arXiv subjects

Lunhua He

Publications and source records attributed to Lunhua He.

At least 19 recordsLinked to original sources

arXiv2026

Magneto-Structural Coupling Enables Cryogenic Cation Redistribution in a Spinel Oxide

Ionic transport in oxides is generally frozen at cryogenic temperatures, where thermal energy lies far below typical cation-migration barriers. Neutron powder diffraction reveals progressive Fe/Mg redistribution between tetrahedral (A) and octahedral (B) sites in the spinel Mg0.5Fe0.5TiFeO4 upon cooling from 200 K to 5 K. A-site Fe occupancy increases toward near completion at 5 K within Rietveld resolution, while Ti remains on the B site. This exchange coincides with complex magnetic correlations rather than a classical thermally activated window. Low-temperature magnetostrictive volume changes indicate strong spin-lattice coupling, but do not identify magnetostriction as the sole thermodynamic driver. Room-temperature high-pressure X-ray diffraction produces the opposite occupancy trend, showing that volume contraction alone cannot explain the cryogenic site exchange. These results point to magneto-structural free-energy minimization as a plausible mechanism for unlocking cryogenic cation mobility in a correlated spinel oxide.

cond-mat.mtrl-sci↗

arXiv2026

Regulating oxygen content and superconductivity in La$_3$Ni$_2$O$_{7+\delta}$

The synthesis of high-quality Ruddlesden-Popper (RP) nickelates remains challenging due to variations in oxygen content and the prevalence of intergrown RP phases. Precisely controlling the stoichiometry and characterizing the resulting physical properties are essential for understanding the mechanism of high-$T_c$ superconductivity in these materials. In this work, we synthesize a series of La$_3$Ni$_2$O$_{7+\delta}$ samples with systematically controlled oxygen content and perform comprehensive structural and compositional analyses. Precise oxygen tuning enables us to tailor the microstructure, yielding a pure bilayer phase, a mixture of bilayer and hybrid single-layer-bilayer phases, and a predominantly bilayer phase containing trilayer intergrowths. High-pressure transport measurements reveal distinct superconducting transitions with contrasting $T_c$ values, corresponding to the bilayer phase, the hybrid phase, and trilayer inclusions. Notably, we find that oxygen content not only governs the phase purity$-$i.e., the presence of intergrowth phases$-$but also directly modulates the upper critical field ($H_{c2}$) of the bilayer superconductivity. By establishing a phase diagram of $T_c$ and $H_{c2}$ as functions of oxygen content in La$_3$Ni$_2$O$_{7+\delta}$, this work advances synthetic control and provides new insights into the superconducting mechanism of RP nickelates.

cond-mat.supr-con↗

arXiv2026

G-type antiferromagnetic structure in Rb1-xV2Te2O

Altermagnetism, known for its non-relativistic spin-split band structures with yet compensated moments, is being intensively investigated. Discovering new altermagnetic materials with characteristics suitable for practical use remains an important ongoing task. Recently a metallic room-temperature altermagnet candidate Rb1-xV2Te2O with a layered structure and d-wave spin symmetry has been reported based on experimental results from the spin-resolved photoemission spectroscopy and scanning tunnelling microscopy/spectroscopy (STM/STS) measurements. Here we report neutron powder diffraction (NPD) investigations on the magnetic structure of Rb1-xV2Te2O, which shows a G-type antiferromagnetic structure below the transition temperature of 337 K. The result is different from the original theoretical expectation, which might lead to new insights on the physics of this altermagnet candidate.

cond-mat.mtrl-sci↗

arXiv2025

Canted ferromagnetic order in a distorted triangular-lattice magnet Na$_2$SrCo(VO$_4$)$_2$

Triangular-lattice cobaltates with glaserite-type $X_2Y$Co($T$O$_4)_2$ structure provide an ideal platform to investigate intriguing quantum magnetism. Here we report a comprehensive study of the structural and magnetic properties of a triangular-lattice cobalt vanadate $\rm Na_2SrCo(VO_4)_2$. Room-temperature x-ray and neutron powder diffraction confirm that $\rm Na_2SrCo(VO_4)_2$ crystallizes in the monoclinic $P2_1/c$ space group with slightly distorted triangular layers of $\rm Co^{2+}$ ions. Magnetization measurements reveal a ferromagnetic transition at $T\rm_C \approx 3.4~{\rm K}$, where a sharp $\lambda$-type anomaly is observed in the specific heat. The magnetic entropy recovered up to 55 K approaches 90$\%$ of $R{\rm ln}2$, supporting an effective spin-1/2 state of Co$^{2+}$ ions at low temperature. Neutron diffraction at 2.3 K (below $T_{\rm C}$) further confirms a long-range canted ferromagnetic order with the Co$^{2+}$ moments aligned in the $ac$ plane and the ordered moment size of $\sim$ 2.6 $\mu\rm_{B}$. Comparing with its sister compounds with a trigonal symmetry, $\rm Na_2BaCo(VO_4)_2$ with a collinear ferromagnetic structure and the recently discovered spin supersolid candidate $\rm Na_2BaCo(PO_4)_2$ with a distinct Y-like antiferromagnetic ground state, this study indicates the decisive role of the $T{\rm O_4}$ tetrahedra in tuning exchange interactions and contrasting magnetic behaviors of these glaserite-structure compounds.

cond-mat.str-el↗

arXiv2025

Single-crystal growth, structural characterization, and physical properties of a decorated square-kagome antiferromagnet KCu$_7$TeO$_4$(SO$_4$)$_5$Cl

The square-kagome lattice, composed of two-dimensional corner-sharing triangles, provides a novel platform for studying frustrated magnetism. However, material realizations of the square-kagome lattice remain scarce. Here, we report the single-crystal growth, structural characterization, magnetic and electric properties of KCu$_7$TeO$_4$(SO$_4$)$_5$Cl, a nabokoite compound featuring a distorted and decorated square-kagome lattice. Weak anomalies near 4 K are observed in both magnetization and specific heat, indicating the onset of a magnetic transition.The formation of a long-range antiferromagnetic state below 4.5 K is further confirmed by $^{35}$Cl nuclear magnetic resonance (NMR) measurements. Magnetic susceptibility data reveal nearly isotropic Curie-Weiss temperatures ($\sim-145$ K) and $g$-factors ($\sim2.4$) for both in-plane and out-of-plane magnetic fields. Moreover, we observe two successive ferroelectric transitions at $T_\mathrm{FE1}\sim30$ K and $T_\mathrm{FE2}\sim27$ K, driven by inversion-symmetry breaking, most likely associated with distortions in the Cu2O$_4$Cl$_1$ pyramids and the adjacent SO$_4$ tetrahedra. These results suggest that a three-dimensional model incorporating interlayer couplings via decorating Cu2 sites is essential for capturing the magnetic and electric behaviors in KCu$_7$TeO$_4$(SO$_4$)$_5$Cl.

cond-mat.str-el↗

arXiv2025

Observation of Orbital-Selective Dual Modulations in an Anisotropic Antiferromagnetic Kagome Metal TbTi3Bi4

Orbital selectivity is pivotal in dictating the phase diagrams of multiorbital systems, with prominent examples including the orbital-selective Mott phase and superconductivity, etc. The intercalation of anisotropic layers represents an effective method for enhancing orbital selectivity and, thereby shaping the low-energy physics of multiorbital systems. Despite its potential, related experimental studies remain limited. In this work, we systematically examine the interplay between orbital selectivity and magnetism in the newly discovered anisotropic kagome TbTi3Bi4 single crystal, and report a unidirectional, orbital-selective band reconstruction within the antiferromagnetic (AFM) state. By combining soft X-ray and vacuum ultraviolet angle-resolved photoemission spectroscopy (ARPES) measurements with orbital-resolved density functional theory (DFT) calculations, we identify that the band reconstruction is a manifestation of the AFM order, driven by a 1/3 nesting instability of the intercalated Tb 5dxz orbitals. Such an orbital-selective modulation leads the unusual momentum-dependent band folding and the emergence of symmetry-protected Dirac cones only at the M1 point. More importantly, the discovery of orbital-selective 3 x 1 AFM order offers crucial insights into the underlying mechanism of the fractional magnetization plateau in this Kagome AFM metal. Our findings not only underscore the essential role of both conducting and localized electrons in determining the magnetic orders of LnTi3Bi4 (Ln = Lanthanide) kagome metals but also offer a pathway for manipulating magnetism through selective control of anisotropic electronic structures.

cond-mat.str-el↗

arXiv2025

On the ambient conditions crystal structure of AgSbTe2

We present a combined X-ray and neutron diffraction, Raman spectroscopy, and 121Sb NMR studies of AgSbTe2, supported by first-principles calculations aiming to elucidate its crystal structure. While diffraction methods cannot unambiguously resolve the structure, Raman and NMR data, together with electric field gradient calculations, strongly support the rhombohedral R-3m phase. Moreover, the agreement between experimental and calculated Raman spectra further corroborates this result, resolving the 60-year sold debate about the exact crystal structure of the AgSbTe2 compound.

cond-mat.mtrl-sci↗

arXiv2025

Ground state and magnetic transitions of the orthorhombic antiferromagnet CaCo$_2$TeO$_6$

We report the systematic synthesis, crystal structure, magnetization, and powder neutron diffraction of single crystalline and polycrystalline CaCo$_2$TeO$_6$ samples. CaCo$_2$TeO$_6$ crystallizes in an orthorhombic structure with $Pnma$ space group, featuring chains of edge-shared CoO$_6$ octahedra arranged in a honeycomb pattern. Two antiferromagnetic transitions are observed at $T$$_{N1}$ = 14.4 K and $T$$_{N2}$ = 16.2 K, corresponding to two long-range magnetic orders with propagation vectors of $\bf{k}$$_1$ = (0, 0, 0) and $\bf{k}$$_2$ = (0.125, 0, 0.25), respectively. The ground state is determined as a canted up-up-down-down zigzag spin configuration along the $c$ axis, wherein the magnetic moments of Co1 and Co2 ions are 3.4(1) and 2.1(1)$μ$$_B$, respectively. Successive spin-flop transitions appear with the increasing magnetic field applied along the easy axis ($c$ axis), accompanied by depression of the antiferromagnetic orders and enhancement of residual magnetic entropy. The field-induced spin-disordered state suggests that CaCo$_2$TeO$_6$ may be an ideal candidate for studying frustrated magnetism.

cond-mat.str-el↗

arXiv2025

Pyrochlore NaYbO2: A potential Quantum Spin Liquid Candidate

The search for quantum spin liquids (QSL) and chemical doping in such materials to explore superconductivity have continuously attracted intense interest. Here, we report the discovery of a potential QSL candidate, pyrochlore-lattice beta-NaYbO2. Colorless and transparent NaYbO2 single crystals, layered alpha-NaYbO2 (~250 um on edge) and octahedral beta-NaYbO2 (~50 um on edge), were grown for the first time. Synchrotron X-ray single crystal diffraction unambiguously determined that the newfound beta-NaYbO2 belongs to the three-dimensional pyrochlore structure characterized by the R-3m space group, corroborated by synchrotron X-ray and neutron powder diffraction and pair distribution function. Magnetic measurements revealed no long-range magnetic order or spin glass behavior down to 0.4 K with a low boundary spin frustration factor of 17.5, suggesting a potential QSL ground state. Under high magnetic fields, the potential QSL state was broken and spins order. Our findings reveal that NaYbO2 is a fertile playground for studying novel quantum states.

cond-mat.str-el↗

arXiv2024

Discovery of a metallic room-temperature d-wave altermagnet KV2Se2O

Beyond conventional ferromagnetism and antiferromagnetism, altermagnetism is a recently discovered unconventional magnetic phase characterized by time-reversal symmetry breaking and spin-split band structures in materials with zero net magnetization. This distinct magnetic phase not only enriches the understanding of fundamental physical concepts but also has profound impacts on condense-matter physics research and practical device applications. Spin-polarized band structures have been recently observed in semiconductors MnTe and MnTe2 with vanishing net magnetization, confirming the existence of this unconventional magnetic order. Metallic altermagnets have unique advantages for exploring novel physical phenomena related to low-energy quasiparticle excitations and for applications in spintronics as electrical conductivity in metals allows the direct manipulation of spin current through electric field. Here, through comprehensive characterization and analysis of the magnetic and electronic structures of KV2Se2O, we have unambiguously demonstrated a metallic room-temperature altermaget with d-wave spin-momentum locking. The highly anisotropic spin-polarized Fermi surfaces and the spin-density-wave order emerging in the altermagnetic phase make it an extraordinary platform for designing high-performance spintronic devices and studying many-body effects coupled with the unconventional magnetism.

cond-mat.mtrl-sci↗

arXiv2024

Lattice and magnetic structure in the van der Waals antiferromagnet VBr3

We report a comprehensive investigation of the lattice and magnetic structure in van der Waals antiferromagnet VBr3, characterized by a BiI3-type structure at room temperature. Neutron diffraction experiments were performed on both polycrystalline and single-crystalline VBr3 samples, revealing clear magnetic Bragg peaks emerging below the Néel temperature of TN = 26.5 K. These magnetic Bragg peaks can be indexed by k = (0, 0.5, 1) in hexagonal notation. Our refinement analysis suggests that the antiferromagnetic order in VBr3 manifests as a zigzag structure. Moreover, we observed peak splitting for nuclear Bragg peaks in the HK-plane below the structure transition temperature of Ts = 94 K, indicating the breaking of 3-fold symmetry within the ab-plane.

cond-mat.str-el↗

arXiv2024

Insights into the defect-driven heterogeneous structural evolution of Ni-rich layered cathode in lithium-ion batteries

Recently, considerable efforts have been made on research and improvement for Ni-rich lithium-ion batteries to meet the demand from vehicles and grid-level large-scale energy storage. Development of next-generation high-performance lithium-ion batteries requires a comprehensive understanding on the underlying electrochemical mechanisms associated with its structural evolution. In this work, advanced operando neutron diffraction and four-dimensional scanning transmission electron microscopy techniques are applied to clarify the structural evolution of electrodes in two distinct full cells with identical LiNi0.8Co0.1Mn0.1O2 cathode but different anode counterparts. It is found that both of cathodes in two cells exhibit non-intrinsic two-phase-like behavior at the early charge stage, indicating selective Li+ extraction from cathodes. But the heterogeneous evolution of cathode is less serious with graphite-silicon blended anode than that with graphite anode due to the different delithiation rate. Moreover, it is revealed that the formation of heterogeneous structure is led by the distribution of defects including Li/Ni disordering and microcracks, which should be inhibited by assembling appropriate anode to avoid potential threaten on cell performance. The present work unveils the origin of inhomogeneity in Ni-rich lithium-ion batteries and highlights the significance of kinetics control in electrodes for batteries with higher capacity and longer life.

cond-mat.mtrl-sci↗

arXiv2024

Neutron Scattering Studies on the High-$T_c$ Superconductor La$_3$Ni$_2$O$_{7-δ}$ at Ambient Pressure

After several decades of studies of high-temperature superconductivity, there is no compelling theory for the mechanism yet; however, the spin fluctuations have been widely believed to play a crucial role in forming the superconducting Cooper pairs. The recent discovery of high-temperature superconductivity near 80 K in the bilayer nickelate La$_3$Ni$_2$O$_7$ under pressure provides a new platform to elucidate the origins of high-temperature superconductivity. We perform elastic and inelastic neutron scattering studies on a polycrystalline sample of La$_3$Ni$_2$O$_{7-δ}$ at ambient pressure. No magnetic order can be identified down to 10 K. The absence of long-range magnetic order in neutron diffraction measurements may be ascribed to the smallness of the magnetic moment. However, we observe a weak flat spin-fluctuation signal at $\sim$ 45 meV in the inelastic scattering spectra. The observed spin excitations could be interpreted as a result of strong interlayer and weak intralayer magnetic couplings for stripe-type antiferromagnetic orders. Our results provide crucial information on the spin dynamics and are thus important for understanding the superconductivity in La$_3$Ni$_2$O$_7$.

cond-mat.supr-con↗

arXiv2024

C-type antiferromagnetic structure of topological semimetal CaMnSb$_2$

Determination of the magnetic structure and confirmation of the presence or absence of inversion ($\mathcal{P}$) and time reversal ($\mathcal{T}$) symmetry is imperative for correctly understanding the topological magnetic materials. Here high-quality single crystals of the layered manganese pnictide CaMnSb$_2$ are synthesized using the self-flux method. De Haas-van Alphen oscillations indicate a nontrivial Berry phase of $\sim$ $π$ and a notably small cyclotron effective mass, supporting the Dirac semimetal nature of CaMnSb$_2$. Neutron diffraction measurements identify a C-type antiferromagnetic (AFM) structure below $T\rm_{N}$ = 303(1) K with the Mn moments aligned along the $a$ axis, which is well supported by the density functional theory (DFT) calculations. The corresponding magnetic space group is $Pn'm'a'$, preserving a $\mathcal{P}\times\mathcal{T}$ symmetry. Adopting the experimentally determined magnetic structure, band crossings near the Y point in momentum space and linear dispersions of the Sb $5p_{y,z}$ bands are revealed by the DFT calculations. Furthermore, our study predicts the possible existence of an intrinsic second-order nonlinear Hall effect in CaMnSb$_2$, offering a promising platform to study the impact of topological properties on nonlinear electrical transports in antiferromagnets.

cond-mat.str-el↗

arXiv2024

High-Entropy Enhanced Negative Thermal Expansion Perfomance in Antiperovkites

The negative thermal expansion (NTE) materials, which can act as thermal-expansion compensators to counteract the positive thermal expansion, have great applications merit in precision engineering. However, the exploration of NTE behavior with a wide temperature range has reached its upper ceiling through traditional doping strategies due to composition limitations. The unique sluggish characteristic in phase transition and extended optimization space in recent high entropy systems has great potential to broaden the temperature range in electronic transitions-induced NTE materials. Mn-based anti-perovskites offer an ideal platform for the exploration of high entropy NTE material due to their abundant element selection and controllable NTE performance. In this paper, the high entropy strategy is first introduced to broaden the NTE temperature range by relaxing the abrupt phase transition in Mn-based anti-perovskite nitride. We propose an empirical screening method to synthesize the high-entropy anti-perovskite (HEAP). it is found that magnetic phase separation from anti-ferromagnetic CII to paramagnetic CI surviving in an ultra-wide temperature range of 5K<=T<=350K (Delta_T=345K), revealing a unique sluggish characteristic. Consequently, a remarkable NTE behavior (up to Delta_T=235K, 5K<=T<=240K) with a coefficient of thermal expansion of -4.7x10-6/K, has been obtained in HEAP. It is worth noting that the temperature range is two/three times wider than that of low-entropy systems. The sluggish characteristic has been further experimentally proved to come from disturbed phase transition dynamics due to distortion in atomic spacing and chemical environmental fluctuation observed by the spherical aberration-corrected electron microscope. Our demonstration provides a unique paradigm for broadening the temperature range of NTE materials induced by phase transition through entropy engineering.

cond-mat.mtrl-sci↗

arXiv2023

Observation of a 1/3 Magnetisation Plateau Phase as Evidence for the Kitaev Interaction in a Honeycomb-Lattice Antiferromagnet

Fractional magnetisation plateaus, in which the magnetisation is pinned at a fraction of its saturated value within a range of external magnetic field, are spectacular macroscopic manifestations of the collective quantum behaviours. One prominent example of the plateau phase is found in spin-1/2 triangular-lattice antiferromagnets featuring strong geometrical frustration, and is often interpreted as quantum-fluctuation-stabilised state in magnetic field via the "order-by-disorder" mechanism. Here, we observe an unprecedented 1/3 magnetisation plateau between 5.2 and 7.4 T at 2 K in a spin-1 antiferromagnet Na$_3$Ni$_2$BiO$_6$ with a honeycomb lattice, where conventionally no geometrical frustration is anticipated. By carrying out elastic neutron scattering measurements, we propose the spin structure of the plateau phase to be an unusual partial spin-flop ferrimagnetic order, transitioning from the zigzag antiferromagnetic order in zero field. Our theoretical calculations show that the plateau phase is stabilised by the bond-anisotropic Kitaev interaction. These results provide a new paradigm for the exploration of rich quantum phases in frustrated magnets and exotic Kitaev physics in high-spin systems.

cond-mat.str-el↗

arXiv2023

Phase transitions associated with magnetic-field induced topological orbital momenta in a non-collinear antiferromagnet

Resistivity measurements are widely exploited to uncover electronic excitations and phase transitions in metallic solids. While single crystals are preferably studied to explore crystalline anisotropies, these usually cancel out in polycrystalline materials. Here we show that in polycrystalline Mn3Zn0.5Ge0.5N with non-collinear antiferromagnetic order, changes in the diagonal and, rather unexpected, off-diagonal components of the resistivity tensor occur at low temperatures indicating subtle transitions between magnetic phases of different symmetry. This is supported by neutron scattering and explained within a phenomenological model which suggests that the phase transitions in magnetic field are associated with field induced topological orbital momenta. The fact that we observe transitions between spin phases in a polycrystal, where effects of crystalline anisotropy are cancelled suggests that they are only controlled by exchange interactions. The observation of an off-diagonal resistivity extends the possibilities for realising antiferromagnetic spintronics with polycrystalline materials.

cond-mat.mtrl-sci↗

arXiv2023

Robust 3.7 V-Na$_{2/3}$[Cu$_{1/3}$Mn$_{2/3}$]O$_2$ Cathode for Na-ion Batteries

Na-ion batteries (NIBs), which are recognized as a next-generation alternative technology for energy storage, still suffer from commercialization constraints due to the lack of low-cost, high-performance cathode materials. Since our first discovery of Cu$^{3+}$/Cu$^{2+}$ electrochemistry in 2014, numerous Cu-substituted/doped materials have been designed for NIBs. However for almost ten years, the potential of Cu$^{3+}$/Cu$^{2+}$ electrochemistry has been grossly underappreciated and normally regarded as a semielectrochemically active redox. Here, we re-synthesized P2-Na$_{2/3}$[Cu$_{1/3}$Mn$_{2/3}$]O$_2$ and reinterpreted it as a high-voltage, cost-efficient, air-stable, long-life, and high-rate cathode material for NIBs, which demonstrates a high operating voltage of 3.7 V and a completely active Cu$^{3+}$/Cu$^{2+}$ redox reaction. The 2.3 Ah cylindrical cells exhibit excellent cycling (93.1% capacity after 2000 cycles), high rate (97.2% capacity at 10C rate), good low-temperature performance (86.6% capacity at -30$^\circ$C), and high safety, based on which, a 56 V-11.5 Ah battery pack for E-bikes is successfully constructed, exhibiting stable cycling (96.5% capacity at the 800th cycle) and a long driving distance (36 km, tester weight 65 kg). This work offers a commercially feasible cathode material for low-cost, high-voltage NIBs, paving the way for advanced NIBs in power and stationary energy storage applications.

cond-mat.mtrl-sci↗