We study the Andreev reflections and the quantum transport in the proximitized graphene/superconductor junction. The proximitized graphene possesses the pseudospin staggered potential and the intrinsic spin-orbit coupling induced by substrate, which are responsible for the spin-valley dependent double Andreev reflections and the anomalous transport properties in the junction. The pure specular Andreev reflection can happen in the superconducting gap for the $K\uparrow$ and $K^{\prime }\downarrow$ electrons, while the pure retro-Andreev reflection happens for the $K\downarrow$ and $K^{\prime }\uparrow$ electrons. The coexisting two types of Andreev reflections related to the fixed spin-valley indices strongly depend on the chemical potential of the proximitized graphene. The condition of the emergence of the specific type of Andreev reflection for the electrons with the fixed spin-valley index is clarified. The spin-valley dependent Andreev reflections bring about the peculiar conductance spectra of the junction, which can help determine the values of the pseudospin staggered potential and the intrinsic spin-orbit coupling induced in graphene. Hence our research results not only provide an experimental method to detect the induced potential and coupling in graphene but also establish the foundation of the superconductor electronics based on the spin-valley indices.

We investigate the role of the Bloch functions and superconducting gap symmetries on the formation and properties of impurity-induced resonances in a two-dimensional superconductor, and elucidate their manifestation in scanning tunneling spectra. We use and extend a recently developed scattering approach, conveniently formulating the results in terms of the phase shifts of electron scattering off the impurity. We find that the discrete subgap states in a nodeless-gap superconductor are insensitive to the potential scattering phase shift (common for the two spin species) if time-reversal symmetry (TRS) is preserved. The independence of potential scattering is exact for $s$-wave superconductors. It remains an accurate approximation over a broad range of subgap energies when the gap function breaks the lattice point symmetry, except for a narrow region below the gap edge. Breaking of TRS makes potential scattering capable of creating spin-degenerate subgap states, which may be further split by spin-dependent scattering. In nodal-gap superconductors, impurity-induced resonances are broadened by coupling to the quasiparticle continuum. We identify the conditions allowing for the formation of narrow resonances. In addition to finding the energy spectrum, we evaluate the spin-resolved differential conductance for all of the considered symmetries and gap structures.

Resistive-switching-based memory is a popular research area for majorly neuromorphic, nonvolatile memory design and in-memory computing. ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x{\mathrm{MnO}}_{\text{3}}$ [$\mathrm{PCMO}(x$)] is one of the most explored perovskite materials for resistive random access memory (RRAM) switching based on electric-field-induced charged oxygen vacancy (${\mathrm{V}}_{\text{O}}^q$) migration. Thus $\mathrm{PCMO}(x=0,0.3,1)$ has been studied thoroughly on the experimental as well as theoretical front for resistive switching properties. However, the experimentally observed large variation ($\sim 33\%$) in activation energies for the ${\mathrm{V}}_{\text{O}}^q$ migration in these materials makes it difficult to understand the ${\mathrm{V}}_{\text{O}}^q$ migration in $\mathrm{PCMO}(x$)-based RRAM. In this paper, first-principles-based density functional theory (DFT) calculations are employed to calculate the stability and migration dynamics of the ${\mathrm{V}}_{\text{O}}^q$ as observed in our experiments performed for $\mathrm{PCMO}(x$). Taking into account the electronic and structural relaxation, the spin polarization, and the effect of strong electronic correlation, DFT calculations are carefully benchmarked to reproduce the experimental lattice parameters and electronic band gap for the end members, i.e., for ${\mathrm{PrMnO}}_{\text{3}}$ (PMO) and ${\mathrm{CaMnO}}_{\text{3}}$ (CMO). The defect formation energy calculated for a single ${\mathrm{V}}_{\text{O}}^q$ reveals the oxygen vacancies to be charged with ${\mathrm{V}}_{\text{O}}^{\text{+2}}$ broadly remaining stable in PMO and CMO. The nudged elastic band calculations are performed for migrating ${\mathrm{V}}_{\text{O}}^{\text{+2}}$, which reveal activation energies $E_{\text{mig,a}}$ of 0.96 and 0.83 eV for the migration of the charged oxygen vacancy in PMO and CMO, respectively, which is in the range of our experimentally observed $E_{\text{mig,a}}$. This provides much-needed insights for the fundamental understanding and modeling of the vacancy dynamics in PMO- and CMO-based RRAM devices.

We discuss a semiclassical approach to solve the quantum impurity model within nonequilibrium dynamical mean-field theory for electron-lattice models. The effect of electronic fluctuations on the phonon is kept beyond Ehrenfest dynamics, leading to a stochastic phonon evolution with damping and noise terms that are self-consistently determined by the electronic correlation functions in the fluctuating phonon field. Together with a solution of the electronic model based on a nonperturbative quantum Boltzmann equation, the approach can be used to address the coupled dynamics of the electrons and the lattice during photoinduced phase transitions. Results for the Anderson-Holstein model are benchmarked against numerically exact quantum Monte Carlo data. We find good agreement for the phonon distribution function at temperatures comparable to the charge ordering temperature. The general formulation can be extended to models with electron-electron interactions or multiorbital systems.

While local unitary transformations are used for identifying quantum states that are in the same topological class, nonlocal unitary transformations are also important for studying the transition between different topological classes. In particular, it is an important task to find suitable nonlocal transformations that systematically sweep different topological classes. Here, regarding the role of dimension in the topological classes, we introduce partially local unitary transformations, namely Greenberger-Horne-Zeilinger (GHZ) disentanglers, which reduce the dimension of the initial topological model by a layer-by-layer disentangling mechanism. We justify the importance of such a mechanism for characterizing topological phases by considering two important examples. To this end, we apply GHZ disentanglers to two-dimensional (2D) topological quantum codes and show that they are converted to many copies of Kitaev's ladders. It implies that the above disentanglers cause a transition from an intrinsic topological phase to a symmetry-protected topological phase. Then, we show that while Kitaev's ladders are building blocks of both color code and toric code, there are different patterns of entangling ladders in 2D color code and toric code. It shows that different topological features of these topological codes are reflected in different patterns of entangling ladders. In this regard, we propose that the layer-by-layer disentangling mechanism can be used as a systematic method for classification of topological orders based on finding different patterns of long-range entanglement in topological lattice models.

We study the linear, second-order nonlinear (NL) current and voltage responses of a two-dimensional gapped semi-Dirac system with merging Dirac nodes along the $x$ direction under the influence of a weak magnetic field $\left(B\right)$, using the semiclassical Boltzmann formalism. We investigate the effect of band geometric quantities such as Berry curvature and orbital magnetic moment in the responses up to linear order in $B$. We derive exact analytical expressions of the linear magnetoconductivities, second-harmonic NL anomalous Hall (NAH), and NL anomalous velocity and Lorentz force induced (NAL) conductivities, unveiling their dependence on Fermi energy and a gap parameter ${\delta }_0$. For ${\delta }_0>0$, the Fermi surface topology changes at a particular Fermi energy, which is reflected in the nature of conductivities through a kink. The ratio of NAL and NAH conductivities is found to be independent of ${\delta }_0$ and inversely related to Fermi energy. The NL dc current exhibits distinct orientations depending on the Fermi energy, magnetic field, and polarization of the electromagnetic wave. In the presence of magnetic field, the NL dc current vector can be rotated through large angles upon variation of Fermi energy. For high Fermi energies, the NL dc current is directed nearly along the $y$-axis for $x$-polarized and low-frequency circularly polarized light, whereas it aligns close to the $x$-axis for high-frequency circularly polarized light. These orientations of the NL dc current are predominantly governed by the mirror symmetry of the system along the $x$ direction. Additionally, we also study the NL voltage responses of the system by applying current along the $x$ and $y$ directions. The system exhibits asymmetry in the $B$-dependencies of the NL resistivities for the two current directions, which may serve as an experimentally relevant signature for band geometric quantities and merging Dirac nodes in such systems.

Diamond is a solid-state platform used to develop quantum technologies, but it has been a long-standing problem that the current understanding of quantum states of nitrogen vacancy (NV) centers in diamond is mostly limited to single-electron pictures. Here, we combine the full configuration interaction quantum Monte Carlo method and the density-matrix functional embedding theory to achieve an accurate description of the many-body quantum states of such defects in diamond. More than 30 electrons and 130 molecular orbitals are correlated, revealing the multiconfigurational nature of the wave functions of the many-body quantum states therein. Such a description explains puzzling experimental measurements in intersystem crossing and charge-state transitions at NV centers in diamond. The calculations not only reproduce the available experimental measurements of the energy gaps between quantum states but also provide benchmarks for states that are still subject to considerable uncertainty.

Motivated by the recent experimental detection of superconductivity in Bernal bilayer (AB) and rhombohedral trilayer (ABC) graphene, we study the emergence of superconductivity in multilayer graphene based on a Kohn-Luttinger (KL) -like mechanism in which the pairing glue is the screened Coulomb interaction. We find that electronic interactions alone can drive superconductivity in AB bilayer graphene and ABC trilayer graphene with the critical temperatures in good agreement with the experimentally observed ones, allowing us to further predict superconductivity from electronic interactions in Bernal ABA trilayer, ABAB tetralayer, and rhombohedral ABCA tetralayer graphene. By comparing the critical temperatures $\left(T_c\right)$ of these five nontwisted graphene stacks, we find that the ABC trilayer graphene possesses the highest $T_c\sim 100$ mK. After considering the enhancement of superconductivity due to Ising spin-orbit coupling, we observe that the AB bilayer graphene has the largest enhancement in the critical temperature, increasing from 23 to 143 mK. The superconducting behaviors in these nontwisted graphene stacks could be explained by the order parameters (OPs). The OPs of Bernal stacks preserve intravalley $C_3$ symmetry, whereas rhombohedral stacks break it. In all stacks, the OPs have zeros and change sign between valleys, which means that these multilayers of graphene are nodal spin-triplet superconductors. Moreover, dressing the purely electronic interaction with acoustic phonons, we observe minor changes of the critical temperatures in these five stacks. We adopt the KL-like mechanism to investigate the tendency of superconductivity in multilayer graphene without fitting parameters, which could provide guidance to future experiments exploring superconductivity in nontwisted graphene.

The discovery of chiral anomaly in Weyl semimetals, the nonconservation of chiral charge and energy across two opposite chirality Weyl nodes, has sparked immense interest in understanding its impact on various physical phenomena. Here, we demonstrate the existence of electrical, thermal, and gravitational quantum chiral anomalies in three-dimensional (3D) spin-orbit coupled systems. Notably, these anomalies involve chiral charge transfer across two Fermi surfaces linked to a single Weyl-type point, rather than across opposite chirality Weyl nodes as in Weyl semimetals. Our findings reveal that the Berry curvature flux piercing the Fermi surface plays a critical role in distinguishing the “chirality” of the carriers and the corresponding chiral charge and energy transfer. Importantly, we demonstrate that these quantum chiral anomalies lead to interesting thermal spin transport such as the spin Nernst effect. Our results suggest that 3D spin-orbit coupled metals offer a promising platform for investigating the interplay between quantum chiral anomalies and charge and spin transport in nonrelativistic systems.

We describe the construction of a wide-ranged multiphase equation of state for elemental iron, consisting of four solid phases and one fluid phase. The free-energy models for the phases are constrained by fitting to a broad swath of ambient, static, and dynamic high-pressure experimental data, including recent ramp-compression and shock–ramp recrystallization experiments. In order to better describe the conditions near Earth's inner-core boundary, ab initio electronic-structure calculations of various types have been carried out for the hcp and liquid phases and used as additional constraints. Given the variability of reported iron melting temperatures in the 1–3 Mbar range, we construct EOS model variations based on two separate $T_{\mathrm{melt}}\left(P\right)$ curves: lower-$T_{\mathrm{melt}}$ and higher-$T_{\mathrm{melt}}$. Our analysis shows that only the higher-$T_{\mathrm{melt}}\left(P\right)$ data are consistent with reported shock-melting pressure measurements. Furthermore, our examination of the recrystallization data from a recent shock–ramp x-ray diffraction study motivates a family of EOS models that address experimental uncertainties and provide a range of possible $T_{\mathrm{melt}}\left(P\right)$ curves that have a somewhat lower melt temperature above 5 Mbar than the one reported in that study. This family of multiphase EOS models allow us to suggest new experiments to further reduce the uncertainties on the high-pressure melt curve, and they should also enable more accurate predictions of complex high-$P,T$ processes involving iron. We show their potential utility by applying these EOSs to the investigation of properties of interest to planetary science, such as the vaporization entropy, critical temperature and density, Earth inner-core boundary temperature plus inner- and outer-core density deficits.

Using quantum Monte Carlo simulations, we study spin-1/2 diagonal ladders coupled by ferromagnetic Heisenberg interactions. The model can also be viewed as usual ladders with ferromagnetic rung couplings coupled by antiferromagnetic diagonal couplings. We show that the model hosts a striped magnetic ordered phase and two topological nontrivial Haldane phases separated by two quantum critical points. We also show that the two quantum critical points are all in the three-dimensional O(3) universality class irrelevant to the topological properties of the Haldane phases. The properties of the surface formed by ladder ends in the two Haldane phases are studied, showing that the surface states are both gapless due to the symmetry-protected topological bulk states. We further demonstrate that extraordinary surface critical behaviors occur at both critical points on such gapless surfaces without enhancing the surface coupling. Notably, the surface is not expected to be ordered in the three-dimensional classical O(3) critical point, suggesting that the topological properties of the Haldane phases are responsible for this surface critical behavior.

Neodymium nickelates have attracted research interest due to their strongly correlated behavior and remarkable magnetic properties. More importantly, superconductivity has recently been confirmed in thin-film samples of Sr-doped ${\mathrm{NdNiO}}_2$, bringing the layered rare earth nickel oxides into the research spotlight. In this report, we present results on a series of ${\mathrm{NdNiO}}_2$ analogs, ${\mathrm{Nd}}_x{\mathrm{Pr}}_{4-x}{\mathrm{Ni}}_3{\mathrm{O}}_8$ ($x=0.1$, 0.25, 1, 2, and 4) obtained by topotactic reduction, in which we observe systematic changes in the magnetic behavior. As the ${\mathrm{Nd}}^{3+}$ content increases, the initially large spin-freezing region with magnetic frustration becomes smaller and gradually shifts to low temperatures, while the magnetic response gradually increases. The muon-spin spectroscopy measurements on ${\mathrm{Nd}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ show that this phenomenon is likely due to the enhancement of spin fluctuations in ${\mathrm{Nd}}_x{\mathrm{Pr}}_{4-x}{\mathrm{Ni}}_3{\mathrm{O}}_8$, which weakens the spin frustration behavior for high ${\mathrm{Nd}}^{3+}$ contents and at low temperatures. These spin fluctuations can be caused by both Nd and Ni ions and could be one of the factors determining the occurrence of possible superconductivity.

We analyze the propagation of acoustic waves in a space-time (ST) modulated grating moving at constant velocity and surrounded by air. By means of asymptotic techniques, we derive in the subwavelength regime a homogenized nonreciprocal model in which the grating is replaced by an equivalent bianisotropic slab at the boundaries of which effective jump conditions apply, that encapsulate the effect of the evanescent fields. This effective framework allows to characterize analytically the properties of ST modulated metagratings in terms of scattering properties and guided wave dispersion. First we derive the closed-form dispersion relation of spoof surface plasmon polaritons (SPPs) and show the appearance of multiple redshifted or blueshifted branches due to the ST modulation. Next, we provide in the radiative region closed-form expressions for the Brewster angle and Fabry-Pérot resonances and show how the ST modulation heavily modifies the complex spectra. Finally, we illustrate the potential of such a system to achieve negative refraction or perfect transparency by playing on the modulation. Throughout the study, our analysis is validated by comparison with direct numerical simulations.

Intrinsic van der Waals layered magnets have attracted much attention. Herein, we carry out a comprehensive investigation of the air-stable two-dimensional van der Waals semiconducting magnet ${\mathrm{CrPS}}_4$ using first-principles calculations. With the first-principles linear response method and Monte Carlo simulations, we estimate the magnetic exchange constants and magnetic transition temperatures of both bulk and monolayer ${\mathrm{CrPS}}_4$. We find that the main intralayer interactions of bulk ${\mathrm{CrPS}}_4$ are all ferromagnetic coupling, while the dominant interlayer interactions of bulk ${\mathrm{CrPS}}_4$ are antiferromagnetic coupling. Using the calculated magnetic exchange constants, our Monte Carlo simulations show that monolayer ${\mathrm{CrPS}}_4$ has Curie temperature $T_C$ = 34 K. Moreover, we study the effect of uniaxial strain of monolayer ${\mathrm{CrPS}}_4$, and find that a uniaxial compressive strain greater than 6% along the $b$ axis would cause the magnetic ground state to change into an up-up-down-down magnetic order along the $b$ axis.

Photoinduced nonequilibrium states in the Mott insulators reflect the fundamental nature of competition between itinerancy and localization of the charge degrees of freedom. The spin degrees of freedom will also contribute to the competition in a different manner depending on lattice geometry. We investigate pulse-excited optical responses of a half-filled two-leg Hubbard ladder and compare them with those of a one-dimensional extended Hubbard chain. Calculating the time-dependent optical conductivity, we find that strong monocycle pulse inducing quantum tunneling gives rise to anomalous suppression of photo-induced in-gap weight, leading to negative weight. This is in contrast to finite positive weight in the Hubbard chain. The origin of this anomalous behavior in the two-leg ladder is attributed to photoinduced localized exciton that reflects strong spin-singlet dimer correlation in the ground state.

Tuning the band gap in ternary III-V semiconductors via modification of the composition or the strain in the material is a major approach for the design of optoelectronic materials. Experimental approaches screening a large range of possible target structures are hampered by the tremendous effort to optimize the material synthesis for every target structure. We present an approach based on density functional theory efficiently capable of providing the band gap as a function of composition and strain. Using a specific density functional designed for accurate band gap computation (TB09) together with a band unfolding procedure and special quasirandom structures, we develop a computational protocol to predict band gaps. The approach's accuracy is validated by comparison to selected experimental data. We thus map the band gap over the phase space of composition and strain (we call this the “band gap phase diagram”) for several important III-V compound semiconductors: GaAsP, GaAsN, GaPSb, GaAsSb, GaPBi, and GaAsBi. We show the application of these diagrams for identifying the most promising materials for device design. Furthermore, our computational protocol can easily be generalized to explore the vast chemical space of III-V materials with all other possible combinations of III and V elements.

The lack of the theory of the crystalline electric field (CEF) in rare-earth-based quasicrystal (QC) and approximant crystal (AC) has prevented us from understanding the electronic states. Recent success of the formulation of the CEF theory on the basis of the point charge model has made it possible to analyze the CEF microscopically. Here, by applying this formulation to the QC Au-SM-Dy ($\mathrm{SM}=\mathrm{Si}$, Ge, Al, and Ga) and AC, we theoretically analyze the CEF. In the ${\mathrm{Dy}}^{3+}$ ion with $4f^9$ configuration, the CEF Hamiltonian is diagonalized by the basis set for the total angular momentum $J=15/2$. The ratio of the valences of the screened ligand ions $\alpha =Z_{\mathrm{SM}}/Z_{\mathrm{Au}}$ plays an important role in characterizing the CEF ground state. For $0\le \alpha <0.30$, the magnetic easy axis for the CEF ground state is shown to be perpendicular to the mirror plane. On the other hand, for $\alpha >0.30$, the magnetic easy axis is shown to be lying in the mirror plane and as $\alpha$ increases, the easy axis rotates to the clockwise direction in the mirror plane at the Dy site and tends to approach the pseudo-fold-fold axis. Possible relevance of these results to experiments is discussed.

A degraded mobility of $5.2\times {10}^5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ but a long quantum scattering time of 2.3 ps at the hole density of $2.25\times {10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$ were obtained from a two-dimensional hole gas in a processed undoped Ge/SiGe heterostructure Hall-bar field-effect device. The heterostructure was grown by the reduced pressure chemical vapor deposition method and the device fabrications were compatible with well-established semiconductor technology. The percolation density of $0.69\times {10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$ indicated a very low disorder potential landscape experienced by holes in the strained Ge quantum well. In addition to integer quantum Hall effects (IQHEs) of consecutive filling factors from $\nu =8$ down to 1, we also observed the quantum states at filling factors between $\nu =1$ and 2, and between $\nu =2$ and 3 at 1.71 K. The observation of quantum Hall effects at the inferior mobility confirmed the mobility/quality dichotomy in the ultraclean undoped Ge/SiGe heterostructure. The study explicitly indicated that the device fabrication process likely compromised the transport mobility, whereas the quantum quality was less influenced, and established the heterostructure as an ideal platform for quantum device implementations.