Monirul Shaikh, Alison Klein, Aleksander L. Wysocki
Electronic structure and magnetic interactions of a Tb adatom on graphene are investigated from first principles using combination of density functional theory and multiconfigurational quantum chemistry techniques including spin-orbit coupling. We determine that the six-fold symmetry hollow site is the preferred adsorption site and we investigate electronic spectrum for different adatom oxidation states including Tb$^{3+}$, Tb$^{2+}$, Tb$^{1+}$, and Tb$^{0}$. For all charge states, the Tb $4f^8$ configuration is retained with other adatom valence electrons being distributed over $5d_{xy}$, $5d_{x2+y2}$, and $6s/5d_0$ single-electron orbitals. We find strong intra-site adatom exchange coupling that ensures that the $5d6s$ spins are parallel to the $4f$ spin. For Tb$^{3+}$, the energy levels can be described by the $J=6$ multiplet split by the graphene crystal field. For other oxidation states, the interaction of $4f$ electrons with spin and orbital degrees of freedom of $6s5d$ electrons in the presence of spin-orbit coupling results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene crystal field. Stable magnetic moment is predicted for Tb$^{3+}$ and Tb$^{2+}$ adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm$^{-1}$ controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb$^{1+}$ and Tb$^{0}$ adatoms. Our results indicate that the occupation of the $6s5d$ orbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms.
{"title":"Low-Energy Magnetic States of Tb Adatom on Graphene","authors":"Monirul Shaikh, Alison Klein, Aleksander L. Wysocki","doi":"arxiv-2409.08079","DOIUrl":"https://doi.org/arxiv-2409.08079","url":null,"abstract":"Electronic structure and magnetic interactions of a Tb adatom on graphene are\u0000investigated from first principles using combination of density functional\u0000theory and multiconfigurational quantum chemistry techniques including\u0000spin-orbit coupling. We determine that the six-fold symmetry hollow site is the\u0000preferred adsorption site and we investigate electronic spectrum for different\u0000adatom oxidation states including Tb$^{3+}$, Tb$^{2+}$, Tb$^{1+}$, and\u0000Tb$^{0}$. For all charge states, the Tb $4f^8$ configuration is retained with\u0000other adatom valence electrons being distributed over $5d_{xy}$, $5d_{x2+y2}$,\u0000and $6s/5d_0$ single-electron orbitals. We find strong intra-site adatom\u0000exchange coupling that ensures that the $5d6s$ spins are parallel to the $4f$\u0000spin. For Tb$^{3+}$, the energy levels can be described by the $J=6$ multiplet\u0000split by the graphene crystal field. For other oxidation states, the\u0000interaction of $4f$ electrons with spin and orbital degrees of freedom of\u0000$6s5d$ electrons in the presence of spin-orbit coupling results in the\u0000low-energy spectrum composed closely lying effective multiplets that are split\u0000by the graphene crystal field. Stable magnetic moment is predicted for\u0000Tb$^{3+}$ and Tb$^{2+}$ adatoms due to uniaxial magnetic anisotropy and\u0000effective anisotropy barrier around 440 cm$^{-1}$ controlled by the temperature\u0000assisted quantum tunneling of magnetization through the third excited doublet.\u0000On the other hand, in-plane magnetic anisotropy is found for Tb$^{1+}$ and\u0000Tb$^{0}$ adatoms. Our results indicate that the occupation of the $6s5d$\u0000orbitals can dramatically affect the magnetic anisotropy and magnetic moment\u0000stability of rare earth adatoms.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142224493","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Moir'e lattices have served as the ideal quantum simulation platform for exploring novel physics due to the flat electronic bands resulting from the long wavelength moir'e potentials. However, the large sizes of this type of system challenge the first-principles methods for full calculations of their electronic structures, thus bringing difficulties in understanding the nature and evolution of the flat bands. In this study, we investigate the electronic structures of moir'e patterns of MoSe$_2$/WSe$_2$ by combining ab initio and machine learning methods. We find that a flat band with a bandwidth of about 5 meV emerges below the valence band edge at the K point for the H-stacking at a twist angle of 3.89$^{circ}$. Then, it shifts dramatically as the twist angle decreases and becomes about 20 meV higher than the valence band maximum for the twist angle of 3.15$^{circ}$. Multiple ultra-flat bands emerge as the twist angle is reduced to 1.7$^{circ}$. Band unfolding reveals that the ultra-flat bands formed by the $Gamma$ and K valleys show distinct behaviors. The $Gamma$-valley flat bands are sensitive to the interlayer coupling, thus experiencing dramatic changes as the twist angle decreases. In contrast, the K-valley flat band, which shows a weak dependence on the interlayer coupling, is mainly modulated by the long wavelength moir'e potential. Therefore, a relatively small angle (2.13$^{circ}$) is required to generate the K-valley flat band, which experiences a transition from the honeycomb to the triangular lattice as the twist angle decreases.
由于长波长摩尔电势产生的平坦电子带，摩尔晶格已成为探索新物理学的理想量子模拟平台。然而，这类系统的巨大尺寸对全面计算其电子结构的第一性原理方法提出了挑战，从而给理解平坦带的性质和演化带来了困难。在本研究中，我们结合 ab initio 和机器学习方法研究了 MoSe$_2$/WSe$_2$ 的 Moir'e 模式的电子结构。我们发现，在扭曲角度为 3.89$^{circ}$ 时，H-叠层在 K 点价带边缘下方出现了一个带宽约为 5meV 的平带。然后，它随着扭曲角度的减小而急剧移动，并在扭曲角度为 3.15$^{circ}$ 时比价带最大值高出约 20 meV。当扭转角减小到 1.7$^{circ}$ 时，出现了多个超平带。波段展开显示，由 $Gamma$ 和 K 谷形成的超扁平波段表现出截然不同的行为。Gamma谷平带对层间耦合很敏感，随着扭转角的减小而发生剧烈变化。相比之下，K 谷平带对层间耦合的依赖性较弱，主要受长波长摩尔电势的调制。因此，产生 K 谷平带需要很小的角度（2.13^{circ}$），随着扭转角的减小，K 谷平带经历了从蜂巢到三角晶格的转变。
{"title":"Evolution of flat bands in MoSe$_2$/WSe$_2$ moiré lattices: A study combining machine learning and band unfolding methods","authors":"Shengguo Yang, Jiaxin Chen, Chao-Fei Liu, Mingxing Chen","doi":"arxiv-2409.07987","DOIUrl":"https://doi.org/arxiv-2409.07987","url":null,"abstract":"Moir'e lattices have served as the ideal quantum simulation platform for\u0000exploring novel physics due to the flat electronic bands resulting from the\u0000long wavelength moir'e potentials. However, the large sizes of this type of\u0000system challenge the first-principles methods for full calculations of their\u0000electronic structures, thus bringing difficulties in understanding the nature\u0000and evolution of the flat bands. In this study, we investigate the electronic\u0000structures of moir'e patterns of MoSe$_2$/WSe$_2$ by combining ab initio and\u0000machine learning methods. We find that a flat band with a bandwidth of about 5\u0000meV emerges below the valence band edge at the K point for the H-stacking at a\u0000twist angle of 3.89$^{circ}$. Then, it shifts dramatically as the twist angle\u0000decreases and becomes about 20 meV higher than the valence band maximum for the\u0000twist angle of 3.15$^{circ}$. Multiple ultra-flat bands emerge as the twist\u0000angle is reduced to 1.7$^{circ}$. Band unfolding reveals that the ultra-flat\u0000bands formed by the $Gamma$ and K valleys show distinct behaviors. The\u0000$Gamma$-valley flat bands are sensitive to the interlayer coupling, thus\u0000experiencing dramatic changes as the twist angle decreases. In contrast, the\u0000K-valley flat band, which shows a weak dependence on the interlayer coupling,\u0000is mainly modulated by the long wavelength moir'e potential. Therefore, a\u0000relatively small angle (2.13$^{circ}$) is required to generate the K-valley\u0000flat band, which experiences a transition from the honeycomb to the triangular\u0000lattice as the twist angle decreases.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142188158","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
A. Tikhanovskii, V. Yu. Ivanov, A. Kuzmenko, E. Constable, A. Pimenov, A. Mukhin
Magnetic field-induced electric polarization has been observed in trigonal non-centrosymmetric paramagnetic Pr3Ga5SiO14 langasite. We detected quadratic electric polarization along the a-axis in the basal ab plane for various magnetic-field orientations. Electric polarization along the c-axis is only evident starting from the fourth power of magnetic field, in accordance with the trigonal symmetry. The magnetic properties of Pr3Ga5SiO14 primarily stem from the local anisotropic magnetic moment of the two lowest Pr3+ singlets (quasi-doublet) in the crystal electric field. The random distribution of Ga/Si in the 2d positions leads to a local distortion of C2 symmetry and to a splitting distribution of the quasi-doublet. By considering the interactions of local moments among different Pr3+ positions within a phenomenological approach for the allowed magnetoelectric coupling, we derive the electric polarization in terms of symmetry-allowed combinations of local magnetic susceptibilities and field components. The magnetic field dependence of electric polarization in the basal plane, P_{a,b*}, is mainly determined by the accumulation of effective local susceptibilities, exhibiting similar behavior in low fields, while polarization along the c-axis, P_c, arises from the non-equivalence of local effective magnetic susceptibilities in different Pr3+ positions. Our findings suggest that the temperature dependencies of magnetic and magnetoelectric susceptibilities are highly sensitive to the distribution of the quasi-doublet splitting, which reflects the local symmetry breaking.
在三方非中心对称顺磁 Pr3Ga5SiO14 长石中观察到了磁场诱导的电极化。我们在不同磁场方向的基底 ab 平面上检测到了沿 a 轴的四级电极化。只有从磁场的四次方开始才会出现沿 c 轴的电极化，这与三重对称性相符。Pr3Ga5SiO14 的磁性主要源于晶体电场中两个最低的 Pr3+ 单晶体（准双晶体）的局部各向异性磁矩。Ga/Si 在 2d 位置的随机分布导致了 C2 对称性的局部变形和准双子的分裂分布。通过在允许磁电耦合的现象学方法中考虑不同 Pr3+ 位置之间局部矩的相互作用，我们从对称性允许的局部磁感应强度和磁场分量的组合中推导出了电极化。基底面电极化 P_{a,b*} 的磁场依赖性主要由局部有效磁感应强度的累积决定，在低磁场中表现出相似的行为，而沿 c 轴的极化 P_c 则源于不同 Pr3+ 位置的局部有效磁感应强度的非等效性。我们的发现表明，磁感应强度和磁电感应强度的温度依赖性对准双分裂的分布非常敏感，而准双分裂反映了局部对称性的破缺。
{"title":"The effect of local distortions on magnetic and magnetoelectric properties of paramagnetic Pr3Ga5SiO14 langasite","authors":"A. Tikhanovskii, V. Yu. Ivanov, A. Kuzmenko, E. Constable, A. Pimenov, A. Mukhin","doi":"arxiv-2409.07897","DOIUrl":"https://doi.org/arxiv-2409.07897","url":null,"abstract":"Magnetic field-induced electric polarization has been observed in trigonal\u0000non-centrosymmetric paramagnetic Pr3Ga5SiO14 langasite. We detected quadratic\u0000electric polarization along the a-axis in the basal ab plane for various\u0000magnetic-field orientations. Electric polarization along the c-axis is only\u0000evident starting from the fourth power of magnetic field, in accordance with\u0000the trigonal symmetry. The magnetic properties of Pr3Ga5SiO14 primarily stem\u0000from the local anisotropic magnetic moment of the two lowest Pr3+ singlets\u0000(quasi-doublet) in the crystal electric field. The random distribution of Ga/Si\u0000in the 2d positions leads to a local distortion of C2 symmetry and to a\u0000splitting distribution of the quasi-doublet. By considering the interactions of\u0000local moments among different Pr3+ positions within a phenomenological approach\u0000for the allowed magnetoelectric coupling, we derive the electric polarization\u0000in terms of symmetry-allowed combinations of local magnetic susceptibilities\u0000and field components. The magnetic field dependence of electric polarization in\u0000the basal plane, P_{a,b*}, is mainly determined by the accumulation of\u0000effective local susceptibilities, exhibiting similar behavior in low fields,\u0000while polarization along the c-axis, P_c, arises from the non-equivalence of\u0000local effective magnetic susceptibilities in different Pr3+ positions. Our\u0000findings suggest that the temperature dependencies of magnetic and\u0000magnetoelectric susceptibilities are highly sensitive to the distribution of\u0000the quasi-doublet splitting, which reflects the local symmetry breaking.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142188160","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Tyler C. Sterlinga, Feng Ye, Seohyeon Jo, Anish Parulekar, Yu Zhang, Gang Cao, Rishi Raj, Dmitry Reznik
In-situ diffuse neutron scattering experiments revealed that when electric current is passed through single crystals of rutile TiO2 under conditions conducive to flash sintering, it induces the formation of parallel planes of oxygen vacancies. Specifically, a current perpendicular to the c-axis generates planes normal to the (132) reciprocal lattice vector, whereas currents aligned with the c-axis form planes normal to the (132) and to the (225) vector. The concentration of defects increases with incresing current. The structural modifications are linked to the appearance of signatures of interacting Ti3+ moments in magnetic susceptibility, signifying a structural collapse around the vacancy planes. Electrical conductivity measurements of the modified material reveal several electronic transitions between semiconducting states (via a metal-like intermediate state) with the smallest gap being 27 meV. Pristine TiO2 can be restored by heating followed by slow cooling in air. Our work suggests a novel paradigm for achieving switching of electrical conductivity related to the flash phenomenon
原位弥散中子散射实验表明，当电流在适合闪烁烧结的条件下通过金红石二氧化钛单晶时，会诱导氧空位平行平面的形成。具体来说，垂直于 c 轴的电流会产生法线指向 (132) 倒晶格矢量的平面，而与 c 轴对齐的电流则会形成法线指向 (132) 和 (225) 矢量的平面。缺陷的浓度随着电流的增大而增加。结构改性与磁感应强度中出现的相互作用 Ti3+ 标记有关，这表明空位平面周围出现了结构坍塌。对改性材料的电导率测量揭示了半导体态（通过类金属中间态）之间的若干电子跃迁，最小间隙为 27 meV。通过在空气中加热并缓慢冷却，可以恢复原始二氧化钛。我们的工作为实现与闪烁现象有关的导电性切换提供了一种新的范例
{"title":"Structural and electronic transformations in TiO2 induced by electric current","authors":"Tyler C. Sterlinga, Feng Ye, Seohyeon Jo, Anish Parulekar, Yu Zhang, Gang Cao, Rishi Raj, Dmitry Reznik","doi":"arxiv-2409.08223","DOIUrl":"https://doi.org/arxiv-2409.08223","url":null,"abstract":"In-situ diffuse neutron scattering experiments revealed that when electric\u0000current is passed through single crystals of rutile TiO2 under conditions\u0000conducive to flash sintering, it induces the formation of parallel planes of\u0000oxygen vacancies. Specifically, a current perpendicular to the c-axis generates\u0000planes normal to the (132) reciprocal lattice vector, whereas currents aligned\u0000with the c-axis form planes normal to the (132) and to the (225) vector. The\u0000concentration of defects increases with incresing current. The structural\u0000modifications are linked to the appearance of signatures of interacting Ti3+\u0000moments in magnetic susceptibility, signifying a structural collapse around the\u0000vacancy planes. Electrical conductivity measurements of the modified material\u0000reveal several electronic transitions between semiconducting states (via a\u0000metal-like intermediate state) with the smallest gap being 27 meV. Pristine\u0000TiO2 can be restored by heating followed by slow cooling in air. Our work\u0000suggests a novel paradigm for achieving switching of electrical conductivity\u0000related to the flash phenomenon","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142188154","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Jesper Byggmästar, Flyura Djurabekova, Kai Nordlund
Refractory high-entropy alloys show promising resistance to irradiation, yet little is known about the fundamental nature of radiation-induced defect formation. Here, we simulate threshold displacement energies in equiatomic MoNbTaVW using an accurate machine-learned interatomic potential, covering the full angular space of crystal directions. The effects of local chemical ordering is assessed by comparing results in randomly ordered and short-range-ordered MoNbTaVW. The average threshold displacement energy in the random alloy is $44.3 pm 0.15$ eV and slightly higher, $48.6 pm 0.15$ eV, in the short-range-ordered alloy. Both are significantly lower than in any of the constituent pure metals. We identify the mechanisms of defect creation and find that they are mainly dependent on the masses of the recoiling and colliding elements. Low thresholds are generally found when heavy atoms (W, Ta) displace and replace the lightest atoms (V). The average threshold energies when separated by recoiling element are consequently ordered inversely according to their mass, opposite to the trend in the pure metals where W has by far the highest thresholds. However, the trend in the alloy is reversed when considering the cross sections for defect formation in electron irradiation, due to the mass-dependent recoil energies from the electrons.
{"title":"Threshold displacement energies in refractory high-entropy alloys","authors":"Jesper Byggmästar, Flyura Djurabekova, Kai Nordlund","doi":"arxiv-2409.08030","DOIUrl":"https://doi.org/arxiv-2409.08030","url":null,"abstract":"Refractory high-entropy alloys show promising resistance to irradiation, yet\u0000little is known about the fundamental nature of radiation-induced defect\u0000formation. Here, we simulate threshold displacement energies in equiatomic\u0000MoNbTaVW using an accurate machine-learned interatomic potential, covering the\u0000full angular space of crystal directions. The effects of local chemical\u0000ordering is assessed by comparing results in randomly ordered and\u0000short-range-ordered MoNbTaVW. The average threshold displacement energy in the\u0000random alloy is $44.3 pm 0.15$ eV and slightly higher, $48.6 pm 0.15$ eV, in\u0000the short-range-ordered alloy. Both are significantly lower than in any of the\u0000constituent pure metals. We identify the mechanisms of defect creation and find\u0000that they are mainly dependent on the masses of the recoiling and colliding\u0000elements. Low thresholds are generally found when heavy atoms (W, Ta) displace\u0000and replace the lightest atoms (V). The average threshold energies when\u0000separated by recoiling element are consequently ordered inversely according to\u0000their mass, opposite to the trend in the pure metals where W has by far the\u0000highest thresholds. However, the trend in the alloy is reversed when\u0000considering the cross sections for defect formation in electron irradiation,\u0000due to the mass-dependent recoil energies from the electrons.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142188157","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
In this study, we considered a large set of materials that are closely related to orthorhombic Fe$_3$C (cementite) with the aim of characterizing trends in their intrinsic magnetic properties and identifying alloys that are optimal for applications. A comprehensive analysis was conducted on the full concentration ranges of hexagonal ($epsilon$) and orthorhombic ($theta$) phases of (Fe-Co)$_3$C, (Fe-Co)$_3$(B-C), (Fe-Co)$_3$(C-N), and their alloys with 3$d$, 4$d$ and 5$d$ transition metals. The calculations were performed using the density functional theory implemented in the full-potential local-orbital code (FPLO). Calculated properties included formation energies, Curie temperatures, magnetic moments, magnetocrystalline anisotropy energies (MAE), and magnetic hardnesses. The considered compositions exhibit a range of magnetic properties, including soft, semi-hard, and hard magnetic. The materials most promising for hard-magnetic applications are orthorhombic Co$_3$C compound, together with selected Co-rich orthorhombic (Fe,Co)$_3$C and hexagonal (Fe,Co)$_3$C alloys. The calculation results do not indicate that substituting with transition metals increases the potential of the alloys for permanent magnet applications. A significant drawback of alloying orthorhombic $theta$-Fe$_3$C (cementite) with transition metals is the notable decline in the Curie temperature. We found that a considerable proportion of the orthorhombic Co$_3$(B-C-N) alloys are magnetically hard, of which boron substitution raises the Curie temperature and improves stability. By mapping the dependence of MAE on the concentration of elements covering both the 3$d$ (from Fe to Co) and 2$p$ (from B, through C, to N) positions, we have demonstrated for the first time the near isoelectronic nature of MAE. The latter observation may be particularly useful in designing compositions of new magnetically hard materials.
在这项研究中，我们考虑了大量与正方体 Fe$_3$C（水泥石）密切相关的材料，目的是描述它们内在磁性能的变化趋势，并确定最适合应用的合金。对(Fe-Co)$_3$C、(Fe-Co)$_3$(B-C)、(Fe-Co)$_3$(C-N)的六方相（$epsilon$）和正方相（$theta$）以及它们与 3d、4d 和 5d 过渡金属的合金的全浓度范围进行了全面分析。计算采用了全势能局域轨道代码（FPLO）中的密度泛函理论。计算得出的特性包括形成能、居里温度、磁矩、磁晶各向异性能（MAE）和磁硬度。所考虑的成分表现出一系列磁性能，包括软磁、半硬磁和硬磁。最有希望用于硬磁性应用的材料是正交钴$_3$C 化合物，以及选定的富钴正交（Fe,Co）$_3$C 和六方（Fe,Co）$_3$C 合金。计算结果并没有表明添加过渡金属会增加合金在永磁应用中的潜力。将正交θ-Fe$_3$C（雪明石）与过渡金属进行合金化的一个显著缺点是居里温度明显下降。我们发现，相当一部分正方晶 Co$_3$(B-C-N)合金具有磁硬性，其中硼的加入可提高居里温度并改善稳定性。通过绘制 MAE 与涵盖 3$d$（从铁到钴）和 2$p$（从 B 到 C 再到 N）位置的元素浓度的关系图，我们首次证明了 MAE 的近似等电子性质。这一观察结果可能对设计新的磁性硬材料成分特别有用。
{"title":"Magnetic hardness of hexagonal and orthorhombic Fe$_{3}$C, Co$_{3}$C, (Fe-Co)$_{3}$C, and their alloys with boron, nitrogen, and transition metals: A first-principles study","authors":"Justyn Snarski-Adamski, Mirosław Werwiński, Justyna Rychły-Gruszecka","doi":"arxiv-2409.07058","DOIUrl":"https://doi.org/arxiv-2409.07058","url":null,"abstract":"In this study, we considered a large set of materials that are closely\u0000related to orthorhombic Fe$_3$C (cementite) with the aim of characterizing\u0000trends in their intrinsic magnetic properties and identifying alloys that are\u0000optimal for applications. A comprehensive analysis was conducted on the full\u0000concentration ranges of hexagonal ($epsilon$) and orthorhombic ($theta$)\u0000phases of (Fe-Co)$_3$C, (Fe-Co)$_3$(B-C), (Fe-Co)$_3$(C-N), and their alloys\u0000with 3$d$, 4$d$ and 5$d$ transition metals. The calculations were performed\u0000using the density functional theory implemented in the full-potential\u0000local-orbital code (FPLO). Calculated properties included formation energies,\u0000Curie temperatures, magnetic moments, magnetocrystalline anisotropy energies\u0000(MAE), and magnetic hardnesses. The considered compositions exhibit a range of\u0000magnetic properties, including soft, semi-hard, and hard magnetic. The\u0000materials most promising for hard-magnetic applications are orthorhombic\u0000Co$_3$C compound, together with selected Co-rich orthorhombic (Fe,Co)$_3$C and\u0000hexagonal (Fe,Co)$_3$C alloys. The calculation results do not indicate that\u0000substituting with transition metals increases the potential of the alloys for\u0000permanent magnet applications. A significant drawback of alloying orthorhombic\u0000$theta$-Fe$_3$C (cementite) with transition metals is the notable decline in\u0000the Curie temperature. We found that a considerable proportion of the\u0000orthorhombic Co$_3$(B-C-N) alloys are magnetically hard, of which boron\u0000substitution raises the Curie temperature and improves stability. By mapping\u0000the dependence of MAE on the concentration of elements covering both the 3$d$\u0000(from Fe to Co) and 2$p$ (from B, through C, to N) positions, we have\u0000demonstrated for the first time the near isoelectronic nature of MAE. The\u0000latter observation may be particularly useful in designing compositions of new\u0000magnetically hard materials.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142187916","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Ferroelectrics are widely used for a broad array of technological applications due to their attractive electrical and electromechanical properties. In order to obtain large functional properties, material compositions are often designed to favor a coexistence of multiple ferroelectric phases. For such compositions, the macroscopically observed enhanced properties are variously attributed to easier domain switching and/or phase transition. Nevertheless, modelling of concurrent domain switching and phase transition in mixed phase ferroelectrics remains a challenging task. Here, a methodology is presented to quantitatively evaluate the volume fractions of different domain variants in a mixed phase ferroelectric under complex electromechanical loading. The methodology combines the phenomenology of Landau free energy of ferroelectric phases with Boltzmann statistical analysis, and is presented for Pb(Zr,Ti)O3 near morphotropic phase boundary (MPB). It is shown that specific grain orientation has a significant effect on how proximity to phase boundary affects microscopic phenomena, and consequently functional responses.
{"title":"A Boltzmann statistical approach for the analysis of polarization states in mixed phase ferroelectric materials","authors":"Abhijit Pramanick, Laurent Daniel","doi":"arxiv-2409.07177","DOIUrl":"https://doi.org/arxiv-2409.07177","url":null,"abstract":"Ferroelectrics are widely used for a broad array of technological\u0000applications due to their attractive electrical and electromechanical\u0000properties. In order to obtain large functional properties, material\u0000compositions are often designed to favor a coexistence of multiple\u0000ferroelectric phases. For such compositions, the macroscopically observed\u0000enhanced properties are variously attributed to easier domain switching and/or\u0000phase transition. Nevertheless, modelling of concurrent domain switching and\u0000phase transition in mixed phase ferroelectrics remains a challenging task.\u0000Here, a methodology is presented to quantitatively evaluate the volume\u0000fractions of different domain variants in a mixed phase ferroelectric under\u0000complex electromechanical loading. The methodology combines the phenomenology\u0000of Landau free energy of ferroelectric phases with Boltzmann statistical\u0000analysis, and is presented for Pb(Zr,Ti)O3 near morphotropic phase boundary\u0000(MPB). It is shown that specific grain orientation has a significant effect on\u0000how proximity to phase boundary affects microscopic phenomena, and consequently\u0000functional responses.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142187915","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
We study the Berry curvature and Chern number of a non-collinear spin state on a honeycomb lattice that evolves from coplanar to ferromagnetic with a magnetic field applied along the $z$ axis. The coplanar state is stabilized by nearest-neighbor ferromagnetic interactions, single-ion anisotropy along $z$, and Dzyalloshinskii-Moriya interactions between next-nearest neighbor sites. Below the critical field $H_c$ that aligns the spins, the magnetic unit cell contains $M=6$ sites and the spin dynamics contains six magnon subbands. Although the classical energy is degenerate wrt the twist angle $phi $ between nearest-neighbor spins, the dependence of the free energy on $phi $ at low temperatures is dominated by the magnon zero-point energy, which contains extremum at $phi =pi l/3$ for integer $l$. The only unique ground states GS($phi )$ have $l=0$ or 1. For $H < H_c'$, the zero-point energy has minima at even $l$ and the ground state is GS(0). For $H_c' < H < H_c$, the zero-point energy has minima at odd $l$ and the ground state is GS($pi/3$). In GS(0), the magnon density-of-states exhibits five distinct phases with increasing field associated with the opening and closing of energy gaps between the two or three magnonic bands, each containing between 1 and 4 four magnon subbands. While the Berry curvature vanishes for the coplanar $phi=0$ phase in zero field, the Berry curvature and Chern numbers exhibit signatures of the five phases at nonzero fields below $H_c'$. If $phi ne pi l/3$, the Chern numbers of the two or three magnonic bands are non-integer. We also evaluate the inelastic neutron-scattering spectrum $S(vk ,omega )$ produced by the six magnon subbands in all five phases of GS(0) and in GS($pi/3$).
{"title":"Topological and Magnetic Properties of a Non-collinear Spin State on a Honeycomb Lattice in a Magnetic Field","authors":"Randy S. Fishman, Daniel M. Pajerowski","doi":"arxiv-2409.07319","DOIUrl":"https://doi.org/arxiv-2409.07319","url":null,"abstract":"We study the Berry curvature and Chern number of a non-collinear spin state\u0000on a honeycomb lattice that evolves from coplanar to ferromagnetic with a\u0000magnetic field applied along the $z$ axis. The coplanar state is stabilized by\u0000nearest-neighbor ferromagnetic interactions, single-ion anisotropy along $z$,\u0000and Dzyalloshinskii-Moriya interactions between next-nearest neighbor sites.\u0000Below the critical field $H_c$ that aligns the spins, the magnetic unit cell\u0000contains $M=6$ sites and the spin dynamics contains six magnon subbands.\u0000Although the classical energy is degenerate wrt the twist angle $phi $ between\u0000nearest-neighbor spins, the dependence of the free energy on $phi $ at low\u0000temperatures is dominated by the magnon zero-point energy, which contains\u0000extremum at $phi =pi l/3$ for integer $l$. The only unique ground states\u0000GS($phi )$ have $l=0$ or 1. For $H < H_c'$, the zero-point energy has minima\u0000at even $l$ and the ground state is GS(0). For $H_c' < H < H_c$, the zero-point\u0000energy has minima at odd $l$ and the ground state is GS($pi/3$). In GS(0), the\u0000magnon density-of-states exhibits five distinct phases with increasing field\u0000associated with the opening and closing of energy gaps between the two or three\u0000magnonic bands, each containing between 1 and 4 four magnon subbands. While the\u0000Berry curvature vanishes for the coplanar $phi=0$ phase in zero field, the\u0000Berry curvature and Chern numbers exhibit signatures of the five phases at\u0000nonzero fields below $H_c'$. If $phi ne pi l/3$, the Chern numbers of the\u0000two or three magnonic bands are non-integer. We also evaluate the inelastic\u0000neutron-scattering spectrum $S(vk ,omega )$ produced by the six magnon\u0000subbands in all five phases of GS(0) and in GS($pi/3$).","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142187914","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Florian P. Lindner, Nina Strasser, Martin Schultze, Sandro Wieser, Christian Slugovc, Kareem Elsayad, Kristie J. Koski, Egbert Zojer, Caterina Czibula
The mechanical properties of metal-organic frameworks (MOFs) are of high fundamental and also practical relevance. A particularly intriguing technique for determining anisotropic elastic tensors is Brillouin scattering, which so far has rarely been used for highly complex materials like MOFs. In the present contribution, we apply this technique to study a newly synthesized MOF-type material, referred to as GUT2. We show that when combining the experiments with state-of-the-art simulations of elastic properties and phonon bands (based on machine-learned force fields and dispersion-corrected density-functional theory). This provides a comprehensive understanding of the experimental signals, which are correlated with the longitudinal and transverse sound velocities. Moreover, even when dealing with comparably small single crystals, which limit the range of accessible experimental data, combining the insights from simulations and experiments allows the determination of approximate values for the components of the elastic tensor of the studied material.
{"title":"Combining Brillouin spectroscopy and machine learned interatomic potentials to probe mechanical properties of metal organic frameworks","authors":"Florian P. Lindner, Nina Strasser, Martin Schultze, Sandro Wieser, Christian Slugovc, Kareem Elsayad, Kristie J. Koski, Egbert Zojer, Caterina Czibula","doi":"arxiv-2409.07039","DOIUrl":"https://doi.org/arxiv-2409.07039","url":null,"abstract":"The mechanical properties of metal-organic frameworks (MOFs) are of high\u0000fundamental and also practical relevance. A particularly intriguing technique\u0000for determining anisotropic elastic tensors is Brillouin scattering, which so\u0000far has rarely been used for highly complex materials like MOFs. In the present\u0000contribution, we apply this technique to study a newly synthesized MOF-type\u0000material, referred to as GUT2. We show that when combining the experiments with\u0000state-of-the-art simulations of elastic properties and phonon bands (based on\u0000machine-learned force fields and dispersion-corrected density-functional\u0000theory). This provides a comprehensive understanding of the experimental\u0000signals, which are correlated with the longitudinal and transverse sound\u0000velocities. Moreover, even when dealing with comparably small single crystals,\u0000which limit the range of accessible experimental data, combining the insights\u0000from simulations and experiments allows the determination of approximate values\u0000for the components of the elastic tensor of the studied material.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142187917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Nicholas Beaver, Aniruddha Dive, Marina Wong, Keita Shimanuki, Ananya Patil, Anthony Ferrell, Mohsen B. Kivy
In an effort to develop a rapid, reliable, and cost-effective method for predicting the structure of single-phase high entropy alloys, a Graph Neural Network (ALIGNN-FF) based approach was introduced. This method was successfully tested on 132 different high entropy alloys, and the results were analyzed and compared with density functional theory and valence electron concentration calculations. Additionally, the effects of various factors, including lattice parameters and the number of supercells with unique atomic configurations, on the prediction accuracy were investigated. The ALIGNN-FF based approach was subsequently used to predict the structure of a novel cobalt-free 3d high entropy alloy, and the result was experimentally verified.
{"title":"Rapid Assessment of Stable Crystal Structures in Single Phase High Entropy Alloys Via Graph Neural Network Based Surrogate Modelling","authors":"Nicholas Beaver, Aniruddha Dive, Marina Wong, Keita Shimanuki, Ananya Patil, Anthony Ferrell, Mohsen B. Kivy","doi":"arxiv-2409.07664","DOIUrl":"https://doi.org/arxiv-2409.07664","url":null,"abstract":"In an effort to develop a rapid, reliable, and cost-effective method for\u0000predicting the structure of single-phase high entropy alloys, a Graph Neural\u0000Network (ALIGNN-FF) based approach was introduced. This method was successfully\u0000tested on 132 different high entropy alloys, and the results were analyzed and\u0000compared with density functional theory and valence electron concentration\u0000calculations. Additionally, the effects of various factors, including lattice\u0000parameters and the number of supercells with unique atomic configurations, on\u0000the prediction accuracy were investigated. The ALIGNN-FF based approach was\u0000subsequently used to predict the structure of a novel cobalt-free 3d high\u0000entropy alloy, and the result was experimentally verified.","PeriodicalId":501234,"journal":{"name":"arXiv - PHYS - Materials Science","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-09-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142224520","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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