Session J4: Materials Science II

Ferromagnetism with Continuum Excitations in the Geometrically Frustrated Pyrochlore Yb$_2$Ti$_2$O$_7$

In the frustrated pyrochlore magnet Yb$_2$Ti$_2$O$_7$, Yb$^{3+}$ forms (effective) S=1/2 moments that are subject to anisotropic exchange interactions. The ground state of this material is rather perplexing and controversial; one proposal is that Yb$_2$Ti$_2$O$_7$ behaves as a quantum version of ``spin ice'', where quantum fluctuations enable tunneling between different ice-rules obeying spin states. This model includes a quantum spin liquid ground state hosting three exotic quasi-particle excitations that are analogs of magnetic monopoles, electric monopoles, and photons; i.e., it would host a full ``emergent electrodynamics''. However, there are varying reports about the true nature of the ground state in Yb$_2$Ti$_2$O$_7$, in part due to a known sample-dependence involving non-stoichiometry on the 1\% level, which is particularly prevalent in single crystal samples. After introducing the key prior measurements on Yb$_2$Ti$_2$O$_7$, I will present neutron scattering results from a polycrystalline sample which is known to be stoichiometric. This sample was found to develop magnetic Bragg peaks at low temperatures, consistent with moments forming a long range ordered ferromagnetic state. Despite this signature of a long range ordered ground state, the spin excitations remain gapless and continuum-like (unlike the expectation for conventional magnons), and are generally insensitive to the first order transition at 260 mK which is observed via specific heat measurements.   

Study of the Structural, Electronic, and Vibrational Properties of Some Type-II Sn-Based Clathrates.

In addition to the $\alpha $-Sn and $\beta $-Sn crystal structures, Sn can crystallize in the clathrate structures. These contain 20, 24, or 28-atom ``cages'' which allow for the introduction of loosely bound guest atoms. Due to their low thermal conductivity, some of the Sn-based materials are of interest because of their thermoelectric properties. Motivated by the recent synthesis of the Type II clathrates K$_{8}$Ba$_{16}$Ga$_{40}$Sn$_{96}$, Rb$_{8}$Ba$_{16}$Ga$_{40}$Sn$_{96}$, and Cs$_{8}$Ba$_{16}$Ga$_{40}$Sn$_{96}$ [1, 2], we are performing a systematic, first principles, computational study of the properties of these materials. Our calculations are based on density functional theory and utilize the VASP code. We present results for the structural, electronic and vibrational properties of these compounds. Our predictions include lattice parameters, bulk moduli, electronic densities of states, and vibrational modes. The lattice thermal conductivity and other transport properties of these materials will also be discussed. [1] Sch\"{a}fer, M. C., Bobev, S.; \textit{J. Am. Chem. Soc. }135, 1696 (2013) [2] Koda, S., Kishimoto, K., Akai, K., Asada, H., Koyanagi, T.; \textit{J. Appl. Phys.}116, 023710 (2014)   

Phase transitions in advanced materials responding to ultrafast laser pulses: review of some experiments and a new theoretical approach

This talk will review some experimental studies of advanced materials responding to fast intense laser pulses, including light-induced superconductivity in cuprates [1]. A new method will be introduced for treating ultrafast phase transitions, such as those involving superconductivity, magnetism, charge density waves, and spin density waves. Illustrative results will be presented for a toy model, with the electronic temperature immediately after the laser pulse calculated as a function of the fluence. We wish to thank Ayman Abdullah-Smoot, Michelle Gohlke, David Lujan, and James Sharp for many helpful discussions and other contributions. [1] D. Fausti, R. I. Tobey, N. Dean, S. Kaiser, A. Dienst, M. C. Hoffmann, S. Pyon, T. Takayama, H. Takagi, and A. Cavalleri, ``Light-Induced Superconductivity in a Stripe-Ordered Cuprate'', Science 331, 189 (2011).   

Electron Spin Coherence in Silicon Carbide

Spin is an important quantum mechanical effect intrinsic in electrons which contributes to the particle's overall qualitative behavior. When contained in a magnetic field, electrons have energy levels determined by the alignment of their spins. Moreover, if the spins are aligned perpendicular to the magnetic field they start to precess and become de-coherent over time. In our experiment, laser pulses align the spins in the up direction, and then a series of focused microwave pulses rotate the spins and allow their coherence to be studied through a technique known as ``spin-echo''. In our group, we seek to better understand what factors contribute to an electron's spin coherence over time. For this conference, I will be presenting on electron spin coherence times found in crystal lattice defects of electron-irradiated silicon carbide. Specifically, I will focus on the experimental set-up and procedure that we use for optically analyzing the spin-states of electrons in SiC. I will also focus on the effects of irradiation (proton vs electron) in overall coherence times in the sample.   

Electronic and optical properties of VSc2N@C68 fullerene

We report a detailed investigation on structural, electronic, and spectroscopic properties of the VSc2N@C68. The candidate structures for the ground state are obtained using a systematic approach. Our results indicate that the isomer 6079 of C68 yields the lowest energy structure of VSc2N@C68 at DFT level. The doping by V leads to an overall spin magnetic moment of 1 muB for the cluster. The ionization energy, electron affinity, the quasi- particle gap, and vibrational analysis of the lowest energy isomer indicate a stable molecule. The calculated infrared, Raman and optical spectra of the most stable fullerene can help in experimental identification.