Review Article | Open Access
Electrodynamics of Bechgaard Salts: Optical Properties of One-Dimensional Metals
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.
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 . 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 .
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 scientific interest, the crucial importance of these phenomena in nanotechnology might not lie ahead too far.
After a short overview of some milestones in the development of quasi-one-dimensional crystalline structures, the electrodynamic properties of the Bechgaard salts are discussed in detail, as they serve as the superior model systems of quasi-one-dimensional conductors. More than 30 years after the first optical spectra on the quasi-one-dimensional organic superconductor were published by Jacobsen et al. , the electrodynamic properties of the Bechgaard salts are still puzzling and subject to continuous experimental and theoretical efforts. Several issues make the materials and their physical properties so interesting and challenging.(i)The metallic properties are extremely anisotropic; this makes these organic salts the prime examples to study one-dimensional physics. (ii)The optical reflectivity and conductivity deviate from the simple Drude model; while there is a plasma edge present like in regular metals, the low-frequency properties exhibit very unusual behavior, not all of them are well understood by now. (iii)The conduction electrons cannot be described by the Fermi liquid theory but exhibit the scaling behavior of a Tomonaga-Luttinger liquid. (iv)A pseudogap seems to open in the far-infrared frequency range at low temperatures; this reduced spectral weight might correspond to the extremely low density of electronic states observed by photoemission spectroscopy. (v)The broken-symmetry ground states—such as charge density wave, spin density wave, or superconducting states—exhibit their particular fingerprint in the electrodynamic properties, single-particle gaps as well as collective excitations. (vi)But even in the metallic state, there are indications of collective modes observed in the THz range of frequency.
1.1. One-Dimensional Metal: Pt-Chains
The low-frequency electrodynamic properties of one-dimensional metals in general and organic conductors in particular have puzzled researches for decades. Although theoretically discussed for a long time , it all started in the early 70s of the last century with the Krogmann salt PtO, known as KCP, that consists of a chain of platinum ions with overlapping orbitals [8, 9]. In Figure 1, the optical reflectivity and conductivity of KCP are displayed for different temperatures and polarizations [4, 5]. The room temperature conductivity of KCP along the chain direction is very high (Ω cm)−1; the anisotropy ratio is . Due to the anisotropic nature, the reflectivity shows a plasma edge only for the electric field along the chains () while it remains low and basically frequency independent perpendicular to it, as known from dielectrics and other insulators. At low temperatures, the single-particle gap around 1000 becomes more pronounced, and an additional structure is observed in the far-infrared conductivity which is assigned to the pinned-mode resonance induced by the charge density wave (Figure 1(b)). A detailed investigation of the pinned-mode resonance, its center frequency and lineshape, and furthermore its dependence on temperature and impurity content turned out to be extremely difficult because it commonly occurs in the range of 3 to 300 GHz (0.1 to 10 ); that is, it falls right into the gap between high-frequency experiments using contacts and optical measurements by freely travelling waves . Microwave technique based on resonant cavities and quasi-optical THz spectroscopy was advanced over the years in order to bridge this so-called THz gap [11–16].
1.2. First One-Dimensional Organic Conductor: TTF-TCNQ
The breakthrough in the field of one-dimensional organic conductors happened around the same time with the synthesis of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), which exhibits a room temperature conductivity of and an anisotropy of more than a factor of 100 [7, 17–19]. In Figure 2, the optical properties of TTF-TCNQ are presented . Clear deviations from the Drude behavior of a conventional metal  are observed due to the one-dimensional nature. The most surprising fact is the somewhat low reflectivity and conductivity in the far-infrared range. This discrepancy between the high dc conductivity and the low far-infrared conductivity seems to be a generic feature found in all one-dimensional conductors. Although the dc conductivity increases by more than an order of magnitude by cooling from room temperatures down to K [20–22], the infrared conductivity remains low (Figure 2(b)). When the temperature is reduced below the K, the low-frequency reflectivity drops because an energy gap opens at the Fermi level.
In their first infrared transmission and reflection studies on TTF-TCNQ films and single crystals, Tanner et al. observed that within the energy gap of 1050 a collective mode develops in the optical conductivity at zero energy that moves to finite frequencies as K [20, 21, 23–30]. They conclude that a collective charge-density-wave mode develops below that is centered around 80 due to pinning by impurities and contains an oscillator strength of 500; this corresponds to an effective mass of with a bandmass . The issue is not settled yet, but the interpretation seems questionable since the center frequency is too high, implying unreasonably strong coupling. The situation in TTF-TCNQ contains the particular complication of two stacks (TTF and TCNQ), both developing a charge density wave with some interaction. An alternative interpretation of the 80 peak would be optical excitations across the Mott gap.
It is quite interesting that the extremely high dc conductivity in the metallic regime above the charge-density-wave transition could barely be reconciled with the optical properties. Some spectral weight is shifted toward low frequencies upon cooling, which infers improved metallic conductivity. Nevertheless, there is a positive slope in at any temperature, indicating a pseudogap and effectively reducing the far-infrared conductivity. It was suggested that the Drude-like mode with a plasma frequency of approximately exhibits a width of only as depicted in Figure 3. This is confirmed by the dielectric constant (or the imaginary part of the conductivity ) that evidences considerable spectral weight at very low frequencies, that is, below 10. It should be noted here that the data are taken at a temperature K, where such a narrow response can only be imagined if phonons scattering does not contribute considerable to the broadening, a fact rather common to one-dimensional metals.
1.3. Fabre and Bechgaard Salts (TMTTF) and (TMTSF)
In late 1970s, Fabre and Bechgaard succeeded to suppress the charge-density-wave metal-insulator transition by enlarging the organic molecule from TTF to TMTTF and TMTSF, which stands for tetramethyltetrathiofulvalene and tetramethyltetraselenafulvalene, respectively [19, 31–35]. Single crystals are usually grown by electrochemical methods and reach a size of several millimeters in length and less than a millimeter in width. All compounds of the TMTTF and TMTSF family are isostructural. Due to the triclinic symmetry, denotes the projection of the axis perpendicular to , and is normal to the plane.
Because of the stronger coupling between the stacks, the TMTSF salts are electronically more two dimensional, as seen by the appreciable warping of the Fermi surface. Metallic behavior is reached down to lower temperatures (Figure 4), and even enters the superconducting state at K when cooled slowly, as presented later in Figure 7(b). undergoes a spin-density-wave transition at 12 K, which can be suppressed by hydrostatic pressure . Since 30 years both compounds serve as prime examples of one-dimensional metals, and enormous effort was put in exploring the low-energy electrodynamics in the search for the pseudogap, the Drude-like component, the collective mode, Luttinger- or Fermi-liquid behavior, dimensional crossover, and so forth. In Figure 5, the various results are compiled by plotting the absorption as a function of frequency on a double logarithmic scale, in order to highlight the low-frequency and high-reflectivity part.
Even after a quarter of a century, the extended reviews by Jacobsen [51–53] still give the best overview on the optical properties of Bechgaard salts, the theoretical background, and thorough discussion of their implication.
2. Dimensional Crossover
In their seminal paper, Jacobsen et al.  already realized that exhibits a plasma edge in the optical reflectivity not only along the stacking direction , that becomes more pronounced as the temperatures decreases below K. At low temperatures a plasma edge is also seen for , although at frequencies (2360) lower by a factor of 5 compared to (11400). Similar results are displayed in Figure 6. They conclude a dimensional crossover from a one-dimensional to a two-dimensional metal as the temperature is lowered. The transverse coupling becomes effective below 100 K, while the smearing of the Fermi-Dirac function exceeds the transverse bandwidth of approximately meV. These conclusions are in perfect agreement with transport measurements along all three directions [58, 59] where a crossover from two-dimensional Fermi-liquid to a one-dimensional Luttinger liquid behavior was identified around K (Figure 7). At exactly the same temperature, the -axis transport turns from an insulating to a metallic one; the systems are actually three-dimensional metals at low temperatures. Jacobsen et al. made similar observation on the optical properties of , , and [40, 60], where the latter compound undergoes a metal-insulator transition at K due to the anion ordering . The missing plasma edge in the third direction ()  was later confirmed by Henderson et al. . It is interesting to look at electronic band structure and how it changes with pressure : the bands dispersion along the -direction varies most, while basically no change is seen for the -axis. This is in perfect agreement with the increasing plasma frequency observed perpendicular to the stacks.
The issue of the dimensional crossover was finally explored in more detail by pressure and temperature-dependent optical investigation of , , and [64–66]. The pressure-induced deconfinement transition in the Mott insulator occurs at approximately 2 GPa. This critical pressure is basically temperature independent and characterized by the rapid onset of the interstack electronic transport (along the direction). In Figure 8, the transverse hopping integral is depicted as a function of pressure, calculated from the transverse optical conductivity () via where denotes the unit cell volume, the separation of the stacks, and the transfer integral along the stacks. The values of for and perfectly agree with each other, taking into account a pressure offset of 3 GPa between these compounds. Above the deconfinement transition (i.e., GPa) and up to 4 GPa, the increase in for is almost linear with a slope of 5 meV/GPa, in accord with our earlier results for [64, 65]. Above 4 GPa the pressure-induced increase in becomes weaker (2 meV/GPa) for and comparable to the low-pressure behavior of . The quantitative criterion for the deconfinement transition  can be verified by comparing the pressure dependence of the charge gap with that of 2 (Figure 8). The onset of the coherent electronic transport normal to the stacks occurs at around 2 GPa, where . The size of the Mott gap, , rapidly decreases as the transition point is approached and stabilizes at a finite value upon further pressure increase. These findings are in accordance with theoretical predictions [2, 55, 56, 67–72] and earlier experiments [59, 73–75] which utilized the effect of chemical pressure.
A quantitative analysis of the dimensional crossover can be obtained from the degree of coherence of the charge transport. The coherence parameter of the Drude response is given by where and are the plasma frequency and the scattering rate, respectively. Coherent transport corresponds to , while incoherent transport occurs for , where is a critical value of the dimensional crossover which should be close to unity.
The evolution of the dimensionality of is summarized in Figure 9(b), where the coherence parameter is plotted as a function of temperature and pressure, determined from the Drude fits of the interstack reflectivity spectra. The highest degree of coherence is achieved for high pressures and low temperatures (lower right corner of the diagrams). It gradually decreases as the pressure is released and the temperature is simultaneously raised towards the upper left corner of the diagram. The lines of constant coherence level are almost linear with pressure for , however, for they become sublinear. The critical values that characterize the dimensional crossover are defined from resistivity measurements of Moser et al. . The constant level line , that is, the “crossover line,” is depicted in Figure 9. For pressures below 1.2 GPa, the crossover line exhibits a remarkable strong slope, not expected in the naive picture of noninteracting electrons where . According to Figure 8, the interstack transfer integral in increases only by about 30% when pressure increases by 4 GPa. Thus, one would expect the crossover temperature of about 130 K at 4 GPa for noninteracting electrons, in contrast to the experimental observation. This provides evidence that electronic correlations play a decisive role in the renormalization of the dimensional crossover in the Bechgaard salts, leading to the very fast suppression of the one-dimensional state in favor of a high-dimensional metallic state.
A corresponding temperature-pressure diagram of the coherence parameter is depicted in Figure 9(a) for in the deconfined state, that is, for GPa. The coherence parameter demonstrates a temperature and pressure dependence similar to that of the salt. Thus, the chemical pressure offset of 3 GPa between the two studied salts in the generic phase diagram also holds for the occurrence of the dimensional crossover.
3. Mott Physics
The low-frequency properties of and resemble features previously observed in KCP and TTF-TCNQ (cf. Section 1). From Figures 6(c) and 10, it can be seen that a strong deviation from the simple Drude behavior is observed below 1000 in the reflectivity for ; for lower frequencies actually drops to a minimum between 50 and 100 and only then rises rapidly in order to approach unity. This behavior was first observed by Jacobsen et al. [3, 40, 60, 76], later confirmed by Ng et al. , Degiorgi et al. [42, 43], and eventually explained by the UCLA group [44, 47] when the optical reflection experiments in the infrared spectral range could be complemented with THz measurements and microwave experiments down to 0.1.
As can be seen in Figure 11 for , the frequency-dependent conductivity consisting of two distinct features: a narrow zero-energy mode at low energy containing a very small part of the spectral weight (approximately ) and a finite-energy mode centered around 200 . Similar observations have been made for , , and , as demonstrated in Figure 12. The general idea now is that these systems resemble a one-dimensional Mott system with filling slightly above one half due to internal doping as a result of the interchain coupling. The isolated stack is effectively half filled (due to the dimerization of the quarter-filled system), leading to a Mott gap in the charge degrees of freedom. This is still seen in the excitation maximum around 200 where the one-dimensional physics is found. The interaction between the stacks results in the Drude-like term and is solely responsible for the finite dc conductivity; here we find the higher-dimensional physics.
These are the general characteristics of a highly anisotropic interacting electron system, with either a half- or quarter-filled band. Coulomb repulsion leads to a Mott gap and, at frequencies above the effective interchain transfer integral, to a Luttinger liquid state. The finite energy feature can be described as the absorption above the Mott gap in such a one-dimensional Luttinger liquid. In particular, above the gap, the optical conductivity behaves as a power law of the frequency , in a way characteristic of a Tomonaga-Luttinger liquid. As demonstrated in Figure 13, the exponent is determined experimentally to be for , , and . Comparison of the data with the Tomonaga-Luttinger theory along with the value of the exponent suggests that the dominant mechanism responsible for the opening of the Mott gap is the quarter-filling of the band; is the order of the commensurability, that is, electrons per site: for half-filling and 2 for quarter-filling. This conclusion leads to a Luttinger liquid parameter , corresponding to very strong repulsion. In a general way, is the noninteracting point with corresponding to attraction and to repulsion. The fact that the one-dimensional theory is unable to account quantitatively for the data below the high-energy peak seems to suggest that the crossover to a two-dimensional regime occurs at relatively high energies. This is in accord to the temperature-dependent resistivity displayed in Figure 7. There we observe a linear temperature dependence of the resistivity at elevated temperatures, that crosses over to a behavior at low temperatures. It is interesting that this power-law behavior extends over about an order of magnitude in temperature if the thermal contraction is properly taken into account .
The remaining zero-energy mode of the itinerant electrons carries only 1% ofthespectralweight and shows deviations from a simple Drude response; it can be adequately described with a frequency-dependent mass and relaxation rate with quadratic frequency-dependence, suggestive of a Fermi liquid. The fact that the resistivity is dominated by electronic scattering basically up to room temperature evidences the negligible contribution of phonons. A similar conclusion is drawn from the extremely narrow zero-energy contribution, that exhibits a width of less than 1 at K. As mentioned above, this seems to be a general property of low-dimensional metals.
4. Spin-Density-Wave State
As can be seen from the temperature-dependent resistivity (Figures 4 and 7) of , also and , and other salts of the Bechgaard family, a spin-density-wave ground state develops at low temperature (K), where the systems become insulating due to the gap in the density of states at the Fermi energy that is caused by the instability of the Fermi surface due to nesting [10, 78]. The low-field dc resistivity shows an activated behavior similar to a standard semiconductor with a gap value K [42, 59]. In analogy to the resistivity, the gap is expected to show up also in the optical conductivity, however, it turned out to be quite challenging to unambiguously measure the spin-density-wave gap by optical means, due to the high reflectivity, augmented by the reduced spectral weight in the relevant range of frequency.
Based on far-infrared reflection measurements on , a spin-density-wave gap of 180 was suggested by Ng et al. ; following the arguments previously developed for , for K, the semiconducting spin-density-wave state coexists with a metallic state, leading to a high conductivity background. The extracted ratio seems to be rather large. However, it is known from incommensurate Peierls transitions that the mean-field temperatures are several times the actual transition temperature due to three-dimensional ordering .
The Vancouver group of Eldridge tried to overcome the mentioned experimental problem by directly probing the absorptivity , that is, the power absorbed by the sample [41, 46, 82]. They point out that the reflectivity of , for instance, evaluated via the directly probed absorption is almost 10% higher than previous reflection measurements on mosaics by Ng et al. . Applying a novel composite-bolometric technique, the far-infrared properties of , , and were measured at low temperatures . For , they find a conductivity peak at with a gap at that corresponds to the thermal gap obtained from magnetotransport measurements by Chaikin et al. . Along the chain direction () an additional sharp feature was observed at . Interestingly for the perpendicular direction also a peak is found at that also follows the dependence. A gap value of is rather close to the mean field ratio .
It was Degiorgi et al.  who could clearly identify the spin-density-wave gap in and later in [77, 84] by looking in the perpendicular direction (). Using a Fourier-transform spectrometer for infrared and far-infrared reflection measurements (15– ) and a THz coherent source spectrometer for lower frequencies (7–20 ), they probed an extremely broad spectral range. Below K and 6 K, the reflectivity is significantly reduced below 70 and 100 , respectively, leading to an energy gap in the optical conductivity. In Figure 14, the frequency-dependent reflectivity and conductivity of is plotted for different temperatures. A gaplike feature developing at about 70 for , which is rather sharp, moves to slightly lower frequencies and decreases in intensity as the temperature increases. It nicely follows the mean-field behavior also extracted from magnetic measurements [85, 86]. This feature seems to persist at temperatures slightly above .
Watanabe et al. measured the reflection of using a THz-time-domain spectroscopic system  that had sufficient resolution in phase shift to observe the transition from the metallic to the insulating spin-density-wave state. A distinct reduction of the conductivity below approximately 65 gives a clear indication of the spin-density-wave gap formation in accord to . In addition, they were able to investigate the photoexcited dynamics of the spin-density-wave state in . After the ultrashort laser-pulse excitation (90 fs at nm), a metallic phase is generated within 3 ps. The sudden closing of the gap (observed in the far-infrared spectrum) is followed by a recovery of the spin-density-wave gap; the recovery time is found to diverge toward , while the gap remains open at [88, 89].
Compared to the two compounds discussed above, the experimental work on and is less substantial . For , a very narrow Drude-like contribution is identified in the metallic state above the spin-density-wave transition (K), that vanishes completely as . Although the low-frequency conductivity ( ) is slightly reduced in the spin-density-wave state (according to a decrease in reflectivity below 63 ), no clear evidence of a spin-density wave gap was identified .
The spin-density-wave ground state is characterized by a complete and more or less isotropic energy gap in the single-particle excitation spectrum that is seen in the temperature-dependent dc resistivity in all three directions (Figure 7). In addition, collective excitations of the many-body ground state are expected, as known from the pinned-mode resonance in the case of charge density waves. In a comprehensive series of microwave experiments, Donovan et al. [42, 90–93] succeeded to probe the frequency-dependent conductivity down to 3 GHz (corresponding to 0.1 ), complemented by radio frequency and standard optical experiments. As displayed in Figure 15, a Drude-like metallic behavior was found above the spin-density wave transition together with a temperature-independent feature at higher frequencies. The later one corresponds to the excitations across the Mott gap discussed above [44, 47]. A fit of the data by the Drude model yields a , which places the material well into the clean limit. The ac response shows a strong frequency-dependence, and most importantly, two subgap modes can be identified: a very broad one in the radio frequency range, due to internal deformations of the spin density wave and a narrow mode near , which is interpreted as the response of the phason. No evidence for a single-particle gap is seen in the infrared spectral range, but this is in full accord with a material in the clean limit. Later Petukhov and Dressel performed [94–96] microwave experiments along all three directions of the spin-density-wave model compound and could demonstrate that the pinned-mode resonance is present along the and axes. Since the collective transport of the spin density wave is considered to be the fingerprint of the condensate, this unambiguously proves that the density wave also slides in the perpendicular direction. The collective response, however, is absent along the least conducting direction.
When a magnetic field of up to 10 Tesla is applied along the axis, no change in the 16.5 GHz microwave response is observed for and [97–99]. For direction, however, striking changes are detected below in the dielectric constant and conductivity. The local maximum near K in is accompanied by a sharp decrease in ; orbital effects might be important for the Fermi-surface nesting properties. This leads to changes in the behavior of the condensate. Based upon anomalies in and , an ambient pressure diagram is suggested by the Sherbrooke group .
5. Very Low-Frequency Behavior
The discussion on the collective mode and pseudogap in one-dimensional conductors raised in the 1970 by measurements of KCP and TTF-TCNQ (cf. Section 1) was revived in the case of the Bechgaard salts. Several aspects come together are often entangled and confused; however, also the materials might not be in a well-defined ground state, but electronically phase separated between metallic, spin-density-wave and even superconducting state. It should be pointed out that the single crystals are extremely clean, with a very low amount of impurities and defects. The coexistence on a microscopic scale is subject to intense investigations [100, 101]. Unfortunately, real features are also mixed with experimental artefact due to a variety of reasons, some of them discussed above. It should be noted, however, that most groups were well aware of the experimental insufficiencies and pitfalls: they present and discuss their findings with the appropriate caution. Strong claims have been rare in all those years.
Due to the ambient pressure superconductivity has drawn most attention starting from the early days [60, 102–104] until the nice reviews of Timusk in the 1990s [80, 105, 106]. In order to allow the anions to order near 24 K, the sample has to be cooled down very slowly through ; only then the metallic state is preserved and superconductivity reached at K. Fast cooling results in the insulating spin-density-wave ground state (Figure 7(b)0. Unfortunately the actual situation for most optical experiments will be a mixture of both states, making the interpretation of the spectra difficult .
The challenge of the low-frequency electrodynamic properties is presented in Figure 16, following the review of Cao et al. . Although the overall reflectivity obtained from various groups agrees quite well, the absolute values differ by up to 10% [80, 108]. As nicely demonstrated by Eldridge and Bates , an increase of by only 1.5% doubles the peak height in conductivity by half the width. The lowest frequency extrapolation of the reflectivity is commonly done by a Hagen-Rubens assumption, with or without taking the actual dc conductivity into account. In Figure 16(b), different extrapolations are plotted, where the solid line corresponds to a Drude-like term with a plasma frequency of and a scattering rate of only 0.034 . The corresponding optical conductivity obtained from a Kramers-Kronig analysis is shown in Figure 16(c). Not only the zero-frequency mode is influenced by the assumed extrapolation, but also the amplitude of the low-lying phonon line at 7 .
As can be seen in Figures 16(c) and 17, the far-infrared optical conductivity is rather small in contrast to the dc conductivity with no sign of a Drude absorption directly visible. Instead the optical conductivity is dominated by a broadband centered around 300 , that can be assigned to excitations across the Mott gap , as discussed above. An energy gap might be identified in the spectra at for K. There is an overall shift of spectral weight to lower energies as the temperature is reduced.
Challener et al. measured the far-infrared properties of between 4 and 40 down to K [109, 110]. They identified two phonon-like peaks in the reflectivity at 7 and 29 ; the upper one does not change with magnetic fields up to 4 Tesla, but vanishes upon irradiation and temperature increase to 20 K. In contrast, the McMaster group [81, 104] found this mode to be sensitive to magnetic field and thus rules out direct phonon absorption. Two far-infrared features, one at 7 and one at 25 seem to be robust, where the later one is suppressed as a field Tesla is applied and shifts by 2 when the temperature is raised to 24 K. However, it is obvious that this feature cannot be identified with the superconducting gap nor with the spin-density-wave state .
It was pointed out by Eldridge et al. [111–114] that the TMTSF salts with centrosymmetric anions, such as , behave quite differently than the one with tetrahedral anions, for instance , or , since the zone folding at the anion order TAO causes a sharp structure in the far-infrared spectral range. Recently a complete vibrational study was published  including Raman and infrared modes, intramolecular as well as lattice vibrations. The X salts crystallize in triclinic space, but the stack dimerization can be neglected and a pseudo-monoclinic unit cell approximated with symmetry . In the spectral region below 200 at most, 15 lattice modes are expected: 6 due to the rigid translations and rotations of the anions and 9 due to the dimer degrees of freedom. The anion translations are infrared active and have symmetry (translation along -axis), and symmetry (translations perpendicular to the axis) while the three anion rotations are Raman active with one and two symmetry modes. When the TMTSF salts with tetrahedral anions undergo an anion order transition at , the unit cell is doubled in all three directions: ; implying that the there are eight times as many phonons present below . Eldridgeetal. interpret the 30 feature—that actually exists already above the anion order as a very broadband in their powder absorption spectra—as a transverse acoustic zone-boundary phonon which is coupled very strongly with the electrons . Note, however, that Ng et al.  found this phonon mode to be sensitive to magnetic field in , although not confirmed by the Richards’ group . By extending the experiments up to magnetic fields of 20 Tesla, Janssen et al. confirmed that this zone-folding phonon mode actually couples to the field-induced spin-density-wave state .
These far-infrared spectroscopic measurements (in reflection and transmission mode) in very high magnetic fields ( Tesla and K) made it possible to explore the field-induced spin-density-wave states. Perel et al.  identified a spin-density-wave gap at 12 from their reflection measurements at K, slightly lower than the one reported by Janssen et al.. A partial gap around 14 is most pronounced in all field-induced spin-density-wave phases. Two sharp features appear around 17 and 32 , and a smaller one at 23 ; none of them shift with magnetic field. A peak shows up at 2.4 that becomes stronger with increasing field and is thought to be a collective mode excitation , although its eigenfrequency is considerably lower than the pinned-mode resonance observed by the UCLA group [42, 90].
In order to probe the superconducting state Reedyk and collaborators  measured the low-temperature reflectivity () of in the spectral range from 10–70 down to 0.45 K. When they find a shift of the low-frequency reflectivity step (associated to the plasma edge of the Drude contribution) to higher frequencies. Also the 30 mode exhibits an upward shift by about 2 . These observations are interpreted as coupling to quasiparticle excitations across the superconducting gap. As already pointed out previously , no direct evidence of the superconducting energy gap can be found in optical experiments.
In Figure 18, recent transmission measurements are presented for the spectral range from 1.6 to 46  obtained by employing a coherent source THz spectrometer . In order to avoid the well-known difficulties with the high reflection values, the sample surface and problems due to mosaic sample, finely ground crystals were dispersed in an appropriate oil and pressed in free-standing pellets. Although this procedure mixes the different crystallographic directions and causes uncertainties in the absolute values, it is free of spurious peaks due to diffraction and leakage. The sample was cooled by approximately 0.8 K per min in the temperature range from K down to 5 K in order to arrive at the relaxed metallic state. At low temperatures, we can clearly identify a peak in the conductivity centered around 30 and a smaller mode at 7 . In addition, there always remains a Drude-like contribution necessary to describe the metallic conductivity. In Figure 19, the temperature dependences of those parameters are plotted that were used to fit these three contributions. The static conductivity obtained from a zero-frequency extrapolation corresponds to a mixture of all three crystallographic directions and thus cannot be simply compared to direct four-probe dc measurements (Figure 4). If we confine ourselves to the optical data, we obtain 2–10 , as plotted in Figure 19(b), which is different than the very narrow Drude-like contribution as previously discussed by Timusk et al. [80, 81] for the results for . They used values of , in order to fit the dc conductivity along the chains and suggest even smaller one (Figure 16(b)). It is interesting to note, that the dielectric constant is still positive and increases as the frequency is reduced, indicating the dominance of the least-conducting direction and a gaplike feature.
Part of it comes from the low-frequency mode seen around 7 . It was already discovered by several groups three decades ago [81, 109], without a clear assignment. The fit by a Lorentzian line is not really satisfactory, implying that it might reflect a gaplike feature. From the temperature dependence of its spectral weight displayed in Figure 19(c), we see a strong increase of strength below 20 K, which infers a connection to the anion order at K rather than to spin-density-wave state that develops in the quenched state only below 7 K and should not be present in our sample. It should be noted here that a similar mode is not observed for centrosymmetric anions (Figure 21). This supports our assignment to an acoustic phonon mode that becomes activated by the back-folding of the Brillouin zone below .
Most pronounced in the spectra (Figure 18) is the strong mode at 30 that can be identified up to elevated temperatures but strongly decreases its width (Figure 19(f)). Below K a second peak seems to appear close by and a satisfactory fit can only be obtained by introducing an additional Lorentz term, indicated by the open symbols in Figures 19(d)–19(f). The constant frequency and decreasing line width presented in Figure 19(f) confirm previous findings by Eldridge et al. , also based on powder transmission measurements in the far-infrared. Finding similar modes in other Bechgaard salts with octahedral anions, such as and , makes them conclude that this feature has to be assigned to a transverse-acoustic zone-boundary phonon that couples strongly with the electrons.
Applying their novel composite-bolometric technique to various Bechgaard salts, the far-infrared properties of , , and were measured by the UBC group [41, 46, 82] at low temperatures . For , they find a conductivity peak at with a gap at that corresponds to the thermal gap obtained from magnetotransport measurements by Chaikin et al. . Along the chain direction () an additional sharp feature was observed at . Interestingly for the perpendicular direction a peak is found at that also follows the dependence.
In Figure 20, the low-temperature (K) optical transmission of is shown for the extremely low-frequency range spanning from 1.6 to 12.6 . The experiments are performed on a large number of ground crystals pressed into pellets of different thickness (mm and 0.115 mm) in order to optimize the transmission for low and high frequencies. In the thin sample Fabry-Pérot interferences are present due to the multireflection within the pellet. These are strongly damped for the thick sample. Analyzing the spectra at different temperatures with Fresnel’s equations [10, 16], we obtain the conductivity and dielectric constant as a function of frequencies for various temperatures. As an example, the temperature-dependent parameters are plotted in Figure 21. According to the metallic behavior, the low-frequency transmission ( ) strongly decreases as the temperature is reduced down to the spin-density-wave transition at K; in the insulating state, the transmission rapidly increases (Figure 21(a)). As seen from the overview of Figure 11, at we still probe the Drude-like contribution that collapses at the metal-to-insulator transition. In Figure 21(b), we consider the parameters right between the Drude term and the band assigned to the excitations across the Mott gap. As the temperature is lowered, there is only a minor variation of around , that is also seen in the dielectric constant. This implies that the change in spectral weight is small due to the spin-density-wave transition.
After 30 years of enormous efforts from various groups all around the globe, the low-frequency properties of one-dimensional conductors remain a challenge with many open questions. In particular, the GHz and THz range is still unexplored, except some isolated points obtained from the cavity perturbation method. What is the width of the zero-frequency mode? Does it exhibit a Drude shape, or does the scattering rate follow a dependence expected for a Fermi liquid? What is the development with temperature? Is the spectral weight conserved? Up to which frequency the spectrum is involved in a redistribution with temperature? How does the reduced spectral density observed in photoemission experiments reflect in the optical data? Can we probe the superconducting gap by optical means? Is there a coexistence of the collective excitations of the spin-density-wave and superconducting ground state? Hopefully it will not take another 30 years to clarify these issues.
Important contributions of L. Degiorgi, S. Donovan, B. Gorshunov, G. Grüner, S. Kaiser, T. Knoblauch, C. Kuntscher, M. Masino, A. Pashkin, K. Petukhov, E. Rose, B. Salameh, A. Schwartz, G. Untereiner, and P. Zornoza over the last decade are sincerely acknowledged. The project was supported by the Deutsche Forschungsgemeinschaft (DFG).
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