Sharon Pecker

A universal critical density underlying the physics of electrons at the LaAlO3/SrTiO3 interface

The two-dimensional electron system at the interface between the insulating oxides LaAlO(3) and SrTiO(3) exhibits ferromagnetism, superconductivity and a range of unique magnetotransport properties. An open experimental challenge is to identify, out of the multitudinous energy bands predicted to exist at the interface, the key ingredients underlying its emergent transport phenomena. Here we show, using magnetotransport measurements, that a universal Lifshitz transition between d orbitals of different symmetries lies at the core of the observed phenomena. We find that LaAlO(3)/SrTiO(3) systems generically switch from one- to two-carrier transport at a universal carrier density, which is independent of the LaAlO(3) thickness and electron mobility. Interestingly, the maximum superconducting critical temperature occurs also at the Lifshitz density, indicating a possible connection between the two phenomena. A simple band model, allowing for spin-orbit coupling at the atomic level, connects the observed transition to a variety of previously reported properties. Our results demonstrate that the fascinating behaviour observed so far in these oxides follows from a small but fundamental set of bands.

Gate-tunable polarized phase of two-dimensional electrons at the LaAlO3/SrTiO3 interface

Controlling the coupling between localized spins and itinerant electrons can lead to exotic magnetic states. A novel system featuring local magnetic moments and extended 2D electrons is the interface between LaAlO3 and SrTiO3. The magnetism of the interface, however, was observed to be insensitive to the presence of these electrons and is believed to arise solely from extrinsic sources like oxygen vacancies and strain. Here we show the existence of unconventional electronic phases in the LaAlO3/SrTiO3 system pointing to an underlying tunable coupling between itinerant electrons and localized moments. Using anisotropic magnetoresistance and anomalous Hall effect measurements in a unique in-plane configuration, we identify two distinct phases in the space of carrier density and magnetic field. At high densities and fields, the electronic system is strongly polarized and shows a response, which is highly anisotropic along the crystalline directions. Surprisingly, below a density-dependent critical field, the polarization and anisotropy vanish whereas the resistivity sharply rises. The unprecedented vanishing of the easy axes below a critical field is in sharp contrast with other coupled magnetic systems and indicates strong coupling with the moments that depends on the symmetry of the itinerant electrons. The observed interplay between the two phases indicates the nature of magnetism at the LaAlO3/SrTiO3 interface as both having an intrinsic origin and being tunable.

Realization of pristine and locally tunable one-dimensional electron systems in carbon nanotubes

The ability to tune local parameters of quantum Hamiltonians has been demonstrated in experimental systems including ultracold atoms, trapped ions, superconducting circuits and photonic crystals. Such systems possess negligible disorder, enabling local tunability. Conversely, in condensed-matter systems, electrons are subject to disorder, which often destroys delicate correlated phases and precludes local tunability. The realization of a disorder-free and locally-tunable condensed-matter system thus remains an outstanding challenge. Here, we demonstrate a new technique for deterministic creation of locally-tunable, ultralow-disorder electron systems in carbon nanotubes suspended over complex electronic circuits. Using transport experiments we show that electrons can be localized at any position along the nanotube and that the confinement potential can be smoothly moved from location to location. The high mirror symmetry of transport characteristics about the nanotube centre establishes the negligible effects of electronic disorder, thus allowing experiments in precision-engineered one-dimensional potentials. We further demonstrate the ability to position multiple nanotubes at chosen separations, generalizing these devices to coupled one-dimensional systems. These capabilities could enable many novel experiments on electronics, mechanics and spins in one dimension.

A multiphase equation of state of three solid phases, liquid, and gas for titanium

A multiple-phase equation of state of the α phase, β phase, ω phase, liquid, and gas for titanium is presented. This equation of state is thermodynamically consistent, based on a three-term semiempirical model for the Helmholtz free energy. The parameters of the free energy are first evaluated from the experimental data and solid-state theoretical calculations. Then, the values of the parameters are adjusted using a numerical minimization scheme based on the simplex algorithm, to values that best reproduce measured phase diagrams and other experimental data. The predicted phase diagram shows a compression-induced β-ω transition, up to a β-ω-liquid triple point at ∼45GPa and ∼2200K. For pressures above this triple point, the melting occurs from the ω phase. Moreover, no β-ω transition is predicted along the Hugoniot curve starting at STP conditions.

Equation of state for aluminum containing helium bubbles

A theoretical model for equation of state (EOS) of aluminum with helium bubbles is presented. Based on this EOS, the influence of helium bubbles on shock loading is examined. The Hugoniot curve (temperature versus pressure as well as shock velocity versus particle velocity) for aluminum containing bubbles is calculated for various bubbles mass, bubbles percentage, and helium EOS models. The bubble mass and concentration seem to affect the measurably Hugoniot curve. The EOS model, implied for the helium in the bubbles, has minor significance, which means our model is not sensitive to the details of the helium EOS. Our findings are consistent with experiments available in the literature.