15 research outputs found
Theory of THz Conductivity in the Pseudogap Phase of the Cuprates: A Pre-Formed Pair Perspective
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
measurements in the underdoped high temperature superconductors and arrive at
reasonable agreement between theory and recent experiment for both
and above and below .Comment: 8 pages, 2 figure
Contrasting Nodal and Anti-Nodal Behavior in the Cuprates Via Multiple Gap Spectroscopies
Using a precursor superconductivity scenario for the cuprates we present a
theory for the temperature dependent behavior of the spectral gaps associated
with four distinct spectroscopies: angle resolved photoemission (ARPES),
differential conductance , quasi-particle interference spectroscopy, and
the autocorrelated ARPES pattern. We find good agreement for a range of
existing experiments and make predictions for others. Our theory, which
incorporates the necessary (observed) contrast between the nodal and anti-nodal
response, shows how different nodal gap shapes are associated with these
alternative spectroscopies.Comment: 4 pages, 3 figure
Perfect Fluids and Bad Metals: Transport Analogies Between Ultracold Fermi Gases and High Superconductors
In this paper, we examine in a unified fashion dissipative transport in
strongly correlated systems. We thereby demonstrate the connection between "bad
metals" (such as the high temperature superconductors) and "perfect fluids"
(such as the ultracold Fermi gases, near unitarity). One motivation of this
work is to communicate to the high energy physics community some of the central
unsolved problems in high superconductors. Because of interest in the
nearly perfect fluidity of the cold gases and because of new tools such as the
AdS/CFT correspondence, this better communication may lead to important
progress in a variety of different fields. A second motivation is to draw
attention to the great power of transport measurements which more directly
reflect the excitation spectrum than, say, thermodynamics and thus strongly
constrain microscopic theories of correlated fermionic superfluids. Our
calculations show that bad metal and perfect fluid behavior is associated with
the presence of a normal state excitation gap which suppresses the effective
number of carriers leading to anomalously low conductivity and viscosity above
the transition temperature . Below we demonstrate that the
condensate collective modes ("phonons") do not couple to transverse probes such
as the shear viscosity. As a result, our calculated shear viscosity at low
becomes arbitrarily small as observed in experiments. In both homogeneous and
trap calculations we do not find the upturn in or (where is
the entropy density) found in most theories. In the process of these studies we
demonstrate compatibility with the transverse sum rule and find reasonable
agreement with both viscosity and cuprate conductivity experiments.Comment: 21 pages, 11 figure
Theory of Diamagnetism in the Pseudogap Phase: Implications from the Self energy of Angle Resolved Photoemission
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.Comment: 15 pages, 8 figure
The Two Component Optical Conductivity in the Cuprates: A Necessary Consequence of Preformed Pairs
We address how the finite frequency real conductivity in the
underdoped cuprates is affected by the pseudogap, contrasting the behavior
above and below . 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.Comment: 5 pages, 3 figure
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 can disagree with the expected
behavior of a Fermi liquid (FL) while the spin susceptiblity(following the
experimental protocols) is consistent with a Fermi liquid picture. We show that
the experimental protocols for extracting are based on a FL
presumption; relaxing this leads to consistency within (but not proof of) a
pseudogap-based approach. Our tranport calculations yield insight into the
measured strong suppression of the spin diffusion constant at lower .Comment: 4 pages, 2 figure