Dirk K. Morr

Imaging Cooper pairing of heavy fermions in CeCoIn5

By pushing scanning tunnelling spectroscopy down to millikelvin temperatures, it is now possible to image a heavy fermion superconductor and measure the superconducting gap symmetry, with gap nodes in unexpected momentum-space locations.

Novel neutron resonance mode in d x 2 y 2-wave superconductors

We show that a new resonant magnetic excitation at incommensurate momenta, observed recently by inelastic neutron scattering experiments on YBa2Cu3O6.85 and YBa2Cu3O6.6, is a spin exciton. Its location in the 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 Q=(pi,pi).

The Resonance Peak in Cuprate Superconductors

We pursue the consequences of a theory in which the resonance peak observed in inelastic neutron scattering experiments on underdoped and optimally doped YBa{sub 2}Cu {sub 3}O{sub 6+x} compounds arises from a spin-wave excitation. We find that it is heavily damped in the normal state and only becomes visible in the superconducting state due to the drastic decrease in spin damping. We show that a spin-fermion model correctly describes the doping dependence of the peak position and of the integrated intensity. Finally, we make several predictions concerning resonance peaks in other cuprate superconductors. {copyright} {ital 1998} {ital The American Physical Society}

Electronic structure of underdoped cuprates

Abstract We consider a two-dimensional Fermi-liquid coupled to low-energy commensurate spin fluctuations. At small coupling, the hole Fermi surface is large and centered around Q = ( π , π ). We show that as the coupling increases, the shape of the quasiparticle Fermi surface and the spin-fermion vertex undergo a substantial evolution. At strong couplings, g ⪢ ω 0 , where ω 0 is the upper cutoff in the spin susceptibility, the hole Fermi surface consists of small pockets centered at ( ± π 2 , ± π 2 ). The full spin-fermion vertex is much smaller than the bare one, and scales nearly linearly with ¦q − Q¦ , where q is the momentum of the susceptibility. At intermediate couplings, there exist both, a large hole Fermi surface centered at (π,π), and four hole pockets, but the quasiparticle residue is small everywhere except for the pieces of the pockets which face the origin of the Brillouin zone. The relevance of these results for recent photoemission experiments in YBCO and Bi2212 systems is discussed.

Differential conductance and quantum interference in kondo systems

We present a large-N theory for the differential conductance, dI/dV, in Kondo systems measured via scanning tunneling spectroscopy. We demonstrate that quantum interference between tunneling processes into the conduction band and into the magnetic f-electron states is crucial in determining the experimental Fano line shape of dI/dV. This allows one to uniquely extract the Kondo coupling and the ratio of the tunneling amplitudes from the experimental dI/dV curve. Finally, we show that dI/dV directly reflects the strength of the antiferromagnetic interaction in Kondo lattice systems.

Local defect in metallic quantum critical systems

We present a theory of a single point, line, or plane defect coupling to the square of the order parameter in a metallic system near a quantum critical point at or above its upper critical dimension. At criticality, a spin droplet is nucleated around the defect with its core size determined by the strength of the defect potential. Outside the core a universal slowly decaying tail of the droplet is found, leading to many dissipative channels coupling to the droplet and to a complete suppression of quantum tunneling. We propose an NMR experiment to measure the impurity-induced changes in the local spin susceptibility.

Interplay of ferromagnetism and triplet superconductivity in a Josephson junction

In this work we extend our earlier analysis of the novel Josephson effect in triplet superconductor-ferromagnet-triplet superconductor (TFT) junctions [Kastening et al., Phys. Rev. Lett. 96, 047009 (2006)]. In our more general formulation of the TFT junction, we allow for potential scattering at the barrier and an arbitrary orientation of the ferromagnetic moment. Several new effects are found upon the inclusion of these extra terms: for example, we find that a Josephson current can flow even when there is vanishing phase difference between the superconducting condensates on either side of the barrier. The critical current for a barrier with magnetization parallel to the interface is calculated as a function of the junction parameters, and is found to display strong nonanalyticities. Furthermore, the Josephson current switches identified in our previous work are found to be robust features of the junction, while the unconventional temperature dependence of the current is very sensitive to the extra terms in the barrier Hamiltonian.

First Order Superconducting Transition Near a Ferromagnetic Quantum Critical Point

We address the issue of how triplet superconductivity emerges in an electronic system near a ferromagnetic quantum critical point (FQCP). Previous studies found that the superconducting transition is of second order, and T(c) is strongly reduced near the FQCP due to pair-breaking effects from thermal spin fluctuations. In contrast, we demonstrate that near the FQCP, the system avoids pair-breaking effects by undergoing a first order transition at a much larger T(c). A second order superconducting transition emerges only at some distance from the FQCP.

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 pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.

Direct evidence for a magnetic f-electron–mediated pairing mechanism of heavy-fermion superconductivity in CeCoIn5

Significance In heavy-fermion materials, the magnetic moment of an f-electron atom, such as Ce, is screened via the Kondo effect resulting in the splitting of a conventional light band into two heavy bands within few millielectron volts of the Fermi energy. For decades it has been hypothesized that Cooper pairing and superconductivity of the resulting heavy electrons are mediated by the f-electron magnetism. By extracting the magnetic interactions of CeCoIn5 from heavy-fermion scattering interference, and by then predicting quantitatively a variety of characteristics expected for unconventional superconductivity driven by them, we provide direct evidence that the heavy-fermion Cooper pairing in this material is indeed mediated by f-electron magnetism. To identify the microscopic mechanism of heavy-fermion Cooper pairing is an unresolved challenge in quantum matter studies; it may also relate closely to finding the pairing mechanism of high-temperature superconductivity. Magnetically mediated Cooper pairing has long been the conjectured basis of heavy-fermion superconductivity but no direct verification of this hypothesis was achievable. Here, we use a novel approach based on precision measurements of the heavy-fermion band structure using quasiparticle interference imaging to reveal quantitatively the momentum space (k-space) structure of the f-electron magnetic interactions of CeCoIn5. Then, by solving the superconducting gap equations on the two heavy-fermion bands Ekα,β with these magnetic interactions as mediators of the Cooper pairing, we derive a series of quantitative predictions about the superconductive state. The agreement found between these diverse predictions and the measured characteristics of superconducting CeCoIn5 then provides direct evidence that the heavy-fermion Cooper pairing is indeed mediated by f-electron magnetism.