arXiv subjects

Liviu Ungur

Publications and source records attributed to Liviu Ungur.

9 recordsLinked to original sources

arXiv2024

New tool for extraction of $^{187}$Os Mössbauer parameters with biologically relevant detection sensitivity

A large number of osmium complexes with osmium in different oxidation states (II, III, IV, VI) have been reported recently to exhibit good antiproliferative activity in cancer cell lines. Herein, we demonstrate new opportunities offered by $^{187}$Os nuclear forward scattering (NFS) and nuclear inelastic scattering (NIS) of synchrotron radiation for characterization of hyperfine interactions and lattice dynamics in a benchmark Os(VI) complex K$_2$[OsO$_2$(OH)$_4$], by accurate extraction of Mössbauer parameters and the determination of Os-projected density of phonon states confirmed by first-principles phonon calculations. The values of isomer shift ($δ$ = 3.3(1) mm/s) relative to [Os$^{IV}$ Cl$_{6}$]$^{2-}$ and quadrupole splitting ($ΔE_Q$ = 12.0(2) mm/s) were determined with NFS, while the Lamb-Mössbauer factor (0.55(1)), the density of phonon states (DOS), and a full thermodynamics characterization was carried out using the NIS data combined with first principle theoretical calculations. In more general terms, this study provides strong evidence that $^{187}$Os nuclear resonance scattering is a reliable technique for the investigation of hyperfine interactions and Os specific vibrations in osmium(VI) species, which might be potentially applicable for measuring such interactions in osmium complexes of other oxidation states, including those with anticancer activity such as Os(III) and Os(IV).

cond-mat.mtrl-sci↗

arXiv2023

Unique determination of localized basis in molecular spin

Localized basis plays an important role in comprehending the magnetic dynamics in molecular spins from a physics perspective. Nonetheless, the uniqueness and rigor of its determination have received limited attention. In this study, we propose a new determination of the localized basis applicable to both non-Kramers and Kramers molecular spin systems, leveraging the time-reversal symmetry of the spin Hamiltonian and the molecular spin's main magnetic axis. By introducing this, we establish a distinct and practical means of determining the localized basis, enabling the association of a molecular spin wave function with either an "up" or "down" magnetic moment orientation in molecular spins. This finding facilitates a comprehensive interpretation of magnetic dynamics and simplifies the construction of theoretical models for materials analysis.

cond-mat.mtrl-sci↗

arXiv2022

Non-monotonic temperature dependence and first-order phase transition of relaxation times in molecular spin

We derive a simple system of equations to describe the magnetization relaxation of a molecular spin in weak interaction with a thermal bath for the whole temperature domain. Using this for the intermediate temperature domain where the transition from coherent to incoherent relaxation occurs, we find that the slowest relaxation mode shows a first-order phase transition. Associated with this transition, an unusual non-monotonic temperature-dependence of the relaxation rate of this mode is also demonstrated. Contrary to the popular belief, this non-monotony gives rise to a peculiar but observable behavior where increasing temperature will not only result in a smaller rate of the slowest relaxation mode but also may lead to a slower decaying of the magnetization after some relaxing time. Additionally, it is also shown that magnetization relaxation in this intermediate temperature domain can only be accurately described by a bi- or tri-exponential form. The physical reason underlying these features can be attributed to the role of the quantum tunneling effect and different but comparative relaxation modes. A simple experiment to confirm our findings on the first-order phase transition and the non-monotony of the relaxation rate is accordingly proposed.

cond-mat.mtrl-sci↗

arXiv2022

Quantum tunneling of magnetization in molecular spin

We examine the quantum tunneling of magnetization in molecular spin in weak interaction with a bath subject to Redfield master equation. By designing a microscopic model for a multilevel spin system using only a generic Hamiltonian and applying stationary approximation for excited doublets/singlets, we derive a key equation of motion for the quantum tunneling of magnetization process which is applicable in the whole temperature domain. From this equation, we find that in general three tunneling rates are needed to accurately describe the quantum tunneling process. More importantly, behavior of the quantum tunneling in the intermediate temperature domain where there exists a transition between incoherent and coherent quantum tunneling is also unraveled for the first time. Limiting cases at low and high temperature and/or low magnetic field are also worked out where some popular well-known results are reproduced. Last but not least, a new interpretation of the quantum tunneling of magnetization is proposed where we reveal the similarity between this relaxation process with a driven damped harmonic oscillator.

cond-mat.mtrl-sci↗

arXiv2022

Coherence/incoherence transition temperature in molecular spin

We examine the coherence/incoherence transition temperature of a generic molecular spin. Our results demonstrates that a molecular spin with a high coherence/incoherence transition temperature should possess a low spin number and low axiality, or high spin number and high axiality. Interestingly, the latter is better protected from the magnetic noises than the former and thus be the best candidate for a robust electron-based molecular spin qubit/qudit. The transition temperature can be further optimized if a large non-axial component of the spin Hamiltonian exists.

cond-mat.mtrl-sci↗

arXiv2022

Dissipative Landau-Zener transition with decoherence rate

An innovative microscopic model with a minimal number of parameters: tunneling splitting gap, external field sweeping velocity, and decoherence rate is used to describe dynamics of the dissipative Landau-Zener transition in the presence of the decoherence. In limiting cases, the derived equation of motion gives rise to the well-known Landau-Zener and Kayanuma formula. In a general case, the description demonstrates a non-monotonic flipping probability with respect to the sweeping velocity, which is also found in some other models. This non-monotony can be explained by considering the competition and timescale of the quantum tunneling, crossing period, and decoherence process. The simplicity and robustness of the theory offer a practical and novel description of the Landau-Zener transition. In addition, it promises an alternative method to the electron paramagnetic resonance in measuring the effective decoherence rate of relevant quantum systems.

quant-ph↗

arXiv2018

${\tilde{J}}$-pseudospin states and the crystal field of cubic systems

Theory of $\tilde{J}$-pseudospin for $f$ element in cubic environment is developed. By fulfilling the symmetry requirements and the adiabatic connection to atomic limit, the crystal-field states are uniquely transformed into $\tilde{J}$-pseudospin states. In terms of the pseudospin operators, both the total angular momentum and the crystal-field Hamiltonian contain higher-rank tensor terms than the traditional ones do, which means the present framework naturally include the effects such as the covalency and $J$-mixing beyond the $f$-shell model. Combining the developed theory with {\it ab initio} calculations, the $\tilde{J}$-pseudospin states for Nd$^{3+}$ and Np$^{4+}$ ions in octahedral sites of insulators are derived.

cond-mat.str-el↗

arXiv2017

Zeeman interaction and Jahn-Teller effect in $Γ_8$ multiplet

We present a thorough analysis of the interplay of magnetic moment and the Jahn-Teller effect in the $Γ_8$ cubic multiplet. We find that in the presence of dynamical Jahn-Teller effect, the Zeeman interaction remains isotropic, whereas the $g$ and $G$ factors can change their signs. The static Jahn-Teller distortion also can change the sign of these $g$ factors as well as the nature of the magnetic anisotropy. Combining the theory with state-of-the-art {\it ab initio} calculations, we analyzed the magnetic properties of Np$^{4+}$ and Ir$^{4+}$ impurity ions in cubic environment. The calculated $g$ factors of Np$^{4+}$ impurity agree well with experimental data. The {\it ab initio} calculation predicts strong Jahn-Teller effect in Ir$^{4+}$ ion in cubic environment and the strong vibronic reduction of $g$ and $G$ factors.

cond-mat.mtrl-sci↗

arXiv2016

Giant exchange interaction in mixed lanthanides

Combining strong magnetic anisotropy with strong exchange interaction is a long standing goal in the design of quantum magnets. The lanthanide complexes, while exhibiting a very strong ionic anisotropy, usually display a weak exchange coupling, amounting to only a few wavenumbers. Recently, an isostructural series of mixed Ln$^{3+}$-N$_2^{3-}$-Ln$^{3+}$ (Ln $=$ Gd, Tb, Dy, Ho, Er) have been reported, in which the exchange splitting is estimated to reach hundreds wavenumbers. The microscopic mechanism governing the unusual exchange interaction in these compounds is revealed here by combining detailed modeling with density-functional theory and {\it ab initio} calculations. We find it to be basically kinetic and highly complex, involving non-negligible contributions up to seventh power of total angular momentum of each lanthanide site. The performed analysis also elucidates the origin of magnetization blocking in these compounds. Contrary to general expectations the latter is not always favored by strong exchange interaction.

cond-mat.str-el↗