We investigate the phenomenon of fermionic pairing with mismatched Fermi surfaces in a two-species system in the presence of Feshbach resonance, where the resonantly paired fermions combine to form bosonic molecules. We observe that the Feshbach parameters control the critical temperature of the gapped BCS superfluid state, and also determine the range over which a gapless breached pair state may exist. Demanding the positivity of the superfluid density, it is shown that although a breached pair state with two Fermi surfaces is always unstable, its single Fermi-surface counterpart can be stable if the chemical potentials of the two pairing species have opposite signs. This condition is satisfied only over a narrow region in the BEC side, characterized by an upper and a lower limit for the magnetic field. We estimate these limits for a mixture of two hyperfine states of $^{6}\text{L}\text{i}$ using recent experimental data.We investigate the phenomenon of fermionic pairing with mismatched Fermi surfaces in a two-species system in the presence of Feshbach resonance, where the resonantly paired fermions combine to form bosonic molecules. We observe that the Feshbach parameters control the critical temperature of the gapped BCS superfluid state, and also determine the range over which a gapless breached pair state may exist. Demanding the positivity of the superfluid density, it is shown that although a breached pair state with two Fermi surfaces is always unstable, its single Fermi-surface counterpart can be stable if the chemical potentials of the two pairing species have opposite signs. This condition is satisfied only over a narrow region in the BEC side, characterized by an upper and a lower limit for the magnetic field. We estimate these limits for a mixture of two hyperfine states of $^{6}\text{L}\text{i}$ using recent experimental data.

arXiv:0812.5028Physical Review ADec 15, 20095 citations

Pair Susceptibilities and Gap Equations in Non-Fermi Liquids

Asle Sudbø

The pair susceptibility and superconducting gap equation are obtained when a branch-cut spectrum, with either spin-charge separation or an anomalous Fermi surface exponent $\ensuremath{\alpha}>0$ is introduced for the normal state. For $\mathbf{k}$ -nondiagonal pairing interactions, spin-charge separation leads to an enhancement of ${T}_{c}$ and ${\ensuremath{\Delta}}_{\mathbf{k}}(0)$ compared to a Fermi liquid. For $\ensuremath{\alpha}>0$, a critical coupling is required for a solution. For spin-charge separation or $0<\ensuremath{\alpha}<\frac{1}{2}$, an arbitrarily small $\mathbf{k}$ -diagonal contribution to the pairing interaction still leads to a solution, as in a Fermi liquid.The pair susceptibility and superconducting gap equation are obtained when a branch-cut spectrum, with either spin-charge separation or an anomalous Fermi surface exponent $\ensuremath{\alpha}>0$ is introduced for the normal state. For $\mathbf{k}$-nondiagonal pairing interactions, spin-charge separation leads to an enhancement of ${T}_{c}$ and ${\ensuremath{\Delta}}_{\mathbf{k}}(0)$ compared to a Fermi liquid. For $\ensuremath{\alpha}>0$, a critical coupling is required for a solution. For spin-charge separation or $0<\ensuremath{\alpha}<\frac{1}{2}$, an arbitrarily small $\mathbf{k}$-diagonal contribution to the pairing interaction still leads to a solution, as in a Fermi liquid.

Physical Review LettersMar 27, 199532 citations

Fate of a multiple-band Fermi liquid that is coupled with critical ϕ4 bosons

Zhiming Pan, Ryuichi Shindou

Multiple-band nature of electronic energy bands leads to novel physical effects in solids. In this paper, we clarify physical properties of a Fermi system with a pair of electron and hole Fermi surfaces (FSs), whose coupling is mediated by a critical U(1) boson field. By using a one-loop renormalization group analysis, we show that when the boson field undergoes a quantum phase transition with broken U(1) symmetry, the multiple-band Fermi system shows a non-Fermi liquid (non-FL) behavior in its thermodynamic and magnetic properties. At a quantum critical point (QCP), Fermi velocities of the two FSs are renormalized into the same critical velocity as a boson velocity, and the fermion's density of states (DOS) shows a pseudogap behavior with a logarithmic energy dependence at the QCP.Multiple-band nature of electronic energy bands leads to novel physical effects in solids. In this paper, we clarify physical properties of a Fermi system with a pair of electron and hole Fermi surfaces (FSs), whose coupling is mediated by a critical U(1) boson field. By using a one-loop renormalization group analysis, we show that when the boson field undergoes a quantum phase transition with broken U(1) symmetry, the multiple-band Fermi system shows a non-Fermi liquid (non-FL) behavior in its thermodynamic and magnetic properties. At a quantum critical point (QCP), Fermi velocities of the two FSs are renormalized into the same critical velocity as a boson velocity, and the fermion's density of states (DOS) shows a pseudogap behavior with a logarithmic energy dependence at the QCP.

arXiv:2008.00359Physical review. B./Physical review. BDec 28, 20200 citations

^{6}\text{L}\text{i}

using recent experimental data.

\mathbf{k}

-nondiagonal pairing interactions, spin-charge separation leads to an enhancement of

{T}_{c}

and ${\ensuremath{\Delta}}_{\mathbf{k}}(0)$ compared to a Fermi liquid. For $\ensuremath{\alpha}>0$, a critical coupling is required for a solution. For spin-charge separation or $0<\ensuremath{\alpha}<\frac{1}{2}$, an arbitrarily small

\mathbf{k}

-diagonal contribution to the pairing interaction still leads to a solution, as in a Fermi liquid.