On the particle excitations in the XXZ spin chain

We continue to study the excited states for the XXZ spin chain corresponding to the complex roots of the Bethe Ansatz equations with the imaginary part equal to π/2$\pi /2$. We propose the particle–hole symmetry which relates the eigenstates build up from the two different pseudovacuum states. We find the XXX spin chain limit for the eigenstates with the complex roots. We also comment on the low-energy excited states for the XXZ spin chain.     

Inelastic quasiparticle lifetimes of the Shockley surface state band on Ni(111)

We present a study of the low-energy quasiparticle lifetimes of the Shockley surface state on the Ni(111) surface with scanning tunnelling spectroscopy. By measuring the coherence length of the decaying standing wave pattern at straight step edges electron and hole lifetimes have been determined. The values of the lifetime measured on this ferromagnetic surface show to be considerable smaller than the values obtained from noble metal surfaces. This is explained by differences in the electron density of states at the Fermi energy but has to include substantial spin-flip scattering. Furthermore hole lifetimes appear to be larger than electron lifetimes with the same excitation energy. Although only results for the majority spin component are presented, a spin-dependent selfenergy is expected.     

High-spin polaron in lightly doped CuO2 planes

We derive and investigate numerically a minimal yet detailed spin-polaron model that describes lightly doped CuO2 layers. The low-energy physics of a hole is studied by total-spin-resolved exact diagonalization on clusters of up to 32 CuO2 unit cells, revealing features missed by previous studies. In particular, spin-polaron states with total spin 3/2 are the lowest eigenstates in some regions of the Brillouin zone. In these regions, and also at other points, the quasiparticle weight is identically zero indicating orthogonal states to those represented in the one electron Green's function. This highlights the importance of the proper treatment of spin fluctuations in the many-body background.     

Fast optical preparation, control, and readout of a single quantum dot spin

We propose and demonstrate the sequential initialization, optical control, and readout of a single spin trapped in a semiconductor quantum dot. Hole spin preparation is achieved through ionization of a resonantly excited electron-hole pair. Optical control is observed as a coherent Rabi rotation between the hole and charged-exciton states, which is conditional on the initial hole spin state. The spin-selective creation of the charged exciton provides a photocurrent readout of the hole spin state.     

Surface bound states and spin currents in noncentrosymmetric superconductors

We investigate the ground state properties of a noncentrosymmetric superconductor near a surface. We determine the spectrum of Andreev bound states due to surface-induced mixing of bands with opposite spin helicities for a Rashba-type spin-orbit coupling. We find that the order parameter suppression qualitatively changes the bound state spectrum. The spin structure of Andreev states leads to a spin supercurrent along the interface, which is strongly enhanced compared to the normal state spin current. Particle and hole coherence amplitudes show Faraday-like rotations of the spin along quasiparticle trajectories.     

Chiral metal as a ferromagnetic super spin chain

The electrons on the surface of a disordered multi-layer integer quantum Hall system constitute an unusual chiral metal with ballistic motion transverse to the field, and diffusive motion parallel to it. We present a non-perturbative analytic treatment of an appropriate model, consisting of disordered chiral fermions in two dimensions. A supersymmetric generating functional is set up for the correlation functions of this system. The strong disorder limit is mapped into a supersymmetric spin chain, with ferromagnetic exchange coupling, reflecting the electron's chiral motion. The ferromagnetic ground state and the spin wave excitations, corresponding to the diffusion modes of the chiral metal, are found exactly. The parametric density of states correlator in the ergodic limit is computed from a Boltzmann-weighted sum over low-energy spin states. The result is of a universal form and coincides with that for a hermitian random matrix.     

Ferromagnetism in the Hubbard model: Spin waves and instability of the Nagaoka state

We discuss two single spin flip variational wave functions describing spin wave excitations which were proposed earlier by Shastry, Krishnamurthy and Anderson (SKA) and by Basile and Elser (BE), respectively, in order to investigate the instability of the fully polarized ferromagnetic state (Nagaoka state) in the infinite U Hubbard model. We calculate the energy of these variational states for the square lattice and for multiple chains. At the zone boundary in the vicinity of the point (0, π) the spin wave energy is reduced substantially by the binding of the spin up hole to the flipped down spin. For the square lattice this leads to a critical hole density of δ_cr = 0.407 for the SKA spin wave and of δ_cr = 0.322 for the BE spin wave which implies remarkable improvements in comparison to the corresponding scattering states investigated previously.     

Quench dynamics of edge states in 2-D topological insulator ribbons

We study the dynamics of edge states of the two dimensional BHZ Hamiltonian in a ribbon geometry following a sudden quench to the quantum critical point separating the topological insulator phase from the trivial insulator phase. The effective edge state Hamiltonian is a collection of decoupled qubit-like two-level systems which get coupled to bulk states following the quench. We notice a pronounced collapse and revival of the Lochschmidt echo for low-energy edge states illustrating the oscillation of the state between the two edges. We also observe a similar collapse and revival in the spin Hall current carried by these edge states, leading to a persistence of its time-averaged value.     

Organization and energy properties of metastable states for the random-field Ising model

Random-field Ising model (RFIM) systems are characterized by a large number of metastable states corresponding to local minima of the system energy with respect to single spin flip. We classified the minima in a hierarchical way based on the possibility of a given state to escape from a basin of mutually reachable states. We investigate the energy properties of the metastable states in relation to the basin they belong to: states of particularly high energy, obtained by fast-quenching randomly initial spin configurations, tend to have access to a complex structure of correlated basins, opposite to what is found for low-energy states. The purpose of this paper is to investigate the connection between the properties of the basin oriented graph and the energy of the corresponding states.     

Tuning of exciton states in a magnetic quantum ring

The exciton states in a CdTe quantum ring subjected to an external magnetic field containing a single magnetic impurity are investigated. We have used the multiband approximation which includes the heavy hole–light hole coupling effects. The electron–hole spin interactions and the s, p–d interactions between the electron, the hole and the magnetic impurity are also included. The exciton energy levels and optical transitions are evaluated using the exact diagonalization scheme. We show that due to the spin interactions it is possible to change the bright exciton state into the dark state and vice versa with the help of a magnetic field. We propose a new route to experimentally estimate the s, p–d spin interaction constants.     

基于量子费希尔信息的纠缠判据

对于几类常见的量子态, 对自旋压缩和量子Fisher信息进行了比较研究.得到量子Fisher信息的表达式,以Fisher信息为基础,给出了量子纠缠的判据,结果表明:比较于自旋压缩,该判据有明显的优势.(1).对于两个Qubit的对称态,该判据与自旋压缩,concurrence完全等价;(2).对于两个Qubit的非对称态的纠缠,自旋压缩不能检验其纠缠,但对于部分态,该判据也能检验;(3).对于Dicke态, 自旋压缩不能检验其纠缠,但该判据能检验且完全等价于concurrence.     

基于量子费希尔信息的纠缠判据

对于几类常见的量子态, 对自旋压缩和量子Fisher信息进行了比较研究.得到量子Fisher信息的表达式,以Fisher信息为基础,给出了量子纠缠的判据,结果表明:比较于自旋压缩,该判据有明显的优势.(1).对于两个Qubit的对称态,该判据与自旋压缩,concurrence完全等价;(2).对于两个Qubit的非对称态的纠缠,自旋压缩不能检验其纠缠,但对于部分态,该判据也能检验;(3).对于Dicke态, 自旋压缩不能检验其纠缠,但该判据能检验且完全等价于concurrence.     

Spin excitations and spin wave gap in the ferromagnetic Weyl semimetal Co_3Sn_2S_2

We report a comprehensive neutron scattering study on the spin excitations in the magnetic Weyl semimetal Co3Sn2S2 with a quasi-two-dimensional structure.Both in-plane and out-of-plane dispersions of the spin waves were revealed in the ferromagnetic state.Similarly,dispersive but damped spin excitations were found in the paramagnetic state.The effective exchange interactions were estimated using a semi-classical Heisenberg model to consistently reproduce the experimental TCand spin stiffness.However,a full spin wave gap below Eg=2.3 meV was observed at T=4 K.This value was considerably larger than the estimated magnetic anisotropy energy(~0.6 meV),and its temperature dependence indicated a significant contribution from the Weyl fermions.These results suggest that Co3Sn2S2 is a three-dimensional correlated system with a large spin stiffness,and the low-energy spin dynamics can interplay with the topological electron states.     

The effect of zero-point spin wave motions on the hole propagation in the strongly correlated Hubbard model

The kinetic energy of holes doped in a two dimensional antiferromagnetic state is dicussed, taking into account the effect of zero-point spin wave motions. The lower band edge of the hole density of states is estimated from its lower order moments in the strong electron correlation limit. A relatively large narrowing of the density of states is obtained, nearly the same as that of the RVB case. Consequences of the result are discussed in connection with the relative stability between the antiferromagnetic state and the RVB state.     

Scattering models for ultracold atoms

We present a review of scattering models that can be used to describe the low-energy behavior of identical bosonic atoms. In the simplest models, the only degrees of freedom are atoms in the same spin state. More elaborate models have other degrees of freedom, such as atoms in other spin states or diatomic molecules. The parameters of the scattering models are specified by giving the S-wave phase shifts for scattering of atoms in the spin state of primary interest. The models are formulated as local quantum field theories and the renormalization of their coupling constants is determined. Some of the parameters can be constrained by renormalizability or by the absence of negative-norm states. The Green’s functions that describe the evolution of two-atom states are determined analytically. They are used to determine the T-matrix elements for atom-atom scattering and the binding energies of diatomic molecules. The scattering models all exhibit universal behavior as the scattering length in a specific spin state becomes large.     

Correlation effects in photoelectron spectra of high-Tc superconductive oxides: the hole-induced redistribution of Cu3d states in La2CuO4

In the transition state model using the LMTO-Green function method an investigation is performed of the changes in energy distribution of Cu3d and O2p states in the valence band of La₂CuO₄ which are caused by the presence of the photoelectron hole. It is shown that taking the hole into account leads to a sharp decrease in the density of Cu3d states on the Fermi level and to a low-energy shift of the d band edge relative to the Fermi level. The grounds for the suggestion of the purely oxygenic nature of hole carriers in high-T_c superconductors are discussed.     

基于量子点-CBP混合层的量子点LED的制备

采用一锅法制备出高质量的具有核壳结构的Cd Se@Zn S、Cd Zn S/Zn S量子点。将量子点混入空穴传输材料CBP中形成复合的有源材料,经过几步简单的旋涂操作,制备出相应的绿光、蓝光量子点LED器件。这种方法利用了油溶性量子点和CBP材料的相容性,减少了旋涂操作的步骤,有利于快速制备基于量子点的电致发光器件。基于两步旋涂操作制备的量子点LED,由于阴极与复合有源层之间的能级差较大,导致需要较高的开启电压。在CBP材料中,注入的载流子有可能会被量子点表面缺陷捕获,形成表面态的发光。表面态发光的相对强度依赖于载流子浓度。     

Interacting antiferromagnetic droplets in quantum critical CeCoIn5

The heavy fermion superconductor CeCoIn5 can be tuned between superconducting and antiferromagnetic ground states by hole doping with Cd. Nuclear magnetic resonance data indicate that these two orders coexist microscopically with an ordered moment approximately 0.7 microB. As the ground state evolves, there is no change in the low-frequency spin dynamics in the disordered state. These results suggest that the magnetism emerges locally in the vicinity of the Cd dopants.     

Hole core in superconductors and the origin of the Spin Meissner effect

It is proposed that superconductors possess a hidden ‘hole core’ buried deep in the Fermi sea. The proposed hole core is a small region of the Brillouin zone (usually at the center of the zone), where the lowest energy states in the normal state reside. We propose that in the superconducting state these energy states become singly occupied with electrons of a definite spin helicity. In other words, that holes of a definite spin helicity condense from the top to the bottom of the band in the transition to superconductivity, and electrons of that spin helicity ‘float’ on top of the hole core, thus becoming highly mobile. The hole core has radius q₀ = 1/2λ_L, with λ_L the London penetration depth, and the electrons expelled from the hole core give an excess negative charge density within a London penetration depth of the real space surface of the superconductor. The hole core explains the development of a spin current in the transition to superconductivity (Spin Meissner effect) and the associated negative charge expulsion from the interior of metals in the transition to superconductivity, effects we have proposed in earlier work to exist in all superconductors and to be at the root of the Meissner effect.