Antiferromagnetic spintronic devices could offer ultrafast dynamics and a higher data density than conventional ferromagnetic devices. One of the challenges in designing such devices is the control and detection of the magnetization of the antiferromagnet due to its lack of stray fields, and this is often achieved through the exchange bias effect. In exchange biased systems, the pinned spins are known to comprise a small fraction of the total number of interface spins, yet their exact nature and physical origin has so far been elusive. Here we show that in the technologically important disordered $\gamma \text{−}{\mathrm{IrMn}}_3$–CoFe structure, the pinned spins arise from the small imbalance in the number of spins in each magnetic sublattice in the antiferromagnet due to the naturally occurring atomic disorder. These pinned spins are strongly coupled to the bulk antiferromagnet, explaining their stability. Moreover, we find that the ferromagnet strongly distorts the interface spin structure of the antiferromagnet, causing a large reversible interface magnetization that does not contribute to exchange bias but does increase the coercivity. We find that the uncompensated spins are not localized spins which occur due to point defects or domain walls but instead constitute a small motion of every antiferromagnet spin at the interface. This unexpected finding resolves one of the long-standing puzzles of exchange bias and provides a route to developing optimized nanoscale antiferromagnetic spintronic devices.

We investigate Anderson localization of two particles moving in a two-dimensional (2D) disordered lattice and coupled by contact interactions. Based on transmission-amplitude calculations for relatively large strip-shaped grids, we find that all pair states are localized in lattices of infinite size. In particular, we show that previous claims of an interaction-induced mobility edge are biased by severe finite-size effects. The localization length of a pair with zero total energy exhibits a nonmonotonic behavior as a function of the interaction strength, characterized by an exponential enhancement in the weakly interacting regime. Our findings also suggest that the many-body mobility edge of the 2D Anderson-Hubbard model disappears in the zero-density limit, irrespective of the (bosonic or fermionic) quantum statistics of the particles.

We construct a field-theoretic description of spin waves in hexagonal antiferromagnets with three magnetic sublattices and coplanar ${120}^{\circ }$ magnetic order. The three Goldstone modes can be separated by point-group symmetry into a singlet ${\alpha }_0$ and a doublet $\mathit{\beta }$. The ${\alpha }_0$ singlet is described by the standard theory of a free relativistic scalar field. The field theory of the $\mathit{\beta }$ doublet is analogous to the theory of elasticity of a two-dimensional isotropic solid with distinct longitudinal and transverse “speeds of sound.” The well-known Heisenberg models on the triangular and kagome lattices with nearest-neighbor exchange turn out to be special cases with accidental degeneracy of the spin-wave velocities. The speeds of sound can be readily calculated for any lattice model. We apply this approach to the compounds of the ${\mathrm{Mn}}_{3}X$ family with stacked kagome layers.

${\mathrm{Pb}}_2\mathrm{Co}\mathrm{Os}{\mathrm{O}}_6$ is a newly synthesized polar metal in which inversion symmetry is broken by the magnetic frustration in an antiferromagnetic ordering of Co and Os sublattices. The coupled magnetic and structural transition occurs at 45 K at ambient pressure. Here, we perform transport measurements and first-principles calculations to study the pressure effects on the magnetic/structural coupled transition of ${\mathrm{Pb}}_2\mathrm{Co}\mathrm{Os}{\mathrm{O}}_6$. Experimentally, we monitor the resistivity anomaly at $T_N$ under various pressures up to 11 GPa in a cubic anvil cell apparatus. We find that $T_N$ determined from the resistivity anomaly first increases quickly with pressure in a large slope of $d{T}_N/dP=+6.8\left(8\right)\mathrm{K}/\mathrm{GPa}$ for $P<4\mathrm{GPa}$ and, then, increases with a much reduced slope of 1.8(4) K/GPa above 4 GPa. Our first-principles calculations suggest that the observed discontinuity of $d{T}_N/dP$ around 4 GPa may be attributed to the vanishing of the Os magnetic moment under pressure. Pressure substantially reduces the Os moment and completely suppresses it above a critical value, which relieves the magnetic frustration in the antiferromagnetic ordering of ${\mathrm{Pb}}_2\mathrm{Co}\mathrm{Os}{\mathrm{O}}_6$. The Co and Os polar distortions decrease with the increasing pressure and simultaneously vanish at the critical pressure. Therefore, above the critical pressure, a new centrosymmetric antiferromagnetic state emerges in ${\mathrm{Pb}}_2\mathrm{Co}\mathrm{Os}{\mathrm{O}}_6$, distinct from the one under ambient pressure, thus, showing a discontinuity in $d{T}_N/dP$.

In-plane anisotropy of electrical resistivity was studied in samples of the hole-doped ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ in the composition range $0.21\le x\le 0.26$ where anisotropy changes sign. Low-temperature ($\sim 20$ K) irradiation with relativistic 2.5 MeV electrons was used to control the level of disorder and residual resistivity of the samples. Modification of the stress-detwinning technique enabled measurements of the same samples before and after irradiation, leading to the conclusion of anisotropic character of predominantly inelastic scattering processes. Our main finding is that the resistivity anisotropy is of the same sign irrespective of residual resistivity, and remains the same in the orthorhombic $C_2$ phase above the reentrant tetragonal transition. Unusual $T$-linear dependence of the anisotropy $\mathrm{\Delta }\rho \equiv {\rho }_a\left(T\right)-{\rho }_b\left(T\right)$ is found in pristine samples with $x=0.213$ and $x=0.219$, without similar signatures in either ${\rho }_a\left(T\right)$ or ${\rho }_b\left(T\right)$. We show that this feature can be reproduced by a phenomenological model of R. M. Fernandes et al. [Phys. Rev. Lett. 107, 217002 (2011)]. We speculate that onset of fluctuations of nematic order on approaching the instability towards the reentrant tetragonal phase contributes to this unusual dependence.

We investigate the Josephson transport properties in a Josephson junction consisting of a conventional $s$-wave superconductor coupled to a multiorbital noncentrosymmetric superconductor marked by an orbitally driven inversion asymmetry and isotropic interorbital spin-triplet pairing. Contrary to the canonical single band noncentrosymmetric superconductor, we demonstrate that the local interorbital spin-triplet pairing is tied to the occurrence of sign-changing spin-singlet pair amplitude on different bands with $d$-wave symmetry. Such multiband $d^{\pm }$-wave state is a unique superconducting configuration that drives unexpected Josephson effects with 0-$\pi$ transitions displaying a high degree of electronic control. Remarkably, we find that the phase state of a noncentrosymmetric/$s$-wave Josephson junction can be toggled between 0 and $\pi$ in multiple ways through a variation of electron filling, strength of the spin-orbital coupling, amplitude of the inversion asymmetry interaction, and junction transparency. These results highlight an intrinsic orbital and electrical tunability of the Josephson response and provide unique paths to unveil the nature of unconventional multiorbital superconductivity as well as inspire innovative designs of Josephson quantum devices.

The study of electronic and magnetic properties of a kagome lattice has been an active research area searching for topological phases of matters. In particular, the kagome system with transition metal stannides exhibit interesting anomalous Hall effects driven by ferromagnetic or noncollinear magnetic ordering. In this paper, motivated by these pioneer works, we study strongly correlated spin-orbit coupled electrons in a kagome lattice at a 1/3 filling. Using both the Hartree-Fock approach and effective model analysis, we report quantum phase transitions accompanied by the distinct charge and magnetic ordered phases. Especially, for strongly interacting limit, we discover new types of 1(2)-pinned metallic states, which are understood by effective localized electron models. Furthermore, when the spin-orbit coupling is present, it turns out that such pinned metallic states open a wide region of the Chern insulating phase with chiral spin ordering. Thus the quantum anomalous Hall effect is expected with the emergent scalar spin chirality. Our theory provides a theoretical platform of strongly interacting kagome metal which is applicable to transition metal stannides.

Using the numerical renormalization group method, we study a magnetic impurity coupled to a quantum wire with Rashba and Dresselhaus spin-orbit coupling (SOC) in an external magnetic field. We consider the low-filling regime with the Fermi energy close to the bottom of the band and report the results for local static and dynamic properties in the Kondo regime. In the absence of the field, local impurity properties remain isotropic in spin space despite the SOC-induced magnetic anisotropy of the conduction band. In the presence of the field, clear fingerprints of anisotropy are revealed through the strong field-direction dependence of the impurity spin polarization and spectra, in particular of the Kondo peak height. The detailed behavior depends on the relative magnitudes of the impurity and band $g$ factors. For the case of an impurity $g$ factor somewhat lower than the band $g$ factor, the maximal Kondo peak suppression is found for a field oriented along the effective SOC field axis, while for a field perpendicular to this direction we observe a compensation effect (revival of the Kondo peak): The SOC counteracts the Kondo peak splitting effects of the local Zeeman field. We demonstrate that the SOC-induced anisotropy, measurable by tunneling spectroscopy techniques, can help to determine the ratio of Rashba and Dresselhaus SOC strengths in the wire.

Charge density wave (CDW) instability and pressure-induced superconductivity in bulk $1T-\mathrm{Nb}{\mathrm{S}}_2$ are predicted theoretically by first-principles calculations. We reveal a CDW instability toward the formation of a stable commensurate CDW order, resulting in a $\sqrt{13}\times \sqrt{13}$ structural reconstruction featured with star-of-David clusters. The CDW phase exhibits one-dimensional metallic behavior with in-plane flatband characteristics and coexists with an orbital-density-wave order predominantly contributed by the $4d_{z^2-r^2}$ orbital from the inner Nb atoms of the star-of-David cluster. By doubling the cell of the commensurate CDW phase along the layer stacking direction, a metal-insulator transition may be realized in the CDW phase in cases where the interlayer antiferromagnetic ordering and Coulomb correlation effect have been considered simultaneously. Bare electron susceptibility, phonon linewidth, and electron-phonon coupling calculations suggest that the CDW instability is driven by softened phonon modes due to the strong electron-phonon coupling interactions. CDW order can be suppressed by pressure, concomitant with the appearance of the superconductivity. Our theoretical predictions call for experimental investigations to further clarify the transport and magnetic properties of $1T-\mathrm{Nb}{\mathrm{S}}_2$. Furthermore, it would also be very interesting to explore the possibility to realize the CDW order coexisting with the superconductivity in bulk $1T-\mathrm{Nb}{\mathrm{S}}_2$.

Fermions with hourglass dispersions have been hotly studied in a nonmagnetic system currently. However, magnetic ones have been rarely discussed because of the scarcity of excellent candidate materials. Here, promoted by first-principles and symmetry analysis, we propose the presence of novel hourglass nodal networks in a nonsymmorphic half-metal ${\mathrm{Mg}}_2{\mathrm{VO}}_4$ that is constructed by three touching hourglass nodal lines at mutually intersecting planes in the extended Brillouin zone. Unlike previous hourglass semimetals in nonmagnetic materials, the material proposed here has a ferromagnetic half-metallic state; thus, the hourglass nodal networks are fully spin polarized. We also show that the hourglass nodal networks manifest spin-polarized drumhead surface states. We demonstrate the hourglass nodal networks are enforced by nonsymmorphic symmetry, and effective models are constructed. We further show that the novel topological phase of ${\mathrm{Mg}}_2{\mathrm{VO}}_4$ is robust against lattice strain and electron correlation effects. Our work provides a good platform to investigate novel hourglass fermions from nonsymmorphic symmetries in magnetic system.

In this paper, we propose an energy functional at the level of $\mathrm{DFT}+U+V$ that allows us to compute self-consistently the values of the onsite interaction, Hubbard $U$ and Hund $J$, as well as the intersite interaction $V$. This functional extends the previously proposed ACBN0 functional [L. A. Agapito et al., Phys. Rev. X 5, 011006 (2015)] including both onsite and intersite interactions. We show that this ab initio self-consistent functional yields improved electronic properties for a wide range of materials, ranging from $sp$ materials to strongly correlated materials. This functional can also be seen as an alternative general and systematic way to construct parameter-free hybrid functionals, based on the extended Hubbard model and a selected set of Coulomb integrals, and might be used to develop novel approximations. By extending the $\mathrm{DFT}+U$ method to materials where strong local and nonlocal interactions are relevant, this work opens the door to the ab initio study the electronic, ionic, and optical properties of a larger class of strongly correlated materials in and out of equilibrium.

We investigate the spin valley polarization in $\mathrm{Mo}{\mathrm{Te}}_2$ monolayer on (111) and (001) surfaces of ferromagnetic semiconductor EuO based on first-principles calculations. We consider surface reconstructions for EuO(111). We find that there is no direct chemical bonding between the reconstructed EuO(111) and the $\mathrm{Mo}{\mathrm{Te}}_2$ overlayer, in contrast to the case of the ideal EuO(111). However, there is a strong hybridization between the states of $\mathrm{Mo}{\mathrm{Te}}_2$ and the substrate states, which has a substantial impact on the valleys. The valley polarization due to the magnetic proximity effect is dependent on the detail of the interface structure, which is in the range of a few meV to about 40 meV. These values are at least one order of magnitude smaller than that induced by the ideal EuO(111). When the $\mathrm{Mo}{\mathrm{Te}}_2$ monolayer is interfaced with EuO(001), the valley polarization is about 3.2 meV, insensitive to the interface structure. By a low-energy effective Hamiltonian model, the effective Zeeman field induced by EuO(001) is about 27 T, comparable to that for $\mathrm{W}{\mathrm{Se}}_2/\mathrm{Eu}\mathrm{S}$ obtained by experiment.

Unique electronic band structure of graphene with its semimetallic features near the charge neutrality point is sensitive to impurity effects. Using the Lifshitz and Anderson impurity models, we study in detail the disorder induced spectral phenomena in the electronic band structure of graphene, namely, the formation of resonances, quasigaps, bound states, impurity subbands, and their overall impact on the electronic band restructuring and the associated Mott-like metal-insulator transitions. We perform a systematic analytical and numerical study for realistic impurities, both substitutional and adsorbed, focusing on those effects that arise from the impurity adatoms locations (top, bridge, and hollow positions), concentration, unequal occupations of the graphene host sublattices, perturbation strengths, etc. Possible experimental and practical implications are discussed as well.

Over the past several years, reliable quantum Monte Carlo results for the charge density wave transition temperature $T_{\mathrm{cdw}}$ of the half-filled two-dimensional Holstein model in square and honeycomb lattices have become available for the first time. Exploiting the further development of numerical methodology, here we present results in three dimensions, which are made possible through the use of Langevin evolution of the quantum phonon degrees of freedom. In addition to determining $T_{\mathrm{cdw}}$ from the scaling of the charge correlations, we also examine the nature of charge order at general wave vectors for different temperatures, couplings, and phonon frequencies, and the behavior of the spectral function and specific heat.

We use the exact analytical technique introduced by us [Phys. Rev. B 101, 115405 (2020)] to recover the surface Green's functions and the corresponding Fermi-arc surface states for various lattice models of Weyl semimetals. For these models we use the $T$-matrix formalism to calculate the quasiparticle interference patterns generated in the presence of impurity scattering. In particular, we consider the models introduced by Kourtis et al. [Phys. Rev. B 93, 041109(R) (2016)] (model A) and Lau et al. [Phys. Rev. Lett. 119, 076801 (2017)] (model B), and we find that, as opposed to observations previously obtained via joint density of states and spin-dependent scattering probability, the interarc scattering in the quasiparticle interference features is fully suppressed in model A, and is very small in model B. Our findings indicate that these models may not correctly describe materials such as ${\mathrm{MoTe}}_2$, since for such materials interarc scattering is clearly visible experimentally, e.g., by Deng et al. [Nat. Phys. 12, 1105 (2016)]. We also focus on the minimal models proposed by McCormick et al. [Phys. Rev. B 95, 075133 (2017)], which indeed recover significant interarc scattering features.

Materials with tunable topological features, simple crystal structure, and flexible synthesis are in extraordinary demand towards the technological exploitation of unique properties of topological nodal points. The controlled design of the lattice geometry of light elements is determined by utilizing density functional theory and the effective Hamiltonian model together with the symmetry analysis. This provides an intriguing venue for reasonably achieving various distinct types of novel fermions. We, therefore, show that a nodal line (types I and II), Dirac fermion, and triple point fermionic excitation can potentially appear as a direct result of a band inversion in group-I nitrides with $\alpha -{\mathrm{Li}}_3\mathrm{N}$-type crystal structure. The imposed strain is exclusively significant for these compounds, and it invariably leads to the considerable modification of the nodal line type. Most importantly, a type-II nodal loop can be realized in the system under strain. These unique characteristics make $\alpha -{\mathrm{Li}}_3\mathrm{N}$-type crystal structure an ideal playground to achieve various types of novel fermions well-suited for technological applications.

A formula for the corner charge in terms of the bulk quadrupole moment is derived for two-dimensional periodic systems. This is an analog of the formula for the surface charge density in terms of the bulk polarization. In the presence of an $n$-fold rotation symmetry with $n=3$, 4, and 6, the quadrupole moment is quantized and is independent of the spread or shape of Wannier orbitals, depending only on the location of Wannier centers of filled bands. In this case, our formula predicts the fractional part of the quadrupole moment purely from the bulk property. The system can contain many-body interactions as long as the ground state is gapped and topologically trivial in the sense it is smoothly connected to a product state limit. An extension of these results to three-dimensional systems is also discussed. In three dimensions, in general, even the fractional part of the corner charge is not fully predictable from the bulk perspective even in the presence of point group symmetry.

In this paper, we study how the probability of presence of a particle is distributed between the two parts of a composite fermionic system. We uncover that the difference of probability depends on the energy in a striking way and show the pattern of this distribution. We discuss the main features of the latter and explain analytically those that we understand. In particular, we prove that it is a nonperturbative property and we find out a large/small coupling constant duality. We also find and study features that may connect our problem with certain aspects of nonlinear classical dynamics, such as the existence of resonances and sensitive dependence on the state of the system. We show that the latter has, indeed, a similar origin than in classical mechanics: the appearance of small denominators in the perturbative series. Inspired by the proof of the Kolmogorov-Arnold-Moser theorem, we are able to deal with this problem by introducing a cutoff in energies that eliminates these small denominators. We also formulate some conjectures that we are not able to prove at present but can be supported by numerical experiments.

Despite the advances in the development of numerical methods analytical approaches still play the key role on the way towards a deeper understanding of many-particle systems. In this regard, diagonalization schemes for Hamiltonians represent an important direction in the field. Among these techniques the method, presented here, might be that approach with the widest range of possible applications: We demonstrate that both stepwise and continuous unitary transformations to diagonalize the many-particle Hamiltonian as well as perturbation theory and also nonperturbative treatments can be understood within the same theoretical framework. The new method is based on the introduction of generalized projection operators and allows us to develop a renormalization scheme which is used to evaluate directly the physical quantities of a many-particle system. The applicability of this approach is shown for two important elementary many-particle problems.

Tricritical Ising (TCI) phase transition is known to occur in several interacting spin and Majorana fermion models and is described in terms of a supersymmetric conformal field theory (CFT) with central charge $c=7/10$. The field content of this CFT is highly nontrivial and includes among its primary fields the Fibonacci anyon, making it of potential interest to strategies seeking to implement fault-tolerant topological quantum computation with non-Abelian phases of matter. In this paper we explore the possibility that a TCI CFT can occur at the edge of a gapped two-dimensional topological state as a stable phase. We discuss a possible realization of this 2D phase based on a coupled-wire construction using the Grover-Sheng-Vishwanath chain model of Majorana zero modes coupled to Ising spins which is known to undergo the TCI phase transition. From the combined analysis using mean-field theory, conformal field theory, and density matrix renormalization group (DMRG) on two- and four-leg ladders, we find that the left- and right-moving gapless TCI modes become spatially separated and reside on two opposite edges of the system, forming a precursor of the required 2D topological phase.