arXiv subjects

C. S. Liu

Publications and source records attributed to C. S. Liu.

35 records · Page 2Linked to original sources

arXiv2012

Interplay between periodicity and nonlinearity of indirect excitons in coupled quantum wells

Inspired by a recent experiment of localization-delocalization transition (LDT) of indirect excitons in lateral electrostatic lattices [M. Remeika \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 186803 (2009)], we investigate the interplay between periodic potential and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that the model involving both attractive two-body and repulsive three-body interactions can lead to a natural account for the LDT of excitons across the lattice when reducing lattice amplitude or increasing particle density. In addition, the observations that the smooth component of the photoluminescent energy increases with increasing exciton density and exciton interaction energy is close to the lattice amplitude at the transition are also qualitatively explained. Our model provides an alternative way for understanding the underlying physics of the exciton dynamics in lattice potential wells.

cond-mat.quant-gas↗

arXiv2012

Gap solitons of a super-Tonks-Girardeau gas in a one-dimensional periodic potential

We study the stability of gap solitons of the super-Tonks-Girardeau bosonic gas in one-dimensional periodic potential. The linear stability analysis indicates that increasing the amplitude of periodic potential or decreasing the nonlinear interactions, the unstable gap solitons can become stable. In particular, the theoretical analysis and numerical calculations show that, comparing to the lower-family of gap solitons, the higher-family of gap solitons are easy to form near the bottoms of the linear Bloch band gaps. The numerical results also verify that the composition relations between various gap solitons and nonlinear Bloch waves are general and can exist in the super-Tonks-Girardeau phase.

cond-mat.quant-gas↗

arXiv2011

Role of Alloying-Atom Size Factor and System Shape Factor in Energetics of bcc Fe under Macroscopic Deformation

We present an \emph{ab initio} study of the effect of macroscopic deformation on energetics of twelve alloying elements in bcc Fe under three specially designed strain modes. We find that there exists a universal linear relation of describing the volume dependence of substitutional energy of alloying elements via introducing two factors --- the system shape factor ($f_{\scriptsize{ss}}$) and the size factor of alloying element $M$ ($Ω^{M}_{\scriptsize{sf}}$): $E_{\scriptsize{sub}} \sim f_{\scriptsize{ss}}Ω^{M}_{\scriptsize{sf}}V$. $Ω^{M}_{\scriptsize{sf}}$ well describes the effect of intrinsic alloying-atom size and the influence of chemical interaction with matrix atom, and $f_{\scriptsize{ss}}$ characterizes the degree of system lattice distortion under deformation. This relation is further validated using the published data of stained-modulated doping in GaP

cond-mat.mtrl-sci↗

arXiv2011

Two-band model of Raman scattering on iron pnictide superconductors

Based on a two-band model, we study the electronic Raman scattering intensity in both normal and superconducting states of iron-pnictide superconductors. For the normal state, due to the match or mismatch of the symmetries between band hybridization and Raman vertex, it is predicted that overall $B_{1g}$ Raman intensity should be much weaker than that of the $B_{2g}$ channel. Moreover, in the non-resonant regime, there should exhibit a interband excitation peak at frequency $ω\simeq 7.3 t_1 (6.8t_1)$ in the $B_{1g}$ ($B_{2g}$) channel. For the superconducting state, it is shown that $β$-band contributes most to the $B_{2g}$ Raman intensity as a result of multiple effects of Raman vertex, gap symmetry, and Fermi surface topology. Both extended $s$- and $d_{xy}$-wave pairings in the unfolded BZ can give a good description to the reported $B_{2g}$ Raman data [Muschler {\em et al.}, Phys. Rev. B. {\bf 80}, 180510 (2009).], while $d_{x^2-y^2}$-wave pairing in the unfolded BZ seems to be ruled out.

cond-mat.supr-con↗

arXiv2011

Gap solitons and Bloch waves of interacting bosons in one-dimensional optical lattices: From the weak to the strong interaction limits

We study the gap solitons and nonlinear Bloch waves of interacting bosons in one-dimensional optical lattices, taking into account the interaction from the weak to the strong limits. It is shown that composition relation between the gap solitons and nonlinear Bloch waves exists for the whole span of the interaction strength. The linear stability analysis indicates that the gap solitons are stable when their energies are near the bottom of the linear Bloch band gap. By increasing the interaction strength, the stable gap solitons can turn into unstable. It is argued that the stable gap solitons can easily be formed in a weakly interacting system with energies near the bottoms of the lower-level linear Bloch band gaps.

cond-mat.quant-gas↗

arXiv2011

Fresnel aperture diffraction: a phase-sensitive probe for superconducting pairing symmetry

Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is shown that in case of SC gap symmetry $Δ(-k_x,\mathbf{k}_\parallel)=Δ(k_x,\mathbf{k}_\parallel)$ with $k_x$ and $\mathbf{k}_\parallel$ respectively the normal and parallel component of electron Fermi wavevector, quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if $k_x x=nπ$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $Δ(-k_x,\mathbf{k}_\parallel)=-Δ(k_x, \mathbf{k}_\parallel)$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multi-band pairings.

cond-mat.supr-con↗

arXiv2010

First principle study of hydrogen behavior in hexagonal tungsten carbide

Understanding the behavior of hydrogen in hexagonal tungsten carbide (WC) is of particular interest for fusion reactor design due to the presence of WC in the divertor of fusion reactors. Therefore, we use first-principles calculations to study the hydrogen behavior in WC. The most stable interstitial site for the hydrogen atom is the projection of the octahedral interstitial site on tungsten basal plane, followed by the site near the projection of the octahedral interstitial site on carbon basal plane. The binding energy between two interstitial hydrogen atoms is negative, suggesting that hydrogen itself is not capable of trapping other hydrogen atoms to form a hydrogen molecule. The calculated results on the interaction between hydrogen and vacancy indicate that the hydrogen atom is energetically trapped by vacancy and the hydrogen molecule can not be formed in mono-vacancy. In addition, the hydrogen atom bound to carbon is only found in tungsten vacancy. We also study the migrations of hydrogen in WC and find that the interstitial hydrogen atom prefers to diffusion along the c axis. Our studies on the hydrogen behavior in WC provide some explanations for the experimental results of the thermal desorption process of energetic hydrogen ion implanted into WC.

cond-mat.mtrl-sci↗

arXiv2010

Contraction and expansion effects on the substitution-defect properties of thirteen alloying elements in bcc Fe

Proposed as blanket structural materials for fusion power reactors, reduced activation ferritic/martensitic (RAFM) steel undergoes volume expanding and contracting in a cyclic mode under service environment. Particularly, being subjected to significant fluxes of fusion neutrons RAFM steel suffers considerable local volume variations in the radiation damage involved regions. It is necessary to study the structure properties of the alloying elements in contraction and expansion states. In this paper we studied local substitution structures of thirteen alloying elements Al, Co, Cr, Cu, Mn, Mo, Nb, Ni, Si, Ta, Ti, V, and W in bcc Fe and calculated their substitutional energies in the volume variation range from -1.0% to 1.0%. From the structure relaxation results of the first five neighbor shells around the substitutional atom we find the relaxation in each neighbor shell keeps approximately uniform within the volume variation from -1.0% to 1.0% except those of Mn and the relaxation of the fifth neighbor shell is stronger than that of the third and forth, indicating that the lattice distortion due to the substitution atom is easier to spread in <111> direction than in other direction. The relaxation pattern and intensity are related to the size and electron structure of the substitutional atom. For some alloying elements, such as Mo, Nb, Ni, Ta, Ti and W, the substitutional energy decreases noticeably when the volume increases. Further analysis show that the substitutional energy comprises the energy variation originated from local structure relaxation and the chemical potential difference of the substitutional atom between its elemental crystalline state and the solid solution phase in bcc Fe. We think the approximately uniform relaxation of each neighbor shell around a substitutional atom give rise to a linear decrease in the substitutional energy with the increasing volume.

cond-mat.mtrl-sci↗

arXiv2010

First principle study of intrinsic defects in hexagonal tungsten carbide

The characteristics of intrinsic defects are important for the understanding of self-diffusion processes, mechanical strength, brittleness, and plasticity of tungsten carbide, which present in the divertor of fusion reactors. Here, we use first-principles calculations to investigate the stability of point defects and their complexes in WC. Our calculation results confirm that the formation energies of carbon defects are much lower than that of tungsten defects. The outward relaxations around vacancy are found. Both interstitial carbon and interstitial tungsten atom prefer to occupy the carbon basal plane projection of octahedral interstitial site. The results of isolated carbon defect diffusion show that the carbon vacancy stay for a wide range of temperature because of extremely high diffusion barriers, while carbon interstitial migration is activated at lower temperatures for its considerable lower activation energy. These results provide evidence for the presumption that the 800K stage is attributed by the annealing out of carbon vacancies by long-range migration.

cond-mat.mtrl-sci↗

arXiv2009

Interplay between antiferromagnetic order and spin polarization in ferromagnetic metal/electron-doped cuprate superconductor junctions

Recently we proposed a theory of point-contact spectroscopy and argued that the splitting of zero-bias conductance peak (ZBCP) in electron-doped cuprate superconductor point-contact spectroscopy is due to the coexistence of antiferromagnetic (AF) and d-wave superconducting orders [Phys. Rev. B {\bf 76}, 220504(R) (2007)]. Here we extend the theory to study the tunneling in the ferromagnetic metal/electron-doped cuprate superconductor (FM/EDSC) junctions. In addition to the AF order, the effects of spin polarization, Fermi-wave vector mismatch (FWM) between the FM and EDSC regions, and effective barrier are investigated. It is shown that there exits midgap surface state (MSS) contribution to the conductance to which Andreev reflections are largely modified due to the interplay between the exchange field of ferromagnetic metal and the AF order in EDSC. Low-energy anomalous conductance enhancement can occur which could further test the existence of AF order in EDSC. Finally, we propose a more accurate formula in determining the spin polarization value in combination with the point-contact conductance data.

cond-mat.supr-con↗

arXiv2008

New Quantum Limits in Plasmonic Devices

Surface plasmon polaritons (SPPs) have recently been recognized as an important future technique for microelectronics. Such SPPs have been studied using classical theory. However, current state-of-the-art experiments are rapidly approaching nanoscales, and quantum effects can then become important. Here we study the properties of quantum SPPs at the interface between an electron quantum plasma and a dielectric material. It is shown that the effect of quantum broadening of the transition layer is most important. In particular, the damping of SPPs does not vanish even in the absence of collisional dissipation, thus posing a fundamental size limit for plasmonic devices. Consequences and applications of our results are pointed out.

cond-mat.other↗

arXiv2007

Theory of point contact spectroscopy in electron-doped cuprates

In the hole-doped $d_{x^{2}-y^{2}}$-wave cuprate superconductor, due to the midgap surface state (MSS), a zero bias conductance peak (ZBCP) is widely observed in [110] interface point contact spectroscopy (PCS). However, ZBCP of this geometry is rarely observed in the electron-doped cuprates, even though their pairing symmetry is still likely the $d_{x^{2}-y^{2}}$-wave. We argue that this is due to the coexistence of antiferromagnetic (AF) and the superconducting (SC) orders. Generalizing the Blonder-Tinkham-Klapwijk (BTK) formula to include an AF coupling, it is shown explicitly that the MSS is destroyed by the AF order. The calculated PCS is in good agreement with the experiments.

cond-mat.supr-con↗

arXiv2007

Pattern formation of indirect excitons in coupled quantum wells

Using a nonlinear Schrödinger equation including short-range two-body attraction and three-body repulsion, we investigate the spatial distribution of indirect excitons in semiconductor coupled quantum wells. The results obtained can interpret the experimental phenomenon that annular exciton cloud first contracts then expands when the number of confined excitons is increased in impurity potential well, as observed by Lai \emph{et al.} [Lai $et al.$, Science \textbf{303}, 503 (2004)]. In particular, the model reconciles the patterns of exciton rings reported by Butov \emph{et al.} [Butov $et al.$, Nature \textbf{418}, 751 (2002)]. At higher densities, the model predicts much richer patterns, which could be tested by future experiments.

cond-mat.mes-hall↗

arXiv2007

Gap function symmetry and spin dynamics in electron-doped cuprate superconductor

An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic d_{x^2-y^2}-wave and explains why the observed gap function has a nonmonotonic d_{x^2-y^2}-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model.

cond-mat.supr-con↗

arXiv2006

Theory of Raman scattering on electron-doped high-$T_c$ superconductor

We have analyzed the $B_{1g}$ and $B_{2g}$ Raman spectra of electron-doped cuprate superconductors Nd$_{2-x}$Ce$_{x}$CuO$_4$ and Pr$_{2-x}$Ce$_{x}$CuO$_4$ using a weakly coupled two-band model. One of these two bands is centered around $(\pm π/2, \pm π/2)$ and couples more strongly with the $B_{2g}$ mode, while the other is centered around $(\pm π, 0)$ and $(0, \pm π)$ and couples more strongly with the $B_{1g}$ mode. This model explains in a natural way why the $B_{2g}$ Raman peak occurs at a higher frequency than the $B_{1g}$ one at optimal doping, and how these two peaks change with doping in agreement with experiments. Our result suggests that the superconducting pairing in electron-doped high-$T_c$ cuprates has the $d_{x^2-y^2}$-wave symmetry and results from the same mechanism as for hole doped materials.

cond-mat.supr-con↗

arXiv2005

Thermodynamic properties of tetrameric bond-alternating spin chains

Thermodynamic properties of a tetrameric bond-alternating Heisenberg spin chain with ferromagnetic-ferromagnetic-antiferromagnetic-antiferromagnetic exchange interactions are studied using the transfer-matrix renormalization group and compared to experimental measurements. The temperature dependence of the uniform susceptibility exhibits typical ferrimagnetic features. Both the uniform and staggered magnetic susceptibilities diverge in the limit $T\to 0$, indicating that the ground state has both ferromagnetic and antiferromagnetic long-range orders. A double-peak structure appears in the temperature dependence of the specific heat. Our numerical calculation gives a good account for the temperature and field dependence of the susceptibility, the magnetization, and the specific heat for Cu(3-Clpy)$_{2}$(N$_{3}$)$_{2}$ (3-Clpy=3-Chloroyridine).

cond-mat.str-el↗

arXiv2000

On the Role of Dust Particles in Decoupling of Plasma from Magnetic Field in Laboratory and Space

The objective of this paper is to apply the recent achievements in understanding of the non-MHD effects in plasma (acquired both in laboratory experiments, as well as in theory), to the interstellar phenomena. Applied to the space plasma, these effects can significantly change the picture of plasma dynamics. Charged dust particles are always present in the interstellar medium and can easily remain not magnetized, even when the plasma electrons and ions are strongly magnetized. In such a medium, the magnetic field can propagate with the super-Alfven velocity. This breaks the local "frozen-in" law for the magnetic field, which can significantly affect the gravitational collapse phenomena, the space plasma turbulence spectrum, magnetic dynamo and the rate of the Fermi acceleration of cosmic rays.

astro-ph↗