Xiaohang Zhang

Using three-body recombination to extract electron temperatures of ultracold plasmas.

Three-body recombination, an important collisional process in plasmas, increases dramatically at low electron temperatures, with an accepted scaling of Te(-9/2). We measure three-body recombination in an ultracold neutral xenon plasma by detecting recombination-created Rydberg atoms using a microwave-ionization technique. With the accepted theory (expected to be applicable for weakly coupled plasmas) and our measured rates, we extract the plasma temperatures, which are in reasonable agreement with previous measurements early in the plasma lifetime. The resulting electron temperatures indicate that the plasma continues to cool to temperatures below 1 K.

Anomalous magnetoresistance in the spinel superconductor LiTi2O4.

LiTi2O4 is a unique compound in that it is the only known spinel oxide superconductor. The lack of high quality single crystals has thus far prevented systematic investigations of its transport properties. Here we report a careful study of transport and tunnelling spectroscopy in epitaxial LiTi2O4 thin films. An unusual magnetoresistance is observed which changes from nearly isotropic negative to prominently anisotropic positive as the temperature is decreased. We present evidence that shows that the negative magnetoresistance likely stems from the suppression of local spin fluctuations or spin-orbit scattering centres. The positive magnetoresistance suggests the presence of an orbital-related state, also supported by the fact that the superconducting energy gap decreases as a quadratic function of magnetic field. These observations indicate that the spin-orbital fluctuations play an important role in LiTi2O4 in a manner similar to high-temperature superconductors.

Observation of an ultracold plasma instability.

We present the first observation of an instability in an expanding ultracold plasma. We observe periodic emission of electrons from an ultracold plasma in weak, crossed magnetic and electric fields, and a strongly perturbed electron density distribution in electron time-of-flight projection images. We identify this instability as a high-frequency electron drift instability due to the coupling between the electron drift wave and electron cyclotron harmonic, which has large wave numbers corresponding to wavelengths close to the electron gyroradius.

Ultracold plasma expansion in a magnetic field.

We measure the expansion of an ultracold plasma across the field lines of a uniform magnetic field. We image the ion distribution by extracting the ions with a high-voltage pulse onto a position-sensitive detector. Early in the lifetime of the plasma (<20 micros), the size of the image is dominated by the time-of-flight Coulomb explosion of the dense ion cloud. For later times, we measure the 2D Gaussian width of the ion image, obtaining the transverse expansion velocity as a function of the magnetic field (up to 70 G). We observe that the expansion velocity scales as B(-1/2), explained by a nonlinear ambipolar diffusion model with anisotropic diffusion in two different directions.

Influence of oxygen isotope exchange on the ground state of manganites

We report a study of oxygen isotope effects on the low-temperature specific heat, magnetization, and resistivity of

Reversible electrochemical modulation of the superconducting transition temperature of LiTi2O4 ultrathin films by ionic liquid gating

{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}

Nonlinear Hall Effect as a Signature of Electronic Phase Separation in the Semimetallic Ferromagnet EuB6

and

Using Charged Particle Imaging to Study Ultracold Plasma Expansion

({\mathrm{La}}_{1\ensuremath{-}y}{\mathrm{Pr}}_{y}{)}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}.

Probing the thiol-gold planar interface by spin polarized tunneling

For metallic compositions of

Signatures of electronic phase separation in the Hall effect of anisotropically strained La0.67Ca0.33MnO3 films

{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}