Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B38: Metal-Insulator Transitions - Experiment I |
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Sponsoring Units: DCMP Chair: John Tranquada, Brookhaven NL Room: LACC 513 |
Monday, March 21, 2005 11:15AM - 11:27AM |
B38.00001: The metal-insulator transition in LaxWO3 and YxWO3 Michael Hoch, Charles Kasl The metal-insulator (MI) transition in the trivalent-ion doped cubic tungsten bronzes, LaxWO3 and YxWO3 ( 0.05 $<$ x $<$ 0.20), has been investigated using low temperature electrical transport, NMR and magnetization measurements on sintered samples.The critical concentration is estimated to be xc = 0.07 with some uncertainty because a mixed phase appears below x = 0.085. The trivalent ions donate their electrons into the WO3 (t2g) conduction band. Hall effect measurements as a function of T indicate that partial ‘freeze-out’ of carriers occurs at low temperatures for x $<$ 0.12. However, no evidence for local moments is obtained. The results suggest that for 0.07 $<$ x $<$ 0.12 an intermediate just-metallic phase, with properties that evolve with x, is important. The findings are compared with available theory for the MI transition in tungsten - bronzes. [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B38.00002: High Pressure Structure and Transport Studies of LaxMnO3 Zhiqiang Chen, Trevor Tyson High pressure structural and transport measurements on the self- doped system LaxMnO$_{3 }$(x=0.9 and x=0.8) were performed. The monoclinic space group is maintained to pressures of 11 Gpa. The compression of the unit cell is not isotropioc. While these space group places constraints on the Mn-O-Mn bond angle and Mn-O bond lengths significant compression of the Mn- O1 distance is found to occur with pressure (tracking the reduction of the a lattice parameter). Transport measurements are being conducted in order to correlate the changes in structure with changes in the resistivity. Comparison with large and small bandwidth manganites will be made. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B38.00003: Metal-insulator transition and electronic structure of quasi-one-dimensional BaVS$_3$ studied by photoemission S.-K. Mo, J.W. Allen, H.D. Kim, E.-J. Cho, W. Tian, R. Jin, D. Mandrus, H. H\"{o}chst, M. Tsunekawa, A. Sekiyama, S. Suga BaVS$_3$ is a quasi-one-dimensional conductor which undergoes multiple phase transitions at 240K (hexagonal to orthorhombic), 70K (``metal to insulator"), and 35K (paramagnetic to antiferromagnetic). Electrical resistivity shows that one-dimensionality is only marginal in the metallic phase in that the conductivity along the chain direction is only 4 times larger than along the interchain direction. The resistivity is almost temperature independent in the metallic phase with a weak minimum at ~130K. We present angle-resolved photoemission (ARPES), high-photon-energy bulk-sensitive PES, and resonant PES spectra of BaVS$_3$. Our data contradict band calculations in which a band crossing the Fermi energy ($E_{\rm F}$) provides a sharp density of states at $E_{\rm F}$. No distinct Fermi edge was observed in the metallic phase spectra, even with the high photon energy where the surface sensitivity of the spectra is minimized. Rather, from 20K to 150K, an $E_{\rm F}$ gap of $\approx$ 70meV is filled by spectral weight transfer over a range 0.5eV below $E_{\rm F}$. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:03PM |
B38.00004: Optical spectroscopy study on the electronic structure of Eu1-x Ca xB6 Jungho Kim, S. -J. Oh, Yongwoo Lee, M. G. Kim, E. J. Choi, Jong-Soo Rhyee, B. K. Cho, C. C. Homes The divalent hexaboride compounds RB$_{6}$ have drawn much attention for the last decade due to their interesting electrical and magnetic properties. In EuB$_{6}$, the optical spectroscopy measurement showed an unusual shift of the plasma frequency across the ferromagentic transition. While Ca~bears no magnetic moment, CaB$_{6}$~exhibits~weak ferromagnetism at high temperature. Although the~band structure is a~prerequisite to the understanding of these unconventional phenomena,~the consensus among the theoretical and experimental works has not been reached. In this work, the optical conductivity of Eu$_{1-x }$Ca$_{x}$B$_{6}$ has been obtained from reflectivity and ellipsometry measurements for series of compositions, 0 $<$ x $<$1. The interband part of $\sigma _{1}(\omega )$ shifts continuously to higher frequency as x increases. Also the intraband spectral weight decreases rapidly and essentially vanishes for x$\ge $x$_{c}$=0.35. These results show that the valence band and the conduction band of Eu$_{1-x }$Ca$_{x}$B$_{6 }$move away from each other such that their band overlap decreases with increasing Ca. As a result, the electronic state evolves from the semimetallic structure of EuB$_{6}$ to the insulating CaB$_{6}$ where the two bands are separated to open a finite gap ($\sim $0.25 eV) at the X-point of the Brillouin zone. [Preview Abstract] |
Monday, March 21, 2005 12:03PM - 12:15PM |
B38.00005: Direct observation of the mass renormalization in SrVO$_3$ by angle resolved photoemission spectroscopy Teppei Yoshida, Kiyohisa Tanaka, Hajime Yagi, Akihiro Ino, Hiroshi Eisaki, Atsushi Fujimori, Zhi-Xun Shen We have performed an angle-resolved photoemission study of the perovskite-type SrVO$_{3}$. Observed spectral weight distribution in the momentum space shows cylindrical Fermi surfaces consisting of the $d_{xy}$ orbitals as predicted by local-density-approximation (LDA) band-structure calculation. The observed energy dispersion shows a moderately enhanced effective mass near the Fermi level compared to the LDA results, corresponding to the effective mass enhancement seen in the thermodynamic properties. Surface effects on the spectra are also discussed. [Preview Abstract] |
Monday, March 21, 2005 12:15PM - 12:27PM |
B38.00006: Ultrafast evolution of the surface plasmon resonance in vanadium dioxide nanoparticles Matteo Rini, Andrea Cavalleri, Robert Schoenlein, Rene Lopez, Leonard Feldman, Richard Haglund, Lynn Boatner, Tony Haynes Vanadium dioxide nanoparticles undergo a metal-insulator transition (MIT) at approximately 340K that has been shown to depend on nanoparticle size and shape. Here we report on the temporal evolution of the metallic phase in VO$_{2}$ nanoparticles following ultrafast laser excitation. The signature of the MIT is the appearance of the surface plasmon resonance (SPR) in the near-infrared. Femtosecond measurements of the temporal evolution of the SPR in VO$_{2}$ nanoparticles show that it is fully formed no later than 200 fs following excitation by a 100 fs laser pulse that initiates a band-to-band excitation. The wavelength dependence of the SPR generally follows Mie theory and agrees with the SPR spectrum observed when the MIT is initiated by slow heating. The rapidity of the transition, as well as its dependence on nanoparticle size, suggests that electronic mechanisms are responsible at least for the initiation of the MIT. A possible coupled electronic-vibronic mechanism for the MIT will be disussed in this context. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B38.00007: Phonons and the metal-insulator transition in VO$_2$ Sung Chang, Y. Janssen, P. C. Canfield, J. W. Kim, B. Sieve, A. I. Goldman, R. J. McQueeney, A. Alatas, H. Sinn VO$_2$ undergoes a metal-insulator transition (MIT) at $T_C = 340 $~K, which is accompanied by a structural phase transition from a high temperature rutile phase to a low temperature monoclinic phase. Although VO$_2$ has been studied extensively for over 40 years, a clear understanding of the origin of the phase transition has not been forthcoming. Still at issue is the relative importance of electron-lattice and electron-electron interactions as driving mechanisms for the MIT. Here, we report the phonon dispersion of VO$_2$, measured along the rutile $\Gamma$-R direction using high resolution inelastic X-ray scattering. Unusual phonon behavior at the R point, as the MIT is approached, suggests significant electron-phonon coupling. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B38.00008: Optical and microscopic studies on metal-insulator transition of the VO$_{2}$ thin film C. H. Koo, J. S. Lee, Y. J. Chang, T. W. Noh, B. G. Chae, H.-T. Kim We investigated the temperature dependent optical spectra of VO$_{2}$, which exhibits an abrupt metal-insulator transition (MIT) around 340 K, in the wide energy range of 0.5-5.0 eV. We observed four absorption peaks around 1 eV, 2 eV, 3 eV, and 4 eV in the insulating state and a large spectral weight redistribution among them as the MIT occurs. We argue that the peaks could be assigned based on the orbitally degenerate Hubbard model suggesting the importance of the electron correlation in VO$_{2}$. And we understand the spectral changes occurring with the MIT in terms of the variation of orbital states induced by the structure distortions. Based on our optical analysis, we will address which plays a dominant role in the MIT between electron correlation and electron--lattice interaction. Additionally, we will present the scanning tunneling spectroscopy results confirming that the MIT of VO$_{2}$ occurs in a percolation process, which is proposed by a previous optical study [1]. [1] H. S. Choi \textit{et al}., Phys. Rev. B \textbf{54}, 4621 (1996). [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B38.00009: Fermi surfaces and band dispersions in 4$d$ compound Sr$_2$RhO$_4$ B.J. Kim, H. Koh, I. Nagai, S.I. Ikeda, J.J. Yu, H.-D. Kim, S. -J. Oh, C. Kim Fermi surface (FS) topology and band dispersions in 4$d$ compound Sr$_2$RhO$_4$ are studied by angle-resolved photoemission spectroscopy (ARPES) and compared with the band structure calculation within the local-density approximation (LDA). The measured FS deviates significantly from the calculation, suggesting the electronic correlation effects are not negligible in this system. Core-level X-ray photoemission spectrum of Rh 3$d$ shows correlation-induced satellite, from which we estimate the strength of the correlation, by simulating the spectrum with the recently proposed model calculation of core-level spectra incorporating the dynamical mean-field theory (DMFT). [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B38.00010: Spatially Resolved Nano-Scale Characterization of Electronic States in Bi-layer Ruthenates by STM/STS K. Iwaya, T. Hanaguri, H. Takagi, S. Satow, Y. Kohsaka, J.P. He, R. Mathieu, Y. Kaneko, Y. Tokura, Y. Yoshida, S.I. Ikeda We have performed low temperature scanning tunneling microscopy/spectroscopy (STM/STS) on bi-layer ruthenates AE$_3 $Ru$_2$O$_7$ (AE = Ca, Sr). Sr$_3$Ru$_2$O$_7$ is a paramagnetic metal very close to the itinerant ferromagnet, while Ca$_3$Ru$_2 $O$_7$ is a poor conductor (2-dimensional metal with antiferromagnetic order). Atomic lattice images were obtained in both systems for the first time. In Sr$_3$Ru$_2$O$_7$, STM image is independent of tip-sample bias voltage from -700 mV to +700 mV. On the contrary, in Ca$_3$Ru$_2$O$_7$, different atomic sites and complicated internal structures within the unit cell are observed depending on the bias voltage. We speculate that the specific orbital states of ruthenium and/or oxygen are responsible for the STM image at each bias voltage. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B38.00011: Electronic structure and Evolution of Orbital State in Metallic Ca$_{2-x}$Sr$_x$RuO$_4$ Han-Jin Noh, S.-J. Oh, B.-G. Park, J.-Y. Kim, H.-D. Kim, J.-H. Park, T. Mizokawa, L. H. Tjeng, H.-J. Lin, C. T. Chen, S. Schuppler, S. Nakatsuji, H. Fukazawa, Y. Maeno We investigated electronic structure of a layered Ca$_{2-x}$ Sr$_x$RuO$_4$ in a metallic regime (0.15 $\le$ x $\le$ 2.0) using polarization dependent O 1s x-ray absorption spectroscopy. The spectrum shows strong variation with the polarization especially in Ru 4d region, which enables us to identify the electronic states. The spectral line shape gradually changes with increase of Sr concentration, and agrees well with the unrestricted Hartree-Fock analysis. The analysis suggests evolution of the orbital states and provides clues for how the lattice distortion affects the orbital occupations. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B38.00012: Optical Investigation on the Electronic Structures of Multi-band Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ S. J. Moon, J. S. Lee, T. W. Noh, S. Nakatsuji, Y. Maeno We investigated the polarization dependent optical spectra of the quasi-two-dimensional multi-band Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ (0.0$<$x$\le $2.0) system. Recently, it becomes an important issue to understand how the electronic structure evolves from a multi-band metal Sr$_{2}$RuO$_{4}$ to a Mott-insulator Ca$_{2}$RuO$_{4}$ in the Ca$_{2-x}$Sr$_{x}$RuO$_{4}$ system, especially whether the orbital selective Mott-transition occurs at x=0.5 or not. In the in-plane optical spectra, we observed an intriguing behavior in the electrodynamics, i.e., the decrease of the mass enhancement accompanying the reduction of the plasma frequency. We found that such result could not be attributed to the Mott-gap opening. Instead, with the detailed understanding of the c-axis optical spectra, we suggested that the critical behaviors near x=x$_{c}$ should be closely related with the renormalization of the d$_{xy}$ band, which is strongly affected by the tilting of the RuO$_{6}$ octahedra. [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B38.00013: Low Frequency Conductivity Scaling in CaRuO$_3$ Saeid Kamal, Steve Dodge, Dong-Min Kim, Chang-Beom Eom We present low frequency conductivity measurements of CaRuO$_3$ thin films using time domain terahertz spectroscopy. The complex conductivity is measured in the frequency range of 200~GHz to 1.4~THz and temperature range of 10~K to 290~K. Below 50~K a Drude-like peak develops which is sufficiently narrow that it is only observable in our frequency window. We also observe a universal scaling of conductivity in the form of $\rm{\sigma_1\omega^{1/2}\propto g(\omega/T)}$ for $1<\rm{\hbar\omega / k_BT<5}$, in agreement with infrared observations of Lee et al.\footnote[1]{Y.S. Lee et al. Phys Rev B {\bf66}, 041104 (2002)} However, our low frequency range allows us to identify deviations from such scaling for $\rm {\hbar\omega /k_BT<1}$. [Preview Abstract] |
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