Nanometer-scale quantum structures are attractive systems for studying new phenomenology arising from the interactions between small numbers of quantum objects. They also present challenges and opportunities in the realm of device design. My group’s research primarily utilizes the unique imaging, manipulation, and spectroscopy capabilities of low-temperature scanning tunneling microscopes (STMs) to explore the magnetic and electronic properties of quantum nanosystems at the atomic scale. We also collaborate with other groups to study these systems using additional theoretical and experimental techniques.
A brief description of this work is provided below. For more information, please visit the Hirjibehedin Research Group's web site.
In addition to its remarkable ability to both image and manipulate individual atoms, STMs can also be used to probe the spectroscopic features of nanostructures at the atomic scale. Using STM-based spin excitation spectroscopy , we can probe the collective spin excitations of magnetic nanostructures. By using the STM to position magnetic atoms next to each other with atomic-scale precision, we have also explored the evolution of Heisenberg coupling in 1D antiferromagnetic spin chains . More recently, this technique has been used to examine the interplay between Kondo screening, magnetic anisotropy, and spin coupling for high-spin atoms on surfaces [3,4,5]. We are now also exploring more complex systems, such as magnetic molecules, to understand how their interactions with the surface affect their magnetic properties .
Dopants in Semiconductors
The power of semiconductor materials, which have been at the heart of the information technology industry for more than half a century, comes from the ability to modify their electronic (and more recently magnetic) properties through the addition of impurity atoms. In the past, these dopants were added to semiconductors in bulk quantities. However, the decades-long march of Moore’s Law in shrinking the size of semiconductor devices now requires us to focus on the properties of these dopants at the single atom scale. In collaboration with a number of other groups, we are exploring the influence of dopants in conventional semiconductors like silicon and GaAs to understand their potential applications for the ultimate limit of electronic and spintronic devices.
Low-Dimensional Quantum Systems
Low dimensional electron systems, in which one or more spatial dimensions are small enough to restrict the quantum mechanical wavefunction of electrons contained inside, exhibit some of the most diverse and intriguing physical phenomena seen in all of condensed matter physics. One low-dimensional material that has received considerable interest since its discovery less than a decade ago is graphene. Made from a two-dimensional network of carbon atoms just one atom thick, this simple structure has truly remarkable properties. One of the key factors in the advancing the use of graphene in commercial devices is the ability to produce it at an industrial scale, and one of the most potentially useful processes for this is chemical vapor depositions. In collaborations with a number of other groups, we have studied the nucleation and growth of graphene on Cu surfaces .
As with other semiconductors, the properties of graphene can be readily tuned by adding charge carriers, a process known as doping. The usual method for doping graphene is via the electric-field effect. However, ten times as many charge carriers can be added by chemical doping, i.e., decorating the graphene surface with atoms. Recently, in collaboration with a number of other groups, we used STM to investigate the graphene-terminated surface of CaC6, a graphite intercalate compound (GIC) that superconducts above 10K. Surprisingly, when we imaged the surface of CaC6 above the superconducting transition temperature, we found nanometer-scale one-dimensional stripes that correspond to a charge density wave (CDW) , a low-dimensional charge-ordered state that is often found in materials that exhibit superconductivity.
Another way to form a low dimensional electron system is to confine charge carriers in the layers of a semiconductor heterostructure. One of the most powerful ways to study the collective excitations of such systems is with inelastic light scattering (often called Raman scattering). This technique can provide a unique way to probe the emergent states that arise in these systems from many-body interactions, and has been used to explore the collective excitations of fractional Quantum Hall liquids [9,10,11], as well as the strongly correlated regime in ultra-low-density electron gases .
 A.J. Heinrich et al., Single-Atom Spin-Flip Spectroscopy, Science 306, 466 (2004)
 C.F. Hirjibehedin et al., Spin Coupling in Engineered Atomic Structures, Science 312, 1021 (2006)
 C.F. Hirjibehedin et al., Large Magnetic Anisotropy of a Single Atomic Spin Embedded in a Surface Molecular Network, Science 317, 1199 (2007)
 A.F. Otte et al., The role of magnetic anisotropy in the Kondo effect, Nature Physics 4, 847 (2008)
 A. F. Otte et al., Spin Excitations of a Kondo-Screened Atom Coupled to a Second Magnetic Atom, Physical Review Letters 103, 107203 (2009)
 T. Palamarciuc et al., Spin crossover materials evaporated in clean high vacuum and ultra-high vacuum conditions: from thin films to single molecules, Journal of Materials Chemistry 22, 9690 (2012)
 H.K. Kim et al., Activation energy paths for graphene nucleation and growth on Cu, ACS Nano 6, 3614 (2012)
 K.C. Rahnejat et al., Charge density waves in the graphene sheets of the superconductor CaC6, Nature Communications 2, 558 (2011)
 C.F. Hirjibehedin et al., Crossover and Coexistence of Quasiparticle Excitations in the Fractional Quantum Hall Regime at nu <= 1/3, Physical Review Letters 91, 186802 (2003)
 C.F. Hirjibehedin et al., Splitting of Long-Wavelength Modes of the Fractional Quantum Hall Liquid at nu <= 1/3, Physical Review Letters 95, 066803 (2005)
 T.D. Rhone et al., Higher-Energy Composite Fermion Levels in the Fractional Quantum Hall Effect, Physical Review Letters 106, 096803 (2011)
 C.F. Hirjibehedin et al., Evidence of Electron Correlations in Plasmon Dispersions of Ultralow Density Two-Dimensional Electron Systems, Physical Review B 65, 161309(R) (2002)