Voltage-controlled magnetic anisotropy (VCMA) has attracted broad interest due to its high efficiency in switching magnetization. Large VCMA was experimentally observed in Cr/Fe/MgO junction with ultrathin Fe layer [Nozaki et al., Phys. Rev. Appl. 5, 044006 (2016)], whose underlying mechanism was still not clear however. The Cr/Fe/MgO/Fe magnetic tunnel junction (MTJ) is also well known for its quantum-well (QW) states and as-induced spin-dependent resonant tunneling [Greullet et al., Phys. Rev. Lett. 99, 187202 (2007)]. Here, in order to uncover the relation between the large VCMA and the QW states, we developed a $k$-resolved VCMA calculation method combined with the second-order perturbation theory to investigate it. We find the VCMA coefficient reaches $-297$ fJ/V m matching well with the previous experiment with three monolayers (MLs) of Fe. The coefficient oscillates strongly and even changes its sign with increasing the number of Fe MLs. Comparing the $k$-resolved VCMA with the Fermi surface of the interfacial Fe atom, the screening charges theory for VCMA was verified. For $2–9$ MLs Fe, interestingly, the QW states of ${\mathrm{\Delta }}_1$ electron at the $\mathrm{\Gamma }$ point provide large (no) contribution to the VCMA with odd (even) MLs. Moreover, the change of the orbital-resolved Fermi surface at the interfacial Fe atom also plays an important role on VCMA oscillation, which as well as the QW states results in the largest VCMA for 3-ML Fe. Our results deepen the understanding of the large VCMA in the Cr/Fe/MgO junction, which would be helpful to design a practical MTJ with large VCMA.

Magnetic bimeron is a topological counterpart of skyrmions in in-plane magnets, which can be used as a spintronic information carrier. We report the static properties of bimerons with different topological structures in a frustrated ferromagnetic monolayer, where the bimeron structure is characterized by the vorticity $Q_{\text{v}}$ and helicity $\eta$. It is found that the bimeron energy increases with $Q_{\text{v}}$, and the energy of an isolated bimeron with $Q_{\text{v}}=\pm 1$ depends on $\eta$. We also report the dynamics of frustrated bimerons driven by the spin-orbit torques, which depend on the strength of the dampinglike and fieldlike torques. We find that the isolated bimeron with $Q_{\text{v}}=\pm 1$ can be driven into linear or elliptical motion when the spin polarization is perpendicular to the easy axis. We numerically reveal the damping dependence of the bimeron Hall angle driven by the dampinglike torque. Besides, the isolated bimeron with $Q_{\text{v}}=\pm 1$ can be driven into rotation by the dampinglike torque when the spin polarization is parallel to the easy axis. The rotation frequency is proportional to the driving current density. In addition, we numerically demonstrate the possibility of creating a bimeron state with a higher or lower topological charge by the current-driven collision and merging of bimeron states with different $Q_{\text{v}}$. Our results could be useful for understanding the bimeron physics in frustrated magnets.

We report the observation of critical behavior in $\mathrm{M}{\mathrm{n}}_{1-x}\mathrm{C}{\mathrm{r}}_x\mathrm{Si}\left(0\le x\le 1\right)$ close to a $T=0\mathrm{K}$ quantum critical point, consistent with the Belitz-Kirkpatrick-Vojta (BKV) theory of disordered metallic ferromagnets. The critical exponents are in good agreement with the theoretical predictions of the BKV theory in the preasymptotic limit. A non-Fermi liquidlike behavior is seen down to 200 mK in the transport and thermodynamic properties around the critical concentration $x_{\mathrm{C}}=0.2$. Quantum criticality and self-consistency of the exponents is further confirmed using a scaling analysis of the magnetization and heat capacity data. A recovery to Fermi liquidlike behavior is displayed on moving away from the critical composition, as well as with the application of a magnetic field.

Two-dimensional second-order topological superconductors host zero-dimensional Majorana bound states at their boundaries. In this work, focusing on rotation-invariant crystalline topological superconductors, we establish a bulk-boundary correspondence linking the presence of such Majorana bound states to bulk topological invariants introduced by Benalcazar et al. [Phys. Rev. B 89, 224503 (2014)]. We thus establish when a topological crystalline superconductor protected by rotational symmetry displays second-order topological superconductivity. Our approach is based on stacked Dirac Hamiltonians, using which we relate transitions between topological phases to the transformation properties between adjacent gapped boundaries. We find that, in addition to the bulk rotational invariants, the presence of Majorana boundary bound states in a given geometry depends on the interplay between weak topological invariants and the location of the rotation center relative to the lattice. We provide numerical examples for our predictions and discuss possible extensions of our approach.

We present an ab initio study of the excitonic states of a prototypical high-temperature superconductor ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and compare them to the isostructural single-layer nickelate ${\mathrm{La}}_2{\mathrm{NiO}}_4$. A key difference in the low-energy electronic structure leads to very different excitonic behavior. Excitons in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ are delocalized and can freely move in the ${\mathrm{CuO}}_2$ plane without disturbing the antiferromagnetic order. In contrast, in ${\mathrm{La}}_2{\mathrm{NiO}}_4$, we find the low-lying excitonic states to be extremely localized, producing a nearly flat dispersion. The theoretically obtained excitonic dispersion and behavior are in excellent agreement with RIXS observations. To classify the excitons, we project the electron-hole coupling onto each atomic site including the full manifold of atomic orbitals. We find the excitons to be composed of a linear combination of exciton classes, including Mott-Hubbard, $d\text{−}d$, and charge-transfer. The implication of these excitations to the high-$T_c$ pairing mechanism is also discussed.

Fluctuations can change the phase transition properties drastically. An example is the fermion-induced quantum critical point (FIQCP), in which fluctuations of the massless Dirac fermions turn a putative Landau–de Gennes first-order phase transition (FOPT) with a cubic boson interaction into a continuous one. However, for the Landau-Devonshire theory, which characterizes another very large class of FOPTs, its fate under the coupling with extra fluctuations has not been explored. Here, we discover another type of FIQCP, in which the Dirac fermion fluctuations round the boson Landau-Devonshire FOPT into a continuous phase transition. By using the functional renormalization group analyses, we determine the condition for the appearance of this FIQCP. Moreover, we point out that the present FIQCP can be a supersymmetric critical point. We finally show that the low-temperature phase diagram can provide distinct experimental evidences to detect this FIQCP.

Many common crystal structures can be described by a single (or very few) repeated structural motif (“monomorphous structures”) such as octahedron in cubic halide perovskites. Interestingly, recent accumulated evidence suggests that electronic structure calculations based on such macroscopically averaged monomorphous cubic ($Pm\text{−}3m$) halide perovskites obtained from x-ray diffraction, show intriguing deviations from experiment. These include systematically too small band gaps, dielectric constants dominated by the electronic, negative mixing enthalpy of alloys, and significant deviations from the measured pair distribution function. We show here that a minimization of the systems $T=0$ internal energy via density functional theory reveals a distribution of different low-symmetry local motifs, including tilting, rotations, and B-atom displacements (“polymorphous networks”). This is found only if one allows for larger-than-minimal cell size that does not geometrically exclude low symmetry motifs. As the (super) cell size increases, the energy is lowered relative to the monomorphous cell, and stabilizes after ∼32 formula units (≥160 atoms) are included. Being a result of nonthermal energy minimization in the internal energy without entropy, this correlated set of displacements must represent the intrinsic geometry preferred by the underlying chemical bonding (lone pair bonding), and as such has a different origin than the normal, dynamic thermal disorder modeled by molecular dynamics. Indeed, the polymorphous network, not the monomorphous ansatz, is the kernel structure from which high temperature thermal agitation develops. The emerging physical picture is that the polymorphous network has an average structure with high symmetry, yet the local structural motifs have low symmetries. We find that, compared with monomorphous counterparts, the polymorphous networks have significantly lower predicted total energies, larger band gaps, and ionic dominated dielectric constants, and agree much more closely with the observed pair distribution functions. An analogous polymorphous situation is found in the paraelectric phase of a few cubic oxide perovskites where local polarization takes the role of local displacements in halide perovskites, and in the paramagnetic phases of a few $3d$ oxides where the local spin configuration takes that role.

The emergence of unconventional superconductivity is generally considered to be related to spin fluctuations. Unveiling the intriguing behaviors of spin fluctuations in parent compounds with layered transition-metal ions may shed light on the search for exotic unconventional superconductors. Here, based on the framework of the first-principles calculations, we theoretically propose that ${\mathrm{LaCoSb}}_2$ is a weak antiferromagnetic layered metal with an in-plane ferromagnetic moment of $0.88{\mu }_B$ at the Co sites, as a candidate parent compound of the cobalt-based superconductors. Importantly, this theoretical finding is experimentally supported by our magnetization measurements on polycrystalline samples of ${\mathrm{LaCo}}_{0.78}{\mathrm{Sb}}_2$. Following the symmetry analysis, we suggest a possible $p$-wave superconductivity hosted in doped ${\mathrm{LaCoSb}}_2$ emerging at the verge of ferromagnetic spin fluctuations, which implies potential applications in topological quantum computing in future.

The extremely large magnetoresistance (XMR) material LaBi was reported to become superconducting under pressure accompanying with suppressed magnetoresistance. However, the underlying mechanism is unclear. By using first-principles electronic structure calculations in combination with a semiclassical model, we have studied the electron-phonon coupling and magnetoresistance of LaBi in the pressure range from 0 to 18 GPa. Our calculations show that LaBi undergoes a structural phase transition from a face-centered cubic lattice to a primitive tetragonal lattice at $\sim 7\mathrm{GPa}$, verifying previous experimental results. Meanwhile, LaBi remains topologically nontrivial across the structural transition. Under all pressures that we have studied, the phonon-mediated mechanism based on the weak electron-phonon coupling cannot account for the observed superconductivity in LaBi, and the calculated magnetoresistance for LaBi does not show a suppression. The distinct difference between our calculations and experimental observations suggests either the existence of extra Bi impurities in the real LaBi compound or the possibility of other unknown mechanism.

A recent experimental paper by Du et al. [Phys. Rev. Lett. 123, 076602 (2019)] had shown colossal values of thermopower at 40 K. The Seebeck coefficient values decrease with the application of magnetic field. We found that the colossal Seebeck coefficient is due to carrier correlation and large increase in effective mass. The reduction of thermopower with magnetic field is explained by including spin-orbit interaction terms in our calculations. By tuning atomistic magnetic moment, the power factor of thermoelectric materials can be further optimized. We have shown that a ${10}^7$ times increase in power factor is possible by increasing magnetic field.

We consider the problem of multilayer graphene on a Haldane sphere and determine the Landau level spectrum for this family of systems. This serves as a generalization of the Landau quantization problem of ordinary nonrelativistic Haldane sphere and spherical graphene, or Dirac-like particles on a sphere. The Hamiltonian is diagonalized in a concise algebraic fashion exploiting two mutually commuting SU(2) algebras of the problem. Additionally, using exact wave functions we demonstrate computation of Haldane pseudopotentials in the second Landau level. These exact solutions add to the current toolkits of the numerical studies on fractional quantum Hall effects in systems of graphite multilayers.

We have investigated the effect of potassium (K) intercalation on $2H\text{−}{\mathrm{MoS}}_2$ using transmission electron energy-loss spectroscopy. For K concentrations up to approximately 0.4, the crystals appear to be inhomogeneous with a mix of structural phases and irregular potassium distribution. Above this intercalation level, ${\mathrm{MoS}}_2$ exhibits a $2a\times 2a$ superstructure in the $ab$ plane and unit cell parameters of $a=3.20\text{Å}$ and $c=8.23\text{Å}$ indicating a conversion from the $2H$ to the $1T^{\prime }$ or $1T^{″}$ polytypes. The diffraction patterns also show a $\sqrt{3}a\times \sqrt{3}a$ and a much weaker $2\sqrt{3}a\times 2\sqrt{3}a$ superstructure that is very likely associated with the ordering of the potassium ions. A semiconductor-to-metal transition occurs signified by the disappearance of the excitonic features from the electron energy-loss spectra and the emergence of a charge carrier plasmon with an unscreened plasmon frequency of 2.78 eV. The plasmon has a positive, quadratic dispersion and appears to be superimposed with an excitation arising from interband transitions. The behavior of the plasmon peak energy positions as a function of potassium concentration shows that potassium stoichiometries of less than $\sim 0.3$ are thermodynamically unstable while higher stoichiometries up to $\sim 0.5$ are thermodynamically stable. Potassium concentrations greater than $\sim 0.5$ lead to the decomposition of ${\mathrm{MoS}}_2$ and the formation of ${\mathrm{K}}_2\mathrm{S}$. The real part of the dielectric function and the optical conductivity of ${\mathrm{K}}_{0.41}{\mathrm{MoS}}_2$ were derived from the loss spectra via Kramers-Kronig analysis.

We present a strain-dependent time-resolved reflectometry experiment on $\mathrm{Ba}{\left({\mathrm{Fe}}_{0.955}{\mathrm{Co}}_{0.045}\right)}_2{\mathrm{As}}_2$ in the nematic state. Uniaxial external pressure has usually been applied to obtain a single nematic domain. However, reflectivity with and without pumping under uniaxial strain along ${\left[110\right]}_{\text{tetr}}$ reveals an unexpected nematicity of which $C_4$ symmetry-breaking orientation is rotated by ${20}^{\circ }$. Our observation suggests a microscopic coexistence of $B_{2g}$ and $B_{1g}$ nematicity under uniaxial strain and possibly of an $XY$-type nematic order in the electron-doped iron pnictide.

We obtained the spin-wave spectrum based on a first-principles method of exchange constants, calculated the phonon spectrum by the first-principles phonon calculation method, and extracted the broadening of the magnon spectrum $\mathrm{\Delta }\omega$, induced by magnon-phonon interactions in gadolinium iron garnet (GdIG). Using the obtained exchange constants, we reproduce the experimental Curie temperature and the compensation temperature from spin models using metropolis Monte Carlo (MC) simulations. In the lower-frequency regime, the fitted positions of the magnon-phonon dispersion crossing points are consistent with the inelastic neutron scattering experiment. We found that the $\mathrm{\Delta }\omega$ and magnon wave vector $k$ have a similar relationship in YIG. The broadening of the acoustic spin-wave branch ($\alpha$ mode) is proportional to $k^2$, while that of the optical branch ($\beta$ and $\gamma$ modes) is a constant. At a specific $k$, the magnon-phonon thermalization times of ${\tau }_{\mathrm{mp}}$ are approximately ${10}^{-9}$, ${10}^{-13}$, and ${10}^{-14}\mathrm{s}$ for $\alpha$, $\beta$, and $\gamma$ modes, respectively. The results show the importance of the higher frequency modes at room temperature. This research provides specific and effective information for developing a clear understanding of the spin-wave mediated spin Seebeck effect and complements the lack of lattice dynamics calculations of GdIG.

We describe the ground- and excited-state electronic structure of bulk MnO and NiO, two prototypical correlated electron materials, using coupled cluster theory with single and double excitations (CCSD). As a corollary, this work also reports an implementation of unrestricted periodic ab initio equation-of-motion CCSD. Starting from a Hartree-Fock reference, we find fundamental gaps of 3.46 and 4.83 eV for MnO and NiO, respectively, for the 16-unit supercell, slightly overestimated compared to experiment, although finite-size scaling suggests that the gap is more severely overestimated in the thermodynamic limit. From the character of the correlated electronic bands we find both MnO and NiO to lie in the intermediate Mott/charge-transfer insulator regime, although NiO appears as a charge transfer insulator when only the fundamental gap is considered. While the lowest quasiparticle excitations are of metal $3d$ and O $2p$ character in most of the Brillouin zone, near the $\mathrm{\Gamma }$ point, the lowest conduction band quasiparticles are of $s$ character. Our study supports the potential of coupled cluster theory to provide high-level many-body insights into correlated solids.

Motivated by recent low-temperature magnetoresistance measurements in twisted bilayer graphene aligned with hexagonal boron nitride substrate, we perform a systematic study of possible symmetry breaking orders in this device at a filling of two electrons per moiré unit cell. We find that the surprising nonmonotonic dependence of the resistance on an out-of-plane magnetic field is difficult to reconcile with particle-hole charge carriers from the low-energy bands in symmetry broken phases. We invoke the nonzero Chern numbers of the twisted bilayer graphene flat bands to argue that skyrmion textures provide an alternative for the dominant charge carriers. Via an effective field theory for the spin degrees of freedom, we show that the effect of spin Zeeman splitting on the skyrmion excitations provides a possible explanation for the nonmonotonic magnetoresistance. We suggest several experimental tests, including the functional dependence of the activation gap on the magnetic field, for our proposed correlated insulating states at different integer fillings. We also discuss possible exotic phases and quantum phase transitions that can arise via skyrmion pairing on doping such an insulator.

Torsion can cause various anomalies in various dimensions, including the $(3+1)$-dimensional $\left[\right(3+1\left)\mathrm{D}\right]$ Nieh-Yan anomaly, the $\left(2+1\right)\text{D}$ Hughes-Leigh-Fradkin (HLF) parity anomaly, and the $\left(3+1\right)\mathrm{D}$, $\left(1+1\right)\mathrm{D}$ chiral energy-momentum anomaly. We study these anomalies from the Hamiltonian approach. We derive the $\left(1+1\right)\mathrm{D}$ chiral energy-momentum anomaly from the single-body Hamiltonian. We then show how other torsional anomalies can be related to the $\left(1+1\right)\mathrm{D}$ chiral energy-momentum anomaly in a straightforward way. Finally, the Nieh-Yan anomaly and the $\left(3+1\right)\mathrm{D}$ chiral energy-momentum anomaly are obtained from the parity anomaly and the HLF effective action, respectively. Hence, we have constructed the dimensional ladder for the torsional anomalies from the single-body Hamiltonian picture.