Fractionally filled Chern bands with strong interactions may give rise to fractional Chern insulator (FCI) states, the zero-field analog of the fractional quantum Hall effect. Recent experiments have demonstrated the existence of FCIs in twisted bilayer ${\mathrm{MoTe}}_2$ without external magnetic fields—most robust at $\nu =-2/3$—as well as Chern insulators (CIs) at $\nu =-1$. Although the appearance of both of these states is theoretically natural in an interacting topological system, experiments repeatedly observe nonmagnetic (or weakly magnetic) states (lacking FCIs) at $\nu =-1/3$ and $-4/3$, a puzzling result, which has not been fully theoretically explained. In this paper, we perform Hartree-Fock and exact diagonalization calculations to test whether the standard ${\mathrm{MoTe}}_2$ moiré model with the (greatly varying) parameter values available in the literature can reproduce the nonmagnetic/weakly magnetic states at $\nu =-1/3$ and $-4/3$ in unison with the FCI at $\nu =-2/3$ and CI state at $\nu =-1$. We focus on the experimentally relevant twist angles and, crucially, include remote bands. We find that the parameters proposed in Wang et al. [arXiv:2306.02501] can nearly capture the experimental phenomena at $\nu =-1/3,-2/3,-1,-4/3$ simultaneously, although the predicted ground states at $\nu =-1/3$ are still mostly FCIs and a larger dielectric constant $\epsilon >10$ than is typical of hexagonal boron nitride (h-BN) substrate $\epsilon \sim 6$ is required. Our results show the importance of remote bands in identifying the competing magnetic orders and lay the groundwork for further study of the realistic phase diagram.

The fascination with semimetals, especially Dirac and Weyl semimetals, is given by their surprisingly strong response to magnetic fields. In particular, the extremely large magnetoresistance (XMR), i.e., the change in electrical resistivity as a function of the applied magnetic field, has attracted interest because of its deviation by several orders of magnitude from the behavior of normal metals, and its potential for technological applications. To date, it is unclear if the XMR in topological semimetals is inherently correlated to the very high electron mobility and electron-hole compensation, or to other exotic mechanisms. Here, we show that the relativistic and topological nature of charge carriers of the Weyl semimetal niobium phosphide (NbP) are only indirect causes of the XMR. Instead, the XMR can be explained by the very long mean free path $l_e\left(4\text{K}\right)\approx 8\mu \mathrm{m}$ in combination with the small cyclotron orbits emerging in the presence of a magnetic field $r_c\left(9\text{T}\right)\approx 20$ nm of the NbP's Weyl electrons. More precisely we find $\text{MR}=c{l}_e/r_c$, where $c$ is a parameter independent of temperature and angle between the magnetic field and the crystal. To demonstrate, we use temperature and angle-dependent magnetoresistance measurements, and extract the mean free path and cyclotron radius from an analysis of the Shubnikov–de Haas oscillation.

Although the topological properties of type-II Weyl semimetal ${\mathrm{WP}}_2$ have been widely studied by both the experiments and the theoretical calculations, the dominant electron-phonon scattering and the effect of Fermi pockets on the electronic transport still remain elusive. In this work, we investigate the electronic transport of Weyl semimetal ${\mathrm{WP}}_2$ by the first-principles calculations and semiclassical transport theory. The results well reproduce the resistivity from the experiments. Carriers in the Fermi pockets favor much less scattering rates. Holes are found to possess much lower scattering rates than electrons at low temperatures, while both types of carriers have close scattering rates at high temperatures. At low temperatures, the scattering rates are mainly contributed by the acoustic modes, especially the transverse acoustic mode, while the rates at high temperatures are jointly contributed by acoustic modes and low-frequency optical modes, especially $B_2^{\left(1\right)}$ and $B_1^{\left(2\right)}$. Compared to other modes, the $A_1$ modes are found to possess larger linewidths and exhibit significant chemical potential dependent behaviors. The low-temperature resistivity is found to increase monotonously as the chemical potential increases. Moreover, both the carrier density and the resistivity can be well tuned by applying the uniaxial or volume strains. The carrier density is enhanced under the tensile strains along the $a$ and $c$ axes, which leads to the decreasing resistivity. In contrast, the tensile strain along the $b$ axis suppresses the carrier density and thus increases the resistivity. The behaviors of carrier density and resistivity can be attributed to the shift of Fermi pockets and the variation of electron-phonon coupling strength under the strains.

We show that a classical spin liquid phase can emerge from an ordered magnetic state in the two-dimensional frustrated Shastry-Sutherland Ising lattice due to lateral confinement. Two distinct classical spin liquid states are stabilized: (i) long-range spin-correlated dimers, and (ii) exponentially decaying spin-correlated disordered states, depending on widths of $\mathrm{W}=3n$, $3n+1$ or $\mathrm{W}=3n+2,n$ being a positive integer. Stabilization of spin liquids in a square-triangular lattice moves beyond the conventional geometric paradigm of kagome, triangular, or tetrahedral arrangements of antiferromagnetic ions, where spin liquids have been discussed conventionally.

We propose how to create, control, and read out real-space localized spin qubits in proximitized finite graphene nanoribbon (GNR) systems using purely electrical methods. Our proposed nanoqubits are formed of in-gap singlet-triplet states that emerge through the interplay of Coulomb and relativistic spin-dependent interactions in GNRs placed on a magnetic substrate. Application of an electric field perpendicular to the GNR heterostructure leads to a sudden change in the proximity couplings, i.e., a quantum quench, which enables us to deterministically rotate the nanoqubit to any arbitrary point on the Bloch sphere. We predict these spin qubits to undergo Rabi oscillations with optimal visibility and frequencies in excess of 10 GHz. Our findings open up an avenue for the realization of graphene-based quantum computing with ultrafast all-electrical methods.

Perovskite-based materials $\left({ABX}_3\right)$ show promise for photovoltaic applications; however, commercial use still encounters challenges, specifically $\mathrm{Pb}$ toxicity and structural instabilities of the active phases. A potential lead-free compound is ${\mathrm{CsSnI}}_3$; nonetheless, the photoinactive yellow phase is energetically more favorable than the photoactive black phase. An undesired $\text{black}\to \text{yellow}$ phase transition implies structural instabilities, possibly explained by the Goldschmidt tolerance factor of $0.84$. In this study, we employ theoretical calculations to investigate the impact of substitutional bivalent doping in the $B$ sites of ${\mathrm{CsSnI}}_3$ with $3d$ transition metals $\left(TM\right)$ on tuning the relative energetic stability between the black and yellow phases. Our calculations are based on density functional theory, incorporating on-site Hubbard corrections for the $TM$ $3d$ states. We observe an inversion of the relative stability between the yellow and black phases for dopings with $\mathrm{Co}$, $\mathrm{Cu}$, and $\mathrm{Zn}$. We attribute the interphase relative energy to Jahn-Teller distortions at the doped octahedra, as the black and yellow phases exhibit distinct octahedral interconnections. For instance, the black phase has corner-shared octahedra, resulting in lower elastic costs for accommodating geometric distortions compared to the edge-shared octahedra in the yellow phase. The presented results highlight the influence of symmetry breaks on the stability of perovskite materials and have implications for other perovskite compositions.

Simulating quantum spin systems at finite temperatures is an open challenge in many-body physics. This work studies the temperature-dependent spin dynamics of a pivotal compound, ${\mathrm{FeI}}_2$, to determine if universal quantum effects can be accounted for by a phenomenological renormalization of the dynamical spin structure factor $S(\mathbf{q},\omega )$ measured by inelastic neutron scattering. Renormalization schemes based on the quantum-to-classical correspondence principle are commonly applied at low temperatures to the harmonic oscillators describing normal modes. However, it is not clear how to extend this renormalization to arbitrarily high temperatures. Here we introduce a temperature-dependent normalization of the classical moments, the magnitude of which is determined by imposing the quantum sum rule, e.g., $\int d\omega d\mathbf{q}S(\mathbf{q},\omega )=N_{S}S(S+1)$ for $N_S$ dipolar magnetic moments. We show that this simple renormalization scheme significantly improves the agreement between the calculated and measured $S(\mathbf{q},\omega )$ for ${\mathrm{FeI}}_2$ at all temperatures. Due to the coupled dynamics of dipolar and quadrupolar moments in that material, this renormalization procedure is extended to classical theories based on SU(3) coherent states, and by extension, to any $\mathrm{SU}\left(N\right)$ coherent state representation of local multipolar moments.

Magnetic molecules on ferromagnetic metallic substrates have been widely explored to exploit the potential for molecular magnetic storage and spintronics applications. Recent advances in these hybrid interfaces integrated with two-dimensional materials have been proposed as a flexible platform for realizing new spin-related effects. Herein, the impact of inserting graphene on the electronic and magnetic properties of a family of transition metal phthalocyanines (TMPcs, TM = Cr, Mn, Fe, Co, and Cu) deposited on ferromagnetic Ni(111) surfaces have been systematically rationalized by density functional theory analysis. Our calculations reveal that the magnetic exchange interaction across the molecule-substrate interfaces can be significantly mediated by the introduction of a graphene interlayer. Interestingly, these TMPcs exhibit ferromagnetic coupling with the Ni substrate. However, the strength of this coupling is reduced in the presence of a graphene decoupling layer, with the exception of CoPc. In the case of CoPc, the original ferromagnetic coupling with Ni(111) can be altered to antiferromagnetic when a graphene interlayer is introduced. By analyzing the different channels of communication involved in the spin interaction between the molecule and the magnetic substrate, we attribute these significant differences to the varied influences on the exchange interaction caused by the intermediary graphene layer. The presence of the inserted graphene layer may block the direct exchange interaction between the TMPc molecule and substrate, while the indirect superexchange interaction facilitated by the nitrogen atoms of the organic ligands is only reduced. Our study thus demonstrates that the inserted graphene can serve as an optimal intermediary layer for mediating the magnetic couplings across the molecule-substrate interfaces while allowing effective spin communication between them. These findings provide important insights into relevant experiments and offer a promising strategy to control the magnetic exchange interactions via utilizing graphene at metal-molecule interfaces.

We investigate resonance phenomena in a system consisting of a nanomagnet coupled to a Josephson junction under external periodic drive. The coupling in the system leads to the appearance of additional resonance peaks whose properties depend on the periodic signal and Josephson junction dynamics. In the linear regime, we derive an analytical description of the resonance phenomena which are then confirmed by numerical simulations. This analytical method is universal and can also be applied to Josephson junctions with an anomalous phase shift in the current phase relation. This work provides a method for controlling the resonances of hybrid structures that might be interesting for applications.

Two-dimensional (2D) magnetic materials are currently of great interest for their promising applications in spintronics. Strong magnetic coupling and anisotropy are both highly desirable for the achievement of high-temperature magnetic order. Here we propose the unusual high-valence ${\mathrm{FeS}}_2$ hexagonal monolayer as such a candidate for a strong Ising 2D ferromagnet by spin-orbital state analyses, first-principles calculations, and the renormalized spin-wave theory (RSWT). We find that, very importantly, the high-valence ${\mathrm{Fe}}^{4+}$ ion is in the low-spin state ($t_{2g}^4$, $S=1)$ with degenerate $t_{2g}$ orbitals rather than the high-spin state ($t_{2g}^3{e}_g^1$, $S=2)$. It is the low-spin state that allows us to carry a large perpendicular orbital moment and then produces a huge single-ion anisotropy (SIA) of 25 meV/Fe. Moreover, the negative charge-transfer character associated with the unusual high valence, strong Fe $3d$-S $3p$ hybridization, wide bands, and a small band gap all help to establish a strong superexchange. Indeed, our first-principles calculations confirm the strong ferromagnetic superexchange and the huge perpendicular SIA, both of which are further enhanced by a compressive strain. Then, our RSWT calculations predict that the ferromagnet $T_{\mathrm{C}}$ is 261 K for the pristine ${\mathrm{FeS}}_2$ monolayer and could be increased to 409 K under −5% compressive strain. The high $T_{\mathrm{C}}$ is also reproduced by our Monte Carlo simulations. Therefore, it is worth exploring high-$T_{\mathrm{C}}$ Ising ferromagnets in high-valence 2D magnetic materials with degenerate orbitals.

Owing to the strong spin-orbit coupling and the related fascinating physical properties, heavy $5d$ transition metals exhibit desirable application prospects. However, up to now, the $5d$ magnetic materials are still very limited, especially very rare for tungsten. In this work, we theoretically predict a two-dimensional multiferroic ${\mathrm{W}}_3{\mathrm{Cl}}_8$ monolayer. Intrinsic $5d$ magnetism of tungsten is activated by the W ions’ fractional valence in a breathing kagome lattice of reduced effective dimension. A coplanar $Y$-type antiferromagnetism composed by ferromagnetic ${\mathrm{W}}_3$ trimers is confirmed as the magnetic ground state. The spontaneous ferroelectric polarization mainly originates from the ion displacement induced by the breathing distortion of kagome lattice. An intrinsic magneto-optical Kerr effect with sizable Kerr angle can be observed to detect this trimeric $Y$-type antiferromagnetism, and it depends strongly on the detailed magnetic order. Thereby, we propose a general scheme for realizing more $5d$ magnetism in two-dimensional multiferroic systems.

The structural phase transitions of $\mathrm{Pr}{\left({\mathrm{Sr}}_{0.1}{\mathrm{Ca}}_{0.9}\right)}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$ have been studied by synchrotron radiation x-ray powder diffraction. Three different charge order (CO) phases are identified as a function of temperature. The three phases have a checkerboard arrangement of compressed and expanded $\mathrm{Mn}{\mathrm{O}}_6$ octahedra in the perovskite bilayers. The structural changes associated with each transition are described in terms of the mode decomposition analysis with respect to the parent tetragonal structure. The high-temperature phase exhibits an orthorhombic symmetry (Amam) due to cooperative tilts of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedra that can be ascribed to the irreducible representation (irrep) X3−. On cooling, the CO1 phase with Pbnm symmetry arises from the condensation of modes belonging to the irrep DT2 together with breathing (X1+) and antiferrodistortive (M5− and Y2) modes. The Mn site is split into two nonequivalent Mn atoms (Mn1 and Mn2) with a small charge segregation, and the asymmetric environment of Mn1 atoms form zigzag chains along the doubled $b$ axis. Further cooling leads to the formation of the CO2 phase that comes from a relative change between the directions of previous X3− and DT2 distortions, producing the simultaneous condensation of new DT1 and ferrodistortive GM5− modes. In the CO2 structure (Am2m symmetry), Mn2 atoms are split into two nonequivalent sites but with similar oxidation states. Then the checkerboard arrangement between Mn1 and Mn2 atoms is preserved but with a reorientation of the zigzag chains along the $a$ axis. At lower temperatures, the CO3 phase arises from a new direction change of irrep X3−, leading to the simultaneous condensation of new secondary modes. Its symmetry is $Pn{2}_{1}m$, and the Mn1 site is now split, which leads to a total of four nonequivalent Mn sites in the unit cell. In this phase, more tilts of the $\mathrm{Mn}{\mathrm{O}}_6$ octahedra are allowed following the $a^{-}{b}^{-}{c}^{+}$ tilt schema.

Temperature-induced polymorphic structural crossover in metallic liquid indium (In), tin (Sn), and antimony (Sb) is investigated by in situ high-energy x-ray diffraction and ab initio molecular dynamics simulations. The results demonstrate the existence of a temperature-induced reversible and gradual structural crossover in these three liquids, reflecting from both “static” structures, e.g., peak positions of the structure factor and pair distribution function, bond angle distribution, number of tetrahedra, cluster connection, free volume, and dynamical behaviors, including the relaxation time, diffusion coefficient, and non-Gaussian parameter. The main difference from liquid In, Sn, to Sb is that not only the large-sized free volume but also the anharmonicity and the dynamical heterogeneity increase significantly with the appearance of strong covalent bonds. These findings will deepen the understanding of liquid structure and the liquid-liquid transition in metallic liquids.

We study from first-principles calculations the ferroelectric structural phase transition of ${\mathrm{Pb}}_5{\mathrm{Ge}}_3{\mathrm{O}}_{11}$ crystal. The calculations of phonons and Born effective charges of the paraelectric phase allow us to identify a polar instability that is unstable in both transverse-optic and longitudinal-optic versions, giving rise to an entire branch of instability along a propagation vector parallel to the mode polarization (the hexagonal axe). This is the hint of hyperferroelectricity and the stable head-to-head and tail-to-tail domain, as recently reported from both experiments and theory. Then, our analysis of the ferroelectric phase shows that the polarization of ${\mathrm{Pb}}_5{\mathrm{Ge}}_3{\mathrm{O}}_{11}$ is uniaxial along the hexagonal axes and with small in-plane components due to a piezoelectric effect. The symmetry-adapted mode analysis shows that the total ferroelectric ground-state distortion comes mainly from polar distortions of the unstable polar phonon mode but also from an invariant, cooperative mode that amplifies the polar deformation. We also build a phenomenological model that highlights how the coupling between these modes is at play and helps us understand how to reproduce the second-order phase transition. Lastly, we also quantify the structural chirality through the continuous symmetry measure method and trace its origin to the polar unstable mode itself. By extending our approach to the phonon states, we further show that the chirality is poorly affected by the relaxation but could also be enhanced by activating high-frequency modes with polar symmetry. Finally, we study the phonon angular momentum (AM) distribution in both phases and identify trends in the AM behavior across the Brillouin zone.

A magnon-assisted electrically driven “mirror” with tuned frequency of the reflected light is created. The effect of in-plane electric current on asymmetry of Stokes–anti-Stokes Brillouin intensities in NiFe/IrMn exchange-biased patterned structures is found. Electric current directed along external magnetic field suppresses anti-Stokes component in Brillouin light-scattering spectra, while the opposite direction of current results in suppression of the Stokes component. We show that spin currents arising in the IrMn layer induce spin-orbit torque affecting uncompensated antiferromagnetic moments at the interface, rotate the antiferromagnetic moments, change bias field, and finally determine direction of dominating spin-wave propagation. The possibility of electric control of spin-wave direction and manipulation over Stokes and anti-Stokes intensities opens a way for design of electro-optical devices switching light frequency.

We report a detailed investigation of the magnetic and electrical transport properties in an antiferromagnetic CeGaSi crystal. It orders ferromagnetically within the $ab$ plane and antiferromagnetically along the $c$ axis below $T_N(\sim 12\mathrm{K})$. The magnetotransport in CeGaSi shows anisotropy with the emergence of negative magnetoresistance (MR) within the $ab$ plane and along the $c$ axis. At low temperatures, the initially negative MR within the $ab$ plane changes sign and becomes positive with $B$ further increasing and approaching saturation. When the field is applied along the $c$ axis, the MR remains negative, as a result of the suppression of spin scattering in the antiferromagnetic state. Interestingly, different Hall responses are also demonstrated. A prominent anomalous Hall effect (AHE) is observed with $B\parallel ab$ in CeGaSi with a maximum value of anomalous Hall conductivity $|{\sigma }_{xz}^A|\sim 128{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$. The scaling behaviors of anomalous Hall resistivity and conductivity indicate the skew scattering as the dominant mechanism of AHE in CeGaSi. However, the AHE is absent and only the normal Hall effect is observed with $B\parallel c$. These fascinating findings in MR and Hall measurements not only establish CeGaSi as a new platform to study AHE, but also deepen our understanding on the interesting physical phenomena associated with magnetic properties.

In this paper, we establish a connection between the bulk topological structure and the magnetic properties of drumhead surface states of nodal loop semimetals. We identify the magnetic characteristics of the surface states and compute the system's magnon spectrum by treating electron-electron interactions on a mean-field level. We draw attention to a subtle connection between a Lifshitz-like transition of the surface states driven by mechanical distortions and the magnetic characteristics of the system. Our findings may be experimentally verified, e.g., by spin-polarized electron energy loss spectroscopy of nodal semimetal surfaces.

The Jordan-Wigner map in two dimensions is an exact lattice regularization of the $2\pi$-flux attachment to a hard-core boson (or spin $\frac{1}{2}$) leading to a composite fermion particle. When the spin-$\frac{1}{2}$ model obeys ice rules this map preserves locality, namely, local Rohkshar-Kivelson (RK) models of spins are mapped onto local models of Jordan-Wigner composite fermions. Using this composite fermion dual representation of RK models we construct spin-liquid states by projecting Slater determinants onto the subspaces of the ice rules. Interestingly, we find that these composite fermions behave as “dipolar” partons for which the projective implementations of symmetries are very different from standard “pointlike” partons. We construct interesting examples of these composite Fermi-liquid states that respect all microscopic symmetries of the RK model. In the six-vertex subspace, we constructed a time-reversal and particle-hole-invariant state featuring two massless Dirac nodes, which is a composite fermion counterpart to the classic $\pi$-flux state of Abrikosov fermions in the square lattice. This state is a good ground-state candidate for a modified RK–type Hamiltonian of quantum spin ice. In the dimer subspace, we constructed a state featuring a composite Fermi surface but with nesting instabilities towards ordered phases such as the columnar state. We have also analyzed the low-energy emergent gauge structure. If one ignores confinement, the system would feature a $\text{U}\left(1\right)\times \text{U}\left(1\right)$ low-energy gauge structure with two associated gapless photon modes, but with the composite fermion carrying gauge charge only for one photon and behaving as a gauge-neutral dipole under the other. These states are also examples of pseudoscalar $\text{U}\left(1\right)$ spin liquids where mirror and time-reversal symmetries act as composite fermion particle-hole conjugations, and the emergent magnetic fields are even under such time-reversal or lattice mirror symmetries.

We theoretically study gate-defined one-dimensional channels in planar Ge hole gases as a potential platform for non-Abelian Majorana zero modes. We model the valence band holes in the Ge channel by adding appropriate confinement potentials to the 3D Luttinger-Kohn Hamiltonian, additionally taking into account a magnetic field applied parallel to the channel, an out-of-plane electric field, as well as the effect of compressive strain in the parent quantum well. Assuming that the Ge channel is proximitized by an $s$-wave superconductor (such as Al) we calculate the topological phase diagrams for different channel geometries, showing that sufficiently narrow Ge hole channels can indeed enter a topological superconducting phase with Majorana zero modes at the channel ends. We estimate the size of the topological gap and its dependence on various system parameters such as channel width, strain, and the applied out-of-plane electric field, allowing us to critically discuss under which conditions Ge hole channels may manifest Majorana zero modes. Since ultraclean Ge quantum wells with hole mobilities exceeding one million and mean-free paths on the order of many microns already exist, gate-defined Ge hole channels may be able to overcome some of the problems caused by the presence of substantial disorder in more conventional Majorana platforms.