docExperiments
download 
Reclamo Transcript
Experiments
Pressure-induced metal-insulator transition on solid Tellurium and liquid iodine Daniele Chermisi Dottorato in fisica XXIII ciclo Tutor: Prof. Paolo Postorino DANIELE CHERMISI Outline • Introduction to metal-insulator transition • Samples Solid tellurium and liquid iodine • Experiments Raman and Infrared spectroscopy • DFT calculations Dynamical properties using the espresso and cpmd code • Conclusion and perspectives DANIELE CHERMISI Introduction to metal-insulator transition (MIT) • Pressure and condensed matter physics, earth and planetary science and material science • Microscopic mechanism of MIT still lacking • Optical spectroscopy and diamond anvil cell (DAC) • Ab-initio technique applied to high pressure physics Optical spectroscopy on solid tellurium and liquid iodine supported by DFT calculation DANIELE CHERMISI Metal-insulator transition (2) Tellurium resistivity Liquid iodine conductivity Barden Phys. Rev. Lett 75,1777 (1949) V.V. Brazhkin High Pressure Res., 06 (1992) 363 DANIELE CHERMISI Samples: solid tellurium Trigonal, insulator Triclinic,metal Monoclinic (incommensurate) Hejny et al. Phys. Rev. B 74,174119 (2006) DANIELE CHERMISI Experiments Diamond anvil cell Structural simmetrization P~25 GPa Sample diameter ~ 50 μm Reduction reticular distortion Polarizability, dipole moment, electronic properties changes Raman experiments Dip. Fisica “Sapienza” Infrared experiments Sincrotrone ELETTRA (Trieste) DANIELE CHERMISI Pressure measurements Rubin fluorescence P=α(ν−ν0) 1/α=0.7539 cm-1/GPa B Λ ∆λ − 1 P = 1 + B λ0 Λ = 19.04Mbar B = 7.665 λ0 = 649.28nm H.K.Mao, P.M.Bell, J.W.Shaner, D.J.Steinberg J. Appl. Phys. 49, 3276 (1978). λ (nm) Gradient control DANIELE CHERMISI Raman experiment on tellurium Γraman=A1+2E rotation Antisymmetric breathing Symmetric breathing DANIELE CHERMISI Raman v.s pressure in two different hydrostatic media Raman experiment on tellurium DHO fit DANIELE CHERMISI Raman experiment on tellurium 0-4 Gpa: softening of A1 mode IMT transition @ 4GPa 4-7 Gpa: softening of E mode @ 140 cm-1 Te I Te II coexistence till 8 Gpa 7-15 Gpa: hardening of A1 mode incommensurate modulation Integrated areas Charge transfer DANIELE CHERMISI DFT Calculation on tellurium Simulazioni a cella variabile Teoria perturbativa DFT DANIELE CHERMISI Dimamica reticolare v.s pressione In collaboration with Prof. S Scandolo of ICTP Trieste Samples: liquid iodine Iodine phase diagram Intramolecular distance (EXAFS) solid liquid Buontempo et al. Phys. Rev. Lett. 80,1912 (1998) Molecular > Atomic Insulator > Metal Gap closure DANIELE CHERMISI Solid 21 GPa 16 GPa Contiuous Liquid 23 GPa 4 GPa Abrupt Configurational disorder Liquid iodine: percolation mechanism? Likalter supposing a percolation mechanism predict a abrupt change in conductivity at density of 4.4 g/cm3 EXAFS measurements show a marked expansion of the intramolecular bond length Charge transfer drive the MIT Raman Raman could be crucial in demonstrating the connection betwen charge transfer and bond length expansion DANIELE CHERMISI Experiments: liquid iodine • High temperature diamond anvil cell (HTDAC) properly designed and constructed by LOTO (Florence) has to be used • The diamond have to be covered with grahte-glue mixture coil to avoid the presence in Raman spectra of black body radiation emitted from coil • The cell has to be kept in vacuum in a steel vessel equipped with thin quartz window • Liquid iodine has to be put in a LiF pellet to avoid direct contact with gasket in order to prevent contamination of sample and corrosion of the gasket DANIELE CHERMISI Experiments: liquid iodine (2) Preliminary Raman measurements In analogy with solid tellurium and crystal iodine the softening of the intrachain Raman active mode is a consequence of the charge trasfer precurring the IMT. DANIELE CHERMISI Iodio liquido: ab-initio molecular dynamics • Local structure of liquid iodine could be very useful to verify the hypothesis of a IMT originated by the insurgence of dynamic percolative path along weakened molecular units • Very few experimental data on liquid iodine at high pressure • Check the simulation calculating the Raman active mode • Investigate the local structure, pair correlation function, atomic coordination function and the molecular orientational correlation DANIELE CHERMISI In collaboration with L. Guidoni “Sapienza” Prospective • Reflectivity measurements on tellurium with synchrotron radiation @ ELETRA •Raman measurements on liquid iodine as function of temperature and pressure •ab-initio molecular dynamics on liquid iodine @ CASPUR in collaboration with L. Guidoni University of Rome “Sapienza” DANIELE CHERMISI Acknowledgements Prof. P. Postorino Dot. C. Marini Prof. S. Scandolo University of Rome “Sapienza” Abdus Salam International Center for Theoretical Physics of Trieste Dot. L. Guidoni University of Rome “Sapienza” DANIELE CHERMISI
