Unconventional spin-density wave in Bechgaard salt $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$

Among many Bechgaard salts, $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$, where TMTSF denotes tetramethyltetraselenafulvalene, exhibits very anomalous low-temperature properties. Unlike the case of the conventional spin-density wave (SDW), $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ undergoes the SDW transition at $T_C\approx 9.5\mathrm{K}$ and the low-temperature quasiparticle excitations are gapless. Also, it is known that $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ does not exhibit superconductivity even under pressure, while a field-induced SDW is found in $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ only for $P=8.5\mathrm{kbar}$ and $B>20\mathrm{T}$. Here we show that both the angle-dependent magnetoresistance data and the nonlinear Hall resistance of $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ at ambient pressure are interpreted satisfactorily in terms of an unconventional spin-density wave. Based on these facts, we propose a new phase diagram for Bechgaard salts.

Figure 1

(Color online) The angular dependence of the normalized resistance $R(B,\theta )∕R_0$ at $T=4.2$K (full lines, experimental data; dashed lines, fits to the theory). Magnetic field is rotated in ${\mathbf{b}}^{\prime }\text{−}{\mathbf{c}}^{\star }$ plane, and $\theta =0°$ corresponds to $\mathbf{B}\parallel {\mathbf{c}}^{\star }$. Data are from Ref. 17 $(B\ge 8\mathrm{T})$ and Ref. 24 $(B=6\mathrm{T})$.

Figure 2

(Color online) The magnetic field $B$ dependence of Hall resistance $R_{xy}$ at several temperatures (points, experimental data; dashed lines, fit to the theory). Data are from Ref. 18.

Figure 3

The temperature dependence of the fitting parameters $D_2$ $(∎)$—see text. Dashed line shows temperature dependence of the resistance $R_{xx}$ of $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ for $B=0$.

Figure 4

The schematic pressure-temperature phase diagram for Bechgaard salts. Arrows denote position of $(\mathrm{TMTSF}{)}_2{\mathrm{PF}}_6$, $(\mathrm{TMTSF}{)}_2{\mathrm{ClO}}_4$, and $(\mathrm{TMTSF}{)}_2{\mathrm{NO}}_3$ in the phase diagram at ambient pressure.