Phonon response of the high‐temperature spin‐density‐wave superconductors

The phonon response of a spin density wave superconductor (SD^W-SC) is calculated. The phonon self energy due to electron-phonon interaction, which involves the electronic density response function, is evaluated explicitly for the coexistent SDW-SC state. It has a square-root singularity at a frequency corresponding to the superconducting gap 2Δ. As a consequence, the spectral density function of the SDW-phonon will show a peak at 2Δ The possibility of observing the 2Δ-peak by Raman scattering is discussed. The relevance of the results to high-temperature superconductors is pointed out. The available Raman data for YBa₂Cu₃O_7-δare compared with the results of the calculation.     

System-parameter dependence of the metallic phase of the non-doped 2D Hubbard model

Naito et al. reported that some non-doped T′-214-type compounds drive high-T_c superconductivity. The compounds are considered to be metallic since on-site Coulomb energy U is moderate and the Fermi surface is much deformed in these compounds. In order to confirm this picture and extract electronic structure information, we have examined the phase diagram of the metallic state of the 2D Hubbard model as a function of U and t′ (with t″ we fixed at − t′/2 here; t′ and t″ are the second- and third-neighbor transfer energies, respectively) by means of the variational Monte–Carlo method. We employed a Jastrow-type Gutzwiller trial wave function. In the studied range of U = 2–12, the boundary value for |t′| at which SDW disappears increases almost linearly with U. Jump-wise transition to the Mott insulator state was not observed. Using the boundary curve and experimental band parameter values, we estimate U  5 for T′-214 compounds. Preceding works are discussed in the last part.     

Charge stripe structure in FeTe

We report the STM study on a single-crystalline sample of FeTe at 7.8 K. FeTe is one of the iron-based superconductor. We measured the resistivity and the magnetization of FeTe. FeTe shows SDW transition at 58 K on these measurements. We study the electronic state of FeTe by using STM/STS for observing FeTe from a microscopic viewpoint. We observed the iron layer and the tellurium layer with atomic resolution. Moreover, we discover the charge stripe structure on STM/STS measurement. We find the charge stripe structure is caused by iron atoms from the analysis. The gap structure of 9 meV was observed in tunneling spectra. This gap size is consistent with the SDW gap which is expected from mean field theory with T_N=58 K.     

DMRG Studies on Properties of Undoped and Doped Molecule-Based Ferromagnetic Chain

By using density matrix renormalization group （DMRG） method a model for organic molecule-based ferromagnetic chain is proposed. It is found that the ground states of Undoped and doped systems both exhibit ferrimagnetic ordering. The e-e repulsion plays an important role in the stability of the ferromagnetic state either in doped system or undoped system. For the undoped system, each unit cell coatains half of the total spins, which is consistent with Lieb＇s theorem. It is convinced that when the system is doped with one electron, a charge density wave is excited, which decreases the amplitude of spin density wave,therefore acting against the stability of ferromagnetic state.     

Study of Pseudogap in Underdoped Cuprate

A mean-field spin-density wave (SDW) analysis of pseudogap in the underdoped cuprates is proposed on the basis of the t-t^ˊ-U Hubbard model. It is surprised to find that a simple t^ˊ term will do the trick to introduce the momentum dependence of the energy gap which mimics the pseudogap near (π,0) point at least. It implies that the pseudogap structure near (π,0) is not sensitive to the long-range order and will survive leading to the pseudogap phenomenon in the underdoped metallic phase. On the other hand, in the long-range ordering antiferromagnetic region, the mean-field SDW theory holds and the pseudogap structure predicated by the theory should be observable experimentally. Then one prediction is that the pseudogap would smoothly extrapolate between itinerant antiferromagnetic phase and underdoped metallic phase.     

SDW in the 2D Compound CuFeTe2

In a previous work (ICAME'97) we presented the Mössbauer results for a non-stoichiometric sample of the quasi-two-dimensional (2D) dichalcogenide CuFeTe₂, where a Spin Density Wave (SDW) ground state with T _SDW=256±15 K was proposed. Here we report the study of the magnetic and electric properties determined by magnetic susceptibility, Mössbauer spectroscopy and resistance measurements, of an almost stoichiometric sample prepared by the vertical Bridgman growth technique. The SDW behavior is supported by the results obtained by the following different techniques: Magnetic susceptibility: A magnetic transition is observed at T _SDW=308 K with a Pauli paramagnetic behavior above this temperature. Mössbauer effect: The shape of the spectra and the thermal evolution of the hyperfine field are characteristic of the SDW's in quasi-2D systems. Electrical resistance: There is a metal–semiconductor transition along the layers as the temperature decreases indicating the opening of a gap at the Fermi level.     

The competition between CDW and SDW in quasi-one-dimensional organic magnetic polymer with interchain coupling interaction

Based on a theoretical model proposed for interchain-coupled quasi-one-dimensional organic magnetic polymer, the effects of the interchain couplings and electron–electron interactions on the charge density wave (CDW) and spin density wave (SDW) that exist in the system are studied. It is found that the amplitude of the SDW along the main chain will decrease with increasing of the oscillatory term of the interchain couplings in the system, which is unfavorable to the ferromagnetic ground state of the system. Moreover, with different interchain couplings, there will all exist a critical value of the inter-site electron–electron Coulomb repulsion, and at this value, the system will experience a transformation from strong SDW state to strong CDW one, which will weaken the mediating function of the antiferromagnetic SDW along the main chain. As a result, the ferromagnetic correlation intensity between the spins of the side radicals will be affected and consequently the stability of the ferromagnetic state in the system will be weakened.     

Mixed density wave state in quasi-2D organic conductor

The density wave phase of α-$\alpha \mathrm{-}$(BEDT-TTF)₂KHg(SCN)₄ was investigated by transport properties and magnetic susceptibility. The density wave transition was observed as a broad increase at T_DW$T_{\mathrm{DW}}$=9 K by resistance measurement. Temperature dependence of the static magnetic susceptibility χ$\chi$ shows a large Curie tail below 100 K. By subtracting the Curie component, we found that the magnetic susceptibility increases like weak ferromagnetism with decreasing temperature below 7.4 K. The gradual increase of χ$\chi$ below T_DW$T_{\mathrm{DW}}$ is not expected in simple CDW or SDW, where the magnetic susceptibility decreases with decreasing temperature due to the reduction of Pauli paramagnetic component. To explain the weak ferromagnetic behavior, we consider the coexistence of CDW and SDW. We propose a model of the mixed density wave, where CDW exists with antiferromagnetically coupled canting spins.     

电荷转移型Hubbard模型的相图

用玻色化技术和高斯波泛函变分理论研究了电荷转移型Hubbard模型.通过自旋密度波和电荷密度波的位相结构的变化，并结合其相应能隙的变化，得到以下结论：系统的Ising相变与Mott相变不重合，中间有一个SDI(spontaneouslydimerizedinsulating)的过渡相.在BI(band-insulator)相，自旋密度波与电荷密度波都具有能隙，而在MI(Mott-insulator)相，电荷密度波具有能隙，自旋密度波没有能隙. 关键词： Hubbard模型 电荷密度波 自旋密度波 相图     

Persistent current and the existence of a metallic phase flanked by two insulating phases in a quantum ring with both electron–electron and electron–phonon interactions

The persistent current in the ground state of a quantum ring threaded by a magnetic flux is calculated within the framework of the Holstein-Hubbard model. It is found that the persistent current is suppressed by both the electron–electron and electron–phonon interactions. Calculation of Drude weight reveals that the persistent current is diamagnetic in nature. It is observed that as the number of atoms in the quantum ring increases, the persistent current decays in a continuous way. It is finally predicted that there exists an intervening metallic phase flanked in real time by two insulating phases, the SDW phase and the CDW phase.