In situ x-ray diffraction measurements and inverse Monte Carlo simulations of pair distribution functions were used to study the structural response of $\mathrm{Pb}{\mathrm{O}}_2$ under pressure and to characterize the local structure of liquid $\mathrm{Pb}{\mathrm{O}}_2$. Two phase transitions are observed upon room-temperature compression of crystalline $\mathrm{Pb}{\mathrm{O}}_2$ up to ∼65 GPa. The starting mixture of rutile structured $\beta \text{−}\mathrm{Pb}{\mathrm{O}}_2$ and orthorhombic $\alpha \text{−}\mathrm{Pb}{\mathrm{O}}_2$ undergoes a transition to a ${\mathrm{ZrO}}_2$-type orthorhombic phase with space group $Pbca$ at ∼21 GPa. Above 42 GPa, the $Pbca$ phase transforms to a cotunnite-type phase with space group $Pnam$. In this paper, we also report short-range order in liquid $\mathrm{Pb}{\mathrm{O}}_2$, showing ∼8–9-fold and 4–5-fold coordination around Pb and O atoms, respectively.

The purpose of this work is to understand the fundamental connection between structural correlations and light localization in three-dimensional (3D) open scattering systems of finite size. We numerically investigate the transport of vector electromagnetic waves scattered by resonant electric dipoles spatially arranged in 3D space by stealthy hyperuniform disordered point patterns. Three-dimensional stealthy hyperuniform disordered systems are engineered with different structural correlation properties determined by their degree of stealthiness $\chi$. Such fine control of exotic states of amorphous matter enables the systematic design of optical media that interpolate in a tunable fashion between uncorrelated random structures and crystalline materials. By solving the electromagnetic multiple scattering problem using Green's matrix spectral method, we establish a transport phase diagram that demonstrates a distinctive transition from a diffusive to a weak localization regime beyond a critical scattering density that depends on $\chi$. The transition is characterized by studying the Thouless number and the spectral statistics of the scattering resonances. In particular, by tuning the $\chi$ parameter, we demonstrate large spectral gaps and suppressed subradiant proximity resonances, facilitating light localization. Moreover, consistently with previous studies, our results show a region of the transport phase diagram where the investigated scattering systems become transparent. Our work provides a systematic description of the transport and weak localization properties of light in stealthy hyperuniform structures and motivates the engineering of photonic systems with enhanced light-matter interactions for applications to both classical and quantum devices.

The collective light-matter interaction in microcavities gives rise to the intriguing phenomena of cavity-mediated transport that can potentially overcome the Anderson localization. Yet, an accurate theoretical treatment is challenging as the matter (e.g., molecules) is subject to large energetic disorder. In this paper, we develop the Green's function solution to the Fano-Anderson model and use the exact analytical solution to quantify the effects of energetic disorder on the spectral and dynamical properties in microcavities. Starting from the microscopic equations of motion, we derive an effective non-Hermitian Hamiltonian and predict a set of scaling laws: (i) The complex eigenenergies of the effective Hamiltonian exhibit an exceptional point, which leads to underdamped coherent dynamics in the weak disorder regime, where the decay rate increases with disorder, and overdamped incoherent dynamics in the strong disorder regime, where the slow decay rate decreases with disorder. (ii) The total density of states of disordered ensembles can be exactly partitioned into the cavity, bright-state, and dark-state local density of states, which are determined by the complex eigensolutions and can be measured via spectroscopy. (iii) The cavity-mediated relaxation and transport dynamics are intimately related such that both the energy-resolved relaxation and transport rates are proportional to the cavity local density of states. The ratio of the disorder-averaged relaxation and transport rates equals the molecule number, which can be interpreted as a result of a quantum random walk. (iv) A turnover in the rates as a function of disorder or molecule density can be explained in terms of the overlap of the disorder distribution function and the cavity local density of states. These findings reveal the significant impact of the dark states on the local density of states and consequently their crucial role in optimizing spectroscopic and transport properties of disordered ensembles in cavities.

We report on element-resolved ultrafast magnetization dynamics in multiferroic ${\mathrm{CoCr}}_2{\mathrm{O}}_4$ and ${\mathrm{Co}}_{0.975}{\mathrm{Ge}}_{0.025}{\mathrm{Cr}}_2{\mathrm{O}}_4$ after optical excitation above the electronic band gap. We observe demagnetization dynamics in the range of several picoseconds, up to two orders of magnitude faster than previously reported demagnetization in other ferrimagnetic insulators. Moreover, we find that the dynamics of the two magnetic ions differ significantly just below the Curie point. The dynamics of the low-temperature multiferroic phase are almost two times slower than those in the ferrimagnetic phase. This suggests that the additional magnetic cycloidal component, which is coupled to electric polarization at low temperatures, might influence the ultrafast magnetization dynamics.

We show theoretically that double photoemission ($2e$-ARPES) may be used to identify the pairing state in superconductors in which the Cooper pairs have a nonzero center-of-mass momentum, ${\mathbf{q}}_{\mathrm{cm}}$. We theoretically evaluate the $2e$-ARPES counting rate $P^{\left(2\right)}$ for the cases of a $d_{x^2-y^2}$-wave superconductor, a pair-density-wave (PDW) phase, and a Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) phase. We show that $P^{\left(2\right)}$ provides direct insight into the center-of-mass momentum and spin state of the superconducting condensate, and thus can distinguish between these three different superconducting pairing states. In addition, $P^{\left(2\right)}$ can be used to map out the momentum dependence of the superconducting order parameter. Our results identify $2e$-ARPES as an ideal tool for identifying and probing ${\mathbf{q}}_{\mathrm{cm}}\ne 0$ superconducting pairing states in superconductors.

We report a high-throughput ab initio study of the thermodynamic and superconducting properties of the recently synthesized $X{\mathrm{B}}_3{\mathrm{C}}_3$ clathrates. These compounds, in which boron and carbon form a spongelike network of interconnected cages each enclosing a central $X$ atom, are attractive candidates to achieve high-$T_c$ conventional superconductivity at ambient pressure, due to the simultaneous presence of a stiff B-C covalent network and a tunable Fermi energy due to the $X$ atom acting as a charge reservoir. Ternary compounds like ${\mathrm{CaB}}_3{\mathrm{C}}_3, {\mathrm{SrB}}_3{\mathrm{C}}_3$, and ${\mathrm{BaB}}_3{\mathrm{C}}_3$ are predicted to exhibit $T_c \lesssim 50$ K at moderate or ambient pressures, which may further increase up to 77 K if the original compounds are hole doped, by replacing the divalent alkaline earth with a monovalent alkali metal to form an ordered $XY{\mathrm{B}}_6{\mathrm{C}}_6$ alloy.

We examine spectral properties of a recently proposed theory of the intermediate temperature pseudogap metal phase of the cuprates. We show that this theory can be obtained from the familiar paramagnon theory of nearly antiferromagnetic metals by fractionalizing the paramagnon into two “hidden” layers of $S=1/2$ spins. The first hidden layer of spins hybridizes with the electrons as in a Kondo lattice heavy Fermi liquid, whereas the second hidden layer of spins forms a spin liquid with fractionalized spinon excitations. We compute the imaginary part of the electronic self-energy induced by the spinon excitations. The energy and momentum dependence of the photoemission spectrum across the Brillouin zone provides a good match to observations by He et al. [Science 331, 1579 (2011)] in $({\mathrm{Pb}}_x,{\mathrm{Bi}}_{2-x})\left({\mathrm{La}}_y{\mathrm{Sr}}_{2-y}\right){\mathrm{CuO}}_{6+\delta }$ and by Chen et al. [Science 366, 1099 (2019)] in ${(\mathrm{Bi},\mathrm{Pb})}_2{\mathrm{Sr}}_2\mathrm{Ca}{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$.

We study quantum phases and phase transitions in a one-dimensional interacting fermion system with a Lieb-Schultz-Mattis (LSM) type anomaly. Specifically, the inversion symmetry enforces any symmetry-preserving gapped ground state of the system to be a Kitaev chain, following an LSM type theorem that we prove. Alternatively, via the Jordan-Wigner transformation, this system describes a spin system whose gapped ground states must break either the inversion or the Ising symmetry associated with fermion parity. We obtain a phase diagram using analytical methods and variational matrix product state simulations and study the critical behaviors of the quantum phase transitions therein using entanglement entropy, energy variance, and finite-size scaling of order parameters. In particular, we observe a continuous phase transition between two ordered phases that are beyond the Ginzburg-Landau-Wilson paradigm, in analogy to the deconfined quantum critical points in two spatial dimensions. We show this type of a 1D deconfined quantum critical point is described by the Tomonaga-Luttinger liquid theory and extract its Luttinger parameter and critical exponents. We also identify a gapless phase that emerges from a deconfined quantum critical point between two ordered phases, which cannot be described by a U(1) Luttinger liquid.

Semiconductor quantum dots are considered to be promising candidates for the hardware of quantum information technology and optoelectronic devices. Herein, motivated by a tetrahedrally shaped colloidal quadruple quantum dot structure made from In-based III–V semiconductors, which has been synthesized very recently by Leemans et al. [J. Am. Chem. Soc., 143, 4290 (2021)], we provide timely insight into the electronic transport and the quantum phase transition (QPT) for such an architecture. When the interdot hopping between different side dots ($t_2$) is absent, a singular two-stage Kondo effect is revealed for small central-side coupling $t_1$. The two screening processes are separated by an energy scale of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, and the fitting parameters deviate from the regular ones of the side-coupled double dot system. The RKKY interaction and temperature are well illustrated by functions of $t_1^4/\left(U{T}_{K1}^2\right)$, where $U$ and $T_{K1}$ are the on-site electron-electron repulsion and the first Kondo temperature, respectively. When $t_2$ turns on, the ground state of the side dots transits from a spin quadruplet to a magnetic frustration phase, and then to a singlet, through two first-order QPTs. In the frustration phase, another new two-stage Kondo effect is demonstrated, which includes the process of screening the local spin on the central dot firstly, and then that on the neighborless side dot is screened at a lower temperature. Both the Kondo temperatures are found to be rather sensitive to $t_2$. When $t_2$ is large enough, the reappearance of the regular Kondo effect is found. With fixed $t_2=0$, charging the central dot triggers a transition from an antiferromagnetic correlation among the side dots to a ferromagnetic one, accompanied by a Kondo behavior in the central dot. We adopt the state-of-the-art numerical renormalization group method to implement the above behaviors, combined with analytical arguments.

Using density functional theory combined with the Boltzmann transport equation, the charge, thermal transport, and thermoelectric properties in two-dimensional (2D) ${\mathrm{Ge}}_2{Y}_2$ ($Y=\text{N}$, P, As, or Sb) monolayers characterized by two structural phases, i.e., $\alpha \text{−}{\mathrm{Ge}}_2{\mathrm{Y}}_2$ and $\beta \text{−}{\mathrm{Ge}}_2{\mathrm{Y}}_2$, have been studied systematically. Our theoretical results demonstrate that the lone-pair electrons have remarkable influences on their lattice thermal conductivity. By performing comparative studies on the two different structures of ${\mathrm{Ge}}_2{\mathrm{Sb}}_2$, we uncover that the above influences not only originate from the interactions between the lone-pair electrons around Sb atoms and the bonding electrons of the adjacent Ge atom, but also from the interlayer Coulomb repulsive forces of lone-pair electrons distributed in different layers. The latter leads to a strong anharmonicity, which greatly suppresses the lattice thermal conductivity. Thus, $\alpha \text{−}{\mathrm{Ge}}_2{\mathrm{Sb}}_2$ monolayer has an ultralow thermal conductivity with 0.19 W/mK, while $\beta \text{−}{\mathrm{Ge}}_2{\mathrm{Sb}}_2$ monolayer with 5.1 W/mK at the temperature 300 K. Owing to the ultralow lattice thermal conductivity induced by lone-pair electrons, the predicted maximum value of the thermoelectric figure of merit ($ZT$) reaches 1.2 for $p$-type and 1.18 for $n$-type doping $\alpha \text{−}{\mathrm{Ge}}_2{\mathrm{Sb}}_2$. Our theoretical results put forward another effective mechanism to design and optimize 2D thermoelectric materials with high thermoelectric conversion efficiency.

We study the emergence of superconductivity in rhombohedral trilayer graphene due purely to the long-range Coulomb repulsion. This repulsive-interaction-driven phase in rhombohedral trilayer graphene (RTG) is significantly different from those found in twisted bilayer and trilayer graphenes. In the latter case, the nontrivial momentum-space geometry of the Bloch wave functions contributes to an effective attractive electron–electron interaction; this allows for less modulated order parameters and for spin-singlet pairing. In RTG, we instead find spin-triplet superconductivity with critical temperatures up to $\sim 0.15$ K. The critical temperatures strongly depend on electron filling and peak where the density of states diverge. The order parameter shows a significant modulation within each valley pocket of the Fermi surface.

In this work, we investigate whether the Kitaev honeycomb model can serve as a starting point to realize the intriguing physics of the Sachdev-Ye-Kitaev (SYK) model. The starting point is to strain the system, which leads to flat bands reminiscent of Landau levels, thereby quenching the kinetic energy. The presence of weak residual perturbations, such as Heisenberg interactions and the $\gamma$ term, creates effective interactions between the Majorana modes when projected into the flux-free sector. We assume the resulting interactions to be effectively random. This leads to a bipartite Sachdev-Ye-Kitaev model (b-SYK) with very similar properties as the SYK model. We also hypothesize under which conditions one would expect the standard SYK model in such a setup.

In polar semiconductors and insulators, the Fröhlich interaction between electrons and long-wavelength longitudinal optical phonons induces a many-body renormalization of the carrier effective masses and the appearance of characteristic phonon sidebands in the spectral function, commonly dubbed “polaron satellites.” The simplest model that captures these effects is the Fröhlich model, whereby electrons in a parabolic band interact with a dispersionless longitudinal optical phonon. The Fröhlich model has been employed in a number of seminal papers, from early perturbation-theory approaches to modern diagrammatic Monte Carlo calculations. One limitation of this model is that it focuses on undoped systems, thus ignoring carrier screening and Pauli blocking effects that are present in real experiments on doped samples. To overcome this limitation, we here extend the Fröhlich model to the case of doped systems, and we provide exact solutions for the electron spectral function, mass enhancement, and polaron satellites. We perform the analysis using two approaches, namely, Dyson's equation with the Fan-Migdal self-energy, and the second-order cumulant expansion. We find that these two approaches provide qualitatively different results. In particular, Dyson's approach yields better quasiparticle masses and worse satellites, while the cumulant approach provides better satellite structures, at the price of worse quasiparticle masses. Both approaches yield an anomalous enhancement of the electron effective mass at finite doping levels, which in turn leads to a breakdown of the quasiparticle picture in a significant portion of the phase diagram.

Several semilocal exchange potentials usually employed in the framework of density-functional theory are tested and compared with their exact counterpart, the exchange optimized effective potential (OEP), as applied to the jellium-slab model of a metal-vacuum interface. Driven by their explicit dependence on the ground-state density, its gradient, and its kinetic-energy density, the three analyzed semilocal exchange potentials approach their respective asymptotic limits faster than in the case of the OEP, all of them having an asymptotic scaling of the form $-\alpha e^2/z+V_{\infty }$, with $\alpha <1$. Here, we provide the leading analytic asymptotics of the three model potentials under study, and we find that none of them exhibits the exact OEP slab asymptotics $-e^2/z$. While the so-called Becke-Roussel potential's leading asymptote is close to its exact OEP counterpart, the other two model potentials under study approach a material-dependent positive constant value far into the vacuum, resulting in considerably overestimated ionization potentials.

Triplet-triplet annihilation (TTA) in phosphorescent emission layers of modern organic light-emitting diodes compromises their performance and device lifetime. TTA can occur by a Förster-type interaction between two triplets, leading to a loss of one of them. The TTA process gives rise to correlations in the positions of the surviving triplets, which complicate its study. These correlations can in principle be accounted for exactly in kinetic Monte Carlo (KMC) simulations, but such simulations are computationally expensive. Here, we present master equation modeling of TTA that accounts for correlations in a computationally efficient way. Cases without and with triplet diffusion, which partly washes out correlations, are considered. We calculate the influence of TTA on transient photoluminescence experiments, where it leads to a deviation from exponential decay, and on steady-state emission efficiency. A comparison with KMC simulations shows that our master equation modeling is an accurate and computationally competitive alternative.

Many interesting experimental systems, such as cavity QED or central spin models, involve global coupling to a single harmonic mode. Out of equilibrium, it remains unclear under what conditions localized phases survive such global coupling. We study energy-dependent localization in the disordered Ising model with transverse and longitudinal fields coupled globally to a $d$-level system (qudit). Strikingly, we discover evidence for an inverted mobility edge where high-energy states are localized whereas low-energy states are delocalized. This prediction is supported by shift-and-invert eigenstate targeting and Krylov time evolution up to $L=13$ and 18, respectively. We argue for a critical energy of the localization phase transition which scales as $E_c\propto L^{1/2}$, consistent with finite-size numerics. We also show evidence for a reentrant many-body localization phase at even lower energies despite the presence of strong effects of the central mode in this regime. Similar results should occur in the central spin-$S$ problem at large $S$ and in certain models of cavity QED.

Time-domain magneto-THz spectroscopy is utilized to study the frustrated magnet averievite ${\mathrm{Cu}}_{5-x}{\mathrm{Zn}}_x{\mathrm{V}}_2{\mathrm{O}}_{10}$(CsCl). Pronounced THz resonances are observed in unsubstituted samples ($x=0$) when cooling below the onset of short-range magnetic correlations. The influence of external magnetic effects confirms the magnetic origin of these resonances. Increasing Zn substitution suppresses the resonances, as frustration effects dominate, reflecting the nonmagnetic phases for $x>0.25$ compounds. The temperature evolution of the THz spectra is complemented with electron spin resonance spectroscopy. This comparison allows a direct probe of the different contributions from magnetic order, frustration, and structural properties in the phase diagram of averievite. Our results illustrate the effect of magnetic interactions in THz spectra of frustrated magnets.

TaAs is a prime example of a topological semimetal with two types of Weyl nodes, ${\mathrm{W}}_1$ and ${\mathrm{W}}_2$, whose bulk signatures have proven elusive. We apply Landau level spectroscopy to crystals with multiple facets and identify—among other low-energy excitations between parabolic bands—the response of a cone extending over a wide energy range. Comparison with density functional theory studies allows us to associate this conical band with nearly isotropic ${\mathrm{W}}_2$ nodes. In contrast, ${\mathrm{W}}_1$ cones, which are more anisotropic and less extended in energy, appear to be buried too deep beneath the Fermi level. They cannot be accessed directly. Instead, the excitations in their vicinity give rise to an optical response typical of a narrow-gap semiconductor rather than a Weyl semimetal.

The influence of the geometrical curvature of chiral magnetic films on the static and dynamic properties of hosted skyrmions is studied theoretically. We predict the effects of the curvature-induced drift of skyrmions under the action of the curvature gradients without any external stimuli. The strength of the curvature-induced driving force essentially depends on the skyrmion type, Néel or Bloch, while the trajectory of motion is determined by the type of magnetic ordering: ferromagnetic or antiferromagnetic. When moving on the surface, skyrmions undergo deformations that depend on the type of skyrmion. In the small-curvature limit, using the collective-variable approach we show that the driving force acting on a Néel skyrmion is linear in the gradient of the mean curvature. The driving acting on a Bloch skyrmion is much smaller: it is proportional to the product of the mean curvature and its gradient. In contrast to the fast Néel skyrmions, the dynamics of the slow Bloch skyrmions is essentially affected by the skyrmion profile deformation. For the sake of simplicity, we restrict ourselves to the case of zero Gaussian curvature and consider cylindrical surfaces of general type. Equations of motion for ferromagnetic and antiferromagnetic skyrmions in curved magnetic films are obtained in terms of collective variables. All analytical predictions are confirmed by numerical simulations.

We investigate the ground-state properties of the spin-$1/2$ pyrochlore Heisenberg antiferromagnet using pseudofermion functional renormalization group techniques. The first part of our analysis is based on an enhanced parton mean-field approach, which takes into account fluctuation effects from renormalized vertex functions. Our implementation of this technique extends earlier approaches and resolves technical difficulties associated with a diagrammatic overcounting. Using various parton ansätze for quantum spin liquids, dimerized and nematic states our results indicate a tendency for lattice symmetry breaking in the ground state. While overall quantum spin liquids seem unfavorable in this system, the recently proposed monopole state still shows the strongest support among all spin liquid ansätze that we have tested, which is further confirmed by our complementary variational Monte Carlo calculations. In the second part of our investigation, we probe lattice symmetry breaking more directly by applying the pseudofermion functional renormalization group to perturbed systems. Our results from this technique confirm that the system's ground state either exhibits broken $C_3$ rotation symmetry, or a combination of inversion and $C_3$ symmetry breaking.