We have grown Sr${}_2$IrO${}_4$ (SIO) epitaxial thin films on SrTiO${}_3$ (STO) and NdGaO${}_3$ (NGO) substrates by a pulsed laser deposition method and characterized their structures and magnetic properties. We find that SIO films grown on STO substrates display tetragonal structure with a tensile strain of 0.13%, while SIO films grown on NGO substrates exhibit slightly orthorhombic structure with anisotropic biaxial tensile strains of 0.39% and 0.51% along the in-plane crystallographic axes. Although both films display insulating properties as bulk SIO does, their magnetic properties are distinct from that of bulk SIO. The ferromagnetic (FM) component of the $J_{\mathrm{eff}}$ = 1/2 canted antiferromagnetic order, which emerges below ∼240 K in bulk SIO, is significantly weakened in both films, with a greater weakening appearing in the SIO/NGO film. From structural and magnetoresistance anisotropy analyses for both films, we reveal that the weak FM component in SIO films is dependent on the epitaxial strain. The greater tensile strain leads to a smaller octahedral rotation: The rotation angle is ∼9.7(1)° for the SIO/NGO film and ∼10.7(2)° for the SIO/STO film. These findings indicate that the $J_{\mathrm{eff}}$ = 1/2 moment orientation in SIO follows the IrO${}_6$ octahedral rotation due to strong spin-orbit interaction.

We rigorously derived an effective model to describe the plasmonic couplings between nanoparticles of general shape. In addition to usual electric/magnetic dipolar interactions, the model also contains a radiation-correction term and an electric/magnetic cross-interacting term. The effective model was justified by full-wave simulations in different coupled plasmonic systems, for which the interplays among these terms generate fascinating behaviors. As an application of our theory, we show that the coupling strength between certain plasmonic nanoparticles can be tuned through varying the orientations of nanoparticles, leading to interesting phenomena such as ultraslow-wave plasmon propagation.

We study theoretically the ground states of topological defects in a spinor four-component condensate of cold indirect excitons. We analyze possible ground-state solutions for different configurations of vortices and half vortices. We show that if only Rashba or Dresselhaus spin-orbit interaction (SOI) for electrons is present the stable states of topological defects can represent a cylindrically symmetric half vortex or half-vortex-antivortex pairs or a nontrivial pattern with warped vortices. In the presence of both Rashba and Dresselhaus SOI the ground state of a condensate represents a stripe phase and vortex type solutions become unstable.

We study spin and charge diffusion in metallic-ferromagnet/topological-insulator junctions. The diffusive theory is constructed for the coupled transport of the spin-dependent electron densities in the ferromagnet and the charge density in the topological insulator. The diffusion equations for the coupled transport are derived perturbatively with respect to the strength of the interlayer tunneling. We analytically calculate spin accumulation in the ferromagnet and junction magnetoresistance associated with a current bias along the interface. It is found that due to the helical spin texture of the surface Dirac fermion, the spin accumulation and the junction magnetoresistance depend on the angle between the magnetization and the current-induced spin polarization on the surface of the topological insulator.

The decoherence of mixed electron-nuclear spin qubits is a topic of great current importance, but understanding is still lacking: While important decoherence mechanisms for spin qubits arise from quantum spin bath environments with slow decay of correlations, the only analytical framework for explaining observed sharp variations of decoherence times with magnetic field is based on the suppression of classical noise. Here we obtain a general expression for decoherence times of the central spin system which exposes significant differences between quantum-bath decoherence and decoherence by classical field noise. We perform measurements of decoherence times of bismuth donors in natural silicon using both electron spin resonance (ESR) and nuclear magnetic resonance (NMR) transitions, and in both cases find excellent agreement with our theory across a wide parameter range. The universality of our expression is also tested by quantitative comparisons with previous measurements of decoherence around “optimal working points” or “clock transitions” where decoherence is strongly suppressed. We further validate our results by comparison to cluster expansion simulations.

The mechanisms of how alloying elements and oxygen influence the stability and elastic properties of binary Ti-$X$ ($X$ = Nb, Zr, or Sn) and Ti2448 (Ti-24Nb-4Zr-8Sn in wt.%) alloys are studied via first principles calculations. In addition to the fully disordered solid solution phase, we consider 44 quasirandom configurations to search for the possible distributions of the alloying elements in Ti2448. Our results show that all alloying elements considered here are good β-stabilizers for Ti, and the formation energies are greatly affected by their distributions. The site preference of oxygen and its concentration dependence in binary Ti alloys and in Ti2448 are also investigated. Oxygen prefers to occupy the octahedral site regardless of the concentrations of the alloys and strongly interacts with Ti and Nb in Ti-Nb. The elastic properties of Ti2448 alloy and the influence of oxygen on the elastic parameters are evaluated. The calculated polycrystalline Young's modulus of the Ti2448 alloy is very close to that of the human bone (10–40 GPa). We find that oxygen has a weak effect on the elastic moduli of Ti2448. The electronic structures are analyzed to reveal how the alloying elements and oxygen influence the stability of binary Ti-$X$ and Ti2448 alloys.

The effects of dynamical diffraction in single crystals engender many unique diffraction phenomena that cannot be interpreted by the kinematical-diffraction theory, yet knowledge of them is vital to resolving a vast variety of scientific problems ranging from crystal optics to strain measurements in crystalline specimens. Although the fundamental dynamical-diffraction theory was established decades ago, modeling it remains a challenge in a general case wherein the crystal has complex boundaries and mixed diffraction geometries (Bragg or Laue). Here, we propose a universal approach for modeling x-ray dynamical diffraction from a single crystal with arbitrary shape and strain field that is based on the integral representation of the Takagi-Taupin equations. Using it, we can construct the solution iteratively via a converging series, independent of the diffraction geometry. Moreover, the integral equations offer additional insights into the diffraction physics that are not readily apparent in its differential counterparts. To demonstrate this approach, we studied the dynamical diffraction from a slab of single crystal with both Bragg and Laue diffraction excited on the entrance boundaries, a problem that is difficult to model by other methods. We also explored the mirage effect caused by the presence of a linear strain field and compared it to the Eikonal theory. Lastly, we derived a dynamical-diffraction equation correlating the structural properties of a particle to its far-field Bragg-diffraction pattern, shedding light on how dynamical diffraction affects these kinematical-diffraction-based inverse techniques for reconstructing the shape and the strain field.

We report on the microscopic magnetic model of a spin-$\frac{1}{2}$ magnet BaV${}_3$O${}_8$. In contrast to earlier phenomenological analysis, our density-functional band-structure calculations combined with quantum Monte Carlo simulations establish a relatively simple and nonfrustrated model of weakly coupled spin chains with intrachain coupling $J\simeq 38$ K and Néel temperature $T_N\simeq 6$ K, both in excellent agreement with experiment. The intrachain coupling between spin-$\frac{1}{2}$ V${}^{+4}$ ions takes place via two contiguous V${}^{+5}$O${}_4$ tetrahedra forming an extended superexchange pathway with the V${}^{+4}$–V${}^{+4}$ distance of 7.44 Å. Surprisingly, this pathway is preferred over shorter V${}^{+4}$–V${}^{+4}$ connections, owing to peculiarities in the interacting orbitals of the magnetic V${}^{+4}$ ions and V${}^{+5}$ ions that are nonmagnetic, but feature low-lying $3d$ states contributing to the superexchange process. We also note that the crystal structure of BaV${}_3$O${}_8$ supports the long-sought uniform arrangement of Dzyaloshinsky-Moriya (DM) couplings on a spin-$\frac{1}{2}$ chain. While our calculations yield only a weak DM anisotropy in BaV${}_3$O${}_8$, the crystal structure of this compound provides a suitable framework for the search of spin chains with the uniform DM anisotropy in other compounds of the vanadate family.

We demonstrate that the magnonic band structure, including the band gap of a ferromagnetic antidot waveguide, can be significantly tuned by a relatively weak modulation of its structural parameters. We study the magnonic band structure in nanoscale spin-wave waveguides with periodically distributed small antidots along their central line by two independent computational methods, namely, a micromagnetic simulation and a plane-wave method. The calculations were performed with consideration of both the exchange and dipolar interactions. For the exchange dominated regime, we discuss, in details, the impact of the changes of the lattice constant, size, and shape of the antidots on the spin-wave spectra. We have shown that a precise choice of these parameters is crucial for achieving desired properties of antidot waveguides, i.e., a large group velocity and filtering properties due to existence of magnonic band gaps. We discuss different mechanisms of magnonic gap opening resulting from Bragg scattering or anticrossing of modes. We have shown that the dipolar interactions start to assert their role in the spin-wave spectrum when the waveguide is scaled up, but even for a period of few hundreds of nanometers, the magnonic band structure preserves qualitatively the properties found in the exchange dominating regime. The obtained results are important for future development of magnonic crystal based devices.

The paper presents a comprehensive characterization of well-developed vortex tangles in a turbulent counterflow in quantum fluids (with a laminar normal fluid component). We perform and analyze extensive numerical simulations using the vortex filament method, solving the full Biot-Savart equations for the vortex dynamics in a wide range of temperatures and counterflow velocities. We start with the analysis of the macroscopic characteristics of the quantum vortex tangle such as vortex line density, its mean anisotropic and curvature parameters, the mean friction force between normal and superfluid components, the drift velocity of the vortex tangle, etc. Next we proceed to the main goal of the paper and move from the traditional macroscopic approach in terms of mean characteristics of the vortex tangle to the microscopic statistical and kinetic levels of description of quantum turbulence. These include objects that are much less studied or even totally neglected such as the vortex reconnection rates, the correlations and probability distribution functions (PDFs) of the vortex loop lengths, of the line curvature, of the mean curvatures of individual loops, the cross-correlation function between the loop length and its mean curvature, and the autocorrelation function of the vortex-line orientations. This detailed statistical information is required for a deeper understanding of quantum turbulence and for the development of its advanced theoretical description. In addition, we identify which of the studied properties are strongly affected by the choice of the reconnection criteria that are traditionally used in the vortex filament method and which of them are practically insensitive to the reconnection procedure. We conclude that the vortex filament method is sufficiently robust and well-suited for the description of the steady-state vortex tangle in the quantum counterflow.

The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron-spin impurities, but the frequency and temperature dependence observed in experiments do not agree with the usual $1/f$ models. We present theoretical calculations and experimental measurements of flux noise in rf SQUID flux qubits that show how these observations, and previous reported measurements, can be interpreted in terms of a spin-diffusion constant that increases with temperature. We fit measurements of flux noise in 16 devices, taken in the $20\text{--}80$ mK temperature range, to the spin-diffusion model. This allows us to extract the spin-diffusion constant and its temperature dependence, suggesting that the spin system is close to a spin-glass phase transition.

Current-induced magnetization dynamics is governed by a subtle combination of damping, adiabatic, and nonadiabatic spin-transfer torques (STTs). A precise determination of these three key parameters is difficult since they have to be determined in the very same nanostructured sample. In this study, we experimentally determine the spin-tansfer torque parameters in a fully self-consistent approach by optically accessing current-induced spin-wave dynamics. Our technique allows a precise access to spin-wave characteristics and their current-induced changes, especially the change in decay length which carries the information about the nonadiabaticity. Accessing this quantity allows the implementation of an analytical model which leads to a direct and separate extraction of the three STT key parameters without resorting to micromagnetic simulations.

Following the discovery of the potentially very high-temperature superconductivity in monolayer FeSe, we investigate the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy-centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore, we find that the disorder-induced broadening causes an effective hole doping. The surprising net result is that, in terms of the Fe-$d$ bands, Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measured by angle-resolved photoemission spectroscopy. Furthermore, the unexpected doping effects not only lead to numerous consequences for the debated role of anion vacancies in the iron-based superconductors, but also demonstrate the surprisingly rich physics of vacancies in materials in general.

A systematic investigation of the dynamic behavior of Ni${}_{80}$Fe${}_{20}$ ring arrays using broadband ferromagnetic resonance spectroscopy as a function of inter-ring spacing and ring thickness is presented. Four distinct resonance modes were found for rectangular rings compared to the two modes seen in circular rings of identical width due to the presence of sharp corners and nonuniform demagnetization field distribution. The resonance peaks were sensitive to the inter-ring spacing and the ring thickness due to magnetostatic coupling. Micromagnetic simulations and analytical calculations are compared with the experiment results.

We discuss the nonequilibrium dynamics of a quantum Ising chain following a quantum quench of the transverse field and in the presence of a Gaussian time-dependent noise. We discuss the probability distribution of the work done on the system both for static and dynamic noise. While the effect of static noise is to smooth the low energy threshold of the statistic of the work, appearing for sudden quenches, a dynamical noise protocol affects also the spectral weight of such features. We also provide a detailed derivation of the kinetic equation for the Green's functions on the Keldysh contour and the time evolution of observables of physical interest, extending previously reported results [Marino and Silva, Phys. Rev. B 86, 060408 (2012)], and discussing the extension of the concept of prethermalization which can be used to study noisy quantum many-body Hamiltonians driven out of equilibrium.

The application of Carr-Purcell-Meiboom-Gill (CPMG) $\pi$ trains for dynamically decoupling a system from its environment has been extensively studied in a variety of physical systems. When applied to dipolar solids, recent experiments have demonstrated that CPMG pulse trains can generate long-lived spin echoes. In this work, we develop a theory to describe the spin dynamics in a dipolar coupled spin-$\frac{1}{2}$ system under a CPMG(${\varphi }_1,{\varphi }_2$) pulse train, where ${\varphi }_1$ and ${\varphi }_2$ are the phases of the $\pi$ pulses. From our theoretical framework, the propagator for the CPMG(${\varphi }_1,{\varphi }_2$) pulse train is equivalent to an effective “pulsed” spin locking of single-quantum coherences with phase $\pm \frac{{\varphi }_2-3{\varphi }_1}{2}$, which generates a periodic quasiequilibrium that corresponds to the long-lived echoes. Numerical simulations, along with experiments on both magnetically dilute, random spin networks found in C${}_{60}$ and C${}_{70}$ and in nondilute spin systems found in adamantane and ferrocene, were performed and confirm the predictions from the proposed theory.

Structural, magnetic, and thermal measurements performed on ${\mathrm{CeCo}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ alloys are reported. Three regions can be recognized: (i) Co rich ($x\le 0.20$) with a decreasing long-range antiferromagnetic order which vanishes at finite temperatures, (ii) an intermediate region ($0.20<x\le 0.30$) showing a broad magnetic anomaly ($C_A$) in specific heat, and (iii) the nonmagnetic region progressively changing from a non-Fermi-liquid-type behavior towards a Fermi-liquid one as Fe concentration increases. The $C_A$ anomaly emerges as an incipient contribution above $T_N$ already at $x=0.10$, which indicates that this contribution is related to short-range correlations likely of quasi-two-dimensional type. Both $T_N$ transition and $C_A$ anomaly are practically unaffected by an applied magnetic field up to $B\approx 10\mathrm{T}$.

We analyze the propagation of electromagnetic waves along the surface of a nonlinear dielectric medium covered by a graphene layer. We reveal that this system can support and stabilize nonlinear transverse electric (TE) plasmon polaritons. We demonstrate that these nonlinear TE modes have a subwavelength localization in the direction perpendicular to the surface, with the intensity much higher than that of an incident wave which excites the polariton.

The Casimir interaction between two thin material films, between a film and a thick plate, and between two films deposited on substrates is considered at large separations (high temperatures) which correspond to the classical limit. It is shown that the free energy of the classical Casimir interaction between two insulating films with no free charge carriers and between an insulating film and a material plate depends on film thicknesses and decreases with separation more rapidly than the classical limit for two thick plates. The free energy of thin films characterized by the metallic-type dielectric permittivity decreases as the second power of separation, i.e., demonstrates the standard classical limit. The obtained results shed light on the possibility to describe dispersion interaction between two graphene sheets and between a graphene sheet and a material plate by modeling graphene as a thin film possessing some dielectric permittivity. It is argued that the most reliable results are obtained by describing the reflection properties on graphene by means of the polarization tensor in (2 + 1)-dimensional space-time.

Heavy-fermion physics deals with the ground state formation and interactions in $f$-electron materials where the electron effective masses are extremely large, more than 100 times the rest mass of an electron. The details of how the $f$ electrons correlate at low temperature to become so massive lacks a coherent theory, partially because so few materials display this “heavy” behavior and thus global trends remain unclear. UHg${}_3$ is now found experimentally to be a heavy-fermion antiferromagnet—just as are all the other U${}_x$M${}_y$ compounds with the metal $M$ being in column IIB (filled $d$-electron shells) in the periodic table (Zn/Cd/Hg) and the spacing between uranium ions $d_{\mathrm{U}\text{−}\mathrm{U}}$ being greater than the Hill limit of 3.5 Å. This result, that—independent of the structure of these U${}_x$M${}_y$, $M$ = Zn/Cd/Hg, compounds and independent of the value of their $d_{\mathrm{U}\text{−}\mathrm{U}}$ (ranging from 4.39 to 6.56 Å)—all exhibit heavy-fermion antiferromagnetism, is a clear narrowing of the parameters important for understanding the formation of this ground state. The sequence of antiferromagnetic transition temperatures $T_N$ of 9.7, 5.0, and 2.6 K for U${}_x$M${}_y$ as the metal $M$ varies down column IIB (Zn/Cd/Hg) indicates an interesting regularity for the antiferromagnetic coupling strength.