We compare the frequency-resolved frozen phonon multislice (FRFPMS) method [introduced in P. M. Zeiger and J. Rusz, Phys. Rev. Lett. 124, 025501 (2020)] with other theoretical approaches used to account for the inelastic scattering of high-energy electrons, namely, the first-order Born approximation and the quantum excitation of phonons model. We show that these theories lead to similar expressions for the single inelastically scattered intensity as a function of momentum transfer for an anisotropic quantum harmonic oscillator in a weak phase object approximation of the scattered waves, except for a too small smearing of the scattering potential by the effective Debye-Waller factor (DWF) in the FRFPMS method. We propose that this issue can be fixed by including an explicit DWF smearing into the potential and demonstrate numerically that in any realistic situation, a FRFPMS approach revised in this way correctly accounts for the single inelastically scattered intensity and the correct elastic scattering intensity. Furthermore, our simulations illustrate that the only requirement for such a revised FRFPMS method is the smallness of mean-squared displacements for all atomic species in all frequency bins. The analytical considerations for the FRFPMS method also explain the $1/{\omega }^2$ scaling of FRFPMS spectra observed by P. M. Zeiger and J. Rusz [Phys. Rev. B 104, 104301 (2021)] by the use of classical statistics in the molecular dynamics simulation. Moreover, we find that the FRFPMS method inherently adds the contributions of phonon loss and gain within each frequency bin. Both of these issues related to the frequency scaling can be fixed by a system-independent postprocessing step.

The anomalous Hall effect (AHE) is essential for developing highly tunable spintronic devices. An interesting phenomenon with AHE is the appearance of an additional hump on the respective magnetic films. However, it is still being disputed whether it originates from an intrinsic mechanism of the Berry phase accumulation in real space or is artificially generated by inhomogeneities. This work systematically investigates the magnetism, electronic transport, and Hall effect of $\mathrm{Ni}{\mathrm{Co}}_2{\mathrm{O}}_4$ (NCO) films with different thicknesses. The investigation reveals that the strong perpendicular magnetic anisotropy, metallic transport, and AHE are coexisting in NCO films. Especially, the sharp spikes in anomalous Hall effect are observed in thinner NCO films. The two-channel AHE model is used to explain the origin of the sharp spikes associated with anomalous Hall effect. It indicates that the phase separation is the reason for the sharp spikes. In addition, the mixed valence state, ferrimagnetic order, and the electronic configuration in NCO films are further analyzed. This study is essential for a comprehensive understanding of NCO films.

Probing the superconducting order parameter symmetry is a crucial step towards understanding the pairing mechanism in unconventional superconductors. Inspired by the recent discoveries of superconductivity in various van der Waals materials and the availability of the relative twist angle as a continuous tuning knob in these systems, we propose a general setup for probing the order parameter symmetry of two-dimensional superconductors in twisted Josephson junctions. The junction is composed of an anisotropic $s$-wave superconductor as a probe and another superconductor with an unknown order parameter symmetry. Assuming momentum-resolved tunneling, we investigate signatures of different order parameter symmetries in the twist angle dependence of the critical current, the current-phase relations, and magnetic field dependence. As a concrete example, we study a twisted Josephson junction between ${\mathrm{NbSe}}_2$ and magic angle twisted bilayer graphene.

We develop an exactly solvable model with Kitaev-type interactions and study its phase diagram on the dual lattice of the quasicrystalline Ammann-Beenker lattice. Our construction is based on the $\mathrm{\Gamma }$-matrix generalization of the Kitaev model and utilizes the cut-and-project correspondence between the four-dimensional simple cubic lattice and the Ammann-Beenker lattice to designate four types of bonds. We obtain a rich phase diagram with gapped (chiral and abelian) and gapless spin liquid phases via Monte Carlo simulations and variational analysis. We show that the ground state can be further tuned by the inclusion of an onsite term that selects 21 different vison configurations while maintaining the integrability of the model. Our results highlight the rich physics at the intersection of quasicrystals and quantum magnetism.

The magnetic and transport properties of ${\mathrm{BiFeO}}_3/{\mathrm{La}}_2{\mathrm{NiMnO}}_6$ (BFO/LNMO) composite were investigated both experimentally and theoretically. Unlike the normal rhombohedral $\left(R3c\right)$ phase, BFO in composites is crystallized in the triclinic phase $\left(P1\right)$. Interestingly, the composites demonstrate a significant enhancement in magnetization and magnetoelectric coupling and show higher resistivity than regular BFO $\left(R3c\right)$. As LNMO has its Curie temperature at 280 K, the room temperature and above room temperature magnetic contribution in the composites is expected to be from the triclinic BFO phase. Experimentally observed enhancement in magnetization is validated using classical Monte Carlo simulation and density functional theory (DFT) calculations. The calculations reveal higher magnetic moments in triclinic BFO as compared to the rhombohedral BFO. Overall, this study reveals triclinic BFO as a promising room temperature multiferroic phase which is helpful to optimize the multiferroicity of BFO and achieve wider applications in future.

We investigate the possibility to employ magnetic Josephson junctions as magnetic noise detectors. To illustrate our idea, we consider a system consisting of a quantum dot coupled to superconducting leads in the presence of an external magnetic field. Under appropriate assumptions, we relate the noise in the Josephson current to magnetization noise. At the magnetic field driven 0-$\pi$ transition, the junction sensitivity as magnetic noise detector is strongly enhanced and it diverges in the zero temperature limit. Moreover, we demonstrate that, if also dot energy is affected by fluctuations, only the magnetic noise channel contributes to Josephson current noise response when the quantum dot is tuned in resonance with superconducting leads.

We investigate the effects of the curved geometry on a massless relativistic electron constrained to a graphene strip with a Möbius strip shape. The anisotropic and parity violating geometry of the Möbius band produces a geometric potential that inherits these features. By considering wires along the strip width and the strip length, we find exact solutions for the Dirac equation and the effects of the geometric potential on the electron were explored. In both cases, the geometric potential yields to a geometric phase on the wave function. Along the strip width, the density of states depends on the direction chosen for the wire, a consequence of the lack of axial symmetry. Moreover, the breaking of the parity symmetry enables the electronic states to be concentrated on the inner or on the outer portion of the strip. For wires along the strip length, the nontrivial topology influences the eigenfunctions by modifying their periodicity. It turns out that the ground state has a period of $4\pi$ whereas the first excited state is a $2\pi$ periodic function. Moreover, we found that the energy levels are half-integer multiples of the energy of the ground state.

We extend and benchmark the recently developed Effective-Hamiltonian (EFFH) method [PRX Quantum 4, 020307 (2023)] as an approximation to the equilibrium state (“mean-force Gibbs state”) of a quantum system at strong coupling to a thermal bath. The EFFH method is an approximate framework. Through a combination of the reaction-coordinate mapping, a polaron transformation, and a controlled truncation, it imprints the system-bath coupling parameters into the system's Hamiltonian. First, we develop a variational EFFH technique. In this method, the system's parameters are renormalized by both the system-bath coupling parameters (as in the original EFFH approach) and bath temperature. Second, adopting the generalized spin-boson model, we benchmark the equilibrium state from the EFFH treatment against numerically exact simulations and demonstrate a good agreement for both polarization and coherences using the Brownian spectral function. Third, we contrast the (normal and variational) EFFH approach with the familiar (normal and variational) polaron treatment. We show that the two methods predict a similar structure for the equilibrium state, albeit the EFFH approach offers the advantage of simpler calculations and closed-form analytical results. Altogether, we argue that for temperatures comparable to the system's frequencies, the EFFH methodology provides a good approximation for the mean-force Gibbs state in the full range of system-bath coupling, from ultraweak to ultrastrong.

The quantum anomalous Hall effect (QAHE) has attracted significant attention as it provides intriguing platforms for exploring prominent physical phenomena and applications of low-dissipation devices. Here, we put forward that, unlike previously reported QAH insulators emerging with either out-of-plane or in-plane magnetizations, robust QAHE can be obtained in a two-dimensional ferromagnet regardless of the magnetization directions. In particular, we identify the intrinsic ferromagnetic ${\mathrm{NiBiO}}_3$ as a feasible candidate material with the nontrivial topology explicitly confirmed by nonzero integer Chern numbers $\mathcal{C}$ and the emergence of chiral edge states. Moreover, we proclaim that the Chern numbers can be tuned and a high-Chern-number QAHE, characterized by $\mathcal{C}=\pm 3$, is obtained by tuning the magnetization from the in-plane to out-of-plane direction. Our findings not only advance the general understanding of QAHE but also put forward potential applications in topotronics.

Twisted bilayer (t-BL) transition-metal dichalcogenides (TMDCs) have attracted considerable attention in recent years due to their distinctive electronic properties, which arise due to the moiré superlattices that lead to the emergence of flat bands and correlated electron phenomena. Also, these materials can exhibit interesting thermal properties, including a reduction in thermal conductivity. In this paper, we report the thermal conductivity of monolayer (1L) and t-BL $\mathrm{Mo}{\mathrm{Se}}_2$ at some specific twist angles around two symmetric stacking $\mathit{AB} \left(0^{\circ }\right)$ and $A{B}^{\prime }$ (${60}^{\circ }$) and one intermediate angle $\left({31}^{\circ }\right)$ using the optothermal Raman technique. The observed thermal conductivity values are found to be $13\pm 1, 23\pm 3$, and $30\pm 4{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1}$ for twist angle $\left[\theta \right]={58}^{\circ }, {31}^{\circ }$, and $3^{\circ }$, respectively, which are well supported by our first-principles calculation results. The reduction in thermal conductivity in t-BL $\mathrm{Mo}{\mathrm{Se}}_2$ compared to monolayer $(38\pm 4{\mathrm{Wm}}^{-1}{\mathrm{K}}^{-1})$ can be explained by the occurrence of phonon scattering caused by the formation of a moiré superlattice. Herein, the emergence of multiple folded phonon branches and modification in the Brillouin zone caused by in-plane rotation are also accountable for the decrease in thermal conductivity observed in t-BL $\mathrm{Mo}{\mathrm{Se}}_2$. The theoretical phonon lifetime study and electron localization function analysis further reveals the origin of angle-dependent thermal conductivity in t-BL $\mathrm{Mo}{\mathrm{Se}}_2$. This work paves the way towards tuning the angle-dependent thermal conductivity for any bilayer TMDCs system.

This paper is concerned with the physics of parametrized gapped quantum many-body systems, which can be viewed as a generalization of conventional topological phases of matter. In such systems, rather than considering a single Hamiltonian, one considers a family of Hamiltonians that depend continuously on some parameters. After discussing the notion of phases of parametrized systems, we formulate a bulk-boundary correspondence for an important bulk quantity, the Kapustin-Spodyneiko higher Berry curvature, first in one spatial dimension and then in arbitrary dimension. This clarifies the physical interpretation of the higher Berry curvature, which in one spatial dimension is a flow of (ordinary) Berry curvature. In $d$ dimensions, the higher Berry curvature is a flow of $(d-1)$-dimensional higher Berry curvature. Based on this, we discuss one-dimensional systems that pump Chern number to/from spatial boundaries, resulting in anomalous boundary modes featuring isolated Weyl points. In higher dimensions, there are pumps of the analogous quantized invariants obtained by integrating the higher Berry curvature. We also discuss the consequences for parametrized systems of Kitaev's proposal that invertible phases are classified by a generalized cohomology theory, and emphasize the role of the suspension isomorphism in generating new examples of parametrized systems from known invertible phases. Finally, we present a pair of general quantum pumping constructions, based on physical pictures introduced by Kitaev, which take as input a $d$-dimensional parametrized system, and produce new $(d+1)$-dimensional parametrized systems. These constructions are useful for generating examples, and we conjecture that one of the constructions realizes the suspension isomorphism in a generalized cohomology theory of invertible phases.

Magnetism plays a key role in the emergence of topological phenomena in the Weyl semimetal ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$, which exhibits ferromagnetic interactions along the $c$-axis of the crystal and antiferromagnetic (AFM) interactions within the $ab$ plane. Extensive studies on the temperature dependence of the magnetism with the magnetic field along the $c$-axis have uncovered a number of magnetic phases. Currently, the nature and origins of the reported magnetic phases are under debate. Here we report on magnetic field orientation effects on the magnetism in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$. The shape of the hysteresis loop of the Hall resistance at a fixed temperature is found to change from rectangular to bow tie-like as the magnetic field is tilted from the $c$-axis toward the $ab$ plane, resembling that reported for magnetic fields along the $c$-axis as the temperature approaches the Curie temperature from below. Unlike their temperature-dependent counterparts, the newly observed bow tie-like hysteresis loops show exchange bias. Our results showcase the contribution of the in-plane AFM interactions to the magnetism in ${\mathrm{Co}}_3{\mathrm{Sn}}_2{\mathrm{S}}_2$ and demonstrate a new way to tune its magnetic phases. They also shed light on the temperature-dependent magnetic phases occurring in the magnetic field along the $c$-axis of the crystal.

Quasi-one-dimensional systems, having tendencies toward density-wave order in competition with superconductive pairing in their ground states, may give rise to unconventional superconductivity, a central theme in condensed matter physics. Partial density-wave gapping of electronic bands at the Fermi surface in such systems can yield superconductivity at very low carrier density that challenges Bardeen-Cooper-Schrieffer (BCS) theory since the pairing energy scale may approach or exceed the Fermi energy and render screening of the Coulomb interaction ineffective. Here we present low-$T$ magnetotransport measurements on the quasi-one-dimensional conductor ${\mathrm{Li}}_{0.9}{\mathrm{Mo}}_6{\mathrm{O}}_{17}$ showing the metallic state from which superconductivity emerges ($T_c\simeq 2$ K) to possess among the lowest known carrier densities, $\sim {10}^{17}{\mathrm{cm}}^{-3}$, and a ratio of $T_c$ to Fermi temperature within the BCS–Bose-Einstein-condensation crossover regime. A semimetallic state caused by a density-wave-induced Fermi surface reconstruction with highly anisotropic electron and hole pockets is implied. The degree of interpocket nesting appears to determine whether the extreme low-density ground state is superconducting or undergoes additional Fermi surface gapping.

A combined study employing density functional theory (DFT) using the experimentally determined modulated structures in the martensite phase and bulk-sensitive hard x-ray photoelectron spectroscopy of stoichiometric single-crystalline ${\mathrm{Ni}}_2\mathrm{MnGa}$ is presented in this paper. The experimental valence band (VB) features closely match the theoretical VB calculated by DFT using generalized gradient approximation for both the martensite and austenite phases. We establish the existence of a charge density wave (CDW) state in the martensite phase from the shape of the VB near the Fermi level ($E_F$). This shows (i) a transfer of spectral weight from the near $E_F$ region to the higher binding energy side resulting in a dip-peak structure in the difference spectrum that is in excellent agreement with DFT and (ii) presence of a pseudogap at $E_F$ that is portrayed by fitting the near $E_F$ region with a power-law function. The present paper emphasizes the electronic origin and the role of the atomic modulation in hosting the CDW state in the martensite phase of stoichiometric ${\mathrm{Ni}}_2\mathrm{MnGa}$.

The thermal conductivity of monolayer graphene is widely believed to surpass that of diamond even for few-micron-size samples and was proposed to diverge with system size. Here, we predict the thermal conductivity from first principles by considering four-phonon scattering, phonon renormalization, an exact solution to the phonon Boltzmann transport equation (BTE), and a dense enough sampling grid. We show that at room temperature, the thermal conductivity saturates at 10 $\mu \mathrm{m}$ system size and converges to 1300 W/($\mathrm{m}\mathrm{K}$), which is lower than that of diamond. This indicates that four-phonon scattering overall contributes 57% to the total thermal resistance and becomes the leading phonon scattering mechanism over three-phonon scattering. On the contrary, considering three-phonon scattering only yields higher-than-diamond values and divergence with size due to the momentum-conserving normal processes of flexural phonons.

The intensive study of noncollinear magnets promotes an urgent demand for the quantitative characterization of the noncollinear magnetic structures, which host numerous exotic phenomena. Here we systematically study the noncollinear magnetic structure of an artificial ferrimagnetic multilayer. The anisotropic magnetoresistance (AMR) measurements reveal two distinct twisted states whose magnetic structures can be quantitatively characterized with the assistance of micromagnetic simulations. Our results manifest AMR as an ideal probe of the noncollinear magnetic structure in artificial ferrimagnets.

It is found that single-cycle THz electromagnetic fields efficiently excite a GHz spin resonance mode in ferrimagnetic ${\mathrm{Tm}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$, despite the near absence of GHz spectral components in the exciting THz pulse. By analyzing how the efficiency of excitation depends on the orientation and strength of the THz electric field, we show that it can be explained in terms of the nonlinear THz inverse Cotton-Mouton effect. Here, the THz electric field gets effectively rectified and acts on the ferrimagnetic spins as a unipolar effective magnetic field pulse. This interpretation is confirmed by a theoretical model based on the phenomenological analysis of the effective magnetic field, combined with the equations of motion derived from the effective Lagrangian for a ferrimagnet. Moreover, by using the outcome of two-dimensional THz spectroscopy, we conjecture a quantum-mechanical interpretation of the observed effect in terms of impulsive stimulated Raman scattering of THz photons by the crystal-field split $f\text{−}f$ electronic transitions of ${\mathrm{Tm}}^{3+}$.

We performed magnetization, resistivity, and neutron diffraction measurements under uniaxial stress applied along the $\left[1\overline{1}0\right]$ direction on the tetragonal magnet ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ with commensurate magnetic orders. ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$ has two successive antiferromagnetic (AFM) orders in zero magnetic field. The high temperature phase (AFM1 phase) has the magnetic modulation wave vector of $q=(\frac{1}{2},\frac{1}{2},0)$, and the low temperature phase (AFM2 phase) is characterized by the four $q$-vectors of $q=(\frac{1}{2},\frac{1}{2},0),(\frac{1}{2},-\frac{1}{2},0),(\frac{1}{2},\frac{1}{2},\frac{1}{2})$, and $(\frac{1}{2},-\frac{1}{2},\frac{1}{2})$. By measuring the uniaxial stress dependence of the magnetization, resistivity and the intensities of magnetic Bragg reflections, we confirmed that the AFM1 phase has the single-$q$ magnetic order with twofold rotational symmetry and the AFM2 phase has the multi-$q$ magnetic order with fourfold rotational symmetry. To understand the origin of multi-$q$ order of ${\mathrm{CeRh}}_2{\mathrm{Si}}_2$, we also performed inelastic neutron scattering measurement on the single crystal samples. We found a magnetic excitation at the transfer energy $\hslash \omega \sim 8\mathrm{meV}$. By applying the linear spin-wave theory, we found that the nearest- and next-nearest neighbor exchange interactions on the $ab$ plane, $J_1$ and $J_2$, are dominant in the AFM2 phase. However, the $J_1\text{−}{J}_2$ model cannot lift the degeneracy between the single-$q$ (AFM1) and multi-$q$ (AFM2) phases. We suggest that it can be lifted by taking into account the biquadratic interaction derived from the perturbative expansion for the Kondo lattice Hamiltonian [S. Hayami et al., Phys. Rev. B 95, 224424 (2017)].

We report a sizable Dzyaloshinskii-Moriya interaction (DMI) originating at an epitaxial ${\mathrm{Fe}}_3\mathrm{Si}\text{/}\mathrm{Ge}$ interface. Using spin-wave spectroscopy, we show that the magnitude of the interfacial DMI of the Heusler-ferromagnet/semiconductor hybrid structure is comparable to that of ferromagnet/Pt structures, despite the absence of heavy elements. We find that the observed DMI at the ${\mathrm{Fe}}_3\mathrm{Si}\text{/}\mathrm{Ge}$ interface is consistent with the prediction of an antisymmetric exchange interaction induced by the interfacial Bychkov-Rashba spin-orbit interaction. These results demonstrate that ferromagnet/semiconductor hybrid structures are a promising class of systems for chiral spintronics.