44 research outputs found
Theory of Room Temperature Ferromagnet V(TCNE)_x (1.5 < x < 2): Role of Hidden Flat Bands
Theoretical studies on the possible origin of room temperature ferromagnetism
(ferromagnetic once crystallized) in the molecular transition metal complex,
V(TCNE)_x (1.5<x<2) have been carried out. For this family, there have been no
definite understanding of crystal structure so far because of sample quality,
though the effective valence of V is known to be close to +2. Proposing a new
crystal structure for the stoichiometric case of x=2, where the valence of each
TCNE molecule is -1 and resistivity shows insulating behavior, exchange
interaction among d-electrons on adjacent V atoms has been estimated based on
the cluster with 3 vanadium atoms and one TCNE molecule. It turns out that
Hund's coupling among d orbitals within the same V atoms and antiferromagnetic
coupling between d oribitals and LUMO of TCNE (bridging V atoms) due to
hybridization result in overall ferromagnetism (to be precise, ferrimagnetism).
This view based on localized electrons is supplemented by the band picture,
which indicates the existence of a flat band expected to lead to ferromagnetism
as well consistent with the localized view. The off-stoichiometric cases (x<2),
which still show ferromagnetism but semiconducting transport properties, have
been analyzed as due to Anderson localization.Comment: Accepted for publication in J. Phys. Soc. Jpn. Vol.79 (2010), No. 3
(March issue), in press; 6 pages, 8 figure
Magnetic Bistability and Nucleation of Magnetic Bubbles in a Layered 2D Organic-Based Magnet [Fe(TCNE)(NCMe)(2)][FeCl(4)]
The 2D layered organic-based magnet [Fe(TCNE)(NCMe)2][FeC 14] (TCNE=tetracyanoethylene) exhibits a unique macroscopic magnetic bistability between the field-cooled and zero-field-cooled states, which cannot be explained by either superparamagnetic behavior or spin freezing due to spin glass order. This magnetic bistability is described through consideration of the ensemble of uncoupled 2D Ising layers and their magnetization reversal initiated by a field-induced nucleation of magnetic bubbles in individual layers. The bubble nucleation rate strongly depends on the external field and temperature resulting in anomalous magnetic relaxation.open7
Suppression of a charge density wave ground state in high magnetic fields: spin and orbital mechanisms
The charge density wave (CDW) transition temperature in the quasi-one
dimensional (Q1D) organic material of (Per)Au(mnt) is relatively low
(TCDW = 12 K). Hence in a mean field BCS model, the CDW state should be
completely suppressed in magnetic fields of order 30 - 40 T. To explore this
possibility, the magnetoresistance of (Per)Au(mnt) was investigated in
magnetic fields to 45 T for 0.5 K < T < 12 K. For fields directed along the Q1D
molecular stacking direction, TCDW decreases with field, terminating at about ~
37 T for temperatures approaching zero. Results for this field orientation are
in general agreement with theoretical predictions, including the field
dependence of the magnetoresistance and the energy gap, .
However, for fields tilted away from the stacking direction, orbital effects
arise above 15 T that may be related to the return of un-nested Fermi surface
sections that develop as the CDW state is suppressed. These findings are
consistent with expectations that quasi-one dimensional metallic behavior will
return outside the CDW phase boundary.Comment: 12 pages, 5 figure
Optical excitations in quarter-filled Ni(dmit)2 based compound described by a dimerized octamer model
The reflectance spectra of [SMe3][Ni(dmit)(2) (dmit=1,3-dithiole-2 thione-4,5-dithiolato), quarter-filled organic conductor with strongly dimerized structure, will be discussed in the framework of the new Raman results for this compound. The spectra are interpreted on the basis of a dimerized Hubbard octamer model, which accounts for the low energy electronic transitions and for the coupling of the electronic excitations with the intramolecular vibrations
Modeling of the Process of Welding Aerosol Formation Taking Place During Mining Equipment Fabrication
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Electrospun a-Si using Liquid Silane/Polymer Inks
Amorphous silicon nanowires (a-SiNWs) were prepared by electrospinning cyclohexasilane (Si{sub 6}H{sub 12}) admixed with polymethylmethacrylate (PMMA) in toluene. Raman spectroscopy characterization of these wires (d {approx} 50-2000 nm) shows 350 C treatment yields a-SiNWs. Porous a-SiNWs are obtained using a volatile polymer
Multiple photonic responses in films of organic-based magnetic semiconductor V(TCNE)(x), x similar to 2
Concomitant photoinduced magnetic and electrical phenomena are reported for the organic-based magnetic semiconductor V(TCNE)(x) (x similar to 2; TCNE=tetracyanoethylene; magnetic ordering temperature T-c similar to 400 K). Upon optical excitation (457.9 nm), the system can be trapped in a thermally reversible photoexcited state, which exhibits reduced magnetic susceptibility and increased conductivity with a simultaneous change in IR absorption spectrum. The multiple photonic effects in V(TCNE)(x) are proposed to originate from structural changes induced by internal excitation in (TCNE)(-) anions, which lead to relaxation to a long-lived metastable state.open242