We theoretically investigate the crystalline anisotropy of topological phase transitions in phase-controlled planar Josephson junctions (JJs) subject to spin-orbit coupling and in-plane magnetic fields. It is shown how topological superconductivity (TS) is affected by the interplay between the magnetic field and the orientation of the junction with respect to its crystallographic axes. This interplay can be used to electrically tune between different symmetry classes in a controlled fashion and thereby optimize the stability and localization of Majorana bound states in planar Josephson junctions. Our findings can be used as a guide for achieving the most favorable conditions when engineering TS in planar JJs and can be particularly relevant for setups containing noncollinear junctions which have been proposed for performing braiding operations on multiple Majorana pairs.

Recently, it was shown that the multipolar Kondo problem, wherein a quantum impurity carrying higher-rank multipolar moments interacts with conduction electrons, leads to novel non-Fermi liquid states. Because of the multipolar character of the local moments, the form of the interaction with conduction electrons is strongly dependent on the orbital symmetry of the conduction electrons via crystalline symmetry constraints. This suggests that there may exist a variety of different non-Fermi liquid states in generic multipolar Kondo problems depending on the character of conduction electrons. In this work, using renormalization group analysis, we investigate a model where the multipolar local moment is coupled to conduction electrons with two different orbital-symmetry components, namely, $p$-wave and $f$-wave symmetries. When each orbital-symmetry component is present alone, non-Fermi liquid states with exactly the same thermodynamic singularities appear. When both orbital-symmetry components are allowed, however, a completely different non-Fermi liquid state arises via the quantum fluctuations in the mixed scattering channels. This remarkable result suggests that the multipolar Kondo problem presents unique opportunities for the discovery of unexpected non-Fermi liquid states.

The propulsion of living microorganisms ultimately relies on viscous drag for body-fluid interactions. The self-locomotion in superfluids such as ${}^4\mathrm{He}$ is deemed impossible due to the apparent lack of viscous resistance. Here, we investigate the self-propulsion of a Janus (two-face) light-absorbing particle suspended in superfluid helium ${}^4\mathrm{He}$ (He-II). The particle is energized by the heat flux due to the absorption of light from an external source. We show that a quantum mechanical propulsion force originates due to the transformation of the superfluid to a normal fluid on the heated particle face. The theoretical analysis is supported by the numerical solution of the Ginzburg-Landau-Khalatnikov model for a superfluid. Our results shed light on the dynamics of inclusions in a superfluid and stimulate experiments.

With a view toward a fracton theory in condensed matter, we introduce a higher-moment polynomial degree-p global symmetry, acting on complex scalar/vector/tensor fields (e.g., ordinary or vector global symmetry for $\mathrm{p}=0$ and $\mathrm{p}=1$ respectively). We relate this higher-moment global symmetry of $n$-dimensional space, to a lower degree [either ordinary or higher-moment, e.g., degree-(p-$\ell$)] subdimensional or subsystem global symmetry on layers of $(n-\ell )$-submanifolds. These submanifolds are algebraic affine varieties (i.e., solutions of polynomials). The structure of layers of submanifolds as subvarieties can be studied via mathematical tools of embedding, foliation, and algebraic geometry. We also generalize Noether's theorem for this higher-moment polynomial global symmetry. We can promote the higher-moment global symmetry to a local symmetry and derive a new family of higher-rank-m symmetric tensor gauge theory by gauging, with $\mathrm{m}=\mathrm{p}+1$. By further gauging a discrete ${\mathbb{Z}}_2^C$ charge conjugation (particle-hole) symmetry, we derive a general class of rank-m tensor non-Abelian gauge field theory (the gauge structure is noncommutative thus non-Abelian but not an ordinary group): a hybrid class of (symmetric or nonsymmetric) higher-rank-m tensor gauge theory and antisymmetric tensor topological field theory, generalizing Wang and Xu [Ann. Phys. 424, 168370 (2021)], interplaying between gapless and gapped sectors.

Dirac-Weyl semimetals are unique three-dimensional (3D) phases of matter with gapless electrons and novel electrodynamic properties believed to be robust against weak perturbations. Here, we unveil the crucial influence of the disorder statistics and impurity diversity in the stability of incompressible electrons in 3D semimetals. Focusing on the critical role played by rare impurity configurations, we show that the abundance of low-energy resonances in the presence of diluted random potential wells endows rare localized zero-energy modes with statistical significance, thus lifting the nodal density of states. The strong nonperturbative effect here reported converts the 3D Dirac-Weyl semimetal into a compressible metal even at the lowest impurity densities. Our analytical results are validated by high-resolution real-space simulations in record-large 3D lattices with up to 536 000 000 orbitals.

What is the fate of the ground state of a two-dimensional electron system at very low Landau level filling factors $\left(\nu \right)$ where interaction reigns supreme? An ordered array of electrons, the so-called Wigner crystal, has long been believed to be the answer. It was in fact the search for the elusive Wigner crystal that led to the discovery of an unexpected, incompressible liquid state, namely the fractional quantum Hall state at $\nu =1/3$. Understanding the competition between the liquid and solid ground states has since remained an active field of fundamental research. Here we report experimental data for a new two-dimensional system where the electrons are confined to an AlAs quantum well. The exceptionally high quality of the samples and the large electron effective mass allow us to determine the liquid-solid phase diagram for the two-dimensional electrons in a large range of filling factors near $\simeq 1/3$ and $\simeq 1/5$. The data and their comparison with an available theoretical phase diagram reveal the crucial role of Landau level mixing and finite electron layer thickness in determining the prevailing ground states.

The electron-doped cuprates are usually characterized by a more robust antiferromagnetic phase and a much narrower superconducting (SC) dome than those of the hole-doped counterparts. Recently, bulk single crystals of ${\text{Pr}}_{1.3-x}{\text{La}}_{0.7}{\text{Ce}}_x{\text{CuO}}_{4-\delta }$ (PLCCO) prepared by the protect annealing method have been studied extensively and revealed many intriguing properties that were different from those obtained from samples annealed by the conventional methods. Here we report on a systematic angle-resolved photoemission spectroscopy study of PLCCO single crystals after protect annealing. The results indicate that the actual electron concentration ($n_{\text{FS}}$) estimated from the Fermi-surface area is significantly larger than the Ce concentration $x$ and the new $n_{\text{FS}}$-based SC dome of PLCCO is more extended towards the overdoped side than the $x$-based SC dome derived for samples prepared using the conventional annealing method.

Material realization of the non-Abelian Kitaev spin liquid phase—an example of Ising topological order (ITO)—has been the subject of intense research in recent years. The $4d$ honeycomb Mott insulator $\alpha \text{−}{\mathrm{RuCl}}_3$ has emerged as a leading candidate, as it enters a field-induced magnetically disordered state where a half-integer quantized thermal Hall conductivity ${\kappa }_{xy}$ was reported. Further, a recent report of a sign change in the quantized ${\kappa }_{xy}$ across a certain crystallographic direction is strong evidence for a topological phase transition between two ITOs with opposite Chern numbers. Although this is a fascinating result, independent verification remains elusive, and one may ask if there is a thermodynamic quantity sensitive to the phase transition. Here we propose that the magnetotropic coefficient $k$ would serve such a purpose. We report a singular feature in $k$ that indicates a topological phase transition across the $\widehat{\mathit{b}}$-axis where ITO is prohibited by a $C_2$ symmetry. If the transition in $\alpha \text{−}{\mathrm{RuCl}}_3$ is indeed a direct transition between ITOs, or is a more broad realization of a topological phase transition, then this feature in $k$ should be observable. Implications of $C_2$ symmetry on other experimental quantities are also discussed.

We systematically construct flatbands (FB) for tight-binding models on simple Bravais lattices in space dimension $d\ge 2$ in the presence of a static uniform DC field. Commensurate DC field directions yield irreducible Wannier-Stark (WS) bands in perpendicular dimension $d-1$ with $d$-dimensional eigenfunctions. The irreducible bands turn into dispersionless flatbands in the absence of nearest neighbor hoppings between lattice sites in any direction perpendicular to the DC field. The number of commensurate directions which yield flatbands is of measure one. We arrive at a complete halt of transport, with the DC field prohibiting transport along the field direction, and the flatbands prohibiting transport in all perpendicular directions as well. The anisotropic flatband eigenstates are localizing at least factorially (faster than exponential).

We study the zero-temperature quantum phase diagram for a two-component Bose-Einstein condensate in an optical cavity. The two atomic spin states are Raman coupled by two transverse orthogonally polarized, blue-detuned plane-wave lasers inducing a repulsive cavity potential. For a weak pump the lasers favor a state with homogeneous density and predefined uniform spin direction. When one pump laser is polarized parallel to the cavity mode polarization, the photons coherently scattered into the resonator induce a polarization gradient along the cavity axis, which mediates long-range density-density, spin-density, and spin-spin interactions. We show that the coupled atom-cavity system implements central aspects of the $t\text{−}J\text{−}V\text{−}W$ model with a rich phase diagram. At the mean-field limit we identify at least four qualitatively distinct density- and spin-ordered phases including ferromagnetic and antiferromagnetic order along the cavity axis, which can be controlled via the pump strength and detuning. Real-time observation of amplitude and phase of the emitted fields bears strong signatures of the realized phase and allows for real-time determination of phase transition lines. Together with measurements of the population imbalance, most properties of the phase diagram can be reconstructed.

We theoretically study correlations present deep in the spectrum of many-body-localized systems. An exact analytical expression for the spectral form factor of Poisson spectra can be obtained and is shown to agree well with numerical results on two models exhibiting many-body localization: a disordered quantum spin chain and a phenomenological $l$-bit model based on the existence of local integrals of motion. We also identify a universal regime that is insensitive to the global density of states as well as spectral edge effects.

In this paper we provide an exhaustive study of the photoemission spectrum of paramagnetic FeO under pressure using a refined version of our recently derived many-body effective energy theory (MEET). We show that, within a nonmagnetic description of the paramagnetic phase, the MEET gives an overall good description of the photoemission spectrum at ambient pressure as well as the changes it undergoes by increasing pressure. In particular at ambient pressure the band gap opens between the mixed Fe $t_{2g}$ and O $2p$ states and the Fe $4s$ states and, moreover, a $d\text{−}d$ gap opens, which is compatible with a high-spin configuration (hence nonzero local magnetic moments as observed in experiment), whereas with decreasing pressure the band gap tends to close, $t_{2g}$ states tend to become fully occupied, and $e_g$ states tend to become fully unoccupied, which is compatible with a low-spin configuration (hence a collapse of the magnetic moments as observed in experiment). This is a remarkable result, since, within a nonmagnetic description of the paramagnetic phase, the MEET is capable to correctly describe the photoemission spectrum and the spin configuration at ambient as well as high pressure. For comparison we report the band-gap values obtained using density-functional theory with a hybrid functional containing screened exchange (HSE06) and a variant of the $GW$ method (self-consistent COHSEX), which are reliable for the description of the antiferromagnetic phase. Both methods open a gap at ambient pressure, although, by construction, they give a low-spin configuration; increasing pressure they correctly describe the band-gap closing. We also report the photoemission spectrum of the metallic phase obtained with one-shot fully dynamical $GW$ on top of the local-density approximation, which gives a spectrum very similar to dynamical mean-field theory results from literature.

The energy landscape of a single electron in a triple quantum dot can be tuned such that the energy separation between ground and excited states becomes a flat function of the relevant gate voltages. These so-called sweet spots are beneficial for charge coherence since the decoherence effects caused by small fluctuations of gate voltages or surrounding charge fluctuators are minimized. We propose a new operation point for a triple quantum dot charge qubit, a so-called $C{Q}_3$-qubit, having a third-order sweet spot. We show strong coupling of the qubit to single photons in a frequency tunable high-impedance SQUID-array resonator. In the dispersive regime, we investigate the qubit linewidth in the vicinity of the proposed operating point. In contrast to the expectation for a higher-order sweet spot, we there find a local maximum of the linewidth. We find that this is due to a non-negligible contribution of noise on the quadrupolar detuning axis not being in a sweet spot at the proposed operating point. While the original motivation to realize a low-decoherence charge qubit was not fulfilled, our analysis provides insights into charge decoherence mechanisms relevant also for other qubits.

A compound with large anions is known to show large compressibility, whereby a novel response may emerge from the degenerated state by a uniaxial pressure. Neutron scattering study of ${\mathrm{BaCo}}_{0.9}{\mathrm{Ni}}_{0.1}{\mathrm{S}}_{1.9}$ crystal reveals that the tetragonal insulating state has two magnetic domains with in-plane anisotropic antiferromagnetic wave vectors ${\mathbf{Q}}_{\mathbf{1}}=(\pi ,0)$ and ${\mathbf{Q}}_{\mathbf{2}}=(0,\pi )$. The magnetic order with in-plane broken $C_4$ symmetry is realized as a twin of these two domains in the tetragonal state without any strain. One magnetic domain with ${\mathbf{Q}}_{\mathbf{2}}$ becomes dominant under a weak strain without any appreciable structural distortion. Correspondingly, the in-plane broken $C_4$ symmetry is also observed in the in-plane magnetic excitation of the tetragonal state.

Due to the absence of periodic length scale, electronic states and their topological properties in quasicrystals have been barely understood. Here, we focus on one-dimensional quasicrystals and reveal that their electronic critical states are topologically robust. Based on tiling space cohomology, we exemplify the case of one-dimensional aperiodic tilings especially Fibonacci quasicrystals and prove the existence of topological critical states at zero energy. Furthermore, we also show exotic electronic transmittance behavior near such topological critical states. Within the perturbative regime, we discuss the lack of translational symmetries and that the presence of topological critical states lead to an unconventional scaling behavior in transmittance. Considering both analytic analysis and numerics, electronic transmittance is computed in cases where the system is placed in air or is connected by semi-infinite periodic leads. Finally, we also discuss a generalization of our analysis to other quasicrystals. Our findings open a new class of topological quantum states which solely exist in quasicrystals due to exotic tiling patterns in the absence of periodic length scale, and their anomalous electronic transport properties applicable to many experiments.

Nuclear magnetic resonance (NMR) was recently shown to measure the bulk band inversion of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ through changes in the ${}^{209}\mathrm{Bi}$ nuclear quadrupole interaction, and the corresponding tensor of the local electric field gradient was found to follow, surprisingly, the direction of the external magnetic field if the sample is rotated. This manifests a hidden property of the charge carriers in the bulk of this topological insulator, which is explored here with another material, ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. It is found that two electric field gradients appear to be present at ${}^{209}\mathrm{Bi}$, one rests with the lattice, as usual, while a second follows the external field if it is rotated with respect to the crystal axes. These electronic degrees of freedom and their lifetime is believed to be responsible for an alternate quadrupolar relaxation that should lead to other special properties including the electronic specific heat.

A softening of phonon dispersion has been observed experimentally in underdoped cuprate superconductors at the charge-density-wave (CDW) ordering wave vector. Interestingly, the softening occurs below the superconducting (SC) transition temperature $T_c$, in contrast to the metallic systems, where the softening occurs usually below the CDW onset temperature $T_{\text{CDW}}$. An understanding of the “anomalous” nature of the phonon softening and its connection to the pseudogap phase in underdoped cuprates remain open questions. Employing a perturbative approach, we find that a complex interplay among the ubiquitous CDW order, SC order, and an unusually connected thermal fluctuations of these orders can explain the anomalous phonon softening below $T_c$. Furthermore, our formalism captures different characteristics of the low-temperature phonon softening, depending on material specificity.

Material realizations of the bond-dependent Kitaev interactions with $S=1/2$ local moments have vitalized the research in quantum spin liquids. Recently, it has been proposed that higher-spin analogs of the Kitaev interactions may also occur in a number of materials with strong spin-orbit coupling. In contrast to the celebrated $S=1/2$ Kitaev model on the honeycomb lattice, the higher-spin Kitaev models are not exactly solvable. Hence, the existence of quantum spin liquids in these systems remains an outstanding question. In this paper, we use the density matrix renormalization group methods to numerically investigate the $S=1$ Kitaev model with both ferromagnetic (FM) and antiferromagnetic (AFM) interactions. Using results on quasi-one-dimensional finite-size cylindrical geometries with circumferences of up to six legs, we conclude that the ground state of the $S=1$ Kitaev model is a quantum spin liquid with a ${\mathbb{Z}}_2$ gauge structure. We are also able to put an upper bound on the excitation gap. The magnetic field responses for the FM and AFM models are similar to those of the $S=1/2$ counterparts. In particular, in the AFM $S=1$ model, a gapless quantum spin-liquid state emerges in an intermediate window of magnetic field strength, before the system enters a trivial polarized state.

We consider a high-Q microresonator with ${\chi }^{\left(2\right)}$ nonlinearity under conditions when the coupling rate between the sidebands around the pump and its second harmonic exceeds the damping rates, implying the strong coupling regime (SC). Using the dressed-resonator approach we demonstrate that this regime leads to the dominance of the Hermitian part of the operator, driving the sideband dynamics over its non-Hermitian part responsible for the parametric gain. This has allowed us to introduce and apply the cross-area concept of the polariton quasiparticles and their effective masses in the context of ${\chi }^{\left(2\right)}$ ring microresonators. We further use polaritons to predict the modified spectral response of the resonator to the weak probe field, and to reveal the splitting of the bare-resonator resonances, avoided crossings, and Rabi dynamics. Polariton basis also allows the deriving of a discrete sequence of the parametric thresholds for generation of sidebands of different orders.

Ferromagnetic spin-1 Bose-Einstein condensates in the broken-axisymmetric phase support polar-core spin vortices (PCVs), which are intimately linked to the nonequilibrium dynamics of the system. For a purely transversely magnetized system, the Turner point-vortex model predicts that PCVs behave like massive charged particles interacting via a two-dimensional Coulomb potential. We test the accuracy of the Turner model for two oppositely charged PCVs, via comparisons with numerical simulations. While the bare Turner model shows discrepancies with our numerical results, we find that a simple rescaling of the PCV mass gives much better agreement. This can be explained via a phenomenological damping arising from coupling to modes extrinsic to the point-vortex phase space. We also identify the excitations produced following PCV annihilation, which help elucidate recent phase ordering results. We extend the Turner model to cases where the system is magnetized both transversally and axially, identifying a crossover to scalar vortex dynamics for increasing external Zeeman field.