A higher-angular-momentum ($\ell =2$) $d$ density wave, a mixed triplet and a singlet, interestingly, admits skyrmionic textures. The skyrmions carry charge $2e$ and can condense into a spin-singlet $s$-wave superconducting state. In addition, a charge current can be induced by a time-dependent inhomogeneous spin texture, leading to quantized charge pumping. The quantum phase transition between this mixed triplet $d$ density wave and the skyrmionic superconducting condensate likely leads to deconfined quantum critical points. We suggest connections of this exotic state to electronic materials that are strongly correlated, such as the heavy-fermion ${\mathrm{URu}}_2{\mathrm{Si}}_2$. At the very least, we provide a concrete example in which topological order and broken symmetry are intertwined, which can give rise to non-BCS superconductivity.

The Hubbard model with large orbital degeneracy has recently gained relevance in the context of ultracold earth alkali-like atoms. We compute its static properties in the SU($2M$) symmetric limit for up to $M=8$ bands at half filling within dynamical mean-field theory, using the numerically exact multigrid Hirsch-Fye quantum Monte Carlo approach. Based on these unbiased data, we establish scaling laws which predict the phase boundaries of the paramagnetic Mott metal-insulator transition at arbitrary orbital degeneracy $M$ with high accuracy.

We investigate electron transport across a complex oxide heterointerface of La${}_{0.67}$Sr${}_{0.33}$MnO${}_3$ (LSMO) on Nb:SrTiO${}_3$ (Nb:STO) at different temperatures. For this, we employ the conventional current-voltage method as well as the technique of ballistic electron emission microscopy (BEEM), which can probe lateral inhomogeneities in transport at the nanometer scale. From current-voltage measurements, we find that the Schottky barrier height (SBH) at the LSMO/Nb:STO interface decreases at low temperatures accompanied by a larger than unity ideality factor. This is ascribed to the tunneling dominated transport caused by the narrowing of the depletion width at the interface. However, BEEM studies of such unbiased interfaces do not exhibit SBH lowering at low temperatures, implying that this is triggered by the modification of the interface due to an applied bias and is not an intrinsic property of the interface. Interestingly, the SBH at the nanoscale, as extracted from BEEM studies, at different locations in the device is found to be spatially homogeneous and similar both at room temperature and at low temperatures. Our results highlight the application of BEEM in characterizing electron transport and its homogeneity at such unbiased complex oxide interfaces and yield insights into the origin of the temperature dependence of the SBH at biased interfaces.

Building on the many existing algorithms for calculating the dc transport properties of quantum tight-binding models, we develop a systematic approach that expresses finite-frequency observables in terms of the stationary Green's function of the system, i.e., the natural output of most dc numerical codes. Our framework allows one to extend the simulation capabilities of existing codes to a large class of observables including, for instance, ac conductance, quantum capacitance, quantum pumping, spin pumping, or photoassisted shot noise. The theory is developed within the framework of Keldysh formalism and we provide explicit links with the alternative (and equivalent) scattering approach. We illustrate the formalism with a study of the ac conductance in a quantum point contact and an electronic Mach-Zehnder interferometer in the quantum Hall regime.

We have carried out first-principles simulations, based on density functional theory, to obtain the atomic and electronic structure of (001) BaTiO${}_3$/KNbO${}_3$ interfaces in an isolated slab geometry. We tried different types of structures including symmetric and asymmetric configurations and variations in the thickness of the constituent materials. The spontaneous polarization of the layer-by-layer non-neutral material (KNbO${}_3$) in these interfaces cancels out almost exactly the “built-in” polarization responsible for the electronic reconstruction. As a consequence, the so-called polar catastrophe is quenched and all the simulated interfaces are insulating. A model, based on the modern theory of polarization and basic electrostatics, allows an estimation of the critical thickness for the formation of the two-dimensional electron gas between 33 and 36 KNbO${}_3$ unit cells. We also demonstrate the presence of an unexpected in-plane polarization in BaTiO${}_3$ localized at the $p$-type TiO${}_2$/KO interface, even under in-plane compressive strains. We expect this in-plane polarization to remain hidden due to angular averaging during quantum fluctuations unless the symmetry is broken with small electric fields.

In this article we report our experimental observations that the repeated polytypism of wurtzite (WZ) and zinc-blende (ZB) phases induced by the fluctuation of the density of stacking faults can lead to the self-organized formation of ZnO hierarchical nanostructures characterized by a hexagonal central trunk decorated with thin blades. The blades epitaxially nucleate on the ZB stripes assisted by the reentrant corners at the ZB-WZ interfaces. Consequently, the blades keep a fixed angle with respect to the central trunk and resemble two sets of mutually intercalated propellers, each set possessing its own threefold rotational symmetry and being rotated for 60${}^{\circ }$ with respect to each other. We also study the optical properties of such novel structures, and show through numerical simulations that the blades provide the essential boundary conditions to establish the resonant electromagnetic responses in the hierarchical nanostructures.

The effect of crystalline order on the anisotropy of spin accumulation in Si/oxide/ferromagnet tunnel devices has been investigated. The spin accumulation induced electrically in the silicon changes when the magnetization of the ferromagnet is rotated either from in-plane to perpendicular to the tunnel interface or when it is rotated within the plane of the magnetic layer. A fourfold in-plane anisotropy, which reflects the crystalline nature of the tunnel contact, is observed not only for crystalline MgO/Fe contacts, but also for devices with amorphous Al${}_2$O${}_3$ tunnel barrier and polycrystalline ferromagnetic electrode. The in-plane anisotropy is attributed to the direct coupling of states from the ferromagnet to those in the Si, as in coherent tunneling, causing anisotropy in devices in which only the nonmagnetic (Si) electrode is crystalline.

Gd atom diffusion in 30-nm-diameter Fe quantum corrals is studied utilizing scanning tunneling microscopy and kinetic Monte Carlo (KMC) simulations. The Gd adatom probability distribution inside the corral forms several trajectories and is closely related to oscillations of the local density of states at the Fermi level, as revealed by spectroscopy measurements. With increasing coverage, the Gd adatoms form a ringlike structure within the vicinity of the quantum corrals. The results are explained with KMC calculations utilizing experimentally determined long-range interactions and quantum confinement. The findings demonstrate that the diffusion of Gd adatoms is significantly influenced by the quantum corrals and that novel quantum ring structures can be created via control of the coverage.

Studies of the mechanical properties of single-walled carbon nanotubes are hindered by their availability only as ensembles of tubes with a range of diameters. However, tunable Raman spectroscopy is capable of identifying individual tubes from such ensembles. Interestingly, both the radial breathing mode and, surprisingly, the $G$-mode pressure coefficients exhibit strong environmental effects, which are largely independent of the nature of the environment. We show that the $G$-mode pressure coefficient varies with diameter, consistent with the thick-wall tube model. Reappraisal of literature data for graphene and graphite suggests revision of both the $G$-mode Grüneisen parameter γ and the shear deformation parameter β toward the value of 1.34.

Using first-principles calculations we demonstrate the feasibility of band-gap engineering in two-dimensional crystalline graphene monoxide (GMO), a recently reported graphene-based material with a 1:1 carbon/oxygen ratio. The band gap of GMO, which can be switched between direct and indirect, is tunable over a large range (0–1.35 eV) for accessible strains. Electron and hole transport occurs predominantly along the zigzag and armchair directions (armchair for both) when GMO is a direct- (indirect-) gap semiconductor. A band gap of ∼0.5 eV is also induced in graphene at the $K$′ points for GMO/graphene hybrid systems.

Angular distribution of backward re-emitted positrons from a W(100) single crystal have been measured for incident positron energies of 250 and 600 eV. The width of the angular distribution was broader than that predicted by a model of re-emission that assumes complete thermalization of positrons in the solid. Angle-resolved energy distribution of re-emitted positrons showed that the mean energy of the emitted positrons increased with angular deviation from normal. The increase is explained in terms of energy-dependent refraction of positrons traversing the potential step at the surface. The results are in qualitative agreement with a one-dimensional step model of positron re-emission. The observed angular distribution and energy distributions qualitatively matched calculated data if a sample effective temperature of four times the actual temperature is assumed. This suggests the emission of incompletely thermalized positrons at low energy of incident positrons.

The structures of the (001) surface of potassium tantalate doped with nominally 2$\%$, 4$\%$, and 7$\%$ lithium have been investigated using high-resolution helium atom scattering. The surfaces were prepared by cleaving single-crystal samples in situ under UHV conditions. Diffraction measurements in the region around the He specular reflection angle soon after cleaving yielded specular peaks initially with broad shoulders. However, over a period of about an hour, the shoulders diminished into the background. Drift spectra measurements of the stabilized surfaces revealed that the step heights separating surface terraces were predominantly multiples of the approximately 4-Å unit-cell dimension rather than multiples of the approximately 2-Å half-unit-cell dimension expected from the cleaving of these crystals. Together, these results suggest that these surfaces are rapidly modified after cleaving by migration of ions to the surface from the near surface. Further, half-order diffraction peaks were observed in the $\langle 100\rangle$ azimuth a short time after cleaving samples at room temperature, indicating that domains with ($2\times 1$) structure had formed.

Individual dopant atoms in silicon devices gain active roles as channel dimensions move into the nanoscale. A single donor can work as an atomic quantum dot, mediating single-electron-tunneling transport from source to drain. However, the tunneling operation of single-donor transistors has so far been reported only at low temperatures below ∼15 K, while at high temperatures tunneling features are expected to be smeared out. For a higher-temperature single-electron-tunneling operation, the donor's tunnel barriers must be considerably higher than $k_{\mathrm{B}}T$. Here, we use a special design of a nanoscale Si channel with a central stub region, in which a phosphorus donor's ground state becomes deeper due to the dielectric confinement effect. In these stub-channel devices, electron tunneling via one donor atom survives even at temperatures above 100 K. Results of ab initio atomistic simulations provide further insights into the fundamental properties of individual donors in ultrasmall nanopatterned structures.

In this paper, a formalism for the activation volume of glass-forming materials is suggested. An isothermal equation of state for the activation volume is formulated, which is extended to a generalized equation of state that describes the activation volume as a function of temperature and pressure. Both the equations of state are very successfully validated by using experimental and simulation data collected for supercooled Kob-Andersen binary Lennard-Jones liquid and materials from various material groups such as van der Waals liquids, polymers, protic ionic liquids, and strongly hydrogen-bonded liquids. Some predictions based on these equations of state for the activation volume are also very satisfactorily verified in the case of each considered system, especially a kind of activation volume scaling is confirmed, which is possible by using the scaling exponent that also constitutes the slope of the expected linear pressure dependence of the isothermal bulk modulus for the activation volume. The until recently unexpected negative value of the slope is explained in the case of the systems that obey the thermodynamic scaling law, at least to a good approximation.

The full mean-field solution of spin glass models with a continuous order-parameter function is not directly available and approximate schemes must be used to assess its properties. One of the authors recently proposed a representation of the free energy generating this solution via an evolution operator parametrized by attainable values of overlap of magnetizations between different states. Here, we introduce a perturbation expansion for the evolution operator that we use to derive all thermodynamic characteristics via the standard methods of statistical mechanics. We obtain a generic scheme for an approximate calculation of physical quantities of different mean-field spin-glass models at all temperatures. The small expansion parameter is a difference between the continuous order-parameter function and the corresponding order parameter from the solution with one level of replica-symmetry breaking. The first correction beyond the approximation with one level of replica-symmetry breaking is explicitly evaluated in the glassy phase of the Sherrington-Kirkpatrick model.

The current dependence of the exponent of the spin torque switching rate of an in-plane magnetized system was investigated by solving the Fokker-Planck equation with low temperature and small damping and current approximations. We derived the analytical expressions of the critical currents, $I_{\mathrm{c}}$ and $I_{\mathrm{c}}^{*}$. At $I_{\mathrm{c}}$, the initial state parallel to the easy axis becomes unstable while at $I_{\mathrm{c}}^{*}(\simeq 1.27I_{\mathrm{c}})$ the switching occurs without the thermal fluctuation. The current dependence of the exponent of the switching rate is well described by ${(1-I/I_{\mathrm{c}}^{*})}^b$, where the value of the exponent $b$ is approximately unity for $I\le I_{\mathrm{c}}$ while $b$ rapidly increases up to $\sim 2.2$ with increasing current for $I_{\mathrm{c}}\le I\le I_{\mathrm{c}}^{*}$. The linear dependence for $I\le I_{\mathrm{c}}$ agrees with the other works, while the nonlinear dependence for $I_{\mathrm{c}}\le I\le I_{\mathrm{c}}^{*}$ was newly found by the present work. The nonlinear dependence is important for analysis of the experimental results, because most experiments are performed in the current region of $I_{\mathrm{c}}\le I\le I_{\mathrm{c}}^{*}$.

We consider a simple integrable model of a spin chain exhibiting the magnetoelectric effect (MEE). Starting from the periodic $S=1/2$ XXZ chain with Dzyaloshinskii-Moriya terms, which we consider as a local electric polarization in the spirit of the Katsura-Nagaosa-Baladsky (KNB) mechanism, we perform the mapping onto the conventional XXZ chain with twisted boundary conditions. Using the techniques of quantum transfer matrix and nonlinear integral equations we obtain the magnetization, electric polarization, and magnetoelectric tensor as functions of magnetic and electric field for arbitrary temperatures. We investigate these dependencies as well as the thermal behavior of the above-mentioned physical quantities, especially in the low-temperature regime. We found several regimes of polarization. Adjusting the magnetic field one can switch the system from one regime to another. The features of the critical properties connected with the MEE are also illustrated.

In Lorentz-invariant systems spontaneously broken gauge symmetry results in three types of fundamental excitations: density excitations, Higgs bosons (amplitude modes), and Goldstone bosons (phase modes). The density and phase modes are coupled by electromagnetic interactions while the amplitude modes are not. In $s$-wave superconductors, the Higgs mode, which can be observed only under special conditions, has been detected. We show that unconventional $d$-wave superconductors, such as the high-temperature cuprate superconductors, should have a rich assortment of Higgs bosons, each in a different irreducible representation of the point-group symmetry of the lattice. We also show that these modes have a characteristic singular spectral structure and discuss conditions for their observability.

Interfaces play a crucial role in controlling the thermal conductivity of a material, as evidenced by the major ongoing research in nanoscale precipitation engineering to improve the performance of thermoelectric materials. To understand their influence on phonon scattering, and thus thermal conductivity, we explored the effect of interfacial strain fields (ISFs) on interfaces with a different misfit between the two dissimilar lattices comprising the interface. We found that phonon scattering depends strongly on the frequency of the incident phonon wave; phonons with frequencies below a critical value pass transparently through the strain zone of the interfaces. Our calculations suggest that the classical acoustic mismatch model (AMM) and the diffusive mismatch model (DMM) represent the two extreme limits of phonon scattering, and that, in reality, scattering lies between these limits. For long-wavelength (low-frequency) phonons, the AMM adequately describes the thermal conductivity of the interfaces. As the frequency of the incident phonons increases, phonon scattering becomes progressively more diffuse to the point where the DMM represents the dominant mechanism for thermal conductivity. Furthermore, we noted that phonon scattering from the interfacial strain field can lower thermal conductivity effectively at high temperatures. We discuss the implications of our findings in explaining the experimental observations of thermal conductivity across precipitates and grain boundaries in thermoelectric materials.

We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO${}_3$. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment; we hence deduce that tetragonal EuTiO${}_3$ enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials.