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Outline Blue Bronze Electrodynamics of CDW Optical Response of the Sliding CDW

Outline Blue Bronze Electrodynamics of CDW Optical Response of the Sliding CDW Conclusion and Outlook. Universit ät Stuttgart B. Gorshunov, S. Haffner Universit ät Frankfurt B. Lommel, F. Ritter, W. Assmuss. Rossendorf 19. Januar 2001.

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Outline Blue Bronze Electrodynamics of CDW Optical Response of the Sliding CDW

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  1. Outline Blue Bronze Electrodynamics of CDW Optical Response of the Sliding CDW Conclusion and Outlook Universität Stuttgart B. Gorshunov, S. Haffner Universität Frankfurt B. Lommel, F. Ritter, W. Assmuss Rossendorf 19. Januar 2001 Optical Response of Sliding Charge Density WavesMartin Dressel1. Physikalisches Institut, Universität Stuttgart, Germany

  2. Blue Bronze K0.3MoO3 conductivity(at T = 300 K) = 1500 (cm)-1 • quasi one-dimensional conductor lattice parametera = 16.25 Åb = 7.56 Åc = 9.87 Å • charge density wave ground state transition temperatureTCDW = 183 K threshold fieldET = 50 – 500 mV/cm

  3. Blue Bronze K0.3MoO3charge density wave formation room temperature conductivity = 1500 (cm)-1 anisotropyb : c’ : a’ = 1000 : 30 : 1 polarized optical reflectivitystrong anisotropymetallic behavior parallel to chainsweak metallic behavior perpendicular to chains Rouxel et al. 1989 Gorshunov et al. 1993

  4. Charge Density Wavesin one dimensional conductors metallic state constant charge distributionparabolic energy bandsfilled up to the Fermi wavevector metallic conductivity CDW state spatially modulated charge densityenergy gap at the Fermi energy semiconducting conductivity

  5. Non-Linear Conductivity field dependent electrical transport depinning of collective mode the CDW condensate is pinned to impurities for E > ET the CDW is depinned andmoves coherently K0.3MoO3 T = 4.2 K Grüner et al. 1994

  6. Low-Frequency Excitations of the CDW Ground State frequency dependent conductivity internal deformations of the density wave distortion due to interaction with impuritiesinternal polarization, relaxation Mihaly et al. 1989

  7. Current Oscillations in the CDW Ground State narrow-band noise ac response to a driving dc current frequency f = vd/ with  = /kF f = 1/h 2vFvd proportional to the excess current Dumas et al. 1983

  8. Optical Properties of the Density Wave Ground State frequency dependent conductivity • two feature can be distinguished: • pinned mode resonance in the GHz rangecollective response of the charge density wave pinned to imperfections • single particle gap in the FIRexcitations across the energy gap at the Fermi edge

  9. Optical Properties of K0.3MoO3 fluctuation effects: paraconductivity Conductivity due to phase fluctuations of the electrons condensed into correlated CDW segments. This is observed in the temperature intervall 183 K = TCDW < T < TMF = 330 K diffusive motion free electrons oscillatory motion pinned to impurities

  10. Collective CDW Excitationpinned mode resonance decreasing conductivity as temperature lowers below TCDW = 180 K development of a pinned mode around 4.5 cm-1 band width 1 cm-1 strong increase in dielectric constant fluctuating CDW seeneven at room temperature

  11. Joint AC-DC Experiments Does the ac (optical) response change if a dc voltage is applied? How do the collective CDW excitations change when an electric field is applied? Does the pinned mode resonance vary? Do we see the amplitude mode? Do other modes appear?

  12. Amplitude and Phase Excitations of a CDW • There are two distinctively • different collective excitations • possible in the CDW ground state. • Amplitude modeis related with a change in kinetic energy and thus A2(q=0) = 1/22kF • Phase modetranslational motion of the undistorted condensatein the ideal case it is gapless:(q=0) = 0

  13. Optical Response of the Collective CDW Excitations • major contributions: • phason mode in the GHz range • amplitude mode in the FIR • phonons in the FIR phason mode amplitude mode phonons

  14. Optical Properties of the CDW Ground State collective response Travaglini and Wachter, 1984 Ng et al. 1986 Degiorgi and Grüner, 1991

  15. Optical Properties of the CDW Ground State Raman scattering amplitude mode of the CDW at around 50 cm-1 in K0.3MoO3 Travaglini et al. 1983

  16. Non-Linear Tansport field dependent conductivity threshold field at 77 K ET = 6 V/cm depinning of charge density wave collective charge transport I V

  17. Joint AC-DC Experiments change of the optical properties by applying an electric field I • How can we be sure that • we do not heat the sample? • measurements in both directions of polarization • measurements at different temperature • measurements at different applied dc field: • We observe a sudden change of the optical properties when E>ET. V focus of optical beam

  18. Optical Response of the Sliding CDW • Experimental procedure: • measure the un-biased reflectivity of K0.3MoO3 • without moving the samplemeasurements of the ratio bias - non-biasfor increasing fields • significant changes as the electric field exceeds the threshold field • calculation of the bias-reflectivity

  19. Optical Response of the Sliding CDW dependence on applied dc field • Experimental procedure: • measure the un-biased reflectivity of K0.3MoO3 • without moving the samplemeasurements of the ratio bias - non-biasfor increasing fields • the effect increases continuous as the dc bias field increases

  20. Optical Response of the Sliding CDW • major contributions: • phason mode in the GHz range • amplitude mode in the FIR • phonons in the FIR • sliding CDW mode phason mode amplitude mode phonons sliding CDW mode

  21. Optical Response of the Sliding CDW • major findings: • the phason mode does not change in frequency,in width, nor in spectral weight • the amplitude mode does not change in frequency,but slightly looses spectral weight • the additional modeconsists of a number of equally spaced ripples,which are due to interaction of the CDW condensate with the underlying lattice

  22. Optical Response of Sliding CDW collective response • ac response to a driving dc current • internal deformationscurrent oscillations, interference effectsinterference of CDW and pinning centerskHz and MHz range f = 1/h 2vFvd • collective responseno change of phase excitationsmodifications of amplitude excitationsripple modeinterference of CDW and lattice

  23. Conclusions We have measured the optical response of K0.3MoO3 single crystals in the charge density wave ground state. In the millimeter wave and far-infrared range we found two optical modes, which we assign to the amplitude and the phason modelocated at 2 cm-1 and 10 cm-1, respectively. Applying an external electric field, which exceeds the threshold of non-linear conductivity,an new mode appears around 4 cm-1,which exhibits an additional fine structure.We assign this mode to the sliding charge density condensatewhich interacts with the underlying lattice. We observe no change in the phason mode (pinned mode resonance) and only a small loss of spectral weight in the amplitude mode.

  24. Fourier Transform Spectrometermodified Bruker IFS 113 v Spectral range 10 cm-1 – 10 000 cm-1 Resolution 0.03 cm-1 3 Sources 6 Detectors Sample chamber 77 KMCT 1.2 KBolometer 6Beamsplitter 4.2 K Bolometer 77 KInSb DTGS Genzel-type Michelson interferometer Reflection unit

  25. Coherent Source Spectrometer50 GHz - 1500 GHz , 2 cm-1 – 50 cm-1 sources: backward wave oscillator monochromatic, coherent tunable, powerful lenses: polyethylene beamsplitter, polarizer: free standing wire grids detector: Golay cell, He-cooled bolometer cryostat: 1.5 K – 300 K magnet: 0 – 8 Tesla split ring, superconducting Voigt, Faraday geometry Mach-Zehnder Interferometer

  26. 1. Physikalisches Institut der Universität StuttgartMartin Dressel und Boris Gorshunov collaborations: P. Haas, S. Haffner, C. Kuntscher, T. Rõõmmaterials:W. Aßmus (Frankfurt) K0.3MoO3, LiCuVO4J. Akimitsu (Tokyo) (Ca,Sr)14Cu24O41M.Greenblatt (Rutgers) Li0.9Mo6O17P. Monceau (Grenoble) (TaSe4)2IC. Schlenker (Grenoble)KxP4W8O32C.S. Jacobsen (Lyngby) TTF-TCNQ experimental:R. Claessen (Augsburg) PhotoemissionC. Thomsen (Berlin) RamanS. Tomic (Zagreb) transport, ac-response theory:A. Muramatsu (Stuttgart) spin chain, spin-laddersJ. Voit (Bayreuth) low-dim. conductorsV.I. Torgashev (Rostov-on-Don) group analysis on-going projects LiCuVO4 finished B.Gorshunov et al., Euro. Phys. J B 23, 427 (2001) KxP4W8O32finished S. Haffner et al., Euro. Phys. J B 24, 123 (2001) (Ca,Sr)14Cu24O41to be analyzed charge density wave TTF-TCNQ to be analyzed unusual metallic behavior, shift in spectral weight, energy gap K0.3MoO3, (TaSe4)2I to continue optical response of the sliding charge density wave intended projects (Ca,Sr)14Cu24O41magnetic field dependence, ac-dc response, shift in spectral weight, energy gap SrCu2(BO3)2B field dependence of low-energy modes Li0.9Mo6O17 non-Fermi liquid behavior CuBi2O4 spin chains, correlations CuNb2O6 phonons

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