Publications by Amalia Coldea


Angle-dependent magnetoresistance oscillations due to magnetic breakdown orbits

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

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


Tuning electronic ground states by using chemical pressure on quasi-two dimensional β″-(BEDT-TTF) <inf>4</inf>[(H <inf>3</inf> O)M(C <inf>2</inf>O <inf>4</inf>) <inf>3</inf>]·Y

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

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

We report high-field magnetotransport studies on quasi-two dimensional β″-(BEDT-TTF) 4[(H 3O)M(C 2 O 4) 3]·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 6H 5NO 2 and C 6 H 5CN) to metallic (for smaller solvents, Y = C 5 H 5N and CH 2Cl 2). 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.


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.


A spin resonance investigation of magnetism and dynamics in the charge-transfer salts β″-(BEDT-TTF)4[(H3O)M(C2O4)3]S

Journal of Low Temperature Physics Springer Science and Business Media LLC 142 (2006) 581-584

A Narduzzo, A Coldea, A Ardavan, J Singleton, L Pardi, V Bercu, A Akutsu-Sato, H Akutsu, S Turner, P Day


Tuning electronic ground states by using chemical pressure on quasi-two dimensionalβ″-(BEDT-TTF)4[(H3O)M(C2O4)3]·Y

Journal of Low Temperature Physics Springer Science and Business Media LLC 142 (2006) 253-256

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


Tuning electronic ground states by using chemical pressure on quasi-two dimensional beta ''-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]center dot Y

J LOW TEMP PHYS 142 (2006) 253-256

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

We report high-field magnetotransport studies on quasi-two dimensional beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]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=C6H5NO2 and C6H5CN) to metallic (for smaller solvents, Y=C5H5N and CH2O2). These changes in the ground state are connected with modifications 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.


Electron paramagnetic resonance studies of the high-spin molecule Cr-10(OMe)(20)(O2CCMe3)(10) - art. no. 03250

APPLIED PHYSICS LETTERS 86 (2005) ARTN 032507

S Sharmin, A Ardavan, SJ Blundell, AI Coldea, EJL McInnes, D Low


Electron paramagnetic resonance studies of the high-spin molecule Cr10 (OMe) 20 (O2 CCMe3) 10

Applied Physics Letters 86 (2005) 1-3

S Sharmin, A Ardavan, SJ Blundell, AI Coldea, EJL Mcinnes, D Low

We report millimeter-wave magneto-optical measurements on the high-spin molecule, Cr10 (OMe) 20 (O2 CCMe3) 10. The dependence of the electron paramagnetic resonance as a function of orientation and temperature demonstrates that this compound behaves as a single molecule magnet, and exhibits one of the smallest zero-field splittings (D=-0.045±0.004 K) yet reported for such a system. © 2005 American Institute of Physics.


Angle-dependent magneto-transport measurements on kappa-(BEDT-TTF)(2)Cu(NCS)(2) under pressure

SYNTHETIC MET 153 (2005) 449-452

AF Bangura, PA Goddard, S Tozer, AI Coldea, RD McDonald, J Singleton, A Ardavan, J Schleuter

Magnetotransport measurements have been performed on single crystals Of kappa-(BEDT-TTF)(2)Cu(NCS)(2) in fields of up to 33 T at temperatures between 500 mK and 4.2K. Using a diamond anvil cell mounted on a goniometer, measurements of the angle and temperature dependence of the interlayer resistance, R-zz, under hydrostatic pressures between 1.1 kbar and 17.3 kbar were performed. For the first time we have been able to measure angle-dependent magnetoresistance oscillations under pressure due to both the 1D and 2D Fermi surfaces in addition to Shubnikov de Haas oscillations. The results show that the shape of the elliptical quasi-2D Fermi-pocket is more elongated under a hydrostatic pressure of 9.8 kbar compared with ambient pressure. When the magnetic field B is close to parallel to the highly conductive plane, bc, a peak in R-zz is observed with an angular width determined by the ratio of the maximum inter- and intra-layer Fermi velocities. The width of this peak is found to increase with pressure suggesting that the Fermi surface becomes more three-dimensional upon application of pressure.


Millimetre-wave studies on the high-spin molecules Cr-10(OMe)(20)(O2CCMe3)(10) and Cr12O9(OH)(3)(O2CCMe3)(15)

SYNTHETIC MET 154 (2005) 305-308

S Sharmin, A Ardavan, SJ Blundell, AI Coldea

We report millimetre-wave electron spin resonance (ESR) measurements on single crystals of the high-spin molecules Cr-10(OMe)(20)(O2CCMe3)(10) and Cr12O9(OH)(3)(O2CCMe3)(15) within a temperature range of 1.4 K to 50 K and in magnetic fields of up to 5 Tesla. In our experiments it is possible to vary the orientation of the magnetic field with respect to the crystal axes, and thus to study the ESR lineshapes as a function of both temperature and angle. Our results confirm that Cr-10(OMe)(20)(O2CCMe3)(10) behaves as a single-molecule magnet with S = 15 and D = -0.03 K, while Cr12O9(OH)(3)(O2CCMe3)(15) has S = 6 and D similar to 0.1 K. A comparison of the experimental spectra with numerical simulations gives good agreement at low temperatures. At higher temperatures, we observe a narrowing of the ESR spectrum that is not explained by simple models.


Robust superconducting state in the low-quasiparticle-density organic metals β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y: Superconductivity due to proximity to a charge-ordered state

Physical Review B - Condensed Matter and Materials Physics 72 (2005)

AF Bangura, AI Coldea, J Singleton, A Ardavan, A Akutsu-Sato, H Akutsu, SS Turner, P Day, T Yamamoto, K Yakushi

We report magnetotransport measurements on the quasi-two-dimensional charge-transfer salts β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, with Y=C6H5NO2 and C6H5CN using magnetic fields of up to 45 T and temperatures down to 0.5 K. A surprisingly robust superconducting state with an in-plane upper critical field Bc2 33T, comparable to the highest critical field of any BEDT-TTF superconductor, and critical temperature Tcâ 7K is observed when M=Ga and Y=C6H5NO2. The presence of magnetic M ions reduces the in-plane upper critical field to 18T for M=Cr and Y=C6H5NO2 and M=Fe and Y=C6H5CN. Prominent Shubnikov-de Haas oscillations are observed at low temperatures and high magnetic fields, showing that the superconducting salts possess Fermi surfaces with one or two small quasi-two-dimensional pockets, their total area comprising 6% of the room-temperature Brillouin zone; the quasiparticle effective masses were found to be enhanced when the ion M was magnetic (Fe or Cr). The low effective masses and quasiparticle densities, and the systematic variation of the properties of the β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts with unit-cell volume points to the possibility of a superconducting groundstate with a charge-fluctuation-mediated superconductivity mechanism such as that proposed by Merino and McKenzie [Phys. Rev. Lett. 87, 237002 (2001)], rather than the spin-fluctuation mechanism appropriate for the κ-(BEDT-TTF)2X salts. © 2005 The American Physical Society.


The role of magnetic ions on the magnetotransport properties of the charge-transfer salts β″-BEDT-[()()] where =, Cr3+ or Fe3+

Journal of Magnetism and Magnetic Materials Elsevier BV 272-276 (2004) 1062-1064

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


Millimetre-wave and magnetic studies on a high-spin molecule, Cr-10(OMe)(20)(O2CCMe3)(10)

J PHYS IV 114 (2004) 645-647

S Sharmin, A Ardavan, SJ Blundell, AI Coldea, EJL McInnes, D Low

We report millimetre-wave and magnetic measurements on single crystals of the high-spin molecular magnet, Cr-10(OMe)(20)(O2CCMe3)(10). The susceptibility data obtained using a SQUID magnetometer indicate that this molecule has a total spin of S = 15. The temperature dependence of the magnetisation allows us to estimate the intermolecular exchange constant. The EPR spectra were obtained at different frequencies, angles and temperatures. The results are compared with numerical simulations allowing determination of the Hamiltonian parameters.


Structural and magnetic properties of polypyrrole nanocomposites

MOLECULAR CRYSTALS AND LIQUID CRYSTALS 417 (2004) 719-727

R Turcu, I Peter, O Pana, L Giurgiu, N Aldea, B Barz, MN Grecu, A Coldea


Fermiology of new charge-transfer salts, beta ''-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]center dot solvent where M = Ga, Cr and Fe

J PHYS IV 114 (2004) 205-209

AI Coldea, AF Bangura, A Ardavan, J Singleton, A Akutsu-Sato, H Akutsu, SS Turner, R Day

We report high-field magnetotransport measurements oil beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)], solvent, where M=Ga3+, Cr3+ and Fe3+ and solvent=C5H5N. In spite of their differing transition metal-ions, M, the three compounds exhibit similar magnetic quantum oscillation spectra superimposed on a positive magnetoresistance. At least four independent quantum oscillation frequencies have been identified, corresponding to two different hole and electron pockets of the Fermi surface which follow the rules of a compensated metal. Observation of the small pockets could be the result of the Fermi surface reconstruction induced by a possible density wave. The effective masses are very similar for different samples and for different pockets range between m(eff) approximate to 0.5 - 1.1 m(e) whereas the Dingle temperatures varies between T-D approximate to 1.4 - 4 K. At low temperature, the longitudinal magnetoresistance violates Kohler's rule, suggesting that the interlayer transport in these quasi-2D systems cannot be related to a single scattering time and that the disorder plays an important role.


Fermiology of new charge-transfer salts, β″-(BEDT-TTF) <inf>4</inf>[(H<inf>3</inf>O)M(C<inf>2</inf>O<inf>4</inf>)<inf>3</inf>]-solvent where M = Ga, Cr and Fe

Journal De Physique. IV : JP 114 (2004) 205-209

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

We report high-field magnetotransport measurements on β″-(BEDT- TTF)4[(H3O)M(C2O4) 3]· solvent, where M=Ga3+, Cr3+ and Fe3+ and solvent=C5H5N. In spite of their differing transition metal-ions, M, the three compounds exhibit similar magnetic quantum oscillation spectra superimposed on a positive magnetoresistance. At least four independent quantum oscillation frequencies have been identified, corresponding to two different hole and electron pockets of the Fermi surface which follow the rules of a compensated metal. Observation of the small pockets could be the result of the Fermi surface reconstruction induced by a possible density wave. The effective masses are very similar for different samples and for different pockets range between meff ≈ 0.5-1.1 me whereas the Dingle temperatures varies between TD ≈ 1.4-4 K. At low temperature, the longitudinal magnetoresistance violates Kohler's rule, suggesting that the interlayer transport in these quasi-2D systems cannot be related to a single scattering time and that the disorder plays an important role. © EDP Sciences.


The effect of magnetic ions and disorder on superconducting β″-(BEDT-TTF)<inf>4</inf>[(H<inf>3</inf>O)M(C<inf>2</inf>O <inf>4</inf>)<inf>3</inf>]· C<inf>6</inf>H<inf>5</inf>NO<inf>2</inf> salts, where M = Ga and Cr

Journal De Physique. IV : JP 114 (2004) 285-287

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

We report magnetotransport measurements performed in magnetic fields of up to 33 T and at low temperatures (0.45 K<T<4.2 K) on two iso-structural superconducting charge-transfer salts, β″-(BEDT-TTF) 4[(H3O)M(C2O4)3]· C6H5NO2, where the 3d metal ion M is either non-magnetic Ga3+ or magnetic Cr3+, Quantum oscillations suggest that the salt with M = Ga has two two-dimensional Fermi surface pockets (45 T and 230 T) whereas when M = Cr only one pocket (230 T) is observed. Replacing Ga with Cr has the effect of enhancing the effective mass of the 230 T pocket from 1.2 ± 0.2 me to 2.2 ± 0,3 me and reducing the superconducting critical temperature from Tc ≈ 8 K to Tc ≈ 5 K. © EDP Sciences.


The effect of magnetic ions and disorder on superconducting beta ''-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)]center dot C6H5NO2 salts, where M = Ga and Cr

J PHYS IV 114 (2004) 285-287

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

We report in magnetotransport measurements performed in magnetic fields of up to 33 T and at low temperatures (0.45 K<T<4.2 K) on two iso-structural Superconducting charge-transfer salts, beta"-(BEDT-TTF)(4)[(H3O)M(C2O4)(3)] . C6H5NO2, where the 3d metal ion M is either non-magnetic Ga3+ or magnetic Cr3+. Quantum oscillations suggest that the salt with M = Ga has two two-dimensional Fermi surface pockets (45 T and 230 T) whereas when M = Cr only one pocket (230 T) is observed. Replacing Ga with Cr has the effect of enhancing the effective mass of the 230 T pocket from 1.2 +/- 0.2 m(e) to 2.2 +/- 0.3 m(e) and reducing the superconducting critical temperature from T-c approximate to 8 K to T-c approximate to 5 K.


The role of magnetic ions on the magnetotransport properties of the charge-transfer salts beta ''-BEDT-TTF4[(H3O)M(C2O4)(3)]C5H5N where M = Ga3+, Cr3+ or Fe3+

J MAGN MAGN MATER 272-76 (2004) 1062-1064

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

We report high-field magneto transport measurements on beta''- BEDT-TTF4[(H3O)M(C2O4)(3)]C5H5N, where M = Ga3+, Cr3+ or Fe3+. In spite of the differing M ions, these compounds have very similar Fermi surfaces. We observe four distinct Shubnikov-de Haas frequencies, corresponding to four Fermi-surface pockets; the frequencies exhibit the additive relationship expected for a compensated semimetal. The compounds show paramagnetic behaviour and no superconductivity down to 0.5 K, in contrast to other materials of the same family with different solvent molecules. (C) 2003 Elsevier B.V. All rights reserved.


Fermi-surface topology and the effects of intrinsic disorder in a class of charge-transfer salts containing magnetic ions: β″-(BEDT-TTF)<inf>4</inf>[(H<inf>3</inf>O)M(C<inf>2</inf>O<inf>4</inf>)3]Y (M = Ga, Cr, Fe; Y = C<inf>5</inf>H<inf>5</inf>N)

Physical Review B - Condensed Matter and Materials Physics 69 (2004)

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

We report high-field magnetotransport measurements on β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y, where M = Ga, Cr, and Fe and Y = C5H5N. We observe similar Shubnikov-de Haas oscillations in all compounds, attributable to four quasi-two-dimensional Fermi-surface pockets, the largest of which corresponds to a cross-sectional area 8.5% of the Brillouin zone. The cross-sectional areas of the pockets are in agreement with the expectations for a compensated semimetal, and the corresponding effective masses are me, rather small compared to those of other BEDT-TTF salts. Apart from the case of the smallest Fermi-surface pocket, varying the M ion seems to have little effect on the overall Fermi-surface topology or on the effective masses. Despite the fact that all samples show quantum oscillations at low temperatures, indicative of Fermi liquid behavior, the sample and temperature dependence of the interlayer resistivity suggest that these systems are intrinsically inhomogeneous. It is thought that intrinsic tendency to disorder in the anions and/or the ethylene groups of the BEDT-TTF molecules leads to the coexistence of insulating and metallic states at low temperatures. A notional phase diagram is given for the general family of β″-(BEDT-TTF)4[(H3O)M(C2O4)3]Y salts. © 2004 The American Physical Society.

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