The zero-temperature value of the in-plane London penetration depth, ${\lambda }_{ab}\left(0\right)$, has been measured in single crystals of $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ as a function of the Co concentration, $x$, across both the underdoped and overdoped superconducting regions of the phase diagram. For $x\gtrsim 0.047$, ${\lambda }_{ab}\left(0\right)$ has been found to have values between $120\pm 50$ and $300\pm 50\text{nm}$. A pronounced increase in ${\lambda }_{ab}\left(0\right)$, to a value as high as $950\pm 50\text{nm}$, has been observed for $x\lesssim 0.047$, corresponding to the region of the phase diagram where the itinerant antiferromagnetic and superconducting phases coexist and compete. Direct determination of the doping-dependent ${\lambda }_{ab}\left(0\right)$ has allowed us to track the evolution of the temperature-dependent superfluid density, from which we infer the development of a pronounced superconducting gap anisotropy at the edges of the superconducting dome.

We consider the variation in the field-induced component of the specific heat $C\left(\mathbf{H}\right)$ with the direction of the applied field in Fe pnictides within quasiclassical Doppler-shift approximation with special emphasis to recent experiments on ${\text{FeSe}}_{0.4}{\text{Te}}_{0.6}$. We show that for extended $s$-wave gap with no nodes, $C\left(\mathbf{H}\right)$ has $\text{cos}4\varphi$ component, where $\varphi$ is the angle between $\mathbf{H}$ and the direction between hole and electron Fermi surfaces. The maxima of $C\left(\mathbf{H}\right)$ are at $\pi /4$, $3\pi /4$, etc., if the applied field is smaller than $H_0\le 1\text{T}$, and at $\varphi =0$, $\pi /2$, etc., if the applied field is larger than $H_0$. The angle dependence of $C\left(\mathbf{H}\right)$, the positions of the maxima, and the relative magnitude of the oscillating component are consistent with the experiments performed in the field of $9\text{T}⪢H_0$. We show that the observed $\text{cos}4\varphi$ variation does not hold if the $s$-wave gap has accidental nodes along the two electron Fermi surfaces.

The formation of well-ordered monolayers of hexagonal boron nitride on the surface of a Rh/YSZ/Si(111) multilayer substrate via a three-step boration-oxidation-nitration process was investigated by x-ray photoelectron spectroscopy (XPS), x-ray photoelectron diffraction (XPD) and low-energy electron diffraction (LEED). The chemical vapor deposition (CVD) of trimethylborate $\text{B}{\left({\text{OCH}}_3\right)}_3$ results in a selective decomposition of the precursor, leading to a dilute distribution of boron within the interstitials of the Rh lattice. After oxidation, the layer of a boron oxygen species of about 1 nm thickness can be transformed into a hexagonal monolayer of BN by annealing in ${\text{NH}}_3$ atmosphere. The results of the present study clearly show that the formation of BN monolayers is also possible when boron and nitrogen are provided successively from separate sources. This procedure represents an alternative routine for the preparation of well-ordered BN monolayers, which benefits from a strong reduction of hazardous potential and economic costs compared to the use of borazine as the current standard precursor.

We propose a plasmon-based reconfigurable antenna to controllably distribute emission from single quantum emitters in spatially separated channels. Our calculations show that crossed particle arrays can split the stream of photons from a single emitter into multiple narrow beams. We predict that beams can be switched on and off by switching host refractive index. The design method is based on engineering the dispersion relations of plasmon chains and is generally applicable to traveling wave antennas. Controllable photon delivery has potential applications in classical and quantum communication.

X-ray powder-diffraction experiments on solid hydrogen are performed at pressures up to 183 GPa and low temperature of 100 K. A transition into phase III is confirmed by in situ Raman-scattering measurements. Two diffraction peaks corresponding to the 100 and 101 lines of the hcp structure are observed above the II-III phase-transition pressure of 160 GPa, indicating that the hydrogen molecules in phases II and III are still in the vicinity of the hcp lattice point up to 183 GPa. Assuming a hexagonal structure for these phases, the lattice parameter, $c$ decreases by 1.5% at the II-III phase transition. The behavior is consistent with a theoretically proposed feature of classical orientational ordering of rotating hydrogen molecules for phase III.

Some time ago, it has been observed that vibrationally highly excited $\text{NO}\left(v\right)$ molecules (with typical vibrational quantum numbers $v\approx 15$) lose substantial amounts of vibrational energy when scattering off a Au(111) surface [H. Huang, C. Rettner, D. Auerbach, and A. Wodtke, Science 290, 111 (2000)]. This has been interpreted as a sign for the breakdown of the Born-Oppenheimer approximation due to vibration-electron coupling. It has been argued that this process cannot be understood on the basis of single-quantum transitions which are typical for “electronic friction” models based on a perturbative treatment of weak vibration-electron couplings. Rather, multiple-quanta transitions characteristic for strong nonadiabatic effects are needed according to recent classical surface hopping calculations involving multiple potential-energy surfaces and model Hamiltonians [N. Shenvi, S. Roy, and J. C. Tully, Science 326, 829 (2009)]. Here we address the importance and magnitude of electronic friction for $\text{NO}@\text{Au}\left(111\right)$ by using fully quantum-mechanical, parameter-free first-principles theories in reduced dimensions. Periodic density-functional theory calculations are performed to obtain a ground-state potential-energy surface along the desorption and NO-vibration coordinates, and coordinate-resolved, finite NO vibrational lifetimes due to vibration-electron coupling. Using this input, the scattering event is modeled by open-system density-matrix theory in the frame of the coupled-channel-density-matrix method, which allows for the inclusion of energy relaxation of the scattering NO molecules. It is found that within this model at least, electronic friction accounts for the observed vibrational deactivation of NO scattering from gold.

We present a formulation of the density-functional $\text{theory}+\text{Hubbard}$ model $\left(\text{DFT}+U\right)$ method that is self-consistent over the choice of Hubbard projectors used to define the correlated subspaces. In order to overcome the arbitrariness in this choice, we propose the use of nonorthogonal generalized Wannier functions (NGWFs) as projectors for the $\text{DFT}+U$ correction. We iteratively refine these NGWF projectors and, hence, the $\text{DFT}+U$ functional, such that the correlated subspaces are fully self-consistent with the $\text{DFT}+U$ ground state. We discuss the convergence characteristics of this algorithm and compare ground-state properties thus computed with those calculated using hydrogenic projectors. Our approach is implemented within, but not restricted to, a linear-scaling DFT framework, opening the path to $\text{DFT}+U$ calculations on systems of unprecedented size.

We consider the proximity effect between a singlet $s$-wave superconductor and the edge of a quantum spin Hall (QSH) topological insulator. We establish that Andreev reflection at a QSH edge state/superconductor interface is perfect while nonlocal Andreev processes through the superconductor are totally suppressed. We compute the corresponding conductance and noise.

We study an Abelian gauge theory in $2+1$ dimensions which has surprising theoretical and phenomenological features. The theory has a vanishing coefficient for the square of the electric field $e_i^2$, characteristic of a quantum critical point with dynamical critical exponent $z=2$, and a level-$k$ Chern-Simons coupling, which is marginal at this critical point. For $k=0$, this theory is dual to a free $z=2$ scalar field theory describing a quantum Lifshitz transition, but $k\ne 0$ renders the scalar description nonlocal. The $k\ne 0$ theory exhibits properties intermediate between the (topological) pure Chern-Simons theory and the scalar theory. For instance, the Chern-Simons term does not make the gauge field massive. Nevertheless, there are chiral edge modes when the theory is placed on a space with boundary and a nontrivial ground-state degeneracy $k^g$ when it is placed on a finite-size Riemann surface of genus $g$. The coefficient of $e_i^2$ is the only relevant coupling; it tunes the system through a quantum phase transition between an isotropic fractional quantum Hall state and an anisotropic fractional quantum Hall state. We compute zero-temperature transport coefficients in both phases and at the critical point and comment briefly on the relevance of our results to recent experiments.

We report the magnetization, ac susceptibility, and specific heat of optically float-zoned single crystals of ${\text{Mn}}_{1-x}{\text{Fe}}_x\text{Si}$ and ${\text{Mn}}_{1-x}{\text{Co}}_x\text{Si}$ for temperatures down to $\sim 2\text{K}$ and magnetic fields up to 9 T. The suppression of the helimagnetic transition temperature $T_1$ above a critical composition $x_1$, as seen in the magnetization, ac susceptibility, and specific heat, suggests the existence of a quantum phase transition at $x_1$. A Vollhardt invariance at a temperature $T_2>T_1$, which may be attributed to the Dzyaloshinsky-Moriya (DM) spin-orbit interactions, is also suppressed with increasing $x$ and vanishes above a concentration $x_2$, where $x_2>x_1$. When suppressing the effects of the DM interactions in an applied magnetic field, the magnetization for sufficiently large fields shares the signatures expected of an underlying putative ferromagnetic quantum critical point for a critical concentration $x_c$, where $x_1<x_c<x_2$. As a function of normalized concentration $x/x_c$, where $x_c^{\text{Co}}\approx 0.084$ and $x_c^{\text{Fe}}\approx 0.192$, the properties of ${\text{Mn}}_{1-x}{\text{Fe}}_x\text{Si}$ and ${\text{Mn}}_{1-x}{\text{Co}}_x\text{Si}$ are essentially identical with $x_1/x_c\approx 0.78$ and $x_2/x_c\approx 1.17$. Taken together, our study identifies ${\text{Mn}}_{1-x}{\text{Fe}}_x\text{Si}$ and ${\text{Mn}}_{1-x}{\text{Co}}_x\text{Si}$ as model systems in which the influence of DM interactions on ferromagnetic quantum criticality may be studied.

Initialization, two-spin coherent manipulation, and readout of a three-spin qubit are demonstrated using a few-electron triple quantum dot. The three-spin qubit is designed to allow all operations for full qubit control to be tuned via nearest-neighbor exchange interaction. Fast readout of charge states takes advantage of multiplexed reflectometry. Decoherence measured in a two-spin subspace is found to be consistent with predictions based on gate voltage noise with a uniform power spectrum. The theory of the exchange-only qubit is developed and it is shown that initialization of only two spins suffices for operation. Requirements for full multiqubit control using only exchange and electrostatic interactions are outlined.

We propose a second version of the van der Waals density functional of Dion et al. [Phys. Rev. Lett. 92, 246401 (2004)], employing a more accurate semilocal exchange functional and the use of a large-$N$ asymptote gradient correction in determining the vdW kernel. The predicted binding energy, equilibrium separation, and potential-energy curve shape are close to those of accurate quantum chemical calculations on 22 duplexes. We anticipate the enabling of chemically accurate calculations in sparse materials of importance for condensed matter, surface, chemical, and biological physics.

Ultrasound attenuation measurements in the $B$-like phase of superfluid ${}^3\text{H}\text{e}$ embedded in 98% porosity aerogel have been performed at four frequencies between 3.6 and 11.3 MHz. At all of the pressures studied (14, 25, and 33 bar), the attenuation exhibits nontrivial frequency dependences that progressively deviate from the hydrodynamic ${\omega }^2$ behavior as the temperature decreases. This behavior is related to the structure of the impurity states inside the gap and could offer a tool to profile the gap structure in this system.

Nearly nanometer-thick cuprates are an appealing platform for devices as well as exploring the roles of dimensionality, disorder, and free carrier density in these compounds. To this end we have produced exfoliated crystals of ${\text{Bi}}_2{\text{Sr}}_2{\text{Dy}}_x{\text{Ca}}_{1-x}{\text{Cu}}_2{\text{O}}_{8+\delta }$ on oxidized silicon substrates. The exfoliated crystals were characterized via atomic force and polarized Raman microscopies. Proper procedures for production, handling, and monitoring of these thin oxides are described. We observe a significant change in the effective exchange constant $J$ of these exfoliated crystals.

The thermal conductivity $\kappa$ of the iron-arsenide superconductor $\text{Ba}{\left({\text{Fe}}_{1-x}{\text{Co}}_x\right)}_2{\text{As}}_2$ was measured down to 50 mK for a heat current parallel $\left({\kappa }_c\right)$ and perpendicular $\left({\kappa }_a\right)$ to the tetragonal $c$ axis for seven Co concentrations from underdoped to overdoped regions of the phase diagram $\left(0.038\le x\le 0.127\right)$. A residual linear term ${\kappa }_{c0}/T$ is observed in the $T\to 0$ limit when the current is along the $c$ axis, revealing the presence of nodes in the gap. Because the nodes appear as $x$ moves away from the concentration of maximal $T_c$, they must be accidental, not imposed by symmetry, and are therefore compatible with an $s_{\pm }$ state, for example. The fact that the in-plane residual linear term ${\kappa }_{a0}/T$ is negligible at all $x$ implies that the nodes are located in regions of the Fermi surface that contribute strongly to $c$-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g., $H_{c2}/4$) excites quasiparticles that conduct heat along the $a$ axis just as well as the nodal quasiparticles conduct along the $c$ axis. This shows that the gap must be very small (but nonzero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong $\mathbf{k}$ dependence of the gap $\Delta \left(\mathbf{k}\right)$ which produces nodes on a Fermi-surface sheet with pronounced $c$-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.

We have systematically investigated the doping and the directional dependence of the gap structure in the 122-type iron pnictide superconductors by point contact Andreev reflection spectroscopy. The studies were performed on single crystals of ${\text{Ba}}_{1-x}{\text{K}}_x{\text{Fe}}_2{\text{As}}_2$ ($x=0.29$, 0.49, and 0.77) and ${\text{SrFe}}_{1.74}{\text{Co}}_{0.26}{\text{As}}_2$ with a sharp tip of Pb or Au pressed along the $c$-axis or the $ab$-plane direction. The conductance spectra obtained on highly transparent contacts clearly show evidence of a robust superconducting gap. The normalized curves can be well described by the Blonder-Tinkham-Klapwijk model with a lifetime broadening. The determined gap value scales very well with the transition temperature, giving the $2\Delta /k_B{T}_C$ value of $\sim 3.1$. The results suggest the presence of a universal coupling behavior in this class of iron pnictides over a broad doping range and independent of the sign of the doping. Moreover, conductance spectra obtained on $c$-axis junctions and $ab$-plane junctions indicate that the observed gap is isotropic in these superconductors.

In a graphene bilayer with Bernal stacking, both $n=0$ and $n=1$ orbital Landau levels have zero kinetic energy. An electronic state in the $N=0$ Landau level consequently has three quantum numbers in addition to its guiding center label: its spin, its valley index $K$ or $K^{\prime }$, and an orbital quantum number $n=0,1$. The two-dimensional electron gas (2DEG) in the bilayer supports a wide variety of broken-symmetry states in which the pseudospins associated with these three quantum numbers order in a manner that is dependent on both filling factor $\nu$ and the electric potential difference between the layers. In this paper, we study the case of $\nu =-1$ in an external field strong enough to freeze electronic spins. We show that an electric potential difference between layers drives a series of transitions, starting from an interlayer-coherent state (ICS) at small potential and leading to an orbitally coherent state (OCS) that is polarized in a single layer. Orbital pseudospins carry electric dipoles with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya interactions that can lead to spiral instabilities. We show that the microwave absorption spectra of ICSs and OCSs are sharply distinct.

We report on (magneto)transport experiments in chemically derived narrow graphene nanoribbons under high magnetic fields (up to 60 T). Evidences of field-dependent electronic confinement are given and allow estimating the possible ribbon edge symmetry. A large positive magnetoconductance indicates a strong suppression of backscattering induced by the magnetic field. Such scenario is supported by tight-binding calculations which consider different types of underlying disorders (smooth edge disorder and long-range Coulomb scatterers).

Spin-blockaded quantum dots provide a unique setting for studying nuclear-spin dynamics in a nanoscale system. Despite recent experimental progress, observing phase-sensitive phenomena in nuclear spin dynamics remains challenging. Here we point out that such a possibility opens up in the regime where hyperfine exchange directly competes with a purely electronic spin-flip mechanism such as the spin-orbital interaction. Interference between the two spin-flip processes, resulting from long-lived coherence of the nuclear-spin bath, modulates the electron-spin-flip rate, making it sensitive to the transverse component of nuclear polarization. In a system repeatedly swept through a singlet-triplet avoided crossing, nuclear precession is manifested in oscillations and sign reversal of the nuclear-spin pumping rate as a function of the waiting time between sweeps. This constitutes a purely electrical method for the detection of coherent nuclear-spin dynamics.

The binary phase diagrams of ${\text{O}}_2/\text{rare}$ gas (He, Ne, Ar, and Xe) mixtures have been measured at 296 K up to 20 GPa in a diamond-anvil cell. The boundary lines were determined by visual observation and by the Raman frequency of the ${\text{O}}_2$ vibron mode. The ${\text{O}}_2\text{-He}$ phase diagram is of eutectic type with a liquid-liquid miscibility gap and complete immiscibility in the solid phase. The ${\text{O}}_2\text{-Ne}$ phase diagram is of eutectic type with partial immiscibility in the solid phase. The ${\text{O}}_2\text{-Ar}$ phase diagram is of azeotrope type and three different structures of $\text{Ar}/{\text{O}}_2$ solid solutions have been identified. The ${\text{O}}_2\text{-Xe}$ phase diagram is of peritectic with eutectic type. The stoichiometric compound $\text{Xe}{\left({\text{O}}_2\right)}_2$ is observed and characterized as a laves phase with a cubic structure isomorphous to ${\text{MgCu}}_2$. A qualitative understanding of these various diagrams is reached by the comparison of the effective hard-sphere diameters of the various species in the 10 GPa range. Some of the usefulness of the knowledge of these binary phase diagrams is highlighted: as a reference data set to test theoretical calculations on mixtures; to grow single crystal of ${\text{O}}_2$ in a rare-gas pressure medium; and to synthesize rare-gas oxides at high pressure.