arXiv subjects

Arno P. Kampf

Publications and source records attributed to Arno P. Kampf.

At least 37 records · Page 2Linked to original sources

arXiv2011

Pairing Theory of Striped Superconductivity

Striped high-$T_{\rm c}$ superconductors such as La$_{2-y-x}$Nd$_{y}$Sr$_x$CuO$_4$ and La$_{2-x}$Ba$_{x}$CuO$_4$ near $x=1/8$ show a fascinating competition between spin and charge order and superconductivity. A theory for these systems therefore has to capture both the spin correlations of an antiferromagnet and the pair correlations of a superconductor. For this purpose we present here an effective Hartree-Fock theory incorporating both electron pairing with finite center-of-mass momentum and antiferromagnetism. We show that this theory reproduces the key experimental features such as the formation of the antiferromagnetic stripe patterns at 7/8 band filling or the quasi one-dimensional electronic structure observed by photoemission spectroscopy.

cond-mat.supr-con↗

arXiv2010

Spin-spiral states in undoped manganites

The experimental observation of multiferroic behavior in perovskite manganites with a spiral spin structure demands to clarify the origin of these magnetic states and their relation to ferroelectricity. We show that spin-spiral phases with diagonal wavevector and also the E-type phase exist for intermediate values of the Hund's rule and the Jahn-Teller coupling in the orbitally ordered and insulating state of the standard two-band model Hamiltonian for manganites. Our results support the spin-current mechanism for ferroelectricity and present an alternative view to earlier conclusions where frustrating superexchange couplings were crucial to obtain spin-spiral states.

cond-mat.str-el↗

arXiv2010

Effects of impurities and vortices on the low-energy spin excitations in high-Tc materials

We review a theoretical scenario for the origin of the spin-glass phase of underdoped cuprate materials. In particular it is shown how disorder in a correlated d-wave superconductor generates a magnetic phase by inducing local droplets of antiferromagnetic order which eventually merge and form a quasi-long range ordered state. When correlations are sufficiently strong, disorder is unimportant for the generation of static magnetism but plays an additional role of pinning disordered stripe configurations. We calculate the spin excitations in a disordered spin-density wave phase, and show how disorder and/or applied magnetic fields lead to a slowing down of the dynamical spin fluctuations in agreement with neutron scattering and muon spin rotation (muSR) experiments.

cond-mat.supr-con↗

arXiv2010

Flux periodicities in loops and junctions with d-wave superconductors

The magnetic flux periodicity in superconducting loops is reviewed. Whereas quantization of the magnetic flux with hc/2e prevails in sufficiently thick loops with current free interior, the supercurrent in narrow loops is either hc/2e or hc/e periodic with the external magnetic flux. The periodicity depends on the properties of the condensate state, in particular on the Doppler shift of the energy spectrum. For an s-wave superconductor in a loop with diameter larger than the coherence length, the Doppler shift is small with respect to the energy gap, and the hc/2e periodic behavior of its flux dependent thermodynamic properties is maintained. However, for smaller s-wave loops and, more prominently, narrow d-wave loops of any diameter R, the Doppler shift has a strong effect on the supercurrent carrying state; as a consequence, the fundamental flux periodicity is in fact hc/e. It is shown analytically and numerically that the hc/e periodic component in the supercurrent decays only algebraically as 1/R for large d-wave loops. For nodal superconductors the discrete nature of the eigenergies close to the Fermi energy has to be respected in the evaluation of the Doppler shift. Furthermore, we investigate, whether the Doppler shift modifies the supercurrent through Josephson junctions with d-wave superconductors. For transparent junctions, the Josephson current behaves similar to the persistent supercurrent in a loop. These distinct physical phenomena can be compared, if the magnetic flux in units of hc/e is identified with the phase variation of the order parameter divided by 4 pi.

cond-mat.supr-con↗

arXiv2009

Disorder- and Field-Induced Antiferromagnetism in Cuprate Superconductors

The underdoped high-Tc materials are characterized by a competition between Cooper pairing and antiferromagnetic (AF) order. Important differences between the superconducting (SC) state of these materials and conventional superconductors include the d-wave pairing symmetry and a remarkable magnetic response to nonmagnetic perturbations, whereby droplets of spin-density wave (SDW) order can form around impurities and the cores of vortices. In a simple picture, whenever SC is suppressed locally, SDW order is nucleated. Within a mean-field theory of d-wave SC in an applied magnetic field including disorder and Hubbard correlations, we show in fact that the creation of SDW order is not simply due to suppression of the SC order parameter, but rather due to a correlation-induced splitting of the electronic bound state created by the perturbation. Since the bound state exists because of the sign change of the order parameter along quasiparticle trajectories, the induced SDW order is a direct consequence of the d-wave symmetry. Furthermore the formation of anti-phase domain walls is important for obtaining the correct temperature dependence of the induced magnetism as measured by neutron diffraction.

cond-mat.supr-con↗

arXiv2009

Material-Specific Investigations of Correlated Electron Systems

We present the results of numerical studies for selected materials with strongly correlated electrons using a combination of the local-density approximation and dynamical mean-field theory (DMFT). For the solution of the DMFT equations a continuous-time quantum Monte-Carlo algorithm was employed. All simulations were performed on the supercomputer HLRB II at the Leibniz Rechenzentrum in Munich. Specifically we have analyzed the pressure induced metal-insulator transitions in Fe2O3 and NiS2, the charge susceptibility of the fluctuating-valence elemental metal Yb, and the spectral properties of a covalent band-insulator model which includes local electronic correlations.

cond-mat.str-el↗

arXiv2009

Correlations in a band insulator

We study a model of a covalent band insulator with on-site Coulomb repulsion at half-filling using dynamical mean-field theory. Upon increasing the interaction strength the system undergoes a discontinuous transition from a correlated band insulator to a Mott insulator with hysteretic behavior at low temperatures. Increasing the temperature in the band insulator close to the insulator-insulator transition we find a crossover to a Mott insulator at elevated temperatures. Remarkably, correlations decrease the energy gap in the correlated band insulator. The gap renormalization can be traced to the low-frequency behavior of the self-energy, analogously to the quasiparticle renormalization in a Fermi liquid. While the uncorrelated band insulator is characterized by a single gap for both charge and spin excitations, the spin gap is smaller than the charge gap in the correlated system.

cond-mat.str-el↗

arXiv2009

Superconductivity with Finite-Momentum Pairing in Zero Magnetic Field

In the BCS theory of superconductivity, one assumes that all Cooper pairs have the same center of mass momentum. This is indeed enforced by self consistency, if the pairing interaction is momentum independent. Here, we show that for an attractive nearest neighbor interaction, this is different. In this case, stable solutions with pairs with momenta q and -q coexist and, for a sufficiently strong interaction, one of these states becomes the groundstate of the superconductor. This finite-momentum pairing state is accompanied by a charge order with wave vector 2q. For a weak pairing interaction, the groundstate is a d-wave superconductor.

cond-mat.supr-con↗

arXiv2009

Insulating behavior with spin and charge order in the ionic Hubbard model

Paramagnetic solutions of the ionic Hubbard model at half-filling in dimensions $D>2$ indicate that the band and the Mott insulator phases are separated by a metallic phase. We present zero-temperature dynamical mean-field theory solutions, which include antiferromagnetic long-range order, and show that the one-particle spectral functions always possess an energy gap and therefore the system is insulating for all interaction strengths. The staggered charge density modulation coexists with antiferromagnetic long-range order of Néel type.

cond-mat.str-el↗

arXiv2008

Flux Periodicities in Loops of Nodal Superconductors

Supercurrents in superconducting flux threaded loops are expected to oscillate with the magnetic flux with a period of hc/2e. This is indeed true for s-wave superconductors larger than the coherence length xi_0. Here we show that for superconductors with gap nodes, there is no such strict condition for the supercurrent to be hc/2e rather than hc/e periodic. For nodal superconductors, the flux induced Doppler shift of the near nodal states leads to a flux dependent occupation probability of quasi-particles circulating clockwise and counter clockwise around the loop, which leads to an hc/e periodic component of the supercurrent, even at zero temperature. We analyze this phenomenon on a cylinder in an approximative analytic approach and also numerically within the framework of the BCS theory. Specifically for d-wave pairing, we show that the hc/e periodic current component decreases with the inverse radius of the loop and investigate its temperature dependence.

cond-mat.supr-con↗

arXiv2008

Combined Effect of Bond- and Potential-Disorder in Half-Doped Manganites

We analyze the effects of both bond- and potential-disorder in the vicinity of a first-order metal insulator transition in a two-band model for manganites using a real-space Monte Carlo method. Our results reveal a novel charge-ordered state coexisting with spin-glass behavior. We provide the basis for understanding the phase diagrams of half-doped manganites, and contrast the effects of bond- and potential-disorder and the combination of both.

cond-mat.str-el↗

arXiv2008

Disorder-induced orbital ordering in doped manganites

We study the effect of quenched disorder on the ordering of orbital and magnetic degrees of freedom in a two-dimensional, two-band double-exchange model for $e_g$ electrons coupled to Jahn-Teller distortions. Using a real-space Monte Carlo method, we find that disorder can induce a short-range ordering of the orbital degrees of freedom near 30% hole doping. The most striking consequence of this short range ordering is a strong increase in the low temperature resistivity. The real-space approach allows to analyze the spatial patterns of the charge, orbital, and magnetic degrees of freedom, and the correlations among them. The magnetism is inhomogeneous on the nanoscale in the short-range orbitally ordered state.

cond-mat.str-el↗

arXiv2007

Disorder-Induced Static Antiferromagnetism in Cuprate Superconductors

Using model calculations of a disordered d-wave superconductor with on-site Hubbard repulsion, we show how dopant disorder can stabilize novel states with antiferromagnetic order. We find that the critical strength of correlations or impurity potential necessary to create an ordered magnetic state in the presence of finite disorder is reduced compared to that required to create a single isolated magnetic droplet. This may explain why in cuprates like LSCO low-energy probes have identified a static magnetic component which persists well into the superconducting state, whereas in cleaner systems like YBCO it is absent or minimal. Finally we address the case of nominally clean LSCO samples at optimal doping, where such ordered magnetic moments are absent, but where they can be induced by small concentrations of strong scatterers.

cond-mat.supr-con↗

arXiv2006

Spin transport in Heisenberg antiferromagnets

We analyze spin transport in insulating antiferromagnets described by the XXZ Heisenberg model in two and three dimensions. Spin currents can be generated by a magnetic-field gradient or, in systems with spin-orbit coupling, perpendicular to a time-dependent electric field. The Kubo formula for the longitudinal spin conductivity is derived analogously to the Kubo formula for the optical conductivity of electronic systems. The spin conductivity is calculated within interacting spin-wave theory. In the Ising regime, the XXZ magnet is a spin insulator. For the isotropic Heisenberg model, the dimensionality of the system plays a crucial role: In d=3 the regular part of the spin conductivity vanishes linearly in the zero frequency limit, whereas in d=2 it approaches a finite zero frequency value.

cond-mat.str-el↗

arXiv2006

The Effect of Disorder in an Orbitally Ordered Jahn-Teller Insulator

We study a two dimensional, two-band double-exchange model for $e_g$ electrons coupled to Jahn-Teller distortions in the presence of quenched disorder using a recently developed Monte-Carlo technique. In the absence of disorder the half-filled system at low temperatures is an orbitally ordered ferromagnetic insulator with a staggered pattern of Jahn-Teller distortions. We examine the finite temperature transition to the orbitally disordered phase and uncover a qualitative difference between the intermediate and strongly coupled systems, including a thermally driven insulator to metal crossover in the former case. Long range orbital order is suppressed in the presence of disorder and the system displays a tendency towards metastable states consisting of orbitally disordered stripe-like structures enclosing orbitally ordered domains.

cond-mat.str-el↗

arXiv2006

Domain Formation and Orbital Ordering Transition in a Doped Jahn-Teller Insulator

The ground state of a double-exchange model for orbitally degenerate $e_g$ electrons with Jahn-Teller lattice coupling and weak disorder is found to be spatially inhomogeneous near half filling. Using a real space Monte-Carlo method we show that doping the half-filled orbitally ordered insulator leads to the appearance of hole-rich disordered regions in an orbitally ordered environment. The doping driven orbital order to disorder transition is accompanied by the emergence of metallic behavior. We present results on transport and optical properties along with spatial patterns for lattice distortions and charge densities, providing a basis for an overall understanding of the low doping phase diagram of La$_{1-x}$Ca$_x$MnO$_3$.

cond-mat.dis-nn↗

arXiv2005

Mirage phenomena in superconducting quantum corrals

We investigate the local density of states and the order parameter structure inside an elliptic quantum corral on surfaces of isotropic and anisotropic superconductors. The Bogoliubov-de Gennes equations are solved in the presence of non-magnetic and magnetic impurities. We observe and discuss a variety of mirage and anti-mirage phenomena, which specifically reflect the nature of the superconducting pairing state.

cond-mat.supr-con↗

arXiv2003

Mirages, anti-mirages, and further surprises in quantum corrals with non-magnetic impurities

We investigate the local density of states (LDOS) for non-interacting electrons in a hard wall ellipse in the presence of a single non-magnetic scattering center. Using a T-matrix analysis we calculate the local Green's function and observe a variety of quantum mirage effects for different impurity positions. Locating the impurity near positions with LDOS maxima for the impurity free corral can either lead to a reduction or an enhancement of the LDOS at the mirror image point, i.e. a mirage or anti-mirage effect, or even suppress LDOS maxima in the entire area of the corral.

cond-mat.mes-hall↗