We study the one-dimensional non-Hermitian lattices with linearly varying nonreciprocal hopping, where the non-Hermitian skin effect (NHSE) is found to be dissolved gradually as the strength of nonreciprocity increases. The energy spectrum under the open boundary condition is composed of real and imaginary eigenenergies when the nonreciprocal hopping is weak. Interestingly, the real eigenenergies form an equally spaced ladder, and the corresponding eigenstates are localized at the boundary with a Gaussian distribution due to NHSE. By increasing the nonreciprocity, the number of real eigenenergies will decrease while more and more eigenenergies become imaginary. Accompanied by the real-imaginary transition in the spectrum, the eigenstates are shifted from the boundary into the bulk of the lattice. When the nonreciprocity gets strong enough, the whole spectrum will be imaginary and the NHSE disappears completely in the system; i.e., all the eigenstates become Gaussian bound states localized inside the bulk. Our work unveils the exotic properties of non-Hermitian systems with spatially varying nonreciprocal hopping.

We develop a classical shadow tomography protocol utilizing the randomized measurement scheme based on hybrid quantum circuits, which consist of layers of two-qubit random Clifford unitary gates mixed with single-qubit random Pauli measurements. Unlike conventional protocols that perform all measurements by the end of unitary evolutions, our protocol allows measurements to occur at any space-time position throughout the quantum evolution. We provide a universal classical post-processing strategy to approximately reconstruct the original quantum state from intermittent measurement outcomes given the corresponding random circuit realizations over repeated experiments. We investigated the sample complexity for estimating different observables at different measurement rates of the hybrid quantum circuits. Our result shows that the sample complexity has an optimal scaling at the critical measurement rate when the hybrid circuit undergoes the measurement-induced transition.

We study the universal real-time relaxation behaviors of a long-range Kitaev chain following a quench, and the results are compared to a short-range quantum XY chain. Our research includes both noncritical and critical quenches. In the case of noncritical quench, i.e., neither the initial state nor the postquench Hamiltonian is at a critical point of the equilibrium phase transition, a quench to the commensurate phase or incommensurate phase gives a scaling of $t^{-3/2}$ or $t^{-1/2}$, respectively, which is the same as the counterpart of the short-range quantum XY model. However, for a quench to the boundary line between the commensurate and incommensurate phases, the scaling law $t^{-\mu }$ may be different from the $t^{-3/4}$ law of the counterpart of the short-range quantum XY model. More interestingly, the decaying exponent $\mu$ may depend on the choice of the parameters of the postquench Hamiltonian because of the different asymptotic behaviors of the energy spectrum. Furthermore, in certain cases, the scaling behavior may be outside the range of predictions made by the stationary phase approximation, because an inflection point emerges in the energy spectrum. For the critical quench, i.e., the initial state or the postquench Hamiltonian is at a critical point of equilibrium phase transition, the aforementioned scaling law $t^{-\mu }$ may be changed because of the gap-closing property of the energy spectrum of the critical point.

We report a combined theoretical and experimental study of the quaternary Heusler alloy CoFeVAl hosting various interesting electronic/magnetic features including an extended Griffiths phase–like behavior. The room temperature x-ray diffraction pattern confirms a cubic structure (space group No. 216) with a B2 disorder between V and Al atoms. The dc magnetization measurements indicate a ferromagnetic-like behavior with a magnetic transition temperature $\left(T_C\right)$ $\sim 50$ K. Above $T_C$ an unusual magnetic phase (extended Griffiths like) is observed which is carefully analyzed from the dc and ac magnetic susceptibility data. Longitudinal resistivity data confirm a metallic-like behavior with a negative nonsaturating magnetoresistance in the magnetically ordered region. The anomalous Hall effect mediated by a non-negligible intrinsic mechanism is found in the total Hall resistivity whereas negative slope in the ordinary Hall resistivity indicates the electron-mediated conduction in CoFeVAl. First-principles calculated results corroborate well with the experimentally observed crystal structure and nearly half-metallic ferromagnetic (FM) behavior. A high spin polarization $(\sim 86\%)$ is observed in the FM ground state. Inclusion of B2 disorder retains the nearly half-metallic behavior of CoFeVAl. Such combined studies for the discovery of new quantum materials are the backbone of a future path toward the spintronic-based device applications.

We performed inelastic neutron scattering (INS) experiments on ${\mathrm{KCu}}_4{\mathrm{P}}_3{\mathrm{O}}_{12}$ powder and compared the experimental results with those calculated for the spin model (an eight-spin zigzag chain with $S=\frac{1}{2}$) using the data-driven technique based on machine learning. We observed magnetic excitations at approximately 3.0, 4.1, 5.9, and 8.8 meV at 5.5 K and at approximately 3.8 and 5.9 meV at 49 K. The excitations corresponding to 3.0, 4.1, and 8.8 meV were magnetic excitations from the ground state to the first, second, and fourth excited states (2.87, 4.23, and 8.53 meV from the calculations), respectively. The excitations corresponding to 3.8 and 5.9 meV were magnetic excitations from the first excited state to the third and fourth excited states (3.78 and 5.67 meV from the calculations), respectively. An excitation was likely to exist between the first and second excited states at approximately 1.35 meV in the experimental results. The excitation energies obtained from the INS experiments were almost consistent with those calculated from the exchange interaction values via the data-driven technique (data-driven values). The experimental $I\left(Q\right)$ curves could not be reproduced. We found that $I\left(Q\right)$ curves could be changed largely by small changes of exchange-interaction values. Therefore, we expect that exchange-interaction values, which can explain not only the magnetic susceptibility, magnetization curves, and excitation energies but also INS intensity, are in the vicinity of the data-driven values.

Inducing superconductivity in systems with unconventional band structures is a promising approach for realizing unconventional superconductivity. Of particular interest are single-interface or Josephson junction architectures involving Weyl semimetals (WSM), which are predicted to host odd-parity, potentially topological, superconducting states. These expectations rely crucially on the tunneling of electronic states at the interface between the two systems. In this study, we revisit the question of induced superconductivity in an inversion-broken WSM via quantum tunneling, treating the interface as an effective potential barrier. We determine the conditions under which the gap function couples to the Weyl physics and its properties within the WSM. Our simulations show that the mismatch in the nature of the low-energy electronic states leads to a rapid decay of the superconductivity within the semimetal.

Understanding the effects of coexisting electron correlations and spin-orbit coupling on the electronic ground state is the crux of emergent quantum phenomena in complex oxide systems. Here, we explore the critical role of such host interactions on the semimetallic state in $\mathrm{CaIr}{\mathrm{O}}_3$ epitaxial thin film using steady-state and time-resolved terahertz (THz) spectroscopies, and dc magneto transport measurements. We observe a THz optical gap of ∼1.6 meV in the Dirac bands along with a theoretically predicted topological semimetallic phase. The ultrafast dynamics of this gapped semimetallic phase, as studied by an optical-pump THz-probe tool, reveal a gigantic change in transmission that is unusual for a metal but typical of an insulating phase. As observed, this photoinduced change in THz transmission in the semimetallic phase of $\mathrm{CaIr}{\mathrm{O}}_3$ (bandgap of a few meV) is of the same order as in the insulating phase of other correlated perovskites having a bandgap of a few eV. An extremely slow recovery time compared to that of metals and other Dirac semimetallic compounds is another anomalous feature. These observations along with anisotropy in THz optical conductivity and dc magnetoconductance along the in-plane [100] and [010] crystal axes, place $\mathrm{CaIr}{\mathrm{O}}_3$ in a new category. This unprecedented combination of ultrafast THz dynamical properties provides valuable insights into the unified control of electron correlations and strong spin-orbit coupling in $\mathrm{CaIr}{\mathrm{O}}_3$, along with new room-temperature ultrafast THz modulation functionalities.

${\mathrm{LiCu}}_3{\mathrm{O}}_3$ is a novel two-dimensional $S=1/2$ antiferromagnet with randomly depleted square lattice. The crystal structure contains two types of square planes with different ${\mathrm{Cu}}^{2+}\to {\mathrm{Li}}^{+}$ substitution rates (20% and 40%). ${}^7\mathrm{Li}$ NMR and magnetization measurements performed on single crystals of ${\mathrm{LiCu}}_3{\mathrm{O}}_3$ revealed the occurrence of magnetic order at $T_{c1}=123\mathrm{K}$ and the change of the magnetic state at $T_{c2}\approx 30\mathrm{K}$. The high-temperature transition can be attributed to establishment of magnetic order in planes with higher concentration of magnetic ions and the low-temperature transition to the magnetic ordering in planes with lower concentration of magnetic ions. Broad continuous NMR spectra below $T_{c1}$ reflect a continuous distribution of values or directions of magnetic moments in ${\mathrm{LiCu}}_3{\mathrm{O}}_3$ typical for spiral, spin-modulated magnetic structures or structures with frozen disorder. Magnetization measurements revealed a spin-flop transition which indicates weak uniaxial anisotropy of the spin structure. The relatively small magnetic susceptibility at all orientations of applied magnetic field shows that the magnetic structure is rigid since the estimated value of saturation field derived from differential susceptibility measured at ${\mu }_{0}H=5\mathrm{T}$ is ${\mu }_0{H}_{\mathrm{sat}}\approx 200\mathrm{T}$.

For a parameterized family of invertible states (short-range-entangled states) in $(1+1)$ dimensions, we discuss a generalization of the Berry phase. Using translationally invariant, infinite matrix product states (MPSs), we introduce a gerbe structure, a higher generalization of complex line bundles, as an underlying mathematical structure describing topological properties of a parameterized family of MPSs. Furthermore, we introduce a generalization of a quantum mechanical inner product, which we call the “triple inner product,” defined for three matrix product states. The triple inner product proves to extract a topological invariant, the Dixmier-Douady class over the parameter space.

We present a detailed study of the dielectric response of ZnO using a combination of low-loss momentum-resolved electron energy-loss spectroscopy (EELS) and first-principles calculations at several levels of theory, from the independent particle and the random phase approximation with different variants of density functional theory (DFT), including hybrid and $\mathrm{DFT}+U$ schemes; to the Bethe-Salpeter equation (BSE). We use a method based on the $f$-sum rule to obtain the momentum-resolved experimental loss function and absorption spectra from EELS measurements. We characterize the main features in the direct and inverse dielectric functions of ZnO and their dispersion, associating them to single-particle features in the electronic band structure, while highlighting the important role of many-body effects such as plasmons and excitons. We discuss different signatures of the high anisotropy in the response function of ZnO, including the symmetry of the excitonic wave functions.

Determining surface structures is a substantial challenge due to the limitations of experimental techniques and the complexity of theoretical models. In this work, we use a method called the exhaustive search of asymmetric unit (ESAU), designed to identify surface structures through a detailed analysis of the asymmetric unit. This method employs two main strategies: narrowing the research area by focusing on a smaller, unique segment rather than the entire unit cell, and transforming the infinite possibilities of atomic positions into discrete, manageable units. These strategies allow us to systematically enumerate all potential surface structures. Employing the ESAU method, we have successfully replicated a variety of known surface structures, including two-dimensional materials on substrates, and have also uncovered some previously unknown structures. Importantly, the ESAU method shows significant promise not only in utilizing experimental data but also in predicting surface and crystal structures without prior experimental evidence. Our results affirm that the ESAU approach provides a comprehensive and efficient tool for uncovering material structures, paving the way for more in-depth studies on the properties and behaviors of materials.

This study explores the impact of a strong perpendicular laser field on the electronic structure and optical conductivity of bilayer graphene. Employing the Floquet-Bloch theorem and a four-band Hamiltonian model, we calculate the optical conductivity, unveiling modified optical properties due to the altered band structure. We investigate the effects of both circularly and linearly polarized dressing fields on the electronic structure and optical conductivity in the system. Under linear polarization, we observe a notable anisotropy in the band dispersion and optical conductivity, resulting in linear dichroism. In the case of circular polarization, we anticipate the emergence of induced Berry curvature and circular dichroism, especially close to the dynamical gaps. When circularly polarized light is applied alongside a bias potential, the band structure differs for right- and left-handed polarization. In this case, the longitudinal optical conductivity remains the same for both, while the transversal optical conductivity exhibits distinct results. Furthermore, the induced Berry curvature and valley asymmetry introduce the potential for generating a valley-polarized current, enabling valley-selective pumping and leading to circular dichroism.

The design and fabrication of hyperbolic metamaterials require precise control over the arrangement and dimensions of constituent components, such as nanosheets or nanowires, to achieve the desired hyperbolic dispersion. However, the ongoing challenge lies in further downsizing these components to broaden the hyperbolic regime and enhance the maximal wave vector. In this study, we propose a linear quasi-one-dimensional electron gas array model as a promising category of type-I natural hyperbolic materials (NHMs). The hyperbolic properties in these NHMs remain insensitive to the position of the Fermi level, owing to the linear dispersion relation near the Fermi level. Through first-principles calculations, we have identified a highly promising candidate material, the $\mathrm{RuOC}{\mathrm{l}}_2$ crystal, for this model. Our research demonstrates that the $\mathrm{RuOC}{\mathrm{l}}_2$ crystal exhibits a broad type-I hyperbolic region that spans from the infrared to the entire visible spectrum, and this property is nearly independent of the Fermi level's position, or equivalently, the carrier density. We also investigate the exceptional negative refraction effects and directional-propagating surface plasmon polaritons that arises from the hyperbolic equifrequency contour. These findings offer a universal approach to designing type-I NHMs, as well as a compelling foundation for development of cutting-edge optoelectronic devices.

We present a symbolic algorithm for the fully analytic treatment of perturbative expansions of Hamiltonians with general two-body interactions. The method merges well-known analytics with the recently developed symbolic integration tool, algorithmic Matsubara integration, that allows for the evaluation of the imaginary frequency/time integrals. By symbolically constructing Wick contractions at each order of the perturbative expansion we order by order construct the fully analytic solution of the Green's function and self-energy expansions. A key component of this process is the assignment of momentum/frequency conserving labels for each contraction that motivates us to present a fully symbolic Fourier transform procedure which accomplishes this feat. These solutions can be applied to a broad class of quantum chemistry problems and are valid at arbitrary temperatures and on both the real- and Matsubara-frequency axis. To demonstrate the utility of this approach, we present results for simple molecular systems as well as model lattice Hamiltonians. We highlight the case of molecular problems where our results at each order are numerically exact with no stochastic uncertainty.

We consider an effective Hubbard model with spin- and direction-dependent complex hoppings $t$, applied to twisted homobilayer ${\mathrm{WSe}}_2$ using a variational Monte Carlo approach. The electronic correlations are taken into account by applying the Gutzwiller on-site correlator as well as long-range Jastrow correlators subjected to noninteracting part being of Pfaffian form. Our analysis shows the emergence of the Mott insulating state at the critical value of Hubbard interaction $U_{c1}\approx 6.5\left|t\right|\sim 7\left|t\right|$ estimated by extrapolating the density-density equal-time two-particle Green's functions. The signatures of an intermediate insulating phase between $U_{c1}$ and $U_{c2}\approx 9.5\left|t\right|\sim 10\left|t\right|$ are also discussed. Furthermore, we report the formation of the ${120}^{\circ }$ in-plane Néel state indicated by the detailed analysis of the spin-spin correlation functions. As shown, switching between antiferromagnetic phases characterized by opposite chirality could be experimentally realized by the change of perpendicular electric field. In a proper range of electric fields, also a transition to the in-plane ferromagnetic state appears.

Conventional spin-degenerate surface electrons have been effectively manipulated by means of self-organized nano-arrays. Of particular interest are one-dimensional, step superlattices at vicinal surfaces, because their fundamental behavior can be understood through simple model theory, and they can be utilized to imprint strong surface anisotropies in electron transport devices. In this work, the realization of periodic resonator arrays on the ${\mathrm{BiAg}}_2$ atom-thick surface alloy with atomic precision is presented, and their potential ability for tailoring the giant-split helical Rashba states is demonstrated. By employing curved crystals to select local vicinal planes, two sharply defined arrays of ${\mathrm{BiAg}}_2$ monoatomic steps with distinct spacing are fabricated, as experimentally determined from scanning tunneling microscopy and low-energy electron diffraction. The interaction of the Rashba helical states with the step arrays is assessed by scanning the photon beam across the ${\mathrm{BiAg}}_2$ curved surface in angle-resolved photoemission experiments and comparing these results with density functional theory. Remarkably, strong orbital-selective renormalization of bands perpendicular to the step superlattice, as well as spin mixing of Rashba bands, are induced by the coherent scattering of the periodic step potential. These results pave the way to fabricate atomically precise coupled arrays of electron resonators to engineer spin-orbital textures.

Scattering dynamics influences the graphene's transport properties and inhibits the charge carrier deterministic behavior. The intra/interband scattering mechanisms are vital for graphene's optical conductivity response under specific considerations of doping. In this study, we systematically explored the impact of scattering on optical conductivity using a semiclassical multiband Boltzmann equation, incorporating both electron-electron and electron-phonon interactions collectively with a single phenomenological relaxation time constant. We found unconventional characteristics of linear optical response with a significant deviation from the universal conductivity $({\mathit{e}}^{\mathit{2}}/4\hslash )$ in doped monolayer graphene. This is explained through phenomenological relaxation rates under low doping regime with dominant intraband scattering. Such novel optical responses vanish at high temperatures or overdoping conditions due to strong Drude behavior. With the aid of approximations around Dirac points we have developed analytical formalism for many-body interactions and are in good agreement with the Kubo approaches.

Technical advancements in oxide molecular beam epitaxy (MBE) have opened new avenues for studying various quantum transport phenomena in correlated transition-metal oxides, as exemplified by the exotic superconductivity of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and quantum oscillations of ${\mathrm{SrRuO}}_3$. On the other hand, film research of another Ruddlesden-Popper strontium ruthenate ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$, which exhibits a unique quantum phase related to metamagnetism in bulk systems, has not progressed well. Here, we report the fabrication of high-quality ${\mathrm{Sr}}_3{\mathrm{Ru}}_2{\mathrm{O}}_7$ thin films by oxide MBE and the observation of a strain-induced ferromagnetic ground state. The change in magnetic exchange coupling evaluated by first-principles calculations indicates a systematic relation between the compression of the $c$-axis length and induced ferromagnetism. Giant epitaxial strain in high-quality films will be a key to a comprehensive understanding of the magnetism in Ruddlesden-Popper strontium ruthenates ${\mathrm{Sr}}_{n+1}{\mathrm{Ru}}_n{\mathrm{O}}_{3n+1}$, which sensitively depends on the ratio of in-plane to out-of-plane Ru-Ru distances.