Optical spectroscopy at UNF
Research by Dr. Lev Gasparov
Dr. Gasparov’s laboratory at the
University of North Florida focuses on the spectroscopic studies of the
Correlated Electron Materials. These materials represent an intermediate case
of solids between those that have itinerant electrons and those with localized
electrons. A complete description of systems with strong electron correlations
is an ongoing challenge to the solid state physics community. However
correlated electron materials may have myriads of industrial applications.
Recent expansion in the field of memory storage such as solid state hard drives
is an example of applications that came from correlated electron system
The main goal of the Dr.
Gasparov’s research is to reveal, investigate, and explain many unconventional
phase transitions that are found in the correlated electron materials. Low
temperature-, ambient-, and high hydrostatic pressure- Raman spectroscopy is
the major tool used in the research.
Research Assistantship for Students
Enquires may be directed to Dr. Gasparov.
UNF students during 2010
TLO trip to the II. Institute of Physics of the RWTH University in Aachen,
T64000 triple Raman spectrometer equipped with the liquid nitrogen cooled CCD Detector. We use Innova 70 Argon ion laser to excite the Raman signal.
Low temperature capabilities are provided by the Oxford Instruments Variox helium bath optical cryostat and R.G. Hansen and Associated high-tran helium flow optical cryostat. UNF high pressure diamond anvil cell is capable of producing hydrostatic pressure up to 20 GPa (~200,000 atm).
Magnetite (Fe3O4) is a naturally occurring mineral interesting for remarkably different fields of science. It is the first magnetic material known to mankind and it is the earliest compound known to manifest charge-ordering transition discovered by Verwey in 1939. At the same time magnetite is an integral part of many live objects. For instance, magnetotactic bacteria and pigeons use it for navigation along the Earth’s magnetic field. Furthermore, it was reported that magnetite occurs in human brains and it may play a role in pathogenesis of the neurodegenerative diseases such as Alzheimer.
Magnetite undergoes a first-order transition (the Verwey transition) at TV = 120 K, with changes of crystal structure, latent heat, and a two-order of magnitude decrease of the DC-conductivity. In spite of decades of research, the nature of this transition is still an open question. The project seeks to establish a correlation between the state dependence of the elementary excitations and the properties of the magnetite through the Verwey transition.
properties were known to mankind since antiquity. A compass made out
of magnetite was used by the ancient Chinese to navigate the seas (Li Shu-hua,
Isis, 45, 175 (1954)). In condensed
matter physics, magnetite is considered to be the earliest compound manifesting
a metal to insulator transition, known as Verwey transition (E. J. W. Verwey,
Nature (London) 144, 327 (1939)). In pure stoichiometric magnetite the
transition takes place around 120K. It affects electric, magnetic and
structural properties of the compound. In spite of decades of research, the
mechanism of transition remains an unsolved puzzle.
The ability to change
temperature and pressure allows one to reveal a comprehensive
pressure-temperature phase diagram of magnetite that is critical for the
understanding of the transition’s mechanism. Existing literature offers
diverging opinions on the effect of pressure on the transition with some
publications suggesting the altogether disappearance of the transition above
certain pressure and other reports indicating a gradual decrease of the transition
temperature with pressure without the disappearance of the transition. The
reasons for such controversy have been debated in a number of publications
without any convergence of the opinions.
The ambiguous nature of
existing experimental results could be explained by doping: the intentional or inadvertent
introduction of impurities. Metal dopants can be easily introduced into the
magnetite during crystal growth. This idea was tested in a recent UNF study of
pure and aluminum doped magnetite (Gasparov et al., J. Appl. Phys. 112,
The following figure summarizes the results. In particular, it displays
how the Verwey transition temperature changes with pressure in a pure and 2%
Al-doped magnetite. Open symbols indicate low temperature (“insulator”) phase
of magnetite; filled symbols indicate the high temperature (“metal”) phase of
magnetite. Squares indicate pure magnetite data, circles correspond to Al-doped
sample, and the stars represent the X-ray data of Rosenberg et al. (Phys. Rev.
Lett. 96, 045705-1 (2006)).
The figure clearly
displays two dramatically different pressure dependencies of the transition
temperature for pure magnetite (short dash line) and Al-doped magnetite (dash
Strong doping effect on the
Pressure-vs.-Transition temperature (PT) line is an indicator of important role
played by charge ordering because doping is the major disruption of such order.
This in turn is an argument in favor of a new charge-ordering based mechanism
of transition. New data on the magnetite samples with different degree of
doping or different doping elements are critical to corroborate such charge
ordering mechanism. This is the subject of ongoing research at the UNF
spectroscopy lab of Dr. Gasparov.
- Geophysical laboratory, Carnegie
Institution, Washington DC, USA
- University of Florida, Gainesville, USA
- University of Arkansas, Fayetteville, USA
- University of Technology, Troyes, France
- II. Institute of
Physics (II. Physikalisches Institut) RWTH Aachen, Germany
- Swiss Federal Institute of Technology (Ecole
Polytechnique Federal De Lausanne), Lausanne, Switzerland