Flat bands and enigma of metamagnetic quantum critical regime in Sr3Ru2O7

Understanding the nature of field-tuned metamagnetic quantum criticality in the ruthenate Sr₃Ru₂O₇ has presented a significant challenge within condensed matter physics. It is known from experiments that the entropy within the ordered phase forms a peak, and is unexpectedly higher than that outside, while the magnetoresistivity experiences steep jumps near the ordered phase. We find a challenging connection between Sr₃Ru₂O₇ and heavy-fermion metals expressing universal physics that transcends microscopic details. Our construction of the T–B$T\text{–}B$ phase diagram of Sr₃Ru₂O₇ permits us to explain main features of the experimental one, and unambiguously implies an interpretation of its extraordinary low-temperature thermodynamic in terms of fermion condensation quantum phase transition leading to the formation of a flat band at the restricted range of magnetic fields B. We show that it is the flat band that generates both the entropy peak and the resistivity jumps at the QCPs.