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Harry Westfahl Jr

Publications and source records attributed to Harry Westfahl Jr.

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Conductivity of electronic liquid-crystalline mesophases

We investigate the connection between the transport properties and the thermodynamics of electronic systems with a tendency to form broken-symmetry mesophases evocative of the physics of liquid crystals. Through a hydrodynamic approach to the electronic transport in inhomogeneous systems, we develop a perturbative expansion for the macroscopic conductivity to study the transport of two-dimensional smectic and nematic phases. At the fluctuation induced first order phase transition expected for the smectic to isotropic transition, a jump in the macroscopic conductivity is predicted, with a directional dependence that reflects the fluctuation spectrum of the order parameter. When elastic fluctuation modes melt the smectic phase into a nematic phase, the resultant nematic order parameter is shown to be linearly proportional to the conductivity anisotropy. We also outline qualitative comparisons with recent experimental works on strongly correlated materials that show evidences of electronic liquid-crystalline mesophases.

cond-mat.str-el

Fluctuation induced first-order phase transitions in a dipolar Ising ferromagnet slab

We investigate the competition between the dipolar and the exchange interaction in a ferromagnetic slab with finite thickness and finite width. From an analytical approximate expression for the Ginzburg-Landau effective Hamiltonian, it is shown that, within a self-consistent Hartree approach, a stable modulated configuration arises. We study the transition between the disordered phase and two kinds of modulated configurations, namely, striped and bubble phases. Such transitions are of the first-order kind and the striped phase is shown to have lower energy and a higher spinodal limit than the bubble one. It is also observed that striped configurations corresponding to different modulation directions have different energies. The most stable are the ones in which the modulation vanishes along the unlimited direction, which is a prime effect of the slab's geometry together with the competition between the two distinct types of interaction. An application of this model to the domain structure of MnAs thin films grown over GaAs substrates is discussed and general qualitative properties are outlined and predicted, like the number of domains and the mean value of the modulation as functions of temperature.

cond-mat.stat-mech