Uniaxial Strain Induced Topological Phase Transition in Bismuth-Tellurohalide-Graphene Heterostructures

We explore the electronic structure and topological phase diagram of heterostructures formed of graphene and ternary bismuth tellurohalide layers. We show that mechanical strain inherently present in fabricated samples could induce a topological phase transition in single-sided heterostructures, turning the sample into a novel experimental realisation of a time reversal invariant topological insulator. We construct an effective tight binding description for low energy excitations and fit the model's parameters to ab initio band structures. We propose a simple approach for predicting phase boundaries as a function of mechanical distortions and hence gain a deeper understanding on how the topological phase in the considered system may be engineered.Comment: 20 pages, 7 figures, Accepted Manuscrip

Új szén nanorendszerek elméleti vizsgálata = Theoretical Investigation of Novel Carbon Nanosystems

Bevezettünk egy lineáris/exponenciális skálázást és DFT számolásokkal meghatároztuk rövid és hosszú lineáris szénláncok Raman aktív LO frekvenciáját; DFT módszerrel tanulmányoztuk a rétegek közötti kölcsönhatást kettősfalú szén nanocsövekben. Töltésátvitelt tapasztaltunk a külső és belső csövek között, továbbá keveredést a két réteg állapotai között; Tanulmányoztuk pikocső kölcsönhatását szén nanocsővel; Tanulmányoztuk a 13C izotópdúsítás hatását szén nanocsövek lélegző módusára; Megmagyaráztuk a nagy görbületű egyfalú szén nanocsövek Raman spektrumában a D és 2D sávok kísérletileg megfigyelt anomális diszperzióját a görbület által okozott fonon puhulás és az optikai átmeneti energiáknak a vöröseltolódása segítségével; Kiszámoltuk kis átmérőjű szén nanocsövek fonon diszperzióját DFT szinten és a helikális szimmetria kihasználásával; Megmagyaráztuk kis átmérőjű szén nanocsöveken végzett in situ Raman spektroelektrokémiai vizsgálatok eredményeit CoMoCat szén nanocsövek elektronszerkezetének és teljesen szimmetrikus rezgéseinek DFT szintű számolása segítségével; Kiszámítottuk dópolt fullerén-kubán kokristályok sávszerkezetét DFT módszerrel; Kiszámoltuk bambusz hibahelyeket tartalmazó szén nanocsövek geometriáját, állapotsűrűségét és ballisztikus transzportját; Kiszámítottuk 4d és 5d átmeneti fématomok kötési energiáját egy grafén síkhoz; Elméletileg megmutattuk, hogy egy királis külső csőnek egy fix belső cső körüli forgatásával elektronok pumpálhatók a belső cső mentén. | We introduced a linear/exponential scaling scheme and calculated with DFT methods the Raman active LO frequencies for olygoynes and polyyne; We studied the intershell interaction in double walled carbon nanotubes. We observed charge transfer between the inner and outer tubes and also orbital mixing between the states of the layers; We studied the interaction of a picotube with carbon nanotubes; We studied the effect of 13C isotope enrichment on the radial breathing mode of carbon nanotubes; We explained the experimentally observed unusual Raman dispersion for D and 2D lines in high-curvature single-walled carbon nanotubes by a curvature-induced phonon softening and the red shift of the optical transition energies; We calculated the phonon dispersion of small diameter carbon nanotubes on DFT level exploiting the screw axis symmetry; We explained the results of in situ Raman spectroelectrochemical studies on small diameter carbon nanotubes by performing DFT calculations on the electronic structure and the totally symmetric vibrations of selected CoMoCat carbon nanotubes; We calculated the band structure of doped fullerene-cubane cocrystals; We calculated the geometry, DOS and ballistic transport for carbon nanotubes with bamboo defect; We studied the strength of the binding of 4d and 5d transition metal atoms on a graphene sheet; We showed theoretically that by rotating a chiral outer tube around a fixed inner tube it is possible to pump electrons along the inner tube

Topological Phase Diagram of BiTeX–GrapheneHybrid Structures

Combining graphene with other novel layered materials is a possible way for engineering the band structure of charge carriers. Strong spin-orbit coupling in BiTeX compounds and the recent fabrication of a single layer of BiTeI points towards a feasible experimental realization of a Kane–Mele phase in graphene-based heterostructures. Here, we theoretically demonstrate the tunability of the topological phase of hybrid systems built from graphene and BiTeX (X = I, Br, Cl) layers by uniaxial in-plane tensile and out-of plane compressive strain. We show that structural stress inherently present in fabricated samples could induce a topological phase transition, thus turning the sample in a novel experimental realization of a time reversal invariant topological insulator

Multiband k⋅pk \cdot p theory for hexagonal germanium

The direct bandgap found in hexagonal germanium and some of its alloys with silicon allows for an optically active material within the group-IV semiconductor family with various potential technological applications. However, there remain some unanswered questions regarding several aspects of the band structiure, including the strength of the electric dipole transitions at the center of the Brillouin zone. Using the $\mathbf{k\cdot p}$ method near the $\Gamma$ point, including 10 bands, and taking spin-orbit coupling into account, we obtain a self-consistent model that produces the correct band curvatures, with previously unknown inverse effective mass parameters, to describe 2H-Ge via fitting to {\it ab initio} data and to calculate effective masses for electrons and holes. To understand the weak dipole coupling between the lowest conduction band and the top valance band, we start from a spinless 12-band model and show that when adding spin-orbit coupling, the lowest conduction band hybridizes with a higher-lying conduction band, which cannot be explained by the spinful 10-band model. With the help of L\"owdin's partitioning, we derive the effective low-energy Hamiltonian for the conduction bands for the possible spin dynamics and nanostructure studies and in a similar manner, we give the best fit parameters for the valance-band-only model that can be used in the transport studies. Finally, using the self-consistent 10-band model, we include the effects of a magnetic field and predict the electron and hole g-factor of the conduction and valance bands.Comment: 11 pages, 4 figure

Exfoliation of single layer BiTeI flakes

Spin orbit interaction can be strongly boosted when a heavy element is embedded into an inversion asymmetric crystal field. A simple structure to realize this concept in a 2D crystal contains three atomic layers, a middle one built up from heavy elements generating strong atomic spin-orbit interaction and two neighboring atomic layers with different electron negativity. BiTeI is a promising candidate for such a 2D crystal, since it contains heavy Bi layer between Te and I layers. Recently the bulk form of BiTeI attracted considerable attention due to its giant Rashba interaction, however, 2D form of this crystal was not yet created. In this work we report the first exfoliation of single layer BiTeI using a recently developed exfoliation technique on stripped gold. Our combined scanning probe studies and first principles calculations show that SL BiTeI flakes with sizes of 100 $\mu$m were achieved which are stable at ambient conditions. The giant Rashba splitting and spin-momentum locking of this new member of 2D crystals open the way towards novel spintronic applications and synthetic topological heterostructures.Comment: 20 pages, 5 figure

Observation of conduction electron spin resonance in boron doped diamond

We observe the electron spin resonance of conduction electrons in boron doped (6400 ppm) superconducting diamond (Tc =3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent ESR signal intensity and its magnitude which is in good agreement with that expected from the density of states through the Pauli spin-susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature which can be understood in the framework of the Elliott-Yafet theory of spin-relaxation. An anomalous and yet unexplained relation is observed between the g-factor, CESR linewidth, and the resistivity using the empirical Elliott-Yafet relation.Comment: 10 pages, 11 figures, submitted to Phys. Rev.