Showing 1–50 of 94 results for author: Jiang, P

Data-Efficient Adaptation of DPA-4 Force Fields to DFT+U Energetics: A Case Study in NiO

Authors: Fengyu Xie, Peiheng Jiang, Zhicheng Zhong

Abstract: Foundation machine-learned force fields (MLFFs) are often pretrained on broad materials datasets whose electronic-structure conventions may not reproduce the phase energetics required for a specific correlated material. Using NiO as a case study, we examine whether incorrect source-level phase energetics can be corrected efficiently through target-level fine-tuning. Along a common structural inter… ▽ More Foundation machine-learned force fields (MLFFs) are often pretrained on broad materials datasets whose electronic-structure conventions may not reproduce the phase energetics required for a specific correlated material. Using NiO as a case study, we examine whether incorrect source-level phase energetics can be corrected efficiently through target-level fine-tuning. Along a common structural interpolation, non-spin-polarized PBE and ferromagnetic PBE+U predict opposite energetic orderings of the octahedral Oct and square-planar Sqr phases. Pretrained DPA-4 models adapt rapidly to the NiO PBE+U surface, reaching energy and force root-mean-square errors (RMSEs) of approximately 0.5 meV/atom and 30 meV/Å, respectively, with approximately 170 PBE+U labels. Crucially, models previously fine-tuned to the opposing no-U surface recover the qualitative PBE+U phase ordering with nearly the same target-data efficiency as models fine-tuned directly from their respective pretrained initializations. Our results show that incorrect source-level phase energetics can be reversed through target-level fine-tuning, and suggest a practical multi-fidelity strategy in which pretraining prioritizes broad, consistent, and affordable data, while compact target-level datasets impose energetics through application-specific fine-tuning. △ Less

Submitted 12 August, 2026; originally announced August 2026.

Reversible fully spin polarization in strain-engineered two-dimensional fully compensated magnets

Authors: Xiuli Zhang, Peng Jiang, Yurui Ma, Xiaodong Zhou, Linlin Liu, Hong-Mei Huang, San-Dong Guo, Tengfei Cao, Yan-Ling Li

Abstract: Achieving controllable spin polarization and its reversal in symmetry-compensated magnets. Here we demonstrate, using symmetry analysis and a minimal tight-binding model, that uniaxial strain removes these constraints by inducing inequivalence between magnetic sublattices in two-dimensional (2D) system, driving an altermagnetic (AM) state into a fully compensated ferrimagnetic (fFIM) state and ena… ▽ More Achieving controllable spin polarization and its reversal in symmetry-compensated magnets. Here we demonstrate, using symmetry analysis and a minimal tight-binding model, that uniaxial strain removes these constraints by inducing inequivalence between magnetic sublattices in two-dimensional (2D) system, driving an altermagnetic (AM) state into a fully compensated ferrimagnetic (fFIM) state and enabling fully spin polarization. Furthermore, strain along orthogonal directions gives rise to two energetically degenerate fFIM states with opposite spin polarization, enabling reversible spin switching. More importantly, the two symmetry-related fFIM states can be regarded as distinct ferroelastic variants, suggesting that this model or mechanism can be extended to ferroelastic fFIM systems. The generality of this mechanism is confirmed by combining spin-group analysis, first-principles calculations, and Boltzmann transport theory in representative candidates, including AM Mn$_2$SeO and ferroelastic fFIM V$_2$SO. Our results reveal a universal symmetry-driven framework for strain-controlled and -reversible fully spin-polarized transport and identify strain-engineered AM and ferroelastic fFIM systems as a promising platform for volatile and nonvolatile spintronic applications. △ Less

Submitted 3 May, 2026; originally announced May 2026.

Comments: 6 pages, 4 figures

A Variable-Spot-Size and Multi-Frequency Square-Pulsed Source (SPS) Approach for Comprehensive Characterization of Anisotropic Thermal Transport Properties in Multilayered Thin Films

Authors: Kexin Zhang, Tao Chen, Jinlong Ma, Puqing Jiang

Abstract: Multilayered thin-film structures are frequently encountered in industrial applications, where accurate thermal property characterization is essential for performance optimization. These films, typically ranging from nanometers to micrometers in thickness, often exhibit anisotropic thermal conductivity and non-bulk heat capacity, which are challenging to measure. In this study, we introduce a vari… ▽ More Multilayered thin-film structures are frequently encountered in industrial applications, where accurate thermal property characterization is essential for performance optimization. These films, typically ranging from nanometers to micrometers in thickness, often exhibit anisotropic thermal conductivity and non-bulk heat capacity, which are challenging to measure. In this study, we introduce a variable-spot-size and multi-frequency square-pulsed source (SPS) method for the simultaneous determination of anisotropic thermal conductivities, heat capacities, and interfacial thermal conductance in multilayered systems. By leveraging a broad modulation frequency range (1 Hz to 10 MHz) and tunable laser spot sizes, the SPS method enhances sensitivity to different thermal parameters across layers. We validate this approach on a silicon-on-insulator (SOI) sample comprising a 1.59 um Si layer, 1.03 um SiO2 layer, and a silicon substrate with a 122 nm aluminum (Al) transducer. The SPS method successfully extracts seven key thermal parameters, including the in-plane and cross-plane thermal conductivities and heat capacity of the Si film, the thermal conductivity and heat capacity of the SiO2 layer, the thermal conductivity of the substrate, and the interfacial thermal conductance between Al and Si. Temperature-dependent measurements from 80 to 500 K showed excellent agreement with literature values and first-principles predictions, confirming the method's accuracy and reliability. These results demonstrate the SPS method as a powerful tool for comprehensive thermal characterization of complex multilayered structures, with implications for both fundamental research and practical applications. △ Less

Submitted 15 April, 2026; originally announced April 2026.

Universal thermometry of solid-liquid interfacial thermal conductance

Authors: Tao Chen, Puqing Jiang

Abstract: Solid-liquid interfacial thermal conductance (ITC) critically influences heat transport in microfluidic, electronic, and energy systems, yet most optical thermometry techniques are limited to specific metal-liquid interfaces. In this work, we introduce a universal broadband square-pulsed thermometry method that enables simultaneous quantification of ITC across a wide range of arbitrary solid-liqui… ▽ More Solid-liquid interfacial thermal conductance (ITC) critically influences heat transport in microfluidic, electronic, and energy systems, yet most optical thermometry techniques are limited to specific metal-liquid interfaces. In this work, we introduce a universal broadband square-pulsed thermometry method that enables simultaneous quantification of ITC across a wide range of arbitrary solid-liquid interfaces, while also providing accurate measurements of nanoscale liquid-film thickness. To validate the method, we applied it to Al-water interfaces, yielding ITC values in the range of 50-55 MW m^(-2) K^(-1), consistent with prior studies. The technique also reveals markedly lower ITCs for glass-water (9.9 MW m^(-2) K^(-1)) and Si-water (5.7 MW m^(-2) K^(-1)), and further measurements on Al-silicone oil (~10 MW m^(-2) K^(-1)) and PMMA-silicone oil (~0.4 MW m^(-2) K^(-1)) extend the validation to highly viscous nonpolar liquids and polymer-liquid interfaces. These results highlight the capability of the method to capture thermal transport differences across diverse solid-liquid combinations. Further comparisons with acoustic/diffuse mismatch models and molecular dynamics simulations, together with theoretical analysis, highlight the influence of vibrational mismatch, wettability, and surface condition on interfacial thermal transport. This broadly applicable technique enables rapid, quantitative characterization of solid-liquid interfacial thermal transport, with broad implications for interfacial heat transfer science and technology. △ Less

Submitted 15 April, 2026; originally announced April 2026.

Thermal Characterization of Buried Interfaces in Multilayer Heterostructures via TDTR with Periodic Waveform Analysis

Authors: Mingzhen Zhang, Puqing Jiang, Ronggui Yang

Abstract: Accurate evaluation of buried thermal interfaces is vital for understanding and optimizing heat dissipation in wide- and ultra-wide-bandgap (WBG/UWBG) semiconductor devices. Conventional time-domain thermoreflectance (TDTR) typically probes only near-surface transport due to its restricted modulation frequency range. Here, we employ a frequency-tunable periodic waveform analysis TDTR (PWA-TDTR) te… ▽ More Accurate evaluation of buried thermal interfaces is vital for understanding and optimizing heat dissipation in wide- and ultra-wide-bandgap (WBG/UWBG) semiconductor devices. Conventional time-domain thermoreflectance (TDTR) typically probes only near-surface transport due to its restricted modulation frequency range. Here, we employ a frequency-tunable periodic waveform analysis TDTR (PWA-TDTR) technique to perform depth-resolved thermal measurements on three representative systems: epitaxial ε-Ga2O3/SiC, GaN/Si, and mechanically bonded GaN/diamond. By combining broadband multi-frequency probing with sensitivity-guided joint fitting, we quantitively determine interfacial thermal conductance, layer-specific thermal conductivity, and volumetric heat capacity, without requiring destructive sample preparation. The results reveal that the buried Ga2O3/SiC interface exhibits weak phonon transmission due to acoustic mismatch; the transition layers in GaN/Si act as phonon-impedance gradients that redistribute heat flux; and the GaN/diamond boundary remains the dominant thermal bottleneck despite diamond's ultrahigh bulk conductivity. These findings demonstrate that the modulation frequency in PWA-TDTR functions as a tunable probe of depth-dependent phonon transport, directly linking frequency-domain thermal response to interfacial energy transmission. Overall, this work positions PWA-TDTR as a versatile platform for investigating buried nonmetal-nonmetal interfaces in next-generation high-power and optoelectronic materials. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Anisotropic Thermal Characterization of Suspended and Spin-Coated Polyimide Films Using a Square-Pulsed Source Method

Authors: Bingjiang Zhang, Dihui Wang, Tao Chen, Heng Ban, Puqing Jiang

Abstract: Polyimide (PI) thin films are widely used in advanced technologies, yet accurate characterization of their thermal properties remains challenging, as evidenced by significant inconsistencies in reported data and an incomplete understanding of heat transfer mechanisms. In this study, we employ an optical Square-Pulsed Source (SPS) technique to simultaneously measure the in-plane and cross-plane the… ▽ More Polyimide (PI) thin films are widely used in advanced technologies, yet accurate characterization of their thermal properties remains challenging, as evidenced by significant inconsistencies in reported data and an incomplete understanding of heat transfer mechanisms. In this study, we employ an optical Square-Pulsed Source (SPS) technique to simultaneously measure the in-plane and cross-plane thermal conductivities, as well as the volumetric heat capacity, of PI thin films. SPS is a pump-probe method that utilizes a square-wave-modulated pump laser to induce periodic heating and a probe laser to detect the thermoreflectance response. Thermal properties are extracted by analyzing amplitude signals across multiple modulation frequencies and laser spot sizes. Measurements were conducted on both suspended commercial PI films and spin-coated PI films on fused silica substrates. The results show that spin-coated films exhibit higher cross-plane thermal conductivity and lower anisotropy compared to suspended films, which we attribute to differences in molecular orientation and substrate interactions. These findings provide new physical insights into anisotropic heat transport in polymer thin films and demonstrate the SPS technique as a robust tool for probing microscale thermal phenomena in soft materials. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Depth-Resolved Thermal Conductivity of HFCVD Diamond Films via Square-Pulsed Thermometry

Authors: Kexin Zhang, Xiaosong Han, Ershuai Yin, Xin Qian, Junjun Wei, Puqing Jiang

Abstract: The integration of high-thermal-conductivity diamond films onto silicon carbide (SiC) substrates offers a promising pathway for thermal management in high-power electronic devices. Here, we investigate the depth-dependent thermal conductivity of a ~5 μm-thick diamond film grown on SiC by hot-filament chemical vapor deposition (HFCVD) using square-pulsed source (SPS) thermometry. Electron backscatt… ▽ More The integration of high-thermal-conductivity diamond films onto silicon carbide (SiC) substrates offers a promising pathway for thermal management in high-power electronic devices. Here, we investigate the depth-dependent thermal conductivity of a ~5 μm-thick diamond film grown on SiC by hot-filament chemical vapor deposition (HFCVD) using square-pulsed source (SPS) thermometry. Electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) reveal pronounced grain coarsening from the nucleation interface to the film surface. By combining frequency-dependent thermal penetration with a depth-resolved thermal transport model, we quantitatively reconstruct the thermal conductivity profile. The thermal conductivity increases sharply from ~60 W m^(-1) K^(-1) near the nucleation region to ~200 W m^(-1) K^(-1) at the surface, directly reflecting the underlying microstructural evolution. These results provide a physically grounded understanding of graded heat transport in HFCVD diamond and offer practical guidance for engineering diamond-based thermal management layers for next-generation power devices. △ Less

Submitted 14 April, 2026; originally announced April 2026.

Strain-released epitaxy of GaN enabled by compliant single-crystalline metal foils

Authors: Yaqing Ma, Junwei Cao, Huaze Zhu, Yijian Song, Huicong Chen, Menglin He, Jun Yang, Ping Jiang, Tong Jiang, Han Chen, Xiang Xu, Yuqiao Zheng, Hao Wang, Muhong Wu, Yu Zou, Xiaochuan Chen, Tongbo Wei, Kaihui Liu, Wei Kong

Abstract: Heteroepitaxy conventionally relies on rigid crystalline substrates, implicitly assuming that lattice and thermal mismatch must be accommodated within the epitaxial layer, leading to residual strain and defects that worsen with increasing substrate size. Here we demonstrate a substrate-mediated strain-partitioning regime in which lattice and thermal mismatch are preferentially partitioned into the… ▽ More Heteroepitaxy conventionally relies on rigid crystalline substrates, implicitly assuming that lattice and thermal mismatch must be accommodated within the epitaxial layer, leading to residual strain and defects that worsen with increasing substrate size. Here we demonstrate a substrate-mediated strain-partitioning regime in which lattice and thermal mismatch are preferentially partitioned into the substrate rather than stored in the epitaxial layer. We report the epitaxial growth of single-crystalline GaN on mechanically compliant yet crystallographically ordered single-crystalline copper foils. Atomic-resolution microscopy, geometric phase analysis and density functional theory reveal that mismatch-induced stress is primarily screened by elastic deformation of the Cu lattice, accompanied by localized interfacial slip confined to a few atomic layers, leaving the AlN and GaN epilayers nearly strain-free despite large nominal mismatch. Leveraging this strain-released epitaxial platform, we further demonstrate dense GaN micro-light-emitting diode arrays that benefit from efficient vertical electrical conduction and thermal dissipation enabled by the metallic substrate. By establishing compliant single-crystal metal foils as a new substrate class, this work identifies mechanical contrast as an underexplored governing parameter in heteroepitaxial design, with implications extending beyond GaN. △ Less

Submitted 27 March, 2026; originally announced March 2026.

Simultaneous measurement of pressure-dependent bulk and interfacial thermal properties in thermal interface materials using square-pulsed source thermoreflectance

Authors: Tao Chen, Bingjia Xiao, Xin Qian, Puqing Jiang

Abstract: Thermal interface materials (TIMs) critically regulate heat dissipation from electronic chips to heat spreaders, yet their thermal conductivity (k), volumetric heat capacity (C), and interfacial thermal resistance (ITR) evolve with mechanical pressure and cannot be determined simultaneously using existing steady-state or transient techniques. As a result, the coupled roles of bulk compaction and i… ▽ More Thermal interface materials (TIMs) critically regulate heat dissipation from electronic chips to heat spreaders, yet their thermal conductivity (k), volumetric heat capacity (C), and interfacial thermal resistance (ITR) evolve with mechanical pressure and cannot be determined simultaneously using existing steady-state or transient techniques. As a result, the coupled roles of bulk compaction and interfacial contact in governing heat transport in TIM assemblies remain poorly resolved. Here, we present a square-pulsed source (SPS) thermoreflectance method that enables simultaneous determination of k, C, and ITR in TIM stacks under controlled mechanical loading. By spanning square-wave modulation frequencies from 1 Hz to 10 MHz, SPS probes a broad range of thermal penetration depths, enabling distinction between heat diffusion in the TIM bulk and interfacial heat transfer at the Al/TIM contact. Measurements on a thermally conductive gel, a thermal pad, and a high-vacuum grease during compression-unloading cycles reveal distinct pressure-dependent thermal transport mechanisms. The gel and pad exhibit increases in k and C, reduced ITR, and pronounced hysteresis, indicating coupled bulk densification and persistent interfacial conformity during loading cycles. In contrast, the grease shows nearly pressure-independent bulk properties but a strong pressure dependence of ITR, consistent with an interface-dominated response. These results resolve the long-standing challenge of simultaneously quantifying bulk and interfacial thermal transport in mechanically loaded TIM assemblies, enabling experimentally constrained thermal management and reliability analysis in electronic packaging. △ Less

Submitted 29 March, 2026; v1 submitted 23 March, 2026; originally announced March 2026.

Weak Coupling of Diffusional and Phonon-like Modes in Liquids Revealed by Dynamic Kapitza Length

Authors: Tao Chen, Puqing Jiang

Abstract: Understanding heat transfer across solid-liquid interfaces is central to thermal management and energy technologies, yet whether the interfacial thermal conductance (ITC) depends on the timescale of heating remains unclear. Here we use square-pulsed source thermoreflectance, which combines time-resolved detection with broadband modulation, to probe Al-water and Al-octane interfaces. We observe a r… ▽ More Understanding heat transfer across solid-liquid interfaces is central to thermal management and energy technologies, yet whether the interfacial thermal conductance (ITC) depends on the timescale of heating remains unclear. Here we use square-pulsed source thermoreflectance, which combines time-resolved detection with broadband modulation, to probe Al-water and Al-octane interfaces. We observe a reproducible increase of the apparent ITC with modulation frequency. A control Al-silica interface shows no measurable frequency dependence, indicating that the effect is specific to liquids rather than a generic feature shared by all amorphous materials. We explain the data with a two-channel liquid picture in which diffusional and phonon-like modes exchange energy weakly over a finite nonequilibrium length. From the relative magnitude of the thermal penetration depth and the nonequilibrium length, we identify three transport regimes. These findings challenge the common assumption of fully equilibrated liquid modes and provide experimental constraints for modeling dynamic energy exchange at liquid interfaces. △ Less

Submitted 23 March, 2026; originally announced March 2026.

External-field-induced transition from altermagnetic metal to fully-compensated ferrimagnetic metal in monolayer $\mathrm{Cr_2O}$

Authors: San-Dong Guo, Qiqi Luo, Shi-Hao Zhang, Peng Jiang

Abstract: Altermagnets and fully-compensated ferrimagnets are two canonical classes of zero-net-moment magnets. An altermagnetic (AM) half-metal cannot exist due to its AM spin splitting, while a fully-compensated ferrimagnetic (FC-FIM) metal seems impossible to realize because both spin channels remain gapless. Here, we propose that an FC-FIM metal can be realized by breaking the rotational or mirror symme… ▽ More Altermagnets and fully-compensated ferrimagnets are two canonical classes of zero-net-moment magnets. An altermagnetic (AM) half-metal cannot exist due to its AM spin splitting, while a fully-compensated ferrimagnetic (FC-FIM) metal seems impossible to realize because both spin channels remain gapless. Here, we propose that an FC-FIM metal can be realized by breaking the rotational or mirror symmetry that links two spin-opposite magnetic atoms in an AM metal. We further demonstrate that charge-carrier doping is fundamentally unable to generate a net magnetic moment in an altermagnet, whereas such a net moment can be readily induced in a fully-compensated ferrimagnet. We use the AM monolayer $\mathrm{Cr_2O}$ as a concrete example to validate our proposal. Either electric field or uniaxial strain can break the $S_{4z}$ symmetry of $\mathrm{Cr_2O}$, thereby inducing a transition from an AM metal to an FC-FIM metal. Uniaxial strain plus carrier doping creates a net moment in an altermagnet, and the so-called piezomagnetism is essentially a strain-driven switch from altermagnetism to fully-compensated ferrimagnetism. By analogy, we advance the concept of electromagnetism: an electric field drives the transition from altermagnetism to fully-compensated ferrimagnetism, and subsequent charge-carrier doping stabilizes a net magnetization. Our work provides a roadmap for further exploring the connection and distinction between altermagnet and fully-compensated ferrimagnet, and confirms the feasibility of FC-FIM metal. △ Less

Submitted 4 December, 2025; originally announced December 2025.

Comments: 8 pages, 6 figures

Dual instability of superconductivity from oxygen defects in La$_3$Ni$_2$O$_{7+δ}$

Authors: Peiheng Jiang, Jie Li, Yu-Han Cao, Xiaodong Cao, Zhicheng Zhong, Yi Lu, Qiang-Hua Wang

Abstract: We uncover a dual mechanism by which oxygen defects suppress superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_{7+δ}$ using density functional theory, dynamical mean-field theory, and functional renormalization group analysis. Apical vacancies and interbilayer interstitials emerge as the dominant low-energy defect species and are further stabilized by orthorhombic domain walls. These two d… ▽ More We uncover a dual mechanism by which oxygen defects suppress superconductivity in the bilayer nickelate La$_3$Ni$_2$O$_{7+δ}$ using density functional theory, dynamical mean-field theory, and functional renormalization group analysis. Apical vacancies and interbilayer interstitials emerge as the dominant low-energy defect species and are further stabilized by orthorhombic domain walls. These two defect classes drive the electronic structure in opposing directions. Vacancy-induced disorder generates local magnetic moments and promotes Anderson localization at moderate concentrations, whereas periodic interstitial ordering yields a coherent but weakly correlated metallic background that fails to support superconductivity. These findings highlight the decisive role of oxygen defects in shaping the superconducting and provide microscopic guidance for improving superconductivity through controlled defect engineering. △ Less

Submitted 28 November, 2025; originally announced December 2025.

Cryogenic temperature dependence and hysteresis of surface-trap-induced gate leakage in GaN high-electron-mobility transistors

Authors: Ching-Yang Pan, Shi-Kai Lin, Yu-An Chen, Pei-hsun Jiang

Abstract: This work provides a detailed mapping of various mechanisms of surface-trap-induced gate leakage in GaN HEMTs across a temperature range from room to cryogenic levels. Two-dimensional variable-range hopping is observed at small gate bias. Under higher reverse gate bias, the leakage is dominated by the Poole--Frenkel emission above 220 K, but gradually transitions to the trap-assisted tunneling bel… ▽ More This work provides a detailed mapping of various mechanisms of surface-trap-induced gate leakage in GaN HEMTs across a temperature range from room to cryogenic levels. Two-dimensional variable-range hopping is observed at small gate bias. Under higher reverse gate bias, the leakage is dominated by the Poole--Frenkel emission above 220 K, but gradually transitions to the trap-assisted tunneling below 220 K owing to the frozen-trap effect. The trap barrier height extracted from the gate leakage current under the upward gate sweep is 0.65 V, which is 12\% higher than that from the downward sweep. The gate leakage current as a function of the gate bias exhibits clockwise hysteresis loops above 220 K but counterclockwise ones below 220 K. This remarkable opposite hysteresis phenomenon is thoroughly explained by the trap mechanisms. △ Less

Submitted 16 October, 2025; originally announced October 2025.

Comments: Accepted by Phys. Rev. Applied

Single crystal growth, structural and physical properties, and absence of a charge density wave in Ti_{0.85}Fe6Ge6

Authors: Dechao Cheng, Nour Maraytta, Xiuhua Chen, Xizhi Li, Xueliang Wu, Xiangxiang Jing, Yong Hu, Youpin Gong, Mingquan He, Yisheng Chai, Xiaoyuan Zhou, Pengfei Jiang, Yilin Wang, Michael Merz, Aifeng Wang

Abstract: Kagome materials with charge density waves (CDWs) are fascinating quantum systems, offering an ideal platform to explore intertwined orders and to uncover novel mechanisms behind CDW formation. Chemical models have been developed and applied to predict CDW in $AM_6X_6$-type kagome materials, such as the rattling chain model based on ScV6Sn6 and the magnetic energy-saving model based on FeGe. In th… ▽ More Kagome materials with charge density waves (CDWs) are fascinating quantum systems, offering an ideal platform to explore intertwined orders and to uncover novel mechanisms behind CDW formation. Chemical models have been developed and applied to predict CDW in $AM_6X_6$-type kagome materials, such as the rattling chain model based on ScV6Sn6 and the magnetic energy-saving model based on FeGe. In this study, we successfully synthesized Ti_{0.85}Fe6Ge6 single crystals using the vapor transport method. As predicted by the rattling chain model, these crystals are expected to exhibit kagome CDW behavior. Magnetization measurements indicate that Ti_{0.85}Fe6Ge6 is an easy-axis antiferromagnet with T_N = 488 K and transport measurements reveal metallic behavior primarily driven by electron-type carriers. However, no clear signatures of a CDW were observed in Ti_{0.85}Fe6Ge6. Density functional theory calculations demonstrate a markedly distinct electronic structure compared to related compounds: instead of a carrier-doping-induced rigid shift, the density of states shifted away from the Fermi level. Consistent with our structural investigations, the absence of a CDW and the unusual band structure can be attributed to the bonding characteristic within Ti_{0.85}Fe6Ge6. The strong covalent bonds of Ti-Ge1b, along with the solid Ge1b-Ge1b dimers, prevent the Ti-Ge1b-Ge1b-Ti chain from rattling. The presence of Fe-Fe antibonding state at the Fermi level enhances the spin polarization and depletes the electronic density around the Fermi level. Our results suggest that both the ionic radius and the bonding characteristics of the filler atom are crucial for the formation of CDWs in kagome materials. These factors can serve as supplementary terms to the rattling chain model, providing new insights for the discovery of novel kagome CDW materials. △ Less

Submitted 24 September, 2025; originally announced September 2025.

Comments: 12 pages, 8 figures

Journal ref: Phys. Rev. B 113, 035133 (2026)

Mechanistic Insight into BEOL Thermal Transport via Optical Metrology and Multiphysics Simulation

Authors: Yang Shen, Shangzhi Song, Tao Chen, Kexin Zhang, Yu Chen, Lu Zhao, Puqing Jiang

Abstract: As integrated circuits continue to scale down and adopt three-dimensional (3D) stacking, thermal management in the back-end-of-line (BEOL) has emerged as a critical design constraint. In this study, we present a combined experimental and simulation framework to quantitatively characterize and mechanistically understand thermal transport in BEOL multilayers. Using the Square-Pulsed Source (SPS) met… ▽ More As integrated circuits continue to scale down and adopt three-dimensional (3D) stacking, thermal management in the back-end-of-line (BEOL) has emerged as a critical design constraint. In this study, we present a combined experimental and simulation framework to quantitatively characterize and mechanistically understand thermal transport in BEOL multilayers. Using the Square-Pulsed Source (SPS) method, a time-resolved optical metrology technique, we measure cross-plane thermal resistance and areal heat capacity in semiconductor chips at nanometer resolution. Two fabricated chip samples, polished to the M4 and M6 interconnection layers, are analyzed to extract thermal properties of distinct multilayer stacks. Results show that thermal resistance follows a series model, while areal heat capacity scales linearly with metal content. To uncover the underlying physical mechanisms, we perform finite element simulations using COMSOL Multiphysics, examining the influence of via connectivity and dielectric thermal conductivity on effective cross-plane heat transport. The simulations reveal that dielectric materials, due to their large volume fraction, are the primary limiting factor in BEOL thermal conduction, while the via structure plays a secondary but significant role. This combined experimental-simulation approach provides mechanistic insight into heat transport in advanced IC architectures and offers practical guidance for optimizing thermal pathways in future high-performance 3D-stacked devices. △ Less

Submitted 11 August, 2025; originally announced August 2025.

Comments: 16 pages, 6 figures

Hybrid Particle Swarm Optimization for Fast and Reliable Parameter Extraction in Thermoreflectance

Authors: Bingjia Xiao, Tao Chen, Wenbin Zhang, Xin Qian, Puqing Jiang

Abstract: Frequency-domain thermoreflectance (FDTR) is a widely used technique for characterizing thermal properties of multilayer thin films. However, extracting multiple parameters from FDTR measurements presents a nonlinear inverse problem due to its high dimensionality and multimodal, non-convex solution space. This study evaluates four popular global optimization algorithms: Genetic Algorithm (GA), Qua… ▽ More Frequency-domain thermoreflectance (FDTR) is a widely used technique for characterizing thermal properties of multilayer thin films. However, extracting multiple parameters from FDTR measurements presents a nonlinear inverse problem due to its high dimensionality and multimodal, non-convex solution space. This study evaluates four popular global optimization algorithms: Genetic Algorithm (GA), Quantum Genetic Algorithm (QGA), Particle Swarm Optimization (PSO), and Fireworks Algorithm (FWA), for extracting parameters from FDTR measurements of a GaN/Si heterostructure. However, none achieve reliable convergence within 60 seconds. To improve convergence speed and accuracy, we propose an AI-driven hybrid optimization framework that combines each global algorithm with a Quasi-Newton local refinement method, resulting in four hybrid variants: HGA, HQGA, HPSO, and HFWA. Among these, HPSO outperforms all other methods, with 80% of trials reaching the target fitness value within 60 seconds, showing greater robustness and a lower risk of premature convergence. In contrast, only 30% of HGA and HQGA trials and 20% of HFWA trials achieve this threshold. We then evaluate the worst-case performance across 100 independent trials for each algorithm when the time is extended to 1000 seconds. Only HPSO, PSO, and HGA consistently reach the target accuracy, with HPSO converging five times faster than the others. HPSO provides a general-purpose solution for inverse problems in thermal metrology and can be readily extended to other model-fitting techniques. △ Less

Submitted 29 July, 2025; originally announced July 2025.

Comments: 28 pages, 8 figures

Journal ref: International Journal of Heat and Mass Transfer 256, 128109 (2026)

Two-dimensional non-van der Waals niobium nitride nanosheets with high-temperature two-gap superconductivity

Authors: Si-Yi Xiong, Peng Jiang, Yiming Wang, Yan-Ling Li

Abstract: The exploration of the superconductivity in two-dimensional materials has garnered significant attention due to their promising low-power applications and fundamental scientific interest. Here, we report some novel stable non-van der Waals Nb$_x$N$_{x+1}$ ($x$ = 1-4) monolayers derived from the NbN bulk exfoliated along the (001) plane, as identified through first-principles calculations. Among th… ▽ More The exploration of the superconductivity in two-dimensional materials has garnered significant attention due to their promising low-power applications and fundamental scientific interest. Here, we report some novel stable non-van der Waals Nb$_x$N$_{x+1}$ ($x$ = 1-4) monolayers derived from the NbN bulk exfoliated along the (001) plane, as identified through first-principles calculations. Among these monolayers, Nb$_2$N$_3$, which crystallizes in the $P \overline{6} m2$ symmetry, stands out with an exceptional superconducting transition temperature of 77.8 K, setting a new high-$T_c$ benchmark for two-dimensional transition metal nitrides and binary compounds. Our detailed analysis reveals that the strong superconductivity in Nb$_2$N$_3$ is driven by phonon modes dominated by N vibrations, with significant electron-phonon coupling contributions from N-$p$ and Nb-$d$ electronic states. Using the anisotropic Migdal-Eliashberg framework, we further determine the two-gap nature of the superconductivity in the Nb$_2$N$_3$ monolayer, characterized by pronounced electron-phonon coupling and anisotropic energy gaps. These results advance our understanding of superconductivity in 2D transition metal nitride and highlight their potential for nanoscale superconducting applications. △ Less

Submitted 29 April, 2025; originally announced April 2025.

Comments: 8 pages, 4 figures

Journal ref: Phys. Rev. B 111, 205426 (2025)

Robust cross-chain surface interstitial electronic states and doping-enhanced superconductivity in monolayer $M_2$N ($M$= Ti, Zr, Hf) electrides

Authors: Da-Bao Zha, Peng Jiang, Yan-Ling Li, Hai-Qing Lin

Abstract: The exploration of electrides holds great promise for advancing both fundamental physics and chemistry, owing to their unique characteristics arising from loosely bound interstitial anionic electrons. Here we report a class of cross-chain electrides, distinguished by two distinct anionic electron subchannels forming alternating chains in real space. Through structural symmetry analysis and first-p… ▽ More The exploration of electrides holds great promise for advancing both fundamental physics and chemistry, owing to their unique characteristics arising from loosely bound interstitial anionic electrons. Here we report a class of cross-chain electrides, distinguished by two distinct anionic electron subchannels forming alternating chains in real space. Through structural symmetry analysis and first-principles calculations, we identify two-dimensional $M_2$N ($M$ = Ti, Zr, Hf) materials as prototypical systems exhibiting these unique features. The anionic electron channels on the upper and lower surfaces of these materials display a vertically alternating pattern, with their projected bands revealing momentum-dependent splitting behavior in the reciprocal space, protected by a crystal symmetry operation $\mathcal{O}$. Notably, the cross-chain electride characteristic in the $M_2$N monoalyers is independent of the layer number and remains robust on the upper and lower surfaces of layered structures, presenting a pronounced and robust surface interstitial electronic state. Additionally, we have explored the superconductivity of these systems, and found that both Ti$_2$N and Zr$_2$N are intrinsic superconductors with superconducting transition temperatures below 1.0 K. Further results show that appropriate hole doping can significantly enhance their superconducting transition temperatures and can induce the Hf$_2$N monolayer to exhibit superconductivity. Our findings provide valuable insights into the design and tuning of novel electrides with enhanced superconducting properties, offering another pathway for deeply understanding the interplay between electride behavior and superconductivity in novel materials. △ Less

Submitted 7 September, 2025; v1 submitted 27 April, 2025; originally announced April 2025.

Comments: 8 Pages, 7 figures

Journal ref: Phys. Rev. B 112, 115416 (2025)

Extending the Low-Frequency Limit of Time-Domain Thermoreflectance via Periodic Waveform Analysis

Authors: Mingzhen Zhang, Tao Chen, Shangzhi Song, Yunjia Bao, Ruiqiang Guo, Weidong Zheng, Puqing Jiang, Ronggui Yang

Abstract: Time-domain thermoreflectance (TDTR) is a powerful technique for characterizing the thermal properties of layered materials. However, its effectiveness at modulation frequencies below 0.1 MHz is hindered by pulse accumulation effects, limiting its ability to accurately measure in-plane thermal conductivities below 6 W/(m K). Here, we present a periodic waveform analysis-based TDTR (PWA-TDTR) metho… ▽ More Time-domain thermoreflectance (TDTR) is a powerful technique for characterizing the thermal properties of layered materials. However, its effectiveness at modulation frequencies below 0.1 MHz is hindered by pulse accumulation effects, limiting its ability to accurately measure in-plane thermal conductivities below 6 W/(m K). Here, we present a periodic waveform analysis-based TDTR (PWA-TDTR) method that extends the measurable frequency range down to 50 Hz with minimal modifications to the conventional setup. This advancement greatly enhances measurement sensitivity, enabling accurate measurements of in-plane thermal conductivities as low as 0.2 W/(m K). We validate the technique by measuring polymethyl methacrylate (PMMA) and fused silica, using PWA-TDTR to obtain in-plane thermal diffusivity and conventional TDTR to measure cross-plane thermal effusivity. Together, these allow the extraction of both thermal conductivity and volumetric heat capacity, with results in excellent agreement with literature values. We further demonstrate the versatility of PWA-TDTR through (1) thermal conductivity and heat capacity measurements of thin liquid films and (2) depth-resolved thermal conductivity profiling in lithium niobate crystals, revealing point defect-induced inhomogeneities at depths up to 100 um. By overcoming frequency and sensitivity constraints, PWA-TDTR significantly expands the applicability of TDTR, enabling detailed investigations of thermal transport in materials and conditions that were previously challenging to study. △ Less

Submitted 20 April, 2025; originally announced April 2025.

Comments: 19 pages, 4 figures

Journal ref: J. Appl. Phys. 138, 055101 (2025)

Temperature-driven structural phase transitions in SmNiO$_3$: insights from deep potential molecular dynamics simulations

Authors: Guoyong Shi, Fenglin Deng, Ri He, Dachuan Chen, Xuejiao Chen, Peiheng Jiang, Zhicheng Zhong

Abstract: The metal-insulator transition (MIT) in rare-earth nickelates exemplifies the intricate coupling between lattice dynamics and electronic effects. This strong interplay makes it challenging to disentangle their individual roles in driving the transition in RNiO3. Here, we isolate the structure response from electronic effect by employing molecular dynamics (MD) simulations based on a machine-learne… ▽ More The metal-insulator transition (MIT) in rare-earth nickelates exemplifies the intricate coupling between lattice dynamics and electronic effects. This strong interplay makes it challenging to disentangle their individual roles in driving the transition in RNiO3. Here, we isolate the structure response from electronic effect by employing molecular dynamics (MD) simulations based on a machine-learned interatomic potential. Taking SmNiO3 as a prototypical system, our simulations show that the structural phase transition is intrinsically temperature-driven and occurs spontaneously via collective lattice distortions. The simulated critical temperature is 340 K and can be further tuned by pressure. These findings provide atomistic insights into the understanding of structural evolution in triggering the phase transition and hence the MIT in RNiO3. △ Less

Submitted 28 June, 2026; v1 submitted 7 March, 2025; originally announced March 2025.

Comments: 16pages, 5 figues

Three-dimensional (3D) tensor-based methodology for characterizing 3D anisotropic thermal conductivity tensor

Authors: Dihui Wang, Heng Ban, Puqing Jiang

Abstract: The increasing complexity of advanced materials with anisotropic thermal properties necessitates more generic and efficient methods to determine three-dimensional (3D) anisotropic thermal conductivity tensors with up to six independent components. Current methods rely on a vector-based framework that can handle only up to four independent components, often leading to inefficiencies and inaccuracie… ▽ More The increasing complexity of advanced materials with anisotropic thermal properties necessitates more generic and efficient methods to determine three-dimensional (3D) anisotropic thermal conductivity tensors with up to six independent components. Current methods rely on a vector-based framework that can handle only up to four independent components, often leading to inefficiencies and inaccuracies. We introduce Three-Dimensional Spatially Resolved Lock-In Micro-Thermography (3D SR-LIT), a novel optical thermal characterization technique combining a 3D tensor-based framework with an efficient area-detection experimental system. For simple tensors (e.g., x-cut quartz, k_xz=k_yz=0), our method reduces uncertainty by over 50% compared to vector-based methods. For complex tensors with six independent components (e.g., AT-cut quartz), 2σ uncertainties remain below 12% for all components. A novel adaptive mapping approach enables high-throughput data acquisition (40 seconds to 3 minutes, depending on tensor complexity), over 35 times faster than current methods, and accommodates samples with 200 nm surface roughness. Extensive numerical validation on 1,000 arbitrary anisotropic tensors ranging from 1 to 1,000 Wm^(-1) K^(-1) further validates the robustness of this methodology. This work highlights significant advancements in thermal characterization of complex anisotropic materials. △ Less

Submitted 11 January, 2025; originally announced January 2025.

Journal ref: Int J Heat Mass Transf 242, 126886 (2025)

Simultaneous Measurement of Thermal Conductivity, Heat Capacity, and Interfacial Thermal Conductance by Leveraging Negative Delay-Time Data in Time-Domain Thermoreflectance

Authors: Mingzhen Zhang, Tao Chen, Ao Zeng, Jialin Tang, Ruiqiang Guo, Puqing Jiang

Abstract: Time-domain thermoreflectance (TDTR) is a widely used technique for characterizing the thermal properties of bulk and thin-film materials. Traditional TDTR analyses typically focus on positive delay time data for fitting, often requiring multiple-frequency measurements to simultaneously determine thermal conductivity and heat capacity. However, this multiple-frequency approach is cumbersome and ma… ▽ More Time-domain thermoreflectance (TDTR) is a widely used technique for characterizing the thermal properties of bulk and thin-film materials. Traditional TDTR analyses typically focus on positive delay time data for fitting, often requiring multiple-frequency measurements to simultaneously determine thermal conductivity and heat capacity. However, this multiple-frequency approach is cumbersome and may introduce inaccuracies due to inconsistencies across different frequency measurements. In this study, we propose a novel solution to these challenges by harnessing the often-overlooked negative delay time data in TDTR. By integrating these data points, we offer a streamlined, single-frequency method that simultaneously measures thermal conductivity, heat capacity, and interface thermal conductance for both bulk and thin-film materials, enhancing measurement efficiency and accuracy. We demonstrate the effectiveness of this method by measuring several bulk samples including sapphire, silicon, diamond, and Si0.992Ge0.008, and several thin-film samples including a 1.76-μm-thick gallium nitride (GaN) film epitaxially grown on a silicon substrate, a 320-nm-thick gallium oxide (ε-Ga2O3) film epitaxially grown on a silicon carbide substrate, and a 330-nm-thick tantalum nitride (TaN) film deposited on a sapphire substrate, all coated with an aluminum (Al) transducer layer on the surface. Our results show that the new method accurately determines the thermal conductivity and heat capacity of these samples as well as the Al/sample interface thermal conductance using a single modulation frequency, except for the Si0.992Ge0.008 sample. This study sheds light on the untapped potential of TDTR, offering a new, efficient, and accurate avenue for thermal analysis in material science. △ Less

Submitted 24 November, 2024; originally announced November 2024.

Journal ref: Progress in Natural Science: Materials International, 35 (2025) 375-384

Decoupling Thermal Properties in Multilayer Systems for Advanced Thermoreflectance Techniques

Authors: Tao Chen, Puqing Jiang

Abstract: Thermoreflectance techniques, including time-domain thermoreflectance (TDTR), frequency-domain thermoreflectance (FDTR), and the square-pulsed source (SPS) method, are powerful tools for characterizing the thermal properties of bulk and thin-film materials. However, accurately interpreting their signals remains challenging due to intricate interdependencies among experimental variables. This study… ▽ More Thermoreflectance techniques, including time-domain thermoreflectance (TDTR), frequency-domain thermoreflectance (FDTR), and the square-pulsed source (SPS) method, are powerful tools for characterizing the thermal properties of bulk and thin-film materials. However, accurately interpreting their signals remains challenging due to intricate interdependencies among experimental variables. This study introduces a systematic framework based on singular value decomposition (SVD) to decouple these interdependent parameters and enhance the reliability of thermal property extraction. By applying SVD to the sensitivity matrix, we identify key parameter combinations and establish essential dimensionless numbers that govern thermoreflectance signals. The framework is applied to a GaN/Si heterostructure, where the performance of TDTR, FDTR, and SPS is evaluated and compared. The results demonstrate a high degree of consistency across all three techniques. Notably, with the intricate relationships of parameters unraveled, TDTR, FDTR, and SPS demonstrate significant potential to simultaneously and accurately extract five to seven key thermal properties, including thermal conductivity, heat capacity, and interfacial thermal conductance of the GaN/Si multilayer system. This framework not only improves the precision of thermoreflectance measurements but also lays a foundation for advanced thermal metrology in research and industrial applications. △ Less

Submitted 10 January, 2025; v1 submitted 10 October, 2024; originally announced October 2024.

Journal ref: Physical Review Applied, 23 (2025) 044004

Charge and spin instabilities in superconducting La$_3$Ni$_2$O$_7$

Authors: Xuejiao Chen, Peiheng Jiang, Jie Li, Zhicheng Zhong, Yi Lu

Abstract: Motivated by the recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ under high pressure, we explore its potential charge and spin instabilities through combined model analysis and first-principles calculations. Taking into account the small charge-transfer nature of high valence nickel, a fully correlated two-cluster model identifies a lattice-coupled charge instability characterized by su… ▽ More Motivated by the recent discovery of superconductivity in La$_3$Ni$_2$O$_7$ under high pressure, we explore its potential charge and spin instabilities through combined model analysis and first-principles calculations. Taking into account the small charge-transfer nature of high valence nickel, a fully correlated two-cluster model identifies a lattice-coupled charge instability characterized by substantial short-range fluctuations of oxygen holes. This instability is corroborated by density-functional-theory plus $U$ calculations that also reveal a strong tendency towards concurrent antiferromagnetic ordering. The charge, spin, and associated lattice instabilities are significantly suppressed with increasing external pressure, contributing to the emergence of superconductivity in pressurized La$_3$Ni$_2$O$_7$. Carrier doping is found to effectively suppress these instabilities, suggesting a viable strategy to stabilize a superconducting phase under ambient pressure. △ Less

Submitted 2 September, 2024; v1 submitted 14 July, 2023; originally announced July 2023.

Journal ref: Phys. Rev. B 111, 014515 (2025)

Zero-Point Quantum Diffusion of Proton in Hydrogen-rich Superconductor $LaH_{10}$

Authors: Xuejian Qin, Hongyu Wu, Guyong Shi, Chao Zhang, Peiheng Jiang, Zhicheng Zhong

Abstract: $LaH_{10}$, as a member of hydrogen-rich superconductors, has a superconducting critical temperature of 250 K at high pressures, which exhibits the possibility of solving the long-term goal of room temperature superconductivity. Considering the extreme pressure and low mass of hydrogen, the nuclear quantum effects in $LaH_{10}… ▽ More $LaH_{10}$, as a member of hydrogen-rich superconductors, has a superconducting critical temperature of 250 K at high pressures, which exhibits the possibility of solving the long-term goal of room temperature superconductivity. Considering the extreme pressure and low mass of hydrogen, the nuclear quantum effects in $LaH_{10}$ should be significant and have an impact on its various physical properties. Here, we adopt the method combines deep-potential (DP) and quantum thermal bath (QTB), which was verified to be able to account for quantum effects in high-accuracy large-scale molecular dynamics simulations. Our method can actually reproduce pressure-temperature phase diagrams of $LaH_{10}$ consistent with experimental and theoretical results. After incorporating quantum effects, the quantum fluctuation driven diffusion of proton is found even in the absence of thermal fluctuation near 0 K. The high mobility of proton is found to be compared to liquid, yet the structure of $LaH_{10}$ is still rigid. These results would greatly enrich our vision to study quantum behavior of hydrogen-rich superconductors. △ Less

Submitted 2 June, 2023; originally announced June 2023.

Comments: 7 pages, 6 figures

Magnetism in doped infinite-layer NdNiO2 studied by combined density functional theory and dynamical mean-field theory

Authors: Dachuan Chen, Peiheng Jiang, Liang Si, Yi Lu, Zhicheng Zhong

Abstract: The recent observation of superconductivity in infinite-layer nickelates has brought intense debate on the established knowledge of unconventional superconductivity based on the cuprates. Despite many similarities, the nickelates differ from the cuprates in many characteristics, the most notable one among which is the magnetism. Instead of a canonical antiferromagnetic Mott insulator as the undope… ▽ More The recent observation of superconductivity in infinite-layer nickelates has brought intense debate on the established knowledge of unconventional superconductivity based on the cuprates. Despite many similarities, the nickelates differ from the cuprates in many characteristics, the most notable one among which is the magnetism. Instead of a canonical antiferromagnetic Mott insulator as the undoped cuprates, from which the superconductivity is generally believed to arise upon doping, the undoped nickelates show no sign of magnetic ordering in experiments. Through a combined density functional theory, dynamical mean-field theory, and model study, we show that although the increased energy splitting between O-$p$ orbital and Cu/Ni-$d$ orbital ($Δ_{dp}$) results in larger magnetic moment in nickelates, it also leads to stronger antiferromagnetism/ferromagnetism competition, and weaker magnetic exchange coupling. Meanwhile, the self-doping effect caused by Nd-$d$ orbital screens the magnetic moment of Ni. The Janus-faced effect of $Δ_{dp}$ and self-doping effect together give a systematic understanding of magnetic behavior in nickelates and explain recent experimental observations. △ Less

Submitted 10 July, 2022; v1 submitted 20 June, 2022; originally announced June 2022.

Comments: 10 pages, 9 figures

Journal ref: Phys. Rev. B.106,045105(2022)

Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films

Authors: Wei-Hsiang Wang, Ching-Yang Pan, Chak-Ming Liu, Wen-Chin Lin, Pei-hsun Jiang

Abstract: Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits strong vertical and… ▽ More Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomenon exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue towards more sophisticate control of canted spin textures in future spintronic devices. △ Less

Submitted 26 April, 2022; originally announced April 2022.

Comments: Supplementary materials available at https://pubs.acs.org/doi/10.1021/acsami.1c23276#notes-1

Investigation of Deformation and Fracture Mechanisms in Two-dimensional Gallium Telluride Multilayers Using Nanoindentation

Authors: Yan Zhou, Shi Zhou, Penghua Ying, Qinghua Zhao, Yong Xie, Mingming Gong, Pisu Jiang, Hui Cai, Bin Chen, Sefaattin Tongay, Wanqi Jie, Jin Zhang, Tao Wang, Dong Liu, Martin Kuball

Abstract: Two-dimensional (2D) materials possess great potential for flexible devices, ascribing to their outstanding electrical, optical, and mechanical properties. However, their mechanical deformation property and fracture mechanism, which are inescapable in many applications like flexible optoelectronics, are still unclear or not thoroughly investigated due methodology limitations. In light of this, suc… ▽ More Two-dimensional (2D) materials possess great potential for flexible devices, ascribing to their outstanding electrical, optical, and mechanical properties. However, their mechanical deformation property and fracture mechanism, which are inescapable in many applications like flexible optoelectronics, are still unclear or not thoroughly investigated due methodology limitations. In light of this, such mechanical properties and mechanisms are explored on example of gallium telluride (GaTe), a promising optoelectronic candidate with an ultrahigh photo-responsibility and a high plasticity within 2D family. Considering the driving force insufficient in atomic force microscopy (AFM)-based nanoindentation method, here the mechanical properties of both substrate-supported and suspended GaTe multilayers were systematically investigated through full-scale Berkovich-tip nanoindentation, micro-Raman spectroscopy, AFM, and scanning electron microscopy. An unusual concurrence of multiple pop-in and load-drop events in loading curve was observed. By further correlating to molecular dynamics calculations, this concurrence was unveiled originating from the interlayer sliding mediated layers-by-layers fracture mechanism within GaTe multilayers. The van der Waals force between GaTe multilayers and substrates was revealed much stronger than that between GaTe interlayers, resulting in the easy sliding and fracture of multilayers within GaTe. This work provides new insights into the deformation and fracture mechanisms of GaTe and other similar 2D multilayers in flexible applications. △ Less

Submitted 23 April, 2022; originally announced April 2022.

Comments: 21 pages, 5 figures

Journal ref: J. Phys. Chem. Lett. 13, 3831-3839 (2022)

A new spatial-scan thermoreflectance method to measure a broad range of anisotropic in-plane thermal conductivity

Authors: Puqing Jiang, Dihui Wang, Zeyu Xiang, Ronggui Yang, Heng Ban

Abstract: In-plane thermal conductivities of small-scale samples are hard to measure, especially for the lowly conductive ones and those lacking in-plane symmetry (i.e., transversely anisotropic materials). State-of-the-art pump-probe techniques including both the time-domain and the frequency-domain thermoreflectance (TDTR and FDTR) are advantageous in measuring the thermal conductivity of small-scale samp… ▽ More In-plane thermal conductivities of small-scale samples are hard to measure, especially for the lowly conductive ones and those lacking in-plane symmetry (i.e., transversely anisotropic materials). State-of-the-art pump-probe techniques including both the time-domain and the frequency-domain thermoreflectance (TDTR and FDTR) are advantageous in measuring the thermal conductivity of small-scale samples, and various advanced TDTR and FDTR techniques have been developed to measure transversely anisotropic materials. However, the measurable in-plane thermal conductivity (k_in) is usually limited to be >10 W/(m K). In this work, a new spatial-scan thermoreflectance (SSTR) method has been developed to measure a broad range of k_in of millimeter-scale small samples, including those lacking in-plane symmetry, extending the current limit of the measurable k_in to as low as 1 W/(m K). This SSTR method establishes a new scheme of measurements using the optimized laser spot size and modulation frequency and a new scheme of data processing, enabling measurements of in-plane thermal conductivity tensors of a broad range of k_in values with both high accuracy and ease of operation. Some details such as the requirement on the sample geometry, the effect of the transducer layer, and the effect of heat loss are also discussed. As a verification, the k_in of some transversely isotropic reference samples with a wide range of k_in values including fused silica, sapphire, silicon, and highly ordered pyrolytic graphite (HOPG) have been measured using this new SSTR method. The measured k_in agree perfectly well with the literature values with a typical uncertainty of 5%. As a demonstration of the unique capability of this method, the in-plane thermal conductivity tensor of x-cut quartz, an in-plane anisotropic material, has also been measured. △ Less

Submitted 28 January, 2022; originally announced January 2022.

Comments: 26 pages, 9 figures, patent-pending technique

Journal ref: Int. J. Heat Mass Transfer, 191 (2022) 122849

Record high $T_{\rm c}$ and robust superconductivity in transition metal $δ$-Ti phase at megabar pressure

Authors: Xuqiang Liu, Peng Jiang, Yiming Wang, Mingtao Li, Nana Li, Qian Zhang, Yandong Wang, Yan-ling Li, Wenge Yang

Abstract: We report a record high superconducting transition temperature ($T_{\rm c}$) up to 23.6 K under high pressure in the elemental metal Ti, one of the top ten most abundant elements in Earth's crust. The $T_{\rm c}$ increases monotonically from 2.3 K at 40.3 GPa to 23.6 K at 144.9 GPa, which surpasses all known records from elemental metals reported so far. With further compression, a robust… ▽ More We report a record high superconducting transition temperature ($T_{\rm c}$) up to 23.6 K under high pressure in the elemental metal Ti, one of the top ten most abundant elements in Earth's crust. The $T_{\rm c}$ increases monotonically from 2.3 K at 40.3 GPa to 23.6 K at 144.9 GPa, which surpasses all known records from elemental metals reported so far. With further compression, a robust $T_{\rm c}$ of ~23 K is observed between 144.9 and 183 GPa in the $δ$-Ti phase. The pressure-dependent $T_{\rm c}$ can be well described by the conventional electron-phonon coupling (EPC) mechanism. Density Functional Theory calculations show the Fermi nesting and the phonon softening of optical branches at the $γ$-Ti to $δ$-Ti phase transition pressure enhance EPC, which results in the record high $T_{\rm c}$. We attribute the robust superconductivity in $δ$-Ti to the apparent robustness of its strong EPC against lattice compression. These results provide new insight into exploring new high-$T_{\rm c}$ elemental metals and Ti-based superconducting alloys. △ Less

Submitted 21 December, 2021; originally announced December 2021.

Exchange-bias dependent diffusion rate of hydrogen discovered from evolution of hydrogen-induced noncollinear magnetic anisotropy in FePd thin films

Authors: Wei-Hsiang Wang, Yu-Song Cheng, Hwo-Shuenn Sheu, Wen-Chin Lin, Pei-hsun Jiang

Abstract: Hydrogenation-induced noncollinear magnetic anisotropy is observed from the evolution of the magnetic domains in FePd alloy thin films using magneto-optic Kerr effect (MOKE) microscopy. MOKE images reveal complicated competitions between different magnetic anisotropies during hydrogen diffusion into the film. An intriguing enhancement of the hydrogen diffusion rate due to the presence of an initia… ▽ More Hydrogenation-induced noncollinear magnetic anisotropy is observed from the evolution of the magnetic domains in FePd alloy thin films using magneto-optic Kerr effect (MOKE) microscopy. MOKE images reveal complicated competitions between different magnetic anisotropies during hydrogen diffusion into the film. An intriguing enhancement of the hydrogen diffusion rate due to the presence of an initial exchange bias induced by a high magnet field is thereby discovered, pointing to an additional scope of controllability of magnetic metal hydrides as potential future hydrogen sensing and storage materials. △ Less

Submitted 9 December, 2021; originally announced December 2021.

Comments: Accepted by Phys. Rev. B

Dependence of magnetic domain patterns on plasma-induced differential oxidation of CoPd thin films

Authors: Wei-Hsiang Wang, Chak-Ming Liu, Tzu-Hung Chuang, Der-Hsin Wei, Wen-Chin Lin, Pei-hsun Jiang

Abstract: We demonstrate the evolution of the micro-patterned magnetic domains in CoPd thin films pretreated with e-beam lithography and O2 plasma. During the days-long oxidation, significantly different behaviors of the patterned magnetic domains under magnetization reversal are observed via magneto-optic Kerr effect microscopy on different days. The evolution of the magnetic behaviors indicate critical ch… ▽ More We demonstrate the evolution of the micro-patterned magnetic domains in CoPd thin films pretreated with e-beam lithography and O2 plasma. During the days-long oxidation, significantly different behaviors of the patterned magnetic domains under magnetization reversal are observed via magneto-optic Kerr effect microscopy on different days. The evolution of the magnetic behaviors indicate critical changes in the local magnetic anisotropy energies due to the Co oxides that evolve into different oxide forms, which are characterized by micro-area X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. The coercive field of the area pre-exposed to plasma can decrease to a value 10 Oe smaller than that unexposed to plasma, whereas after a longer duration of oxidation the coercive field can instead become larger in the area pre-exposed to plasma than that unexposed, leading to an opposite magnetic pattern. Various forms of oxidation can therefore provide an additional dimension for magnetic-domain engineering to the current conventional lithographies. △ Less

Submitted 10 November, 2021; originally announced November 2021.

Comments: Supplementary materials available at https://www.sciencedirect.com/science/article/pii/S2468023021006544#ec-research-data

Antiferromagnetism in Ni-Based Superconductors

Authors: Xiaorong Zhou, Xiaowei Zhang, Jiabao Yi, Peixin Qin, Zexin Feng, Peiheng Jiang, Zhicheng Zhong, Han Yan, Xiaoning Wang, Hongyu Chen, Haojiang Wu, Xin Zhang, Ziang Meng, Xiaojiang Yu, Mark B. H. Breese, Jiefeng Cao, Jingmin Wang, Chengbao Jiang, Zhiqi Liu

Abstract: Due to the lack of any magnetic order down to 1.7 K in the parent bulk compound NdNiO2, the recently discovered 9-15 K superconductivity in the infinite-layer Nd0.8Sr0.2NiO2 thin films has provided an exciting playground for unearthing new superconductivity mechanisms. In this letter, we report the successful synthesis of a series of superconducting Nd0.8Sr0.2NiO2 thin films ranging from 8 to 40 n… ▽ More Due to the lack of any magnetic order down to 1.7 K in the parent bulk compound NdNiO2, the recently discovered 9-15 K superconductivity in the infinite-layer Nd0.8Sr0.2NiO2 thin films has provided an exciting playground for unearthing new superconductivity mechanisms. In this letter, we report the successful synthesis of a series of superconducting Nd0.8Sr0.2NiO2 thin films ranging from 8 to 40 nm. We observe the large exchange bias effect between the superconducting Nd0.8Sr0.2NiO2 films and a thin ferromagnetic layer, which suggests the existence of the antiferromagnetic order. Furthermore, the existence of the antiferromagnetic order is evidenced by X-ray magnetic linear dichroism measurements. These experimental results are fundamentally critical for the current field. △ Less

Submitted 28 October, 2021; originally announced October 2021.

Comments: 25 pages, 22 figure, 1 table. The manuscript is in press at Advanced Materials

Journal ref: Adv. Mater. 34, 2106117 (2022)

Chirality locking charge density waves in a chiral crystal

Authors: Geng Li, Haitao Yang, Peijie Jiang, Cong Wang, Qiuzhen Cheng, Shangjie Tian, Guangyuan Han, Hechang Lei, Chengmin Shen, Xiao Lin, Wei Ji, Ziqiang Wang, Hong-Jun Gao

Abstract: In Weyl semimetals, charge density wave (CDW) order can spontaneously break the chiral symmetry, gap out the Weyl nodes, and drive the material into the axion insulating phase. Investigations have however been limited since CDWs are rarely seen in Weyl semimetals. Here, using scanning tunneling microscopy/spectroscopy, we report the discovery of a novel unidirectional CDW order on the (001) surfac… ▽ More In Weyl semimetals, charge density wave (CDW) order can spontaneously break the chiral symmetry, gap out the Weyl nodes, and drive the material into the axion insulating phase. Investigations have however been limited since CDWs are rarely seen in Weyl semimetals. Here, using scanning tunneling microscopy/spectroscopy, we report the discovery of a novel unidirectional CDW order on the (001) surface of chiral crystal CoSi - a unique Weyl semimetal with unconventional chiral fermions. The CDW is incommensurate with both lattice momentum and crystalline symmetry directions, and exhibits an intra unit cell π phase shift in the layer stacking direction. The tunneling spectrum shows a particle-hole asymmetric V-shaped energy gap around the Fermi level that modulates spatially with the CDW wave vector. Combined with first-principle calculations, we identify that the CDW is locked to the crystal chirality and is related by a mirror reflection between the two enantiomers of the chiral crystal. Our findings reveal a novel correlated topological quantum state in chiral CoSi crystals and raise the potential for realizing an axion insulator and exploring the unprecedented physical behaviors of unconventional chiral fermions. △ Less

Submitted 19 October, 2021; v1 submitted 14 October, 2021; originally announced October 2021.

Dynamic Behaviors and Training Effects in TiN/Ti/HfO$_x$/TiN Nanolayered Memristors with Controllable Quantized Conductance States: Implications for Quantum and Neuromorphic Computing Devices

Authors: Min-Hsuan Peng, Ching-Yang Pan, Hao-Xuan Zheng, Ting-Chang Chang, Pei-hsun Jiang

Abstract: Controllable quantized conductance states of TiN/Ti/HfO$_x$/TiN memristors are realized with great precision through a pulse-mode reset procedure, assisted with analytical differentiation of the condition of the set procedure, which involves critical monitoring of the measured bias voltage. An intriguing training effect that leads to faster switching of the states is also observed during the opera… ▽ More Controllable quantized conductance states of TiN/Ti/HfO$_x$/TiN memristors are realized with great precision through a pulse-mode reset procedure, assisted with analytical differentiation of the condition of the set procedure, which involves critical monitoring of the measured bias voltage. An intriguing training effect that leads to faster switching of the states is also observed during the operation. Detailed analyses on the low- and high-resistance states under different compliance currents reveal a complete picture of the structural evolution and dynamic behaviors of the conductive filament in the HfO$_x$ layer. This study provides a closer inspection on the quantum-level manipulation of nanoscale atomic configurations in the memristors, which helps to develop essential knowledge about the design and fabrication of the future memristor-based quantum devices and neuromorphic computing devices. △ Less

Submitted 22 September, 2021; originally announced September 2021.

Comments: Accepted by ACS Applied Nano Materials

Lateral modulation of magnetic anisotropy in tricolor 3d-5d oxide superlattices

Authors: Zengxing Lu, Jingwu Liu, Lijie Wen, Jiatai Feng, Shuai Kong, Xuan Zhen, Sheng Li, Peiheng Jiang, Zhicheng Zhong, Junfa Zhu, Xianfeng Hao, Zhiming Wang, Run-Wei Li

Abstract: Manipulating magnetic anisotropy (MA) purposefully in transition metal oxides (TMOs) enables the development of oxide-based spintronic devices with practical applications. Here, we report a pathway to reversibly switch the lateral magnetic easy-axis via interfacial oxygen octahedral coupling (OOC) effects in 3d-5d tricolor superlattices, i.e. [SrIrO3,mRTiO3,SrIrO3,2La0.67Sr0.33MnO3]10 (RTiO3: SrTi… ▽ More Manipulating magnetic anisotropy (MA) purposefully in transition metal oxides (TMOs) enables the development of oxide-based spintronic devices with practical applications. Here, we report a pathway to reversibly switch the lateral magnetic easy-axis via interfacial oxygen octahedral coupling (OOC) effects in 3d-5d tricolor superlattices, i.e. [SrIrO3,mRTiO3,SrIrO3,2La0.67Sr0.33MnO3]10 (RTiO3: SrTiO3 and CaTiO3). In the heterostructures, the anisotropy energy (MAE) is enhanced over one magnitude to ~106 erg/cm3 compared to La0.67Sr0.33MnO3 films. Moreover, the magnetic easy-axis is reversibly reoriented between (100)- and (110)-directions by changing the RTiO3. Using first-principles density functional theory calculations, we find that the SrIrO3 owns a large single-ion anisotropy due to its strong spin-orbit interaction. This anisotropy can be reversibly controlled by the OOC, then reorient the easy-axis of the superlattices. Additionally, it enlarges the MAE of the films via the cooperation with a robust orbital hybridization between the Ir and Mn atoms. Our results indicate that the tricolor superlattices consisting of 3d and 5d oxides provide a powerful platform to study the MA and develop oxide-based spintronic devices. △ Less

Submitted 5 September, 2021; originally announced September 2021.

Journal ref: ACS Applied Electronic Materials 3, 4210 (2021)

Photo-induced plasmon-phonon coupling in PbTe

Authors: M. P. Jiang, M. Trigo, S. Fahy, A. Hauber, É. D. Murray, I Savić, C. Bray, J. N. Clark, T. Henighan, M. Kozina, M. Chollet, J. M. Glownia, M. C. Hoffmann, D. Zhu, O. Delaire, A. F. May, B. C. Sales, A. M. Lindenberg, P. Zalden, T. Sato, R. Merlin, D. A. Reis

Abstract: We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray scattering at the Linac Coherent Light Source (LCLS). We measure the near-zone-center dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following ab… ▽ More We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray scattering at the Linac Coherent Light Source (LCLS). We measure the near-zone-center dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above band gap photoexcitation. △ Less

Submitted 3 September, 2021; originally announced September 2021.

Comments: 5 pages, 2 figures

The FAST Galactic Plane Pulsar Snapshot survey: I. Project design and pulsar discoveries

Authors: J. L. Han, Chen Wang, P. F. Wang, Tao Wang, D. J. Zhou, Jing-Hai Sun, Yi Yan, Wei-Qi Su, Wei-Cong Jing, Xue Chen, X. Y. Gao, Li-Gang Hou, Jun Xu, K. J. Lee, Na Wang, Peng Jiang, Ren-Xin Xu, Jun Yan, Heng-Qian Gan, Xin Guan, Wen-Jun Huang, Jin-Chen Jiang, Hui Li, Yun-Peng Men, Chun Sun , et al. (12 additional authors not shown)

Abstract: Discovery of pulsars is one of the main goals for large radio telescopes. The Five-hundred-meter Aperture Spherical radio Telescope (FAST), that incorporates an L-band 19-beam receiver with a system temperature of about 20~K, is the most sensitive radio telescope utilized for discovering pulsars. We designed the {\it snapshot} observation mode for a FAST key science project, the Galactic Plane Pul… ▽ More Discovery of pulsars is one of the main goals for large radio telescopes. The Five-hundred-meter Aperture Spherical radio Telescope (FAST), that incorporates an L-band 19-beam receiver with a system temperature of about 20~K, is the most sensitive radio telescope utilized for discovering pulsars. We designed the {\it snapshot} observation mode for a FAST key science project, the Galactic Plane Pulsar Snapshot (GPPS) survey, in which every four nearby pointings can observe {\it a cover} of a sky patch of 0.1575 square degrees through beam-switching of the L-band 19-beam receiver. The integration time for each pointing is 300 seconds so that the GPPS observations for a cover can be made in 21 minutes. The goal of the GPPS survey is to discover pulsars within the Galactic latitude of $\pm10^{\circ}$ from the Galactic plane, and the highest priority is given to the inner Galaxy within $\pm5^{\circ}$. Up to now, the GPPS survey has discovered 201 pulsars, including currently the faintest pulsars which cannot be detected by other telescopes, pulsars with extremely high dispersion measures (DMs) which challenge the currently widely used models for the Galactic electron density distribution, pulsars coincident with supernova remnants, 40 millisecond pulsars, 16 binary pulsars, some nulling and mode-changing pulsars and rotating radio transients (RRATs). The follow-up observations for confirmation of new pulsars have polarization-signals recorded for polarization profiles of the pulsars. Re-detection of previously known pulsars in the survey data also leads to significant improvements in parameters for 64 pulsars. The GPPS survey discoveries are published and will be updated at http://zmtt.bao.ac.cn/GPPS/ . △ Less

Submitted 18 May, 2021; originally announced May 2021.

Comments: 38 pages, 22 figures. See http://www.raa-journal.org/docs/Supp/2021Newsonline.pdf for News and Views

Journal ref: Research in Astronomy and Astrophysics, 2021, Vol. 21, 107

Observation of an Unusual Colossal Anisotropic Magnetoresistance Effect in an Antiferromagnetic Semiconductor

Authors: Huali Yang, Qing Liu, Zhaoliang Liao, Liang Si, Peiheng Jiang, Xiaolei Liu, Yanfeng Guo, Junjie Yin, Meng Wang, Zhigao Sheng, Yuxin Zhao, Zhiming Wang, Zhicheng Zhong, Run-Wei Li

Abstract: Searching for novel antiferromagnetic materials with large magnetotransport response is highly demanded for constructing future spintronic devices with high stability, fast switching speed, and high density. Here we report a colossal anisotropic magnetoresistance effect in an antiferromagnetic binary compound with layered structure rare-earth dichalcogenide EuTe2. The AMR reaches 40000%, which is… ▽ More Searching for novel antiferromagnetic materials with large magnetotransport response is highly demanded for constructing future spintronic devices with high stability, fast switching speed, and high density. Here we report a colossal anisotropic magnetoresistance effect in an antiferromagnetic binary compound with layered structure rare-earth dichalcogenide EuTe2. The AMR reaches 40000%, which is 4 orders of magnitude larger than that in conventional antiferromagnetic alloys. Combined magnetization, resistivity, and theoretical analysis reveal that the colossal AMR effect is attributed to a novel mechanism of vector-field tunable band structure, rather than the conventional spin-orbit coupling mechanism. Moreover, it is revealed that the strong hybridization between orbitals of Eu-layer with localized spin and Te-layer with itinerant carriers is extremely important for the large AMR effect. Our results suggest a new direction towards exploring AFM materials with prominent magnetotransport properties, which creates an unprecedented opportunity for AFM spintronics applications. △ Less

Submitted 3 March, 2021; originally announced March 2021.

Impact of Band Structure on Wave Function Dissipation in Field Emission Resonance

Authors: Wei-Bin Su, Shin-Ming Lu, Ho-Hsiang Chang, Horng-Tay Jeng, Wen-Yuan Chan, Pei-Cheng Jiang, Kung-Hsuan Lin, Chia-Seng Chang

Abstract: We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar pr… ▽ More We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar projected bulk band structures and the ensemble interpretation of quantum mechanics, according to which all resonant electrons are governed by a single wave function. Ag(100) has an energy gap above its vacuum level, whereas Ag(111) does not. Consequently, the dissipation rate of the wave function, which is relevant to the FER linewidth, on Ag(111) was almost stable, whereas that on Ag(100) fluctuated. The fluctuation revealed that the quantum trapping effect and surface dipole layer on Ag(100) surface can be investigated through FER. △ Less

Submitted 29 December, 2020; originally announced December 2020.

Comments: 16 pages, 4 figures

Formation of buried domain walls in the ultrafast transition of SmTe$_3$

Authors: M. Trigo, P. Giraldo-Gallo, J. N. Clark, M. E. Kozina, T. Henighan, M. P. Jiang, M. Chollet, I. R. Fisher, J. M. Glownia, T. Katayama, P. S. Kirchmann, D. Leuenberger, H. Liu, D. A. Reis, Z. X. Shen, D. Zhu

Abstract: We study ultrafast x-ray diffraction on the charge density wave (CDW) of SmTe$_3$ using an x-ray free electron laser. The CDW peaks show that photoexcitation with near-infrared pump centered at 800 nm generates domain walls of the order parameter propagating perpendicular to the sample surface. These domain walls break the CDW long range order and suppress the diffraction intensity of the CDW for… ▽ More We study ultrafast x-ray diffraction on the charge density wave (CDW) of SmTe$_3$ using an x-ray free electron laser. The CDW peaks show that photoexcitation with near-infrared pump centered at 800 nm generates domain walls of the order parameter propagating perpendicular to the sample surface. These domain walls break the CDW long range order and suppress the diffraction intensity of the CDW for times much longer than the $\sim 1$~ps recovery of the local electronic gap. We reconstruct the spatial and temporal dependence of the order parameter using a simple Ginzburg-Landau model and find good agreement between the experimental and model fluence dependences. Based on the model we find that at long times, depending on the pump fluence, multiple domain walls remain at distances of few nm from the surface. △ Less

Submitted 15 June, 2020; originally announced June 2020.

Journal ref: Phys. Rev. B 103, 054109 (2021)

Polarity induced electronic and atomic reconstruction at NdNiO2/SrTiO3 interfaces

Authors: Ri He, Peiheng Jiang, Yi Lu, Yidao Song, Mingxing Chen, Mingliang Jin, Lingling Shui, Zhicheng Zhong

Abstract: Superconductivity has recently been observed in Sr-doped NdNiO2 films grown on SrTiO3. Whether it is caused by or related to the interface remains an open question. To address this issue, we use density functional theory calculation and charge transfer self-consistent model to study the effects of polar discontinuity on the electronic and atomic reconstruction at the NdNiO2/SrTiO3 interface. We fi… ▽ More Superconductivity has recently been observed in Sr-doped NdNiO2 films grown on SrTiO3. Whether it is caused by or related to the interface remains an open question. To address this issue, we use density functional theory calculation and charge transfer self-consistent model to study the effects of polar discontinuity on the electronic and atomic reconstruction at the NdNiO2/SrTiO3 interface. We find that sharp interface with pure electronic reconstruction only is energetically unfavorable, and atomic reconstruction is unavoidable. We further propose a possible interface configuration that contain residual apical oxygen. These oxygen atoms lead to hybrids of dz2 and dx2-y2 states at the Fermi level, which weaken the single-band feature and may be detrimental to superconductivity. △ Less

Submitted 31 May, 2020; originally announced June 2020.

Journal ref: Phys. Rev. B 102, 035118 (2020)

Plasma-induced magnetic patterning of FePd thin films without and with exchange bias

Authors: Wei-Hsiang Wang, Po-Chun Chang, Pei-hsun Jiang, Wen-Chin Lin

Abstract: We demonstrate control of magnetic domain structures in continuous FePd thin films by patterning their surfaces with plasma treatment. The Fe-oxide layer formed on the surface upon ambient exposure of the FePd alloy thin film grown on an Al$_2$O$_3$(0001) substrate was patterned into microstructures by e-beam lithography followed by O$_2$- or Ar-plasma treatment. Microscopic pinning of magnetic do… ▽ More We demonstrate control of magnetic domain structures in continuous FePd thin films by patterning their surfaces with plasma treatment. The Fe-oxide layer formed on the surface upon ambient exposure of the FePd alloy thin film grown on an Al$_2$O$_3$(0001) substrate was patterned into microstructures by e-beam lithography followed by O$_2$- or Ar-plasma treatment. Microscopic pinning of magnetic domain walls in the thin films is then observed by magneto-optic Kerr effect microscopy, with the magnetic field needed to reverse the magnetization of the plasma-treated areas being larger than that for the untreated areas. An intriguing competition between the uniaxial anisotropy and the exchange bias is also observed in the system. This study demonstrates that patterning of the film surface with plasma treatment can be an easy and efficient method for sophisticated engineering of magnetic structures in thin films, and therefore has potential application in developing future data-storage and spintronic devices. △ Less

Submitted 29 May, 2020; originally announced May 2020.

Comments: Accepted for publication in Applied Surface Science

Electric-field-controllable high-spin SrRuO3 driven by a solid ionic junction

Authors: Jingdi Lu, Liang Si, Xiefei Yao, Chengfeng Tian, Jing Wang, Qinghua Zhang, Zhengxun Lai, Iftikhar Ahmed Malik, Xin Liu, Peiheng Jiang, Kejia Zhu, Youguo Shi, Zhenlin Luo, Lin Gu, Karsten Held, Wenbo Mi, Zhicheng Zhong, Ce-Wen Nan, Jinxing Zhang

Abstract: Controlling magnetism and spin structures in strongly correlated systems by using electric field is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chemical junction at SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers of oxygen vacancies, analogue to electronic band alignment in semiconductor heterojunction.… ▽ More Controlling magnetism and spin structures in strongly correlated systems by using electric field is of fundamental importance but challenging. Here, a high-spin ruthenate phase is achieved via a solid ionic chemical junction at SrRuO3/SrTiO3 interface with distinct formation energies and diffusion barriers of oxygen vacancies, analogue to electronic band alignment in semiconductor heterojunction. Oxygen vacancies trapped within this interfacial SrRuO3 reconstruct Ru-4d electronic structure and orbital occupancy, leading to an enhanced magnetic moment. Furthermore, an interfacial magnetic phase can be switched reversibly by electric-field-rectifying oxygen migration in a solid-state ionic gating device, providing a framework for atomic design of functionalities in strongly correlated oxides using a way of solid chemistry. △ Less

Submitted 24 March, 2020; originally announced March 2020.

Journal ref: Phys. Rev. B 101, 214401 (2020)

Tunable Anisotropic Thermal Transport in Super-Aligned Carbon Nanotube Films

Authors: Wei Yu, Xinpeng Zhao, Puqing Jiang, Changhong Liu, Ronggui Yang

Abstract: Super-aligned carbon nanotube (CNT) films have intriguing anisotropic thermal transport properties due to the anisotropic nature of individual nanotubes and the important role of nanotube alignment. However, the relationship between the alignment and the anisotropic thermal conductivities was not well understood due to the challenges in both the preparation of high-quality super-aligned CNT film s… ▽ More Super-aligned carbon nanotube (CNT) films have intriguing anisotropic thermal transport properties due to the anisotropic nature of individual nanotubes and the important role of nanotube alignment. However, the relationship between the alignment and the anisotropic thermal conductivities was not well understood due to the challenges in both the preparation of high-quality super-aligned CNT film samples and the thermal characterization of such highly anisotropic and porous thin films. Here, super-aligned CNT films with different alignment configurations are designed and their anisotropic thermal conductivities are measured using time-domain thermoreflectance (TDTR) with an elliptical-beam approach. The results suggest that the alignment configuration could tune the cross-plane thermal conductivity k_z from 6.4 to 1.5 W/mK and the in-plane anisotropic ratio from 1.2 to 13.5. This work confirms the important role of CNT alignment in tuning the thermal transport properties of super-aligned CNT films and provides an efficient way to design thermally anisotropic films for thermal management. △ Less

Submitted 4 February, 2020; originally announced February 2020.

Comments: 16 pages, 4 figures

Transient and Steady-State Temperature Rise in Three-Dimensional Anisotropic Layered Structures in Pump-Probe Thermoreflectance Experiments

Authors: Puqing Jiang, Heng Ban

Abstract: Recent developments of the pump-probe thermoreflectance methods (such as the beam-offset and elliptical-beam approaches of the time-domain and frequency-domain thermoreflectance techniques) enabled measurements of the thermal conductivities of in-plane anisotropic materials. Estimating the temperature rise of anisotropic layered structures under surface heating is critically important to make sure… ▽ More Recent developments of the pump-probe thermoreflectance methods (such as the beam-offset and elliptical-beam approaches of the time-domain and frequency-domain thermoreflectance techniques) enabled measurements of the thermal conductivities of in-plane anisotropic materials. Estimating the temperature rise of anisotropic layered structures under surface heating is critically important to make sure that the temperature rise is not too high to alias the signals in these experiments. However, a simple formula to estimate the temperature rise in three-dimensional (3D) anisotropic layered systems heated by a non-circular laser beam is not available yet, which is the main problem we aim to solve in this work. We first re-derived general formalisms of the temperature rise of a multilayered structure based on the previous literature work by solving the 3D anisotropic heat diffusion equation in the frequency domain. These general formalisms normally require laborious numerical evaluation; however, they could be reduced to explicit analytical expressions for the case of semi-infinite solids. We then extend the analytical expressions to multilayered systems, taking into account the effect of the top layers. This work not only enhances our understanding of the physics of temperature rise due to surface laser heating but also enables quick estimation of the peak temperature rise of 3D anisotropic layered systems in pump-probe thermoreflectance experiments and thus greatly benefits the thermoreflectance experiments in choosing the appropriate heating power intensity for the experiments. △ Less

Submitted 31 August, 2020; v1 submitted 3 December, 2019; originally announced December 2019.

Electronic structure of rare-earth infinite-layer ReNiO2 (Re=La, Nd)

Authors: Peiheng Jiang, Liang Si, Zhaoliang Liao, Zhicheng Zhong

Abstract: The discovery of infinite layer nickelate superconductor marks the new era in the field of superconductivity. In the rare-earth (Re) nickelates ReNiO2, although the Ni is also of d9 electronic configuration, analogous to Cu d9 in cuprates, whether electronic structures in infinite-layer nickelate are the same as cuprate and possess the single band feature as well are still open questions. To illus… ▽ More The discovery of infinite layer nickelate superconductor marks the new era in the field of superconductivity. In the rare-earth (Re) nickelates ReNiO2, although the Ni is also of d9 electronic configuration, analogous to Cu d9 in cuprates, whether electronic structures in infinite-layer nickelate are the same as cuprate and possess the single band feature as well are still open questions. To illustrate the electronic structure of rare-earth infinite-layer nickelate, we perform first principle calculations of LaNiO2 and NdNiO2 compounds and compare them with that of CaCuO2 using hybrid functional method together with Wannier projection and group symmetry analysis. Our results indicate that the Ni-dx2-y2 in the LaNiO2 has weak hybridization with other orbitals and exhibits characteristic single band feature, whereas in NdNiO2, the Nd-f orbital hybridizes with Ni-dx2-y2 and is a non-negligible ingredient for transport and even high-temperature superconductivity. Given that the Cu-dx2-y2 in cuprate strongly hybridizes with O-2p, the calculated band structures of nickelate imply some new band characters which is worth to gain more attentions. △ Less

Submitted 30 September, 2019; originally announced September 2019.

Journal ref: Phys. Rev. B 100, 201106 (2019)

Measurements of Nonequilibrium Interatomic Forces in Photoexcited Bismuth

Authors: Samuel W. Teitelbaum, Thomas C. Henighan, Hanzhe Liu, Mason P. Jiang, Diling Zhu, Matthieu Chollet, Takahiro Sato, Éamonn D. Murray, Stephen Fahy, Shane O'Mahony, Trevor P. Bailey, Ctirad Uher, Mariano Trigo, David A. Reis

Abstract: We determine experimentally the excited-state interatomic forces in photoexcited bismuth. The forces are obtained by a constrained least-squares fit of the excited-state dispersion obtained by femtosecond time-resolved x-ray diffuse scattering to a fifteen-nearest neighbor Born-von Karman model. We find that the observed softening of the zone-center $A_{1g}$ optical mode and transverse acoustic mo… ▽ More We determine experimentally the excited-state interatomic forces in photoexcited bismuth. The forces are obtained by a constrained least-squares fit of the excited-state dispersion obtained by femtosecond time-resolved x-ray diffuse scattering to a fifteen-nearest neighbor Born-von Karman model. We find that the observed softening of the zone-center $A_{1g}$ optical mode and transverse acoustic modes with photoexcitation are primarily due to a weakening of three nearest neighbor forces along the bonding direction. This provides a more complete picture of what drives the partial reversal of the Peierls distortion previously observed in photoexcited bismuth. △ Less

Submitted 20 August, 2019; originally announced August 2019.

Comments: 6 pages, 3 figures, plus 3 pages, 3 figures of supplemental information

Journal ref: Phys. Rev. B 103, 180101 (2021)

Coherent order parameter dynamics in SmTe$_3$

Authors: M. Trigo, P. Giraldo-Gallo, M. E. Kozina, T. Henighan, M. P. Jiang, H. Liu, J. N. Clark, M. Chollet, J. M. Glownia, D. Zhu, T. Katayama, D. Leuenberger, P. S. Kirchmann, I. R. Fisher, Z. X. Shen, D. A. Reis

Abstract: We present a combined ultrafast optical pump-probe and ultrafast x-ray diffraction measurement of the CDW dynamics in SmTe$_3$ at 300 K. The ultrafast x-ray diffraction measurements, taken at the Linac Coherent Light Source reveal a $\sim 1.55$ THz mode that becomes overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, these… ▽ More We present a combined ultrafast optical pump-probe and ultrafast x-ray diffraction measurement of the CDW dynamics in SmTe$_3$ at 300 K. The ultrafast x-ray diffraction measurements, taken at the Linac Coherent Light Source reveal a $\sim 1.55$ THz mode that becomes overdamped with increasing fluence. We identify this oscillation with the lattice component of the amplitude mode. Furthermore, these data allow for a more clear identification of the frequencies present in the optical pump-probe data. In both, reflectivity and diffraction, we observe a crossover of the response from linear (for small displacements) to quadratic in the amplitude of the order parameter displacement. Finally, a time-dependent Ginzburg-Landau model captures the essential features of the experimental observations. △ Less

Submitted 26 September, 2018; originally announced September 2018.

Comments: 7 pages, 5 figures

Journal ref: Phys. Rev. B 99, 104111 (2019)

Three-Dimensional Anisotropic Thermal Conductivity Tensor of Single Crystalline \b{eta}-Ga2O3

Authors: Puqing Jiang, Xin Qian, Xiaobo Li, Ronggui Yang

Abstract: \b{eta}-Ga2O3 has attracted considerable interest in recent years for high power electronics, where thermal properties of \b{eta}-Ga2O3 play a critical role. The thermal conductivity of \b{eta}-Ga2O3 is expected to be three-dimensionally (3D) anisotropic due to the monoclinic lattice structure. In this work, the 3D anisotropic thermal conductivity tensor of a (010)-oriented \b{eta}-Ga2O3 single cr… ▽ More \b{eta}-Ga2O3 has attracted considerable interest in recent years for high power electronics, where thermal properties of \b{eta}-Ga2O3 play a critical role. The thermal conductivity of \b{eta}-Ga2O3 is expected to be three-dimensionally (3D) anisotropic due to the monoclinic lattice structure. In this work, the 3D anisotropic thermal conductivity tensor of a (010)-oriented \b{eta}-Ga2O3 single crystal was measured by using a novel time-domain thermoreflectance (TDTR) method with a highly elliptical pump beam. Our measured results suggest that at room temperature, the highest in-plane thermal conductivity is along a direction between [001] and [102], with a value of 13.3+/-1.8 W/mK, and the lowest in-plane thermal conductivity is close to the [100] direction, with a value of 9.5+/-1.8 W/mK. The through-plane thermal conductivity, which is along the [010] direction, has the highest value of 22+/-2.5 W/mK among all the directions. Temperature-dependent thermal conductivity of \b{eta}-Ga2O3 was also measured and compared with a modified Callaway model calculation to understand the temperature dependence and the role of impurity scattering. △ Less

Submitted 7 November, 2018; v1 submitted 13 September, 2018; originally announced September 2018.

Comments: 14 pages, 4 figures

Journal ref: Appl. Phys. Lett. 113, 232105 (2018)