248 research outputs found
Phase separation and electron pairing in repulsive Hubbard clusters
Exact thermal studies of small (4-site, 5-site and 8-site)
Hubbard clusters with local electron repulsion yield intriguing insight into
phase separation, charge-spin separation, pseudogaps, condensation, in
particular, pairing fluctuations away from half filling (near optimal doping).
These exact calculations, carried out in canonical (i.e. for fixed electron
number N) and grand canonical (i.e. fixed chemical potential ) ensembles,
monitoring variations in temperature T and magnetic field h, show rich phase
diagrams in a T- space consisting of pairing fluctuations and signatures
of condensation. These electron pairing instabilities are seen when the onsite
Coulomb interaction U is smaller than a critical value U(T) and they point
to a possible electron pairing mechanism. The specific heat, magnetization,
charge pairing and spin pairing provide strong support for the existence of
competing (paired and unpaired) phases near optimal doping in these clusters as
observed in recent experiments in doped LaSrCuO high T
superconductors.Comment: 5 pages, 5 figure
Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model
We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.Comment: 30 pages, 24 figure
Magnetic impurities coupled to quantum antiferromagnets in one dimension
Magnetic impurities coupled antiferromagnetically to a one-dimensional
Heisenberg model are studied by numerical diagonalization of chains of finite
clusters. By calculating the binding energy and the correlation function, it is
shown that a local singlet develops around each impurity. This holds true for
systems with a single impurity, with two impurities, and for impurities forming
a lattice. The local character of the singlet is found to be little affected by
the presence of other impurity spins. A small effective interaction is found
between a pair of impurity spins, which oscillates depending on impurity
distances. For impurity lattices, the energy spectrum shows a gap which is
found to be much smaller than the binding energy per impurity if the coupling
constants are small. For larger coupling constants, it increases to the same
order of magnitude as the binding energy, indicating that a local singlet is
broken to create excited states. Impurity lattices with ferromagnetic couplings
are also studied and their connection to the Haldane problem is discussed.Comment: 25 pages, plain TeX, 17 figures available on request, to be publised
in Phys. Rev.
Optical Spectra in the Ferromagnetic States near the Charge Ordering
The optical conductivity is studied numerically for the ferromagnetic
metallic state close to the charge ordering observed in perovskite manganites.Comment: 11 pages, Latex, 6 ps figure
RKKY interaction and Kondo screening cloud for correlated electrons
The RKKY law and the Kondo screening cloud around a magnetic impurity are
investigated for correlated electrons in 1D (Luttinger liquid). We find slow
algebraic distance dependences, with a crossover between both types of
behavior. Monte Carlo simulations have been developed to study this crossover.
In the strong coupling regime, the Knight shift is shown to increase with
distance due to correlations.Comment: 5 pages REVTeX, incl two figures, to appear in Phys.Rev.
Long-range correlations in the mechanics of small DNA circles under topological stress revealed by multi-scale simulation
It is well established that gene regulation can be achieved through activator and repressor proteins that bind to DNA and switch particular genes on or off, and that complex metabolic networks deter- mine the levels of transcription of a given gene at a given time. Using three complementary computa- tional techniques to study the sequence-dependence of DNA denaturation within DNA minicircles, we have observed that whenever the ends of the DNA are con- strained, information can be transferred over long distances directly by the transmission of mechanical stress through the DNA itself, without any require- ment for external signalling factors. Our models com- bine atomistic molecular dynamics (MD) with coarse- grained simulations and statistical mechanical calcu- lations to span three distinct spatial resolutions and timescale regimes. While they give a consensus view of the non-locality of sequence-dependent denatura- tion in highly bent and supercoiled DNA loops, each also reveals a unique aspect of long-range informa- tional transfer that occurs as a result of restraining the DNA within the closed loop of the minicircles
Thermodynamics of doped Kondo insulator in one dimension: Finite Temperature DMRG Study
The finite-temperature density-matrix renormalization-group method is applied
to the one-dimensional Kondo lattice model near half filling to study its
thermodynamics. The spin and charge susceptibilities and entropy are calculated
down to T=0.03t. We find two crossover temperatures near half filling. The
higher crossover temperature continuously connects to the spin gap at half
filling, and the susceptibilities are suppressed around this temperature. At
low temperatures, the susceptibilities increase again with decreasing
temperature when doping is finite. We confirm that they finally approach to the
values obtained in the Tomonaga-Luttinger (TL) liquid ground state for several
parameters. The crossover temperature to the TL liquid is a new energy scale
determined by gapless excitations of the TL liquid. The transition from the
metallic phase to the insulating phase is accompanied by the vanishing of the
lower crossover temperature.Comment: 4 pages, 7 Postscript figures, REVTe
- …