By combining transport and photoemission measurements on $({\mathrm{Bi}}_{1-x}{\mathrm{In}}_x{)}_2{\mathrm{Se}}_3$ thin films, we report that this system transforms from a topologically nontrivial metal into a topologically trivial band insulator through three quantum phase transitions. At $x\approx 3\%–7\%$, there is a transition from a topologically nontrivial metal to a trivial metal. At $x\approx 15\%$, the metal becomes a variable-range-hopping insulator. Finally, above $x\approx 25\%$, the system becomes a true band insulator with its resistance immeasurably large even at room temperature. This material provides a new venue to investigate topologically tunable physics and devices with seamless gating or tunneling insulators.

The emission of plasmonic light from a single ${\mathrm{C}}_{60}$ molecule on Cu(111) is probed in a scanning tunneling microscope from the weak-coupling, tunneling range to strong coupling of the molecule to the electrodes at contact. At positive sample voltage the photon yield decreases owing to shot-noise suppression in an increasingly transparent quantum contact. At reversed bias an unexpected nonlinear increase occurs. First-principles transport calculations reveal that ultrafast charge fluctuations on the molecule give rise to additional noise at optical frequencies beyond the shot noise of the current that is injected to the tip.

We show that a direct current in a tantalum microstrip can induce steady-state magnetic oscillations in an adjacent nanomagnet through spin torque from the spin Hall effect (SHE). The oscillations are detected electrically via a magnetic tunnel junction (MTJ) contacting the nanomagnet. The oscillation frequency can be controlled using the MTJ bias to tune the magnetic anisotropy. In this 3-terminal device, the SHE torque and the MTJ bias therefore provide independent controls of the oscillation amplitude and frequency, enabling new approaches for developing tunable spin torque nano-oscillators.

We present an experimental study of $S\mathrm{\text{−}}N\mathrm{\text{−}}S$ junctions, with $N$ being a quantum spin Hall insulator made of $\mathrm{InAs}/\mathrm{GaSb}$. A front gate is used to vary the Fermi level into the minigap, where helical edge modes exist [Phys. Rev. Lett. 107, 136603 (2011)]. In this regime we observe a $\sim 2e^2/h$ Andreev conductance peak, consistent with a perfect Andreev reflection on the helical edge modes predicted by theories. The peak diminishes under a small applied magnetic field due to the breaking of time-reversal symmetry. This work thus demonstrates the helical property of the edge modes in a quantum spin Hall insulator.

This work proposes a new type of optoelectronic switch, the phenyl-acetylene-macrocycle-based single-molecule transistor, which utilizes photon-assisted tunneling and destructive quantum interference. The analysis uses single-particle Green’s functions along with Floquet theory. Without the optical field, phenyl-acetylene-macrocycle exhibits a wide range of strong antiresonance between its frontier orbitals. The simulations show large on–off ratios (over ${10}^4$) and measurable currents ($\sim {10}^{-11}\mathrm{A}$) enabled by photon-assisted tunneling in a weak optical field ($\sim 2\times {10}^5\mathrm{V}/\mathrm{cm}$) and at a small source-drain voltage ($\sim 0.05\mathrm{V}$). Field amplitude power scaling laws and a range of field intensities are given for operating one- and two-photon assisted tunneling in phenyl-acetylene-macrocycle-based single-molecule transistors. This development opens up a new direction for creating molecular switches.

We studied the low-energy states of spin-$1/2$ quantum dots defined in $\mathrm{InAs}/\mathrm{InP}$ nanowires and coupled to aluminum superconducting leads. By varying the superconducting gap $\Delta$ with a magnetic field $B$ we investigated the transition from strong coupling $\Delta \ll T_K$ to weak-coupling $\Delta \gg T_K$, where $T_K$ is the Kondo temperature. Below the critical field, we observe a persisting zero-bias Kondo resonance that vanishes only for low $B$ or higher temperatures, leaving the room to more robust subgap structures at bias voltages between $\Delta$ and $2\Delta$. For strong and approximately symmetric tunnel couplings, a Josephson supercurrent is observed in addition to the Kondo peak. We ascribe the coexistence of a Kondo resonance and a superconducting gap to a significant density of intragap quasiparticle states, and the finite-bias subgap structures to tunneling through Shiba states. Our results, supported by numerical calculations, own relevance also in relation to tunnel-spectroscopy experiments aiming at the observation of Majorana fermions in hybrid nanostructures.

Quantum fluctuations in quasi-one-dimensional superconducting channels leading to spontaneous changes of the phase of the order parameter by $2\pi$, alternatively called quantum phase slips (QPS), manifest themselves as the finite resistance well below the critical temperature of thin superconducting nanowires and the suppression of persistent currents in tiny superconducting nanorings. Here we report the experimental evidence that in a current-biased superconducting nanowire the same QPS process is responsible for the insulating state—the Coulomb blockade. When exposed to rf radiation, the internal Bloch oscillations can be synchronized with the external rf drive leading to formation of quantized current steps on the $I\mathrm{\text{−}}V$ characteristic. The effects originate from the fundamental quantum duality of a Josephson junction and a superconducting nanowire governed by QPS—the QPS junction.

Using terahertz (THz) pump-THz probe spectroscopy, we have investigated the dynamics of the nonequilibrium BCS state in a superconducting NbN film after the impulsive photoinjection of high-density Bogoliubov quasiparticles. The superconducting state rapidly changes within the duration of the monocycle THz pump pulse (1.6 ps). The complex optical conductivity spectrum in the nonequilibrium BCS state significantly deviates from that in the equilibrium state. The observed spectral features are qualitatively well described by the effective medium theory that assumes the formation of normal state patches embedded in a superconducting matrix.

Unconventional superconductors host a plethora of interesting physical phenomena. However, the standard theory of superconductors suggests that unconventional pairing is highly sensitive to disorder, and hence can only be observed in ultraclean systems. We find that due to an emergent chiral symmetry, spin-orbital locking can parametrically suppress pair decoherence introduced by impurity scattering in odd-parity superconductors. Our work demonstrates that disorder is not an obstacle to realize odd-parity superconductivity in materials with strong spin-orbit coupling.

Several classical analogues of electromagnetically induced transparency in metamaterials have been demonstrated. A simple two-resonator model can describe their absorption spectrum qualitatively, but fails to provide information about the scattering properties—e.g., transmission and group delay. Here we develop an alternative model that rigorously includes the coupling of the radiative resonator to the external electromagnetic fields. This radiating two-oscillator model can describe both the absorption spectrum and the scattering parameters quantitatively. The model also predicts metamaterials with a narrow spectral feature in the absorption larger than the background absorption of the radiative element. This classical analogue of electromagnetically induced absorption is shown to occur when both the dissipative loss of the radiative resonator and the coupling strength are small. These predictions are subsequently demonstrated in experiments.

Under optical excitation, coupled quantum wells are known to reveal fascinating features in the photoluminescence pattern originating from dipole orientated indirect excitons. The appearance of an external ring has been attributed to macroscopic charge separation in the quantum well plane. We present a classical model of nonlinear diffusion to account for the observed fragmentation of the external ring into a periodic array of islands. The model incorporates the Coulomb interactions between electrons, holes, and indirect excitons. At low temperatures, these interactions lead to pattern formation similar to the experimentally observed ring fragmentation. The fragmentation is found to persist to temperatures above the quantum degeneracy temperature of indirect excitons.

We theoretically study the phase relaxation of excitons in doped single-walled carbon nanotubes caused by elastic scattering between an exciton and an electron. Our calculation of the spectral linewidth, that is the inverse of the scattering rate, demonstrates that this dephasing process is unexpectedly suppressed in doped single-walled carbon nanotubes, giving rise to anomalous nonlinear behavior as a function of the electron density. A detailed investigation reveals that the logarithmic correction of the exciton self-energy plays a critical role in the exciton-carrier scattering dynamics.

We demonstrate experimentally a subdiffraction light pattern, with a period down to 150 nm, at the surface of an optimized silicon nanostructured thin film. We show, using near-field and far-field characterization, that this subdiffraction pattern can be translated and rotated just by changing the illumination angle. The movable high frequency light pattern paves the way for subdiffraction resolution surface imaging microscopy without scanning near-field probes.

A marked change in anelastic properties, namely, elastic softening accompanied by increased damping, has been observed in a single crystal of ${\mathrm{SrTiO}}_3$ below $\sim 50\mathrm{K}$ by resonant ultrasound spectroscopy. This correlates with other subtle changes in structure and properties which have been explained in the past in terms of a novel quantum state and the formation of polar clusters in an incipient ferroelectric structure. Comparison of the new data, obtained at frequencies near 1 MHz, with mechanical spectroscopy data collected at a few Hz or a few kHz, reveals a distinct dispersion with frequency and is interpreted in terms of an acoustic loss mechanism which depends primarily on the mobility under stress of ferroelastic twin walls. In most ferroelastic materials, it is found that the twin walls become immobile below some low-temperature interval due to the pinning effects of defects. It is proposed instead for ${\mathrm{SrTiO}}_3$ that associated with the local atomic displacements within the incipient ferroelectric clusters is a change in structure of the twin walls such that their mobility becomes enhanced. We propose that the structural change is not correlated with structural changes of the bulk material but relates to increasing polarity of the walls. This interpretation implies that ferroelastic domain walls in ${\mathrm{SrTiO}}_3$ become ferroelectric at low temperatures.

We demonstrate a new type of alternating current (ac) interfacial polarization and frequency-dependent fluid displacement phenomenon at a liquid-liquid electrical interface. Two fluid streams—one with a greater electrical conductivity and the other a greater dielectric constant—are made to flow side by side in a microfluidic channel. An ac electric field is applied perpendicular to the interface formed between the liquid lamellae, and fluid is observed to displace across the liquid-liquid interface. The direction and magnitude of this displacement is frequency dependent. At low ac frequency, below the interfacial inverse charge relaxation time, the high-conductivity fluid displaces into the high-dielectric stream. At high frequency the direction of liquid displacement reverses, and the high-dielectric stream injects into the high-conductivity stream. The interfacial crossover frequency where the liquid displacement direction reverses is dependent on differences in electrical properties between the two fluid streams, and is well explained by Maxwell-Wagner polarization mechanics.

Using electron paramagnetic resonance (EPR), energy levels of the carbon vacancy ($V_C$) in $4H$-SiC and its negative-$U$ properties have been determined. Combining EPR and deep-level transient spectroscopy we show that the two most common defects in as-grown $4H$-SiC—the $Z_{1/2}$ lifetime-limiting defect and the $E{H}_7$ deep defect—are related to the double acceptor ($2-|0$) and single donor ($0|+$) levels of $V_C$, respectively.

We report numerical simulations of membrane tubulation driven by large colloidal particles. Using Monte Carlo simulations we study how the process depends on particle size and binding strength, and present accurate free energy calculations to sort out how tube formation compares with the competing budding process. We find that tube formation is a result of the collective behavior of the particles adhering on the surface, and it occurs for binding strengths that are smaller than those required for budding. We also find that long linear aggregates of particles forming on the membrane surface act as nucleation seeds for tubulation by lowering the free energy barrier associated to the process.

How nanoparticles interact with biomembranes is central for understanding their bioactivity. In this Letter, we report novel tubular membrane structures induced by adsorbed spherical nanoparticles, which we obtain from energy minimization. The membrane tubules enclose linear aggregates of particles and protrude into the vesicles. The high stability of the particle-filled tubules implies strongly attractive, membrane-mediated interactions between the particles. The tubular structures may provide a new route to encapsulate nanoparticles reversibly in vesicles.

Combining extensive molecular dynamics simulations of lipid bilayer systems of varying chemical compositions with single-trajectory analyses, we systematically elucidate the stochastic nature of the lipid motion. We observe subdiffusion over more than 4 orders of magnitude in time, clearly stretching into the submicrosecond domain. The lipid motion depends on the lipid chemistry, the lipid phase, and especially the presence of cholesterol. We demonstrate that fractional Langevin equation motion universally describes the lipid motion in all phases, including the gel phase, and in the presence of cholesterol. The results underline the relevance of anomalous diffusion in lipid bilayers and the strong effects of the membrane composition.

We study the influence of complex graphs on the metastability and fixation properties of a set of evolutionary processes. In the framework of evolutionary game theory, where the fitness and selection are frequency dependent and vary with the population composition, we analyze the dynamics of snowdrift games (characterized by a metastable coexistence state) on scale-free networks. Using an effective diffusion theory in the weak selection limit, we demonstrate how the scale-free structure affects the system’s metastable state and leads to anomalous fixation. In particular, we analytically and numerically show that the probability and mean time of fixation are characterized by stretched-exponential behaviors with exponents depending on the network’s degree distribution.