Publications


Several kinds of aminoxyl radicals and their metal ion complexes

MOL CRYST LIQ CRYS A 334 (1999) 477-486

T Sugano, SJ Blundell, FL Pratt, T Jestadt, BW Lovett, W Hayes, P Day

Recent results of magnetization, magnetic susceptibility and muon spin rotation/relaxation (mu SR) measurements of some organic neutral radicals based on aminoxyl and their anion radical complexes with all;ali and transition metal cations are reported. Ferromagnetic intermolecular interactions, coexistent with antiferromagnetic ones, in several carboxyaryl nitronyl nitroxide radicals are affected by inserting alkali metal ions, while the incorporation of transition metal ions result in complex behavior suggesting in part ferromagnetic interactions. Temperature dependence of spontaneous magnetization of TANOL suberate in the ordered state, obtained through the results of mu SR measurements, is characteristic of a two-dimensional magnetic lattice, consistent with that of magnetic susceptibility above the ordering temperature.


Josephson effect and a π-state in superfluid <sup>3</sup>He [7] (multiple letters)

Nature 397 (1999) 484-485

O Avenel, Y Mukharsky, E Varoquaux, S Backhaus, RW Simmonds, A Loshak, JC Davis, RE Packard


Bi-state superfluid <sup>3</sup>he weak links and the stability of josephson π States

Physical Review Letters 83 (1999) 3860-3863

A Marchenkov, RW Simmonds, S Backhaus, A Loshak, JC Davis, RE Packard

We have discovered new features of the current-phase relation, If, in superfluid 3He weak links. Firstly, we find that at any given temperature there are two distinct If functions that characterize the weak link. Secondly, both functions continuously develop an unusual form that ultimately leads to the previously reported p state. The observed form of If has recently been predicted for unconventional quantum fluids such as 3He, high- Tc superconductors, and Bose-Einstein condensates. The two distinct states are likely to originate from the textural degree of freedom in superfluid 3He. © 1999 The American Physical Society.


One-and two-dimensional angle-dependent magnetoresistance oscillations(AMROs) in kappa-(BEDT-TTF)(2)Cu(SCN)(2) in fields of up to 33 T

SYNTHETIC MET 103 (1999) 1905-1906

MS Nam, MM Honold, C Proust, N Harrison, CH Mielke, SJ Blundell, J Singleton, W Hayes, M Kurmoo, P Day

Angle-dependent magnetoresistance oscillations (AMROs) in kappa-(BEDT-TTF)(2)Cu(SCN)(2) have been observed in fields of up to 33 T. AMROs due to both open and closed Fermi surface sections have been measured simultaneously for the first time in an organic metal. This provides information concerning the low temperature Fermi surface and demonstrates the absence of a spin-density wave state in this material.


Muon-spin-relaxation studies of magnetic order and dynamics of the (formula presented) ruddlesden-popper phases (formula presented) (formula presented) nd, sm, eu, gd, tb, dy, and ho)

Physical Review B - Condensed Matter and Materials Physics 60 (1999) 12286-12293

RI Bewley, SJ Blundell, BW Lovett, T Jestädt, FL Pratt, KH Chow, W Hayes

Zero field muon spin relaxation (μSR) has been used to study the magnetic properties of (Formula presented) Ruddlesden-Popper phases (Formula presented) where (Formula presented) Nd, Sm, Eu, Gd, Tb, Dy, and Ho. The results show that the size of the lanthanide ion is crucial in determining the magnetic state and dynamics of the system. Because muons are implanted throughout the bulk of the sample, impurity phases contribute only according to their volume fraction. Hence in the case of biphasic samples the data are dominated by the majority phase. Although none of our samples has a ferromagnetic ground state, colossal magnetoresistance (CMR) is observed over a wide temperature range, (Formula presented) for both the Pr and Nd compounds. The μSR results show that the magnetic transition in both these samples is broad. Ordered, but fluctuating, regions form at (Formula presented) the reported onset of CMR, with the fluctuation rates gradually decreasing with temperature. Even at 5 K, fluctuations are still observed. The ferromagnetic double exchange between Mn ions becomes weaker as the size of the lanthanide ion decreases. (Formula presented) shows weak clustering at a much reduced temperature of 30 K whereas (Formula presented) shows spin-glass-like behavior. For all lanthanide ions smaller than Eu no long range magnetic ordering of the spins is observed and the observed relaxation rates follow an activated dependence. The technique allows us to extract the effective activation energy associated with the magnetic fluctuations of the lanthanide moments in samples with (Formula presented) Eu, Gd, Tb, Dy, and Ho. CMR is only observed where μSR measurements show a broad magnetic transition associated with fluctuations. We therefore believe that these fluctuating ordered regions are responsible for the extended temperature regime in which CMR has been observed in these nonferromagnetic (Formula presented) Ruddlesden-Popper phases. © 1999 The American Physical Society.


Stability of the vortex lattice in ET superconductors studied by mu SR

SYNTHETIC MET 103 (1999) 1925-1928

SJ Blundell, SL Lee, FL Pratt, CM Aegerter, T Jestadt, BW Lovett, C Ager, T Sasaki, VN Laukhin, E Laukhina, EM Forgan, W Hayes

Muon-spin rotation (mu SR) measurements have been used to study the vortex lattice and its instabilities in the organic superconductors kappa-ET2Cu(SCN)(2) and beta-ET2IBr2. We ascribe the field- and temperature-dependent destruction of the vortex lattice to the large anisotropy found in these materials.


Fermi-surface traversal resonance in alpha-(BEDT-TTF)(2)KHg(SCN)(4)

SYNTHETIC MET 103 (1999) 2038-2039

A Ardavan, JM Schrama, A Semeno, SJ Blundell, J Singleton, P Goy, M Kurmoo, P Day

We experimentally demonstrate a mechanism for a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR). This is caused by the periodic traversal of carriers across quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in an external magnetic field. Owing to the warping of the Q1D Fermi sheets, the real space velocities of the carriers oscillate as they cross the FS, generating resonances in the high frequency conductivity of the material. The results contain information about the FS, including the direction and harmonic content of the warping components. Using a rotating resonant-cavity system, FTRs have been observed in alpha-(BEDT-TTF)(2)KHg(SCN)(4).


ESR investigation of the low temperature state in alpha-(BEDT-TTF)(2)KHg(SCN)(4)

SYNTHETIC MET 103 (1999) 1949-1950

A Semeno, J Singleton, A Ardavan, M Schrama, M Kurmoo, P Day

We have performed ESR measurements on an alpha-(BEDT-TTF)(2)KHg(SCN)(4) Single crystal at temperatures down to 0.9 K. Two regions where the ESR intensity decreases were observed. One corresponds to the proposed SDW transition (T approximate to 9 K); the other takes place at T less than or equal to 2 K. A new resonance line appears below the transition temperature T approximate to 9 K; its intensity grows continuously as the temperature is lowered. We regard the decrease of the ESR line at very low temperatures as evidence of a further phase transition.


Layered transition metal molecular magnets studied with implanted muons

SYNTHETIC MET 103 (1999) 2325-2326

T Jestadt, M Kurmoo, SJ Blundell, BW Lovett, FL Pratt, W Hayes

We have carried out magnetic measurements on the layered magnets Ni-2(OH)(3)(N(CN)(2)) (NiAM), Co-5 (OH)(8)(N(CN)(2))(2). 6H(2)O (CoAM), Co-5(OH)(8) (C12H25SO4)(2). 2NH(3) (CoDOS), and Co-5 (OH)(8) (C7H15CO2)(2). 4H(2)O ( CoCAP), using various experimental methods, including muon spin relaxation (mu SR). We find phase transitions at temperatures T-M in all the samples, characterized by a change in the magnetic susceptibilty and loss of initial muon spin polarization.


Nature of the spin state in TmNi2B2C

PHYSICA B 261 (1999) 588-589

R Nagarajan, E Alleno, SJ Blundell, C Mazumdar, DW Cooke, SP Cottrell, SFJ Cox, C Godart, LC Gupta, Z Hossain, WL Hults, T Jestadt, EJ Peterson, FL Pratt, JHL Smith

We had observed earlier that in the quaternary borocarbide magnetic superconductor TmNi2B2C (T-c = 11K, T-N = 1.5 K) quasistatic magnetic correlations persist well above TN (up to similar to 20 K), which is rather unusual. To further probe the nature of its spin state, we used magnetically oriented powder samples of TmNi2B2C and the mu-SR technique. We observe that the quasistatic internal held has a well defined direction. The spontaneous muon precession signal has three times the amplitude when the c-axis alignment is perpendicular to the initial muon beam polarization than when parallel. The transverse configuration enhances the oscillatory signal. This has enabled us to show that the quasistatic magnetic correlations persist even up to 50 K. Furthermore, we find a peak around 7 G in the field dependence of the longitudinal relaxation, which we attribute to the presence of a level crossing resonance arising from quadrupole levels of the boron nuclei. (C) 1999 Published by Elsevier Science B.V. All rights reserved.


Millimetre-wave response of the organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2)

SYNTHETIC MET 103 (1999) 1947-1948

JM Schrama, A Ardavan, J Singleton, SJ Blundell, W Hayes, M Kurmoo, P Day

A study of the electronic and superconducting properties of the organic superconductor kappa-(BEDT-TTF)(2)Cu(SCN)(2) in the millimetre-wave range is presented. The measurments are carried out using a single crystal in an unique rotating rectangular cavity. This novel technique provides information about the anisotropy of the magnetoresistance at GHz freqencies.


Anisotropic polaron motion in conducting polymers studied by muon spin relaxation

SYNTHETIC MET 101 (1999) 323-324

FL Pratt, SJ Blundell, BW Lovett, K Nagamine, K Ishida, W Hayes, T Jestadt, AP Monkman

Muon spin relaxation has been used to study the anisotropic motion of muon-generated negative polarons in polyaniline and polypyridine. Fast on-chain diffusion is found to become limited by 10 meV phenylene librations above about 150 K in polyaniline, whereas the rapid on-chain diffusion is maintained to much higher temperatures in polypyridine due to its more rigid structure.


Several kinds of aminoxyl radicals and their metal ion complexes

MOL CRYST LIQ CRYST 334 (1999) 477-486

T Sugano, SJ Blundell, FL Pratt, T Jestadt, BW Lovett, W Hayes, P Day

Recent results of magnetization, magnetic susceptibility and muon spin rotation/relaxation (mu SR) measurements of some organic neutral radicals based on aminoxyl and their anion radical complexes with alkali and transition metal cations are reported. Ferromagnetic intermolecular interactions, coexistent with antiferromagnetic ones, in several carboxyaryl nitronyl nitroxide radicals are affected by inserting alkali metal ions, while the incorporation of transition metal ions result in complex behavior suggesting in part ferromagnetic interactions. Temperature dependence of spontaneous magnetization of TANOL suberate in the ordered state, obtained through the results of ASR measurements, is characteristic of a two-dimensional magnetic lattice, consistent with that of magnetic susceptibility above the ordering temperature.


Fermi surface traversal resonance in metals: Two theories and an experiment

P SOC PHOTO-OPT INS 3828 (1999) 366-377

A Ardavan, JM Schrama, SJ Blundell, J Singleton, A Semeno, P Goy, M Kurmoo, P Day

Fermi-surface traversal resonance (FTR) is caused by the periodic motion of carriers in a magnetic field across open sections of Fermi surface (FS). Owing to the warping of the FS, the real space velocities of the carriers oscillate, generating resonances in the high frequency conductivity which may be described by a semiclassical model. A rectangular resonant cavity, oscillating at 70 GHz, which can rotate in the external magnetic field, has been used to confirm the existence of the effect in the organic metal alpha-(BEDT-TTF)(2)KHg(SCN)(4). The data contain a great deal of information about the FS, including the direction and anharmonicity of warping components. A quantum mechanical model is presented which predicts all of the features of FTR appearing in the semiclassical model. This confirms that FTR is a fundamental property of few-dimensional systems, existing under a very wide range of conditions.


Spin-polarized muons in condensed matter physics

Contemporary Physics 40 (1999) 175-192

SJ Blundell

A positive muon is a spin-1/2 particle. Beams of muons with all their spins polarized can be prepared and subsequently implanted in various types of condensed matter. The subsequent precession and relaxation of their spins can then be used to investigate a variety of static and dynamic effects in a sample and hence to deduce properties concerning magnetism, superconductivity and molecular dynamics. Though strictly a lepton, and behaving essentially like a heavy electron, it is convenient to think of a muon as a light proton, and it is often found with a captured electron in a hydrogen-like atom known as muonium. This article outlines the principles of various experimental techniques which involve implanted muons and describes some recent applications. The use of muons in condensed matter physics has shed new light on subjects as diverse as passivation in semiconductors, frustrated spin systems, vortex lattice melting, and quantum diffusion of light particles.


Muon study of the spin dynamics in the organic spin-peierls compound MEM(TCNQ)(2)

SYNTHETIC MET 103 (1999) 2034-2037

BW Lovett, SJ Blundell, T Jestadt, FL Pratt, M Kurmoo, S Tagaki, W Hayes

We report a muon spin relaxation study of the organic spin-Peierls system MEM(TCNQ)(2) over the temperature range 39 mK to 200 K. Our results show a crossover from Gaussian to exponential relaxation below the spin-Peierls transition temperature. We attribute this change to the effect of a dilute set of static electronic defect spins resulting from the majority of spins freezing into a non-magnetic spin singlet state.


Temperature and doping-level dependence of magnetic order in (formula presented) studied by muon spin rotation

Physical Review B - Condensed Matter and Materials Physics 59 (1999) 3775-3782

T Jestädt, KH Chow, SJ Blundell, W Hayes, FL Pratt, BW Lovett

We report muon-spin-rotation (Formula presented)SR) measurements on a series of compounds with composition (Formula presented) with strontium doping level x between 0 and 1. A magnetic transition is found in all the samples studied, which occurs at a composition-dependent temperature, (Formula presented) Below (Formula presented) clear precession signals are observed in zero applied magnetic field for all (Formula presented), indicating the existence of at least short-range magnetic order on a time scale longer than (Formula presented) s. Above (Formula presented) the magnetic correlation times decrease by several orders of magnitude. At (Formula presented), we find peaks in both (Formula presented) and the zero-temperature staggered magnetization, an observation which we attribute to the higher degree of localization of the holes at this doping level. The measurements of (Formula presented) as a function of x extend the determination of Néel temperatures by previous neutron diffraction and (Formula presented)SR measurements. © 1999 The American Physical Society.


Quantum-mechanical model of fermi-surface traversal resonance

Physical Review B - Condensed Matter and Materials Physics 60 (1999) 15500-15503

A Ardavan, SJ Blundell, J Singleton

We describe a quantum-mechanical model of Fermi-surface traversal resonance (FTR), a magneto-optical resonance that occurs in quasi-one-dimensional metals. We show that the predictions of this model are in quantitative agreement with earlier semiclassical models of FTR. The agreement between the two approaches, whose starting assumptions are very different, demonstrates that it is a fundamental property of quasi-one-dimensional systems. © 1999 The American Physical Society.


Millimetre-wave magneto-optical determination of the anisotropic superconducting order parameter in the molecular superconductor (kappa)-(BEDT-TTF)(2)Cu(NCS)(2)

P SOC PHOTO-OPT INS 3828 (1999) 311-314

JM Schrama, E Rzepniewski, A Ardavan, R Edwards, J Singleton, M Kurmoo, P Day

We have used a novel millimetre-wave magneto-optical technique to study the angle-dependence of the high-frequency conductivity of the molecular superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). The data, strongly suggest that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in kappa-(BEDT-TTF)(2)Cu(NCS)(2) is d-wave in nature, and is mediated by spin fluctuations.


Muon-spin-rotation studies of organic magnets

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 357 (1999) 2923-2937

SJ Blundell

A muon is an unstable spin-1/2 particle with a lifetime of 2.2 μs. Beams of spinpolarized positive muons can be prepared at accelerator facilities and then subsequently implanted in various types of condensed matter. Both the time and direction dependence of the subsequent positron emission can be monitored. This allows the precession and relaxation of the average muon-spin polarization to be measured and the local magnetic field in the sample to be directly inferred. The muon thus behaves essentially as a 'microscopic magnetometer' and is used to follow the magnetic order at a local level and to investigate both static and dynamic effects. This article outlines the principles of various experimental techniques that involve implanted muons, and reviews some recent experimental data on organic and molecular magnets.

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