The theoretical analysis of many problems in physics, astronomy, and applied mathematics requires an efficient numerical exploration of multimodal parameter spaces that exhibit broken ergodicity. Monte Carlo methods are widely used to deal with these classes of problems, but such simulations suffer from a ubiquitous sampling problem: The probability of sampling a particular state is proportional to its entropic weight. Devising an algorithm capable of sampling efficiently the full phase space is a long-standing problem. Here, we report a new hybrid method for the exploration of multimodal parameter spaces exhibiting broken ergodicity. Superposition enhanced nested sampling combines the strengths of global optimization with the unbiased or athermal sampling of nested sampling, greatly enhancing its efficiency with no additional parameters. We report extensive tests of this new approach for atomic clusters that are known to have energy landscapes for which conventional sampling schemes suffer from broken ergodicity. We also introduce a novel parallelization algorithm for nested sampling.

We study the ground state of $N$ weakly interacting electrons (with $N\le 10$) in a two-dimensional parabolic quantum dot with strong Rashba spin-orbit coupling. Using dimensionless parameters for the Coulomb interaction $\lambda \lesssim 1$ and the Rashba coupling $\alpha \gg 1$, the low-energy physics is characterized by an almost flat single-particle dispersion. From an analytical approach for $\alpha \to \infty$ and $N=2$, and from numerical exact-diagonalization and Hartree-Fock calculations, we find a transition from a conventional unmagnetized ground state (for $\lambda <{\lambda }_c$) to an orbital ferromagnet (for $\lambda >{\lambda }_c$), with a large magnetization and a circulating charge current. We show that the critical interaction strength ${\lambda }_c={\lambda }_c(\alpha ,N)$ vanishes in the limit $\alpha \to \infty$.

A common method for extracting true correlations from large data sets is to look for variables with unusually large coefficients on those principal components with the biggest eigenvalues. Here, we show that even if the top principal components have no unusually large coefficients, large coefficients on lower principal components can still correspond to a valid signal. This contradicts the typical mathematical justification for principal component analysis, which requires that eigenvalue distributions from relevant random matrix ensembles have compact support, so that any eigenvalue above the upper threshold corresponds to signal. The new possibility arises via a mechanism based on a variant of the Feynman-Hellmann theorem, and leads to significant correlations between a signal and principal components when the underlying noise is not both independent and uncorrelated, so the eigenvalue spacing of the noise distribution can be sufficiently large. This mechanism justifies a new way of using principal component analysis and rationalizes recent empirical findings that lower principal components can have information about the signal, even if the largest ones do not.

We propose a waveguiding mechanism based on the effective gauge potential for photons. The waveguide geometry consists of core and cladding regions with the same underlying dispersion relation, but subject to different gauge potentials. This geometry can be realized in a dynamically modulated resonator lattice and provides a conceptually straightforward and dynamically reconfigurable mechanism for generating a one-way waveguide.

We show that a spatially well-defined layer of boron dopants in a hydrogen-enriched silicon target allows the production of a high yield of alpha particles of around ${10}^9$ per steradian using a nanosecond, low-contrast laser pulse with a nominal intensity of approximately $3\times {10}^{16}\mathrm{W}{\mathrm{cm}}^{-2}$. This result can be ascribed to the nature of the long laser-pulse interaction with the target and with the expanding plasma, as well as to the optimal target geometry and composition. The possibility of an impact on future applications such as nuclear fusion without production of neutron-induced radioactivity and compact ion accelerators is anticipated.

We analyze the nature of the statistics of the work done on or by a quantum many-body system brought out of equilibrium. We show that, for the sudden quench and for an initial state that commutes with the initial Hamiltonian, it is possible to retrieve the whole nonequilibrium thermodynamics via single projective measurements of observables. We highlight, in a physically clear way, the qualitative implications for the statistics of work coming from considering processes described by operators that either commute or do not commute with the unperturbed Hamiltonian of a given system. We consider a quantum many-body system and derive an expression that allows us to give a physical interpretation, for a thermal initial state, to all of the cumulants of the work in the case of quenched operators commuting with the unperturbed Hamiltonian. In the commuting case, the observables that we need to measure have an intuitive physical meaning. Conversely, in the noncommuting case, we show that, although it is possible to operate fully within the single-measurement framework irrespectively of the size of the quench, some difficulties are faced in providing a clear-cut physical interpretation to the cumulants. This circumstance makes the study of the physics of the system nontrivial and highlights the nonintuitive phenomenology of the emergence of thermodynamics from the fully quantum microscopic description. We illustrate our ideas with the example of the Ising model in a transverse field showing the interesting behavior of the high-order statistical moments of the work distribution for a generic thermal state and linking them to the critical nature of the model itself.

On theoretical grounds, we show that orbital domain walls (ODWs), which are known to exist in the charge and orbital ordered layered manganite ${\mathrm{LaSr}}_2{\mathrm{Mn}}_2{\mathrm{O}}_7$, should be partially charged as a result of competition between orbital-induced strain and Coulomb repulsion. This unexpected result provides the necessary condition for the known thermodynamic stability of these ODWs, which are unlike the more typical domain walls that arise only from an external field. We offer experimental data consistent with this theoretical framework through a combined transport and x-ray-diffraction study. In particular, our transport data on this charge and orbital ordered manganite exhibit abrupt transformations to higher conductance at a threshold electric field. As transport phenomena closely resemble effects found for sliding charge-density waves (SCDWs) in pseudo-one-dimensional (1D) materials, a SCDW along such pseudo-1D ODWs provides a natural explanation of our data. Importantly, x-ray-diffraction data eliminate heating and melting of charge order as tenable alternative explanations of our data.

Driving a quantum system periodically in time can profoundly alter its long-time dynamics and trigger topological order. Such schemes are particularly promising for generating nontrivial energy bands and gauge structures in quantum-matter systems. Here, we develop a general formalism that captures the essential features ruling the dynamics: the effective Hamiltonian, but also the effects related to the initial phase of the modulation and the micromotion. This framework allows for the identification of driving schemes, based on general $N$-step modulations, which lead to configurations relevant for quantum simulation. In particular, we explore methods to generate synthetic spin-orbit couplings and magnetic fields in cold-atom setups.

Borromean rings and Borromean binding, a class of intriguing phenomena as three objects are linked (bound) together while any two of them are unlinked (unbound), widely exist in nature and have been found in systems of biology, chemistry, and physics. Previous studies have suggested that the occurrence of such a binding in physical systems typically relies on the microscopic details of pairwise interaction potentials at short range and is, therefore, nonuniversal. Here, we report a new type of Borromean binding in ultracold Fermi gases with Rashba spin-orbit coupling, which is universal against short-range interaction details, with its binding energy only dependent on the $s$-wave scattering length and the spin-orbit-coupling strength. We show that the occurrence of this universal Borromean binding is facilitated by the symmetry of the single-particle dispersion under spin-orbit coupling and is, therefore, symmetry selective rather than interaction selective. The state is robust over a wide range of mass ratios between composing fermions, which are accessible by Li-Li, K-K, and K-Li mixtures in cold-atom experiments. Our results reveal the importance of single- particle spectral symmetry in few-body physics and shed light on the emergence of new quantum phases in a many-body system with exotic few-body correlations.

We describe photonic crystal microcavities with very strong light-matter interaction to realize room-temperature, equilibrium, exciton-polariton Bose-Einstein condensation (BEC). This goal is achieved through a careful balance between strong light trapping in a photonic band gap (PBG) and large exciton density enabled by a multiple quantum-well (QW) structure with a moderate dielectric constant. This approach enables the formation of a long-lived, dense $10-\mu \mathrm{m}-1-\mathrm{cm}$- scale cloud of exciton polaritons with vacuum Rabi splitting that is roughly 7% of the bare exciton-recombination energy. We introduce a woodpile photonic crystal made of ${\mathrm{Cd}}_{0.6}{\mathrm{Mg}}_{0.4}\mathrm{Te}$ with a 3D PBG of 9.2% (gap-to-central-frequency ratio) that strongly focuses a planar guided optical field on CdTe QWs in the cavity. For 3-nm QWs with 5-nm barrier width, the exciton-photon coupling can be as large as $\hslash \mathrm{\Omega }=55\mathrm{meV}$ (i.e., a vacuum Rabi splitting of $2\hslash \mathrm{\Omega }=110\mathrm{meV}$). The exciton-recombination energy of 1.65 eV corresponds to an optical wavelength of 750 nm. For $N=106$ QWs embedded in the cavity, the collective exciton-photon coupling per QW ($\hslash \mathrm{\Omega }/\sqrt{N}=5.4\mathrm{meV}$) is much larger than the state-of-the-art value of 3.3 meV, for the CdTe Fabry-Pérot microcavity. The maximum BEC temperature is limited by the depth of the dispersion minimum for the lower polariton branch, over which the polariton has a small effective mass of approximately $10^{-5}{m}_0$, where $m_0$ is the electron mass in vacuum. By detuning the bare exciton-recombination energy above the planar guided optical mode, a larger dispersion depth is achieved, enabling room-temperature BEC. The BEC transition temperature ranges as high as 500 K when the polariton density per QW is increased to $(11a_B{)}^{-2}$, where $a_B\simeq 3.5\mathrm{nm}$ is the exciton Bohr radius and the exciton-cavity detuning is increased to 30 meV. A high-quality PBG can suppress exciton radiative decay and enhance the polariton lifetime to beyond 150 ps at room temperature, sufficient for thermal equilibrium BEC.

We introduce and theoretically investigate here a novel analytical method that we have called focal molography, in which molecular interactions are made visible through scattering of coherent light by a coherent pattern of molecules. The scattered light quantifies the presence of molecules at molecular interaction sites. It is separated from noncoherent background scatter by a combination of local dark-field illumination, interference enhancement, and spatial filtering. The latter is achieved by holographic focusing of the wave field generated by the coherently assembled molecules onto an Airy disk and by subtraction of the noncoherent irradiance in the focal plane outside the disk from the irradiance in the disk. This new microscopic method allows distinct detection of low-refractive-index contrast in the nanoenvironment of biomolecules from which information on the interaction of the coherently assembled molecules with molecules in a liquid or gaseous sample may be deduced. The noncoherent surroundings of the coherently assembled molecules consist of freely diffusing solvent and solute molecules. The surroundings, as well as changes in temperature, do not contribute to the coherent signal in the diffraction focus. Interference lithography or high-resolution-imaging lithography can be used to synthesize the coherent pattern of molecules on a monolithic substrate. The coherent pattern of molecules constitutes a synthetic phase hologram that creates a diffraction-limited light wave. We suggest the term “mologram” for the coherent assembly of functional nanostructures and the term “focal molography” for label-free or labeled analysis of molecular interactions through the measurement of the properties of light in the focus of the mologram. We derive analytical formulas that express the detection signal and the sensitivity of focal molography on the surface of a high-refractive-index thin-film optical waveguide in terms of known parameters. We discuss the implementation of a readout system for molograms on a thin-film optical waveguide by adapting a confocal laser-scanning microscope to a bifocal laser-scanning microscope.

Inspired by the recent demonstration that ${\mathrm{Ca}}_2\mathrm{N}$ is a two-dimensional electride in which electrons near the Fermi level float between ionic layers, we have conducted a systematic search for 2D electrides by an approach combining crystal structure databases and ab initio electronic structure calculations. Starting from a set of working hypotheses, we have identified six 2D metallic electrides, of which one is nonmagnetic and five are nearly ferromagnetic ferrimagnets with a distinct spin texture. The latter five materials are the first magnetic electrides reported at ambient pressure.

Physics and information are intimately connected, and the ultimate information processing devices will be those that harness the principles of quantum mechanics. Many physical systems have been identified as candidates for quantum information processing, but none of them are immune from errors. The challenge remains to find a path from the experiments of today to a reliable and scalable quantum computer. Here, we develop an architecture based on a simple module comprising an optical cavity containing a single negatively charged nitrogen vacancy center in diamond. Modules are connected by photons propagating in a fiber-optical network and collectively used to generate a topological cluster state, a robust substrate for quantum information processing. In principle, all processes in the architecture can be deterministic, but current limitations lead to processes that are probabilistic but heralded. We find that the architecture enables large-scale quantum information processing with existing technology.

Radiationless electromagnetic interference (REI) has been used to achieve focusing below Abbe’s diffraction limit. Here, we demonstrate an approach to REI that uses the Fano resonance of subwavelength slits to achieve subdiffraction focusing. Two main features of the Fano resonance are critical: (1) The Fano resonance suppresses radiation by destructive interference, thereby allowing for REI, and (2) the Fano resonance creates a resonant field enhancement allowing one to overcome evanescent decay, which is different from past approaches to REI. An analytic theory is introduced to explain these results. While the analytic theory is formulated for a perfect electric conductor, comprehensive numerical simulations show the applicability in the visible regime, where losses and plasmonic effects play a role.

The dimensionality of a system can fundamentally impact the behavior of interacting quantum particles. Classic examples range from the fractional quantum Hall effect to high-temperature superconductivity. As a general rule, one expects confinement to favor the binding of particles. However, attractively interacting bosons apparently defy this expectation: While three identical bosons in three dimensions can support an infinite tower of Efimov trimers, only two universal trimers exist in the two-dimensional case. Here, we reveal how these two limits are connected by investigating the problem of three identical bosons confined by a harmonic potential along one direction. We show that the confinement breaks the discrete Efimov scaling symmetry and successively destroys the weakest bound trimers. However, the deepest bound trimers persist even under strong confinement. In particular, the ground-state Efimov trimer hybridizes with the two-dimensional trimers, yielding a superposition of trimer configurations that effectively involves tunneling through a short-range repulsive barrier. Our results suggest a way to use strong confinement to engineer more stable Efimov-like trimers, which have so far proved elusive.

This work is concerned with thermal quantum states of Hamiltonians on spin- and fermionic-lattice systems with short-range interactions. We provide results leading to a local definition of temperature, thereby extending the notion of “intensivity of temperature” to interacting quantum models. More precisely, we derive a perturbation formula for thermal states. The influence of the perturbation is exactly given in terms of a generalized covariance. For this covariance, we prove exponential clustering of correlations above a universal critical temperature that upper bounds physical critical temperatures such as the Curie temperature. As a corollary, we obtain that above the critical temperature, thermal states are stable against distant Hamiltonian perturbations. Moreover, our results imply that above the critical temperature, local expectation values can be approximated efficiently in the error and the system size.

The normal-state in-plane resistivity below the zero-field superconducting transition temperature $T_c$ and the upper critical field ${\mu }_0{H}_{c2}(T)$ was measured by suppressing superconductivity in pulsed magnetic fields for ${\mathrm{K}}_{0.70}{\mathrm{Fe}}_{1.46}{\mathrm{Se}}_{1.85}{\mathrm{Te}}_{0.15}$. The normal-state resistivity ${\rho }_{ab}$ is found to increase logarithmically with decreasing temperature as $(T/T_c)\to 0$. Similar to granular metals, our results suggest that a superconductor–insulator transition below zero-field $T_c$ may be induced in high magnetic fields. This is related to the intrinsic real-space phase-separated states common to all inhomogeneous superconductors.

The enigmatic pseudogap phase in underdoped cuprate high-$T_c$ superconductors has long been recognized as a central puzzle of the $T_c$ problem. Recent data show that the pseudogap is likely a distinct phase, characterized by a medium range and quasistatic charge ordering. However, the origin of the ordering wave vector and the mechanism of the charge order is unknown. At the same time, earlier data show that precursive superconducting fluctuations are also associated with this phase. We propose that the pseudogap phase is a novel pairing state where electrons on the same side of the Fermi surface are paired, in strong contrast with conventional Bardeen-Cooper-Schrieffer theory which pairs electrons on opposite sides of the Fermi surface. In this state the Cooper pair carries a net momentum and belongs to a general class called pair density wave. The microscopic pairing mechanism comes from a gauge theory formulation of the resonating valence bond (RVB) picture, where spinons traveling in the same direction feel an attractive force in analogy with Ampere’s effects in electromagnetism. We call this Amperean pairing. Charge order automatically appears as a subsidiary order parameter even when long-range pair order is destroyed by phase fluctuations. Our theory gives a prediction of the ordering wave vector which is in good agreement with experiment. Furthermore, the quasiparticle spectrum from our model explains many of the unusual features reported in photoemission experiments. The Fermi arc, the unusual way the tip of the arc terminates, and the relation of the spanning vector of the arc tips to the charge ordering wave vector also come out naturally. Finally, we propose an experiment that can directly test the notion of Amperean pairing.

Graphene on metal substrates often shows different electronic properties from isolated graphene because of graphene-substrate interactions. One needs to remove the metals with acids and then to transfer graphene to weakly interacting substrates to recover electrical properties inherent in graphene. This process is not easy and besides causes undesirable tears, defects, and impurities in graphene. Here, we report a method to recover the electronic structure of graphene from a strongly interacting Ni substrate by spontaneous Na intercalation. In order to characterize the intercalation process, the density-functional-theory calculations and angle-resolved photoemission-spectroscopy (ARPES) and scanning-tunneling-microscopy (STM) measurements are carried out. From the density-functional-theory calculations, Na atoms energetically prefer interface intercalation to surface adsorption for the graphene/Ni(111) surface. Unlike most intercalants, Na atoms intercalate spontaneously at room temperature due to a tiny diffusion barrier, which is consistent with our temperature-dependent ARPES and core-level photoemission spectroscopy, and with our submonolayer ARPES and STM results at room temperature. As a result of the spontaneous intercalation, the electronic structure of graphene is almost recovered, as confirmed by the Dirac cone with a negligible band gap in ARPES and the sixfold symmetry in STM.

We provide a unified thermodynamic formalism describing information transfers in autonomous as well as nonautonomous systems described by stochastic thermodynamics. We demonstrate how information is continuously generated in an auxiliary system and then transferred to a relevant system that can utilize it to fuel otherwise impossible processes. Indeed, while the joint system satisfies the second law, the entropy balance for the relevant system is modified by an information term related to the mutual information rate between the two systems. We show that many important results previously derived for nonautonomous Maxwell demons can be recovered from our formalism and use a cycle decomposition to analyze the continuous information flow in autonomous systems operating at a steady state. A model system is used to illustrate our findings.