arXiv subjects

Guangxin Ni

Publications and source records attributed to Guangxin Ni.

9 recordsLinked to original sources

arXiv2026

Revealing Intrinsic Anisotropy of Collective Magnetic Excitations in Twinned Crystals of a Kitaev-Heisenberg Quantum Magnet

Quantum magnets with competing interactions often emerge from delicate balances among microscopic parameters, making it essential to disentangle intrinsic spin dynamics from extrinsic disorder effects. Here, we introduce a multimodal optical approach combining magneto-infrared spectroscopy with domain-resolved micro-Raman spectroscopy at high magnetic fields to reconstruct the intrinsic magnetic excitation spectrum of twinned crystals of the Kitaev-Heisenberg quantum magnet Na$_3$Co$_2$SbO$_6$. Far-infrared spectroscopy reveals multiple field-tunable magnetic excitations, but the intrinsic response is obscured by replica features arising from twin domains. By correlating magneto-infrared and domain-resolved Raman spectra, we isolate the single-domain magnon response and uncover a pronounced twofold in-plane magnon anisotropy. This anisotropy far exceeds that expected from the measured in-plane g-factor anisotropy and is instead dominated by anisotropic bond-dependent exchange interactions. By unifying high-field, high-resolution and spatially selective optical probes, our work establishes a broadly applicable framework for revealing intrinsic spin dynamics and constraining the spin Hamiltonian in multidomain quantum magnets.

cond-mat.mtrl-sci↗

arXiv2026

All-Altermagnetic Tunnel Junction of RuO2/NiF2/RuO2

Emerging altermagnets with zero net magnetic moment and moment-dependent spin splitting offer a promising avenue for antiferromagnetic spintronic devices, yet their integration into magnetic tunnel junctions has been hindered by reliance on ferromagnetic electrodes (introducing stray fields) or by limited functionality (non-tunable magnetoresistance without spin filtering). Here, we propose an all-altermagnetic tunnel junction (AAMTJ) paradigm composed exclusively of altermagnets, exemplified by experimentally feasible RuO2/NiF2/RuO2. By introducing an altermagnetic NiF2 barrier, the achieved tunneling magnetoresistances of 11,704%, 2,496% and 1,892% for RuO2/NiF2/RuO2 are much higher than that of 221% for RuO2/TiO2/RuO2 with a nonmagnetic TiO2 barrier. High spin filtering efficiencies of ~90% are also obtained. This architecture unlocks multistate high magnetoresistance and spin filtering via magnetization control of the electrodes and barrier, stemming from their synergistic and antagonistic alignments of momentum-dependent altermagnetic spin-splitting. Importantly, high tunneling magnetoresistances are still achieved in the AAMTJ with TiO2 spacer of RuO2/TiO2/NiF2/TiO2/RuO2. Our AAMTJ inherently exhibits low consumption and zero stray field, with nonrelativistic spin splitting and vanishing magnetic moment, combining the advantages of both ferromagnetic and antiferromagnetic tunnel junctions. This AAMTJ paradigm opens an interesting avenue within the area of high-performance altermagnet-based tunnel junctions.

cond-mat.mtrl-sci↗

arXiv2025

Lithium and Vanadium Intercalation into Bilayer V2Se2O: Ferrimagnetic-Ferroelastic Multiferroics and Anomalous and Spin Transport

Spin splitting in emerging altermagnets is non-relativistic and momentum-dependent, yet energy-independent, and localized in momentum space, posing challenges for practical applications. Here, we propose an intercalation-driven paradigm for altermagnets to attain ameliorative electronic structures, multiferroic characteristics, and anomalous and spin transport functionalities. As a representative system, we investigate electrochemistry- and self-intercalated V2Se2O bilayers, building on the recently reported room-temperature K- and Rb-intercalated V2Se2O family [Nat. Phys. 2025, 21, 754; Nat. Phys. 2025, 21, 760], utilizing density functional theory, Wannier function analyses, Monte Carlo simulations, and non-equilibrium Green function methods. Intercalation induces room-temperature intralayer ferrimagnetic and interlayer ferromagnetic order (358 K for Li-intercalation and 773 K for V-intercalation), ferroelasticity (~1 % signal intensity), in-plane uniaxial magnetic anisotropy, and metallization, while also modifying the anomalous Hall effect. Notably, Li- and V-intercalated V2Se2O bilayers exhibit enhanced spin splitting and half-metallic behavior, respectively, yielding near-perfect spin filtering efficiency. Intercalation substantially enhances spin transport in V2Se2O-based devices, enabling giant magnetoresistance (877 %), ultra-high thermal tunneling magnetoresistance (~12000 %), and observable spin Seebeck and temperature negative differential resistance effects. This intercalation-driven paradigm expands altermagnetic functionalities through multifunctional integration, offering promising avenues for advanced, miniaturized, room-temperature exploitation of anomalous, electron, and spin transport properties.

cond-mat.mtrl-sci↗

arXiv2025

Above room temperature multiferroic tunnel junction with the altermagnetic metal CrSb

Altermagnets with nonrelativistic momentum-dependent spin splitting and compensated net magnetic moments have recently garnered significant interest in spintronics, particularly as pinning layers in magnetic tunnel junctions (MTJs). However, room temperature (RT) altermagnet-based MTJs with tunable tunneling magnetoresistance (TMR) or electroresistance (TER) modulated by multiferroicity remain largely unexplored. Here, we propose an experimentally fabricable above-RT multiferroic MTJ, comprising an altermagnetic metal, ferroelectric barrier, and ferromagnetic metal-epitomized by a CrSb/In2Se3/Fe3GaTe2 heterostructure. Our calculations with first-principles and nonequilibrium Green function method indicate that the architecture enables magnetically switchable TER, electrically tunable TMR, and dual-mode controllable spin filtering. To disentangle the roles of ferroelectricity and the tunnel barrier, nonferroelectric Sb2Se3 and a vacuum gap are exploited as control cases. Remarkably, the system achieves TMR up to 2308%, TER of 707%, and near-perfect spin filtering efficiency. Both TMR and TER are considerable for CrSb/In2Se3/Fe3GaTe2 with either Cr or Sb interface. The transport performance is robust under bias voltage. These findings demonstrate the above-RT multiferroic altermagnet-based MTJs and highlight their exciting potential as a versatile platform for next-generation spin dynamics, magnetic sensing, and quantum logic nanodevices.

cond-mat.mtrl-sci↗

arXiv2023

Visualizing structure of correlated ground states using collective charge modes

The variety of correlated phenomena in moiré systems is incredibly rich, spanning effects such as superconductivity, a generalized form of ferromagnetism, or even charge fractionalization. This wide range of quantum phenomena is partly enabled by the large number of internal degrees of freedom in these systems, such as the valley and spin degrees of freedom, which interplay decides the precise nature of the ground state. Identifying the microscopic nature of the correlated states in the moiré systems is, however, challenging, as it relies on interpreting transport behavior or scanning-tunneling microscopy measurements. Here we show how the real-space structure of collective charge oscillations of the correlated orders can directly encode information about the structure of the correlated state, focusing in particular on the problem of generalized Wigner crystals in moiré transition metal dichalcogenides. Our analysis builds upon our earlier result [10.1126/sciadv.adg3262] that the presence of a generalized Wigner crystal modifies the plasmon spectrum of the system, giving rise to new collective modes. We focus on scanning near-field optical microscopy technique (SNOM), fundamentally a charge-sensing-based method, and introduce a regime under which SNOM can operate as a probe of the spin degree of freedom.

cond-mat.mes-hall↗

arXiv2021

Dual-gated graphene devices for near-field nano-imaging

Graphene-based heterostructures display a variety of phenomena that are strongly tunable by electrostatic local gates. Monolayer graphene (MLG) exhibits tunable surface plasmon polaritons, as revealed by scanning nano-infrared experiments. In bilayer graphene (BLG), an electronic gap is induced by a perpendicular displacement field. Gapped BLG is predicted to display unusual effects such as plasmon amplification and domain wall plasmons with significantly larger lifetime than MLG. Furthermore, a variety of correlated electronic phases highly sensitive to displacement fields have been observed in twisted graphene structures. However, applying perpendicular displacement fields in nano-infrared experiments has only recently become possible (Ref. 1). In this work, we fully characterize two approaches to realizing nano-optics compatible top-gates: bilayer $\text{MoS}_2$ and MLG. We perform nano-infrared imaging on both types of structures and evaluate their strengths and weaknesses. Our work paves the way for comprehensive near-field experiments of correlated phenomena and plasmonic effects in graphene-based heterostructures.

cond-mat.mes-hall↗

arXiv2020

Nano-photocurrent mapping of local electronic structure in twisted bilayer graphene

We report a combined nano-photocurrent and infrared nanoscopy study of twisted bilayer graphene (TBG) enabling access to the local electronic phenomena at length scales as short as 20 nm. We show that the photocurrent changes sign at carrier densities tracking the local superlattice density of states of TBG. We use this property to identify domains of varying local twist angle by local photo-thermoelectric effect. Consistent with the photocurrent study, infrared nano-imaging experiments reveal optical conductivity features dominated by twist-angle dependent interband transitions. Our results provide a fast and robust method for mapping the electronic structure of TBG and suggest that similar methods can be broadly applied to probe electronic inhomogeneities of moiré superlattices in other van der Waals heterostructures.

cond-mat.mes-hall↗

arXiv2016

Tunable plasmonic reflection by bound 1D electron states in a 2D Dirac metal

We show that surface plasmons of a two-dimensional Dirac metal such as graphene can be reflected by line-like perturbations hosting one-dimensional electron states. The reflection originates from a strong enhancement of the local optical conductivity caused by optical transitions involving these bound states. We propose that the bound states can be systematically created, controlled, and liquidated by an ultranarrow electrostatic gate. Using infrared nanoimaging, we obtain experimental evidence for the locally enhanced conductivity of graphene induced by a carbon nanotube gate, which supports this theoretical concept.

cond-mat.mes-hall↗

arXiv2013

An innovative way of etching MoS2: Characterization and mechanistic investigation

We report a systematic study of the etching of MoS2 crystals by using XeF2 as a gaseous reactant. By controlling the etching process, monolayer MoS2 with uniform morphology can be obtained. The Raman and photoluminescence spectra of the resulting material were similar to those of exfoliated MoS2. Utilizing this strategy, different patterns such as a Hall bar structure and a hexagonal array can be realized. Furthermore, the etching mechanism was studied by introducing graphene as an etching mask. We believe our technique opens an easy and controllable way of etching MoS2, which can be used to fabricate complex nanostructures, such as nanoribbons, quantum dots and transistor structures. This etching process using XeF2 can also be extended to other interesting two-dimensional crystals.

cond-mat.mes-hall↗