Kondo breakdown is one of the most intriguing problems in strongly correlated electron systems, as it is rooted in many anomalous electron behaviors found in heavy-fermion materials. In Kondo lattice systems, Kondo breakdown can arise from either strong magnetic frustrations or critical fluctuations of collective modes. Here, we reveal another type of Kondo breakdown with a fully different origin in interacting topological Kondo insulators. By employing numerically exact quantum Monte Carlo simulations, we show that with open boundary conditions, Kondo screening is inexorably destroyed by interaction effects on edges or corners in these systems. We argue that the Kondo breakdown is enforced by the symmetries of the system, because the ground states are symmetry-protected Haldane phases.

The massless Dirac fermions and the ease to introduce spatial and magnetic confinement in graphene provide us unprecedented opportunity to explore confined relativistic matter in this condensed-matter system. Here we report the interplay between the confinement induced by external electric fields and magnetic fields of the massless Dirac fermions in graphene. When the magnetic length $l_B$ is larger than the characteristic length of the confined electric potential $l_V$, the spatial confinement dominates and a relatively small critical magnetic field splits the spatial-confinement induced atomiclike shell states by switching on a π Berry phase of the quasiparticles. When the $l_B$ becomes smaller than the $l_V$, the transition from spatial confinement to magnetic confinement occurs and the atomiclike shell states condense into Landau levels (LLs) of the Fock-Darwin states in graphene. Our experiment demonstrates that the spatial confinement dramatically changes the energy spacing between the LLs and generates large electron-hole asymmetry of the energy spacing between the LLs. These results shed light on puzzling observations in previous experiments, which hitherto remained unaddressed.

Nonreciprocal plasmons in current-driven, isotropic, and homogenous graphene with proximal metallic gates are theoretically explored. Nearby metallic gates screen the Coulomb interactions, leading to linearly dispersive acoustic plasmons residing close to its particle-hole continuum counterpart. We show that the applied bias leads to spectral broadband focused plasmons whose resonance linewidth is dependent on the angular direction relative to the current flow due to Landau damping. We predict that forward focused nonreciprocal plasmons are possible with accessible experimental parameters and setup.

The scenario of a metal-insulator transition driven by the onset of antiferromagnetic order in ${\mathrm{NaOsO}}_3$ calls for a trustworthy derivation of the underlying effective spin Hamiltonian. To determine the latter we rely on ab initio electronic-structure calculations, linear spin-wave theory, and comparison to experimental data of the corresponding magnon spectrum. We arrive this way to Heisenberg couplings that are $\lesssim$45 $\mathrm{to}\lesssim$63% smaller than values presently proposed in the literature and Dzyaloshinskii-Moriya interactions in the region of 15% of the Heisenberg exchange $J$. These couplings together with the symmetric anisotropic exchange interaction and single-ion magnetocrystalline anisotropy successfully reproduce the magnon dispersion obtained by resonant inelastic x-ray scattering measurements. In particular, the spin-wave gap fully agrees with the measured one. We find that the spin-wave gap is defined from a subtle interplay between the single-ion anisotropy, the Dzyaloshinskii-Moriya exchange, and the symmetric anisotropic exchange interactions. The results reported here underpin the local-moment description of ${\mathrm{NaOsO}}_3$, when it comes to analyzing the magnetic excitation spectra. Interestingly, this comes about from a microscopic theory that describes the electron system as Bloch states, adjusted to a mean-field solution to Hubbard-like interactions.

We have investigated the magnetic structure of ${\mathrm{Li}}_2\mathrm{Co}{\left({\mathrm{WO}}_4\right)}_2$ via magnetic susceptibility, and neutron diffraction measurements. Two magnetic transitions are observed in magnetic susceptibility at $T_{N1}\sim 9.5\mathrm{K}$ and $T_{N2}\sim 7.2\mathrm{K}$. Neutron diffraction reveals an incommensurate magnetic order with a wave vector $k_{\text{ICM}}=(\sim 0.46,\sim 0.27,\sim 0.24$) between $T_{N1}$ and $T_{N2}$ and a commensurate magnetic order with a wave vector $k_{\text{CM}}=(0.5,0.25,0.25)$ below $T_{N2}$. The magnetic periodicity in the commensurate phase is four times larger than the nuclear unit cell length along the $b$- and $c$-axis directions with a saturated magnetic moment equal to $\approx 2.92{\mu }_B$. Below $T_{N1}$, the ICM wave vector for ${\mathrm{Li}}_2\mathrm{Co}{\left({\mathrm{WO}}_4\right)}_2$ varies with decreasing temperature and locked into commensurate at $T_{N2}$, whereas weaker temperature dependence of $k_{\text{ICM}}=(0.46,0.17,0.33)$ is observed for ${\mathrm{Li}}_2\mathrm{Ni}{\left({\mathrm{WO}}_4\right)}_2$.

Layered three-dimensional centrosymmetric crystals can exhibit characteristics of noncentrosymmetric materials. This happens when each individual layer alone lacks inversion but, when combined, inversion symmetry is restored; hence the designation locally noncentrosymmertic superconductors (LNCSs). In LNCSs, the effects of impurities and subdominant magnetic field induced pairing channels remain unexplored. Using a minimal model, we examine all pairing channels and show that there is always a subdominant superconducting instability that is favored at high magnetic fields, which can substantially alter the magnetic field–temperature phase diagram. Also, we find that the phase diagram responds to disorder in a nonmonotonic way, which can be subjected to experimental verification. We apply these ideas to the recently unveiled two-phase superconducting phase diagram of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$. We identify the two phases as singlet-triplet mixed even- and odd-parity states at low and high-fields, respectively. Furthermore, we predict the presence of two superconducting phases also for in-plane magnetic fields in cleaner samples, since a high-field phase could have been so far hindered by impurity effects.

Quantum anomalous Hall insulator (QAH)/$s$-wave superconductor (SC) hybrid systems are known to be an ideal platform for realizing two-dimensional topological superconductors with chiral Majorana edge modes. In this paper we study QAH/unconventional SC hybrid systems whose pairing symmetry is $p$ wave, $d$ wave, chiral $p$ wave, or chiral $d$ wave. The hybrid systems are a generalization of the QAH/$s$-wave SC hybrid system. In view of the symmetries of the QAH and pairings, we introduce three topological numbers to classify the topological phases of the hybrid systems. One is the Chern number, which characterizes chiral Majorana edge modes, and the others are topological numbers associated with crystalline symmetries. We numerically calculate the topological numbers and associated surface states for three characteristic regimes that feature the influence of unconventional SCs on QAHs. Our calculation shows a rich variety of topological phases and unveils the following topological phases that are not counterparts of the $s$-wave case: crystalline symmetry-protected helical Majorana edge modes, a line node phase (a crystalline-symmetry-protected Bogoliubov Fermi surface), and multiple chiral Majorana edge modes. The phenomena result from a nontrivial topological interplay between the QAH and unconventional SCs. Finally, we discuss tunnel conductance in a junction between a normal metal and the hybrid systems and show that the chiral and helical Majorana edge modes are distinguishable in terms of the presence or absence of a zero-bias conductance peak.

We investigate one-dimensional charge conserving, spin-singlet (SSS), and spin-triplet (STS) superconductors in the presence of boundary fields. In systems with open boundary conditions (OBC) it has been demonstrated that STS display a fourfold topological degeneracy, protected by the ${\mathbb{Z}}_2$ symmetry, which reverses the spins of all fermions, whereas SSS are topologically trivial. In this paper we show that it is not only the type of the bulk superconducting instability that determines the eventual topological nature of a phase, but rather the interplay between bulk and boundary properties. In particular we show by means of the Bethe ansatz technique that SSS may as well be in a ${\mathbb{Z}}_2$-protected topological phase provided suitable “twisted” open boundary conditions $\widehat{\mathrm{OBC}}$ are imposed. More generally, we find that depending on the boundary fields, a given superconductor, either SSS or STS, may exhibit several types of phases such as topological, mid-gap, and trivial phases; each phase being characterized by a boundary fixed point, which we determine. Of particular interest are the mid-gap phases, which are stabilized close to the topological fixed point. They include fractionalized phases where spin-$1/4$ bound states are localized at the two edges of the system and unfractionalized phases where a spin-$1/2$ bound state is localized at either the left or the right edge.

Understanding the formation of topological defects in connected magnetic nanowires such as in artificial spin ices (ASIs) is critical to controlling their behavior. Moreover, emergent frustration behavior of these defects can be expected due to complex underlying lattice geometry of the ASIs. We have explored in situ thermalization and quenching of quasicrystalline ASIs, which exhibit additional frustration due to aperiodicity from near the Curie temperature, and we observe the formation of two sets of topological vortex defects: within the magnetic bars and within the vertices at which the bars meet. By varying the laser fluence, we show that the number of defects in the vertices varies as the quenching rate changes following a power law relation, which we interpret as being related to the ferromagnetic-to-paramagnetic phase transition. Conversely, the defects formed within the bars are metastable states related to the spontaneous magnetization reversal of the magnetic bars and are therefore strongly dependent on underlying lattice geometry. These results provide insight into the emergence and control of topological defects in confined frustrated magnetic systems.

Spin pumping across ferromagnet/superconductor (F/S) interfaces has attracted much attention lately, yet the focus has been mainly on $s$-wave superconductor based systems whereas (high-temperature) $d$-wave superconductors such as $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\text{−}\mathrm{d}}$ (YBCO) have received scarce attention despite their fundamental and technological interest. Here we use wideband ferromagnetic resonance to study spin-pumping effects in bilayers that combine a soft metallic ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ (Py) ferromagnet and YBCO. We evaluate the spin conductance in YBCO by analyzing the magnetization dynamics in Py. We find that the Gilbert damping exhibits a drastic drop as the heterostructures are cooled across the normal-superconducting transition and then, depending on the S/F interface morphology, either stays constant or shows a strong upturn. This unique behavior is explained considering quasiparticle density of states at the YBCO surface, and is a direct consequence of zero-gap nodes for particular directions in the momentum space. Besides showing the fingerprint of $d$-wave superconductivity in spin pumping, our results demonstrate the potential of high-temperature superconductors for fine tuning of the magnetization dynamics in ferromagnets using $k$-space degrees of freedom of $d$-wave/F interfaces.

The magnetic field of vortices in anisotropic superconducting films is considered in the framework of an anisotropic London approach. It is found that at distances large relative to the core size, the magnetic field normal to the film surface may change sign. We find that the magnetic field attenuates at large distances as $1/r^3$ as it does in isotropic films, but the anisotropy induces an angular dependence to the supercurrents which causes the sign of the field to change for anisotropy parameters $\gamma ={\lambda }_2/{\lambda }_1>\sqrt{2}$ in some parts of the $(x,y)$ plane.

The spin transport inside an odd-frequency spin-triplet superconductor differs from that of a conventional superconductor due to its distinct symmetry properties. We study spin transport inside an emergent odd-frequency superconductor by replacing the spin-singlet gap matrix in the Usadel equation with a matrix representing spin-triplet pairing that is odd under inversion of energy. We show that the peculiar nature of the density of states allows for an even larger spin injection than in the normal state. Moreover, when the odd-frequency pairing inherits its temperature dependence from a conventional superconductor through the proximity effect, the density of states can transition from gapless to gapped as the temperature decreases. At the transition point, the spin accumulation inside the odd-frequency superconductor is peaked and larger than in the normal state. While the spin-flip scattering time is known to decrease below the superconducting transition temperature in conventional superconductors, we find that the same is true for the spin-orbit scattering time in odd-frequency superconductors. This renormalization is particularly large for energies close to the gap edge, if such a gap is present.

Despite their dispersionless character, flat bands (FBs) are often at the heart of remarkable physical phenomena. Because FBs may be responsible for unconventional quantum electronic transport, our purpose is to investigate their impact on the Ruderman-Kittel-Kasuya-Yosida (RKKY) couplings $\left[J\right(\mathbf{R}\left)\right]$. As a good candidate, we choose the Lieb lattice which consists of one fourfold coordinated atom (A) and two twofold coordinated atoms (B,C) per unit cell. As in graphene, at the neutrality point, $J\left(\mathbf{R}\right)$ is found to fall off as $1/R^3$. The coupling between a pair of impurities on sublattice A is found ferromagnetic and isotropic. In contrast, for impurities on the twofold coordinated sites, $J\left(\mathbf{R}\right)$ reveals (i) a strong angular anisotropy and (ii) can be both ferromagnetic (F) or antiferromagnetic (AF). The AF character introduced by the FB can largely dominate, and introduces frustration effects. The FB is found to have a drastic impact on $J\left(\mathbf{R}\right)$ for impurities located on (B,C) sublattices. We have also addressed the effect of tuning the carrier density that reveals remarkable features as well.

Complex oxide heterostructures such as the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ (LAO/STO) interface are paradigmatic platforms to explore emerging multidegree of freedom coupling and the associated exotic phenomena. In this paper, we reveal the effects of multiorbital and magnetic ordering on Rashba spin-orbit coupling (SOC) at the LAO/STO (001) interface. Based on first-principles calculations, we show that the Rashba spin splitting near the conduction band edge can be tuned substantially by the interfacial insulator-metal transition due to the varied multiorbital effect of the lowest $\mathrm{Ti}\text{−}3d{t}_{2g}$ bands. We further unravel an interplay between Rashba SOC and intrinsic magnetism, in which the Rashba SOC-induced spin polarization is suppressed significantly by the presence of interfacial magnetic ordering. These results deepen our understanding of SOC-related intricate electronic and magnetic reconstruction at the perovskite oxide interfaces and shed light on engineering of oxide heterostructures for all-oxide-based spintronic devices.

High-mobility graphene hosting massless charge carriers with linear dispersion provides a promising platform for electron optics phenomena. Inspired by the physics of dielectric optical microcavities where the photon emission characteristics can be efficiently tuned via the cavity shape, we study corresponding mechanisms for trapped Dirac fermionic resonant states in deformed microdisk graphene billiards and directed emission from those. In such graphene devices a back-gate voltage provides an additional tunable parameter to mimic different effective refractive indices and thereby the corresponding Fresnel laws at the boundaries. Moreover, cavities based on single-layer and double-layer graphene exhibit Klein- and anti-Klein tunneling, respectively, leading to distinct differences with respect to dwell times and resulting emission profiles of the cavity states. Moreover, we find a variety of different emission characteristics depending on the position of the source where charge carriers are fed into the cavities. Combining quantum mechanical simulations with optical ray tracing and a corresponding phase-space analysis, we demonstrate strong confinement of the emitted charge carriers in the midfield of single-layer graphene systems and can relate this to a lensing effect. For bilayer graphene, trapping of the resonant states is more efficient and the emission characteristics do depend less on the source position.

In this work we developed an extension to the transfer-matrix method (TMM) to include a current source term, with the aim of describing both the transmission and emission of THz pulses in spintronics THz emitters. The TMM with source is derived from the Maxwell equations with a volume free current term. This extension to the TMM allows for the study of realistic spintronics THz emitters as multilayers of different materials with different thicknesses. We use this to describe the drop rate of the THz emission amplitude at higher heavy metal layer thicknesses, and use it as a chance to remind that is not related and does not provide information about the spin diffusion length. It is instead the effect of the increase in the parasitic absorption of the generated radiation by the conducting parts of the spintronics THz emitter itself.

It is commonly assumed that the photocurrent in two-dimensional systems with a centrosymmetric lattice is generated at structural inhomogeneities, such as $p-n$ junctions. Here, we study an alternative mechanism of photocurrent generation associated with the inhomogeneity of the driving electromagnetic field, termed “plasmonic drag.” It is associated with direct momentum transfer from the field to conduction electrons and can be characterized by a nonlocal nonlinear conductivity ${\sigma }^{\left(2\right)}(\mathbf{q},\omega )$. By constructing a classical kinetic model fully accounting for nonlocality, we show that the nonlinear conductivity is resonantly enhanced for wave phase velocity coinciding with electron Fermi velocity. The enhancement is interpreted as phase locking between electrons and the wave. We discuss a possible experiment where a nonuniform field is created by a propagating graphene plasmon and find an upper limit of the current responsivity vs plasmon velocity. This limit is set by a competition between resonantly growing ${\sigma }^{\left(2\right)}(\mathbf{q},\omega )$ and the diverging kinetic energy of electrons as the wave velocity approaches the Fermi velocity.

The recently developed concept of time-varying metamaterials paves the way for advanced methods of electromagnetic property modulation, such as magnet-free isolators, frequency converters, and other nonreciprocal photonic components. Here, we analyze the metamaterial based on the periodically arranged temporally modulated graphene layers via a steplike graphene Fermi level. The material properties are described by the effective medium theory. The electromagnetic wave temporal scattering phenomenon caused by rapid steplike graphene Fermi level switching is considered. It is demonstrated that the frequency of scattered waves increases as the Fermi level grows rapidly and vice versa. The relation between the amplitudes of forward- and backward-scattered waves is found using the Laplace transform technique. The proposed concept allows us to efficiently generate frequency harmonics under instant switching of the graphene Fermi level at a few meV. Our investigation provides a basement for the realization of temporally modulated multilayer structures based on functional tunable materials as power-efficient frequency converters.