We describe a technique, using thin lines of triplet-state $\mathrm{He}_2^{*}$ molecular tracers created by femtosecond-laser field ionization of helium atoms, for visualizing the flow of the normal fluid in superfluid ${}^4\mathrm{He}$, together with its application to thermal counterflow in a channel. We show that, at relatively small velocities, where the superfluid is already turbulent, the flow of the normal fluid remains laminar, but with a distorted velocity profile, while at a higher velocity there is a transition to turbulence. The form of the structure function in this turbulent state differs significantly from that found in types of conventional turbulence. This visualization technique also promises to be applicable to other fluid dynamical problems involving cryogenic helium.

Resolving the high-field superconducting phase (HFSP), often called the $Q$ phase, and the antiferromagnetic or spin-density-wave (SDW) order appearing in the phase remains a crucial issue on the superconductor ${\mathrm{CeCoIn}}_5.$ It is shown that a switching of the SDW domain due to a tiny change of the magnetic field direction in HFSP, reported and interpreted as evidence of the presence of a $\pi$-triplet pairing inducing the SDW order [S. Gerber et al., Nat. Phys. 10, 126 (2014)], can be explained with no triplet pairing component if the $d$-wave superconducting order in HFSP includes the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulation parallel to the field. This result corroborates the idea that HFSP of ${\mathrm{CeCoIn}}_5$ and the SDW order found only in the phase are consequences of the strong paramagnetic pair breaking in this $d$-wave superconductor.

The residual low-angle grain boundary (GB) network is still the most important current-limiting mechanism operating in biaxially textured rare-earth barium-copper-oxide (REBCO) coated conductors. While Ca doping is well established to improve supercurrent flow across low-angle GBs in weak fields at high temperatures, Ca doping also depresses $T_c$, making it so far impractical for high-temperature applications of REBCO coated conductors. On the other hand, high-field-magnet applications of REBCO require low temperatures. Here we systematically evaluate the effectiveness of Ca doping in improving the GB transparency, $r^{\mathrm{GB}}=J_c^{\mathrm{GB}}/J_c^{\mathrm{grain}}$, of low-angle $\mathrm{Y}{\mathrm{b}}_{1-x}\mathrm{C}{\mathrm{a}}_x\mathrm{BaCuO}$ [001] tilt bicrystal films down to 10 K and with magnetic fields perpendicular and parallel to the film surfaces, while varying the Ca and oxygen doping level. Using low-temperature scanning laser microscopy and magneto-optical imaging, we found $r^{\mathrm{GB}}$ to strongly depend on the angle between magnetic field and the GB plane and clearly identified regimes in which $J_c^{\mathrm{GB}}$ can exceed $J_c^{\mathrm{grain}}\left(r^{\mathrm{GB}}>1\right)$ where the GB pinning is optimized by the field being parallel to the GB dislocations. However, even in this favorable situation, we found that $r^{\mathrm{GB}}$ became much smaller at lower temperatures. Calculations of the GB Ca segregation profile predict that the high-$J_c$ channels between the GB dislocation cores are almost Ca free. It may be therefore that the positive effects of Ca doping seen by many authors near $T_c$ are partly a consequence of the higher $T_c$ of these Ca-free channels.

We have implemented the dynamical vertex approximation $\left(\mathrm{D}\Gamma \mathrm{A}\right)$ in its full parquet-based version to include spatial correlations on all length scales and in all scattering channels. The algorithm is applied to study the electronic self-energies and the spectral properties of finite-size one-dimensional Hubbard models with periodic boundary conditions (nanoscopic Hubbard rings). From a methodological point of view, our calculations and their comparison to the results obtained within dynamical mean-field theory, plain parquet approximation, and the exact numerical solution allow us to evaluate the performance of the $\mathrm{D}\Gamma \mathrm{A}$ algorithm in the most challenging situation of low dimensions. From a physical perspective, our results unveil how nonlocal correlations affect the spectral properties of nanoscopic systems of various sizes in different regimes of interaction strength.

Pure hydride of the $\alpha \text{−}\mathrm{U}{\mathrm{H}}_3$ type without any $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$ admixture was prepared by high-pressure hydrogenation of bcc U stabilized by Zr. Such material, characterized by a general formula ${\left(\mathrm{U}{\mathrm{H}}_3\right)}_{1\text{−}x}\mathrm{Z}{\mathrm{r}}_x$, is stable in air at ambient and elevated temperatures. H release is observed in the range of 400–600 °C similar to $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$. Its stability allowed us to measure the magnetic properties, specific heat, and electrical resistivity in a wide temperature range. Despite the rather different crystal structure and inter-U spacing, the electronic properties are almost identical to $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$. Its ferromagnetic ground state with Curie temperature $T_{\mathrm{C}}\approx 180\mathrm{K}$ (weakly and nonmonotonously dependent on Zr concentration) and U moments of $1.0{\mu }_{\mathrm{B}}$ indicate why mixtures of α- and $\beta \text{−}\mathrm{U}{\mathrm{H}}_3$ exhibited only one transition. Magnetic ordering leads to a large spontaneous magnetostriction ${\omega }_{\mathrm{s}}=3.2\times {10}^{-3}$, which can be explained by the increase of the spin moment between the paramagnetic (disordered local moment) and the ferromagnetic states. The role of orbital moments in magnetism is indicated by fully relativistic electronic structure calculations.

We study potential and electron density depth profiles in accumulation, inversion, and depletion layers in crystals with a large and nonlinear dielectric response such as ${\mathrm{SrTiO}}_3$. We describe the lattice dielectric response using the Landau-Ginzburg free energy expansion. In accumulation and inversion layers we arrive at new nonlinear dependencies of the width $d$ of the electron gas on an applied electric field $D_0$. Particularly important is the predicted electron density profile of accumulation layers (including the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface) $n\left(x\right)\propto {(x+d)}^{-12/7}$, where $d\propto D_0^{-7/5}$. We compare this profile with available data and find satisfactory agreement. For a depletion layer we find an unconventional nonlinear dependence of the capacitance on voltage. We also evaluate the role of spatial dispersion in the dielectric response by adding a gradient term to the Landau-Ginzburg free energy.

Propagation-invariant beams that pull objects towards a light source are commonly known as tractor beams. Here, an efficient, linearly polarized tractor beam with improved stability is introduced. The beam consists of a superposition of transverse-electric and transverse-magnetic polarized Bessel beams of orders $m=+1$ and $m=-1$. It is shown that this beam can stably pull a wide range of dielectric microparticles arbitrarily long distances, independent of ambient conditions. Next, a straightforward method of generating these high-performance beams is proposed. A Si metasurface transforms an incident linearly polarized Gaussian beam into the desired tractor beam. Full-wave simulations demonstrate that it is possible for this simple geometry to pull a polystyrene sphere a distance equal to the nondiffracting range of the Bessel beam. The simplicity of the setup and the robust performance of the proposed tractor beam significantly enhance the ability to manipulate matter with light.

The positron surface state and the energetics for positron reemission are investigated using two-component density functional theory (TC-DFT) in the projector augmented-wave framework. Trapping of positrons by the surface image potential and the effect of the positron band-shift energy in the surface region are appropriately described by the corrugated mirror model and the ramp potential, respectively, without empirical parameters. The results obtained for various physical quantities of positron states on a clean Al(100) surface, i.e., the affinity, work function, life-time, binding energy, and activation energy, are in good agreement with the experimental results. The positron states on Li-adsorbed Al(100) surfaces are highly dependent on the Li coverage. In particular, the work function of positronium negative ions $\left({\mathrm{Ps}}^{-}\right)$ becomes negative at low Li coverage, which indicates the possible emission of ${\mathrm{Ps}}^{-}$ from the adsorbed surface. The present study not only elucidates the key energetics that are responsible for positron reemission from the surface, but also emphasizes the excellent performance of TC-DFT for prediction of the positron state on real surfaces.

Motivated by experiments in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ [A. E. Böhmer et al., arXiv:1412.7038], we analyze the type of spin-density wave (SDW) order in doped iron-pnictides and the discontinuities of the superconducting transition temperature $T_c$ in the coexistence phase with SDW magnetism. We find a sequence of transitions, upon lowering the temperature, from the stripe-orthorhombic $\left(C_2\right)$ SDW order to the tetragonal $\left(C_4\right)$ order and then back to the $C_2$ order. We argue that $T_c$ has two discontinuities—it jumps to a smaller value upon entering the coexistence region with the $C_4$ magnetic phase, and then jumps to a larger value inside the SDW state when it crosses the boundary between the $C_4$ and $C_2$ SDW orders. We argue that the agreement with the experimental phase diagram provides a strong indication that the itinerant approach is adequate for weakly/moderately doped iron pnictides.

Previous studies of electron-phonon interaction in impure graphene have found that disorder can give rise to an enhancement of electronic cooling at high temperatures. We investigate the effect of lattice vacancy in both mono- and bilayer graphene and observe an order of magnitude suppression of electronic cooling at low temperatures compared with clean graphene. The dependence of the coupling constant on the phonon temperature implies its link to the dynamics of disorder. Our study highlights the effect of disorder on electron-phonon interaction in graphene. In addition, the suppression of electronic cooling holds great promise for improving the performance of graphene-based bolometer and photodetector devices.

Similarly to a previous work on the homogeneous electron gas [Y. Todorov, Phys. Rev. B 89, 075115 (2014)], we apply the Power-Zienau-Wooley (PZW) formulation of the quantum electrodynamics to the case of an electron gas quantum confined by one-dimensional potential. We provide a microscopic description of all collective plasmon modes of the gas, oscillating both along and perpendicular to the direction of quantum confinement. Furthermore, we study the interaction of the collective modes with a photonic structure, planar metallic waveguide, by using the full expansion of the electromagnetic field into normal modes. We show how the boundary conditions for the electromagnetic field influence both the transverse light-matter coupling and the longitudinal particle-particle interactions. The PZW descriptions appear thus as a convenient tool to study semiconductor quantum optics in geometries where quantum-confined particles interact with strongly confined electromagnetic fields in microresonators, such as the ones used to achieve the ultrastrong light-matter coupling regime.

For decades it has been a well-known fact that among the few ferroelectric compounds in the perovskite family, namely, ${\mathrm{BaTiO}}_3$, ${\mathrm{KNbO}}_3$, ${\mathrm{PbTiO}}_3$, and ${\mathrm{Na}}_{1/2}{\mathrm{Bi}}_{1/2}{\mathrm{TiO}}_3$, the dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_3$ are considerably higher than the others in polycrystalline form at room temperature. Further, similar to ferroelectric alloys exhibiting morphotropic phase boundary, single crystals of ${\mathrm{BaTiO}}_3$ exhibit anomalously large piezoelectric response when poled away from the direction of spontaneous polarization at room temperature. These anomalous features in ${\mathrm{BaTiO}}_3$ remained unexplained so far from the structural standpoint. In this work, we have used high-resolution synchrotron x-ray powder diffraction, atomic resolution aberration-corrected transmission electron microscopy, in conjunction with a powder poling technique, to reveal that at 300 K (i) the equilibrium state of ${\mathrm{BaTiO}}_3$ is characterized by coexistence of metastable monoclinic Pm and orthorhombic (Amm2) phases along with the tetragonal phase, and (ii) strong electric field switches the polarization direction from the [001] direction towards the [101] direction. These results suggest that ${\mathrm{BaTiO}}_3$ at room temperature is within an instability regime, and that this instability is the fundamental factor responsible for the anomalous dielectric and piezoelectric properties of ${\mathrm{BaTiO}}_3$ as compared to the other homologous ferroelectric perovskite compounds at room temperature. Pure ${\mathrm{BaTiO}}_3$ at room temperature is therefore more akin to lead-based ferroelectric alloys close to the morphotropic phase boundary where polarization rotation and field induced ferroelectric-ferroelectric phase transformations play a fundamental role in influencing the dielectric and piezoelectric behavior.

We calculate the thermomechanical properties of $\alpha$-iron, and in particular its isothermal and adiabatic elastic constants, using first-principles total-energy and lattice-dynamics calculations, minimizing the quasiharmonic vibrational free energy under finite strain deformations. Particular care is made in the fitting procedure for the static and temperature-dependent contributions to the free energy, in discussing error propagation for the two contributions separately, and in the verification and validation of pseudopotential and all-electron calculations. We find that the zero-temperature mechanical properties are sensitive to the details of the calculation strategy employed, and common semilocal exchange-correlation functionals provide only fair to good agreement with experimental elastic constants, while their temperature dependence is in excellent agreement with experiments in a wide range of temperature almost up to the Curie transition.

We investigate time-independent disorder on several two-dimensional discrete-time quantum walks. We find numerically that, contrary to claims in the literature, random onsite phase disorder, spin-dependent or otherwise, cannot localize the Hadamard quantum walk; rather, it induces diffusive spreading of the walker. In contrast, split-step quantum walks are generically localized by phase disorder. We explain this difference by showing that the Hadamard walk is a special case of the split-step quantum walk, with parameters tuned to a critical point at a topological phase transition. We show that the topological phase transition can also be reached by introducing strong disorder in the rotation angles. We determine the critical exponent for the divergence of the localization length at the topological phase transition, and find $\nu =2.6$, in both cases. This places the two-dimensional split-step quantum walk in the universality class of the quantum Hall effect.

We studied surface morphology induced changes of magnetic anisotropy, magnetization reversal, and symmetry of the anisotropic magnetoresistance (AMR) in ion sputtered Ni films grown on MgO (001). Grazing-incidence ion sputtering generally develops anisotropic surface roughness of the Ni films, i.e., nanometer wide ripples parallel to the ion beam direction, giving rise to uniaxial magnetic anisotropy with the easy axis along the ion beam direction. The formed ripples act as domain wall nucleation and pinning sites during magnetization reversal, while two-jump domain wall motion dominates in the as-grown Ni films. More importantly, the azimuthal angular dependence of the AMR indicates a superposition of twofold symmetry and fourfold symmetry. By relying on grazing-incidence ion sputtering along specific crystallographic directions, we are able to tailor the relative weight of twofold and fourfold symmetry of AMR. We demonstrate that in contrast to the bulk case, the symmetry of the AMR becomes also sensitive to the surface morphology in thin films, which is in particular relevant for the design of magnetotransport based sensors.

We study the combined effects of lattice deformation, $e\text{−}e$ interaction, and spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt different kinds of hopping modulation—generalized dimerization and a Kekulé distortion—and calculate topological invariants for the noninteracting system and for the interacting system. We identify the parameter range (Hubbard $U$, hopping modulation, spin-orbit coupling) where the 2D system behaves as a trivial insulator or quantum spin Hall insulator.

Inspired by the parity-time symmetry concept, we show that a judicious spatial modulation of gain and loss in $\epsilon$-near-zero metamaterials can induce the propagation of exponentially bound interface modes characterized by zero attenuation. With specific reference to a bilayer configuration, via analytical studies and parametrization of the dispersion equation, we show that this waveguiding mechanism can be sustained in the presence of moderate gain/loss levels, and it becomes leaky (i.e., radiative) below a gain/loss threshold. Moreover, we explore a possible rod-based metamaterial implementation, based on realistic material constituents, which captures the essential features of the waveguiding mechanism, in good agreement with our theoretical predictions. Our results may open up possibilities for the design of optical devices and reconfigurable nanophotonics platforms.

The advent of a new class of high-mobility semiconducting polymers opens up a window to address fundamental issues in electrical transport mechanism such as transport between localized states versus extended state conduction. Here, we investigate the origin of the ultralow degree of disorder $(E_a\sim 16\mathrm{meV})$ and the “bandlike” negative temperature ($T$) coefficient of the field effect electron mobility: ${\mu }_{\mathrm{FET}}^e\left(T\right)$ in a high performance $({\mu }_{\mathrm{FET}}^e>2.5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$ diketopyrrolopyrrole based semiconducting polymer. Models based on the framework of mobility edge with exponential density of states are invoked to explain the trends in transport. The temperature window over which the system demonstrates delocalized transport was tuned by a systematic introduction of disorder at the transport interface. Additionally, the Hall mobility $\left({\mu }_{\mathrm{Hall}}^e\right)$ extracted from Hall voltage measurements in these devices was found to be comparable to field effect mobility $\left({\mu }_{\mathrm{FET}}^e\right)$ in the high $T$ bandlike regime. Comprehensive studies with different combinations of dielectrics and semiconductors demonstrate the effectiveness of rationale molecular design, which emphasizes uniform-energetic landscape and low reorganization energy.

Electronic properties of surface areas decoupled from graphite are studied using scanning tunneling microscopy and spectroscopy. We show that it is possible to identify the decoupled graphene monolayer, the Bernal bilayer, and the Bernal trilayer on a graphite substrate according to their tunneling spectra in a high magnetic field. The decoupled monolayer and bilayer exhibit Landau quantization of massless and massive Dirac fermions, respectively. The substrate generates a sizable band gap $\sim 35\mathrm{meV}$ in the Bernal bilayer, therefore, the eightfold degenerate Landau level at the charge neutrality point is split into two valley-polarized quartets polarized on each layer. In the decoupled Bernal trilayer, we find that both massless and massive Dirac fermions coexist and its low-energy band structure can be described quite well by taking into account only the nearest-neighbor intra- and interlayer hopping parameters. A strong correlation between the Fermi velocity of the massless Dirac fermions and the effective mass of the massive Dirac fermions is observed in the graphene trilayer. Our result demonstrates that the surface of graphite provides a natural ideal platform to probe the electronic spectra of graphene layers.

We obtain a fluctuation-dissipation theorem describing thermal electromagnetic fluctuation effects in nonlinear media that we exploit in conjunction with a stochastic Langevin framework to study thermal radiation from Kerr $\left({\chi }^{\left(3\right)}\right)$ photonic cavities coupled to external environments at and out of equilibrium. We show that, in addition to thermal broadening due to two-photon absorption, the emissivity of such cavities can exhibit asymmetric, non-Lorentzian line shapes due to self-phase modulation. When the local temperature of the cavity is larger than that of the external bath, we find that the heat transfer into the bath exceeds the radiation from a corresponding linear blackbody at the same local temperature. We predict that these temperature-tunable thermal processes can be observed in practical, nanophotonic cavities operating at relatively small temperatures.