quasi-2D System Research Articles

RETRACTED ARTICLE: Quasi-dimensional models applied to kinetic and exchange energy density functionals

We investigate the behavior of three-dimensional 3D exchange energy functional of density-functional theory in anisotropic systems with two-dimensional 2D character and 1D character. The local density approximation (LDA), the generalized gradient approximation (GGA), and the meta-GGA behave as functions of quantum well width. We use the infinite-barrier model (IBM) for the quantum well. In the first section, we describe the problem of three-dimensional exchange functional, in the second section we introduce the quasi-2D IBM system, in the third section we introduce the quasi-1D IBM system. Using that an exact-exchange functional provides the correct approach to the true two-dimensional limit, we want to show that the 2D limit can be considered as a constraint on approximate functionals. For the 1D limit case we also propose a new functional obtained with methods completely similar to those of 2D limit.

Collective Dynamics in Quasi-One-Dimensional Hard Disk System

We present the results of molecular dynamic studies of collective dynamics in a system of hard disks confined to a narrow quasi-one-dimensional (quasi-1D) channel. The computer simulations have been performed for the specific channel width of 3/2 of disk diameter in which the disk arrangement at close packing resembles zigzag ordering characteristic of a vertically oriented two-dimensional (2D) triangular lattice. In such a quasi-1D system, which is intermediate between 1D and 2D arrays of hard disks, the transverse excitations obey very specific dispersion law typical of the usual optical transverse modes. This is in a sharp contrast both to the 1D case, where transverse excitations are not possible, and to the 2D case, where the regular shear waves with a propagation gap were observed. Other peculiarities of the dispersion of collective excitations as well as some results of disk structuring and thermodynamics of the quasi-1D hard disk system are presented and discussed for a range of hard disk densities typical for fluid and distorted crystal states.

Dark-soliton-like magnetic domain walls in a two-dimensional ferromagnetic superfluid

We report a stable magnetic domain wall in a uniform ferromagnetic spin-1 condensate, characterized by the magnetization having a dark soliton profile with nonvanishing superfluid density. We find exact stationary solutions for a particular ratio of interaction parameters with and without magnetic fields, and develop an accurate analytic solution applicable to the whole ferromagnetic phase. In the absence of magnetic fields, this domain wall relates various distinct solitary excitations in binary condensates through $\textrm{SO}(3)$ spin rotations, which otherwise are unconnected. Remarkably, studying the dynamics of a quasi-two-dimensional (quasi-2D) system we show that standing wave excitations of the domain wall oscillate without decay, being stable against the snake instability. The domain wall is dynamically unstable to modes that cause the magnetization to grow perpendicularly while leaving the domain wall unchanged. Real time dynamics in the presence of white noise reveals that this "spin twist" instability does not destroy the topological structure of the magnetic domain wall.

Parameters of Tomonaga-Luttinger liquid in a quasi-one-dimensional material with Coulomb interactions

In this work we derive a new scheme to calculate Tomonaga-Luttinger liquid (TLL) parameters and holon (charge modes) velocities in a quasi-1D material that consists of two-leg ladders coupled through Coulomb interactions. Firstly, we obtain an analytic formula for electron-electron interaction potential along the conducting axis for a generalized charge distribution in a plane perpendicular to it. In the second step we introduce many-body screening that is present in a quasi-1D material. To this end we propose a new approximation for the charge susceptibility. Based on this we are able to find the TLL's parameters and velocities. We then show how to use these to validate the experimental ARPES data measured recently in p-polarization in $NbSe_3$. Although we focus our study on this specific material it is applicable for any quasi-1D system that consists of two-leg ladders as basic units.

Dynamics of transmission in disordered topological insulators

The discovery of topological insulators has opened new prospects for robust signal transport for electronic, phononic, and photonic devices. Though transport of topological protected edge states is robust to disorder, large fluctuations and lengthened average delay time are observed. Here, we consider a quasi-1d system following the Haldane model and generalize the idea of eigenchannel time delay to the topological system. Eigenchannel time delay indicates the excited density of states for the configuration and relates to the intensity integral inside the system. Taking advantage of this property, we point out a practical way to extract the central frequency and linewidth of localized modes excited in the topological system. This work links the fluctuation of time delay to the strength of disorder and discusses the scaling of time delay.

First-principles characterization of the magnetic properties of Cu2(OH)3Br

Low dimensional spin-1/2 transition metal oxides and oxyhalides continue to be at the forefront of research investigating nonclassical phases such as quantum spin liquids. In this study, we examine the magnetic properties of the oxyhalide $\text{Cu}_2\text{(OH)}_3\text{Br}$ in the botallackite structure using first-principles density functional theory, linear spin-wave theory, and exact diagonalization calculations. This quasi-2D system consists of $\text{Cu}^{2+}$ $\mathrm{S} = 1/2$ moments arranged on a distorted triangular lattice. Our exact diagonalization calculations, which rely on a first-principles-based magnetic model, generate spectral functions consistent with inelastic neutron scattering (INS) data. By performing computational experiments to disentangle the chemical and steric effects of the halide ions, we find that the dominant effect of the halogen ions is steric in the $\text{Cu}_2\text{(OH)}_3\text{X}$ series of compounds.

Wetting and wrapping of a floating droplet by a thin elastic filament.

We study the wetting of a thin elastic filament floating on a fluid surface by a droplet of another, immiscible fluid. This quasi-2D experimental system is the lower-dimensional counterpart of the wetting and wrapping of a droplet by an elastic sheet. The simplicity of this system allows us to study the phenomenology of partial wetting and wrapping of the droplet by measuring angles of contact as a function of the elasticity of the filament, the applied tension and the curvature of the droplet. We find that a purely geometric theory gives a good description of the mechanical equilibria in the system. The estimates of applied tension and tension in the filament obey an elastic version of the Young-Laplace-Dupré relation. However, curvatures close to the contact line are not captured by the geometric theory, possibly because of 3D effects at the contact line. We also find that when a highly-bendable filament completely wraps the droplet, there is continuity of curvature at the droplet-filament interface, leading to seamless wrapping as observed in a 3D droplet.

Mean-field model for the junction of two quasi-1-dimensional quantum Coulomb systems

Junctions appear naturally when one studies surface states or transport properties of quasi one dimensional materials such as carbon nanotubes, polymers and quantum wires. These materials can be seen as 1D systems embedded in the 3D space. In this article, we first establish a mean--field description of reduced Hartree--Fock type for a 1D periodic system in the 3D space (a quasi 1D system), the unit cell of which is unbounded. With mild summability condition, we next show that a quasi 1D system in its ground state can be described by a mean--field Hamiltonian. We also prove that the Fermi level of this system is always negative. A junction system is described by two different infinitely extended quasi 1D systems occupying separately half spaces in 3D, where Coulombic electron-electron interactions are taken into account and without any assumption on the commensurability of the periods. We prove the existence of the ground state for a junction system, the ground state is a spectral projector of a mean--field Hamiltonian, and the ground state density is unique.

Possible Phason-Polaron Effect on Purely One-Dimensional Charge Order of Mo6Se6 Nanowires

In one-dimensional (1D) metallic systems, the diverging electron susceptibility and electron-phonon coupling collaboratively drive the electrons into a charge density wave (CDW) state. However, strictly 1D system is unstable against perturbations, whose effect on CDW order requires clarification ideally with altered coupling to surroundings. Here, we fabricate such a system with nanowires of Mo6Se6 bundles, which are either attached to edges of monolayer MoSe2 or isolated freely, by post-annealing the preformed MoSe2. Using scanning tunneling microscopy (STM), we visualized charge modulations and CDW gaps with prominent coherent peaks in the edge-attached nanowires. Astonishingly, the CDW order becomes suppressed in the isolated nanowires, showing CDW correlation gaps without coherent peaks. The contrasting behavior, as revealed with theoretical modeling, is interpreted as the effect of phason-polarons on the 1D CDW state. Our work elucidates a possibly unprecedented many body effect that may be generic to strictly 1D system but undermined in quasi-1D system.

Dynamic phase transition and universality in a quasi 2D system: Bilayer Ising/Blume-Capel ferromagnet on a honeycomb lattice

Monte Carlo simulation technique is applied on a magnetic bilayer system composed of spin-1/2 Ising and spin-1 Blume-Capel spin variables distributed on a honeycomb lattice. Assuming an indirect exchange interaction between adjacent monolayers, we investigate the influence of single ion anisotropy and number of nonmagnetic layers between the magnetic sheets on the critical behavior of the system. Equilibrium phase diagram in a reduced pseudo critical temperature (Tc/J) versus reduced single ion anisotropy (DB/J) plane has been determined. In the presence of alternating magnetic field, dynamic phase transition properties have been elucidated. The form of the oscillating magnetic field is assumed to be in a square wave form. By performing a detailed finite-size scaling analysis, we have calculated the critical half period t1/2c at which the system undergoes a dynamic phase transition between dynamically ordered and disordered states. Extracted critical exponent ratios β/ν and γ/ν suggest that observed dynamic phase transition for the present multilayer system falls into the same universality class as the equilibrium two-dimensional Ising model.

Experimental validation of the theoretical prediction for the optical S matrix

Scattering of waves is omnipresent in nature in systems with sizes varying from $10^{-15}$ to $10^{25}$ m. Within this 40 orders of magnitude, in a great number of systems, the scattering can be separated in an averaged response that crosses rapidly the scattering region and a fluctuating delayed response. This fact is the basis of the optical model; the averaged response, represented by the optical matrix $\langle S\rangle$, is composed with the fluctuating part that can be taken as a random matrix. Although the optical model was developed more than 60 years ago, a theoretical prediction for the optical matrix was obtained until very recently. The validity of such prediction is experimentally demonstrated here. This is done studying the scattering of torsional waves in a quasi-1D elastic system in which a locally periodic system is built; the distribution of the scattering matrix is calculated completely free of parameters. In contradistinction to all previous works, in microwaves and in elasticity, in which the value of $\langle S\rangle$ is obtained from the experiment, here the theoretical prediction is used to compare with the experiment. Numerical simulations show that the theoretical value is still valid when strong disorder is present. Several applications of the theoretical expression for the optical matrix in other areas of physics are proposed. Possible extensions of this work are also discussed.

Brownian dynamics simulations of a cubic haematite particle suspension with a more effective treatment of steric layer interactions

We have proposed a new repulsive layer model for describing the interaction between steric layers of coated cubic particles. This approach is an effective technique applicable to particle-based simulations such as a Brownian dynamics simulation of a suspension composed of cubic particles. 3D Brownian dynamics simulations employing this repulsive interaction model have been performed in order to investigate the equilibrium aggregate structures of a suspension composed of cubic haematite particles. It has been verified that Brownian dynamics employing the present steric interaction model are in good agreement with Monte Carlo results with respect to particle aggregate structures and particle orientational characteristics. From the viewpoint of developing a surface modification technology, we have also investigated a regime change in the aggregate structure of cubic particle in a quasi-2D system by means of Brownian dynamics simulations. If the magnetic particle–particle interaction strength is relatively strong, in zero applied magnetic field the particles aggregate in an offset face-to-face configuration. As the magnetic field strength is increased, the offset face-to-face structure is transformed into a more direct face-to-face contact configuration that extends throughout the whole simulation region.

Impact of inversion symmetry on a quasi-1D S = 1 system

Here, we report the synthesis and magnetic properties of a novel, centrosymmetric, quasi-1D spin chain system La3VWS3O6, with hexagonal crystal structure (P63/m, a = 9.460 76(3), c = 5.518 09(2) Å). Pure powders were obtained by solid-state reactions from La2O3, WO3 and metal powders of V and W. X-ray powder diffraction, specific heat, magnetization, 139La-nuclear magnetic resonance (NMR), and electric resistivity measurements indicate that the compound is a low dimensional magnet with an S = 1 spin chain that exhibits no sign of magnetic ordering above 2 K. A single ion anisotropy (D/kB ~ 10 K), caused by magneto-crystalline effects, is probably responsible for a thermodynamic entropy release at lower temperatures, which concurs with 139La-NMR data. By detailed comparison with non-centrosymmetric Ba3V2S4O3, having a very similar magnetic lattice, it is obvious that the presence of crystallographic inversion symmetry has an effect on the behaviour of the magnetic chains.

Nonautonomous matter wave bright solitons in a quasi-1D Bose-Einstein condensate system with contact repulsion and dipole-dipole attraction

We investigate the existence and propagation properties of envelope solitons of an extended nonlinear Schr ödinger equation with the time-modulated dispersion, quadratic-cubic nonlinearities and linear gain or loss, which govern the nonlinear wave propagation in a nonautonomous quasi-1D Bose-Einstein condensate system with contact repulsion and dipole-dipole attraction. A novel class of nonautonomous bright soliton solutions on continuous-wave background is identified for the first time. It is shown that these localized structures possess interesting features that differ from the usual bright solitons. A rich variety of evolution behaviors, which include snakelike and periodic oscillating bright soliton dynamics, is revealed. The constraints of the system parameters to form these nonlinear localized structures are also suggested.

Hubbard model on a triangular lattice: Pseudogap due to spin density wave fluctuations

We calculate the fermionic spectral function $A_k (\omega)$ in the spiral spin-density-wave (SDW) state of the Hubbard model on a quasi-2D triangular lattice at small but finite temperature $T$. The spiral SDW order $\Delta (T)$ develops below $T = T_N$ and has momentum ${ \bf K} = (4\pi/3,0)$. We pay special attention to fermions with momenta ${\bf k}$, for which ${\bf k}$ and ${\bf k} + {\bf K}$ are close to Fermi surface in the absence of SDW. At the mean field level, $A_k (\omega)$ for such fermions has peaks at $\omega = \pm \Delta (T)$ at $T < T_N$ and displays a conventional Fermi liquid behavior at $T > T_N$. We show that this behavior changes qualitatively beyond mean-field due to singular self-energy contributions from thermal fluctuations in a quasi-2D system. We use a non-perturbative eikonal approach and sum up infinite series of thermal self-energy terms. We show that $A_k (\omega)$ shows peak/dip/hump features at $T < T_N$, with the peak position at $\Delta (T)$ and hump position at $\Delta (T=0)$. Above $T_N$, the hump survives up to $T = T_p > T_N$, and in between $T_N$ and $T_p$ the spectral function displays the pseudogap behavior. We show that the difference between $T_p$ and $T_N$ is controlled by the ratio of in-plane and out-of-plane static spin susceptibilities, which determines the combinatoric factors in the diagrammatic series for the self-energy. For certain values of this ratio, $T_p = T_N$, i.e., the pseudogap region collapses. In this last case, thermal fluctuations are logarithmically singular, yet they do not give rise to pseudogap behavior. Our computational method can be used to study pseudogap physics due to thermal fluctuations in other systems.

Crystal structures and sign reversal Hall resistivities in iron-based superconductors Lix(C3H10N2)0.32FeSe )**Project supported by the National Natural Science Foundation of China (Grant Nos. 51472266, 51532010, 91422303, and 51772323), the National Key Research and Development Program of China (Grant No. 2016YFA0300301), and the Key Research Program of Frontier Sciences, Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH013).

Heavy electron-doped FeSe-derived materials have attracted attention due to their uncommon electronic structures with only ‘electron pockets’, and they are different from other iron-based superconductors. Here, we report the crystal structures, superconductivities and normal state properties of two new Li-doped FeSe-based materials, i.e., Li0.15(C3H10N2)0.32FeSe (P-4) and Lix(C3H10N2)0.32FeSe (P4/nmm, ) with superconducting transition temperatures ranging from 40 K to 46 K. The determined crystal structures reveal a coupling between Li concentration and the orientation of 1,3-diaminopropane molecules within the largely expanded FeSe layers. Superconducting fluctuations appear in the resistivity of the two superconductors and are fitted in terms of the quasi two-dimensional (2D) Lawrence–Doniach model. The existence of a crossing point and scaling behavior in the T-dependence of diamagnetic response also suggests that the two superconductors belong to the quasi-2D system. Interestingly, with the increase of temperature, a sign of Hall coefficient (RH) reversing from negative to positive is observed at ∼185 K in both phases, suggesting that ‘hole pockets’ emerge in these electron-doped FeSe materials. First principle calculations indicate that the increase in FeSe layer distance will lift up a ‘hole band’ associated with character and increase the hole carriers. Our findings suggest that the increase in two dimensionalities may lead to the sign-reversal Hall resistivity in Lix(C3H10N2)0.32FeSe at high temperature.

Critical region phase transitions in the quasi-2D magnet CrCl3

CrCl3 has been known for a long time to be a quasi-2D ferromagnet (FM) with a specific-heat peak at 16.8K. Its 3D ordering is antiferromagnetic (AFM) with alternating 2D FM sheets perpendicular to the c-axis. Kuhlow [Phys. Stat. Sol. A 72, 161 (1982)] carried out Faraday rotation measurements on CrCl3 and concluded, on fairly tenuous grounds, that there are actually two closely spaced magnetic transitions, one at “16.7K” at which 2D FM sheets of in-plane spins start ordering, but do so with random alignment relative to the c-axis (the latter being perpendicular to the 2D FM sheets), and one in which a 3D AFM alignment of the 2D sheets of spins takes place, that being at TN ∼15.5K. We have carried out an extensive series of precision SQUID magnetometer measurements as a function of temperature, from very low fields (≥2.5G) and up to ∼2 kG on a high-purity single-crystal platelet, with the magnetic field both parallel and perpendicular to the c axis. The measurements provide more definitive evidence that there are two transitions, possibly three (at ∼16.8K, 16.0 and ∼14.3K, in our measurements). These data provide some support to Kuhlow’s conjecture that the initial phase transition is to a phase consisting of 3D-disordered, 2D-ordered, planes of spins. The very low field measurements provide the strongest support for this picture. To clarify the situation further neutron scattering (NS) measurements were carried out on a batch of the same material. The NS temperature-dependence measurements appear to provide additional support by showing a change in curvature in the magnetization (M) curve near 14K, and then an apparent linear decrease in M above 14K. Additionally, the NS data show a possible hysteresis near 16K during cooling. Possibly (though unlikely) the critical-region phase-transition behaviors observed in CrCl3 are unique to this quasi-2D system.

Transport Properties of a Quasi-1D Wigner Solid on Liquid Helium Confined in a Microchannel with Periodic Potential

We present transport measurements in a quasi-1D system of surface electrons on liquid helium confined in a 101-upmu m-long and 5-upmu m-wide microchannel where an electrostatic potential with periodicity of 1-upmu m along the channel is introduced. In particular, we investigate the influence of such a potential on the nonlinear transport properties of a quasi-1D Wigner solid (WS) by varying the amplitude of the periodic potential in a wide range. At zero and small values of amplitude, the quasi-1D WS in the microchannel shows expected features such as the Bragg–Cherenkov scattering of ripplons and re-entrant melting. As the amplitude of the potential increases, the above features are strongly suppressed. This behavior suggests the loss of long-range positional order in the electron system, which is reminiscent of re-entrant melting behavior due to lateral confinement of the WS in the channel.

Thermalization of dipole oscillations in confined systems by rare collisions

We study the relaxation of the center-of-mass, or dipole oscillations in the system of interacting fermions confined spatially. With the confinement frequency $\omega_{\perp}$ fixed the particles were considered to freely move along one (quasi-1D) or two (quasi-2D) spatial dimensions. We have focused on the regime of rare collisions, such that the inelastic collision rate, $1/\tau_{in} \ll \omega_{\perp}$. The dipole oscillations relaxation rate, $1/\tau_{\perp}$ is obtained at three different levels: by direct perturbation theory, solving the integral Bethe-Salpeter equation and applying the memory function formalism. As long as anharmonicity is weak, $1/\tau_{\perp} \ll 1/ \tau_{in}$ the three methods are shown to give identical results. In quasi-2D case $1/\tau_{\perp} \neq 0$ at zero temperature. In quasi-1D system $1/\tau_{\perp} \propto T^3$ if the Fermi energy, $E_F$ lies below the critical value, $E_F < 3 \omega_{\perp}/4$. Otherwise, unless the system is close to integrability, the rate $1/\tau_{\perp}$ has the temperature dependence similar to that in quasi-2D. In all cases the relaxation results from the excitation of particle-hole pairs propagating along unconfined directions resulting in the relationship $1/\tau_{\perp} \propto 1/\tau_{in}$, with the inelastic rate $1/\tau_{in} \neq 0$ as the phase-space opens up at finite energy of excitation, $\hbar \omega_{\perp}$. While $1/\tau_{\perp} \propto \tau_{in}$ in the hydrodynamic regime, $\omega_{\perp} \ll 1/\tau_{in}$, in the regime of rare collisions, $\omega_{\perp} \gg 1/\tau_{in}$, we obtain the opposite trend $1/\tau_{\perp} \propto 1/\tau_{in}$.

Second generation Car-Parrinello MD: application to the h-BN/Rh(111) nanomesh

Hexagonal boron nitride sp2 layers grown and supported on the Rh(111) metal surface attracted quite some interest thanks to the structural and electronic peculiarities of this quasi-2D system. The honeycomb regular corrugation is the key feature at the origin of several properties and applications in nanotechnology, e.g., the selective adsorption and functionalisation related to the modulation of the electronic structure. Atomistic simulations play an important role, since they can shed light on the nature of such a complex interface, providing resolution of details that cannot be achieved experimentally. However, the studies by electronic structure calculations have been mostly limited to static models of the optimized system. The sampling of configurations at finite temperature by ab-initio molecular dynamics requires significantly larger computational effort, and can become unfeasible for large scale and metallic models, as it is the case of h-BN/Rh(111). In this work, we employ a recently developed Car-Parrinello-like approach to overcome the performance limitations of the standard Born-Oppenheimer molecular dynamics scheme, thus obtaining a speed-up of 17×. We report on the set-up and the application of this approach to simulate the h-BN/Rh(111) interface at different temperatures and discuss the thermal stability of the corrugated pattern.