Publications by Amalia Coldea

An ideal Weyl semimetal induced by magnetic exchange


J-R Soh, FD Juan, Vergniory, NBM Schröter, MC Rahn, DY Yan, M Bristow, PA Reiss, JN Blandy, YF Guo, YG Shi, TK Kim, A McCollam, SH Simon, Y Chen, AMALIA Coldea, AT Boothroyd

Weyl semimetals exhibit exceptional quantum electronic transport due to the presence of topologically-protected band crossings called Weyl nodes. The nodes come in pairs with opposite chirality, but their number and location in momentum space is otherwise material specific. Following the initial discoveries there is now a need for better material realizations, ideally comprising a single pair of Weyl nodes located at or very close to the Fermi level and in an energy window free from other overlapping bands. Here we propose the layered intermetallic EuCd$_2$As$_2$ to be such a system. We show that Weyl nodes in EuCd$_2$As$_2$ are magnetically-induced via exchange coupling, emerging when the Eu spins are aligned by a small external magnetic field. The identification of EuCd$_2$As$_2$ as a model magnetic Weyl semimetal, evidenced here by ab initio calculations, photoemission spectroscopy, quantum oscillations and anomalous Hall transport measurements, opens the door to fundamental tests of Weyl physics.

Evolution of the Fermi surface of the nematic superconductors FeSe1-xSx

npj Quantum Materials Nature Research (part of Springer Nature) (0)

AI Coldea, SF Blake, S Kasahara, AA Haghighirad, MD Watson, W Knafo, ES Choi, A McCollam, P Reiss, T Yamashita, M Bruma, S Speller, Y Matsuda, T Wolf, T Shibauchi, AJ Schofield

We investigate the evolution of the Fermi surfaces and electronic interactions across the nematic phase transition in single crystals of FeSe1-xSx using Shubnikov-de Haas oscillations in high magnetic fields up to 45 tesla in the low temperature regime. The unusually small and strongly elongated Fermi surface of FeSe increases monotonically with chemical pressure, x, due to the suppression of the in-plane anisotropy except for the smallest orbit which suffers a Lifshitz-like transition once nematicity disappears. Even outside the nematic phase the Fermi surface continues to increase, in stark contrast to the reconstructed Fermi surface detected in FeSe under applied external pressure. We detect signatures of orbital-dependent quasiparticle mass renomalization suppressed for those orbits with dominant dxz=yz character, but unusually enhanced for those orbits with dominant dxy character. The lack of enhanced superconductivity outside the nematic phase in FeSe1-xSx suggest that nematicity may not play the essential role in enhancing Tc in these systems.

de Haas–van Alphen study of role of4felectrons in antiferromagneticCeZn11as compared to its nonmagnetic analogLaZn11

Physical Review B American Physical Society (APS) 94 (0) 235103

SF Blake, H Hodovanets, A McCollam, SL Bud'ko, PC Canfield, AI Coldea

Field-induced nematic-like magnetic transition in an iron pnictide superconductor, Ca$_{10}$(Pt$_{3}$As$_{8}$)((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$

ArXiv (0)

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

We report a high magnetic field study up to 55 T of the nearly optimally doped iron-pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8}$) ((Fe$_{1-x}$Pt$_{x}$)$_{2}$As$_{2}$)$_{5}$ (x=0.078(6)) with a Tc 10 K using magnetic torque, tunnel diode oscillator technique and transport measurements. We determine the superconducting phase diagram, revealing an anisotropy of the irreversibility field up to a factor of 10 near Tc and signatures of multiband superconductivity. Unexpectedly, we find a spin-flop like anomaly in magnetic torque at 22 T, when the magnetic field is applied perpendicular to the ab planes, which becomes significantly more pronounced as the temperature is lowered to 0.33 K. As our superconducting sample lies well outside the antiferromagnetic region of the phase diagram, the observed field-induced transition in torque indicates a spin-flop transition not of long-range ordered moments, but of nematic-like antiferromagnetic fluctuations.

Interplay between localized and itinerant d electrons in a frustrated metallic antiferromagnet, 2H-AgNiO2

ArXiv (0)

AI Coldea, A Carrington, R Coldea, L Malone, AF Bangura, MD Johannes, II Mazin, EA Yelland, JG Analytis, JAAJ Perenboom, C Jaudet, D Vignolles, T Sorgel, M Jansen

We report the electronic and magnetic behaviour of the frustrated triangular metallic antiferromagnet 2H-AgNiO2 in high magnetic fields (54 T) using thermodynamic and transport measurements. Here localized d electrons are arranged on an antiferromagnetic triangular lattice nested inside a honeycomb lattice with itinerant d electrons. When the magnetic field is along the easy axis we observe a cascade of field-induced transitions, attributed to the competition between easy-axis anisotropy, geometrical frustration and coupling of the localized and itinerant system. The quantum oscillations data suggest that the Fermi surface is reconstructed by the magnetic order but in high fields magnetic breakdown orbits are possible. The itinerant electrons are extremely sensitive to scattering by spin fluctuations and a significant mass enhancement (~ 3) is found.

Enhancement of the magnetoresistance at the curie temperature of the ferromagnetic insulator La<inf>1.5</inf>Sr<inf>0.5</inf>MnRhO<inf>6</inf>

Physical Review B - Condensed Matter and Materials Physics 62 (2000) R6077-R6080

AI Coldea, IM Marshall, SJ Blundell, J Singleton, LD Noailles, PD Battle, MJ Rosseinsky

We report a study of the magnetic and electrical properties of the ferromagnetic insulator La1.5Sr0.5MnRhO6. A significant magnetoresistance is found in this system which is largest at the Curie temperature (Tc) even though there is no metal-insulator transition. The electrical transport is found to be activated above Tc and described by a variable range hopping law below Tc. Above Tc the carriers are magnetic polarons with a size which increases as the temperature approaches the magnetic transition. Rh substitution preserves ferromagnetic ordering, in contrast with the effect of other dopants on the B sites, but modifies the electrostatic potential leading to carrier localization. We attribute the peak in the magnetoresistance at Tc to the field-induced suppression of critical spin fluctuations which modulate the energetic barriers seen by the carriers.

Rapid synthesis of colossal magnetoresistance manganites by microwave dielectric heating


KE Gibbons, MO Jones, SJ Blundell, AI Mihut, I Gameson, PP Edwards, Y Miyazaki, NC Hyatt, A Porch

Structural chemistry and electronic properties of the n=3 Ruddlesden-Popper phases Ca4Mn2FeO9.75 and Sr4Mn2FeO9.80

CHEMISTRY OF MATERIALS 11 (1999) 674-683

PD Battle, WR Branford, A Mihut, MJ Rosseinsky, J Singleton, J Sloan, LE Spring, JF Vente

Ferromagnetic ordering in the perovskite La1.5Sr0.5RhMnO6


B Bakowski, PD Battle, EJ Cussen, LD Noailles, MJ Rosseinsky, AI Coldea, J Singleton

Magnetotransport studies on the Ruddlesden Popper phases Sr2RMn2O7 (R = Nd, Pr, Ho, Y) and Sr2-xNd1+xMn2O7 (x = 0, 0.1, 0.2, 0.5)


AI Coldea, LE Spring, SJ Blundell, J Singleton, W Hayes

FTIR reflectance studies of electrochemically prepared polypyrrole films


R Turcu, M Brie, G Leising, V Tosa, A Mihut, A Niko, A Bot

Chemistry of naturally layered manganites

JOURNAL OF APPLIED PHYSICS 83 (1998) 6379-6384

PD Battle, N Kasmir, JE Millburn, MJ Rosseinsky, RT Patel, LE Spring, JF Vente, SJ Blundell, W Hayes, AK Klehe, A Mihut, J Singleton

Physical properties of the n=3 Ruddlesden-Popper compound Ca4Mn3O10


AI Mihut, LE Spring, RI Bewley, SJ Blundell, W Hayes, T Jestadt, BW Lovett, R McDonald, FL Pratt, J Singleton, PD Battle, J Lago, MJ Rosseinsky, JF Vente

Magnetoresistance in high oxidation state iron oxides


PD Battle, MA Green, J Lago, A Mihut, MJ Rosseinsky, LE Spring, J Singleton, JF Vente

Chemistry of naturally layered manganites (invited)

Journal of Applied Physics 83 (1998) 6379-6384

PD Battle, N Kasmir, JE Millburn, MJ Rosseinsky, RT Patel, LE Spring, JF Vente, SJ Blundell, W Hayes, AK Klehe, A Mihut, J Singleton

Experiments on three double-layer (n=2) Ruddlesden-Popper (RP) systems are reported. Doping Sr1.8La1.2Mn2O7 (Tc = 126 K) with Nd to form Sr1.8La1.2- xNdxMn2O7 leads to a reduction in Curie temperature for low doping levels (x = 0.2), and to behavior reminiscent of Sr1.8Nd1.2Mn2O7 for x≥0.7. This suggests that it may be possible to control the temperature of maximum magnetoresistance chemically in these phases. The application of pressure (0<P/GPa≤1.8) is shown to modify the magnetotransport properties of Sr2NdMn2O7 to resemble those of Sr 1.9Nd1.1Mn2O7. The changes can be explained by considering the relative strength of ferromagnetic and antiferromagnetic interactions within the material. Finally, the need for careful phase analysis of n = 2 RP materials is demonstrated by the misleading magnetization data recorded for a sample of Sr1.8Sm 1.2Mn2O7 containing ∼2.8% of an n = ∞ perovskite phase. © 1998 American Institute of Physics.

Correlation between the electrochemical synthesis conditions and the optical properties of polypyrrole

SYNTHETIC METALS 84 (1997) 825-826

R Turcu, M Brie, G Leising, A Niko, V Tosa, A Mihut, A Bot

Stability study of conducting polypyrrole films and polyvinylchloride-polypyrrole composites doped with different counterions


M Brie, R Turcu, A Mihut

Reply to Comment by Borisenko et al. on article `A de Haas-van Alphen study of the Fermi surfaces of superconducting LiFeP and LiFeAs'

ArXiv (0)

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

Recently, Borisenko et al have posted a Comment (arXiv:1108.1159) where they suggest an alternative interpretation of our de Haas-van Alphen (dHvA) measurements on the superconductor LiFeAs. In our original paper (arXiv:1107.4375) we concluded that our measurements of the bulk Fermi surface were not consistent with the surface bands observed thus far by ARPES. Borisenko et al dispute this and suggest the two measurements are consistent if some of the orbits we observe are due to magnetic breakdown. We argue here that this scenario is inconsistent with the experimental data and therefore that our original conclusion stands.