Modern network science has revealed fundamental aspects of normal brain-network organization, such as small-world and scale-free patterns, hierarchical modularity, hubs and rich clubs. The next challenge is to use this knowledge to gain a better understanding of brain disease. Recent developments in the application of network science to conditions such as Alzheimer's disease, multiple sclerosis, traumatic brain injury and epilepsy have challenged the classical concept of neurological disorders being either 'local' or 'global', and have pointed to the overload and failure of hubs as a possible final common pathway in neurological disorders.

Modern network science of neurological disorders

The success of new scientific areas can be assessed by their potential for contributing to new theoretical approaches and in applications to real-world problems. Complex networks have fared extremely well in both of these aspects, with their sound theoretical basis developed over the years and with a variety of applications. In this survey, we analyze the applications of complex networks to real-world problems and data, with emphasis in representation, analysis and modeling, after an introduction to the main concepts and models. A diversity of phenomena are surveyed, which may be classified into no less than 22 areas, providing a clear indication of the impact of the field of complex networks.

Analyzing and Modeling Real-World Phenomena with Complex Networks: A Survey of Applications

The success of new scientific areas can be assessed by their potential in contributing to new theoretical approaches and in applications to real-world problems. Complex networks have fared extremely well in both of these aspects, with their sound theoretical basis being developed over the years and with a variety of applications. In this survey, we analyze the applications of complex networks to real-world problems and data, with emphasis in representation, analysis and modeling. A diversity of phenomena are surveyed, which may be classified into no less than 11 areas, providing a clear indication of the impact of the field of complex networks.

Analyzing and modeling real-world phenomena with complex networks: a survey of applications

Ir1−xPtxTe2 is an interesting system showing competing phenomenon between structural instability and superconductivity. Due to the large atomic numbers of Ir and Te, the spin-orbital coupling is expected to be strong in the system which may lead to nonconventional superconductivity. We grew single crystal samples of this system and investigated their electronic properties. In particular, we performed optical spectroscopic measurements, in combination with density function calculations, on the undoped compound IrTe2 in an effort to elucidate the origin of the structural phase transition at 280 K. The measurement revealed a dramatic reconstruction of band structure and a significant reduction of conducting carriers below the phase transition. We elaborate that the transition is not driven by the density wave type instability but caused by the crystal field effect which further splits/separates the energy levels of Te (px, py) and Te pz bands.

/pdf/structural-phase-transition-in-irte2-a-combined-study-of-2px372d4yn.pdf

Structural phase transition in IrTe2: A combined study of optical spectroscopy and band structure calculations

Since interface superconductivity was discovered at the interface between two insulating layers LaAlO3 and SrTiO3, such interface-induced superconducting systems have been a research hotspot in superconductivity. Here, we report homogeneous interfaces formed by stacking chemically exfoliated monolayer TaS2 nanosheets randomly. Enhanced superconductivity of Tc = 3 K is observed, compared with 0.8 K of parent 2H-TaS2. The measurement of heat capacity shows the increase of electronic specific-heat coefficient γ of restacked TaS2 nanosheets compared to parent 2H-TaS2 crystals. Density functional theory calculations indicate that increase and delocalization of electron states near the Fermi surface due to the homogeneous interfaces effects could account for the enhanced superconductivity.

Enhanced Superconductivity in Restacked TaS2 Nanosheets

We report an optical study of transition metal dichalcogenide $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ and the Na intercalated superconductor ${\mathrm{Na}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ over a broad frequency range at various temperatures A clear gap feature was observed for $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ when it undergoes the charge-density wave (CDW) transition The existence of a Drude component in ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ below ${T}_{\mathit{CDW}}$ indicates that the Fermi surface of $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ is only partially gapped in the CDW state The spectral evolution of two different ${\mathrm{Na}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ crystals further confirms that the partial gap structure observed in $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ has a CDW origin The CDW mechanism for $2H\text{\ensuremath{-}}\mathrm{Ta}{\mathrm{S}}_{2}$ and the competition between CDW and superconductivity in the ${\mathrm{Na}}_{x}\mathrm{Ta}{\mathrm{S}}_{2}$ system are discussed

Optical study of the charge-density-wave mechanism in 2H-TaS2 and NaxTaS2

Because disorders are inevitable in realistic nanodevices, the capability to quantitatively simulate the disorder effects on electron transport is indispensable for quantum transport theory. Here, we report a unified and effective first-principles quantum transport method for analyzing effects of chemical or substitutional disorder on transport properties of nanoelectronics, including averaged transmission coefficient, shot noise, and disorder-induced device-to-device variability. All our theoretical formulations and numerical implementations are worked out within the framework of the tight-binding linear muffin tin orbital method. In this method, we carry out the electronic structure calculation with the density functional theory, treat the nonequilibrium statistics by the nonequilbrium Green's function method, and include the effects of multiple impurity scattering with the generalized nonequilibrium vertex correction (NVC) method in coherent potential approximation (CPA). The generalized NVC equations are solved from first principles to obtain various disorder-averaged two-Green's-function correlators. This method provides a unified way to obtain different disorder-averaged transport properties of disordered nanoelectronics from first principles. To test our implementation, we apply the method to investigate the shot noise in the disordered copper conductor, and find all our results for different disorder concentrations approach a universal Fano factor $1/3$. As the second test, we calculate the device-to-device variability in the spin-dependent transport through the disordered Cu/Co interface and find the conductance fluctuation is very large in the minority spin channel and negligible in the majority spin channel. Our results agree well with experimental measurements and other theories. In both applications, we show the generalized nonequilibrium vertex corrections play a determinant role in electron transport simulation. Our results demonstrate the effectiveness of the first-principles generalized CPA-NVC for atomistic analysis of disordered nanoelectronics, extending the capability of quantum transport simulation.

First-principles quantum transport method for disordered nanoelectronics: Disorder-averaged transmission, shot noise, and device-to-device variability

Electron transport properties of nanoelectronics can be significantly influenced by the inevitable and randomly distributed impurities/defects. For theoretical simulation of disordered nanoscale electronics, one is interested in both the configurationally averaged transport property and its statistical fluctuation that tells device-to-device variability induced by disorder. However, due to the lack of an effective method to do disorder averaging under the nonequilibrium condition, the important effects of disorders on electron transport remain largely unexplored or poorly understood. In this work, we report a general formalism of Green's function based nonequilibrium effective medium theory to calculate the disordered nanoelectronics. In this method, based on a generalized coherent potential approximation for the Keldysh nonequilibrium Green's function, we developed a generalized nonequilibrium vertex correction method to calculate the average of a two-Keldysh-Green's-function correlator. We obtain nine nonequilibrium vertex correction terms, as a complete family, to express the average of any two-Green's-function correlator and find they can be solved by a set of linear equations. As an important result, the averaged nonequilibrium density matrix, averaged current, disorder-induced current fluctuation, and averaged shot noise, which involve different two-Green's-function correlators, can all be derived and computed in an effective and unified way. To test the general applicability of this method, we applied it to compute the transmission coefficient and its fluctuation with a square-lattice tight-binding model and compared with the exact results and other previously proposed approximations. Our results show very good agreement with the exact results for a wide range of disorder concentrations and energies. In addition, to incorporate with density functional theory to realize first-principles quantum transport simulation, we have also derived a general form of conditionally averaged nonequilibrium Green's function for multicomponent disorders.

Generalized nonequilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

The exact muffin tin orbital (EMTO) method features high efficiency and accuracy for first-principles simulations with density functional theory. In this paper we report our implementation of the EMTO method for electronic-structure and quantum transport simulation of device materials. We consider a device-material structure with a central device region in contact with different semi-infinite electrodes. Based on the Green's function method, the infinite device, nonperiodic in transport direction, is transformed into a calculable finite material system by treating the semi-infinite electrodes with electrode self-energies, and the Green's function of the device region is calculated with an efficient recursive technique. In the present implementation we adopt the spherical cell approximation to treat the electrostatics, and we solve the electrostatic potential of the finite device region by enforcing the boundary conditions to the known potential of electrode materials. The coherent potential approximation is incorporated for treating the atomic disorders inevitable in realistic materials, and the effects of multiple disorder scattering on electron transport are accounted for by vertex correction for simulating disordered electronic devices. To demonstrate the capability of the present implementation, we calculate the monolayer two-dimensional material MoS2 and black phosphorus, and study the spin-dependent tunneling in the Fe/MgO/Fe magnetic tunneling junction. We find the EMTO electronic structures of the calculated systems agree well with the results of the projector augmented wave method. The EMTO transport simulation produces the important spin-filtering effect of the Fe/MgO/Fe junction and the important influence of the interfacial disorders on the spin-dependent tunneling, agreeing well with previous theoretical and experimental studies. The implementation of the EMTO based device simulator provides an effective simulation tool for simulating both ordered and disordered device materials, extending the capability for theoretical design of electronic devices from first principles.

Exact muffin tin orbital based first-principles method for electronic-structure and electron-transport simulation of device materials

We present first-principles analysis of interfacial disorder effects on spin-dependent tunneling statistics in thin Fe/MgO/Fe magnetic tunnel junctions. We find that interfacial disorder scattering can significantly modulate the tunneling statistics in the minority spin of the parallel configuration (PC) while all other spin channels remain dominated by the Poissonian process. For the minority-spin channel of PC, interfacial disorder scattering favors the formation of resonant tunneling channels by lifting the limitation of symmetry conservation at low concentration, presenting an important sub-Poissonian process in PC, but is destructive to the open channels at high concentration. We find that the important modulation of tunneling statistics is independent of the type of interfacial disorder. A bimodal distribution function of transmission with disorder dependence is introduced and fits very well our first-principles results. The increase of MgO thickness can quickly change the tunneling from a sub-Poissonian to Poissonian dominated process in the minority spin of PC with disorder. Our results provide a sensitive detection method of an ultralow concentration of interfacial defects.

Jiawei Yan

Papers

Optical study of the charge-density-wave mechanism in 2H-TaS2 and NaxTaS2

First-principles quantum transport method for disordered nanoelectronics: Disorder-averaged transmission, shot noise, and device-to-device variability

Generalized nonequilibrium vertex correction method in coherent medium theory for quantum transport simulation of disordered nanoelectronics

Exact muffin tin orbital based first-principles method for electronic-structure and electron-transport simulation of device materials

Anomalous spin-dependent tunneling statistics in Fe/MgO/Fe junctions induced by disorder at the interface