SEARCH · Searcharxiv
Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
We reconsider monolayer graphene in the presence of elastic deformations. It is described by the tight - binding model with varying hopping parameters. We demonstrate, that the fermionic quasiparticles propagate in the emergent 2D Weitzenbock geometry and in the presence of the emergent U(1) gauge field. Both emergent geometry and the gauge field are defined by the elastic deformation of graphene.
Madsen et al. [arXiv:1307.2577] claim that one-dimensional insulating crystals and one-dimensional insulating quasicrystals are topologically equivalent and, thus, trivial. In this comment, we clarify that in topological classification of one-dimensional systems, quasiperiodicity plays a role similar to that of a symmetry. Hence, periodic systems are out of the quasiperiodic topological space, and the equivalence between quasicrystals and crystals becomes immaterial.
The Bi-O pockets problem, namely, the appearance in theoretical ab initio calculations of the electronic band structure of Bi-cuprates of a pocket of states at the Fermi energy ($E_F$) that is attributed to states belonging to the Bi-O plane is an issue that still calls for more study. The Bi-O pockets are in contradiction with experiments. We have investigated the possible reasons for the disagreement. We checked that by using the experimental lattice and internal parameters without any optimization procedure, the Bi-O pockets do not appear at $E_F$ in agreement with experiment. Nevertheless, as pointed out by R. Kouba et al. [{\em Phys. Rev. B} {\bf 60}, 9321 (1999)] optimization is compulsory to a band structure calculation that will describe appropriately the electronic properties. But starting with the experimental parameters a further optimization procedure previous to the actual ab initio calculation leads to the Bi-O pockets. Doping with 25% of Pb they disappear. From the several configurations that we have considered, we found two very simple ways in which the Bi-O pockets disappear without avoiding an optimization procedure previous to the calculation and without including a doping of any kind. In this paper, we report the effect of the slight displacement of the oxygen atom associated to the Sr-plane (O3) in the electronic properties of $Bi_2Sr_2Ca_2Cu_3O_{10}$ (Bi-2223) with tetragonal structure ($I4/mmm$) using the Local Density Approximation (LDA). The slight displacement is performed after the system has been optimized. We determined the intervals of the O3 atomic positions for which calculations of the band structures show that the Bi-O bands emerge towards higher energies in agreement with the experimental results, thereby solving the Bi-O pockets problem (continue).
We study the spin-1/2 $J_{1}$-$J_{2}$ Heisenberg model on a square lattice using the cluster mean-field theory. We find a rapid convergence of phase boundaries with increasing cluster size. By extrapolating the cluster size $L$ to infinity, we obtain accurate phase boundaries $J_{2}^{c1} \approx 0.42$ (between the N$\acute{e}$el antiferromagnetic phase and nonmagnetic phase), and $J_{2}^{c2} \approx 0.59$ (between nonmagnetic phase and the collinear antiferromagnetic phase). The transitions are identified unambiguously as second order at $J_{2}^{c1}$ and first order at $J_{2}^{c2}$. At finite temperature, we present a complete phase diagram with stable, meta-stable and unstable states near $J_{2}^{c2}$, being relevant to that of the anisotropic $J_1-J_2$ model. The uniform as well as staggered magnetic susceptibilities are also discussed.
Monoclinic crystal structure of the undoped BaBiO3 can be described as a cubic perovskite distorted by a frozen breathing and tilting phonon modes of BiO6 octahedra. The phonon mode softening is experimentally observed [M. Braden et al., Europhysics Letters (EPL) 34, 531 (1996)] in Ba1-xKxBiO3 through potassium doping followed by a transition into an ideal cubic perovskite structure at x = 0.37 close to the appearance of superconductivity. In our previous paper [D. Korotin et al., Journal of Physics: Condensed Matter 24, 415603 (2012)] we demonstrated that it is necessary to take into account correlation effects by DFT+U method in Wannier functions basis to obtain a good agreement between the calculated and experimental values of crystal structure distortion and energy gap in BaBiO3. In the present work with the same method we calculated the breathing mode phonon frequencies as a function of potassium doping level in Ba1-xKxBiO3. Obtained frequencies are in a good agreement with experimental values and the breathing mode softening with doping effect is reproduced while calculations without consideration of correlation effects failed to do so. We shown that the cubic crystal structure becomes stable at x = 0.30 in agreement with the experimental transition to cubic perovskite at x = 0.37. The possible connections between the correlation effects, phonon mode softening, and superconductivity in Ba1-xKxBiO3 are discussed.
Skyrmions are topologically protected field configurations with particle-like properties that play important roles in various fields of science. Recently, skyrmions have been directly observed in chiral magnets. Here, we investigate the effects of nonmagnetic impurities (structural point-like defects) on the different initial states (random or helical states) and on the formation of the skyrmion crystal in a discrete lattice. By using first-principle calculations and Monte Carlo techniques, we have shown that even a small percentage of spin vacancies present in the chiral magnetic thin film affects considerably the skyrmion order. The main effects of impurities are somewhat similar to thermal effects. The presence of these spin vacancies also induces the formation of compact merons (bimerons) in both the helical and skyrmion states. We have also investigated the effects of adjacent impurities producing only one hole (with an almost circular shape) intentionally inserted into the plate (forming a non-simply connected manifold) on the skyrmion crystal.
Non-equilibrium quantum phenomena are ubiquitous in nature. Yet, theoretical predictions on the real-time dynamics of many-body quantum systems remain formidably challenging, especially for high dimensions, strong interactions or disordered samples. Here we consider a notable paradigm of strongly correlated Fermi systems, the Mott phase of the Hubbard model, in a setup resembling ultracold-gases experiments. We study the three-dimensional expansion of a cloud into an optical lattice after removing the confining potential. We use time-dependent density-functional theory combined with dynamical mean-field theory, considering interactions below and above the Mott threshold, as well as disorder effects. At strong coupling, we observe multiple timescales in the melting of the Mott wedding-cake structure, as the Mott plateau persist orders of magnitude longer than the band insulating core. We also show that disorder destabilises the Mott plateau and that, compared to a clean setup, localisation can decrease, creating an interesting dynamic crossover during the expansion.
Recently, application of electric field (E-field) has received considerable attention as a new method to induce novel quantum phenomena since application of E-field can tune the electronic states directly with obvious scientific and industrial advantages over other turning methods. However, E-field-induced Mott transitions are rare and typically require high E-field and low temperature. Here we report that the multiband Mott insulator Ca2RuO4 shows unique insulator-metal switching induced by applying a dry-battery level voltage at room temperature. The threshold field Eth ~40 V/cm is much weaker than the Mott gap energy. Moreover, the switching is accompanied by a bulk structural transition. Perhaps the most peculiar of the present findings is that the induced metal can be maintained to low temperature by a weak current.
In this paper we obtain an exact periodic solution for the system of equations of state corresponding to the spiral phase of crystalline ordering. We used method of integral rational basis of invariants to construct an inhomogeneous Landau potential. As a result the inhomogeneous Landau potential takes account of anisotropic invariants consisting of OP components as well as anisotropic invariants that comprise space derivatives of OP components. It is demonstrated that it is taking into account the latter that leads to the exact periodic solution for the system of equations of state that describes the spiral phase. A phase diagram of states is built for active Lifshitz representations at the second-order phase transition point. Analogy between inhomogeneous states of A-phase and second-order superconductors in the magnetic field is discussed. It is suggested to describe the vortex state in the magnetic field as a deformation of the magnetic sublattice with dislocations.
We report on detailed low temperature measurements of the magnetization, the specific heat and the electrical resistivity on high quality CeRuSn single crystals. The compound orders antiferromagnetically at $T_{\rm N} = 2.8$ K with the Ce$^{3+}$ ions locked within the $a-c$ plane of the monoclinic structure. Magnetization shows that below $T_{\rm N}$ CeRuSn undergoes a metamagnetic transition when applying a magnetic field of 1.5 and 0.8 T along the $a$ and $c$--axis, respectively. This transition manifests in a tremendous negative jump of $\sim 25$% in the magnetoresistance. The value of the saturated magnetization along the easy magnetization direction ($c$--axis) and the magnetic entropy above $T_{\rm N}$ derived from specific heat data correspond to the scenario where only one third of the Ce ions in the compound being trivalent and carrying a stable Ce$^{3+}$ magnetic moment, whereas the other two thirds of the Ce ions are in a nonmagnetic tetravalent and/or mixed valence state. This is consistent with the low temperature CeRuSn crystal structure i.\,e.\,, a superstructure consisting of three unit cells of the CeCoAl-type piled up along the $c$--axis, and in which the Ce$^{3+}$ ions are characterized by large distances from the Ru ligands while the Ce-Ru distances of the other Ce ions are much shorter causing a strong 4{\it f}-ligand hybridization and hence leading to tetravalent and/or mixed valence Ce ions.
In this paper, we study the influence of spatial fluctuations in a two-dimentional Kondo-Lattice model (KLM) with anti-ferromagnetic couplings. To accomplish this, we first present an implementation of the dual-fermion (DF) approach based on the hybridization expansion continuous-time quantum Monte Carlo impurity solver (CT-HYB), which allows us to consistently compare the local and non-local descriptions of this model. We find that, the inclusion of non-locality restores the self-energy dispersion of the conduction electrons, {\it i.e.} the $\vec{k}$ dependence of $Σ(\vec{k}, iω_{n})$. The anti-ferromagnetic correlations result in an additional symmetry in $Σ(\vec{k}, iω_{n})$, which is well described by the Néel antiferromagnetic wave-vector. A "metal"-"anti-ferromagnetic insulator"-"Kondo insulator" transition is observed at finite temperatures, which is driven by the competition of the effective RKKY interaction (at the weak coupling regime) and the Kondo singlet formation mechanism (at the strong coupling regime). Away from half-filling, the anti-ferromagnetic phase becomes unstable against hole doping. The system tends to develop a ferromagnetic phase with the spin susceptibility $χ_{s}(Q)$ peaking at $Q=Γ$. However, for small $J/t$, no divergence of $χ_{s}(Γ)$ is really observed, thus, we find no sign of long-range ferromagnetism in the hole-doped two-dimension KLM. The ferromagnetism is found to be stable at larger $J/t$ regime. Interestingly, we find the local approximation employed in this work, {\it i.e.} the dynamical mean-field theory (DMFT), is still a very good description of the KLM, especially in the hole-doped case. However, at half-filling, the non-local fluctuation effect is indeed pronounced. We observe a strong reduction of the critical coupling strength for the onset of the Kondo insulating phase.
Angle resolved photoemission (ARPES) experiments reveal the coherent electronic excitation in the energy spectrum of under- and optimally doped cuprates at temperatures above their temperature of transition into superconducting state only near nodal points on the parts of the "bare" Fermi surface known as the Fermi arcs. That raises questions about the meaning and possible interpretations of the experimental data. Below we derive expressions for the Hall coefficient for the Fermi arc model both in weak and strong magnetic fields.
In the review article the influence of the Rashba spin-orbit coupling on the two-dimensional electrons and holes in a strong perpendicular magnetic field is described in Landau gauge using the spinor-type two-component wave functions with different numbers of the Landau quantization levels in different spin projections, their difference being determined by the order of the chirality terms. In this representation the two-particle integral operators such as the electron and hole densities ${{\hatρ}_{e}}(\vec{Q})$, and ${{\hatρ}_{h}}(\vec{Q})$ as well as the magnetoexciton creation and annihilation operators $\hatΨ_{ex}^{\dagger}({{F}_{n}},\vec{k})$ and $\hatΨ_{ex}^{}({{F}_{n}},\vec{k})$ in different combinations ${{F}_{n}}$ of the electron and hole spinor states were introduced. On this base the Hamiltonians of the Coulomb electron-electron and electron-radiation interactions were deduced. The properties of the magnetoexcitons in these conditions were discussed.
We study monopole operators at the infrared fixed points of Abelian and non-Abelian gauge theories with N_f fermion flavors in three dimensions. At large N_f, independent monopole operators can be defined via the state-operator correspondence only for stable monopole backgrounds. In Abelian theories, every monopole background is stable. In the non-Abelian case, we find that many (but not all) backgrounds are stable in each topological class. We calculate the infrared scaling dimensions of the corresponding operators through next-to-leading order in 1/N_f. In the case of U(N_c) QCD with N_f fundamental fermions (and in particular in the QED case, N_c =1), we find that the monopole operators transform as non-trivial irreducible representations of the SU(N_f) flavor symmetry group.
We report a large persistent photoconductivity (PPC) in oxygen reduced PLD grown YBa2Cu3O7-δ/La2/3Ca1/3MnO3-y (YBCO/LCMO) superlattices scaling with the oxygen deficiency and being similar to that observed in single layer YBa2Cu3O7-δ films. These results contradict previous observations where in sputtered bilayer samples only a transient photoconductivity (TPC) was found. We argue that the PPC effect in superlattices is caused by the PPC effect due to the YBa2Cu3O7-x layers with limited charge transfer to La2/3Ca1/3MnO3-y. The discrepancy arises from the different permeability of charges across the interface and shed light on the sensitivity of oxide interface properties on details of their preparation.
We report on far- and mid-infrared reflectivity of NdMnO3 from 4 K to 300K. Two main features are distinguished in the infrared spectra: active phonons in agreement with the expected for orthorhombic D2h 16-Pbnm (Z=4) space group remaining constant down to 4 K and a well-defined collective excitation in the THz region due to eg electrons in a d-orbital fluctuating environment. We trace its origin to the NdMnO3 high temperature orbital disordered intermediate phase not being totally dynamically quenched at lower temperatures. This results in minute orbital misalignments that translate in randomize non-static eg electrons within orbitals yielding a room temperature collective excitation. Below TN~78 K, electrons gradually localize inducing long-range magnetic order as the THz band condenses into two modes that emerge pinned to the A-type antiferromagmetic order. They harden simultaneously down to 4 K obeying power laws with TN as the critical temperature and exponents β~0.25 and β~0.53, as for a tri-critical point and Landau magnetic ordering, respectively. At 4K they match known zone center spin wave modes. The power law dependence is concomitant with a second order transition in which spin modes modulate orbital instabilities in a magnetoelectric hybridized orbital/charge/spin/lattice scenario. We also found that phonon profiles also undergo strong changes at TN~78 K due to magnetoelasticity.
The pyrochlore compounds Ho2Ti2O7 and Dy2Ti2O7 show an exotic form of magnetism called the spin ice state, resulting from the interplay between geometrical frustration and ferromagnetic coupling. A fascinating feature of this state is the appearance of magnetic monopoles as emergent excitations above the degenerate ground state. Over the past years, strong effort has been devoted to the investigation of these monopoles and other properties of the spin ice state in bulk crystals. A tantalising prospect is to incorporate spin ice materials into devices for spintronics and devices that can manipulate the magnetic monopoles. This would require the availability of spin ice thin films. Here, we report the fabrication of Ho2Ti2O7 thin films using pulsed laser deposition. These films not only show a high crystalline quality, but also exhibit the hallmarks of a spin ice: a pronounced magnetic anisotropy and an intermediate plateau in the magnetisation along the [111] crystal direction.
Former results for a Tight-Binding (TB) model of CuO planes in La2CuO4 are reinterpreted here to underline their wider implications. It is noted that physical systems being appropriately described by the TB model can exhibit the main strongly correlated electron system (SCES) properties, when they are solved in the HF approximation, by also allowing crystal symmetry breaking effects and non-collinear spin orientations of the HF orbitals. It is argued how a simple 2D square lattice system of Coulomb interacting electrons can exhibit insulator gaps and pseudogap states, and quantum phase transitions as illustrated by the mentioned former works. A discussion is also presented here indicating the possibility of attaining room temperature superconductivity, by means of a surface coating with water molecules of cleaved planes of graphite, being orthogonal to its c-axis. The possibility that 2D arrays of quantum dots can give rise to the same effect is also proposed to consideration. The analysis also furnishes theoretical insight to solve the Mott-Slater debate, at least for the La2CuO4 and TMO band structures. The idea is to apply a properly non-collinear GW scheme to the electronic structure calculation of these materials. The fact is that the GW approach can be viewed as a HF procedure in which the screening polarization is also determined. This directly indicates the possibility of predicting the assumed dielectric constant in the previous works. Thus, the results seem to identify that the main correlation properties in these materials are determined by screening. Finally, the conclusions also seem to be of help for the description of the experimental observations of metal-insulator transitions and Mott properties in atoms trapped in planar photonic lattices.