Wonkee Kim

Quasiparticle States around a Nonmagnetic Impurity in a d-Density-Wave State of High-Tc Cuprates

The electronic structure around a single nonmagnetic impurity in the d-density-wave (DDW) ordering is studied. It is found that a single subgap resonance peak appears in the local density of states around the impurity. In the unitary limit, the position of this resonance peak shifts away from the Fermi energy in accord with the chemical potential. This result is dramatically different from the case of the pure superconducting state for which the impurity resonant energy is approximately pinned at the Fermi level. This can be used to probe the existence of the DDW ordering in cuprates.

c-axis response of a high-Tc superconductor with d-density-wave order

The microscopic origin of the pseudogap state that exists in the underdoped cuprates remains unknown. The c-axis properties in the pseudogap regime are particularly anomalous. We use a recently proposed model of a d density wave that leads to staggered currents and the doubling of the unit cell to investigate the c-axis kinetic energy and the optical sum rule. The density of states expected in the model is also considered.

Hidden symmetries of electronic transport in a disordered one-dimensional lattice

Correlated, or extended, impurities play an important role in the transport properties of dirty metals. Here, we examine, in the framework of a tight-binding lattice, the transmission of a single electron through an array of correlated impurities. In particular we show that particles transmit through an impurity array in identical fashion, regardless of the direction of transversal. The demonstration of this fact is straightforward in the continuum limit, but requires a detailed proof for the discrete lattice. We also briefly demonstrate and discuss the time evolution of these scattering states, to delineate regions (in time and space) where the aforementioned symmetry is violated.

How many electrons are needed to flip a local spin

Considering the spin of a local magnetic atom as a quantum-mechanical operator, we illustrate the dynamics of a local spin interacting with a ballistic electron represented by a wave packet. This approach improves the semi-classical approximation and provides a complete quantum-mechanical understanding for spin transfer phenomena. Sending spin-polarized electrons towards a local magnetic atom one after another, we estimate the minimum number of electrons needed to flip a local spin.

Quantum mechanics of spin transfer in coupled electron-spin chains

The manner in which spin-polarized electrons interact with a magnetized thin film is currently described by a semi-classical approach. This in turn provides our present understanding of the spin transfer, or spin torque phenomenon. However, spin is an intrinsically quantum-mechanical quantity. Here, we make the first strides towards a fully quantum-mechanical description of spin transfer through spin currents interacting with a Heisenberg-coupled spin chain. Because of quantum entanglement, this requires a formalism based on the density matrix approach. Our description illustrates how individual spins in the chain time-evolve as a result of spin transfer.

Impurity Scattering of Wave Packets on a Lattice

Quantum transport in a lattice is distinct from its counterpart in continuum media. Even a free wave packet travels differently in a lattice than in the continuum. We describe quantum scattering in a one dimensional lattice using three different formulations and illustrate characteristics of quantum transport such as resonant transmission. We demonstrate the real time propagation of a wave packet and its phase shift due to impurity configurations. Spin-flip scattering is also taken into account in a spin chain system. We show how individual spins in the chain evolve as a result of a spin-flip interaction between an incoming electron and a spin chain.

Spin torque and its relation to spin filtering

The spin torque exerted on a magnetic moment is a reaction to spin filtering when spin-polarized electrons interact with a thin ferromagnetic film. We show that, for certain conditions, a spin transmission resonance (STR) gives rise to a failure of spin filtering. As a consequence, no spin is transfered to the ferromagnet. The condition for STR depends on the incoming energy of electrons and the thickness of the film. For a simple model we find that when the STR condition is satisfied, the ferromagnetic film is transparent to the incoming electrons.

Hall conductivity of a spin-triplet superconductor.

We calculate the Hall conductivity for a spin-triplet superconductor, using a generalized pairing symmetry dependent on an arbitrary phase phi. A promising candidate for such an order parameter is Sr2RuO4, whose superconducting order parameter symmetry is still subject to investigation. The value of this phase can be determined through Kerr rotation and dc Hall conductivity measurements. Our calculations impose significant constraints on phi.

Microwave conductivity of a high-purity d-wave superconductor

The cusp-like behavior of the microwave conductivity observed in clean ortho-II YB

Phase Fluctuations of s-Wave Superconductors on a Lattice

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