An Analytical Approach for the Pseudogap in the Spin Fluctuations Model

The electronic self-energy due to the electron–spin fluctuation interaction is calculated using the one loop approximation for a two-dimensional system and quasi-two-dimensional (anisotropic) model. We analyzed the relevance of the diffusive modes and the temperature dependence of the magnetic correlation length for a possible temperature dependence of the pseudogap.     

Stripes with Spin Canting in the Three-Band Hubbard Model

In underdoped cuprates, both stripes and spiral states may account for the incommensurate spin response observed by elastic neutron scattering experiments. Here, we investigate the respective stability of both textures within the framework of the three-band Hubbard model which we treat within the unrestricted Gutzwiller approximation. Our calculations indicate that for parameter sets appropriate for lanthanum cuprates and small doping nor purely longitudinal stripes nor uniform spirals are stable but stripes with significant spin canting. Indeed at small doping uniform spirals are unstable toward nanoscale phase separation.     

Effect of Normal-State Pseudogap on Physical Quantities in Hubbard Model for Underdoped High-T c Cuprates

We develop the strong-coupling theory of coexisting charge-density-wave (CDW) and superconductivityd-wave gaps within the framework of the FLEX (fluctuation exchange) approximation for the two-dimensional Hubbard model. For nested sections of the Fermi surface these equations reduce to the previous FLEX equations for superconductivity where the squared energy gap _s ² in the denominator of the Green's function is replaced by ( _s ² + _c ² ) (here _s is the superconductivity and _c the CDW gap). We solve these equations by taking for _c a phenomenologicald-wave gap. The resulting neutron scattering intensity, spin-lattice relaxation rate 1/T₁ , Knight shift, resistivity, and photoemission intensity are in qualitative agreement with the data on underdoped high-T_c cuprates. TheT_c for superconductivity decreases and the crossover temperature T_* for 1/T₁Tincreases with increasing gap amplitude of _c which is in qualitative agreement with the phase diagram for underdoped cuprates.     

On Magnetic Relaxation in Spin Clusters

We summarize some of the significant features of the exotic behavior in the relaxation of molecular spin clusters and comment on aspects of some theories intended to account for observations. We point out that while the peaks in the relaxation rate and the hysteresis steps are connected with what may be termed resonant transitions at crossings of Zeeman levels, and are probably due to quantum tunneling, what is lacking is a direct confirmation through controllable parameters that are connected with theory. We discuss the need to match theoretical calculations with experimental conditions. This author and others have presented a theory for the hysteresis steps based upon quantum tunneling. However, the size of the observed steps far exceeds that predicted by theory. The results indicate that a combination of other fields - such as nuclear hyperfine fields, dipole fields, and anisotropy fields due to optical phonons - are important.     

Anisotropic Spin Splitting and Spin Relaxation in AlGaAs/GaAs Quantum Structures

Spin relaxation due to the D'yakonov–Perel' mechanism is intimately related with the spin splitting of the electronic states. We calculate the spin relaxation rates from anisotropic spin splittings of electron subbands in n-(001)-AlGaAs/GaAs quantum structures obtained in a self-consistent multiband approach. The giant anisotropy of spin relaxation rates found for different spin components in the (001) plane can be ascribed to a mutual compensation of terms because of the asymmetry of the bulk crystal and the quantum well structure.     

Exciton Spin Manipulation in ZnMnSe-Based Structures

Strong effect of structural design on spin functionality is observed in quantum structures based on II–VI semiconductors. Spin switching is realized when using a thin layer of Zn^0.95Mn^0.05Se diluted magnetic semiconductor (DMS) as a spin manipulator. This is evident from the polarization of photoluminescence related to a spin detector (an adjacent nonmagnetic quantum well (QW)) measured under the resonant excitation of the spin-up and spin-down states of the DMS, which is identical in value but opposite in sign. The achieved spin switching is suggested to reflect fast carrier diffusion from the DMS due to the absence of an energy barrier between the upper spin state of the DMS layer and the QW. On the other hand, the spin alignment is accomplished in the tunneling structures where the presence of the energy barrier inserted between a spin manipulator (i.e., a ZnMnSe/CdSe DMS superlattice) and a spin detector ensures a slow escape rate from the DMS layer.     

Pseudogap and Transport in HTS

Perpendicular transport is one of the key factors to HTS superconductivity, sampling the quasi-insulating blocking layer, separating the conducting CuO-planes, and driving the metal–insulator transition (MIT) that is induced by disorder and underdoping. Various measurements have been carried out to study the transport, the MIT, and the in-plane Fermi surface especially by surface methods via the blocking layer, and these depend sensitively on surface quality. ARXPS results on UHV cleaving show that at 300 K and 10^–10 Torr, a Bi hydroxide layer occurs in 30 min, followed by H₂O or C_y H_x OH chemisorption. Consequences of this result on STS, ARPES, perpendicular transport, Coulomb charging, and pseudogap are analyzed, yielding scenario for HTS superconductivity, where static and dynamic charge exchange via and with the blocking layer initiates plaques of preformed pairs of d-wave symmetry weakening the inplane Coulomb repulsion yielding by this plasmonic mechanism, finally, HTS. Consequences of this scenario on anisotropic transport with its strong Fermi velocity v_F anisotropy and its strong in-plane scattering rate (T) const. at (, 0) in k-space with pseudo gap kT^* _{_P}/3 and superconducting gap _S 3 kT_c maxima and the strongly decreasing rate (T) T at 0.4 (, ) with pseudo gap _{_p} (k) node and superconducting gap _s (k node are given.     

Influence of Spin Ordering on Superconducting Correlations in the Spin-One-Half Falicov-Kimball Model with Hund and Hubbard Coupling

The generalized spin-one-half Falicov-Kimball model with Hund and Hubbard coupling is used to examine effects of spin ordering on superconducting correlations in the strongly correlated electron and spin systems. It is found that the ferromagnetic spin clusters (lines, bands, domains) suppress the superconducting correlations in the d-wave channel, while the antiferromagnetic ones have the fully opposite effect. The enhancement of the superconducting correlations due to the antiferromagnetic spin ordering is by factor 3 in the axial striped phase and even by the factor 8 in the phase segregated phase.     

Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

Spin Properties of Electrons in Low-Dimensional SiGe Structures

Using ESR, we investigate g-factor and spin coherence time of electrons confined in 2D Si_1–xGe_x {channels} (x < 0.1) by barriers with x > 0.2 and in SiGe quantum dots grown on prepatterned Si substrates. The quantum wells exhibit 2D-anisotropy of both g and which can be explained in terms of the Bychkov–Rashba field. The latter increases with increasing Ge content in the well indicating that the increasing spin-orbit coupling is more important than interface properties. The narrow ESR permits selective spin manipulation already for x > 0.02. Large, regular arrays of Ge quantum dots (about 10⁹) were grown on prepatterned substrates. Strain in the Si capping layer lowers the conduction band relative to that of Ge causing confinement. The g-shift observed implies the possibility of g-tuning by confinement. The line width shows substantial inhomogeneous broadening whereas the longitudinal spin lifetime is hardly changed with respect to 2D structures.     

The Electronic Structures and Photographic Properties of Nanometer-sized Silver Clusters

The electronic structures and spectroscopic properties of nanometer-sized silver clusters Ag_n (n = 1 to 15) have been examined in the framework of self-consistent-field local density theory SCC-DV-X_α. The results show that there is a quantum size effect in Agn clusters, which can be observed as a red shift in the absorption threshold and semiconductor-conductor transition with increasing n. Based on the molecular orbital energies we studied the microscopical mechanisms of the transition point from lower efficiency step to higher efficiency step in latent image growth. The results also show that there are positive charges on the surface silver atoms of some large Ag" clusters, which is favourable for the latent image particles to adsorb developer anion in development.     

Electronic Inhomogeneity and Possible Pseudogap Behavior of Spin Susceptibility in the Electron-Doped Superconductor Sr0.93La0.07CuO2: 63Cu NMR Study

The magnetic susceptibility, NMR spectra, nuclear spin-lattice relaxation rate (T ₁ ^?1)_α and the echo-decay rate (T ₂ ^?1) of ⁶³Cu were measured for the electron-doped infinite-layer superconductor Sr_0.93La_0.07CuO₂/T _c ^onset = 42.4 K). The results obtained revealed a clear tendency toward frustrated phase separation in this nominally underdoped high-T _c material. Above T _c the ⁶³Cu Knight shift is found to decrease upon cooling giving an evidence for a pseudogap-like decrease of the spin susceptibility. It is shown that unusual anisotropy of the ⁶³Cu Knight shift in the electron-doped CuO₂ layer can be understood as a “compensation effect” between the isotropic hyperfine coupling, mediated by the 4s Fermi-contact and 3d core-polarization exchange interactions, and the anisotropic on-site spin-dipolar hyperfine interaction of the Cu nuclei with the itinerant carriers, whose states near the Fermi energy have a sizeable admixture of Cu(4p_z) and/or Cu(3d_z ²) orbitals.     

Spin Transport in (110) GaAs-Based Cavity Structures

The anisotropic spin dephasing of optically generated electrons in an undoped (110) GaAs quantum well inside a microcavity structure is investigated by means of spatially resolved photoluminescence experiments at a temperature of T=80 K. The dynamic type-II potential modulation induced by a surface acoustic wave (SAW) is used to transport electrons spatially separated from holes. Thus, the D’yakonov–Perel’ (DP) and the Bir–Aronov–Pikus spin dephasing mechanisms are suppressed, and electron spins can be transported over long distances of about 24 μm, which correspond to spin lifetimes of at least 8 ns. The spin vector can be rotated by an external in-plane magnetic field or by the effective in-plane field resulting from the structural inversion anisotropy induced by an intense SAW. This rotation generates an in-plane spin component that is subject to the DP spin dephasing mechanism. By means of Hanle effect measurements the lifetime of this in-plane spin component is found to be of 0.7 ns.     

Pseudogap in the Normal State of Cuprates

We consider underdoped cuprates as disordered conductors. The diffusion coefficient D can be as low as 10^–5 m² s^–1. In these conditions, Coulomb interaction between electrons must be taken into account. The main effect is to open a dip and even a gap in the density of state (DOS) near the Fermi level (FL). We show that this model explains most of the observed features of the so-called pseudogap in the normal state and in particular its value, anisotropy, and variation with doping.     

The Origin of the Pseudogap in Underdoped HTSC

Here, the origin of the pseudogap in HTSC is attributed to the modulated antiferromagnetic (AFM) phase, whose preliminary version has been sketched recently by the present author (in J. Super. Nov. Mag. 22:517, 2009). Starting from the t-J Hamiltonian, I show that the formal failure of the perturbation theory leads to a transformation to the pseudogap phase. This phase is characterized by the aggregation of the holes into rows and columns, which in turn results in two internal fields. The first is the modulated AFM field, whose main evidence comes from Neutron scattering experiments. The second internal field is made up by the checkerboard charge density waves that have been observed by scanning tunneling measurements. The present paper deals mainly with the internal field of the first type, and discusses the second type only tentatively. Formalism is derived that yields the ground state, the internal field, the Hamiltonian, and the propagators of the condensed phase. Our results resolve the presumably inherent self-contradictory concept of pseudogap. It is shown that the excitation energy spectrum is gapless despite the order parameter that is inherent to the condensed system. In addition, it is shown qualitatively that our model predicts “Fermi surface” that is in agreement with the experiment.     

Pseudogap in Boron-Doped Diamond and Cuprates

The Marcus model, well known as a general model for electron transfer, is applied to electron pair formation and transfer in systems characterized by neighboring superconducting and local phases, particularly boron-doped diamond and cuprates. Hubbard-U is identified with the adiabatic free energy difference between charged and spin-coupled configurations. Typical for the Marcus model is the coupling of electronic and nuclear coordinates due to structural changes with number of electrons during electron transfer or excitation.     

Fermi Arcs and Pseudogap in Cuprate Superconductors

Earlier, we have proposed [arXiv:1110.0227] the model of HTSC electronic structure modification under doping. In this model, doping by localized charges plays the key role, being responsible for local closing of the gap ??_ct between excitonic-like 3d¹⁰L^? state of cation and electronic 3d⁹L state of anion and formation (at a certain dopant concentration) of the percolation cluster with the Fermi surface located in the cation-anion band of peculiar nature. This electronic structure is favorable for the formation of diatomic negative-U centers (NUCs) and realization of an unusual mechanism of electron?Celectron interaction. Here, the nature of normal state of cuprates as well as the mechanism of pseudogap and Fermi arcs formation are considered in the framework of this model by example YBCO.     

Geometric and Electronic Structures of Tc and Mn Clusters by Density Functional Calculations

The geometric structures of Tc₂-Tc₅ and Mn₂-Mn₈ clusters were optimized by the Amsterdam density functional method. The trimeric, tetrameric, and pentameric Tc clusters exhibit the equilibrium structures of isosceles triangle, tetrahedron, and square pyramid, respectively. The structures of the Mn clusters up to Mn₄ are similar to those of the respective Tc clusters, while the Mn₅, Mn₆, Mn₇, and Mn₈ clusters have the distorted trigonal bipyramidal, distorted octahedral, pentagonal bipyramidal, and C2v structures, respectively. The Tc clusters are paramagnetic, while the Mn clusters are basically ferromagnetic.