We theoretically studied the phonon properties of the triangular‐, stripe‐, and honeycomb‐type electronic crystals recently found in two‐dimensional semiconductor moiré patterns. By analyzing the ...phonon dispersions, we found the interaction induced lattice deformation in the zigzag‐stripe crystal results in a much higher dynamical stability than in the linear‐stripe crystal. Moreover, chiral phonons with finite magnetizations and large Berry curvatures can emerge in triangular and honeycomb crystals under time‐reversal or inversion symmetry breaking. The small effective mass of the electrons allows the selective and efficient generation of chiral phonons from the optical activity of zone‐center phonons combined with the anharmonicity, facilitating the realization of the phonon Hall effect. These findings point to an exciting new platform for exploring chiral phonons and related topological phononic devices.
Key points
The phonons of the electronic crystals in semiconductor moiré systems are systematically analyzed, which can lead to the infrared absorption, Raman scattering, and Hall effect of chiral phonons.
The small effective mass of the electrons as compared to their ionic counterparts facilitates tuning the chiral phonons through external fields, which may stimulate further studies about elementary excitations of the intriguing correlated insulators in moiré systems.
Our study connects the two active research fields of strongly correlated physics and chiral phonons.
In electronic crystals recently discovered in layered semiconductor moiré systems, the strong and long‐range Coulomb interaction leads to collective vibrations of the electronic sites, i.e., the phonon modes. Chiral phonons with finite magnetizations and large Berry curvatures can emerge in triangular or honeycomb crystals under time‐reversal or inversion symmetry breaking, which can be optically generated and realize the phonon Hall effect.
Charge and spin density waves are typical symmetry broken states of quasi one-dimensional electronic systems. They demonstrate such common features of all incommensurate electronic crystals as a ...spectacular non-linear conduction by means of the collective sliding and susceptibility to the electric field. These phenomena ultimately require for emergence of static and transient topological defects: there are dislocations as space vortices and space-time vortices known as phase slip centers, i.e., a kind of instantons. Dislocations are statically built-in under a transverse electric field; their sweeping provides a conversion among the normal carriers and condensate which ensures the onset of the collective sliding. A special realization in a high magnetic field, when the density wave is driven by the Hall voltage, originated by quantized normal carriers, reveals the dynamic vorticity serving to annihilate compensating normal and collective currents. Spin density waves, with their rich multiplicative order parameter, bring to life complex objects with half-integer topologically bound vorticities in charge and spin degrees of freedom. We present the basic concepts and modelling results of the stationary states and their transient dynamics involving vorticity. The models take into account multiple fields in their mutual non-linear interactions: the complex order parameter, the self-consistent electric field, and the reaction of normal carriers. We explore the traditional time-dependent Ginzburg–Landau approach and introduce its generalization allowing the treatment of intrinsic normal carriers. The main insights and illustrations come from numerical solutions to partial differential equations for the dissipative dynamics of one and two space dimensions.
Metastable states appear in many areas of physics as a result of symmetry-breaking phase transitions. An important challenge is to understand the microscopic mechanisms which lead to the formation of ...the energy barrier separating a metastable state from the ground state. In this paper, we describe an experimental example of the hidden metastable domain state in 1T-TaS2, created by photoexcitation or carrier injection. The system is an example of a charge density wave superlattice in the Wigner crystal limit displaying discommensurations and domain formation when additional charge is injected either through contacts or by photoexcitation. The domain walls and their crossings in particular display interesting, topologically entangled structures, which have a crucial role in the metastability of the system. We model the properties of experimentally observed thermally activated dynamics of topologically protected defects—dislocations—whose annihilation dynamics can be observed experimentally by scanning tunnelling microscopy as emergent phenomena described by a doped Wigner crystal. The different dynamics of trivial and non-trivial topological defects are quite striking. Trivial defects appear to annihilate quite rapidly at low temperatures on the timescale of the experiments, while non-trivial defects annihilate rarely, if at all.
The dimensionless thermoelectric figure of merit and magnetic field production ability of “natural” nanostructures–layered ternary alloys (TA) of the family (Ge, Sn, Pb)(Te, Se)
m
(Bi,Sb)
2
(Te,Se)3
...n
, with non-isovalent cationic substitution (Ge, Sn, Pb ↔ Bi, Sb) are investigated. In the transition from binary alloys (BA) to TA, we observed the formation of the phase “phonon glass–electronic crystal” (PGEC) and its subsequent degeneracy, accompanied by sharp increase in the carrier densities in the samples. As a result, the size
ZT
of samples went down, and the size
X
substantially increased, which speaks in the work to formation of a degenerated PGEC phase under non-isovalent cationic substitution in the samples. Comparison with known thermoelectric materials (ТEMs) (metals, semimetals, and semiconductors) used for production of magnetic fields
H
in contours of short-circuited ТC has shown that the investigated TA forms a new class of TEMs for magnetic field production with raised values of parameters
X
and
Y
.
We report on studies of stationary states and their transient dynamic for an incommensurate charge density wave (ICDW) in a restricted geometry of two spatial dimensions. The model takes into account ...multiple fields in mutual nonlinear interactions: the amplitude and the phase of the complex order parameter, and distributions of the electric and chemical potentials, of the density and the current of normal carriers. We observed spontaneous formation of vortices (the ICDW dislocations), and followed events of their creation and the subsequent evolution. The vortices appear when the voltage across, or the current through, the sample exceed a threshold. The number of vortices remnant in the reconstructed stationary state increases stepwise – in agreement with experiments, while a much greater number of vortices appears during the intermediate transient states. The vortex core concentrates the electric dipole leading to sharp drops of the electric and chemical potentials across the core. That can lead to enhanced inter-layer tunneling making the core to be a self-tuned microscopic tunneling junction. The results are applied to experiments on nano-fabricated mesa-junctions. They also appeal to modern efforts of the field-effect transformations in correlated electronic systems.
Charge density wave (CDW) under an applied electric field in constraint geometry experience stresses, which can easily exceed a plastic threshold. The stress is resolved by the ground state ...reconstruction which proceed via creation of topological defects like solitons and dislocations—the CDW vortices. These states can be observed experimentally either in average at macroscopic scales of X-ray and multijunction space resolved studies, at mesoscopic scales of coherent X-ray micro-diffraction and nano-junctions or individually as by the STM. Here, we report numerical modeling taking into account multiple fields in their mutual nonlinear interactions: the phase and the amplitude of the CDW order parameter, distributions of the electric field, of the density and the current of normal carriers. Following events of creation and the subsequent evolution of dislocations, we find that vortices are formed in the junction when the voltage across, or the current through, exceed a threshold. The number of vortices remnant in the reconstructed ground state increases stepwise—in agreement with experiments. The vortex core concentrates the voltage drop across the junction giving rise to observed peaks of the interlayer tunneling. The studied reconstruction in junctions of CDWs may be relevant to modern efforts of the field-effect transformations in other correlated electronic systems.
Superconductivity and structure OESTERREICHER, H
Journal of superconductivity and novel magnetism,
04/2007, Letnik:
20, Številka:
3
Journal Article
Recenzirano
The degree of the isolation of the CuO2 planes (e.g. distance or bond valence to the apical coordination) has been shown by quantitative algorithms to be the major factor in determining aspects of ...the doping curves. They include the magnitude of the optimal number of doped holes (hop) and the corresponding T cop. It is shown that the roots of these phenomenological laws lie in a related structural dependence of super-exchange. The latter is expressed in the pseudo-gap or Neel temperature of the undoped parent compound. A fruitful language can be developed which deals with a buildup of complex quantum chemical features by bringing two holes into vicinity of a super-exchange O, forming a 'local' Cu2O7 pair. Structural considerations also dictate that stress is relieved by alternate orthogonal pair orientation. This leads to plaid patterns with primary and secondary channels of charge. The presence of these two types of charge channels is involved in the mechanism of superconducting charge transport. Similar structuring of doped charge into plaid patterns of 'local' pairs has been proposed for 'all' high T c superconductivity. STM now gives pictorial representation of the remnants of such an electronic crystal structure. The response of these bond-ordering motifs to structural details is further discussed. These ideas supply organization to the manifold experimental situation and provide opportunities for a unifying theory for high T c superconductivity in terms of real space structuring of 'local' pairs, largely on crystal-chemical principles.
La formation des ondes de densité de charge (ODC) est un phénomène de brisure de symétrie qui apparaît dans systèmes électroniques, et particulièrement dans les conducteurs quasi-unidimensionnels. ...Elle est observée aussi bien dans les matériaux très anisotropes que les isotropes comme par exemple les supraconducteurs pnictures. L'ODC peut être vue comme une déformation sinusoïdale de la densité électronique et de la modulation du réseau, ou également comme un cristal de singulets électroniques. Dans un état d'ODC, les cellules élémentaires peuvent être modifiées en absorbant ou en rejetant des paires d'électrons. Un tel processus passe par des configurations topologiquement non triviales: des solitons et des dislocations, ou plus généralement des vortex d'ODC. Ces états inhomogènes peuvent être obtenus expérimentalement dans des nano-produits appelés ''mésa-jonctions'', et observés à l'aide d'un microscope à effet tunnel ou d’une radiographie par micro-diffraction. Afin de simuler ces expériences, nous avons réalisé un programme modélisant les états stationnaires d'ODC ainsi que leur dynamique transitoire à travers des géométries restreintes auxquelles sont appliquées une tension ou un courant. Le modèle prend en compte plusieurs champs en interaction non linéaire: le paramètre d'ordre complexe d'ODC, la distribution de champ électrique, ainsi que la densité et le courant des autres porteurs de charge. Nous avons utilisé une approche de type Ginzburg-Landau ainsi qu'une extension basée sur une propriété d'invariance chirale. A l'aide de ce programme, nous avons observé la création progressive de dislocations statiques dans les jonctions. La dynamique transitoire est alors très riche avec notamment des créations, des annihilations et des balayages de vortex multiples. Des chutes de tension apparaissent au centre des dislocations, créant ainsi des jonctions tunnel microscopiques à travers lesquelles transitent des paires électron-trou. Les résultats qualitatifs obtenus sont en très bon accord avec les observations expérimentales. Ce model peut aussi être étendu à tout type de cristaux électronique comme les cristaux de Wigner dans les hétéro-jonctions et les nano-fils, les ODC dans les composés de chaîne, les ondes de densité de spin dans les conducteurs organiques, ou encore les structures de bandes dans les oxydes dopés. La reconstruction des ODC dans les jonctions tunnel peut aussi trouver son importance dans l'étude des effets de champs sur les matériaux fortement corrélés induisant des brisures spontanées de symétries.
Formation of charge density waves (CDW) is a symmetry breaking phenomenon found in electronic systems, which is particularly common in quasi-one-dimensional conductors. It is widely observed from highly anisotropic materials to isotropic ones like the superconducting pnictides. The CDW is seen as a sinusoidal deformation of coupled electronic density and lattice modulation; it can be also viewed as a crystal of singlet electronic pairs. In the CDW state, the elementary units can be readjusted by absorbing or rejecting pairs of electrons. Such a process should go via topologically nontrivial configurations: solitons and dislocations - the CDW vortices. An experimental access to these inhomogeneous CDW states came from studies of nano-fabricated mesa-junctions, from the STM and from the X-ray micro-diffraction. Following these requests, we have performed a program of modeling stationary states and of their transient dynamic for the CDW in restricted geometries under applied voltage or at passing normal currents. The model takes into account multiple fields in mutual nonlinear interactions: the two components of the CDW complex order parameter, and distributions of the electric field, the density and the current of normal carriers. We were using the Ginzburg-Landau type approach and also we have derived its extension based on the property of the chiral invariance. We observed the incremental creation of static dislocations within the junctions. The transient dynamics is very rich showing creation, annihilation and sweeping of multiple vortices. The dislocations cores concentrate the voltage drops thus providing self-tuned microscopic junctions where the tunneling creation of electron-hole pairs can take place. The results obtained from this model agree with experiment observations. The methods can be extended to other types of charge organization known under the general name of the Electronic Crystal. It takes forms of Wigner crystals at hetero-junctions and in nano-wires, CDWs in chain compounds, spin density waves in organic conductors, and stripes in doped oxides. The studied reconstruction in junctions of the CDW may be relevant also to modern efforts of the field-effect transformations in strongly correlated materials with a spontaneous symmetry breaking.
