819 research outputs found
Attribution of Water-Exchange Mechanisms of Transition-Metal Hexaaqua Ions Using Quantum Chemical Methods
The mechanism for the water-exchange reaction with the transition-metal aqua ions from ScIII through ZnII has been investigated. The exchange mechanisms are analyzed on a model that involves the metal ion with six or seven water molecules. The structures of the reactants/products, transition states, and penta- or heptacoordinated intermediates have been computed with Hartree-Fock or CAS-SCF methods. Each type of mechanism, associative, concerted, or dissociative, proceeds via a characteristic transition state. The calculated activation energies agree with the experimental ΔG‡298 or ΔH‡298 values, and the computed structural changes indicate whether an expansion or compression takes place during the transformation of the reactant into the transition state. These changes are in perfect agreement with the changes deduced from the experimental volumes of activation. The dissociative mechanism is always feasible, but it is the only possible pathway for high-spin d8, d9, and d10 systems. In contrast, the associative mechanism requires that the transition-metal ion does not have more than seven 3d electrons. Thus, ScIII, TiIII, and VIII react via the A, NiII, CuII, and ZnII via the D (or Id) mechanism, whereas all pathways are feasible for the elements in the middle of the periodic table
Analog quantum simulation of the Rabi model in the ultra-strong coupling regime
The quantum Rabi model describes the fundamental mechanism of light-matter
interaction. It consists of a two-level atom or qubit coupled to a quantized
harmonic mode via a transversal interaction. In the weak coupling regime, it
reduces to the well-known Jaynes-Cummings model by applying a rotating wave
approximation (RWA). The RWA breaks down in the ultra-strong coupling (USC)
regime, where the effective coupling strength is comparable to the energy
of the bosonic mode, and remarkable features in the system dynamics
are revealed. We demonstrate an analog quantum simulation of an effective
quantum Rabi model in the USC regime, achieving a relative coupling ratio of
. The quantum hardware of the simulator is a superconducting
circuit embedded in a cQED setup. We observe fast and periodic quantum state
collapses and revivals of the initial qubit state, being the most distinct
signature of the synthesized model.Comment: 20 pages, 13 figure
Case Report: Intrapulmonary Arteriovenous Anastomoses in COVID-19-Related Pulmonary Vascular Changes: A New Player in the Arena?
Up to now, COVID-19-related vascular changes were mainly described as thrombo-embolic events. A handful of researchers reported another type of vascular abnormality referred to as "vascular thickening" or "vascular enlargement," without specifying whether the dilated vessels are arteries or veins nor providing a physiopathological hypothesis. Our observations indicate that the vascular dilatation occurs in the venous compartment, and underlying mechanisms might include increased blood flow due to inflammation and the activation of arteriovenous anastomoses
Transmon Qubit in a Magnetic Field: Evolution of Coherence and Transition Frequency
We report on spectroscopic and time-domain measurements on a fixed-frequency
concentric transmon qubit in an applied in-plane magnetic field to explore its
limits of magnetic field compatibility. We demonstrate quantum coherence of the
qubit up to field values of , even without an optimized
chip design or material combination of the qubit. The dephasing rate
is shown to be not affected by the magnetic field in a broad
range of the qubit transition frequency. For the evolution of the qubit
transition frequency, we find the unintended second junction created in the
shadow angle evaporation process to be non-negligible and deduce an analytic
formula for the field-dependent qubit energies. We discuss the relevant
field-dependent loss channels, which can not be distinguished by our
measurements, inviting further theoretical and experimental investigation.
Using well-known and well-studied standard components of the superconducting
quantum architecture, we are able to reach a field regime relevant for quantum
sensing and hybrid applications of magnetic spins and spin systems.Comment: 9 pages, 8 figure
Vena Cava Filters: Toward Optimal Strategies for Filter Retrieval and Patients' Follow-Up.
Mortality rates associated with venous thromboembolism (VTE) are high. Inferior vena cava filters (IVCFs) have been frequently placed for these patients as part of their treatment, albeit the paucity of data showing their ultimate efficacy and potential risk of complications. Issues regarding long-term filter dwell time are accounted for in society guidelines. This topic has led to an FDA mandate for filter retrieved as soon as protection from pulmonary embolism is no longer needed. However, even though most are retrievable, some were inadvertently left as permanent, which carries an incremental lifetime risk to the patient. In the past decade, attempts have aimed to determine the optimal time interval during which filter needs to be removed. In addition, distinct strategies have been implemented to boost retrieval rates. This review discusses current conflicts in indications, the not uncommon complications, the rationale and need for timely retrieval, and different quality improvement strategies to fulfill this aim
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