433 research outputs found
Classical Phase Fluctuations in Incommensurate Peierls Chains
In the pseudogap regime of one-dimensional incommensurate Peierls systems,
fluctuations of the phase of the order parameter prohibit the emergence of
long-range order and generate a finite correlation length. For classical phase
fluctuations, we present exact results for the average electronic density of
states, the mean localization length, the electronic specific heat and the spin
susceptibility at low temperatures. Our results for the susceptibility give a
good fit to experimental data.Comment: 4 Revtex pages, 4 figures, submitted to Phys. Rev. Let
Competing orders II: the doped quantum dimer model
We study the phases of doped spin S=1/2 quantum antiferromagnets on the
square lattice, as they evolve from paramagnetic Mott insulators with valence
bond solid (VBS) order at zero doping, to superconductors at moderate doping.
The interplay between density wave/VBS order and superconductivity is
efficiently described by the quantum dimer model, which acts as an effective
theory for the total spin S=0 sector. We extend the dimer model to include
fermionic S=1/2 excitations, and show that its mean-field, static gauge field
saddle points have projective symmetries (PSGs) similar to those of `slave'
particle U(1) and SU(2) gauge theories. We account for the non-perturbative
effects of gauge fluctuations by a duality mapping of the S=0 dimer model. The
dual theory of vortices has a PSG identical to that found in a previous paper
(L. Balents et al., cond-mat/0408329) by a duality analysis of bosons on the
square lattice. The previous theory therefore also describes fluctuations
across superconducting, supersolid and Mott insulating phases of the present
electronic model. Finally, with the aim of describing neutron scattering
experiments, we present a phenomenological model for collective S=1 excitations
and their coupling to superflow and density wave fluctuations.Comment: 22 pages, 10 figures; part I is cond-mat/0408329; (v2) changed title
and added clarification
Putting competing orders in their place near the Mott transition
We describe the localization transition of superfluids on two-dimensional
lattices into commensurate Mott insulators with average particle density p/q
(p, q relatively prime integers) per lattice site. For bosons on the square
lattice, we argue that the superfluid has at least q degenerate species of
vortices which transform under a projective representation of the square
lattice space group (a PSG). The formation of a single vortex condensate
produces the Mott insulator, which is required by the PSG to have density wave
order at wavelengths of q/n lattice sites (n integer) along the principle axes;
such a second-order transition is forbidden in the Landau-Ginzburg-Wilson
framework. We also discuss the superfluid-insulator transition in the direct
boson representation, and find that an interpretation of the quantum
criticality in terms of deconfined fractionalized bosons is only permitted at
special values of q for which a permutative representation of the PSG exists.
We argue (and demonstrate in detail in a companion paper: L. Balents et al.,
cond-mat/0409470) that our results apply essentially unchanged to electronic
systems with short-range pairing, with the PSG determined by the particle
density of Cooper pairs. We also describe the effect of static impurities in
the superfluid: the impurities locally break the degeneracy between the q
vortex species, and this induces density wave order near each vortex. We
suggest that such a theory offers an appealing rationale for the local density
of states modulations observed by Hoffman et al. (cond-mat/0201348) in STM
studies of the vortex lattice of BSCCO, and allows a unified description of the
nucleation of density wave order in zero and finite magnetic fields. We note
signatures of our theory that may be tested by future STM experiments.Comment: 35 pages, 16 figures; (v2) part II is cond-mat/0409470; (v3) added
new appendix and clarifying remarks; (v4) corrected typo
Algorithm for obtaining the gradient expansion of the local density of states and the free energy of a superconductor
We present an efficient algorithm for obtaining the gauge-invariant gradient
expansion of the local density of states and the free energy of a clean
superconductor. Our method is based on a new mapping of the semiclassical
linearized Gorkov equations onto a pseudo-Schroedinger equation for a
three-component wave-function psi(x), where one component is directly related
to the local density of states. Because psi(x) satisfies a linear equation of
motion, successive terms in the gradient expansion can be obtained by simple
linear iteration. Our method works equally well for real and complex order
parameter, and in the presence of arbitrary external fields. We confirm a
recent calculation of the fourth order correction to the free energy by
Kosztin, Kos, Stone and Leggett [Phys. Rev. B 58, 9365 (1998)], who obtained a
discrepancy with an earlier result by Tewordt [Z. Phys. 180, 385 (1964)]. We
also give the fourth order correction to the local density of states, which has
not been published before.Comment: 12 preprint pages, added remark concerning Eilenberger equation,
accepted for publication in Phys. Rev.
Recycling the purpose of old drugs to treat ovarian cancer
The main challenge in ovarian cancer treatment is the management of recurrences. Facing this scenario, therapy selection is based on multiple factors to define the best treatment sequence. Target therapies, such as bevacizumab and polymerase (PARP) inhibitors, improved patient survival. However, despite their achievements, ovarian cancer survival remains poor; these therapeutic options are highly costly and can be associated with potential side effects. Recently, it has been shown that the combination of repurposed, conventional, chemotherapeutic drugs could be an alternative, presenting good patient outcomes with few side effects and low costs for healthcare institutions. The main aim of this review is to strengthen the importance of repurposed drugs as therapeutic alternatives, and to propose an in vitro model to assess the therapeutic value. Herein, we compiled the current knowledge on the most promising non-oncological drugs for ovarian cancer treatment, focusing on statins, metformin, bisphosphonates, ivermectin, itraconazole, and ritonavir. We discuss the primary drug use, anticancer mechanisms, and applicability in ovarian cancer. Finally, we propose the use of these therapies to perform drug efficacy tests in ovarian cancer ex vivo cultures. This personalized testing approach could be crucial to validate the existing evidences supporting the use of repurposed drugs for ovarian cancer treatment.Funding: This manuscript was funded by HOPE: Improving ovarian cancer patients’ survival—donation from an ovarian cancer patient and by the FCT (Fundação para a Ciência e a Tecnologia) project PTDC/MEC-ONC/29503/2017
Ferromagnetic Luttinger Liquids
We study weak itinerant ferromagnetism in one-dimensional Fermi systems using
perturbation theory and bosonization. We find that longitudinal spin
fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the
Fermi velocity. This leads to a large anomalous dimension in the spin-channel
and strong algebraic singularities in the single-particle spectral function and
in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F,
where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure
Exact Results for the Crossover from Gaussian to Non-Gaussian Order Parameter Fluctuations in Quasi One-Dimensional Electronic Systems
The physics of quasi one-dimensional Peierls systems is dominated by order
parameter fluctuations. We present an algorithm which allows for the first time
to exactly calculate physical properties of the electrons gas coupled to
classical order parameter fluctuations. The whole range from the Gaussian
regime dominated by amplitude fluctuations to the non-Gaussian regime dominated
by phase fluctuations is accessible. Our results provide insight into the
'pseudogap' phenomenon occurring in underdoped high-temperature
superconductors, quasi one-dimensional organic conductors and liquid metals.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Some remarks about pseudo gap behavior of nearly antiferromagnetic metals
In the antiferromagnetically ordered phase of a metal, gaps open on parts of
the Fermi surface if the Fermi volume is sufficiently large. We discuss simple
qualitative and heuristic arguments under what conditions precursor effects,
i.e. pseudo gaps, are expected in the paramagnetic phase of a metal close to an
antiferromagnetic quantum phase transition. At least for weak interactions, we
do not expect the formation of pseudo gaps in a three dimensional material.
According to our arguments, the upper critical dimension d_c for the formation
of pseudo gaps is d_c=2. However, at the present stage we cannot rule out a
higher upper critical dimension, 2 < d_c <= 3. We also discuss briefly the role
of statistical interactions in pseudo gap phases.Comment: 6 pages, accepted in PRB, relevant references added, several small
change
Subgap tunneling through channels of polarons and bipolarons in chain conductors
We suggest a theory of internal coherent tunneling in the pseudogap region
where the applied voltage is below the free electron gap. We consider quasi 1D
systems where the gap is originated by a lattice dimerization like in
polyacethylene, as well as low symmetry 1D semiconductors. Results may be
applied to several types of conjugated polymers, to semiconducting nanotubes
and to quantum wires of semiconductors. The approach may be generalized to
tunneling in strongly correlated systems showing the pseudogap effect, like the
family of High Tc materials in the undoped limit. We demonstrate the evolution
of tunneling current-voltage characteristics from smearing the free electron
gap down to threshold for tunneling of polarons and further down to the region
of bi-electronic tunneling via bipolarons or kink pairs.Comment: 14 pages, 8 postscript figure
The zero-dimensional O(N) vector model as a benchmark for perturbation theory, the large-N expansion and the functional renormalization group
We consider the zero-dimensional O(N) vector model as a simple example to
calculate n-point correlation functions using perturbation theory, the large-N
expansion, and the functional renormalization group (FRG). Comparing our
findings with exact results, we show that perturbation theory breaks down for
moderate interactions for all N, as one should expect. While the
interaction-induced shift of the free energy and the self-energy are well
described by the large-N expansion even for small N, this is not the case for
higher-order correlation functions. However, using the FRG in its one-particle
irreducible formalism, we see that very few running couplings suffice to get
accurate results for arbitrary N in the strong coupling regime, outperforming
the large-N expansion for small N. We further remark on how the derivative
expansion, a well-known approximation strategy for the FRG, reduces to an exact
method for the zero-dimensional O(N) vector model.Comment: 13 pages, 13 figure
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