Using femtosecond optical pulses, we have investigated the ultrafast magnetization dynamics induced in a dielectric film of bismuth-substituted yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We show that exciting the sample from the Pt surface launches an acoustic strain pulse propagating into the garnet film. We discovered that this strain pulse induces a coherent magnetization precession in the Bi-YIG at the frequency of the ferromagnetic resonance. The observed phenomena can be explained by strain-induced changes of magnetocrystalline anisotropy via the inverse magnetostriction effect. These findings open perspectives toward the control of the magnetization in magnetic garnets embedded in complex heterostructure devices.

We report angle-dependent spin-wave spectroscopy on aperiodic quasicrystalline magnetic lattices, i.e., Ammann, Penrose P2 and P3 lattices made of large arrays of interconnected ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanobars. Spin-wave spectra obtained in the nearly saturated state contain distinct sets of resonances with characteristic angular dependencies for applied in-plane magnetic fields. Micromagnetic simulations allow us to attribute detected resonances to mode profiles with specific mirror symmetries. Spectra in the reversal regime show systematic emergence and disappearance of spin-wave modes indicating reprogrammable magnonic characteristics.

We propose a magnetic measurement method based on combining depth sectioning and electron magnetic circular dichroism in scanning transmission electron microscopy. Electron vortex beams with large convergence angles, as those achievable in current state-of-the-art aberration correctors, could produce atomic lateral resolution and depth resolution below 2 nm.

We present inelastic light scattering data on a single crystal of magnetite (${\mathrm{Fe}}_3{\mathrm{O}}_4$) across the Verwey transition. We identify anomalies of the lattice vibrations that originate from strong coupling to electronic excitations. We reveal spectroscopic signatures of diffusive modes in the electronic contribution to the Raman response function. We thereby provide information on the critical dynamics and the hierarchy of the structural and electronic modes in the mechanism of the Verwey transition.

We discuss theoretically frustrated Heisenberg spiral magnets in magnetic field $\mathbf{H}$. We demonstrate that small anisotropic spin interactions (single-ion biaxial anisotropy or dipolar forces) select the plane in which spins rotate (spiral plane) and can lead to the spiral plane flop upon in-plane field increasing. Expressions for the critical fields $H_{flop}$ are derived. It is shown that measuring of $H_{flop}$ is an efficient and simple method of quantifying the anisotropy in the system (as the measurement of spin-flop fields in collinear magnets with axial anisotropy). Corresponding recent experiments are considered in spiral magnets some of which are multiferroics of spin origin.

In shape memory materials the fine twinned microstructure plays a fundamental role. Here we show that in the martensite phase of Ni-Mn-Ga Heusler alloy, the fine features of electronic structure are not caused by intrinsic electronic changes but mesoscopically different orientations of ferroelastic domains, i.e., twins. The cuts of Fermi surfaces of a (pseudo) tetragonally distorted $\mathrm{N}{\mathrm{i}}_{49.7}\mathrm{M}{\mathrm{n}}_{29.1}\mathrm{G}{\mathrm{a}}_{21.2}$ Heusler single crystal with a (100) surface orientation were measured using angle-resolved photoemission spectroscopy and compared with first-principles calculations. In this work we demonstrated that the measured photoelectron spectrum was a projection of three separate electronic structures originating from single a-a and two perpendicular a-c ferroelastic domains with a twinned relationship. The twinning results in pseudosplitting of the experimentally observed bands at Fermi level.

Based on a simple model of a layered superconductor with strong attractive interaction, we find that the separation of the pair-condensation temperature from the pair-formation temperature becomes more remarkable as the interlayer hopping gets smaller. We propose from this result the BCS-BEC crossover induced by the change in dimensionality, for instance, due to insertion of additional insulating layers or application of uniaxial pressure. The emergence of a pseudogap in the electronic density of states, which supports the idea of the dimensionality-induced BCS-BEC crossover, is also verified.

We investigate the ground state and low-energy spin-orbital excitations of a single iron(II) phthalocyanine molecule in isolation and on an oxidized Cu(110) surface. Considering the subspace spanned by the three lowest spin-triplet states of ${}^{3}A_{2g}$ and ${}^{3}E_g$ symmetry, we diagonalize the Hamiltonian made of the anisotropic spin-orbit interaction and the ligand field splitting $\mathrm{\Delta }$, defined as the energy difference between ${}^{3}E_g$ and ${}^{3}A_{2g}$. We find that the ground state switches from a ${}^{3}E_g$-like state with large orbital moment and out-of-plane easy axis for $\mathrm{\Delta }<-60$ meV to a ${}^{3}A_{2g}$-like singlet state with in-plane easy axis for $\mathrm{\Delta }>-60$ meV. The analysis of the first excited states in the two regimes explains the zero-field splitting data reported for $\beta$-FePc as well as for FePc molecules adsorbed on an oxidized Cu(110) surface [N. Tsukahara et al., Phys. Rev. Lett. 102, 167203 (2009)]. Importantly, the calculated magnetic susceptibility obtained with the ab initio value $\mathrm{\Delta }=93$ meV compares remarkably well with the experimental data of $\beta$-FePc in the whole available temperature range of 1–300 K.

Hyperbolic metasurfaces have gained significant attention due to their extraordinary electromagnetic properties to control propagating plane waves, but the excitation and propagation of surface plasmon-polaritons at hyperbolic metasurfaces, called hyperbolic plasmons, have been experimentally observed only recently. However, the advantages of the hyperbolic plasmons, such as hybrid TE-TM polarization discussed below, are not yet fully revealed and analyzed. In this paper, we focus on the numerical and experimental characterization of surface waves in the frequency range from 2 to 8 GHz supported by a hyperbolic metasurface composed of anisotropic metallic Jerusalem crosses printed on a thin dielectric substrate. We show different shapes of equal frequency contours, which correspond to a plethora of excitation and propagation regimes of surface waves. The principal novelty of this work consists in the experimental demonstration of surface waves with hybrid, i.e., mixed TE-TM, polarization. Surface waves with hybrid polarization are a promising tool in a number of applications and phenomena, including polarization converters, plasmonic sensors, plasmon steering over a surface, optical forces, and spin-orbit photonics, and they highlight the impact of on-chip and planar networks.

The measured electric resistance of carbon nanotubes wrapped with DNA molecules depends strongly on the spin of the injected electrons. Motivated by these experiments, we study the effect of helix-shaped potentials on the electronic spectrum of carbon nanotubes. We find that in combination with the curvature-induced spin-orbit coupling inherent to nanotubes, such a perturbation opens helicity-dependent gaps. Within these partial gaps, left-moving electrons carry a fixed spin-projection that is reversed for right-moving electrons, and the probability of electrons to transfer through the nanotube correlates with their helicity. We explain the origin of this effect and show that it can alternatively be induced by twisting the nanotube. Our findings suggest that carbon nanotubes hold great potential for implementing spin filters and may form an ideal platform to study the physical properties of one-dimensional helical liquids.

The effects of anisotropy and splitting of the phonon dispersions in the Raman spectrum of nanocrystals are investigated. We describe the different branches of the optical phonon dispersion curves along each high-symmetry direction by fitting a simple empirical model to experimental data. These curves are then used to calculate the Raman spectrum in the framework of the phonon confinement model and the results are compared with a wide range of available experimental data as well as with the spectra predicted by currently available models based on a single isotropic dispersion curve. We show that by considering the anisotropy and splitting of rigorously obtained optical phonon dispersions, the commonly observed deviations between experimental and theoretical data are strongly reduced. Our work enables the extraction of significantly more accurate information about relevant physical properties of nanocrystals from Raman spectroscopy.

We report resistivity, magnetization, and optical spectroscopy study on single-crystal sample of ${\mathrm{Sr}}_3{\left({\mathrm{Ru}}_{0.985}{\mathrm{Fe}}_{0.015}\right)}_2{\mathrm{O}}_7$. An upturn is observed in resistivity at about 30 K. Below 30 K, the dip in resistivity $R(\omega$), the suppression in scattering rate $1/\tau \left(\omega \right)$, the peaklike feature in optical conductivity ${\sigma }_1\left(\omega \right)$, and the remainder of spectral weight all suggest the formation of a pseudogap. In addition, one phonon peak at about $600{\mathrm{cm}}^{-1}$ is distinguished at all temperatures, which has asymmetric line shape. Such asymmetric line shape can be fit by a Fano function, and the resulting Fano factor $1/q^2$ and linewidth $\gamma$ show significant increases below 30 K, giving further evidence for the formation of a pseudogap, which might originate from the partial $k$-space gap opening due to density wave instability.

We study the robustness of the quantization of the Hall conductivity in the Harper-Hofstadter model towards the details of the protocol with which a longitudinal uniform driving force $F_x\left(t\right)$ is turned on. In the vector potential gauge, through Peierls substitution, this involves the switching on of complex time-dependent hopping amplitudes $e^{-\frac{i}{\hslash }{\mathcal{A}}_x\left(t\right)}$ in the $\widehat{\mathbf{x}}$ direction such that ${\partial }_t{\mathcal{A}}_x\left(t\right)=F_x\left(t\right)$. The switching on can be sudden, $F_x\left(t\right)=\theta \left(t\right)F$, where $F$ is the steady driving force, or more generally smooth $F_x\left(t\right)=f(t/t_0)F$, where $f(t/t_0)$ is such that $f\left(0\right)=0$ and $f\left(1\right)=1$. We investigate how the time-averaged (steady-state) particle current density $j_y$ in the $\widehat{\mathbf{y}}$ direction deviates from the quantized value $j_{y}h/F=n$ due to the finite value of $F$ and the details of the switching-on protocol. Exploiting the time periodicity of the Hamiltonian $\widehat{H}\left(t\right)$, we use Floquet techniques to study this problem. In this picture the (Kubo) linear response $F\to 0$ regime corresponds to the adiabatic limit for $\widehat{H}\left(t\right)$. In the case of a sudden quench $j_{y}h/F$ shows $F^2$ corrections to the perfectly quantized limit. When the switching on is smooth, the result depends on the switch-on time $t_0$: For a fixed $t_0$ we observe a crossover force $F^{*}$ between a quadratic regime for $F<F^{*}$ and a nonanalytic exponential $e^{-\gamma /\left|F\right|}$ for $F>F^{*}$. The crossover $F^{*}$ decreases as $t_0$ increases, eventually recovering the topological robustness. These effects are in principle amenable to experimental tests in optical lattice cold atomic systems with synthetic gauge fields.

Fermi liquid theory works very well in most normal metals, but is found violated in many strongly correlated electron systems, such as cuprate and heavy-fermion superconductors. A widely accepted criterion is that the Fermi liquid theory is valid when the interaction-induced fermion damping rate approaches zero more rapidly than the energy. Otherwise, it is invalid. Here, we demonstrate that this criterion breaks down in topological double- and triple-Weyl semimetals. Renormalization group analysis reveals that, although the damping rate of double- and triple-Weyl fermions induced by the Coulomb interaction approaches zero more rapidly than the energy, the quasiparticle residue vanishes and the Fermi liquid theory is invalid. This behavior indicates a weaker-than-marginal violation of the Fermi liquid theory. Such an unconventional non-Fermi liquid state originates from the special dispersion of double- and triple-Weyl fermions, and is qualitatively different from all the other Fermi liquid and non-Fermi liquid states. The predicted properties of the fermion damping rate and the spectral function can be probed by the angle-resolved photoemission spectroscopy. The density of states, specific heat, and conductivities are also calculated and analyzed after incorporating the corrections induced by the Coulomb interaction.

Large Rashba-like spin splitting has been recently found in the band structure of certain bulk ferroelectrics. In contrast to the relativistic Rashba effect, the chiral spin texture and large spin splitting of the electronic bands depend strongly on the character of the band and atomic spin-orbit coupling. We establish that this can be traced back to an interplay between the orbital angular momentum and ferroelectricity. The additional dependence on the orbital composition of the bands is crucial to the complete picture of the effect. Results from first-principles calculations on ferroelectic GeTe verify the key predictions of the model.

Gyroscopic metamaterials, mechanical structures composed of interacting spinning tops, have recently been found to support one-way topological edge waves. In these structures, the time-reversal symmetry breaking that enables their topological behavior emerges directly from the lattice geometry. Here we show that variations in the lattice geometry can give rise to more complex band topology than has been previously described. A “spindle” lattice (or truncated hexagonal tiling) of gyroscopes possesses both clockwise and counterclockwise edge modes distributed across several band gaps. Tuning the interaction strength or twisting the lattice structure along a Guest mode opens and closes these gaps and yields bands with Chern numbers of $\left|C\right|>1$ without introducing next-nearest-neighbor interactions or staggered potentials. A deformable honeycomb structure provides a simple model for understanding the role of lattice geometry in constraining the effects of time-reversal symmetry and inversion symmetry breaking. Last, we find that topological band structure generically arises in gyroscopic networks, and a simple protocol generates lattices with topological excitations.

The effects of nonmagnetic impurity doping on magnetic and ferroelectric properties of multiferroic delafossite ${\mathrm{CuCrO}}_2$ are investigated by means of density functional theory calculations and Monte Carlo simulations. Density functional theory calculations show that replacing up to $30\%$ of ${\mathrm{Cr}}^{3+}$ ions by ${\mathrm{Ga}}^{3+}$ ones does not significantly affect the remaining Cr-Cr superexchange interactions. Monte Carlo simulations show that ${\mathrm{CuCr}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_2$ preserves its magnetoelectric properties up to $x\simeq 0.15$ with a spiral ordering, while it becomes disordered at higher fractions. Antiferromagnetic transition shifts towards lower temperatures with increasing $x$ and eventually disappears at $x\ge 0.2$. Our simulations show that ${\mathrm{Ga}}^{3+}$ doping increases the Curie-Weiss temperature of ${\mathrm{CuCr}}_{1-x}{\mathrm{Ga}}_x{\mathrm{O}}_2$, which agrees well with experimental observations. Moreover, our results show that the incommensurate ground-state configuration is destabilized by ${\mathrm{Ga}}^{3+}$ doping under zero applied field associated with an increase of frustration. Finally, coupling between noncollinear magnetic ordering and electric field is reported for $x\le 0.15$ through simulating $P\text{−}E$ hysteresis loops, which leads to ferroelectricity in the extended inverse Dzyaloshinskii-Moriya model.

We report the thermodynamic and muon spin relaxation $\left(\mu \mathrm{SR}\right)$ evidences for a possible gapless spin liquid in ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$, with the rare-earth ions ${\mathrm{Tm}}^{3+}$ forming a two-dimensional kagomé lattice. We extract the magnetic specific heat of ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ by subtracting the phonon contribution of the nonmagnetic isostructural material ${\mathrm{La}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ and obtain a clear linear-$T$ temperature dependence of magnetic specific heat at low temperatures. No long-range magnetic order was observed down to 0.35 K in the heat capacity measurements, and no signature of spin freezing down to 50 mK was observed in A.C. susceptibility measurements. The absence of magnetic order is further confirmed by the $\mu \mathrm{SR}$ measurements down to 20 mK. We find that the spin-lattice relaxation time remains constant down to the lowest temperature. We point out that the physics in ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ is fundamentally different from the Cu-based herbertsmithite and propose spin liquid ground states with non-Kramers doublets on the kagomé lattice to account for the experimental results. However, we cannot rule out that these exotic properties are induced by the Tm/Zn site-mixing disorder in ${\mathrm{Tm}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$.

We present the electrical resistivity data under application of pressures up to $\sim 26$ GPa and down to 50 mK on ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$. We find a pressure induced magnetic phase transition with an onset at $p_c=18.2\pm 0.8$ GPa. At ambient pressure, ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ manifests a heavy fermion, nonmagnetic ground state and the Fermi liquid behavior at low temperatures. As pressure is increased, the power law exponent in resistivity, $n$, deviates significantly from Fermi liquid behavior and tends to saturate with $n=1$ near $p_c$. A pronounced resistivity maximum $T_{\max }$, which scales with the Kondo temperature, is observed. $T_{\max }$ decreases with increasing pressure and flattened out near $p_c$ indicating the suppression of Kondo exchange interaction. For $p>p_c,T_{\max }$ shows a sudden upward shift, most likely becoming associated with crystal electric field scattering. Application of magnetic field for $p>p_c$ broadens the transition and shifts it toward the higher temperature, which is a typical behavior of a transition towards a ferromagnetic state, or a state with a significant ferromagnetic component. The magnetic transition appears to abruptly develop above $p_c$, suggesting probable first-order (with changing pressure) nature of the transition; once stabilized, the ordering temperature does not depend on pressure up to $\sim 26$ GPa. Taken as a whole, these data suggest that ${\mathrm{YbFe}}_2{\mathrm{Zn}}_{20}$ has a quantum phase transition at $p_c=18.2$ GPa associated with the avoided quantum criticality in metallic ferromagnets.

We report the crystal structure of ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{CoTe}}_2{\mathrm{O}}_{12}$ determined by Rietveld analysis using x-ray powder-diffraction data. It was found from magnetic measurements that ${\mathrm{Ba}}_2{\mathrm{La}}_2{\mathrm{CoTe}}_2{\mathrm{O}}_{12}$ can be described as a spin-$\frac{1}{2}$ triangular-lattice antiferromagnet with easy-plane anisotropy at low temperatures. This compound undergoes a magnetic phase transition at $T_{\mathrm{N}}=3.26\mathrm{K}$ to an ordered state with the ${120}^{\circ }$ structure. The magnetization curve exhibits the one-third plateau characteristic of triangular-lattice quantum antiferromagnets. The antiferromagnetic exchange interaction and the $g$ factors parallel and perpendicular to the $c$ axis were evaluated to be $J/k_{\mathrm{B}}=22\mathrm{K},g_{\parallel }=3.5$, and $g_{\perp }=4.5$, respectively.