Nearest-neighbour Spin Research Articles

Metamagnetism in a coordination polymer built of trimeric cobalt units and melamine.

A coordination polymer of linear trimeric cobalt units and melamine has been synthesized. The magnetic isotherms of violet coloured crystals as long as 400 μm show a field-induced transition in an external field of about 2 T at temperatures approximately below 2 K. It is addressed that by assuming the coexistent positive and negative exchange between the nearest-neighbour spins in the linear trimer, this metamagnetism can be interpreted as a transition from antiferromagnetic to ferromagnetic exchange within each trimeric spin cluster. Although weak inter-cluster or inter-chain exchange to yield a long-range magnetic order is another possible and often attributed origin of metamagnetism in low-dimensional spin systems, this study demonstrates the significance of the exchange flip within each cluster in clustered spin networks.

Flux noise in disordered spin systems

Impurity spins randomly distributed at the surfaces and interfaces of superconducting wires are known to cause flux noise in Superconducting Quantum Interference Devices, providing a mechanism for decoherence in superconducting qubits. While flux noise is well characterised experimentally, the microscopic model underlying spin dynamics remains unknown. First-principles theories are too computationally expensive to capture spin diffusion over large length scales, third-principles approaches lump spin dynamics into a single phenomenological spin-diffusion operator that is not able to describe the quantum noise regime and connect to microscopic models and disorder scenarios. Here we propose an intermediate "second principles" method to describe general spin dissipation and flux noise in the quantum regime. It leads to the interpretation that flux noise arises from the density of paramagnon excitations at the edge of the wire, with paramagnon-paramagnon interactions leading to spin diffusion, and interactions between paramagnons and other degrees of freedom leading to spin energy relaxation. At high frequency we obtain an upper bound for flux noise, showing that the (super)Ohmic noise observed in experiments does not originate from interacting spin impurities. We apply the method to Heisenberg models in two dimensional square lattices with random distribution of vacancies and nearest-neighbour spins coupled by constant exchange. Numerical calculations of flux noise show that it follows the observed power law $A/\omega^{\alpha}$, with amplitude $A$ and exponent $\alpha$ depending on temperature and inhomogeneities. These results are compared to experiments in niobium and aluminium devices. The method establishes a connection between flux noise experiments and microscopic Hamiltonians identifying relevant microscopic mechanisms and guiding strategies for reducing flux noise.

Orthogonal antiferromagnetism to canted ferromagnetism in CaCo3Ti4O12 quadruple perovskite driven by underlying kagome lattices

AA′3B4O12 quadruple perovskites, with magnetic A′ and non-magnetic B cations, are characterized by a wide range of complex magnetic structures. These are due to a variety of competing spin-exchange interactions up to the fourth nearest neighbours. Here, we synthesize and characterize the magnetic behaviour of the CaCo3Ti4O12 quadruple perovskite. We find that in the absence of an external magnetic field, the system undergoes antiferromagnetic ordering at 9.3 K. This magnetic structure consists of three interpenetrating mutually orthogonal magnetic sublattices. Under an applied magnetic field, this antiferromagnetic structure evolves into a canted ferromagnetic structure. In explaining these magnetic structures, as well as the seemingly unrelated magnetic structures found in other quadruple perovskites, we suggest a crucial role played by the underlying kagome lattices in these systems. All observed magnetic structures of these materials represent indeed one of the three possible ways to reduce spin frustration in the A′ site kagome layers. More specifically, our survey of the magnetic structures observed for quadruple perovskites AA′3B4O12 reveals the following three ways to reduce spin frustration, namely to make each layer ferromagnetic, to adopt a compromise 120° spin arrangement in each layer, or to have a magnetic structure with a vanishing sum of all second nearest-neighbour spin exchanges.

Chiral Waves and Topological Novel States in Fermi

This first show was that the spin system, the two nearest neighbour spin in the Entanglement of measuring compliance by the ground conditions are achieved Berry stage to the relation when it is a closed path developed is. It noted the need to share the stage in a kind of geometric phase as explained to go to which any deformations involved is not and it’s the first Stiefel-Whitney class which is to Z_z–cohomology value takes. However for polarized fermions we can relate the exchange phase as the celebrated Berry phase as in this case the Z_z-cohomology becomes irrelevant and is consistent with the first Chern class which involves curvature. This follows from the depiction of a fermion as a scalar particle attached with a magnetic flux line. As in this framework the measure of entanglement of two nearest neighbor spins in a spin system given by concurrence is found to be associated with the Berry phase acquired by a spin state when it evolves in a closed path we can consider that entanglement is a consequence of Fermi statistics. It has been noted that in terms of quantum field theory, the berry phase is related to the perpetual inconsistency caused by the breaking of the perpetual symmetry. As mentioned earlier the quantization procedure of a fermion in the framework of Nelson’s stochastic quantization procedure introduces an internal variable which appears as a direction vector and gives rise to spin degrees of freedom. It's pointing to is that when a spin- 1 state of two spin 1/2 state is a Entangle system as considered to be that, the most widely covered state longitudinal elements with the matching it.

Quantum Critical Lines in the Ground State Phase Diagram of Spin‐1/2 Frustrated Transverse‐Field Ising Chains

AbstractThis paper focuses on the ground state phase diagram of a 1D spin‐1/2 quantum Ising model with competing first and second nearest neighbour interactions known as the axial next nearest neighbour Ising model in the presence of a transverse magnetic field. Here, using quantum correlations, both numerically and analytically, some evidence is provided to clarify the identification of the ground state phase diagram. Local quantum correlations play a crucial role in detecting the critical lines either revealed or hidden by symmetry‐breaking. A non‐symmetry‐breaking disorder transition line can be identified by the first derivative of both entanglement of formation and quantum discord between nearest neighbour spins. In addition, the quantum correlations between the second neighbour spins can also be used to reveal Kosterlitz–Thouless phase transition when their interaction strength grows and becomes closer to the first nearest neighbour one. The results obtained using the Jordan–Wigner transformation confirm the accuracy of the numerical case.

Finding the ground state of spin Hamiltonians with reinforcement learning

Reinforcement learning (RL) has become a proven method for optimizing a procedure for which success has been defined, but the specific actions needed to achieve it have not. We apply the so-called "black box" method of RL to what has been referred as the "black art" of simulated annealing (SA), demonstrating that an RL agent based on proximal policy optimization can, through experience alone, arrive at a temperature schedule that surpasses the performance of standard heuristic temperature schedules for two classes of Hamiltonians. When the system is initialized at a cool temperature, the RL agent learns to heat the system to "melt" it, and then slowly cool it in an effort to anneal to the ground state; if the system is initialized at a high temperature, the algorithm immediately cools the system. We investigate the performance of our RL-driven SA agent in generalizing to all Hamiltonians of a specific class; when trained on random Hamiltonians of nearest-neighbour spin glasses, the RL agent is able to control the SA process for other Hamiltonians, reaching the ground state with a higher probability than a simple linear annealing schedule. Furthermore, the scaling performance (with respect to system size) of the RL approach is far more favourable, achieving a performance improvement of one order of magnitude on L=14x14 systems. We demonstrate the robustness of the RL approach when the system operates in a "destructive observation" mode, an allusion to a quantum system where measurements destroy the state of the system. The success of the RL agent could have far-reaching impact, from classical optimization, to quantum annealing, to the simulation of physical systems.

Information and Decoherence in a Muon-Fluorine Coupled System.

The unitary evolution of a quantum system preserves its coherence, but interactions between the system and its environment result in decoherence, a process in which the quantum information stored in the system becomes degraded. A spin-polarized positively charged muon implanted in a fluoride crystal realizes such a coherent quantum system, and the entanglement of muon and nearest-neighbor fluorine nuclear spins gives rise to an oscillatory time dependence of the muon polarization that can be detected and measured. Here we show that the decohering effect of more distant nuclear spins can be modelled quantitatively, allowing a very detailed description of the decoherence processes coupling the muon-fluorine "system" with its "environment," and allowing us to track the system entropy as the quantum information degrades. These results show how to precisely quantify the spin relaxation of muons implanted into quantum entangled states in fluoride crystals.

Theory of robust multiqubit nonadiabatic gates for trapped ions

The prevalent approach to executing quantum algorithms on quantum computers is to break-down the algorithms to a concatenation of universal gates, typically single and two-qubit gates. However such a decomposition results in long gate sequences which are exponential in the qubit register size. Furthermore, gate fidelities tend to decrease when acting in larger qubit registers. Thus high-fidelity implementations in large qubit registers is still a prominent challenge. Here we propose and investigate multi-qubit entangling gates for trapped-ions. Our gates couple many qubits at once, allowing to decrease the total number of gates used while retaining a high gate fidelity. Our method employs all of the normal-modes of motion of the ion chain, which allows to operate outside of the adiabatic regime and at rates comparable to the secular ion-trapping frequency. Furthermore we extend our method for generating Hamiltonians which are suitable for quantum analog simulations, such as a nearest-neighbour spin Hamiltonian or the Su-Schrieffer-Heeger Hamiltonian.

Short-range cation and spin ordering in the relaxor ferromagnet La3Ni2SbO9 studied by polarized-neutron scattering and Monte-Carlo methods

Spin-polarised neutron scattering experiments have facilitated the separation of the nuclear and magnetic diffuse scattering from the partially-ordered cation distribution in the perovskite La3Ni2SbO9. The nuclear component of the data collected at 110 and 175 K could be modelled using Monte-Carlo methods when it was assumed that the distribution of Ni2+ and Sb5+ cations over their shared six-coordinate site is that which minimises the repulsive interactions between Sb5+ cations ~5.5 Å apart. Analysis of the diffuse magnetic scattering showed that nearest-neighbour and next-nearest neighbour spins show short-range antiferromagnetic and ferromagnetic coupling, respectively, at 110 K. The former persists to 175 K but the latter weakens with increasing temperature.

On the uniqueness of Gibbs measure in the Potts model on a Cayley tree with external field**To the memory of H-O Georgii.

The paper concerns the q-state Potts model (i.e. with spin values in ) on a Cayley tree of degree (i.e. with k + 1 edges emanating from each vertex) in an external (possibly random) field. We construct the so-called splitting Gibbs measures (SGM) using generalized boundary conditions on a sequence of expanding balls, subject to a suitable compatibility criterion. Hence, the problem of existence/uniqueness of SGM is reduced to solvability of the corresponding functional equation on the tree. In particular, we introduce the notion of translation-invariant SGMs and prove a novel criterion of translation invariance. Assuming a ferromagnetic nearest-neighbour spin–spin interaction, we obtain various sufficient conditions for uniqueness. For a model with constant external field, we provide in-depth analysis of uniqueness versus non-uniqueness in the subclass of completely homogeneous SGMs by identifying the phase diagrams on the ‘temperature–field’ plane for different values of the parameters q and k. In a few particular cases (e.g. q = 2 or k = 2), the maximal number of completely homogeneous SGMs in this model is shown to be 2q − 1, and we make a conjecture (supported by computer calculations) that this bound is valid for all and .

Magnetization plateaus and phase diagrams of the extended Ising model on the Shastry-Sutherland lattice: effects of long-range interactions

Magnetization plateaus and phase diagrams of the extended Ising model on the Shastry-Sutherland lattice with the first $(J_1)$, second $(J_2)$, third $(J_3)$ fourth $(J_4)$ and fifth $(J_5)$ nearest-neighbour spin couplings are studied by the classical Monte Carlo method. It is shown that switching on $J_4$ and $J_5$ interactions (in addition to usually considered $J_1, J_2$ and $J_3$ interactions) changes significantly the picture of magnetization processes found for $J_4=J_5=0$ and leads to stabilization of new macroscopic magnetic phases (plateaus) with fractional magnetization. In particular, it is found that combined effects of $J_4$ and $J_5$ interactions generate the following sequence of plateaus with the fractional magnetization: $m/m_s$=1/9, 1/6, 2/9, 1/3, 4/9, 1/2, 5/9 and 2/3. The results obtained are consistent with experimental measurements of magnetization curves in selected rare-earth tetraborides.

Microscopic study of the role of second nearest neighbour spin density wave coupling and electron hopping on superconducting gap

We report here a tight binding single band model Hamiltonian to study the role of spin density wave interaction and superconductivity. The Hamiltonian consists of nearest and next nearest neighbour electron hopping between the copper sides. The spin density wave interaction also includes nearest-neighbour and the next-nearest-neighbour spin interactions within a mean field approximation. BCS type superconducting interaction is considered taking d-wave pairing symmetry. By using Zubarev's Green function technique the Hamiltonian is solved. The antiferromagnetic and the superconducting gap equations are derived from the correlation functions and are solved self-consistently technique 100 × 100 grid points of the electron momentum. The evolution of these two order parameters are investigated by varying hole doping concentration, superconducting coupling, spin density wave coupling and second nearest neighbour electron hopping integrals.

Microscopic study of the role of second nearest neighbour spin density wave coupling and electron hopping on superconducting gap

We report here a tight binding single band model Hamiltonian to study the role of spin density wave interaction and superconductivity. The Hamiltonian consists of nearest and next nearest neighbour electron hopping between the copper sides. The spin density wave interaction also includes nearest-neighbour and the next-nearest-neighbour spin interactions within a mean field approximation. BCS type superconducting interaction is considered taking d-wave pairing symmetry. By using Zubarev's Green function technique the Hamiltonian is solved. The antiferromagnetic and the superconducting gap equations are derived from the correlation functions and are solved self-consistently technique 100 × 100 grid points of the electron momentum. The evolution of these two order parameters are investigated by varying hole doping concentration, superconducting coupling, spin density wave coupling and second nearest neighbour electron hopping integrals.

Asymptotic Behaviour of Ground States for Mixtures of Ferromagnetic and Antiferromagnetic Interactions in a Dilute Regime

We consider randomly distributed mixtures of bonds of ferromagnetic and antiferromagnetic type in a two-dimensional square lattice with probability $1-p$ and $p$, respectively, according to an i.i.d. random variable. We study minimizers of the corresponding nearest-neighbour spin energy on large domains in ${\mathbb Z}^2$. We prove that there exists $p_0$ such that for $p\le p_0$ such minimizers are characterized by a majority phase; i.e., they take identically the value $1$ or $-1$ except for small disconnected sets. A deterministic analogue is also proved.

Chaotic Dynamical Ferromagnetic Phase Induced by Nonequilibrium Quantum Fluctuations.

We investigate the robustness of a dynamical phase transition against quantum fluctuations by studying the impact of a ferromagnetic nearest-neighbor spin interaction in one spatial dimension on the nonequilibrium dynamical phase diagram of the fully connected quantum Ising model. In particular, we focus on the transient dynamics after a quantum quench and study the prethermal state via a combination of analytic time-dependent spin wave theory and numerical methods based on matrix product states. We find that, upon increasing the strength of the quantum fluctuations, the dynamical critical point fans out into a chaotic dynamical phase within which the asymptotic ordering is characterized by strong sensitivity to the parameters and initial conditions. We argue that such a phenomenon is general, as it arises from the impact of quantum fluctuations on the mean-field out of equilibrium dynamics of any system which exhibits a broken discrete symmetry.

Exact solutions to plaquette Ising models with free and periodic boundaries

An anisotropic limit of the 3d plaquette Ising model, in which the plaquette couplings in one direction were set to zero, was solved for free boundary conditions by Suzuki (1972) [1], who later dubbed it the fuki-nuke, or “no-ceiling”, model. Defining new spin variables as the product of nearest-neighbour spins transforms the Hamiltonian into that of a stack of (standard) 2d Ising models and reveals the planar nature of the magnetic order, which is also present in the fully isotropic 3d plaquette model. More recently, the solution of the fuki-nuke model was discussed for periodic boundary conditions, which require a different approach to defining the product spin transformation, by Castelnovo et al. (2010) [2].We clarify the exact relation between partition functions with free and periodic boundary conditions expressed in terms of original and product spin variables for the 2d plaquette and 3d fuki-nuke models, noting that the differences are already present in the 1d Ising model. In addition, we solve the 2d plaquette Ising model with helical boundary conditions. The various exactly solved examples illustrate how correlations can be induced in finite systems as a consequence of the choice of boundary conditions.

Quantum decoherence dynamics of divacancy spins in silicon carbide.

Long coherence times are key to the performance of quantum bits (qubits). Here, we experimentally and theoretically show that the Hahn-echo coherence time of electron spins associated with divacancy defects in 4H–SiC reaches 1.3 ms, one of the longest Hahn-echo coherence times of an electron spin in a naturally isotopic crystal. Using a first-principles microscopic quantum-bath model, we find that two factors determine the unusually robust coherence. First, in the presence of moderate magnetic fields (30 mT and above), the 29Si and 13C paramagnetic nuclear spin baths are decoupled. In addition, because SiC is a binary crystal, homo-nuclear spin pairs are both diluted and forbidden from forming strongly coupled, nearest-neighbour spin pairs. Longer neighbour distances result in fewer nuclear spin flip-flops, a less fluctuating intra-crystalline magnetic environment, and thus a longer coherence time. Our results point to polyatomic crystals as promising hosts for coherent qubits in the solid state.

Single-spin-flip dynamics of the Ising chain

We consider the most general single-spin-flip dynamics for the ferromagnetic Ising chain with nearest-neighbour influence and spin reversal symmetry. This dynamics is a two-parameter extension of Glauber dynamics corresponding respectively to non-linearity and irreversibility. The associated stationary measure is given by the usual Boltzmann–Gibbs distribution for the ferromagnetic Hamiltonian of the chain. We study the properties of this dynamics both at infinite and at finite temperature, all over its parameter space, with particular emphasis on special lines and points.

Fractional magnetization plateaus in the extended Ising model on the Shastry-Sutherland lattice: Application to rare-earth metal tetraborides

The classical Monte Carlo method is used to examine possibilities of formation of magnetization plateaus in the extended Ising model with the first , second , third , and fourth nearest-neighbour spin couplings on the Shastry–Sutherland lattice. It is shown that in the limit of ferromagnetic interaction the combined effects of and lead to the stabilization of the magnetization plateau against the standard Ising plateau (where is the saturation magnetization). At some critical values of and interactions the plateau phase is fully removed from the ground-state phase diagram providing a possible explanation of the fact that in some rare-earth tetraborides this phase is present and in some completely absent. In the opposite limit of antiferromagnetic interaction the model provides a reach spectrum of magnetic solutions, where in addition to the standard magnetization plateau one can find the following sequence of plateaus with the fractional magnetization: =1/10, 1/9, 1/6, 1/5, 2/5, 4/9, 7/15, and 5/9. The results obtained are consistent with experimental measurements of magnetization curves in selected rare-earth tetraborides.

Ground-State Phase Diagram of the Extended Ising Model on the Shastry-Sutherland Lattice

on the Shastry-Sutherland Lattice H. ƒen£arikovaa,∗, P. Farka2ovský and P. Puchala Institute of Experimental Physics SAS, Watsonova 47, 040 01 Ko2ice, Slovakia Institute of Physics, Faculty of Sciences, P. J. ’afarik University, Park Angelinum 9, 041 54 Ko2ice, Slovakia The ground-state phase diagram of the Ising model on the Shastry-Sutherland lattice with the rst (J1), second (J2), third (J3) and fourth (J4) nearest neighbour spin couplings is studied using the exact and well controlled numerical method. It is shown that switching on of the J3 and J4 interactions (in addition to usually considered J1 and J2 interactions) leads to stabilization of the following dominant magnetic phases with m/ms = 1/9, 1/6, 4/9, 1/2 and 5/9, which is manifested in appearance of new magnetization plateaus on the magnetization curve of the model at given values of m/ms. The obtained results are consistent with experimental measurements of magnetization curves in selected rare-earth tetraborides.