Office: NPL Building , Room 5 Email: aluican [at] physics [dot] rutgers [dot] edu Phone: (732)445-5936 Fax: (732)445-4343
Department of Physics and Astronomy
Rutgers, The State University of New Jersey
136 Freylinghuysen Rd
Piscataway, NJ 08854 USA

# Research Projects (...under construction)

• Low Temperature Scanning Tunneling Microscopy and Spectroscopy (STM/STS)
In our experiments we use STM to characterize the topography of the surface and STS to probe the electronic properties and obtain the local density of states. The special features of the STM are low temperature (4K) and high magnetic field (12T).

• Landau Quantization in graphene on insulating substrates
For more details see:

Because of the symmetry of the honeycomb lattice the electronic spectrum in the low energy regime describes massless charge carriers obeying a Dirac-like equation and they have a conical dispersion. In the presence of magnetic field perpendicular to the 2D system, the electrons travel in circular orbits at discrete energies - Landau levels. In graphene, the energies of these levels are given by $$E=\pm v_F\sqrt{2e\hbar\left|N\right|\cdot B}$$ where B is the magnetic field, and $$n = \ldots,2,-1,0,1,2,\ldots$$is the level index

The sketch of the experimental set up is showing a graphene flake deposited on $$Si$$/$$SiO_2$$, the STM tip above and the back gate connected to the graphene.

By using STM/STS in magnetic field we have shown here that for graphene supported on chlorinated $$SiO_2$$ substrates, the substrate disorder is weak enough to allow observation of quantized LLs already at moderate fields. We note that the LL at the Dirac point (DP) where N=0, is 200 meV above the Fermi level, indicating that the system is hole doped even though the gate is at ground potential. The typical line-width for LLs is 20-30 meV, corresponds to carrier lifetimes of approx. 22 fs - 32 fs.

To study the effect of gating on the LLs we record the differential conductance spectra, dI/dV(E), at a fixed value of the magnetic field (12T) for a sequence of gate voltages. Our results for B=12 T are summarized as a map of the spectra versus gate voltage. Each vertical line in the map corresponds to the spectrum taken at a particular gate voltage. Upon changing the carrier density we find abrupt jumps in the Fermi level after each Landau level is filled. Qualitatively, one can understand the step-like features as follows: The LL spectrum consists of peaks where the DOS is large separated by minima with low DOS. It takes a large change in the charge carrier density to fill the higher DOS regions, resulting in plateaus where the Fermi level is "pinned" to a particular Landau level. In contrast, filling the low DOS region in between the LLs does not require a large change in carrier density therefore the jumps (changes in slope) in between plateaus.

• Tuning electronic properties of graphene by changing the stacking order
For more details see:

Graphene layers stack to form graphite in such a way that one triangular sublattice of the top layer has atoms underneath, however the other does not, and that is called Bernal or AB stacking. When the top layer is rotated with respect to the one below, for certain rotation angles a commensurate lattice is formed creating the so called Moire patterns.

By rotating the top layer the electronic properties become completely different than single layer, bilayer or multilayer and they are strongly dependent on the twisting angle. An important aspect is the existence of two saddle points in the band structure of the twisted bilayer, which are the origin of two Van Hove singularities in the density of states.

Also the Fermi velocity of the charge carriers changes, and this renormalization is strongly dependent on the twisting angle. By using scanning tunneling spectroscopy on twisted graphene in magnetic field we have demonstrated the presence of quantized Landau levels specific to massless Dirac fermions in the low energy spectrum. We find that the Fermi velocity of the quasiparticle excitations is dramatically reduced at low rotation angles and that it recovers to the value observed in unrotated layers at higher angles as expected from theoretical considerations.