786 research outputs found
ARPES on HTSC: simplicity vs. complexity
A notable role in understanding of microscopic electronic properties of high
temperature superconductors (HTSC) belongs to angle resolved photoemission
spectroscopy (ARPES). This technique supplies a direct window into reciprocal
space of solids: the momentum-energy space where quasiparticles (the electrons
dressed in clouds of interactions) dwell. Any interaction in the electronic
system, e.g. superconducting pairing, leads to modification of the
quasi-particle spectrum--to redistribution of the spectral weight over the
momentum-energy space probed by ARPES. A continued development of the technique
had an effect that the picture seen through the ARPES window became clearer and
sharper until the complexity of the electronic band structure of the cuprates
had been resolved. Now, in an optimal for superconductivity doping range, the
cuprates much resemble a normal metal with well predicted electronic structure,
though with rather strong electron-electron interaction. This principal
disentanglement of the complex physics from complex structure reduced the
mystery of HTSC to a tangible problem of interaction responsible for
quasi-particle formation. Here we present a short overview of resent ARPES
results, which, we believe, denote a way to resolve the HTSC puzzle.Comment: A review written for a special issue of FN
Phenomenological theory of the underdoped phase of a high-T superconductor
We model the Fermi surface of the cuprates by one-dimensional nested parts
near and and unnested parts near the zone diagonals.
Fermions in the nested regions form 1D spin liquids, and develop spectral gaps
below some , but superconducting order is prevented by 1D phase
fluctuations.
We show that the Josephson coupling between order parameters at and
locks their relative phase at a crossover scale . Below
, the system response becomes two-dimensional, and the system displays
Nernst effect. The remaining total phase gets locked at , at
which the system develops a (quasi-) long-range superconducting order.Comment: 4 pages, 1 figure; typos corrected, references adde
Signatures of non-monotonic d-wave gap in electron-doped cuprates
We address the issue whether the data on optical conductivity and Raman
scattering in electron-doped cuprates below support the idea that the
wave gap in these materials is non-monotonic along the Fermi surface. We
calculate the conductivity and Raman intensity for elastic scattering, and find
that a non-monotonic gap gives rise to several specific features in optical and
Raman response functions. We argue that all these features are present in the
experimental data on NdCeCuO and PrCeCuO
compounds.Comment: 7 pages, 6 figure
Self-energy of a nodal fermion in a d-wave superconductor
We re-consider the self-energy of a nodal (Dirac) fermion in a 2D d-wave
superconductor. A conventional belief is that Im \Sigma (\omega, T) \sim max
(\omega^3, T^3). We show that \Sigma (\omega, k, T) for k along the nodal
direction is actually a complex function of \omega, T, and the deviation from
the mass shell. In particular, the second-order self-energy diverges at a
finite T when either \omega or k-k_F vanish. We show that the full summation of
infinite diagrammatic series recovers a finite result for \Sigma, but the full
ARPES spectral function is non-monotonic and has a kink whose location compared
to the mass shell differs qualitatively for spin-and charge-mediated
interactions.Comment: 4pp 3 eps figure
Optical Integral and Sum Rule Violation
The purpose of this work is to investigate the role of the lattice in the
optical Kubo sum rule in the cuprates. We compute conductivities, optical
integrals W, and \Delta W between superconducting and normal states for 2-D
systems with lattice dispersion typical of the cuprates for four different
models -- a dirty BCS model, a single Einstein boson model, a marginal Fermi
liquid model, and a collective boson model with a feedback from
super-conductivity on a collective boson. The goal of the paper is two-fold.
First, we analyze the dependence of W on the upper cut-off w_c placed on the
optical integral because in experiments W is measured up to frequencies of
order bandwidth. For a BCS model, the Kubo sum rule is almost fully reproduced
at w_c equal to the bandwidth. But for other models only 70%-80% of Kubo sum
rule is obtained up to this scale and even less so for \Delta W, implying that
the Kubo sum rule has to be applied with caution. Second, we analyze the sign
of \Delta W. In all models we studied \Delta W is positive at small w_c, then
crosses zero and approaches a negative value at large w_c, i.e. the optical
integral in a superconductor is smaller than in a normal state. The point of
zero crossing, however, increases with the interaction strength and in a
collective boson model becomes comparable to the bandwidth at strong coupling.
We argue that this model exhibits the behavior consistent with that in the
cuprates.Comment: 16 pp, 23 figures, submitted to PRB, typo corrected, reference adde
Ginzburg-Landau Like Theory for High Temperature Superconductivity in the Cuprates: Emergent d-wave Order
High temperature superconductivity in the cuprates remains one of the most
widely investigated, constantly surprising, and poorly understood phenomena in
physics. Here, we describe briefly a new phenomenological theory inspired by
the celebrated description of superconductivity due to Ginzburg and Landau and
believed to describe its essence. This posits a free energy functional for the
superconductor in terms of a complex order parameter characterizing it. We
propose, for superconducting cuprates, a similar functional of the complex, in
plane, nearest neighbor spin singlet bond (or Cooper) pair amplitude psi_ij. A
crucial part of it is a (short range) positive interaction between nearest
neighbor bond pairs, of strength J'. Such an interaction leads to nonzero long
wavelength phase stiffness or superconductive long range order, with the
observed d-wave symmetry, below a temperature T_c\simzJ' where z is the number
of nearest neighbours; it is thus an emergent, collective consequence. Using
the functional, we calculate a large range of properties, e.g. the pseudogap
transition temperature T* as a function of hole doping x, the transition curve
T_c(x), the superfluid stiffness rho_s(x,T), the specific heat (without and
with a magnetic field) due to the fluctuating pair degrees of freedom, and the
zero temperature vortex structure. We find remarkable agreement with
experiment. We also calculate the self energy of electrons hopping on the
square cuprate lattice and coupled to electrons of nearly opposite momenta via
inevitable long wavelength Cooper pair fluctuations formed of these electrons.
The ensuing results for electron spectral density are successfully compared
with recent ARPES experiments, and comprehensively explain strange features
such as temperature dependent Fermi arcs above T_c and the 'bending' of the
superconducting gap below T_c .Comment: 22 pages, 14 figures, to appear in Int J Mod Phys
Electronic structure of strongly correlated d-wave superconductors
We study the electronic structure of a strongly correlated d-wave
superconducting state. Combining a renormalized mean field theory with direct
calculation of matrix elements, we obtain explicit analytical results for the
nodal Fermi velocity, v_F, the Fermi wave vector, k_F, and the momentum
distribution, n_k, as a function of hole doping in a Gutzwiller projected
d-wave superconductor. We calculate the energy dispersion, E_k, and spectral
weight of the Gutzwiller-Bogoliubov quasiparticles, and find that the spectral
weight associated with the quasiparticle excitation at the antinodal point
shows a non monotonic behavior as a function of doping. Results are compared to
angle resolved photoemission spectroscopy (ARPES) of the high temperature
superconductors.Comment: final version, comparison to experiments added, 4+ pages, 4 figure
Doped carrier formulation of the t-J model: the projection constraint and the effective Kondo-Heisenberg lattice representation
We show that the recently proposed doped carrier Hamiltonian formulation of
the t-J model should be complemented with the constraint that projects out the
unphysical states. With this new important ingredient, the previously used and
seemingly different spin-fermion representations of the t-J model are shown to
be gauge related to each other. This new constraint can be treated in a
controlled way close to half-filling suggesting that the doped carrier
representation provides an appropriate theoretical framework to address the t-J
model in this region. This constraint also suggests that the t-J model can be
mapped onto a Kondo-Heisenberg lattice model. Such a mapping highlights
important physical similarities between the quasi two-dimensional heavy
fermions and the high-T superconductors. Finally we discuss the physical
implications of our model representation relating in particular the small
versus large Fermi surface crossover to the closure of the lattice spin gap.Comment: corrected and enlarged versio
Bare electron dispersion from photoemission experiments
Performing an in-depth analysis of the photoemission spectra along the nodal
direction of the high temperature superconductor Bi-2212 we have developed a
procedure to determine the underlying electronic structure and established a
precise relation of the measured quantities to the real and imaginary parts of
the self-energy of electronic excitations. The self-consistency of the
procedure with respect to the Kramers-Kronig transformation allows us to draw
conclusions on the applicability of the spectral function analysis and on the
existence of well defined quasiparticles along the nodal direction even for the
underdoped Bi-2212 in the pseudogap state.Comment: 4 pages 3 figures revtex, corrected misprint
Optical sum in Nearly Antiferromagnetic Fermi Liquid Model
We calculate the optical sum (OS) and the kinetic energy (KE) for a tight
binding band in the Nearly Antiferromagnetic Fermi Liquid (NAFFL) model which
has had some success in describing the electronic structure of the high
cuprates. The interactions among electrons due to the exchange of spin
fluctuations profoundly change the probability of occupation of states of momentum {\bf k} and spin which is the
central quantity in the calculations of OS and KE. Normal and superconducting
states are considered as a function of temperature. Both integrals are found to
depend importantly on interactions and an independent electron model is
inadequate.Comment: 9 Pages, 5 Figures Accepted for publication in Phys. Rev.
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