125 research outputs found
The Non-Abelian Density Matrix Renormalization Group Algorithm
We describe here the extension of the density matrix renormalization group
algorithm to the case where Hamiltonian has a non-Abelian global symmetry
group. The block states transform as irreducible representations of the
non-Abelian group. Since the representations are multi-dimensional, a single
block state in the new representation corresponds to multiple states of the
original density matrix renormalization group basis. We demonstrate the
usefulness of the construction via the one-dimensional Hubbard model as the
symmetry group is enlarged from , up to .Comment: Revised version discusses the Hubbard model with SO(4) symmetr
Ground-state Properties of a Supersolid in RPA
We investigate the newly discovered supersolid phase by solving in
random-phase approximation the anisotropic Heisenberg model of the hard-core
boson He lattice at zero temperature. We include nearest and next-nearest
neighbor interactions and calculate exactly all pair correlation functions in a
cumulant decoupling scheme. We demonstrate the importance of vacancies and
interstitials in the formation of the supersolid phase. The supersolid phase is
characterised by strong quantum fluctuations which are taken into account
rigorously. Furthermore we confirm that the superfluid to supersolid transition
is triggered by a collapsing roton minimum however is stable against
spontaneously induced superflow, i.e. vortex creation.Comment: 10 pages, 8 figure
Issues for countries considering introducing the "fourth hurdle": The case of Hungary
Objectives: This study outlines the needs and current development of the "fourth hurdle" (i.e., requirement of effectiveness and cost-effectiveness data for drug coverage policy decisions) in Hungary, describes the legal background and seeks to address some of the most important questions in this field. Methods: The study draws on international experiences and discusses five issues that a given jurisdiction needs to consider before introducing the "fourth hurdle" for pharmaceuticals. Results: The "fourth hurdle" is very relevant in Hungary because many existing drugs are unevaluated and many new, expensive drugs are becoming available. On the other hand, the existing resources for health technology assessment, including economic evaluation, are quite limited. All the five issues are relevant in the Hungarian setting and were helpful in determining exactly how the "fourth hurdle" should be applied. Conclusions: The most important issue seems to be that the implementation of the "fourth hurdle" needs to be achieved in a way consistent with the limited resources for HTA in Hungary. Specifically this means that, in setting priorities for drugs to evaluate, additional criteria need to be applied. In particular, priority should be given to assessing drugs that have been evaluated in other countries, because this affords the opportunity to adapt existing studies or models to the Hungarian situation
Competing interactions of spin and lattice in the Kondo lattice model
The magnetic properties of a system of coexisting localized spins and
conduction electrons are investigated within an extended version of the one
dimensional Kondo lattice model in which effects stemming from the
electron-lattice and on-site Coulomb interactions are explicitly included.
After bosonizing the conduction electrons, is it observed that intrinsic
inhomogeneities with the statistical scaling properties of a Griffiths phase
appear, and determine the spin structure of the localized impurities. The
appearance of the inhomogeneities is enhanced by appropriate phonons and acts
destructively on the spin ordering. The inhomogeneities appear on well defined
length scales, can be compared to the formation of intrinsic mesoscopic
metastable patterns which are found in two-fluid systems.Comment: 9 pages, to appear in Jour. Superconductivit
Phase Diagram of the 1D Kondo Lattice Model
We determine the boundary of the fully polarized ferromagnetic ground state
in the one dimensional Kondo lattice model at partial conduction electron band
filling by using a newly developed infinite size DMRG method which conserves
the total spin quantum number. The obtained paramagnetic to ferromagnetic phase
boundary is below for the whole range of band filling. By this
we solve the controversy in the phase diagram over the extent of the
ferromagnetic region close to half filling.Comment: 6 pages, 4 EPS figures. Presented at MOS9
Pairing mechanism in Fe pnictide superconductors
By applying an exact unitary transformation to a two-band hamiltonian which
also includes the effects due to large pnictogen polarizabilities, we show that
an attractive spin-mediated Hubbard term appears in the ,
nearest-neighbour channel. This pairing mechanism implies a singlet
superconducting order parameter in iron pnictides.Comment: 4 pages, 3 figure
Incomplete Protection of the Surface Weyl Cones of the Kondo Insulator SmB: Spin Exciton Scattering
The compound SmB is a Kondo Insulator, where the lowest-energy bulk
electronic excitations are spin excitons. It also has surface states that are
subjected to strong spin-orbit coupling. It has been suggested that SmB is
also a topological insulator. Here we show that, despite the absence of
time-reversal symmetry breaking and the presence of strong spin-orbit coupling,
the chiral spin texture of the Weyl cone is not completely protected. In
particular, we show that the spin-exciton mediated scattering produces features
in the surface electronic spectrum at energies separated from the surface Fermi
energy by the spin-exciton energy. Despite the features being far removed from
the surface Fermi energy, they are extremely temperature dependent. The
temperature variation occurs over a characteristic scale determined by the
dispersion of the spin exciton. The structures may be observed by electron
spectroscopy at low temperatures.Comment: 7 pages, 5 figure
DMRG and the Two Dimensional t-J Model
We describe in detail the application of the recent non-Abelian Density
Matrix Renormalization Group (DMRG) algorithm to the two dimensional t-J model.
This extension of the DMRG algorithm allows us to keep the equivalent of twice
as many basis states as the conventional DMRG algorithm for the same amount of
computational effort, which permits a deeper understanding of the nature of the
ground state.Comment: 16 pages, 3 figures. Contributed to the 2nd International Summer
School on Strongly Correlated Systems, Debrecen, Hungary, Sept. 200
Magnetism in the dilute Kondo lattice model
The one dimensional dilute Kondo lattice model is investigated by means of
bosonization for different dilution patterns of the array of impurity spins.
The physical picture is very different if a commensurate or incommensurate
doping of the impurity spins is considered. For the commensurate case, the
obtained phase diagram is verified using a non-Abelian density-matrix
renormalization-group algorithm. The paramagnetic phase widens at the expense
of the ferromagnetic phase as the -spins are diluted. For the incommensurate
case, antiferromagnetism is found at low doping, which distinguishes the dilute
Kondo lattice model from the standard Kondo lattice model.Comment: 11 pages, 2 figure
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