The electronic structure of $\alpha$-Sn (001) thin films strained compressively in-plane was studied both experimentally and theoretically. A new topological surface state (TSS) located entirely within the gapless projected bulk bands is revealed by ab initio-based tight-binding calculations as well as directly accessed by soft x-ray angle-resolved photoemission. The topological character of this state, which is a surface resonance, is confirmed by unravelling the band inversion and by calculating the topological invariants. In agreement with experiment, electronic structure calculations show the maximum density of states in the subsurface region, while the already established TSS near the Fermi level is strongly localized at the surface. Such varied behavior is explained by the differences in orbital composition between the specific TSS and its associated bulk states, i.e., their hybridization, respectively.

We report on the electronic band structure, structural, magnetic, and thermal properties of ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Sn}}_5$. Ce $L_{\mathrm{III}}$-edge XAS spectra give direct evidence for an intermediate valence behavior. Thermodynamic measurements reveal magnetic transitions at $T_{\mathrm{N}1}\approx 2.9$ K and $T_{\mathrm{N}2}\approx 2.4$ K. Electrical resistivity shows behavior typical for the Kondo lattices. The coexistence of magnetic order and valence fluctuations in a Kondo lattice system we attribute to a peculiar crystal structure in which Ce ions occupy two distinct lattice sites. Analysis of the structural features of ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Sn}}_5$, together with results of electronic band structure calculations and thermodynamic and spectroscopic data indicate that at low temperatures only Ce ions from the Ce1 sublattice adopt a stable trivalent electronic configuration and show local magnetic moments that give rise to the magnetic ordering. By contrast, our study suggests that Ce2 ions exhibit a nonmagnetic Kondo-singlet ground state. Furthermore, the valence of Ce2 ions estimated from the Ce $L_{\mathrm{III}}$-edge XAS spectra varies between +3.18 at 6 K and +3.08 at room temperature. Thus our joined experimental and theoretical investigations classify ${\mathrm{Ce}}_2{\mathrm{Rh}}_3{\mathrm{Sn}}_5$ as a multivalent charge-ordered system.

Dark excitons (DEs) confined in epitaxial quantum dots (QDs) are interesting because of their long lifetime compared to bright excitons (BEs). For the same reason they are usually difficult to access in optical experiments. Here we report on the observation of vertically polarized light emission from DEs confined in high-quality epitaxial GaAs/AlGaAs QDs located in proximity of a cleaved facet of the QD specimen. Calculations based on the eight-band k·p method and configuration interaction allow us to attribute the brightening of the DE to the anisotropic strain present at the sample edge, which breaks the symmetry of the system and enhances valence-band mixing. The mechanism of DE brightening is discussed in detail by inspecting both the Bloch and envelope wave functions of the involved hole states. In addition, by investigating experimentally and theoretically QDs with different sizes, we find that the energy separation between DE and BEs tends to decrease with increasing QD height. Finally, the presence of a cleaved facet is found also to enhance the BE fine structure splitting. This work provides a simple method to optically probe dark excitonic states in QDs and shows that the properties of QDs can be significantly affected by the presence of nearby edges.

Recent research suggests that the supercritical state consists of liquidlike and gaslike states where particle dynamics and key system properties are qualitatively different. We report experimental evidence of the structural crossover in supercritical neon at pressure and temperature conditions significantly exceeding the critical point values: $250P_c$ and $6.6T_c$. The experimental results show a crossover of the medium-range order structure evidenced by the change of the structure factor with pressure. We also observe the crossover of the short-range order structure indicated by changes in the coordination number. The relative width of the crossover is fairly narrow and is smaller than 10–12% in pressure and temperature. By comparing our experimental results with molecular dynamics simulations, we suggest that the observed crossover can be attributed to the Frenkel line and discuss the relationship between the structural crossover and qualitative changes of dynamical and thermodynamic properties of supercritical matter.

The structural and dynamical properties of FePt nanoparticles were studied within the density functional theory. The effect of size and chemical composition on the dynamical stability of nanoparticles was investigated for the cuboctahedral and icosahedral symmetries. In cuboctahedra, a structural distortion is observed, which for systems with odd number of Pt layers leads to lowering of the tetragonal symmetry. Significant differences between the vibrational properties of FePt particles and bulk crystals are observed, but similarly to the FePt crystal, cuboctahedral particles exhibit a strong anisotropy of atomic vibrations. The icosahedral particles with perfect shell geometry are unstable due to enlarged distances between Fe atoms. They can be stabilized by removing a central atom or replacing it by a smaller one. The heat capacity and entropy of nanoparticles show typical enhancement due to low-energy vibrations at the surface layers.

The impact of the longitudinal spin Seebeck effect (LSSE) on the magnon damping in magnetic-insulator/nonmagnetic-metal bilayers was recently discussed in several reports. However, results of those experiments can be blurred by multimode excitation within the measured linewidth. In order to avoid possible intermodal interference, we investigated the damping of a single magnon group in a platinum covered yttrium iron garnet (YIG) film by measurement of the threshold of its parametric excitation. Both dipolar and exchange spin-wave branches were probed. It turned out that the LSSE-related modification of spin-wave damping in a micrometer-thick YIG film is too weak to be observed in the entire range of experimentally accessible wave vectors. At the same time, the change in the absolute temperature of the YIG layer, which can appear by applying a temperature gradient, strongly modifies the damping value.

The field-driven magnetization dynamics of a multiskyrmion cluster in a nanodisk is investigated by micromagnetic simulation and analytical calculation. Under a weak in-plane static magnetic field, the multiskyrmion cluster shows an oscillatory motion around an equilibrium position, which resembles the dynamical behavior of the conventional torsional pendulum. We show that this oscillation is induced by restoring torque acting on the skyrmion generated by the potential energy determined by the angle of the skyrmion orientation. Moreover, the multiskyrmion cluster can be driven to rotate by an in-plane rotating magnetic field. The rotation directions and frequencies are fully determined by the number of the skyrmions.

We report cryogenic magnetic force microscopy (MFM) studies of a 200-nm-thick hexagonal ($h$) ${\mathrm{LuFeO}}_3$ film grown by molecular-beam epitaxy on a (111)-oriented yttria-stabilized cubic-zirconia substrate. Labyrinthlike domains $\sim 1.8\mu \mathrm{m}$ in size were observed after zero-field cooling below the Néel temperature, $T_{\mathrm{N}}\approx 147$ K, where weak ferromagnetic order ($P{6}_3$cm) with a canted moment of $M_{\mathrm{S}}\approx 0.02{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ exists. At 6 K, MFM images of the magnetization reversal process reveal a typical domain behavior of a pinning-dominated hard magnet. The pinning strength is substantially reduced at elevated temperature. The temperature dependence of the domain contrast demonstrates that our MFM is able to detect the domain contrast of magnets with tiny magnetic moments ($\sim 0.002{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$). An upper limit of the linear magnetoelectric coefficient of $h-{\mathrm{LuFeO}}_3$ (${\alpha }_{zz}<6\mathrm{ps}/\mathrm{m}$) is estimated by magnetoelectric force microscopy measurements.

The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multimode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unusual dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators.

The superconducting and normal-state properties of the filled-skutterudite system ${\mathrm{Pr}}_{1-x}{\mathrm{Eu}}_x{\mathrm{Pt}}_4{\mathrm{Ge}}_{12}$ were studied. Polycrystalline samples were investigated via x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific-heat measurements. Upon Eu substitution, we observed a crossover from superconducting to antiferromagnetic states with a region where both states coexist. In the superconducting region, the specific-heat data exhibit a change of temperature dependence, suggesting an evolution from a nodal to a nodeless superconducting energy gap or a suppression of multiband superconductivity. This change is relatively slower than those reported for different substituent ions, suggesting that paramagnetic impurities have a weaker pair-breaking effect on unconventional superconductivity in ${\mathrm{PrPt}}_4{\mathrm{Ge}}_{12}$. In the normal state, an evolution from Fermi-liquid to non-Fermi-liquid behavior was observed, accompanied by the coexistence of superconductivity and antiferromagnetism, suggesting that the underlying electronic structure is primarily responsible for the complex physical phenomena found in this system.

We calculate the admittance of two types of Josephson weak links—the first is a one-dimensional superconducting wire with a local suppression of the order parameter, and the second is a short S-c-S structure, where S denotes a superconducting reservoir and c is a constriction. The systems of the first type are analyzed on the basis of time-dependent Ginzburg-Landau equations derived by Gor'kov and Eliashberg for gapless superconductors with paramagnetic impurities. It is shown that the impedance $Z\left(\mathrm{\Omega }\right)$ has a maximum as a function of the frequency $\mathrm{\Omega }$, and the electric field $E_{\mathrm{\Omega }}$ is determined by two gauge-invariant quantities. One of them is the condensate momentum $Q_{\mathrm{\Omega }}$ and another is a potential $\mu$ related to charge imbalance. The structures of the second type are studied on the basis of microscopic equations for quasiclassical Green's functions in the Keldysh technique. For short S-c-S contacts (the Thouless energy $E_{\text{Th}}=D/L^2\gg \mathrm{\Delta })$, we present a formula for admittance $Y$ valid frequencies $\mathrm{\Omega }$ and temperatures $T$ less than the Thouless energy $E_{\text{Th}}$ $\left(\hslash \mathrm{\Omega },T\ll E_{\text{Th}}\right)$ but arbitrary with respect to the energy gap $\mathrm{\Delta }$. It is shown that, at low temperatures, the absorption is absent $\left[\mathrm{Re}\left(Y\right)=0\right]$ if the frequency does not exceed the energy gap in the center of the constriction $(\mathrm{\Omega }<\mathrm{\Delta }cos{\phi }_0$, where $2{\phi }_0$ is the phase difference between the S reservoirs). The absorption gradually increases with increasing the difference $(\mathrm{\Omega }-\mathrm{\Delta }cos{\phi }_0)$ if $2{\phi }_0$ is less than the phase difference $2{\phi }_{\text{c}}$ corresponding to the critical Josephson current. In the interval $2{\phi }_{\text{c}}<2{\phi }_0<\pi$, the absorption has a maximum. This interval of the phase difference is achievable in phase-biased Josephson junctions. Close to $T_{\text{c}}$ the admittance has a maximum at low $\mathrm{\Omega }$, which is described by an analytical formula.

We suggest a class of two-dimensional lattice spin Hamiltonians describing non-Abelian SU(2) chiral spin liquids—spin analogs of fractional non-Abelian quantum Hall states—with gapped bulk and gapless chiral edge excitations described by the SU(2)${}_n$ Wess-Zumino-Novikov-Witten conformal field theory. The models are constructed from an array of generalized spin-$n/2$ ladders with multi-spin-exchange interactions which are coupled by isolated spins. Such models allow a controllable analytic treatment starting from the one-dimensional limit and are characterized by a bulk gap and non-Abelian SU(2)${}_n$ gapless edge excitations.

The equation of state (EOS) of carbon in its high-pressure solid and liquid phases is of interest to planetary astrophysics and inertial confinement fusion. Of particular interest are the high-pressure shock and release responses of diamond as these provide rigorous constraints on important paths through the EOS. This paper presents experimental Hugoniot and release data for both single-crystal diamond (SCD) and nanocrystalline diamond (NCD), which is comprised of nanometer-scale diamond grains and is $\sim 5\%$ less dense than SCD. We find that NCD has a stiffer Hugoniot than SCD that can be attributed to porosity. A Grüneisen parameter of $\sim 1$ was derived from the data, which suggests increased coordination in the high-pressure fluid carbon compared to ambient diamond.

We study the quantum melting of stripe phases in models with competing short-range and long-range interactions decaying with distance as $1/r^{\sigma }$ in two space dimensions. At zero temperature we find a two step disordering of the stripe phases with the growth of quantum fluctuations. A quantum critical point separating a phase with long-range positional order from a phase with long-range orientational order is found when $\sigma \le 4/3$, which includes the Coulomb interaction case $\sigma =1$. For $\sigma >4/3$ the transition is first order, which includes the dipolar case $\sigma =3$. Another quantum critical point separates the orientationally ordered (nematic) phase from a quantum disordered phase for any value of $\sigma$. Critical exponents as a function of $\sigma$ are computed at one loop order in an $\epsilon$ expansion and, whenever available, compared with known results. For finite temperatures it is found that for $\sigma \ge 2$ orientational order decays algebraically with distance until a critical Kosterlitz-Thouless line. Nevertheless, for $\sigma <2$ it is found that long-range orientational order can exist at finite temperatures until a critical line which terminates at the quantum critical point at $T=0$. The temperature dependence of the critical line near the quantum critical point is determined as a function of $\sigma$.

We show that the peculiarities of the electron band structure strongly affect the spin-valve effect in heterostructures consisting of a superconductor (S) and two ferromagnetic layers ${\mathrm{F}}_1$ and ${\mathrm{F}}_2$. For the $\mathrm{S}/{\mathrm{F}}_1/{\mathrm{F}}_2$ systems the energy shift between the electron bands in the S and ${\mathrm{F}}_2$ layers determines whether the critical temperature $T_c$ of the superconductor increases or decreases as a function of the angle $\theta$ between the magnetic moments in the ferromagnets. In the case of the half-metallic ${\mathrm{F}}_2$ layer the type of the spin-valve effect becomes dependent on the position of the only occupied spin band and the minimum of $T_c$ may correspond to the antiparallel, parallel, or noncollinear orientations of magnetic moments. Also, for the first time, we analytically demonstrate the possibility of the triplet spin-valve effect in the F/S/half-metal structures.

We study Andreev reflection and the Josephson effect in a ballistic monolayer of black phosphorous, known as phosphorene. Due to the anisotropic band structure of this system, the supercurrent changes with an order of magnitude when comparing tunneling along two perpendicular directions in the monolayer. We show that the main reason for this effect is a large difference in the number of transverse modes in Andreev bound states. The oscillatory behavior of the supercurrent as a function of the length and chemical potential of the junction also differs substantially depending on the orientation of the superconducting electrodes deposited on the phosphorene sheet. For Andreev reflection, we show that gate voltaging controls the probability of this process and that the anisotropic behavior found in the supercurrent case is also present for conductance spectra.

The temperature-dependent electronic structure of the heavy-fermion compound ${\mathrm{CeB}}_6$ was investigated thoroughly by means of the combination of density functional theory and single-site dynamical mean-field theory. The band structure, density of states, and $4f$ valence state fluctuation of ${\mathrm{CeB}}_6$ were calculated in a broad temperature range of 10–120 K. Overall, the $4f$ electrons remain incoherent, approximately irrespective of environmental temperature. However, we find that these observables exhibit some unusual features near 20 K. In addition, the evolutions of $4f$ orbital occupancy, total angular momentum, and total energy with respect to temperature show apparent nonmonotonic behaviors around 20 K. The possible explanations for these tantalizing characteristics are discussed concisely.

We study numerically the reflectivity of three-dimensional (3D) photonic crystals with a complete 3D photonic band gap. We employ the finite element method to study crystals with the cubic diamondlike inverse woodpile structure. The high-index backbone has a dielectric function similar to silicon. We study crystals with a range of thicknesses up to ten unit cells $\left(L\le 10c\right)$. The crystals are surrounded by vacuum, and have a finite support as in experiments. The polarization-resolved reflectivity spectra reveal Fabry-Pérot fringes related to standing waves in the finite crystal, as well as broad stop bands with nearly $100\%$ reflectivity, even for thin crystals. The frequency ranges of the stop bands change little with angle of incidence, which is plausible since the stop bands are part of the 3D band gap. Moreover, this result supports the previous assertion that intense reflection peaks measured with a large numerical aperture provide a faithful signature of the 3D photonic band gap. For $p$-polarized waves, we observe an intriguing hybridization between the Fabry-Pérot resonances and the Brewster angle that remains to be observed in experiments. From the strong reflectivity peaks, it is inferred that the maximum reflectivity observed in experiments is not limited by finite size. The frequency ranges of the stop bands agree very well with stop gaps in the photonic band structure that pertain to infinite and perfect crystals. The angle-dependent reflectivity spectra provide an improved interpretation of the reflectivity measurements performed with a certain numerical aperture and a new insight in the crystal structure, namely unequal pore radii in $X$ and $Z$ directions. The Bragg attenuation lengths $L_B$ are found to be smaller by a factor 6 to 9 than earlier estimates that are based on the width of the stop band. Hence, crystals with a thickness of 12 unit cells studied in experiments are in the thick crystal limit $\left(L\gg L_B\right)$. Our reflectivity calculations suggest that the 3D silicon photonic band gap crystals are interesting candidates for back reflectors in a solar cell in order to enhance the photovoltaic efficiency.

Optical pump-THz probe spectroscopy is used to investigate the exciton formation dynamics and its intensity dependence in bulk Ge. Associated with the intraexcitonic $1s\text{−}2p$ transition, the gradual buildup of an absorption peak around 3.1 meV (0.75 THz) signifies a delayed exciton formation after optical pump which is accelerated for higher excitation densities. Analyzing the spectral shape of this THz absorption resonance, two distinct resonances are found which are attributed to the mass anisotropy of $L$-valley electrons via a microscopic theory.

Large room-temperature magnetoresistance observed for devices composed of self-assembled monolayers of different oligophenylene thiols sandwiched between gold contacts has recently been reported [Z. Xie, S. Shi, F. Liu, D. L. Smith, P. P. Ruden, and C. D. Frisbie, ACS Nano 10, 8571 (2016)]. The transport mechanism through the organic molecules was determined to be nonresonant tunneling. To explain this kind of magnetoresistance, we develop an analytical model based on the interaction of the tunneling charge carrier with an unpaired charge carrier populating a contact-molecule interface state. The Coulomb interaction between carriers causes the transmission coefficients to depend on their relative spin orientation. Singlet and triplet pairing of the tunneling and the interface carriers thus correspond to separate conduction channels with different transmission probabilities. Spin relaxation enabling transitions between the different channels, and therefore tending to maximize the tunneling current for a given applied bias, can be suppressed by relatively small magnetic fields, leading to large magnetoresistance. Our model elucidates how the Coulomb interaction gives rise to transmission probabilities that depend on spin and how an applied magnetic field can inhibit transitions between different spin configurations.