arXiv subjects

Yi Gao

Publications and source records attributed to Yi Gao.

108 records · Page 6Linked to original sources

arXiv2014

"Nodal gap" induced by the incommensurate diagonal spin density modulation in underdoped high-$T_c$ superconductors

Recently it was revealed that the whole Fermi surface is fully gapped for several families of underdoped cuprates. The existence of the finite energy gap along the $d$-wave nodal lines ("nodal gap") contrasts the common understanding of the $d$-wave pairing symmetry, which challenges the present theories for the high-$T_c$ superconductors. Here we propose that the incommensurate diagonal spin-density-wave order can account for the above experimental observation. The Fermi surface and the local density of states are also studied. Our results are in good agreement with many important experiments in high-$T_c$ superconductors.

cond-mat.supr-con↗

arXiv2014

Topological quantum phase transitions and edge states in spin-orbital coupled Fermi gases

We study the superconducting state in the presence of spin-orbital coupling and the Zeeman field. It is found that a phase transition from the Fulde-Ferrell-Larkin-Ovchinnikov state to the topological superconducting state occurs upon increasing the spin-orbital coupling. The nature of this topological phase transition and its critical property are investigated numerically. Physical properties of topological superconducting phase are also explored. Moreover, the local density of states is calculated, through which the topological feature may be tested experimentally.

cond-mat.supr-con↗

arXiv2014

Breaking the theoretical scaling limit for predicting quasi-particle energies: The stochastic GW approach

We develop a formalism to calculate the quasi-particle energy within the GW many-body perturbation correction to the density functional theory (DFT). The occupied and virtual orbitals of the Kohn-Sham (KS) Hamiltonian are replaced by stochastic orbitals used to evaluate the Green function, the polarization potential, and thereby the GW self-energy. The stochastic GW (sGW) relies on novel theoretical concepts such as stochastic time-dependent Hartree propagation, stochastic matrix compression and spatial/temporal stochastic decoupling techniques. Beyond the theoretical interest, the formalism enables linear scaling GW calculations breaking the theoretical scaling limit for GW as well as circumventing the need for energy cutoff approximations. We illustrate the method for silicon nanocrystals of varying sizes with over 3000 electrons.

cond-mat.mes-hall↗

arXiv2014

Testing the $d^{*}_{x^{2}-y^{2}}$-wave pairing symmetry by quasiparticle interference in BiS$_{2}$-based superconductors

The quasiparticle interference (QPI) patterns in BiS$_{2}$-based superconductors are theoretically investigated by taking into account the spin-orbital coupling and assuming the recently proposed $d^{*}_{x^{2}-y^{2}}$-wave pairing symmetry. We found two distinct scattering wave vectors whose evolution can be explained based on the evolution of the constant-energy contours. The QPI spectra presented in this paper can thus be compared with future scanning tunneling microscopy experiments to test whether the pairing symmetry is $d^{*}_{x^{2}-y^{2}}$-wave in BiS$_{2}$-based superconductors.

cond-mat.supr-con↗

arXiv2013

A guided stochastic energy-domain formulation of the second order Møller-Plesset perturbation theory

We develop an alternative formulation in the energy-domain to calculate the second order Møller-Plesset (MP2) perturbation energies. The approach is based on repeatedly choosing four random energies using a non-separable guiding function, filtering four random orbitals at these energies, and averaging the resulting Coulomb matrix elements to obtain a statistical estimate of the MP2 correlation energy. In contrast to our time-domain formulation, the present approach is useful for both quantum chemistry and real-space/plane wave basis sets. The scaling of the MP2 calculation is roughly linear with system size, providing a useful tool to study dispersion energies in large systems. This is demonstrated on a structure of 64 fullerenes within the SZ basis as well as on silicon nanocrystals using real-space grids.

physics.chem-ph↗

arXiv2013

Probing active/passive bands by quasiparticle interference in Sr$_{2}$RuO$_{4}$

The quasiparticle interference (QPI) in Sr$_{2}$RuO$_{4}$ is theoretically studied based on two different pairing models in order to propose an experimental method to test them. For a recently proposed two-dimensional model with pairing primarily from the $γ$ band, we found clear QPI peaks evolving with energy and their locations can be determined from the tips of the constant-energy contour (CEC). On the other hand, for a former quasi-one-dimensional model with pairing on the $α$ and $β$ bands, the QPI spectra are almost dispersionless and may involve off-shell contributions to the scatterings beyond the CEC. The different behaviors of the QPI in these two models may help to resolve the controversy of active/passive bands and whether Sr$_{2}$RuO$_{4}$ is a topological superconductor.

cond-mat.supr-con↗

arXiv2013

Pairing symmetry in BiS$_{2}-$based superconductors

The possible pairing symmetries for BiS$_{2}-$based superconductors is investigated by using a minimal two-orbital model with onsite and nearest-neighbor intraorbital attractions $V_{0}$ and $V_{1}$, respectively. By using the mean-field approximation and solving the self-consistent equations, the phase diagram of the pairing symmetry is obtained. It is shown that the model allows three possible pairing symmetries, depending on the values of $V_{0}$ and $V_{1}$: the isotopic $s-$wave pairing [$Δ_{\mathbf{k}}=Δ_{s}$], the anisotropic $s-$wave pairing [$Δ_{\mathbf{k}}=Δ_{s}+\frac{Δ_{xs}}{2}(\cos k_{x}+\cos k_{y})$] and the $d-$wave pairing [$Δ_{\mathbf{k}}=\frac{Δ_{d}}{2}(\cos k_{x}-\cos k_{y})$]. Furthermore the density of states for these pairing symmetries exhibit different behaviors which can be used to distinguish them.

cond-mat.supr-con↗

arXiv2013

Mixed-State Effect on the Low-Energy Spin Dynamics in Optimally-doped Iron Pnictide Superconductors

Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry, the mixed-state effect on the low-energy spin dynamics in optimally-doped iron pnictide superconductors is studied by solving Bogoliubov-de Gennes equations. Our results of the spin susceptibility at $\mathbf{q}=\mathbf{Q}$ in the normal, superconducting and mixed states agree qualitatively with recent neutron scattering experiments. We also propose that the field-induced intensity change shows different behaviors between the $s_{\pm}$ and s-wave symmetries in both momentum and real space, thus it can be used to distinguish these two pairing symmetries.

cond-mat.supr-con↗

arXiv2012

Superfluid density in the $s_{\pm}$-wave state of clean iron-based superconductors

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density $ρ_s(T)$ and then the penetration depth $λ(T)$ of the iron-based superconductors in the coexistence region of the spin-density-wave(SDW) and superconductivity, and also in the over-doped region. Our calculations show a dramatic increase of $λ(0)$ with the decrease of the doping concentration $x$ below $x=0.1$. This result is consistent with the experimental observations. At low temperatures, $ρ(T)$ shows an exponential-law behavior, while at higher temperatures, the linear-in-$T$ behavior is dominant before it trends to vanish. The evolution of $Δλ(T)$ can be roughly fitted by a exponential function at overdoped levels while in other doping range it is a power-law function with the exponent depending on the doping concentration. We show that the Uemura relation holds for the iron-based superconductors only at very low doping levels.

cond-mat.supr-con↗

arXiv2012

Theory of Mixed-State Effect on Quasiparticle Interference in Iron-based Superconductors

Based on a phenomenological model with $s_{\pm}$ or s-wave pairing symmetry, the mixed-state effect on quasiparticle interference in iron-based superconductors is investigated by solving large-scale Bogoliubov-de Gennes equations based on the Chebyshev polynomial expansion. Taking into account the presence of magnetic field, our result for the $s_{\pm}$ pairing is in qualitative agreement with recent scanning tunneling microscopy experiment while for the s-wave pairing, the result is in apparent contradiction with experimental observations, thus excluding the s-wave pairing. Furthermore, we treat the effect of vortices rigorously instead of approximating the vortices as magnetic impurities, thus our results are robust and should be more capable of explaining the experimental data.

cond-mat.supr-con↗

arXiv2012

Volumetric Mapping of Genus Zero Objects via Mass Preservation

In this work, we present a technique to map any genus zero solid object onto a hexahedral decomposition of a solid cube. This problem appears in many applications ranging from finite element methods to visual tracking. From this, one can then hopefully utilize the proposed technique for shape analysis, registration, as well as other related computer graphics tasks. More importantly, given that we seek to establish a one-to-one correspondence of an input volume to that of a solid cube, our algorithm can naturally generate a quality hexahedral mesh as an output. In addition, we constrain the mapping itself to be volume preserving allowing for the possibility of further mesh simplification. We demonstrate our method both qualitatively and quantitatively on various 3D solid models

cs.CG↗

arXiv2011

Impurity-induced quasiparticle interference in the parent compounds of iron-pnictide superconductors

The impurity-induced quasiparticle interference(QPI) in the parent compounds of iron-pnictide superconductors is investigated based on a phenomenological two-orbital four-band model and T-matrix method. We find the QPI is sensitive to the value of the magnetic order which may vary from one compound to another. For small value of the magnetic order, the pattern of oscillation in the local density of states (LDOS) induced by the QPI exhibits two-dimensional characteristics, consistent with the standing wave state observed in the 1111 compound. For larger value of the magnetic order, the main feature of the spatial modulation of the LDOS is the existence of one-dimensional stripe structure which is in agreement with the nematic structure in the parent compound of the 122 system. In both cases the system shows $C_2$ symmetry and only in the larger magnetic order case, there exist in-gap bound states. The corresponding QPI in $q$-space is also presented. The patterns of modulation in the LDOS at nonzero energies are attributed to the interplay between the underlying band structure and Fermi surfaces.

cond-mat.supr-con↗

arXiv2011

Consistent picture for the electronic structure around a vortex core in iron-based superconductors

Based on a two-orbital model and taking into account the presence of the impurity, we studied theoretically the electronic structure in the vortex core of the iron-Pnictide superconducting materials. The vortex is pinned when the impurity is close to the vortex core. The bound states shows up for the unpinned vortex and are wiped out by a impurity. Our results are in good agreement with recent experiments and present a consistent explanation for the different electronic structure of vortex core revealed by experiments on different materials.

cond-mat.supr-con↗

arXiv2011

Quasiparticle states around a nonmagnetic impurity in electron-doped iron-based superconductors with spin-density-wave order

The quasiparticle states around a nonmagnetic impurity in electron-doped iron-based superconductors with spin-density-wave (SDW) order are investigated as a function of doping and impurity scattering strength. In the undoped sample, where a pure SDW state exists, two impurity-induced resonance peaks are observed around the impurity site and they are shifted to higher (lower) energies as the strength of the positive (negative) scattering potential (SP) is increased. For the doped samples where the SDW order and the superconducting order coexist, the main feature is the existence of sharp in-gap resonance peaks whose positions and intensity depend on the strength of the SP and the doping concentration. In all cases, the local density of states exhibits clear $C_2$ symmetry. We also note that in the doped cases, the impurity will divide the system into two sublattices with distinct values of magnetic order. Here we use the band structure of a two-orbital model, which considers the asymmetry of the As atoms above and below the Fe-Fe plane. This model is suitable to study the properties of the surface layers in the iron-pnictides and should be more appropriate to describe the scanning tunneling microscopy experiments.

cond-mat.supr-con↗

arXiv2010

Vortex states in hole-doped iron-pnictide superconductors

Based on a phenomenological model with competing spin-density-wave (SDW) and extended $s-$wave superconductivity, the vortex states in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.

cond-mat.supr-con↗

arXiv2010

Spin dynamics in electron-doped pnictide superconductors

The doping dependence of spin excitations in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ is studied based on a two-orbital model under RPA approximation. The interplay between the spin-density-wave (SDW) and superconductivity (SC) is considered in our calculation. Our results for the spin susceptibility are in good agreement with neutron scattering (NS) experiments in various doping ranges at temperatures (T) above and below the superconducting transition temperature T$_{c}$. For the overdoped sample where one of the two hole pockets around $Γ$ point disappears according to ARPES, we show that the imaginary part of the spin susceptibility in both SC and normal phases exhibits a gap-like behavior. This feature is consistent with the "pseudogap" as observed by recent NMR and NS experiments.

cond-mat.supr-con↗

arXiv2007

Hydrogen storage in pillared Li-dispersed boron carbide nanotubes

Ab initio density-functional theory study suggests that pillared Li-dispersed boron carbide nanotubes is capable of storing hydrogen with a mass density higher than 6.0 weight% and a volumetric density higher than 45 g/L. The boron substitution in carbon nanotube greatly enhances the binding energy of Li atom to the nanotube, and this binding energy (~ 2.7 eV) is greater than the cohesive energy of lithium metal (~1.7 eV), preventing lithium from aggregation (or segregation) at high lithium doping concentration. The adsorption energy of hydrogen on the Li-dispersed boron carbide nanotube is in the range of 10 ~24 kJ/mol, suitable for reversible H2 adsorption/desorption at room temperature and near ambient pressure.

cond-mat.mtrl-sci↗

arXiv1996

Scaling Properties of Hadron Production in the Ising Model for Quark-Hadron Phase Transition

Earlier study of quark-hadron phase transition in the Ginzberg-Landau theory is reexamined in the Ising model, so that spatial fluctuations during the transition can be taken into account. Although the dimension of the physical system is 2, as will be argued, both d=2 and d=4 Ising systems are studied, the latter being theoretically closer to the Ginzberg-Landau theory. The normalized factorial moments F_q are used to quantify multiplicity fluctuations, and the scaling exponent νis used to characterize the scaling properties. It is found by simulation on the Ising lattice that νbecomes a function of the temperature T near T_c. The average value of νover a range of T<T_c agrees with the value of 1.3 derived analytically from the Ginzberg-Landau theory. Thus the implications of the mean-field theory are not invalidated by either the introduction of spatial fluctuations or the restriction to a 2D system.

nucl-th↗