Over the past years a lot of theoretical and experimental effort has been made to find states with broken time-reversal symmetry (BTRS) in multiband superconductors. In particular, it was proposed theoretically that in the ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ system either an $s+is$ or an $s+id$ BTRS state may exist at high doping levels in a narrow region of the phase diagram. Here we report the observation of an enhanced zero-field muon spin-relaxation rate below the superconducting transition temperature for a high quality crystalline sample with $x\approx 0.73$. This indicates that indeed the time-reversal symmetry is broken in superconducting ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ at this doping level.

Temperature-dependent reflectivity measurements in the frequency range 75–8000 ${\mathrm{cm}}^{-1}$ were performed on ${\mathrm{BaFe}}_2{\left({\mathrm{As}}_{0.77}{\mathrm{P}}_{0.23}\right)}_2$ single crystals under pressure up to $\sim 5\mathrm{GPa}$. The obtained optical conductivity spectra have been analyzed to extract the electron-boson spectral density ${\alpha }^{2}F\left(\mathrm{\Omega }\right)$. A sharp resonance peak was observed in ${\alpha }^{2}F\left(\mathrm{\Omega }\right)$ upon the superconducting transition, persisting throughout the applied pressure range. The energy and temperature dependences of this peak are consistent with the superconducting gap opening. Furthermore, several similarities with other experimental probes such as inelastic neutron scattering (INS) [D. S. Inosov et al., Nat. Lett. 6, 178 (2010)] give evidence for the coupling to a bosonic mode, possibly due to spin fluctuations. Moreover, electronic correlations have been calculated via spectral weight analysis, which revealed that the system stays in the strongly correlated regime throughout the applied pressure range. However, a comparison to the parent compound showed that the electronic correlations are slightly decreased with P doping. The investigation of the phase diagram obtained by our optical study under pressure also revealed the coexistence of the spin-density wave and the superconducting regions, where the coexistence region shifts to the lower pressure range with increasing P content. Moreover, the optimum pressure range, where the highest superconducting transition temperature has been obtained, shows a nonlinear decrease with increasing P content.

Taking pure Mg, Mg-Al, and Mg-Zn as prototypes, the effects of strain on the stacking fault energies (SFEs), dislocation core structure, and Peierls stress were systematically investigated by means of density functional theory and the semidiscrete variational Peierls-Nabarro model. Our results suggest that volumetric strain may significantly influence the values of SFEs of both pure Mg and its alloys, which will eventually modify the dislocation core structure, Peierls stress, and preferred slip system, in agreement with recent experimental results. The so-called “strain factor” that was previously proposed for the solute strengthening could be justified as a major contribution to the strain effect on SFEs. Based on multivariate regression analysis, we proposed universal exponential relationships between the dislocation core structure, the Peierls stress, and the stable or unstable SFEs. Electronic structure calculations suggest that the variations of these critical parameters controlling strength and ductility under strain can be attributed to the strain-induced electronic polarization and redistribution of valence charge density at hollow sites. These findings provide a fundamental basis for tuning the strain effect to design novel Mg alloys with both high strength and ductility.

A combination of in situ volumetric measurements, Raman spectroscopy, x-ray diffraction, differential thermal analysis, and transmission electron microscopy was used to investigate the crystalline-amorphous transformation of zirconium tungstate at high pressure and room temperature. Zirconium tungstate transformed at about 1.2 GPa to an amorphous phase which was recoverable to ambient conditions. The volume contraction and temperature increase in the sample during pressure-induced amorphization indicate that it was a first-order phase transition. The kinetics data are consistent with a model where an amorphous phase occurs by the formation of nuclei without noticeable growth. However, our results conflict with the three-dimensional interface-controlled growth of a new phase in conventional solid-state polymorphic changes.

A systematic investigation of the magnetization reversal and the dynamic behaviors of uncoupled ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ nanowires (NWs) with artificial continuous width modulation is presented. In contrast with the single resonance mode observed in the homogeneous NWs from the broadband ferromagnetic resonance spectroscopy, the NWs with continuous width modulation display three to five distinct resonance modes with increasing wire thickness in the range from 5 to 70 nm due to the nonuniform demagnetizing field. The highest frequency mode and the frequency difference between the two distinct highest modes are shown to be markedly sensitive to the NW thickness. Interestingly, we found that these modes can be described in terms of the quantization of the standing spin waves due to confined varied width. In addition, the easy axis coercive field for the width modulated NWs is much higher than homogeneous NWs of the same thickness when less than 70 nm. Our experimental results are in good qualitative agreement with the micromagnetic simulations. The results may find potential applications in the design and optimization of tunable magnonic filters.

${}^{13}\mathrm{C}$ nuclear magnetic resonance measurements were performed for a single-component molecular material Zn(tmdt)${}_2$, in which tmdt's form an arrangement similar to the so-called $\kappa$-type molecular packing in quasi-two-dimensional Mott insulators and superconductors. A detailed analysis of the powder spectra uncovered local spin susceptibility in the tmdt $\pi$ orbitals. The obtained shift and relaxation rate revealed singlet-triplet excitations of the $\pi$ spins, indicating that Zn(tmdt)${}_2$ is a spin-gapped Mott insulator with exceptionally large electron correlations compared to conventional molecular Mott systems.

Motivated by recent thermal conductivity measurements in the superconductor ${\mathrm{CeCoIn}}_5$, we theoretically examine a possible staggered spin-triplet superconducting order to be induced by the coupled spin-density-wave (SDW) and $d$-wave superconducting (SC) orders in the high-field and low-temperature (HFLT) SC phase peculiar to this material with strong paramagnetic pair-breaking (PPB). It is shown that one type of the $\pi$-triplet order is consistent with that explaining the thermal conductivity data and can naturally be incorporated in the picture that the HFLT phase is a consequence of the strong PPB effect inducing the SDW order and the FFLO spatial modulation parallel to the applied magnetic field.

Electromagnetic scattering from moving bodies, being an inherently time-dependent phenomenon, gives rise to a generation of new frequencies, which can be used to characterize the motion. Whereas an ordinary motion along a linear path produces a constant Doppler shift, an accelerated scatterer can generate a micro-Doppler frequency comb. The spectra produced by rotating objects were studied and observed in a bistatic lock-in detection scheme. The internal geometry of a scatterer was shown to determine the spectrum, and the degree of structural asymmetry was suggested to be identified via signatures in the micro-Doppler comb. In particular, hybrid magnetoelectric particles, showing an ultimate degree of asymmetry in forward and backward scattering directions, were investigated. It was shown that the comb in the backward direction has signatures at the fundamental rotation frequency and its odd harmonics, whereas the comb of the forward scattered field has a prevailing peak at the doubled frequency and its multiples. Additional features of the comb were shown to be affected by the dimensions of the particle and by the strength of the magnetoelectric coupling. Experimental verification was performed with a printed circuit board antenna based on a wire and a split ring, while the structure was illuminated at a 2 GHz carrier frequency. Detailed analysis of micro-Doppler combs enables remote detection of asymmetric features of distant objects and could find use in a span of applications, including stellar radiometry and radio identification.

We explore the effects of asymmetry of hopping parameters between double parallel quantum dots and the leads on the conductance and a possibility of local magnetic moment formation in this system using functional renormalization group approach with the counterterm. We demonstrate a possibility of a quantum phase transition to a local moment regime [so-called singular Fermi liquid (SFL) state] for various types of hopping asymmetries and discuss respective gate voltage dependencies of the conductance. We show that, depending on the type of the asymmetry, the system can demonstrate either a first-order quantum phase transition to an SFL state, accompanied by a discontinuous change of the conductance, similarly to the symmetric case, or the second-order quantum phase transition, in which the conductance is continuous and exhibits Fano-type asymmetric resonance near the transition point. A semianalytical explanation of these different types of conductance behavior is presented.

In III-V semiconductors it was shown theoretically that under optical cooling the nuclear-spin polaron bound to neutral donors would form below some critical nuclear-spin temperature $T_c$ [Merkulov, Phys. Solid State 40, 930 (1998)]. The predicted critical behavior is a direct consequence of the use of the mean-field approximation. It is known however that in any finite-size system a critical behavior must be absent. Here we develop a model of the optically cooled nuclear polaron, which goes beyond the mean-field approximation. An expression of the generalized free energy of the optically cooled nuclear polaron, valid for a finite, albeit large, number of spins, is derived. This model permits us to describe the continuous transition from the fluctuation dominated regime to the collective regime, as the nuclear-spin temperature decreases. It is shown that due to the finite number of nuclear spins involved in the polaron, the critical effects close to $T_c$ are smoothed by the spin fluctuations. Particularly, instead of a divergence, the nuclear-spin fluctuations exhibit a sharp peak at $T_c$, before being depressed well below $T_c$. Interestingly, the formation of the nuclear polaron can, in certain conditions, boost the nuclear polarization beyond the value obtained solely by optical pumping. Finally, we suggest that the nuclear polaron could be detected by spin noise spectroscopy or via its superparamagnetic behavior.

Emergence of a classical particle trajectory concept from the full quantum description is a key feature of quantum mechanics. Recent progress of solid-state on-demand sources has brought single-electron manipulation into the quantum regime; however, the quantum-to-classical crossover remains unprobed. Here we describe theoretically a mechanism for generating single-electron wave packets by tunneling from a driven localized state, and we show how to tune the degree of quantumness. Applying our theory to existing on-demand sources, we demonstrate the feasibility of an experimental investigation of quantum-to-classical crossover for single electrons, and we open up yet unexplored potential for few-electron quantum technology devices.

The propagation of surface waves supported by a finite array of slots perforated on a zero thickness perfect electrically conducting screen is studied both experimentally and theoretically. To generate numerical results, the integral equation satisfied by the electric field in the slots is efficiently solved by means of Galerkin's method, treating the metal as perfectly conducting. The finite size of the array along the direction of propagation creates a family of states of higher momentum and lower amplitude than the single mode for the corresponding infinite array. These modes are spaced in momentum with a periodicity inversely proportional to the length of the array. In addition, the finite width in the transverse direction produces a set of higher frequency modes due to this additional quantization. Both effects arising from finite sample length and width are explained by the theoretical model and validated experimentally.

The coherent high-fidelity generation of nuclear spins in long-lived singlet states, which may find application as quantum memory or sensor, represents a considerable experimental challenge. Here, we propose a dissipative scheme that achieves the preparation of pairs of nuclear spins in long-lived singlet states by a protocol that combines the interaction between the nuclei and a periodically reset electron spin of an nirogen-vacancy center with local radio-frequency control of the nuclear spins. The final state of this protocol is independent of the initial preparation of the nuclei, is robust to external field fluctuations, and can be operated at room temperature. We show that a high-fidelity singlet pair of a ${}^{13}\mathrm{C}$ dimer in a nuclear bath in diamond can be generated under realistic experimental conditions.

In topological semimetals the Dirac points can form zero-dimensional and one-dimensional manifolds, as predicted for Dirac/Weyl semimetals and topological nodal line semimetals, respectively. Here, based on first-principles calculations, we predict a topological nodal line semimetal phase in the two-dimensional compounds $X_{2}Y$ ($X$ = Ca, Sr, and Ba; $Y$ = As, Sb, and Bi) in the absence of spin-orbit coupling (SOC) with a band inversion at the M point. A nontrivial ${\mathbb{Z}}_2$ invariant of ${\mathbb{Z}}_2=1$ remains although a tiny gap appears at the nodal line when SOC is included. The mirror symmetry as well as the electrostatic interaction, which can be engineered via strain, are responsible for the nontrivial phase. In addition, the nontrivial phase is further explicitly confirmed via the existence of exotic edge states without and with SOC.