arXiv subjects

J. Keller

Publications and source records attributed to J. Keller.

At least 37 records · Page 2Linked to original sources

arXiv2002

Superlattice with hot electron injection: an approach to a Bloch oscillator

A semiconductor superlattice with hot electron injection into the miniband is considered. The injection changes the stationary distribution function and results in a qualitative change of the frequency behaviour of the differential conductivity. In the regime with Bloch oscillating electrons and injection into the upper part of the miniband the region of negative differential conductivity is shifted from low frequencies to higher frequencies. We find that the dc differential conductivity can be made positive and thus the domain instability can be suppressed. At the same time the high-frequency differential conductivity is negative above the Bloch frequency. This opens a new way to make a Bloch oscillator operating at THz frequencies.

cond-mat.mes-hall↗

arXiv2002

Charge-imbalance effects in intrinsic Josephson systems

We report on two types of experiments with intrinsic Josephson systems made from layered superconductors which show clear evidence of nonequilibrium effects: 1. In 2-point measurements of IV-curves in the presence of high- frequency radiation a shift of the voltage of Shapiro steps from the canonical value hf/(2e) has been observed. 2. In the IV-curves of double-mesa structures an influence of the current through one mesa on the voltage measured on the other mesa is detected. Both effects can be explained by charge-imbalance on the superconducting layers produced by the quasi-particle current, and can be described successfully by a recently developed theory of nonequilibrium effects in intrinsic Josephson systems.

cond-mat.supr-con↗

arXiv2001

Nonequilibrium effects due to charge fluctuations in intrinsic Josephson systems

Nonequilibrium effects in layered superconductors forming a stack of intrinsic Josephson junctions are investigated. We discuss two basic nonequilibrium effects caused by charge fluctuations on the superconducting layers: a) the shift of the chemical potential of the condensate and b) charge imbalance of quasi-particles, and study their influence on IV-curves and the position of Shapiro steps.

cond-mat.supr-con↗

arXiv2001

The spectral and magnetic properties of $α$- and $γ$-Ce from the Dynamical Mean-Field Theory and Local Density Approximation

We have calculated ground state properties and excitation spectra for Ce metal with the {\it ab initio} computational scheme combining local density approximation and dynamical mean-field theory (LDA+DMFT). We considered all electronic states, i.e. correlated f-states and non-correlated s-, p- and d-states. The strong local correlations (Coulomb interaction) among the f-states lead to typical many-body resonances in the partial f-density, such as lower and upper Hubbard band. Additionally the well known Kondo resonance is observed. The s-, p- and d-densities show small to mediate renormalization effects due to hybridization. We observe different Kondo temperatures for $α$- and $γ$-Ce ($T_{K,α}\approx 1000 K$ and $T_{K,γ}\approx 30 K$), due to strong volume dependence of the effective hybridization strength for the localized f-electrons. Finally we compare our results with a variety of experimental data, i.e. from photoemission spectroscopy (PES), inverse photoemission spectroscopy (BIS), resonant inverse photoemission spectroscopy (RIPES) and magnetic susceptibility measurements.

cond-mat.str-el↗

arXiv2000

Static Charge Coupling of Intrinsic Josephson Junction

A microscopic theory for the coupling of intrinsic Josephson oscillations due to charge fluctuations on the quasi two-dimensional superconducting layers is presented. Thereby in close analogy to the normal state the effect of the scalar potential on the transport current is taken into account consistently. The dispersion of collective modes is derived and an estimate of the coupling constant is given. It is shown that the correct treatment of the quasiparticle current is essential in order to get the correct position of Shapiro steps. In this case the influence of the coupling on dc-properties like the $I-V$-curve is negligible.

cond-mat.supr-con↗

arXiv2000

Phonons in intrinsic Josephson systems with parallel magnetic field

Subgap resonances in the I-V curves of layered superconductors are explained by the coupling between Josephson oscillations and phonons with dispersion in c-direction. In the presence of a magnetic field applied parallel to the layers additional structures due to fluxon motion appear. Their coupling with phonons is investigated theoretically and a shift of the phonon resonances in strong magnetic fields is predicted.

cond-mat.supr-con↗

arXiv2000

d-wave Superconductivity in the Hubbard Model

The superconducting instabilities of the doped repulsive 2D Hubbard model are studied in the intermediate to strong coupling regime with help of the Dynamical Cluster Approximation (DCA). To solve the effective cluster problem we employ an extended Non Crossing Approximation (NCA), which allows for a transition to the broken symmetry state. At sufficiently low temperatures we find stable d-wave solutions with off-diagonal long range order. The maximal $T_c\approx 150K$ occurs for a doping $δ\approx 20%$ and the doping dependence of the transition temperatures agrees well with the generic high-$T_c$ phase diagram.

cond-mat.supr-con↗

arXiv2000

Theory for the coupling between longitudinal phonons and intrinsic Josephson oscillations in layered superconductors

In this publication a microscopic theory for the coupling of intrinsic Josephson oscillations in layered superconductors with longitudinal c-axis-phonons is developed. It is shown that the influence of lattice vibrations on the c-axis transport can be fully described by introducing an effective longitudinal dielectric function. Resonances in the I-V-characteristic appear at van Hove singularities of both acoustical and optical longitudinal phonon branches. This provides a natural explanation of the recently discovered subgap structures in the I-V-characteristic of highly anisotropic cuprate superconductors. The effect of the phonon dispersion on the damping of these resonances and the coupling of Josephson oscillations in different resistive junctions due to phonons are discussed in detail.

cond-mat.supr-con↗

arXiv2000

Microscopic theory of the coupling of intrinsic Josephson oscillations and phonons

A microscopic theory for the coupling of intrinsic Josephson oscillations and dispersive phonon branches in layered superconductors is developed. Thereby the effect of phonons on the electronic c-axis transport enters through an effective longitudinal dielectric function. This coupling provides an explanation of recently observed subgap resonances in the $I_{dc}$-$V_{dc}$- curve of anisotropic cuprate superconductors forming a stack of short Josephson junctions. Due to the finite dispersion these resonances can appear at van-Hove-singularities of both optical and acoustical phonon branches, explaining low-voltage structures in the I-V-characteristic, which are not understood in phonon models without dispersion. In long junctions the dispersion of collective electron-phonon modes parallel to the layers is investigated.

cond-mat.supr-con↗

arXiv1999

LDA+DMFT study for $La_{1-x}Sr_{x}TiO_{3}$

The dynamical mean-field theory together with the non-crossing approximation is used to set up a novel scheme to study the electronic structure of strongly correlated electron systems. The non-interacting band structure is obtained from a density functional calculation within the local density approximation. With this method the doped Mott insulator $\rm La_{1-x}Sr_x Ti O_3$ is studied. Starting from first-principle calculations for a cubic and an orthorhombic system we determined the one-particle spectrum. Both one-particle spectra show a lower Hubbard band (seen as $d^1 \to d^o$ transitions in photo emission experiments), a Kondo resonance near the Fermi energy and the upper Hubbard band ($d^1\to d^2$ transitions in an inverse photoemission experiment). The upper Hubbard band develops a multipeak structure, a consequence of the consideration of all local two-particle correlations, which leads to the full multiplet structure in the atomic limit. The calculation for the orthorhombic system shows qualitative good agreement when compared with experimental photoemission spectra.

cond-mat.str-el↗

arXiv1999

A Non-Crossing Approximation for the Study of Intersite Correlations

We develop a Non-Crossing Approximation (NCA) for the effective cluster problem of the recently developed Dynamical Cluster Approximation (DCA). The DCA technique includes short-ranged correlations by mapping the lattice problem onto a self-consistently embedded periodic cluster of size $N_c$. It is a fully causal and systematic approximation to the full lattice problem, with corrections ${\cal{O}}(1/N_c)$ in two dimensions. The NCA we develop is a systematic approximation with corrections ${\cal{O}}(1/N_c^3)$. The method will be discussed in detail and results for the one-particle properties of the Hubbard model are shown. Near half filling, the spectra display pronounced features including a pseudogap and non-Fermi-liquid behavior due to short-ranged antiferromagnetic correlations.

cond-mat.str-el↗

arXiv1998

Coupling of intrinsic Josephson oscillations in layered superconductors by charge fluctuations

The coupling of Josephson oscillations in layered superconductors is studied with help of a tunneling Hamiltonian formalism. The general form of the current density across the barriers between the superconducting layers is derived. The induced charge fluctuations on the superconducting layers lead to a coupling of the Josephson oscillations in different junctions. A simplified set of equations is then used to study the non-linear dynamics of the system. In particular the influence of the coupling on the current-voltage characteristics is investigated and upper limits for the coupling strength are estimated from a comparison with experiments on cuprate superconductors.

cond-mat.supr-con↗

arXiv1998

Electronic properties of the three-band Hubbard model

We study the electronic band-structure and transport properties of a CuO2-plane within the three-band Hubbard model. The Dynamical Mean-Field Theory (DMFT) is used to solve the many particle problem. The calculations show that the optical gap Delta_opt is given by excitations from the lower Hubbard band into the so called Zhang-Rice singlet band. The optical gap Delta_opt turns out to be considerably smaller than the charge transfer energy Delta (Delta=ep-ed) for a typical set of parameters, which is in agreement with experiment. For the two-dimensional CuO2-plane we investigated the dependency of the shape of the Fermi surface on the different hopping parameters t_CuO and t_OO. A value t_OO/t_CuO >0$ leads to a Fermi surface surrounding the M point. An additional different static shift of the oxygen energies is also considered to calculate the electronic response due to a displacement of the oxygen atoms given by a frozen phonon. The density-density correlation for the oxygen orbitals is linear in doping for both hole and electron doping but shows a different temperature dependency in the two regimes. In the first case it is temperature independent and increases upon doping, which leads to an increasing electron-phonon coupling for the B_1g-mode in high-Tc superconductors.

cond-mat.str-el↗

arXiv1998

Magnetic properties of the three-band Hubbard model

We present magnetic properties of the three-band Hubbard model in the para- and antiferromagnetic phase on a hypercubic lattice calculated with the Dynamical Mean-Field Theory (DMFT). To allow for solutions with broken spin-symmetry we extended the approach to lattices with AB-like structure. Above a critical sublattice magnetization m_d=0.5 one can observe rich structures in the spectral-functions similar to the t-J model which can be related to the well known bound states for one hole in the Neel-background. In addition to the one-particle properties we discuss the static spin-susceptiblity in the paramagnetic state at the points q=0 and q=(pi,pi,pi,...) for different dopings delta. The delta-T-phase-diagram exhibits an enhanced stability of the antiferromagnetic state for electron-doped systems in comparison to hole-doped. This asymmetry in the phase diagram is in qualitative agreement with experiments for high-T_c materials.

cond-mat.str-el↗

arXiv1997

Coupling between phonons and intrinsic Josephson oscillations in cuprate superconductors

The recently reported subgap structures observed in the current-voltage characteristic of intrinsic Josephson junctions in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+δ} and Bi_2Sr_2CaCu_2O_{8+δ} are explained by the coupling between c-axis phonons and Josephson oscillations. A model is developed where c-axis lattice vibrations between adjacent superconducting multilayers are excited by the Josephson oscillations in a resistive junction. The voltages of the lowest structures correspond well to the frequencies of longitudinal c-axis phonons with large oscillator strength in the two materials, providing a new measurement technique for this quantity.

cond-mat.supr-con↗

arXiv1997

Subgap structures in the current-voltage characteristic of the intrinsic Josephson effect due to phonons

A modified RSJ-model for the coupling of intrinsic Josephson oscillations and c-axis phonons in the high-T_c superconductors Tl_2Ba_2Ca_2Cu_3O_{10+δ} and Bi_2Sr_2CaCu_2O_{8+δ} is deveoped. This provides a very good explanation for recently reported subgap structures in the I-V-characteristic of the c-axis transport. It turns out that the voltages of these structures coincide with the eigenfrequencies of longitudinal optical phonons, providing a new measurement technique for this quantity. The significantly enhanced microwave emission at the subgap structures in both the GHz and THz region is discussed.

cond-mat.supr-con↗

arXiv1996

Prime rings with PI rings of constants

It is shown that if the ring of constants of a restricted differential Lie algebra with a quasi-Frobenius inner part satisfies a polynomial identity (PI) then the original prime ring has a generalized polynomial identitiy (GPI). If additionally the ring of constants is semiprime then the original ring is PI. The case of a non-quasi-Frobenius inner part is also considered.

alg-geom↗

arXiv1996

Effects of spin fluctuations in the t-J model

Recent experiments on the Fermi surface and the electronic structure of the cuprate-supercondutors showed the importance of short range antiferromagnetic correlations for the physics in these systems. Theoretically, features like shadow bands were predicted and calculated mainly for the Hubbard model. In our approach we calculate an approximate selfenergy of the $t$-$J$ model. Solving the $U=\infty$ Hubbard model in the Dynamical Mean Field Theory (DMFT) yields a selfenergy that contains most of the local correlations as a starting point. Effects of the nearest neighbor spin interaction $J$ are then included in a heuristical manner. Formally like in $J$-perturbation theory all ring diagrams, with the single bubble assumed to be purely local, are summed to get a correction to the DMFT-self engergy This procedure causes new bands and can furnish strong deformation of quasiparticle bands. % Our results are finally compared with %former approaches to the Hubbard model.

cond-mat↗