Quantum and thermal phase transitions in the Bechgaard salts and their analogs

Using a combination of Density Functional Theory, mean-field analysis and exact diagonalization calculations we reveal the emergence of a dimerized charge ordered state in TMTTF$_2$-PF$_6$ organic crystal. The interplay between charge and spin order leads to a rich phase diagram. Coexistence of charge ordering with a structural dimerization results in a ferroelectric phase, which has been observed experimentally. The tendency to the dimerization is magnetically driven reveali...

We review the current understanding of superconductivity in the quasi-one-dimensional organic conductors of the Bechgaard and Fabre salt families. We discuss the interplay between superconductivity, antiferromagnetism, and charge-density-wave fluctuations. The connection to recent experimental observations supporting unconventional pairing and the possibility of a triplet-spin order parameter for the superconducting phase is also presented.

In this tutorial we will tackle the problem of electronic correlations in quasi-one-dimensional organic superconductors. We will go through different pieces of experimental evidence showing the range of applicability of the Fermi and Luttinger liquid descriptions of the normal phase of the Bechgaard salts series and their sulfur analogs.

We present a detailed low-temperature investigation of the statics and dynamics of the anions and methyl groups in the organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$AsF$_6$ (TMTSF : tetramethyl-tetraselenafulvalene). The 4 K neutron scattering structure refinement of the fully deuterated (TMTSF)$_2$PF$_6$-D12 salt allows locating precisely the methyl groups at 4 K. This structure is compared to the one of the fully hydrogenated (TMTSF)$_2$PF$_6$-H12 salt previously dete...

We discuss the temperature dependence of the Hall conductivity $\sigma_{xy}$ in the magnetic-field-induced spin-density-wave (FISDW) state of the quasi-one-dimensional Bechgaard salts (TMTSF)_2X. Electronic thermal excitations across the FISDW energy gap progressively destroy the quantum Hall effect, so $\sigma_{xy}(T)$ interpolates between the quantized value at zero temperature and zero value at the transition temperature T_c, where FISDW disappears. This temperature depend...

We report results of high-resolution measurements of the \emph{c$^*$}-axis expansivity ($\alpha_{c^{*}}$) at the charge-ordering (CO) transition for the quasi-1D (TMTTF)$_{2}$X compounds with X = SbF$_6$ and Br and make a comparison with previous results for the X = PF$_6$ and AsF$_6$ salts. For X = SbF$_6$, due to the screening of the long-range Coulomb forces, a sharp $\lambda$-type anomaly is observed at $T_{CO}$, which contrasts with the step-like mean-field anomaly at $T...

(TMTTF)$_2$SbF$_6$ is known to undergo a charge ordering (CO) phase transition at $T_{CO}\approx156K$ and another transition to an antiferromagnetic (AF) state at $T_N\approx 8K$. Applied pressure $P$ causes a decrease in both $T_{CO}$ and $T_N$. When $P>0.5 GPa$, the CO is largely supressed, and there is no remaining signature of AF order. Instead, the ground state is a singlet. In addition to establishing an expanded, general phase diagram for the physics of TMTTF salts, we...

An $ab$ $initio$ investigation of the family of molecular compounds TM$_2$$X$ is conducted, where TM is either TMTSF or TMTTF and $X$ takes centrosymmetric monovalent anions. By deriving the extended Hubbard-type Hamiltonians from first-principles band calculations and evaluating not only the intermolecular transfer integrals but also the Coulomb parameters, we discuss their material dependence in the unified phase diagram. Furthermore, we apply the many-variable variational ...

It is known that within the interacting electron model Hamiltonian for the one-dimensional 1/4-filled band, the singlet ground state is a Wigner crystal only if the nearest neighbor electron-electron repulsion is larger than a critical value. We show that this critical nearest neighbor Coulomb interaction is different for each spin subspace, with the critical value decreasing with increasing spin. As a consequence, with the lowering of temperature, there can occur a transitio...

We theoretically investigate the effects of charge order and spin frustration on the spin ordering in TMTTF salts. Using first-principles band calculations, we find that a diagonal inter-chain transfer integral $t_{q1}$, which causes spin frustration between the inter-chain dimers in the dimer-Mott insulating state, strongly depends on the choice of anion. Within the numerical Lanczos exact diagonalization method, we show that the ferroelectric charge order changes the role o...