Charge Density Modulation Research Articles

Effect of oxygen adsorption on the local properties of epitaxial graphene on SiC (0001)

The effect of oxygen adsorption on the local structure and electronic properties of monolayer graphene grown on SiC (0001) has been studied by means of low-energy electron microscopy (LEEM), microprobe low-energy electron diffraction (\ensuremath{\mu}LEED), and microprobe angle resolved photoemission (\ensuremath{\mu}ARPES). We show that the buffer layer of epitaxial graphene on SiC (0001) is partially decoupled after oxidation. The monitoring of the oxidation process demonstrates that the oxygen saturates the Si dangling bonds, breaks some Si--C bonds at the interface, and intercalates the graphene layer. Accurate control over the oxidation parameters enables us to tune the charge density modulation in the layer.

Pumping current of a Luttinger liquid with finite length

We study transport properties in a Tomonaga-Luttinger liquid in the presence of two time-dependent point like weak impurities, taking into account finite-length effects. By employing analytical methods and performing a perturbation theory, we compute the backscattering pumping current (I_bs) in different regimes which can be established in relation to the oscillatory frequency of the impurities and to the frequency related to the length and the renormalized velocity (by the electron-electron interactions) of the charge density modes. We investigate the role played by the spatial position of the impurity potentials. We also show how the previous infinite length results for I_bs are modified by the finite size of the system.

Imaging the coexistence of a superconducting phase and a charge-density modulation in the K0.73Fe1.67Se2superconductor using a scanning tunneling microscope

We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K${}_{0.73}$Fe${}_{1.67}$Se${}_{2}$ with ${T}_{\mathrm{C}}$ $=$ 32 K. On the atomically resolved FeSe surface, we observe a well-defined superconducting gap and the microscopic coexistence of a charge-density modulation with $\sqrt{2}\ifmmode\times\else\texttimes\fi{}\sqrt{2}$ periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block-antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides.

Theoretical analysis of the color tuning mechanism of oxyluciferin and 5-hydroxyoxyluciferin

Firefly luciferase exhibits a multicolor bioluminescence, which is caused by the modulation of the emission of oxyluciferin by intermolecular interactions. One of the objectives of the present paper was analyze the possible effect of the charge density of the emitter in the color tuning. Theoretical calculations on oxyluciferin, and its respective moieties, demonstrated that no correlation between charge density and light emission can be found once solvent effects are considered. Further computational calculations demonstrate that intermolecular interactions modulate the emission by affecting the geometry of oxyluciferin, which controls the energy gap between the excited and ground state. Direct intermolecular interactions and polarity also affect the color of emission of oxyluciferin, by the same mechanism. Also, the effect of charge density modulation of Keto-(−1). A novel emitter, 5-hydroxyoxyluciferin, was considered and demonstrated to be more red-shifted that oxyluciferin.

Mechanical strains modulate the carrier behaviors of organic field effect transistors

We theoretically analyze carrier transports in an organic field effect transistor that is subjected to mechanical strains. These strains change the density of carriers that are accumulated at the channel by the Maxwell-Wagner mechanism and thus modulate drain currents. These effects of strains are characterized by three parameters α, μ1, and μ2, which represent the changes of the dielectric constant of gate insulators and channel mobilities. These charge density modulations are different from piezoelectric effects, and accumulated charge density depends on both applied gate voltages and applied strains.

Polar charge and orbital order in2H-TaS2

It was recently discovered that in spite of the scalar nature of its order parameter, the charge order in $1T$-TiSe${}_{2}$ can be chiral. This is made possible by the emergence of orbital order in conjunction with the charge-density modulations. Here we show that a closely related charge and orbital ordered state arises in $2H$-TaS${}_{2}$. In both materials, the microscopic mechanism driving the transition is based on the interaction between three differently polarized displacement waves. The relative phase shifts between these waves lead both to the formation of orbital order and to the breakdown of inversion symmetry. In contrast to $1T$-TiSe${}_{2}$, however, the presence of a mirror plane in the lattice of $2H$-TaS${}_{2}$ prevents the distorted structure in this material from being chiral, and a polar charge and orbital ordered state arises instead. It is stressed that bulk experiments are indispensable in differentiating between the chiral and polar phases.

The Stripe State in Cuprates

Since the discovery of high temperature superconductors (HTS) two decades ago, many anomalous properties have been reported. One of the most interesting properties is the possible existence of the stripe state consisting of one dimensional charge-density modulation coupled with some kind of spin ordering. X-ray and neutron scattering experiments and recently high resolution scanning tunneling microscopy have reported direct evidences of such a structure. In particular it has found in the La-Sr-Cu-O (LSCO) family the existence of the half-doped stripe with average of half a hole in one charge modulation period below and about 1/8 hole density. These results have fueled the idea about the presence of these charge or spin density wave states competing with the superconducting phase in underdoped HTS. They may even contribute to the pairing mechanism. In this talk, we will demonstrate that the presence of these stripes is actually a natural consequence of the strongly interacting t-J model by using a variational approach which provides a good enough accuracy to address the subtle result. Furthermore we show that half-doped stripes could be stabilized in hole-doped systems if we assume a simple electron-phonon interaction to renormalize the electron mass. However we have not found any evidence to support half-doped stripes in electron-doped systems.

Dual top gated graphene transistor in the quantum Hall regime

We study the effect of local modulation of charge density and carrier type in graphene field effect transistors using a double top gate geometry. The two top gates lead to the formation of multiple p–n junctions. Electron transport measurements at low temperature and in the presence of magnetic field show various integer and fractionally quantized conductance plateaus. We explain these results based on the mixing of the edge channels and find that inhomogeneity plays an important role in defining the exact quantization of these plateaus, an issue critical for applications of p–n junctions.

Functional polymeric nanoparticles for dexamethasone loading and release

Poly(phenylacetylene) (PPA) and poly(phenylacetylene-co-acrylic acid) (P(PA-co-AA)), nanoparticles bioconjugated with dexamethasone (DXM) during the synthesis, named PPA@DXM and P(PA-co-AA)@DXM, were prepared by a modified surfactant free emulsion method. The loading was studied as a function of different functionality grades of the copolymer and different amounts of drug, obtaining up to 90% of drug loading for P(PA-co-AA)@DXM with 8/1 PA/AA monomer ratio. The SEM images and DLS measurements showed spheres with average diameters in the range 190–500nm, depending on the content of acrylic acid monomer units in the copolymer and of DXM loading. ζ-potential and surface charge density of DXM loaded nanoparticles were also investigated and confirm the charge density modulation in the range 0.62–2.68σ (μC/m2). The results highlight the enhanced capability of our copolymer of hosting DXM, with the advantage of a control of size, surface functionality, charge and release. Moreover we demonstrate for the first time the ability of P(PA-co-AA) DXM loaded nanoparticles to be used in the apoptosis inhibition of human tumor cells (HeLa). On the basis of the results obtained by comparing the effects elicited in HeLa cells by free DXM versus DXM loaded nanoparticles we confirmed the biological efficacy of our preparation.

On the charge density modulation in the bunch by the interaction with electromagnetic wave

The efficiency of the charge density modulation of an electron bunch in the field of a linearly polarized monochromatic electromagnetic wave propagating oppositely to the bunch motion is considered. It is shown that as a result of the bunch-wave interaction the coherently radiating bunches can be obtained.

Formation of unconventional standing waves at graphene edges by valley mixing and pseudospin rotation.

We investigate the roles of the pseudospin and the valley degeneracy in electron scattering at graphene edges. It is found that they are strongly correlated with charge density modulations of short-wavelength oscillations and slowly decaying beat patterns in the electronic density profile. Theoretical analyses using nearest-neighbor tight-binding methods and first-principles density-functional theory calculations agree well with our experimental data from scanning tunneling microscopy. The armchair edge shows almost perfect intervalley scattering with pseudospin invariance regardless of the presence of the hydrogen atom at the edge, whereas the zigzag edge only allows for intravalley scattering with the change in the pseudospin orientation. The effect of structural defects at the graphene edges is also discussed.

Highly Anisotropic Anomaly in the Dispersion of the Copper-Oxygen Bond-Bending Phonon in SuperconductingYBa2Cu3O7from Inelastic Neutron Scattering

Motivated by predictions of a substantial contribution of the "buckling" vibration of the CuO(2) layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa(2)Cu(3)O(7). The data reveal a pronounced softening of the phonon at the in-plane wave vector q=(0,0.3) upon cooling below ~105 K, but no corresponding anomaly at q=(0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity.

Visualization of nano-plasmons in graphene

We study localized plasmons at the nanoscale (nano-plasmons) in graphene. The collective excitations of induced charge density modulations in graphene are drastically changed in the vicinity of a single impurity compared to graphene's bulk behavior. The dispersion of nano-plasmons depends on the number of electrons and the sign, strength and size of the impurity potential. Due to this rich parameter space the calculated dispersions are intrinsically multidimensional requiring an advanced visualization tool for their efficient analysis, which can be achieved with parallel rendering. To overcome the problem of analyzing thousands of very complex spatial patterns of nano-plasmonic modes, we take a combined visual and quantitative approach to investigate the excitations on the two-dimensional graphene lattice. Our visual and quantitative analysis shows that impurities trigger the formation of localized plasmonic excitations of various symmetries. We visually identify dipolar, quadrupolar and radial modes, and quantify the spatial distributions of induced charges.

Vortex dynamics and Hall conductivity of hard-core bosons

Magneto-transport of hard core bosons (HCB) is studied using an XXZ quantum spin model representation, appropriately gauged on the torus to allow for an external magnetic field. We find strong lattice effects near half filling. An effective quantum mechanical description of the vortex degrees of freedom is derived. Using semiclassical and numerical analysis we compute the vortex hopping energy, which at half filling is close to magnitude of the boson hopping energy. The critical quantum melting density of the vortex lattice is estimated at 6.5x10-5 vortices per unit cell. The Hall conductance is computed from the Chern numbers of the low energy eigenstates. At zero temperature, it reverses sign abruptly at half filling. At precisely half filling, all eigenstates are doubly degenerate for any odd number of flux quanta. We prove the exact degeneracies on the torus by constructing an SU(2) algebra of point-group symmetries, associated with the center of vorticity. This result is interpreted as if each vortex carries an internal spin-half degree of freedom ('vspin'), which can manifest itself as a charge density modulation in its core. Our findings suggest interesting experimental implications for vortex motion of cold atoms in optical lattices, and magnet-transport of short coherence length superconductors.

Charge disproportionation in TlGaSe2 crystals detected by dielectric spectroscopy

The electrical properties of TlGaSe2 ferroelectric-semiconductor with layer crystalline structure in the direction parallel to the layers plane is studied by impedance spectroscopy in the frequency range 30 Hz–15 MHz. The impedance spectra were analyzed together with earlier results for perpendicular direction. Low frequency dispersion due to the interfacial polarization from Maxwell–Wagner type space charge effect has been observed. It was established that anomalous rise of relaxation time in the 145–190 K temperature range far from the known phase transitions temperatures is characteristic only for the polarization perpendicular to the layers plane. The model of a specific phase transition taking place in 145–190 K temperature region assuming carriers accumulation was proposed. Charge disproportination is the main characteristic feature of this phase transition which leads to the charge density modulation in one crystallographic direction perpendicular to the layers plane of the crystal.

Bond-located spin density wave phase in the two-dimensional (2D) ionic Hubbard model

We investigate the quantum phase transitions in the half-filled ionic Hubbard model on a two-dimensional (2D) square lattice using the variational cluster approach (VCA). We present explicit evidence for the tendency toward a novel intermediate phase in this model. This phase is characterized by bond-located magnetization. For weak Coulomb repulsion U, the system is a band insulator, and then undergoes a transition to the intermediate phase at the first-phase boundary U=Uc1. As U is increased beyond the second transition point Uc2, there occurs a Mott insulator accompanied by a long-range antiferromagnetic (AF) order. The bond-located spin density wave competes with the antiferromagnetism while the charge-density modulation exists all the way due to the staggered potential Δ.

Mechanism of formation of half-doped stripes in underdoped cuprates

Using a variational Monte-Carlo approach with a recently proposed stripe wave function, we showed that the strong correlation included in a t-J-type model has essentially all the necessary ingredients to form these stripes with modulations of charge density, spin magnetization, and pair field. If a perturbative effect of electron-phonon coupling to renormalize the effective mass or the hopping rate of holes is considered with the model, we find the half-doped stripes, which has on the average one half of a hole in one period of charge modulation, to be most stable, energetic wise in the underdoped region, $1/12\leq\delta\leq1/8$. This is in good agreement with the observation in the neutron scattering experiments. We also find long range Coulomb interaction to be less effective in the formation of half-doped stripes.

Observation of molecular orbital gating

The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.

Linear response and the Thomas-Fermi approximation in undoped graphene

We analyze the range of validity of Thomas-Fermi theory for describing charge-density modulations induced by external potentials in neutral graphene. We compare exact results obtained from a tight-binding calculation with those of linear-response theory and the Thomas-Fermi approximation. For experimentally interesting ranges of size and density amplitudes (electron densities less than $\ensuremath{\sim}{10}^{11}\text{ }{\text{cm}}^{\ensuremath{-}2}$, and spatial length scales below $\ensuremath{\sim}20\text{ }\text{nm}$), linear response is significantly more accurate than Thomas-Fermi theory.

Insulating behavior with spin and charge order in the ionic Hubbard model

Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions $Dg2$ indicate that the band and the Mott insulator phases are separated by a metallic phase. We present zero-temperature dynamical mean-field theory solutions, which include antiferromagnetic long-range order and show that the one-particle spectral functions always possess an energy gap, and therefore the system is insulating for all interaction strengths. The staggered charge-density modulation coexists with antiferromagnetic long-range order of N\'eel type.