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Light-Matter-Coupling formalism for magnons: probing quantum geometry with light
Authors:
Ying Shing Liu,
Emil Viñas Boström,
Michael A. Sentef,
Silvia Viola Kusminskiy
Abstract:
Nontrivial quantum geometry is a key feature of the wavefunctions of collective magnetic excitations in topological systems, but accessing it experimentally remains an open challenge. While Raman circular dichroism (RCD) has emerged as a promising probe, the fundamental link between the RCD and magnon quantum geometry has remained unsettled, and complicated by the fact that magnons are charge neut…
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Nontrivial quantum geometry is a key feature of the wavefunctions of collective magnetic excitations in topological systems, but accessing it experimentally remains an open challenge. While Raman circular dichroism (RCD) has emerged as a promising probe, the fundamental link between the RCD and magnon quantum geometry has remained unsettled, and complicated by the fact that magnons are charge neutral. Here, we identify when and why this link exists. We show that, under broad conditions, the Fleury-Loudon Raman vertex can be obtained directly from a light-matter coupling expansion of the effective magnon Hamiltonian, bypassing the conventional microscopic derivation based on virtual electronic processes. This yields an analytical connection between the RCD and the Berry curvature of magnon bands. Applied to monolayer CrI\textsubscript{3}, our theory predicts finite temperature signatures of topological magnons in the RCD. These results establish a general route to quantum-geometry sensitive optical probes in magnonic systems.
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Submitted 28 April, 2026; v1 submitted 13 April, 2026;
originally announced April 2026.
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Integrability Breaking and Coherent Dynamics in Hermitian and Non-Hermitian Spin Chains with Long-Range Coupling
Authors:
Y. S. Liu,
X. Z. Zhang
Abstract:
Unraveling the mechanisms of ergodicity breaking in complex quantum systems is a central pursuit in nonequilibrium physics. In this work, we investigate a one-dimensional spin model featuring a tunable long-range hopping term, $H_{n}$, which introduces nonlocal interactions and bridges the gap between Hermitian and non-Hermitian regimes. Through a systematic analysis of level-spacing statistics, K…
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Unraveling the mechanisms of ergodicity breaking in complex quantum systems is a central pursuit in nonequilibrium physics. In this work, we investigate a one-dimensional spin model featuring a tunable long-range hopping term, $H_{n}$, which introduces nonlocal interactions and bridges the gap between Hermitian and non-Hermitian regimes. Through a systematic analysis of level-spacing statistics, Krylov complexity, and entanglement entropy, we demonstrate that $H_{n}$ acts as a universal control parameter driving the transition from integrability to quantum chaos. Specifically, increasing the strength of $H_{n}$ induces a crossover from Poissonian to Gaussian Orthogonal Ensemble statistics in the Hermitian limit, and similarly triggers chaotic dynamics in the non-Hermitian case. Most remarkably, despite the onset of global chaos, we identify a tower of exact nonthermal eigenstates that evade thermalization. These states survive as robust quantum many-body scars, retaining low entanglement and coherent dynamics even under strong non-Hermitian perturbations. Our findings reveal a universal mechanism by which long-range and non-Hermitian effects reshape quantum ergodicity, offering new pathways for preserving quantum coherence in complex many-body systems.
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Submitted 15 December, 2025;
originally announced December 2025.
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The out-of-plane magnetoresistance in a Van der Waals thin film of WTe2
Authors:
Y. S. Liu,
H. Xiao,
C. Zhang,
C. W. Zhang,
Y. G. Shi,
T. Hu,
C. M. Schneider
Abstract:
We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a Van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes.At low temperatures, we found a large unsaturated in-plane magnetoresistance and a saturated out-of-plane magnetore…
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We report the magneto-transport measurements of thin film devices of the topological Weyl semimetal WTe2 with the applied current along and vertical to the in-plane directions. The device is composed of a Van der Waals thin film of WTe2 sandwiched between top and bottom Au electrodes.At low temperatures, we found a large unsaturated in-plane magnetoresistance and a saturated out-of-plane magnetoresistance when the external magnetic fields are applied perpendicular to the plane. By analysis of Shubnikov-de Haas oscillations, one oscillation peak is found in the out-of-plane magnetoresistance, in contrast to four oscillation peaks in the in-plane magnetoresistance.Our work provides new insight into the origin of the unsaturated magnetoresistance in WTe2 and may inspire non-planar engineering to reach higher integration in spintronics.
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Submitted 6 February, 2023;
originally announced February 2023.
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Giant magnetoresistance and spin Seebeck coefficient in zigzag a-graphyne nanoribbons
Authors:
M. X. Zhai,
X. F. Wang,
P. Vasilopoulos,
Y. S. Liu,
Y. J. Dong,
L. Zhou,
Y. J. Jiang,
W. L. You
Abstract:
We investigate the spin-dependent electric and thermoelectric properties of ferromagnetic zigzag-graphyne nanoribbons (ZGNRs) using the density-functional theory combined with the non-equilibrium Green's function method. A giant magnetoresistance is obtained in the pristine even-width ZGNRs and can be as high as 10e6 %. However, for the doped systems, a large magnetoresistance behavior may appear…
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We investigate the spin-dependent electric and thermoelectric properties of ferromagnetic zigzag-graphyne nanoribbons (ZGNRs) using the density-functional theory combined with the non-equilibrium Green's function method. A giant magnetoresistance is obtained in the pristine even-width ZGNRs and can be as high as 10e6 %. However, for the doped systems, a large magnetoresistance behavior may appear in the odd-width ZGNRs rather than the even-width ones. This suggests that the magnetoresistance can be manipulated in a wide range by the dopants on edges of ZGNRs. Another interesting phenomenon is that in the B- and N-doped even-width ZGNRs the spin Seebeck coefficient is always larger than the charge Seebeck coefficient, and a pure-spin-current thermospin device can be achieved at specific temperatures.
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Submitted 28 August, 2014;
originally announced August 2014.
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Vacancy Effects on Electric and Thermoelectric Properties of Zigzag Silicene Nanoribbons
Authors:
R. L. An,
X. F. Wang,
P. Vasilopoulos,
Y. S. Liu,
A. B. Chen,
Y. J. Dong,
M. X. Zhai
Abstract:
We study the crystal reconstruction in the presence of monovacancies (MVs), divacancies (DVs) and linear vacancies (LVs) in a zigzag silicene nanoribbon (ZSiNR) with transversal symmetry. Their influence on the electric and thermoelectric properties is assessed by the density functional theory combined with the nonequilibrium Green's functions. In particular, we focus on the spin resolved conducta…
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We study the crystal reconstruction in the presence of monovacancies (MVs), divacancies (DVs) and linear vacancies (LVs) in a zigzag silicene nanoribbon (ZSiNR) with transversal symmetry. Their influence on the electric and thermoelectric properties is assessed by the density functional theory combined with the nonequilibrium Green's functions. In particular, we focus on the spin resolved conductance, magnetoresistance and current-voltage curves. A 5-atom-ring is formed in MVs, a 5-8-5 ring structure in DVs, and a 8-4-8-4 ring structure in LVs. The linear conductance becomes strongly spin dependent when the transversal symmetry is broken by vacancies especially if they are located on the ribbon's edges. The giant magnetoresistance can be smeared by asymmetric vacancies. Single spin negative differential resistance may appear in the presence of LVs and asymmetric MVs or DVs. A strong spin Seebeck effect is expected at room temperature in ZSiNRs with LVs.
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Submitted 28 August, 2014;
originally announced August 2014.
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Spin-dependent ballistic transport properties and electronic structures of pristine and edge-doped zigzag silicene nanoribbons: large magnetoresistance
Authors:
A. B. Chen,
X. F. Wang,
P. Vasilopoulos,
M. X. Zhai,
Y. S. Liu
Abstract:
The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magne…
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The electronic structure and conductance of substitutionally edge-doped zigzag silicene nanoribbons (ZSiNRs) are investigated using the nonequilibrium Green's function method combined with the density functional theory. Two-probe systems of ZSiNRs in both ferromagnetic and antiferromagnetic states are considered. Doping effects of elements from groups III and V, in a parallel or antiparallel magnetic configuration of the two electrodes, are discussed. Switching on and off the external magnetic field, we may convert the metallic ferromagnetic ZSiNRs into insulating antiferromagnetic ZSiNRs. In the ferromagnetic state, even- or odd-width ZSiNRs exhibit a drastically different magnetoresistance. In an odd-width edge-doped ZSiNR a large magnetoresistance occurs compared to that in a pristine ZSiNR. The situation is reversed in even-width ZSiNRs. These phenomena result from the drastic change of the conductance in the antiparallel configuration.
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Submitted 28 August, 2014;
originally announced August 2014.
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Influence of itinerancy on the magnetic properties of the single crystals of Sr2YRu1-xCuxO6 solid solutions
Authors:
Chi Liang Chen,
S. M. Rao,
C. L. Dong,
J. L. Chen,
Y. S. Liu,
C. L. Chang,
T. S. Chan,
H. L. Liu,
H. H. Shieh,
M. K. Wu
Abstract:
X-ray absorption spectroscopy(XAS) Ru L-edge and O K-edge measurements on single crystals of Sr2YRu1-xCuxO6 (x=0.01-0.05) solid solutions show that Cu replaces Ru in the lattice which results in a lattice distortion and itinerant holes that increase with increasing x. Powder X-ray diffraction (XRD) measurements confirm the lattice distortion through changes in the lattice parameters. Magnetic meas…
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X-ray absorption spectroscopy(XAS) Ru L-edge and O K-edge measurements on single crystals of Sr2YRu1-xCuxO6 (x=0.01-0.05) solid solutions show that Cu replaces Ru in the lattice which results in a lattice distortion and itinerant holes that increase with increasing x. Powder X-ray diffraction (XRD) measurements confirm the lattice distortion through changes in the lattice parameters. Magnetic measurements reveal an antiferromagnetic transition at 32 K in the crystals with x=0 and a diamagnetic transition at 32 K for x \geq 0.01indicative of superconductivity in the zero field cooled (ZFC) curves. The field cooled (FC) curves show a weak ferromagnetic character with x=0 and a ferromagnetic transition at 30 K and an antiferromagnetic transition at 23 K in the presence of Cu. The peak at 30 K in the ac susceptibility measurements indicates a spin-glass behavior. The low field part of the M-H curve for x=0.05 crystal indicates weak superconductivity. The shifting of the M-H curve on FC indicates spin-glass state. In conjunction with the XAS results the magnetic behavior is explained in terms of a spontaneous vortex state as a result of the non-freezing Ru5+ spins.
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Submitted 15 November, 2010;
originally announced November 2010.