Large Interlayer Separation Research Articles

Eliashberg Theory for Dynamical Screening in Bilayer Exciton Condensation.

We study the effect of dynamical screening of interactions on the transition temperatures (T_{c}) of exciton condensation in a symmetric bilayer of quadratically dispersing electrons and holes by solving the linearized Eliashberg equations for the anomalous interlayer Green's functions. We find that T_{c} is finite for the range of density and layer separations studied, decaying exponentially with interlayer separation. T_{c} is suppressed well below that predicted by a Hartree Fock mean field theory with unscreened Coulomb interaction, but is above the estimates from the statically screened Coulomb interaction. Furthermore, using a diagrammatic framework, we show that the system is always an exciton condensate at zero temperature but T_{c} is exponentially small for large interlayer separation.

Drag Resistivity of Hole-Hole Static Interactions with the Effect of Non- Homogeneous Dielectric Medium

BACKGROUND: We have study the Coulomb drag phenomena for hole-hole static potentials theoretically and measured numerically using the random phase approximation (RPA) method OBJECTIVE: The drag resistivity is evaluated at low temperature, large interlayer separation limit and weakly screening regime, with the geometry of two atomically thin materials, such as, BLG/GaAs based multilayer system, is a promising systems in nanomaterials and technology METHOD: Static local field corrections (LFC) are considered to take into account the Exchange-correlations (XC) and mutual interaction effects with varying concentrations of active and passive layer RESULT: It is found that the drag resistivity is found enhanced on using the LFC effects and increases on increasing the effective mass. In Fermi-Liquid regime, drag resistivity is directly proportional to T^2, n^(-3), d^(-4) and ϵ^2 with respect to temperature (T), density (n), interlayer separation (d~nm) and dielectric constant (ϵ_2), respectively. CONCLUSION: Dependency of drag resistivity is measured and compared to 2D e-e and e-h coupled-layer systems with and without the effect of non-homogeneous dielectric medium.

Na0.67Mn0.33Ni0.33Co0.33O2: Effect of synthesis technique on competing P3 and P2 phases

In layered NaxTMO2 (TM = transition metal) common understanding attributes the emergence of P2 and P3 phases to the synthesis temperature. In this work, we show that the synthesis technique is the main responsible for these phases. We synthesize Na0.67Mn0.33Ni0.33Co0.33O2 with two techniques, solid-state and electrospinning, using two temperatures, 700 °C and 900 °C. Both electrospun samples sintered at 900 °C and 700 °C show single phase P2 and P3, respectively. Both solid-state synthesized samples at 900 °C and 700 °C show mixture of P2/P3. Electrochemical properties of the sample with only P3 phase has the highest initial capacity of 130 mAhg−1 at C/3 rate. The sample with only P2 phase has very stable cycle performance with 87 % capacity retention over 100 cycles consistent with its large interlayer separation. Our results show that the synthesis technique plays a crucial role in the crystal structure and the electrochemical performance of the layered cathode materials for Na-ion batteries.

Two-Dimensional Negative Thermal Expansion in a Crystal of LiBO2

Negative thermal expansion (NTE), violating the common sense of “thermal expansion and cold contraction” effects, is a novel temperature-responding behavior of great scientific and technical significance. Herein, we report a two-dimensional (2D) NTE behavior in a crystal of LiBO2, which is constructed by graphite-like [LiBO2]∞ layers. This intriguing thermal property originates from the synergistic effect of the distortion of in-plane [LiO3] bases in [LiO4] tetrahedra and the rotation of [BO3] triangles in the [LiBO2]∞ layer, driven by the force perpendicular to the layer owing to the large interlayer separation as temperature increases. Remarkably, the in-plane and out-of-plane Li–O bonds within the [LiO4] tetrahedra have nearly the same bond strength and exhibit the similar variation with respect to temperature, and this is quite different from the common sense on the 2D NTE behavior in layered structures that the intralayer atomic interaction must be much stronger than the interlayer ones. Our study deepens the understanding of the 2D NTE mechanism and would promote the exploration for NTE materials.

Quantum halo states in two-dimensional dipolar clusters

A halo is an intrinsically quantum object defined as a bound state of a spatial size which extends deeply into the classically forbidden region. Previously, halos have been observed in bound states of two and less frequently of three atoms. Here, we propose a realization of halo states containing as many as six atoms. We report the binding energies, pair correlation functions, spatial distributions, and sizes of few-body clusters composed by bosonic dipolar atoms in a bilayer geometry. We find two very distinct halo structures, for large interlayer separation the halo structure is roughly symmetric and we discover an unusual highly anisotropic shape of halo states close to the unbinding threshold. Our results open avenues of using ultracold gases for the experimental realization of halos composed by atoms with dipolar interactions and containing as many as six atoms.

Fingerprinting triangular-lattice antiferromagnet by excitation gaps

CeCd$_3$As$_3$ is a rare-earth triangular-lattice antiferromagnet with large inter-layer separation. Our field-dependent heat capacity measurements at dilution fridge temperatures allow us to trace the field-evolution of the spin-excitation gaps throughout the antiferromagnetic and paramagnetic regions. The distinct gap evolution places strong constraints on the microscopic pseudo-spin model, which, in return, yields a close {\it quantitative} description of the gap behavior. This analysis provides crucial insights into the nature of the magnetic state of CeCd$_3$As$_3$, with a certainty regarding its stripe order and low-energy model parameters that sets a compelling paradigm for exploring and understanding the rapidly growing family of the rare-earth-based triangular-lattice systems.

Coulomb drag study in graphene/GaAs bilayer system with the effect of local field correction and dielectric medium

The drag resistivity is measured numerically for electron-electron (e-e) and electron-hole (e-h) coupled layer system composed of single layer graphene and two-dimensional (2D) GaAs layer separated by a barrier. The system is studied within the model of random phase approximation (RPA) and taking into account the static local field correction (LFC) and layer thickness effects in the Ballistic/Boltzmann regime. We have shown analytical expressions of non-linear susceptibility function and effective interlayer Coulomb interaction for non homogeneous dielectric environment at low temperature, high density and large interlayer separation limit. Exchange and correlation effects enhanced the drag resistivity by considering the static local field correction. Width of the layer and interlayer distance dependent local form factors (LFF) are obtained from the solution of the Poisson equation for a multi-layer dielectric medium. The effects of LFC and LFF are the function of bare inter- and intra-layer potential, which enhances the drag resistivity compare to simply measured RPA results. Enhanced drag resistivity also measured for e-h bilayer system where the hole layer is considered for 2D GaAs (drag layer), cause of larger effective mass of hole compare to the electron. Similarly, e-e and e-h bilayer system measured the enhanced drag resistivity due to LFC effects.

Coulomb drag study in electron-electron bilayer system with a dielectric medium

Abstract We have theoretically investigated the drag resistivity in electron-electron bilayer system by using the random phase approximation (RPA) for weakly interacting regime, at low temperature, high density, and large interlayer separation limit. We show analytical expressions by using long wavelength (low frequency regime) expansion of response function in Boltzmann regime and bare Coulomb interaction for non homogeneous dielectric environment which make the system interested and different compare to others. The influence of electron-electron bare inter and intra layer interaction are considered static cause of static local form factor (LFF) approximation in coupled layer system of non homogeneous dielectric background. The effective dielectric function and LFF are obtained from the solution of the Poisson equation of a three-layer dielectric medium. The effects of local form factors are included in bare inter and intralayer potential, yields the dependency of layer separation and dielectric constants of the materials. Drag resistivity dependency is measured numerically with respect to temperature T and density n and compared to other 2DES, which yields to possess the same qualitative and quantitative behaviour compare to bilayer system of homogeneous dielectric environment.

Theoretical phase diagram of two-component composite fermions in double-layer graphene

Theory predicts that double layer systems realize "two-component composite fermions," which are formed when electrons capture both intra- and inter-layer vortices, to produce a wide variety of new strongly correlated liquid and crystal states as a function of the layer separation. Recent experiments in double layer graphene have revealed a large number of layer-correlated fractional quantum Hall states in the lowest Landau level, many of which have not been studied quantitatively in previous theoretical works. We consider the competition between various liquid and crystal states at several of these filling factors (specifically, the states at total filling factors $\nu=3/7$, $4/9$, $6/11$, $4/7$, $3/5$, $2/3$, and $4/5$) to determine the theoretical phase diagram as a function of the layer separation. We compare our results with experiments and identify various observed states. In particular, we show that at small layer separations the states at total fillings $\nu=3/7$ and $\nu=3/5$ are partially pseudospin polarized, where pseudospin refers to the layer index. For certain fractions, such as $\nu=3/7$, interlayer correlations are predicted to survive to surprisingly large interlayer separations.

Progressive Micromodulation of Interlayer Coupling in Stacked WS2/WSe2 Heterobilayers Tailored by a Focused Laser Beam.

When a vertically stacked heterobilayer comprising of a WSe2 monolayer on a WS2 monolayer is first fabricated, the heterobilayer behaves like two independent monolayers because of the presence of a large interlayer separation. However, after the stacked heterobilayer is subjected to a focused laser treatment, the interlayer separation between the two monolayers becomes progressively reduced which transforms the WS2/WSe2 heterostructure from the noncoupling to the strongly coupling regime. This strong coupling induces the charge transfer between two layers and thus lowers the exciton recombination rate in the individual layer. This changes the optical properties of the heterobilayer from a fluorescence-active species into one where the fluorescence is quenched. The focused laser beam scanning method is therefore able to serve as a localized annealing tool to progressively modulate the interlayer separation and enable the micropatterning of the heterobilayer to achieve distinct regions with different degrees of fluorescence quenching. Systematic studies are carried out to gain an insight into the mechanism involved in the onset of the interlayer coupling in the material. Our method is also successfully extended to a WS2/WS2 homobilayer structure.

Exotic bilayer crystals in a strong magnetic field

Electron bilayers in a strong magnetic field exhibit insulating behavior for a wide range of interlayer separation $d$ for total Landau level fillings $\nu\leq 1/2$, which has been interpreted in terms of a pinned crystal. We study theoretically the competition between many strongly correlated liquid and crystal states and obtain the phase diagram as a function of quantum well width and $d$ for several filling factors of interest. We predict that three crystal structures can be realized: (a) At small $d$, the Triangular Ising AntiFerromagnetic (TIAF) crystal is stabilized in which the particles overall form a single-layer like triangular crystal while satisfying the condition that no nearest-neighbor triangle has all three particles in the same layer. (b) At intermediate $d$, a Correlated Square (CS) crystal is stabilized, in which particles in each layer form a square lattice, with the particles in one layer located directly across the centers of the squares of the other. (c) At large $d$, we find a Bilayer Graphene (BG) crystal in which the A and B sites of the graphene lattice lie in different layers. All crystals that we predict are strongly correlated crystals of composite fermions; a theory incorporating only electron Hartree-Fock crystals does not find any crystals besides the `trivial' ones occurring at large interlayer separations for total filling factor $\nu\leq1/3$ (when layers are uncorrelated and each layer is in the long familiar single-layer crystal phase). The TIAF, CS and BG crystals come in several varieties, with different flavors of composite fermions and different interlayer correlations. The appearance of these exotic crystal phases adds to the richness of the physics of electron bilayers in a strong magnetic field, and also provides insight into experimentally observed bilayer insulator as well as transitions within the insulating part of the phase diagram.

A many-electron perturbation theory study of the hexagonal boron nitride bilayer system*

In this article we explore methods to reduce the computational cost in many-electron wave function expansions including explicit correlation and compact one-electron basis sets for the virtual orbitals. These methods are applied to the calculation of the interlayer binding energy of the h-BN bilayer system. We summarize the optimized interlayer distances as well as their binding energies for various stacking faults on different levels of theory including second-order Moller-Plesset perturbation theory and the random phase approximation. Furthermore, we investigate the asymptotic behavior of the binding energy at large interlayer separation and find that it decays as D -4 in agreement with theoretical predictions, where D is the interlayer distance.

Boltzmann-Langevin theory of Coulomb drag

We develop a Boltzmann-Langevin description of the Coulomb drag effect in clean double-layer systems with large interlayer separation $d$ as compared to the average interelectron distance ${\ensuremath{\lambda}}_{F}$. Coulomb drag arises from density fluctuations with spatial scales of order $d$. At low temperatures, their characteristic frequencies exceed the intralayer equilibration rate of the electron liquid, and Coulomb drag may be treated in the collisionless approximation. As temperature is raised, the electron mean free path becomes short due to electron-electron scattering. This leads to local equilibration of electron liquid, and consequently drag is determined by hydrodynamic density modes. Our theory applies to both the collisionless and the hydrodynamic regimes, and it enables us to describe the crossover between them. We find that drag resistivity exhibits a nonmonotonic temperature dependence with multiple crossovers at distinct energy scales. At the lowest temperatures, Coulomb drag is dominated by the particle-hole continuum, whereas at higher temperatures of the collision-dominated regime it is governed by the plasmon modes. We observe that fast intralayer equilibration mediated by electron-electron collisions ultimately renders a stronger drag effect.

Film Structure of Epitaxial Graphene Oxide on SiC: Insight on the Relationship Between Interlayer Spacing, Water Content, and Intralayer Structure

Chemical oxidation of multilayer graphene grown on silicon carbide yields films exhibiting reproducible characteristics, lateral uniformity, smoothness over large areas, and manageable chemical complexity, thereby opening opportunities to accelerate both fundamental understanding and technological applications of this form of graphene oxide films. Here, we investigate the vertical inter‐layer structure of these ultra‐thin oxide films. X‐ray diffraction, atomic force microscopy, and IR experiments show that the multilayer films exhibit excellent inter‐layer registry, little amount (<10%) of intercalated water, and unexpectedly large interlayer separations of about 9.35 Å. Density functional theory calculations show that the apparent contradiction of “little water but large interlayer spacing in the graphene oxide films” can be explained by considering a multilayer film formed by carbon layers presenting, at the nanoscale, a non‐homogenous oxidation, where non‐oxidized and highly oxidized nano‐domains coexist and where a few water molecules trapped between oxidized regions of the stacked layers are sufficient to account for the observed large inter‐layer separations. This work sheds light on both the vertical and intra‐layer structure of graphene oxide films grown on silicon carbide, and more in general, it provides novel insight on the relationship between inter‐layer spacing, water content, and structure of graphene/graphite oxide materials.

Structure and functionality of bromine doped graphite

First-principles calculations are used to study the enhanced in-plane conductivity observed experimentally in Br-doped graphite, and to study the effect of external stress on the structure and functionality of such systems. The model used in the numerical calculations is that of stage two doped graphite. The band structure near the Fermi surface of the doped systems with different bromine concentrations is compared to that of pure graphite, and the charge transfer between carbon and bromine atoms is analyzed to understand the conductivity change along different high symmetry directions. Our calculations show that, for large interlayer separation between doped graphite layers, bromine is stable in the molecular form (Br2). However, with increased compression (decreased layer-layer separation) Br2 molecules tend to dissociate. While in both forms, bromine is an electron acceptor. The charge exchange between the graphite layers and Br atoms is higher than that with Br2 molecules. Electron transfer to the Br atoms increases the number of hole carriers in the graphite sheets, resulting in an increase of conductivity.

Breakdown of the Interlayer Coherence in Twisted Bilayer Graphene

Coherent motion of electrons in Bloch states is one of the fundamental concepts of charge conduction in solid-state physics. In layered materials, however, such a condition often breaks down for the interlayer conduction, when the interlayer coupling is significantly reduced by, e.g., a large interlayer separation. We report that complete suppression of coherent conduction is realized even in an atomic length scale of layer separation in twisted bilayer graphene. The interlayer resistivity of twisted bilayer graphene is much higher than the c-axis resistivity of Bernal-stacked graphite and exhibits strong dependence on temperature as well as on external electric fields. These results suggest that the graphene layers are significantly decoupled by rotation and incoherent conduction is a main transport channel between the layers of twisted bilayer graphene.

Photoelectron Diffraction and Holographic Reconstruction of Graphite

A series of two-dimensional photoelectron angular distributions from a graphite single crystal for different kinetic energies from 311 to 908 eV using circularly polarized soft X-ray was measured. We realized that the directions of the prominent peaks arranged in a hexagon appearing in the photoelectron diffraction patterns and the directions from a photoelectron emitter atom to the surrounding atoms, which we previously assigned as the corresponding scatterer atoms [Appl. Phys. Lett. 85 (2004) 3737], do not match. From the comparison of the photoelectron diffraction patterns with multiple scattering calculations for each atomic site, we corrected our earlier assignments and revealed that the signatures of such peaks are even found in the photoelectron diffraction pattern of monolayer graphene. In the case of graphite that has a large interlayer separation, the diffraction rings due to in-plane scattering become dominant over the forward focusing peaks owing to interlayer scattering. Furthermore, a holographic reconstruction algorithm based on the fitting of elemental diffraction patterns for various C–C bond distances was used to analyze the full set of measured data. Clear nanometer-scale real-space images of several graphite layers were reconstructed.

Enhancement of Coulomb drag in double-layer graphene structures by plasmons and dielectric background inhomogeneity

The drag of massless fermions in graphene double-layer structures is investigated over a wide range of temperatures and interlayer separations. We show that the inhomogeneity of the dielectric background in such graphene structures, for experimentally relevant parameters, results in a significant enhancement of the drag resistivity. At intermediate temperatures the dynamical screening via plasmon-mediated drag enhances the drag resistivity and results in an upturn in its behavior at large interlayer separations. In a range of interlayer separations, corresponding to the crossover from strong to weak coupling of graphene layers, we find that the decrease of the drag resistivity with interlayer spacing is approximately quadratic. This dependence weakens below this range of interlayer spacing while for larger separations we find a cubic (quartic) dependence at intermediate (low) temperatures.

Coulomb drag between massless and massive fermions

We theoretically investigate the frictional drag induced by the Coulomb interaction between spa- tially separated massless and massive fermions at low temperatures. As a model system, we use a double-layer structure composed of a two-dimensional electron gas (2DEG) and a n-doped graphene layer. We analyze this system numerically and also present analytical formulae for the drag re- sistivity in the limit of large and small interlayer separation. Both, the temperature and density dependence are investigated and compared to 2DEG-2DEG and graphene-graphene double-layer structures. Whereas the density dependence of the transresistivity for small interlayer separation differs already in the leading order for each of those three structures, we find the leading order con- tribution of density dependence in the large interlayer separation limit to exhibit the same density dependence in each case. In order to distinguish between the different systems in the large interlayer separation limit, we also investigate the subleading contribution to the transresistivity. Furthermore, we study the Coulomb drag in a double-layer structure consisting of n-doped bilayer and monolayer graphene, which we find to possess the same qualitative behavior as the 2DEG-graphene system.

Structure and Magnetism of the Topotactically Reduced Oxychloride Sr4Mn3O6.5Cl2

Reaction of the n = 3 Ruddlesden-Popper oxychloride Sr(4)Mn(3)O(7.5)Cl(2) with calcium hydride yields the topotactically reduced phase Sr(4)Mn(3)O(6.5)Cl(2). The deintercalation of oxide ions from the central MnO(1.5) layer of the starting phase is accompanied by a rearrangement of the anion lattice, resulting in a layer of composition MnO(0.5) in the reduced material, consisting of chains of MnO(4) tetrahedra connected by edge and corner sharing. Magnetization and low-temperature neutron diffraction data are consistent with antiferromagnetic coupling of manganese spins, but no long-range magnetic order is observed down to 5 K, presumably due to the large interlayer separation in the reduced phase. The influence of anion substitution on the structural selectivity of low-temperature reduction reactions is discussed.