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At least 433 records · Page 24

Electronic Properties and Magnetocaloric Effect of GdNi{sub 4}Si: Ab Initio Calculations, Mean Field Approximation, and Monte Carlo Simulation

We investigated the magnetic properties and magnetocaloric effect of GdNi{sub 4}Si by using a combination of first-principle density functional theory (DFT) calculations and Monte Carlo Simulation. Ab initio calculations show the stability of the ferromagnetic phase in this material. The Monte Carlo simulations combined with ab initio exchange coupling constants as input parameters lead to discuss the transition temperature, critical exponents, and isothermal entropy changes. The measured critical temperature and the isothermal magnetic entropy changes for a field change of 14T in good agreement with the experimental data.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Ferrimagnetism in an Ising Bilayer Film with a Transverse Field and Nonmagnetic Interlayers

The phase diagram and total magnetization of an Ising bilayer film with nonmagnetic interlayers are investigated by the use of the effective field theory with correlations. The system is consisted of two spin-1/2 magnetic layers where one of them is canted from the easy-axis (or z-) direction due to an applied transverse field, and they are coupled by an indirect exchange interaction. It is examined whether the system may exhibit some characteristic features of ferrimagnetism, such as a compensation point. The effects of nonmagnetic layers on them are clarified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Effects of Indirect Exchange Interactions in a Mixed-Spin Bilayer Film with Nonmagnetic Layers

The magnetic properties (phase diagrams and magnetizations) of a mixed-spin Ising ferrimagnetic bilayer film on the graphene honeycomb lattice where two (A and B) layers separated by a number of nonmagnetic layers are investigated by the use of the effective field theory with correlations. The systems are consisted of two (spin-1/2 and spin-1) magnetic atoms where spin-1 atoms have a single-ion anisotropy. The effects of an indirect exchange interaction between the A and B layers are examined. Some characteristic behaviors have been found in the system, when the single-ion anisotropy takes a large negative value. Some characteristic phenomena of ferrimagnetism, have been found in the thermal variation of magnetization.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Investigation of Size and Shape Dependencies of Phase Diagrams of the Ising Nanofilms and Nanotubes on the Honeycomb Lattice Using Cellular Automata Simulation Approach

Size and shape dependencies of phase diagrams of the Ising nanofilms and nanotubes on the honeycomb lattice are investigated by means of probabilistic cellular automata simulation based on Glauber algorithm. The values of reduced critical temperature, K{sub c} = k{sub B}T{sub c}/J (where k{sub B} and J are the Boltzmann constant and nearest-neighbor coupling, respectively), for both nanofilms and nanotubes, are obtained at the different sizes of the lattices and the dependency of K{sub c} to the number of layers is studied. By increasing the number of layers K{sub c} increases but for number of layer more than 8, the critical temperature increases very slowly. We have shown that between two isotropic nanotubes with the same number of spins, the ones with greater diameter (more spins on the edge) have larger critical temperature. For equal size of lattices, the obtained values of K{sub c} for nanotube are greater than the nanofilm, but for large sizes, this difference disappears.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Phase Transition in a Spin-1/2 and Spin-1 Ising Bilayer Film with Non-magnetic Inter-layers

The phase transition, phase diagrams, and magnetizations of an Ising bilayer film with non-magnetic inter-layers are investigated by the use of the effective field theory with correlations. The system is consisted of two magnetic layers where upper and lower layers are consisted of spin-1/2 atoms and spin-1 atoms with a single-ion anisotropy. It is examined how the system may exhibit the tricritical behavior and some characteristic features of ferrimagnetism. The effects of non-magnetic layers on the magnetic properties are clarified.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

The Magnetic Properties of the Spin-1 Ising Fullerene Cage with a Core-Shell Structure

Magnetic properties of Ising-type endohedral fullerene (IEF), which are formed by an atom encapsulated inside a diluted magnetic spherical cage, have been investigated theoretically as a function of its system parameters, and magnetism calculations by using effective field theory (EFT) were performed. Theoretical studies have been carried out to understand the effect on magnetic behaviors by the diluted magnetic surface, crystal field, and exchange couplings. The results display the first second-order phase transitions and tricritical point. The reentrant phenomena have been observed within a certain parameter range.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Monte Carlo Study of the Mixed-Spin (1/2, 2) Ferrimagnetic Ising System on a Honeycomb Lattice

By means of the Monte Carlo method, we have simulated the mixed-spin-1/2 and spin-2 Ising ferrimagnetic system on a honeycomb lattice. We studied the variations of the magnetization as functions of the crystal field and the temperature and obtained the phase diagram of the system for various crystal field values. We have found that the system cannot display a point of compensation if we consider only nearest-neighbor interactions and the crystal field effect. Also, the tricritical point does not exist. To the best of our knowledge, this system has never been studied using the Monte Carlo method.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

The Effects of an Applied Transverse Field in a Graphene-Like Ising Bilayer Film with Nonequivalent Planes

Using the effective field theory with correlations, we have studied the phase diagrams and the thermal variations of total magnetization (m{sub T}) in a graphene-like Ising bilayer film with nonequivalent planes, consisting of two A (spin-1/2) and B (spin-1) planes with an applied transverse field in the B plane. We have investigated the effects of an applied transverse field and the interplane exchange interaction on the magnetic properties. Many interesting behaviors, such as various types of m{sub T} curves and the re-entrant phenomena, have been obtained, depending on the selections of physical parameters.

77 NANOSCIENCE AND NANOTECHNOLOGY↗

Reentrant Phenomena in an Antiferromagnetic Ising Nanoparticle and a Ladder-Type Ising System under an Applied Transverse Field

The phase diagrams and magnetization curves of antiferromagnetic nanoparticle and ladder-type system are investigated by using the effective-field theory with correlations. They have exhibited some characteristic features, such as the reentrant phenomena, when a finite transverse field h is applied and even when h = 0.0.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Shape-preserving machining produces gradient nanolaminate medium entropy alloys with high strain hardening capability

A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. In this work, we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. Furthermore, we applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ~50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 k, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperature-dependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material.

36 MATERIALS SCIENCE↗

Geometry and speed of evolution for a spin-s system with long-range zz-type Ising interaction

We study the evolution of a spin-s system described by the long-range zz-type Ising interaction. The Fubini–Study metric of the quantum state manifold defined by this evolution is obtained. We explore the topology of this manifold and show that it corresponds to a sphere. Exploration of the Riemannian curvature allows us to determine the manifold geometry. Also we calculate the speed of evolution of the system and represent the curvature by means of this speed. This is important for an experimental measurement of the curvature. The conditions for achieving the minimal and maximal values of the speed of evolution are obtained. Also we examine the geometry of state manifold and speed of evolution of spin system in the thermodynamic limit. We propose the physical system of methane molecule for application of our considerations. Finally, we study the influence of an external magnetic field on the metric of state manifold and on the speed of evolution. In this case we obtain the conditions for achieving the minimal possible speed of evolution. For some predefined initial states the orientations of magnetic fields to reach the minimal and maximal values of the speed are found.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Particle track classification using quantum associative memory

Pattern recognition algorithms are commonly employed to simplify the challenging and necessary step of track reconstruction in sub-atomic physics experiments. Aiding in the discrimination of relevant interactions, pattern recognition seeks to accelerate track reconstruction by isolating signals of interest. In high collision rate experiments, such algorithms can be particularly crucial for determining whether to retain or discard information from a given interaction even before the data is transferred to tape. As data rates, detector resolution, noise, and inefficiencies increase, pattern recognition becomes more computationally challenging, motivating the development of higher efficiency algorithms and techniques. Quantum associative memory is an approach that seeks to exploits quantum mechanical phenomena to gain advantage in learning capacity, or the number of patterns that can be stored and accurately recalled. Here, we study quantum associative memory based on quantum annealing and apply it to the particle track classification. We focus on discrimination models based on Ising formulations of quantum associative memory model (QAMM) recall and quantum content-addressable memory (QCAM) recall. We characterize classification performance of these approaches as a function detector resolution, pattern library size, and detector inefficiencies, using the D-Wave 2000Q processor as a testbed. Discrimination criteria is set using both solution-state energy and classification labels embedded in solution states. We find that energy-based QAMM classification performs well in regimes of small pattern density and low detector inefficiency. In contrast, state-based QCAM achieves reasonably high accuracy recall for large pattern density and the greatest recall accuracy robustness to a variety of detector noise sources.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

A Generalized Law of Corresponding States for the Physisorption of Classical Gases with Cooperative Adsorbate–Adsorbate Interactions

The Law of Corresponding States for classical gases is well established. Recent attempts at developing an analogous Law of Corresponding States for gas physisorption, however, have had limited success, in part due to the omission of relevant adsorption considerations such as the adsorbate volume and cooperative adsorbate–adsorbate interactions. In this work, we modify a prior Law of Corresponding States for gas physisorption to account for adsorbate volume, and test it with experimental data and a generalized theoretical approach. Furthermore, we account for the recently reported cooperative adsorbate–adsorbate interactions on the surface of zeolite-templated carbon (ZTC) with an Ising-type model and in doing so show that the Law of Corresponding States for gas physisorption remains valid even in the presence of atypically enhanced adsorbate–adsorbate interactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Using Hyperoptimized Tensor Networks and First-Principles Electronic Structure to Simulate the Experimental Properties of the Giant {Mn 84 } Torus

The single-molecule magnet {Mn 84 } is a challenge to theory because of its high nuclearity. Here, we directly compute two experimentally accessible observables, the field-dependent magnetization up to 75 T and the temperature-dependent heat capacity, using parameter-free theory. In particular, we use first-principles calculations to derive short- and long-range exchange interactions and compute the exact partition function of the resulting classical Potts and Ising spin models for all 84 Mn S = 2 spins to obtain observables. The latter computation is made possible by using hyperoptimized tensor network contractions, a technique developed to simulate quantum supremacy circuits. We also synthesize the magnet and measure its heat capacity and magnetization, observing qualitative agreement between theory and experiment and identifying an unusual bump in the heat capacity and a plateau in the magnetization. Our work also identifies some limitations of current theoretical modeling in large magnets, such as sensitivity to small, long-range exchange couplings.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Large-scale simulations of Floquet physics on near-term quantum computers

Abstract Periodically driven quantum systems exhibit a diverse set of phenomena but are more challenging to simulate than their equilibrium counterparts. Here, we introduce the Quantum High-Frequency Floquet Simulation (QHiFFS) algorithm as a method to simulate fast-driven quantum systems on quantum hardware. Central to QHiFFS is the concept of a kick operator which transforms the system into a basis where the dynamics is governed by a time-independent effective Hamiltonian. This allows prior methods for time-independent simulation to be lifted to simulate Floquet systems. We use the periodically driven biaxial next-nearest neighbor Ising (BNNNI) model, a natural test bed for quantum frustrated magnetism and criticality, as a case study to illustrate our algorithm. We implemented a 20-qubit simulation of the driven two-dimensional BNNNI model on Quantinuum’s trapped ion quantum computer. Our error analysis shows that QHiFFS exhibits not only a cubic advantage in driving frequency ω but also a linear advantage in simulation time t compared to Trotterization.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Spin-lattice relaxation within a dimerized Ising chain in a magnetic field

A qualitative study of the spin-lattice relaxation within a dimerized Ising chain in a magnetic field is presented. We have first determined the time dependence of the deviation of the lattice distortion parameter δΔ from the equilibrium state within framework of a technique combining the statistical equilibrium theory based on the transfer matrix method and the linear theory of irreversible thermodynamics. We have shown that the time dependence of the lattice distortion parameter is characterized by a single time constant (τ) which diverges around the critical point in both dimerized (Δ≠0) and uniform (Δ=0) phase regions. When the temperature and magnetic field are fixed to certain values, the time τ depends only on exchange coupling between the spins. It is a characteristic time associated with the long wavelength fluctuations of distortion. We have also taken into account the effects of spatial fluctuations on the relaxation time using the full Landau-Ginzburg free energy functional. We have found an explicit expression for the relaxation time as a function of temperature, coupling constant and wave vector (q) and shown that the critical mode corresponds to the case q=0. Finally, our results are found to be in good qualitative agreement with the results obtained in recent experimental study on synchrotron x-ray scattering and muon spin relaxation in diluted material Cu{sub 1−y}Mg{sub y}GeO{sub 3} where the composition y is very close to 0.0209. These results can be considered as natural extensions of some previous works on static aspects of the problem.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

A spin-1 representation for dual-funnel energy landscapes

The interconversion between the left- and right-handed helical folds of a polypeptide defines a dual-funneled free energy landscape. In this context, the funnel minima are connected through a continuum of unfolded conformations, evocative of the classical helix-coil transition. Physical intuition and recent conjectures suggest that this landscape can be mapped by assigning a left- or right-handed helical state to each residue. In this study, we explore this possibility using all-atom replica exchange molecular dynamics and an Ising-like model, demonstrating that the energy landscape architecture is at odds with a two-state picture. A three-state model—left, right, and unstructured—can account for most key intermediates during chiral interconversion. Additionally, competing folds and excited conformational states still impose limitations on the scope of this approach. However, the improvement is stark: Moving from a two-state to a three-state model decreases the fit error from 1.6 k B T to 0.3 k B T along the left-to-right interconversion pathway.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗