[Paper Review] Scaling Behavior of the Longitudinal and Transverse Transport in Quasi One-Dimensional Organic Conductors

Martin Dressel, K. Petukhov+3 more|arXiv (Cornell University)|Sep 13, 2004

Organic and Molecular Conductors Research59 references49 citations

TL;DR

This study investigates the anisotropic electrical transport in quasi-one-dimensional organic conductors (TMTSF)2PF6 and (TMTSF)2ClO4 via dc resistivity and contactless microwave measurements along the a, b′, and c∗ axes. It reveals a dimensional crossover at ~100 K in (TMTSF)2PF6, with power-law scaling consistent with Luttinger liquid behavior along a and b′, while (TMTSF)2ClO4 exhibits unexpectedly low anisotropy and weak one-dimensional signatures only above 200 K, challenging expectations from transfer integrals.

ABSTRACT

We report on dc and microwave experiments of the low-dimensional organic conductors (TMTSF)$_2$PF$_6$ and (TMTSF)$_2$ClO$_4$ along the $a$, $b^{\prime}$, and $c^*$ directions. In the normal state of (TMTSF)$_2$PF$_6$ below T=70 K, the dc resistivity follows a power-law with $ ho_a$ and $ ho_{b^{\prime}}$ proportional to $T^2$ while $ ho_{c^*}\propto T$. Above $T = 100$ K the exponents extracted from the data for the $a$ and $c^*$ axes are consiste1nt with what is to be expected for a system of coupled one-dimensional chains (Luttinger liquid) and a dimensional crossover at a temperature of about 100 K. The $b^\prime$ axis shows anomalous exponents that could be attributed to a large crossover between these two regimes. The contactless microwave measurements of single crystals along the $b^{\prime}$-axis reveal an anomaly between 25 and 55 K which is not understood yet. The organic superconductor (TMTSF)$_2$ClO$_4$ is more a two-dimensional metal with an anisotropy $ ho_a/ ho_{b^{\prime}}$ of approximately 2 at all temperatures. Such a low anisotropy is unexpected in view of the transfer integrals. Slight indications to one-dimensionality are found in the temperature dependent transport only above 200 K. Even along the least conducting $c^*$ direction no region with semiconducting behavior is revealed up to room temperature.

Motivation & Objective

• To investigate the scaling behavior of longitudinal and transverse transport in quasi-one-dimensional organic conductors (TMTSF)2PF6 and (TMTSF)2ClO4.
• To determine the extent to which Luttinger liquid behavior emerges in these materials, particularly in the normal state and across different crystallographic directions.
• To resolve discrepancies in anisotropy by measuring transport along all three crystallographic axes (a, b′, c∗) with minimal contact resistance and surface effects.
• To probe the dimensional crossover temperature in (TMTSF)2PF6 and assess the role of interchain coupling in the observed transport anomalies.
• To compare the transport behavior of (TMTSF)2PF6 and (TMTSF)2ClO4 to test the universality of their electronic response under varying chemical and pressure-like tuning.

Proposed method

• Performed low-temperature dc four-probe resistivity measurements along the a, b′, and c∗ directions using high-quality single crystals of (TMTSF)2PF6 and (TMTSF)2ClO4.
• Employed contactless microwave spectroscopy at 24, 33.5, and 60 GHz to probe the dielectric response and conductivity without contact resistance artifacts.
• Used needle-shaped and mosaic-arranged single crystals to achieve directional selectivity in microwave measurements, especially for the c∗ direction.
• Applied a 3He cryostat to cool samples below 1.1 K to access the superconducting state and studied both quenched and relaxed thermal histories.
• Analyzed temperature-dependent resistivity data using power-law fits to extract critical exponents and identify crossover regimes.
• Compared results with theoretical expectations for coupled Luttinger chains and dimensional crossover at T ≈ 100 K, based on transfer integrals (ta : tb : tc ≈ 250 : 20 : 1 meV).

Experimental results

Research questions

• RQ1 What is the scaling behavior of dc resistivity in (TMTSF)2PF6 and (TMTSF)2ClO4 along the a, b′, and c∗ directions, and does it reflect Luttinger liquid physics?
• RQ2 At what temperature does a dimensional crossover from one-dimensional to three-dimensional behavior occur in (TMTSF)2PF6, and how does it compare to theoretical predictions?
• RQ3 Why does (TMTSF)2ClO4 exhibit such low anisotropy (ρa/ρb′ ≈ 2) despite strong one-dimensional character in the transfer integrals?
• RQ4 What causes the anomalous resistivity anomaly in (TMTSF)2PF6 between 25 and 55 K observed in microwave measurements along the b′-axis?
• RQ5 How do the transport properties of (TMTSF)2PF6 and (TMTSF)2ClO4 compare under chemical tuning, and what does this imply for universality in quasi-1D systems?

Key findings

• In (TMTSF)2PF6 below 70 K, ρa and ρb′ scale as T², while ρc∗ scales as T, indicating distinct one-dimensional transport behavior along different axes.
• Above 100 K, the resistivity exponents along the a and c∗ axes are consistent with Luttinger liquid predictions, indicating a dimensional crossover at ~100 K.
• The b′-axis resistivity in (TMTSF)2PF6 shows anomalous exponents, likely due to a large crossover between one-dimensional and three-dimensional regimes.
• Contactless microwave measurements reveal an unexplained anomaly in the b′-axis response between 25 and 55 K, not yet understood.
• In (TMTSF)2ClO4, the anisotropy ρa/ρb′ remains approximately 2 across all temperatures, contradicting expectations from transfer integrals and indicating strong interchain coupling.
• No semiconducting behavior is observed along the c∗ direction in (TMTSF)2ClO4 up to room temperature, and only weak indications of one-dimensionality appear above 200 K.