The influence of heat treatment on properties of lead-free solders

The article is focused on the analysis of degradation of properties of two eutectic lead-free solders SnCu0.7 and SnAg3.5Cu0.7. The microstructures of the intermetallic compound (IMC) layers at the copper substrate - solder interface were examined before and after heat treatment at 150°C for 50, 200, 500 and 1000 hours. The thickness of IMC layers of the Cu6Sn5 phase was growing with the increasing time of annealing and shown the typical scallops. For the heat treatment times of 200 hours and longer, the Cu3Sn IMC layers located near the Cu substrate were also observed. The experiments showed there is a link between the thickness of IMC layers and decrease of the shear strength of solder joints. In general, the joints made of the ternary solder showed higher shear strength before and after heat treatment in comparison to joints from solder SnCu0.7

Unconventional order/disorder behaviour in Al–Co–Cu–Fe–Ni multi-principal element alloys after casting and annealing

The effect of Cu concentration on the order/disorder behaviour of the AlCoCuxFeNi (x = 0.6 to 3.0) multi-principal element alloys was investigated. BCC and/or FCC phases were observed in the microstructures of the alloys after casting and annealing at 1050 ◦C followed by slow cooling. Interesting is that the alloys form ordered structures after casting and disordered structures after annealing and slow cooling, while the opposite would be expected. The ordering in the as-cast state is explained by the strong affinity of Al to transition metals, which results in the formation of supercell structures having sublattices occupied by certain elements only. Disordering after annealing has two reasons. Either the phase is composed of nearly pure element (Cu) and is disordered by default or it is composed of randomly distributed nano-segregated regions within a single phase resulting in a uniform distribution of all elements in the sublattices and therefore appearing to be macroscopically disordered. The reason for the formation of such nano-segregated areas might reside in the reduction of Gibbs free energy due to the annealing by the interplay between enthalpy and entropy

Magnetotransport in heterostructures of transition metal dichalcogenides and graphene

We use a van der Waals pickup technique to fabricate different heterostructures containing WSe2(WS2) and graphene. The heterostructures were structured by plasma etching, contacted by one-dimensional edge contacts, and a top gate was deposited. For graphene/WSe2/SiO2 samples we observe mobilities of similar to 12 000 cm(2) V-1 s(-1). Magnetic-field-dependent resistance measurements on these samples show a peak in the conductivity at low magnetic fields. This dip is attributed to the weak antilocalization (WAL) effect, stemming from spin-orbit coupling. Samples where graphene is encapsulated between WSe2(WS2) and hexagonal boron nitride show a much higher mobility of up to similar to 120 000 cm(2) V-1 s(-1). However, in these samples noWAL peak can be observed. We attribute this to a transition from the diffusive to the quasiballistic regime. At low magnetic fields a resistance peak appears, which we ascribe to a size effect due to boundary scattering. Shubnikov-de Haas oscillations in fully encapsulated samples show all integer filling factors due to complete lifting of the spin and valley degeneracies

Commensurability oscillations in one-dimensional graphene superlattices

We report the experimental observation of commensurability oscillations (COs) in 1D graphene superlattices. The widely tunable periodic potential modulation in hBN encapsulated graphene is generated via the interplay of nanopatterned few layer graphene acting as a local bottom gate and a global Si back gate. The longitudinal magneto-resistance shows pronounced COs, when the sample is tuned into the unipolar transport regime. We observe up to six CO minima, providing evidence for a long mean free path despite the potential modulation. Comparison to existing theories shows that small angle scattering is dominant in hBN/graphene/hBN heterostructures. We observe robust COs persisting to temperature exceeding T=150 K. At high temperatures, we find deviations from the predicted T-dependence, which we ascribe to electron-electron scattering

Giant ratchet magneto-photocurrent in graphene lateral superlattices

We report on the observation of the magnetic quantum ratchet effect in graphene with a lateral dual-grating top gate (DGG) superlattice. We show that the THz ratchet current exhibits sign-alternating magneto-oscillations due to the Shubnikov–de Haas effect. The amplitude of these oscillations is greatly enhanced as compared to the ratchet effect at zero magnetic field. The direction of the current is determined by the lateral asymmetry which can be controlled by variation of gate potentials in DGG. We also study the dependence of the ratchet current on the orientation of the terahertz electric field (for linear polarization) and on the radiation helicity (for circular polarization). Notably, in the latter case, switching from right- to left-circularly polarized radiation results in an inversion of the photocurrent direction. We demonstrate that most of our observations can be well fitted by the drift-diffusion approximation based on the Boltzmann kinetic equation with the Landau quantization fully encoded in the oscillations of the density of states

Moire superlattice effects in graphene/boron-nitride van der Waals heterostructures

Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moir\'e superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly.Comment: 25 pages, 7 figure

Übergittereffekte in eindimensional moduliertem Graphen

Diese Arbeit behandelt elektronischen Transport in Graphen unter Einfluss eines eindimensionalen, periodischen Potentials. Das Übergitter wird dem Graphen über ein strukturiertes Top- bzw. Bottomgate aufgeprägt, wobei sich plasmageätztes Mehrlagengraphen als optimale Gateelektrode für das in hexagonales Bornitrid eingekapselte, hochbewegliche Graphen herausgestellt hat. Aufgrund der Ambipolarität von Graphen können über den Feldeffekt sowohl unipolare, als auch bipolare Übergitter erzeugt werden. Zunächst wird der Einfluss eines lateralen Übergitters auf den ballistischen Transport in Monolagengraphen untersucht. In Abhängigkeit von der Gatespannung werden im bipolaren Regime ausgeprägte Fabry-Perot-Oszillationen des longitudinalen Widerstands, sowie bei zunehmender Beweglichkeit der Proben, eine Feinstruktur der Oszillationen beobachtet. Unipolare Potentialstufen in Graphen führen im Quanten-Hall Regime zu Bereichen unterschiedlicher Füllfaktoren und damit ausgeprägten Plateaus im longitudinalen Widerstand. Im Rahmen der Arbeit wird die Abhängigkeit der Plateauwerte vom Magnetfeld und von der Anzahl der Potentialstufen untersucht. In hochbeweglichen Übergittern mit geringer Modulationsamplitude werden im unipolaren Regime zum ersten Mal Weiss-Oszillationen des Magnetowiderstands in Graphen nachgewiesen. Die Oszillationen stellen sich hierbei als sehr robust gegenüber der Temperatur heraus und bleiben bis zu 150K beobachtbar. Die Messergebnisse werden mit der Theorie verglichen und Unterschiede diskutiert. Insbesondere darf im moderaten Temperaturregime ein Einfluss der Elektron-Elektronwechselwirkung auf den Widerstand nicht vernachlässigt werden