Two-dimensional networks of ordered quantum dots beyond the percolation threshold are studied, as a typical example of conducting nanostructures with quenched random disorder. Theory predicts anomalous diffusion with stretched-exponential relaxation at short distances, and computer simulations on lattices of crossing, straight paths of random length confirm such a behavior. Anomalous diffusion is interpreted as resulting from the higher probability of taking straight, or ballistic, paths, when the traveled distance is comparable or shorter than the lattice characteristic length. Diffusion turns over to normal for longer traveled distances, whence all paths tend to become equiprobable. Such random lattice structures may represent a model for realistic quantum dot networks, with potential applications in optoelectronics, photovoltaics, or spintronics.

We report neutron inelastic scattering experiments on single-crystal ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_3$ doped with 32% ${\mathrm{PbTiO}}_3$, a relaxor ferroelectric that lies close to the morphotropic phase boundary. When cooled under an electric field $\mathbf{E}\parallel$ [001] into tetragonal and monoclinic phases, the scattering cross section from transverse acoustic (TA) phonons polarized parallel to $\mathbf{E}$ weakens and shifts to higher energy relative to that under zero-field-cooled conditions. Likewise, the scattering cross section from transverse optic (TO) phonons polarized parallel to $\mathbf{E}$ weakens for energy transfers $4\le \hslash \omega \le 9$ meV. However, TA and TO phonons polarized perpendicular to $\mathbf{E}$ show no change. This anisotropic field response is similar to that of the diffuse scattering cross section, which, as previously reported, is suppressed when polarized parallel to $\mathbf{E}$ but not when polarized perpendicular to $\mathbf{E}$. Our findings suggest that the lattice dynamics and dynamic short-range polar correlations that give rise to the diffuse scattering are coupled.

Antiferromagnetic (AFM) thin films were recently proposed to be an alternative to conventional materials for achieving perpendicular magnetic anisotropy (PMA) in ferromagnetic thin films, because AFM thin films exhibit an advantage of flexible control. Here, we report that antiferromagnet-induced PMA is highly sensitive to interfacial moments of AFM thin films as well as the magnetic interaction of such moments with volume moments, determined according to the vertical interlayer distance. Magnetic hysteresis loops and x-ray magnetic domain imaging revealed the establishment of perpendicular magnetization on face-centered tetragonal (fct)-like Mn/Co/Ni films when covered with monolayered Mn films. A cover of Mn films that exhibit contracted fct- [vertical-to-in-plane lattice constant ratio $\left(\mathrm{c}/\mathrm{a}\right)=0.95]$ and expanded fct-like $(\mathrm{c}/\mathrm{a}=1.05)$ structures at different thickness levels induced in-plane magnetic anisotropy and PMA in Co/Ni films, respectively, confirming that the interlayer distance is a crucial parameter for establishing perpendicular magnetization.

We study the optical properties of arrays of ultrathin cobalt nanowires by means of the Brillouin scattering of light on magnons. We employ the Stokes/anti-Stokes scattering asymmetry to probe the circular polarization of a local electric field induced inside nanowires by linearly polarized light waves. We observe the anomalous polarization conversion of the opposite sign than that in a bulk medium or thick nanowires with a great enhancement of the degree of circular polarization attributed to the unconventional refraction in a nanowire medium. A rigorous simulation of the electric field polarization as a function of the wire diameter and spacing reveals the reversed polarization for a thin sparse wire array, in full quantitative agreement with experimental results.

The ferrimagnetic insulator yttrium iron garnet (YIG) is an important material in the field of magnon spintronics, mainly because of its low magnetic losses. YIG also has very low acoustic losses, and for this reason the conversion of a state of magnetic excitation (magnons) into a state of lattice vibration (phonons), or vice versa, broadens its possible applications in spintronics. Since the magnetic parameters can be varied by some external action, the magnon-phonon interconversion can be tuned to perform a desired function. We present a quantum theory of the interaction between magnons and phonons in a ferromagnetic material subject to a dynamic variation of the applied magnetic field. It is shown that when the field gradient at the magnetoelastic crossover region is much smaller than a critical value, an initial elastic excitation can be completely converted into a magnetic excitation, or vice versa. This occurs with conservation of linear momentum and spin angular momentum, implying that phonons created by the conversion of magnons have spin angular momentum and carry spin current. It is shown further that if the system is initially in a quantum coherent state, its coherence properties are maintained regardless of the time dependence of the field.

A nondissipative supercurrent state of a Josephson junction is metastable with respect to the formation of a finite-resistance state. This transition is driven by fluctuations, thermal at high temperatures and quantum at low temperatures. We evaluate the lifetime of such a state due to quantum fluctuations in the limit when the supercurrent is approaching the critical current. The decay probability is determined by the instanton action for the superconducting phase difference across the junction. At low temperatures, the dynamics of the phase is massive and is determined by the effective capacitance, which is a sum of the geometric and intrinsic capacitance of the junction. We model the central part of the Josephson junction either by an arbitrary short mesoscopic conductor described by the set of its transmission coefficients, or by a diffusive wire of an arbitrary length. The intrinsic capacitance can generally be estimated as $C_{*}\sim G/E_g$, where $G$ is the normal-state conductance of the junction and $E_g$ is the proximity minigap in its normal part. The obtained capacitance is sufficiently large to qualitatively explain the hysteretic behavior of the current-voltage characteristic even in the absence of overheating.

We study surface zero-energy flat bands in three-dimensional chiral superconductors with $p_z{(p_x+i{p}_y)}^{\nu }$-wave pairing symmetry ($\nu$ is a nonzero integer), based on topological arguments and tunneling conductance. It is shown that the surface flat bands are fragile against (i) the surface misorientation and (ii) the surface Rashba spin-orbit interaction. The fragility of (i) is specific to chiral SCs, whereas that of (ii) happens for general odd-parity SCs. We demonstrate that these flat-band instabilities vanish or suppress a zero-bias conductance peak in a normal/insulator/superconductor junction, which behavior is clearly different from high-$T_c$ cuprates and noncentrosymmetric superconductors. By calculating the angle-resolved conductance, we also discuss a topological surface state associated with the coexistence of line and point nodes.

It was found recently that multiferroic $\mathrm{Pb}\left({\mathrm{Fe}}_{1/2}{\mathrm{Nb}}_{1/2}\right){\mathrm{O}}_3$ (PFN) has a complex crystal structure containing nanoscale regions of Fe ions segregated into chemical clusters. This unusual property may have exciting applications, for instance, in magnetic, electric, or magnetoelectric devices, and it definitely has a fundamental significance. However, this challenge will be unclaimed until one can find the way how to control this property. In the present study, we investigate the possibility of using an epitaxial strain for these purposes. We have performed first-principles calculations of the energies of different chemical configurations of PFN for a wide range of the misfit strains. We found that the misfit strain does influence the energy, magnetic moments, crystal shape, ferroelectric, and even antiferroelectric polarization of PFN, due to a different response to the strain of different chemical configurations. This makes the epitaxial strain one of the most valuable candidates to control the chemical clustering in PFN.

The temperature dependence of the spin diffusion length typically reflects the scattering mechanism responsible for spin relaxation. Within nonmagnetic metals it is reasonable to expect the Elliot-Yafet mechanism to play a role and thus the temperature dependence of the spin diffusion length might be inversely proportional to resistivity. In lateral spin valves, measurements have found that at low temperatures the spin diffusion length unexpectedly decreases. By measuring the transport properties of lateral Py/Cu/Py spin valves, fabricated from Cu with magnetic impurities of $<1$ ppm and $\sim 4$ ppm, we extract a spin diffusion length which shows this suppression below 30 K only in the presence of the Kondo effect. We have calculated the spin-relaxation rate and isolated the contribution from magnetic impurities. We find the spin-flip probability of a magnetic impurity to be 34%. Our analysis demonstrates the dominant role of Kondo scattering in spin relaxation, even in low concentrations of order 1 ppm, and hence illustrates its importance to the reduction in spin diffusion length observed by ourselves and others.

Small-angle soft x-ray scattering in resonance with Fe $L$ absorption edge has been investigated for helical magnetic order and magnetic skyrmion crystal (SkX) in B20-type cubic FeGe. Transformation of magnetic structures among helical, conical, SkX, and field-polarized spin-collinear forms is observed with the application of a magnetic field parallel to the incident soft x-ray. The resonant soft x-ray scattering with high $\mathit{q}$-resolution revealed a transient dynamics of SkX, such as rotation of SkX and variation of the SkX lattice constant, upon the change of magnetic field.

We present synchrotron x-ray diffraction, neutron powder diffraction, and time-of-flight inelastic neutron scattering measurements on the rare earth pyrochlore oxide ${\mathrm{Nd}}_2{\mathrm{Zr}}_2{\mathrm{O}}_7$ to study the ordered state magnetic structure and cystal-field states. The structural characterization by high-resolution synchrotron x-ray diffraction confirms that the pyrochlore structure has no detectable O vacancies or Nd/Zr site mixing. The neutron diffraction reveals long-range all-in/all-out antiferromagnetic order below $T_{\mathrm{N}}\approx 0.4$ K with propagation vector k = (0 0 0) and an ordered moment of $1.26\left(2\right){\mu }_{\mathrm{B}}$/Nd at 0.1 K. The ordered moment is much smaller than the estimated moment of $2.65{\mu }_{\mathrm{B}}$/Nd for the local $\langle 111\rangle$ Ising ground state of ${\mathrm{Nd}}^{3+}$ $\left(J=9/2\right)$ suggesting that the ordering is partially suppressed by quantum fluctuations. The inelastic neutron scattering experiment further confirms the Ising anisotropic ground state of ${\mathrm{Nd}}^{3+}$ and also reveals its dipolar-octupolar character which possibly induces the quantum fluctuation. The crystal-field level scheme and ground state wave function have been determined.

Magnetic and magneto-transport properties of [Co/Pt]/MgO/[Co/Pt]CoTb magnetic tunnel junctions have been investigated. Depending on the thickness of the Pt layer, temperature, and field history, the [Co/Pt]CoTb hard layer shows complex behaviors that can be reproduced by micromagnetic calculations. The magnetic tunnel junctions appear to exhibit conventional behavior but also exchange bias and spring magnet and domain duplication phenomena that have been observed combining magnetometry and interface-sensitive spin-dependent tunnel transport.

We numerically study the behavior of two-dimensional skyrmions in the presence of a quasi-one-dimensional sinusoidal substrate under the influence of externally applied dc and ac drives. In the overdamped limit, when both dc and ac drives are aligned in the longitudinal direction parallel to the direction of the substrate modulation, the velocity-force curves exhibit classic Shapiro step features when the frequency of the ac drive matches the washboard frequency that is dynamically generated by the motion of the skyrmions over the substrate, similar to previous observations in superconducting vortex systems. In the case of skyrmions, the additional contribution to the skyrmion motion from a nondissipative Magnus force shifts the location of the locking steps to higher dc drives, and we find that the skyrmions move at an angle with respect to the direction of the dc drive. For a longitudinal dc drive and a perpendicular or transverse ac drive, the overdamped system exhibits no Shapiro steps; however, when a finite Magnus force is present, we find pronounced transverse Shapiro steps along with complex two-dimensional periodic orbits of the skyrmions in the phase-locked regimes. Both the longitudinal and transverse ac drives produce locking steps whose widths oscillate with increasing ac drive amplitude. We examine the role of collective skyrmion interactions and find that additional fractional locking steps occur for both longitudinal and transverse ac drives. At higher skyrmion densities, the system undergoes a series of dynamical order-disorder transitions, with the skyrmions forming a moving solid on the phase locking steps and a fluctuating dynamical liquid in regimes between the steps.

Spring magnets based on GdCo multilayers have been prepared to study the nucleation and evolution of interfacial domain walls (iDWs) depending on layer composition and interlayer coupling. GdCo alloy compositions in each layer were chosen so that their net magnetization aligns either with the Gd ($\mathrm{G}{\mathrm{d}}_{35}\mathrm{C}{\mathrm{o}}_{65}$) or $\mathrm{Co}\left(\mathrm{G}{\mathrm{d}}_{11}\mathrm{C}{\mathrm{o}}_{89}\right)$ sublattices. This condition forces an antiparallel arrangement of the layers’ net magnetization and leads to nucleation of iDWs above critical magnetic fields whose values are dictated by the interplay between Zeeman and exchange energies. By combining x-ray resonant magnetic scattering with Kerr magnetometry, we provide detailed insight into the nucleation and spatial profile of the iDWs. For strong coupling (GdCo/GdCo′ bilayer), iDWs are centered at the interface but with asymmetric width depending on each layer magnetization. When interlayer coupling is weakened by introducing a thin Gd interlayer, the exchange spring effect becomes restricted to a lower temperature and field range than observed in the bilayer structure. Due to the ferromagnetic alignment between the high magnetization $\mathrm{G}{\mathrm{d}}_{35}\mathrm{C}{\mathrm{o}}_{65}$ layer and the Gd interlayer, the iDW shrinks and moves into the lower exchange Gd interlayer, causing a reduction of iDW energy.

We use the coupled cluster method implemented to high orders of approximation to investigate the frustrated $\text{spin-}\frac{1}{2}{J}_1\text{−}{J}_2\text{−}{J}_3$ antiferromagnet on the honeycomb lattice with isotropic Heisenberg interactions of strength $J_1>0$ between nearest-neighbor pairs, $J_2>0$ between next-nearest neighbor pairs, and $J_3>0$ between next-next-nearest-neighbor pairs of spins. In particular, we study both the ground-state (GS) and lowest-lying triplet excited-state properties in the case $J_3=J_2\equiv \kappa J_1$, in the window $0\le \kappa \le 1$ of the frustration parameter, which includes the (tricritical) point of maximum classical frustration at ${\kappa }_{\mathrm{cl}}=\frac{1}{2}$. We present GS results for the spin stiffness ${\rho }_s$ and the zero-field uniform magnetic susceptibility $\chi$, which complement our earlier results for the GS energy per spin $E/N$ and staggered magnetization $M$ to yield a complete set of accurate low-energy parameters for the model. Our results all point towards a phase diagram containing two quasiclassical antiferromagnetic phases, one with Néel order for $\kappa <{\kappa }_{c_1}$, and the other with collinear striped order for $\kappa >{\kappa }_{c_2}$. The results for both $\chi$ and the spin gap $\mathrm{\Delta }$ provide compelling evidence for a disordered quantum paramagnetic phase that is gapped over a considerable portion of the intermediate region ${\kappa }_{c_1}<\kappa <{\kappa }_{c_2}$, especially close to the two quantum critical points at ${\kappa }_{c_1}$ and ${\kappa }_{c_2}$. Each of our fully independent sets of results for the low-energy parameters is consistent with the values ${\kappa }_{c_1}=0.45\pm 0.02$ and ${\kappa }_{c_2}=0.60\pm 0.02$, and with the transition at ${\kappa }_{c_1}$ being of continuous (and hence probably of the deconfined) type and that at ${\kappa }_{c_2}$ being of first-order type.

The logarithmic velocity profile is the most important statistical law of classical turbulence affected by channel walls. This paper demonstrates numerically that the logarithmic velocity profile of a superfluid flow appears in quantum turbulence under pure normal flow in a channel. We investigated the configuration and dynamics of an inhomogeneous vortex tangle affected by the walls, and found the characteristic behavior of the log-law.

Recent experiments on certain Fe-based superconductors have hinted at a role for paired electrons in “incipient” bands that are close to, but do not cross, the Fermi level. Related theoretical works disagree on whether or not strong-coupling superconductivity is required to explain such effects, and whether a critical interaction strength exists. In this work, we consider various versions of the model problem of pairing of electrons in the presence of an incipient band, within a simple multiband weak-coupling BCS approximation. We categorize the problem into two cases: case (i), where superconductivity arises from the “incipient band pairing” alone, and case (ii), where it is induced on an incipient band by pairing due to Fermi-surface-based interactions. Negative conclusions regarding the importance of incipient bands have been drawn so far largely based on case (i), but we show explicitly that models under case (ii) are qualitatively different, and can explain the nonexponential suppression of $T_c$, as well as robust large gaps on an incipient band. In the latter situation, large gaps on the incipient band do not require a critical interaction strength. We also model the interplay between phonon and spin fluctuation driven superconductivity and describe situations in which they can enhance each other rather than compete. Finally, we discuss the effect of the dimensionality of the incipient band on our results. We argue that pairing on incipient bands may be significant and important in several Fe-based materials, including LiFeAs, FeSe intercalates, and FeSe monolayers on strontium titanate, and indeed may contribute to high critical temperatures in some cases.

We present a theoretical study of the dynamical spin susceptibility for the intriguing Fe-based superconductor FeSe, based on a tight-binding model developed to account for the temperature-dependent band structure in this system. The model allows for orbital ordering in the $d_{xz}/d_{yz}$ channel below the structural transition and presents a strongly $C_4$-symmetry-broken Fermi surface at low temperatures which accounts for the nematic properties of this material. The calculated spin excitations are peaked at wave vector $(\pi ,0)$ in the 1-Fe Brillouin zone, with a broad maximum at energies of order a few meV. In this range, the occurrence of superconductivity sharpens this peak in energy, creating a $(\pi ,0)$ “neutron resonance” as seen in recent experiments. With the exception of the quite low energy scale of these fluctuations, these results are roughly similar to standard behavior in Fe pnictide systems. At higher energies, however, intensity increases and shifts to wave vectors along the $(\pi ,0)\text{−}(\pi ,\pi )$ line. We compare with existing inelastic neutron experiments and NMR data, and give predictions for further studies.

Orbital-based exchange $\left(x\right)$ correlation $\left(c\right)$ energy functionals, leading to the optimized effective potential (OEP) formalism of density-functional theory (DFT), are gaining increasing importance in ground-state DFT, as applied to the calculation of the electronic structure of closed systems with a fixed number of particles, such as atoms and molecules. These types of functionals prove also to be extremely valuable for dealing with solid-state systems with reduced dimensionality, such as is the case of electrons trapped at the interface between two different semiconductors, or narrow metallic slabs. In both cases, electrons build a quasi-two-dimensional electron gas, or Q2DEG. We provide here a general DFT-OEP formal scheme valid both for Q2DEGs either isolated (closed) or in contact with a particle bath (open), and show that both possible representations are equivalent, being the choice of one or the other essentially a question of convenience. Based on this equivalence, a calculation scheme is proposed which avoids the noninvertibility problem of the density response function for closed systems. We also consider the case of spontaneously spin-polarized Q2DEGs, and find that far from the region where the Q2DEG is localized, the exact $x\text{-only}$ exchange potential approaches two different, spin-dependent asymptotic limits. As an example, aside from these formal results, we also provide numerical results for a spin-polarized jellium slab, using the new OEP formalism for closed systems. The accuracy of the Krieger-Li-Iafrate approximation has been also tested for the same system, and found to be as good as it is for atoms and molecules.

We study the impact of the fermionic self-energy on one-loop functional renormalization group flows of the two-dimensional $t\text{−}{t}^{\prime }$ Hubbard model, with emphasis on electronic densities away from van Hove filling. In the presence of antiferromagnetic hot spots, antiferromagnetic fluctuations lead to a flattening of the Fermi surface, shift magnetic phase boundaries, and significantly enhance critical scales. We trace back this effect to the presence of a magnetic first-order transition. For some parameters, the first-order character of the latter is reduced by self-energy effects. For reliably determining phase diagrams, the fermionic self-energy should be taken into account in functional renormalization group studies if scattering between hot spots is important.