Current Projects

Hybrid Nanocrystals

The advancement of nanotechnology lies in the design of new materials that exhibit novel or superior properties to the systems currently available. Colloidal nanocrystals (NCs) represent very attractive building blocks to create new materials with tuneable chemical and physical properties and as such they have a great potential to produce multi- functional systems with enhanced technological added value. Demonstrated examples entail novel nanoscale architectures of core/shell or dimer type fluorescent and magnetic particles, metal and semiconductor tipped nanorods, as well as bimetallic particles. We investigate the chemical specificity of molecule-based, structure directing agents in allowing control over the nanocrystal size/shape and study the finite-size effects on the physical properties of such nanoscale systems. On another account NCs are assembled into secondary structures (superlattices) with the aim to harness size-dependent properties and tune collective phenomena arising from subunit interactions.

Geometrical Frustration

Frustration arises when a system cannot minimize all the pair-wise interactions simultaneously because of local geometric constraints. Competing or frustrated interactions extend beyond the condensed matter problems and into biological materials. In that respect, nature has the ability to "resolve" frustrated interactions in order to perform specific biological activity.

Frustration may give-rise to novel and complex phenomena that motivate us to (i) develop new class of materials (ii) study cooperative phenomena in magnetism that provide fertile ground for testing theories of interacting systems that possess different spatial dimensions, ranges, and sign of interactions, and that exhibit local anisotropy of the basic interacting unit, the spin.

Molecule-Based Materials

The versatility of the structure and bonding in molecule-based systems enables one to synthesise materials exhibiting dual properties, such as superconductivity and magnetism, or coupling of properties e.g. optical sensitivity and magnetism, that is currently finding use in magneto-optical devices. Of both fundamental and applied interest to the materials science community is the search for alternative molecular components to create efficient magnetic linkages between paramagnetic lattice sites. We explore such possibilities by investigating the crystal chemistry of novel materials based on shared bridging "cyano moieties" and their derivatives.

Chemical models: M[(N(CN₂)]₂, M^II= transition metal.

Valence Manipulation

The reductive intercalation of chemically diverse systems with alkali metals has demonstrated that a spectacular transformation of the electronic ground state, from a magnetic insulator to a metal or superconductor, is possible on certain occasions. Valence manipulation by topotactic insertion methods can lead to chemical compounds displaying novel physical behaviour. The interplay of structural and electronic properties finds important examples among solids with topologies ranging from zero- (0D) to three- (3D) dimensional lattices. Their capacity for multifunctional capability depends on the host lattice's potential to experience variable perturbations with respect to its crystal chemistry and its electronic properties. We combine the effects of "intrinsic" chemical pressure induced by cation/anion substitution against "extrinsic" parameters such as the applied hydrostatic pressure on the parent (undoped) compound. This allows shifting the phase boundary between localised/magnetic and itinerant behaviour through the control of the on-site Coulomb repulsion (U) and bandwidth (W) in strongly correlated electron systems.

Chemical model: Li_xMo₂SbS₂.

The Power of Light

An all optical pump-probe method, using ultrashort photo-magnetic pulses (170 fs, at 800 nm), has been developed in order to study the ultrafast spin dynamics of single crystalline antiferromagnets over a broad temperature range. We envisage the technique as a flexible tool, as possible to adapt to the needs of fast physico-chemical processes involving classic and quantum states of chemically diverse systems.

Our results on orthoferrites (LnFeO₃, Ln= Er, Y) show that this method is an efficient means for thermal control of the ultrafast oscillations of the antiferromagnetic spins on the local scale; the effects are manifested via the changes in the time-resolved (ps time-scale) linear magnetic birefringence of the materials. We demonstrate that a variant of this methodology, pertaining to the inverse Faraday effect, also achieves non-thermal control of the light-induced spin dynamical phenomena. We discuss how such manipulation of the magnetisation on the nanoscale, combined with the exchange-bias effect can play a key role towards the design of ultrafast magneto-optical switches in future storage devices.

Magnetism in Low-Dimensions

One of the new frontiers in condensed-matter physics lies in the field of critical behaviour, especially in "dirty" low-dimensional spin-gap systems involving quantum impurities. Their fascinating and adverse properties allows one to address unresolved but interrelated issues, for example, the nature of the quasi-particle excitations in strongly correlated electronic solids, such as the high-temperature superconductors. Unfortunately only a small number of inorganic chemical compounds with such structural features have been examined so far. We search for chemical compounds with unusual low-dimensional cation arrangement. The influence of either the spin or charge degrees of freedom and the effect of their lattice topology are examined as crucial parameters for the fundamental understanding of the phase transformations involved and deviations from the possibly gapped ground state.

Chemical models: PbNi₂V₂O₈ (1D Ni-chains), SrCu₂(BO₃)₂ (2D Cu-planes).

Recent Publications

[1] C. Stock, L. C. Chapon, O. Adamopoulos, A. Lappas, M. Giot, J.W. Taylor, M. A. Green, C. M. Brown, and P. G. Radaelli

"One-Dimensional Magnetic Fluctuations in the Spin-2 Triangular Lattice α-NaMnO₂"

Phys. Rev. Lett. 2009, 103, 077202-4.

[2] M. Giot, L.C. Chapon, J. Androulakis, M.A. Green, P.G. Radaelli, and A. Lappas,

“Magnetoelastic Coupling and Symmetry Breaking in the Frustrated Antiferromagnet α-NaMnO₂”

Phys. Rev. Lett. 2007, 99, 247211-4.

[3] A. Lappas, C.J. Nuttall, Z. Fthenakis, V. Pomjakushin, and M.A. Roberts,

"Topotactic Intercalation of a Metallic Dense Host Matrix Chalcogenide with Large Electron-Phonon Coupling: Crystal Structures and Electronic Properties of Li_xMo₂SbS₂ (0<=x<0.7)"

Chem. Mater. 2007, 19, 69-78.

[4] A. Zorko, D. Arcon, A. Lappas, and Z. Jaglicic,

"Magnetic versus Non-magnetic Doping Effects in the Haldane Chain Compound PbNi₂V₂O₈"

New J.  Phys. 2006, 8, 60-77.

[5] A. Lappas, A. Zorko, E. Wortham, R.N. Das, E.P. Giannnelis, P. Cevc, and D. Arcon,

"Low-Energy Magnetic Excitations and Morphology in Layered Hybrid Perovskite-Poly(dimethylsiloxane) Nanocomposites"

Chem. Mater.  2005, 17, 1199-1207.

[6] S. Mistry, D.C. Arnold, C.J. Nuttall, A. Lappas, and M.A. Green,

"A New Series of Sodium Cobalt Oxyhydrates"

Chem. Commun. 2004, 2440-2441.

[7] I. Mastoraki, A. Lappas, J. Giapintzakis, D. Tobbens, and J. Hernandez-Velasco,

"Relations of Crystal Structure to Magnetic Properties in the Quasi-one-dimensional Compound PbNi_1.88Mg_0.12V₂O₈"

J. Solid State Chem. 2004, 177, 2404-2414.

[8] A. Lappas, A. Schenck, and K. Prassides,

"Spin-Freezing in the S=1/2 Two-Dimensional Spin-Gap Systems SrCu_2-xMg_x(BO₃)₂ (x=0, 0.04, 0.12)"

Physica B 2003, 326, 431-435.

[9] D. Arcon, A. Zorko, and A. Lappas,

"⁵¹V NMR Study of the Doped Chain Compounds PbNi_2-xMg_xV₂O₈"

Europhys. Lett. 2004, 65, 109-115.

[10] K. Papagelis, J. Arvanitidis, I. Margiolaki, K. Brigatti, K. Prassides, A.Schenck, A. Lappas, A. Amato, Y. Iwasa, and T. Takenobu,

"mSR Studies of the Superconducting MgB_1.96C_0.04"

Physica B 2003, 326, 346-349.

[11] A. Lappas, A.S. Wills, M.A. Green, K. Prassides, and M. Kurmoo,

"Magnetic Ordering in the Rutile Molecular Magnets M^II[N(CN)₂]₂ (M= Ni, Co, Fe, Mn, Ni_0.5Co_0.5 and Ni_0.5Fe_0.5) "

Phys. Rev. B 2003, 67, 144406-8.

[12] A. Lappas, V. Alexandrakis, J. Giapintzakis, V. Pomjakushin, K. Prassides, and A. Schenck

"Impurity-Induced Antiferromagnetic Order in the Haldane-Gap Compound PbNi_2-xMg_xV₂O₈ (x=0.24)"

Phys. Rev. B 2002, 66, 14428-8.

[13] E. Wortham, A. Zorko, D. Arcon, and A. Lappas,

"Organic-Inorganic Perovskites for Magnetic Nanocomposites"

Physica B 2002, 318, 387-391.

[14] A. Lappas, J.E.L. Waldron, M.A. Green, and K. Prassides,

"Magnetic Ordering in the Charge-Ordered Nb₁₂O₂₉"

Phys. Rev. B 2002, 65; 134405-7.

[15] A. Zorko, D. Arcon, A. Lappas, and J. Giapintzakis,

"Near Critical Behaviour in the Two-dimensional Spin-Gap System SrCu₂(BO₃)₂"

Phys. Rev. B 2001, 65, 024417-6.

[16] A. Lappas, K. Prassides, F.N. Gygax, and A. Schenck,

"Magnetic and Structural Instabilities in the Stripe-Phase Region of in La_1.875Ba_0.125-ySr_yCuO₄ (0<=y<0.1)"

J. Phys.: Condens. Matter 2000, 12, 3401-3422.