Ultrafast optical excitation of a metal ferromagnetic film results in a modification of the magnetocrystalline anisotropy and induces the magnetization precession. We consider two main contributions to these processes: an effect of noncoherent phonons, which modifies the temperature dependent parameters of the magnetocrystalline anisotropy and coherent phonons in the form of a strain contributing via inverse magnetostriction. Contrary to earlier experiments with high-symmetry ferromagnetic structures, where these mechanisms could not be separated, we study the magnetization response to femtosecond optical pulses in the low-symmetry magnetostrictive galfenol film so that it is possible to separate the coherent and noncoherent phonon contributions. By choosing certain experimental geometry and external magnetic fields, we can distinguish the contribution from a specific mechanism. Theoretical analysis and numerical calculations are used to support the experimental observations and proposed model.

We present a systematical study on the roles of interactions among electron correlation, electron-phonon coupling, and spin-orbit coupling in the isovalent Pd-substituted superconductor ${\mathrm{SrPt}}_3\mathrm{P}$. By using the solid state reaction method, the Pd element with the $4d$ orbital was successfully substituted in the strong spin-orbit coupling superconductors $\mathrm{Sr}{\left({\mathrm{Pt}}_{1-x}{\mathrm{Pd}}_x\right)}_3\mathrm{P}$. As increasing the isovalent Pd concentrations without introducing any extra electron/hole carriers, the superconducting transition temperature $T_c$ decreases monotonously. In addition, combining the data of resistivity and specific heat, as well as electronic band structure calculations, we found that the electron correlation is enhanced while the electron-phonon coupling and the spin-orbit coupling are suppressed by Pd substitution. Our results may provide significant insights in the natures of the interplay among the electron correlation, electron-phonon coupling, and spin-orbit coupling in superconductivity, and may also pave a route for understanding the mechanism of superconductivity in heavily $5d$-based superconductors.

We propose a realization of the electric-field-induced antiferromagnetic resonance. We consider three-dimensional antiferromagnetic insulators with spin-orbit coupling characterized by the existence of a topological term called the $\theta$ term. By solving the Landau-Lifshitz-Gilbert equation in the presence of the $\theta$ term, we show that, in contrast to conventional methods using ac magnetic fields, the antiferromagnetic resonance state is realized by ac electric fields along with static magnetic fields. This mechanism can be understood as the inverse process of the dynamical chiral magnetic effect, an alternating current generation by magnetic fields. In other words, we propose a way to electrically induce the dynamical axion field in condensed matter. We discuss a possible experiment to observe our proposal, which utilizes the spin pumping from the antiferromagnetic insulator into a heavy metal contact.

We study the disordered Heisenberg spin chain, which exhibits many-body localization at strong disorder, in the weak to moderate disorder regime. A continued fraction calculation of dynamical correlations is devised, using a variational extrapolation of recurrents. Good convergence for the infinite chain limit is shown. We find that the local spin correlations decay at long times as $C\sim t^{-\beta }$, whereas the conductivity exhibits a low-frequency power law $\sigma \sim {\omega }^{\alpha }$. The exponents depict subdiffusive behavior $\beta <1/2,\alpha >0$ at all finite disorders and convergence to the scaling result $\alpha +2\beta =1$ at large disorders.

The behavior of the nonlinear susceptibility ${\chi }_3$ and its relation to the spin-glass transition temperature $T_f$ in the presence of random fields are investigated. To accomplish this task, the Sherrington-Kirkpatrick model is studied through the replica formalism, within a one-step replica-symmetry-breaking procedure. In addition, the dependence of the Almeida-Thouless eigenvalue ${\lambda }_{\mathrm{AT}}$ (replicon) on the random fields is analyzed. Particularly, in the absence of random fields, the temperature $T_f$ can be traced by a divergence in the spin-glass susceptibility ${\chi }_{\mathrm{SG}}$, which presents a term inversely proportional to the replicon ${\lambda }_{\mathrm{AT}}$. As a result of a relation between ${\chi }_{\mathrm{SG}}$ and ${\chi }_3$, the latter also presents a divergence at $T_f$, which comes as a direct consequence of ${\lambda }_{\mathrm{AT}}=0$ at $T_f$. However, our results show that, in the presence of random fields, ${\chi }_3$ presents a rounded maximum at a temperature $T^{*}$ which does not coincide with the spin-glass transition temperature $T_f$ (i.e., $T^{*}>T_f$ for a given applied random field). Thus, the maximum value of ${\chi }_3$ at $T^{*}$ reflects the effects of the random fields in the paramagnetic phase instead of the nontrivial ergodicity breaking associated with the spin-glass phase transition. It is also shown that ${\chi }_3$ still maintains a dependence on the replicon ${\lambda }_{\mathrm{AT}}$, although in a more complicated way as compared with the case without random fields. These results are discussed in view of recent observations in the ${\mathrm{LiHo}}_x{\mathrm{Y}}_{1-x}{\mathrm{F}}_4$ compound.

A two-band model with repulsive interband coupling and interband potential scattering is considered to elucidate their effects on material properties. In agreement with previous work, we find that the bands' order parameters ${\mathrm{\Delta }}_{1,2}$ differ, and the larger is at the band with a smaller normal density of states (DOS), $N_{n2}<N_{n1}$. However, the bands' energy gaps, as determined by the energy dependence of the DOS, are equal due to scattering. For each temperature, the gaps become zero at a certain critical interband scattering rate; i.e., for strong enough scattering the model material becomes gapless. In the gapless state, the DOS at band 2 is close to the normal state value, whereas at band 1 it has a V shape with nonzero minimum. When the normal bands DOS are mismatched, $N_{n1}\ne N_{n2}$, the critical temperature $T_c$ is suppressed even in the absence of interband scattering, $T_c\left(N_{n1}\right)$ has a domelike shape. With increasing interband scattering, the London penetration depth at low temperatures evolves from being exponentially flat to a power law and even to near linear behavior in the gapless state, the latter being easily misinterpreted as caused by order parameter nodes.

We provide a theory of triplon dynamics in the valence bond solid ground state of the coupled spin ladders modeled for ${\mathrm{BiCu}}_2{\mathrm{PO}}_6$. Utilizing the recent high-quality neutron scattering data [K. W. Plumb et al., Nat. Phys. 12, 224 (2016)] as guides and a theory of interacting triplons via the bond operator formulation, we determine a minimal spin Hamiltonian for this system. It is shown that the splitting of the low-energy triplon modes and the peculiar magnetic field dependence of the triplon dispersions can be explained by including substantial Dzyaloshinskii-Moriya and symmetric anisotropic spin interactions. Taking into account the interactions between triplons and the decay of the triplons to the two-triplon continuum via anisotropic spin interactions, we provide a theoretical picture that can be used to understand the main features of the recent neutron scattering experimental data.

We study how the stability of the fractional quantum Hall effect is influenced by the geometry of band structure in lattice Chern insulators. We consider the Hofstadter model, which converges to continuum Landau levels in the limit of small flux per plaquette. This gives us a degree of analytic control not possible in generic lattice models, and we are able to obtain analytic expressions for the relevant geometric criteria. These may be differentiated by whether they converge exponentially or polynomially to the continuum limit. We demonstrate that the latter criteria play a dominant role in predicting the physics of interacting particles in Hofstadter bands in this low flux density regime. In particular, we show that the many-body gap depends monotonically on a band-geometric criterion related to the trace of the Fubini-Study metric.

We compute the terahertz third-order nonlinear conductance of metallic armchair graphene nanoribbons using time-dependent perturbation theory. Significant enhancement of the intrinsic third-order conductance over the result for instrinsic 2D single-layer graphene is observed over a wide range of temperatures. We also investigate the nonlinear response of extrinsic metallic acGNR with $|E_F|\ll 200\mathrm{meV}$. We find that the third-order conductance exhibits a strong Fermi level dependence at low temperatures. A third-order critical field strength of between $\sim 1$ and $5\mathrm{kV}/\mathrm{m}$ is computed for the Kerr conductance as a function of temperature. For the third-harmonic conductance, the minimum critical field is computed to be $\sim 5\mathrm{kV}/\mathrm{m}$.

We investigate the influence of the photon statistics on the excitation dynamics of a single two-level system. A single semiconductor quantum dot represents the two-level system and is resonantly excited either with coherent laser light, or excited with chaotic light, with photon statistics corresponding to that of thermal radiation. Experimentally, we observe a reduced absorption cross section under chaotic excitation in the steady state. In the transient regime, the Rabi oscillations observable under coherent excitation disappear under chaotic excitation. Likewise, in the emission spectrum, the well-known Mollow triplet, which we observe under coherent drive, disappears under chaotic excitation. Our observations are fully consistent with theoretical predictions based on the semiclassical Bloch equation approach.

The results are presented for systematic heat capacity measurements of π-$d$ interacting systems of $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{Fe}{\mathrm{Br}}_4$ and $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{FeC}{\mathrm{l}}_4$ [BETS = bis(ethylenedithio)tetraselenafulvalene] performed under in-plane magnetic fields. We observed sharp thermal anomalies at 2.47 K for $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{FeB}{\mathrm{r}}_4$ and at 0.47 K for $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{FeC}{\mathrm{l}}_4$ at 0 T that are associated with antiferromagnetic transitions of the $3d$ electrons in the anion layers. From analyses of the magnetic heat capacity data, we indicate that the two compounds show unconventional thermodynamic behaviors inherent in the π-d interacting layered system. In the case of $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{FeB}{\mathrm{r}}_4$, a small hump structure was observed in the magnetic heat capacity below the transition temperature when a magnetic field was applied parallel to the $a$ axis. In the case of $\kappa -{\left(\mathrm{BETS}\right)}_2\mathrm{FeC}{\mathrm{l}}_4$, a similar hump structure was observed at 0 T that remained in the data with magnetic fields applied parallel to the $a$ axis. We demonstrate that the temperature dependencies of the magnetic heat capacities scale well by normalizing the temperatures with dominant one-dimensional direct interactions $(J_{\mathrm{dd}}/k_{\mathrm{B}})$ of each compound. The field dependencies of the transition temperatures and the hump structures are elucidated in one simple magnetic field vs temperature ($H-T$) phase diagram. These results indicate that the thermodynamic features of both κ-type BETS salts are essentially equivalent, and the observed hump structures are derived from the one-dimensional $J_{\mathrm{dd}}$ interaction characters, which are still influential for magnetic features even in the long-range magnetic ordered states.

The evolution of the local structure of ${\mathrm{VO}}_2$ was investigated across the pressure-induced insulator to metal transition (IMT) by means of pair distribution function measurements. The pressure behavior of the V-V and V-O bond lengths have been determined. The data demonstrated that the pressure-driven IMT is not activated by the suppression of the Peierls-type distortion. A clear octahedra symmetrization is observed in the metallic phase, suggesting a link between structural degree of freedom and the metallization process.

The excitation of cavity standing waves in double-slit structures in thin gold films, with slit lengths between 400 and 2560 nm, was probed with a strongly focused electron beam in a transmission electron microscope. The energies and wavelengths of cavity modes up to the $11\mathrm{th}$ mode order were measured with electron energy loss spectroscopy to derive the corresponding dispersion relation. For all orders, a significant redshift of mode energies accompanied by a wavelength elongation relative to the expected resonator energies and wavelengths is observed. The resultant dispersion relation is found to closely follow the well-known dispersion law of surface-plasmon polaritons (SPPs) propagating on a gold/air interface, thus providing direct evidence for the hybridized nature of the detected cavity modes with SPPs.

We present first-principles calculations of the vibrational properties of the transition-metal dichalcogenide $1T\text{−}{\mathrm{TaS}}_2$ for various thicknesses in the high-temperature (undistorted) phase and the low-temperature commensurate charge density wave (CDW) phase. We also present measurements of the Raman spectra for bulk, few-layer, and monolayer samples at temperatures well below that of the bulk transition to the commensurate phase. Through our calculations, we identify the low-frequency folded-back acoustic modes as a convenient signature of the commensurate CDW structure in vibrational spectra. In our measured Raman spectra, this signature is clearly evident in all of the samples, indicating that the commensurate phase remains the ground state as the material is thinned, even down to a single layer. This is in contrast to some previous studies, which suggest a suppression of the commensurate CDW transition in thin flakes. We also use polarized Raman spectroscopy to probe $c$-axis orbital texture in the low-$T$ phase, which has recently been suggested to play a role in the metal-insulator transition that accompanies the structural transition to the commensurate CDW phase.

The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron-scattering measurements for the multichiral ${\mathrm{Ba}}_{3}M{\mathrm{Fe}}_3{\mathrm{Si}}_2{\mathrm{O}}_{14} \left(M=\mathrm{Nb},\mathrm{Ta}\right)$ langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the $c$ axis and the in-plane distortion of the ${120}^{\circ }$ Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian and provide the link to the magnetically induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground.

We report the study of the skyrmion state near the surface of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ using soft resonant elastic x-ray scattering (REXS) at the Cu $L_3$ edge. Within the lateral sampling area of $200\times 200\mu {\mathrm{m}}^2$, we found a long-range-ordered skyrmion lattice phase as well as the formation of skyrmion domains via the multiple splitting of the diffraction spots. In a recent REXS study of the skyrmion phase of ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ [M. C. Langner, S. Roy, S. Mishra, J. Lee, X. Shi, M. Hossain, Y.-D. Chuang, S. Seki, Y. Tokura, S. Kevan, and R. Schoenlein, Phys. Rev. Lett. 112, 167202 (2014)], the authors reported the observation of the unexpected existence of two distinct skyrmion sublattices that arise from inequivalent Cu sites, and that the rotation and superposition of the two periodic structures lead to a moiré pattern. However, we find no energy splitting of the Cu peak in x-ray-absorption measurements and, instead, discuss alternative origins of the peak splitting. In particular, we find that for magnetic field directions deviating from the major cubic axes a multidomain skyrmion lattice state is obtained, which consistently explains the splitting of the magnetic spots into two—and more—peaks.

We show how a superconducting region (S), sandwiched between two normal leads (N), in the presence of barriers, can act as a lens for propagating electron and hole waves by virtue of the so-called crossed Andreev reflection (CAR). The CAR process, which is equivalent to Cooper pair splitting into two N electrodes, provides a unique possibility of constructing entangled electrons in solid state systems. When electrons are locally injected from an N lead, due to the CAR and normal reflection of quasiparticles by the insulating barriers at the interfaces, sequences of electron and hole focuses are established inside another N electrode. This behavior originates from the change of momentum during electron-hole conversion beside the successive normal reflections of electrons and holes due to the barriers. The focusing phenomena studied here are fundamentally different from the electron focusing in other systems, such as graphene p-n junctions. In particular, due to the electron-hole symmetry of the superconducting state, the focusing of electrons and holes is robust against thermal excitations. Furthermore, the effects of the superconducting layer width, the injection point position, and barrier strength are investigated on the focusing behavior of the junction. Very intriguingly, it is shown that by varying the barrier strength, one can separately control the density of electrons or holes at the focuses.

Magnetic properties of dirty multiband superconductors near the upper critical field are studied. The parameter ${\kappa }_2$ characterizing magnetization slope is shown to have a significant temperature variation which is quite sensitive to the pairing interactions and relative strengths of intraband impurity scattering. In contrast to single-band superconductors the increase of ${\kappa }_2$ at low temperatures can be arbitrarily large determined by the ratio of maximal and minimal diffusion coefficients in different bands. Temperature dependencies of ${\kappa }_2\left(T\right)$ in two-band ${\mathrm{MgB}}_2$ and iron-based superconductors are shown to be much more sensitive to the multiband effects than the upper critical field $H_{c2}\left(T\right)$.

The stability of the magnetization auto-oscillations of the ferromagnetic free layer of a cylindrical nanopillar structure is studied theoretically using a classical Hamiltonian formalism for weakly interacting nonlinear waves, in a weakly dissipative system. The free layer corresponds to a very thin circular disk, made of a soft ferromagnetic material like Permalloy, and it is magnetized in plane by an externally applied magnetic field. There is a dc electric current that traverses the structure, becomes spin polarized by a fixed layer, and excites the modes of the free layer through the transfer of spin angular momentum. If this current exceeds a critical value, it is possible to generate a large amplitude periodic auto-oscillation of a dynamic mode of the magnetization. We separate our theoretical study into two parts. First, we consider an approximate expression for the demagnetizing field in the disk, i.e., ${\vec{H}}_D=-4\pi M_z\widehat{z}$ or a very thin film approximation, and secondly we consider the effect of the full demagnetizing field, where one sees important effects due to the edges of the disk. In both cases, as the applied current density is increased, we determine the modes that will first auto-oscillate and when these become unstable to the growth of other modes, i.e., their ranges of “isolated” auto-oscillation.