Bulk Limit Research Articles

A sudden collapse in the transport lifetime across the topological phase transition in (Bi1−xInx)2Se3

The quantum phase transition from a topological to a conventional insulator in In-doped Bi2Se3 occurs when the topological phase is destroyed by the hybridization of states on opposite surfaces. This is characterized by a sudden change in the transport lifetime, measured by means of optical spectroscopy. Topological insulators are newly discovered states of matter with robust metallic surface states protected by the topological properties of the bulk wavefunctions1,2,3,4,5,6. A quantum phase transition from a topological insulator to a conventional insulator and a change in topological class can occur only when the bulk bandgap closes3. In this work, we have used time-domain terahertz spectroscopy to investigate the low-frequency conductance in (Bi1−xInx)2Se3 as we tune through this transition by In substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit x≈0.06 where a maximum in the mid-infrared absorption is exhibited. This absorption can be identified with the bulk bandgap closing and change in topological class. The correlation length associated with the quantum phase transition appears as the evanescent length of the surface states. The observation of the thickness-dependent collapse of the transport lifetime shows the unusual role that finite-size effects play in this topological quantum phase transition.

First-Order Dyson Coordinates and Geometry

The mathematical constructs of the Dyson coordinates and geometry are introduced. The former are a unitary transformation of the normal coordinates and the anharmonic vibrational counterpart of the Dyson orbitals in electronic structure theory. The first-order Dyson coordinates bring the sums of the harmonic force constants and their first-order diagrammatic perturbation corrections (the first-order Dyson self-energy) to a diagonal form. The first-order Dyson geometry has no counterpart in electronic structure theory. It is the point on the potential energy surface at which the sums of the energy gradients and their first-order diagrammatic perturbation corrections vanish. It agrees with the vibrationally averaged geometry of vibrational self-consistent field (VSCF) theory in the bulk limit. These constructs provide a unified view of the relationship of VSCF and its diagrammatically size-consistent modifications as well as the self-consistent phonon method widely used in solid-state physics.

Electronic and magnetic reconstructions in manganite superlattices

We investigate the electronic reconstruction at the interface between ferromagnetic metallic (FM) and antiferromagnetic insulating (AFI) manganites in superlattices using a two-orbital double-exchange model including superexchange interactions, Jahn-Teller lattice distortions, and long range Coulomb interactions. The magnetic and the transport properties critically depend on the thickness of the AFI layers. We focus on superlattices where the constituent parent manganites have the same electron density n = 0.6. The induced ferromagnetic moment in the AFI layers decreases monotonically with increasing layer width, and the electron-density profile and the magnetic structure in the center of the AFI layer gradually return to the bulk limit. The width of the AFI layers and the charge-transfer profile at the interfaces control the magnitude of the magnetoresistance and the metal-insulator transition of the FM/AFI superlattices.

Orbital magnetism of graphene flakes

Orbital magnetism is studied for graphene flakes with various shapes and edge configurations using the tight-binding approximation. In the low-temperature regime where the thermal energy is much smaller than the energy-level spacing, the susceptibility rapidly changes between diamagnetism and paramagnetism as a function of Fermi energy, in accordance with the energy-level structure. The susceptibility at the charge neutral point is generally larger in armchair flake than in zigzag flake, and larger in hexagonal flake than in triangular flake. As the temperature increases, the discrete structures due to the quantum confinement are all gone, and the susceptibility approximates the bulk limit independently of the atomic configuration. The diamagnetic current circulates entirely on the graphene flake at zero temperature, while in increasing temperature it is localized near the edge with the characteristic depth proportional to $1/T$. We predict that the diamagnetism of graphene can be observed using the alignment of graphene flakes in a feasible range of magnetic field.

(Invited) Optical Spin Orientation in SiGe Heterostructures

Ge is emerging as an interesting material for spintronic applications. We demonstrate that in SiGe heterostructures strain and quantum confinement effects can be used to tailor spin related properties enhancing the spin polarization of injected carriers above the bulk limit. Moreover the fast Γ-L electron scattering and the strain induced removal of the heavy-hole, light-hole degeneracy, make SiGe heterostructures an ideal material platform for the study of hole dynamics.

Non-perturbative effects of laser illumination on the electrical properties of graphene nanoribbons

Floquet theory combined with a realistic description of the electronic structure of illuminated graphene and graphene nanoribbons is developed to assess the emergent non-adiabatic and non-perturbative effects on the electronic properties. Here we introduce an efficient computational scheme and illustrate its use by applying it to graphene nanoribbons in the presence of both linear and circular polarization. The interplay between confinement due to the finite sample size and laser-induced transitions is shown to lead to sharp features in the average conductance and density of states. Particular emphasis is given to the emergence of the bulk limit response.

Topological phase transition and Dirac fermion transfer in Bi2Se3 films

Topological surface states, while protected by time-reversal symmetry in the bulk limit, can be missing in films with thicknesses much greater than the decay lengths of the surface states. This novel effect is demonstrated theoretically in Bi2Se3, the best known topological insulator. When the spin-orbit-coupling strength is tuned through the quantum critical point (realizable experimentally by low-Z element substitution), there is a wide dead zone where the film is topological but without topological surface states within the projected bulk gap. This dead zone can be suppressed by interfacial bonding.

Solution-processed small molecule:fullerene bulk-heterojunction solar cells: impedance spectroscopy deduced bulk and interfacial limits to fill-factors

Using impedance spectroscopy, we demonstrate that the low fill factor (FF) typically observed in small molecule solar cells is due to hindered carrier transport through the active layer and hindered charge transfer through the anode interfacial layer (IFL). By carefully tuning the active layer thickness and anode IFL in BDT(TDPP)2 solar cells, the FF is increased from 33 to 55% and the PCE from 1.9 to 3.8%. These results underscore the importance of simultaneously optimizing active layer thickness and IFL in small molecule solar cells.

Nuclear magnetic resonance predictions for graphenes: concentric finite models and extrapolation to large systems

Nuclear magnetic resonance (NMR) data for graphenes are mainly lacking in the literature. We provide quantitative first-principles quantum-chemical calculations of NMR chemical shifts and shielding anisotropies as well as spin-spin couplings and anisotropies for increasingly large, hexagon-like fragments of graphene, hydrogenated graphene (graphane) and fluorinated graphene (fluorographene). Due to the rapid convergence of finite molecular model results, the parameter values in the innermost region of large flakes of these materials approach the bulk limit. For nuclear shieldings in the finite band-gap graphane and fluorographene systems, as well as deuterium quadrupole couplings in graphane, these limiting values are verified by periodic gauge-including projector augmented wave (PAW) calculations at corresponding theoretical levels. The periodic PAW wave method was used for all systems to obtain periodic structures. A quantum-chemical cluster approach was used with novel completeness-optimised basis sets to calculate both the shielding and coupling tensors for planar carbon nanoflakes of increasing size. The geometry of the innermost part of the nanoflakes as well as the nuclear shieldings converge toward the periodic counterparts. The cluster method allows the calculation of the spin-spin coupling tensors of all the graphenes and--in contrast to the periodic approach--all the NMR properties for the zero-band-gap graphene itself. The obtained parameters provide a plausible starting point for experimental NMR investigations of graphenes.

Permittivity and Permeability Measurement of Spin-Spray Deposited Ni-Zn-Ferrite Thin Film Sample

Permittivity and permeability values of thin film Nickel-Zinc ferrite samples deposited on a glass substrate have been derived in the higher microwave frequency range. This thin film has been fabricated by the spin spray process. The thickness of the thin film has been measured to be 0.6 μm. It is deposited on to the 0.2 mm glass substrate. In the lower GHz frequency range, this sample has been observed to exhibit higher permeability than the Snoek's limit of bulk Ni-Zn-ferrite film. The transmission-reflection method based in-waveguide measurement technique has been employed to obtain the scattering parameters in the frequency range from 18 to 40 GHz. The evaluation method for calculating the material properties has been modified to obtain reliable results for thin film based samples. The in-waveguide based measurement technique has been successfully employed for the measurement of the constitutive properties of thin film ferrite samples.

Thermally Driven Crossover from Indirect toward Direct Bandgap in 2D Semiconductors: MoSe2 versus MoS2

Layered semiconductors based on transition-metal chalcogenides usually cross from indirect bandgap in the bulk limit over to direct bandgap in the quantum (2D) limit. Such a crossover can be achieved by peeling off a multilayer sample to a single layer. For exploration of physical behavior and device applications, it is much desired to reversibly modulate such crossover in a multilayer sample. Here we demonstrate that, in a few-layer sample where the indirect bandgap and direct bandgap are nearly degenerate, the temperature rise can effectively drive the system toward the 2D limit by thermally decoupling neighboring layers via interlayer thermal expansion. Such a situation is realized in few-layer MoSe(2), which shows stark contrast from the well-explored MoS(2) where the indirect and direct bandgaps are far from degenerate. Photoluminescence of few-layer MoSe(2) is much enhanced with the temperature rise, much like the way that the photoluminescence is enhanced due to the bandgap crossover going from the bulk to the quantum limit, offering potential applications involving external modulation of optical properties in 2D semiconductors. The direct bandgap of MoSe(2), identified at 1.55 eV, may also promise applications in energy conversion involving solar spectrum, as it is close to the optimal bandgap value of single-junction solar cells and photoelechemical devices.

Inferring bulk self-assembly properties from simulations of small systems with multiple constituent species and small systems in the grand canonical ensemble

In this paper, we generalize a methodology [T. E. Ouldridge, A. A. Louis, and J. P. K. Doye, J. Phys.: Condens. Matter 22, 104102 (2010)] for dealing with the inference of bulk properties from small simulations of self-assembling systems of characteristic finite size. In particular, schemes for extrapolating the results of simulations of a single self-assembling object to the bulk limit are established in three cases: for assembly involving multiple particle species, for systems with one species localized in space and for simulations in the grand canonical ensemble. Furthermore, methodologies are introduced for evaluating the accuracy of these extrapolations. Example systems demonstrate that differences in cluster concentrations between simulations of a single self-assembling structure and bulk studies of the same model under identical conditions can be large, and that convergence on bulk results as system size is increased can be slow and non-trivial.

Temperature and size dependency of thermal conductivity of aluminum nanocluster

Molecular dynamics simulation has been applied for investigation of coefficient thermal conductivity (CTC) of aluminum nanocluster and its bulk limit via Green–Kubo formalism. The dependence of CTC on size range $$ 256 \le N \le 1,372 $$ is investigated. Temperature dependence of CTC quantity is considered for aluminum nanocluster and its bulk limit in range $$ 300 \le T \le 1,100 $$ K. At low temperature, CTC quantity for aluminum nanocluster is greater than its bulk value. Our results regarding the CTC quantity as a function of size and temperature of aluminum nanocluster show that there is a peak in the thermal conductivity. It is worthwhile to notice that trend and the value of our result for CTC quantity in the bulk of aluminum is in agreement with experimental results.

Dependence of self-diffusion coefficient, surface energy, on size, temperature, and Debye temperature on size for aluminum nanoclusters

Molecular dynamics simulations are performed to investigate the surface energy and self-diffusion coefficient in aluminum nanoclusters (AlN) as a function of temperature, T (T=300–1100K), and size (N=108–4000 atoms, with N the number of atoms in the cluster), with the self-diffusion coefficient compared with the bulk limit. Debye temperature, cohesive energy, and average coordination number of AlN are also explored as a function of the cluster size. The surface energy decreases as a function of size as well as temperature, and becomes very small at sizes larger than N≥2000, while the average coordination number and the Debye temperature increase as a function of nanocluster size. The self-diffusion coefficient decreases with increasing size as N−2/3, while increasing as a function of temperature, and exhibits values substantially larger than in the bulk, quantitatively confirming the much greater structural freedom encountered of nanoscale systems.

Scalable properties of metal clusters: A comparative study of modern exchange-correlation functionals

The performance of eight generalized gradient approximation exchange-correlation (xc) functionals is assessed by a series of scalar relativistic all-electron calculations on octahedral palladium model clusters Pd(n) with n = 13, 19, 38, 55, 79, 147 and the analogous clusters Au(n) (for n up through 79). For these model systems, we determined the cohesive energies and average bond lengths of the optimized octahedral structures. We extrapolate these values to the bulk limits and compare with the corresponding experimental values. While the well-established functionals BP, PBE, and PW91 are the most accurate at predicting energies, the more recent forms PBEsol, VMTsol, and VT{84}sol significantly improve the accuracy of geometries. The observed trends are largely similar for both Pd and Au. In the same spirit, we also studied the scalability of the ionization potentials and electron affinities of the Pd clusters, and extrapolated those quantities to estimates of the work function. Overall, the xc functionals can be classified into four distinct groups according to the accuracy of the computed parameters. These results allow a judicious selection of xc approximations for treating transition metal clusters.

Spectral weight in holographic scaling geometries

We compute the low energy spectral density of transverse currents in theories with holographic duals that exhibit an emergent scaling symmetry characterized by dynamical critical exponent $z$ and hyperscaling violation exponent $\theta$. For any finite $z$ and $\theta$, the low energy spectral density is exponentially small at nonzero momentum. This indicates that any nonzero momentum low energy excitations of putative hidden Fermi surfaces are not visible in the classical bulk limit. We furthermore show that if the limit $z \to \infty$ is taken with the ratio $\eta = - \theta/z > 0$ held fixed, then the resulting theory is locally quantum critical with an entropy density that vanishes at low temperatures as $s \sim T^\eta$. In these cases the low energy spectral weight at nonzero momentum is not exponentially suppressed, possibly indicating a more fermionic nature of these theories.

Enhanced orbital mixing in the valence band of strained germanium

We give a theoretical explanation of the very high electron-spin polarization measured in compressively strained Ge layers through spin-polarized photoemission experiments, based on the enhancement of the band orbital mixing between light-hole (LH) and split-off (SO) bands along the growth direction of Ge thin films. We deduce an increased valence-band mixing, with respect to the bulk case, from the measured spin-polarization values, which exceed the bulk limit when optical transitions away from the $\ensuremath{\Gamma}$ point of the tetragonally distorted Brillouin zone are taken into account. Furthermore, we show that the degree of LH-SO band orbital mixing is directly proportional to the degree of compressive strain in the Ge layer.

Melting profile of cesium metal nanoclusters by molecular dynamics simulation

Metallic nanoclusters have been attracted considerable attention owing to their unique catalytic and electronic properties as well as their great potential for industrial application. Using molecular dynamics simulations with the Gupta type many-body potential, we investigated the melting of cesium metal nanocluster (CsN) with N=55 up to 6750 atoms for the first time. The internal energy, heat capacity, enthalpy, entropy and surface energy of the nanoclusters were calculated at different temperatures. These properties are used to characterize the physical phase and also to determine the melting transition profile of each nanocluster. The melting temperatures of nanoclusters conform to each other and are increased with the cluster size, approaching to the bulk limit for the largest one. The size dependence of the melting temperature is consistent with both experimental results and theories of metal nanoclusters.

Orbital magnetic susceptibility of finite-sized graphene

We study the orbital magnetism of graphene ribbon in the effective-mass approximation, to figure out the finite-size effect on the singular susceptibility known in the bulk limit. We find that the susceptibility at T = 0 oscillates between diamagnetism and paramagnetism as a function of Fermi energy, in accordance with the subband structure formed by quantum confinement. In increasing T, the oscillation rapidly disappears once the thermal broadening energy exceeds the subband spacing, and the susceptibility approaches the bulk limit i.e., a thermally broadened diamagnetic peak centered at zero energy point. The electric current supporting the diamagnetism is found to flow near the edge with a depth which proportional to reciprocal of T, with v being the band velocity, while at T = 0 the current distribution spreads entirely in the sample reflecting the absence of the characteristic wavelength in graphene. The result is applied to estimate the three-dimensional random-stacked multilayer graphene, where we show that the external magnetic field is significantly screened inside the sample in low temperatures, in a much stronger manner than in graphite.

Optical Spin Injection and Spin Lifetime in Ge Heterostructures

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5ns below 150K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.