arXiv subjects

Zheyong Fan

Publications and source records attributed to Zheyong Fan.

107 records · Page 6Linked to original sources

arXiv2017

Nonlinear Conductivity of a Holographic Superconductor Under Constant Electric Field

The dynamics of a two-dimensional superconductor under a constant electric field $E$ is studied by using the gauge/gravity correspondence. The pair breaking current induced by $E$ first increases to a peak value and then decreases to a constant value at late time, where the superconducting gap goes to zero, corresponding to a normal conducting phase. The peak value of the current is found to increase linearly with respect to the electric field. Moreover, the nonlinear conductivity, defined as an average of the conductivity in the superconducting phase, scales as $\sim E^{-2/3}$ for large $E$ when the system is close to the critical temperature, which agrees with predictions from solving the time dependent Ginzburg-Landau equation.

hep-th↗

arXiv2016

Scattering from spin-polarized charged impurities in graphene

We study the spin relaxation of charge carriers in graphene in the presence of spin-polarized charged impurities by calculating the time evolution of initially polarized state. The spin relaxation time shows completely different energy behaviour for short-ranged and long-ranged spin scatterers and can be used to identify the dominant source of spin scattering. Our results agree well with recent experimental findings and indicate that their spin relaxation is likely caused by long-ranged scatterers.

cond-mat.mes-hall↗

arXiv2016

Multiscale modeling of polycrystalline graphene: A comparison of structure and defect energies of realistic samples from phase field crystal models

We extend the phase field crystal (PFC) framework to quantitative modeling of polycrystalline graphene. PFC modeling is a powerful multiscale method for finding the ground state configurations of large realistic samples that can be further used to study their mechanical, thermal or electronic properties. By fitting to quantum-mechanical density functional theory (DFT) calculations, we show that the PFC approach is able to predict realistic formation energies and defect structures of grain boundaries. We provide an in-depth comparison of the formation energies between PFC, DFT and molecular dynamics (MD) calculations. The DFT and MD calculations are initialized using atomic configurations extracted from PFC ground states. Finally, we use the PFC approach to explicitly construct large realistic polycrystalline samples and characterize their properties using MD relaxation to demonstrate their quality.

cond-mat.mes-hall↗

arXiv2016

Dominant source of disorder in graphene: Charged impurities or ripples?

Experimentally produced graphene sheets exhibit a wide range of mobility values. Both extrinsic charged impurities and intrinsic ripples (corrugations) have been suggested to induce long-range disorder in graphene and could be a candidate for the dominant source of disorder. Here, using large-scale molecular dynamics and quantum transport simulations, we find that the hopping disorder and the gauge and scalar potentials induced by the ripples are short-ranged, in strong contrast with predictions by continuous models, and the transport fingerprints of the ripple disorder are very different from those of charged impurities. We conclude that charged impurities are the dominant source of disorder in most graphene samples, whereas scattering by ripples is mainly relevant in the high carrier density limit of ultraclean graphene samples (with a charged impurity concentration < 10 ppm) at room and higher temperatures.

cond-mat.mes-hall↗

arXiv2015

Silicon and silicon-nitrogen impurities in graphene: structure, energetics and effects on electronic transport

We theoretically study the atomic structure and energetics of silicon and silicon-nitrogen impurities in graphene. Using density-functional theory, we get insight into the atomic structures of the impurities, evaluate their formation energies and assess their abundance in realistic samples. We find that nitrogen, as well as oxygen and hydrogen, are trapped at silicon impurities, considerably altering the electronic properties of the system. Furthermore, we show that nitrogen doping can induce local magnetic moments resulting in spin-dependent transport properties, even though neither silicon nor nitrogen impurities are magnetic by themselves. To simulate large systems with many randomly distributed impurities, we derive tight-binding models that describe the effects of the impurities on graphene π electron structure. Then by using the linear-scaling real-space Kubo-Greenwood method, we evaluate the transport properties of large-scale systems with random distribution of impurities, and find the fingerprint-like scattering cross sections for each impurity type. The transport properties vary widely, and our results indicate that some of the impurities can even induce strong localization in realistic graphene samples.

cond-mat.mes-hall↗

arXiv2015

Impurity-induced quantum phase transition in finite Heisenberg spin chains: Criteria for existence and stability

A quantum phase transition may occur in a system at zero temperature when a controlling parameter is tuned towards a critical point. An important question is whether such a critical point exists in a particular system and how stable it is. Here, we identify the critical point of a quantum phase transition as a singular point in the affine algebraic variety of the characteristic equation for the Hamiltonian describing the system, with an unstable critical point being associated with an isolated singular point which has a finite Tjurina number. The theory is illustrated by studying a model system of zero-dimensional (finite) Heisenberg spin chain with an impurity, which exhibits a nontrivial first-order quantum phase transition. Both analytical and numerical calculations show that the quantum phase transition always exists when the impurity has a $Z_2$ symmetry but only remains in systems with an even number of spin sites when the $Z_2$ symmetry is broken.

cond-mat.str-el↗

arXiv2015

Force and heat current formulas for many-body potentials in molecular dynamics simulation with applications to thermal conductivity calculations

We derive expressions of interatomic force and heat current for many-body potentials such as the Tersoff, the Brenner, and the Stillinger-Weber potential used extensively in molecular dynamics simulations of covalently bonded materials. Although these potentials have a many-body nature, a pairwise force expression that follows Newton's third law can be found without referring to any partition of the potential. Based on this force formula, a stress applicable for periodic systems can be unambiguously defined. The force formula can then be used to derive the heat current formulas using a natural potential partitioning. Our heat current formulation is found to be equivalent to most of the seemingly different heat current formulas used in the literature, but to deviate from the stress-based formula derived from two-body potential. We validate our formulation numerically on various systems descried by the Tersoff potential, namely three-dimensional silicon and diamond, two-dimensional graphene, and quasi-one-dimensional carbon nanotube. The effects of cell size and time used in the simulation are examined.

cond-mat.mes-hall↗

arXiv2014

Electronic and transport properties in geometrically disordered graphene antidot lattices

A graphene antidot lattice, created by a regular perforation of a graphene sheet, can exhibit a considerable band gap required by many electronics devices. However, deviations from perfect periodicity are always present in real experimental setups and can destroy the band gap. Our numerical simulations, using an efficient linear-scaling quantum transport simulation method implemented on graphics processing units, show that disorder that destroys the band gap can give rise to a transport gap caused by Anderson localization. The size of the defect induced transport gap is found to be proportional to the radius of the antidots and inversely proportional to the square of the lattice periodicity. Furthermore, randomness in the positions of the antidots is found to be more detrimental than randomness in the antidot radius. The charge carrier mobilities are found to be very small compared to values found in pristine graphene, in accordance with recent experiments.

cond-mat.mes-hall↗

arXiv2014

Anderson localization in two-dimensional graphene with short-range disorder: One-parameter scaling and finite-size effects

We study Anderson localization in graphene with short-range disorder using the real-space Kubo-Greenwood method implemented on graphics processing units. Two models of short-range disorder, namely, the Anderson on-site disorder model and the vacancy defect model, are considered. For graphene with Anderson disorder, localization lengths of quasi-one-dimensional systems with various disorder strengths, edge symmetries, and boundary conditions are calculated using the real-space Kubo-Greenwood formalism, showing excellent agreement with independent transfer matrix calculations and superior computational efficiency. Using these data, we demonstrate the applicability of the one-parameter scaling theory of localization length and propose an analytical expression for the scaling function, which provides a reliable method of computing the two-dimensional localization length. This method is found to be consistent with another widely used method which relates the two-dimensional localization length to the elastic mean free path and the semiclassical conductivity. Abnormal behavior at the charge neutrality point is identified and interpreted to be caused by finite-size effects when the system width is comparable to or smaller than the elastic mean free path. We also demonstrate the finite-size effect when calculating the two-dimensional conductivity in the localized regime and show that a renormalization group beta function consistent with the one-parameter scaling theory can be extracted numerically. For graphene with vacancy disorder, we show that the proposed scaling function of localization length also applies. Lastly, we discuss some ambiguities in calculating the semiclassical conductivity around the charge neutrality point due to the presence of resonant states.

cond-mat.mes-hall↗

arXiv2014

Obtaining localization properties efficiently using the Kubo-Greenwood formalism

We establish, through numerical calculations and comparisons with a recursive Green's function based implementation of the Landauer-Büttiker formalism, an efficient method for studying Anderson localization in quasi-one-dimensional and two-dimensional systems using the Kubo-Greenwood formalism. Although the recursive Green's function method can be used to obtain the localization length of a mesoscopic conductor, it is numerically very expensive for systems that contain a large number of atoms transverse to the transport direction. On the other hand, linear-scaling has been achieved with the Kubo-Greenwood method, enabling the study of effectively two-dimensional systems. While the propagating length of the charge carriers will eventually saturate to a finite value in the localized regime, the conductances given by the Kubo-Greenwood method and the recursive Green's function method agree before the saturation. The converged value of the propagating length is found to be directly proportional to the localization length obtained from the exponential decay of the conductance.

cond-mat.mes-hall↗

arXiv2013

Efficient linear-scaling quantum transport calculations on graphics processing units and applications on electron transport in graphene

We implement, optimize, and validate the linear-scaling Kubo-Greenwood quantum transport simulation on graphics processing units by examining resonant scattering in graphene. We consider two practical representations of the Kubo-Greenwood formula: a Green-Kubo formula based on the velocity auto-correlation and an Einstein formula based on the mean square displacement. The code is fully implemented on graphics processing units with a speedup factor of up to 16 (using double-precision) relative to our CPU implementation. We compare the kernel polynomial method and the Fourier transform method for the approximation of the Dirac delta function and conclude that the former is more efficient. In the ballistic regime, the Einstein formula can produce the correct quantized conductance of one-dimensional graphene nanoribbons except for an overshoot near the band edges. In the diffusive regime, the Green-Kubo and the Einstein formalisms are demonstrated to be equivalent. A comparison of the length-dependence of the conductance in the localization regime obtained by the Einstein formula with that obtained by the non-equilibrium Green's function method reveals the challenges in defining the length in the Kubo-Greenwood formalism at the strongly localized regime.

cond-mat.mes-hall↗

arXiv2013

Accelerated molecular dynamics force evaluation on graphics processing units for thermal conductivity calculations

In this paper, we develop a highly efficient molecular dynamics code fully implemented on graphics processing units for thermal conductivity calculations using the Green-Kubo formula. We compare two different schemes for force evaluation, a previously used thread-scheme where a single thread is used for one particle and each thread calculates the total force for the corresponding particle, and a new block-scheme where a whole block is used for one particle and each thread in the block calculates one or several pair forces between the particle associated with the given block and its neighbor particle(s) associated with the given thread. For both schemes, two different classical potentials, namely, the Lennard-Jones potential and the rigid-ion potential are implemented. While the thread-scheme performs a little better for relatively large systems, the block-scheme performs much better for relatively small systems. The relative performance of the block-scheme over the thread-scheme also increases with the increasing of the cutoff radius. We validate the implementation by calculating lattice thermal conductivities of solid argon and lead telluride.

cond-mat.mtrl-sci↗

arXiv2012

A molecular dynamics investigation of the mechanical properties of graphene nanochain

In this paper, we investigate, by molecular dynamics simulations, the mechanical properties of a new carbon nanostructure, termed graphene nanochain, constructed by sewing up pristine or twisted graphene nanoribbons (GNRs) and interlocking the obtained nanorings. The obtained tensile strength of defect-free nanochain is a little lower than that of pristine GNRs and the fracture point is earlier than that of the GNRs. The effects of length, width and twist angle of the constituent GNRs on the mechanical performance are analyzed. Furthermore, defect effect is investigated and in some high defect coverage cases, an interesting mechanical strengthening-like behavior is observed. This structure supports the concept of long-cable manufacturing and advanced material design can be achieved by integration of nanochain with other nanocomposites. The technology used to construct the nanochain is experimentally feasible, inspired by the recent demonstrations of atomically precise fabrications of GNRs with complex structures [Phys. Rev. Lett,2009,\textbf{102},205501; Nano Lett., 2010, \textbf{10},4328; Nature,2010,\textbf{466},470]

cond-mat.mtrl-sci↗

arXiv2010

Constructing quantum circuits for maximally entangled multi-qubit states using the genetic algorithm

Numerical optimization methods such as hillclimbing and simulated annealing have been applied to search for highly entangled multi-qubit states. Here the genetic algorithm is applied to this optimization problem -- to search not only for highly entangled states, but also for the corresponding quantum circuits creating these states. Simple quantum circuits for maximally (highly) entangled states are discovered for 3, 4, 5, and 6-qubit systems; and extension of the method to systems with more qubits is discussed. Among other results we have found explicit quantum circuits for maximally entangled 5 and 6-qubit circuits, with only 8 and 13 quantum gates respectively. One significant advantage of our method over previous ones is that it allows very simple construction of quantum circuits based on the quantum states found.

quant-ph↗

arXiv2010

Braid Matrices and Quantum Gates for Ising Anyons Topological Quantum Computation

We study various aspects of the topological quantum computation scheme based on the non-Abelian anyons corresponding to fractional quantum hall effect states at filling fraction 5/2 using the Temperley-Lieb recoupling theory. Unitary braiding matrices are obtained by a normalization of the degenerate ground states of a system of anyons, which is equivalent to a modification of the definition of the 3-vertices in the Temperley-Lieb recoupling theory as proposed by Kauffman and Lomonaco. With the braid matrices available, we discuss the problems of encoding of qubit states and construction of quantum gates from the elementary braiding operation matrices for the Ising anyons model. In the encoding scheme where 2 qubits are represented by 8 Ising anyons, we give an alternative proof of the no-entanglement theorem given by Bravyi and compare it to the case of Fibonacci anyons model. In the encoding scheme where 2 qubits are represented by 6 Ising anyons, we construct a set of quantum gates which is equivalent to the construction of Georgiev.

cond-mat.mes-hall↗

arXiv2009

Time Reversal Symmetry Breaking Holographic Superconductor in Constant External Magnetic Field

It is known that a classical SU(2) Einstein-Yang-Mills theory in 3+1 dimensional anti-de Sitter spacetime can provide a holographic dual to a 2+1 dimensional time reversal symmetry breaking superconductor with a pseudogap. We study the properties of this holographic superconductor in the presence of an applied constant external magnetic field, neglecting backreaction on the geometry. The superconductor is immersed into a constant external magnetic field by adding a radially (the extra dimension) dependent magnetic field to the black hole. As for real superconductors, there is a critical magnetic field above which no superconductivity can appear. The continuity of the first derivative of the free energy difference between the superconducting phase and the normal phase at the critical temperature suggests that the superconducting phase transition with applied magnetic field is of second order.

hep-th↗

arXiv2009

A no-ghost theorem for the bosonic Nappi-Witten string

We prove a no-ghost theorem for a bosonic string propagating in Nappi-Witten spacetime. This is achieved in two steps. We first demonstrate unitarity for a class of NW/U(1) modules: the norm of any state which is primary with respect to a chosen timelike U(1) is non-negative. We then show that physical states - states satisfying the Virasoro constraints - in a class of modules of an affinisation of the Nappi-Witten algebra are contained in the NW/U(1) modules. Similar to the case of strings on $AdS_3$, in order to saturate the spectrum obtained in light-cone quantization we are led to include modules with energy not bounded from below, which are related to modules with energy bounded from below by spectral flow automorphisms.

hep-th↗