We report large negative electron affinity (NEA) on diamond (100) using magnesium adsorption on a previously oxygen-terminated surface. The measured NEA is up to $(-2.01\pm 0.05)$ eV, the largest reported negative electron affinity to date. Despite the expected close relationship between the surface chemistry of Mg and Li species on oxygen-terminated diamond, we observe differences in the adsorption properties between the two. Most importantly, a high-temperature annealing step is not required to activate the Mg-adsorbed surface to a state of negative electron affinity. Diamond surfaces prepared by this procedure continue to possess negative electron affinity after exposure to high temperatures, air, and even immersion in water.

We perform detailed magnetotransport studies on two-dimensional electron gases (2DEGs) formed in undoped Si/SiGe heterostructures in order to identify the electron mobility limiting mechanisms. By analyzing data from 26 different heterostructures, we observe a strong correlation between the background oxygen concentration in the Si quantum well and the maximum mobility. The highest-quality wafer supports a 2DEG with mobility $\mu =160000 \mathrm{cm}{}^2/\mathrm{Vs}$ at a density $n=2.17\times {10}^{11}/\mathrm{cm}{}^2$ and exhibits a metal-to-insulator transition at a critical density $n_c=0.46\times {10}^{11}/\mathrm{cm}{}^2$. We extract a valley splitting ${\mathrm{\Delta }}_v\sim 150\mu \mathrm{eV}$ at a magnetic field $B=1.8$ T. These results provide evidence that undoped Si/SiGe heterostructures are suitable for the fabrication of few-electron quantum dots.

We report on pure-quantum-state polariton condensates in optical annular traps. The study of the underlying mechanism reveals that the polariton wave function always coalesces in a single pure quantum state that, counterintuitively, is always the uppermost confined state with the highest overlap with the exciton reservoir. The tunability of such states combined with the short polariton lifetime allows for ultrafast transitions between coherent mesoscopic wave functions of distinctly different symmetries, rendering optically confined polariton condensates a promising platform for applications such as many-body quantum circuitry and continuous-variable quantum processing.

Grain boundaries are commonly observed in carbon nanostructures, but their influence on thermal and electric properties is still not completely understood. Using a combined approach of density functional tight-binding theory and nonequilibrium Green functions we investigate electron and phonon transport in carbon-based systems. In this work, quantum transport and thermoelectric properties are summarized for graphene sheets, graphene nanoribbons, and carbon nanotubes with a variety of grain boundary types in a wide temperature range. Motivated by previous findings that disorder scatters phonons more effectively than electrons, a significant improvement in the thermoelectric performance for polycrystalline systems is expected. As the effect is marginally sensitive to the grain boundary type, we demonstrate that grain boundaries are a viable tool to greatly enhance the figure of merit, paving the way for the design of new thermoelectric materials.

We analyze the scaling theory of two-dimensional metallic electron systems in the presence of critical bosonic fluctuations with small wave vectors, which are either due to a $\text{U}\left(1\right)$ gauge field or generated by an Ising nematic quantum critical point. The one-loop dynamical exponent $z=3$ of these critical systems was shown previously to be robust up to three-loop order. We show that the cancellations preventing anomalous contributions to $z$ at three-loop order have special reasons, such that anomalous dynamical scaling emerges at four-loop order.

We have investigated the electronic structure of ${\mathrm{BiS}}_2$-based ${\mathrm{CeO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ superconductor using polarization-dependent angle-resolved photoemission spectroscopy (ARPES), and succeeded in elucidating the orbital characters on the Fermi surfaces. In the rectangular Fermi pockets around the $X$ point, the straight portion parallel to the $k_y$ direction is dominated by Bi $6p_x$ character. The orbital polarization indicates the underlying quasi-one-dimensional electronic structure of the ${\mathrm{BiS}}_2$ system. Moreover, distortions on tetragonally aligned Bi could give rise to the band Jahn-Teller effect.

The nesting of the Fermi surfaces of an electron and a hole pocket separated by a nesting vector $\mathbf{Q}$ and the interaction between electrons gives rise to itinerant antiferromagnetism. The order can gradually be suppressed by mismatching the nesting and a quantum critical point is obtained as the Néel temperature tends to zero. If the vector $\mathbf{Q}$ is commensurate with the lattice (umklapp with $\mathbf{Q}=\mathbf{G}/2$), pairs of electrons can be transferred between the pockets and a superconducting dome above the quantum critical point may arise. If the vector $\mathbf{Q}$ is not commensurate with the lattice, there are eight phases that need to be considered: commensurate and incommensurate spin and charge density waves and four superconductivity phases, two of them with a modulated order parameter of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) type. The renormalization group equations are studied and numerically integrated. The phase diagram is obtained as a function of the mismatch of the Fermi surfaces and the magnitude of $|\mathbf{Q}-\mathbf{G}/2|$.

Scanning tunneling microscopy has been used to real-time study the growth of silicene on $\mathrm{Ag}\left(111\right)$ in the 200–533-K temperature range. We show that the growth mode depends strongly on the deposition temperature $T$. At $T=300\mathrm{K}$ and above, the formation of silicene results from the exchange between surface Ag atoms and Si atoms, which are inserted in the substrate top layer. Density functional theory calculations confirm that Si insertion is thermodynamically favored, and we propose an energetic model for explaining the observed growth processes as a function of $T$. For $T\ge 400\mathrm{K}$, ordered structures are observed as soon as silicene domains are large enough. With increasing coverage, disordered and dotted phases progressively transform into stable $(\sqrt{13}\times \sqrt{13})R13.9^{\circ }$ and $(4\times 4)$ structures.

We derive the kinetic equations for a hopping transport that take into account an electron spin and the possibility of double occupation. In the Ohmic regime, the equations are reduced to the generalized Miller-Abrahams resistor network. We apply these equations to the problem of the magnetic moment relaxation due to the interaction with the random hyperfine fields. It is shown that in a wide range of parameters the relaxation rate is governed by the hops with the similar rates as spin precession frequency. It is demonstrated that at the large time scale spin relaxation is nonexponential. We argue that the nonexponential relaxation of the magnetic moment is related to the spin of electrons in the slow-relaxing traps. Interestingly, the traps can significantly influence the spin relaxation in the infinite conducting cluster at large times.

The magnetic-field dependence of the longitudinal spin Seebeck effect (LSSE) in a $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ (YIG)-slab junction system was found to deviate from a bulk magnetization curve of the YIG slab in a low field range. In this paper, we show that the deviation originates from the difference between surface and bulk magnetization processes in the YIG slab and that it appears even when removing possible extrinsic magnetic anisotropy due to surface roughness and replacing the Pt layer with different materials. This result indicates that the anomalous field dependence of the LSSE is due to an intrinsic magnetic property of the YIG surface. Our numerical calculation based on the Landau-Lifshitz-Gilbert equation shows that the deviation between the LSSE and bulk magnetization curves is qualitatively explained by introducing easy-axis perpendicular magnetic anisotropy near the surface of YIG.

Strontium intercalation between van der Waals bonded layers of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ is found to induce superconductivity with a maximum $T_c$ of 2.9 K. Transport measurement on a single crystal of the optimally doped sample ${\mathrm{Sr}}_{0.1}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ shows weak anisotropy $(\mathrm{\Gamma }\sim 1.5)$ and an upper critical field $H_{c2}\left(0\right)$ equal to 2.1 T for a magnetic field applied perpendicular to the $c$ axis of the sample. The Ginzburg-Landau coherence lengths are found to be ${\xi }_{ab}=15.3$ nm and ${\xi }_c=10.2$ nm. The lower critical field $H_{c1,ab}\left(0\right)$ and zero temperature penetration depth ${\lambda }_{ab}\left(0\right)$ are estimated to be $0.39\pm 0.02$ mT and $1221\pm 36$ nm, respectively. Hall and Seebeck measurements confirm the dominance of electronic conduction, and the carrier concentration is surprisingly low $(n=1.85\times {10}^{19}{\mathrm{cm}}^{-3})$ at 10 K, indicating the possibility of unconventional superconductivity.

A range of compelling information on the local structure and dynamics of the ferroelectric perovskite-type $\left({\mathrm{ABO}}_3\right)$ system $(1-x){\mathrm{PbTiO}}_3-x{\mathrm{BiScO}}_3$ has been revealed through analyzing room-temperature x-ray pair distribution functions and temperature-dependent Raman scattering data for compositions ranging across the morphotropic phase boundary (MPB). Raman scattering data have provided exclusive evidence for distinct dynamical responses from the A-site Pb/Bi and the B-site Ti/Sc cations as a function of composition and temperature. Both pair distribution function and Raman scattering analyses indicate improved coherence between the A-site and B-site off-center displacements for $x$ above the MPB composition. The distinguishable dynamical features were spotted between two sets of compositional range from which $x=0.34$ could be assigned as a critical composition of “local” MPB: the composition at which the pathway of mesoscopic-scale temperature-driven coupling and transformation processes changes. This differs from the previous reports based on the analysis of the average structure, suggesting the response function of a system may not necessarily follow the evolution of the average structure from which however the MPB has so far been ascribed for a particular system. The individual features of the cations revealed here will further help in understanding the structure-property correlations for similar ferroelectric solid solutions.

We develop a hydrodynamic description of the collective modes of interacting liquids in a quasi-one-dimensional confining potential. By solving Navier-Stokes equation we determine analytically the excitation spectrum of sloshing oscillations. For parabolic confinement, the lowest frequency eigenmode is not renormalized by interactions and is protected from decay by the Kohn theorem, which states that center of mass motion decouples from internal dynamics. We find that the combined effect of potential anharmonicity and interactions results in a frequency shift and final lifetime of the Kohn mode. All other excited modes of sloshing oscillations thermalize with the parametrically faster rate. Our results are significant for the interpretation of recent experiments with trapped Fermi gases that observed a weak violation of the Kohn theorem.

A theoretical method is used to find, in an accurate and systematic way, all the possible equilibrium magnetic states of a finite-size one-dimensional chain of classical planar spins, interacting via ferromagnetic nearest-neighbor symmetric exchange and subjected to a uniaxial anisotropy which favors the orientation of the spins along an easy axis within the plane perpendicular to the chain. For a chain with $N$ spins, the energy is a function of $N$ absolute orientation angles $({\theta }_i$, $i=1,...,N)$ and, owing to the absence of energy competition, the ground state is collinear ferromagnetic and doubly degenerate (either ${\theta }_i=0$ $\forall i$, or ${\theta }_i=\pi$ $\forall i$). The ground state corresponds to a global minimum of the energy in the $N$-dimensional space, while metastable states correspond to local minima. As a function of $N$, we have numerically determined the minimal value ${\gamma }_{\min }$ to be assumed by the ratio $\gamma$ between the anisotropy and the exchange constant for the first noncollinear metastable state to appear. For $N\le 13$, all the noncollinear equilibrium spin configurations of the system were calculated with great accuracy at two different values, $\gamma =0.18$ and 0.21. A stability analysis was subsequently performed, showing that metastable noncollinear states can appear in the model even in the absence of energy competition. The various kinds of metastable states can be classified, in order of increasing energy, on the basis of the increasing number of $\pi$ walls (including both domains and antidomains) which can be inserted into the discrete finite chain with open ends when $N$ and/or $\gamma$ are increased. The effects of open boundary conditions and finite size on the Peierls-Nabarro barrier (i.e., the difference between the energy of an unstable onsite-centered domain wall and the energy of a stable intersite-centered domain wall) were also investigated.

We present a theoretical framework for evaluating effective interactions between localized spins mediated by itinerant electrons in double-exchange models. Performing the expansion with respect to the spin-dependent part of the electron hopping terms, we show a systematic way of constructing the effective spin model in the large Hund's coupling limit. As a benchmark, we examine the accuracy of this method by comparing the results with the numerical solutions for the spin-ice type model on a pyrochlore lattice. We also discuss an extension of the method to the double-exchange models with Heisenberg and $XY$ localized spins.

High-aspect-ratio Fe nanostrips are studied with real-space micromagnetic imaging methods. We experimentally demonstrate reversible switching from essentially homogeneous single-domain states at room temperature to multidomain diamond states at elevated temperature. This temperature-dependent magnetic bistability can be understood and modeled by accounting for the temperature dependence of the magnetocrystalline, shape, and magnetoelastic anisotropies. These results show how the transition temperature between two magnetic domain states can be tailored by controlling epitaxial strain and particle geometry, which may generate new opportunities for magnetic memory and logic device design.

Using first-principles density functional theory calculations, a systematic study of the lattice dynamics and related (e.g., dielectric and anharmonic) properties of BiOCuSe (bismuth-copper oxyselenide), along with a comparison with its isostructural analog LaOCuSe, is performed to find the origin of the ultralow thermal conductivity $\kappa$ in BiOCuSe. From the marked differences in some of these properties of the two materials, the reasons why BiOCuSe is a better thermal insulator than LaOCuSe are elucidated. For this class of oxychalcogenide thermoelectrics, phonon frequencies with symmetries, characters, spectroscopic activities, displacement patterns, and pressure coefficients of different zone-center modes, dielectric constants, dynamical charges, and phonon and Grüneisen dispersions are also determined.

The spectra of plasma and magnetoplasma excitations in a two-dimensional system of anisotropic heavy fermions are investigated. The spectrum of microwave absorption by disklike samples of stressed AlAs quantum wells at low electron densities shows two plasma resonances separated by a frequency gap. These two plasma resonances correspond to electron mass principle values of $(1.10\pm 0.05)m_0$ and $(0.20\pm 0.01)m_0$. The observed results correspond to the case of a single valley strongly anisotropic Fermi surface. It is established that an increase in electron density results in the population of the second valley, manifesting itself as a drastic modification of the plasma spectrum. We directly determine the electron densities in each valley and the intervalley splitting energy from the ratio of the two plasma frequencies.