While the on-chip processing power in circuit QED devices is growing rapidly, an open challenge is to establish high-fidelity quantum links between qubits on different chips. Here, we show entanglement between transmon qubits on different cQED chips with $49\%$ concurrence and $73\%$ Bell-state fidelity. We engineer a half-parity measurement by successively reflecting a coherent microwave field off two nearly identical transmon-resonator systems. By ensuring the measured output field does not distinguish $|01\rangle$ from $|10\rangle$, unentangled superposition states are probabilistically projected onto entangled states in the odd-parity subspace. We use in situ tunability and an additional weakly coupled driving field on the second resonator to overcome imperfect matching due to fabrication variations. To demonstrate the flexibility of this approach, we also produce an even-parity entangled state of similar quality, by engineering the matching of outputs for the $|00\rangle$ and $|11\rangle$ states. The protocol is characterized over a range of measurement strengths using quantum state tomography showing good agreement with a comprehensive theoretical model.

We present the results of ${}^{75}\mathrm{As}$ nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and resistivity measurements in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ under pressure ($p$). The temperature dependence of the NMR shift, nuclear spin-lattice relaxation time ($T_1$), and resistivity show a crossover between a high-temperature incoherent, local-moment behavior and a low-temperature coherent behavior at a crossover temperature ($T^{*}$). $T^{*}$ is found to increase monotonically with pressure, consistent with increasing hybridization between localized $3d$ orbital-derived bands with the itinerant electron bands. No anomaly in $T^{*}$ is seen at the critical pressure $p_{\mathrm{c}}=1.8$ GPa where a change of slope of the superconducting (SC) transition temperature $T_{\mathrm{c}}\left(p\right)$ has been observed. In contrast, $T_{\mathrm{c}}\left(p\right)$ seems to correlate with antiferromagnetic spin fluctuations in the normal state as measured by the NQR $1/T_1$ data, although such a correlation cannot be seen in the replacement effects of $A$ in the $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\text{K}$, Rb, Cs) family. In the superconducting state, two $T_1$ components are observed at low temperatures, suggesting the existence of two distinct local electronic environments. The temperature dependence of the short $T_{1\mathrm{s}}$ indicates a nearly gapless state below $T_{\mathrm{c}}$. On the other hand, the temperature dependence of the long component $1/T_{1\mathrm{L}}$ implies a large reduction in the density of states at the Fermi level due to the SC gap formation. These results suggest a real-space modulation of the local SC gap structure in ${\mathrm{KFe}}_2{\mathrm{As}}_2$ under pressure.

Whereas in the familiar Kondo effect the exchange interaction is dipolar, there are systems in which the exchange interaction is multipolar, as has been realized in a recent experiment. Here, we study multipolar Kondo effect in a Fermi gas of cold ${}^{173}\mathrm{Yb}$ atoms. Making use of different ac polarizabilities of the electronic ground state $\mathrm{Yb}({}^1\mathrm{S}_0)$ and the long-lived metastable state ${\mathrm{Yb}}^{*}({}^3\mathrm{P}_2)$, it is suggested that the latter atoms can be localized and serve as a dilute concentration of magnetic impurities while the former ones remain itinerant. The exchange mechanism between the itinerant Yb and the localized ${\mathrm{Yb}}^{*}$ atoms is analyzed and shown to be antiferromagnetic. The quadrupole and octupole interactions act to enhance the Kondo temperature $T_K$ that is found to be experimentally accessible. The bare exchange Hamiltonian needs to be decomposed into dipole $\left(\text{d}\right)$, quadrupole $\left(\text{q}\right)$, and octupole $\left(\text{o}\right)$ interactions in order to retain its form under renormalization group (RG) analysis, in which the corresponding exchange constants $({\lambda }_{\text{d}},{\lambda }_{\text{q}}$, and ${\lambda }_{\text{o}})$ flow independently. Numerical solution of the RG scaling equations reveals a few finite fixed points. Arguments are presented that the Fermi-liquid fixed point at low temperature is unstable, indicating that the impurity is overscreened, which suggests a non-Fermi-liquid phase. The impurity contributions to the specific heat, entropy, and the magnetic susceptibility are calculated in the weak coupling regime $\left(T\gg T_K\right)$, and are compared with the analogous results obtained for the standard case of dipolar exchange interaction (the $s-d$ Hamiltonian).

Topological nodal-line semimetals are exotic conductors that host symmetry-protected conducting nodal lines in their bulk electronic spectrum and nontrivial drumhead states on the surface. Based on first-principles calculations and an effective model analysis, we identify the presence of topological nodal-line semimetal states in the low crystalline symmetric $T{T}^{\prime }X$ family of compounds ($T,T^{\prime }$ = transition metal, $X$= Si or Ge) in the absence of spin-orbit coupling (SOC). Taking ZrPtGe as an exemplar system, we show that owing to small lattice symmetry this material harbors a single nodal line on the $k_y=0$ plane with large energy dispersion and unique drumhead surface state with a saddlelike energy dispersion. When the SOC is included, the nodal line gaps out and the system transitions to a strong topological insulator state with $Z_2=(1;000)$. The topological surface state evolves from the drumhead surface state via the sharing of its saddlelike energy dispersion within the bulk energy gap. These features differ remarkably from those of the currently known topological surface states in topological insulators such as ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ with Dirac-cone-like energy dispersions.

Motivated by recent experiments on $\alpha -{\mathrm{RuCl}}_3$, we investigate a possible quantum spin liquid ground state of the honeycomb-lattice spin model with bond-dependent interactions. We consider the $K-\mathrm{\Gamma }$ model, where $K$ and $\mathrm{\Gamma }$ represent the Kitaev and symmetric-anisotropic interactions between spin-1/2 moments on the honeycomb lattice. Using the infinite density matrix renormalization group, we provide compelling evidence for the existence of quantum spin liquid phases in an extended region of the phase diagram. In particular, we use transfer-matrix spectra to show the evolution of two-particle excitations with well-defined two-dimensional dispersion, which is a strong signature of a quantum spin liquid. These results are compared with predictions from Majorana mean-field theory and used to infer the quasiparticle excitation spectra. Further, we compute the dynamical structure factor using finite-size cluster computations and show that the results resemble the scattering continuum seen in neutron-scattering experiments on $\alpha -{\mathrm{RuCl}}_3$. We discuss these results in light of recent and future experiments.

Using finite-temperature determinantal quantum Monte Carlo calculations, we reexamine the pairing susceptibilities in the Hubbard model on the honeycomb lattice, focusing on doping levels onto and away from the Van Hove singularity (VHS) filling. For this purpose, electronic densities of 0.75 (at the hole-doping VHS) and 0.4 (well below the VHS) are considered in detail, where due to a severe sign problem at strong coupling strengths, we focus on the weak-interaction region of the Hubbard model Hamiltonian. By analyzing the temperature dependence of pairing susceptibilities in various symmetry channels, we find the singlet $d+id$ wave to be the dominant pairing channel both at and away from the VHS filling. We furthermore investigate the electronic susceptibility to a specific chiral spin density wave (SDW) order, which we find to be similarly relevant at the VHS, while it extenuates upon doping away from the VHS filling.

Defects are almost inevitably present in realistic materials and defective materials are expected to exhibit very different properties than their nondefective (perfect) counterparts. Here, using a combination of the tight-binding approach and the scattering matrix formalism, we investigate the electronic transport properties of defective phosphorene nanoribbons (PNRs) containing atomic vacancies. We find that for both armchair PNRs (APNRs) and zigzag PNRs (ZPNRs), single vacancies can create quasilocalized states, which can affect their conductance. With increasing vacancy concentration, three different transport regimes are identified: ballistic, diffusive, and Anderson localized ones. In particular, ZPNRs that are known to be metallic due to the presence of edge states become semiconducting: edge conductance vanishes and transport gap appears due to Anderson localization. Moreover, we find that for a fixed vacancy concentration, both APNRs and ZPNRs of narrower width and/or longer length are more sensitive to vacancy disorder than their wider and/or shorter counterparts, and that for the same ribbon length and width, ZPNRs are more sensitive to vacancy disorder than APNRs.

We model the magnetic ratchet effect in bilayer graphene in which a dc electric current is produced by an ac electric field of frequency $\omega$ in the presence of a steady in-plane magnetic field and inversion-symmetry breaking. In bilayer graphene, the ratchet effect is tunable by an external metallic gate which breaks inversion symmetry. For zero in-plane magnetic field, we show that trigonal warping and inversion-symmetry breaking are able to produce a large dc valley current, but not a nonzero total dc charge current. For the magnetic ratchet in a tilted magnetic field, the perpendicular field component induces cyclotron motion with frequency ${\omega }_c$ and we find that the dc current displays cyclotron resonance at ${\omega }_c=\omega$, although this peak in the current is actually smaller than its value at ${\omega }_c=0$. Second harmonic generation, however, is greatly enhanced by resonances at ${\omega }_c=\omega$ and ${\omega }_c=2\omega$ for which the current is generally much larger than at ${\omega }_c=0$.

Relating the band structure of correlated semimetals to their transport properties is a complex and often open issue. The partial occupation of numerous electron and hole bands can result in properties that are seemingly in contrast with one another, complicating the extraction of the transport coefficients of different bands. The $5d$ oxide ${\mathrm{SrIrO}}_3$ hosts parabolic bands of heavy holes and light electrons in gapped Dirac cones due to the interplay between electron-electron interactions and spin-orbit coupling. We present a multifold approach relying on different experimental techniques and theoretical calculations to disentangle its complex electronic properties. By combining magnetotransport and thermoelectric measurements in a field-effect geometry with first-principles calculations, we quantitatively determine the transport coefficients of different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients, yielding a holelike response under field-effect and thermoelectric measurements and a linear electronlike Hall effect up to 33 T.

X-ray diffraction pressure studies at room temperature demonstrate that the spinel $\mathrm{FeA}{\mathrm{l}}_2{\mathrm{O}}_4$ transforms to a tetragonal phase at ∼18 GPa. This tetragonal phase has a highly irregular unit-cell volume versus pressure dependence up to ∼45 GPa, after which a transformation to a Cmcm postspinel phase is onset. This is attributable to pressure driven Fe↔Al site inversion at room temperature, corroborated by signatures in the ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy pressure data. At the tetragonal→postspinel transition, onset in the range 45–50 GPa, there is a concurrent emergence of a nonmagnetic spectral component in the Mössbauer data at variable cryogenic temperatures. This is interpreted as spin crossover at sixfold coordinated Fe locations emanated from site inversion. Spin crossover commences at the end of the pressure range of the tetragonal phase and progresses in the postspinel structure. There is also a much steeper volume change Δ$V/V$ ∼ 10% in the range 45–50 GPa compared to the preceding pressure regime, from the combined effects of the structural transition and spin crossover electronic change. At the highest pressure attained, ∼106 GPa, the Mössbauer data evidence a diamagnetic Fe low-spin abundance of ∼50%. The rest of the high-spin Fe in eightfold coordinated sites continue to experience a relatively small internal magnetic field of ∼33 T. This is indicative of a magnetic ground state associated with strong covalency, as well as substantive disorder from site inversion and the mixed spin-state configuration. Intriguingly, magnetism survives in such a spin-diluted postspinel lattice at high densities. The $R$ (300 K) data decrease by only two orders of magnitude from ambient pressure to the vicinity of ∼100 GPa. Despite a ∼26% unit-cell volume densification from the lattice compressibility, structural transitions, and spin crossover, $\mathrm{FeA}{\mathrm{l}}_2{\mathrm{O}}_4$ is definitively nonmetallic with an estimated gap of ∼400 meV at ∼100 GPa. At such high densification appreciable bandwidth broadening and gap closure would be anticipated. Reasons for the resilient nonmetallic behavior are briefly discussed.

Motivated by the existence of mobile low-energy excitations like domain walls in one dimension (1D) or gauge-charged fractionalized particles in higher dimensions, we compare quantum dynamics in the presence of weak Markovian dephasing for a particle hopping on a chain and for an Ising domain wall whose motion leaves behind a string of flipped spins. Exact solutions show that the two models have near identical transport responses in the bulk. On the other hand, in finite-length chains, the broadening of discrete spectral lines is much more noticeable in the case of a domain wall. These results may be of relevance to a broad class of systems including quasi-1D antiferromagnets, polymer chains, and even retinal systems.

Using an approximate expression for the Landau levels of the electrons located near a nodal line of a topological line-node semimetal, we obtain formulas for the magnetization of this semimetal at an arbitrary shape of its line. It is also shown that the dependence of the chemical potential on the magnetic field can be strong in these materials, and this dependence can essentially influence the de Haas-van Alphen oscillations. The obtained results are applied to the rhombohedral graphite, which is one of the line-node semimetals. For this material, we find temperature and magnetic field dependencies of its magnetic susceptibility.

Tight-binding Hamiltonians with single and multiple orbitals exhibit an intriguing array of magnetic phase transitions. In most cases the spin ordered phases are insulating, while the disordered phases may be either metallic or insulating. In this paper we report a determinant quantum Monte Carlo study of interacting electrons in a geometry which can be regarded as a two-dimensional periodic Anderson model with depleted interacting ($f$) orbitals. For a single depletion, we observe an enhancement of antiferromagnetic correlations and formation of localized states. For half of the $f$ orbitals regularly depleted, the system exhibits a ferrimagnetic ground state. We obtain a quantitative determination of the nature of magnetic order, which we discuss in the context of Tsunetsugu's theorem, and show that, although the dc conductivity indicates insulating behavior at half filling, the compressibility remains finite.

The magnetism and spin-induced ferroelectricity of perovskite $R{\mathrm{MnO}}_3$ ($R=\text{La}$-Lu) epitaxial films grown on ${\mathrm{YAlO}}_3$ substrate have been investigated by means of first-principles density-functional-theory calculations. We show that the epitaxial strain leads to an $E$-type magnetic ground state for all of the rare-earth manganites, which explains the large ferroelectric polarization observed in ${R\mathrm{MnO}}_3$ ($R=\text{Gd}$-Lu) epitaxial films. The transition of the ground state from the $A$-type or spiral antiferromagnetic phase to the $E$-type phase can be attributed mainly to the significant weakening of ferromagnetism in the in-plane nearest-neighbor exchange interaction, which changes from ferromagnetic to antiferromagnetic with the increase of $R$ ionic radius. The enhancement of Jahn-Teller distortion caused by the epitaxial strain plays an important role in the change of exchange interactions. The predicted $E$-type phase in strained films for larger $R$ ions ($R=\text{La}$-Dy) can greatly improve the ferroelectric polarization. However, for the smaller $R$ ions ($R=\text{Ho}$-Lu) whose ground state of the bulk is the $E$-type phase, the ferroelectric polarization of strained films is slightly reduced compared to the bulks, resulting from the competition between the opposite trends of electronic and ion polarization. The ferroelectric distortion caused by the magnetostriction effect is enhanced by the epitaxial strain, while the polarization induced by the pure electronic mechanism is significantly suppressed. The introduction of Coulomb repulsion $U$ in the calculations overestimates the tendency of exchange interactions toward ferromagnetism, leading to the incorrect ground states.

We study the two-dimensional square-lattice Hubbard model for small to moderate interaction strengths $1\le U/t\le 4$ by means of the ladder dual fermion approach. The nonlocal correlations beyond dynamical mean-field theory lower the potential energy, lead to a maximum in the uniform susceptibility, and induce a pseudogap in the density of states. While the self-energy exhibits precursors of a possible insulating phase linked to the appearance of long-range fluctuations, the metallic phase persists within the accessible temperature range. Finite-size effects affect results qualitatively. Upper bounds on the crossover temperature are found to be significantly lower than previously reported dynamical vertex approximation results at $U/t=1$.

Using semiclassical Boltzmann transport theory, we predict a current-induced spin polarization in Weyl semimetals, similar to the Edelstein effect of surface states in Rashba systems or in topological insulators. The theory is applied to the Weyl semimetal TaAs, for which we estimate the magnitude of the effect. The main contribution comes from the topological surface states, i.e., the Fermi arcs, which provide an enormous current-induced spin polarization enhanced by at least one order of magnitude in comparison to Rashba systems and the surface states of topological insulators.

We have identified a two-dimensional (2D) beryllium diphosphide (${\mathrm{BeP}}_2$) structure using a global structure search combined with first-principles calculations. Phonon calculations and molecular dynamics simulation confirm that the structure is dynamically and thermally stable. Electronic structure calculations show that the 2D sheet is a direct band gap semiconductor with a small band gap of 0.15 eV, and the intrinsic acoustic-phonon-limited carrier mobility of the structure can reach $\sim {10}^4{\text{cm}}^2{\text{V}}^{-1}{\text{s}}^{-1}$ for both electrons and holes with anisotropic features in the $x$ and $y$ directions. More interestingly, both mechanical and chemical compression can close the band gap and the structure turns to a Dirac semimetal with the Dirac cones located exactly at the Fermi level. The emerged Dirac semimetal state is direction dependent, with a linear band dispersion in the $x$ direction and a quadratic one in the $y$ direction. Moreover, it is demonstrated that the Dirac point is symmetry protected in the absence of spin-orbit coupling (SOC). In ${\mathrm{BeP}}_2$, the SOC is too weak to alter the semimetal feature except for the cases at extremely low temperatures. The band gap closing mechanism is further clarified by using the tight-binding (TB) method.

The thermal lines method for the evaluation of vibrational expectation values of electronic observables [B. Monserrat, Phys. Rev. B 93, 014302 (2016)] was recently proposed as a physically motivated approximation offering balance between the accuracy of direct Monte Carlo integration and the low computational cost of using local quadratic approximations. In this paper we reformulate thermal lines as a stochastic implementation of quadrature-grid integration, analyze the analytical form of its bias, and extend the method to multiple-point quadrature grids applicable to any factorizable harmonic or anharmonic nuclear wave function. The bias incurred by thermal lines is found to depend on the local form of the expectation value, and we demonstrate that the use of finer quadrature grids along selected modes can eliminate this bias, while still offering an $\sim 30\%$ lower computational cost than direct Monte Carlo integration in our tests.

A hyperuniform many-body system is characterized by a structure factor $S\left(\mathit{k}\right)$ that vanishes in the small-wave-number limit or equivalently by a local number variance ${\sigma }_N^2\left(R\right)$ associated with a spherical window of radius $R$ that grows more slowly than $R^d$ in the large-$R$ limit. Thus, the hyperuniformity implies anomalous suppression of long-wavelength density fluctuations relative to those in typical disordered systems, i.e., ${\sigma }_N^2\left(R\right)\sim R^d$ as $R\to \infty$. Hyperuniform systems include perfect crystals, quasicrystals, and special disordered systems. Disordered hyperuniform systems are amorphous states of matter that lie between a liquid and crystal [S. Torquato et al., Phys. Rev. X 5, 021020 (2015)], and have been the subject of many recent investigations due to their novel properties. In the same way that there is no perfect crystal in practice due to the inevitable presence of imperfections, such as vacancies and dislocations, there is no “perfect” hyperuniform system, whether it is ordered or not. Thus, it is practically and theoretically important to quantitatively understand the extent to which imperfections introduced in a perfectly hyperuniform system can degrade or destroy its hyperuniformity and corresponding physical properties. This paper begins such a program by deriving explicit formulas for $S\left(\mathit{k}\right)$ in the small-wave-number regime for three types of imperfections: (1) uncorrelated point defects, including vacancies and interstitials, (2) stochastic particle displacements, and (3) thermal excitations in the classical harmonic regime. We demonstrate that our results are in excellent agreement with numerical simulations. We find that “uncorrelated” vacancies or interstitials destroy hyperuniformity in proportion to the defect concentration $p$. We show that “uncorrelated” stochastic displacements in perfect lattices can never destroy the hyperuniformity but it can be degraded such that the perturbed lattices fall into class III hyperuniform systems, where ${\sigma }_N^2\left(R\right)\sim R^{d-\alpha }$ as $R\to \infty$ and $0<\alpha <1$. By contrast, we demonstrate that certain “correlated” displacements can make systems nonhyperuniform. For a perfect (ground-state) crystal at zero temperature, increase in temperature $T$ introduces such correlated displacements resulting from thermal excitations, and thus the thermalized crystal becomes nonhyperuniform, even at an arbitrarily low temperature. It is noteworthy that imperfections in disordered hyperuniform systems can be unambiguously detected. Our work provides the theoretical underpinnings to systematically study the effect of imperfections on the physical properties of hyperuniform materials.