By employing ab initio molecular dynamics simulations at constant pressure we investigated the behavior of amorphous carbon dioxide between 0 and 100 GPa and 200 and 500 K. We focused on the evolution of the high-pressure polymeric amorphous form known as a-carbonia on its way down to zero pressure, where it eventually converts into a molecular state. During the simulations we observed a spectrum of amorphous forms between two limiting polymeric forms with different proportions of three- and four-coordinated carbon atoms. Besides that we also found a mixed molecular-polymeric form that shows pronounced metastability at certain conditions. The observed behavior suggests ${\mathrm{CO}}_2$ as a possible candidate for polyamorphism. We discuss the structural and physical properties of the observed amorphous forms as well as their relation to crystalline phases.

Recent data reported on magnetoelectric coupling parameter α for hexaferrite materials are very high; hence, these materials are expected to potentially impact future technologies in a major way. At this juncture, it is imperative to address the fundamental question, “What does α depend on?” in order to advance the design of new hexaferrite materials with even higher α. A complete mathematical formulation of a microscopic model would be rather complicated, especially for complex crystal structures such as the $Z$-type hexaferrites. Herein we present a simple, yet elegant thermodynamic argument by which we derive a functional relationship between α and the material parameters. We show that α is best described as a tensor proportional to the product of magnetostriction and piezoelectric strain tensors. Using this relationship, quantitative values for α are estimated and compared to experimentally measured values for various hexaferrites, composites, and ${\mathrm{Cr}}_2{\mathrm{O}}_3$.

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide $K_y{\mathrm{Fe}}_{1.6+x}{\mathrm{Se}}_2$. In the high temperature paramagnetic state, both materials have a single phase with a crystal structure similar to that of the BaFe${}_2$As${}_2$ family of iron pnictides. While the insulating $K_y{\mathrm{Fe}}_{1.6+x}{\mathrm{Se}}_2$ forms a $\sqrt{5}\times \sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5}\times \sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K${}_z$Fe${}_2$Se${}_2$ and BaFe${}_2$As${}_2$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K${}_z$Fe${}_2$Se${}_2$ in the matrix of the insulating block AF phase.

We investigate the effect of a direct current on propagating spin waves in a CoFeB/Ta bilayer structure. Using the micro-Brillouin light scattering technique, we observe that the spin-wave damping and amplitude may be attenuated or amplified depending on the direction of the current and the applied magnetic field. Our work suggests an effective approach for electrically controlling the propagation of spin waves in a magnetic waveguide and may be useful in a number of applications such as phase-locked nano-oscillators and hybrid information-processing devices.

Superconducting condensation energy $U_0^{\text{int}}$ has been determined by integrating the electronic entropy in various iron pnictide/chalcogenide superconducting systems. It is found that $U_0^{\text{int}}\propto T_c^n$ with $n=3$–4, which is in sharp contrast to the simple BCS prediction $U_0^{\text{BCS}}=1/2N_F{\Delta }_s^2$, with $N_F$ the quasiparticle density of states at the Fermi energy and ${\Delta }_s$ the superconducting gap. A similar correlation holds if we compute the condensation energy through $U_0^{\text{cal}}=3{\gamma }_n^{\text{eff}}{\Delta }_s^2/4{\pi }^2{k}_B^2$, with ${\gamma }_n^{\text{eff}}$ the effective normal state electronic specific heat coefficient. This indicates a general relationship ${\gamma }_n^{\text{eff}}\propto T_c^m$ with $m=1$–2, which is not predicted by the BCS scheme. A picture based on quantum criticality is proposed to explain this phenomenon.

We report ${}^{195}\mathrm{Pt-NMR}$ and ${}^{75}\mathrm{As}$ nuclear quadrupole resonance measurements for the locally noncentrosymmetric superconductor SrPtAs where the As-Pt layer breaks inversion symmetry while globally the compound is centrosymmetric. The nuclear spin-lattice relaxation rate $1/T_1$ shows a well-defined coherence peak below $T_c$ and decreases exponentially at low temperatures. The spin susceptibility measured by the Knight shift also decreases below $T_c$ down to $T<T_c/6$. These data, together with the penetration depth obtained from the NMR spectra, can be consistently explained by a spin-singlet superconducting state with a full gap. Our results suggest that the spin-orbit coupling due to the local inversion-symmetry breaking is not large enough to bring about an exotic superconducting state, or the interlayer hopping interaction is larger than the spin-orbit coupling.

Hydrogen solubility and interaction with vacancies and divacancies are investigated in 12 fcc metals by density functional theory. We show that in all studied fcc metals, vacancies trap H very efficiently and multiple H trapping is possible. H is stronger trapped by divacancies and even stronger by surfaces. We derive a condition for the maximum number of trapped H atoms as a function of the H chemical potential. Based on this criterion, the possibility of a dramatic increase of vacancy concentration (superabundant vacancy formation) in the studied metals is discussed.

Field cooling of a transition metal-rare earth (TM-RE) Fe/Tb-multilayer system is shown to form a double hysteresis loop with exchange-bias-like shifts along and opposite to the field cooling axis below the ordering temperature of the RE. The measurement of the polarized neutron reflectivity at various applied fields confirms an antiferromagnetic alignment between the individual layers of Fe and Tb associated with a significant value of the magnetic moment for the Tb layers, even at room temperature. We attribute the shifts of the hysteresis loops to the formation of 2$\pi$-domain walls by the interface moments that are pinned by the magnetically hard Tb layers forming bidomainlike states in this layered artificial ferrimagnetic system. We conclude that the exchange bias in Fe/Tb-multilayers, the RE layers being on either sides of the TM layers, is caused by the formation of 2$\pi$-domain walls in the Fe layers thus excluding an explanation in terms of $\pi$-domain walls, which are believed to be responsible for the exchange bias in other RE-TM bilayer systems.

The temperature dependence of the point defects in 6$H$-SiC induced by 12-MeV proton irradiation was studied by means of isochronal annealing followed by both positron annihilation spectroscopy and electron paramagnetic resonance measurements. The formation energies and positron lifetimes of various vacancy clusters were calculated to help in the interpretation of the experiments. The combination of the experiments and calculations enabled the identification of a negative silicon vacancy, with the lifetime of 218 ps, which is annealed between 400 ${}^{\circ }$C and 700 ${}^{\circ }$C. This process involves vacancy migration and formation of the $V_{\mathrm{C}}+V_{\mathrm{Si}}$ cluster, with a lifetime of 235 ps. In addition, our calculations confirm the identification of several clusters proposed in previous experimental studies.

We study surface plasmon polaritons on a topological insulator-vacuum interface. When the time-reversal symmetry is broken due to ferromagnetic coupling, the surface states exhibit a magneto-optical Kerr effect. This effect gives rise to a novel transverse-type surface plasmon polariton, in addition to the longitudinal type. In specific, these two types contain three different channels, corresponding to the pole of the determinant of the Fresnel reflection matrix. All three channels of the surface plasmon polaritons display tight confinement and a long lifetime and show strong light-matter coupling with a dipole emitter.

We use photon correlation measurements to study blinking in single, epitaxially grown self-assembled InAs quantum dots situated in circular Bragg grating and microdisk cavities. The normalized second-order correlation function $g^{\left(2\right)}\left(\tau \right)$ is studied across 11 orders of magnitude in time, and shows signatures of blinking over time scales ranging from tens of nanoseconds to tens of milliseconds. The $g^{\left(2\right)}\left(\tau \right)$ data is fit to a multilevel system rate equation model that includes multiple nonradiating (dark) states, from which radiative quantum yields significantly less than 1 are obtained. This behavior is observed even in situations for which a direct histogramming analysis of the emission time-trace data produces inconclusive results.

By varying the front-gate and the substrate voltages in a short silicon-on-insulator trigate field-effect transistor, we control the ionization state of three arsenic donors. We obtain good quantitative agreement between three-dimensional electrostatic simulations and experiment for the control voltages at which the ionization takes place. It allows us to observe the three doubly occupied states As${}^{-}$ at strong electric field in the presence of nearby source-drain electrodes.

We investigate paramagnetic metal-insulator transitions in the infinite-dimensional ionic Hubbard model at finite temperatures. By means of the dynamical mean-field theory with an impurity solver of the continuous-time quantum Monte Carlo method, we show that an increase in the interaction strength brings about a crossover from a band insulating phase to a metallic one, followed by a first-order transition to a Mott insulating phase. The first-order transition turns into a crossover above a certain critical temperature, which becomes higher as the staggered lattice potential is increased. Further, analysis of the temperature dependence of the energy density discloses that the intermediate metallic phase is a Fermi liquid. It is also found that the metallic phase is stable against strong staggered potentials even at very low temperatures.

Using a first-principles Green's function approach we study magnetic properties of the magnetic binary tetradymite chalcogenides ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$, and ${\mathrm{Sb}}_2{\mathrm{Te}}_3$. The magnetic coupling between transition-metal impurities is long range, extends beyond a quintuple layer, and decreases with increasing number of $d$ electrons per $3d$ atom. We find two main mechanisms for the magnetic interaction in these materials: the indirect exchange interaction mediated by free carriers and the indirect interaction between magnetic moments via chalcogen atoms. The calculated Curie temperatures of these systems are in good agreement with available experimental data. Our results provide deep insight into exchange interactions in magnetic binary tetradymite chalcogenides and open a way to design new materials for promising applications.

Finite-bias spectroscopy measurements of a three-terminal graphene quantum dot are presented. Numerous lines of enhanced differential conductance are observed outside the Coulomb diamonds. In the single-level transport regime such lines are often associated with transport through excited states. Here the system is in the multilevel transport regime. We argue that the lines are most likely a result of strong coupling to only a few of the excited states available in the bias window. We also discuss the option that fluctuations of the density of states in the leads are fully or partly responsible for the appearance of the lines. Such a detailed analysis requires the presence of three leads to the dot.

By using broadband linear and nonlinear dielectric spectroscopy we studied the magnetoelectric dynamics in the chiral antiferromagnet ${\mathrm{MnWO}}_4$. In the multiferroic phase the dielectric response is dominated by the dynamics of domains and domain walls which is strongly dependent on the stimulating electric field. The mean switching time reaches values in the minute range in the middle of the multiferroic temperature regime at $T\approx 10$ K but unexpectedly decays again on approaching the lower, first-order phase boundary at $T_{N1}\approx 7.6$ K. The switchability of the ferroelectric domains denotes a pinning-induced threshold and can be described considering a growth-limited scenario with an effective growth dimension of $d\approx 1.8$. The rise of the effective dynamical coercive field on cooling below the $T_{N2}$ is much stronger compared to the usual ferroelectrics and can be described by a power law $E_c\propto {\nu }^{1/2}$. The latter questions the feasibility of fast-switching devices based on this type of material.

$\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$, a $p$-type metallic conductor with a small thermopower, and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$, a $p$-type insulator with a large thermopower in its hyperstoichiometric state, have been reported to make a complete solid solution. It is, thus, suggested that the thermoelectric power factor, conductivity times thermopower squared (σθ), may be best optimized by mixing these two extreme oxides. In order to explore this possibility, we examined σ and θ of the solution $\mathrm{Sr}\left({\mathrm{Ru}}_{1–x}{\mathrm{Ti}}_x\right){\mathrm{O}}_3$ against composition (0 $\le$ $x$ $\le$ 1), temperature (300 $\le$ T/K < 1000) and oxygen activity ($-4\le log{a}_{O_2}<0$), and subsequently its crystallographic structure against $x$ at room temperature. It has been found that, contrary to literature, there falls a miscibility gap over the composition range 0.1 < $x$ < 0.6. While there occurs a metal-to-insulator transition crossing the miscibility gap, θ remains little dependent on $T$ and $a_{O_2}$ for the compositions up to $x$ = 0.9. Unexpectedly, however, θ changes its sign twice with increasing $x$, indicating the majority carrier type changes from $p$ to $n$ at a composition in 0.6 < $x$ < 0.9 and back to $p$ at a composition in 0.9 < $x$ < 1.0, whereas σ decreases monotonically. The highest power factor is observed, contrary to the expectation, at $x$ = 0 (pure $\mathrm{Sr}\mathrm{Ru}{\mathrm{O}}_3$) on the order of 10${}^{-4}$ W/mK${}^2$. The origin of the repeated carrier-type changes is discussed in terms of electronic-structure change.

In MnSi thin films the magnetic properties of the B20 compound are influenced by induced uniaxial anisotropy. In comparison to bulk MnSi the critical magnetic fields are enhanced and the Skyrmion phase is found to be enlarged within the magnetic phase diagram. Furthermore the ordering temperature depends on the film thickness reaching 43 K for films of around 10 nm and is considerably higher than in bulk crystals ($T_{c,\mathrm{bulk}}=29$ K). In bulk MnSi the ordering temperature can be reduced by pressure, where at 1.46 GPa the magnetic order is completely suppressed and a non-Fermi-liquid behavior characterized by a $T^{3/2}$ law of the resistivity is observed. We present resistance measurements on MnSi thin films under applied pressure of up to 3.44 GPa. Qualitatively, the behavior is similar to bulk MnSi. However, the critical pressure is considerably enhanced to 3.1 GPa, which is assumed to be a consequence of strain. At high pressure non-Fermi-liquid behavior evidenced by a $T^{3/2}$ behavior of the resistance is observed up to $T_{\mathrm{lin}}=30$ K, i. e., in a larger temperature range than for bulk MnSi. Uniaxial anisotropy might play an important role in this breakdown of Fermi-liquid behavior, since it stabilizes nontrivial spin structures.

The magnetic entropy change ($\Delta {\mathrm{S}}_{\mathrm{M}}$) in a single crystal of the geometrically frustrated spin chain system ${\mathrm{Ca}}_3{\mathrm{Co}}_2{\mathrm{O}}_6$ has been determined under the influence of a wide range of temperatures and magnetic field variations. Our findings are consistent with the spin-density wave description of the zero field order, and with the suppression of the modulated state at low field. Metamagnetic transitions to the ferrimagnetic up-up-down configuration and full ferromagnetic alignment are observed upon the application of moderate magnetic fields in the $c$ direction. At low temperatures, an increase in $\Delta {\mathrm{S}}_{\mathrm{M}}$ supports the presence of short-range magnetic correlations coexisting with long-range order up to large fields. Depending on the temperature regime, local maxima or minima in $\Delta {\mathrm{S}}_{\mathrm{M}}\left(H\right)$ can be found within the ferrimagnetic phase, which are identified with intermediate configurations among the chains. A new magnetic phase diagram has been constructed from the magnetic field and temperature dependence of magnetic entropy change.