M. P. M. Dean

Ferroelectric quantum criticality

Materials tuned to the neighbourhood of a zero temperature phase transition often show the emergence of novel quantum phenomena. Much of the effort to study these new effects, like the breakdown of the conventional Fermi-liquid theory of metals has been focused in narrow band electronic systems. Ferroelectric crystals provide a very different type of quantum criticality that arises purely from the crystalline lattice. In many cases the ferroelectric phase can be tuned to absolute zero using hydrostatic pressure or chemical or isotopic substitution. Close to such a zero temperature phase transition, the dielectric constant and other quantities change into radically unconventional forms due to the quantum fluctuations of the electrical polarization. The simplest ferroelectrics may form a text-book paradigm of quantum criticality in the solid-state as the difficulties found in metals due to a high density of gapless excitations on the Fermi surface are avoided. We present low temperature high precision data demonstrating these effects in pure single crystals of SrTiO3 and KTaO3. We outline a model for describing the physics of ferroelectrics close to quantum criticality and highlight the expected 1/T2 dependence of the dielectric constant measured over a wide temperature range at low temperatures. In the neighbourhood of the quantum critical point we report the emergence of a small frequency independent peak in the dielectric constant at approximately 2K in SrTiO3 and 3K in KTaO3 believed to arise from coupling to acoustic phonons. Looking ahead, we suggest that in ferroelectric materials supporting mobile charge carriers, quantum paraelectric fluctuations may mediate new effective electron-electron interactions giving rise to a number of possible states such as superconductivity.

Spin excitations in a single La2CuO4 layer

Cuprates and other high-temperature superconductors consist of two-dimensional layers that are crucial to their properties. The dynamics of the quantum spins in these layers lie at the heart of the mystery of the cuprates. In bulk cuprates such as La(2)CuO(4), the presence of a weak coupling between the two-dimensional layers stabilizes a three-dimensional magnetic order up to high temperatures. In a truly two-dimensional system however, thermal spin fluctuations melt long-range order at any finite temperature. Here, we measure the spin response of isolated layers of La(2)CuO(4) that are only one-unit-cell-thick. We show that coherent magnetic excitations, magnons, known from the bulk order, persist even in a single layer of La(2)CuO(4), with no evidence for more complex correlations such as resonating valence bond correlations. These magnons are, therefore, well described by spin-wave theory (SWT). On the other hand, we also observe a high-energy magnetic continuum in the isotropic magnetic response that is not well described by two-magnon SWT, or indeed any existing theories.

High-Energy Magnetic Excitations in the Cuprate Superconductor Bi2Sr2CaCu2O8+delta: Towards a Unified Description of Its Electronic and Magnetic Degrees of Freedom

Bi2Sr2CaCu2O8+δ: Towards a unified description of its electronic and magnetic degrees of freedom M. P. M. Dean, ∗ A. J. A. James, R. S. Springell, 3 X. Liu, † C. Monney, K. J. Zhou, ‡ R. M. Konik, J. S. Wen, Z. J. Xu, G. D. Gu, V. N. Strocov, T. Schmitt, and J. P. Hill § Department of Condensed Matter Physics and Materials Science, Brookhaven National Laboratory, Upton, New York 11973, USA Royal Commission for the Exhibition of 1851 Research Fellow, Interface Analysis Centre, University of Bristol, Bristol BS2 8BS, UK London Centre for Nanotechnology and Department of Physics and Astronomy, University College London, London WC1E 6BT, United Kingdom Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland (Dated: May 5, 2014)

Nonadiabatic phonons within the doped graphene layers of XC6 compounds

We report Raman-scattering measurements of BaC6, SrC6, YbC6, and CaC6, which permit a systematic study of the phonons and the electron-phonon interaction within the doped graphene layers of these compounds. The out-of-plane carbon phonon softens as the spacing of the graphene layers is reduced in the series BaC6, SrC6, YbC6, and CaC6. This is due to increasing charge in the pi* electronic band. Electron-phonon interaction effects between the in-plane carbon modes at approximate to 1500 cm(-1) and the pi* electrons cause a strong nonadiabatic renormalization. As charge is transferred into the pi* band, these nonadiabatic effects are found to increase concurrent with a reduction in the phonon lifetime.

Magnetic excitations in stripe-ordered La_{1.875}Ba_{0.125}CuO_{4} studied using resonant inelastic x-ray scattering

The charge and spin correlations in La

Itinerant effects and enhanced magnetic interactions in Bi-based multilayer cuprates

_{1.875}

Static charge-density-wave order in the superconducting state of La2−xBaxCuO4

Ba

Neutron scattering study of the high-energy graphitic phonons in superconducting CaC6

_{0.125}

Doping Dependence of Collective Spin and Orbital Excitations in the Spin-1 Quantum Antiferromagnet La2-xSrxNiO4 Observed by X Rays

CuO

Comparison of charge modulations in

_4