Nonzero Magnetic Field Research Articles

Fluctuation Relations for Dissipative Systems in Constant External Magnetic Field: Theory and Molecular Dynamics Simulations

We illustrate how, contrary to common belief, transient Fluctuation Relations (FRs) for systems in constant external magnetic field hold without the inversion of the field. Building on previous work providing generalized time-reversal symmetries for systems in parallel external magnetic and electric fields, we observe that the standard proof of these important nonequilibrium properties can be fully reinstated in the presence of net dissipation. This generalizes recent results for the FRs in orthogonal fields—an interesting but less commonly investigated geometry—and enables direct comparison with existing literature. We also present for the first time a numerical demonstration of the validity of the transient FRs with nonzero magnetic field via nonequilibrium molecular dynamics simulations of a realistic model of liquid NaCl.

Non-local terahertz photoconductivity in the topological phase of Hg1\u2212xCdxTe

We report on observation of strong non-local photoconducitivity induced by terahertz laser pulses in non-zero magnetic field in heterostructures based on Hg1−xCdxTe films being in the topological phase. While the zero-field non-local photoconductivity is negligible, it is strongly enhanced in magnetic fields ~ 0.05 T resulting in appearance of an edge photocurrent that exceeds the respective dark signal by orders of magnitude. This photocurrent is chiral, and the chirality changes every time the magnetic field or the electric bias is reversed. Appearance of the non-local terahertz photoconductivity is attributed to features of the interface between the topological film and the trivial buffer.

Исследование шумовых характеристик вертикально-излучающих лазеров с ромбовидной токовой апертурой для применения в компактном квантовом цезиевом магнитометре

The possibility of using vertical-emitting lasers with intracavity contacts (IC-VCSEL) and a rhomboidal oxide current aperture for creating a non-zero magnetic field optically pumped atomic magnetometers (OPM) with a 133Cs vapor cell for magnetoencephalographic (MEG) systems were demonstrated. Relative intensity noise (RIN) and polarization resolved RIN of the IC-VCSEL in the 895 nm range with different mirror losses (linewidth) in the frequency range from 1 Hz to 100 kHz were experimentally investigated. Lasers with low mirror loss (narrow linewidth) have polarization resolved RIN comparable to amplitude noise. For IC-VCSEL with an output optical power of 0.8 mW and a linewidth of 55 MHz, the noise level measured is 148 dB/Hz in 1 Hz bandwidth at 40 kHz frequency. The ultimate sensitivity of OPM based on two-beam MX scheme with studied VCSELs was estimated as ~ 11 fT/√Hz.

Molecular and Polymer Ln2M2 (Ln = Eu, Gd, Tb, Dy; M = Zn, Cd) Complexes with Pentafluorobenzoate Anions: The Role of Temperature and Stacking Effects in the Structure; Magnetic and Luminescent Properties.

Varying the temperature of the reaction of [{Cd(pfb)(H2O)4}+n·n(pfb)−], [Ln2(pfb)6(H2O)8]·H2O (Hpfb = pentafluorobenzoic acid), and 1,10-phenanthroline (phen) in MeCN followed by crystallization resulted in the isolation of two type of products: 1D-polymers [LnCd(pfb)5(phen)]n·1.5nMeCN (Ln = Eu (I), Gd (II), Tb (III), Dy (IV)) which were isolated at 25 °C, and molecular compounds [Tb2Cd2(pfb)10(phen)2] (V) formed at 75 °C. The transition from a molecular to a polymer structure becomes possible because of intra- and intermolecular interactions between the aromatic cycles of phen and pfb from neighboring tetranuclear Ln2Cd2 fragments. Replacement of cadmium with zinc in the reaction resulted in molecular compounds Ln2Zn2 [Ln2Zn2(pfb)10(phen)2]·4MeCN (Ln = Eu (VI), Tb (VIII), Dy (IX)) and [Gd2Zn2(pfb)10(H2O)2(phen)2]·4MeCN (VII). A new molecular EuCd complex [Eu2Cd2(pfb)10(phen)4]·4MeCN (X)] was isolated from a mixture of cadmium, zinc, and europium pentafluorobenzoates (Cd:Zn:Ln = 1:1:2). Complexes II-IV, VII and IX exhibit magnetic relaxation at liquid helium temperatures in nonzero magnetic fields. Luminescent studies revealed a bright luminescence of complexes with europium(III) and terbium(III) ions.

A search for neutron to mirror-neutron oscillations using the nEDM apparatus at PSI

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: τnn′>352 s at B′=0 (95% C.L.), τnn′>6s for 0.4μT<B′<25.7μT (95% C.L.), and τnn′/cos⁡β>9s for 5.0μT<B′<25.4μT (95% C.L.), where β is the fixed angle between the applied magnetic field and the local mirror magnetic field, which is assumed to be bound to the Earth. These new constraints are the best measured so far around B′∼10μT and B′∼20μT.

Optically pumped non-zero field magnetometric sensor for the magnetoencephalographic systems using intra-cavity contacted VCSELs with rhomboidal oxide current aperture

We demonstrate the possibility of using vertical-cavity surface emitting lasers with intracavity contacts and a rhomboidal oxide current aperture for creating compact optically pumped 133Cs atomic magnetometer operating in non-zero magnetic fields, which is promising to use in magnetoencephalographic systems. The magnetic resonance parameters were studied in a two-beam MX optically pumped magnetic field sensor scheme based on the effect of the magnetic resonance line narrowing at high laser pumping and high concentrations of alkali-metal atoms. The ultimate sensitivity of OPM was estimated by the ratio of the magnetic resonance steepness to the probe light shot noise level. It is shown that a magnetic sensor based on the vertical-cavity surface emitting lasers and a compact (0.125 cm3 volume) Cs vapor cell can achieve a shot noise-limited sensitivity better than 15 fT in 1 Hz bandwidth. Developed intracavity contacted VCSELs suitable for use in compact atomic magnetometers for magnetoencephalographic systems.

NQR and NMR spectra in the odd-parity multipole material CeCoSi

We theoretically study NQR and NMR spectra in the presence of odd-parity multipoles originating from staggered antiferromagnetic and antiferroquadrupole orderings. For the $f$-electron metal, CeCoSi, which is a candidate hosting odd-parity multipoles, we derive an effective hyperfine field acting on Co nucleus generated from electronic origin multipole moments at Ce ion under zero and nonzero magnetic fields. We elucidate that emergent odd-parity multipoles give rise to sublattice-dependent spectral splittings in NQR and NMR through the effective hyperfine coupling in the absence of the global inversion symmetry. We mainly examine behaviors of the NQR and NMR spectra in three odd-parity multipole ordered states: a $y$-type magnetic toroidal dipole order with a staggered $x$-type antiferromagnetic structure, an $xy$-type electric toroidal quadrupole order with a staggered $x^2-y^2$-type antiferroquadrupole structure, and a $z$-type electric dipole order with a staggered $3z^2-r^2$-type antiferroquadrupole structure. We show that different odd-parity multipole orders lead to different field-dependent spectral splittings in NMR, while only the $xy$-type electric toroidal quadrupole order exhibits the NQR spectral splitting. We also present possible sublattice-dependent spectral splittings for all the odd-parity multipole orders potentially activated in low-energy crystal-field levels, which will be useful to identify odd-parity order parameters in CeCoSi by NQR and NMR measurements.

On superintegrability of 3D axially-symmetric non-subgroup-type systems with magnetic fields

We extend the investigation of three-dimensional Hamiltonian systems of non-subgroup type admitting non-zero magnetic fields and an axial symmetry, namely the circular parabolic case, the oblate spheroidal case and the prolate spheroidal case. More precisely, we focus on linear and some special cases of quadratic superintegrability. In the linear case, no new superintegrable system arises. In the quadratic case, we found one new minimally superintegrable system that lies at the intersection of the circular parabolic and cylindrical cases and another one at the intersection of the cylindrical, spherical, oblate spheroidal and prolate spheroidal cases. By imposing additional conditions on these systems, we found for each quadratically minimally superintegrable system a new infinite family of higher-order maximally superintegrable systems. These two systems are linked respectively with the caged and harmonic oscillators without magnetic fields through a time-dependent canonical transformation.

Unconventional strengthening of the bipartite entanglement of a mixed spin-(1/2,1) Heisenberg dimer achieved through Zeeman splitting

The bipartite quantum and thermal entanglement is quantified within pure and mixed states of a mixed spin-(1/2,1) Heisenberg dimer with the help of negativity. It is shown that the negativity, which may serve as a measure of the bipartite entanglement at zero as well as nonzero temperatures, strongly depends on intrinsic parameters as for instance exchange and uniaxial single-ion anisotropy in addition to extrinsic parameters such as temperature and magnetic field. It turns out that a rising magnetic field unexpectedly reinforces the bipartite entanglement due to the Zeeman splitting of energy levels, which lifts a two-fold degeneracy of the quantum ferrimagnetic ground state. The maximal bipartite entanglement is thus reached within a quantum ferrimagnetic phase at sufficiently low but nonzero magnetic fields on assumption that the gyromagnetic g-factors of the spin-1/2 and spin-1 magnetic ions are equal and the uniaxial single-ion anisotropy is a half of the exchange constant. It is suggested that the heterodinuclear complex [Ni(dpt)(H$_2$O)Cu(pba)]$\cdot$2H$_2$O (pba=1,3-propylenebis(oxamato) and dpt=bis-(3-aminopropyl)amine), which affords an experimental realization of the mixed spin-(1/2,1) Heisenberg dimer, remains strongly entangled up to relatively high temperatures (about 140~K) and magnetic fields (about 140~T) being comparable with the relevant exchange constant.

Detection of human auditory evoked brain signals with a resilient nonlinear optically pumped magnetometer

Optically Pumped Magnetometers (OPMs) have been hailed as the future of human magnetoencephalography, as they enable a level of flexibility and adaptability that cannot be obtained with systems based on superconductors. While OPM sensors are already commercially available, there is plenty of room for further improvements and customization. In this work, we detected auditory evoked brain fields using an OPM based on the nonlinear magneto-optical rotation (NMOR) technique. Our sensor head, containing only optical and non-magnetizable elements, is connected to an external module including all the electronic components, placed outside the magnetically shielded room. The use of the NMOR allowed us to detect the brain signals in non-zero magnetic field environments. In particular, we were able to detect auditory evoked fields in a background field of 70 nT. We benchmarked our sensor with conventional SQUID sensors, showing comparable performance. We further demonstrated that our sensor can be employed to detect modulations of brain oscillations in the alpha band. Our results are a promising stepping-stone towards the realization of resilient OPM-based magnetoencephalography systems that do not require active compensation.

Magnetic catalysis and the chiral condensate in holographic QCD

We investigate the effect of a non-zero magnetic field on the chiral condensate using a holographic QCD approach. We extend the model proposed by Iatrakis, Kiritsis and Paredes in [1] that realises chiral symmetry breaking dynamically from 5d tachyon condensation. We calculate the chiral condensate, magnetisation and susceptibilities for the confined and deconfined phases. The model leads, in the probe approximation, to magnetic catalysis of chiral symmetry breaking in both confined and deconfined phases. In the chiral limit, mq = 0, we find that in the deconfined phase a sufficiently strong magnetic field leads to a second order phase transition from the chirally restored phase to a chirally broken phase. The transition becomes a crossover as the quark mass increases. Due to a scaling in the temperature, the chiral transition will also be interpreted as a transition in the temperature for fixed magnetic field. We elaborate on the relationship between the chiral condensate, magnetisation and the (magnetic) free energy density. We compare our results at low and moderate temperatures with lattice QCD results.

Jet quenching parameters in strongly coupled anisotropic plasmas in the presence of magnetic fields

I use the holographic gauge/gravity duality to systematically calculate the jet quenching parameters in strongly coupled anisotropic plasmas in the presence of external magnetic fields. The magnetic field breaks down spatial rotation symmetry from $SO(3)$ to $SO(2)$, leading to the presence of multiple anisotropic jet quenching parameters, which are evaluated here in two quite different holographic settings. One of them corresponds to a top-down deformation of the strongly coupled $\mathcal{N} = 4$ Super Yang-Mills plasma triggered by an external magnetic field, while the other one is a bottom-up Einstein-Maxwell-Dilaton model of phenomenological relevance for high energy peripheral heavy ion collisions, since it is able to provide a quantitative description of $(2+1)$-flavors lattice QCD thermodynamics with physical quark masses at zero and nonzero magnetic fields. I find for both models an overall enhancement of all the anisotropic jet quenching parameters with increasing magnetic fields. Moreover, I also conclude that for both models transverse momentum broadening is larger in transverse directions than in the direction of the magnetic field. Since these conclusions are shown to hold for two rather different holographic setups at finite temperature and magnetic fields, they are suggested as fairly robust features of strongly coupled anisotropic magnetized plasmas.

A new approach to search for free neutron-antineutron oscillations using coherent neutron propagation in gas

Coherent forward neutron propagation in gas is discussed as a new approach to search for neutron-antineutron oscillations (n−n¯), which violate both B and B−L conservation. We show that one can increase the probability of neutron - antineutron transitions in the presence of a nonzero external magnetic field to essentially free neutron oscillation probability by tuning the density of an appropriate mixture of gases so that the neutron optical potential of the gas cancels that from the magnetic field.

Anisotropic transport properties of a hadron resonance gas in a magnetic field

An intense transient magnetic field is produced in high energy heavy-ion collisions mostly due to the spectator protons inside the two colliding nucleus. The magnetic field introduces anisotropy in the medium and hence the isotropic scalar transport coefficients become anisotropic and split into multiple components. Here we calculate the anisotropic transport coefficients shear, bulk viscosity, electrical conductivity, and the thermal diffusion coefficients for a multicomponent Hadron- Resonance-Gas (HRG) model for a non-zero magnetic field by using the Boltzmann transport equation in a relaxation time approximation (RTA). The anisotropic transport coefficient component along the magnetic field remains unaffected by the magnetic field, while perpendicular dissipation is governed by the interplay of the collisional relaxation time and the magnetic time scale, which is inverse of the cyclotron frequency. We calculate the anisotropic transport coefficients as a function of temperature and magnetic field using the HRG model. The neutral hadrons are unaffected by the Lorentz force and do not contribute to the anisotropic transports, we estimate within the HRG model the relative contribution of isotropic and anisotropic transports as a function of magnetic field and temperature. We also give an estimation of these anisotropic transport coefficients for the hadronic gas at finite baryon chemical potential.

Radio constraints on dark matter annihilation in Canes Venatici I with LOFAR†

ABSTRACT Dwarf galaxies are dark matter (DM) dominated and therefore promising targets for the search for weakly interacting massive particles (WIMPs), which are well-known candidates for DM. The annihilation of WIMPs produces ultrarelativistic cosmic ray electrons and positrons that emit synchrotron radiation in the presence of magnetic fields. For typical magnetic field strengths (few μG) and $\mathcal {O}$(GeV–TeV) WIMP masses, this emission peaks at hundreds of MHz. Here, we use the non-detection of 150-MHz radio continuum emission from the dwarf spheroidal galaxy Canes Venatici I with the Low-Frequency Array to derive constraints on the annihilation cross-section of WIMPs into primary electron–positron and other fundamental particle–antiparticle pairs. Our main underlying assumption is that the transport of the cosmic rays can be described by the diffusion approximation, thus requiring a non-zero magnetic field strength with small-scale structure. In particular, by adopting magnetic field strengths of $\mathcal {O}(1\, \mu$G) and diffusion coefficients $\sim \!10^{27}~\rm cm^2\, s^{-1}$, we obtain limits that are comparable with those set by the Fermi Large Area Telescope using gamma-ray observations of this particular galaxy. Assuming s-wave annihilation and WIMPs making up 100 per cent of the DM density, our benchmark limits exclude several thermal WIMP realizations in the [2, 20]-GeV mass range. We caution, however, that our limits for the cross-section are subject to enormous uncertainties that we also quantitatively assess. In particular, variations on the propagation parameters or on the DM halo can shift our limits up by several orders of magnitude (in the pessimistic scenario).

A Search for Neutron to Mirror-Neutron Oscillations

It has been proposed that there could be a mirror copy of the standard model particles, restoring the parity symmetry in the weak interaction on the global level. Oscillations between a neutral standard model particle, such as the neutron, and its mirror counterpart could potentially answer various standing issues in physics today. Astrophysical studies and terrestrial experiments led by ultracold neutron storage measurements have investigated neutron to mirror-neutron oscillations and imposed constraints on the theoretical parameters. Recently, further analysis of these ultracold neutron storage experiments has yielded statistically significant anomalous signals that may be interpreted as neutron to mirror-neutron oscillations, assuming nonzero mirror magnetic fields. The neutron electric dipole moment collaboration performed a dedicated search at the Paul Scherrer Institute and found no evidence of neutron to mirror-neutron oscillations. Thereby, the following new lower limits on the oscillation time were obtained: $\tau_{nn'} > 352~$s at $B'=0$ (95% C.L.), $\tau_{nn'} > 6~\text{s}$ for all $0.4~\mu\text{T} 9~\text{s}$ for all $5.0~\mu\text{T}<B'<25.4~\mu\text{T}$ (95% C.L.), where $\beta$ is the fixed angle between the applied magnetic field and the local mirror magnetic field which is assumed to be bound to the Earth. These new constraints are the best measured so far around $B'\sim10~\mu$T, and $B'\sim20~\mu$T.

Dynamical phase transitions for the activity biased Ising model in a magnetic field

We consider large deviations of the dynamical activity—defined as the total number of configuration changes within a time interval—for mean-field and one-dimensional Ising models, in the presence of a magnetic field. We identify several dynamical phase transitions that appear as singularities in the scaled cumulant generating function of the activity. In particular, we find low-activity ferromagnetic states and a novel high-activity phase, with associated first- and second-order phase transitions. The high-activity phase has a negative susceptibility to the magnetic field. In the mean-field case, we analyse the dynamical phase coexistence that occurs on first-order transition lines, including the optimal-control forces that reproduce the relevant large deviations. In the one-dimensional model, we use exact diagonalisation and cloning methods to perform finite-size scaling of the first-order phase transition at non-zero magnetic field.

The effect of magnetic susceptibility and magnetic field strength on porosity estimates determined from low-field nuclear magnetic resonance

The presence of materials with high magnetic susceptibility are known to have an impact on 1H nuclear magnetic resonance (NMR) measurements and in laboratory data this often results in poor estimates of porosity from NMR data. To quantify and understand the cause of the poor NMR porosity estimations, in this laboratory study, we examine the effect of magnetic susceptibility, NMR measurement parameters, and NMR instrument design on NMR measurements. Data were collected with two instruments, each with a different Larmor frequency (2 MHz and 485 kHz), on water-saturated unconsolidated sediments with magnetic susceptibility values ranging from 3.6 × 10−6 to 7020 × 10−6 SI. The results show that for materials with low magnetic susceptibility (<378 × 10−6 SI), the gravimetric porosity is accurately predicted from the NMR measurements. For the samples with high magnetic susceptibility (>987 × 10−6 SI) the gravimetric porosity is poorly predicted from the 2 MHz NMR measurements made at all echo times (from 0.2 to 3.0 ms). In contrast, the gravimetric porosity is more accurately predicted at an echo time of 0.2 ms for measurements made using the 485 kHz instrument, although at larger echo times (>1.0 ms), the porosity estimate becomes poor. The 485 kHz NMR instrument has non-zero internal magnetic field gradients, similar to those found in borehole instruments, in contrast to the 2 MHz NMR instrument, which has a homogeneous applied magnetic field. We conclude that differences is the magnetic field strength and higher magnetic field inhomogeneities in the 485 kHz NMR instrument contribute to a reduction of the impact of inhomogeneities in the magnetic field caused by materials with high magnetic susceptibility, allowing for improved porosity estimation. These results indicate that NMR measurements collected at short echo times in low, inhomogeneous static fields, e.g., borehole instruments, may provide accurate estimates of porosity in water saturated sediments, even in the presence of magnetic minerals.

One-step replica-symmetry-breaking phase below the de Almeida-Thouless line in low-dimensional spin glasses.

The de Almeida-Thouless (AT) line is the phase boundary in the temperature-magnetic field plane of an Ising spin glass at which a continuous (i.e., second-order) transition from a paramagnet to a replica-symmetry-breaking (RSB) phase occurs, according to mean-field theory. Here, using field-theoretic perturbative renormalization group methods on the Bray-Roberts reduced Landau-Ginzburg-type theory for a short-range Ising spin glass in space of dimension d, we show that at nonzero magnetic field the nature of the corresponding transition is modified as follows: (a) For d-6 small and positive, with increasing field on the AT line, first, the ordered phase just below the transition becomes the so-called one-step RSB, instead of the full RSB that occurs in mean-field theory; the transition on the AT line remains continuous with a diverging correlation length. Then at a higher field, a tricritical point separates the latter transition from a quasi-first-order one, that is one at which the correlation length does not diverge, and there is a jump in part of the order parameter, but no latent heat. The location of the tricritical point tends to zero as d→6^{+}. (b) For d≤6, we argue that the quasi-first-order transition could persist down to arbitrarily small nonzero fields, with a transition to full RSB still expected at lower temperature. Whenever the quasi-first-order transition occurs, it is at a higher temperature than the AT transition would be for the same field, preempting it as the temperature is lowered. These results may explain the reported absence of a diverging correlation length in the presence of a magnetic field in low-dimensional spin glasses in some simulations and in high-temperature series expansions. We also draw attention to the similarity of the "dynamically frozen" state, which occurs at temperatures just above the quasi-first-order transition, and the "metastate-average state" of the one-step RSB phase, and discuss the issue of the number of pure states in either.

Exploring the worldline formulation of the Potts model

We revisit the issue of worldline formulations for the q-state Potts model and discuss a worldline representation in arbitrary dimensions which also allows for magnetic terms. For vanishing magnetic field we implement a Hodge decomposition for resolving the constraints with dual variables, which in two dimensions implies self-duality as a simple corollary. We present exploratory 2-d Monte Carlo simulations in terms of the worldlines, based on worm algorithms. We study both, vanishing and non-zero magnetic field, and explore q between q=2 and q=30, i.e., Potts models with continuous, as well as strong first order transitions.