# Showing papers in "Physical Review B in 2020"

••

TL;DR: In this article, the authors present a theoretical framework to understand collective effects in the dynamics of quantum entanglement and information, using the tools of statistical mechanics, in order to identify a measurement-induced phase transition in the information content of the system.

Abstract: The interplay of entanglement, measurement, and noise in near-term quantum devices may lead to novel emergent phenomena. This work presents a theoretical framework to understand collective effects in the dynamics of quantum entanglement and information, using the tools of statistical mechanics. The new effective description of generic quantum circuits lends insight into a measurement-induced phase transition in the information content of the system and points toward novel schemes to identify this transition in experiments.

302 citations

••

TL;DR: In this paper, it was shown that two conservation laws are sufficient to break ergodicity by "shattering" Hilbert space into exponentially many dynamically disconnected sectors, which represents a new paradigm for localization, which does not rely on disorder or energy conservation.

Abstract: Known routes to ergodicity breaking -- integrability and many-body localization -- require extensively many explicit or emergent conservation laws. Here we show that two conservation laws are sufficient to provably and robustly break ergodicity by ``shattering'' Hilbert space into exponentially many dynamically disconnected sectors. This represents a new paradigm for localization, which does not rely on disorder or energy conservation, which works in any number of spatial dimensions, and which should be realizable in near-term ultracold atom experiments.

300 citations

••

TL;DR: In this paper, the authors propose a theory for the area-law to volume-law entanglement transition in many-body systems that undergo both random unitary evolutions and projective measurements.

Abstract: A new class of quantum entanglement transitions separating phases with different entanglement entropy scaling has been observed in recent numerical studies. Despite the numerical efforts, an analytical understanding of such transitions has remained elusive. Here, the authors propose a theory for the area-law to volume-law entanglement transition in many-body systems that undergo both random unitary evolutions and projective measurements. Using the replica method, the authors map analytically this entanglement transition to an ordering transition in a classical statistical mechanics model. They derive the general entanglement scaling properties at the transition and show a solvable limit where this transition can be mapped onto two-dimensional percolation.

285 citations

••

TL;DR: In this article, the parent compound of the newly discovered superconducting nickelate Nd${}_{1\ensuremath{-}x}$Sr${}$NiO${}{2}$ is a self-doped Mott insulator, in which the low-density Nd $5d$ conduction electrons couple to localized Ni 3${d}_{{x}^{2}\ensureMath{-}{y}€2}}$ electrons to form Kondo spin singlets at low temperature.

Abstract: Motivated by analyses of the reported electrical resistivity and Hall coefficient data in the normal state, the authors propose that the parent compound of the newly discovered superconducting nickelate Nd${}_{1\ensuremath{-}x}$Sr${}_{x}$NiO${}_{2}$ is a self-doped Mott insulator, in which the low-density Nd $5d$ conduction electrons couple to localized Ni 3${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ electrons to form Kondo spin singlets at low temperature. The antiferromagnetic long-range order is suppressed and an insulator-metal transition results.

177 citations

••

TL;DR: In this paper, higher-order versions of the non-Hermitian skin effect have been discovered in two-dimensional systems with the system size L×L, where the conventional (first-order) skin effect accompanies O(L2) skin modes, while the second-order skin effect accompanied O (L) corner skin modes.

Abstract: The non-Hermitian skin effect is a unique feature of non-Hermitian systems, in which an extensive number of boundary modes appear under the open boundary conditions. Here, we discover higher-order counterparts of the non-Hermitian skin effect that exhibit new boundary physics. In two-dimensional systems with the system size L×L, while the conventional (first-order) skin effect accompanies O(L2) skin modes, the second-order skin effect accompanies O(L) corner skin modes. This also contrasts with Hermitian second-order topological insulators, in which only O(1) corner zero modes appear. Moreover, for the third-order skin effect in three dimensions, O(L) corner skin modes appear from all O(L3) modes. We demonstrate that the higher-order skin effect originates from intrinsic non-Hermitian topology protected by spatial symmetry. We also show that it accompanies the modification of the non-Bloch band theory in higher dimensions.

166 citations

••

TL;DR: In this article, the authors numerically study the phase transition of Haar-random quantum circuits in 1 + 1 dimensions and extract estimates for the associated bulk critical exponents that are consistent with the values for percolation, as well as those for stabilizer circuits, but differ from previous estimates.

Abstract: We numerically study the measurement-driven quantum phase transition of Haar-random quantum circuits in $1+1$ dimensions. By analyzing the tripartite mutual information we are able to make a precise estimate of the critical measurement rate ${p}_{c}=0.17(1)$. We extract estimates for the associated bulk critical exponents that are consistent with the values for percolation, as well as those for stabilizer circuits, but differ from previous estimates for the Haar-random case. Our estimates of the surface order parameter exponent appear different from those for stabilizer circuits or percolation, but we cannot definitively rule out the scenario where all exponents in the three cases match. Moreover, in the Haar case the prefactor for the entanglement entropies ${S}_{n}$ depends strongly on the R\'enyi index $n$; for stabilizer circuits and percolation this dependence is absent. Results on stabilizer circuits are used to guide our study and identify measures with weak finite-size effects. We discuss how our numerical estimates constrain theories of the transition.

164 citations

••

TL;DR: In this paper, the authors performed a comprehensive theoretical analysis at weak and strong coupling of infinite-layer Nd-doped NiO$ nickelates and found that they form a $d$-wave superconductor with three-dimensional fermiological features.

Abstract: Cuprate superconductors have shaped the contemporary state of condensed matter physics. Recently, infinite-layer Nd-doped NiO${}_{2}$ nickelates have initiated a new era of unconventional superconductivity, of which this paper constitutes the first comprehensive theoretical analysis at weak and strong coupling. From a combined perspective of $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ studies, random phase approximation, and $t$-$J$ model analysis, the authors find that the infinite-layer nickelates form a $d$-wave superconductor with three-dimensional fermiological features. Several experiments are proposed to confirm these theoretical predictions.

162 citations

••

Iowa State University

^{1}, Ames Laboratory^{2}, Oak Ridge National Laboratory^{3}, University of Tennessee^{4}TL;DR: In this article, the authors demonstrate that the Dirac surface state remains gapless well below the N ¼ eel temperature, due to the complexity of the magnetic ordering at the surface.

Abstract: A topological insulator can be converted to an axion insulator when time reversal symmetry is broken, which leads to the opening of a gap in the Dirac surface state. Recently, MnBi${}_{2}$Te${}_{4}$ was predicted to host such a state upon antiferromagnetic ordering. Collecting and analyzing very detailed photoemission data, the authors demonstrate that this does not occur and that the Dirac surface state remains gapless well below the N\'eel temperature. Most likely, this is due to the complexity of the magnetic ordering at the surface. The quest for the discovery of an axion insulator is, therefore, still on.

152 citations

••

TL;DR: In this article, the correlated electronic structure of infinite-layer compounds with finite hole doping was compared from an advanced first-principles many-body perspective, showing that the self-doped nickelate remains non-insulating even for large interaction strength.

Abstract: The correlated electronic structure of the infinite-layer compounds ${\mathrm{NdNiO}}_{2}$ and ${\mathrm{SrCuO}}_{2}$ at stoichiometry and with finite hole doping is compared. Key differences are elucidated from an advanced first-principles many-body perspective. Contrary to the charge-transfer insulating cuprate, the self-doped nickelate remains noninsulating even for large interaction strength, though the Ni-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ spectral weight is also gapped in that limit. Hybridization between $\mathrm{Ni}(3d)$ and $\mathrm{Nd}(5d)$ is crucial for the appearance of the self-doping band. Upon realistic hole doping, ${\mathrm{Sr}}_{1\ensuremath{-}y}{\mathrm{CuO}}_{2}$ shows the expected mixed oxygen-Cu-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ (Zhang-Rice) states at low energy. In the case of ${\mathrm{Nd}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{2}$, the self-doping band is shifted to higher energies and a doping-dependent ${d}_{{z}^{2}}$-versus-${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ competition on Ni is revealed. The absence of prominent Zhang-Rice physics in infinite-layer nickelates might be relevant to understand the notable difference in the superconducting ${T}_{\mathrm{c}}$'s.

152 citations

••

TL;DR: In this article, a one-dimensional non-Hermitian Aubry-Andr\'e-Harper (AAH) model with imaginary periodic or quasiperiodic modulations is studied.

Abstract: Topological phases have recently witnessed rapid progress in non-Hermitian systems. Here we study a one-dimensional non-Hermitian Aubry-Andr\'e-Harper (AAH) model with imaginary periodic or quasiperiodic modulations. We demonstrate that the non-Hermitian off-diagonal AAH models can host zero-energy modes at the edges. In contrast to the Hermitian case, the zero-energy mode can be localized only at one edge. Such a topological phase corresponds to the existence of a quarter winding number defined by eigenenergy in momentum space. We further find the coexistence of a zero-energy mode located only at one edge and topological nonzero-energy edge modes characterized by a generalized Bott index. In the incommensurate case, a topological non-Hermitian quasicrystal is predicted where all bulk states and two topological edge states are localized at one edge. Such topological edge modes are protected by the generalized Bott index. Finally, we propose an experimental scheme to realize these non-Hermitian models in electric circuits. Our findings add another direction for exploring topological properties in Aubry-Andr\'e-Harper models.

152 citations

••

TL;DR: In this article, the authors considered the class of dual-unitary quantum circuits in 1 + 1 dimensions and introduced a notion of "solvable" matrix product states (MPSs), defined by a specific condition which allows us to tackle their time evolution analytically.

Abstract: We consider the class of dual-unitary quantum circuits in 1 + 1 dimensions and introduce a notion of “solvable” matrix product states (MPSs), defined by a specific condition which allows us to tackle their time evolution analytically. We provide a classification of the latter, showing that they include certain MPSs of arbitrary bond dimension, and study analytically different aspects of their dynamics. For these initial states, we show that while any subsystem of size l reaches infinite temperature after a time t ∝ l, irrespective of the presence of conserved quantities, the light cone of two-point correlation functions displays qualitatively different features depending on the ergodicity of the quantum circuit, defined by the behavior of infinite-temperature dynamical correlation functions. Furthermore, we study the entanglement spreading from such solvable initial states, providing a closed formula for the time evolution of the entanglement entropy of a connected block. This generalizes recent results obtained in the context of the self-dual kicked Ising model. By comparison, we also consider a family of nonsolvable initial mixed states depending on one real parameter β, which, as β is varied from zero to infinity, interpolate between the infinite-temperature density matrix and arbitrary initial pure product states. We study analytically their dynamics for small values of β, and highlight the differences from the case of solvable MPSs.

••

TL;DR: In this article, the Coulomb interaction was used to study the ground states of twisted bilayer graphene with a Hartree-Fock approximation, and the results provided good reference points for further study of the rich correlated physics in the twisted bilayers graphene.

Abstract: Motivated by the recently observed insulating states in twisted bilayer graphene, we study the nature of the correlated insulating phases of the twisted bilayer graphene at commensurate filling fractions. We use the continuum model and project the Coulomb interaction onto the flat bands to study the ground states by using a Hartree-Fock approximation. In the absence of the hexagonal boron nitride substrate, the ground states are the intervalley coherence states at charge neutrality (filling $\ensuremath{
u}=0$, or four electrons per moir\'e cell) and at $\ensuremath{
u}=\ensuremath{-}1/4$ and $\ensuremath{-}1/2$ (three and two electrons per cell, respectively) and the ${C}_{2}\mathcal{T}$ symmetry-broken state at $\ensuremath{
u}=\ensuremath{-}3/4$ (one electron per cell). The hexagonal boron nitride substrate drives the ground states at all $\ensuremath{
u}$ into ${C}_{2}\mathcal{T}$ symmetry broken-states. Our results provide good reference points for further study of the rich correlated physics in the twisted bilayer graphene.

••

TL;DR: In this article, it was shown that Ni-Ni exchange coupling can cause spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of the Ni and Ni exchange coupling.

Abstract: Atomic $4f$ states have been found to be essential players in the physical behavior of lanthanide compounds, at the Fermi level ${E}_{F}$ as in the proposed topological Kondo insulator ${\mathrm{SmB}}_{6}$, or further away as in the magnetic superconductor system $R{\mathrm{Ni}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ ($R$ = rare-earth ion) and in ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pr}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$, where the $4f$ shell of Pr has a devastating effect on superconductivity. In hole-doped $R{\mathrm{NiO}}_{2}$, the $R$ = Nd member is found to be superconducting while $R$ = La is not, in spite of the calculated electronic structures being nearly identical. We report first-principles results that indicate that the Nd $4f$ moment affects states at ${E}_{F}$ in infinite-layer ${\mathrm{NdNiO}}_{2}$, an effect that will not occur for ${\mathrm{LaNiO}}_{2}$. Treating 20% hole doping in the virtual crystal approach indicates that 0.15 holes empty the $\mathrm{\ensuremath{\Gamma}}$-centered Nd-derived electron pocket while leaving the other electron pocket unchanged; hence Ni only absorbs 0.05 holes; the La counterpart would behave similarly. However, coupling of $4f$ states to the electron pockets at ${E}_{F}$ arises through the Nd intra-atomic $4f\text{\ensuremath{-}}5d$ exchange coupling $K\ensuremath{\approx}0.5\mathrm{eV}$ and is ferromagnetic (FM), i.e., anti-Kondo, in sign. This interaction causes spin-disorder broadening of the electron pockets and should be included in models of the normal and superconducting states of ${\mathrm{Nd}}_{0.8}{\mathrm{Sr}}_{0.2}{\mathrm{NiO}}_{2}$. The Ni moments differ by $0.2{\ensuremath{\mu}}_{B}$ for FM and antiferromagnetic alignment (the latter are larger), reflecting some itineracy and indicating that Heisenberg coupling of the moments may not provide a quantitative modeling of Ni-Ni exchange coupling.

••

University of Arkansas

^{1}, Fudan University^{2}, Hefei Normal University^{3}, Nanjing University^{4}TL;DR: In this article, the Dzyaloshinskii-Moriya interactions are used to identify strong enough DMIs, which leads to not only helical cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in the Janus monolayers, and the magnetic-field induced bimerons in the top-layer Br or Cl atoms.

Abstract: Topological magnetic states are promising for ultradense memory and logic devices. Recent progress in two-dimensional magnets encourages the idea to realize topological states, such as skyrmions and merons, in freestanding monolayers. However, monolayers such as ${\mathrm{CrI}}_{3}$ lack Dzyaloshinskii-Moriya interactions (DMIs) and thus do not naturally exhibit skyrmions/merons but rather a ferromagnetic state. Here we propose the fabrication of $\mathrm{Cr}{(\mathrm{I},X)}_{3}$ Janus monolayers, in which the Cr atoms are covalently bonded to the underlying I ions and top-layer Br or Cl atoms. By performing first-principles calculations and Monte Carlo simulations, we identify strong enough DMIs, which leads to not only helical cycloid phases, but also to topologically nontrivial states, such as the intrinsic domain wall skyrmions in $\mathrm{Cr}{(\mathrm{I},\mathrm{Br})}_{3}$ and the magnetic-field-induced bimerons in $\mathrm{Cr}{(\mathrm{I},\mathrm{Cl})}_{3}$. Microscopic origins of such spin textures are revealed as well.

••

TL;DR: In this article, the authors demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY (X/Y = S, Se, Te, X ≠ Y) in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry.

Abstract: The Dzyaloshinskii-Moriya interaction (DMI), which only exists in noncentrosymmetric systems, is responsible for the formation of exotic chiral magnetic states. The absence of DMI in most two-dimensional (2D) magnetic materials is due to their intrinsic inversion symmetry. Here, using first-principles calculations, we demonstrate that significant DMI can be obtained in a series of Janus monolayers of manganese dichalcogenides MnXY (X/Y = S, Se, Te, X ≠ Y) in which the difference between X and Y on the opposites sides of Mn breaks the inversion symmetry. In particular, the DMI amplitudes of MnSeTe and MnSTe are comparable to those of state-of-the-art ferromagnet/heavy metal (FM/HM) heterostructures. In addition, by performing Monte Carlo simulations, we find that at low temperatures the ground states of the MnSeTe and MnSTe monolayers can transform from ferromagnetic states with worm-like magnetic domains into the skyrmion states by applying external magnetic field. At increasing temperature, the skyrmion states starts fluctuating above 50 K before an evolution to a completely disordered structure at higher temperature. The present results pave the way for new device concepts utilizing chiral magnetic structures in specially designed 2D ferromagnetic materials.

••

TL;DR: In this article, the effect of Hund coupling and crystal field splitting in a simple model system was considered and it was shown that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime.

Abstract: Superconductivity has recently been reported in Sr-doped NdNiO${}_{2}$ thin films. This work considers the effect of Hund coupling and crystal field splitting in a simple model system and shows that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. An analysis of this system, inspired by the spin-freezing theory of unconventional superconductivity, furthermore reveals that Nd${}_{0.8}$Sr${}_{0.2}$NiO${}_{2}$ has strongly enhanced local spin fluctuations due to the interplay of two spin-freezing crossovers.

••

TL;DR: If the numerical results on small lattice sizes are representative of the somewhat larger lattices accessible to near-term quantum hardware, they suggest that optimising over quantum circuits with a gate depth less than a thousand could be sufficient to solve instances of the Hubbard model beyond the capacity of classical exact diagonalisation.

Abstract: One of the main applications of near-term quantum computers is expected to be solving many-body quantum systems. The present study reports on extensive optimization and numerical investigation of the Variational Quantum Eigensolver method for the iconic Fermi-Hubbard model, including the effect of realistic measurements and noise. The presented optimized circuits use up to an order of magnitude fewer gates than previously reported, suggesting that quantum circuits with a gate depth substantially below 1000 could be sufficient to solve instances beyond the capacity of classical exact diagonalization.

••

TL;DR: In this paper, the authors show that TBG flat-band superconductivity relies on such quantum geometric properties of the band as its quantum metric and Berry curvature, which can be used to reveal the yet unknown pairing symmetry.

Abstract: Superconductivity in twisted bilayer graphene (TBG) has inspired fervent research to explain the origin of the phenomenon and to find whether moir\'e materials featuring flat bands offer a route to room-temperature superconductivity. Here, the authors show that TBG flat-band superconductivity relies on such quantum geometric properties of the band as its quantum metric and Berry curvature. The authors predict qualitative differences between different pairing potentials, which can be used to reveal the yet unknown pairing symmetry.

••

TL;DR: In this article, the authors provide a common framework to understand and prove known exact towers of scars in non-integrable models, by evaluating the commutator of the Hamiltonian and a ladder operator.

Abstract: Quantum many-body scar states are many-body states with finite energy density in non-integrable models that do not obey the eigenstate thermalization hypothesis. Recent works have revealed “towers” of scar states that are exactly known and are equally spaced in energy, specifically in the AKLT and spin-1 XY models, and a spin-1/2 model that conserves the number of domain walls. We provide a common framework to understand and prove known exact towers of scars in these systems, by evaluating the commutator of the Hamiltonian and a ladder operator. In particular, we provide a simple proof of the scar towers in the integer-spin 1D AKLT models by studying two-site spin projectors. Through this picture we deduce a family of Hamiltonians that share the scar tower with the AKLT model, and also find common parent Hamiltonians for the AKLT and XY model scars. We also introduce new towers of exact states, organized in a “pyramid” structure, in the spin-1/2 model through the successive application of a nonlocal ladder operator.

••

TL;DR: In this article, the authors uncover a new exactly solvable example of many-body scars in a spin-textonehalf{} model and show that the scarred dynamics in this model evades such an interpretation.

Abstract: Strongly interacting systems with quantum many-body scars exhibit persistent fidelity oscillations when prepared in a certain class of initial states, but otherwise undergo ergodic dynamics. Previous examples of scars have interpreted the characteristic periodic dynamics in terms of the precession of a macroscopic SU(2) spin in an effective magnetic field. Here, the authors uncover a new exactly solvable example of many-body scars in a spin-\textonehalf{} model and show that the scarred dynamics in this model evades such an interpretation. This unusual coherent dynamics arises due to an emergent kinetic constraint that endows the time-evolving many-body state with constant area-law entanglement. The model studied in this work is relevant to experiments on Rydberg-atom quantum simulators in the antiblockade regime.

••

TL;DR: In this paper, the authors studied the disorder-induced ergodicity breaking transition in high-energy eigenstates of interacting spin-1/2 chains using exact diagonalization.

Abstract: We study disorder-induced ergodicity breaking transition in high-energy eigenstates of interacting spin-1/2 chains. Using exact diagonalization, we introduce a cost function approach to quantitatively compare different scenarios for the eigenstate transition. We study ergodicity indicators such as the eigenstate entanglement entropy and the spectral level spacing ratio, and we consistently find that an (infinite-order) Berezinskii-Kosterlitz-Thouless transition yields a lower cost function when compared to a finite-order transition. Interestingly, we observe that the ergodicity breaking transition in systems studied by exact diagonalization (with around 20 lattice sites) takes place at disorder values lower than those reported in previous works. As a consequence, the crossing point in finite systems exhibits nearly thermal properties, i.e., ergodicity indicators at the transition are close to the random matrix theory predictions.

••

TL;DR: In this article, the authors used the density matrix renormalization group (DMRG) to study the correlated electron states favored by the Coulomb interaction projected onto the narrow bands of twisted bilayer graphene within a spinless one-valley model.

Abstract: We use the density matrix renormalization group (DMRG) to study the correlated electron states favored by the Coulomb interaction projected onto the narrow bands of twisted bilayer graphene within a spinless one-valley model. The Hilbert space of the narrow bands is constructed from a pair of hybrid Wannier states with opposite Chern numbers, maximally localized in one direction and Bloch extended in another direction. Depending on the parameters in the Bistritzer-Macdonald model, the DMRG in this basis determines the ground state at one particle per unit cell to be either the quantum anomalous Hall (QAH) state or a state with zero Hall conductivity which is nearly a product state. Based on this form, we then apply the variational method to study their competition, thus identifying three states: the QAH, a gapless ${C}_{2}\mathcal{T}$-symmetric nematic, and a gapped ${C}_{2}\mathcal{T}$-symmetric stripe. In the chiral limit, the energies of the two ${C}_{2}\mathcal{T}$-symmetric states are found to be significantly above the energy of the QAH. However, all three states are nearly degenerate at the realistic parameters of the Bistritzer-Macdonald model. The single-particle spectrum of the nematic contains either a quadratic node or two close Dirac nodes near $\mathrm{\ensuremath{\Gamma}}$. Motivated by the Landau level degeneracy found in this state, we propose it to be the state observed at the charge neutrality point once spin and valley degeneracies are restored. The optimal period for the ${C}_{2}\mathcal{T}$ stripe state is found to be two unit cells. In addition, using the fact that the topological charge of the nodes in the ${C}_{2}\mathcal{T}$-nematic phase is no longer described simply by their winding numbers once the translation symmetry is broken, but rather by certain elements of a non-Abelian group that was recently pointed out, we identify the mechanism of the gap opening within the ${C}_{2}\mathcal{T}$ stripe state. Although the nodes at the Fermi energy are locally stable, they can be annihilated after braiding with other nodes connecting them to adjacent (folded) bands. Therefore, if the translation symmetry is broken, the gap at one particle per unit cell can open even if the system preserves the ${C}_{2}\mathcal{T}$ and valley $\text{U}(1)$ symmetries, and the gap to remote bands remains open.

••

TL;DR: In this article, the authors revisited the notion of ''ensuremath{\eta}$-pairing states in Hubbard models and explored their connections to quantum many-body scars to discover a universal scars mechanism.

Abstract: We revisit the $\ensuremath{\eta}$-pairing states in Hubbard models and explore their connections to quantum many-body scars to discover a universal scars mechanism. $\ensuremath{\eta}$-pairing occurs due to an algebraic structure known as a spectrum generating algebra (SGA), giving rise to equally spaced towers of eigenstates in the spectrum. We generalize the original $\ensuremath{\eta}$-pairing construction and show that several Hubbard-like models on arbitrary graphs exhibit SGAs, including ones with disorder and spin-orbit coupling. We further define a restricted spectrum generating algebra (RSGA) and give examples of perturbations to the Hubbard-like models that preserve an equally spaced tower of the original model as eigenstates. The states of the surviving tower exhibit a subthermal entanglement entropy, and we analytically obtain parameter regimes for which they lie in the bulk of the spectrum, showing that they are exact quantum many-body scars. The RSGA framework also explains the equally spaced towers of eigenstates in several well-known models of quantum scars, including the Affleck-Kennedy-Lieb-Tasaki model.

••

TL;DR: In this paper, the effect of the long-range Coulomb interaction on twisted graphene bilayers near a magic angle was investigated, and the results suggest that the nonsuperconducting broken symmetry phases observed experimentally are induced by the long range exchange interaction.

Abstract: We analyze the phase diagram of twisted graphene bilayers near a magic angle. We consider the effect of the long-range Coulomb interaction, treated within the self-consistent Hartree-Fock approximation, and we study arbitrary band fillings. We find a rich phase diagram, with different broken symmetry phases, although they do not show necessarily a gap at the Fermi energy. There are nontrivial effects of the electrostatic potential on the shape and the gaps of the bands in the broken symmetry phases. The results suggest that the nonsuperconducting broken symmetry phases observed experimentally are induced by the long-range exchange interaction.

••

TL;DR: In this article, the authors investigated the transition from volume-law to area-law entanglement in a quantum many-body state under continuous position measurement on the basis of the quantum trajectory approach and found the signatures of the transitions as peak structures in the mutual information as a function of measurement strength.

Abstract: We investigate entanglement phase transitions from volume-law to area-law entanglement in a quantum many-body state under continuous position measurement on the basis of the quantum trajectory approach. We find the signatures of the transitions as peak structures in the mutual information as a function of measurement strength, as previously reported for random unitary circuits with projective measurements. At the transition points, the entanglement entropy scales logarithmically and various physical quantities scale algebraically, implying emergent conformal criticality, for both integrable and nonintegrable one-dimensional interacting Hamiltonians; however, such transitions have been argued to be absent in noninteracting regimes in some previous studies. With the aid of $U(1)$ symmetry in our model, the measurement-induced criticality exhibits a spectral signature resembling a Tomonaga-Luttinger liquid theory from symmetry-resolved entanglement. These intriguing critical phenomena are unique to steady-state regimes of the conditional dynamics at the single-trajectory level and are absent in the unconditional dynamics obeying the Lindblad master equation, in which the system ends up with the featureless, infinite-temperature mixed state. We also propose a possible experimental setup to test the predicted entanglement transition based on the subsystem particle-number fluctuations. This quantity should readily be measured by the current techniques of quantum gas microscopy and is in practice easier to obtain than the entanglement entropy itself.

••

TL;DR: In this paper, it was demonstrated that the standard non-Bloch band theory breaks down in the symplectic class, in which non-Hermitian Hamiltonians exhibit Kramers degeneracy because of reciprocity.

Abstract: Non-Hermitian Hamiltonians are generally sensitive to boundary conditions, and their spectra and wave functions under open boundary conditions are not necessarily predicted by the Bloch band theory for periodic boundary conditions. To elucidate such a non-Bloch feature, recent works have developed a non-Bloch band theory that works even under arbitrary boundary conditions. Here, it is demonstrated that the standard non-Bloch band theory breaks down in the symplectic class, in which non-Hermitian Hamiltonians exhibit Kramers degeneracy because of reciprocity. Instead, a modified non-Bloch band theory for the symplectic class is developed in a general manner, as well as illustrative examples. This nonstandard non-Bloch band theory underlies the ${\mathbb{Z}}_{2}$ non-Hermitian skin effect protected by reciprocity.

••

Abstract: Rare-earth polyhydrides formed under pressure are promising conventional superconductors, with the critical temperature ${T}_{c}$ in compressed ${\mathrm{LaH}}_{10}$ almost reaching room temperature. Here, we report a systematic computational investigation of the structural and superconducting properties of rare-earth (RE) polyhydrides formed under pressure across the whole lanthanide series. Analyses of the electronic and dynamical properties and electron-phonon coupling interaction for the most hydrogen-rich hydrides ${\mathrm{REH}}_{n}$ ($n=8,9,10$) that can be stabilized below 400 GPa show that enhanced ${T}_{c}$ correlates with a high density of H $s$ states and low number of RE $f$ states at the Fermi level. In addition to previously predicted and measured ${\mathrm{LaH}}_{10}$ and ${\mathrm{CeH}}_{9}$, we suggest ${\mathrm{YbH}}_{10}$ and ${\mathrm{LuH}}_{8}$ as additional potential high-${T}_{c}$ superconducters. They form a ``second island'' of high-${T}_{c}$ superconductivity amongst the late lanthanide polyhydrides, with an estimated ${T}_{c}$ of 102 K for ${\mathrm{YbH}}_{10}$ at 250 GPa.

••

TL;DR: In this paper, the authors introduce the notion of statistically localized integrals of motion (SLIOM) to characterize the properties of disorder-free Hamiltonians, which can lead to topological string order for certain highly excited eigenstates as well as infinitely long-lived edge magnetization along with a thermalizing bulk.

Abstract: Certain disorder-free Hamiltonians are nonergodic due to a $s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ $f\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n$ of the Hilbert space. Here, the authors introduce the notion of ``statistically localized integrals of motion'' (SLIOM) to characterize these systems. Despite SLIOMs being nonlocal operators, they become spatially localized to subextensive regions when their expectation value is taken in typical states. These can also result in statistically localized $s\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g$ $z\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o$ $m\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}d\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$, leading to topological string order for certain highly excited eigenstates as well as infinitely long-lived edge magnetization along with a thermalizing bulk.

••

TL;DR: In this article, a combined theoretical and experimental study of solid-state spin decoherence in an electronic spin bath is presented, focusing specifically on ensembles of nitrogen-vacancy (NV) centers in diamond and the associated substitutional nitrogen spin bath.

Abstract: We present a combined theoretical and experimental study of solid-state spin decoherence in an electronic spin bath, focusing specifically on ensembles of nitrogen-vacancy (NV) centers in diamond and the associated substitutional nitrogen spin bath. We perform measurements of NV spin free-induction decay (FID) times ${T}_{2}^{*}$ and spin-echo coherence times ${T}_{2}$ in 25 diamond samples with nitrogen concentrations [N] ranging from 0.01 to 300 ppm. We introduce a microscopic model and perform numerical simulations to quantitatively explain the degradation of both ${T}_{2}^{*}$ and ${T}_{2}$ over four orders of magnitude in [N]. Our analysis enables us to describe the NV ensemble spin coherence decay shapes as emerging consistently from the contribution of many individual NV centers.

••

TL;DR: In this article, a quasi-BIC resonance was proposed to control light absorption at critical coupling through the quasi BIC resonance, and the maximum absorption of 0.5 was achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene.

Abstract: Enhancing the light-matter interaction in two-dimensional (2D) materials with high $Q$ resonances in photonic structures has boosted the development of optical and photonic devices. Herein we intend to build a bridge between the radiation engineering and the bound states in the continuum (BIC), and present a general method to control light absorption at critical coupling through the quasi-BIC resonance. In a single-mode, two-port system composed of graphene, coupled with silicon nanodisk metasurfaces, the maximum absorption of 0.5 can be achieved when the radiation rate of the magnetic dipole resonance equals to the dissipate loss rate of graphene. Furthermore, the absorption bandwidth can be adjusted more than two orders of magnitude, from 0.9 nm to 94 nm, by simultaneously changing the asymmetric parameter of metasurfaces, the Fermi level, and the layer number of graphene. This work reveals the essential role of BIC in radiation engineering and provides promising strategies in controlling light absorption of 2D materials for the next-generation optical and photonic devices, e.g., light emitters, detectors, modulators, and sensors.