Tensor network states form a powerful framework for both the analytical and numerical study of strongly correlated phases. Vital to their analytical utility is that they appear as the exact ground states of associated parent Hamiltonians, where canonical proof techniques guarantee a controlled ground space structure. Yet, while those Hamiltonians are local by construction, the known techniques often yield complex Hamiltonians which act on a rather large number of spins. In this paper, we present an algorithm to systematically simplify parent Hamiltonians, breaking them down into any given basis of elementary interaction terms. The underlying optimization problem is a semidefinite program, and thus the optimal solution can be found efficiently. Our method exploits a degree of freedom in the construction of parent Hamiltonians—the excitation spectrum of the local terms—over which it optimizes such as to obtain the best possible approximation. We benchmark our method on the AKLT model and the toric code model, where we show that the canonical parent Hamiltonians (acting on 3 or 4 and 12 sites, respectively) can be broken down to the known optimal two-body and four-body terms. We then apply our method to the paradigmatic resonating valence bond (RVB) model on the kagome lattice. Here, the simplest previously known parent Hamiltonian acts on all the 12 spins on one kagome star. With our optimization algorithm, we obtain a vastly simpler Hamiltonian: we find that the RVB model is the ground state of a parent Hamiltonian whose terms are all products of at most four Heisenberg interactions, and whose range can be further constrained, providing a major improvement over the previously known 12-body Hamiltonian.

Visual perception relies on light scattering at the object's surface in the direction of observation. By engineering the surface scattering properties, it is possible to realize arbitrary visual percepts. Here, we address theoretically this problem of electromagnetic field transition conditions at conformal interfaces to achieve surface-topography-dependent transmitted and reflected fields. Our analysis, supported by two- and three-dimensional finite-element simulations, provides a solid theoretical framework to design metasurfaces for cloaking, wearable optics, and next-generation freeform imaging systems.

Since the discovery of superconductivity in infinite-layer nickelates ${\mathrm{RNiO}}_2$ (R = La, Pr, Nd), great research efforts have been made to unveil its underlying superconducting mechanism. However, the physical origin of the intriguing hole-doped superconductivity phase diagram, characterized by a superconductivity dome sandwiched between two weak insulators, is still unclear. Here we present a microscopic theory for the electronic structure of nickelates from a fundamental model-based perspective. We found that the appearance of weak insulator phase in lightly and heavily hole-doped regime is dominated by Mottness and Hundness, respectively, exhibiting a unique orbital-selective doping originated from the competition of Hund interaction and crystal field splitting. Moreover, the superconducting phase can also be created in the “mixed” transition regime between Mott-insulator and Hund-induced correlated state, exactly reproducing the experimentally observed superconducting phase diagram. Our findings not only demonstrate the orbital-dependent strong-correlation physics in Ni $3d$ states but also provide a unified understanding of superconducting phase diagram in hole-doped infinite-layer nickelates, which are distinct from the well-established paradigms in cuprates and iron pnictides.

Ferrimagnetic nodal-line semiconductor ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$ exhibits exotic magnetotransport behavior, termed colossal angular magnetoresistance. Here, by investigating the pressure effects on the electronic and magnetic properties of single-crystal ${\mathrm{Mn}}_3{\mathrm{Si}}_2{\mathrm{Te}}_6$, we demonstrate a pressure-induced semiconductor-metal transition between 1.5 and 2.5 GPa as well as a possible structural transition/amorphization at ∼12 GPa. Both DC magnetization and density-functional theory calculation indicate the steady stabilization of ferrimagnetism upon compression up to at least 10 GPa, as evidenced by the monotonic increment of Curie temperature $T_{\mathrm{C}}$. However, the colossal magnetoresistance occurs only in the semiconducting state and is dramatically suppressed and eventually absent in the metallic state. These findings provide important insight into the microscopic interplay between non-trivial band topology and intrinsic magnetic order.

Recently, the hexagonal compound SrAgBi has been theoretically predicted to host linearly dispersing type-II and nonlinearly dispersing type-IV Dirac fermions near the Fermi level. Here, we report the results of magnetotransport measurements performed on SrAgBi single crystals in the temperature range 2–300 K, with electric current flowing within the crystallographic $ab$ plane and magnetic field up to 9 T oriented along the $c$ axis. The electrical resistivity shows a metallic behavior, and an onset of filamentary like, type-II superconductivity below 2.9 K. The transverse magnetoresistance is positive and very small (approximately 4% at 5 K and 9 T). In weak fields and at low temperatures, it shows the existence of weak antilocalization effect, which signifies the presumed Weyl semimetallic character of SrAgBi. The Hall measurements show holes to be major charge carriers in the entire temperature range investigated. At 5 K, their density equals $\sim 5.4\times {10}^{20}\mathrm{c}{\mathrm{m}}^{\text{–}3}$ and their mobility is $\sim 3.6\times {10}^2\mathrm{c}{\mathrm{m}}^2/\left(\mathrm{V}\mathrm{s}\right)$. Based on the collected magnetotransport data, SrAgBi can be categorized as an uncompensated semimetal with the electrical properties describable using the simplistic single-band approach. The experimental results, in conjunction with the theoretical calculations, suggest that the topological surface states in SrAgBi are overshadowed by the large hole pocket which governs the transport behavior in this material.

Spin-polarized electrons confined in low-dimensional structures are of high interest for spintronics applications. Here, we investigate the electronic structure of an ordered array of Bi monomer and dimer chains on the Ag(110) surface. By means of spin-resolved photoemission spectroscopy, we find Rashba-Bychkov split bands crossing the Fermi level with one-dimensional constant energy contours. These bands are up-spin polarized for positive wave vectors and down-spin polarized for negative wave vectors, at variance with the Rashba-Bychkov model that predicts a pair of states with opposite spin in each half of the surface Brillouin zone. Density functional theory shows that spin-selective hybridization with the Ag bulk bands originates this unconventional spin texture.

The broad tunability of the proximity exchange effect between transition-metal dichalcogenides (TMDCs) and chromium iodide (${\mathrm{CrI}}_3$) heterostructures offers intriguing possibilities for the use of TMDCs in two-dimensional magnetoelectrics. In this work, the influence of the twist angle and the gate electric field on the electric and thermal transport in a $\mathrm{TMDC}/{\mathrm{CrI}}_3$ junction is investigated using the Dirac-Bogoliubov-de Gennes equation. We show that significant amounts can be controlled by spin-splitting of band structures due to spin-orbit interaction, and that the exchange-splitting of bands arises from the proximity effect. The property of the Andreev reflection (AR) process is highly dependent on the spin valley polarized states due to spin-orbit coupling. Remarkably, perfect spin valley polarized AR is possible over a wide bias range by using a gate voltage to tune the local Fermi energy and varying the type of charge doping. The proposed structure with $p$-type doping is found to have larger spin valley polarized Andreev conductance and high thermal conductance. We further show that, depending on the TMDC material and chemical potential of the $\mathrm{TMDC}/{\mathrm{CrI}}_3$ layer, twisting can lead to suppression or a significant increase in Andreev conductance as well as enhancement of thermal conductance for chemical potentials smaller than that of the superconducting regime.

Mean-field theories have since long predicted edge magnetism in graphene nanoribbons, where the order parameter is given by the local magnetization. However, signatures of edge magnetism appear elusive in the experiments, suggesting another class of solutions. We employ a self-consistent mean-field approximation within a multi-orbital tight-binding model and obtain particle-hole symmetry broken solutions, where the local filling plays the role of the order parameter. Unlike the magnetic edge solutions, these are topologically nontrivial and show zero local magnetization. A small and a large doping regime are studied, and a free energy minimum for finite hole doping is encountered, which may serve as an explanation for the absence of experimental evidence for magnetic edge states in zigzag graphene nanoribbons. The electronic interaction may increase the finite $d$-orbital occupation, which leads to a change of the effective Coulomb interaction of the dominant $p_z$ orbitals. Our findings indicate that the nonmagnetic solution for finite hole doping becomes energetically preferred, compared to the magnetic phases at half filling, once thermal fluctuations or unintentional doping from the substrate are considered. This result persists even in the presence of the $d$ orbitals and the Coulomb interaction therein.

We study the field-temperature phase diagram of the two-dimensional dipolar Ising ferromagnet on a kagome lattice with a specific ratio between the exchange and dipolar constants, $\delta =1$. Using the stochastic cutoff $O\left(N\right)$ Monte Carlo method, we calculated order parameters for stripe and bubble phases and other thermodynamical quantities. We find two kinds of stripe phases at low fields, where the arrangement of the branch spins neighboring the stripe frame varies, and two bubble phases at high fields, in which three-spin domains (bubbles) form a regular triangular lattice but the triangular array of bubbles changes on the kagome lattice. We also find that with increasing field, a disordered phase exists between the stripe and bubble phases and between the two bubble phases. We discuss the details of the features of these phases and phase transitions.

We investigate pairing and superconductivity in the attractive Fermi-Hubbard model on the one-dimensional sawtooth lattice, which exhibits a flat band by fine-tuning the hopping rates. We first solve the two-body problem, both analytically and numerically, to extract the binding energy and the effective mass of the pairs. Based on the density matrix renormalization group method, we address the ground-state properties of the many-body system, assuming equal spin populations. We compare our results with those available for a linear chain, where the model is integrable by the Bethe ansatz, and we show that the multiband nature of the system substantially modifies the physics of the BCS-BEC crossover. We find that near a flat band, the chemical potential remains always close to its zero-density limit predicted by two-body physics. In contrast, the pairing gap exhibits a remarkably strong density dependence and, differently from the pair binding energy, it is no longer peaked at the flat-band point. We show that these results can be interpreted in terms of polarization screening effects, due to an anomalous attraction between pairs in the medium and single fermions. Importantly, we unveil that three-body bound states (trimers) exist in the sawtooth lattice, in sharp contrast with the linear chain geometry, and we compute their binding energy. The nature of these states is investigated via a strong-coupling variational approach, revealing that they originate from tunneling-induced exchange processes.

We demonstrate the locking of magnetic precession in the ${\phi }_0$ Josephson junction by external electromagnetic radiation through the locking of the Josephson oscillations in the ferromagnetic resonance region. This leads to a step in the dependence of the magnetization on the bias current. The step's position is determined by the radiation frequency and the shape of the resonance curve. In junctions with a strong spin-orbit coupling, states with negative differential resistance appear on the IV characteristic, resulting in an additional locking step. We show that the corresponding oscillations have the same frequency as the oscillations at the first step, but they have a different amplitude and a different dependence on the radiation frequency. This makes it possible to control not only the frequency but also the amplitude of the magnetic precession in the locking region. It opens up unique perspectives for the control and manipulation of magnetic moment in hybrid superconducting systems.

We report on the observation of superconductivity in $\mathrm{La}{\mathrm{Rh}}_2{\mathrm{As}}_2$, which is the analog without $f$ electrons of the heavy-fermion system with two superconducting phases $\mathrm{Ce}{\mathrm{Rh}}_2{\mathrm{As}}_2$ [S. Khim et al., Science 373, 1012 (2021)]. A zero-resistivity transition, a specific-heat jump, and a drop in magnetic ac susceptibility consistently point to a superconducting transition at a temperature of $T_c=0.28$ K. The magnetic-field temperature superconducting phase diagrams determined from field-dependent ac-susceptibility measurements reveal small upper critical fields ${\mu }_0{H}_{c2}\approx 12$ mT for $H\parallel ab$ and ${\mu }_0{H}_{c2}\approx 9$ mT for $H\parallel c$. The observed $H_{c2}$ is larger than the estimated thermodynamic critical-field $H_c$ derived from the heat-capacity data, suggesting that $\mathrm{La}{\mathrm{Rh}}_2{\mathrm{As}}_2$ is a type-II superconductor with Ginzburg-Landau parameters ${\kappa }_{GL}^{ab}\approx 1.9$ and ${\kappa }_{GL}^c\approx 2.7$. The microscopic Eliashberg theory indicates superconductivity to be in the weak-coupling regime with an electron-phonon-coupling constant ${\lambda }_{e-ph}\approx 0.4$. Despite a similar $T_c$ and the same crystal structure as the Ce compound, $\mathrm{La}{\mathrm{Rh}}_2{\mathrm{As}}_2$ displays conventional superconductivity, corroborating the substantial role of the $4f$ electrons for the extraordinary superconducting state in $\mathrm{Ce}{\mathrm{Rh}}_2{\mathrm{As}}_2$.

Silica, a main mineral inside the Earth and super-Earth, is generally assumed to be resistant to react with noble gas elements at ambient conditions. Here, combining crystal structural prediction and first-principles calculation, we predict a $\mathrm{Si}{\mathrm{O}}_2\mathrm{He}$ compound that becomes stable at high pressure. Each Si (He) atom in the $\mathrm{Si}{\mathrm{O}}_2\mathrm{He}$ is bonded with seven O atoms and two He (Si) atoms, forming a Si-centered $\mathrm{Si}{\mathrm{O}}_7{\mathrm{He}}_2$ (He-centered $\mathrm{He}{\mathrm{O}}_7{\mathrm{Si}}_2$) polyhedron. Further calculations indicate that the $\mathrm{Si}{\mathrm{O}}_2\mathrm{He}$ compound remains solid over a wide range of pressures exceeding 607 GPa and temperatures of 0–9000 K, covering the extreme conditions of the core-mantle boundary in super-Earth exoplanets or even in the ice giants of our solar system. Our results may provide theoretical guidance for the discovery of other silicides at high pressures, which promote the exploration of materials at planetary core-mantle boundaries, and enable planetary models to be refined as well.

Despite recent interest in Bi-containing layered-perovskite ferroelectrics due to high $T_c$, their rational design is constrained by a poor understanding of local atomic structures. Here, we use neutron total scattering and first-principles theory to examine the local atomic structure in Dion-Jacobson layered-perovskite ferroelectric $\mathrm{CsBi}{\mathrm{Nb}}_2{\mathrm{O}}_7$. We show that the instantaneous short-range atomic structure can be described by a Debye-Einstein model that accommodates atomic motions due to acoustic and low-energy optic phonon modes. We determine structural distortions due to localized atomic dynamics with extremely high resolution. These are in addition to ground-state relative ionic displacements. The magnitude of dynamic variations in ionic positions points toward strong electron-phonon coupling in this material.

Two-level systems are at the heart of quantum information and quantum computation based on qubits. The manipulation of Zeeman-type energy-level splittings is crucial for utilizing two-level systems hosted by electronic spin or pseudospin in crystalline materials. Of particular interest are Zeeman-type energy level splittings controlled by the electric field, most of which were realized in materials involving both spin and nonspin degrees of freedom that are entangled strongly via spin-orbital interaction. Here, we provide a strategy enabling the electric-field control of Zeeman-type energy-level splitting rooted in the nonspin degree of freedom of electrons—essentially from sublattice/atomic orbitals—in materials containing nonsymmorphic symmetry elements (e.g., glide plane or screw axis). The physical origin of such a Zeeman-type splitting is revealed as the breakdown of the specific nonsymmorphic symmetry elements by electric field. We further propose, via first-principles simulations, a platform system of $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_3$ under tensile strain (ferroelectric compound) that accommodates a large Zeeman-type energy-level splitting of $\sim 124\mathrm{meV}$, controllable by the electric field.

The pseudogap regime of the underdoped cuprates arguably remains one of the most enigmatic phenomena of correlated quantum matter. Recent theoretical ideas suggest that a pair density wave (PDW) or a “fractionalized PDW” could be a key ingredient for the understanding of the pseudogap physics. These ideas are to be contrasted to the scenario where charge density wave order and superconductivity coexist at low temperatures. In this paper, we present a few tests to compare the two scenarios in a Josephson junction setup. For a PDW scenario, we observe a beatlike structure of AC Josephson current. The additional frequencies for the AC Josephson current appear at the half-odd integer multiple of the standard Josephson frequency. We can extract the modulation wave vector of the PDW state by studying the average Josephson current. Furthermore, the usual sharp Shapiro steps break down. In contrast, these signatures are absent for the simple coexistence of orders. Any detection of such signatures in a similar experimental setup will strongly support the PDW scenario for the pseudogap phase.

We systematically investigate the topological properties of spin polarized Rydberg-dressed fermionic atoms loaded in a bilayer optical lattice. Through tuning the Rydberg coupling strength and the interlayer tunneling amplitude, we identify different types of topological superfluid states generated from the interlayer pairing and relative pairing phase modulation of the coupled two-dimensional $p$-wave superfluids. These phases include gapped/gapless with/without time-reversal symmetry. One of the most interesting states is a gapless paired topological superfluid with both the time-reversal symmetry and particle-hole symmetry, which is a realization of a DIII gapless topological superconductor. The flexibility of experimental manipulation in such Rydberg-dressed fermionic systems therefore becomes a promising system for realizing interesting topological superfluids.

We present the $O\left({\lambda }^3\right)$ results from the $\lambda$ expansion in the extremely correlated Fermi-liquid theory applied to the infinite-dimensional $t-J$ model (with $J=0$) and compare the results with the earlier $O\left({\lambda }^2\right)$ results as well as the results from the dynamical mean-field theory. We focus attention on the $T$ dependence of the resistivity $\rho \left(T\right)$, the Dyson self-energy, and the quasiparticle weight $Z$ at various densities. The comparison shows that all the methods display quadratic-in-$T$ resistivity followed by a quasilinear-in-$T$ resistivity characterizing a strange metal and gives an estimate of the different scales of these variables relative to the exact results.

A series of III-V digital alloy avalanche photodiodes have been recently seen to exhibit very low excess noise. These alloys have low hole ionization coefficients due to enhanced effective mass, a large separation between light-hole and split-off bands in the valence band, and in one case (InAlAs), potentially the creation of a small minigap in computed band structures. Whereas such minigaps would indeed provide a reliable way to suppress hole transport and reduce excess noise, their physical origin is explored here. In this paper, we provide an explanation for the formation of the minigaps as arising from oscillations in certain orbital overlaps. We demonstrate that decreasing the substrate lattice constant would increase the minigap size and mass in the transport direction. This leads to reduced quantum tunneling and phonon scattering of the holes. Finally, we illustrate the band-structure modification with substrate lattice constant for other III-V digital alloys. We, thus, provide a recipe for deterministic engineering of sizable valence-band minigaps as a potential recipe for high gain avalanche photodiodes.

The recent observation of superconductivity in infinite-layer nickelates has brought intense debate on the established knowledge of unconventional superconductivity based on the cuprates. Despite many similarities, the nickelates differ from the cuprates in many characteristics, the most notable of which is the magnetism. Instead of a canonical antiferromagnetic (AFM) Mott insulator as the undoped cuprates, from which the superconductivity is generally believed to arise upon doping, the undoped nickelates show no sign of magnetic ordering in experiments. Through a combined density functional theory, dynamical mean-field theory, and model study, we show that although the increased energy splitting between O-$p$ orbital and Cu/Ni-$d$ orbital (${\mathrm{\Delta }}_{dp}$) results in a larger magnetic moment in nickelates, it also leads to stronger AFM/ferromagnetism competition and weaker magnetic exchange coupling. Meanwhile, the self-doping effect caused by Nd-$d$ orbital screens the magnetic moment of Ni. The Janus-faced effect of ${\mathrm{\Delta }}_{dp}$ and self-doping effect together give a systematic understanding of magnetic behavior in nickelates and explain recent experimental observations.