We study theoretically the polarization dynamics in a ring-shape bosonic condensate of exciton-polaritons confined in a rotating trap. The interplay between the rotating potential and TE-TM splitting of polariton modes offers a tool of control over the spin state and the angular momentum of the condensate. Specific selection rules describing the coupling of pseudospin and angular momentum are formulated. The resonant coupling between states having linear and circular polarizations leads to the polarization beats. The effect may be seen as a polariton analogy to the electronic magnetic resonance in the presence of constant and rotating magnetic fields. Remarkably, spin beats are induced by a purely mechanical rotation of the condensate.

We theoretically study magnetic ground states of doped zigzag graphene nanoribbons and the emergence of topological domain walls. Using the Hartree-Fock mean-field approach and an effective continuum model, we demonstrated that the carrier doping stabilizes a magnetic structure with alternating antiferromagnetic domains, where the doped carriers are accommodated in topological bound states localized at the domain wall. The energy spectrum exhibits a Hofstadter-type fractal spectral evolution as a function of the carrier density, where minigaps are characterized by the Chern number associated with the adiabatic charge pump in moving domain walls. A systematic analysis for nanoribbons with different widths revealed that the ferromagnetic domain-wall phase emerges in relatively wide ribbons, while the collinear domain-wall phase arises in narrower ribbons.

In this work, a generalized force-field methodology for the relaxation of large moiré heterostructures is proposed. The force-field parameters are optimized to accurately reproduce the structural degrees of freedom of some computationally manageable cells relaxed using density functional theory. The parameters can then be used to handle large moiré systems. We specialize in the case of 2H-phased twisted transition-metal dichalcogenide homo- and heterobilayers using a combination of the Stillinger-Weber intralayer and the Kolmogorov-Crespi interlayer potential. Force-field parameters are developed for all combinations of ${MX}_2$ for $M\in \{\text{Mo},\text{W}\}$ and $X\in \{\text{S},\text{Se},\text{Te}\}$. The results show agreement within 20 meV in terms of band structure between density functional theory and force-field relaxation. Using the relaxed structures, a simplified and systematic scheme for the extraction of the interlayer moiré potential is presented for both R- and H-stacked systems. We show that in-plane and out-of-plane relaxation effects on the moiré potential, which is made both deeper and wider after relaxation, are essential. An interpolation based methodology for the calculation of the interlayer binding energy is also proposed. Finally, we show that atomic reconstruction, which is captured by the force-field method, becomes especially prominent for angles below $4^{\circ }–5^{\circ }$, when there is no mismatch in lattice constant between layers.

One of the recent surprising discoveries is the crystal-axis-dependent anisotropic magnetoresistance (CAMR) that depends on two magnetization components perpendicular to the current differently, in contrast to the conventional anisotropic magnetoresistance that predicts no change in resistance when the magnetization varies in the plane perpendicular to the current. Using density functional theory and Boltzmann transport equation calculations for bcc Fe, hcp Co, and bcc FeCo alloys, we show that CAMR can be accounted for by the magnetization-dependent spin-orbit interactions (SOI): Magnetization-dependent SOI modifies electron energy bands that, in turn, changes resistance. A phenomenological model reveals the intrinsic connection between SOI and order parameters. Such a mechanism is confirmed by the strong biaxial stain effect on CAMR. Our findings provide an efficient way of searching and optimizing materials with large CAMR, important in the design of high-performance spintronic devices.

In a metal with multiple Fermi pockets, the formation of $s$-wave superconductivity can be conventional due to electron-phonon coupling or unconventional due to spin fluctuations. We analyze the hexagonal diboride ${\text{CrB}}_2$, which is an itinerant antiferromagnet at ambient conditions and turns superconducting upon increasing pressure. While the high-pressure behavior of $T_c$ suggests conventional $s$-wave pairing, we find that spin fluctuations promoting unconventional $s$-wave pairing become important in the vicinity of the antiferromagnetic dome. As the symmetry class of the $s$-wave state is independent of its underlying mechanism, we argue that ${\text{CrB}}_2$ is a realization of a hybrid $s$-wave superconductor where unconventional and conventional $s$-wave mechanisms team up to form a joint superconducting dome.

We investigate the total energies of spontaneous spin-valley polarized states in bi-, tri-, and tetralayer rhombohedral graphene where the long-range Coulomb correlations are accounted for within the random phase approximation. Our analysis of the phase diagrams for varying carrier doping and perpendicular electric fields shows that the exchange interaction between chiral electrons is the main driver of spin-valley polarization, while the presence of Coulomb correlations brings the flavor polarization phase boundaries to carrier densities close to the complete filling of the Mexican hat shape top at the Dirac points. We find that the tendency towards spontaneous spin-valley polarization is enhanced with the chirality of the bands and therefore with increasing number of layers.

We developed an effective procedure to single out the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction from the other exchange mechanisms in metallic two-dimensional (2D) magnetic materials, and applied this procedure to study two prototypical systems, 2D $\mathrm{Cr}{\mathrm{S}}_2$ and ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$, using first-principles calculations. In particular, the RKKY interaction is nearly independent of the lattice size in a reasonably large range and the number of $\mathrm{Cr}{\mathrm{S}}_2$ layers. In contrast, the other magnetic interactions such as superexchange strongly depend on the lattice size and thickness of $\mathrm{Cr}{\mathrm{S}}_2$ films. In ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$, the RKKY interaction is relatively weak and does not play a leading role in the magnetic structure, whereas the strong intralayer antiferromagnetic superexchange and interlayer ferromagnetic direct exchange dominate the magnetic configuration of ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$. Our work uncovers the universal role of the 2D RKKY interaction and proposes an effective way to analyze and control the magnetic coupling of 2D magnetic materials.

Zero-dimensional (0D) lead-free halide perovskites doped with ${\mathit{ns}}^2$-metal ions have emerged as an interesting research topic, due to their broad luminescence spectra and large Stokes shift for both fundamental research and future optoelectronic applications. Herein, we report an abnormal phenomenon of pressure-induced blue-shifted and enhanced emission in ${\mathrm{Sb}}^{3+}$-doped ${\mathrm{Cs}}_2{\mathrm{InBr}}_5·{\mathrm{H}}_2\mathrm{O}$. The pressure-induced photoluminescence enhancement (<0.6 GPa) of ${\mathrm{Sb}}^{3+}$:${\mathrm{Cs}}_2{\mathrm{InBr}}_5·{\mathrm{H}}_2\mathrm{O}$ can be ascribed to the less energy difference between ${}^3{P}_1$ of ${\mathrm{Sb}}^{3+}$ and self-trapped exciton (STE) state, and the reduced nonradiative energy loss. Meanwhile, the pressure-induced anisotropic contraction of inhomogeneously coordinated [${\mathrm{SbBr}}_5\mathrm{O}]{\mathrm{H}}_2^{2-}$/[${\mathrm{InBr}}_5\mathrm{O}]{\mathrm{H}}_2^{2-}$ octahedra is highly responsible for the unusual blue-shifted STE emission and the variation of the band gap. These findings provide a basis for understanding the structure-properties correlation of ${\mathit{ns}}^2$-metal ions doped 0D perovskite, and shed light on the effects of the pressure in emission enhancement and tuning the optical properties of halide perovskites.

Recent neutron-diffraction experiments in honeycomb ${\mathrm{CrI}}_3$ quasi-2D ferromagnets have evinced the existence of a gap at the Dirac point in their spin-wave spectra. The existence of this gap has been attributed to strong in-plane Dzyaloshinskii-Moriya or Kitaev (DM/K) interactions and suggested to set the stage for topologically protected edge states to sustain non-dissipative spin transport. We perform state-of-the-art simulations of the spin-wave spectra in monolayer ${\mathrm{CrI}}_3$, based on time-dependent density-functional perturbation theory (TDDFpT) and fully accounting for spin-orbit couplings (SOC) from which DM/K interactions ultimately stem. While our results are in qualitative agreement with experiments, the computed TDDFpT magnon gap at the Dirac point is found to be 0.47 meV, roughly six times smaller than the most recent experimental estimates, so questioning that intralayer anisotropies alone can explain the observed gap. Lattice-dynamical calculations, performed within density-functional perturbation theory (DFpT), indicate that a substantial degeneracy and a strong coupling between vibrational and magnetic excitations exist in this system, providing a possible additional gap-opening mechanism in the spin-wave spectra. To pursue this path, we introduce an interacting magnon-phonon Hamiltonian featuring a linear coupling between lattice and spin fluctuations, enabled by the magnetic anisotropy induced by SOC. Upon determination of the relevant interaction constants by DFpT and supercell calculations, this model allows us to propose magnon-phonon interactions as an important microscopic mechanism responsible for the enhancement of the gap in the range of $\approx 4\mathrm{meV}$ around the Dirac point of the ${\mathrm{CrI}}_3$ monolayer.

Vortex lines in superconducting Dirac semimetals with $s$-wave pairing realize crystal symmetry protected gapless vortex line phases in which gapless excitations propagate inside a vortex line, in the presence of appropriate crystal symmetry, spin-orbit coupling, and multiband structures. Here we present a general scheme to classify possible gapless vortex line phases in $s$-wave superconducting states of Dirac semimetals with rotation (or screw) symmetry and inversion symmetry, assuming that the rotation (screw) axis is parallel to the vortex line. The rotation (screw) symmetry protected gapless modes are stable as long as they have different rotation (screw) eigenvalues. The underlying mechanism for the formation of gapless vortex bound states depends on irreducible representations of rotation (screw) symmetry subject to a vortex field and is classified into three types: (i) accidental band crossing of two vortex bound-state modes under rotation symmetry; (ii) accidental and (iii) enforced band crossing of four vortex bound-state modes under screw symmetry. We present a tight-binding model of screw symmetry protected Dirac semimetal with an $s$-wave pair potential, demonstrating a gapless vortex line phase of type (ii). We obtain four gapless modes of vortex bound states whose gapless points (Majorana zero modes) pinned at a time-reversal invariant momentum (TRIM) when the Fermi energy is close to the Dirac points. As the Fermi energy is moved away from the Dirac points, the four gapless modes are split into a pair of two gapless modes with vanishing excitation energy at non-TRIMs. In closing, we discuss ${\mathrm{Nb}}_3\mathrm{Pt}$ as a candidate material with the fourfold screw symmetry protected Dirac cones that can host a gapless vortex line phase.

We investigate hyperfine interaction (HFI) using density-functional theory for several open-shell planar $s{p}^2$-carbon nanostructures displaying $\pi$ magnetism. Our prototype structures include both benzenoid ($\left[n\right]\mathrm{triangulenes}$ and a graphene nanoribbon) as well as nonbenzenoid (indene, fluorene, and indene[2,1-b]fluorene) molecules. Our results obtained with orca indicate that isotropic Fermi contact and anisotropic dipolar terms contribute in comparable strength, rendering the HFI markedly anisotropic. We find that the magnitude of HFI in these molecules can reach more than 100 MHz, thereby opening up the possibility of experimental detection via methods such as electron spin resonance-scanning tunneling microscopy (ESR-STM). Using these results, we obtain empirical models based on $\pi$-spin polarizations at carbon sites. These are defined by generic $s{p}^2$ HFI fit parameters which are derived by matching the computed HFI couplings to $\pi$-spin polarizations computed with methods such as Orca, Siesta, or mean-field Hubbard (MFH) models. This approach successfully describes the Fermi contact and dipolar contributions for ${}^{13}\mathrm{C}$ and ${}^1\mathrm{H}$ nuclei. These fit parameters allow to obtain hyperfine tensors for large systems where existing methodology is not suitable or computationally too expensive. As an example, we show how HFI scales with system size in $\left[n\right]\mathrm{triangulenes}$ for large $n$ using MFH. We also discuss some implications of HFI for electron-spin decoherence and for coherent nuclear dynamics.

Exchange coupling in an interfacial context is crucial for spin-torque nano-oscillator (STNO) that consists of a nonmagnetic spacer which is alloyed with a ferromagnetic material. Currently, investigations on the dynamics of the free-layer magnetization and frequency enhancement in the STNO with bilinear coupling are still being actively pursued. In this work, we investigate the dynamics of the STNO in the presence of bilinear coupling but in the absence of an external magnetic field by analyzing the associated Landau-Lifshitz-Gilbert-Sloncewski (LLGS) equation, and consequently the impact of the bilinear coupling on the dynamics of the magnetization of the free layer is studied. It is observed that the frequency of the oscillations in the magnetization component along the direction of the pinned layer polarization can be enhanced above 300 GHz by positive bilinear coupling and up to around 30 GHz by negative bilinear coupling. We further reveal a transition from in-plane to out-of-plane precession both for positive and negative bilinear couplings. We also analyze the switching of the magnetization for different values of current and bilinear coupling. Our detailed investigations of STNO with bilinear coupling aim at the possibilities of high-frequency devices by considering the applied current and bilinear coupling in the absence of a magnetic field.

We analyzed the distribution of the Meissner shielding currents in a flat superconducting ring and quantitatively described the penetration of magnetic avalanches (dendrites) inside it. Using a recurrent procedure, the external field $H_{\mathrm{ext}}$, in which a perforating (edge to edge crossing) avalanche appears, is calculated. A staircase dependence of the mean field trapped inside a ring, $\langle H\rangle$ vs $H_{\mathrm{ext}}$, is comprehensively described. A staircase slope appears to be a universal function of a ring shape (inner to outer diameter ratio). The heat released by a penetrating dendrite grows linearly with each next perforation. Flux pinning, if present, modifies a staircase dependence and makes steps smaller, but does not change the staircase slope. Our theoretical results are in a good accordance with the experimental data.

The coexistence of nontrivial topological properties and superconductivity in a material offers the potential to achieve topological superconductivity and Majorana zero modes. However, research on ideal topological semimetals with superconductivity has been limited. In this paper, we utilize first-principles calculations to predict the existence of three two-dimensional potassium borides (${\mathrm{K}}_2{\mathrm{B}}_9, {\mathrm{KB}}_9$, and ${\mathrm{KB}}_{18}$) with a hidden kagome-like lattice structure, inspired by the arrangement of the ${\mathrm{K}}_2{\mathrm{B}}_9{}^{-}$ cluster in an inverse sandwich configuration. Our findings reveal that all three materials exhibit phonon-mediated superconductivity, with transition temperatures ($T_c$) of 12.56, 14.46, and 10.50 K, respectively. Furthermore, ${\mathrm{K}}_2{\mathrm{B}}_9$ and ${\mathrm{KB}}_{18}$ are identified as topological nodal-line semimetals, while ${\mathrm{KB}}_9$ demonstrates the characteristics of an ideal Dirac semimetal due to the broken ${\sigma }_h$ mirror symmetry and appropriate potassium content. The topological properties arise from the band inversion of the boron atom's $p{}_x+p{}_y$ and $p{}_z$ orbitals within the ${\mathrm{B}}_9$ layer. Intriguingly, ${\mathrm{KB}}_9$ exhibits coexisting Dirac points and van Hove singularities near the Fermi level, which are a consequence of the hidden kagome-like lattice, thereby enhancing the superconducting transition temperature ($T_c$). This research not only sheds light on the pivotal role played by the orbital character of the ${\mathrm{B}}_9$ layer in the kagome-like lattice of potassium borides but also presents a strategy for achieving the coexistence of topological properties and superconductivity, thereby opening up possibilities for the realization of exotic physics.

We calculate the contribution to the doping ($n$) and temperature ($T$) dependence of the electrical resistivity of twisted bilayer graphene (TBLG) due to scattering by acoustic phonons. Our calculation retains the full Bistritzer-MacDonald (BM) band structure, with a focus on understanding the role of the complicated geometric features present in the BM band structure on electronic transport theory. We find that the band geometry plays an important role in determining the resistivity, giving an intricate dependence on both $n$ and $T$ that mirrors features in the band structure and complicates the Bloch-Grüneisen (BG) crossover. Our calculations predict pronounced departures from the standard simplistic expectation of a linear $T$ dependence above the BG crossover. In particular, we are able to explain the presence of the resistance peaks that have been observed in experiment, as well as quantitatively predict the temperatures at which they occur. Our calculated theoretical results are germane to an ongoing debate over the existence of a strange metal state in TBLG by providing a quantitatively accurate theory for the TBLG resistivity at finite temperatures.

Solving the Bethe-Salpeter equation (BSE) for the optical polarization functions is a first principles means to model optical properties of materials including excitonic effects. One almost ubiquitously used approximation neglects the frequency dependence of the screened electron-hole interaction. This is commonly justified by the large difference in the magnitude of electronic plasma frequency and exciton binding energy. We incorporated dynamical effects into the screening of the electron-hole interaction in the BSE using two different approximations as well as exact diagonalization of the exciton Hamiltonian. We compare these approaches for a naphthalene organic crystal, for which the difference between exciton binding energy and plasma frequency is only about a factor of ten. Our results show that in this case, corrections due to dynamical screening are about 15% of the exciton binding energy. We analyze the effect of screening dynamics on optical absorption across the visible spectral range and use our data to establish an effective screening model as a computationally efficient approach to approximate dynamical effects in complex materials in the future.

In this paper, we extend the method implementing Virasoro operators in a tensor network we proposed in Wang et al. [Phys. Rev. B 106, 115116 (2022)] and test it on the Fibonacci model, which is known to suffer from far more finite-size effects. To pick up the “seed” state that would flow to the stress tensor in the thermodynamic limit, we make use of the topological idempotent that projects the transfer matrix to the trivial sector. Combined with an optimization method, the seed state can be identified. We demonstrate that the descendant states in the Fibonacci model can be correctly generated with this approximate stress tensor, giving further evidence that the method applies more generally.

We present results of the magnetic susceptibility on high-quality single crystals of ${\mathrm{EuPd}}_2{\left({\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x\right)}_2$ for Ge concentrations $0\le x\le 0.105$ performed under varying hydrostatic (He-gas) pressure $0\le p\le 0.5$ GPa. The work extends recent studies at ambient pressure demonstrating the drastic change in the magnetic response from valence-crossover behavior for $x=0$ and 0.058, to long-range antiferromagnetic (AFM) order below $T_{\text{N}}=47$ K for $x=0.105$. The valence-crossover temperature $T_{\text{V}}^{\prime }$ shows an extraordinarily strong pressure dependence of $d{T}_{\text{V}}^{\prime }/dp=+(80\pm 10)$ K/GPa. In contrast, a very small pressure dependence of $d{T}_{\text{N}}/dp\le +(1\pm 0.5)$ K/GPa is found for the AFM order upon pressurizing the $x=0.105$ crystal from $p=0$ to 0.05 GPa. Remarkably, by further increasing the pressure to 0.1 GPa, a drastic change in the ground state from AFM order to valence-crossover behavior is observed. Estimates of the electronic entropy related to the Eu $4f$ electrons, derived from analyzing susceptibility data at varying pressures, indicate that the boundary between AFM order and valence crossover represents a first-order phase transition. Our results suggest a particular type of second-order critical end point of the first-order transition for $x=0.105$ at $p_{\text{cr}}\approx 0.06$ GPa and $T_{\text{cr}}\approx 39$ K where intriguing strong-coupling effects between fluctuating charge, spin, and lattice degrees of freedom can be expected.

We study equilibrium crystal shapes of a topological insulator (TI), a topological crystalline insulator (TCI) protected by mirror symmetry, and a second-order topological insulator (SOTI) protected by inversion symmetry. By adding magnetic fields to the three-dimensional TI, we can realize the mirror-symmetry-protected TCI and the inversion-symmetry-protected SOTI. They each have topological boundary states in different positions: The TCI has gapless states on the surfaces that are invariant under the symmetry operation, and the SOTI has gapless states at the intersections between certain surfaces. In this paper, we discuss how these boundary states affect the surface energies and the equilibrium crystal shapes in terms of the calculations of the simple tight-binding model by using the Wulff construction. By comparing the changes in the shapes of the TI with those of the trivial insulator through the process of applying the magnetic fields, we show that the presence or absence of the topological boundary states affects the emergence of the specific facets in a different way from the trivial insulator.

The cerium hexaboride and its alloyed compounds are strongly correlated materials, hosting many-body hidden order or nontrivial band topology. The fine electronic structure of the Ce $4f^1$ state, derived from spin-orbit coupling and crystal electric field splitting (CEF), plays a vital role in forming the exotic quantum state in Ce-based compounds. By using the high-resolution resonant inelastic x-ray scattering technique, we determined the fine electronic excitation energies of the Ce $4f^1$ state in the Kondo material ${\mathrm{Ce}}_{0.3}{\mathrm{Sm}}_{0.7}{\mathrm{B}}_6$. The extracted energy levels show an overall consistency with other scattering techniques, except that CEF splitting in the total angular momentum $J=7/2$ state is identified to be $(89.5\pm 1)$ meV, clearly larger than the value of 82 meV drawn from Raman spectroscopy. A detailed discussion is made to reconcile the mismatch. This work provides key information on the Ce $4f^1$ orbital in ${\mathrm{CeB}}_6$, which would also put a constraint on the related theoretical model, helping to address the challenges for theoretical dealings with many-body correlations in Ce $4f^1$ compounds.