The question of when and how dense granular materials start to flow under stress, despite many industrial and geophysical applications, remains largely unresolved. We develop and test a simple equation for the onset of quasistatic flows of granular materials which is based on the frictional aging of the granular packing. The result is a nonmonotonic stress-strain relation which—akin to classical friction—is independent of the shear rate. This relation suffices to understand the quasistatic deformations of aging granular media, and its solid-to-liquid transition. Our results also elucidate the (flow) history dependence of the mechanical properties, and the sensitivity to the initial preparation of granular media.

Underscreening is a collective term for charge correlations in electrolytes decaying slower than the Debye length. Anomalous underscreening refers to phenomenology that cannot be attributed alone to steric interactions. Experiments with concentrated electrolytes and ionic fluids report anomalous underscreening, which so far has not been observed in simulation. We present Molecular Dynamics simulation results exhibiting anomalous underscreening that can be connected to cluster formation. A theory that accounts for ion pairing confirms the trend. Our results challenge the classic understanding of dense electrolytes impacting the design of technologies for energy storage and conversion.

Certain Feynman integrals are associated to Calabi-Yau geometries. We demonstrate how these integrals can be computed with the method of differential equations. The four-loop equal-mass banana integral is the simplest Feynman integral whose geometry is a nontrivial Calabi-Yau manifold. We show that its differential equation can be cast into an $ϵ$-factorized form. This allows us to obtain the solution to any desired order in the dimensional regularization parameter $ϵ$. The method generalizes to other Calabi-Yau Feynman integrals. Our calculation also shows that the four-loop banana integral is only minimally more complicated than the corresponding Feynman integrals at two or three loops.

This Letter examines the effectiveness of the Dyson-Schwinger (DS) equations as a calculational tool in quantum field theory. The DS equations are an infinite sequence of coupled equations that are satisfied exactly by the connected Green’s functions $G_n$ of the field theory. These equations link lower to higher Green’s functions and, if they are truncated, the resulting finite system of equations is underdetermined. The simplest way to solve the underdetermined system is to set all higher Green’s function(s) to zero and then to solve the resulting determined system for the first few Green’s functions. The $G_1$ or $G_2$ so obtained can be compared with exact results in solvable models to see if the accuracy improves for high-order truncations. Five $D=0$ models are studied: Hermitian ${\varphi }^4$ and ${\varphi }^6$ and non-Hermitian $i{\varphi }^3$, $-{\varphi }^4$, and $i{\varphi }^5$ theories. The truncated DS equations give a sequence of approximants that converge slowly to a limiting value but this limiting value always differs from the exact value by a few percent. More sophisticated truncation schemes based on mean-field-like approximations do not fix this formidable calculational problem.

We examine the effect of a parasitic rf magnetic field, attributed to ion trapping, on the highly anticipated nuclear clock based on ${{}^{229}\mathrm{Th}}^{3+}$ [C. J. Campbell et al., Phys. Rev. Lett. 108, 120802 (2012)]. The rf magnetic field induces an ac Zeeman shift to the clock frequency. As we demonstrate, this shift threatens to be the dominant systematic frequency shift for the clock, exceeding other systematic frequency shifts and the projected systematic uncertainty of the clock by orders of magnitude. We propose practical means to suppress or eliminate this shift.

We show that the simplest of existing molecules—closed-shell diatomics not interacting with one another—host topological charges when driven by periodic far-off-resonant laser pulses. A periodically kicked molecular rotor can be mapped onto a “crystalline” lattice in angular momentum space. This allows us to define quasimomenta and the band structure in the Floquet representation, by analogy with the Bloch waves of solid-state physics. Applying laser pulses spaced by $1/3$ of the molecular rotational period creates a lattice with three atoms per unit cell with staggered hopping. Within the synthetic dimension of the laser strength, we discover Dirac cones with topological charges. These Dirac cones, topologically protected by reflection and time-reversal symmetry, are reminiscent of (although not equivalent to) that seen in graphene. They—and the corresponding edge states—are broadly tunable by adjusting the laser strength and can be observed in present-day experiments by measuring molecular alignment and populations of rotational levels. This paves the way to study controllable topological physics in gas-phase experiments with small molecules as well as to classify dynamical molecular states by their topological invariants.

In a Hermitian system, bound states must have quantized energies, whereas free states can form a continuum. We demonstrate how this principle fails for non-Hermitian systems, by analyzing non-Hermitian continuous Hamiltonians with an imaginary momentum and Landau-type vector potential. The eigenstates, which we call “continuum Landau modes” (CLMs), have Gaussian spatial envelopes and form a continuum filling the complex energy plane. We present experimentally realizable 1D and 2D lattice models that host CLMs; the lattice eigenstates are localized and have other features matching the continuous model. One of these lattices can serve as a rainbow trap, whereby the response to an excitation is concentrated at a position proportional to the frequency. Another lattice can act a wave funnel, concentrating an input excitation onto a boundary over a wide frequency bandwidth. Unlike recent funneling schemes based on the non-Hermitian skin effect, this requires a simple lattice design with reciprocal couplings.

Tunnel spectroscopy data for the detection of Majorana bound states (MBS) is often criticized for its proneness to misinterpretation of genuine MBS with low-lying Andreev bound states. Here, we suggest a protocol removing this ambiguity by extending single shot measurements to sequences performed at varying system parameters. We demonstrate how such sampling, which we argue requires only moderate effort for current experimental platforms, resolves the statistics of Andreev side lobes, thus providing compelling evidence for the presence or absence of a Majorana center peak.

Squeezing of the quadratures of the electromagnetic field has been extensively studied in optics and microwaves. However, previous works focused on the generation of squeezed states in a low impedance ($Z_0\approx 50\mathrm{\Omega }$) environment. We report here on the demonstration of the squeezing of bosonic edge magnetoplasmon modes in a quantum Hall conductor whose characteristic impedance is set by the quantum of resistance ($R_K\approx 25\mathrm{k}\mathrm{\Omega }$), offering the possibility of an enhanced coupling to low-dimensional quantum conductors. By applying a combination of dc and ac drives to a quantum point contact, we demonstrate squeezing and observe a noise reduction 18% below the vacuum fluctuations. This level of squeezing can be improved by using more complex conductors, such as ac driven quantum dots or mesoscopic capacitors.

Understanding the molecular and electronic structure of solvated ions at surfaces requires an analysis of the interactions between the surface, the ions, and the solvent environment on equal footing. Here, we tackle this challenge by exploring the initial stages of ${\mathrm{Cs}}^{+}$ hydration on a Cu(111) surface by combining experiment and theory. Remarkably, we observe “inside-out” solvation of ${\mathrm{Cs}}^{+}$ ions, i.e., their preferential location at the perimeter of the water clusters on the metal surface. In addition, water-Cs complexes containing multiple ${\mathrm{Cs}}^{+}$ ions are observed to form at these surfaces. Established models based on maximum ion-water coordination and conventional solvation models cannot account for this situation, and the complex interplay of microscopic interactions is the key to a fundamental understanding.

We propose and study a two-dimensional phase of shifted charge density waves (CDW), which is constructed from an array of weakly coupled 1D CDW wires whose phases shift from one wire to the next. We show that the fully gapped bulk CDW has topological properties, characterized by a nonzero Chern number, that imply edge modes within the bulk gap. Remarkably, these edge modes exhibit spectral pseudoflow as a function of position along the edge, and are thus dual to the chiral edge modes of Chern insulators with their spectral flow in momentum space. Furthermore, we show that the CDW edge modes are stable against interwire coupling. Our predictions can be tested experimentally in quasi-1D CDW compounds such as ${\mathrm{Ta}}_2{\mathrm{Se}}_8\mathrm{I}$.

We combine spin-polarized scanning tunneling microscopy with quantum master equation analysis to investigate the spin dynamics of the single atom magnet Dy on graphene/Ir(111). By performing reading and writing experiments, we show that the strongly spin polarized $5d6s$ valence shells, as well as their intra-atomic exchange coupling to the $4f$ shell, determine the pathways for magnetization relaxation and thus the spin dynamics. The good quantum number that determines which states are stable and which mechanisms for reversal exist in a given crystal field is the atomic total angular momentum $J_z^{\mathrm{tot}}$ and not the commonly considered $J_z^{4f}$ of the $4f$ shell only.

${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is an excitonic insulator candidate showing the semiconductor or semimetal-to-insulator (SI) transition below $T_c=326\mathrm{K}$. However, since a structural transition accompanies the SI transition, deciphering the role of electronic and lattice degrees of freedom in driving the SI transition has remained controversial. Here, we investigate the photoexcited nonequilibrium state in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ using pump-probe Raman and photoluminescence spectroscopies. The combined nonequilibrium spectroscopic measurements of the lattice and electronic states reveal the presence of a photoexcited metastable state where the insulating gap is suppressed, but the low-temperature structural distortion is preserved. We conclude that electron correlations play a vital role in the SI transition of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$.

We introduce a new model consisting of globally coupled high-dimensional generalized limit-cycle oscillators, which explicitly incorporates the role of amplitude dynamics of individual units in the collective dynamics. In the limit of weak coupling, our model reduces to the $D$-dimensional Kuramoto phase model, akin to a similar classic construction of the well-known Kuramoto phase model from weakly coupled two-dimensional limit-cycle oscillators. For the practically important case of $D=3$, the incoherence of the model is rigorously proved to be stable for negative coupling ($K<0$) but unstable for positive coupling ($K>0$); the locked states are shown to exist if $K>0$; in particular, the onset of amplitude death is theoretically predicted. For $D\ge 2$, the discrete and continuous spectra for both locked states and amplitude death are governed by two general formulas. Our proposed $D$-dimensional model is physically more reasonable, because it is no longer constrained by fixed amplitude dynamics, which puts the recent studies of the $D$-dimensional Kuramoto phase model on a stronger footing by providing a more general framework for $D$-dimensional limit-cycle oscillators.

Spectral functions of non-Hermitian Hamiltonians can reveal the existence of topologically nontrivial line gaps and the associated topological edge modes. However, the computation of spectral functions in a non-Hermitian many-body system remains an open challenge. Here, we put forward a numerical approach to compute spectral functions of a non-Hermitian many-body Hamiltonian based on the kernel polynomial method and the matrix-product state formalism. We show that the local spectral functions computed with our algorithm reveal topological spin excitations in a non-Hermitian spin model, faithfully reflecting the nontrivial line gap topology in a many-body model. We further show that the algorithm works in the presence of the non-Hermitian skin effect. Our method offers an efficient way to compute local spectral functions in non-Hermitian many-body systems with tensor networks, allowing us to characterize line gap topology in non-Hermitian quantum many-body models.

By considering the quantum Oppenheimer-Snyder model in loop quantum cosmology, a new quantum black hole model whose metric tensor is a suitably deformed Schwarzschild one is derived. The quantum effects imply a lower bound on the mass of the black hole produced by the collapsing dust ball. For the case of larger masses where the event horizon does form, the maximal extension of the spacetime and its properties are investigated. By discussing the opposite scenario to the quantum Oppenheimer-Snyder, a quantum Swiss Cheese model is obtained with a bubble surrounded by the quantum universe. This model is analogous to black hole cosmology or fecund universes where the big bang is related to a white hole. Thus our models open a new window to cosmological phenomenology.

The study of the gravitational field produced by a spatially nonlocal, superposed quantum state of a massive particle is an interesting and active area of research. One outstanding issue is whether the gravitational field behaves like the classical superposition of the gravitational field of two particles separated by a spatial distance with half the mass at each position. Alternatively, does the gravitational field behave as a quantum superposition with a far more interesting and subtle behavior than a simple classical superposition? Quantum field theory is ideally suited to probe exactly this kind of question. We study the scattering of a massless scalar on a spatially nonlocal quantum superposition of a massive particle. We compute the differential scattering cross section corresponding to one-graviton exchange. We find that the scattering cross section disagrees with the Newton-Schrödinger picture of potential scattering from two localized sources with half the mass at each source. This suggests that experimental observation of gravitational scattering could inform the viability of the semiclassical treatment of the gravitational field, as in the Newton-Schrödinger description, vs the fully quantum mechanical treatment adopted here. We comment on the experimental feasibility of observing such effects in systems with many particles such as Bose-Einstein condensates.

We demonstrate an efficient optical guiding technique for coupling cold atoms in the near field of a planar nanophotonic circuit, and realize large atom-photon coupling to a whispering-gallery mode in a microring resonator with a single-atom cooperativity $C\gtrsim 8$. The guiding potential is created by diffracted light on a nanophotonic waveguide that smoothly connects to a dipole trap in the far field for atom guiding with subwavelength precision. We observe atom-induced transparency for light coupled to a microring, characterize the atom-photon coupling rate, extract guided atom flux, and demonstrate on-chip photon routing by single atoms. Our demonstration promises new applications with cold atoms on a nanophotonic circuit for chiral quantum optics and quantum technologies.