Search

Motivated by the idea of using simple macroscopic examples to illustrate the physics of complex systems, we modify a historic experimental setup in which interacting floating magnets spontaneously self-assemble into ordered clusters. By making the cluster components mechanically stable against their natural tendency to flip and coalesce, we can monitor the torque experienced by individual magnets through the macroscopic tilt angles they acquire when exposed to non-collinear external fields. A mathematical model that reproduces the empirical observations is introduced, enabling us to go beyond the experimental cases considered. The model confirms the existence of alternative orderings as the number of objects increases. Furthermore, a simpler and more mathematically transparent version of our model enables us to establish the conditions under which the cluster structure is stable and when it will collapse. We argue that our simple experimental setup combined with the accompanying models may be useful to describe general features seen in systems composed of mutually repulsive particles in the presence of modulating confining fields., Comment: 14 pages, 8 figures, submitted to PRE

General Relativity (GR) is not the only way gravity can be geometrised. Instead of curvature, the Teleparallel Theory attributes gravity to torsion $T$, which is related to the antysimmetric part of connection, and the Symmetric Teleparallel theory no longer preserves metricity, describing gravity through the non-metricity tensor $Q_{\alpha\mu\nu}\equiv \nabla_\alpha g_{\mu\nu}.$ These descriptions give form to what is known as geometrical trinity of gravity. Recently, the extensions of GR have been intensively investigated in order to solve the theoretical impasses which have arisen. In this sense, it is also useful to investigate the extensions of alternative descriptions of gravity, which leads us to the so-called $f(T)$ and $f(Q)$ gravities. In this paper, we consider a family of $f(Q)$ models and obtain their corresponding Tolman-Oppenheimer-{Volkoff} equations applied to {polytropic} stars. Using numerical integration, it is possible to solve a system of differential equations and calculate, among other things, the maximum mass and mass-radius relation allowed. In addition, we explicitly show the non-metricity behavior inside and outside the star.

In 1935, Einstein, Podolsky, and Rosen argued that quantum mechanics is incomplete, based on the assumption that local actions cannot influence elements of reality at a distant location (local realism). In this work, using a recently defined quantum reality quantifier, we show that Alice's local quantum operations can be correlated with the erasure of the reality of observables in Bob's causally disconnected laboratory. To this end, we implement a modified optical quantum eraser experiment, ensuring that Alice's and Bob's measurements remain causally disconnected. Using an entangled pair of photons and quantum state tomography, we experimentally verify that, even with the total absence of any form of classical communication, the choice of quantum operation applied by Alice on her photon is correlated with the erasure of a spatial element of reality of Bob's photon. In this case, it is shown that Bob's photon can entangle two extra non-interacting degrees of freedom, thus confirming that Bob's photon path is not an element of physical reality.

This study presents the results of a search for rotation signature in 250 Gaia DR3 Ultra-Cool Dwarfs (UCDs) with TESS light curves. We identified 71 targets with unambiguous periodicities, of which 61 present rotation signatures and a single source behavior, with periods between 0.133 and 5.81 days. Five UCDs show double-dip features, namely variations with two periods, one approximately double or half the other. The remaining ten UCDs with unambiguous variability present a likely non-single behavior. We also found 20 UCDs showing complex behavior in their light curves, with noticeable fluctuations and irregular structure, with a few exhibiting apparent changes in their temporal structure. The remaining 159 targets show noisy light curves corresponding to low-amplitude signals, whose temporal variation cannot be easily identified. The distribution of the UCDs with rotation signature in the CMD diagram points to a lack of rotating objects within about $11.5

There has been a growing interest within the astrophysics community in highly magnetized and fast-spinning white dwarfs (WDs), commonly referred to as HMWDs. WDs with these characteristics are quite uncommon and possess magnetic fields $\geqslant 10^6$ G, along with short rotation periods ranging from seconds to just a few minutes. Based on our previous work, we analyze the emission of Gravitational Waves (GWs) in HMWDs through two mechanisms: matter accretion and magnetic deformation, which arise due to the asymmetry surrounding the star's rotational axis. Here, we perform a thorough self-consistent analysis, accounting for rotation and employing a realistic equation of state to investigate the stability of stars. Our investigation focuses on the emission of gravitational radiation from six rapidly spinning WDs: five of them are situated within binary systems, while one is an AXP, proposed as a magnetic accreting WD. Furthermore, we apply the matter accretion mechanism alongside the magnetic deformation mechanism to assess the influence of one process on the other. Our discoveries indicate that these WDs could potentially act as GW sources for BBO and DECIGO, depending on specific parameters, such as their mass, the angle ($\alpha$) between the magnetic and rotational axes, and the accumulated mass ($\delta m$) at their magnetic poles, which is influenced by the effect of matter accretion. However, detecting this particular class of stars using the LISA and TianQin space detectors seems unlikely due to the challenging combination of parameters such as a large $\delta m$, a large $\alpha$ angle and a small WD mass value., Comment: Accepted for publication in MNRAS

Perturbed angular correlation spectroscopy combined with $ab-initio$ electronic structure calculations is used to unravel the structural phase transition path from the low-temperature polar structure to the high-temperature structural phase in $Ca_3Ti_2O_7$, a hybrid improper ferroelectric. This procedure explores the unique features of a local probe environment approach by monitoring the evolution of the electric field gradient tensor at the calcium sites. The local environments, observed above 1057 K, confirm a structural phase transition from the $A2_1am$ symmetry to an orthorhombic $Acaa$ symmetry in the $Ca_3Ti_2O_7$ crystal lattice, disagreeing with the frequently reported avalanche structural transition from the polar $A2_1am$ phase to the aristotype $I4/mmm$ phase. Moreover, the EFG temperature dependency, within the $A2_1am$ temperature stability, is shown to be sensitive to the recently proposed $Ca_3Ti_2O_7$ ferroelectric polarization decrease within the 500-800~K temperature range.

Double white-dwarf (DWD) mergers are relevant astrophysical sources expected to produce massive, highly-magnetized WDs, supernovae (SNe) Ia, and neutron stars (NSs). Although they are expected to be numerous sources in the sky, their detection has evaded the most advanced transient surveys. This article characterizes the optical transient expected from DWD mergers in which the central remnant is a stable (sub-Chandrasekhar) WD. We show that the expansion and cooling of the merger's dynamical ejecta lead to an optical emission peaking at $1$-$10$ d post-merger, with luminosities of $10^{40}$-$10^{41}$ erg s$^{-1}$. We present simulations of the light-curves, spectra, and the color evolution of the transient. We show that these properties, together with the estimated rate of mergers, are consistent with the absence of detection, e.g., by The Zwicky Transient Facility (ZTF). More importantly, we show that the Legacy Survey of Space and Time (LSST) of the Vera C. Rubin Observatory will likely detect a few/several hundred per year, opening a new window to the physics of WDs, NSs, and SN Ia., Comment: Accepted for publication in The Astrophysical Journal

We present an \textit{ab-initio} study of the quasi-2D layered perovskite Sr$_3$Hf$_2$O$_7$ com\-pound, performed within the framework of the Density Functional Theory and lattice dynamics analysis. At high temperatures, this compound takes a \textit{I4/mmm} centrosym\-met\-ric structure (S.G. n. 139); as the temperature is lowered, the symmetry is broken into other intermediate polymorphs before reaching the ground state structure, which is the \textit{Cmc2$_1$} ferroelectric phase (S.G. n. 36). One of these intermediate polymorphs is the \textit{Ccce} structural phase (S.G. n. 68). Additionally, we have probed the \textit{C2/c} system (S.G n. 15), which was obtained by following the atomic displacements corresponding to the eigenvectors of the imaginary frequency mode localized at the $\mathbf{\Gamma}$-point of the \textit{Ccce} phase. By observing the enthalpies at low pressures, we found that the \textit{Cmc2$_1$} phase is thermodynamically the most stable. Our results show that the \textit{I4/mmm} and \textit{C2/c} phases never stabilize in the 0-20 GPa range of pressure values. On the other hand, the \textit{Ccce} phase becomes energetically more stable at around 17 GPa, surpassing the \textit{Cmc2$_1$} structure. By considering the effect of entropy and the constant-volume free energies, we observe that the \textit{Cmc2$_1$} polymorph is energetically the most stable phase at low temperature; however, at 350 K the \textit{Ccce} system becomes the most stable. By probing the volume-dependent free energies at 19 GPa, we see that \textit{Ccce} is always the most stable phase between the two structures and also throughout the studied temperature range. When analyzing the phonon dispersion frequencies, we conclude that the \textit{Ccce} system becomes dynamically stable only around 19-20 GPa, and that the \textit{Cmc2$_1$} phase, is metastable up to 30 GPa., Comment: 32 pages, 11 figures

Stressed soft materials commonly present viscoelastic signatures in the form of power-law or exponential decay. Understanding the origins of such rheologic behaviors is crucial to find proper technological applications. Using an elastic network model of macromolecules immersed in a viscous fluid, we numerically reproduce those characteristic viscoelastic relaxations and show how the microscopic interactions determine the rheologic response. We find that exponential relaxations are indeed the most common behavior. However, power laws may arise when drag forces between the macromolecules and the fluid are sublinear, which is related to micro-deformations of the macromolecules.

In this work, for a given oriented graph $D$, we study its interval and hull numbers, respectively, in the oriented geodetic, P3 and P3* convexities. This last one, we believe to be formally defined and first studied in this paper, although its undirected version is well-known in the literature. Concerning bounds, for a strongly oriented graph D, and the oriented geodetic convexity, we prove that $ohng(D)\leq m(D)-n(D)+2$ and that there is at least one such that $ohng(D) = m(D)-n(D)$. We also determine exact values for the hull numbers in these three convexities for tournaments, which imply polynomial-time algorithms to compute them. These results allow us to deduce polynomial-time algorithms to compute $ohnp(D)$ when the underlying graph of $D$ is split or cobipartite. Moreover, we provide a meta-theorem by proving that if deciding whether $oing(D)\leq k$ or $ohng(D)\leq k$ is NP-hard or W[i]-hard parameterized by $k$, for some $i\in\mathbb{Z_+^*}$, then the same holds even if the underlying graph of $D$ is bipartite. Next, we prove that deciding whether $ohnp(D)\leq k$ or $ohnps(D)\leq k$ is W[2]-hard parameterized by $k$, even if $D$ is acyclic and its underlying graph is bipartite; that deciding whether $ohng(D)\leq k$ is W[2]-hard parameterized by $k$, even if $D$ is acyclic; that deciding whether $oinp(D)\leq k$ or $oinps(D)\leq k$ is NP-complete, even if $D$ has no directed cycles and the underlying graph of $D$ is a chordal bipartite graph; and that deciding whether $oinp(D)\leq k$ or $oinps(D)\leq k$ is W[2]-hard parameterized by $k$, even if the underlying graph of $D$ is split. Finally, also argue that the interval and hull numbers in the oriented P3 and P3* convexities can be computed in cubic time for graphs of bounded clique-width by using Courcelle's theorem.

Double white dwarf (DWD) mergers are possibly the leading formation channel of massive, rapidly rotating, high-field magnetic white dwarfs (HFMWDs). However, the direct link connecting a DWD merger to any observed HFMWD is still missing. We here show that the HFMWDs SDSS J221141.80+113604.4 (hereafter J2211+1136) and ZTF J190132.9+145808.7 (hereafter J1901+1458), might be DWD merger products. J2211+1136 is a $1.27\, M_\odot$ WD with a rotation period of $70.32$ s and a surface magnetic field of $15$ MG. J1901+1458 is a $1.327$--$1.365\, M_\odot$ WD with a rotation period of $416.20$ s, and a surface magnetic field in the range $600$--$900$ MG. With the assumption of single-star evolution, the currently measured WD masses and surface temperatures, the cooling ages of J2211+1136 and J1901+1458 are, respectively, $2.61$--$2.85$ Gyr and $10$--$100$ Myr. We hypothesize that these WDs are DWD merger products and compute the evolution of the post-merged configuration formed by a central WD surrounded by a disk. We show that the post-merger system evolves through three phases depending on whether accretion, mass ejection (propeller), or magnetic braking dominates the torque onto the central WD. We calculate the time the WD spends in each of these phases and obtain the accretion rate and disk mass for which the WD rotational age, i.e., the total time elapsed since the merger to the instant where the WD central remnant reaches the current measured rotation period, agrees with the estimated WD cooling age. We infer the mass values of the primary and secondary WD components of the DWD merger that lead to a post-merger evolution consistent with the observations.

We report a simple, efficient manner to assess magnetic interactions, more specifically the exchange interaction, in systems as blocked magnetic nanoparticles. Specifically, we investigate what is the theoretical limit for the external magnetic susceptibility in a system described by the plain old Stoner-Wohlfarth model. We go beyond and introduce a general mean field theory for interacting systems, thus estimating how the magnetic susceptibility is affected due to the dipolar and exchange interactions inside the system. We disclose a fundamental inequality for the magnetic susceptibility and show its violation is an unambiguous signature of the existence of exchange interactions between nanoparticles in a collective system. To test the robustness of our theoretical achievements, we examine magnetization measurements and Henkel plots for blocked magnetic nanopowders. The agreement between experiment and theory provides evidence to confirm the validity of our findings., Comment: 8 pages, 4 figures

We report two fundamental inequalities in the Stoner-Wohlfarth model. Specifically, we investigate the theoretical limit for the initial magnetic susceptibility in a system described by the Stoner-Wohlfarth approach. We also find analytical solutions for the magnetization in the low-field regime and obtain the borderline value for the uniaxial anisotropy constant in such an ideal Stoner-Wohlfarth system. We go beyond and introduce a general mean-field theory for interacting Stoner-Wohlfarth-like systems, thus estimating how the initial magnetic susceptibility is affected by the dipolar and exchange interactions inside the system. By means of a simple insight from a fundamental inequality for the magnetic susceptibility of an ideal Stoner-Wohlfarth system, we show its violation is a signature of the existence of exchange interactions between nanoparticles in an interacting Stoner-Wohlfarth-like system., Comment: arXiv admin note: text overlap with arXiv:2110.04139

We consider a general quantum field relativistic scattering involving two half spin fermions, $A$ and $B$, which are initially entangled with another fermion $C$ that does not participate in the scattering dynamics. We construct general expressions for the reduced spin matrices for the out-state considering a general tripartite spin-entangled state. In particular we study an inelastic QED process at tree-level, namely $e^-e^+\rightarrow \mu^- \mu^+$ and a half spin fermion $C$ as an spectator particle which can be entangled to the $AB$ system in the following ways: W state, GHZ state, $|\text{A}^\alpha \rangle \otimes |\Psi^{\pm} \rangle_{\text{BC}}$ and $|\text{A}^\alpha \rangle \otimes |\Phi^{\pm} \rangle_{\text{BC}}$, where $\{|\Psi^{\pm} \rangle,|\Phi^{\pm} \rangle\}$ are the Bell basis states and $|\text{A}^\alpha \rangle$ is a spin superposition state of system $A$. We calculate the von-Neumann entropy variation before and after the scattering for the particle $C$ and show that spin measurements in $C$ contain numerical information about the total cross section of the process. We compare the initial states W and GHZ as well as study the role played by the parameter $\alpha$ in the evaluation of the entropy variations and the cross section encoded in the spectator particle., Comment: 12 pages, 10 figures

Heat, work and entropy production: the statistical distribution of such quantities are constrained by the fluctuation theorems (FT), which reveal crucial properties about the nature of non-equilibrium dynamics. In this paper we report theoretical and experimental results regarding two FT for a new quantity, named coherent energy, which is an energy form directly associated with the coherences of the quantum state. We also demonstrate that this quantity behaves as a thermodynamic arrow of time for unitary evolutions, that is, in the absence of entropy production. The experiment is implemented in an all-optical setup in which the system is encoded in the polarization of one photon of a pair. The FT are demonstrated using the two-point measurement protocol, executed using the other photon of the pair, allowing to assess the probability distributions directly from the outcomes of the experiment., Comment: comments are welcome

Quantum correlations encoded in photonic Laguerre-Gaussian modes were shown to be related to the Gouy phase shifts (D. Kawase et al., Phys. Rev. Lett. 101, 050501 (2008)) allowing for a non-destructive manipulation of photonic quantum states. In this work we exploit the relation between phase space correlations of biphotons produced by spontaneously parametric down conversion (SPDC) as encoded in the logarithmic negativity (LN) and the Gouy phase as they are diffracted through an asymmetrical double slit setup. Using an analytical approach based on a double-gaussian approximation for type-I SPDC biphotons, we show that measurements of Gouy phase differences provide information on their phase space entanglement variation, governed by the physical parameters of the experiment and expressed by the LN via covariance matrix elements.

The Teleparallel Theory is an alternative theory of gravity equivalent to General Relativity (GR) and with non-vanishing torsion $T$. Some extensions of this theory, the so-called $f(T)$ models, have been subject of many recent works. The purpose of our work in the end is to consider recent results for a specific family of $f(T)$ models by using their corresponding Tolman-Oppenheimer-Volkof to describe compact objects such as neutron stars. By performing numerical calculations, it is possible to find, among other things, the maximum mass allowed by the model for a neutron star for a given equation of state (EOS), which would also allow us to evaluate which models are in accordance with observations. To begin with, the present work, the second in the series, considers polytropic EOSs since they can offer a simpler and satisfactory description for the compact objects. In addition, with these EOSs, we can already assess how different the $f(T)$ theories are in relation to GR with respect to the stellar structure. The results already known to GR must be reproduced to some extent and, eventually, we can find models that allow higher maximum masses than Relativity itself, which could explain, for example, the secondary component of the event GW190814. This particular issue will be subject of a forthcoming paper, the third in the series, where realistic EOSs are considered.

The torsion models have stood out among the proposals for an alternative description of gravity. The simplest of them, the Teleparallel theory, is equivalent to General Relativity and there are many studies that seek to study its extension to more general functions of the torsion $T$. The purpose of {our study } is to consider a family of $f(T)$ models and apply their corresponding Tolman-Oppenheimer-Volkof equations to compact objects such as neutron stars. Consequently, through a numerical analysis, calculate, among other things, the maximum mass allowed by the model for a neutron star, which would also allow us to evaluate which models are in accordance with observations. In the present paper, the first in the series, we show explicitly the set of equations that must be solved, and how to solve it, in order to model compact stars in $f(T)$ gravity without the need to adopt any particular form for the metric functions or consider any perturbative approach, as has been done in some works in the literature.

We investigate the magnetic nanoparticles (fluid) hyperthermia in non-adiabatic conditions through the calorimetric method. Specifically, we propose a theoretical approach to magnetic hyperthermia from a thermodynamic point of view. To test the robustness of the approach, we perform hyperthermia experiments and analyze the thermal behavior of magnetite and magnesium ferrite magnetic nanoparticles dispersed in water submitted to an alternating magnetic field. From our findings, besides estimating the specific loss power value from a non-adiabatic process, thus enhancing the accuracy in the determination of this quantity, we provide physical meaning to parameters found in literature that still remained not fully understood, and bring to light how they can be obtained experimentally., Comment: 3 figures

We propose experimental measurements of the logarithmic negativity, which quantifies quantum correlations using Gouy phase measurements in an asymmetric double-slit interference experiment for twin photons. This is possible because both quantities have analogous dependence with the spatial confinement by the slits and enables one to manipulate the portion of entanglement by the Gouy phase. In order to obtain those measurements, we need to work in a regime where the position correlations between particles are strong, therefore we investigate such correlations for biphotons. Since we would like to handle entanglement quantifiers through the Gouy phase, we analyze the Gouy phase difference for two entangled photons in an asymmetric double-slit interference experiment.

We systematically investigated and quantified how gold (Au) metal nanoparticles (NPs) optical spectra change upon introduction into biological tissue phantoms environment, in which the AuNPs can agglomerate. Quantitative knowledge of how the AuNPs spectra and plasmon resonance wavelength change inside a phantom environment can provide many in-vitro and in-vivo plasmonic NPs-mediated applications. Because the plasmonic properties of metal NPs are dependent on their size, morphology, concentration and local environment, tuning the incident photon wavelength may increase the AuNPs plasmonic properties, on which applications such as plasmonic photothermal therapy and photonic gene circuits are based. Quantitatively analyzing optical absorption and scattering data, we were able to observe changes in the resonance peak positions and breadths, which are related to the distribution of the spherical AuNPs within the chitosan phantoms as a function of the particles size and concentration., Comment: 17 pages, 3 figures, 1 table

Rheological responses are the most relevant features to describe soft matter. So far, such constitutive relations are still not well understood in terms of small scale properties, although this knowledge would help the design of synthetic and bio-materials. Here, we investigate, computational and analytically, how mesoscopic-scale interactions influence the macroscopic behavior of viscoelastic materials. We design a coarse-grained approach where the local elastic and viscous contributions can be controlled. Applying molecular dynamics simulations, we mimic real indentation assays. When elastic forces are dominant, our model reproduces the hertzian behavior. However, when friction increases, it restores the Standard Linear Solid model. We show how the response parameters depend on the microscopic elastic and viscous contributions. Moreover, our findings also suggest that the relaxation times, obtained in relaxation and oscillatory experiments, obey a universal behavior in viscoelastic materials.

Finding suitable p-type dopants, as well as reliable doping and characterization methods for the emerging wide bandgap semiconductor $\beta$-$Ga_2O_3$ could strongly influence and contribute to the development of the next generation of power electronic. In this work, we combine easily accessible ion implantation, diffusion and nuclear transmutation methods to properly incorporate the Cd dopant into the $\beta$-$Ga_2O_3$ lattice, being subsequently characterized at the atomic scale with the Perturbed Angular Correlation (PAC) technique and Density Functional Theory (DFT) simulations. The acceptor character of Cd in $\beta$-$Ga_2O_3$ is demonstrated, with Cd sitting in the octahedral Ga site in the negative charge state, showing no evidence of polaron deformations nor extra point defects nearby. Furthermore, thermally activated free electrons were observed for temperatures above ~648 K with an activation energy of 0.54(1) eV. At lower temperatures the local electron transport is dominated by a tunneling process between defect levels and the Cd probe., Comment: 11 pages, 7 figures, including Supplementary Material; Corrected affiliations

We synthesized and characterized the colloidal suspensions of $FeO \cdot Fe_{(2-x)}Nd_{x}O_{3}$ nanoparticles with $x=0.00, 0.02, 0.04, 0.06 \text{ and }0.1.$ The effect of the $Fe^{3+}$ ion replacement by $Nd^{3+}$ on the crystal structure is in-depth studied, through X-ray diffraction (XRD) and the obtained cation distribution. The magnetic properties of the synthesized $FeO \cdot Fe_{(2-x)}Nd_{x}O_{3}$ nanoparticles also were investigated and corroborated by other physical methods. A remarkable saturation magnetization of 105 $Am^{2}/kg$ was achieved for $x=0.06$., Comment: 28 pages, 8 figures, 9 tables, Last submission to Physical Review E

The goal of this paper is to study primitive groups that are contained in the union of maximal (in the symmetric group) imprimitive groups. The study of types of permutations that appear inside primitive groups goes back to the origins of the theory of permutation groups. However, this is another instance of a situation common in mathematics in which a very natural problem turns out to be extremely difficult. Fortunately, the enormous progresses of the last few decades seem to allow a new momentum on the attack to this problem. In this paper we prove that there are infinite families of primitive groups contained in the union of imprimitive groups and propose a new hierarchy for primitive groups based on that fact. In addition to the previous results and hierarchy, we introduce some algorithms to handle permutations, provide the corresponding GAP implementation, solve some open problems, and propose a large list of open problems.

The presence of multifractality in a time series shows different correlations for different time scales as well as intermittent behaviour that cannot be captured by a single scaling exponent. The identification of a multifractal nature allows for a characterization of the dynamics and of the intermittency of the fluctuations in non-linear and complex systems. In this study, we search for a possible multifractal structure (multifractality signature) of the flux variability in the quasar 3C 273 time series for all electromagnetic wavebands at different observation points, and the origins for the observed multifractality. This study is intended to highlight how the scaling behaves across the different bands of the selected candidate which can be used as an additional new technique to group quasars based on the fractal signature observed in their time series and determine whether quasars are non-linear physical systems or not. The Multifractal Detrended Moving Average algorithm (MFDMA) has been used to study the scaling in non-linear, complex and dynamic systems. To achieve this goal, we applied the backward ({\theta} = 0) MFDMA method for one-dimensional signals. We observe weak multifractal (close to monofractal) behaviour in some of the time series of our candidate except in the mm, UV and X-ray bands. The non-linear temporal correlation is the main source of the observed multifractality in the time series whereas the heaviness of the distribution contributes less., Comment: 12 pages, 51 figures, 1 table, accepted for publication in MNRAS. arXiv admin note: text overlap with arXiv:1312.3434, arXiv:1506.00586, arXiv:1506.08847 by other authors

With the ever increasing power dissipation in electrical devices, new thermal management solutions are in high demand to maintain an optimal operating temperature and efficient performance. In particular, recently developed magnetically-activated thermal switches (MATSs) provide an alternative to existing devices, using the magnetic and thermal properties of superparamagnetic nanofluids to dissipate heat in a controlled manner. However, the presence of moving parts is a major drawback in those systems that must still be addressed. Herein, we present a compact and automatized MATS composed by an encapsulated superparamagnetic nanofluid and an electromagnet allowing to activate the MATS without any moving part. We investigate the effect of different temperature gradients (40, 26 and 10 C) and powers applied to the coil (6.5, 15, 25 and 39 W) on the performance of this novel MATS. The results show that the highest (44.4$) and fastest (0.6 C/s) temperature variation occur for the highest studied temperature gradient. On the other hand, with increasing power, there is also an increase in the efficiency of the heat exchange process between the two surfaces. These results remove one of the main barriers preventing the actual application of magnetic thermal switches and opens new venues for the design of efficient thermal management devices.

Magnetic frustration and disorder are key ingredients to prevent the onset of magnetic order. In the disordered hexagonal double perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, Mn$^{4+}$ cations, with $S=3/2$ spins, can either form highly correlated states of magnetic trimers or dimers or remain as weakly interacting orphan spins. At low temperature ($T$), the dimer response is negligible, and magnetism is dominated by the trimers and orphans. To explore the role of magnetic frustration, disorder and possibly of quantum fluctuations, the low-$T$ magnetic properties of the remaining magnetic degrees of freedom of BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$ are investigated. Heat-capacity data and magnetic susceptibility display no evidence for a phase transition to a magnetically ordered phase but indicate the formation of a correlated spin state. The low-temperature spin dynamics of this state is then explored by $\mu$SR experiments. The zero field $\mu^{+}$ relaxation rate data show no static magnetism down to $T=19$ mK and longitudinal field experiments support as well that dynamic magnetism persists at low $T$. Our results are interpreted in terms of a spin glass state which stems from a disordered lattice of orphans spins and trimers. A spin liquid state in BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, however, is not excluded and is also discussed., Comment: Main Text: 6 pages, 4 figures; Supplemental: 11 pages, 7 figures

The attempt to describe the recent accelerated expansion of the Universe includes different propositions for dark energy models and modified gravity theories. Establish their features in order to discriminate and even rule out part of these models using observational data is a fundamental issue of cosmology. In the present work we consider a class of extended theories of gravity (ETG) that are minimally coupled to the ordinary matter fields. In this context, and assuming a homogeneous and isotropic spacetime, we derive the energy conditions for this ETG class. We then put constraints on these conditions using a model-independent approach to reconstruct the deceleration function along with the Joint Light-curve Analysis (JLA) supernova sample, 11 baryon acoustic oscillation and 22 cosmic-chronometer measurements. We also consider an additional condition imposing the strong energy condition only on the ordinary matter. This is to guarantee the presence of only attractive matter in the energy-momentum tensor, at least in the redshift range of the observations, i.e., the recent accelerated expansion of the Universe is due solely to the modifications in the gravity theory. The main result of this work is a general reconstruction of the energy conditions valid for every considered ETG., Comment: 20 pages, 5 figures

Among the various possibilities to probe the theory behind the recent accelerated expansion of the universe, the energy conditions (ECs) are of particular interest, since it is possible to confront and constrain the many models, including different theories of gravity, with observational data. In this context, we use the ECs to probe any alternative theory whose extra term acts as a cosmological constant. For this purpose, we apply a model-independent approach to reconstruct the recent expansion of the universe. Using Type Ia supernova, baryon acoustic oscillations and cosmic-chronometer data, we perform a Markov Chain Monte Carlo analysis to put constraints on the effective cosmological constant $\Omega^0_{\rm eff}$. By imposing that the cosmological constant is the only component that possibly violates the ECs, we derive lower and upper bounds for its value. For instance, we obtain that $0.59 < \Omega^0_{\rm eff} < 0.91$ and $0.40 < \Omega^0_{\rm eff} < 0.93$ within, respectively, $1\sigma$ and $3\sigma$ confidence levels. In addition, about 30\% of the posterior distribution is incompatible with a cosmological constant, showing that this method can potentially rule it out as a mechanism for the accelerated expansion. We also study the consequence of these constraints for two particular formulations of the bimetric massive gravity. Namely, we consider the Visser's theory and the Hassan and Roses's massive gravity by choosing a background metric such that both theories mimic General Relativity with a cosmological constant. Using the $\Omega^0_{\rm eff}$ observational bounds along with the upper bounds on the graviton mass we obtain constraints on the parameter spaces of both theories., Comment: 11 pages, 4 figures, 1 table

We have revisited the mean-field treatment for the Blume-Capel model under the presence of a discrete random magnetic field as introduced by Kaufman and Kanner. The magnetic field ($H$) versus temperature ($T$) phase diagrams for given values of the crystal field $D$ were recovered in accordance to Kaufman and Kanner original work. However, our main goal in the present work was to investigate the distinct structures of the crystal field versus temperature phase diagrams as the random magnetic field is varied because similar models have presented reentrant phenomenon due to randomness. Following previous works we have classified the distinct phase diagrams according to five different topologies. The topological structure of the phase diagrams is maintained for both $H-T$ and $D-T$ cases. Althoug the phase diagrams exhibit a richness of multicritical phenomena we did not found any reentrant effect as have been seen in similar models., Comment: 21 pages, 5 figures

We report perturbed angular correlation measurements with 111mCd/111Cd and 111In/111Cd probes, at the ISOLDE-CERN facility, in the manganite compounds BaMnO3 , with the 6H and 15R polymorphs, and SrMnO3 , with the 4H polymorph. The electric field gradient (EFG) is measured, and found approximately constant in a large temperature range for all the compounds. The EFG is also calculated from first principles with density functional theory, and compared with experimental results by considering diluted substitutional Cd impurities. Based on the results, we assign as sites for the probes the Ba (for BaMnO3-6H, 15R) and Sr (for SrMnO3-4H) sites, apart from fractions of undetermined origin in the case of BaMnO3-6H. We predict the hyperfine parameters in the recently synthesized multiferroic manganite Sr0.5Ba0.5MnO3 , and its variation with the structure and electric polarization, which is found to be very small., Comment: 13 pages, 9 figures

In this work, we derive an expression for the correction in the Newton's law of gravitation due to the gravitational Kaluza-Klein states in a general thick string-like braneworld scenario in six dimensions. In order to analyze corrections to Newton's law we study the gravity fluctuations in a 3-brane placed in a transverse resolved conifold and use suitable numerical methods to attain the massive spectrum and the corresponding eigenfunctions. Such braneworld model has a resolution parameter which removes the conical singularity. The correction has an exponentially suppressed mass term and depends on the values of the eigenfunctions and warp factors computed at the core peak of the brane. The spectrum is real and monotonically increased, as desired. However, the resolution parameter must assume moderate values to have physically acceptable states. Moreover, the trapped massless mode regains the 4D gravity and it is displaced from the origin, sharing similar profile with the energy density of brane for small values of resolution parameter. Finally, for the singular conifold, we found that a non-first eigenstate is a resonant mode. Such excited state is the largest contributor to corrections in the Newtonian potential., Comment: 21 pages, 12 figures. 5 new figures. Section V modified in order to include more details and analysis of experimental bounds. Some references added

We use Monte Carlo simulations and finite-size scaling theory to investigate the phase transition and critical behavior of the $S$-state block voter model on square lattices. It is shown that the system exhibits an order-disorder phase transition at a given value of the noise parameter, which changes from a continuous transition for $S\le4$ to a discontinuous transition for $S\ge5$. Moreover, for the cases of continuous transition, the calculated critical exponents indicate that the present studied nonequilibrium model system is in the same universality class of its counterpart equilibrium two-dimensional S-state Potts model. We also provide a first estimation of the long-range exponents governing the dependence on the range of interaction of the magnetization, the susceptibility, and the derivative of Binder's cumulant., Comment: 6 pages, 6 figures, 2 table

We examine a new class of CPT-even and dimension-five nonminimal interactions between fermions and photons, deprived of higher-order derivatives, yielding electric dipole moment and magnetic dipole moment in the context of the Dirac equation. These couplings are Lorentz-violating nonminimal structures, composed of a rank-2 tensor, the electromagnetic tensor, and gamma matrices, being addressed in its axial and non-axial hermitian versions. We use the electron's anomalous magnetic and electric dipole moment measurements to reach upper bounds of $1$ part in $10^{11}$ and $10^{16}$ GeV$^{-1}$., Comment: Presented at the Seventh Meeting on CPT and Lorentz Symmetry, Bloomington, Indiana, June 20-24, 2016

There is a host of alternative theories of gravitation in the literature, among them the $f(R,T)$ recently elaborated by Harko and collaborators. In these theories the $R$ and $T$ are respectively the Ricci scalar and the trace of the energy momentum tensor. There is already in literature a series of studies of different forms of the $f(R,T)$ functions as well as their cosmological consequences. However, there is not so far in the literature studies related to the gravitational waves in $f(R,T)$ gravity. Here we consider such an issue, in particular studying the putative extra polarization models that can well appear in such theories. To do that, we consider different functional forms for $f(R,T)$., Comment: 7 pages, no figures, accepted for publication in Physical Review D

We study a binary system composed of point particles of unequal masses in eccentric orbits in the linear regime of the characteristic formulation of general relativity, generalising a previous study found in the literature in which a system of equal masses in circular orbits is considered. We also show that the boundary conditions on the time-like world tubes generated by the orbits of the particles can be extended beyond circular orbits. Concerning the power lost by the emission of gravitational waves, it is directly obtained from the Bondi's News function. It is worth stressing that our results are completely consistent, because we obtain the same result for the power derived by Peters and Mathews, in a different approach, in their seminal paper of 1963. In addition, the present study constitutes a powerful tool to construct extraction schemes in the characteristic formalism to obtain the gravitational radiation produced by binary systems during the inspiralling phase., Comment: 16 pages, 2 figures, DOI 10.1007/s10714-016-2038-1

Control of the magnetization vector in ferromagnetic films and heterostructures by using electric tools instead of external magnetic fields can lead to low-power memory devices. We observe the robust changes in magnetization states of a thin (about 30 nm) film of alpha-Fe covered by the naturally formed layer ( about 6 nm in thickness) of iron ohyhydroxides (FeOOH) under discharging a capacitor through the film. Strikingly, the magnetization vector is switchable by the discharge even with no any biasing field at room temperatures. In this electrically induced magnetization switching (EIMS) we reveal the key role of the FeOOH layer. We demonstrate experimental evidences that not the discharge current itself but the electric field (of the order of 10 kV/m) generated by this current is responsible for EIMS. The results reported here provide a plausible explanation of the observed phenomenon in terms of electric-field-induced weak ferromagnetism in the FeOOH layer and its coupling with the underlying alpha-Fe.

From the field equations in the linear regime of the characteristic formulation of general relativity, Bishop, for a Schwarzschild's background, and M\"adler, for a Minkowski's background, were able to show that it is possible to derive a fourth order ordinary differential equation, called master equation, for the $J$ metric variable of the Bondi-Sachs metric. Once $\beta$, another Bondi-Sachs potential, is obtained from the field equations, and $J$ is obtained from the master equation, the other metric variables are solved integrating directly the rest of the field equations. In the past, the master equation was solved for the first multipolar terms, for both the Minkowski's and Schwarzschild's backgrounds. Also, M\"adler recently reported a generalisation of the exact solutions to the linearised field equations when a Minkowski's background is considered, expressing the master equation family of solutions for the vacuum in terms of Bessel's functions of the first and the second kind. Here, we report new solutions to the master equation for any multipolar moment $l$, with and without matter sources in terms only of the first kind Bessel's functions for the Minkowski, and in terms of the Confluent Heun's functions (Generalised Hypergeometric) for radiative (nonradiative) case in the Schwarzschild's background. We particularize our families of solutions for the known cases for $l=2$ reported previously in the literature and find complete agreement, showing the robustness of our results., Comment: 19 pages

CoFe2O4 nanoparticles were synthesized using gelatin as a polymerizing agent. Structural, morphological and magnetic properties of samples treated at different temperatures were investigated by X-ray diffraction, scanning electron microscopy, Mossbauer spectroscopy and magnetization measurements. Our results revealed that the samples annealed at 623 K and temperatures above 973 K have a cation distributions given by (Co0.19Fe0.81)[Co0.81Fe1.19]O4 and (Co0.06Fe0.94)[Co0.94Fe1.06]O4, respectively. The particle sizes varied from 73 to 296 nm and the magnetocrystalline anisotropy, K1, has values ranging from 2.60x10^6 to 2.71x10^6 J/m3, as determined from the law of approach to saturation applied to the MxH data at high field. At 5 K, the saturation magnetization, coercive field and (BH)max varied from 76 to 95 Am2/kg, 479.9 to 278.5 kA/m and 9.7 to 20.9 kJ/m3, respectively. The reported values are in good agreement with near-stoichiometric cobalt ferrite samples., Comment: To be published in Ceramics International: 10.1016/j.ceramint.2015.05.148

In the lifetime of a magnetic refrigerator the materials are subjected to millions of thermal magnetic cycles, so it is of the utmost importance to predict the behavior of such materials when subjected to related work conditions. Thus, in this work we investigated the influence of thermal cycling in the microstructure, magnetic phase transition and magnetic entropy change of a Gd5Si1.3Ge2.7 thin film up to 1000 cycles. In a first stage, till 450 cycles, the hysteresis area of the magnetization curves as a function of temperature was found to decrease 16% with thermal cycling, due to an arresting of the O(II) phase caused by internal strain. Nonetheless, after 1000 cycles there is a clear loss of the magneto-structural transition. For all thermal cycles the O(I) phase remains unchanged. Therefore, prolonged thermal cycling leads to a strong reduction of the O(II) phase, attributed to an internal pressure build-up, caused by the large number of expansion/compression cycles that the O(II) phase undergoes across the magnetostructural transition., Comment: 12 pages, 3 figures, Submitted to Journal of Physics D: Applied Physics

The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 DSm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 minutes of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process., Comment: 15 pages, 6 figures, Submitted to Materials & Design

We investigate the cosmological production of gravitational waves in a nonsingular flat cosmology powered by a "running vacuum" energy density described by $\rho_{\Lambda}\equiv\rho_{\Lambda}(H)$, a phenomenological expression potentially linked with the renormalization group approach in quantum field theory in curved spacetimes. The model can be interpreted as a particular case of the class recently discussed by Perico et al. (Phys. Rev. D {\bf 88}, 063531, 2013) which is termed complete in the sense that the cosmic evolution occurs between two extreme de Sitter stages (early and late time de Sitter phases). {The gravitational wave equation is derived and its time-dependent part numerically integrated since the primordial de Sitter stage. The generated spectrum of gravitons is also compared with the standard calculations where an abrupt transition, from the early de Sitter to the radiation phase, is usually assumed.} It is found that the stochastic background of gravitons is very similar to the one predicted by the cosmic concordance model plus inflation except at higher frequencies ($\nu \gtrsim 100$ kHz). This remarkable signature of a "running vacuum" cosmology combined with the proposed high frequency gravitational wave detectors and measurements of the CMB polarization (B-modes) may provide a new window to confront more conventional models of inflation., Comment: 13 pages, 4 figures, uses latex, title changed. Other corrections in agreement with the accepted version in Astroparticle Physics