Interplay Of Charge Research Articles

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.

Cross-plane thermoelectric transport in p-type La0.67Sr0.33MnO3/LaMnO3 oxide metal/semiconductor superlattices

The cross-plane thermoelectric transport properties of La0.67Sr0.33MnO3 (LSMO)/LaMnO3 (LMO) oxide metal/semiconductor superlattices were investigated. The LSMO and LMO thin-film depositions were performed using pulsed laser deposition to achieve low resistivity constituent materials for LSMO/LMO superlattice heterostructures on (100)-strontium titanate substrates. X-ray diffraction and high-resolution reciprocal space mapping indicate that the superlattices are epitaxial and pseudomorphic. Cross-plane devices were fabricated by etching cylindrical pillar structures in superlattices using inductively, this coupled-plasma reactive-ion etching. The cross-plane electrical conductivity data for LSMO/LMO superlattices reveal a lowering of the effective barrier height to 223 meV as well as an increase in cross-plane conductivity by an order of magnitude compared to high resistivity superlattices. These results suggest that controlling the oxygen deficiency in the constituent materials enables modification of the effective barrier height and increases the cross-plane conductivity in oxide superlattices. The cross-plane LSMO/LMO superlattices showed a giant Seebeck coefficient of 2560 μV/K at 300 K that increases to 16 640 μV/K at 360 K. The giant increase in the Seebeck coefficient with temperature may include a collective contribution from the interplay of charge, spin current, and phonon drag. The low resistance oxide superlattices exhibited a room temperature cross-plane thermal conductivity of 0.92 W/m K, this indicating that the suppression of thermal conductivities due to the interfaces is preserved in both low and high resistivity superlattices. The high Seebeck coefficient, the order of magnitude improvement in cross-plane conductivity, and the low thermal conductivity in LSMO/LMO superlattices resulted in a two order of magnitude increase in cross-plane power factor and thermoelectric figure of merit (ZT), compared to the properties of superlattices with higher resistivity that were reported previously. The temperature dependence of the cross-plane power factor in low resistance superlattices suggests a direction for further investigations of the potential LSMO/LMO oxide superlattices for thermoelectric devices.

Interplay of charge and heat transport in a nano-junction in the out-of-equilibrium cotunneling regime

We study the charge transport and heat transfer through a nano-junction composed of a small metallic grain weakly coupled to two metallic leads. We focus on the cotunneling regime out-of-equilibrium, where the bias voltage and the temperature gradient between the leads strongly drive electron and phonon degrees of freedom in the grain, which in turn have a strong feedback on the transport through the grain. We derive and solve the heat balance equation for electron and phonon degrees of freedom in the grain and self-consistently find the current–voltage characteristics. We demonstrate that the transport in the nano-junction is very sensitive to the spectrum of the bosonic modes in the grain.

Inelastic X-ray Scattering Studies of Electronic Excitations

Inelastic x-ray scattering (IXS) has developed into one of the most powerful momentum-resolved spectroscopies. Especially in the last decade, it has achieved significant progress utilizing brilliant x-rays from third-generation synchrotron radiation facilities. Simultaneously, theoretical efforts have been made to predict or interpret the experimental spectra. One of the scientific fields studied intensively by IXS is strongly correlated electron systems, where the interplay of charge, spin, and orbital degrees of freedom determines their physical properties. IXS can provide a new insight into the electron dynamics of the systems through the observation of charge, spin, and orbital excitations. Focusing on the momentum-resolved electronic excitations in strongly correlated electron systems, we review IXS studies and the present capabilities of IXS for the study of the dynamics of materials. With nonresonant inelastic x-ray scattering (NIXS), one can directly obtain dynamical charge correlation and we disc...

Interplay of charge and spin in quantum dots: The Ising case

The physics of quantum dots is depicted succinctly by the universal Hamiltonian, where only zero-mode interactions are included. In the case in which the latter involve charging and isotropic spin-exchange terms, this would lead to a non-Abelian action. Here we address an Ising spin-exchange interaction, which leads to an Abelian action. The analysis of this simplified yet nontrivial model shed light on a more general case of charge and spin entanglement. We present a calculation of the tunneling density of states and dynamic magnetic susceptibility. We explain how the latter can be used for an experimental determination of the exchange interaction strength.

Influence of asymmetry on bias behavior of spin torque

We report calculations, based on the tight-binding model and the nonequilibrium Keldysh formalism, of the effect of band-filling (BF) asymmetry between the ferromagnetic leads on the bias behavior of the spin torque and the tunneling magnetoresistance (TMR) in magnetic tunnel junctions. The underlying mechanism for the asymmetry-induced change in the bias dependence of TMR and the spin-transfer component, ${T}_{\ensuremath{\parallel}}$, is the interplay of charge and spin currents in the ferromagnetic (FM) and antiferromagnetic (AF) configurations. The BF asymmetry has a dramatic effect on the low-bias behavior of the fieldlike component, ${T}_{\ensuremath{\perp}}$, which can vary from linear to quadratic bias dependence with positive or negative curvature, thus reconciling the apparently contradictory experimental results. A general expression is derived relating ${T}_{\ensuremath{\perp}}$ with four independent nonequilibrium interlayer exchange couplings (NEIECs), ${J}_{\text{FM}}^{\ensuremath{\sigma}{\ensuremath{\sigma}}^{\ensuremath{'}}}$, associated with the majority- and minority-spin channels, $\ensuremath{\sigma},{\ensuremath{\sigma}}^{\ensuremath{'}}=\ensuremath{\uparrow},\ensuremath{\downarrow}$, of the two leads solely in the FM configuration. The bias behavior of the NEIEC components can be selectively tuned with the BF of the free and pinned FM layers, thus opening a new avenue for controlling experimentally ${T}_{\ensuremath{\perp}}$.

THERMAL HYSTERESIS IN RESISTIVITY AND MAGNETIZATION OF PrCa(Sr)MnO

Thermal hysteresis in resistivity and magnetization of Pr 0.65 Ca 0.35-x Sr x MnO 3(x = 0–0.35) manganites has been systematically studied to establish the interplay of their charge and spin-ordering. The increasing Sr concentration (x) transforms the charge-ordered (CO)/antiferromagnetic (AFM) insulating system (for x = 0) into a mixed-phased one (for x = 0.1) with sharp metal–insulator (MI) transition and finally leads to a ferromagnetic (FM) metallic (for x = 0.35) system. It has been found that the interplay of charge and spin-ordering is higher in mixed-phased state and the thermal hysteresis loop area is bigger. It increases with the increase of sharpness of MI transition. Interplay of charge and spin-ordering decreases with the increase of either CO/AFM insulating or FM metallic phase and is almost absent in completely CO/AFM insulating (with x = 0) or FM metallic (with x = 0.35) states bringing about zero thermal hysteresis loop.

Cuprate-Manganite Interfaces-What Controls the Properties of FM/SC Oxide Superlattices?

This is a copy of the slides presented at the meeting but not formally written up for the volume. The physical properties of complex oxides are characterized by the interplay of charge-,spin-,orbital and lattice degrees of freedom. Combining oxides with different spin-,charge- or orbital order opens generate an additional degree of freedom that can give rise to novel physical phenomena. We have studied the interface of the system La0.67.Ca.33MnO3 and YBa2Cu3O7 prepared by conventional PLD as an example representing the combination of ferromagnetic and superconducting oxides by a variety of techniques including neutron diffraction and XMCD analysis. It turns out that at the interface interaction effects are taking place at two different length scales. One is based on the self ? injection of spin-polarized quasiparticles with a length scale of ~ 10nm, the other is a short range exchange interaction with a length scale of ~ 3 nm. The short range interaction gives rise to an antiferromagnetic coupling of the Mn and Cu spins across the interface. The results are discussed in the frame of orbital interactions across the interface. Furthermore, it could be shown that in FM/SC/FM trilayer systems a so far unknown coupling of adjacent magnetic layers occurs when the superlattices are cooled through the superconducting transition temperature. It is well established that two ferromagnetic layers are coupled through an intervening metallic layer by the Ruddermann-Kittel-Kasuya-Yoshida (RKKY) mechanism. The sign of the coupling oscillates from positive to negative with the thickness of the metallic layer. On the other hand, the interaction between ferromagnetic (FM) and superconducting (SC) layers in a FM/SC/FM heterostructure is not well understood. Tentatively, we propose a coupling mechanism mediated by London screening currents to explain our observations.

Introduction of Localized Spins to Strongly Correlated One-Dimensional Electronic System at Quarter-Filling

We discuss an interesting interplay of charge and spin degrees of freedom under the combination of the localized spins and the quarter-filled one-dimensional electronic system. The electronic system has on-site and intersite Coulomb interactions, U and V, respectively, which cooperate with the Kondo coupling, J, leading to a rich phase diagram with charge ordering and the ferromagnetism. The origin of the ferromagnetism is being discussed in comparison with the double exchange mechanism.

Ultrafast four-wave mixing in strongly correlated electron systems

We theoretically investigate the dynamics of photoexcited states in strongly correlated low-dimensional electron systems by analyzing the transient four-wave mixing (TFWM). We adopt the effective Hamiltonian of the Hubbard model for the photoexcited states in the strong correlation case, and TFWM intensity is obtained by numerically calculating the time development of the photoexcited state. We find that TFWM intensity sensitively reflects the relaxation due to the diffusion of the photogenerated charges and the relaxation due to the spin structure rearrangement to the diffused charge distribution. This enables us to clarify the interplay of charge and spin degrees of freedom. In the two-dimensional case, a well-separated two-step relaxation is observed; the slower spin relaxation follows the faster charge relaxation as a result of the coupling between the spin and charge degrees of freedom. This shows that the spin-charge separation approximately holds and the coupling between the spin and charge degrees of freedom is weak. In the one-dimensional case, only the charge relaxation is observed. Furthermore, there is no correlation between the relaxations before and after the second pulse. These are characteristic behaviors inherent in the spin-charge separation in strongly correlated electron systems.

Electronic transport through quantum dots coupled to ferromagnetic leads: from Coulomb blockade to Kondo effect

Recent experiments on magnetic nanostructured materials revealed new phenomena related to the interplay of ferromagnetism and discrete charging effects. Typical example of such systems is a double ferromagnetic tunnel junction with a quantum dot as the central electrode . Interplay of charge and spin degrees of freedom in such systems is analyzed theoretically in the sequential tunneling, cotunneling and strong coupling regimes.

Phase separation in manganites induced by orbital-ordering strains.

Doped manganite perovskites AMnO(3) exhibit a rich variety of electronic properties, resulting from the interplay of charge (Mn(3+)/Mn(4+)), spin (Mn magnetic moment) and orbital (Mn(3+) Jahn-Teller distortion) degrees of freedom. Magnetisation measurements and ESR spectra have been used to study a series of eight AMnO(3) perovskites, in which the A cation sites are occupied by a distribution of 70% trivalent lanthanide and 30% divalent Ca, Sr or Ba ions. These all have a mean A cation radius of 1.20 Angstrom but different values of the cation size variance sigma(2). A change from orbital disorder to order (cooperative Jahn-Teller distortions) was previously found in the insulating regime at sigma(2) = approximately 0.005 Angstrom(2). This work has shown that co-existence of the orbitally ordered and disordered phases is found in sigma(2)= 0.0016-0.0040 Angstrom(2) samples, with a difference of 40 K between their Curie temperatures. This is ascribed to competition between orbital ordering and microstructural lattice strains. At larger sigma(2) > 0.005 Angstrom(2), the orbital ordering strains are dominant and only this phase is observed. This intermediate temperature phase segregation is one of many strain-driven separation phenomena in manganites.

Interplay of charge, spin, orbital and lattice correlations in colossal magnetoresistance manganites

We derive a realistic microscopic model for doped colossal magnetoresistance manganites, which includes the dynamics of charge, spin, orbital and lattice degrees of freedom on a quantum mechanical level. The model respects the SU(2) spin symmetry and the full multiplet structure of the manganese ions within the cubic lattice. Concentrating on the hole doped domain ($0\le x\le 0.5$) we study the influence of the electron-lattice interaction on spin and orbital correlations by means of exact diagonalisation techniques. We find that the lattice can cause a considerable suppression of the coupling between spin and orbital degrees of freedom and show how changes in the magnetic correlations are reflected in dynamic phonon correlations. In addition, our calculation gives detailed insights into orbital correlations and demonstrates the possibility of complex orbital states.

Metal substitution in transferrins: specific binding of cerium(IV) revealed by the crystal structure of cerium-substituted human lactoferrin

Proteins of the transferrin family play a key role in iron homeostasis through their extremely strong binding of iron, as Fe3+. They are nevertheless able to bind a surprisingly wide variety of other metal ions. To investigate how metal ions of different size, charge and coordination characteristics are accommodated, we have determined the crystal structure of human lactoferrin (Lf) complexed with Ce4+. The structure, refined at 2.2 A resolution (R=20.2%, Rfree=25.7%) shows that the two Ce4+ ions occupy essentially the same positions as do Fe3+, and that the overall protein structure is unchanged; the same closed structure is formed for Ce2Lf as for Fe2Lf. The larger metal ion is accommodated by small shifts in the protein ligands, made possible by the presence of water molecules adjacent to each binding site. The two Ce4+ sites are equally occupied, indicating that the known difference in the pH-dependent release of Ce4+ arises from a specific protonation event, possibly of the His ligand in one of the binding sites. Comparing the effects of binding Ce4+ with those for the binding of other metal ions, we conclude that the ability of transferrins to accommodate metal ions other than Fe3+ depends on an interplay of charge, size, coordination and geometrical preferences of the bound metal ion. However, it is the ability to accept the six-coordinate, approximately octahedral, site provided by the protein that is of greatest importance.

Roles of orbital in magnetoelectronic properties of colossal magnetoresistive manganites

Extensive studies on the colossal magnetoresistive (CMR) manganites have revealed important interplay of charge, spin, orbital, and lattice degrees of freedom not only in CMR characteristics but also in anomalous charge and spin dynamics. Here, we overview our latest experimental investigations of the CMR manganites in light of the important role of the orbital degree of freedom.

Fermi surface and gap parameter in high- Tc superconductors: the Stripe Quantum Critical Point scenario

We study the single-particle spectral properties of electrons coupled to quasicritical charge and spin fluctuations close to a stripe-phase, which is governed by a Quantum Critical Point near optimum doping. We find that spectral weight is transferred from the quasiparticle peak to incoherent dispersive features. As a consequence, the distribution of low-laying spectral weight is modified with respect to the quasiparticle Fermi surface. The interplay of charge and spin fluctuations reproduces features of the observed Fermi surface, such as the asymmetric suppression of spectral weight near the M points of the Brillouin zone. Within the model, we also analyze the interplay between repulsive spin and attractive charge fluctuations in determining the symmetry and the peculiar momentum dependence of the superconducting gap parameter. When both spin and charge fluctuations are coupled to the electrons, we find d x 2− y 2 -wave gap symmetry in a wide range of parameter. A crossover d- vs. s-wave symmetry of the gap may occur when the strength of charge fluctuations increases with respect to spin fluctuations.

Interplay of charge, orbital and magnetic order in Pr1−xCaxMnO3

ABSTRACTWe report resonant x-ray scattering studies of charge and orbital order in Prl−xCaxMnO3 with x=0.4 and 0.5. Below the ordering temperature, To=245 K, the charge and orbital order intensities follow the same temperature dependence, including an increase at the antiferromagnetic ordering temperature, TN. High resolution measurements reveal, however, that long range orbital order is never achieved. Rather, an orbital domain state is formed. Above To, the charge order fluctuations are more highly correlated than the orbital fluctuations. We conclude that the charge order drives the orbital order at the transition.

Interplay of charge and orbital ordering in manganese perovskites

A model of localized classical electrons coupled to lattice degrees of freedom and, via the Coulomb interaction, to each other, has been studied to gain insight into the charge and orbital ordering observed in lightly doped manganese perovskites. Expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. The expressions are analyzed for several hole configurations, including that experimentally observed by Yamada et al. in La_{7/8}Sr_{1/8}MnO_3. We find that, although the preferred charge and orbital ordering depend sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are on the order of the observed charge ordering transition temperature. The effects of additional strains are also studied. Some results for La_{1/2}Ca_{1/2}MnO_3 are presented, although our model may not adequately describe this material because the high temperature phase is metallic.

Interplay of charge and spin correlations in nickel perovskites

Analyzing the motion of low-spin (s=1/2) holes in a high-spin (S=1) background, we derive a sort of generalized t-J Hamiltonian for the NiO2 planes of Sr-doped nickelates. In addition to the rather complex carrier-spin and spin-spin couplings we take into account the coupling of the doped holes to in-plane oxygen breathing modes by a Holstein-type interaction term. Because of strong magnetic confinement effects the holes are nearly entirely prelocalized and the electron-phonon coupling becomes much more effective in forming polarons than in the isostructural cuprates. In the light of recent experiments on La2−xSrxNiO4 we discuss how the variety of the observed transport and charge/spin-ordering phenomena can be qualitatively understood in terms of our model Hamiltonian.

Magnetic field dependence of the current-voltage curve of a superconducting single electron transistor in a high impedance environment

A superconducting single electron transistor has been investigated in a regime where its behavior is governed by the interplay of charging and Josephson effects with the corresponding energies Ec and EJ, respectively. Using a SQUID geometry, we were able to change the ratio of EJ/Ec (from EJ/Ec≈3.5 to EJ/Ec≈0.44). In zero magnetic field, we observed a sharp Coulomb blockade at low current followed by a negative slope of the I–V curve at higher current. Applying an external magnetic field perpendicular to the substrate and thus decreasing EJ/Ec we observed an increase of the blockade voltage from 10 μV up to 45 μV and a decrease of the crossover current from 5 nA down to 0.5 nA. Such a behavior of the transistor can be explained using a Bloch band picture.