673 research outputs found
Proposal for an Experiment to Test a Theory of High Temperature Superconductors
A theory for the phenomena observed in Copper-Oxide based high temperature
superconducting materials derives an elusive time-reversal and rotational
symmetry breaking order parameter for the observed pseudogap phase ending at a
quantum-critical point near the composition for the highest . An
experiment is proposed to observe such a symmetry breaking. It is shown that
Angle-resolved Photoemission yields a current density which is different for
left and right circularly polarized photons. The magnitude of the effect and
its momentum dependence is estimated. Barring the presence of domains of the
predicted phase an asymmetry of about 0.1 is predicted at low temperatures in
moderately underdoped samples.Comment: latex, 2 figure
Pseudogap and photoemission spectra in the attractive Hubbard model
Angle-resolved photoemission spectra are calculated microscopically for the
two-dimensional attractive Hubbard model. A system of self-consistent T-matrix
equations are solved numerically in the real-time domain. The single-particle
spectral function has a two-peak structure resulting from the presense of bound
states. The spectral function is suppressed at the chemical potential, leading
to a pseudogap-like behavior. At high temperatures and densities the pseudogap
diminishes and finally disappears; these findings are similar to experimental
observations for the cuprates.Comment: 5 pages, 4 figures, published versio
Instability of a Landau Fermi liquid as the Mott insulator is approached
We examine a two-dimensional Fermi liquid with a Fermi surface which touches
the Umklapp surface first at the 4 points as the
electron density is increased. Umklapp processes at the 4 patches near lead the renormalization group equations to scale to strong
coupling resembling the behavior of a 2-leg ladder at half-filling. The
incompressible character of the fixed point causes a breakdown of Landau theory
at these patches. A further increase in density spreads the incompressible
regions so that the open Fermi surface shrinks to 4 disconnected segments. This
non-Landau state, in which parts of the Fermi surface are truncated to form an
insulating spin liquid, has many features in common with phenomenological
models recently proposed for the cuprate superconductors.Comment: Minor changes. LaTeX2e, 12 pages, 5 figures. J. Phys. CM 10 (1998)
L38
Quasiparticles and Energy Scaling in BiSrCaCuO (=1-3): Angle-Resolved Photoemission Spectroscopy
Angle-resolved photoemission spectroscopy (ARPES) has been performed on the
single- to triple-layered Bi-family high-{\it T} superconductors
(BiSrCaCuO, =1-3). We found a sharp
quasiparticle peak as well as a pseudogap at the Fermi level in the
triple-layered compound. Comparison among three compounds has revealed a
universal rule that the characteristic energies of superconducting and
pseudogap behaviors are scaled with the maximum {\it T}.Comment: 4 pages, 4 figure
High-Resolution Photoemission Study of MgB2
We have performed high-resolution photoemission spectroscopy on MgB2 and
observed opening of a superconducting gap with a narrow coherent peak. We found
that the superconducting gap is s-like with the gap value of 4.5 meV at 15 K.
The temperature dependence (15 - 40 K) of gap value follows well the BCS form,
suggesting that 2Delta/kBTc at T=0 is about 3. No pseudogap behavior is
observed in the normal state. The present results strongly suggest that MgB2 is
categorized into a phonon-mediated BCS superconductor in the weak-coupling
regime.Comment: 3 pages, 3 figures, accepted in Physical Review Letter
Truncation of a 2-dimensional Fermi surface due to quasiparticle gap formation at the saddle points
We study a two-dimensional Fermi liquid with a Fermi surface containing the
saddle points and . Including Cooper and Peierls channel
contributions leads to a one-loop renormalization group flow to strong coupling
for short range repulsive interactions. In a certain parameter range the
characteristics of the fixed point, opening of a spin and charge gap and
dominant pairing correlations are similar to those of a 2-leg ladder at
half-filling. An increase of the electron density we argue leads to a
truncation of the Fermi surface with only 4 disconnected arcs remaining.Comment: Reference added. RevTeX 4 pages incl. 4 ps file
Detecting fractions of electrons in the high- cuprates
We propose several tests of the idea that the electron is fractionalized in
the underdoped and undoped cuprates. These include the ac Josephson effect, and
tunneling into small superconducting grains in the Coulomb blockade regime. In
both cases, we argue that the results are qualitatively modified from the
conventional ones if the insulating tunnel barrier is fractionalized. These
experiments directly detect the possible existence of the chargon - a charge
spinless boson - in the insulator. The effects described in this paper
provide a means to probing whether the undoped cuprate (despite it's magnetism)
is fractionalized. Thus, the experiments discussed here are complementary to
the flux-trapping experiment we proposed in our earlier work(cond-mat/0006481).Comment: 7 pages, 5 figure
Coherent quasiparticle weight and its connection to high-T_c superconductivity from angle-resolved photoemission
In conventional superconductors, the pairing energy gap (\Delta) and
superconducting phase coherence go hand-in-hand. As the temperature is lowered,
both the energy gap and phase coherence appear at the transition temperature
T_c. In contrast, in underdoped high-T_c superconductors (HTSCs), a pseudogap
appears at a much higher temperature T^*, smoothly evolving into the
superconducting gap at T_c. Phase coherence on the other hand is only
established at T_c, signaled by the appearance of a sharp quasiparticle (QP)
peak in the excitation spectrum. Another important difference between the two
types of superconductors is in the ratio of 2\Delta / T_c=R. In BCS theory,
R~3.5, is constant. In the HTSCs this ratio varies widely, continuing to
increase in the underdoped region, where the gap increases while T_c decreases.
Here we report that in HTSCs it is the ratio z_A\Delta_m/T_c which is
approximately constant, where \Delta_m is the maximum value of the d-wave gap,
and z_A is the weight of the coherent excitations in the spectral function.
This is highly unusual, since in nearly all phase transitions, T_c is
determined by an energy scale alone. We further show that in the
low-temperature limit, z_{\it A} increases monotonically with increasing doping
x. The growth is linear, i.e. z_A(x)\propto x, in the underdoped to optimally
doped regimes, and slows down in overdoped samples. The reduction of z_A with
increasing temperature resembles that of the c-axis superfluid density.Comment: 11 pages, 5 figures, revised versio
Nesting Induced Precursor Effects: a Renormalization Group Approach
We develop a controlled weak coupling renormalization group (RG) approach to
itinerant electrons. Within this formalism we rederive the phase diagram for
two-dimensional (2D) non-nested systems. Then we study how nesting modifies
this phase diagram. We show that competition between p-p and p-h channels,
leads to the manifestation of unstable precursor fixed points in the RG flow.
This effect should be experimentally measurable, and may be relevant for an
explanation of pseudogaps in the high temperature superconductors (HTC), as a
crossover phenomenon.Comment: 4 pages, 4 figures, 1 tabl
The Unusual Superconducting State of Underdoped Cuprates
There is increasing experimental evidence that the superconducting energy gap
in the underdoped cuprates is independent of doping concentration
while the superfluid density is linear in . We show that under these
conditions, thermal excitation of the quasiparticles is very effective in
destroying the superconducting state, so that is proportional to
and part of the gap structure remains in the normal state. We then
estimate and predict it to be proportional to . We also discuss
to what extent the assumptions that go into the quasiparticle description can
be derived in the U(1) and SU(2) formulations of the t-J model.Comment: 4 pages RevTe
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