We investigate the ground-state energy of a one-dimensional Fermi gas with two bosonic impurities. We consider spinless fermions with no fermion-fermion interactions. The fermion-impurity and impurity-impurity interactions are modeled with Dirac delta functions. First, we study the case where impurity and fermion have equal masses, and the impurity-impurity two-body interaction is identical to the fermion-impurity interaction, such that the system is solvable with the Bethe ansatz. For attractive interactions, we find that the energy of the impurity-impurity subsystem is below the energy of the bound state that exists without the Fermi gas. We interpret this as a manifestation of attractive boson-boson interactions induced by the fermionic medium, and refer to the impurity-impurity subsystem as an in-medium bound state. For repulsive interactions, we find no in-medium bound states. Second, we construct an effective model to describe these interactions, and compare its predictions to the exact solution. We use this effective model to study nonintegrable systems with unequal masses and/or potentials. We discuss parameter regimes for which impurity-impurity attraction induced by the Fermi gas can lead to the formation of in-medium bound states made of bosons that repel each other in the absence of the Fermi gas.

We propose a method to realize a broad class of tunable fermionic Hamiltonians in graphene bilayer. For that matter, we consider graphene bilayer functionalized with ${\mathrm{sp}}^3$ defects that induce zero-energy resonances hosting an individual electron each. The application of an off-plane electric field opens up a gap, so that the zero-energy resonance becomes an in-gap bound state whose confinement scales inversely with the gap. Controlling both the distance among the defects and the applied electric field, we can define fermionic models, even lattices, whose hoppings and Coulomb interactions can be tuned. We consider in detail the case of triangular and honeycomb artificial lattices and we show how, for a given arrangement of the ${\mathrm{sp}}^3$ centers, these lattices can undergo an electrically controlled transition between the weak and strong coupling regimes.

Using position averaged convergent beam electron diffraction (PACBED) the Jahn-Teller distortion in $\mathrm{LaMn}{\mathrm{O}}_3$ is quantitatively measured using a straightforward pattern-matching approach. The fit between experimental patterns and PACBED patterns simulated using the quantum excitation of phonons model allows a three-dimensional measure of octahedral distortion and rotation information from the near transmitted disk region with picometer precision. The effects of plasmon and other inelastic components on quantification using this method are investigated and discussed. The results provide an avenue for accurate local studies of the crystal structure origins of emergent physics in parallel with high-resolution annular dark field scanning transmission electron microscopy imaging at interfaces and defects in quantum materials.

The optical excitation of $\mathrm{Nb}{\mathrm{O}}_2$ is known to induce a transient metallicity that partially recovers on a subpicosecond speed and only fully recovers after several nanoseconds. Here, through low-fluence pump-probe spectroscopy, the short-lived and long-lived components of $\mathrm{Nb}{\mathrm{O}}_2$’s optically induced modulation are found to be linearly dependent on pump fluence in the low-fluence regime. Through an in-depth analysis, the short-lived and long-lived modulations are found to be caused by photocarrier-induced refractive index changes and photothermally induced changes in the lattice temperature of $\mathrm{Nb}{\mathrm{O}}_2$, respectively, and not an insulator-metal transition.

Nodal semimetals (e.g., Dirac, Weyl, and nodal-line semimetals, graphene, etc.) and systems of pinned particles with power-law interactions (trapped ultracold ions, nitrogen defects in diamonds, spins in solids, etc.) are presently at the center of attention of large communities of researchers working in condensed-matter and atomic, molecular, and optical physics. Although seemingly unrelated, both classes of systems are abundant with novel fundamental thermodynamic and transport phenomena. In this Rapid Communication, we demonstrate that low-energy field theories of quasiparticles in semimetals may be mapped exactly onto those of pinned particles with power-law hopping excitations. The duality between the two classes of systems, which we establish, trades strong disorder in one class for weak disorder in the other, and allows one to describe the transport and thermodynamics of each class of systems using the results established for the other class. In particular, using the duality mapping, we establish the existence of another class of disorder-driven transition in systems with the power-law hopping $\propto 1/r^{\gamma }$ of excitations with $d/2<\gamma <d$, different from the conventional Anderson-localization transition. Non-Anderson disorder-driven transitions have been studied broadly for nodal semimetals. Our work extends the class of systems exhibiting such transitions to systems with long-range hopping (interactions) with $\gamma <d$.

We analyze charge fluctuations in a parasitic state strongly coupled to a superconducting Josephson-junction-based charge detector. The charge dynamics of the state resembles that of electron transport in a quantum dot with two charge states, and hence we refer to it as a two-level fluctuator. By constructing the distribution of waiting times from the measured detector signal and comparing it with a waiting time theory, we extract the electron in- and out-tunneling rates for the two-level fluctuator, which are severely asymmetric.

Interface-driven magnetic properties such as exchange bias and inverted hysteresis are highly sought after in modern functional materials, where at least two magnetically active layers are required. The ability to achieve these functionalities in a single-layer thin film (monolithic) reduces the dimensionality while enriching the magnetism. Here we uncover a previously unseen part of the phase diagram of a monolithic epitaxial thin film of $\mathrm{L}{\mathrm{a}}_{0.67}\mathrm{S}{\mathrm{r}}_{0.33}\mathrm{Mn}{\mathrm{O}}_3$ on $\mathrm{SrTi}{\mathrm{O}}_3$ which exhibits inverted hysteresis, spontaneous magnetic reversal, and exchange bias due to a structural gradient in the oxygen-octahedral network. Varying the growth conditions, we have mapped the phase diagram of this material and discovered that at a specific oxygen pressure and above a critical thickness, a complex magnetic behavior appears. Atomic-scale characterization shows that this peculiar magnetism is closely linked to a continuous structural gradient that creates three distinct regions within the monolithic film, each with a different magnetism onset. Extracting oxygen-octahedral geometry by electron microscopy, we found that the Curie temperature is directly correlated with the metal-oxygen bond angle. This study illustrates the importance of octahedral geometry in shaping the physical properties of the materials.

Employing a combination of first-principles density functional theory and a solution of a generalized spin Hamiltonian, we predict finite-temperature ferromagnetism in a yet-to-be synthesized two-dimensional monolayer of Cu spins, derived out of the layered structure of inorganic-organic hybrid perovskites. The computed cleavage energies are found to be a factor of 2–3 smaller than that of graphite, thereby making the synthesis of such monolayers by exfoliation of their bulk counterparts probable. The ferromagnetic exchanges, together with in-plane magnetic anisotropy, give rise to the possibility of a finite-temperature long-range ordering of Cu spins, as established through the solution of a generalized model Hamiltonian. Our study should motivate future experiments in this exciting class of compounds.

We report on the formation of heteronuclear quantum droplets in an attractive bosonic mixture of ${}^{41}\mathrm{K}$ and ${}^{87}\mathrm{Rb}$. We observe long-lived self-bound states, both in free space and in an optical waveguide. In the latter case, the dynamics under the effect of a species-dependent force confirms their bound nature. By tuning the interactions from the weakly to the strongly attractive regime, we study the transition from expanding to localized states, in both geometries. We compare the experimental results with beyond-mean-field theory and we find good agreement in the full range of explored interactions. Our findings provide access to the production of long-lived droplets with important implications for further research.

We have systematically studied the effects of in-plane uniaxial pressure $p$ on the superconducting transition temperature $T_c$ in many iron-based superconductors. The change of $T_c$ with $p$ is composed of linear and nonlinear components. The latter can be described as a quadratic term plus a much smaller fourth-order term. In contrast to the linear component, the nonlinear $p$ dependence of $T_c$ displays a pronounced in-plane anisotropy, which is similar to the anisotropic response of the resistivity to $p$. As a result, it can be attributed to the coupling between the superconducting and nematic orders, in accordance with the expectations of a phenomenological Landau theory. Our results provide direct evidences for the interplay between nematic fluctuations and superconductivity, which may be a common behavior in iron-based superconductors.

Rutile $\mathrm{Ti}{\mathrm{O}}_2$ is a paradigmatic transition-metal oxide with applications in optics, electronics, photocatalysis, etc., that are subject to pervasive electron-phonon interaction. To understand how energies of its electronic bands, and in general semiconductors or metals where the frontier orbitals have a strong $d$-band character, depend on temperature, we perform a comprehensive theoretical and experimental study of the effects of electron-phonon $\left(e-p\right)$ interactions. In a two-photon photoemission (2PP) spectroscopy study we observe an unusual temperature dependence of electronic band energies within the conduction band of reduced rutile $\mathrm{Ti}{\mathrm{O}}_2$, which is contrary to the well-understood $sp$-band semiconductors and points to a so far unexplained dichotomy in how the $e-p$ interactions affect differently the materials where the frontier orbitals are derived from the $sp$- and $d$ orbitals. To develop a broadly applicable model, we employ state-of-the-art first-principles calculations that explain how phonons promote interactions between the $\mathrm{Ti}-3d$ orbitals of the conduction band within the octahedral crystal field. The characteristic difference in $e-p$ interactions experienced by the $\mathrm{Ti}-3d$ orbitals of rutile $\mathrm{Ti}{\mathrm{O}}_2$ crystal lattice are contrasted with the more familiar behavior of the $\mathrm{Si}-2s$ orbitals of stishovite $\mathrm{Si}{\mathrm{O}}_2$ polymorph, in which the frontier $2s$ orbital experiences a similar crystal field with the opposite effect. The findings of this analysis of how $e-p$ interactions affect the $d$- and $sp$-orbital derived bands can be generally applied to related materials in a crystal field. The calculated temperature dependence of $d$-orbital derived band energies agrees well with and explains the temperature-dependent inter-$d$-band transitions recorded in 2PP spectroscopy of $\mathrm{Ti}{\mathrm{O}}_2$. The general understanding of how $e-p$ interactions affect $d$-orbital derived bands is likely to impact the understanding of temperature-dependent properties of highly correlated materials.

The ground state of the spin-1/2 kagome antiferromagnet remains uncertain despite decades of active research. Here we step aside from this debated question to address the ground-state nature of a related, and potentially just as rich, system made of corner-sharing triangles: the square kagome lattice (SKL). Our work is motivated by the recent synthesis of a distorted SKL compound mentioned by Morita and Tohyama [J. Phys. Soc. Jpn. 87, 043704 (2018)]. We have studied its spin-1/2 $J_1-J_2$ phase diagram with an unrestricted Schwinger boson mean-field theory (SBMFT). Our results agree with previous observations of a plaquette phase ($J_2\ll J_1$) and a ferrimagnet ($J_1\ll J_2$). In addition, three original phases appear: two incommensurate orders and a topological quantum spin liquid with weak nematicity. The topological order is characterized by fluxes on specific gauge-invariant quantities and the phase is stable under anisotropic perturbations relevant for experiments. Finally, we provide dynamical structure factors of the reported phases that could be observed in inelastic neutron scattering.

We quantify the role of many-body elastic scattering effects on exciton dephasing rates in two-dimensional hybrid metal-halide perovskites by means of nonlinear coherent excitation spectroscopy at a temperature of 5 K. We find that the exciton-density dependence of excitation-induced dephasing (EID) is two to three orders of magnitude lower than in other atomic monolayer semiconductors such as transition metal dichalchogenides. Furthermore, EID is different for the multiple excitons evident in the excitation line shape, as is their phonon-mediated temperature dependence. We ascribe these observations to screening effects due to polaronic dressing by the lattice.

We study superconducting quantum interference in InSb flake Josephson junctions. An even-odd effect in the amplitude and periodicity of the superconducting quantum interference pattern is found. Interestingly, the occurrence of this pattern coincides with enhanced conduction at both edges of the flake, as is deduced from measuring a superconducting quantum interference device (SQUID) pattern at reduced gate voltages. We identify the specific crystal facet of the edge with enhanced conduction, and confirm this by measuring multiple devices. Furthermore, we argue the even-odd effect is due to crossed Andreev reflection, a process where a Cooper pair splits up over the two edges and recombines at the opposite contact. An entirely $h/e$ periodic SQUID pattern, as well as the observation of both even-odd and odd-even effects, corroborates this conclusion. Crossed Andreev reflection could be harnessed for creating a topological state of matter or performing experiments on the nonlocal spin entanglement of spatially separated Cooper pairs.

We propose an exact construction for atypical excited states of a class of nonintegrable quantum many-body Hamiltonians in one, two, and three dimensions that display area-law entanglement entropy. These examples of many-body “scar” states have, by design, other properties, such as topological degeneracies, usually associated with the gapped ground states of symmetry-protected topological phases or topologically ordered phases of matter.

Transverse field Ising model is a common model in quantum magnetism and is often illustrated as an example for quantum phase transition. Its physical origin in quantum magnets, however, is actually not quite well-understood. The quantum mechanical properties of this model on frustrated systems are not well-understood either. We here clarify the physical origin, both extrinsic one and intrinsic one, for the transverse field of the quantum Ising model, and then explain the quantum effects in the kagome system. We discuss the quantum plaquette order and the quantum phase transition out of this ordered state in the rare-earth kagome magnets. Our specific results can find their relevance in the rare-earth tripod kagome magnets.

In materials with strong electron-phonon ($e\text{−}\mathrm{ph}$) interactions, the electrons carry a phonon cloud during their motion, forming quasiparticles known as polarons. Predicting charge transport and its temperature dependence in the polaron regime remains an open challenge. Here, we present first-principles calculations of charge transport in a prototypical material with large polarons, ${\mathrm{SrTiO}}_3$. Using a cumulant diagram-resummation technique that can capture the strong $e\text{−}\mathrm{ph}$ interactions, our calculations can accurately predict the experimental electron mobility in ${\mathrm{SrTiO}}_3$ between 150–300 K. They further reveal that for increasing temperature the charge transport mechanism transitions from bandlike conduction, in which the scattering of renormalized quasiparticles is dominant, to a beyond-quasiparticle transport regime governed by incoherent contributions due to the interactions between the electrons and their phonon cloud. Our work reveals long-sought microscopic details of charge transport in ${\mathrm{SrTiO}}_3$, and provides a broadly applicable method for predicting charge transport in materials with strong $e\text{−}\mathrm{ph}$ interactions and polarons.

A method of ultrafast switching of ferroelectric polarization is suggested. The method is based on the interaction of a ferroelectric sample with the feedback field of a resonator in which the sample is inserted. The polarization reversal time can be of order of femtoseconds. The polarization switching produces a coherent electromagnetic pulse.

We show that the light conelike structures that form in spin chains after a quench are quantum caustics. Their natural description is in terms of catastrophe theory and this implies (1) a hierarchy of light cone structures corresponding to the different catastrophes, (2) dressing by characteristic wave functions that obey scaling laws determined by the Arnol'd and Berry indices, and (3) a network of vortex-antivortex pairs in space-time inside the cone. We illustrate the theory by giving explicit calculations for the transverse field Ising model and the XY model, finding fold catastrophes dressed by the Airy functions and cusp catastrophes dressed by the Pearcey functions; multisite correlation functions are described by higher catastrophes such as the hyperbolic umbilic. Furthermore, we find that the vortex pairs created inside the cone are sensitive to phase transitions in these spin models with their rate of production being determined by the dynamical critical exponent. More broadly, this work illustrates how catastrophe theory can be applied to singularities in quantum fields.

We consider zero-sound collective excitations of a two-dimensional Fermi liquid. For each value of the angular momentum $l$, we study the evolution of longitudinal and transverse collective modes in the charge ($c$) and spin ($s$) channels with the Landau parameter $F_l^{c\left(s\right)}$, starting from positive $F_l^{c\left(s\right)}$ and all the way to the Pomeranchuk transition at $F_l^{c\left(s\right)}=-1$. In each case, we identify a critical zero-sound mode, whose velocity vanishes at the Pomeranchuk instability. For $F_l^{c\left(s\right)}<-1$, this mode is located in the upper frequency half-plane, which signals an instability of the ground state. In a clean Fermi liquid, the critical mode may be either purely relaxational or almost propagating, depending on the parity of $l$ and on whether the response function is longitudinal or transverse. These differences lead to qualitatively different types of time evolution of the order parameter following an initial perturbation. A special situation occurs for the $l=1$ order parameter that coincides with the spin or charge current. In this case, the residue of the critical mode vanishes at the Pomeranchuk transition. However, the critical mode can be identified at any distance from the transition, and is still located in the upper frequency half-plane for $F_1^{c\left(s\right)}<-1$. The only peculiarity of the charge- and spin-current order parameter is that its time evolution occurs on longer scales than for other order parameters. We also analyze collective modes away from the critical point, and find that the modes evolve with $F_l^{c\left(s\right)}$ on a multisheet Riemann surface. For certain intervals of $F_l^{c\left(s\right)}$, the modes either move to an unphysical Riemann sheet or stay on the physical sheet but away from the real frequency axis. In that case, the modes do not give rise to peaks in the imaginary parts of the corresponding susceptibilities.