arXiv subjects

Weiwei Xie

Publications and source records attributed to Weiwei Xie.

At least 55 records · Page 3Linked to original sources

arXiv2024

Giant Uniaxial Magnetocrystalline Anisotropy in SmCrGe$_3$

Magnetic anisotropy is a crucial characteristic for enhancing spintronic device performance. The synthesis of SmCrGe$_3$ single crystals through a high-temperature solution method has led to the determination of uniaxial magnetocrystalline anisotropy. Phase verification was achieved using scanning transmission electron microscopy (STEM), powder, and single-crystal X-ray diffraction techniques. Electrical transport and specific heat measurements indicate a Curie temperature ($T_C$) of approximately 160 K, while magnetization measurements were utilized to determine the anisotropy fields and constants. Curie-Weiss fitting applied to magnetization data suggests the contribution of both Sm and Cr in the paramagnetic phase. Additionally, density functional theory (DFT) calculations explored the electronic structures and magnetic properties of SmCrGe$_3$, revealing a significant easy-axis single-ion Sm magnetocrystalline anisotropy of 16 meV/f.u.. Based on the magnetization measurements, easy-axis magnetocrystalline anisotropy at 20 K is 13 meV/f.u..

cond-mat.mtrl-sci↗

arXiv2024

Pseudosymmetry in Tetragonal Perovskite SrIrO$_3$ Synthesized under High Pressure

In this study, we report a tetragonal perovskite structure of SrIrO$_3$ (P4/mmm, a = 3.9362(9) Å, c = 7.880(3) Å) synthesized at 6 GPa and 1400 $°$C, employing the ambient pressure monoclinic SrIrO$_3$ with distorted 6H structure as a precursor. The crystal structure of tetragonal SrIrO3 was evaluated on the basis of single crystal and powder X-ray diffraction. A cubic indexing was observed attributed to overlooked superlattice reflections. Weak fractional peaks in the H and K dimensions suggest possible structure modulation by oxygen defects. Magnetization study reveals weak paramagnetic behavior down to 2 K, indicative of the interplay between spin-orbit coupling, electron correlations, and crystal electric field. Additionally, measurements of electrical resistivity display metallic behavior with an upturn at about 54 K, ascribed to weak electron localization and possible structural defects.

cond-mat.mtrl-sci↗

arXiv2024

Structural Constraint Integration in Generative Model for Discovery of Quantum Material Candidates

Billions of organic molecules are known, but only a tiny fraction of the functional inorganic materials have been discovered, a particularly relevant problem to the community searching for new quantum materials. Recent advancements in machine-learning-based generative models, particularly diffusion models, show great promise for generating new, stable materials. However, integrating geometric patterns into materials generation remains a challenge. Here, we introduce Structural Constraint Integration in the GENerative model (SCIGEN). Our approach can modify any trained generative diffusion model by strategic masking of the denoised structure with a diffused constrained structure prior to each diffusion step to steer the generation toward constrained outputs. Furthermore, we mathematically prove that SCIGEN effectively performs conditional sampling from the original distribution, which is crucial for generating stable constrained materials. We generate eight million compounds using Archimedean lattices as prototype constraints, with over 10% surviving a multi-staged stability pre-screening. High-throughput density functional theory (DFT) on 26,000 survived compounds shows that over 50% passed structural optimization at the DFT level. Since the properties of quantum materials are closely related to geometric patterns, our results indicate that SCIGEN provides a general framework for generating quantum materials candidates.

cond-mat.mtrl-sci↗

arXiv2024

Pressure Tuning the Mixture of Eu$^{2+}$ and Eu$^{3+}$ in Eu$_4$Bi$_6$Se$_{13}$

The investigation of crystallographic, electronic, and magnetic characteristics, especially the mixed valences of Eu$^{2+}$ and Eu$^{3+}$ under pressure of a novel europium-based bismuth selenide compound, Eu$_4$Bi$_6$Se$_{13}$, presented. This new compound adopts a monoclinic crystal structure classified under the P$2_1$/m space group (#11). It exhibits distinctive structural features, including substantial Eu-Se coordination numbers, Bi-Se ladders, and linear chains of Eu atoms that propagate along the b-axis. Electronic resistivity assessments indicate that Eu$_{4}$Bi$_{6}$Se$_{13}$ exhibits weak metallic behaviors. Magnetic characterization reveals uniaxial magnetic anisotropy, with a notable spin transition at approximately 1.2 T when the magnetic field is oriented along the b-axis. This behavior, coupled with the specific Eu-Eu interatomic distances and the magnetic saturation observed at low fields, supports the identification of metamagnetic properties attributable to the flipping of europium spins. The Curie-Weiss analysis of the magnetic susceptibility measured both perpendicular and parallel to the b-axis and high-pressure partial fluorescence yield (PFY) results detected by X-ray absorption spectroscopy (XAS) reveal the tendency of the material to enter a mixed valent state where the trivalent state becomes more prominent with the pressure increase or temperature decrease.

cond-mat.mtrl-sci↗

arXiv2024

Effect of Halogen Substituents on Charge Transport Properties of n-type Organic Semiconductors: A Theoretical Study

Organic semiconductors (OSCs) have received much attention as promising materials for electronic devices. In this study, we investigate the impact of halogen groups on the charge transport properties of n-type OSC-6,13 bis ((triisopropylsilyl) ethynyl)-5,7,12,14-tetraazapentacene (TIPS-TAP). The computed mobilities for TAPs substituted with F and Cl exhibit excellent agreement with the experimental values, while the simulation overestimates the electron mobility for TAP. Interestingly, the mobility of TIPS-TAP-4F is significantly lower than that of TIPS-TAP-4Cl/Br, despite their similar packing structures. This discrepancy can be attributed to the strong electron-withdrawing effect of fluoride, leading to reduced electron transfer integrals and increased reorganization energy. While molecular packing is widely accepted as a dominant factor in charge transport in OSCs, our study highlights the essential role of electronic effects in OSC charge transport. This study provides new insights into the understanding of the charge transport mechanism in OSCs.

cond-mat.mtrl-sci↗

arXiv2024

Temperature-dependent Structural Evolution of Ruddlesden-Popper Bilayer Nickelate La$_3$Ni$_2$O$_7$

A recent $J. Am. Chem. Soc.$ Article (DOI: 10.1021/jacs.3c13094) details a pressure-temperature ($P$-$T$) phase diagram for the Ruddlesden-Popper bilayer nickelate La$_3$Ni$_2$O$_7$ (LNO-2222) using synchrotron X-ray diffraction. This study identifies a phase transition from $Amam$ (#63) to $Fmmm$ (#69) within the temperature range of 104 K to 120 K under initial pressure and attributes the $I\rm{4/}$$mmm$ (#139) space group to the structure responsible for the superconductivity of LNO-2222. Herein, we examine the temperature-dependent structural evolution of LNO-2222 single crystals at ambient pressure. Contrary to symmetry increase and the established $Amam$-$Fmmm$ phase boundary, we observe an enhancement in the $Cmcm$ reflections as temperature decreases. This work not only establishes a benchmark method for single crystal structure studies of LNO-2222 using laboratory X-rays, but also enhances the understanding of the complex crystallographic behavior of this system, contributing insights to further experimental and theoretical explorations.

cond-mat.supr-con↗

arXiv2024

Pressure induced metallization and loss of surface magnetism in FeSi

Single crystalline FeSi samples with a conducting surface state (CSS) were studied under high pressure ($\textit{P}$) and magnetic field ($\textit{B}$) by means of electrical resistance ($\textit{R}$) measurements to explore how the bulk semiconducting state and the surface state are tuned by the application of pressure. We found that the energy gap ($Δ$) associated with the semiconducting bulk phase begins to close abruptly at a critical pressure ($P_{cr}$) of ~10 GPa and the bulk material becomes metallic with no obvious sign of any emergent phases or non-Fermi liquid behavior in $\textit{R}$($\textit{T}$) in the neighborhood of $P_{cr}$ above 3 K. Moreover, the metallic phase appears to remain at near-ambient pressure upon release of the pressure. Interestingly, the hysteresis in the $\textit{R}$($\textit{T}$) curve associated with the magnetically ordered CSS decreases with pressure and vanishes at $P_{cr}$, while the slope of the $\textit{R}$($\textit{B}$) curve, d$\textit{R}$/d$\textit{B}$, which has a negative value for $\textit{P}$ < $P_{cr}$, decreases in magnitude with $\textit{P}$ and changes sign at $P_{cr}$. Thus, the CSS and the corresponding two-dimensional magnetic order collapse at $P_{cr}$ where the energy gap $Δ$ of the bulk material starts to close abruptly, revealing the connection between the CSS and the semiconducting bulk state in FeSi.

cond-mat.str-el↗

arXiv2024

Layered Kagome Compound Na$_2$Ni$_3$S$_4$ with Topological Flat Band

We report structural and electronic properties of Na$_2$Ni$_3$S$_4$, a quasi-two-dimensional compound composed of alternating layers of [Ni$_3$S$_4$]$^{2-}$ and Na$^{+}$. The compound features a remarkable Ni-based kagome lattice with a square planar configuration of four surrounding S atoms for each Ni atom. Magnetization and electrical measurements reveal a weak paramagnetic insulator with a gap of about 0.5 eV. Our band structure calculation highlights a set of topological flat bands of the kagome lattice derived from the rotated d$_{xz}$-orbital with $C_\mathrm{3}$ + $T$ symmetry in the presence of crystal-field splitting.

cond-mat.str-el↗

arXiv2024

Long-Range Structural Order in a Hidden Phase of Ruddlesden-Popper Bilayer Nickelate La$_3$Ni$_2$O$_7$

The recent discovery of superconductivity in Ruddlesden-Popper bilayer nickelate, specifically La$_3$Ni$_2$O$_7$, has generated significant interest in the exploration of high-temperature superconductivity within this material family. In this study, we present the crystallographic and electrical resistivity properties of two distinct Ruddlesden-Popper nickelates: the bilayer La$_3$Ni$_2$O$_7$ (referred to as 2222-phase) and a previously uncharacterized phase, La$_3$Ni$_2$O$_7$ (1313-phase). The 2222-phase is characterized by a pseudo $F$-centered orthorhombic lattice, featuring bilayer perovskite [LaNiO$_3$] layers interspaced by rock salt [LaO] layers, forming a repeated ...2222... sequence. Intriguingly, the 1313-phase, which displays semiconducting properties, crystallizes in the $Cmmm$ space group and exhibits a pronounced predilection for a $C$-centered orthorhombic lattice. Within this structure, the perovskite [LaNiO$_3$] layers exhibit a distinctive long-range ordered arrangement, alternating between single- and trilayer configurations, resulting in a ...1313... sequence. This report contributes to novel insights into the crystallography and the structure-property relationship of Ruddlesden-Popper nickelates, paving the way for further investigations into their unique physical properties.

cond-mat.supr-con↗

arXiv2024

Anomalous Neutron Nuclear-Magnetic Interference Spectroscopy

The electron-phonon interaction plays a critical role in materials electrical, thermal, optical, and superconducting properties. However, measuring the phonon mode-resolved electron-phonon interaction has been challenging. Here we propose neutron-scattering-based Anomalous Neutron nUclear-Magnetic Interference Spectroscopy (ANUBIS), where the co-existence of neutron nuclear scattering and magnetic scattering leads to anomalous dynamical structure factor under the presence of the electron-phonon interaction. Such anomalous structure factor is linear in electron-phonon coupling constant at the phonon wavevector, and is directly proportional to the momentum and energy-resolved dielectric function. The experimental configuration can be achieved using existing polarized inelastic neutron scattering setup, and an order-of-magnitude estimate shows the viability to observe the anomalous scattering signal is around $10^{-4}$ to $10^{-3}$ relative to phonon scattering, which is achievable at emerging neutron facilities. Our proposal offers an alternative neutron-based metrology to probe the crucial electronic properties.

cond-mat.mtrl-sci↗

arXiv2024

Insulator to Metal Transition, Spin-Phonon Coupling, and Potential Magnetic Transition Observed in Quantum Spin Liquid Candidate LiYbSe$_2$ under High Pressure

Metallization of quantum spin liquid (QSL) materials has long been considered as a potential route to achieve unconventional superconductivity. Here we report our endeavor in this direction by pressurizing a three-dimensional QSL candidate, LiYbSe$_2$, with a previously unreported pyrochlore structure. High-pressure X-ray diffraction and Raman studies up to 50 GPa reveal no appreciable changes of structural symmetry or distortion in this pressure range. This compound is so insulating that its resistance decreases below 10$^5$ $Ω$ only at pressures above 25 GPa in the corresponding temperature range accompanying the gradual reduction of band gap upon compression. Interestingly, an insulator-to-metal transition takes place in LiYbSe$_2$ at about 68 GPa and the metallic behavior remains up to 123.5 GPa, the highest pressure reached in the present study. A possible sign of magnetic or other phase transition was observed in LiYbSe$_2$. The insulator-to-metal transition in LiYbSe$_2$ under high pressure makes it an ideal system to study the pressure effects on QSL candidates of spin-1/2 Yb$^{3+}$ system in different lattice patterns.

cond-mat.str-el↗

arXiv2023

Pressure-dependent "Insulator-Metal-Insulator" Behavior in Sr-doped La$_3$Ni$_2$O$_7$

Recently, superconductivity at high temperatures has been observed in bulk La$_3$Ni$_2$O$_{7-δ}$ under high pressure. However, the attainment of high-purity La$_3$Ni$_2$O$_{7-δ}$ single crystals, exhibiting controlled and homogeneous stoichiometry through the post-annealing process in an oxygen-rich floating zone furnace, remains a formidable challenge. Here, we report the crystal structure and physical properties of single crystals of Sr-doped La$_3$Ni$_2$O$_7$ synthesized at high pressure (20 GPa) and high temperature (1400 °C). Through single crystal X-ray diffraction, we showed that high-pressure-synthesized paramagnetic Sr-doped La$_3$Ni$_2$O$_7$ crystallizes in an orthorhombic structure with Ni-O-Ni bond angles of 173.4(2)°out-of-plane and 175.0(2)°and 176.7(2)°in plane. The substitution of Sr alters in band filling and the ratio of Ni$^{2+}$/Ni$^{3+}$ in Sr-doped La$_3$Ni$_2$O$_7$, aligning them with those of "La$_3$Ni$_2$O$_{7.05}$", thereby leading to significant modifications in properties under high pressure relative to the unsubstituted parent phase. At ambient pressure, Sr-doped La$_3$Ni$_2$O$_7$ exhibits insulating properties, and the conductivity increases as pressure goes up to 10 GPa. However, upon further increasing pressure beyond 10.7 GPa, Sr-doped La$_3$Ni$_2$O$_7$ transits back from a metal-like behavior to an insulator. The insulator-metal-insulator trend under high pressure dramatically differs from the behavior of the parent compound La$_3$Ni$_2$O$_{7-δ}$, despite their similar behavior in the low-pressure regime. These experimental results underscore the considerable challenge in achieving superconductivity in nickelates.

cond-mat.supr-con↗

arXiv2023

Impersonating a Superconductor: High-Pressure BaCoO$_3$, an Insulating Ferromagnet

We report the high-pressure synthesis (6 GPa, 1200 $^{\circ}$C) and ambient pressure characterization of hexagonal HP-BaCoO$_3$. The material (with the 2H crystal structure) has a short intrachain Co-Co distance of about 2.07 $\text{Å}$. Our magnetization investigation revealed robust diamagnetic behavior below approximately 130 K when exposed to weak applied magnetic fields (10 Oe) and a distinct half-levitation phenomenon below that temperature, such as is often observed for superconductors. Its field-dependent magnetization profile, however, unveils the characteristics of ferromagnetism, marked by a substantial magnetic retentivity of 0.22(1) $μ_B$/Co at a temperature of 2 K. Electrical resistivity measurements indicate that HP-BaCoO$_3$ is a ferromagnetic insulator, not a superconductor.

cond-mat.mtrl-sci↗

arXiv2023

Incommensurate Spiral Spin Order in CaMn$_2$Bi$_2$ observed via High Pressure Neutron Diffraction

High pressure neutron diffraction is employed to investigate the magnetic behavior of CaMn$_2$Bi$_2$ in extreme conditions. In contrast to antiferromagnetic ordering on Mn atoms reported at ambient pressure, our results reveal that at high pressure, incommensurate spiral spin order emerges due to the interplay between magnetism on the Mn atoms and strong spin-orbit coupling on the Bi atoms: sinusoidal spin order is observed at pressures as high as 7.4 GPa. Competing antiferromagnetic order is observed at different temperatures in the partially frustrated lattice. This research provides a unique toolbox for conducting experimental magnetic and spin dynamics studies on magnetic quantum materials via high pressure neutron diffraction.

cond-mat.mtrl-sci↗

arXiv2023

Is La3Ni2O6.5 a Bulk Superconducting Nickelate?

Superconducting states onsetting at moderately high temperatures have been observed in epitaxially-stabilized RENiO2-based thin films. However, recently it has also been reported that superconductivity at high temperatures is observed in bulk La3Ni2O7-δ at high pressure, opening further possibilities for study. Here we report the reduction profile of La3Ni2O7 in a stream of 5% H2/Ar gas and the isolation of the metastable intermediate phase La3Ni2O6.45, which is based on Ni2+. Although this reduced phase does not superconduct at ambient or high pressures, it offers insights into the Ni-327 system and encourages the future study of nickelates as a function of oxygen content.

cond-mat.mtrl-sci↗

arXiv2023

The Pressure-Stabilized Polymorph of Indium Triiodide

A layered rhombohedral polymorph of indium (III) triiodide is synthesized at high pressure and temperature. The unit cell symmetry and approximate dimensions are determined by single crystal X-ray diffraction. Its R-3 crystal structure, with a = 7.217 Å, and c = 20.476 Å, is refined by the Rietveld method on powder X-ray diffraction data. The crystal structure is based on InI6 octahedra sharing edges to form honeycomb lattice layers, though with considerable stacking variations. Different from ambient pressure InI3, which has a monoclinic molecular structure and a light-yellow color, high pressure InI3 is layered and has an orange color. The band gaps of both the monoclinic and rhombohedral variants of InI3 are estimated from diffuse reflectance measurements.

cond-mat.mtrl-sci↗

arXiv2023

Hidden Hydroxides in KOH-Grown BaNiO3 Crystals: A Potential Link to Their Catalytic Behavior

The hexagonal perovskite BaNiO3, prepared via non-ceramic approaches, is known to act as a good catalyst for the oxygen-evolution reaction (OER) in alkaline media. Here we report our observation that BaNiO3 synthesized via KOH flux growth and high O2 pressure ceramic synthesis have different magnetic properties. We show that this is because the KOH flux-grown crystals made in open-air are actually a hydroxide-containing form of BaNiO3 that can be dried upon annealing in O2 flow. This work not only unveils a previously unknown aspect of the BaNiO3 OER catalyst and offers some insights into the underlying mechanism, but also suggests that hydroxide ions may be present in other hexagonal perovskite oxides prepared in wet conditions.

cond-mat.mtrl-sci↗

arXiv2023

Stuffed Rare Earth Garnets

We report the synthesis and magnetic characterization of stuffed rare earth gallium garnets, RE3+xGa5-xO12 (RE=Lu, Yb, Er, Dy, Gd), for x up to 0.5. The excess rare earth ions partly fill the octahedral sites normally fully occupied by Ga3+, forming disordered pairs of corner-shared face-sharing magnetic tetrahedra. The Curie-Weiss constants and observed effective moments per rare earth are smaller than are seen for the unstuffed gallium garnets. No significant change in the field-dependent magnetization is observed but missing entropy is seen when integrating the low-temperature heat capacity to 0.5 K.

cond-mat.mtrl-sci↗