Publications by Amalia Coldea


Fermi surface of IrTe2 in the valence-bond state as determined by quantum oscillations

PHYSICAL REVIEW B 91 (2015) ARTN 121105

SF Blake, MD Watson, A McCollam, S Kasahara, RD Johnson, A Narayanan, GL Pascut, K Haule, V Kiryukhin, T Yamashita, D Watanabe, T Shibauchi, Y Matsuda, AI Coldea


Linear magnetoresistance caused by mobility fluctuations in n-doped Cd(3)As(2).

Physical review letters 114 (2015) 117201-

A Narayanan, MD Watson, SF Blake, N Bruyant, L Drigo, YL Chen, D Prabhakaran, B Yan, C Felser, T Kong, PC Canfield, AI Coldea

Cd(3)As(2) is a candidate three-dimensional Dirac semimetal which has exceedingly high mobility and nonsaturating linear magnetoresistance that may be relevant for future practical applications. We report magnetotransport and tunnel diode oscillation measurements on Cd(3)As(2), in magnetic fields up to 65 T and temperatures between 1.5 and 300 K. We find that the nonsaturating linear magnetoresistance persists up to 65 T and it is likely caused by disorder effects, as it scales with the high mobility rather than directly linked to Fermi surface changes even when approaching the quantum limit. From the observed quantum oscillations, we determine the bulk three-dimensional Fermi surface having signatures of Dirac behavior with a nontrivial Berry phase shift, very light effective quasiparticle masses, and clear deviations from the band-structure predictions. In very high fields we also detect signatures of large Zeeman spin splitting (g∼16).


Emergence of the nematic electronic state in FeSe

Physical Review B American Physical Society 91 (2015) ARTN 155106

MD Watson, TK Kim, AA Haghighirad, NR Davies, A McCollam, A Narayanan, SF Blake, YL Chen, S Ghannadzadeh, AJ Schofield, M Hoesch, C Meingast, T Wolf, AI Coldea


Field-induced magnetic transitions in Ca10(Pt3As8)((Fe1−xPtx)2As2)5 compounds

Phys. Rev. B 89, 205136 (2014) American Physical Society 89 (2014) 205136-205136

MD Watson, A McCollam, SF Blake, D Vignolles, L Drigo, II Mazin, D Guterding, HO Jeschke, R Valentí, N Ni, R Cava, AI Coldea


X-ray magnetic spectroscopy of MBE-grown Mn-doped Bi2Se3 thin films

AIP ADVANCES 4 (2014) ARTN 127136

LJ Collins-McIntyre, MD Watson, AA Baker, SL Zhang, AI Coldea, SE Harrison, A Pushp, AJ Kellock, SSP Parkin, G van der Laan, T Hesjedal


Cascade of field-induced magnetic transitions in a frustrated antiferromagnetic metal

PHYSICAL REVIEW B 90 (2014) ARTN 020401

AI Coldea, L Seabra, A McCollam, A Carrington, L Malone, AF Bangura, D Vignolles, PG van Rhee, RD McDonald, T Soergel, M Jansen, N Shannon, R Coldea


Quasiparticle Mass Enhancement Close to the Quantum Critical Point in BaFe2(As1-xP_x)_2

Phys. Rev. Lett. American Physical Society 110 (2013) 257002-257002

P Walmsley, C Putzke, L Malone, I Guillamón, D Vignolles, C Proust, S Badoux, AI Coldea, MD Watson, S Kasahara, Y Mizukami, T Shibauchi, Y Matsuda, A Carrington


Study of the structural, electric and magnetic properties of Mn-doped Bi 2 Te 3 single crystals

New Journal of Physics 15 (2013) 10

MD Watson, LJ Collins-McIntyre, LR Shelford, AI Coldea, D Prabhakaran, SC Speller, T Mousavi, CRM Grovenor, Z Salman, SR Giblin, GVD Laan, T Hesjedal

Breaking the time reversal symmetry of a topological insulator, for example by the presence of magnetic ions, is a prerequisite for spin-based electronic applications in the future. In this regard Mn-doped Bi 2 Te 3 is a prototypical example that merits a systematic investigation of its magnetic properties. Unfortunately, Mn doping is challenging in many host materials—resulting in structural or chemical inhomogeneities affecting the magnetic properties. Here, we present a systematic study of the structural, magnetic and magnetotransport properties of Mn-doped Bi 2 Te 3 single crystals using complimentary experimental techniques. These materials exhibit a ferromagnetic phase that is very sensitive to the structural details, with T C varying between 9 and 13 K (bulk values) and a saturation moment that reaches4.4(5) μ B per Mn in the ordered phase. Muon spin rotation suggests that the magnetism is homogeneous throughout the sample. Furthermore, torque measurements in fields up to 33 T reveal an easy axis magnetic anisotropy perpendicular to the ab -plane. The electrical transport data show an anomaly around T C that is easily suppressed by an applied magnetic field, and also anisotropic behavior due to the spin-dependent scattering in relation to the alignment of the Mn magnetic moment. Hall measurements on different crystals established that these systems are n -doped with carrier concentrations of ∼ 0.5–3.0 × 10 20 cm −3 . X-ray magnetic circular dichroism (XMCD) at the Mn L 2,3 edge at 1.8 K reveals a large spin magnetic moment of4.3(3) μ B /Mn, and a small orbital magnetic moment of0.18(2) μ B /Mn. The results also indicate a ground state of mixed d 4 –d 5 –d 6 character of a localized electronic nature, similar to the diluted ferromagnetic semiconductor Ga 1− x Mn x As. XMCD measurements in a field of 6 T give a transition point at T ≈ 16 K, which is ascribed to short range magnetic order induced by the magnetic field. In the ferromagnetic state the easy direction of magnetization is along the c -axis, in agreement with bulk magnetization measurements. This could lead to gap opening at the Dirac point, providing a means to control the surface electric transport, which is of great importance for applications.


Iron-based superconductors in high magnetic fields

Comptes Rendus Physique 14 (2013) 94-105

AI Coldea, D Braithwaite, A Carrington

Here we review measurements of the normal and superconducting state properties of iron-based superconductors using high magnetic fields. We discuss the various physical mechanisms that limit superconductivity in high fields, and the information on the superconducting state that can be extracted from the upper critical field, but also how thermal fluctuations affect its determination by resistivity and specific heat measurements. We also discuss measurements of the normal state electronic structure focusing on measurement of quantum oscillations, particularly the de Haas-van Alphen effect. These results have determined very accurately, the topology of the Fermi surface and the quasi-particle masses in a number of different iron-based superconductors, from the 1111, 122 and 111 families. © 2012 Académie des sciences.


De Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs

Physical Review Letters 108 (2012)

C Putzke, AI Coldea, I Guillamón, D Vignolles, A McCollam, D Leboeuf, MD Watson, II Mazin, S Kasahara, T Terashima, T Shibauchi, Y Matsuda, A Carrington

We report a de Haas-van Alphen oscillation study of the 111 iron pnictide superconductors LiFeAs with T c18K and LiFeP with T c5K. We find that for both compounds the Fermi surface topology is in good agreement with density functional band-structure calculations and has almost nested electron and hole bands. The effective masses generally show significant enhancement, up to ∼3 for LiFeP and ∼5 for LiFeAs. However, one hole Fermi surface in LiFeP shows a very small enhancement, as compared with its other sheets. This difference probably results from k-dependent coupling to spin fluctuations and may be the origin of the different nodal and nodeless superconducting gap structures in LiFeP and LiFeAs, respectively. © 2012 American Physical Society.


Nesting of electron and hole Fermi surfaces in nonsuperconducting BaFe2P2

PHYSICAL REVIEW B 83 (2011) ARTN 220504

BJ Arnold, S Kasahara, AI Coldea, T Terashima, Y Matsuda, T Shibauchi, A Carrington


Evolution of the Fermi surface of BaFe2(As1-xPx){2} on entering the superconducting dome.

Phys Rev Lett 104 (2010) 057008-

H Shishido, AF Bangura, AI Coldea, S Tonegawa, K Hashimoto, S Kasahara, PMC Rourke, H Ikeda, T Terashima, R Settai, Y Onuki, D Vignolles, C Proust, B Vignolle, A McCollam, Y Matsuda, T Shibauchi, A Carrington

Using the de Haas-van Alphen effect we have measured the evolution of the Fermi surface of BaFe2(As1-xPx){2} as a function of isoelectric substitution (As/P) for 0.41<x<1 (T{c} up to 25 K). We find that the volumes of electron and hole Fermi surfaces shrink linearly with decreasing x. This shrinking is accompanied by a strong increase in the quasiparticle effective mass as x is tuned toward the maximum T{c}. These results are not explained by simple band structure calculations, and it is likely that these trends originate from the same many-body interactions which give rise to superconductivity.


A chiral ferromagnetic molecular metal.

J Am Chem Soc 132 (2010) 9271-9273

JR Galán-Mascarós, E Coronado, PA Goddard, J Singleton, AI Coldea, JD Wallis, SJ Coles, A Alberola

The first molecular material with the coexistence of ferromagnetism, metal-like conductivity, and chirality has been prepared using an organic/inorganic approach. In this case, a two-dimensional packing of chiral organic radical cations (responsible for both the electrical conductivity and optical activity) was assembled with a layered bimetallic oxalate-based anionic network (responsible for the magnetic properties). Shubnikov-de Haas oscillations confirmed the presence of a Fermi surface even when the transport properties suggested "insulating"-type behavior at very low temperatures.


Quantum oscillations probe the normal electronic states of novel superconductors.

Philos Trans A Math Phys Eng Sci 368 (2010) 3503-3517

AI Coldea

In 2008, new classes of high-temperature superconductors containing iron have been discovered. These iron pnictides offer a new area of exploration and understanding of superconductivity. Quantum oscillations is a bulk probe that allows us to map out the full Fermi surface of a superconducting system in its normal metallic state. These oscillations are determined by the Landau quantization in high magnetic fields and are usually observed at very low temperatures and in very clean samples. By knowing the exact nature of the quasi-particles in the normal state and the degree of electronic correlations, one can simplify and restrict theoretical models required to understand the pairing mechanism in superconductors. I will discuss the current understanding of the Fermi surface studies in iron-based superconductors as determined from quantum oscillations.


Dimensionality-driven spin-flop transition in quasi-one-dimensional PrBa<inf>2</inf> Cu<inf>4</inf> O<inf>8</inf>

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

X Xu, A Carrington, AI Coldea, A Enayati-Rad, A Narduzzo, S Horii, NE Hussey

In the quasi-one-dimensional cuprate PrBa2 Cu4 O 8, the Pr cations order antiferromagnetically at 17 K in zero field. Through a combination of magnetic susceptibility, torque magnetometry, specific heat, and interchain transport measurements, the anisotropic temperature-magnetic-field phase diagram associated with this ordering has been mapped out. A low-temperature spin-flop transition in the Pr sublattice is found to occur at the same magnetic field strength and orientation as a dimensional crossover in the ground state of the metallic-CuO chains. This coincidence suggests that the spin reorientation is driven by a change in the anisotropic Rudermann-Kittel-Kasuya-Yosida interaction induced by a corresponding change in effective dimensionality of the conduction electrons. © 2010 The American Physical Society.


Anisotropic fluctuations and quasiparticle excitations in FeSe <inf>0.5</inf> Te<inf>0.5</inf>

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

A Serafin, AI Coldea, AY Ganin, MJ Rosseinsky, K Prassides, D Vignolles, A Carrington

We present data for the temperature dependence of the magnetic penetration depth λ (T), heat capacity C (T), resistivity ρ (T), and magnetic torque τ for highly homogeneous single-crystal samples of Fe1.0 Se0.44 (4) Te0.56 (4). λ (T) was measured down to 200 mK in zero field. We find λ (T) follows a power law Δλ∼ Tn with n=2.2±0.1. This is similar to some 122 iron arsenides and likely results from a sign-changing pairing state combined with strong scattering. Magnetic fields of up to B=55 or 14 T were used for the τ (B) and C (T) /ρ (T) measurements, respectively. The specific heat, resistivity, and torque measurements were used to map out the (H,T) phase diagram in this material. All three measurements were conducted on exactly the same single-crystal sample so that the different information revealed by these probes is clearly distinguished. Heat-capacity data strongly resemble those found for the high- Tc cuprates, where strong fluctuation effects wipe out the phase transition at Hc2. Unusually, here we find the fluctuation effects appear to be strongly anisotropic. © 2010 The American Physical Society.


Quantum oscillation studies of the Fermi surface of LaFePO

Physica C: Superconductivity and its Applications 469 (2009) 459-468

A Carrington, AI Coldea, JD Fletcher, NE Hussey, CMJ Andrew, AF Bangura, JG Analytis, JH Chu, AS Erickson, IR Fisher, RD McDonald

We review recent experimental measurements of the Fermi surface of the iron-pnictide superconductor LaFePO using quantum oscillation techniques. These studies show that the Fermi surface topology is close to that predicted by first principles density functional theory calculations, consisting of quasi-two-dimensional electron-like and hole-like sheets. The total volume of the two hole sheets is almost equal to that of the two electron sheets, and the hole and electron Fermi surface sheets are close to a nesting condition. No evidence for the predicted three-dimensional pocket arising from the Fe dz2 band is found. Measurements of the effective mass suggest a renormalisation of around two, close to the value for the overall band renormalisation found in recent angle resolved photoemission measurements. © 2009 Elsevier B.V.


Topological change of the Fermi surface in ternary iron pnictides with reduced c/a ratio: a de Haas-van Alphen study of CaFe2P2.

Phys Rev Lett 103 (2009) 026404-

AI Coldea, CMJ Andrew, JG Analytis, RD McDonald, AF Bangura, J-H Chu, IR Fisher, A Carrington

We report a de Haas-van Alphen effect study of the Fermi surface of CaFe2P2 using low-temperature torque magnetometry up to 45 T. This system is a close structural analog of the collapsed tetragonal nonmagnetic phase of CaFe2As2. We find the Fermi surface of CaFe2P2 to differ from other related ternary phosphides in that its topology is highly dispersive in the c axis, being three dimensional in character and with identical mass enhancement on both electron and hole pockets ( approximately 1.5). This suggests that when the bonding between pnictogen layers becomes important nesting conditions are not fulfilled.


Fermi surface of SrFe2P2 determined by the de Haas-van Alphen effect.

Phys Rev Lett 103 (2009) 076401-

JG Analytis, CMJ Andrew, AI Coldea, A McCollam, J-H Chu, RD McDonald, IR Fisher, A Carrington

We report measurements of the Fermi surface (FS) of the ternary iron-phosphide SrFe2P2 using the de Haas-van Alphen effect. The calculated FS of this compound is very similar to SrFe2As2, the parent compound of the high temperature superconductors. Our data show that the Fermi surface is composed of two electron and two hole sheets in agreement with band-structure calculations. Several of the sheets show strong c-axis warping emphasizing the importance of three dimensionality in the nonmagnetic state of the ternary pnictides. We find that the electron and hole pockets have a different topology, implying that this material does not satisfy a (pi, pi) nesting condition.


Fermi surface of superconducting LaFePO determined from quantum oscillations.

Phys Rev Lett 101 (2008) 216402-

AI Coldea, JD Fletcher, A Carrington, JG Analytis, AF Bangura, J-H Chu, AS Erickson, IR Fisher, NE Hussey, RD McDonald

We report extensive measurements of quantum oscillations in the normal state of the Fe-based superconductor LaFePO, (T(c) approximately 6 K) using low temperature torque magnetometry and transport in high static magnetic fields (45 T). We find that the Fermi surface is in broad agreement with the band-structure calculations with the quasiparticle mass enhanced by a factor approximately 2. The quasi-two-dimensional Fermi surface consists of nearly nested electron and hole pockets, suggesting proximity to a spin or charge density wave instability.

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