Ferromagnetic systems under the influence of spin-polarized currents exhibit rich spatiotemporal dynamics at nanoscales. We study spin-transfer nano-oscillators driven by the combination of alternating and direct spin-polarized electric currents. We show here that the alternating current induces parametric instabilities on spin valves, that is, the magnetization responses at half the forcing frequency. A spatial self-organization emerges as a result of the oscillatory current, which includes dissipative solitons and Faraday-type waves. The parametric regime is described analytically by means of the Landau-Lifshitz-Gilbert-Slonczewski equation, in good agreement with micromagnetic simulations including the full dipolar field.

We show that the Fulde-Ferrell (FF) phase may appear as a sole result of the orbital effect in a cylindrical metallic nanowire. Namely, in the external magnetic field the twofold degeneracy with respect to the orbital magnetic quantum number $m$ is lifted, which leads to a Fermi wave vector mismatch between the subbands with opposite orbital momenta in the paired state. This mismatch can be compensated by the nonzero total momentum of the Cooper pairs created by electrons from the split subbands which results in the formation of the FF phase. In this manner, a transformation of the orbital motion into a linear supercurrent parallel to the applied field is taking place. With the increasing magnetic field a series of FF stability regions appear, in between which the standard BCS superconducting phase is stable. For the sake of completeness, we show, that the inclusion of the Zeeman term in the model does not change the picture qualitatively, particularly if larger $m$ states contribute essentially to the Fermi-surface splitting. A brief but important note concerning the possibility of steering the supercurrent by an applied magnetic field parallel to it, is also provided.

The electronic properties of one-dimensional graphene superlattices strongly depend on the atomic size and orientation of the 1D external periodic potential. Using a tight-binding approach, we show that the armchair and zigzag directions in these superlattices have a different impact on the renormalization of the anisotropic velocity of the charge carriers. For symmetric potential barriers, the velocity perpendicular to the barrier is modified for the armchair direction while remaining unchanged in the zigzag case. For asymmetric barriers, the initial symmetry between the forward and backward momentum with respect to the Dirac cone symmetry is broken for the velocity perpendicular (armchair case) or parallel (zigzag case) to the barriers. At last, Dirac cone multiplication at the charge neutrality point occurs only for the zigzag geometry. In contrast, band gaps appear in the electronic structure of the graphene superlattice with barrier in the armchair direction.

Asymmetric Fano resonance arises from quantum interference between discrete and continuum states. The characteristic asymmetry has attracted strong interests in understanding light-induced optoelectronic responses and corresponding applications. In conventional solids, however, the tunability of Fano resonance is generally limited by a material's intrinsic property. Topological insulators are unique states of matter embodying both conducting Dirac surface and underlying bulk. If it is possible to manipulate the two coexisting states, then it should form an ideal laboratory for realizing a tunable topological Fano system. Here, with the recently discovered topological phase transition in ${\left(\mathrm{B}{\mathrm{i}}_{1-x}\mathrm{I}{\mathrm{n}}_x\right)}_2\mathrm{S}{\mathrm{e}}_3$, we report tunable Fano interference phenomena. By engineering the spatial overlap between surface Dirac electrons (continuous terahertz transitions) and bulk phonon (discrete mode at ∼2 terahertz), we continuously tune, abruptly switch, and dynamically modulate the Fano resonance. Eliminating the topological surface via decreasing spin-orbit coupling―that is, across topological and nontopological phases, we find that the asymmetric Fano spectra return to the symmetric profile. Laser-excited ultrafast terahertz spectroscopy reveals that the controlled spatial overlap is responsible for the picosecond tunability of the Fano resonance, suggesting potentials toward optically controllable topological Fano systems.

We experimentally demonstrate a self-organized metasurface with a polarization dependent transmittance that can be dynamically controlled by optical means. The configuration consists of tightly packed plasmonic nanowires with a large dispersion of width and height produced by the defocused ion-beam sputtering of a thin gold film supported on a silica glass. Our results are quantitatively interpreted according to a theoretical model based on the thermomodulational nonlinearity of gold and a finite-element numerical analysis of the absorption and scattering cross-sections of the nanowires. We found that the polarization sensitivity of the metasurface can be strongly enhanced by pumping with ultrashort laser pulses, leading to potential applications in ultrafast all-optical modulation and switching of light.

A $\beta$-NMR study was performed on a ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ crystal in the metallic state using a beam of spin-polarized ${}^8\mathrm{Li}^{+}$ implanted at a mean depth of 90 nm. The ${}^8\mathrm{Li}^{+}$ spin-lattice relaxation rate is strongly influenced by the onset of incommensurate spin fluctuations. The nuclear relaxation rate can be explained using previously published bulk ${}^{17}\mathrm{O}$ NMR and inelastic neutron spectroscopy measurements of the dynamic magnetic susceptibility to model the hyperfine coupling. A well-resolved quadrupolar-split NMR for ${}^8\mathrm{Li}^{+}$ implies a static stopping position in an interstitial site. The ${}^8\mathrm{Li}^{+}$ Knight shift is highly sensitive to the anisotropic static susceptibility.

A long spin relaxation time $\left({\tau }_{\mathrm{sf}}\right)$ is the key for the applications of graphene to spintronics but the experimental values of ${\tau }_{\mathrm{sf}}$ have been generally much shorter than expected. We show that the usual determination by the Hanle method underestimates ${\tau }_{\mathrm{sf}}$ if proper account of the spin absorption by contacts is lacking. By revisiting series of experimental results and taking into account the spin absorption, we find that the corrected ${\tau }_{\mathrm{sf}}$ are longer and, for series of graphene samples of the same fabrication, less dispersed, which leads to a more unified picture of the ${\tau }_{\mathrm{sf}}$.

We use homotopy theory to extend the notion of strong and weak topological insulators to the nonstable regime (low numbers of occupied/empty energy bands). We show that for strong topological insulators in $d$ spatial dimensions to be “truly $d$-dimensional,” i.e., not realizable by stacking lower-dimensional insulators, a more restrictive definition of “strong” is required outside the stable regime. However, this does not exclude weak topological insulators from being “truly $d$-dimensional,” which we demonstrate by an example. Additionally, we prove some useful technical results, including the homotopy theoretic derivation of the factorization of invariants over the torus into invariants over spheres in the stable regime, as well as the rigorous justification of the parameter space replacements $T^d\to S^d$ and $T^{d_k}\times S^{d_x}\to S^{d_k+d_x}$ used widely in the current literature.

Due to underlying symmetries, the ground states of magnetic adatoms may be highly stable, which opens perspectives for application as single-atom memory. A specific example is a single holmium atom (with $J=8$) on a platinum (111) surface for which exceptionally long lifetimes were observed in recent scanning tunneling microscopy studies. For control and read-out, the atom must be coupled to electronic contacts. Hence the spin dynamics of the system is governed by a quantum master equation. Our analysis shows that, in general, it cannot be reduced to a classical master equation in the basis of the unperturbed crystal-field Hamiltonian. Rather, depending on parameters and control fields, “environment-induced superselection” principles choose the appropriate set of basis states, which in turn determines the specific relaxation channels and lifetimes. Our simulations suggest that in ideal situations the lifetimes should be even longer than observed in the experiment. We, therefore, investigate the influence of various perturbations. We also study the initialization process of the state of the Ho atom by applied voltage pulses and conclude that fast, high fidelity preparation, on a $100\text{-ns}$ time scale, should be possible.

The Raman spectrum of ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$, a tetragonal insulator which develops Dzyaloshinskii-Moriya helical magnetism below $T_N=3.2\mathrm{K}$, has been detected at temperatures varying from 300 to 80 K in a single crystal, with the radiation polarized either in the $ab$ plane or along the $c$ axis of its tetragonal cell. Twenty-nine phonon lines out of the 35 allowed by the Raman selection rules for the present geometry were observed, and their vibrational frequencies were found in overall good agreement with those provided by shell-model calculations. Together with the previous report on the infrared-active phonons [A. Nucara et al., Phys. Rev. B 90, 014304 (2014)], the present study provides an exhaustive description, both experimental and theoretical, of the lattice dynamics in ${\mathrm{Ba}}_2{\mathrm{CuGe}}_2{\mathrm{O}}_7$.

Thermal irreversibility of the magnetization in ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$ epitaxial thin films on Ge(111) is reported. The frequency dependence of the ac susceptibility demonstrates a spin-glass-like behavior, despite the fact that the thin film is a single crystal. Such glassy behavior is attributed to the presence of a “ferromagnetically dead” layer with spin-glass-like properties at the ${\mathrm{Mn}}_5{\mathrm{Ge}}_3/\mathrm{Ge}$ interface, which results in frustrated interactions with the ferromagnetic ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$. Moreover, it is shown that the magnetic phase diagram in the $H\text{−}T$ plane of the glassy state is very sensitive to the growth conditions. Variation in the growth temperature and film thickness changes the spin-glass-like properties and the Curie temperature. The sensitivity of the glassy state to the growth conditions is related to the variation in the properties of the interfacial layer.

We have synthesized single crystals of ternary intermetallic ${\mathrm{Yb}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ with a distorted kagome lattice structure, and investigated the low-temperature resistivity, specific heat, magnetization, and magnetic phase transitions. ${\mathrm{Yb}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ is the first $4f$ system that has a ${\mathrm{Gd}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$-type crystal structure where antiferromagnetic interaction acts on the spin. The crystal electric field (CEF) ground state of this compound is determined as a well isolated twofold degenerate state that is subjected to a strong easy-plane-type magnetic anisotropy. In the present study, the spin system of ${\mathrm{Yb}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ is regarded as an AFM $XY$ model of $S=1/2$. This compound undergoes successive magnetic phase transitions at 1.5 and 1.6 K, and the resistivity exhibits $T^2$ behavior below 1 K. The ratio of the coefficient of the $T^2$ term in the resistivity $A$, and that of the electronic specific heat coefficient ${\gamma }_0$, deviates from the Kadowaki-Woods (KW) law. The successive phase transitions and low-temperature properties of ${\mathrm{Yb}}_3{\mathrm{Ru}}_4{\mathrm{Al}}_{12}$ where geometrical frustration and heavy fermion behavior occur are discussed.

The origin of the nematic state is an important puzzle to be solved in iron pnictides. Iron superconductors are multiorbital systems and these orbitals play an important role at low energy. The singular $C_4$ symmetry of $d_{zx}$ and $d_{yz}$ orbitals has a profound influence at the Fermi surface since the $\mathrm{\Gamma }$ pocket has vortex structure in the orbital space and the $X/Y$ electron pockets have $yz/zx$ components, respectively. We propose a low-energy theory for the spin-nematic model derived from a multiorbital Hamiltonian. In the standard spin-nematic scenario, the ellipticity of the electron pockets is a necessary condition for nematicity. In the present model, nematicity is essentially due to the singular $C_4$ symmetry of $yz$ and $zx$ orbitals. By analyzing the ($\pi ,0$) spin susceptibility in the nematic phase, we find a spontaneous generation of orbital splitting, extending previous calculations in the magnetic phase. We also find that the ($\pi ,0$) spin susceptibility has an intrinsic anisotropic momentum dependence due to the nontrivial topology of the $\mathrm{\Gamma }$ pocket.

We have conducted extensive investigations on the magnetization and its dynamical relaxation on a ${\mathrm{Ba}}_{0.66}{\mathrm{K}}_{0.32}{\mathrm{BiO}}_{3+\delta }$ single crystal. It is found that the magnetization-relaxation rate is rather weak compared to that in cuprate superconductors, indicating a higher collective vortex pinning potential (or activation energy), although the intrinsic pinning potential $U_{\mathrm{c}}$ is weaker. A detailed analysis leads to the following discoveries: (1) A second-peak effect on the magnetization-hysteresis loop was observed in a very wide temperature region, ranging from 2 to 24 K. Its general behavior is similar to that in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$. (2) Associated with the second-peak effect, the magnetization-relaxation rate is inversely related to the transient superconducting current density $J_{\mathrm{s}}$, revealing a quite general and similar mechanism for the second-peak effect in many high-temperature superconductors. (3) A detailed analysis based on the collective creep model reveals a large glassy exponent $\mu$ and a small intrinsic pinning potential $U_{\mathrm{c}}$. (4) Investigation on the volume pinning force density shows that the data can be scaled to the formula $F_p\propto b^p{(1-b)}^q$, with $p=2.79$ and $q=3.14$, where $b$ is the reduced magnetic field to the irreversible magnetic field. The maximum normalized pinning force density appears near $b\approx 0.47$. Finally, a vortex phase diagram is drawn to show the phase transitions or crossovers between different vortex phases.

Renormalization-group methods provide a viable approach for investigating the emergent collective behavior of classical and quantum statistical systems in both equilibrium and nonequilibrium conditions. Within this approach we investigate here the dynamics of an isolated quantum system represented by a scalar ${\varphi }^4$ theory after a global quench of the potential close to a dynamical critical point. We demonstrate that, within a prethermal regime, the time dependence of the relevant correlations is characterized by a short-time universal exponent, which we calculate at the lowest order in a dimensional expansion.

The intermetallic compound ZnSb is an interesting thermoelectric material largely due to its low lattice thermal conductivity. The origin of the low thermal conductivity has so far been speculative. Using multitemperature single crystal x-ray diffraction (9–400 K) and powder x-ray diffraction (300–725 K) measurements, we characterized the volume expansion and the evolution of structural properties with temperature and identified an increasingly anharmonic behavior of the Zn atoms. From a combination of Raman spectroscopy and first principles calculations of phonons, we consolidate the presence of low-energy optic modes with wave numbers below $60{\mathrm{cm}}^{-1}$. Heat capacity measurements between 2 and 400 K can be well described by a Debye-Einstein model containing one Debye and two Einstein contributions with temperatures ${\mathrm{\Theta }}_{\mathrm{D}}=195\mathrm{K}, {\mathrm{\Theta }}_{\mathrm{E}1}=78\mathrm{K}$, and ${\mathrm{\Theta }}_{\mathrm{E}2}=277\mathrm{K}$ as well as a significant contribution due to anharmonicity above 150 K. The presence of a multitude of weakly dispersed low-energy optical modes (which couple with the acoustic, heat carrying phonons) combined with anharmonic thermal behavior provides an effective mechanism for low lattice thermal conductivity. The peculiar vibrational properties of ZnSb are attributed to its chemical bonding properties, which are characterized by multicenter bonded structural entities. We argue that the proposed mechanism to explain the low lattice thermal conductivity of ZnSb might also control the thermoelectric properties of other electron poor semiconductors, such as ${\mathrm{Zn}}_4{\mathrm{Sb}}_3$, CdSb, ${\mathrm{Cd}}_4{\mathrm{Sb}}_3, {\mathrm{Cd}}_{13-x}{\mathrm{In}}_y{\mathrm{Zn}}_{10}$, and ${\mathrm{Zn}}_5{\mathrm{Sb}}_4{\mathrm{In}}_{2-\delta }$.