Pressure not only provides a powerful way to tune the crystal structure of transition-metal dichalcogenides (TMDCs) but also promotes the discovery of exotic electronic states and intriguing phenomena. Structural transitions from the quasi-two-dimensional layered orthorhombic phase to three-dimensional cubic pyrite phase, metallization, and superconductivity under high pressure have been observed experimentally in TMDC materials ${\mathrm{PdS}}_2$ and ${\mathrm{PdSe}}_2$. We report a theoretical prediction of the pressure-induced evolutions of crystal structure and electronic structure of PdSSe, an isomorphous intermediate material of the orthorhombic ${\mathrm{PdS}}_2$ and ${\mathrm{PdSe}}_2$. A series of pressure-induced structural phase transitions from the layered orthorhombic structure into an intermediate phase and then to a cubic phase is revealed. The intermediate phase features the same structure symmetry as the ambient orthorhombic phase, except for drastic collapsed interlayer distances and striking changes of the coordination polyhedron. Furthermore, the structural phase transitions are accompanied by electronic structure variations from semiconductor to semimetal, which are attributed to bandwidth broadening and orbital-selective mechanisms. Specifically, the cubic phase PdSSe is distinct from the cubic ${\mathrm{PdS}}_2$ and ${\mathrm{PdSe}}_2$ materials by breaking inversion and mirror-plane symmetries but showing similar superconductivity under high pressure, which originates from strong electron-phonon coupling interactions concomitant with topologically nontrivial Weyl and high-fold fermions. The intricate interplay between lattice, charge, and orbital degrees of freedom as well as the topologically nontrivial states in these compounds will further stimulate wide interest to explore the exotic physics of the TMDC materials.

We present a study of the two-dimensional circular quantum dot model Hamiltonian using a range of quantum chemical ab initio methods. Ground and excited state energies are computed on different levels of perturbation theories, including the coupled cluster method. We outline a scheme to compute the required Coulomb integrals in real space and utilize a semianalytic solution to the integral over the Coulomb kernel in the vicinity of the singularity. Furthermore, we show that the remaining basis set incompleteness error for two-dimensional quantum dots scales with the inverse number of virtual orbitals, allowing us to extrapolate to the complete basis set limit energy. By varying the harmonic potential parameter we tune the correlation strength and investigate the predicted ground and excited state energies.

We investigate the charging process of quantum battery (QB) systems in an extended Dicke model with both atomic interactions and an external driving field. We focus on the effects of the atomic interaction and the external driving field on the charging performance of the QB and find that the maximum stored energy of the QB has a critical phenomenon. We analyze the critical behavior and obtain the analytical expression of the critical atomic interaction. The dependence of the maximum stored energy, the energy quantum fluctuations, and the maximum charging power on the number $N$ of the two-level systems are also discussed. In particular, for the maximum charging power, we obtain the quantum advantage of the QB, which approximately satisfies a superlinear scaling relation $P_{\text{max}}\propto N^{\alpha }$, where scaling exponent $\alpha$ varies with the number $N$ of the two-level systems. In the ultrastrong coupling regime, the atomic interaction can lead to faster battery charging, and the quantum advantage $\alpha =1.88$ can be achieved. While in the deep-strong coupling regime, the quantum advantage of the QB's maximum charging power is the same as that of the Dicke QB, i.e., $\alpha =1.5$.

We construct and study a chiral Sachdev-Ye (SY) model consisting of $N$ chiral $\mathrm{SU}{\left(M\right)}_1$ Wess-Zumino-Witten (WZW) models with current-current interactions among each other, which generalizes the $0+1\mathrm{d}$ quantum chaotic SY spin model into $1+1\mathrm{d}$ chiral system with anyon excitations. Each WZW model hosts Abelian anyons as charge excitations, and may arise as the chiral edge theory of $2+1\mathrm{d}$ gapped topological phases. We solve the chiral SY model in two limits which show distinct quantum dynamics. The first limit is the case with uniform interactions at any integers $N$ and $M$, which is integrable and decomposes into a chiral $\mathrm{SU}{\left(M\right)}_N$ WZW model and its coset with a different speed of light. When $N=M=2$, the model maps to a free Majorana fermion model. The second limit is the large $N$ and $M$ limit with random interactions, which is solvable to the leading $\frac{1}{NM}$ order, and exhibits many-body quantum chaos in the out-of-time-ordered correlation of anyons. As the interaction strength approaches the upper limit preserving the chirality, the leading velocity-dependent Lyapunov exponent of the model saturates the maximal chaos bound $2\pi /\beta$ at temperature ${\beta }^{-1}$.

Methods of attosecond science originally developed to investigate systems in the gas phase are currently being adapted to obtain temporal information on the electron dynamics that takes place in condensed-matter systems. In particular, streaking measurements have recently been performed to determine photoemission time delays from the ${\mathrm{WSe}}_2$ dichalcogenide. In this work we present a fully atomistic description of the photoemission process in ${\mathrm{WSe}}_2$ and provide angularly resolved photoemission cross sections and time delays from the W $4f$, Se $3d$ and Se $4s$ core states of the system. Since these states are spatially localized, we propose a cluster approach in which we build up from smaller to larger clusters, so that we can assess the importance of scattering effects by each new layer of neighboring atoms. We use a static-exchange density functional theory method with $B$-spline functions, where a one-center angular-momentum expansion is supplemented by off-center expansions with fewer partial waves. This enhances convergence in comparison with a one-center expansion, which would require very high angular momenta to characterize the localized fast oscillations near each off-center atomic core. We find that the photoemission delays and fully differential cross sections are strongly affected by scattering events that take place off the neighboring atoms, implying the need to consider their effects for quantitative descriptions of the photoemission process.

Recently discovered superconductivity in hole-doped nickelate ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_2$ has attracted intensive attention in the field. An immediate question is how to improve its superconducting properties. Guided by the key characteristics of electronic structures of the cuprates and the nickelates, we propose that nickel chalcogenides with a similar lattice structure should be a promising family of materials. Using ${\mathrm{NdNiS}}_2$ as an example, through first-principle structural optimization and phonon calculation, we find this particular crystal structure to be a stable one. We justify our proposal by comparing ${\mathrm{NdNiS}}_2$ with ${\mathrm{CaCuO}}_2$ and ${\mathrm{NdNiO}}_2$ with regard to the strength of the charge-transfer characteristics and the trend in their low-energy many-body effective Hamiltonians of doped hole carriers. This analysis indicates that nickel chalcogenides host low-energy physics closer to that of the cuprates, with stronger magnetic interaction than the nickelates, and thus they deserve further experimental exploration. Our proposal also opens up the possibility of a wide range of parameter tuning through ligand substitution among chalcogenides, to further improve superconducting properties.

Kramers' degeneracy theorem underpins many interesting effects in quantum systems with time-reversal symmetry. We show that the generator of dynamics for Markovian open fermionic systems can exhibit an analogous degeneracy, protected by a combination of time-reversal symmetry and the microreversibility (detailed balance) property of systems at thermal equilibrium—the degeneracy is lifted if either condition is not met. We provide simple examples of this phenomenon and show that the degeneracy is reflected in the single-particle Green's functions. Furthermore, we show that certain experimental signatures of topological edge modes in open many-body systems can be protected by microreversibility in the same way. Our results highlight the importance of detailed balance in characterizing open topological matter.

Fermion-exciton condensates (FECs)—computationally and theoretically predicted states that simultaneously exhibit the character of superconducting states and exciton condensates—are novel quantum states whose properties may involve a hybridization of superconductivity and the dissipationless flow of energy. Here, we exploit prior investigations of superconducting states and exciton condensates on quantum devices to identify a tuneable quantum state preparation entangling the wave functions of the individual condensate states. Utilizing this state preparation, we prepare a variety of FEC states on quantum computers—realizing strongly correlated FEC states on current, noisy intermediate-scale quantum devices—and verify the presence of the dual condensate via postmeasurement analysis. This confirmation of the previously predicted condensate state on quantum devices as well as the form of its wave function motivates further theoretical and experimental exploration of the properties, applications, and stability of FECs.

Neutron diffraction, magnetization, and muon spin relaxation measurements, supplemented by density functional theory (DFT) calculations are employed to unravel temperature-driven magnetization reversal in inverse spinel ${\mathrm{Co}}_2{\mathrm{VO}}_4$. All measurements show a second-order magnetic phase transition at $T_{\mathrm{C}}=168\mathrm{K}$ to a collinear ferrimagnetic phase. Neutron diffraction measurements reveal two antiparallel ferromagnetic (FM) sublattices, belonging to magnetic ions on two distinct crystal lattice sites, where the relative balance between the two sublattices determine the net FM moment in the unit cell. As the evolution of the ordered moment with temperature differs between the two sublattices, the net magnetic moment reaches a maximum at $T_{\mathrm{NC}}=138\mathrm{K}$ and reverses its sign at $T_{\mathrm{MR}}=65\mathrm{K}$. The DFT results suggest that the underlying microscopic mechanism for the reversal is a delocalization of the unfilled $3d$-shell electrons on one sublattice just below $T_{\mathrm{C}}$, followed by a gradual localization as the temperature is lowered. This delocalized-localized crossover is supported by muon spectroscopy results, as strong $T_1$ relaxation observed below $T_{\mathrm{C}}$ indicates fluctuating internal fields.

Finding and probing the ground states of spin lattices, such as the antiferromagnetic Heisenberg model on the kagome lattice (KAFH), is a very challenging problem on classical computers and only possible for relatively small systems. We propose using the variational quantum eigensolver (VQE) to find the ground state of the KAFH on a quantum computer. We find efficient ansatz circuits and show how physically interesting observables can be measured efficiently. To investigate the expressiveness and scaling of our ansatz circuits we used classical, exact simulations of VQE for the KAFH for different lattices ranging from 8 to 24 qubits. We find that the fidelity with the ground state approaches one exponentially in the circuit depth for all lattices considered, except for a 24-qubit lattice with an almost degenerate ground state. We conclude that VQE circuits that are able to represent the ground state of the KAFH on lattices inaccessible to exact diagonalization techniques may be achievable on near-term quantum hardware. However, for large systems, circuits with many variational parameters are needed to achieve high fidelity with the ground state.

We explore the eigenenergy distribution, localization transition, and topological property of a one-dimensional non-Hermitian lattice with a slowly varying potential, which does not possess parity-time ($\mathcal{PT}$) symmetry. We found that, in contrast to $\mathcal{PT}$-symmetric systems whose eigenenergies can stay real until the $\mathcal{PT}$-symmetry breaking point, the eigenenergies in the present system are fully complex and their distribution in the complex plane depends on the parameters. As non-Hermiticity is introduced into the system, also depending on the parameters, the eigenstates may show a crossover behavior from fully extended to fully localized as non-Hermiticity is increased, or there will be a sharp localization transition at finite non-Hermiticity. Nevertheless, this non-Hermitian system is proven to be topologically trivial, thus we conclude that in general non-Hermitian systems, the localization transition is not always of a topological nature.

Spin pumping is a technique widely used to generate pure spin current and characterize the spin-charge conversion in various systems. The reversing sign of the symmetric Lorentzian charge current with respect to the opposite magnetic field is generally accepted as the key criterion to identifying its pure spin current origin. However, we herein find that the rectified voltage due to the planar Hall effect can exhibit a similar spurious signal, complicating and even misleading the analysis. The distribution of microwave magnetic field and induction current has a strong influence on the magnetic field symmetry and line shape of the obtained signal. We further demonstrate a geometry where the spin-charge conversion and the rectified voltage can be readily distinguished with a straightforward symmetry analysis.

We present an analytical model to compute the subterahertz (sub-THz) spin current transients injected from the insulating uniaxial antiferromagnet (AFM) into the adjacent nonmagnetic layer excited by spin pumping under the antiferromagnetic resonance condition, where both intra- and cross-sublattice damping parameters are treated on an equal footing. As expected, the sub-THz spin-pumping signal decreases with larger intra-sublattice damping dissipation. Interestingly, it is found that the amplitude of the spin current transient is enhanced with increasing cross-sublattice damping. On the other hand, the spin pumping is reduced by increasing the cross-sublattice spin-mixing conductance. These trends indicate that the intrinsic origin of the cross-sublattice damping in the bulk AFM enhances the spin current transients while its extrinsic origin, directly related to the interfacial cross-sublattice spin-mixing conductance, has the opposite effect. Our results suggest the important role of the previously neglected cross-sublattice damping in modulating the sub-THz spin current pulses for ultrafast spintronic applications.

The oxide superconductor ${\mathrm{Li}}_{1\text{−}x}\mathrm{Nb}{\mathrm{O}}_2$ has two-dimensional (2D) $\mathrm{Nb}{\mathrm{O}}_2$ layers consisting of edge-shared triangular prisms. This structure is a unique oxide analog to $2H$-type transition-metal dichalcogenides (TMDs), yet its electronic properties have not received any significant attention. Using ${\mathrm{Li}}_{1\text{−}x}\mathrm{Nb}{\mathrm{O}}_2$ epitaxial films, we find the 2D superconductivity associated with the Berezinskii-Kosterlitz-Thouless transition and a large anisotropy of the upper critical field. The temperature-independent anisotropy strongly suggests single-band superconductivity, in contrast to TMDs. Correspondingly, the largely isolated single $d_{z2}$ state revealed by synchrotron-radiation photoelectron spectroscopy and density functional theory is explained in terms of strong ligand field splitting. These results indicate that superconductivity in ${\mathrm{Li}}_{1\text{−}x}\mathrm{Nb}{\mathrm{O}}_2$ occurs in 2D $\mathrm{Nb}{\mathrm{O}}_2$ layers with a narrow and correlated single band, reminiscent of superconductivity in cuprates. Our study provides insight into the unconventional correlation in the Hubbard system with a 2D triangular lattice.

We propose a method for computing space-resolved correlation properties of the two-dimensional Hubbard model within a quantum-classical embedding strategy that uses a noisy, intermediate scale quantum (NISQ) computer to solve the embedded model. While previous approaches were limited to purely local, one-impurity embedded models, requiring at most four qubits and relatively shallow circuits, we solve a two-impurity model requiring eight qubits with an advanced hybrid scheme on top of the variational quantum eigensolver algorithm. This iterative scheme, dubbed natural orbitalization (NOization), gradually transforms the single-particle basis to the approximate natural-orbital basis, in which the ground state can be minimally expressed, at the cost of measuring the one-particle reduced density matrix of the embedded problem. We show that this transformation tends to make the variational optimization of existing (but too deep) Ansatz circuits faster and more accurate, and we propose an Ansatz, the multireference excitation-preserving (MREP) Ansatz, that achieves great expressivity without requiring a prohibitive gate count, thus bridging the gap between hardware-efficient and physically motivated strategies in variational Ansatz design. The one-impurity version of the Ansatz has only one parameter, making the ground-state preparation a trivial step, which supports the optimal character of our approach. Within a rotationally invariant slave boson embedding scheme that requires a minimal number of bath sites and does not require computing the full Green's function, the NOization combined with the MREP Ansatz allow us to compute accurate, space-resolved quasiparticle weights and static self-energies for the Hubbard model even in the presence of noise levels representative of current NISQ processors. This paves the way to a controlled solution of the Hubbard model with larger and larger embedded problems solved by quantum computers.

Symmetry-protected topological semimetals are at the focus of solid-state research due to their unconventional properties, for example, regarding transport. By investigating local two-band Bloch Hamiltonians in the spin-1/2 basis for the 122 magnetic point groups, we classify twofold-degenerate band touchings such as Weyl points, robust nodal lines on axes and in mirror planes, and fragile nodal lines. We find that all magnetic point groups that lack the product of inversion and time-reversal symmetries can give rise to topologically nontrivial band touchings. Hence, such nodes are the rule rather than the exception and, moreover, do not require any complicated multiband physics. Our classification is applicable to every momentum in the Brillouin zone by considering the corresponding little group and provides a powerful tool to identify magnetic and nonmagnetic topological semimetals.

To describe a spin-$\frac{1}{2}$ particle on the Bloch sphere with a radial magnetic field and topological states of matter from the reciprocal space, we introduce $C$ square ($C^2$) as a local formulation of the global topological invariant. For the Haldane model on the honeycomb lattice, this $C^2$ can be measured from the Dirac points through circularly polarized light related to the high-symmetry $M$ point(s). For the quantum spin Hall effect and the Kane-Mele model, the ${\mathbb{Z}}_2$ topological number robust to interactions can be measured locally from a correspondence between the Pfaffian and light. We address a relation with a spin pump and the quantum spin Hall conductance. The analogy between light and magnetic nuclear resonance may be applied for imaging, among other applications.