Topological Phase Transition and Phase Diagrams in a Two-Leg Kitaev Ladder System

A scheme to investigate the topological properties in a two-leg Kitaev ladder system composed of two Kitaev chains is proposed. In the case of two identical Kitaev chains, it is found that the interchain hopping amplitude plays a significant role in the separation of the energy spectrum and in inducing a topologically nontrivial phase, while the interchain pairing strength only affects the size of the energy gap. Moreover, another situation that the system consists of two non-identical Kitaev chains is also investigated and the corresponding phase diagram is calculated. It is found that two pairs of degenerate nonzero edge modes will, respectively, appear in the upper and lower energy gaps when the interchain hopping amplitude or the interchain pairing strength is large enough. Furthermore, it is pointed out that the winding number is quantitatively equivalent to half of the number of zero energy edge modes in our system.     

Josephson current versus potential strength of the interface in ferromagnetic superconductors

Based on the scattering theory, we calculate the Josephson current in a junction between two ferromagnetic superconductors as a function of the interface potential z. We consider the ferromagnetic superconductor(FS) in three different Cooper pairing states: spin singlet s-wave pairing(SWP) state, spin triplet opposite spin pairing(OSP) state, and spin triplet equal spin pairing(ESP) state. We find that the critical Josephson current as a function of z shows clear differences among the SWP, OSP, and ESP states. The obtained results can be used as a useful tool for determining the pair symmetry of the ferromagnetic superconductors.     

156Gd基态SU(3)→O(6)相变的一种微观理解

γ射线能量自旋曲线指认156Gd核基态具有SU(3)和O(6)两种对称性。 基于微观sdIBM-max方案和单粒子能量实验值, 用两组核子之间的对作用、 四极对作用、 四极 四极作用的等效强度参数, 都很好地再现了这两种能谱及其演化过程。 计算结果揭示出对基态相变的一种新理解: SU(3)的基准态是低激发 低有序态, 而O(6)基准态则是高激发 高有序的, 它们有临界区6+1—8+1态; 当核退耦到临界区时, 高有序基准态释放多余的有序结构能, 导致低有序基准态重组, 实现减速旋转驱动高有序核向着低有序核过渡的量子相变。最后用156Gd核的势能曲面作了直观说明。 The γ ray energy over spin curves identifies that there are the SU(3) and O(6) symmetries in the ground states of the 156Gd nucleus; by means of the microscopic sdIBM max approach and signal particle experimental energies the spectra of those two symmetries and their transient process are successfully reproduced through two parameters of nucleon nucleon effective interaction with pairing plus quadrupole pairing plus quadrupole quadrupole forces. The calculated results reveal a new way to recognize ground states quantum phase transition, in which the basic state of the SU(3) symmetry is a low lying and low ordered state, while one of the O(6) are a high lying and high ordered state, their critical region is between 6+1—8+1 states, the high ordered basic state releases spare ordered structure energy, reducing rotation speed, thus causing the restructure of low ordered basic state and accomplishing the quantum phase transition from the high ordered phase into the low ordered phase, the shape phase transition takes place along the yrast line of nucleus when it de excited to the critical region. Because the structural phase transition takes place by no obvious charge of boson structure constants in the critical region it is a benignancy and calm transition with respect to its macroscopic behave. The potential energy surface of 156Gd nucleus has been illustrated to visualize.     

The structural, electronic, and magnetic properties of Fe-Ir, Co-Ir and Ni-Ir linear and zigzag nanowires: First-principles calculations

The magnetic and electronic properties of both linear and zigzag bimetallic chains of Fe-Ir, Co-Ir and Ni-Ir have been calculated based on density functional theory and using the generalized gradient approximation. It is found that all considered zigzag chains form a twisted two-legged ladder, look like a corner-sharing triangle ribbon, and have a lower total energy than the corresponding linear chains. All the Fe-Ir, Co-Ir and Ni-Ir linear and zigzag chains have stable or metastable ferromagnetic (FM) states. The bond lengths in bimetallic Fe-Ir, Co-Ir and Ni-Ir at ferromagnetic state are larger than those in the corresponding structures at nonmagnetic state. Interestingly, the Ni-Ir zigzag nanowire has two energy minimum states, both ferromagnetic and nonmagnetic (NM), indicating a possible stable condition for mechanically controllable break-junction experiments.     

Electronic and magnetic structures of chain structured iron selenide compounds

Electronic and magnetic structures of iron selenide compounds Ce2O2FeSe2 （2212＊） and BaFe2Se3 （123＊） are studied by the first-principles calculations. We find that while all these compounds are composed of one-dimensional （1D） Fe chain （or ladder） structures, their electronic structures are not close to be quasi-lD. The magnetic exchange couplings between two nearest-neighbor （NN） chains in 2212＊ and between two NN two-leg-ladders in 123＊ are both antiferromagnetic （AFM）, which is consistent with the presence of significant third NN AFM coupling, a common feature shared in other iron-chalcogenides, FeTe （11＊） and KyFe2-xSe2 （122＊）. In magnetic ground states, each Fe chain of 2212＊ is ferromagnetic and each two-leg ladder of 123＊ form a block-AFM structure. We suggest that all magnetic structures in iron-selenide compounds can be unified into an extended J1-J2-J3 model. Spin-wave excitations of the model are calculated and can be tested by future experiments on these two systems.     

Electron-phonon interaction in carbon clathrate hex- $\mathsf{C_{40}}$

We investigate an extended spin ladder with diagonal frustrated exchanges in a wide parameter regime. By representing the model as a sum of semidefinite positive projection operators, we prove that this model has exactly a dimer ground state. Smoothly changing parameters may lead the model cover several exactly known models. Starting from this ladder model, we proposed two two-dimensional net models with exact ground states. The quantum phase transition of the ground state, due to the change of exchange strengths along perpendicular rungs, is also discussed. Received 13 October 2002 Published online 27 January 2003 RID="a" ID="a"e-mail: schen@thphy.uni.duesseldorf.de     

Relation between initial conditions and entanglement sudden death for two-qubit extended Werner-like states

In this paper, the dynamical behavior of entanglement of an uncoupled two-qubit system, which interacts with inde- pendent identical amplitude damping environments and is initially prepared in the extended Werner-like （EWL） states, is investigated. The results show that whether entanglement sudden death （ESD） of an EWL state will occur or not depends on initial purity and concurrence. The boundaries between ESD states and ESD-free states for two kinds of EWL states are found to be different. Furthermore, some regions are shown where ESD states can be transformed into ESD-free states by local unitary operations.     

Delocalization of quasiparticles in quasi-two-dimensional disordered superconductors with s-wave pairing

We investigate localization behavior of quasiparticles in disordered multi-plane superconductors with s-wave pairing. By introducing disorder with random site energies, the spatial fluctuations of Bogoliubov-de Gennes pairing potential are self-consistently determined. The size dependence of rescaled localization length for a long bar is calculated by using the transfer-matrix method. From the finite-size scaling analysis we show that there exists a critical point of the disorder strength W_c which separates the extended and localized quasiparticle states in such quasi-two-dimensional systems. The associated critical behavior is studied and the relationship of the results to the number of planes is discussed.     

Metamagnetic transitions in frustrated spin-ladders

Variational calculations of the magnetization curve at zero temperature are reported for two models of frustrated ladder spin systems with ferro-and antiferromagnetic interactions. The ground state of the models is either ferro-or antiferromagnetic depending on model parameters. The character of the transition from the ferro-to the antiferromagnetic state differs from that of the corresponding transition in the XXZ model and is characterized by the appearance of bound multimagnon states. The existence of these states is shown to result in magnetization jumps at certain external field values. The region of the phase diagram where such jumps occur was determined, and the corresponding critical field values were found.     

Spin fluctuations and the superconducting state in doped insulators

We propose a model of electron pairing via spin fluctuations in doped insulators. The bare states for the superconducting condensate correspond to impurity bands in the original band gap of the undoped material. We obtain a complete set of equations for the superconducting state. We show that fermion pairing in impurity bands of extended states is possible, and thus so is superconductivity, if localized spin-0 bosons are produced. The latter are necessarily accompanied by localized spin-1 bosons, which are responsible for the relationship between singlet and triplet pairing channels of quasiparticles. Zh. éksp. Teor. Fiz. 114, 1765–1784 (November 1998)     

链间耦合对聚乙炔多链体系中电子极化子再激发态的影响

采用拓展紧束缚Su-Schrieffer-Heeger(SSH)模型,研究了链间耦合对反式聚乙炔多链体系中电子极化子再激发态的晶格位形、净电荷密度、局域能级波函数和态密度的影响.结果发现:对于两条链体系,当链间耦合很小(t⊥≤0.01 e V)时,注入到系统中的电子只会在第一条链上诱发产生一个晶格缺陷,形成电子极化子再激发态,这和单链体系是一致,而第二条链仍是二聚化基态.随着链间耦合的增大,第一条链上缺陷的局域度减少而第二条链上的缺陷局域度相应增加,直至两条链上的位形相同;对于多条链(5条链和6条链)体系,当耦合很小(t⊥≤0.05 e V)时,电子极化子再激发态也只会存在于一条链上,当链间耦合较强时,极化子再激发态会在链间层次性地扩展开来,并不会出现多条链位形相同;从两条链的能级图上可以看到随着链间耦合t⊥的增大,体系的带隙不断的增大和电子态密度显示的是完全吻合的,体系的导电性减弱.通过分析两条链体系在t⊥=0 e V和t⊥=0.1 e V的能级态密度,发现链间耦合越强,则中间局域能级的态密度越小,最后没有中间局域态.     

链间耦合对聚乙炔多链体系中电子极化子再激发态的影响

从拓展紧束缚模型出发,研究了链间耦合对反式聚乙炔多链体系中电子极化子再激发态的晶格位形、净电荷密度、局域能级波函数和态密度的影响。结果发现：对于两条链体系,当链间耦合很小（eV）时,注入到系统中的电子只会在第一条链上诱发产生一个晶格缺陷,形成电子极化子再激发态,这和单链体系是一致,而第二条链仍是二聚化基态。随着链间耦合的增大,第一条链上缺陷的局域度减少而第二条链上的缺陷局域度相应增加,直至两条链上的位形相同。对于多条链（5条链和6条链）体系,当耦合很小（0.05eV）时,电子极化子再激发态也只会存在于一条链上,当链间耦合较强时,极化子再激发态会在链间层次性地扩展开来,并不会出现多条链位形相同。从两条链的能级图上可以看到随着链间耦合的增大,体系的带隙不断的增大和电子态密度显示的是完全吻合的,体系的导电性减弱。通过分析两条链体系在eV和eV的能级态密度,发现链间耦合越强,则中间局域能级的态密度越小,最后没有中间局域态。     

Pairing correlations on t-U-J ladders

We find that the pairing correlations on the usual t-U Hubbard ladder are significantly enhanced by the addition of a nearest-neighbor exchange interaction J. Likewise, these correlations are also enhanced for the t-J model when the on-site Coulomb interaction is reduced from infinity. Moreover, the pairing correlations are larger on a t-U-J ladder than on a t-J(eff) ladder in which J(eff) has been adjusted so that the two models have the same spin gap at half filling. This enhancement of the pairing correlations is associated with an increase in the pair-binding energy and the pair mobility in the t-U-J model and points to the importance of the charge-transfer nature of the cuprate systems.     

具有p波超流的一维非公度晶格中迁移率边研究

研究了具有p波超流的一维非公度晶格中迁移率边的性质. 发现适当的p波超流可以增加体系中的迁移率边的数目, 并且通过多分形分析确定了迁移率边所在的位置.     

Nucleon pairing in atomic nuclei

The nucleon pairing effect that is analyzed in the present paper is one of the striking manifestations of nuclear dynamics. Nucleon pairing for different chains of nuclei dependent upon the number of protons or neutrons in the nucleus allows one to explain the emergence of a great number of positive-parity states, which form a ground state multiplet, in even-even nuclei in the excitation energy range E* < 4 MeV. The interaction of paired nucleons with vibrational and rotational degrees of freedom of a nucleus produces a wide variety of excitation spectra of positive-parity states in even-even nuclei.     

Nuclear shell structure in the systematics of nuclear properties

Existing experimental data allow us to study the behavior of nuclear shell structure by analyzing characteristics of different nature. In this work, one of the most striking phenomena of nuclear dynamics is examined: nucleon pairing. Nucleon pairing for different nuclei chains as a function of the number of protons or neutrons in a nucleus explains why high numbers of states with positive parity in even-odd nuclei are observed for excitation energies E* < 4 MeV that form the multiplet of a nucleus’s ground state.     

Correlation-hole induced paired quantum Hall states in the lowest Landau level

A theory is developed for the paired even-denominator fractional quantum Hall states in the lowest Landau level. We show that electrons bind to quantized vortices to form composite fermions, interacting through an exact instantaneous interaction that favors chiral p-wave pairing. There are two canonically dual pairing gap functions related by the bosonic Laughlin wave function (Jastrow factor) due to the correlation holes. We find that the ground state is the Moore-Read Pfaffian in the long-wavelength limit for weak Coulomb interactions, a new Pfaffian with an oscillatory pairing function for intermediate interactions, and a Read-Rezayi composite Fermi liquid beyond a critical interaction strength. Our findings are consistent with recent experimental observations of the 1/2 and 1/4 fractional quantum Hall effects in asymmetric wide quantum wells.     

High-temperature superfluidity of fermionic atoms in optical lattices

Fermionic atoms confined in a potential created by standing wave light can undergo a phase transition to a superfluid state at a dramatically increased transition temperature. Depending upon carefully controlled parameters, a transition to a superfluid state of Cooper pairs, antiferromagnetic states or d-wave pairing states can be induced and probed under realistic experimental conditions. We describe an atomic physics experiment that can provide critical insight into the origin of high-temperature superconductivity in cuprates.     

Structural stabilities and electronic structures of Ti atomic chains

The present first principles density-functional calculations reveal that titanium can form one-dimensional chains in linear, dimer, zigzag and ladder structures. The most stable structure is a zigzag chain with a unit cell rather close to an equilateral triangular geometry with four nearest neighbors. Two intermediary chains between the linear and zigzag ones have the ladder and dimer structure, respectively. Titanium can also form a metastable zigzag structure with an obtuse bond angle. It is important and interesting to find that during the elongation of the zigzag chain, the bond angle will shift abruptly from a sharp angle to an obtuse one at a critical point, and the bonding character also transforms from mainly metallic to more covalent. This is the first time that such a structural transition is discussed in the atomic chain system. The electronic structures of these one-dimensional titanium chains are also discussed.     

Effect of Nucleon Pairing in the Spectra of <Emphasis Type="Italic">N</Emphasis> = 50 Isotones

The emergence of the pairing effect of identical nucleons in the j = 9/2 state in low-lying excited states of nuclei near ⁹⁰Zr (N = 50, Z = 40) is discussed. Multiplets of states with seniority s ≥ 2, the splitting of which is determined by the proton pairing energy, are clearly visible in the nuclear spectra for a chain of N = 50 isotones. A comparison of the spectra of ground state multiplets, calculated in the δ-interaction approximation, with experimental data and results from other theoretical calculations shows this approach can be used to describe the structure of spectra and level positions with high J values.