We present a novel approach for computing the surface roughness-limited thermal conductivity of silicon nanowires with diameter $D<100\mathrm{nm}$. A frequency-dependent phonon scattering rate is computed from perturbation theory and related to a description of the surface through the root-mean-square roughness height $\Delta$ and autocovariance length $L$. Using a full phonon dispersion relation, we find a quadratic dependence of thermal conductivity on diameter and roughness as $(D/\Delta {)}^2$. Computed results show excellent agreement with experimental data for a wide diameter and temperature range (25–350 K), and successfully predict the extraordinarily low thermal conductivity of $2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at room temperature in rough-etched 50 nm silicon nanowires.

Optical absorption measurements are used to probe the spin polarization in the integer and fractional quantum Hall effect regimes. The system is fully spin polarized only at filling factor $\nu =1$ and at very low temperatures ($\sim 40\mathrm{mK}$). A small change in filling factor ($\delta \nu \approx \pm 0.01$) leads to a significant depolarization. This suggests that the itinerant quantum Hall ferromagnet at $\nu =1$ is surprisingly fragile against increasing temperature, or against small changes in filling factor.

We report measurements of turbulent heat transport in samples of ethane (${\mathrm{C}}_2{\mathrm{H}}_6$) heated from below while the applied temperature difference $\Delta T$ straddled the liquid-vapor coexistence curve $T_{\varphi }(P)$. When the sample top temperature $T_t$ decreased below $T_{\varphi }$, droplet condensation occurred and the latent heat of vaporization $H$ provided an additional heat-transport mechanism. The effective conductivity ${\lambda }_{\mathrm{eff}}$ increased linearly with decreasing $T_t$, and reached a maximum value ${\lambda }_{\mathrm{eff}}^{*}$ that was an order of magnitude larger than the single-phase ${\lambda }_{\mathrm{eff}}$. As $P$ approached the critical pressure, ${\lambda }_{\mathrm{eff}}^{*}$ increased dramatically even though $H$ vanished. We attribute this phenomenon to an enhanced droplet-nucleation rate as the critical point is approached.

We report an in-plane solid-liquid-solid (IPSLS) mode for obtaining self-avoiding lateral silicon nanowires (SiNW) in a reacting-gas-free annealing process, where the growth of SiNWs is guided by liquid indium drops that transform the surrounding $a\mathrm{\text{−}}\mathrm{Si}:\mathrm{H}$ matrix into crystalline SiNWs. The SiNWs can be $\sim \mathrm{mm}$ long, with the smallest diameter down to $\sim 22\mathrm{nm}$. A high growth rate of $>{10}^2\mathrm{nm}/\mathrm{s}$ and rich evolution dynamics are revealed in a real-time in situ scanning electron microscopy observation. A qualitative growth model is proposed to account for the major features of this IPSLS SiNW growth mode.

We discover in simulations of sliding coaxial nanotubes an unanticipated example of dynamical symmetry breaking taking place at the nanoscale. While both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero angular momentum of phonon origin appears spontaneously at a series of critical sliding velocities, in correspondence with large peaks of the sliding friction. The nonlinear equations governing this phenomenon resemble the rotational instability of a forced string. However, several new elements, exquisitely “nano” appear here, with the crucial involvement of umklapp and of sliding nanofriction.

In ultraperipheral relativistic heavy-ion collisions, a photon from the electromagnetic field of one nucleus can fluctuate to a quark-antiquark pair and scatter from the other nucleus, emerging as a ${\rho }^0$. The ${\rho }^0$ production occurs in two well-separated (median impact parameters of 20 and 40 F for the cases considered here) nuclei, so the system forms a two-source interferometer. At low transverse momenta, the two amplitudes interfere destructively, suppressing ${\rho }^0$ production. Since the ${\rho }^0$ decays before the production amplitudes from the two sources can overlap, the two-pion system can only be described with an entangled nonlocal wave function, and is thus an example of the Einstein-Podolsky-Rosen paradox. We observe this suppression in 200 GeV per nucleon-pair gold-gold collisions. The interference is $87\%\pm 5\%(\mathrm{stat}.)\pm 8\%(\mathrm{syst}.)$ of the expected level. This translates into a limit on decoherence due to wave function collapse or other factors of 23% at the 90% confidence level.

Under compression, the As-S liquids are subject to transformations, including polymerization and metallization. We have measured the viscosity of the As-S liquids under high pressures. As a result, large viscosity variations by 4–5 orders of magnitude have been revealed. The viscosity values of the As-S liquids are moderate in the molecular state, very high in the covalent state, and low in the metallic state. Therefore, predicting the viscosity behavior in other melts under pressure is possible, which is of considerable importance for the physics of glass transition, geophysics, and material science.

Using $\mathrm{Co}\mathrm{\text{−}}{L}_{2,3}$ and $\mathrm{O}\mathrm{\text{−}}K$ x-ray absorption spectroscopy, we reveal that the charge ordering in ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ involves high spin ($S=3/2$) ${\mathrm{Co}}^{2+}$ and low spin ($S=0$) ${\mathrm{Co}}^{3+}$ ions. This provides evidence for the spin-blockade phenomenon as a source for the extremely insulating nature of the ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CoO}}_4$ series. The associated $e_g^2$ and $e_g^0$ orbital occupation accounts for the large contrast in the Co-O bond lengths and, in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the nonmagnetic ($S=0$) ${\mathrm{Co}}^{3+}$ ions. From the identification of the bands we infer that ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoO}}_4$ is a narrow band material.

We demonstrate the existence of a novel superconducting state in high quality two-component ${\mathrm{MgB}}_2$ single crystalline superconductors where a unique combination of both type-1 (${\lambda }_1/{\xi }_1<1/\sqrt{2}$) and type-2 (${\lambda }_2/{\xi }_2>1/\sqrt{2}$) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations.

We report studies of novel self-assembled magnetic surface swimmers (magnetic snakes) formed from a dispersion of magnetic microparticles at a liquid-air interface and energized by an alternating magnetic field. We show that under certain conditions the snakes spontaneously break the symmetry of surface flows and turn into self-propelled objects. Parameters of the driving magnetic field tune the propulsion velocity of these snakelike swimmers. We find that the symmetry of the surface flows can also be broken in a controlled fashion by attaching a large bead to a magnetic snake (bead-snake hybrid), transforming it into a self-locomoting entity. The observed phenomena have been successfully described by a phenomenological model based on the amplitude equation for surface waves coupled to a large-scale hydrodynamic mean flow equation.

Heat convection in water can be suppressed by adding a small amount of highly thermophilic nanoparticles. We show that such suppression is not effective when a suspension with uniform concentration of nanoparticles is suddenly heated from below. At Rayleigh numbers smaller than a sample dependent threshold ${\mathrm{Ra}}^{*}$ we observe transient oscillatory convection. Unexpectedly, the duration of convection diverges at ${\mathrm{Ra}}^{*}$. Above ${\mathrm{Ra}}^{*}$ oscillatory convection becomes permanent and the heat transferred exhibits bistability. Our results are explained only partially and qualitatively by existing theories.

Thermal or finite-size scaling analyses of importance sampling Monte Carlo computer simulations in the vicinity of phase transition points often combine different estimates for the same quantity, such as a critical exponent, with the intent to reduce statistical fluctuations. We point out that the origin of such estimates in the same time series often results in pronounced cross correlations which are usually ignored even in high-precision studies, generically leading to significant underestimation of statistical fluctuations. We suggest to use a simple extension of the conventional analysis taking correlation effects into account, which leads to improved estimators with often substantially reduced statistical fluctuations at almost no extra cost in terms of computation time.

We measure up to $2\times {10}^{10}$ positrons per steradian ejected out the back of $\sim \mathrm{mm}$ thick gold targets when illuminated with short ($\sim 1\mathrm{ps}$) ultraintense ($\sim 1\times {10}^{20}\mathrm{W}/{\mathrm{cm}}^2$) laser pulses. Positrons are produced predominately by the Bethe-Heitler process and have an effective temperature of 2–4 MeV, with the distribution peaking at 4–7 MeV. The angular distribution of the positrons is anisotropic. Modeling based on the measurements indicate the positron density to be $\sim {10}^{16}\mathrm{\text{positrons}}/{\mathrm{cm}}^3$, the highest ever created in the laboratory.

We report the results of a new experimental search for a permanent electric dipole moment of ${}^{199}\mathrm{Hg}$ utilizing a stack of four vapor cells. We find $d({}^{199}\mathrm{Hg})=(0.49\pm {1.29}_{\mathrm{stat}}\pm {0.76}_{\mathrm{syst}})\times {10}^{-29}e\mathrm{cm}$, and interpret this as a new upper bound, $|d({}^{199}\mathrm{Hg})|<3.1\times {10}^{-29}e\mathrm{cm}$ (95% C.L.). This result improves our previous ${}^{199}\mathrm{Hg}$ limit by a factor of 7, and can be used to set new constraints on $CP$ violation in physics beyond the standard model.

The response of a noisy nonlinear system to deterministic input signals can be enhanced by cooperative phenomena. We show that when one presents two square waves as input to a two-state system, the response of the system can produce a logical output ($\mathrm{NOR}/\mathrm{OR}$) with a probability controlled by the noise intensity. As one increases the noise (for fixed threshold or nonlinearity), the probability of the output reflecting a $\mathrm{NOR}/\mathrm{OR}$ operation increases to unity and then decreases. Changing the nonlinearity (or the thresholds) of the system changes the output into another logic operation ($\mathrm{NAND}/\mathrm{AND}$) whose probability displays analogous behavior. The interplay of nonlinearity and noise can yield logic behavior, and the emergent outcome of such systems is a logic gate. This “logical stochastic resonance” is demonstrated via an experimental realization of a two-state system with two (adjustable) thresholds.

Long-lived, nondispersing circular, or Bohr, wave packets are produced starting from Li Rydberg atoms by exposing them first to a linearly polarized microwave field at the orbital frequency, 17.6 GHz at principal quantum number $n=72$, which locks the electron’s motion into an approximately linear orbit in which the electron oscillates in phase with the microwave field. The microwave polarization is changed to circular polarization slowly compared to the orbital frequency, and the electron’s motion follows, resulting in a nondispersing Bohr wave packet.

Spatially resolved Raman spectra of individual pristine suspended carbon nanotubes are observed under electrical heating. The Raman $G_{+}$ and $G_{-}$ bands show unequal temperature profiles. The preferential heating is more pronounced in short nanotubes ($2\mu \mathrm{m}$) than in long nanotubes ($5\mu \mathrm{m}$). These results are understood in terms of the decay and thermalization of nonequilibrium phonons, revealing the mechanism of thermal transport in these devices. The measurements also enable a direct estimate of thermal contact resistances and the spatial variation of thermal conductivity.

We demonstrate, via scanning tunneling microscopy (STM) measurements performed at 48 K, the existence of “bright beaches” at the edges of K islands (diameter $\sim 5–500\mathrm{nm}$) on the graphite surface. The enhanced tunneling current is only observed in monolayer-high islands on graphite, and not in islands of similar geometry on top of a K monolayer film. First-principles density functional calculations and STM simulations suggest that this is an STM field effect, which appears as the positive tip attracts donated electrons back to the metallic K islands. The restored charge accumulates preferentially at the island edges.

We use a Feshbach resonance to tune the scattering length $a$ of a Bose-Einstein condensate of ${}^7\mathrm{Li}$ in the $|F=1,m_F=1⟩$ state. Using the spatial extent of the trapped condensate, we extract $a$ over a range spanning 7 decades from small attractive interactions to extremely strong repulsive interactions. The shallow zero crossing in the wing of the Feshbach resonance enables the determination of $a$ as small as 0.01 Bohr radii. Evidence of the weak anisotropic magnetic dipole interaction is obtained by comparison with different trap geometries for small $a$.

We report on a new mechanism for heat conduction in the neutron star crust. We find that collective modes of superfluid neutron matter, called superfluid phonons, can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to the magnetic field when the magnetic field $B\gtrsim {10}^{13}\mathrm{G}$. At a density of $\rho \simeq {10}^{12}–{10}^{14}\mathrm{g}/{\mathrm{cm}}^3$, the conductivity due to superfluid phonons is significantly larger than that due to lattice phonons and is comparable to electron conductivity when the temperature $\simeq {10}^8\mathrm{K}$. This new mode of heat conduction can limit the surface anisotropy in highly magnetized neutron stars. Cooling curves of magnetized neutron stars with and without superfluid heat conduction could show observationally discernible differences.