Long-range Coulomb Repulsion Research Articles

Multiferroicity and Topology in Twisted Transition Metal Dichalcogenides.

Van der Waals heterostructures have recently emerged as an exciting platform for investigating the effects of strong electronic correlations, including various forms of magnetic or electrical orders. Here, we perform an unbiased exact diagonalization study of the effects of interactions on topological flat bands of twisted transition metal dichalcogenides (TMDs) at odd integer fillings. For hole-filling ν_{h}=1, we find that the Chern insulator phase, expected from interaction-induced spin-valley polarization of the bare bands, is quite fragile, and gives way to spontaneous multiferroic order-coexisting ferroelectricity and ferromagnetism, in the presence of long-range Coulomb repulsion. We provide a simple real-space picture to understand the phase diagram as a function of interaction range and strength. Our findings establish twisted TMDs as a novel and highly tunable platform for multiferroicity, and we outline a potential route towards electrical control of magnetism in the multiferroic phase.

Voltage sensitive ion channels, minimal size of ordered phases and the evolution of metazoa.

The complex electrical, mechanical and optical behavior of a voltage sensitive ion channel (VSIC) requires that the macromolecule possess at least one ordered phase. For an object to support order, it must be of a minimum size to overcome thermal fluctuations. The limiting size of a stable phase, the correlation length, depends on temperature. Converging lines of experimental evidence indicate that the VSIC is a ferroelectric liquid crystal of the blue phase type. Reaching beyond the analogy of a gated pore, the Channel Activation by Electrostatic Repulsion (CAbER) hypothesis (Leuchtag 2022 Biophys J 121(3):92a) postulates that voltage sensitivity arises from polarization due to the induction of dipoles in the branched sidechains of the numerous isoleucine, leucine and valine residues, producing a polar order in the excitable (closed) phase and making it an insulator by compressing and crowding the ion-conducting segments. On threshold depolarization the dipoles collapse into thermal chaos, reducing the macromolecule's dielectric permittivity and thereby increasing the long-range Coulomb repulsions between the conserved positive charges of the four S4 segments, pushing them apart. The consequent relaxation of the S5 and S6 segments allows their core hydrogen bonds to widen and allow permeant ions to enter and percolate stochastically through the membrane. The critical role of VSIC size in the CAbER model has implications for the beginning of multicellularity during the emergence of the Metazoa in the genomic revolution. At about 300 K, the size effect underlies VSIC activation as an order–disorder transition. This property of the size effect implies that the evolution of Metazoa became possible only after the organism's DNA had grown extensive enough to code for ion channels with dimensions larger than the correlation length to sustain an ordered polar, bioferroelectric, phase.

Loop-current charge density wave driven by long-range Coulomb repulsion on the kagomé lattice

Recent experiments on vanadium-based nonmagnetic kagom\'e metals $A$V$_3$Sb$_5$ ($A=$ K, Rb, Cs) revealed evidence for possible spontaneous time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered state. The long-sought-after quantum order of loop currents has been suggested as a candidate for the TRS breaking state. However, a microscopic model for the emergence of the loop-current CDW due to electronic correlations is still lacking. Here, we calculate the susceptibility of the real and imaginary bond orders on the kagom\'e lattice near van Hove filling, and reveal the importance of next-nearest-neighbor Coulomb repulsion $V_2$ in triggering the instability toward imaginary bond ordered CDW. The concrete effective single-orbital $t$-$V_1$-$V_2$ model on the kagom\'e lattice is then studied, where $t$ and $V_1$ are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram in the plane spanned by $V_1$ and $V_2$ at van Hove filling. The region dominated by $V_1$ is occupied by a $2a_0 \times 2a_0$ real CDW insulator with the inverse of Star-of-David (ISD) bond configuration. Increasing $V_2$ indeed drives a first-order transition from ISD to stabilized loop-current insulators that exhibit four possible current patterns of different topological properties, leading to orbital Chern insulators. We then extend these results away from van Hove filling and show that electron doping helps the stabilization of loop currents, and gives rise to doped orbital Chern insulators with emergent Chern Fermi pockets carrying large Berry curvature and orbital magnetic moment. Our findings provide a concrete model realization of the loop-current Chern metal at the mean-field level for the TRS breaking normal state of the kagom\'e superconductors.

Revisiting Anderson-Higgs mechanism: application of Lieb-Schultz-Mattis theorem

We consider an electron model of superconductivity on a three-dimensional lattice where there are on-site attractive Hubbard interaction and long-range repulsive Coulomb interaction. It is claimed that fully gapped s-wave superconductivity within this model, if present, exhibits spontaneous translation symmetry breaking possibly related to a charge order. Our discussions are based on an application of the Lieb-Schultz-Mattis theorem under some physical assumptions. The inconsistency between the proposed supersolid and experiments can impose some constraints on a reasonable choice of a theoretical model.

Superconductivity from repulsive interactions in rhombohedral trilayer graphene: A Kohn-Luttinger-like mechanism

We study the emergence of superconductivity in rhombohedral trilayer graphene due purely to the long-range Coulomb repulsion. This repulsive-interaction-driven phase in rhombohedral trilayer graphene is significantly different from those found in twisted bilayer and trilayer graphenes. In the latter case, the nontrivial momentum-space geometry of the Bloch wavefunctions leads to an effective attractive electron-electron interaction; this allows for less modulated order parameters and for spin-singlet pairing. In rhombohedral trilayer graphene, we instead find spin-triplet superconductivity with critical temperatures up to 0.15 K. The critical temperatures strongly depend on electron filling and peak where the density of states diverge. The order parameter shows a significant modulation within each valley pocket of the Fermi surface.

Sequence Blockiness Controls the Structure of Polyampholyte Necklaces.

A scaling theory of statistical (Markov) polyampholytes is developed to understand how sequence correlations, that is, the blockiness of positive and negative charges, influences conformational behavior. An increase in the charge patchiness leads to stronger correlation attractions between oppositely charged monomers, but simultaneously, it creates a higher charge imbalance in the polyampholyte. A competition between effective short-range attractions and long-range Coulomb repulsions induces globular, pearl-necklace, or fully stretched chain conformations, depending on the average length of the block of like charges. The necklace structure and the underlying distribution of the net charge are also controlled by the sequence. Sufficiently long blocks allow for charge migration from globular beads (pearls) to strings, thereby providing a nonmonotonic change in the number of necklace beads as the blockiness increases. The sequence-dependent structure of polyampholyte necklaces is confirmed by molecular dynamics simulations. The findings presented here provide a framework for understanding the sequence-encoded conformations of synthetic polyampholytes and intrinsically disordered proteins (IDPs).

Momentum Space Quantum Monte Carlo on Twisted Bilayer GrapheneSupported by the RGC of Hong Kong SAR of China (Grant Nos. 17303019, 17301420, and AoE/P-701/20), the National Key Research and Development Program of China (Grant No. 2016YFA0300502), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB33000000 and XDB28000000), the National Natural Science Foundation of China (Grant

We report an implementation of the momentum space quantum Monte Carlo (QMC) method on the interaction model for the twisted bilayer graphene (TBG). The long-range Coulomb repulsion is treated exactly with the flat bands, spin and valley degrees of freedom of electrons taking into account. We prove the absence of the minus sign problem for QMC simulation when either the two valleys or the two spin degrees of freedom are considered. By taking the realistic parameters of the twist angle and interlayer tunnelings into the simulation, we benchmark the QMC data with the exact band gap obtained at the chiral limit, to reveal the insulating ground states at the charge neutrality point (CNP). Then, with the exact Green’s functions from QMC, we perform stochastic analytic continuation to obtain the first set of single-particle spectral function for the TBG model at CNP. Our momentum space QMC scheme therefore offers the controlled computation pathway for systematic investigation of the electronic states in realistic TBG model at various electron fillings.

Intramolecular micellization and nanopatterning in pH- and thermo-responsive molecular brushes.

Conformational transitions and nanoscale self-organization triggered in double pH- and thermo-responsive molecular brushes by varying environmental conditions are studied by means of analytical mean-field theory and numerical Scheutjens-Fleer self-consistent field modelling. Such molecular brushes are composed of multiple thermo-responsive side chains end-grafted onto the main chain (backbone) and are capable of acquiring ionic charges via reversible (de)protonation of the monomer units. Competition of long-range Coulomb repulsion with short-range solvophobic interactions leads to complex patterns in the intramolecular self-organization of molecular brushes. In particular, we observed formation of pearl necklace-like structures with multiple dense nanodomains formed by weakly ionized collapsed side chains and stabilized by a fraction protruding into the solution and strongly ionized ones. Such structures are thermodynamically stable in a certain parameter range and can be termed as intramolecular micelles. The stimuli-induced intramolecular nanopatterning occurs via a sequence of quasi-first order phase transitions corresponding to splitting/fusion of collapsed domains accompanied by jumps in the average degree of ionization and macromolecular dimensions. A re-entrant sequence of transitions is observed when the salt concentration is used as a control parameter. These theoretical predictions provide guidelines for design of smart unimolecular devices, for example multicompartment nanocarriers of active substances or nanosensors.

Structural investigation of La2SrDyCu2Oy complexities

Abstract The structural complexities in lanthanum cuprates family were revisited with the aim of understanding factors that structurally triggers long-range repulsive Coulomb interactions. In this study, polycrystalline samples of La2SrDyCu2Oy (LSDCO) were prepared via solid-state synthesis using high purity chemicals. The X-ray diffraction experiment revealed an unusual structural anomaly in the [2 0 5] and [2 1 3] planes of the crystal lattice. The lattice system was further probed using the Niggli-reduced cell at gamma = 6.0678. It was observed that grain boundaries leading to electron trapping originates from the CuO2 plane while the mesoscopic phase separation is controlled by the cell type and axial value in the x- and y-axes of the crystal lattice. Although the research partly supports popular findings that the main positive lobes of LSDCO are centered on the z-axis, it observed that the negative lobe is located in a ring-like structure along the X Y plane. This result is particularly interesting because it shows the likely origin of broken symmetry in LSDCO sample. The ion bombardment analysis shows that the LSDCO electron-phonon ratio was 8:5. The microstructural analysis of the LSDCO sample was observed to have magnetic field strength of 1.5 kA/m. LSDCO sample possess enormous structural mystery that may interest further studies beyond superconductivity.

Influence of disorder on the superconducting critical temperature in indium-opal nanocomposites

Transport properties of bulk indium-opal and indium-porous glass superconducting nanocomposites possessing moderate and strong disorder are investigated. A strongly nonmonotonous dependence of the global critical temperature Tc versus normal state conductivity of samples is found. The maximum, which is observed at moderate disorder, has Tc higher than that of clean bulk indium. The increasing part can be explained by the Eliashberg equations with disorder and an additional mechanism of interaction between superconducting and dielectric granules. The descending part of the maximum at higher disorder can be explained by the increasing of long-range Coulomb repulsion due to diffusion of charges. Negative slope in magnetic field dependence of resistivity and a peak in the temperature dependence of resistivity, observed in the sample near the proximity to the disorder-induced superconductor–insulator transition (SIT). A large difference between the onset temperature of superconducting fluctuations, Tcon, and global critical temperature Tc is found and considered in the framework of the weak multifractal theory. Slow time-logarithmic relaxation of the resistivity between Tc and Tcon is observed, which assumes existence of the precursor state near the SIT. This unusual state is discussed in the scope of the many-body localization theory.

Scaling laws for migrating cloud of low-Reynolds-number particles with Coulomb repulsion

We investigate the evolution of spherical clouds of charged particles that migrate under the action of a uniform external electrostatic field. Hydrodynamic interactions are modelled by Oseen equations and the Coulomb repulsion is calculated through pairwise summation. It is shown that strong long-range Coulomb repulsion can prevent the breakup of the clouds covering a wide range of particle Reynolds number $Re_{p}$ and cloud-to-particle size ratio $R_{0}/r_{p}$. A dimensionless charge parameter $\unicode[STIX]{x1D705}_{q}$ is constructed to quantify the effect of the repulsion, and a critical value $\unicode[STIX]{x1D705}_{q,t}$ is deduced, which successfully captures the transition of a cloud from hydrodynamically controlled regime to repulsion-controlled regime. Our results also reveal that, with sufficiently strong repulsion, the cloud undergoes a universal self-similar expansion. Scaling laws of cloud radius $R_{cl}$ and particle number density $n$ are obtained by solving a continuum convection equation.

Effects of interedge scattering on the Wigner crystallization in graphene nanoribbons

We investigate the effects of coupling between the two zigzag edges of graphene nanoribbons on the Wigner crystallization of electrons and holes using a combination of tight-binding, mean field Hubbard and many-body configuration interaction methods. We show that the thickness of the nanoribbon plays a crucial role in the formation of Wigner crystal. For ribbon widths smaller than 16 \mbox{\AA}, increased kinetic energy overcomes the long-range Coulomb repulsion and suppresses the Wigner crystallization. For wider ribbons up to 38 \mbox{\AA} wide, strong Wigner localization is observed for even number of electrons, revealing an even-odd effect also found in Coulomb blockade addition spectrum. Interedge correlations are found to be strong enough to allow simultaneous crystallization on both edges, although an applied electric field can decouple the two edges. Finally, we show that Wigner crystallization can also occurs for holes, albeit weaker than for electrons.

Evolution of Clouds of Migrating Micro-particles with Hydrodynamic and Electrostatic Interactions

The evolution of a migrating cloud of particles with electrohydrodynamic interactions is numerically investigated. The hydrodynamic interaction is modelled by Oseen dynamics in the limit of small-but-finite particle Reynolds number. The effects of external field and the particle-particle Coulomb repulsion, calculated through pairwise summation, are quantified by a charge parameter, κ q . With a small or zero κ q , the cloud is seen to flatten into a toroidal configuration and eventually breaks up into two small clouds, indicating that the external electrostatic field has a similar effect with gravity. Increasing the long-range Coulomb repulsion can delay or even totally prevent the breakup. With sufficiently strong repulsion, the cloud undergoes a self-similar expansion. Finally, we show that the breakup of the cloud is strongly related to the initial inhomogeneity of the particle concentration. This chaotic characteristic, however, is dramatically depressed by the long-range Coulomb repulsion.

Effect of long-range electrostatic repulsion on pore clogging during microfiltration.

We perform computer simulations based on adhesive contact mechanics to demonstrate the clogging process of charged microparticles at the single-pore level. The effect of long-range Coulomb repulsion on clogging is characterized in terms of bulk permeability, the number of penetrating particles, and particle capture efficiency. Results indicate that the repulsion among particles delays or even totally prevents the formation of clogs. A clogging phase diagram, in the form of the driving pressure and a proposed charge parameter κ_{q}, is constructed to quantify the clogging-nonclogging transition. In addition, a critical state, where the capture efficiency of particles decreases to its minimum, is identified as a clogging-nonclogging criterion for repulsive particles. The distributions of the local volume fraction show that the structure of clogs is mainly determined by short-range adhesion. With relatively strong adhesion, a loose clog will be formed and it is easier for particles to penetrate. Finally, a schematic representation of the clogging process, considering both long-range repulsion and adhesion, is proposed to show the relationship between the clogging results and the interparticle interactions.

Metal-atom Interactions and Clustering in Organic Semiconductor Systems

The interatomic interactions and clustering of metal atoms have been studied by first-principles calculations in graphene, pentacene, and polyacetylene as representative organic systems. It is shown that long-range repulsive Coulomb interaction appears between metal atoms with small electronegativity such as Al due to their ionization on host organic molecules, inducing their scattered distribution in organic systems. On the other hand, metal atoms with large electronegativity such as Au are weakly bonded to organic molecules, easily diffuse in molecular solids, and prefer to combine with each other owing to their short-range strong metallic-bonding interaction, promoting metal cluster generation in organic systems.

A disorder-enhanced quasi-one-dimensional superconductor

A powerful approach to analysing quantum systems with dimensionality d>1 involves adding a weak coupling to an array of one-dimensional (1D) chains. The resultant quasi-1D (q1D) systems can exhibit long-range order at low temperature, but are heavily influenced by interactions and disorder due to their large anisotropies. Real q1D materials are therefore ideal candidates not only to provoke, test and refine theories of strongly correlated matter, but also to search for unusual emergent electronic phases. Here we report the unprecedented enhancement of a superconducting instability by disorder in single crystals of Na2−δMo6Se6, a q1D superconductor comprising MoSe chains weakly coupled by Na atoms. We argue that disorder-enhanced Coulomb pair-breaking (which usually destroys superconductivity) may be averted due to a screened long-range Coulomb repulsion intrinsic to disordered q1D materials. Our results illustrate the capability of disorder to tune and induce new correlated electron physics in low-dimensional materials.

Local density of states and its mesoscopic fluctuations near the transition to a superconducting state in disordered systems

We develop a theory of the local density of states (LDOS) of disordered superconductors, employing the non-linear sigma-model formalism and the renormalization-group framework. The theory takes into account the interplay of disorder and interaction couplings in all channels, treating the systems with short-range and Coulomb interactions on equal footing. We explore 2D systems that would be Anderson insulators in the absence of interaction and 2D or 3D systems that undergo Anderson transition in the absence of interaction. We evaluate both the average tunneling density of states and its mesoscopic fluctuations which are related to the LDOS multifractality in normal disordered systems. The obtained average LDOS shows a pronounced depletion around the Fermi energy, both in the metallic phase (i.e., above the superconducting critical temperature $T_c$) and in the insulating phase near the superconductor-insulator transition (SIT). The fluctuations of the LDOS are found to be particularly strong for the case of short-range interactions -- especially, in the regime when $T_c$ is enhanced by Anderson localization. On the other hand, the long-range Coulomb repulsion reduces the mesoscopic LDOS fluctuations. However, also in a model with Coulomb interaction, the fluctuations become strong when the systems approaches the SIT.

Effect of vertex corrections on the possibility of chiral symmetry breaking induced by long-range Coulomb repulsion in graphene

In this paper, we consider the possibility of chiral (charge or spin density wave) symmetry breaking in graphene due to long-range Coulomb interaction by comparing the results of the Bethe-Salpeter and functional renormalization-group approaches. The former approach performs a summation of ladder diagrams in the particle-hole channel and reproduces the results of the Schwinger-Dyson approach for the critical interaction strength of the quantum phase transition. The renormalization-group approach combines the effect of different channels and allows to study the role of vertex corrections. The critical interaction strength, which is necessary to induce the symmetry breaking in the latter approach, is found in the static approximation to be ${\ensuremath{\alpha}}_{c}={e}^{2}/(\ensuremath{\epsilon}{v}_{F})\ensuremath{\approx}1.05$ without considering the Fermi velocity renormalization, and ${\ensuremath{\alpha}}_{c}=3.7$ with accounting the renormailzation of the Fermi velocity. The latter value is expected to be, however, reduced, when the dynamic screening effects are taken into account, yielding the critical interaction, which may be comparable to the one in freely suspended graphene. We show that the vertex corrections are crucially important to obtain the mentioned values of critical interactions.

Effect of long-range repulsive Coulomb interactions on packing structure of adhesive particles.

The packing of charged micron-sized particles is investigated using discrete element simulations based on adhesive contact dynamic model. The formation process and the final obtained structures of ballistic packings are studied to show the effect of interparticle Coulomb force. It is found that increasing the charge on particles causes a remarkable decrease of the packing volume fraction ϕ and the average coordination number 〈Z〉, indicating a looser and chainlike structure. Force-scaling analysis shows that the long-range Coulomb interaction changes packing structures through its influence on particle inertia before they are bonded into the force networks. Once contact networks are formed, the expansion effect caused by repulsive Coulomb forces are dominated by short-range adhesion. Based on abundant results from simulations, a dimensionless adhesion parameter Ad*, which combines the effects of the particle inertia, the short-range adhesion and the long-range Coulomb interaction, is proposed and successfully scales the packing results for micron-sized particles within the latest derived adhesive loose packing (ALP) regime. The structural properties of our packings follow well the recent theoretical prediction which is described by an ensemble approach based on a coarse-grained volume function, indicating some kind of universality in the low packing density regime of the phase diagram regardless of adhesion or particle charge. Based on the comprehensive consideration of the complicated inter-particle interactions, our findings provide insight into the roles of short-range adhesion and repulsive Coulomb force during packing formation and should be useful for further design of packings.

Nanofriction in Cavity Quantum Electrodynamics.

The dynamics of cold trapped ions in a high-finesse resonator results from the interplay between the long-range Coulomb repulsion and the cavity-induced interactions. The latter are due to multiple scatterings of laser photons inside the cavity and become relevant when the laser pump is sufficiently strong to overcome photon decay. We study the stationary states of ions coupled with a mode of a standing-wave cavity as a function of the cavity and laser parameters, when the typical length scales of the two self-organizing processes, Coulomb crystallization and photon-mediated interactions, are incommensurate. The dynamics are frustrated and in specific limiting cases can be cast in terms of the Frenkel-Kontorova model, which reproduces features of friction in one dimension. We numerically recover the sliding and pinned phases. For strong cavity nonlinearities, they are in general separated by bistable regions where superlubric and stick-slip dynamics coexist. The cavity, moreover, acts as a thermal reservoir and can cool the chain vibrations to temperatures controlled by the cavity parameters and by the ions' phase. These features are imprinted in the radiation emitted by the cavity, which is readily measurable in state-of-the-art setups of cavity quantum electrodynamics.