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Quantum oscillations

Amalia Coldea

Professor of Physics

Research theme

  • Quantum materials

Sub department

  • Condensed Matter Physics

Research groups

  • Quantum matter in high magnetic fields
amalia.coldea@physics.ox.ac.uk
Telephone: 01865 (2)82196
Clarendon Laboratory, room 251,265,264,166
orcid.org/0000-0002-6732-5964
  • About
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  • Publications

Anisotropy of the zigzag order in the Kitaev honeycomb magnet α-RuBr3

Physical Review B (condensed matter and materials physics) American Physical Society 110 (2024) 214404

Authors:

John S Pearce, David AS Kaib, Zeyu Ma, Danrui Ni, Rj Cava, Roser Valenti, Radu Coldea, Amalia Coldea

Abstract:

Kitaev materials often order magnetically at low temperatures due to the presence of non-Kitaev interactions. Torque magnetometry is a very sensitive technique for probing the magnetic anisotropy, which is critical in understanding the magnetic ground state. In this work, we report detailed single-crystal torque measurements in the proposed Kitaev candidate honeycomb magnet α-RuBr3, which displays zigzag order below 34 K. Based on angular-dependent torque studies in magnetic fields up to 16 T rotated in the plane normal to the honeycomb layers, we find an easy-plane anisotropy with a temperature dependence of the torque amplitude following closely the behaviour of the powder magnetic susceptibility. The torque for field rotated in the honeycomb plane has a clear six-fold periodicity with a saw-tooth shape, reflecting the three-fold symmetry of the crystal structure and stabilization of different zigzag domains depending on the field orientation, with a torque amplitude that follows an order parameter form inside the zigzag phase. By comparing experimental data with theoretical calculations we highlight the importance of relevant anisotropic interactions and the role of the competition between different zigzag domains in this candidate Kitaev magnet.

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Anisotropic magnetic interactions in a candidate Kitaev spin liquid close to a metal-insulator transition

Communications Physics Nature Research 7:1 (2024) 390

Authors:

Zeyu Ma, Danrui Ni, David AS Kaib, Kylie MacFarquharson, John S Pearce, Robert J Cava, Roser Valentí, Radu Coldea, Amalia I Coldea

Abstract:

In the Kitaev honeycomb model, spins coupled by strongly-frustrated anisotropic interactions do not order at low temperature but instead form a quantum spin liquid with spin fractionalisation into Majorana fermions and static fluxes. The realization of such a model in crystalline materials could lead to major breakthroughs in understanding entangled quantum states, however achieving this in practice is a very challenging task. The recently synthesized honeycomb material RuI3 shows no long-range magnetic order down to the lowest probed temperatures and has been theoretically proposed as a quantum spin liquid candidate material on the verge of an insulator to metal transition. Here we report a comprehensive study of the magnetic anisotropy in un-twinned single crystals via torque magnetometry and detect clear signatures of strongly anisotropic and frustrated magnetic interactions. We attribute the development of sawtooth and six-fold torque signal to strongly anisotropic, bond-dependent magnetic interactions by comparing to theoretical calculations. As a function of magnetic field strength at low temperatures, torque shows an unusual non-parabolic dependence suggestive of a proximity to a field-induced transition. Thus, RuI3, without signatures of long-range magnetic order, displays key hallmarks of an exciting candidate for extended Kitaev magnetism with enhanced quantum fluctuations.
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Details from ORA

Collapse of metallicity and high- T c superconductivity in the high-pressure phase of FeSe 0.89 S 0.11

npj Quantum Materials Nature Research 9:1 (2024) 73

Authors:

Pascal Reiss, Alix McCollam, Zachary Zajicek, Amir A Haghighirad, Amalia I Coldea

Abstract:

We investigate the high-pressure phase of the iron-based superconductor FeSe0.89S0.11 using transport and tunnel diode oscillator studies using diamond anvil cells. We construct detailed pressure-temperature phase diagrams that indicate that the superconducting critical temperature is strongly enhanced by more than a factor of four towards 40 K above 4 GPa. The resistivity data reveal signatures of a fan-like structure of non-Fermi liquid behaviour which could indicate the existence of a putative quantum critical point buried underneath the superconducting dome around 4.3 GPa. With further increasing the pressure, the zero-field electrical resistivity develops a non-metallic temperature dependence and the superconducting transition broadens significantly. Eventually, the system fails to reach a fully zero-resistance state, and the finite resistance at low temperatures becomes strongly current-dependent. Our results suggest that the high-pressure, high-Tc phase of iron chalcogenides is very fragile and sensitive to uniaxial effects of the pressure medium, cell design and sample thickness. This high-pressure region could be understood assuming a real-space phase separation caused by nearly concomitant electronic and structural instabilities.
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Anisotropic magnetic interactions in a candidate Kitaev spin liquid close to a metal-insulator transition

(2024)

Authors:

Zeyu Ma, Danrui Ni, David AS Kaib, Kylie MacFarquharson, John S Pearce, Robert J Cava, Roser Valenti, Radu Coldea, Amalia I Coldea
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Details from ArXiV

Anisotropy of the zigzag order in the Kitaev honeycomb magnet $\alpha$-RuBr$_3$

(2024)

Authors:

John S Pearce, David AS Kaib, Zeyu Ma, Danrui Ni, RJ Cava, Roser Valenti, Radu Coldea, Amalia I Coldea
More details from the publisher
Details from ArXiV

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