Cyrus Hirjibehedin

Spectroscopy of quantum nanostructures
Low-temperature scanning tunneling microscopy
Spin-coupling, anisotropy, and frustration at the atomic scale
Crossover in magnetism from quantum to classical regimes
Low dimensional electron systems, including graphene
Kondo screening and quantum interactions at the atomic scale

Contact details:
Office: 2C3
Telephone: +44 (0)20 7679 2617
Fax: +44(0)20 7679 0595
Email: c.hirjibehedin

ucl.ac.uk

Research interest
I am fascinated by the physics and chemistry of nanometer-scale quantum systems. These structures are attractive for studying new phenomenology arising from the interactions between small numbers of quantum objects and 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, such as density functional theory (DFT) and x-ray magnetic circular dichroism (XMCD).

Other activities
Member of the American Physical Society

Recent publications
In addition to its remarkable ability to both image and manipulate individual atoms, a scanning tunnelling microscope (STM) can also be used to probe the spectroscopic features of nanostructures at the atomic scale. The results presented in these papers are obtained using a technique called spin-excitation spectroscopy, first reported by A.J. Heinrich et al. in Science 306, 466 (2004) [PDF file]. By utilizing the inelastic tunnelling capabilities of an STM to probe the collective spin excitations of magnetic nanostructures, we have found that individual magnetic atoms adsorbed on a thin copper nitride layer display very large magnetic anisotropy, similar to that seen in molecular magnets. 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.

Cyrus F. Hirjibehedin, Chiung-Yuan Lin, Alexander F. Otte, Markus Ternes, Christopher P. Lutz, Barbara A. Jones, Andreas J. Heinrich, Large Magnetic Anisotropy of a Single Atomic Spin Embedded in a Surface Molecular Network, Science 317, 1199 (2007) [PDF file]

Cyrus F. Hirjibehedin, Christopher P. Lutz, and Andreas J. Heinrich, Spin Coupling in Engineered Atomic Structures, Science 312, 1021 (2006) [PDF file]

A more general overview of this work can be found in the reviews on the footnote on this page*.

Another form of inelastic spectroscopy that can be used to probe quantum structures is inelastic light scattering (often called Raman scattering). For strongly interacting low dimensional electron systems in semiconductor heterostructures, this probe provides a unique way to study the emergent states that arise from many-body interactions. This has been used to explore the collective excitations of fractional Quantum Hall liquids, as well as the strongly correlated regime in ultra-low-density electron gases.

C.F. Hirjibehedin, Irene Dujovne, A. Pinczuk, B.S. Dennis, L.N. Pfeiffer, K.W. West, Splitting of Long-Wavelength Modes of the Fractional Quantum Hall Liquid at nu <= 1/3, Physical Review Letters 95, 066803 (2005) [PDF file]

C.F. Hirjibehedin, A. Pinczuk, B.S. Dennis, L.N. Pfeiffer, K.W. West, Crossover and Coexistence of Quasiparticle Excitations in the Fractional Quantum Hall Regime at nu <= 1/3, Physical Review Letters 91, 186802 (2003) [PDF file]

C.F. Hirjibehedin, A. Pinczuk, B.S. Dennis, L.N. Pfeiffer, K.W. West, Evidence of Electron Correlations in Plasmon Dispersions of Ultralow Density Two-Dimensional Electron Systems, Physical Review B 65, R161309 (2002) [PDF file]

In the future, it may be possible to perform similar studies of interacting electron systems at the atomic scale on novel surface-exposed low dimensional electron systems, such as graphene.

Biography

Lecturer, University College London (2007-present)
Postdoctoral Scientist, IBM Almaden Research Center (2004-2007)
Ph.D. in Physics, Columbia University (2004)
M.Phil. in Physics, Columbia University (2000)
M.A. in Physics, Columbia University (1999)
B.S. in Physics and Computer Science, Stanford University (1997)

Research
An STM image of a single chain of 10 Mn atoms on a copper nitride patch, with a single Mn atom on the side for reference [after C.F. Hirjibehedin et al., Science 312, 1021 (2006)]. The chain was built one atom at a time, with each atom placed precisely by the STM. The magnetic coupling between the atoms can then be studied along the chain as it is built.

Density functional theory calculation of the charge density of two cross-sections of slab of Copper Nitride on a Copper surface with a single Fe atom placed on top [after C.F. Hirjibehedin et al., Science 317, 1199 (2007)]. The scale for the charge density is shown on the bottom. The system is reminiscent of molecular magnets, but has the added advantage of being controllable in-situ at the atomic-scale with scanning probes.

Visualization of the spin excitations for an Iron atom on a copper nitride surface [image courtesy IBM].

* A more general overview of this work can be found in the following reviews:

Harald Brune, News and Views: Probing magnetism at the nanoscale, Nature Nanotechnology 2, 674 (2007) [PDF file]
Mark Wilson, Search and Discovery: Scanning tunneling microscope measures the spin-excitation spectrum of atomic-scale magnets, Physics Today 59, 13 (2006).
Pietro Gambardella, News and Views: Quantum chains with a spin, Nature Materials 5, 431 (2006) [PDF file]
Harald Brune, Perspectives: Assembly and Probing of Spin Chains of Finite Size, Science 312, 1005 (2006) [PDF file]

Positions
There are currently openings for Ph.D. candidates. For more information, please contact Dr. Hirjibehedin directly.