Topological superconductors represent one of the key hosts of Majorana-based topological quantum computing. Typical scenarios for one-dimensional (1D) topological superconductivity assume a broken gauge symmetry associated to a superconducting state. However, no interacting 1D many-body system is known to spontaneously break gauge symmetries. Here, we show that zero modes emerge in a many-body system without gauge symmetry breaking and in the absence of superconducting order. In particular, we demonstrate that Majorana zero modes of the symmetry-broken superconducting state are continuously connected to these zero-mode excitations, demonstrating that zero-bias anomalies may emerge in the absence of gauge symmetry breaking. We demonstrate that these many-body zero modes share the robustness features of the Majorana zero modes of symmetry-broken topological superconductors. We further show that the interface between the interacting model and a 1D topological superconductor does not support Majorana modes. We introduce a bosonization formalism to analyze these excitations and show that a ground state analogous to a topological superconducting state can be analytically found in a certain limit. Our results demonstrate that robust Majorana-like zero modes may appear in a many-body system without gauge symmetry breaking, thus introducing a family of protected excitations with no single-particle analogs.

We investigate the Hall conductance of a two-dimensional Chern insulator coupled to an environment causing gain and loss. Introducing a biorthogonal linear response theory, we show that sufficiently strong gain and loss lead to a characteristic nonanalytical contribution to the Hall conductance. Near its onset, this contribution exhibits a universal power law with a power 3/2 as a function of Dirac mass, chemical potential, and gain strength. Our results pave the way for the study of non-Hermitian topology in fermionic transport experiments.

In the electron dynamics in quantum matter, the Berry curvature of the electronic wave function provides the artificial magnetic field in momentum space, which leads to nontrivial contributions to transport coefficients. It is known that in the presence of electron-electron and/or electron-phonon interactions, there is an extra contribution to the electron dynamics due to the artificial electric field (AEF) in momentum space. In this work we construct hydrodynamic equations for the electrons in time-reversal invariant but inversion-breaking systems and find the novel hydrodynamic coefficients related to the AEF. Furthermore, we investigate the novel linear and nonlinear transport coefficients in the presence of the AEF.

We introduce Weyl Josephson circuits: small Josephson junction circuits that simulate Weyl band structures. We first formulate a general approach to design circuits that are analogous to Bloch Hamiltonians of a desired dimensionality and symmetry class. We then construct and analyze a six-junction device that produces a three-dimensional (3D) Weyl Hamiltonian with broken inversion symmetry and in which topological phase transitions can be triggered in situ. We argue that currently available superconducting circuit technology allows experiments that probe topological properties inaccessible in condensed matter systems.

We image equilibrium and nonequilibrium transport through a two-dimensional electronic cavity using scanning gate microscopy. Injecting electrons into the cavity through a quantum point contact close to equilibrium, we raster scan a weakly invasive tip above the cavity region and measure the modulated conductance through the cavity. Varying the electron injection energy between $\pm 2\mathrm{meV}$, we observe that conductance minima turn into maxima beyond an energy threshold of $\pm 0.6\mathrm{meV}$. This observation bears similarity to previous measurements by Jura et al. [Phys. Rev. B 82, 155328 (2010)], who used a strongly invasive tip potential to study electron injection into an open two-dimensional electron gas. This resemblance suggests a similar microscopic origin based on electron-electron interactions.

Localized surface plasmon and Klein tunneling resonances are two phenomena that were previously thought to be unrelated, where the former plays an important role in subwavelength optics while the latter is fundamental to relativistic quantum mechanics and physics of Dirac materials. We develop a rigorous theory for spin-1 Dirac-Weyl particles, which establishes a striking analogy between the two phenomena and unveils a deep connection between the distinct physical contexts, paving the way for gate-controlled surface plasmon mimetic electronics as well as for realizing localized spoof plasmons in a scope wider than previously thought. A possible experimental scheme is articulated.

One-particle reduced density matrix functional theory would potentially be the ideal approach for describing Bose-Einstein condensates. It namely replaces the macroscopically complex wave function by the simple one-particle reduced density matrix, and therefore provides direct access to the degree of condensation and still recovers quantum correlations in an exact manner. We initiate and establish this theory by deriving the respective universal functional $\mathcal{F}$ for homogeneous Bose-Einstein condensates with arbitrary pair interaction. Most importantly, the successful derivation necessitates a particle-number conserving modification of Bogoliubov theory and a solution of the common phase dilemma of functional theories. We then illustrate this approach in several bosonic systems such as homogeneous Bose gases and the Bose-Hubbard model. Remarkably, the general form of $\mathcal{F}$ reveals the existence of a universal Bose-Einstein condensation force which provides an alternative and more fundamental explanation for quantum depletion.

By means of first-principles calculations and crystal structure searching techniques, we predict that a different NaCaBi family crystallized into the ZrBeSi-type structure (i.e., $P{6}_3/mmc$) are strong topological insulators (STIs). Taking $P{6}_3/mmc$ NaCaBi as an example, the calculated band structure indicates that there is a band inversion between two opposite-parity bands at the $\mathrm{\Gamma }$ point. In contrast to the well-known ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family, the band inversion in the NaCaBi family has already occurred even without spin-orbit coupling (SOC), giving rise to a nodal ring surrounding $\mathrm{\Gamma }$ in the $k_z=0$ plane (protected by $M_z$ symmetry). With time-reversal symmetry $\left(\mathcal{T}\right)$ and inversion symmetry $\left(\mathcal{I}\right)$, the spinless nodal-line metallic phase protected by ${\left[\mathcal{TI}\right]}^2=1$ is the weak-SOC limit of the spinful topological insulating phase. Upon including SOC, the nodal ring is gapped, driving the system into a STI. Besides inversion symmetry, the nontrivial topology of NaCaBi can also be indicated by $\overline{6}$ symmetry. More surprisingly, the SOC-induced band gap in NaCaBi is about 0.34 eV, which is larger than the energy scale of room temperature. Four other compounds (KBaBi, KSrBi, RbBaBi, and RbSrBi) in the family are stable at ambient pressure, both in thermodynamics and lattice dynamics, even though their gaps are smaller than that of NaCaBi. Thus, they provide good platforms to study topological states both in theory and experiments.

The Edelstein effect produces a homogeneous magnetization in nonmagnetic materials with broken inversion symmetry which is generated and tuned exclusively electrically. Often the spin Edelstein effect—that is, a spin density in response to an applied electric field—is considered. In this paper we report on the electrically induced magnetization that comprises contributions from the spin and the orbital moments. Our theory for these spin and orbital Edelstein effects is applied to the topologically nontrivial two-dimensional electron gas at ${\mathrm{SrTiO}}_3$ interfaces. In this particular system the orbital Edelstein effect exceeds the spin Edelstein effect by more than one order of magnitude. This finding is explained mainly by orbital moments of different magnitude in the Rashba-like split band pairs, while the spin moments are of almost equal magnitude.

Recent progress in optically trapped ultracold atomic gases is now making it possible to access microscopic observables in doped Mott insulators, which are the parent states of high-temperature superconductors. This makes it possible to address longstanding questions about the temperature scales at which attraction between charge carriers are present, and their mechanism. Controllable theoretical results for this problem are not available at low temperature due to the sign problem. In this work, we make important progress with this problem by employing worm-algorithm Monte Carlo, which allows us to obtain completely unbiased results for two charge carriers in a Mott insulator. Our method gives access to lower temperatures than what is currently possible in experiments, and provides evidence for attraction between dopants at a temperature scale that is now feasible in ultracold atomic systems. We also report on spin correlations in the presence of charge carriers, which are directly comparable to experiments.

${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$, a quasi-one-dimensional (1D) crystal, shows a characteristic temperature-driven metal-insulator phase transition. Above the charge density wave (CDW) temperature $T_c$, ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ has been predicted to harbor a Weyl semimetal phase. Below $T_c$, it becomes an axion insulator. Here, we performed angle-resolved photoemission spectroscopy measurements on the (110) surface of ${\left(\mathrm{Ta}{\mathrm{Se}}_4\right)}_2\mathrm{I}$ and observed two sets of Dirac-like energy bands in the first Brillouin zone, which agree well with our first-principles calculations. Moreover, we found that each Dirac band exhibits an energy splitting of hundreds of millielectron volts under certain circumstances. In combination with core level measurements, our theoretical analysis showed that this Dirac band splitting is a result of surface charge polarization due to the loss of surface iodine atoms. Our findings here shed light on the interplay between band topology and CDW order in Peierls compounds and will motivate more studies on topological properties of strongly correlated quasi-1D materials.

Due to the lack of full rotational symmetry in condensed matter physics, solids exhibit new excitations beyond Dirac and Weyl fermions, of which the sixfold excitations have attracted considerable interest owing to the presence of maximum degeneracy in bosonic systems. Here, we propose that a single linear dispersive sixfold excitation can be found in the electride ${\mathrm{Li}}_{12}{\mathrm{Mg}}_3{\mathrm{Si}}_4$ and its derivatives. The sixfold excitation is formed by the floating bands of elementary band representation $A@12a$ originating from the excess electrons centered at the vacancies (i.e., the $12a$ Wyckoff sites). There exists a unique topological bulk-surface-edge correspondence for the spinless sixfold excitation, resulting in trivial surface “Fermi arcs” but topological hinge arcs. All gapped $k_z$ slices belong to a two-dimensional higher-order topological insulating phase, which is protected by a combined symmetry $\mathcal{T}{\tilde{S}}_{4z}$ and characterized by a quantized fractional corner charge $Q_{\text{corner}}=\frac{3\left|e\right|}{4}$. Consequently, the hinge arcs are obtained in the hinge spectra of the ${\tilde{S}}_{4z}$-symmetric rod structure. The state with a single sixfold excitation, stabilized by both nonsymmorphic crystalline symmetries and time-reversal symmetry, is located at the phase boundary and can be driven into various topologically distinct phases by explicit breaking of symmetries, making these electrides promising platforms for the systematic studies of different topological phases.

The planar Hall effect (PHE), wherein a rotating magnetic field in the plane of a sample induces oscillating transverse voltage, has recently garnered attention in a wide range of topological metals and insulators. The observed twofold oscillations in ${\rho }_{yx}$ as the magnetic field completes one rotation are the result of chiral, orbital, and/or spin effects. The antiperovskites $A_{3}B\mathrm{O}$ ($A=\mathrm{Ca}$, Sr, Ba; $B=\mathrm{Sn}$, Pb) are topological crystalline insulators whose low-energy excitations are described by a generalized Dirac equation for fermions with total angular momentum $J=\frac{3}{2}$. We report unusual sixfold oscillations in the PHE of ${\mathrm{Sr}}_3\mathrm{Sn}\mathrm{O}$, which persisted nearly up to room temperature. Multiple harmonics (twofold, fourfold, and sixfold), which exhibited distinct field and temperature dependencies, were detected in ${\rho }_{xx}$ and ${\rho }_{yx}$. These observations are more diverse than those in other Dirac and Weyl semimetals and point to a richer interplay of microscopic processes underlying the PHE in the antiperovskites.

The engineering of artificial systems hosting topological excitations is at the heart of current condensed matter research. Most of these efforts focus on single-particle properties, neglecting possible engineering routes via the modifications of the fundamental many-body interactions. Interestingly, recent experimental breakthroughs have shown that Coulomb interactions can be efficiently controlled by substrate screening engineering. Inspired by this success, we propose a simple platform in which topologically nontrivial many-body excitations emerge solely from dielectrically engineered Coulomb interactions in an otherwise topologically trivial single-particle band structure. Furthermore, by performing a realistic microscopic modeling of screening engineering, we demonstrate how our proposal can be realized in one-dimensional systems such as quantum-dot chains. Our results put forward Coulomb engineering as a powerful tool to create topological excitations, with potential applications in a variety of solid-state platforms.

We explore the photon-number distribution of bimodal quantum-dot micropillar lasers with a two-channel transition-edge sensor (TES) detection system. The two channels of the photon-number-resolving TES system simultaneously detect light emission of two orthogonal components of the micropillar's fundamental emission mode. The applied experimental scheme provides unprecedented access to the joint photon-number distribution and enables a profound insight into the dynamics and photon statistics of the gain-coupled mode components. In particular, the two-channel TES measurements reveal an optical bistability of the correlated laser modes leading to temporal hopping between emission associated with Poissonian and thermal-like emission statistics. The experimental data and theoretical modeling based on Monte Carlo simulations are in good agreement and reveal the anticorrelated behavior of the mode hopping, which results in intensity fluctuations and superthermal values of the autocorrelation function. Our investigations clearly demonstrate the great benefit of using photon-number-resolving detectors in nanophotonics to explore the rich physics of multimode micro- and nanolasers.

Quasiperiodicity has long been known to be a potential platform to explore exotic phenomena, realizing an intricate middle point between ordered solids and disordered matter. In particular, quasiperiodic structures are promising playgrounds to engineer critical wave functions, a powerful starting point to engineer exotic correlated states. Here we show that systems hosting a quasiperiodic modulation of antiferromagnetism and spin-singlet superconductivity, as realized by atomic chains in twisted van der Waals materials, host a localization-delocalization transition as a function of the coupling strength. Associated with this transition, we demonstrate the emergence of a robust quasiperiodic critical point for arbitrary incommensurate potentials, which appears for generic relative weights of the spin-singlet superconductivity and antiferromagnetism. We show that inclusion of residual electronic interactions leads to an emergent spin-triplet superconducting state, which gets dramatically enhanced at the vicinity of the quasiperiodic critical point. Our results put forward quasiperiodicity as a powerful knob to engineer robust superconducting states, providing an alternative pathway towards artificially designed unconventional superconductors.

Based on first-principles calculations including a Coulomb repulsion term, we identify trends in the electronic reconstruction of $A{\mathrm{NiO}}_2/{\mathrm{SrTiO}}_3\left(001\right)$ ($A=\mathrm{Pr}$, La) and $A{\mathrm{CuO}}_2/{\mathrm{SrTiO}}_3\left(001\right)$ ($A=\mathrm{Ca}$, Sr). Common to all cases is the emergence of a quasi-two-dimensional electron gas (q2DEG) in ${\mathrm{SrTiO}}_3$(001), albeit the higher polarity mismatch at the interface of nickelates versus cuprates to the nonpolar ${\mathrm{SrTiO}}_3\left(001\right)$ substrate ($3+/0$ versus $2+/0$) results in an enhanced q2DEG carrier density. The simulations reveal a significant dependence of the interfacial Ti $3d_{xy}$ band bending on the rare-earth-metal ion in the nickelate films, being $20–30\%$ larger for ${\mathrm{PrNiO}}_2$ and ${\mathrm{NdNiO}}_2$ than for ${\mathrm{LaNiO}}_2$. Contrary to expectations from the formal polarity mismatch, the electrostatic doping in the films is twice as strong in cuprates as in nickelates. We demonstrate that the depletion of the self-doping rare-earth-metal $5d$ states enhances the similarity of nickelate and cuprate Fermi surfaces in film geometry, reflecting a single hole in the Ni and Cu $3d_{x^2-y^2}$ orbitals. Finally, we show that ${\mathrm{NdNiO}}_2$ films grown on a polar ${\mathrm{NdGaO}}_3\left(001\right)$ substrate feature a simultaneous suppression of q2DEG formation as well as Nd $5d$ self-doping.

We demonstrate transport rectification in a Hermitian Hamiltonian quantum ratchet by a dissipative, dynamic impurity. While the bulk of the ratchet supports transport in both directions, the properly designed loss function of the local impurity acts as a direction-dependent filter for the moving states. We analyze this scheme theoretically by making use of Floquet $S$-matrix theory. In addition, we provide direct experimental observation of one-way transmittance in periodically modulated plasmonic waveguide arrays containing a local impurity with engineered losses.

Recently a few 2D AKLT models have been shown to be gapped, including the one on the hexagonal lattice, whose local spin magnitude is spin-3/2. Here we report the existence of a nonzero spectral gap on several nontrivial AKLT models having either solely spin-2 or mixed spin-2 and spin-1 degrees of freedom. The hybrid models we consider are defined on the 3D diamond and 2D kagome lattices, where lattice sites are spin-2 and one single spin-1 degree of freedom is added on some or all of the edges. Although the spectral gap problem for the uniformly spin-2 AKLT models on the diamond, kagome, and square lattices is still open, we are able to establish the existence of the gap for spin-2 AKLT models on two planar lattices, which we call the inscribed square lattice and the triangle-octagon lattice, respectively. So far these latter two are the only two uniformly spin-2 AKLT models that have a provable nonzero gap above the ground state. We also discuss some attempts to prove the gap existence on both the square and kagome lattices. In addition, we show that if one can show that the gap of a finite-size, weighted AKLT Hamiltonian is larger than a certain threshold, then the original AKLT model on the square lattice is gapped in the thermodynamic limit. The threshold of the gap we obtain scales inversely with the linear size of the problem.