arXiv subjects

Le Tuan Anh Ho

Publications and source records attributed to Le Tuan Anh Ho.

12 recordsLinked to original sources

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

Universal secular form of non-secular master equation

We develop a more accurate secular form for the Redfield non-secular master equation, which is insensitive to the choice of the basis. This completely solves the ambiguity on which basis should be used when the secular approximation of the Redfield master equation is applied.

quant-ph↗

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↗

arXiv2021

Rectangle-like hysteresis in a Dysprosium Metallacrown Magnet with Linear F-Dy-F Anisotropic Moiety

Single-molecule magnets (SMMs) exhibiting open hysteresis loops may potentially apply to molecule-based information processing and storage. However, the capacity to retain magnetic memory is always limited by zero-field quantum tunneling of magnetization (QTM). Herein, a well-designed dysprosium metallacrown SMM, consisting of an endohedral approximate linear F-Dy-F strong anisotropic moiety in a peripheral [15-MCNi-5] metallacrown (MC), is reported with the largest reversal barrier of 1060 cm-1 among d-f SMMs. Rectangle-like hysteresis loops are observed with the huge squareness (remanence/saturation magnetization) up to 97% at 2 K. More importantly, zero-field QTM step is phenomenologically removed by minimizing the dipole coupling and hyperfine interactions. The results demonstrate for the first time that zero-field QTM step can be eliminated via manipulating the ligand field and vanishing the external magnetic perturbations, which illuminates a promising blueprint for developing high-performance SMMs.

cond-mat.mes-hall↗

arXiv2020

Field-induced oscillation of magnetization blocking in holmium metallacrown magnet

Single-molecule magnets (SMMs) are promising elements for quantum informatics. In the presence of strong magnetic anisotropy, they exhibit magnetization blocking - a magnetic memory effect at the level of a single molecule. Recent studies have shown that the SMM performance scales with the height of magnetization blocking barrier. By employing molecular engineering this can be significantly modified, remaining independent from other external factors such as magnetic field. Taking advantage of hyperfine coupling of electronic and nuclear spins further enhances their functionality, however, a poor understanding of relaxation mechanisms in such SMMs limits the exploitation of nuclear-spin molecular qubits. Here we report the opening discovery of field-dependent oscillation of the magnetization blocking barrier in a new holmium metallacrown magnet driven by the switch of relaxation mechanisms involving hyperfine interaction. Single-crystal magnetic hysteresis measurements combined with first-principles calculations reveal an activated temperature dependence of magnetic relaxation dominated either by incoherent quantum tunneling of magnetization at anti-crossing points of exchange-hyperfine states or by Orbach-like processes at crossing points. We demonstrate that these relaxation mechanisms can be consecutively switched on and off by increasing the external field, which paves a way for manipulating the magnetization dynamics of SMMs using hyperfine interaction.

cond-mat.mes-hall↗

arXiv2018

Intermolecular mechanism for multiple maxima in molecular dynamic susceptibility

A simple two-level system is introduced to demonstrate the existence of the intermolecular mechanism of the appearance/disappearance of multiple maxima in molecular dynamic susceptibility at low temperature. With minimum two relaxation processes, quantum tunneling induced by the nuclear spin bath and the direct process due to the spin-phonon interaction, we prove that the existence of a sufficiently wide dipolar field distribution in the experimental sample is the main cause of this phenomenon in zero or weak applied dc field. The correlations between the phenomenon and the applied dc field, temperature, or magnetic dilution of the sample are also investigated. Application to several experimental systems has shown a good agreement between the proposed theory and experiments. Cases with more complex multiplet spectrum, more relaxation processes,and possibility of more maxima in the molecular dynamic susceptibility are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Spin-lattice relaxation of magnetic centers in molecular crystals at low temperature

We study the spin-phonon relaxation rate of both Kramers and non-Kramers molecular magnets in strongly diluted samples at low temperature. Using the "rotational" contribution to the spin-phonon Hamiltonian, universal formulae for the relaxation rate are obtained. Intriguingly, these formulae are all entirely expressed via measurable or \emph{ab initio} computable physical quantities. Moreover, they are also independent of the energy gaps to excited states involved in the relaxation process. These obtained expressions for direct and Raman processes offer an easy way to determine the lowest limit of the spin-phonon relaxation of any spin system based on magnetic properties of the ground doublet only. In addition, some intriguing properties of Raman process are also found. Particularly, Raman process in Kramers system is found dependent on the magnetic field's orientation but independent of its magnitude, meanwhile the same process in non-Kramers system is significantly reduced out of resonance, i.e. for an applied external field. Interestingly, Raman process is demonstrated to vary as $T^{9}$ for both systems. Application of the theory to a recently investigated cobalt(II) complex shows that it can provide a reasonably good description for the relaxation. Based on these findings, a strategy in developing efficient single-molecule magnets by enhancing the mechanical rigidity of the molecular unit is proposed.

cond-mat.str-el↗

arXiv2016

Multiple relaxation times in single-molecule magnets

Multiple relaxation times detected in the ac magnetic susceptibility of several single-molecule magnets have been always assigned to extrinsic factors, such as nonequivalent magnetic centers or effects of intermolecular interactions in the crystal. By solving quantum relaxation equations, we prove that the observed multiple relaxation times can be of intramolecular origin and can show up even in single-ion metal complexes. For the latter a remarkably good description of the coexistent two relaxation times is demonstrated on several experimental examples. This proves the relevance of the intramolecular mechanism of multiple relaxation times in such systems, which is even easier justified in polynuclear magnetic complexes.

cond-mat.mtrl-sci↗

arXiv2009

Cosmological Model with a Local Void: New Supernova Constraints

A simple inhomogeneous cosmological model with a local void is constrained with the latest Union supernova compilation. To fit the supernova data, a large local void on the scales of 1 Gpc is found, contrary to the small scales of 200 Mpc in the previous finding. A more realistic inhomogeneous cosmological model may be required to fit the supernova data. Alternatively, a clumpy universe with clumpiness parameter < 1 can fit the supernova data with reduced local void scales.

astro-ph.CO↗