Charge Correlations Research Articles

Noise and thermodynamic uncertainty relation in "underwater" molecular junctions.

We determine the zero-frequency charge current noise in a metal-molecule-metal junction embedded in a thermal environment, e.g., a solvent, dominated by sequential charge transmission described by a classical master equation, and we study the dependence of specific model parameters, i.e., the environmental reorganization energy and relaxation behavior. Interestingly, the classical current noise term has the same structure as its quantum analog, which reflects a charge correlation due to the bridging molecule. We further determine the thermodynamic uncertainty relation (TUR) defininig a bound on the relationship between the average charge current, its fluctuation, and the entropy production in an electrochemical junction in the Marcus regime. In the second part, we use the same methodology to calculate the current noise and the TUR for a protoype photovoltaic cell in order to predict its upper bound for the efficiency of energy conversion into useful work.

Finite-temperature tensor network study of the Hubbard model on an infinite square lattice

The Hubbard model is a longstanding problem in the theory of strongly correlated electrons and a very active one in the experiments with ultracold fermionic atoms. Motivated by current and prospective quantum simulations, we apply a two-dimensional tensor network, an infinite projected entangled pair state, evolved in imaginary time by the neighborhood tensor update algorithm working directly in the thermodynamic limit. With U(1)xU(1) symmetry and the bond dimensions up to 29, we generate thermal states down to the temperature of 0.17 times the hopping rate. We obtain results for spin and charge correlators, unaffected by boundary effects. The spin correlators, measurable in prospective ultracold atoms experiments attempting to approach the thermodynamic limit, provide evidence of disruption of the antiferromagnetic background with mobile holes in a slightly doped Hubbard model. The charge correlators reveal the presence of hole-doublon pairs near half filling and signatures of hole-hole repulsion on doping. We also obtain specific heat in the slightly doped regime.

Charge and spin correlations in insulating and incoherent metal states of twisted bilayer graphene

We study electronic, charge, and magnetic properties of twisted bilayer graphene with fillings $2\leq n\leq 6$ per moire unit cell within the recently introduced formulation of extended dynamical mean-field theory (E-DMFT) for two-sublattice systems. We use previously obtained hopping parameters between the states, described by Wannier functions, centered at the lattice spots of AB and BA stacking, and the long-range Coulomb interaction, obtained within cRPA analysis. We show that account of spin exchange between AB and BA nearest neighbour spots is crucial to introduce charge and spin correlations between these spots. Account of this exchange yields preferable concentration of electrons in the same valley, with the tendency of parallel spin alignment of electrons in AB and BA spots, in agreement with earlier results of the strong coupling analysis, suggested SU(2)$\times$SU(2) emergent spin-valley symmetry. The local spectral functions show almost gapped state at the fillings $n=2,4,6$, and incoherent metal state for the other fillings. We find that in both cases the local states of electrons have rather long lifetime. At the same time, the non-local charge- and spin susceptibilities, obtained within the ladder approximation, are peaked at incommensurate wave vectors, which implies that the above discussed ordering tendencies are characterised by an incommensurate pattern.

Long-range charge transport in homogeneous and alternating-rigidity chains.

We study the interplay of intrinsic-electronic and environmental factors in long-range charge transport across molecular chains with up to N ∼ 80 monomers. We describe the molecular electronic structure of the chain with a tight-binding Hamiltonian. Thermal effects in the form of electron decoherence and inelastic scattering are incorporated with the Landauer-Büttiker probe method. In short chains of up to ten units, we observe the crossover between coherent (tunneling, ballistic) motion and thermally-assisted conduction, with thermal effects enhancing the current beyond the quantum coherent limit. We further show that unconventional (nonmonotonic with size) transport behavior emerges when monomer-to-monomer electronic coupling is made large. In long chains, we identify a different behavior, with thermal effects suppressing the conductance below the coherent-ballistic limit. With the goal to identify a minimal model for molecular chains displaying unconventional and effective long-range transport, we simulate a modular polymer with alternating regions of high and low rigidity. Simulations show that, surprisingly, while charge correlations are significantly affected by structuring environmental conditions, reflecting charge delocalization, the electrical resistance displays an averaging effect, and it is not sensitive to this patterning. We conclude by arguing that efficient long-range charge transport requires engineering both internal electronic parameters and environmental conditions.

Random singlet-like phase of disordered Hubbard chains

Local moment formation is ubiquitous in disordered semiconductors such as Si:P, where it is observed both in the metallic and the insulating regimes. Here, we focus on local moment behavior in disordered insulators, which arises from short-ranged, repulsive electron-electron interactions. Using density matrix renormalization group and strong-disorder renormalization group methods, we study paradigmatic models of interacting insulators: one dimensional Hubbard chains with quenched randomness. In chains with either random fermion hoppings or random chemical potentials, both at and away from half-filling, we find exponential decay of charge and fermion 2-point correlations but power-law decay of spin correlations that are indicative of the random singlet phase. The numerical results can be understood qualitatively by appealing to the large-interaction limit of the Hubbard chain, in which a remarkably simple picture emerges.

Impact parameter dependence of color charge correlations in the proton

The impact parameter dependence of color charge correlators in the proton is obtained from the light front formalism in light cone gauge. We include NLO corrections due to the |qqqg\rangle|qqqg⟩ Fock state via light-cone perturbation theory. Near the center of the proton, the bb-dependence of the correlations is very different from a ``transverse profile function’’. The resulting tt-dependence of exclusive J/\PsiJ/Ψ photoproduction transitions from exponential to power law at |t| \approx 1|t|≈1~GeV^22. This prediction could be tested at upcoming DIS facilities or in nucleus-proton ultraperipheral collisions (UPCs).

Unifying Weak and Strong Charge Correlations within the Random Phase Approximation: Polyampholytes of Various Sequences

We consider the problem of charge correlations in self-coacervate phases of polyampholytes and disordered proteins with different monomer sequences. An analytical approach consistently describing both weak and strong correlations is proposed, which is based on the improvement of the random phase approximation (RPA) by taking into account (i) discreteness of charges in polymer chains and (ii) a finite number of wave modes of the charge density fluctuations. These modifications are an essential element of the particle-to-field transformation. For strong Coulomb interactions, the generalized RPA reproduces the free energy of the strongly correlated Wigner liquid of disjointed charges. The developed theory is applied to describe coil-to-globule transitions in single-chain polyampholytes as a function of the monomer sequence. Comparison with results of molecular simulations confirms that the theory reproduces the observed scaling laws for globule size at weak charge correlations, and in addition, it provides a quantitative description of electrostatic interactions when correlations are strong.

Anomalous Hall effect and perpendicular magnetic anisotropy in ultrathin ferrimagnetic NiCo2O4 films

The inverse spinel ferrimagnetic NiCo2O4 possesses high magnetic Curie temperature TC, high spin polarization, and strain-tunable magnetic anisotropy. Understanding the thickness scaling limit of these intriguing magnetic properties in NiCo2O4 thin films is critical for their implementation in nanoscale spintronic applications. In this work, we report the unconventional magnetotransport properties of epitaxial (001) NiCo2O4 films on MgAl2O4 substrates in the ultrathin limit. Anomalous Hall effect measurements reveal strong perpendicular magnetic anisotropy for films down to 1.5 unit cell (1.2 nm), while TC for 3 unit cell and thicker films remains above 300 K. The sign change in the anomalous Hall conductivity (σxy) and its scaling relation with the longitudinal conductivity (σxx) can be attributed to the competing effects between impurity scattering and band intrinsic Berry curvature, with the latter vanishing upon the thickness driven metal–insulator transition. Our study reveals the critical role of film thickness in tuning the relative strength of charge correlation, Berry phase effect, spin–orbit interaction, and impurity scattering, providing important material information for designing scalable epitaxial magnetic tunnel junctions and sensing devices using NiCo2O4.

Superconducting-like and magnetic transitions in oxygen-implanted diamond-like and amorphous carbon films, and in highly oriented pyrolytic graphite

In our previously published work, we have reported colossal magnetoresistance, Andreev oscillations, ferromagnetism, and granular superconductivity in oxygen-implanted carbon fibers, graphite foils, and highly oriented pyrolytic graphite (HOPG). In this follow-up research, more results on these oxygen-implanted graphite samples are presented. We show results from transport measurements on oxygen-implanted diamond-like carbon thin coatings, amorphous carbon films, and HOPG. Significantly, a three-order magnitude drop in the electrical resistance of the oxygen-implanted diamond-like carbon films is observed at the 50 K temperature that we have previously reported for the transition to the superconducting state. Below 50 K, the films’ resistance oscillates between the high and low resistance states, less when the sample is under a transverse magnetic field. This metastability between the insulating and superconducting-like states possibly reflects the evolution of the amplitude for the superconducting order parameter also known as the longitudinal Higgs mode. Transitions to low resistance state and metastability are also observed for amorphous carbon films. Finally, the HOPG samples’ resistance have a thermally activated term that can be understood on the basis of the Langer–Ambegaokar–McCumber–Halperin model applied to narrow SC channels in which thermal fluctuations can cause phase slips. We also find that in oxygen-implanted carbon materials, the electron charge and spin correlations do not compete and their interplay rather facilitates the emergence of high-temperature superconductivity, and thus, additional unexpected effects like Heisenberg spin waves and magneto-structural transitions are observed.

Enhancement of d -wave pairing in the striped phase with nearest neighbor attraction

Recently, the experimental results by the angle-resolved photoemission spectroscopy suggested that an additional strong nearest neighbor attraction in the Hubbard model might be significant to describe the properties of doped cuprates more accurately. The stripe-ordered patterns, formed by the inhomogeneous distribution of spin, charge and pairing correlations in the CuO$_{2}$ planes, is a known feature of doped cuprates. In this work, the effect of the nearest neighbor attraction and the stripe phase are examined by using the constrained path quantum Monte Carlo method within the repulsive Hubbard model on two-dimensional square lattice. The ground state spin correlations along and cross the stripe regions, and the $d$-wave pairing correlation are calculated. It is found that the spin-spin correlation is the highest when the interstripe region is fairly close to half-filling, and $d$-wave superconducting correlation on neighboring sites could be enhanced in the presence of stripe pattern and strong nearest neighbor attraction, which reveals their crucial roles on superconductivity in the doped cuprates.

Stochastic Density Functional Theory on Lane Formation in Electric-Field-Driven Ionic Mixtures: Flow-Kernel-Based Formulation.

Simulation and experimental studies have demonstrated non-equilibrium ordering in driven colloidal suspensions: with increasing driving force, a uniform colloidal mixture transforms into a locally demixed state characterized by the lane formation or the emergence of strongly anisotropic stripe-like domains. Theoretically, we have found that a linear stability analysis of density dynamics can explain the non-equilibrium ordering by adding a non-trivial advection term. This advection arises from fluctuating flows due to non-Coulombic interactions associated with oppositely driven migrations. Recent studies based on the dynamical density functional theory (DFT) without multiplicative noise have introduced the flow kernel for providing a general description of the fluctuating velocity. Here, we assess and extend the above deterministic DFT by treating electric-field-driven binary ionic mixtures as the primitive model. First, we develop the stochastic DFT with multiplicative noise for the laning phenomena. The stochastic DFT considering the fluctuating flows allows us to determine correlation functions in a steady state. In particular, asymptotic analysis on the stationary charge-charge correlation function reveals that the above dispersion relation for linear stability analysis is equivalent to the pole equation for determining the oscillatory wavelength of charge–charge correlations. Next, the appearance of stripe-like domains is demonstrated not only by using the pole equation but also by performing the 2D inverse Fourier transform of the charge–charge correlation function without the premise of anisotropic homogeneity in the electric field direction.

Stripes and spin-density waves in the doped two-dimensional Hubbard model: Ground state phase diagram

We determine the spin and charge orders in the ground state of the doped two-dimensional (2D) Hubbard model in its simplest form, namely with only nearest-neighbor hopping and on-site repulsion. At half-filling, the ground state is known to be an antiferromagnetic Mott insulator. Doping Mott insulators is believed to be relevant to the superconductivity observed in cuprates. A variety of candidates have been proposed for the ground state of the doped 2D Hubbard model. A recent work employing a combination of several state-of-the-art numerical many-body methods established the stripe order as the ground state near $1/8$ doping at strong interactions. In this paper, we apply one of these methods, the cutting-edge constrained-path auxiliary field quantum Monte Carlo (AFQMC) method with self-consistently optimized gauge constraints, to systematically study the model as a function of doping and interaction strength. With careful finite size scaling based on large-scale computations, we map out the ground state phase diagram in terms of its spin and charge order. We find that modulated antiferromagnetic order persists from near half-filling to about $1/5$ doping. At lower interaction strengths or larger doping, these ordered states are best described as spin-density waves, with essentially delocalized holes and modest oscillations in charge correlations. When the charge correlations are stronger (large interaction or small doping), they are best described as stripe states, with the holes more localized near the node in the antiferromagnetic spin order. In both cases, we find that the wavelength in the charge correlations is consistent with so-called filled stripes.

Dynamics of the O(4) critical point in QCD

We perform a Langevin simulation of the $O(4)$ critical point, which lies in the dynamic universality class of ``model G.'' This is the dynamic universality class of the chiral phase transition in QCD with two massless flavors. The axial charge and the order parameter ${\ensuremath{\phi}}_{a}=(\ensuremath{\sigma},\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\pi}})$ exhibit a rich dynamical interplay, which reflects the qualitative differences in the hydrodynamic effective theories above and below ${T}_{c}$. From the axial charge correlators on the critical line we extract a dynamical critical exponent of $\ensuremath{\zeta}=1.47\ifmmode\pm\else\textpm\fi{}0.01(\text{stat.})$, which is compatible with the theoretical expectation of $\ensuremath{\zeta}=d/2$ (with $d=3$) when systematic errors are taken into account. At low temperatures, we quantitatively match the $O(4)$ simulations to the superfluid effective theory of soft pions.

π phase shift across stripes in a charge density wave system

Many strongly correlated materials are characterized by deeply intertwined charge and spin order. Besides their high superconducting transition temperatures, one of the central features of these complex patterns in cuprates is a phase shift which occurs across lines of decreased hole density. That is, when doped away from their antiferromagnetic phase, the additional charge is not distributed uniformly, but rather in ``stripes.'' The sublattices preferentially occupied by up and down spin are reversed across these stripes, a phenomenon referred to as a ``$\ensuremath{\pi}$ phase shift.'' Many of the spin-charge patterns, including the $\ensuremath{\pi}$ phase shift, are reproduced by density matrix renormalization group and quantum Monte Carlo calculations of simplified tight binding (repulsive Hubbard) models. In this paper we demonstrate that this sublattice reversal is generic by considering the corresponding phenomenon in the attractive Hubbard Hamiltonian, where a charge density wave phase forms at half filling. We introduce charge stripes via an appropriate local chemical potential; measurements of charge correlation across the resulting lines of lowered density reveal a clear $\ensuremath{\pi}$ phase.

Probing hadronization with flavor correlations of leading particles in jets

We study nonperturbative flavor correlations between pairs of leading and next-to-leading charged hadrons within jets at the Electron-Ion Collider (EIC). We introduce a charge correlation ratio observable $r_c$ that distinguishes same- and opposite-sign charged pairs. Using Monte Carlo simulations with different event generators, $r_c$ is examined as a function of various kinematic variables for different combinations of hadron species, and the feasibility of such measurements at the EIC is demonstrated. The precision hadronization study we propose will provide new tests of hadronization models and hopefully lead to improved quantitative, and perhaps eventually analytic, understanding of nonperturbative QCD dynamics.

Cubic color charge correlator in a proton made of three quarks and a gluon

The three point correlation function of color charge densities is evaluated explicitly in light cone gauge for a proton on the light cone. This includes both $C$-conjugation even and odd contributions. We account for perturbative corrections to the three-quark light cone wave function due to the emission of an internal gluon which is not required to be soft. We verify the Ward identity as well as the cancellation of UV divergences in the sum of all diagrams so that the correlator is independent of the renormalization scale. It does, however, exhibit the well known soft and collinear singularities. The expressions derived here provide the $C$-odd contribution to the initial conditions for high-energy evolution of the dipole scattering amplitude to small $x$. Finally, we also present a numerical model estimate of the impact parameter dependence of quantum color charge three-point correlations in the proton at moderately small $x$.

Intertwined spin, charge, and pair correlations in the two-dimensional Hubbard model in the thermodynamic limit

The high-temperature superconducting cuprates are governed by intertwined spin, charge, and superconducting orders. While various state-of-the-art numerical methods have demonstrated that these phases also manifest themselves in doped Hubbard models, they differ on which is the actual ground state. Finite-cluster methods typically indicate that stripe order dominates, while embedded quantum-cluster methods, which access the thermodynamic limit by treating long-range correlations with a dynamical mean field, conclude that superconductivity does. Here, we report the observation of fluctuating spin and charge stripes in the doped single-band Hubbard model using a quantum Monte Carlo dynamical cluster approximation (DCA) method. By resolving both the fluctuating spin and charge orders using DCA, we demonstrate that they survive in the doped Hubbard model in the thermodynamic limit. This discovery also provides an opportunity to study the influence of fluctuating stripe correlations on the model's pairing correlations within a unified numerical framework. Using this approach, we also find evidence for pair-density-wave correlations whose strength is correlated with that of the stripes.

PACIAE 2.2.1: An updated issue of the parton and hadron cascade model PACIAE 2.2

The parton and hadron cascade model PACIAE 2.2 (cf. Yan et al. (2018) [11]) has been upgraded to the new issue of PACIAE 2.2.1 with the chiral magnetic effect (CME). The charge correlation parameter, γss and γos, which is typically used to “detect” the CME, is calculated by PACIAE 2.2.1 in Au+Au and Pb+Pb collisions, and the result is consistent with the STAR and ALICE data. As an example, PACIAE 2.2.1 model is employed to mimic the pseudorapidity distribution (dNch/dη) and transverse momentum spectra (pT) of charged particles in non-central Au+Au collisions at RHIC energy and Pb+Pb collisions at LHC energy. The chiral magnetic effect is found to partly affect the final-state basic observables in relativistic nuclear collisions. By this new issue a more realistic dynamical evolution of relativistic nuclear collisions can be depicted. Program summaryProgram Title: PACIAE version 2.2.1CPC Library link to program files:https://doi.org/10.17632/w3g68dj4d9.3Code Ocean capsule:https://codeocean.com/capsule/3695338Licensing provisions: CC By 4.0Programming language: FORTRAN 77 or GFORTRANJournal reference of previous version: Comput. Phys. Comm. 224 (2018) 417Does the new version supersede the previous version?: No.Reasons for the new version: In order that PACIAE 2.2.1 model is now able to simulate the CME-related physics and a more realistic dynamical evolution scenario in the relativistic nucleus-nucleus collisions.Summary of revisions: In PACIAE 2.2.1 model the chiral magnetic effect is introduced. The CME-induced charge separation mechanism [7] into the initial partonic states is artificially added by switching py values for a fraction f of the downward moving partons with those of the corresponding upward moving antipartons.Nature of problem: PACIAE is based on PYTHIA, where it has no any mechanisms to generate the chiral magnetic effect (CME). The CME is a novel transport phenomenon arising from the interaction between quantum anomalies and strong magnetic fields in the ultra-relativistic nuclear collisions. Charge separation is an important consequence of the CME. In order to depict the CME, the corresponding upgrade about introducing a global charge separation scheme into the initial partonic states is highly required.Solution method: The global CME-induced charge separation mechanism [7] into the initial partonic states is introduced by switching py values for a fraction f of the downward moving partons with those of the corresponding upward moving antipartons.Additional comments including restrictions and unusual features: PACIAE 2.2.1 has three versions of PACIAE 2.2.1a, 2.2.1b, and 2.2.1c. PACIAE 2.2.1a is for the elementary collision (here no upgraded from PACIAE 2.2a). PACIAE 2.2.1b and PACIAE 2.2.1c are for the nucleus-nucleus collision of A + A with input file of usu.dat. PACIAE 2.2.1b and PACIAE 2.2.1c are similar in the physical contents but are different in the topological structure [8].•Using the attached input file of usux.dat to run 1000 events for the s = 200 GeV Non Single Diffractive pp collision by PACIAE 2.2.1a takes ≈0.5 min.•Using the attached input file of usu.dat to run 10 events for the 35%-45% non-central Au+Au collisions at sNN = 200 GeV by PACIAE 2.2.1b takes ≈6 min.•Using the attached input file of usu.dat to run 10 events for the 35%-45% non-central Au+Au collisions at sNN = 200 GeV by PACIAE 2.2.1c takes ≈8 min.

The symmetry-preserving mean field condition for electrostatic correlations in bulk.

Accurate simulations of a condensed system of ions or polar molecules are concerned with proper handling of the involved electrostatics. For such a Coulomb system at a charged planar interface, the Coulomb interaction averaged over the lateral directions with preserved symmetry serves as a necessary constraint in building any accurate handling that reconciles a simulated singlet charge density with the corresponding macroscopic charge/dielectric response. At present, this symmetry-preserving mean field (SPMF) condition represented in the reciprocal space is conjectured to be necessary for a simulated bulk system to reproduce correctly the charge structure factor of the macroscopic bulk as well. In this work, we further examine analytically the asymptotic behavior of the charge structure factor at small wavenumbers for an arbitrary charge-charge interaction. In light of our theoretical predictions, simulations with lengths of nearly 0.1μm are carried out to demonstrate that typical efficient methods violating the SPMF condition, indeed, fail to capture the exact charge correlations at small wavenumbers for both ionic and polar systems. However, for both types of systems, these existing methods can be simply amended to match the SPMF condition and subsequently to precisely probe the electrostatic correlations at all length scales.

Effect of strong intersite Coulomb interaction on the topological properties of a superconducting nanowire

For superconducting nanowire with the pairing of extended s-type symmetry, Rashba spin-orbit interaction in a magnetic field, the influence of strong intersite charge correlations on single-particle Majorana excitations is analyzed. This problem is investigated on the basis of the density matrix renormalization group numerical method. It is shown that with an increase in the repulsion intensity of electrons located at the neighboring sites, two subbands emerge in the lower Hubbard band of the open system. Based on calculations of the Majorana polarization and degeneracy of the entanglement spectrum, it was found that a topologically nontrivial phase with one edge state survives at the edge of each of the subbands where the concentration of electrons or holes is minimal. Keywords: superconducting nanowires, Majorana zero modes, strong electron correlations.