A note on the path integral representation for Majorana fermions

Majorana fermions are currently of huge interest in the context of nanoscience and condensed matter physics. Different to usual fermions, Majorana fermions have the property that the particle is its own anti-particle thus, they must be described by real fields. Mathematically, this property makes nontrivial the quantization of the problem due, for instance, to the absence of a Wick-like theorem. In view of the present interest on the subject, it is important to develop different theoretical approaches in order to study problems where Majorana fermions are involved. In this note we show that Majorana fermions can be studied in the context of field theories for constrained systems. Using the Faddeev-Jackiw formalism for quantum field theories with constraints, we derived the path integral representation for Majorana fermions. In order to show the validity of the path integral we apply it to an exactly solvable problem. This application also shows that it is rather simple to perform systematic calculations on the basis of the present framework.Comment: 7 pages, to be published in Journal of Physics A: Mathematical and Theoretica

Competition between spin-induced charge instabilities in underdoped cuprates

We study the static charge correlation function in a one-band model on a square lattice. The Hamiltonian consists of effective hoppings of the electrons between the lattice sites and the Heisenberg Hamiltonian. Approximating the irreducible charge correlation function by a single bubble yields the ladder approximation for the charge correlation function. In this approximation, one finds, in general, three charge instabilities - two of them are due to nesting, the third one is the flux phase instability. Since these instabilities cannot explain the experiments in hole-doped cuprates, we have included in the irreducible charge correlation function also Aslamasov-Larkin (AL) diagrams where charge fluctuations interact with products of spin fluctuations. We then find at high temperatures a nematic or d-wave Pomeranchuk instability with a very small momentum. Its transition temperature decreases roughly linearly with doping in the underdoped region and vanishes near optimal doping. Decreasing the temperature further, a secondary axial charge-density wave (CDW) instability appears with mainly d-wave symmetry and a wave vector somewhat larger than the distance between nearest-neighbor hot spots. At still lower temperatures, the diagonal flux phase instability emerges. A closer look shows that the AL diagrams enhance mainly axial and not diagonal charge fluctuations in our one-band model. This is the main reason why axial and not diagonal instabilities are the leading ones in agreement with experiment. The two instabilities due to nesting vanish already at very low temperatures and do not play any major role in the phase diagram. Remarkable is that the nematic and the axial CDW instabilities show a large reentrant behavior.Fil: Zeyher, Roland. Max Planck Institute For Solid State Research; AlemaniaFil: Greco, Andres Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentin

Strong particle-hole asymmetry of charge instabilities in doped Mott insulators

We study possible charge instabilities in doped Mott insulators by employing the two-dimensional t-J model with a positive value of the next nearest-neighbor hopping integral t' on a square lattice, which is applicable to electron-doped cuprates. Although the d-wave charge density wave (flux phase) and d-wave Pomeranchuk instability (nematic order) are dominant instabilities for a negative t' that corresponds to hole-doped cuprates, we find that those instabilities are strongly suppressed and become relevant only rather close to half filling. Instead, various types of bond orders with modulation vectors close to (pi,pi) are dominant in a moderate doping region. Phase separation is also enhanced, but it can be suppressed substantially by the nearest-neighbor Coulomb repulsion without affecting the aforementioned charge instabilities.Comment: 16 pages, 7 figure

Pseudogap in cuprates driven by d-wave flux-phase order proximity effects: A theoretical analysis from Raman and ARPES experiments

One of the puzzling characteristics of the pseudogap phase of high-$T_c$ cuprates is the nodal-antinodal dichotomy. While the nodal quasiparticles have a Fermi liquid behaviour, the antinodal ones show non-Fermi liquid features and an associated pseudogap. Angle-resolved photoemission spectroscopy and electronic Raman scattering are two valuable tools which have shown universal features which are rather material-independent, and presumably intrinsic to the pseudogap phase. The doping and temperature dependence of the Fermi arcs and the pseudogap observed by photoemission near the antinode correlates with the non-Fermi liquid behaviour observed by Raman for the B$_{1g}$ mode. In contrast, and similar to the nodal quasiparticles detected by photoemission, the Raman B$_{2g}$ mode shows Fermi liquid features. We show that these two experiments can be analysed, in the context of the $t$-$J$ model, by self-energy effects in the proximity to a d-wave flux-phase order instability. This approach supports a crossover origin for the pseudogap, and a scenario of two competing phases. The B$_{2g}$ mode shows, in an underdoped case, a depletion at intermediate energy which has attracted a renewed interest. We study this depletion and discuss its origin and relation with the pseudogap.Comment: 12 pages, 6 figure

Close inspection of plasmon excitations in cuprate superconductors

Recently resonant inelastic x-ray scattering experiments reported fine details of the charge excitations around the in-plane momentum q∥=(0,0) for various doping rates in electron-doped cuprates La2−xCexCuO4. We find that those new experimental data are well captured by acousticlike plasmon excitations in a microscopic study of the layered t−J model with the long-range Coulomb interaction. The acousticlike plasmon is not a usual plasmon typical to the two-dimensional system, but has a small gap proportional to the interlayer hopping tz.Fil: Greco, Andres Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Max-Planck-Institut für Festkörperforschung; AlemaniaFil: Yamase, Hiroyuki. National Institute for Materials Science; Japón. Hokkaido University; JapónFil: Bejas, Matias Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentin

Superconductivity with and without glue and the role of the double-occupancy forbidding constraint in the t-J-V model

The occurrence of retarded (with glue) and unretarded (without glue) pairing is thoroughly discussed in cuprates. We analyze some aspects of this problem in the context of the t − J − V model in a large- N approximation. When 1 / N renormalizations are neglected the mean-field result is recovered, where the unretarded d -wave superconducting pairing triggered by the spin-exchange interaction J is obtained. However, the presence of a nonnegligible nearest-neighbors Coulomb interaction V ( q ) kills superconductivity. If the non-double-occupancy constraint and its fluctuations are considered, the situation changes drastically. In this case, V ( q ) is screened making d -wave superconductivity very robust. In addition, we show that the early proposal for the presence of an unretarded pairing contribution triggered by the spin-exchange interaction J can be discussed in this context.Fil: Zinni, Luciano Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Bejas, Matias Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; ArgentinaFil: Greco, Andres Francisco. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentin

Low-energy plasmon excitations in infinite-layer nickelates

The discovery of superconductivity in infinite-layer nickelates is presently an important topic in condensed-matter physics, and potential similarities to and differences from cuprates are under intense debate. We determine general features of the charge excitation spectrum in nickelates from two opposite viewpoints: (i) Nickelates are regarded as strongly correlated electron systems like cuprate superconductors and thus can be described by the t-J model, and (ii) electron correlation effects are not as strong as in cuprates, and thus, random-phase approximation (RPA) calculations may capture the essential physics. We find that in both cases, plasmon excitations are realized around the momentum transfer q=(0,0,qz), although they tend to be damped more strongly in the RPA. In particular, this damping is enhanced by the relatively large interlayer hopping expected in nickelates. Besides reproducing the optical plasmon at q=(0,0,0) observed in Nd0.8Sr0.2NiO2, we obtain low-energy plasmons with gaps of ∼360 and ∼560 meV at q=(0,0,qz) for finite qz in cases (i) and (ii), respectively. The present work offers a possible theoretical hint to answer whether nickelates are cupratelike or not and contributes to the general understanding of the charge dynamics in nickelates.Fil: Zinni, Luciano Sebastian. Universidad Nacional de Rosario. Facultad de Ciencias Exactas, Ingeniería y Agrimensura; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario; ArgentinaFil: Bejas, Matias Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Yamase, Hiroyuki. Tsukuba University. National Institute For Materials Science; JapónFil: Greco, Andres Francisco. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentin

Extracellular matrix composition modulates the responsiveness of differentiated and stem pancreatic cancer cells to lipophilic derivate of gemcitabine

Background: Pancreatic ductal adenocarcinoma (PDAC) is a highly lethal disease. Gemcitabine (GEM) is used as the gold standard drug in PDAC treatment. However, due to its poor efficacy, it remains urgent to identify novel strategies to overcome resistance issues. In this context, an intense stroma reaction and the presence of cancer stem cells (CSCs) have been shown to influence PDAC aggressiveness, metastatic potential, and chemoresistance. Methods: We used three-dimensional (3D) organotypic cultures grown on an extracellular matrix composed of Matrigel or collagen I to test the effect of the new potential therapeutic prodrug 4-(N)-stearoyl-GEM, called C18GEM. We analyzed C18GEM cytotoxic activity, intracellular uptake, apoptosis, necrosis, and autophagy induction in both Panc1 cell line (P) and their derived CSCs. Results: PDAC CSCs show higher sensitivity to C18GEM treatment when cultured in both two-dimensional (2D) and 3D conditions, especially on collagen I, in comparison to GEM. The intracellular uptake mechanisms of C18GEM are mainly due to membrane nucleoside transporters' expression and fatty acid translocase CD36 in Panc1 P cells and to clathrin-mediated endocytosis and CD36 in Panc1 CSCs. Furthermore, C18GEM induces an increase in cell death compared to GEM in both cell lines grown on 2D and 3D cultures. Finally, C18GEM stimulated protective autophagy in Panc1 P and CSCs cultured on 3D conditions. Conclusion: We propose C18GEM together with autophagy inhibitors as a valid alternative therapeutic approach in PDAC treatment

Genome-Wide Association Analyses in 128,266 Individuals Identifies New Morningness and Sleep Duration Loci

Disrupted circadian rhythms and reduced sleep duration are associated with several human diseases, particularly obesity and type 2 diabetes, but until recently, little was known about the genetic factors influencing these heritable traits. We performed genome-wide association studies of self-reported chronotype (morning/evening person) and self-reported sleep duration in 128,266 white British individuals from the UK Biobank study. Sixteen variants were associated with chronotype (P<5x10(-8)), including variants near the known circadian rhythm genes RGS16 (1.21 odds of morningness, 95% CI [1.15, 1.27], P = 3x10(-12)) and PER2 (1.09 odds of morningness, 95% CI [1.06, 1.12], P = 4x10(-10)). The PER2 signal has previously been associated with iris function. We sought replication using self-reported data from 89,283 23andMe participants;thirteen of the chronotype signals remained associated at P<5x10(-8) on meta-analysis and eleven of these reached P< 0.05 in the same direction in the 23andMe study. We also replicated 9 additional variants identified when the 23andMe study was used as a discovery GWAS of chronotype (all P<0.05 and meta-analysis P<5x10(-8)). For sleep duration, we replicated one known signal in PAX8 (2.6 minutes per allele, 95% CI [1.9, 3.2], P = 5.7x10(-16)) and identified and replicated two novel associations at VRK2 (2.0 minutes per allele, 95% CI [1.3, 2.7], P = 1.2x10(-9);and 1.6 minutes per allele, 95% CI [1.1, 2.2], P = 7.6x10(-9)). Although we found genetic correlation between chronotype and BMI (rG = 0.056, P = 0.05);undersleeping and BMI (rG = 0.147, P = 1x10(-5)) and over-sleeping and BMI (rG = 0.097, P = 0.04), Mendelian Randomisation analyses, with limited power, provided no consistent evidence of causal associations between BMI or type 2 diabetes and chronotype or sleep duration. Our study brings the total number of loci associated with chronotype to 22 and with sleep duration to three, and provides new insights into the biology of sleep and circadian rhythms in humans