-plane fields, a definitive signature of interlayer coherence, remains to Ts exceeding the Anderson criterion for incoherent transport by a factor approximately 30. Angle-dependent magnetoresistance oscillations are modeled using an approach based on field-induced quasiparticle paths on a 3D Fermi surface, to yield the T dependence of the scattering rate tau(-1). The results suggest that tau(-1) does not vary strongly over the Fermi surface, and that it has a T(2) dependence due to electron-electron scattering.


Toward controlled spacing in one-dimensional molecular chains: alkyl-chain-functionalized fullerenes in carbon nanotubes.

J Am Chem Soc 129 (2007) 8609-8614

TW Chamberlain, A Camenisch, NR Champness, GA Briggs, SC Benjamin, A Ardavan, AN Khlobystov

A range of fullerenes (C60) functionalized with long alkyl chains have been synthesized and inserted into single-walled carbon nanotubes. The impact of the alkyl chain length and of the type of linker between the addend and the fullerene cage on the geometry of molecular arrays in nanotube has been studied by high-resolution transmission electron microscopy. In the presence of functional groups the mean interfullerene separations are significantly increased by 2-8 nm depending on the length of the alkyl chain, but the periodicity of the fullerene arrays is disrupted due to the conformational flexibility of the alkyl groups.


Arbitrary precision composite pulses for NMR quantum computing.

J Magn Reson 189 (2007) 114-120

WG Alway, JA Jones

We discuss the implementation of arbitrary precision composite pulses developed using the methods of Brown et al. [K.R. Brown, A.W. Harrow, I.L. Chuang, Arbitrarily accurate composite pulse sequences, Phys. Rev. A 70 (2004) 052318]. We give explicit results for pulse sequences designed to tackle both the simple case of pulse length errors and the more complex case of off-resonance errors. The results are developed in the context of NMR quantum computation, but could be applied more widely.


Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

SURF SCI 601 (2007) 2750-2755

DF Leigh, C Norenberg, D Cattaneo, JHG Owen, K Porfyrakis, AL Bassi, A Ardavan, GAD Briggs

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of Er3N@C-80 and SC3N@C-80 on Ag/Si(111), Au(111)/mica, Si(111), and Si(001) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(I 1 1) whereas annealing at elevated temperatures (250-300 degrees C) is necessary for the self-assembly of close-packed islands on An(111). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(111) and already at room temperature on Si(001), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly vander-Waals on the other surfaces. (C) 2006 Elsevier B.V. All rights reserved.


Environmental effects on electron spin relaxation in N@C-60

PHYSICAL REVIEW B 76 (2007) ARTN 085418

JJL Morton, AM Tyryshkin, A Ardavan, K Porfyrakis, SA Lyon, GAD Briggs


Will Spin-Relaxation Times in Molecular Magnets Permit Quantum Information Processing?

Physical Review Letters 98 (2007) 057201 4pp-

A Ardavan, John J.L. Morton, Olivier Rival, Stephen J. Blundell


Morphology of the nonspherically decaying radiation beam generated by a rotating superluminal source.

J Opt Soc Am A Opt Image Sci Vis 24 (2007) 2443-2456

H Ardavan, A Ardavan, J Singleton, J Fasel, A Schmidt

We consider the nonspherically decaying radiation field that is generated by a polarization current with a superluminally rotating distribution pattern in vacuum, a field that decays with the distance R(P) from its source as R(P)(-1/2), instead of R(P)(-1). It is shown (i) that the nonspherical decay of this emission remains in force at all distances from its source independently of the frequency of the radiation, (ii) that the part of the source that makes the main contribution toward the value of the nonspherically decaying field has a filamentary structure whose radial and azimuthal widths become narrower (as R(P)(-2) and R(P)(-3), respectively) the farther the observer is from the source, (iii) that the loci on which the waves emanating from this filament interfere constructively delineate a radiation subbeam that is nondiffracting in the polar direction, (iv) that the cross-sectional area of each nondiffracting subbeam increases as R(P), instead of R(P)(2), so that the requirements of conservation of energy are met by the nonspherically decaying radiation automatically, and (v) that the overall radiation beam within which the field decays nonspherically consists, in general, of the incoherent superposition of such coherent nondiffracting subbeams. These findings are related to the recent construction and use of superluminal sources in the laboratory and numerical models of the emission from them. We also briefly discuss the relevance of these results to the giant pulses received from pulsars.


Self-assembly of trimetallic nitride template fullerenes on surfaces studied by STM

Surface Science 601 (2007) 2750-2755

DF Leigh, C Nörenberg, K Porfyrakis, GAD Briggs, D Cattaneo, JHG Owen, A Li Bassi, A Ardavan

Trimetallic nitride template fullerenes have been deposited onto a variety of substrates in order to elucidate the substrate-fullerene interactions. We have investigated self-assembled island formation and molecular detail of ErN@C and ScN@C on Ag/Si(1 1 1), Au(1 1 1)/mica, Si(1 1 1), and Si(0 0 1) using variable temperature scanning tunnelling microscopy (STM). At room temperature, the fullerenes self-assemble into monolayer-high hexagonal close-packed islands on Ag-passivated Si(1 1 1) whereas annealing at elevated temperatures (250-300 °C) is necessary for the self-assembly of close-packed islands on Au(1 1 1). Intra-molecular resolution of the fullerenes has been achieved at liquid nitrogen temperature on Ag/Si(1 1 1) and already at room temperature on Si(0 0 1), when the rotation of the fullerenes is frozen. Whereas the bonding between the fullerenes and Si surfaces is mainly covalent, it appears to be mainly van-der-Waals on the other surfaces. © 2006 Elsevier B.V. All rights reserved.


Fluctuating superconductivity in organic molecular metals close to the Mott transition.

Nature 449 (2007) 584-587

MS Nam, A Ardavan, SJ Blundell, JA Schlueter

On cooling through the transition temperature T(c) of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed. Here we argue that a key pseudogap phenomenon-fluctuating superconductivity occurring substantially above the transition temperature-could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the kappa-(BEDT-TTF)2X organic molecular metals can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases.


Observation of Berry's phase in a solid-state qubit

SCIENCE 318 (2007) 1889-1892

PJ Leek, JM Fink, A Blais, R Bianchetti, M Goeppl, JM Gambetta, DI Schuster, L Frunzio, RJ Schoelkopf, A Wallraff


Rapid oscillations in (TMTSF) PF

Journal of Low Temperature Physics 142 (2006) 309-314

AV Kornilov, VM Pudalov, A-K Klehe, A Ardavan, J Singleton, JS Qualls

In order to clarify the origin of the "Rapid Oscillation" (RO) in (TMTSF) PF , we studied the magnetoresistance anisotropy in the Field-Induced Spin Density Wave (FISDW) phase. We have found that in the FISDW insulating state, the Fermi surface is not totally gapped; the remaining 2D metallic pockets are quantized in magnetic field and give rise to the RO. Decreasing temperature does not change the size and orientation of the closed pockets, rather, it causes depopulation of the delocalized states in favor of the localized ones, resulting in the disappearance of the RO. © Springer Science+Business Media, LLC 2006.


Origin of rapid oscillations in low-dimensional (TMTSF)(2)PF6

PHYSICAL REVIEW B 76 (2007) ARTN 045109

AV Kornilov, VM Pudalov, A-K Klehe, A Ardavan, JS Qualls, J Singleton


Manipulation of quantum information in N@C-60 using electron and nuclear magnetic resonance

PHYS STATUS SOLIDI B 244 (2007) 3874-3878

A Ardavan, JJL Morton, SC Benjamin, K Porfyrakis, G Andrew, D Briggs, AM Tyryshkin, SA Lyon

We review recent progress towards implementing quantum information processing protocols using electron and nuclear magnetic resonance in the molecule N-14@C-60. Using conventional spectrometers, we measure the errors inherent in quantum manipulations of spins. By applying composite pulses, we correct the dominant error to a high degree. Identifying a two qubit computational basis out of the electron and oF nuclear spin levels of N-14@C-60, we implement an ultrafast phase gate on the nuclear qubit by exciting an electron spin transition. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


On the existence of rapid oscillations in various phases of quasi-one-dimensional (TMTSF)(2)PF6

JETP LETTERS 84 (2007) 628-631

AV Kornilov, VM Pudalov, A-K Klehe, A Ardavan, JS Qualls


Environmental effects on electron spin relaxation in N@ C60

Physical Review B - Condensed Matter and Materials Physics 76 (2007)

JJL Morton, A Ardavan, K Porfyrakis, GAD Briggs, AM Tyryshkin, SA Lyon

We examine environmental effects of surrounding nuclear spins on the electron spin relaxation of the N@ C60 molecule (which consists of a nitrogen atom at the center of a fullerene cage). Using dilute solutions of N@ C60 in regular and deuterated toluene, we observe and model the effect of translational diffusion of nuclear spins of the solvent molecules on the N@ C60 electron spin relaxation times. We also study spin relaxation in frozen solutions of N@ C60 in C S2, to which small quantities of a glassing agent, S2 Cl2, are added. At low temperatures, spin relaxation is caused by spectral diffusion of surrounding nuclear Cl 35,37 spins in the S2 Cl2, but, nevertheless, at 20 K, T2 as long as 0.23 ms is observed. © 2007 The American Physical Society.


Tuning electronic ground states by using chemical pressure on quasi-two dimensional β″-(BEDT-TTF) [(H O)M(C O ) ]·Y

Journal of Low Temperature Physics 142 (2006) 257-260

AI Coldea, AF Bangura, A Ardavan, J Singleton, A Akutsu-Sato, H Akutsu, P Day

We report high-field magnetotransport studies on quasi-two dimensional β″-(BEDT-TTF) [(H O)M(C O ) ]·Y where Y is a solvent in the anionic layer. By changing the size of the solvent the low temperatures electronic behaviour varies from superconducting (for larger solvents, Y = C H NO and C H CN) to metallic (for smaller solvents, Y = C H N and CH Cl ). These changes in the ground state are connected with modications of the Fermi surface, which varies from having one or two pockets for the superconducting charge- transfer salts to at least four pockets in the case of metallic ones. When superconducting, the materials have very large in-plane critical fields (up to 32 T) and enhanced effective masses compared with the metallic compounds. The role of the charge-order fluctuations in stabilizing the superconducting ground state and the effects of intrinsic local disorder is discussed. © 2007 Springer Science+Business Media, Inc.


Fluctuating superconductivity in organic molecular metals close to the Mott transition

Nature 449 (2007) 584-587

M-S Nam, A Ardavan, SJ Blundell, JA Schlueter

On cooling through the transition temperature T of a conventional superconductor, an energy gap develops as the normal-state charge carriers form Cooper pairs; these pairs form a phase-coherent condensate that exhibits the well-known signatures of superconductivity: zero resistivity and the expulsion of magnetic flux (the Meissner effect). However, in many unconventional superconductors, the formation of the energy gap is not coincident with the formation of the phase-coherent superfluid. Instead, at temperatures above the critical temperature a range of unusual properties, collectively known as 'pseudogap phenomena', are observed. Here we argue that a key pseudogap phenomenon-fluctuating superconductivity occurring substantially above the transition temperature-could be induced by the proximity of a Mott-insulating state. The Mott-insulating state in the κ-(BEDT-TTF) X organic molecular metals can be tuned, without doping, through superconductivity into a normal metallic state as a function of the parameter t/U, where t is the tight-binding transfer integral characterizing the metallic bandwidth and U is the on-site Coulomb repulsion. By exploiting a particularly sensitive probe of superconducting fluctuations, the vortex-Nernst effect, we find that a fluctuating regime develops as t/U decreases and the role of Coulomb correlations increases. ©2007 Nature Publishing Group.


Dissipation in the superconducting state of kappa-(BEDT-TTF)(2)Cu(NCS)(2)

PHYSICAL REVIEW B 76 (2007) ARTN 014506

L Yin, M-S Nam, JG Analytis, SJ Blundell, A Ardavan, JA Schlueter, T Sasaki


Efficient dynamic nuclear polarization at high magnetic fields

Physical Review Letters 98 (2007)

GW Morley, J Van Tol, A Ardavan, K Porfyrakis, J Zhang, GAD Briggs

By applying a new technique for dynamic nuclear polarization involving simultaneous excitation of electronic and nuclear transitions, we have enhanced the nuclear polarization of the nitrogen nuclei in N15@C60 by a factor of 103 at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice the maximum enhancement reported to date. This methodology will allow the initialization of the nuclear qubit in schemes exploiting N@C60 molecules as components of a quantum information processing device. © 2007 The American Physical Society.


Single qubit gates with jump and return sequences

PHYSICAL REVIEW A 74 (2006) ARTN 052324

MD Bowdrey, JA Jones