We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.

We experimentally demonstrate a topological transition of classical light in “photonic graphene”: an array of waveguides arranged in the honeycomb geometry. As the system is uniaxially strained (compressed), the two unique Dirac points (present in the spectrum of conventional graphene) merge and annihilate each other, and a band gap forms. As a result, edge states are created on the zigzag edge and destroyed on the bearded edge. These results are applicable for any 2D honeycomb-type structure, from carbon-based graphene to photonic lattices and crystals.

We have developed an interferometric implementation of coherent anti-Stokes Raman scattering which enables broadband coherent Raman spectroscopy free from the nonresonant background, with a signal strength proportional to concentration. Spectra encode mode symmetry information into the amplitude response, which can be directly compared to polarized spontaneous Raman spectra. The method requires only passive polarization optics and is suitable for a wide range of laser linewidths and pulse durations. The method’s application to Raman spectral imaging is demonstrated.

Nature has successfully evolved the mushroom-shaped contact geometry in many organisms in order to solve the attachment problem. We studied the detachment process of individual bioinspired artificial mushroom-shaped adhesive microstructures (MSAMSs) resolving the failure dynamics at high spatiotemporal resolution. The experimental data provide strong evidence for a homogeneous stress distribution in MSAMS, which was recently proposed. Our results allow us to explain the advantage of such contact geometry and provide a suggestion for the widely observed mushroom-shaped contact geometry.

The Courant-Snyder (CS) theory for one degree of freedom is generalized to the case of coupled transverse dynamics in general linear focusing lattices with quadrupole, skew-quadrupole, dipole, and solenoidal components, as well as torsion of the fiducial orbit and variation of beam energy. The envelope function is generalized into an envelope matrix, and the phase advance is generalized into a 4D sympletic rotation. The envelope equation, the transfer matrix, and the CS invariant of the original CS theory all have their counterparts, with remarkably similar expressions, in the generalized theory.

We present a generalized model to explain the spatial and temporal evolution of photoinduced surface structure in photosensitive amorphous thin films. The model describes these films as an incompressible viscous fluid driven by a photoinduced pressure originating from dipole rearrangement. This derivation requires only the polarizability, viscosity and surface tension of the system. Using values of these physical parameters, we check the validity of the model by fitting to experimental data of ${\mathrm{As}}_2{\mathrm{S}}_3$ and demonstrating good agreement.

We measured the adsorption geometry of single molecules with intramolecular resolution using noncontact atomic force microscopy with functionalized tips. The lateral adsorption position was determined with atomic resolution, adsorption height differences with a precision of 3 pm, and tilts of the molecular plane within 0.2°. The method was applied to five $\pi$-conjugated molecules, including three molecules from the olympicene family, adsorbed on Cu(111). For the olympicenes, we found that the substitution of a single atom leads to strong variations of the adsorption height, as predicted by state-of-the-art density-functional theory, including van der Waals interactions with collective substrate response effects.

We perform semilocal and hybrid density-functional theory (DFT) studies of the $\mathrm{Sn}/\mathrm{Ge}(111)$ surface to identify the origin of the observed insulating $\sqrt{3}\times \sqrt{3}$ phase below $\sim 30\mathrm{K}$. In contrast with the semilocal DFT calculation predicting a metallic $3\times 3$ ground state, the hybrid DFT calculation including van der Waals interactions shows that the insulating ferrimagnetic structure with a $\sqrt{3}\times \sqrt{3}$ structural symmetry is energetically favored over the metallic $3\times 3$ structure. It is revealed that the correction of the self-interaction error with a hybrid exchange-correlation functional gives rise to a band gap opening induced by a ferrimagnetic order. The results show that the observed insulating phase is attributed to the Slater mechanism via itinerant magnetic order rather than the hitherto accepted Mott-Hubbard mechanism via electron correlations.

We present NMR measurements of a strong-leg spin-$1/2$ Heisenberg antiferromagnetic ladder compound $({\mathrm{C}}_7{\mathrm{H}}_{10}\mathrm{N}{)}_2{\mathrm{CuBr}}_4$ under magnetic fields up to 15 T in the temperature range from 1.2 K down to 50 mK. From the splitting of NMR lines, we determine the phase boundary and the order parameter of the low-temperature (three-dimensional) long-range-ordered phase. In the Tomonaga-Luttinger regime above the ordered phase, NMR relaxation reflects characteristic power-law decay of spin correlation functions as $1/T_1\propto T^{1/2K-1}$, which allows us to determine the interaction parameter $K$ as a function of field. We find that field-dependent $K$ varies within the $1<K<2$ range, which signifies attractive interaction between the spinless fermions in the Tomonaga-Luttinger liquid.

Quantum degeneracy is an important concept in quantum mechanics with large implications to many processes in condensed matter. Here, we show the consequences of electron energy level degeneracy on the conductance and the chemical force between two bodies at the atomic scale. We propose a novel way in which a scanning probe microscope can detect the presence of degenerate states in atomic-sized contacts even at room temperature. The tunneling conductance $G$ and chemical binding force $F$ between two bodies both tend to decay exponentially with distance in a certain distance range, usually maintaining direct proportionality $G\propto F$. However, we show that a square relation $G\propto F^2$ arises as a consequence of quantum degeneracy between the interacting frontier states of the scanning tip and a surface atom. We demonstrate this phenomenon on the $\mathrm{Si}(111)\mathrm{\text{−}}(7\times 7)$ surface reconstruction where the Si adatom possesses a strongly localized dangling-bond state at the Fermi level.

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal ${\mathrm{LaNiO}}_3$. We also report reference experiments on thin films of ${\mathrm{PrNiO}}_3$ and ${\mathrm{NdNiO}}_3$. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni $d$-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions.

High-resolution laser-based angle-resolved photoemission measurements have been carried out on ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ (Bi2212) superconductors to investigate momentum dependence of electron coupling with collective excitations (modes). Two coexisting energy scales are clearly revealed over a large momentum space for the first time in the superconducting state of the overdoped Bi2212 superconductor. These two energy scales exhibit distinct momentum dependence: one keeps its energy near 78 meV over a large momentum space while the other changes its energy from $\sim 40\mathrm{meV}$ near the antinodal region to $\sim 70\mathrm{meV}$ near the nodal region. These observations provide a new picture on momentum evolution of electron-boson coupling in Bi2212 that electrons are coupled with two sharp modes simultaneously over a large momentum space in the superconducting states. Their unusual momentum dependence poses a challenge to our current understanding of electron-mode-coupling and its role for high-temperature superconductivity in cuprate superconductors.

We use polarized neutron scattering to demonstrate that in-plane spin excitations in electron doped superconducting ${\mathrm{BaFe}}_{1.904}{\mathrm{Ni}}_{0.096}{\mathrm{As}}_2$ ($T_c=19.8\mathrm{K}$) change from isotropic to anisotropic in the tetragonal phase well above the antiferromagnetic (AFM) ordering and tetragonal-to-orthorhombic lattice distortion temperatures ($T_N\approx T_s=33\pm 2\mathrm{K}$) without an uniaxial pressure. While the anisotropic spin excitations are not sensitive to the AFM order and tetragonal-to-orthorhombic lattice distortion, superconductivity induces further anisotropy for spin excitations along the [110] and $[1\overline{1}0]$ directions. These results indicate that the spin excitation anisotropy is a probe of the electronic anisotropy or orbital ordering in the tetragonal phase of iron pnictides.

We propose to use an ancilla fluxonium qubit to interact with a Majorana qubit hosted by a topological one-dimensional wire. The coupling is obtained using the Majorana qubit-controlled $4\pi$ Josephson effect to flux bias the fluxonium qubit. We demonstrate how this coupling can be used to sensitively identify topological superconductivity, to measure the state of the Majorana qubit, to construct 2-qubit operations, and to implement quantum memories with topological protection.

We have built a hybrid system composed of a superconducting flux qubit (the processor) and an ensemble of nitrogen-vacancy centers in diamond (the memory) that can be directly coupled to one another, and demonstrated how information can be transferred from the flux qubit to the memory, stored, and subsequently retrieved. We have established the coherence properties of the memory and succeeded in creating an entangled state between the processor and memory, demonstrating how the entangled state’s coherence is preserved. Our results are a significant step towards using an electron spin ensemble as a quantum memory for superconducting qubits.

Topological phases are unique states of matter which support nonlocal excitations which behave as particles with fractional statistics. A universal characterization of gapped topological phases is provided by the topological entanglement entropy (TEE). We study the finite size corrections to the TEE by focusing on systems with a $Z_2$ topological ordered state using density-matrix renormalization group and perturbative series expansions. We find that extrapolations of the TEE based on the Renyi entropies with a Renyi index of $n\ge 2$ suffer from much larger finite size corrections than do extrapolations based on the von Neumann entropy. In particular, when the circumference of the cylinder is about ten times the correlation length, the TEE obtained using von Neumann entropy has an error of order ${10}^{-3}$, while for Renyi entropies it can even exceed 40%. We discuss the relevance of these findings to previous and future searches for topological ordered phases, including quantum spin liquids.

We report femtosecond stimulated Raman spectroscopy measurements of lattice dynamics in semiconductor nanocrystals and characterize longitudinal optical (LO) phonon production during confinement-enhanced, ultrafast intraband relaxation. Stimulated Raman signals from unexcited CdSe nanocrystals produce a spectral shape similar to spontaneous Raman signals. Upon photoexcitation, stimulated Raman amplitude decreases owing to experimentally resolved ultrafast phonon generation rates within the lattice. We find a $\sim 600\mathrm{fs}$, particle-size-independent depletion time attributed to hole cooling, evidence of LO-to-acoustic down-conversion, and LO phonon mode softening.

We study the effect of confinement on glassy liquids using random first order transition theory as a framework. We show that the characteristic length scale above which confinement effects become negligible is related to the point-to-set length scale introduced to measure the spatial extent of amorphous order in supercooled liquids. By confining below this characteristic size, the system becomes a glass. Eventually, for very small sizes, the effect of the boundary is so strong that any collective glassy behavior is wiped out. We clarify similarities and differences between the physical behaviors induced by confinement and by pinning particles outside a spherical cavity (the protocol introduced to measure the point-to-set length). Finally, we discuss possible numerical and experimental tests of our predictions.

We propose a mechanism for the initiation of cell motility that is based on myosin-induced contraction and does not require actin polymerization. The translocation of a cell is induced by symmetry breaking of the motor-driven flow, and the ensuing asymmetry gives rise to a steady motion of the center of mass of a cell. The predictions of the model are consistent with observations on keratocytes.

In various single-molecule experiments, a chiral polymer, such as DNA, is simultaneously pulled and twisted. We address an elementary but fundamental question raised by various authors: does the molecule overwind or unwind under tension? We show that within the context of the classic Kirchhoff-Love rod model of elastic filaments, both behaviors are possible, depending on the precise constitutive relations of the polymer. More generally, our analysis provides an effective linear response theory for helical structures that relates axial force and axial torque to axial translation and rotation.