The electrodynamic properties of the quasi-one-dimensional organic conductors (TMTSF)2X are discussed, with particular emphasis on important deviations from the simple Drude model, the transition from a Luttinger-liquid to a Fermi-liquid behavior at the dimensional crossover when pressure is applied or temperature reduced, indications of a pseudogap as well as a low-frequency collective mode. Superconductivity and spin-density-wave ground states breaking the symmetry and gaps should occur in the excitation spectra. The previous literature is summarized and the current status of our understanding presented. Novel THz experiments on (TMTSF)2PF6 and (TMTSF)2ClO4 not only shine light into some of the open questions, but also pose new ones. 1. Introduction Physics in one dimension is a fascinating topic for theory and challenging for experiments. One-dimensional models are simpler compared to three-dimensional ones; in many cases, analytical solutions exist only in one dimension, while numerical approaches have to be used in higher dimensions [1]. Often the reduction of dimensionality does not really matter because the essential physics remains unaffected. But there are also a number of phenomena in condensed matter which only or mostly occur in one dimension. In general, the dominance of the lattice is reduced and electronic interactions become superior. This implies that physics in reduced dimensions is physics of low energies; the relevant effects do not occur in the electron-volt range but at millielectron volts and below. Quantum mechanical effects are essential as soon as the confinement approaches the electron wavelength. Fundamental concepts of physics, like the Fermi liquid theory of interacting particles breaks down in one dimension and has to be replaced by alternative concepts based on collective excitations [2]. One-dimensional structures are intrinsically unstable for thermodynamic reasons. Hence various kinds of ordering phenomena may take place which break the translational symmetry of the lattice, charge, or spin degrees of freedom: phase transitions occur as a function of temperature or some order parameter. On the other hand, fluctuations suppress long-range order at any finite temperature in one (and two) dimension. The ordered ground state is only stabilized by the fact that real systems consist of one-dimensional chains, which are coupled to some degree. The challenge now is to extract the one-dimensional physics from experimental investigations of quasi-one-dimensional systems and to check the theoretical predictions. Besides pure
References
[1]
E. H. Lieb and D. C. Mattis, Eds., Mathematical Physics in One Dimension, Academic Press, New York, NY, USA, 1966.
[2]
T. Giamarchi, Quantum Physics in One Dimen-Sion, Oxford University Press, Oxford, UK, 2004.
[3]
C. S. Jacobsen, D. B. Tanner, and K. Bechgaard, “Dimensionality crossover in the organic superconductor tetramethyltetraselenafulvalene hexafluorophosphate [(TMTSF)2PF6],” Physical Review Letters, vol. 46, no. 17, pp. 1142–1145, 1981.
[4]
P. Brüuesch, “Optical properties of the one-dimensional Pt complex compounds,” in One-Dimensional Conductors, H. G. Schuster, Ed., p. 194, Springer, Berlin, Germany, 1975.
[5]
P. Brüesch, S. Str？ssler, and H. R. Zeller, “Fluctuations and order in a one-dimensional system. A spectroscopical study of the Peierls transition in ,” Physical Review B, vol. 12, no. 1, pp. 219–225, 1975.
[6]
H. Basista, D. A. Bonn, T. Timusk, J. Voit, D. Jérome, and K. Bechgaard, “Far-infrared optical properties of tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ),” Physical Review B, vol. 42, no. 7, pp. 4088–4099, 1990.
[7]
S. Kagoshima, H. Nagasawa, and T. Sambongi, One-Dimensional Conductors, vol. 72 of Springer Series in Solid-State Sciences, Spinger, Berlin, Germany, 1989.
[8]
H. R. Zeller, “Electronic properties of one-dimensional solid state systems,” in Festk？rperprobleme (Advances in Solid State Physics), H. J. Queisser, Ed., vol. 13, p. 31, Pergamon Press, New York, NY, USA, 1973.
[9]
H. R. Zeller, “Electrical transport and spectroscopical studies of the Peierls transition ,” in Low Dimensional Cooperative Phenomena, H. J. Keller, Ed., pp. 215–233, Plenum Press, New York, NY, USA, 1975.
[10]
M. Dressel and G. Grüner, Electrodynamics of Solids, Cambridge University Press, Cambridge, UK, 2002.
[11]
O. Klein, S. Donovan, M. Dressel, and G. Grüner, “Microwave cavity perturbation technique: part I: principles,” International Journal of Infrared and Millimeter Waves, vol. 14, no. 12, pp. 2423–2457, 1993.
[12]
S. Donovan, O. Klein, M. Dressel, K. Holczer, and G. Grüner, “Microwave cavity perturbation technique: part II: experimental scheme,” International Journal of Infrared and Millimeter Waves, vol. 14, no. 12, pp. 2459–2487, 1993.
[13]
M. Dressel, S. Donovan, O. Klein, and G. Grüner, “Microwave cavity perturbation technique: part III: applications,” International Journal of Infrared and Millimeter Waves, vol. 14, no. 12, pp. 2489–2517, 1993.
[14]
A. Schwartz, M. Dressel, A. Blank et al., “Resonant techniques for studying the complex electrodynamic response of conducting solids in the millimeter and submillimeter wave spectral range,” Review of Scientific Instruments, vol. 66, no. 4, pp. 2943–2953, 1995.
[15]
M. Dressel, O. Klein, S. Donovan, and G. Grüner, “High frequency resonant techniques for studying the complex electrodynamic response in solids,” Ferroelectrics, vol. 176, no. 1–4, pp. 285–308, 1996.
[16]
B. P. Gorshunov, A. Volkov, I. E. Spektor et al., “Terahertz BWO-spectrosopy,” International Journal of Infrared and Millimeter Waves, vol. 26, no. 9, pp. 1217–1240, 2005.
[17]
L. B. Coleman, M. J. Cohen, D. J. Sandman, F. G. Yamagishi, A. F. Garito, and A. J. Heeger, “Superconducting fluctuations and the peierls instability in an organic solid,” Solid State Communications, vol. 12, no. 11, pp. 1125–1132, 1973.
[18]
M. J. Cohen, L. B. Coleman, A. F. Garito, and A. J. Heeger, “Electrical conductivity of tetrathiofulvalinium tetracyanoquinodimethan (TTF) (TCNQ),” Physical Review B, vol. 10, no. 4, pp. 1298–1307, 1974.
[19]
D. Jérome and H. J. Schulz, “Organic conductors and superconductors,” Advances in Physics, vol. 31, no. 4, pp. 299–490, 1982.
[20]
L. B. Coleman, C. R. Fincher, A . F. Garito, and A. J. Heeger, “Far-infrared single crystal studies of TTF-TCNQ,” Physica Status Solidi (b), vol. 75, no. 1, pp. 239–246, 1976.
[21]
M. J. Cohen, L. B. Coleman, A. F. Garito, and A. J. Heeger, “Electronic properties of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ),” Physical Review B, vol. 13, no. 11, pp. 5111–5116, 1976.
[22]
G. A. Thomas, D. E. Schafer, F. Wudl et al., “Electrical conductivity of tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ),” Physical Review B, vol. 13, no. 11, pp. 5105–5110, 1976.
[23]
D. B. Tanner, C. S. Jacobsen, A. F. Garito, and A. J. Heeger, “Infrared conductivity of tetrathiofulvalene tetracyanoquinodimethane (TTF- TCNQ) films,” Physical Review Letters, vol. 32, no. 23, pp. 1301–1305, 1974.
[24]
C. S. Jacobsen, D. B. Tanner, and A. J. Heeger, “Single-crystal reffectance studies of tetrathiafulvalene tetracyanoquinodimethane,” Physical Review Letters, vol. 32, pp. 1559–1562, 1974.
[25]
D. B. Tanner, C. S. Jacobsen, A. F. Garito, and A. J. Heeger, “Infrared studies of the energy gap in tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ),” Physical Review B, vol. 13, no. 8, pp. 3381–3404, 1976.
[26]
J. E. Eldridge and F. E. Bates, “Far-infrared optical properties of semiconducting tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ), including the pinned charge-density wave,” Physical Review B, vol. 28, no. 12, pp. 6972–6981, 1983.
[27]
J. E. Eldridge, “Improved measurement of the far-infrared optical properties of semiconducting tetrahiafulvalene tetracyanoquinodimethane,” Physical Review B, vol. 31, no. 8, pp. 5465–5467, 1985.
[28]
D. B. Tanner, K. D. Cummings, and C. S. Jacobsen, “Far-Infrared study of the charge density wave in tetrathiafulvalene tetracyanoquinodimethane (TTF-TCNQ),” Physical Review Letters, vol. 47, no. 8, pp. 597–600, 1981.
[29]
D. B. Tanner and C. S. Jacobsen, “Low-temperature infrared studies of TTF-TCNQ,” Molecular crystals and liquid crystals, vol. 85, no. 1–4, pp. 137–145, 1982.
[30]
B. P. Gorshunov, G. V. Kozlov, A. A. Volkov, V. ？elezny, J. Petzelt, and C. S. Jacobsen, “Dielectric function of TTF-TCNQ in the submillimetre range,” Solid State Communications, vol. 60, no. 9, pp. 681–687, 1986.
[31]
D. Jérome, “The physics of organic superconductors,” Science, vol. 252, no. 5012, pp. 1509–1514, 1991.
[32]
J. P. Farges, Ed., Organic Conductors, Marcel Dekker, New York, NY, USA, 1994.
[33]
T. Ishiguro, K. Yamaji, and G. Saito, Organic Supercon-Ductors, Springer, Berlin, Germany, 2nd edition, 1998.
[34]
N. Toyota, M. Lang, and J. M. Müller, Low-Dimensional Molecular Metals, vol. 154 of Springer Series in Solid-State Science, Springer, Berlin, Germany, 2007.
[35]
A. Lebed, The Physics of Organic Superconductors and Conductors, vol. 110 of Springer Series in Materials Science, Springer, Berlin, Germany, 2008.
[36]
M. Dressel, S. Kirchner, P. Hesse et al., “Charge and spin dynamics of TMTSF and TMTTF salts,” Synthetic Metals, vol. 120, no. 1–3, pp. 719–720, 2001.
[37]
M. Dressel, “Spin-charge separation in quasi one-dimensional organic conductors,” Naturwissenschaften, vol. 90, no. 8, pp. 337–344, 2003.
[38]
M. Dressel, “Ordering phenomena in quasi-one-dimensional organic conductors,” Naturwissenschaften, vol. 94, no. 7, pp. 527–541, 2007.
[39]
B. Kóhler, E. Rose, M. Dumm, G. Untereiner, and M. Dressel, “Comprehensive transport study of anisotropy and ordering phenomena in quasi-one-dimensional (TMTTF)2X salts ( ),” Physical Review B, vol. 84, no. 3, pp. 035124-1–035124-13, 2011.
[40]
C. S. Jacobsen, D. B. Tanner, and K. Bechgaard, “Optical and infrared properties of tetramethyltetraselenafulvalene [(TMTSF)2X] and tetramethyltetrathiafulvalene [(TMTTF)2X] compounds,” Physical Review B, vol. 28, no. 12, pp. 7019–7032, 1983.
[41]
J. E. Eldridge and G. S. Bates, “The far-infrared properties of (TMTSF)2PFg and (TMTSF)2ClO4 at 6K,” Molecular Crystals and Liquid Crystals, vol. 119, pp. 183–190, 1985.
[42]
S. Donovan, Y. Kim, L. Degiorgi, M. Dressel, G. Grüner, and W. Wonneberger, “Electrodynamics of the spin-density-wave ground state: optical experiments on (TMTSF)2PF6,” Physical Review B, vol. 49, no. 5, pp. 3363–3377, 1994.
[43]
L. Degiorgi, M. Dressel, A. Schwartz, B. Alavi, and G. Grüner, “Direct observation of the spin-density-wave gap in (TMTSF)2PF6,” Physical Review Letters, vol. 76, no. 20, pp. 3838–3841, 1996.
[44]
M. Dressel, A. Schwartz, G. Grüner, and L. Degiorgi, “Deviations from drude response in low-dimensional metals: electrodynamics of the metallic state of (TMTSF)2PF6,” Physical Review Letters, vol. 77, no. 2, pp. 398–401, 1996.
[45]
H. K. Ng, T. Timusk, D. Jérome, and K. Bechgaard, “Far-infrared spectrum of di-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF)2AsF6],” Physical Review B, vol. 32, no. 12, pp. 8041–8045, 1985.
[46]
J. E. Eldridge and G. S. Bates, “Far-infrared spectra of bis-(tetramethyltetraselenafulvalene) hexafluoroarsenate [(TMTSF)2AsF6] and hexafluoroantimonate [(TMTSF)2SbF6] in their spin-density-wave state,” Physical Review B, vol. 34, no. 10, pp. 6992–7002, 1986.
[47]
A. Schwartz, M. Dressel, G. Grüner, V. Vescoli, L. Degiorgi, and T. Giamarchi, “On-chain electrodynamics of metallic (TMTSF)2X salts: observation of Tomonaga-Luttinger liquid response,” Physical Review B, vol. 58, no. 3, pp. 1261–1271, 1998.
[48]
H. K. Ng, T. Timusk, and K. Bechgaard, “Far-infrared study of bis(tetramethyltetraselenafulvalene) hexafluoroantimonate [(TMTSF)2SbF6]: coexistence of metallic and semiconducting states,” Physical Review B, vol. 30, no. 10, pp. 5842–5846, 1984.
[49]
S. Donovan, A. Schwartz, M. Dressel et al., “Effects of anion disorder on the electrodynamical response of a spin density wave,” Ferroelectrics, vol. 176, no. 1–4, pp. 343–352, 1996.
[50]
D. Jérome, A. Mazaud, M. Ribault, and K. Bechgaard, “Superconductivity in a synthetic organic conductor (TMTSF)2PF6,” Journal de Physique Lettres, vol. 41, no. 4, pp. L95–L98, 1980.
[51]
C. S. Jacobsen, “Infrared studies on the electronic structure of organic conductors,” Mat. Fys. Medd. Dan. Vidensk. Selsk, vol. 41, pp. 251–290, 1985.
[52]
C. S. Jacobsen, “Infrared properties of organic conductors,” in Low-Dimensional Conductors and Superconductors, D. Jérome and L. G. Caron, Eds., vol. 155 of NATO ASI B Series, Physics, pp. 253–2274, Plenum Press, London, UK, 1987.
[53]
C. S. Jacobsen, “Optical properties,” in Highly Conducting Quasi-One-Dimensional Organic Conductors, E. Conwell, Ed., vol. 27 of Semiconductors and Semimetals, pp. 293–384, Academic Press, Boston, Mass, USA, 1988.
[54]
S. Donovan, L. Degiorgi, and G. Grüner, “Electro-dynamics of one-dimensional metals-optical experiments on (TMTSF)2PF6,” in Europhysics Letters, vol. 19, pp. 433–438, 1992.
[55]
S. Biermann, A. Georges, A. Lichtenstein, and T. Giamarchi, “Deconfinement transition and Luttinger to Fermi liquid crossover in quasi-one-dimensional systems,” Physical Review Letters, vol. 87, no. 27, pp. 276405-1–276405-4, 2001.
[56]
T. Giamarchi, “Theoretical framework for quasi-one dimensional systems,” Chemical Reviews, vol. 104, no. 11, pp. 5037–5055, 2004.
[57]
D. N. Basov, R. D. Averitt, D. van der Marel, M. Dressel, and K. Haule, “Electrodynamics of correlated electron materials,” Reviews of Modern Physics, vol. 83, no. 2, pp. 471–541, 2011.
[58]
C. S. Jacobsen, K. Mortensen, M. Weger, and K. Bechgaard, “Anomalous magnetoresistance in an organic conductor: (TMTSF)2PF6,” Solid State Communications, vol. 38, no. 5, pp. 423–428, 1981.
[59]
M. Dressel, K. Petukhov, B. Salameh, P. Zornoza, and T. Giamarchi, “Scaling behavior of the longitudinal and transverse transport in quasi-one-dimensional organic conductors,” Physical Review B, vol. 71, no. 7, pp. 075104-1–075104-10, 2005.
[60]
C. S. Jacobsen, D. B. Tanner, and K. Bechgaard, “Optical properties of some (TMTSF)2X compounds,” Molecular Crystals and Liquid Crystals, vol. 79, pp. 25–38, 1982.
[61]
C. S. Jacobsen, H. J. Pedersen, K. Mortensen, J. B. Torrance, and K. Bechgaard, “An unusual metal-insulator transition: bis(tetramethyltetraselenafulvalenium)-perrhenate (TMTSF2ReO4),” Journal of Physics C, vol. 15, no. 12, article 014, pp. 2651–2663, 1982.
[62]
W. Henderson, V. Vescoli, P. Tran, L. Degiorgi, and G. Grüner, “Anisotropic electrodynamics of low dimensional metals: optical studies of (TMTSF)2ClO4,” European Physical Journal B, vol. 11, no. 3, pp. 365–368, 1999.
[63]
E. Rose, C. Loose, J. Kortus et al., “Pressure-dependent structural and electronic properties of quasi-one-dimensional (TMTTF)2PF6,” Journal of Physics. In press.
[64]
A. Pashkin, M. Dressel, and C. A. Kuntscher, “Pressure-induced deconfinement of the charge transport in the quasi-one-dimensional Mott insulator (TMTTF)2AsF6,” Physical Review B, vol. 74, no. 16, pp. 165118-1–165118-9, 2006.
[65]
A. Pashkin, M. Dressel, and C. A. Kuntscher, “Pressure-induced changes in the optical response of the quasi-1D organic salt (TMTTF)2AsF6,” Journal of Low Temperature Physics, vol. 142, no. 3-4, pp. 563–566, 2006.
[66]
A. Pashkin, M. Dressel, M. Hanfland, and C. A. Kuntscher, “Deconfinement transition and dimensional crossover in the bechgaard-fabre salts: pressure- and temperature-dependent optical investigations,” Physical Review B, vol. 81, no. 12, pp. 125109-1–125109-11, 2010.
[67]
Y. Suzumura, M. Tsuchiizu, and G. Grüner, “Confinement of interchain hopping by umklapp scattering in two coupled chains,” Physical Review B, vol. 57, no. 24, pp. R15040–R15043, 1998.
[68]
M. Tsuchiizu, Y. Suzumura, and T. Giamarchi, “Renormalized inter-chain hopping versus charge gap in two coupled chains,” Progress of Theoretical Physics, vol. 101, no. 3, pp. 763–768, 1999.
[69]
M. Tsuchiizu and Y. Suzumura, “Confinement-deconfinement transition in two coupled chains with umklapp scattering,” Physical Review B, vol. 59, no. 19, pp. 12326–12337, 1999.
[70]
M. Tsuchiizu, P. Donohue, Y. Suzumura, and T. Gia- marchi, “Commensurate-incommensurate transition in two-coupled chains of nearly half-filled electrons,” The European Physical Journal B, vol. 19, no. 2, pp. 185–193, 2001.
[71]
K. le Hur, “Weakly coupled Hubbard chains at half-filling and confinement,” Physical Review B, vol. 63, no. 16, pp. 165110-1–165110-11, 2001.
[72]
T. Giamarchi, “From luttinger to fermi liquids in organic conductors,” in The Physics of Organic Superconductors and Conductors, pp. 719–7743, Springer, Berlin, Germany, 2008.
[73]
J. Moser, M. Gabay, P. Auban-Senzier, D. Jérome, K. Bechgaard, and J. M. Fabre, “Transverse transport in (TM)2X organic conductors: possible evidence for a Luttinger liquid,” European Physical Journal B, vol. 1, no. 1, pp. 39–46, 1998.
[74]
V. Vescoli, L. Degiorgi, W. Henderson, G. Grüner, K. P. Starkey, and L. K. Montgomery, “Dimensionality-driven insulator-to-metal transition in the Bechgaard salts,” Science, vol. 281, no. 5380, pp. 1181–1184, 1998.
[75]
P. Auban-Senzier, D. Jérome, C. Carcel, and J. M. Fabre, “Longitudinal and transverse transport of the quasi-one dimensional organic conductor TMTTF2PF6 studied under high pressure,” Journal de Physique, vol. 114, pp. 41–44, 2004.
[76]
C. S. Jacobsen, D. B. Tanner, and K. Bechgaard, “In-frared and optical properties of (TMTSF)2X,” Journal de Physique, vol. 44, pp. C3-859–C3-865, 1983.
[77]
V. Vescoli, L. Degiorgi, M. Dressel et al., “Spin-density-wave gap in the Bechgaard salts (TMTSF)2X,” Physical Review B, vol. 60, no. 11, pp. 8019–8027, 1999.
[78]
G. Grüner, Density Waves in Solids, Addison-Wesley, Reading, Mass, USA, 1994.
[79]
L. J. Azevedo, J. E. Schirber, and E. M. Engler, “Se77 nuclear magnetic resonance in di-tetramethyltetraselenafulvalene phosphorous hexafluoride [(TMTSF)2PF6] under pressure,” Physical Review B, vol. 27, no. 9, pp. 5842–5845, 1983.
[80]
N. Cao, T. Timusk, and K. Bechgaard, “Unconventional electrodynamic response of the quasi-one-dimensional organic conductor (TMTSF)2ClO4,” Journal de Physique I, vol. 6, no. 12, pp. 1719–1726, 1996.
[81]
H. K. Ng, T. Timusk, and K. Bechgaard, “Far-infrared properties of (TMTSF)2ClO4 at low temperatures,” Journal de Physique, vol. 44, pp. C3-867–C3-872, 1983.
[82]
K. Kornelsen, J. E. Eldridge, and G. S. Bates, “Far-infrared reflectivity of bis-tetramethyltetraselenafulvalene hexafluoroarsenate [(TMTSF)2AsF6] through the spin-density-wave phase transition,” Physical Review B, vol. 35, no. 17, pp. 9162–9167, 1987.
[83]
P. M. Chaikin, P. Haen, E. M. Engler, and R. L. Greene, “Magnetoresistance and Hall effect in tetramethyl-tetraselenafulvalene-phosphorus hexafloride [(TMTSF)2PF6],” Physical Review B, vol. 24, no. 12, pp. 7155–7161, 1981.
[84]
V. Vescoli, L. Degiorgi, B. Alavi, and G. Grüner, “The spin-density-wave gap in (TMTSF)2ClO4,” Physica B, vol. 244, pp. 121–124, 1998.
[85]
M. Dressel, L. Degiorgi, J. Brinckmann, A. Schwartz, and G. Grüner, “Optical response of the spin-density-wave ground state,” Physica B, vol. 230-232, pp. 1008–1010, 1997.
[86]
M. Dumm, A. Loidl, B. Alavi, K. P. Starkey, L. K. Montgomery, and M. Dressel, “Comprehensive ESR study of the antiferromagnetic ground states in the one-dimensional spin systems (TMTSF)2PF6, (TMTSF)2AsF6, and (TMTTF)2Br,” Physical Review B, vol. 62, no. 10, pp. 6512–6520, 2000.
[87]
S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano, “Spin-density-wave gap in (TMTSF)2PF6 probed by reflection-type terahertz time-domain spectroscopy,” Physica Status Solidi (B), vol. 245, no. 12, pp. 2688–2691, 2008.
[88]
S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano, “Observation of ultrafast photoinduced closing and recovery of the spin-density-wave gap in (TMTSF )2PF6,” Physical Review B, vol. 80, no. 22, pp. 2204081-1–220408-4, 2009.
[89]
S. Watanabe, R. Kondo, S. Kagoshima, and R. Shimano, “Ultrafast photo-induced insulator-to-metal transition in the spin density wave system of (TMTSF)2PF6,” Physica B, vol. 405, no. 11, pp. S360–S362, 2010.
[90]
H. H. S. Javadi, S. Sridhar, G. Grüner, L. Chiang, and F. Wudl, “Giant conductivity resonance in the spin-density-wave state of an organic conductor,” Physical Review Letters, vol. 55, no. 11, pp. 1216–1219, 1985.
[91]
S. Donovan, M. Dressel, L. Degiorgi, A. Schwartz, A. Virosztek, and G. Grüner, “Electrodynamic properties of (TMTSF)2PF6,” Synthetic Metals, vol. 86, no. 1, pp. 2181–2182, 1997.
[92]
A. Schwartz, S. Donovan, M. Dressel, L. Degiorgi, and G. Grüner, “Normal state electrodynamics of compounds with charge- and spin-density-wave ground states,” Physica B, vol. 230–232, pp. 1005–1007, 1997.
[93]
M. Dressel, “On the order parameter of Bechgaard salts,” Physica C, vol. 317-318, pp. 89–97, 1999.
[94]
K. Petukhov and M. Dressel, “Collective spin-density-wave response perpendicular to the chains of the quasi-one-dimensional conductor (TMTSF)2PF6,” Physical Review B, vol. 71, no. 7, pp. 073101-1–073101-3, 2005.
[95]
P. Zornoza, K. Petukhov, M. Dressel, N. Biskup, T. Vuleti？, and S. Tomi？, “Anisotropy and field-dependence of the spin-density-wave dynamics in the quasi one-dimensional conductor (TMTSF)2PF6,” European Physical Journal B, vol. 46, no. 2, pp. 223–230, 2005.
[96]
M. Dressel, K. Petukhov, and M. Scheffler, “Anisotropic SDW dynamics in (TMTSF)2PF6,” Journal of Low Temperature Physics, vol. 142, no. 3-4, pp. 133–136, 2006.
[97]
J. L. Musfeldt, M. Poirier, P. Batail, and C. Lenoir, “Microwave dielectric studies of the spin-density-wave state in (TMTSF)2PF6,” Physical Review B, vol. 51, no. 13, pp. 8347–8356, 1995.
[98]
J. L. Musfeldt, M. Poirier, P. Batail, and C. Lenoir, “H-T behavior of the spin density wave condensate in (TMTSF)2AsF6,” Europhysics Letters, vol. 30, no. 2, pp. 105–110, 1996.
[99]
J. L. Musfeldt, M. Poirier, P. Batail, and C. Lenoir, “Magnetic-field behavior of the spin-density-wave state in (TMTSF)2AsF6,” Physical Review B, vol. 52, no. 22, pp. 15983–15991, 1995.
[100]
T. Vuleti？, P. Auban-Senzier, C. Pasquier et al., “Coexistence of superconductivity and spin density wave orderings in the organic superconductor (TMTSF)2PF6,” European Physical Journal B, vol. 25, no. 3, pp. 319–331, 2002.
[101]
N. Kang, B. Salameh, P. Auban-Senzier, D. Jérome, C. R. Pasquier, and S. Brazovskii, “Domain walls at the spin-density-wave endpoint of the organic superconductor (TMTSF)2PF6 under pressure,” Physical Review B, vol. 81, no. 10, pp. 100509-1–100509-4, 2010.
[102]
R. Bozio, C. Pecile, K. Bechgaard, F. Wudl, and D. Nalewajek, “Infrared study on the formation of charge density waves in (TMTSF)2X ( and at atmospheric pressure,” Solid State Communications, vol. 41, no. 12, pp. 905–910, 1982.
[103]
K. Kikuchi, I. Ikemoto, K. Yakushi, H. Kuroda, and K. Kobayashi, “Temperature dependence of the reflectance spectrum of (TMTSF)2ClO4,” Solid State Communications, vol. 42, no. 6, pp. 433–435, 1982.
[104]
H. K. Ng, T. Timusk, J. M. Delrieu, D. Jérome, K. Bechgaard, and J. M. Fabre, “Observation of a gap in the farinfrared magneto-absorption of (TMTSF)2ClO4: possibility of one-dimensional fluctuating superconductivity,” Journal de Physique Lettres, vol. 43, pp. 513–519, 1982.
[105]
T. Timusk, “Quasi one-dimensinoal conductors: the far infrared problem,” in Low-Dimensional Conductors and Superconductors, D. Jérome and L. G. Caron, Eds., vol. 155 of NATO ASI B Series, Physics, pp. 275–2284, Plenum Press, London, UK, 1987.
[106]
T. Timusk, “Infrared properties of exotic superconductors,” Physica C, vol. 317-318, pp. 18–29, 1999.
[107]
H. Schwenk, K. Andres, and F. Wudl, “Resistivity of the organic superconductor ditetramethyltetraselenafulvalenium perchlorate, (TMTSF)2ClO4, in its relaxed, quenched, and intermediate state,” Physical Review B, vol. 29, no. 1, pp. 500–502, 1984.
[108]
D. Pedron, R. Bozio, M. Meneghetti, and C. Pecile, “Electronic interactions in the organic conductors (TMTSF)2X ( and PF6) and (TMTTF)2X ( and PF6) from their infrared spectra,” Physical Review B, vol. 49, no. 16, pp. 10893–10907, 1994.
[109]
W. A. Challener, P. L. Richards, and R. L. Greene, “Far infrared properties of (TMTSF)2ClO4,” Journal de Physique, vol. 44, pp. C3-873–C3-878, 1983.
[110]
W. A. Challener, P. L. Richards, and R. L. Greene, “Far infrared measurements of (TMTSF)2ClO4,” Solid State Communications, vol. 51, no. 10, pp. 765–768, 1984.
[111]
J. E. Eldridge, C. C. Homes, F. E. Bates, and G. S. Bates, “Far-infrared powder absorption measurements of some tetramethyltetraselenafulvalene salts [(TMTSF)2X],” Physical Review B, vol. 32, no. 8, pp. 5156–5162, 1985.
[112]
C. C. Homes and J. E. Eldridge, “Lattice-mode coupling to the charge-density wave in (TMTSF)2ReO4 (where TMTSF is bis-tetramethyltetraselenafulvalene),” Physical Review B, vol. 40, no. 9, pp. 6138–6143, 1989.
[113]
C. C. Homes and J. E. Eldridge, “Infrared optical properties of (TMTSF)2ReO4 and (TMTSF)2BF4 (where TMTSF is tetramethyltetraselenafulvalene) compared with several model calculations,” Physical Review B, vol. 42, no. 15, pp. 9522–9533, 1990.
[114]
J. E. Eldridge and C. C. Homes, “Vibrational assignments in the conductivity spectra of semiconducting (TMTSF)2ReO4 and (TMTSF)2BF4 (where TMTSF is tetramethyltetraselenafulvalene) for radiation polarized perpendicular to the chains,” Physical Review B, vol. 43, no. 17, pp. 13971–13977, 1991.
[115]
M. Dressel, M. Dumm, T. Knoblauch, and M. Masino, “Comprehensive optical investigations of charge order in organic chain compounds (TMTTF)2X,” Crystals, vol. 2, pp. 528–578, 2012.
[116]
M. Krauzman, H. Poulet, and R. M. Pick, “Resonant Raman scattering in a bis-tetramethyltetraselenafulvalene- hexafluorophosphate [(TMTSF)2PF6] single crystal,” Physical Review B, vol. 33, no. 1, pp. 99–105, 1986.
[117]
G. Rindorf, H. Soling, and N. Throup, “Di( -tetramethyl- -bi-1,3-diselenolyliden)ium perrhenate, C20H24 . , (TMTSF)2ReO4. Detailed superstructure at 120？K,” Acta Crystallographica C, vol. 40, pp. 1137–11139, 1984.
[118]
T. J. B. M. Janssen, A. S. Perel, A. M. Gerrits et al., “Far-infrared spectroscopy of the field-induced spin-density-wave gap in (TMTSF)2ClO4,” Physical Review B, vol. 46, no. 13, pp. 8663–8666, 1992.
[119]
A. S. Perel, J. S. Brooks, C. J. G. N. Langerak et al., “Magnetic-field-dependent energy levels in a highly anisotropic electronic material,” Physical Review Letters, vol. 67, no. 15, pp. 2072–2075, 1991.
[120]
R. Ellison, M. Reedyk, and K. Behnia, “Far-infrared electrodynamic response of (TMTSF)2ClO4 in the normal and superconducting states,” Physical Review B, vol. 66, no. 1, pp. 125081–125084, 2002.
[121]
B. Gorshunov, S. Kaiser, and M. Dressel, “THz optical properties of (TMTSF)2ClO4 and (TMTSF)2PF6,” in press.
