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Zhibing Li

Publications and source records attributed to Zhibing Li.

54 records · Page 3Linked to original sources
arXiv2009

Strong spin-oscillation of small spin-1 condensates caused by an inclined weak magnetic field

When a magnetic field is applied along a direction deviated from the quantization $Z$-axis, the conservation of total magnetization holds no more. In this case the inclined field can cause a strong spin-evolution via the linear Zeeman term even the field is as weak as a percentage of $mG$. An approach beyond the mean field theory is proposed to study the evolution of small $^{87}$Rb condensates under the weak inclined fields. The time-dependent populations of spin-components are given in analytical forms. The evolution is found to be highly sensitive to the magnitude and direction of the field.

cond-mat.other↗
arXiv2009

Modification of spin mixing of spinor BEC by cavity QED coupling

Dressed states of spinor Bose-Einstein condensates of spin-1 atoms coupling with optical cavity modes with far off resonance frequency are investigated. The exact solution of time evolution of population of spin component is derived, and the numerical result shows that the evolution is different from spin mixing without the coupling. Due to the coupling with the atoms, the photon state also evolute to different optical cavity modes.

physics.atm-clus↗
arXiv2009

Simulation for field emission images of micrometer-long SWCNTs

The electron distribution of open-ended single-walled carbon nanotubes with chirality indexes (7,0) and (5,5) in the field emission conditions was calculated via a multi-scaled algorithm. The field emission images were produced numerically. It was found that the emission patterns change with the applied macroscopic field. Especially, the symmetry of the emission pattern of the (7,0) carbon nanotube is breaking in the lower field but the breaking is less obvious in the higher field. The enlargement factor increases with the applied macroscopic field.

cond-mat.mtrl-sci↗
arXiv2009

Rotation Symmetry Spontaneous Breaking of Edge States in Zigzag Carbon Nanotubes

Analytical solutions of the edge states were obtained for the (N, 0) type carbon nanotubes with distorted ending bonds. It was found that the edge states are mixed via the distortion. The total energies for N=5 and N>=7 are lower in the asymmetric configurations of ending bonds than those having axial rotation symmetry. Thereby the symmetry is breaking spontaneously. The results imply that the symmetry of electronic states at the apex depends on the occupation; the electron density pattern at the apex could change dramatically and could be controlled by applying an external field.

cond-mat.mtrl-sci↗
arXiv2008

Interference of the evolutions of Rb and Na Bose-Einstein condensates

Spin evolution of a mixture of two species of spin-1 Bose-Einstein condensates (Rb and Na) has been investigated beyond the mean field theory. Both analytical expression and numerical results on the populations of spin-components are obtained. The interference is found to depend on the initial states and the inter-species interaction sensitively. A number of phenomena arising from the interference have been predicted. In particular, periodic behavior and the alternate appearances of a zone of oscillation and a long quiet zone are found.

physics.atm-clus↗
arXiv2008

Image potential of single-wall carbon nanotubes in the field emission condition

We calculated the image potentials of single-walled carbon nanotubes of various structures in the condition of field emission with a quantum chemistry method. The image potential of the single-walled carbon nanotubes can be well fitted with the image potential of an ideal metal sphere in a size comparable to an atom. The image potential is not sensitive to both the applied field and the structure of the tube. When the image potential is included, the emission current would be one order larger.

cond-mat.mtrl-sci↗
arXiv2008

Effect of magnetic fields on the spin evolution of non-polarized 87Rb Bose-Einstein condensates

The spin mixing dynamics of spin-1 Bose-Einstein condensates with zero magnetization and under an external magnetic field is investigated. The time-dependent solutions are obtained via a diagonalization of the Hamiltonian, which has a simple form under the single mode approximation. The features of evolution are compared in detail with those with the field removed so as to emphasize the effect of the field, which can induce strong oscillation in population of atoms in spin component 0 . A new mode of oscillation characterized by a high frequency and a low frequency, is found when the field is sufficiently strong.

cond-mat.other↗
arXiv2007

Screening effects on field emission from arrays of (5,5) carbon nanotubes: Quantum-mechanical simulation

The simulation of field electron emission from arrays of micrometer-long open-ended (5, 5) carbon nanotubes is performed in the framework of quantum theory of many electrons. It is found that the applied external field is strongly screened when the spacing distance is shorter than the length of the carbon nanotubes. The optimal spacing distance is two to three times of the nanotube length, slightly depending on the applied external fields. The electric screening can be described by a factor that is a exponential function of the ratio of the spacing distance to the length of the carbon nanotubes. For a given length, the field enhancement factor decreases sharply as the screening factor larger than 0.05. The simulation implies that the thickness of the array should be larger than a value but it does not help the emission much by increasing the thickness a great deal.

cond-mat.mtrl-sci↗
arXiv2007

Spin evolution of spin-1 Bose-Einstein condensates

An analytical formula is obtained to describe the evolution of the average populations of spin components of spin-1 atomic gases. The formula is derived from the exact time-dependent solution of the Hamiltonian $H_{S}=c mathbf{S}^{2}$ without using approximation. Therefore it goes beyond the mean field theory and provides a general, accurate, and complete description for the whole process of non-dissipative evolution starting from various initial states. The numerical results directly given by the formula coincide qualitatively well with existing experimental data, and also with other theoretical results from solving dynamic differential equations. For some special cases of initial state, instead of undergoing strong oscillation as found previously, the evolution is found to go on very steadily in a very long duration.

physics.atm-clus↗
arXiv2007

Characteristic frequency of the magnetic radiation of spinor condensates

The magnetic radiation of the fully-condensed states of $^{23}$Na condensates have been studied. A narrow characteristic spectral line with a wave length proportional to N^(-2/5) (N is the number of particles) and with a probability of transition proportional to N^(17/5) emitted (absorbed) by the condensate was found. It implies that short wave radiation with a huge probability of transition can be obtained if numerous atoms are trapped. A new technique developed by the authors, namely, the analytical forms of the fractional parentage coefficients, was used to calculate analytically the matrix elements between the total spin-states.

cond-mat.other↗
arXiv2007

Atomic decoration for improving the efficiency of field electron emission of carbon nanotubes

The field electron emission from the single-walled carbon nanotubes with their open ends terminated by -BH, -NH, and -O has been simulated. The apex-vacuum barrier and the emission current have been calculated. It has been found that -BH and -NH suppress the apex-vacuum barrier significantly and lead to higher emission current in contrast to the -O terminated structure in the same applied field. The calculated binding energy implies that the carbon nanotubes terminated with -BH and -NH are more stable than those saturated by oxygen atoms or by hydrogen atoms.

cond-mat.mtrl-sci↗
arXiv2006

The roles of apex dipoles and field penetration in the physics of charged, field emitting, single-walled carbon nanotubes

A 1 $μ$m long, field emitting, (5,5) single-walled, carbon nanotube (SWCNT) closed with a fullerene cap, and a similar open nanotube with hydrogen-atom termination, have been simulated using the MNDO (Modified Neglect of Diatomic Overlap) quantum-mechanical method. Both contain about 80 000 atoms. It is found that field penetration and band-bending, and various forms of chemically and electrically induced apex dipole, play roles. Field penetration may help to explain electroluminescence associated with field emitting carbon nanotubes. Charge-density oscillations, induced by the hydrogen adsorption, are also found. Many of the effects can be related to known effects that occur with metallic or semiconductor field emitters; this helps both to explain the effects and to unify our knowledge about field electron emitters. However, it is currently unclear how best to treat correlation-and-exchange effects when defining the CNT emission barrier. A new form of definition for the field enhancement factor (FEF) is used. Predicted FEF values for these SWCNTs are significantly less than values predicted by simple classical formulae. The FEF for the closed SWCNT decreases with applied field; the FEF for the H-terminated open SWCNT is less than the FEF for the closed SWCNT, but increases with applied field. Physical explanations for this behavior are proposed. Curved Fowler-Nordheim plots are predicted. Overall, the predicted field emission performance of the H-terminated open SWCNT is slightly better than that of the closed SWCNT, essentially because a C-H dipole is formed that reduces the height of the tunnelling barrier. In general, the physics of a charged SWCNT seems more complex than hitherto realised.

cond-mat.mtrl-sci↗
arXiv2005

Field-Induced Insulator to Semimetal Transition and Field Electron Emission of Nanorods of Semiconductors of Wide Energy Band Gaps

Significant field emission is found theoretically possible from nanorods of semiconductors of wide energy band gaps. If the nanorod has a thin surface layer containing a large number of localized states, a part of nanorod can exhibit an insulator-to-semimetal transition under high enough fields of direction parallel to its axis, so that field emission occurs at the apex of the metal like tip. The field emission property of silicon carbide nanorods is studied as an example and found to be in qualitative agreement with the experimental findings.

cond-mat.other↗
arXiv1999

The short-time behavior of kinetic spherical model with long-ranged interactions

The kinetic spherical model with long-ranged interactions and an arbitrary initial order m_{0} quenched from a very high temperature to T < T_{c} is solved. In the short-time regime, the bulk order increases with a power law in both the critical and phase-ordering dynamics. To the latter dynamics, a power law for the relative order m_{r} ~ -t^{-k} is found in the intermediate time-regime. The short-time scaling relation of small m_{0} are generalized to an arbitrary m_{0} and all the time larger than t_{mic}. The characteristic functions $ϕ(b,m_{0})$ for the scaling of m_{0} and $ε(b,T')$ for T'=T/T_{c} are obtained. The crossover between scaling regimes is discussed in detail.

cond-mat.soft↗