We demonstrate that magnetocaloric measurements can rapidly reveal details of the phase diagrams of high-temperature skyrmion hosts, concurrently yielding quantitative latent heats of the field-driven magnetic phase transitions. Our approach addresses an outstanding issue in the phase diagram of the skyrmion host FeGe by showing that dc magnetic anomalies can be explained in terms of entropic signatures consistent with a phase diagram containing a single pocket of skyrmionic order and a Brazovskii transition.

We measure and analyze the effective spin Hall angle of platinum in the low-residual resistivity regime by second-harmonic measurements of the spin-orbit torques for a multilayer of $\mathrm{Pt}\left|\mathrm{Co}\right|{\mathrm{AlO}}_x$. An angular-dependent study of the torques allows us to extract the effective spin Hall angle responsible for the damping-like torque in the system. We observe a strikingly nonmonotonic and reproducible temperature dependence of the torques. This behavior is compatible with recent theoretical predictions which include both intrinsic and extrinsic (impurities and phonons) contributions to the spin Hall effect at finite temperatures.

Exchange coupling at the interface of an antiferromagnet and a ferromagnet is the major route toward the future magnetoelectric applications of multiferroic bismuth ferrite at room temperature. By using angle-resolved longitudinal magneto-optic Kerr effect microscopy, we have investigated the exchange anisotropy in a ferromagnetic Co layer adjacent to electrically aligned straight-stripe mixed-phase-boundary (MPB) regions of La-5%-doped $\mathrm{BiFe}{\mathrm{O}}_3$. We have found that the magnetic easy axis of the exchange-coupled Co layer becomes parallel to the in-plane crystallographic axis nearest to the adjacent MPB elongation axis. The coercive field of the exchange-coupled Co layer along the magnetic easy axis has prominently increased by $\sim 66$ Oe, i.e., 30 times larger than that of a control sample (2.3 Oe), wherein a 5-nm-thick nonmagnetic Ta spacer is placed between the antiferromagnetic and ferromagnetic layers and thus it is intended to have only the shape anisotropy arising from the surface nanostructure of the MPB regions. The finding opens a promising avenue for magnetoelectric applications at room temperature.

As a strongly spin-orbit-coupled metallic model with ferromagnetism, we have considered an extended Stoner model to the relativistic regime, named Dirac ferromagnet in three dimensions. In a previous paper [J. Fujimoto and H. Kohno, Phys. Rev. B 90, 214418 (2014)], we studied the transport properties giving rise to the anisotropic magnetoresistance (AMR) and the anomalous Hall effect (AHE) with the impurity potential being taken into account only as the self-energy. The effects of the vertex corrections (VCs) to AMR and AHE are reported in this paper. AMR is found not to change quantitatively when the VCs are considered, although the transport lifetime is different from the one-electron lifetime and the charge current includes additional contributions from the correlation with spin currents. The side-jump and the skew-scattering contributions to AHE are also calculated. The skew-scattering contribution is dominant in the clean case as can be seen in the spin Hall effect in the nonmagnetic Dirac electron system.

Thin ferromagnetic films with an interfacially induced DMI exhibit nontrivial asymmetric dispersion relations that lead to unique and useful magnonic properties. Here we derive an analytical expression for the magnon propagation angle within the micromagnetic framework and show how the dispersion relation can be approximated with a comprehensible geometrical interpretation in the $k$ space of the propagation of spin waves. We further explore the refraction of spin waves at DMI interfaces in heterochiral magnetic films, after deriving a generalized Snell's law tunable by an in-plane magnetic field, that yields analytical expressions for critical incident angles. The found asymmetric Brewster angles at interfaces of regions with different DMI strengths, adjustable by magnetic field, support the conclusion that heterochiral ferromagnetic structures are an ideal platform for versatile spin-wave guides.

We investigate the anisotropic nature of magnetocrystalline coupling between the crystallographic and skyrmion crystal (SKX) lattices in the chiral magnet MnSi by magnetic field-angle resolved resonant ultrasound spectroscopy. Abrupt changes are observed in the elastic moduli and attenuation when the magnetic field is parallel to the [011] crystallographic direction. These observations are interpreted in a phenomenological Ginzburg-Landau theory that identifies switching of the SKX orientation to be the result of an anisotropic magnetocrystalline coupling potential. Our paper sheds new light on the nature of magnetocrystalline coupling potential relevant to future spintronic applications.

We investigate superconductivity in ${\mathrm{In}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ ($x\le 0.5$) synthesized at high pressures of up to 2 GPa and observe an enhancement of the superconducting transition temperature $T_{\mathrm{c}}$ for increasing tin concentration $x$. These compounds have not been accessible in rocksalt structure via conventional ambient pressure synthesis. While the lattice constant smoothly increases with $x$, $T_{\mathrm{c}}$ saturates around $x=0.4$. Electronic structure calculations indicate that the $T_{\mathrm{c}}$ modulation is brought on by the change of the density of states in the vicinity of the Fermi energy $\left[N\left(E_{\mathrm{F}}\right)\right]$. However, differences between the calculated $N\left(E_{\mathrm{F}}\right)$ and the observed electronic specific-heat coefficient indicate that the phonon dispersion plays an important role in the system and that the mechanism of superconductivity may not be the same in the entire doping range.

We show that bringing into proximity two topologically trivial systems can give rise to a topological phase. More specifically, we study a 1D metallic nanowire proximitized by a 2D superconducting substrate with a mixed $s$-wave and $p$-wave pairing, and we demonstrate both analytically and numerically that the phase diagram of such a setup can be richer than reported before. Thus apart from the two “expected” well-known phases (i.e., where the substrate and the wire are both simultaneously trivial or topological), we show that there exist two peculiar phases in which the nanowire can be in a topological regime while the substrate is trivial and vice versa.

The Chern topological numbers of a material system are traditionally written in terms of the Berry curvature which depends explicitly on the material band structure and on the Bloch eigenwaves. Here, we demonstrate that it is possible to calculate the gap Chern numbers of a photonic platform without having any detailed knowledge of its band structure, relying simply on the system photonic Green function. It is shown that the gap Chern number is given by an integral of the photonic Green function along a line of the complex frequency plane parallel to the imaginary axis. Our theory applies to arbitrary frequency dispersive fully three-dimensional photonic crystals, as well as to the case of electromagnetic continua with no intrinsic periodicity.

In the antiferromagnetic heavy-fermion compound ${\mathrm{U}}_2{\mathrm{Zn}}_{17}$, the Sommerfeld coefficient $\gamma$ can be enhanced if all Zn atoms are replaced by a combination of Cu and Al or Cu and Ga. In the former ternary phase, glassy behavior was observed, while for the latter, conflicting ground-state reports suggest material quality issues. In this work, we investigate the ${\mathrm{U}}_2{\mathrm{Cu}}_{17-x}{\mathrm{Ga}}_x$ substitutional series for $4.5\le x\le 9.5$. In the homogeneity range of the phase with the ${\mathrm{Th}}_2{\mathrm{Zn}}_{17}$-type of crystal structure, all samples exhibit glassy behavior with $0.6\mathrm{K}\le T_{\text{f}}\le 1.8\mathrm{K}$. The value of the electronic specific heat coefficient $\gamma$ in this system exceeds 900 mJ/${\mathrm{mol}}_{\mathrm{U}}{\mathrm{K}}^2$. Such a drastic effective-mass enhancement can possibly be attributed to the effects of structural disorder, since the role of electron concentration and lattice compression is likely minimal. Crystallographic disorder is also responsible for the emergence of non-Fermi-liquid behavior in these spin-glass materials, as evidenced by logarithmic divergence of magnetic susceptibility, specific heat, and electrical resistivity.

Using the tight-binding treatment for the spin-asymmetric Hubbard model we explore the effect of electronic interactions in the ferromagnetic, partially filled Lieb lattice. As a key result we demonstrate the formation of correlation satellites in the minority spin channel. In addition, we consider the role played by transverse-field spin fluctuations in metallic ferromagnets. We quantify the degree of electronic demagnetization, showing that the half-metallic state is rather robust to local spin flips. Not being restricted to the case of a partially filled Lieb lattice, our findings are expected to advance the general understanding of spin-selective electronic reconstruction in strongly correlated quantum ferromagnets.

We study the Hubbard model on the honeycomb lattice in the vicinity of the quantum critical point by means of a multiband formulation of the dual fermion approach. Beyond the strong local correlations of the dynamical mean field, critical fluctuations on all length scales are included by means of a ladder diagram summation. Analysis of the susceptibility yields an estimate of the critical interaction strength of the quantum phase transition from a paramagnetic semimetal to an antiferromagnetic insulator, in good agreement to other numerical methods. We further estimate the crossover temperature to the renormalized classical regime. Our data imply that, at large interaction strengths, the Hubbard model on the honeycomb lattice behaves like a quantum nonlinear $\sigma$ model, while displaying signs of non-Fermi-liquid behavior.

Despite the superiority of two-dimensional (2D) topological insulators (TIs) over their three-dimensional (3D) counterparts in various aspects and the essential distinction between them in structural symmetry, the variation of the topological one-dimensional (1D) edge states upon magnetic interaction and their application for spintronic devices have not been sufficiently illuminated. Here, we reveal that 1D edge states of 2D TIs have a unique magnetic response never observed in 2D surface states of 3D TIs, and using this exotic nature we propose a way to utilize the spin-polarized channel for spintronic applications. We investigate the effects of width and magnetic decoration on the 1D topological edge state of Bi bilayer nanoribbons (BNRs). Through the Zak phase, we find that the zero-energy states are enforced at the magnetic domain boundaries in the Cr-decorated BNR and directly examine their robustness using short-range magnetic domain structures. We also demonstrate that 1D edge states of BNRs can be selectively and reversibly controlled by the combination of magnetic reorientation and electric field without compromising their structural integrity. Our work provides a fundamental understanding of 1D topological edge states and shows the opportunity of using these features in spintronic devices.

We report on magneto-optical imaging and the temperature dependency of the upper critical fields $H_{c2}^c\left(T\right)$ parallel to the $c$ axis and $H_{c2}^{ab}\left(T\right)$ parallel to the $ab$ plane in ${\mathrm{Ba}}_2{\mathrm{Ti}}_2{\mathrm{Fe}}_2{\mathrm{As}}_4\mathrm{O}$ single crystals. These data were inferred from the measurements of the temperature-dependent resistance in static magnetic fields up to 14 T and magnetoresistance in pulsed fields up to 60 T. $H_{c2}$ values are found to be 52 and 50 T for $H\parallel ab$ and $H\parallel c$, respectively. These values are 1.2–1.35 times larger than the weak-coupling Pauli paramagnetic limit $\left(H_p\sim 1.84T_c\right)$, indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. Our observations of strong bending in the $H_{c2}^{ab}\left(T\right)$ curves and a nearly isotropic maximum upper critical field $H_{c2}^{ab}\left(0\right)\approx H_{c2}^c\left(0\right)$ support the presence of a strong Pauli paramagnetic effect. We show that the Werthamer-Helfand-Hohenberg (WHH) formula that includes the spin-orbit scattering can effectively describe the $H_{c2}^{ab}\left(T\right)$ curve, whereas $H_{c2}$ deviates from the conventional WHH theoretical model without considering the spin paramagnetic effect for the $H\parallel c$ and $H\parallel ab$ directions. For $H\parallel c$, a two-band model is required to fully reproduce the behavior of $H_{c2}$, while for $H\parallel ab$ the spin paramagnetic effect is responsible for the behavior of $H_{c2}$. The anisotropy of $H_{c2}$ is close to 3 near $T_c$ and decreases rapidly at lower temperatures.

It is demonstrated that fermionic/bosonic symmetry-protected topological (SPT) phases across different dimensions and symmetry classes can be organized using geometric constructions that increase dimensions and symmetry-reduction maps that change symmetry groups. Specifically, it is shown that the interacting classifications of SPT phases with and without glide symmetry fit into a short exact sequence, so that the classification with glide is constrained to be a direct sum of cyclic groups of order 2 or 4. Applied to fermionic SPT phases in the Wigner-Dyson class AII, this implies that the complete interacting classification in the presence of glide is ${\mathbb{Z}}_4\oplus {\mathbb{Z}}_2\oplus {\mathbb{Z}}_2$ in three dimensions. In particular, the hourglass-fermion phase recently realized in the band insulator KHgSb must be robust to interactions. Generalizations to spatiotemporal glide symmetries are discussed.

Femtosecond time-resolved midinfrared reflectivity is used to investigate the electron and phonon dynamics occurring at the direct band gap of the excitonic insulator ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ below the critical temperature of its structural phase transition. We find that the phonon dynamics show a strong coupling to the excitation of free carriers at the $\mathrm{\Gamma }$ point of the Brillouin zone. The optical response saturates at a critical excitation fluence $F_C=0.30\pm 0.08$ mJ/${\mathrm{cm}}^2$ due to optical absorption saturation. This limits the optical excitation density in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ so that the system cannot be pumped sufficiently strongly to undergo the structural change to the high-temperature phase. We thereby demonstrate that ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ exhibits a blocking mechanism when pumped in the near-infrared regime, preventing a nonthermal structural phase transition.

Non-centrosymmetric $\mathrm{Eu}T{\mathrm{Ge}}_3$ ($T$= Co, Ni, Rh, and Ir) possesses magnetic ${\mathrm{Eu}}^{2+}$ ions, and antiferromagnetic ordering appears at low temperatures. Transition-metal substitution leads to changes in the unit-cell volume and in the magnetic ordering. However, the magnetic ordering temperature does not scale with the volume change, and the Eu valence is expected to remain divalent. Here we study the bulk electronic structure of non-centrosymmetric $\mathrm{Eu}T{\mathrm{Ge}}_3$ ($T$= Co, Ni, Rh, and Ir) by hard x-ray photoelectron spectroscopy. The Eu $3d$ core-level spectrum confirms the robust ${\mathrm{Eu}}^{2+}$ valence state against the transition-metal substitution with a small contribution from ${\mathrm{Eu}}^{3+}$. The estimated Eu mean valence is around 2.1 in these compounds, as confirmed by multiplet calculations. In contrast, the Ge $2p$ spectrum shifts to higher binding energy upon changing the transition metal from $3d$ to $4d$ to $5d$ elements, hinting at a change in the Ge-$T$ bonding strength. The valence bands of the different compounds are found to be well reproduced by ab initio band structure calculations.

Cadmium arsenide $\left({\mathrm{Cd}}_3{\mathrm{As}}_2\right)$ has recently became conspicuous in solid-state physics due to several reports proposing that it hosts a pair of symmetry-protected 3D Dirac cones. Despite vast investigations, a solid experimental insight into the band structure of this material is still missing. Here we fill one of the existing gaps in our understanding of ${\mathrm{Cd}}_3{\mathrm{As}}_2$, and based on our Landau-level spectroscopy study, we provide an estimate for the energy scale of 3D Dirac electrons in this system. We find that the appearance of such charge carriers is limited—contrary to a widespread belief in the solid-state community—to a relatively small energy scale (below 40 meV).

In the present work, the electronic and vibrational properties of both pristine ${\mathrm{V}}_2\mathrm{C}$ and fully terminated ${\mathrm{V}}_2\mathrm{C}{T}_2$ (where $T=\mathrm{F},$ O, OH) two-dimensional monolayers are investigated using density functional theory. First, the atomic structures of ${\mathrm{V}}_2\mathrm{C}$-based MXene phases are optimized, and their respective dynamical stabilities are discussed. Second, electronic band structures are computed indicating that ${\mathrm{V}}_2\mathrm{C}$ is metallic as well as all the corresponding functionalized systems. Third, the vibrational properties (phonon frequencies and spectra) of ${\mathrm{V}}_2\mathrm{C}$-based MXenes are computed with the density functional perturbation theory and reported. Both Raman-active ($E_g$, $A_{1g}$) and infrared-active ($E_u$, $A_{2u}$) vibrational modes are predicted ab initio with the aim to correlate the experimental Raman peaks with the calculated vibrational modes and to assign them to specific atomic motions. The effect of the terminal groups on the vibrational properties is emphasized, along with the effect on the presence and position of the corresponding Raman peaks. Our results provide insights for the identification and characterization of ${\mathrm{V}}_2\mathrm{C}$-based samples using Raman spectroscopy.

Quantum size effects on the permittivity of metal nanoparticles are investigated using the quantum box model. Explicit upper and lower bounds are derived for the permittivity and relaxation rates due to quantum confinement effects. These bounds are verified numerically, and the size dependence and frequency dependence of the empirical Drude size parameter is extracted from the model. Results suggest that the common practice of empirically modifying the dielectric function can lead to inaccurate predictions for highly uniform distributions of finite-sized particles.