3,483 research outputs found
Landau mapping and Fermi liquid parameters of the 2D t-J model
We study the momentum distribution function n(k) in the 2D t-J model on small
clusters by exact diagonalization. We show that n(k) can be decomposed
systematically into two components with Bosonic and Fermionic doping
dependence. The Bosonic component originates from the incoherent motion of
holes and has no significance for the low energy physics. For the Fermionic
component we exlicitely perform the one-to-one Landau mapping between the low
lying eigenstates of the t-J model clusters and those of an equivalent system
of spin-1/2 quasiparticles. This mapping allows to extract the quasiparticle
dispersion, statistics, and Landau parameters. The results show conclusively
that the 2D t-J model for small doping is a Fermi liquid with a `small' Fermi
surface and a moderately strong attractive interaction between the
quasiparticles.Comment: Revtex file, 5 pages with 5 embedded eps-files, hardcopies of figures
(or the entire manuscript) can be obtained by e-mail request to:
[email protected]
First Experiences Integrating PC Distributed I/O Into Argonne's ATLAS Control System
First Experiences Integrating PC Distributed I/O Into Argonne's ATLAS Control
System The roots of ATLAS (Argonne Tandem-Linac Accelerator System) date back
to the early 1960s. Located at the Argonne National Laboratory, the accelerator
has been designated a National User Facility, which focuses primarily on
heavy-ion nuclear physics. Like the accelerator it services, the control system
has been in a constant state of evolution. The present real-time portion of the
control system is based on the commercial product Vsystem [1]. While Vsystem
has always been capable of distributed I/O processing, the latest offering of
this product provides for the use of relatively inexpensive PC hardware and
software. This paper reviews the status of the ATLAS control system, and
describes first experiences with PC distributed I/O.Comment: ICALEPCS 2001 Conference, PSN WEAP027, 3 pages, 1 figur
Dynamics of an SO(5) symmetric ladder model
We discuss properties of an exactly SO(5) symmetric ladder model. In the
strong coupling limit we demonstrate how the SO(3)-symmetric description of
spin ladders in terms of bond Bosons can be upgraded to an SO(5)-symmetric
bond-Boson model, which provides a particularly simple example for the concept
of SO(5) symmetry. Based on this representation we show that antiferro-
magnetism on one hand and superconductivity on the other hand can be understood
as condensation of either magnetic or charged Bosons into an RVB vacuum. We
identify exact eigenstates of a finite cluster with general multiplets of the
SO(5) group, and present numerical results for the single particle spectra and
spin/charge correlation functions of the SO(5)-symmetric model and identify
`fingerprints' of SO(5) symmetry in these. In particluar we show that SO(5)
symmetry implies a `generalized rigid band behavior' of the photoemission
spectrum, i.e. spectra for the doped case are rigorously identical to spectra
for spin-polarized states at half-filling. We discuss the problem of adiabatic
continuity between the SO(5) symmetric ladder and the actual t-J ladder and
demonstrate the feasibility of a `Landau mapping' between the two models.Comment: Revtex-file, 16 pages with 15 eps-figures. Hardcopies of Figures (or
the entire manuscript) obtainable by e-mail request to
[email protected]
Excitation spectrum of the homogeneous spin liquid
We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivityComment: RevTex-file, 16 PRB pages with 13 embedded eps figures. Hardcopies of
figures (or the entire manuscript) can be obtained by e-mail request to:
[email protected]
Spin bags in the doped t-J model
We present a nonperturbative method for deriving a quasiparticle description
of the low-energy excitations in the t-J model for strongly correlated
electrons. Using the exact diagonalization technique we evaluated exactly the
spectral functions of composite operators which describe an electron or hole
dressed by antiferromagnetic spin fluctuations as expected in the string or
spin bag picture. For hole doping up to , use of the composite operators
leads to a drastic simplification of the single particle spectral function: at
half-filling it takes free-particle form, for the doped case it resembles a
system of weakly interacting Fermions corresponding to the doped holes. We
conclude that for all doping levels under study, the elementary electronic
excitations next to the Fermi level are adequately described by the
antiferromagnetic spin fluctuation picture and show that the dressing of the
holes leads to formation of a bound state with d(x^2-y^2) symmetry.Comment: Remarks: Revtex file + 4 figures attached as compressed postscript
files Figures can also be obtained by ordinary mail on reques
Spectral density for a hole in an antiferromagnetic stripe phase
Using variational trial wave function based on the string picture we study
the motion of a single mobile hole in the stripe phase of the doped
antiferromagnet. The holes within the stripes are taken to be static, the
undoped antiferromagnetic domains in between the hole stripes are assumed to
have alternating staggered magnetization, as is suggested by neutron scattering
experiments. The system is described by the t-t'-t''-J model with realistic
parameters and we compute the single particle spectral density.Comment: RevTex-file, 9 PRB pages with 15 .eps and .gif files. To appear in
PRB. Hardcopies of figures (or the entire manuscript) can be obtained by
e-mail request to: [email protected]
Validity of the rigid band picture for the t-J model
We present an exact diagonalization study of the doping dependence of the
single particle Green's function in 16, 18 and 20 site clusters of t-J model.
We find evidence for rigid-band behaviour starting from the half-filled case:
upon doping, the topmost states of the quasiparticle band observed in the
photoemisson spectrum at half-filling cross the chemical potential and reappear
as the lowermost states of the inverse photoemission spectrum. Features in the
inverse photoemission spectra which are inconsistent with rigid-band behaviour
are shown to originate from the nontrivial point group symmetry of the ground
state with two holes, which enforces different selection rules than at
half-filling. Deviations from rigid band behaviour which lead to the formation
of the `large Fermi surface' in the momentum distribution occur only at
energies far from the chemical potential. A Luttinger Fermi surface and a
nearest neighbor hopping band do not exist.Comment: Remarks: Revtex file + 7 figures attached as compressed postscript
files Figures can also be obtained by ordinary mail on reques
Quasiparticle dispersion of the t-J and Hubbard models
The spectral weight of the two dimensional and Hubbard models has been calculated using exact diagonalization and
quantum Monte Carlo techniques, at several densities . The photoemission region contains two
dominant distinct features, namely a low-energy quasiparticle peak with
bandwidth of order J, and a broad valence band peak at energies of order t.
This behavior away from half-filling, as long as the
antiferromagnetic (AF) correlations are robust. The results give support to
theories of the copper oxide materials based on the behavior of holes in
antiferromagnets, and it also provides theoretical guidance for the
interpretation of experimental photoemission data for the cuprates.Comment: (minor changes) RevTeX, 4 figures available on reques
Spin currents in diluted magnetic semiconductors (extended version)
Spin currents resulting in the zero-bias spin separation have been observed
in unbiased diluted magnetic semiconductor structures (Cd,Mn)Te/(Cd,Mg)Te. The
pure spin current generated due to the electron gas heating by terahertz
radiation is converted into a net electric current by application of an
external magnetic field. We demonstrate that polarization of the magnetic ion
system enhances drastically the conversion due to the spin-dependent scattering
by localized Mn(2+) ions and the giant Zeeman splitting.Comment: 6 pages, 4 figure
Interrelation of Superconducting and Antiferromagnetic Gaps in High-Tc Compounds: a Test Case for a Microscopic Theory
Recent angle resolved photoemission (ARPES) data, which found evidence for a
d-wave-like modulation of the antiferromagnetic gap, suggest an intimate
interrelation between the antiferromagnetic insulator and the superconductor
with its d-wave gap. This poses a new challenge to microscopic descriptions,
which should account for this correlation between, at first sight, very
different states of matter. Here, we propose a microscopic mechanism which
provides a definite correlation between these two different gap structures: it
is shown that a projected SO(5) theory, which aims at unifying
antiferromagnetism and d-wave superconductivity via a common symmetry principle
while explicitly taking the Mott-Hubbard gap into account, correctly describes
the observed gap characteristics. Specifically, it accounts for both the
dispersion and the order of magnitude difference between the antiferromagnetic
gap modulation and the superconducting gap.Comment: 8 pages, 5 figure
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