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1. Low-energy magnetic states of Tb adatom on graphene

   https://doi.org/10.1088/1361-648X/ad8fe9  

   Shaikh, Monirul; Klein, Alison; Wysocki, Aleksander L (November 2024, Journal of Physics: Condensed Matter)

   Abstract 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 (SOC) . We determine that the six-fold symmetry hollow site is the preferred adsorption site and investigate electronic spectrum for different adatom oxidation states including Tb³⁺, Tb²⁺, Tb¹⁺, and Tb⁰. 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 the4fspin. For Tb³⁺, the energy levels can be described by theJ = 6 multiplet split by the graphene crystal field (CF). For other oxidation states, the interaction of4felectrons with spin and orbital degrees of freedom of $6s5d$electrons in the presence of SOC results in the low-energy spectrum composed closely lying effective multiplets that are split by the graphene CF. Stable magnetic moment is predicted for Tb³⁺and Tb²⁺adatoms due to uniaxial magnetic anisotropy and effective anisotropy barrier around 440 cm⁻¹controlled by the temperature assisted quantum tunneling of magnetization through the third excited doublet. On the other hand, in-plane magnetic anisotropy is found for Tb¹⁺and Tb⁰adatoms. Our results indicate that the occupation of the $6s5d$orbitals can dramatically affect the magnetic anisotropy and magnetic moment stability of rare earth adatoms. 

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2. Observation of quantum entanglement in top quark pair production in proton–proton collisions at $\sqrt{s}=13$  TeV

   https://doi.org/10.1088/1361-6633/ad7e4d  

   CMS_Collaboration, The (October 2024, Reports on Progress in Physics)

   Abstract Entanglement is an intrinsic property of quantum mechanics and is predicted to be exhibited in the particles produced at the Large Hadron Collider. A measurement of the extent of entanglement in top quark-antiquark ( $\mathrm{t}\overline{\mathrm{t}}$) events produced in proton–proton collisions at a center-of-mass energy of 13 TeV is performed with the data recorded by the CMS experiment at the CERN LHC in 2016, and corresponding to an integrated luminosity of 36.3 fb⁻¹. The events are selected based on the presence of two leptons with opposite charges and high transverse momentum. An entanglement-sensitive observableDis derived from the top quark spin-dependent parts of the $\mathrm{t}\overline{\mathrm{t}}$production density matrix and measured in the region of the $\mathrm{t}\overline{\mathrm{t}}$production threshold. Values of $D<-1/3$are evidence of entanglement andDis observed (expected) to be $-{0.480}_{-0.029}^{+0.026}$( $-{0.467}_{-0.029}^{+0.026}$) at the parton level. With an observed significance of 5.1 standard deviations with respect to the non-entangled hypothesis, this provides observation of quantum mechanical entanglement within $\mathrm{t}\overline{\mathrm{t}}$pairs in this phase space. This measurement provides a new probe of quantum mechanics at the highest energies ever produced. 

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3. Spin stiffness and spin excitation gap of van der Waals ferromagnetic ${\mathrm{Fe}}_{3+\delta }{\mathrm{GeTe}}_2$

   https://doi.org/10.1088/1361-648X/ad581f  

   Shu, Zhixue; Zhang, Shufeng; Kong, Tai (June 2024, Journal of Physics: Condensed Matter)

   Abstract ${\mathrm{Fe}}_{3+\delta }{\mathrm{GeTe}}_2$(FGT) has proved to be an interesting van der Waals (vdW) ferromagnetic compound with a tunable Curie temperature ( $T_{\mathrm{C}}$). However, the underlying mechanism for varying $T_{\mathrm{C}}$remains elusive. Here, we systematically investigate and compare low-temperature magnetic properties of single crystalline FGT samples that exhibit $T_{\mathrm{C}}$s ranging from 160 K to 205 K. Spin stiffness (D) and spin excitation gap (Δ) are extracted using Bloch’s theory for crystals with varying Fe content. Compared to Cr-based vdW ferromagnets, FGT compounds have higher spin stiffness values but lower spin wave excitation gaps. We discuss the implication of these relationships in Fe–Fe ion magnetic interactions in FGT unit cells. The itinerancy of magnetic electrons is measured and discussed under the Rhodes–Wohlfarth ratio (RWR) and the Takahashi theory. 

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4. Electron impact excitation of allowed and forbidden transitions between fine-structure levels in Ti III

   https://doi.org/10.1088/1361-6595/ad95b8  

   Wang, Yang; Cao, Qiu-Yao; Zhu, Xi-Ming; Du, Chun-Meng; Bartschat, Klaus (November 2024, Plasma Sources Science and Technology)

   Abstract Introduction: We present an extensive theoretical investigation of the electron impact excitation of doubly-ionized titanium (Ti III) to meet the needs of spectral analysis and plasma modeling. OBJECTIVES: The main objective of this work is to extend the currently scarce database of both structure and collision data for Ti III. METHODS: The calculation was performed in the close-coupling approximation using theB-splineR-matrix method. The multi-configuration Hartree–Fock method in combination withB-spline configuration interaction expansions and the non-orthogonal orbitals technique is employed for accurate descriptions of the target wave functions and adequate accounts of the various interactions between the target states. Relativistic effects are treated at the semi-relativistic Breit-Pauli approximation level. RESULTS: The present close-coupling expansion includes 138 fine-structure levels of Ti III belonging to the $3d^2$, $4s^2$, $4s4p$, $3d4l$( $l=0-3$), $3d5l$( $l=0-3$), $3d6s$, and $3d6p$configurations. Comprehensive sets of radiative and electron collisional data are reported for all of the possible transitions between the 138 fine-structure levels. Thermally averaged collision strengths are determined using a Maxwellian distribution for a wide range of temperatures from ${10}^2$K to ${10}^5$K. The accuracy of the calculated radiative parameters is validated by comparing with available values from the NIST database and previous literature. CONCLUSION: Given the lack of sufficient currently available experimental and theoretical data, the electron impact excitation cross sections of the Ti III fine-structure levels presented here are systematic, extensive, and internally consistent, thus making them suitable for many modeling applications. 

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5. Experimental investigation of circumnutation-inspired penetration in sand

   https://doi.org/10.1088/1748-3190/ad8c89  

   Anilkumar, Riya; Martinez, Alejandro (November 2024, Bioinspiration & Biomimetics)

   Probes that penetrate soil are used in fields such as geotechnical engineering, agriculture, and ecology to classify soils and characterize their propertiesin situ. Conventional tools such as the Cone Penetration Test (CPT) often face challenges due to the lack of reaction force needed to penetrate stiff or dense soil layers, necessitating the use of large drill rigs. This paper investigates more efficient means of penetrating soil by taking inspiration from a plant-root motion known as circumnutation. Experimental penetration tests on sands are performed with circumnutation-inspired (CI) probes that advance at a constant vertical velocity ( $v$) while simultaneously rotating at a constant angular velocity ( $\omega$). These probes have bent tips with a given bent angle ( $\alpha$) and bent length ( $L_1$). The variation of the mobilized vertical force ( $F_z$), torque ( $T_z$.), and the mechanical work components with the ratio of tangential to vertical velocity (ωR/ν, whereRis the distance of the tip of the probe from the vertical axis of rotation) is investigated along with the effects of probe geometry, vertical velocity, and soil relative density ( $D_R$). The results show that the soil penetration resistance does not vary with $v$, but it increases as $\alpha$, $L_1$, and $D_R$are increased. $F_z$decays exponentially with increasing $\omega R/v$, $T_z$initially increases and then plateaus, while total work ( $W_T$) shows little magnitude changes initially but later increases monotonically. The mechanisms leading to these trends are identified as the changes in the probe projected areas and mobilized normal stresses due to differences in probe geometry and the effects of $\omega R/v$on the resultant force direction and soil disturbance. The results show that CI penetration within a specific range of $\omega R/v$leads to small increases in $W_T$(i.e., $\text{⩽}$25%), yet mobilizes $F_z$magnitudes that are 50%–80% lower than that mobilized during non-rotational penetration (i.e., CPT). This indicates that CI penetration can be adopted forin situcharacterization or sensor placement with smaller vertical forces, allowing for use of lighter rigs. 

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