AC calorimetry of Quasi-one-dimensional materials

There has been continuing interest in quasi-one-dimensional materials for decades. Their one-dimensional nature gives rise to many unusual properties such as spin-density waves, charge-density waves, spin-Peierls transitions, glassy behavior, and large fluctuation effects. We study these phase transitions by measuring resistance, specific heat, and Young's moduli, among other properties.; We have measured the specific heat of small (∼1 mg) single crystalline flakes of α′-NaV₂O₅. The size of the peak (Δc_p ∼ R) is more than an order of magnitude greater than the value estimated from mean-field theory. The specific heat and Young's modulus anomalies are very large and sample dependent. Subsequent work has shown that NaV₂O₅ does not go through a spin-Peierls transition as was first thought. It is generally believed that this is a charge order phase transition, with a transformation into a spin gap singlet state; We have measured the specific heat of single crystals of TSeF-TCNQ. Anomalies are observed at T_a = 28K and T_b = 33K, indicating that there are two phase transitions, presumably reflecting the separate condensation of the charge-density wave on the TSeF and TCNQ conducting chains.; We built an ac Joule calorimeter to measure the specific heats of materials from 1K to 10K. Pumping on liquid helium allowed us to operate at any temperatures between 1K and 4.2K. Oscillating power was applied to the sample with a thin film resistor attached directly to the sample. The magnitude of the oscillating temperature is inversely proportional to the heat capacity when certain experimental conditions are met.; We have searched for dynamic thermal effects in (TMTSF)₂PF ₆ by measuring its specific heat with ac calorimetry. Near its spin-density wave transition (T_c = 12K), the magnitude of the specific heat is the same as that measured in quasi static measurements, indication that there are no kinetic contributions to the specific heat at these temperatures. No anomalies are observed in a pellet of (TMTSF)₂PF₆ mixed with KBr at T_c or the expected “spin-density wave glass” transition (T_g ∼ 3.5K), suggesting that residual disorder in the pellet freezes in a glassy state at T_c.