Lambda DASH

The mysteries of the helium phase diagram further deepen at the strange A-line that divides the two liquid phases. In certain respects, this coexistence curve (dashed line) exhibits characteristics of a line of critical points, with divergences of heat capacity and other properties that are normally associated with critical-point limits (so-called second-order transitions, in Ehrenfest s classification). Sidebar 7.5 explains some aspects of the Ehrenfest classification of phase transitions and the distinctive features of A-transitions (such as the characteristic lambda-shaped heat-capacity curve that gives the transition its name) that defy classification as either first-order or second-order. Such anomalies suggest that microscopic understanding of phase behavior remains woefully incomplete, even for the simplest imaginable atomic components. 

Figure 3.16 Enthalpy and heat capacity as a function of temperature for (a) first and (b) second order and lambda transformations [5][8]. Lambda transformation behavior is shown with dot-dot-dashed lines.

Figure 3.17 Heat-flux DSC trace at 10°C/min of the ferromagnetic to paramagnetic lambda transformation at 354°C in nickel (dotted line). The Curie temperature indicated by the DSC trace is 346°C (dot-dashed line).

The phase diagram for " He is shown in Fig. 4.3. There are two liquid phases, Hel and Hell, separated in the phase diagram by a line known as the lambda line (X line) and shown dashed in Fig. 4.3. The change that occurs at the X line is marked by a very characteristic anomaly in the specific heat capacity c of the liquid the specific heat capacity rises to a very high value at the lambda... 

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