Showing papers on "Spin states published in 1994"
TL;DR: The light-induced excited spin state trapping (LIESST) as mentioned in this paper phenomenon is well understood within the theoretical context of radiationless transitions and applications of the LIESST effect in optical information technology can be envisaged.
Abstract: Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mossbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping - acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.
1,796 citations
428 citations
TL;DR: In this paper, the spin adaptation described in previous work was applied to remove the spin contamination in both coupled cluster amplitudes and energy, both for linear and nonlinear versions of the single and double excitation coupled cluster (CCSD) method.
Abstract: The coupled clusters (CC) method for effective calculations of open shell systems with the single restricted Hartree–Fock (ROHF) reference determinants is formulated We apply the spin adaptation described in our previous work, aimed at removing the spin contamination in both coupled cluster (CC) amplitudes and CC energy, both for the linear and nonlinear versions of the single and double excitation coupled cluster (CCSD) method We give a comparison of results with adapted and nonadapted methods Together with the elimination of the spin contamination, our spin adaptation yields CC equations, which reduce the number of CC amplitudes and the number of arithmetical operations, which results in computational time comparable with analogous closed shell calculations The complication in the full spin adapted CC method is that individual spin states require specific spin adapted excitations, so that for every spin state, a different implementation is needed It is possible to define a very effective approximation to the full spin adapted model that is common to all half‐spin states The method is less complicated in implementation, while the computational requirements increase in comparison with the full spin adapted method only slightly We also suggest how to introduce spin adaptation in a very simple way to existing ROHF CCSD programs
76 citations
Patent•
IBM1
TL;DR: The spin flip-flop as discussed by the authors is a quantum logic and memory element that is suitable for use as a quantum computer, and it can be used to store the up-down and down-up spin states of two single electrons placed into two neighboring electronic quantum dots.
Abstract: There is provided by this invention logic and memory elements of atomic or near-atomic scale useful in computer central processing units. These elements consist of two quantum dots having opposite states and a third quantum dot situated between the two quantum dots and in physical contact with them. The third quantam dot is of a material which makes the opposite states of the first two quantum dots energetically favorable. In particular, there is provided by the invention a spin flip-flop suitable for use as electronic logic and memory in a quantum computer. The spin flip-flop is designed to have two highly stable states, encoded entirely in the arrangement of electronic spins in the structure. Switching between the two states is accomplished by fast electromagnetic pulsing generally and by optical pulsing in the case of the spin flip-flop. The two stable states are the up-down and the down-up spin states of two single electrons placed into two neighboring electronic quantum dots typically by doping or by a field effect. The operation of the device is facilitated and stabilized by the presence of a small particle or dot of an antiferromagnetic material placed between the two electronic dots, and in physical contact with both of them.
54 citations
TL;DR: In this paper, the magnetic behavior of cobalt and copper in the 112-phase YBa (Co2-xCux)O5 has been determined from a XANES analysis: copper is mainly in the Cu(II) state whereas cobalt appears in a mixed valent state Co(II)/Co(III) in agreement with the oxygen stoichiometry.
53 citations
52 citations
TL;DR: In this paper, the influence of excited levels on the nonlinear transport properties of a quantum dot weakly coupled to ideal leads is studied using a master-equation approach using a quantum-mechanical model for interacting electrons.
Abstract: The influence of excited levels on the non-linear-transport properties of a quantum dot weakly coupled to ideal leads is studied using a master-equation approach. A quantum-mechanical model for interacting electrons is used to determine the spectrum of the dot. The current-voltage characteristic shows Coulomb blockade and additional fine structure that is related to the excited states of the correlated electrons. Asymmetric coupling to the leads causes asymmetric conductance peaks. It is demonstrated that spin selection rules can lead to regions of negative differential conductance.
47 citations
TL;DR: Analysis of the core-level spectrum in terms of a configuration interaction model suggests that both low- and high-spin states coexist at the low temperature and that there is a decrease in the low-spin contribution at 573 K related to local structural changes.
Abstract: We report the valence-band and Co 2p photoemission spectra of $LaCoO_3$ obtained at different temperatures (100, 300, and 573 K). Analysis of the core-level spectrum in terms of a configuration interaction model suggests that both low- and high-spin states coexist at the low temperature (100 K). It also indicates that there is a decrease in the low-spin contribution at 573 K related to local structural changes. Photoemission spectra of the valence-band region further support this interpretation.
40 citations
TL;DR: An interpretation of recent cyclotron resonance experiments on the two-dimensional electron gas in GaAs/AlGaAs heterostructures is presented and an $f$-sum rule is derived which allows spin polarisation to be determined directly from resonance spectra.
Abstract: We present an interpretation of recent cyclotron resonance experiments on the two-dimensional electron gas in GaAs/AlGaAs heterostructures. We show that the observed dependence of the resonance spectrum on Landau level occupancy and temperature arises from the interplay of three factors: spin splitting of the cyclotron frequency, thermal population of the two spin states, and coupling of the resonances for each spin orientation by Coulomb interactions. In addition, we derive an f-sum rule which allows spin polarization to be determined directly from resonance spectra.
34 citations
TL;DR: In this article, the machinery of the q-deformation of su(2) was used to obtain generalisations of spin chains with exact valence-bond ground states, such as the spin-1 VBS model and spin-half Majumdar-Ghosh model.
Abstract: Quantum spin chains with exact valence-bond ground states are of great interest in condensed-matter physics. A class of such models was proposed by Affleck et al., each of which is su(2)-invariant and constructed as a sum of projectors onto definite total spin states at neighboring sites. We propose to use the machinery of the q-deformation of su(2) to obtain generalisations of such models, and work out explicitly the two simplest examples. In one case we recover the known anisotropic spin-1 VBS model while in the other we obtain a new anisotropic generalisation of the spin-½ Majumdar-Ghosh model.
33 citations
TL;DR: In this paper, the magnetic coupling constant has been obtained by computing ab initio wave functions for different spin states, which explicitly include internal and external correlation effects, including the delocalization of magnetic orbitals, the external correlation, and the collective effects normally hidden in the two body operator of the Heisenberg Hamilt.
Abstract: Cluster models of increasing complexity have been used to model magnetic interactions in KNiF3. These clusters contain two or four magnetic centers plus the bridge F− anions and different representations of the remaining of the crystal. The magnetic coupling constant has been obtained by computing ab initio wave functions for different spin states. These wave functions explicitly include internal and external correlation effects. Several sets of Gaussian functions have been tested and many sets of molecular orbitals have been considered in order to study the physical origin of magnetism in KNiF3. The calculated magnetic coupling constant differs from model to model but shows a fairly good convergence to the experimental result. The use of different cluster models permits to separate the magnetic coupling constant in several contributions. These are the delocalization of magnetic orbitals, the external correlation, and the collective effects normally hidden in the two body operator of the Heisenberg Hamilt...
TL;DR: By comparing results of Mossbauer spectroscopy and X-ray diffraction investigations of ultrafine Fe and Fe-Ni particles, a formerly stated supposition on the existence of two spin states of the fcc phase in pure iron and Fe -Ni alloys has been verified as mentioned in this paper.
TL;DR: In this article, the photoinduced short circuit current I SC was found to be dependent on the spin polarization in the space charge region of the A1/PPV/ITO-photodiode, and a large decrease in I SC of up to ten percent was observed in the temperature range between 1.5 and 293 K when ESR conditions were fulfilled.
TL;DR: The Fe II (TRIM) 2 (PhCO 2 )(ClO 4 ) spin crossover complex has been synthesized and investigated by Mossbauer spectroscopy and magnetic susceptibility measurements as a function of temperature as mentioned in this paper.
TL;DR: In this article, a new Haldane state chain compound with a magnetic excitation gap Δmin/kB of 100±5K and Δmin|J|≈0.3 was identified.
Abstract: We have identified Y2BaNiO5 as a new Haldane state chain compound with a magnetic excitation gap Δmin/kB of 100±5K and Δmin/|J|≈0.3. Single crystal results of χ(T→0) reveal a splitting of the excited magnetic state. The S=1 chains have been severed in a controlled way by the substitution of Zn or Mg for Ni, and the resulting modifications of χ(T) are presented.
TL;DR: In this article, the nuclear spin-lattice relaxation in paramagnetic systems is treated using the classic expression for transition probability between the coupled electron and nuclear spin states, and the rate equations governing the incoherent occupancies of these states are solved analytically and numerically (where not) to construct the relaxation function.
Abstract: Nuclear spin-lattice relaxation in paramagnetic systems is treated using the classic expression for transition probability between the coupled electron and nuclear spin states. The rate equations governing the incoherent occupancies of these states are solved analytically (where possible) and numerically (where not) to construct the relaxation function for the nuclear spin. The method is illustrated for muonium, and the muonium-substituted molecular radicals, for the case of perturbation due to fluctuation of the local field,i.e. modulation of the interaction with a third spin. A slight departure from single exponential behaviour is demonstrated for slow fluctuations.
TL;DR: In this article, the odd-odd nucleus 214 Fr has been studied using γ-ray and electron spectroscopy following 208 Pb(11 B, 5n) and 205 Tl(13 C, 4n) reactions.
TL;DR: High spin states in the neutron deficient isotopes Ru90 and Ru91 have been studied in the 2p2n and 2pn exit channels of the reaction Ar-36 + Ni-58 using the 149 MeV Ar- 36 beam at VICKSI and the OSIRIS array by gating the gammagamma coincidences with evaporated neutrons and protons.
Abstract: High spin states in the neutron deficient isotopes Ru90 and Ru91 have been studied in the 2p2n and 2pn exit channels of the reaction Ar-36 + Ni-58 using the 149 MeV Ar-36 beam at VICKSI and the OSIRIS array. By gating the gammagamma coincidences with evaporated neutrons and protons, excited states in Ru90 were identified for the first time and the level scheme of Ru-91 was extended. In both nuclei, the level energies and branching ratios follow the predictions of shell model calculations performed within the (1g9/2,2p1/2) single-particle space.
TL;DR: In this article, a modified Hartmann-Hahn type of cross-polarization experiment is performed with varying amplitudes of the radiofrequency field in a descending or ascending order, and the initial spin state evolves quasi-adiabatically in a zero-quantum space to a final state with a larger polarization transfer than that under the matched Hartmann Hahn condition, which is predicted by a spin flip-flop model to have the most efficient polarization transfer.
TL;DR: In this paper, it was shown that the highest occupied energy levels are well separated from empty levels, and up to a maximum of eight electrons can be unpaired, giving a maximum spin state with S = 4.
Abstract: Spin-polarized Xα–SW calculations of [Fe6(μ3−S)8(PH3)6]2+ as a model of the cluster [Fe6(μ3−S)8(PEt3)6] (BPh4)2 have been performed. The highest occupied energy levels are well separated from empty levels, and up to a maximum of eight electrons can be unpaired, giving a maximum spin state with S = 4. This electronic state is consistent with the magnetic data of [Fe6(μ3−S)8(PEt3)6](BPh 4)2, which have been interpreted using the Heisenberg–Dirac–Van Vleck exchange spin Hamiltonian. The S = 4 state arises from the magnetic coupling between five low-spin (Si = 1/2) and one intermediate-spin (S = 3/2) iron(III) center. © 1994 John Wiley & Sons, Inc.
TL;DR: In this article, a continuum field theory for the Majumdar-Ghosh model in the large-S limit was derived for the SO(3) manifold, and the field theory is not Lorentz-invariant.
TL;DR: In this article, the thermal and light-induced spin transition in [FexZn1 -x(mtz)6][ClO4]2 (mtz = 1-methyl-1H-tetrazole) was studied by 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements.
Abstract: The thermal and light-induced spin transition in [FexZn1 –x(mtz)6][ClO4]2(mtz = 1-methyl-1H-tetrazole) was studied by 57Fe Mossbauer spectroscopy and magnetic susceptibility measurements. The pure iron compound possesses two different lattice sites A and B for the complex molecules. At temperatures below 100 K a high spin (h.s.)→ low spin (l.s.) transition of the complexes at site A was observed, whereas the B molecules stay in the h.s. state at all temperatures down to 4.2 K. On isotypic dilution with zinc the spin transition was shifted to lower temperatures and became more gradual with decreasing iron content x of the mixed-metal crystals. At T⩽ 40 K the l.s. (A) state can be converted by green light into the long-lived metastable h.s. (A) state [light-induced excited spin state trapping (LIESST)(l → h)] and the h.s.(B) state can be converted into a metastable long-lived l.s.(B) state using red light [LIESST (h → l)]. Reverse LIESST using red and green light, respectively, is possible in both cases.
TL;DR: In this article, the authors consider a nearest-neighbor-interaction ±J Ising spin glass in a square lattice and design a dynamic rule that includes selection, randomness, and multibranch exploration.
Abstract: We consider a nearest-neighbor-interaction ±J Ising spin glass in a square lattice. Inspired by natural evolution, we design a dynamic rule that includesselection, randomness, andmultibranch exploration. Following this rule, we succeed in walking along the space of states between local energy maxima and minima alternately. During the walk, we store various information about the spin states corresponding to these minima and maxima for later statistical analysis. In particular, we plot a histogram displaying how many times each minimum (or maximum) energy is visited as a function of the corresponding density value. Through finite-size scaling analysis, we conclude that a nonvanishing fraction of bonds remains unsatisfied (satisfied) at these energy minimum (maximum) states in the thermodynamic limit. This fraction measures the degree of unavoidable frustration of the system. Also in this limit, the width of these histograms vanishes, meaning that almost all metastable states occur at the same energy density value, with no dispersion.
TL;DR: In this article, diamagnetic muon species in a spin-crossover complex diisothiocyanatobisphenanthrolineiron(II) were investigated: two species with different spin relaxation functions and rates were observed over the range from 21 to 293 K.
Abstract: Diamagnetic muon species in a spin-crossover complex diisothiocyanatobisphenanthrolineiron(II) were investigated: two species with different spin relaxation functions and rates were observed over the range from 21 to 293 K The sum of their yields changed at about the spin transition temperature of the complex, where the spin state of the complex changes from paramagnetic to diamagnetic when it is cooled down across the temperature The results suggest that paramagnetic muon species could exhibit diamagnetic muon-like μSR signals in paramagnetic environment
TL;DR: In this paper, the reaction of WCl 4 (PMe 2 Ph) 2 with 2 equiv of TlOCH 2 CF 3 gives W(OR f ) 2 Cl 2 (Pme 2 PH) 2, which is shown by solution spectroscopic and solid-state (X-ray diffraction) methods to have a trans,cis, cis stereochemistry.
Abstract: The reaction of WCl 4 (PMe 2 Ph) 2 with 2 equiv of TlOCH 2 CF 3 gives W(OR f ) 2 Cl 2 (PMe 2 Ph) 2 , which is shown by solution spectroscopic and solid-state (X-ray diffraction) methods to have a trans,cis,cis stereochemistry. The resulting ligand field leaves the two d electrons spin-paired in a single orbital, which enhances O→W π-donation and tends to increase the metal valence electron count to (or near) 18. The electron configuration leaves the molecule surprisingly unreactive toward PMe 2 Ph, H 2 , H 2 O, and even O 2
TL;DR: In this article, high spin states in 106Cd were populated by the reactions Zr-94(O-17, 5n) and Ge(S-32, 4n) at 80 MeV and at 148 MeV, respectively.