1,604 research outputs found
Disproportionation and Metallization at Low-Spin to High-Spin Transition in Multiorbital Mott Systems
We study the thermally driven spin state transition in a two-orbital Hubbard
model with crystal field splitting, which provides a minimal description of the
physics of LaCoO3. We employ the dynamical mean-field theory with quantum
Monte-Carlo impurity solver. At intermediate temperatures we find a spin
disproportionated phase characterized by checkerboard order of sites with small
and large spin moments. The high temperature transition from the
disproportionated to a homogeneous phase is accompanied by vanishing of the
charge gap. With the increasing crystal-field splitting the temperature range
of the disproportionated phase shrinks and eventually disappears completely.Comment: 4+ pages, 4 figure
Identification of Mackinawite and Constraints on Its Electronic Configuration Using Mössbauer Spectroscopy
The Fe(II) monosulfide mineral mackinawite (FeS) is an important phase in low temperature iron and sulfur cycles, yet it is challenging to characterize since it often occurs in X-ray amorphous or nanoparticulate forms and is extremely sensitive to oxidation. Moreover, the electronic configuration of iron in mackinawite is still under debate. Mössbauer spectroscopy has the potential to distinguish mackinawite from other FeS phases and provide clarity on the electronic configuration, but conflicting results have been reported. We therefore conducted a Mössbauer study at 5 K of five samples of mackinawite synthesized through different pathways. Samples show two different Mössbauer patterns: a singlet that remains unsplit at all temperatures studied, a sextet with hyperfine magnetic field of 27(1) T at 5 K, or both. Our results suggest that the singlet corresponds to stoichiometric mackinawite (FeS), while the sextet corresponds to mackinawite with excess S (FeS1+x). Both phases show center shifts near 0.5 mm/s at 5 K. Coupled with observations from the literature, our data support non-zero magnetic moments on iron atoms in both phases, with strong itinerant spin fluctuations in stoichiometric FeS. Our results provide a clear approach for the identification of mackinawite in both laboratory and natural environments
Quantum information analysis of electronic states at different molecular structures
We have studied transition metal clusters from a quantum information theory
perspective using the density-matrix renormalization group (DMRG) method. We
demonstrate the competition between entanglement and interaction localization.
We also discuss the application of the configuration interaction based
dynamically extended active space procedure which significantly reduces the
effective system size and accelerates the speed of convergence for complicated
molecular electronic structures to a great extent. Our results indicate the
importance of taking entanglement among molecular orbitals into account in
order to devise an optimal orbital ordering and carry out efficient
calculations on transition metal clusters. We propose a recipe to perform DMRG
calculations in a black-box fashion and we point out the connections of our
work to other tensor network state approaches
Giant resistance change across the phase transition in spin crossover molecules
The electronic origin of a large resistance change in nanoscale junctions
incorporating spin crossover molecules is demonstrated theoretically by using a
combination of density functional theory and the non-equilibrium Green's
functions method for quantum transport. At the spin crossover phase transition
there is a drastic change in the electronic gap between the frontier molecular
orbitals. As a consequence, when the molecule is incorporated in a two terminal
device, the current increases by up to four orders of magnitude in response to
the spin change. This is equivalent to a magnetoresistance effect in excess of
3,000 %. Since the typical phase transition critical temperature for spin
crossover compounds can be extended to well above room temperature, spin
crossover molecules appear as the ideal candidate for implementing spin devices
at the molecular level
Pressure and Temperature Spin Crossover Sensors with Optical Detection
Iron(II) spin crossover molecular materials are made of coordination centres switchable between two states by temperature, pressure or a visible light irradiation. The relevant macroscopic parameter which monitors the magnetic state of a given solid is the high-spin (HS) fraction denoted nHS, i.e., the relative population of HS molecules. Each spin crossover material is distinguished by a transition temperature T1/2 where 50% of active molecules have switched to the low-spin (LS) state. In strongly interacting systems, the thermal spin switching occurs abruptly at T1/2. Applying pressure induces a shift from HS to LS states, which is the direct consequence of the lower volume for the LS molecule. Each material has thus a well defined pressure value P1/2. In both cases the spin state change is easily detectable by optical means thanks to a thermo/piezochromic effect that is often encountered in these materials. In this contribution, we discuss potential use of spin crossover molecular materials as temperature and pressure sensors with optical detection. The ones presenting smooth transitions behaviour, which have not been seriously considered for any application, are spotlighted as potential sensors which should stimulate a large interest on this well investigated class of materials
Electric field control of valence tautomeric interconversion in Cobalt dioxolene
We demonstrate that the critical temperature for valence tautomeric
interconversion in Cobalt dioxolene complexes can be significantly changed when
a static electric field is applied to the molecule. This is achieved by
effectively manipulating the redox potential of the metallic acceptor forming
the molecule. Importantly our accurate density functional theory calculations
demonstrate that already a field of 0.1 V/nm, achievable in Stark spectroscopy
experiments, can produce a change in the critical temperature for the
interconversion of 20 K. Our results indicate a new way for switching on and
off the magnetism in a magnetic molecule. This offers the unique chance of
controlling magnetism at the atomic scale by electrical means
An Octanuclear Metallosupramolecular Cage Designed To Exhibit Spin-Crossover Behavior.
By employing the subcomponent self-assembly approach utilizing 5,10,15,20-tetrakis(4-aminophenyl)porphyrin or its zinc(II) complex, 1H-4-imidazolecarbaldehyde, and either zinc(II) or iron(II) salts, we were able to prepare O-symmetric cages having a confined volume of ca. 1300â
Ă
3 . The use of iron(II) salts yielded coordination cages in the high-spin state at room temperature, manifesting spin-crossover in solution at low temperatures, whereas corresponding zinc(II) salts led to the corresponding diamagnetic analogues. The new cages were characterized by synchrotron X-ray crystallography, high-resolution mass spectrometry, and NMR, Mössbauer, IR, and UV/Vis spectroscopy. The cage structures and UV/Vis spectra were independently confirmed by state-of-the-art DFT calculations. A remarkably high-spin-stabilizing effect through encapsulation of C70 was observed. The spin-transition temperature T1/2 is lowered by 20â
K in the host-guest complex
Spin Transition Sensors Based on ÎČ-Amino-Acid 1,2,4-Triazole Derivative
A ÎČ-aminoacid ester was successfully derivatized to yield to 4H-1,2-4-triazol-4-yl-propionate (ÎČAlatrz) which served as a neutral bidentate ligand in the 1D coordination polymer [Fe(ÎČAlatrz)3](CF3SO3)2·0.5H2O (1·0.5H2O). The temperature dependence of the high-spin molar fraction derived from 57Fe Mossbauer spectroscopy recorded on cooling below room temperature reveals an exceptionally abrupt single step transition between high-spin and low-spin states with a hysteresis loop of width 4 K (Tcâ = 232 K and Tcâ = 228 K) in agreement with magnetic susceptibility measurements. The material presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen, and acts as an alert towards temperature variations. The phase transition is of first order, as determined by differential scanning calorimetry, with transition temperatures matching the ones determined by SQUID and Mössbauer spectroscopy. The freshly prepared sample of 1·0.5H2O, dried in air, was subjected to annealing at 390 K, and the obtained white compound [Fe(ÎČAlatrz)3](CF3SO3)2 (1) was found to exhibit a similar spin transition curve however much temperature was increased by (Tcâ = 252 K and Tcâ = 248 K). The removal of lattice water molecules from 1·0.5H2O is not accompanied by a change of the morphology and of the space group, and the chain character is preserved. However, an internal pressure effect stabilizing the low-spin state is evidenced
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