Room-Temperature Ferromagnetism in Epitaxial Bilayer FeSb/SrTiO3(001) Terminated with a Kagome Lattice.

Two-dimensional (2D) magnets exhibit unique physical properties for potential applications in spintronics. To date, most 2D ferromagnets are obtained by mechanical exfoliation of bulk materials with van der Waals interlayer interactions, and the synthesis of single- or few-layer 2D ferromagnets with strong interlayer coupling remains experimentally challenging. Here, we report the epitaxial growth of 2D non-van der Waals ferromagnetic bilayer FeSb on SrTiO3(001) substrates stabilized by strong coupling to the substrate, which exhibits in-plane magnetic anisotropy and a Curie temperature above 390 K. In situ low-temperature scanning tunneling microscopy/spectroscopy and density-functional theory calculations further reveal that an Fe Kagome layer terminates the bilayer FeSb. Our results open a new avenue for further exploring emergent quantum phenomena from the interplay of ferromagnetism and topology for application in spintronics.

A valence balancing rule for the design of bimetallic phosphides targeting high thermoelectric performance.

Two-dimensional (2D) materials with outstanding electronic and mechanical properties have attracted considerable attention as efficient thermoelectric materials. Here, we propose a generalized eight-valence electron rule for designing 2D semiconductor materials, i.e., metal-shrouded bimetallic phosphides ABP (A: group IA element, B: group IIA element). Following this rule, we screen out ten stable semiconductors (LiMgP, LiCaP, LiSrP, NaBeP, NaMgP, KMgP, KCaP, RbMgP, RbCaP and RbSrP) with tunable bandgaps in the range of 0.35-2.40 eV by comprehensive first-principles calculations. Among them, the electron mobility of RbMgP can be as high as 2.3 × 104 cm2 V-1 s-1, and the hole mobility of KMgP is estimated to be 9.9 × 103 cm2 V-1 s-1. Moreover, KMgP, KCaP, RbCaP and RbSrP exhibit an ultralow thermal conductivity of 0.02, 0.14, 0.08 and 0.14 W m-1 K-1, respectively. As a result, KMgP and RbCaP monolayers are p-type or n-type thermoelectric materials with a figure of merit of 2.25 and 1.13 at room temperature, respectively. The underlying mechanism of high electron conductivity and low thermal conductivity has been correlated with their unique bonding characteristics, narrow phonon band gap and the scattering from low-frequency phonons. This work demonstrates not only a guiding electron principle to design stable 2D semiconductors, but also a powerful metal-shrouded strategy for discovering high performance thermoelectric materials by decoupling electronic and thermal transport properties.

HIV/AIDS Knowledge, Depression, and HIV-Related Stigma Among Elderly Men in Rural China: A Hierarchical Regression Analysis.

This study explored the current status and influencing factors of HIV-related stigma among elderly men (≥50 years old) in rural Chengdu, China. A structured face-to-face interview survey was conducted among 286 elderly males from three towns in Chengdu using convenience sampling, 240 men (83.9%) who had heard of HIV/AIDS were included in the analysis. Hierarchical regression was used to examine the associated factors of HIV-related stigma, including demographic variables, HIV/AIDS knowledge level, receiving HIV/AIDS-related health education in the past year, depression, and anxiety, and to examine the moderating effect of educational level on HIV/AIDS knowledge and HIV-related stigma. Hierarchical regression analysis showed that men with lower HIV/AIDS knowledge scores, primary school or below educated, and depression had higher HIV-related stigma total score and social stigma dimensional scores than their counterparts. In addition, living alone was associated with higher HIV-related stigma, and elderly men with lower monthly income and those without HIV/AIDS-related health education in the past year had higher levels of social stigma. Higher HIV/AIDS knowledge score was significantly associated with lower HIV-related stigma level among those with middle school or above education level, but no such effect in those with primary school or below. In conclusion, the HIV-related stigma level among elderly men in rural Chengdu was high and positively associated with depression. HIV/AIDS education should target elderly men with low education, living alone, and low income, and interventions to promote mental health may work together to reduce HIV-related stigma in the rural elderly population.

Electric Field-Controlled Magneto-Optical Kerr Effect in A-Type Antiferromagnetic Fe2CX2 (X = F, Cl) and Its Janus Monolayer.

The magneto-optical Kerr effect (MOKE) is a powerful probe of magnetism and has recently gained new attention in antiferromagnetic (AFM) materials. Through extensive first-principles calculations and group theory analysis, we have identified Fe2CX2 (X = F, Cl) and Janus Fe2CFCl monolayers as ideal A-type collinear AFM materials with high magnetic anisotropy and Néel temperatures. By applying a vertical external electrical field (Ef) of 0.2 V/Å, the MOKE is activated for Fe2CF2 and Fe2CCl2 monolayers without changing their magnetic ground state, and the maximum Kerr rotation angles are 0.13 and 0.08°, respectively. Due to the out-of-plane spontaneous polarization, the intrinsic and nonvolatile MOKE is found in the Janus Fe2CFCl monolayer and the maximal Kerr rotation angle without external electronic field is 0.25°. Moreover, the intrinsic built-in electronic field also gives origin to more robust A-type AFM ordering and reversible Kerr angle against external Ef. Our study suggests that Ef is an effective tool for controlling MOKE in two-dimensional (2D) AFM materials. This research opens the possibility of related studies and applications in AFM spintronics.

Alloying two-dimensional NbSi2N4: a new strategy to realize half-metallic antiferromagnets.

Finding two-dimensional (2D) materials with both 100% spin polarization and zero net magnetic moment is essential for next-generation spintronics. Half-metallic antiferromagnets (HMAFs) are ideal materials to satisfy these exigent needs, but such a system has never been found among 2D inorganic materials. In this paper, we theoretically demonstrate that intrinsic 2D HMAFs can be realized by alloying Nb with Mn in 2D septuple-atomic-layer NbSi2N4. By continuously incorporating Mn, the stronger Mn-N hybridization relative to Nb-N induces a metal to half-metal to semiconductor transition. The competitive coupling between the Nb-d itinerant electron spin and the Nb-Mn d-d direct interaction drives the ferromagnetic to antiferromagnetic phase transition. For the first time in 2D inorganic materials, the exact cancellation of local magnetic moments and band gap opening in one spin channel is obtained simultaneously at a Nb/Mn ratio of 3 : 1, as demonstrated by our first-principles calculations. The present results would not only inspire materials design of more 2D HMAFs in the future but also impel the advancement of next-generation antiferromagnetic spintronic devices.

FeSi2: a two-dimensional ferromagnet containing planar hexacoordinate Fe atoms.

As an unconventional bonding pattern different from conventional chemistry, the concept of planar hypercoordinate atoms was first proposed in the molecular system, and it has been recently extended to 2D periodic systems. Using first-principles calculations, herein we predict a stable FeSi2 monolayer with planar hexacoordinate Fe atoms. Due to its abundant multicenter bonds, the FeSi2 monolayer shows excellent thermal and kinetic stability, anisotropic mechanical properties and room-temperature ferromagnetism (T C ∼360 K). Furthermore, we have demonstrated the feasibility of directly growing an FeSi2 monolayer on a Si (110) substrate while maintaining the novel electronic and magnetic properties of the freestanding monolayer. The FeSi2 monolayer synthesized in this way would be compatible with the mature silicon semiconductor technology and could be utilized for spintronic devices.

A Study on the Association Between Family Support and High-Risk Sexual Behavior of Elderly Men in Rural China.

Our study aimed to understand the relationship between the status of high-risk sexual behaviors of male ≥50 years old (elderly men) and their family support in a rural county-level city in Chengdu. Multi-stage sampling was used to select rural elderly men from six towns to conduct an interview questionnaire survey. Ordinal logistic regression was used to analyze the relationship between high-risk sexual behavior and family factors. A total of 790 samples were included, and the prevalence of high-risk sexual behavior was 16.2%. Two-hundred thirty-nine men (30.3%) had three close family members. More than half of the men (n = 397) had never been provided financial support by family members (50.3%). More than half of men (n = 406) never communicate deeply with family members (51.4%). Logistic analysis reported that 50-59 years old (odds ratio [OR] = 1.928, 95% confidence interval [CI] = [1.070, 3.477]), unmarried, divorced/widowed, married and separated (OR = 8.232, 95% CI = [2.640, 25.673]; OR = 3.589, 95% CI = [1.713, 7.520]; OR = 3.003, 95% CI = [1.238, 7.280]) elderly men were more likely to be involved in commercial sex. Meanwhile, either never or often family financial support (OR = 0.435, 95% CI = [0.228, 0.830]; OR = 0.288, 95% CI = [0.095, 0.876]) helped elderly men to avoid commercial sex. This study predicts family factors may be affected by loneliness, life satisfaction, disposable economic condition, family responsibilities as the middle path, thus affect high-risk sexual behaviors in elderly men.

Multiferroicity in a Two-Dimensional Non-van der Waals Crystal of AgCr2X4 (X = S or Se).

Two-dimensional (2D) intrinsic multiferroics have long been pursued not only for their potential technological applications but also as model systems for studying emergent quantum phenomena and coupling mechanisms between various order parameters in low-dimensional space. However, the realization of 2D multiferroics is still a challenge. In this paper, we reveal that 2D AgCr2X4 (X = S or Se) crystals, which have been synthesized from the non-van der Waals (non-vdW) AgCrX2 bulk phase, are type I half-metallic/metallic multiferroics in which ferroelectricity and ferromagnetism coexist. The off-centering displacement of the Ag ion introduces out-of-plane polarization, and the magnetism originates from the interactions between Cr atoms. Remarkably, AgCr2Se4 shows topologically nontrivial spin textures, such as Meron pairs and Néel-type skyrmions, under suitable temperatures and magnetic fields. Our findings demonstrate that 2D multiferroics can be achieved from non-vdW materials and in turn open a new avenue for 2D multiferroics.

Effects of spin-phonon coupling on two-dimensional ferromagnetic semiconductors: a case study of iron and ruthenium trihalides.

The contributions of spin-phonon coupling (SPC) to spin and thermal transport properties are important in emerging two-dimensional (2D) magnetic semiconductors and are relevant to the data security and working stability of spin-based devices. By evaluating the dynamical stability of both ferromagnetic (FM) and paramagnetic (PM) states at 0 K, six transition-metal trihalides, namely FeCl3, FeBr3, FeI3, RuCl3, RuBr3, and RuI3, are identified as ideal 2D magnetic semiconductors for investigating SPC. For these compounds, we perform first-principles calculations to evaluate the spin-relative lattice constants, bond angles, phonon dispersions, lattice thermal conductivity, as well as phonon-relative magnetic coupling parameters, magnetic moment, and Curie temperature. Our results suggest weaker SPC effects on Ru trihalides and stronger SPC effects on Fe trihalides. To explain these observations, we further analyze the SPC induced heat capacity, group velocity, scattering possibility, phonon anharmonicity, and phonon-magnon scattering. More importantly, we also provide a unified explanation for the SPC role in causing anomalous lattice thermal conductivity and stabilizing the ferromagnetic/antiferromagnetic ordering. The present study offers not only in-depth knowledge about the mechanisms of SPC regarding the spin and thermal transport behavior of 2D FM semiconductors, but also a better understanding of thermal management and control in magnetic quantum materials.

2D tetragonal transition-metal phosphides: an ideal platform to screen metal shrouded crystals for multifunctional applications.

Two-dimensional (2D) metal shrouded crystals, a new kind of conceptional material, have attracted remarkable attention due to their unique properties. Here, we propose a novel class of 2D metal shrouded materials, tetragonal transition-metal phosphides (TM2Ps), which show peculiar features of coexistence of in-plane TM-P covalent bonds and TM-TM interlayer metallic bonds. From a combination of high throughput searching and first-principles calculations, Fe2P, Co2P, Ni2P, Ru2P, and Pd2P monolayer sheets stand out because they simultaneously have high thermal, dynamical, and mechanical stability. All these five TM2P materials are metals, especially Pd2P, which can be a promising catalyst for the hydrogen evolution reaction with a very low overpotential. Moreover, these 2D TM2Ps show good ductility since they can withstand a tensile strain of up to 45%. Even in the large strain range, the strengthened interlayer TM-TM metallic bonds dominate the deformation behavior, and the corresponding metallicity of 2D TM2Ps is well preserved. Due to the competition between the d-d direct exchange and d-p-d superexchange interactions, Fe2P behaves as an antiferromagnetic material with a TN of 23 K, while Co2P is a ferromagnetic material with a TC of 580 K. Our results not only enrich the database of 2D metal shrouded crystals, but also provide novel 2D materials as promising candidates for multifunctional applications in nanoelectronics, spintronics and electrocatalysis.

Two-Dimensional AXenes: A New Family of Room-Temperature d0 Ferromagnets and Their Structural Phase Transitions.

The recent discovery of two-dimensional (2D) magnetic order in monolayer CrI3 and bilayer Fe3GeTe2 has stimulated intense experimental and theoretical activities to expand the family of 2D magnets. Most 2D magnets reported to date are transition metal compounds with unpaired d electrons. Novel 2D intrinsic magnets with long-range p state coupling are also highly desirable. Here, we propose that nonstoichiometry is a feasible and universal strategy to realize long-range p electron magnetic order in 2D metal-shrouded AXenes (Na2N, K2N, and Rb2N), supported by our first-principles calculations. Taking K2N as a representative, three series of cation-deficient K2N (T, H, and I phases) have been predicted as stable ferromagnetic half-metal/metal with a Curie temperature of 480-1180 K. Their robust ferromagnetism is ascribed to the coexistence of carrier-mediated exchange and N-K-N superexchange interaction. Moreover, mechanical deformation can trigger reversible phase transformation by choosing their 3D layered counterpart as the intermediate phase.