Lattice Effects Research Articles

Janus Pyrrolopyrrole Aza-dipyrrin: Hydrogen-Bonded Assemblies and Slow Magnetic Relaxation of the Cobalt(II) Complex in the Solid State.

A novel pyrrolopyrrole azadipyrrin (Janus-PPAD) with Janus duality was synthesized by a Schiff base-forming reaction of diketopyrrolopyrrole. The orthogonal interactions of the hydrogen-bonding ketopyrrole and metal-coordinating azadipyrrin moieties in Janus-PPAD enabled the metal ions to be arranged at regular intervals: zinc(II) and cobalt(II) coordination provided metal-coordinated Janus-PPAD dimers, which can subsequently form hydrogen-bonded one-dimensional arrays both in solution and in the solid state. The supramolecular assembly of the zinc(II) complex in solution was investigated by 1 H NMR spectroscopy based on the isodesmic model, in which a binding constant for the elongation of assemblies is constant. Owing to the tetrahedral coordination, in the solid state, the cobalt(II) complex exhibited a slow magnetic relaxation due to the negative D value of -27.1 cm-1 with an effective relaxation energy barrier Ueff of 38.0 cm-1 . The effect of magnetic dilution on the relaxation behavior is discussed. The relaxation mechanism at low temperature was analyzed by considering spin lattice interactions and quantum tunneling effects. The easy-axis magnetic anisotropy was confirmed, and the relevant wave functions were obtained by ab initio CASSCF calculations.

Solid-state lattice effects on high-spin Mn(III) complexes with hexadentate Schiff-base ligand

ABSTRACT This paper reports the syntheses, crystal structures, magnetics and spectroscopic properties of high-spin (HS) salts of formula [Mn(salk-N-1,5,8,12)]Y·S (Y = AsF6 for 1, Y = BF4, S = CH3OH for 2, Y = PF6, S = CH3OH for 3), where (salk-N-1,5,8,12)2− (2,2ʹ-((2E,14E)-3,7,10,14-tetraazahexadeca-2,14-diene-2,15-diyl)diphenolate) is a hexadentate Schiff-base ligand. Crystal structural analysis indicates that the extensive hydrogen bonds play a primary role in stabilisation of their HS states. As an aid to understanding this behaviour, the crystal structure of the known spin crossover compound [Mn(sal-N-1,5,8,12)]AsF6 (4) has been re-examined. To understand the impact of intermolecular forces to the different magnetic behaviours of salts 1 and 4, we evaluate of their supramolecular structures and confirm that intermolecular interactions can significantly affect the cooperativity. Furthermore, electrochemical studies on 1 and 4 shows that the choice of spin state is controlled by the solid-state lattice effects, i.e. cooperativity rather than molecular electronic effect.

Structure and isotropy of lattice pressure tensors for multirange potentials.

We systematically analyze the tensorial structure of the lattice pressure tensors for a class of multiphase lattice Boltzmann models (LBM) with multirange interactions. Due to lattice discrete effects, we show that the built-in isotropy properties of the lattice interaction forces are not necessarily mirrored in the corresponding lattice pressure tensor. This finding opens a different perspective for constructing forcing schemes, achieving the desired isotropy in the lattice pressure tensors via a suitable choice of multirange potentials. As an immediate application, the obtained LBM forcing schemes are tested via numerical simulations of nonideal equilibrium interfaces and are shown to yield weaker and less spatially extended spurious currents with respect to forcing schemes obtained by forcing isotropy requirements only. From a general perspective, the proposed analysis yields an approach for implementing forcing symmetries, never explored so far in the framework of the Shan-Chen method for LBM. We argue this will be beneficial for future studies of nonideal interfaces.

Moiré lattice effects on the orbital magnetic response of twisted bilayer graphene and Condon instability

We analyze the orbital magnetic susceptibility from the band structure of twisted bilayer graphene. Close to charge neutrality, the out-of-plane susceptibility inherits the strong diamagnetic response from graphene. Increasing the doping, a crossover from diamagnetism to paramagnetism is obtained and a logarithmic divergence develops at the van Hove singularity of the Moir\'e lattice in the first band. The enhanced paramagnetism at the van Hove singularity is stronger for relatively large angle but gets suppressed by the flat spectrum towards the vicinity of the first magic angle. A diverging paramagnetic susceptibility indicates an instability towards orbital ferromagnetism with an orbital out-of-plane magnetization and a Landau level structure. The region of instability is however found to be practically very small, parametrically suppressed by the ratio of the electron velocity to the speed of light. We also discuss the in-plane orbital susceptibility at charge neutrality where we find a paramagnetic response and a logarithmic divergence at the magic angle. The paramagnetic response is associated with negative counterflow current in the two layers and does not admit a semiclassical description.

Critical charge fluctuations and emergent coherence in a strongly correlated excitonic insulator

Excitonic insulator is a coherent electronic phase that results from the formation of a macroscopic population of bound particle-hole pairs—excitons. With only a few candidate materials known, the collective excitonic behavior is challenging to observe, being obscured by crystalline lattice effects. Here we use polarization-resolved Raman spectroscopy to reveal the quadrupolar excitonic mode in the candidate zero-gap semiconductor Ta2NiSe5 disentangling it from the lattice phonons. The excitonic mode pronouncedly softens close to the phase transition, showing its electronic character, while its coupling to noncritical lattice modes is shown to enhance the transition temperature. On cooling, we observe the gradual emergence of coherent superpositions of band states at the correlated insulator gap edge, with strong departures from mean-field theory predictions. Our results demonstrate the realization of a strongly correlated excitonic state in an equilibrium bulk material.

Segregation‐controlled densification and grain growth in rare earth‐doped Y2O3

AbstractCation doping of Y2O3 is an established approach for tailoring densification and grain growth during sintering. However, the segregation of doped cations to the grain boundary and their impact on processing are still not completely understood. Segregation can be driven by electrostatic effects due to charge mismatch with the host lattice or elastic effects induced by ion size mismatch. While segregation is caused by thermodynamics, it impacts diffusion and the kinetics of grain boundaries during densification and microstructure evolution. In this study, we utilize two isovalent dopants (La3+ and Gd3+), that is we focus on the elastic component of segregation. We investigate the densification as well as the grain growth kinetics of both doped and undoped Y2O3 during field‐assisted sintering/spark plasma sintering (FAST/SPS). While Gd3+ is showing no significant effect on densification, La3+ resulted in a strongly reduced sintering activity. Furthermore, the analysis of the grain growth behavior during sintering and on predensified samples revealed a decrease in the grain growth coefficient, with La3+ having the strongest impact. The structure and chemistry at the grain boundary were observed by aberration‐corrected TEM. While no structural change was caused by doping, the chemical analysis showed a strong segregation of La3+ to the grain boundary, which could not be observed for Gd3+. The results indicate that segregated La3+ causes a drastic decrease in grain boundary migration rates through solute drag as well as much slower sintering kinetics, likely caused by a decrease in the grain boundary self‐diffusion due to segregation. This study further underlines the importance of the elastic contribution to cation segregation and establishes a clear relationship to grain growth and sintering kinetics, which are both decreased by segregation.

Quantum transport in a single molecular transistor at finite temperature

We study quantum transport in a single molecular transistor in which the central region consists of a single-level quantum dot and is connected to two metallic leads that act as a source and a drain respectively. The quantum dot is considered to be under the influence of electron–electron and electron–phonon interactions. The central region is placed on an insulating substrate that acts as a heat reservoir that interacts with the quantum dot phonon giving rise to a damping effect to the quantum dot. The electron–phonon interaction is decoupled by applying a canonical transformation and then the spectral density of the quantum dot is calculated from the resultant Hamiltonian by using Keldysh Green function technique. We also calculate the tunneling current density and differential conductance to study the effect of quantum dissipation, electron correlation and the lattice effects on quantum transport in a single molecular transistor at finite temperature.

Lattice relaxation, mirror symmetry and magnetic field effects on ultraflat bands in twisted trilayer graphene

Twisted graphene multilayers exhibit strongly correlated insulating states and superconductivity due to the presence of ultraflat bands near the charge neutral point. In this paper, the response of ultraflat bands to lattice relaxation and a magnetic field in twisted trilayer graphene (tTLG) with different stacking arrangements is investigated by using a full tight-binding model. We show that lattice relaxations are indispensable for understanding the electronic properties of tTLG, in particular, of tTLG in the presence of mirror symmetry. Lattice relaxations renormalize the quasiparticle spectrum near the Fermi energy and change the localization of higher energy flat bands. Furthermore, different from the twisted bilayer graphene, the Hofstadter butterfly spectrum can be realized at laboratory accessible strengths of magnetic field. Our work verifies tTLG as a more tunable platform than the twisted bilayer graphene in strongly correlated phenomena.

High-Dimensional Quantum Dynamics Study on Excitation-Specific Surface Scattering Including Lattice Effects of a Five-Atom Surface Cell.

In this work, high-dimensional (21D) quantum dynamics calculations on the mode-specific surface scattering of a carbon monoxide molecule on a copper(100) surface with lattice effects of a five-atom surface cell are performed through the multilayer multiconfiguration time-dependent Hartree (ML-MCTDH) method. We employ a surface model in which five surface atoms near the impact site are treated as fully flexible quantum particles, while all other more distant atoms are kept at fixed locations. To efficiently perform the 21D ML-MCTDH wave packet propagation, the potential energy surface is transferred to a canonical polyadic decomposition form with the aid of a Monte Carlo-based method. Excitation-specific sticking probabilities of CO on Cu(100) are computed, and lattice effects caused by the flexible surface atoms are demonstrated by comparison with sticking probabilities computed for a rigid surface. The dependence of the sticking probability of the initial state of the system is studied, and it is found that the sticking probability is reduced when the surface atom on the impact site is initially vibrationally excited.

Lattice dynamics effects on finite-temperature stability of R1−xFex (R = Y, Ce, Nd, Sm, and Dy) alloys from first principles

We report the effects of lattice dynamics on the thermodynamic stability of binary R1−xFex(0 < x < 1) compounds (R: rare-earth elements, Y, Ce, Nd, Sm, and Dy) at finite temperature predicted by first-principles calculation based on density functional theory (DFT). We first demonstrate that the thermodynamic stability of R1−xFex(0 < x < 1) alloys cannot be predicted accurately by the conventional approach, where only the static DFT energy at T = 0 K is used. This issue can be overcome by considering the entropy contribution, including electronic and vibrational free energies, and we obtained convex hull plots at finite temperatures that successfully explain the thermodynamic stability of various known compounds. Our systematic calculation indicates that vibrational entropy helps stabilize various R1−xFex compounds with increasing temperature. In particular, experimentally reported R2Fe17 compounds are predicted to become thermodynamically stable above ~ 800 K. We also show that thermodynamic stability is rare-earth dependent and discuss its origin. Besides the experimentally reported structures, the stability of two new monoclinic RFe12 structures found by Ishikawa et al. [Phys. Rev. Mater. 4, 104408 (2020)] based on a genetic algorithm are investigated. These monoclinic phases are found to be dynamically stable and have larger magnetization than the ThMn12-type RFe12. Although they are thermodynamically unstable, the formation energies decrease significantly with increasing temperature, indicating the possibility of synthesizing these compounds at high temperatures.

The Lennard-Jones Potential Revisited: Analytical Expressions for Vibrational Effects in Cubic and Hexagonal Close-Packed Lattices.

Analytical formulas are derived for the zero-point vibrational energy and anharmonicity corrections of the cohesive energy and the mode Grüneisen parameter within the Einstein model for the cubic lattices (sc, bcc, and fcc) and for the hexagonal close-packed structure. This extends the work done by Lennard-Jones and Ingham in 1924, Corner in 1939, and Wallace in 1965. The formulas are based on the description of two-body energy contributions by an inverse power expansion (extended Lennard-Jones potential). These make use of three-dimensional lattice sums, which can be transformed to fast converging series and accurately determined by various expansion techniques. We apply these new lattice sum expressions to the rare gas solids and discuss associated critical points. The derived formulas give qualitative but nevertheless deep insight into vibrational effects in solids from the lightest (helium) to the heaviest rare gas element (oganesson), both presenting special cases because of strong quantum effects for the former and strong relativistic effects for the latter.

Visualizing the Kondo lattice crossover in YbRh2Si2 with Compton scattering

In a Kondo lattice, the Luttinger theorem requires the $f$ electron degree of freedom to enter the Fermi volume in the strong-coupling regime, while the $f$ electron remains nearly localized. A common hypothesis assumes that the Fermi volume shrinks with increasing temperature when the $f$ electron decouples from the Fermi surface with the breakdown of the Kondo lattice effect. However, experimental evidence for this phenomen is still very scarce. Here, the authors demonstrate the reconstruction of the Fermi surface between 14 and 300 K from $l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}e$ to $s\phantom{\rule{0}{0ex}}m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}l$ in the prototypical Kondo lattice YbRh${}_{2}$Si${}_{2}$ by Compton scattering.

Combining the core-shell construction with an alloying effect for high efficiency ethanol electrooxidation

Summary Combining core-shell construction with an alloying effect is effective for optimizing the electronic and lattice strain effects for achieving highly catalytic efficiency for a given electrochemical reaction. Here, we report the use of core-shell Au@FePd nanoparticles with an Au core and a sub-nano FePd alloy shell to demonstrate this concept. The Au core modifies the electronic configuration of Pd atoms, while the Fe component in thin shells reduces the lattice expansion of Pd induced by the Au core, both of which endow the Pd shell with an appropriate d-band center, favorable for the electrooxidation of ethanol through the C1 pathway. In particular, the as-prepared core-shell Au@FePd nanoparticles at an optimized Fe:Pd ratio exhibit a mass activity of 13.3 A mg−1 and a specific activity of 20.2 mA cm−2 for the ethanol electrooxidation in an alkaline medium, which outperform those of a majority of the recently reported Pd-based electrocatalysts.

Multipole lattice effects in high refractive index metasurfaces

In this Perspective, we outline the recent progress, primary achievements, and further directions in the development of high refractive index nanostructures and metasurfaces. In particular, we review the role of multipole lattice effects in resonant properties of underlying nanostructures and nanophotonic elements in detail. Planar optical designs with efficient light control at the nanoscale can be engineered based on photonic lattices that operate in the translational regime between two and three dimensions. Such transdimensional lattices include 3D-engineered nanoantennas supporting multipole Mie resonances and arranged in the 2D arrays to harness collective effects in the nanostructure. Lattice effects in the periodic nanoparticle arrays have recently attracted a lot of attention as they enable not only spectrally narrow resonant features but also resonance position tuning over a broad range. The recent results indicate that different nanoparticle multipoles not only produce resonant spectral features but are also involved in the cross-multipole coupling, and these effects need to be accounted for in photonic designs. Multipole lattice phenomena provide an effective way to control nanoparticle resonances, facilitate excitation of additional multipoles through a cross-multipole coupling, and enable light localization in planar photonic elements. We review different effects related to the same- and cross-multipole interactions in the arrays. Both infinite and finite arrays, as well as lattices of complex-shape nanoparticles, which allow out-of-plane multipole excitations, are considered.

Lattice vibration as a knob on exotic quantum criticality

Control of quantum coherence in a many-body system is one of the key issues in modern condensed matter, and conventional wisdom is that lattice vibration is an innate source of decoherence. Much research has been conducted to eliminate lattice effects. Challenging this wisdom, we show that lattice vibration may not be a decoherence source but an impetus of a novel coherent quantum many-body state. We demonstrate the possibility by studying the transverse-field Ising model on a chain with renormalization group and density-matrix renormalization group methods and theoretically discover a stable $\mathcal{N}=1$ supersymmetric quantum criticality with central charge $c=3/2$. Thus, we propose an Ising spin chain with strong spin-lattice coupling as a candidate to observe supersymmetry. Generic precursor conditions of novel quantum criticality are obtained by generalizing the Larkin-Pikin criterion of thermal transitions. Our work provides the perspective that lattice vibration may be a knob for exotic quantum many-body states.

Micropolar homogenization of wavy tetra-chiral and tetra-achiral lattices to identify axial–shear coupling and directional negative Poisson's ratio

Chiral lattices are generally considered to possess auxetic properties with negative Poisson's ratios and high compressibility. However, the effects of anisotropy and handedness (chirality) on their mechanical properties, such as directional moduli, Poisson's ratio, and other coupling effects, are not clearly understood due to the lack of analytical methods to handle both anisotropy and chirality. Herein, we construct a generalized micropolar homogenization method to characterize the elastic constants of tetra-chiral and tetra-achiral lattices having different joint types. The developed method can identify the directional moduli and Poisson's ratio and coupling effects in the anisotropic wavy lattices. The generalized micropolar homogenization of the wavy square lattices reveals i) the axial–shear coupling effect of tetra-chiral structures, ii) the weak correlation of the chirality of tetra-chiral lattices to auxeticity, and iii) the directional positive and negative Poisson's ratio of tetra-achiral lattices. This work offers a powerful platform for metamaterials' design via geometric reconfiguration of lattice structures to adjust both anisotropy and chirality.

Lattice effects on the multiferroic characteristics of (La, Ho) co-substituted BiFeO3

The influence of La3+ and Ho3+ co-substitution on the structural, electrical and magnetic properties of rhombohedrally distorted polycrystalline BiFeO3 is reported. The presence of a tiny residual secondary phase of Bi2Fe4O9, Bi25FeO40 is evident in all the samples, while La doping suppresses the impurity phase and favors the grain growth (size of 1–2 µm) of the pristine compound. A fivefold increase in the dielectric constant (230 at 1 MHz) is achieved for the samples with the highest La content with a remarkable reduction in the dielectric loss. LaxHoyBi1-(x+y)FeO3 ceramics exhibit lossy ferroelectric loops and the magnitude of polarization reduces upon the co-substitution. The antiferromagnetic BiFeO3 transforms to ferromagnetic at low temperatures due to the annihilation of the cycloid-spin structure. The co-substitution effectively improved the magnetic properties. ‘Ho’ substitution helps in achieving the enhanced magnetization values while ‘La’ substitution helps in increasing the magnetic coercivity, HC. The variation of the HC with the temperature is interpreted by means of the two sublattice model.

Hydrates of adenosine 3′,5′-cyclic monophosphate sodium and their transformation

The difference of crystal water behaviors for cAMPNa hydrates was attributed mainly to the steric effects in lattices. Excessive loss of crystal water would destroy the host structure, and result in the disability of reversible transformation.

Exploitation of Triangular Lattice Arrays for Improved Spectral Efficiency in Massive MIMO 5G Systems

A thorough analysis of the performance of planar arrays with a regular periodic lattice is carried out and applied to massive multiple-input-multiple-output (MIMO) systems operating within 5G NR n257 and n258 frequency band. It is shown that, among different arrangements with uniform spacing, a triangular lattice guarantees the reduction of the Average Side Lobe Level (ASLL), a better angular scan resolution of the main beam within a predefined angular sector and a lower mutual coupling level among elements. Moreover, single beam and multibeam application scenarios are considered for the performance comparison and both cases assess the remarkable features offered by a triangular arrangement. Particular attention is paid to illustrate, for different propagation channel scenarios, the effects of the array lattice on overall system performance including average gain as well as Signal-to-Interference plus Noise Ratio (SINR) and Sum Spectral Efficiency (SSE). The obtained results prove that a regular and periodic triangular lattice is appealing for arrays to be adopted in massive MIMO 5G systems.

Different Lattice Distortion Effects on the Tensile Properties of Ni-W Dilute Solutions and CrFeNi and CoCrFeMnNi Concentrated Solutions

The lattice distortion from a solute is mainly due to atomic size and chemical bonding differences with neighboring atoms. The lattice distortion effects in conventional alloys and high-entropy alloys are known to be different but lack of clarification. In this study, FCC-structured dilute solid solutions of Ni, Ni–2at.%W, and Ni–4at.%W and concentrated solutions of equiatomic CrFeNi and CoCrFeMnNi metals were designed to compare their tensile properties. The lattice distortion of both W-containing alloys was only around the large and strong W atoms but that of the two concentrated solutions with similar inter-element atomic size and shear modulus prevailed at all lattice sites. Results showed that the trends in both types of lattice distortion were significantly different on tensile properties. The combination with strength and ductility from high concentration of distorted lattice points was superior over that from low concentration ones even the latter had larger nominal atomic size difference and shear modulus difference. The evolution mechanisms on different mechanical properties from dilute alloys to concentrated alloys were elucidated and clarified under the large contrast of lattice distortion. The universal solid solution strengthening mechanism had been established.