Dan Wulin

Microscopic approach to shear viscosities of unitary Fermi gases above and below the superfluid transition.

We compute the shear viscosity, η, at general temperatures T , in a BCS-BEC crossover scheme which is demonstrably consistent with conservation laws. The study of η is important because it constrains microscopic theories by revealing the excitation spectra. The onset of a normal state pairing gap and the contribution from pair degrees of freedom imply that η at low T becomes small, rather than exhibiting the upturn predicted by most others. Using the local density approximation, we find quite reasonable agreement with just-published experiments.

Spin transport in cold Fermi gases: a pseudogap interpretation of spin diffusion experiments at unitarity

We address recent spin transport experiments in ultracold unitary Fermi gases. We provide a theoretical understanding for how the measured temperature dependence of the spin diffusivity at low T can disagree with the expected behavior of a Fermi liquid (FL), while the spin susceptiblity, following the experimental protocols, is consistent with a FL picture. We show that the experimental protocols for extracting the spin susceptibility implicitly reflect a FL viewpoint; relaxing this leads to consistency within but not proof of a pseudogap-based theory. Our transport calculations yield insight into the observed suppression of the spin diffusion constant at lower T.

Two-component optical conductivity in the cuprates: A necessary consequence of preformed pairs

We address how the finite frequency real conductivity

Theory of diamagnetism in the pseudogap phase of high-temperature superconductors: Implications from the self-energy of angle-resolved photoemission

\sigma(\omega)

Theory of terahertz conductivity in the pseudogap phase of the cuprates: A preformed pair perspective

in the underdoped cuprates is affected by the pseudogap, contrasting the behavior above and below

Contrasting nodal and antinodal behavior in the cuprates via multiple gap spectroscopies

T_c

Perfect Fluids and Bad Metals: Insights from Ultracold Fermi Gases

. The f-sum rule is analytically shown to hold. Here we presume the pseudogap is associated with non-condensed pairs arising from stronger-than-BCS attraction. This leads to both a Drude and a mid infrared (MIR) peak, the latter associated with the energy needed to break pairs. These general characteristics appear consistent with experiment. Importantly, there is no more theoretical flexibility (phenomenology) here than in BCS theory; the origin of the two component conductivity we find is robust.

Model for the temperature dependence of the quasiparticle interference pattern in the measured scanning tunneling spectra of underdoped cuprate superconductors

In this paper we apply the emerging- consensus understanding of the fermionic self energy deduced from angle resolved photoemisssion spectroscopy (ARPES) experiments to deduce the implications for orbital diamagnetism in the underdoped cuprates. Many theories using many different starting points have arrived at a broadened BCS-like form for the normal state self energy associated with a d-wave excitation gap, as is compatible with ARPES data. Establishing compatibility with the f-sum rules, we show how this self energy, along with the constraint that there is no Meissner effect in the normal phase are sufficient to deduce the orbital susceptibility. We conclude, moreover, that diamagnetism is large for a d-wave pseudogap. Our results should apply rather widely to many theories of the pseudogap, independent of the microscopic details.

Microscopic Approach to Viscosities in Superfluid Fermi Gases: From BCS to BEC

In this paper we deduce transport properties in the presence of a pseudogap associated with precursor superconductivity. Our theoretical analysis is based on the widely adopted self energy expression reflecting this normal state gap, which has appeared in interpretations of photoemission and in other experiments. Thus, it should be generally applicable. Here we address THz conductivity

Pseudogap effects of Fermi gases in the presence of a strong effective magnetic field

\sigma (\omega) = \sigma_1(\omega) + i \sigma_2(\omega)