Field induced multiple order-by-disorder state selection in antiferromagnetic honeycomb bilayer lattice

In this paper we present a detailed study of the antiferromagnetic classical Heisenberg model on a bilayer honeycomb lattice in a highly frustrated regime in presence of a magnetic field. This study shows strong evidence of entropic order-by-disorder selection in different sectors of the magnetization curve. For antiferromagnetic couplings $J_1=J_x=J_p/3$, we find that at low temperatures there are two different regions in the magnetization curve selected by this mechanism with different number of soft and zero modes. These regions present broken $Z_2$ symmetry and are separated by a not fully collinear classical plateau at $M=1/2$. At higher temperatures, there is a crossover from the conventional paramagnet to a cooperative magnet. Finally, we also discuss the low temperature behavior of the system for a less frustrated region, $J_1=J_x<J_p/3$.Comment: revised version - accepted for publication in Physical Review B - 12 pages, 11 figure

Magnetization plateaux and jumps in a frustrated four-leg spin tube under a magnetic field

We study the ground state phase diagram of a frustrated spin-1/2 four-leg spin tube in an external magnetic field. We explore the parameter space of this model in the regime of all-antiferromagnetic exchange couplings by means of three different approaches: analysis of low-energy effective Hamiltonian (LEH), a Hartree variational approach (HVA) and density matrix renormalization group (DMRG) for finite clusters. We find that in the limit of weakly interacting plaquettes, low-energy singlet, triplet and quintuplet states play an important role in the formation of fractional magnetization plateaux. We study the transition regions numerically and analytically, and find that they are described, at first order in a strong- coupling expansion, by an XXZ spin-1/2 chain in a magnetic field; the second-order terms give corrections to the XXZ model. All techniques provide consistent results which allow us to predict the existence of fractional plateaux in an important region in the space of parameters of the model.Comment: 10 pages, 7 figures. Accepted for publication in Physical Review

Metastable and scaling regimes of a one-dimensional Kawasaki dynamics

We investigate the large-time scaling regimes arising from a variety of metastable structures in a chain of Ising spins with both first- and second-neighbor couplings while subject to a Kawasaki dynamics. Depending on the ratio and sign of these former, different dynamic exponents are suggested by finite-size scaling analyses of relaxation times. At low but nonzero-temperatures these are calculated via exact diagonalizations of the evolution operator in finite chains under several activation barriers. In the absence of metastability the dynamics is always diffusive.Comment: 18 pages, 8 figures. Brief additions. To appear in Phys. Rev.

Machine learning techniques to construct detailed phase diagrams for skyrmion systems

Recently, there has been an increased interest in the application of machine learning (ML) techniques to a variety of problems in condensed matter physics. In this regard, of particular significance is the characterization of simple and complex phases of matter. Here, we use a ML approach to construct the full phase diagram of a well known spin model combining ferromagnetic exchange and Dzyaloshinskii-Moriya (DM) interactions where topological phases emerge. At low temperatures, the system is tuned from a spiral phase to a skyrmion crystal by a magnetic field. However, thermal fluctuations induce two types of intermediate phases, bimerons and skyrmion gas, which are not as easily determined as spirals or skyrmion crystals. We resort to large scale Monte Carlo simulations to obtain low temperature spin configurations, and train a convolutional neural network (CNN), taking only snapshots at specific values of the DM couplings, to classify between the different phases, focusing on the intermediate and intricate topological textures. We then apply the CNN to higher temperature configurations and to other DM values, to construct a detailed magnetic field-temperature phase diagram, achieving outstanding results. We discuss the importance of including the disordered paramagnetic phases in order to get the phase boundaries, and finally, we compare our approach with other ML algorithms.Comment: 9 pages, 10 figures; accepted for publication in Physical Review

Spin-phonon induced magnetic order in Kagome ice

We study the effects of lattice deformations on the Kagome spin ice, with Ising spins coupled by nearest neighbor exchange and long range dipolar interactions, in the presence of in-plane magnetic fields. We describe the lattice energy according to the Einstein model, where each site distortion is treated independently. Upon integration of lattice degrees of freedom, effective quadratic spin interactions arise. Classical MonteCarlo simulations are performed on the resulting model, retaining up to third neighbor interactions, under different directions of the magnetic field. We find that, as the effect of the deformation is increased, a rich plateau structure appears in the magnetization curves.Comment: 7 pages, 8 figure

An algorithm to represent inbreeding trees

Recent work has proven the existence of extreme inbreeding in a European ancestry sample taken from the contemporary UK population \cite{nature_01}. This result brings our attention again to a math problem related to inbreeding family trees and diversity. Groups with a finite number of individuals could give a variety of genetic relationships. { In previous works \cite{PhysRevE.92.052132, PhysRevE.90.022125, JARNE20191}, we have addressed the issue of building inbreeding trees for biparental reproduction using Markovian models. Here, we extend these studies by presenting an algorithm to generate and represent inbreeding trees with no overlapping generations. We explicitly assume a two-gender reproductory scheme, and we pay particular attention to the links between nodes. We show that even for a simple case with a relatively small number of nodes in the tree, there are a large number of possible ways to rearrange the links between generations. We present an open-source python code to generate the tree graph, the adjacency matrix, and the histogram of the links for each different tree representation. We show how this mapping reflects the difference between tree realizations, and how valuable information may be extracted upon inspection of these matrices. The algorithm includes a feature to average several tree realizations, obtain the connectivity distribution, and calculate the average and mean value. We used this feature to compare trees with a different number of generations and nodes. The code presented here, available in Git-Hub, may be easily modified to be applied to other areas of interest involving connections between individuals, extend the study to add more characteristics of the different nodes, etc

Chiral phase transition and thermal Hall effect in an anisotropic spin model on the kagome lattice

We present a study of the thermal Hall effect in the extended Heisenberg model with $XXZ$ anisotropy in the kagome lattice. This model has the particularity that, in the classical case, and for a broad region in parameter space, an external magnetic field induces a chiral symmetry breaking: the ground state is a doubly degenerate $q=0$ order with either positive or negative net chirality. Here, we focus on the effect of this chiral phase transition in the thermal Hall conductivity using Linear-Spin-Waves theory. We explore the topology and calculate the Chern numbers of the magnonic bands, obtaining a variety of topological phase transitions. We also compute the magnonic effect to the critical temperature associated with the chiral phase transition ($T_c^{SW}$). Our main result is that, the thermal Hall conductivity, which is null for $T>T_c^{SW}$, becomes non-zero as a consequence of the spontaneous chiral symmetry breaking at low temperatures. Therefore, we present a simple model where it is possible to "switch" on/off the thermal transport properties introducing a magnetic field and heating or cooling the system.Comment: 9 pages, 6 figures, Accepted for publication in Phys. Rev.

From chiral spin liquids to skyrmion fluids and crystals, and their interplay with itinerant electrons

We present an in-depth study of the competition between skyrmions and a chiral spin liquid in a model on the kagome lattice that was recently proposed by some of the authors [H. D. Rosales, et al. Phys. Rev. Lett. 130, 106703 (2023)]. We present an analytical overview of the low-energy states using the Luttinger-Tisza approximation. Then we add thermal fluctuations thanks to large-scale Monte-Carlo simulations, and explore the entire parameter space with a magnetic field $B$, in-plane $D^{xy}$ and out-of-plane $D^z$ Dzyaloshinskii-Moriya interactions, using the ferromagnetic strength as unit of energy. While skyrmions and the chiral spin liquid live in different regions of parameter space, we show how to bring them together, stabilizing a skyrmion fluid in between; a region where the density of well-defined skyrmions can be tuned from quasi-zero (gas) to saturated (liquid) before ordering of the skyrmions (solid). In particular, we investigate the two-dimensional melting of the skyrmion solid. Our analysis also brings to light a long-range ordered phase with Z$_3$ symmetry. To conclude, when conduction electrons are coupled to the local spins, different chiral magnetic textures stabilized in this model (skyrmion solid, liquid and gas \& chiral spin liquid) induce anomalous Quantum Hall effect in the magnetically disordered skyrmion liquid for specific band-filling fractions. Landau levels persist even in the skyrmion-liquid regime in absence of broken translational symmetry and gradually disappear as the skyrmion density decreases to form a gas.Comment: 18 pages, 16 figure

Intermediate magnetization state and competing orders in Dy2Ti2O7 and Ho2Ti2O7

We thank R. Moessner, C. Castelnovo and M. Gingras for helpful discussions, and the financial support of ANPCYT through PICT 2013-2004 and PICT 2014-2618 and CONICET (Argentina), the EPSRC and the Royal Society (UK).Among the frustrated magnetic materials, spin-ice stands out as a particularly interesting system. Residual entropy, freezing and glassiness, Kasteleyn transitions and fractionalization of excitations in three dimensions all stem from a simple classical Hamiltonian. But is the usual spin-ice Hamiltonian a correct description of the experimental systems? Here we address this issue by measuring magnetic susceptibility in the two most studied spin-ice compounds, Dy2Ti2O7 and Ho2Ti2O7, using a vector magnet. Using these results, and guided by a theoretical analysis of possible distortions to the pyrochlore lattice, we construct an effective Hamiltonian and explore it using Monte Carlo simulations. We show how this Hamiltonian reproduces the experimental results, including the formation of a phase of intermediate polarization, and gives important information about the possible ground state of real spin-ice systems. Our work suggests an unusual situation in which distortions might contribute to the preservation rather than relief of the effects of frustration.Publisher PDFPeer reviewe

Proteomic and Physiological Responses of Kineococcus radiotolerans to Copper

Copper is a highly reactive, toxic metal; consequently, transport of this metal within the cell is tightly regulated. Intriguingly, the actinobacterium Kineococcus radiotolerans has been shown to not only accumulate soluble copper to high levels within the cytoplasm, but the phenotype also correlated with enhanced cell growth during chronic exposure to ionizing radiation. This study offers a first glimpse into the physiological and proteomic responses of K. radiotolerans to copper at increasing concentration and distinct growth phases. Aerobic growth rates and biomass yields were similar over a range of Cu(II) concentrations (0–1.5 mM) in complex medium. Copper uptake coincided with active cell growth and intracellular accumulation was positively correlated with Cu(II) concentration in the growth medium (R2 = 0.7). Approximately 40% of protein coding ORFs on the K. radiotolerans genome were differentially expressed in response to the copper treatments imposed. Copper accumulation coincided with increased abundance of proteins involved in oxidative stress and defense, DNA stabilization and repair, and protein turnover. Interestingly, the specific activity of superoxide dismutase was repressed by low to moderate concentrations of copper during exponential growth, and activity was unresponsive to perturbation with paraquot. The biochemical response pathways invoked by sub-lethal copper concentrations are exceptionally complex; though integral cellular functions are preserved, in part, through the coordination of defense enzymes, chaperones, antioxidants and protective osmolytes that likely help maintain cellular redox. This study extends our understanding of the ecology and physiology of this unique actinobacterium that could potentially inspire new biotechnologies in metal recovery and sequestration, and environmental restoration