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Abstract:
The behavior of matter near zero temperature continuous phase
transitions, or "quantum critical points" is a central topic of study in
condensed matter physics. In fermionic systems, fundamental questions
remain unanswered: the nature of the quantum critical regime is unclear
because of the apparent breakdown of the concept of the quasiparticle, a
cornerstone of existing theories of strongly interacting metals. Even
less is known experimentally about the formation of ordered phases from
such a quantum critical "soup." Here, we report a study of the specific
heat across the phase diagram of the model system Sr3Ru2O7, which
features an anomalous phase whose transport properties are consistent
with those of an electronic nematic. We show that this phase, which
exists at low temperatures in a narrow range of magnetic fields, forms
directly from a quantum critical state, and contains more entropy than
mean-field calculations predict. Our results suggest that this extra
entropy is due to remnant degrees of freedom from the highly entropic
state above T-c. The associated quantum critical point, which is
"concealed" by the nematic phase, separates two Fermi liquids, neither
of which has an identifiable spontaneously broken symmetry, but which
likely differ in the topology of their Fermi surfaces.
