While it is becoming apparent that organic semiconductor/metal interfaces may exhibit a variety of different structural phases, it is at present unclear to what extent these different thin-film structures determine the interfacial electronic structure. Here, we observe large changes in the interfacial electronic structure for the case of copper(II) phthalocyanine (CuPc) on $\mathrm{Cu}\left(110\right)-(2\times 1)\mathrm{O}$. This striking evolution of the interfacial electronic structure occurs beyond the first monolayer of CuPc and is particularly evident in the frontier orbital region. Using scanning tunneling microscopy in conjunction with photoemission spectroscopy, we characterize ultrathin films of CuPc grown on oxygen reconstructed Cu(110). We propose that the observed unique changes to the electronic structure result from an abrupt transition in film structure between the first and second layers: An interface layer of ordered, face-on molecules templates a largely vertical, edge-on orientation of molecules in the subsequent layer. The quadrupole moment of the molecule accounts for the sizable and unusual change in ionization energy between molecules in the two layers. Our results demonstrate that the precise structure of the organic semiconductor film exerts an important role in determining the interfacial electronic structure that must be considered and may be harnessed for tailoring energy level alignment at such interfaces.

We develop an alternative time-dependent numerical renormalization group (TDNRG) formalism for multiple quenches and implement it to study the response of a quantum impurity system to a general pulse. Within this approach, we reduce the contribution of the NRG approximation to numerical errors in the time evolution of observables by a formulation that avoids the use of the generalized overlap matrix elements in our previous multiple-quench TDNRG formalism [Nghiem et al., Phys. Rev. B 89, 075118 (2014); Phys. Rev. B 90, 035129 (2014)]. We demonstrate that the formalism yields a smaller cumulative error in the trace of the projected density matrix as a function of time and a smaller discontinuity of local observables between quenches than in our previous approach. Moreover, by increasing the switch-on time, the time between the first and last quench of the discretized pulse, the long-time limit of observables systematically converges to its expected value in the final state, i.e., the more adiabatic the switching, the more accurately is the long-time limit recovered. The present formalism can be straightforwardly extended to infinite switch-on times. We show that this yields highly accurate results for the long-time limit of both thermodynamic observables and spectral functions, and overcomes the significant errors within the single quench formalism [Anders et al., Phys. Rev. Lett. 95, 196801 (2005); Nghiem et al., Phys. Rev. Lett. 119, 156601 (2017)]. This improvement provides a first step towards an accurate description of nonequilibrium steady states of quantum impurity systems, e.g., within the scattering states NRG approach [Anders, Phys. Rev. Lett. 101, 066804 (2008)].

In this paper we report on the emergence of a high energy band at high optically excited carrier densities in the low temperature photoluminescence spectra from polar InGaN/GaN single quantum well structures. This high energy band emerges at carrier densities when the emission from the localized ground states begins to saturate. We attribute this high energy band to recombination involving higher energy less strongly localized electron and hole states that are populated once the localized ground states become saturated; this assignment is supported by the results from an atomistic tight-binding model. A particular characteristic of the recombination at the high carrier densities is that the overall forms of the photoluminescence decay curves bear great similarity to those from semiconductor quantum dots. The decay curves consist of plateaus where the photoluminescence intensity is constant with time as a result of Pauli state blocking in the high energy localized states followed by a rapid decrease in intensity once the carrier density is sufficiently low that the states involved are no longer saturated.

The concept of the Zak phase lies at the core of the modern theory of electric polarization. It is defined using the components of the Bloch wave functions in a certain basis, which is not captured by the standard expression for Berry potential. We provide a consistent geometric interpretation of the Zak phase in terms of projected connections. In the context of Bloch states, we relate the transformation law of projected Berry potential with classical currents that contribute to the time derivative of the electric polarization. This gives a new argument for the Zak phase formula for the electronic contribution to the polarization. We demonstrate that the Wannier functions play a key role in the description of an adiabatic current in a periodic system.

We investigate the nature of doped Mott insulators using exact diagonalization and density matrix renormalization group methods. Persistent spin currents are revealed in the ground state, which are concomitant with a nonzero total momentum or angular momentum associated with the doped hole. The latter determines a nontrivial ground state degeneracy. By further making superpositions of the degenerate ground states with zero or unidirectional spin currents, we show that different patterns of spatial charge and spin modulations will emerge. Such anomaly persists for the odd numbers of holes, but the spin current, ground state degeneracy, and charge/spin modulations completely disappear for even numbers of holes, with the two-hole ground state exhibiting a $d$-wave symmetry. An understanding of the spin current due to a many-body Berry-like phase and its influence on the momentum distribution of the doped holes will be discussed.

Ultrafast pump-probe spectroscopy with high temporal and spectral resolutions provides new insight into ultrafast nonequilibrium phenomena. We propose that transient interference between pump and probe pulses is realized in pump-probe spectroscopy of band and Mott insulators, which can be observed only after recent developments of ultrafast spectroscopic techniques. A continuum structure in the excitation spectrum of band insulators is found to act as a medium for storing the spectral information of the pump pulse, and the spectrum detected by the probe pulse is interfered with by the medium, generating the transient interference in the energy domain. We also demonstrate the transient interference in the presence of electron correlations in a one-dimensional half-filled Hubbard model. Furthermore, bosons coupled to electrons additively contribute to the interference. Our finding will provide an interpretation of probe-energy-dependent oscillations recently observed in the pump-probe spectrum for a two-dimensional Mott insulator.

Using the decorated domain wall procedure, we construct finite-depth local unitaries that realize fermionic symmetry-protected topological (SPT) phases. This results in explicit full commuting projector Hamiltonians, where “full” implies the fact that the ground state as well as all excited states of these Hamiltonians realize the nontrivial SPT phase. We begin by constructing explicit examples of 1+1D phases protected by the symmetry group $G={\mathbb{Z}}_2^T\times {\mathbb{Z}}_2^F$, which also has a free fermion realization in class BDI, and by the symmetry group $G={\mathbb{Z}}_4\times {\mathbb{Z}}_4^F$, which does not. We then turn to 2+1D, and construct the square roots of the Levin-Gu bosonic SPT phase, protected by ${\mathbb{Z}}_2\times {\mathbb{Z}}_2^F$ symmetry, in a concrete model of fermions and spins on the triangular lattice. Edge states and the anomalous symmetry action on them are explicitly derived. Although this phase has a free fermion representation as two copies of $p+ip$ superconductors combined with their $p-ip$ counterparts with a different symmetry charge, the full set of commuting projectors is only realized in the strongly interacting version, which also implies that it admits a many-body localized realization.

We have investigated scaling properties of the Aubry–André model and related one-dimensional quasiperiodic Hamiltonians near their localization transitions. We find numerically that the scaling of characteristic energies near the ground state, usually captured by a single dynamical exponent, does not obey a power law relation. Instead, the scaling behavior depends strongly on the correlation length in a manner governed by the continued fraction expansion of the irrational number $\beta$ describing incommensurability in the system. This dependence is, however, found to be universal between a range of models sharing the same value of $\beta$. For the Aubry–André model, we explain this behavior in terms of a discrete renormalization group protocol which predicts rich critical behavior. This result is complemented by studies of the expansion dynamics of a wave packet under the Aubry–André model at the critical point. Anomalous diffusion exponents are derived in terms of multifractal (Rényi) dimensions of the critical spectrum; non-power-law universality similar to that found in ground state dynamics is observed between a range of critical tight-binding Hamiltonians.

We report the crystal field levels of several newly discovered rare-earth kagome compounds: ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$, ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$, and ${\mathrm{Pr}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$. We determine the crystal electric field (CEF) Hamiltonian by fitting to neutron scattering data using a point charge Hamiltonian as an intermediate fitting step. The fitted Hamiltonians accurately reproduce bulk susceptibility measurements, and the results indicate easy-axis ground-state doublets for ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$ and ${\mathrm{Nd}}_3{\mathrm{Sb}}_3{\mathrm{Zn}}_2{\mathrm{O}}_{14}$ and a singlet ground state for ${\mathrm{Pr}}_3{\mathrm{Sb}}_3{\mathrm{Mg}}_2{\mathrm{O}}_{14}$. These results provide the groundwork for future investigations of these compounds and a template for CEF analysis of other low-symmetry materials.

We report an electric-field-induced in-plane magnetoresistance of an atomically flat paramagnetic insulator|platinum (Pt) interface at low temperatures with an ionic liquid gate. Transport experiments as a function of applied magnetic field strength and direction obey the spin Hall magnetoresistance phenomenology with perpendicular magnetic anisotropy. Our results establish the utility of ionic gating as an alternative method to control spintronic devices without using ferromagnets.

The modular (or entanglement) Hamiltonian correspondent to the half-space bipartition of a quantum state uniquely characterizes its entanglement properties. However, in the context of lattice models, its explicit form is analytically known only for the two spin chains and certain free theories in one dimension. In this work, we provide a thorough investigation of entanglement Hamiltonians in lattice models obtained via the Bisognano-Wichmann theorem, which provides an explicit functional form for the entanglement Hamiltonian itself in quantum field theory. Our study encompasses a variety of one- and two-dimensional models, supporting diverse quantum phases and critical points, and, most importantly, scanning several universality classes, including Ising, Potts, and Luttinger liquids. We carry out extensive numerical simulations based on the density matrix renormalization group method, exact diagonalization, and quantum Monte Carlo. In particular, we compare the exact entanglement properties and correlation functions to those obtained applying the Bisognano-Wichmann theorem on the lattice. We carry out this comparison on both the eigenvalues and eigenvectors of the entanglement Hamiltonian, and expectation values of correlation functions and order parameters. Our results evidence that as long as the low-energy description of the lattice model is well captured by a Lorentz-invariant quantum field theory, the Bisognano-Wichmann theorem provides a qualitatively and quantitatively accurate description of the lattice entanglement Hamiltonian. The resulting framework paves the way to direct studies of entanglement properties utilizing well-established statistical mechanics methods and experiments.

Polarization-induced spin switching of atoms in magnetic materials opens the possibilities to design and develop advanced spintronic devices, in particular, storage devices where the magnetic state can be controlled by an electric field. We employ density functional theory calculations to study the magnetic properties of a perovskite strontium cobalt oxyfluoride ${\mathrm{Sr}}_2{\mathrm{CoO}}_3\mathrm{F}$ (SCOF) in a hybrid perovskite heterostructure, where SCOF is sandwiched between two ferroelectic ${\mathrm{BaTiO}}_3$ (BTO) layers. Our calculations show that the spin state of the central Co atom in SCOF can be controlled by altering the polarization direction of the BTO, specifically, to switch from a high-spin state to a low-spin state by changing the relative orientation of the ferroelectric polarization of BTO with respect to SCOF, leading to an unexpected, giant magnetoelectric coupling, ${\alpha }_s\approx 21\times {10}^{-10}{\mathrm{G}\mathrm{cm}}^2/\mathrm{V}$.

Photonic topological insulator plates violate Lorentz reciprocity, which leads to a directionality of surface-guided modes. This in-plane directionality can be imprinted via an applied magnetic field. On the basis of macroscopic quantum electrodynamics in nonreciprocal media, we show that two photonic topological insulator surfaces are subject to a tunable, magnetic-field-dependent Casimir torque. Due to the directionality, this torque exhibits a unique $2\pi$ periodicity, in contradistinction to the Casimir torques encountered for reciprocal uniaxial birefringent media or corrugated surfaces which are $\pi$ periodic. Remarkably, the torque direction and strength can be externally driven in situ by simply applying a magnetic field on the system, and we show that this can be exploited to induce a control of the rotation of small objects. Our predictions are relevant for nano-optomechanical experiments and devices.

Finite-temperature infrared and Raman spectra of theoretically proposed stable structures $C2/c$ and $Pc$ of high-pressure solid hydrogen are calculated from time correlation functions of dipole moments and polarizabilities extracted from first-principles molecular dynamics simulations. Calculated spectra are much improved compared with those obtained from density functional perturbation theory at zero temperature, which suggests the significance of finite-temperature effects in both spectra. The excellent agreement between the calculated spectra of the $C2/c$ structure and experimental results supports the theory that $C2/c$ is the structure of phase III. The high-frequency Raman vibron mode of the $Pc$ structure is also well reproduced compared with experimental spectra. However, the energy of the low-frequency Raman vibron mode of the $Pc$ structure is underestimated up to 16%. This suggests that the atomic structure of the strongly bonded layer in phase IV is well predicted, while the weakly bonded layer still differs from the real structure somehow. In addition, we find that diffusion in the weakly bonded layer of the $Pc$ structure is strong and the layer displays several features of a two-dimensional liquid.

We have investigated the electronic properties of ${\mathrm{BiS}}_2$-based superconductors by using x-ray photoemission spectroscopy (XPS). In going from $x=0.3$ to 0.5 in ${\mathrm{PrO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, the Pr $3d$ and Pr $4d$ peaks are shifted by $\sim 0.10\pm 0.05$ eV from the Fermi level, partially consistent with the electron doping. In ${\mathrm{PrO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, the ${\mathrm{Pr}}^{3+}-{\mathrm{Pr}}^{4+}$ mixed valence remains unchanged with the electron doping from $x=0.3$ to 0.5. In ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, the doped electrons for $x=0.5$ almost suppress the ${\mathrm{Ce}}^{3+}-{\mathrm{Ce}}^{4+}$ valence fluctuation. Although the core-level peaks are also shifted by $\sim 0.10\pm 0.05$ eV towards the higher-binding-energy side with the electron doping from $x=0$ to 0.5 in ${\mathrm{CeO}}_{1-x}{\mathrm{F}}_x{\mathrm{BiS}}_2$, the Bi $4f_{7/2}$ binding-energy shift is higher in the Pr system compared with the Ce system. The present results suggest that the doped electrons increase orbital occupations in the rare-earth $4f$ orbitals at the valence band and show valence fluctuations differently in the two systems.

We study weakly interacting Bose gases using the functional renormalization group with a hydrodynamic effective action. We use a scale-dependent parametrization of the boson fields that interpolates between a Cartesian representation at high momenta and an amplitude-phase one for low momenta. We apply this to Bose gases in two and three dimensions near the superfluid phase transition where they can be described by statistical O(2) models. We are able to give consistent physical descriptions of the infrared regime in both two and three dimensions. In particular, and in contrast to previous studies using the functional renormalization group, we find a stable superfluid phase at finite temperatures in two dimensions. We compare our results for the superfluid and boson densities with Monte-Carlo simulations, and we find they are in reasonable agreement.

Kitaev's honeycomb-lattice spin-$1/2$ model has become a paradigmatic example for ${\mathbb{Z}}_2$ quantum spin liquids, both gapped and gapless. Here we study the fate of these spin-liquid phases in differently stacked bilayer versions of the Kitaev model. Increasing the ratio between the interlayer Heisenberg coupling $J_{\perp }$ and the intralayer Kitaev couplings $K^{x,y,z}$ destroys the topological spin liquid in favor of a paramagnetic dimer phase. We study phase diagrams as a function of $J_{\perp }/K$ and Kitaev coupling anisotropies using Majorana-fermion mean-field theory, and we employ different expansion techniques in the limits of small and large $J_{\perp }/K$. For strongly anisotropic Kitaev couplings, we derive effective models for the different layer stackings that we use to discuss the quantum phase transition out of the Kitaev phase. We find that the phase diagrams depend sensitively on the nature of the stacking and anisotropy strength. While in some stackings and at strong anisotropies we find a single transition between the Kitaev and dimer phases, other stackings are more involved. Most importantly, we prove the existence of two novel macrospin phases, which can be understood in terms of Ising chains that can be either coupled ferromagnetically or remain degenerate, thus realizing a classical spin liquid. In addition, our results suggest the existence of a flux phase with spontaneous interlayer coherence. We discuss prospects for experimental realizations.

Despite nearly a century of study of the $S=1/2$ Heisenberg model on the square lattice, there is still disagreement on the nature of its high-energy excitations. By tuning toward the Heisenberg model from the exactly soluble Ising limit, we find that the strongly attractive magnon interactions of the latter naturally account for a number of spectral features of the Heisenberg model. This claim is backed up both numerically and analytically. Using the density matrix renormalization group method, we obtain the dynamical structure factor for a cylindrical geometry, allowing us to continuously connect both limits. Remarkably, a semiquantitative description of certain observed features arises already at the lowest nontrivial order in perturbation theory around the Ising limit. Moreover, our analysis uncovers that high-energy magnons are localized on a single sublattice, which is related to the entanglement properties of the ground state.

Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic $\mathrm{LaNi}{\mathrm{O}}_3$ and antiferromagnetic $\mathrm{CaMn}{\mathrm{O}}_3$ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in $\mathrm{LaNi}{\mathrm{O}}_3$. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the $\mathrm{LaNi}{\mathrm{O}}_3/\mathrm{CaMn}{\mathrm{O}}_3$ interface. Our measurements reveal an increased concentration of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the $\mathrm{M}{\mathrm{n}}^{4+}-\mathrm{M}{\mathrm{n}}^{3+}$ ferromagnetic double exchange and $\mathrm{N}{\mathrm{i}}^{2+}-\mathrm{O}-\mathrm{M}{\mathrm{n}}^{4+}$ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

We study two-component electrons in the lowest Landau level at total filling factor ${\nu }_T=1/2$ with anisotropic mass tensors and principal axes rotated by $\pi /2$ as realized in aluminum arsenide (AlAs) quantum wells. Combining exact diagonalization and the density matrix renormalization group we demonstrate that the system undergoes a quantum phase transition from a gapless state in which both flavors are equally populated to another gapless state in which all the electrons spontaneously polarize into a single flavor beyond a critical mass anisotropy of $m_x/m_y\sim 7$. We propose that this phase transition is a form of itinerant Stoner transition between a two-component and a single-component composite Fermi sea states and describe a set of trial wave functions which successfully capture the quantum numbers and shell filling effects in finite size systems as well as providing a physical picture for the energetics of these states. Our estimates indicate that the composite Fermi sea of AlAs is the analog of an itinerant Stoner magnet with a finite spontaneous valley polarization. We pinpoint experimental evidence indicating the presence of Stoner magnetism in the Jain states surrounding $\nu =1/2$.