arXiv subjects

Dirk K. Morr

Publications and source records attributed to Dirk K. Morr.

At least 91 records · Page 5Linked to original sources

arXiv2004

Molecular Nanostructures on the Surface of a d_(x2-y2)-Superconductor

We study molecular nanostructures on the surface of a d_(x2-y2)-wave superconductor. We show that the interplay between the molecular nanostructure's internal excitation spectrum and quantum interference of the scattered host electrons leads to a series of novel effects in the local density of states. We demonstrate that these effects give insight into both the nature of the superconducting pairing correlations and of the intermolecular interactions.

cond-mat.supr-con↗

arXiv2004

Novel neutron resonance mode in $d_{x^2-y^2}$ superconductors

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa$_2$Cu$_3$O$_{6.85}$ and YBa$_2$Cu$_3$O$_{6.6}$, is a {\it spin exciton}. Its location in the magnetic Brillouin zone and its frequency are determined by the momentum dependence of the particle-hole continuum. We identify several features that distinguish this novel mode from the previous resonance mode observed near ${\bf Q}=(π,π)$, such as its intensity maximum which occurs in a different part of the magnetic Brillouin zone.

cond-mat.supr-con↗

arXiv2004

Quantum Imaging and Selection Rules in Triangular Quantum Corrals

We study quantum imaging in a triangular quantum corral that is embedded in a superconducting host system with s-wave symmetry. We show that the corral acts as a quantum copying machine by creating multiple images of a quantum candle. We obtain new selection rules for the formation of quantum images that arise from the interplay of the corral's geometry and the location of quantum candles. In more complex corral structures, we show that quantum images can be projected "around the corner".

cond-mat.supr-con↗

arXiv2003

Quantum Corrals, Eigenmodes and Quantum Mirages in s-wave Superconductors

We study the electronic structure of magnetic and non-magnetic quantum corrals embedded in s-wave superconductors. We demonstrate that a quantum mirage of an impurity bound state peak can be projected from the occupied into the empty focus of a non-magnetic quantum corral via the excitation of the corral's eigenmodes. We observe an enhanced coupling between magnetic impurities inside the corral, which can be varied through oscillations in the corral's impurity potential. Finally, we discuss the form of eigenmodes in magnetic quantum corrals.

cond-mat.str-el↗

arXiv2003

Localized Bosonic Modes in Superconductors

We show that a localized bosonic mode acts as a new type of "defect" in s- and d_{x^2-y^2}-wave superconductors. The mode induces bound or resonance states, whose spectral signature are peaks in the superconductor's density of states (DOS). We study the peaks' shape and energy as a function of temperature and the mode's frequency and lifetime. We identify several characteristic signatures of the localized mode that qualitatively distinguishes its effects from those of magnetic or non-magnetic impurities.

cond-mat.str-el↗

arXiv2002

Quantum interference between non-magnetic impurities in d_x2-y2-wave superconductors

We study quantum interference of electronic waves that are scattered by multiple non-magnetic impurities in a d_x2-y2-wave superconductor. We show that the number of resonance states in the density-of-states (DOS), as well as their frequency and spatial dependence change significantly as the distance between the impurities or their orientation relative to the crystal lattice is varied. Since the latter effect arises from the momentum dependence of the superconducting gap, we argue that quantum interference is a novel tool to identify the symmetry of unconventional superconductors.

cond-mat.supr-con↗

arXiv2002

Quantum Interference between Impurities: Creating Novel Many-Body States in s-wave Superconductors

We demonstrate that quantum interference of electronic waves that are scattered by multiple magnetic impurities in an s-wave superconductor gives rise to novel bound states. We predict that by varying the inter-impurity distance or the relative angle between the impurity spins, the states' quantum numbers, as well as their distinct frequency and spatial dependencies, can be altered. Finally, we show that the superconductor can be driven through multiple local crossovers in which its spin polarization, $ $, changes between $ =0, 1/2$ and 1.

cond-mat.str-el↗

arXiv2001

Electronic Structure near Impurities in the Chains of YBa$_2$Cu$_3$O$_{6+x}$

We study the electronic structure near impurities in the one-dimensional chains of YBa$_2$Cu$_3$O$_{6+x}$. Assuming chain superconductivity below $T_c$ due to a proximity coupling, we show that only a magnetic impurity induces resonance states in the local density of states. Its spatial structure reflects the particle-hole nature of chain superconductivity and thus distinguishes it from other broken symmetry phases. For a sufficiently large scattering strength, or due to interference effects arising from a second impurity, the chains undergo a quantum phase transition to a polarized state.

cond-mat.str-el↗

arXiv2001

The Resonance Peak in Sr$_2$RuO$_4$: Signature of Spin Triplet Pairing

We study the dynamical spin susceptibility, $χ({\bf q}, ω)$, in the normal and superconducting state of Sr$_2$RuO$_4$. In the normal state, we find a peak in the vicinity of ${\bf Q}_i\simeq (0.72π,0.72π)$ in agreement with recent inelastic neutron scattering (INS) experiments. We predict that for spin triplet pairing in the superconducting state a {\it resonance peak} appears in the out-of-plane component of $χ$, but is absent in the in-plane component. In contrast, no resonance peak is expected for spin singlet pairing.

cond-mat.supr-con↗

arXiv2001

Resonant Impurity States in the D-Density-Wave Phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudo-gap region of the high-temperature superconductors. We show that the local DOS near a non-magnetic impurity in the DDW state is {\it qualitatively} different from that in a superconductor with $d_{x^2-y^2}$-symmetry. Since this result is a robust feature of the DDW phase, it can help to identify the nature of the two different phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.

cond-mat.str-el↗

arXiv2000

One-dimensional Surface Bound States in d-wave Superconductors

We present an {\it exact} quantum theory for the bound states in the vicinity of an edge or a line of impurities in a $d_{x^2-y^2}$ superconductor. For a (110)-surface we show that a finite dispersion of the one dimensional band of bound states leads to a two peak structure in the density of states (DOS). We study the effect of an applied magnetic field and a subdominant $id_{xy}$ order parameter on the DOS and discuss the implications of our results for tunneling experiments.

cond-mat.supr-con↗

arXiv2000

Proximity Effects and Quantum Dissipation in the Chains of YBa$_2$Cu$_3$O$_{6+x}

We argue that the results of recent scanning tunneling microscopy, angle-resolved photoemission and infrared spectroscopy experiments on the CuO chains of YBa$_2$Cu$_3$O$_{6+x}$ are consistently explained within a proximity model by the interplay of a coherent chain-plane and an incoherent interchain coupling. We show that the CuO$_2$ planes act as an ohmic heat bath for the electronic degrees of freedom in the chains which induces a substantial quantum dissipation. Below the planar $T_c$, charge excitations in the chains acquire a universal superconducting gap whose phase and magnitude are momentum dependent. We predict that the magnitude of this gap varies non-monotonically with the hole concentration in the chains.

cond-mat.supr-con↗

arXiv2000

Spin-lattice relaxation in the mixed state of the high-$T_c$ cuprates: electronic spin-flip scattering versus spin-fluctuations

Recent experimental and theoretical studies have established that the spin-lattice relaxation rate, $1/T_1$, measured in nuclear magnetic resonance (NMR) experiments is a site-sensitive probe for the electronic spectrum in the mixed state of the high-$T_c$ cuprates. While some groups suggested that $1/T_1$ is solely determined by spin-flip scattering of BCS-like electrons, other groups stressed the importance of antiferromagnetic spin-fluctuations. We show that these two relaxation mechanisms give rise to a {\it qualitatively} different temperature and frequency dependence of $^{17}$O $1/T_1$. A comparison of our results with the experimental $^{17}$O $1/T_1$ data provides support for a relaxation mechanism dominated by antiferromagnetic spin-fluctuations.

cond-mat.supr-con↗

arXiv2000

Magnetic Coherence as a Universal Feature of Cuprate Superconductors

Recent inelastic neutron scattering (INS) experiments on La$_{2-x}$Sr$_x$CuO$_4$ have established the existence of a {\it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $χ''$, in the superconducting phase. We show, using the spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that the magnetic coherence effect establishes the ability of INS experiments to probe the electronic spectrum of the cuprates, in that the effect arises from the interplay of an incommensurate magnetic response, the form of the underlying Fermi surface, and the opening of the d-wave gap in the fermionic spectrum. In particular, we find that the magnetic coherence effect observed in INS experiments on La$_{2-x}$Sr$_x$CuO$_4$ requires that the Fermi surface be closed around $(π,π)$ up to optimal doping. We present several predictions for the form of the magnetic coherence effect in YBa$_2$Cu$_3$O$_{6+x}$ in which an incommensurate magnetic response has been observed in the superconducting state.

cond-mat.supr-con↗

arXiv2000

Magnetic Coherence in Cuprate Superconductors

Recent inelastic neutron scattering (INS) experiments on La$_{2-x}$Sr$_x$CuO$_4$ observed a {\it magnetic coherence effect}, i.e., strong frequency and momentum dependent changes of the spin susceptibility, $χ''$, in the superconducting phase. We show that this effect is a direct consequence of changes in the damping of incommensurate antiferromagnetic spin fluctuations due to the appearance of a d-wave gap in the fermionic spectrum. Our theoretical results provide a quantitative explanation for the weak momentum dependence of the observed spin-gap. Moreover, we predict {\bf (a)} a Fermi surface in La$_{2-x}$Sr$_x$CuO$_4$ which is closed around $(π,π)$ up to optimal doping, and {\bf (b)} similar changes in $χ''$ for all cuprates with an incommensurate magnetic response.

cond-mat.supr-con↗

arXiv2000

Spin and charge inhomogeneities in high-T_c cuprates: Evidence from NMR and neutron scattering experiments

In this communication we consider the doping dependence of the strong antiferromagnetic spin fluctuations in the cuprate superconductors. We investigate the effect of an incommensurate magnetic response, as recently observed in inelastic neutron scattering (INS) experiments on several YBa_2Cu_3O_{6+x} compounds, on the spin-lattice and spin-echo relaxation rates measured in nuclear magnetic resonance (NMR) experiments. We conclude that a consistent theoretical description of INS and NMR can be reached if one assumes spatially inhomogeneous but locally commensurate spin correlations and that NMR and INS experiments can be described within a single theoretical scenario. We discuss a simple scenario of spin and charge inhomogeneities which includes the main physical ingredients required for consistency with experiments.

cond-mat.supr-con↗

arXiv2000

Effect of phase fluctuations on INS and NMR experiments in the pseudo-gap regime of the underdoped cuprates

We present a theory for inelastic neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments in the pseudo-gap regime of the underdoped high-$T_c$ cuprates. We show that superconducting phase fluctuations greatly affect the temperature and frequency dependence of the spin-susceptibility, $χ''$, probed by both experimental techniques. This result explains the appearance of a {\it resonance peak}, observed in INS experiments, below a temperature $T_0 > T_c$. In the same temperature regime, we find that the $^{63}$Cu spin-lattice relaxation rate, $1/T_1$, measured in NMR experiments, is suppressed. Our results are in qualitative agreement with the available experimental data.

cond-mat.supr-con↗

arXiv1999

Theory of NMR as a local probe for the electronic structure in the mixed state of the high-$T_c$ cuprates

We argue that nuclear magnetic resonance experiments are a site-sensitive probe for the electronic spectrum in the mixed state of the high-$T_c$ cuprates. Within a spin-fermion model, we show that the Doppler-shifted electronic spectrum arising from the circulating supercurrent changes the low-frequency behavior of the imaginary part of the spin-susceptibility. For a hexagonal vortex lattice, we predict that these changes lead to {\it (a)} a unique dependence of the $^{63}$Cu spin lattice relaxation rate, $1/T_1$, on resonance frequency, and {\it (b)} a temperature dependence of $T_1$ which varies with frequency. We propose a nuclear quadrupole experiment to study the effects of a uniform supercurrent on the electronic structure and predict that $T_1$ varies with the direction of the supercurrent.

cond-mat.supr-con↗