Quasi Two-dimensional Systems Research Articles

Mapping the nonequilibrium order parameter of a quasi-two dimensional charge density wave system

The driving force of a charge density wave (CDW) transition in quasi-two dimensional systems is still debated, while being crucial in understanding electronic correlation in such materials. Here we use femtosecond time- and angle-resolved photoemission spectroscopy combined with computational methods to investigate the coherent lattice dynamics of a prototypical CDW system. The photo-induced temporal evolution of the periodic lattice distortion associated with the amplitude mode reveals the dynamics of the free energy functional governing the order parameter. Our approach establishes that optically-induced screening rather than CDW melting at the electronic level leads to a transiently modified potential which explains the anharmonic behaviour of the amplitude mode and discloses the structural origin of the symmetry-breaking phase transition.

Crystal Structure Design and Synthesis of Noncentrosymmetric Superconductors in Intercalation-Induced Transition Metal Dichalcogenides.

In this paper, we have performed a crystal structure screening and properties prediction framework within the noncentrosymmetric AMX2 system, which arises from the intercalation of elements in transition metal dichalcogenides. After rigorous evaluations of thermodynamic and dynamic stability, we have refined our initial structure pool of 504 crystals to a focused set of 48 promising candidates. Analysis of their electronic properties has revealed that 23 of these crystals exhibit semiconducting behavior. The results on electron-phonon coupling and band calculations indicate that most of the remaining 25 crystals are superconducting topological nodal line or Weyl semimetals. Additionally, we find that β-InNbSe2 in the AMX2 family can serve as an excellent candidate to explore topological nodal lines and Lifshitz transitions. Furthermore, we also extended the thermodynamic stability analysis to higher temperatures. Ultimately, we have successfully synthesized β-InNbSe2 with a stoichiometric ratio of 1:1:2 experimentally and characterized its superconducting properties. This study represents a systematic foray into the identification of new superconducting and topological materials within the noncentrosymmetric AMX2 family. This screening framework can be extended to other quasi two-dimensional systems.

Charge-Density Waves vs. Superconductivity: Some Results and Future Perspectives

Increasing experimental evidence suggests the occurrence of filamentary superconductivity in different (quasi) two-dimensional physical systems. In this piece of work, we discuss the proposal that under certain circumstances, this occurrence may be related to the competition with a phase characterized by charge ordering in the form of charge-density waves. We provide a brief summary of experimental evidence supporting our argument in two paradigmatic classes of materials, namely transition metal dichalcogenides and cuprates superconductors. We present a simple Ginzburg–Landau two-order-parameters model as a starting point to address the study of such competition. We finally discuss the outcomes of a more sophisticated model, already presented in the literature and encoding the presence of impurities, and how it can be further improved in order to really address the interplay between charge-density waves and superconductivity and the possible occurrence of filamentary superconductivity at the domain walls between different charge-ordered regions.

A fast magnetic vector characterization method for quasi two-dimensional systems and heterostructures

The use of magnetic vector tomography/laminography has opened a 3D experimental window to access the magnetization at the nanoscale. These methods exploit the dependence of the magnetic contrast in transmission to recover its 3D configuration. However, hundreds of different angular projections are required leading to large measurement times. Here we present a fast method to dramatically reduce the experiment time specific for quasi two-dimensional magnetic systems. The algorithm uses the Beer-Lambert equation in the framework of X-ray transmission microscopy to obtain the 3D magnetic configuration of the sample. It has been demonstrated in permalloy microstructures, reconstructing the magnetization vector field with a reduced number of angular projections obtaining quantitative results. The throughput of the methodology is × 10–× 100 times faster than conventional magnetic vector tomography, making this characterization method of general interest for the community.

Phonon-induced instabilities in correlated electron Hamiltonians

Studies of Hamiltonians modeling the coupling between electrons as well as to local phonon excitations have been fundamental in capturing the novel ordering seen in many quasi-one dimensional condensed matter systems. Extending studies of such Hamiltonians to quasi-two dimensional systems is of great current interest, as electron-phonon couplings are predicted to play a major role in the stabilization or enhancement of novel phases in 2D material systems. In this work, we study model systems that describe the interplay between the Hubbard coupling and the phonon modes in the Holstein (H) and Su-Schrieffer-Heeger (SSH) Hamiltonians using the functional renormalization group (fRG). For both types of electron phonon couplings, we find the predicted charge density wave phases in competition with anti-ferromagnetic ($AF$) ordering. As the system is doped, the transition shifts, with both orders showing incommensurate peaks. We compare the evolution of the quasiparticle weight for the Holstein model with that of the SSH model as the systems transition from antiferromagnetic to charge-ordered ground states. Finally, we calculate the self-energy of the phonon and capture the impact of charge ordering on the phonon modes.

Quasi-two-dimensional bacterial swimming around pillars: Enhanced trapping efficiency and curvature dependence.

Microswimmers exhibit more diverse behavior in quasi-two dimensions than in three dimensions. Such behavior remains elusive due to the analytical difficulty of dealing with two parallel solid boundaries. The existence of additional obstacles in quasi-two dimensional systems further complicates the analysis. Combining experiments and hydrodynamic simulations, we investigate how the spatial dimension affects the interactions between microswimmers and obstacles. We fabricated microscopic pillars in quasi-two dimensions by etching glass coverslips and observed bacterial swimming among the pillars. Bacteria got trapped around the circular pillars and the trapping efficiency increased as the quasi-two-dimensionality was increased or as the curvature of the pillars was decreased. Numerical simulations of the simplest situation of a confined squirmer showed anomalous increase of hydrodynamic attractions, establishing that the enhanced interaction is a universal property of quasi-two-dimensional microhydrodynamics. We also demonstrated that the local curvature of the obstacle controls the trapping efficiency by experiments with elliptic pillars.

Editorial

Editorial

Studies on water transport in quasi two-dimensional porous systems using neutron radiography

Abstract The spontaneous wetting and drying of flat porous samples of linen, cotton and synthetic textiles were studied using dynamic neutron radiography (DNR). The progress of the wetting process of the media was delineated from the obtained neutron dynamical radiography images. The results of the investigation reveal a non-classical behaviour of kinetics of wicking of these materials. The character of the wetting kinetics is discussed in terms of the fractal character of the tortuosity of fabric capillaries.

Charge doping zirconium nitride halide monolayers

The observation of superconducting phases in transition metal nitride halides has sparked interest due to its unconventional and not yet fully understood mechanism. These materials have a layered structure, and can be exfoliated into quasi-two dimensional systems. The thinnest form of these materials are of particular interest for electric-double-layer field-effect transistor configurations. Here, we have studied the effect of charge doping on the structural and electronic structure of Zirconium-based nitride halides. We find that upon electron doping, the effective masses and the bands close to the Fermi energy change considerably. The effect of considering quasiparticles, with self energies within the G0W0 approximation, has also been addressed. This work aims to help build better models for understanding the superconducting properties of ZrNXs, and TMNXs in general.

Tuning Ginzburg–Landau theory to quantitatively study thin ferromagnetic materials

Along with experiments, numerical simulations are key to gaining insight into the underlying mechanisms governing domain wall motion in thin ferromagnetic systems. However, a direct comparison between numerical simulation of model systems and experimental results still represents a great challenge. Here, we present a tuned Ginzburg–Landau model to quantitatively study the dynamics of domain walls in quasi two-dimensional ferromagnetic systems with perpendicular magnetic anisotropy. This model incorporates material and experimental parameters and the micromagnetic prescription for thermal fluctuations, allowing us to perform material-specific simulations and at the same time recover universal features. We show that our model quantitatively reproduces previous experimental velocity-field data in the archetypal perpendicular magnetic anisotropy Pt/Co/Pt ultra-thin films in the three dynamical regimes of domain wall motion (creep, depinning and flow). In addition, we present a statistical analysis of the domain wall width parameter, showing that our model can provide detailed nano-scale information while retaining the complex behavior of a statistical disordered model.

Improved transition metal surface energies from a generalized gradient approximation developed for quasi two-dimensional systems.

Nonuniform density scaling in the quasi-two-dimensional (quasi-2D) regime is an important and challenging aspect of the density functional theory. Semilocal exchange-correlation energy functionals, developed by solving the dimensional crossover criterion in the quasi-2D regime, have great theoretical and practical importance. However, the only semilocal generalized gradient approximation (GGA) that has been designed to satisfy this criterion is the Q2D-GGA [L. Chiodo et al., Phys. Rev. Lett. 108, 126402 (2012)]. Here, we establish the applicability, broadness, and accuracy of the Q2D-GGA functional by performing an extensive assessment of this functional for transition metal surface energies. The important characteristic of the surface density localization and oscillation due to the rearrangement of the d electrons is also shown for different metal surfaces.

A machine learning approach to analyze the structural formation of soft matter via image recognition

ABSTRACT A novel method is developed to analyze the structural formation of colloidal particles based on image recognition via a convolutional neural network (CNN). This makes it possible to analyze various complex structures that are difficult to study using a traditional bond-order parameter analysis. Molecular dynamics simulations on soft colloidal systems are performed in quasi two-dimensional and three-dimensional systems, and the efficiency of the proposed method is demonstrated.

Physical properties of Ruddlesden-Popper (n = 3) nickelate: La4Ni3O10

Nickelates, just like their isoelectronic cousins (the cuprate superconductors), are candidates for high temperature superconductivity. As charge density waves (CDW) and superconductivity might share a similar microscopic origin and that CDW behaviour is earlier suggested for La4Ni3O10, we here provide a detailed physical characterization of the magnetic, electronic and thermodynamic properties of stoichiometric polycrystalline La4Ni3O10 at and below 300 K. By variable temperature, synchrotron powder X-ray diffraction, we observe noticeable expansion in the b-axis and a contraction in c-axis coinciding with the electronic transition. Magnetic measurements indicate Pauli paramagnetic behaviour for the entire temperature range of 4–300 K. Transport data suggest electron–phonon interaction based conduction from 150 to 300 K, at 138 K the we observe an anomaly of second order nature, previously reported as a CDW anomaly. Below this anomaly, we observe no indications of electron–phonon interaction, but rather, Fermi liquid (FL) based T2 dependence with resistivity minima at 20 K, below which T1/2 dependence occurs for the resistivity indicating pure electron–electron interactions (EEI) based transport. Specific heat data indicates that mass enhancement of La4Ni3O10 similar to that of the normal state of the cuprates and the entropy change at anomaly suggests close similarities to other quasi-two dimensional systems, such as molybdenum bronzes and manganites.

The influence of long-range interaction on the structure of a two-dimensional multi scale potential system

We present the results of a computer simulation study of a finite temperature phase diagram of two-dimensional and quasi two-dimensional core-softened systems both taking into account long-range Coulomb-like forces and ignoring them. The system structure was determined from analysis of the behavior of radial distribution functions, order parameters and number of nearest neighbors. The system has been shown to have a large number of different phases. We have found that long-range forces substantially affected the structure and the melting point of the system at low and moderate densities, while at high densities the effect of long-range forces was negligible.

Nematic topological defects positionally controlled by geometry and external fields.

Using a Landau–de Gennes approach, we study the impact of confinement topology, geometry and external fields on the spatial positioning of nematic topological defects (TDs). In quasi two-dimensional systems we demonstrate that a confinement-enforced total topological charge of m > 1/2 decays into elementary TDs bearing a charge of m = 1/2. These assemble close to the bounding substrate to enable essentially bulk-like uniform nematic ordering in the central part of a system. This effect is reminiscent of the Faraday cavity phenomenon in electrostatics. We observe that in certain confinement geometries, varying the correlation length size of the order parameter could trigger a global rotation of an assembly of TDs. Finally, we show that an external electric field could be used to drag the boojum fingertip towards the interior of the confinement cell. Assemblies of TDs could be exploited as traps for appropriate nanoparticles, opening several opportunities for the development of functional nanodevices.

Optical and Excitonic Properties of Atomically Thin Transition-Metal Dichalcogenides

Starting with the isolation of a single sheet of graphene, the study of layered materials has been one of the most active areas of condensed matter physics, chemistry, and materials science. Single-layer transition-metal dichalcogenides are direct-gap semiconducting analogs of graphene that exhibit novel electronic and optical properties. These features provide exciting opportunities for the discovery of both new fundamental physical phenomena as well as innovative device platforms. Here, we review the progress associated with the creation and use of a simple microscopic framework for describing the optical and excitonic behavior of few-layer transition-metal dichalcogenides, which is based on symmetry, band structure, and the effective interactions between charge carriers in these materials. This approach provides an often quantitative account of experiments that probe the physics associated with strong electron–hole interactions in these quasi two-dimensional systems and has been successfully employed by many groups to both describe and predict emergent excitonic behavior in these layered semiconducting systems.

Flat Optical Conductivity in ZrSiS due to Two-Dimensional Dirac Bands.

ZrSiS exhibits a frequency-independent interband conductivity σ(ω)=const(ω)≡σ_{flat} in a broad range from 250 to 2500 cm^{-1} (30-300meV). This makes ZrSiS similar to (quasi-)two-dimensional Dirac electron systems, such as graphite and graphene. We assign the flat optical conductivity to the transitions between quasi-two-dimensional Dirac bands near the Fermi level. In contrast to graphene, σ_{flat} is not universal but related to the length of the nodal line in the reciprocal space, k_{0}. Because of spin-orbit coupling, the discussed Dirac bands in ZrSiS possess a small gap Δ, for which we determine an upper bound max(Δ)=30 meV from our optical measurements. At low temperatures the momentum-relaxation rate collapses, and the characteristic length scale of momentum relaxation is of the order of microns below 50K.

Using quasi two-dimensional systems to understand large-scale vortical structures and dynamics

An experimental physicist, surveying the research, describes how quasi 2D flows on curved, thin liquid films provide insight into the macroscopic properties of turbulent flows.

Magnetic oscillations measure interlayer coupling in cuprate superconductors

The magnetic oscillations in YBCO high-temperature superconductors have been widely studied over the last decade and consist of three equidistant low frequencies with a central frequency several times more intense than its two shoulders. This remains a puzzle in spite of numerous attempts to explain the corresponding small Fermi-surface pockets. Furthermore the ARPES data indicate only four Fermi-arcs with bilayer splitting, and show no sign of such small areas in the Fermi surface. Here we argue that the magnetic oscillations measured in under-doped bilayer high temperature superconductors, in particular YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, provide a measure of the interplanar electronic coupling rather than the areas of fine-grain reconstruction of the Fermi surfaces coming from induced charge density waves. This identification is based on the relative intensities of the different peaks, as well as their angular dependence, which points to an effective Fermi surface that is larger than the oscillation frequencies, and is compatible with several indications from ARPES. The dominance of such frequencies with respect to the fundamental frequencies from the Fermi surface is natural for a strongly correlated quasi-two dimensional electronic systems where non-linear mixings of frequencies are more resistant to sample inhomogeneity.

Multicontrol Over Graphene-Molecule Hetereojunctions.

The vertical configuration is a powerful tool recently developed experimentally to investigate field effects in quasi two-dimensional systems. Prototype graphene-based vertical tunneling transistors can achieve an extraordinary control over current density utilizing gate voltages. In this work, we study theoretically vertical tunneling junctions that consist of a monolayer of photoswitchable aryl azobenzene molecules sandwiched between two sheets of graphene. Azobenzene molecules transform between trans and cis conformations upon photoexcitation, thus adding a second knob that enhances the control over physical properties of the junction. Using first-principles methods within the density functional framework, we perform simulations with the inclusion of field effects for both trans and cis configurations. We find that the interference of interface states resulting from molecule–graphene interactions at the Fermi energy introduces a dual-peak pattern in the transmission functions and dominates the transport properties of gate junctions, shedding new light on interfacial processes.