Andrei Sushkov

• e-mail: sushkov@physics.umd.edu
• Office:  (301) 405-6162
• Lab:       (301) 405-7278
• FAX:     (301) 314-9465
• Address: Department of Physics, University of Maryland, College Park, MD 20742


Research Associate

 

• M. S.   Moscow Physical and Technical Institute, Department of General and Applied Physics, Russia 1983
• Ph.D.   Kapitsa Institute for Physical Problems, Low-temperature Physics, Moscow, Russia 1994

Research Project: Optical properties of systems with the strongly correlated electrons: ferroelectricity in spiral magnets (multiferroics), spinels, hexa-manganites (multiferroics), high-temperature superconductors; electron-phonon and spin-(optical)phonon coupling in these materials. 

Project Overview:  

TbMn₂O₅ is a representative of a group of multiferroic materials where ferroelectricity is induced by spiral magnetic ordering at low temperatures. These materials demonstrate several weak structural and magnetic transitions reflecting complex interplay between magnetic order and the lattice. Phonons serve as a probe of lattice changes and, via spin-phonon coupling, of magnetic ordering. We measured the reflectivity spectra in a- and b-polarizations of an orthorhombic single crystal. The Tb-dominating phonons are active in both polarizations but only b-polarization shows a magnetic shift below 24 K where Tb moments start to order. We made lattice dynamics calculations using popular GULP program to understand why a particular infrared phonon is strongly coupled to spin ordering.

Geometrically frustrated magnets can resist magnetic ordering and remain in a strongly correlated paramagnetic state well below the Curie-Weiss temperature. The spin-lattice coupling can play an important role in relieving the frustration in these systems. In ZnCr₂O₄, an excellent realization of the Heisenberg antiferromagnet on the pyrochlore network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at Tc=12.5 K with a lowering of the symmetry from cubic to tetragonal. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of the Cr optical phonon triplet in ZnCr₂O₄ tracks the nearest-neighbor spin correlations above Tc. Below Tc, the triplet splits into a singlet and a doublet, separated by 11 cm^-1. This splitting gives a direct measurement of the spin-Peierls order parameter.  From analysis of the ion displacements in the phonon modes we can conclude that direct Cr-Cr exchange dominates in ZnCr₂O₄. These experiments result in a clear understanding of spin-phonon coupling in ZnCr₂O₄ in contrast to other oxide magnets. Recent ab initio calculations confirm the magnetic origin of both the phonon splitting in ZnCr₂O₄ and the frequency shifts in the ferromagnetic insulating spinel CdCr₂S₄.

The colossal magnetoresistance compounds based on doped pseudo-cubic LaMnO₃ have excited much attention because of their interesting physical properties and potential applications. YMnO₃ and LuMnO₃ are members of another series of RMnO₃ materials with smaller radius R³⁺ ions that crystallize in a hexagonal lattice. The hexagonal manganites are interesting as examples of multiferroics — they are both ferroelectric (Tc ~ 900K) and antiferromagnetic (T_N ~ 90 K) with frustration. The coupling between ferroelectric and magnetic order parameters in multiferroics opens the possibility to manipulate electric properties through magnetic fields and vice versa which, in turn, gives these materials potential for applications in spintronics and as read/write heads. These materials are also interesting because of their non-linear optical properties.

 

Papers: 

•  A.B. Sushkov, O. Tchernyshyov, W. Ratcliff II, S.W. Cheong, and H.D. Drew, Probing spin correlations with phonons in the strongly frustrated magnet ZnCr₂O₄, Phys. Rev. Lett. 94 (2005) 137202.
• A.B. Souchkov, J.R. Simpson, M. Quijada, H. Ishibashi, N. Hur, J.S. Ahn, S.W. Cheong, A.J. Millis, and H.D. Drew, Exchange interaction effects on the optical properties of LuMnO₃,  PRL 91 (2003) 027203.

Other papers: here

Invited talks:

• Probing spin correlations with phonons in the strongly frustrated magnet: ZnCr₂O₄, March Meeting of the American Physical Society, Baltimore, 2006.
• Antiferromagnetism and ferroelectricity in the hexagonal manganites: an optical study, March Meeting of the American Physical Society, Montreal, 2004.

Collaborators:

MRSEC Materials Research Science and Engineering Center at the University of Maryland, University of Maryland
Group of Prof. Sang Cheong, Rutgers University
Group of Prof. Andrew Millis, Columbia University
 

Last Modified: January 17, 2006