Publications


Configuration-selective spectroscopic studies of Er3+ centers in ErSc2N@C80 and Er2ScN@C80 fullerenes.

J Chem Phys 127 (2007) 194504-

A Tiwari, G Dantelle, K Porfyrakis, RA Taylor, AA Watt, A Ardavan, GA Briggs

Low temperature photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of high purity ErSc(2)N@C(80) and Er(2)ScN@C(80) fullerenes reveal at least two metastable configurations of the Er(3+) ion within the cage, consistent with previous observations from x-ray diffraction. Using PLE measurements at a number of different emission wavelengths we have characterized the ground state, (4)I(152), and the first excited state, (4)I(132), of the various Er(3+) configurations and their crystal-field splitting. We present detailed energy level diagrams for the ground and excited states of the two dominant configurations of ErSc(2)N@C(80) and Er(2)ScN@C(80).


Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits

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

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

We present experimental evidence for a hitherto unconfirmed type of angle-dependent magnetoresistance oscillation caused by magnetic breakdown. The effect was observed in the organic superconductor κ- (BEDT-TTF)2 Cu (NCS)2 using hydrostatic pressures of up to 9.8 kbar and magnetic fields of up to 33 T. In addition, we show that similar oscillations are revealed in ambient-pressure measurements, provided that the Shubnikov-de Haas oscillations are suppressed either by elevated temperatures or filtering of the data. These results provide a compelling validation of Pippard's semiclassical picture of magnetic breakdown. © 2007 The American Physical Society.


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.


Will spin-relaxation times in molecular magnets permit quantum information processing?

Physical Review Letters 98 (2007)

A Ardavan, O Rival, JJL Morton, SJ Blundell, AM Tyryshkin, GA Timco, REP Winpenny

Using X-band pulsed electron-spin resonance, we report the intrinsic spin-lattice (T1) and phase-coherence (T2) relaxation times in molecular nanomagnets for the first time. In Cr7M heterometallic wheels, with M=Ni and Mn, phase-coherence relaxation is dominated by the coupling of the electron spin to protons within the molecule. In deuterated samples T2 reaches 3μs at low temperatures, which is several orders of magnitude longer than the duration of spin manipulations, satisfying a prerequisite for the deployment of molecular nanomagnets in quantum information applications. © 2007 The American Physical Society.


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


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


Exploiting the Poincare-Bloch symmetry to design high-fidelity broadband composite linear retarders

NEW JOURNAL OF PHYSICS 9 (2007) ARTN 24

A Ardavan


Efficient dynamic nuclear polarization at high magnetic fields.

Phys Rev Lett 98 (2007) 220501-

GW Morley, J van Tol, A Ardavan, K Porfyrakis, J Zhang, GA 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 15N@C60 by a factor of 10(3) 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.


Synthesis of fullerene dimers with controllable length

PHYS STATUS SOLIDI B 244 (2007) 3849-3852

K Porfyrakis, MR Sambrook, TJ Hingston, J Zhang, A Ardavan, GAD Briggs

Fullerenes have attracted interest as materials for next-generation electronic devices. Considerable effort has been focused on the chemical functionalization of fullerenes. Fullerene dimers possess interesting photophysical effects. Herein we present an overview of our efforts on the synthesis and characterization of different types of fullerene dimers such as directly bonded dimers, a short chain C-60 dimer and other dimers with bridge molecules of varying length. By choosing the bridge-molecule one can control the interfullerene spacing and thus tune the electronic interaction between the fullerenes. (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.


Synthesis of fullerene dimers with controllable length

Physica Status Solidi (B) Basic Research 244 (2007) 3849-3852

K Porfyrakis, MR Sambrook, TJ Hingston, J Zhang, GAD Briggs, A Ardavan

Fullerenes have attracted interest as materials for next-generation electronic devices. Considerable effort has been focused on the chemical functionalization of fullerenes. Fullerene dimers possess interesting photophysical effects. Herein we present an overview of our efforts on the synthesis and characterization of different types of fullerene dimers such as directly bonded dimers, a short chain C dimer and other dimers with bridge molecules of varying length. By choosing the bridge-molecule one can control the interfullerene spacing and thus tune the electronic interaction between the fullerenes. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.


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


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

Physica Status Solidi (B) Basic Research 244 (2007) 3874-3878

A Ardavan, JJL Morton, SC Benjamin, K Porfyrakis, GAD 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@C. 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 nuclear spin levels of N@C, we implement an ultrafast phase gate on the nuclear qubit by exciting an electron spin transition. © 2007 WILEY-VCH Verlag GmbH & Co. KGaA.


Quantum information processing with delocalized qubits under global control.

Phys Rev Lett 99 (2007) 030501-

J Fitzsimons, L Xiao, SC Benjamin, JA Jones

Conventional quantum computing schemes are incompatible with nanometer-scale "hardware," where the closely packed spins cannot be individually controlled. We report the first experimental demonstration of a global control paradigm: logical qubits delocalize along a spin chain and are addressed via the two terminal spins. Using NMR studies on a three-spin molecule, we implement a globally clocked quantum mirror that outperforms the equivalent swap network. We then extend the protocol to support dense qubit storage and demonstrate this experimentally via Deutsch and Deutsch-Jozsa algorithms.


PL, magneto-PL and PLE of the trimetallic nitride template fullerene ErN@C

Physica Status Solidi (B) Basic Research 243 (2006) 3037-3041

MAG Jones, JJL Morton, GAD Briggs, RA Taylor, A Ardavan

ErN@C exhibits sharp optical emission lines in the near-infrared attributed to fluorescence from the Er ion. Here we demonstrate that high magnetic fields cause this spectrum to split, corresponding to transitions from the lowest field-split Kramers doublet of the I manifold to the four lowest field-split levels of the I manifold. The internal structure of these fullerenes can be spatially aligned with a preferred orientation under high magnetic field; the effect of alignment is to reduce the broadening associated with the isotropic spatial averaging characteristic of powder or frozen-solution spectra. Using a tunable 1.5 μm laser, we directly observe non-cage-mediated optical interactions with the Er ion. This spectroscopic method provides the opportunity to map the energy level structure of the incarcerated ion and to coherently control its quantum state. These qualities suggest that rare-earth endohedral fullerenes have characteristics that could be employed as a readout pathway for fullerene-based quantum information processing. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA.