Siddharth S. Saxena

Superconductivity on the border of itinerant-electron ferromagnetism in UGe2

The absence of simple examples of superconductivity adjoining itinerant-electron ferromagnetism in the phase diagram has for many years cast doubt on the validity of conventional models of magnetically mediated superconductivity. On closer examination, however, very few systems have been studied in the extreme conditions of purity, proximity to the ferromagnetic state and very low temperatures required to test the theory definitively. Here we report the observation of superconductivity on the border of ferromagnetism in a pure system, UGe 2, which is known to be qualitatively similar to the classic d-electron ferromagnets. The superconductivity that we observe below 1 K, in a limited pressure range on the border of ferromagnetism, seems to arise from the same electrons that produce band magnetism. In this case, superconductivity is most naturally understood in terms of magnetic as opposed to lattice interactions, and by a spin-triplet rather than the spin-singlet pairing normally associated with nearly antiferromagnetic metals.

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.

Pressure-induced insulating state in (La,Sr)CoO3

We have investigated the effect of pressure on the electronic, magnetic, and structural properties on a single crystal of conducting, ferromagnet (TC=157 K) La0.82Sr0.18CoO3 located near the boundary of the metal-insulator transition. Contrary to the results reported on related systems, we find a transition from the conducting state to an insulating state and a decrease of TC with increasing pressure while the lattice structure remains unchanged. We show that this unusual behavior is driven by a gradual change of the spin state of Co3+ ions from magnetic intermediate-spin (t2g5eg1;S=1) to a nonmagnetic low-spin (t2g6eg0;S=0) state.

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.

Superconductivity: Symmetry not required

Superconductivity is a complex phenomenon. And now theres something else to think about: a magnetic material whose structure is not mirror symmetric and yet, unexpectedly, superconducts.

Anomalous low temperature states in CeNi2Ge2 and CePd2Si2

High purity samples of the paramagnetic 4f-electron metal CeNi2Ge2 exhibit a non-Fermi-liquid form of the resistivity Δρ~Tx with x<1.5 and decreasing towards 1 with increasing sample purity. Measurements of ρ versus T as a function of magnetic field and pressure show that this strange metallic phase is connected to the proximity of an antiferromagnetic quantum critical point as in the isoelectronic relative CePd2Si2 near 2.8 GPa. The anomalous power-law dependence is surprisingly stable over extended ranges in temperature and pressure and challenges current theory of magnetic quantum phase transitions.

Superconductivity. Iron cast in exotic role.

Conventional wisdom says that superconductivity and magnetism are incompatible bedfellows. So the idea of iron as a superconductor is ruled out — or is it?

Non-Fermi-liquid behaviour in magnetic d- and f-electron systems

Abstract A growing number of metals have been found to show systematic deviations from the predictions of Landau Fermi-liquid theory. The most straightforward examples are magnetic metals in which the Curie ( T C ) or Neel ( T N ) temperature has been suppressed to 0 K by the application of hydrostatic pressure. Such systems are discussed here within a spin fluctuation framework which provides quantitatively accurate descriptions of the d-electron systems MnSi and ZrZn 2 near their ‘quantum critical points’, but which appears to be less successful (in its simplest form) in antiferromagnetic f-electron systems, particularly CePd 2 Si 2 , which at its critical pressure has a resistivity of the form Δρ ∝ T 1.2 = 0.1 over a temperature range extending from around 40 K to below 1 K. CeNi 2 Ge 2 , which is believed to be close to a quantum critical point at ambient pressure, shows similar behaviour. CeIn 3 , CePd 2 Si 2 and CeNi 2 Ge 2 exhibit superconductivity, in the first two cases limited to a narrow region near the critical pressure, making these the first unambiguous examples of magnetically mediated superconductivity.

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.

Bulk evidence for single-Gap s-wave superconductivity in the intercalated graphite superconductor C6Yb.

We report measurements of the in-plane electrical resistivity rho and thermal conductivity kappa of the intercalated graphite superconductor C6Yb down to temperatures as low as Tc/100. When a field is applied along the c axis, the residual electronic linear term kappa0/T evolves in an exponential manner for Hc1<H<Hc2/2. This activated behavior is compelling evidence for an s-wave order parameter, and is a strong argument against the possible existence of multigap superconductivity.