A model of the T-dependent pseudogap and its competition with superconductivity in copper oxides

Results for pseudogaps are obtained from a band model, where the stability of the gap depends on the amplitudes of vibrational displacements, or magnetic moments, and their coupling to electrons. A one-particle gap is favored by normal thermal excitations of phonons or spin waves. Another gap can be generated by spontaneous waves at lower temperature, if the electronic energy gain overcomes the elastic/magnetic energy needed for increased amplitudes of the oscillations. This state is characterized by charge or spin density waves. The pseudogap has many features in common with the superconducting gap, and the model lends support to the interpretation that the pseudogap is a precursor of, and competes with, superconducting pairing.     

Non-BCS superconductivity for underdoped cuprates by spin-vortex attraction

Within a gauge approach to the t-J model, we propose a new, non-BCS mechanism of superconductivity for underdoped cuprates. The gluing force of the superconducting mechanism is an attraction between spin vortices on two different Néel sublattices, centered around the empty sites described in terms of fermionic holons. The spin fluctuations are described by bosonic spinons with a gap generated by the spin vortices. Due to the no-double occupation constraint, there is a gauge attraction between holon and spinon binding them into a physical hole. Through gauge interaction the spin vortex attraction induces the formation of spin-singlet (RVB) spinon pairs with a lowering of the spinon gap. Lowering the temperature, the approach exhibits two crossover temperatures: at the higher crossover a finite density of incoherent holon pairs are formed leading to a reduction of the hole spectral weight, while at the lower crossover a finite density of incoherent spinon RVB pairs are also formed, giving rise to a gas of incoherent preformed hole pairs, and magnetic vortices appear in the plasma phase. Finally, at a even lower temperature the hole pairs become coherent, the magnetic vortices becoming dilute and superconductivity appears. The superconducting mechanism is not of BCS-type since it involves a gain in kinetic energy (for spinons) coming from the spin interactions.     

Temperature dependence of the gap in the S=1/2 antiferromagnetic chain in the presence of a staggered field

We calculate the temperature dependent energy gap in the magnetic excitation spectrum of the one-dimensional spin 1/2 antiferromagnetic Heisenberg model in the presence of a staggered magnetic field, using the phase Hamiltonian, a self consistent harmonic approximation, and the thermal-Green function technique.     

Dynamic spin and charge susceptibilities in thet−J model

Based on the spin-rotation-invariant formulation of the Kotliar Ruckenstein slave-boson representation, the paramagnetic spin and charge susceptibilities in thet-J model are calculated. Analyzing the static spin susceptibility, the instability of the paraphase towards incommensurate magnetic order is in agreement, with the saddle-point phase diagram recently obtained by some of the authors. The spin dynamics at arbitrary frequencies, wave vectors and band fillings is calculated, where the Fermi-surface and correlation effects are studied. The magnetic instability region is investigated with respect to the formation of a collective spin-fluctuation mode. Near the transition point, a kinetic gap and a sharp peak in the spectral weight ((1,0) paramagnon) are obtained.     

( Sr / Ca ) 14 Cu 24 O 41 spin ladders studied by NMR under pressure

⁶³Cu-NMR measurements have been performed on two-leg hole-doped spin ladders Sr_14-xCa_xCu₂₄O₄₁ single crystals 0 ? x ? 12 at several pressures up to the pressure domain where the stabilization of a superconducting ground state can be achieved. The data reveal a marked decrease of the spin gap derived from Knight shift measurements upon Ca substitution and also under pressure and confirm the onset of low lying spin excitations around P _c as previously reported. The spin gap in Sr ₂ Ca ₁₂ Cu ₂₄ O ₄₁ is strongly reduced above 20 kbar. However, the data of an experiment performed at P = 36 kbar where superconductivity has been detected at 6.7 K by an inductive technique have shown that a significant amount of spin excitations remains gapped at 80 K when superconductivity sets in. The standard relaxation model with two and three-magnon modes explains fairly well the activated relaxation data in the intermediate temperature regime corresponding to gapped spin excitations using the spin gap data derived from Knight shift experiments. The data of Gaussian relaxation rates of heavily doped samples support the limitation of the coherence length at low temperature by the average distance between doped holes. We discuss the interplay between superconductivity and the spin gap and suggest that these new results support the exciting prospect of superconductivity induced by the interladder tunneling of preformed pairs as long as the pressure remains lower than the pressure corresponding to the maximum of the superconducting critical temperature. Received 8 March 2001 and Received in final form 27 July 2001     

An NMR approach to the superconducting regime of the spin ladder compound Sr2Ca12Cu24O41

⁶³Cu-NMR experiments of Knight shift and relaxation time T₁ have been performed on the two-leg spin ladders of a Sr₂Ca₁₂Cu₂₄O₄₁ single crystal at several pressures up to the critical pressure for the stabilization of a superconducting ground state. The data confirm the onset of low-lying spin excitations at observed previously [Science 279, 345 (1998)] and reveal a marked decrease of the spin gap under pressures above 20 kbar although a significant fraction of the spin excitations remains gapped at kbar. A comparison between NMR and transport data under pressure suggests that the depression of the spin gap can be ascribed to an increase in the interladder exchange coupling, possibly mediated by the ladder-chain interaction along the b-direction. Received 21 October 1999     

Creation and destruction of a spin gap in weakly coupled quarter-filled ladders

We investigate weakly coupled quarter-filled ladders with model parameters relevant for NaV(2)O(5) using density-matrix renormalization group calculations on an extended Hubbard model coupled to the lattice. NaV(2)O(5) exhibits super-antiferroelectric charge order with a zigzag pattern on each ladder. We show that this order causes a spin dimerization along the ladder and is accompanied by a spin gap of the same magnitude as that observed experimentally. The spin gap is destroyed again at large charge order due to a restructuring of the spins. An analysis of an effective spin model predicts a recreation of the gap by interladder singlets when the charge order increases further.     

Spin reorientation and magnon-phonon hybridization in a ferromagnet

A model is presented for the magnetic excitations and magnon-phonon coupling in a localised moment ferromagnet in which spins can reorientate by application of a magnetic field. The model is suitable for those materials which possess a spin wave gap at zero wave vector and therefore the magnon and acoustic phonon branches can intersect. A magnon-phonon coupling linear in both spin and phonon operators is employed which has proved successful for the ferrous salts. The main effect of the applied field is to modify the spin wave gap, and to introduce a critical value for the coupling constant which enables the system to remain stable as the gap goes to zero. Furthermore the wave vector of the anticrossing point decreases as the spin wave gap increases and therefore the value of the sound velocity determined by high resolution inelastic neutron experiments is dependent on the gap.     

Quantum spin dynamics of the bilayer ferromagnet La Sr Mn O

We construct a theory of spin wave excitations in the bilayer manganite La_1.2Sr_1.8Mn₂O₇ based on the simplest possible double-exchange model, but including leading quantum corrections to the spin wave dispersion and damping. Comparison is made with recent inelastic neutron scattering experiments. We find that quantum effects account for some part of the measured damping of spin waves, but cannot by themselves explain the observed softening of spin waves at the zone boundary. Furthermore a doping dependence of the total spin wave dispersion and the optical spin wave gap is predicted. Received 15 January 2002 Published online 6 June 2002     

Magnetic excitations in charge ordered a' - {N_a}{V_2}{O_5}

An investigation of the spin excitation spectrum of charge ordered (CO) NaV₂O₅ is presented. We discuss several different exchange models which may be relevant for this compound, namely in-line and zig-zag chain models with weak as well as strong inter-chain coupling and also a ladder model and a CO/MV (mixed valent) model. We put special emphasis on the importance of large additional exchange across the diagonals of V-ladders and the presence of exchange anisotropies on the excitation spectrum. It is shown that the observed splitting of transverse dispersion branches may both be interpreted as anisotropy effect as well as acoustic-optic mode splitting in the weakly coupled chain models. In addition we calculate the field dependence of excitation modes in these models. Furthermore we show that for strong inter-chain coupling, as suggested by recent LDA + U results, an additional high energy optical excitation appears and the spin gap is determined by anisotropies. The most promising CO/MV model predicts a spin wave dispersion perpendicular to the chains which agrees very well with recent results obtained by inelastic neutron scattering. Received 30 April 1999 and Received in final form 5 October 1999     

Interchain impurity substitution effects on the magnetoelastic coupling in CuGeO3

We report the polarized far-infrared transmittance of Si-doped CuGeO₃ single crystals as a function of impurity concentration and applied magnetic field at low temperature. We use the behavior of the 44 cm⁻¹ spin gap excitation and the 98 cm⁻¹ zone-folding mode structure to investigate the interaction between the magnetic system and the lattice distortion. We find that interchain impurity substitution collapses the spin gap before it suppresses the lattice dimerization, a result that is understood in terms of the relative length scales for lattice distortion and spin singlet formation as well as criteria for magnetoelastic coupling in chains.     

Response of spin-accumulated ferromagnet/superconductor/ferromagnet double tunnel junction to applied magnetic field

Tunneling current in a ferromagnet/superconductor/ferromagnet double tunnel junction induces a nonequilibrium spin accumulation in the superconductor. We study theoretically the response of such a system to applied magnetic field. We show that the interplay between the magnetic field and the spin accumulation could lead to novel bias voltage dependence and magnetic field dependence of the superconducting gap function, and bring in anomalous asymmetry in the spin-dependent transport. Our study also indicates a possible application of the spin injection.     

Thermodynamics and optical conductivity of unconventional spin density waves

We consider the possibility of formation of an unconventional spin density wave (USDW) in quasi-one-dimensional electronic systems. In analogy with unconventional superconductivity, we develop a mean field theory of SDW allowing for the momentum dependent gap Δ() on the Fermi surface. Conditions for the appearance of such a low temperature phase are investigated. The excitation spectrum and basic thermodynamic properties of the model are found to be very similar to those of d-wave superconductors in spite of the different topology of their Fermi surfaces. Several correlation functions are calculated, and the frequency dependent conductivity is evaluated for various gap functions. The latter is found to reflect the maximum gap value, however with no sharp onset for absorbtion. Received 19 February 2001     

Density-dependent spin polarization in ultra-low-disorder quantum wires

There is controversy as to whether a one-dimensional (1D) electron gas can spin polarize in the absence of a magnetic field. Together with a simple model, we present conductance measurements on ultra-low-disorder quantum wires supportive of a spin polarization at B=0. A spin energy gap is indicated by the presence of a feature in the range (0.5-0.7)x2e(2)/h in conductance data. Importantly, it appears that the spin gap is not constant but a function of the electron density. Data obtained using a bias spectroscopy technique are consistent with the spin gap widening further as the Fermi level is increased.     

A new semiconducting organic-inorganic nanocomposite, 1,5-diaminonphthalene-saponite intercalation compound

We synthesized a new intercalation compound, 1,5-diaminonaphathalene(DAN)-saponite where intercalated DAN molecules were shown to have a formal charge of +0.67 The measurement of optical diffuse reflectance spectra revealed the formation of electronic bands with a gap of ca. 1 eV suggesting semiconducting behaviour of this system. From ESR measurements, the radical formation in DAN-saponite was confirmed and the spin concentration was determined to be 1 spin per 200 and 300 DAN-molecules at 290 and 7.9 K, respectively. This temperature dependence of the spin density also implies the semiconductive nature of DAN-saponite.     

Non-equivalence between Heisenberg XXZ spin chain and Thirring model

The Bethe ansatz equations for the spin 1/2 Heisenberg XXZ spin chain are numerically solved, and the energy eigenvalues are determined for the antiferromagnetic case. We examine the relation between the XXZ spin chain and the massless Thirring model, and show that the spectrum of the XXZ spin chain has a gapless excitation while the regularized Thirring model calculated with the Bogoliubov transformation method has a finite gap. This finite gap spectrum is also confirmed by the Bethe ansatz solution of the massless Thirring model.Received: 28 October 2004, Published online: 21 January 2005PACS: 10.Kk, 03.70. + k, 11.30.-j, 11.30.Rd     

First excitations of the spin 1/2 Heisenberg antiferromagnet on the kagomé lattice

We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets. Received: 27 November 1997 / Accepted: 29 January 1998     

On the origin of biquadratic exchange in spin 1 chains

One-dimensional spin 1 systems may have a rich phase diagram including Haldane gap and dimerized phases if the usually very small biquadratic exchange becomes significant. We show that this unlikely condition may be fulfilled in electron systems with quasi-degenerate orbitals. This mechanism may have been experimentally realized in the spin 1 chain LiVGe₂O₆. The implications for the exploration of the physics and quantum chemistry of spin 1 chains are discussed. Received 4 January 2000     

Dynamics of the anisotropic two-dimensional XY model

In this paper we study the dynamics of the two-dimensional XY model with single-ion anisotropy, and spin S = 1, in the large D phase, and low temperatures, using the bond operator formalism. The in-plane structure factor is a delta function. The out of plane shows a three peak structure, which merges in a single peak at the Brillouin zone boundary. We analyze also spin currents generated by a magnetic field gradient. The spin conductivity is calculated, at finite temperature, using the Kubo formula. The model shows unconventional ballistic spin transport at finite temperature. The computed spin conductivity exhibits a nonzero Drude weight at finite temperature. For ω< 2m, where m is the energy gap, the spin conductivity is described solely by the Drude weight. There is a regular contribution to the spin conductivity for ω> 2m, which persist in the zero temperature limit. The conductivity at the critical point, and for small frequencies, is (gμ_B)²/ħ times a universal scaling function of ħω/k_B T.