Kalobaran Maiti

Doping of Graphene by Low-Energy Ion Beam Implantation: Structural, Electronic, and Transport Properties

We investigate the structural, electronic, and transport properties of substitutional defects in SiC-graphene by means of scanning tunneling microscopy and magnetotransport experiments. Using ion incorporation via ultralow energy ion implantation, the influence of different ion species (boron, nitrogen, and carbon) can directly be compared. While boron and nitrogen atoms lead to an effective doping of the graphene sheet and can reduce or raise the position of the Fermi level, respectively, (12)C(+) carbon ions are used to study possible defect creation by the bombardment. For low-temperature transport, the implantation leads to an increase in resistance and a decrease in mobility in contrast to undoped samples. For undoped samples, we observe in high magnetic fields a positive magnetoresistance that changes to negative for the doped samples, especially for (11)B(+)- and (12)C(+)-ions. We conclude that the conductivity of the graphene sheet is lowered by impurity atoms and especially by lattice defects, because they result in weak localization effects at low temperatures.

Short-range ordering of ion-implanted nitrogen atoms in SiC-graphene

We perform a structural analysis of nitrogen-doped graphene on SiC(0001) prepared by ultra low-energy ion bombardment. Using scanning tunneling microscopy, we show that nitrogen atoms are incorporated almost exclusively as graphitic substitution in the graphene honeycomb lattice. With an irradiation energy of 25 eV and a fluence of approximately 5 × 1014 cm−2, we achieve a nitrogen content of around 1%. By quantitatively comparing the position of the N-atoms in the topography measurements with simulated random distributions, we find statistically significant short-range correlations. Consequently, we are able to show that the dopants arrange preferably at lattice sites given by the 6 × 6-reconstruction of the underlying substrate. This selective incorporation is most likely triggered by adsorbate layers present during the ion bombardment. This study identifies low-energy ion irradiation as a promising method for controlled doping in epitaxial graphene.

Unusual spectral renormalization in hexaborides.

Employing high resolution photoemission spectroscopy, we studied the evolution of the spectral features in rare earth hexaboride single crystals as a function of temperature and 4f binding energy, where the variation of the 4f binding energy is obtained by changing the rare earth element. High energy resolution helped to reveal the distinct features corresponding to the various photoemission final states. Experimental results of CeB(6), a dense Kondo system, exhibit the growth of the features near the Fermi level with the decrease in temperature relative to the uncompensated local moment contributions. The valence band spectra of the antiferromagnetic compounds, PrB(6) and NdB(6), exhibit multiple features-the 4f ionization peaks (poorly screened features) appear at higher binding energies and the features in the vicinity of the Fermi level possessing significant 4f character are due to the well-screened photoemission final states. These results indicate finite hybridization between the 4f and B 2s2p conduction electronic states. Interestingly, the well-screened features in PrB(6) and NdB(6) exhibit unusual enhancement in intensity at low temperature.

Complex evolution of the electronic structure of Cr with temperature

Employing state-of-the-art high resolution photoemission spectroscopy, we studied the electronic structure evolution of Cr with temperature. Experimental results reveal signature of a pseudogap much below the spin density wave transition temperature. A sharp peak appears near the Fermi level at low temperatures presumably related to the orbital Kondo effect. These results provide possible origin of the complex electronic properties observed in this system.

Anomalies of a topologically ordered surface

Bulk insulators with strong spin orbit coupling exhibit metallic surface states possessing topological order protected by the time reversal symmetry. However, experiments show vulnerability of topological states to aging and impurities. Different studies show contrasting behavior of the Dirac states along with plethora of anomalies, which has become an outstanding problem in material science. Here, we probe the electronic structure of Bi2Se3 employing high resolution photoemission spectroscopy and discover the dependence of the behavior of Dirac particles on surface terminations. The Dirac cone apex appears at different binding energies and exhibits contrasting shift on Bi and Se terminated surfaces with complex time dependence emerging from subtle adsorbed oxygen-surface atom interactions. These results uncover the surface states behavior of real systems and the dichotomy of topological and normal surface states important for device fabrication as well as realization of novel physics such as Majorana Fermions, magnetic monopole, etc.

High Resolution Photoemission Study of Cr–A Classic SDW‐type Antiferromagnetic Metal

We studied the temperature evolution of the electronic structure of Chromium across the spin density wave‐type antiferromagnetic transition using high resolution photoemission spectroscopy. The spectral density of states (SDOS) remain almost unchanged across the magnetic transition. Signature of a pseudogap appears at low temperatures. In addition, SDOS exhibits growth of a peak just above the Fermi level at low temperatures similar to that found in various Kondo lattice systems.

Interesting spectral evolution in Fe-based superconductors

Fe-based superconductors are studied extensively during past decade to understand the interplay of superconductivity and magnetism. We studied the electronic structure of some of these fascinating systems exhibiting antiferromagnetic ordering and superconductivity, employing high resolution photoemission spectroscopy. We observed signature of finite hybridization of the electronic states corresponding to the local moment and the conduction electrons. The electronic states near Fermi level exhibit significant pnictogen/chalcogen p character. Signature of Kondo like features are observed near M-point in correlated Fe-compound.

Kondo resonance in magnetic and non‐magnetic Ce‐intermetallics

Ce2CoSi3 is a Kondo lattice compound possessing non‐magnetic ground state and Ce2RhSi3 exhibits antiferromagnetic ordering below 7 K. A substitution of Co at Rh sites leads the system to evolve from magnetically ordered phase to a non‐magnetic phase due to Kondo effect via quantum critical point (QCP). Here, we review the electronic structure of these two compounds studied using high resolution photoemission spectroscopy. The valence band spectra of Ce2CoSi3 exhibit distinct Kondo resonance features as expected. Interestingly, the high resolution spectra of Ce2RhSi3 also reveal similar features, which are sensitive to temperature. The linewidth of the core level spectra are found to be influenced by the Kondo effect. All these results indicate the persistence of Kondo compensation below QCP (Kondo temperature, TK≠0 at QCP).

Evidence of unusual spin polarization of the surface states of W(110) surface

We studied the surface electronic structure of W(110) surface employing spin and angle resolved photoemission spectroscopy. Experimental results exhibit highly dispersive linear bands corresponding to the surface states and signature of Dirac cones. Spin resolved spectra exhibit unusual spin character of the energy bands and signature of time reversal symmetry breaking in some cases.

Study of the surface electronic structure of Si(111) surface using spin and angle resolved photoemission spectroscopy

We studied the surface electronic structure of 7×7 reconstructed Si(111) surface employing spin and angle resolved photoemission spectroscopy. Experimental spectra exhibit distinct surface bands. One of these bands crosses the Fermi level indicating metallicity of the surface of this insulating material. Spin resolved spectra do not show spin polarization of these surface states in contrast to that found recently in other systems such as topological insulators.