46 research outputs found
Raman Response in Doped Antiferromagnets
The resonant part of the electronic Raman scattering response is
calculated within the model on a planar lattice as a function of
temperature and hole doping, using a finite-temperature diagonalization method
for small systems. Results, directly applicable to experiments on cuprates,
reveal on doping a very pronounced increase of the width of the two-magnon
Raman peak, accompanied by a decrease of the total intensity. At the same time
the peak position does not shift substantially in the underdoped regime.Comment: 11 pages revtex, 3 postscript figures. Minor corrections and changes
from previous version, to be published in Phys. Rev.
Finite temperature properties of the triangular lattice t-J model, applications to NaCoO
We present a finite temperature () study of the t-J model on the
two-dimensional triangular lattice for the negative hopping , as relevant
for the electron-doped NaCoO (NCO). To understand several aspects of
this system, we study the -dependent chemical potential, specific heat,
magnetic susceptibility, and the dynamic Hall-coefficient across the entire
doping range. We show systematically, how this simplest model for strongly
correlated electrons describes a crossover as function of doping () from a
Pauli-like weakly spin-correlated metal close to the band-limit (density )
to the Curie-Weiss metallic phase () with pronounced
anti-ferromagnetic (AFM) correlations at low temperatures and Curie-Weiss type
behavior in the high-temperature regime. Upon further reduction of the doping,
a new energy scale, dominated by spin-interactions () emerges (apparent both
in specific heat and susceptibility) and we identify an effective interaction
, valid across the entire doping range. This is distinct from
Anderson's formula, as we choose here , hence the opposite sign of the
usual Nagaoka-ferromagnetic situation. This expression includes the subtle
effect of weak kinetic AFM - as encountered in the infinitely correlated
situation (). By explicit computation of the Kubo-formulae, we
address the question of practical relevance of the high-frequency expression
for the Hall coefficient . We hope to clarify some open questions
concerning the applicability of the t-J model to real experimental situations
through this study
Pseudo-gap behavior in dynamical properties of high-Tc cuprates
Dynamical properties of 2D antiferromagnets with hole doping are investigated
to see the effects of short range local magnetic order on the temperature
dependence of the dynamical magnetic susceptibility. We show the pseudo-gap
like behavior of the temperature dependence of the NMR relaxation rate. We also
discuss implications of the results in relations to the observed spin gap like
behavior of low-doped copper oxide high- superconductors.Comment: 3 pages, Revtex, with 2 eps figures, to appear in J.Phys.Soc.Jpn.
Vol.67 No.
Application of the finite-temperature Lanczos method for the evaluation of magnetocaloric properties of large magnetic molecules
We discuss the magnetocaloric properties of gadolinium containing magnetic
molecules which potentially could be used for sub-Kelvin cooling. We show that
a degeneracy of a singlet ground state could be advantageous in order to
support adiabatic processes to low temperatures and simultaneously minimize
disturbing dipolar interactions. Since the Hilbert spaces of such spin systems
assume very large dimensions we evaluate the necessary thermodynamic
observables by means of the Finite-Temperature Lanczos Method.Comment: 7 pages, 10 figures, invited for the special issue of EPJB on "New
trends in magnetism and magnetic materials
Anomalous low doping phase of the Hubbard model
We present results of a systematic Quantum-Monte-Carlo study for the
single-band Hubbard model. Thereby we evaluated single-particle spectra (PES &
IPES), two-particle spectra (spin & density correlation functions), and the
dynamical correlation function of suitably defined diagnostic operators, all as
a function of temperature and hole doping. The results allow to identify
different physical regimes. Near half-filling we find an anomalous `Hubbard-I
phase', where the band structure is, up to some minor modifications, consistent
with the Hubbard-I predictions. At lower temperatures, where the spin response
becomes sharp, additional dispersionless `bands' emerge due to the dressing of
electrons/holes with spin excitatons. We present a simple phenomenological fit
which reproduces the band structure of the insulator quantitatively. The Fermi
surface volume in the low doping phase, as derived from the single-particle
spectral function, is not consistent with the Luttinger theorem, but
qualitatively in agreement with the predictions of the Hubbard-I approximation.
The anomalous phase extends up to a hole concentration of 15%, i.e. the
underdoped region in the phase diagram of high-T_c superconductors. We also
investigate the nature of the magnetic ordering transition in the single
particle spectra. We show that the transition to an SDW-like band structure is
not accomplished by the formation of any resolvable `precursor bands', but
rather by a (spectroscopically invisible) band of spin 3/2 quasiparticles. We
discuss implications for the `remnant Fermi surface' in insulating cuprate
compounds and the shadow bands in the doped materials.Comment: RevTex-file, 20 PRB pages, 16 figures included partially as gif. A
full ps-version including ps-figures can be found at
http://theorie.physik.uni-wuerzburg.de/~eder/condmat.ps.gz Hardcopies of
figures (or the entire manuscript) can also be obtained by e-mail request to:
[email protected]
The pseudogap in underdoped high Tc superconductors in the framework of the Boson Fermion model
The question of whether the pseudogap in high cuprates is related to
super conducting precursor effects or to the existence of extrinsic bosonic
massive excitations is investigated on the basis of the Boson-Fermion model.
The characteristic three peak structure of the electronic spectral function and
the temperature dependent Fermi vector derived here are signatures for a two
component scenario which can be tested by ARPES and BIS experiments.Comment: revtex version with 3 eps figures. Revised version to appear in Phys.
Rev. B. 4 c programs adde
Pseudogap Induced Antiferromagnetic Spin Correlation in High-Temperature Superconductors
The pseudogap phenomena observed on cuprate high temperature superconductors
are investigated based on the exact diagonalization method on the finite
cluster t-J model. The results show the presence of the gap-like behavior in
the temperature dependence of various magnetic properties; the NMR relaxation
rate, the neutron scattering intensity and the static susceptibility. The
calculated spin correlation function indicates that the pseudogap behavior
arises associated with the development of the antiferromagnetic spin
correlation with decreasing the temperature. The numerical results are
presented to clarify the model parameter dependence, that covers the realistic
experimental situation. The effect of the next-nearest neighbor hopping is
also studied.Comment: 7 pages, Revtex, with 10 eps figures, to appear in J. Phys. Soc. Jpn.
(Vol. 70, No. 1
Saturation of electrical resistivity
Resistivity saturation is observed in many metallic systems with a large
resistivity, i.e., when the resistivity has reached a critical value, its
further increase with temperature is substantially reduced. This typically
happens when the apparent mean free path is comparable to the interatomic
separations - the Ioffe-Regel condition. Recently, several exceptions to this
rule have been found. Here, we review experimental results and early theories
of resistivity saturation. We then describe more recent theoretical work,
addressing cases both where the Ioffe-Regel condition is satisfied and where it
is violated. In particular we show how the (semiclassical) Ioffe-Regel
condition can be derived quantum-mechanically under certain assumptions about
the system and why these assumptions are violated for high-Tc cuprates and
alkali-doped fullerides.Comment: 16 pages, RevTeX, 15 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/andersen/saturation
Finite temperature mobility of a particle coupled to a fermion environment
We study numerically the finite temperature and frequency mobility of a
particle coupled by a local interaction to a system of spinless fermions in one
dimension. We find that when the model is integrable (particle mass equal to
the mass of fermions) the static mobility diverges. Further, an enhanced
mobility is observed over a finite parameter range away from the integrable
point. We present a novel analysis of the finite temperature static mobility
based on a random matrix theory description of the many-body Hamiltonian.Comment: 11 pages (RevTeX), 5 Postscript files, compressed using uufile
Specific heat and high-temperature series of lattice models: interpolation scheme and examples on quantum spin systems in one and two dimensions
We have developed a new method for evaluating the specific heat of lattice
spin systems. It is based on the knowledge of high-temperature series
expansions, the total entropy of the system and the low-temperature expected
behavior of the specific heat as well as the ground-state energy. By the choice
of an appropriate variable (entropy as a function of energy), a stable
interpolation scheme between low and high temperature is performed. Contrary to
previous methods, the constraint that the total entropy is log(2S+1) for a spin
S on each site is automatically satisfied. We present some applications to
quantum spin models on one- and two- dimensional lattices. Remarkably, in most
cases, a good accuracy is obtained down to zero temperature.Comment: 10 pages (RevTeX 4) including 11 eps figures. To appear in Phys. Rev.