arXiv subjects

Liviu F. Chibotaru

Publications and source records attributed to Liviu F. Chibotaru.

At least 19 recordsLinked to original sources

arXiv2026

Coupled-cluster study of dynamic Jahn-Teller effect in a $5d^2$ W antifluorite

In correlated insulators, the interplay among coexisting charge, spin, orbital, and lattice degrees of freedom gives rise to rich quantum phenomena, while unraveling the interplay is not straightforward. In the family of cubic $5d^2$ double perovskites, the ground spin-orbit coupled electronic states of $5d$ metal sites are degenerate and couple to the Jahn-Teller active vibrations, whereas no experimental evidence of the symmetry-lowering in the low-temperature ordered phases has been reported. To quantitatively unravel the nature of $5d^2$ centers, we apply equation-of-motion coupled cluster (EOM-CC) theory to analyze the vibronic and magnetic properties of $5d^2$ W sites of Cs$_2$WCl$_6$. We derive the electronic and vibronic model Hamiltonians, calculate the W $L_3$ edge resonant inelastic x-ray scattering (RIXS) spectra, and determine the effective magnetic moment. The simulated RIXS spectra show that vibronic coupling makes several peaks asymmetric. The effective magnetic moments exhibit a temperature dependence similar to that observed experimentally, confirming the validity of the calculated distribution of low-energy levels. Our calculations indicate that the Jahn-Teller effect in Cs$_2$WCl$_6$ is in a weak regime, and noticeable deformation would not occur, whereas the dynamic Jahn-Teller effect modulates the shapes of the RIXS spectra and affects the magnetic moment. This work demonstrates the usefulness of the EOM-CC method for predicting physical phenomena on metal sites in correlated insulating materials.

cond-mat.str-el↗

arXiv2026

Coupled-cluster approach to vibronic effects in resonant inelastic x-ray scattering of quantum materials: Application to a $5d^1$ rhenium oxide

First-principles analysis of the spectroscopic signatures of correlated quantum materials poses significant challenges due to the interplay between spin-orbit and vibronic couplings, as well as the need to describe both dynamic and static electron correlation to reach decent accuracy. In this work, we apply the equation-of-motion coupled-cluster (EOM-CC) method to derive the spin-orbit-lattice entangled vibronic states and predict the Re $L_3$ edge resonant inelastic x-ray scattering (RIXS) spectra of Ba$_2$MgReO$_6$. The EOM-CC yields interaction parameters in close agreement with those extracted from RIXS spectra, with errors of less than 5\%. In particular, the EOM-CC method allowed us to determine the weak vibronic coupling to the $T_{2g}$ vibrations, which is difficult to address experimentally. The simulated spectra indicate that vibronic coupling to the $T_{2g}$ modes gives rise to a shoulder on the elastic peak. Going beyond the conventional treatment, which focuses solely on $E_g$ modes, we show that vibronic couplings to both $T_{2g}$ and $E_g$ modes are required to account for the fine structure of the RIXS spectra. This work demonstrates that the EOM-CC method is a powerful tool for accurately predicting the complex local states at metal sites and spectroscopic signatures of correlated insulating materials.

cond-mat.str-el↗

arXiv2025

Breakdown of broken-symmetry approach to exchange interaction

Broken-symmetry (BS) approaches are widely employed to evaluate Heisenberg exchange parameters, primarily in combination with DFT calculations. For many magnetic materials, BS-DFT calculations give reasonable estimations of exchange parameters, although systematic failures have also been reported. While the latter were attributed to deficiencies of approximate exchange-correlation functional, we prove here by treating a simple model system that the broken-symmetry methodology has serious problems. Detailed analysis clarifies the intrinsic issue with the broken-symmetry treatment of low-spin states. It shows, in particular, that the error in the BS calculation of exchange parameter scales with the degree of covalency between the magnetic and the bridging orbitals. This is due to the constraint on the form of multiconfigurational state imposed by the BS determinant, a feature common to other single-reference methods too. As a possible tool to overcome this intrinsic drawback of single-determinant BS approaches, we propose their extension to a minimal multiconfigurational version.

physics.chem-ph↗

arXiv2024

First-principles derivation of elastic interaction between Jahn-Teller centers in crystals via lattice Green's functions

Jahn-Teller (JT) systems with strong and intermediate vibronic coupling are described in terms of local JT active vibrational modes. In JT crystals, the elastic interaction of these modes at different JT centers plays a crucial role, for instance, in determining critical temperature of structural phase transitions. Despite their importance, the parameters of elastic interaction between JT centers have not been accessed yet by first-principles calculations. In this paper, we develop an effective Hamiltonian methodology for the thorough description of the elastic interactions in cooperative Jahn-Teller problems, which treats the interactions with both phonons and uniform strains. All the microscopic parameters, such as lattice Green's functions, elastic modulus, can be obtained or calculated based on first-principles the calculation. The method has been applied to a series of 5d1 double perovskites. Such effective Hamiltonian methodology can be, in general, used to investigate the APES of any type of the local distortions, such as impurities, defects, etc.

cond-mat.mtrl-sci↗

arXiv2023

Vibronic order and emergent magnetism in cubic $d^1$ double perovskites

The synergistic interplay of different interactions in materials leads to the emergence of novel quantum phenomena. Spin-orbit and vibronic couplings usually counteract each other, however, in cubic $d^1$ double perovskites they coexist and give rise to spin-orbit-lattice entanglement with unquenched dynamic Jahn-Teller effect on the metal sites. The correlation of these entangled states induced by intersite interactions was not assessed so far. Here, we investigate the joint cooperative effect of spin-orbit and vibronic interactions on the formation of the ordered phases in $d^1$ double perovskites. We found that the magnetic ordered states in these systems coexist with a dynamic vibronic order characterized by the ordering of vibronic quadrupole moments on sites. This treatment allows the rationalization of a number of unexplained features of experimentally investigated phases.

cond-mat.str-el↗

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↗

arXiv2022

Multipolar exchange interaction and complex order in insulating lanthanides

In insulating lanthanides, unquenched orbital momentum and weak crystal-field (CF) splitting of the atomic $J$ multiplet at lanthanide ions result in a highly ranked (multipolar) exchange interaction between them and a complex low-temperature magnetic order not fully uncovered by experiment. Explicitly correlated {\it ab initio } methods proved to be highly efficient for an accurate description of CF multiplets and magnetism of individual lanthanide ions in such materials. Here we extend this {\it ab initio } methodology and develop a first-principles microscopic theory of multipolar exchange interaction between $J$-multiplets in $f$ metal compounds. The key point of the approach is a complete account of Goodenough's exchange mechanism along with traditional Anderson's superexchange and other contributions, the former being dominant in many lanthanide materials. Application of this methodology to the description of the ground-state order in the neodymium nitride with rocksalt structure reveals the multipolar nature of its ferromagnetic order. We found that the primary and secondary order parameters (of $T_{1u}$ and $E_g$ symmetry, respectively) contain non-negligible $J$-tensorial contributions up to the ninth order. The calculated spin-wave dispersion and magnetic and thermodynamic properties show that they cannot be simulated quantitatively by confining to the ground CF multiplet on the Nd sites. Our results demonstrate that the {\it ab initio } approach to the low-energy Hamiltonian represents a powerful tool for the study of materials with complex magnetic order.

cond-mat.str-el↗

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↗

arXiv2021

Jahn-Teller effect in cubic fullerides A$_{3}$C$_{60}$

Compared to isolated C$_{60}^{3-}$ ions, characterized by a threedimensional equipotential trough at the bottom of the lowest adiabatic potential energy surface (APES), the Jahn-Teller (JT) effect in cubic fullerides is additionally influenced by the interaction of JT distortions at C$_{60}$ sites with vibrational modes of the lattice. This leads to modification of JT stabilization energy and to the warping of the trough at each fullerene site, as well as to the interaction of JT distortions at different sites. Here we investigate these effects in three fcc fullerides with A=K,Rb,Cs and in Cs$_3$C$_{60}$ with bcc (A15) structure. DFT calculations of orbital vibronic coupling constants at C$_{60}$ sites and of phonon spectra have been done for fully ordered lattices (1 C$_{60}$/u.c.). Based on them the elastic response function for local JT distortions has been evaluated and the lowest APES investigated. To this end an expression for the latter in function of trough coordinates of all sites has been derived. The results show that the JT stabilization energy slightly increases compared to an isolated C$_{60}^{3-}$ and a warping of the trough of few meV occurs. The interaction of JT distortions on nearest- and next-nearest-neighbor fullerene sites is of similar order of magnitude. These effects arise first of all due to the interaction of C$_{60}$ sites with the displacements of neighbor alkali atoms and are more pronounced in fcc fullerides than in the A15 compound. The results of this study support the picture of weakly hindered independent rotations of JT deformations at C$_{60}$ sites in cubic A$_3$C$_{60}$.

cond-mat.str-el↗

arXiv2020

Ferromagnetic kinetic exchange interaction in magnetic insulators

The superexchange theory predicts dominant antiferromagnetic kinetic interaction when the orbitals accommodating magnetic electrons are covalently bonded through diamagnetic bridging atoms/groups. Here we show that explicit consideration of magnetic and (leading) bridging orbitals, together with the electron transfer between the former, reveals a strong ferromagnetic kinetic exchange contribution. First principle calculations show that it is comparable in strength with antiferromagnetic superexchange in a number of magnetic materials with diamagnetic metal bridges. In particular, it is responsible for a very large ferromagnetic coupling ($-10$ meV) between the iron ions in a Fe$^{3+}$-Co$^{3+}$-Fe$^{3+}$ complex.

cond-mat.str-el↗

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↗

arXiv2020

Yu-Shiba-Rusinov bands in ferromagnetic superconducting diamond

The combination of different exotic properties in materials paves the way for the emergence of their new potential applications. An example is the recently found coexistence of the mutually antagonistic ferromagnetism and superconductivity in hydrogenated boron-doped diamond, which promises to be an attractive system with which to explore unconventional physics. Here, we show the emergence of Yu-Shiba-Rusinov (YSR) bands with a spatial extent of tens of nanometers in ferromagnetic superconducting diamond using scanning tunneling spectroscopy. We demonstrate theoretically how a two-dimensional (2D) spin lattice at the surface of a three-dimensional (3D) superconductor gives rise to the YSR bands, and how their density-of-states profile correlates with the spin lattice structure. The established strategy to realize new forms of the coexistence of ferromagnetism and superconductivity opens a way to engineer the unusual electronic states and also to design better performing superconducting devices.

cond-mat.supr-con↗

arXiv2020

Broken symmetry $G_0W_0$ approach for the evaluation of exchange coupling constants

The applicability of a broken symmetry version of the $G_0W_0$ approximation to the calculation of isotropic exchange coupling constants has been studied. Using a simple H--He--H model system the results show a significant and consistent improvement of the results over both broken symmetry Hartree--Fock and broken symmetry density functional theory. In the case of more realistic bimetallic Cu(II) complexes, inclusion of the $G_0W_0$ correction does not lead to obvious improvement in the results. The discrepancies are explained by improved description of the interactions within the magnetic orbital space upon inclusion of the $G_0W_0$ corrections but deterioration of the description of charge- and spin-polarization effects outside the magnetic orbital space. Overall the results show that computational methods based on the $GW$ method have a potential to improve computational estimates of exchange coupling constants.

cond-mat.str-el↗

arXiv2018

Manifestation of vibronic dynamics in infrared spectra of Mott insulating fullerides

The fine structure and temperature evolution of infrared spectra have been intensively used to probe the nature of Jahn-Teller dynamics in correlated materials. At the same time, theoretical framework to adequately extract the information on the complicated vibronic dynamics from infrared spectra is still lacking. In this work, the first-principles theory of the infrared spectra of dynamical Jahn-Teller system is developed and applied to the Mott-insulating Cs$_3$C$_{60}$. With the calculated coupling parameters for Jahn-Teller and infrared active vibrational modes, the manifestation of the dynamical Jahn-Teller effect in infrared spectra is elucidated. In particular, the temperature evolution of the infrared line shape is explained. The transformation of the latter into Fano resonance type in metallic fulleride is discussed on the basis of obtained results.

cond-mat.str-el↗