Boron carbide (${\mathrm{B}}_4\mathrm{C}$) is superhard, but its engineering applications are limited by the abnormal brittle failure arising from amorphous shear band formation. Mitigating the local amorphization is essential to improve the ductility of ${\mathrm{B}}_4\mathrm{C}$. Here, we carried out ab initio molecular dynamics (AIMD) simulations to examine the response of ${\mathrm{B}}_4\mathrm{C}$ to shear along two plausible slip systems $\left(001\right)/\left[100\right]$ and $\left(111\right)/\left[11\overline{2}\right]$ at finite temperature. We found that the icosahedra of ${\mathrm{B}}_4\mathrm{C}$ gradually deconstruct at finite temperature, resulting in local amorphization, thereby giving rise to a lower stress barrier than that of density functional theory simulations at zero temperature. The deconstruction of the icosahedral clusters arises from the interaction with the neighboring chains. The failure mechanism at the finite temperature suggested that the local amorphization can be suppressed by altering the structure of the 12-atom icosahedron and the 3-atom chain of ${\mathrm{B}}_4\mathrm{C}$. To demonstrate this, we replaced the ${\mathrm{B}}_{11}{\mathrm{C}}^{\mathrm{P}}$ icosahedron in ${\mathrm{B}}_4\mathrm{C}$ with ${\mathrm{B}}_{12}$ icosahedron to form boron-rich boron carbide (${\mathrm{B}}_{13}{\mathrm{C}}_2$) and then performed the same shear deformations. We found that local amorphization significantly decreases, which results from the modified icosahedral interaction. We also altered the 3-atom C-B-C chain to the 2-atom P-P chain and found that the accumulated shear stress can be released through icosahedral slipping, which is achieved by breaking the chains. The icosahedral slipping then prevents the destruction of icosahedra under shear deformations, expected to improve the ductility. Our results demonstrate two design strategies toward improved ductility of ${\mathrm{B}}_4\mathrm{C}$: (1) boron enrichment can alleviate amorphous shear band formation in boron carbide, and (2) altering the 3-atom chain to a 2-atom chain and meanwhile decreasing the strength of the chain to make it less stable than the icosahedron cage may lead to an icosahedral-slipping-dominated mechanism, thereby avoiding icosahedral fracture.

We drive periodically a two-dimensional diamond-octagon lattice model by switching between two Hamiltonians corresponding two different magnetic flux piercing through diamond plaquette to investigate the generation of topological flat bands. We show that in this way, the flatness and topological nature of all the bands of the model can be tuned and Floquet quasibands can be made topologically flat while its static counterpart does not support the topological band and flat band together. We define here a measure of flatness of quasienergy bands from their microscopic details that has been justified using the numerical calculations of the Floquet density of states. In the context of Floquet dynamics, we indeed have a better control on the engineering of flat bands. Interestingly, we find the generation of flux current due to periodic drives. We systematically analyze the work done and flux current in the asymptotic limit as a function of system's parameters to show that topology and flatness both share a close connection to the flux current and work done, respectively. We finally extend our investigation to the aperiodic array of step Hamiltonian, where we find that the heating up problem can be significantly reduced if the initial state is substantially flat as the initial large degeneracy of states prevents the system from absorbing energy easily from the aperiodic driving. In addition, we show that the heating can be reduced if the values of the magnetic flux in the step Hamiltonians are made small and, also the duration of these fluxes become unequal. Finally, we successfully explain our finding by plausible analytical arguments.

The observation of large swings in the magnetic anisotropy in ligated ${\mathrm{Co}}_2$ dimers has motivated further calculations on single-center Co qubits in quasitetrahedral and quasioctahedral symmetries. In all cases our results indicate that it is the creation of an unquenched orbital moment due to a half-occupied frontier state at the Co center that directly drives large changes in the magnetic anisotropy barriers regardless of whether it is through symmetry breaking, a change in the charge state, or orbital energy reordering. While these observations are computationally demonstrated through our results on both monomers and dimers, the origin of orbital reordering at the Fermi level is not describable in terms of single physical changes. For example, in ligated ${\mathrm{Co}}_2$ the appearance of an unquenched orbital moment in one of the Co centers is correlated with a change from antiferromagnetic to ferromagnetic ordering at an energy that is uncharacteristically high for such transitions. Because this raises the spector of electromagnetically switchable quantum devices that operate at higher temperatures relative to that of the first-generation magnetic qubits, we carefully discuss subtle details associated with single-center qubits in realistic atomistic environments.

Competition between antiferromagnetic (AFM), paramagnetic (PM), and ferromagnetic (FM) states in ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_6$ compound is investigated by means of temperature- and magnetic field-dependent x-ray diffraction, magnetization, linear thermal expansion, and magnetostriction experiments. It is shown that both AFM and PM phases get converted into the FM phase via a first-order metamagnetic transition, which is accompanied by a huge forced-volume magnetostriction $\mathrm{\Delta }V/V\left(25\mathrm{K},6\mathrm{T}\right)=1.15\%$. X-ray powder diffraction reveals a magnetic field-induced crystallographic phase transition from a $R\overline{3}m$ rhombohedral (AFM, PM) to a $C2/m$ monoclinic (FM) structure. A peculiarly anisotropic lattice expansion as well as giant negative thermal expansion with a volumetric thermal expansion coefficient ${\alpha }_V=-193\times {10}^{–6}{\mathrm{K}}^{–1}$ are observed. These findings point to the significance of magnetoelastic effects in this metamagnet and illustrate the strength of the coupling between lattice and spin degrees of freedom in the ${\mathrm{La}}_{0.9}{\mathrm{Ce}}_{0.1}{\mathrm{Fe}}_{12}{\mathrm{B}}_6$ intermetallic compound.

We report magnetic and electronic properties of $\gamma \text{−}{\mathrm{Fe}}_2\mathrm{W}{\mathrm{O}}_6$ via neutron powder-diffraction measurements and first-principles density-function theory calculations. We reveal a magnetic ground state with two phases coexisting in which the minor phase is the same as an earlier report [Pinto et al., Acta Crystallogr., Sect. A 33, 663 (1977).], despite the fact that both materials studied have the same space group but with slightly different atomic positions. Interestingly, both magnetic phases are well captured by first-principles calculations. Furthermore, we show that these two magnetic phases are correlated with electronic properties, with one being insulating and the other being metallic. These features suggest that $\gamma \text{−}{\mathrm{Fe}}_2\mathrm{W}{\mathrm{O}}_6$ exhibits competing energetic states in which spin, charge, and lattice degrees of freedom are strongly coupled to each other.

The superconductivities and topological properties of ${\mathrm{MgB}}_2$-type diborides are investigated by means of first-principles calculations with different exchange-correlation functionals. Functionals with the van der Waals (vdW) correction (such as OptB88-vdW) may predict critical temperature ($T_c$) comparable with experimental results for several ${\mathrm{MgB}}_2$-type superconductors, particularly for the pristine ${\mathrm{MgB}}_2$ (39.3 vs 39 K). Interestingly, the spin-fluctuation is found to play a significant role in the superconducting behavior of diborides with transition metal elements, and their $T_c$ can be enhanced monotonically by applying tensile strains. Furthermore, Dirac surface states of $\mathrm{Ta}{\mathrm{B}}_2$ and ${\mathrm{NbB}}_2$ are revealed, suggesting their potential use as topological superconducting materials. This paper provides a useful guideline for ab initio studies of superconductivities and topological properties of vdW layered materials.

We studied structural and magnetic phase transitions under applied pressure for the doped cobaltite ${\mathrm{La}}_{0.75}{\mathrm{Ba}}_{0.25}{\mathrm{CoO}}_{2.9}$. Neutron and x-ray diffraction experiments established the coexistence of rhombohedral and cubic phases in the sample. The magnetic state at 2 K is best described as a long-range ordered antiferromagnet (AFM) with small ferromagnetic (FM) clusters. With application of pressure, the rhombohedral phase gradually transforms into a cubic one. At room temperature and the highest applied pressure of 16 GPa, the cubic phase accounts for 70% of the sample volume. Quantum mechanical modeling confirmed the experimental findings and provided more insights into the structural and magnetic phase transitions at pressures exceeding 16 GPa. While the cubic crystal structure was preserved above 10 GPa, the AFM to FM phase transition was found at around 16 GPa. Further increase of the pressure resulted in suppression of magnetic order above 45 GPa. Using density functional theory (DFT)$+\mathrm{U}$ calculations, we were able to relate macroscopic magnetic properties induced by pressure with corresponding spin-state transitions in Co ions.

We revisit the theory of magnetic Raman scattering in Mott insulators with strong spin-orbit coupling, with a major focus on Kitaev materials. We show that Kitaev materials with bond-anisotropic interactions are generally expected to show both one- and two-magnon responses. It is further shown that, in order to obtain the correct leading contributions to the Raman vertex operator $\mathcal{R}$, one must take into account the precise, photon-assisted microscopic hopping processes of the electrons and that, in systems with multiple hopping paths, $\mathcal{R}$ contains terms beyond those appearing in the traditional Loudon-Fleury theory. Most saliently, a numerical implementation of the revised formalism to the case of the three-dimensional hyperhoneycomb Kitaev material $\beta \text{−}{\mathrm{Li}}_2\mathrm{Ir}{\mathrm{O}}_3$ reveals that the non-Loudon-Fleury scattering terms actually dominate the Raman intensity. In addition, they induce a qualitative modification of the polarization dependence, including, e.g., the emergence of a sharp one-magnon peak at low energies, which is not expected in the traditional Loudon-Fleury theory. This peak is shown to arise from microscopic photon-assisted tunneling processes that are of similar type with the ones leading to the symmetric off-diagonal interaction $\mathrm{\Gamma }$ (known to be present in many Kitaev materials), but take the form of a bond-directional magnetic dipole term in the Raman vertex. These results are expected to apply across all Kitaev materials and mark a drastic change of paradigm for the understanding of Raman scattering in materials with strong spin-orbit coupling and multiple exchange paths.

We utilize transverse ac susceptibility measurements to characterize magnetic anisotropy in archetypal exchange-bias bilayers of ferromagnet permalloy (Py) and antiferromagnet CoO. Unidirectional anisotropy is observed for thin Py but becomes negligible at larger Py thicknesses, even though the directional asymmetry of the magnetic hysteresis loop remains significant. Additional magnetoresistance measurements, magneto-optical imaging, as well as micromagnetic simulations show that these surprising behaviors are likely associated with asymmetry of spin-flop distribution created in CoO during Py magnetization reversal, which facilitates the rotation of the latter back into its field-cooled direction. Our findings suggest possibilities for efficient realization of multistable nanomagnetic systems for neuromorphic applications

We performed systematic angle-resolved photoemission spectroscopy measurements in situ on $T^{\prime }\text{−}{\mathrm{La}}_{2-x}{\mathrm{Ce}}_x\mathrm{Cu}{\mathrm{O}}_{4\pm \delta }$ (LCCO) thin films over the extended doping range prepared by the refined ozone/vacuum annealing method. Electron doping level ($n$), estimated from the measured Fermi surface (FS) volume, varied from 0.05 to 0.23, fully encompassing the superconducting dome. We observed an absence of the insulating behavior around $n\sim 0.05$ and the shift of FS reconstruction to $n\sim 0.11$ in LCCO from $n\sim 0.15$ in other electron-doped cuprates, suggesting that the antiferromagnetic order is strongly suppressed in this material. The possible explanation may lie in the enhanced next-nearest-neighbor hopping in LCCO because of the largest ${\mathrm{La}}^{3+}$ ionic radius among all the lanthanide elements.

The complex polar oxide heterojunction of ${\mathrm{SrTiO}}_3/{\mathrm{LaAlO}}_3$ (STO/LAO) is of great interest due to the emergent physical phenomena observed at the interface. STO and LAO separately are wide band-gap insulators. However, upon joining them at the 001, 110, and 111 crystallographic planes, the interface undergoes a transition to a conductive state. Although first-principles modeling of the 001 plane interface has been widely studied, there is a lack of reports regarding the 110 and 111. This paper expands the theoretical model of the STO/LAO heterointerface to the three crystallographic planes (001, 110, and 111) where the conductivity has been experimentally reported. The calculations showed that whereas at the 001 interface the conductivity appears at a critical thickness of 4 monolayers of LAO, the 110 and 111 planes have no clear critical thickness; these two interfaces were always conductive. Nevertheless, the number of conductive electrons per unit cell increases with the thickness of the LAO layer in the 110 and 111 interfaces. This is related to the energy levels downshifting due to the electrostatic potential buildup (which was in the opposite direction respect to the 001 interface), increasing the number of conductive sates below the Fermi level. Given the absence of a critical thickness and the fact that chemical intermixing and oxygen vacancies at the interface were not considered, the main mechanism responsible for the conductivity in the 110 and 111 planes was attributed to the large structure reconstruction that locally changes the energy levels at the interface causing charge transfer and accumulation at the layers close to the interface.

A thermoelectric effect of the noncentrosymmetric electron-phonon coupled system is proposed, in which a temperature difference between electrons and phonons, instead of a spatial temperature gradient, induces an electric current. This is a realization of the Seebeck ratchet in solids with the asymmetric energy dispersion of electrons, and the dynamical phonons act as the fluctuation of the scalar potential and induce the dc current. Possible realizations of this mechanism are also discussed.

The kagome Hubbard model (KHM) is a paradigmatic example of a frustrated two-dimensional model. While its strongly correlated regime, described by a Heisenberg model, is of topical interest due to its enigmatic prospective spin-liquid ground state, the weakly and moderately correlated regimes remain largely unexplored. Motivated by the rapidly growing number of metallic kagome materials (e.g., ${\mathrm{Mn}}_3\mathrm{Sn}, {\mathrm{Fe}}_3{\mathrm{Sn}}_2$, FeSn, ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2, {\mathrm{Gd}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$, and $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ with $A=\mathrm{K}$, Rb, Cs), we study the respective regimes of the KHM by means of three complementary numerical methods: the dynamical mean-field theory, the dynamical vertex approximation, and determinant quantum Monte Carlo. In contrast to the archetypal square lattice, we find no tendencies toward magnetic ordering, as magnetic correlations remain short-range. Nevertheless, the magnetic correlations undergo a remarkable crossover as the system approaches the metal-to-insulator transition. The Mott transition itself does not affect the magnetic correlations. Our equal-time and dynamical structure factors can be used as a reference for inelastic neutron scattering experiments on the growing family of metallic kagome materials.

We studied flux pinning in exfoliated ${\mathrm{FeTe}}_{0.6}{\mathrm{Se}}_{0.4}$ thin-film devices with a thickness $d$ from 30 to 150 nm by measuring the critical current density $J_{\mathrm{c}}$. In bulk ${\mathrm{FeTe}}_{0.6}{\mathrm{Se}}_{0.4}$, the flux pinning has been discussed in the framework of weak collective pinning, while there is little knowledge on the pinning mechanism in the thin-film region. From the thickness $d$ dependence of $J_{\mathrm{c}}$ at a fixed magnetic field of 1 T, we found that the strong pinning is dominant below $d\approx 70$ nm, while the weak collective pinning becomes more important above $d\approx 100$ nm. This crossover thickness can be explained by the theoretical model proposed by van der Beek et al. [Phys. Rev. B 66, 024523 (2002)].

Ideal Sharvin contact in a multimode regime shows the conductance $G\approx G_{\mathrm{Sharvin}}=g_0{k}_{F}W/\pi$ (with $g_0$ the conductance quantum, $k_F$ the Fermi momentum, and $W$ the contact width) accompanied by strongly suppressed shot noise quantified by small Fano factor $F\approx 0$. For ballistic graphene away from the charge-neutrality point, sub-Sharvin transport occurs, characterized by suppressed conductance $G\approx (\pi /4)G_{\mathrm{Sharvin}}$ and enhanced shot noise $F\approx 1/8$. All these results can be derived from a basic model of quantum scattering, involving assumptions of infinite height and a perfectly rectangular shape of the potential barrier in the sample. Here we have carried out the numerical analysis of the scattering on a family of smooth barriers of finite height interpolating between parabolic and rectangular shapes. We find that, tuning the barrier shape, one can modify the asymmetry between electron- and hole-doped systems. For electronic dopings, the system crosses from the Sharvin to sub-Sharvin transport regime (indicated by both the conductance and the Fano factor) as the potential becomes closer to the rectangular shape. In contrast, for hole dopings, the conductivity is strongly suppressed when the barrier is parabolic and slowly converges to $G\approx (\pi /4)G_{\mathrm{Sharvin}}$ as the potential evolves toward a rectangular shape. In such a case, the Sharvin transport regime is inaccessible, shot noise is generically enhanced (with much slower convergence to $F\approx 1/8$) compared to the electron-doped case, and aperiodic oscillations of both $G$ and $F$ are prominent due to the formation of quasibound states.

Perpendicular magnetic anisotropy (PMA) in ferromagnet/oxide systems has attracted considerable attention owing to its significant potential in spintronic applications; however, an understanding of the causes of PMA enhancement in such systems remains inadequate. This study investigates the influence of epitaxial strain on PMA in the Fe/MgO system. The PMA energy increases monotonically as the in-plane lattice constant of Fe decreases, and increases further after annealing. We find that the interface component of the PMA energy is largely unaffected by the epitaxial strain and annealing process, and that changes in its bulk component play an important role. This study leads to a comprehensive understanding of PMA and voltage-controlled magnetic anisotropy effects in ultrathin magnetic films.

We theoretically describe a mechanism of low-field linear magnetoconductivity in helical magnetic metals. Two ingredients for the mechanism in three-dimensional metals are identified to be the spin-orbit coupling and momentum-dependent ferromagnetic exchange interaction. We propose and study a number of minimal theoretical models which have linear magnetoconductivity and discuss their implications for recent experiments.

We report on the measured thermal conductivity of silicon irradiated with an array of ions: ${\mathrm{C}}^{2+},$ ${\mathrm{N}}^{2+},$ ${\mathrm{Al}}^{2+},$ ${\mathrm{Si}}^{2+},$ ${\mathrm{P}}^{2+},$ and ${\mathrm{Ge}}^{2+}$. Results are analyzed in consideration of ion mass, radius, and induced displacement damage. Recrystallization of select samples via annealing demonstrates that structural disorder imparted by incident ions, rather than the ions themselves, drive the reduction in thermal conductivity. This, in turn, is dictated by the mass of the ion. A generalized model is provided to relate thermal conductivity to displacement damage. Knowledge of the displacements-per-atom profile can thus be used to predict thermal conductivity reduction for silicon devices in extreme environments.

Atoms with principal quantum number smaller than 2 are traditionally regarded to be nonmagnetic, but recent appealing advances in the understanding of ferromagnetism in materials such as graphene nanoribbons with zigzag edges and triangular graphene nanoflakes pave a novel way to investigate magnetism in materials merely with $s$ and $p$ orbitals. Opening the $p$ shell turns out to be essential for the production of unpaired electrons, and the normal chemical treatment is to modify the molecules with various atoms and groups such as hydrogen. In this paper, we combine first-principles calculations with the Hubbard model to investigate magnetic properties in hydrogenated ${\mathrm{C}}_{60}$ (${\mathrm{C}}_{60}{\mathrm{H}}_n$). For the enumerated ${\mathrm{C}}_{60}{\mathrm{H}}_2$ structures, there are nine configurations with ferromagnetic ground states and one configuration with an antiferromagnetic ground state. We have proposed a pair magnetic interaction model to predict the magnetic order, as well as a general rule to screen the maximum local magnetic moments for given $n$. When $n=3$ and 4, our model can give a good prediction of magnetic order, which is confirmed by the first-principles calculations. We find that the maximum number of unpaired electrons in ${\mathrm{C}}_{60}{\mathrm{H}}_n$ is smaller than the number of hydrogen atoms when $n\ge 6$.

Single crystals of the hexagonal triangular lattice compound ${\mathrm{AgCrSe}}_2$ have been grown by chemical vapor transport. The crystals have been carefully characterized and studied by magnetic susceptibility, magnetization, specific heat, and thermal expansion. In addition, we used Cr-electron spin resonance and neutron diffraction to probe the Cr $3d^3$ magnetism microscopically. To obtain the electronic density of states, we employed x-ray absorption and resonant photoemission spectroscopy in combination with density functional theory calculations. Our studies evidence an anisotropic magnetic order below $T_N=32\mathrm{K}$. Susceptibility data in small fields of about 1 T reveal an antiferromagnetic (AFM) type of order for $H\perp c$, whereas for $H\parallel c$ the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for $H\perp c$, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at $H^{+}=5\mathrm{T}$, which is absent for $H\parallel c$. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fanlike arrangement above $H^{+}$. A fully ferromagnetically polarized state is obtained above the saturation field of $H_{\perp S}=23.7\mathrm{T}$ at 2 K with a magnetization of $M_s=2.8{\mu }_{\mathrm{B}}/\mathrm{Cr}$. For $H\parallel c$, $M\left(H\right)$ monotonically increases and saturates at the same $M_s$ value at $H_{\parallel S}=25.1\mathrm{T}$ at 4.2 K. Above $T_N$, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about $T^{*}=45\mathrm{K}$, which is a clear fingerprint of short range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possibly related to the noncentrosymmetric polar space group $R3m$. Due to these competing interactions, the magnetism in ${\mathrm{AgCrSe}}_2$, in contrast to the oxygen-based delafossites, can be tuned by relatively small, experimentally accessible magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.