X-ray method to study temperature-dependent stripe domains in MnAs/GaAs(001)

MnAs films grown on GaAs (001) exhibit a progressive transition between hexagonal (ferromagnetic) and orthorhombic (paramagnetic) phases at wide temperature range instead of abrupt transition during the first-order phase transition. The coexistence of two phases is favored by the anisotropic strain arising from the constraint on the MnAs films imposed by the substrate. This phase coexistence occurs in ordered arrangement alternating periodic terrace steps. We present here a method to study the surface morphology throughout this transition by means of specular and diffuse scattering of soft x-rays, tuning the photon energy at the Mn 2p resonance. The results show the long-range arrangement of the periodic stripe-like structure during the phase coexistence and its period remains constant, in agreement with previous results using other techniques.Comment: 4 pages, 4 figures, submitted to Applied Physics Letter

Solution of the X-ray edge problem for 2D electrons in a magnetic field

The absorption and emission spectra of transitions between a localized level and a two-dimensional electron gas, subjected to a weak magnetic field, are calculated analytically. Adopting the Landau level bosonization technique developed in previous papers, we find an exact expression for the relative intensities of spectral lines. Their envelope function, governed by the interaction between the electron gas and the core hole, is reminescent of the famous Fermi edge singularity, which is recovered in the limit of a vanishing magnetic field.Comment: 4 pages, 1 figur

Magnetic reconfiguration of MnAs/GaAs(001) observed by Magnetic Force Microscopy and Resonant Soft X-ray Scattering

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft X-ray scattering and magnetic force microscopy. The results of the diffuse satellite X-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane mono-domain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9 times the film thickness d. This is in good agreement with the expected theoretical value of L = 2.6d.Comment: 16 pages in PD

Bosonization for Wigner-Jordan-like Transformation : Backscattering and Umklapp-processes on Fictitious Lattice

We analyze the asymptotic behavior of the exponential form in the fermionic density operators as the function of ruling parameter Q. In the particular case Q=\pi this exponential associates with the Wigner-Jordan transformation for XY spin chain model. We compare the bosonization approach and the evaluation via the Toeplitz determinant. The use of Szego-Kac theorem suggests that at Q>\pi/3 the divergent series for intrinsic logarithm provides a bosonized solution and faster decaying one, found as the logarithm's value on another sheet of the complex plane. The second solution is revealed as umklapp-process on the fictitious lattice while originates from backscattering terms in bosonized density. Our finding preserves in a wide range of fermion filling ratios.Comment: 8 pages, REVTEX, 3 eps figures, accepted to Phys.Rev.

Dynamical Mean Field Theory for Self-Generated Quantum Glasses

We present a many body approach for non-equilibrium behavior and self-generated glassiness in strongly correlated quantum systems. It combines the dynamical mean field theory of equilibrium systems with the replica theory for classical glasses without quenched disorder. We apply this approach to study a quantized version of the Brazovskii model and find a self-generated quantum glass that remains in a quantum mechanically mixed state as T -> 0. This quantum glass is formed by a large number of competing states spread over an energy region which is determined within our theory.Comment: 10 pages, 4 figure

Hidden Order in the Cuprates

We propose that the enigmatic pseudogap phase of cuprate superconductors is characterized by a hidden broken symmetry of d(x^2-y^2)-type. The transition to this state is rounded by disorder, but in the limit that the disorder is made sufficiently small, the pseudogap crossover should reveal itself to be such a transition. The ordered state breaks time-reversal, translational, and rotational symmetries, but it is invariant under the combination of any two. We discuss these ideas in the context of ten specific experimental properties of the cuprates, and make several predictions, including the existence of an as-yet undetected metal-metal transition under the superconducting dome.Comment: 12 pages of RevTeX, 9 eps figure

Theory of microphase separation on side-chain liquid-crystalline polymers with flexible spacers

We model a melt of monodisperse side-chain liquid-crystalline polymers as a melt of comb copolymers in which the side groups are rod-coil diblock copolymers. We consider both excluded-volume and Maier-Saupe interactions. The first acts among any pair of segments while the latter acts only between rods. Using a free-energy functional calculated from this microscopic model, we study the spinodal stability of the isotropic phase against density and orientational fluctuations. The phase diagram obtained in this way predicts nematic and smectic instabilities as well as the existence of microphases or phases with modulated wave vector but without nematic ordering. Such microphases are the result of the competition between the incompatibility among the blocks and the connectivity constraints imposed by the spacer and the backbone. Also the effects of the polymerization degree and structural conformation of the monomeric units on the phase behavior of the side-chain liquid-crystalline polymers are studied

Bosonization Of A 2d Electron Gas In A Magnetic Field

Starting from a Landau level description of a 2D fermion gas subject to a perpendicular uniform magnetic field, under the condition that there is a large integer number of filled Landau levels, we introduce a bosonization scheme for the low energy excitations of the system. We give an explicit construction of the fermion operator in terms of the bosons and show that the description of the elementary neutral excitations within a bosonic language provides a quadratic bosonic Hamiltonian for the interacting electron system which can be easily diagonalized. © Springer-Verlag 1997.1032279281MacDonald, A.H., (1989) The Quantum Hall Effect: A Perspective, , Dordrecht: Kluwer Academic(1990) The Quantum Hall Effect, 2nd Ed., , R.E. Prange, S.M. Girvin eds., New York: SpringerKarlhede, A., Kivelson, S.A., Sondhi, S.L., The Quantum Hall Effect: The article (1992) 9th Jerusalem Winter School on Theor. Phys., , Lectures presKallin, C., Halperin, B.I., (1984) Phys. Rev. B, 30, p. 5655Tomonaga, S., (1950) Prog. Theor. Phys. (Kyoto), 5, p. 544Pegg, D.T., Barnet, S.M., (1988) Europhysics Letters, 6, p. 483Susskind, L., Glogower, J., (1964) Physics, 1, p. 49Thirring, W., (1958) Ann. Phys. (N. Y.), 3, p. 91Luttinger, J.M., (1963) J. Math. Phys., 4, p. 1154Mattis, D., Lieb, E., (1965) J. Math. Phys., 6, p. 304Luther, A., (1975) Phys. Rev. B, 14, p. 2153Coleman, S., (1975) Phys. Rev. D., 11, p. 2088Haldane, F.D.M., (1981) J. Phys. C, 14, p. 2585H. Westfahl Jr., A.H. Castro Neto, A.O. Caldeira, cond-mat/9611004Mandelstam, S., (1975) Phys. Rev. D, 11, p. 3026Castro Neto, A.H., Fradkin, E., (1993) Phys. Rev. Lett., 72, p. 1393(1995) Phys. Rev. B, 51, p. 4048Lopez, A., Fradkin, E., (1991) Phys. Rev. B, 44, p. 5246(1992) Phys. Rev. Lett., 69, p. 2126(1993) Nucl. Phys. B, 33 C, p. 67Luther, A., (1979) Phys. Rev. B, 19, p. 320Haldane, F.D.M., Varenna Lectures (1992) (1992) Helv. Phys. Acta., 65, p. 152Houghton, A., Marston, B., (1993) Phys. Rev. B, 48, p. 7790Kwon, H.J., Houghton, A., Marston, B., (1995) Phys. Rev. B, 52, p. 800