Condensed Matter

Close to a magical angle, twisted bilayer graphene (TBLG) systems exhibit isolated flat electronic bands and, accordingly, strong electron localization. TBLGs have hence been ideal platforms to explore superconductivity, correlated insulating states, magnetism, and quantized anomalous Hall states in reduced dimension. Below a threshold twist angle ( ??1.1 ??), the TBLG superlattice undergoes lattice reconstruction, leading to a periodic moiré structure which presents a marked atomic corrugation. Using a tight-binding framework, this research demonstrates that superlattice reconstruction affects significantly the electronic structure of small-angle TBLGs. The first magic angle at ??1.1 ??is found to be a critical case presenting globally maximized electron localization, thus separating reconstructed TBLGs into two classes with clearly distinct electronic properties. While low-energy Dirac fermions are still preserved at large twist angles > 1.1 ??, small-angle ( ??1.1 ??) TBLG systems present common features such as large spatial variation and strong electron localization observed in unfavorable AA stacking regions. However, for small twist angles below 1.1 ??, the relative contribution of the local AA regions is progressively reduced, thus precluding the emergence of further magic angles, in very good agreement with existing experimental evidence.

Shot noise is typically associated with the random partitioning of a current. Recently, charge current shot noise due to a temperature bias, in the absence of an average current, has attracted interest and was dubbed delta- T noise. Here, we show that this concept is much more general, occurring in different nonequilibrium constellations and for different types of currents. We derive a fundamental bound for the zero-current charge shot noise at thermovoltage, while such a bound does not exist for heat shot noise in the absence of a heat current.

The effects of second-neighbor interactions in Kekule patterned graphene electronic properties are studied starting from a tight-binding Hamiltonian. Thereafter, a low-energy effective Hamiltonian is obtained by projecting the high energy bands at the Gamma point into the subspace defined by the Kekule wave vector. The spectrum of the low energy Hamiltonian is in excellent agreement with the one obtained from a numerical diagonalization of the full tight-binding Hamiltonian. The main effect of the second-neighbour interaction is that a set of bands gains an effective mass and a shift in energy, thus lifting the degeneracy of the conduction bands at the Dirac point. This band structure is akin to a spin-one Dirac cone, a result expected for honeycomb lattices with a distinction between one third of the atoms in one sublattice. Finally, we present a study of Kekule patterned graphene nanoribbons. This shows that the previous effects are enhanced as the width decreases. Moreover, edge states become dispersive, as expected due to second neighbors interaction, but here the Kek-Y bond texture results in an hybridization of both edge states. The present study shows the importance of second neighbors in realistic models of Kekule patterned graphene, specially at surfaces.

The recent advances in nanowire (NW) growth technology have made possible the growth of more complex structures such as core-multi-shell (CMS) NWs. We propose the approach for calculation of electron subbands in cylindrical CMS NWs within the simple effective mass approximation. Numerical results are presented for GaAs/A l 0.3 G a 0.7 As radial heterostructure with AlGaAs-core and 4 alternate GaAs and AlGaAs shells. The influence of an effective mass difference in heterolayers is discussed.

The electron spectrum in a uniform nanowire with a hexagonal cross-section is calculated by means of a numerical diagonalization of the effective-mass Hamiltonian. Two basis sets are utilized. The wave-functions of low-lying states are calculated and visualized. The approach has an advantage over mesh methods based on finite-differences (or finite-elements) schemes: non-physical solutions do not arise. Our scheme can be easily generalized to the case of multi-band (Luttinger or Kane) k?�p Hamiltonians. The external fields (electrical, magnetic or strain) can be consistently introduced into the problem as well.

The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which follows a universal thermal distribution with the bias voltage as the chemical potential of the photon population. A significant deviation between electronic and lattice temperatures is found below 30 K for tunnel currents larger than 15 nA. This deviation is rationalized as the result of a two-electron process in which the second electron excites plasmon modes with an energy distribution that reflects the higher temperature following the first tunneling event. These results dispel a long-standing controversy on the nature of overbias emission in tunnel junctions and adds a new method for the determination of electronic temperatures and quasiparticle dynamics.

Polar skyrmions are theoretically predicted to emerge resulting from the interplay of elastic, electrostatic and gradient energies, in contrast to the key role of the anti-symmetric Dzyalozhinskii-Moriya interaction in magnetic skyrmions. With the discovery of topologically stable polar skyrmions reported by Das et al., (Nature 568, 368, 2019), it is of both fundamental and practical interest to understand the microscopic nature and the possibility of temperature- and strain-driven phase transitions in ensembles of such polar skyrmions. Here, we explore the emergence of a two-dimensional, tetratic lattice of merons (with topological charge of +1/2) from a skyrmion state (topological charge of +1) upon varying the temperature and elastic boundary conditions in [(PbTiO 3 ) 16 /(SrTiO 3 ) 16 ] 8 lifted-off membranes. Such a topological phase transition is accompanied by a change in chirality, e.g. from left-handed to zero-net chirality, as measured by four-dimensional scanning transmission electron microscopy (4D-STEM). We show how 4D-STEM provides a robust measure of the local polarization simultaneously with the strain state at sub-nm resolution, while directly revealing the origins of chirality in each skyrmion. Using this, we demonstrate strain as a crucial order parameter to drive isotropic-to-anisotropic structural transitions of chiral polar skyrmions to non-chiral merons, validated with X-ray reciprocal space mapping and theoretical phase-field simulations. These results provide the first illustration of systematic control of rich variety of topological dipole textures by altering the mechanical boundary conditions, which may offer a promising way to control their functionalities in ferroelectric nanodevices using the local and spatial distribution of chirality and order for potential applications.

Twisted van der Waals materials have been shown to host a variety of tunable electronic structures. Here we put forward twisted trilayer graphene (TTG) as a platform to emulate heavy fermion physics. We demonstrate that TTG hosts extended and localized modes with an electronic structure that can be controlled by interlayer bias. In the presence of interactions, the existence of localized modes leads to the development of local moments, which are Kondo coupled to coexisting extended states. By electrically controlling the effective exchange between local moments, the system can be driven from a magnetic into a heavy fermion regime, passing through a quantum critical point. Our results put forward twisted graphene multilayers as a platform for the realization of strongly correlated heavy fermion physics in a purely carbon-based platform.

The past decade has witnessed a surge of interest in exploring emergent particles in condensed matter systems. Novel particles, emerged as excitations around exotic band degeneracy points, continue to be reported in real materials and artificially engineered systems, but so far, we do not have a complete picture on all possible types of particles that can be achieved. Here, via systematic symmetry analysis and modeling, we accomplish a complete list of all possible particles in time reversal-invariant systems. This includes both spinful particles such as electron quasiparticles in solids, and spinless particles such as phonons or even excitations in electric-circuit and mechanical networks. We establish detailed correspondence between the particle, the symmetry condition, the effective model, and the topological character. This obtained encyclopedia concludes the search for novel emergent particles and provides concrete guidance to achieve them in physical systems.

In natural and artificial light-harvesting complexes (LHC) the resonant energy transfer (RET) between chromophores enables an efficient and directional transport of solar energy between collection and reaction centers. The detailed mechanisms involved in this energy funneling are intensely debated, essentially because they rely on a succession of individual RET steps that can hardly be addressed separately. Here, we developed a scanning tunnelling microscopy-induced luminescence (STML) approach allowing visualizing, addressing and manipulating energy funneling within multi-chromophoric structures with sub-molecular precision. We first rationalize the efficiency of the RET process at the level of chromophore dimers. We then use highly resolved fluorescence microscopy (HRFM) maps to follow energy transfer paths along an artificial trimer of descending excitonic energies which reveals a cascaded RET from high- to low-energy gap molecules. Mimicking strategies developed by photosynthetic systems, this experiment demonstrates that intermediate gap molecules can be used as efficient ancillary units to convey energy between distant donor and acceptor chromophores. Eventually, we demonstrate that the RET between donors and acceptors is enhanced by the insertion of passive molecules acting as non-covalent RET bridges. This mechanism, that occurs in experiments performed in inhomogeneous media and which plays a decisive role in fastening RET in photosynthetic systems, is reported at the level of individual chromophores with atomic-scale resolution. As it relies on organic chromophores as elementary components, our approach constitutes a powerful model to address fundamental physical processes at play in natural LHC.