A theory of spin fluctuations of localized electrons interacting with an optically cooled nuclear spin bath has been developed. Since nuclear spin temperature may stay low enough for a macroscopically long time, the effect of dynamic nuclear polarization on electron spin dynamics can be directly revealed by means of spin noise spectroscopy. We have shown that in the case of weak fields/relatively high nuclear spin temperature, a small degree of nuclear spin polarization affects the electron spin fluctuations in the same way as an additional external magnetic field. By contrast, the high degree of nuclear polarization realized in relatively strong magnetic field and at low nuclear spin temperature leads to a suppression of hyperfine field fluctuations and to a dramatic narrowing of the precession-induced peak in the spin noise spectrum. The experimental possibilities of nuclear spin system investigation by means of spin noise spectroscopy are discussed.

Understanding the ionic conduction in solid electrolytes in contact with electrodes is vitally important to many applications, such as lithium ion batteries. The problem is complex because both the internal properties of the materials (e.g., electronic structure) and the characteristics of the externally contacting phases (e.g., voltage of the electrode) affect defect formation and transport. In this paper, we developed a method based on density functional theory to study the physics of defects in a solid electrolyte in equilibrium with an external environment. This method was then applied to predict the ionic conduction in lithium fluoride (LiF), in contact with different electrodes which serve as reservoirs with adjustable Li chemical potential $\left({\mu }_{\mathrm{Li}}\right)$ for defect formation. LiF was chosen because it is a major component in the solid electrolyte interphase (SEI) formed on lithium ion battery electrodes. Seventeen possible native defects with their relevant charge states in LiF were investigated to determine the dominant defect types on various electrodes. The diffusion barrier of dominant defects was calculated by the climbed nudged elastic band method. The ionic conductivity was then obtained from the concentration and mobility of defects using the Nernst-Einstein relationship. Three regions for defect formation were identified as a function of ${\mu }_{\mathrm{Li}}$: (1) intrinsic, (2) transitional, and (3) $p$-type region. In the intrinsic region (high ${\mu }_{\mathrm{Li}}$, typical for LiF on the negative electrode), the main defects are Schottky pairs and in the $p$-type region (low ${\mu }_{\mathrm{Li}}$, typical for LiF on the positive electrode) are Li ion vacancies. The ionic conductivity is calculated to be approximately ${10}^{-31}\text{S}{\text{cm}}^{-1}$ when LiF is in contact with a negative electrode but it can increase to ${10}^{-12}\text{S}{\text{cm}}^{-1}$ on a positive electrode. This insight suggests that divalent cation (e.g., ${\text{Mg}}^{2+}$) doping is necessary to improve Li ion transport through the engineered LiF coating, especially for LiF on negative electrodes. Our results provide an understanding of the influence of the environment on defect formation and demonstrate a linkage between defect concentration in a solid electrolyte and the voltage of the electrode.

Recent experimental and theoretical interest in the superconducting phase of the heavy-fermion material ${\mathrm{URu}}_2{\mathrm{Si}}_2$ has led to a number of proposals in which the superconducting order parameter breaks time-reversal symmetry (TRS). In this study we measure the polar Kerr effect (PKE) as a function of temperature for several high-quality single crystals of ${\mathrm{URu}}_2{\mathrm{Si}}_2$. We find an onset of PKE below the superconducting transition that is consistent with a TRS-breaking order parameter. This effect appears to be independent of an additional, possibly extrinsic, PKE generated above the hidden order transition at $T_{\text{HO}}=17.5$ K, and contains a structure below $T_c$ suggestive of additional physics within the superconducting state.

Single crystal neutron diffraction is combined with synchrotron x-ray scattering to identify the different superlattice phases present in ${\text{Cs}}_{0.8}{\text{Fe}}_{1.6}{\text{Se}}_2$. A combination of single crystal refinements and first principles modeling are used to provide structural solutions for the $\sqrt{5}\times \sqrt{5}$ and $\sqrt{2}\times \sqrt{2}$ superlattice phases. The $\sqrt{5}\times \sqrt{5}$ superlattice structure is predominantly composed of ordered Fe vacancies and Fe distortions, whereas the $\sqrt{2}\times \sqrt{2}$ superlattice is composed of ordered Cs vacancies. The Cs vacancies only order within the plane, causing Bragg rods in reciprocal space. By mapping x-ray diffraction measurements with narrow spatial resolution over the surface of the sample, the structural domain pattern was determined, consistent with the notion of a majority antiferromagnetic $\sqrt{5}\times \sqrt{5}$ phase and a superconducting $\sqrt{2}\times \sqrt{2}$ phase.

Among composite materials that layer constituent substances of nanoscale thicknesses, ${\left[\right(\mathrm{SnSe})}_{1+y}{\mathrm{]}}_m\left({\mathrm{VSe}}_2\right){}_n$ emerges as an example where the constituents retain incommensurate lattice structures. Perpendicular to the stacking direction, the system exhibits random translations and random rotations on average, i.e., turbostratic disorder, with local regions showing twelvefold diffraction patterns. Earlier theoretical work on these structures showed that combining density functional theory with an empirical treatment of the van der Waals interaction gave structural parameters in good agreement with experiment, but no attempt was made to examine the relative orientations. Here we approximate the extended system with one extended constituent and one finite constituent, which allows the treatment of all relative orientations on equal footing. The calculations show how the twelvefold periodicity follows from how the ions of the SnSe layer lock in with favored positions relative to the ${\mathrm{VSe}}_2$ layer, and the associated energy scale supports arguments for the overall turbostratic disorder. The success of this approximation in describing the structural parameters of the extended ${\left[\right(\mathrm{SnSe})}_{1+y}{\mathrm{]}}_m\left({\mathrm{VSe}}_2\right){}_n$ system encourages its use for other properties and for other similar systems with other chemistries.

Atomistic spin model simulations are immensely useful in determining temperature-dependent magnetic properties but are known to give the incorrect dependence of the magnetization on temperature compared to experiment owing to their classical origin. We find a single-parameter rescaling of thermal fluctuations which gives quantitative agreement of the temperature-dependent magnetization between atomistic simulations and experiment for the elemental ferromagnets Ni, Fe, Co, and Gd. Simulating the subpicosecond magnetization dynamics of Ni under the action of a laser pulse, we also find quantitative agreement with experiment in the ultrafast regime. This enables the quantitative determination of temperature-dependent magnetic properties, allowing for accurate simulations of magnetic materials at all temperatures.

We consider physical properties of a superconductor with a recently proposed type of odd-frequency pairing that exhibits diamagnetic Meissner response (“odd-dia state”). Such a state was suggested in order to address stability issues arising in an odd-frequency superconducting state with paramagnetic Meissner response (“odd-para state”). Assuming the existence of an odd-dia state (due to a proper retarded interaction), we study its coexistence with an odd-para state. The latter is known to be generated as an induced superconducting component in, e.g., singlet superconductor/ferromagnet proximity structures or triplet superconductor/normal metal systems. Calculating the superfluid density of the mixed odd-para/odd-dia state and the Josephson current between the odd-para and odd-dia states, we find that the expressions for the currents in both cases have nonvanishing imaginary contributions and are therefore unphysical. We show that a realization of the odd-dia state implies the absence of a Hamiltonian description of the system, and suggest that there exists no physically realizable perturbation that could give rise to the spontaneous symmetry breaking necessary for an actual realization of the odd-dia superconducting state.

Motivated by recent experimental measurements, we study the quasiparticle spectra and the collective excitations in doped ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, in which the interesting interplay between the electronic correlations and strong spin-orbital coupling (SOC) exists. To include the SOC, we use the Hugenholtz diagrams to extend the fluctuation exchange (FLEX) approach to the case where the SU(2) symmetry can be broken. By using this generalized FLEX method, we find a weak pseudogap behavior near $(\pi ,0)$ in the slightly electron-doped system, with the corresponding Fermi arc formed by the partial destruction of Fermi surface. Similar features also appear in the hole-doped system; however, the position of the Fermi arc is rotated ${45}^{\circ }$ with respect to the former. These results are consistent with the recent angle-resolved photoemission spectra in ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. We suggest that these anomalous phenomena are caused by the scatterings of quasiparticles off the isospin fluctuation derived from the effective $J_{\text{eff}}=1/2$ doublet.

The interplay of nonlocal Coulomb repulsion and Hund's coupling in the $d$-orbital manifold in frustrated triangular lattices is analyzed by a multiband extended Hubbard model. We find a rich phase diagram with several competing phases, including a robust pinball liquid phase, which is an unconventional metal characterized by threefold charge order, bad metallic behavior, and the emergence of high-spin local moments. Our results naturally explain the anomalous charge-ordered metallic state observed in the triangular layered compound ${\mathrm{AgNiO}}_2$. The potential relevance to other triangular transition-metal oxides is discussed.

Superconducting ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ has been of current interest due to indications of a characteristic phase transition associated with structural distortions in its normal state. To further shed light on the nature of the phase transition, we performed a detailed study of single crystalline ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ by means of the thermal expansion, electrical resistivity, Hall coefficient, Seebeck coefficient, thermal conductivity, as well as ${}^{119}\mathrm{Sn}$ nuclear magnetic resonance (NMR) measurements, mainly focusing on the signatures around the phase transition temperature $T^{*}=137$ K. The phase transition has been characterized by marked features near $T^{*}$ in all measured physical quantities. In particular, the NMR characteristics provide microscopic evidence for the reduction in the electronic Fermi-level density of states (DOSs) below $T^{*}$. Based on the analysis of the ${}^{119}\mathrm{Sn}$ NMR spin-lattice relaxation rate, we clearly demonstrated that the Sn $5s$ partial Fermi-level DOS in ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ is reduced by 13% across the phase transition. In this respect, it points to the strong association between electronic and structural instability for the peculiar phase transition in ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$.

We generalize the screened Korringa-Kohn-Rostoker method for solving the corresponding Kohn-Sham-Bogoliubov–de Gennes equations for surfaces and interfaces. As an application of the theory, we study the quasiparticle spectrum of Au overlayers on a Nb(100) host. We find that within the superconducting gap region, the quasiparticle spectrum consists of Andreev bound states with a dispersion which is closely connected to the underlying electronic structure of the overlayer. We also find that the spectrum has a strongly $k$-dependent induced gap. The properties of the gap are discussed in relation to the thickness of the overlayer, and it is shown that certain states do not participate in the Andreev scattering process.

Codoping of Ce-doped ${\text{LaBr}}_3$ with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes this enhancement on the basis of first-principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral $V_{\text{Br}}\text{−}{\text{Sr}}_{\text{La}}$ complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different ${\text{Ce}}_{\text{La}}\text{−}{\text{Sr}}_{\text{La}}\text{−}{V}_{\text{Br}}$ triple complex configurations. Codoping with other alkaline as well as alkaline-earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline-earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline-earth elements (Ca, Sr, Ba) combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that codoping of wide-gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but also the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this mechanism are therefore not specific to the material considered here but can be adapted for controlling charge carrier populations and recombination in other wide-gap materials.

We have measured the electric signal downconverted from a terahertz frequency by an unbiased high mobility two-dimensional electron-gas (2DEG) device. The 2DEG was confined in an asymmetric plasmonic microcavity, and the radiation frequency was continuously tuned in the 0.2–0.4 THz range. The presence of resonant peaks at three frequencies corresponding to three plasma oscillation modes of the ungated 2DEG clearly points to the intrinsic nature of the hydrodynamic nonlinearity responsible for the downconversion as opposed to previously proposed plasmonic cavity configurations where the 2DEG oscillates under the metal gate that also acts as the source of the nonlinearity.

One important feature of surface states in topological insulators is the so-called “spin-momentum locking,” which means that electron spin is oriented along a fixed direction for a given momentum and forms a texture in the momentum space. In this work, we study spin textures of two typical topological insulators in Hg-based chalcogenides, namely, HgTe and HgS, based on both the first-principles calculation and the eight-band Kane model. We find opposite helicities of spin textures between these two materials, originating from the opposite signs of spin-orbit couplings. Based on the effective Kane model, we present a physical picture to understand opposite spin textures in these two materials with the help of the relationship between spin textures and mirror Chern numbers. We also reveal the existence of gapless states at the interface between HgTe and HgS due to the opposite spin textures and opposite mirror Chern numbers.

Investigation of the wavelength dependence (725–1025 nm) of the threshold for nanosecond optical breakdown in water revealed steps consistent with breakdown initiation by multiphoton ionization, with an initiation energy of about 6.6 eV. This value is considerably smaller than the autoionization threshold of about 9.5 eV, which can be regarded as band gap relevant for avalanche ionization. Breakdown initiation is likely to occur via excitation of a valence band electron into a solvated state, followed by rapid excitation into the conduction band. Theoretical analysis based on these assumptions suggests that the seed electron density required for initiating avalanche ionization amounts to $2.5\times {10}^{15}\mathrm{c}{\mathrm{m}}^{-3}$ at $725\mathrm{nm}$ and drops to $1.1\times {10}^{12}\mathrm{c}{\mathrm{m}}^{-3}$ at 1025 nm. These results demand changes of future breakdown modeling for water, including the use of a larger band gap than previously employed, the introduction of an intermediate energy level for initiation, and consideration of the wavelength dependence of seed electron density.

Neutron scattering from high-quality ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.33}\left(\text{YBCO6.33}\right)$ single crystals with a $T_c$ of 8.4 K shows no evidence of a coexistence of superconductivity with long-range antiferromagnetic order at this very low, near-critical doping of $p\sim 0.055$. However, we find short-range three-dimensional spin correlations that develop at temperatures much higher than $T_c$. Their intensity increases smoothly on cooling and shows no anomaly that might signify a Néel transition. The system remains subcritical with spins correlated over only one and a half unit cells normal to the planes. At low energies, the short-range spin response is static on the microvolt scale. The excitations out of this ground state give rise to an overdamped spectrum with a relaxation rate of 3 meV. The transition to the superconducting state below $T_c$ has no effect on the spin correlations. The elastic interplanar spin response extends over a length that grows weakly but fails to diverge as doping is moved towards the superconducting critical point. Any antiferromagnetic critical point likely lies outside the superconducting dome. The observations suggest that conversion from Néel long-range order to a spin-glass texture is a prerequisite to formation of paired superconducting charges. We show that while $p_c=0.052$ is a critical doping for superconducting pairing, it is not for spin order.

In contrast to the extensive study of domain reversal in ferromagnetic materials, the domain switching process in antiferromagnets is much less studied due to the difficulty of probing antiferromagnetic spins. Using a combination of hysteresis loop, Kerr microscope, and x-ray magnetic linear dichroism measurements, we investigated the antiferromagnetic (AFM) domain switching process in single crystalline Fe/CoO bilayers on MgO(001). We demonstrate that the CoO AFM switching is a Kolmogorov-Avrami process in which the thermal activation energy creates AFM domain nucleation centers which further expand by domain wall propagation. From the temperature- and thickness-dependent measurements, we are able to retrieve quantitatively the important parameter of the CoO AFM activation energy, which is shown to increase linearly with CoO thickness.