arXiv subjects

Heejun Yang

Publications and source records attributed to Heejun Yang.

At least 19 recordsLinked to original sources

arXiv2026

Probing intrinsic magnetic phases in low-dimensional nearly twin-free NiPS$_3$ single crystals

We report the intrinsic thermal and magnetic properties of the low-dimensional van der Waals (vdW) antiferromagnet NiPS$_3$ and explore its emergent magnetic phases by controlling crystallographic twinning. Using nearly twin-free crystals, we resolve intrinsic properties that are typically obscured by multidomain effects in bulk samples. Magnetization results reveal a highly anisotropic, sharp spin-flop transition, confirming the high domain purity of our crystals. Furthermore, high-precision thermodynamic and transport data reveal a broad fluctuation regime around the N\'eel temperature ($T_{\mathrm{N}}$ = 157.5 K), with a heat capacity anomaly and a concurrent suppression of thermal conductivity. Field-dependent thermal transport shows a small but distinct contribution from spin-lattice coupling, as evidenced by the dip at the spin-flop transition. We develop a theoretical model to explain these properties reported in this paper, with good agreement between experiment and theory. Our work establishes a definitive baseline for bulk properties of NiPS$_3$ and demonstrates the feasibility of resolving intrinsic anisotropies by addressing crystallographic twinning in vdW magnets.

cond-mat.mtrl-sci↗

arXiv2026

Unconventional and Fragile Magnetic Exciton in a van der Waals Quantum Magnet

The recently discovered magnetic exciton in the van der Waals (vdW) antiferromagnet NiPS3 exemplifies these phenomena, exhibiting several distinctive characteristics. Despite extensive investigation, much of its physics remains unresolved, with key questions about why the NiPS3 magnetic exciton is so sharp and optically bright despite the nominally spin-forbidden transition, posing significant challenges to a proper understanding and practical manipulation of the exciton. An urgent question is to what extent it is due to chemical disorder, magnetic weakening, lattice modification, or intrinsic instability of the bright exciton itself: answers to which will put stringent constraints on possible theoretical models. Here we address these questions using hydrostatic pressure as a clean, continuous, reversible, and in-situ tuning parameter. We find that the sharp photoluminescence peak is drastically suppressed by as little as 0.4 GPa and completely quenched by 1.5 GPa, with demonstrating its reversibility. Crucially, this bright-to-dark conversion occurs without magnetic, crystallographic, or electronic reconstruction despite an increase in the Neel temperature, as established by Raman, X-ray absorption, nuclear magnetic resonance spectroscopy, and first-principles many-body calculations. Our results demonstrate that the optical brightness of the magnetic exciton is independent of chemical disorder, lattice expansion, and weakening of magnetic order, indicating that a higher-order correlated mechanism governs the bright exciton. We further propose experimentally constrained microscopic scenarios involving exciton pairing, crystal-field-controlled spin-orbit mixing, and symmetry breaking, providing a framework for future tests of entangled magnetic exciton in correlated quantum magnets.

cond-mat.mtrl-sci↗

arXiv2025

Spatial correlations of charge density wave order across the transition in 2H-NbSe2

Charge density waves (CDWs) involve coupled amplitude and phase degrees of freedom, but direct access to local amplitude correlations remains experimentally challenging. Here, we report cryogenic four-dimensional scanning transmission electron microscopy (4D-STEM) measurements of CDW ordering in a 2H-NbSe2 flake of 24 nm thickness, enabled by liquid helium-based cooling. By mapping the spatial distribution of CDW superlattice intensities at nanometer-scale resolution and analyzing their autocorrelations, we extract the temperature-dependent correlation length associated with the local amplitude of the CDW order parameter, independent of global phase coherence. Our results reveal that a finite local CDW amplitude is already established well above the transition temperature. When the system is cooled below the transition temperature down to 20 K, the correlation length extends to nearly 110 nm, and the local CDW amplitude is found to strongly anticorrelate with the local strain field.

cond-mat.str-el↗

arXiv2025

Polymorphic spin ordering in a single-crystalline cobalt-doped Fe3GaTe2

A single crystalline system typically stabilizes a unique state for spin ordering below a critical temperature. Certain materials exhibit multiple magnetic states, driven by structural phase transitions under varying thermodynamic conditions. Recently, van der Waals magnets have demonstrated subtle interlayer exchange interactions, offering a promising approach to electrically control spin states without structural transformation. Here, we report the emergence of three distinct magnetic states, ferromagnetic ordering and both collinear and non-collinear antiferromagnetic orderings, in a layered single crystalline magnet, cobalt-doped Fe3GaTe2 ((Co, Fe)3GaTe2). These three magnetic phases occur without structural phase transitions, a phenomenon we designate as polymorphic spin ordering in the material. The introduction of 16% Co-doping in Fe3GaTe2 modulates the interlayer magnetic interaction, enabling multiple spin orderings within the same lattice system with three critical temperatures: a Curie temperature for a ferromagnetic state (Tc=210 K) and two Neel temperatures for the collinear (TN1=110 K) and non-collinear (TN2=30 K) antiferromagnetic states. Our findings are supported by magnetic force microscopy, first-principles calculations, and circular dichroism angular photoemission spectroscopy, which reveals varying spin ordering and changes in the topological band structure and Berry curvature at different temperatures within the single-crystalline (Co, Fe)3GaTe2.

cond-mat.mtrl-sci↗

arXiv2024

Exchange striction induced thermal Hall effect in van der Waals antiferromagnet MnPS$_3$

The thermal Hall effect has emerged as an ideal probe for investigating topological phenomena of charge-neutral excitation. Notably, it reveals crucial aspects of spin-lattice couplings that have been difficult to access for decades. However, the exchange striction mechanism from a lattice-induced change in exchange interaction has often been ignored in thermal Hall experiments. MnPS3 can offer a platform to study exchange striction on the thermal Hall effect due to its significant spin-lattice coupling and field-induced non-collinear spin configuration. Our thermal transport data show distinct temperature and field dependence of longitudinal thermal conductivity ($\kappa_{xx}$) and thermal Hall effect ($\kappa_{xy}$). By using detailed theoretical calculation, we found that the inclusion of the exchange striction is essential for a better description of both $\kappa_{xx}$ and $\kappa_{xy}$. Our result demonstrates the importance of the exchange striction mechanism for a complete understanding of the magnon-phonon-driven thermal Hall effect.

cond-mat.str-el↗

arXiv2024

Topological magnon-polarons in honeycomb antiferromagnets with spin-flop transition

We theoretically investigate the thermal Hall transport of magnon-polarons in a two-dimensional honeycomb antiferromagnetic insulator under the influence of a perpendicular magnetic field, varying in strength. The application of a perpendicular magnetic field induces a magnetic phase transition from the collinear antiferromagnetic phase to the spin-flop phase, leading to a significant alteration in Hall transport across the transition point. In this paper, our focus is on the intrinsic contribution to thermal Hall transport arising from the magnetoelastic interaction. To facilitate experimental verification of our theoretical results, we present the dependence of thermal Hall conductivity on magnetic field strength and temperature.

cond-mat.mes-hall↗

arXiv2023

Singular Hall response from a correlated ferromagnetic flat nodal-line semimetal

Topological quantum phases have been largely understood in weakly correlated systems, which have identified various quantum phenomena such as spin Hall effect, protected transport of helical fermions, and topological superconductivity. Robust ferromagnetic order in correlated topological materials particularly attracts attention, as it can provide a versatile platform for novel quantum devices. Here, we report singular Hall response arising from a unique band structure of flat topological nodal lines in combination with electron correlation in an itinerant, van der Waals ferromagnetic semimetal, Fe3GaTe2, with a high Curie temperature of Tc=360 K. High anomalous Hall conductivity violating the conventional scaling, resistivity upturn at low temperature, and a large Sommerfeld coefficient are observed in Fe3GaTe2, which implies heavy fermion features in this ferromagnetic topological material. Our circular dichroism in angle-resolved photoemission spectroscopy and theoretical calculations support the original electronic features in the material. Thus, low-dimensional Fe3GaTe2 with electronic correlation, topology, and room-temperature ferromagnetic order appears to be a promising candidate for robust quantum devices.

cond-mat.str-el↗

arXiv2023

Thermal Hall effects due to topological spin fluctuations in YMnO$_3$

The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted, which could open the possibility of a non-zero thermal Hall effect. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO$_3$ produces a highly unusual thermal Hall effect due to topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction under an applied magnetic field. We conclude the thermal Hall conductivity arises from the system's topological nature of spin fluctuations. Our theoretical calculations demonstrate that the thermal Hall conductivity is also related in this material to the splitting of the otherwise degenerate two chiralities, left and right, of its 120$^{\circ}$ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken $Z_2$ symmetry of the chirality in the supposedly paramagnetic state of YMnO$_3$, with strong topological spin fluctuations. These new mechanisms in this important class of materials are crucial in exploring new thermal Hall physics and exotic excitations.

cond-mat.str-el↗

arXiv2023

Sizable suppression of magnon Hall effect by magnon damping in Cr$_2$Ge$_2$Te$_6$

Two-dimensional (2D) Heisenberg honeycomb ferromagnets are expected to have interesting topological magnon effects as their magnon dispersion can have Dirac points. The Dirac points are gapped with finite second nearest neighbor Dzyaloshinskii-Moriya interaction, providing nontrivial Berry curvature with finite magnon Hall effect. Yet, it is unknown how the topological properties are affected by magnon damping. We report the thermal Hall effect in Cr$_2$Ge$_2$Te$_6$, an insulating 2D honeycomb ferromagnet with a large Dirac magnon gap and significant magnon damping. Interestingly, the thermal Hall conductivity in Cr$_2$Ge$_2$Te$_6$ shows the coexisting phonon and magnon contributions. Using an empirical two-component model, we successfully estimate the magnon contribution separate from the phonon part, revealing that the magnon Hall conductivity was 20 times smaller than the theoretical calculation. Finally, we suggest that such considerable suppression in the magnon Hall conductivity is due to the magnon damping effect in Cr$_2$Ge$_2$Te$_6$.

cond-mat.str-el↗

arXiv2022

Significant thermal Hall effect in the 3$d$ cobalt Kitaev system $Na_2Co_2TeO_6$

Kitaev physics has recently attracted attention in condensed matter for its anticipated novel quantum spin liquid state. The thermal transport measurement is crucial for probing the novel features of charge-neutral quasiparticles. In this letter, we report a significant thermal Hall effect in $Na_2Co_2TeO_6$ (NCTO), a Kitaev quantum spin liquid candidate, when the magnetic field is applied along the out-of-plane direction of the honeycomb plane. The thermal conductivity ($\kappa_{xx}$) and thermal Hall conductivity ($\kappa_{xy}$) in NCTO reveal distinct magnetic field dependences below and above the Neel temperature ($T_N$) of 27 K. For $T>T_N$, $\kappa_{xx}$ has a monotonic decrease in the field dependence, while $\kappa_{xy}$ persists up to $T^*$ = 150 K. On the other hand, both $\kappa_{xx}$ and $\kappa_{xy}$ exhibit complex field dependence for $T<T_N$.

cond-mat.str-el↗

arXiv2022

Terahertz spin-to-charge current conversion in stacks of ferromagnets and the transition-metal dichalcogenide NbSe$_2$

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

cond-mat.mes-hall↗

arXiv2022

Diagonal and off-diagonal thermal conduction with resonant phonon scattering in $Ni_3TeO_6$

The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that Ni3TeO6 exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity ($\kappa_{xx}$) and thermal Hall coefficient ($\kappa_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain the measured magnetic field dependence of $\kappa_{xx}(H)$: phonon scattering from spin fluctuation in the paramagnetic phase and additional scattering due to magnon-phonon coupling in the collinear antiferromagnetic phase. We further suggest that a similar approach could be applied to understand the finite $\kappa_{xy}$ values in $Ni_3TeO_6$.

cond-mat.mtrl-sci↗

arXiv2021

Reshaped Weyl fermionic dispersions driven by Coulomb interactions in MoTe2

We report the direct evidence of impacts of the Coulomb interaction in a prototypical Weyl semimetal, MoTe2, that alter its bare bands in a wide range of energy and momentum. Our quasiparticle interference patterns measured using scanning tunneling microscopy are shown to match the joint density of states of quasiparticle energy bands including momentum-dependent self-energy corrections, while electronic energy bands based on the other simpler local approximations of the Coulomb interaction fail to explain neither the correct number of quasiparticle pockets nor shape of their dispersions observed in our spectrum. With this, we predict a transition between type-I and type-II Weyl fermions with doping and resolve its disparate quantum oscillation experiments, thus highlighting the critical roles of Coulomb interactions in layered Weyl semimetals.

cond-mat.mes-hall↗

arXiv2020

Sizeable suppression of thermal Hall effect upon isotopic substitution in strontium titanate

We report measurements of the thermal Hall effect in single crystals of both pristine and isotopically substituted strontium titanate. We discovered a two orders of magnitude difference in the thermal Hall conductivity between $SrTi^{16}O_3$ and $^{18}O$-enriched $SrTi^{18}O_3$ samples. In most temperature ranges, the magnitude of thermal Hall conductivity ($\kappa_{xy}$) in $SrTi^{18}O_3$ is proportional to the magnitude of the longitudinal thermal conductivity ($\kappa_{xx}$), which suggests a phonon-mediated thermal Hall effect. However, they deviate in the temperature of their maxima, and the thermal Hall angle ratio ($|\kappa_{xy}/\kappa_{xx}|$) shows anomalously decreasing behavior below the ferroelectric Curie temperature $T_c$ ~$25 K$. This observation suggests a new underlying mechanism, as the conventional scenario cannot explain such differences within the slight change in phonon spectrum. Notably, the difference in magnitude of thermal Hall conductivity and rapidly decreasing thermal Hall angle ratio in $SrTi^{18}O_3$ is correlated with the strength of quantum critical fluctuations in this displacive ferroelectric. This relation points to a link between the quantum critical physics of strontium titanate and its thermal Hall effect, a possible clue to explain this example of an exotic phenomenon in non-magnetic insulating systems.

cond-mat.str-el↗

arXiv2019

Symmetry dictated grain boundary state in a two-dimensional topological insulator

Structural imperfections such as grain boundaries (GBs) and dislocations are ubiquitous in solids and have been of central importance in understanding nature of polycrystals. In addition to their classical roles, advent of topological insulators (TIs) offers a chance to realize distinct topological states bound to them. Although dislocation inside three-dimensional TIs is one of the prime candidates to look for, its direct detection and characterization are challenging. Instead, in two-dimensional (2D) TIs, their creations and measurements are easier and, moreover, topological states at the GBs or dislocations intimately connect to their lattice symmetry. However, such roles of crystalline symmetries of GBs in 2D TIs have not been clearly measured yet. Here, we present the first direct evidence of a symmetry enforced Dirac type metallic state along a GB in 1T'-MoTe$_2$, a prototypical 2D TI. Using scanning tunneling microscope, we show a metallic state along a grain boundary with non-symmorphic lattice symmetry and its absence along the other boundary with symmorphic one. Our large scale atomistic simulations demonstrate hourglass like nodal-line semimetallic in-gap states for the former while the gap-opening for the latter, explaining our observation very well. The protected metallic state tightly linked to its crystal symmetry demonstrated here can be used to create stable metallic nanowire inside an insulator.

cond-mat.mes-hall↗

arXiv2015

Spin to charge conversion in MoS$_{2}$ monolayer with spin pumping

Layered transition-metal dichalcogenides (TMDs) family are gaining increasing importance due to their unique electronic band structures, promising interplay among light, valley (pseudospin), charge and spin degrees of freedom. They possess large intrinsic spin-orbit interaction which make them most relevant for the emerging field of spin-orbitronics. Here we report on the conversion of spin current to charge current in MoS2 monolayer. Using spin pumping from a ferromagnetic layer (10 nm of cobalt) we find that the spin to charge conversion is highly efficient. Analysis in the frame of the inverse Rashba-Edelstein (RE) effect yields a RE length in excess of 4 nm at room temperature. Furthermore, owing to the semiconducting nature of MoS$_{2}$, it is found that back-gating allows electrical field control of the spin-relaxation rate of the MoS$_{2}$-metallic stack.

cond-mat.mes-hall↗

arXiv2014

Graphene-Passivated Nickel as an Oxidation-Resistant Electrode for Spintronics

We report on graphene-passivated ferromagnetic electrodes (GPFE) for spin devices. GPFE are shown to act as spin-polarized oxidation-resistant electrodes. The direct coating of nickel with few layer graphene through a readily scalable chemical vapour deposition (CVD) process allows the preservation of an unoxidized nickel surface upon air exposure. Fabrication and measurement of complete reference tunneling spin valve structures demonstrates that the GPFE is maintained as a spin polarizer and also that the presence of the graphene coating leads to a specific sign reversal of the magneto-resistance. Hence, this work highlights a novel oxidation-resistant spin source which further unlocks low cost wet chemistry processes for spintronics devices.

cond-mat.mtrl-sci↗

arXiv2011

Band gap opening by two-dimensional manifestation of Peierls instability in graphene

Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been attributed usually to quantum confinement effects, can be understood with the chiral symmetry breaking.

cond-mat.mtrl-sci↗