We demonstrate experimentally a relatively simple yet powerful all-optical enhancement and control technique for high harmonic generation. This is achieved by using as a pump beam two different spatial optical modes interfering together to realize tunable periodic quasi-phase matching of the interaction. With this technique, we demonstrate on-the-fly quasi-phase matching of harmonic orders 29–41 at ambient gas pressure levels of 50 and 100 Torr, where an up to 100-fold enhancement of the emission is observed. The technique is scalable to different harmonic orders and ambient pressure conditions.

The microcanonical Gross-Pitaevskii (also known as the semiclassical Bose-Hubbard) lattice model dynamics is characterized by a pair of energy and norm densities. The grand canonical Gibbs distribution fails to describe a part of the density space, due to the boundedness of its kinetic energy spectrum. We define Poincaré equilibrium manifolds and compute the statistics of microcanonical excursion times off them. The tails of the distribution functions quantify the proximity of the many-body dynamics to a weakly nonergodic phase, which occurs when the average excursion time is infinite. We find that a crossover to weakly nonergodic dynamics takes place inside the non-Gibbs phase, being unnoticed by the largest Lyapunov exponent. In the ergodic part of the non-Gibbs phase, the Gibbs distribution should be replaced by an unknown modified one. We relate our findings to the corresponding integrable limit, close to which the actions are interacting through a short range coupling network.

We investigate, both experimentally and theoretically, how the spectral distribution of laser accelerated carbon ions can be filtered by charge exchange processes in a double foil target setup. Carbon ions at multiple charge states with an initially wide kinetic energy spectrum, from 0.1 to 18 MeV, were detected with a remarkably narrow spectral bandwidth after they had passed through an ultrathin and partially ionized foil. With our theoretical calculations, we demonstrate that this process is a consequence of the evolution of the carbon ion charge states in the second foil. We calculated the resulting spectral distribution separately for each ion species by solving the rate equations for electron loss and capture processes within a collisional radiative model. We determine how the efficiency of charge transfer processes can be manipulated by controlling the ionization degree of the transfer matter.

The ability to measure single quanta allows the complete characterization of small quantum systems known as full-counting statistics. Quantum gas microscopy enables one to observe many-body systems at the single-atom precision. We extend the idea of full-counting statistics to nonequilibrium open many-particle dynamics and apply it to discuss the quench dynamics. By way of illustration, we consider an exactly solvable model to demonstrate the emergence of unique phenomena such as nonlocal and chiral propagation of correlations, leading to a concomitant oscillatory entanglement growth. We find that correlations can propagate beyond the conventional maximal speed, known as the Lieb-Robinson bound, at the cost of probabilistic nature of quantum measurement. These features become most prominent at the real-to-complex spectrum transition point of an underlying parity-time-symmetric effective non-Hermitian Hamiltonian. A possible experimental situation with quantum gas microscopy is discussed.

Time-dependent density-functional theory molecular dynamics reveals an unexpected effect of optical excitation in the experimentally observed rhombohedral-to-cubic transition of GeTe. The excitation induces coherent forces along [001], which may be attributed to the unique energy landscape of Peierls-distorted solids. The forces drive the $A_{1g}$ optical phonon mode in which Ge and Te move out of phase. Upon damping of the $A_{1g}$ mode, phase transition takes place, which involves no atomic diffusion, defect formation, or the nucleation and growth of the cubic phase.

Low-angle grain boundaries generally exist in the form of dislocation arrays, while high-angle grain boundaries (misorientation angle $>15°$) exist in the form of structural units in bulk metals. Here, through in situ atomic resolution aberration corrected electron microscopy observations, we report size-dependent grain-boundary structures improving both stabilities of electrical conductivity and mechanical properties in sub-10-nm-sized gold crystals. With the diameter of a nanocrystal decreasing below 10 nm, the high-angle grain boundary in the crystal exists as an array of dislocations. This size effect may be of importance to a new generation of interconnects applications.

The vortex, a fundamental topological excitation featuring the in-plane winding of a vector field, is important in various areas such as fluid dynamics, liquid crystals, and superconductors. Although commonly existing in nature, vortices were observed exclusively in real space. Here, we experimentally observed momentum-space vortices as the winding of far-field polarization vectors in the first Brillouin zone of periodic plasmonic structures. Using homemade polarization-resolved momentum-space imaging spectroscopy, we mapped out the dispersion, lifetime, and polarization of all radiative states at the visible wavelengths. The momentum-space vortices were experimentally identified by their winding patterns in the polarization-resolved isofrequency contours and their diverging radiative quality factors. Such polarization vortices can exist robustly on any periodic systems of vectorial fields, while they are not captured by the existing topological band theory developed for scalar fields. Our work provides a new way for designing high-$Q$ plasmonic resonances, generating vector beams, and studying topological photonics in the momentum space.

Wear is the inevitable damage process of surfaces during sliding contact. According to the well-known Archard’s wear law, the wear volume scales with the real contact area and as a result is proportional to the load. Decades of wear experiments, however, show that this relation only holds up to a certain load limit, above which the linearity is broken and a transition from mild to severe wear occurs. We investigate the microscopic origins of this breakdown and the corresponding wear transition at the asperity level. Our atomistic simulations reveal that the interaction between subsurface stress fields of neighboring contact spots promotes the transition from mild to severe wear. The results show that this interaction triggers the deep propagation of subsurface cracks and the eventual formation of large debris particles, with a size corresponding to the apparent contact area of neighboring contact spots. This observation explains the breakdown of the linear relation between the wear volume and the normal load in the severe wear regime. This new understanding highlights the critical importance of studying contact beyond the elastic limit and single-asperity models.

The phenomenon of charge fractionalization describes the emergence of novel excitations with fractional quantum numbers, as predicted in strongly correlated systems such as spin liquids. We elucidate that precisely such an unusual effect may occur in the simplest possible non-Fermi liquid, the two-channel Kondo effect. To bring this concept down to experimental test, we study nonequilibrium transport through a device realizing the charge two-channel Kondo critical point in a recent experiment by Iftikhar et al. [Nature (London) 526, 233 (2015)]. The shot noise at low voltages is predicted to result in a universal Fano factor $e^{*}/e=1/2$. This allows us to experimentally identify elementary transport processes of emergent fermions carrying half-integer charge.

We study the phase diagram of a model quantum spin Hall system as a function of band inversion and band-coupling strength, demonstrating that when band hybridization is weak, an interaction-induced nematic insulator state emerges over a wide range of band inversion. This property is a consequence of the long-range Coulomb interaction, which favors interband phase coherence that is weakly dependent on momentum and therefore frustrated by the single-particle Hamiltonian at the band inversion point. For weak band hybridization, interactions convert the continuous gap closing topological phase transition at inversion into a pair of continuous phase transitions bounding a state with broken time-reversal and rotational symmetries. At intermediate band hybridization, the topological phase transition proceeds instead via a quantum anomalous Hall insulator state, whereas at strong hybridization interactions play no role. We comment on the implications of our findings for InAs/GaSb and HgTe/CdTe quantum spin Hall systems.

Unconventional superconductivity and magnetism are intertwined on a microscopic level in a wide class of materials. A new approach to this most fundamental and hotly debated issue focuses on the role of interactions between superconducting electrons and bosonic fluctuations at the interface between adjacent layers in heterostructures. Here we fabricate hybrid superlattices consisting of alternating atomic layers of the heavy-fermion superconductor ${\mathrm{CeCoIn}}_5$ and antiferromagnetic (AFM) metal ${\mathrm{CeRhIn}}_5$, in which the AFM order can be suppressed by applying pressure. We find that the superconducting and AFM states coexist in spatially separated layers, but their mutual coupling via the interface significantly modifies the superconducting properties. An analysis of upper critical fields reveals that, upon suppressing the AFM order by applied pressure, the force binding superconducting electron pairs acquires an extreme strong-coupling nature. This demonstrates that superconducting pairing can be tuned nontrivially by magnetic fluctuations (paramagnons) injected through the interface.

The Holstein model describes the interaction between fermions and a collection of local (dispersionless) phonon modes. In the dilute limit, the phonon degrees of freedom dress the fermions, giving rise to polaron and bipolaron formation. At higher densities, the phonons mediate collective superconducting (SC) and charge-density wave (CDW) phases. Quantum Monte Carlo (QMC) simulations have considered both these limits but have not yet focused on the physics of more general phonon spectra. Here we report QMC studies of the role of phonon dispersion on SC and CDW order in such models. We quantify the effect of finite phonon bandwidth and curvature on the critical temperature $T_{\mathrm{cdw}}$ for CDW order and also uncover several novel features of diagonal long-range order in the phase diagram, including a competition between charge patterns at momenta $\mathbf{q}=(\pi ,\pi )$ and $\mathbf{q}=(0,\pi )$ which lends insight into the relationship between Fermi surface nesting and the wave vector at which charge order occurs. We also demonstrate SC order at half filling in situations where a nonzero bandwidth sufficiently suppresses $T_{\mathrm{cdw}}$.

We present polar Kerr effect measurements of the filled skutterudite superconductor ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$. Simultaneous ac susceptibility measurements allow us to observe the superconducting transition under the influence of heating from the optical beam. A nonzero Kerr angle ${\theta }_K$ develops below the superconducting transition, saturating at $\sim 300\mathrm{nrad}$ at low temperatures. This result is repeated across several measurements of multiple samples. By extrapolating the measured ${\theta }_K(T)$ to zero optical power, we are able to show that the Kerr angle onset temperature in one set of measurements is consistent with the transition to the $B$ phase at $T_{C2}$. We discuss the possible explanations for this result and its impact on the understanding of multiphase and inhomogeneous superconductivity in ${\mathrm{PrOs}}_4{\mathrm{Sb}}_{12}$.

We study the fundamental question of the lattice dynamics of a metallic ferromagnet in the regime where the static long-range magnetic order is replaced by the fluctuating local moments embedded in a metallic host. We use the ab initio density functional theory $+$ embedded dynamical mean-field theory functional approach to address the dynamic stability of iron polymorphs and the phonon softening with an increased temperature. We show that the nonharmonic and inhomogeneous phonon softening measured in iron is a result of the melting of the long-range ferromagnetic order and is unrelated to the first-order structural transition from the bcc to the fcc phase, as is usually assumed. We predict that the bcc structure is dynamically stable at all temperatures at normal pressure and is thermodynamically unstable only between the bcc-$\alpha$ and the bcc-$\delta$ phases of iron.

We present magneto-Raman spectroscopy measurements on suspended graphene to investigate the charge carrier density-dependent electron-electron interaction in the presence of Landau levels. Utilizing gate-tunable magnetophonon resonances, we extract the charge carrier density dependence of the Landau level transition energies and the associated effective Fermi velocity $v_F$. In contrast to the logarithmic divergence of $v_F$ at zero magnetic field, we find a piecewise linear scaling of $v_F$ as a function of the charge carrier density, due to a magnetic-field-induced suppression of the long-range Coulomb interaction. We quantitatively confirm our experimental findings by performing tight-binding calculations on the level of the Hartree-Fock approximation, which also allow us to estimate an excitonic binding energy of $\approx 6\mathrm{meV}$ contained in the experimentally extracted Landau level transitions energies.

Three-dimensional shells can be synthesized from the spontaneous self-folding of two-dimensional templates of interconnected panels, called nets. However, some nets are more likely to self-fold into the desired shell under random movements. The optimal nets are the ones that maximize the number of vertex connections, i.e., vertices that have only two of its faces cut away from each other in the net. Previous methods for finding such nets are based on random search, and thus, they do not guarantee the optimal solution. Here, we propose a deterministic procedure. We map the connectivity of the shell into a shell graph, where the nodes and links of the graph represent the vertices and edges of the shell, respectively. Identifying the nets that maximize the number of vertex connections corresponds to finding the set of maximum leaf spanning trees of the shell graph. This method allows us not only to design the self-assembly of much larger shell structures but also to apply additional design criteria, as a complete catalog of the maximum leaf spanning trees is obtained.

We investigate the collective behavior of magnetic swimmers, which are suspended in a Poiseuille flow and placed under an external magnetic field, using analytical techniques and Brownian dynamics simulations. We find that the interplay between intrinsic activity, external alignment, and magnetic dipole-dipole interactions leads to longitudinal structure formation. Our work sheds light on a recent experimental observation of a clustering instability in this system.

In recent years, intriguing hints for the violation of lepton flavor universality (LFU) have been accumulated in semileptonic $B$ decays, both in the charged-current transitions $b\to c{\ell }^{-}{\overline{\nu }}_{\ell }$ (i.e., $R_D$, $R_{D^{*}}$, and $R_{J/\psi }$) and the neutral-current transitions $b\to s{\ell }^{+}{\ell }^{-}$ (i.e., $R_K$ and $R_{K^{*}}$). Hints for LFU violation in $R_{D^{(*)}}$ and $R_{J/\psi }$ point at large deviations from the standard model (SM) in processes involving tau leptons. Moreover, LHCb has reported deviations from the SM expectations in $b\to s{\mu }^{+}{\mu }^{-}$ processes as well as in the ratios $R_K$ and $R_{K^{*}}$, which together point at new physics (NP) affecting muons with a high significance. These hints for NP suggest the possibility of huge LFU-violating effects in $b\to s{\tau }^{+}{\tau }^{-}$ transitions. In this Letter, we predict the branching ratios of $B\to K{\tau }^{+}{\tau }^{-}$, $B\to K^{*}{\tau }^{+}{\tau }^{-}$, and $B_s\to \varphi {\tau }^{+}{\tau }^{-}$, taking into account NP effects in the Wilson coefficients $C_{9{(}^{\prime })}^{\tau \tau }$ and $C_{10{(}^{\prime })}^{\tau \tau }$. Assuming a common NP explanation of $R_D$, $R_{D^{(*)}}$, and $R_{J/\psi }$, we show that a very large enhancement of $b\to s{\tau }^{+}{\tau }^{-}$ processes, of around 3 orders of magnitude compared to the SM, can be expected under fairly general assumptions. We find that the branching ratios of $B_s\to {\tau }^{+}{\tau }^{-}$, $B_s\to \varphi {\tau }^{+}{\tau }^{-}$, and $B\to K^{(*)}{\tau }^{+}{\tau }^{-}$ under these assumptions are in the observable range for LHCb and Belle II.