579 research outputs found
Exact Drude weight for the one-dimensional Hubbard model at finite temperatures
The Drude weight for the one-dimensional Hubbard model is investigated at
finite temperatures by using the Bethe ansatz solution. Evaluating finite-size
corrections to the thermodynamic Bethe ansatz equations, we obtain the formula
for the Drude weight as the response of the system to an external gauge
potential. We perform low-temperature expansions of the Drude weight in the
case of half-filling as well as away from half-filling, which clearly
distinguish the Mott-insulating state from the metallic state.Comment: 9 pages, RevTex, To appear in J. Phys.
Bioprocess considerations for T cell therapy: Investigating the impact of agitation, dissolved oxygen and pH on T cell expansion and differentiation
Adoptive T‐cell therapy (ACT) has emerged as a promising new way to treat systemic cancers such as acute lymphoblastic leukemia. However, the robustness and reproducibility of the manufacturing process remains a challenge. Here, a single‐use 24‐well microbioreactor (micro‐Matrix) was assessed for its use as a high‐throughput screening tool to investigate the effect and the interaction of different shaking speeds, dissolved oxygen (DO), and pH levels on the growth and differentiation of primary T cells in a perfusion‐mimic process. The full factorial design allowed for the generation of predictive models, which were used to find optimal culture conditions. Agitation was shown to play a fundamental role in the proliferation of T cells. A shaking speed of 200 rpm drastically improved the final viable cell concentration (VCC), while the viability was maintained above 90% throughout the cultivation. VCCs reached a maximum of 9.22 × 106 cells/ml. The distribution of CD8+ central memory T cells (TCM), was found to be largely unaffected by the shaking speed. A clear interaction between pH and DO (p < .001) was established for the cell growth and the optimal culture conditions were identified for a combination of 200 rpm, 25% DO, and pH of 7.4. The combination of microbioreactor technology and Design of Experiment methodology provides a powerful tool to rapidly gain an understanding of the design space of the T‐cell manufacturing process
Resonating Valence Bond Theory of Superconductivity for Dopant Carriers: Application to the Cobaltates
Within the -- model Hamiltonian we present a RVB mean field theory
directly in terms of dopant particles. We apply this theory to
and show that the
resulting phase diagram versus doping is in qualitative agreement with
the experimental results
Tunneling in the topological mechanism of superconductivity
We compute the two-particle matrix element and Josephson tunneling amplitude
in a two-dimensional model of topological superconductivity which captures the
physics of the doped Mott insulator. The hydrodynamics of topological
electronic liquid consists of the compressible charge sector and the
incompressible chiral topological spin liquid. We show that ground states
differing by an odd number of particles are orthogonal and insertion of two
extra electrons is followed by the emission of soft modes of the transversal
spin current. The orthogonality catastrophe makes the physics of
superconductivity drastically different from the BCS-theory but similar to the
physics of one-dimensional electronic liquids. The wave function of a pair is
dressed by soft modes. As a result the two particle matrix element forms a
complex d-wave representation (i.e., changes sign under degree
rotation), although the gap in the electronic spectrum has no nodes. In
contrast to the BCS-theory the tunneling amplitude has an asymmetric broad peak
(much bigger than the gap) around the Fermi surface. We develop an operator
algebra, that allows one to compute other correlation functions.Comment: 18 pages, 2 eps figures, revtex, psfig, significant changes have been
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Multichannel Kondo Screening in a One-Dimensional Correlated Electron System
We present the exact Bethe Ansatz solution of a multichannel model of one-
dimensional correlated electrons coupled antiferromagnetically to a magnetic
impurity of arbitrary spin S. The solution reveals that interactions in the
bulk make the magnetic impurity drive both spin and charge fluctuations,
producing a mixed valence at the impurity site, with an associated effective
spin S_eff > S in the presence of a magnetic field. The screening of the
impurity spin is controlled by its size independently of the number of
channels, in contrast to the multichannel Kondo effect for free electrons.Comment: 5 pages Revtex. Final revised version to appear in Europhys. Let
Lattice path integral approach to the one-dimensional Kondo model
An integrable Anderson-like impurity model in a correlated host is derived
from a gl(21)-symmetric transfer matrix by means of the
Quantum-Inverse-Scattering-Method (QISM). Using the Quantum Transfer Matrix
technique, free energy contributions of both the bulk and the impurity are
calculated exactly. As a special case, the limit of a localized moment in a
free bulk (Kondo limit) is performed in the Hamiltonian and in the free energy.
In this case, high- and low-temperature scales are calculated with high
accuracy.Comment: 26 pages, 9 figure
Commensurate-Incommensurate Phase Transitions for Multichain Quantum Spin Models: Exact Results
The behavior in an external magnetic field is studied for a wide class of
multichain quantum spin models. It is shown that the magnetic field together
with the interchain couplings cause commensurate-incommensurate phase
transitions between the gapless phases in the ground state. The conformal limit
of these models is studied and it is shown that the low-lying excitations for
the incommensurate phases are not independent. A scenario for the transition
from one to two space dimensions for the integrable multichain models is
proposed. The similarities in the external field behavior for the quantum
multichain spin models and a wide class of quantum field theories are
discussed. The exponents for the gaps caused by relevant perturbations of the
models are calculated.Comment: 23 pages, LaTeX, typos correcte
Bosonization for disordered and chaotic systems
Using a supersymmetry formalism, we reduce exactly the problem of electron
motion in an external potential to a new supermatrix model valid at all
distances. All approximate nonlinear sigma models obtained previously for
disordered systems can be derived from our exact model using a coarse-graining
procedure. As an example, we consider a model for a smooth disorder and
demonstrate that using our approach does not lead to a 'mode-locking' problem.
As a new application, we consider scattering on strong impurities for which the
Born approximation cannot be used. Our method provides a new calculational
scheme for disordered and chaotic systems.Comment: 4 pages, no figure, REVTeX4; title changed, revision for publicatio
Semiclassical Solution of One Dimensional Model of Kondo Insulator
The model of Kondo chain with -fold degenerate band of conduction
electrons of spin 1/2 interacting with localized spins is studied for the
case when the electronic band is half filled. It is shown that the spectrum of
spin excitations in the continuous limit is described by the O(3) nonlinear
sigma model with the topological term with . For a case
(even) the system is an insulator and single electron excitations
at low energies are massive spin polarons. Otherwise the density of states has
a pseudogap and vanishes only at the Fermi level. The relevance of this picture
to higher dimensional Kondo insulators is discussed.Comment: 10 pages, LaTe
Spinons in more than one dimension: Resonance Valence Bond state stabilized by frustration
For two spatially anisotropic, SU(2)-invariant models of frustrated magnets
in arbitrary space dimension we present a non-perturbative proof of the
existence of neutral spin-1/2 excitations (spinons). In one model the
frustration is static and based on fine tuning of the coupling constants,
whereas in the other it is dynamic and does not require adjusting of the model
parameters. For both models we derive a low-energy effective action which does
not contain any constraints. Though our models admit the standard gauge theory
treatment, we follow an alternative approach based on Abelian and non-Abelian
bosonization. We prove the existence of propagating spin-1/2 excitations
(spinons) and consider in detail certain exactly solvable limits. A qualitative
discussion of the most general case is also presented.Comment: 42 pages, 7 figures, replaced with revised versio
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