We study the ground state of a gapped quantum many-body system whose entanglement entropy $S_A$ can be expressed as $S_A=a\left|\partial A\right|-\gamma$, where $a,\gamma$ are some constants and $\left|\partial A\right|$ is an area of the subsystem $A$. By using a recently proved operator extension of strong subadditivity of entropy [I. H. Kim, J. Math. Phys. 53, 122204 (2012)], we show that a certain linear combination of the real-space entanglement spectrum has a small correlation with almost any local operator. Our result implies that there exists a structure relating the real-space entanglement spectrum over different subsystems. Further, this structure is inherited from the generic property of the ground state alone, suggesting that the locality of the entanglement spectrum may be attributed to the area law of entanglement entropy.

We have investigated the electronic structure of CaCu${}_3$Ru${}_4$O${}_{12}$ and LaCu${}_3$Ru${}_4$O${}_{12}$ using soft x-ray photoelectron and absorption spectroscopy together with band structure and cluster configuration interaction calculations. We found the Cu to be in a robust divalent ionic state while the Ru is more itinerant in character and stabilizes the metallic state. Substitution of Ca by La predominantly affects the Ru states. We observed strong correlation effects in the Cu $3d$ states affecting the valence band line shape considerably. Using resonant photoelectron spectroscopy at the Cu $L_3$ edge we were able to unveil the position of the Zhang-Rice singlet states in the one-electron removal spectrum of the Cu with respect to the Ru-derived metallic bands in the vicinity of the chemical potential.

We investigate the effects of bulk impurities on the electronic spectrum of Weyl semimetals, a recently identified class of Dirac-type materials. Using a $T$-matrix approach, we study resonant scattering due to a localized impurity in tight-binding versions of the continuum models recently discussed by [Burkov, Hook, and Balents, Phys. Rev. B 84, 235126 (2011)], describing perturbed four-component Dirac fermions in the vicinity of a critical point. The impurity potential is described by a strength $g$ as well as a matrix structure $\Lambda$. Unlike the case in $d$-wave superconductors, where a zero energy resonance can always be induced by varying the scalar and/or magnetic impurity strength, we find that for certain types of impurity ($\Lambda$), the Weyl node is protected and that a scalar impurity will induce an intragap resonance over a wide range of scattering strength. A general framework is developed to address this question, as well as to determine the dependence of resonance energy on the impurity strength.

Bonding in the crystalline phase of GeTe-based phase-change alloys is believed to be purely $p$ type. In this work we demonstrate that in the absence of long-range order, despite the preserved bonding angles of $\sim {90}^{\circ }$, $s{p}^3$ hybridization is more favorable with a pair of nonbonding electrons localized on a Ge $s{p}^3$ orbital, i.e., in contrast to the crystalline phase, amorphous GeTe is a lone-pair (LP) semiconductor. Upon disordering, tetrahedral Ge sites are formed due to unpairing of LP electrons and subsequent formation of additional Ge-Ge bonds, a process generating additional free electrons. Recombination of these electrons during the structure relaxation may be the underlying reason for the conductivity drift in the amorphous phase making the latter process analogous to persistent photoconductivity. Implications for the stability of the amorphous phase are also discussed.

Noise measurements are a probe beyond simple electronic transport that can reveal additional information about electronic correlations and inelastic processes. Here we report noise measurements in individual electromigrated nanojunctions, examining the evolution from the many-channel regime to the tunneling regime, using a radio frequency technique. While we generally observe the dependence of noise on bias expected for shot noise, in approximately 12$\%$ of junction configurations we find discrete changes in the bias dependence at threshold values of the bias, consistent with electronic excitation of local vibrational modes. Moreover, with some regularity we find significant mesoscopic variation in the magnitude of the noise in particular junctions even with small changes in the accompanying conductance. In another $\sim$17$\%$ of junctions we observe pronounced asymmetries in the inferred noise magnitude as a function of bias polarity, suggesting that investigators should be concerned about current-driven ionic motion in the electrodes even at biases well below those used for deliberate electromigration.

The electronic structure of the vacancy-ordered K${}_{0.5}$Fe${}_{1.75}$Se${}_2$ iron-selenide compound (278 phase) is studied using the first-principles density functional method. The ground state of the 278 phase is stripelike antiferromagnetic, and its bare electron susceptibility shows a large peak around $(\pi ,\pi )$ in the folded Brillouin zone. Near the Fermi level, the density of states is dominated by the Fe-3$d$ orbitals, and both electronlike and holelike Fermi surfaces appear in the Brillouin zone. An unfolded band structure shows limited similarities to a hole-doped 122 phase. With 0.1$e$ electron doping, the susceptibility peak is quickly suppressed and broadened, while the two dimensionality of the electronlike Fermi surfaces is greatly enhanced, resulting in a better nesting behavior. Our study should be relevant to the recently reported superconducting phase K${}_{0.5+x}$Fe${}_{1.75+y}$Se${}_2$ with both $x$ and $y$ being very tiny.

During heteroepitaxial growth, coherently strained islands form. These “self-assembled quantum dots” then undergo a series of shape transitions with increasing size. The best-known examples are the transitions of Ge on Si(001) and InAs on GaAs(001) from pyramidal islands to multifaceted domes. Here we examine the transition pathway, using a simple two-dimensional model. We find that the transition occurs via sequential nucleation of individual facets. While the stable states are symmetrical, the transition states are highly asymmetrical. The calculated transition path can pass through a metastable half-dome island shape, consistent with experimental observations. The broken symmetry of the transition state can be “locked in” by intermixing with substrate material, leading to asymmetrical islands.

We present an effective tight-binding Hamiltonian for Li${}_2$IrO${}_3$ based on maximally localized Wannier functions for states near the Fermi level as obtained from first-principles electronic structure calculations. The majority of the Wannier orbitals are positioned on the center site with dominant $j_{\mathrm{eff}}=1/2$ character, while relatively small $j_{\mathrm{eff}}=3/2$ tails lie on the three nearest-neighbor sites. Interestingly, the spin quantization axis of the $j_{\mathrm{eff}}=1/2$ components deviates from the local octahedral axis and points toward the nearest-neighbor Ir direction. In our tight-binding model, there are relatively strong next-nearest- and the third-nearest-neighbor hopping terms within the two-dimensional Ir honeycomb lattice in addition to the relatively small but significant interlayer hopping terms. The ratio between the nearest-neighbor and the third-nearest-neighbor hoppings, which can be controlled by the lattice strain, plays a critical role in determinating the $Z_2$-invariant character of Li${}_2$IrO${}_3$. From our tight-binding model, we also derive an effective Hamiltonian and its parameters for the magnetic exchange interactions. Due to the complex spin-dependent next-nearest-neighbor hopping terms, our pseudospin Hamiltonian includes significant next-nearest-neighbor antiferromagnetic Kitaev terms as well as Dzyaloshinskii-Moriya and Heisenberg interactions. From our model Hamiltonian we estimate classical energies of collinear magnetic configurations as functions of the Hund's coupling of the Ir atom, from which zigzag-type magnetic order gives the lowest energy.

To facilitate reliable and accurate modeling of the late transition metal oxides from first principles, we present detailed and systematic calculations of the dynamical screened Coulomb interactions (Hubbard $U$) of MnO, FeO, CoO, and NiO within the constrained random-phase approximation. The matrix elements of the screened interactions are calculated in maximally localized Wannier functions. We consider the screened interactions not only for conventional models that include only the $d$-like bands but also for models that include both the transition metal $d$ bands and the oxygen $p$ bands. The screened interaction is found to be sensitive to the screening channels subtracted from the polarization function. The frequency dependence of the screened interactions of these oxides is characterized by the two sharp peaks in low-energy region that may be called “subplasmons,” which arise from the particle-hole transitions between $d$ and oxygen $p$ states.

We introduce and study a class of anyon models that are a natural generalization of Ising anyons and Majorana fermion zero modes. These models combine an Ising anyon sector with a sector associated with $\mathit{\text{SO}}{\left(m\right)}_2$ Chern-Simons theory. We show how they can arise in a simple scenario for electron fractionalization and give a complete account of their quasiparticle types, fusion rules, and braiding. We show that the image of the braid group is finite for a collection of $2n$ fundamental quasiparticles and is a proper subgroup of the metaplectic representation of $Sp(2n-2,{\mathbb{F}}_m)⋉H(2n-2,{\mathbb{F}}_m)$, where $Sp(2n-2,{\mathbb{F}}_m)$ is the symplectic group over the finite field ${\mathbb{F}}_m$ and $H(2n-2,{\mathbb{F}}_m)$ is the extra special group (also called the $(2n-1)$-dimensional Heisenberg group) over ${\mathbb{F}}_m$. Moreover, the braiding of fundamental quasiparticles combined with a restricted set of measurements can be efficiently simulated classically. However, computing the result of braiding a certain type of composite quasiparticle followed by fusion into the identity is $\#P$-hard. It is not universal for quantum computation because it has a finite braid group image. This is a rare example of a topological phase that is not universal for quantum computation through braiding but nevertheless has $\#P$-hard link invariants. We argue that our models are closely related to recent analyses finding non-Abelian anyonic properties for defects in quantum Hall systems, generalizing Majorana zero modes in quasi-1D systems.

Using Landau–Ginzburg–Devonshire theory and phase-field modeling, we explore the complex interplay between a structural order parameter (oxygen octahedron rotation) and polarization in Eu${}_x$Sr${}_{1-x}$TiO${}_3$ thin films. Under a biaxially tensile strain, a low-symmetry monoclinic phase with in-plane ferroelectric polarization is found to be stabilized by antiferrodistortive oxygen octahedra tilts. The monoclinic phase is stable over a wide temperature range. It is characterized by a large number of energetically equivalent polar and structural twin domains. This work demonstrates the development of a spontaneous polarization and piezo- and pyroelectricity in a ferroelastic twin boundary arising from flexoelectric coupling and rotostriction.

We report neutron scattering and ac susceptibility measurements on the pyrochlore Yb${}_2$Sn${}_2$O${}_7$. The appearance of magnetic diffuse scattering, low-energy spin excitations, and a nearly temperature-independent relaxation time are signatures that the system enters a short-range-ordered regime with quantum dynamics beneath 2 K. ac susceptibility data further suggest that Yb${}_2$Sn${}_2$O${}_7$ enters a long-range magnetically ordered state below 0.11 K. These results suggest that Yb${}_2$Sn${}_2$O${}_7$ is a ferromagnet with persistent spin dynamics down to 0.05 K. Referencing the phase diagram for Yb pyrochlores, Yb${}_2$Sn${}_2$O${}_7$ is approaching a quantum phase transition near the ferromagnetic ordered critical point.

We carefully consider the interplay between ferromagnetism and the Kondo screening effect in the conventional Kondo lattice systems at finite temperatures. Within an effective mean-field theory for small conduction electron densities, a complete phase diagram has been determined. In the ferromagnetic ordered phase, there is a characteristic temperature scale to indicate the presence of the Kondo screening effect. We further find two distinct ferromagnetic long-range ordered phases coexisting with the Kondo screening effect: spin fully polarized and partially polarized states. A continuous phase transition exists to separate the partially polarized ferromagnetic ordered phase from the paramagnetic heavy Fermi liquid phase. These results may be used to explain the weak ferromagnetism observed recently in the Kondo lattice materials.

The magnetic and electronic properties of the cubic Laves phase intermetallic compound NpCo${}_2$ have been investigated by magnetization, specific heat, and electrical resistivity measurements. Magnetization data confirm previous results, i.e., an antiferromagneticlike order at 12.5 K and a metamagnetic transition at $B_m$ ∼ 4.3 T at 3 K with a saturation moment of ∼0.6 ${\mu }_{\mathrm{B}}$. Although extensive neutron diffraction studies failed to establish the nature of the antiferromagnetic order, its occurrence is confirmed by specific heat measurements that also highlight the presence of magnetic fluctuations suppressed by the application of an external field larger than $B_m$. The observed high value of the electronic specific heat, ${\gamma }_0$ ∼ 330 mJ/mol K${}^2$ as $T$ → 0, and the minute magnetic entropy, <0.1$R$ln2, classify NpCo${}_2$ as an itinerant, moderately-heavy-fermion antiferromagnet. The electrical resistivity data agree with the presence of an antiferromagnetic order below $B_m$ and suggest a non-Fermi liquid behavior at low temperature. The electronic structure of NpCo${}_2$ was examined theoretically and compared to experimental data. Local spin density approximation (LSDA) calculations show that this material is close to a magnetic instability and that ferromagnetic alignment of the Np moments is the most stable. LSDA+$U$ calculations in the fully localized limit with $U$ $=$ 0 give reasonable agreement with the experimental Np moment value and their orbital and spin contributions.

We review the magnetic form factor deduced by Delrieu from Gorkov's equation for a Bardeen-Cooper-Schrieffer (BCS) type-II superconductor near its $B_{\mathrm{c}2}$ phase boundary, i.e., when its magnetization is small. A numerical study of the form factor, field map, and field distribution follows. The characteristics of the transition from the low-temperature BCS to the high-temperature Ginzburg-Landau vortex lattices is studied. The exotic shape of the component field distribution and the form factor at low temperature and as a function of the external field intensity are discussed. Our numerical work should be helpful for the analyzing of small-angle neutron scattering and muon spin rotation vortex-lattice data recorded for BCS superconductors and may be other superconductors in the clean limit.

Using highly efficient GPU-based simulations of the tight-binding Bogoliubov–de Gennes equations we solve self-consistently for the pair correlation in rhombohedral (ABC) and Bernal (ABA) multilayer graphene by considering a finite intrinsic $s$-wave pairing potential. We find that the two different stacking configurations have opposite bulk/surface behavior for the order parameter. Surface superconductivity is robust for ABC stacked multilayer graphene even at very low pairing potentials for which the bulk order parameter vanishes, in agreement with a recent analytical approach. In contrast, for Bernal stacked multilayer graphene, we find that the order parameter is always suppressed at the surface and that there exists a critical value for the pairing potential below which no superconducting order is achieved. We considered different doping scenarios and find that homogeneous doping strongly suppresses surface superconductivity while nonhomogeneous field-induced doping has a much weaker effect on the superconducting order parameter. For multilayer structures with hybrid stacking (ABC and ABA) we find that when the thickness of each region is small (few layers), high-temperature surface superconductivity survives throughout the bulk due to the proximity effect between ABC/ABA interfaces where the order parameter is enhanced.

We consider a tight-binding model on the regular honeycomb lattice with uncorrelated on-site disorder. We use two independent methods (recursive Green's function and self-consistent Born approximation) to extract the scattering mean-free path, the scattering mean-free time, the density of states, and the localization length as a function of the disorder strength. The two methods give excellent quantitative agreement for these single-particle properties. Furthermore, a finite-size scaling analysis reveals that all localization lengths for different lattice sizes and different energies (including the energy at the Dirac points) collapse onto a single curve, in agreement with the one-parameter scaling theory of localization. The predictions of the self-consistent theory of localization however fail to quantitatively reproduce these numerically extracted localization lengths.

Motivated by recent experiments on chemically synthesized magnetic molecular chains, we investigate the lowest-lying energy band of short spin-$s$ antiferromagnetic Heisenberg chains focusing on effects of open boundaries. By numerical diagonalization we find that the Landé pattern in the energy levels, i.e., $E\left(S\right)\propto S(S+1)$ for total spin $S$, known from, e.g., ring-shaped nanomagnets, can be recovered in odd-membered chains, while strong deviations are found for the lowest excitations in chains with an even number of sites. This particular even-odd effect in the short Heisenberg chains cannot be explained by simple effective Hamiltonians and symmetry arguments. We go beyond these approaches, taking into account quantum fluctuations by means of a path-integral description and the valence bond basis, but the resulting quantum edge-spin picture which is known to work well for long chains does not agree with the numerical results for short chains and cannot explain the even-odd effect. Instead, by analyzing also the classical chain model, we show that spatial fluctuations dominate the physical behavior in short chains, with length $N\lesssim e^{\pi s}$, for any spin $s$. Such short chains are found to display a unique behavior, which is not related to the thermodynamic limit and cannot be described well by theories developed for this regime.

We use spin density functional theory ab initio calculations to theoretically explore the possibility of achieving useful gate control over exchange coupling between cobalt clusters placed on a graphene sheet. By applying an electric field across supercells, we demonstrate that the exchange interaction is strongly dependent on gate voltage, and find that it is also sensitive to the relative sublattice registration of the cobalt clusters. We use our results to discuss strategies for achieving strong and reproducible magnetoelectric effects in graphene/transition-metal hybrid systems.

Here we report a study on the unoccupied states of the half-doped Pr${}_{0.5}$Ca${}_{1.5}$MnO${}_4$ (PCMO). Our investigation, based on temperature dependent x-ray absorption linear dichroism (XLD) and density-functional theory discloses the role of the charge-orbital ordering-disordering mechanisms on the unoccupied density of states. In particular, the lowest unoccupied band has a Mn $e_g$ $d_{3z^2-r^2}$ character, proving that the physical properties of the two-dimensional (2D) PCMO are also determined by the out-of-plane orbital. Yet, the difference in energy between the $d_{3z^2-r^2}$ and $d_{x^2-y^2}$ states is observed to increase when a charge-orbital ordering is established, hence revealing that the Mn $3d$ electronic hopping is frustrated when the MnO${}_6$ cluster orthorhombic strain is increased. This finding addresses the question concerning the complex interplay between the in-plane and out-of plane orbitals in these 2D half-doped single layered manganites.