Intersite Interactions Research Articles

Single Platinum Atom Catalysts with HighDensity.

As a new frontier, the rapid development of single-atom catalysts (SACs) in heterogeneous catalysis has attracted extensive attention. However, a fundamental understanding of the dynamic formation process of active metal atom sites is lacking. The rational design of high-performance SACs now becomes possible byexplicitly addressing the conundrum of constructing high-density SACs and elucidating the evolution of SACs during the synthesis.

Lattice control of nonergodicity in a polar lattice gas

Inter-site interactions in polar lattice gases may result, due to Hilbert-space fragmentation, in a lack of ergodicity even in absence of disorder. We show that the inter-site interaction in a one-dimensional dipolar gas in an optical lattice departs from the usually considered $1/r^3$ dependence, acquiring a universal form that depends on the transversal confinement and the lattice depth. Due to the crucial role played by the nearest- and next-to-nearest neighbors, the Hilbert-space fragmentation and particle dynamics are very similar to that of a power-law model $1/r^{\beta_{\mathrm{eff}}<3}$, where $\beta_{\mathrm{eff}}$ is experimentally controllable by properly tailoring the transversal confinement. Our results are of direct experimental relevance for experiments on dipolar gases in optical lattices, and show that the particle dynamics may be remarkably different if the quasi-1D lattice model is realized in a strong 3D lattice, or by means of a strong transversal harmonic confinement.

Unraveling the effects of inter-site Hubbard interactions in spinel Li-ion cathode materials.

Accurate first-principles predictions of the structural, electronic, magnetic, and electrochemical properties of cathode materials can be key in the design of novel efficient Li-ion batteries. Spinel-type cathode materials LixMn2O4 and LixMn1.5Ni0.5O4 are promising candidates for Li-ion battery technologies, but they present serious challenges when it comes to their first-principles modeling. Here, we use density-functional theory with extended Hubbard functionals-DFT+U+V with on-site U and inter-site V Hubbard interactions-to study the properties of these transition-metal oxides. The Hubbard parameters are computed from first-principles using density-functional perturbation theory. We show that while U is crucial to obtain the right trends in properties of these materials, V is essential for a quantitative description of the structural and electronic properties, as well as the Li-intercalation voltages. This work paves the way for reliable first-principles studies of other families of cathode materials without relying on empirical fitting or calibration procedures.

Real-time dynamics of a critical resonating valence bond spin liquid

Implementation of the hardcore-dimer Hilbert space in cold Rydberg-atom simulators opens a new route of investigating real-time dynamics of dimer liquids under Hamiltonian quench. Here, we consider an initial Resonating Valence Bond (RVB) state on the square lattice realizing a critical Coulomb phase with algebraic and dipolar correlations. Using its representation as a special point of a broad manifold of SU($2$)-symmetric, translationnally invariant, Projected Entangled Pair States (PEPS), we compute its non-equilibrium dynamics upon turning on inter-site Heisenberg interactions. We show that projecting the time-evolution onto the PEPS manifold remains accurate at small time scales. We also find that the state evolves within a PEPS sub-manifold characterized by a U($1$) gauge symmetry, suggesting that the Coulomb phase is stable under such unitary evolution.

The Dynamic Formation from Metal-Organic Frameworks of High-Density Platinum Single-Atom Catalysts with Metal-Metal Interactions.

Single-atom catalysts (SACs) hold great promise for highly efficient heterogeneous catalysis, yet the practical applications require the development of high-density active sites with flexible geometric structures. The lack of understanding in the dynamic formation process of single atoms in the host framework has been plaguing the controllable synthesis of next generation SACs. Here using Co-based metal-organic frameworks (MOFs) as a starting substrate, we fully elucidated the formation of high-density Pt single atoms with inter-site interactions in derived Co3 O4 host. The cation exchange process and dynamic evolution of Pt-Pt interactions, organic ligand cleavage and Pt-oxygen coordination formation during the pyrolysis process have been unambiguously interpreted by a series of in situ/ex situ spectroscopic measurements and theoretical computation. These findings would direct the synthesis of high-density SACs with metal-metal interactions, which demonstrate significantly enhanced structural flexibility and catalytic properties.

Euclidean-to-Lorentzian wormhole transition and gravitational symmetry breaking in the Sachdev-Ye-Kitaev model

We study a two-site Sachdev-Ye-Kitaev model with complex couplings and a weak intersite interaction. At low temperatures, the system is dual to a Euclidean wormhole in Jackiw-Teitelboim gravity plus matter. Interestingly, the energy spectrum becomes real for sufficiently strong intersite coupling despite the Hamiltonian being non-Hermitian. In gravity, this complex-to-real transition corresponds to a Euclidean-to-Lorentzian transition: a dynamical restoration of the gravitational $\mathrm{SL}(2,\mathbb{R})$ symmetry of the Lorentzian wormhole, broken to U(1) in the Euclidean wormhole. We show this by identifying an order parameter for the symmetry breaking and by matching the oscillating patterns of the Green's functions. Above the transition, the system can be continued to Lorentzian signature and is dual to an eternal traversable wormhole. Additionally, we observe a thermal phase transition from the wormhole to two black holes and provide a detailed matching of the associated physical quantities. The analysis of level statistics reveals that in a broad range of parameters the dynamics is quantum chaotic in the universality class of systems with time reversal invariance.

Mott versus Hybridization Gap in the Low-Temperature Phase of 1T-TaS_{2}.

We address the long-standing problem of the ground state of 1T-TaS_{2} by computing the correlated electronic structure of stacked bilayers using the GW+EDMFT method. Depending on the surface termination, the semi-infinite uncorrelated system is either band insulating or exhibits a metallic surface state. For realistic values of the on-site and inter-site interactions, a Mott gap opens in the surface state, but it is smaller than the gap originating from the bilayer structure. Our results are consistent with recent scanning tunneling spectroscopy measurements for different terminating layers, and with our own photoemission measurements, which indicate the coexistence of spatial regions with different gaps in the electronic spectrum. By comparison to exact diagonalization data, we clarify the interplay between Mott insulating and band insulating behavior in this archetypal layered system.

Crucial role of out-of-plane Sb p orbitals in Van Hove singularity formation and electronic correlations in the superconducting kagome metal CsV3Sb5

First-principles density functional theory calculations are performed to understand the electronic structure and interaction parameters for recently discovered superconducting Kagome metal CsV$_3$Sb$_5$. A systematic analysis of the tight-binding parameters based on maximally localized Wannier function method demonstrates that the out-of-plane Sb$^{\rm out}$-$p$ orbital is a key element in complete description of the three Van Hove singularity structures known in this material at $M$ point near the Fermi level. Further, the correlation strengths are also largely determined by Sb$^{\rm out}$-$p$ states. Based on constrained random phase approximation, we find that on-site and inter-site interaction parameter are both significantly affected by the screening effect of Sb$^{\rm out}$-$p$ orbitals. As the role of this previously unnoticed orbital state can be tuned or controlled by out-of-plane lattice parameters, we examine the electronic structure and particularly the evolution of Van Hove singularity points as a function of strain and pressure, which can serve as useful knobs to control the material properties.

Superconductivity in the extended Hubbard chain with three-electron density interaction

At weak coupling and at half filling, we analytically study effects of an intersite three-electron density interaction (P) of the one-dimensional extended Hubbard model in addition to the usual two-electron density interactions (U,V). By the bosonization and renormalization group approaches, we obtain the ground-state phase diagram of the model. In particular, we find that even in the presence of repulsive U and V, the attractive P may give rise to the superconducting phases for 2V≤min{2|P|;4|P|−U}.

Metal-metal interactions in correlated single-atom catalysts

Single-atom catalysts (SACs) include a promising family of electrocatalysts with unique geometric structures. Beyond conventional ones with fully isolated metal sites, an emerging class of catalysts with the adjacent metal single atoms exhibiting intersite metal-metal interactions appear in recent years and can be denoted as correlated SACs (C-SACs). This type of catalysts provides more opportunities to achieve substantial structural modification and performance enhancement toward a wider range of electrocatalytic applications. On the basis of a clear identification of metal-metal interactions, this review critically examines the recent research progress in C-SACs. It shows that the control of metal-metal interactions enables regulation of atomic structure, local coordination, and electronic properties of metal single atoms, which facilitate the modulation of electrocatalytic behavior of C-SACs. Last, we outline directions for future work in the design and development of C-SACs, which is indispensable for creating high-performing new SAC architectures.

Impact of non-Hermitian mode interaction on inter-cavity light transfer

Understanding inter-site mutual mode interaction in coupled physical systems is essential to comprehend large compound systems, as this local interaction determines the successive multiple inter-site energy transfer efficiencies. In the present study, we demonstrate that only the non-Hermitian coupling can correctly account for the light transfer between two coupled optical cavities. We also reveal that the non-Hermitian coupling effect becomes crucial as the system dimension decreases. Our results provide important insight for handling general-coupled devices in the subwavelength regime.

Crystal-field excitations and quadrupolar fluctuations of 4f-electron systems studied by polarized light scattering

We present Raman-scattering results for three materials, CeB6, TbInO3, and YbRu2Ge2, to illustrate the essential aspects of crystal-field (CF) excitations and quadrupolar fluctuations of 4f-electron systems. For CF excitations, we illustrate how the 4f orbits are split by spin-orbit coupling and CF potential by presenting spectra for inter- and intra-multiplet excitations over a large energy range. We discuss identification of the CF ground state and establishment of low-energy CF level scheme from the symmetry and energy of measured CF excitations. In addition, we demonstrate that the CF linewidth is a sensitive probe of electron correlation by virtue of self-energy effect. For quadrupolar fluctuations, we discuss both ferroquadrupolar (FQ) and antiferroquadrupolar (AFQ) cases. Long-wavelength quadrupolar fluctuations of the same symmetry as the FQ order parameter persists well above the transition temperature, from which the strength of electronic intersite quadrupolar interaction can be evaluated. The tendency towards AFQ ordering induces ferromagnetic correlation between neighboring 4f-ion sites, leading to long-wavelength magnetic fluctuations.

Hidden breathing kagome topology in hexagonal transition metal dichalcogenides

A kagome lattice, formed by triangles of two different directions, is known to have many emergent quantum phenomena. Under the breathing anisotropy of bond strengths, this lattice can become a higher-order topological insulator (HOTI), which hosts topologically protected corner states. Experimental realizations of HOTI on breathing kagome lattices have been reported for various artificial systems, but not for simple natural materials with an electronic breathing kagome lattice. Here we prove that a breathing kagome lattice and HOTI are hidden inside the electronic structure of hexagonal transition metal dichalcogenides (h-TMD). Due to the trigonal prismatic symmetry, $s{p}^{2}$-like hybrid $d$ orbitals create an electronic kagome lattice with anisotropic intersite and on-site hopping interactions. We demonstrate that HOTI h-TMD triangular nanoflakes host topologically protected corner states, which could be quantum-mechanically entangled with triple degeneracy. Because $\mathrm{h}\text{\ensuremath{-}}\mathrm{TM}{\mathrm{D}}_{\mathrm{S}}$ are easily synthesizable and stable with a large bandgap at ambient conditions, our findings open an avenue for quantum physics based on simple condensed matter systems.

Hyperuniform electron distributions controlled by electron interactions in quasicrystals

We study how the electron-electron interactions influence the charge distributions in the metallic state of quasicrystals. As a simple theoretical model, we introduce an extended Hubbard model on the Penrose lattice, and numerically solve the model (up to $\ensuremath{\sim}1.4$ million sites) within the Hartree-Fock approximation. Because each site on the quasiperiodic lattice has a different local geometry, the Coulomb interaction, in particular the intersite one, works in a site-dependent way, leading to a nontrivial redistribution of the charge. The resultant charge distribution patterns are not multifractal but characterized by hyperuniformity, which offers a measure to distinguish various inhomogeneous but ordered distributions. We clarify how the electron interactions alter the order metric of the hyperuniformity, revealing that the intersite interaction considerably affects the hyperuniformity in particular on the electron-rich side.

Hilbert Space Shattering and Disorder-Free Localization in Polar Lattice Gases.

Emerging dynamical constraints resulting from intersite interactions severely limit particle mobility in polar lattice gases. Whereas in absence of disorder hard-core Hubbard models with only strong nearest-neighbor interactions present Hilbert space fragmentation but no many-body localization for typical states, the 1/r^{3} tail of the dipolar interaction results in Hilbert space shattering, as well as in a dramatically slowed down dynamics and eventual disorder-free localization. Our results show that the study of the intriguing interplay between disorder- and interaction-induced many-body localization is within reach of future experiments with magnetic atoms and polar molecules.

Ferro-octupolar Order and Low-Energy Excitations ind2Double Perovskites of Osmium

Conflicting interpretations of experimental data preclude the understanding of the quantum magnetic state of spin-orbit coupled $d^2$ double perovskites. Whether the ground state is a Janh-Teller-distorted order of quadrupoles or the hitherto elusive octupolar order remains debated. We resolve this uncertainty through direct calculations of all-rank inter-site exchange interactions and inelastic neutron scattering cross-section for the $d^2$ double perovskite series Ba$_2M$OsO$_6$ ($M$= Ca, Mg, Zn). Using advanced many-body first principles methods we show that the ground state is formed by ferro-ordered octupoles coupled by superexchange interactions within the ground-state $E_g$ doublet. Computed ordering temperature of the single second-order phase-transition is consistent with experimentally observed material-dependent trends. Minuscule distortions of the parent cubic structure are shown to qualitatively modify the structure of gaped magnetic excitations.

Optical lattice for a tripodlike atomic level structure

Standard optical potentials use off-resonant laser standing wave induced AC-Stark shift. In a recent development [Phys. Rev. Lett. {\bf 117}, 233001 (2016)] a three-level scheme in $\Lambda$ configuration coupled coherently by resonant laser fields was introduced leading to an effective lattice with subwavelength potential peaks. Here as an extension of that work to a four level atomic setup in the tripod configuration is used to create spin $1/2$-like two-dimensional dark-space with 1D motion and the presence of external gauge fields. Most interestingly for a possible application, the lifetime for a dark subspace motion is up to two orders of magnitude larger than for a similar $\Lambda$ system. The model is quite flexible leading to lattices with significant nearest, next-nearest, or next-next-nearest hopping rates, $J_1,J_2,J_3$ opening up new intriguing possibilities to study, e.g. frustrated systems. The characteristic Wannier functions lead also to new type of inter-site interactions not realizable in typical optical lattices.

Interplay between singlet and triplet pairings in multiband two-dimensional oxide superconductors

We theoretically study the superconducting properties of multi-band two-dimensional transition metal oxide superconductors by analyzing not only the role played by conventional singlet pairings, but also by the triplet order parameters, favored by the spin-orbit couplings present in these ma- terials. In particular, we focus on the two-dimensional electron gas at the (001) interface between LaAlO3 and SrTiO3 band insulators where the low electron densities and the sizeable spin-orbit couplings affect the superconducting features. Our theoretical study is based on an extended su- perconducting mean-field analysis of the typical multi-band tight-binding Hamiltonian, as well as on a parallel analysis of the effective electronic bands in the low-momentum limit, including static on-site and inter-site intra-band attractive potentials under applied magnetic fields. The presence of triplet pairings is able to strongly reduce the singlet order parameters which, as a result, are no longer a monotonic function of the charge density. The interplay between the singlet and the triplet pairings affects the dispersion of quasi-particle excitations in the Brillouin zone and also induces anisotropy in the superconducting behavior under the action of an in-plane and of an out- of-plane magnetic fields. Finally, non-trivial topological superconducting states become stable as a function of the charge density, as well as of the magnitude and of the orientation of the magnetic field. In addition to the chiral, time-reversal breaking, topological superconducting phase, favored by the linear Rashba couplings and by the on-site attractive potentials in the presence of an out- of-plane magnetic field, we find that a time-reversal invariant topological helical superconducting phase is promoted by not-linear spin-orbit couplings and by the inter-site attractive interactions in the absence of magnetic field.

Charge-order on the triangular lattice: Effects of next-nearest-neighbor attraction in finite temperatures

The extended Hubbard model in the atomic limit, which is equivalent to lattice S=1/2 fermionic gas, is considered on the triangular lattice. The model includes onsite Hubbard U interaction and both nearest-neighbor (W1) and next-nearest-neighbor (W2) density–density intersite interactions. The variational approach treating the U term exactly and the Wl terms in the mean-field approximation is used to investigate thermodynamics of the model and to find its finite temperature (T>0) phase diagrams (as a function of particle concentration) for W1>0 and W2<0. Two different types of charge-order (i.e., DCO and TCO phases) within 3×3 unit cells as well as the nonordered (NO) phase occur on the diagram. Moreover, several kinds of phase-separated (PS) states (NO/DCO, DCO/DCO, DCO/TCO, and TCO/TCO) are found to be stable for fixed concentration. Attractive W2<0 stabilizes PS states at T=0 and it extends the regions of their occurrence at T>0. The evolution of the diagrams with increasing of |W2|/W1 is investigated. It is found that some of the PS states are stable only at T>0. Two different critical values of |W2|/W1 are determined for the PS states, in which two ordered phases of the same type (i.e., two domains of the DCO or TCO phase) coexist.

Bloch's band structures of a pair of interacting electrons in simple one- and two-dimensional lattices

We have applied the Bloch's theorem to a pair of interacting electrons in periodic simple one-dimensional lattice and two-dimensional square lattice with both intra-site and nearest neighbor inter-site interactions. The electronic structures can be classified as binding bands and unbinding continuum states of a pair of electrons. In addition to the usual intra-site binding band, we found the nearest-neighbor inter-site binding bands in both spin singlet and triplet states irrespective of interaction nature. The binding bands are either above or below the unbinding continuum states separated by gaps for a repulsive or attractive interaction potential.