Publications

Physical Review Letters (1 article)

Unusual Behavior of the Ferroelectric Polarization in PbTiO3/SrTiO3 Superlattices

by M. Dawber / DPMC - University of Geneva (picture), C. Lichtensteiger, M. Cantoni, M. Veithen, P. Ghosez, K. Johnston, K. M. Rabe, and J.-M. Triscone - PRL 95, 177601 (2005)

A personal resume by Matt Dawber : 'More bites from a ferroelectric sandwich'
With todays advanced deposition techniques one of the most popular approaches in the search for exceptional material properties is to fabricate multilayers of two or more materials grown epitaxially one on top of each other, not entirely dissimilar to layers of meat and bread in a very large sandwich (though perhaps a little more expensive). Our interest is in ferroelectric materials, which posess an electrical polarization in the absence of an applied electric field and are used in applications ranging from non-volatile memories to a host of piezoelectric devices. Specifically we made a series of 20 bi-layer superlattices, in which each bi-layer was composed of a 3 unit cell thick layer of strontium titanate (a paraelectric material) and layer of lead titanate (a ferroelectric material) which had a thickness varied from 1 unit cell to 54 unit cells. The expectation was that as the volume fraction of the lead titanate was reduced the polarization of the superlattice structure should decrease, and indeed so long as the ratio of lead titanate to strontium titanate was greater than one this is what we observed. However when the ratio of lead titanate was further reduced we observed an unexpected recovery of the polarization, which we were unable to explain either on the basis of an electrostatic model or from first principles calculations. This counter-intuitive result suggests that on the smallest length scales ferroelectricity behaves in an highly intriguing manner, a property that is ripe for future exploitation in nanoscale devices.

Official abstract
Artificial PbTiO3/SrTiO3 superlattices were constructed using off-axis rf magnetron sputtering. X-ray diffraction and piezoelectric atomic force microscopy were used to study the evolution of the ferroelectric polarization as the ratio of PbTiO3 to SrTiO3 was changed. For PbTiO3 layer thicknesses larger than the 3-unit cell SrTiO3 thickness used in the structure, the polarization is found to be reduced as the PbTiO3 thickness is decreased. This observation confirms the primary role of the depolarization field in the polarization reduction in thin films. For the samples with ratios of PbTiO3 to SrTiO3 of less than one, a surprising recovery of ferroelectricity that cannot be explained by electrostatic considerations was observed.

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Nature Materials (1 article)

Three-dimensionality of field-induced magnetism in a high-temperature superconductor

by B. Lake, K. Lefmann, N. B. Christensen, G. Aeppli, D. F. McMorrow, H. M. Ronnow / LNS - PSI - ETHZ (picture), P. Vorderwisch, P. Smeibidl, N. Mangkorntong, T. Sasagawa, M. Nohara, H. Takagi - Nature Materials 4, 658-662 (01 Sep 2005) Letters

Abstract
Many physical properties of high-temperature superconductors are two-dimensional phenomena derived from their square-planar CuO2 building blocks. This is especially true of the magnetism from the copper ions. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours1, evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge-carrier densities, quantum fluctuations are sufficiently suppressed to yield static long-period order2-6, and external magnetic fields also induce such order7-12. Here we show that, in contrast to the chemically controlled order in superconducting samples, the field-induced order in these same samples is actually three-dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three-dimensional magnetic couplings that survive into the superconducting state, and coexist with the crucial inter-layer couplings responsible for three-dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which requires zero electrical resistance in an applied magnetic field and depends on vortex dynamics, and the underlying antiferromagnetism of the cuprates.

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Physical Review B (1 article)

Temperature-modulation analysis of superconductivity-induced transfer of in-plane spectral weight in Bi2Sr2CaCu2O8
by A. B. Kuzmenko, H. J. A. Molegraaf, F. Carbone, and D. van der Marel - Phys. Rev. B 72, 144503 (2005)

Abstract
We examine the superconductivity-induced redistribution of optical spectral weight in Bi2Sr2CaCu2O8 near optimal doping using a detailed Kramers-Kronig consistency analysis of the kink (slope change) at Tc of the temperature-dependent optical spectra, published earlier [H. J. A. Molegraaf et al., Science 295, 2239 (2002)]. We demonstrate that the temperature dependence of the complex dielectric function at high frequencies (above 0.75 eV) imposes the most stringent limits on the possible changes of the low-frequency integrated spectral weight. The presented calculations provide additional arguments, supporting the previous conclusion about a superconductivity-induced increase of the integrated low-frequency spectral weight below Tc. The Ferrell-Glover-Tinkham sum rule is not satisfied well above 2.5 eV, which indicates that this increase is caused by the transfer of spectral weight from the interband to the intraband region and only partially by the narrowing of the Drude peak.

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Journal of Physics : Condensed Matter (1 article)

Novel spin lattice in Cu3TeO6: an antiferromagnetic order and domain dynamics
by M. Herak, H. Berger / EPFL (picture), M. Prester, M. Miljak, I. Zivkovic, O. Milat, D. Drobac, S. Popovic and O. Zaharko - J. Phys.: Condens. Matter 17 7667-7679

Abstract
We report on the magnetic properties of an insulating cubic compound Cu3TeO6 studied by ac and dc susceptibility, torque magnetometry and neutron powder diffraction. A novel three-dimensional magnetic lattice composed of almost planar regular hexagons of Cu2+ S = 1/2 spins is present in Cu3TeO6. The magnetic susceptibility in the paramagnetic state obeys the Curie–Weiss law in the 200–330 K regime with ΘCW = -148 K and at TN = 61 K system undergoes an antiferromagnetic phase transition. Above TN the susceptibility is isotropic. Below TN a large anisotropy develops in fields H≥500 Oe. Torque measurements reveal the presence of antiferromagnetic domains below TN. In a rather low magnetic field ( Oe) switching of domains is observed. The dynamics related to movement of domain walls is very slow at low temperatures (of the order of 102 s) and interferes with all torque measurements. The presence of domains is a consequence of the symmetry of the underlying magnetic lattice. Neutron powder diffraction reveals that antiferromagnetic long-range order is associated with the wavevector . The dominant component of the magnetic moment is along one of the space diagonals of the cubic unit cell, but it is not possible to resolve whether the structure is collinear or canted.

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Thermochimica Acta (1 article)

Tantalum and niobium perovskite oxynitrides: Synthesis and analysis of the thermal behaviour
by A. Rachela, S.G. Ebbinghausa, M. Güngerichb, P.J. Klarb, J. Hanssa, A. Weidenkaff / EMPA (picture), and A. Rellera - Thermochimica Acta 438 (2005) 134–143

Abstract
Ta5+ and Nb5+-based oxynitride perovskites of the ABO2N type (A = Ca, Sr, Ba) were synthesised by ammonolysis of complex oxide precursors. These precursors were either crystalline perovskites or amorphous xerogels prepared by solid–solid reaction and by soft chemistry methods, respectively. Phase purity of the oxynitrides was verified by X-ray diffraction (XRD) and their crystal structures were determined by Rietveld refinements. The morphology of the obtained powders was characterised by scanning electron microscopy (SEM). Thermal stability was investigated by thermogravimetric analysis (TGA) coupled with mass spectroscopy. Oxidation studies reveal an intermediate product that gives rise to a characteristic weight gain in the TG curve. This intermediate was found for all the examined oxynitrides in oxidising atmosphere. Investigations by Raman scattering revealed the presence of dinitrogen (NN) loosely bound to B and NB bonds (B = transition metal) in the intermediate compounds. Mass spectral analysis confirmed molecular nitrogen evolution indicating that N2 is retained during the oxidation reaction. At higher temperatures (T = 800–1000 °C) the dinitrogen is released leading to the formation of the corresponding oxides.

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