-
Field evolution of the magnetic structure and spin Hamiltonian in Cs$_2$RuO$_4$
Authors:
S. D. Nabi,
E. Ressouche,
D. G. Mazzone,
J. Lass,
R. Sibille,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report neutron diffraction under applied magnetic fields and complementary zero-field neutron spectroscopy measurements on Cs$_2$RuO$_4$. Previous work [S. D. Nabi et al., Phys. Rev. B 112, 134436 (2025)] identified a spin-flop-like transition accompanied by a quantum critical point within the ordered phase, attributed to strong frustration between alternating single-ion anisotropy planes. Here…
▽ More
We report neutron diffraction under applied magnetic fields and complementary zero-field neutron spectroscopy measurements on Cs$_2$RuO$_4$. Previous work [S. D. Nabi et al., Phys. Rev. B 112, 134436 (2025)] identified a spin-flop-like transition accompanied by a quantum critical point within the ordered phase, attributed to strong frustration between alternating single-ion anisotropy planes. Here, we quantitatively confirm the predicted field evolution of the magnetic structure using neutron diffraction. Furthermore, analysis of the excitation spectrum within an SU(3) spin-wave framework resolves previously undetermined parameters of the minimal spin Hamiltonian and lifts the associated degeneracies.
△ Less
Submitted 11 August, 2026; v1 submitted 22 May, 2026;
originally announced May 2026.
-
Magnetic phase diagram and spin Hamiltonian of antiferromagnet Cs$_2$CoI$_4$
Authors:
S. D. Nabi,
L. Facheris,
V. Romerio,
V. Kocsis,
K. Yu. Povarov,
D. Sheptyakov,
J. Lass,
D. G. Mazzone,
H. Kikuchi,
T. Masuda,
S. A. Barnett,
D. R. Allan,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. Th…
▽ More
We report comprehensive thermodynamic and neutron scattering measurements on the $S$ = 3/2 antiferromagnet Cs$_2$CoI$_4$, a member of the thoroughly studied family of frustrated magnets Cs$_2MX_4$ ($M$ = Cu, Co, Ru, $X$ = Br, Cl, I, O). Unlike previously studied members, Cs$_2$CoI$_4$ undergoes a structural phase transition, for which we determine the low-temperature crystallographic structure. The resulting symmetry reduction strongly affects both the magnetic exchange interactions and single-ion anisotropy. Despite the large parameter space, we propose a minimal magnetic Hamiltonian that reasonably captures the observed excitation spectrum, analyzed using extended SU(4) linear spin-wave theory.
△ Less
Submitted 29 May, 2026; v1 submitted 9 March, 2026;
originally announced March 2026.
-
Spin-flop-like transition as quantum critical point in Cs$_2$RuO$_4$
Authors:
S. D. Nabi,
M. Zhu,
K. Yu. Povarov,
D. G. Mazzone,
J. Lass,
Y. Wu,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs$_2$RuO$_4$, a member of the celebrated family of frustrated magnets Cs$_2$MX$_4$ (M = Cu, Co, X = Br, Cl). Unlike the previously studied members, it is based on $4d$ transition metal ions with $S=1$. Mapping out the $H-T$ magnetic phase diagram reveals an unusual continuous spin-flop-like phase transi…
▽ More
We report thermodynamic, neutron diffraction, and inelastic neutron scattering measurements on Cs$_2$RuO$_4$, a member of the celebrated family of frustrated magnets Cs$_2$MX$_4$ (M = Cu, Co, X = Br, Cl). Unlike the previously studied members, it is based on $4d$ transition metal ions with $S=1$. Mapping out the $H-T$ magnetic phase diagram reveals an unusual continuous spin-flop-like phase transition associated with a quantum critical point within the antiferromagnetically ordered phase. A quantitative analysis of the complex magnetic excitation spectrum measured in zero field allows us to derive a model magnetic Hamiltonian for this compound. Its main feature is a frustration of magnetic anisotropy on a level that is much higher than in any of the previously studied species. This frustration naturally explains the peculiar phase transition observed.
△ Less
Submitted 21 October, 2025; v1 submitted 26 July, 2025;
originally announced July 2025.
-
Continuum excitations in a spin-supersolid on a triangular lattice
Authors:
M. Zhu,
V. Romerio,
N. Steiger,
S. D. Nabi,
N. Murai,
S. Ohira-Kawamura,
K. Yu. Povarov,
Y. Skourski,
R. Sibille,
L. Keller,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the…
▽ More
Magnetic, thermodynamic, neutron diffraction and inelastic neutron scattering are used to study spin correlations in the easy-axis XXZ triangular lattice magnet K2Co(SeO3)2. Despite the presence of quasi-2D "supersolid" magnetic order, the low-energy excitation spectrum contains no sharp modes and is instead a broad and structured multi-particle continuum. Applying a weak magnetic field drives the system into an m = 1/3 fractional magnetization plateau phase and restores sharp spin wave modes. To some extent, the behavior at zero field can be understood in terms of spin wave decay. However, the presence of clear excitation minima at the M-points of the Brillouin zone suggest that the spinon language may provide a more adequate description, and signals a possible proximity to a Dirac spin liquid state.
△ Less
Submitted 20 August, 2024; v1 submitted 29 January, 2024;
originally announced January 2024.
-
Magnetic field-induced phases and spin Hamiltonian in Cs2CoBr4
Authors:
L. Facheris,
S. D. Nabi,
K. Yu. Povarov,
Z. Yan,
A. Glezer Moshe,
U. Nagel,
T. Rõõm,
A. Podlesnyak,
E. Ressouche,
K. Beauvois,
J. R. Stewart,
P. Manuel,
D. Khalyavin,
F. Orlandi,
A. Zheludev
Abstract:
Magnetic structures and spin excitations are studied across the phase diagram of the geometrically frustrated S = 3/2 quantum antiferromagnet Cs2CoBr4 in magnetic fields applied along the magnetic easy axis, using neutron diffraction, inelastic neutron scattering and THz absorption spectroscopy. The data are analyzed, where appropriate, using extended SU (4) linear spin wave theory. A minimal magn…
▽ More
Magnetic structures and spin excitations are studied across the phase diagram of the geometrically frustrated S = 3/2 quantum antiferromagnet Cs2CoBr4 in magnetic fields applied along the magnetic easy axis, using neutron diffraction, inelastic neutron scattering and THz absorption spectroscopy. The data are analyzed, where appropriate, using extended SU (4) linear spin wave theory. A minimal magnetic Hamiltonian is proposed based on measurements in the high field polarized state. It deviates considerably from the previously considered models. Additional dilatometry experiments highlight the importance of magnetoelastic coupling in this system.
△ Less
Submitted 14 March, 2024; v1 submitted 17 November, 2023;
originally announced November 2023.
-
Confinement of fractional excitations in a triangular lattice antiferromagnet
Authors:
L. Facheris,
S. D. Nabi,
A. Glezer Moshe,
U. Nagel,
T. Rõõm,
K. Yu. Povarov,
J. R. Stewart,
Z. Yan,
A. Zheludev
Abstract:
High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs$_2$CoBr$_4$, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [Phys. Rev. Lett. 129, 087201 (2022)] is shown to be a series of dispersive bound states reminiscent of "Zeeman ladders" in quasi-one-dimensi…
▽ More
High-resolution neutron and THz spectroscopies are used to study the magnetic excitation spectrum of Cs$_2$CoBr$_4$, a distorted-triangular-lattice antiferromagnet with nearly XY-type anisotropy. What was previously thought of as a broad excitation continuum [Phys. Rev. Lett. 129, 087201 (2022)] is shown to be a series of dispersive bound states reminiscent of "Zeeman ladders" in quasi-one-dimensional Ising systems. At wave vectors where inter-chain interactions cancel at the Mean Field level, they can indeed be interpreted as bound finite-width kinks in individual chains. Elsewhere in the Brillouin zone their true two-dimensional structure and propagation are revealed.
△ Less
Submitted 23 June, 2023; v1 submitted 31 January, 2023;
originally announced January 2023.
-
Spin Density Wave versus Fractional Magnetization Plateau in a Triangular Antiferromagnet
Authors:
L. Facheris,
K. Yu. Povarov,
S. D. Nabi,
D. G. Mazzone,
J. Lass,
B. Roessli,
E. Ressouche,
Z. Yan,
S. Gvasaliya,
A. Zheludev
Abstract:
We report an excellent realization of the highly non-classical incommensurate spin-density wave (SDW) state in the quantum frustrated antiferromagnetic insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case, here the SDW is stabilized by virtue of competing planar in-chain anisotropies and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m = 1/3$ magneti…
▽ More
We report an excellent realization of the highly non-classical incommensurate spin-density wave (SDW) state in the quantum frustrated antiferromagnetic insulator Cs$_2$CoBr$_4$. In contrast to the well-known Ising spin chain case, here the SDW is stabilized by virtue of competing planar in-chain anisotropies and frustrated interchain exchange. Adjacent to the SDW phase is a broad $m = 1/3$ magnetization plateau that can be seen as a commensurate locking of the SDW state into the up-up-down (UUD) spin structure. This represents the first example of long-sought SDW-UUD transition in triangular-type quantum magnets.
△ Less
Submitted 18 August, 2022; v1 submitted 22 April, 2022;
originally announced April 2022.
-
Quasiparticle trapping by orbital effect in a hybrid superconducting-semiconducting circuit
Authors:
Willemijn Uilhoorn,
James G. Kroll,
Arno Bargerbos,
Syed D. Nabi,
Chung-Kai Yang,
Peter Krogstrup,
Leo P. Kouwenhoven,
Angela Kou,
Gijs de Lange
Abstract:
The tunneling of quasiparticles (QPs) across Josephson junctions (JJs) detrimentally affects the coherence of superconducting and charge-parity qubits, and is shown to occur more frequently in magnetic fields. Here we demonstrate the parity lifetime to survive in excess of 50$\,\mathrmμ$s in magnetic fields up to 1$\,$T, utilising a semiconducting nanowire transmon to detect QP tunneling in real t…
▽ More
The tunneling of quasiparticles (QPs) across Josephson junctions (JJs) detrimentally affects the coherence of superconducting and charge-parity qubits, and is shown to occur more frequently in magnetic fields. Here we demonstrate the parity lifetime to survive in excess of 50$\,\mathrmμ$s in magnetic fields up to 1$\,$T, utilising a semiconducting nanowire transmon to detect QP tunneling in real time. We exploit gate-tunable QP filters and find magnetic-field-enhanced parity lifetimes, consistent with increased QP trapping by the ungated nanowire due to orbital effects. Our findings highlight the importance of QP trap engineering for building magnetic-field compatible hybrid superconducting circuits.
△ Less
Submitted 23 May, 2021;
originally announced May 2021.