We have studied a charge-orbital driven metal-insulator transition (MIT) in hollandite-type ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$ by means of hard x-ray photoemission spectroscopy (HAXPES). The Ti $2p$ HAXPES indicates strong ${\mathrm{Ti}}^{3+}$/${\mathrm{Ti}}^{4+}$ charge fluctuation in the metallic phase above the MIT temperature. The metallic phase is characterized by a power-law spectral function near the Fermi level which would be a signature of bad metal with non-Drude polaronic behavior. The power-law spectral shape is associated with the large Seebeck coefficient of the metallic phase in ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$.

Anomaly cancellation has been shown to occur in broken time-reversal symmetry Weyl metals, which explains the existence of a Fermi arc. We extend this result in the case of broken inversion symmetry Weyl metals. Constructing a minimal model that takes a double pair of Weyl points, we demonstrate the anomaly cancellation explicitly. This demonstration explains why a chiral pair of Fermi arcs appear in broken inversion symmetry Weyl metals. In particular, we find that this pair of Fermi arcs gives rise to either “quantized” spin Hall or valley Hall effects, which corresponds to the “quantized” version of the charge Hall effect in broken time-reversal symmetry Weyl metals.

The nature of the glass transition is a fundamental and long-standing intriguing issue in the condensed-matter physics and materials science community. In particular, the structural response by which a liquid is arrested dynamically to form a glass or amorphous solid upon approaching its freezing temperature [the glass transition temperature ($T_g$)] remains unclear. Various structural scenarios in terms of the percolation theory have been proposed recently to understand such a phenomenon; however, there is still no consensus on what the general percolation entity is and how the entity responds to the sudden slowdown dynamics during the glass transition. In this paper, we demonstrate that one-dimensional local linear ordering (LLO) is a universal structural motif associated with the glass transition for various metallic glasses. The quantitative evolution of LLO with temperature indicates that a percolating LLO network forms to serve as the backbone of the rigid glass solid when the temperature approaches the freezing point, resulting in the frozen-in dynamics accompanying the glass transition. The percolation transition occurs by pinning different LLO networks together, which only needs the introduction of a small number of “joint” atoms between them, and therefore the energy expenditure is very low.

We present a combined analytical and numerical micromagnetic study of the equilibrium energy, size, and shape of antiskyrmionic magnetic configurations. Antiskyrmions can be stabilized when the Dzyaloshinskii-Moriya interaction has opposite signs along two orthogonal in-plane directions, breaking the magnetic circular symmetry. We compare the equilibrium energy, size, and shape of antiskyrmions and skyrmions that are stabilized in environments with anisotropic and isotropic Dzyaloshinskii-Moriya interactions, respectively, but with the same strength of the magnetic interactions. When the dipolar interactions are neglected, the skyrmion and the antiskyrmion have the same energy, shape, and size in their respective environments. However, when dipolar interactions are considered, the energy of the antiskyrmion is strongly reduced, and its equilibrium size increases with respect to that of the skyrmion. While the skyrmion configuration shows homochiral Néel magnetization rotations, antiskyrmions show partly Néel and partly Bloch rotations. The latter do not produce magnetic charges and thus cost less dipolar energy. Both magnetic configurations are stable when the magnetic energies almost cancel each other, which means that a small variation of one parameter can drastically change their configurations, sizes, and energies.

Using first-principles calculations, we study the magnetic properties of a Co monolayer on a Pt(111) surface with a capping monolayer of selected $5d$ elements (Re, Os, Ir, Pt, and Au). First we determine the tensorial exchange interactions and magnetic anisotropies characterizing the Co monolayer for all considered systems. We find a close relationship between the magnetic moment of the Co atoms and the nearest-neighbor isotropic exchange interaction, which is attributed to the electronic hybridization between the Co and the capping layers, in the spirit of the Stoner picture of ferromagnetism. The Dzyaloshinskii-Moriya interaction is decreased for all overlayers compared to the uncapped Co/Pt(111) system, while even the sign of the Dzyaloshinskii-Moriya interaction changes in the case of the Ir overlayer. We conclude that the variation of the Dzyaloshinskii-Moriya interaction is well correlated with the change of the magnetic anisotropy energy and of the orbital moment anisotropy. The unique influence of the Ir overlayer on the Dzyaloshinskii-Moriya interaction is traced by scaling the strength of the spin-orbit coupling of the Ir atoms in Ir/Co/Pt(111) and by changing the Ir concentration in the ${\mathrm{Au}}_{1-x}{\mathrm{Ir}}_x$/Co/Pt(111) system. Our spin dynamics simulations indicate that the magnetic ground state of Re/Co/Pt(111) thin film is a spin spiral with a tilted normal vector, while the other systems are ferromagnetic.

We investigated static and dynamic magnetoelectric properties of single crystalline ${\mathrm{BaSrCoZnFe}}_{11}{\mathrm{AlO}}_{22}$, which is a room-temperature multiferroic with Y-type hexaferrite crystal structure. Below 300 K, a purely electric-dipole-active electromagnon at $\approx 1.2$ THz with the electric polarization oscillating along the hexagonal axis was observed by THz and Raman spectroscopies. We investigated the behavior of the electromagnon with applied dc magnetic field and linked its properties to static measurements of the magnetic structure. Our analytical calculations determined selection rules for electromagnons activated by the magnetostriction mechanism in various magnetic structures of Y-type hexaferrite. Comparison with our experiment supports that the electromagnon is indeed activated by the magnetostriction mechanism involving spin vibrations along the hexagonal axis.

Broken symmetries in diverse systems generate a number of intriguing phenomena and the analysis on such broken symmetries often provides decisive clues for exploring underlying physics in the systems. Recently, in magnetic thin-film systems, the Dzyaloshinskii-Moriya interaction (DMI)—induced by the broken symmetry of structural inversion—accounts for various chiral phenomena, which are of timely issues in spintronics. Here, we report an experimental observation on unexpected tilting of magnetic domain walls (DWs) due to the broken symmetry under the application of the magnetic field transverse to the magnetic wire systems. It has been predicted that the DMI possibly causes such DW tilting in the direction of the energy minimization. However, very interestingly, experimental observation reveals that the DW tilting does not follow the prediction based on the energy minimization, even for the tilting direction. Instead, the DW tilting is governed by the DW speed asymmetry that is initiated by the DW pinning at wire edges. A simple analytic model is proposed in consideration of the DW speed asymmetry at wire edges, which successfully explains the experimental observation of the DW tilting directions and angles, as confirmed by numerical simulation. The present study manifests the decisive role of the DW pinning with the DW speed asymmetry, which determines the DW configuration and consequently, the dynamics.

Theoretical proposals for spin-ice analogs based on nanostructured superconductors have suggested larger flexibility for probing the effects of fluctuations and disorder than in the magnetic systems. In this paper, we unveil the particularities of a vortex ice system by direct observation of the vortex distribution in a kagome lattice of paired antidots using scanning Hall probe microscopy. The theoretically suggested vortex ice distribution, lacking long-range order, is observed at half matching field ($H_1/2$). Moreover, the vortex ice state formed by the pinned vortices is still preserved at $2H_1/3$. This unexpected result is attributed to the introduction of interstitial vortices at these magnetic-field values. Although the interstitial vortices increase the number of possible vortex configurations, it is clearly shown that the vortex ice state observed at $2H_1/3$ is less prone to defects than at $H_1/2$. In addition, the nonmonotonic variations of the vortex ice quality on the lattice spacing indicates that a highly ordered vortex ice state cannot be attained by simply reducing the lattice spacing. The optimal design to observe defect-free vortex ice is discussed based on the experimental statistics. The direct observations of a tunable vortex ice state provides new opportunities to explore the order-disorder transition in artificial ice systems.

The temperature dependence of the in-plane magnetic penetration depth (${\lambda }_{ab}$) in an extensively characterized sample of superconducting ${\mathrm{CaKFe}}_4{\mathrm{As}}_4(T_{\mathrm{c}}\simeq 35\mathrm{K})$ was investigated using muon-spin rotation ($\mu \mathrm{SR}$). A comparison of ${\lambda }_{ab}^{-2}\left(T\right)$ measured by $\mu \mathrm{SR}$ with the one inferred from angle-resolved photoemission spectroscopy (ARPES) data confirms the presence of multiple gaps at the Fermi level. An agreement between $\mu \mathrm{SR}$ and ARPES requires the presence of additional bands, which are not resolved by ARPES experiments. These bands are characterized by small superconducting gaps with an average zero-temperature value of ${\mathrm{\Delta }}_0=2.4\left(2\right)\mathrm{meV}$. Our data suggest that in ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ the $s^{\pm }$ order parameter symmetry acquires a more sophisticated form by allowing a sign change not only between electron and hole pockets, but also within pockets of similar type.

We show that spin-polarized local density of states (LDOS) measurements can uniquely determine the chiral nature of topologically protected edge states surrounding a ferromagnetic island embedded in a conventional superconductor with spin-orbit coupling. The spin-polarized LDOS show a strong spin polarization directly tied to the normal direction of the edge with opposite polarizations on opposite sides of the island and with a distinct oscillatory pattern in energy.

We propose to raise the critical temperature $T_c$ for superconductivity in doped ${\mathrm{C}}_{60}$ molecular crystals by increasing the electronic density of states at the Fermi level $N\left(E_F\right)$ and thus the electron-phonon coupling constant in low-dimensional ${\mathrm{C}}_{60}$ nanoarrays. We consider both electron and hole dopings and present numerical results for $N\left(E_F\right)$, which increases with the decreasing bandwidth of the partly filled $h_u$- and $t_{1u}$-derived frontier bands with the decreasing coordination number of ${\mathrm{C}}_{60}$. Whereas a significant increase in $N\left(E_F\right)$ occurs in two-dimensional (2D) arrays of doped ${\mathrm{C}}_{60}$ intercalated in-between graphene layers, we propose that the highest-$T_c$ values approaching room temperature may occur in bundles of nanotubes filled by one-dimensional (1D) arrays of externally doped ${\mathrm{C}}_{60}$ or $\mathrm{La}@{\mathrm{C}}_{60}$ or in diluted three-dimensional (3D) crystals where quasi-1D arrangements of ${\mathrm{C}}_{60}$ form percolation paths.

We consider the $O\left(3\right)$ nonlinear sigma model with the $\theta$ term and its linear counterpart in 1+1D. The model has discrete time-reflection and space-reflection symmetries at any $\theta$, and enjoys the periodicity in $\theta \to \theta +2\pi$. At $\theta =0,\pi$ it also has a charge-conjugation $C$ symmetry. Gauging the discrete space-time reflection symmetries is interpreted as putting the theory on the nonorientable $\mathbb{R}{P}^2$ manifold, after which the $2\pi$ periodicity of $\theta$ and the $C$ symmetry at $\theta =\pi$ are lost. We interpret this observation as a mixed 't Hooft anomaly among charge-conjugation $C$, parity $P$, and time-reversal $T$ symmetries when $\theta =\pi$. Anomaly matching implies that in this case the ground state cannot be trivially gapped, as long as $C,P$, and $T$ are all good symmetries of the theory. We make several consistency checks with various semiclassical regimes, and with the exactly solvable XYZ model. We interpret this anomaly as an anomaly of the corresponding spin-half chains with translational symmetry, parity, and time reversal [but not involving the SO(3)-spin symmetry], requiring that the ground state is never trivially gapped, even if SO(3) spin symmetry is explicitly and completely broken. We also consider generalizations to $\mathbb{C}{P}^{N-1}$ models and show that the $C\text{−}P\text{−}T$ anomaly exists for even $N$.

We implement an extension of the pseudofermion functional renormalization group method for quantum spin systems that takes into account two-loop diagrammatic contributions. An efficient numerical treatment of the additional terms is achieved within a nested graph construction which recombines different one-loop interaction channels. In order to be fully self-consistent with respect to self-energy corrections, we also include certain three-loop terms of Katanin type. We first apply this formalism to the antiferromagnetic $J_1\text{−}{J}_2$ Heisenberg model on the square lattice and benchmark our results against the previous one-loop plus Katanin approach. Even though the renormalization group (RG) equations undergo significant modifications when including the two-loop terms, the magnetic phase diagram, comprising Néel ordered and collinear ordered phases separated by a magnetically disordered regime, remains remarkably unchanged. Only the boundary position between the disordered and the collinear phases is found to be moderately affected by two-loop terms. On the other hand, critical RG scales, which we associate with critical temperatures $T_{\text{c}}$, are reduced by a factor of $\sim 2$ indicating that the two-loop diagrams play a significant role in enforcing the Mermin-Wagner theorem. Improved estimates for critical temperatures are also obtained for the Heisenberg ferromagnet on the three-dimensional simple cubic lattice where errors in $T_{\text{c}}$ are reduced by $\sim 34\%$. These findings have important implications for the quantum phase diagrams calculated within the previous one-loop plus Katanin approach which turn out to be already well converged.

We investigate the effects of strain on superconductivity with particular reference to ${\mathrm{SrTiO}}_3$. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviors under uniaxial and biaxial strain can be understood.

We investigate spin-triplet exciton condensation in the two-orbital Hubbard model close to half-filling by means of dynamical mean-field theory. Employing an impurity solver that handles complex off-diagonal hybridization functions, we study the behavior of excitonic condensate in stoichiometric and doped systems subject to external magnetic field. We find a general tendency of the triplet order parameter to lie perpendicular with the applied field and identify exceptions from this rule. For solutions exhibiting $\mathbf{k}$-odd spin textures, we discuss the Bloch theorem, which, in the absence of spin-orbit coupling, forbids the appearance of spontaneous net spin current. We demonstrate that the Bloch theorem is not obeyed by the dynamical mean-field theory.

We study the dynamic response of a two-dimensional system of itinerant fermions in the vicinity of a uniform $\left(\mathrm{Q}=0\right)$ Ising nematic quantum critical point of $d\text{−}\text{wave}$ symmetry. The nematic order parameter is not a conserved quantity, and this permits a nonzero value of the fermionic polarization in the $d\text{−}\text{wave}$ channel even for vanishing momentum and finite frequency: $\mathrm{\Pi }(\mathrm{q}=0,{\mathrm{\Omega }}_m)\ne 0$. For weak coupling between the fermions and the nematic order parameter (i.e., the coupling is small compared to the Fermi energy), we perturbatively compute $\mathrm{\Pi }(\mathrm{q}=0,{\mathrm{\Omega }}_m)\ne 0$ over a parametrically broad range of frequencies where the fermionic self-energy $\mathrm{\Sigma }\left(\omega \right)$ is irrelevant, and use Eliashberg theory to compute $\mathrm{\Pi }(\mathrm{q}=0,{\mathrm{\Omega }}_m)$ in the non-Fermi-liquid regime at smaller frequencies, where $\mathrm{\Sigma }\left(\omega \right)>\omega$. We find that $\mathrm{\Pi }(\mathrm{q}=0,\mathrm{\Omega })$ is a constant, plus a frequency-dependent correction that goes as $\left|\mathrm{\Omega }\right|$ at high frequencies, crossing over to ${\left|\mathrm{\Omega }\right|}^{1/3}$ at lower frequencies. The ${\left|\mathrm{\Omega }\right|}^{1/3}$ scaling holds also in a non-Fermi-liquid regime. The nonvanishing of $\mathrm{\Pi }(q=0,\mathrm{\Omega })$ gives rise to additional structure in the imaginary part of the nematic susceptibility ${\chi }^{″}(q,\mathrm{\Omega })$ at $\mathrm{\Omega }>v_{F}q$, in marked contrast to the behavior of the susceptibility for a conserved order parameter. This additional structure may be detected in Raman scattering experiments in the $d\text{−}\text{wave}$ geometry.

Obtaining quantitative ground-state behavior for geometrically-frustrated quantum magnets with long-range interactions is challenging for numerical methods. Here, we demonstrate that the ground states of these systems on two-dimensional lattices can be efficiently obtained using state-of-the-art translation-invariant variants of matrix product states and density-matrix renormalization-group algorithms. We use these methods to calculate the fully-quantitative ground-state phase diagram of the long-range interacting triangular Ising model with a transverse field on six-leg infinite-length cylinders and scrutinize the properties of the detected phases. We compare these results with those of the corresponding nearest neighbor model. Our results suggest that, for such long-range Hamiltonians, the long-range quantum fluctuations always lead to long-range correlations, where correlators exhibit power-law decays instead of the conventional exponential drops observed for short-range correlated gapped phases. Our results are relevant for comparisons with recent ion-trap quantum simulator experiments that demonstrate highly-controllable long-range spin couplings for several hundred ions.

The calculation of optical force density distribution inside a material is challenging at the nanoscale, where quantum and nonlocal effects emerge and macroscopic parameters such as permittivity become ill-defined. We demonstrate that the microscopic optical force density of nanoplasmonic systems can be defined and calculated using the microscopic fields generated using a self-consistent hydrodynamics model that includes quantum, nonlocal, and retardation effects. We demonstrate this technique by calculating the microscopic optical force density distributions and the optical binding force induced by external light on nanoplasmonic dimers. This approach works even in the limit when the nanoparticles are close enough to each other so that electron tunneling occurs, a regime in which classical electromagnetic approach fails completely. We discover that an uneven distribution of optical force density can lead to a light-induced spinning torque acting on individual particles. The hydrodynamics method offers us an accurate and efficient approach to study optomechanical behavior for plasmonic systems at the nanoscale.

We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound $\mathrm{Sm}{\mathrm{B}}_6$, a prototype Kondo insulator. Temperature-dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at $\sim T^{*}=20\mathrm{K}$, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20 K) suggests the emergence of a surface state with an effective electron mass of $0.1m_e$. The conductivity dynamics following optical excitation is also measured and interpreted using Rothwarf-Taylor (R-T) phenomenology, yielding a hybridization gap energy of 17 meV. However, R-T modeling of the conductivity dynamics reveals a deviation from the expected thermally excited quasiparticle density at temperatures below 20 K, indicative of another channel opening up in the low-energy electrodynamics. Taken together, these results are consistent with the onset of a surface state well below the crossover temperature (100 K) after long-range coherence of the $f$-electron Kondo lattice is established.