We consider a two-impurity Kondo system with spin-exchange coupling within the conduction band. Our numerical renormalization group calculations show that for strong intraband spin correlations the competition of these nonlocal correlations with local Kondo spin screening stabilizes a phase exhibiting features of a metallic quantum spin liquid, namely nonlocal impurity-spin entanglement and fractionalized charge excitations, without the need for geometric frustration. For weak Kondo coupling, the spin-liquid-like and the Kondo singlet phases are separated by two quantum phase transitions and an intermediate RKKY spin-dimer phase, while beyond a critical coupling they are connected by a crossover. The results suggest how a quantum spin liquid may be realized in heavy-fermion systems with strong magnetic correlations in the conduction band, e.g., near a spin-density-wave instability.

We present a classical molecular-spin dynamics (MSD) methodology that enables accurate computations of the temperature dependence of the magnetocrystalline anisotropy as well as magnetoelastic properties of magnetic materials. The nonmagnetic interactions are accounted for by a spectral neighbor analysis potential (SNAP) machine-learned interatomic potential, whereas the magnetoelastic contributions are accounted for using a combination of an extended Heisenberg Hamiltonian and a Néel pair interaction model, representing both the exchange interaction and spin-orbit-coupling effects, respectively. All magnetoelastic potential components are parameterized using a combination of first-principles and experimental data. Our framework is applied to the $\alpha$ phase of iron. Initial testing of our MSD model is done using a 0 K parametrization of the Néel interaction model. After this, we examine how individual Néel parameters impact the $B_1$ and $B_2$ magnetostrictive coefficients using a moment-independent $\delta$ sensitivity analysis. The results from this study are then used to initialize a genetic algorithm optimization which explores the Néel parameter phase space and tries to minimize the error in the $B_1$ and $B_2$ magnetostrictive coefficients in the range of 0–1200 K. Our results show that while both the 0 K and genetic algorithm optimized parametrization provide good experimental agreement for $B_1$ and $B_2$, only the genetic algorithm optimized results can capture the second peak in the $B_1$ magnetostrictive coefficient which occurs near approximately 800 K.

We have fabricated NbN/Au nanogaps and bridged them with an Al superconductor using Ti as an interlayer. The nanodevices show a critical current density at 300 mK as high as $3\times {10}^6{\text{A/cm}}^2$, which is $30\%$ higher than that of Al nanowires with the same lateral dimensions as the NbN-based devices. The response of the critical current as a function of the external magnetic field clearly showed a Fraunhofer-like behavior, indicating a Josephson coupling between the NbN electrodes through the Al barrier. The superconducting transport evolves into different transport regimes as a function of the temperature. These findings demonstrate the importance of using superconducting barriers in hybrid nanodevices to achieve very high Josephson current in nanodevices of great relevance in superconducting circuits requiring high integration density.

We consider a non-Hermitian superconducting system by coupling a conventional superconductor to a ferromagnet lead and demonstrate the emergence of exceptional points when an external Zeeman field is applied. We discover that, depending on the non-Hermiticity and the Zeeman field, the exceptional points mark the ends of lines with zero real energy, thus giving rise to topologically protected and highly tunable bulk Fermi arcs, which we coin bulk Bogoliubov Fermi arcs due to their superconducting nature. We show that these bulk Bogoliubov Fermi arcs are the non-Hermitian counterparts of the Hermitian topological phase transition but are much more prevalent and also experimentally detectable through large spectral signatures.

We have studied the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state in Mg-doped ${\mathrm{Ti}}_2{\mathrm{O}}_3$ using hard x-ray photoemission spectroscopy. The Ti $2p$ spectrum for the corundum-type ${\mathrm{Ti}}_2{\mathrm{O}}_3$ revealed the ${\mathrm{Ti}}^{3+}$ configuration with strong electronic coupling in the $c$-axis Ti-Ti pairs whereas the data for the ilmenite-type ${\mathrm{MgTiO}}_3$ confirmed the ${\mathrm{Mg}}^{2+}\text{−}{\mathrm{Ti}}^{4+}$ charge state in the $c$-axis cation pairs. In ${\mathrm{Mg}}_{0.29}{\mathrm{Ti}}_{1.71}{\mathrm{O}}_3$, the Ti $2p$ spectrum hardly showed the ${\mathrm{Ti}}^{4+}$ peak, which was in ${\mathrm{MgTiO}}_3$ indicating that the $c$-axis pairing of the Mg is with a ${\mathrm{Ti}}^{3+}$ ion rather than a ${\mathrm{Ti}}^{4+}$ and that the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state is materialized within the Ti-Ti $c$-axis pairs. In ${\mathrm{Mg}}_{0.63}{\mathrm{Ti}}_{1.37}{\mathrm{O}}_3$, we detected the presence of ${\mathrm{Mg}}^{2+}\text{−}{\mathrm{Ti}}^{4+}$ pairs. The results indicate the important role of hybridization within the $c$-axis pairs not only in the ${\mathrm{Ti}}^{3+}\text{−}{\mathrm{Ti}}^{3+}$ configuration, but also above all in the ${\mathrm{Ti}}^{3+}/{\mathrm{Ti}}^{4+}$ mixed valence state, superseding the Madelung energy gain of the ${\mathrm{Mg}}^{2+}\text{−}{\mathrm{Ti}}^{4+} c$-axis pair formation.

Dissipation in open systems enriches the possible symmetries of the Hamiltonians beyond the Hermitian framework, allowing the possibility of novel non-Hermitian topological phases which exhibit long-living end states that are protected against disorder. So far, non-Hermitian topology has been explored in settings where probing genuine quantum effects has been challenging. We theoretically show that a non-Hermitian topological quantum phase can be realized in a reservoir-engineered transmon chain. The spatial modulation of dissipation is obtained by coupling each transmon to a quantum circuit refrigerator, allowing in situ tuning of dissipation strength in a wide range. By solving the many-body Lindblad master equation using a combination of the density matrix renormalization group and Prosen-Seligman third quantization approaches, we show that the topological end modes and the associated phase transition are visible in simple reflection measurements with experimentally realistic parameters. Finally, we demonstrate that genuine quantum effects are observable in this system via robust and slowly decaying long-range quantum entanglement of the topological end modes, which can be generated passively starting from a locally excited transmon.

In most cases, the formation of topological quantum states requires intrinsic crystalline symmetry to protect either discrete or continuous degeneracies in real materials, which greatly hinders their realization in practice. Patterned two-dimensional electron gas (2DEG), on the other hand, has become a very effective external means to manipulate the symmetry to whatever we want. Here, taking nonsymmorphic symmetries as the focus of attention, based on patterned 2DEG decoration, we reveal rich band-crossing features in two-dimensional systems. It is demonstrated that in the presence of intrinsic spin-orbital coupling (SOC), wallpaper groups $p2mg,p2gg$, and $p4mg$ possess fourfold-degenerate Dirac nodal lines, and if Rashba SOC is further considered, the fourfold-degenerate nodal lines disappear and hourglass Weyl fermions then emerge. Our results not only afford an attractive route for designing robust nodal-line and hourglass Weyl semimetals in reality, but also pave the way for designing ideal macroscale materials through rational extrinsic symmetry engineering.

In materials, certain approximated symmetry operations can exist in a lower-order approximation of the effective model but are good enough to influence the physical responses of the system, and these approximated symmetries were recently dubbed “quasisymmetries” [Nat. Phys. 18, 813 (2022)]. In this paper, we reveal a hierarchy structure of the quasisymmetries and the corresponding nodal structures that they enforce via two different approaches of the perturbation expansions for the effective model in the chiral crystal material CoSi. In the first approach, we treat the spin-independent linear momentum ($k$) term as the zero-order Hamiltonian. Its energy bands are fourfold degenerate due to an $\mathrm{SU}\left(2\right)\times \mathrm{SU}\left(2\right)$ quasisymmetry. We next consider both the k-independent spin-orbit coupling (SOC) and full quadratic $k$ terms as the perturbation terms and find that the first-order perturbation leads to a model described by a self-commuting “stabilizer code” Hamiltonian with a U(1) quasisymmetry that can protect nodal planes. In the second approach, we treat the SOC-free linear $k$ term and $k$-independent SOC term as the zero order. They exhibit an SU(2) quasisymmetry, which can be reduced to U(1) quasisymmetry by a choice of quadratic terms. Correspondingly, a twofold degeneracy for all the bands due to the SU(2) quasisymmetry is reduced to twofold nodal planes that are protected by the U(1) quasisymmetry. For both approaches, including higher-order perturbation will break the U(1) quasisymmetry and induce a small gap $\sim 1\mathrm{meV}$ for the nodal planes. These quasisymmetry protected near degeneracies play an essential role in understanding recent quantum oscillation experiments in CoSi.

Recently, ${\mathrm{LaWN}}_3$ nitride perovskite with polar symmetry has been synthesized and has shown a large piezoelectric response. In addition to experimentally measured piezoelectricity and theoretically predicted ferroelectricity, the thermal transport properties of ${\mathrm{LaWN}}_3$ remains unexplored. In this work, the phonon dispersion, lattice thermal conductivity, group velocity, phonon lifetime, phase space, and Grüneisen parameters of ${\mathrm{LaWN}}_3$ are systematically studied using first-principles calculations. The results show that the lattice thermal conductivity of ${\mathrm{LaWN}}_3$ is 3.33 and 3.71 W/(mK) along the $a$ and $c$ axes at 300 K, respectively, which is comparable to some typical ferroelectric oxide perovskites such as ${\mathrm{PbTiO}}_3$ and ${\mathrm{BaTiO}}_3$. Further analyses imply that over 60% of the lattice thermal conductivity of ${\mathrm{LaWN}}_3$ is contributed by the optical phonons. It is found that there exists the coupling effect between the acoustic and low-lying optical phonons which limits the group velocity and the contribution to lattice thermal conductivity of acoustic phonons. Additionally, the four-phonon scattering is found to play an important role in suppressing the lattice thermal conducitivty, which is reduced by ∼50% compared to the values where only three-phonon scattering is considered.

Recently, Floquet systems have attracted a great deal of interest as they offer the unprecedented ability to engineer topological states through the tuning of an external time-periodic drive. Consequentially, seeking new driving protocols that allow for more exotic topological phases and transitions becomes imperative for the Floquet engineer. In this paper, we study the Su-Schrieffer-Heeger model driven by two time-dependent periodic sources with commensurate frequencies and an amplitude modulation. Imposing more than one driving frequency allows us to realize even more exotic topological phases resulting from new couplings appearing in the Fourier space representation. Moreover, we find an experimentally practical method for sweeping the system through a topological phase transition by varying the amplitude mixture of the commensurate sources. We employ the local Chern marker, a real-space representation of the Chern number, to simulate topological phase diagrams of the two-drive Floquet Hamiltonian in a variety of driving cases.

At high temperature, generic strongly interacting spin systems are expected to display hydrodynamics: local transport of conserved quantities, governed by classical partial differential equations like the diffusion equation. I argue that the emergence of this dissipative long-wavelength dynamics from the system's unitary microscopic dynamics is controlled by the structure of the Liouvillian graph of the system's Hamiltonian, that is, the graph induced on Pauli strings by commutation with that Hamiltonian. The Liouvillian graph decomposes naturally into subgraphs of Pauli strings of constant diameter, and the coherent dynamics of these subgraphs determines the rate at which operator weight spreads to long operators. This argument provides a quantitative theory of the emergence of a dissipative effective dynamics from unitary microscopic dynamics; it also leads to an effective model with Hilbert space dimension linear in system size and exponential in the UV cutoff for diffusion.

We present a systematic study of the Hall conductance in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_x$ thin films over a large range of doping. We find that, in a large part of the phase diagram, the Hall coefficient changes sign as a function of temperature in the flux-flow regime. By comparing data from many samples, we show that the sign reversal is tied to the superconducting transition and is not a result of a competing order. We then compare our data to the predictions of the Bardeen-Stephen model and show that, in all samples, there is an additional negative contribution to the Hall conductivity. We extract from the negative excess Hall a vortex charge that is found to be strongly doping dependent.

Intersite dipolar interactions induce, even in absence of disorder, an intriguing nonergodic dynamics for dipolar bosons in an optical lattice. We show that the inherent dipole-induced density-dependent tunneling, typically neglected, plays a crucial role in this dynamics. For shallow-enough lattices, the delocalization stemming from the interaction-induced hopping overcomes the localization induced by intersite interactions. As a result, in stark contrast to the more studied case of hard-core bosons, delocalization is counterintuitively strengthened when the dipolar strength increases. Furthermore, the quasicancellation between bare and interaction-induced tunneling may lead, near a lattice-depth-dependent value of the dipole strength, to an exact decoupling of the Hilbert space between ergodic hard-core states and strongly nonergodic soft-core ones. Our results show that interaction-induced hopping should play a crucial role in future experiments on the dynamics of polar lattice gases.

Within this paper we show the importance of accurate implementations of the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions for antiferromagnetically coupled ferromagnetic layers with thicknesses exceeding the exchange length. In order to evaluate the performance of different implementations of RKKY interaction, we develop a benchmark problem by deriving the analytical formula for the saturation field of two infinitely thick magnetic layers that are antiparallelly coupled. This benchmark problem shows that state of the art implementations in commonly used finite-difference codes lead to errors of the saturation field that amount to more than 20% for mesh sizes of 2 nm which is well below the exchange length of the material. In order to improve the accuracy, we develop higher order cell-based and nodal-based finite-difference codes that significantly reduce the error compared to state of the art implementations. For the second order cell-based approach and first order nodal-based approach the error of the saturation field is reduced by about a factor of 10 (2% error) for the same mesh size of 2 nm.

The spin-1/2 Heisenberg antiferromagnet on the frustrated diamond-decorated square lattice is known to feature various zero-field ground-state phases, consisting of extended monomer-dimer and dimer-tetramer ground states as well as a ferrimagnetic regime. Using a combination of analytical arguments, density matrix renormalization group (DMRG), exact diagonalization as well as sign-problem-free quantum Monte Carlo (QMC) calculations, we investigate the properties of this system and the related Lieb lattice in the presence of a finite magnetic field, addressing both the ground-state phase diagram as well as several thermodynamic properties. In addition to the zero-field ground states, we find at high magnetic field a spin-canted phase with a continuously rising magnetization for increasing magnetic field strength as well as the fully polarized paramagnetic phase. At intermediate field strength, we identify a first-order quantum phase transition line between the ferrimagnetic and the monomer-dimer regime. This first-order line extends to finite temperatures, terminating in a line of critical points that belong to the universality class of the two-dimensional Ising model.

Hund's metal is one kind of correlated metal, in which the electronic correlation is strongly influenced by the Hund's interaction. At high temperatures, while the charge and orbital degrees of freedom are quenched, the spin degrees of freedom can persist in terms of frozen moments. As temperature decreases, a coherent electronic state with characteristic orbital differentiation always emerges at low temperatures through an incoherent-to-coherent crossover, which has been widely observed in iron-based superconductors [e.g., iron selenides and $A{\mathrm{Fe}}_2{\mathrm{As}}_2$ ($A=\mathrm{K}$, Rb, Cs)]. Consequently, the above frozen moments are “screened” by coupling to orbital degrees of freedom, leading to an emergent Fermi-liquid state. In contrast, the coupling among frozen moments should impede the formation of the Fermi-liquid state by competitive magnetic ordering, which is still unexplored in Hund's metal. Here, in the iron-based Hund's metal ${\mathrm{CsFe}}_2{\mathrm{As}}_2$, we adopt a chemical substitution at iron sites by Cr/Co atoms to explore the competitive magnetic ordering. By a comprehensive study of resistivity, magnetic susceptibility, specific heat, and nuclear magnetic resonance, we demonstrate that the Fermi-liquid state is destroyed in Cr-doped ${\mathrm{CsFe}}_2{\mathrm{As}}_2$ by a spin-freezing transition below $T_g\sim 22$ K. Meanwhile, the evolution of charge degrees of freedom measured by angle-resolved photoemission spectroscopy also supports the competition between the Fermi-liquid state and spin-glass state.