Strings in strongly correlated electron systems

It is shown that strongly correlated electrons on frustrated lattices like pyrochlore, checkerboard or kagomè lattice can lead to the appearance of closed and open strings. They are resulting from nonlocal subsidiary conditions which propagating strongly correlated electrons require. The dynamics of the strings is discussed and a number of their properties are pointed out. Some of them are reminiscent of particle physics.     

Superconductivity in weakly correlated electron systems

We study the influence of the short-ranged Hubbard correlation U between the conduction electrons on the Cooper pair formation in normal (s-wave) superconductors. The Coulomb correlation is considered within the standard second order perturbation theory, which becomes exact in the weak coupling limit but goes beyond the simple Hartree-Fock treatment by yielding a finite lifetime of the quasiparticles at finite temperature. An attractive pairing interaction V, which may be mediated by the standard electron-phonon mechanism, is considered between nearest neighbor sites. A critical value for the attractive interaction is required to obtain a superconducting state. For finite temperature a gapless superconductivity is obtained due to the finite lifetime of the quasiparticles, i.e. the Coulomb correlation has a pair-breaking influence. The energy gap and depend very sensitively on U, V and band filling n and develop a maximum away from half filling as function of n. The ratio varies with n, being higher than the BCS value near half filling and reaching the BCS value for lower n. Received 17 February 1999     

Nonlinear electrostatic response of strongly correlated one-dimensional electron systems

We investigate the nonlinear response of the strongly correlated one-dimensional electron systems to the static electric field with using the one-dimensional Hubbard model in the half-filled case. We adopt the variational Monte Carlo method with the Gutzwiller wave function to describe the strong correlation effects. In the weak correlation region U/t≤4, where U is the one-site Coulomb repulsion energy and t is the transfer integral between the nearest neighbor sites, the response can be described within the band picture, and the third order nonlinear susceptibility χ⁽³⁾ increases slowly with increasing U/t. For U/t≤4, χ⁽³⁾ increases rapidly with increasing U/t, and χ⁽³⁾ at U/t=10 is more than ten times larger than that at U/t=2. This large value of χ⁽³⁾ originates from the exotic properties of carriers in the strongly correlated one-dimensional electron systems.     

Odd-frequency superconductivity on the Hubbard model using the FLEX approximation

It is an important issue to clarify whether the odd-frequency superconducting state can be derived from microscopic Hamiltonian or not, where gap function has an odd-parity in frequency. We study the instability of following four superconducting states: (1) even-frequency spin-singlet, (2) even-frequency spin-triplet, (3) odd-frequency spin-singlet and (4) odd-frequency spin-triplet. By using the fluctuation exchange (FLEX) approximation on a triangular and square lattice, we find that the odd-frequency spin-triplet pairing can become dominant at a certain region where the suppression of the antiferromagnetic fluctuation due to a geometric frustration becomes prominent.     

Effective action for strongly correlated electron systems

The su(2|1) coherent-state path-integral representation of the partition function of the t-J model of strongly correlated electrons is derived at finite doping. The emergent effective action is compared to the one proposed earlier on phenomenological grounds by Shankar to describe holes in an antiferromagnet [R. Shankar, Nucl. Phys. B 330 (1990) 433]. The t-J model effective action is found to have an important “extra” factor with no analogue in Shankar?s action. It represents the local constraint of no double electron occupancy and reflects the rearrangement of the underlying phase-space manifold due to the presence of strong electron correlation. This important ingredient is shown to be essential to describe the physics of strongly correlated electron systems.     

Reproducible resistive switching effect for memory applications in heterocontacts based on strongly correlated electron systems

The transitional processes in heterocontacts based on strongly correlated electron systems (SCES) are studied for analyzing of the effect of resistive switching (ERS). It has been shown that the process is asymmetric with respect to switching into “on” and “off” states, the switching time is controlled by a voltage level, this time can be less than microseconds, on the other hand, relaxation processes can reach tens seconds. The switching is controlled by two processes: a change in the resistance state of the normal metal/SCES interface under effect of electric current field and by electrodiffusion of oxygen to vacancies, at that the doping level of the contact area and resistive properties of the heterocontact change. In particular, electrodiffusion of mobile oxygen induced by the electric field makes it possible to use a device with ERS as a memristor. On the other hand, a possibility to control the switching time and ON and OFF parameters show the possibilities to use these devices as memory elements “RAM”.     

Spin-triplet superconductivity induced by off-site Coulomb interaction in a triangular lattice

In order to study how superconductivity emerges when the charge fluctuation coexists with the spin fluctuation in a triangular lattice, we obtain a phase diagram against the nearest-neighbor Coulomb repulsion V and band filling n on the extended Hubbard model using the fluctuation exchange (FLEX) approximation. We find that a charge density wave (CDW) phase exists in the region where the value of V is large, and the f-wave spin-triplet pairing mediated by a strong charge fluctuation is dominant near the CDW phase.     

Remote state preparation with classically correlated state

We experimentally demonstrate the first remote state preparation with no shared entanglement, employing classically correlated state (CCS). CCS is verified with quantum state tomography process, and the fidelity is 0.99. The states chosen from a special diameter of the Poincaré sphere can be remotely prepared with unit efficiency at the cost of one cbit, if one classically correlated state is shared. The scheme can remotely prepare the other states on the Poincaré sphere with other CCS using the same experimental setup. The efficiency is 50% in general.     

Non-perturbative approaches to magnetism in strongly correlated electron systems

The microscopic basis for the stability of itinerant ferromagnetism in correlated electron systems is examined. To this end several routes to ferromagnetism are explored, using both rigorous methods valid in arbitrary spatial dimensions, as well as Quantum Monte Carlo investigations in the limit of infinite dimensions (dynamical mean-field theory). In particular we discuss the qualitative and quantitative importance of (i) the direct Heisenberg exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a high spectral density near the band edges caused by an appropriate lattice structure and/or kinetic energy of the electrons. We furnish evidence of the stability of itinerant ferromagnetism in the pure Hubbard model for appropriate lattices at electronic densities not too close to half-filling and large enough U. Already a weak direct exchange interaction, as well as band degeneracy, is found to reduce the critical value of U above which ferromagnetism becomes stable considerably. Using similar numerical techniques the Hubbard model with an easy axis is studied to explain metamagnetism in strongly anisotropic antiferromagnets from a unifying microscopic point of view.     

Spin transition in strongly correlated bilayer two-dimensional electron systems

Using a combination of heat pulse and nuclear magnetic resonance techniques, we demonstrate that the phase boundary separating the interlayer phase coherent quantum Hall effect at nu(T) = 1 in bilayer electron gases from the weakly coupled compressible phase depends upon the spin polarization of the nuclei in the host semiconductor crystal. Our results strongly suggest that, contrary to the usual assumption, the transition is attended by a change in the electronic spin polarization.     

Dynamics of photoexcited states in strongly correlated electron systems

We theoretically investigate the dynamics of the photoexcited state in the strongly correlated low-dimensional electron systems. In the two-dimensional case, the ultrafast relaxation originating from the transfer of photogenerated charges in the antiferromagnetic background is followed by the slower one originating from the spin structure rearrangement with the new charge distributions. This clearly shows the difference in the relaxation between charge and spin degrees of freedom. In the one-dimensional case, spin-charge separation holds and the mechanical coherence is preserved.     

Spin-incoherent behavior in the ground state of strongly correlated systems

It is commonly believed that strongly interacting one-dimensional Fermi systems with gapless excitations are effectively described by Luttinger liquid theory. However, when the temperature of the system is high compared to the spin energy, but small compared to the charge energy, the system becomes "spin incoherent." We present numerical evidence showing that the one-dimensional "t-J-Kondo" lattice, consisting of a t-J chain interacting with localized spins, displays all the characteristic signatures of spin-incoherent physics, but in the ground state. We argue that similar physics may be present in a wide range of strongly interacting systems.     

Quasiparticle properties of strongly correlated electron systems with itinerant metamagnetic behavior

A brief account of the zero temperature magnetic response of a system of strongly correlated electrons in strong magnetic field is given in terms of its quasiparticle properties. The scenario is based on the paramagnetic phase of the half-filled Hubbard model, and the calculations are carried out with the dynamical mean field theory (DMFT) together with the numerical renormalization group (NRG). As well known, in a certain parameter regime one finds a magnetic susceptibility which increases with the field strength. Here, we analyze this metamagnetic response based on Fermi liquid parameters, which can be calculated within the DMFT-NRG procedure. The results indicate that the metamagnetic response can be driven by field-induced effective mass enhancement. However, also the contribution due to quasiparticle interactions can play a significant role. We put our results in context with experimental studies of itinerant metamagnetic materials.     

Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF)2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF)₂X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2k_F spin and 2k_F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF)₂X compounds, a magnetic field induced singlet-triplet transition is consistent with above mechanism in (TMTSF)₂ClO₄.     

Non-BCS pairing in anisotropic strongly correlated electron systems in solids

The problem of pairing in anisotropic electronic systems possessing patches of fermion condensate in the vicinity of the Van Hove points is analyzed. Attention is directed to opportunities for the occurrence of non-BCS pairing correlations between the states belonging to the fermion condensate. It is shown that the physical emergence of such pairing correlations would drastically alter the behavior of the single-particle Green function, the canonical pole of Fermi-liquid theory being replaced by a branch point.     

Nonequilibrium photo dynamics of low-dimensional strongly correlated electron systems

One of the outstanding contemporary challenges in condensed matter physics is to understand the dynamics of interacting quantum systems exposed to an external perturbation. We theoretically examine nonequilibrium photo dynamics and its interplay of charge, spin, and lattice degrees of freedom on a Hubbard-Holstein chain in one dimension and a t-J-Holstein square lattice in two dimensions. In the chain, performing dynamical density-matrix renormalization group calculations, we find that many phonons generated dynamically after photo irradiation in Mott insulators cause initial relaxation process. On the other hand, in the square lattice with model parameters as relevant for cuprates, a Lanczos-type exact diagonalization calculation shows that the majority of absorbed energy flows into spin subsystem rather than phonon subsystem.     

Momentum dependence of quasiparticle spectrum and Bogoliubov angle in cuprate superconductors

The momentum dependence of the low energy quasiparticle spectrum and the related Bogoliubov angle in cuprate superconductors are studied within the kinetic energy driven superconducting mechanism. By calculation of the ratio of the low energy quasiparticle spectra at positive and negative energies, it is shown that the Bogoliubov angle increases monotonically across the Fermi crossing point. The results also show that the superconducting coherence of the low energy quasiparticle peak is well described by a simple d-wave Bardeen-Cooper-Schrieffer formalism, although the pairing mechanism is driven by the kinetic energy by exchanging spin excitations.