We propose an energy-scale correspondence between the Mott physics and the Kondo lattice physics and construct a tentative phase diagram of their correlated electrons with two characteristic energy scales ${\omega }^{*}$ and $\mathrm{\Omega }$ marking the upper boundary of a low-energy regime with well-developed long-range coherence and the lower boundary of localized spectral weight in the energy space, respectively. In between, there exists a crossover region with emergent but damped quasiparticle excitations. We argue that the presence of two separate energy scales is a generic property of correlated electrons on a lattice and reflects an intrinsic two-stage process of the quasiparticle dynamics to build up the lattice coherence. For the Hubbard model, they correspond to the kink and waterfall structures on the dispersion, while for the periodic Anderson model, they are associated with the indirect and direct hybridization gaps. Our work reveals a deep connection between the Mott and Kondo lattice physics and provides a basic ingredient for the study of many-body systems.

Single-molecule magnets (SMMs) possessing bistable states have been considered as promising candidates to realize zero-dimensional (0D) ferroelectrics (FE) and multiferroics (MF) with high storage density. However, how to read or manipulate the FE states with low-energy-dissipation strategy is still a hard challenge. Here, we intercalated a magnetic metal porphyrin molecule, such as TiP with switchable vertical electric polarization, within the ${\mathrm{MoS}}_2$ bilayer to realize 0D FE states. First-principles calculations show that the ${\mathrm{MoS}}_2$ monolayer contacted by SMMs is spin polarized to provide two spin-dependent transport tunnels. As a temperature gradient is applied along the transport channels, a well-defined spin-Seebeck effect (SSE) occurs in the spin-polarized ${\mathrm{MoS}}_2$ layer, helping to generate a pure thermal spin current. More importantly, the spin-Seebeck signal is associated tightly with the FE state in the same layer, and both of them can be switched simultaneously to another layer by an external electric field. The theoretical results not only put forwards a low-energy-dissipation strategy to read the SMM-based FE states, but also develop a new research field of spin-ferroelecto-caloritronics, which focuses on the interplay of electrons' spin and FE states in the presence of a temperature gradient.

We present a comprehensive investigation of lattice dynamics in the double-helix antiferromagnet FeP by means of high-resolution time-of-flight neutron spectroscopy and ab initio calculations. Phonons can hybridize with the magnetic excitations in noncollinear magnets to significantly influence their properties. We observed a rich spectrum of phonon excitations, which extends up to $\sim 50$ meV. We performed detailed analysis of the observed and calculated spectra for all high-symmetry points and high-symmetry directions of the Brillouin zone. We show that the DFT calculations quantitatively capture the essential features of the observed phonons, including both dispersions and scattering intensities. By making use of the detailed intensity comparison between the theory and the data, we were able to identify displacement vectors for the majority of the observed modes. The overall excellent agreement between the DFT predictions and the experimental results breaks down for the lowest mode at the $Y$ point, whose energy is lower than calculated by $\sim 13\%$. The present study provides vital information on the lattice dynamics in FeP and demonstrates applicability of the DFT to novel pressure-induced phenomena in related materials, such as MnP and CrAs.

The study of entanglement spectra is a powerful tool to detect or elucidate universal behavior in quantum many-body systems. We investigate the scaling of the entanglement (or Schmidt) gap $\delta \xi$, i.e., the lowest-laying gap of the entanglement spectrum, at a two-dimensional quantum critical point. We focus on the paradigmatic quantum spherical model, which exhibits a second-order transition and is mappable to free bosons with an additional external constraint. We analytically show that the Schmidt gap vanishes at the critical point, although only logarithmically. For a system on a torus and the half-system bipartition, the entanglement gap vanishes as ${\pi }^2/ln\left(L\right)$, with $L$ the linear system size. The entanglement gap is nonzero in the paramagnetic phase and exhibits a faster decay in the ordered phase. The rescaled gap $\delta \xi ln\left(L\right)$ exhibits a crossing for different system sizes at the transition, although logarithmic corrections prevent a precise verification of the finite-size scaling. Interestingly, the change of the entanglement gap across the phase diagram is reflected in the zero-mode eigenvector of the spin-spin correlator. At the transition quantum fluctuations give rise to a nontrivial structure of the eigenvector, whereas in the ordered phase it is flat. We also show that the vanishing of the entanglement gap at criticality can be qualitatively but not quantitatively captured by neglecting the structure of the zero-mode eigenvector.

Atomistic van der Waals heterostacks are ideal systems for high-temperature exciton condensation because of large exciton binding energies and long lifetimes. Charge transport and electron energy-loss spectroscopy showed first evidence of excitonic many-body states in such two-dimensional materials. Pure optical studies, the most obvious way to access the phase diagram of photogenerated excitons, have been elusive. We observe several criticalities in photogenerated exciton ensembles hosted in $\mathrm{Mo}{\mathrm{Se}}_2\text{−}\mathrm{W}{\mathrm{Se}}_2$ heterostacks with respect to photoluminescence intensity, linewidth, and temporal coherence pointing towards the transition to a coherent many-body quantum state, consistent with the predicted critical degeneracy temperature. For this state, the estimated occupation is approximately 100% and the phenomena survive above 10 K.

We present a general framework, based on Lagrange duality, for computing physical bounds on a wide array of electromagnetic scattering problems. Namely, we show that, via projections into increasingly localized spatial clusters, the central equality of scattering theory—the definition of the $\mathbb{T}$ operator—can be used to generate a hierarchy of increasingly accurate mean-field approximations (enforcing local power conservation) that naturally complement the standard design problem of optimizing some objective with respect to structural degrees of freedom. Utilizing the systematic control over the spatial extent of local violations of physics offered by the approach, proof-of-concept application to maximizing radiative Purcell enhancement for a dipolar current source in the vicinity of a structured medium, an effect central to many sensing and quantum technologies, yields bounds that are often more than an order of magnitude tighter than past results, highlighting the need for a theory capable of accurately handling differing domain and field-localization length scales. Similar to related domain decomposition and multigrid notions, analogous constructions are possible in any branch of wave physics, providing a unified approach for investigating fundamental limits.

By means of time- and angle-resolved photoelectron spectroscopy, we give evidence of a remarkable reduction of the Fermi velocity of out-of-equilibrium Dirac bands in the quasi-two-dimensional semimetal ${\mathrm{BaNiS}}_2$. This effect is accompanied by a nonrigid shift of the bands at the center of the Brillouin zone. Analytical and first-principles calculations indicate that this band renormalization is ascribed to a change in nonlocal electron correlations driven by a photoinduced enhancement of screening properties. Our results are explained by a scaling relation between inverse screening length and electronic temperature that is of general relevance for the description of out-of-equilibrium dynamics and screening effects in all layered Dirac materials.

The behavior of electrons in condensed matter systems is mostly determined by the repulsive Coulomb interaction. However, under special circumstances, the Coulomb interaction can be effectively attractive, giving rise to electron pairing in unconventional superconductors and specifically designed mesoscopic setups. In quantum Hall systems, electron interactions can play a particularly important role due to the huge degeneracy of Landau levels, leading, for instance, to the emergence of quasiparticles with fractional charge and anyonic statistics. Quantum Hall Fabry-Pérot interferometers (FPI) have attracted increasing attention due to their ability to probe such exotic physics. In addition, such interferometers are affected by electron interactions themselves in interesting ways. Recently, experimental evidence for electron pairing in a quantum Hall FPI was found [Choi et al., Nat. Commun. 6, 7435 (2015)]. Theoretically describing an FPI in the limit of strong backscattering and under the influence of a screened Coulomb interaction, we compute electron shot noise and indeed find a two-fold enhanced Fano factor for some parameters, indicative of electron pairing. This result is explained in terms of an electron interaction due to exchange of neutral interedge plasmons, so-called neutralons.

We systematically study the effect of disorder and interactions on a quasi-one-dimensional diamond chain possessing flat bands. Disorder localizes all the single-particle eigenstates, while at low disorder strengths, we obtain weak flat-band based localization (FBL), at high disorder strengths, we see conventional Anderson localization (AL). The compactly localized (CL) eigenstates of flat bands show a persisting oscillatory recurrence in the study of single-particle wave-packet dynamics. For low disorder, a damped oscillatory recurrence behavior is observed which is absent for high disorder. Noninteracting many-particle fermion states also follow the same trend except showing a delocalizing tendency at intermediate disorder due to the fermionic statistics in the system. As interactions are switched on, for the finite sizes that we are able to study, a nonergodic “mixed phase” is observed at low disorder which is separated from the MBL phase at high disorder by a thermal phase at intermediate disorder. A study of many-body nonequilibrium dynamics reinforces these findings.

We report a combined theoretical and experimental investigation of magnetic proximity and Hall transport in Pt/Cr bilayers. Density-functional theory indicates that interfacial magnetic moments can be induced in the Pt layer and a strong magnetocrystalline anisotropy with an out-of-plane easy axis arises in the antiferromagnet. A signal ascribed to the anomalous Hall effect is detected and associated to the interface between Pt and Cr layers. We discuss how this effect originates from the Berry curvature of the electronic structure at the interface and the correlation with the character of the proximity-induced magnetic moments.

We performed neutron resonance spin echo measurements on MnSi to investigate how the magnetic skyrmions crystalize into a lattice on cooling in a magnetic field. We found that the skyrmions aggregate into small skyrmion-lattice (SkL) domains with orientational disorder upon the transition from the paramagnetic phase to the SkL phase. These domains are nearly static in time, and coexist with chiral spin fluctuations with a characteristic relaxation time of ${10}^{-9}\mathrm{s}$ near the transition temperature. As the temperature is further decreased, the SkL domains merge into the long-range-ordered SkL. These observations provide a microscopic picture of the crystallization of the magnetic skyrmions.

Superconductivity is often found in a dome around quantum critical points, i.e., second-order quantum phase transitions. Here, we show that an enhancement of superconductivity is avoided at the critical pressure of the charge-density-wave (CDW) state in $2H\text{−}{\mathrm{NbSe}}_2$. We present comprehensive high-pressure Hall effect and magnetic susceptibility measurements of the CDW and superconducting state in $2H\text{−}{\mathrm{NbSe}}_2$. Initially, the second-order CDW transition is suppressed smoothly but it drops to zero abruptly at $P_{\text{CDW}}=4.4\mathrm{GPa}$ thus indicating a change to first order, while the superconducting transition temperature $T_{\text{c}}$ rises continuously up to $P_{\text{CDW}}$ but is constant above. The putative first-order nature of the CDW transition is suggested as the cause for the absence of a superconducting dome at $P_{\text{CDW}}$. Indeed, we show that the suppression of the superconducting state at low pressures is due to the loss of density of states inside the CDW phase, while the initial suppression of the CDW state is accounted for by the stiffening of the underlying bare phonon mode.

Films produced by assembling bare gold clusters well beyond the electrical percolation threshold show a resistive switching behavior whose investigation has started only recently. Here we address the challenge to charaterize the resistance of a nanogranular film starting from limited information on the structure at the microscopic scale by the means of Bruggeman's approach to multicomponent media, within the framework of effective medium approximations. The approach is used to build a model that proves that the observed resistive switching can be explained by thermally regulated local structural rearrangements.

We theoretically describe a thermoelectric effect that is entirely due to Andreev processes involving the formation of Cooper pairs through the coupling of electrons and holes. The Andreev thermoelectric effect can occur in ballistic ferromagnet-superconductor junctions with a dominant superconducting proximity effect on the ferromagnet, and it is very sensitive to surface states emerging in unconventional superconductors. We consider hybrid junctions in two and three dimensions to demonstrate that the thermoelectric current is always reversed in the presence of low-energy Andreev bound states at the superconductor surface. A microscopic analysis of the proximity-induced pairing reveals that the thermoelectric effect only arises if even- and odd-frequency Cooper pairs coexist in mixed singlet and triplet states. Our results are an example of the richness of emergent phenomena in systems that combine magnetism and superconductivity, and they open a pathway for exploring exotic surface states in unconventional superconductors.

We discuss distinctive features of spiral states in bulk chiral magnets such as MnSi and ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$ that stem from the effect of the cubic magnetocrystalline anisotropy. First of all, at both the helical-to-conical and the conical-to-ferromagnetic transitions, taking place at $H_{c1}$ and $H_{c2}$, respectively, the cubic anisotropy leads to reversible or irreversible jumplike reorientations of the spiral wave vectors. The subtle interplay between the easy and hard anisotropy axes gives rise to a phase transition between elliptically distorted conical states almost without any detectable change in the period. We show that the competition between on-site cubic and exchange anisotropy terms can also lead to oblique spiral states. Our work gives clear directions for further experimental studies to reveal theoretically predicted spiral states in cubic helimagnets beyond the aforementioned well-established states and thus it can help to understand the magnetic phase diagram of these archetypal skyrmion hosts. In addition, we show that properties of isolated skyrmions such as interskyrmion attraction, orientation, and/or nucleation are also rooted in the properties of host spiral states, in which skyrmions are stabilized.

We have found an unusual competition of two frustration mechanisms in the 2D quantum antiferromagnet ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$. The key actors are the alternation of single-ion planar anisotropy direction of the individual magnetic ${\mathrm{Co}}^{2+}$ ions, and their arrangement in a distorted triangular lattice structure. In particular, the uniquely oriented Ising-type anisotropy emerges from the competition of easy-plane ones, and for a magnetic field applied along this axis one finds a cascade of five ordered phases at low temperatures. Two of these phases feature magnetization plateaux. The low-field one is supposed to be a consequence of a collinear ground state stabilized by the anisotropy, while the other plateau bears characteristics of an “up-up-down” state exclusive for lattices with triangular exchange patterns. Such coexistence of the magnetization plateaux is a fingerprint of competition between the anisotropy and the geometric frustration in ${\mathrm{Cs}}_2{\mathrm{CoBr}}_4$.

Coulomb interaction has important consequences for the physics of quantum spin Hall edge states, weakening the topological protection via two-particle scattering and renormalizing both the velocity and the charge of collective plasmon modes compared to that of free electrons. Despite these effects, interactions remain difficult to quantify. We propose here simple and robust edge resonator geometries to characterize Coulomb interaction by means of high-frequency measurements. They rely on a transmission line approach and take advantage of the impedance mismatch between the edge states and their microwave environment.

We use scanning gate microscopy to demonstrate the presence of localized states arising from potential inhomogeneities in a $50\mathrm{nm}$ wide, gate-defined conducting channel in encapsulated bilayer graphene. When imaging the channel conductance under the influence of a local tip-induced potential, we observe ellipses of enhanced conductance as a function of the tip position. These ellipses allow us to infer the location of the localized states and to study their dependence on the displacement field. For large displacement fields, we observe that localized states tend to occur halfway into the channel. All our observations can be well explained within the framework of stochastic Coulomb blockade.

We report the observation of a two-dimensional (2D) checkerboard charge density wave (CDW) in the low-dimensional superconductor ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$. By determining its CDW properties across the temperature-pressure ($T-P$) phase diagram and comparing with prototypical CDW materials, we conclude that ${\mathrm{Ta}}_4{\mathrm{Pd}}_3{\mathrm{Te}}_{16}$ features (a) an incommensurate CDW with a mixed character of dimensions [quasi-1D (Q1D) considering its needlelike shape along the $\mathrm{b}$ axis, Q2D as the CDW has checkerboard wave vectors, and 3D because of CDW projections along all three axes], and (b) one of the weakest CDWs compared to its superconductivity (SC), i.e., enhanced SC with respect to CDW, suggesting an interesting interplay of the two orders.

We develop a method that uses truncation-order-dependent reexpansions constrained by generic strong-coupling information to extrapolate perturbation series to the nonperturbative regime. The method is first benchmarked against a zero-dimensional model field theory and then applied to the dilute Fermi gas in one and three dimensions. Overall, our method significantly outperforms Padé and Borel extrapolations in these examples. The results for the ground-state energy of the three-dimensional Fermi gas are robust with respect to changes in the form of the reexpansion and compare well with quantum Monte Carlo simulations throughout the BCS regime and beyond.