We demonstrate that superconducting aluminium nanobridges can be driven into a state with complete suppression of the critical supercurrent via electrostatic gating. Probing both in- and out-of-plane magnetic field responses in the presence of electrostatic gating can unveil the mechanisms that primarily cause the superconducting electric field effects. Remarkably, we find that a magnetic field, independently of its orientation, has only a weak influence on the critical electric field that identifies the transition from the superconducting state to a phase with vanishing critical supercurrent. This observation points to the absence of a direct coupling between the electric field and the amplitude of the superconducting order parameter or $2\pi$-phase slips via vortex generation. The magnetic field effect observed in the presence of electrostatic gating is described within a microscopic model where a spatially uniform interband $\pi$ phase is stabilized by the electric field. Such an intrinsic superconducting phase rearrangement can account for the suppression of the supercurrent, as well as for the weak dependence of the critical magnetic fields on the electric field.

Motivated by the recent work on ${\mathrm{QED}}_3$-Chern-Simons quantum critical points of fractional Chern insulators [Phys. Rev. X 8, 031015 (2018)], we study its non-Abelian generalizations, namely, ${\mathrm{QCD}}_3$-Chern-Simons quantum phase transitions of fractional Chern insulators. These phase transitions are described by Dirac fermions interacting with non-Abelian Chern-Simons gauge fields [$U\left(N\right), SU\left(N\right), USp\left(N\right)$, etc.]. Utilizing the level-rank duality of Chern-Simons gauge theory and non-Abelian parton constructions, we discuss two types of ${\mathrm{QCD}}_3$ quantum phase transitions. The first type happens between two Abelian states in different Jain sequences, as opposed to the ${\mathrm{QED}}_3$ transitions between Abelian states in the same Jain sequence. A good example is the transition between ${\sigma }^{xy}=1/3$ state and ${\sigma }^{xy}=-1$ state, which has $N_f=2$ Dirac fermions interacting with a $U\left(2\right)$ Chern-Simons gauge field. The second type is naturally involving non-Abelian states. For the sake of experimental feasibility, we focus on transitions of Pfaffian-like states, including the Moore-Read Pfaffian, anti-Pfaffian, particle-hole Pfaffian, etc. These quantum phase transitions could be realized in experimental systems such as fractional Chern insulators in graphene heterostructures.

We set out a general protocol for steering the state of a quantum system from an arbitrary initial state toward a chosen target state by coupling it to auxiliary quantum degrees of freedom. The protocol requires multiple repetitions of an elementary step: During each step, the system evolves for a fixed time while coupled to auxiliary degrees of freedom (which we term “detector qubits”) that have been prepared in a specified initial state. The detectors are discarded at the end of the step, or equivalently, their state is determined by a projective measurement with an unbiased average over all outcomes. The steering harnesses backaction of the detector qubits on the system, arising from entanglement generated during the coupled evolution. We establish principles for the design of the system-detector coupling that ensure steering of a desired form. We illustrate our general ideas using both few-body examples (including a pair of spins-1/2 steered to the singlet state) and a many-body example (a spin-1 chain steered to the Affleck-Kennedy-Lieb-Tasaki state). We study the continuous time limit in our approach and discuss similarities to (and differences from) drive-and-dissipation protocols for quantum state engineering. Our protocols are amenable to implementations using present-day technology. Obvious extensions of our analysis include engineering of other many-body phases in one and higher spatial dimensions, adiabatic manipulations of the target states, and the incorporation of active error correction steps.

By generalizing the Landau quantization Boltzmann equation to a finite-size system, we investigate the magnetotransport in disordered Weyl semimetals (WSMs) for both the ballistic and diffusive regimes. It is found that, in the diffusive limit, the chiral chemical potential would drive extra electrical current to flow through the chiral channels in the $n=0$ Landau levels, leading to emergence of the positive longitudinal magnetoconductivity (LMC). The positive LMC will exhibit the angular narrowing phenomenon if $\hslash {\omega }_c\gg E_{\mathrm{F}}\sqrt{l_{\mathrm{a}}/l_{\mathrm{e}}}$, where ${\omega }_c$, $E_{\mathrm{F}}$, and $l_{\mathrm{a}\left(\mathrm{e}\right)}$ denote, respectively, the cyclotron frequency, the Fermi energy, and the intravalley (intervalley) relaxation length. The anomalous magnetic field dependence of the angular narrowing, i.e., the width and height of the positive LMC peak could increase or decrease with increasing the magnetic field, can be attributable to quantum oscillations of the chiral anomaly. In addition, the quantum oscillations are sensitive to the temperature and impurity scattering, which may also cause some other anomalous properties for the LMC. Our findings are helpful to understand the anomalous behaviors of the positive LMC in WSMs.

We study the boundary charge $Q_B$ of generic semi-infinite one-dimensional insulators with translational invariance and show that nonlocal symmetries (i.e., including translations) lead to rational quantizations $p/q$ of $Q_B$. In particular, we find that (up to an unknown integer) the quantization of $Q_B$ is given in integer units of $\frac{1}{2}\overline{\rho }$ and $\frac{1}{2}(\overline{\rho }-1)$, where $\overline{\rho }$ is the average charge per site (which is a rational number for an insulator). This is a direct generalization of the known half-integer quantization of $Q_B$ for systems with local inversion or local chiral symmetries to any rational value. Quite remarkably, this rational quantization remains valid even in the presence of short-ranged electron-electron interactions as well as static random disorder (breaking translational invariance). This striking stability can be traced back to the fact that local perturbations in insulators induce only local charge redistributions. We establish this result with complementary methods including density matrix renormalization group calculations, bosonization methods, and exact solutions for particular lattice models. Furthermore, for the special case of half-filling $\overline{\rho }=\frac{1}{2}$, we present explicit results in single-channel and nearest-neighbor hopping models and identify Weyl semimetal physics at gap closing points. Our general framework also allows us to shed new light on the well-known rational quantization of soliton charges at domain walls.

We have performed angle-resolved photoemission spectroscopy on ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ which is predicted to be an axion insulator in the antiferromagnetic state. By utilizing soft-x-ray and vacuum-ultraviolet photons, we revealed a three-dimensional hole pocket centered at the $\mathrm{\Gamma }$ point of the bulk Brillouin zone together with a heavily hole-doped surface state in the paramagnetic phase. Upon entering the antiferromagnetic phase, the band structure exhibits a marked reconstruction characterized by the emergence of an “M”-shaped bulk band near the Fermi level. The qualitative agreement with first-principles band-structure calculations suggests the occurrence of bulk-band inversion at the $\mathrm{\Gamma }$ point in the antiferromagnetic phase. We suggest that ${\mathrm{EuIn}}_2{\mathrm{As}}_2$ provides a good opportunity to study the exotic quantum phases associated with a possible axion-insulator phase.

We investigate the propagation of magnetic skyrmions on elastically deformable geometries by employing imaginary time quantum field theory methods. We demonstrate that the Euclidean action of the problem carries information of the elements of the surface space metric, and develop a description of the skyrmion dynamics in terms of a set of collective coordinates. We reveal that curvature-driven effects emerge in geometries with nonconstant curvature, which explicitly break the translational invariance of flat space. In particular, for a skyrmion stabilized by a curvilinear defect, an inertia term and a pinning potential are generated by the varying curvature, while both of these terms vanish in the flat-space limit.

We demonstrate the creation of robust localized zero-energy states that are induced into topologically trivial systems by the insertion of a $\mathcal{PT}$-symmetric defect with local gain and loss. A pair of robust localized states induced by the defect turns into zero-energy modes when the gain-loss contrast exceeds a threshold, at which the defect states encounter an exceptional point. Our approach can be used to obtain robust lasing or perfectly absorbing modes in any part of the system.

Strain effect on circularly polarized electroluminescence (EL) is theoretically analyzed for a p-i-n junction of transition metal dichalcogenides. The electrically controllable circularly polarized EL without the strain effect is understood by valley polarization combined with the electron–hole asymmetry due to the trigonal warping effect in the valence band. The strain enhances circularly polarized EL more efficiently than the trigonal warping effect since the asymmetry occurs in the same direction as the direction of the applied electric field. The calculated result shows that the circular polarization in the strained $\mathrm{Mo}{\mathrm{S}}_2$ becomes $100\%$ without losing the EL intensity.

Shortly after the discovery of superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, spin-triplet pairing was proposed and further corroborated by a constant Knight shift $\left(K\right)$ across the transition temperature $\left(T_c\right)$. However, a recent experiment observed a drop in $K$ at $T_c$ which becomes larger under uniaxial strain, ruling out several spin-triplet scenarios. Here we show that even parity interorbital spin-triplet pairing can feature a $d$ vector that rotates when uniaxial strain is applied, leading to a larger drop in the spin polarization perpendicular to the strain direction, distinct from spin-singlet pairing. We propose that anisotropic spin polarization under strain will ultimately differentiate triplet versus singlet pairing.

In the spin Hall effect (SHE), the vector cross-product relations among charge current, spin current, and spin polarization are strictly followed, so that only the in-plane spin accumulation can be induced at the film surface. But, recently this restriction has been lifted by the symmetry breaking of a magnet, where spin polarization can be additionally manipulated by the magnetization directions. In this work, we demonstrated the magnetization-dependent spin-to-charge signals in yttrium iron garnet/Pt/Co/Pt heterostructure. Without the complications in all-metallic structures, we explicitly identified the magnetization-dependent spin Hall effect. We compared its size with the SHE, and further estimated the magnetization-dependent spin Hall angle. Our approach provides an explicit route in exploring the spin-to-charge conversion with arbitrary and controllable orientations of spin polarization, which offers significant advantages in developing electrically controlled energy-efficient spintronic devices.

We provide theory and formal insight on the Green function quantization method for absorptive and dispersive spatial-inhomogeneous media in the context of dielectric media. We show that a fundamental Green function identity, which appears, e.g., in the fundamental commutation relation of the electromagnetic fields, is also valid in the limit of nonabsorbing media. We also demonstrate how the zero-point field fluctuations yields a nonvanishing surface term in configurations without absorption, when using a more formal procedure of the Green function quantization method. We then apply the presented method to a recently developed theory of photon quantization using quasinormal modes [Franke et al., Phys. Rev. Lett. 122, 213901 (2019)] for finite nanostructures embedded in a lossless background medium. We discuss the strict dielectric limit of the commutation relations of the quasinormal mode operators and present different methods to obtain them, connected to the radiative loss for nonabsorptive but open resonators. We show exemplary calculations of a fully three-dimensional photonic crystal beam cavity, including the lossless limit, which supports a single quasinormal mode and discuss the limits of the commutation relation for vanishing damping.

We introduce a model of three-dimensional (3D) topological order enriched by planar subsystem symmetries. The model is constructed starting from the 3D toric code, whose ground state can be viewed as an equal-weight superposition of two-dimensional (2D) membrane coverings. We then decorate those membranes with 2D cluster states possessing symmetry-protected topological order under linelike subsystem symmetries. This endows the decorated model with planar subsystem symmetries under which the looplike excitations of the toric code fractionalize, resulting in an extensive degeneracy per unit length of the excitation. We also show that the value of the topological entanglement entropy is larger than that of the toric code for certain bipartitions due to the subsystem symmetry enrichment. Our model can be obtained by gauging the global symmetry of a short-range entangled model which has symmetry-protected topological order coming from an interplay of global and subsystem symmetries. We study the nontrivial action of the symmetries on boundary of this model, uncovering a mixed boundary anomaly between global and subsystem symmetries. To further study this interplay, we consider gauging several different subgroups of the total symmetry. The resulting network of models, which includes models with fracton topological order, showcases more of the possible types of subsystem symmetry enrichment that can occur in 3D.

We derive the definition of the Berry phase for the adiabatic transport of a composite fermion (CF) in a half-filled composite Fermi liquid (CFL). It is found to be different from that adopted in previous investigations by Geraedts et al. For the standard CFL wave function, we analytically show that the Berry curvature is uniformly distributed in the momentum space. For the Jain-Kamilla wave function, we numerically show that its Berry curvature has a continuous distribution inside the Fermi sea and vanishes outside. We conclude that the CF with respect to both the microscopic wave functions is not a massless Dirac particle.

We present a theoretical study of the response of $\beta \text{−}{\text{Li}}_2{\text{IrO}}_3$ under external magnetic fields in the $ab, bc$, and $ac$ crystallographic planes. The results are based on the minimal nearest-neighbor $J\text{−}K\text{−}\mathrm{\Gamma }$ model and reveal a rich intertwining of field-induced phases and magnetic phase transitions with distinctive signatures that can be probed directly via torque magnetometry. Most saliently, we observe (i) an unusual reentrance of the incommensurate counterrotating order for fields in the $ab$ plane and (ii) a set of abrupt torque discontinuities which are particularly large for fields rotating in the $bc$ plane and whose characteristic shape resembles closely the ones observed in the three-dimensional (3D) Kitaev magnet $\gamma \text{−}{\text{Li}}_2{\text{IrO}}_3$. An experimental confirmation of these predictions will pave the way for an accurate determination of all relevant microscopic parameters of this 3D Kitaev magnet.

We study a three-dimensional chiral second-order topological insulator (SOTI) subject to a magnetic field. Via its gauge field, the applied magnetic field influences the electronic motion on the lattice, and via the Zeeman effect, the field influences the electronic spin. We compare two approaches to the problem: an effective surface theory, and a full lattice calculation. The surface theory predicts a massive Dirac spectrum on each of the gapped surfaces, giving rise to Landau levels once the surfaces are pierced by magnetic flux. The surface theory qualitatively agrees with our lattice calculations, accurately predicting the surface gap as well as the spin and orbital components of the states at the edges of the surface Dirac bands. In the context of the lattice theory, we calculate the spectrum with and without magnetic field and find a deviation from the surface theory when a gauge field is applied. The energy of the lowest-lying Landau level is found closer to zero than is predicted by the surface theory, which leads to an observable magnetotransport signature: inside the surface gap, there exist different energy regions where either one or two chiral hinge modes propagate in either direction, quantizing the differential conductance to either one or two conductance quanta.

We consider the prospects for quantum simulation of condensed matter models exhibiting strong electron-phonon coupling using a hybrid platform of trapped laser-cooled ions interacting with an ultracold atomic gas. This system naturally possesses a phonon structure, in contrast to the standard optical lattice scenarios usually employed with ultracold atoms in which the lattice is generated by laser light and thus it remains static. We derive the effective Hamiltonian describing the general system and discuss the arising energy scales, relating the results to commonly employed extended Hubbard-Holstein models. Although for a typical experimentally realistic system the coupling to phonons turns out to be small, we provide the means to enhance its role and reach interesting regimes with competing orders. Extended Lang-Firsov transformation reveals the emergence of phonon-induced long-range interactions between the atoms, which can give rise to both localized and extended bipolaron states with low effective mass, indicating the possibility of fermion pairing.

Central spin models describe a variety of quantum systems in which a spin-$\frac{1}{2}$ qubit interacts with a bath of surrounding spins, as realized in quantum dots and defect centers in diamond. We show that the fully anisotropic central spin Hamiltonian with (XX) Heisenberg interactions is integrable. Building on the class of integrable Richardson-Gaudin models, we derive an extensive set of conserved quantities and obtain the exact eigenstates using the Bethe ansatz. These states divide into two exponentially large classes: bright states, where the qubit is entangled with the bath, and dark states, where it is not. We discuss how dark states limit qubit-assisted spin bath polarization and provide a robust long-lived quantum memory for qubit states.

We study the thermodynamics of short-range-interacting, two-dimensional bosons constrained to the lowest Landau level. When the temperature is higher than other energy scales of the problem, the partition function reduces to a multidimensional complex integral that can be handled by classical Monte Carlo techniques. This approach takes the quantization of the lowest Landau level orbits fully into account. We observe that the partition function can be expressed in terms of a function of a single combination of thermodynamic variables, which allows us to derive exact thermodynamic relations. We determine the asymptotic behavior of this function and compute some thermodynamic observables numerically.

The orbital of electrons in a solid affects their mutual interaction, which is a key to emergent phenomena. Therefore, control of the interplay between distinct orbitals makes the behavior of solids fertile. Such a case is substantiated by the aggregation of the molecule, $M{\left(\mathrm{tmdt}\right)}_2$, which contains $\pi$ orbitals extended over the organic ligand, tmdt, and a level-tunable $d$ orbital centered at the metal ion $M$. Among them, $\mathrm{Au}{\left(\mathrm{tmdt}\right)}_2$ exemplifies the critical $\pi \text{−}d$ mixing, which arguably causes an over-100 K antiferromagnetic metal whose nature remains elusive. Here, we track the $\pi \text{−}d$ interplay in $\mathrm{Au}{\left(\mathrm{tmdt}\right)}_2$ with dual orbital-resolved probes, ${}^{13}\mathrm{C}$ NMR and synchrotron x-ray diffraction. We find substantial intramolecular (interorbital) redistribution of electrons on cooling, triggering a commensurate $\pi \text{−}d$ antiferromagnetic metal to emerge, which invokes an orbital-selective doped Mott insulator caused by intramolecular self-doping. This demonstrates that vital traffic of electrons between distinct orbitals brings about an unique correlated phase.