We investigate the occurrence of multicritical points under pressure and magnetic field in a model that describes two $5f$ bands (of either $\alpha$ or $\beta$ characters), which hybridize with a single itinerant conduction band. The $5f$ electrons interact through Coulomb and exchange terms. The antiferromagnetic (AF) order parameter is a Néel vector, which is assumed to be fixed by an Ising anisotropy. The applied magnetic field is transverse to the anisotropy axis. Without field, our results for the temperature-pressure phase diagram show that at low temperatures a first-order phase transition occurs between two distinct antiferromagnetic phases, ${\mathrm{AF}}_1$ and ${\mathrm{AF}}_2$, as the pressure is increased. The two phases are characterized by the gaps of bands $\alpha$ and $\beta$ given by ${\mathrm{\Delta }}_{\alpha }$ and ${\mathrm{\Delta }}_{\beta }$, respectively. The ${\mathrm{AF}}_1$ phase occurs when ${\mathrm{\Delta }}_{\beta }>{\mathrm{\Delta }}_{\alpha }>0$, while in the ${\mathrm{AF}}_2$ phase the gaps satisfy ${\mathrm{\Delta }}_{\alpha }>{\mathrm{\Delta }}_{\beta }>0$. The application of a magnetic field produces a drastic change in the phase diagram. The ${\mathrm{AF}}_1$ and ${\mathrm{AF}}_2$ phases separate, with the latter acquiring a dome shape that is eventually suppressed for large values of the applied field. The evolution of the phase diagram under pressure, without and with magnetic field, shows the presence of multicritical points. Our results show that the evolution of these multicritical points by the simultaneous application of pressure and magnetic field is also drastic, with the suppression of some multicritical points and the emergence of others. We believe that these results may have relevance for the growing field of multicritical points (classical and quantum) in the physics of uranium compounds.

In this paper we discuss, within the Gross-Pitaevskii framework, superfluidity, soliton-like patterns, and instabilities in a nonequilibrium polariton fluid injected by a spatially localized and continuous-wave coherent pump and flowing against a defect located outside the pump spot. In contrast to equilibrium condensates of ultracold atoms or liquid helium, the steady-state solutions of the driven-dissipative equations in this specific geometry hardly show a clean superfluid flow around the defect and rather feature a crossover from shallow to deep soliton-like perturbation. This is explained in terms of the properties of one-dimensional flows, in particular their weak dependence on the pump parameters and their rapid transition to a supersonic regime under the effect of the quantum pressure. The role of disorder and of an incoherent reservoir in inducing nonstationary behaviors with moving phase singularities is also highlighted. Such complex and highly nonlinear behaviors call for quantitative experimental tests of the underlying Gross-Pitaevskii equation.

We formulate and solve the Eliashberg equations on the imaginary frequency axis at temperatures below $T_c$ in the weak-coupling limit. We find an excellent scaling at all temperatures, for a fixed coupling strength, and the normalized order parameter exhibits a BCS-like temperature dependence. The hybrid real-imaginary axis equations are also solved to obtain numerically exact analytic continuations from the imaginary frequency axis to the real frequency axis. This provides a determination of the gap edge, which, in the weak-coupling limit, is identical to the order parameter from the imaginary axis. The analytical result for the zero-temperature gap edge deviates from the BCS result by a factor of $1/\sqrt{e}$, which was also obtained for the transition temperature $T_c$. We show that the normalized gap function on both the real and imaginary frequency axes, for an electron-phonon Einstein spectrum $(\delta$ function) of a fixed strength, is a universal function of frequency, independent of temperature. The $1/\sqrt{e}$ correction is a result of this nontrivial frequency dependence in the gap function. This modification, in the gap edge and in $T_c$, serves to preserve various dimensionless ratios to their BCS values.

Novel materials incorporating electronic degrees of freedom other than charge, including spin, orbital, and valley degrees of freedom, have shown themselves to be of great interest and applicable potential. Recently, the multipolar degrees of freedom have attracted remarkable attention in the electronic correlated effects. In this work, we systematically study the transport, magnetic, and thermodynamic properties of the topological semimetal candidate PrBi in the framework of crystalline electric field theory. Our results demonstrate the ${\mathrm{\Gamma }}_3$ non-Kramers doublet as the ground state of ${\mathrm{Pr}}^{3+}$ ($4f^2$) ions. This ground state is nonmagnetic but carries a nonzero quadrupolar moment $\langle {\widehat{O}}_2^0\rangle$. A quadrupolar phase transition is inferred below 0.08 K. No obvious quadrupolar Kondo effect can be identified. Ultrahigh-field quantum oscillation measurements confirm PrBi as a semimetal with a nontrivial Berry phase and low total carrier density 0.06/f.u. We discuss the interplay between low carrier density and $4f^2$ quadrupolar moment, and ascribe the weak quadrupolar ordering and Kondo effect to consequences of the low carrier density. PrBi, thus, opens a new window to the physics of topology and strongly correlated effects with quadrupolar degrees of freedom in the low-carrier-density limit, evoking the need for a reexamination of the Nozières exhaustion problem in the context of the multichannel Kondo effect.

Spin-dependent recombination in GaAsN offers many interesting possibilities in the design of spintronic devices mostly due to its astounding capability to reach conduction band electron spin polarization close to 100% at room temperature. The mechanism behind the spin selective capture of electrons in ${\mathrm{Ga}}^{2+}$ paramagnetic centers is revisited in this paper to address inconsistencies common to most previously presented models. Primarily, these errors manifest themselves as major disagreements with the experimental observations of two key characteristics of this phenomenon: the effective Overhauser-like magnetic field and the width of the photoluminescence Lorentzian-like curves as a function of the illumination power. These features are not only essential to understand the spin-dependent recombination in GaAsN, but are also key to the design of novel spintronic devices. Here we demonstrate that the particular structure of the electron capture expressions introduces spurious electron-nucleus correlations that artificially alter the balance between the hyperfine and the Zeeman contributions. This imbalance strongly distorts the effective magnetic field and width characteristics. In this work we propose an alternative recombination mechanism that preserves the electron-nucleus correlations and, at the same time, keeps the essential properties of the spin selective capture of electrons. This mechanism yields a significant improvement to the agreement between experimental and theoretical results. In particular, our model gives results in very good accord with the experimental effective Overhauser-like magnetic field and width data, and with the degree of circular polarization under oblique magnetic fields.

Exciting solids with intense femtosecond laser pulses prompts electrons of the interrogated material to respond in a highly nonlinear manner, as is evident in the emission of high-order harmonic radiation and photoelectrons with kinetic energies well above that of the driving photons. Such high-field interactions can be resolved, for example, in above-threshold multiphoton photoemission (ATP) spectroscopy. In this work, we interrogate the nonlinear photoelectric responses of the pristine copper, silver, and gold noble metal surfaces in (111) and (100) crystal orientations in the perturbative regime. Using multiphoton photoemission spectroscopy (mPP) excited by finely tuned optical fields, we characterize enhancement of the mPP and ATP yields from (111) surfaces in selected $k_{\left|\right|}$-momentum ranges when the occupied Shockley surface (SS) states are (near-)resonantly coupled by multiphoton transitions to image potential (IP) intermediate states in the excitation process. The ATP signal from the IP states of (111) surfaces is largely defined by their formation through polarization of SS electrons; this observation is contrasted with ATP experiments from the Ag(100) surface, for which the SS becomes an unoccupied resonance and the IP states can only be excited from bands with significantly more bulk character. In addition, based on the optical power and nonlinear order-dependent mPP spectra, we provide evidence for ATP being a one-step, rather than a sequential process, as previously postulated.

Fractons emerge as charges with reduced mobility in a class of gauge theories. Here, we generalize fractonic theories of $\text{U}\left(1\right)$ type to what we call $(k,n)$-fractonic Maxwell theory, which employs symmetric rank-$n$ tensors of $k$ forms (rank-$k$ antisymmetric tensors) as “vector potentials.” The generalization, valid in any spatial dimension $d$, has two key manifestations. First, the objects with mobility restrictions extend beyond simple charges to higher-order multipoles (dipoles, quadrupoles, etc.) all the way to $(n-1)\mathrm{th}$-order multipoles, which we call the order-$n$ fracton condition. Second, these fractonic charges themselves are characterized by tensorial densities of $(k-1)$-dimensional extended objects. For any $(k,n)$, the theory can be constructed to have a gapless “photon modes” with dispersion $\omega \sim {\left|q\right|}^z$, where the integer $z$ can range from 1 to $n$.

The origin of the metal to insulator transition in ${\mathrm{Fe}}_3{\mathrm{O}}_4$ remains a challenge due to the complexity of the system: it is a mixed valent, strongly correlated system where many interactions such as Jahn-Teller distortion, exchange, and phonons are very close in energetics. A recent interpretation of the Verwey transition as an ordering of a three-site magnetic polaron, the trimeron, has been put forward. Here we investigate the existence of the trimeron correlations in the high-temperature phase of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ using high-resolution iron $2p3d$ resonant inelastic scattering magnetic linear dichroism. Guided by theoretical simulations, we reveal that the polarization dependence of the low-energy spin-orbital excitations is incompatible with tetragonal Jahn-Teller trimeron-type distortion. We conclude that the lowest-energy state of the high-temperature phase of ${\mathrm{Fe}}_3{\mathrm{O}}_4$ arises from an intricate interplay between trigonal crystal-field, exchange, and spin-orbit interactions.

We propose a platform for robust and tunable nonreciprocal phonon transport based on arrays of optomechanical microtoroids. In our approach, time-reversal symmetry is broken by the interplay of photonic spin-orbit coupling, engineered using a state-of-the-art geometrical approach, and the optomechanical interaction. We demonstrate the topologically protected nature of this system by investigating its robustness to imperfections. This type of system could find application in phonon-based information storage and signal-processing devices.

Solid solubility (SS) is one of the most important features of alloys, which is difficult to be largely tuned in the entire alloy concentrations by external approaches, e.g., the case for monolayer BNC $\left[{\left(\mathrm{BN}\right)}_{1-x}{\left({\mathrm{C}}_2\right)}_x\right]$ alloys. Here a novel mechanism of selective orbital coupling between wrong-bond states and surface states mediated by the specific substrate has been proposed to stabilize the wrong bonds and in turn significantly enhance the SS of ${\left(\mathrm{BN}\right)}_{1-x}{\left({\mathrm{C}}_2\right)}_x$. Surprisingly, we demonstrate that five ordered alloys can spontaneously be formed when ${\left(\mathrm{BN}\right)}_{1-x}{\left({\mathrm{C}}_2\right)}_x$ is grown on hcp-phase Cr. These ordered alloys can exhibit largely tunable quasiparticle band gaps (1.45–3.91 eV) and exciton binding energies (0.5–1.6 eV) in their free-standing forms. Importantly, the disordered ${\left(\mathrm{BN}\right)}_{1-x}{\left({\mathrm{C}}_2\right)}_x$ can be achieved on Cr substrate with a hugely reduced miscibility temperature from 5600 to 1200 K. These disordered alloys exhibit continuously tunable band gaps from 0 to 6 eV in their free-standing forms. Our discovery could potentially resolve the long-standing SS problem of BNC alloys and significantly extend their optoelectronic applications from infrared to deep ultraviolet.

We present measurements of the nucleation rate into a diamond lattice in dynamically compressed polystyrene obtained in a pump-probe experiment using a high-energy laser system and in situ femtosecond x-ray diffraction. Different temperature-pressure conditions that occur in planetary interiors were probed. For a single shock reaching 70 GPa and 3000 K no diamond formation was observed, while with a double shock driving polystyrene to pressures around 150 GPa and temperatures around 5000 K nucleation rates between ${10}^{29}$ and ${10}^{34}{\mathrm{m}}^{-3}$ ${\mathrm{s}}^{-1}$ were recorded. These nucleation rates do not agree with predictions of the state-of-the-art theoretical models for carbon-hydrogen mixtures by many orders of magnitude. Our data suggest that there is significant diamond formation to be expected inside icy giant planets like Neptune and Uranus.

Motivated by recent experiments and density functional theory calculations on choloalite ${\mathrm{PbCuTe}}_2{\mathrm{O}}_6$, which possesses a Cu-based three-dimensional hyperkagome lattice, we propose and study a $J_1\text{−}{J}_2\text{−}{J}_3$ antiferromagnetic Heisenberg model on a hyperkagome lattice. In the classical limit, possible ground states are analyzed by two triangle rules, i.e., the “hyperkagome triangle rule” and the “isolated triangle rule,” and classical Monte Carlo simulations are exploited to identify possible classical magnetic ordering and explore the phase diagram. In the quantum regime, Schwinger boson theory is applied to study possible quantum spin liquid states and long-range magnetically ordered states on an equal footing. These quantum states with bosonic partons are classified and analyzed by using projective symmetry groups (PSGs). It is found that there are only four types of algebraic PSGs allowed by the space group $P{4}_{1}32$ on a hyperkagome lattice. Moreover, there are only two types of PSGs that are compatible with the $J_1\text{−}{J}_2\text{−}{J}_3$ Heisenberg model. These two types of $Z_2$ bosonic states are distinguished by the gauge-invariant flux on the elementary ten-site loops on the hyperkagome network, called zero-flux state and $\pi$-flux state respectively. Both the zero-flux state and the $\pi$-flux state are able to give rise to quantum spin liquid states as well as magnetically ordered states, and the zero-flux states and the $\pi$-flux states can be distinguished by the lower and upper edges of the spectral function $S(\mathit{q},\omega )$, which can be measured by inelastic neutron scattering experiments.

We systematically study the indirect interaction between a magnon mode and a cavity photon mode mediated by traveling photons of a waveguide. From a general Hamiltonian, we derive the effective coupling strength between two separated modes, and obtain the theoretical expression of the system's transmission. Accordingly, we design an experimental setup consisting of a shield cavity photon mode, a microstrip line, and a magnon system to test our theoretical predictions. From measured transmission spectra, indirect interaction, as well as mode hybridization, between two modes can be observed. All experimental observations support our theoretical predictions. In this work we clarify the mechanism of traveling photon mediated interactions between two separate modes. Even without spatial mode overlap, two separated modes can still couple with each other through their correlated dissipations into a mutual traveling photon bus. This conclusion may help us understand the recently discovered dissipative coupling effect in cavity magnonics systems. Additionally, the physics and technique developed in this work may benefit us in designing new hybrid systems based on the waveguide magnonics.

The $f\text{−}d$ magnetic exchange interaction is considered to be a key ingredient for many exotic topological phases in pyrochlore iridates. Here, we have investigated the evolution of structural, magnetic, and electronic properties in doped pyrochlore iridate, ${\left({\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x\right)}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$. Apart from geometrical frustration, pyrochlore iridates are well known for their active spin-orbit-coupling effect. The substitution of ${\mathrm{Pr}}^{3+}\left(4f^2\right)$ for the nonmagnetic ${\mathrm{Y}}^{3+}\left(4d^0\right)$ acts as a magnetic doping, which provides an ideal platform to study $f\text{−}d$ exchange interaction without altering the Ir sublattice. With Pr substitution, the system retains its original cubic structural symmetry, but the local structural parameters show an evolution with the doping concentration $x$. The robust magnetic-insulating state in ${\mathrm{Y}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ is drastically weakened, while ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7\left(x=1.0\right)$ shows a paramagnetic-metallic behavior. A metal-insulator transition is observed for the $x=0.8$ sample. This evolution of magnetic and electronic properties is believed to be induced by an exchange interaction between localized Pr-$4f$ and itinerant Ir-$5d$ electrons, as well as by an increased hybridization between Ir-$t_{2g}$ and (basal) O-$p$ orbitals, as observed in x-ray absorption spectroscopy study. The resistivity in insulating materials follows a power-law behavior with a decreasing exponent with $x$. A negative magnetoresistance is observed for the present series of samples at low temperature and where the magnetoresistance shows a quadratic field dependence at higher fields.

Understanding properties of quantum matter is an outstanding challenge in science. In this paper, we demonstrate how machine-learning methods can be successfully applied for the classification of various regimes in single-particle and many-body systems. We realize neural network algorithms that perform a classification between regular and chaotic behavior in quantum billiard models with remarkably high accuracy. We use the variational autoencoder for autosupervised classification of regular/chaotic wave functions, as well as demonstrating that autoencoders could be used as a tool for detection of anomalous quantum states, such as quantum scars. By taking this method further, we show that machine-learning techniques allow us to pin down the transition from integrability to many-body quantum chaos in Heisenberg XXZ spin chains. For both cases, we confirm the existence of universal W shapes that characterize the transition. Our results pave the way for exploring the power of machine-learning tools for revealing exotic phenomena in quantum many-body systems.

We report numerical simulations of large-amplitude oscillations of a trapped vortex line under a strong ac magnetic field $H\left(t\right)=Hsin\omega t$ parallel to the surface. The power dissipated by an oscillating vortex segment driven by the surface ac Meissner currents was calculated by taking into account the nonlinear vortex line tension, vortex mass, and a nonlinear Larkin-Ovchinnikov (LO) viscous drag coefficient $\eta \left(v\right)$. We show that the LO decrease of $\eta \left(v\right)$ with the vortex velocity $v$ can radically change the field dependence of the surface resistance $R_i\left(H\right)$ caused by trapped vortices. At low frequencies, $R_i\left(H\right)$ exhibits a conventional increase with $H$, but as $\omega$ increases, the surface resistance becomes a nonmonotonic function of $H$ which decreases with $H$ at higher fields. The effects of frequency, pin spacing, and the mean-free path $l_i$ on the field dependence of $R_i\left(H\right)$ were calculated. It is shown that, as the surface gets dirtier and $l_i$ decreases, the anomalous drop of $R_i\left(H\right)$ with $H$ shifts to lower fields which can be much smaller than the lower critical magnetic field. Our numerical simulations also show that the LO decrease of $\eta \left(v\right)$ with $v$ can cause a vortex bending instability at high field amplitudes and frequencies, giving rise to the formation of dynamic kinks along the vortex. Measurements of $R_i\left(H\right)$ caused by sparse vortices trapped perpendicular to the surface can offer opportunities to investigate an extreme nonlinear dynamics of vortices driven by strong current densities up to the depairing limit at low temperatures. The behavior of $R_i\left(H\right)$, which can be tuned by varying the rf frequency or concentration of nonmagnetic impurities, is not masked by strong heating effects characteristic of dc or pulse transport measurements.

Recent experiments have focused attention on the properties of chains of atoms in which the atoms are either in their ground states or in highly excited Rydberg states which block similar excitations in their immediate neighbors. As the low-energy Hilbert space of such chains is isomorphic to that of a chain of Fibonacci anyons, they have been proposed as a platform for topological quantum computation and for simulating anyon dynamics. We show that generic local operators in the Rydberg chain correspond to nonlocal anyonic operators that do not preserve a topological symmetry of the physical anyons. Consequently, we argue that Rydberg chains do not yield Fibonacci anyons and quantum computation with Rydberg atoms is not topologically protected.

Derivation of a reduced effective model allows for a unified treatment and discussion of the $J_1\text{−}{J}_{2}S=1/2$ Heisenberg model on triangular and kagome lattices. Calculating thermodynamic quantities, i.e., the entropy $s\left(T\right)$ and uniform susceptibility ${\chi }_0\left(T\right)$, numerically on systems up to effectively $N=42$ sites, we show by comparing to full-model results that low-$T$ properties are qualitatively well represented within the effective model. Moreover, we find in the spin-liquid regime similar variation of $s\left(T\right)$ and ${\chi }_0\left(T\right)$ in both models down to $T\ll J_1$. In particular, studied spin liquids appear to be characterized by the Wilson ratio vanishing at low $T$, indicating that the low-lying singlets are dominating over the triplet excitations.

We probe ultrastrong light-matter coupling between metallic terahertz metasurfaces and Landau-level transitions in high-mobility two-dimensional electron and hole gases. We utilize heavy-hole cyclotron resonances in strained Ge and electron cyclotron resonances in InSb quantum wells, both within highly nonparabolic bands, and compare our results to well-known parabolic AlGaAs/GaAs quantum well systems. Tuning the coupling strength of the system by two methods, lithographically and by optical pumping, we observe a behavior clearly deviating from the standard Hopfield model previously verified in cavity quantum electrodynamics: an opening of a lower polaritonic gap.