Hamiltonian Parameters Research Articles

Magnetic properties of bond-diluted nanoribbons with hexagonal and rectangular shape

An exploration of the magnetic behavior of bond-diluted mixed spin-3/2 and spin-1/2 Ising model on hexagonal and rectangular nanoribbons is considered by means of the finite cluster approximation. The effects of the bond concentration p and the crystal field D on the phase diagrams yield satisfactory results. In particular, it is found that a horizontal reentrance appears for both nanosystems; while the tricritical phenomenon may occur by the inclusion of the crystal field according to specific Hamiltonian parameters for the rectangular nanoribbon. The dependence on temperature of the total, σ- and S- sublattice magnetizations, mainly at the ground state has also emphasized the significance of the dilution in dominating the magnetic and critical characters.

Quantifying the effects of dissipation and temperature on dynamics of a superconducting qubit-cavity system

The superconducting circuits involving Josephson junction offer macroscopic quantum two-level system (qubit) which are coupled to cavity resonators and are operated via microwave signals. In this work, we study the dynamics of superconducting qubits coupled to a cavity with including dissipation in a subKelvin temperature domain. In the first step, a classical finite element method is used to simulate the cavities and basic circuit elements to model Josephson junctions. Then, the quantization of the circuit is done to obtain the full Hamiltonian of the system using energy participation ratios of the junctions. Once the parameters of Hamiltonian are obtained, the dynamics is studied via the Lindblad equation for an open quantum system using a realistic set of dissipative parameters and include temperature effects. Finally, we get frequency spectra and/or dynamics of the system with time which have quantum imprints. Such devices work at tens of milliKelvins and we search for a set of parameters which could enable to observe quantum behaviour at temperatures as high as 1 K.

Open quantum rotors: Connecting correlations and physical currents

We consider a finite one-dimensional chain of quantum rotors interacting with a set of thermal baths at different temperatures. When the interaction between the rotors is made chiral, such a system behaves as an autonomous thermal motor, converting heat currents into non-vanishing rotational ones. Such a dynamical response is strongly pronounced in the range of the Hamiltonian parameters for which the ground state of the system in the thermodynamic limit exhibits a quantum phase transition. Such working points are associated with large quantum coherence and multipartite quantum correlations within the state of the system. This suggests that the optimal operating regime of such quantum autonomous motor is one of maximal quantumness.

Analytic Linear Vibronic Coupling Method for First-Principles Spin-Dynamics Calculations in Single-Molecule Magnets.

Accurate modeling of vibronically driven magnetic relaxation from ab initio calculations is of paramount importance to the design of next-generation single-molecule magnets (SMMs). Previous theoretical studies have been relying on numerical differentiation to obtain spin-phonon couplings in the form of derivatives of spin Hamiltonian parameters. In this work, we introduce a novel approach to obtain these derivatives fully analytically by combining the linear vibronic coupling (LVC) approach with analytic complete active space self-consistent field derivatives and nonadiabatic couplings computed at the equilibrium geometry with a single electronic structure calculation. We apply our analytic approach to the computation of Orbach and Raman relaxation rates for a bis-cyclobutadienyl Dy(III) sandwich complex in the frozen-solution phase, where the solution environment is modeled by electrostatic multipole expansions, and benchmark our findings against results obtained using conventional numerical derivatives and a fully electronic description of the whole system. We demonstrate that our LVC approach exhibits high accuracy over a wide range of coupling strengths and enables significant computational savings due to its analytic, "single-shot" nature. Evidently, this offers great potential for advancing the simulation of a wide range of vibronic coupling phenomena in magnetism and spectroscopy, ultimately aiding the design of high-performance SMMs. Considering different environmental representations, we find that a point charge model shows the best agreement with the reference calculation, including the full electronic environment, but note that the main source of discrepancies observed in the magnetic relaxation rates originates from the approximate equilibrium electronic structure computed using the electrostatic environment models rather than from the couplings.

Non-Kramers iron S = 2 ions in β-Ga2O3 crystals: High-frequency low-temperature EPR study

Using high-frequency electron paramagnetic resonance (EPR), we have observed non-Kramers ions with giant fine structure splitting of the order of 100 GHz in n-type β-Ga2O3 crystals. These EPR spectra were assigned to Fe2+ ions 5D (3d 6) with S = 2. This interpretation was supported by experiments on Fermi level displacement induced by high-energy electron irradiation and photoexcitation of irradiated samples with 405-nm laser light. The values and signs of the basic parameters of the spin Hamiltonian for ions, namely Cr3+ (S = 3/2) and Fe3+ (S = 5/2), were identified, and the order of their spin levels was established.

Transient Behavior of Damage Spreading in the Two-Dimensional Blume–Capel Ferromagnet

We study the transient behavior of damage propagation in the two-dimensional spin-$1$ Blume-Capel model using Monte Carlo simulations with Metropolis dynamics. We find that, for a particular region in the second-order transition regime of the crystal field--temperature phase diagram of the model, the average Hamming distance decreases exponentially with time in the weakly damaged system. Additionally, its rate of decay appears to depend linearly on a number of Hamiltonian parameters, namely the crystal field, temperature, applied magnetic field, but also on the amount of damage. Finally, a comparative study using Metropolis and Glauber dynamics indicates a slower decay rate of the average Hamming distance for the Glauber protocol.

Analysis of the Magnetic Coupling in a Mn(II)-U(V)-Mn(II) Single Molecule Magnet.

[{Mn(TPA)I}{UO2(Mesaldien)}{Mn(TPA)I}]I formula (here TPA=tris(2-pyridylmethyl)amine and Mesaldien=N,N'-(2-aminomethyl)diethylenebis(salicylidene imine)) reported by Mazzanti and coworkers (Chatelain et al. Angew. Chem. Int. Ed. 2014, 53, 13434) is so far the best Single Molecule Magnet (SMM) in the {3d-5f} class of molecules exhibiting barrier height of magnetization reversal as high as 81.0 K. In this work, we have employed a combination of ab initio CAS and DFT methods to fully characterize this compound and to extract the relevant spin Hamiltonian parameters. We show that the signs of the magnetic coupling and of the g-factors of the monomers are interconnected. The central magnetic unit [UV O2 ]+ is described by a Kramers Doublet (KD) with negative g-factors, due to a large orbital contribution. The magnetic coupling for the {Mn(II)-U(V)} pair is modeled by an anisotropic exchange Hamiltonian: all components are ferromagnetic in terms of spin moments, the parallel component JZ twice larger as the perpendicular one J⊥ . The spin density distribution suggests that spin polarization on the U(V) center favors the ferromagnetic coupling. Further, the JZ /J⊥ ratio, which is related to the barrier height, was found to correlate to the corresponding spin contribution of the g-factors of the U(V) center. This correlation established for the first time offers a direct way to estimate this important ratio from the corresponding gS -values, which can be obtained using traditional ab initio packages and hence has a wider application to other {3d-5f} magnets. It is finally shown that the magnetization barrier height is tuned by the splitting of the [UV O2 ]+ 5 f orbitals.

Investigations on the local structure and the spin Hamiltonian parameters for Cu2+ in xCuO-(68-x)V2 O5 -32TeO2 glasses.

The defect structure, spin Hamiltonian parameters (SHPs: anisotropic factors and and the hyperfine structure constants and ), and their compositional dependence of in ( = 5, 10, 20, 30 mol%) glasses are quantitatively analyzed by using the higher-order perturbation formula of octahedral complex with tetrahedral elongation distortion. Due to the Jahn-Teller effect, the group is subjected to tetragonal elongation distortion of varying degrees. , and show nonlinear changes with the concentrations of . When = 10 mol% CuO, the degree of distortion ( ) is the smallest; when = 30 mol% CuO, the degree of distortion ( ) is the largest, which indicates that excessive distortion leads to the appearance of -axis oxygen vacancies and the coordination number of copper ions from six to four. The increasing tendency of the evaluated and reveals decreasing covalency of the whole glass system. Present theoretical studies would be useful to the explore the structural properties and optical applications of glass with different concentrations.

Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein.

A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}7 intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

Study some of the nuclear characters of Samarium 148,150,152,154Sm isotopes in the IBM-1 framework

The interacting boson model IBM-1 has been applied to some 62Sm even-even isotopes .This study has included calculation of the positive-parity collectiv g-band and β,γ –degenerate bands of 148 62Sm, 150 62Sm, 152 62Sm and 154 62Sm isotopes and compared with experimental values of energy level. The used Hamiltonian parameters correspond to description close to the U(5)-SU(3). The potential energy surfaces E(β,γ) as function of geometrical parameter β and γ for these isotopes can be obtained where they were represented by contour lines and the minimum value of deformation factor βmin was fixed , which was in front of Emin, and so the γ factor. The transition from the spherical to the rotational shape can be observed in these isotopes. In the framework of IBM-1 , what we conclude from the energy spectra study and the geometrical picture of potential energy surface for these nuclei is that these nuclei are spherical shape (U(5)symmetry), prolate rotational shape (SU(3)symmetry) and deformed shape between these two symmetry limits in the U(5)-SU(3)transitional region .

Field-Induced Slow Magnetic Relaxation in CoII Cyclopropane-1,1-dicarboxylates.

New CoII substituted malonate field-induced molecular magnets {[Rb6Co3(cpdc)6(H2O)12]∙6H2O}n (1) and [Cs2Co(cpdc)2(H2O)6]n (2) (where cpdc2− stands for cyclopropane-1,1-dicarboxylic acid dianions) were synthesized. Both compounds contain mononuclear bischelate fragments {CoII(cpdc)2(H2O)2}2− where the quasi-octahedral cobalt environment (CoO6) is complemented by water molecules in apical positions. The alkali metal atoms play the role of connectors between the bischelate fragments to form 3D and 2D polymeric structures for 1 and 2, respectively. Analysis of dc magnetic data using the parametric Griffith Hamiltonian for high-spin CoII supported by ab initio calculations revealed that both compounds have an easy axis of magnetic anisotropy. Compounds 1 and 2 exhibit slow magnetic relaxation under an external magnetic field (HDC = 1000 and 1500 Oe, respectively).

Engineering Floquet dynamical quantum phase transitions

Floquet dynamical quantum phase transitions (FDQPTs) are signified by recurrent nonanalytic behaviors of observables in time. In this work, we introduce a quench-free and generic approach to engineer and control FDQPTs for both pure and mixed Floquet states. By applying time-periodic modulations with two commensurate driving frequencies to a general class of spin chain model, we find multiple FDQPTs within each driving period. The nonanalytic cusps of return probability form sublattice structures in time domain. Notably, the number and time-locations of these cusps can be flexibly controlled by tuning the Hamiltonian parameter and the higher frequency of the drive. We further employ the dynamical topological order parameter (DTOP), which shows a quantized jump whenever a DQPT happens, to identify the topological feature of FDQPTs. Our findings reveal the advantage of engineering nonequilibrium phase transitions with multi-frequency driving fields.

Quantum computation of molecular structure using data from challenging-to-classically-simulate nuclear magnetic resonance experiments.

We propose a quantum algorithm for inferring the molecular nuclear spin Hamiltonian from time-resolved measurements of spin-spin correlators, which can be obtained via nuclear magnetic resonance (NMR). We focus on learning the anisotropic dipolar term of the Hamiltonian, which generates dynamics that are challenging to classically simulate in some contexts. We demonstrate the ability to directly estimate the Jacobian and Hessian of the corresponding learning problem on a quantum computer, allowing us to learn the Hamiltonian parameters. We develop algorithms for performing this computation on both noisy near-term and future fault-tolerant quantum computers. We argue that the former is promising as an early beyond-classical quantum application since it only requires evolution of a local spin Hamiltonian. We investigate the example of a protein (ubiquitin) confined on a membrane as a benchmark of our method. We isolate small spin clusters, demonstrate the convergence of our learning algorithm on one such example, and then investigate the learnability of these clusters as we cross the ergodic to non-ergodic phase transition by suppressing the dipolar interaction. We see a clear correspondence between a drop in the multifractal dimension measured across many-body eigenstates of these clusters, and a transition in the structure of the Hessian of the learning cost function (from degenerate to learnable). Our hope is that such quantum computations might enable the interpretation and development of new NMR techniques for analyzing molecular structure.

Periodic dynamics in superconductors induced by an impulsive optical quench

A number of experiments have evidenced signatures of enhanced superconducting correlations after photoexcitation. Initially, these experiments were interpreted as resulting from quasi-static changes in the Hamiltonian parameters, for example, due to lattice deformations or melting of competing phases. Yet, several recent observations indicate that these conjectures are either incorrect or do not capture all the observed phenomena, which include reflectivity exceeding unity, large shifts of Josephson plasmon edges, and appearance of new peaks in terahertz reflectivity. These observations can be explained from the perspective of a Floquet theory involving a periodic drive of system parameters, but the origin of the underlying oscillations remains unclear. In this paper, we demonstrate that following incoherent photoexcitation, long-lived oscillations are generally expected in superconductors with low-energy Josephson plasmons, such as in cuprates or fullerene superconductor K3C60. These oscillations arise from the parametric generation of plasmon pairs due to pump-induced perturbation of the superconducting order parameter. We show that this bi-plasmon response can persist even above the transition temperature as long as strong superconducting fluctuations are present. Our analysis offers a robust framework to understand light-induced superconducting behavior, and the predicted bi-plasmon oscillations can be directly detected using available experimental techniques.

Glassy disorder-induced effects in noisy dynamics of Bose–Hubbard and Fermi–Hubbard systems

We address the effects of quenched disorder averaging in the time-evolution of systems of ultracold atoms in optical lattices in the presence of noise, imposed by an environment. For bosonic systems governed by the Bose–Hubbard Hamiltonian, we quantify the response of disorder in Hamiltonian parameters in terms of physical observables, including bipartite entanglement in the ground state, and report the existence of disorder-induced enhancement in weakly interacting cases. For systems of two-species fermions described by the Fermi–Hubbard Hamiltonian, we find similar results. In both cases, our dynamical calculations show no appreciable change in the effects of disorder from that of the initial state of the evolution. We explain our findings in terms of the statistics of the disorder in the parameters and the behaviour of the observables with the parameters.

Spin-Lattice Relaxation Decoherence Suppression in Vanishing Orbital Angular Momentum Qubits.

Multifrequency electron paramagnetic resonance spectroscopy on oriented single crystals of magnetically dilute Gd(III) ions in Gd0.004Y0.996(trensal) is used to determine the Hamiltonian parameters of the ground 8S7/2 term and its phase memory time, Tm, characterizing its coherent spin dynamics. The vanishing orbital angular momentum of the 8S7/2 term makes it relatively insensitive to spin-lattice relaxation mediated by magnetoelastic coupling and leads to a Tm of 12 μs at 3 K, which is not limited by spin-lattice relaxation.

Time oscillating magnetic field effect on critical behavior and dynamic phase transition of monolayer nanographene structure

ABSTRACT Dynamic magnetic properties of a ferrimagnetic mixed (1/2,1) Ising spin monolayer nanographene structure with a crystal-field interaction in a time oscillating magnetic field are studied by the use of the kinetic Monte Carlo simulation based on the Metropolis algorithm. Dynamic phase transitions are obtained, the effect of the next-nearest neighbor interactions, the crystal field, the amplitude, and the period of the external dynamic magnetic field on the critical and compensation behaviors and on the thermal variations of the dynamic order parameters are investigated. We have found that in the presence of the time oscillating field, the system may exhibit a dynamic phase transition from the ferrimagnetic to the paramagnetic phase at a critical point. The system may even present two dynamic compensation temperatures for appropriate values of the hamiltonian parameters. This behavior of dynamic compensation temperature has not been observed previously in published papers with kinetic Monte Carlo simulation.

Riemann-Encircling Exceptional Points for Efficient Asymmetric Polarization-Locked Devices.

Dynamically encircling exceptional points (EPs) have unveiled intriguing chiral dynamics in photonics. However, the traditional approach based on an open manifold of Hamiltonian parameter space fails to explore trajectories that pass through an infinite boundary. Here, by mapping the full parameter space onto a closed manifold of the Riemann sphere, we introduce a framework to describe encircling-EP loops. We demonstrate that an encircling trajectory crossing the north vertex can realize near-unity asymmetric transmission. An efficient gain-free, broadband asymmetric polarization-locked device is realized by mapping the encircling path onto L-shaped silicon waveguides.

Machine learning for excitation energy transfer dynamics

A well-known approach to describe the dynamics of an open quantum system is to compute the master equation evolving the reduced density matrix of the system. This approach plays an important role in describing excitation transfer through photosynthetic light harvesting complexes (LHCs). The hierarchical equations of motion (HEOM) was adapted by Ishizaki and Fleming (J. Chem. Phys., 2009) to simulate open quantum dynamics in the biological regime. We generate a set of time dependent observables that depict the coherent propagation of electronic excitations through the LHCs by solving the HEOM. We solve the inverse problem using classical machine learning (ML) models as this is a computationally intractable problem. The objective here is to determine whether a trained ML model can perform Hamiltonian tomography by using the time dependence of the observables as inputs. We demonstrate the capability of convolutional neural networks to tackle this research problem. The models developed here can predict Hamiltonian parameters such as excited state energies and inter-site couplings of a system up to 99.28\% accuracy and mean-squared error as low as 0.65.

Influence of Multi‐Orbitals and Hund's Coupling Induced Pseudogap on Specific Heat Jump in Iron‐Pnictide High T c Superconductors

A three-orbital per site theoretical model study is attempted to analyze nature of specific heat in iron-based superconductors. Based on recent experimental data in these systems, it is found that Fe five 3d orbitals, Hund’s coupling and intra- and inter-electron orbital correlations play significant role in electronic properties of these systems. Thus, in present work, we employed model Hamiltonian, containing hopping between orbitals, Hund’s coupling and intra- and inter- orbital Coulomb interactions for iron pnictide. Employing Green’s function technique within BCS mean field approximation, expressions of superconducting energy gap parameter and quasiparticle energies are obtained. Considering these parameters, we calculated specific heat and analyzed its variation with temperature and other parameters of model Hamiltonian. Also, a pseudogap (PG) parameter, arising out of multi-orbitals’ Hund’s coupling, as supported by ARPES and STM experiments, is introduced. The expression of specific heat is numerically analyzed to pinpoint its variation below and above transition temperature (T c ). A self-consistent numerical analysis of the specific heat jump as a function of multi-orbital hopping, electron interactions and Hund’s coupling in terms of PG parameter is shown in detail. Finally, we compared outcomes with recent theoretical and experimental data available on specific heat variations in iron superconductors like LaOFeAs system. This article is protected by copyright. All rights reserved.