We revisit the issue of the temperature dependence of the specific heat
C(T) for interacting fermions in one dimension. The charge component
Cc(T) scales linearly with
T, but the spin component
Cs(T) displays a more complex behavior with
T as it depends on the backscattering amplitude,
g1, which scales down under renormalization group transformation and eventually behaves as ${g}_{1}(T)\ensuremath{\sim}1∕\mathrm{log}\phantom{\rule{0.2em}{0ex}}T$. We show, however, by direct perturbative calculations that
Cs(T) is strictly linear in
T to order
g12 as it contains the renormalized backscattering amplitude not on the scale of
T, but at the cutoff scale set by the momentum dependence of the interaction around
2kF. The running amplitude
g1(T) appears only at third order and gives rise to an extra
T∕log3T term in
Cs(T). This agrees with the results obtained by a variety of bosonization techniques. We also show how to obtain the same expansion in
g1 within the sine-Gordon model.
The nature of the order as $T\ensuremath{\rightarrow}0$ in the one-dimensional interacting electron gas is investigated. We consider both low-momentum-transfer (
g2 and
g4) and large-momentum-transfer (
g1 and
g3) electron-electron interactions, focusing upon the particular value of
g1 for which Luther and Emery have recently found an exact solution. For this value of ${g}_{1}=\ensuremath{-}\frac{6}{5\ensuremath{\pi}{\ensuremath{\nu}}_{F}}$, we calculate the low-frequency behavior of the zero-temperature response functions explicitly. For the singlet superconducting and charge-density-wave responses, our results are consistent with those of Luther and Emery, implying that these response functions are divergent as $\ensuremath{\omega}\ensuremath{\rightarrow}0$ for certain values of
g2. For the triplet superconducting and spin-density-wave responses, however, our results differ from those of Luther and Emery, as we show explicitly that these responses have a gap for low frequencies, as was predicted by Lee. Furthermore, they do not diverge at the gap edge for any value of
g2. We also consider the effect of interactions between electrons on the same side of the Fermi "surface," and find that the low-momentum-transfer process of that type (
g4) does not change the response behavior qualitatively. For the large-momentum-transfer process between electrons on the same side of the Fermi "surface" (
g3), we may solve the problem exactly for
g2=0 and ${g}_{1}=\ensuremath{-}\frac{6}{5\ensuremath{\pi}{\ensuremath{\nu}}_{F}}$, and find that the ground state of the system exhibits only long-range charge-density-wave order. By a mapping onto the classical two-dimensional Coulomb gas problem, we may extend those results for ${g}_{3}\ensuremath{\ne}0$ to the region
g1<0 and ${g}_{1}\ensuremath{-}2{g}_{2}<|{g}_{3}|$.
A recent solution by Luther and Emery of a one-dimensional interacting Fermi-gas model contains a flaw in its final step which when corrected yields qualitatively different behavior. In particular spin-density wave and triplet superconducting pairing excitations are not divergent. The result is consistent with a gap in the uniform spin susceptibility and the gap can now be given a simple physical interpretation. The bulk of the Luther-Emery solution including exponents for the charge-density wave and singlet pairing remains correct.
We consider a model of TTF-TCNQ (tetrahiafulvalenium-tetracyanoquinodimethanide) and similar charge-transfer linear conductors in which there are two chains. The electrons on the acceptor (TCNQ) chains have a usual metallic band, but the electrons on the donor (TTF) chains have an inverted or "hole" band. The electrons on each chain interact via the forward and backward scattering
g2 and
g1 processes, respectively. Electrons on different chains interact via the analogous processes
W2 and
W1. The model is also applicable to two metallic chains, with the signs of
W1 and
W2 reversed. Using the techniques of Luther and Emery and of Chui and Lee we show that the problem may be mapped onto a similar problem of three coupled classical two-dimensional Coulomb gases. In addition to gaps in the spin degrees of freedom that were present in the single-chain problem, we find that the antisymmetric form of the resultant charge degrees of freedom may have a gap. We then investigate the predominant two-, four-, six-, and eight-particle response functions and calculate their temperature dependence. In addition to the four divergent two-particle response functions present in the single chain, there is a region for weak interaction strengths in which the
2kF interchain Cooper pairing response is the most divergent as $T\ensuremath{\rightarrow}0$. The excitonic insulator response may also be important at high temperatures. For the single chain, either the
4kF response or a four-particle
q=0 response is the most divergent for strong coupling and repulsive
g1, depending upon the sign of ${g}_{2}\ensuremath{-}\frac{1}{2}{g}_{1}$. For the two-chain system, the most divergent response as $T\ensuremath{\rightarrow}0$ for strongly attractive interactions (
g1 and
g2) is the response to the formation of excitonic "molecules" or exciton pairs. For strong coupling and repulsive
g1 either
8kF or octet responses may be observable. We discuss these results in conjunction with our previous renormalization-group treatment, which indicates that for certain values of
g2,
W2, and
W1 the line
g1=0 may be crossed. We use these results to interpret the recent x-ray data on TTF-TCNQ showing a crossover from
4kF to
2kF behavior above the crossover from one-dimensional to three-dimensional behavior.
Using an exactly soluble model, the decay rate of a current-carrying state of one-dimensional fermions is calculated in the presence of random scatterers at finite temperature and the dc conductivity thereby inferred. For interacting fermions it is modified by a factor ${(\frac{T}{T}+\ensuremath{\theta})}^{g}$, where
g is a positive (negative) coupling constant for repulsive (attractive) two-body forces. While the conductivity could be greatly enhanced for
g<0 and $T\ensuremath{\ll}\ensuremath{\theta}$, one-dimensional superconductivity appears ruled out at any finite temperature.