L. J. Spalek

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.

Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

Up to now the crystallographic structure of the magnetoelectric perovskite EuTiO3 was considered to remain cubic down to low temperature. Here we present high resolution synchrotron X-ray powder diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle indicates a second-order phase transition with an estimated Tc=235K. This critical temperature is well below the recent anomaly reported by specific heat measurement at TA\sim282K. By performing atomic pair distribution function analysis on diffraction data we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated Tc, the average model symmetry is fully compatible with the local environment distortion but the former is characterized by a reduced value of the tilting angle compared to the latter. At T=240K data show the presence of local octahedra tilting identical to the low temperature one, while the average crystallographic structure remains cubic. On this basis, we propose intrinsic lattice disorder to be of fundamental importance in the understanding of EuTiO3 properties.

Electric field modulation of the tetragonal domain orientation revealed in the magnetic ground state of quantum paraelectric EuTiO3

A.P. Petrović, Y. Kato, S.S. Sunku, T. Ito, P. Sengupta, L. Spalek, 5 M. Shimuta, T. Katsufuji, C.D. Batista, S. Saxena, and C. Panagopoulos 4, 5 Division of Physics and Applied Physics, Nanyang Technological University, 637371 Singapore Theoretical Division, Los Alamos National Laboratory, CNLS and T-4, Los Alamos, New Mexico 87545, USA Correlated Electron Engineering Group, AIST, Ibaraki 305-8562, Japan Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom Department of Physics, University of Crete and FORTH, 71003 Heraklion, Greece Department of Physics, Waseda University, Tokyo 169-8555, Japan (Dated: May 10, 2014)

Magnetoelastic relaxations in EuTiO3

The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300 K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below . Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls.

Novel metallic states at low temperatures

We present an overview of unconventional metallic states arising close to magnetic quantum critical points with a focus on d-electron systems. The applicability and potential breakdowns of traditional self-consistent field theories of such materials are discussed as well as related phenomena in other systems.

Verwey transition in Fe3O4 at high pressure: Quantum critical point at the onset of metallization

We provide evidence for the existence of a quantum critical point at the metallization of magnetite

Verwey transition inFe3O4at high pressure: Quantum critical point at the onset of metallization

{\text{Fe}}_{3}{\text{O}}_{4}

Verwey transition in Fe

at pressure

_{3}

{p}_{c}\ensuremath{\approx}8\text{ }\text{GPa}

O

. We show that the present ac magnetic-susceptibility data support earlier resistivity data. The Verwey temperature scales with pressure